Antiviral and Antimicrobial Smart Coatings: Fundamentals and Applications 0323992919, 9780323992916

Table of contents :
Front Matter
Copyright
Contributors
The world of microbes and its medical significance
Introduction
History of microbiology
Classification of bacteria
Strain development
Primary screening
Secondary screening of microorganisms
Testing the quality of novel antibiotics
Identification of microorganisms by biochemical tests
Host-pathogen interaction
Biofilm formation and associated diseases
Mechanism of biofilm formation
Diseases due to biofilm formation
Dental plaque
Corneal infections
Urinary tract infection
Medical implant diseases
Uses of bacterial biofilms
References
Biomaterials and biomimetics
Introduction
Methods or techniques used for the development of biomimetic surfaces or coatings
Electrophoretic deposition (EPD)
Plasma electrolytic oxidation (PEO)
Pulsed laser deposition (PLD)
Hydrothermal deposition
Electrospinning technique
Smart coatings
Antibacterial smart coatings
Antiviral smart coatings
Antifungal coatings
Recent advances in biomaterials and biomimetics
Outlook and conclusion
References
Further reading
Current scenario on the microbial world and associated diseases
Introduction
Impacts of bacteria on life
Common strategies of bacterial pathogens to cause infection
Adhesins
Host invasion
Collagenase
Spreading factor
Overcoming the immune system
Toxins
Quorum sensing
References
Growth of microbes and biofilm formation on various materials
Introduction
Surface characteristics features in feasible microbial adhesion
Differential charge on the surface
Surface wettability
Surface roughness
Surface topography
Surface stiffness
Perseverance of microbes on solid surfaces
Biofilm formation
Initiation of solid-liquid interface
Substratum compo sum
Conditioning film
Hydrodynamics
Aqueous medium milieu
Properties of the cell in biofilm formation
Biofilm structure
Mechanism of action of attachment to solid surfaces
Bacterial attachment
Growth of viruses on solid surfaces
Viral adsorption
References
Identification and culture test
Introduction
Staining techniques
Simple staining
Negative staining
Differential staining
Gram staining
Acid-fast staining
Albert staining
Different culture techniques
Methods of culture
Streak culture
Lawn or carpet culture
Stroke culture
Stab culture
Liquid culture
Pour-plate method
Spread-plate method
Types of culture media
Biochemical tests
Automated identification technique
MALDI-TOF
VITEK 2
Phoenix
MicroScan
Molecular methods
Molecular detection using PCR
Biofire Film Array
Loop-mediated isothermal amplification
Flow cytometry
Conclusions
References
Industrial backgrounds and microbes growth
Introduction
Biofilm formation
Reversible adherence
Irreversible adherence
Microcolony formation
Maturation
Dispersion
Biofilm resistance
Failure of antibiotics to penetrate biofilm
Oxygen gradients
Decreased growth rate
Persister cells
High rate of genetic material exchange
Extracellular DNA
Stress
Mutation
Quorum sensing
Efflux pumps and membrane protein
Human health and biofilms
Cystic fibrosis
Endocarditis
Periodontitis
Osteomyelitis
Rhinosinusitis
Infection in chronic wound
Medical device-related biofilm infections
Central venous catheters
Urinary catheters
Endotracheal tubes
Prosthetic joints
Pacemakers and heart valves
Contact lenses
Orthopedic implants
Biofilm formation in food-processing environments
Factors influencing biofilm formation in food industries
Properties of the attachment surface
Food matrix constituents
Properties of the microbial cells
Environmental conditions
Biofilm in food industries
Dairy industries
Fish processing industry
Poultry industry
Meat industry
Ready-to-eat (RTE) food industry
Control of biofilm formation in food industries
Microbiological corrosion
Common bacteria in MIC process
Sulfate-reducing bacteria
Metal-oxidizing bacteria
Iron-oxidizing bacteria (IOB)
Slime-forming bacteria
Acid-producing bacteria (APB)
Iron-reducing bacteria (IRB)
Methanogens
Oxygen-free MIC mechanisms
Extracellular electron transfer-MIC (EET-MIC)
Metabolite-MIC (M-MIC)
Biodegradation-MIC (BD-MIC)
MIC mechanisms in the presence of oxygen
MIC caused by metal-oxidizing bacteria
Oxygen concentration cell
Secretion of corrosive metabolites
Mitigation of biocorrosion
Biocides
d-Amino acids
Norspermidine
Chelators
Bacteriophage treatment
Control of biofilms and limiting their related complications
Physical control methods
Ultrasonication
Chemical control methods
Sodium hypochlorite
Hydrogen peroxide (H2O2)
Ozone
Peracetic acid
Biological control methods
Enzymes
Protease
Polysaccharide-hydrolyzing enzymes
Deoxyribonuclease I (DNase I)
Lysostaphin
Lyase
Other enzymes
Limitation of enzymatic eradication of biofilms
Phages
Inhibitors of QS system
QS inhibition
Preventing the biosynthesis of the AHL signal molecules
Biodegradation or alteration of AHL signal molecule
Interfering via analog compounds
Suppressing alarmone scheme
Antibiofilm mechanism of nanoparticles
Silver nanoparticles
Zinc oxide nanoparticles
Titanium oxide nanoparticles
Copper oxide nanoparticles
Gold nanoparticles
Organic nanoparticles
Liposomes
Dendrimers
Disruption of the cell membrane by antimicrobial peptides (AMPs)
Antimicrobial lipids (AML) as biofilm inhibitors
References
Introduction of smart coatings in various directions
Introduction
Corrosion-resistant coatings
Self-cleaning coatings
Photocatalysis-assisted self-cleaning surface
Antiicing surfaces
Chromic-based smart coatings for textiles
Corrosion sensing coatings
Antigraffiti coatings
Antifouling coatings
Intumescent coatings
Antimicrobial coatings
Smart window coatings
Summary
References
Working principles of various smart coatings on microbes/virus growth
Introduction
What are smart coatings?
Why smart coatings?
Strategies used by smart coatings to combat microbes
Antiadhesive surfaces
Contact killing of adhered microbes
Drawback of contact-killing mechanism
Biocidal release mechanism
Physical strategies
Topographic modifications
Smart and synergistic antimicrobial coatings
Self-defensive antibacterial coatings
pH-responsive self-defensive antibacterial coatings
Bacteria-secreted substance-responsive self-defensive antibacterial coatings
Synergistic antibacterial coatings
Smart kill and release antibacterial coatings
Different materials used for smart coatings
Nanoparticles
Antimicrobial polymers
Metal ion- and oxide-based antimicrobial coatings
Application of smart coating
Conclusions and future work
References
Biomimetics in smart coatings
Introduction
Acknowledged biomimetics and their applications
The lotus effect-superhydrophobic-dust repellant
The shark's skin drag reduction and bioactive coatings
Structural color coating inspired by blue butterfly wings
Pitcher plant-inspired coating-Antifouling, natural cleaner
Moth eye-inspired optically active surface coating
Honeycomb-inspired superhydrophobic and robust coatings
Gecko-inspired reversible adhesive coatings
Cicada wings-self-cleaning and antibacterial surfaces
Recent developments
Color-changing film inspired by chameleon skin
Diamond-resembling carbon coatings
Slippery liquid-infused porous coating (SLIPS)-A special kind
Summary
References
Self-cleaning coating materials
Basics of self-cleaning surfaces (SCSs)
Self-cleaning surfaces from nature
Mechanism of wettability in hydrophobic surfaces
Young's equation
Cassie-Baxter model
Wenzel model
Recent advances in superhydrophobic coatings
Superhydrophobic coatings from natural waxes
Superhydrophobic coatings from proteins
Superhydrophobic coatings from fatty acids
Superhydrophobic coatings from cellulose and its derivatives
Approaches for growing durable self-cleaning surfaces
Dip-coating technique
Electrospray/electrospinning coating
Chemical etching
Applications of self-cleaning surfaces
Blood repellent
Solar cell and water harvesting
Fabrics and textiles
Antibacterial coatings
Anticorrosion
Medical industry
Antiicing protection
Antireflective and transparent coatings
Oil water sorption and separation
Conclusions
References
Antimicrobial coatings based on polymeric materials
Introduction
Antimicrobial polymer synthesis and coating fabrication strategies
Structurally modified polymers
Antimicrobial agent incorporated polymers
Applications of the antimicrobial polymer coatings
Antimicrobial polymer coatings in the food industry
Antimicrobial polymer coatings in textile industry
Antimicrobial polymer coatings in biomedical applications
Conclusions
References
Nano based technologies for antibacterial, antifungal, and antiviral coatings
Introduction
Role of nanotechnology in smart coatings
Nanotechnology against COVID-19
An indispensable nanocoating
Nanomaterial in medicine
Nanotechnology in therapeutic and health-care applications
Nanotechnology for antibacterial coating
Physical vapor deposition (PVD) coating
Physical vapor deposition process and its principle
Plasma-assisted antibacterial coating
Process of deposition
Thermal evaporation (TE)
Chemical vapor deposition (CVD)
Sol-gel process
Biocidal sol-gel coating
Smart antibacterial coatings
Nanotechnology for antifungal coating
Layer-by-layer (LbL) technique
Antifungal edible coatings-on postharvest loss
Coatings vs films
Tablet coating
Advantages of tablet coating
Long-term antimicrobial device coatings
Nanotechnology for antiviral coating
Nanomaterials-Viral entry inhibitors
Metal nanoparticle-based antiviral strategies
Multimechanism antiviral nanomaterial-Graphene oxide
Applications: Antiviral coatings and materials
Coatings empower viricidal and antiviral properties
Antiviral polymer
Antiviral metal oxides/metal ions
Antiviral functional nanomaterials
Conclusion and future perspective
Acknowledgment
References
Further reading
Nanomaterial-based smart coatings as antimicrobials
Introduction
Strategies of antimicrobial coatings
Smart antimicrobial coatings: A bright perspective
Nanomaterials as antimicrobial coating
Smart coatings and stimuli
Some examples of smart antimicrobial coatings
Silver nanoparticles in smart coatings
Zinc nanoparticles in smart coatings
Titanium-based smart antimicrobial coatings
Chitosan-based smart antimicrobial coatings
Other nonmaterial-based smart antimicrobial coatings
Conclusions and future outlook
References
Hybrid antibacterial, antifungal, and antiviral smart coatings
Introduction
Antibacterial coatings
Antifungal coatings
Antiviral coatings
Conclusions and future perspectives
Acknowledgment
References
Edible and food-safe antiviral and antimicrobial smart coatings
Introduction
Need for edible films and coatings
Importance of antimicrobial and antiviral coatings
Edible coatings and films
Protein-based edible films
Casein
Whey proteins (WP)
Gelatin-based films
Polysaccharide-based films
Cellulose and derivatives
Starch and derivatives
Chitosan and its derivatives
Alginates
Pectin
Lipids
Wax
Free fatty acids
Resins
Conclusions
Future perspectives
References
Plant extract-based antibacterial coating: An introduction
Introduction
Antibacterial/antimicrobial properties of plant extract
Plant-based edible coating having antimicrobial/antibacterial effect
Methods of extraction
Incorporation route
Conclusions
References
Bioengineered metal-based antimicrobial nanomaterials for surface coatings
Introduction
Green nanotechnology: An overview of bioengineering of metal-based nanomaterials
Green synthesis of nanomaterials using plants
Green synthesis of nanomaterials using fungi
Green synthesis of nanomaterials using bacteria
Green synthesis of nanomaterials using algae
Antimicrobial potential of green synthesized metal-based nanomaterials
Antimicrobial potential of green synthesized silver nanomaterials
Antimicrobial potential of green synthesized gold nanomaterials
Antimicrobial potential of green synthesized selenium nanomaterials
Antimicrobial potential of green synthesized copper nanomaterials
Antimicrobial potential of green synthesized zinc nanomaterials
Recent advances in green synthesized metal-based nanomaterials for surface coatings
Conclusions and future outlook
References
Green antibacterial and antifungal smart coating
Bacterial and bacterial contamination
Green metal nanoparticles
Antifungal properties of the coatings
Smart antibacterial coatings
Types of antibacterial coatings
Smart antibacterial and antifungal coatings for various applications
Medical devices
Health care and light activated
Textile
Food packaging
Smart antiadhesive coatings
References
Green synthesis of metal nanoparticles and its antibacterial study
Introduction
Classification of nanoparticles
Preparation of metal nanoparticles (MNPs)
Green synthesis of noble MNPs
Characterization of synthesized MNPs
Antibacterial test
Applications in MNPs in everyday life
Smart nanomaterials
Summary
References
Bioactivity prospection, antimicrobial, nutraceutical, and pharmacological potentialities of Carica papaya
Introduction
Nutraceutical potentialities of Carica papaya
Antimicrobial properties
Anthelmintic property
Bacteriostatic property
Antitrichomonal activity
Pharmaceutical properties
Anticancer activity
Antidiabetic activity
Antioxidant activity
Rejuvenation activity
Contraceptive activity
Renal activity
Effect on the gastrointestinal track
Immunity booster
Antimalarial activity
Antidengue agent
Obesity control
Sickle cell anemia
Heart diseases
Hepatoprotective activity
Challenges and concluding remarks
References
Index
A
B
C
D
E
F
G
H
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M
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P
Q
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S
T
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V
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Citation preview

ANTIVIRAL AND ANTIMICROBIAL SMART COATINGS

ANTIVIRAL AND ANTIMICROBIAL SMART COATINGS Fundamentals and Applications Edited by

ADITYA KUMAR Department of Chemical Engineering, Indian Institute of Technology (IIT-ISM) Dhanbad, Dhanbad, Jharkhand, India

AJIT BEHERA Metallurgical and Materials Engineering Department, National Institute of Technology, Rourkela, Odisha, India

TUAN ANH NGUYEN Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

MUHAMMAD BILAL Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Poznan, Poland

RAM K. GUPTA Department of Chemistry, Pittsburg State University, Pittsburg, KS, United States

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-99291-6 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Stephen Jones Editorial Project Manager: Franchezca A. Cabural Production Project Manager: Maria Bernard Cover Designer: Christian Bilbow Typeset by STRAIVE, India

Contributors Muhammad Aamir Functional Nanomaterials Laboratory (FNL), Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur (AJK), Pakistan K. Abhitha Department of Polymer Science and Ruber Technology; Inter University Centre for Nanomaterials and Devices, CUSAT, Kochi, India Bilal Ahmad Department of Horticulture, MNS-University of Agriculture, Multan, Pakistan Javeed Akhtar Functional Nanomaterials Laboratory (FNL), Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur (AJK), Pakistan Mehvish Ashiq Department of Chemistry, The Women University Multan, Multan, Pakistan Sukesh Kumar Bajire Division of Microbiology and Biotechnology, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India Kiran Bala School of Biotechnology, University of Jammu; Department of Biotechnology, GGM Science College, Jammu, India Hamed Barabadi Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran Ajit Behera Department of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, India Madhulika Bhagat School of Biotechnology, University of Jammu, Jammu, India Iman Khosravi Bigdeli Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran Negar Bozorgchami Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran Lorena Cornejo-Ponce Department of Mechanical Engineering, Faculty of Engineering, University of Tarapaca, Avda, General Velasquez, Arica, Chile

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Vibha Devi Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee; Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India V. Devika Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Abhishek S. Dhoble School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India A. Geetha Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India Abdul Ghaffar Department of Biochemistry, Government College University, Faisalabad, Pakistan Sougata Ghosh Department of Physics, Faculty of Science, Kasetsart University, Bangkok, Thailand; Department of Microbiology, School of Science, RK University, Rajkot, Gujarat, India Nellaiah Hariharan Bangalore Biotech Labs Private Limited (BiOZEEN), Bangalore, India Uzma Jabeen Faculty of Basic Sciences, Sardar Bahadur Khan Women’s University, Quetta, Pakistan Muhammad Jahangeer Food and Biotechnology Research Center, PCSIR Laboratories Complex, Ferozpur Road, Lahore, Pakistan Honey John Department of Polymer Science and Ruber Technology; Inter University Centre for Nanomaterials and Devices, CUSAT, Kochi, India Renjith P. Johnson Polymer Nanobiomaterial Research Laboratory, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India Kuruvilla Joseph Department of Chemistry, Indian Institute of Space Science and Technology, Valiyamala, Thiruvananthapuram, Kerala, India Sherin Joseph Department of Polymer Science and Ruber Technology, CUSAT, Kochi, India Kamyar Jounaki Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Contributors

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Aditya Kumar Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India Preeti Kumari Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India Jeetesh Kushwaha School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Meegle S. Mathew School of Energy Materials, Mahatma Gandhi University, Kerala, India Shalini Mohan Department of Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India Hamed Morad Department of Pharmaceutics, Faculty of Pharmacy, Mazandaran University of Medical Sciences, Sari; Ramsar Campus, Mazandaran University of Medical Sciences, Ramsar, Iran Ebrahim Mostafavi Stanford Cardiovascular Institute; Department of Medicine, Stanford University School of Medicine, Stanford, CA, United States Bushra Munir Institute of Chemistry, University of Sargodha, Sargodha, Pakistan Lakshmanan Muthulakshmi Department of Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India Ehsan Nazarzadeh Zare School of Chemistry, Damghan University, Damghan, Iran Rekha Pachaiappan Department of Mechanical Engineering, Faculty of Engineering, University of Tarapaca, Avda, General Velasquez, Arica, Chile Kailash Pati Pandey School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Mukesh Pasupuleti Division of Microbiology, Central Drug Research Institute, Lucknow, India Anmiya Peter Department of Polymer Science and Ruber Technology, CUSAT, Kochi, India

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Elaheh Pishgahzadeh Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran Madhumita Priyadarsini School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Sarmad Ahmad Qamar Department of Biochemistry, University of Agriculture, Faisalabad, Pakistan Vincent Femilaa Rajan Department of Sustainable Energy Management, Stella Maris College (Autonomous), Chennai, Tamil Nadu, India Jyoti Rani School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India Sandhya Sadanandan Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India Hina Sahar Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan Prakash Kumar Sahoo Department of Chemistry, Centurion University of Technology and Management, Gajapati, Odisha, India Fatemeh Salimi Department of Cellular and Molecular Biology, School of Biology, Damghan University, Damghan, Iran Sandesh G. Sanjeeva Polymer Nanobiomaterial Research Laboratory, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India Appukuttan Saritha Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Bishwarup Sarkar College of Science, Northeastern University, Boston, MA, United States M. Rehan H. Shah Gilani Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan Rajesh P. Shastry Division of Microbiology and Biotechnology, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India Yashpal Singh School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India

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Apurba Sinhamahapatra Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India M.M. Sreejaya Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India P.J. Sreelekshmi Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Srishti Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India Sabu Thomas School of Energy Materials, Mahatma Gandhi University, Kerala, India Sirikanjana Thongmee Department of Physics, Faculty of Science, Kasetsart University, Bangkok, Thailand Hossein Vahidi Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran Kushal Yadav Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India Mahdi Yeganeh Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

CHAPTER 1

The world of microbes and its medical significance Shalini Mohana, Mukesh Pasupuletib, Ajit Beherac, Lakshmanan Muthulakshmia, and Nellaiah Hariharand a

Department of Biotechnology, Kalasalingam Academy of Research and Education, Srivilliputhur, India Division of Microbiology, Central Drug Research Institute, Lucknow, India c Department of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, India d Bangalore Biotech Labs Private Limited (BiOZEEN), Bangalore, India b

1. Introduction Microbes are regarded as ancestors of almost all living species, striving for their survival with ceaseless adaptations through evolution. Microorganisms have established enormous associations with other organisms, driven by both biotic and abiotic factors. They have been characterized as having both beneficial and harmful roles, such as serving as bio-factories for the production of various useful products, associating as commensals with plants, and infecting various hosts, both microbial and nonmicrobial. Exploratory studies in earlier times helped elucidate basic cellular function, cellular morphology and unique cellular features, besides providing tools for cellular studies and the classification scheme for microbes (Black & Black, 2018; Pelczar et al., 2001). Research into microbes was greatly accelerated by developments and improvements in microscopic techniques. After the formulation of the Germ Theory of disease, the causative agents of many diseases were identified and studied. Pure cultures of pathogenic microorganisms were simultaneously developed and characterized to determine the pathophysiology of infections. Later, the serendipitous discovery of penicillin led to a boom in the identification and development of various antibiotics, which were used as part of a rational approach to the treatment of various infections (Willey et al., 2008). One of the common mechanisms of microorganisms for establishing infection is biofilm formation. A biofilm is a community or group of microorganisms thriving together to enhance their capabilities for survival in adverse environmental conditions. Biofilms may be comprised of single or multiple species of microorganism (Table 1). Biofilms are responsible for the majority of nosocomial infections occurring in hospitals. These infections are difficult to treat because of the extensive secretion of Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00002-5

Copyright © 2023 Elsevier Inc. All rights reserved.

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Table 1 Anti-biofilm compounds reported for bacterial pathogens. Sl. No.

Bacterial pathogen

Anti-biofilm compound

Reference

1.

Stenotrophomonas maltophilia

2. 3. 4.

Streptococcus dysgalactiae P. aeruginosa P. aeruginosa

Epigallocatechin-3 gallate Ellagic acid Quercetin Halogenated furanone

5. 6. 7. 8. 9. 10. 11.

Klebsiella pneumoniae Klebsiella pneumoniae Klebsiella pneumoniae Klebsiella pneumoniae P.aeruginosa E.coli Staphylococcus epidermidis

(Vidigal et al., 2014) (D€ urig et al., 2010) (Gopu et al., 2015) (Hentzer et al., 2002) (Magesh et al., 2013)

12.

Methicillin-resistant S. aureus (MRSA) P.aeruginosa E.coli E.coli Salmonella typhimurium Streptococcus mutans Vibrio harveyi Methicillin-resistant S. aureus (MRSA)

13. 14. 15. 16. 17. 18. 19.

20. 21. 22. 23. 24.

25.

S. aureus E. coli, S. aureus, and MRSA S. aureus and P. aeruginosa Staphylococcus epidermidis Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumonaie A. baumannii

Reserpine Linoleic acid Berberine Curcumin Ciprofloxacin Tetracycline LL-37 Antimicrobial peptide Oritavancin 2-aminobenzimidazoles 7-hydroxyindole Isolimonic acid Carvacrol 7-Epiclusianone Isolimonic acid Superparamagnetic iron oxide nanoparticles (SPIONs) Gold nanoparticles Silver nanoparticles Fmoc phenylalanine Ageloxime-D Manolides

(Verderosa et al., 2019)

(Rabin et al., 2015a, 2015b)

(Bi et al., 2021)

(Singh et al., 2021)

Dihydrosventrin (DHS)

extracellular polysaccharides that inhibit penetration of antibiotic agents into the biofilm community. Pathogenic biofilms are capable of inducing chronic infections and causing serious damage to host cells. Hence, it becomes necessary to identify substances with the potential to combat biofilm formation (Costerton, 1999). This chapter describes the evolution of microbiology, classification and screening of microorganisms, biofilm formation and some diseases associated with biofilms.

World of microbes and medical significance

5

2. History of microbiology The historical construct often holds those notable accomplishments that were prospective for the panoptic evolution of the scientific arena. Hence, mostly those findings capable of widely persuading the community were commemorated, while those that led to improvisations of the former were neglected. This could be colligated with the ideas of Charles Darwin who proposed that the adaptive species were fitter for survival than the stronger and more intelligent ones. Likewise, a lot of small and big innovations contributed to the boom in what is currently called the field of microbiology (Pelczar et al., 2001). Around 400 BCE, the Greek physician Hippocrates and historian Thucydides proposed the dangers of communicable infections such as plague. People’s belief in the existence of microbes dates back to the period 98–55 BCE, when the Roman philosopher Lucretius suspected the role of microbes in the onset of diseases. The four humors, including the blood, phlegm, yellow bile and black bile, were then considered to cause diseases, as proposed by the Greek philosopher Galen (129–199). Later, the English philosopher Roger Bacon (1220–1292), the Italian physician Girolamo Fracastoro (1478–1553) and the Slovenian physician Anton von Plenciz (1762) laid emphasis on the persona of invisible living creatures responsible for diseases. In 1658, the German Jesuit scholar Athanasius Kircher referred to these invisible creatures as “worms.” Meanwhile, the bubonic plague killed millions of people in Europe in 542 and later, in 1347, all along the trade routes, creating panic among the people and causing them to remain isolated from one another. Yet, the origins of these disease could not be scientifically traced to these invisible creatures due to a lack of the tools and techniques for visualizing, analyzing, observing and identifying them. Therefore, the evolution of studies on microorganisms studies came to be inseparable from the development and application of microscopic techniques as these served as a means for proving several hypotheses concerning microorganisms (Aneja, 2007; Pelczar et al., 2001). A milestone in the field of microscopy was achieved in 1665 by the English scientist Robert Hooke, with striking microscopic observations of a slice of cork revealing hollow compartments, which he termed "cells". Hooke also made an exquisite drawing of the fungus Mucor in his book Micrographia. These served as the baseline for the emergence of the Cell theory, formulated by Matthias Schleiden and Theodore Schwann (1838–1839), which proposed that all living organisms are composed of cells that develop

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from other cells. Hooke’s microscope also served as a prototype for the many amateur compound microscopes built by Antonie van Leeuwenhoek of Holland (1632–1723). From 1673, van Leeuwenhoek communicated detailed descriptions of his microscopic observations to the Royal Society in London, terming the microorganisms he observed “very little animalcules”. A belief in spontaneous generation (or Abiogenesis) dominated the thinking of the scientific community during the early days of microbiology. This was largely due to demonstrations involving the appearance of maggots in meat exposed to air. Still, the Italian physician Francesco Redi (1626–1697) was sceptical towards the idea of spontaneous generation. He, therefore, conducted experiments in which the meat was covered with a piece of cloth to prevent flies from laying their eggs in the meat. As Redi had expected, there were no maggots in the meat, which disproved the theory of spontaneous generation. In 1749 the English priest John Needham postulated the origin of bacteria from meat exposed to hot ashes, which was refuted by the Italian naturalist Lazzaro Spallanzani (1729–1799), when he demonstrated that beef broth, sealed in their containers after being boiled, did not show any microbial growth, while the broth that was exposed to the air after boiling showed the presence of microbes, thereby proving that air was the source of the microorganisms. But the adherents of spontaneous generation claimed that growth did not occur in some of Spallanzani’s containers as air was excluded from these by sealing, implying that air was necessary for microbial growth. In order to silence them, Theodor Schwann (1810–1882) performed an experiment involving the passage of air through a red-hot tube into meat broth. As expected by Schwann, the broth showed no sign of microbial growth; but the champions of spontaneous generation maintained that heating of the air had destroyed its capacity to support life. Georg Friedrich Schroder (1810–1882) and Theodor von Dusch (1824–1890) allowed the passage of air into heat-sterilized broth in a container that was sealed using a cotton plug; microbial growth did not appear in the broth, disproving the doctrine of spontaneous generation. The French naturalist Felix Pouchet made a final attempt to prove the validity of spontaneous generation, claiming that microbial growth occurred without air, provoking Louis Pasteur (1822–1895) to perform his famous swan-neck flask experiment. Pasteur designed swan-neck flasks in which he boiled meat broth. After boiling the broth, he left the mouths of the necks open so that air could pass freely into the flasks. The meat broth in the flasks remained sterile indefinitely. However, when the necks of some of these

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Fig. 1 Pasteur’s swan-neck flask experiment.

flasks were broken, he observed that microbial growth appeared rapidly in these flasks. From these observations, Pasteur stated that air carries microorganisms, and that. the intact flasks remained sterile because the microorganisms in the air could not reach the sterile broth in the flasks as they were trapped in the long, curved necks of the flasks. This experiment of Pasteur’s settled, once and for all, the debate over spontaneous generation (Fig. 1). The findings of Pasteur were confirmed by John Tyndall (1820–1893) who demonstrated that airborne dust particles carry germs. Pasteur’s contributions to microbiology were enormous in terms of developing techniques like pasteurization in winemaking; developing an attenuated cholera vaccine with Peirre Roux (1853–1933); developing the anthrax vaccine in collaboration with Chamberland; and preparing an attenuated vaccine against rabies. Pasteur’s discoveries paved the way for the accelerated growth of microbiology. Joseph Lister (1827–1912) extended Pasture’s findings to humans and was the first to apply the Germ Theory to surgical wound infections. The infectious nature of microorganisms was proposed by the German physician Robert Koch (1843–1910) through his postulates describing the isolation, culturing, and inoculation of infectious pathogens responsible for diseases (Fig. 2). Koch worked on tuberculosis to prove his postulates and was one of the pioneers in the development of pure culture techniques. He formulated the widely used nutrient broth and nutrient agar media and, along with Walther Hesse, introduced agar as a solidifying agent for microbiological media, which is widely used even today. He also pioneered the

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Fig. 2 Illustration of Koch’s postulates.

use of the Petri dish, designed by Richard Petri (1852–1921), for developing pure or axenic cultures of microorganisms. Over the years, many diseases and their causative agents were studied in order to develop better treatments for these diseases. This period, commonly called the Golden Age of Microbiology, flourished with rapid advances in knowledge about various diseases and their pathogens. It also triggered the birth of several other areas of microbiological applications such as antibiotics, vaccines, biofertilizers, biopesticides, and many others that are indispensable to the everyday life of humans (Black & Black, 2018; Pelczar et al., 2001; Willey et al., 2008).

3. Classification of bacteria Initially, microbes were classified based on phenetic observations or those that involve phenotypic characteristics. The classification of bacteria based on the shapes of their cells is an example of this approach (Fig. 3). With the evolution of staining techniques, the categorization of microorganisms was accelerated and became more elaborate. Meanwhile, flagellation patterns were also well studied using staining techniques (Fig. 4). With the evolution of nucleic acid sequencing techniques, rRNA sequencing came to be the tool of choice for the classification of organisms. Woese, Kandler, and Wheelis (1990) used

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Fig. 3 Shapes of bacterial cells: (A) Coccus, (B) Bacillus, (C) Vibrio, (D) Spirochete, and (E) Spirillum.

Fig. 4 Flagellation pattern in bacteria: (A) Atrichous, (B) Monotrichous, (C) Amphitrichous, (D) Cephalotrichous, (E) Lophotrichous, and (F) Peritrichous.

this approach to classify cellular life-forms into three domains, namely, Archaea, Bacteria and Eukarya. In 1923, the American bacteriologist David Hendricks Bergey collected all the available information about bacteria and published the famous Bergey’s Manual of Determinative Bacteriology, which is followed even to this day for bacterial classification (Costerton, 1999; Pelczar et al., 2001).

4. Strain development Bacterial strains are subgroups of a species represented by a different name, number, and genus. They are screened by primary and secondary screening methods for isolation of the desired microorganism from soil, probiotic sources, industrial plants and other natural sources (Stanbury et al., 2013).

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4.1 Primary screening This type of screening method involves the isolation of organisms from a group based on their ability to produce certain organic compounds or antibiotics, or by using auxanography and enrichment culture techniques (Fig. 5) (Qiu et al., 2009). • Crowded plate method: Microorganisms capable of producing antibiotics could be screened by this method. Repeated subculturing or streak culturing on suitable solid media would lead to the development of zones of growth inhibition, usually clear and colorless, around specific colonies with antibiotic activity. These colonies can further be isolated and confirmed for antibiotic production (Basavaraj et al., 2010). • Indicator Dye method: Acid- or amine-synthesizing microorganisms can be detected using the Indicator Dye method. In this technique, certain pH indicator dyes like neutral red or bromothymol blue are incorporated into nutrient agar medium. The growth of producers of acids or bases leads to a change in the color of the particular indicator dye (Santoshkumar et al., 2010). • Enrichment Culture method: Microorganisms capable of producing certain useful substances but difficult to cultivate due to their special nutrient requirements are screened by the Enrichment Culture method. In order to enhance their growth, the nutrients specific to them are supplied in excess to increase their numbers to a

Fig. 5 Primary screening methods.

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detectable level. These could then be screened further for commercial use (Selvi et al., 2017). • Auxanotrophic method: Microorganisms capable of producing certain valuable extracellular substances, such as growth-stimulating factors, are screened by the auxanographic method. Minimal media are employed for the identification of these organisms (Pontecorvo, 1949). • Method of supplementing volatile organic substances: Microorganisms capable of utilizing carbon sources from volatile substrates, such as hydrocarbons and similar carbon sources, are screened by this technique. The volatile substrate in question is supplied to the lid of the petri dishes, which are incubated in an inverted position. Organisms utilizing the volatile substrate and growing on the solid nutrient medium are isolated, purified and stock cultures of these prepared, which could be utilized for further screening tests (Bakand et al., 2006). • Giant Colony method: Microorganisms capable of diffusing their products into a solid medium can be screened by this technique. The culture isolated from primary screening is inoculated into the centre of petri dishes containing sterile nutrient agar and the petri dishes incubated under appropriate conditions. Antibiotic sensitivity is measured by streaking the test organism from the edge of the agar medium to a point close to the centre, and incubating the petri dishes for growth to occur. The relative inhibition of growth of different test organisms by the antibiotic is called the inhibition spectrum, measured as the distance from the centre outward to which the clear zone extends (Richards, 1967).

4.2 Secondary screening of microorganisms Secondary screening provides information on the production potential or yield of the microorganism, confirmed by experimentation. It can provide a broad range of information pertaining to the ability of the microorganism to produce metabolites, the quality of the product, the appropriate fermentation method, and the elimination of contaminants. To evaluate the suitability of the isolated microorganism, both qualitative and quantitative analyses are generally conducted (Phothichitto et al., 2006). • Filtration method: Antibiotics that are sparingly soluble in water or improperly diffuse through solid media are screened by this method. As an example, a broth culture of Streptomyces is filtered to obtained the culture filtrate, following which

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various dilutions of the filtrate are prepared and each of these mixed separately with a volume of molten agar medium for plating. After solidification of the agar media in the petri dishes, test organisms are streaked in parallel lines and the petri dishes incubated appropriately. The inhibitory effect of antibiotics against the test organisms is measured from the clear zones obtained (Sharpe, 1995). • Liquid medium method: The determination of the exact amounts of antibiotics produced by microorganisms could be made by this method. The microorganism in question is cultured at the appropriate temperature in a nutritive medium under vigorous aeration and agitation, followed by testing for antibiotic activity (Arai & Mikami, 1972).

4.3 Testing the quality of novel antibiotics Samples are periodically withdrawn aseptically for routine analysis to learn about the following: (1) Suitability of different media for maximum antibiotic production. (2) pH for maximum growth and antibiotic production. (3) Presence of contamination. (4) Categorization of the antibiotic as traditional or novel. (5) Stability of the antibiotic at various pH levels and temperatures. (6) Solubility of the antibiotic. (7) Toxicity of the antibiotic in vivo (Gajda´cs, 2019). For greater insight, information on the following aspects needs to be obtained: (1) Influence of incubation temperature and antifoams on the fermentation. (2) Level of resistance acquired by the test organisms. (3) Effect of the antibiotic on the target organism. (4) Necessity of inclusion of a precursor for antibiotic production. (5) Adaptability to mutational changes, and other genetic studies. After the primary and secondary screening, the following criteria could be considered for evaluating the industrial potential of each microorganism isolated. • Novelty of the product obtained. • Product yields. • Physicochemical parameters and their levels for optimum growth and product formation.

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Genetic stability. Potential for chemically altering or destroying fermentation products through accumulation of adaptive enzymes at high concentrations. Suitability of medium for growth and product formation. Physical and chemical stability of the product. Toxicity of the product. Different forms, if any, of the fermentation product and the concentration of each. Species-level identification of the isolate to facilitate comparisons of growth and product yields with known microorganisms. Economic feasibility of the fermentation process (Derkx et al., 2014; Lee et al., 2005).

4.4 Identification of microorganisms by biochemical tests Both Gram-positive and Gram-negative bacteria may be further characterized using standard biochemical tests. The following are the common tests performed for identification of bacteria: • Test for catalase: The ability of the isolate to produce the enzyme catalase is analyzed in this test using hydrogen peroxide. Obligate aerobes and, sometimes, facultative anaerobes are known to produce catalase. • Test for coagulase: The ability of the isolate to produce the enzyme coagulase is analyzed in this test using blood plasma as the coagulation reaction indicator. • Test for oxidase: The ability of the isolate to produce the enzyme oxidase is analyzed in this test using a culture medium with an artificial electron donor to check for the electron transfer potential of oxidase. • Test for indole digestion: The ability of the isolate to produce tryptophanase is analyzed in this test using Kovac’s reagent, which converts indole coupled with a color change in the indicator dye. • Test for sulfur: The ability of the organism to produce cysteine desulfurase is analyzed in this test by the change in the color of the medium to black due to the production of hydrogen sulfide. • Test for urease: The ability of the organism to produce urease is analyzed in this test using urea broth, with a change in color in half an hour indicating a positive result. • Triple sugar iron test: The ability of the isolate to utilize different sources of sugar such as glucose, lactose and sucrose may be analyzed by this test using indicators like phenol red and ferrous sugar. The nature of

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the microorganism, whether aerobic or anaerobic, may also be studied using this test. Test for nitrate reduction: The ability of the microorganism to reduce nitrate to nitrite by the production of the enzyme nitrate reductase could be analyzed using indicators such as sulfanilic acid and zinc under fermentative conditions. The test is also helpful in distinguishing Grampositive from Gram-negative bacteria. Test for starch hydrolysis: The ability of the microorganism to produce the enzyme amylase is analyzed using starch medium and iodine as indicator, with the color change to dark purple indicating a positive reaction. Methyl red test: The ability of the isolate to convert sugars to acids is analyzed in this test using methyl red as an indicator to detect the change in pH. Voges-Proskauer test: The ability of the isolate to utilize the butylene glycol pathway is analyzed in this test using the production of acetoin as an indicator. Test for citrate utilization: The ability of the isolate to produce the enzyme citrate permease that helps in the uptake of citrate as a carbon source is analyzed in this test. This reaction is confirmed using the indicator bromothymol blue, which changes in color with the progress of the reaction (Shoaib et al., 2020).

5. Host-pathogen interaction The interaction of the host and pathogen plays a vital role in the infection cycle of the pathogen. The pathogen strives to establish infection by creating several points of contact or communication with the host organism. These interactions could be analyzed at the population, organism or molecular levels. The general strategy employed by microbes for the establishment of infection includes (1) Invasion of the host by attacking its primary defense barriers. (2) Reactions leading to the compromising of the host’s defenses. (3) Multiplication of the pathogen inside the host. (4) Resistance to the immunological reactions of the host to eliminate the pathogen (Sen et al., 2016).

5.1 Biofilm formation and associated diseases Many years ago, Antonie van Leeuwenhoek observed bacteria from the plaque scraped from his own teeth. A biofilm is an uncontrolled growth

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of microorganisms on living and nonliving surfaces. Biofilms have enormous adverse impacts in health care, industry, and many other domains. In hospitals, a number of pathogenic bacteria cause severe diseases, including plaque, urinary infection, pneumonia and osteomyelitis, among others; and in industrial manufacturing settings, microorganisms thrive predominantly as biofilms.

5.2 Mechanism of biofilm formation Microbial colonies easily adhere to surfaces and cell attachment depends on the number of cells (microcolony) and extracellular substances, which facilitate the formation of a mature colony and dispersal to surrounding regions. The steps in biofilm formation are shown in Fig. 6. Biofilm formation is closely related to communication between bacterial cells using chemical signaling molecules and receptor proteins in a process called quorum sensing (Bispo et al., 2015; Passos da Silva et al., 2017). The stages in biofilm formation are (1) Attachment: This step involves both reversible and irreversible attachment of bacteria to the support material. Several cellular appendages help in the attachment or adhesion of these microcolonies, which is mainly due to van der Waals forces of attraction. Meanwhile, flagellar motility is halted and cells attach themselves securely to the exopolysaccharide (EPS) matrix. The factors influencing attachment of cells include the nature of the surface, properties of the medium and the cell surface of the microbes (Rasamiravaka et al., 2015). (2) Maturation: After attachment, the bacteria produce an extracellular matrix as a bulk fluid to ensure their survival. The cells then multiply within the vicinity and even recruit other cells from the surroundings,

Fig. 6 Stages of biofilm formation

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leading to the enlargement of the colonies into a three-dimensional structure, with channels for the passage of nutrients to the cells. (3) Dispersal: This step begins with the spread of mature clusters from the three-dimensional structure into other locations for further establishment of biofilms. This happens as the result of the necessity to overcome nutrient deficiency, intense competition and many other factors (Berhe et al., 2017; Lo´pez et al., 2010; Rabin et al., 2015a, 2015b). The components of a typical biofilm include • The Exopolysaccharide matrix, which consists of polysaccharides that help in water and nutrient retention within the biofilm. • Extracellular DNA, providing structural integrity to the threedimensional structure of the biofilm along with protection from DNase enzymes. • Adhesive proteins present in the EPS, playing a vital role in cellular communication and offering structural support to the biofilm (Kalesinskas et al., 2014).

5.3 Diseases due to biofilm formation Biofilms lead to the formation of serious infections in many parts of the body due to their resistance to many potential antibiotics. Given below are descriptions of some diseases caused by biofilm formation.

6. Dental plaque Oral infection is one of the most notable health problems as the causative agents for dental plaque generally comprise organisms that are part of the normal microflora of the host. They are considered a threat when a number of organisms form a floc that eventually turns into a biofilm comprising a community of about 500 species of bacteria colonizing the teeth. The bacterial biofilm formed on the surface of teeth cause dental caries and develop resistance to antibiotics. Among those forming biofilms, Streptococcus species, Eikenella spp., Haemophilus spp., some Prevotella spp., Capnocytophaga spp., Priopionibacterium spp., and Veillonella spp. are early colonizers, while A. actinomycetemcomitans, Prevotella intermedia, Eubacterium spp., Treponema spp., and Porphyromonas gingivalis are late colonizers that stabilize the microbial niche already established. Biofilm formation in teeth, as shown in Fig. 7, it depicts the formation of a microbial community over the enamel crown of the teeth, creating symptoms like

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Fig. 7 Biofilm in dental plaque

toothache, pain, gum weakening, and scars. Quorum sensing is enabled, which ensures effective communication between cells for the production of necessary components that aid the survival of the organisms and their infective potential. The bacteria communicate with each other using the competence-stimulating peptide, which induces genetic and protein components such as adhesins and co-adhesins for developing tolerance to adverse conditions in the bacterial colony ( Jacob, 2006; Marsh, 2004; Rosan & Lamont, 2000).

7. Corneal infections Microbial biofilms are attached to contact lenses in cluster forms and secrete extracellular substances for their stability on living and nonliving surfaces. Nowadays people use contact lenses to correct visual aberrations; yet, microbes residing on the surfaces of contact lenses could cause damage to the corneal epithelium, eventually leading to loss of vision. After the cornea is damaged, the environment becomes conducive to further invasion by pathogens and trapping of other organisms in the vicinity, leading to the formation of biofilms, as shown in Fig. 8. The slow deposition of microbes over the contact lens surface leads to the formation of a biofilm over the corneal layer closer to it. This might lead to further deterioration of the corneal layer. The most common pathogens are Pseudomonas aeruginosa, Serratia marcescens, Staphylococcus aureus, Acanthamoeba spp., and Fusarium spp. (Bispo et al., 2015).

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Fig. 8 Biofilm in corneal infection.

8. Urinary tract infection Colonization by bacteria, viruses and fungi of the linings, or the surfaces of the organs, of the urinary system is responsible for urinary tract infection (UTI). Every year, around 150 million people suffer from UTI and nearly 6 billion dollars are spent on direct health care for UTI. The uropathogens responsible for infection include E. coli, K. pneumoniae, Staphylococcus sp., Proteus sp., Pseudomonas sp., Enterococcus sp., and Enterobacter sp. (Bono & Reygaert, 2021; Odoki et al., 2019). UTI is one of the most prevalent infections occurring due to hospital sickness or nosocomial infection. Based on the severity of the infection UTI is classified as Complicated and Uncomplicated types. In an uncomplicated UTI, infection occurs in the bladder and invades the bladder mucosal lining, followed by accumulation along the urethra. The presence of organic acids, high level of urea and the prevalent pH make urine a nonideal source for the growth of any pathogen. Yet, uropathogens resist these condition by using their adhesins to bind themselves to the mucosal surface and producing toxins and extracellular enzymes to ensure colonization (Tan & Chlebicki, 2016). These pathogens are capable of forming biofilms as shown in Fig. 9. Deposition of microbial cells in the bladder may lead to the development of a biofilm capable of dispersing to other organs in the urinary system, creating symptoms such as painful and irritative urination, and increasing the probability of recurrent infection. The common symptoms of UTI include painful and frequent urination, inability to urinate and bleeding during urination (Flores-Mireles et al., 2015).

9. Medical implant diseases When patients are admitted to hospital, they are prone to suffer from biofilm- related infections. Implants, contact lenses, heart valves, intravascular catheters and artificial joints could be colonized by bacterial biofilms. Generally, a medical implant is a tool that is used for a wide range of

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Fig. 9 Biofilm in urinary tract infection.

applications like prognosis, diagnosis, treatment and rehabilitation of a disease. Based on the level of control required for handling these implants, they are classified into four classes associated with different levels of risk. The contamination of these implants could lead to the failure of these devices, infections of a chronic nature and increased morbid and mortality effects. One of the most serious bacterial infections in postoperative complications is the infection occurring at the surgical site, contributing to other nosocomial infections. Prosthetic joints, dental implants and orthopaedic devices are also prone to colonization by bacterial biofilms (Arciola et al., 2018). During treatment of implant infections, antibiotics kill the majority of the susceptible bacterial population; however, the small surviving remains as persister cells. These persister cells are capable of causing a relapsed infection and even developing into a biofilm under favorable conditions (Khatoon et al., 2018).

10. Uses of bacterial biofilms Biofilm colonies are also beneficial in cases where the communities possess potential for useful applications, as discussed below • Biofilms play an important role as biocontrol agents in the treatment of plant diseases due to their ability to enter into symbiotic relationships; they could also be used as biofertilizers for plants; and compounds such as surfactin produced by bacteria in biofilms protect plants against pathogens.

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Biofilms are exploited in bioremediation for the metabolic conversion of pollutants to nontoxic compounds. Due to their increased tolerance to contaminants compared to sessile forms, they are used for both on-site and off-site treatment of pollutants such as pesticides and oil spills, among others. • Granular activated carbon (GAC) encapsulated with bacterial biofilms is widely employed for wastewater treatment. A microbial fuel cell is one such major application utilizing bacterial biofilms as catalysts to produce energy (Edwards & Kjellerup, 2013; Muhammad et al., 2020).

References Aneja, K. R. (2007). Experiments in microbiology, plant pathology and biotechnology. India: New Age International. Arai, T., & Mikami, Y. (1972). Chromogenicity of streptomyces. Applied Microbiology, 23(2), 402–406. Arciola, C. R., Campoccia, D., & Montanaro, L. (2018). Implant infections: Adhesion, biofilm formation and immune evasion. Nature Reviews Microbiology, 16(7), 397–409. Bakand, S., Winder, C., Khalil, C., & Hayes, A. (2006). A novel in vitro exposure technique for toxicity testing of selected volatile organic compounds. Journal of Environmental Monitoring, 8(1), 100–105. Basavaraj, K. N., Chandrashekhara, S., Shamarez, A. M., Goudanavar, P. S., & Manvi, F. V. (2010). Isolation and morphological characterization of antibiotic producing actinomycetes. Tropical Journal of Pharmaceutical Research, 9(3). Berhe, N., Tefera, Y., & Tintagu, T. (2017). Review on biofilm formation and its control options. International Journal of Advanced Research in Biological Sciences, 8(4), 122–133. Bi, Y., Xia, G., Shi, C., Wan, J., Liu, L., Chen, Y., & Liu, R. (2021). Therapeutic strategies against bacterial biofilms. Fundamental Research. Bispo, P. J., Haas, W., & Gilmore, M. S. (2015). Biofilms in infections of the eye. Pathogens, 4(1), 111–136. Published 2015 Mar 23 https://doi.org/10.3390/pathogens4010111. Black, J. G., & Black, L. J. (2018). Microbiology: Principles and explorations. United Kingdom: Wiley. Bono, M. J., & Reygaert, W. C. (2021). Urinary tract infection. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/ books/NBK470195/. Costerton, J. W. (1999). Introduction to biofilm. International Journal of Antimicrobial Agents, 11(3-4), 217–221. Derkx, P. M., Janzen, T., Sørensen, K. I., Christensen, J. E., Stuer-Lauridsen, B., & Johansen, E. (2014). The art of strain improvement of industrial lactic acid bacteria without the use of recombinant DNA technology. Microbial Cell Factories, 13(1), 1–13. D€ urig, A., Kouskoumvekaki, I., Vejborg, R. M., & Klemm, P. (2010). Chemoinformaticsassisted development of new anti-biofilm compounds. Applied Microbiology and Biotechnology, 87(1), 309–317. Edwards, S. J., & Kjellerup, B. V. (2013). Applications of biofilms in bioremediation and biotransformation of persistent organic pollutants, pharmaceuticals/personal care products, and heavy metals. Applied Microbiology and Biotechnology, 97(23), 9909–9921. Flores-Mireles, A. L., et al. (2015). Urinary tract infections: Epidemiology, mechanisms of infection and treatment options. Nature Reviews. Microbiology, 13(5), 269–284. https:// doi.org/10.1038/nrmicro3432.

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Rosan, B., & Lamont, R. J. (2000). Dental plaque formation. Microbes and Infection, 2(13), 1599–1607. Santoshkumar, M., Nayak, A. S., Anjaneya, O., & Karegoudar, T. B. (2010). A plate method for screening of bacteria capable of degrading aliphatic nitriles. Journal of Industrial Microbiology and Biotechnology, 37(1), 111. Selvi, K. B., Paul, J. J. A., Vijaya, V., & Saraswathi, K. (2017). Analyzing the efficacy of phosphate solubilizing microorganisms by enrichment culture techniques. Biochemistry & Molecular Biology Journal, 3(1), 1–7. Sen, R., Nayak, L., & De, R. K. (2016). A review on host-pathogen interactions: Classification and prediction. European Journal of Clinical Microbiology & Infectious Diseases, 35(10), 1581–1599. Sharpe, A. N. (1995). Development and evaluation of membrane filtration techniques in microbial analysis. In Rapid analysis techniques in food microbiology (pp. 29–60). Boston, MA: Springer. Shoaib, M., Muzammil, I., Hammad, M., Bhutta, Z. A., & Yaseen, I. (2020). A mini-review on commonly used biochemical tests for identification of bacteria. Biology and Life Sciences, 54(1), 8. Singh, H., Gahane, A., Singh, V., Ghosh, S., & Thakur, A. (2021). Antibiofilm activity of Fmoc-phenylalanine against Gram-positive and Gram-negative bacterial biofilms. The Journal of Antibiotics, 74(6), 407–416. Stanbury, P. F., Whitaker, A., & Hall, S. J. (2013). Principles of fermentation technology. Elsevier. Tan, C. W., & Chlebicki, M. P. (2016). Urinary tract infections in adults. Singapore Medical Journal, 57(9), 485–490. https://doi.org/10.11622/smedj.2016153. Verderosa, A. D., Totsika, M., & Fairfull-Smith, K. E. (2019). Bacterial biofilm eradication agents: A current review. Frontiers in Chemistry, 7, 824. Vidigal, P. G., et al. (2014). Effects of green tea compound epigallocatechin-3-gallate against Stenotrophomonas maltophilia infection and biofilm. PLoS One, 9(4), e92876. Willey, J. M., Sherwood, L., & Woolverton, C. J. (2008). Prescott, Harley, and Klein’s microbiology. Woese, C. R., Kandler, O., & Wheelis, M. L. (1990). Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences of the United States of America, 87(12), 4576–4579.

CHAPTER 2

Biomaterials and biomimetics P.J. Sreelekshmia, V. Devikaa, M.M. Sreejayaa, Sandhya Sadanandand, Meegle S. Mathewc, Appukuttan Sarithaa, Kuruvilla Josephb, and Sabu Thomasc a

Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Department of Chemistry, Indian Institute of Space Science and Technology, Valiyamala, Thiruvananthapuram, Kerala, India c School of Energy Materials, Mahatma Gandhi University, Kerala, India d Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India b

1. Introduction Biomaterials are the pillars of the industry of medical devices. The Clemson University Advisory Board for Biomaterials defined biomaterial as “a pharmacologically and systematically inert substance which is designed for implantation within or incorporation with living systems.” It can be any material that can be used to design devices, which can work as a substitute for function of the body in a safe, dependable economic and physiologically acceptable manner (Nair & Laurencin, 2007). These biomaterials can help the body to fix and recover damaged tissues. They also help in different pharmaceutical preparations including various devices ranging from contact lenses to implants such as cardiac pacemakers (Dai et al., 1988). The essential requirement for a material to be a biomaterial is the capability of the material to implement an appropriate host response in a specific application, i.e., biocompatibility of the material (Nair & Laurencin, 2007). It includes synthetic polymer materials to biological materials such as proteins, cells, and tissues. Common biomaterials for musculoskeletal tissue engineering are shown in Fig. 1 (del Bakhshayesh et al., 2019). The biomaterials have found potential applications in tissue engineering (Shin et al., 2003) and in the making of medical devices for implantation (Teo et al., 2016). The field of biomaterials has now been concentrating on the construction and design of intelligent materials (Sakiyama-Elbert & Hubbell, 2001). Thus, several materials have been generated which can respond to their biological environment to improve device integration and tissue regeneration. Therefore, biomimetics have gained much attention in the research community. Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00007-4

Copyright © 2023 Elsevier Inc. All rights reserved.

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Fig. 1 Common biomaterials for musculoskeletal tissue engineering. (Adapted with permission from Springer. del Bakhshayesh, A. R., Asadi, N., Alihemmati, A., Tayefi Nasrabadi, H., Montaseri, A., Davaran, S., Saghati, S., Akbarzadeh, A., & Abedelahi, A. (2019). An overview of advanced biocompatible and biomimetic materials for creation of replacement structures in the musculoskeletal systems: Focusing on cartilage tissue engineering. Journal of Biological Engineering, 13 (1). https://doi.org/10.1186/s13036019-0209-9.)

The term biomimetics originated in 1950s and involves the study and mimicking of natural processes (Vincent, 2009). Biomimetics find vast application in the biomedical field. Wan and coworkers synthesized hydroxyapatite and bacterial cellulose nanocomposites by a biomimetic process, which are promising for application in tissue engineering (Wan et al., 2007). There are peptide- or protein-directed biomimetic synthesis of inorganic nanoparticles for various biomedical applications such as biosensing, ion detection, and biolabeling (W. Yang et al., 2017). Another study by Yuvakumar et al. reported that rambutan extract assisted the biomimetic synthesis of zinc oxide nanochains and their application in the treatment of liver cancer disease (Yuvakkumar et al., 2015). In addition to biomedical applications, biomimetics also find application in the making of biomimetic robots, which will be more stable than current robots and which will take advantage of new developments in materials, fabrication technologies, sensors, and actuators (Habib, Watanabe, & Izumi, 2007).

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In this chapter, we focus on various types of biomaterials used for biomimetic coatings, methods used for development of biomimetic surfaces, smart coatings, and recent advances in biomaterials and biomimetics.

2. Methods or techniques used for the development of biomimetic surfaces or coatings 2.1 Electrophoretic deposition (EPD) EPD is one of the most commonly used methods for the development of biomimetic surfaces. EPD involves the movement of powdered charged particles suspended or dispersed in a liquid medium toward an oppositely charged electrode, which then gets deposited on to it (Besra & Liu, 2007). It is classified into two types: (a) cathodic EPD, where the particles that are positively charged get deposited onto the cathode and (b) anodic EPD, where the particles that are negatively charged get deposited onto the anode (Fig. 2). EPD depends on the charge as well as on the electrophoretic mobility of the particle. Bartmanski et al. carried out the EPD of an antibacterial chitosan-silver composite coating onto a nanotubular TiO2 layer (Bartma
nski et al., 2020). For this, Ti-13Zr-13Nb alloy that possessed a TiO2 nanotubular layer was chosen as the cathode and platinum was chosen as the anode. The electrolyte

Fig. 2 Diagrammatic representation of (A) cathodic EPD and (B) anodic EPD. (Adapted with permission from Elsevier. Besra, L., & Liu, M. (2007). A review on fundamentals and applications of electrophoretic deposition (EPD). Progress in Materials Science, 52 (1), 1–61. https://doi.org/10.1016/j.pmatsci.2006.07.001.)

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consisted of chitosan and powdered Ag nanoparticles (NPs) dispersed in acetic acid. Polysorbate 20 was also added to the electrolyte in order to produce a more stable, less aggregated suspension of metallic NPs. When a direct current (DC) was applied across the electrodes, EPD of a chitosanAg nanocomposite occurred at the TiO2 layer. The formed coating was gently washed with distilled water and kept for air-drying for about 48 h at room temperature. Akhtar et al. conducted the EPD of an antibacterial and bioactive ferulic acid (FA)-loaded bioactive glass (BG)-chitosan (CS) composite coating (Akhtar et al., 2021). Here, alternating current (AC)-EPD was employed. The stainless steel AISI 316L foils were chosen as electrodes. Before performing EPD, the electrodes were first washed in ethanol-water mixture, then with deionized water and then dried. The suspension was prepared in the following manner: CS (0.5 g/L) was dissolved in a mixture of distilled water (20%) and acetic acid (1%) via constant stirring. The hydrolysis of the CS solution during EPD was prevented by adding ethanol to the solution. Then, FA (0.3, 0.5, 1.0 g/L) was added to the CS solution followed by stirring for approximately 5 min. Finally, to this CS-FA solution, BG (1.0 g/L) was added to get the suspension. The electrodes were separated by a distance of 10 mm. On applying an AC of 50 V across the electrodes EPD takes places at the cathode. The deposited coatings were dried for about 24 h at room temperature. The morphology of the coating was analyzed by scanning electron microscope (SEM) analysis. SEM revealed that the coating consisted of a CS polymer matrix with BG particles uniformly distributed onto it. Humayun et al. reported the EPD of gentamicin (GS)-loaded Zn halloysite nanotubes (HNTs)-chitosan coatings onto the surface of titanium (Ti) (Humayun et al., 2020). Gentamicin is an antibiotic for bacterial infections. GS was first loaded onto HNTs. It was then blended with CS to form CS-ZnHNTs-GS. The coating was then formed on Ti via EPD. Here, the Pt mesh electrode was taken as the anode and titanium foil was chosen as the cathode. Before performing EPD, in order to remove the surface oil, the Ti foil was ground using silicon carbide paper and then washed with acetone and distilled water. After that, it was treated with 4 N NaOH for increasing surface hydrophilicity. The suspension was prepared by first dissolving 0.1 g of CS in 100-mL distilled water. Then, 1-mL acetic acid was added to it in order to reduce the pH of the solution that is essential for chitosan protonation, which in turn favored cathodic EPD. To this solution, 0.01 g GS-ZnHNTs was added and constantly stirred for 30 min to get the suspension. The two electrodes were separated by a distance of 1 in

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Fig. 3 SEM image of (A) chitosan and (B) CS-ZnHNTs-GS. (Adapted with permission from MDPI. Humayun, A., Luo, Y., & Mills, D. K. (2020). Electrophoretic deposition of gentamicinloaded znhnts-chitosan on titanium. Coatings, 10 (10), 1–18. https://doi.org/10.3390/ coatings10100944.)

and were connected to a DC source maintained at 40 V. The EPD was performed at room temperature for about 10 min. The deposited coating was then dried. The morphology of the coating was analyzed using SEM (Fig. 3). It revealed that the coating consisted of a thin layer of chitosan on Ti with ZnHNTs uniformly distributed onto it. Singh et al. reported the EPD of iron oxide-chitosan-hydroxyapatite (HA) composite coatings on Mg alloys that were used for making biomedical implants in order to make them antibacterial and bioactive (Singh et al., 2021). Here, Mg alloy was taken as the functioning electrode and 315-L stainless steel was taken as the opposite electrode. The suspension was prepared as follows: The suspensions having different concentrations of Fe3O4 (1, 3, 5 g; wt%) were prepared. First, Fe3O4 particles were added to 2% (w/v) acetic acid solution and it was vigorously stirred for about 45 min. After that, the suspension of powdered HA was mixed with the above solution and was stirred for about 60 min. The CS solution was obtained by mixing 0.5 g/L CS with 1 vol% acetic acid, which was then followed by stirring for about 24 h. Finally, the dark brown suspension of HA and Fe3O4 was added to a mixture of ethanol (70 mL) and CS solution (30 mL) to form an Fe3O4-HA-CS suspension. The electrodes were placed at a distance of about 10 mm and a potential of about 15 V was applied across the electrodes for about 6 min. The formed coatings were kept for approximately 24 h at room temperature. These Fe3O4-HA-CS coatings exhibited

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better crystallinity when compared with HA coating. The SEM images displayed a crack-free and uniform coating having recognizable micropores. Mehnath et al. carried out the EPD of bioactive sponge minerals incorporated calcium silicate (CaSiO3) onto a Ti alloy implant, Ti-6Al-4 V, for enhancing bone regeneration (Mehnath et al., 2020). The minerals were incorporated into the calcium silicate by adding Zn, Mg, and Ce into a Ca-Si system. The bioactive sponge-mineral incorporated CaSiO3 dispersions for EPD was prepared by mixing the marine sponge, Hyatella cribriformis and mineral-incorporated CaSiO3 in two different ratios (7:3; 3:7). Each of these mixtures were added to 20 mL of tetraethyl orthosilicate (TEOS) to get the corresponding suspensions. Here, Ti plate was chosen as the cathode and platinum electrode as the anode and they were separated by a distance of 1 cm. A total of 20 mL of the prepared dispersion was used. The deposition was carried out at different potentials (10–60 V) for 2 min. Finally, the deposited samples were dried. The coating enhanced the bioactivity, cell proliferation, biocorrosion of a Ti alloy implant and also exhibited an antibacterial nature. SEM images revealed the formation of a continuous and even layer of coating on the Ti-6Al-4V substrate. Wang et al. developed mesoporous graphitic carbon nitride (g-C3N4)coated surfaces exhibiting antibacterial properties via EPD (X. Wang et al., 2020). Here, indium tin oxide (ITO) or Au film-coated glass substrates were selected as the anode. The dispersion was prepared by mixing g-C3N4 in deionized water at a concentration of 5 mg/mL. Pt mesh was taken as the cathode. A DC of 10–30 V was applied across the electrodes for about 10–30 min for the EPD of g-C3N4 onto the surface. The g-C3N4-coated surfaces exhibited more than 90% antibacterial activity against Pseudomonas aeruginosa and more than 80% antibacterial activity against Staphylococcus aureus.

2.2 Plasma electrolytic oxidation (PEO) Plasma electrolytic oxidation (PEO) or microarc oxidation (MAO) or plasma chemical oxidation (PCO) is a complex electrochemical surface-treatment process employed for producing dense, hard, and thick ceramic-like coatings on Al, Ti, Mg, and alloys having light weight (Sikdar et al., 2021). PEO is an enhanced form of anodization method in which coatings are produced on to the substrate by performing chemical, thermal, and plasma reactions on the substrate surface by applying high potentials. The PEO coatings are noticed to increase the oxidation and corrosion resistance, bioactivity, dielectric

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property, thermal stability, and biocompatibility of the coated material. PEO involves the following process: the metal substrate to which coating has to be performed is taken as the anode or working electrode and an opposite electrode usually made from graphite or stainless steel is taken as the cathode. The electrodes are dipped in a weakly alkaline electrolyte and are connected to an external electric supply. During the PEO process, evolution of oxygen (Eq. 1) and oxidation of metal (Eq. 2) occurs at the anode while evolution of hydrogen (Eq. 4) and reduction of cations (Eq. 5) occurs at the cathode. After anodic generation of oxygen, O2
anions move toward the anode and form metal oxide with the substrate. This then results in the surface dissolution or anodic oxide film formation (Eq. 3) depending upon the composition of the electrolyte, substrate, and chemical activity. The process of PEO can be represented by 2H2 O
4e
! O2 " +4H+

(1)

Me
ne
! Me+n

(2)

xMe+n +

3 x n  H2 O ! Mex On # +x n H3 Ο+ 2

(3)

2H+ + 2e
! H2 "

(4)

Cat+n + ne
! Cat0 #

(5)

Ti ! Ti4+ + 4e


(6)

Al ! Al3+ + 3e


(7)

V ! V5+ + 5e
KOH ! K + OH +

(8)





4OH ! O2 + 2H2 O + 4e

(9)


(10)

Ti4+ + 2O2
! TiO2

(11)

Ti + 4OH
! TiO2 + 2H2 O + 4e


(12)

Al3+ + O2
! Al2 O3

(13)

2V5+ + 5O2
! V2 O5

(14)

Zakaria et al. developed a titanium dioxide (TiO2) layer onto the surface of Ti-6Al-4V by PEO in order to incorporate bioactive elements such as hydroxyapatite (HAp) and fluorapatite (FAp) onto the surface (Fig. 4) (Zakaria et al., 2020). Ti-6Al-4V was taken as the anode and it was dipped

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Fig. 4 Diagrammatic representation of PEO. (Adapted with permission from MDPI. Zakaria, A., Shukor, H., Todoh, M., & Jusoff, K. (2020). Bio-functional coating on Ti6Al4V surface produced by using plasma electrolytic oxidation. Metals, 10 (9), 1–16. https:// doi.org/10.3390/met10091124.)

in the electrolyte solution containing tri-sodium orthophosphate (5 g), calcium fluoride (3 g), and potassium hydroxide (3 g) dissolved in distilled water (1000 mL). The electrolyte solution was maintained at a pH of 12.55. The electrolyte was taken in the stainless-steel beaker, which itself acted as the cathode. The PEO was performed for 10 min. During the process, dissolution of Ti, Al, and V on the substrate occurred (Eqs. 6–8). Also, ionization of KOH in the electrolyte occurred forming OH
ions (Eq. 9), which further underwent oxidation to evolve gaseous oxygen (Eq. 10). The O2 appears as either gas or O2
ions. The O2
ions will react with Ti4+ to form a TiO2 layer (Eq. 11). The reaction of Ti4+ with OH
(Eq. 12) will be less since Ti4+ is attracted more toward O2
than toward OH
. Apart from TiO2, Al2O3 and V2O5 were also formed in smaller amounts by the reaction of Al3+ (Eq. 13) and V5+ (Eq. 14) formed by dissolution with O2
. This TiO2 layer formed by PEO then promoted the incorporation of bioactive elements such as HAp and FAp onto the surface. Costa et al. reported the PEO of bioactive glass onto the surface of titanium implants in order to improve its biocompatibility, protein adsorption, antimicrobial activity, and corrosion resistance (Costa et al., 2020). The chemical constituents of the bioactive glass were SiO2, CaO, NaO, and P2O5. The PEO experimental setup was as follows: commercially pure Ti disc (cpTi) was taken as the anode and a steel tank with a cooling system was taken as the cathode. The electrolyte solution consisted of sodium

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metasilicate (0.014 M), calcium acetate (0.20 M), sodium nitrate (0.50), and sodium glycerolphosphate (0.0010 M), which provided the constituents for the formation of bioactive glass coating. The chelating agent Na2-EDTA. 2H2O was also added to the electrolyte solution. PEO was carried out for 420 s under an anodic pulse voltage of 500 V. The bioactive glass-coated Ti substrates were then washed using distilled water and dried in air. Kyrylenko et al. had employed a PEO process in order to produce bioactive coatings based on Ca and P on titanium dental implants with antibacterial activity (Kyrylenko et al., 2021). The experimental setup was as follows: cpTi was taken as the anode here. The polishing of the PEO substrate was done using silicon carbide paper and degreased using isopropanol. The electrolyte solution consisted of ethylene diamine tetraacetic acid (EDTA) as the chelating agent, calcium hydroxide or calcium formate as the calcium supplier and, potassium dihydrogen phosphate as the phosphorus supplier. The PEO process was then carried out under high-voltage power supply. Nikoomanzari et al. reported that the addition of ZrO2/TiO2 coatings via PEO had improved the antibacterial property of the Ti-6Al-4 V substrate against Gram-negative bacteria such as P. aeruginosa and E. coli (Nikoomanzari et al., 2020). For performing PEO, Ti-6Al-4 V sheet was taken as the anode and cylindrical stainless-steel container that contained the electrolyte solution was taken as the cathode. Before performing PEO, the sample was polished using 60–800 grit grinding papers, washed using distilled water, and dried with cold air blasting. The electrolyte solution consisted of distilled water, Na3PO4. 12H2O and ZrO2 NPs. The PEO process was carried out using DC pulsed power supply having maximum voltage 700 V. To analyze the processes taking place at the time of coating, voltage variation was recorded as a function of processing time. After the process, the samples were washed with distilled water and dried with cold air blasting. Thukkaram et al. introduced an antibacterial coating of porous silverdoped TiO2 coated onto the surface of Ti (Thukkaram et al., 2020). For that, Ti disc was taken as the anode and the stainless steel beaker containing the electrolyte solution chosen as the cathode The electrolytic solution comprised of NaOH (0.4 g/L), Na2HPO4(4.0 g/L), and silver nanoparticles (0.1, 0.5, 1.0 g/L). The electrolyte temperature was controlled at 25°C in order to prevent the chemical dissolution of the coating. The PEO was carried out for 5 min at a DC voltage of 400 V. Then, the coatings obtained were washed using distilled water and dried in air.

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Nagay et al. developed a TiO2 coating codoped with nitrogen and bismuth onto the implant surfaces in order to prevent bacterial infection (Nagay et al., 2019). For that, a cpTi disc was taken as the anode and stainless steel consisting of a cooling system was taken as the anode. Ammonium acetate and nitric acid were used as the base electrolytes. N-TiO2 coating was obtained by adding urea onto the electrolyte and Bi-TiO2 coating was prepared by adding bismuth nitrate onto the electrolyte. An N, Bi-TiO2 coating was prepared by adding both urea and bismuth nitrate into the electrolyte. The process was carried out using a pulsed voltage supply of 250 V for 7 min. The treated samples were washed using deionized water and dried in air. Zhou et al. introduced an antibacterial Ca/P/Al coating onto the surface of Ti-6Al-4 V (T. Zhou, Zhang, et al., 2020). The substrate was taken as the anode. The cathode was connected with a stainless-steel container. The electrolyte solution consisted of (NaPO3)6, NaAlO2, Ca(CH3COO)2H2O, and EDTA-4Na2H2O. The uniformity of the solution throughout the process was ensured by continuous stirring. The cooling system was employed to maintain the temperature of the solution below 40°C. A bipolar pulse source power equipment was used to carry out the PEO.

2.3 Pulsed laser deposition (PLD) In PLD, the irradiation of a target contained in a vacuum chamber using a laser with nanosecond pulses caused the explosive removal of the material from the target by a process called ablation (Fig. 5) (Schou, 2009). The plasma produced by laser irradiation then expanded at right angles to the surface and finally deposited onto the substrate placed in the holder system. PLD enables the stoichiometric transfer of materials from the target to the substrate film. The thickness of the films formed may lie in subnanometer or micrometer range. The films are produced in the presence of high vacuum or background gases such as oxygen or nitrogen. The entire PLD process can be explained in five steps: (a) absorption of light by the solid, (b) illumination using laser caused the plume expansion of the ablated material along 1-D, (c) free 3-D expansion into vacuum or background gas, (d) plume then retards and finally stops in a background gas, and (e) the ablated materials got deposited onto the substrate enabling film formation. Menazea et al. synthesized antibacterial chitosan/polyvinyl alcoholembedded gold nanoparticle (CS/PVA/AuNPs) films for wound-healing

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Fig. 5 Experimental setup for PLD. (Adapted with permission from MDPI. de Bonis, A., & Teghil, R. (2020). Ultra-short pulsed laser deposition of oxides, borides and carbides of transition elements. Coatings, 10 (5). MDPI AG. https://doi.org/10.3390/COATINGS10050501.)

activity (Menazea & Ahmed, 2020). The gold plate was taken as the substrate for deposition. The gold plate was kept at the bottom of the vessel containing 20 mL of CS/PVA solution. The Nd: YAG pulsed nanosecond laser was used here. The ablation was carried out by focusing the laser beam coming from the laser source at right angles to the surface of the copper plate by using a convex lens. The formed CS/PVC/AuNPs films were then transferred to a polypropylene dish. It was then kept for 5 days in a furnace for drying at 40°C temperature. Popesu-Pelin et al. conducted the deposition of fish-bone-derived biphasic calcium phosphate coatings for biomedical applications (PopescuPelin, Ristoscu, Duta, Pasuk, et al., 2020). The target taken by them was bioceramic obtained from the bones of the fishes Spartus aurata and Salmo salar. The chemically etched titanium disks or flat silicon wafers were taken as the substrate. The process was conducted inside a stainless-steel container in vapors of water under 35 Pa residual pressure. The reactant mixture was allowed to rotate continuously throughout the process in order to obtain a uniform distribution of the film. The temperature of the substrate was kept constant at 500 °C during deposition. The laser source was allowed to irradiate the sample surface at an angle of 45 degrees.The films deposited on the surface of the substrate were then subjected to thermal annealing at 500°C

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continuously for 6 h in atmospheric air enriched with water vapor in order to enhance crystallinity and regain stoichiometry. The coatings could be used as fast osteointegration implants and exhibit antibacterial activity against E. coli. Popescu-Pelin et al. introduced a cytocompatible and antimicrobial bovine hydroxyapatite-alumina-clinoptilolite composite coatings (BHA-Al2O3CLIN-5 and BHA-Al2O3-CLIN-15) via PVD (Popescu-Pelin, Ristoscu, Duta, Stan, et al., 2020). BHA-Al2O3-CLIN-5 was obtained by combining BHA, Al2O3, and CLIN powders in 90/5/5 w% ratio and for BHA-Al2O3-CLIN-15 in the weight % ratio 70/15/15. The targets were prepared by pressing the combined powders at 6 MPa in 20 mm mold followed by sintering at 500 °C for 6 h. Ti disks and Si wafers were taken as the substrate for deposition. The distance between the target and the substrate was 50 nm and the temperature of the substrate was maintained at 35°C throughout the deposition process. The coating was carried out using a pulsed laser source under water vapor pressure of 35 Pa. The deposited coatings were then subjected to thermal treatment in water vapor-enriched environment for 6 h at 500°C in order to regain the structural characteristics of the coating. Both the obtained BHA-Al2O3-CLIN coatings exhibited antibacterial activity against E. coli. Oyane et al. had introduced an antibacterial fluoride-incorporated apatite (FCP) coating on the dentin surfaces by PVD process (Oyane et al., 2020). NaCl, K2HPO4, and 1 M HCl were dissolved in ultrapure water and buffered at pH 7.4 at 25°C with tris(hydroxymethyl)aminomethane and sufficient amount of 1 M HCl to get the CP solution. The prepared CP solution was then kept in a sealed polystyrene container for about 30 days at 4°C. The solution was then filtered and 1.0 mM NaF was added to it. The solution was allowed to stir for 20 min to obtain FCP solution. The human dentin was taken as the substrate here. The substrate was placed in a poly(tetrafluoroethylene) holder and was kept at the bottom of a glass bottle. The bottle containing the substrate was then filled with the FCP solution. The bottle was placed in a water bath maintained at 25°C followed by irradiating the sample surface using a pulsed laser light source. The coated substrate was then cleaned using ultrapure water and dried in air. SEM images showed micron-sized dentinal tubules on the dentin surface before performing PVD (Fig. 6). The smoothness of the dentin surface remains unchanged after 1 min of processing. The surface started becoming rough after 5 min of processing and it increased with processing time to 30 min. It was observed that densely assembled micron subparticles covered the dentin surface.

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Fig. 6 SEM image of dentin substrate surface at different time intervals of the coating process. (Adapted with permission from Elsevier. Oyane, A., Sakamaki, I., Koga, K., Nakamura, M., Shitomi, K., & Miyaji, H. (2020). Antibacterial tooth surface created by laser-assisted pseudo-biomineralization in a supersaturated solution. Materials Science and Engineering C, 116. https://doi.org/10.1016/j.msec.2020.111170.)

Hidalgo-Robatto et al. introduced a fluor-carbonated hydroxyapatite coating via PLD for promoting cell viability and antibacterial properties (Hidalgo-Robatto et al., 2018). The target used for PLD was synthesized with a bio-ceramic produced from the enameloid of the teeth of the shark. Ti64AlV discs were used as the substrate. The atmosphere of the PLD chamber was kept at 10
4 mbar and the chamber was filled with air containing H2O vapor. The target was irradiated using a laser source at frequency of

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Antiviral and antimicrobial smart coatings

10 Hz for 120 min for carrying out PLD process. These films exhibited antibacterial activity against S. aureus and Staphylococcus epidermidis.

2.4 Hydrothermal deposition The method consisted of an aqueous solution placed in a special closed vessel maintained at high temperature and pressure (G. Yang & Park, 2019). The process involves dissolving and recrystallizing less soluble or insoluble substance under normal conditions. The experimental setup and the general steps involved in a hydrothermal method are depicted in Fig. 7. Yang et al. introduced antibacterial gentamicin (GS)-loaded polyelectrolyte multilayers and strontium-doped hydroxyapatite composite coatings on Ti-6Al-4 V alloy (K. Yang et al., 2021). First, GS/alginate (ALG) multilayers were introduced into the Ti implant via layer-by-layer assembly to form GS/ALG-coated Ti. HA doped with Sr was then coated onto this via hydrothermal deposition. For this, a solution containing 14 mmol L
1 CaNO34H2O, 1.4 mmol L
1 SrCl26H2O, 4 mmol L
1 NaHCO3, and 8.4 mmol L
1 NaH2PO4 was taken in a stainless-steel autoclave lined using Teflon. The substrate was dipped in this solution. The pH of the solution was made 8.5 by adding NaOH. Now, it was heated at 150°C for 8 h. After that, the Hap-coated substrate was washed with water and air dried.

Fig. 7 Experimental setup and the steps involved in a hydrothermal process. (Adapted with permission from MDPI. Yang, G., & Park, S. J. (2019). Conventional and microwave hydrothermal synthesis and application of functional materials: A review. Materials, 12 (7). MDPI AG. https://doi.org/10.3390/ma12071177.)

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Qianqian Li et al. introduced a composite coating of hydroxyapatite/ stearic acid, which was superhydrophobic, onto the surface of Mg alloys via hydrothermal deposition (Li et al., 2021). The coatings possessed antibacterial activity and resistance to corrosion. The Mg alloy plates having dimensions 10 mm  10 mm  2 mm was taken as the substrate. The substrate was polished using sandpaper, ultrasonicated in deionized water and ethanol, and then dried in oven. The substrate was then placed in NaOH solution (1.5 M) and then heated at 80 °C in a water bath for 1 h. Then, EDTA-2NaCa (0.2 M) and KH2PO4 (0.2 M) were configured in deionized water under magnetic stirring to form a homogeneous mixed solution. The pH of the solution was then made basic (8.5) using NaOH solution. The substrate was then placed in an autoclave holding the prepared solution and was heated at 120°C for 1 day to get Mg alloy coated with HA. The HA-coated substrate was then dipped in 0.1-M stearic acid ethanol solution at different temperatures (60°C, 80°C, 100°C) to obtain corresponding HA/stearic acid coatings. Zhou et al. performed the coating of nanohydroxyapatite/ZnO (HA/ZnO) composites onto the surface of Mg-Zn-Ca bulk metallic glass (BMG) by hydrothermal process ( J. Zhou, Li, et al., 2020). The Mg68Zn28Ca4 alloy was prepared by melting Mg, Zn, and Mg-30Ca alloy in a BN crucible subjected to an atmosphere of high argon in a highfrequency induction furnace. The Mg68Zn28Ca4 BMG rod was then formed by remelting the alloy followed by injection into a copper mold. In order to obtain the coating, an aqueous solution (pH ¼ 4) was prepared by mixing 0.1 M Ca(NO3)24H2O and 0.06 M NaH2PO42H2O. The HA/ZnO composite was then coated onto the BMG by placing the Mg68Zn28Ca4BMGs and aqueous solution in a Teflon-lined autoclave followed by heating at 125°C for 2 h. Feng et al. fabricated a nanoprism-like hydroxyapatite (HAP) coating on a porous titanium substrate by a combined biomimetic-hydrothermal method (Feng et al., 2016). Before the hydrothermal process, a HAP-seeded layer was formed on the Ti substrate via a biomimetic process. The Ti substrate was then placed vertically at the center of a reaction vessel lined using Teflon. Then, two solutions were prepared: one by mixing Ca(NO3)24H2O and chelating ligand Na.2EDTA in 25-mL deionized water and the other by mixing (NH4)2HPO4 in 25-mL deionized water. NH4OH was added to each solution to increase its pH to 9.5. The two solutions were then combined together and added to the reaction vessel sealed in a stainless-steel autoclave. The hydrothermal process was carried out for 1 day at 180°C. The coated specimen was then cooled, rinsed, and dried in an oven at 60°C.

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2.5 Electrospinning technique This technique uses electrostatic forces to produce fine fibers having thin diameters and large surface areas from polymer solutions or melt (Bhardwaj & Kundu, 2010). The electrospinning apparatus consisted of three main parts: a high voltage source, a spinneret, and a collector plate (usually a metal) (Fig. 8). The polymer solution or melt taken for electrospinning is pushed through a syringe pump, thus forming a pendant drop of the polymer at the capillary tip (Subbiah et al., 2005). Then, the polymer solution in the syringe is subjected to high voltage potential with an immersed electrode. This has led to the creation of free charges in the polymer solution. In the presence of the electric field, the pendant hemispherical polymer drop takes a cone-like projection. When the applied electric field becomes greater than the surface tension of the liquid, a jet of liquid is released from the cone tip and moves toward the collector having an opposite charge. As the liquid travels through the atmosphere the solvent will get evaporated leaving behind the polymer fiber on the collector. Lei et al. fabricated a TPU nanofiber membrane or coating containing Cu-loaded zeolite, possessing antibacterial activity via electrospinning (Lei et al., 2019). The TPU particles were combined with a solvent containing DMF and dichloromethane. The solution was then stirred for 4 h for the TPU particles to get completely dissolved. To this solution, CuX or NaX zeolites were added and stirred for 20 h. The solution was then forced through a syringe. The collector was kept at a distance of 15 cm from the

Fig. 8 Experimental setup for the electrospinning process. (Adapted with permission from Elsevier. Bhardwaj, N., & Kundu, S. C. (2010). Electrospinning: A fascinating fiber fabrication technique. Biotechnology Advances, 28 (3), 325–347. https://doi.org/10. 1016/j.biotechadv.2010.01.004.)

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needle tip. The process was carried out at a high voltage of 18 kV to produce nanofibers on the aluminum foil. Bakhsheshi-Rad et al. introduced the coating of Mg alloys with poly-Llactic acid (PLLA)-akermanite (AKT)-doxycycline (DOXY) nanofibers for antibacterial activity and corrosion resistance (Bakhsheshi-Rad et al., 2019). The AKT powders were prepared via a sol–gel method mixing tetraethyl orthosilicate, magnesium nitrate hexahydrate, calcium nitrate tetrahydrate, and nitric acid. The reaction mixture was stirred at room temperature for approximately 5 h. It was then heated for approximately 1 day at 60 °C and dried for 2 days at 120°C. The product was then ground to produce AKT nanoparticles. The solution for electrospinning was prepared by mixing 4 wt% PLLA in dichloromethane followed by the addition of AKT powders with 3 wt% PLLA solution to it. The PLLA-AKT-DOXY solution was then formed by adding DOXY to the above-prepared solution. The conditions for electrospinning are ambient temperature and 10 kV voltage. The fibers were collected on the Mg alloy (substrate) kept on the aluminum foil with 12 cm distance from the tip of the needle. SEM images revealed that the diameter of the PLLA fibers decreased on adding AKT and DOXY due to the conductivity increase or viscosity change of the electrospinning solution (Fig. 9). Tonda-Turo et al. synthesized an antibacterial gelatin (GL) cross-linked nanofiber mats that mimic the extracellular matrix for wound healing via electrospinning (green electrospinning) (Tonda-Turo et al., 2018). The nanofibrous mats were doped with gentamicin sulfate (GS) or silver nanoparticles (AgNPs) in order to make it antibacterial. The GL/AgNP solution for electrospinning was prepared by dissolving 2.5% AgNO3 in demineralized water followed by the addition of GL. The solution was then stored in an amber glass bottle with stirring for 18 h at 50°C. The GS/GL solutions were obtained at 50°C by mixing GS and GL in distilled water. The solution was then stirred for 40 min at 50°C. The electrospinning system consisted of a generator of high voltage, a volumetric pump, a mobile support syringe, and an aluminum foil as the collector. The voltage was kept at 30 kV. In order to avoid the gelation of GL with the needle, the electrospinning system was kept in a chamber that was maintained at 50°C. Jin et al. introduced an antibacterial electrospun silver ion-loaded calcium phosphate/chitosan (CS) composite fibrous membrane ( Jin et al., 2018). First, the CS solution was prepared at 40°C by mixing CS with TFA. To this, Ag-CaP was added and mixed via ultrasonication for half an hour. Electrospinning was conducted and the spinneret needle was kept at a potential of 26 kV. An aluminum foil kept at a distance of 20 cm under the needle was taken as the collector.

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Antiviral and antimicrobial smart coatings

Fig. 9 SEM image of PLLA-AKT-DOXY nanofibers. (Adapted with permission from Elsevier. Bakhsheshi-Rad, H. R., Akbari, M., Ismail, A. F., Aziz, M., Hadisi, Z., Pagan, E., Daroonparvar, M., & Chen, X. (2019). Coating biodegradable magnesium alloys with electrospun poly-L-lactic acid-åkermanite-doxycycline nanofibers for enhanced biocompatibility, antibacterial activity, and corrosion resistance. Surface and Coatings Technology, 377. https://doi.org/10.1016/j.surfcoat.2019.124898.)

Sl. no.

Material used for coating

Material on which coating is done

Method of coating

Reference

1

Chitosan-silver composite Ferulic acid bioactive glass chitosan composites Zn halloysite nanotubes-chitosan coating Iron oxide-chitosan hydroxyapatite composite Bioactive sponge minerals incorporated calcium silicate

Ti13Zr13Nb alloy Stainless steel AISI316L foils

Electrophoretic deposition Electrophoretic deposition

Bartma
nski et al. (2020) Akhtar et al. (2021)

Ti

Electrophoretic deposition

Humayun et al. (2020)

Mg alloy

Electrophoretic deposition

Singh et al. (2021)

Ti-6Al-4V alloy

Electrophoretic deposition

Mehnath et al. (2020)

2

3

4

5

Biomaterials and biomimetics

41

Sl. no.

Material used for coating

Material on which coating is done

Method of coating

Reference

6

Mesoporous graphitic carbon nitride

Electrophoretic deposition

X. Wang et al. (2020)

7

Hydroxyapatite/ graphene oxide/ collagen bioactive composites Titanium dioxide layer

Indium tin oxide or Au filmcoated glass substrates Ti16Nb alloy

Electrophoretic deposition

Yılmaz et al. (2019)

Ti-6Al-4V alloy

Zakaria et al. (2020)

9

Bioactive glass (SiO2, CaO, NaO, P2O5)

Commercially pure Ti

10

Porous silver-doped TiO2

Ti

11

Bioactive coating based Ti dental implant on Ca and P

12

ZrO2/TiO2 Coating

Ti-6Al-4V alloy

13

Commercially pure Ti

14

TiO2 coating co-doped with Nitrogen and Bismuth Ca/P/Al coating

Plasma electrolytic oxidation Plasma electrolytic oxidation Plasma electrolytic oxidation Plasma electrolytic oxidation Plasma electrolytic oxidation Plasma electrolytic oxidation

Ti-6Al-4V alloy

15

MgO/ZnO composite

AZ91 Mg alloy

16

Nano-hydroxyapatitematrix coatings containing graphene nanosheets Hydroxyapatite coatings containing Zn and Si Chitosan/ Polyvinyl alcohol

Ti6Al7Nb alloy

Plasma electrolytic oxidation Plasma electrolytic oxidation Plasma electrolytic oxidation

T. Zhou, Zhang, et al. (2020) BordbarKhiabani et al. (2019) Yigit et al. (2021)

Plasma electrolytic oxidation Pulsed laser deposition

Hwang and Choe (2018)

8

17

18

Ti-6Al-4V alloy

Au nanoparticles

Costa et al. (2020) Thukkaram et al. (2020) Kyrylenko et al. (2021) Nikoomanzari et al. (2020) Nagay et al. (2019)

Menazea and Ahmed (2020) Continued

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Antiviral and antimicrobial smart coatings

Sl. no.

Material used for coating

Material on which coating is done

Method of coating

19

Fish-bone derived biphasic calcium phosphate

Chemically etched Ti discs or flat silicon wafers

Pulsed laser ablation

20

Fluor-carbonated hydroxyapatite

Ti6Al4V

Pulsed laser deposition

21

Bovine hydroxyapatitealumina-zeolite composites

Ti disks and Si wafers

Pulsed laser ablation

22

Fluoride-incorporated apatite layer GS-loaded polyelectrolyte multilayers and strontium-doped hydroxyapatite composites Hydroxyapatite/stearic acid coating Nano prism-like hydroxyapatite coating Nano-hydroxyapatite/ ZnO coating TPU nanofiber coating containing Cu-loaded Zeolite Silver-ion-loaded calcium phosphate / chitosan composite membrane PLLA-AKT-DOXY

Dentin surface

Pulsed laser ablation Hydrothermal method

23

24 25

26 27

28

29

30

Gelatin cross-linked nanofiber mats

Ti6Al4V alloy

Mg Alloy

Reference

Popescu-Pelin, Ristoscu, Duta, Pasuk, et al. (2020) HidalgoRobatto et al. (2018) Popescu-Pelin, Ristoscu, Duta, Stan, et al. (2020) Oyane et al. (2020) K. Yang et al. (2021)

Hydrothermal method Hydrothermal method

Feng et al. (2016)

Mg-Zn-Ca bulk metallic glass Al foil

Hydrothermal method Electrospinning

J. Zhou, Li, et al. (2020) Lei et al. (2019)

Al foil

Electrospinning

Jin et al. (2018)

Mg alloy

Electrospinning

Al foil

Electrospinning

BakhsheshiRad et al. (2019) Tonda-Turo et al. (2018)

Porous Ti

Li et al. (2021)

Biomaterials and biomimetics

43

3. Smart coatings Smart coatings have captured a great deal of progress and interest in this era. In 2011, Professor Jamil Baghdachi defined smart materials as, “Materials that are capable of adapting their properties dynamically to an external stimulus.” The term smart coating refers to the concept of coatings being able to sense the environment and make an appropriate response to that stimulus. The driving force behind the development of smart coating is the continuous demand for higher performance, reduction of maintenance cost, and extension of product lifetimes. The use of smart coatings is extended to medical, transport, textile, construction, electronics, military, and so on. There are various types of smart coatings including self-healing coatings, corrosion coatings, antimicrobial coatings, etc. In this chapter, our focus is on antimicrobial coatings. Antimicrobial coating is the process of coating a surface with chemical agents that can restrict the growth of microorganisms on the surface. Diseases due to microorganisms are increasing by every hour. This plethora of microbes constitutes an arising threat to the public. This made the researchers think of antimicrobial surface coatings, which can kill microbes. There are no specific surfaces for microbes to live on; they can be anywhere including metal surfaces, our body, and even on the medical devices such as implants and contact lenses. The development of these microbes in such devices results in the formation of biofilm, which is the main reason for microbial infections. The treatment for such infections is localized. There are many different strategies or the localized treatment of infections, which includes the prevention of accumulation of pioneer bacteria, eliminating the adhering bacteria by contact with the surface, and killing of all-encompassing microbes by drug discharge from the surface. The release of antimicrobials is done even if there is no infection. This results in the damage of tissues, resistant strains of bacteria, and cytotoxicity; studies have reported that the use of stimuli-responsive antimicrobial coating can put an end to the bacterial infections and their consequences. Smart coatings have the capacity to adapt the release of antiinfection agents specifically by enhancing the local drug accumulation at the site of infection. This method prevents the aggregation of drug in host tissues and decreases the rate of cytotoxicity.

3.1 Antibacterial smart coatings Antibacterial coatings are quickly arising as an essential part of the global mitigation strategy of microbial pathogens. These types of antibacterial coatings

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Antiviral and antimicrobial smart coatings

are used to prevent bacterial infections in medical devices as well as in our body after surgery. They are also used in the packaging industry. The formation of biofilm is the major reason behind the infections, which has the ability to act as a reservoir for pathogens. A stimulus-triggered system offers a great signal and is not restricted by diffusion. Also, it has the ability to produce on-demand antibacterial effects and lead to the extension of lifetime of coatings (Cloutier et al., 2015). Various external stimuli like electrical, magnetic, ultrasonic, photothermal, and pH are reported. In addition to single stimulus, dual stimuli-responsive and antibiotic-loaded smart coatings are also reported. Several coatings like hydroxyapatite (HA), used for implantable device coatings, lack antibacterial activity; so, to upgrade the antibacterial potential of HA, Loftali et al. designed a pH- and redox-sensitive HA nanocomposite with photolytic activity (Lotfali & Meshkini, 2021). The implantable devices were coated with zinc oxide/HA with Ag nanoparticles with assisted gallic acid using an UV irradiation method. Then, the surface of the nanocomposite was conjugated by an antibacterial enzyme through a redox responsive linker. They studied the release of this lysozyme (Lyso. CAGZ@HA) from nanocomposites at pH 7.4 with and without the presence of GSH. The content of reducing sugar decreased in the presence of GSH when compared to the absence of GSH, which resulted in the reduction of the disulfide bond in nanocomposites and the release of lysozyme. The lysozyme (Lyso. CAGZ@HA) had high antibacterial activity than HA alone. The efficacy of Lyso CAGZ@HA for the irradiation of E. coli was studied and observed that there was an effective destruction and eradication of E. coli biofilm. The activity increased when the bacteria was subjected to UV irradiation. This causes biofilm degradation through bacterias’ inability to remain attached to the surface. Also, Lyso. CAGZ@HA caused cell death and cell membrane damage in the treated bacteria. Several studies showed that hydrogels modified with polydopamine nanoparticles exerted a good antibacterial activity. Gao et al. used NIR light irradiation to generate a thermosensitive hydrogel drug reservoir for wound healing and bacterial infections (G. Gao et al., 2019). They used polydopamine nanoparticles due to their high transparency, noninvasive manipulation, and accurate remote control which convert NIR light to heat indicating their excellent photothermal activity. First, they cross-linked polydopamine nanoparticles (PDANPs) with amine-rich glycol chitosan (GC) through Schiff’s base reaction and Michael addition, which results in the formation of an injectable hydrogel (PDANPs/GC hydrogel). Then,

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ciprofloxacin (Cip) was loaded on the PDANPs via hydrophobic and π-π stacking interactions and this complex was mixed with GC, which resulted in the formation of PDANP-Cip/GC hydrogel (Gel-Cip). NIR light irradiation enhanced the release of Cip from Gel-Cip and also activated the photothermal PDANPs. The treatment of Gel-Cip under NIR light irradiation can destroy both Gram-positive and Gram-negative bacteria. Also, in vivo tests confirmed that the combination of Gel-Cip and NIR light irradiation had an effective antibacterial efficacy, promoted wound healings of bacteria-infected mice. Another work by Andoy and coworkers reported that the surface modification of PDANPs with an antimicrobial peptide CWR11 under NIR-laser irradiation was responsive against the bacterial activity of E. coli (Andoy et al., 2020). Marturano et al. reported that photoresponsive nanocapsule-coated polymers can be used for antimicrobial active packaging (Marturano et al., 2019). They used thyme essential oil for coating the nanocapsules for polyethylene and polylactic acid films. The thymol concentration increased upon UV irradiation. There are cases where postsurgical infections occur; so, to avoid those, Kohler et al. proposed a new alternative method for the coating of mesh materials for hernia repair with antimicrobial compounds after implantation to avoid postsurgical infections (P
erez-K€ ohler et al., 2020). The surface of the mesh materials was coated with HApN (poly(N-isopropylacrylamide) hyaluronan) and rifampicin-loaded HApN. Results showed that drug-free hydrogel did not show any antibacterial activity while rifampicin-loaded thermoresponsive hydrogel possessed strong antibacterial activity. When coated with a drug-free HApN hydrogel alone, bacteria were found in the areas of the connective tissues near to the mesh materials whereas rifHApN possessed no bacteria in the areas of connective tissues. This antimicrobial coating completely blocked implant infection and allowed an optimal tissue integration. Manouras et al. developed an effective antimicrobial surface coating on biocidal films using chitosan modified with 3-bromo-N,N,N-trimethyl propan-1-aminium bromide in order to bear quaternary ammonium salts (Manouras et al., 2021). The coating consisted of not only modified chitosan but also TiO2 nanoparticles modified with reduced graphene oxide. Then, an acetal-based cross-linker was used to cross-link the chitosan chains and this formed a water-insoluble polymer network. The assessment of the antimicrobial action of modified chitosan and rGO-modified TiO2 under dark and visible light irradiation showed that modified chitosan increased the antimicrobial activity under dark and visible light irradiation while

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Antiviral and antimicrobial smart coatings

rGO-modified TiO2 was able to possess antimicrobial activity under visible light irradiation. The polishing behavior of the coating was assessed by varying pH. The thickness of the film was found to decrease on increasing the pH. In highly acidic media, the films were totally eliminated from the substrates. Another work by Xiao and coworkers reported that zeolitic imidazolate framework-8 (ZIF-8) encapsulated with vancomycin when modified with PDA was able to generate a dual stimulus (NIR/pH)-responsive antibacterial formulation (Xiao et al., 2021). This type of coating enhanced the stability and dispersion of ZIF-8 and could act as a potential photothermal agent under NIR light irradiation. Also, the ability of this type of coating to release vancomycin was studied under different pH values. Results showed that lower pH possessed a stronger drug release under NIR irradiation than higher pH. The antibacterial assessment showed that this coating under NIR irradiation and at pH ¼ 4.7 exhibited strongest activity. Ye and coworkers reported that a dextran-coated, stimuli-responsive nanoparticle could effectively treat serious antimicrobial-resistant infections (Ye et al., 2021). This nanoparticle consists of a biodegradable poly(β-amino ester)-guanidine-phenyl boronic acid (PBAE-G-B) polymer inside the hydrophobic core, which encapsulated the drug rifampicin. This polymer under low pH and high ROSa can be converted into a hydrophilic cation polymer that exhibits antimicrobial activity. This conversion required a sudden release of rifampicin and the resulting cationic polymer formed a linkage with rifampicin, which effectively destroyed the bacteria. The composition and the antibacterial mechanism of dual-responsive nanoparticles is represented in Fig. 10. In another study by Song and coworkers, redox and photoresponsive polydopamine ferrocene-functionalized TiO2 molecules exhibited antibacterial activity (Song et al., 2020). From previous studies, it is known that PDA coatings are redox active, thereby donating electron to oxygen to generate H2O2. This H2O2 will be channeled into hydroxyl radical through the Fenton reaction. This led to the generation of ROS. Thus, a small amount of hydroxyl radical generated from oxygen was able to kill bacteria. In addition to redox-active PDA coatings under NIR irradiation it enhances antibacterial activity. Photoresponsive PDA coatings could effectively kill 99% of bacteria and even suppress biofilm formation. Initially, the coating localized ROS production to the implant surface; then, it limited the hydroxyl generation activities and, finally, enabled the use of a second synergistic antimicrobial system based on exogenously triggered photothermal hyperthermia.

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Fig. 10 Illustration of composition and antibacterial mechanism of dual-responsive nanoparticles. (Adapted with permission from Wiley. Ye, M., Zhao, Y., Wang, Y., Zhao, M., Yodsanit, N., Xie, R., Andes, D., & Gong, S. (2021). A dual-responsive antibioticloaded nanoparticle specifically binds pathogens and overcomes antimicrobial-resistant infections. Advanced Materials, 33 (9). https://doi.org/10.1002/adma.202006772.)

3.2 Antiviral smart coatings Viruses replicate by infecting a host cell and then multiplying in great numbers, causing serious diseases, whereas bacteria restrict host cell growth in a localized area causing infections on the specific part of the human body. Thus, the bacterial infections are much easy to remove via antibacterial coatings. The initial aim of antiviral coating is to neutralize the virus before it destroys the host cell (Basak & Packirisamy, 2020). By a known virusreceptor interaction mechanism, nanomaterials, composites, biopolymer, and polymer-based coating can be developed. Butot and coworkers evaluated the antiviral activity of three coatings by contrasting the existence of HCoV-229E (Butot et al., 2021). The three coatings include quaternary ammonium compounds (QACs), ROS, and copper compounds. The ROS-based coating had no antiviral activity, whereas copper compounds and QACs induced a reduction of HCov-229E.

48

Antiviral and antimicrobial smart coatings

The antiviral activity of other two coatings against SARS-CoV-2 were then analyzed. With the increase in time, the reduction of SARS-CoV-2 was observed. The antiviral activity of copper-based coatings was due to the interaction between the surface of the virus and copper compounds, which caused a denaturation of biomolecules and thereby the inactivation of virus. The mechanism of QAC-based coating is due to the positively charged quaternized nitrogen and carbon chain spikes. The spikes attract and disrupt the bacterial cell wall, thereby leading to viral inactivation. The QAC-based coating was removed by cleaning while copper-based coating remained even after one round of cleaning and lasted up to 90 rounds of cleaning. Also, the antiviral potential of copper coatings remained even after 10 times of finger touching. Muller and coworkers presented an anti-COVID mask, which could attract droplets of aerosol and kill the virus (M€ uller et al., 2021). This should consist of a hydrophilic biodegradable polyester namely, polycaprolactone (PCL), to which a biocompatible, biodegradable antivirally active polymer, an inorganic phosphate (polyP) namely, sodium polyP (Na-polyP-NP) and calcium polyP (Ca-polyP-NP) nanoparticles are attached (Fig. 11). A virus matrix protein envelope (E) is needed for the formation and pathogenesis of virus envelope. This E protein was grouped to the ionguiding viroporins, which then oligomerized to ion channels and these channels were bound to potential antiviral drugs. There were three

Fig. 11 Three targets of innovative ingredients of PCL spun masks. (Adapted with permission from RSC. M€ uller, W. E. G., Neufurth, M., Lieberwirth, I., Muñoz-Espí, R., Wang, S., Schro€der, H. C., & Wang, X. (2021). Triple-target stimuli-responsive anti-COVID19 face mask with physiological virus-inactivating agents. Biomaterials Science, 9 (18), 6052–6063. https://doi.org/10.1039/d1bm00502b.)

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principles, which were used to remove SARS-CoV-2 present in aerosol droplets. (i) The virus particles were trapped and stored onto the PCL spun fibers during the development of a coacervate, (ii) polyP inactivated the receptor-binding domain of the virus spike protein, and (iii) the viral envelope protein destroyed the viral particles. Choi et al. reported a new visible light-induced photocatalyst based on several transition metals-doped TiO2 and evaluated for the deactivation of influenza virus H1N1 under the visible light irradiation (Choi & Cho, 2018). This photocatalyst-coated TiO2 deactivated influenza virus H1N1 to more than 99% under visible light. This was rigidly attached to the polyester fabric surface and remained even after 25 washes. So, this photocatalyst is considered to be semipermanent if it is not eliminated by scratch. Also, this is an effective agent for viral inactivation; therefore, this controlled the influenza infection effectively. This could be activated even with a very weak light source.

3.3 Antifungal coatings Gao and coworkers reported a pH- or cellulase-responsive carrier based on mesoporous silica nanoparticles (Y. Gao et al., 2021). These were designed by implanting hydroxypropyl cellulose into pyraclostrobin-loaded hollow MSN via an ester linkage. They examined the release behavior of PYR-HMS-HPC under different values of pH (pH 3, 5, and 7). The results suggested that the release was higher at low pH. Also, in the presence of cellulase, the release behavior was greater compared to pH. This HMS-coated HPC helped in the prevention of several crop diseases.

4. Recent advances in biomaterials and biomimetics Denisa Druvari and coworkers, after blending and cross-linking, synthesized biocide coatings based on quaternized ammonium copolymers (Druvari et al., 2018). Through free radical copolymerization, two sets of copolymers with compatible reactive groups were synthesized and further modified, yielding electrostatically attached (hexadecyltrimethylammonium 4-styrene sulfonate, SSAmC16) units and covalently linked (4-vinyl benzyl dimethyl hexadecyl ammonium chloride, VBCHAM). Potential applications of quaternary ammonium groups include contact-based action, released-based action, and a combination of the two was focused in this research. Copolymers containing a high concentration of electrostatically linked ammonium groups demonstrated strong biocide activity.

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Antiviral and antimicrobial smart coatings

When coupled with ultraviolet-A light, edible hydrogel coating exhibit significant antibacterial activity and such coatings were developed by Juliano V. Tosati and coworkers (Tosati et al., 2018). The hydrogel coatings were made from either turmeric residue and gelatin hydrogels (TGH) or cassava starch and gelatin hydrogels with added pure curcumin (CGH). The coatings were applied on cooked sausages and tested for their ability to avoid bacterial cross contaminations. After short light treatments, UV-A light-exposed hydrogel coatings were found to be capable of inactivating Listeria innocua. Lignin has vast potential for biomedical use. Eneko Larraneta and coworkers utilized the esterification procedure to synthesize hydrogels by mixing lignin with poly(methyl vinyl ether-co-maleic acid) and poly(ethylene glycol) (Larran˜eta et al., 2018). The hydrogels thus produced had lignin concentrations ranging from 40% to 24%, as well as water uptake capacities of up to 500%. Furthermore, the hydrophobic characteristic of lignin made it easier to load a hydrophobic drug (curcumin). The hydrogels were able to deliver this drug for up to 4 days. The products displayed large reductions in Staphylococcus aureus and Proteus mirabilis adhesion up to 5 days when compared to the commonly used medicinal material polyvinyl chloride (PVC). Galvao and coworkers synthesized poly(methylmethacrylate) (PMMA) hybrid nanoparticles by emulsion polymerization in the presence of poly(diallyl dimethyl ammonium) chloride (PDDA) (Galva˜o et al., 2018). The coatings formed by casting or spin-coating the nanoparticle dispersions onto silicon wafers were hydrophilic, with contact angles increasing as the amount of cationic polymer in the nanoparticles increased. Bacterial cell counts were reduced significantly after interaction with the coatings. The increased water repellence of the two methyl groups on the quaternary nitrogen of the PDDA molecule over the cationic glycosylated moiety provided an advantage for efficient microbicide activity. The relatively poor function of PDDA at the interface of methyl methacrylate droplets and the surrounding aqueous phase during nanoparticle production was a key downside of PMMA/PDDA nanoparticle synthesis. Antibacterial hydrogels based on sodium alginate (SA) were designed and manufactured by Gang Wang and coworkers (G. Wang et al., 2018). The hydrogels were initially generated with good mechanical strength using Diels-Alder click chemistry, and then the antimicrobial peptide with cysteine terminal was grafted into the hydrogel utilizing the thiol-ene reaction between the thiol group and oxy-norbornene group. Antimicrobial hydrogels demonstrated high antibacterial activity as well as good biocompatibility.

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Amit Nautiyal et al. studied the antibacterial action of N-halamines (Nautiyal et al., 2018). The oxidation state of halide atoms in nitrogenhalogen bonds contributes to N-halamines’ antibacterial action. After halide atoms have been depleted, the antibacterial action can be restored by using halogenation agents like chlorine bleach. This “recharging” method is simple to incorporate into current food sector hygienic practices. N-halamines are therefore an appealing antibacterial solution for the food industry. The use of N-halamine polymers for machinery coatings or packaging in the food sector has a lot of potential. N-halamine polymers were grafted onto stainless steel surfaces. However, in order to preserve N-halamine antimicrobial functional groups (e.g., >NdCl), the coatings must be treated with chlorine bleach on a regular basis, which can corrode stainless steel. To address implant-associated infections, Qiang Zeng et al. created antibacterial and antifouling branched polymeric agents using gentamicin, a broad-spectrum antimicrobial agent, and ethylene glycol (Zeng et al., 2018). After implantation and exposure to bodily fluids, titanium implants are prone to protein adsorption and bacterial fouling. Due to the abundance of primary amine groups that can be easily coated on Ti discs, such polymeric coatings will have robust antibacterial activity and considerably prevent biofilm formation. Surface functionalization of medical implants is a promising strategy for reducing implant-associated infections. Hung and coworkers studied the built-in self-sterilizing properties of antimicrobial coatings in food safety (Hung et al., 2018). The surface properties were modified by incorporating cross-linking agents with larger molecular weights and varying chemical compositions. High molecularweight molecules indicate that there are more active sites for interaction. Due to the general high reactivity of the anhydride units, styrene-maleic anhydride (SMA) could cross-link polyethyleneimine (PEI) to generate antimicrobial polymer coatings. Coatings were tested in their pure form (cationic) and then chlorinated as N-halamines. Dimitra Aslanidou and coworkers developed a multifunctional coating for silk protection that is superhydrophobic, superoleophobic, and antimicrobial (Aslanidou & Karapanagiotis, 2018). An aqueous dispersion of alkoxysilanes, silica nanoparticles, organic fluoropolymer, and silane quaternary ammonium salt were used to synthesize the coating. The multifunctional coating reduces the vapor permeability of the treated silk moderately, has extremely effective mechanical abrasion resistance, and has a small visible effect on the aesthetic appearance of silk.

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Mariana Barbosa and coworkers created highly antibacterial ultrathin films by tethering antimicrobial peptides (AMP) onto ground chitosan (Barbosa et al., 2019). Powdered AMP-tethered chitosan proves to be an alternative approach for the manufacturing of antimicrobial devices since it is ideal for extensive production, is easier to manage for the fabrication of various coatings and materials with microbicidal properties, and does not cause toxicity. Dhvar-5, a synthetic AMP with N-terminal hydrophobic and C-terminal cationic domains was previously found to have antifungal action against Candida albicans. Dhvar-5 attributes its head-to-tail amphiphilicity to the creation of transient pores on the phospholipid bilayer of the cells, resulting in cell content leakage. T. Kruk and coworkers developed biocompatible polyelectrolytecopper nanocomposite films with poly(sodium 4-styrene sulfonate) (PSS) as the polyanion, poly(diallyl dimethylammonium chloride) (PDADMAC) as the polycation, and negatively charged copper nanoparticles (CuNPs) (Kruk et al., 2019). S. aureus is commonly detected in biofilms produced for medical devices or human tissue, and it is the most common reason for orthopedic implant-associated infections. In addition, it can be found on catheters, heart implants, and cosmetic surgical implants. Antibacterial activity against S. aureus was demonstrated using films composed of PDADMAC/CuNPs bilayer containing silver nanoparticles. Cell lysis generated by interaction with the multilayer film’s surface was responsible for the antibacterial action. Ilya Shlar and colleagues created an antimicrobial coating derived from a curcumin-cyclodextrin inclusion complex and a support matrix of polyethylene terephthalate (PET) film (Shlar et al., 2018). Following a pretreatment to ensure that the PET surface had a suitable electric charge, it was electrostatically coated with successive multilayers of alternatively deposited negatively charged poly-L-glutamic acid (PLGA), positively charged poly-Llysine (PLL) and carboxymethyl—cyclodextrin (CMBCD). The coatings featured a (PLL-PLGA)6-(PLL-PLGAPLL-CMBCD)n architecture, with the number of repeating multilayers ranging from 5 to 20. The CMBCD molecules were either left unbound or covalently cross-linked using carbodiimide cross-linker chemistry. T. Ferreira and coworkers utilized solution blow spraying to create microbicidal coatings of poly(dimethylsiloxane) with silver nanoparticles (PDMS/AgNPs) (Ferreira et al., 2019). The PDMS/AgNPs coating was created in a two-step process. The first step was to deposit a PDMS solution in hexane over the substrate, followed by five depositions of colloidal

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dispersion of AgNPs onto a moderately cured PDMS surface at 7.5-min intervals. AgNPs were attached on the exterior of the PDMS covering, ensuring antimicrobial action against E. coli and S. aureus. For S. aureus and E. coli, the antibacterial activity of the AgNPs on the PDMS coating was significantly greater than in the test specimens free of silver. Silver nanoparticles promote the scaling down of cell adherence and the production of biofilms. Copper targets several bacterial cell processes, resulting in pleiotropic effects and potentially slowing resistance evolution (Mitra et al., 2020). Cu ions are harmful to bacteria because they depolarize their membranes. Cu ions can bind to negatively charged domains on both the outer and inner bacterial cell membranes reducing the potential difference between the outside and inside of the cell and causing membrane depolarization, which leads to membrane rupture when the difference in potential reaches zero. Increased reactive oxygen species levels cause oxidative damage, which includes lipid peroxidation and deoxyribonucleic acid (DNA) damage in membranes. Copper ions can potentially decrease intracellular enzyme action by displacement of critical cofactors from metalloenzymes, metalcatalyzed oxidation of amino acid residue, or site-specific inactivation of iron sulfur clusters, which are found in various proteins. B. Alkan Tas and coworkers used a layer-by-layer assembly to coat polyethylene films with an antibacterial thin film that can be used in food packaging (Alkan Tas et al., 2019). Halloysite nanotubes (HNTs) were used to encapsulate and release carvacrol, the active component in thyme oil. Aeromonas hydrophila’s vitality was reduced significantly, and the aerobic count on the surface of chicken was also lowered, when coated films were used. Furthermore, as compared to control polyethylene films, coated film surfaces revealed lesser bacterial attachment, suggesting their antibiofilm nature. Antimicrobial coatings produced in this work were made of natural and benign components, and they offer an innovative and effective technique of obtaining microbicidal food-packaging materials that can significantly improve food safety. Derya Boyaci developed antimicrobial coatings incorporating essential oils for the suppression of bacterial pathogen contamination on fruit skins (Boyacı et al., 2019). L. innocua and Escherichia inoculated at their surfaces were inhibited by all essential oil-containing films. Zein films were made utilizing various essential oils, including thymol (THY), eugenol (EUG), and carvacrol (CAR). The antibacterial activity of THY-, CAR-, and EUG-loaded films was investigated in sterile environments using E. coli

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Antiviral and antimicrobial smart coatings

and L. innocua as test microorganisms. CAR and THY-containing films had better antibacterial efficiency than EUG-loaded films. Antimicrobial surfaces that limit biofilm formation are the greatest strategy to reduce pathogen spread and material damage. To do this, materials must inhibit living planktonic microbial cells from adhering to their surroundings on a primary level. This can be accomplished in general by repelling or killing incoming germs. Hydrogel coatings based on PEG (polyethylene glycol) or similar hydrogel-producing polymers, highly negatively charged polymers, or superhydrophobic changes can all be used to create bacteriostatic titanium surfaces. Chouirfa H created bacteriostatic surfaces by layering polycationic and polyanionic polymers (Chouirfa et al., 2019). After that, titanium surfaces become both bacteriostatic and bactericidal when an antibiotic is added (Fig. 12). Antimicrobial coatings comprised of phytosynthesized nanoparticles were examined by Irina Fierascu and coworkers for the development of antibacterial textiles, other biomedical uses, food safety, and other sorts of applications based on their antimicrobial properties (Fierascu et al., 2019).

Fig. 12 Bacteriostatic and bactericidal surfaces. (Adapted with permission from Elsevier. Chouirfa, H., Bouloussa, H., Migonney, V., & Falentin-Daudr
e, C. (2019). Review of titanium surface modification techniques and coatings for antibacterial applications. Acta Biomaterialia, 83, 37–54. Acta Materialia Inc. https://doi.org/10.1016/j.actbio.2018.10.036.)

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The phytosynthesized nanoparticles are easily incorporated into the paint/ epoxy matrix and have no impact on the finished goods’ mechanical or aesthetic properties. Silver and copper nanoparticles increase the shelf-life of fruits. They are effective against E. coli and Staphylococcus aureus. Akbar Bahrami and coworkers studied active packagings based on the incorporation of nanocarriers containing natural antibacterial agents into food packaging (Bahrami et al., 2020). Active packaging (AP) refers to packaging that has better features such as managing the package’s environment and microbe development, which may be accomplished through novel engineering materials. The efficiency of active packaging has been improved by nanoencapsulation techniques, in which antimicrobial-loaded nanocarriers might enable a stable release of antimicrobial active packaging for preserving food quality during storage. Essential oils (EOs) are promising and beneficial antibacterial chemicals found throughout nature. Another type of natural antimicrobial element utilized in the food sector is antimicrobial peptides (AMPs). They have a wide range of activity, quick action, and have antiinflammatory properties. Organic acids’ principal antibacterial strategy for preventing bacterial growth is to lower the pH of food. The entry of nonionized organic acids into pH-sensitive bacteria’s cells disrupt their critical functions. Furthermore, the anionic portion of organic acids accumulate in bacteria, causing disruptions in critical bacterial processes. One major method for producing controlled release AP is the coating, which involves coating food products with a thin layer of embedded biopolymers containing antimicrobial-loaded nanoparticles. Nanoliposomes, nanostructured lipid carriers (NLCs) and solid lipid nanoparticles (SLNs), and cyclodextrin (CD) nanocarrier biopolymer-based systems were used for microbicidal encapsulation. Stephanie Elisabeth Klein and coworkers synthesized bioactive ligninbased polyurethane coatings using lignins isolated from black liquor at different pH values (Klein et al., 2019). The presence of double bonds and methyl groups in the phenolic fragments of lignin gives them maximum potency against microbes. Additional antimicrobial substances like triphenylmethane derivatives (brilliant green and crystal violet) were used. The synthesized coating showed significant microbial reduction against bacteria. The microbial activity was observed to increase for coatings with lignins isolated at pH 2–5. This could be due to the improved homogeneity achieved by the higher cross-linking density of the polyurethane with increase in hydroxyl groups.

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Using a chemical reduction technique at 90 °C, Bang-on Nokkrut and coworkers synthesized silver nanoparticles (AgNPs) (Nokkrut et al., 2019). The phytopathogen Colletotrichum gloeosporioides causes black anther, which reduces the postharvest life of cut orchid flowers, particularly during the rainy season. Antimicrobial substances such as silver nanoparticles (AgNPs), zinc pyrithione, benzimidazole, organic acids, borate, and plant extracts have been suggested as viable coating materials in paper boxes to limit C. gloeosporioides development. The synthesis of AgNPs was carried out using the chemical reduction of NaBH4 and Na3C6H5O7. The AgNPs were utilized as an antifungal coating on packaging paper to prevent Colletotrichum gloeosporioides from growing in cut orchid blossoms during transportation. When compared to uncoated paper, coated paper showed improved water resistance and mechanical qualities. In addition, when orchid inflorescence anthers infected by C. gloeosporioides were housed in the coated boxes, there was a considerable reduction in the quantity of infected anthers. T. Mokabber and coworkers developed a calcium phosphate coating containing silver (Ag/Ca-P) on titanium substrates through electrochemical deposition, enabling regulated coating growth on complicated geometries and porous surfaces (Mokabber et al., 2020). Two deposition procedures were investigated: one-step Ag/Ca-P (1) deposition coatings with Ag ions implanted as micronized silver phosphate particles in the calcium phosphate matrix, and a two-step Ag/Ca-P (2) deposition method with silver deposited as metallic silver nanoparticles on the calcium phosphate coating. The antibacterial assessment against S. aureus demonstrates that the Ag/Ca-P (1) coating’s high release rate of silver ions resulted in leaching death. The Ag/Ca-P (2) coating’s antibacterial action is primarily contact killing. Tal Zada and colleagues investigated the prevention of biofouling by thin films based on several sol–gel predecessors (Zada et al., 2020). Spin-coating on glass was used to create films from phenyltrimethoxysilane (PTMOS), methyltrimethoxysilane (MTMOS), 3,3,3-trifluropropyltrimethoxysilane (FTMOS), or 3-aminopropyltrimethoxysilane (APTMS). Sol-gel coatings were heavily used to reduce fouling in marine environments. Sol-gel precursors are commercially accessible, providing appealing building blocks for the formation of antifouling and antibacterial coatings. These coatings establish siloxane linkages with hydroxylated surfaces by binding to the alkoxy group of the sol-gel precursor. As a result, sol-gel films are covalently linked to the surface and have high stability when compared to other adsorbed coatings.

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Antibacterial activity, toxicity, and bioavailability of AMPs are all influenced by their structural and physicochemical features (Magana et al., 2020). AMPs prefer topical application over systemic administration. Broadening the range of mapped chemical space is a viable antibacterial resistance treatment. To identify and advance novel, antimicrobial drugs to clinical usage, interdisciplinary knowledge is required. Aside from their capacity to synergize with antibiotics, AMPs’ antimicrobial effectiveness can be strengthened by investigating synergies among various AMPs, lowering the likelihood of antimicrobial resistance development. Sara M. Imani and coworkers investigated antiviral substances and coatings that could be used to prevent the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) through infected surfaces (Imani et al., 2020). Infectious viruses that remain alive on surfaces, such as SARSCoV-2, represent a significant risk of spread via the surface route. Since viruses lack the repair mechanisms present in bacteria and fungi, they are vulnerable to copper-induced damage. Silver is another antiviral substance that deactivates viruses by interacting with the viral sheath and viral surface proteins, preventing its penetration into cells, obstructing cellular passage, interacting with the viral genome, and interacting with viral replication factors. TiO2 has photocatalytic characteristics and subsequent applications in the deactivation of bacteria and viruses. The positive charge of polycations in polymers (polyethylenimine) attracts viruses with an intrinsic negative charge, interferes with their genetic content or structural components, and causes viral disintegration. With its high yield of singlet oxygen generation, a C60-based sensitizer was designed to test virus deactivation in air. Ming Hong Lin and coworkers developed a biocomposite coating on porous titanium oxide, which consisted of chitosan and ZnO (Lin et al., 2021). This coating could prevent infections related to orthopedic and dental implants. The outer layer of chitosan matrix was zinc oxide nanoparticles and the inner layer was nanoporous TiO2. Being a cationic molecule, chitosan could bind to negatively charged bacterial cell walls. As a result, it is effective against a wide range of bacterial strains. However, it cannot be used as such since it cannot withstand mechanical scratches. Metal nanoparticles like TiO2 was used to increase mechanical resistance. ZnO nanoparticles increased the antibacterial activity, corrosion resistance and adhesive properties of the coating. Mahsa Zabara and coworkers developed an antimicrobial coating based on the self-assembly of the food-grade amphiphilic lipid glycerol

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monooleate with the antimicrobial peptide LL-37 derived from human cathelicidin (Zabara et al., 2021). The antimicrobial peptides (AMPs) were encapsulated with amphiphilic lipids like oleic acid (OA) and glycerol monooleate (GMO). This protected the AMPs from degradation. The self-assembly was prepared on silicon segment. Inside the coating, highly geometrically organized lamellar and nonlamellar liquid crystalline nanostructures were developed. The microbicidal activity against both Gramnegative and Gram-positive bacterial strains increased significantly. The self-assembly of lipid and AMP modified the antibacterial action of the human cathelicidin-derived AMP LL-37. Antimicrobial surface coatings have arisen as a viable means to controlling infections caused by medical devices. Bacterial contamination and the formation of biofilms on medical devices can result in infection. Surfaces coated with antimicrobial peptides may help to avoid such outbreaks. Yasir et al. worked on designing antimicrobial biomedical devices capable of resisting Pseudomonas attachment (Yasir et al., 2020). Antimicrobial peptides (AMPs) are potential bioactive compounds that are extremely biocompatible and resistant to bacterial resistance development. Franziska Pietsch et al. studied the properties of various antimicrobial coating (AMC) materials and their potential in eradicating antimicrobialresistant (AMR) bacterial infections (Pietsch et al., 2020). They studied the evolution and mechanism of resistance of AMR species against antimicrobial substances like silver, copper, and antimicrobial peptide (AMP). The AMR species evolve by gene transfer and mutation. Copper has been used extensively as an AMC in textiles, door handles, etc. Copper gets its antimicrobial character from redox properties and transfer from Cu(II) to Cu(III) oxidation states. The antimicrobial properties of silver are attributed to its ability to bind FedS clusters in iron-containing enzymes. But, many bacterial strains have developed resistance against these antimicrobial materials. That is the use of antimicrobial coatings has resulted in the selection of antibacterial resistance and the spread of infectious diseases through crossresistance. Krukiewicz and coworkers investigated the use of an electrically responsive poly(3,4-ethylenedioxytiophene) (PEDOT)-based matrix as a surface capable of controlling bacterial growth (Czerwi
nska-Gło´wka et al., 2021). Platinum films were chosen as the surface. During an electrochemical polymerization method, a PEDOT coating was produced on a platinum-coated substrate and was employed as a tetracycline carrier (Tc). Tetracycline is a strong antibiotic with a broad antibacterial range that has a bactericidal effect

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by interfering with the synthesis of protein. Tetracycline is effective against aerobic and anaerobic bacteria, both Gram-negative and Gram-positive, as well as protozoan parasites. The use of a conjugated polymer as a Tc carrier enabled the development of a robust antibacterial system, which electrically brought about biological performance, laying the groundwork for future applications, especially in implantology. Electroactive PEDOT/Tc systems would serve as multifaceted coatings for biomedical devices utilized in neural tissue and cardiac engineering as they combine high electrochemical activity with antibacterial properties. VelaCano and coworkers evaluated the antibacterial capabilities of numerous materials used as pipeline coatings containing varying concentrations of silver, as well as the prevention of the biofilm formation process in metal pipes (Vela-Cano et al., 2021). Coating metal pipes with silver zeolite paint at a concentration of 2000 mg L
1 significantly inhibits the production of microbial biofilms and prevents environmental biodeterioration processes. In this regard, silver zeolite demonstrated stronger protective capability than other silver preparations, suggesting that silver’s antibacterial activity is enhanced as silver zeolite. Silvia Gonza´lez and coworkers demonstrated the performance of a nanostructuring and functional method that produces a possible antibacterial impact on food-grade stainless steel pipes (Silvia Gonza´lez et al., 2021). Bacterial attachment to a surface and the subsequent creation of a biofilm can cause serious health issues such as microbial contamination and persistent illnesses. Biofilms can spread by detaching tiny or large clumps of cells and releasing individual cells. The combination of silver nanoparticles and a thin coating of TiO2 placed over nanostructured stainless steel suppresses the establishment of all forms of biofilms by causing protein inactivation and cellular death. Sl. no.

Material used

Application

Reference

1.

Quaternized ammonium copolymers

Biocide

2.

Gelatin hydrogels (TGH) or cassava starch and gelatin hydrogels with added pure curcumin (CGH) Lignin with poly(methyl vinyl etherco-maleic acid) and poly(ethylene glycol)

Food safety

Druvari et al. (2018) Tosati et al. (2018)

3.

Biomedical uses

Larran˜eta et al. (2018) Continued

60

Sl. no.

Antiviral and antimicrobial smart coatings

Material used

Application

Reference

Antibacterial

5.

Poly(methylmethacrylate) (PMMA) hybrid nanoparticles Sodium alginate, antimicrobial peptide

Antibacterial

6.

N-halamines

Food safety

7.

Gentamicin and ethylene glycol

8.

Styrene-maleic anhydride (SMA) and polyethyleneimine (PEI) Alkoxysilanes, silica nanoparticles, organic fluoropolymer and silane quaternary ammonium salt Antimicrobial peptides (AMP)

Medical implant safety Food safety

Galva˜o et al. (2018) G. Wang et al. (2018) Nautiyal et al. (2018) Zeng et al. (2018)

4.

9.

10.

Silk protection

Antifungal

11.

Polyelectrolyte-copper nanocomposite films

12.

14.

Curcumin-cyclodextrin inclusion complex and polyethylene terephthalate (PET) film Poly(dimethylsiloxane) with silver nanoparticles Copper

15.

Halloysite nanotubes

16.

Essential oils

Food packaging Antibacterial

17.

PEG (polyethylene glycol)

Antibacterial

18.

Phytosynthesized nanoparticles

19.

Essential oils, antimicrobial peptides

Antibacterial textiles, biomedical uses, food safety Food safety

20.

Lignin-based polyurethane coatings

Antimicrobial

13.

Medical implant safety Antimicrobial

Antibacterial Antibacterial

Hung et al. (2018) Aslanidou and Karapanagiotis (2018) Barbosa et al. (2019) Kruk et al. (2019)

Shlar et al. (2018)

Ferreira et al. (2019) Mitra et al. (2020) Alkan Tas et al. (2019) Boyacı et al. (2019) Chouirfa et al. (2019) Fierascu et al. (2019)

Bahrami et al. (2020) Klein et al. (2019)

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Sl. no.

Material used

Application

Reference

21.

Silver nanoparticles

Packaging

22.

Antibacterial

24.

Calcium phosphate coating containing silver Phenyltrimethoxysilane (PTMOS), methyltrimethoxysilane (MTMOS), 3,3,3-trifluropropyltrimethoxysilane (FTMOS), or 3-aminopropyltrimethoxysilane (APTMS) Silver, TiO2, polyethylenimine

Nokkrut et al. (2019) Mokabber et al. (2020) Zada et al. (2020)

25. 26.

TiO2, chitosan and ZnO Antimicrobial peptide

Antibacterial Food safety

27.

Antimicrobial peptides

28.

Silver, copper, antimicrobial peptide

Biomedical use Antimicrobial

29.

Poly (3,4-ethylenedioxytiophene)

Biomedical uses

30

Silver zeolite paint

31.

Silver nanoparticles and of TiO2

Pipeline coatings Food safety

23.

Prevention of biofouling

Antiviral

Imani et al. (2020) Lin et al. (2021) Zabara et al. (2021) Yasir et al. (2020) Pietsch et al. (2020) Czerwi
nskaGło´wka et al. (2021) Vela-Cano et al. (2021) Silvia Gonza´lez et al. (2021)

5. Outlook and conclusion The chapter gives an overview of the various preparative techniques used to fabricate biomimetic coatings and their applications in various fields. The antiviral as well as antibacterial nature of these coatings have been elaborately discussed but there is not much work reported on antifungal coatings. Thus, there are enough prospects for the development of biomimetic antifungal coatings in the near future. Superhydrophobic coating is another area where biomimetics can contribute a lot. The development of lotus-like or rose petal-like surfaces can promote the self-cleaning property in coatings. Amalgamation of the self-cleaning property and antimicrobial properties can give rise to superior coatings with high-end applications in the biomedical arena.

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Introduction of two-dimensional materials like transition metal dichalcogenides, MXenes, carbon nanomaterials, and their surface-modified hybrid materials will extend the application window of biomimetic coatings.

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Further reading de Bonis, A., & Teghil, R. (2020). Ultra-short pulsed laser deposition of oxides, borides and carbides of transition elements. Coatings, 10(5). https://doi.org/10.3390/ COATINGS10050501.

CHAPTER 3

Current scenario on the microbial world and associated diseases Fatemeh Salimia and Ehsan Nazarzadeh Zareb a

Department of Cellular and Molecular Biology, School of Biology, Damghan University, Damghan, Iran School of Chemistry, Damghan University, Damghan, Iran

b

1. Introduction We are living in a microbial world. Microorganisms were the first organisms on the Earth. Various microorganisms including bacteria, archaea, fungi, protozoa, and microalgae are present in different aquatic and terrestrial atmospheric habitats, or living hosts more than 3.0 billion years before the appearance of plants and animals. Despite the very small size of microorganisms, they have impacts on our life including positive (like playing important role in enhanced production of food, medicine, bioremediation of hazardous materials, and agriculture) and negative (various infectious diseases) impacts (Staley et al., 1996). In fact, the geochemical activities of microorganisms have formed the biosphere of the Earth and consequently provided conditions for the evolution of plants and animals. They can rapidly adjust themselves to diverse environmental conditions, including extreme temperature, alkaline or acidic conditions, various carbon and nitrogen sources, high salt content, low oxygen levels and the presence of toxic compounds. According to recent evidence, millions of microorganisms have not yet been discovered. It has been stated that less than 1% of the bacterial species and less than 5% of fungal species are known now (Gunatilaka, 2006).

1.1 Impacts of bacteria on life Microorganisms are the most abundant living thing on our planet. In fact, we live in a world run by microbes. Microorganisms including bacteria, viruses, fungi, and protozoa have been found everywhere, like aquatic ecosystems (oceans, ponds, lakes, streams, ice, and hot springs), terrestrial ecosystems (surface soils, deep subsurface, and solid rock), on or in the other organisms, such as plants and animals or ecosystems with harsh conditions Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00006-2

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like acidic or alkaline pH or so salty condition. Microbial cells can live in the deepest parts of the ocean and in boiling hot springs. Through the years many microorganisms and their functions have been discovered. This knowledge has greatly increased the beneficial effects of microorganisms and limited many of their harmful effects. Some animaland plant-associated microorganisms provide essential nutrients, vitamins, or hormones for them and protect them from diseases. Beneficial microorganisms assist humans to digest foods, and also they are involved in the production of vitamin K, immune system development, and detoxification of hazardous chemicals. It has been declared that intestinal microflora has an important role in our obtained calories from food. In fact, they can enhance nutrient harvest and modify appetite signaling. Actually, there are more than 100 trillion symbiotic microorganisms and microbiota on or in the skin, the mucosae of buccal and nasal cavities, vagina, and the gastrointestinal tract of human beings. They play an important role in human health and disease. They are associated with obesity, metabolic syndrome, atherosclerosis, diabetes mellitus, liver diseases, and inflammatory bowel disease. Also, the microbiota makes a physical barrier, produces compounds with antimicrobial activities, and are involved in the development of the immune system of the host which protect the host against foreign pathogens (Wang et al., 2017). In addition, many industries like food, pharmaceutical, and agricultural industries are relying on microorganisms. Many commercial antibiotics, anticancer, and antioxidative drugs are microbial-derived compounds. The essential elements of oxygen, carbon, nitrogen, and sulfur are made by microbial communities. Microorganisms via their decomposing activities revive life. Photosynthetic microorganisms enhance oxygen levels and decrease carbon dioxide (Stark, 2010) (Fig. 1). However, some microorganisms are detrimental agents. They cause infectious diseases that are the leading cause of death worldwide. In fact, the emergence of new infectious diseases, reemergence of deadly infectious diseases and also antimicrobial-resistant infectious agents are considered a formidable threat to public health and welfare. Various fatal pandemics have been caused by pathogenic microorganisms throughout history. Italian plague, great plague of Seville, great plague of London, great plague of Marseille (due to Yersinia pestis), tuberculosis (due to Mycobacterium tuberculosis), cholera pandemics (due to Vibrio cholerae), diphtheria (Corynebacterium diphtheriae), syphilis (due to Treponema pallidum), typhus (due to Rickettsia prowazekii), sepsis, and tetanus (due to Clostridium tetani), global flu pandemic, Spanish

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Fig. 1 Beneficial activities of microorganisms.

flu, Asian flu, Hong Kong flu, H1N1 pandemic, severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory syndrome coronavirus (MERS-CoV) are some of these infectious diseases. We are currently facing the new coronavirus disease 2019 (COVID-19) pandemic. Global monitoring programs of water-borne pathogens, vector-borne diseases, and zoonotic spillovers at the animal-human interface have great importance in quickly detecting the emergence of infectious threats and preventing their spread (Piret & Boivin, 2021) (Fig. 2).

1.2 Common strategies of bacterial pathogens to cause infection Accurate detection or diagnostic tests, and new platforms for the development and production of vaccines or therapeutic agents are needed to limit potential pandemics. In this regard, the distinguishment between pathogenic and nonpathogenic microorganisms and identification of their pathogenesis is a perquisite to prevent or treat infections. Studies showed that some pathogenic bacteria have unique virulence factors and determinants while some noticeably diverse microbial pathogens (different species or even genera) apply common strategies to cause infection. These common mechanisms are their abilities to adhere, invade, and cause damage to host cells and tissues, to survive in the presence of host defenses and establish infection (Wilson et al., 2002).

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Fig. 2 The history of pandemics.

1.2.1 Adhesins Adherence of pathogens to host surfaces is a main stage in host-pathogen interaction, colonization, infection, and disease production. In other words, colonization accelerates the establishment of microorganisms in a host body. When pathogens adhere to the host cell surface, they can start their proliferation, toxin secretion, host cell invasion, and activation of the host cell signaling cascade (Di Martino, 2018). Pathogens can adhere to various surfaces including skin, mucous membranes such as the oral cavity, nasopharynx, urogenital tract, and deeper tissues like lymphoid tissue, gastric and intestinal epithelia, alveolar lining, as well as endothelial tissue. Membrane-spanning proteins, surface immunoglobulin, glycolipids, glycoproteins, and extracellular matrix proteins like fibronectin and collagen are molecules of host cells which are used as host receptors for microbes (Wilson et al., 2002). Host applied various mechanical forces to remove pathogens from these surfaces. These mechanisms include saliva secretion, coughing, sneezing, mucous flow, peristalsis, and blood flow (Bischoff & Kr€amer, 2007; Sheehan et al., 2007; Wilson et al., 2002). The surface of microbial pathogens is a highly specialized organelle equipped with different adherence factors, and adhesins that facilitate attachment to molecules on various host tissue cells and resist them against mechanical washing forces. In fact, there are numerous surface structures

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on microbial cells capable of mediating specific or nonspecific adhesion to host surfaces (Fletcher, 1996). Adhesins interact with surface molecules of a host cell or the extracellular matrix. These interactions result in intermolecular linkages that are not easily broken. The role of adhesive structures on microbial cells relies on their biochemical structures. It is possible that they are involved in initial, weak, and nonspecific adhesion via generating hydrophobic linkage, Brownian movement, and applying biofilm polymers. Through this interaction, microbial cells can overcome the electrostatic repulsion between bacterial and host surfaces (An et al., 2000; Gilbert et al., 1997; Ukuku & Fett, 2002). Some adhesins are involved in significantly specific interactions with host surface receptors. Therefore, high-affinity and stable interactions can be formed between pathogens and the surfaces of the host (Kline et al., 2009). The microbial adhesins can be made from polypeptides or polysaccharides. Protein adhesins are separated into two groups: fimbrial (pili dependent) and afimbrial (nonpilus adhesins). Fimbriae are hairlike structures on the bacterial surface. Frequently, the fimbrial tip serves to bind the host receptor. Gram-negative bacterial pathogens in particular rely on fimbriae for adherence. Examples include E. coli, V. cholerae, P. aeruginosa, and Neisseria species (Zaini et al., 2015). The tip of the fimbrial structure is involved in adherence. For example, uropathogenic E. coli that colonize the urinary tract create kidney infections, and possess pyelonephritis-associated pili at their surface. PapG subunits constitute the tip of this pili that attaches to glycosphingolipids of the kidney epithelium. E. coli strains bind to mannose receptors on host cells through Type 1 pili (Mulvey, 2002). Also, some UPEC strains display Type I pili which can especially bind to D-mannosylated receptors, such as the uroplakins of the bladder (Terlizzi et al., 2017). Some Gram-negative bacteria have Type IV pili. These pili can provide twitching motility that is important for effective colonization on host surfaces. In addition, type IV pili on Neisseria meningitidis are involved in the formation of microcolonies attached to vascular endothelial cells (Coureuil et al., 2012). There are some Gram-positive bacteria that have pili structures, including sortase-assembled pili and type IV-like pili. Two types of pili have been described so far in these species (Kline et al., 2010; Melville & Craig, 2013). Afimbrial adhesins are proteins that serve as adherence factors. These proteins do not form a fimbrial structure. The afimbrial adhesins generally mediate more intimate contact with the host cell. Gram-negative, including Yersinia pseudotuberculosis, enteropathogenic E. coli, Neisseria spp., and

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Gram-positive such as Staphylococcus spp., Streptococcus spp. and mycobacterial pathogens express and apply afimbrial adhesins (Donnenberg, 2000; Joh et al., 1999; Johnston Jr, 1991; Merz & So, 2000). Components of the bacterial cell membrane, cell wall, or capsule can act as polysaccharide adhesins. The teichoic acids act as adhesins for Staphylococcus spp. and Streptococcus spp. The capsular polysaccharides of Mycobacteria spp. (glucan and mannan) are also applied to bind to host cell receptors like complement receptor and mannose receptor. Poly-N-acetylglucosamine or polysaccharide intercellular adhesion is involved in Staphylococcal adherence as the extracellular polysaccharide adhesin (Wilson et al., 2002). It should be noted that a single pathogen can express or apply more than one adhesin. Researchers are currently trying to develop vaccines or therapeutic agents to block the adherence step in the infection cycle (Asadi et al., 2019). There are a lot of bacterial surface factors that possess adhesive properties besides pili. They can bind to different classes of host molecules (cell adhesion molecules-CAMs) such as integrins, cadherins, immunoglobulin superfamily cell adhesion molecules, selectins, receptor protein tyrosine phosphatases, syndecans and hyaluronate receptors, collagen, fibronectin, laminin, or elastin. On adherence, some of these adhesins trigger the internalization of bacteria inside host cells (Golias et al., 2011; Ono & Uede, 2018). Some bacteria apply an interesting strategy to bind to the host cells. For example, the enteropathogenic E. coli and enterohemorrhagic E. coli via injecting a Tir effector into the plasma membrane host cell create exogenous receptors for themselves to bind to the host cell through their intimin proteins. After this interaction, Wiskott-Aldrich syndrome protein and the actin-related protein 2/3 complex are recruited as host cytoskeleton regulators. Through this remodeling Tir effectors make a direct connection between the bacteria and cytoskeleton of the host (Mao et al., 2017; Ngoenkam et al., 2021). 1.2.2 Host invasion On attachment, pathogen invades the host. Invasion of pathogens to epithelia mainly mucous membranes is easier than invasion of the skin. Because the former is a single cell layer while the latter is tough and multilayered which just trauma, insect bites or other damages make invasion possible. Although the invasion to mucosal surfaces is not very easy due to the presence of mucosal and submucosal glands which secrete a protective network of carbohydrate-rich glycoproteins called mucin. Mucin traps bacteria and

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limits their accessibility to mucosal cells. Nevertheless, there are some pathogens which reside within the mucus layer or penetrate the mucus and adhere to epithelial cells through various strategies like degradation of mucin. Also, pathogens with no mucin-binding surface proteins or carbohydrates can pass the mucin layer without limitation (Kohler, 2002; Lewis, Richards, & Mulvey, 2016). Infection occurs once the microorganisms invade the host and multiply in close association with the host tissues. Some invasive bacteria are obligate intracellular pathogens while most are facultative intracellular pathogens. Various gene products are mostly involved in the invasion. The invader pathogens should repel the host defenses (Thakur et al., 2019). Some pathogens through switching their surface antigens confound the immune system of the host (Deitsch et al., 2009). Also, pathogens express enzymes with the ability to deactivate or even annihilate immunity cells such as phagocytes. Through this strategy and attenuating immunity of surrounding host tissues, pathogens can spread easily. Some of these invader pathogens showed antibiotic resistance and their associated infections are not easily eradicated. In normal conditions, after the passage of microorganisms from the epithelial barrier macrophages, including mononuclear phagocytes and neutrophils in the tissues identify them through their receptors. Then they engulf the pathogens. The internalized pathogens are enclosed in a membrane vesicle or phagosome and their pH becomes acidic after their fusion with lysosomes, in the formation of phagolysosome. This process leads to the initiation of an antimicrobial response to eliminate the pathogens. This immune response can kill most bacteria (Weiss & Schaible, 2015). Although there are bacterial pathogens that can tolerate or avoid this condition, there are pathogens that can adapt themselves to withstand the antimicrobial activity of the fused phagolysosome. It has been suggested that the surface components of the bacteria or their extracellular products which interfere with the mechanisms of phagocytic killing can be responsible for this ability (Uribe-Querol & Rosales, 2017). For example, Brucella abortus and Staphylococcus aureus by producing various catalase and superoxide dismutase can neutralize the toxic reactive oxygen species (ROS) that are produced by oxidative enzymes of the host and consequently prevent phagocytosis(Das & Bishayi, 2010; Kim, 2015). It has been reported that S. aureus by producing carotenoids neutralizes ROS (Donegan et al., 2019). Acidic conditions of phagolysosome are a perquisite for Coxiella burnetti, Brucella suis, and S. typhimurium to express products for intracellular persistence (Hackstadt & Williams, 1981; Porte et al., 1999; Rathman et al., 1996).

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Some pathogens like Mycobacterium, Legionella, and Chlamydia spp. evade destruction by preventing phagosome-lysosome fusion and consequently the discharge of lysosomal contents into the phagosome environment. Also, some pathogens like Rhodococcus spp. and Yersinia pseudotuberculosis via attenuating the acidification of phagolysosomes can survive under this harsh condition. It has been shown that Y. pseudotuberculosis attenuates vacuolarATPase activity (Tsukano et al., 1999). Disrupting the vesicle membrane to give access to the cytoplasm is another coping strategy that is applied by Actinobacillus spp., Listeria spp., Rickettsia spp., and Shigella spp. (Gouin et al., 1999). Phospholipase A is a bacterial enzyme that is applied by pathogens to dissolute phagosome membrane. Listeriolysin O (LLO) is a bacterial cytolysin that is used by Listeria monocytogenes to escape into the cytoplasm. Also, phagosomal vacuole can be lysed by Shigella spp. to evade phagosome. Subsequently, it induces cytoskeletal actin polymerization to move intracellularly and spread among cells (K€ uhn et al., 2020; Schnupf & Portnoy, 2007). Pathogens like Streptococcus pneumoniae (using streptolysin) and components of Mycobacterium spp. via suppressing the inflammatory response inhibit chemotaxis phagocytes (Pinkney et al., 1995; Uribe-Querol & Rosales, 2017). Also hiding the antigens with components that are recognized as “self” by the host phagocytes is another evading strategy. This strategy is applied by Staphylococcus aureus, Treponema pallidum, and Group A streptococci. Sometimes pathogens by penetrating areas where phagocytes have no access to them avoid phagocytosis (Cameron et al., 2004; Valderrama & Nizet, 2018). Some bacteria enter the host cells that are not naturally phagocytic such as epithelial and endothelial cells lining mucosal surfaces and blood vessels. These bacteria induce their internalization through zipper and trigger mechanisms. In zipper mechanisms, the pathogens bind to host surface receptors which have a role involved in cell-matrix or cell-cell adherence (Finlay & Cossart, 1997). With this binding extension of the host membrane subsequently bacterial uptake are occurred (Cossart & Sansonetti, 2004). For example, Yersinia spp. by its invasin adheres to β1 integrins as host cell adhesion receptors which induces bacterial uptake (Clark et al., 1998). Other bacteria such as Helicobacter pylori (Kwok et al., 2002), Listeria monocytogenes (Lecuit et al., 1997), Neisseria spp. (McCaw et al., 2004), and some streptococci apply this mechanism. In latter mechanism pathogens directly interact with the cellular machinery without the need to initial adhesion to the surface of host cells. In trigger mechanism pathogens such as Salmonella spp. (Hayward & Koronakiss, 2002) and Shigella flexneri (Van Der Goot et al., 2004) inject effectors via a

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type III secretory system which consequently results in cytoskeletal rearrangements and entrance of pathogens through micropinocytosis (Finlay & Cossart, 1997). Some bacterial pathogens have unique invasion mechanisms. For example, Streptococcus pyogenes strains apply SfbI protein (Streptococcal fibronectin binding protein) for attachment, uptake, and invasion. In this condition actin is not recruited while in S. pyogenes strain A8 with no SfbI gene cytoskeletal proteins are rearranged and cellular leaflets are generated (Molinari et al., 1997, 2000). To create successful infections microorganisms should multiply within the host after overcoming the host defense or its circumvention. So pathogens need a specific time to multiply and fight with the immune system. The infective dose (ID) shows the potential of a pathogen to cause a successful infection. Various factors, including the nature of the pathogen, exposure route, age and the immune status of the host affect ID (Schmid-Hempel & Frank, 2007). The necessary nutrients and factors should be obtained by pathogens to multiply. Some of these elements such as iron are limited within eukaryotic hosts. So, pathogens such as Salmonella spp. and E. coli synthesize iron chelating agents with high-affinity, siderophores, to extract iron in Fe3+ from lactoferrin (Naidu et al., 1993). 1.2.3 Collagenase Clostridium histolyticum and Clostridium perfringens by producing collagenase degrade collagen of the host and are mainly involved in the pathology of gas gangrene by which the connective tissue barriers are destroyed (Legat et al., 1994; Rood, 1998). 1.2.4 Spreading factor Hyaluronidase of streptococci, staphylococci, and clostridia is considered as a “spreading factor.” It adversely influences tissue matrices and intercellular spaces by degrading hyaluronic acid. Therefore, these pathogens can spread easily. Some pathogens like streptococci and staphylococci produce streptokinase and staphylokinase, respectively. By these enzymes, pathogens transform inactive plasminogen to plasmin that by digesting fibrin prevent blood clotting. This event makes rapid diffusion of pathogens. Other bacteria such as Mannheimia haemolytica use sialidases or neuraminidases breakdown sialic acid residues and enhance their spread and invasion (Corona Torres, 2017; Gladysheva et al., 2003).

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1.2.5 Overcoming the immune system Once the entrance of pathogens, innate immune systems can detect them using recognition receptors like toll-like receptors (TLRs). These receptors recognize pathogen-associated molecular patterns such as peptidoglycan, teichoic acids, LPS, mycolic acid, and mannose that are unique to microbial cells. This recognition results in the production and secretion of a variety of cytokines by defense cells. Consequently, the promotion of innate immune defenses, activation of proinflammatory signaling cascades, and the complement and coagulation cascades, phagocytosis as well as apoptosis occurred (El-Zayat et al., 2019; Wilson et al., 2002). 1.2.6 Toxins Toxins are proteinaceous or nonproteinaceous molecules (like LPS and teichoic acid in Gram-negative and positive bacteria, respectively) produced by bacteria to destroy or damage the host cell. Usually proteinaceous or nonproteinaceous are exotoxins and endotoxins, respectively. The proteinaceous toxins are generally enzymes which are delivered to eukaryotic cells by two different methods: (1) secretion into the surrounding milieu or (2) direct injection into the host cell cytoplasm via type III secretion systems or other mechanisms. There are four types of bacterial exotoxins according to their amino acid composition and function, including (1) A-B toxins, (2) proteolytic toxins, (3) pore-forming toxins, and (4) other toxins. Several different bacterial species can produce A-B toxins including P. aeruginosa, E. coli, Vibrio cholerae, Corynebacterium diphtheriae, and Bordetella pertussis (Odumosu et al., 2010). These toxins possess two components: the A subunit with enzymatic activity; and the B subunit with the ability to bind and deliver the toxin into the host cell. There are various A subunits with various enzymatic activities, including proteolytic activity (e.g., botulinum from Clostridium botulinum, tetanus from Clostridium tetani, elastase, and protease IV from P. aeruginosa), and ADP-ribosylating activity (e.g., cholera, pertussis, diphtheria, and P. aeruginosa) (Simon et al., 2014). A subunits proteolyze specific host proteins which result in clinical manifestations of the disease. For example, botulinum and tetanus toxins act on synaptobrevins and disrupt the release of neurotransmitters and paralysis (Dolly, 2003). P. aeruginosa strains by applying elastase and protease IV degrade cellular matrix proteins that lead to infection spread (Schmidtchen et al., 2003). Some toxins disrupt the host cell membrane and subsequently lyse cell by forming pores in it or enzymatic attack by lecithinases, phospholipases

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(produced by Clostridium perfringens), and hemolysins (produced by staphylococci and streptococci) on phospholipids, that disrupt the membrane (Flores-Dı´az & Alape-Giro´n, 2003; Marchlewicz & Duncan, 1980; Wilson et al., 2002). Also, Leukocidin which is produced by staphylococci lyses phagocytes and their granules (Spaan et al., 2017). There are other toxins which can degrade immunoglobulin A, activate guanylate cyclase, and modify the host cell cytoskeleton (Henderson & Owen, 1999). 1.2.7 Quorum sensing Pathogenic bacteria should coordinate virulence factors and determinants to survive, colonize, and establish an infection. In this regard, bacteria apply a complicated process to regulate the expression of their genes according to environmental conditions. For this purpose, bacteria communicate with each other through a phenomenon known as quorum sensing (QS) which is a cell density-dependent process. It has been shown that QS mutants possess less virulence compared to their wild-type counterparts in some of the infection models (Kalia et al., 2018). Many bacterial pathogens, including Escherichia coli, Pseudomonas aeruginosa, Bacteroides, Yersinia, Burkholderia, and Enterococcus spp., and many clinically important staphylococcal and streptococcal pathogens were shown to contain QS genes (Lupp et al., 2003; Miller & Bassler, 2001; Parsek & Greenberg, 2000; Qazi et al., 2006; Shiner et al., 2005; Williams, 2007). The bacterial QS system consists of two main components, including autoinducers (AIs) and their membrane-bound cognate receptors. Bacteria synthesize small diffusible, extracellular signaling molecules during their growth known as AIs. Using them, bacteria can suppress or induce various genes including virulence ones in a concentration-dependent manner. For example, using QS pathogens activate their virulence genes whenever their cell density is high enough that they can overcome the immune system. Otherwise, the immune system easily removes them. In other words, QS determines a suitable time for pathogens to make an infection. In fact, QS by helping pathogens to colonize, invade, produce the virulent factor or resist against immune system or antibiotics regulates microbial pathogenesis (Rutherford & Bassler, 2012). For this purpose, AIs should be produced, released, and detected. Whenever the threshold concentration of the signal molecule is achieved, the signal molecules interact with a transcriptional regulator, allowing the expression of specific genes (Bassler, 2002). There are AIs with various molecular-weight. N-Acylhomoserine lactones (AHLs) are the most common and well-studied AIs (Coquant et al., 2020).

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QS in Gram-positive bacteria utilizes short and posttranslationally modified peptides and is relay on phosphorylation/dephosphorylation cascades that result in transcriptional regulation of target genes. The ATP-binding cassette transporter exports these peptides out of the cell where they interact with the external domain’s sensor proteins in the cell membrane (Ng & Bassler, 2009). This interaction leads to the expression of target genes through a two-component signal transduction system. QS in Gram-negative bacteria is relying on N-acyl-L-homoserine lactones. It is composed of fatty acyl chains (4–18 carbons) linked to lactonized homoserine through an amide bond. LuxR-like proteins are the major regulatory proteins found in Gram-negative bacteria. There are LuxI-type synthases in many bacterial species which produce AHLs (Case et al., 2008). Generally, LuxR-type receptors, cytoplasmic transcription factors, detect AHLs in Gram-negative bacteria. LuxR proteins bind to ligands and target genes through their amino-terminal a carboxy-terminal domains, respectively. Most LuxR-type receptors cannot fold in the absence of the cognate autoinducer and subsequently are degraded. While binding the LuxR proteins to their autoinducers makes them stable. So, they can dimerize and bind to upstream of target genes which are called lux boxes (Smith et al., 2006).

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CHAPTER 4

Growth of microbes and biofilm formation on various materials Sukesh Kumar Bajirea, Sandesh G. Sanjeevab, Renjith P. Johnsonb, and Rajesh P. Shastrya a

Division of Microbiology and Biotechnology, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India b Polymer Nanobiomaterial Research Laboratory, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India

1. Introduction Microorganisms were reported to have evolved 3.5–3.8 billion years ago as suggested by molecular phylogeny and geobiology after the evolution of the Earth (Cavalier-Smith et al., 2006). Since evolution and with various scientific revolutions, microorganisms made a tremendous contribution either in developing an infection or making a new path for various scientific conducts. It is estimated that a total of 4 to 6  1030 diverse microbial communities advanced in every terrestrial ecosystem sampled to date (Mendes et al., 2017). A dominant and well-known hypothesis addressed in microbiology is that most of the evolved microbes in the ecosystem tend to survive in communities that are closely “associated” with the surfaces, since the “association” is a major key in the successful initiation of bacterial colonization on the various surfaces (Dang & Lovell, 2015). Interestingly, proliferating cell growth into communities, various material surfaces will have a broad range of key structures which saves the cell from various environmental predisposing stress conditions, thus facilitating the microbial population to reserve the genotypic variations (Kim et al., 2010). Microbial adhesion to solid surfaces has been a hot trend for long which is the current topic of the chapter. Reports regarding the activity of the attachment of microbial communities to the surfaces were repeatedly given by PS Meadows, the first person to determine the effect of salinity on the adhesion of both marine water and freshwater microbes on a glass slide (Wolinska et al., 2017). Most of the studies were focused on the adhesion of the microbial communities on the biotic surfaces, especially on the teeth and epithelial layers, followed by the increased report showcased on abiotic surfaces ( Jakubovics, 2015;

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Lamont et al., 2018). Solid surfaces that are in direct contact with the microbial communities were said to colonize easily in the natural environment, unescapable adherence of the microbiota either by the development of biofilm or surface adhesion has a major implication in the ecological nurture, industrial growth, and human health (Goodrich-Blair, 2021). As said earlier, surface association with the microorganism is a key feature in the regulation of nutrient cycling and degradation of xenobiotic compounds and is hence responsible for biofouling of the man-made developments and infections persisted with the indwelling medical systems (Lebeaux et al., 2014). The physicochemical characteristic features of the substratum include net surface charge, surface hydrophobicity, free energy residing on the surface, critical surface tension, surface wettability, surface molecular topography, etc. (Zheng et al., 2021). It is known that the alteration of the colonization on the solid surfaces can be made by modifications followed on the surface physicochemical interactions. The action of the microbial attachment on the solid surface in the case of health-care setting is a considerable issue, microbes tend to attach to human tissues, lungs, urinary tract, eyes, and chronic wounds (Zachary, 2017). Common implantations such as catheters, prosthetic cardiac valves, and intrauterine devices reveal serious threats to humankind and decrease the lifespan of the intradevices by microbial attachment. Since the issue raised around the surface modifications contributed by the microorganism, biofilm formations take the major negative initiative in environmental, industrial, and biomedical areas. Biofilm formation is not affected by the environment where it will take place, but by the interference of nutrient feasibility, hydrodynamics, temperature, pH, and microbial surface structures (Lorenzetti et al., 2015). Biofilms were known to affect the efficiency of the industrial organization, aquaculture, and coastal industries (Pommet et al., 2008). Ensnarling behavior showed by the organism increases the surface roughness resistance, thus causing uninvited friction between the hull and water. In addition, reports on the biofilm-dependent friction generated with the immersed offshore structures (cages, nets), and onshore equipment such as pumps, pipelines, and filters, leading to high drag and speeded biocorrosion of the same (Galarce et al., 2020). The Center for Disease Control and Prevention (CDC) estimated to be total of 48 million infectious cases, 128,000 hospitalizations, and 3000 deaths were reported due to the formation of biofilm in food industries, in a study between 1996 and 2010 (Abdallah, Benoliel, Drider, Dhulster, & Chihib, 2014). The sessile, ubiquitous ABCs of the biofilm and increasing resistance to most

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of the antibiotic therapies imposed a greater challenge in the health-care setting. Nosocomial infections were reported to be common in most hospitals, with developing resistance to antimicrobial therapies and planktonic cell counterparts with the obscured susceptibility to the host defense structure (Bai et al., 2021). National Institute of Health (NIH) revealed that 65% of the microbial infections enhanced its virulence activity by the biofilm, which was known to grow on indwelling medical instruments, estimated cost burden of 7 billion EUR in European countries (Fair & Tor, 2014). Thus, developed biofilms may lead to a generation of new infections in the case of an immunosuppressed individual; with the surface structure modifications either by material degradation or affecting the normal function of the medical setting. Surface modifications are one of the novel, the simplest, and most reasonable strategy put forward to inhibit the surface colonization of the microbial populations and either to prevent the biofilm formation or alterations leading to delayed development of biofilm formation (Bazaka et al., 2015). However, recent advances in surface modification of solid surfaces have put in touch, who evaluated the effectiveness of the developed surface modifiers, which mimic the real scenario, especially in the field of hydrodynamics. Many in vitro platforms were put forward to assess microbial adhesion and biofilm formation such as Robbins’s devices, flow chambers, rotating biofilm devices, microplates, and microfluidic devices whose pros and cons were discussed (Gomes & Mergulha˜o, 2021; Kim et al., 2012). These assessment strategies were constantly used in various industries as well as medical fields. These devices mainly aim in evaluating the effects of different surface substratum characters, microbial strains, shear forces on communicating molecules in the organism, biofilm formation and the ability to control the flow in hydrodynamics (Azeredo et al., 2017). Interestingly, reports have been shown the activity of altered solid surface by polyurethane coating presupplemented potent anti-QS compound 6-methylcoumarin (Bajire et al., 2021). The study reported the enhanced inhibition of biofilm formation and other virulence factors on the solid surfaces, which is validated using Caenorhabditis elegans host-pathogen interaction. Similarly, antimicrobial coatings were prepared using PDMS (polydimethylsiloxane), which is supplemented with antibiotics used to inhibit most of the pathogenic microorganisms such as Escherichia coli, Proteus mirabilis, Staphylococcus aureus, Staphylococcus epidermidis, etc. (Armugam et al., 2021). These coatings were found efficient in inhibiting the biofilm formation and virulence factors associated with the direct contact killing mechanism. In addition, biocompatible nanomaterial coatings such as graphene conjugated with silicon

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and silicon dioxide were found efficient in killing Gram-positive and Gramnegative bacteria such as Salmonella typhimurium, E. coli, Staphylococcus aureus, etc. (Wang et al., 2017). The toxicity of these compounds was found to be very low and suggested to use in medical personnel settings. In this regard, this book chapter explains the mechanism of microbial adhesion on the various surfaces and facilitates the understanding of the surface characteristic features of the adhering microorganisms.

2. Surface characteristics features in feasible microbial adhesion 2.1 Differential charge on the surface Surface charge and its density are the key factors to be considered during the attachment of microbes to the surfaces (Fig. 1). Reports suggest that considering the van der Waals force and electrostatic force of interactions generated between the adhering microbe to the material surface drives the rest of the mechanism such as biofilm formation (Feng et al., 2015). Bacteria secrete some chemical substances such as carboxyl, amino, and phosphate groups which contain a net negative charge on its cell wall, which will help in the initiation of the adherence to the surface (Vollmer et al., 2008). Initial adhesion and growth of Pseudomonas aeruginosa on poly(methacrylates), a positively charged material, was studied which gives confident information about the attraction of the charged particle on the Pseudomonas aeruginosa to the positively charged ions on the surface (Gottenbos et al., 1999). A study reported positively charged poly(allylamine hydrochloride) (PAH) multilayers collectively attracted to the negative charge present on Pseudomonas aeruginosa (Alba et al., 2014). They also added the binding affinity of some Gram-negative organisms such as E. coli and Gram-positive bacteria Staphylococcus aureus to the polyelectrolyte multilayer controlled the concentration of the microorganism by modifying the surface charge on the material. Since the mechanism of development of biofilm formation is also initiated by the surface charge, but not a coincident (Guzma´n-Soto et al., 2021). The statement was supported by a study who’s the surface activity was studied by E. coli, the negatively charged ions in the bacteria attach to the positively charged surfaces, but the increased charge on the surface affect the lower viability and in turn also reduces the biofilm formation later on. Negative charge on the Gram-negative bacteria is also because of the lipopolysaccharide, presence of surface appendages such as fimbriae which is able to guide the adhesion to the oppositely charged surfaces. Thus, a need in research to

Fig. 1 Schematic representation of major surface properties on the solid revealing impact by bacterial motility, hydrodynamics, bacterial surface sensing, and its process of initial adhesion.

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demonstrate the relationship between the charged surfaces with the bacteria and its biofilm formation (Bertani & Ruiz, 2018).

2.2 Surface wettability Surface wettability is an important property and plays a swivel role in interactions between the surface and microbe which governs the solid-liquid interphase (Fig. 2). The liquid phase of the microbes wets the solid surface which in turn increases the surface area required and also increases the physical interactions bound between the solid and liquid (Krawczyk, 2018). Surface energy bound also takes the major turn, surface with low surface energy and the culture-bound liquid with high surface energy tends to reduce the wettability property, surfaces with low surface tension and liquid bound with low surface tension increases the surface wettability property in turn facilitates the microbial adhesion (Kung et al., 2019). There are conflicts raised by the reports published on the initial adhesion of the microbes on surfaces with moderate hydrophobicity and hydrophilicity. Since the engineered surface modified materials showed the maximized water in contact angle with the surface, referred to as superhydrophobic or super hydrophilic which is reported to reduce the microbial adhesion on the surfaces. The statement was supported by the work, the adhesion of E. coli on the super hydrophilic surfaces was technically impaired due to the altered surface contact angles as compared with the control surface (Yuan et al., 2017).

2.3 Surface roughness Development of the high indexed surface wettability material surface can be achieved by the application of surface roughness (Fig. 2). Surface roughness is known to increase the surface area available and enables the luxurious initiation of bacteria adhesion thus by providing a scaffold (Yu et al., 2016). It also acts as protective barrier for the colonizing microbe against shear forces bound, thus preventing the microbe from detaching. Thus, the increase in surface roughness of the structures also increases the microbial adhesion and thus concluding the first step in microbial colonizations leading to biofilm formation (Limoli et al., 2015). Many reports published regarding the surface roughness modifications acted against Staphylococcus aureus, Sepidermidis epidermidis, Pseudomonas aeruginosa, and Ralstonia picketti, shown to develop resistive biofilm on the rougher surfaces as compared with smoother surfaces (Zheng et al., 2021). Similar work has supported the mechanism in the case of oral bacteria where the development of the virulence and biofilm

Fig. 2 Pictorial representation of bacterial biofilm formation on an efficient solid surface. Pictures highlighted the major processes of bacterial biofilm formation with the help of quorum sensing and exopolysaccharide.

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formation is enhanced with the increased roughness of the surface independent of the material used for the surface. In addition to surface roughness, other variables were also tending to affect the microbial activity, attraction, and attachment toward the surface roughness, different microbial species tested and the specific medium used to create an artificial environment (Cheng et al., 2019). Such criteria were also put into an account for the appropriate evaluation of the microbial adhesion regulated surface roughness.

2.4 Surface topography Surface topography is the structural arrangement of the natural as well as artificial properties in a selected area (Fig. 2). Microbes were proved to say that they are sensible enough to identify the mechanical cues related to the surfaces such as attachment of the microbes through the hydrodynamics, nanostructure associated with the surface may alter the adherence of the microbes on the surface (Holban et al., 2021). Noncovalent bonds generated between the microbe and substrate may be directly influenced by the topographic characters (Wong et al., 2021). The topographic characteristics of the larger surface than the attaching microbe facilitate the greater contact area and thus, in turn, favoring the microbe by providing shelter and also increasing the probability of attaching to the surface. It is necessary to understand, many therapeutics have been put in advance to prevent the biofilm formation by the modification lead in the surface topographic features, microstructures also have shown to affect the initial adhesion of microbes to the surface wisely (Wu et al., 2018). The broad spectrum of therapeutics has been introduced to inhibit the biofilm formation and its application in an industrial setup. Reports published on topographic modification studies revealed the potent feature in preventing biofilm formation by Pseudomonas aeruginosa, Staphylococcus aureus and E. coli, etc. (Koo et al., 2017).

2.5 Surface stiffness Young’s modules are a unit for the measurement of elasticity of a material and its factors come into consideration when the subject is about surface stiffness counter related to microbial adhesion and biofilm formation. Young’s modules are the common parameter measured by the ratio of stress to strain (Pacha-Olivenza et al., 2019). Parameters such as low Young’s module are said to have material that is sifter and more elastic (Fig. 2). Various studies have been reported regarding the stiffness variation resulting in

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the altered bacterial adhesion which in turn varied microbial population after the speculated interval. Using biocompatible polymers and hydrogels such as poly(ethylene glycol) Di methacrylate as substrate acted against the attachment of microorganisms such as E. coli, Staphylococcus aureus; and the adhesion of the respective microbes increases as the regulated increase of the material stiffness, irrespective of the hydrogel nanocomposite structure and adhesive mechanism (Li et al., 2011). Similar studies have been reported with the help of material polydimethylsiloxane exhibiting a negative interaction with the surface used and the bacterial population selected (Ren et al., 2015). Comparative studies have been put into work giving the contrasting conclusion, effect of various surface stiffness properties tested against both the Gram-negative and Gram-positive bacteria. There has been an interesting positive conclusion revealed when the surface stiffness correlated with the Staphylococcus epidermidis and found a negative correlation with the E. coli tested (Khatoon et al., 2018). In addition, no correlation was observed for Lactobacillus lactis on the same material used, the reason behind the varying adhesion is due to the altered elastic modules selected in the material. Besides all the experimental data reported, the density of the polymer used, chemical properties of the cross-linker in the surface, and cross-linker used, material hydrophobicity, viscosity, and topographic stiffness conjugate integrated with the same surface also took the major part of the responsibility on the colonization (Thanabalasuriar et al., 2019). Further potential mechanism of the bacterial adhesion with the surface associated stiffness is still in query; research regarding the intrinsic material properties needed to assist with the adhesion. 2.5.1 Perseverance of microbes on solid surfaces Most of the microbial persistence were studied on dry surfaces as well as the nutrient-rich aquatic conditions using specific artificial contamination with the optimized prototype in the laboratory conditions. Gram-negative organisms such as E. coli, Klebsiella pneumonia including VRE (vancomycin-resistant enterococcus), and Gram-positive organisms such as Staphylococcus aureus especially MRSA found to survive on dry surfaces for months (Zulkifli et al., 2018). Reports available say the survival difference between multiresistance and susceptible strains of Staphylococcus aureus and E. coli was limited (Chambers & DeLeo, 2009). Many Gram-negative bacteria such as Pseudomonas aeruginosa, Acinetobacter boumannii, Serratia marcescens, and Shigella sp., were able to survive on inanimate surfaces for months, which are also frequently isolated from patients with hospital-acquired infections ( Jung et al., 2021). However, other

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rarer Gram-negative organisms such as Bordetella pertussis, Haemophilus influenzae, Proteus vulgaris, and Vibrio cholerae were able to survive on the solid surfaces only for limited days. Some mycobacterial species such as Mycobacterium tuberculosis and spore-forming bacteria Clostridium defficile were reported to survive for more than a month (Kramer et al., 2006). Interestingly, bacterial persistence in the paper is pervasive in the case of health-care settings, scientists worldwide isolated many Gram-negative and Gram-positive bacteria after contaminating the office papers with the standard inoculum size. Collectively, the organism was stable at room temperature, and survive on the paper system at a reduced count by 3 log10 after a week (R€att€ o et al., 2005). Furthermore, the transmissibility of the microbe from the hand to paper and back to the hand was demonstrated for selected bacteria, revealing paper money could anchorage the pathogenic bacteria thus may lead to an outbreak. Persistence and transmissibility of the viruses on the solid surface is a peculiar mechanism but with a survival rate limited to a few days. It is found that most of the viruses could be isolated from the respiratory tract were able to isolate Corona, Coxsackie, Influenzae virus, SARS, and rhinovirus survived for only a few days when they are in direct contact with solid surfaces. Herpes viruses such as Cytomegalovirus and herpes simplex virus were found very sensitive to the natural environment and survived only up to a few hours (Leung, 2021). Besides the viruses found in the respiratory tract, viruses such as Astrovirus, Hepatitis A virus, Picornaviridae, and Rotavirus which were commonly found in the gastrointestinal tract would harbor the viruses for up to 2 months (Witkowski et al., 2017). Some of the important bloodborne pathogens such as the human immune deficiency virus and hepatitis B virus could last its availability up to 2 weeks (Sun et al., 2014).

2.5.2 Biofilm formation Biofilm is a pack of irreversible association of microbial cell components compacted toward any solid surface, enclosed with the help of polysaccharide matrices. Materials including minerals, crystals, corrosive components or slit particles were also found in the matrix-bound biofilm formation irrespective of the surrounding environment (Kooli et al., 2018). Biofilm formation is wisely associated with the freely suspended cell components called planktonic cells which counter changes with the genes transcribed. As said, biofilm formation is readily developed in living tissues, medical instruments, and portable water systems, variable nature of biofilm formation can be characterized with the help of scanning electron micrographs (Mosharaf et al., 2018).

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The biofilm developments on the solid surface are composed of a single, coccoid organism and complexed with the extracellular polymeric substance’s matrices (Fig. 2).

2.6 Initiation of solid-liquid interface The solid-liquid interface between the surface and aqueous medium facilitates the optimum attachment of the microbial population. The clear picture of the attaching microbial communities on the surface is highly dependent on substratum compo sum, the conditioned film developed during the process of attachment, hydrodynamics of the aqueous medium, chemical makeup of the media, and also various morphological properties of the cells (Khatoon et al., 2018; Subbiahdoss & Reimhult, 2020).

2.7 Substratum compo sum The solid surface in media takes the majority of the considerations which are said to be important during the attachment of microbes. Microbial colonization increases eventually as a rise in the potential surface roughness, due to increased surface area and diminished shear forces (Palmer et al., 2007). The extent of the microbial attachment eventually varies with the physicochemical properties presented on the surfaces. There have been reports that microbial population rapidly and readily attach to hydrophobic, nonpolar material such as Teflon, hydrophilic substances such as glass or metals, etc. But the reports given were in dilemma due to the insufficient standardized methods to prove the surface hydrophobicity (Krasowska & Sigler, 2014).

2.8 Conditioning film Any surface when exposed to an aqueous medium will develop an inevitable layer and immediately develop a conditioned layer by the copolymers from the media, thus having a substrate effect and extent of the microbial adhesion on the surface (Desmond et al., 2018). Loeb and Neihof (1975) were the first in the list to say that biofilm formation is facilitated by the conditioned film extracted from marine water. The Loeb and Neihof’s (1975) study reported the physicochemical properties of the conditioned film, and added that films were organic in nature, and developed immediately after exposure to marine water (Lorite et al., 2011). Structural and chemical properties of the conditioning film may vary extensively as changes in surface properties when exposed to human hosts.

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The sentence supported by the report, protein-rich conditioned film developed is referred to as “acquired pellicle” which readily formed in the tooth enamel surfaces in the oral cavity. Pellicles thus developed composed of albumin, lysozyme, glycoproteins, phosphoproteins, lipids, and some pleuritic fluids (Bhagwat et al., 2021). When the microorganism was exposed to aqueous media, the acquired pellicle-conditioned surfaces were developed within short period of time. Many reports have given the idea of the vas host produced and conditioned film by blood, tears, urine, saliva, pleuritic fluids, vascular fluids, and respiratory secretions enhances the spectrum of the adhesion of microbes (Derlon et al., 2012).

2.9 Hydrodynamics According to the basic principles of hydrodynamics, the velocity of the flow of the liquid which is adjacent to the substratum/liquid interface is always negligible. The null value created by the slow is considered as a hydrodynamics boundary layer, whose media depend on the linear velocity; when the velocity rises, the decrease in the boundary level is seen (Krsmanovic et al., 2021). There is a region outside the boundary layer meant for substantial mixing or to have space for sufficient turbulence. The cell-substratum compo-sum interactions were primarily affected by the substantial hydrodynamic boundary layer as the subresult of minimum or the limited turbulence (Fernandez-Rodriguez et al., 2021). In addition, the velocity of freely suspended particles in the cell’s media is directly associated with the settling period with the submerged surfaces. Thus, with decreased velocities of the cell particles in the media, the cell will have a seizable hydrodynamic boundary layer and the association and attachment of the cell to the surface will have a greater impact by the cell due to varying size, motility offered, etc. In simple, the rise in cell velocity is inversely proportional to the decrease in the boundary layer, which in turn cells in the boundary layer are subjected to greater turbulence and mixing (Lawrence & Caldwell, 1987). Greater linear velocities in the submerged layer would help in the rapid association with the surface, clear enough to substantiate the shear forces on the attaching cell and surface. The viral adsorption on the surface is quite peculiar, and also affected by the surface hydrophobicity, specifically the outer surface layer proteins such as capsid, etc. (Gerba, 1984). Reports available to understand the viral adsorption with the solid surface with the example of hydrophobic viruses which allowed to react with the surface modified by hydrophobic sorbents

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hydrophilic surface interacted with the hydrophilic surface proteins (ElHadidy et al., 2013). The statement suggests the fundamental principle in understanding the potential functions of the hydrophobic surface proteins and compatible surfaces. The study used both computational and experimental analysis and proved the theory of surface protein interaction with the solid surface (Mi et al., 2014).

2.10 Aqueous medium milieu Chemical properties of the medium such as pH, nutrient levels, ionic strength, and temperature will have a major role in the rate of the attachment of the microbe to the surface. Vast studies have reported the substantial seasonal effect on the conditioned layer formed followed by the biofilm formation in varying the chemical compositions of the media (Chen et al., 2020). This is due to the water temperature, pH, and other unmeasured seasonal parameters. The study reported by fletcher showed cationic variations such as sodium, calcium, lanthanum, and ferric ions shown to affect the Pseudomonas fluorescens on the glass surface, due to reduced repulsive forces between the negative charge on the bacteria cell and the same on the glass surface. In addition, nutrient composition in the medium is also directly proportional to the number of bacteria cells adhered on the solid surface (Meadows, 1971).

2.11 Properties of the cell in biofilm formation Some of the basic cell structural components such as hydrophobicity associated with the cell surface, fimbriae, and sex pilus including flagella and EPS production were said to have a significant influence on the rate of microbial adhesion. Hydrophobic interactions tend to increase as an increase in the nonpolar nature of the surfaces participated (Donlan, 2002). Secondly, most of the microorganisms were negatively charged due to the presence of cations in the cell surface which tends to attract the oppositely charged surfaces. The structures involved in the motility and conjugations are directly involved in the transfer of bacterial as well as viral nucleic acids which are involved highly in cell surface hydrophobicity (Dussud et al., 2018). The presence of proteolytic enzymes in the cell contributes to a majority of the modifications during the biofilm formation, mycolic acid rich bacteria such as Corynebacterium, Nocardia, and Mycobacterium were reported to be highly hydrophobic in nature as compared with the nonmycolic acid contented bacteria. Similarly cell somatic antigen “O” readily mediated by liposaccharide shown to counter with hydrophilic properties, especially

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in the Gram-negative bacteria. Sentence supported by a prime example, Pseudomonas fluorescens which lacks somatic antigen shown to have a higher affinity toward the hydrophobic surfaces (Lam et al., 2011). Some of the important functional protein-carbohydrate conjugate, a lectin from the cell surface shown to have enhanced activity toward the inhibition of the cells but not the attachment (Mishra et al., 2019). Similarly, structural amino acids such as Glucosidase and N-acetyl glucosaminidase present in the Pseudomonas fluorescence reduced the attachment on the solid surfaces while, N-acetylglucosamide in Desulfovibrio desulfuricans reduced the attachment on the surfaces (Banar et al., 2019). Lectin competitively binds to the lipopolysaccharide or the exopolysaccharide on the cell surface, which in turn would turns down the bacterial active attachment site, which is proved by the study conducted on Pseudomonas aeruginosa (Ma et al., 2012). Motility features associated with the cell surface of Pseudomonas strains showed enhanced adhesion to the surfaces and also attached against the flow (Lecuyer et al., 2011). Flagella take a major part in the initial attachment of the microbial colonization on the surfaces. Microbial adhesion to the substratum is showcased as complex, with “n” number of regulatory factors affecting the outcome (Haiko & Westerlund-Wikstr€ om, 2013). Attachment is seen very readily and rapidly on surfaces with greater surface stiffness as well as surface roughness, hydrophobic nature and finely polymeric coated by conditioned film. Interestingly, eventual rise in the velocity, temperature of the media and nutritive conditions may also have an immense effect on enhance attachment till the critical level (Paulsen et al., 1997). Specialized cells surface structures such as fimbriae, flagella, polysaccharide, and proteins give a competitive advantage wisely for the polymicrobial communities involved.

2.12 Biofilm structure A composite structure of biofilm is formed of a conjugate of microbial cells and exopolysaccharides. The majority of the conjugate is composed of organic EPS, which acts as a primary matrix material for the biofilm. EPS is mainly composed of polysaccharide electrically neutral in charge sometimes poly-ionic (Ostapska et al., 2018). Uranic acid derivatives such as D-glucuronic, D-galacturonic and mannuronic acid, and ketal-linked pyruvates exhibit anionic release. The divalent cations developed between the calcium and magnesium are shown to cross link between the polymer strands of the media, developing a finite layer of biofilm (Stender et al., 2019). Teichoic acid is the main structural component of the

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coagulase-negative bacteria such as CONS, which were shown to have diverse EPS production in chemical properties, facilitated in the cationic attachment of the microbial cells on the surfaces. The hydrated structure of the EPS has the capability of incorporating a vast number of hydrogen ions (Brauge et al., 2016). EPS structure may vary for each bacterium most of the EPS extracted is both hydrophilic and hydrophobic. Sutherland (2001) reported on biofilm structure and composition and highlighted two important features of biofilm. The primary conformational picture of the biofilm was determined by the composition and structure of polysaccharides (Sutherland, 2001). The basic structure of the EPS consisted of a backbone of 1,3- or 1,4-β-linked hexose residual contents, giving a rigid, less deformable, and solubility property. The spatial arrangements of the EPS in a biofilm are always uniform but temporal arrangements may vary (Cerantola et al., 1999). The statement is supported by research conducted by a group of scientists, different microorganisms were structurally arranged to produce a different number of EPS, which may interrelate with the age of the biofilm (Brady et al., 2008). In addition, the EPS is also supported by the micronutrients such as metal ions, divalent cations, proteins, DNAs, lipid, etc. Functionally, EPS was found to have a rivalry relationship between the nutrient medium supplied, the presence of luxurious carbon, limited amino groups, and potassium contents proven to enhance the growth of the organisms (Dhar & Han, 2020). Since the EPS is a hydrating substance, slow growth will be seen in the system, which in turn enhanced the production of biofilm, is seen. Importantly, antimicrobial resistance offered by the biofilm toward antibiotics, obstruct in mass transportation of antibiotics by biofilm binding directly to the therapeutic agents. As discussed earlier by Klausen et al. (2003), contribution to microbial diversity in biofilm formation, structural attributes conjugated stay universal and constant (Klausen et al., 2003). Even though various researches are carried out on the biofilm formed, the minute knowledge regarding the same still remains contradictory. Since the biofilm structure formed of continuous monolayer deposition of EPS with micronutrients (Harmsen et al., 2010). Biofilm are considered as heterogeneous in property, composed of microcolonies of microbial cells adhered to each other by EPS and separated by interstitial voids. Biofilm structures of most of the Gram-negative bacteria such as Pseudomonas aeruginosa and K. pneumonia on steel surfaces predicted the insight structures of water channels and heterogenicity characters (Ciofu & Tolker-Nielsen, 2019). The continuous flow of the liquids on the water channels in the biofilm facilitates the nutrients, oxygen

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requirements, and therapeutic particles. Motility is also considered as a key point in luxurious biofilm formation which was investigated by TolkerNielsen et al. (2003) in Pseudomonas aeruginosa and Pseudomonas putida with the help of confocal laser scanning microscopic analysis. Polymicrobial colonization was studied with the two species of Pseudomonas; both organisms initially had developed small microcolonies on the surface. As time passed, the two microcolonies were shown to be intermixed and migration of the cells from one microcolony to another microcolony is seen. Thus, the cells in the one compact microcolony structure transferred to the polymicrobial structure. The motility observed inside the cells were ultimately dispersed out of the biofilm composite and later in stage leading to the dissolution of the biofilm architecture (Klausen et al., 2003).

3. Mechanism of action of attachment to solid surfaces 3.1 Bacterial attachment As the microbial cells were found freely suspended in the bulk liquid interface just before attaching the surface, the motile organism initiates the mechanism of attachment to the surfaces in three regions. (a) Bulk liquid noninterface with the surface, cells do not experience any effects with the surfaces; (b) near contact with the solid-surface interface, cells experience hydrodynamic effects with the surface; (c) In close contact with the solid surface interface, cells experience both hydrodynamic and physicochemical effects from the solid surface-liquid conjugate (Cheng et al., 2019). As discussed earlier, the rate of attachment of the microbial communities is directly proportional to the fluid velocity, at low velocity the nonmotile microorganism readily adheres to the active surface, whereas at high fluid velocity, there is a slight decrease in the velocity of the adhering microorganism to the active surface. There is a small gap between the differential adhesion of the motile and nonmotile microbes, Due to the presence of the functional flagella which actuate the motility, but in some cases of the organism nonfunctional flagella which failed to adhere to the surface just like the cells lacking flagella (Pizarro-Cerda´ & Cossart, 2006). Once after the initiation of the mechanical contact of the cells with the surface, the attachment of the cell takes place in two phases. The first step of the attachment is rapid and can be reversed with the hydrodynamic and electrostatic interactions. At this stage, this involves the enhanced adhesive force between the adhering cells and the active surface. The phenomenon was observed in bacterial attachment to polystyrene beads surfaces, which is

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due to the physicochemical interactions generated during (a) the loss of solidliquid interface water; (b) structural modifications on the surface of the molecules; (c) restructuring of the cell components to maximized adherence. The rate of attachment also varies according to the different phases of microbial growth, especially during the stationary phase most of the microbial cell walls contain negatively charged ions which counteract the positively charged ions on the surface, giving a rapid and enhanced microbial adhesion (Morisaki et al., 1999). But media concentration is also one of the key factors that fluctuate the rate of attachment and the rate of adherence will decrease as the iconic strength of the media increases. Similarly, quorum sensing in E. coli is responsible for the varying charge on the cell surface, as quorum sensing increases the negative charge on the surface which luxuriously facilitates the adherence of the microbes to the active surface (Goulter et al., 2009). As continuing after the first step, the second step is always irreversible, a prolonged event from minutes to several hours with the van der Waals force of attraction between the activated surface and hydrophobic region of the outer wall of the cell. Several proteins such as SadB specific to Pseudomonas aeruginosa have a major interaction with the attachment. In the case of E. coli, both lipopolysaccharide and sex pili are responsible for the enhanced rate of the initial attachment of the microbes to the activated surface. Irreversible mode of attachment of the microbial colonies on the surface is moderated by the EPS action and is capable of monitoring the surface properties of the active surface. The polymicrobial ecosystem is a crucial factor to be considered, most of the microorganisms interact specifically with the specific ecosystem (Roy et al., 2019). The statement was proved by the study, streptococcus mutants are capable of forming biofilm on the teeth surface but not on the tough surface, but in the case of Streptococcus salivarius shows the opposite behavioral action in attachment. As said earlier, microbes have a different class of the extracellular cell organelles which bridges the specific interactions between the surface and microbes. These extracellular cell organelles such as flagella, sex pili, and curli fiber, were said to be readily exempt from the specific proteins to form an adhesin, which facilitates steady attachment with the surface of the hosts. The hypothesis was explained with the example of type I pili in E. coli binds to the alpha-D-mannose specifically glycoproteins. And type IV pili of the E. coli binds to the phosphatidylethanolamine. Reversible attachment by the microbes does not need compulsorily transferred to the irreversible mode of attachment, a phenomenon well explained by the example of adhesin in E. coli promoted by pilus tip adhesin FimH helps as a surfactant and facilitated adhesion (Fletcher, 1976).

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4. Growth of viruses on solid surfaces 4.1 Viral adsorption As the bacterial surfaces, viruses are also electrically charged particles which are attracted toward the oppositely charged surfaces, which may have a major impact on the behavioral changes in the viral environment (Gerba, 1984). Since there is a rise in water pollution, and viruses contribute to most part of human diseases, improved and advanced research is required to understand the fate of the viral particles. As reported, the surfaces have innumerable colloidal particles such as clay particles and organic substances. Several studies published the mechanism of viral attachment to the surfaces with the bacteriophage-cell model, advantageous in quantitative measurements and to understand the flow kinetics of the interactive attachment (Armanious et al., 2016). The viral particle incorporates various strategies in order to attach to the solid surfaces, infecting a human host requires a bacterial host cell incorporation and then attachment to the surface. The event is regulated by the first order kinetics and the diffusion of the smaller sized viral particles incorporation into the larger bacterial cells, thus achieving the relative collision between the attaching bacteria and incorporated viruses. The viral colloidal particles are negatively charged at neutral pH since the attachment to the biological and nonbiological surfaces is entirely regulated by the nature of the surrounding media, physicochemical properties of the attaching surface and ionic composition and concertation of the media where the viruses reside. Adsorption of the viral particles on the latex substances was seen including nitrocellulose, carbon, aluminum, and gold particles (Zerda et al., 1985). At the continuous flow cycle rate of the media, as the viral particles make direct contact with the surface, it is assumed that the interference of the Brownian movement at the point of collision, but the theory does not relate to aluminum surface since the surface is positively charged and the attachment of the viral particles managed by the presence of cellular cations in the media including sodium and calcium. Due to the rise in the ionic concentration of the residing media, the thickness of the double-layered wall around the attaching virus is reduced and the vicinity between the solid-liquid interfaces were bound and attracted by each other by van der Waals force of interactions. Activated carbon materials were reported to be used as water purifiers in the removal of organic substances in case of water pollution. Since the organic matters were readily present in the natural flow water reservoirs, viruses were found prominent in competitive binding to the carbon surfaces. The study also reported the electrostatic force of interaction of the carbon material with the viruses with the reversible adhesion property (Gerba, 1984).

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CHAPTER 5

Identification and culture test Kailash Pati Pandey, Jeetesh Kushwaha, Madhumita Priyadarsini, Jyoti Rani, Yashpal Singh, and Abhishek S. Dhoble School of Biochemical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh, India

1. Introduction Microbiology includes detecting viral, fungal, bacterial, and parasitic microbes that cause disease in humans, offering diagnosis and therapy assistance for the healthcare system, and stopping infectious disease transmission in the community as well as in the healthcare system. Since the 1940s, the count of newly recognized pathogens are continuously increasing ( Jones et al., 2008). So, in the earlier 21st century, techniques such as PCR (Fournier & Raoult, 2011), sequencing, MALDI-TOF, and culture methodologies started transforming the diagnosis system. This scientific advancement, which started in environmental microbiology, has uncovered a significantly bigger microbiological world than was previously believed. Approximately 1800 valid microbial species were reported in 1980; however, more than 500 distinct species are reported every year presently ( Janda & Abbott, 2007). In 1985 and 2000, significant human microorganisms, including Helicobacter pylori, which causes stomach ulcers and tumors, and Tropheryma whipplei, which causes Whipple’s disease, were discovered. Significant improvement has been made in the case of viruses, SARS-associated coronavirus (Marra et al., 2003) the arenavirus Lujo virus (Briese et al., 2009), which cause hemorrhagic fever, were discovered in 2003 and 2009, respectively. Robert Koch initially stated that pure culture is the cornerstone of all pathogenic microbe studies (Marshall et al., 1985). Microbial culture permits researchers to investigate microbial antibiotic sensitivity, the first step in the development of therapeutic interventions (Boulos et al., 2004, 2005). Developing a sterile microbial culture allows the strain genomes to be sequenced. Protein expression research emphasizes specific proteins and immunoproteomic methods to examine their immunogenicity, enabling synthesizing these proteins as antigens for serological tests (Fournier et al., 2013). Conventional culture methods require time consuming cultivation processes, proceeded by the disc diffusion method ( Jorgensen & Ferraro, 2009) Antiviral and Antimicrobial Smart Coatings http://doi.org/10.1016/B978-0-323-99291-6.00014-1

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and broth dilution sensitivity testing (Wiegand et al., 2008), which takes many days to evaluate and deliver MIC values. Antibiotic susceptibility tests (ASTs) are used to assess antibiotic resistance characteristics of microbes, lead therapeutic approaches, and predict the response (Daniels, 2011; Jorgensen & Ferraro, 2009). AST is currently conducted at a therapeutic microbiology lab involving patient samples delivered from the health professional to the research facility. An isolated culture of the problematic pathogen is required for antibiotic susceptibility and can take many days. Because of this patient does not receive the appropriate antibiotic at a suitable time, and delay in administering suitable medicines result in a higher patient fatality, poor clinical results (Daniels, 2011), and a wide range of antibiotic use, which leads to antibiotic resistance. Clinical microbiology laboratories primarily depend on phenotypic approaches like culture and biochemical assays to identify, detect, and characterize pathogen virulence. Meanwhile, challenges to deploying molecular methods for diagnosis of pathogenic microbes’ are quickly being addressed, enabling laboratory testing and the adoption of a broad range of technologies for early diagnosis. Since the invention of polymerase chain reaction, molecular approaches have been extensively used for the diagnosis of common and complex infections. Several automated tools for identifying bacteria have been established, with their primary advantages and weaknesses (Sader et al., 2006). As a result, it is critical to use an automated system to identify and test susceptibility efficiently. It would be beneficial to obtain the results of a critical appraisal of the systems utilizing a range of microbial pathogens while choosing an automated system with the capability of unique identification of species of most microbes identified in diagnostic laboratories. Few scientific articles also have been published on the new Phoenix system have gotten a lot more attention even though the VITEK and MicroScan systems have been around for a long time (Bosshard et al., 2006; Fanjat et al., 2007; Zbinden et al., 2007).

2. Staining techniques Staining techniques are used to observe the structural properties of bacteria at the microscopic level with the help of staining dyes. Because structural details of bacteria cannot be observed under a light microscope because of lack of contrast, staining methods must be used to generate color contrast and thus increase visibility. The smear is fixed to the slide

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before staining. Fixation is preserving and protecting the internal and external structures of the specimen. It deactivates enzymes which could affect the morphology of cells and toughen cellular structure so they do not change while staining and observation. A microorganism is usually killed during fixation and permanently affixed to the microscope slide. There are mainly two types of fixations; one is heat fixation, and the second one is chemical fixation (Dettmeyer, 2018; Hillman, 2000; Penney et al., 2002). The heat fixation method is frequently used to study bacteria and archaea. Usually, a smear of cells is gently heated. Heat fixation maintains overall morphology while neutralizing enzymes. However, it destroys proteins in subcellular structures, which may cause them to appear distorted (Beveridge et al., 2007). Chemical fixation is used to protect both perfect cellular substructure and morphological characteristics. Many electron microscopy techniques use this method to analyze microorganisms. Chemical fixatives enter the cell and react with cellular components, most commonly proteins and lipids, proving them inactive, insoluble, and immobile. Ethanol, acetic acid, mercuric chloride, formaldehyde, and glutaraldehyde are widespread fixative mixtures used (Alturkistani et al., 2015; Dettmeyer, 2018). There are three types of staining techniques.

2.1 Simple staining This method is known as simple staining when only one dye is applied. Bacterial morphology is typically evaluated using heat fixed smears stained with basic dyes. The utility of simple staining comes from its simplicity. The fixed smear is stained for a small duration, the excess stain is washed with water, and the slide is dried. Simple staining with basic dyes such as methylene blue, crystal violet, and carbol fuchsin is generally used for the determination of shape, size, and arrangement of the archaeal and bacterial cells (Beveridge et al., 2007; Dettmeyer, 2018).

2.2 Negative staining Negative staining involves staining the background rather than the cell; the unstained cells appear as luminous objects against a dark background. In this method, India ink and nigrosine dyes are used, which stain the cell or area of interest directly (Beveridge et al., 2007).

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2.3 Differential staining In this method, two stains are applied. Each one provides a different color to discriminate bacteria or bacterial structure, which assists in bacterial identification (Dettmeyer, 2018). Some differential staining methods are: • Gram staining • Acid-fast staining • Albert staining 2.3.1 Gram staining Gram staining is frequently used in bacteriology; it was developed by the Danish scientist Christian Gram in 1884. The cells are stained using crystal violet dye, and subsequently, an iodine solution is used to mordant them (Beveridge et al., 2007). Ethanol is subsequently used to wash the purplestained cells. Gram-positive cell walls retain the stain, but Gram-negative cell walls wipe out the stain with ethanol. After that, cells are counterstained with safranin dyes and washed with ethanol. When observed under the microscope, Gram-positive bacteria show violet or purple color and Gram-negative bacteria show pink or red color (Fig. 1). The combination of crystal violet and iodine, and the cell wall’s thickness and complexity, particularly the molecular composition of the peptidoglycan (murein) associated protein of bacteria, define the Gram staining reaction and form a crystal violet-iodine complex. Due to the formation of a large crystal violet-iodine complex in a thick cell wall, it is not extracted after decolorization in Gram-positive bacteria; that’s why it retains the stain. But in Gramnegative bacteria peptidoglycan layer is thin, so it is wiped out with stain

Fig. 1 Gram staining steps.

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after being rinsed with alcohol (Hucker & Conn, 1923; Sizemore et al., 1990). Some Gram-positive bacteria are Streptococcus pneumoniae, Staphylococcus aureus, Enterococci, Corynebacterium diphtheria, Clostridia tetani, etc. And examples of Gram-negative bacteria are Escherichia coli, Pseudomonas aeruginosa, Klebsiella, Proteus, Salmonella typhi, Providencia, etc. 2.3.2 Acid-fast staining This method is used for the Mycobacterium species such as Mycobacterium tuberculosis and Mycobacterium leprae, which cause the disease tuberculosis and leprosy, respectively. Mycobacterium cell wall contains lipids that are composed of mycolic acid. Mycolic acid is a family of branched-chain hydroxy fatty acids that inhibits dye from attaching to cells efficiently. Due to this normal stain cannot penetrate the cell wall of Mycobacterium. For delivering the stain carbol fuchsin into mycobacterial cells, the cold Ziehl-Neelsen method uses carbol fuchsin and phenol in high quantities and is also a wetting agent. When this color has been applied, the cells are difficult to decolorize once they penetrate (Reynolds et al., 2009). They are acid-fast because they are made of acidified alcohol (acid-alcohol). Acid alcohol decolorizes nonacidfast bacteria; thus, a second dye termed a counterstain is used to stain bacteria with a different color. Acid-fast bacteria are red colored, and nonacid-fast bacteria show blue color. Actinomycetes and mycobacteria are examples of acid-fast bacteria. Hibernating endospores are also acid-fast, but the reason for this is uncertain (Beveridge et al., 2007; Fukunaga et al., 2002). 2.3.3 Albert staining It is defined as a special staining method because it is used to display a unique structure in bacteria. It is most commonly used to show the metachromatic granules which are found in Corynebacterium diphtheriae, the causal organism of diphtheria. Granules are known as metachromatic granules because they exhibit the property of metachromasia. This method displays a color that is different from the staining color. The granules show a violet color when stained by polychrome methylene blue, whereas the remainder of the bacillus looks blue. It happened because the granules are made up of polymetaphosphates but are also known by various other names such as volutin bodies, Babe-Ernst granules, or polar bodies. The bacillus stain shows green color with Albert’s stain, but the granules stain is seen as blue-black (D’mello et al., 2016).

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3. Different culture techniques Louis Pasteur was the scientist to culture bacteria for the first time in 1860. Despite the development of genomic approaches, methods based on culturing are essential because of their applications in detection and enumeration for phenotypic analysis and viability determination. Culturing of microbes started with the discovery of culture media in the 19th century (Wainwright & Lederberg, 1992). Different culture techniques and media are used to grow microbes for a specific purpose.

3.1 Methods of culture Specific culture methods are used according to the type of experiments. Some of them are mentioned below. 3.1.1 Streak culture Streaking is performed to isolate pure microbial strains from a mixed culture (Austin, 2017). Streaking methods that are currently used were developed by Robert Hook. Streaking is the process of sample dilution from higher to lower concentration, and finally, a colony that is believed to grow from a single bacterium is isolated. It is performed by a sterile inoculation loop or cotton swab. T streak and quadrant methods are most commonly used. When streaking is done in three phases, called T streak, four sections of equal size are streaked in quadrant streaking (Fig. 2).

Fig. 2 Different types of streaking.

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3.1.2 Lawn or carpet culture Lawn culturing makes an even layer of bacterial growth on the culture media plate. Lawn culturing is done by swabbing and flooding the media plates. In the swab method, a sterile swab is soaked in liquid culture media, inoculated onto the solid agar plate, and inoculated at 37°C overnight. In the flood method, a plate containing solid media is flooded with inoculum, and excess culture can be removed by pipetting. The lawn culture method is generally used to study antibiotic action, bacteriophage activity, or inhibition of one microbe over another (Wright et al., 1981). 3.1.3 Stroke culture Stroke culturing is performed in test tubes with slanted agar. The sample is inoculated by an inoculation loop in a zig-zag manner. This is used to obtain pure bacterial culture for diagnostic purposes. 3.1.4 Stab culture In Stab culture, the agar surface in the tube is stabbed by an inoculation needle at the center of the tube and incubated at 37°C. Bacterial growth occurs in the punctured area. Stab culture is used for maintenance of stock culture, motility test, and demonstration of oxygen requirement (Syed et al., 2009). 3.1.5 Liquid culture Liquid culture is done in test tubes, flasks, or screw cap tubes. Media can be inoculated by touching the surface with an inoculation loop, needle or thorough pipetting. In liquid cultures, bacterial growth can be detected through observation, as growth is proportional to turbidity. Turbidity can be uniform or granular. Liquid culture is used for sterility testing, blood culture, and water analysis. This is also done when a large amount of bacterial product is required. 3.1.6 Pour-plate method Pour-Plate method is a quantitative method used for the determination of the viable bacterial count. The sample is serially diluted, and then the sample from each dilution is added to molten agar (temp around 45°C), then incubated (Fig. 3), and colonies can be counted after overnight incubation; the number of colonies is inversely proportional to the number of dilutions. The pour-plate method is used for sampling the heterogeneous microbial population.

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Fig. 3 Serial dilution and pour plate method.

3.1.7 Spread-plate method Spread-Plate method is similar to the pour-plate method; this sample is serially diluted as in the pour-plate. After dilution sample is spread on agar surface and overnight incubated at 37°C. After overnight incubation number of colonies is counted. This is also used for the quantification of bacterial cells.

3.2 Types of culture media Culture media is an integral part of conventional microbiology. It essentially contains all the nutrients required for microbial growth. Different microbes have different requirements so the media optimization is done on the basis of the microbe to be cultivated. Media optimized for a particular type of microbe does not allow the growth of other microbes and promotes the effective growth of selective microbes (Chapin & Lauderdale, 2003). On the basis of the origin of its constituents, culture media can be classified into complex media and defined media. If the exact composition of a media is not known, it is termed Complex media. Complex media consists of digest of animal, microbial or plant products, e.g., beef extract, yeast extract, and tryptic soy broth. Pasteur used complex media containing yeast extract in his early studies. Defined media consists of the exact amount of organic or inorganic chemicals thus their exact composition in qualitative and quantitative is

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known as Czepak Dox Agar media. Moreover, based on their application, functional classification is done; in Table 1 brief description has been given.

4. Biochemical tests In conventional microbiology biochemical tests are really significant for the identification of bacteria. The result of these tests shows the capability of bacteria to utilize some nutrients or change in pH, which depend on enzymes present in particular bacterial species. Some common tests are briefly explained in Table 2.

5. Automated identification technique Microbiology has been significantly improved since the development of automated identification techniques. These techniques do not only save time but also give accurate results. Some of the most used techniques are briefly discussed below.

5.1 MALDI-TOF Anhalt and Fenselau (1975) were the first to offer the use of mass spectrometry (MS) to identify microbes; this discovery has distinctive spectral data recorded from microbial samples of various species (Anhalt & Fenselau, 1975). The most popular ionization procedures in current mass spectrometry are electrospray ionization (ESI) and MALDI. It is a “soft” ionization type that keeps the sample unchanged throughout MS examination. Laser light produces the analyte ions, desorption, and ionization. The matrix is crucial because that absorbs the laser light energy and induces the sample to evaporate. Almost predominantly, the MALDI method produces individually charged ions. The emergence of MALDI-TOF MS for pathogenic microbes has substantially impacted microbial diagnostics, changing traditional biochemical detection approaches (Fig. 4). MALDI-TOF MS offers the advantages of detecting microorganisms and yeasts immediately from colonies cultured on Petri plates for their primary isolation in very little time and with significant resource and time savings compared to presently deployed detection approaches. Many research studies have proven that MALDI-TOF MS is precise and trustworthy in identifying clinically significant pathogens and yeast strains. Its efficiency is consistently greater than that of phenotypic methodological approaches

Table 1 Composition of different types of media and their significance. Type of media

Enriched media

General character

Composition (in 1 L of distilled water)

Significance

Supports the growth of fastidious and nonfastidious bacteria [basal media + additional nutrients (blood, serum, etc.)]

Blood agar: Peptone 5 g, beef extract/yeast extract 3 g, agar 1.5 g NaCl 0.5 g, sheep blood 5 mL. pH ¼ 7.2  0.2 Loeffler’s serum slope: Dextrose 2.5 g, proteose peptone: 2.5 g, beef extract 2.5 g, sodium chloride 1.25 g, horse serum 750 mL. pH ¼ 7.6  0.2 Chocolate agar: Casein/animal tissue digest 15 g, corn starch 1 g, potassium phosphate, dibasic 4 g, potassium phosphate, monobasic 1 g, sodium chloride 5 g, agar 10 g, hemoglobin solution (2%) 500 mL, isovitox enrichment 10 mL. pH ¼ 7.2  0.2

α and β hemolytic property of bacteria

Isolation of Corynebacterium diphtheriae

To isolate fastidious pathogens such as Neisseria and Haemophilus

Enrichment broth

Liquid media allows growth of certain microorganisms while inhibiting others (liquid media + inhibitory agent)

Tetrathionate broth: Meat extract 0.9 g, peptone 4.5 g, yeast extract 1.8 g, sodium chloride 4.5 g, calcium carbonate 25g sodium thiosulfate 40.7 g, pH ¼ 7.8  0.2 Gram-negative broth: Tryptose 20 g, dextrose 1 g, mannitol 2 g, sodium citrate 5 g, sodium deoxycholate 0.5g dipotassium phosphate 4 g, monopotassium phosphate 1.5 g, sodium chloride 5 g. pH ¼ 7  0.2 Alkaline peptone water: Peptone 10 g, sodium chloride 10 g, pH ¼ 8.6  0.2 Alkaline peptone water: Peptone 10 g, sodium chloride 10 g, pH ¼ 8.6  0.2

Used for the isolation of Salmonella typhi

Used to isolate Shigella

Used to isolate Vibrio cholerae

To isolate Salmonella and Shigella from urine, stool, water, and food Continued

Table 1 Composition of different types of media and their significance—cont’d Type of media

Selective media

General character

Composition (in 1 L of distilled water)

Significance

Solid media allows growth of certain microorganisms while inhibiting others

Lowenstein-Jensen media: L-Asparagine 3.6 g, monopotassium phosphate 2.4 g, magnesium sulfate 0.24 g magnesium citrate 0.6 g, potato flour 30 g, malachite green 0.4 g. pH ¼ 7.2  0.2 Deoxycholate citrate agar: Heart infusion solids 10 g, proteose peptone 10 g, lactose 10 g, sodium deoxycholate 5 g, neutral red 0.02 g, sodium citrate 20 g, ferric ammonium citrate 2 g, agar 13.5 g. Thiosulfate citrate bile salt sucrose agar: Proteose peptone 10 g, yeast extract 5 g, sodium thiosulfate 10 g, sodium citrate 10 g, bile 8 g, sucrose 20 g, sodium chloride 10 g, ferric citrate 1 g, bromo thymol blue 0.04 g, thymol blue 0.04 g, agar 15 g. pH 5 8.6 ± 0.2 Potassium tellurite agar: Biopeptone: 10 g, sodium chloride: 5 g, dipotassium hydrogen phosphate: 4 g, corn starch: 1 g, monopotassium phosphate: 1 g, agar: 10 g. hemoglobin powder, one vial of vitamin growth supplement and potassium tellurite 1%. pH ¼ 7.2  0.2

Used to isolate Mycobacterium tuberculosis

To isolate enteric pathogens from stool such as Salmonella Enteritidis, Salmonella Typhimurium

To isolate Vibrio cholerae, Vibrio parahaemolyticus and other enteropathogenic species

To isolate Vibrio cholerae and other enteropathogenic species

Transport media

Used to transport clinical specimens

Differential media

It uses an indicator to differentiate between two different groups of bacteria through a change in color

Anaerobic culture media

Used to culture anaerobic microorganisms

Amies medium: Sodium chloride 3 g, potassium chloride 0.2 g, calcium chloride 0.1 g, magnesium chloride 0.1 g, monopotassium phosphate 0.2 g, disodium phosphate 1.15 g, sodium thioglycolate 1 g, charcoal 10 g, agar 4 g. pH ¼ 7.2  0.2 Cary-Blair transport medium: Sodium thioglycolate, di-sodium hydrogen phosphate, sodium chloride, calcium chloride, Agar. pH ¼ 8.3  0.2 MacConkey agar: Bile salts mixture 2.5 g, lactose 10 g, neutral red 0.075 g, peptone 20 g, sodium chloride 5 g, agar 15 g. pH ¼ 7.4  0.3 Robertson’s cooked meat broth: Peptic digest of animal tissue 17.5 g, cooked meat medium 250 g, iron filings 10 g, sodium chloride 5 g, dextrose 5 g, yeast extract 5 g, hemin 10 mL, vitamin K 10 mL. pH 6.8  0.3 Thioglycolate broth: L-cystine 0.5 g, sodium chloride 2.5 g, glucose 5.5 g, yeast extract 5 g, pancreatic digest of casein 15 g, sodium thioglycolate 0.5 g. pH ¼ 7.1  0.2

For transport of Neisseria, Hemophilus influenzae

For transport of Shigella and Salmonella

Differentiate between lactose fermentative such as E. coli (pink color colonies) and nonlactose fermentative, e.g., Shigella (colorless colonies) Widely used to culture anaerobes (Clostridium spp.)

Widely used to culture anaerobes (Clostridium spp.) To maintain stock culture

Table 2 Differential biochemical tests for bacterial identification. Type of bacteria

Name of test

Media

Significance

Gram-positive

Coagulase test

–

DNase test

DNase agar is used with indicator Methyl green

Bile esculin hydrolysis test PYR test

Bile Esculin agar

Bile solubility test Inulin fermentation test Indole test

Blood agar media and sodium deoxycholate –

To differentiate between coagulase-positive Staphylococcus aureus and coagulase-negative Staphylococci (CoNS) DNase test is used to differentiate DNase positive S. aureus from other DNase negative Staphylococci. Moreover, this also distinguishes M. catarrhalis from all other Gram-negative diplococci such as Neisseria gonorrhoeae and Neisseria meningitidis This test differentiates group Enterococci and D Streptococci from nongroup D viridans group Streptococci on the basis of esculin hydrolysis PYR broth is used to differentiate between group D Streptococci and Enterococcus spp. and to identify group A beta hemolytic Streptococci Used to differentiate Streptococcus pneumoniae from other alphahemolytic Streptococci Differentiate other Streptococci from S. pneumoniae

Gram-negative

Citrate utilization test

PYR broth

Nutrient peptone or SIM (sulfide, indole, motility) Cimon citrate agar media

To differentiate various members of the family Enterobacteriaceae, such as E. coli from Klebsiella and Enterobacter based on production of indole from tryptophan by the action of tryptophanase enzyme Used for identification Gram negative pathogens of the family Enterobacteriaceae

Urea hydrolysis test Triple Sugar Iron agar test Decarboxylase (LAO) test Methyl Red

Christensen’s urea medium Triple Sugar Iron agar

VogesProskauer test

Glucose phosphate broth

Decarboxylase medium Glucose phosphate broth

To differentiate urease-positive Proteus from other members of the family Enterobacteriaceae To differentiate different genera of the Enterobacteriaceae family To differentiate members of Enterobacteriaceae based on enzyme decarboxylase production For the identification of bacteria producing stable acids by mixed acid fermentation of glucose, the result is observed on the basis of color change. E. coli gives MR a negative result, and Klebsiella pneumoniae gives a positive result To determine the production of acetylmethylcarbinol from glucose fermentation

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Fig. 4 MALDI-TOF.

used to identify bacteria. It can be upgraded even more by software and database updates. Apart from bacterial detection from culture plates, MALDI-TOF MS has been used to investigate additional possibilities such as detecting pathogens straight through blood cultures and subspecies typing, including detection of antibiotic resistance determinants. Unfortunately, intense ionization allows only the diagnosis of pathogenic lipids, restricting the ability to differentiate microorganisms at the level of species. After the emergence of soft ionization technology in the latter part of the 1980s, the characterization of higher molecular weight compounds such as polypeptides becomes feasible (Tanaka et al., 1988). Holland et al. (1996) reported that by using MALDI-TOF MS assessment, particular spectral patterns can be extracted from entire microbial species, opening the door for bacterial identification by mass spectroscopy. Following that, software and database advancements have mainly worked to develop MALDI-TOF MS, a more precise and efficient technique for microbiological diagnosis (Kliem & Sauer, 2012).

5.2 VITEK 2 Microbiological laboratories should be dynamic and adaptive to deliver the appropriate, timely information in the context of today’s worldwide health concerns, such as multidrug-resistant microbes. The VITEK was created to meet the increased need for quality standards in the food sector, and the Gram-Negative (GN) test card was designed to identify microbes in just a few hours. Conventional approaches to recognizing microbe’s significance

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incidentally or deliberately from contaminated origins usually depend on time consuming processes like isolation, biochemical identification, and serological tests (Manual, 2001)(). This system meets such demand by automating the screening of microbes in commodities, construction environments, and final goods. The VITEK 2 system allows rapid detection, thus providing results in few hours instead of days as in conventional techniques (Biom
erieux, 2002). The VITEK 2 is an automated system for microbiological identification that uses the VITEK 2 Gram-Negative (GN) card to detect Gram-negative bacilli, including Burkholderia mallei, Francisella tularensis, Brucella melitensis, Yersinia pestis, and B. pseudomallei. This card is primarily built on 47 biochemistry tests that measure carbon source consumption, inhibitory and resistivity, and enzyme activities. In 10 h or less, we can get our results of identification. The Gram-Negative Identification Card is used for the identification of Gram-negative bacteria and other organisms. Fermenting as well as nonfermenting Gram-negative microbes can be easily identified using this card designed to work with the VITEK 2 system for automatic identification. It is a use and throw card. It works based on previous microbe analysis and data analysis and identifies the test microbe. A significant amount of data has been stored through identified strains to predict the claimed species’ specific reactions. The VITEK 2 program analyses the test set of responses with the predicted set of reactions for every microbe in the identification procedure. An evaluative number described as the percentage possibility of the identified results is obtained and provided. This number indicates how much the obtained reactions match the microbes’ complicated reactions (Renaud et al., 2005). The identification accuracy of Phoenix and VITEK 2 for culture isolates is very much similar to one another and comparatively better than MicroScan.

5.3 Phoenix Antibiotic susceptibility testing (AST) and rapid identification (ID) of therapeutically related microbial isolates are critical to successful care for infected patients (Carroll et al., 2006). It has become even more critical in past decades, owing to the worrying increase of antibiotic resistance among pathogenic microbes. The Phoenix Automated Microbial Technology is a 21st century, completely automated system for Gram-negative and Grampositive microbe’s identification and testing antibiotic susceptibility. It could

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also evaluate up to 100 ID and antibiotic susceptibility testing integrated panels at the same time. The time required to get a broad list of ID/AST reports is a minimum of 8–12 h and varies depending on the analyzed microbes. Several tests in the Phoenix ID panels are adaptations of traditional approaches. The Phoenix method uses chromogenic, fluorogenic, and single carbon source substrates to identify microbes (Kampfer et al., 1991; Manafi et al., 1991). The Phoenix device can execute up to 100 detections and AST at once, employing Phoenix ID/AST combo panels. Its consumables consist of a sealed and self-inoculating molded tray of polystyrene with 136 microwells filled with dried reagents constituting 85-well AST side and 51-well ID side. An ID side has dried substrates for the detection of microbes, an AST side with variable doses of antimicrobials, plus proliferation and fluorescence controls at suitable well locations are all included in the combo panel. For AST, the Phoenix system uses a spectrometric redox signal that is refined and several spectrometric and fluorometric markers for ID. AST Broth contains Ca2+ and Mg2+ ions for cation adjustment to improve rapid screening efficiency. On the ID side, there are 45 dry biochemical substrate wells and two fluorescent control wells, and the other AST side has 84 dried bactericidal and one growth control well. Then, they are inoculated with standard inoculum and put in the device, where they are constantly incubated at 35°C. Panels are tested every 20 min and read by the instrument.

5.4 MicroScan This is an automated, easily accessible device for quick detection and susceptibility testing of Gram-negative microbes that have acquired positive feedback in terms of microbial identification (Bascomb et al., 1997; McGregor et al., 1995). It works by identifying bacterial enzymes that have been produced. In the 20th century, MicroScan also introduced combined negative type 1S panels to the customers to detect aerobic and nonaerobic facultative Gram-negative microbes. The pH indicator and fluorogenic substrates are used to measure microbial enzyme activities. MicroScan works by identifying bacterial enzymes that have been produced. MicroScan panel consists of 36 biochemical assays that employ fluorometric technology to distinguish between fermentation and nonfermentation Gram-negative species useful for diagnosis. For detecting H. alvei strains, the MicroScan WalkAway system with a combination of negative type 1S panels is effective (Rodrı´guez et al., 1999).

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6. Molecular methods In general, molecular methods verify and confirm the culture based method’s findings. One can quantify the culture negative microbes using molecular techniques along with the culture-positive microbes. More broadly, the molecular techniques can be subdivided based on amplification techniques. In general, it can be divided into amplification-based methods and nonamplification-based techniques. Amplification-based techniques are classified into molecular detection using PCR, Biofire Film Array, loop mediated isothermal amplification, cartridge-based nucleic acid amplification, etc. Nonamplification based techniques include flow cytometry techniques, etc. All-inclusive, these methods give consistent and concordant results.

6.1 Molecular detection using PCR PCR stands for polymerase chain reaction. Using this technique, one can generate a billion copies of DNA from a single copy of DNA. It is a type of DNA replication in which synthetic primers, thermostable DNA polymerases, dNTPs, etc., are used to amplify the desired DNA fragment. This process is continued cyclically to get the required amplification. PCR detection techniques are generally used to detect, identify, and differentiate microorganisms rapidly. Kary B. Mullis developed PCR in 1985 for the first time. He was also awarded the Noble prize in 1993 in chemistry along with Michael Smith (Shampo & Kyle, 2002). PCR includes three basic steps. Firstly, denaturation, then annealing, and lastly, extension. Every step has its specific temperature and time period. In denaturation, the temperature used is more than the melting temperature. So that the DNA double stand can break and generate DNA single strands. Afterward, in the extension step, the temperature is reduced. As a result, the primers can anneal to DNA single strands. Then in the extension step, the temperature is slightly increased to stop the anonymous binding of the primers and to start the extension step. In this step, the heat stable DNA polymerases play really important role. They help in the synthesis of the new strand taking the denatured strand as a template. As the temperature in these three steps varies rapidly, the standard polymerases may be denatured or not work efficiently. Thus, in PCR, we use thermostable DNA polymerases generally extracted from thermophiles of hot vents. So that their DNA polymerase can resist the varied temperature of the amplification procedure and work efficiently in such diverse temperature ranges; in this way, each newly synthesized strand works as a template

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Fig. 5 Steps involved in PCR.

in the subsequent cycle, and the reaction ends, resulting in multiple copies of the required DNA sample (Fig. 5). The PCR procedure takes less time than the conventional culture techniques. So, nowadays, it can outcompete these techniques. For this reason, PCR is used more widely than other traditional methods. According to the human need and amplifying and quantifying microbes more efficiently, this PCR method evolves and different types of PCR sequel. The different types of PCR include real-time PCR, RT-PCR, nested PCR, multiplex PCR, etc. (Adzitey et al., 2013). Conventional PCR techniques have been used to assay microorganisms for the last 15 years, but real-time analysis has become more critical nowadays. Using real-time PCR (rt-PCR), the real-time analysis of amplicon

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formation and its quantification can be traced after each amplification cycle of PCR (Buszewski et al., 2017). The fungal real-time quantitative assay can be done using real-time PCR (Walsh et al., 2011). Unlike conventional PCR, a different thermocycler was used in rt-PCR. The rate of contamination is meager. Different fluorogenic molecules such as SYBR Green etc., are used in the detection step of rt-PCR. RT-PCR is the short form of reverse transcriptase polymerase chain reaction. In this PCR type, a reverse transcriptase enzyme is used. This enzyme is responsible for the synthesis of DNA from an RNA template. Thus, mainly for RNA amplification, RT-PCR is used. This PCR type is commonly employed for RNA viruses and 16s rRNA gene detection. During the COVID19 outburst, this method was used worldwide to detect the SARS CoV-II virus in the patient’s body (Han et al., 2021; Zhang et al., 2021). The viral load kinetics and CT value can be detected using RT-PCR (Han et al., 2021). Nested PCR is the type of modification in which two different sequences of the same microorganisms can be targeted at once. To do so, two different primers are used. The yield of nested PCR is comparatively higher than conventional PCR as two rounds of amplification are done here. Nested PCR detects various disease causing microorganisms like Mycobacterium tuberculosis (Kamra & Mehta, 2021). As there are two rounds of amplification, the chances of contamination are a little higher. The limitation of this technique is sometimes false positive reports might result. Multiplex PCR is the advanced form of nested PCR in which multiple primers are used to detect several organisms in a single reaction run (Rajapaksha & Elbourne, 2019). It is very much convenient in the field of disease diagnostics, mainly for diseases that are caused by more than one pathogenic microorganism. Like nested PCR, the chances of contamination are also high in this type of PCR.

6.2 Biofire Film Array It is an entirely automated system. It is a modification of the multiplex, nested PCR system. In this sample preparation, its amplification, detection, analysis, etc., are carried out in an automated mechano-electric way. It is an effective technique in which, within 1 h, the results can be assessed. Biofire Film Array comprises four different panels that target various diseases. The four available panels are the respiratory panel (Leber et al., 2018), gastrointestinal panel (Buss et al., 2015), meningitis-encephalitis panel (Tansarli &

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Chapin, 2020), and the blood culture identification panel (Salimnia et al., 2016). In each panel, primers are there that target 20–30 common infectious pathogens of the respective system. All the panels can determine the pathogenic microbes directly from the specimen except for the blood panel. This technique is more expensive than the previously mentioned methods, which is the only cause that limits its wide use (Soucek et al., 2019).

6.3 Loop-mediated isothermal amplification Loop-mediated isothermal amplification also known as LAMP is a technique for amplification of nucleic acid. Unlike PCR, the total amplification step in LAMP is performed at a constant temperature of 60–65°C. It is a novel isothermal amplification technique which uses four to six primers that are bound to six to eight separate regions on the targeted DNA to amplify the finite amount of DNA and to produce its many copies (Wong et al., 2018). The primers used in this technique comprise two outer primers and two inner primers. The two inner primers are forward inner primers (FIP) and backward inner primers (BIP). The two outer primers are forward outer primer (F3) and backward outer primer (B3) (Garg et al., 2022). Since its discovery, it is widely used to detect and amplify a wide range of microorganisms, starting from bacteria like E. coli to the virus such as SARS CoV II. Using this technique very low level of Plasmodium can also be detected in malaria patients (Selvarajah et al., 2020). This technique can analyze samples within 1 h. So, it is widely used for diagnosis of various microorganisms with high specificity.

6.4 Flow cytometry Flow cytometry is an analytical method through which a single cell can be ´ lvarez-Barrientos et al., 2000). In a single run, multiple paramanalyzed (A eters can be analyzed within less period of time. The main principle of flow cytometry is based on hydrodynamic focusing. The cells are suspended in a solution and focused in such a way that it looks like a bead on a string. It is based on light scattering and fluorescent emission. The cells are fluorescent labeled and then hydrodynamically focused inside the flow cytometer. Different types of dyes such as SYBR Green, DAPI, etc. (Guo et al., 2020) are used to stain the cells and/or their nucleic acids. The cells are then focused with different LASER and the light scattering and the emitted fluorescence are measured. From the generated data the graphs are plotted and analyzed further. Generally, two types of scattering are obtained. One is forward

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scattering and the other is side scatter. Forward scattering gives the data about the size and shape of the microorganism. The cell composition can be analyzed through side scattering (Koch et al., 2014). Flow cytometry can be utilized to detect a varied range of samples starting from bacteria and fungus (Cao et al., 2021) to animal cells. This method is widely used these days due to its specificity and high throughput in microbial identification and detection.

7. Conclusions Microbes are omnipresent and affect human lives in various ways. They are beneficial to humankind in food, industrial, and health aspects by helping in fermentation, medicines, etc., and harm us as well by food spoilage, disease, etc. These qualities of microbes make their identification and culturing extremely important. Since the development of the first identification and culture technique, a lot of new methods have developed which makes microbial identification less time consuming. These developments are playing a significant role in the food, industrial, and health aspects of microbiology.

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Manafi, M., Kneifel, W., & Bascomb, S. (1991). Fluorogenic and chromogenic substrates used in bacterial diagnostics. Microbiological Reviews, 55(3), 335–348. https://doi.org/ 10.1128/mmbr.55.3.335-348.1991. Manual, B. A. (2001). BAM: Rapid Methods for Detecting Foodborne Pathogens. Food and Drug Administration US. Marra, M. A., Jones, S. J. M., Astell, C. R., Holt, R. A., Brooks-wilson, A., Butterfield, Y. S. N., Khattra, J., Asano, J. K., Barber, S. A., Chan, S. Y., Cloutier, A., Coughlin, S. M., Freeman, D., Girn, N., Griffith, O. L., Leach, S. R., Mayo, M., Mcdonald, H., Montgomery, S. B., … Roper, R. L. (2003). The genome sequence of the SARSassociated coronavirus. Science, 300(May), 1399–1405. Marshall, B. J., Armstrong, J. A., McGechie, D. B., & Glancy, R. J. (1985). Attempt to fulfil Koch’s postulates for pyloric Campylobacter. Medical Journal of Australia, 142(8), 436–439. https://doi.org/10.5694/j.1326-5377.1985.tb113443.x. McGregor, A., Schio, F., Beaton, S., Boulton, V., Perman, M., & Gilbert, G. (1995). The microscan walkaway diagnostic microbiology system—An evaluation. Pathology, 27(2), 172–176. https://doi.org/10.1080/00313029500169822. Penney, D. P., Powers, J. M., Frank, M., Willis, C., & Churukian, C. (2002). Analysis and testing of biological stains—The Biological Stain Commission Procedures. Biotechnic & Histochemistry, 77(5-6), 237–275. Rajapaksha, P., & Elbourne, A. (2019). A review of methods for the detection of pathogenic microorganisms. Analyst, 396–411. https://doi.org/10.1039/c8an01488d. Renaud, F. N. R., Bergeron, E., Tigaud, S., Fuhrmann, C., Gravagna, B., & Freney, J. (2005). Evaluation of the new Vitek 2 GN card for the identification of Gram-negative bacilli frequently encountered in clinical laboratories. European Journal of Clinical Microbiology and Infectious Diseases, 24(10), 671–676. https://doi.org/10.1007/s10096-0050026-6. Reynolds, J., Moyes, R. B., & Breakwell, D. P. (2009). Differential staining of bacteria: Acid Fast stain. Current Protocols in Microbiology, 15(1). https://doi.org/10.1002/ 9780471729259.mca03hs15. Rodrı´guez, L. A., Vivas, J., Gallardo, C. S., Acosta, F., Barbeyto, L., & Real, F. (1999). Identification of Hafnia alvei with the MicroScan WalkAway system. Journal of Clinical Microbiology, 37(12), 4186–4188. https://doi.org/10.1128/jcm.37.12.4186-4188.1999. Sader, H. S., Fritsche, T. R., & Jones, R. N. (2006). Accuracy of three automated systems (MicroScan WalkAway, VITEK, and VITEK 2) for susceptibility testing of Pseudomonas aeruginosa against five broad-spectrum beta-lactam agents. Journal of Clinical Microbiology, 44(3), 1101–1104. https://doi.org/10.1128/JCM.44.3.1101-1104.2006. Salimnia, H., Fairfax, M. R., Lephart, P. R., Schreckenberger, P., DesJarlais, S. M., Johnson, J. K., Robinson, G., Carroll, K. C., Greer, A., Morgan, M., Chan, R., Loeffelholz, M., Valencia-Shelton, F., Jenkins, S., Schuetz, A. N., Daly, J. A., Barney, T., Hemmert, A., & Kanack, K. J. (2016). Evaluation of the FilmArray blood culture identification panel: Results of a multicenter controlled trial. Journal of Clinical Microbiology, 54(3), 687–698. https://doi.org/10.1128/JCM.01679-15. Selvarajah, D., Naing, C., Htet, N. H., & Mak, J. W. (2020). Loop-mediated isothermal amplification (LAMP) test for diagnosis of uncomplicated malaria in endemic areas: A meta-analysis of diagnostic test accuracy. Malaria Journal, 19(1), 1–10. https://doi. org/10.1186/s12936-020-03283-9. Shampo, M. A., & Kyle, R. A. (2002). Kary B. Mullis—Nobel Laureate for procedure to replicate DNA. Mayo Clinic Proceedings. Mayo Clinic, 77(7), 606. https://doi.org/ 10.4065/77.7.606. Sizemore, R. K., Caldwell, J. J., & Kendrick, A. S. (1990). Alternate Gram staining technique using a fluorescent lectin. Applied and Environmental Microbiology, 56(7).

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Soucek, D. K., Dumkow, L. E., VanLangen, K. M., & Jameson, A. P. (2019). Cost justification of the BioFire FilmArray meningitis/encephalitis panel versus standard of care for diagnosing meningitis in a community hospital. Journal of Pharmacy Practice, 32(1), 36–40. https://doi.org/10.1177/0897190017737697. Syed, M. A., Khalid, A., & Shukor, Y. (2009). A simple method to screen for azo-dyedegrading bacteria. Journal of Environmental Biology, 30(1), 89–92. Tanaka, K., Waki, H., Ido, Y., Akita, S., Yoshida, Y., Yoshida, T., & Matsuo, T. (1988). Protein and polymer analyses up to m/z 100 000 by laser ionization time-of-flight mass spectrometry. Rapid Communications in Mass Spectrometry, 2(8), 151–153. https://doi. org/10.1002/rcm.1290020802. Tansarli, G. S., & Chapin, K. C. (2020). Diagnostic test accuracy of the BioFire® FilmArray® meningitis/encephalitis panel: A systematic review and meta-analysis. Clinical Microbiology and Infection, 26(3), 281–290. https://doi.org/10.1016/j.cmi.2019.11.016. Wainwright, M., & Lederberg, J. (1992). History of microbiology. Ahschools.Us. Retrieved from https://www.ahschools.us/cms/lib08/MN01909485/Centricity/Domain/4774/ history%20of%20micro.pdf (Accessed 23 February 2022). Walsh, T. J., Wissel, M. C., Grantham, K. J., Petraitiene, R., Petraitis, V., Kasai, M., Francesconi, A., Cotton, M. P., Hughes, J. E., Greene, L., Bacher, J. D., Manna, P., Salomoni, M., Kleiboeker, S. B., & Reddy, S. K. (2011). Molecular detection and species-specific identification of medically important aspergillus species by real-time PCR in experimental invasive pulmonary aspergillosis. Journal of Clinical Microbiology, 49(12), 4150–4157. https://doi.org/10.1128/JCM.00570-11. Wiegand, I., Hilpert, K., & Hancock, R. E. W. (2008). Agar and broth dilution methods to determine the minimal inhibitory concentration (MIC) of antimicrobial substances. Nature Protocols, 3(2), 163–175. https://doi.org/10.1038/nprot.2007.521. Wong, Y. P., Othman, S., Lau, Y. L., Radu, S., & Chee, H. Y. (2018). Loop-mediated isothermal amplification (LAMP): A versatile technique for detection of micro-organisms. Journal of Applied Microbiology, 124(3), 626–643. https://doi.org/10.1111/jam.13647. Wright, S. J. L., Redhead, K., & Maudsley, H. (1981). Acanthamoeba castellanii, a predator of cyanobacteria. Journal of General Microbiology, 125(2), 293–300. https://doi.org/ 10.1099/00221287-125-2-293/CITE/REFWORKS. Zbinden, A., B€ ottger, E. C., Bosshard, P. P., & Zbinden, R. (2007). Evaluation of the colorimetric VITEK 2 card for identification of Gram-negative nonfermentative rods: Comparison to 16S rRNA gene sequencing. Journal of Clinical Microbiology, 45(7), 2270–2273. https://doi.org/10.1128/JCM.02604-06. Zhang, Z., Bi, Q., Fang, S., Wei, L., Wang, X., He, J., Wu, Y., Liu, X., Gao, W., Zhang, R., Gong, W., Su, Q., Azman, A. S., Lessler, J., & Zou, X. (2021). Insight into the practical performance of RT-PCR testing for SARS-CoV-2 using serological data: A cohort study. The Lancet Microbe, 2(2), e79–e87. https://doi.org/10.1016/S2666-5247(20) 30200-7.

CHAPTER 6

Industrial backgrounds and microbes growth Fatemeh Salimia and Ehsan Nazarzadeh Zareb a

Department of Cellular and Molecular Biology, School of Biology, Damghan University, Damghan, Iran School of Chemistry, Damghan University, Damghan, Iran

b

1. Introduction Microorganisms, including bacteria, fungi, and yeasts, frequently adhere to biotic, abiotic, hydrophobic, and hydrophilic surfaces through secreting hydrated extracellular polymeric substances (EPS) and form well-multifaceted structure of communities which is known as biofilms. Also, biofilms can be found on liquid surfaces as a floating mat and in submerged state. They have a complicated structure, being filled with mature macrocolonies surrounded by fluid-filled channels. The biofilm contains microbial cells, water, polysaccharides, proteins, enzymes, lipids, and nucleic acid (Vishwakarma, 2020). Extracellular polysaccharide matrix has a structural role and is critical as it forms a scaffold and holds the biofilm together and protects the biofilm against harsh environmental conditions. It contains polysaccharides, proteins, and extracellular microbial DNA. The formation of the matrix is a dynamic process and depends on the type of available nutrients and the colonizing species. The matrix of EPS trap nutrients and water via hydrogen bonding with its hydrophilic polysaccharides which are used by microbial cells in the biofilms (Kostakioti et al., 2013). The biofilm due to the EPS can be considered as technically hydrogels that exhibit viscoelastic behavior (Hall-Stoodley et al., 2004). The EPS helps biofilms to withstand mechanical stress. In fact, processes inside the biofilm and interactions with the external world are defined by the matrix. Matrix generates complex gradients because of nutrients and oxygen diffusion, contains extracellular enzymes used for nutritional purposes, allows for the transfer of cell communication molecules, and protects the embedded cells against poisonous compounds (H.-C. Flemming et al., 2016). The biofilms consist of one (homogenous communities of microbial cells) or more (heterogeneous communities of microbial cells) microbial (various bacterial or fungal) species. The presence of more than one Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00011-6

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microbial species in a biofilm has significant ecological benefits as it can facilitate the biofilm’s attachment to a surface or provide higher resistance to toxic substances like disinfectants. Biofilms have high cell densities, ranging from 108 to 1011 cells g
1 wet weight. They can usually grow on any organic or inorganic surface which can serve as nutrient source. The formation of biofilm enhances resistance of microbial cells toward nutrient limitation; physical (desiccation), mechanical (liquid streams in pipelines), and chemical (chemicals, antimicrobials and disinfectants) stresses, and also protect them from immunologic defense system of host (H.-C. Flemming et al., 2016). The biofilm lifestyle is clearly distinct from that of free-living microbial cells. Therefore, biofilm communities have emerging characteristics like enhanced antibiotic resistance, higher metabolic diversity, and metabolic cooperation in comparison with planktonic cells. This lifestyle is not predictable from the study of free-living bacterial cells (Konopka, 2009). Microbial communities of the biofilms usually are involved in beneficial process, including the sewage purification, groundwater treatment, nitrification process, heavy metal detoxification, and remediation. Microbial biofilm in rhizospheric soil improves the soil fertility and plant growth. In extreme acidic environment, microbial biofilms are involved in sulfur cycle. Apart from these beneficial effects, microbial biofilms cause serious problems for food, dairy, medical, marine, water, gas and oil industries, and community-related settings. In this regard, microbial biofilms are related to hard-to-treat infections, decreased quality of food products, reduced efficiency of industrial equipment, corrosions, and fouling (Banerjee et al., 2020).

2. Biofilm formation The formation of biofilm is a progressive process. This process relies on environmental stimuli and a series of genetic and phenotypic changes in planktonic cells. Biofilm growth is actually a series of physical, chemical, and biological events which develop through a series of steps. The biofilm formation on any substratum/layer is a stepwise and dynamical process consisting of multiple process, including initial attachment, irreversible attachment, early development of biofilm architecture (or microcolony formation and 3D biofilm formation), maturation, and dispersion (dissemination) (Banerjee et al., 2020; Gupta et al., 2016; Jamal et al., 2018; Taraszkiewicz et al., 2013; Verderosa et al., 2019).

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2.1 Reversible adherence Primarily, free-swimming planktonic cells swim along surfaces. These movements can be performed actively via their flagella or passively through the gravitational transportation of their planktonic form, diffusion, or shear force of the surrounding fluid phase (Kostakioti et al., 2013; Kumar & Anand, 1998; Yin et al., 2019). Then they attach reversibly to the biotic or abiotic surfaces via weak interactions such as acid base, hydrophobic, Van der Waals, steric interactions, and electrostatic forces (Garrett et al., 2008). Bacterial appendages such as fimbriae, pili, or flagella accelerate adhesion (Kumar & Anand, 1998). The initial electrostatic repulsion barrier between the surfaces and microbial cells can be overcome via flagella-mediated motility and pili-associated structures. The flagella through involving in transport and initial cell–surface interactions promote biofilm formation. Finally, a monolayer of cells forms on the surface (Sauer & Camper, 2001). Generally, the initial attachment affected by several factors, including bacterial cell surface (presence or absence of pili, fimbriae, and glycocalyx on the surface), available nutrients, pH, temperature, and pressure of the environment as well as texture (rough or smooth), charge, and hydrophobicity of living or nonliving surfaces (Abdallah et al., 2009; R. M. Donlan, 2002; Dunne Jr, 2002; Gerstel & R€ omling, 2001; Nilsson et al., 2011). Commonly, any surface is vulnerable to biofilm development including plastic, glass, metal, wood, and food products (Srey et al., 2013). Mainly, rough and hydrophobic surfaces are more susceptible to microbial cell attachment. Because hydrophobicity of the surfaces via reducing the force of repulsion between the bacteria and the surface strengthen microbial cell attachment (Kumar & Anand, 1998; Tribedi & Sil, 2014). Therefore, microorganisms attach more likely to the hydrophobic and nonpolar surfaces like Teflon and other plastics, than to hydrophilic and polar surface like metals and glass (Fletcher & Loeb, 1979; Pringle & Fletcher, 1983). For example, Sinde and Carballo showed that Salmonella and Listeria can attach in higher numbers to hydrophobic surfaces than the hydrophilic ones (Sinde & Carballo, 2000). Also, the presence of conditioning film comprising macromolecules like organic substances on surfaces enhances the attachment of bacterial cells (X. Tang, Flint, Bennett, et al., 2009; X. Tang, Flint, Brooks, & Bennett, 2009). In this step, the adhesion is reversible because the attached microbial cells are not yet ready to undergo differentiation process which include morphological changes that results in biofilm formation. So, many of the cells may

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detach from the surface and return to the planktonic lifestyle (Stoodley, Cargo, et al., 2002; Stoodley, Sauer, et al., 2002). At this stage, the bacteria are still sensitive to antibiotics. The importance of the first attachment step was confirmed by experiments with surface attachment-defective mutant strains of Pseudomonas aeruginosa, which are unable to form biofilms (G. A. O’Toole & Kolter, 1998).

2.2 Irreversible adherence In the next stage, some of the reversibly attached cells remain immobilized and become irreversibly adhered when the attractive forces are greater than repulsive forces (Garrett et al., 2008). The shift from reversible to irreversible attachment is due to replacement of weak interaction with a permanent bonding (Stoodley, Cargo, et al., 2002; Stoodley, Sauer, et al., 2002). Besides the significant role of flagella in adherence, the attachment is strengthened by strong chemical attachments of the matrix polymer (Ramasamy & Lee, 2016). The adherent cells secrete EPS and become irreversibly attached to the surface, which results in cell aggregation and matrix formation (Schachter, 2003; Stoodley, Cargo, et al., 2002; Stoodley, Sauer, et al., 2002). At this stage, several genes coding surface proteins such as porins are expressed. Porins allow the transport of the polysaccharides. They are used in the formation of EPS layer. Consequently, the polymer matrix is synthesized around the microcolonies and generally a mixture of polymeric compounds, mainly polysaccharides, exists in it (Peirano et al., 2014). The biofilm matrix contains polymers secreted by microorganisms, water, absorbed nutrients and metabolites, and cell lysis products. All major classes of macromolecules, including proteins, polysaccharides, and nucleic acids, are present in biofilm matrix (Cochrane et al., 1988). Therefore, initially loosely attachment and subsequently specific and strong adhesion to microbial cells occurred in the first and second stages of biofilm formation. After this irreversible attachment, strong shear force or chemical breaking of the attachment forces by enzymes, detergents, surfactants, sanitizers, or heat is needed for biofilm removal (Hall-Stoodley et al., 2004; Kumar & Anand, 1998; Sinde & Carballo, 2000).

2.3 Microcolony formation After stable attachment of microbial cells to a living or a nonliving surface, process of division and multiplication of adhered microorganisms initiate through particular chemical signaling within the EPS. In this stage,

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microorganisms exude a matrix of EPS to stabilize the biofilm network (Costerton et al., 1999; McKenney et al., 1998). For example, P. aeruginosa synthesizes and releases three polysaccharides, including alginate, Pel, and Psl, which improve biofilm stability. Alginate via interacting with nutrients and water provides nutrients to the biofilm (Rasamiravaka et al., 2015). Pel (glucose-rich polysaccharide) and Psl (pentasaccharide) are applied as a scaffold for the structure of the biofilm (Colvin et al., 2011; Franklin et al., 2011). In some cases, DNA has important role in cellular communication and stabilization of biofilm (Gloag et al., 2013). In this phase, microbial cells communicate with each other via producing autoinducer (AI) signals that lead to the expression of biofilm-specific genes (Vasudevan, 2014). This process then results in the formation of microcolonies. Microcolony formation results from concurrent accumulation and growth of microorganisms (Chmielewski & Frank, 2003). In this stage, the biofilm becomes multilayered and their thickness increases up to 10 μm (Davey & O’toole, 2000).

2.4 Maturation In biofilm maturation step, biofilm develops into an organized structure that can be flat or mushroom-shaped, relying on the nutrient source. The mature biofilms possess maximum cell density and are considered a threedimensional community. In this stage, microcolonies and water channels develop (Fey & Olson, 2010; Schachter, 2003; Verderosa et al., 2019). Ten days or more are required for biofilm maturation that substantially lead to increased biofilm thickness (100 μm) (Davey & O’toole, 2000). Mature biofilm can contain 100 billion bacterial cells per milliliter. These cells are grouped into various communities (Rasamiravaka et al., 2015). Each community is responsible for a special function. Therefore, cell-to-cell communication and coordination is vital for a mature biofilm. The cells communicate among themselves by the secretion of AI signals (Davey & O’toole, 2000; Thoendel et al., 2011). QS is facilitated by autoinducers. In maturation stage, specific genes are expressed which are considered important for the formation of EPS. Within the EPS matrix the required molecules for communication may accumulate at concentrations high enough to be effective. The EPS is the major component in the biofilm’s three-dimensional structure, interstitial voids are then generated in the matrix. These channels are filled with water and act as a circulatory system. They are involved in the distribution of important nutrients, oxygen, and

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other elements as well as the removal of waste products and debris from the communities of microcolonies in the biofilm (Federle & Bassler, 2003; Parsek & Singh, 2003). In other words, biofilm provides a perfect environment for the establishment of syntrophic association in which metabolically distinct bacteria profit each other. Biofilms in this stage showed significant resistance to hostile conditions like presence of antibiotics (Davey & O’toole, 2000).

2.5 Dispersion In final step of biofilm formation microbial cells within the biofilm quickly multiply and ultimately dispersion occurs due to various reasons, including starvation, external perturbation, such as increased fluid shear, activity of endogenous saccharolytic enzymes, and surface-binding protein (Kaplan et al., 2003; G. O’Toole et al., 2000; Otto, 2013). N-Acetyl-heparosan lyase, alginate lyase, and hyaluronidase are some of the degrading enzymes produced by Escherichia coli, P. aeruginosa, Pseudomonas fluorescens, and Streptococcus equi, to lysis the EPS matrix in detachment phase (Otto, 2013; Stoodley, Cargo, et al., 2002; Stoodley, Sauer, et al., 2002; Sutherland, 1999). They degrade the biofilm stabilizing polysaccharides. These factors accelerate microbial cell release into a new area for colonization. Dispersion allows microbial cells to convert from sessile into planktonic form and search for a nutrient-rich environment and colonize new niches (Otto, 2013; Ramasamy & Lee, 2016). Therefore, in this phase, microbial cells upregulate the expression of proteins related to flagella formation, to let the bacteria move to a new site. Finally, at the end of biofilm formation cycle biofilm spreads and establishes itself at new surfaces and forms colonies through disruptive forces (Otto, 2013) (Fig. 1).

3. Biofilm resistance Today, antibiotic resistance is the most significant cause of noneffective therapy of biofilm-associated bacterial infections and the patient is exposed to the risk of recurrence (M. E. Lewis, 2007). Planktonic bacteria may be eradicated by combined action of host immune responses and antibiotics. However, biofilm-related infections are not easily eradicated and the alive cells can trigger the recurrence of infection (Lebeaux et al., 2014). Microorganisms in biofilms exhibited reduced susceptibility to antimicrobial agents (up to 1000-fold) compared to planktonic cells (Costerton, 1999; Costerton et al., 1999). Although, the mechanisms by which planktonic bacteria resist

Fig. 1 Biofilm formation steps and mechanisms of biofilm resistance against antimicrobial compounds.

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antibiotics are well understood, those same strategies, including mutations, efflux pumps, and antibiotic modifying enzymes, do not appear to be the main cause of biofilm-mediated antibiotic tolerance, because inherently drug-susceptible bacterial strains often show considerable antibiotic tolerance in the biofilm mode of life (Mah, 2012; Verderosa et al., 2019). However, their antimicrobial susceptibility is quickly restored when biofilm-residing cells are dispersed from the main community. Thus, it has been suggested that alternative mechanisms are involved in biofilm antibiotic tolerance. This tolerance can be grouped into two categories: innate (resulting from growth in a biofilm) and induced (resulting as a response to antimicrobial treatment) (Costerton et al., 1999; R. M. Donlan & Costerton, 2002; Dunne Jr, 2002; Høiby et al., 2010; K. Lewis, 2001; Stewart, 2002). The underlying resistance mechanisms may be affected by the type of antibiotic treatment and the immunity system of host, growth rate of biofilm, transformed metabolism, and the presence of an oxygen gradient that prevents the action of some antibiotics (Cheng et al., 2019; Y. Li, Jia, et al., 2016; W. Li, Yao, et al., 2016). In the following, the antimicrobial tolerance mechanisms are discussed in detail.

3.1 Failure of antibiotics to penetrate biofilm The EPS of a biofilm has long been considered as one the major causes of antibiotic resistance. The biofilm matrix consists of extracellular polysaccharide, DNA, proteins, and lipids (Annous et al., 2009). It has structural role and protects cells in the biofilm. Also, the matrix by keeping extracellular enzymes close to the cells acts as an external digestive system (H.-C. Flemming et al., 2007). The rate of antibiotics penetration may be retarded by matrix acting as a physical and chemical barrier (Taraszkiewicz et al., 2013). In fact, antibiotics can be prevented from penetrating if they bind to components of the biofilm matrix or to bacterial membranes (Chiang et al., 2013; Walters III et al., 2003). For example, ampicillin failed to penetrate the biofilm due to its inactivation by the wild-type biofilm. The rate of ampicillin inactivation is faster than its diffusion rate into the film (Anderl et al., 2003). Electrostatic interaction between negatively charged matrix and positively charged antibiotics, including aminoglycosides, tobramycin, and polypeptides, is resulted in delayed penetration of these antibiotics into the biofilm (Bagge et al., 2004; Laza˜r & Chifiriuc, 2010). Actually negatively charged EPS act as an ion-exchange resin with ability to bind to a large number of antibiotics trying to reach cells in the biofilm (Bagge et al., 2004).

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The extent of binding may vary with the type of the antimicrobial agents, the age of the biofilm, and its heterogeneity (Mah, 2012). Relatively uncharged agents such as β-lactam antibiotics are improbable to bind to EPS with electrostatic interactions (Van Acker et al., 2014). Retarded rate of penetration of antimicrobials can also induce expression of genes that mediate antibiotic resistance within the biofilm ( Jefferson et al., 2005). However, this phenomenon is not common among the EPS of all biofilm-forming species and also it seems to be antibiotic specific. For example, it has been found that ciprofloxacin effectively penetrates and diffuse through Klebsiella pneumoniae and P. aeruginosa biofilms, finally reaching distal cells (Anderl et al., 2000; Walters III et al., 2003). Besides, tetracycline can effectively reach all cells within E. coli biofilms (Stone et al., 2002). Unfortunately, many of these antibiotics are yet unsuccessful to eradicate the biofilm. While restricted penetration may be a major contributing factor for antimicrobial agents, their effects are certainly not universal. Thus, it is possible that further or complementary strategies that facilitate antibiotic tolerance be present (Verderosa et al., 2019).

3.2 Oxygen gradients Penetration of oxygen into the biofilms is limited. It has been shown that oxygen can penetrate about 50μm into biofilm with an average thickness of 210 μm (Borriello et al., 2004). The studies indicated that enhanced hypoxia in P. aeruginosa caused increased antibiotic resistance of microbial cells to the biofilm via modifying the composition of multidrug efflux pumps (Mah, 2012). It has been suspected that hypoxia by shifting expression of the multidrug efflux pump linker protein toward a dominance of the MexEF-OprN increases multidrug resistance in P. aeruginosa. This mechanism may be involved in antibiotic resistance of P. aeruginosa infections in patients with cystic fibrosis (Borriello et al., 2004). Oxygen limitation within the biofilm most probably influences the function of some antibiotics such as aminoglycosides due to the downregulation of energy metabolism genes and modifications in gene expression (Kindrachuk et al., 2011; Taylor et al., 2014).

3.3 Decreased growth rate It has been observed that bacteria in the outer part of the biofilm have high metabolic activity, while the ones in the inner part have low metabolic activity. Therefore, microbial cells within biofilms exhibit different levels of physiologic activities. In fact, the complex internal structure of a biofilm

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generates microenvironments in which oxygen and nutrients are deprived (Walters III et al., 2003; Werner et al., 2004). Microbial cells in these microenvironments especially the ones located deep inside the biofilm structure exist in a slow-growing or starved state (M. R. Brown et al., 1988). These nutrients and oxygen-depleted microenvironments can result in a stationary phase-like dormancy that influences the function of antibiotics (Y. Li, Jia, et al., 2016; W. Li, Yao, et al., 2016). Slow-growing or nongrowing cells are not very sensitive to many antimicrobial agents. In this condition the microbial cells divide infrequently (M. R. Brown et al., 1988). So, antibiotics that act on various processes in rapidly replicating cells, including replication, transcription, translation, and cell wall synthesis are ineffective on them. Slow-growing or dormant cells in nutrient- and oxygen-deficient zones within a biofilm would be unaffected by these antibiotics and thus exhibit high levels of antibiotic tolerance compared to active bacteria that have exponential growth (Ciofu et al., 2015; Verderosa et al., 2019). The starvation of most amino acids, mainly leucine, cysteine, and lysine, as well as glucose-induced biofilm tolerance to ofloxacin have been reported (Bernier et al., 2013). Dividing cells were more susceptible to bactericidal action of tobramycin and ciprofloxacin, while cells in inner parts of the biofilm were resistant and showed slow-growth state (Van Acker et al., 2014). It seems that limited penetration of nutrients has a more considerable role compared to restricted access of antibiotics in the antibiotic tolerance of biofilms. However, there are antibiotics that do not require rapidly replicating cells to facilitate their mode of action, and many of these agents are still highly tolerated by biofilms. Therefore, decreased growth rate is not enough to confer antibiotic tolerance property (Hall-Stoodley & Stoodley, 2009).

3.4 Persister cells It has been shown that whenever a biofilm is treated by antibiotics, a small cell population will be unaffected regardless of the concentration of utilized antibiotics. These cells are known as persister cells. They are a minute subpopulation of bacterial cells in a slow-growing, starving state or dormant state and exhibit extreme antimicrobial tolerance (K. Lewis, 2012). At the end of antibiotic therapy when antibiotic concentration reduces, the persister cells revert to a growing state and the biofilm was repopulated by the action of persister cells as a nucleation point. Subsequently, the infection is relapsed. So, the contribution of persister cells to pathogenesis especially in chronic diseases is undeniable (Olsen, 2015). Interestingly, a larger number

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of these cells in repopulated biofilm exhibit no additional antimicrobial tolerance in comparison with the original cells that were eradicated. This observation strongly confirmed that the persister state is a phenotypic variant rather than a genetic alteration. In other word, persisters do not directly carry antibiotic resistance genes, but they undeniably provide a perfect platform for the development of resistant mutants (Verderosa et al., 2019). Persisters exhibit enhanced toxin/antitoxin (TA) systems induced by starvation or DNA damage. The TA systems act through suppressing protein synthesis by phosphorylation of elongation factor, causing translational suppression and subsequent tolerance to antibiotics; expression of TA molecules, which produce an anion channel in the membrane, results in reduction in proton motive force and ATP levels; and degradation of mRNA by suppressing translation. These mechanisms lead to the emergence of multidrug-tolerant persister cells (Conlon, 2014; Jolivet-Gougeon & Bonnaure-Mallet, 2014). However, studies on P. aeruginosa mutants have suggested that additional genes including rpoS, spoT, relA, dksA, dinG, spuC, algR, pilH, ycgM, and pheA other than those encoding toxin/antitoxins can be involved in persister cell formation (Ciofu et al., 2015). This indicates that persister cells may be produced by multiple pathways (Drenkard & Ausubel, 2002). As a result, many research groups are investigating the mechanisms of persister cell formation to find antibacterial agents effective on persister cells. Recently it has been shown that a particular compound, ADEP4, can eradicate Staphylococcus aureus persister cells (Riool et al., 2017). ADEP4 is a synthetic acyldepsipeptide antibiotic that activates a ClpP protease and causes dysregulation of proteolysis (Br€ otz-Oesterhelt et al., 2005). The dysregulated proteolysis is ATP-independent process. The ClpP protease activation forces self-digestion of persister cells. By combining ADEP4 with rifampicin complete eradication of persisters can be achieved (D€ orr et al., 2010; Kirstein et al., 2009).

3.5 High rate of genetic material exchange The microbial cells are located close to each other within biofilms. This arrangement and also presence of the extracellular matrix accelerate horizontal gene transfer especially when the cells are exposed to a sublethal dose of antibiotics. Microbial cells in the biofilm exhibited up to 10-fold enhancement in the transfer efficiency of the plasmid DNA (H. Ma & Bryers, 2013; Olsen et al., 2013). The transfer of DNA can occur through transformation, transduction, and conjugation. Besides, outer membrane vesicles can transfer DNA in biofilm (Kadurugamuwa & Beveridge, 1999). The

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transferred DNA can be integrated or recombined in the recipient’s chromosome or remain as an extrachromosomal inheritable element. This can make biofilm a reservoir for antimicrobial resistance genes (Olsen, 2015).

3.6 Extracellular DNA Large amounts of extracellular DNA (eDNA) exist in the extracellular matrix of biofilms. The eDNA in the matrix of biofilms creates a cationlimited environment. P. aeruginosa biofilms are rich in eDNA. It has been shown that LPS modification operon, PA3552-PA3559, is induced in P. aeruginosa under this. The expression of this operon leads to 2560- and 640-fold enhanced resistance to cationic antimicrobial peptides and aminoglycosides, respectively. However, susceptibility to β-lactam and fluoroquinolone is not affected by this condition. Consequently, the reduced sensitivity of biofilm to some of positively charged antibiotics may also be related to the cation chelating properties of eDNA (Mulcahy et al., 2008).

3.7 Stress Bacterial life in biofilms is stressful form of life because of the deprivation of nutrients, excess waste products, and hypoxia. Indeed, biofilm resistance may reflect response of cells within biofilm to stress, which may enhance rate of mutations in biofilms. Stress-related genes are more extensively expressed in the cells located on top of a thick biofilm (Williamson et al., 2012). The cells in the biofilm protect themselves against oxidative stress which at the same time results in their tolerance to cidal drugs through overexpression of some efflux proteins, causing DNA damage via suppressing topoisomerase. Cleavage of LexA occurs in this condition which modulates drug resistance and stimulates the formation of persisters or dormant cells in biofilm ( Jolivet-Gougeon & Bonnaure-Mallet, 2014). RpoS-mediated stress responses result in increased antibiotic resistance of E. coli in mature biofilm (Ito et al., 2009). Also, high tolerance to the fluoroquinolone ofloxacin is induced by the SOS stress response in microenvironments of heterogeneous and nutrient-limited biofilm (Bernier et al., 2013).

3.8 Mutation The cells located deep inside the biofilm structure are exposed to sub-MIC levels of antibiotics during antimicrobial treatment because of declined penetration, dilution, or diffusion gradients of antibiotics in the biofilm. In this condition mutagenesis is accelerated ( Jørgensen et al., 2013). When

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P. aeruginosa populations, including normomutable and hypermutable ones, were constantly exposed to sub-MICs of ciprofloxacin, rapid development of high-level ciprofloxacin resistance happened because of mutations in the genes gyrA and gyrB, along with cross-resistance to β-lactam antibiotics ( Jørgensen et al., 2013). As mentioned previously, the biofilm mode of growth leads to oxidative stress, which may cause an increase in mutation rate in biofilms. Mutability of cells within P. aeruginosa biofilm is 105-fold higher than that of planktonic cells (Driffield et al., 2008). Mutations lead to modification of antibiotic targets, enhanced expression of drug efflux pumps, and reduced permeability of the cells because of changes in the outer membrane and in the action of modifying enzymes. It has been shown that mutator (mutS) derivative strains of P. aeruginosa took over the whole biofilm after ciprofloxacin treatment for 2 days (Mah, 2012).

3.9 Quorum sensing Microbial cells within biofilm communicate with each other. This communication occurs through small diffusible molecules called as autoinducers (AIs). This communication is known as QS which regulates or coordinates physiological status of the microbial population and various behaviors of cells such as biofilm development and virulence. Generally, QS has important role in cell maintenance and division, horizontal gene transfer, host-microorganism interactions, and behavior, including movement and biofilm formation. This integrated response allows the biofilm to behave like a multicellular organism, which enables the bacterial community to adapt to changing environmental conditions (Banerjee et al., 2020; Carniol & Gilmore, 2004; Kong et al., 2006; Labbate et al., 2004; Raguraman et al., 2018). Also, this cell-to-cell communication as a complex regulatory process prevents biofilm cell density from reaching an unsustainable level (Nadell et al., 2008). The AI molecules differ depending on the type of organisms. These signal molecules called N-acyl-homoserine lactones (AHLs) or autoinducing peptides (AIPs) in Gram-negative and Gram-positive bacteria, respectively (Bhatt, 2018; Papenfort & Bassler, 2016; Rama Devi et al., 2016). These AIs are continuously being synthesized by the bacterial cells, thus cell density and the level of AIs enhance simultaneously. Whenever the concentration of AIs reaches a critical threshold concentration (the quorum level), AI receptor binding results in the repression or activation of several target genes (Annous et al., 2009). For example, luxI is AHL encoding gene

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which is transcriptionally expressed to the basic level at low population density (Rasmussen & Givskov, 2006). The AHLs can freely diffuse across the inner and outer membranes (Annous et al., 2009). As the cell density is increased and the concentration of AHL reaches a sufficiently high level, the AHL molecules are diffused back to the cell and sensed by LuxR family of receptor proteins. Then, the AHL binds to the receptor protein and the activated LuxR-AHL complex forms multimers with other activated LuxR-AHL complexes. Finally, these multimers control the transcription of QS regulated genes. Gram-positive bacteria apply small posttranslationally processed peptide signal molecules, AIPs. After production in the cell, the AIPs are first processed and subsequently secreted. When the AIP reaches high extracellular concentration, it attaches to its associated membranebound two-component histidine kinase receptor. Consequently, autophosphorylation occurs that passes phosphate to the associated cytoplasmic response regulator which further activates the receptor kinase activity. This event activates transcription of the genes involved in QS regulation (Rasmussen & Givskov, 2006; Thoendel et al., 2011). It has been found that QS contributes to the tolerance of P. aeruginosa biofilms to tobramycin, kanamycin, and hydrogen peroxide (Bjarnsholt et al., 2005; Hassett et al., 1999; Shih & Huang, 2002). This may be connected to the role of QS in the production of eDNA that suppresses the penetration of some antibiotics into the biofilm. There is no data that QS induces antibiotic tolerance in planktonic cells. So, it seems that this mechanism could be biofilm-specific mechanism (Ciofu et al., 2015).

3.10 Efflux pumps and membrane protein Efflux pumps exist in the periplasmic space and by their action inhibit the accumulation of antibiotics. They have a significant role in multidrug resistance ( J. Sun et al., 2014). It seems that overexpression of efflux pumps in biofilm contributes to antibiotic resistance of P. aeruginosa biofilms (Kassen & Rainey, 2004). Porins are outer membrane channel proteins in Gramnegative bacteria. They are involved in transporting hydrophilic molecules from outer environment to the periplasmic space. Mutation in porins lead to the decreased permeability of hydrophobic molecules (L. Wang et al., 2000).

4. Human health and biofilms Although the level of clinical care has advanced significantly over the few years, bacterial biofilm-associated infections continue to pose a considerable

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threat to public health. It is estimated that significant part (65%) of all bacterial infections are associated with bacterial biofilms. These include both device- and nondevice-associated infections. This phenomenon highlights the significance of microbial adhesion and aggregation from medical aspect. Microorganisms can adhere to various natural (skin, mucosa) and artificial (catheters and implants) substrates that is required for their colonization and consequently their pathogenicity. After colonization, multilayered biofilm occurs and make colonization of other species possible. Pathogenicity of bacteria increases with the formation of biofilms since it protects bacteria from being eradicated by external treatment. Biofilm protects the bacterial cells from various cleaning procedures such as chemical exposure, UV radiation, phagocytosis, pH stress, antibiotics, and dehydration. It is thought that biofilm formation is a prime factor that causes a number of chronic and recurrent infections. Therefore, biofilm-related infections are hard to treat because it possesses several defense mechanisms mentioned previously. Biofilm-related infections frequently occur in immunocompromised patients and in the patients who suffer from chronic illness such as cardiovascular disease, diabetes, skin barrier breakage, and cancer. It was shown that the usage of different forms of embedded medical devices, including intravascular implants, cardiovascular implants, neurosurgical implants, orthopedic implants, ophthalmic implants, dental implants, gynecological implants, urinary catheters and intravenous, and respiratory assist devices increase the chance of biofilm-related infections. Microorganisms reach the surface of medical devices through several mechanisms, including direct contamination, adjacent paths, and blood. Microbial cells attach to biomaterials due to Van der Waals forces and hydrophobic interactions. The macromolecular components existing in the body fluids (urine, blood, and saliva) adsorb on the surface of the medical devices and thus create a conditioning film, which plays an important role in microbial adherence. Some of these macromolecules can act as receptors for microbial cells. Although negative charge on the microbial cells and biomaterial surface creates repulsion, van der Waals forces overcome the repulsive forces. Therefore, the microorganisms are being held at about 10 nm away from the surface of the biomaterials, rendering possible their initial attachment. Also, if cell surface of microorganisms and biomaterials is hydrophobic, hydrophobic forces play an important role in microbial attachment. Hydrophobic forces are 10–100 times stronger than Van der Waals forces at a distance of 10 nm from the biomaterial surface.

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Various properties of surfaces such as surface texture, electrostatic loading, and surface hydrophobicity affect microbial adherence. Biomaterials with rough, hydrophobic, and positively charged surfaces are more prone to biofilm formation than ones with smooth, hydrophilic, and negatively charged surfaces ( Jamal et al., 2018).

4.1 Cystic fibrosis Cystic fibrosis (CF) is a genetic disease (autosomal recessive). In CF lungs cannot function normally because of a defect in cystic fibrosis transmembrane conductance regulator protein. This defect results in the production of viscous mucus on respiratory epithelium making breathing difficult. The lungs in this condition are prone to bacterial infections especially by S. aureus, Haemophilus influenza, and P. aeruginosa. The P. aeruginosa biofilms in cystic fibrosis adversely affects lungs, and their presence is revealed by detecting homoserine lactone (HSL) secreted by the bacteria in the sputum of patients (Koch & Hoiby, 1993; Lyczak et al., 2002; P. K. Singh et al., 2000).

4.2 Endocarditis Native valve endocarditis results from the interaction between the bacteria in blood stream and surfaces of the mitral, aortic, tricuspid, and the pulmonic valves of the heart. The endocarditis-associated microorganisms are species of Staphylococcus, Candida, Pneumococci, Streptococcus, and few other Gramnegative bacteria (Kokare, Chakraborty, Khopade, & Mahadik, 2009). These microbial cells first pass into the blood stream through oropharynx, genitourinary tract, and gastrointestinal tract (Kokare et al., 2009). Bacterial association is weak in early steps. But wound emergence or damaged epithelium enables opportunistic bacteria to form a strong biofilm colonization that hurts the heart valves (Kokare et al., 2009). In this situation, red blood cells, platelets, and fibrin accumulate at the site of injury. Endothelial cells secrete fibronectin that bind to collagen, fibrin, human cell as well as bacteria. Microorganisms such as Staphylococcus and Streptococcus sp. have fibronectin receptors which can form biofilms at the site of injury as well as damage the tissue of the valves. This biofilmassociated infection disrupts normal function of heart valves which leads to their leakage. Subsequently bloodstream can be infected and recurrent fever, chronic systemic inflammation, and other severe complications may occur (Kokare et al., 2009; Long & Koyfman, 2018).

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4.3 Periodontitis Periodontitis is extensively studied as a biofilm infection in gum. In this disease the soft tissues and bones that support our teeth are damaged. The bacteria, including Fusobacterium nucleatum and Pseudomonas aerobicus, responsible for this infection can colonize on mucosal surfaces in oral cavity (Lamont & Jenkinson, 1998). After colonizing, the microbes alter the calcium flux, invade mucosal cells, and release toxins. Plaque which is a biofilm community appear 2–3 weeks after bacterial infection. With increase in the amount of plaque, saliva (which has bactericidal properties) cannot penetrate or reach the whole bacteria in the biofilm (Lamont & Jenkinson, 1998; Overman, 2000).

4.4 Osteomyelitis Osteomyelitis is a bacterial or fungal infection of the bone. Osteomyelitis is a serious bone infection that infrequently occurs (only 2 out of 10,000 people). Bones can become infected when infection spread through bloodstream, trauma, or an earlier infection. In this condition, the leucocytes in the bloodstream enter the infected tissue and try to engulf the bacteria through secreting enzymes. These enzymes lead to bone lysis. S. aureus is the predominant bacterium which is isolated from all forms of osteomyelitis. S. aureus has fibrin receptors and thus bind to fibrinogen present in the bone matrix and can accelerate biofilm formation (Banerjee et al., 2020; Ciampolini & Harding, 2000; Lew & Waldvogel, 2004).

4.5 Rhinosinusitis Rhinosinusitis is a condition with symptoms such as nasal irritation, thick mucus discharge, plugged nose, and pain in face. Chronic rhinosinusitis, an inflammatory disorder, is caused by the presence of bacterial biofilms such as Streptococcus pneumoniae, S. aureus, H. influenza, and Moraxella catarrhalis (Foreman et al., 2010; Stephenson et al., 2010).

4.6 Infection in chronic wound Chronic wounds such as diabetic foot ulcer, pressure sores, and venous ulcers are resilient to treatment by current drugs because of the presence of biofilm-associated bacteria such as S. aureus biofilms. It has been found that S. aureus exists in 88%–98% of wound infections. The infectious agents such as P. aeruginosa delay reepithelialization of wound tissues (Alhede & Alhede, 2014; Bjarnsholt, 2013; Gjødsbøl et al., 2006; Hansson et al., 1995).

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4.7 Medical device-related biofilm infections Medical device-related infections mainly occur due to the formation of biofilm on them. Central venous catheters, urinary catheters, endotracheal tubes, pacemakers, joint prosthetics, prosthetic heart valves, intravenous catheters, orthopedic implants, and contact lenses are medical devices that are susceptible to biofilm formation (R. M. Donlan, 2001). Medical devices can be contaminated by microorganisms transferred via the skin of patients or health-care workers, contaminated water, or other external environmental factors (R€ omling et al., 2014). Among biofilm-forming microorganisms Staphylococcus epidermidis, S. aureus, Acinetobacter baumannii, E. coli, K. pneumoniae, and P. aeruginosa are most commonly associated with biofilms formed on medical devices (Niveditha et al., 2012). 4.7.1 Central venous catheters Vascular catheters placed more than 30 days are colonized by microbial cells and subsequently biofilm forms on them. Consequently, health of patients with bone marrow transplant who need a long-term vascular catheter for intravenous access is more compromised due to increased risk of biofilm infections. To decrease infection rate, vascular catheters are replaced regularly. This replacement greatly increases health-care costs (R. Donlan et al., 2001; Raad et al., 1993). 4.7.2 Urinary catheters Urinary catheterization is usually used to collect urine during surgery, monitor urine output, and prevent urine retention in patients in intensive care unit. Urinary catheters are made of either latex or silicone. Proteus, Pseudomonas, and Klebsiella as urease producing bacterial genera enhance urinary pH by making an alkaline condition that promotes the crystalline biofilm formation (struvite). Consequently, deposits can be formed on the outer surfaces, tips, and balloons of catheters and results in severe difficulty, like damage to the urinary bladder. Short-term catheterization when it is necessary or regular replacement of urinary catheter may prevent urinary catheter-associated infections. Nevertheless, repeated replacement causes disruption that poses severe complications specially the spread of bacteria to uncontaminated sites due to biofilm shedding (Hessen et al., 2000; Kokare et al., 2009; Neethirajan et al., 2014; Stickler & Morgan, 2008).

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4.7.3 Endotracheal tubes It has been shown that numerous microorganisms, including microflora in oral cavity and also pathogenic microorganisms can colonize on endotracheal tubes and form biofilms. Methicillin-resistant S. aureus, E. coli, K. pneumoniae, P. aeruginosa, and Acinetobacter spp. are key factors for the development of ventilator-associated pneumonia (Bauer et al., 2002; Vandecandelaere & Coenye, 2015; Vandecandelaere et al., 2012). 4.7.4 Prosthetic joints S. epidermidis or Propionibacterium acnes can colonize and form biofilm on prosthetic joints and induce severe complications and significant mortality after joint replacement surgery. These infections lead to loosening of joint prostheses (McMinn et al., 2012; Trampuz & Widmer, 2006). 4.7.5 Pacemakers and heart valves Mechanical valves and bioprostheses are prone to microbial colonization and subsequent biofilm formation. It has been reported that S. aureus, S. epidermidis, P. aeruginosa, A. baumannii, K. pneumonia, E. coli, P. acnes, Enterococcus, Candida spp., Enterococci, and yeasts are the most common causative agents of cardiac implant infections on pacemakers, prosthetic valves, defibrillators, and coronary artery bypass grafts. Microbial cells via developing biofilms on the surrounding tissues of the prosthesis colonize on these surfaces with higher affinity (Kokare et al., 2009; Viola & Darouiche, 2011). Created biofilms on heart valve decrease blood flow, cause hematogenous spread, and infect other organs. Primarily, formation of blood clots on heart valves after injury makes them susceptible to infections because blood clots provide a favorable surface for bacterial adhesion (Bosio et al., 2012). 4.7.6 Contact lenses There are two classes of contact lenses, soft contact lenses and hard contact lenses. The first is made of silicone and hydrogel and the latter is made of polymethylmethacrylate. P. aeruginosa and staphylococcal species are main infectious agents in lens carriers. Microbial contamination and its related biofilms cause ulcers which is known as microbial keratitis (Stoica et al., 2017). To prevent these complications various types of materials to make polymeric contact lens have been developed. For example, contact lenses have been developed from hydrogels. They release ceragenin and can prevent P. aeruginosa and S. aureus colonization for 2 and 4 weeks, respectively. Although, some microorganisms, including Candida, P. aeruginosa, and

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Fusarium, are resistant to the biocides in standard contact lens (X. Gu et al., 2013; Szczotka-Flynn et al., 2009). 4.7.7 Orthopedic implants Usage of hip implant has increased in the last decade to enhance life quality. Biofilm formation (1.5%–2.5%) on hip implant results in inflammation and tissue destruction around implants and makes its replacement necessary. Infections related to orthopedic implants are frequent and have high morbidity and mortality rates, and cause devastating complications. Microbial contamination may originate from direct contamination of the prosthesis, contaminated wound, or adjacent infections. S. epidermidis and S. aureus, Staphylococcus hominis, Staphylococcus haemolyticus, P. aeruginosa, and Enterococcus faecalis can cause orthopedic infections. The infections are hard to treat, sometimes the removal of the prosthesis is necessary (Campoccia et al., 2006; Guggenbichler et al., 2011).

5. Biofilm formation in food-processing environments Safe food with good quality is essential for growing population. Therefore, food spoilage and deterioration due to microbial contamination should be prevented. Foodborne pathogens selected modern food processing lines as favorable environments to form biofilm (Lindsay & Von Holy, 2006). They attach to various surfaces, including the food surfaces and processing equipment during food handling or processing, and form biofilm. These bacteria compromise human health through secreting toxins or other pathogenesis mechanisms. Their devastating effects mainly rely on the bacterial species involved in the formation of biofilms (Meireles et al., 2016). Food-processing environments are compromised by persistence of these pathogens. Some properties such as high moisture, prevalence of food contact surfaces, the complexity of processing facilities, lengthy production cycles, mass production of products, and the vast areas, make foodprocessing environments susceptible to biofilm development and their associated complications, which are a main concern for public health (Awad et al., 2018; Lindsay & Von Holy, 2006). Water, milk and other liquid pipelines, pasteurizer plates, reverse osmosis membranes, tables, employee gloves, animal carcasses, contact surfaces, storage silos for raw materials and additives, dispensing tubing, and packing material are the main locations for biofilm development in various food factories (Camargo et al., 2017).

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Biofilm formation in food products decreases their shelf life, encourages food spoilage and foodborne diseases, causes corrosion of metal surfaces, damages equipment, and alters of organoleptic properties due to the secretion of lipases or proteases (Awad et al., 2018). Based on the estimation, 80% of bacterial infections in the United States is directly related to foodborne pathogens residing in biofilms (Srey et al., 2013). Gastrointestinal, neurological, gynecological, and immunological systems can be affected by foodborne pathogens. The World Health Organization (WHO) estimated that 420,000 deaths occur annually due to foodborne pathogens. Biofilm provide nutrients for the microorganisms and protect them against dehydration, starvation, and antimicrobial attacks (Meireles et al., 2016). E. coli O157:H7 (Al-Shabib et al., 2017), S. aureus (Millezi et al., 2012), Listeria monocytogenes (Daneshvar Alavi & Truelstrup Hansen, 2013), Campylobacter jejuni (Ica et al., 2012), Vibrio parahaemolyticus (Mizan et al., 2018), and Salmonella enterica (Liu et al., 2016) are some of the foodborne pathogens with the ability to attach to different food contact surfaces and form biofilm. They can cause hemorrhagic colitis, hemolytic uremic syndrome (Rangel et al., 2005), listeriotic (Bruchmann et al., 2015), and bacterial gastroenteritis (L. Li et al., 2019). Also, spoilage-associated microorganisms belonging to the genera of Pseudomonas, Acinetobacter, Serratia, Shewanella, Chryseobacterium, Flavobacterium, and Enterococcus have been described to attach to food contact surfaces and form biofilms (Vilanova et al., 2015; Yuan et al., 2018).

5.1 Factors influencing biofilm formation in food industries Several biotic and abiotic factors, including properties of the attachment surface (X. Tang, Flint, Bennett, et al., 2009; X. Tang, Flint, Brooks, & Bennett, 2009), the food matrix constituents (Van Houdt & Michiels, 2010), environmental factors (Govaert et al., 2018; Ibusquiza et al., 2012), and the bacterial cells affect biofilm formation in food industries. 5.1.1 Properties of the attachment surface Various bacteria can attach to different surfaces used in food-processing environments. This phenomenon leads to unhygienic status that finally causes economic losses and health problems owning to food contamination and spoilage. Subsequently, the shelf life of the affected food products declines, and finally food products are denied by consumers. Also, biofilm aggregation and accumulation inside pipelines often result in the mechanical

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blockage of water circulation and pipes, corrosion of surfaces, reduced capacity, and enhanced energy losses (Chmielewski & Frank, 2003). The formation of biofilm on membranes used in filtration process in the dairy and beverage industries drastically reduces their efficiency and results in the premature replacement of the membrane (Anand et al., 2014). Also, the food products are exposed to cross-contamination with L. monocytogenes, Yersinia enterocolitica, C. jejuni, Salmonella spp. Staphylococcus spp., Bacillus cereus, and E. coli O157:H7 (Anand et al., 2014). Therefore, orderly sanitization is vital to minimize the risk of contamination and to have a hygienic environment in food processing plants. Foodborne pathogens are able to attach and form biofilms on different surfaces common in food industry, such as stainless steel, polyethylene, wood, glass, polypropylene, and rubber. The surface characteristics, including electrostatic charge, hydrophobicity, interface roughness, and topography affect the rate of biofilm formation (Yuan et al., 2020). The effect of these parameters is not the same under different experimental conditions. For example, it has been reported that rougher surfaces with more hydrophobicity enhance bacterial attachment (Dhowlaghar et al., 2018), while some studies showed that there is no relationship between roughness or hydrophobicity and bacterial attachment (Niu et al., 2017). These inconsistencies can be due to the application of different methods and bacterial strains, and also due to the fact that overall attachment is affected by multiple factors. Therefore, it is impossible to anticipate the rate of biofilm formation based on any single factor, because various factors and their interactions are involved in biofilm formation (Yuan et al., 2020). Stepanovic reported that Salmonella spp. and L. monocytogenes can produce high amounts of biofilms on plastic surfaces. It has been reported that L. monocytogenes can form biofilm on 17 different surfaces (Beresford et al., 2001; Stepanovic et al., 2004). Surfaces that provide more nutrients are more susceptible to colonization by biofilm-forming microorganisms. For example, Adetunji and Isola (2011) evaluated the biofilm formation on various materials, including wood, stainless steel, and glass surfaces. They reported that wood encouraged biofilm formation due to its porosity and absorbency, properties that lead to entrapment of organic material and bacteria. In this regard glass is the preferred food contact surface due to its smooth surface and corrosion resistance properties. Also, stainless steel show more resistance to the damage. It is chemically inert at a variety of processing temperatures, resistant to corrosion, and easy to clean. However, cracks and crevices appear on this material generally due to the continuous use. These cracks

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reduce efficiency of mechanical cleaning methods and sanitizing treatments as well as protect the bacteria (Adetunji & Isola, 2011; Petkoska & Nasov, 2014). 5.1.2 Food matrix constituents Food matrix constituents are one of the factors effective on bacterial attachment in food-processing environments. For example, food residues such as milk and meat exudate promote the growth and proliferation of microorganisms because they are rich sources of proteins, fats, and carbohydrates. Under this condition it is likely that biofilm with mixed species will be formed. For example, lactose in milk encourages biofilm formation by both Bacillus subtilis, and S. aureus through activating the LuxS-mediated QS system and promoting polysaccharide intercellular adhesion, respectively (Duanis-Assaf et al., 2016; Xue et al., 2014). Also, considerable concentrations of free Ca2+ and Mg2+ in milk lead to increased biofilm formation by Geobacillus spp. (Somerton et al., 2015). Spoilage of the fruits, vegetables, meat surfaces, and low-acid dairy foodstuffs is mainly due to bacteria belong to Pseudomonas genus (Vishwakarma, 2020). 5.1.3 Properties of the microbial cells Some properties of the microbial cells, especially cell surface hydrophobicity; presence of appendages such as flagella, pili, and fimbriae; components of the cellular membrane such as protein and lipopolysaccharide, and the EPS secretion play significant roles in enhancing biofilm formation (VanHoudt & Michiels, 2010). According to these properties, difference in biofilm-forming capacity among species or strains of different serotypes and genotypes have been described (R. Wang et al., 2015). In this regard, the presence of different species in a mixed microbial community provides ecological advantages and can promote cocolonization of various species. For example, coexistence of Pseudomonas, Listeria, Salmonella, and other pathogens can form biofilms more stable and resistant to disinfectants like quaternary ammonium compounds and other biocides (Giaouris et al., 2015). The presence of mixed microbial community consisting of Aeromonas hydrophila, L. monocytogenes, S. enterica, and Vibrio spp. has been reported in fresh fish products that led to considerable health and economic burden. In mixed-species biofilms the psychrotrophic bacteria such as Pseudomonas spp. can be found along with the thermophilic Geobacillus stearothermophilus (Giaouris et al., 2015).

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5.1.4 Environmental conditions Furthermore, environmental conditions, such as nutrient composition, pH, temperature, O2, and shear forces, also play important roles in the process of biofilm formation (Yuan et al., 2020).

5.2 Biofilm in food industries 5.2.1 Dairy industries Milk is very susceptible to microbial contamination and subsequent spoilage. Dairy industry equipment especially milk storage tanks and milk process lines is very prone to microbial contamination due to milk protein remaining in them which provides a source of nutrients. Microorganisms, especially the ones belonging to Enterobacter (Salo et al., 2006), Listeria (Waak et al., 2002), Lactobacillus, Micrococcus, Streptococcus, Bacillus (M. Sharma & Anand, 2002), and Pseudomonas genera can survive and probably form biofilm. For example, in this condition Streptococcus thermophilus can develop biofilms at short period of time (Dharmadhikari, 1992). The thermoduric and thermophilic microorganisms can be alive throughout the heat treatment and often colonize the stainless-steel surfaces (Vishwakarma, 2020). Sporeforming bacteria including Bacillus such as B. subtilis and B. cereus and Clostridium such as Clostridium tyrobutyricum resist very high heat level treatments and then in favorable conditions, they germinate and contaminate the equipment. Enterobacter sakazakii grows well in biofilms. So, it is considered as a major health concern in the milk powder industry. Microbial contamination results in unfavorable changes in the flavor and bitter taste of dairy products as well as declines their shelf life (Vishwakarma, 2020). 5.2.2 Fish processing industry Equipment and water are considered to be contamination sources in the fish processing industry. Vibrio cholera (Faruque et al., 2006), V. parahaemolyticus, Vibrio vulnificus ( Joseph & Wright, 2004), and Vibrio alginolyticus (Kogure et al., 1998) frequently contaminate fishes and form biofilm. L. monocytogenes, Salmonella spp., Bacillus spp., Aeromonas, and Pseudomonas spp. are also known to form biofilm in fish and seafood processing. These biofilm-forming bacteria can be found in many locations on the seafood processing lines despite applying regular cleaning and disinfection strategies (Enos-Berlage et al., 2005; Rajkowski, 2009). Using seawater instead of fresh water for economic reason is another important risk factor. Combined chlorination and ultraviolet radiation system are used to decontaminate seawater. Nevertheless, the treated seawater contains Vibrio spp.

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Various microorganisms especially Pseudomonas putida and P. fluorescens, Aeromonas spp., Enterobacteriaceae, and yeasts are the causes of spoilage in fresh or chilled fish (Gram & Huss, 1996; Shikongo-Nambabi, Kachigunda, & Venter, 2010). The presence of various microbial strains enhances biofilm formation. For example, the presence of Pseudomonas spp. would considerably enhance the colonization of L. monocytogenes on stainless steel (Guobjoernsdottir et al., 2005). 5.2.3 Poultry industry The important risk factors in the poultry processing industry are dust, surfaces, feces, poultry feed, and transportation. These risk factors enhance probability of Salmonella contamination (Marin et al., 2009). It has been shown that almost half of the isolated strains in poultry farms were able to produce biofilms (Marin et al., 2009). Salmonella (like Salmonella sofia) as the most frequent genus can easily attach to commonly used materials in poultry industry, including polytetrafluoroethylene, stainless steel, rubber, and polyurethane. It was suggested that high adhering ability of S. sofia serovar may be due to the presence of high number of pili or fimbriae (Chia et al., 2009). Campylobacter spp. (like C. jejuni) are another genus that occur frequently in poultry and poultry processing (Sanders et al., 2007). 5.2.4 Meat industry Organic residues in food processing create a favorable niche for the accumulation of microbial cells that subsequently form biofilm and can compromise public health. At first, microorganisms such as E. coli O157:H7 attach to the surface and their population increases through the time which can result from the migration of cells toward the surface by flagella-mediated motility and Brownian motion (Van Houdt & Michiels, 2005). After surface attachment the microorganisms can survive on food contact surfaces, and even increase in population through the time even at low temperature (Dourou et al., 2011). The presence of other microbial strains promotes E. coli O157:H7 attachment on the surfaces (Marouani-Gadri et al., 2009). For example, it has been indicated that E. coli O157:H7 cells were embedded by a Acinetobacter calcoaceticus biofilm under both static and dynamic growth conditions (Habimana et al., 2010; Srey et al., 2013). 5.2.5 Ready-to-eat (RTE) food industry RTE foods are a remarkably high risk food because they are prone to crosscontamination during processing and handling and no bactericidal processes

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are applied before their consumption. Also, the storage duration and conditions are known to be factors effectual on quality and safety of RTE foods (Sofos & Geornaras, 2010). Contamination can occur in loading, conveying, weighing, and packaging steps. One of the main sources of crosscontamination is slicing materials used in processing. Di Pinto et al. found that smoked salmon is the most L. monocytogenes contaminated product (Di Pinto et al., 2010). Another concerning strain in RTE food is E. coli O157:H7 because it can form biofilms on food contact surfaces during food processing (Srey et al., 2013).

5.3 Control of biofilm formation in food industries To ensure the quality and safety of food products, biofilm formation should be prevented or formed biofilms must be eradicated by biological (like enzymes, bacteriophages, bacteriocins) or chemical cleaning compounds (like chlorine, hydrogen peroxide, iodine, isothiazolinones, ozone, peracetic acid, acidic compounds, aldehyde-based biocides, phenolics, biguanides, surfactants, halogens, and quaternary ammonium compounds) (Zhao et al., 2017). Impairment of established biofilm can also be achieved by applying physical treatments like ultrasonication and ultraviolet radiation (Sadekuzzaman et al., 2015). Ultrasonic treatment using high acoustic power effectively detach biofilms from affected surfaces; however, it can decrease food quality, modify its physical compositions, and change the taste (Majid et al., 2015). Also, chemical treatments are not sufficient for the complete eradication of target biofilm. Moreover, these chemicals are not ecofriendly and emit toxic and carcinogenic residues that even accelerate the corrosion of applied equipment, processing surfaces, and machinery (Gilbert et al., 2003). In this regard, biological and nanomaterial (such as titanium, silver, gold, palladium)-based strategies seem to be ecofriendly and effective strategies to combat biofilm-related complications in food industries (Nahar et al., 2018; Vishwakarma, 2020).

6. Microbiological corrosion There are different types of corrosion. Various microorganisms can induce corrosion which is also known as bacterial corrosion or biocorrosion. The term microbially induced corrosion (MIC) is also used to describe corrosion resulting from the presence and activities of microbial cells within biofilms at metal or nonmetal surfaces such as concrete. In other words, the synergistic

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interaction between microbial communities and metal or nonmetal surfaces lead to MIC (Beech & Sunner, 2004). The MIC process initiates with formation of biofilm on a metal or nonmetal substrates. Initially cells attach to the substrate surfaces, grow and reproduce on them, and also synthesize an EPS. All these events together result in a formation of complex biofilm. The biofilm formation occurs during three different stages. In the first stage, macromolecules, including protein, lipids, polysaccharides, and humic acids, are absorbed on the surfaces and act as conditioners. With the help of these macromolecules the physical and chemical properties of the interface, including hydrophobicity and electrical charge, change. In this stage, the extent of bacterial transfer rate, adhesion, and size of the formed biofilm is determined by microorganisms, surface, and aqueous medium properties. The attachment of microbial cells which is vital step in biofilm formation process happens during the second stage. In fact, microorganisms move from the bulk phase to the surface. The first bacteria attachment is a reversible adsorption; electrostatic attraction, physical forces, and hydrophobic interactions are involved in it. The attached bacteria that are known as sessile bacteria are more important to the MIC process compared to the planktonic bacteria. These sessile cells reside on a metal surface and produce metabolic products that introduce multiple cathodic reactions. These reactions promote corrosion. The microbial transport to the interface is mediated by (1) diffusion by Brownian motion, (2) convection by system flow, and (3) motile movement (Mishra & Apelian, 2014). EPS production is performed at the third stage of the MIC process, by which the colonization process on the surface is promoted. EPS makes attachment of negatively charged bacteria such as SRB attach to either negatively or positively charged surfaces. The rate of the microorganism’s colonization determines further growth of the biofilm. Biofilms affect the MIC process through three different strategies, including creation of differential aeration cells, modification of the chemical species diffusion at the interfaces, and production of corrosive substances like EPS and other organic acids (Mishra & Apelian, 2014). MIC may be defined as the microbial degradation of materials like metal. Depending on the metal substrate and the environment concerned, it can appear as localized corrosion such as pitting or as general corrosion. Most MIC cases appear as localized corrosion since most organisms do not create a continuous film on the metal surface. In other words, microbial cells tend to settle on surfaces in the form of discrete colonies or at least spotty, rather

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than continuous, films (Muyzer & Stams, 2008). MIC can occur in any aqueous environments, because the microorganisms are frequent in all ecosystems. Different microorganisms can survive and grow in oil, gas, and water injection systems because the essential elements including a carbon source (like hydrocarbons and other organic compounds), water, an electron donor, and an electron acceptor for their lives are present in these environments (Mishra & Apelian, 2014). MIC is one of the several problematic issues in the oil and gas industry. It has been reported that 20%–40% of corrosion cases in oil and gas treatment facilities like refineries, gas fractionating plants, pipeline systems, and exporting terminals is due to the MIC ( Javaherdashti, 2008; B. J. Little & Lee, 2007). Biological organisms present in the aqueous medium often have the potential to increase or decrease oxygen transport to the surface; consequently, these organisms have a role in increasing or decreasing general corrosion. There are two types of biological corrosion: anaerobic corrosion and aerobic corrosion. Sulfate-reducing bacteria (SRB) from the genus Desulfovibrio and aerobic sulfur-oxidizing bacteria of the type Thiobacillus are involved in anaerobic and aerobic corrosion, respectively (Loto, 2017). Many microorganisms including bacteria, archaea, and fungi are involved in the corrosion. Most published research studies on MIC focus on bacteria. Within bacteria, SRB have the main impact on corrosion because sulfate is widely distributed in anoxic environments (Lv & Du, 2018). Archaea are important microorganisms that can induce corrosion in extreme environments like reservoirs with high temperature (Stetter et al., 1993). Also, fungi are the microorganisms frequently involved in MIC in warm and humid climates (Qu et al., 2015). Bacteria and fungi tend to produce organic acids that can cause corrosion of carbon steel. It was reported that these acids lead to corrosion on other metals such as magnesium alloy and zinc. The presence of complex microbial interactions in MIC make investigations, predictions, and mitigations difficult ( Juzeli unas et al., 2007; Qu et al., 2017).

6.1 Common bacteria in MIC process SRB, metal-reducing bacteria (MRB), acid-producing bacteria (APB), and metal-oxidizing bacteria (MOB) (iron-oxidizing bacteria), slime-forming bacteria, and acid-producing bacteria are common bacteria that are involved in MIC of metals in pipeline systems. Mixed population of bacteria in a biofilm are the most corrosively damaging ( Jia et al., 2019).

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6.1.1 Sulfate-reducing bacteria It has been speculated that the SRB are the main cause of MIC and therefore many researchers have focused on corrosion by these bacteria. SRB are significant and frequent bacteria in many ecosystems, including marine sediments, oil field environments, deep-sea hydrothermal vents as well as oil and gas production facilities like wells, pipeline distribution, treatment facilities and refineries, transport, and delivery infrastructure. SRB are typically 3–10 μm in size. They are anaerobic microorganisms and can grow in soil and fresh water and almost in any environment containing traces of water and nutrients. In the presence of oxygen, SRB may survive for a remarkable time until they access anaerobic conditions in a habitat like small cracks, crevices, and the interior of tubercles. In mixed-culture biofilms, SRB tend to be bottom dwellers. Top-layer aerobic and facultative microbes provide a locally anaerobic environment that is required for SRB to grow. All or most of the cell carbon of the SRB is derived from organic substances, so they are heterotrophic bacteria (Loto, 2017). The SRB use sulfate ions as a terminal electron acceptor to produce sulfide in a microbial energetically favorable eight electron transfer reaction. Then, hydrogen sulfide acts as a corrosive agent when it comes in contact with any metals. Besides sulfate, SRB can also use other sulfur compounds with a valence above
2 as terminal electron acceptors. They include bisul2
fite (HSO
3 ), thiosulfate (S2O3 ), and elemental sulfur (Thauer et al., 2007). Furthermore, SRB also have other metabolic abilities, including the ability to reduce nitrate and thiosulfate and direct electron uptake from the metal surfaces. Also, SRB can obtain their carbon and energy from organic acids and nutrients, such as lactate, acetate, and pyruvate (Alabbas & Mishra, 2013). Desulfovibrio and Desulfotomaculum are the only known genera of SRB. They are commonly detected in the oil field (Loto, 2017).

6.1.2 Metal-oxidizing bacteria MOB are significant microorganisms that are often cause MIC in oil and gas pipelines. MOB are classified into three categories: microbes that catalyze the metal oxidation, microbes that accumulate oxidized metal precipitates, and microbes that derive energy by oxidizing metals. MOB are microaerophilic microorganisms, so need minimal oxygen to survive and grow. In this regard, MOB require the presence and assistance of other microorganisms to create and sustain this condition in the environment. Gallionella, Sphaerotilus, and Leptothrix are the common MOB that cause MIC. Also, B. cereus can degrade aliphatic protons and aromatic protons in diesel and oxidize

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ferrous/manganese into oxides. Therefore, B. cereus can cause API 5LX steel corrosion (Ghiorse, 1984; B. Little et al., 1992; Rajasekar et al., 2007). 6.1.3 Iron-oxidizing bacteria (IOB) IOB are able to extracellularly deposit iron hydroxides. They gain energy by oxidizing ferrous ions to ferric ions. IOB like Siderocapsa, Gallionella, Sphaerotilus, and Hyphomicrobium can oxidize iron from the ferrous to the ferric state and precipitate it as a layer. This hydroxide coating and the slime masses associated with it set up oxygen concentration cells that result in corrosion and establish an anaerobic condition in fresh water and also brine. IOB grow in ecosystems having less than 0.5 ppm oxygen. Under this condition, they usually are heavily involved in slime formation. IOB with their metabolism can provide an oxygen-free local environment for SRB growth (Loto, 2017). 6.1.4 Slime-forming bacteria There are various types of slime-forming bacteria that are mostly aerobic and heterotrophic microorganisms. Therefore, they gain their energy from organic sources such as alcohols, sugars, and acids. They can be found in fresh water or brine, but are more common in waters with low salinity. In the oil fields, they are more commonly found in open ponds, supply wells, fillers, lines and equipment, injection wells, storage tanks or ion-exchange resins in steam floods, and in free-water knockouts. They have been isolated from environments with a numerous total bacterial count. Pseudomonadaceae, Enterobacteriaceae, Micrococcaceae, and Bacillaceae are frequently slime-forming bacteria. They are problematic microorganisms whenever they are present in sufficient number to cause plugging or contribute to the corrosion process by producing organic acids (Loto, 2017; TPC, 1990). 6.1.5 Acid-producing bacteria (APB) APB can metabolize organic compounds like ethanol, lactate, aromatic hydrocarbon, and produce organic and inorganic acids like acetic acid. APB can cause MIC due to organic acid production that decrease the pH underneath their biofilms (Xu et al., 2016). In this condition, proton attack is thermodynamically desirable when coupled with iron oxidation. Dissimilar to sulfate reduction, proton reduction can occur extracellularly on a metal surface because it does not require biocatalysis (T. Gu, 2014). Planktonic cells through producing protons can promote corrosion. In fact, they help to maintain an acidic environment. Although the organic acids are

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typically weak acids, at the same pH they are significantly more corrosive in comparison with strong acids like sulfuric acid. This is because organic acids have a buffering power to provide additional protons (Kryachko & Hemmingsen, 2017). Moreover, in an acidic environment protective corrosion product films such a siderite film cannot be formed (Olsson & Landolt, 2003). Acetobacter aceti by producing acetic acid accelerates corrosion of stainless steel and also destroys a protective calcareous film formed through cathodic polarization (Mishra & Apelian, 2014). Acidithiobacillus caldus can cause more severe corrosion of a high-grade stainless steel (Dong et al., 2018). Clostridium acetitum through producing an organic acid promote corrosion. Clostridia and Butyribacteria are considered as main causes of internal corrosion of carbon steel natural gas pipelines. Other species, such as Thiobacillus, can produce sulfuric acid with high corrosive activity. Overall, the generated biogenic organic acids promote corrosion by supplying additional cathodic reactants, binding metal ions and degrading the passive film and hampering passivation that enhance the metal dissolution (Mishra & Apelian, 2014). 6.1.6 Iron-reducing bacteria (IRB) IRB can reduce insoluble ferric iron to soluble ferrous iron. Moreover, other electron acceptors, including Mn(IV), NO3
, NO2
, S2O2
3 , and SO3 can be metabolized by them. Understanding the role of IRB in biocorrosion is difficult because of their environmental requirements (facultative anaerobic), and also the metabolic diversity of different IRB. Therefore, their involvement in MIC is still arguable. IRB accelerate corrosion by several actions, including the reduction of insoluble ferric ion compounds to soluble ferrous ions, by which remove the protective corrosion scales formed on exposed surfaces and formation of concentration cells among the biofilm. Pseudomonas sp. through anodic polarization due to its ability to remove the protective ferric compounds cause corrosion of mild steel under micro-aerobic conditions (Herrera & Videla, 2009; Nealson & Saffarini, 1994; Videla & Herrera, 2009). 6.1.7 Methanogens Methanogens are involved in iron corrosion in oxygen-free environments. Methanogens can cause pipeline pitting corrosion. They usually consume H2 as an electron donor through their respiration that is coupled with reduction of CO2. The utilization of H2 results in cathodic depolarization, thus promote corrosion by CO2. When there is limited amount of H2,

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methanogen biofilms switch to use iron as an electron donor, thus causing more severe corrosion. However, confirmation of this phenomenon is difficult in lab condition because CO2 in the methanogen growth ecosystems seems to be more a corrosive factor and a methanogen biofilm can actually be a barrier for CO2 diffusion and slows down corrosion. Similar to the role of an aerobic biofilm that act as an oxygen barrier and slows down oxygen corrosion (Boopathy & Daniels, 1991; Larsen et al., 2010). MIC can occur either in the absence (extracellular electron transferMIC, metabolite-MIC, and biodegradation-MIC) or presence of oxygen. The mechanisms of these processes are described below.

6.2 Oxygen-free MIC mechanisms 6.2.1 Extracellular electron transfer-MIC (EET-MIC) According to Gu classification, MIC can occur through three different types under oxygen-free conditions (Gu, 2012). In Type I MIC, an energetic metal like elemental iron is used as an electron donor by sessile cells in a biofilm. A nonoxygen oxidant like sulfate and nitrate serves as the electron acceptor. The reduction occurs intracellularly because intracellular enzymes as biocatalysis are needed. However, oxidation reaction like iron oxidation occurs extracellularly because of the insolubility of the metal matrix. Therefore, extracellular electrons can be transported to the cytoplasm for the reduction reaction. Such cross-cell wall electron transport is known as EET. Type I MIC is also called EET-MIC. Whenever SRB use an organic carbon as the electron donor, it does not need to do EET because oxidation of a diffusible organic carbon releases electrons in the SRB cytoplasm where sulfate reduction also occurs. However, when elemental iron is used, EET is required. Xu and Gu hypothesized under carbon source starvation, elemental iron is used by SRB as an electron donor (Huang et al., 2018; Jia, Li, Al-Mahamedh, & Gu, 2017; Jia, Yang, Al-Mahamedh, & Gu, 2017; Jia, Yang, Xu, & Gu, 2017a; Jia, Yang, Xu, & Gu, 2017b). 6.2.2 Metabolite-MIC (M-MIC) Type II MIC is known as metabolite-MIC (M-MIC) and occurs due to microbial-derived corrosive metabolites. Microbes such as APB can produce organic acids that generate a sufficiently acidic environment underneath an APB biofilm. Proton reduction itself does not need biocatalysis unlike sulfate or nitrate reduction in EET-MIC. Thus, M-MIC typically has a counterpart in abiotic corrosion. For example, abiotic acetic acid corrosion is similar to M-MIC by biofilm-secreted acetic acid. In both cases the acidity is provided

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on the metal surface. Besides bacteria, fungi can also cause M-MIC, e.g., A. niger biofilms secret organic acids that cause M-MIC on aluminum alloy 2024. Both aerobic and anaerobic microbes can produce enough organic acids to cause MIC (Dai et al., 2016). 6.2.3 Biodegradation-MIC (BD-MIC) Type III MIC is occurred by microorganisms that attack organic materials such as plasticizers and polymers. Type III MIC is known as biodegradationMIC (BD-MIC). In BD-MIC, microbes by secreting enzymes degrade organic matters like plasticizers and polymers obtain small organic molecules as nutrients. BD-MIC can be caused by anaerobes or aerobes depending on the type of microbes involved in biodegradation ( J.-D. Gu, 2003; T. Gu, 2014; Jia et al., 2018).

6.3 MIC mechanisms in the presence of oxygen MIC in the presence of oxygen is more intricate since the oxygen is terminal electron acceptor. Moreover, if an oxygen-free environment be provided underneath the aerobic biofilm, anaerobic sessile cells can grow. Therefore, the MIC mechanisms on the metal surface will be the same as those in the absence of oxygen. It is very likely that in natural environments, mixedculture biofilms are involved in MIC process, where aerobic IOB support anaerobic SRB growth. If a biofilm is thick enough, oxygen will be depleted before reaching the bottom of the biofilm. Thus, sessile cells at the bottom will switch to anaerobic growth (Thauer et al., 2007). 6.3.1 MIC caused by metal-oxidizing bacteria Aerobic metal-oxidizing bacteria can be divided into two main groups: IOB and manganese-oxidizing bacteria (MOB). Aerobic IOB catalyze the oxidation of ferrous iron to ferric iron by using oxygen as the terminal electron acceptor in respiration to gain energy. Acidithiobacillus ferrooxidans accelerates the corrosion of carbon steel through oxidation of Fe2+ to Fe3+ to fix CO2 and produces organic carbon (H. Wang et al., 2014). MOB catalyze the oxidation of Mn(II) to Mn(IV) and deposits manganese dioxide (MnO2) (Linhardt, 2010). B. cereus ACE4 oxidizes Fe2+ and Mn2+ to Fe3+ and Mn3+, respectively, to gain electrons for energy metabolism and subsequently ferric oxide (Fe2O3) and manganese oxide (MnO2) are formed in the presence of oxygen. This phenomenon promotes pitting corrosion of steel API 5LX. Also, Pseudoxanthomonas sp. through extracellularly catalyzing Mn(II) oxidation accelerates the corrosion rate of carbon

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steel and MnO2 is deposited in a slime layer of biofilm biomass. Moreover, IOB and MOB by consuming oxygen provide oxygen-free environment that is necessary for the growth of SRB and nitrate-reducing bacteria (Linhardt, 2010; Rajasekar et al., 2007). 6.3.2 Oxygen concentration cell The aerobic slime-producing bacteria form biofilm with a patchy distribution on various surfaces like metal surfaces in marine environments. The aerobic bacteria consume oxygen during respiration and the areas underneath the biofilms are sites with low oxygen concentration. Consequently, these areas act as anodic sites and the sites with a less dense biofilm or no biofilm coverage that have higher oxygen concentrations serve as cathodic sites and this phenomenon results in localized oxygen corrosion. In fact, differential aeration in this process lead to the biocorrosion which is known as the oxygen concentration (Abdolahi et al., 2015; J. S. Lee, 2017). 6.3.3 Secretion of corrosive metabolites Some microorganisms like APB through secreting corrosive metabolites lead to corrosion of various metals like stainless steels and nonmetallic surfaces. For example, Acetobacter spp. oxidize ethanol to acetic acid through aerobic fermentation and accelerate the pitting corrosion of steel and copper alloys. Also, bacteria belonging to the genus Acidithiobacillus promote the corrosion of metals and alloys via synthesizing inorganic metabolic products like sulfuric acid in thiosulfate oxidation (Sowards & Mansfield, 2014). B. subtilis also can accelerate the corrosion of cold rolled steel through producing organic acids that reduce pH values. Also, abundant amounts of organic acids like acetic, oxalic, citric, and glutaric are secreted by growth of fungi. So, they cause metal corrosion as well as biodeterioration of monuments and cultural relics (Sadiki et al., 2015). It has been reported that A. niger and Trichoderma harzianum through producing acidic metabolites decline the pH of the culture medium, thus accelerating the corrosion of AZ31B magnesium alloy (Qu et al., 2017).

6.4 Mitigation of biocorrosion 6.4.1 Biocides Biocide treatment is usually applied to mitigate biofilm-related complications. This section focuses on specific points about suitable biocides to limit corrosion. For example, the biocides applied in fuel systems should not react

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with fuels or additives. Also, they should not promote equipment corrosion. In the oil and gas industry both oxidizing (such as chlorine) and nonoxidizing [tetrakis hydroxymethyl phosphonium sulfate (THPS), glutaraldehyde, quaternary ammonium salts, isothiazolones, organobromines, oxazolidines, and triazines] biocides are used owning to their broad-spectrum efficacy, biodegradability, safety, and cost effectiveness. Oxidizing biocides by releasing free radicals attack cellular components. They are commonly used in water utilities and wastewater treatment (Kahrilas et al., 2015; Xu et al., 2017). Chlorination is a common method for controlling biofilms in water utilities. This is low cost method and possesses significant efficacy (Fagerlund et al., 2017). Chlorine through reacting with organic components and EPS of the biofilms deactivate microorganisms or destroys the biofilm integrity (Oliveira et al., 2016). Although its high concentration can lead to corrosion of equipment and also adversely affect the environment after discharge (Rubio, Casanueva, & Nebot, 2015). Also, some of nonoxidizing biocides like THPS, glutaraldehyde, isothiazoline, and quaternary ammonium salts are also applied in water utilities ( Jia, Li, Al-Mahamedh, & Gu, 2017; Jia, Yang, Al-Mahamedh, & Gu, 2017; Jia, Yang, Xu, & Gu, 2017a; Jia, Yang, Xu, & Gu, 2017b; Miller & Koebel, 2006). Some of these biocides like quaternary ammonium/amine compounds and alkyldimethylbenzylammonium chloride (ADBAC) are known as lytic biocides and damage cell membranes (Ioannou et al., 2007). Quaternary ammonium/amine compounds also act as corrosion suppressor and cationic surfactants (Kahrilas et al., 2015). Methylisothiazolinone (MIT) infrequently used biocide can interact with amino acids and suppress critical metabolic processes (Kahrilas et al., 2015). Organobromines like 2,2-dibromo-2-nitrilopropionamide (DBNPA) and bronopol via reacting with sulfur-containing nucleophiles disrupt biological actions (Paulus, 2005). Researchers are now trying to find natural compounds with biocide activity to mitigate biofilms in water utilities. For instance, it has found that ginger extract can act as a green biocide to control MIC of mild steel 1010 in cooling systems (Narenkumar et al., 2017). But high expensive and low efficiency of these compounds limit their application. It should be noted that in flow systems in the oil and gas industry and water utilities, one-step biocide treatment is not enough because microorganisms will be introduced by fluid flow. Therefore, biofilms will be formed again. In this regard, cyclic or repeated biocide treatment is necessary in these systems. It is likely that, this type of treatment with the same biocide

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results in selection of tolerant species to the biocide. So, to overcome this problem higher dose of biocide should be applied over time (Vance & Thrasher, 2005). 6.4.2 D-Amino acids Various organism, including plants, microorganisms, and humans produce D-amino acids to regulate biological functions (Aliashkevich et al., 2018). It has been reported that D-amino acids, including D-tyrosine (D-tyr), D-leucine (D-leu), D-methionine (D-met), and D-tryptophan (D-trp) disperse S. aureus, B. subtilis, P. aeruginosa, and Desulfovibrio sp. biofilms (Emma, 2011; Kolodkin-Gal et al., 2010; E. Li et al., 2018). However, some studies have reported that tested D-amino acids did not inhibit S. aureus, B. subtilis, and S. epidermidis biofilms (Sarkar & Pires, 2015). Kao et al. have showed that D-alanine (D-ala), D-leu, D-met, D-trp were effective only against P. aeruginosa biofilm formation (Kao et al., 2017). D-Amino acids exhibited suitable compatibility with chemicals used in enhanced oil recovery. It has been reported that a membrane with immobilized D-tyr considerably suppressed the primary bacterial absorbance and formation of biofilm. Therefore, it can decrease membrane biofouling in nanofiltration application ( Jiang et al., 2017; C. Yu et al., 2018). D-Amino acids have less efficacy against recalcitrant biofilms, so a biocidal stress is necessary. It has been found that D-met and D-tyr are not effective enough to disperse biofilm of sulfatereducing D. vulgaris on carbon steel. Although, their combination with a biocide (e.g., THPS and ADBAC) is far more efficient ( Jia, Yang, Xu, & Gu, 2017b; Xu, Li, & Gu, 2012, 2014). Moreover, D-phenylalanine (500 ppm) increased efficiency of THPS (80 ppm) in reducing biofilm of a sulfate-reducing D. vulgaris on carbon steel (Xu, Jia, Yang, Sun, & Gu, 2019). Also, D-amino acids enhance efficiency of antibiotics against biofilms (Sanchez Jr et al., 2014; Zilm et al., 2017). Jia et al. have reported that D-tyr (2 ppm) increases ciprofloxacin activity (30 ppm) against P. aeruginosa ( Jia, Li, Al-Mahamedh, & Gu, 2017; Jia, Yang, Al-Mahamedh, & Gu, 2017; Jia, Yang, Xu, & Gu, 2017a; Jia, Yang, Xu, & Gu, 2017b). In order to achieve the desired results in the mitigation of industrial biofilm consortia D-amino acid mixture are needed (Y. Li, Jia, et al., 2016; W. Li, Yao, et al., 2016). Combination of D-tyr, D-met, D-leu, and D-trp increase biocide activity of THPS and ADBAC against recalcitrant oilfield biofilm consortia ( Jia, Li, Al-Mahamedh, & Gu, 2017; Jia, Yang, Al-Mahamedh, & Gu, 2017; Jia, Yang, Xu, & Gu, 2017a; Jia, Yang, Xu, & Gu, 2017b; Y. Li, Jia, et al., 2016; W. Li, Yao, et al., 2016).

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The exact mode action of D-amino acids is not clear. It has been suggested that the substitution of D-ala terminus in the bacterial cell wall by other D-amino acids results in biofilm dispersal (Cava et al., 2011; Kolodkin-Gal et al., 2010). Also, it has been thought that D-amino acids suppress biofilm formation by interfering with protein synthesis (Leiman et al., 2013). 6.4.3 Norspermidine Algae, bacteria, and plants can produce norspermidine which can inhibit biofilm formation (Si & Quan, 2017). It has been reported that norspermidine (25 μM) suppressed S. epidermidis biofilm formation. It via attaching to negatively charged or neutral sugar residues collapses the exopolysaccharides (Ramo´n-Perez et al., 2015). Mature biofilm of S. epidermidis can be disassembled by norspermidine (100 μM). Also, Streptococcus mutans biofilm can be inhibited by norspermidine (5 mM) (Ou & Ling, 2017). Si, Quan, Li, and Wu (2014) reported that a combination of norspermidine (500 μM) and D-tyr (500 μM) disassembled a 6-month old wastewater system biofilm. Norspermidine exert its biofilm dispersal activity through changing the structure of polysaccharide matrix. Moreover, norspermidine can enhance activity of other antibiofilm agents. It has been reported that efficiency of silver and copper ions on biofilm in wastewater treatment system and also on reverse osmosis membranes was increased by norspermidine. One advantage of norspermidine is its cost-effectiveness. Nevertheless, its compatibility with other oxidizing or nonoxidizing biocides should be evaluated (H.-J. Lee et al., 2017; Wu et al., 2016). 6.4.4 Chelators Chelators can enhance biocide activity to suppress formation of biofilm (Pereira de Mello et al., 2017). For example, the antibiofilm efficiency of antibiotics and antimicrobial peptides can be strengthened by ethylenediaminetetraacetic acid (EDTA) (Maisetta et al., 2017). To avoid the corrosive effect of EDTA due to its acidic nature at a high concentration it should be used in a sodium salt form, or the solution’s pH must be adjusted to nearneutral pH before application. Ethylenediamine disuccinate (EDDS) is another chelating agent whose biodegradation rate is faster than that of EDTA (Schowanek et al., 1997). So, it is suitable for industrial applications like fresh water systems. Like EDTA, biocide enhancer activity of trisodium salt of EDDS has been reported. It (2000 ppm) enhanced the inhibitory activity of glutaraldehyde (30 ppm) on biofilm of a sulfate-reducing D. desulfuricans on carbon steel (Wen et al., 2009). The presence of many ions

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in the fluid under treatment decreases the chelator’s availability. So, chelating agents should be applied at high concentrations. 6.4.5 Bacteriophage treatment Bacteriophages can be considered as antibiofilm agents, especially for biofilm-induced corrosion (Akanda et al., 2018). Bacteriophages use their own depolymerases to lyse cells (Parasion et al., 2014). The formed bacterial biofilm like Delftia tsuruhatensis ARB-1 biofilm on membranes in wastewater treatment systems can be removed by bacteriophages (Goldman et al., 2009). This treatment increases the water flux through the membrane (Bhattacharjee et al., 2015). One of the important properties of bacteriophages is that they are host-specific infecting agents. Therefore, to achieve phage treatment with board spectrum activity a combination of several phages or mixture of phages and other antimicrobial agents should be used (Kamal & Dennis, 2015). Although in industrial systems like the oil and gas industry and water utilities applying bacteriophages is not simple due to biodiversity of biofilm-forming cells (Calendar, 2006). For example, phages with inhibitory effect on D. vulgaris or Desulfovibrio aespoeensis may have no effect on other species of Desulfovibrio (Eydal et al., 2009; Summer et al., 2011). So, treatment with cocktail of phages should be used in systems with a variety of bacteria to gain better efficacy (Chatain-Ly, 2014). In conclusion, the application of phage treatment to mitigate mixed-culture biofilm-induced corrosion is very expensive and it is possible that unaffected species readily fill the void and establish new biofilm.

7. Control of biofilms and limiting their related complications Developing controlling strategies to prevent biofilm formation, eradicating formed biofilms, or reducing biofilm-related complications is one of the most important, difficult, and challenging functions in various industries, including medical, food, and oil and gas industries. In this regard, various strategies including mechanical, physical, chemical, and biological methods are being developed to control biofilms.

7.1 Physical control methods The physical methods, including superhigh magnetic fields (Pothakamury et al., 1993), ultrasound treatment (Qian et al., 1997), high pulsed electric fields, low electrical fields, low currents, and use of silver, carbon, and

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platinum electrodes are applied to control biofilms. Antibiotics and low electrical currents can be applied simultaneously to control biofilm (Satpathy et al., 2016). Biofilm formation by S. epidermis can be inhibited using nanoplasma trimethyl silane (TMS) coating on stainless steel or hydrophilic surfaces (Y. Ma et al., 2012). Superhydrophobic surfaces can be provided by silane xerogel coating that act as antiadhesion agent against biofilm-forming bacteria (Privett et al., 2011). 7.1.1 Ultrasonication Ultrasonication is one of the efficient physical techniques that is used in various processes like freezing, cutting, drying, tempering, bleaching, sterilization, and extraction (Chemat, Zill-e-Huma, & Khan, 2011). According to the studies, it is an efficient biofilm removal method (Oulahal-Lagsir et al., 2000). For example, its antibiofilm activity against the industrial milk biofilm on stainless steel has been reported. Also, it has been revealed that sonication with lower frequency is more effective as compared to sonication with higher frequency in declining the survival of cells in the biofilm. Ultrasound showed synergy with other treatments like ozonation, EDTA, antibiotics, or enzymes in eliminating bacteria (Baumann et al., 2009). Ultrasound can improve oxygen diffusion into the biofilm matrix and activates the cells in the biofilms. So, antibiotics can affect them. Also, ultrasound can disrupt the cells before they can develop resistance to the antimicrobial agents.

7.2 Chemical control methods Antibiotics, biocides, and chemical coatings are commonly applied as chemical strategies to inhibit biofilm formation or mitigate its related complications. Chemicals through acting on EPS complex or modifying surface properties like hydrophobicity, charge, or roughness make biofilm formation difficult and accelerate its eradication via other antibiofilm strategies like mechanical antibiofilm methods. Simultaneous application of iron-chelating compounds and aminoglycosides can effectively disrupt P. aeruginosa biofilms (Moreau-Marquis et al., 2009). Also, sodium citrate and N-acetyl cysteine can inhibit biofilm formation by several Staphylococci species and S. epidermis, respectively (Perez-Giraldo et al., 1997; Shanks et al., 2008). Best antibiofilm efficiency can be attained through applying both chemical and mechanical treatment (Exner et al., 1987).

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7.2.1 Sodium hypochlorite Sodium hypochlorite (NaClO) is currently used for bleaching or disinfecting the nonliving surfaces. It showed antibiofilm activities against S. aureus, Prevotella intermedia, Peptostreptococcus micros, Streptococcus intermedius, F. nucleatum, and Enterococcus faecalis biofilms (Ozdemir et al., 2010). It showed more activity against acidic pH compared to alkaline pH (Arau´jo et al., 2011). It has been reported that NaClO was effective against both planktonic cells and cells in the biofilms. For example, NaClO (10 ppm, for 30 s) eliminated all planktonic L. monocytogenes cells and it (1000 ppm, for 20 min) also reduced the number of cells in the biofilm (2 log) (Norwood & Gilmour, 2000). 7.2.2 Hydrogen peroxide (H2O2) H2O2 is a frequently used disinfectant with significant oxidizing ability. It can generate free radicals which affect the biofilm matrix (De Carvalho, 2007). It (500 mg/L) shows antibiofilm activity against four strains of Vibrio spp. in seawater. Being safe and causing no allergic reactions are the advantages of H2O2 (Rideout et al., 2005). It has no adverse effect on product quality in food industries even in its high concentration. 7.2.3 Ozone Ozone is another antibiofilm agent with oxidizing characteristics (Horvath et al., 1985). It showed antimicrobial activity on bacteria, fungi, viruses, protozoa, and bacterial and fungal spores (Khadre et al., 2001). It acts through disrupting the cell membrane, which leads to the leakage of the cell and kill microorganisms. Therefore, it can act faster than antimicrobial agents with intracellular targets. Moreover, microorganisms cannot develop resistance against ozone. Also, its antibiofilm activity has been reported against P. fluorescens and P. aeruginosa biofilms. By forming biofilm their resistance was enhanced against ozone by 3000 and 10 times, respectively. It can due to the reaction of ozone with the biofilm matrix. Nevertheless, their survival was less than 1% after ozone treatment (Tachikawa et al., 2009). 7.2.4 Peracetic acid Peracetic acid (in concentrations of 5%–15%) is produced by the reaction between hydrogen peroxide and acetic acid, or by the acetaldehyde oxidation. It acts as disinfectant in the purification of water. Catalase and peroxidase enzymes that can degrade H2O2 has no inactivating effect on peracetic acid. This chemical is safe because it was decomposed into safe and

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environmental friendly residues, including acetic acid and hydrogen peroxide. So, it can be applied in food industries without the need for subsequent rinsing and its efficacy is not affected by protein residues. It has been shown that L. monocytogenes biofilm can be reduced by peracetic acid treatment (0.50% w/v, 24 h) (Cabec¸a et al., 2008). Also, peracetic acid treatment (2.0 mL/L, 10 min) can reduce L. monocytogenes biofilms on stainless steel in the presence of fat and protein soil. Peracetic acid reduces viable S. aureus (98%) and P. aeruginosa (98%) after short exposure (1 min) (Srey et al., 2013).

7.3 Biological control methods Due to the extensive effect of biofilms on human health and various industries, more effective, safe, and cost-effective strategies are needed. Following are some of the bio-based technologies, including control strategies using enzymes, bacteriophage, and microbial interactions and metabolite molecules, to fight against bacterial biofilm. 7.3.1 Enzymes Enzymes can be an important bio-based strategy to remove biofilm especially in the food industry. This strategy can be considered as a new and environmental friendly antibiofilm approach based on its nontoxic and biodegradability properties. Enzymes mainly target constituents of the extracellular polymeric matrix of biofilms, including polysaccharide, eDNA, and proteins. By their disruption or degradation enzymes biofilms can be eradicated. EPS is a heterogenic matrix. So, in order to degrade this complex, a combination of enzymes is required (Simo˜es et al., 2010). It has been demonstrated that enzymes can efficiently be applied as enzymatic cleaning products against biofilm of foodborne pathogens, including Lactobacillus bulgaricus, Lactobacillus lactis, and Streptococcus thermophiles (Augustin, Ali-Vehmas, & Atroshi, 2004). Protease

Proteins are involved in cell adhesion and subsequently biofilm formation (Hinsa & O’Toole, 2006). It has been shown that protease enzymes were effective in the degradation of P. fluorescens biofilm’s EPS, while efficiency of amylase enzymes was lower (Molobela et al., 2010). Proteinase K and trypsin are proteases with broad specificity, the treatment of biofilms with these proteases leads to biofilm disassembly like S. aureus biofilm (Gilan & Sivan, 2013; Shukla & Rao, 2013).

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Since various cells are present in the biofilms, a mixture of proteases and amylases is commonly used to eradicate biofilm formation. A significant biofilm inhibitory effect of proteases has been extensively described by Mukherji et al. (2015). It has been reported that Esperase HPF, a protease, prevented the biofilm formation of the species such as Dokdonia donghaensis, Shewanella japonica, Microbacterium phyllosphaerae, and A. lwoffi (Hangler et al., 2009). Artini et al. showed the efficiency of proteinase K, trypsin, chymotrypsin, serratiopeptidase, and carboxypeptidase in the inhibition of Staphylococcus spp. biofilm (Artini et al., 2013). Polysaccharide-hydrolyzing enzymes

Polysaccharide-degrading enzymes via hydrolyzing the glycosidic bond of carbohydrates disassemble biofilms. Among polysaccharide-hydrolyzing enzyme α-amylases are currently used due to their thermostability (Craigen et al., 2011). Inhibitory activity of α-amylases on S. aureus and S. epidermidis biofilms has been investigated. Craigen et al. showed that α-amylases efficiently declined formed S. aureus biofilm (79% during 5-min treatment and 89% during 30-min treatment) and showed no antibiofilm effect of the S. epidermidis biofilm. Amylase treatment showed dosedependent manner. In such way that, it reduced biofilms by 72%, 89%, and 90% at dose of 10, 20, and 100 mg/mL, respectively. In addition, the antibiofilm activities of amylases from different biological sources have been evaluated. The best antibiofilm effect was observed for α-amylase isolated from B. subtilis. Although enzymes derived from human saliva and sweet potato were ineffective on preformed biofilms, all of the investigated enzymes were highly effective in inhibiting biofilm formation (Craigen et al., 2011). It has been found that a mixture of α-amylase, β-glucanase, and protease was effective in eradicating an industrial biofilm formed during paper pulp manufacture. Deoxyribonuclease I (DNase I)

Extracellular-DNA (e-DNA) plays an important role in the primary attachment and aggregation of the EPS onto the surfaces. Therefore, formation and stability of the biofilm is related to e-DNA (Chagnot et al., 2013). So, DNase can impose antibiofilm activity on Gram-positive and Gramnegative bacteria (Tetz et al., 2009). DNase involve in the hydrolysis of phosphodiester bond in the backbone of e-DNA molecules in the formed biofilm. In the presence of DNase I, bacterial susceptibility to azithromycin, rifampin, levofloxacin, ampicillin, amphotericin B, and cefotaxime was

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increased. DNase with metronidazole showed synergistic action in the disintegration of Gardnerella vaginalis biofilm. The fungicidal efficiency of caspofungin and fluconazole reduced in the presence of DNase I, which shows that the synergistic effect between the antibiotics and DNase I relies on the fungicidal agent used. It has been reported that DNase suppressed P. aeruginosa, Helicobacter pylori, Enterococcus faecalis, E. faecium, S. aureus, A. baumannii, H. influenza, K. pneumoniae, E. coli, Streptococcus pyogenes, Streptococcus pneumonia, and Candida albicans biofilms (Bugli et al., 2016; Eckhart et al., 2007; Tetz et al., 2009; Torelli et al., 2017). Lysostaphin

Lysostaphin is considered as promising antibiofilm agent. Lysostaphin is a natural staphylococcal endopeptidase with ability to penetrate bacterial biofilms. Lysostaphin increased efficiency of antibiotics like doxycycline and levofloxacin (Ceotto-Vigoder et al., 2016). Walencka et al. have evaluated the antimicrobial characteristics of lysostaphin and reported the biofilm inhibitory concentration (BIC) of this enzyme for 13 S. aureus and 12 S. epidermidis clinical strains. The BIC of lysostaphin on eight S. aureus strains was 4–32 μg/mL, and the BIC value for the remaining five strains was higher than the maximum tested concentration (>64 μg/mL). The majority of the studied S. epidermidis strains were more resistant to lysostaphin than the S. aureus strains. Lysostaphin (128 μg/mL or 16 μg/mL) reduced biofilm formation of only 2 of the 12 S. epidermidis strains. For the remaining 10 strains, the BIC value was greater than 254 μg/mL. Also, it has been reported that combined application of lysostaphin with oxacillin increased the susceptibility of the biofilm-growing bacteria to the antibiotic (Walencka et al., 2008). Also, lysostaphin (10 mg/kg) effectively prevent bacterial infection on indwelling catheters (Kokai-Kun et al., 2009). Lyase

Alginate lyase is a promising enzyme in the dissolution of biofilm of certain bacteria (Germoni et al., 2016). According to the studies, the marine bacteria, Pseudoalteromonas, serves as a renowned source for the production of alginate lyase that can inhibit biofilm formation of P. aeruginosa, E. coli, and S. enterica (Daboor et al., 2019; Dheilly et al., 2010; Dufourcq et al., 2014). It can due the fact that marine bacteria are exposed to substantial quantities of the alginate present in their surroundings. Complete eradication of the biofilm structure and living bacteria can be achieved by treating biofilm with lyase and gentamycin (96 h) (Zhu et al., 2017).

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Other enzymes

A combination of polysaccharide-hydrolyzing enzymes, proteases, and oxidoreductases were recommended for bacteria biofilm removal due to a wide range of polysaccharide-hydrolyzing enzymes and proteases activities which make them useful for the efficient degradation of biofilm matrix. Also, the bactericidal effect of oxidoreductases has been reported ( Johansen et al., 1997). It has been reported that pectin esterase, pectin lyase, and cellulose were not effective on mature biofilms of P. fluorescens. However, the efficacy of this enzymatic mixture was increased when pronase, detachment promoting-agent, was added in the treatment (Orgaz et al., 2007). Antibiofilm activity of colanic acid-degrading enzymes of a Streptomyces isolate has been reported (Boone et al., 1996). Cellulase derived from Penicillium funiculosum was effective in degrading mature biofilms of P. aeruginosa; and it was also found to be useful in degrading the exopolysaccharides (Loiselle & Anderson, 2003; Vickery et al., 2004). Enzyme efficiency can be enhanced by the application of physical or chemical methods. For example, it was found that as a result of synergistical application (for 10 s) of ultrasonic waves, proteolytic and glycolytic enzymes exhibited antibiofilm activity on E. coli biofilm (61%–96%) on stainless steel (Oulahal-Lagsir et al., 2003; Parkar et al., 2004). The combination of proteolytic enzymes with surfactants increased the wettability of biofilms formed by a thermophilic Bacillus species and, therefore, enhanced the cleaning efficiency (Parkar et al., 2004). Limitation of enzymatic eradication of biofilms

There are yet many limitations with enzymatic eradication of biofilms. It has been shown that efficiency of enzymes in biofilm inhibition or eradication may vary according to the species of bacteria (Lequette et al., 2010). Also, various bacteria possess EPS with different composition which the reason for the enzyme inefficiency. The specific activity of the enzymes increases the difficulty of finding effective enzymes against all the different types of biofilms. Mixtures containing several different enzymes are considered to be pivotal to achieve a successful biofilm control strategy. Basically, a mixture of proteases and polysaccharide hydrolyzing enzymes may be useful (Meyer, 2003). 7.3.2 Phages Bacterial cells within a biofilm can develop resistance to chemicals such as biocides and antibiotics. In this regard, as mentioned earlier, bacteriophages

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seem to be suitable alternatives. Phages attach to receptor sites on host cells in the biofilms. Lytic bacteriophages lysis the cells in the biofilm. So, it is possible that biofilm integrity might be destroyed. Treatment with lytic bacteriophages can be considered a bio-based, extremely specific, nontoxic, and accessible approach to inhibit several biofilm-forming microorganisms (Kudva et al., 1999). Bacteriophage therapy was applied to treat bacterial infections especially the multidrug-resistant bacteria in the early 20th century. It has been reported that bacteriophage therapy declined biofilm formation (Curtin & Donlan, 2006; Merril et al., 2003). Also, using genetic engineering bacteriophages can be designed to produce biofilm-degrading enzymes. It has been reported that the engineered phages can kill bacterial cells in biofilms (approximately 99.997% removal), as well as degrade the biofilm matrix. Eventually, biofilm was collapsed (Lu & Collins, 2007). An E. coli phage, T7, was engineered to express dispersin B, an enzyme that break down β-1,6-N-acetylglucosamine, as an important component of biofilm (Lu & Collins, 2007). The efficiency of bacteriophage K on S. epidermidis planktonic cells and biofilms was studied. It has been shown that efficiency of phage K lysis relies on the bacteria growth phase (Cerca et al., 2007). Effectiveness of bacteriophages in eliminating P. fluorescens cells has been investigated. According to the results bacteriophages can inhibit (80%) 5-day-old biofilm (in the early stage of development) under optimal conditions (Sillankorva et al., 2004). Sillankorva et al. (2008) showed that the phage philBB-PF7A could be a significant antibiofilm agent because of its lysing ability. This phage can be used in controlling the dual species biofilm of P. fluorescens and Staphylococcus lentus. Inactivation of L. monocytogenes, E. coli, and P. aeruginosa biofilm can be achieved through treatment with L. monocytogenes phage ATCC 23074B1, bacteriophage T4, and E27 respectively (Doolittle et al., 1995, 1996; Hibma et al., 1997). It has been suggested that successful phage treatment can be severely affected via phage concentration and the environmental factors, including temperature, growth stage, and media (Chaignon et al., 2007). Also, the technology of bacteriophage therapy has not yet been successfully developed and there is little information on the action of bacteriophages on biofilms (Sutherland et al., 2004). 7.3.3 Inhibitors of QS system QS inhibition

Bacteria use QS to communicate with each other and coordinate their gene expression based on their density, which acts as a decision-making process to

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regulate the motility, sporulation, antibiotic resistance, production of bioluminescence, virulent factors, pathogenicity, and biofilm synthesis factors. Therefore, many researches tried to find compounds that can block QS system in a phenomenon that is known as downregulating, silencing the QS system, or quorum quenching. The inhibition of QS makes bacterial pathogens more sensitive to the host immune system and antibiotics (Grandclement et al., 2016). The most studied QS systems are AHL-based signaling system in Gram-negative bacteria, luxS-encoded autoinducer 2 (AI-2)-based QS system in both Gram-negative and Gram-positive bacteria (Amara, Krom, Kaufmann, & Meijler, 2011), and oligopeptide-based QS system in Gram-positive bacteria. QS inhibition (QSI) is effective in controlling biofilm formation (Christiaen, Matthijs, et al., 2014; Christiaen, O’Connell Motherway, et al., 2014; Keshavan et al., 2006). QS can be interrupted through different strategies: (1) inhibit or reduce the activity of the genes that control the synthesis of signal molecule of QS, (2) degrade the signal molecule, (3) modulate the binding of the signal to receptor sites, and (4) block receptor sites with antagonistic signal analogs (Lade et al., 2014; Rasmussen & Givskov, 2006). The most studied QSI strategy is enzymatic degradation of QS signal molecules (e.g., AHL) (Kalia & Purohit, 2011). Many natural and chemically synthesized inhibitors have been reported to disrupt QS mechanisms. Bacterial, fungal, animal, plant, marine organism, and antibody based QS inhibitors have been used as natural inhibitors (K. Tang & Zhang, 2014). Many naturally occurring substances such as iberin from horseradish, ajoene from garlic and vanillin have shown to possess QSI abilities against P. aeruginosa ( Jacobsen et al., 2012). Plant-derived compounds such as alkaloids, coumarins, phenolics, quinones, saponins, tannins, and terpenoids have been tested for QSI (Paczkowski et al., 2017). However, their production in very small quantities, high cost, and toxicity limit their application. Therefore, researchers are trying to design chemically synthesized inhibitors (Kalia, 2013). In this regard, it has been found that a large number of synthetic compounds can regulate QS ( Janssens et al., 2008). Utilization of synthetic inhibitors can be more practical and a more costeffective way for QSI. However, in industrial systems, microbial species are diverse and the inhibition of their signal molecules is difficult ( Jia et al., 2019). Preventing the biosynthesis of the AHL signal molecules

QS suppressors that target biosynthesis of AHL molecules can be developed using knowledge of signal generation. The S-adenosyl methionine, an

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adequately charged acyl carrier protein (ACP), and S-adenosyl-Lmethionine are used to synthesize homoserine lactone ring moiety, as the precursor of the acyl side chain of the AHL signal or the primary methyl donor for several methylation processes (Hentzer & Givskov, 2003; Y. Li, Jia, et al., 2016; W. Li, Yao, et al., 2016). Therefore, hindrance of these molecules using their analogs or antagonists can inhibit the biofilm formation in various Gram-negative bacterial pathogens. It has been shown that macrolide antibiotics like azithromycin and erythromycin at their subinhibitory concentrations suppressed AHL molecule biosynthesis by P. aeruginosa and consequently blocked its virulence factors and biofilm formation (Sofer et al., 1999; PechE`re, 2001). Further, Christensen et al. (2013) performed high-throughput screening to find the QS inhibitors against biosynthesis of AHL molecule in Proteobacteria such as Burkholderia mallei and Yersinia pestis (Christensen et al., 2013; Tateda et al., 2001). Biodegradation or alteration of AHL signal molecule

Enzymes with ability to degrade the AHL signal molecules are promising candidate to eradicate the biofilms and reduce their related complications. These enzymes prevent AHL accumulation. Six major classes of enzymes can degrade or modify AHL signal molecules. The AHL lactonases (like AiiA 24B1 from Bacillus spp. 24B1) and decarboxylases via opening the HSL ring degrade it. Moreover, AHL acylase hydrolyzes the AHL signal molecules (Christiaen, Matthijs, et al., 2014; Christiaen, O’Connell Motherway, et al., 2014; Paul et al., 2009; Shastry et al., 2019; Utari et al., 2017). Deaminase enzymes also can break down the acyl side chain of the HSL ring (Kose-Mutlu et al., 2019). Some enzymes alter the AHL molecule by modifying the binding efficacy of receptor proteins with signal molecules. For example, the AHL oxidase catalyzes the carbon atoms in acyl chains of AHL signal molecules (Gao et al., 2013). The AHL oxidoreductases oxidize or reduce the carboxyl group of the third carbon to attack the side chain of AHL molecules and exhibited inhibitory potential on biofilm formation of Gram-negative (Weiland-Br€auer et al., 2016). Interfering via analog compounds

By using the antagonistic molecules, the receptors become unavailable for attachment of AHL signaling molecules. In this condition, no signal is recognized. So, biofilm formation, pathogenicity, or antibacterial resistance can be reduced. In this regard, furanone is an analog compound which is known

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as the potent QS inhibitor. Furanone successfully inhibited the biofilm formation by S. epidermidis (Hume et al., 2004). It has been reported that antagonistic molecule produced by Halobacillus salinus has silenced the QS system of Vibrio harveyi by inhibiting the activation of lux gene (Teasdale et al., 2009). Also, honaucins of Leptolyngbya crosbyana and Penicillium derived compounds like patulin and penicillic acid inhibit QS and subsequently hinder cell-to-cell communication. The penicillic acid and patulin as well as phytol inhibited the QS in P. aeruginosa and Serratia marcescens, respectively (Abraham, 2005; Alexpandi et al., 2019; Rasmussen et al., 2005; Srinivasan et al., 2016; Wagner et al., 2004). Arunachalam et al. (2018) have revealed the biofilm inhibitory potential of phytol and geraniol on S. marcescens associated acute pyelonephritis infection and S. epidermidis associated endocarditis infection in animal models, respectively. Some antimicrobial peptides suppress the QS system by affecting the transport of signal molecules within or outside the cell, thereby affecting the signal transduction cascade and biofilm formation (S. Sharma et al., 2019; Zuberi et al., 2017). Suppressing alarmone scheme

The stringent response has been exhibited by bacteria under stress conditions such as nutrient depletion. In this status it is known as alarmone scheme, specific molecules, including guanidine diphosphate triphosphate (pppGpp) and guanidine 30 ,50 -bis-diphosphate (ppGpp) are expressed through the cassette of RelA and SpoT (Metzger et al., 1989). Bacteria RelA and SpoT regulate the stringent response through the production and hydrolysis of (p) ppGpp as the signal molecule (Wendrich & Marahiel, 1997). Stringent response regulates the biofilm formation in various bacteria like E. coli ˚ berg et al., 2006; Lemos et al., 2004). So, compounds with and S. mutans (A ability to degrade alarmone [(p)ppGpp] or inhibit its synthesis can act as biofilm inhibitor (De la Fuente-Nu´n˜ez et al., 2014). Compounds with inhibitory effect on RelA also can reduce biofilm formation because it prompts the stringent response (Wexselblatt et al., 2012). LL-37 peptide, proteaseresistant D-enantiomeric peptides DJK-5, and DJK-6 impose their antibiofilm activity through hindering the alarmone system (De La Fuente-Nu´n˜ez et al., 2012, 2014; Overhage et al., 2008). 7.3.4 Antibiofilm mechanism of nanoparticles Nanoparticles such as silver, gold, titanium, and zinc nanoparticles via damaging DNA or creating oxidative stress can exert their inhibitory effects on

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biofilms. Some of their characteristics such as size, shape, charge, and composition affect their antibiofilm activities. For example, nanoparticles with smaller size probably due to superior penetration ability into the bacterial cell membrane have more antibacterial activity. Truncated triangular silver nanoplates with a basal lattice plane displayed more biocidal action compared to spherical- and rod-shaped silver nanoparticles (Pal et al., 2007). The NPs with more zeta potential bind more effectively to the negatively charged bacterial cell walls compared to ones with less zeta potential. This attachment consequently causes cell membrane disruption (El Badawy et al., 2011). Antimicrobial and antibiofilm activities of titanium oxide (TiO), copper oxide (CuO), zinc oxide (ZnO), iron oxide (Fe2O3), magnesium oxide, aluminum oxide (Al2O3), and cerium oxide (CeO) have been reported. Nanoparticles seems as a suitable alternative to antibiotics because they do not result in resistance development (Banerjee et al., 2020). Silver nanoparticles

It has been demonstrated that silver nanoparticles have antimicrobial properties owning to their effective binding with DNA, cell membrane receptor, also enzymes that control respiration and many other crucial cell functionalities. Moreover, they have shown remarkable antibiofilm effects because they disrupt biofilm matrices by damaging the intermolecular forces. Silver nanoparticles showed suppressing effect on biofilm formation of E. coli, P. aeruginosa, S. aureus, S. epidermidis, C. albicans, and C. glabrata strains (Ubhayawardana et al., 2015; Vickers, 2017). Zinc oxide nanoparticles

Zinc nanoparticles exhibit antibacterial activities with lower toxicity to mammalian cells. Their antibiofilm activities against E. faecalis, S. aureus, S. epidermidis, B. subtilis, and E. coli have been reported ( J.-H. Lee et al., 2014). Dhillon et al. reported inhibitory activity of chitosan-coated Zn-NPs on M. luteus and S. aureus biofilm formation. It is likely that ZnO NPs by generating ROS exert their antibiofilm activity (Dhillon et al., 2014). Titanium oxide nanoparticles

Titanium oxide nanoparticles are also nontoxic antibacterial and antifungal NPs because of its inert behavior. It photocatalytically inhibits biofilm formation. TiO NPs through producing ROS, peroxidation of lipid, and oxidation of internal enzymes impairs the respiratory activity and cause cell

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death (Ohko et al., 2009). Khan et al. evaluated efficiency of TiO NPs in the eradication of the biofilm infections. It has been shown that TiO NPs via reducing extracellular polysaccharide formation and generating ROS disrupt cell membrane and exert their antibiofilm activity (Khan et al., 2016). Copper oxide nanoparticles

Copper oxide nanoparticles have broad spectrum antimicrobial activity. However, their antimicrobial effect is less than silver or zinc nanoparticles. These nanoparticles showed effective activity at higher concentrations that may be cytotoxic to mammalian cells. Combination of ZnO NPs and CuO NPs exhibited considerable biofilm suppression in tooth model. Lewis Oscar et al. reported antibiofilm activity of CuO NPs against P. aeruginosa. According to confocal micrographs CuO NPs destroy the biofilm at the initial stages of development (Comeau et al., 2011). CuO NPs also exhibited the effective eradication of MRSA, S. mutans, S. aureus, and E. coli biofilms. In fact, CuO NPs by generating ROS exert their antimicrobial effect (Eshed et al., 2014; S.-M. Kang et al., 2014; A. Singh et al., 2015). Gold nanoparticles

Gold nanoparticles are nontoxic to cells and possess negligible or no antibacterial activity alone. So, they should bind with antibiotics or biologically active compounds to show efficient bactericidal and biofilm inhibitory activities against a wide variety of pathogens. Treatment of Candida spp. biofilm with peptide indolicidin-coated Au-NPs (48 h) caused its disruption. Also, antibiofilm activity of AHL lactonase-coated Au-NPs on Proteus spp. was also reported (de Alteriis et al., 2018; Vinoj et al., 2015; Q. Yu et al., 2016). Organic nanoparticles

Polymer-based nanoparticles are considered as effective antibiofilm nanobased material. For example, the polycationic groups of these polymers via ion-exchange interactions bind to the bacterial membranes. Subsequently, this binding results in the disruption of bacterial membranes and consequently cell damage and death (Lichter & Rubner, 2009). Polystyrene NPs exert their action by binding to the polysaccharides present in EPS through interacting with its sulfate groups. This binding lead to disruption of bacterial biofilm. Also, biodegradable hydrogels like cationic functionalized polycarbonates showed antibacterial and antifungal activities. Moreover, cell viability of S. aureus, E. coli, and C. albicans biofilms are

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diminished by these hydrogels. According to study of Lee et al. both planktonic and sessile formed in the biofilm can be removed by these polycarbonate hydrogels (A. L. Lee et al., 2013). Liposomes

Liposomes also can be applied to suppress biofilm formation. They are bilayered structures and are used to carry therapeutic agents. They are biocompatible, possess significant capacity to carry both hydrophobic and hydrophilic drugs and protect them from unfavorable environmental conditions, thus enhance the efficiency of drug delivery. Liposomes can be used against the microbial colonization by delivering compounds with antibiofilm activities. A strategy has been developed by DiTizio et al. to deliver ciprofloxacin by a liposomal hydrogel. The results show that bacterial adhesion reduced considerably in a rat model infected with persistent P. aeruginosa biofilm infection (Banerjee et al., 2020; DiTizio et al., 1998). Dendrimers

Both hydrophilic and hydrophobic antibiofilm compounds can be encapsulated into voids of highly branched structures called dendrimers (Ramalingam et al., 2013). Moreover, low molecular weight dendrimers with no loaded antibiotics onto them exhibited antibacterial properties. Antimicrobial activity of dendrimers has been observed on E. coli and S. aureus ( Janiszewska et al., 2003). Fucose-peptide dendrimer through disrupting attachment prevented P. aeruginosa biofilms ( Johansson et al., 2008). 7.3.5 Disruption of the cell membrane by antimicrobial peptides (AMPs) AMPs are introduced as promising biofilm eradication agents and seems to be amazing alternatives to antibiotics (Baltzer & Brown, 2011). All organisms, including plant, animal, and microbial species are able to produce these ubiquitous compounds. AMPs through various mode actions, including carpet, barrelstave, or toroidal-pore, can disrupt cell membrane integrity, which results in cellular leakage and cause cell death. Moreover, they can inhibit protein folding or enzyme activities (Bechinger & Gorr, 2017). Multiple factors including cationic charge, amphipathicity, amino acid composition, and size influence AMP attachment, translocation, and membrane permeability. AMPs with various sizes (5 to over 90 amino acids) and molecular mass (between 1 and 5 kDa) have been discovered. Charge moiety and hydrophobic interactions are among influential factors in the interaction of peptide with the cell membrane. RT2, KT2, and

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magainin II are membrane-targeting peptides which through their hydrophobic part interact with the anionic moiety in the lipid head of E. coli cell membrane (Anunthawan et al., 2015). Most of the AMPs discovered are cationic peptides (K. L. Brown & Hancock, 2006); however, anionic forms have also been reported (Harris et al., 2009). It has been reported that cationic peptides interact with lipopolysaccharide’s layer or specific divalent cationic binding site at the lipopolysaccharides on the outer membrane of Gram-negative bacteria (Zhang et al., 2000). Inhibition of biofilm formation or its dispersion is more frequent than biofilm eradication activity (K. Flemming et al., 2009). Therefore, combination of AMPs and antimicrobial agents like antibiotics are usually applied to eradicate biofilms (Eckert et al., 2006). For example, synergism of G10KHc and tobramycin was observed against P. aeruginosa biofilms (Eckert et al., 2006). Inhibitory effect of LL-37, that is, a human cathelicidin-derived AMP with amphipathic property has been studied extensively (Overhage et al., 2008). It exists in most bodily fluids (Burton & Steel, 2009). It has been reported that LL-37 (0.11 μM) reduced attachment of P. aeruginosa to plastic surfaces. Moreover, it (0.9 μM) decreased the thickness of established biofilms (40%) (Overhage et al., 2008). Also, Chennupati et al. revealed efficiency of LL-37 (556 μM) in the eradication of P. aeruginosa biofilms in an in vivo animal model (Chennupati et al., 2009). Also, LL-37 inhibited S. epidermidis cell attachment and prevented the establishment of mature biofilm at concentrations 0.22 μM and 0.22–7.12 μM, respectively (Hell et al., 2010). Biofilm eradication activity of LL-37 on S. aureus biofilm has been reported by Kang (X. Kang et al., 2019). Oritavancin is another AMP with antibiofilm activity. It is a semisynthetic lipoglycopeptide. It has been developed to treat infections related to Gram-positive bacteria like methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and vancomycin-resistant S. aureus (VRSA) (Allen, 2010). It (0.3 and 4.5 μM) showed eradication activity (99.9%) on MSSA, MRSA, and VRSA biofilms (Belley et al., 2009). The advantage of oritavancin is that it has lower toxicity toward humans than other lipoglycopeptides like vancomycin and telavancin (Darpo et al., 2010). Because of the promising bioactivities of AMP, it is necessary to modify the structure, shape, and charge of AMPs to achieve better electrostatic interaction in order to interrupt cell membrane and biofilm. The biofilm is disturbed by different AMPs through creating the transmembrane pores, which results in cell death. The study of Pulido et al.

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(2016) reveals that the permeabilization effect of RN3 (5-17P22-36) on biofilm cellular population was visualized by confocal laser scanning microscopy (CLSM) and Sytox green permeabilization assay. 7.3.6 Antimicrobial lipids (AML) as biofilm inhibitors AML, which include fatty acids and monoglycerides, are known as singlechain lipid amphiphiles (Verderosa et al., 2019). Koch et al. reported antimicrobial activity of AML on causative agent of anthrax, B. anthracis (Thormar, 2011). AML act through various strategies, including increasing membrane permeability, creating temporary or permanent membrane pores, targeting the bacterial surface signal transduction system, destroying electron transport chain, causing cell lysis, and inhibiting enzymatic activity (Schlievert & Peterson, 2012; Verderosa et al., 2019). It has been stated that AML can inhibit various bacterial biofilm formations at low doses. Glycerol monolaurate (GML) showed inhibitory activity on biofilm formation of L. monocytogenes, H. influenza, P. aeruginosa, S. mutans and S. aureus (Lopes et al., 2019; Vetter & Schlievert, 2005; Witcher et al., 1996). Also, unsaturated fatty acids such as oleic acid, linoleic acid, and palmitoleic acid exhibited suppress activity on S. epidermidis, S. aureus, and S. mutans (Yuyama et al., 2020). Sun et al. revealed that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) decreased biofilm formation of F. nucleatum, Porphyromonas gingivalis, and S. mutans (M. Sun et al., 2016).

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CHAPTER 7

Introduction of smart coatings in various directions Kushal Yadav and Aditya Kumar Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India

1. Introduction Smart coatings also sometimes called intelligent or auto-responsive coatings have gained major attention in the past 25 years. These coatings dynamically adapt to the external stimulus by giving an appropriate response. The conventional coating remains passive against these external disturbances and is unresponsive in nature (Ulaeto et al., 2017). These external changes include pH, acoustics, surface tension, light, electric field, magnetic field, pressure, temperature, and mechanical forces. Controlling the coating composition at the molecular level and tuning the surface morphology are the current trends in the development of such materials. The multifunctional characteristics of these smart coatings find their applications in medical devices, electronics, textiles, aviation, and marine engineering. In addition to this, property enhancement, controlling coating formulation, increasing durability, and longevity remains the major area of scientific research (Montemor, 2014). For designing these coating surfaces two actions must happen selectively. Initiation of stimulus and signal must be received by the material. These external stimuli must induce some chemical or physical change. These signals can be momentary or continuous in nature. These signals must be strong enough to switch the property of the substrate from one state to another state. In addition, the signals must be clear and unambiguous for the response to be predictable and functional. The stimulus can be further classified into physical stimuli and chemical stimuli. Physical signals include electromagnetic waves, pressure, acoustics, temperature, and light. The chemical signal constitutes acid–base, redox, electrochemical, and photochemical. In this chapter, we basically report different types of smart coatings, their external stimuli, mechanism of response, and their applications.

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2. Corrosion-resistant coatings Corrosion has been a source of major concern for the metals such as iron, aluminum, copper, steel, etc. Metallic objects lying in nature are prone to develop multiple pits and cracks on the surface because of prolonged exposure to water and atmospheric moisture. This causes the metal to become corroded with time losing its mechanical strength and making it useless for future purposes (Harsimran et al., 2021). So corrosion resistance has become a source of great interest in order to prolong the life of metal objects. At the time of corrosion, an irreversible surface reaction takes place between the metal which is mainly anode (oxidation) and the corrosive media. This reaction basically involves the transfer of an electron from the metal surface through the solution to a less active site called a cathode (Sebastian et al., 2018). It can be inferred that in order to prevent the corrosion to take place, either the reaction at the anode or the reaction at cathode needs to be partially or completely ceased off. Some techniques such as painting the surface and coating the surface with other metals are reported (Gonc¸alves & Margarido, 2015). However, coating the surface with superhydrophobic coating can be thought of as another alternative method for preventing corrosion. Introducing the coating on the surface prevents the contact between the metal surface and corrosive media, which can avoid corrosion from taking place. On the basis of the surface-water interaction, the surfaces can be broadly classified into two categories, i.e., hydrophilic and hydrophobic (Drelich & Marmur, 2014). Hydrophilic surfaces have great affinity toward the water and are called water loving with a contact angle (CA) of less than 90°, whereas hydrophobic surfaces are known to be water repelling which have CA > 90°. Superhydrophobic surfaces have a high contact angle greater than 150° and a sliding angle of less than 10° (Law, 2014; Paras & Kumar, 2021). Fig. 1 illustrates the contact angle on

Fig. 1 Illustration of water contact angle.

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different types of surfaces. The geometric nanostructures and low surface energy material present on the surface basically influence the wettability of the surface. These ordered structures trap a layer of air between the solid substrate and the water droplet which result in the isolation and prevention of direct contact between the substrate and corrosive species. In the recent work conducted by Gao and Guo on steel, they modified the surface with 1H,1H,2H,2H-perfluoroalkyltriethoxysilane and tested the anticorrosion property of the surface. The superhydrophobic sample exhibited enhanced anticorrosive property with a more positive value of corrosion potential and decrement in the value of corrosion current (Gao & Guo, 2018). Wang et al. fabricated the surface of steel with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17) after the treatment with HCl and HNO3 and further treating it with H2O2. The potentiodynamic polarization test was done to investigate the corrosion barrier of the surface and the superhydrophobic surface exhibited excellent resistance to corrosion (Wang et al., 2017). Glover et al. prepared a primer coating using graphene nanoplatelets and polyvinylbutyral on the iron surface. The surface exhibited excellent mechanical durability and showed great resistance to corrosion-based material failures (Glover et al., 2017). Tudu et al. fabricated the aluminum surface using a facile chemical etching technique and further coating the surface with low surface energy hexadecyltrimethoxysilane. The obtained aluminum surface showed excellent resistance to corrosion (Tudu et al., 2019). Meena et al. modified the steel surface using SiO2 nanoparticles and polyurethane polymeric matrix. The coating showed excellent mechanical durability and corrosion-restricting nature (Meena et al., 2020). Zhang et al. spray-coated the copper surface with fluorinated hydrophobic SiO2 nanoparticles and PDMS solution. The surface showed a water contact angle of more than 150°. The potentiodynamic polarization test showed that the superhydrophobic film was effective in resisting the corrosion (Zhang et al., 2016). Liu et al. developed a superhydrophobic aluminum surface using the facile anodic oxidation method and low surface energy Heptadecafluoro-1,1,2,2-tetradecyltriethoxysilane. The Tafel plots obtained showed a positive shift in corrosion potential and a significant decrease in corrosion current. This shows that the superhydrophobic surface was helpful in restricting corrosion (Liu et al., 2014). Endowing the bare aluminum surface with superhydrophobicity immensely improves the corrosion resistance making it suitable for various industrial applications where corrosion becomes a major source of concern.

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3. Self-cleaning coatings Inspired by nature, scientists have developed a lot of self-cleaning surfaces. It is observed that the water droplet rolls off the lotus leaf without wetting the surface and trapping all the dust accumulated on the surface. In the past few decades, superhydrophobic self-cleaning surfaces have been developed which have surface morphology similar to a lotus leaf, rose petals, reed, and Salvinia molesta leaves (Barthlott et al., 2017; Neinhuis & Barthlott, 1997). These surfaces are known to have micro-nano scale roughness and low surface energy. Lotus leaf has a layer of epicuticular wax which has low surface energy. The water droplets reside on the top of nanostructures as the space between the valleys is filled with air pockets. Therefore, these leaves exhibit considerable superhydrophobicity and extremely low contact angle hysteresis (Zorba et al., 2008). The lotus leaf shows a water contact angle of 164° and contact angle hysteresis of 3°. A lot of self-cleaning surfaces are designed nowadays which have multifarious applications in solar panels, self-cleaning windows, windshields, and exterior paints in homes and navigation ships. Various superhydrophobic coatings have been developed which possess excellent properties of self-cleaning (Geyer et al., 2020). These coatings also exhibit the properties of antiicing, antifogging (Gurumukhi et al., 2020), and drag-reduction (Tuo et al., 2019). In order to remove dust from the surface manual cleaning is mostly done using soft brushes and certain cleaning agents. In the long-term cleaning with these methods will cause certain wear and abrasion of surface (Moharram et al., 2013). In order to overcome the problem caused by wear and tear, passive cleaning methods can be used as an alternative. In the recent work, it was noted that dust accumulation was considerably reduced by fabricating the surface with superhydrophobic coating. Fig. 2 clearly shows the dust being trapped by the water droplet while it rolls off the surface. Pan et al. checked

Fig. 2 Schematic representation of self-cleaning superhydrophobic surface.

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the dust deposition density on various glass surfaces (Pan et al., 2019). Liang et al. explored the efficiency of the superhydrophobic photovoltaic power generation system and found 14.21% higher conversion efficiency than the uncoated surface. Therefore, the accumulation of dust also causes a lower conversion rate in the case of photovoltaic cell (Liang et al., 2020). Fu et al. achieved superhydrophobicity on glass by etching with HF and FAS-17 modification. WCA angle of 170.3° and sliding angle of 2° were obtained with excellent self-cleaning properties (Fu et al., 2020). Varshney et al. studied the self-cleaning behavior of superhydrophobic aluminum fabricated by facile wet chemical etching method and simple immersion in ethanolic lauric acid solution. The surface exhibited a water contact angle exceeding 150° and a sliding angle of about 5° (Varshney et al., 2016). Xu et al. obtained a superhydrophobic copper mesh by simple immersion method which had a dendritic rough morphology. The contact angle measured for the obtained copper mesh was about 155.5°. Furthermore, the self-cleaning, mechanical durability, and anticorrosion test were performed which showed promising results (Xu et al., 2019).

3.1 Photocatalysis-assisted self-cleaning surface The quality of air is deteriorating year by year. Air quality becomes a source of major concern, as it directly impacts human health and climate change and also contributes to the denigration of buildings and monuments around us. The various air pollutants include polycyclic aromatic hydrocarbons, nitrogen oxides (NOx), heavy metals such as Ni, Se, Cr, Cu, Cd, Pb, and volatile organic compounds (VOCs). These pollutants can deposit on buildings, glazing, and other architectural objects and lead to some irreversible losses such as chemical degradation of concrete, paints, glazing, and other building materials (Rabajczyk et al., 2021). Coatings containing photocatalytic additives help in the decomposition of these organic impurities (Hashimoto et al., 2005). A number of materials have been investigated which helps in imparting photocatalytic behavior include WO3, ZrO2, SrTiO2, and CeO2. Titanium oxide is one of the most promising compounds among all the abovementioned chemicals because of its high photocatalytic activity, nontoxicity, chemical stability, and low cost (Hamidi & Aslani, 2019). TiO2 exists in various crystalline forms such as anatase and rutile but only anatase is known to have higher photocatalytic activity in comparison to rutile ( Jing et al., 2008). These photocatalytic layers are impregnated on the various materials such as ceramic surfaces, construction materials, sound

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absorbing screens, bathroom tiles, etc. These photocatalytic compounds help in the decomposition of noxious impurities into less harmful products such as water and CO2. Materials such as Ag3PO4, WO3, Fe(III)-FexTi1-xO2 are used in modifying building materials which can reduce gaseous organic pollutants when visible light is irradiated on them (Guo et al., 2013; Liu et al., 2013).

4. Antiicing surfaces The formation of ice on many engineering surfaces can lead to catastrophic and unusual consequences (Dou et al., 2014). Ice formation on aircraft surfaces can alter the dynamics parameters. The accretion of ice on the surface of aircraft results in the elevated drag force experienced by the aircraft which can cause tragic loss of lift force. It is known to hamper about 50% performance of aircrafts (Zhou, Sun, & Liu, 2021). Ice accumulation on heat transfer equipments such as heat exchangers can decrease the equipment efficiency by up to 70%. So various methods have been developed to combat the threats and problems caused by ice adhesion on these surfaces. Methods such as coating the surface with liquid lubricants can reduce the ice adhesion considerably (Wong et al., 2011). Chen et al. designed a self-lubricated coating of dopamine and hyaluronic acid to control ice formation. Different surfaces of metal and polymers were incorporated with such lubricated coatings and the ice adhesion is known to reduce by an order of magnitude of one (Chen et al., 2014). Wong et al. synthesized a slippery liquid infused porous surface which exhibited excellent ice repellency. The high mobility of liquid lubricant makes it difficult for the ice to get adhered to the surface (Wong et al., 2011). Superhydrophobic surfaces which are basically water repellent surfaces with a contact angle of larger than 150° have gained considerable attention in the past two decades. Accretion of water is significantly reduced on such surfaces which eventually delays the formation of ice. Li et al. developed a nanosilica-based polydimethylsiloxane superhydrophobic coating which showed a great reduction in the ice deposition on the surface (Li et al., 2012). Boinovich et al. fabricated the surface of stainless steel using a facile chemical etching technique followed by the deposition of SiO2 nanoparticles. The obtained superhydrophobic surface exhibited excellent durability even after 100 icing and deicing cycles (Boinovich et al., 2013). Photothermal and electrothermal functional coatings have been developed which are known to impart unique antiicing properties to the

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substrate. These coatings when irradiated with light or power melt the ice completely and the water is further detached due to the superhydrophobicity exhibited by the surface. Wu et al. fabricated a superhydrophobic surface with the candle soot and further tested the antiicing characteristic by illuminating the surface with sunlight. Heating is induced on the surface due to the irradiation of light which raises the surface temperature above 0°C and finally helps in the melting of ice (Wu et al., 2020). Another type of functional Electromechanical piezoelectric deicing system vibrates in the presence of an electric field, this generates the stresses which are helpful in removing or preventing of adhesion of ice on the substrate (Shamshiri et al., 2021). Commonly used piezoelectric materials are lithium tetraborate (LiB4O7), quartz, and barium titanate (BaTiO3). Pommier-Budinger et al. investigated the working of piezoelectric materials with icephobic coatings on the aluminum surface. The ice adhesion strength was measured using the lap shear test (Pommier-Budinger et al., 2016).

5. Chromic-based smart coatings for textiles Smart chromic textiles are known for their innovative and self-functioning abilities which make them suitable for various technological applications (Ramlow et al., 2021). Smart textiles are the ones which can sense and respond to stimuli from the environment. These smart materials have the characteristics of responding to the physical stimulus such as temperature, electricity, pH, light, chemicals, and magnetic field (Khattab, 2018). Smart textiles find their application in power generation, fashion products, energy storage, and medical products. Chromic textiles are also known as chameleon textiles which can change their color under the influence of external stimuli. These smart textiles adhere to the phenomenon of chromism which basically means reversible color change due to a change in electron density of a substance. As visual signals are generated from these surfaces, it becomes easy to sense the change occurring in the environment (Stoppa & Chiolerio, 2014). On the basis of external stimulus, chromism can be classified as photochromism (sunlight as external stimuli), thermochromism (changes in temperature), electrochromism (induced by a change in electricity), and halochromism (changes in pH). The different forms of chromism along with the induced stimuli are shown in Fig. 3. The phenomenon of chromism includes processes such as heterolytic cleavage, isomerization, and redox reaction involving the transfer of electron and pericyclic reactions. Photochromic textiles change their color reversibly when triggered by

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Fig. 3 Schematic representation of types of Chromism.

electromagnetic radiation (Schneegass & Amft, 2017). These textiles are prepared by simply introducing photochromic molecules or microcapsules of photochromic dye into the fibers of textiles. The well-known classes of photochromic dyes include naphthopyrans, naftopyrans, spirooxazines, azobenzene, and fulgids. Khattab et al. using screen printing technique introduced strontium aluminate on the cotton surface. The surface showed excitation peaks at 272, 325, and 365 nm. The emission peaks were obtained at 418, 495, and 520 nm which were explained by fluorescent optical microscopy by energy dispersion (Khattab et al., 2018). The authors pointed out that fabric stained with strontium aluminate showed better color resistance in comparison to other organic pigments. Although photochromic textiles can find their way into multifarious applications, the degradation of photochromic compounds still remains a major drawback which hinders its large-scale industrial production (Cherenack & van Pieterson, 2012). Thermochromic textile responds to the variation in external temperature by the alteration in the supramolecular structure. This type of smart textile is made using three different components such as leuco dye, solvent and a

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developer. Tozum et al. developed a thermochromic system using the emulsion polymerization method. The crystal violet lactone leuco dye along with bisphenol as developer and 1-tetradecanol (solvent) were incorporated into this thermochromic system (T€ oz€ um et al., 2018).

6. Corrosion sensing coatings The corrosion sensing smart coatings are usually referred to as pH-responsive coatings which are used in monitoring the onset of corrosion in the given substrate (Ulaeto et al., 2017). The presence of color-changing dyes in the matrix of the film fluorescence exhibits a change in color when the oxidation takes place at high pH. There are color dyes which are formulated with anticorrosive species that are released when the system senses the presence of corrosive ions in its close vicinity (Maia et al., 2013). Schiff bases, bromothymol, 7-diethylamino-4-methylcoumarin, fluorescein are among some of the color changing compounds which are incorporated in film matrix. Li et al. developed a fluorescent coating on the aluminum substrate with phenylfluorene as the color-changing compound (Li et al., 2006). Calle et al. developed a corrosion sensing material which detects the onset of corrosion and helps in reducing the further deterioration of material (Li, 2007). These functional coatings reduce the timely repairing and maintenance cost thus elevating the lifetime of material.

7. Antigraffiti coatings Graffiti basically refers to drawings scribbled or sprayed on public walls, architectural buildings, and monuments. About 3,500,000 heritage buildings in Europe are affected by graffiti. In order to preserve them from getting denigrated a large amount of capital is spent yearly by the government (Sanmartı´n et al., 2014). Among the commonly used graffiti components includes paints, waxy substances, chalk, crayons, spray paints, and posters (Lettieri & Masieri, 2014). Fig. 4 shows the spray-painted graffiti on a public wall. Most of the graffiti paints are xenobiotic which are not produced naturally. Some of the components in graffiti paints are biodegradable xenobiotic substances which are decomposed by the actions of several organisms present in our environment. In addition, recalcitrant xenobiotic compounds are also known which are nonbiodegradable in nature. Halocarbons used in spray paints come under the category of the abovementioned xenobiotic compound. Pigments such as Sb2O3, PbSO4, and TiO2 are among the

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Fig. 4 Spray painted Graffiti on the public wall.

commonly used pigments in graffiti (Casati & Jansen Van Vuuren, 1999). Titanium oxide (TiO2) is one of the most important pigments present in paints. A large number of methods have been proposed to clean the surface impacted by painting or scribbling illicitly. Chemical methods such as using a suitable solvent or paint strippers are used for the removal of graffiti (Goidanich et al., 2010). But antigraffiti coatings such as fluorinating the surface with fluoropolymers impart it with low surface energy. This reduces the adherence of graffiti and other staining agents on the surface as the newly obtained surface is hydrophobic in nature. Siloxane and silanes are commonly added to the commercially available antigraffti paints (Bayer, 2017).

8. Antifouling coatings Antifouling coatings are of great importance in the field of marine engineering because the biofouling which occurs on the hull of ships gradually increases the consumption of fuel. Marine biofouling is an area of intensive research as a large amount of money is spent to combat this issue (Poornima Vijayan et al., 2022). Fouling can be categorized into two parts namely microfouling and macrofouling. In microfouling, the formation of biofilm takes place and bacteria get attached to the surface whereas in macrofouling larger organisms get adhered to the surface (Buskens et al., 2013). Antifouling coatings work by preventing the growth of such microorganisms on the exterior of the ship. Most of the antifouling coatings release toxin

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compounds such as organotins and cuprous oxide. However, the use of such compounds is banned due to their detrimental impact on the environment (Evans et al., 1995). Tributyltin was banned in 2008 due to its undesirable impact on nature. Antifouling coatings that are toxin free are commonly suggested nowadays. Hydrophilic antifouling coatings mainly constitutes of polymer brushes which are hydrophilic in nature. Polyethylene oxide (PEO) is a commonly used material in brush coating which is grafted on the surface using linkers such as siloxanes (Kang et al., 2012). Thiols are another type of linkers which are typically used. These coatings strongly prevent the adherence of bacteria and protein on the surface. But the mechanical durability of such coatings still remains a matter of concern. Another type of antifouling coating present is hydrophobic foul release coatings. Hydrophobic coatings imply that the surface has low energy and low surface energy implies unspontaneous interaction. Webster et al. investigated the antifouling properties by combining polyurethane and siloxane resin (Sommer et al., 2010). Polyurethane matrix provides the required toughness and framework whereas siloxane imparts low surface energy to the substrate. Silanes along with fluoroalkyl materials are the commonly used low surface energy agents.

9. Intumescent coatings Fire poses a serious threat to infrastructures and the inhabitants occupying them. A catastrophic fire accident can lead to loss of life, wealth, and structural collapses (Saxena & Gupta, 1990). Intumescent coatings are the fireproof coatings which get swelled upon exposure to heat and fire (Puri & Khanna, 2017). A carbonaceous char is formed which acts as a heat transfer barrier and protect the steel surface from structural collapses. This type of coating can swell up to 100 times its original thickness. Fig. 5 shows the swelling up of intumescent coating on exposure to fire. Intumescent Intumescent coating before heat and fire exposure Substrate

Expansion of Char on heating

Fig. 5 Schematic representation of Intumescent coating on exposure to fire.

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coatings have been widely used in skyscrapers, auditoriums, stadiums, offices, and workplaces. Intumescent coatings generally consist of the following component such as carbon donor or char forming materials such as pentaerythritol, blowing agents such as melamine and acid donor-like ammonium phosphate (Kandare et al., 2010). Commonly used binders for such coating includes polyvinyl acetate, epoxy and vinyl acetate/ vinyl ester synthesized using versatic acid. The binder basically influences the insulation performance of the fire-resistant coatings. Various additives such as montmorillonite, saponite, and hectorite are known for enhancing the thermal shielding efficiency of intumescent coatings. Li et al. investigated the fire resistance of the intumescent using rutile-TiO2 and anatase-TiO2and showed that the intumescent coating incorporated with rutile-TiO2 has longer resistance to fire (Li et al., 2015).

10. Antimicrobial coatings The rapid transmission of microorganisms such as fungi, bacteria, and viruses through air, water, household surfaces in contact, doors, and medical devices are the major cause of infections and epidemic (Smith et al., 2019). The attachment of bacteria to the surfaces can cause undesirable consequences such as biofouling, corrosion, and cross-contamination. Endowing these surfaces with the ability to immobilize or completely kill the microbes is gaining huge attention in the past few decades. Therefore, these antimicrobial coatings are an effective way to control the spread of infections (Elbourne et al., 2017). These antimicrobial coatings find their application in food packaging, water treatment, health-care products, marine industries, and cosmetics. The conventional approaches to managing the infections are preventing contact with the surfaces where the footfall is high, disinfecting the surface using chemicals, protective equipments, and sanitization. In order to avoid the spread of SARS-CoV-2 virus disinfecting the surfaces was considered one of the efficacious methods (Nicola et al., 2020). It is presented in some recent work that SARS-CoV-2 virus can remain active on surfaces for up to 7 days (Pan et al., 2020). However, repeated use of chemicals such as sodium hypochlorite, chlorine, ethanol, and hydrogen peroxide for disinfecting the surface can be economically challenging in the long run (Rai et al., 2020). The antimicrobial coatings prevent the fouling of surfaces by pathogens in two ways. The first approach involves the directly killing (cidal activity) of microorganisms in order to prevent their transmission. The second approach is to hinder the adherence of microorganisms and

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furthermore the formation of biofilm (static activity) (Tuson & Weibel, 2013). On the basis of the interaction of the surface with microbes, a number of antimicrobial surfaces have been designed. The designed surfaces are mainly classified into three major sections. 1. Patterned surface, 2. Functionalized surface, 3. Superwetting surface. Our nature presents the example of a variety of surfaces with nanostructures or structured roughness ( Jaggessar et al., 2017). Taro leaf, gecko skin, and cicada wings (Kelleher et al., 2016) are known to have structures which exhibit microbicidal activity and prevent the adherence of microbes on the surface. Fig. 6 illustrates a patterned antimicrobial surface which inhibits the adhesion of microbes on the surface. Inspired by the nature, Yang et al. developed an antibacterial surface with a honeycomb pattern on a silicon surface using photolithography and showed that the pattern of 1 μm was effective to prevent the adhesion of bacteria on the surface (Yang et al., 2015). The proliferation and growth of biofilm of S. aureus and E. Coli were significantly hindered due to physical confinement. Rosenzweig et al. synthesized the nanopillar of polymethylmethacrylate and investigated its fungicidal property (Rosenzweig et al., 2019). Another type of antimicrobial surface called functionalized surface is developed by modifying the surface chemically which actively kills the pathogens on adhesion to the surface. These surfaces contain nonleachable material such as polycations which enhance the electrostatic interaction between the negatively charged surfaces of microbes. This causes the leakage of genetic material and disruption of the activity of microbes Wong

Microbes unable to adhere Patterned Structure

Fig. 6 Schematic representation of patterned antimicrobial surface.

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et al. developed an antimicrobial surface using N,N-dodecyl methylpolyethylenimine which was effective against S. aureus, H1N1 virus, and E. coli (Wong et al., 2010). Donskyi et al. studied the antiviral activity of graphene nanosheets functionalized with polyglycerol sulfate and fatty amine against HSV-1 virus. The functionalized surfaces are effective against a large spectrum of Gram-positive and Gram-negative bacteria along with viruses and fungi (Donskyi et al., 2019). Apart from the chemical mode of action, photothermal functionalized coatings are known to generate heat on exposure to light which results in rupturing of the cell membrane and microbial death due to hyperthermia ( Jin et al., 2019). Lei et al. developed a polydopamine antimicrobial coating which showed 93% killing efficiency against C. albicans (Lei et al., 2016). Microbial degradation by photocatalysis using UV active metal oxides have also been reported. Commonly used metal oxides include TiO2 and ZnO. In the past decades, photocatalytic activity and its mechanism have been examined against strains such as Aspergillus niger, Streptococcus pneumonia, E. coli, etc. The bombardment of photons on the surface of the photocatalyst results in the excitation of electrons to the conduction band leaving behind a hole in the valence band. The excited electron in the conduction band reacts with O2 and the formation of superoxide radicals and peroxide radicals takes place as a result of this. The degradation of pollutants is assisted in the presence of these reactive oxygen species (ROS) (Mahanta et al., 2021). Kim et al. studied the photocatalytic activity of TiO2 against murine norovirus and influenza virus (Kim et al., 2021). Changing the wettability of the surface is also an efficacious way to control the biofilm formation on the surface due to their enhanced antiadhesion property (Zhang et al., 2013). Ellinas et al. developed a superhydrophobic antibacterial surface using polymethyl methacrylate which showed a water contact angle (WCA) of about 155° and exhibited low bacterial adhesion against cyanobacteria Synechococcus sp. (Ellinas et al., 2017). Kim et al. synthesized a superhydrophobic aluminum which showed a WCA of about 169° and effective against fungi such as Cladosporium and aspergillus (Kim & Hwang, 2015). Chauhan et al. fabricated the cotton surface with hexadecyltrimethoxysilane and showed antibacterial property against E. coli strain (Chauhan et al., 2019).

11. Smart window coatings The smart window coatings were first designed by Lampert and Granqvist in the 1980s. Smart window coatings aim at dynamically adjusting the

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transmittance of light in order to save energy consumption (Zhou, Fan, et al., 2021). In comparison to the conventional static windows these smart windows reduce energy consumption by up to 10%. Humidity-triggered smart windows adjust their transmittance dynamically when the atmospheric humidity changes. These can be further categorized into wet and dry transparent. Castellon et al. designed humidity responsive smart window by sol–gel method using deliquescent materials such as calcium chloride incorporated in a silica-titania framework. At low relative humidity, the transmittance was about 6% and the transmittance crossed 65% at high relative humidity of about 51%. In addition, humidity responsive color changing coatings are also developed (Castello´n et al., 2018). Xiao et al. synthesized a bioinspired melanin nanoparticle which exhibited dynamic color-changing characteristics in response to humidity (Xiao et al., 2016).

12. Summary The book chapter aims at providing a vivid description of some of the currently developed smart coatings along with their mechanism of work and application. Recent developments in the area of smart coatings are gradually leading ways for the development of coatings with multiple functionalities. The sustainability and durability of such functional coatings still remain a source of great concern. Efforts to scale-up these functional material remains a great challenge because of their high cost which makes them commercially unviable for industrial application. In future, these innovative coatings are expected to boost market demands and bridge the gap between technological advancement and market requirements.

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CHAPTER 8

Working principles of various smart coatings on microbes/virus growth Preeti Kumari and Aditya Kumar Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India

1. Introduction Microbes are tiny organisms that are present everywhere in biosphere and cannot be detected by the naked eye (Noone, 2012). Their presence affects both human beings and environment either in a beneficial or harmful way: it entirely depends on the type of microbes and their interaction with the environment. The most common types of microbes are bacteria, fungi, protozoa, and virus. Bacteria are single-celled organisms that do not have a nucleus. Most of the bacteria are not injurious to health; only a few of them cause infection in the form of diarrhea, cold, tuberculosis, tonsillitis, etc. (Noone, 2012). These bacterial infections are generally treated by antibiotics, which are medicines that kill or inhibit the growth of bacteria. Viruses are biological entities that require a host (living organism) for their survival. They are generally a small collection of genetic code, either DNA or RNA, surrounded by a protein coat. They have a multitude of architectures that include enveloped or nonenveloped, RNA-based or DNA-based and can be further classified as positive- or negative-sense or single- or double-stranded, respectively (Imani et al., 2020). Unlike bacteria, most viruses cause disease once they invade host cell and multiply themselves (Noone, 2012). Some of the diseases caused by virus are chickenpox, flu (influenza), herpes, and human immunodeficiency virus (HIV/AIDS). Fungi are eukaryotic organisms that include microorganisms such as yeast, molds, and mushrooms. They mostly cause infections such as ringworm, athlete’s foot, toenail fungus, yeast infections, and jock itch. Fungal diseases are called mycoses, which affect our skin, nails, hair, internal organs (lungs), and the nervous

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system (Walker & Medical, 2014). Protozoa are unicellular, eukaryotic, and heterotrophic organisms. They can be either parasites or predators and cause disease such as malaria (Wasson, 2007). Proliferation and transmission of these microbes have adverse consequences on the environment as well as on human health. They have become a major cause of chronic infections and dreadful diseases, which has increased the mortality rate. Even the World Health Organization (WHO) has declared antimicrobial resistance (AMR) as one of the top 10 global health and development threats. Presently, treatment of these diseases is facing a great crisis, as therapeutic actions against these pathogens are limited. The major hindrances are antibiotic resistance, in the case of bacteria, and design of safe and effective antiviral drugs, in case of viruses. The application of antiviral drugs is also challenging because they use the host’s cells to replicate, which makes it difficult to target drugs without harming the host cells (Warnes et al., 2015). These obstacles led to the quest for alternative methods to minimize the dependency on antibiotics and antiviral drugs, which led to the development of smart coatings.

2. What are smart coatings? Smart coatings are the coatings, which determine and respond dynamically to changes in the environment in a functional and anticipated manner. Smart coatings are rapidly emerging as a global mitigation strategy for pathogens. Development of “smart coatings” with properties such as intelligence, reliability, longer life span, and a resistant protective coating are the primary focus of current research. The main aim is to ensure the delivery of antimicrobial agents only at sites where microbes are present rather than targeting the healthy cells (Xiao et al., 2020). These coatings are commonly based on the utilization of smart materials via its application on the surface, which will make it able to respond to changes in environmental conditions (for example, chemical conditions—pH, salt concentration, presence of biomolecules, or physical conditions—temperature, electrical potential, light, etc.) rapidly and reversibly (Urban, 2005; Vladimir & Winnik, 2010). They must possess the ability to adjust the release of antimicrobial drug according to microbial contamination at the specific site. They should be able to increase the local drug concentration at the targeted site to increase the therapeutic efficacy of the drug. These coatings should minimize the accumulation of drug at a healthy site in order to decrease the toxicity and drug resistance as well as damage to the surroundings (P
erez-Cobas et al., 2013). They generally reduce the adhesion on surface and provide biocidal activity against the microbes.

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3. Why smart coatings? Smart coatings are required for microbes (bacteria or viruses) because of the ineffectiveness of the traditional treatment methods, which includes the use of antibiotics and antiviral drugs. The traditional antimicrobial treatment includes the usage of antibiotics due to its cost-effectiveness and powerful outcomes. However, several studies have provided direct evidence that the widespread use of antibiotics has given birth to a new problem of multidrug resistance, which has become a problem of global concern. However, in the case of viruses, antiviral drugs development is a tedious process as viruses rely on the host cell metabolism to copy themselves; so, it becomes challenging to kill or inhibit the growth of viruses without damaging the working of healthy host cells. These circumstances led us to the development of smart coating with the motto to overcome the past issues and as a solution to emerging problems such as multidrug resistance, biofilm formation, etc.

4. Strategies used by smart coatings to combat microbes Smart coatings use the following chemical and physical strategies (as given in Fig. 1) to inhibit the growth of microbes, which are discussed in detail in the next sections. 1. Chemical strategies. 2. Physical strategies.

4.1 Antiadhesive surfaces Antiadhesive surfaces are the surfaces that reduce the adhesion between the microbes and solid surface, which assists in the removal of microbes before

Fig. 1 Various strategies used to combat the microbes.

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Fig. 2 Mechanism of antimicrobial surfaces.

adherence and further growth and transmission. These surfaces are generally made up via the use of passive action polymers, hydrogels, and polyzwitterionic polymers. The preliminary mechanism used for repelling the microbes is based on prohibition of microbes from the surface due to steric hindrance; it generally occurs due to the attachment of polymers on the coating surface. These polymers provide a physical barrier to proteins of the bacterial cell wall and thus inhibit its attachment (Statz et al., 2005). Another mechanism is based on electrostatic repulsion, in which repulsion occurs due to presence of similar charges on coating surface as well as on the microbial surface, which prohibits the adherence of microbes on the surface (Kumar et al., 2008). The last technique is the use of superhydrophobic surfaces in which low surface energy material is used, which reduce the external microbial adhesion on the coated surface (Fig. 2). The antiadhesive surfaces are obtained by the application of the following materials as described below: A. Passive action polymers: A passive polymer layer generally aims at reduction of protein adsorption on the surface, and thus preventing the adhesion of microbes on its surface. Passive polymers generally include self-healing, slippery liquid-infused porous surface (SLIPS) (Barthlott et al., 1997; Gao & Mccarthy, 2006), charged polyampholytes, uncharged polymers (Statz et al., 2005), and zwitterionic polymers (Shang et al., 2005). Passive polymers should be either (1) hydrophilic; (2) negatively charged; or (3) have a low surface free energy, which helps in repelling bacteria as bacteria are generally negatively charged and have a hydrophilic nature (Shang et al., 2005). Poly(ethylene glycol) (PEG) is the most

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commonly used polymer due to its impeccable features such as antifouling properties, biocompatibility, and safety in various applications of the medical field (Qu et al., 2018). B. Hydrogels Hydrogels have emerged as a promising antimicrobial material due to their unique three-dimensional structure cross-linked by physical or covalent bonds, which provides them sufficient capability to accommodate different materials such as small molecules, polymers, and particles. Substrates coated with antibacterial hydrogel layers have been demonstrated as an effective strategy against bacterial colonization (Li et al., 2018). Hydrogels are highly hydrated material and are being used in diversified medical application due to their general biocompatibility properties. The main advantage of using hydrogels is that they can control the number of cross-linkers and monomers in the hydrogel network and thus can control the surface morphology. They can also influence the antibacterial activity of the surface by their ability to disperse the loaded agents in the hydrogel network. Hydrogels with inherent antibacterial properties include hydrogels made of chitosan, peptides, and polymers (Li et al., 2018). (a) Chitosan-based hydrogel: Chitosan (CS) is a linear poly-saccharide, obtained by the deacetylation of chitin. It is a modified natural biopolymer and can be used as a coating material. The basic mechanism for the antimicrobial action of chitosan involves the interaction between the negatively charged carboxylate (-COO-) groups present on the surface of bacterial cell membranes and positively charged amino (dNH3) groups of chitosan, which leads to bacterial cell membrane disruption (Xing et al., 2016). Chitosanbased hydrogels possess many properties such as self-healing, antibacterial activity, biocompatibility, and biodegradability (Sharma et al., 2019), which makes it a good source for smart coating. (b) Nanoparticle-incorporated hydrogels: They are used for antibacterial surface applications, as the mode of action of nanoparticles is direct contact with the microbe cell wall such as in the case of wound dressings, that is when the surface is in contact with the body. The hydrogel is loaded with NPs which, in turn, increase the antibacterial efficacy of the coating. The most commonly used metal NPs is silver NPs (Ag-NPs) due to its antimicrobial properties. Silver NPs possess a large surface area to volume ratio, which provides it ample area to attach on the surface of microbes, and thus

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enhances its permeability in the cell membrane. Increase in permeability boost the process of cell disruption as NPs can easily enter the cell membrane and damage the intracellular organelles such as ribosomes and mitochondria, and biomolecules (Pangli et al., 2021). C. Poly-zwitterionic polymers: Zwitterionic polymers are the materials, which consist of equal numbers of cations and anions along their polymer chains. They are highly hydrophilic antifouling compounds due to the presence of positive and negative charged groups in their structure (Laschewsky, 2014). The zwitterionic groups, based on the anions, can be classified into sulfobetaine (SB), carboxybetaine (CB), and phosphorylcholine (PC). Various studies were made to test the antifouling activity of this material. Zheng et al. reported favorable antifouling capabilities due to the presence of chemical groups on zwitterionic materials (Zheng et al., 2017). These materials were also widely used in biomedical devices, implants, drug delivery, separation membranes, and marine coating.

4.2 Contact killing of adhered microbes The contact killing approach emphasizes on adhering the antibacterial agents onto the surface through strong covalent bonds such that they are immobile and kill the microbes in contact. Different approaches based on the principle of active contact killing include the use of active action polymers, quaternary ammonium cations, and antimicrobial proteins and peptides, which are discussed below: A. Active action polymers: Active polymers quickly kill the bacteria that come in their contact or adhere to their surface. It is basically due to their inherent antibacterial activity that reduces the metabolism of bacteria and helps in minimizing their adverse effect on the environment (Huang et al., 2016). Presently, the most commonly used active antimicrobial polymer is functionalized with positively charged quaternary ammonium compounds. (a) Quaternary ammonium compounds: Quaternary ammonium compounds (QACs) are positively charged polyatomic ions with properties such as low toxicity, broad antimicrobial spectrum, nonvolatility, and chemical stability (Rajkowska et al., 2016). They are one of the most widely used disinfectants in the food industry due to their stability and effectiveness against microbial biofilms. QACs consist of four alkyl groups attached to a central cationic nitrogen

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atom. It was also found that the antibacterial activity of QACs depends on the chain length. Studies showed that the optimum chain length for Gram-positive bacteria was 12–14 carbons whereas it was 14–16 carbons in the case of Gram-negative bacteria (Li & Weir, 2013). QACs are covalently integrated on the biomaterial surface via various methods, which include the sol–gel process via covalent hydrolysable ester linkage, atom transfer radical polymerization, plasma polymerization, and layer-by-layer deposition (Qiao & Liu, 2014; Thebault et al., 2009). These techniques were able to effectively impart a permanent antimicrobial activity on the surface based on the contact killing mechanism (Huang et al., 2007; Xue et al., 2015). The application of surface-immobilized QACs is yet not fully developed. Therefore, the toxicity and safety application in medical settings remains uncertain and more precise and optimized results are required for future research. (b) Surface-attached antimicrobial peptides: Antimicrobial peptides (AMPs) are one of the emerging class of natural and synthetic peptides with a wide range of targets that include bacteria, viruses, fungi, and parasites. They are the oligopeptides composed of cationic and hydrophobic amino acids (Peptides, 2013). Antimicrobial peptides are categorized by their secondary structure in four groups, which include β-sheet, α-helix, extended, and loop (Powers & Hancock, 2003). Among the most promising antibacterial strategies, surface-attached AMPs exhibit maximum potential, as they inhibit biofilm formation and possess good biocompatibility. AMPs kill cells by disrupting membrane integrity (via interaction with negatively charged cell membrane), by inhibiting proteins, DNA and RNA synthesis, or by interacting with certain intracellular targets. They are highly sensitive to surface-attachment techniques and have set a new standard due to their durability and long-lasting antimicrobial properties. Their impeccable properties and intrinsic ability has led to its widespread application in commercial purpose to combat biofilms. (c) Quorum-sensing inhibition: Quorum sensing is a cell-to-cell bacterial communication process, which depends on the production, sensing, and response to external signals. It is a way used by bacteria to convey the information about their changing environment (Qvortrup et al., 2019; Rutherford & Bassler, 2012; Samanta et al., 2011). QS is also exploited for antimicrobial purposes, as it

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minimizes microbial colonization and biofilm formation by hindering bacterial communication. They also alter specific gene expression in a population. QS shows immense potential as an antibacterial agent due to its immobile nature, which helps in the prevention of biofilm formation and development of multidrug resistance (Li & Nair, 2012). 4.2.1 Drawback of contact-killing mechanism A major intrinsic drawback of contact-killing coating is the aggregation of dead bacteria and other residues on the coating surface. This results in the blockage of the active biocidal activity centers on the surface and results in decreased efficacy against the bacteria adhered post accumulation of debris. The long-term usage of antibacterial surfaces, blocks the active biocidal centers and thus makes it vulnerable to bacterial attachment, which leads to the formation of biofilms. Biofilms are complex, three-dimensional communities of microorganisms attached to the surface and enclosed in a protective extracellular polymeric substance (EPS). Formation of biofilms due to microbial colonization takes place on virtually all kinds of surfaces (Hall-Stoodley et al., 2004). Biofilms derived from the colonization of microbes on the surface pose serious problems to the society from both economical perspective as well as health concern. Therefore, the harmful effects of biofilms on human society are manifold (Garibo et al., 2022). Biofilms show a significant role in infections caused by bacteria. Different ways that have been used to combat the growth of microbes include (a) surfacemodification techniques, which include topographical changes, chemical modification, or use of stimuli-responsive surfaces to prevent bacterial adhesion, (b) penetration of biofilms through molecular agents, which release drug and kill the bacteria, and (c) disruption of cell membrane by mechanical stresses such as shear stresses, cold plasma, photodynamic/photothermal therapy, interfacial tension, or pressure/electric waves (Makvandi et al., 2020).

4.3 Biocidal release mechanism This mechanism includes both the attachment of microbes on the surface as well as release of biocidal agents (Sorzabal-Bellido et al., 2019). Many strategies have been reported in the literature, for the attachment of biocides and eradication of bacteria from the surface. These biocidal agents (such as polymers) are grouped into three broad categories, namely antibiotic-releasing polymers, polymeric antibiotics, and antibiotic polymers. Antibiotic

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polymers either in the form of solution or coatings showcase their efficacy against bacteria, with different mechanisms of action. The efficiency of these biocidal polymers can be enhanced by the integration of single or combined biocidal and antibiofouling actions (Chen et al., 2017). Biocidal polymers are self-evident (Bryers, 2008) and exert a prolonged resistance against bacteria with a high concentration of active components (Kenawy et al., 2007). They also possess longer shelf-life, lower toxicity, and have less chances of inducing drug-resistant bacterial species (Sovadinova et al., 2011).

4.4 Physical strategies 4.4.1 Topographic modifications It includes the modification of the surface to inhibit biofilm formation through superhydrophobic/hydrophobic interaction. A lot of work has been done to investigate the feasibility of using a superhydrophobic surface for reducing the adhesion of microbes on the surface. It has been found that a reduced protein adsorption plays an integral role in inhibiting the growth of microbes on the surface. Privett et al. (2011) demonstrated that the adhesion of Staphylococcus aureus and Pseudomonas aeruginosa reduced significantly on the surface coated with fluorinated silica colloid; it was due to the superhydrophobic behavior (water contact angle of 167°) induced on the surface. Crick et al. (2011) also checked and demonstrated a reduced adhesion of Staphylococcus aureus and Escherichia coli on three surfaces: (i) AACVD (aerosol-assisted chemical vapor deposition)-coated superhydrophobic surface showed a water contact angle of 165°, (ii) an uncoated plain glass reported a water contact angle of 60°, and (iii) a dip-coated elastomer glass reported a water contact angle of 95°. The majority of the current research is still focused on investigating the phenomenon and the potential of applying superhydrophobic surfaces to reduce bacterial adhesion. Due to a different procedure of bacterial adhesion test and variation of test conditions in various experiments, contradictive results were obtained. So, for better understanding of the mechanism of bacterial adhesion on superhydrophobic surfaces, more bacterium types should be tested and a standard cell-adhesion test procedure should be followed. Further parameters, such as surface roughness, morphology, and free energy of the superhydrophobic surfaces, should also be determined to get the accurate results (Zhang, Wang, & Lev€anen, 2013).

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5. Smart and synergistic antimicrobial coatings Currently, work is done on the evolution of “smart” and “synergistic” antimicrobial coatings. In comparison to the “traditional” antibacterial coatings, which have a perpetual and monotonous killing mechanism, smart and synergistic coatings show enhanced capability to activate their activity in response to pathogen-infected microenvironments. For the effective removal of microbes, positive interaction of multiple mechanism is used and the dead bacteria are released from the surface by the trigger through external stimuli. Such types of coatings are long-lasting and also inhibit biofilm formation due to multi-drug resistant (MDR) bacteria. The development of smart and “synergistic” antibacterial coatings includes (i) selfdefensive antibacterial coatings, which activates its biocidal activity in response to an environment-containing bacteria; (ii) synergistic antibacterial coatings, which is a positive integration of two or more killing mechanisms; and (iii) smart “kill-and-release” antibacterial coatings, which can change its functionality by either killing bacteria or releasing the dead bacteria from the surface (Wei et al., 2019).

5.1 Self-defensive antibacterial coatings The coatings based on a contact-killing mechanism show a degradation in their efficacy due to biofilm formation. It happens due to long-term exposure of biocidal groups to the contaminated environment, which is even toxic for healthy cells and tissues. Also, the coatings based on a release-killing mechanism lack the controlled release of biocides, which leads to adverse consequences such as development of multidrug resistance, premature depletion of biocides, and many other undesired side effects. Therefore, an appropriate approach would be to prepare antibacterial coatings with bacteria-responsive property such that it activates its biocidal activity in the presence of bacteria only. In recent years, self-defensive antibacterial coatings that activate via an external trigger due to the presence of bacteria have attracted a lot of attention. On the basis of external triggers, these coatings can be categorized into: (i) pH-responsive self-defensive antibacterial coatings and (ii) bacteria-secreted substance-responsive self-defensive antibacterial coatings (Fig. 3). 5.1.1 pH-responsive self-defensive antibacterial coatings The infection caused by bacteria is always associated with acidification of the environment as the pH at the site of infection is reduced to a minimum due to

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Fig. 3 Response of a self-defensive antimicrobial coating.

low oxygen fermentation. This leads to the production of organic acids such as lactic acid secreted by Staphylococci and acetic acid secreted by Escherichia coli (Lavalle et al., 2015). The decrease in pH results in the development of a variety of pH-responsive self-defensive antibacterial coatings used to combat the infections caused by bacteria. Sukhishvili and coworkers developed various types of pH-responsive multilayered coatings by a layer-by-layer (LBL) assembly technique, constituting antibacterial agents (Albright et al., 2018). These multilayered coatings were prepared by either assembly of negatively charged montmorillonite (MMT) clay nanoplatelets and polyacrylic acid (PAA) with a further layer formed by adsorption of cationic antibiotic gentamicin or alternative deposition of negatively charged tannic acid (TA) and cationic antibiotics (such as tobramycin, gentamycin, and polymyxin B). Under normal physical conditions, the antibiotics remain stably incorporated within the coatings but when enclosed in a place surrounded by bacteria, the acidic environment lead to the protonation of the anionic components (PAA or TA) and further disruption of charge balance occurs due to counter-ion pairing between the formed anionic components and cationic antibiotics, resulting in the release of antibiotics that eradicate several types of infectious bacteria (such as Staphylococcus aureus, Staphylococcus epidermidis, or E. coli). These bacteria-triggered antibiotic-releasing coatings due to the varying pH have proven to be a promising solution for the problems associated with traditional antimicrobial coatings (Wei et al., 2019).

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5.1.2 Bacteria-secreted substance-responsive self-defensive antibacterial coatings The secretion of substances such as enzymes (e.g., phosphatase, phospholipase, hyaluronidase, chymotrypsin, and extracellular lipases) and toxins by bacteria during their cell metabolism acts as an external trigger, which activates their biocidal activity (Zelzer et al., 2013). Recently, a series of antibacterial coatings with enzyme-triggered biocide-release capability has been developed due to the bacterial infection that occurs on the biomaterial surfaces (Cado et al., 2013). Cado et al. (2013) designed biocompatible and biodegradable polysaccharide multilayer films with the conception of devising an integrated antibacterial and antifungal coating. The coating, based on functionalized hyaluronic acid by cateslytin (CTL), an endogenous host-defensive antimicrobial peptide, and chitosan (HA-CTL-C/CHI) were deposited on a planar surface. After an incubation of 24 h, it was observed that HA-CTL-C/ CHI films fully inhibit the development of Gram-positive Staphylococcus aureus bacteria and Candida albicans yeasts, which are one of the most common pathogenic agents encountered in infectious diseases.

5.2 Synergistic antibacterial coatings Biocidal agents such as antibiotics, metal nanoparticles, cationic polymers, QACs (quaternary ammonium compounds), antibacterial peptides, and antibacterial enzymes have some constraints such as the fact that antibiotics are ineffective against multidrug-resistant bacteria (MDR) and agents such as quaternary ammonium compounds (QACs), metal nanoparticles, and cationic polymers are more toxic to normal cells than infected cells. Therefore, presently drug-free biocidal methods such as phototherapy, radiofrequency therapy, microwave therapy, and ultrasound therapy have emerged as efficient alternatives for fighting against bacteria-induced infections and fouling. Photothermal therapies such as antibacterial photothermal therapy (APTT) (Gharatape et al., 2016; Tim, 2015) and antibacterial photodynamic therapy (APDT) (Spagnul et al., 2015; Tim, 2015) are seeking more attention due to their high efficacy and yield. The combinatory effect of these methods enhances the killing efficiency due to the synergism.

5.3 Smart kill and release antibacterial coatings Smart antimicrobial coatings were developed by incorporation of both the features, which are killing of bacteria as well as releasing the killed bacteria from the

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surface under a suitable stimulus. This smart “kill and release” mechanism helped to enhance the longevity and durability of the antimicrobial performance of these coatings. A common example of smart antimicrobial coating is based on the switchability of the charges of the coating prepared from zwitterionic compound and polymer derivatives. The chemical structure changes from the zwitterionic form, which repels the bacteria to a cationic form that exhibit bacterial killing ability. Cheng et al. prepared a hydrogel with sustained release of antimicrobial agent to kill and inhibit the growth of infectious bacteria. The hydrogel was incorporated with a mild antimicrobial agent (salicylate) into poly(N,N-dimethyl-N-(ethylcarbonylmethyl)N-[2-(methacryloyloxy)-ethyl]ammonium salicylate) (pCBMA-1 C2 SA) hydrogel, which served as an anionic counterion. This resulted in the formation of a nonfouling surface that prevented the accumulation of bacteria and adsorption of protein on the surface due to the hydrolysis of carboxybetaine esters into zwitterionic groups. The pCBMA-1 C2 SA hydrogel showed 99.9%. efficacy by inhibiting the growth of both Gram-negative (Escherichia coli K12) and Gram-positive (Staphylococcus epidermidis) bacteria. This hydrogel also holds great potential in applications such as wound dressing and surface coatings for medical devices (Cheng et al., 2010).

6. Different materials used for smart coatings 6.1 Nanoparticles Nanoparticles are being widely used to target microbes as a substitute to antibiotics because the mode of action of nanoparticles is direct contact with the microbial cell wall without the necessity to penetrate the cell. So, they are less prone in developing resistance in bacteria in comparison to antibiotics. The contact between the nanoparticles and the bacteria can be in the form of

Fig. 4 Schematic representation of antimicrobial mechanism of various nanoparticles.

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electrostatic attraction (Wang et al., 2017), van der Waals forces (Armentano et al., 2014), receptor-ligand (Gao et al., 2014), or hydrophobic interactions (Gao et al., 2014). Thereafter, nanoparticles cross the bacterial cell membrane and influence the functioning of cell metabolism via interaction with cell components such as DNA, mitochondria, ribosomes, lysosomes and enzymes. This leads to the generation of oxidative stress, alteration of the cell permeability, disbalance of electrolytes, inhibition of enzymes, changes in genetic code, and protein deactivation (Fig. 4) (Shrivastava et al., 2007; Xu et al., 2016). The basic mechanism involved in the antibacterial activity of nanoparticles: (a) Interaction of nanoparticles with the cell membrane: Bacteria can be divided in two groups on the basis of their cell wall structure— Gram-positive and Gram-negative bacteria. Gram-positive bacteria include a thick layer of peptidoglycan in their cell walls, whereas Gram-negative bacteria consist of a thin peptidoglycan layer with an additional outer membrane made up of lipopolysaccharide (Slavin et al., 2017). Nanoparticles generally release positive ions, which can be countered by the lipopolysaccharide, as it provides a negatively charged membrane, which has a higher affinity for the positive ions. This interaction results in an increased ionic exchange, which causes intracellular damage. Several studies have unveiled that nanoparticles have a greater affinity for Gram-positive bacteria as the cell wall of Gram-negative bacteria consists of lipopolysaccharide, lipoproteins, and phospholipids, which forms a penetration barrier, thus allowing permeability to only macromolecules (Slavin et al., 2017). Grampositive bacteria consists of a thin layer of peptidoglycan and teichoic acid with abundant pores that allow the invasion and disruption of the cell membrane by the nanoparticles. The presence of negative charges on the cell wall surface attracts the nanoparticles, enhancing the bactericidal activity (Godoy-Gallardo et al., 2021). (b) Penetration mechanism of nanoparticles: Nanoparticles can penetrate the cell membrane through various ways such as diffusion, adsorption, etc. Zhang et al. studied the reactive oxygen species generation kinetics of uncoated silver, gold, nickel, and silicon nanoparticles in aqueous suspension under UV irradiation. The result showed the generation of superoxide and hydroxyl radicals by Ag nanoparticles, whereas Au, Ni, and Si generated only singlet oxygen. These reactive oxygen species penetrated cell membrane to produce an antibacterial effect.

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The diffusion coefficient of these species was reported nearly about 105 cm2/s (Zhang, Li, et al., 2013). (c) Interaction with the cell components of microbes: Nanoparticles interact with the cell components (DNA, lysosomes, ribosomes, and enzymes) through various mechanism such as (i) oxidative stress (Gurunathan et al., 2012), (ii) metal ion release (Zakharova et al., 2015), and (iii) nonoxidative mechanism (Leung et al., 2014). Nanoparticles attack the bacterial cell through various mechanisms involving the damage of protein, DNAs and membrane through ROS formation or production of metal ions based on metal nanoparticles with dissolution leading to inhibition of electron transport chain (Wang et al., 2017). (i) Oxidative stress: When nanoparticles interact with cell components, then reactive oxygen species (ROS) are formed due to reduction of oxygen. The four ROS types are the superoxide radical (O2 ), the hydroxyl radical (
OH), hydrogen peroxide (H2O2), and singlet oxygen (O2), which exhibit different levels of dynamics and activity. The main causes of ROS production are restructuring, defect sites, and oxygen vacancies in the crystal (Malka et al., 2013). ROS induced by nanoparticles have a very strong oxidation potential, which results in the destruction of membrane structure, alteration of DNA/RNA, lipid peroxidation, damage of cell organelles, and protein oxidative carbonylation, which further results in necrosis or mutagenesis of the bacterial cell (Yu et al., 2020). Studies have shown that the intensity of stress reactions caused by O2 and H2O2 are acute and can be neutralized by antioxidants, such as superoxide enzymes and catalase, whereas reactive stress caused by
OH and O2 can lead to acute microbial death. Nanoparticles form the reactive oxygen species through various mechanisms and the most common route is photocatalysis. When light is irradiated on metal nanoparticles with an energy greater than or equal to band gap then the electron in the valence band moves to the conduction band, leaving behind the hole in the vacuum band. H+ adheres to surface leaving behind OH , which is oxidized to the hydroxyl radical (
OH), with further oxidation to the superoxide radical (O2 ) (Yu et al., 2015). (ii) Metal ion release: Metal ions are released from the metal oxide and are absorbed by the cell membrane, leading to damage to cell structure, inactivation of enzymes, and also adversely affecting

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the cell metabolism. This occurs due to the interaction with the functional groups of proteins and nucleic acids, such as mercapto (–SH), amino (–NH), and carboxyl (–COOH) groups with the metal ions, leading to the inhibition of microbes (Hussein-AlAli et al., 2014). (iii) Nonoxidative mechanism: MgO has a great potential as an antimicrobial agent. Its antimicrobial effects were studied using Escherichia coli (E. coli) bacteria. Upon investigation of the toxicity of three different MgO nanoparticle samples, the results clearly showed the toxicity toward Escherichia coli bacterial cells in the absence of ROS production for two MgO nanoparticle samples. The data also clearly demonstrated the absence of oxidative stress and indicated that the primary mechanism of cell death is related to the cell membrane damage, which was not due to lipid peroxidation. Therefore, it can be concluded that there exists a nonROS-mediated toxicity mechanism for MgO nanoparticles. This result clearly demonstrates an nonoxidative mechanism of metal nanoparticles (Leung et al., 2014).

6.2 Antimicrobial polymers Polymers pursue admirable antimicrobial properties and show repellancy to the microbes adhered on the surface. Their hydrophobic nature imparts them water-repelling properties, which assist in rolling off virus- or bacteria-contaminated water from the surface. An antimicrobial polymer is a polymer that has the capability to inhibit or inactivate the development or accumulation of microbes on the surface due to the presence of certain chemical structures such as amphiphilic structures or poly-cation groups on the polymer backbone or side chain (Siedenbiedel & Tiller, 2012). The mechanisms of action of antimicrobial polymers for inhibition of growth of pathogens are nonspecific in nature. They use bacterial lysis, which includes disruption of the cell membrane due to the presence of different charges whereas molecular antibiotics inhibit the synthesis of DNA via receptor binding. Thus, there is no development of microbial resistance against antimicrobial polymers ( Jain et al., 2014). Antimicrobial polymers (natural or synthetic) have the ability to impede the growth and colonization of bacteria due to the presence of chemical structures such as amphiphilic structures or poly-cation groups (Nasri et al., 2021). The most-used antimicrobial agent is the cationic polymer, due to the

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presence of positively charged groups in the polymer chain. Examples of some of the cationic polymers that are currently used as antimicrobial polymers are poly(ethyleneimine) (PEI), poly [2-(N, N-dimethylamino)ethylmethacrylate] (PDMAEMA), and polydimethylsiloxanes (PDMS). The mechanism used by these cationic groups involves the adsorption of microbes onto the surface due to electrostatic interaction between the negatively charged group of microbes and positively charged polymers. The positively charged cationic polymer inserts into a phospholipid membrane bilayer and interacts with anionic lipids causing cell apoptosis (Azevedo et al., 2014). Natural cationic polymers which are derived from natural sources such as chitosan, cellulose, dextran, and others can also be used (Samal et al., 2012). Chitosan has positively charged, functional amino groups, which is then chemically modified by quaternary ammonium salts to increase chargeability, and results in enhancement of its antibacterial activity. One of the most effective antibacterial coating agents is an amphiphilic polymer, due to the presence of hydrophobic and hydrophilic components in the same polymer structure. A polymer consisting of a halogen element such as fluorine, bromine, chlorine, and iodine also act as a potential antimicrobial agent, due to their properties, such as strong electronegativity and superhydrophobic nature. Halogen, being an oxidizing agent, exhibits antimicrobial and antiviral activity due their contact-killing action and antibiofilm formation ( Jain et al., 2014).

6.3 Metal ion- and oxide-based antimicrobial coatings Metal ions or oxides of metals such as silver, copper, titanium, etc. hold the potential to inhibit the growth of microbes and can be used in antibacterial coatings. Their basic mode of action generally includes the metal reduction potential or metal donor atom selectivity/speciation (Palza, 2015). (a) Metal reduction potential: Redox-active metals can be used as catalyzing agent for the generation of reactive oxygen species (ROS), which generates oxidative stress inside the cell. This oxidative stress can damage cellular proteins, lipids, and DNA. (b) Metal donor atom selectivity/speciation: Metal ions accept electrons and form complexes via strong and specific interactions. These electrons are obtained by some donor ligands such as oxygen, nitrogen, and sulfur (Lemire et al., 2013). The metal atoms or ions present inside the microbial cell selectively displace any original metal present inside the cell,

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Fig. 5 Schematic illustration of antiviral mechanism of metal ion.

which disturb the cell functioning mechanism leading to the phenomenon of cellular dysfunction (Fig. 5) (Nasri et al., 2021).

7. Application of smart coating (1) Medical devices and health-care facilities: Smart coatings are a powerful weapon against health-care-associated infections (HAI). These coatings inhibit the growth, and spread of bacteria and germs on surfaces of medical instruments, surgical tools, and implants. The contamination of medical devices caused by microbes (such as bacteria and fungi) possesses a severe threat to the patient’s health in hospitals. Also, there is decline in the efficacy of the traditional antibiotic treatment due to the growing resistance of the microbes to antibiotics. So, smart coating provides the best alternative for the protection of these medical devices against microbial adhesion. (2) They also work as an alternative to chemical disinfectants, as they reduce the need for frequent disinfection of surfaces. These coatings are a cost-effective solution and have also increased infrastructure value by preventing staining and discoloration and have also improved the longevity and durability of the surface. (3) Textiles: Textiles with smart coating are widely used as fabrics for hospital bed linen, uniforms for patient and staff, curtains, and wound dressings. The coating layer resists the microbial adhesion and thus reduces the chances of transmission of disease in hospitals. It has been also observed that textile degrades with time, but due to application of smart coating on textile the lifespan of material increases. (4) Food packaging: Antimicrobial packaging in films prevents the growth of microbes on the food surface via direct contact of the packaging material

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with the surface of foods. Due to the direct contact of the packaging film with the food surface, the diffusion of bacteriocins takes place, which inhibit the growth of bacteria on the food surface (Kharkwal, 2015). (5) Water treatment: Biofouling is one of the major problems in the water-treatment industry and research shows that antimicrobial coatings and films are a promising preventative measure for biofouling in water-treatment systems. These coatings on the surface of equipment extend its durability and lifespan by preventing the biological contamination on the surface. It also prevents the phenomenon of biocorrosion and biofouling of the machinery used in treatment plants (de Kwaadsteniet et al., 2011).

8. Conclusions and future work In this chapter, we have discussed smart coatings and their working mechanism against microbes. A basic introduction with the meaning of smart coating and its necessity for suppressing the spread of microbes is discussed. Emphasis is also laid upon the various strategies used by these coatings to inhibit the growth of microbes. Various materials like passive and active polymers, antimicrobial peptides, nanoparticles, zwitterionic polymers, and their mechanism of action is also discussed in detail. Further research in this field stems from the need to develop stimuli-responsive antibacterial coatings that activates only in the presence of external triggers. To overcome the shortcomings of traditional antimicrobial coatings, smart coatings should be synthesized with (1) the right choice of stimuli and (2) coating should be fabricated with efficient stimuli-responsive materials. With development in science, many responsive materials have been recognized, but further research is required to acquire the control on the response behavior in the presence or absence of stimuli. Presently, smart antibacterial coatings are developed at the laboratory scale and in order to use them at a commercial scale, a number of challenges need to be overcome, such as their efficacy under real conditions, long-term durability, fabrication processes, and abidance with human and environmental safety standards. These challenges provide an opportunity to researchers to explore further and devise smart coatings at a commercial scale.

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CHAPTER 9

Biomimetics in smart coatings Srishti, Aditya Kumar, and Apurba Sinhamahapatra Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India

1. Introduction Nature offers essential pieces of knowledge to us—for a more resistant, regenerative, and resplendent world. For years, we have come across many researchers who have successfully mimicked the models, elements, or biological processes already existing in our environment to address our day-today difficulties. This course of gaining knowledge from nature which has occurred/evolved over millions of years, and utilizing the same in practice is termed Biomimetics. In a more discrete form, as an approach for developing new ideas—bioinspiration engineering focuses on using biological phenomena for kindling research in nonbiological science and technology, developing novel solutions to prevailing challenges. Bioinspired research has been a hub for many researchers and rapidly increased with fruitful progression in recent years. It does not have boundaries and applies to various fields such as pharmaceuticals, biomedical, architecture, different industries, etc. In coating technology and research, a vast, diverse application was seen with the distinctive approach. As the name proposes, these bioinspired coatings are the coating whose structures, properties, or functions are inspired by natural existence. There are a variety of organisms, and different strategies, whose exploration and adaptation to changes in the environment have proved successful implementation of those in coating technology termed smart coatings. Superhydrophobicity, self-cleaning, drag reduction in fluid flow, high adhesion, antireflection, antifouling, antiicing, and self-healing are a few examples observed in nature with viable attention. This chapter provides a broad overview of the various fascinating objects and processes found in nature along with their primitive and prevailing application under study.

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2. Acknowledged biomimetics and their applications 2.1 The lotus effect-superhydrophobic-dust repellant Lotus or Nelumbo nucifera is the plant mostly studied under water repellency. It remains dirt-free, an obvious advantage for all aquatic plants living in a typical muddy habitat. Fundamentally, the micro/nanostructures on the lotus leaves provide roughness to the surface where the air gets entrapped, preventing water molecules from adhering to the surface of the leaves. The water in contact rolls right off from the surface, picking up any dirt or oil along its way. Therefore, these leaves exhibit considerable superhydrophobicity empirically defined as the surfaces exhibiting static water contact angle greater than 150 degrees, and contact angle hysteresis less than 5 degree. This phenomenon keeps them clean and also sustainably protects them. The microtextured and superhydrophobic surfaces are mainly responsible for the minimal adhesion of dirt and water with the surface, which cleans off and drains off easily. Superhydrophobicity is stated to be achieved either by coating the surface with low-surface energy materials or by erecting micro/nanohierarchical structures over the surface. Inspired by the lotus structure and the literature, Wang and his team developed a CNT film— a highly stable and robust superhydrophobic coating that can even prevent tiny water droplets from infusing into it. The fabricated film comprises microscale clusters of nanotubes entangled, providing the needed roughness structure while the air pockets within, principally reduce the liquid-solid interface, leading to a superhydrophobic Cassie state (Wang et al., 2017). A significant number of studies presented superhydrophobic, strongly adhesive surface coating on PET textiles, fascinated with the bioinspiration from the hierarchical lotus leaves combined with the adhesion property of mussels present in the sea (Xue et al., 2015). Polydopamine (PDA) with perfluorodecalin trichlorosilane was used as the coating material, turning the textile’s inherent hydrophilic property to superhydrophobic with a water contact angle higher than 150 degrees. In the past decades, lotus leaves have been an inspiration for a vast number of researchers; whether for their inherent superhydrophobic nature or in combination with others, they have been a breeding ground for many of us (Latthe et al., 2014; Wang et al., 2016; Xiao et al., 2017; Yang et al., 2015, 2021). Other examples of similar superhydrophobicity and self-cleaning biomimetics include rice leaves (Rius-Ayra et al., 2019), water striders (Su et al., 2010; Sun, Li, et al., 2018), mosquito eyes (Liu et al., 2021; Song et al., 2017), butterfly wings (Han et al., 2017), rose petal (Zong et al., 2019),

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and duck feathers (Bhushan, 2009) are few of them. Their superhydrophobicity results from the hierarchical structures they carry. These grooved surfaces include air pockets in-between that inhibit water adherence to the surface, allowing water droplets to attract “into” themselves, nearer to the perfect spheres. Recent developments in superhydrophobic coatings can be further discussed with an emphasis on its self-healing characteristics as well. For example, Qian et al. and their team fabricated a novel superhydrophobic selfcurable coating on Q235 carbon steel and studied its corrosion resistance property. The coating here includes benzotriazole (BTA) as a corrosion inhibitor and an epoxy polymer as a memory shaper. Under the diverse thermal condition, the proposed coating induces its intrinsic property without failing. Moreover, the coating presented its practical application when the self-healing was achieved in an actual outdoor environment, along with long-term corrosion protection from external damage (Qian et al., 2017). With a similar perception, Ni et al. and his coworkers presented another smart coating based on biomimetic stimuli-responsive mesoporous polydopamine microspheres displaying excellent NIR and pH-responsive selfhealing properties with antifouling and durable superhydrophobic characteristics (Ni et al., 2021). Likewise, different studies were carried including Manoj et al. (2020), Zhang et al. (2016), Pang et al. (2019), Li et al. (2017), and others.

2.2 The shark’s skin drag reduction and bioactive coatings Shark skin-type coating is known for its low drag surface and antibiofouling nature (Liu & Jiang, 2011). Sharks’ skin is covered with small thousands of minute tooth like scales called dermal denticles with longitudinal microscopic grooves called riblets, oriented parallel to the moving direction. When a shark moves fast, low-pressure regions of turbulent water near the skin are formed, called vortices. These vortices stay attached at the surface, resulting in the efficient movement of water nearby, helping in increasing thrust and drag reduction and enabling high speed underwater. In addition, these dermal denticles—evenly spaced hierarchical structure over the skin inhibit the adherence and colonization of microscopic aquatic organisms such as fungi and algae, inheriting the antibiofouling characteristics. The unique structure of sharks’ skin has attracted researchers to develop similar coatings for watercrafts such as ship hulls, boats and submarines,

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aircraft, and even swimwear for humans (Siddaiah & Menezes, 2016). These coatings diminish the friction between the solid surface and the fluid flowing, enhancing thrust and reducing drag, thereby helping in a more effortless movement. For example, inspired by the morphology of the shark’s skin and its drag-reducing properties of the dermal denticles present, Lauder et al. experimented and stimulated the enhanced performance of aerodynamics mimicking the structure of aerofoil (Domel, Saadat, et al., 2018). Further, Deng and his coworkers demonstrated the sol-gel preparation of ZrO2/ WS2 coatings, biomimetic shark skin-textured surfaces with a low friction coefficient and elevated wear resistance. The coating thus achieved exhibited the potential of a promising wear-resistant layered surface for future use. With similar inspiration, different investigations studies made by Fu et al. (2017), Zhao et al. (2014), Ball (1999), B€ uttner and Schulz (2011), Bixler and Bhushan (2013a, 2013b), Dean and Bhushan (2010), Li et al. (2019), Kim (2014), Domel, Domel, et al. (2018), and others. The presence of dermal denticles over the sharks’ skin serves a range of purposes. Besides the efficient tribological performance, these coatings are inevitable with their biofouling characteristics. As an aquatic animal, the shark is subjected to constant exposure to bacteria, algae, fungi, and other contaminants. However, the presence of dermal denticles prevents these fouling organisms from adhering to the shark skin creating an unstable repelling layer over the skin, a smaller surface area for bacterial contact, and engendering a moving target for the fouling organism with variation in exerting pressures. Another study suggested the reduced drag to prevent biofouling, reducing the time for microorganisms to settle with fast-flowing water near the shark skin surface (Zhang et al., 2017). The proposed reason is that the epidermal mucus of the shark skin inhibits the microorganism attachment, creating a barrier between the surfaces (Bechert et al., 2000). Recently, Chien et al. replicated the polymeric shark’s skin pattern imprinting from PDMS, providing insights into the shark-skin antifouling mechanism (Chien et al., 2020). Before this, Watkin and his team presented the production of large-area high-performance shark’s skin patterned surface coating enabling antibacterial and antifouling properties simultaneously (Dundar Arisoy et al., 2018). The team developed this bioinspired polymer and ceramic-based coated surface with the addition of titanium dioxide nanoparticles as the antibacterial agent via nanoimprint lithography. The surface coated with TiO2 showed 25% for Escherichia coli and 10% for Staphylococcus aureus with more inhibition tendency than the surface without TiO2.

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Furthermore, similar coated surface find other applications under the development of various human-associated products such as underwater devices, healthcare, and water treatment, along with the necessity of more understanding and identification of the possible underlying mechanisms (Liu & Choi, 2021; Thamaraiselvan et al., 2021; Wu et al., 2021; Zhang et al., 2015).

2.3 Structural color coating inspired by blue butterfly wings Natural color found on insects is because of the complex structure of their wings or scale, which sources out the properties like light diffraction, interference, scattering, and optical diffusion (Sun et al., 2013). Natural structural coloration has attracted copious attention lately due to its potential applications in technologies such as automobiles, optical industries, biomedical, and paints industries. About many species known, none more than those of butterflies could be considered much for these properties, as the group shows the widest diversified structures, colors, and visual effects than any other living organisms (Ingram & Parker, 2008). A prevalent species named the Blue Morpho butterfly is known for its bright blue-colored wings. As a myth in art, these wings project blue color not because of their color but the layered microstructure. When the light incident these structures, the rays get diffracted in different directions, interfering with each other. Hence, canceling each other and eventually intensified blue with the lowest wavelength is reflected. This blue irradiance is due to the alternation of the refractive indices of air and the multilayered structured wings. Encouraged by this, a leading company, “Cypris material,” developed a tunable, eco-friendly, and chemically inert self-assembled nanoscaled structural color coating that can quickly reflect the UV, visible and infra-red light when applied on the surface. Further, Morpho butterflies use multiple layers of the cuticle, a kind of microscopic ridges that have cross-ribs just like the profile of a fir tree and, in the presence of air pockets, turn out to be a natural photo producing striking blue color (Vukusic & Sambles, 2003). With these characteristics, they could be helpful in the development of chemical sensors for detecting different acenic crystals of vapor constituents (Rasson et al., 2017). Moreover, their structural color concept directs researchers’ interest toward their basic optical working principle. A team of researchers perceives this bioinspiration as the surface helpful in molding light flow (Siddique et al., 2013). They studied the nanostructured pattern on three different levels. First, the

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alternatively arranged lamellae layers—widen the reflection spectrum; second, the Christmas tree-like shape reduces the directionality of the reflectance; and lastly, the zigzag pattern of the ridges—provides more roughness to the surface. And later, they presented promising results in agreement with their investigation. Fascinatingly, another study reported the changing color mechanism found in Morpho butterfly scales receptive to vapors including water, ethanol, and methanol (Potyrailo et al., 2007). They demonstrated a highly selective vapor response by comparing reflectance as the function of time at a different wavelength, arising to the advanced potential application to optical gas sensors (Potyrailo et al., 2007). Other structures create similar effects; for example, the helical-structured cellulose microfibers in marble berries, sparkle with a flickering metallic shade of blue. However, being so amazed by the inspiration, researchers are still seeking the finer details on the ways structural color manifests in nature and the principle behind the bouncing back of light from the surface with color spectrum generation.

2.4 Pitcher plant-inspired coating—Antifouling, natural cleaner Besides the other superhydrophobic surfaces known, Nepenthes, or pitcher plant, has evolved as a particular version with properties beyond the lotus effect, such as high contact angle with unlimited oleophobicity, a close frictionless surface, and exclusive self-healing properties. The plant is superslippery due to the thin wet film on its surface, providing frictionless contact between the plant and the insect. With this aid, the plants trap insects and other small prey when they land on the rounded edge of the pitcher. Fundamentally, the edge is ultra-slippery, which makes the insects lose hold and later end up in a pool of plant’s digestive juices (Barthlott et al., 2007; Bauer & Federle, 2009; Bohn & Federle, 2004). The slippery sides of the pitcher plant were observed to be lined with heaps of ill-fitting cells, comprised of gaps and ridges. The plant fills these gaps and ridges with a sap (the lubricating liquid), imparting the ability to repelling almost everything. Inspired by their skin structure, researchers seek coating with excellent self-healing properties, damage tolerance, and endurance to severe climatic conditions to different substrates including metals, semiconductors, papers, and cotton fabrics. Wong et al. presented a facile method for creating self-curing and superslippery liquid-infused porous surfaces (SLIPS) with excellent liquid-repelling properties, even for lower surface tension fluids including blood, oil, honey, and ice (Wong et al., 2011).

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In addition, these bioinspired surfaces were able to surpass their counterparts through their exceptional performance at elevated pressures, resisting ice adhesion, and maintaining a low angle of hysteresis (Ahuja et al., 2008; Lee & Kim, 2011). However, the smooth nature of the plant finds its application in diversified objectives such as stain-free optical surfaces, graffitirepelling walls, self-cleaning windows, and pipes for easier and quicker fluids conveyance and biomedical devices.

2.5 Moth eye-inspired optically active surface coating Moths hunt at dusk or night when partial light is available. To maximize the capturing of light, moths’ eyes have a unique subwavelength structured coating, 200 nm in diameter and height, which dramatically minimizes the light reflection over a broader range of the light spectrum (Sun, Wang, et al., 2018). These nanostructured organized optical pillar-looking structures make the eye surface nearly antireflective in direction as the light waves interfere with one another and nullify each, rendering the eyes to stay dark (Bhushan, 2009). Biologically, they have evolved with these characteristics to save themselves from predators. The nanoscale arrayed structure they comprise is so effective that it has inspired researchers to design a new, highly advanced antireflective coating that could be useful for solar panels, smartphones, and tablet computers as its practical applications. Recently, Yano et al. fabricated a moth eye-inspired scalable, transparent mold coating by sputtering glassy carbon with an oxygen ion beam, with a noted low reflectance of around 0.1% in the visiblelight region which, when later converted to film, showed reflectance of about four times the mold (Yano et al., 2020). Likewise, Sun et al. replicated the patterns with enhanced antireflection and outstanding self-cleaning characteristics with an average reflection percentage of around 1.21% in the visible-light region (Sun, Wang, et al., 2018). Further imitating, researchers investigated the 3D nanocone antireflective structure with a vividly curbed reflection rate and improved light transmission (Cui et al., 2018). Interestingly, this 3D conical structure widens up the antireflection with polarization independence (Ingram & Parker, 2008; Vukusic & Sambles, 2003). A similar outlook presented by Liu et al., facilitates the applications of VO2 thermochromic smart windows with bioinspired TiO2 nanocone structured antireflective layer, making it a sustainable option for energy savage (Liu et al., 2020). These innovative window coatings can block NIR radiations and visible radiations from the sun, keeping the indoor

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temperature controlled with customization. Another substantial study presented a multifunctional nanocomposite film comprising KxWO3 and F-TiO as active materials. The designed windows exhibit strong nearinfrared, UV light-shielding ability with good visible light transmittance along with exceptional hydrophilic characteristics and high photocatalytic activity. The build film exhibits improved thermal insulation capacity than ITO and higher photocatalytic activity than titanium dioxide (P25), as recorded (Liu et al., 2016). Similarly, with varying degrees of success, different outcomes were presented by researchers but, the barrier to scalability and manufacturing of these optically active antireflective coatings still prevails (Potyrailo et al., 2007; Siddique et al., 2013). Other biomimetics with similar characteristics includes butterfly wings (Ahmed et al., 2018), mosquito eyes (Shin et al., 2019), and dragon wings (Chen et al., 2022), and cicada wings (Tanahashi & Harada, 2014) as well. Remarkably, inspired by the butterfly species having black wings known as Trogonoptera brookiana, the material designed by the researchers recorded to achieve ideal material characteristics with optical absorption nearest to 100% at a very low reflectance (170 degrees) incombination with low contact angle hysteresis for microliter droplets while exhibiting subsequent bouncing of picoliter water droplets. The study inherits the potential of closed-cell microstructured polymeric coatings with enhanced mechanical and pressure stability, tunable physical and chemical properties, responsive facile replication, and large-scale fabrication. These superhydrophobic surfaces can be further exploited as an antiicing surface evading ice accumulation on different structures especially on aircraft, to avoid accidents. In addition to the excellent water repellency they possess, these microstructures can impart ultra-slippery characteristics to the coating. Recently, a new robust ultra-slippery/slicky coating was introduced by Wyss institute, turning the glass into superglass. Again, the honeycomb-like structure holding lubricant in a tiny, container-like cavity imparts an equally slippery, nonetheless more durable, self-healing, and fully transparent coating. In the study, the researchers confined a collection of tiny spherical particles of polystyrene on a flat glass surface, similar to a ping-pong ball. Subsequently, they poured liquid glass on them until the balls were immensely half-buried in the glass. After that, as the glass solidifies, the beads are burnt away, leaving a network of honeycomb-like bowl structures. Later, coated with the same liquid lubricant is used in SLIPS (discussed in the later section). Further, on top of these adjustments, the honeycomb diameter could be tuned according to the substrate’s transparency (glass) to an extent. Together with these listed advances, the biomimetics of the honeycomb structure is anticipated to include the solution to the long-lasting challenges of crafting commercially valuable materials that repel almost everything with the application of additional properties.

2.7 Gecko-inspired reversible adhesive coatings Various animal species, including insects, spiders, and lizards bear the capability to attach and detach to or from the surfaces for their locomotion on vertical or ceiling walls. Bhushan et al. referred to this dynamic property as reversible adhesion or smart adhesion in surface coatings (Bhushan, 2007; Bhushan et al., 2006). Gecko (Tokyo gecko or Geeko gecko) became a widely investigated species recognized for its size, availability, adequate adhesion, and self-cleaning mechanism. Gecko feet characteristics have acquired the interest of researchers for a quite long time. The origin of the highly adhesive forces it holds could be attributed to the high-density hierarchical nanopillar morphology present in

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their foot. With this strong adhesion, the gecko foot is stated to possess both superhydrophobic and high adhesive force toward the water. As a shred of evidence, Liu et al. proposed polyimide films with gecko-like multiscale structures using anodic aluminum oxide (AAO) templates and reported to exhibit both the qualities as stated before—superhydrophobicity and strong adhesiveness. The resulting gecko-inspired polyimide film could be further proposed to be used as the mechanical hand swiping up a sufficient microliter of liquids (Liu et al., 2012). However, the same gecko further served as an inspiration to build a fast-responsive interface, generating and/or vanishing under external stimuli including force, heat, light, moisture, and even solvent. In nature, various organisms such as insects and crustaceans carry the behavior of utilization of a weak boundary layer in achieving a rapid response to external attack, climate change, or their own growth (Liu et al., 2022). For instance, the gecko has a remarkable capability of constructing a fast tail exclusion through a weak nonossification septum to avoid attack (Gilbert, Payne, & Vickaryous, 2013). Another researcher anticipated the molting of arthropods as the shedding of hormone-facilitated minerals, enzymes, or protein deviations between the old and new shells in tail development (Song et al., 2017). Stirred by this, Liu and his team developed an ultrasensitive robust mechano-optical membrane through a dye-stimulated weak-boundary layer (Liu et al., 2022). The membrane as designed exhibits a high total transmittance 75% with a long-term reversibly tuning of 2000 cycles without losing its firmness and opticality. As described above, extensive efforts have been made to recreate the gecko-inspired coatings that could be used in various diverse applications such as reusable tapes, biomedical bandages, great powered tires, highly efficient brakes, and rapid patch repairs on military vehicles.

2.8 Cicada wings-self-cleaning and antibacterial surfaces Among various insects showing tremendous novel functions, the cicada wing is one—primarily known for its self-cleaning and antibacterial properties. The cicada wings consist of hexagonal nanopillars with heights around 225–250 nm and inter distances of about 110–140 nm. Because of this nanoscale hierarchical structure and waxy coating of the pillars, a water-repellent film is developed on the surface and as raindrops fall over it, they fuse and roll off, naturally removing dirt, imparting the self-cleaning capability. Also, when bacteria land on the wings, their membranes are torn

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apart by the spikes that are the nanopillar it comprises. This property makes cicada wings the first known biomaterial capable of killing bacteria. Cicada wings show self-cleaning and antibacterial properties. For instance, Cicada or Psaltoda claripennis wing’s surface is notorious to be bactericidal against Pseudomonas aeruginosa cells particularly due to the surface nanostructuring rather than its surface chemistry (Ivanova et al., 2012). With a similar point of view, other researchers do successfully practice the same (Elliott et al., 2021; Hasan, Crawford, & Ivanova, 2013; Hasan, Webb, et al., 2013; Morco et al., 2021; Shahali et al., 2019). Furthermore, these wings are intrinsically transparent, showing high transmittance value over the visible light spectrum, resulting from the periodic conical structures covering the entire wing surface. They possess another exciting property known as the antireflecting surface (Chen et al., 2015; Han et al., 2019; Wang et al., 2020). Stimulated by this, a team of researchers fabricated TiO2-based antireflective structures (ARSs) using a simple, reasonable, and highly effective sol-gel process (Zada et al., 2016). The surface prepared exhibited highly receptive and excellent angledependent antireflective properties from normal to 45 degrees. The reflection property of these ARSs is changeable depending on the angle of incidence over the extensive visible light range. This inspiring attribute of an optimized gradient refractive index between the TiO2 and air via ARSs emerges a broader opportunity for those in solar cell industries. Furthermore, similar work was presented using SiO2 as well (Zada et al., 2017). However, researchers studying the extraordinary properties of cicada wings suggested that they could also be admiringly used as the inspiration for new self-cleaning, antiicing, antireflective, antifogging, and waterrepellent surfaces with innumerable applications. This type of surface inspired by cicada wings was widely used in medical and surgical equipment and packaging (Elliott, Wiggins, & Dua, 2021; Soni & Brightwell, 2022). Lastly, with this more generalized understanding of the properties and biomimetics of cicada wings, more artificial, functionalized, and bioinspired surfaces are possible in the future.

3. Recent developments 3.1 Color-changing film inspired by chameleon skin Inherently, chameleon skin comprises crystalline nanostructures made up of cellulose, which is majorly responsible for its color swapping capability to specified external stimuli, whether stretching, pressure, or humidity. Referring to this as the smart skin that changes its color in response to heat and

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sunlight, Dong and his team replicated the identical. They embedded photonic crystals in hydrogels, which change color accordingly on expanding or contracting as its inherited property (Dong et al., 2019). The resulting robustness of the material is subjected to a closer view of the species. They perceived the presence of photonic crystals in a few skin cells and reasoned to be responsible for the change of color with their movement from filled to unfilled skin cells, upon contraction or elongation. Stirred by this natural responsive behavior of photonic crystals, SASS was demonstrated using Fe3O4 and SiO2 nanoparticles together with pNIPAM. The prepared precursor solution is well embedded over mechanically elastic hydrogel in form of a colored and noncolored patterned array. This kind of sensitive film developed is under schemes with potential applications of camouflage, signaling, and anticounterfeiting. Another similar approach was conducted by Chou et al., where they formulated a stretchable electronic skin, or the e-skin as they termed it, which controls the color depending on the amount and duration of pressure applied (Chou et al., 2015). In this, a pyramidal-microstructured PDMS substrate is spray-coated with SWCNT and converted to a stretchable, transparent, and highly tunable resistive PS, which is later integrated with the ultrathin ECD into a circuit. This PS-integrated system with tactile sensing control could be a promising solution to interactive wearable devices, military applications, medical implants, and humanoids. Another group of researchers focused on the development of C3 which is Chameleon Cool Coatings (Gonome et al., 2018). They presented an artificial film inspired by the reptile, which functions depending on the radiation spectra. Interestingly, Chameleon carries two superimposed skin layers—one for the change in color according to the external stimuli and the other skilled them with thermal protection under various climatic conditions. To keep the surface cool, maximum solar energy reflectance is required. The researchers mimicked this in form of a single coating layer comprising two pigment particles (CuO and TiO2) as the superimposed layers with opposing properties: black and cool. One controls the color (visible light), and the other participates in reflecting a substantial proportion of sunlight keeping it cool. This type of coating is widely applicable in keeping surfaces at a lower temperature, such as surfaces exclusively exposed to sunlight, and in other innovative optical techniques.

3.2 Diamond-resembling carbon coatings Diamond-resembling carbon coatings (DLCs) are emerging efficiently with advanced properties of an amorphous carbon atom with other materials

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which include more or less flexibility, robustness, low friction, high conductivity, wear resistance, anticorrosiveness, optical transparency, and chemical inertness. Like graphite and diamond, DLC comprises properties of both. These coatings can be very hard like diamond and, at the same time, can project low friction as graphite. Additionally, the properties can be tuned accordingly by changing process conditions. They have been widely studied in the prevention of thrombus formation in cardiovascular applications with extremely low wear resistance (Hasebe et al., 2006; Hauert, 2003). Moreover, DLCs may find their other usages in biomedical applications and devices as well, which include the production of soft contact lenses, probes, and catheters, in the treatment of complex bone fractures, medical implants, and dental applications (Grill, 2003; Kobayashi et al., 1995). Under pressure, DLCs can impart hardness to the material with lower internal stress causing its slower delamination (Baba et al., 2020). As for biocompatibility, there is evidence of DLC’s successful performance in vitro and in vivo, projecting itself as a desirable biomaterial in the long run (Grill, 2003; Ma et al., 2009). Apart from the medical field, DLCs are commonly used in the transportation sector (Kolawole et al., 2020; Zia, 2020). However, the high cost and inadequate investigation limit its progression and commercialization on a large scale.

3.3 Slippery liquid-infused porous coating (SLIPS)—A special kind Conventionally, SLIPS includes porous, hierarchical structured substrates such as metal, plastics, optics, textiles, and ceramics, capable of immobilizing the liquid layer through their inherent roughness and chemically functionalized surfaces. They are inspired by the structure and functioning of the Nepenthes named pitcher plant. According to the Wyss Institute team at Harvard, these types of coatings are resilient to extreme climatic conditions, highly robust, nonsticking, superrepellent, anticoagulant, antifouling, and self-healing coatings and are widely accepted in different fields. Interestingly, these slippery surfaces can reach beyond the characteristics of the superhydrophobic lotus effect, repealing almost everything, liquid or solid—from bacteria, ice, water, oil, dust, barnacles, and other contaminants. Conceptually, superhydrophobic and SLIPS surfaces are similar except for the fact in the initial, the lubricating air is replaced, which is not in the case of the latter. However, for an ideal SLIPS, the surface must satisfy three criteria. First, the lubricating liquid should completely get absorbed on the surface. Secondly, the water behaving as the working fluid should not

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disturb the lubricating liquid layer. And finally, the involved liquids must not be miscible with each other (Smith et al., 2013). Recently, Geraldi and his team presented the idea of a double-sided SLIPS mesh, highly flexible and robust, enduring the natural flows (laminar and turbulent) for a specific period (Geraldi et al., 2019). Remarkably, Long et al. prepared a superhydrophobic surface infused with silicone oil, showing exceptional liquid repellency, anticorrosive, antiicing, and high durability compared to the original superhydrophobic surface initially prepared (Long et al., 2020). Before this, Barthwal et al., fascinated by the lotus effect and the slipperiness of the pitcher plant, developed a robust, highly durable long-term antiicing coating on the superhydrophobic aluminum surfaces using silicone oil as the lubricant fluid (Barthwal et al., 2019). Especially in medical applications, SLIPS emerges as a promising substitute with antibacterial, antiadhesive, self-healing, and antifouling properties (Habib et al., 2021; Leslie et al., 2014; Rao et al., 2021; Sotiri et al., 2016). As reported by various researchers and with the huge success on a laboratory scale, the SLIPS coating is foresighted to play a vital role in the future biomedical coatings accompanying the advancement in practical applications of various materials.

4. Summary The primary purpose of this chapter is to provide quick insight into the biomimetics in smart coatings, whether its lotus effect—superhydrophobicity, gecko adhesion, pitcher plant’s ultra-slipperiness, moth eyes’ optical activeness, or butterfly’s structural coloration are few of them. All these natural phenomena well existed, only needed attention and their respective relation to existing problems toward the direction of novel development. Henceforth, biomimetics research could be summarized as a revisit of classical science, understanding the simplicity and complexity of things in depth. In coatings, it is a field of observing the living organism and its remarkable functioning, acquiring knowledge, conceptualizing, and imitating the same to develop functional surfaces with superior properties. This emerging field has already gained the interest of researchers to a greater extent— understanding and exercising the natural solutions to practical applications. As applications of smart coatings include superhydrophobic, self-cleaning, antiicing, antifogging, antifouling, antireflective, and antimicrobial surfaces, objects with low drag force, ultra-slippery, super adhesive surfaces, in textiles

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and paint industries, in solar, optical as well as sensory-based devices, in biomedical devices and implants, to mention a few. Future biomimetics in smart coatings could also be addressed as the combination of two or more models from nature converting to multipurpose coatings. Few are reported, but intense investigation and unveiling of the hidden properties are still waiting. By understanding these, we may move toward a more sustainable future—from mother nature to humanity.

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Zhang, S., Ouyang, X., Li, J., Gao, S., Han, S., Liu, L., & Wei, H. (2015). Underwater drag-reducing effect of superhydrophobic submarine model. Langmuir, 31(1), 587–593. https://doi.org/10.1021/la504451k. Zhang, Y., Zhao, W., Chen, Z., Liu, Z., Cao, H., Zhou, C., & Cui, P. (2017). Influence of biomimetic boundary structure on the antifouling performances of siloxane modified resin coatings. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 528, 57–64. Zhao, D., Tian, Q., Wang, M., & Jin, Y. (2014). Study on the hydrophobic property of shark-skin-inspired micro-riblets. Journal of Bionic Engineering, 11(2), 296–302. https:// doi.org/10.1016/S1672-6529(14)60046-9. Zia, A. (2020). New generation carbon particles embedded diamond-like carbon coatings for transportation industry (pp. 307–332). https://doi.org/10.1016/B978-0-12-849870-5.00004-5. Zong, C., Hu, M., Azhar, U., Chen, X., Zhang, Y., Zhang, S., & Lu, C. (2019). Smart copolymer-functionalized flexible surfaces with photoswitchable wettability: From superhydrophobicity with “rose petal” effect to superhydrophilicity. ACS Applied Materials & Interfaces, 11(28), 25436–25444. https://doi.org/10.1021/acsami.9b07767.

CHAPTER 10

Self-cleaning coating materials Hina Sahara, Javeed Akhtarb, Muhammad Aamirb, M. Rehan H. Shah Gilania, and Uzma Jabeenc a

Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan Functional Nanomaterials Laboratory (FNL), Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur (AJK), Pakistan c Faculty of Basic Sciences, Sardar Bahadur Khan Women’s University, Quetta, Pakistan b

1. Basics of self-cleaning surfaces (SCSs) Nature offers best and attractive means to mimic the processes taking place in plants/animals for the development of technological devices. Butterfly wings and plant leaves such as cabbage and lotus are some common examples that show remarkable antifogging, hydrophobic, and anticorrosion properties. The SCSs received a lot of interest in the late 20th century due to various uses, from solar panels to windowpane cleaning, cement to fabrics, and now a great deal of research are being done around the world to achieve a high level of development in SC roofing surfaces, efficient and durable with enhanced optical properties. Aside from having extensive applications, this technology also has shown numerous benefits, such as reduced maintenance costs, low time-consuming, and decreased cleaning time (Dalawai et al., 2020; P. Liu, Niu, et al., 2019; Nakajima et al., 2001; S. Wu et al., 2019). The superhydrophobic surfaces are extremely waterproof and greater than 150° water contact angle was measured for such surfaces. Consequently, the water droplets round up and bits of dirt roll by the surface and are washed away. Such a phenomenon is frequently seen in nature on leaves, particularly on lotus leaves. Two important parameters, roughness and surface energy, control the wettability of the surface. The second approach is based on hydrophilicity, where water is distributed (covered) on a surface while SC is carried out through a process known as photocatalysis. When exposed to light, dirt/stain molecules degrade into simpler molecules such as CO2 and water. Hydrophilic coatings employ appropriate metal oxides, which have the unique function of chemical breakdown of complicated impurities via the solar cleaning procedure (Afzal et al., 2014).

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2. Self-cleaning surfaces from nature Several plants and insects have self-cleaning surfaces. On the basis of surface structures, self-cleaning surfaces may either consist of micro
/hierarchicaltype structures or single structure, which is arranged in a fibrous network (Chinese watermelon, ramee leaves). Generally speaking, superhydrophobic surfaces have a hierarchical structure. The horizontal hairs (Populus sp.) or vertical hairs (Alchemilla vulgaris) present on the surfaces of plant leaves are able to repel water. Salvinia molesta (water fern) possesses hydrophobic hairs that terminate in hydrophilic ends. They maintain an air layer, stabilize the air-water contact beneath the water, and allow “breathing” (Salvini’s paradox). Photosynthesis continues in Oryza sativa (rice), which is submerged, by an air layer floating on the surface of the superhydrophobic leaves. Leaf gas layers improve gas exchange, thereby allowing plants to tolerate floods. Melilotus siculus can delay the absorption of salt by decreasing the penetration of Na+ and Cl
, allowing for short-term survival while facilitating salt water entrance (Avra˘mescu et al., 2018). Similarly, rice leaves, rose petals, pitchers, taro, cricket wings, Namib desert beetles, water arrows, gecko feet, butterfly wings, fish scales, and mosquito eyes can exhibit flawless waterproof behavior. Fig. 1 presents an overview of certain naturally occurring hydrophobic, hydrophilic, oleophobic, and superhydrophilic self-cleaning surfaces (Avra˘mescu et al., 2018).

3. Mechanism of wettability in hydrophobic surfaces Wettability refers to a surface’s ability to be wetted by a liquid; hence, a surface with low wettability repels liquid. The contact angle of the liquid on the surface is an important characteristic for determining the wettability of a hard surface (Bhushan & Jung, 2011). Surface wettability is the very essential feature of hard surfaces, and it has various practical uses in industry, agriculture, and everyday life (L. Feng et al., 2014). On a solid surface wettability of a liquid is controlled by the chemical composition and geometric structure of the solid surface (X. Feng et al., 2004). The capacity of liquid droplets to spread on the surface increases when the surface free energy and surface tension of the liquid is altered. The increase in spread ability represents the adhesion strength of various fluids. Hydrophilicity is caused by high surface energy and low surface tension; conversely, hydrophobicity is caused by low surface energy and high surface tension.

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Fig. 1 An overview of surfaces and properties of some naturally existing self-cleaning surfaces. (Reproduced from reference Nishimoto, S., Bhushan, B. (2013). Bioinspired selfcleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity. RSC Advances, 3 (3), 671–690 with copyright permission from The Royal Society of Chemistry 2013.)

3.1 Young’s equation Traditionally, Young’s equation has been employed to explain the wetting patterns. The static angle of contact can be determined by the interfacial tension of the interfacial surfaces of solid, liquid, and vapor (SLV), according to the young equations given below in Eq. (1). Young’s equation used to represent the angle of contact is as follows: cos θE ¼ ðγ SV
γ SL Þ=γ LV

(1)

where SLV denotes the solid-liquid-vapor phases and γ is the surface energy. If the angle of contact is more than 90 degrees then the solid surface is said to be hydrophobic, and if the angle of contact is under 90 degrees, the surface is said

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to be intrinsically hydrophilic. When the contact angle exceeds 150 degrees, the surface is considered ultra-hydrophobic or superhydrophobic. Likewise, when the angle of contact is close to 0 degrees, the surface becomes superhydrophilic (Fig. 2). Indeed, by integrating a roughness equivalent to the low surface energy material barrier, a surface with an angle of contact with water greater than 150 degrees may be developed (Gowri et al., 2010; Whyman et al., 2008). The degree of hydrophobicity is determined by two factors: surface topography and surface chemistry. The surface topography also influences the degree of hydrophobicity. If a surface is already hydrophobic (low g LG GAS GAS

LIQUID g SL SOLID

g SL Cosq

q

q q < 10∞

g SG

q

Superhydrophilic

10∞ < q < 90∞ Hydrophilic

q q q > 90∞

Hydrophobic

q > 150∞ Superhydrophobic

Fig. 2 Schematic representation of a liquid drop on the solid surface. (From reference Padmanabhan, N. T., John, H. (2020). Titanium dioxide based self-cleaning smart surfaces: A short review. Journal of Environmental Chemical Engineering, 8 (5), 104211 with copyright permission from Elsevier 2020.)

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Fig. 3 Cassie-Baxter and Wenzel model.

surface energy), the surface roughness increases resulting in higher hydrophobicity. Moreover, adhesion/nonadhesion and reduced surface droplet behavior are related to superhydrophobicity. The gliding angle is used to assess rollability (or tilt angle). This is the lowest angle at which the surface may be inclined toward the horizontal plane for a drop of water to roll of. Conversely, rollover behavior is associated with contact angle hysteresis. The contact angle hysteresis is the variation between an increasing and decreasing angle of contact measured on a moving contact line with increasing and decreasing droplet volume, respectively. The hysteresis in the contact angle is the major factor that makes it difficult to understand the wetting phenomenon. Two different angles of contact exist on the same (even smooth) surface, the minimum and maximum of which are, referred to as receding and advancing contact angles respectively (Wong et al., 2013). Moreover, for improved evaporation, the surface should have a small receding contact angle. A wide advancing angle of contact is required to restrict the spread of droplets. Surfaces with smaller receding contact angles and large advancing contact angles are effective for minimizing droplet movement while transporting droplets and manipulating particles. As a result, developing a theoretical model that can anticipate receding and advancing contact angles based on surface properties such as model size and hydrophobicity is critical for superhydrophobic surface design for specialized applications (Lafuma & Qu
er
e, 2003). The rough surface water contact angle (WCA) is explained by two models Wenzel and Cassie-Baxter (Fig. 3).

3.2 Cassie-Baxter model The classic Cassie-Baxter equation (Eq. 2) predicts a contact angle on a superhydrophobic surface based on a solid areal fraction.

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cos θ1 ¼ φ cos θi + φ
1

(2)

where θ1 represents the equilibrium angle of contacts on a partially wetted rough surface and θi represents the equilibrium angle of contacts on a smooth surface, and φ is the solid fraction. The usefulness of the classical model, however, has been disputed, since it is unable to explain the distinctive variety of apparent angles of contact recorded in practical implementation that may range from θR to θA on the surface ( Jiang et al., 2019). Furthermore, the standard model of Cassie-Baxter cannot explain various angles obtained on surfaces with the same solid areal percentage but different morphological features, such as linked pore structures vs distinct pillars. Further analysis of the design or dynamics of a local line of contact at the visible droplet border explains such discrepancies (McHale, 2007).

3.3 Wenzel model Wenzel’s study shows how a rugged surface can affect the actual contact angle between a surface and a liquid. Many examples are given to show the difficulty of rough surface wetting because of the large contact visible angle. The angle of contact derived by Wenzel is expressed as. cos θw ¼ r ðγ SV
γ SL Þ=γ LV ¼ r cos θ

(3)

where θw is the angle of contact of the water droplet on a rough or uneven surface, θ the contact angle on a corresponding smooth or plane surface, r represents the surface roughness coefficient, and θw is stated in terms of the cosine of Young’s angle of contact. Wenzel equations explain that the roughness of the surface has a substantial impact on the surface wettability and is projected to increase by up to 90 degrees. However, when θ > 90 degrees, the air bubbles take up the position in the coarse channel and prevent water droplets from entering the hydrophobic column, thereby making the surface superhydrophobic (Cassie & Baxter, 1944; Wenzel, 1936). On hydrophobic rough surfaces, both the Cassie-Baxter and Wenzel models display significant angles of contact. But the contact angle hysteresis was substantially nonidentical in both of these circumstances. Wenzel model shows that the liquid phase enters the holes, voids, and molds on the solid surface and gets stuck to it which results in the comparatively higher value of contact angle, where larger forces are predicted to pull the stuck droplet off the surface. Cassie-Baxter model, on the other side, demonstrates limited contact between liquid and solids, resulting in reduced hysteresis. As a result,

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the droplets travel at a little slope. This model is likely to be a superior technique to create a superhydrophobic surface for a better SC choice. The higher the surface roughness on the Wenzel model, the greater the hysteresis and contact angle. Nevertheless, if the coefficient of roughness approaches a certain value, the gaps and cavities on the surface cannot be filled by liquid droplets, and thus, transition takes place from Wenzel to Cassie-Baxter model, where multiple air packs are trapped underneath the liquid droplets. In this scenario, increasing the roughness with a larger contact angle can likewise greatly reduce hysteresis (Nakajima et al., 2001; Ragesh et al., 2014; Sas et al., 2012).

3.4 Recent advances in superhydrophobic coatings Scientists have succeeded in mimicking superhydrophobicity by closely observing and examining naturally occurring surfaces. The existence of distinctive waxy structures that may readily trap air underneath the water droplets is also included in such textures. Researchers can now replicate and improve such surface patterns utilizing modern nanofabrication, lithography, and wet chemical methods. Synthetic materials with an extremely low affinity for water are frequently explored. However, perfluorinated chemicals generate plenty of environmental issues. Furthermore, most superhydrophobic coatings and treatment formulas are either not environmentally friendly to sustain large-scale manufacturing or too costly to be common industrial practice. However, recently, significant attempts have been undertaken to create superhydrophobic coatings from natural, biodegradable, and nontoxic food or medicinal ingredients utilizing ecologically friendly solvents or water, chemical procedures, and even solvent-free casting procedures, reducing the impact on environment (Bayer, 2020). 3.4.1 Superhydrophobic coatings from natural waxes Natural waxes are a low surface energy hydrocarbon source that is also ecologically safe for superhydrophobic coatings. One of the greatest evidence of super hydrophobia can be found in the lotus (Nelumbo nucifera), a semiaquatic plant; the lotus leaf surface is comprised of waxy crystals with a hierarchical structure that covers the surface micrometers and nanometers in length. These hierarchical structures are peculiar to lotuses and some plants, particularly tubular, high-density nanoscale structures. Wax, on the other hand, is frequently found as a coating agent in plants with the main hydrocarbon makeup. Some plants, such as palm trees, have very thick wax coverings that

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may be easily removed to manufacture products on an industrial scale (e.g., carnauba wax from Copernicia prunifera). This finding has sparked a lot of interest in the application of this natural waxy substance in the production of superhydrophobic coatings (Torun et al., 2019). 3.4.2 Superhydrophobic coatings from proteins Recent studies have revealed that coated proteins can exhibit consistent surface characteristics. The lysozyme phase transition is one of the most promising techniques. Phase-transited lysozymes, which are composed of amyloid-containing microfiber networks, can tightly attach to metals, oxides, semiconductors, and polymers. A coating of this type can improve the surface’s corrosion resistance. Based on specific chemical interactions, the protein layer may also impart positive charges and enhance carbonhydrogen bonds on the surface, promoting the creation of a variety of functional building blocks such as small molecules, colloids, polymer brushes, and biomacromolecules (Z. Wu & Yang, 2015). The individual collection of certain protein and amino acid supramolecules can come up with an effective way to create several nanostructures/ microhierarchies that can mimic the surface structure of a lotus leaf. Chiral molecules and chiral nanostructures are recognized to be well accepted by biological systems. These modifications can be carried out on the surfaces to turn certain proteins into nonwet layers. Gao et al., for example, created a superhydrophobic layer on the surface by directly depositing the passingphase lysozyme protein. The tape peel test showed that superhydrophobic coatings are heat resistant and mechanically stable and have heat resistance over the 196–200°C temperature range. This surface was termed a superhydrophobic proteinaceous substance. They claimed that such surfaces exhibit the ability to stimulate protein crystallization and that the assembly of protein crystals can be facilitated over large areas with very low protein concentrations (Gao et al., 2016). Liu et al., inspired by the Stenocara beetle which can gather water from wet air in the Namib Desert, constructed a coating for the controlled growth of a soy protein having zeolitic imidazolate crystal structure. On the soy protein coating, micro-/nanohierarchical crystals were formed by varying the precursor concentration and crystal growth period. Because of the different sizes of such crystals, the surface was hydrophobically modified using stearic acid to generate superhydrophobic/hydrophilic patterns in support of effective fog water collection (H. Liu, Xie, et al., 2019).

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3.4.3 Superhydrophobic coatings from fatty acids Fatty acids are organic macromolecules composed of hydrocarbon chains that terminate with carboxylic acid groups. Lipids are primarily composed of fatty acids and their derivatives. Individual fatty acids differ greatly in terms of the degree of saturation and length of their hydrocarbon chains, which determines their related physical characteristics (e.g., flow rate and melting point). Furthermore, practically all fatty acids are hydrophobic in nature and insoluble in water. Stearic acid, for example, is commonly utilized in superhydrophobic coatings ( J. Song et al., 2013; Wang et al., 2018; Wei et al., 2017). Atta et al. also modified nanoparticles of calcium carbonates with different fatty acids and epoxy-based fatty acids. The monodispersity of CaCO3 nanoparticles in epoxy polymer matrices can be increased by epoxidized oleic acid (EOA). The coatings obtained a maximum water contact angle of 160.5 degrees and a much reduced slide angle of 1.3 degrees. Epoxy group CaCO3/EOA nanoparticles may be chemically reacted with epoxycured coatings to yield rough epoxy nonwetting coatings for stainless steel (Atta et al., 2016). Hu and coworker used stearic acid, oleic acid, and precipitated calcium carbonate (PCC) to create superhydrophobic coatings. They observed that coatings formed of PCC modified by oleic acid had around 2 degrees sliding angle. PCC treated with stearic acid, on the other hand, had a sliding angle of around 10 degrees (Hu & Deng, 2010). 3.4.4 Superhydrophobic coatings from cellulose and its derivatives Cellulose is a notable regular polymer that has been used broadly in a variety of utilizations, such as textile materials, domestic devices, cosmetic agents, and clinical purposes. Cellulose has good mechanical qualities, is radiation resistant, and has high stability (Almeida et al., 2018; Du et al., 2019; Fu et al., 2019; Kamel & Khattab, 2020). The polysaccharide cellulose, a plant’s most essential skeletal constituent, is a practically limitless polymeric raw material with fascinating structure and characteristics. The highly functionalized, linear stiff-chain homopolymer is distinguished by its chirality, biodegradability, hydrophilicity, and extensive chemical modification capabilities, and it is generated by the repetitive attachment of D glucose building blocks and the development of diverse semicrystalline fiber morphologies (Habibi et al., 2008). The most prevalent polysaccharide in nature is cellulose, which is composed of b-(1,4)-linked anhydroglucose units (Geissler et al., 2013). By derivatizing the hydroxyl groups with different functional groups, the

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hydrophilic character of cellulose may be modified. Cellulose can be converted into hydrophobic compounds by esterification or etherification, silylation, fluorination, or by binding with polymers. These cellulose derivatives, on the other hand, had just a small amount of hydrophobicity and were still very water-adhesive. A kind of hydrophobic cellulose derivative is long chain of fatty acid esters of cellulose, with a structure that is comparable to other cellulose derivatives. These esters have been used in the production of water-repulsive aerogels and nontoxic biodegradable plastics. Geissler et al. (2013) used micro- or nanostructure cellulose fatty acid esters (cellulose stearoyl ester, CSE) produced through nanoprecipitation to fabricate durable, superhydrophobic, and SC films. Different coating techniques could be used to coat superhydrophobic films/coatings on a variety of surfaces with nonuniform shapes. The degree of substitution of CSE synthesized by the authors was 2.95, which suggests that stearoyl groups replaced almost all hydroxyl groups in the cellulose backbone, making them soluble in various organic solvents (Zhang et al., 2019).

4. Approaches for growing durable self-cleaning surfaces 4.1 Dip-coating technique This is simple in operation and an effective way to grow uniform and conformal coatings of hydrophobic materials on large number of substrates. Ebert and coworkers reported the use of this method to incorporate ZnO on indium tin oxide (ITO) substrates to generate clear superhydrophobic coatings (Ebert & Bhushan, 2012).

4.2 Electrospray/electrospinning coating This method is popular due to its automation and large-scale production of hydrophobic surfaces. The electrostatic spinning method is a recently developed technique for the fabrication of micro-/nanofibers (Ke et al., 2014; Kim & Kim, 2011; Verreck et al., 2003). Adjusting the concentration or ratio of the spinning solution allows it to produce polymer fiber with various morphologies. Therefore, this process is commonly used for preparing superhydrophobic surfaces and is particularly well suited to large-scale applications. Zhu et al. (2006) created a lotus-leaf superhydrophobic biofilm based on a new composite structure of perforated nanofibers and microspheres. The perforated microspheres enhanced surface roughness, resulting in superhydrophobicity; on the other side, nanofibers are intertwined to

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create a three-dimensional network, linking the perforated microspheres together and strengthening the composite thin layer. Moreover, by altering the initiating concentration of the solution, PS films with various morphologies could be made. This approach proposed a new and efficient method for preparing SC surfaces on a wide scale at a low cost. Superhydrophobic surfaces with ZnO nanostructures were reported by Ding et al. (2008). The ZnO fibrous films were superhydrophilic before FAS coating, with a water contact angle of 0 degrees. The fibrous FAS-coated ZnO films, on the other hand, displayed obvious superhydrophobicity with a sliding angle down to 5 degrees and an angle of contact of water up to 165 degrees and an after-FAS coating.

4.3 Chemical etching On brass surface, superhydrophobic coating is produced by using chemical etching technique that combines two acids, namely, HNO3 and HCl. The resistance of this coating is determined by thermal, mechanical, and chemical stability tests, and the coating also exhibits self-cleaning and antifog properties (Xing et al., 2017). Here the mechanical stability and qualities of the yellow superhydrophobic SC can be readily systematized by adjusting the immersion time in HNO3 and displaying the self-cleaning’s mechanical stability and qualities (Ferna´ndez et al., 2017).

5. Applications of self-cleaning surfaces 5.1 Blood repellent The high tendency of blood to form thrombogenesis, or the formation of blood clots as it meets foreign surfaces, is well known. This is all contributed by the intrinsic weather pathways of the blood, stimulation of coagulation, and platelet activation (Nokes et al., 2016). The coagulation action makes the blood highly adherent to the substrate, which makes adhesion to surfaces easier. If blood coagulation develops in vivo on medical equipment, it may have serious adverse effect leading to a deterioration of the patient’s state (Hall, 1994). The adherence of blood to surfaces, such as garments and bandages, may cause waste contamination and even diseases (Schellenberger et al., 2019). The main worry in medical textiles is the exposure to contaminated blood and this can become source of spread of bacterial or viral infections ( J. Liu et al., 2020). As a result, it is not only desirable but also vital to create excellent blood-resistant surfaces. Superhydrophobic surfaces, such as

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enhanced transfer of heat in microfluidics, and ice prevention have previously been investigated and employed in several sectors. Superhydrophobic surfaces are highly capable of repelling blood, according to new studies.

5.2 Solar cell and water harvesting Photovoltaic (PV) is an electricity production system that converts solar energy into electrical energy directly from solar radiations. Photovoltaic power generating consists of multiple cells endowed with photovoltaic active material on solar panels (G. He et al., 2011). Installation of such panels on a microscale is becoming increasingly popular worldwide. The efficiency of such installed solar devices is affected by contaminants in air/moisture/ dust particles. Dry winds and weather sweep dirt into the atmosphere and deposit it on the surface of solar panels. This reduces power of solar panel to absorb solar radiations. According to studies presence of 4 g of dust per square meter decreases the conversion of solar energy by 40% (Son et al., 2012). Therefore, a high quality antidust hydrophobic coating layer on these panels is highly desirable.

5.3 Fabrics and textiles Superhydrophobic coatings are also used in the production of water-repellent fabrics and textiles. Micro- or nanostructured and low-surface energy fabrics can be woven to create superhydrophobic textiles (Drelich & Marmur, 2014). A combination of strong water repellency in fabrics and a low tilt angle of water droplets deliver superhydrophobic and SC qualities to woven textiles. The wetting qualities of textiles can be adjusted while maintaining warmth, comfort, softness, and the unique characteristics of the fabric, including crease-free, nontoxicity, and life span. Ma et al. (2005) synthesized poly(styrene-block-dimethylsiloxane) and poly(caprolactone) (PLC) copolymer fibers using a combination of chemical vapor deposition and electrospinning. PCL electrospinning was used together with chemical vapor deposition of a thin layer of PPFEMA (hydrophobic polymerized perfluoroalkyl ethyl methacrylate). Both oleophobicity and hydrophobicity were present in the PPFEMA-coated PCL composite. With reduced diameter, it was observed that the hydrophobicity of bead-free high-density fibers, which characterizes the wetting feature, progressively increases. Firefighting, military, and other professions related to defense are among the new application areas for superhydrophobic and superoleophobic fabrics. The performance, assessment tests, mechanical and physical qualities,

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durability, personal comfort, and chemical resistance connected with superhydrophobic textiles and fabrics, on the other hand, define their utilization in difficult real-world settings. Most countries are worried about military needs to prevent personnel from becoming soaked by water or chemicals when defending or conducting operations in chemical or biohazardous areas (Cloutier et al., 2015).

5.4 Antibacterial coatings The antibacterial coatings are becoming very popular thanks to the recent progress in biotechnology of materials and a growing awareness of the environment. The tools and strategies developed by microbiology today offer a wide variety of opportunities to create antibacterial surfaces. As a result, developing a targeted release approach is critical for controlling the biological activity of the layer (Cloutier et al., 2015). Silver nanoparticles and their derivatives are the most prevalent form of antibacterial coatings. Silver nanoparticles (AgNPs) have been shown to have strong antibacterial characteristics, capable of killing a wide range of microbes. AgNP’s biological activity can be boosted by lowering particle size and changing its surface area. However, mixing AgNP with other oxides, such as the ferrite nanosilver composition, is a newer technique for increasing the antibacterial activity of AgNP. Heinonen et al. (2013) examined the antibacterial activity of a silver nanoparticle-incorporated superhydrophobic coating at various pH levels. W. He et al. (2016) used a quaternary ammonium salt of polyurethane with a polyethylene glycol/lysine antifouling agent to create a surface-activated antibacterial contact. The developed covering demonstrated superior biological capabilities for microbial growth of clay courts. A reversed surface structure is interspersed into the antibacterial top layer and the antifouling sublayer of the coating. The antibacterial effect of the top coating and the antifouling qualities of the bottom coating enhance the coating’s surface. Contact with this antifouling layer inhibits the development of both Gram-positive and Gram-negative bacteria.

5.5 Anticorrosion Currently, compounds containing chromium are used as corrosion-resistant agents, but they harm people and environment. Another method to improve its anticorrosion properties is to produce a superhydrophobic layer directly on the metal surface. The presence of an air layer between the solution and the substrate surface, which hinders the corrosive ions migration, is the

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primary mechanism for the creation of corrosion-resistant materials. Superhydrophobic coatings have been employed frequently nowadays to increase their corrosion protection on different solid substrates such as Fe, Al, Zn, Ti, Cu, and alloys (Asmatulu, 2012; Ishizaki et al., 2010). Xu et al. (2011)) produced a corrosion-resistant surface of superhydrophobic magnesium alloy using an electrochemical treatment process with fluoroalkylsilane.

5.6 Medical industry In the medical industry, superhydrophobic polymeric nanocoating has been used for dental, drug delivery, and SC practices. Yohe et al. (2012) used a poly(glycerol monostearate-co-caprolactone) hydrophobic polymer dopant to produce 3D superhydrophobic poly(caprolactone) electrospun meshes. These superhydrophobic meshes were used to monitor the distribution rate of tunable drug release activity by displacing air. The air layer that has become entrapped inside superhydrophobic meshes displayed long-lasting durability and efficiency, targeting cancer cells in vitro for more than 60 days in the presence of serum. High aspect ratio of TiO2 nano tubes can act as SC with high sensitivity immunoassays for isolating antibodies for target antigen (Y. Y. Song et al., 2010). Because of photocatalytic properties of TiO2, immune sensors made of TiO2 are reusable and SC. Because of its specific photocatalysis and SC properties, TiO2 has an antibacterial impact (Dastjerdi & Montazer, 2010; Markowska-Szczupak et al., 2011). The incorporation of sulfur into TiO2 improved the photocatalytic property. Sulfur-doped TiO2 showed higher photo-induced superhydrophobicity and photocatalysis than commercial items. These films were commonly used in the UK hospitals and discovered to be an effective facilitator for destroying bacteria (Escherichia coli) using light sources.

5.7 Antiicing protection One of the desirable features of the superhydrophobic coating is its ability to drastically reduce the impact of ice storms and airplane ice sheets. The terminology used to describe such type of layer is ice phobia. There are a number of processes that control the formation/inhibition of ice phobia including water-solid surface interactions and parameters for ice adhesion. The superhydrophobic coating exhibits reduced ice adhesion and low interface energy. This is also required to quickly clear ice using mechanical external forces. A full discussion of anti-icing protection can be found in numerous recent review publications that provide numerous procedures

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for the creation of ice-phobic coating. Most of the approaches proposed are concerned with the reducing forces of the adhesive forces between ice and the foundation (Y. Wu et al., 2021). Aircraft and submarines frequently confront time-consuming obstacles. The superhydrophobic layer creates a gaseous barrier between water and the coated surface, reducing interaction between the water and solid substrate (Kavalenka et al., 2015; Sethi & Manik, 2018).

5.8 Antireflective and transparent coatings Antireflection or optical transparency is an essential requirement for many latest optical instruments such as OLEDs, LCDs, solar cells, touch screens, building windows, optical mirrors, eyeglasses, optical windows, and lenses for highly sophisticated electronic devices. Most of these devices also need SC and water repellency properties. Then there may be a need to include superhydrophobicity with antireflection or optical transparency ability on the same given surface. Light scattering often increases due to surface roughness and, as a result, decreases transparency. To create an effective antireflective or optical transparent superhydrophobic coating, the surface roughness must be adjusted or controlled. To obtain proper material optical transparency, the surface roughness must be lower than the incident light’s wavelength (350–750 nm) (Teshima et al., 2005). In this regard, Hozumi and Takai (1997) produced a transparent hydrophobic layer by oxygen plasma treatment utilizing PET (polyethylene terephthalate). They initially employed a bonded radio frequency dilution system to build a nanotextured PET substrate and then employed an organosilane precursor in a decreased temperature chemical vapor deposition technique to minimize the energy of the surface. The resulting PET (polyethylene terephthalate) substrate provides more than 90% transparency and 150 degrees WCA. Bravo et al. (2007) produced a transparent superhydrophobic film by giving the silicon film a surface roughness in the 30–100 nm range by sublimation of aluminum acetylacetonate Al(C5H7O2)3 during calcination and coating with heptadeca fluoro decyl trimethoxysilane.

5.9 Oil water sorption and separation Use of water-resistant compounds such as long-chain alkyls to cover a certain surface and minimize its energy not just offers it superhydrophobicity but also super-oleophilicity (Pan et al., 2008). Water can flow out of a copper net that has been changed to be superhydrophobic and then tilted,

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but chemicals such as petroleum ether, hexane, diesel oil, and toluene do not resist like water and can pass through the network. The mixture of oily material and water can therefore be separated with greater than 99% efficiency. Superhydrophobic nanoporous polymers have been shown to be particularly efficient organic chemical absorbents. They can absorb items such as oil and leave water behind. They might be used to cope with oil spills at sea (Crick & Parkin, 2010).

6. Conclusions Because of their unique features and possible industrial uses, nature-inspired superhydrophobic surfaces have drawn much interest. The self-cleaning surface idea has several advantages in a variety of businesses. There are a large number of applications of self-cleaning surfaces such as blood repellent, antiicing, anticorrosion, antibacterial coatings, antifogging, and antireflective and transparent coatings. Self-cleaning surfaces can be grown by a wide range of coating techniques.

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CHAPTER 11

Antimicrobial coatings based on polymeric materials Sandesh G. Sanjeevaa, Sukesh Kumar Bajireb, Rajesh P. Shastryb, and Renjith P. Johnsona a

Polymer Nanobiomaterial Research Laboratory, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India b Division of Microbiology and Biotechnology, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, Karnataka, India

1. Introduction The current decade witnessed that global health is always under threat due to the unexpected and rapid emergence of diverse pathogenic microorganisms such as bacteria, fungi, and viruses (Houwenhuyse et al., 2018). The microbes such as bacteria and fungi can form biofilms on various biotic surfaces (wounds, skin, lungs, urinary tract) as well as abiotic surfaces (food, textile, medical devices) leading to various infectious diseases, which are clinically challenging (Lebeaux et al., 2013). Undoubtedly, the development of innovative therapeutic approaches is necessary to eradicate biofilm formation, microbial infections, and minimize environmental hazards such as spoilage of food crops, various food materials, textiles, and disease conditions in domesticated animals. To this end, polymer-derived functional coating materials have been developed to inhibit the action of various microbes and to eradicate biofilm formation (Salwiczek et al., 2014). Notably, various polymeric systems have evolved, which can effectively destroy the microbes with excellent antimicrobial performance. The antimicrobial polymer coatings are functionally excellent, and can inhibit the maturation of microorganisms on various surfaces and neighboring habitats (Mun˜oz-Bonilla & Ferna´ndez-Garcı´a, 2012). These antimicrobial polymers can be classified based on the inherent antimicrobial activity or the activity acquired by incorporating an antimicrobial agent of interest into the polymer systems ( Jain et al., 2014). The antimicrobial polymers composed of quaternary amine or zwitterionic group functionalities, hydrophobic moiety, chitosan, and halamines demonstrate excellent antimicrobial activity as a result of appropriate chemical modifications (C-Cruz et al., 2021).

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On the other hand, polymeric systems with antimicrobial activities can also can be fabricated through the incorporation of antibiotics, antibacterial peptides, graphene oxide, and metal ions or metallic nanoparticles. These polymer systems also offer biocompatibility and stability toward the fabrication of bio-friendly antimicrobial coatings (Qian et al., 2020). Based on the source and origin, polymers can be generally categorized into natural polymers, synthetic polymers, and hybrid or semisynthetic polymers (Kristufek et al., 2017). The design and synthesis of various antimicrobial polymers can be achieved through several modern polymerization methods [reversible addition-fragmentation chain-transfer polymerization (RAFT), ring-opening polymerization (ROP), atom transfer radical polymerization (ATRP), and other macromolecular synthesis tools (click chemistry, Schiff base reaction, amidation, and esterification) (P. Li et al., 2012; Song & Jang, 2014)]. Besides, to obtain antimicrobial activity, the postpolymerization functionalization of polymers toward polycations, quaternary amines, halogen moiety, and zwitterionic functionalities were also reported (Chouirfa et al., 2019). Regardless of the type of polymer system adopted for coating fabrication, the polymer should be biocompatible with an ideal molecular weight and appropriate surface charges. These factors are crucial in microbial adhesion to the surface, and the subsequent antimicrobial action (Timofeeva & Kleshcheva, 2011). Besides, an ideal antimicrobial polymeric coating should hold a long-term antimicrobial activity, with action against a broad spectrum of pathogens, and high stability to various environmental conditions. The possibility of reinducing the antimicrobial activity for a coating is an additional advantage since it might minimize the manufacturing cost and be more affordable (Arora et al., 2013). Mechanistically, antimicrobial polymeric coatings act through either passive action by preventing microbial attachment or through an active action by destructing the adhered microorganisms (Irshad et al., 2020). The passive defense mechanism is comparably ineffective due to the lack of biocidal agents and the possibility of biofilm formations (Balaure & Grumezescu, 2020). The active defense mechanisms are also affected by the dead microbes and debris which shields the functional groups. This can contribute to microbial maturation, thereby causing a significant reduction in the antimicrobial efficacy (Hetrick & Schoenfisch, 2006). Therefore, to overcome the limitations, dual-functional (active and passive) antimicrobial polymer coatings emerged by adding antimicrobial agents into the antifouling polymer through a physical or chemical bonding. Notably, the antimicrobial

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performance of the polymer coatings can be improved through the synergistic action of both active and passive mechanisms. The antimicrobial agents bind to microbes and in turn act as a killing agent, whereas antifouling polymer repels and leaches out the microbes from the surfaces (Maan et al., 2020). Undoubtedly, the antimicrobial polymer coating has a wide range of applications in medical implants (Acosta et al., 2020; Hoque et al., 2019; Xu et al., 2018), food industries (Galotto et al., 2015), as well as in textile industries (Luo et al., 2017). In this chapter, we focused on recent developments in synthesizing antimicrobial polymers, and fabrication of coatings on various surfaces. The mode of antimicrobial action and its applications in various domains were also addressed.

2. Antimicrobial polymer synthesis and coating fabrication strategies Antimicrobial polymers are a promising class of biocidal materials, which contains cationic, hydrophobic functionalities, or antimicrobial agents, which are capable to rupture microbial membranes and inhibit the microbial growth. The antimicrobial properties can be induced by appropriate functionalization of the polymers through specific postpolymerization modifications. As mentioned earlier, the synthesis of antimicrobial polymers can be achieved through various polymerization techniques, by combining other reactions such as click-chemistry, Schiff-base reaction, amidation, and esterification reactions. Notably, these synthesis and functionalization strategies significantly contributed to eradicate antimicrobial resistance with minimal toxicity. The functionalized polymers can be converted as an antimicrobial coating in various substrates through different coating procedures, and these coatings have potential applications in food and textile industries as well in other biomedical domains. In the following section, we briefly discuss the synthesis pathways of antimicrobial polymers and their coating fabrication strategies.

2.1 Structurally modified polymers Antimicrobial polymers can be generally classified as inherently antimicrobial polymers, structurally modified (e.g., quaternization) antimicrobial polymers, or antimicrobial agent (antibiotic drug or peptides)-loaded polymers. Notably, the majority of inherent antimicrobial polymers possess only limited activity in their original form. Thus, it is necessary to synthesize a rationally designed polymer with structural features for better antimicrobial

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activity. The versatility in the chemical structure of various polymers endows antimicrobial activity against a broad spectrum of pathogenic microorganisms. The properties such as electrostatic interaction, chelation, and hydrophobic effect of these polymers contribute to their excellent antimicrobial behavior. For instance, Hoque et al. developed a charge-switchable, antibiofilm coating from hydrophobic as well as cationic polyethyleneimine derivatives for preventing catheter-associated infections. Herein, the N-methyl polyethyleneimine (PEIs) were synthesized from branched and linear PEIs through Eschweiler-Clarke methylation. A series of PEI derivatives (PEI with cationic charge, both cationic and zwitterionic groups, and with fully zwitterionic PEI) were synthesized. In a typical synthesis, the quaternization of N-methyl PEI was achieved by reacting with N-hexadecyl-2bromoethanoate and N-hexadecyl-2-bromoethanamide to obtain cationic quaternary PEIs (QPEIs) containing ester and amide groups in the hexadecyl chain respectively. Subsequently, the hydrolysis of QPEIs side chain was achieved by varying the degree of quaternization resulting in the formation of the above PEIs with cationic and zwitterionic nature. Finally, these polymers were dissolved in chloroform and coated on the surface through brush or dip coating strategies (Hoque et al., 2019). An antibacterial coating based on 3-methylallyloxy-5,5-dimethylhydantoin monomer containing N-halamine moiety on cotton fabrics was developed. Initially, the methacrylate monomer was synthesized with an N-halamine moiety by reacting 5,5-dimethyl hydantoin, trimethylamine, and methacryolyl chloride in dichloromethane. Subsequently, an antibacterial coating on cotton fabric was achieved through mist polymerization. Herein, an aqueous solution of N-halamine functionalized methacrylate monomer crumbled onto the ammonium ceric nitrate, which activated the cotton surface and heated for 4 h at 80°C followed by multiple washing in distilled water and vacuum drying at 60°C for 24 h. Finally, functionalized cotton fabric was chlorinated by dipping it in chlorine bleach under stirring (Luo et al., 2017). Dı´ez-Pascual et al. developed an antimicrobial packaging film, based on poly(butylene adipate-co-terephthalate) (PBAT) and chitosan nanofibers. Herein, the chitosan nanofibers were obtained through electrospinning, by dissolving chitosan in TFA mixed with glutaraldehyde. The PBAT was reinforced with chitosan nanofibers, and the electrospun nanofibers were fabricated through a solution casting technique, followed by vacuum curing (Dı´ez-Pascual & Dı´ez-Vicente, 2015).

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Haldar group has developed a UV curable coating based on quaternary benzophenone-based ester (QBEst) and quaternary benzophenone-based amide (QBAm) active against bacteria, fungi, and influenza viruses. Toward the fabrication of QBEst and QBAm, initially (4-(6-(bromohexyl)-oxy)phenyl)(phenyl)methanone and (4-(6-(dimethylamino)-hexyl)oxy)phenyl) (phenyl)methanone were obtained through Williamson ether synthesis. Parallelly, decyl 2-bromoacetate and 2-bromo-N-decylacetamide were also synthesized by activating bromoacetyl bromide with 1-decanol and 1-decylamine in the presence of potassium carbonate. Finally, QBEst and QBAm were obtained by quternizing the tertiary amine decyl 2-bromoacetate with decyl 2-bromoacetate and 2-bromo-N-decylacetamide respectively. The QBEst and QBAm were coated on various polymer sheets (cotton, polyurethane, poly(vinyl chloride), and polypropylene) by dropcasting method followed by UV irradiation for 10 min (Ghosh et al., 2020). Xu et al. developed a multilayered self-polishing, antifouling as well as antimicrobial coating based on dextran aldehyde and carboxymethyl chitosan. In a typical synthesis, the stainless steel activated in piranha solution to incorporate hydroxyl group and polydopamine (PDA) layer was subsequently deposited. The resulting steel-PDA surface was dipped in aqueous dextran aldehyde for 30 min, and rinsed with distilled water. Subsequently, the steel-dextran layer was further immersed in aqueous carboxymethyl chitosan for 30min, to form the first bilayer through a Schiff-base reaction. In the following, different layers on the substrates were obtained through the layer-bylayer (LBL) coating (Xu, Liu, Pranantyo, Neoh, & Kang, 2018). Jalageri et al. fabricated an antimicrobial coating based on Jeffamine-ED2003. In a typical synthesis, a bifunctional coupler with piperazine containing amino chlorohydrin and azetidinium group was synthesized by reacting epichlorohydrin and piperazine. Subsequently, the bifunctional coupler was used in the postpolymerization modification of Jeffamine-ED-2003 in water at 90°C toward the formation of the waxy white solid polymer. The polymeric thin film coating was developed by dissolving the functionalized Jeffamine-ED-2003 polymer in Milli-Q water and dipping the substrate in solution at 90°C for 1 h followed by vacuum drying for 1 day ( Jalageri et al., 2019). An antifogging/antimicrobial polymer coating based on semi quaternized linear poly(2-(dimethylamino)-ethyl methacrylate-co-methyl methacrylate) (poly(DMAEMA-co-MMA)) and a photopolymerized ethylene glycol dimethacrylate (EGDMA) network was developed by Zhao and coworkers. In a typical synthesis, the poly(DMAEMA-co-MMA) copolymer

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was obtained by the free radical polymerization of DMAEMA and MAA monomers. Subsequently, the partial quaternization of the copolymer was achieved by reacting to various concentrations of 1-bromoundecane to obtain a copolymer with different degrees of quaternization. Herein, the semi-interpenetrating polymer network coating was fabricated by photopolymerization of EGDMA and partially quaternized copolymer in the presence of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone as photoinitiator (Zhao et al., 2016). An antibacterial coating on silicone contact lenses was developed from poly glycidyl methacrylate (GMA)/poly sulfobetaine methacrylate (SBMA) and polyethyleneimine (PEI) against eye-related infections. Initially, the synthesis of chlorine-functionalized poly(ethylene glycol) methacrylate (Cl-PEGMA) was obtained, and the synthesis of PEI-grafted PEGMA were prepared by adding an isopropyl alcohol solution of Cl-PEGMA into the PEI in deionized water and NaOH solution (1:16 ratio) and the mixture was stirred for 24 h at ambient temperature. The antibacterial coating with PEI-PEGMA was fabricated using the thermal method to activate the radical initiation at 90°C. Herein, the silicone contact lenses were subjected to ozone treatment to form an active peroxide group, and then immersing the contact lenses in a coating solution containing different amounts of PEI-PEGMA and SBMA in the absence of oxygen were stirred at 90°C for 4 h. Another coating was also fabricated using GMA, SBMA, and PEI on a silicone lens through a two-step process. The adhesion layer on the silicone lens by ozonolysis forms a peroxide group on the surfaces, and subsequently PEI grafting on the adhesive layer is obtained by dipping in GMA with different concentrations (Pillai et al., 2020). Cuervo-Rodrı´guez et al. reported the fabrication of a contact active antimicrobial film based on cationic block copolymer, polystyrene-b-poly 4-(1(2-(4-methylthiazol-5-yl)ethyl)-1H1,2,3-triazol-4-yl) butyl methacrylate (PS-b-PTTBM). The block copolymer synthesis was achieved through ATRP and “click chemistry” strategies. In the following, quaternization of the block copolymer was conducted by reacting with alkyl iodides to incorporate the cationic groups 1,3-thiazolium and 1,2,3-triazolium into the block copolymer. Finally, the cationic block copolymer film was fabricated through a spin-coating approach (Rodrı´guez et al., 2017). Hung et al. developed an inherent, self-sanitizing antimicrobial polymer coating based on polypropylene (PP) and branched PEI to support food safety and preservation applications. In the typical synthesis, polypropylene pellets were

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pressed into films on a hot press with a 9000 psi load at 180°C. The surface activation of propylene films was achieved by UV-Ozone irradiation, which facilitates the binding nature of polymer coating by the formation of a carboxylic group through photooxidation. The surface-activated PP films were then treated with 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline to convert carboxylic groups into an anhydride function. A three-layer polymer coating was fabricated by a spin coating method, using styrene-maleic anhydride (SMA) as a cross-linker between primary amines of PEI and anhydrides of PP films (Hung et al., 2018). Bastarrachea and Goddard developed a self-healing antimicrobial polymer coating based on PP-PEI-SMA-PEI system against Escherichia coli O157:H7 in the presence of organic matter. Herein, the authors prepared PP/PP-g-MA films by purifying the PP pellets and PP-grafted maleic anhydride (PPg-MA) after sonication with isopropanol, acetone, and deionized (DI) water for two cycles for 10min in each solvent. The cleaned pellets of PP and PP-g-MA were blended, hot-pressed, and self-healing polymer coating was prepared on the PP/PP-g-MA films through spin coating using a solution of PEI and SMA in acetone followed by thermal curing to facilitate the cross-linking of amine and anhydrides. The thermosensitive self-healing nature of polymer coating can be achieved due to the presence of reactive chemical species such as amine, carboxylic acids, carbonyls, and anhydride (Bastarrachea & Goddard, 2016). An antimicrobial and antifouling polymer brush coating was developed from methacrylate-ended polypeptides/polypeptoids (MePPEP/MePSAR) and PDA. The methacrylate-ended peptides and peptoids were synthesized through ROP of N-carboxyanhydrides (NCAs), and the MePPEP was obtained by random copolymerization of Lys(z)-NCA, Phe-NCA, initiated by 2-aminoethyl methacrylate hydrochloride (AEMA) followed by the deprotection of the side chains. Similarly, the MePSAR was synthesized by polymerizing Sar-NCA in the presence of AEMA as an initiator. The polymer brush coatings were prepared on Pristine PDMS films layered with PDA by dipping the Pristine PDMS films in a buffer solution containing DA followed by shaking (200 rpm) at room temperature for 2 h with exposure to air. Subsequently, the PDA-coated substrates were immersed in 2 mL of MePPEP solution or MePPEP /MePSAR mixed solution and placed in a photochemical reactor with UV irradiation for 1 h under nitrogen (Gao et al., 2017). Li et al. developed an antimicrobial coating from a quaternary ammonium silane (QAS) (PMT-5% and PMT-10%) through a thermal-curing process to prevent the adhesion and growth of microorganisms on material

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surfaces for industrial applications. In a typical synthesis, the PMT-5% and PMT-10% QAS copolymers were synthesized by free radical polymerization of (3-(methacryloylamino)propyl)trimethyl ammonium chloride (MAPTAC), 3-trimethylsilylpropyl methacrylate (TMSPMA). The coating was fabricated by dissolving PMT-5% and PMT-10% in acetic acid, and the pH is adjusted to 2.5 to convert the silyl ether into reactive silanol species. Subsequently, the polymer solution was transferred into a glass substrate and spread evenly followed by drying at 130°C to establish the siloxane crosslinking (Li, Bao, et al., 2016). A self-stratified antimicrobial coating was derived from quaternary ammonium methacrylate compounds (QAC-1, QAC-2) containing perfluoroalkyl tail and a methacrylate moiety through EGDMA cross-linking and UV curing. Initially, the QAC-1 and QAC-2 were synthesized through a quaternization reaction. The QAC-containing acrylic coatings were prepared through the spin coating of methyl methacrylate and different concentrations of QAC monomers were cross-linked with EGDMA in the presence of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone as UV initiator. Subsequently, the coating was rinsed with ethanol and deionized water and dried under a vacuum, and the control coating was prepared similarly without any QAC monomers (Zhao et al., 2015). Hoque et al. have described the development of antibacterial and antifungal polymeric paint materials based on organic soluble PEI derivatives. The synthesis was initiated from various PEI derivatives. For instance, a linear and branched N-methyl PEI were prepared through Eschweiler-Clarke methylation of PEI, and the cationic group insertions to the N-methyl PEIs by quaternization with alkyl bromides resulting in the formation of corresponding PEIs. Finally, The film was prepared by dissolving the PEI derivative in ethanol and added to the 96-well plate followed by drying under air and vacuum oven at 50°C. The antimicrobial coating was fabricated by mixing the PEI solution with biocompatible polymers such as polylactic acid through the spin coating method. Besides, the antimicrobial paint was prepared by brush coating different concentrations of PEI with a polymer matrix (Hoque et al., 2015). An antimicrobial edible film coating was fabricated from glucomannan, beeswax, and chitosan as antimicrobial packaging materials. Herein, beeswax emulsion (B) was initially prepared by dissolving with Tween 60 and Span 60, followed by emulsification in water homogenization at 70°C. In the following, glucomannan was dissolved in distilled water to glucomannan gel (G). Similarly, chitosan (C) solution was also prepared in acetic acid, The

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edible film coatings were fabricated by mixing the G, B, and C in different formulations G, GB, GC, and GCB with vigorous stirring for 15 min at 85°C. The coating was prepared on the Salak fruits by peeling the hard scaly skin and dipping for 15 s in G, GB, GC, or GCB solutions (Meindrawan et al., 2020). An antifouling and antimicrobial coating was developed using N-vinylpyrrolidone (NVP) and maleopimaric acid quaternary ammonium cation containing a double bond functionality (GMA-MPA-N+) through RAFT polymerization. Herein, the authors prepared polydimethyl siloxane (PDMS) silicone elastomer samples using SYLGARD 184 elastomer kit and activated the PDMS to surface hydroxylated PDMS (PDMS-g-OH). Subsequently, The PDMS samples were functionalized with triethoxysilane (APTES) to APTES-modified PDMS samples (PDMS-g-APTES). The RAFT agent was modified PDMS by anchoring the RAFT agent (4-cyano-4-(phenyl-carbonothioylthio) pentanoic acid) into the PDMSg-APTES surface through carbodiimide coupling. The PNVP-MPA-N+ coating was then prepared by surface-initiated RAFT polymerization of GMA-MPA-N+ and NVP in different molar ratios (Fig. 1B) (Z. Li et al., 2020). Zhou et al. reported nonleachable antibiofilm and antibacterial coating using (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA), poly(ethylene glycol) dimethacrylate (PEGDMA), and quaternized polyethylenimine methacrylate (Q-PEI-MA) through free radical polymerization. In the typical synthesis, the ATRP initiator (α-bromoisobutyryl bromide (BiBB) was functionalized onto the hydroxyl and amino groups of PDA-coated PDMS catheter. Subsequently, the synthesis of quaternized Q-PEI-MA using branched PEI and glycidyl methacrylate in ethanol and bromoethane solution was conducted at constant stirring for 24 h. Finally, the antibacterial coatings were prepared using AMPTMA, PEGDMA, and Q-PEI-MA as monomers on BiBB functionalized PDMS catheter via the technique of supplemental activator and reducing agent surface-initiated ATRP in the air-free solvent (Zhou et al., 2017). Through the LBL deposition technique on cotton fabric, Liu et al. fabricated a self-assembled antibacterial coating with cationic homopolymer poly((3-acrylamidopropyl) trimethylammonium chloride) (CHP) and an anionic poly(2-acrylamido-2-methylpropane sulfonic acid sodium salt) (AHP). Herein, cationic homopolymer N-halamine CHP and anionic homopolymer AHP was synthesized through free radical polymerization of (3-Acrylamidopropyl)trimethylammonium chloride and 2-Acrylamido2-methylpropane sulfonic acid. The antibacterial coating on the cellulose substrate was developed by soaking the bleached cotton fabrics in positively

Fig. 1 (A). Synthesis of antimicrobial polymer containing PEI, N-halamines, and SMA via UV cross-linking and its coating on polypropylene pellets. (B). Fabrication of dual-functional antimicrobial coating using NVP and GMA-MPA-N+ through RAFT polymerization. ((A) Adapted with permission from Bastarrachea, L. J., Goddard, J. M. (2015). Antimicrobial coatings with dual cationic and N-halamine character: Characterization and biocidal efficacy. Journal of Agricultural and Food Chemistry, 63 (16), 4243–4251. Copyright 2015 American Chemical Society. (B) Adapted with permission from Li, Z., Wang, S., Yang, X., Liu, H., Shan, Y., Xu, X., Shang, S., Song, Z. (2020). Antimicrobial and antifouling coating constructed using rosin acid-based quaternary ammonium salt and N-vinylpyrrolidone via RAFT polymerization. Applied Surface Science, 5 (30), 147193. Copyright 2020 Elsevier.)

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charged CHP homopolymer and negatively charged homopolymer followed by drying at 100°C for 5 min LBL coating of the polymers technique (Liu et al., 2015). In another study, Goddard group developed an antimicrobial coating containing dual cationic and N-Halamines antimicrobial on PP surface using PEI and SMA to reduce the cross-contamination of microorganisms from food-processing equipment. Initially, the PP coupons were prepared from PP pellets, which were cleaned by sonicating with isopropanol, acetone, and DI water and then hot pressed with a load of 9000 lb. at 170°C. The carboxyl group activation on the surface PP coupons was induced by UV-Ozone irradiation and converted the carboxyl group into anhydride, which can react with primary amines. The final antimicrobial coating was prepared by dissolving PEI and SMA in acetone and spin-coated an anhydrideactivated PP coupon for 1 min at 3000 rpm followed by heating for 12 h at 100°C (Fig. 1A) (Bastarrachea & Goddard, 2015).

2.2 Antimicrobial agent incorporated polymers Besides structurally modified antimicrobial polymers, several other polymer systems recently emerged by incorporating potent antimicrobial agents (Polyphenols, antibiotics, peptides, and inorganic metal ions) toward the development of antimicrobial coatings. These antimicrobial agents were either conjugated into the polymer backbone or incorporated through physical interactions. Importantly, the antimicrobial properties of these categories of polymers depend on the surface area and capability of effectively releasing antimicrobial drugs. The active agents can attach to the cell membrane, penetrate inside the cells, and subsequently hamper DNA replication. Besides, the inactivation of the key enzymes and the subsequent damage of the cell cytoplasmic membrane results in the destruction of the pathogenic microorganisms. For instance, Ibrahim et al. developed an edible paper coating based on a polymer and an antimicrobial amino acid conjugate for packaging applications. In a typical fabrication, sodium salt of polystyrene sulfate-lysine conjugate was prepared through the fusion method by mixing PSS with different ratios of lysine, by heating at 250°C for 90 min. The resulting conjugate was then exposed to a microwave, followed by thorough washing with Millipore water and air drying. The paper coating was developed using an automatic film applicator with different road coaters (150 and 200 μm), and the coated sheets were further thermally dried at 80°C under hot air (Galotto et al., 2015). For the prevention of implant-associated infections,

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Acosta et al. have described the fabrication of an antibiofilm coating on implant surfaces based on antimicrobial peptides (AMPs). Herein, initially, the synthesis of hybrid protein polymers (AMP-ELR) was achieved by using polycationic elastin-like recombinamers (ELRs) with a C-terminal Cys-Cys motif for specific site binding on the surface. The AMPs (L-GL13K) were incorporated into the N-terminal of ELR through recombinant DNA techniques, and the resulting hybrid L-GL13K was obtained. Finally, AMP/ ELR/AMP-ELR was anchored into the titanium surfaces through a covalent bond using organosilanes as cross-linking agents (Acosta et al., 2020). Xu et al. developed a bifunctional (antifouling and antimicrobial) polymer brushes based on tannic acid (TA), poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC), and cationic polylysine through RAFT and ROP polymerization and copper-free azide-alkyne cycloaddition reactions. Herein, the TA was surface functionalized with a chain transfer agent with an azide function (CTA-TA-N3). Subsequently, RAFT polymerization of MPC was conducted from CTA-TA-N3 resulting in the formation of p(MPC)-TA-N3. In parallel, alkyne-terminated polylysine (DBp(CbzLys)) was also synthesized through ROP of N-ε-carbobenzoxy-L-lysine N-carboxyanhydride (Cbz-Lys-NCA) from dibenzocyclooctyne-amine initiator. Finally, the azide-terminated p(MPC)-TA-N3 was coupled with carbobenzoxyl group-stabilized polylysine (DB-p(CbzLys)) through azide-alkyne cycloaddition to yield p(MPC)-TA-p(Lys). The aqueous solution of p(MPC)-TA-p(Lys) was deposited on stainless steel surfaces to obtain TA-p(MPC)/p(Lys) as well as CTA-TA-N3 and p(MPC)-TA-N3 modified steel surfaces (Fig. 2B) (Xu et al., 2018). In another study, Peretto et al. developed an edible antimicrobial coating based on sodium alginate, carvacrol, and methyl cinnamate to preserve strawberry fruits. Herein, a clear coating solution was prepared by dissolving sodium alginate powder in DI water at 70°C with constant stirring for 15 min, followed by a dropwise addition of glycerol. Subsequently, the incorporation of 0.98% (w/w) carvacrol and 1.45% (w/w) methyl cinnamate as active agents into the solution by dissolving methyl cinnamate in ethanol, and by subsequent homogenization of the mixture (Peretto et al., 2014). Pishbin et al. fabricated an antimicrobial composite coating derived from bioactive glass and chitosan, loaded with gentamicin through the electrophoretic deposition method. The chitosan solution (pH ¼ 3) was prepared, and the composite suspension was prepared by adding bioactive glass particles into the chitosan solution. Subsequently, gentamicin sulfate solution (1 mL) was added to the composite suspension. The composite coating

Fig. 2 See figure legend on next page

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was prepared on the stainless steel foils (which act as a cathode) using a gold counter electrode in the electrophoretic deposition cell. The electrophoretic deposition of gentamicin-loaded bioglass/chitosan was achieved by passing a constant current into the electrodes placed at a constant distance of 1.5 cm of the composite suspension with different combinations of chitosan, chitosan/ bioglass, and chitosan/bioglass/gentamicin coatings (Pishbin et al., 2014). For food packaging applications, an antimicrobial film was developed using ionic silver-loaded poly(L-lactide) (PLA) polymer through solvent casting technique with the aid of glycerol as a plasticizer in tetrahydrofuran (THF) and dimethylformamide (DMF). Herein, PLA pellets containing D-isomer (Molecular weights about 130,000–150,000 g mol1) were dissolved in THF or a mixture of THF and DMF at 50°C, with stirring to obtain PLA-THF and PLA-DMF films. Further, the PLA-THF and PLA-DMF films were plasticized with glycerol to make the film soft and flexible. Finally, the silver nitrate with different concentrations was loaded into the plasticized films in dry conditions, followed by solvent evaporation (Martı´nez-Abad et al., 2014). Xu and coworkers have described the synthesis of switchable, smart antimicrobial, and antifouling coating from a pH-sensitive poly(2Fig. 2, cont’d (A). Functionalization of nanoparticles with copolymer brushes via SI-ATRP and conjugation of AMPs through iodoacetic acid N-hydroxysuccinimide ester linker. (B). Synthesis route of various tannic acid-scaffolded polymer brushes and the fabrication of coatings on stainless steel surfaces. (C). The one-step deposition of PLYS-TAPDPA-b-PMPC polymer brushes on stainless steel surfaces. (D). Illustration of a PDA/HA/Ag/CS hybrid coatings on Ti implant via LBL method and its disruptive behavior on a bacterial cell membrane. ((A) Adapted with permission from Yu, K., Lo, J. C., Mei, Y., Haney, E. F., Siren, E., Kalathottukaren, M. T., Hancock, R. E., Lange, D., Kizhakkedathu, J. N. (2015). Toward infection-resistant surfaces: Achieving high antimicrobial peptide potency by modulating the functionality of polymer brush and peptide. Applied Materials & Interfaces, 7 (51), 28591–28605. Copyright 2015 American Chemical Society. (B) Adapted with permission from Hoque, J., Ghosh, S., Paramanandham, K., Haldar, J. (2019). Charge-switchable polymeric coating kills bacteria and prevents biofilm formation in vivo. ACS Applied Materials & Interfaces, 11 (42), 39150–39162. Copyright 2019 American Chemical Society. (C) Adapted with permission from Xu, G., Neoh, K. G., Kang, E. T., Teo, S. L. (2020). Switchable antimicrobial and antifouling coatings from tannic acid-scaffolded binary polymer brushes. ACS Sustainable Chemistry & Engineering, 8 (6), 2586–2595. Copyright 2020 American Chemical Society. (D) Adapted with permission from Li, M., Liu, X., Xu, Z., Yeung, K. W., Wu, S. (2016). Dopamine modified organic–inorganic hybrid coating for antimicrobial and osteogenesis. ACS Applied Materials & Interfaces, 8 (49), 33972–33981. Copyright 2016 American Chemical Society.)

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diisopropylaminoethyl methacrylate)-b-poly(2-methacryloyloxyethyl phosphorylcholine) (PDPA-b-PMPC) and cationic polylysine, which are grafted to TA. Herein, the dibenzocyclooctyne (DB) terminated PDPA-b-PMPC block copolymer was synthesized through sequential RAFT polymerization 2-diisopropylaminoethyl methacrylate (DPA), and MPC. The TA-scaffolded copolymers (PLYS-TA-PDPA-b-PMPC) were obtained through azidealkyne ‘click’ reaction of azide functionalized TA, DB-PDPA-b-PMPC, and DB-terminated PLL in PBS at pH 7.4. Finally, the triblock copolymer was functionalized into different surfaces such as stainless steel foils, silicon wafers, and SPR gold chips through the coordination chelation ability of TA (Fig. 2C) (Xu et al., 2020). Grande et al. fabricated an antimicrobial film coating based on chitosan cross-linked graphene oxide nanocomposite (CS-GO). Initially, graphene oxide (GO) was synthesized by reacting graphite flakes with KMnO4 and H2O2, and subsequently, CS-GO solution was prepared by mixing C and different concentrations (0.1, 0.2, 0.4, 0.6 wt%) of GO in a 1% acetic acid aqueous solution. Toward the coating fabrication, the CS-GO solution was sonicated and poured into the glass plate to obtain CS-GO nanocomposite films. Finally, curing of CS-GO nanocomposite film at 120°C overnight under vacuum induces the thermal cross-linking between the epoxy group of GO with amine as well as hydroxyl functionalities of chitosan (Grande et al., 2017). Dong et al. have described the fabrication of an antimicrobial film based on thermosensitive polyurethane loaded with carvacrol and cinnamyl aldehyde for food packaging applications. The thermosensitive polyurethane was synthesized from polycaprolactone and diphenylmethane 4,4-di isocyanate (hard segment), and 1,4-Butanediol as a soft segment in DMF under inert conditions. To fabricate the film, carvacrol and cinnamyl aldehyde (2%–5% w:w) were loaded into the polyurethane, and the mixture was poured into a plastic dish and placed at 60°C for 12 h (Dong et al., 2020). Nguyen et al. synthesized an antimicrobial adhesive coating based on a series of biomimetic polymers obtained by the free radical polymerization of dopamine methacrylamide with other monomers followed by the loading of various antimicrobial agents. The other monomers used are 2-methoxyethyl acrylate (polymer A), ethyl methacrylate (polymer C), and 2-hydroxyethyl methacrylate (polymer D), which are loaded with antimicrobial PEI and nanoparticles such as graphene, graphene oxide, and molybdenum trioxide. In the fabrication of coated slides, the adhesive polymer A-D was dissolved in methanol or DMF followed by the addition of antimicrobial agents in different ratios under sonication for 10 min. Subsequently, the coating on the glass slides was achieved through the spin

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coating technique annealing at 70°C for 16 h (Nguyen et al., 2017). Mei et al. have developed an antimicrobial coating, based on the fluorescent silver nanocluster (AgNCs)-embedded zein films for food packaging applications. Initially, AgNCs were synthesized by inducing ultraviolet-A irradiation at wavelengths ranging from 315 to 400 nm for 7.5 h to a mixture of siliver nitrate (AgNO3)and polymethacrylic acid in deionized water. The AgNC-embedded zein films were prepared by dissolving zein protein in 70% aqueous ethanol followed by the addition of AgNC into ethanol solution (3:7 (v/v)), followed by coating on paper disk through dry casting (Mei et al., 2017). Gu et al. developed an environmentally benign antimicrobial coating based on supramolecular assembled lysozyme protein, enriched on polymeric aggregates through one-step aqueous coating or solvent-free printing. Herein, egg-white lysozyme dissolved in natural buffer solution and mixed with tris(2-carboxyethyl)phosphine buffer at neutral pH to reduce the disulfide bonds of lysozyme to obtain phase-transited lysozyme nanofilm. The resulting amyloid-like protein nanospheres were hierarchically assembled into 2D nanofilm at the solution surface such as Si, Ag, Au, Pt, Ni, Ti, ZrO2, PDMS, polytetrafluoroethylene, poly(methyl methacrylate), polystyrene, and cloth (Gu et al., 2017). Poudel et al. developed a thermoresponsive antimicrobial nanocomposite and coatings from poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinyl caprolactam) incorporated with silver (Ag) nanocomposites with silica nanoparticles (SNPs) or carbon nanotubes (CNT) through the single-pass gas-to-liquid process. Initially, the SNPs and CNT were synthesized through the spark ablation method and injected into the liquid cell containing a biphasic mixture of thermoresponsive polymer (TRP) PDMS, PNIPAM (which act as reducing and thermoresponsive passivating agents), and AgNO3 under ultrasound irradiation to obtain Ag-SNP@TRP and Ag-CNT@TRP nanocomposites. Finally, the transparent coatings of Ag-SNP@TRP and Ag-CNT@TRP nanocomposites were prepared on the touch screen panel through electrohydrodynamic spraying (Poudel et al., 2017). A dual functional coating based on N,N-dimethylacrylamide and N-(3aminopropyl) methacrylamide p(DMA-co-APMA) copolymer and antimicrobial peptides was developed to prevent catheter-associated urinary tract infections. Herein, a nonfouling surface coating was constructed from ATRP synthesized p(DMA-co-APMA), and the copolymer further functionalized with an iodoacetyl linker by reacting with iodoacetic acid N-hydroxysuccinimide ester. The coating on the titanium, polyurethane

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catheters, and 96-well plate surface was achieved by dipping the substrate in the polymer solution. Ultimately, the AMP E6 (RRWRIVV IRVRRC-NH2) was conjugated onto polydopamine (PDA)/uhPDMA and PDA/PDMA-co-APMA-I-coated surfaces through a Michel-type addition reaction followed by adding 1-thioglycerol as a plasticizer (Yu et al., 2021). Recently, Bataglioli et al. developed an antiviral coating on a propylene mask, based on hybrid alginate incorporated with copper sulfate to inactivate coronavirus. The coating formulations were prepared by adding CuSO4 (60 and 240 mM) drop by drop into alginate solution (0.1% and 0.5% (w/v)) with constant stirring, and a three-layer surgical mask made up of 100% polypropylene was used as a substrate for the coating. The coating was prepared either by immersing in the coating formulations or by spraying into the substrate followed by drying at 60°C (Bataglioli et al., 2022). An antibacterial coating based on poly(acrylic acid-co-acrylamide) hydrogel incorporated with the silver nanoparticle (AgNPs), gentamicin, or AMP (Bac2A) were prepared through UV-initiated polymerization. The hydrogel was prepared in polyvinyl chloride tubes, through photopolymerization of acrylic acid and acrylamide through photoinitiation by irradiating with UV light at 365 nm for several hours. Subsequently, the hydrogel-coated PVC tube was soaked in saturated sodium carbonate solution to achieve carboxyl groups (dCOOH) to dCOONa+, and dried at room temperature. Similarly, hydrogel coated on other polymer substrates such as PDMS and polyurethane. Finally, the antimicrobial agent was loaded into the coating by immersing in an aqueous solution of AgNO3 followed by dipping in an aqueous NaBH4 to convert the chelated Ag+ ions to AgNPs. The other antimicrobial agents, gentamicin, and Bac2A were also loaded by dipping the hydrogel-coated substrate in respective aqueous solutions (Zhang et al., 2021). Guo et al. reported an antimicrobial, biodegradable edible coating using a chitosan-based ternary blend consisting of gelatin (GE), and natural cinnamon essential oil. The CS-GE film-forming solution was constructed by dissolving 2 g CS in 200 mL 1% acetic acid, and mixing with GE (2 g in 200 mL deionized water) at 35°C for 3 h. In the following, glycerol was added as a plasticizer and Tween 80 as an emulsifier. The essential oil was loaded into the film-forming solution with continuous stirring and homogenization followed by heating at 60°C. After degassing, the mixture was poured into the polystyrene Petri dishes followed by curring in a constant temperature humidity chamber for 2 days (Guo et al., 2019).

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Li et al. developed a hybrid antimicrobial coating on titanium by using poly(dopamine), hydroxyapatite (HA), AgNPs, and chitosan through an LBL assembly process. Initially, the PDA layer was deposited in the titanium substrate by oxidative polymerization of 3,4-Dihydroxyphenethylamine in 10 mM Tris buffer (pH 8.5), dried under a nitrogen atmosphere, and incubated in 1.5-fold of simulated body fluid at 37°C in a calorstat to grow into HA, resulting in the formation of PDA-assisted HA coating. This solution was incubated with simulated body fluid for 14 days and soaked with different concentrations of AgNO3 solution for 30 min. The sample was then exposed to the UV light of 253 nm for 30 min to obtain various PDA/ HA/Ag combinations. Finally, the sample was mixed with a chitosan solution in acetic acid and coated on the substrate through spin coating results in different PDA/HA/Ag/CS hybrid coatings (Fig. 2D) (Li, Liu, Xu, Yeung, and Wu, 2016). A dual-layer antimicrobial coating was constructed on the urinary catheter using polycaprolactone (PCL) and PEG incorporated with a synthetic peptide (KRWWKWWRR) through dip and syringe coating. Herein, the silicone foley catheter and PDMS surfaces were modified with silane groups followed by heating at 60°C for 5 h along with trimethoxy(2-(7-oxabicyclo[4.1.0]hept3-yl)ethyl) silane solution in toluene. For dip coating, the saline-treated catheter and PDMS samples were added into the coating solution containing 4–6 w/v% of PCL, PEG, and 10–40 mg/mL of AMP, followed by immersing in ethyl cellulose (EC):1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) mixture for 10 s and dried at 25°C to obtain AMP-EC-PCL-coated catheter and PDMS samples. For syringe coating, the saline-treated catheter was connected to the module of the syringe coating instrument and coating solution was pumped into the inner and outer surface of the catheters with a flow rate of 0.1 mL/s, followed by immersing in EC:POPC mixture in ethanol and dried at room temperature (Le et al., 2021). Druvari et al. initially, the block copolymer p(VBC-co-AAx) was synthesized through free radical polymerization of 4-vinylbenzyl chloride (VBC) and poly(acrylic acid) (PAA). The copolymer p(SSNa-co-GMAx) was synthesized using glycidyl methacrylate (GMA) and sodium 4-styrene sulfonate (SSNa). Subsequently, the introduction of quaternized units into the block copolymer p(VBC-co-AAx) through a covalent bond using N,N-dimethylhexadecylamine (HAM) to obtain p(VBCHAM-co-AAx) coating precursor. In parallel, the incorporation of quaternary ammonium cations into p(SSNa-co-GMAx) copolymer was achieved through electrostatic interaction of sodium ions of SSNa units with an excess of quaternary cetyltrimethylammonium cations (AmC16), yield

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p(SSAmC16-co-GMAx). The antibacterial membrane was prepared by mixing the coating solutions of p(VBCHAM-co-AAx) and P(SSAmC16-co-GMAx) in different concentrations followed by thermal curring (Druvari et al., 2016). An antimicrobial coating was constructed on the leather surface using skin collagen, gallic acid (GA) stabilized AgNPs (GA@AgNPs) through chromium(III) cross-linking. Herein, GA@AgNPs were synthesized by reacting an equimolar solution of AgNO3 and GA with NaBH4 and purified by centrifugation followed by dissolving in PBS with pH 7.4 containing 400 ppm silver concentration. The coating on the leather surface was created through spray coating of GA@AgNPs aqueous solution followed by drying at room temperature (Xia et al., 2018). Beech et al. have developed an antimicrobial paint using vinyl acrylic copolymer emulsion incorporated with crystal violet, methylene blue, and safranin O toward a self-sterilizing painted surface. The antimicrobial paint was prepared by introducing a three-dye system (crystal violet, methylene blue, safranin O, and 2-nm gold nanoparticles), a two-dye system (crystal violet, methylene blue, and 2-nm gold nanoparticles), and a one-dye system (safranin O and 2-nm gold nanoparticles) into the acrylic copolymer emulsion. The coating on the glass slides was prepared through dip-coating followed by air-drying (Beech et al., 2015). Yu et al. developed an antimicrobial coating from poly(N,N-dimethylacrylamide) (PDMA), poly(2-(methacryloyloxy) ethyl phosphorylcholine) (PMPC) and poly(3-(methacryloylamido)propyl)-N,N-dimethyl(3-sulfopropyl)ammonium hydroxide) (PMPDSAH), N-(3Aminopropyl)methacrylamide) (APMA) conjugated with an AMP. Initially, the APMA was copolymerized through a surface-initiated ATRP with DMA, MPC, and MPDSAH on the initiator-modified surfaces such as polystyrene (PS) nanoparticle, quartz slides, and Ti-surfaces to obtain PDMA-co-APMA, PMPDSAH-co-APMA, and PMPC-co-APMA brushes. The AMP E6 (RRWRIVVIRVRRC) and Tet20 (KRWRIRVRVIRKC) were conjugated onto the PDMA-co-APMA, PMPDSAH-co-APMA, and PMPC-co-APMA brushes grafted in PS nanoparticle quartz slides and Ti-surfaces using iodoacetic acid N-hydroxysuccinimide ester as a linker (Fig. 2A) (Yu et al., 2015). Jahdkaran et al. reported an antimicrobial film based on methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) incorporated with peppermint (Menthol) and Origanum vulgare (Carvacrol) essential oils on polyethylene films. The polymer formulation was prepared by dissolving MC and HPMC in 95% ethanol at 65°C followed by the addition of PEG in deionized water as a plasticizer. Finally, different concentrations of carvacrol

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and menthol were introduced into the coating formulation and were applied to the PE film ( Jahdkaran et al., 2021). In another work, antimicrobial films and coating were prepared from ethylene polymers such as ethylene-octene (LLDPE), ethylene-vinyl acetate (EVA), ethylene-methacrylic acid (EMA), and ethylene-alkaline methacrylate (ION) incorporated with carvacrol for food packaging applications. Herein, the polymer suspension was prepared using different ethylene polymers and poured into the glass vial with an ultrasonication bath at room temperature for 30 min. The film and coating were subsequently constructed on the propylene films through casting and drying at 80°C (Cerisuelo et al., 2015). Khin et al. developed an antimicrobial surface coating on the fabric, based on bismuth oxyhalide (BiOClBr) along with PVA and PAA as a binder. Initially, the BiOClBr microparticles were synthesized by dissolving bismuth(III) nitrate pentahydrate in acetic acid and deionized water. Under ambient temperature, the different concentrations of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) as a source of Cl and Br were separately added into the bismuth precursor, followed by probe sonication. Finally, for coating on polyester and cotton, the cleaned substrates were dipped in 2 wt% of PVA or PAA containing different concentrations of BiOClBr microparticles for 2 h under sonication (Khin et al., 2021). Qin et al. have developed an antioxidant and antimicrobial coating using two series of multifunctional pH-responsive starch nanoparticles and metalphenolic network (MPN), chelating TA with Fe(III) ions. In the typical synthesis, the debranched starch nanoparticles (DBS-NP) were synthesized by the retrogradation method to form a self-assembled DBS-NP, similarly, the starch nanocrystals (SNC) were also prepared by the one-step assembly method. Finally, metal-phenolic network-coated nanoparticles (MPN@DBS-NP and MPN@SNC) was constructed by adding an appropriate amount of TA and FeCl3∙6H2O solutions into the nanoparticles suspension with different concentrations followed by ultrasonication for 10 min results in different MPN@DBS-NP coatings (Qin et al., 2019). An antimicrobial coating for milk packaging applications using AgNPs incorporated low-density polyethylene film (LDPE) was developed. Herein, the Ag nanoparticle colloidal solution was synthesized by reducing AgNO3 using fructose as a reducing agent, and the reaction was controlled by diammonium hydrogen citrate. The LDPE surface was modified with corona air plasma to increase the adhesion ability. Finally, the silver coating was prepared by soaking the corona-modified LDPE in an AgNPs solution (Bandpey et al., 2017).

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3. Applications of the antimicrobial polymer coatings 3.1 Antimicrobial polymer coatings in the food industry Undoubtedly, various surfaces will be in contact with food materials during food preparation, processing, and packaging. In addition, the food storage conditions can also potentially increase the chance of microbial crosscontaminations. The microbial growth in food items is a serious challenge to maintaining product shelf life and food safety. The development of polymeric antimicrobial coatings is efficient to reduce microbial crosscontamination by inactivating microbial adherence in food materials. To date, a series of polymeric antimicrobial agents have been studied as antimicrobial coatings with quaternary ammonium functionalization, cationic charge, or incorporating antimicrobial peptides, essential oils, N-halamines, or metal nanoparticles. In the following, we describe the major classes of polymer-based antimicrobial coatings employed in the food industry. Ibrahim et al. developed an antimicrobial, edible polymeric packaging material derived from PSS lysine conjugate. The antibacterial activity of this material was significant against Bacillus subtilis, Staphylococcus aureus, E. coli, and Pseudomonas aeruginosa with a minimum inhibitory concentration (MIC) value of 25, 12.5, 50, and 12.5 μg/mL respectively. Besides bacteria, this material showed antifungal activity against Candida albicans with a MIC of 3.1 μg/mL with minimal toxicity. Herein, the antibacterial and antifungal activity was induced by blocking the metabolic pathways as well as the active site of the amino acid binding site due to the presence of bulkier PSS molecules in the conjugate (Galotto et al., 2015). An antimicrobial packaging film based on PBAT and chitosan nanofibers (CS-NF) showed antibacterial activity against foodborne pathogens such as S. aureus, B. subtilis, Salmonella enteritidis, and E. coli at 2.0 and 5.0 wt% of CS-NF concentration. Herein, the chitosan penetrated the microbial cell wall through pervasion and form a new polymer membrane on the surface of the cells. This membrane acts as a barrier for nutrient entry to the cells and disturbs the physiological activity of the cells. Besides, the positively charged amino group interacts with the negatively charged cell membrane through electrostatic interaction also results in the leakage of intracellular components of microorganisms. Overall, the chitosan nanofibers embedded with PBAT films for food packaging applications extended the food shelf life (Dı´ez-Pascual & Dı´ez-Vicente, 2015). An antifogging/antimicrobial coating based on a semi-interpenetrating polymer network of partially quaternized poly(DMAEMA-co-MMA) and polymerized EGDMA network

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was employed as transparent food packaging material. These materials displayed an excellent antifogging capacity even in the presence of different concentrations of hydrophobic quaternary ammonium compounds (7%, 11%, and 13% QAC). The light transmittance was 90% examined by storing at 20°C for 30 min and exposing to the ambient conditions of 20°C, with 50% relative humidity. The antimicrobial activity of the coating incorporated with QAC against Staphylococcus epidermidis and E. coli after 24 h incubation showed an inhibition with a bacterial log reduction of 3.6 and 2, due to the presence of the permanent hydrophobic effect of QAC (Zhao et al., 2016). A contact-active antimicrobial film based on PS-b-PTTBM was studied for food packaging applications. The PS-b-PTTBM film showed an active mode of action due to the presence of polycations and hydrophobic alkyl chains interacting with microbes through electrostatic interactions and resulting in the disruption of the microbial membrane leading to cell death. To investigate roughness and accessibility for electrostatic interactions, the protein absorption capacity of PS-b-PTTBM film was conducted and confirmed an absorption ability around 50 ng mm2 in 24 h of incubation. The PS-b-PTTBM film showed antimicrobial activity against S. aureus, P. aeruginosa, and Candida parapsilosis fungi with a killing efficacy of more than 99.999% with a coating containing 50 wt% of the copolymer (Rodrı´guez et al., 2017). The inherent self-sanitizing antimicrobial PP-PEI-SMA-PEI coating, which supports food safety and processing equipment by reducing cross-contamination pathogens was developed. The hydrophobicity of native PP and PP modified with PEI-SMA-PEI coating was evaluated by measuring the water contact angle and surface energy of the coatings, the modified PP shows an increase in hydrophilicity compared to native PP. The modified PP showed higher surface energy due to the presence of charged groups such as amine, hydroxyl, and carboxyl groups. The PP modified with PEI-SMA-PEI showed effective antimicrobial activity with no significant difference between longer periods and different temperature conditions under atmospheric storage. The chlorinated and unchlorinated PP-PEI-SMA-PEI coating showed excellent antimicrobial activity against foodborne microorganisms such as E. coli 452 O157:H7, Listeria monocytogenes, and Pseudomonas fluorescens with more than 99.99% of killing efficiency, confirmed by scanning electronic microscopy (SEM) images and agar overlay assay. The antimicrobial activity of PP-PEI-SMA-PEI coating showed two modes of action due to the presence of N-halamines and cationic polymers. The N-halamines transfer the

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oxidative chlorine into the microbial cell through the cell membrane, and cationic polymer interacts with cells through electrostatic interactions (Hung et al., 2018). A self-healing antimicrobial polymer coating based on PP-PEI-SMAPEI was studied toward the prevention of microbial cross-contamination. The antimicrobial character of the PP-PEI-SMA-PEI polymer coating was due to the presence of hydrophobic styrene units, cationic primary amine groups and chlorinated N-halamine groups, which oppose the adhesion of protein, lipid, or microorganisms. Notably, the chlorinated and unchlorinated PP-PEI-SMA-PEI polymer coating showed >99.99% inactivation of E. coli O157:H7 and demonstrated antimicrobial efficacy up to the lowest organic concentrations of 500 ppm. The PP-PEI-SMA-PEI coatings toxicity was evaluated by the release of chlorine from N-halamine moiety and which induces direct oxidation of microorganisms, herein coatings release a maximum of 14.1  1.3 ppm chlorine (Bastarrachea & Goddard, 2016). An antimicrobial coating based on quaternary ammonium methacrylate compound (QAC-1, QAC-2), bearing perfluoroalkyl tail was developed. The coatings displayed antimicrobial activity against S. epidermidis and E. coli with a killing potency of 5-log reduction for a QAC-1 concentration of 0.96 mol% and QAC-2 concentration of 0.06 mol% as a result of high positive charge density, and lower surface energy. The coating showed contactkilling antimicrobial mode of action and was confirmed by zone inhibition test (Fig. 3C (i) and (ii)) (Zhao et al., 2015). An antimicrobial edible film coating based on natural polymer was developed for improving shelf life and safety of salak and other varieties of fruits. The film was based on glucomannan (G), beeswax (B), and chitosan (C), Notably, the mechanical property of the GCB polymer is 8.3N, or a 332% increase compared to G film with 35.0% of elongation property. Importantly, the chitosan-based coatings (GCB or GC) coating showed an excellent antibacterial performance against E. coli, however, GB and G films cannot resist E. coli and S. aureus. The performance of GCB-coated salak fruit was maintained below the SNI limit of 5 logs of CFU g1 of the microbial count till 3 days of incubation and it maintained the smallest weight loss, less than 4% during 3 days of storage (Meindrawan et al., 2020). Antimicrobial coatings with dual cationic and N-Halamine moieties were developed against cross-contamination of microorganisms in food-processing equipments. The antimicrobial activity of cationic and N-halamine polymeric coating on PP was tested against L. monocytogenes. In its unchlorinated form, the coating inactivated L. monocytogenes by 3

Fig. 3 (A) A silver fluorescent nanocluster-embedded zein films for food packaging applications. (B) Metal-phenolic surface coatings on DBS-NP and SNC, and its interaction at different pH values against E. coli, and S. aureus. (C) (i) Fabrications of QAC-1 and QAC-2 coating through the spin coating and (ii) antibacterial against S. epidermidis. ((A) Adapted with permission from Mei, L., Teng, Z., Zhu, G., Liu, Y., Zhang, F., Zhang, J., Li, Y., Guan, Y., Luo, Y., Chen, X., Wang, Q. (2017). Silver nanocluster-embedded zein films as antimicrobial coating materials for food packaging. ACS Applied Materials & Interfaces, 9 (40), 35297–35304. Copyright 2017 American Chemical Society. (B) Adapted with permission from Qin, Y., Wang, J., Qiu, C., Hu, Y., Xu, X., Jin, Z. (2109). Self-assembly of metal–phenolic networks as functional coatings for preparation of antioxidant, antimicrobial, and pH-sensitive-modified starch nanoparticles. ACS Sustainable Chemistry & Engineering, 7 (20), 17379–17389. Copyright 2019 American Chemical Society. (C) Adapted with permission from Zhao, J., Millians, W., Tang, S., Wu, T., Zhu, L., Ming, W. (2015). Self-stratified antimicrobial acrylic coatings via one-step UV curing. ACS Applied Materials & Interfaces, 7 (33), 18467–18472. Copyright 2015, American Chemical Society.)

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logarithmic cycles, however, in the form of N-halamines >5 logarithmic cycles were achieved. The antibacterial activity is due to the presence of both cationic and N-halamine, which interact with bacterial cells and destroy the bacteria through the contact-killing process was confirmed by SEM. Importantly, the rechargeable ability of polymer coating was about 10 cycles of chlorination and it retains the antibacterial activities even after 10 cycles of chlorination (Bastarrachea & Goddard, 2015). An edible antimicrobial coating of sodium alginate, carvacrol, and methyl cinnamate was reported for preserving strawberry fruits. Herein, the calcium chloride cross-linked carvacrol, and methyl cinnamate incorporated coating showed significant antifungal and antibacterial activity against Botrytis cinerea and E. coli with MIC of 1.25% (w/w) for carvacrol and 0.5%– 2.5% (w/w) for methyl cinnamate. The Whitish Index study for examining the color of the coating showed from transparent to white, an increase in turbidity, and decrease inviscosity due to the incorporation of the antimicrobial compound, and this property plays an important role in food packaging. The coating was prepared on the strawberry based on the desirable surface model and the coating formulation was optimized with 0.98% (w/w) carvacrol and 1.45% (w/w) methyl cinnamate concentrations for practical applications (Peretto et al., 2014). Silver-loaded poly(L-lactide) films were developed for food coating applications through solvent (THF and DMF) casting techniques. From the color analysis, the films showed completely transparent for blank polymer films, however, after the addition of silver, the color was changed to yellow depending on concentrations as well as the size and shape of silver particles. Silver release kinetics from both films can maintain the shelf life of the food, however, a burst release of silver was observed from the PLA-THF films, compared to PLA-DMF films due to the absence of plasticizer glycerol. Notably, the glycerol controls the silver release rate and the sustained release enhances the initial biocidal activity and prevents recontamination. The 0.1 wt% silver-loaded PLA-THF and PLA-DMF showed long-term antimicrobial activities against the foodborne pathogen Salmonella enterica after 8 weeks of washings (Martı´nez-Abad et al., 2014). An antimicrobial film coating CS-GO was developed for food packaging applications. The color of the CS-GO films appears to be dark-colored film due to the presence of GO and the high opacity of films which prevents the damage to food products from excessive incidence of light. The stability and load-bearing capacity of the films was evaluated by mechanical analysis, the film showed increasing in the mechanical properties as well as a decrease in

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the elongation with the loading of 0.6 wt% of GO. The coating showed significantly higher antibacterial activity against E. coli with a killing efficiency of 22.83% of inactivation and B. subtilis with 54.93% of inactivation at 0.4 and 0.6 wt% MIC of CS-GO when compared to CS film. Here, the antibacterial activity was lowest in chitosan and showed better action in GO because of greater interaction with bacterial cells (Grande et al., 2017). A thermosensitive polyurethane (TSPU) loaded with carvacrol and cinnamyl aldehyde coatings were developed for food packaging applications. The antimicrobial activity of TSPU film-loaded carvacrol and cinnamyl aldehyde showed significant inhibitory effects against E. coli, B. subtilis, and Monascus ruber with an average zone diameter of 30 mm. The antimicrobial activity of TSPU films loaded with carvacrol and cinnamyl aldehyde was dependent on the release of antimicrobial agents, the film showed a release of 0.6%–2.2% of antimicrobial agents, and the release rate increases with an increase in temperature. The TSPU coating on the Cantonese-style moon cakes (prepared with lotus paste, egg yolk, and oil) showed a decrease in the peroxide (which damages cakes) values after 20 days of preservation and also inhibit microbial growth when compared to normal PE packaging (Dong et al., 2020). Antimicrobial coating materials AgNC-loaded zein film were developed for food packaging applications with potent antimicrobial activity against pathogenic E. coli O157:H7 (Fig. 3A). The antimicrobial activity depends on the release rate of Ag ion from the AgNC-loaded zein films. The coating demonstrated significant zone inhibition with an MIC of 0.525– 201.6 μg/cm2 for various formulation-loaded zein films, respectively. Besides, the biocompatibility of coating with different concentrations of AgCNs was examined using HCT116 human colon cancer cells showed significantly lower cytotoxicity than AgNO3 with 80% viability and IC50 for AgNCs was 34.68 μg/mL (Mei et al., 2017). An antimicrobial as well as a biodegradable coating using chitosan-based ternary blend consisting of GE and cinnamon oil were developed for food packaging applications. The films showed good mechanical properties with tensile strengths, excellent water vapor barrier character, and higher transparency. Importantly, the film exhibits excellent antibacterial activity at 98% killing efficiency and zone inhibition of 20.00  1.00 mm and 17.66  1.52 mm against E. coli and S. aureus, respectively, with an MIC of 52.06 μg/mL (Guo et al., 2019). An antimicrobial MC/HPMC film loaded with peppermint and O. vulgare essential oil on polyethylene films was fabricated for food packaging applications. The coating material for food packaging application

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should have good heat sealing capacity. Herein, the coating material on PE films showed an average value of heat-sealing strength and reasonable mechanical properties. The water vapor permeability of coated films also showed a low value of water vapor permeability and high oxygen permeability with increasing concentrations of carvacrol and menthol. The light transmittance of essential oil-loaded PE films was investigated by inducing light of wavelength in the range 200–800 nm, which showed a decrease in the light transmission with increasing concentration of carvacrol and menthol when compared to blank PE film. The microbial growth kinetics of carvacrol and menthol-loaded PE films were investigated against E. coli, S. aureus, and Listeria innocua, which showed a delayed lag phase and decreases the populations in log phase ( Jahdkaran et al., 2021). Antimicrobial films and coating of ethylene polymers incorporated with carvacrol were reported for antimicrobial food packaging applications. The film showed heterogeneity structure decreasing heterogeneity, porosity, and roughness of the coating depending on the type of ethylene polymer in order of LLDPE > EVA ≫ ION > EMA. The higher values of the diffusion coefficient for carvacrol, LLDPE, and EVA and the release of active agents depend on the solubility and diffusivity of coating material, which inhibits microbial activity and improves the long-term antimicrobial activity (Cerisuelo et al., 2015). The pH-responsive, antioxidant, and antimicrobial coating based on starch nanoparticles and MPN, chelating TA with Fe(III) ions were developed. The pH-responsive MPN@DBS-NP and MPN@SNC particles are stable at high pH and destabilize at acidic pH and the release of TA acid was examined under different pH conditions and showed 80% of TA release at pH 1.2 and 3.0, and less than 60% of TA release was observed at pH 5. The antioxidant property of the coating was evaluated based on the release of TA using a DPPH assay showed more than 80% of scavenging activity at acidic pH (1 and 3) and 40%–50% at pH 5 confirming the coating can be used in chronic wound applications. The coating showed excellent antimicrobial activities with MPN@DBS-NP and MPN@SNC against E. coli, and S. aureus (90% killing efficiency at pH 1–3). The in vitro biocompatibility of the coating was evaluated by Caco-2 cells and showed 100% and 80% cell viability in 24 and 48 h of incubation for MPN@DBS-NP coating (Fig. 3B) (Qin et al., 2019). Antimicrobial AgCNs incorporated LDPE film coating was developed for milk packaging applications. The Ag-coated LDPE film showed an increase in surface roughness when compared to blank LDPE film, confirmed by AFM results. The antimicrobial activity of Ag-coated

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LDPE films depended on the release of Ag ions, and the release profile showed a slow release for up to 15 days. The ideal microbiological colony-forming unit for the shelf life of milk is defined as 100,000 CFU/mL, the milk packed with film showed 110,000 CFU/mL microbial load at a corona power of 800 W after 14 days, which enabled an extension of the microbial shelf life of milk to 13 days. A corona input power of 800 W showed microbial growth, which was prevented and confirmed by a microscopic microbial count (Bandpey et al., 2017).

3.2 Antimicrobial polymer coatings in textile industry Various fabric materials are inevitable in daily life, and unfortunately, these fabrics are one of the favorable substrates for microbial growth and colonization under pathogen favorable conditions. The fabric adhered to microorganisms can easily be transmitted to human skin, and it can spread with a high infection rate. The polymer-based antimicrobial coatings on the fabric can protect from biofilm formation, and prevent the spread of microorganisms. Apparently, different synthetic and natural polymers with antimicrobial activity were used as coatings in the textile industry. These antimicrobial textiles can be used in a variety of applications ranging from households to commercial applications. Herein we discuss the action of various synthetic and natural polymer-based coatings in the fabric industry to prevent microbial adhesion and destruction of microorganisms through the release of active compounds. For instance, a coating based on 3-methylallyloxy-5,5dimethylhydantoin monomer composed of N-halamine moiety was used as an antibacterial coating. In a cotton fabric, these coatings showed 100% antibacterial activity against E. coli and S. aureus at 0.10 mol/L of monomer concentration. Herein, as a result of direct transfer of the oxidative halogen to the bacterial cell membrane from the N-halamine structures leads to cell death. Notably, the laundering durability of N-halamine-coated fabrics showed 99% antibacterial activity against E. coli after 30 laundering cycles as well as recharging of antibacterial properties by chlorinating the N-halamine, which increases the long-term antibacterial activity of the N-halamine-coated fabric (Luo et al., 2017). A polymeric antimicrobial coating made of modified Jeffamine-ED2003 showed good antimicrobial action against Gram-negative bacteria (E. coli, Mycobacterium smegmatis) and Gram-positive bacteria (S. aureus), and also against fungus (C. albicans). This coating displayed a significant zone of inhibition, around 6–8 mm according to the concentration of the coating.

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Besides, the antimicrobial activity of Jeffamine-ED-2003 was increased after functionalization with piperazine bifunctional coupler to quaternary amine, hydroxyl, and counter ion groups. The functionalized polymeric system was efficiently interacting with negatively charged microbial cells leading to the leakage of intercellular components resulting in the death of microbes ( Jalageri et al., 2019). Antimicrobial polymeric paint materials based on PEI (linear and branched) derivatives with 100% killing efficiency against air and waterborne as well as drug-resistant pathogenic bacteria and fungi were developed. The polymer with different hydrophobic and hydrophilic chains with optimum alkyl chain length showed significant antimicrobial activity, and especially polymers with branched and linear PEIs were showed high activity at the concentration of 0.4, 12.5, 7.8, 0.24 μg/dm2, against S. aureus, E. coli, Klebsiella pneumoniae, methicillin and vancomycin-resistant species. Notably, the antifungal activity of linear and branched PEI against pathogenic fungi such as Candida spp. and Cryptococcus spp. showed good antifungal activities with a MIC value of 0.63μg/dm2 for the linear polymer and 10 μg/dm2 for the branched polymer. Herein, the polymer showed an active mode of action against various bacteria, the PEI penetrates through the bacterial and fungal cells due to the presence of hydrophobic polycations, leading to cell death. The hemocompatibility of the polymer coating showed negligible hemolysis of human red blood cells at 6.25 μg/dm2 and 25%–35% hemolysis at 31 μg/dm2 (Hoque et al., 2015). Self-assembled, antibacterial multilayered coatings of cationic CHP homopolymer and an anionic homopolymer AHP (CHP/AHP)5, as well as chlorinated (CHP/AHP)5 dCl polymer coatings were established in cotton fabrics (Fig. 4A). The antibacterial efficacy of (CHP/AHP)5 and (CHP/ AHP)5 dCl coated on the fabrics was tested against S. aureus and E. coli O157:H7. The chlorinated polymer coating showed 100% bacterial inactivation of S. aureus and 99.73% of E. coli O157:H7 within 30 min of contact. The inactivation or biocidal activities of chlorinated polymer are due to the presence of oxidative free halogen, which enters into the cell and damages the bacterial cells. The stability and durability of antibacterial activities of unchlorinated and chlorinated polymer coatings on the fabrics showed a 51% of chlorine loss after 50 cycles of washing, and chlorine can be reloaded into the polymer through chlorination. The in vivo skin stimulation of (CHP/AHP)5dCl coated on the fabrics was investigated using New Zealand rabbits and showed negligible skin integrity irritation after 7 days of contact, which indicates the safety of the antimicrobial coatings in textile applications (Fig. 4B) (Liu et al., 2015). Hybrid alginate incorporated with

Fig. 4 (A) LBL deposition of CHP-Cl and AHP-Cl on the cotton. (B) Skin stimulation test of of CHP-Cl and AHP-Cl coated cotton on normal and damaged skin after 7 days. (C) Fabrication of p(VBC-co-AAx), and p(SSNa-co-GMAx) copolymer loaded with VBCHAM or SSAmC16 on aquaculture cage and its antimicrobial activity against P. aeruginosa and S. aureus in saltwater. ((B) Adapted with permission from Liu, Y., Li, J., Cheng, X., Ren, X., Huang, T. S. (2015). Self-assembled antibacterial coating by N-halamine polyelectrolytes on a cellulose substrate. Journal of Materials Chemistry B, 3 (7), 1446–1454. Copyright 2015 Royal society of chemistry. (C) Adapted with permission from Druvari, D., Koromilas, N. D., Lainioti, G. C., Bokias, G., Vasilopoulos, G., Vantarakis, A., Baras, I., Dourala, N., Kallitsis, J. K. (2016). Polymeric quaternary ammonium-containing coatings with potential dual contact-based and release-based antimicrobial activity. ACS Applied Materials & Interfaces, 8 (51), 35593–35605. Copyright 2016 American Chemical Society.)

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copper sulfate and applied as a coating on a propylene mask to inactivate coronavirus by shielding cell adhesion sites of the virus surface. The release of copper from the immersion-coated and the spray-coated mask was investigated by atomic absorption spectroscopy and showed release up to 5 and 20 mg after 24 h in an aqueous solution, respectively. The hybrid alginateCu (II) coating showed excellent antiviral activity against SARS-CoV-2 with a killing efficiency of 99% within 1 min and a higher concentration of biopolymer-copper combination inactivates the virus by 99.99% within 5 min (Bataglioli et al., 2022). A dual contact, release-based antimicrobial coating derived from p(VBCHAM-co-AAx) and p(SSAmC16-co-GMAx) copolymer with quaternary ammonium group was developed for the application on aquaculture nets. The S2-120 and S5-120 membranes, cured at 120°C containing (VBCHAMco-AAx) copolymer showed marginal solubility in pure water and was practically insoluble in salt solution. Notably, p(SSAmC16-co-GMAx) copolymers are limited soluble in pure water and completely soluble in saltwater and both membranes retain their shape in natural and seawater. The membranes initially showed a burst release of AmC16 followed by a steady release for a longer time in pure water, and a minimal release was observed in the salt solution. The long-term antimicrobial effect of the membranes was confirmed against P. aeruginosa and S. aureus with log reduction 5.4 and 5.3, respectively. Besides, the antifouling properties were also confirmed by immersing the copolymer-coated aquaculture cage in saltwater over a period of 35 days compared to the uncoated nets (Fig. 4C) (Druvari et al., 2016). Antimicrobial skin collagen and gallic acid stabilized silver nanoparticles (GA@AgNPs) through chromium (III) cross-linking were fabricated by spray coating on chrome-tanned leather to produce diabetic shoes. The stability of GA@AgNPs spray-coated leather was investigated by washing and rubbing, which showed good stability without color fading after 30 cycles of washing and 1000-cycle friction as well as no leaching of the silver ions from the coatings. The GA@AgNPs spray-coated leather showed water repellence by increasing the hydrophobicity and also showed long-term antimicrobial activity by the sustainable release of Ag ions with a release rate of 0.56% at 30 days. The GA@AgNPs spray-coated leather showed excellent antiadhesive against various bacterial and fungal species due to the hydrophobic nature and electrostatic repulsion. The antimicrobial activity of GA@AgNPs-coated leather against E. coli, S. aureus, Methicillin-resistant S. aureus (MRSA), and C. albicans with zones of inhibition by average 1 mm width and 99% killing ability were observed after 2 h of incubation

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through direct contact as well as Ag ion-mediated inactivation (Xia et al., 2018). Self-sterilizing antimicrobial paint surfaces were prepared using vinyl acrylic copolymer emulsion incorporated with various dyes. The stability of the antimicrobial paint loaded with various dyes (crystal violet, methylene blue, safranin O, and 2-nm gold nanoparticles) was investigated by a leaching study. Herein, rapid release of loosely bound dye from the surface was observed over 120 h in an aqueous solution. The photobacterial activity of antimicrobial paint samples was investigated under a white light source (at 3500  250 lx) and under dark conditions. In white light, the irradiation to all dye samples showed complete photosensitization against E. coli and S. aureus with >4  0.3 log reduction. However, under dark conditions, only three and two dye systems showed significant lethal photosensitization against E. coli and S. aureus (Beech et al., 2015). An ambient light antimicrobial coating based on BiOClBr, PVA, and PAA surface coating was developed on the fabric toward the protection of laboratory and factory clothing. Antimicrobial efficacy of BiOClBr-coated fabrics was evaluated against E. coli and S. aureus under ambient light and showed bacterial inhibition with a log reduction value of less than 4 at 1.25% of BiOClBr concentrations. The leaching of Bi in an aqueous solution was evaluated by leaching test, and cloth showed release of 25.1 and 37.5 mg L1, in 5 and 10-day washings. The in vitro biocompatibility of the coating was evaluated using L929 cells and showed low toxicity without any change in cell morphology. The ambient light antimicrobial mechanism of BiOClBr-coated fabrics showed high reactive oxygen species residue OH*, O 2 *, HO2*, and H2O2 generated on the surface cause oxidative stress on the bacterial cells and cell death (Khin et al., 2021).

3.3 Antimicrobial polymer coatings in biomedical applications The microbial adhesion and biofilm formation on material surfaces is a serious concern since they can lead to a variety of infectious diseases. The polymeric antimicrobial coating approaches can prevent bacterial adhesion and biofilm formation with their antiadhesive and biocidal properties. Besides, microbial killing can be achieved through chemically modified surfaces and antimicrobial agents incorporated in polymers. The biocompatible, as well as antimicrobial polymeric coatings were applied on the surfaces of various biomedical devices such as implants, catheters, contact lenses, and tissue engineering scaffolds. In this section, we summarize different polymeric

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antimicrobial coatings and their applications in biomedical fields. Charge switchable, antibacterial, and antibiofilm coatings based on cationic PEI derivatives for preventing catheter-associated infections. Herein, the antibacterial activity of linear polymer showed a higher bacterial growth inhibition with MIC values of 62.5–125 μg/mL against S. aureus and 500–1000 μg/mL against E. coli when compared to branched PEI derivatives (MIC values 125–500 μg/mL against S. aureus and 1000 μg/mL against E. coli). The linear PEI derivatives showed excellent antibacterial activity against drug-resistant bacterial biofilm with a MIC 125 μg/mL. In the comparative study of cationic PEI, cationic and zwitterionic PEI, and fully zwitterionic PEI, the cationic PEI showed 99.9% bacterial inhibition and negligible hemolysis of red blood cells. Herein, the mode of antibacterial action was through the active mechanism, the positively charged polymer penetrate through the bacterial cell wall resulting in cell death. The in vivo toxicity of cationic PEI-coated catheter implanted in mice showed minimal inflammation of normal skin epidermis and dermis layer and confirmed an excellent in vivo antibiofilm formation against MRSA (Hoque et al., 2019). An antibiofilm coating based on protein-engineered polymers was developed for preventing implant-associated infections. The antibiofilm activity of GL13K peptide-incorporated hybrid coatings were tested against Streptococcus gordonii biofilm formations. The coating displayed antibiofilm and bactericidal activities, confirmed by LIVE/DEAD and SEM imaging. Besides, the antibiofilm activity of the hybrid coatings was also tested against oral microcosm biofilms, of cariogenic patients confirmed by confocal laser scanning microscopy. Notably, the GL13K peptide incorporated hybrid coatings displayed a significant reduction of biofilm about 65%–70% and demonstrated an antimicrobial property for 6 days. The cytocompatibility of the antimicrobial coating was studied in human gingival fibroblasts and showed 100% cell viability indicating the antimicrobial coating is safe to use in dental applications (Acosta et al., 2020). Bifunctional antifouling and antimicrobial polymer brush coatings developed from TA, pMPC, and polylysine on different surfaces such as stainless steel, Au chips, and silicon wafers. The antiadhesion activity of the coatings was evaluated by protein absorption test, and notably Au-TA-p(Lys) surface absorbed protein two times greater than Au-TA-p(MPC) and Au-TA-p(MPC)/p(Lys) surfaces. The antiadhesion and antimicrobial activity were investigated against S. epidermidis and SS-TA-p(MPC)/p(Lys) coating showed 89% bacterial adhesion eradication due to the presence of cationic p(Lys) brushes, which

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induce the cytoplasmic leakages by rupturing the bacterial cell membrane. The antibacterial efficiency of SS-TA-p(MPC)/p(Lys) showed 65% because of the combined effect of cationic p(Lys) chains and hydrophilic zwitterionic p(MPC) brushes. The adhesion capacity of SS-TA-p(MPC)/p(Lys) coatings against Amphora coffeaeformis showed a 69% reduction in bacterial adhesion. The biocompatibility evaluation of SS-TA-p(MPC)/p(Lys) coatings using 3T3 fibroblasts cells showed 88% cell viability and confirmed the biocompatibility can be used in the aquatic as well as healthcare environments (Xu et al., 2018). The rechargeable, antimicrobial covalent coating based on QBEst and QBAm was developed against various microbes for several biomedical applications and for treating implant-related infections. Against S. aureus, E. coli, and MRSA, the coating showed excellent antibacterial activities with MIC values ranging from 0.5–1 μg/mL. The antibacterial activities of QBEst- and QBAm-coated surfaces showed the complete killing of bacteria, by exhibiting almost 5 log reduction in the bacterial count by denaturating bacterial cells due to the presence of hydrophobic aliphatic chains and a polycationic nature of the coating. Notably, the covalent incorporation QBEst and QBAm on the surfaces showed repeated antibacterial activity even after four repeated washing and drying cycles. The QBEst and QBAm coating also displayed antifungal activity against C. albicans because the degradation of the ester/amide linkage of QBEst or QBAm leads to the formation of zwitterionation. The antiviral activity of the coating was also confirmed against A/NWS/33 (H1N1) by plaque assay (Ghosh et al., 2020). A pH-responsive dual-functional (antifouling and antimicrobial) coating was developed from multilayered dextran and chitosan. The coatings showed excellent antifouling properties with the lowest protein adsorption capacity with a response unit of 1.7  102 in the multilayered coating. The bacterial adhesion on the coating was studied using S. aureus and E. coli, and the coating showed 94% of antifouling and antimicrobial activities as increasing the number of coating layers due to the synergetic effects of the two surface properties. The coating showed good antifouling activity against A. coffeaeformi with 7% of attachment on the coating with 11 layers. The LBL-deposited multilayer coating displayed an excellent self-polishing nature due to the presence of increased surface hydrophilicity and surface roughness. The coating exhibits minimal cytotoxicity (82% of the viability) against 3T3 fibroblasts cell line, and the cytotoxicity of the coating was found to be increased according to the number of layers (Xu, Liu, Pranantyo, Neoh, & Kang, 2018). Two series of antibacterial coatings based on PEI-PGMA and zwitterion monomer SBMA were developed on silicon contact lenses for the treatment

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of eye-related infections. Herein, the in vitro antibacterial assays of the coating formulations exhibited an excellent (>99.99%) bacterial killing efficacy against MRSA with an average log reduction of 5.1 and 6.3 respectively. The antibacterial property is because of the presence of hydrophobic polycationic PEI chain and poly SBMA opposes the bacterial adhesion and multiplication. The biocompatibility of the coating formulations was assessed with NIH 3T3 cells fibroblasts displayed 79% and 90% cell viability, which confirms that toxicity increases on higher PEI concentrations (Pillai et al., 2020). An antimicrobial and antifouling polymer brush coating was developed to control infections related to biomedical devices. These coatings showed excellent contact-active bactericidal activity against S. aureus, E. coli, P. aeruginosa, and C. albicans with a killing efficiency of 97.6%, 99.7%, 94.6%, and 95.6%, respectively. Herein, the hydrophobic cationic segments of the polypeptides/polypeptoids showed good resistance to the adhesion of dead cells and the killing of live cells. The coating can prevent the biofilm formation of S. aureus and E. coli and protein absorption is reduced by 83.4% for bovine serum albumin (BSA) and 87.8% for lysozyme. The in vitro biocompatibility of the coating was assessed on L929 cells and showed cell proliferation for up to 4 days. The in vivo antiinfective activity of the coating was examined in the rat subcutaneous S. aureus infection model. The implants with poly(Ppep/Psar) coating showed a reduced bacterial count and inflammation, compared to uncoated implants (Gao et al., 2017). QAS-based coating (PMT-5% and PMT-10%) was fabricated by Li et al. and investigated the antimicrobial activities against bacteria and fungi. The QAS-based coatings showed antimicrobial activity against S. aureus, E. coli, and fungus C. albicans with a MIC of 62.5–125 μg/mL for bacteria and >500 μg/mL for fungus. The QAC copolymer interacts with anionic bacterial cell surfaces results in the disruption of the cytoplasmic membrane, which leads to the killing of the bacteria. The cytotoxicity of the coating was investigated with human dermal fibroblasts (HDF), keratinocytes (HaCaT), and human embryonic kidney HEK293 cells. Notably, the biocompatibility of the various formulations was investigated using cytotoxicity assays with HDF cells, HaCaT keratinocytes, and HEK293 cell lines confirmed minimal toxicity compared to commercial QAS monomeric antimicrobial agents (Li, Bao, et al., 2016). An antifouling and antimicrobial coating based on maleopimaric acid quaternary ammonium cation containing PDMS-g-PNVP-MPA-N+ polymer was developed to prevent infections related to biomedical implants. The coating showed high killing efficiency and antibiofilm activities against pathogenic bacteria including

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S. aureus with a 1.35 log reduction and E. coli with a 1.02 log reduction and P. aeruginosa with a 1.29 log reduction (Fig. 5A(i)). The antibacterial property of the polymeric coating was due to the interaction of cationic MPA-N+ functionality with negatively charged phospholipid bilayer of microbes leading to the cytoplasmic leakage and cell lysis (Fig. 5A(ii)). The antifouling character of the coating was examined by protein absorption assay using BSA, lysozyme, and fibrinogen. Besides, the hemocompatibility and cytotoxicity of coatings showed an acceptable hemolytic rate and demonstrated high cell viability (91.87%) toward human aorta smooth muscle cells even at 5 days of incubation (Fig. 5B(i) and (ii)). The in vivo antiinfective property of PDMS-g-PNVP-MPA-N+ was evaluated using a rat subcutaneous infection model (Fig. 5C (i) and (ii)). The number of adhered bacteria on the PDMS-g-PNVP MPA-N+ implant is significantly lower than that on the pristine PDMS implant, with a log reduction of 4.89 (Fig. 5C(iii)) (Z. Li et al., 2020). A nonleachable, antibiofilm, and antibacterial coating based on AMPTMA, PEGDMA, and Q-PEI-MA were developed for catheterassociated urinary tract infections. The antibiofilm activities of coated PDMS catheter showed a 98.59% reduction of biofilms against MRSA and a 95.63% of reduction against vancomycin-resistant Enterococcus faecalis (VRE) biofilms were confirmed by in vitro antibiofilm assay. The antibacterial capacity of coated PDMS catheter with a reduction of 98.88% against MRSA and 94.51% against VRE was confirmed by in vitro antibacterial assay. The hemolytic activity of coated PDMS catheter displayed less than 2% of hemolysis of red blood cells. The in vitro biocompatibility of the coating was examined on 3T3 fibroblast cells and PDMS-coated catheter showed viability above 95%. Besides, the in vivo antibacterial activities were investigated with mice model infected with MRSA and VRE showed 98.88% and 94.51% reduction of MRSA and VRE respectively (Zhou et al., 2017). Multifunctional composite chitosan/bioglass coatings loaded with gentamicin to prevent infection related to orthopedic implants. The release profile of antibiotic gentamicin from the coating showed initial burst release and a study release of nearly 50% was observed within 28 days. The antibacterial activity of gentamicin-loaded coating against S. aureus (ATCC 25923) displayed a zone of inhibition of about 13 mm in 2 days of incubation, due to the burst release of gentamicin by increasing pH due to the degradation of bioactive glass. The cytotoxicity of the coating was evaluated using MG-63 cells, and the coated material showed low toxicity and supports the cell attachment and proliferation of osteoblast-like cells in 7 days of culture (Pishbin et al., 2014).

Fig. 5 (A) Antimicrobial activity comparison of PDMS, PDMS-g-PNVP, and PDMS-g-PNVP-MPA-N+ antimicrobial coating against S. aureus, E. coli, and P. aeruginosa. (i) Digital images on agar plates and surviving bacterial colonies. (ii) SEM images of the adhered S. aureus and its morphology in various surfaces. (B) (i) and (ii) Live/dead staining of human aorta smooth muscle cells and hemocompatibility of the coatings. (C) (i) and (ii) Photograph of the rat subcutaneous wound model after implantation, and histological study of the peri-implant soft tissues. (iii) The viable colonies and number of colonies recovered from coated implants after 7 days. (Adapted with permission from Li, Z., Wang, S., Yang, X., Liu, H., Shan, Y., Xu, X., Shang, S., Song, Z. (2020). Antimicrobial and antifouling coating constructed using rosin acid-based quaternary ammonium salt and N-vinylpyrrolidone via RAFT polymerization. Applied Surface Science, 5 (30), 147193. Copyright 2020 Elsevier.)

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A pH-responsive antimicrobial and antifouling coatings based on PDPAb-PMPC copolymer for application in biomedical and marine environments. The PDPA-b-PMPC copolymer brushes exhibited pH-responsive reversibility in the surface wettability. At pH -7.4, a water contact angle of 31 degrees was observed and showed a decreased water contact angle of 23 degrees at pH-5.5 due to the hierarchical rearrangement of the Si-TAPDPA-b-PMPC/PLYS surface. The Si-TAPDPA-b-PMPC/PLYS brushes displayed excellent antimicrobial activity against S. epidermidis, and E. coli with 77% killing efficiency as well as, bacterial adhesion. The microalgal (A. coffeaeformis) attachment with only 7% of adhesion on the surfaces due to the pH-responsive switchable transition between antifouling and antimicrobial properties. The fouling resistance of the coating was investigated by microbial and protein absorption assays. These coatings showed good antifouling ability against protein adsorption, bacterial adhesion, and A. coffeaeformis attachment because of low surface roughness and zwitterionic PMPC chain. The biocompatibility of PLYS-TA-PDPAb-PMPC coating was evaluated on 3T3 fibroblast cells and showed 90% cell viability (Xu et al., 2020). A polymer adhesive-antimicrobial coating loaded with PEI, graphene, graphene oxide, and molybdenum trioxide to prevent the growth of pathogenic bacteria on surfaces of medical devices. The stability of the coating was evaluated by leaching test, which showed negligible release of the active substance. In addition, no significant loss of material and no sign of toxicity were observed in human cells as well as bacterial cells during 16 h incubation. The antimicrobial activity of the adhesive coating against E. coli and B. subtilis depends on the concentrations of antimicrobial agents, especially the PEI and GO concentrations. The mechanisms of action of the polymeric coating with antimicrobial agents have confirmed SEM images and showed damaged cell membranes and twisted shapes because of the toxicity of GO and cationic PEI. Notably, the coating showed cytotoxicity against bacterial cells, however, not to human corneal epithelial cell lines (Nguyen et al., 2017). An antimicrobial coating-based supramolecular assembled lysozyme protein material enriched on polymeric aggregates showed antimicrobial activities against S. aureus, E. coli, and fungus C. albicans with killing efficiency of 95%, 92%, and 94% respectively. As a result of synergistic combination amino acids with hydrophobic moieties and positive surface charge contribute to the growth inhibition of the microbes. The coating exhibits long-term contact-killing mechanisms due to the strong interaction of antimicrobial agents with polymeric aggregates, which maintains a sustained release of antimicrobial agents. The coating exhibits low

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hemolysis of red blood cells as well as low cytotoxicity against CCK-8 cell lines. The in vivo evaluation of PTL-coated catheters was examined in a rat model of E. coli infection and showed excellent inhibition of the infection with an average 4–5 Log reduction of bacteria (Gu et al., 2017). A thermoresponsive, AgSNP@TRP and Ag-CNT@TRP antimicrobial nanocomposite coating were developed for a broad range of antimicrobial applications. The nanocomposite coatings showed good antimicrobial activities with 75% inhibition against S. epidermidis and E. coli due to the release of Ag ions. The antimicrobial activity was enhanced by increasing the temperature from 27°C to 37°C by shrinking the TRP layer on the AgNP. The incorporation of TRP into the coating showed controllable antimicrobial activity by shielding the AgCNs and Ag ions entering into the bacterial cells. The mechanism of action on the microorganisms by Ag ions promotes the production of reactive oxygen species in the cells, which blocks the activity of thiol-containing antioxidative enzymes and damages the cell membrane, protein, and intracellular system. The coating showed viability of >80% to HEK-293 cells, as confirmed by the MTS assay (Poudel et al., 2017). Based on dual-functional polymers, (PDA/uhPDMA, PDA/PDMA-coAPMA-I) an AMP conjugated coating was developed against catheterassociated urinary tract infection. The antibiofilm efficacy of the coating was evaluated by live/dead assay against Staphylococcus saprophyticus, the BA-AMP conjugates showed more resistance to bacterial adhesion compared to MA-AMP conjugates. The antibiofilm efficacy of BA-AMP (E6) and MA-AMP(E6) coated on polyurethane catheters demonstrated antiadhesion and antibiofilm activity against S. saprophyticus and P. aeruginosa. The mechanism of action BA-AMP and MA-AMP surfaces was observed through SEM, indicating membrane collapse due to the electrostatic interaction of AMP and bacterial cells. The BA and BA-AMPcoated polyurethanes are hemocompatible and showed cell viability over 90% to human bladder epithelial cells. The antibiofilm activity of AMPtethered coatings in a mouse urinary infection model showed an excellent inhibition of biofilms in BA-AMP and MA-AMP coatings (Yu et al., 2021). Zhang et al. developed an antibacterial hydrogel based on poly(acrylic acid-co-acrylamide) coating for applications in blood-contacting devices. The antimicrobial activity of the coating is due to poly(acrylic acidco-acrylamide), which acts as a barrier against bacterial adhesion and antimicrobial agents. Herein, the release of Ag, gentamicin, or Bac2A from the hydrogel can interact with bacteria through electrostatic interaction, which ruptures the cell. The coating showed efficient antimicrobial activity against

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E. coli and S. aureus, and demonstrated a 4–10 mm zone of inhibition. The coating showed less than 5% of hemolysis of blood cells with 75% blood compatibility and causes serious hemolysis when the concentration increases to a certain extent. The ex vivo blood circulation tests using rabbit models implanted with hydrogel PVC catheters showed no thrombus deposition with the hydrogel coating (Zhang et al., 2021). A hybrid antimicrobial coating based on PDA, HA, AgNPs, and CS was developed by Li and coworkers. Herein, the antimicrobial efficiency of the coating was dependent on the Ag ion release, and the hybrid coating showed CS layer dependent releases. The presence of CS layer coating exhibits a very high release rate for 4 days and subsequently, a sustained release was observed for more than 2 weeks. The hybrid coating showed excellent antimicrobial activity against E. coli and S. aureus with a killing rate of 78.0% and 59.3% for the coating composition PDA/HA/Ag/CS-1. The bacterial morphology after interacting with PDA/HA/Ag/CS-1 was examined by SEM, showing a rough and wrinkled bacterial surface after 12 h of incubation. Besides, the PDA/HA/Ag/CS-1 coating showed cytocompatibility against mouse calvarial cell lines and promoted osteogenic proliferation (Li, Liu, Xu, Yeung and Wu, 2016). A dual-layer antimicrobial coating based on PEG and synthetic peptide (KRWWKWWRR) was developed for efficacious urinary catheter protection. These coatings showed excellent antibacterial activity against uropathogenic bacteria such as E coli, S. aureus, and P aeruginosa in 6 days of incubation time as a result of sustained release of antimicrobial peptide. The coated catheter demonstrated antibiofilm performance, which was confirmed by CLSM observations. Besides, in the in vivo antibacterial activity was investigated in a mouse excision wound model, which indicated good antibacterial activity by reducing wound area 10-fold, and confirmed that the coating can reduce bacterial colonization (Le et al., 2021). An antimicrobial coating based on copolymer brushes was conjugated with AMP to produce infection-resistant surfaces in implants. The antimicrobial activity of AMP-conjugated polymer nanoparticle surface was tested against S. aureus, E. coli, and P. aeruginosa, which showed excellent antimicrobial activity with a lower MIC of 72.3 μM. The E6 conjugated to PDMA, PMPC, and PMPDSAH brushes showed a killing efficiency of 100%, 77.2%, and 65.8%, respectively. The AMPconjugated copolymer showed an active mode of action by transforming into a secondary structure and interacting with bacterial membrane leading to cell death (Yu et al., 2015). Representatively, various antimicrobial polymers, antimicrobial agents, and their applications were summarized in Table 1.

Table 1 Representative examples of antimicrobial polymer coatings used in food, textile, and biomedical applications. Antimicrobial polymer

Antimicrobial agents

Applications

Reference

Jeffamine- ED-2003

Piperazine bifunctional coupler PEI

Textile industry

Jalageri et al. (2019)

Textile industry

Hoque et al. (2015)

Chlorine

Textile industry

Liu et al. (2015)

Copper Quaternary ammonium group AmC16

Textile industry Aquaculture nets

Bataglioli et al. (2022) Druvari et al. (2016)

Silver ions Crystal violet, methylene blue, Safranin O, and gold nanoparticles BiOClBr microparticles

Diabetic shoe Antimicrobial paint

Xia et al. (2018) Beech et al., 2015

Laboratory and factory clothing Food packaging applications

Khin et al. (2021)

Linear and branched polyethylenimine (PEI) Chlorinated poly((3acrylamidopropyl) trimethylammonium chloride) and poly(2acrylamido-2-methylpropane sulfonic acid) Hybrid alginate-Cu(II) Poly(4-vinylbenzyl chloride-coacrylic acid) and poly(sodium 4-styrenesulfonate-co-glycidyl methacrylate) Collagen/GA@AgNPs Vinyl acrylic copolymer

Poly(vinyl alcohol) and poly(acrylic acid) Poly(2-(dimethylamino)-ethyl methacrylate-co-methyl methacrylate) and polyethylene glycol dimethacrylate

Quaternary ammonium compound

Zhao et al. (2016)

Continued

Table 1 Representative examples of antimicrobial polymer coatings used in food, textile, and biomedical applications—cont’d Antimicrobial polymer

Antimicrobial agents

Applications

Reference

Glucomannan/beeswax/chitosan Sodium alginate

Edible coating on fruits Edible coating on fruits

Meindrawan et al. (2020) Peretto et al. (2014)

Poly(L-lactide) films

Chitosan Carvacrol and methyl cinnamate Silver ions

Food packaging applications

MPN@DBS-NP and MPN@SNC Low density polyethylene film Polyethyleneimine derivatives

Tannic acid Silver ion PEI

Protein-engineered polymers TA-p(MPC)/p(Lys)

GL13K peptide p(Lys), p(MPC)

Chitosan/bioglass coatings Poly(acrylic acid-co-acrylamide)

Gentamicin Ag, gentamicin, or Bac2A

Chronic wound applications Milk packaging applications Catheter-associated infections Implant-associated infections Aquatic and healthcare environments Orthopaedic implants Blood contacting devices

Martı´nez-Abad et al. (2014) Qin et al. (2019) Bandpey et al. (2017) Hetrick and Schoenfisch (2006) Maan et al. (2020) Hoque et al. (2019) Pishbin et al. (2014) Zhang et al. (2021)

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4. Conclusions In the past decade, antimicrobial polymer coatings emerged as a class of smart materials, which were extensively investigated for a wide range of applications. The antimicrobial polymers were synthesized by various polymerization techniques, by combining other macromolecular synthesis tools such as click-chemistry, Schiff-base, amidation, and esterification reactions. The fabrication of the coating on various surfaces was achieved by different coating techniques (dip coating, spray coating, etc.) and has been widely employed in environmental protection, biomedical, textile, and food industries. The antimicrobial polymer coatings have proven their efficiency to inhibit biofilm formation, as well as to mitigate the infection spread caused by various microbes such as bacteria, fungi, and viruses through passive or active defense mechanisms. The examples discussed in this chapter have demonstrated the current status of antimicrobial polymer coatings and their applications in food and textile industries, and in biomedical domains. It is expected that there is still a whole field to be explored in terms of existing and new coating platforms to eradicate the spread of infections from potentially pathogenic microorganisms.

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Xu, G., Neoh, K. G., Kang, E. T., & Teo, S. L. (2020). Switchable antimicrobial and antifouling coatings from tannic acid-scaffolded binary polymer brushes. ACS Sustainable Chemistry & Engineering, 8(6), 2586–2595. Yu, K., Alzahrani, A., Khoddami, S., Cheng, J. T., Mei, Y., Gill, A., Luo, H. D., Haney, E. F., Hilpert, K., Hancock, R. E., & Lange, D. (2021). Rapid assembly of infection-resistant coatings: Screening and identification of antimicrobial peptides works in cooperation with an antifouling background. ACS Applied Materials & Interfaces, 13(31), 36784–36799. Yu, K., Lo, J. C., Mei, Y., Haney, E. F., Siren, E., Kalathottukaren, M. T., Hancock, R. E., Lange, D., & Kizhakkedathu, J. N. (2015). Toward infection-resistant surfaces: Achieving high antimicrobial peptide potency by modulating the functionality of polymer brush and peptide. Applied Materials & Interfaces, 7(51), 28591–28605. Zhang, F., Hu, C., Yang, L., Liu, K., Ge, Y., Wei, Y., Wang, J., Luo, R., & Wang, Y. (2021). A conformally adapted all-in-one hydrogel coating: Towards robust hemocompatibility and bactericidal activity. Journal of Materials Chemistry B, 9(11), 2697–2708. Zhao, J., Ma, L., Millians, W., Wu, T., & Ming, W. (2016). Dual-functional antifogging/ antimicrobial polymer coating. ACS Applied Materials & Interfaces, 8(13), 8737–8742. Zhao, J., Millians, W., Tang, S., Wu, T., Zhu, L., & Ming, W. (2015). Self-stratified antimicrobial acrylic coatings via one-step UV curing. ACS Applied Materials & Interfaces, 7(33), 18467–18472. Zhou, C., Wu, Y., Thappeta, K. R., Subramanian, J. T., Pranantyo, D., Kang, E. T., Duan, H., Kline, K., & Chan-Park, M. B. (2017). In vivo anti-biofilm and anti-bacterial non-leachable coating thermally polymerized on cylindrical catheter. ACS Applied Materials & Interfaces, 9(41), 36269–36280.

CHAPTER 12

Nano based technologies for antibacterial, antifungal, and antiviral coatings Vincent Femilaa Rajana, Rekha Pachaiappanb, Lorena Cornejo-Ponceb, and A. Geethac a

Department of Sustainable Energy Management, Stella Maris College (Autonomous), Chennai, Tamil Nadu, India b Department of Mechanical Engineering, Faculty of Engineering, University of Tarapaca, Avda, General Velasquez, Arica, Chile c Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India

1. Introduction The current viral pandemic and the threat we have on antibiotic-resistant bacteria illustrate the ever-increasing need to discover new novel pharmaceutical compounds to conflict against microbial pathogens. Compounds derived from the nature with varied pharmacological properties are a key to overcome the inevitable threats (Zulhendri et al., 2021). Reduction in the effectiveness of the existing industrial products and the increase in the spread of infections have increased the demand for fresh antimicrobial products. However, it is extremely difficult to evolve a new unique product that either regulates or stops the occurrence of infections. Industrial sectors and academic institutions are conducting extensive research work for understanding the origin and attainable regulations of health hazards especially through microbes (Tiwari & Chaturvedi, 2018). Companies contend to develop new products that would limit the spread of infectious microbes. In order to prevent bacteria from existing on the implant surface structure, various methods have been developed and this would reduce bacterial colonization by taking measures to prevent antibacterial adhesion onto the prosthesis surface (Qin et al., 2018). In recent days, antibacterial coatings have emerged as a mitigation strategy for pathogenic bacteria. The strategy would be to develop antibiotic resistance and restrict the nosocomial infection rate with no side effects (L. Wang et al., 2021). For dental implants, designing coatings that control bacterial adhesion and have cell stimulatory behavior

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remain a challenging strategy. One of the main risk factors associated with implant failure is implant-related infections during the early healing period (Cordeiro et al., 2021). Bacterial infections are the main agent of implant failure that necessitates further medical care. By the use of antibacterial materials, we could coat the implant surface to reduce bacterial adhesion and proliferation. An ideal implant surface inhibits the activity of bacteria and promotes the growth of living cells (Song et al., 2021). The antibacterial material coating could help to break the spread of viruses and bacteria via metallic surfaces of biomedical devices, food packages, research laboratories, communication devices, and public infrastructures such as transports, shopping malls, toilets, etc. Antibacterial material coatings will resolve the major engineering problem of biofouling on marine installation and watercraft surfaces via anticorrosion and tribomechanical properties. Nowadays, antibacterial nanocomposite coatings exercise various engineering applications (Owhal et al., 2021). In bioengineering applications, the growth of bacteria on biomedical implants inserted in the human body could be restricted through antibacterial material coatings. Infection caused by bacterial colonization on the surface of implants would bring excessive discomfort among patients who requires frequent replacement of implants. Biofilm formation may cause implant failure, local infection, and in the worst case, it may lead to the death of the patients (Ahmed et al., 2019). Hence, a medical implant can be overlaid with an antibacterial material coating, which is noncytotoxic and biocompatible in nature, to prevent bacterial infection. In recent times, it is apparent that the biofilms on biomedical devices could be formed by fungal species either in conjunction with bacteria or by themselves. The formation of fungal biofilms is similar to that of bacteria; few important differences are apparent. As long time known, microbiologists struggle to detect and identify fungal species for human pathogenic bacteria (Giles et al., 2018). This is the reason why the fungal contributions to device infections were not minded for some time and why the infections do not respond to the functionality of antibacterial drugs at certain times and why the fungal organisms were not obstructed and bloomed out after the bacteria were killed. Therefore, it is clear that fungal colonization has to be prevented in the design of infection-resistant coatings for medical devices (Giles et al., 2018). COVID-19 has become one of the most challenging pandemics of the last century that brought deadly outcomes and an increase in the rate of reproduction number. It highlights the decisive need on designing efficient

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vaccines in order to prevent virus infection and early and fast diagnosis by high selectivity and high sensitivity diagnostic kits. An emphasize is given on effective antiviral and protective therapeutics to decline and eliminate the viral load and its effects from tissue damage. Henceforth, to prevent and treat COVID-19, nontoxic antiviral nanoparticles were developed for clinical application. Nanoparticles show great hope to deliver nanovaccines against viral infections (Tavakol et al., 2021). Nanotechnology is an emerging field with the development of novel materials (of 5–200 nm in size). As with other fields, advancements in nanotechnology are exercised in medicine to treat varied diseases and disorders and are also utilized for therapeutic drug delivery. A dissolved drug is entrapped into nanoparticles that are biodegradable and is designed in a way to absorb the drug and to keep it against the chemical as well the enzymatic degradation. Nanotechnology assists in the delivery of pharmacologically active molecules with an accurate dose for site-specific action. With accurate dose, the nanotechnology is utilized in drug delivery systems which lower the side effects by depositing an active agent in the morbid region. A variety of materials such as polysaccharides, synthetic polymers, and proteins prepare the nanoparticles for drug delivery. An important route of drug delivery and the opting of materials rely on various factors such as the size of nanoparticles, surface characteristics such as permeability and charge, inherent properties of the drug, and degree of biodegradability (Lata et al., 2017). Gold and solid lipid nanoparticles show an appreciable result in cancer therapy and anticancer and antiviral therapy respectively. Nanosuspensions and nanofibers are also in use. Nanotechnology is used to facilitate the delivery of drugs in a suitable amount and in a targeted manner (Lata et al., 2017). Irregular and frequent use of antibiotics is the main cause of bacterial resistance in a public health crisis. An investigation by the Centers for Disease Control and Prevention (CDC) estimated that antibiotic resistance causes millions of illnesses throughout the world per year. It is estimated that by 2050 antibiotic resistance may cause tens of millions of death. Hence, novel antimicrobial agents and their development have become an emergent task (Tan et al., 2021). Nanotechnology is an approach, which is new, that stands for its greater efficiency. The functionalization or the composition of nanometric materials may act as an antimicrobial agent. It may penetrate deep into the skin, which acts with high efficiency and directionality (Souza & dos Santos Rosa, 2021).

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Viruses are still a threat that causes infectious diseases in humans in spite of an immense amount of research in virology and a greater number of developed vaccines. Earlier studies demonstrated that a threat to human health over the globe is caused by infectious viruses that are also responsible for chronic and acute infectious diseases. To reduce the lethal attacks of the new pandemic “Coronavirus Disease 2019” (COVID-19), sustained multidirectional research is being conducted by exploring activities in varied directions. A common route for SARS-CoV-2 to transmit is through viral contaminated surfaces which we humans are exposed frequently in our daily life (Ayub et al., 2021). Antimicrobial agents are widely available which kill or inhibit the growth of pathogenic microorganisms to reduce the spread of infectious diseases and to prevent cross infection. This may also counteract the development of unpleasant odor and the textile materials are also protected against microbial attack (Ibrahim et al., 2021). In spite of the long use of nanomaterials in biomedical applications, the field of nanomedicine has established itself as an individual field from nanotechnology by 2004. For medical purposes, the nanoparticles ranging from 1 to 100 nm are utilized. To make the nanoparticles suitable for biomedical applications, the properties such as the chemical and physical characteristics of the particles should be altered and the surface area should be increased. Nanoparticles with varied size, shape, and composition are consumed to improve the solubility, optical and magnetic properties of formulations, and biodistribution where the surfaces function with different ligands and coatings to enhance the tissue selectivity and to uptake into the targeted cells (Maksoudian et al., 2020). This chapter contextualizes the role of nanotechnology and nanoparticles as antimicrobial agents for varied therapeutic, diagnostic, and health-care applications.

2. Role of nanotechnology in smart coatings Nanotechnology is the use of matter, which is of nanometer size (109 m). It is one billionth of a meter-about a thousand times smaller than a red blood cell (Silva, 2004). Nanotechnology is defined as the research and technology development at the atomic, macromolecular or molecular levels harnessing a length scale of approximately 1–100 nm in any dimension (Yousif, 2017) (Fig. 1). The classical laws of chemistry as well as physics do not apply this at a very small scale for two important reasons. Firstly, the electronic properties of these nanoparticles are very different from their greater cousins. Secondly,

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Fig. 1 Wide range of applications of nanotechnology in various fields.

the ratio of surface area to volume becomes much higher and as the surface atoms are more reactive, the properties of materials could change in unexpected ways. A good example is—when we turn silver into very small particles, it takes antimicrobial properties whereas gold particles take any color we choose (Rokde et al., 2014). Nanotechnology may enhance the primary properties of traditional construction material (e.g., concrete), and may add functionalities to an existing material (e.g., self clean of coatings, paints, pollution-reducing properties, etc.). It may introduce new materials in order to fill an existing need. Nanotechnology is highly capable of reducing the impacts on the environment and energy intensity of structures. Meanwhile, it improves safety and decreases costs related to civil infrastructure. Nanotechnology is used worldwide now, as, it is the most developing one in varied fields such as electrical, mechanical, and electronic instruments. This technology is also used in the applications of the computer that prevent damage by weathering effects, corrosions, water, increasing performance, and many more effects. Importantly, it is cheap and hence the use of nanotechnology is expanded in machining operations such as in turning operations. The nanoparticles

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can be coated in various conventional tools with which we could turn a hard material, which is hard to turn. Nanocoating is an important part of nanotechnology. It is to perform painting or coating the nanoparticles to a destination material. This is a technique to functionalize the nanoparticles for incorporating them into polymer coatings. The coating would protect the underlying materials against the environment, which is gas permeation, thermal barrier, corrosion, and is also economical. In near future, innovations in formulating and developing industrial paints and coatings would be observed especially using nanotechnology. Novel findings of chemical and physical base research will let appear the technical phenomena of coatings in another light. Nanocoating was done utilizing various methods and some important methods include (Chinglenthoiba et al., 2017): • chemical vapor deposition (CVD) • physical vapor deposition (PVD) • atomic layer deposition (ALD) “Nano” in the field of coating—it is to employ a new method, process, and new physical instruments for the production, analysis, and development of nanosized materials. The variations between physics, biology, and chemistry vanish. Particles acquire new atomic properties that influence the mechanical, chemical, and optical properties of the surface (Chinglenthoiba et al., 2005). Nanotechnology-based coatings are extensively considered new intuition into finishing procedures for textiles. Nanostructured materials and nanoparticles are employed for the purposes of smart functionalization and surface modification of textiles (Gashti et al., 2016). To confer the antibacterial properties onto a given product, nanoscale silver is utilized by silver nanotechnology. This associates with the incorporation or application of nanosized silver particles onto the surface of the product. Varied end products and manufacturing processes include the nanosilver technologies. It appears fixed in a coating, which is employed for the product (coating) by the manufacturer. There are few products that come in a liquid form, which is meant to be applied to create a coating (spray and coating) (Fauss, 2008). The latest advances in the field of nanotechnology offer wide opportunities to address the challenges of bacterial infection by killing germs with no use of antibiotics (Y. Wang et al., 2020). Nanotechnology has acquired phenomenal achievement in the application of antibacterial exhibits great ability to reform antibiotic-free strategies. A number of

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novel advantages are offered by nanomaterials as it has a huge variety of compositions (Y. Wang et al., 2020). In recent times, nanoscience enables evolutionary change in certain technologies that eventually brings a wider and more revolutionary impact. Coatings could be created with enhanced properties using new approaches that employ nanoscale effects. An eventual impact of nanotechnology and nanoscience in the field of coatings will rely on our capability to direct the assembly of hierarchical (Makhlouf, 2014) systems that comprises nanostructures. There are two fundamental varied approaches to direct self-assembly is available. Firstly, it includes ordering an existing identifiable component into a desired structure or coating. The latter includes the creation of novel structures during the coating process (Baer et al., 2003). Nanotechnology is the control and manipulation of materials on a small scale that offers huge benefits to metal finishing industries that develops a number of exciting proponents. Since earlier, the nanocoatings have been greatly applied to a varied specialized array of objects right from the parts for a nuclear steam generator to an extravagant golf club (Bjerklie, 2005). The global smart coating market is mutilated in nature because of the presence of a greater number of players who operates the market. A report by Transparency Market Research says that the competition among the players is expected to rise in the near future. The statistics by Transparency Market Research state that the global smart coating market is likely to progress at a staggering 29.8% CAGR between 2017 and 2025—the forecast period. With the rise in this growth rate, the market valuation is expected to reach $9.82 bm by the end of 2025 (see https://www.prnewswire.com/ news-releases/growth-of-smart-coatings-market-to-increase-with-use-ofnanotechnology-in-manufacturing-tmr-895544070.html).

2.1 Nanotechnology against COVID-19 The evolution of severe acute respiratory syndrome (SARS-CoV-2) Coronavirus 2 in early 2020 has quickly led to the global pandemic of COVID-19 (Corona virus disease 2019). Since then scientific and clinical communities have colluded closely to develop active and efficient strategies to keep the ongoing pandemic under control. Nanomedicine and nanotechnology have been a game-changing field in current years which has a greater potential to not only design novel approaches but also to enhance existing methods to fight against (Corona virus 2019).

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Nanomaterials could be utilized for the development of reusable personal protective (PPE) equipment and antiviral nanocoating in a public setting that would forbid the spread of SARS-CoV-2. Smart nanocarriers hastened the design of several prophylactic, immune-mediated or therapeutic approaches against COVID-19 (Hasanzadeh et al., 2021).

2.2 An indispensable nanocoating Thin films and nanocoating have a definite benefit over bulk materials. Nanocoating is not just a layer that protects the surface layer; instead, it is an alteration at the level of the molecule. This is a point where the binding of nanoparticles occurs both chemical as well as physical to the surface to provide the fullest protection against corrosion, abrasion, etc. Due to their small size, an improvement is viable in the nanoparticles that penetrate the substrate surface (Reinosa et al., 2012). Nanocoating acts as a resistance to abrasion, corrosion, UV attack, and temperature change. Their properties are so unique such as low friction, antifogging, antimicrobial, biocompatible, and even more (Behera et al., 2020). Any coating that modifies the material property with regard to environment stimulus is a smart coating. This is a new material where the modifications could be reversed again. It holds coated materials that are adaptive, active, and responsive toward the environment or stimulus. Common technologies and methods utilized for the synthesis of smart coatings for corrosion protection are chemical conversion coating, microcapsule and nanocapsule-based polymer coating, layer-by-layer (LbL) self-assembling molecule deposition, shape memory (SM) and self-healing coatings, carbon nanotubes, clay nanotubes, nanoporous titania interlayer, self-healing ion-permselective conducting polymer coating, self-healing and self-cleaning superhydrophobic coatings, and waterborne smart coatings (Makhlouf, 2014). Products based on nanotechnology are efficient at inhibiting various pathogens including viruses regardless of their biological structure, physiology, and drug-resistant profile. Nanomaterials have been extensively demonstrated to be an alternative for sanitizing surfaces to inactivate viruses. Inhibition of fungi and bacteria that contaminates health-care related facilities reduces the risk of secondary microbial infections in COVID-19 patients via antimicrobial nanomaterials. Henceforth the cost-effective antiviral nanomaterials reduce the burden of COVID-19 in developing countries.

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2.3 Nanomaterial in medicine Nanomaterials are employed in varied health-care related applications such as diagnosis, wound dressing, anticancer therapies, drug delivery, diagnosis techniques, imaging tools, sanitizers, wearable devices, pharmaceuticals, implants, and vaccines. The properties of nanomaterials combat multiple pathogenic microorganisms and hence are widely utilized for the research and development of antimicrobial and antiviral drugs. Antiviral activity can be displayed by multiple nanomaterials. The chemical composition of these nanomaterials can either be inorganic or organic and their shape is polyandric or spheroid with size ranges from 1 to 50 nm. In general, the size of antiviral nanomaterials is smaller than viral particles such as SARS-CoV-2 viral particle, which holds a size of 120 nm. Hence, the interaction of nanomaterials can occur with the surface protein or with viral particles which may lead to the inactivation of viruses (Vazquez-Munoz & Lopez-Ribot, 2020). Currently, nanomaterial-based coatings are utilized for various applications and products especially from metallic elements such as titanium, bismuth, or silver (Sim, Barnard, Blaskovich, & Ziora, 2018). In the water industry, biofouling is a major concern because of the impact it has on the cost of prevention and maintenance of system functioning. Chemical and mechanical methods are utilized now to control biofouling such as biocides, which are not always effective. The demand for alternative methods to prevent biofouling therefore exists. Designing of the antimicrobial surface is an attractive alternate to constraining biofouling (De Kwaadsteniet et al., 2011). To prevent biological contamination and to increase the lifespan of equipment prone to biocorrosion and biofouling, surfaces are manufactured using antifouling and antimicrobial coatings. Spoilage of food can be prevented by the complete expel of air and prooxidants. This can also be performed by totally inactivating the enzymes that have a connection to food degradation. Antimicrobial and antioxidant molecules when added to food can help in protecting the food from spoilage. It is demonstrated that sterols, curcuminoids, vitamins, polyunsaturated lipids, acid polyphenol, flavonoids, and carotenoids, present in vegetables, fruits, and algae have certain extra properties such as considerable antimicrobial and antioxidant activities can be exploited to block, prevent or to delay the spoilage of food (Alfei et al., 2020). In the medical field, treating injuries and illnesses is so important. The ability to accomplish the procedures with no setbacks in a task may and at times cannot happen. In the biomedical field, coatings can be used to save

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lives. The coating on devices for implants required to be biocompatible with the body in order not to reject them. Following is the list of coatings that is being used in the medical field (Makhlouf & Rodriguez, 2020). • Thermal insulation—Prevents overheating by regulating temperature. • Self-healing—This has the ability to repair itself and for the device that was implanted already, the coating equips the device with resistance to corrosion. • Antifouling—Prevention of fouling organisms that grow outside or within the body. • Self-cleaning—Prevents the sticking of unwanted materials onto the surface. Microstructured hard metal can be coated with diamonds and can be processed to enhance the performance of the surgical blades. Neurosurgery and ophthalmic surgery are highly beneficial for this kind of enhancement (Shrikant, 2013).

3. Nanotechnology in therapeutic and health-care applications 3.1 Nanotechnology for antibacterial coating In orthopedic surgery, the preferred metals for permanent implants are titanium alloys, cobalt-chrome alloys, and stainless steel. This is due to their excellent characteristics such as mechanical stability, corrosion resistance, biocompatibility as well as fracture toughness (Hayes & Richards, 2010; Tejwani & Immerman, 2008). The demand for alternative multifunctional biomaterials through research for implants has been increased along with the latest innovations in medical technology and increased life expectancy (Navarro et al., 2008). The ultimate aim and the greatest challenge of current research are to increase the lifetime of implants and to eliminate the issues that limit their lifetime, e.g., infections associated with implants that can never be completely prevented or avoided (Diefenbeck et al., 2006; Prasad et al., 2017; Weinstein & Darouiche, 2001). These impairments may lead to deliberate health issues such that to perform consecutive surgical operations either to remove or to revise implants. This can in turn lead to unusual problems and may compromise the quality of life of patients depicting an expensive and unsatisfactory scenario with a negative socioeconomic impact. In orthopedic surgery, cementless and cemented prostheses are widely used. Significant efforts are taken to research in enhancing the bone-implant interface to

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improve bone anchorage and stimulate bone healing (Albrektsson et al., 1981; Ehrenfest et al., 2010). Biomaterials especially bio ceramics have been modified and developed from being inert to bioactive (Krishnan & Lakshmi, 2013; Sim, Barnard, Blaskovich, & Ziora, 2018). By the late 1960s, Hench presented bioactive glasses (silicate glasses) as material for bone bonding. The bioactive glasses have the feasibility to get bond with living tissues that form an apatite layer, where they put forth an impressive group of materials for the improvement of coatings on metallic implants for orthopedic and for dental applications (Hench, 2015). Several antibacterial agents are utilized to incorporate it into the coating to avoid bacterial colonization (Ahn et al., 2009; Li et al., 2012; Qin et al., 2015; Subramani et al., 2018; J. Wang et al., 2017; J. Zhang, Kong, et al., 2014; L. Zhang, Yan, et al., 2014). Gentamicin is an antibiotic, which is suitable to counter implant and related infections immediately after surgery. Antibiotic usage is controversial as the concentrations below minimum (MIC) inhibitory concentration for each species may generate antibiotic resistance (Moore et al., 1987). Silica-based nanomaterials act as the best candidate for developing functional coating as they are biocompatible, easy to handle, and cost-effective (Ducheyne et al., 2015). They are further categorized as nontoxic and hydrophilic depending on the dose and size (Kim et al., 2015). Silica (a typical nanoparticle spherical in shape) is probed to be a promising carrier system for drug delivery (S. Zhang et al., 2013). J. Wang et al. (2017) have stated that the preparation of silica nanoparticles incorporates gentamicin sulfate to produce an antibacterial carrier for preventing infections in dental or bone implants. In orthopedic applications, the electrophoretic deposition of coatings uses a wide range of materials and substances. To acquire varied coating features, many of them include gelatin or chitosan coating matrices which are either enhanced or modified according to the requirement (Corni et al., 2008; Gebhardt et al., 2012; Patel et al., 2014; Pishbin et al., 2011, 2013; Z. Zhang et al., 2016). Alkaline N-deacetylation of a shrimp obtains chitosan, which is biocompatible, chemically stable, has better film-forming properties, promotes cell adhesion, and has good mechanical properties (Dash et al., 2011). Gelatin is one other vital natural biopolymer that could be applied in varied fields such as the food, pharmaceutical, and medical industries. In recent days, titanium, titanium alloys, and stainless steel are widely used as artificial implant materials (Y. Zhao et al., 2014). Implant

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surface supports colonization, biofilm formation, and bacterial adhesion that produce implant failures very often which is due to implant-associated bacterial infection ( Jin et al., 2014; L. Zhao et al., 2009). One other factor is that it leads to stress shielding due to its high elastic modulus (Nagels et al., 2003; Staiger et al., 2006). As they are nondegradable, a secondary surgical operation for implant removal is essential because that increases the morbidity rate among patients and rises the cost to the health-care system (Erdmann et al., 2011; Kraus et al., 2012). The urgent need for biodegradability and to have a perfect match with that of human bone tissue, the implant materials are possessed as they have low elastic modulus and antibacterial ability. Magnesium metal (Mg) has elastic module and antibacterial properties such as nature bone among varied biodegradable materials (Lock et al., 2012, 2014; Robinson et al., 2010). Surface coating and alloying have been used to decrease the degradation rate of magnesium to make a match with a bone union (Tie et al., 2013). Mg alloys results in low alkaline pH due to high corrosion resistance which compromises the antibacterial activity as the antibacterial property of magnesium is proportional and attributed to alkaline pH (Robinson et al., 2010), which escalates with Mg corrosion (Staiger et al., 2006; Witte et al., 2006; Y. Zhao et al., 2014). The elements that hold antimicrobial property has to be chosen as coating or alloy compositions to develop the corrosion resistance of the magnesium alloy and to enhance/compensate for the antimicrobial activity. In recent times, a novel biodegradable Mg alloy, Mg-Nd-Zn-Zr, has been developed by adding zinc (Zn), neodymium (Nd), and zirconium (Zr) elements in Mg. This exhibits high corrosion resistance and proper mechanical properties (Guan et al., 2014; Niu et al., 2013; J. Zhang, Kong, et al., 2014; L. Zhang, Yan, et al., 2014; X. Zhang et al., 2012). It has been proved that Zn has antibacterial effects, which is one of the necessary elements in the human body. It has been reported that Zr also has antibacterial properties (Y. Zhao et al., 2014). To manufacture maxillofacial, orthopedic prostheses and dental implants, titanium (Ti) and its alloys are extensively used. In the United States, more than 500,000 total hip and knee replacement surgeries are performed with titanium implants every year and the number keeps increasing (see https://consensus.nih.gov/2000/2000medicalimplantsta019html. htm). The use of dental implants has become a mainstay of restorative and rehabilitative dentistry. Nanocomposites with drug-eluting capacity and bone-bioactivity are taken up to be potentially worthy coating materials for metallic bone implants (Patel et al., 2012; Arany, 2013). Commercially,

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titanium alloys with aluminum, iron, molybdenum, vanadium and others are used to manufacture orthopedic reconstructive prostheses like knee implants, hip implants, bone plates, and screws (Burke et al., 1996; Imam & Fraker, 1996) whereas, in dental implants, pure titanium is extensively used. Titanium dental implants may fail because of various factors like a bacterial infection, insufficient early osseointegration, surgical trauma, micromovements, premature overloading caused by improper surgical placement, inadequate prosthesis design, metal fatigue, and inadequate quantity and quality of bone surrounding the implant (H€ammerle et al., 1992; Rosenberg et al., 1991). To promote faster healing time, osseointegration, the longevity of titanium implants, higher bone-to-implant (BIC) contact ratio and also to fabricate implant surfaces, varied techniques of titanium surface modification have been employed. Poor osseointegration and bacterial infection of titanium implants are the two major reasons that hinder their usage in clinical applications. To resolve the issue, titanium dioxide (TiO2) nanotubes were loaded and fabricated with naringin (NA) and were covered using gentamicin sulfate (GS)—gelatin/chitosan bilayers are inserted using layer-by-layer technique. The multilayer-coated NA-loaded titanium dioxide (TiO2) nanotubes exhibit better biocompatibility by cell viability assay, cell morphology, mineralization, alkaline phosphatase activity, and the qRT-PCR analysis of osteogenesis-related genes. It is understood that bifunctional titanium dioxide nanotubes have antibacterial capabilities by antibacterial experiments that examined E. coli and S. aureus. This is an approach that gives a feasible and new strategy to enhance the properties of titanium implants in order to improve osteogenic and antibacterial activity. Bacterial infection is one of the world’s largest public health issues that was reported in millions of people every year. Nanomaterials display a greater potential for the treatment of infections via bacteria, but their effects remain limited by immune clearance and by low antibacterial efficiency. Fabrication was done by facet-dependent nanozymes that were coated with pathogen receptor membranes provided an approach for generating superphotothermal antibacterial nanomaterials with low immune clearance and high biocompatibility (Hou et al., 2021). The developing threat of pathogenic microbes has raised the rate of mortality and morbidity among the human population. Centers for Disease Control has disclosed that the world is entering into a postantibiotic era which would increase the death rate due to bacterial infections rather than cancer and fatal diseases. In order to prevent bacterial growth on biotic and

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abiotic surfaces, the multidrug-resistant strains of bacteria have driven varied novel strategies. Hence, the nanomaterials thrive to be a potential strategy to replace conventional antimicrobial therapy. Nowadays, bacterial pathogens are well known to colonize implants, medical devices, tooth filings, fabrics, lenses, nonmetallic and metallic surfaces. Silver (Ag), zinc oxide (ZnO), copper (Cu), gold (Au), selenium (Se), titanium (Ti), polymer materials and iron (Fe) are significant inorganic and organic nanomaterials, which are highlighted as highly capable coating materials to prevent the attachment as well as the growth of bacteria. Nanoparticles are the latest interest in the studies to encounter bacterial pathogens and thus can be utilized as potential anticoating materials. This led to the progress of new guidelines for the safety evaluation of nanotechnology-enabled medical devices. Till now metal and metal oxide nanoparticles such as Au, Ag, Se, CuO, TiO2, Fe3O4, polymeric NPs, ZnO, and polymer-based NP’s are reported as coating materials suited for biomedical applications. To reduce toxicity, easy degradation or damage of bacterial membranes/enzymes and to target specific delivery nanoparticles were used as drug carriers for the conventional antibiotics. To increase the bioactivity collagen, chitosan, silica NPs, and hydroxyapatite (HAp) are used and are also utilized as nanomatrices in medical devices (Pugazhendhi et al., 2021). Following are the different techniques, which can be used as antibacterial coating via nanotechnology. • Physical vapor deposition (PVD) coating • Plasma-assisted antibacterial coating • Thermal evaporation • Chemical vapor deposition • Sol-gel process 3.1.1 Physical vapor deposition (PVD) coating An application area in the field of nanotechnology is surface coating precisely the physical vapor deposition (PVD). Physical vapor deposition describes the processes of a family of thin film coating that is applied under vacuum conditions at a range of 10 2 to 10 4 Torr. Ultrathin layers can be produced by this technique with a coating of thickness in nanometers range (Luther, 2007). Improved material properties such as abrasion resistance, friction, surface fatigue, and tribological oxidation create a well-known condition that the coated metal surfaces are greatly beneficial rather than any uncoated tool and component. Metal coating brings higher life span of tools and greater resistance of components to deformity as its outcome. This also

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provides economic benefits compared to an uncoated system. Cutters, molds in plastics manufacturing, drills, and gears for automobiles are coated products. PVD coating process (in practice) comprises the following steps: • Cleaning tools and components • Creating vacuum • Charging • Sputtering with argon gas • Coating • Heating up to process temperature • Cooling with helium gas under reaction temperature • Investigation has been accomplished with TiN, TiAlN, Ti + TiAlN using the PVD process of metal coating in view to determine the quantitative impacts on the environment Physical vapor deposition process and its principle

Metals such as titan, chromium, tungsten, and aluminum are coated on a (substrate) base material within the process of PVD. These are combined with nitrogen or reactive hydrocarbon-based gases and form carbonitride and nitride coatings. Titannitride (TiN) is the predominant PVD surface coating. To attain higher hardness, titancarbonitride (TiCN) is further developed. For chipping applications, the modern metal coating comprises titan-aluminum-nitride (TiAlN). Smaller aluminum atoms replace a part of titan atoms here. Chemical stability and higher hardness are its advantages. Process duration determines that 1 and 5 μm are the total layer thickness of deposit on the components and tools. Fig. 2 shows the principle and the components of the PVD process. The solid metal coating material is converted to gas phase by the principle of the PVD process. To condense ionized coating gases onto the substrate, a high vacuum is required. Thermal energy, electric arc, a sputter process or an electron beam are various methods to vaporize metal coating material (target). Once the process of coating begins, the substrate is heated (i.e., molds, drills) in a range of 200–500°C using two heating process steps. During vacuum generation, radiant heaters are activated first. Secondly, plasma of electrons and argon ions are generated. To commence an atomic sputter etching process, the generated plasma needs an electromagnetic field. At sputter etching process, collision of argon molecules with the substrate takes place and this kinetic energy led to cleaning and heating the substrate. In general, an electric arc process is used to vaporize the metal coating

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Fig. 2 Schematic diagram representing the PVD process (Bauer et al., 2008).

material. This commences heat energy that vaporizes the metal target material and converts the solid metal into an ionized gas. The substrate attracts the ions and it works similar to a cathode (Fig. 2). The defined system boundary of a PVD process is provided in Fig. 3. 3.1.2 Plasma-assisted antibacterial coating In various fields, bacterial colonization of the synthetic material surface was found to be a greater issue and hence there have been substantial efforts

Fig. 3 Pictorial representation of the system boundary of the PVD process (Bauer et al., 2008).

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undertaken over many decades in order to determine solutions. Plasma treatment and plasma polymerization have become impressive modalities to develop antibacterial coatings and different strategies have been reported. Plasma polymers with antibacterial agents have been used as reservoirs that are released subsequently. These layers can also serve as a diffusion barrier to control the release rate. Plasma-modified surfaces are used as functional coatings for the attachment of bacteriostatic or biocidal molecules (Vasilev et al., 2011). During film growth, plasma is highly utilized by thin film deposition processes. An energetic deposition species and an in situ source of increased ionization are provided by the plasma. To influence the properties and growth of deposited films, various enhanced chemical and physical processes are suitably used by this technique. In the thin film deposition process, the plasma that is used at the space between the substrate and source is termed to be “plasma enhanced deposition process or plasma-assisted.” Researchers have introduced the latest invention of plasma-enhanced techniques along with earlier existing process developments that occur in chemical vapor deposition (CVD) and physical vapor deposition (PVD). This new technique is termed activated reactive evaporation (ARE) by (Bunshah, 1981; Bunshah & Raghuram, 1972) in which a positive probe electrode is incorporated to draw the electrons that are emitted by a vapor source. This increases the ionization of condensing species. Matthews and Teer (Matthews & Teer, 1980) termed their process as a thermionically assisted triode ion plating process and Thornton (Thornton, 1983) described this technique as a magnetron sputter ion plating (Matthews & Teer, 1980; Thornton, 1983). Process of deposition

Three major steps are involved in all of the deposition processes for the formation of a thin film onto a substrate (Randhawa, 1991). • Creation of flux of condensable species (ions and neutral atoms). • Transport of species created from source to substrate. • Growth of film on the substrate. 3.1.3 Thermal evaporation (TE) For thick and thin film deposition, thermal evaporation is a simple, fast, old, and economic method. Pure atomic elements such as nonmetals, metals, and molecules such as nitrides and oxides are the materials to be applied with the thermal evaporation technique. Semiconductor wafers, optical components or solar cells are the objects to be coated which are termed the substrate.

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The applications that use thermal evaporation are microscale fabrication, astronomical telescope mirrors, and macroscale production. Fig. 4 illustrates the TE technique. The source material is sublimated or melted into a gaseous phase under high vacuum by electrical heating. Condensation of vaporized material onto the substrate surface held at the temperature below the freezing point of the evaporated source material. Residual gas impurities are minimized and the particles from scattering are prevented by the high vacuum. 3.1.4 Chemical vapor deposition (CVD) An extensively used materials-processing technology is chemical vapor deposition (CVD). The applications of the technique are to produce high-purity bulk materials and powders, to apply solid thin film coating to the surface and to fabricate the composite materials through infiltration techniques. The CVD technique can be used to deposit a wide range of materials. Fig. 5 shows the periodic table with elements that have used the CVD technique for deposition. This occurs either in the form of the pure element and more frequently, it combines to form compounds. In a simple manifestation, chemical vapor deposition comprises the flow of precursor gases into a chamber, which holds one or more heated objects that need to be coated. Deposition of thin film on the surface is performed by the occurrence of chemical reactions near and on hot surfaces. This is assisted by the production of chemical by-products that exhausts out of the chamber along with the unreacted precursor gases. A wide range of applications and a

Fig. 4 Schematic diagram representing the thermal evaporation technique (AbuThabit & Makhlouf, 2020).

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Fig. 5 Schematic illustration of typical steps carried out in the CVD process. First, reactant gases (blue circles) are transported into the reactor (step a). Then, there are two possible routes for the reactant gases: directly diffusing through the boundary layer (step b) and adsorbing onto the substrate (step c), or forming intermediate reactants (green circles) and by-products (red circles) via the gas-phase reaction (step d) and being deposited onto the substrate by diffusion (step b) and adsorption (step c). Surface diffusion and heterogeneous reactions (step e) take place on the surface of the substrate before the formation of thin films or coatings. Finally, by-products and unreacted species are desorbed from the surface and forced out of the reactor as exhausts (step f ). CVD, chemical vapor deposition (Sun et al., 2021).

huge variety of materials deposited show that we have diverse variants of chemical vapor deposition (CVD). CVD terminology also has many derivatives such as organo-metallic chemical vapor deposition (OMCVD), metal–organic chemical vapor deposition (MOCVD), and which at times are used to figure out the class of molecules used in the deposition process. The process of CVD can be categorized based on the type (Sun et al., 2021), • The reactor and process used • Application • In chemical reaction a precursor is used. 3.1.5 Sol-gel process A wonderful growth in science is sol-gel technology and for varied applications, it requires a multidisciplinary approach. Sol-gel technology has different emerging applications such as in the areas of nanotechnology, defense, biomedical devices, and environmental monitoring (Chaudhury et al., 2007; Reisfeld, 2001). Generally, this process includes a transition of the system right from a liquid “sol” into a solid “gel” phase. Metal-organic compounds or inorganic metal salts are the starting materials that are used in the preparation of sol such as metal alkoxides [M(OR)n] where M denotes a network

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forming elements such as Ti, Si, Zr, B, Al, etc., and R is an alkyl group. Tetraethyl-orthosilicate (TEOS) and tetramethyl-orthosilicate (TMOS) are the precursors most often used in the sol-gel process (Brinker & Scherer, 2013; Dı´az-Garcı´a & Laı´nn˜o, 2005; Gupta & Chaudhury, 2007). Biocidal sol-gel coating

A serious complication we have on the implanted medical device is bacterial infection such as central venous catheters, prosthetic hip implants, etc. This may lead to premature implant removal that is expensive, traumatic to the patient and can be lethal. An invasive bacterial infection was observed even after the different control measures such as meticulous surgical procedure, sterilization, and ensuring proper infection preventive guidelines. It was reported that the infection rate found to be 90% on the sites of implantation soon after the surgery. Nowadays, to control and prevent microbial contamination of medical devices, latest strategies are exploited for developing materials with antimicrobial activities. Compared to various immobilization methods, the sol-gel dip coating process is found to be an adequate procedure to develop antimicrobial coating (Copello et al., 2006; Haufe et al., 2005; Oakes & Wood, 1986; Pardhi et al., 2020; Parida et al., 2016; Salvati & Brause, 1988; Schierholz & Beuth, 2001). Certain studies show that silver ions or silver have a broad-spectrum antibacterial activity to counter Gram-negative and Gram-positive strains (Feng et al., 2000; Ip et al., 2006; Kang et al., 2000). Stobie et al. (2008) reported a probable solution to the issue of biofilm growth on the short-term indwelling surfaces by low-temperature processed silver-doped phenyltriethoxysilane (PhTEOS) sol-gel coating that reduces the formation of Staphylococcus epidermidis over a period of 10 days. Silverdoped phenyltriethoxysilane coatings against planktonic S. epidermidis are depicted in Fig. 6, where an approximately 100% kill rate is caused by the release of silver ions. 3.1.6 Smart antibacterial coatings Varied applications of smart antibacterial thin films are antifouling membranes, antibacterial textiles, biomedical devices, food packaging, and wound dressing. Using the following mechanisms, the surface properties can be significantly altered. • Upon direct contact, the fabrication of responsive surfaces kills bacteria. • Surfaces that are embedded with the antibacterial agents show good biocide-leaching effect.

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Fig. 6 Antibacterial activity of silver-doped PhTEOS coatings against the planktonic S. epidermidis (CSF 41498) after 24 h (Stobie et al., 2008).

• Bacterial adhesion can be prevented by fabricating antiadhesion surfaces. Photopolymerization and photografting (Xin et al., 2016; Yang et al., 2016), electrospraying (Y. Liu et al., 2018), surface grafting (Roy et al., 2008; Vasilev et al., 2009), spray-coating (Noda et al., 2009), dip-coating (Su et al., 2010), spin-coating (Schwartz et al., 2012), LbL self-assembly (Fu et al., 2006), sol-gel coating (Marini et al., 2007), and bioinspired, antibacterial surface coatings (Glinel et al., 2012) are different functionalization and deposition techniques for a smart antibacterial surface. Bactericidal coatings and bacteria-resistant coatings are the two categories of antibacterial coating available in recent times. Nonspecific interactions with the components of the biological environment are prevented by bacteria-resistant coatings. This occurs by inhibiting the biofilm formation and by decreasing initial bacterial attachment. Bacteria attached to the surface are killed by a bactericidal coating that offered a direct route to prevent biofilm formation. In recent times, synergistic and smart responsive antibacterial coatings are classified into three categories as follows (Wei et al., 2019) (Fig. 7): 1. Synergistic 2. Smart kill and release antibacterial coating 3. Self-defensive

3.2 Nanotechnology for antifungal coating Fungal infections have become an alarming public health issue which has been aggravated due to the increase in the host predisposition factors.

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Fig. 7 Schematic diagram representing the three types of antibacterial coatings with a specific function. (A) Self-defensive antibacterial coating can “turn on” biocidal activity in response to a bacteria-containing microenvironment. (B) Synergistic antibacterial coatings possess two or more killing mechanisms that interact positively. (C) Smart “kill-and-release” antibacterial coatings can switch functionality between bacteria killing and bacteria releasing (Abu-Thabit & Makhlouf, 2020).

Though many drugs are available in the market to treat fungal infections, still their efficiency is questionable and we cannot neglect their side effects. Researchers have investigated the antifungal properties of chlorhexidine coating along with a range of nanoparticle additives that inhibits fungal infestations in dental silicones that are generally used as denture obturators and soft liners (AlKahtani, 2018). Sodium triphosphate (TP), hexametaphosphate (HMP), or trimetaphosphate (TMP) are the nanoparticles used with the solution of chlorhexidine. Hydrophobicity and the water uptake of denture silicons were not affected by the addition of the nanoparticles. Chlorhexidine-HMP coating has proved to be more effective in its

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antifungal activity by inhibition of the metabolic activity of Candida albicans. Maintenance of oral health and the longevity of dental prostheses are potentially possible with coating that becomes clinically essential (AlKahtani et al., 2018). Silver is recognized to be a prehistoric element. More than an inorganic antibacterial agent, silver has been extensively investigated for its antimicrobial property. Silver is well known to fight against a large range of biological processes in microorganisms that comprise the alteration of cell membrane structure and functions (Pal et al., 2007; Sondi & Salopek-Sondi, 2004; Stobie et al., 2008). Silver ions in micromolar doses (1–10 μM) are well enough to kill bacteria in water (Z. Liu et al., 1994) whereas they can be highly toxic to mammals (Conrad et al., 1999; Hirasawa et al., 1997), marine and freshwater organisms (Bianchini et al., 2002; Morgan et al., 2005) when delivered at high dose. Such silver in micromolar concentrations is not harmful to humans (Berger et al., 1976). Hence, silver is widely utilized in the development of various pharmaceutical and biological processes, its products and appliances as a coating material for topical aid for wound repair (Dowsett, 2004), for medical devices (Raad & Hanna, 2002) such as dental grafts or orthopedic (Hotta et al., 1998; Matsuura et al., 1997), water sanitization (Lin et al., 2002; Z. Liu et al., 1994), washing machines ( Jung et al., 2007), and even textile products (Takai et al., 2002). A common site of fungal and bacterial adhesion is the surface of a medical device that frequently leads to chronic infections. To prevent such complications, several chemical and physical modifications of the device surface are required. Layer-by-layer is a technique, which can be employed to experiment with a new kind of topical antifungal coating. 3.2.1 Layer-by-layer (LbL) technique In general, a thin film fabrication technique is a layer-by-layer method (LbL). The formation of films takes place by depositing oppositely charged materials as alternate layers. Dip coating, spray-coating, and spin-coating are a few of the other possible ways to perform the layers. This technique is based on an alternat assembly of oppositely charged polyelectrolytes. It can be enforced for noncovalent alteration of multiple substrates inclusive of medical implants. Extensive research is being carried out on polyelectrolyte multilayers fabricated by layer-by-layer technique for the development of antibacterial coating as they are loaded with antibacterial peptides and nanoparticles along with bactericide action, loaded with antibiotics and are also capable of restricting the adhesion of bacteria on to the surfaces.

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To prevent biofilm formation and bacterial adhesion, LbL assembly technique is employed as it offers a great advantage of large tunability and with that several strategies have been created by the large variety of available building blocks. Assembly is induced by an attractive electrostatic interaction between polyanions and polycations and entropy considerations pertinent to the release of counterions at the time of polyelectrolyte assembly (Farhat & Schlenoff, 2001; Schlenoff et al., 1998). This LbL is a powerful strategy enclosed technique for noncovalent modification of a charged surface. This technique is a reliable and easy method for surface engineering and it has varied potential applications in wide areas such as nanofiltration, catalysis, anticorrosion coatings, and optoelectronics (Holmes et al., 2014; Onitsuka et al., 1996; Ouyang et al., 2008). Polyelectrolyte multilayers (PEM) have also found to have various biomedical and biological applications. These comprise a coating either to prevent cell adhesion or to maintain or direct cellular phenotype and drug encapsulation (Mendelsohn et al., 2003) (Fig. 8). Initially, the LbL technique is developed for synthetic polyelectrolytes and later it has been extended to biopolyelectrolytes and various other

Fig. 8 Pictorial illustration of layer-by-layer technique (Escobar et al., 2020).

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materials that are assembled by hydrogen bonding, electrostatic interactions, or by ion coordination in the sequential protocol. Polyelectrolyte multilayers (PEM) can be developed by combining a wide array of components. To bring out potential applications in multiple areas such as filtration, sensing, energy or drug delivery, etc., expansion work is done by introducing biopolymers such as lipids, nucleic acids and proteins, inorganic particles, and enzymes in multilayer films (Decher et al., 1998; Laurent & Schlenoff, 1997; Lvov et al., 1995). PEM’s in addition can also be produced by spraying polyelectrolyte solutions covering large surfaces. Spray-based layer-by-layer technique is theoretically as easy as the dipping LbL and it can be applied (Dierendonck et al., 2014). Growth kinetics in the LbL technique has been classified into two major types: 1. Linear 2. Supralinear or exponential The amount of polyelectrolyte that is assembled per layer is constant in linear growth while the amount of polymer assembled will increase as the number of layers assembled increases in supralinear or exponential (De Villiers et al., 2011; Xu et al., 2012). 3.2.2 Antifungal edible coatings-on postharvest loss Major postharvest losses of citrus fruits are due to fungal diseases, weight loss, quarantine pests, and physiological disorders. Postharvest treatments and cold storage with conventional synthetic waxes, chemical fungicides, and a combination of both are the most commonly used ways to reduce postharvest losses. However, the continuous application of the above treatments has led to major problems such as environmental and health issues associated with waxes or fungicide residues containing the proliferation of resistant pathogenic fungal strains or ammoniacal compounds. Therefore, occurred an increasing demand for nonpolluting alternatives to be utilized as a part of integrated disease (IDM) management programs to preserve fresh citrus fruit. Among them, a technological challenge here is to develop novel natural edible coatings and films with antimicrobial properties for the industry. The development of antifungal edible coating confronts two important concerns of citrus postharvest handling, the losses due to pathological problems and the losses due to physiological problems. General practice in citrus packing houses is fruit coating that replaces the natural waxes which can be removed at the time of fruit washing and handling in the packing line. The citrus commercial coating is generally known

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to be waxed because of the fact that the composition of initial formulations is based on a combination of varied waxes such as carnauba or beeswax or based on paraffin wax. Commonly they are anionic microemulsions that comprises resins or/and waxes such as wood rosin, shellac, carnauba wax, polyethylene, beeswax, petroleum waxes, or candelilla wax. The ultimate purpose is to limit fruit shrinkage, weight loss, and to improve appearance (Chowdary & Napoleon, 2017; Felton & Porter, 2013). In certain cases, if the coatings restrict the gaseous exchange excessively during the storage of fruit, the fruit coating adversely affects the flavor of the fruit because of an overproduction of volatiles in association with anaerobic conditions (Fukui et al., 2017; Galet et al., 2010). Consumer interest in nutrition, food safety, and health along with environmental concerns has raised the interest of many research groups to create new biodegradable, natural, and edible coating formulations to replace conventional commercial waxes, thus avoiding the usage in the formulations of synthetic components such as ammonia, morpholine, or polyethylene wax. The theory of antifungal edible coating arrives when additional ingredients with properties of antifungal are employed in these biodegradable formulations to replace the use of traditional chemical fungicides for postharvest disease control (Table 1). Coatings vs films

Though the chemical composition of coatings and films are the same, sometimes they are used synonymously; they depict various concepts based on their different utilization and purpose. Films are defined as a stand-alone thin layers of materials that can be used as wraps, covers or separation layers. Films are generally prepared by a casting process and are commonly used for the determination of barrier, solubility, mechanical and other properties given by certain film materials. On the other side, food coatings include the formation of films directly onto the surface of the product to which they are intended to be applied (George, 2010). 3.2.3 Tablet coating Tablet coating is defined as a pharmaceutical technique where film coatings are applied to drug containing tablets or granules with polymer-based thin layer. In order to mix the tablets, rotating pans are used. To coat the film layers on the bed surface of the tablet, polymer solution spray nozzles are responsible. With increasing development in studies and techniques that have been performed to date, the application of an aqueous film coating also

Table 1 Materials used for edible coating (Kapoor et al., 2020). Material class

Examples

Advantages

Polysaccharide films

Chitosan and chitin, cellulose derivatives, starch.

Lipid films

Waxes and paraffin, acetoglyceride, shellac resins.

Protein films

Gelatin films, corn zein films, wheat gluten films, soy protein films.

Composite films

–

Excellent gas permeation property, help in achieving desirable modified atmospheres that enhance product’s shelf life in the absence of a required anaerobic condition, the shelf life of muscle foods can be extended by adding polysaccharide films and coatings. Their hydrophilic property forms a poor water-vapor barrier. In order to raise the resistance against penetration of water, lipids are united with film-forming materials such as polysaccharide, proteins, or multilayer coatings, in order. A flexible and stable barrier for CO2, O2 and ethylene is polar resin films. The presence of either hydrophilic portion or the absence of polar constituents is so small that no feasible interaction is possible with water, which avoids molecules to spread and form a monolayer. Waxes are effective barriers for vapor transfer because of their high hydrophobicity, which makes them water-insoluble and organic solvent-soluble. Proteins are good film formers exhibiting excellent gas and lipidbarrier properties particularly at low relative humidity. To enhance the water-vapor barrier, resistance to proteolysis of film and mechanical properties, the proteins are cross-linked by enzymatic, chemical, and physical means. Ability to enhance mechanical properties. Ability to carry food ingredients. Inhibition of the migration of moisture, oxygen, carbon dioxide, atoms, lipids, and so forth.

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increases in the field of pharmaceutical sciences especially in formulation development. Organic solvent-based coatings, include a wide range of benefits such as nontoxicity, cost-effectiveness and nonhazardous nature that make them comparatively highly beneficial. In recent times, to focus on major issues, novel techniques are developed to improve the overall processes and to refine types of coatings for greater application of pharmaceutical oral dosage forms. To prevent organic solvent-dependent coating hazards, the coating techniques such as magnetically aided impaction coating and electrostatic dry coating are used. The latter type of dry coating includes powdered coating composition and allows coating on either tablet facet along with distinguished colors if needed and desired. Advantages of tablet coating

Sticking tablets can be prevented by tablet coatings. This must be quite stable enough to undergo the handling of tablets. These coatings will improvise the mechanical strength of dosage form, will provide a smooth finish in creating easy swallowable tablets, and should follow the fine curves of symbols and imprinted characters on tablets (Knop & Kleinebudde, 2013). It masks the disagreeable color, odor, and taste of the tablet. It can also control the sustained release of drugs from the dosage form at specific sites of action. The tablets are designed in a way that provides assistance to acid-sensitive drugs that can reach the intestine with no negative enteric impacts. To keep on controlling the drug dissolution rate in the gastrointestinal tract (GI), tablet coating sounds to be the best strategy (Knop et al., 2013). Different merits of tablet coating are (Knop et al., 2013): • It is capable of modifying the drug release • It enhances the quality of the product • Improves manufacturability The polymers used in pharmaceutical coating are (Kapoor et al., 2020): • Cellulose acetate phthalate • Cellulose acetate trimellitate • Polyvinyl acetate phthalate • Methylcellulose • Ethyl cellulose • Hydroxypropyl cellulose • Hydroxyethyl cellulose • Hydroxypropyl methylcellulose • Hydroxypropyl methylcellulose acetate succinate

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Sodium carboxymethyl cellulose Polyvinyl pyrrolidone Polyethylene glycol Shellac Zein Eudragit L-30D Eudragit L-100-55

3.2.4 Long-term antimicrobial device coatings Over the past two decades, the field of antimicrobial medical device coatings has developed nearly to 30-fold with technologies that shift its focus right from the diffusion-based coating (short time antimicrobial eluting) to long-term antimicrobial eluting. To mitigate the risk of any implanted device failures, varied emerging coatings have been developed. The coatings can be grouped into two categories: i. Those which use antibiotics in conjunction with a polymer coating ii. Those that depend on the intrinsic property of the material to repel or kill bacteria which come in contact with the surface. Infections can be evolved surrounding a variety of implanted medical devices such as orthopedic implants, vascular grafts, catheters, and hernia meshes. Latest modifications in the reimbursement policy of health care have shifted the cost of implant infections back to its original service provider as preventable infections, which generates an immediate need to bring out devices that have greater ability to prevent fungal and bacterial infections (U.S. Government Printing Office, 2010). In order to prevent device infection, many coatings depend on the encapsulation of varied antifungal or antibiotics drugs by locally delivering the drugs (Harth et al., 2010; Labay et al., 2015, Nava-Ortiz et al., 2010). To enhance the loading of encapsulated drugs and to extend the duration of release, over the capability of ordinary polymers, many coatings have been started to incorporate high-affinity moieties such as CD (cyclodextrin) (Thatiparti et al., 2010).

3.3 Nanotechnology for antiviral coating In recent times, pandemics and epidemics due to viral infections have led an extraordinary effect on human life that led to financial challenges and severe social challenges. In the community spread of the virus, the nosocomial route of viral transmission plays a significant role. Unfortunately, no strategies exist to prevent the spread of viral infections. To contain the viral

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transmission, the target (principal) would be to stop the virus from reaching healthy individuals. As nanomaterials have unique chemical and physical properties, it has been widely used for the development of novel antiviral agents. Today the world faces unprecedented challenges in combating viral emergencies along with the outbreak of Covid-19. Historically, one of the primary causes of economic losses and morbidity is a viral outbreak that highlights our lack of preparedness to fight against sudden reemergence or emergence of viral diseases (Sportelli et al., 2020; Webby & Webster, 2003; Weiss et al., 2020). Enteric viruses like Rotaviruses (RV), Caliciviruses (Noroviruses), Astroviruses (AV), Hepatitis A Viruses, Enteroviruses (Polioviruses, Kobu viruses), Hepatitis E Virus, Coronaviruses, Enteric Adenoviruses Toroviruses, etc., transmits either through direct contact with an infected person or by the consumption of or contact with the contaminated water or food. In simple terms it is stated that these enteric viruses can transmit by orofecal route ( Javier & Rodrı´guez-Dı´az, 2006; Rodriguez-Diaz & Monedero, 2013). To avoid viral infections that cause an outbreak, the key is early identification (Ducel et al., 2002). But for viruses such as SARS-CoV 2, the period for the onset of symptoms can be high in 2 weeks, as it might become challenging to the spread of the virus (Lauer et al., 2020). In such a situation, nanotechnology has come out to rescue us with functional nanoparticle relied on antiviral agents along with nanomaterial coatings in order to prevent the virus from infecting individuals (Sportelli et al., 2020; Weiss et al., 2020). It drastically reduces viral transmissions by providing easy and affordable solutions. A surprising nature of antiviral nanomaterials is its nonspecific mode of action which has the ability to inactivate varied kinds of microbes just by a single platform. However, the multivalent nature of the nanomaterials brings them out as a suitable candidate that may interfere with viral attachment and meanwhile interfere with viral entry into the cell (Wiehe et al., 2019). For vector-borne viral transmissions such as arthropod Aedes aegypti borne (DEN-2) dengue virus, the control measures would include strategies that bring an effective vector control, nanomaterial intervention as an antimicrobial strategy, i.e., a surface coating which would be of limited use (Sujitha et al., 2015). The knowledge of virus receptor interaction plays an important role in the development of novel antiviral strategies. Research works with various nanotechnological approaches have been undertaken to prevent the virus from entering host cells. These approaches have a greater potential to be

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used as nanomaterial coating that could be utilized to prevent the virus infection and its transmission. 3.3.1 Nanomaterials—Viral entry inhibitors Around the world, several antiviral therapeutics are being developed, based on carbon quantum dots or carbon dots initially discovered for their bright fluorescence and color resemble conventional semiconductor quantum dots. One such approach would be curcumin-derived cationic carbon dots that were utilized as multisite inhibitors for enteric coronaviruses. Investigations were performed with benzoxazine monomer-derived carbon dots to determine the effectiveness in reducing viral infectivity. Metal nanoparticle-based antiviral strategies

Silver nanoparticles have great attraction and attention due to the multifaced mode of antimicrobial activity that led to unprecedented biomedical significance. Investigation on antiviral behavior of silver nanoparticles against nonenveloped viruses, it was perceived that AgNP inhibits infection by direct interaction with it and not allowing the virus to get bind with receptor moiety on the cell surface. Monovalent Ag ions released from nanoparticle acts to be a reservoir that was greatly reported to be an active antimicrobial agent. Irreversible aggregation is caused as the released Ag+ ions have a high affinity toward amine-bearing biomolecules or thiol-bearing biomolecules. The phenomenon is also known as oligodynamic effect. In recent times, copper is also well known to possess antimicrobial properties with approval from American Environmental Protection (EPA) Agency. Copper nanomaterials have thus been widely tested and used for their antimicrobial properties. Antimicrobial properties were also exerted by various other metal nanoparticles such as cobalt, zinc oxide, gold, etc. Investigations have been carried out with the fact that these antimicrobial nanomaterials are potential agents that effectively get used to reduce the risk of the spread of microbial infection by inactivation of microbial load. A substantial drop in the spread of microbe could be made by the use of such functional nanomaterials to be a coating at offices, public places, door knobs, mattresses, touch surfaces, hospital equipment, etc. Suggestions have been made on the use of copper-based nanocoating in medical practices and dentistry to coat the surfaces of clinics, to coat the rotary equipment used in handpieces, dentistry, air water syringes, and other medical equipment in order to examine the spread of SARS-CoV-2 virus.

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Multimechanism antiviral nanomaterial—Graphene oxide

Graphene was procured first right from the mechanical exfoliation of graphite. It is a single atomic plane of graphite and it is constituted of sp2-bonded carbon atoms. By structure, the graphene oxide possesses carbon atoms in a hexagonal lattice arrangement to create a 2D single atomic thick structure with multiple oxygen-carrying functional groups and is depicted in Fig. 9. Graphene oxide can be exfoliated by chemicals from graphite oxide, which is a graphene sheet along with add-on functional groups associated with its structure such as epoxide group on its basal plane, phenol hydroxyl groups, and carboxyl groups at the edges. Meanwhile, the elimination of functional groups on graphene oxide either by chemical treatment or by thermal annealing results in the formation of reduced graphene [rGO] oxide. An excellent nanomaterial for multidimensional applications is graphene and its derivatives as they have unique chemical and physical properties. Ye et al. have investigated the antiviral activity of graphene oxide nanosheet structure using pseudorabies virus (RNA virus) and a DNA virus porcine epidemic diarrhea virus (PEDV). They reported that the negative charge and the layer structure of GO to be the reason for the broad-spectrum antiviral activity of GO. Graphene oxide sheets are wrapped with a neutral polymer (PVP) and a cationic polymer (PDDA) to find out the charge present on GO nanosheets that contributes to antiviral activity. They observed

Fig. 9 Pictorial representation of (A) structure of graphene (G), graphene oxide (GO), and reduced graphene oxide (rGO), (B) antiviral activity of Graphene oxide and reduced graphene oxide nanosheets by interacting with the virus particles in a charge-based mechanism and subsequently inactivating the viruses through PDI (Basak & Packirisamy, 2020).

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that GO-PVP retained the antiviral activity while the GO-PDDA composite totally lost it. Hence, the negative charge on graphene oxide (GO) interacts with viruses is more probable just before the viral entry inside cells that led to viral inactivation. Comparable antiviral activity of reduced graphene oxide (rGO) and GO nanosheets substantiates the fact that functional groups on graphene oxide are not required for its operation. Another experiment was performed where they used graphite oxide (GtO) and graphite flakes with GO sheets where they determined that the antiviral activity was very less for GtO/GO flakes compared to rGO/GO sheets. They also observed that the rGO/GO nanosheets tend to wrap up the virus particles due to their opposite charges, and the sharp edge of the nanosheet ruptures the viral envelope as well the capsid protein layer that led to its inactivation and is illustrated in the figure (Fig. 9). In another study by Du et al., an investigation on graphene oxide Ag nanoparticle composite is performed for its antiviral activity by inhibiting viral invasion using PEDV and PRRSV and was observed that AgNP incorporated with GO sheets provide a better antiviral effect. Silver nanoparticle-graphene oxide (AgNP-GO) composite was determined to inhibit AgNP agglomeration, hence increasing its bioavailability for better antiviral effect, adding to sharp edge induced antiviral activity of graphene oxide (Basak & Packirisamy, 2020). Applications: Antiviral coatings and materials A broad range of applications exists using antiviral coatings and materials right from antiviral food packaging and food contact surfaces to control human enteric viruses, to health products to prevent sexually transmitted infections to personal protective equipment in health-care sector. Application imposes different properties of the materials. For example, in food retail and production, antiviral materials cannot be toxic (Randazzo et al., 2018). Materials used in public transport need to be durable, stable, and nonflammable. Table 2 summarizes and provides an overview of the practical applications of antiviral coatings and materials. Fig. 10 illustrates the application in the context of the hospital setting. In recent days, the outbreak of COVID-19 (coronavirus pandemic) worldwide has drawn a greater interest toward the designing and development of novel viricidal and antiviral agents with a broad-spectrum of antiviral activity. The current imperative challenge lies in the development of a virus repudiation system that is capable of inactivating the pathogens and is reusable which would reduce the risk of transmission and infection.

Table 2 Antiviral surfaces and coatings and their mechanisms of action (Rakowska et al., 2021). Application type

Material

Note

Marks and PPE

Rechargeable antibacterial and antiviral nanofibrous membranes (RNMs)

Marks and PPE

Silver nanoparticles absorbed on a chitin sheet

Face masks

Graphene

Face masks

TiO2-modified hydroxyapatite composite

Face masks

Nanoporous membrane

The photoactive RNMs can store biocidal activity and under light irradiation and readily release ROS to provide biocidal functions under dim light or dark conditions Development of microbicidal/antiviral material, using Ag NPs absorbed on a chitin sheet with a nanoscale fiber-like surface structure is under development Few-layer graphene deposited onto low-melting temperature nonwoven masks. The masks could be reusable after sunlight sterilization Hydroxyapatite modified with anatase titanium dioxide composite shows antiviral activity against H1N1 influenza A virus, which is promising to apply to masks A flexible nanoporous Si-based template on a silicon on-insulator wafer is used as a hard mask during a reactive ion etching process to transfer the patterns onto a flexible and lightweight polymeric membrane. Pores with sizes down to 5 nm were achieved. The mask is antifouling and self-cleaning of surface and as protective clothing

Face masks, PPE and wipes

Catechin polyphenol

Public transport

Silver

Various surfaces

Polycation N,N-dodecyl, methylpolyethylenimine (PEI)

Various surfaces

Silica nanoparticles (SNPs) coated with a quaternary ammonium cationic surfactant, didodecyldimethylammonium bromide (DDAB)

Door frames, window panels and hospital and medical equipment

Nanostructured surfaces

Chemical modification of the surface of nonwoven cellulosic fiber filters (Kimwipes) by fixing polyphenol could confer antiviral properties, which could be potentially used as filters in masks and wipes for the cleaning of surfaces and as protective clothing Seating covers used in public transport system could be treated with silver through the in situ photoreduction of a silver solution The surfaces could be simply coated with this polycation’s butanol solution, which exerts antimicrobial and antiviral properties SNPs coated with a quaternary ammonium cationic surfactant are very effective biocide and can be reused several times without significantly losing their biocidal activity. SNPs can be potentially attached to any substrate by a chemical adhesive formed by polydopamine Nanostructures randomly aligned as ridges on aluminum 6063 alloy surfaces, providing antiviral properties

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Fig. 10 A scenario of a hospital where antiviral surfaces and coatings could provide benefits. Four types of surfaces are shown (A) hard surfaces such as touch screens and door handles, (B) large surfaces and infrastructures like cupboards, doorframes, floors, tables, walls, and window panels, (C) soft surfaces such as textiles, ambulance interior, mattresses, and (D) personal protective equipment including masks, gloves, visors, and coverings (Rakowska et al., 2021).

Antiviral members are classified into metal oxides/metal ions, functional nanomaterials and polymeric materials depending on the type of materials that are used at the virus contamination site. Hence, coatings need to be authenticated and to be tested in order to fabricate a greater range of coated antiviral products such as surgical drapes, masks, gowns, and personal protective equipment aimed at disentangling from COVID-19 pandemic. By nature, the existence of various microorganisms may at times cause a deleterious impact on human health (Howell & D’Souza, 2013). Extreme illnesses are led by viruses, regarded to be a hazard as they impair health as human contact with these microbes right from the environment (Daszak et al., 2000). The breakout of COVID-19 has made a destructive challenge to human health in varied sections of the world (Nicola et al., 2020). It has generated negative massive damage to the economy and social effects globally. Microorganisms constitute both viruses and bacteria (West et al., 2006). They differ from each other in regard to their mode of infection and their size. Viruses cause severe illness as they reproduce by infecting the host cell and then by multiplying in greater numbers while a local infection is caused by bacteria as they restrict their growth in a localized area that creates an impact on a particular part of the human body (Durmus Tekir et al., 2012; Kaufmann et al., 2018). Henceforth, it is easier to target bacterial infections using novel antimicrobial than viruses. Coatings empower viricidal and antiviral properties Many research efforts and assumptions are being administered currently, on the

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Fig. 11 Classification of different antiviral coating materials for the development of antiviral products (Pemmada et al., 2020).

development of vaccines against COVID-19 (Le et al., 2020; Lurie et al., 2020). Thus, attention is much needed to consider the development of antiviral surfaces in sanitizing technologies and equipment. Classification of different viricidal and antiviral coatings are depicted in Fig. 11 and the development and inventions under each heading are summarized in Table 3. Antiviral polymer

A polymer is described to be a chemical compound with molecules that bond together in long and repeating chains. They project unique properties such as ductility, impact resistance, and elasticity that can be tailored for varied biomedical applications. For specific requirements, the antiviral agent is encapsulated in a polymer that creates antiviral composites, while few polymers show excellent antibacterial and antiviral effects owing to their resistance to the adhesion of viruses and bacteria (Mun˜oz-Bonilla & Ferna´ndez-Garcı´a, 2012). Polymers with viricidal and antiviral effects are majorly used as coatings that cover the surface of a wide range of substrates and Fig. 12 clearly shows the schematic demonstration of the antiviral mechanism of polymer coatings. Antiviral metal oxides/metal ions

A number of metal oxides and metal ions have been investigated in the past decades for their viricidal and antiviral activity specifically copper, zinc, and

Table 3 Coatings employed for antiviral and viricidal properties (Pemmada et al., 2020). Type of coating

Polymer

Polymer

Polymer

Coating materials

Mechanism

Effectiveness conditions

Polymers containing both quaternary ammonium groups and hydrocarbon chains Microbicidal polycation N,N-dodecyl, methylpolyethylenimine

Creating localized surfactancy

Inactivation of lipid-enveloped viruses

-NA-

-NA-

Destruction of human bacteria pathogens (Staphylococcus aureus and Escherichia coli and two common strains of influenza virus) -NA-

Up to 5 days

An antiinfective agent selected from the group consisting of chlorhexidine and pharmaceutically acceptable salts of chlorhexidine Methyl-polyethylenimine (N,N-dodecyl,methyl-PEI)

-NA- Blocking adsorption sites of antiinfective agent

-NA-

Polymer

Polyethylene (preferred polyolefin) Polyethylenimine

-NA-

Polymer

PVP-I and N-9

-NA-

Polymer

Polymer

Average time duration to destroy viruses

-NA-

Highly lethal to waterborne Influenza A viruses, including wild-type human and avian strains Porous plastic materials with antiviral agent Antiviral surface coating that can detoxify aqueous solutions containing various viruses Dual or multilayered format antiviral coating that imparts antipathogenic properties to substrate

-NA-

Up to 5 days

2–3 days -NA-

-NA-

Polymer

Polymer

Polymer

Polymer

Metal ions/ metal oxides Metal ions/ metal oxides

A dendrimer such as a polyamidoamine or polylysine having a plurality of terminal groups Porcine gastric mucin polymers

Attachment of Ionic moieties

Antiviral activity against human immunodeficiency (HIV) and other enveloped viruses

-NA-

Shielding effect

Protect an underlying cell layer from infection by small viruses like human papillomavirus (HPV), Merkel cell polyomavirus (MCV), or a strain of influenza A virus Action of Cu2+ particles as antiviral agents

-NA-

Effective against an avian influenza virus

-NA-

Action of single source of both Ag2+ and Cu2+ ions in treating virally contaminated surfaces Action of Cu2+ ions

Less than 4 h

A polymeric component selected from the group consisting of a polyamide, a polyester, an acrylic, a polyalkylene, and mixtures thereof A polymer containing a maleic acid component as a monomer unit in a polymer chain thereof Copper ion, silver ion or both

Action of metallic particles with copolymerized fiber Ion exchange type

Cu in powder

Electrolytic Plating

Precipitation and reduction

Less than 4 h

Less than 4 h

Continued

Table 3 Coatings employed for antiviral and viricidal properties (Pemmada et al., 2020)—cont’d Type of coating

Metal ions/ metal oxides

Coating materials

Mechanism

Effectiveness conditions

Silver, copper, and zinc

Washing with PBS

Action of Ag2+, Zn2+ and Cu2+ ions showed viricidal activity against (HIV-1), and other enveloped viruses Inactivation influenza viruses and Norovirus

Metal ions/ metal oxides

Metal ions/ metal oxides Metal ions/ metal oxides Metal ions/ metal oxides Metal ions/ metal oxides

A copper complex titanium oxide dispersion liquid Cuprous oxide particle dispersion liquid and a binder resin Water soluble metal ions include aluminum, copper, and mixtures thereof An antiviral composition consisting of a thiosulfate complex salt coated with a material layer like silicon dioxide of a metal like silver, copper, and zinc

Reduction/ oxidation reaction on surfaces of photocatalytic particles Dispersion

Average time duration to destroy viruses

Less than 4 h

Less than 4 h

-NA-

Less than 4 h

Coupling effect

-NA-

Less than 4 h

Hydroxide formation

Water soluble meal ion has the ability to kill certain strains of viruses The composition releases its salts into the contaminated sites

4–8 h

Releasing salts and by attaching chemotherapeutic agents to complex

-NA-

Metal ions/ metal oxides Metal ions/ metal oxides

Metal oxides or metalloid oxides, such as, e.g., TiO2, ZrO2, SnO2, ZnO, and SiO2 Antiviral activity of arsenic oxide (As2O3) and antimony oxide (Sb2O3)

Oligodynamic effect

-NA-

4–8 h

Excellent viricidal property on viral stain bacteriophage

-NA-

Nanomaterials

(GO-Ag) nanocomposites

Washing

Other

A quaternary ammonium salt and a polyhydric carboxylic acid having a C6 or more hydrocarbon group and two or more carboxyl groups An antiviral agent contains a powder obtained by baking (calcining) dolomite and hydrating a part thereof An antiviral agent includes an inorganic solid acid having an acid site concentration of more than 0.005 mol/g

Preventing elution of salts in water thus maintaining antiviral effect

CuI particles showed antiviral activity against influenza A virus of swine origin inactivating many viral strains from lipid envelope viruses Antiviral activity of nanomaterials on nonenveloped viruses Water-resistant antiviral coating

Less than 4 h

Nanomaterials

Nanosized copper (I) iodide (CuI) Silver nanoparticles

Hydroxyl radical oxidation, diffusion of disinfectant Hydroxyl radical formation Blocking interaction

Nanomaterials

Filtration

An antiviral coating effective for Coronavirus

-NA-

Dispersion

Excellent in heat resistance and maintains the inactivating effect on viruses

-NA-

Other

Other

Less than 4 h

Less than 4 h

2–3 days

Continued

Table 3 Coatings employed for antiviral and viricidal properties (Pemmada et al., 2020)—cont’d Type of coating

Other

Other

Other Other Other

Other

Coating materials

Mechanism

Effectiveness conditions

Average time duration to destroy viruses

The fluid compositions consist of at least one viricide like lauric acid and essential oils like laurel essential oil, soybean oil A therapeutically effective amount of an essential oil

-NA-

-NA-

2–3 days

Presence of different functional groups in essential oils Drug delivery system (DDS) Filtration

This method prevents a respiratory infection in a mammal Inactivate variety of DNA and RNA viruses Disable influenza virus

-NA-

Filtration

Acting against influenza virus, hepatitis C virus, HIV

-NA-

Higher antibacterial activity against Gram-positive and Gram-negative bacteria

-NA-

Novel antibiotics didemnin A, B, C, D, and E (didemnins) Antiviral filter air cleaner impregnated with tea extract Tea extract, other herbals, and phytochemicals like curcumin Chitosan incorporated neem seed extract (Azadirachta indica)

-NA-NA-

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Fig. 12 Schematic demonstration of the antiviral mechanism of polymer coatings (Pemmada et al., 2020).

copper-based oxides/ions. These metal materials produce high antiviral activity and low toxicological effect in comparison to other metals and hence they are a popular fit for this kind of application. Among different metal ions, copper, and silver ions present the broadest spectrum in terms of antiviral action (Thurman, Gerba, & Bitton, 1989). The use of such materials in filters and coatings accounts well for the elimination of viruses. A hybrid coating containing copper, zinc, and silver cations that were fabricated through radical polymerization using sol-gel method. Viricidal activity is shown against HIV and other viruses including herpes simplex and dengue by these hybrid coatings. A large number of studies have stated that the coating materials that contain a metal ion, i.e., copper, zinc, and silver have excellent antiviral ability to endure persistent effects (Fujimori et al., 2012; Hodek et al., 2016; Lara et al., 2011; Miki et al., 2018). A mixture of metal oxide/ions with other antiviral materials reduces the negative impact on the surface and achieves the viricidal activity. Copper ion (in powder form) to polymeric material was used to form surgical tubing, surgical gloves, etc. (Gabbay et al., 2007). To treat localized infections, metal oxides and metal ions can be used as an antiviral coating (Oka et al., 1997). Antiviral functional nanomaterials

In accordance with the nanoscale properties of the virus, it is feasible to develop definite hybrid nanomaterials with various functionalities in order to achieve viricidal effects as they are considered to be greatly effective in the control of viral infections. By functionalized chemical modification and by the chemical and physical characterization of the materials, the absolute

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Antiviral and antimicrobial smart coatings

biological effect could be determined. By the inhibition of respiratory enzymes, antiviral activity is maintained well by the silver (Ag) nanoparticles thereby controlling the bind between the host cell and virus (Mori et al., 2013) (Figs. 13 and 14). Various functional nanoparticles of metals and their oxides, graphene, and the metals combining with graphene oxides or graphene are currently being explored against Coronavirus (COVID-19). The role of nanotechnology and functional nanomaterials is highly relevant to counter COVID-19 “virus” nanoenemy (Pemmada et al., 2020).

Fig. 13 Schematic illustration of the antiviral mechanism of metal ions (Pemmada et al., 2020).

Fig. 14 Pictorial representation of antiviral mechanisms of functional nanomaterials (Pemmada et al., 2020).

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4. Conclusion and future perspective Antimicrobial coatings of polymeric composite, nanocomposite, and polymer are employed for varied purposes in biomedical applications. For example, the surfaces of many numbers of metallic implants are well modified by the nanomaterial coating for their implementation in orthopedic applications. These coatings are favorable to improvise the host response in long term by assisting proliferation and gene-level regulation, cell migration through the adjustment of stiffness/hydrophobicity of surfaces and also protects the implant from biofilm formation and microbial attack. For commercialization, it is essential to understand the nature of coating materials and their production parameters such as surface geometry, coating thickness, high performance, and functionality. Still, to develop a long-term stable tool for the biomedical industry, it is essential to fabricate and design nanocomposite coating and novel polymers. In recent times, pandemic diseases are looked at as a global public health issue and thus it is required to develop new technologies, which could improve new antiviral and antimicrobial molecules and other therapeutic approaches to bind its spreading. Patients of COVID-19 have been facing difficulty in breathing (exhibits pneumonia-like symptoms). Hence, it is censorious to assist breathing with pertinent medical devices. 3D printing or additive manufacturing with the help of antimicrobial polymer blends could be utilized to develop critical device pieces such as connectors for ventilators or medical devices. This technology would help out to develop alternative options to access medical devices and to speed up their process in production. To summarize, there are many composites/polymers, antimicrobial compounds and nanoparticles with confirmed antifungal, antibacterial, and antiviral activity that could directly be applied onto the surfaces or could be employed into coatings to prevent any risk of spreading. Besides all, combining real-time sensing skills and basic skills to the antimicrobial surface would support in recognizing the pathogens that are present in the environment and would ultimately help public health experts to control infectious disease pandemics. Comprehensively, the application of nanotechnology is highly important to prevent the spread of infectious diseases and would be required for the future and for the long-term success of biomedical devices.

Acknowledgment The authors (LCP and RP) thank the Solar Energy Research Center, SERC-Chile (ANID/ FONDAP/15110019), FONDECYT 1201314.

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Schwartz, V. B., Thetiot, F., Ritz, S., P€ utz, S., Choritz, L., Lappas, A., F€ orch, R., Landfester, K., & Jonas, U. (2012). Antibacterial surface coatings from zinc oxide nanoparticles embedded in poly (n-isopropylacrylamide) hydrogel surface layers. Advanced Functional Materials, 22(11), 2376–2386. Shrikant, M. (2013). Nanotechnology for surgeons. Indian Journal of Surgery, 75(6), 485–492. https://doi.org/10.1007/s12262-012-0726-y. Silva, GA. (2004). Introduction to nanotechnology and its applications to medicine. Surgical neurology, 61(3), 216–220. Sim, W., Barnard, R. T., Blaskovich, M. A., & Ziora, Z. M. (2018). Antimicrobial silver in medicinal and consumer applications: A patent review of the past decade (2007–2017). Antibiotics, 7(4), 93. https://doi.org/10.3390/antibiotics7040093. Sondi, I., & Salopek-Sondi, B. (2004). Silver nanoparticles as antimicrobial agent: A case study on E. coli as a model for Gram-negative bacteria. Journal of Colloid and Interface Science, 275(1), 177–182. Song, X., Xie, L., Zhang, M., Wang, W., Li, L., Lu, X., Lei, P., Liu, D., Chen, Y., Chen, H., & Zhao, C. (2021). Cu-decorated graphene oxide coatings with enhanced antibacterial activity for surface modification of implant. Materials Research Bulletin, 141, 111345. Souza, A. G., & dos Santos Rosa, D. (2021). Nanotechnology in antimicrobial and hygiene materials. In Md. I. H. Mondal (Ed.), Antimicrobial textiles from natural resources (pp. 557–587). Woodhead Publishing. Sportelli, M. C., Izzi, M., Kukushkina, E. A., Hossain, S. I., Picca, R. A., Ditaranto, N., & Cioffi, N. (2020). Can nanotechnology and materials science help the fight against SARS-CoV-2? Nanomaterials, 10(4), 802. Staiger, M. P., Pietak, A. M., Huadmai, J., & Dias, G. (2006). Magnesium and its alloys as orthopedic biomaterials: A review. Biomaterials, 27(9), 1728–1734. Stobie, N., Duffy, B., McCormack, D. E., Colreavy, J., Hidalgo, M., McHale, P., & Hinder, S. J. (2008). Prevention of Staphylococcus epidermidis biofilm formation using a low-temperature processed silver-doped phenyltriethoxysilane sol–gel coating. Biomaterials, 29(8), 963–969. Su, W., Wang, S., Wang, X., Fu, X., & Weng, J. (2010). Plasma pre-treatment and TiO2 coating of PMMA for the improvement of antibacterial properties. Surface and Coatings Technology, 205(2), 465–469. Subramani, K., Mathew, R. T., & Pachauri, P. (2018). Titanium surface modification techniques for dental implants—From microscale to nanoscale. In Karthikeyan Subramani, & Waqar Ahmed (Eds.), Emerging nanotechnologies in dentistry (2nd edn., pp. 99–124). William Andrew Publishing. Sujitha, V., Murugan, K., Paulpandi, M., Panneerselvam, C., Suresh, U., Roni, M., Nicoletti, M., Higuchi, A., Madhiyazhagan, P., Subramaniam, J., & Dinesh, D. (2015). Green-synthesized silver nanoparticles as a novel control tool against dengue virus (DEN-2) and its primary vector Aedes aegypti. Parasitology Research, 114(9), 3315–3325. Sun, L., Yuan, G., Gao, L., Yang, J., Chhowalla, M., Gharahcheshmeh, M. H., Gleason, K. K., Choi, Y. S., Hong, B. H., & Liu, Z. (2021). Chemical vapour deposition. Nature Reviews Methods Primers, 1(1), 1–20. Takai, K., Ohtsuka, T., Senda, Y., Nakao, M., Yamamoto, K., Matsuoka, J., & Hirai, Y. (2002). Antibacterial properties of antimicrobial-finished textile products. Microbiology and Immunology, 46(2), 75–81. Tan, P., Fu, H., & Ma, X. (2021). Design, optimization, and nanotechnology of antimicrobial peptides: From exploration to applications. Nano Today, (39), 101229. Tavakol, S., Zahmatkeshan, M., Mohammadinejad, R., Mehrzadi, S., Joghataei, M. T., Alavijeh, M. S., & Seifalian, A. (2021). The role of nanotechnology in current COVID-19 outbreak. Heliyon, 7(4), e06841.

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Xu, L., Pristinski, D., Zhuk, A., Stoddart, C., Ankner, J. F., & Sukhishvili, S. A. (2012). Linear versus exponential growth of weak polyelectrolyte multilayers: Correlation with polyelectrolyte complexes. Macromolecules, 45(9), 3892–3901. Yang, H., Li, G., Stansbury, J. W., Zhu, X., Wang, X., & Nie, J. (2016). Smart antibacterial surface made by photopolymerization. ACS Applied Materials & Interfaces, 8(41), 28047–28054. Yousif, Q. A. (2017). Nanotechnology: Theory, application and explanation of the most important devices used. https://www.researchgate.net/publication/320267951_Nanotechnology_ Theory_application_and_explanation_of_the_most_important_devices_used. (Accessed 30 October 2022). Zhang, X., Yuan, G., Mao, L., Niu, J., Fu, P., & Ding, W. (2012). Effects of extrusion and heat treatment on the mechanical properties and biocorrosion behaviors of a Mg–Nd– Zn-Zr alloy. Journal of the Mechanical Behavior of Biomedical Materials, (7), 77–86. Zhang, S., Chu, Z., Yin, C., Zhang, C., Lin, G., & Li, Q. (2013). Controllable drug release and simultaneously carrier decomposition of SiO2-drug composite nanoparticles. Journal of the American Chemical Society, 135(15), 5709–5716. Zhang, J., Kong, N., Niu, J., Shi, Y., Li, H., Zhou, Y., & Yuan, G. (2014). Influence of fluoride treatment on surface properties, biodegradation and cytocompatibility of Mg–Nd–Zn–Zr alloy. Journal of Materials Science: Materials in Medicine, 25(3), 791–799. Zhang, L., Yan, J., Yin, Z., Tang, C., Guo, Y., Li, D., Wei, B., Xu, Y., Gu, Q., & Wang, L. (2014). Electrospun vancomycin-loaded coating on titanium implants for the prevention of implant-associated infections. International Journal of Nanomedicine, 9, 3027. Zhang, Z., Cheng, X., Yao, Y., Luo, J., Tang, Q., Wu, H., Lin, S., Han, C., Wei, Q., & Chen, L. (2016). Electrophoretic deposition of chitosan/gelatin coatings with controlled porous surface topography to enhance initial osteoblast adhesive responses. Journal of Materials Chemistry B, 4(47), 7584–7595. Zhao, Y., Jamesh, M. I., Li, W. K., Wu, G., Wang, C., Zheng, Y., Yeung, K. W., & Chu, P. K. (2014). Enhanced antimicrobial properties, cytocompatibility, and corrosion resistance of plasma-modified biodegradable magnesium alloys. Acta Biomaterialia, 10(1), 544–556. Zhao, L., Chu, P. K., Zhang, Y., & Wu, Z. (2009). Antibacterial coatings on titanium implants. Journal of Biomedical Materials Research. Part B, Applied Biomaterials, 91(1), 470–480. Zulhendri, F., Chandrasekaran, K., Kowacz, M., Ravalia, M., Kripal, K., Fearnley, J., & Perera, C. O. (2021). Antiviral, antibacterial, antifungal, and antiparasitic properties of propolis: A review. Food, 10(6), 1360.

Further reading Hu, H., Bandell, M., Petrus, M. J., Zhu, M. X., & Patapoutian, A. (2009). Zinc activates damage sensing TRPA1 ion channels. Nature Chemical Biology, 5, 183–190. Mali, S. (2013). Nanotechnology for surgeons. The Indian Journal of Surgery, 75(6), 485–492. Watt, N. T., Taylor, D. R., Kerrigan, T. L., Griffiths, H. H., Rushworth, J. V., Whitehouse, I. J., & Hooper, N. M. (2012). Prion protein facilitates uptake of zinc into neuronal cells. Nature Communications, 3(1), 1–2.

CHAPTER 13

Nanomaterial-based smart coatings as antimicrobials Kiran Balaa,b and Madhulika Bhagata a

School of Biotechnology, University of Jammu, Jammu, India Department of Biotechnology, GGM Science College, Jammu, India

b

1. Introduction Microbial contamination is a relevant problem throughout the world and is responsible for various unfavorable outcomes. One area of particular significance in the transmission of infectious diseases is the ability of microbes to survive on contaminated surfaces of medical equipment or hospital apparel, food packaging, water purification systems, etc. (Elashnikov et al., 2021). Commonly, antibiotics are used for treating such infections but many microbes are now no longer susceptible to commercially available antibiotics which contribute to the proliferation of multidrug-resistant bacteria. It is estimated that at least 1.27 million deaths per year are directly related to infections caused by these antimicrobial-resistant microbes, this calls for urgent action from policymakers and health communities to avoid further probable deaths (Antimicrobial Resistance Collaborators, 2022). Moreover, the process of development of new compounds is expensive, time consuming, and associated with significant regulatory burdens. For these reasons, almost no new antibiotic compounds have been provided to the market in the last two decades (Miethke et al., 2021). Suggested alternative strategies are the possibility of smart reactions to infections, or antimicrobial effective surfaces to reduce bacterial colonization and attachment. The explored antimicrobial surface or coatings contain antimicrobial agents that inhibit the ability of microorganisms to grow on the surface of a material. Bacterial surface attachment and colonization depends on several factors including (i) general physicochemical surface properties, (ii) the exopolymeric matrix, and (iii) the cell surface biochemical components (Gottenbos et al., 2000). Considering the differences in the molecular physiology of the bacterial cells, the type of bacteria-surface interaction varies from one bacterial strain to another. In order to protect the surface from bacterial attachment, the control of surface hydrophobicity, morphology, charge, or embedding with Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00010-4

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antimicrobial agents and manipulating the gradual release of these agents, can be considered as a potential alternative to reduce bacterial adhesion to the surface (Wang et al., 2020). Researchers are currently progressing toward developing new “smart” materials and coatings that deliver antimicrobial agents in an intelligent way, i.e., only when bacteria are present. This requires the utilization of new and modern devices consolidating antimicrobial agents with regular and engineered lipids or polymers. On the basis of stimulus, two types of smart antimicrobial coatings have been recognized: chemo- and physically responsive. When a change in the chemical composition of surroundings acts as a stimulus and switches a coating on, it is called chemo-responsive coating. Stimuli in such cases are the pH changes of the surroundings due to bacterial presence or the presence of some specific bacterial biomolecule in the environment indicating the presence of bacteria. And when the antibacterial activity of a smart coating is modulated by physical factors like temperature, light, electric field, magnetic field, mechanical stress, etc., the coatings are called physically responsive antibacterial coatings.

2. Strategies of antimicrobial coatings According to a report by Grand View Research global antimicrobial coatings market was valued at USD 7.1 billion in 2019 and is also continuously expected to grow at a compound annual growth rate (CAGR) of 12.8% up to 2027 (Grand View Research, 2021-2028). Smart coatings have acquired acknowledgment in material sciences, colloidal science, biomedical sciences, and polymer science because of their innate properties and the acquired properties on being modified. Different strategies are explored to develop antimicrobial coatings (Fig. 1) for example, bacteria-repelling coating that involve prevention of bacterial binding to the surfaces by using various hydrophilic polymers, for example, polyethylene glycol (PEG), oxazolines, nitroxide radicals, or chlorinated plasma polymers, etc. (Vasilev, 2019). The coating materials have a solid propensity to degrade on exposure to oxygen, high temperature, or sunlight. Thus, effectiveness of hydrophilic polymers depends on the surrounding environment, making it a less favorable surface protection strategy (Hucknall et al., 2021). Another strategy is to develop contact-killing coatings that explore quaternary ammonium compounds, silicone, antibacterial polymers, etc. for coatings that involve killing the microbe on its contact with the surface. This approach does not require the release of antimicrobial agents, thus keeping their surface concentration

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Fig. 1 Common strategies of antimicrobiocidal coatings.

at a constant, bacteria-killing level, and does not lead to pollution of the surroundings. Mostly, contact-killing-based approaches are not commonly associated with the risk of antibiotic-resistant bacteria development. Other strategies include coating surfaces with materials that discharge antibacterial agents including numerous antibacterial mixtures, such as regular antimicrobial, nitric oxide, antibacterial polymers, and peptides. Additionally, a surface-protection approach is also explored, in which superhydrophobic surface properties are combined with water repellency properties to reach the “lotus leaf” effect. Here, when the water droplets roll down on superhydrophobic surfaces, they pick up bacteria or other (bio) debris, leaving behind a cleaner surface. Such a kind of surface functionality can be achieved through a combination of low surface energy materials and high surface roughness (Coulson et al., 2000). Despite different approaches that are explored, a lot of drawbacks generally are related to the subsequent loss of surface functionality by the release or degradation of incorporated antimicrobial agents in due time, screening, adhesion, low specificity, and poor control of antimicrobial action of the coated surface (Tallet et al., 2010). Recently dynamic and responsive strategies referred to as smart coatings are getting attention due their activation at the right time and place to eliminate the bacteria, in response to a stimulus.

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3. Smart antimicrobial coatings: A bright perspective The smart antimicrobial coatings are intelligent, reliable, long-life, and safe surface protection candidate options. The term “smart coating” refers to the concept of coatings that is being able to sense the environment and make an appropriate response to that stimulus. The smart coatings that are used by the end-use industries are based on their functions such as antimicrobial, anticorrosion, antiicing, antifouling, self-cleaning, self-healing, self-dimming, electrochromic, antidrag, smart skin, color-shifting, camouflage coatings, and others. These coatings are made with programmable/responsive materials that respond to changes in light, chemical, thermal, or other stimuli. Developed coating brings new performances such as self-healing, selfcleaning, self-sensors, etc. due to their piezoelectric, thermoelectric, piezoresistive, chemical properties (i.e., surface energy differences), and other properties (Sa´nchez-Romate et al., 2020). The coatings are categorized according to functional components, fabrication methods, their application, etc. Most coatings are commonly based on targeted surface decoration with the utilization of so-called smart materials that are able to rapidly and reversibly change their physicochemical properties in response to little changes in the surrounding environment, for example, chemical changes in the vicinity like pH, salt concentration, presence of biomolecules, or physical changes such as temperature, electrical potential, light, etc. (Buchegger, Kamenac, Fuchs, Herrmann, Houdek, Gorzelanny, et al., 2019; Gao et al., 2019; Hao, Chen, Qin, Zhang, Li, Fan, Wang, et al., 2020; Qu et al., 2018; Zhang et al., 2019) (Fig. 2).

• • • • • • •

Temperature responsive Light responsive MagneƟc field responsive Electric field responsive Mechanically responsive pH responsive Enzyme Responsive

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AnƟbioƟcs Metal and metallic NanoparƟcles Nitric oxide Enzymes AnƟbacterial polymers and pepƟdes Biofilm dispersal compounds Fluorescent dyes

Fig. 2 Design of a smart nanomaterial with embedded antimicrobial agents.

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4. Nanomaterials as antimicrobial coating The nanomaterials form an integral part of innovative coatings with their unique properties that provide protection against microbes and simultaneously permit specific control of coating design on a subatomic scale. They are materials having at least one dimension (1–100 nm) in the nanometer scale range or whose basic unit is in three-dimensional space. In addition to various other benefits, nanomaterial-based approaches are a preferred tool to combat bacterial infections. Owing to their unique size and physical properties they have the capability to target biofilms while overcoming recalcitrant infections and the capacity to evade existing mechanisms associated with acquired drug resistance (Makabenta et al., 2021). The combination of different types of nanomaterials in smart nanocoatings has exhibited superior properties in comparison to traditional coatings. Surface coating with nanomaterials had exhibited improved material functionalities mostly corrosion and friction reduction, antifouling, and antibacterial properties, self-cleaning, heat, and radiation resistance and thermal management (Bayda et al., 2020; Future Markets, 2014; Provder & Baghdachi, 2009). Antimicrobial nanomaterials known so far have multiple modes of action and depend on the size, shape, ζ-potential, ligands, and material used (Huh & Kwon, 2011; Singh et al., 2014; Zazo et al., 2016). Currently accepted mechanisms include (1) direct interaction with the bacteria, leading to the disruption of membrane potential and integrity; (2) generation of reactive oxygen species (ROS); (3) inhibition of biofilm formation; (4) triggering of the host immune responses; and (5) inhibition of RNA and protein synthesis through the induction of intracellular effects (Beyth et al., 2015; Pelgrift & Friedman, 2013) (Fig. 3).

5. Smart coatings and stimuli Intelligent smart coats can actively sense various external environmental stimuli such as pressure, temperature, light, heat, pH, biological factors, polarity, etc. and can react with an appropriate response. They can be tailored in such a way that one or more of the above listed functions can act as “switched on” or “switched off” depending on the type and strength of an external signal. Moreover, sensitive molecules, nanoparticles, and antimicrobial agents can be added as additives, or strategically designed in polymer structures and coatings so as to respond against either internal or external stimuli.

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Fig. 3 Mechanisms of antimicrobial activity of nanoparticles.

In principle, to obtain responsiveness, two actions must happen concomitantly as well as selectively: • A stimulus must be generated and the material should receive the signal. • The signal should produce a chemical or physical change. The signal should be clear, explicit, and unambiguous in order for the response to be anticipated and working and to make sure that the resultant responses do not meddle with or cancel each totally. Those signals that function by triggering responses within the coating itself and with the aim of modifying the bulk properties are called internal stimuli such as anticorrosive coatings. Responses that change the surface properties relative to the environment such as in antimicrobial coatings are mostly responsive to external stimuli (Provder & Baghdachi, 2009). Moreover, signals can be momentary or continuous; the momentary signal (a burst of stimulus) is just long and strong enough and required to switch the properties of the material from one state to another. Subsequently, the material will stay in a changed state

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until a restricting sign returns the properties to the first state. For instance, materials that are pH-responsive will require a sequential change in the value of pH to get back to the first state. Designing smart coatings responsive to such stimuli is more challenging because the modified state needs to be sufficiently stable. The modified properties remain unaltered in the case of continuous stimuli, as long as the signal remains (e.g., pressure or temperature change). Signal(s) acting may induce a smart, unique, and permanent behavioral change in the system preventing it from returning to the original state in any condition (examples self-healing and antimicrobial coatings). On the other hand, a true two-directional system must be able to switch repeatedly from one state to another and perhaps thousands of times during its service life. Examples of such materials are rare and available ones are thermochromic and pressure-triggered coatings. Since the signal must affect some organic or inorganic chemical entity (polymer, pigment, additive, etc.) its nature can be either physical or chemical. The physical signals are abundant, more tunable, and perhaps less complicated than the chemical counterparts. The chemical stimuli encompass the biochemical, electrochemical, photochemical, redox, and acid-base reactions and also involves chemical bond breakage and formation. While there are numerous chemical reactions that can be used as stimuli, the process is not that easy and simple. Understanding the exact reaction mechanism is far more difficult and complex. Changes in the structure or properties are the result of smart or responsive coatings and materials. For example, alteration in the structure and conformation leading to color change of a reversible elementary system containing diazobenzene is restricted to cis-trans conformation, and insignificant structural change is seen in a silver-containing antimicrobial coating (Baghdachi, 2009).

6. Some examples of smart antimicrobial coatings Lots of smart materials and active surfaces are being tailored for antibacterial responses. Commonly explored materials for antimicrobial coating are inorganic (metal-based), organic compounds and derivatives, polymers, etc. Some major nanoparticle/nanomaterials employed recently in smart coatings are discussed below.

6.1 Silver nanoparticles in smart coatings Silver nanoparticles (Ag NPs) have wide acceptance as antimicrobials because of their wide spectrum antimicrobial efficacy against bacteria, yeast, fungi, and viruses (Rai et al., 2009). Numerous approaches had been

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recently applied to adhere Ag metal NPs on fabric surfaces to provide antimicrobial surfaces. For example, antimicrobial coat was prepared using silver nanoparticles (AgNPs) embedded in temperature-responsive poly(di(ethylene glycol)methyl ether methacrylate)(POEGMA188) polymer and also in poly(4-vinylpyridine)—(P4VP) polymer and coated on to a glass surface. This synthesized material demonstrated strong antibacterial activities against both Gram-positive and Gram-negative bacteria (Raczkowska et al., 2019). Similarly, Priyanka and Kumar synthesized multi-stimuli-responsive smart supramolecular hydrogels with antimicrobial properties. For this in situ coating of silver nanoparticles (Ag NPs) on colloidal cytidine-50 monophosphate-capped β-FeOOH nanohybrids (β-FeOOH@50 -CMP) was done under physiological conditions that formed a polycrystalline building block (Ag-coated β-FeOOH@50 -CMP) hydrogel. The synthesized smart hydrogels possess superparamagnetic nanosystems with superior viscoelastic, sensing, and antimicrobial properties, displaying their activation on multiple stimuli (pH/temperature/sonication) (Priyanka & Kumar, 2020). In another study, pH-responsive nanocomposite hydrogel with strong antibacterial activity was prepared via a Schiff base linkage between oxidized polysaccharides (CHO-dextran) and cationic dendrimers (Polyethelenimine, PEI) encapsulated with silver nanoparticles (Dai et al., 2018). Yan et al. created mussel-inspired, silver nanoparticles (AgNPs) that were incorporated in silk fibroin (SF)/gentamicin sulfate (GS) coated upon a porous polyetheretherketone surface. The bactericidal efficiency of the synthesized coat was established against both the Gram-positive and Gram-negative bacteria in response to pH stimuli of the surrounding environment, making it an intelligent self-defensive coating (Yan et al., 2018). Similarly, silver-coated multifunctional smart cotton fabrics are reported with photoluminescence, antimicrobial, and superhydrophobic properties. Atta et al. synthesized silver nanoparticles (Ag NPs) immobilized onto a cotton surface using a facile pad-dry-curing technique with long-lasting antimicrobial properties against E. coli and S. aureus (Atta, 2021). Gautam et al., developed thermoresponsive antibacterial-cellulose papers to exhibit hybrid kill and release properties. In this study, thermoresponsive copolymers [p(NIPAAm-co-AEMA)] were grafted on cellulose papers using a surface-initiated atom transfer radical polymerization approach for bacterial debris release. Further, silver nanoparticles (AgNPs) were immobilized on thermoresponsive copolymer-grafted cellulose papers using electrostatic interactions for antibacterial properties against Gram-negative bacteria Escherichia coli (E. coli DH5-α) (Gautam et al., 2021).

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6.2 Zinc nanoparticles in smart coatings Zinc oxide (ZnO) is a semiconductor-type material, at a nanometric size, it exhibits various properties among antibacterial, antifungal, UV-blocking or photocatalytic properties (Applerot et al., 2012; Dastjerdi & Montazer, 2010; Palanikumar et al., 2014). Smart zinc oxide-based coatings have been produced to combat implant-associated infections, for example, ZnOnanoparticles embedded in diamond-like carbon (DLC) coat was prepared, and the release of Zn2+ ion from coatings in aqueous environments reacts and adapts smartly toward acidosis caused by the bacterial growth, further resulting in antibacterial effect (Buchegger, Kamenac, Fuchs, Herrmann, Houdek, et al., 2019). In another example, smart self-healing coating with superhydrophobicity was formed by blending UV/NIR/acid/base multiple-responsive ZnO-encapsulated mesoporous polydopamine (MPDA) microspheres with silicone latex and hydrophobic nanoparticles. These synthesized coats showed super/high hydrophobicity and antibiofouling properties in the presence of UV, NIR, acid, or base stimuli (Ni et al., 2021). Nanostructured Ag@ZnO were assembled on cotton fabrics as self-cleaning flexible materials and in the presence of visible-light, it demonstrated photocatalysis and antibacterial properties (Manna et al., 2015). In one study, coatings containing titania nanotubes (TiO2-NTs) were consolidated with zinc (NT-Zn) on Ti substrates by anodization and hydrothermal treatment. The NT-Zn implants not only exhibited improved bone formation (shown by both in vitro and in vivo studies) but also enhanced osseointegration between bone and implant and inhibited the growth of bacteria (Wang et al., 2017). Salmus et al. synthesized zinc-based smart coating (ZnO/Na-Montmorillonite hybrid), a nanostructured composite material, with promising antimicrobial activity for food packaging (Salmas et al., 2020).

6.3 Titanium-based smart antimicrobial coatings Titanium dioxide or titania (TiO2) is another inorganic nanomaterial utilized as photocatalysts for commercial antimicrobial coatings owing to its low-cost, reactivity, stability, reusability, durability, biocompatibility, crystallinity, high surface area, and corrosion resistance (Kumaravel et al., 2021). Titanium alloys are the principal component in many responsive and intelligent coating systems of biomedical applications. For example, titanium implants conjugated with antibiotic-peptide moieties were prepared that release on-demand antibiotics in the presence of S. Aureus

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(Zhang et al., 2020). In another example, smart hydrogel nanoparticle suspension is used to spray-coat titanium alloy-based (Ti-6Al-4V) implant materials. The nanosuspension was released only after degradation by specific enzymes thereby causing enzymatically induced drug release (Tolle et al., 2004). Sang et al. coated a layer of silk protein on the surface of titanium, and then loaded gentamicin on it. Bacteriostatic ring tests in vitro and in vivo showed that the coat exhibits a stable release behavior of gentamicin, in an acidic environment demonstrating intelligent release (Sang et al., 2021). Another mechanism for intelligent controlled release is to use the heating effect caused by infection, for example, Li et al. layered a thermosensitive chitosan-glycerol-hydroxypropyl methyl hydrogel (CGHH) on TiO2 nanotubes showed an antibacterial effect established in in vitro and in vivo studies. Moreover, the results of subcutaneous infection animal models showed that under the high temperature caused by an infection in vivo, CGHH could release a large amount of glycerin, thus suggesting excellent antibacterial properties (Li et al., 2021). Daylight bactericidal cotton textiles were developed by Matos et al. using amorphous titania (a-TiO2) and amorphous titania/chitosan complexes (a-TiO2//CS) impregnated on cotton textiles the bactericidal efficacy was established against Staphylococcus aureus strain (ATCC 6538TM)(Matos et al., 2019).

6.4 Chitosan-based smart antimicrobial coatings Chitosan is one of the most used organic nanomaterials used in smart antimicrobial coatings. It has been used in a number of smart food packaging and preservation films because of its antimicrobial activities. Smart and multifunctional chitosan/tannic acid/corn starch has been developed which acts as a pH actuator and can be used in food preservation. The chitosan-based nanocoating exhibited different directional deformation properties in acidbase solutions and can grab and deliver 21 times heavier objects than itself and also prolonged the storage time of bananas (Zhao et al., 2022). Temperature-responsive and antimicrobial core-shell system has been proposed as a coating material for food wherein temperature-sensitivity is due to polyethylene glycol core and the antibacterial activity is imparted by chitosan. The mentioned food coating markedly enhanced the shelf life of ricotta cheese and also warned about the appropriate temperature for its storage by changing the color (Kritchenkov et al., 2021). Further, chitosan/montmorillonite nanocomposites are reported to be potential smart antimicrobial coatings for fruit packaging and demonstrated improvement in the storage

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of William pears (Reis et al., 2021). Chitosan has also been used in antifouling smart coatings in the marine industry where they play an antibiofouling role and the antibacterial effect in a pH-dependent manner. Hao et al. fabricated capsaicin-based pH-triggered polyethylene glycol/capsaicin@chitosan (PEG/CAP@CS), polyvinyl alcohol (PVA)/CAP@CS, and alginate (ALG)/CAP@CS multilayer films in chitosan matrix which exhibited extraordinary pH-responsive properties. The biocide capsaicin in the chitosan matrix was released in a controlled manner in response to pH with low-level delivery in alkaline conditions and fast delivery in an acidic environment. This enhances the longevity of antibiofouling coatings (Hao, Chen, Qin, Zhang, Li, Fan, et al., 2020).

6.5 Other nonmaterial-based smart antimicrobial coatings Graphene is a nonmaterial that has emerged as one of the most promising materials in various applications due to its exceptional strength as well as thermal, mechanical, and optical properties. It is a thin sheet of monoatomic carbon arranged in a crystalline lattice with a hexagonal honeycomb-like structure. The intrinsic material characteristics of graphene allow it to be constructed as micro- and nanostructures for superhydrophobic nanocoating applications (Calori et al., 2020). In a study, Abbas et al. fabricated a stable self-cleaning superhydrophobic surface on copper alloy substrates using fluorinated graphene through a drop coating process. Graphene oxide is an excellent smart coating nanomaterial because of its distinctive properties, including superior solubility and dispersibility in water and other solvents, high biocompatibility, antimicrobial activity, and response to electrical fields in its reduced form. Patarroyo et al. synthesized Graphene oxide-gelatin nanocomposite hydrogel for controlled delivery of probiotics which exhibits pH-dependent swelling ratio. The smart hydrogel can be used as a smart coating due to its changes in the water holding capacity with the change in pH of the surrounding environment (Patarroyo et al., 2021). Carbon-based smart coatings are also proposed for the biomedical field. Diamond-like carbon coatings with embedded nanoparticles were synthesized for the first time by Buchegger et al. which exhibited excellent antimicrobial activities against MRSA and S. epidermis in a pH-responsive manner. They adapt smartly to bacterial acidosis and release the antimicrobial agent on time as requested. The authors proposed its potential use in the design of smart orthopedic implants (Buchegger, Kamenac, Fuchs, Herrmann, Houdek, et al., 2019).

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Antiviral and antimicrobial smart coatings

Silica-based nanomaterials have been widely used in the field of superhydrophobic nanocoating. Although they are intrinsically hydrophilic, they can easily undergo chemical modifications to obtain superhydrophobicity. Excellent optical properties are another advantageous feature of silica-based nanomaterials. Other favorable features of silica-based nanomaterials in the fabrication of superhydrophobic surfaces include low toxicity, mechanical and thermal stability, ease of structural regulation, and low preparation cost. Hegazy et al. synthesized mesoporous silica nanocarrier (MSN)-based delivery system sensitive to the dual stimulus of temperature and CO2. A poly (DMAEMA)-co-poly(MPS) shell on MSN was constructed via a facile one-step reaction and this coating exerted effective antimicrobial effects against E. coli (Hegazy et al., 2020). Francesko et al. developed amino cellulose nanospheres (ACNSs)/HA multilayer films to fight against P. aeruginosa, a bacterial species widely found in infections associated with medical devices. The antibacterial activity of the coating was due to the on-demand release of amino cellulose nanospheres contained in the hyaluronic acid film. The coating is stable in the absence of bacteria while the presence of bacteria triggers the degradation of the coating leading to the release of antibacterial nanospheres and prevent the formation of biofilm (Francesko et al., 2016) (Table 1). Table 1 Common induction stimulus and nanoparticle/nanomaterial involved in smart antimicrobial coatings. Induction stimuli

Nanoparticle/ nanomaterial involved

Reference

Physically responsive smart antimicrobial coatings

Temperature

Silver nanoparticles TiO2 nanotubes TiO2 nanoparticles

(Priyanka & Kumar, 2020; Raczkowska et al., 2019) (Li et al., 2021) (Matos et al., 2019)

Chitosan Silica nanoparticles

Light

Silver nanoparticles Zinc oxide nanoparticles

(Kritchenkov et al., 2021; Li et al., 2021) (Hegazy et al., 2020) (Atta, 2021; Gautam et al., 2021; Manna et al., 2015) (Manna et al., 2015; Ni et al., 2021)

Nanomaterial-based smart coatings as antimicrobials

425

Table 1 Common induction stimulus and nanoparticle/nanomaterial involved in smart antimicrobial coatings—cont’d Induction stimuli

Nanoparticle/ nanomaterial involved

Reference

Chemo responsive smart microbial coatings

pH

Silver Zinc oxide nanoparticles Chitosan Carbon-based nanomaterial Graphene

Bacteria/ bacterial biomolecules

(Dai et al., 2018; Priyanka & Kumar, 2020) (Buchegger, Kamenac, Fuchs, Herrmann, Houdek, et al., 2019; Ni et al., 2021; Wang et al., 2017)

Titania nanotubes Titanium

(Dai et al., 2018; Hao, Chen, Qin, Zhang, Li, Fan, et al., 2020; Zhao et al., 2022) (Buchegger, Kamenac, Fuchs, Herrmann, Houdek, et al., 2019) (Patarroyo et al., 2021)

Titanium

(Wang et al., 2017) (Sang et al., 2021) (Tolle et al., 2004; Zhang et al., 2020)

Amino cellulose nanospheres

(Francesko et al., 2016)

7. Conclusions and future outlook The adherence and development of infectious bacteria on surfaces is an issue of concern worldwide due to the limitations associated with classical approaches of antimicrobial coatings researchers are developing smart antimicrobial surfaces. Such smart frameworks permit the discharge of antimicrobial agents in a spatiotemporal way thereby showing the effect only in the presence of bacteria and the covering otherwise remains in a dormant stage. The smart approach helps to contain the emergence of antimicrobial strains, avoids drug dilution to healthy tissue and confines the treatment region. The two most important features behind the success of any smart coating are the choice of appropriate stimuli and the manufacturing of the responsive material. Integrating nanomaterials as smart antimicrobial coating becomes the choice of the materials due to their unique properties at nanosize. Despite

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Antiviral and antimicrobial smart coatings

the fact that nanomaterial-based smart antimicrobial coatings are the demand of the present, still many challenges need to be overcome. One of the critical challenges of applying nanocoatings is the aggregation of nanoparticles in the coating material and the uniformity of the coatings. More specifically, a better understanding should be gained of the coating thickness, functionality, geometry, stability, and toxicity within in vitro and in vivo conditions. In addition, more efforts are required in developing this innovative technology in terms of the longevity, stability of the coatings, manufacturing strategies, and their conformity to the environmental effects, especially in terms of controlling microbial infections.

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CHAPTER 14

Hybrid antibacterial, antifungal, and antiviral smart coatings Sougata Ghosha,b, Bishwarup Sarkarc, Sirikanjana Thongmeea, and Ebrahim Mostafavid,e a Department of Physics, Faculty of Science, Kasetsart University, Bangkok, Thailand Department of Microbiology, School of Science, RK University, Rajkot, Gujarat, India c College of Science, Northeastern University, Boston, MA, United States d Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA, United States e Department of Medicine, Stanford University School of Medicine, Stanford, CA, United States b

1. Introduction Microbial infections associated with biofilm formation are difficult to treat as the microbial cells are often surrounded by a protective matrix composed of polysaccharides, extracellular DNA, and proteins (Kazemzadeh-Narbat, 2021; Mostafavi et al., 2022). Hence, such biofilms are one of the major causes of nosocomial infections as they are prevalent in clinical settings. Therefore, new avenues and technologies are under extensive research for the effective treatment of microbial biofilms as well as strains that have become resistant to traditional antibiotics (Chaudhary, Mostafavi, & Kaushik, 2022; Ranpariya, 2021). One of the novel approaches includes the application of nanotechnology for the preparation of enhanced antibacterial, antifungal, and antiviral coatings that could be useful in a multitude of medical as well as nonmedical situations (Derakhshi, Mostafavi, & Ashkarran, 2022; Desai, 2021; Ghosh et al., 2018; Mostafavi et al., 2019). Hence, this chapter elaborates on such applications of nanocoatings on various substrates for improved biocidal activities. Several bacterial species are reported to actively form biofilms and colonize on implants or medical devices (Singh et al., 2021). In addition, an alarming increase in multidrug resistance among bacteria has also been highlighted in the recent decade (Wei et al., 2021). Therefore, the formulation of smart antibacterial nanocoatings with distinct nanomaterials could be applied on diverse surfaces to effectively control such bacterial species (Pormohammad et al., 2021). Several nanomaterials such as niosomes, nanocomposite films, metallic nanoparticles (NPs) as well as nanoencapsulation are used as promising microbe-resistant nanocoatings and are discussed in this chapter. Antiviral and Antimicrobial Smart Coatings http://doi.org/10.1016/B978-0-323-99291-6.00016-5

Copyright © 2023 Elsevier Inc. All rights reserved.

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Antiviral and antimicrobial smart coatings

Likewise, antifungal and antiviral nanocoatings are also meant to combat resistant strains of the same. With the recent outbreak of Coronavirus disease 19 (COVID 19), and its newly emerged variant of concerns, extensive research is being carried out to find amicable solutions to mitigate the spread of the virus (Basak & Packirisamy, 2020; Ghosh et al., 2020; Mostafavi et al., 2022). Antiviral nanocoatings are potentially aiding in such measures wherein several nanomaterials effectively inhibit severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) after application on various surfaces such as surgical masks, glass, and porous membrane media (Saravanan et al., 2021). As listed in Table 1, antifungal nanocoatings are also discussed in this chapter that could potentially be used to mitigate and control harmful fungal species. Thus, further exploration of such effective nanocoatings against resistant bacteria, fungi, and viruses could be an effective strategy to control infections and mitigate their spread.

2. Antibacterial coatings Lee et al. (2007) reported the development of silver nanocoated fabric with antibacterial activity. Deposition of silver nanoparticles (AgNPs) onto cotton fabrics was carried out wherein 0.1 mM of butylamine and silver nitrate solution resulted in the formation of AgNPs over the cotton fabric with an average size of 41  7 nm. The size of AgNPs was then increased with a concomitant increase in the concentration of silver nitrate and butylamine, respectively, along with increased aggregation of the particles. X-ray diffraction (XRD) results confirmed the presence of AgNP deposition on the surface of cotton fabrics, which demonstrated four distinct peaks highlighting the crystalline and cubic planes of Ag. The reduction of silver was suggested to involve binding the metal ions onto the anionic oxygen sites of the cotton fabrics, followed by reducing the silver ions using butylamine. Field emission scanning electron (FESEM) microscopy further revealed that these AgNPs provided the nucleation sites for aggregation of the particles. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) further provided quantitative measurement of the AgNPs that were loaded onto the cotton fabrics. Three different prepared cotton nanofabrics revealed the presence of 0.61, 1.4, and 5.3 mg/g of AgNPs that were coated onto the fabrics. Furthermore, the prepared cotton nanofabric was evaluated for its application as an antiseptic dressing or bandage. Initially, the antibacterial activity of the nanofabrics was investigated against Staphylococcus aureus and Escherichia coli, wherein complete growth inhibition was observed. Moreover, growth

Table 1 Biologically active nanocoatings for infection control. Nanomaterial

Coated surface

Application

Reference

AgNPs

Cotton fabric

Antibacterial activity against E. coli and S. aureus

AgNPs

Dentine disc

Antibacterial activity against S. mutans

Chitosan-clove and chitosan-argan oil nanoencapsulations

PLA film

DFNS-NH2@AgNPs

PMMA film

Antibacterial activity against E. coli, L. monocytogenes, S. typhimurium, antioxidant activity Antibacterial activity against E. coli and S. aureus

(Lee et al., 2007) (Besinis et al., 2014) (Munteanu et al., 2018)

Niosomes

Bone plate

Antibacterial activity against S. aureus

PLGA 5% NFX

Ti disc

Antibacterial activity against E. coli

Titania/chitosan/AgNPs composite film

Antibacterial activity against S. aureus and E. coli

Zn-CuONPs

Commercial cellulose material Contact lens

ZnONPs

NiTi wire

AgNPs

Nystatin and clotrimazol

Antibacterial activity against S. epidermidis and P. aeruginosa Antibacterial activity against S. mutans Antifungal activity against Candida albicans, Candida tropicalism, Candida pampislosis, and Candida glabrata

(Wang et al., 2018) (Dwivedi et al., 2018) (Baghdan et al., 2019) (Xiao et al., 2013) (Tuby et al., 2016) (Kachoei et al., 2016) (Alheety & Hameed, 2018) Continued

Table 1 Biologically active nanocoatings for infection control—cont’d Nanomaterial

Coated surface

Application

Reference

CHT/CMC/SGL/MMT

Hydrogel

Cyclodextrin-based emulsions

–

Antifungal activity against T. mentagrophytes and T. rubrum Antifungal activity against C. albicans

Graphene

Grade 4 titanium

Antifungal activity against C. albicans

μTiO2

Acrylic paint film

Ag/SiO2NPs, CuONPs and ZnONPs

Glass and porous membrane media PET fabric

Antifungal activity against M. ruber, photocatalytic activity for methylene blue dye degradation Antiviral activity against SARS-CoV-2

(Bozoglan et al., 2021) (Leclercq & NardelloRataj, 2016) (Agarwalla et al., 2021) (Amorim et al., 2020)

LAP-Cu2+

Polygalactose, guanidine, octane, and coumarin fluorescence attached nanoworms Tea-cinnamaldehyde oil mixed with copper/silver salts

Surgical mask

Nonwoven polypropylene cloths

Effective SARS-CoV-2 trapping efficiency, antibacterial activity against S. aureus and P. aeruginosa Antiviral activity against AAV-HA, VIC01, and B.1.1.7 variants of SARS-CoV-2 Antiviral activity against MHV-A59, antibacterial activity against E. coli and S. aureus

(Merkl et al., 2021) (Choi et al., 2021) (Bobrin et al., 2021) (Cox et al., 2021)

Hybrid antibacterial, antifungal, and antiviral smart coatings

435

retardation of the two bacteria was analyzed in which 50 and 70 mg of silvernanocoated fabrics were able to successfully inhibit S. aureus, whereas 70 mg of silver-coated fabrics demonstrated inhibition of ampicillin-resistant strain of E. coli. A skin irritation experiment was also carried out in guinea pigs that showed no erythema or edema after applying the prepared nanofabric. Silver nanocoating was also demonstrated to possess effective inhibitory activity against biofilms on human dentine discs (Besinis et al., 2014). Silver nanoparticles (AgNPs) solution (10 g/L) was sonicated, followed by coating on the dentine discs. Energy-dispersive X-ray spectroscopy (EDS) results confirmed the presence of Ag, while scanning electron microscopy (SEM) images confirmed the successful baseline coating of AgNPs. Application of AgNPs occluded most of the dentinal tubules, wherein stability of the particles was observed after 24 h of submersion in the medium. Further, low Ag concentration of 0.54  0.17 mg/L was attributed to the limited dissolution of the Ag metal in the medium. Turbidity measurements for analysis of the growth of Streptococcus mutans were further analyzed wherein the cell viability was less than 0.5% as observed using the LIVE/DEAD kit. In addition, the production of lactate by S. mutans after 24 h of exposure with AgNP-coated dentine discs was significantly lowered compared to control discs. Hence, the antibacterial effect of AgNPs coating on the surface of dentine discs was displayed. Moreover, the adhesion of bacteria on the surface of dentine discs coated with AgNPs was less than 2% as compared to 76% adhesion observed in chlorhexidine-coated discs. The natural color of the dentine was also not affected in the presence of AgNPs and chlorhexidine, while AgNO3 coating resulted in significant discoloration of the dentine discs. Munteanu et al. (2018) prepared nanocoated poly(lactic) acid (PLA) films that had antioxidant and antibacterial activity. A coaxial electrospinning system was employed for nanocoating of the wire with nanoencapsulated essential and vegetable (clove and argan) oils. Transmission electron microscopy (TEM) images revealed more efficient macromolecular entanglements in the electrospun solution when a high concentration of chitosan was used for encapsulation of clove (HC) and argan oil (HA). In addition, particle morphology was observed with low concentrations of chitosan (LC and LA), wherein isolated beads of oil were encapsulated by chitosan. In contrast, in the case of high concentrations of chitosan, scattered beads of encapsulated oil were observed along with the main chitosan fiber. Moreover, a fiber diameter of 10–20 nm was observed in the case of HC and HA samples. The atomic force microscopy (AFM) results displayed an increased compact

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arrangement of particles in the case of LA and LC samples. The smaller gaps resulted in reduced porosity along with reduced height distribution as compared to HA and HC samples. The thickness of the coating layer was 100  20 nm, with increased roughness observed in HA and HC samples as compared to LA and LC samples. SEM images then displayed a nanofiber diameter range of 60–70 nm in the case of commercial NATIVIA NTSS 40 μm PLA foils that were nanocoated with high concentrations of chitosan-encapsulated clove oil (HC) samples. Moreover, improved antibacterial activity against E. coli, Listeria monocytogenes, and Salmonella typhimurium was observed in the case of HC and HA samples. The combination of high concentrations of chitosan-encapsulating clove and argan oil was more efficient in inhibiting bacterial growth than high amounts of chitosan alone. Likewise, the antioxidant activity of the chitosan-clove oil combination was also higher than others. Nanocoatings prepared using properly dispersed AgNPs and partially embedding dendritic fibrous nanosilica (DFNS) also exhibited efficient antireflective and antibacterial activities when applied on poly(methyl methacrylate) (PMMA) polymeric glass (Wang et al., 2018). A postsynthetic grafting approach was carried out in the mesoporous surface of DNFS that resulted in aminopropyl functionalization, which further facilitated efficient AgNPs loading. The obtained DFNS-NH2@AgNPs then showed an ultravioletvisible (UV-Vis) absorption peak at around 417 nm, which was attributed to the Mie plasmon resonance of AgNPs localized on the surface of mesoporous walls with a mean particle size of 3.4 1.2 nm as observed through SEM and TEM images as seen in Fig. 1. Moreover, considerable stability was exhibited by DFNS-NH2@AgNPs when suspended in ethanol and water. The dip-coating method was carried out on the surface of PMMA wherein the transmittance at 688 nm increased from 92.0% to 96.9%, with subsequent rounds of dip-coating, with eight successive cycles found to be optimal. Additionally, chloroform vapor treatment was performed to improve the adhesion of the particles on the surface of PMMA slides. Later ultrasonic processing removed all the unembedded particles, which resulted in the formation of a homogenous layer of AgNPs on the slide surface. After ultrasonic processing, SEM images then displayed highly dense particles on the PMMA surface that formed a monolayer. Further, the antibacterial activity of the prepared nanocoatings was investigated in which around 20% and 10% of E. coli and S. aureus cells were viable on the surface of nanocoated PMMA films as compared to 90% and 80% E. coli and S. aureus cell viability in unmodified PMMA films, respectively. Hence, the effective antibacterial activity of the

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Fig. 1 SEM (A,C) and TEM (B,D) images of DFNS (A,B) and DFNS-NH2@AgNPs (C,D). (Reprinted with permission from Wang, Y., et al. (2018). Dendritic silica particles with well-dispersed Ag nanoparticles for robust antireflective and antibacterial nanocoatings on polymeric glass. ACS Sustainable Chemistry & Engineering, 6(11), 14071–14081. Copyright © 2018 American Chemical Society.)

nanocoated films was demonstrated, highlighting its potential applications in optical devices and daily touch plastic glass. In a similar study, Dwivedi et al. (2018) formulated antibacterial niosomes using layer-by-layer nanocoating of vancomycin/poly(lactic acid) (PLA)/ vancomycin. Niosomes loaded with vancomycin were obtained using the evaporation of solvent that was composed of a 3:1:1 (w/w/w) ratio of Span 60 (nonionic surfactant), cholesterol, and vancomycin that resulted in the formation of a thin film that was further hydrated and sonicated, respectively. Characterization of the prepared vancomycin-loaded niosomes was also carried out wherein the size of the niosomes increased from 255.9 14.67 nm to 340.5  2.95nm. Likewise, the zeta potential changed from 43.8  2.97 mV to 45.4 0.77 mV. Moreover, the encapsulation efficiency and drugloading content were 50.47 3.66% and 19 1.77%, respectively.

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The in vitro release of vancomycin was evaluated in phosphate buffer solution (pH 7.4) that showed a sustained and prolonged drug release up to 28 h, whereas more than 90% of free vancomycin was diffused out within 9 h. Layer-by-layer niosome coating was carried out for 40 cycles, which resulted in loading of 6.93 mg of the drug onto the bone plate. A twofold increase in the drug release time was observed in the case of coated bone plates compared to the control. Such a slow rate of vancomycin release was attributed to the uniform distribution of the drug after encapsulation in the niosome. The growth of S. aureus was also inhibited for 2 weeks because of the sustained release of the drug from nanocoated bone plates. SEM images further demonstrated prominent nanosized aggregates on the coated bone plates, thus confirming the presence of niosomes. Bacterial attachment studies also displayed significant reductions in S. aureus counts in bone plates coated with niosomes as the prolonged vancomycin release was above the bacteria’s minimum inhibitory concentration (MIC). The coatings were also noncytotoxic as more than 76% cell viability was obtained when applied on fibroblast (L929) cells. In another study, Baghdan et al. (2019) formulated nanocoatings with antibacterial activity that could be utilized for dental implants. Nanospray drying technology was used for the preparation of nanocoating in which small titanium discs were nanocoated with a feed solution composed of varying concentrations of norfloxacin (NFX) and poly(lactic-co-glycolic acid) (PLGA). The antibacterial activity of the prepared nanocoatings was then evaluated against E. coli. Maximum inhibition zones were obtained in nanocoatings with PLGA 10% NFX that were comparable with PLGA 5% NFX. The spectrophotometric quantification of the nanocoatings revealed an NFX content range of 10.33 μg/cm2 (PLGA 10% NFX) to 0.73 μg/cm2 (PLGA 2.5% NFX). The release profile of NFX from the nanocoatings was also investigated at 37°C for a time period of 15 days wherein an initial burst within the first 48 h followed by a constant release rate from the third day was observed. Scanning electron microscopy (SEM) results then demonstrated spherical particles with a smooth surface and an average size of around 600 nm. Morphological changes in the particles were observed after 15 days of incubation in phosphate buffered saline (PBS). In addition, the surface roughness of the Ti discs increased in the presence of PLGA 5% NFX. Nanocoated Ti discs were then incubated with E. coli suspension for a time period of 24 h after which 99.83% and 95.42% reduction in bacterial cell viability was observed in the top and bottom positions of the disc coated with PLGA 5% NFX,

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respectively whereas no such inhibition was observed in the case of control Ti discs. Furthermore, in vitro biocompatibility of the nanocoated Ti discs was also analyzed using fibroblast (L929) cells. Fluorescence microscopy results showed considerable cell growth on the nanocoated Ti discs after 96 h of incubation. Hence, good biocompatibility of the nanocoated Ti discs was observed, highlighting its potential use in dental implants. Similarly, Xiao et al. (2013) fabricated hybrid materials that were composed of microfibril bundles of cellulose along with titania/chitosan/AgNPs composite films. A layer-by-layer self-assembly process was followed for coating the commercial filter paper with titania/chitosan composite film. Thereafter, silver ions were properly adsorbed onto the surface of the titania/chitosan composite film-coated commercial filter paper followed by in situ reduction using UV irradiation, which resulted in the formation of AgNPs. SEM images of the prepared hybrid material displayed a hierarchical fibrous assembly with densely anchored AgNPs on the composite microfibril bundles with a size range of 4–20 nm. Moreover, TEM image revealed an average single microfibril thickness of 14 nm. Atomic absorption spectroscopy (AAS) analysis further revealed 1.25 wt% of Ag in the hybrid material. UV-Vis spectra also demonstrated absorption peaks at 250 and 450 nm that were attributed to the amorphous titania layer and surface plasmon resonance (SPR) of AgNPs, respectively. The antibacterial activity of the prepared material was then analyzed against E. coli and S. aureus, wherein effective growth inhibition was observed against both the test organisms. The intrinsic biocidal property of deposited titania composition, positively charged chitosan as well as AgNPs was suggested to synergistically promote such improved antibacterial activities. In another interesting report, the contact lenses were nanocoated with zinc-doped copper oxide nanoparticles (Zn-CuONPs) for effective antibacterial activity (Tuby et al., 2016). A 3:1 M ratio of Cu: Zn was maintained for effective formation of ZnO-CuO NPs while 0.01 and 0.05 M of acetate were used during the coating process. XRD analysis then revealed the crystalline nature of the particles while high resolution-scanning electron microscopy (HR-SEM) images demonstrated uniform coating with a homogeneous distribution of the particles on the surface of the lens wherein the average size of the particle was around 65 and 30 nm when high and medium level of concentrations were used, respectively. Moreover, focused ion beam (FIB) analysis showed a thickness layer of around 40 and 20 nm when high and low concentration of particles was used for the coating, respectively. Thereafter, the leaching effect of Zn-CuO NPs was also

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evaluated for its safe use in medical devices in which no change in the morphology of the coating was observed after 72 h of immersion in saline solution for both the concentrations of the particles. In addition, high resolution-transmission electron microscopy (HR-TEM) images also confirmed the absence of Zn-CuO NPs in the leaching solution, which highlighted efficient embedding of the particles onto the surface of the lens. The antibacterial activity of the coated lenses indicated significant reduction in the adhesion of Staphylococcus epidermidis and Pseudomonas aeruginosa compared to uncoated contact lens surface. A mean reduction of 4.64 and 4.95 logs of P. aeruginosa adhesion was achieved in the case of medium and high concentration coatings on the lenses, respectively. Likewise, 2.72 log reduction in S. epidermidis adhesion was achieved using a high concentration of nanocoatings. In another study, Kachoei et al. (2016) fabricated a friction-reducing coating of zinc oxide nanoparticles (ZnO NPs) that had antibacterial activity on nickel-titanium (NiTi) wire. A chemical deposition method was carried out using ZnO NPs to coat the wires at pH 12.0 and temperature of 50°C. SEM and transmission electron microscopy (TEM) images of ZnO NPs revealed its spherical shape with a size range of 16–30 nm while XRD pattern confirmed the crystalline nature of the particles. Characterization of the NiTi wires was also carried out after nanocoating that showed the hardness and elastic modulus of 2.3  0.2 and 61.0  3.6 GPa, respectively, along with increase in the whiteness of the wire. The SEM images showed no major damage or cracks on the nanocoated wire even after bending indicating its durability. Thereafter, atomic force microscopy (AFM) results demonstrated increase in average roughness of the wires from 11.8  2.8 to 18.6  4.6 nm. Decrease in the mean frictional force from 1.412  0.11 to 1.227  0.13 N after nanocoating at an angle of 0°C was noted which further decreased with increase in the angle between the wires and brackets. In addition, the mean amount of modulus of elasticity for the austensite phase of the nanocoated wires was 50.930  3.91 GPa whereas it was 18.898  1.86 GPa in the martensite phase. Thereafter, antibacterial activity of the nanocoated wires was also demonstrated where no growth of S. mutans (ATCC 35668) was observed after 48 h of treatment at 37°C.

3. Antifungal coatings Various nanocoatings are developed for effective inhibition of fungal growth on the surface. AgNP-coated nystatin (NY) and clotrimazol (CM) drugs

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exhibited effective antifungal activity as demonstrated by Hameed and Alheety (Alheety & Hameed, 2018). Fourier transform infrared (FTIR) spectroscopy results of the prepared composite revealed shifts in the stretching vibrational peaks of dOH and CdO groups of NY, which suggested successful coating of the drug with AgNPs by dNH2 and -OH groups. Likewise, functional groups of CM also facilitated AgNP conjugation. XRD pattern of the prepared AgNP-Cm and AgNP-NY composites showed a crystalline size of 34 and 7 nm, respectively, while AgNPs had an increased crystalline size of 44 nm due to aggregation of the particles to form clusters. Thereafter, antifungal activity of the nanodrug formulation was investigated against four different species of Candida, namely Candida albicans, Candida tropicalism, Candida pampislosis, and Candida glabrata. A comparable antifungal activity was observed when 8 mg of the AgNP-CM and AgNP-NY or 50 mg of either AgNPs, CM, or NY was used against different species of Candida. Hence, it was proposed that AgNPs facilitated easy entry of the drugs into the fungal cell making the nanoformulation effective even in low doses. Bozoglan et al. (2021) also recently reported the fabrication of smart nanocomposite hydrogels that had antifungal activity and could be potentially used for onychomycosis treatment. The hydrogel was composed of thermosensitive chitosan/carboxymethylcellulose/scleroglucan/montmorillonite (CHT/CMC/SGL/MMT) that had a highly porous structure. Moreover, the pore size was significantly reduced with concomitant increase in MMT concentration. Differential scanning calorimetry (DSC) analysis also revealed increase in the glass transition temperature (Tg) with subsequent increase in MMT concentration, which suggested a tighter structure of the hydrogel in the presence of higher amounts of MMT. Moreover, mechanical properties were also improved in the presence of 5.0 w/w% MMT, which was evident from 61.5% return of the hydrogel gel containing MMT in its original form after the removal of physical force as compared to 46.2% return in gels without MMT. The compression stress value of hydrogels made up of 5.0 w/w% of MMT was 266.99  6.60 kPa. The contact angle measurement of the nanocomposite material containing 5 w/w% MMT was 77.9 degree, which indicated its hydrophilic nature. The hydrogels were also transparent in nature. Release of oxiconazole nitrate (OXI) from the hydrogel was evaluated at 32°C wherein sustained drug release was observed in the case of hydrogel prepared using 5 w/w% MMT. Moreover, 51.55  1.02% of the drug was released within 6 months when stored at 4°C. Antifungal activity of the prepared nancomposite hydrogel was also

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displayed against Trichophyton mentagrophtyes and Trichophyton rubrum dermatophytes. Diameter of the inhibition zones was significantly reduced when the fungal strains were incubated with the nanocomposite hydrogels at 26°C for a time period of 7 days. Leclercq and Nardello-Rataj (2016) also reported the preparation of cyclodextrin-based emulsions for enhanced antifungal activity. A stable, white oil/water biphasic Pickering emulsion system was obtained using 1 wt% of econazole nitrate wherein the water phase was continuous and the particles were more wetted by water than oil. Optical microscopy revealed proper stability of the Pickering emulsions with an average droplet size range of 9.2–16.1 μm. TEM micrographs of precipitated fraction of the emulsion further showed insoluble inclusion complexes that formed polydisperse nanoparticles with a size range of 30–250 nm. At a cyclodextrin concentration of 1%–5%, the particles self-assembled to form a pseudocrystalline structure. Moreover, the antifungal activity of the prepared emulsions was evaluated in which a clear C. albicans inhibition was obtained after 24 h of incubation as seen in Fig. 2. It was suggested that the Pickering emulsions facilitated steady release of econazole nitrate that can easily diffuse into the media and acts as a biocidal agent. However, it is interesting to note that the encapsulation of econazole nitrate inside γ-cyclodextrin was the limiting factor for its diffusion. In another similar recent study, Agarwalla et al. (2021) formulated graphene nanocoating on titanium for the inhibition of a C. albicans biofilm. A grade 4 titanium was nanocoated twice (TiGD) or 5 times (TiGV) wherein 99% of coverage yield was observed using Raman mappings. Thereafter, inhibition of biofilm formation was evaluated for a time period of 7 days wherein the cell density was significantly lower in TiGD and TiGV samples as compared to control. Similarly, the colony forming unit (CFU) assay also demonstrated higher yeast cell count in the control as compared to the nanocoated titanium. Confocal laser scanning microscopy (CSLM) then revealed proper biofilm formation after 7 days of incubation in the case of control sample whereas TiGD and TiGV displayed only adhered yeast cells without the presence of any biofilm. Hence, such nanocoatings with antifungal activity could prevent proliferation of pathogenic fungi on implantable materials. Amorim et al. (2020) reported antifungal and photocatalytic activity of acrylic paint formulations that were supplemented with titanium dioxide microspheres (μTiO2) for the preparation of smart paints. For the synthesis of μTiO2, a combination of sol-gel method along with a solvothermal and

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Fig. 2 Typical photograph images of the zone of inhibition obtained by the Kirby-Bauer test against Candida albicans (left) or Staphylococcus aureus (right) of the formulated Pickering emulsions with econazole nitrate (F1 to F6) or without econazole nitrate (C1 to C6) compared to the commercial formulation (F0). (Reprinted with permission from Leclercq, L., & Nardello-Rataj, V. (2016). Pickering emulsions based on cyclodextrins: A smart solution for antifungal azole derivatives topical delivery. European Journal of Pharmaceutical Sciences, 82, 126–137. Copyright © 2015 Elsevier B.V.)

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calcination process was carried out after which 10 wt% of μTiO2 was utilized for the formulation of photocatalytic paint. SEM and TEM micrographs displayed spherical morphology of the particles that were rough and irregular with an average pore size of 13 nm. Moreover, a uniform size distribution was achieved within a range of 600–1300 nm. The surface of the formulated paint films was also characterized that revealed large, insoluble particles at the base of the film along with the presence of μTiO2 particles as well. Additionally, almost 8 wt% of dry solids was observed in the formulated paints. Effective antifungal activity of the prepared paint formulation was then demonstrated against Monascus ruber wherein a clear halo of inhibition was seen when coated with the μTiO2-laden paint samples. This was speculated to be a result of oxidizing radical formation such as peroxides due to irradiation that may have inhibited the fungal growth. Moreover, photocatalytic activity of the prepared paint sample was also analyzed that showed effective decolorization of methylene blue dye in the presence of ultraviolet-C (UVC) light. The mechanism of dye degradation by μTiO2-containing acrylic paint was proposed to be a two-stage process in which the initial step was involved with photodegradation of the paint film followed by exposure of the μTiO2 that may facilitate dye degradation.

4. Antiviral coatings Antiviral nanocoatings are most significant for the containment of virus (Saravanan et al., 2021; Truong et al., 2022). Recently, Merkl et al. (2021) demonstrated antiviral activity of metal nanoparticle coatings specifically against severe acute respiratory syndrome coronavirus-2 (SARS-CoV2) highlighting the potential to control the spread of the virus. A flame spray pyrolysis method was employed for the synthesis of AgNPs and copper oxide and zinc oxide nanoparticles (CuO NPs and ZnO NPs) wherein the prepared NPs were then attached to solid glass substrates or porous filter membranes either by thermophoresis or filtration, respectively. TEM images then revealed an average particle diameter size of approximately 5 nm in case of AgNPs whereas CuO NPs and ZnO NPs had average particle size of approximately 10 nm. All three particles were homogeneously distributed on solid glass substrates and porous membrane media. The antiviral capacity of the developed nanocoatings was then analyzed by varying the duration of the treatment. CuO NP film demonstrated 54% and 76% reduction in viral load after 30 and 120 min of incubation, respectively. Likewise, the Ag/SiO2 NP film exhibited 75% of viral reduction within

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5 min and 98% reduction after 120 min of incubation. Hence, such nanosilver coatings were suggested for effective reduction of the viral load and hence can be used in multiple ways to mitigate the spread of the virus. In another similar study, Choi et al. (2021) fabricated antiviral nanocoating against SARS-CoV-2 to limit the spread of the virus via advanced filtration and self-sterilization systems. The nanocoating was prepared using laponite (LAP) and Cu2+ ions in which a silicon wafer served as the substrate. Electrostatic interaction of the negatively charged LAP with the positively charged poly(diallyldimethylammonium chloride) (PDAC) polymer facilitated its deposition on the fabric composed of polyethylene terephthalate (PET). Moreover, CuCl2 solution was used for stacking of LAP to form a multilayer that also facilitated retention of stable Cu2+ ions in its reduced state. Thereafter, the color of the PET fabric changed from white to blue after 10 cycles of nanocoating of PDAC/LAP layers because of the presence of metal ions. A rough surface morphology was attributed to the LAP-Cu2+ nanocoating on the individual fibers as well as between the spaces of the two strands. Later on, the ability of the nanocoating to trap proteins was also evaluated in which bovine serum albumin (BSA) in the form of aerosols with a size range of 100 nm to 5 μm was passed through the nanocoated fabric. The trapping capacity of nanocoated fabric was 3.72 times higher than the control fabric. Additionally, viral trapping efficiency of the nanocoating was also investigated in which the filtering capacity of nanocoated fabric was 9.55 times higher as compared to the control fabric when SARS-CoV-2 was sprayed. It was further proposed that the viral particle exhibited improved interaction with the nanocoating due to the presence of both negatively and positively charged face of LAP that resulted in hydrogen bonding and electrostatic effect with LAP and viral spike proteins. The protein trapping efficiency was 1.7 mg/cm2 in 1 h (99.75%) when the nanocoated silicon wafer was submerged in solution containing excessive amounts of BSA. Moreover, the LAP-Cu2+ nanocoating also demonstrated antibacterial effect with 73% and 74% decrease in cell viability of S. aureus and P. aeruginosa, respectively. Hence, the functionality of the LAP-Cu2+ nanocoating can be effective on commercial filters and facial masks to control the spread of the virus in the environment. Bobrin et al. (2021) also formulated a waterborne spray-on nanocoating with antiviral activities against SARS-CoV-2 along with other viruses as depicted in Fig. 3. Multifunctional nanoworms sensitive to pH and temperature were synthesized using a temperature-directed morphology transformation (TDMT) method wherein three functional and thermoresponsive

Fig. 3 Water-based and responsive nanocoating for disruption of SARS-CoV-2 and other viruses. (A) Decorated nanoworm in extended and collapsed conformation. (B) Illustration for the nanomechanical disruption and inactivation of viral particles; rapid droplet spreading leading to extension, then collapse of nanoworms in response to change in pH. (C) Attachment of a fluorescence probe to the nanoworms for continuous monitoring of coated nanoworms on the surface at 356 nm. (Reprinted with permission from Bobrin, V.A., et al. (2021). Waterborne nanocoating for rapid inactivation of SARS-CoV-2 and other viruses. ACS Nano, 15(9), 14915–14927. Copyright © 2021 American Chemical Society.)

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macrochain transfer agents (MacroCTAs) were used for the same. The diameter of the prepared nanowires was 15 nm while the length was between 300 nm and 2 μm. A polygalactose residue with around 10 sugar units was incorporated that was speculated to facilitate easy access to the membrane-fusion S2 subunit of the SARS-CoV-2 spike proteins and thus, increase its binding affinity to the virus. Quaternarization of dimethylaminoethyl acrylate (DMAEMA) groups, as well as coupling with guanidine, resulted in the formation of positively charged nanoworms that could easily bind to negatively charged viral particles. In addition, hydrophobic octane groups were added into the nanoworm corona for intercalation between the virus and nanoworm. Coumarin fluorescence probes then accurately determined attachment of less than 1500 mg/m2 of nanoworms on the surface. Antiviral activity of the prepared nanoworms was then analyzed on adeno-associated virus (AAV) with expression of hemagglutinin (HA) of influenza A virus. A reduced infectivity of AAV-HA was observed after 30 min exposure to the nanoworm-coated surface. Moreover, quantitative reverse transcription polymerase chain reaction (qRT-PCR) results demonstrated significant reduction in the genome copy number of AAV-HA after exposure to the nanoformulation. Further analysis of the nanoworm components displayed higher viral inactivation when the polygalactose, guanidine, and octane residues are attached on the same polymer chain, which might result in a synergistic effect. Hyperbranched variants of nanoworms demonstrated further reductions in genome copy number of AAV-HA. Similar virucidal activity of the hyperbranched nanoworms was obtained against SARS-CoV-2 with a mean 50% tissue culture infectious dose (TCID50)/mL value of 103.2 in case of the VIC01 isolate. Likewise, the B.1.1.7 variant of SARS-CoV-2 was also highly inhibited by the nanoworms with no detectable viral genome copies observed in the eluate. Thereafter, the prepared antiviral nanoworms were coated on surgical masks followed by the introduction of 50 μL of viral inoculum and incubation at room temperature for 30 min. As compared to control masks, nanowormcoated masks revealed the absence of both VIC01 and B.1.1.7 variants of SARS-CoV-2 in the eluate after 30 min, which highlighted the potential use of the obtained nanoformulation in this study for application on personal protective equipment (PPE). Natural antimicrobial coatings derived from tea and cinnamaldehyde oil were reported to be effective in viral as well as bacterial inactivation when mixed with metal salts (Cox et al., 2021). In this study, tea-cinnamaldehydemetal hybrid nanocoating was prepared using two metal salts namely, copper and silver that were then sprayed on PET film. In the case of nanocoatings

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made up of 10 mg of copper sulfate pentahydrate salts, the color of the film changed to opaque orange-brown. X-ray photoelectron spectroscopy (XPS) results then confirmed the presence of Cu in the nanocoating while TEM images showed the presence of large copper crystallites. Likewise, a tea-cinnamaldehyde nanocoating prepared using 10 mg of silver nitrate demonstrated a visible color change to opaque dark grayish-brown wherein TEM micrographs displayed the presence of individual AgNPs. Leaching tests for both the nanocoatings demonstrated similar results with no change in metal concentration. Thereafter, both the metal hybrid nanocoatings exhibited significant antibacterial activity with the complete killing of E. coli and S. aureus cells. More importantly, copper and silver hybrid nanocoatings on nonwoven polypropylene cloths showed 98.6% and 99.8% reduction in murine coronavirus (MHV-A59) titer after 2 h of incubation, respectively.

5. Conclusions and future perspectives The problem of biofilm-associated bacterial infections has highlighted the need to develop effective treatment strategies to both inhibit and eradicate bacterial biofilms from animate as well as inanimate surfaces (Ghosh et al., 2021; Singh, Ghosh, & Chauhan, 2020; Talank et al., 2022). Hence, fabrication of antibacterial coating is very critical for protecting medical equipments, catheters, artificial teeth, and titanium implants (Singh et al., 2021). Biofilms associated with the contact lenses that are primarily used either to correct vision or for cosmetic or therapeutic reasons can cause severe impairment of vision and can be fatal at times. Thus, polymeric biomaterial impregnated with antimicrobial peptides or antibiotics might help to resist biofilm formation on the surface of contact lenses (Desai et al., 2021; Rashiya et al., 2021). The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infection results in disastrous complications during orthopedic surgery and significantly delays the postsurgical recovery (Ghosh & Webster, 2021a). Orthopedic surgical site infections (SSIs) associated with primary hip and knee arthroplasties and revision arthroplasties are often complicated by periprosthetic joint infection (PJI) resulting in functional loses (Ghosh & Webster, 2021b). Hence, coating orthodontic implants with nanoparticles having promising antibacterial activity can prevent morbidity and mortality associated with the deep infections and implant failure (Ghosh et al., 2018; Pormohammad et al., 2021). Various bacteria and fungus are responsible for food spoilage. Hence, nanocoating of food packages can significantly increase the storage- and shelf-life of food products. Another advantage of such food preservation

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strategy is the intrinsic antimicrobial property of the nanoparticles impregnated in the coating material that overrule the use of synthetic additives (Bloch et al., 2021; Ranpariya et al., 2021). Additionally, nanocoatings can also be applied for developing antifungal nail paints. Such bioactive cosmetics can prevent onychomycosis, which is a fungal nail infection caused by dermatophytes (e.g., Trichophyton, Microsporum, and Epidermophyton), yeasts (e.g., Candida), and nondermatophyte molds such as Scopulariopsis, Aspergillus, and Fusarium (Bozoglan et al., 2021). Slow and sustained release of antifungal agents from the nanocoated surfaces may hinder the development of antimicrobial resistance by impairing synthesis of exopolymeric matrix and expression of efflux pumps (Cuellar-Cruz et al., 2012). The outbreaks of Middle East Respiratory Syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV) and, more recently, the SARS-CoV-2 have emphasized the need to develop more promising antiviral coating for masks, clothing, and work surfaces (Ghosh et al., 2020). Bioactive nanoparticles impregnated polymers can ensure passive prevention of the spreading of the deadly virus by a strategy so-called “eradicate-in-place” measure against the virus (Bloch et al., 2022; Otto & De Villiers, 2020). Such antiviral coating can effectively prevent the attachment and penetration of the virus within the host. In view of the background, nanocoating opens a new branch of nanomedicine that offers significant solution for controlling infections caused by bacteria, fungi, and virus. However, thorough toxicity studies and long-term effect to the host as well as the environment should be carefully investigated before real-time application of nanocoatings for biomedical applications.

Acknowledgment SG acknowledges Kasetsart University, Bangkok, Thailand, for Postdoctoral Fellowship and funding under Reinventing University Program (Ref. No. 6501.0207/10870 dated 9th November 2021 and Ref. No. 6501.0207/9219 dated 14th September, 2022). E.M. would like to acknowledge the support from the National Institute of Biomedical Imaging and Bioengineering (5T32EB009035).

References Agarwalla, S. V., et al. (2021). Persistent inhibition of Candida albicans biofilm and hyphae growth on titanium by graphene nanocoating. Dental Materials, 37(2), 370–377. Alheety, M. A., & Hameed, A. A. (2018). Synthesis, characterization and antifungal activity of coated silver nanoparticles-nystatin and coated silver nanoparticles-clotrimazol. Tikrit Journal of Pure Science, 23(7), 63–70.

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CHAPTER 15

Edible and food-safe antiviral and antimicrobial smart coatings Anmiya Petera, Sherin Josepha, Honey Johna,b, and K. Abhithaa,b a Department of Polymer Science and Ruber Technology, CUSAT, Kochi, India Inter University Centre for Nanomaterials and Devices, CUSAT, Kochi, India

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1. Introduction The food industry has undergone many changes over the passage of time, but its focus always been to deliver good and healthy food to customers without any compromise in its quality, safety, and aesthetics. Hence, the food packaging industry always tries to improve itself, and ideally a good packaging system should be capable of maintaining food quality norms, extending shelf life, and preventing contamination. In addition to the recyclability or environment-friendly nature, the materials should be economic and convenient to handle and process. The rapid growth in global population, shifting of people from rural to urban areas, and climatic changes have affected global food production and changed the food habits of people. Consumers became more aware of the food quality and its nutritional value and hence want the natural freshness to be preserved in the packaged foods. Processed food, in particular, requires special protection from spoilage during preparation, distribution, and storage (Dinika et al., 2020). So, the food industry always looks forward to more innovative and highly efficient packaging materials, which can effectively preserve the quality of packed food items. Moreover, several microorganisms can thrive and create health problems in individuals if food is handled in unsanitary surroundings during manufacturing and distribution. Food packaging is the state of the art, and it is the essential part in the production of food. Our modern food industry also employs intelligent and active packaging solutions that can increase the shelf life and assess its quality. Active packaging solutions with antimicrobial and antioxidant agents, as well as superior barrier capabilities, are available to address a variety of physiological, chemical, physical, microbiological, and pest issues that might degrade packaged food quality (Chawla et al., 2021). Food packaging material should satisfy several factors economically and effectively emphasizing the need for a new food packaging system to manage Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00008-6

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the food supply chain. Traditional packaging materials contain nonbiodegradable paper, plastic, and glass materials, which can cause severe environmental issues. A novel packaging material is the one which can preserve the quality of food, ensure easy portability, processability, have good mechanical properties, be economical, and also biodegradable, which does not cause municipal solid waste. Although extensive research is happening in this area, such a system is a distant dream as most of the materials developed have pros and cons (Umaraw & Verma, 2017) and they are still in the developing stage. In edible packaging, new technologies modify food for getting more palatable, natural, and digestible material. The modified materials have excellent biodegradability and biocompatibility. Hence, humans or animals can easily consume the coating materials along with food. Edible packaging was inspired from nature itself, i.e., from the skin/peel of fruits and vegetables. But nowadays, this edible packaging has been developed in various forms like films, sheets, trays, lids, bags, etc.

1.1 Need for edible films and coatings Edible coatings and films for packaging purposes are emerging fields in the food technology sector. The edible packaging materials do not substitute traditional packaging, but they can reduce the expense of packaging. The easy consumption of films along with food items is the main advantage of edible films and provides economic efficiency (Okcu et al., 2018). Naturally occurring packaging materials can be applied only for wrapping food items, but cannot be applied universally. At the same time, edible materials can control oxygen and moisture permeability, gas aroma, movement of solutes out of food, and proponents of additives and nutrients (Pathak et al., 2020). Moreover, these edible materials can entirely cover the food or act as a persistent layer between the food components. Generally, coatings are made on the food surface. Still, edible films are synthesized separately ultimately coated on the surface of food items. Even though these edible materials provide good protection, nonedible secondary packaging is employed to improve the acceptability. The secondary filling can enhance the ease of handling and hygiene. The main criteria for selecting as an edible packaging material is mainly its edible nature and the capability to form a uniform layer (Venkatachalam & Lekjing, 2020). Generally, proteins, waxes, and polysaccharides can quickly develop films. Furthermore, the films are synthesized from an edible material soluble in alcohol, water, or a mixture of alcohol and water. For improving the elasticity and flexibility, plasticizers

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are included occasionally. For enhancing the functionality and film properties, coloring agents, antimicrobial agents, and flavors can add to the solution. Film casting, the drawdown bar method, and mold castings are standard methods employed for making films. An ideal film not only maintains structural stability but also preserves food from contaminants and foodborne pathogens (Orellana et al., 2020).

1.2 Importance of antimicrobial and antiviral coatings Active packaging systems combine multiple additives, like antioxidants, colors, antimicrobial agents, and nutrients. It may enhance food products’ quality and shelf life. The incorporation of antimicrobial or antiviral agents into biopolymer-based edible films can retard or reduce the growth of microbes on the food surface (Kumari et al., 2021). The specific dosage of antibacterial agents must be included for getting better action. Various approaches have been recognized and studied to develop safe, biodegradable, and antimicrobial packaging systems (Kumar, 2019). The two main approaches are: use a naturally occurring polymer with antibacterial properties, or add antimicrobial chemicals to the biopolymer matrix. Bacteriocins, plant extracts, enzymes, essential oils, organic acids, inorganic or metallic nanoparticles, and other antibacterial agents are among them, as given in Fig. 1. The food product, the headspace packaging atmosphere,

Fig. 1 Various antimicrobial and antiviral agents are selected for constructing edible coatings or films.

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and the packaging material are the three essential components of a food packaging system. Antimicrobials are typically embedded in the nonfood components of the packaging system, such as the internal package atmosphere and the packaging material itself, to inhibit the growth of harmful bacteria on food surfaces (Pathak et al., 2020). These incorporated agents may be released into food items to kill the pathogenic microorganisms and thus improve food safety. Several agents are incorporated into the films for providing antimicrobial properties, which can delay the degeneration of food products.

2. Edible coatings and films Besides the edibility and biodegradability, materials having film-forming capability are selected for creating edible films or coatings. The materials are dispersed or dissolved in suitable solvents and the other ingredients such as coloring agents, flavors, plasticizers, or antimicrobial agents are added to it. The coating is then applied to food products at the appropriate temperature and humidity. Coatings can be applied by various methods includes dipping, spraying, panning, or brushing, and then allowed to dry. Polysaccharides, lipids, and proteins are three types of materials that can be used in edible films (Fig. 2).

2.1 Protein-based edible films Proteins are regarded as one of the most significant biopolymers utilized as renewable raw materials. It can be selected for developing environmentally friendly bioplastics. Proteins are three-dimensional molecules made up of various amino acids. Different functional groups of amino acids can

Fig. 2 Classification of edible film components (Umaraw & Verma, 2017).

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significantly influence food packaging. Edible films with an outstanding gas barrier and mechanical qualities are produced by denaturing proteins and casting them. However, the fundamental drawback of protein films is their low water vapor resistance. Combining hydrophobic materials with chemical and enzymatic processes can improve protein film characteristics. Cross-linking is essential for improving the mechanical strength and water resistance of films or coatings. However, maintaining biodegradability when cross-linking proteins is difficult (Zubair & Ullah, 2019). One of the main sources of proteins selected for making coatings and films are milk proteins. Casein and whey protein are the two key milk proteins that have high nutritional value as well as barrier and film-forming properties. The following are some of the most important proteins used in the manufacture of edible coatings and films as represented in Fig. 3. 2.1.1 Casein Casein is a milk protein having extreme resistance to coagulation or denaturation, and its structure is given in Fig. 4. Hence, it can be stable for a wide temperature range and salt concentrations. Caseinates are salts made from casein by coagulating milk protein, and they are a promising raw material for biodegradable and water-soluble packaging. Sodium caseinate films, for example, can be selected as an edible packaging material. Caseinates are

Fig. 3 Proteins used for making edible films or coatings.

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Fig. 4 The chemical structure of casein milk protein.

highly coiled and can form hydrogen bonds between molecules, allowing films to be prepared rapidly in aqueous solutions (Am et al., 2018). The hydrophilic nature of proteins may reduce moisture barrier, solubility, and swelling. In order to expand the use of casein as a packaging film, specific lipids, starch, etc., can be combined with it. A potassium sorbateincorporated sodium caseinate film containing waxes showed excellent thermomechanical properties, antimicrobial action against E. coli, and water vapor barrier properties. The obtained films can be used for the storage of cheese and dry foods (Chevalier et al., 2018). Pea starch/casein edible films can give good mechanical, physical, and water vapor permeability characteristics. The 3:1 casein/starch combination provided solubility of 52.27%, film thickness of 0.204 nm, tensile strength of 20.9 MPa, and 9.7% elongation at break (Rai & Poonia, 2019). 2.1.2 Whey proteins (WP) Casein coagulation produces whey proteins, a yellowish-green powder that is soluble in water. Various methods can be adopted for converting diary whey material into value-added products. WP-based edible films are prepared by the casting method, by pouring the solution of film on flat trays to make dried gel, which can be used for wrapping food items. Varying concentrations of oregano oil (nearly 0.5%–1.5%) incorporated to whey protein containing sorbitol as plasticizer expressed moisture uptake nature. The increase in oil concentration decreased Young’s modulus and maximum tensile strength was obtained. Essential oils have the highest resistance toward the foodborne pathogens, due to their higher concentration of

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eugenol, carvacrol, and thymol. Owing to their antioxidant and antimicrobial activities, they have pronounced effects. These materials can destroy the cytoplasmic membrane, and disrupt the electron flow and active transport. As a result, essential oils such as garlic, rosemary, and oregano were added to whey protein films and evaluated against Listeria monocytogenes (NCTC 2167), Staphylococcus aureus (ATCC 43300), Lactobacillus plantarum (DSM 20174), and Salmonella enteritidis (ATCC 13076), as shown in Fig. 5 (Seydim & Sarikus, 2006). The combination of glycerol, lemon, and bergamot to WP were found to produce good films and these materials exhibit moisture transfer inhibition, long shelf life, and bactericidal activity toward E. coli and S. aureus (C ¸ akmak et al., 2020). Whey protein combined with beeswax, sorbitol, and potassium sorbate affected the stickiness and appearance of the film. Listeria monocytogenes, Escherichia coli, and Salmonella typhimurium (DT104) could be prevented by using WP in combination with p-aminobenzoic acid (PABA) or sorbic acid (SA). These antimicrobial films protect frankfurters by limiting bacteria development and so ensuring their safety. Owing to the improved mechanical properties including percentage elongation and tensile strength these films can also be used for sausages (Cagri-Mehmetoglu et al., 2001). Soy

Fig. 5 The inhibition zone of 2% (wt/vol) oregano essential added whey protein (A) control (B) S. aureus, (C) E. coli, (D) L. monocytogenes, (E) L. plantarum, and (F) S. enteritidis (Seydim & Sarikus, 2006).

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sauce (SS) can be incorporated to WP for making edible films. SS possess various biological activities, including antihypertensive, anticarcinogenic, and antimicrobial activity against Salmonella typhi, Shigella flexneri, Vibrio cholera, and Escherichia coli. Apart from this, antioxidants like isoflavines, phenolic compounds, melanoids, superoxide dismutase, and free amino acids are also present in it. The addition of the SS to a product can extend its shelf life. After the addition of 5% (v/v) SS to the WP films, the elongation was improved. Furthermore, pathogens such as L. monocytongenes, S. Typhimurium, and E. coli can be inhibited by the films (Garcı´a et al., 2020). 2.1.3 Gelatin-based films The coatings or films developed from gelatin exhibited good barrier and mechanical properties, and also have good transparency. Gelatin containing montmorillonite as the filler and nisin acts as antimicrobial agent can exhibit as a good film. Moreover, it shows antimicrobial activity against Staphylococcus aureus (Ge et al., 2017). The antifungal and physicochemical properties of natamycin-augmented casein/zein (Nata-Z/C) composite nanoparticles incorporated into gelatin film might be improved. The antifungal activity of the composite film was demonstrated against Penicillium citrinum, Botrytis cinerea, and Aspergillus niger. The antifungal activity of pristine gelatin film (CK), gelatin films integrating pure natamycin (GNATA—film with the inclusion of 0.14 mg/mL of natamycin suspension without NPs), and various concentrations of Nata-Z/C NPs (GNP3 corresponds to 0.6 mg/mL of total protein and 0.06 mg/mL of natamycin; GNP5 ¼ 1.4 mg/mL of total protein and 0.14 mg/mL of natamycin) against fungus are shown in Fig. 6 (Mo et al., 2021). Mint oil has been widely selected for antimicrobial applications as it contains volatile components of organic acids, quinones, and flavonoids along with analgesic, antispasmodic, and antiinflammatory properties. Mint essential oil can be used to increase the water vapor barrier, thickness, and mechanical qualities of gelatin films. Oil may have antifungal properties against Rhizopus stolonifer and Botrytis cinerea (Scartazzini et al., 2019). Antimicrobial activity of Aloe vera gel in fish gelatin has been discovered against S. aureus bacteria (Gel et al., 2020). Strawberries with gelatin films containing Mentha pulegium essential oil may have a longer shelf life. In strawberries, gelatin coating in combination with Menthae pulegium effectively reduced total flora, molds, and yeasts (Aitboulahsen et al., 2018).

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Fig. 6 Pictorial representation of the antifungal action of the gelatin film as control and the nanocomposite films GNP3, GNP5, and GNATA counter to (A) Aspergillus niger; (B) Botrytis cinerea, and (C) Penicillium citrinum; (D) the graphical representation of the zone of inhibition (Mo et al., 2021).

2.2 Polysaccharide-based films Polysaccharides are divided into two categories: starch and nonstarch polysaccharides. Nonstarch polysaccharides are more hydrophilic than starch-based compounds. The viscosity of solutions can be improved by using aqueous solutions and dispersions of long-chain hydrophilic polysaccharides. With the inclusion of these polysaccharides, the crispness, tackiness, and hardness of the biopolymer matrices can be customized. Cellulose and its derivatives (e.g., carboxymethyl cellulose, hydroxypropyl cellulose, and methylcellulose), chitosan, alginates, agar, pectin, and gums are all nonstarch polysaccharides (Anis & Pal, 2021). Essential oils blended with polysaccharides have exceptional applications in the field of packaging given in Fig. 7. The polysaccharide-based packaging materials must keep low availability, cost, and functional properties. The primary reason for the damage of food products is the dehydration of foods, which reduces the crispiness. Controlled permeability of oxygen gas is required to ripe the fruits and decline the oxidation of food items. 2.2.1 Cellulose and derivatives Cellulose is a basic polymer that is widely available. It can be developed as a crucial source of sustainable packaging material because it is renewable and biocompatible, and it is found in both animals and plants. The repeating D-glucose units include 1,4-glycosidic linkages, and Fig. 8A depicts the

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Fig. 7 The preparation of edible film by the blending polysaccharides with essential oil.

chemical structure of cellulose (Yaradoddi et al., 2020). Hydroxyl-methyl groups are substituted above and below the chain in the polymer backbone. Coating and film materials based on cellulose and cellulose derivatives have good film-forming capabilities. The resulting goods are translucent, odorless, tasteless, and fat and grease resistant. In deep frying food products, the oil absorption can be blocked by covering them by methylcellulose (MC) and hydroxypropyl methyl cellulose (HPMC) whose structures are given in Fig. 8B and C. The incorporation of Thymus daenensis essential oil (EO) into HPMC improves the mechanical and antimicrobial activities of the films. Thymus daenensis Cleak is one of the Thymus genus and the essential oils present in them are thymol and carvacrol. The antimicrobial activities of these compounds are reported. It possesses good resistance to B. subtilis, E. faecium, S. aureus, S. epidermidis, and E. faecalis (Moghimi et al., 2017). In food industries, carboxymethyl cellulose (CMC) can be adopted as an excellent stabilizer. The main feature of CMC is its efficient barrier properties against CO2, oxygen, and lipids, good mechanical strength packaging applications and heat sealing. Apart from this, high mechanical properties of CMC make it suitable for developing durable films with biocompatibility. The incorporation of glycerol (2.0%) in gelatin/CMC/agar can be selected as a perfect material in food packaging (Yaradoddi et al., 2020). The above material can impart excellent biodegradability and lowest water vapor permeability. Citric acid (CA) coated with hydroxyethyl cellulose (HEC) and sodium carboxymethyl cellulose (NaCMC)-containing probiotic bacteria can be used in the film

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Fig. 8 The chemical structures of (A) cellulose and its derivatives like (B) methyl cellulose and (C) hydroxypropyl methylcellulose.

preparation. The SEM images of cellulose films, cellulose films entrapped with probiotic bacteria along with fluorescence microscopy images are given in Fig. 9A and B (Singh et al., 2019). Owing to the good antioxidant property of rosemary (Rosmarinus officinalis, L), it can be used as food antioxidant. It can be blended with carboxymethyl cellulose to possess desirable properties like high viscosity, water-solubility, biocompatibility, hydrophilicity, and biodegradability. Thus, modified CMC can be used as edible coating for smoked fish to reduce oxidation and improve the quality of products. Rosemary-incorporated CMC can inhibit the growth of lactic acid bacteria and Pseudomonas spp. (Choulitoudi et al., 2017). Lactococcus lactis can be embedded in corn starch and CMC can form edible film and can be a replacement to petroleum-based films. The rigidity of the CMC composite is improved to 5.83 MPa and there is good improvement in elongation at

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Fig. 9 (A) Images of cellulose-based film strips after casting and drying in Petri dishes. (1) Hydroxyethyl cellulose (2 wt%); (2) carboxymethyl cellulose (2 wt%); (3) 1 wt % hydroxyethylcellulose +1 wt% carboxymethyl cellulose; (4) 1 wt% hydroxyethyl cellulose +1wt% carboxymethyl cellulose +5wt% citric acid; (5) HEC (1 wt%) + CMC (1 wt%) + CA (10 wt%) (B) SEM images of probiotic bacteria entrapped in carboxymethyl cellulose-based films (A and B) and the fluorescent microscopy image by using the Dead/Alive kit (D). Lactobacillus rhamnosus GG (LGG) is taken as the control (C) (Singh et al., 2019).

break. The composite film have good bactericidal action against Staphylococcus aureus (nearly 53.53%) even after 8 days (Lan et al., 2021). The edible films of CMC, mucilage of Dioscorea opposite (DOM), glycerol, and ZnO NPs can be considered as an eco-friendly replacement to plastics owing to their biocompatibility and biodegradability. The addition of ZnO NPs improves the resistance counter to both Gram-positive and Gram-negative bacteria (Li et al., 2021). Curcumin-incorporated carboxymethyl cellulose are flexible, homogeneous, and transparent and this can add to the safety of food, thereby improving the food quality (Bourbon et al., 2021). 2.2.2 Starch and derivatives Starch has been considered as a fast-growing packaging material for future active packaging purposes owing to the properties like abundance, thermoplastic behavior, and biodegradability. Starch is a naturally existing carbohydrate polymer used for making edible films and coatings. It is a glucose polymer having a branched chain structure mainly consisting of amylase and amylopectin, as shown in Fig. 10A and B. Starch-based films and coatings are extensively studied owing to properties like transparency, tastelessness, odorless nature, and significant barrier property toward O2 and CO2 ( Jiang et al., 2011). However, it possesses a poor water vapor barrier

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Fig. 10 The chemical structure of (A) amylopectin and (B) amylose.

and high water solubility due to the hydrophilicity of starch. The mechanical properties of starch make it available in the food packaging purposes (Xu et al., 2005). The addition of fats, waxes, or oils can reduce the affinity of starch toward water due to the hydrophobic nature of the above materials. Due to the long-chain alkane contents and high fatty alcohol, waxes are excellent water vapor barriers. The composite obtained from Carnauba wax and starch can be used in packaging applications. The presence of Carnauba wax could improve the light barrier and hydrophobic properties, along with reduction in the moisture, water vapor permeability, and thermal stability of the starch film (Gonc¸alves et al., 2020). Beeswax incorporated in starch films also improved the antifungal activity (toward Aspergillus niger) and water vapor resistance. The above-developed films can be used in biodegradable packaging that maintain the quality of food products (P
erez-vergara et al., 2020). The plant extracts are known to have inherent antimicrobial and antioxidant activities and the incorporation of olive oil onto starch expressed resistance to S. aureus and E. coli (Aguilar, 2020). Aloe vera gel has the ability to control the growth of fungi and can be incorporated into starch for obtaining antifungal coatings. The coatings developed were flexible; hence, they could prevent cracks in the films. Moreover, they could control weight loss in cherry tomatoes and hence used in the preservation of vegetables and fruits. The radial growth of various fungi discs incubated on potato dextrose agar (PDA) with and without Aloe vera gel, and mycelium growth inhibition values on the 7th day of incubation at 25°C is given in Fig. 11 (Santamarina, 2017). The addition of pineapple, pomace and mango puree improved the swelling index, moisture content, thickness, and swelling index of the corn starch/gelatin film. The antimicrobial and antioxidant nature of the films were improved and showed 50% biodegradability after 15 days (Susmitha et al., 2021). The introduction of blackberry pulp into starch caused

Fig. 11 Graphical representation of the growth of different fungi discs incubated on PDA with (●) and without (▪) aloe vera gel. Mycelium growth inhibition (MGI) values on the 7th day of incubation at 25°C are shown (Santamarina, 2017).

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interactions between pulp and starch and hence altered the mechanical properties, microstructure, and water vapor barrier nature. The pulp made the starch film surface irregular, rough, thicker, and more flexible. Along with this, it can transfer anthocyanins and antioxidant capacity to the matrix starch film (Nogueira et al., 2019). The zein nanoparticles introduced in potato starch/glycerol/olive oil can be used in packaging materials. The water vapor permeability was reduced, and improved the barrier properties. The appearance, UV-transmittance, and tensile strength improved after the addition of zein nanoparticle (Farajpour et al., 2020). 2.2.3 Chitosan and its derivatives Chitin is the second-most-abundant biodegradable polymer produced in nature after cellulose. Chitin has poor solubility in water; hence, to improve the solubility the chemical modifications are required mainly by deacetylation to form chitosan as shown in Fig. 12. The washing with NaOH removes the acetyl group from the C2 position of glucosamine, which can further protonate the anionic amine (NHd) to the amine (NH2) group. The subsequent protonation of the amino group can facilitate the chitosan dissolution, and thus, improve the antibacterial and antiviral properties. Chitosan has film-forming ability due to its good solubility in hydrochloric acid and acetic acid. Many methods can be adopted to make films of chitosan by coating, casting, and layer-by-layer assembly which enhanced the properties like barrier property, antioxidant property, antimicrobial property, and mechanical properties, etc. After the advancement in research and technology, chitosan has high applications in packaging by improving the shelf life by dissolving in dilute acid solutions and changed to edible films. The extendibility, strength of the film and flexibility can be increased by adding plasticizers like sorbitol and glycerol in chitosan films. Chitosan is a natural polysaccharide that has antibacterial properties against bacteria, fungus, and yeast. The antibacterial action of this substance

Fig. 12 Conversion of chitin to chitosan.

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is determined by its cationic character, concentration, exposure time, and the test organism. The mechanism for antibacterial activity of chitosan is yet unknown. However, numerous methods have been proposed: • Chitosan produces a cellophane-like protective coating on the food surface, effectively shielding it from microbial attack. • The chitosan layer on food prevents gas passage in between food and the environment. As a result, permeability to oxygen is restricted and, consequently, the growth of aerobic microorganisms is prevented. • The cationic group of chitosan engages with the anionic, bacterial cell wall peptidoglycans, causing the bacterial cell wall to dissolve and intracellular fluids to seep out, resulting in cell death. • Chitosan forms an impenetrable coating around the bacterial cell, preventing the exchange of necessary solutes between the interior and exterior, leading to metabolic failure and cell death. • Low MW chitosan can penetrate the cell nucleus and block DNA translation into RNA by attaching to the DNA molecule. Extracts from plants can be used for food preservation, which are nontoxic in nature. The addition of plant extracts significantly improved the properties of chitosan film, like barrier characteristics, antioxidant nature, mechanical and thermal stability, and antimicrobial action. The antibacterial activities of chitosan films can be improved by adding gelatin (GE) and cinnamon essential oil (CEO), which enhances the thermal stability but reduces the crystallinity and wettability. These ternary composite film had good inhibition to Escherichia coli and Staphylococcus aureus. The minimum inhibitory concentration against E. coli and S. aureus were 52.06 μg/mL. The minimal bactericidal concentration was found to be 104.12 and 52.06 μg/mL (Guo et al., 2019). Apple peel polyphenols (APP) are a good source of antioxidants and can be added to chitosan for developing novel films/coatings. The film has better antimicrobial activities against E. coli, B. cereus, S. aureus, and S. typhimurium (Riaz et al., 2018). Cinnamon bark oil and soybean oil incorporated chitosan films display antimicrobial activities against Gram-positive Listeria monocytogenes, as given in Fig. 13 (Ma et al., 2016). Carvacol and grape seed extracts addition offer improvement in food preservation and shelf life (Rubilar et al., 2013). Edible films formed by dissolving green tea extract in chitosan solutions can resist the food-borne viruses and bacteria like Listeria innocua and Escherichia coli (Amankwaah et al., 2020). Many proteins derived from plants, animals, or microorganism could be mixed with chitosan for preparing films with better properties. Chitosan/ caseinate films were connected by ionic interactions, possessed better water

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Fig. 13 Representative photograph showing inhibition of Listeria monocytogenes. Films were prepared from mixtures containing 1% w/w chitosan and microemulsions at an overall cinnamon bark oil concentration of 1%, 2%, or 3% w/w without soybean oil (Ma et al., 2016).

vapor permeability and antimicrobial activities (Khwaldia et al., 2020). For keeping the freshness of eggs during storage time, lysozymes are added with chitosan, which further improves the shell strength and quality (Yuceer & Caner, 2014). Proteins derived from plants have promising applications in food preservation because of its low cost. Kidney bean protein isolate can be added to chitosan for getting flexible films with low surface energy and high hydrophobicity. Hence, it can be selectively used in antimicrobial packaging (Ma et al., 2013). Proteins isolated from microorganisms also play a pivotal role in food preservation. Amphiphilic cationic peptide, nisin obtained from Lactococcus lactis and blended with chitosan, can extend the shelf life of packaged food products (Imran et al., 2014). Chitosan have good chelating ability with inorganic materials. Silver nanoparticles have broad resistance to antibiotic resistant bacteria. It could be added to the chitosan matrix in order to improve the safety and quality of food (Lo et al., 2013). Zinc oxide is another inorganic filler used for enhancing physicochemical and biological properties of chitosan films. ZnO addition significantly increases the mechanical properties, transparency, and antimicrobial activity (Kamel, 2015). 2.2.4 Alginates Alginates are polysaccharides with gel-forming, film-forming, and thickening properties that are commonly used in edible films and coatings. Brown algae is commonly used to make these. It can be used to improve the quality of fruits and vegetables, as well as to extend the shelf life of meat, seafood, poultry, and cheese by preventing dehydration and improving mechanical

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Fig. 14 Chemical structure of alginate.

qualities. As indicated in Fig. 14, alginates are made up of various proportions of L-guluronic acid (G) and D-mannuronic acid (M). Antimicrobials and antioxidants can be added to alginates to improve food quality. Antimicrobial agents transferred from coatings can help prevent microbiological infection (Quintavalla & Vicini, 2002). Antimicrobial agents can be added to alginate-based films and coatings in a variety of ways. Natural antimicrobials can be found in essential oils. The addition of castor oil to the alginates results in edible films with improved thermal stability and mechanical characteristics, and reduced water vapor permeability. The produced film has a strong antibacterial effect against B. subtilis and S. aureus (Aziz et al., 2018). Essential oils originating from medicinal herbs such as Rosmarinus officinalis L., Artemisia herba-alba, Ocimum basilicum, and Mentha pulegium can be added to sodium alginates. Moisture, water vapor permeability, and oxygen permeability are all reduced as a result. It has substantial DPPH (2,2-diphenyl-1-picryl-hydrazyl-hydrate) radical scavenging potential and has good antibacterial activity against foodborne pathogenic microorganisms (Mahcene et al., 2020). Green tea extract and grape seed extract added to alignates expressed excellent antiviral activity against murine norovirus (MNV) and hepatitis A virus (HAV) (Fabra et al., 2018). Listeria monocytogenes is a prominent food contaminant mainly found in ready-to-eat food products. The shelf life of sliced ham can be improved by covering it with Na-alginate containing oregano EO (Pavli et al., 2019). Clove essential oil added to sodium alginate/carrageenan gives good inhibition to E. coli (Prasetyaningrum et al., 2021). 2.2.5 Pectin Pectin is a nontoxic, anionic polysaccharide that is extracted from plant cell walls. As illustrated in Fig. 15, pectin is the dimethyl ester of polygalacturonic acid. Pectins are classified as high or low methoxyl pectins based on their degree of methoxylation. Pectin is used in the pharmaceutical sector because

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Fig. 15 Chemical structure of pectin.

of its gelling qualities. It has primarily been used to transport drugs such as matrix tablets to the gastrointestinal tract. To create active antimicrobial packaging, edible films produced from pectin were combined with several antimicrobial ingredients (Okcu et al., 2018). This active packaging can help increase the shelf life of food while also reduce the risk of disease growth on food surfaces. The limited release of one or more antimicrobial agents from the packaging material to the product alters the food’s conditions and so preserves it. The ability of edible antimicrobial films to inhibit postprocessing foodborne bacteria on food surfaces by diffusing antimicrobial chemicals into the food product is the significant benefit (Garcı´a et al., 2020). Bacteriocins are a promising alternative to traditional chemical preservatives in the control of rotting bacteria. Nisin is the most extensively researched bacteriocin and is now the only permitted bacteriocin food additive. On a ready-to-eat (RTE) turkey meat, an edible pectin film infused with nisin and its derivatives was employed to suppress Listeria monocytogenes (Guo et al., 2014). L. monocytogenes was examined using a composite film made from pectin, PLA, and nisin. In brain heart infusion (BHI) broth, liquid egg white, and orange juice, the composite can suppress bacterial growth by 2.1, 4.5, and 3.7 log CFU/mL, respectively ( Jin et al., 2009). Similarly, film fabricated using chitosan/poly(vinyl alcohol)/pectin possesses excellent antimicrobial activity against Staphylococcus aureus, Pseudomonas, Candida albicans, Bacillus subtilis, and Escherichia coli. A clear zone of inhibition was obtained for all the organisms (Tripathi et al., 2010). Pectin-based edible films blended with lemongrass, oregano, or cinnamon essential oil have antimicrobial activity against E. coli. Oregano essential oil incorporated edible film possessed the highest antibacterial activity. Carvacol-blended pectin edible film has resistance to E. coli. and shows stability between 5°C and 25°C even for 98 days (Guo et al., 2014).

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2.3 Lipids Lipids have very good moisture barrier ability. An extensive range of combinations increases the hydrophobicity of lipid-based coatings and edible films. Hydrophobic materials selected include fatty acids and waxes (Hagenmaier & Shaw, 1990). Resins are also adopted for imparting brilliance to the entity. Lipids added with polysaccharides and proteins can generate coatings with high barrier property. The effective blocking of moisture can be done by lipids due to its low polarity. However, the hydrophobicity of lipids make the films more brittle and thicker (Hassan et al., 2018). Beeswax and paraffin are the simplest form of lipids. Owing to the low polarity of lipids, it can resist the passage of moisture through the film. 2.3.1 Wax Wax has been considered widely for the shielding of new things. Chemically, wax is an ester formed by combining an aliphatic acid with aliphatic alcohol. Waxes are highly insoluble in water and do not wholly spread on the surface. The hydrophobicity of these materials is very high. It is soluble in organic solvents such as chloroform, benzene, and hexane. The hydrophilic part of wax is tiny; hence, it is buried in the molecule. Therefore, it cannot interact efficiently with water, preventing the molecule from spreading. The edible films fabricated using cassava starch, carnauba wax, stearic acid, and glycerol-based edible coatings/films have good water vapor resistance and respiration rate (Chiumarelli & Hubinger, 2014). Whey protein isolate with Candelilla wax emulsion can also be used for making edible films (Kim & Ustunol, 2001). Beeswax incorporated starch shows resistance to R. stolonifer, C. gloeosporioides, B. cinerea, and of S. Saintapul (Ochoa et al., 2017). 2.3.2 Free fatty acids Fatty acids can be adopted as a natural product. They can be generated from triglycerides or phospholipids, which have a broad spectrum of biological activities. They can hinder or destroy pathogenic bacteria. Fatty acids can boost cell lysis, retard enzyme activity, and toxic peroxidation. Due to the limitations like unpleasant taste, they are conjugated with other biodegradable materials. Fatty acid-modified pectin-based materials are investigated for antimicrobial activity against S. aureus and E. coli. Better results are obtained for pectin-linoleate and pectin-oleate against S. aureus and are represented in Fig. 16 (Calce et al., 2014). Fatty acids like palmitic, lauric, and caprylic acids can be incorporated into basil seed gums, which could

Fig. 16 The percentage of inhibition of pectin functionalized with fatty acids against E. coli and S. aureus and SEM images of pectin and functionalized pectin (Calce et al., 2014).

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reduce the opacity of films. The water vapor permeability and water solubility of the edible films were reduced (Hashemi et al., 2020). Fatty acids like stearic acid, linoleic acid and oleic acid can be combined with potato starch for getting the inherent antibacterial activities of fatty acids in starch films. Compared with the starch films, the composite films possessed higher tensile strength and lower water vapor permeability (Liu et al., 2016). Stearic acid can be combined with hydroxypropyl methylcellulose (HPMC) for making edible films. The addition of 15% stearic acid to HPMC increased the film’s moisture barrier capacity, and the final product demonstrated antibacterial activity against Staphylococcus aureus and Listeria monocytogenes (Oma, 2002). 2.3.3 Resins Shellac resins are obtained from the secretion of the Laccifer lacca insect having esters with carboxylic and hydroxyl groups. These resins are insoluble in water even though they have excellent film-forming abilities. They are not considered as food-safe materials, but can be used as a secondary chemical in edible coatings. A few studies were reported using these resins as edible coating materials. They are mainly used as edible coatings in fruits as they can impart an extra shine to the fruit surfaces. Interaction of shellac resins with gelatin provides good barrier properties toward moisture and gas. The mechanism of shellac-gelatin composite film formation is given in Fig. 17 (Soradech et al., 2012). Citrus fruits coated with resins can provide

Fig. 17 Mechanism diagram of shellac and gelatin composite film formation (Soradech et al., 2012).

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better CO2 in the inner content of the film and less O2 content in the film. The shellac films can be cross-linked with other molecules; this can improve the mechanical as well as functional properties (Hagenmaier & Baker, 1993).

3. Conclusions A vital component of sustainable packaging is edible packaging. Reducing the use of nonrenewable petroleum products and using biodegradable materials will significantly reduce processing waste. Natural, feasible, readily available materials are the current scope of food packaging. Edible polymers are nontoxic, sustainable, biocompatible, and biodegradable. Every natural polymer has its own set of advantageous qualities, as well as certain drawbacks. No single natural polymer can provide all of the desirable edible film features, such as barrier capabilities, coating ability, gelling properties, mechanical-thermal properties, and antibacterial activity. Antioxidants, nutraceuticals, colorants, and flavors are examples of food additives that can be used to provide foods with desired attributes. Polysaccharides, proteins, and lipids are the most commonly utilized edible films/coatings. Researchers are currently using numerous changes to maximize material features such as film-forming, mechanical, barrier properties, and antioxidant and antibacterial activities, taking into account the advantages and limits of these materials. The antibacterial and mechanical qualities of matrix materials can be improved further by adding essential oils, nanomaterials, plant extracts, and other additives. However, more research is needed to optimize antimicrobial concentrations in order to develop materials with suitable antibacterial activities as well as required physical and mechanical properties.

4. Future perspectives Edible food packaging systems are responding to increased demand, and the market for sustainable foods is expanding in lockstep. With this in mind, it is critical to develop innovative, active food packaging that is long-lasting and provides solutions to complex food business difficulties. One of the most serious is antimicrobial resistance in food. In edible packaging, nanocomposites play an important role. Scientists can engineer the nanostructure of packaging materials using nanotechnology. By transporting bioactive substances and performing their intended activities better, desirable barriers and mechanical qualities are provided. However, edible packaging still has a long way to go before it can be used in commercial applications. The use of nanoparticles in packaging materials will have a significant impact on the future.

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€ R. C. (2002). Edible bioactive fatty acid—Cellulosic derivative composites Oma, A. used in food-packaging applications. Journal of Agricultural and Food Chemistry, 50, 4290–4294. Orellana, N., Elizabeth, S., Benavente, D., Prieto, P., Enrione, J., & Acevedo, C. A. (2020). A new edible film to produce in vitro meat. Foods, 9, 1–14. Pathak, S. K. B. V., Arya, T. A. A., & Awasthi, G. B. M. G. (2020). Materiality of edible film packaging in muscle foods: A worthwhile conception. Journal of Packaging Technology and Research, 4, 117–132. https://doi.org/10.1007/s41783-020-00087-9. Pavli, F., Argyri, A. A., Skandamis, P., & Nychas, G. (2019). Antimicrobial activity of oregano essential oil incorporated in sodium alginate edible films: Control of listeria monocytogenes and spoilage in ham slices treated with high pressure processing. Materials, 12, 1–26. P
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erez-cervera, C. E., & Andradepizarro, R. D. (2020). Development and characterization of edible films based on native cassava starch, beeswax, and propolis. NFS Journal, 21, 39–49. https://doi.org/10.1016/ j.nfs.2020.09.002. Prasetyaningrum, A., Utomo, D. P., Raemas, A. F. A., Kusworo, T. D., & Jos, B. (2021). Alginate/κ-carrageenan-based edible films incorporated with clove essential oil: Physico-chemical characterization and antioxidant-antimicrobial activity. Polymers, 13, 1–16. Quintavalla, S., & Vicini, L. (2002). Antimicrobial food packaging in meat industry. Meat Science, 62, 373–380. Rai, S., & Poonia, A. (2019). Formulation and characterization of edible films from pea starch and casein. Journal of Pharmacognosy and Phytochemistry, 8, 317–321. Riaz, A., Lei, S., Muhammad, H., Akhtar, S., Chen, D., Jabbar, S., Abid, M., Hashim, M., & Zeng, X. (2018). Preparation and characterization of chitosan-based antimicrobial active food packaging film incorporated with apple peel polyphenols. International Journal of Biological Macromolecules, 547–555. https://doi.org/10.1016/j.ijbiomac. 2018.03.126. Rubilar, J. F., Cruz, R. M. S., Silva, H. D., Vicente, A. A., Khmelinskii, I., & Vieira, M. C. (2013). Physico-mechanical properties of chitosan films with carvacrol and grape seed extract. Journal of Food Engineering, 115, 466–474. https://doi.org/10.1016/j. jfoodeng.2012.07.009. Santamarina, P. (2017). Antifungal starch-based edible films containing Aloe vera. Food Hydrocolloids, 72, 1–10. https://doi.org/10.1016/j.foodhyd.2017.05.023. Scartazzini, L., Tosati, J. V., Cortez, D. H. C., Rossi, M. J., & Flo, S. H. (2019). Gelatin edible coatings with mint essential oil (Mentha arvensis): Film characterization and antifungal properties. Journal of Food Science and Technology, 56, 4045–4056. https://doi.org/ 10.1007/s13197-019-03873-9. Seydim, A. C., & Sarikus, G. (2006). Antimicrobial activity of whey protein based edible films incorporated with oregano, rosemary and garlic essential oils. Food Research International, 39, 639–644. https://doi.org/10.1016/j.foodres.2006.01.013. Singh, P., Magalha˜es, S., Alves, L., Antunes, F., Miguel, M., Lindman, B., & Medronho, B. (2019). Food hydrocolloids cellulose-based edible films for probiotic entrapment. Food Hydrocolloids, 88, 68–74. https://doi.org/10.1016/j.foodhyd.2018.08.057. Soradech, S., Nunthanid, J., Limmatvapirat, S., & Luangtana-anan, M. (2012). An approach for the enhancement of the mechanical properties and film coating efficiency of shellac by the formation of composite films based on shellac and gelatin. Journal of Food Engineering, 108, 94–102. https://doi.org/10.1016/j.jfoodeng.2011.07.019. Susmitha, A., Sasikumar, K., Rajan, D., Arun Padmakumar, M., & Madhavan, K. (2021). Development and characterization of corn starch-gelatin based edible films incorporated with mango and pineapple for active packaging. Food Bioscience, 41, 100977. https://doi. org/10.1016/j.fbio.2021.100977.

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Tripathi, S., Mehrotra, G. K., & Dutta, P. K. (2010). Preparation and physicochemical evaluation of chitosan/poly (vinyl alcohol)/pectin ternary film for food-packaging applications. Carbohydrate Polymers, 79, 711–716. https://doi.org/10.1016/j.carbpol. 2009.09.029. Umaraw, P., & Verma, A. K. (2017). Comprehensive review on application of edible film on meat and meat products: An eco-friendly approach. Critical Reviews in Food Science and Nutrition, 57, 1270–1279. https://doi.org/10.1080/10408398.2014.986563. Venkatachalam, K., & Lekjing, S. (2020). A chitosan-based edible film with clove essential oil and nisin for improving the quality and shelf life of pork patties in cold storage. RSC Advances, 10, 17777–17786. https://doi.org/10.1039/d0ra02986f. Xu, Y. X., Kim, K. M., Hanna, M. A., & Nag, D. (2005). Chitosan-starch composite film: Preparation and characterization. Industrial Crops and Products, 21, 185–192. https://doi. org/10.1016/j.indcrop.2004.03.002. Yaradoddi, J. S., Banapurmath, N. R., & Ganachari, S. V. (2020). Biodegradable carboxymethyl cellulose based material for sustainable packaging application. Scientific Reports, 10, 1–13. https://doi.org/10.1038/s41598-020-78912-z. Yuceer, M., & Caner, C. (2014). Antimicrobial lysozyme-chitosan coatings affect functional properties and shelf life of chicken eggs during storage. Journal of the Science of Food and Agriculture, 153–162. https://doi.org/10.1002/jsfa.6322. Zubair, M., & Ullah, A. (2019). Recent advances in protein derived bionanocomposites for food packaging applications. Critical Reviews in Food Science and Nutrition, 1–29. https://doi.org/10.1080/10408398.2018.1534800.

CHAPTER 16

Plant extract-based antibacterial coating: An introduction Vibha Devia,b and Aditya Kumarb a

Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, India Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand, India b

1. Introduction Plant-based coating plays the important role not only in food and pharmaceutical applications but also in protecting metallic surfaces from any spoilage and degradation due to the atmospheric oxygen, moisture, dust, bacteria, etc. (Karthik et al., 2021; Ong et al., 2021). Modern and busy lifestyle of the human kind leads to increase in their dependency on the ready to cook and packaged food. To store the food items for prolonged periods, various types of preservatives and coatings are applied. Exotic fruits and vegetables are also imported with food grade coating to increase their shelf life. Medicinal products are also covered with thin layer of coating to preserve their quality. For metal protection, the most commonly used coating materials are acrylic and epoxy polymer resins. However, these coatings lack the sustainability against chemical solvent and sunlight. Also, the small pores of epoxy coating are ineffective in protecting metal surfaces from corrosion and other degradations. Therefore, to increase the effectiveness of coating, some additives are included in the coating. Initially synthetic additives were used due to their effective performance and improved metal protection. However, the hazardous nature of synthetic additives toward human and environment, discouraged their further application in the coating. These synthetic additives are replaced by plant-based additives, which are not only equally efficient but also cheap, natural, and nontoxic (Ong et al., 2021). These plant-based additives offer various properties like antioxidant, antibacterial, antifouling, self-healing, and UV shielding. Chitosan is a natural biopolymer which is used as an antimicrobial coating augmented with plant extracts to increase the shelf life of the packed food items (Karthik et al., 2021).

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In this chapter, the antibacterial/antimicrobial activity of the plant extract coating is discussed. It is applied on the material surface to protect it from bacterial, virus, and fungal attack. These coatings are majorly used in food and pharmaceutical industries to protect the food and medical items from pathogens. These coatings can be directly applied on the bandages to protect the wound from environmental microorganisms. Generally, metal nanoparticles such as silver, copper, titanium, zinc, etc., are synthetic antibacterial agents, which are used in coating. However, these nanoparticles can easily transmit into the body through food, medicines, bandages, etc., and can cause health problems. Therefore, nowadays, plant-based antibacterial additives in coatings are gaining much attention from researchers. These additives can be extracted from different parts of the plant like seeds, buds, leaves, twigs, roots, bark, fruits, herbs, etc. (Ong et al., 2021).

2. Antibacterial/antimicrobial properties of plant extract The extracts of aromatic plants are rich in polyphenols, which are responsible for its antibacterial and antioxidant effects (Suchkova & Hussaineh, 2021). Phenol contains hydroxyl group –OH in the aromatic ring, which helps to disrupt the cell membrane of pathogens (Ong et al., 2021). More than 8000 types of polyphenols exist in the nature, which are classified into four categories such as flavonoids, phenolic acids, polyphenolic amides, and some other polyphenols (Tsao, 2010). Flavonoids hold the highest contribution (approximately 60%) among all polyphenols, which includes anthocyanins, flavan-3-ols, flavones, flavanones, flavonols, etc., which can be extracted from apples, dark chocolate, tea leaves, etc. Phenolic acids contribute second largest share (approximately 30%) among all polyphenols, which are the derivatives of benzoic and cinnamic acids and these are mostly found in fruits, vegetables, grains, and seeds. Polyphenolic amides such as capsaicinoids in chili peppers and avenanthramides in oats are the major components of food. Other polyphenols includes resveratrol in grapes and red wine; ellagic acid in berries and some tree skin; curcumin in turmeric; and lignans in flax, sesame, and whole grains (Tsao, 2010). Panawes and group (Panawes et al., 2017) investigated the antimicrobial effect of mangosteen extract on the wound dressing and observed promising effects against Gram positive (Staphylococcus aureus, Staphylococcus epidermidis) and Gram negative (Enterococcus faecalis, Escherichia coli, Acinetobacter baumannii, Vancomycin-resistant enterococci) bacteria. The extracts of sea buckthorn leaves showed antimicrobial effect against Pseudomonas (Pseudomonas

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aeruginosa) microorganism, which make it as a suitable additive in the coating (Ong et al., 2021). Tannic acid is one of the polyphenols which is used in the coating for antioxidant and antimicrobial properties. Dong and team (Dong et al., 2017) investigated the antimicrobial activity of tannic acid against S. aureus and observed its potential to prohibit biofilm formation by destroying the integrity of cell wall. Curcumin, which is found in the extract of Indian spice turmeric, also exhibits antimicrobial activity against most of the pathogenic microorganisms (Ong et al., 2021). Selvam and group (Mari selvam R, Kalirajan K, Ranjit Singh AJA., 2012) investigated curcumin effect against 10 different bacterial strains and observed promising antibacterial activity against E. coli and Vibrio cholera. Enhancement in antimicrobial effect in the presence of light was also reported in the literature (Shlar et al., 2017). Antimicrobial effects of curcumin such as the effect of curcumin-methyl-β-cyclodextrin complex against E. coli bacteria, and curcumin nanoparticle against S. aureus, Bacillus subtilis, E. coli, P. aeruginosa, Penicillium notatum, and Aspergillus niger (Bhawana et al., 2011; Shlar et al., 2017) were investigated in some more studies. Extracts of blueberry leaves and fruits were also observed to show antimicrobial effect when tested against E. coli, Salmonella typhimurium, Listeria monocytogenes, and S. aureus bacteria (Yang et al., 2014). Santamaria-Echart and group (SantamariaEchart et al., 2018) investigated antibacterial activities of Salvia officinalis L. (sage) and Melissa officinalis L. (lemon balm) extracts synthesized with waterborne polyurethane-urea coating against S. aureus, E. coli, and P. aeruginosa bacteria. Nguyen et al. used piper betel extract on cotton gauge fabric and investigated its effective antimicrobial property against S. aureus and E. coli bacteria, which would be useful to prevent wound dressing (Nguyen et al., 2021). Cistus incanus L. (rockrose) extract exhibited antimicrobial activity against Aspergillus parasiticus (Kalli et al., 2018). Apart from the abovementioned plant extracts, there are large varieties of other plant extracts such as pomegranate peel, wine grape pomace, different parts of wild artichoke (Cynara scolymus), globe artichoke (C. cardunculus), etc., which were explored for antimicrobial activity and found suitable for use in coatings (Deng & Zhao, 2011; Karthik et al., 2021; Kollia et al., 2017; Ong et al., 2021; Rahnemoon et al., 2021; Valencia-Chamorro et al., 2011). To estimate the antimicrobial activity of a coating, there are various methods such as film disk agar diffusion test, film surface inoculation test, antimicrobial gradient method, cross streak method, poisoned food method, dilution methods, and detection though thin-layer chromatographybioautography (Balouiri et al., 2016; Campos et al., 2011).

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3. Plant-based edible coating having antimicrobial/ antibacterial effect Edible coatings are specifically used on food, medicines, capsules, etc., which are directly consumed. These coatings are synthesized from hydrocolloids (proteins and polysaccharides), lipids (waxes, acylglycerols, and fatty acids), and composites (both hydrocolloid and lipid) (Valencia-Chamorro et al., 2011). These coatings are acceptable if they help to preserve the flavor, aroma, and moisture of the food items for a longer time and does not degrade the food quality (Ncama et al., 2018). These coatings are incorporated with natural antibacterial additives to protect the food items from pathogens, which include chitosan, polypeptides, and plant extracts (Karthik et al., 2021; Ncama et al., 2018; Valencia-Chamorro et al., 2011). Chitosan and polypeptides are produced by living organism. Extracts of angelica, anise, carrot, cardamom, cinnamon, cassia, clove, garlic, pimento, thyme, rosemary, lemongrass, Scutellaria, Forsythia suspensa, coriander, dill weed, fennel, nutmeg, oregano, parsley, sage, etc., are observed to show inhibitory activity against various pathogens, bacteria, molds, and yeasts (Campos et al., 2011; Valencia-Chamorro et al., 2011). These coatings are applied on the food and medicinal items by different methods, i.e., dipping, spraying, brushing, and panning followed by drying (Ncama et al., 2018).

4. Methods of extraction The extraction method of these natural products is very critical as it can affect the quality and quantity of the products. The extracts prepared using some separation processes such as supercritical extraction, steam distillation, microwave extraction, solvent extraction, etc., have already shown the antimicrobial effect against various pathogens (Kalli et al., 2018; Kapadiya et al., 2018; Ncama et al., 2018). Different extraction processes were also compared by some authors to identify better antimicrobial activity (Kapadiya et al., 2018; Kollia et al., 2017). Kapadiya and group compared antibacterial activity of clove bud extracts obtained through microwave-assisted extraction and hydrodistillation, and observed better antibacterial activity with microwave extraction against S. aureus, B. subtilis, E. coli, and P. aeruginosa as the better quality oil was obtained with microwave extraction (Kapadiya et al., 2018). Kollia and team compared classical extraction with ultrasound-assisted extraction of wild artichoke and globe artichoke extracts to investigate antifungal activity against A. parasiticus. They observed better performance of both extracts obtained using ultrasound extraction (Kollia et al., 2017).

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5. Incorporation route Some authors have reported that microbial activity is also affected by modes of delivery system or incorporation route. Santamaria-Echart and group (Santamaria-Echart et al., 2018) explored three different incorporation routes, i.e., post-, in situ, and premethods to investigate antibacterial properties of sage and lemon balm extracts in waterborne polyurethane-urea coating against S. aureus, E. coli, and P. aeruginosa bacteria. In postmethod, the extract was dissolved in distilled water and added drop wise to the synthesized coating under mechanical stirring. In in situ method, the dissolved extract was incorporated progressively during the formation of waterborne polyurethane-urea nanoparticles. In premethod, the dissolved extract was added prior to the formation of waterborne polyurethane-urea nanoparticles. They observed different antimicrobial effects for all incorporation routes for similar strains of bacteria. Bhawana and group compared the curcumin nanoparticle (2–40 nm) with relatively large particle size curcumin and observed better aqueous dispersion with nanocurcumin, which showed enhanced antimicrobial activity against S. aureus, B. subtilis, E. coli, P. aeruginosa, P. notatum, and A. niger (Bhawana et al., 2011). Shlar and group observed better antimicrobial activity against E. coli bacteria with curcuminmethyl-β-cyclodextrin complex than with polyelectrolyte-coated monolithic curcumin nanoparticle (Shlar et al., 2017).

6. Conclusions In conclusion, plant-based additives are a natural alternative to improve the performance of coating. However, natural products also have some limitations such as fast degradation, sensitivity toward light, moisture, air, etc. Moreover, the selection of extraction method, solvent, incorporation route, etc., is also crucial as it can affect the antibacterial activity. These gaps and limitations should be addressed in future research for optimized coating performance.

References Balouiri, M., Sadiki, M., & Ibnsouda, S. K. (2016). Methods for in vitro evaluating antimicrobial activity: A review. Journal of Pharmaceutical Analysis, 6, 71–79. https://doi.org/ 10.1016/j.jpha.2015.11.005. Bhawana, Basniwal, R. K., Buttar, H. S., Jain, V. K., & Jain, N. (2011). Curcumin nanoparticles: Preparation, characterization, and antimicrobial study. Journal of Agricultural and Food Chemistry. https://doi.org/10.1021/jf104402t.

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Campos, C. A., Gerschenson, L. N., & Flores, S. K. (2011). Development of edible films and coatings with antimicrobial activity. Food and Bioprocess Technology, 4, 849–875. https:// doi.org/10.1007/s11947-010-0434-1. Deng, Q., & Zhao, Y. (2011). Physicochemical, nutritional, and antimicrobial properties of wine grape (cv. Merlot) pomace extract-based films. Journal of Food Science, 76, E309– E317. https://doi.org/10.1111/j.1750-3841.2011.02090.x. Dong, G., Liu, H., Yu, X., Zhang, X., Lu, H., Zhou, T., et al. (2017). Natural product research formerly natural product letters. In Antimicrobial and anti-biofilm activity of tannic acid against Staphylococcus aureus Taylor & Francis. Kalli, V., Kollia, E., Roidaki, A., Proestos, C., & Markaki, P. (2018). Cistus incanus L. extract inhibits Aflatoxin B1 production by Aspergillus parasiticus in macadamia nuts. Industrial Crops and Products, 111, 63–68. https://doi.org/10.1016/j.indcrop.2017.10.003. Kapadiya, S. M., Parikh, J., & Desai, M. A. (2018). A greener approach towards isolating clove oil from buds of Syzygium aromaticum using microwave radiation. Industrial Crops and Products. https://doi.org/10.1016/j.indcrop.2017.12.060. Karthik, C., Caroline, D. G., & Pandi, P. S. (2021). Nanochitosan augmented with essential oils and extracts as an edible antimicrobial coating for the shelf life extension of fresh produce: A review. Polymer Bulletin. https://doi.org/10.1007/s00289-021-03901-9. Kollia, E., Markaki, P., Zoumpoulakis, P., & Proestos, C. (2017). Comparison of different extraction methods for the determination of the antioxidant and antifungal activity of Cynara scolymus and C. cardunculus extracts and infusions. Natural Product Communications, 12, 423–426. https://doi.org/10.1177/1934578x1701200329. Mari selvam R, Kalirajan K, Ranjit Singh AJA. (2012). Anti-microbial activity of turmeric natural dye against different bacterial strains. Journal of Applied Pharmaceutical Science. https://doi.org/10.7324/JAPS.2012.2624. Ncama, K., Magwaza, L. S., Mditshwa, A., & Tesfay, S. Z. (2018). Plant-based edible coatings for managing postharvest quality of fresh horticultural produce: A review. Food Packaging and Shelf Life. https://doi.org/10.1016/j.fpsl.2018.03.011. Nguyen, T. C. V., Rajeswari, V. D., Al-Kheraif, A. A., & Brindhadevi, K. (2021). Study of antimicrobial properties of Piper betel coated nanozirconium on cotton gauze. Applied Nanoscience. https://doi.org/10.1007/s13204-021-01987-1. Ong, G., Kasi, R., & Subramaniam, R. (2021). A review on plant extracts as natural additives in coating applications. Progress in Organic Coatings, 151, 106091. https://doi.org/ 10.1016/j.porgcoat.2020.106091. Panawes, S., Ekabutr, P., Niamlang, P., Pavasant, P., Chuysinuan, P., & Supaphol, P. (2017). Antimicrobial mangosteen extract infused alginate-coated gauze wound dressing. Journal of Drug Delivery Science and Technology. https://doi.org/10.1016/j.jddst.2017.06.021. Rahnemoon, P., Sarabi-Jamab, M., Bostan, A., & Mansouri, E. (2021). Nano-encapsulation of pomegranate (Punica granatum L.) peel extract and evaluation of its antimicrobial properties on coated chicken meat. Food Bioscience, 43, 101331. https://doi.org/ 10.1016/j.fbio.2021.101331. Santamaria-Echart, A., Fernandes, I., Barreiro, F., Retegi, A., Arbelaiz, A., Corcuera, M. A., et al. (2018). Development of waterborne polyurethane-ureas added with plant extracts: Study of different incorporation routes and their influence on particle size, thermal, mechanical and antibacterial properties. Progress in Organic Coatings. https://doi.org/ 10.1016/j.porgcoat.2018.01.006. Shlar, I., Droby, S., Choudhary, R., & Rodov, V. (2017). The mode of antimicrobial action of curcumin depends on the delivery system: Monolithic nanoparticles: Vs. supramolecular inclusion complex. RSC Advances, 7, 42559–42569. https://doi.org/10.1039/ c7ra07303h.

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Suchkova, E. P., & Hussaineh, R. (2021). Study of the antioxidant properties of some aromatic plant extracts. IOP Conference Series: Earth and Environmental Science, 866. https:// doi.org/10.1088/1755-1315/866/1/012017. Tsao, R. (2010). Chemistry and biochemistry of dietary polyphenols. Nutrients, 2, 1231–1246. https://doi.org/10.3390/nu2121231. Valencia-Chamorro, S. A., Palou, L., Delr´io, M. A., & Perez-Gago, M. B. (2011). Antimicrobial edible films and coatings for fresh and minimally processed fruits and vegetables: A review. Critical Reviews in Food Science and Nutrition, 51, 872–900. https://doi.org/ 10.1080/10408398.2010.485705. Yang, G., Yue, J., Gong, X., Qian, B., Wang, H., Deng, Y., et al. (2014). Blueberry leaf extracts incorporated chitosan coatings for preserving postharvest quality of fresh blueberries. Postharvest Biology and Technology. https://doi.org/10.1016/j.postharvbio. 2014.01.018.

CHAPTER 17

Bioengineered metal-based antimicrobial nanomaterials for surface coatings Hamed Barabadia, Kamyar Jounakia, Elaheh Pishgahzadeha, Hamed Moradb,c, Negar Bozorgchamia, and Hossein Vahidia a

Department of Pharmaceutical Biotechnology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran b Department of Pharmaceutics, Faculty of Pharmacy, Mazandaran University of Medical Sciences, Sari, Iran c Ramsar Campus, Mazandaran University of Medical Sciences, Ramsar, Iran

1. Introduction The lucrative and incredible revolution of nanobiotechnology has been providing various opportunities for scientific approaches (Bhardwaj & Kaushik, 2017). Generally, nanomaterials are materials with at least one dimension in the scale of a nanometer. Nanostructures include inorganic nanoparticles (NPs), polymeric NPs, solid lipid NPs, liposomes, dendrimers, quantum dots, nanocrystals, nanowires, nanotubes, nanoarrays, nanorods, etc. (Morad et al., 2021; Shenderova et al., 2006). NPs have been noticed more than other nanomaterials based on the literature (Buzea et al., 2007). The high surface-area-to-volume ratio of NPs offers various physicochemical properties such as enhanced reactivity (Auffan et al., 2009), which provides applied potentials. Metallic nanoparticles (MNPs) as a branch of inorganic NPs have been merged with lots of beneficial opportunities for different industries. Silver, gold, and platinum NPs are three main metals that have been targeted to be used in the field of health (Bhattacharya & Mukherjee, 2008). Iron oxides, silica, and quantum dots were also biofabricated up to now (Ebrahimnejad et al., 2013; Ladj et al., 2013). The first history of MNP fabrication dates back to the ancient humans who fabricated the Lycurgus Cup with green color illumination from the external side and red color from the internal side. The observed colors were related to the presence of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) (Freestone et al., 2007). Besides, the synthesis of copper, gold, zinc, stannum, and iron NPs was conducted in

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1857 and their optical reactions with light were evaluated (Alaqad & Saleh, 2016; Becker, 2012). Moreover, the first synthesis of magnetic iron oxide NPs was reported from the study of Blakemore who extracted them from different organisms such as insects, bacteria, mammals, birds, and algae (Benelmekki, 2015; Shnoudeh et al., 2019). The MNPs have been under the spotlight of scientists in different fields such as pharmaceuticals, medicines, mechanics, and physics-based on wide properties in the fabrication of catalysis devices (Narayanan & El-Sayed, 2004), sensors (Gomez-Romero, 2001; Shaikh et al., 2016), optoelectronic facilities (Gracias et al., 2000), smart coatings (Kahraman et al., 2009), etc. Their special surface characteristics such as plasmon excitation, dielectric capabilities, and shape provide lots of opportunities for the fabrication of molecular detectors with ultrasensitivity, cellular-labeled drug delivery systems, hyperthermia-mediated devices for tumor removals, mRNA expression transformers, etc. in recent years (V. Kumar et al., 2013). Metal oxides NPs possess a smaller size and enhanced density over the fringes (Pico´ & Blasco, 2012). Quantum dots consist of metallic complexes including magnetic metals, metals, and semiconductors. Their capability in surface coating and fluorescent properties is remarkable (Nair et al., 2011). MNPs bear various advantages over other nanomaterials in biological, chemical, and medical concepts. These characteristics include size-dependent activity, biological suitability, stability, biocompatibility, safety, inert structure, size manipulation potential, low dispersity, surface functionalization capability, catalytic activity, recyclability, and selectivity (Boris et al., 2014; Honary et al., 2012; M.S. Khan et al., 2013; Sengani et al., 2017; Wang et al., 2009). On the contrary, the cellular interactions of polymeric nanostructures are influenced by their chemical composition, size, surface structure, shape, surface charge, physical aggregation, agglomeration, and solubility (Dong et al., 2012). Therefore, considering the properties of MNPs, the capability of manipulation in size, charge, structure, composition, entrapment, and assembling with optical-mediated changings provides significant benefits for them over polymeric NPs (Cobley et al., 2011; Figueiredo et al., 2014; Huang et al., 2007; V. Kumar et al., 2013). Various methods have been conducted for the synthesis of MNPs which could be classified based on bottom-up and top-down approaches (Ahmed et al., 2016; Horikoshi & Serpone, 2013; Pacioni et al., 2015; Rajput, 2015; Swathy, 2014). The beginning materials for bottom-up methods are atoms and molecules while in top-down methods bulk of materials are starting components that should be physically put under tension to be

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ground to nanometer-scale particles (Pacioni et al., 2015; Rajput, 2015). From another perspective, the applied methods could be categorized into three approaches including physical, chemical, and biological. Physical methods like ball milling, thermal evaporation, lithography, and vapor phase disposition could be considered top-down methods (Ghorbani, 2014). In the ball milling method, the bulk powder would be under tension by providing extreme energy destruction from balls to decrease their size (Rajput, 2015; Yadav et al., 2012). Another mechanical method is a mechanochemical approach which would utilize ball milling in the first step and then a reductant would be added to complete the reaction to achieve the desired MNPs. Cobalt, silver, copper, chromium, and aluminum are some examples that were fabricated by this method (Ghorbani, 2014; Paskevicius et al., 2009). The second subgroup of physical methods is vapor methods including laser ablation, exploding wire, and gas evaporation. Laser ablation is based on heat-mediated vaporization by laser pulses and plasma expansion (Ghorbani, 2014). ZndAl and MgdAl bimetal NPs were fabricated by this method (Hur et al., 2009). The exploding wire method is based on providing a pulsimetric high voltage around a metallic wire which makes a periodical huge thermal enhancement for producing MNPs. Silver, copper, iron, aluminum, and gold NPs have been fabricated by this method (Sen et al., 2003; Yap et al., 2008). The gas evaporation method is conducted by metal evaporation and condensation by inert gas (Yap et al., 2008). Chemical methods have many advantages including cost-benefits, great efficiency, and frequency. In this method, the metal ions would be reduced and stabilized by reductants such as Na3C6H5O7, C6H8O6, NaBH4, (CH3)2NC(O) H, Ag (NH3)2OH, polyols, and elemental hydrogens (Evanoff & Chumanov, 2004; Merga et al., 2007; Wiley et al., 2005). Some obstacles against utilizing chemical methods are the risk of remaining any residuals of solvents and hazardous components in the final products and also the probability of entering the violent by-products in the environment (Das, Pachapur, et al., 2017; Inagaki et al., 2013; Thakkar et al., 2010). The biological approach includes utilizing diverse microorganisms or plant derivatives as reducing and capping agents. The applied microorganisms include bacteria, fungi, algae, viruses, and yeasts (Iravani, 2011; Korbekandi et al., 2009). The MNPs could be fabricated by bioreactors or extracts from intracellular or extracellular exudates of microorganisms. This approach is known as green synthesis method since its cheap, eco-friendly, and harmless which inhibits the increasing trend of environmental hazards (Deplanche et al., 2010; Shah et al., 2015). One of the main risks which threatening the human life is

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antibiotics resistance which restricts the performance of antimicrobial treating protocols. The inaccurate and incorrect administration of antibiotics is responsible for causing modification in bacterial genomes leading to the ineffectiveness of antimicrobial agents (Aslam et al., 2018). The consequence of this phenomenon would be extra consumption of antibiotics, huge morbidity, delayed treatment periods, and financial issues for the governments that result from it (Sa´nchez-Lo´pez et al., 2020). Based on clinical findings, it is obvious that the development of resistance is faster than the rate of antibiotic discovery. Although there is an extreme demand for presenting new antibiotics, the world’s preventive policies have inhibited the entrance of new antibacterials into the market (Power, 2006). Despite the activity of about 25 companies in 1980 on formulating antibiotics, recently less than 12 factories are involved with the production of antibiotics like Merck, Pfizer, M oderna, Glaxo Smith Kline, AstraZeneca, Johnson & Johnson, etc. Most companies prefer to dedicate their production lines to anticancer, high-tech agents, and also for chronic diseases which are more reasonable in financial aspects (Fernandes & Martens, 2017). On the other hand, WHO conducted some policies in 2017 for the management of multiresistant bacterial diseases by declaring a Global Priority Pathogens List (PPL) which would provide a vital need for working on new antibacterial agents (WHO, 2017). Based on the mentioned report, Gram-negative bacteria are the most resistant strains and the treatment for them could not be covered by the currently approved drugs (World Health Organization, 2017). Therefore, the companies have been motivated to work on new antimicrobial agents. It has been noticed that most of them are not involved with conventional dosage forms and especially put their effort on novel formulations like nanomaterials which could be noticed for their better efficacy and performance against resistant microbial strains (A˚rdal et al., 2018). MNPs administration priority over conventional antimicrobials against resistant microbial organisms has lots of reasons. However, the main parameter is their multiple and different mechanism of action toward resistant microorganisms (Slavin et al., 2017). Recently, the remarkable antimicrobial properties of green-synthesized and bioengineered MNPs lead scientists to utilize them as antimicrobial coatings. The antimicrobial coatings with bioengineered MNPs could be used for the fabrication of antimicrobial textiles, biomedical devices, food-protecting covers, etc. (Fierascu et al., 2019). Ag, Cu, Au, Pt, TiO2, ZnO, and MgO NPs have been used for surface coatings with great antifungal activity (Van Long et al., 2016). The usage of MNPs for smart coatings are not

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restricted to above indications and more applications in biomedical fields such as orthopedic infections (Gallo et al., 2016), dentistry (Noronha et al., 2017), fruits and vegetables, packaging (Xing et al., 2019), textiles, etc. were newly reported (Burdușel et al., 2018; Lee & Jun, 2019). In this chapter, initially, the green synthesis of metal-based nanomaterials would be explained by classifying the utilized resources for the bioengineering process. Then, detailed information about some kinds of metal-based nanomaterials would be presented by emphasizing their antimicrobial potentials. Finally, recent advances in bioengineered metal-based nanomaterials for surface coatings would be fully discussed.

2. Green nanotechnology: An overview of bioengineering of metal-based nanomaterials Green nanotechnology is known as an attitude that utilizes the potential of nanotechnology in the eco-friendly production of nanomaterials. The advantages of this technology include saving resources, making less environmental pollutions and wastes, biocompatibility, high yield, boosting energy efficiency, cost-benefit, better effectiveness, etc. (Dahoumane, Jeffryes, et al., 2017; Dahoumane, Mechouet, et al., 2017; Hullmann & Meyer, 2003; Zou et al., 2008). The better points of microorganisms compared with other green sources in the fabrication of MNPs are a great growth rate with high production yield, especially in bacteria, and less basal requirements like pH, pressure, or thermal considerations (Ali et al., 2020). Diverse MNPs including Ag, Cu, Au, Zn, Pd, Ti, and Ni could be produced by green nanosynthesis. Various bacteria, viruses, yeasts, fungi, and algae have been utilized for the green synthesis of MNPs (Iravani, 2011; Korbekandi et al., 2009). This method is categorized as a kind of bottom-up approach (Hulkoti & Taranath, 2014; X. Zhang et al., 2011). Considering the microorganisms as the major bioreactors in the green biosynthesis of MNPs, two main approaches are identified for this procedure. The metal ions could be reduced to MNPs by intracellular or extracellular pathways (Hulkoti & Taranath, 2014). The intracellular route is considered as using the intrinsic enzymes of the microorganisms to produce the MNPs from transported metallic ions. The extracellular route consists of the procedure of excreting the enzymes from the cells to reduce the metal ions to MNPs (Hulkoti & Taranath, 2014). Based on the requirement of using extra physical or chemical tensions in an intracellular pathway for disrupting and drainage of MNPs, the extracellular route has more preferences over

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the intracellular route (Dahoumane, Jeffryes, et al., 2017; X. Zhang et al., 2011). The size, dispersity, and shape of the MNPs are related to the kind of microorganism, culture media, and other cultural conditions (X. Zhang et al., 2011).

2.1 Green synthesis of nanomaterials using plants Plants are the most ecofriendly resources for the green synthesis of MNPs. Alkaloids, polyphenols, sugars, phenolic acids, terpenoids, and proteins in the extracts of plants are responsible for conducting reduction reactions and stabilization steps, as reducing and capping agents, in the process of MNPs synthesis (Hadinejad et al., 2021; Shah et al., 2015). The method is clear in which the solution of metallic salts would be exposed to plant extract at ambient temperature. The indicating parameter for detecting and approving the formation of MNPs is the color change (Malik et al., 2014). The size, morphology, and other physical characteristics of MNPs could be manipulated by modification in the concentration of the metallic salt solution, reaction time, concentration of phytoextracts, temperature, and pH (Pantidos & Horsfall, 2014; Shah et al., 2015). The advantages of this method compared to other methods include availability, safety, suitability for production on large scales, and omission of the risk of infection. The need for heating during the procedures may relatively cause a financial issue (Dhayalan et al., 2018; Pantidos & Horsfall, 2014; Rehana et al., 2017). However, considering the fact of no need for cell culture and capping/ stabilizing agents would make it more cost–benefit than using microorganisms (P.V. Kumar et al., 2019; P
erez et al., 2017; Selvan et al., 2018). The total parts of the plant or aerial parts, leaves, peels, seeds, fruits, rhizomes, and petals have been utilized for this purpose (Appapalam & Panchamoorthy, 2017; Arya et al., 2019; Herna´ndez-Morales et al., 2019; Konwarh et al., 2011; Matinise et al., 2017; P
erez et al., 2017). The plant extraction could be done by boiling or using a Soxhlet device (S.A. Khan et al., 2018). There are two main steps in the synthesis of MNPs. Initially, reducing metallic ions to MNPs consists of nucleation and growing procedures and then stabilizing steps that finalize the formation of MNPs (Herna´ndez-Morales et al., 2019; Konwarh et al., 2011; Selvan et al., 2018; Sonia et al., 2017). A point that needs to be noticed about this method is despite less time would be dedicated to performing this procedure; however, the achieved MNPs may be formed polydisperse. Utilizing different compounds like phenols, flavonoids, alkaloids, and also the

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variations which refer to climates and geographical locations are responsible for this fact (Singh et al., 2015). Various metals have been utilized for plant-based green synthesis of MNPs including gold, copper, copper oxide, palladium, silver, selenium, zinc, lead, platinum, indium, and iron (Shah et al., 2015).

2.2 Green synthesis of nanomaterials using fungi The term myconanotechnology is referred to utilizing fungi in the biosynthesis of NPs and considering the related applications (Mahendra et al., 2009). This method is under the category of bottom-up approach with intracellular or extracellular pathways. The biosynthesis of MNPs by fungal resources has lots of beneficial points compared with bacteria. The production yield in fungi is significantly higher than in other microorganisms. The remarkable extracellular production of MNPs is related to the presence of mycelia which provides an enhancement in protein secretion, which is responsible for the reduction process. Moreover, some fungi have a high intracellular capacity for the entrance of metals which induces the production of smaller particles. Straightforward scaling-up procedures utilizing solid-state fermentation (SSF), efficient excretion of enzymes for extracellular productions, cost-benefit viability, facility of controlling produced biomass, and downstream processing are some advantages of using fungal resources over other bioresources (Boroumand Moghaddam et al., 2015; Kitching et al., 2015; Mukherjee et al., 2002). The study of Mukherjee et al. on an intracellular synthesis of AgNPs by the fungus Verticillium showed that the synthesis of MNPs was performed under the surface of the cell wall, which depicted the involved enzymes were present in the cell wall (Mukherjee et al., 2001). On the other hand, the intracellular approach causes tough downstream procedures which put extra financial burdens. The extracellular synthesis of the same AgNPs was conducted by Bhainsa and D’Souza by using Aspergillus fumigatus. The production rate in this pathway was reported to be rapid and achieved MNPs were remarkably stable (Bhainsa & D’souza, 2006). A point that needs to be noticed is the fact of pathogenicity which has been seen by most of the Aspergillus strains. This fact would cause an obstacle against harvesting, purification, and finally marketing of products that are biosynthesized by the majority of Aspergillus species. In the following, the Trichoderma asperellum was utilized as a nonpathogenic fungus for biosynthesis of AgNPs. The result of this study demonstrated that the AgNPs with nanocrystalline structure could be

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biosynthesized by nonpathogenic fungi (Mukherjee et al., 2008). The titania, zirconia, magnetite, gold, and silica are some other examples of MNPs which were fabricated by using fungi like Fusarium sp., and Penicillium sp., Aspergillus sp., etc. (A. Ahmad, Mukherjee, et al., 2003; Bansal et al., 2004; Bansal et al., 2005; Bharde et al., 2006; Chen et al., 2003).

2.3 Green synthesis of nanomaterials using bacteria The utilization of bacteria for the green synthesis of MNPs has reached the highest rate in research studies (Thakkar et al., 2010). The utilized bacteria species in green nanotechnology include Staphylococcus aureus, Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Bacillus subtilis, Sufolobus acido caldarium, Escherichia coli, Pseudomonas aeruginosa, etc. (Gudikandula & Charya, 2016; Narayanan & Sakthivel, 2010). Various kinds of MNPs have been biosynthesized by utilizing the mentioned bacterial resources including gold, silver, palladium, cadmium, etc. (Narayanan & Sakthivel, 2010). Facile manipulation in particle size, shape and morphology, compatibility, sustainability, economic benefits, safety, and high yield of fabrication are some advantages of using bacterial resources ( Javaid et al., 2018; Thakkar et al., 2010). One of the first studies on the biosynthesis of AgNPs demonstrated that the exposure of bacteria to high concentrations of metallic ion solutions could lead to the formation of larger MNPs (Klaus et al., 1999). The influence of physiological parameters on the green synthesis of semiconductor nanocrystals was also reported by the study on cadmium sulfide NPs which was conducted by using E. coli to be incubated with cadmium chloride and sodium sulfide (Sweeney et al., 2004). The sulfate-reductant bacteria could be utilized for the green synthesis of magnetic NPs with particle sizes of around 20 nm. It is noticeable that the production process takes a week and must be conducted under anaerobic conditions (Watson et al., 1999). Another study demonstrated that the Actinobacters could synthesis the magnetic NPs at a higher rate and also under aerobic conditions (Bharde et al., 2005). A study on the production of AuNPs demonstrated that the pH conditions played a crucial role in the formation of the MNPs. Utilizing Rhodopseudomonas capsulate bacterium as the reducing agent for the production of AuNPs demonstrated that spherical NPs with a particle size of 10–20 nm could be achieved at neutral pH 7. Whereas, acidifying the condition by changing the pH value to 4 would lead to the formation of nanoplates (He et al., 2007). A separate study reported that the slightly elevated temperature in alkaline conditions could result in monodisperse AuNPs

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(A. Ahmad, Senapati, et al., 2003). Overall, the particle size, morphology, and dispersity are related to the pH, concentration, and temperature of the initial metal solution. Besides, the type of bacteria, reacting period, and tensions like disturbance and radiation are other influencing factors (Pantidos & Horsfall, 2014). The pH factor is in direct relationship with particle size and maintenance of pH at 3, could lead to formation of MNPs with size less than 10 nm (Gericke & Pinches, 2006; Li et al., 2011; Rac et al., 2014). The exposure time has direct relationship with particle size and monodispersity index of MNPs (Saifuddin et al., 2009). The rate of reduction procedure could be influenced by temperature. The higher the temperature, the more synthesized MNPs (Vanaja et al., 2013).

2.4 Green synthesis of nanomaterials using algae Chlorophyta, Cyanophyta, Bacillariophyta, Dinophyta, Phaeophyta, Chrysophyta, and Rhodophyta belongs to the great family of algae. They play an important role in the biosynthesis of a wide range of nutritions (Bhuyar et al., 2021). Many properties are attributed to algae including antimalignancy, antiinflammation, antioxidant, etc. (Khalid et al., 2017). Furthermore, the consist components of algae could play the role of reducing agents for the green synthesis of MNPs (Khanna et al., 2019). The chemical components with hydroxyl, carboxyl, and amine groups could be responsible for acting as capping and reducing agents for the biosynthesis of MNPs (M. Mahdavi et al., 2013; Namvar et al., 2012). Because of the existence of many useful algae, it seems to be a great potential for studying more on using them as bioreducing agents in the green synthesis of MNPs (El-Sheekh & El-Kassas, 2016). Pithophora oedogonia, Chlorella vulgaris and Caulerpa racemose from Chlorophyta, Turbinaria ornate, Sargassum wightii, and Padina tetrastromatica from Phaeophyta and Gracilaria edulis from Rhodophyta have been utilized for fabrication of AgNPs (Patel et al., 2015; Sharma et al., 2016; Sunitha et al., 2015). AuNPs were also biosynthesized by the extract of Sargassum wightii in the extracellular pathway (Singaravelu et al., 2007).

3. Antimicrobial potential of green synthesized metal-based nanomaterials Nowadays, significant efforts are being undertaken in medical microbiology to develop suitable replacements for routinely used antibiotics, which have become less efficient as a result of increased bacterial resistance reported in the last decade (Ventola, 2015). However, due to the mentioned problem,

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the quantity and severity of infections have grown, and hospital charges in the United States alone have climbed by $125 billion every year (Gao et al., 2014). Therefore, the constant selection of antibiotic resistance traits in bacteria and other pathogens necessitates the continual development of antimicrobial drugs (Schr€ ofel et al., 2014). Metals have been employed as antimicrobial agents in a variety of nations for ages. Copper salt was utilized as a constringent by the Egyptians in 1500 BCE. Silver and copper were utilized by the Greeks, Egyptians, Persians, Romans, and Indians to purify water and preserve food (Gold et al., 2018). However, recent advancements in the field of nanotechnology are elevated by the development of MNPs. It has fascinated scientists and researchers worldwide due to a wide range of applications in biomedical fields such as bioremediation and biosensor applications due to its remarkable antimicrobial, antioxidant, and optical properties, large surface-area-to-volume ratio, and higher efficacy (Gahlawat & Choudhury, 2019). With that being said, biological synthesis of nanomaterials is the best substitute for physical and chemical methods and does not involve any input of toxic chemicals or energy sources, and gives excellent options for dealing with life-threatening conditions such as infectious diseases (A. Saravanan et al., 2021). Hence, bioinspired NPs have appeared as an effective remediation technology (A. Saravanan et al., 2021). The biological fabrication of MNPs has grown as an important field of nanobiotechnology, and bio-agents serve as potential nanofactories to synthesize nanomaterials (Gahlawat & Choudhury, 2019). As mentioned earlier, this green approach uses several reducing and stabilizing agents derived from biological resources to synthesize NP. Because of the action of numerous bioactive molecules involved in capping and stabilizing the NPs, biosynthesized NPs possess stronger antibacterial activity than conventionally synthesized NPs (Gahlawat & Choudhury, 2019). Generally, bacterial vulnerability to antimicrobial compounds can be affected by the structure of the cell wall. Bacteria are divided into two groups depending on the structure of their cell walls: Gram-negative (G ) bacteria have a multilayer cell wall, whereas Gram-positive (G+) bacteria have a single-layer cell wall. The antibacterial activity of biosynthesized NPs for both kinds of bacteria has been examined (Schr€ ofel et al., 2014). Mostly, Gram-negative bacteria are more sensitive to antibacterial agents because they have a thin peptidoglycan layer with a lipopolysaccharide layer encircling it resulting in a lack of stiffness and strength allowing NPs fabricated via biomediated methods to enter cells. Gram-positive bacteria, on the other hand, have a cell wall with a stiff and thick peptidoglycan layer, which

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complicates the entry of biosynthesized NPs into the cell (Paiva-Santos et al., 2021). Worth mentioning that, unlike conventional antibiotics, MNPs do not exert their impacts in a single specific mode (Singh et al., 2015). Even though the exact mode of nanoparticle activity against pathogens is still being discovered, a combination of mechanisms of action of these MNPs briefly consists of (i) additional formation of reactive oxygen species (ROS) inside pathogens and inducing oxidative stress; (ii) disturbing microbial plasma membranes, essential enzymes in the respiratory chain are disrupted leading to lysis; (iii) accumulation of metal ions at a lethal concentration in the microbial cell; (iv) electrostatic attraction among both MNPs and microbial cells alters the cellular structure and enzymatic pathways; (v) suppression of vital microbial enzymes and proteins via amplified generation of H2O2 (Nisar et al., 2019); (vi) blocking DNA replication and protein synthesis by attaching to DNA and inducing DNA condensation, or denaturing ribosomes and inhibiting tRNA binding to ribosome subunits, and (vii) obstructing ATPase activity leading to a drop in cellular ATP levels (Schr€ ofel et al., 2014). Because of these mechanisms, MNPs are excellent for targeting a wide variety of pathogens. This also suggests that microorganisms would need to acquire several mutations at the same time to protect themselves from MNPs (Singh et al., 2015). Combining MNPs with antibiotics minimizes their toxicity to human cells by lowering the needed dosage while improving microbicidal capabilities (Allahverdiyev et al., 2011). Moreover, these combinations restore the ability of the drugs to kill microorganisms that have developed resistance to them (Singh et al., 2013). Consequently, a unique strategy of using biogenic MNPs alone and combined could act as efficient new compounds for sensitizing microbes. Nevertheless, caution should be taken to minimize prolonged exposure of these nanostructures to pathogens, given that research on E. coli revealed that the bacterium might gain AgNP resistance after 225 generations of continuous exposure via genetic changes (Graves et al., 2015). As already mentioned, biogenic MNPs have been thoroughly investigated for antibacterial efficacy against a wide variety of microorganisms including Gram-negative and Gram-positive bacteria, fungi, yeast, and microbial biofilms (Abdeen, Geo, Praseetha, & Dhanya, 2014; S. Gaidhani et al., 2013; Manivasagan et al., 2013; Singh et al., 2013). There are several techniques for assessing microbe inhibition regarding MNP exposure which include evaluating (i) the zone of inhibition using disk diffusion method and agar well diffusion techniques, (ii) the minimum inhibitory concentration (MIC) using broth macrodilution and microdilution assays, (iii) the minimum bactericidal

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concentration (MBC), (iv) the growth pattern, as well as (v) the time to kill curve (Priyadarshini et al., 2013; M. Zhang et al., 2014). Finally, the biosynthesis of the most widely researched metal and metal salt NPs such as silver, gold, selenium, copper, zinc, and zinc oxide is the primary emphasis of this section. These NPs can be employed as antibacterial and antibiofilm agents in pharmaceutical products.

3.1 Antimicrobial potential of green synthesized silver nanomaterials Silver is a noble metal with exceptional capabilities including microbiocidal potential, notably against multidrug-resistant pathogens such as bacteria, protozoa, fungi, and viruses, and its action includes biofilm inhibition (Paiva-Santos et al., 2021). The toxicity of AgNPs and increased antibacterial capabilities against infective bacteria are owing to their tiny size and higher surface area ( Javaid et al., 2018). AgNPs of size 5 nm exhibited the highest antibacterial activity counter to S. aureus, E. coli, and B. subtilis (Agnihotri et al., 2014). The benefit of the small size of silver nanostructures is that they can infiltrate effortlessly into the bacterial cell (Gahlawat et al., 2016). This activity is attained even at low doses and matches or even surpasses the potency presented by conventional antibacterial agents (Dipankar & Murugan, 2012). The antimicrobial potential of bio-AgNPs stems from a multisided mechanism. In addition, a concentration-dependent rise in inhibition of the growth of microbes is regularly observed when microbes are exposed to AgNPs (Priyadarshini et al., 2013); Nevertheless, the responsiveness differs among species based on AgNP morphology, strain type, inherent microbial sensitivity, as well as cell wall construction that acts as a barrier to AgNPs penetration (Abdeen et al., 2014; Kalpana & Lee, 2013). A study revealed that heating AgNPs generated by bacteria for 2 h at 100°C and 300°C reduced their antibacterial activity. The likely cause was nanoparticle aggregation after heat treatment, which increased the size of Ag particles resulting in a loss of penetrating potential (Pourali et al., 2013). Furthermore, a study described a technique for producing AgNPs from sulfated polysaccharides isolated from the marine red alga Porphyra vietnamensis. The dose-dependent antibacterial activity of AgNPs produced in this work exhibited high activity against E. coli but lesser efficacy against S. aureus (Venkatpurwar & Pokharkar, 2011). AgNPs can attach to the exterior of the microbial cell wall, penetrate, and disrupt the cell membrane construction, and finally cause the death of the cell (Nisar et al., 2019). The interaction of nanostructures with microbes begins with

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nanoparticle adherence to the microbial cell wall and membrane, which is based on electrostatic attraction between the negatively charged microbial cell membrane and positively or less negatively charged NPs (Abbaszadegan et al., 2015). After adhesion, the particles cause morphological changes in the cellular membrane, altering membrane permeability and respiratory functions via membrane depolarization and, eventually, damaging cell structure and cell death (Gomaa, 2017). As a result, the degeneration of the cell wall caused by nanoparticle adhesion, which results in the formation of pore-induced intracellular traps (PITs) in the bacterial cell wall, is thought to be the initial mechanism of antimicrobial activity (Ansari & Alzohairy, 2018). As a result of the rupture of the cell structure, cellular components such as enzymes, proteins, DNA, and metabolites begin to leak into the environment (Ravichandran et al., 2018; Yuan et al., 2017). Furthermore, NPs are hypothesized to interact with proteins on the outer surface of the cells, form complexes with oxygen, phosphorous, nitrogen, or sulfur atom-containing electron donors, and induce permanent cell wall damage (Ghosh et al., 2012). Because of their large surface areas, AgNPs and silver ions can adhere to the functional thiol group (–SH) of proteins, causing protein denaturation (Kim et al., 2009). Thiol groups in bacterial respiratory chain enzymes could be Ag+ ion binding sites (Toh et al., 2014). A noteworthy fact is that phosphorus and sulfur are essential constituents of DNA. Because sulfur and phosphorous are soft bases and Ag is a soft acid, they inherently react with one another (Shrivastava et al., 2007). Therefore, these activities will disrupt cell replication machinery, respiration functions, and membrane permeability, resulting in intracellular content leakage, plasmid and chromosomal DNA damage, and cell death (Appapalam & Panchamoorthy, 2017; Konwarh et al., 2011; Nayak et al., 2015). Another possibility is that the NPs can alter bacterial cell signal transduction by phosphorylating protein substrates, destabilize the potential of the plasma membrane, and alter the phosphotyrosine profile of bacterial peptides (key for cell cycle progression and the formation of capsular polysaccharides) (Ghaedi et al., 2015; Konwarh et al., 2011; Rolim et al., 2019). The NPs dephosphorylate the tyrosine residues of the peptide substrates on Gram-negative bacteria. It inhibits growth of bacteria via blocking signal transduction (Shrivastava et al., 2007). One of the hypothesized mechanisms for the antibacterial action of AgNPs is the release of silver ions from the NPs, which damages both DNA and proteins (Gogoi et al., 2006). According to a study, silver ions cause bacterial DNA to deform from its naturally relaxed state to a

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condensed state in which the DNA loses its replication capacity (Feng et al., 2000). However, silver ions have shown limited antibacterial effect due to strong in vivo interactions with ions found in body fluids precluding their function (Paiva-Santos et al., 2021). It has also been noticed that toxicity by MNPs is mostly induced by the formation of free radicals, such as ROS (Soenen et al., 2011). ROS are extremely reactive molecules that may oxidize cellular structures and biomolecules causing DNA denaturation, protein alterations, and lipid peroxidation, all of which contribute to cell death. Based on a study, ROS generation may contribute to silver nanoparticle-induced cytotoxicity in microorganisms. Such nanostructures adhere to the cellular membrane, penetrate it, and generate ROS. Significantly, ROS production destabilizes the integrity of the plasma membrane and lowers intracellular ATP levels, causing cell respiratory chain (cellular enzymes) and DNA damage, as a result, cell lysis, and death (Das, Dash, et al., 2017). Deletions, additions, mutations, single-strand breaks, double-strand breaks, and cross-linking with proteins are all examples of DNA damage (Gahlawat & Choudhury, 2019). Ultimately, when AgNPs enter the cell, they form a low molecular weight region, while bacterial cells congregate by protecting their DNA from AgNPs via the electron transport chain. Bacteria are impacted by Ag ions produced by AgNPs during cell division resulting in cell death (Agnihotri et al., 2013; Sotiriou et al., 2012). Several researchers have reported the antimicrobial activities of AgNPs synthesized by biological resources. In a study, Bacillus brevis was used to fabricate spherical AgNPs with sizes ranging from 41 to 62 nm. In this study, AgNPs showed exceptional antibacterial properties against multidrug-resistant strains of Salmonella typhi and S. aureus (M. Saravanan et al., 2018). In addition, a research group described the extracellular biosynthesis of AgNPs using an endophytic fungus, Pestalotia sp., isolated from Syzygium cumini leaves, as well as the antibacterial activities of AgNPs against S. aureus and S. typhi. When biosynthesized AgNPs were coupled with the commercially available antibiotics gentamycin and sulfamethizole, synergistic effects were observed (Raheman et al., 2011). Also, biofabricated AgNPs from the culture supernatants of S. aureus through a bioreduction of aqueous Ag ion shows bacterial inhibitory effects. These AgNPs were evaluated for antimicrobial effect against methicillin-resistant S. aureus (MRSA), Streptococcus pyogenes, methicillin-resistant Staphylococcus epidermidis (MRSE), Klebsiella pneumoniae, and S. typhi. The findings showed that the S. pyogenes, MRSA, and MRSE were more sensitive to AgNPs than the other microorganisms (Nanda & Saravanan, 2009). Additionally, an example

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of algae-based nanoparticle biogenesis is described by a group of scientists, in which AgNPs with antibacterial capabilities were produced by the microalgal strains Chaetocerus calcitrans, Chlorella salina, Isochrysis galbana, and Tetraselmis gracilis. Using a clearing zone assay, these NPs demonstrated antibacterial activity against K. pneumoniae, Proteus vulgaris, P. aeruginosa, and E. coli (Merin et al., 2010). Furthermore, the interaction that happens during the synthesis of phytoAgNPs between metal ions and plant metabolites, i.e., the electrostatic attractions between positively/neutrally charged NPs and negatively charged phytocomponents, operate synergistically to improve NPs biological activity (Dhayalan et al., 2018; P
erez et al., 2017). For instance, botanical extracts of bitter apple stem callus (Citrullus colocynths) were utilized to synthesis AgNPs and showed activity against biofilm-forming E. coli, Vibrio parahaemolyticus, P. aeruginosa, P. vulgaris, and Listeria monocytogenes. No activity was observed in the case of Proteus mirabilis, Salmonella enteritidis, or S. aureus (Satyavani et al., 2011). In addition, actinomycetes have been generally used to synthesize extracellular enzymes and secondary metabolites. They have also been used in the biosynthesis of NPs due to their unrivaled ability to generate diverse bioactive chemicals and high protein content (Gahlawat & Choudhury, 2019). For example, a study adopted the marine bacterium, Streptomyces sp. to reduce silver ions into AgNPs. The authors concluded that NPs were prepared extracellularly, and NADH-dependent nitrate reductase was primarily accountable for reducing silver ions into stable AgNPs via an electron transfer reaction. AgNPs were found to have potent acaricidal and antiparasitic efficacy against Rhipicephalus microplus and Haemaphysalis bispinosa (Karthik et al., 2014). In a related context, biofilm is an organized consortium of bacteria that is enclosed in a self-generated aqueous medium made up of proteins, DNA, and polysaccharides (Sahu et al., 2014). It is noteworthy to highlight that biofilms are responsible for about 80% of microbial infections in humans (Davies, 2003). These are antibiotic-resistant up to 1000-fold and, therefore, pose a problem in medicine (S.V. Gaidhani et al., 2014). Several studies have demonstrated the use of NPs in the treatment of biofilm-related infections. The research found that AgNPs significantly reduced biofilms of S. epidermidis and P. aeruginosa. AgNPs are capable of penetrating these bacterial biofilms and blocking the formation of the glycocalyx matrix and bacterial adherence (Kalishwaralal et al., 2010). Exposure to AgNPs decreases bacterial biomass and existing cells while also inhibiting exopolysaccharide and protein synthesis (M. Zhang et al., 2014). AgNPs

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have a better adhesion capability, surface/mass ratio, and capability of entering and releasing Ag ions into bacterial biofilms without expelling any AgNPs resulting in biofilm disintegration (Ansari et al., 2015). Besides, antibiotics linked to AgNPs might be a potential method of eradicating these biofilms. The research used AgNPs coupled with gentamicin to suppress biofilm development and break existing biofilms in a synergistic approach (Mu et al., 2016). In addition, although EPS appears to impede antibiotic molecules, it has been suggested that small-sized NPs can pass through biofilm layers indicating the potency of AgNPs to eliminate biofilms (S. Gaidhani et al., 2013). Besides, in one investigation, a marine strain of the ascomycetous yeast Yarrowia lipolytica was used for the bioinspired fabrication of AgNPs in a cell-associated way. The study concluded that brown pigment (melanin) derived from yeast cells was possibly responsible for the biomineralization of metal ions. AgNPs produced from pigments were shown to have antibiofilm action against the pathogenic Salmonella paratyphi (Apte et al., 2013). Additionally, a viral infection is challenging to treat because viruses multiply and spread fast. Antiviral activity of AgNPs has been documented against influenza virus, human immunodeficiency virus (HIV), hepatitis B virus, monkeypox virus, herpes simplex virus, and respiratory syncytial virus. There is a surge in research into metal-based NPs as effective antiviral agents. In general, as reported in numerous studies, the mechanism of antiviral activity of NPs might be intracellular by inhibiting viral replication or extracellular by interacting with viral protein (gp120) and inhibiting entry, which could differ depending on the type of virus ( Jain et al., 2021). Specifically, AgNPs stimulate the synthesis of cytokines and chemokines, which interact with the residues of accessible sulfur-bearing glycoprotein knobs to prevent viral adherence and penetration, lower TNF expression, and inhibit transcription, all of which are involved in HIV-1 pathogenesis (Lawn et al., 2001; Orlowski et al., 2014; Sun et al., 2005). Moreover, AgNPs act as an inhibitor of viral entry or virucidal agents at the first viral replication stage, regardless of cell tropism determinant and mutation resistance (Naganawa et al., 2008; Walter et al., 2010). The antiviral activity of AgNPs formed intracellularly in Aspergillus ochraceus was reported in a research. In this investigation, thermal treatment of the cells yielded AgNPs embedded in a carbonaceous matrix, and the plaque count technique was used to assess their efficiency against the M13 phage (Vijayakumar & Prasad, 2009). Furthermore, AgNPs inhibit the glycoprotein of viral envelope (gp120-CD4) interaction, which facilitates viral penetration, by attaching to protein structures around the viral

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membrane, resulting in denaturation of protein in HIV-1 (Borkow & Lapidot, 2005; Dura´n et al., 2005). Additionally, in a study, AgNPs derived from marine actinomycetes have antiviral efficacy against the new castle viral disease. NPs of 1–10 nm in size are thought to interact with gp120 and may hinder virus binding to cells (Avilala & Golla, 2019). Moreover, several studies have shown that AgNPs formed intracellularly or extracellularly have antifungal properties. Because of their great surface area, AgNPs help in better contact with fungi; the antifungal activity can be linked to the oligodynamic impact, which causes changes in the permeability of cell membrane, denaturation of protein, and inhibition of DNA replication ( Javaid et al., 2018). In addition, the AgNPs are biosynthesized for assessing their antifungal potential against pathogenic Trichosporon asahii. To this target, T. asahii was cultured on a potato dextrose agar with varied silver nanoparticle concentrations, and the antifungal impact was assessed using the MIC. The authors stated that, while the MIC for AgNPs was inferior to that of other antifungal medications (often utilized in clinical practice), it could suppress fungal growth more effectively. They also described the process that prevents T. asahii growth, which includes penetrating the fungi cell, breaking its cell wall, and destroying other vital cellular constitutes (Xia et al., 2016). Besides, a study found that extracellularly mediated fabricated AgNPs with particle sizes of 27 nm have antimicrobial potential against Fusarium oxysporum (Musarrat et al., 2010). Furthermore, insecticides continue to be an important element in managing and controlling a pest; nonetheless, the use of chemically produced insecticides has proven ineffectual due to the development of resistance by insects ( Javaid et al., 2018). Because of the nontoxicity, enhanced penetrating capability, and large surface area of NPs, nanotechnology has emerged as one of the most promising solutions for insect/pest management. Bacterial-mediated synthesized AgNPs possess high larvicidal activity against the dengue vector, Aedes aegypti, as well as acaricidal potential against H. bispinosa and R. microplus, indicating their potential use as an effective pesticide (Debabov et al., 2013). AgNPs exhibit larvicidal capabilities via developing clusters and entering the larval membrane, where they attach to sulfurcontaining proteins or DNA, inducing denaturation of organelles and enzymes (Adesuji et al., 2016; Salunkhe et al., 2011; Shanmugasundaram & Balagurunathan, 2015). Eventually, the increased impact of biogenic silver nanostructures on the antimicrobial action of conventional antibiotics has also been found with inter and intragroup variation in the degree of synergism (Singh et al., 2015). This is determined by the interaction of particles

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with a specific antibiotic and the pathogen under test (Ghosh et al., 2012). A study investigated the effectiveness of a combination of biosynthesized AgNPs and conventional antibiotics using the MIC breakpoints described in the CLSI guidelines (Singh et al., 2013). MIC breakpoints explain the resistance and susceptibility of microbes to a specific antimicrobial agent and are critical when considering antibiotic activity. Surprisingly, Acinetobacter baumannii, which was resistant to 10 of 14 antibiotics, became vulnerable to seven of them in the presence of AgNPs. The cooperative impact of the medication with AgNPs was so powerful that it reduced the MIC and MBC by up to 2000-folds (Singh et al., 2013). As a result, in the near future, this comprehensive information on the diverse array of biologically fabricated AgNPs against pathogenic microbes will be valuable in pharmaceutical design and development.

3.2 Antimicrobial potential of green synthesized gold nanomaterials Gold is inert in nature, has low toxicity, and is used as a medicine (Wu et al., 2019). It also possesses excellent bactericidal and bacteriostatic properties (Gong et al., 2018). Several studies have indicated that AuNPs have antibacterial properties. A study reported biosynthesis of AuNPs from the leaves extract of the plant Mentha peperita. The fabricated AuNPs were investigated using UV-Vis spectroscopy and analyzed using a scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and fourier-transform infrared spectroscopy (FT-IR). These biomediated fabricated NPs were found to have a diameter of 150 nm and to have high bactericidal effects against E. coli, and S. aureus (MubarakAli et al., 2011). Moreover, deciduous tree (Terminalia chebula) extract was exploited for the biogenesis of AuNPs, which was effective against S. aureus and E. coli (K.M. Kumar et al., 2012). In such a manner, Piper nigrum cell extracts were utilized to form spherical AuNPs which were then modified using chitosan, a polysaccharide. The authors stated that the synthesized AuNPs exhibited improved productivity, better size distribution, higher yield, more stability, and greater bioactivity as an antifilarial drug (Saha et al., 2017). In a related context, green-based AuNPs were also produced utilizing a benign solvent extract of Trianthema decandra (saponin with no capping or other particular reducing compounds). The reduction of AuNPs produced in various forms (cubical, spherical, and hexagonal) was studied using ultraviolet–visible spectroscopy. The antibacterial activity of AuNPs was investigated using the Kirby-Bauer technique. These NPs were highly effective against

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E. coli, S. aureus, Streptococcus faecalis, P. vulgaris, and Yersinia enterocolitica (Geethalakshmi & Sarada, 2012). Alternatively, a study disclosed the synthesis of AuNPs with Abelmoschus esculentus extract. X-ray diffraction analysis (XRD) verified the crystalline nature of the green synthesized AuNPs (size; 62 nm). The outcomes indicated that the mentioned NPs had great fungicidal activity against Candida albicans and P. graminis with substantial potency for use in the development of particular antifungal medicines as well ( Jayaseelan et al., 2013). Bio-AuNPs have shown great antifungal and antibacterial activity overcoming antibiotics resistance (Paiva-Santos et al., 2021). A group of scientists are utilizing proteomic and transcriptome techniques to demonstrate the antibacterial mechanism of AuNPs. NPs exploit their antibacterial properties against multidrug-resistant Gram-negative bacteria in two ways: degrading membrane potential to reduce ATP levels by suppressing ATPase function and impeding ribosomal subunit binding with tRNA. AuNPs increase the level of ATP (oxidative phosphorylation pathway), decrease the level of F-type ATP synthase, and inhibit the ribosomal pathways, leading in a transitory increase in chemotaxis (Cui et al., 2012). F-type ATP synthase catalyzes ATP production in the last stage of the oxidative phosphorylation pathway. F-type ATP synthase also acts in a reverse way as an ATPase to provide the transmembrane proton electrochemical gradient required for molecular translocation (Zharova & Vinogradov, 2004). As reported in E. coli, AuNPs suppress the activity of F-type synthase resulting in lower ATP levels and, as a result, metabolic failure. According to the transcriptome analysis, AuNPs were shown to raise the expression of genes involved in flagellar motility and chemotaxis (Wani & Ahmad, 2013). Furthermore, the antibacterial property of AuNPs is highlighted via disrupting and diminishing the biofilm, whereas the fungicidal impact of AuNPs is spontaneous and irreversible (Raghuwanshi et al., 2017). AuNPs have an effect on hydrogen ion extrusion via the plasma membrane H+ATPase. The proton pump (transmembrane H+-ATPase) was inhibited in Candida cells treated with AuNPs (T. Ahmad et al., 2013). The acidification of the extracellular medium caused by glucose in yeast cells is a good indicator of H+-ATPase-mediated proton pumping. In both cases, the enzymes may be in varied conformational states. As a result, AuNPs may interact directly with enzymes that regulate proton gradients through the plasma membrane. H+-ATPase is an ATP-driven enzyme that converts the energy of ATP hydrolysis to electrochemical potential differences of protons across various biological membranes via primary active transport of H+. Interactions between AuNPs and microbial enzymes (ATPases) have

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the potential to alter their regular conformations leading to the loss of function and finally, cell death (T. Ahmad et al., 2013). Finally, the antibacterial efficacy of bioinspired AuNPs is affected by the diverse bacterial cell walls and surface properties. Surface properties are altered by the biomolecules attached, and electrostatic interactions directly interfere with the process of adhesion and penetration through the bacterial cell wall (Benedec et al., 2018; Raghuwanshi et al., 2017). The difference in bacterial cell wall components justifies the increased efficacy against Gram-negative bacteria (Dhayalan et al., 2018). Additionally, the antifungal effects of NPs have been linked to particle size; the smaller the particle size, the greater the fungicidal activity (Nisar et al., 2019). This feature of NPs is due to their larger surface area, which results in greater/increased contact with target sites on plasma membranes. Particles with lower sizes can also effortlessly pass through the cell membrane. Au (soft acid) reacts with soft bases containing sulfur and phosphorus. As a result, AuNPs may react with phosphorus-containing bases in DNA or proteins having sulfur in the membrane. These interactions might disrupt the regular functions of DNA, such as the replication, repair mechanism, and newly strand synthesis, eventually leading to cell death (Tan et al., 2011).

3.3 Antimicrobial potential of green synthesized selenium nanomaterials Numerous researches have reported the antimicrobial potency of selenium nanoparticles (SeNPs). A study stated a green and eco-friendly approach to the biofabrication of SeNPs by exploiting Penicillium expansum with an average size ranging from 4 to 12.7 nm. In the mentioned study, the researchers assessed the antibacterial and antifungal potential and the MIC of mycosynthesized SeNPs against human pathogenic microbial strains. The inhibitory activity of SeNPs was more effective against Gram-positive than Gram-negative, as well as unicellular and multicellular fungi with a zone of inhibition (ZOI) of 36.3  0.88, 30.3  1.09, 28.3  0.33, 26  0.55, 25.6  0.66, 23.7  0.14, and 22.9  0.49 mm for S. aureus, B. subtilis, E. coli, P. aeruginosa, C. albicans, Ammophilus fumigatus, and Aspergillus niger, respectively. The MIC for S. aureus and B. subtilis were found to be 62.5 μg/mL, while it was 125 μg/mL for P. aeruginosa, E. coli, and C. albicans. The MIC for A. niger and A. fumigatus was 250 μg/mL. The authors proposed that the mechanism of SeNPs may be attributed to NPs surface, which is electrostatically attracted to the microbial cell membrane and infiltrate into it, causing physical damage, and subsequently, leakage of cellular elements, which hinders respiratory enzymes causing

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microbial cell death. Additionally, the inhibitory effects of nanomaterials might well be correlated with the disruption of DNA structure or enzyme activity by creating free radicals of hydroxyl (Hashem et al., 2021). Moreover, the authors of a separate study dealt with mycosynthesis and characterization of SeNPs using Penicillium chrysogenum to evaluate their antibacterial activity. The obtained MNPs were fairly uniform with an average hydrodynamic size of 24.65 nm. The P. chrysogenum-mediated SeNPs exhibited antibacterial properties against Gram-positive bacteria including S. aureus and L. monocytogenes through well diffusion assay with ZOI of 10 and 13 mm, respectively. Conversely, no ZOI was observed against Gram-negative bacteria such as E. coli, S. typhimurium, and P. aeruginosa. The authors proposed that because Gram-positive bacteria had a substantially lesser membrane negative surface charge than Gram-negative bacteria, SeNPs deposition on the surface of Gram-positive bacteria and bacterial damage were more likely. As a result of the electrostatic repulsion between SeNPs and bacterial membrane charge, Gram-negative bacteria tend to be resistant to SeNPs. Furthermore, because Gram-positive bacteria lack a lipopolysaccharide membrane, SeNPs could easily penetrate that type of microbes by chemisorption (Vahidi et al., 2020). Furthermore, a study described microfabrication of SeNPs as antivector malaria by employing Penicillium corylophilum in the presence of ascorbic acid as a reducing agent. The SeNPs characterization findings confirmed the capability of P. corylophilum to fabricate SeNPs in a spherical shape with an average size of 29.1–48.9 nm. Results established the high potency of SeNPs against larvae, pupa, and adults of the third instar of Anopheles stephensi mosquitoes even at low concentrations in which by increasing the concentration of SeNPs, the larvicidal activities reached 100% at 100 ppm of SeNPs. Also, LC50 of the biosynthesized SeNPs against mosquito larvae was established at 25 ppm. The activity of SeNPs as insecticidal may be attributed to the reaction of NPs and the amino acid domain-containing dSH group or compounds containing phosphorus such as nucleic acid, which denaturized the larvae organelles. Alternatively, the authors mentioned that NPs initiate the deactivation of enzymes after penetrating through midget membranes, and then releasing peroxidase cause the death of the cell (Salem et al., 2021).

3.4 Antimicrobial potential of green synthesized copper nanomaterials Copper is a transition metal that is less expensive than noble metals such as Ag or Au (Paiva-Santos et al., 2021). Additionally, Cu and CuO have a wide range of applications in both laboratory and industry including use as a

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component in antibacterial formulations and germicides, as well as in cancer therapy (Chakraborty & Basu, 2017; Mukhopadhyay et al., 2018). Copper nanoparticles (CuNPs), among other NPs, have received considerable interest due to their potential applications as antibacterial agents. There are various studies about the antimicrobial potential of biosynthesized CuNPs. The authors of a study explained that CuNPs have potent antibacterial and antifungal properties. CuNPs (size range from 40 to 100 nm) were effectively produced utilizing leaf extract of Mangolia virginiana as a reducing agent mixed with an aqueous solution of CuSO4
5H2O. The antibacterial test was performed against E. coli, which revealed outstanding antibacterial potency (Lee et al., 2011). Additionally, Citrus medica Linn was exploited to synthesize phytogenic NPs (10–60 nm). These copper oxide nanoparticles (CuONPs) demonstrated antibacterial activity against particular human and plant pathogens. The study findings showed significant inhibitory efficacy against a variety of bacteria such as E. coli, K. pneumoniae, P. vulgaris, S. aureus, Shigella flexneri, Propionibacterium acnes, P. aeruginosa, S. typhi, and E. faecalis. Among the mentioned pathogens, E. coli and E. faecalis were found to be very sensitive to CuONPs (Ren et al., 2009). Additionally, an investigation elucidates the antifungal and antibacterial properties of biomediated fabricated CuONPs employing Acalypha indica leaf extract. The biofabricated NPs varied in size from 26 to 30 nm. These NPs were shown to have strong antifungal activity against C. albicans as well as notable antibacterial activity against E. coli and Pseudomonas fluorescens (Sivaraj et al., 2014). Earlier studies on the antibacterial effects of CuNPs stated that the release of Cu2+ ions might alter the local conductivity and pH of the solution resulting in the rupture of the bacterial cell membrane and localization of ions within the cell, causing disruption in functions of cellular enzyme and bacterial cells to be inactivated or destroyed (Ren et al., 2009). For instance, electrospun cellulose acetate complexes containing CuNPs and AgNPs supported in mesoporous silica and sepiolite were developed and utilized as fungistatic membranes. The size of NPs for mesoporous silica and sepiolite ranges between 5 and 8 nm and 3–50 nm, respectively; and both NPs are well disseminated inside the fibers. To create a fungistatic environment, large aggregates in the micrometer range are employed and allowed metals release in a controlled manner. Their outcomes demonstrated that AgNPs and CuNPs inhibited fungus growth when spores were incubated in direct contact with particles or in the cellulose acetate complex membrane. The fungistatic action against A. niger was seen in germinating spores before the formation of hyphae growth conidiophores. Sepiolite

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containing Cu has been shown to inhibit fungal growth via lowering metabolic activity. These metal-loaded complex membranes serve as reservoirs for the controlled release of metal ions, which eventually influence the growth of fungi (Quiro´s et al., 2016). Likewise, another investigation highlighted the possible antifungal mechanism of phytosynthesized CuNPs in fungus based on changes in the structure and function of the fungal cell. It can also induce cell death by altering DNA and interfering with replication and transcription machinery. Furthermore, CuNPs may inactivate proteins by interacting with their sulfhydryl groups, causing growth to cease (Shende et al., 2015).

3.5 Antimicrobial potential of green synthesized zinc nanomaterials Various metal and metal oxide NPs can be used as antimicrobial agents, e.g., zinc (Zn), zinc oxide (ZnO), and titanium oxide (TiO2) nanostructures (Kaviyarasu et al., 2017). Among them, Zn and ZnO have attracted a lot of attention due to low costs of production, simplicity of preparation, and safe handling ( Jayaseelan et al., 2012). Generally, Zn and ZnO are inorganic materials that have several benefits over organic materials owing to intrinsic features such as (photo) stability, the ability to withstand harsh procedures and high temperatures, and a wide spectrum of radiation absorption. Moreover, they are regarded as generally recognized as safe (GRAS) components for humans (Paiva-Santos et al., 2021). ZnO is an n-type semiconductor metal oxide with a wide range of nanoplatform structures and improved biological activities (Szabo´ et al., 2003) such as antifungal and antibacterial activity against a wide range of microorganisms (even in low concentrations), antioxidant activity, wound healing, anticancer action, and UV radiation absorption (Nisar et al., 2019; Paiva-Santos et al., 2021). As a result, zinc nanoparticles (ZnNPs) and zinc oxide nanoparticles (ZnONPs) have piqued the interest of researchers in biomedicine, drug delivery, and cosmetic applications (Paiva-Santos et al., 2021). Furthermore, zinc oxide at the nanoscale format has shown antibacterial characteristics and potential uses in food preservation. ZnONPs have been used in polymeric matrices to enhance packaging while also providing antimicrobial activity to the packaging materials (Espitia et al., 2012). Antimicrobial packaging is vital for the food business because it could inhibit microbial growth, preserve food safety, and increase the shelf life of the product (Neethirajan & Jayas, 2011). Furthermore, ZnONPs have demonstrated

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antimicrobial activity against foodborne pathogens such as E. coli, L. plantarum, S. aureus, and B. subtilis ( Jin & Gurtler, 2011). The effectiveness of these NPs is mostly determined by the metallic concentration, size, and polydispersity of ZnNPs (Sonia et al., 2017). Several studies on the biosynthesis of ZnNPs and ZnONPs have been published. A study, for example, investigated in vitro antibacterial activity of ZnONPs using nanoparticle suspensions called as nanofluids or pure NPs. ZnO inhibits bacterial growth on a broad spectrum of bacteria, mostly via the catalysis of ROS generation from water and oxygen. ZnO nanofluids are ideal formulations for antiseptic compounds in liquid phases; however, because of the hydrophobic nature of ZnONPs, they tend to aggregate into large flocculates in aqueous environments and, as a result, cannot effectively interact with microorganisms. A team of scientists combined iron oxide (FeO) and ZnO to create magnetic nanocomposites with improved colloidal properties and antibacterial potency. The study found that biomediated fabricated nanocomposites exhibited better antibacterial efficacy against S. aureus than E. coli. Nevertheless, the antibacterial activity of nanocomposites is dependent on the Zn and Fe ratio (Gordon et al., 2011). As mentioned earlier, even at low concentrations, ZnNPs and ZnONPs are potent bactericidal, fungicidal, bacteriostatic, and fungistatic agents, which is why they have been widely used as antimicrobial agents in dermopharmaceutical products to treat skin infections (Sonia et al., 2017). The antibacterial potential of these biosynthesized ZnNPs is demonstrated in a variety of methods including morphological alterations in bacteria and biofilms, biofilm synthesis inhibition, biofilm roughness enhancement, and biofilm density reduction (Alavi et al., 2019). The five stages that zinc-derived NPs cause bacterial morphological alterations are interaction with the membrane, integration with the membrane, blebs on the membrane, and membrane clumping, results in membrane damage (Alavi et al., 2019). Bacteriostatic and fungistatic actions of the mentioned ZnNPs may result from the (i) attached surface capping agents since they are effective microbiocidal agents (Alavi et al., 2019; Sonia et al., 2017). An alternative possible mechanism postulated for the antibacterial activity of ZnNPs is the (ii) inhibition of bacterial enzymes such as glutathione reductases, thiol peroxidases, and dehydrogenases. The oxygen in the nanomaterials reacts with proteins in the bacterial cell membrane that have thiol functionalities (sulfhydryl groups) to erase hydrogen atoms in the form of water molecules, forming an RdSdSdR bond with S atoms and preventing bacterial respiration, resulting in the downregulation of DNA replication and protein synthesis, altered

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membrane permeability, and cell lysis (Shao et al., 2018). A different possible explanation for the antibacterial activity of ZnONPs includes the reduction of biofilm formation owing to (iii) generation of ROS, specifically superoxide radical (O2 ), peroxide anions (O22 ), hydroxide (OH ), and hydrogen peroxide (H2O2), which results in lipid peroxidation of the bacterial cell membrane and cell growth inhibition (Alavi et al., 2019; Sonia et al., 2017). The antibacterial activity is also based on (iv) cellular process inhibition and cellular immobilization of bacteria by attaching to its surface. ZnONPs enter both the outer and inner bacterial cell walls, obstructing transport channels and causing intracellular leakage, impaired nuclear functions, and bacterial cell membrane breakage, ultimately leading to cell death (Shao et al., 2018; Sonia et al., 2017). ZnONPs have a specific antibacterial mechanism based on the (v) lethal impact of electrostatic attachment between ZnO (charged positively) and bacterial cell wall (charged negatively) resulting in the release of Zn2+ ions in the cytoplasm. The ions and NPs attach to the bacterial DNA molecule and nucleic acid strands causing them to cross-link. This occurrence results in a DNA with a disrupted helical structure which triggers biochemical processes (such as protein denaturation) and, eventually, bacterial death (Alavi et al., 2019; S.A. Khan et al., 2018; Nadeem et al., 2019). Significantly, (vi) the increased roughness of biofilm might be attributed to the denaturation of macromolecules (nucleic acids, proteins, and polysaccharides) and ZnNPs coagulation in biofilm (Alavi et al., 2019). Furthermore, additional antibacterial mechanism is the (vii) activation of electrons, which results in the formation of ROS and the oxidation of cellular components (Nadeem et al., 2019). The antibacterial potency of biomediated synthesized ZnNPs and ZnONPs was concentration-dependent as reported in all studies including decreasing biofilm formation and swarming motility (B. Mahdavi et al., 2019). However, the antimicrobial activities of these NPs are dependent on many factors including the biochemical, physiological, and morphological differences between Gram-positive and Gram-negative strains, the temperature during fabrication, physiological characteristics, and specific properties of ZnNPs and ZnONPs (such as nanometric size and high surface/volume ratio), the amount of interaction between NPs and bacteria, and surface modification with chemical or biological substances (Alavi et al., 2019; B. Mahdavi et al., 2019; Nadeem et al., 2019; Shao et al., 2018; Sonia et al., 2017). To demonstrate what was previously stated, smaller NPs can penetrate the cell and cause toxicity, and the higher surface-area-to-volume ratio provides NPs with adsorption, absorption, and penetration abilities on the bacterial

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cell surface (Alavi et al., 2019; B. Mahdavi et al., 2019; Shao et al., 2018). Different studies found that ZnNPs and ZnONPs had higher antibacterial impacts on Gram-positive bacteria, which might be explained by differences in growth rates, surface potential neutralization, and cell wall/bacterial stability properties. The walls of Gram-negative bacteria are made up of 90% lipids and two cellular membranes (inner and outer membranes); the outer membrane includes lipopolysaccharide structures, while the inner membrane has a thin layer of peptidoglycan with lipoprotein structures. Gram-positive bacteria, on the other hand, lack an external membrane (missing lipopolysaccharide structures) and have a thick peptidoglycan layer rich in teichoic acids, making them vulnerable to ZnNPs (Nadeem et al., 2019; Sonia et al., 2017). In a study, it was also observed that an inverse link between exposure time and bacterial growth. Bacterial growth was delayed at lower doses of ZnONPs (bacteriostatic action), while total suppression was found at higher concentrations (bactericidal effect) (Alavi et al., 2019). Alternatively, a study demonstrated the formation of monometallic Zn and bimetallic (Ag and Zn) phyto-NPs. Among them, bimetallic NPs demonstrated superior antibacterial action and the ability to avoid the immunological responses commonly associated with biocidal treatments. The greater antibacterial activity of bimetallic NPs might be attributed to the synergism attained by utilizing two metals, as well as enhanced reactivity caused by varying intensities of strength, binding, configuration, and interaction (Nadeem et al., 2019). ZnONPs have antifungal properties as well. Phytosynthesized ZnONPs (size varying from 12 to 32 nm) were obtained from an aqueous extract of Nyctanthes arbor-tristis. They were shown to have strong fungicidal activity against fungal diseases such as Alternaria alternata, P. expansum, F. oxysporum, Botrytis cinerea, and A. niger, signifying that these ZnO particles might be effective antifungal agents in agriculture ( Jamdagni et al., 2018). In a separate investigation, the antifungal activity of ZnONPs against two pathogenic fungal species, P. expansum, and F. oxysporum, was explored. In a concentration-dependent manner, ZnONPs with a size of around 70 nm demonstrated significant inhibitory effects against both mentioned pathogenic funguses. Fungal hyphae may be inhibited by these NPs. As a result, it has the potential to be an effective antimycotic in agriculture and food preservation applications (He et al., 2011). The antifungal activity of ZnONPs has not yet been fully understood. Nevertheless, research on the effects of ZnONPs on the fungus B. cinerea and P. expansum revealed suppression of conidial development through deformation of P. expansum conidiophores. P. expansum conidia were completely repressed, and

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ZnONPs decreased conidial development. While the inhibition of B. cinerea is caused by fungal hyphae deformation (Huang et al., 2008), this deformation might be caused by an increase in the accumulation of carbohydrates and nucleic acids. ZnONPs can disrupt cell activity by creating an abnormally excessive amount of nucleic acids (Espitia et al., 2012). The elevated nucleic acid content in fungal hyphae might be related to a stress response mediated by ZnONPs. Likewise, a rise in carbohydrate levels might be interpreted as a self-protection strategy against ZnONPs, which can result in distorted hyphal cell structures and, finally, cell death. As a result of these findings, it appears that ZnO nanostructures have other modes of action than antibacterial activity. It was also discovered that different fungi are known for producing mycotoxin such as fusaric acid and patulin. It was found that when the quantity of ZnONPs increases, the inhibition rate of these mycotoxins decreases progressively. The ZnONPs may have inhibited mycotoxin development by inactivating particular enzymes involved in the biogenesis pathway of these toxins (Yehia & Ahmed, 2013).

4. Recent advances in green synthesized metal-based nanomaterials for surface coatings Textiles are used in various traditional crafts worldwide, but they can also be utilized as a medium for microorganism growth and transportation. To optimize the qualities of textiles, several solutions have been proposed including surface modification. Nanoengineered functional textiles are a new frontier in garment technology. The benefit of NPs is that they may provide function without affecting the comfort qualities of a substrate. Moreover, traditional approaches to functionalize fabrics do not result in permanent impacts which are a major difficulty in the textile business. As a result, nanotechnology can affect textiles to gain new and permanent functionality. Hydrophobicity, antimicrobial characteristics, conductivity, wrinkle resistance, antistatic properties, and radiation protection are capabilities that may be nanoengineered into the textiles (Syafiuddin et al., 2020; Yetisen et al., 2016). Furthermore, the awareness of health and hygiene is the other reason for the significant development in multifunctional fabrics in recent years. Therefore, the demand for fabrics coated with MNPs and particularly those with antibacterial properties has surged. In fact, part of the oxygen in the air or water is thought to be converted to active oxygen via catalysis with the metallic ion dissolving the organic component and creating a sterilizing effect. The number of particles per unit area is enhanced when nanosized

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particles are used, allowing antibacterial effects to be maximized. NPs were coated on the surface of cotton fabrics using a variety of processes including the sol-gel method, sonochemical approach, and pad dry curing method (Maghimaa & Alharbi, 2020; Wong et al., 2006). In the conventional method, surfactants are commonly used as reducing and capping agents in the chemical production of MNPs. Nevertheless, biological synthesis requires a simple extract containing a reducing agent, a capping agent, energy, and everything else needed for synthesis (Ramkumar et al., 2017). Ma et al. reported the green synthesis of AgNPs induced by soluble soybean polysaccharides (SSPS). These NPs were spherical and the average particle sizes were found to be 6.9  2.9, 3.7  1.1, and 2.9  0.7 nm for AgNO3 concentrations of 40, 80, and 120 mM, respectively. A sprayand-dry approach prepared the coated craft papers. The microstructures of coated Kraft papers analyzed by field emission scanning electron microscope (FESEM) revealed that the coating completely covered the paper, and small particles distributed on the surface uniformly. They also checked the antimicrobial effect of coated Kraft papers via the disk diffusion method against E. coli (ATCC 25922), S. aureus (ATCC 29213), and P. aeruginosa (ATCC27853). The result indicated that the diameter of the ZOI in E. coli and P. aeruginosa was more than S. aureus indicating that the AgNP-coated paper was more effective against E. coli and P. aeruginosa than S. aureus (Ma et al., 2021). In addition, Othman et al., biosynthesized AgNPs by using Aspergillus terreus NRRL265 in an extracellular approach and according to this study, AgNPs have been produced in spherical shape and range of between 13 and 49 nm. Furthermore, textile fabrics were treated by the biosynthesized AgNPs solution, and the antimicrobial activity of AgNPs surface-coated fabrics was analyzed by agar well diffusion technique. The diameter of ZOI could be detected around 20, 19, and 19 mm against Bacillus mycoides, E. coli, and C. albicans, respectively. The major antimicrobial mechanism can be based on the facts including the increase of the cell membrane permeability due to cell wall degradation, releasing the ROS as a result of an interaction between dissolved oxygen and silver ions that gradually excreted into microbial cells. As mentioned earlier, the other significant underlying mechanism is the adsorption of Ag ions into microbial cells that interact with phosphorous and sulfurcontaining proteins and cell components resulting in cell death (Othman et al., 2021).

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Notably, Shaheen et al. synthesized CuONPs by biomass filtrate of Aspergillus terreus AF-1 in the range of 11–47 nm. In this study, the mycosynthesized CuONPs were incorporated into the textile and the SEM analysis demonstrated homogeneous distributions (Fig. 1). These CuONPs surface-coated cotton fabrics showed significant antibacterial activity against B. subtilis (ATCC 6633) and P. aeruginosa (ATCC 9027), but not against S. aureus (ATCC 6538), and E. coli (ATCC 8739) (Shaheen et al., 2021). Moreover, Mia et al. added Tulsi (Ocimum tenuiflorum) extract to silver nitrate solution to biosynthesize AgNPs. The average size of herbal silver nanoparticles (H-AgNPs) was evaluated at around 23  3 nm. The H-AgNPs coated the cotton fabrics through the pad-dry-cure method, and SEM confirmed the deposition of AgNPs on the cotton fabric surface. For further investigation, the antibacterial activity of AgNP-coated fabrics was performed against S. aureus and E. coli bacteria via agar diffusion plate, the diameter of ZOI was 10.5  0.9 mm, and 9.5  0.5 mm for E. coli and S. aureus, respectively. UV protection against UV-A and UV-B was computed as UV transmission through the Ultraviolet Protection Factor (UPF) test resulting in less UV transmission of 1.04% and 3.12% for UV-B and UV-A, respectively (Mia et al., 2021). Remarkably, Pisitsak et al. biosynthesized AuNPs by tannin-rich extract of Xylocarpus granatum. These NPs were formed with spherical shape with an average particle diameter of 17 nm. In this study, cotton fabrics coated with the whey protein

Fig. 1 SEM images of nanoparticle-functionalized fibers (D) native fabrics (untreated) and (E and F) fabrics treated with CuONPs. (Reprinted (adapted) with permission from reference Shaheen T, Fouda A, and Salem S. (2021). Integration of cotton fabrics with biosynthesized CuO nanoparticles for bactericidal activity in the terms of their cytotoxicity assessment. Industrial and Engineering Chemistry Research, 60, 1553–1563. Copyright 2021 American Chemical Society.)

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isolate (WPI) solution were secondary coated with colloidal AuNPs. EDX mapping proved the uniform distribution of elemental gold across the fiber surface. In this study, the catalytic reduction of AuNPs was analyzed using Congo red (CR). The results showed that after 24 h, AuNP-coated fabrics only left a 4.7% CR concentration in the solution indicating that they could be used as a heterogeneous catalyst for CR degradation in an aqueous solution (Pisitsak et al., 2021). Moreover, Maghimaa et al. fabricated AgNPs using the leaf of Curcuma longa L. The AgNPs were formed with spherical morphology and particle size ranging from 15 to 40 nm. Afterward, the C. longa leaves extract containing AgNPs was incorporated into the cotton fabrics. The SEM with EDX analysis confirmed the binding of AgNPs on the fabrics by showing semigranulated patterns on the fabric’s surface. The AgNP-coated cotton fabrics showed antimicrobial efficacy via a well diffusion method against P. aeruginosa, S. aureus, and C. albicans pathogenic microbes compared to normal uncoated fabric. The antimicrobial effects of these surface coatings of NPs can occur via releasing Ag+ ions which oxidase the microbial structures. In addition, in vitro wound scratch assay was conducted on the L929 cells proving the considerable improvement in the migration of cells toward the wounded area (Maghimaa & Alharbi, 2020). Besides, Eid et al. biosynthesized AgNPs with a spherical shape and a range of 7–15 nm by using the actinomycetes strain of Streptomyces laurentii R-1 which was isolated from the plant Achillea fragrantissima. The biofabricated AgNPs were loaded on cotton fabrics, and SEM–EDX demonstrated the smooth distribution of NPs on the surface of treated fabrics. The antimicrobial activity of these surface-coated fabrics was investigated by using inhibition zones. The results indicated that the AgNP-coated fabrics manifested the largest ZOI against B. subtilis (2.63  0.15 mm), whereas P. aeruginosa had a smaller ZOI and significant activity against S. aureus and E. coli (Eid et al., 2020). Remarkably, Turakhia et al. synthesized CuONPs biologically by using Carica papaya leaf extract. The size of NPs was less than 100 nm with a square or rectangle shape. In this study, cotton fabric was loaded with CuONPs, and SEM images depicted the smooth distribution of CuONPs on the surface of the cotton fabric. The disk diffusion method evaluated the antimicrobial activity of CuONPs cottoncoated fabrics. The result indicated good antimicrobial activity against gram-negative E. coli, and it remains after multiple washing cycles of treated cotton fabric (Turakhia et al., 2020). Besides, Salem et al. biosynthesized AgNPs by using Streptomyces antimycoticus L-1 isolated from Mentha longifolia L leaves. According to this study, fabricated NPs had a spherical shape in the

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range of 13–40 nm. Cotton fabric pieces were loaded with biofabricated Ag-NPs (100 ppm), and SEM showed these NPs were homogeneously distributed on cotton fabrics surface. The antibacterial performance of silver-coated fabrics was evaluated against B. subtilis, P. aeruginosa, E. coli, and S. aureus using a well-diffusion assay. The results showed the highest antimicrobial activity of these fabrics against B. subtilis compared to other groups (Salem et al., 2020). In a study, Stular et al. reported the biomimetic synthesis of AgNPs in the presence of sumac leaf extract with an average size of 52–105 nm. In this study, chemical modification of cotton fabrics surface with AgNPs indicated that the bacterial growth against Grampositive S. aureus and Gram-negative E. coli was decreased compared to untreated samples. The potential mechanism may be the releasing of both silver cations and AgNPs from the surface-coated cotton fabrics and interacting with bacterial cell membranes. Furthermore, Ag+ ions and AgNPs may photocatalytically create ROS in the presence of oxygen which was hazardous to the bacterial cells (Sˇtular et al., 2021). Additionally, Rodrigues and colleagues reported the biosynthesis of AgNPs by exploiting epiphytic fungus Bionectria ochroleuca in the range of 8–21 nm and spherical shape. The padding method impregnated the cotton and the polyester fabrics with AgNPs. The antimicrobial activity of surface-coated fabrics was evaluated on the growth of microorganisms including S. aureus, E. coli, C. albicans, C. parapsilosis, and C. glabrata. The findings exhibited that 100% of the microorganisms growth in both S. aureus and E. coli species was inhibited, and also surface coating fabrics were efficient against C. parapsilosis, C. glabrata, and C. albicans (Rodrigues et al., 2019). In a study conducted by Shaheen et al., AgNPs were microsynthesized by endophytic fungi in the range of 5–20 nm. Cotton fabrics were loaded with AgNPs for 72 h, and AgNPs were homogeneously dispersed on the surface. This treated fabric showed antimicrobial activity against Gram-positive bacterium S. aureus (ATCC 29213), Gram-negative bacterium E. coli (ATCC 25922), fungus A. niger (NRC 53), and yeast C. albicans (ATCC 10321) through using the agar diffusion technique (Fig. 2) (Shaheen & Abd El Aty, 2018). Shahid-ul-Islam et al. immersed cotton fabrics in AgNPs colloidal system that was biosynthesized using Punica granatum peel extract to coat fabrics. Transmission electron microscope (TEM) and SEM showed spherical and well-dispersed AgNPs, as well as the presence and uniform distribution of AgNPs on the cotton. Antimicrobial efficacy against bacteria E. coli and S. aureus was evaluated via colony counting method and demonstrated

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Fig. 2 Photos of the antimicrobial effect of untreated and AgNPs treated cotton fabric against (A) S. aureus, (B) E. coli, (C) C. albicans, and (D) A. niger (1–5 refers to the fungal strains that were used for the biosynthesis of AgNPs). (Reprinted (adapted) with permission from reference Shaheen, T. I., and Abd El Aty A. A. (2018). In-situ green myco-synthesis of silver nanoparticles onto cotton fabrics for broad spectrum antimicrobial activity. International Journal of Biological Macromolecules, 118, 2121–2130. Copyright 2018 Elsevier.)

values higher than 93% and 92% against S. aureus and E. coli, respectively. As the authors mentioned, the mechanism of bacterial killing was due to the reaction between silver cations as positively charged species and negatively charged debris of bacterial membranes. Besides, silver cation release caused adherence to the bacterial cell walls resulting in its constantly damaging and catalyzing DNA denaturation and eventually complete cell damage through

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conducting reactions forming ROS (Shahid ul et al., 2019). Boomi et al. reported the biosynthesis of AuNPs by using Coleus aromaticus leaf extract with different shapes (spherical, rod, and triangular) in average crystalline sizes of 18. Coating of AuNPs on cotton fabric was performed by the pad-dry-cure method. Then, coated fabrics were examined via a disk diffusion method for antibacterial activity against S. epidermidis and E. coli bacterial strains. The ZOI was found to be 27 and 22 mm for E. coli (Gram ) and S. epidermidis (Gram +), respectively (Fig. 3). In vitro cytotoxicity and cell viability of AuNPs against human liver cancer HepG2 cell lines were evaluated by MTT assay and showed 50% inhibition in growth. This could be related to the physicochemical interaction of AuNPs with intracellular proteins and DNA or the triggering of death by activation of caspase 3 enzymes as stated by the authors (Boomi et al., 2019). Ganesan et al. biosynthesized AuNPs with spherical shape in a range of 100–500 nm induced by Acorus calamus rhizome extract. Coating of AuNPs on the cotton fabrics was done by the pad-dry-cure method. In this study, antibacterial activity against S. aureus and E. coli bacterial strains was performed by quantitative test method at two different time durations (24 and 28 h) in which percentage inhibition was remarkable after 48 h compared to 24 h. This was related, according to the scientists, to the stability of AuNPs on the surface of cotton fabric via the adsorption of hydroxyl groups present in the extract to the 4-D-glucosepyranose structure of fabrics. Moreover, AuNP-coated cotton fabric displayed a UPF value of 31.9

Fig. 3 Antibacterial activity of green AuNPs against (A) S. epidermidis and (B) E. coli: (1) uncoated cotton, (2) extract-coated cotton, and (3) AuNPs-coated cotton. (Reprinted (adapted) with permission from reference Boomi, P., Ganesan, R., Poorani, G., Prabu, H. G., Ravikumar, S., and Jeyakanthan, J. (2019). Biological synergy of greener gold nanoparticles by using Coleus aromaticus leaf extract. Materials Science and Engineering: C, 99, 202–210. Copyright 2019 Elsevier.)

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representing a very good UV protection efficiency compared to uncoated cotton (Ganesan & Gurumallesh, 2019). Besides, Vasantharaj et al. used Ruellia tuberosa aqueous extract for the biosynthesis of CuONPs which were nanorods with an average size of 83.23 nm. The antibacterial activity of CuONPs-treated cotton fabrics was found to be higher against S. aureus and K. pneumonia than E. coli in a disk diffusion method (Fig. 4). The FESEM analysis revealed that these coated fabrics had a potential active surface with uniform distribution of CuONPs (Vasantharaj et al., 2019). In a study, Jacob et al. reported the effective incorporation of biosynthesized AgNPs using Ocimum sanctum leaf extract into tissue paper. Biosynthesized AgNPs were spherical and ranged from 10 to 50 nm particle size. The surface-coated tissue paper was analyzed against common clinical pathogens such as S. aureus, K. pneumoniae, and E. coli. The results showed a significant inhibitory effect against these bacteria compared to untreated tissue paper. Furthermore, more proteins were adsorbed on AgNP-coated

Fig. 4 Antibacterial properties of green synthesized CuONPs-treated fabrics compared to the negative control against (A) E. coli, (B) K. pneumoniae, and (C) S. aureus. (Reprinted (adapted) with permission from reference Vasantharaj, S., Sathiyavimal, S., Saravanan, M., Senthilkumar, P., Gnanasekaran, K., Shanmugavel, M., Manikandan, E., and Pugazhendhi, A. (2019). Synthesis of ecofriendly copper oxide nanoparticles for fabrication over textile fabrics: Characterization of antibacterial activity and dye degradation potential. Journal of Photochemistry and Photobiology, B: Biology, 191, 143–149. Copyright 2018 Elsevier.)

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surface of paper than on uncoated paper which might be a critical factor in the ability of AgNPs coatings to limit bacterial growth on paper towels ( Jacob et al., 2019). Rajaboopathi et al. biosynthesized AgNPs using Padina gymnospora extract (brown seaweed extract). TEM showed the small spherical morphology of AgNPs ranging from 2 to 20 nm in size. The surface cotton fabrics were functionalized by the pad-dry-cure method. Agar well diffusion method elucidated the antibacterial activity of AgNP-coated fabric against the S. aureus (Gram +) and E. coli (Gram ) with the ZOI of 21 and 19 mm, respectively, which had higher inhibitory zone values than standard antibiotic (Amikacin) (ZOI of 17 mm for both pathogens tested). The effect of washing cycles of AgNP-coated cotton fabric did not demonstrate a large effect on its antimicrobial effects that could be attributed to the strong chemical and physical bonding of AgNPs to the molecular structure of the cotton fabric. The authors suggested that the underlying mechanism of the antibacterial effect of AgNPs was based on the released Ag+ ion which strongly attaches to electron donor groups on biomolecules carrying sulfur, oxygen, or nitrogen atoms in their cell area, and causing bacterial species to be destroyed (Rajaboopathi & Thambidurai, 2018). Balamurugan et al. synthesized AgNPs by adding Peltophorum pterocarpum flower extract as capping/binding and reducing agents into an aqueous AgNO3 solution to convert the silver cations to their nanoform. The phytofabricated AgNPs were formed with almost spherical morphology and an average particle size of around 86 nm. Then, the cotton fabric was coated with the green synthesized AgNPs using dipping and ultrasonication methods. A disk diffusion technique was used to evaluate the antibacterial properties of AgNP-coated cotton fabric against Gram-positive S. aureus strains. For both dip and ultrasonicated samples, the AgNP-coated cotton fabric exhibited a significant size of ZOI around the fabric. The authors declared that the higher diameter of the ZOI for ultrasonicated fabric in comparison to the dip-coated one might be due to the higher amounts of AgNPs in ultrasonicated fabric. As the authors suggested, the observed antibacterial activity of AgNP-coated cotton fabric was due to the activity of AgNP-incorporated fabrics on the membrane of the microorganism (Balamurugan et al., 2017).

5. Conclusions and future outlook The worldwide burden of microbial infectious illnesses is a severe health issue as well as an economic burden. In addition, the multidrug resistance

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of numerous microorganisms has also been a clinical crisis worldwide. Meanwhile, green chemistry provides innovative opportunities to combat microbial infections. Bioengineering of metal-based nanoparticles by employing biological resources such as plants and microorganisms has attracted significant attention owing to their great antimicrobial properties. As discussed in this chapter, the bioengineering of MNPs offers various advantages such as biocompatibility, high yield, boost in energy efficiency, saving resources, as well as creating less environmental pollution and waste. In this chapter, not only the antimicrobial potential of bioengineered MNPs was discussed but also the recent advances of bioengineered metal-based nanomaterials for surface coatings were fully discussed. Although we provided preliminary evidences that biosynthesized MNPs were effective as surface coatings, further studies should address all the concerns related to the use of these NPs in clinical studies such as the safety profile of these NPs as well as their pharmacodynamics and pharmacokinetics.

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CHAPTER 18

Green antibacterial and antifungal smart coating Iman Khosravi Bigdeli and Mahdi Yeganeh Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

1. Bacterial and bacterial contamination Organisms that cause damage in a variety of environments are bacteria, which are microorganisms that are often pathogenic, particularly in the case of industrial processes and human health. Bacterial contamination, among other factors, poses significant challenges to the food manufacturing industry and the water treatment industry. Known as biofouling, bacteria-enhanced corrosion adversely affects vessels and other equipment, causing a reduction in the life span of equipment and an increase in maintenance costs. Biomedical applications, in particular, are especially susceptible to infection and even death caused by bacteria colonizing on the surface of implanted devices (Wei et al., 2019; Yeganeh & Saremi, 2015). In many ecological niches, fungi, a very diverse and large group of organisms, are among the most deleterious organisms. In general, the most common genera found in buildings are Penicillium, Aspergillus, Cladosporium, and Alternaria (Alexopoulos et al., 1996; Li & Yang, 2004; Negrin et al., 2007; Verdier et al., 2014). As a result of bacteria adhering to biomaterial surfaces and forming biofilms, biomaterial-related infections are responsible for 64% of all hospitalacquired infections worldwide (Shankar & Balasubramanium, 2014; Wang et al., 2013, 2017a). In the United States, a total of over 1 million implantrelated infections occur every year, at a cost of over $3 billion every year (Xu et al., 2017). It is estimated that more than 17 million patients die from bacterial infections every year, making it the second leading cause of death for humans (Hu et al., 2017). However, even with advances in aseptic procedures during the surgical procedure, pathogens can still be found on most implant surfaces (Neoh et al., 2012; Vasilev et al., 2009). Bacteria attaching themselves to surfaces and colonizing them can have several unintended consequences. In addition to these negative consequences, hospital-associated infections can have a significant adverse impact Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00019-0

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on patient outcomes, resulting in high mortality and morbidity rates. Approximately one in three hospitalized patients will suffer from a health-associated infection (HAI) on any given day, according to the Centers for Disease Control and Prevention (CDC). The number of hospitalacquired infections (HAIs) in acute care hospitals in the United States in 2015 was estimated to be 687,000 and 72,000 patients died as a consequence of HAIs during their hospitalization (Mitra et al., 2020). The last few years have seen a series of studies suggesting approximately 45% of HAIs are caused by implants and medical devices colonized with bacteria (Schierholz & Beuth, 2001). Biofilms frequently form on the implant surface and cause implant infections, and antibiotics do not cure these infections and assure the host’s security. Thus, the standard treatment for implant infections is the removal of the implant (Schierholz & Beuth, 2001). Furthermore, the inanimate environment in health-care facilities also plays a significant role in spreading HAIs, in addition to the implantation of implants and other invasive surgery procedures (Page et al., 2009). Patients can become infected when come in contact with surfaces that are contaminated with bacteria. Health-care workers may also cross-contaminate patients with these surfaces through direct contact with them. Furthermore, it is well known that hospital textiles are excellent breeding grounds for bacteria under the conditions of the appropriate temperature and humidity Additionally, there are numerous surfaces that may become contaminated (Borkow & Gabbay, 2008; Montali, 2006). During a patient’s stay in the hospital, their clothing and bed linen become contaminated with bacteria, resulting in bacterial proliferation. In addition, as a result of hospital bed making, large quantities of bacteria can be released into the air, which are then passed on to the environment surrounding the patient, contributing to the occurrence of HAIs. The contamination of food by bacteria is a significant health concern. A consequence of the increasing automation of food production is an increase in the amount of food-contacting surfaces and, therefore, the risk of cross-contamination (Bastarrachea et al., 2015).

2. Green metal nanoparticles Coating industries are faced with the challenge of finding sustainable alternatives to conventional chemical additives. The unique optical, electrical, and catalytic properties of metal nanoparticles have attracted much attention in recent years. These properties of nanosized metal particles were influenced greatly by the size, shape, and surface morphology of the particles.

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Chemical, physical, and biological methods can all be used to prepare metal nanoparticles. To synthesize metal nanoparticles, the most eco-friendly and simplest method is biosynthesis. The biosynthesis of nanoparticles using coriander leaves, the leaves of Bischofia javanica (L.), the leaves of Daucus carota, the leaves of Solanum lycopersicum, the flowers of Moringa oleifera, the peels of Citrus unshiu, and the leaves of Citrus cannabinus has been reported (Babu et al., 2013; Basavegowda & Rok Lee, 2013; Bindhu et al., 2013; Bindhu & Umadevi, 2013, 2014; Narayanan & Sakthivel, 2008; Notriawan et al., 2013; Shankar et al., 2004; Umadevi et al., 2012, 2013). In this study, green synthesized nanosilver was used as an additive to water-based interior paints due to the benefits associated with the green synthesis of nanoparticles and the properties of the nanosilver. Compared to traditional coatings, the green nanohybrid formulation also prevents the growth of other pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, Micrococcus luteus, Klebsiella pneumoniae, Bacillus subtilis, Pseudomonas aeruginosa, and the most deadly yeast Candida sp. are expected to be widely used in the future, prompting government bodies and the general public to question their safety (Cardoso et al., 2016; Wu et al., 2013). A study was conducted to synthesize nanoparticles using Couroupita guianensis. To investigate the potential application of the phytosynthesized silver nanoparticles in home products and consumer products such as water-based interior paint, extracts of C. guianensis leaves were tested on mouse embryonic fibroblasts (NIH 3T3) and African green monkey kidney cells (Vero). Chemical compounds are capable of inhibiting or killing a wide range of microorganisms. There are a variety of chemicals that are being used extensively in hospitals and homes, including halogens, phenols, soaps, detergents, ammonia compounds, alcohols, heavy metals, acids, and certain special compounds that can be solids, liquids, and gases (Bernard, 2003). The silver nanoparticles synthesized using an extract from the leaves of C. guianensis demonstrated very low toxicity when tested on mouse embryonic fibroblasts (NIH 3T3) and African green monkey kidney cells (Vero), reflecting their biocompatibility and making them ideal coating ingredients. A simple, lowcost, nontoxic, and environmentally friendly method of producing silver nanoparticles using the leaves of C. guianensis has been shown to be a viable alternative for the development of new antibacterial and antifungal compounds. Compared to antibiotics, silver is less likely to produce resistance in microorganisms as it attacks a broad range of targets in these organisms. Even at low concentrations, the compound demonstrated antibacterial and antifungal activities (Rajarathinam et al., 2014). Eco-friendly alternative

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exploration has led to the promotion of “green” products that are “environmentally friendly.” Silver nanoparticles phyto-synthesized using C. guianensis leaf extracts were evaluated as an environmentally friendly antibacterial and antifungal additive to commercially available water-based interior paints for increasing antibacterial and antifungal properties. As a result of this approach, it seems possible to paint bathrooms, kitchens, and hospitals with this material, which will help to reduce the number of infections caused by pathogenic strains in these environments. Using these products would reduce the impact of poor cleaning and disinfection practices in households and, most importantly, decrease the chances of nosocomial infections (Rajarathinam et al., 2014; Saremi & Yeganeh, 2014). Gold nanoparticles (AuNPs) can be regarded as unique among metallic nanoparticles. A substantial role for gold in nanoscale technologies and devices can be attributed to its chemical inertness and resistance to surface oxidation. Nanoparticles have also been demonstrated to possess antibacterial properties against various pathogens (Lok et al., 2006; Park et al., 2010; Rajeshkumar et al., 2013; Tran et al., 2013).

3. Antifungal properties of the coatings The macroscopic observations were confirmed by scanning electron microscopy. Fig. 1 illustrates the fungus film on MF-Lavandin microcapsules and control paint. As shown in Fig. 1A, fungal colonization covers all control paint surfaces with a broad mycelial development as well as many spores and conidiophores. Darkly pigmented spores cause aesthetic damage to the paint. In addition, spores are well documented as the principal source of fungal bioaerosol that causes allergic and asthmatic reactions (Alexopoulos et al., 1996; Revuelta et al., 2021). In contrast, Fig. 1B shows

Fig. 1 The SEM images shown above are of paints (A) MF-LVO, (B) LVO, and (C) MF-LVO, after being exposed to A. fumigatus after 4 weeks at 28°C, with a magnification of 2000 (Revuelta et al., 2021).

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that the mycelium has developed poorly and there are only spores and conidiophores present. Moreover, Fig. 1C shows that only trace levels of mycelium were observed for Aspergillus fumigatus, thus confirming the MF-LVO paint’s antifungal activity. In addition, Fig. 1C confirms the stability of the microcapsules by displaying them immersed within the paint film. Therefore, the waterborne coating containing MF-LVO microcapsules exhibits optimal antifungal activity against Aspergillus fumigatus as a result of the addition of the microcapsules. Since the antifungal properties of the coatings are dependent on the presence of EO inside the microcapsules, this is imperative in order to prevent fungal growth. By overcoming two types of barriers, the release from the microcapsule’s oily core is diffused. Porosity takes the form of the shell wall microcapsule, and channels take the form of the paint matrix internal channels, eventually reaching the coating’s outer surface. Thus, the sustained release of a substance depends on the ability of the coating matrix to penetrate the microcapsule barrier. In order to achieve the same goal, it may be necessary to use reservoirs with a diameter of micrometers for biocides. As a result, the active could be released more slowly, prevented from being absorbed into the air, and the coating would last longer. When enclosed in a protective coating, oils are protected from rapid degradation and from the effects of light and heat. Encapsulation, on the other hand, uses a higher concentration of biocide, as previously noted (Revuelta et al., 2021). Additionally, silver nanoparticles are likely to be very effective antibacterial and antifungal additives to water-based interior paints, which are very promising and point toward the possibility of using this material for painting bathrooms, kitchens, and hospitals, which will decrease the number of infections caused by pathogenic strains in these environments. Novel technologies, particularly those that utilize nanotechnology, have paved the way for the development of improved personal hygiene products. As a result of poor cleaning and disinfecting practices at home, the use of such products might reduce the risk of nosocomial infections (Rajarathinam et al., 2014). Among the various components of coatings, binders represent the matrix structure, and their amounts relative to pigments and fillers have a significant impact on the structure, hence the barrier property of the coating. As a result of their excellent adhesion characteristics and film-forming capabilities, acrylic polymers and copolymers are commonly used as paint binder materials (Carretti & Dei, 2004). Microorganisms are attracted to waterborne paints, particularly acrylic-based paints containing cellulosic compounds as thickeners. By utilizing these compounds as carbon sources, microorganisms

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can cause the paint to deteriorate (Allwood, 1988; Guiamet & Videla, 1996; Rhoades et al., 2007). A major influence on the practical performance of antimicrobial coatings is the nature and physicochemical properties of the materials used as the main matrix. One of the most important factors of consideration is the affordability of the coatings, their ease of fabrication, their optimal mechanical properties, as well as their nontoxicity and noninflammatory properties concerning mammalian cells. A review of the related literature data suggests that vegetable oil-based polyurethanes possess a great deal of potential for use as polymeric frameworks for coatings with the above-required properties. In our research group, we have prepared various antibacterial polyurethane coatings with quaternary ammonium salt functionality built into soybean oil-based polyols (Bakhshi, Yeganeh, et al., 2013; Bakhshi, Yeganeh, & Mehdipour-Ataei, 2013; Bakhshi, Yeganeh, Mehdipour-Ataei, Solouk, & Irani, 2013). These coatings have been verified to possess antibacterial activity against a variety of bacteria and fungi. Based on the work of Kessler et al., it has been demonstrated that antibacterial polyurethanes can be formulated by the use of quaternary ammonium salts obtained from soybean oil (Xia et al., 2012; Zhang et al., 2014). As a method of preparing polyurethane coatings, quaternizing triethanolamine chains is also employed for the preparation of coatings based on castor oil (Bakhshi et al., 2014). It has also been used in more sophisticated biomedical applications based on vegetable oil-based polyurethane (Miao et al., 2012) such as wound dressing (Gharibi et al., 2015; Yari et al., 2014), cardiac patches (Baheiraei et al., 2016a, 2016b), and tissue engineering (Miao et al., 2016). To synthesize polyurethanes, we have used hydroxylated vegetable oils in combination with isocyanate monomers. In response to the increasing concern about the cytotoxicity and environmental hazards of isocyanate-based compounds, nonisocyanate (isocyanate-free) methods for preparing polyurethanes have been developed (Maisonneuve et al., 2015). As a strategy for advancing protective and functional materials, microcapsule coatings have recently emerged as a promising technological development. Due to the increasing efficiency of smart approaches, the challenge for the development of new materials is to achieve more functionality with less material. A coating-containing biocides can benefit greatly from microencapsulation. It is believed that microcapsules provide protection against degradation and enable proper control over the release process, thus extending the duration of the biocidal activity and reducing waste generation (Chong et al., 2019; Trojer et al., 2015).

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4. Smart antibacterial coatings Over the last few decades, several bactericidal coatings have been developed, and laboratory tests have shown that they protect against a number of bacteria. By definition, the coatings which are traditionally bactericidal are capable of continuing to work continuously, a feature that may not be necessary; it is safe if there are no harmful levels of bacteria. Furthermore, continuous activities may, in some cases, negatively impact the environment as a whole. An implant surface exposed to high concentrations of a biocide may trigger undue immune reactions. An ideal coating would demonstrate activity only when it is needed, such as when there is significant bacterial colonization, and remain “inert” when not required (Mitra et al., 2020; Wei et al., 2019). In addition to their bactericidal properties, traditional bactericidal coats accumulate dead bacteria on their surface, resulting in the formation of the biocidal molecules and progressive loss of efficacy. Antibacterial coatings should have antimicrobial properties or self-cleaning properties to reduce the possibility of dead bacteria accumulating on the surface. In order to achieve such ideal antibacterial coatings, a substantial amount of research interest has been directed toward the development of intelligent coatings. The definition of smart coatings is a broad term that has not yet been standardized; in this chapter, smart antibacterial coatings are defined as those that are able to display or alter their antibacterial activity in response to a stimulus. Smart antibacterial coatings are available in two types. Firstly, there are coatings that are activated by one or more stimuli and subsequently become bactericidal. Most often, this is achieved by triggering the release of biocides that have been loaded or encapsulated. As well as antibiotics, AMPs, metal nanoparticles (NPs), quaternary ammonium compounds (QACs), cationic dendrimers, and hydrogen peroxide are examples of released biocides (Mitra et al., 2020; Wei et al., 2019). Activation may also be achieved through stimulating the production of reactive oxygen species (ROS) or altering the conformation of the grafted biocide. In addition to smart antibacterial coatings, some coatings can “switch” or alter their antibacterial properties in response to external stimuli. The “kill and release” coating is a classic example of such a coating, which can switch between bactericidal and bacteria-releasing functionality when the stimulus triggers it. In addition to pH, temperature, and light, one or more of the following stimuli may be used: magnetic fields, electric fields, mechanical force (pressure or touch), ultrasound, ionic strength, and the presence of biochemical agents (e.g., enzymes, glucose,

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and toxins) present at the area to be treated (Cavallaro et al., 2014). pH is probably the most commonly investigated stimulus when it comes to designing smart antibacterial coatings, followed by temperature, light, and concentration of biochemicals (Mitra et al., 2020). The triggered effect of localized acidification is not only due to acidification, as a result of several factors, such as enzymes, phospholipases, hyaluronidase, chymotrypsin, and extracellular lipases, which are produced by bacteria during their metabolism. Several antibacterial coatings have been developed that release biocide upon release of enzymes, in response to the presence of bacterial infection on biomaterial surfaces. It has become widely accepted that the construction of such coatings can be achieved by the deposition of multilayered films or hydrogels, which may contain enzyme-sensitive components (such as hyaluronic acid and poly-L-lysine) on biomaterial surfaces. In addition, hydrolytic enzymes secreted by bacteria (specifically, hyaluronidase and chymotrypsin) can gradually degrade these films to allow preloaded biocide to kill bacteria (Wei et al., 2019). A bacterial infection can reduce the pH of the environment to as low as 5.5 due to low-oxygen fermentation, which generates organic acids such as lactic acid secreted by staphylococci or acetic acid by Escherichia coli. An array of pH-responsive, self-defending, antibacterial coatings has been developed to combat bacterial contamination in response to the decrease in pH. By a layer-by-layer (LBL) assembly technique, Sukhishvili and coworkers created several types of pH-responsive multilayered coatings containing antibacterial agents (Fig. 2). By assembling negatively charged montmorillonite (MMT) clay nanoplatelets with polyacrylic acid, we prepared multilayered coatings that adsorb gentamicin or tannic acid and cationic antibiotics (such as tobramycin, gentamycin, and polymyxin B). As long as normal physiological conditions are met, antibiotics remain stable within the coatings. An increase in the release of cationic antibiotics to kill the bacterial pathogens (such as Staphylococcus aureus,

Fig. 2 Illustration of an antibacterial multilayer film that releases antibiotics when bacteria are present (Wei et al., 2019).

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Staphylococcus epidermidis, and E. coli) associated with the infection can be attributed to protonation of the anionic components (PAA and TA) as well as the disruption of ionic pairing between these anionic components and the antibiotics (Wei et al., 2019).

5. Types of antibacterial coatings In Fig. 3, you can see the three types of antibacterial coatings that can be used: (i) coatings that are self-defensive and able to control biocidal activity due to microenvironments containing bacteria; (ii) coatings that have multiple killing mechanisms that involve positive interactions, and (iii) coatings that kill and release bacteria at the same time (Wei et al., 2019).

Fig. 3 An illustration of three types of antibacterial coatings, each with its own characteristics. (A) Antibacterial coating that self-defends. (B) Antibacterial coatings that synergizes. (C) Smart “kill-and-release” antibacterial coating (Wei et al., 2019).

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It is possible for smart surfaces to release drugs “on demand” in specific physiological environments or in response to external stimuli (AlvarezLorenzo et al., 2016; Palanisamy et al., 2017; Ramburrun et al., 2017). An antibiofilm agent can either prevent the formation of biofilms or destroy them once they have formed. Using smart surfaces has proven to be a potent method to deliver drugs via controlled delivery, since they are responsive to the microenvironment of bacterial infections, such as enzymes (Cardoso et al., 2016; Wu et al., 2013) and local acidity (Hu et al., 2013; Tripathi et al., 2012), The smart surface construction can also be influenced by external stimuli such as light (Perni et al., 2009; Tomatsu et al., 2011), temperature (Higgins et al., 2016; Karimi et al., 2016), electricity (Cantini et al., 2016; Xue et al., 2016), magnetism, and redox changes. In potential strategies to eliminate biofilms, new antibiofilm agents could inhibit signaling pathways that induce biofilm formation or encourage biofilm dissolution (Li et al., 2018). However, due to the continuous accumulation of dead bacteria, these coatings will eventually become contaminated, causing the inactivation of the bactericidal functional groups, which not only will impede bactericidal activity, but may also stimulate inflammation or adverse immune responses (Yu et al., 2015). A promising “kill-and-release” strategy was proposed to address this issue, in which antibacterial coatings are designed to not only kill bacteria attached to their surfaces, but also release the killed bacteria after a suitable stimulus, thereby providing long-term antimicrobial protection (Wei, Tang, et al., 2017). According to Jiang and coworkers, the first pH-responsive smart antibacterial coating, based on zwitterionic polymers, was demonstrated in 2008 and could release killed bacteria by increasing pH (Cheng et al., 2008). It was Lopez and his coworkers who created a temperature-responsive smart antibacterial coating in 2013 (Cheng et al., 2008). We were able to switch surface functions by changing temperature, which alleviated some of the limitations of zwitterionic polymer-based coatings. After these pioneering works, further research has been devoted to this interesting area and a variety of antibacterial coatings have been developed. It is possible to divide these coatings into three classes based on the methods of fixing bactericidal agents: (i) coatings with permanently fixed bactericidal agents, (ii) coatings with reloading capability of bactericidal agents, and (iii) coatings based on physical killing mechanism without bactericidal agents (Wei et al., 2019). Smart drug-delivery systems can benefit from light-responsive materials for several reasons, including fast response times, noninvasive nature, convenience, high spatial resolution, and accurate time control. In recent years,

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more and more focus has been placed on developing light-responsive materials that provide smart, antibacterial surfaces. By altering the conformation of an enzyme, light can also be used to modulate its activity. An antibacterial coating can be made to inactivate enzymes by conjugating a photo-cleavable polymer, such as azobenzene and its derivatives, spiropyran, and diarylethene containing a photoisomeric group (Li et al., 2018; Liu, Zhang, et al., 2017). For example, Wang et al. (Wang et al., 2016) synthesized covalently grafted o-nitrobenzyl-containing ligands that were able to degrade biofilms and bacteria by releasing proteins such as lysozyme or horseradish peroxide (HRP) (Fig. 4). Additionally, with an increase in irradiation time to 20 min, bacteria proteins were released completely. A reversible photoisomerizable diarylethene scaffold was used by Babii et al. (Babii et al., 2014) to construct the amino acid analog. It exhibited potent antimicrobial activity against Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus xylosus due to its incorporation within the cyclic backbone of the antimicrobial peptide Gramicidin S. Various photo-responsive protein-release systems have been developed as a remedy for irreversible enzyme activation (Li et al., 2018). Currently, the CD/Azo interactions are widely used in the preparation of biomaterials that respond and inhibit the adhesion of proteins (Shen et al., 2014), antibacterial (Wang et al., 2017b), and targeting cancer therapies (Bian et al., 2016), etc. Earlier this year, Zhan and Wei developed a series of photo-switched smart surfaces. Azo groups incorporated into the surface-immobilized CD-QAS molecules produced a highly effective bactericidal effect, killing over 90% of attached bacteria. As a result of the UV irradiation of the Azo groups, the cis form of the Azo groups was generated,

Fig. 4 Using the photocleavable linker to modify BDD NWs, and then linking biomolecules by means of amide bonds (Wang et al., 2016).

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which in turn resulted in the dissociation of the Azo/CD-QAS inclusion complex and the release of the dead bacteria from the inclusions. It was then possible to regenerate the surface by irradiating it with visible light for more killing and releasing cycles. The use of photothermal technologies such as graphene conjugated with polyelectrolytes or inorganic metals can be used as a means of killing bacteria or eliminating biofilms under light illumination (Li et al., 2018). Recently, Ji et al. (Hu et al., 2017) described gold nanoparticles (NPs) with surface-adaptive properties, which act as self-adaptive targets in the acidic microenvironment of biofilms. Under near-infrared (NIR) light irradiation, a novel photothermal procedure has been developed to ablate MRSA biofilms without harming the adjacent healthy tissue. In another example, Kim et al. (Kim et al., 2015) developed a photothermal, antibacterial compound comprised of poly(vinylpyrrolidone)sulfobetaine (PVPS) and polyaniline (PANI) bonded by ionic interactions (PVPS:PANI). In less than 3 min, the antibacterial coating was able to absorb NIR and produce heat, killing 99.9% of E. coli and S. aureus.

6. Smart antibacterial and antifungal coatings for various applications 6.1 Medical devices Among the most important applications for smart antibacterial coatings is the prevention of device- or implant-associated infections. The majority of research on smart coatings has focused on their potential use in medical devices. There is almost always a relationship between bacterial infections and lowered pH conditions due to the secretion of acidic metabolites, which can result in a low pH of as much as 5.5 where the infection takes place (Simmen & Blaser, 1993). As a consequence of this difference between physiological pH (7.4) and acidic pH at the location of infection, the smart coating initiates antimicrobial activity in response. As materials, pH-responsive coatings are typically produced by layer-by-layer (LbL) coatings using carboxyl-containing polymers and cationic moieties or electrostatically complexed hydrogel coatings. Alternatively, the cationic moiety itself may act as a biocide by incorporating biocides into the LbL or hydrogel coating. Carboxyl groups are protonated at pH levels below the pKa of the acid. As a consequence, ionic imbalance occurs, which destabilizes the hydrogel or LbL structure, which then releases the biocide. Using a silane linker between poly(vinyl alcohol) and poly(lactide-glycolide acid) polymers containing carboxylic acids, Liu et al. have reported heteropolyvinyl alcohol/

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poly(lactide-glycolide acid) (PLGA) NPs grafted on biomedical titanium. The coating releases small amounts of Van continuously as the chain segments slow down at physiological pH. It is found that Van release is increased at acidic pH (6.4, 5.4, and 4.5) and the coating exhibits the maximum antibacterial efficacy at pH 4.5. The coating did not affect mammalian cell adhesion or proliferation (Fig. 5) (Liu, Zhu, et al., 2017). Under acidic conditions, Schiff bases can also be used, which cleave into carbonyl and amino groups. It has been demonstrated that a pH-responsive hydrogel can be fabricated using Schiff base linkage between oxidized dextran (dextran-CHO) and cationic polyethyleneimine (PEI) dendrimers with silver nanoparticles (AgNPs) embedded within (Fig. 6). At pH 7.4 and pH 5, respectively, Ag and dendrimer released B6% each; at pH 5, these values increased to B18% and 13%, respectively. As a result, the antibacterial hydrogel exhibited strong antibacterial activity against Gram-negative and Gram-positive bacteria, whereas it showed no signs of hemolytic, cytotoxic, or biochemical toxicity after incubation with cells or after implantation (Dai et al., 2018). Various other mechanisms have been proposed to release biocides, including pH-responsive imines (Amador et al., 2012), boronic ester

Fig. 5 Hybrid PLGA nanoparticles trigger the release of drugs based on pH (Liu, Zhu, et al., 2017).

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Fig. 6 Reduced pH of Schiff base-containing hydrogels leads to dendrimer and Ag release (Dai et al., 2018).

bonds (Ding et al., 2017), or coordination bonds (Wang et al., 2017c). Antibacterial agents and Ag nanoparticles were coated with coordination polymers bonded to metal ions such as zinc (Zn) or silver (Ag). As a result of these metal-polymer coordination bonds, Ag and antibiotic release occurred at acidic pH with enhanced antibacterial efficacy when compared to physiological pH (Wang et al., 2017c). The use of Zn-incorporated silicate glass coatings on orthopedic stainless steel was reported in another study. Based on this hypothesis, the authors speculate that Zn oxide is incorporated into the silicate network, thereby forming acid-hydrolysable bonds Si_O_Zn. When Zn oxide was present in the glasses, Zn release was very low (10%) but increased dramatically under acidic conditions (pH 4.5) (B90%), which was proportional to the degree of Zn oxide content in the glasses (Chen et al., 2014). Despite its antibacterial efficacy, its application potential cannot yet be assessed because its efficacy has not been tested. Additionally, bacterial metabolism involves many other compounds, such as enzymes (especially phosphatases, hyaluronases, and chymotrypsins) and toxins, which are used as triggers by smart coatings for their antibacterial property (Li et al., 2018; Wei et al., 2019).

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A study described self-assembled LbL films composed of montmorillonite/hyaluronic acid/gentamicin that slowly degraded in hyaluronic acid solutions or an infection microenvironment, which resulted in the release of gentamicin. In phosphate-buffered saline (PBS), only 5 wt% of antibiotics was released from the film after 48 h of immersion; however, it increased rapidly to 30 wt% when 105 CFU/mL Escherichia coli were added. It appears unlikely that long-term biofilm inhibition will be achieved by the gradual removal of the biofilms despite the smart release of antibiotics contributing to their efficient antibacterial properties (Li et al., 2018). pH and enzymes are not the only factors that can be used as stimuli for antibacterial coatings on medical devices; body temperature (37°C) could also be applied. A common strategy used to reduce body temperature is the use of poly(N-isopropyl acrylamide) or its copolymers as thermoresponsive polymers. They can be set to a temperature that is just below body temperature (LCST). It has been found that hydrogels made with PNIPA tend to compact at body temperature (LCST) due to their coil-to-globe transition when entrapped biocide molecules are released (Sˇtular et al., 2017). As an instance, Ag NPs trapped inside PNIPA functionalized surfaces with LCSTs below body temperature showed good antibacterial activity against E. coli at 37°C, but surface hydrophobicity caused the surface to foul at this temperature. Scientists observed that when temperatures are lowered below the LCST, PNIPA chains swell, releasing dead bacteria (Yang et al., 2016). The triggering of the release of dead bacteria at temperatures below the body’s temperature is not feasible for medical implants. While the temperature-triggered release of Ag may be applied to medical devices, it is not feasible for implants. A second PNIPA-based surface demonstrating bactericidal activity at room temperature and adhesion-inhibiting properties at body temperature was reported by Wang et al. (Fig. 7). An antiadhesive poly(sulfobetaine methacrylate) (PSBMA) layer was laminated on top of a PNIPA-based copolymer. A PNIPA that was swollen and exposed to room temperature exerted activity. With the temperature raised to 37°C, chains of the PNIPA collapsed, exposing the PSBMA and repelling bacteria (Yang et al., 2016). Despite the novelty of this temperature-triggered smart coating, it is not suitable for implantable devices or medical devices whose temperature cannot be lowered below 37°C. However, poly(N-vinyl caprolactam) (PVCL) has also been shown to undergo phase separation as a result of temperature fluctuation in an aqueous environment (Yang et al., 2016). Two thermodynamic transitions have been observed in aqueous PVCL gels: (1) a low-temperature transition at 31.5°C, attributed to the

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Fig. 7 Changing bactericidal functions based on temperature (Yang et al., 2016).

segregation of hydrophobic domains, and (2) a higher temperature transition at 37.5°C, representing the collapse of the gel volume (Laukkanen et al., 2005). The latter case may lead to the release of antibacterial agents. However, a coating that responds to body temperature would activate immediately, regardless of whether bacteria are present or not, unless the coating is made from polymers that undergo a lower temperature transition at higher temperatures (fever), caused by an infection. Nevertheless, if this were possible, there would be a major limitation, since not all fevers are symptoms of infectious agents, and not all infections are associated with high body temperatures (Laukkanen et al., 2005). Although recent advancements in smart coatings have been made, several challenges are still to be overcome. The first consideration is to select materials that are completely nontoxic to mammals. Secondly, it is extremely difficult to develop coatings that are highly effective only in the presence of harmful concentrations of bacteria, but completely inert otherwise. It should be noted that most coatings release biocides gradually regardless of external stimuli, which may not be desirable for this application. The continued use of antimicrobials in the absence of

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infection may produce adverse immune reactions and toxicity, which are similar to those associated with traditional antibacterial coatings. It has also been suggested that biocides released in sublethal doses might contribute to the emergence of antimicrobial resistance. For such in vivo applications, even the presence of stimuli makes it difficult to achieve fine control of the biocide release pattern. Thirdly, many medical devices are implanted for long-term purposes; so, smart coatings must remain effective for the entire lifespan of the implant. Implanted medical devices can last from a few days, such as a catheter, to a few months, such as orthopedic implants, and up to a lifetime, such as heart implants. There has been little research conducted so far concerning the long-term durability and efficacy of these coatings in a physiologically simulated environment. Further, in vitro tests are limited in their ability to simulate the complex in vivo environment as is the case with any device intended for biomedical use. The cell-surface interactions resulting from a stimulus are complex and extremely difficult to simulate. Accordingly, it is unlikely to be easy to translate successful laboratory results into clinical practice. In addition, scaling-up would also make it more difficult to translate smart coatings into commercial products, regardless of their intended use (Mitra et al., 2020).

6.2 Health care and light activated The spread of HAIs is caused in part by surface contamination in health-care settings. This application of intelligent antibacterial coatings has not been extensively investigated compared to medical devices. At this time, the only stimuli-responsive coatings that have been reported are light activated. Various dyes (such as methylene blue and toluidine blue) can be used for the fabrication of these coatings or photocatalysts (such as titanium dioxide) can also be used, both of which produce ROS in the presence of molecular oxygen when illuminated by light (Amador et al., 2012; Noimark, Dunnill, & Parkin, 2013; Schierholz & Beuth, 2001), as shown in Fig. 8. These coatings have the advantage that, by selecting the correct photoactive materials, they can provide continuous or discontinuous antibacterial activity, depending on the application requirements. By using photosensitizers with wavelengths of absorption within the range of visible light or indoor white light, for example, continuous bacterial killing can be achieved. Coatings with antibacterial properties are especially useful on frequently touched surfaces, such as bedrails and door handles, where bacteria are constantly present. Using Toluidine Blue O and Rose Bengal as the

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Fig. 8 Structures and mechanisms of light-activated antibacterial surfaces manufactured with (A) integrated photosensitizers and (B) photocatalyst-deposited substrates (Amador et al., 2012).

antibacterial agents in this study, light-activated cellulose acetate coatings had continuous antibacterial activity for 6 h following illumination with a 28 W fluorescent lamp (Decraene et al., 2008a). The opposite effect can be achieved by using infrared (IR)- or ultraviolet (UV)-active materials. It is possible to apply these coatings to certain medical equipment, such as stethoscopes and thermometers that must be disinfected only after use. The smart coating was synthesized by sequentially depositing gold nanoparticles and phase-transitioned lysozyme films onto a variety of substrates. Under near-IR laser irradiation, the gold nanoparticles achieved extremely high antibacterial efficacy due to their photocatalytic effect. In addition, the layer of lysozyme can be degraded and removed from the surface of the material by immersion in a solution of vitamin C for a short period of time, thereby removing dead bacteria and revealing the silver nanoparticle layer beneath (Qu et al., 2018). Although this is an interesting laboratory study, the degradation of layers over time will restrict its usefulness over the long term. As a result, design of smart coatings for use in health-care facilities presents a major challenge, as there are few stimuli other than light available indoors. Other stimuli have yet to be tested for this application. Despite the attractiveness of lightresponsive coatings, long-term efficacy of the photoactive materials is very difficult to achieve due to bleaching. Furthermore, coatings used in hospitals ought to be durable against both dry and wet friction, since surfaces are frequently wiped during routine daily cleaning and disinfection procedures.

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Hence, it is necessary to evaluate the efficiency of these coatings after contact with liquid disinfectants. It is important to note that, as with coatings used on medical devices, practical considerations crucial to translating into clinical practice are rarely addressed. The intensity of light plays a significant role in photoactivity as well. So far, most works have been carried out at a light intensity of B3500 lx. In hospital settings, however, light intensity varies from 200 lx in corridors up to 50,000 lx in operating theaters; therefore, coatings should be tailored to the specific location where they will be used (Decraene et al., 2008b; Mitra et al., 2020). In addition, the items in a healthcare setting may range from metals to ceramics to plastics and glass, and they may be small or large. In order to apply smart coatings to a variety of substrates at a reasonable cost, there may be a need for different methods of surface treatment within the health-care industry. It should be noted, however, that a light-responsive coating technology that uses titanium dioxide codoped with fluorine and copper has recently been commercialized (KASTUS Technologies) (Leyland et al., 2016). In addition to its transparency and capability to be activated by indoor, visible light, this coating also possesses a darkening effect due to the presence of copper. Nevertheless, this coating technology uses sol-gel deposition at 550°C and, therefore, can only be used on glass or ceramic substrates. There is currently no information regarding the long-term efficacy of the coating in a clinical setting; however, the coating is claimed to be “permanent” (Mitra et al., 2020).

6.3 Textile Various types of antibacterial textiles are used as bed linen for hospitals, uniforms for patients and staff, curtains, and dressings for wounds. The use of antibacterial textiles has been demonstrated to reduce the microbial burden significantly (Borkow & Gabbay, 2008). Although there have been traditional bactericidal coatings on textiles for many years, little research has been performed into smart antibacterial coatings. The smart textiles that have been studied to date employ hydrogel coatings with embedded biocides that respond to temperature or pH. A pH-sensitive chitosan hydrogel coating has been studied, which imparts antimicrobial activity in addition to pH sensitivity (Hu et al., 2012; Jocic, 2009). In acidic or alkaline solutions, respectively, chitosan chains are extended or coiled, and subsequently chitosan chains in their extended form have greater antibacterial activity. Antibacterial textiles are commonly used in wound dressings. A smart dressing has not yet been commercialized, although dressings releasing biocide and

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containing chitosan are available on the market. As an alternative to conventional wound dressings, cotton modified with pH- and temperaturesensitive microgels containing biocides (made of PNIPA copolymerized with methacrylic acid) has been proposed as a potential smart wound dressing, but it is difficult to argue that it is as effective as a conventional wound dressing (Cornelius et al., 2006). Another study was conducted utilizing PNIPA/chitosan microgel coated with silane_QAC in order to impart pH, temperature, and moisture-responsive antibacterial properties to cotton (Sˇtular et al., 2017). The chitin fibers that occur in crustaceans, crabs, and shrimps, and in the cell walls of fungi are composed of natural macrofibrils, which occur in three forms, namely the α, β, and γ (Azuma et al., 2015; Jayakumar et al., 2011). In addition to being degradable, biocompatible, antimicrobial, and antifungal, chitosan is a polysaccharide, which has been extensively studied. A number of different products are produced from this polymer, including hydrogels, membranes, nanofibers, beads, scaffolds, sponges, and micro-/nanoparticles. With these different applications, production has reached 105 million tons annually (Chaudhary et al., 2020). Smart antibacterial coatings for textiles are difficult to design due to the absence of appropriate stimuli. However, despite the development of temperature and pH-responsive coatings, their use has been limited in textile applications. It may be appropriate to use the body’s temperature as a trigger for wound dressings since the antibacterial component will only be activated once it comes in contact with a skin wound; however, the release will occur regardless of whether the wound is infected or not. The problem, however, is the difficulty of identifying an ideal stimulus for hospital bed linens or patient uniforms. Furthermore, when choosing coating materials for textiles that come into direct contact with the skin, care must be taken to avoid any irritation or sensitization of the skin. In addition to their ability to resist laundering, coated textiles, whether smart or traditional, also pose a challenge. Under appropriate conditions, it has not been well investigated how long coated textiles will endure and how useful they will be in practical applications. The release of biocides from coated medical devices and their longterm effectiveness are also major concerns regarding coatings for medical devices (Mitra et al., 2020).

6.4 Food packaging Research into smart antibacterial coatings for food packaging has received very little attention, although “active,” but not environmentally

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responsive, food packaging (incorporating agents that are activated by the environment) (Werner et al., 2017), and responsive packaging (printed electronics containing sensing interfaces as a result of the manufacturing process) (Han et al., 2005; Vanderroost et al., 2014) for food safety and monitoring are already available. In order to manufacture active packaging with antibacterial functions, “safe” biocides are typically utilized, such as essential oils, naturally occurring organic acids such as citric acid, or polymers such as chitosan derived from plants (Lucera et al., 2012). Chemical sensors incorporated within packaging films typically have responsive coatings composed of self-assembled nanoparticles, hydrogels, grafted polymers, linear block ligands, and supramolecular materials (Brockgreitens & Abbas, 2016; Vanderroost et al., 2014). Currently, some self-assembled monolayers and polymer films are used as sensing surfaces that respond to biological molecules based on temperature, moisture, light, or pH. Moreover, some hydrogel coatings come into contact with moisture or light to shrink, swell, or degrade. In principle, this approach can be further extended to the fabrication of smart antibacterial coatings that respond to pH, moisture, temperature, light, and biogenic compounds, such as enzymes or toxins, or other contaminants present in foods. In the future, these stimuli may be used to trigger the release of biocide from smart antibacterial coatings and provide signals to consumers as to the quality of the food being consumed (Mitra et al., 2020). The most challenging aspect of the process would be to find coating materials and biocides that have no toxicity and have no or minimal effect on taste, smell, and color of the food, while also extending its shelf life. For food to meet stringent safety regulations (as with medical devices) and to prevent unnecessary or untimely release of biocides, precise monitoring of the response to stimuli would be necessary (Mitra et al., 2020; Werner et al., 2017). Biogenic compounds or food analytes may be used as stimuli; however, the coating must be tailored specifically to the types of foods or the types of microorganisms present. It would then be necessary to develop a wide range of coatings designed to respond to such triggers. In the food industry, there are similar challenges in translating from the prototype to the commercial product, mostly due to the complexity of foods. When complex constituents are present (e.g., lipids, proteins, and ions), the biocide release may differ from what is observed in laboratory testing when using standard food simulants. In order to achieve smart antibacterial food packaging, existing technologies on antibacterial packaging and responsive packaging must be integrated cautiously (Mitra et al., 2020).

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Chitosan’s main attributes are biocompatibility, renewable origin, nontoxicity, nonallergenicity, and biodegradability. Furthermore, chitosan is also characterized by attractive biological properties (antifungal, antibacterial, antitumor). A major limitation of the direct application of chitosan as an edible coating is its poor solubility in water due to its rigid crystalline structure. Along with chitosan-based emulsions, the polymer can also be blended with plasticizers, surfactants, cross-linkers, etc., which can improve the coating’s physical and functional properties. In order to form films, a number of mechanisms are involved, including molecular bonds, covalent unions, electrostatic, hydrophobic, and ionic interactions between polymer chains and various functional agents (Butnaru et al., 2019; Harkin et al., 2019; MorinCrini et al., 2019; Shariatinia, 2018).

6.5 Smart antiadhesive coatings Several industries suffer from biofouling of equipment. Even though much research has been conducted on antiadhesive coatings to prevent biofouling, smart antiadhesive coatings designed for this application have yet to be developed. In view of the inherent vulnerability of bactericidal coatings to fouling, surface coatings that can change from bactericidal to bacteriostatic (biofouling detachment) when stimulated may serve as an alternative to self-cleaning or regenerable surfaces. Using hierarchical polymer brush coatings, researchers created pH-sensitive coatings by hydrolyzing the outer layer of poly(methacrylic acid) (PMA) at low pH to release AMPs that have been covalently incorporated into the inner layer (Fig. 9) (Mitra et al., 2020). PMAs with hydrated components are hydrophilic and demonstrate resistance to initial bacterial attachments. The pH decreases when fouling occurs, causing the PMA chains to collapse, exposing the AMP to bacteria, which causes the cells to die. Additionally, when the pH level of the surrounding environment is increased, the dead bacteria will become hydrophilic and be released (Yan et al., 2016). Another study demonstrated the development of a smart supramolecular surface that can be switched between bactericidal and bacteria-releasing properties in response to UV/visible light (Wei, Zhan, et al., 2017). Surfaces were produced by combining azobenzene (Azo) groups with a biocidal cyclodextrin derivative conjugated with quaternary ammonium salts (CD_QAS). Azo groups, in their trans form, have been incorporated into CD-QAS in the form of an inclusion complex that produces a bactericidal surface, which is then dissociated upon exposure to UV light, resulting in the release of dead bacteria (Mitra et al., 2020).

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Fig. 9 Based on the hydration of the PMA layer, pH-triggered switching between bacteria-repellent and bacterial-killing surfaces (Mitra et al., 2020).

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CHAPTER 19

Green synthesis of metal nanoparticles and its antibacterial study Prakash Kumar Sahoo Department of Chemistry, Centurion University of Technology and Management, Gajapati, Odisha, India

1. Introduction From prehistoric times till date, human civilization has achieved dev advancements in different fields of science and technology including the nanosciences. Nanotechnology is the science of the small to smaller to smallest. It is the use and manipulation of matter at a minute scale as at this size, atoms and molecules work differently, and provide a variety of surprising and interesting uses. The word “Nano” comes from the Latin word “Nanus” which means literally “Dwarf.” The particle size of nanoparticles (NPs) is 10–9 m. The sizes of NPs are within molecular and macroscopic dimensions, i.e., greater than 1 nm and less than 100 nm. NPs are very minute particles with sizes below 100 nm (1 nm ¼ 10–9 m). Due to the large application of nanoscience and nanotechnology, it has become one of the major fields of research of this century. The application of nanosciences is mainly in the field of inter-disciplinary sciences. This is mostly applied in the fields of energy technology, catalysis, magnetism, fuel cells, data storage, biomedicine, etc. Nanoparticles have exceptional physical, chemical, and biological properties, which give them a special place in the areas of basic science, materials science, technology, and biology. So, the study of synthesis, characterization, and applications of various NPs is largely needed in different fields of science and technology. Due to various morphologies, sizes, and physical and chemical properties, these have different applications in different fields of medicine, science and technology. The MNPs are synthesized mainly by two routes: conventional route and green route. The conventional route of synthesis is of two types: physical route and chemical route. The physical route involves high energy consumption. In the chemical route, MNPs are synthesized by using different

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toxic chemicals, which release hazardous chemicals or gases into the environment as by-products. To overcome these limitations of physical and chemical methods, green synthesis or biosynthesis is an excellent alternative route for the synthesis of NPs. In general, green synthesis includes the use of plants, algae, or microorganisms under ambient temperature and pressure in an environmentally friendly manner. The use of plants and their different parts is proven to be the most convenient and economic route for the preparation of NPs. The phytochemicals present in different plants and their parts contain different biomolecules. These biomolecules are responsible for the reduction of metal (positively charged) present in different metallic salts to atomic or nanosize particles. These biomolecules in the form of phytochemicals are also responsible for the stabilization of synthesized NPs. The agglomeration of MNPs to a less reactive big size is restricted by the phytochemicals by acting as the capping agent. NPs possess unique properties like adsorption, large surface area, quantum, and releasing properties, which show great application in interdisciplinary sciences and technologies.

2. Classification of nanoparticles There are different approaches for the classification of nanomaterials. (A) Based on their composition Based on its composition, nanoparticles are classified as below: Type

Examples

Carbon based Ceramic NPs Metal NPs (MNPs) Oxide NPs

Graphene, fullerenes carbon nanotubes, etc. SiC, BN, Al2O3, etc. Gold, silver, platinum, copper, etc. Cuprous oxide (Cu2O), zinc oxide (ZnO), titanium oxide (TiO2), etc. GaP, GaAs, GaN, etc.

Semiconductor NPs

(B) Based on their dimension NPs are classified into three types of dimensions. The different types of nanoparticles based on its dimension are as follows: (a) One-dimensional NPs This type of category of NPs includes thin film or manufactured surfaces. These are being used in technology, chemistry, and electronics and engineering. For solar cell and catalytic systems, thin films of sizes

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between 1 and 100 nm and monolayers are commonly used now-adays. Currently, these thin films are frequently used in different fields of science and technologies including chemical and biological sensors, fiber-optic systems, information storage systems and magneto-optic and optical devices. (b) Two-dimensional NPs The two-dimensional (2D) NPs mainly include carbon nanotubes (CNTs). In CNTs, carbon atoms are arranged in a network-like structure with a hexagonal array with a diameter of 1 nm and length of 100 nm. CNTs have an excellent capacity for molecular absorption and can be arranged in a three-dimensional configuration. CNTs have cylindrical shapes rolled up by layer of graphite materials. CNTs are basically of two types, single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). These are unique materials as the small dimensions of CNTs have remarkable physical, mechanical, and electrical properties (Kohler, 2004). These possess specific metallic (conductive) or some semiconductive behavior. The current density of CNTs is extremely high (approximately up to one billion amperes per square meter) which can be used in a superconductor. The mechanical strength of CNTs is sixty times (60) greater than that of steel. Lastly, CNTs are physically and chemically very stable. (c) Three-dimensional nanoparticles These are of various types, which are mentioned below. (i) Fullerenes (Carbon 60) Fullerenes have 3D arrangements with spherical cages containing 28 to more than 100 carbon atoms. The basic unit of fullerenes contains 60 numbers of carbon (C60). These are hollow balls composed of interconnected carbon pentagons and hexagons and which resemble a soccer ball. These have unique physical and chemical properties. They show elastic properties, i.e., when subjected to extreme pressure; they regain the original shape when the pressure is released. These have a wide application in the field of lubricants as these molecules do not combine with each other. These are mostly used in nanoelectronics, data storage, production of solar cells, etc. As these 3D molecules are similar to many biologically active molecules with large empty space, they can be entrapped with different active molecules and find potential medical application (Tomalia, 2004a).

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(ii) Dendrimers These are controlled structured polymers with a 3D nanometric dimension. These are used in targeted drug delivery with imaging of 10–100 nm in diameter (Wiener et al., 1994). These have various active groupings on the surfaces and are well compatible with organic basic structure (like DNA), which have large applications in the field of biomedicals. These encapsulate functional molecules inside their core and are used as therapeutic or diagnostic agents (Li et al., 2007). For the large-scale synthesis of organic and inorganic nanostructures with dimensions of 1–100 nm dendrimers are used as basic elements (Tomalia, 2004b). Dendrimers have a wide application in the field of antimicrobials, antiviral drugs, anticancer agents, antiinflammatory formulations, prodrugs, nonsteroidal drugs, and screening agents in drug discovery (Cheng et al., 2008). Dendrimers are very much compatible with organic molecules (basically DNA or like structures), which are modified to metallic nanotubes or structures (Fu et al., 2007; Pardhi et al., 2020). (iii) Quantum dots (QDs) These are small-scale devices with free electrons. These act as semiconductor nanocrystals in the colloidal state. The diameter ranges from 2 to 10 nm. The synthesis method of QDs is by colloidal synthesis or by electro-synthesis from different semiconductors. Some examples of QDs are indium phosphide (InP), indium arsenide (InAs), cadmium selenide (CdSe), cadmium telluride (CdTe), etc. These contain anything from a single electron to a collection of several thousands. By different methods of its synthesis, the morphology, size, number of electrons, and shape can be controlled precisely. These are mostly synthesized as semiconductors, insulators, metallic oxides, magnetic materials, or metals. These are mostly used in information storage, quantum computing, optical and optoelectronic devices, etc.

3. Preparation of metal nanoparticles (MNPs) The various unique properties of MNPs depend on the method of their synthesis. The size, structure, orientation, and number of MNPs depend on the process of synthesis. Depending upon the method of synthesis, various

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Nano Composites

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Nano Crystals

Nano filters

Nano Cluster

Nano Rod

Nano Disks Nano Materials

Nano tubes Nano Wire Nano Compound coatings

Nano Particles Nano Structured Material

Nano Clays

Fig. 1 Different dimensions of nanomaterials.

PREPARATION METHODS OF NANO-PARTICLES (NPs) BOTTOM UP APPROACH 1. SPINNING 2. TEMPLATE SUPPORT 3. PLASMA OR FLAME SPRAYING 4. LASER PYROLYSIS 5. CVD 6. ATOMIC OR MOLECULAR CONDENSATION

TOP DOWN APPROACH BIOLOGICAL PREPARATION USING PLANTS, BACTERIA, FUNGI,ALGAE etc.

1. 2. 3. 4. 5.

MECHANICAL MILLING CHEMICAL ETCHING SPUTTERING LASER ABLATION ELCETRO-EXPLOSION

Fig. 2 Various approaches to prepare nanoparticles (NPs).

shapes of MNPs like nanocubes, nanotriangles, nanotetrapods, nanostars, nanoclusters, nanobranches, nanorods, nanospheres, nanowires, nanotripods, nanoprisms, nanodisks, nanoshells, and nanoframes are reported and characterized (Fig. 1). Therefore, the selection of the synthesis method and preferable conditions are the most important aspects in the preparation of desired NPs. The two basic routes employed to prepare NPs are, as shown in Fig. 2: (1) top-down method and (2) bottom-up method. In the top-down method, NPs are obtained by mechanical crushing of the source material. Some examples are chemical etching, electro-explosion, high-energy ball milling, and laser ablation. In the bottom-up method, by chemical or physical processes, new structures are built up. Here, NPs are obtained as an addition of atom to atom or molecule to molecule. This route is based on physiochemical principles,

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which follow atomic or molecular self-organization. Some of the examples of this route are the sol-gel process, green synthesis, chemical reduction method, hydrothermal route, chemical vapor deposition (CVD), combustion, aerosol process, and plasma spraying process.

3.1 Green synthesis of noble MNPs Noble MNPs can be prepared by physical or chemical or biological methods. In chemical and physical methods, high energy consumption with the use of toxic chemicals or generation of hazardous by-products occurs. In the chemical method, the toxic capping agents present do not show any biomedical applications. The physical and chemical methods are relatively not environment friendly as well as being economical. In lieu the use of a biosynthesis route (mainly green route), i.e., use of different parts of green plants like leaves, flowers, fruits, seeds, roots, bark, etc., with noble metal salts shows an effective route for the preparation of MNPs. Under spontaneous or under basic conditions at room temperature, a nontoxic environment is maintained along with ecological integrity and eliminating the drawbacks of conventional approaches. In earlier times by conventional routes, toxic chemicals were frequently used for the preparation of MNPs. So, scientists and researchers synthesize MNPs in the field of basic sciences, materials science, or technology by adopting a green synthesis route. The advantages of the green route for the synthesis of MNPs are: it is an eco-friendly, nonhazardous, and economic route. This noble route of synthesis mostly uses green plants and its different parts along with metallic salt solution for the synthesis of MNPs. In spite of this, other various biological methods utilize microorganisms (algae, bacteria, fungi, yeast, etc.) and different plant parts in the synthesis of MNPs. From the economic point of view, the green route of MNP synthesis is widely accepted in a large scale by different fields of scientists and researchers. It is the phytochemicals present in different parts of green plants are responsible for reduction and stabilization of synthesized MNPs. Some common phytochemicals in different parts of green plants have been given in Fig. 3. By different scientific studies since prehistoric times, it is confirmed that green plants are gifted naturally with several phytochemicals like vitamins, sterols, terpenes, polyphenols, and carotenoids, with several beneficial effects. Out of several beneficial effects, one is their reducing property by which metallic salts are reduce resulting in their corresponding MNPs. The

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Fig. 3 Some common phytochemicals in different parts of green plants.

agglomeration of MNPs to a less reactive big size is restricted by the phytochemicals by acting as a capping agent. The synthesis of noble metal NPs like silver (Ag), gold (Au), platinum (Pt), and palladium (Pd) is not an easy task. In order to develop such NPs of various morphologies and sizes, defined production route and reaction conditions are very important. The various factors which affect the particle size, chemical composition, crystallinity and shape are pH, concentration of precursor salt solution, temperature, concentration of capping agent, sonication conditions, etc. The synthesis of MNPs is due to the reduction of noble metal salt by phytochemicals present in different parts of green plants and it has been given in Fig. 4. The color of NPs varies when there is any variation in their size, shape, or both. The color of Au colloids depends on particle aggregation. For example, an Au NPs with 20 nm diameter has characteristic wine-red color. Absorption of light of certain wavelength by Au NPs results in coherent oscillation of valence electrons and thus gives surface plasmon resonance (SPR) (confirmed by UV-Visible data) band in absorption spectrum (plot of absorbance against wavelength). The SPR band also depends on particle size and aggregation (Fig. 5A and B). For example, 13-nm spherical Au NPs give an SPR band at around 520 nm, while 99-nm Au NPs give an SPR band at 600 nm NP with same size and exhibits yellowish gray color.

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Fig. 4 Reduction of noble metal salts by phytochemicals.

Fig. 5 (A) Color of Au-NPs at different particles sizes and (B) surface plasmon resonance band of Au NPs at different particle sizes.

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4. Characterization of synthesized MNPs The synthesized MNPs are confirmed by different spectroscopic studies. The morphology, size, charge, types of NP absorption, interaction with phytochemicals, and microbial studies are studied by the following methods (Figs. 6–11).

Fig. 6 UV-Visible spectra of Au NPs (SPR band near 550 nm shows formation of Au NPs).

Fig. 7 FTIR spectra of Au NPs. Decrease in the C-H stretching band and bending vibration band intensities suggests some interaction between gold NPs and extract molecules.

Fig. 8 SEM micrograph of the plant extract containing Au NPs.

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Fig. 9 Photoluminescence (PL) spectra of Au NPs. Decrease in the emission intensity of plant extract molecules at 330 nm reveals charge transfer from the extract molecule to the gold NP.

Intensity (a.u.)

5.1

3.4

1.7

0.0 30

50 70 Hydrodynamic diameter (nm)

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Fig. 10 Hydrodynamic diameter of a plant extract containing Au NPs.

(-)11.0

(-)15.20

Intensity (arb.units)

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3.5 (b)

0.0 -50

(a)

-25 0 Zeta potential (mV)

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Fig. 11 Zeta potential of plant extract and extract with Au NPs.

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(I) (II) (III) (IV) (V) (VI) (VII)

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Ultraviolet-Visible Spectroscopy (UV-Visible) Fourier Transformation Infra-Red Spectroscopy (FTIR) X-Ray Diffraction (XRD) Scanning Electron Microscope (SEM) Dynamic Light Scattering (DLS) Zeta Potential Antimicrobial study (Antibacterial Test)

5. Antibacterial test Basically, our environment contains billions of bacteria all around it. Bacteria may exist as unicellular (single cell), paired, clusters, or chains. Generally, these are unicellular and microscopic organisms. On the basis of their shapes, bacteria are of five types, such as: spherical shaped (Cocci), rod shaped (Bacilli), spiral shaped (Spirilla), comma shaped (Vibrio), and corkscrew shaped (Spirochaetes). Furthermore, the bacteria are classified on the basis of cell walls by a Gram stain test. This is an important test to distinguish the bacteria on the basis of structural arrangements of the cell wall. In this test, a counterstain, generally safranin, is added after crystal violet. Under the Gram staining experiment, bacteria show two types of effects. When dark blue or violet color appears in Gram staining, it is a Gram-positive bacterium and when it cannot hold the blue or violet color and appears red or pink in a counter stain, it is a Gram-negative bacterium. This happens because an outer membrane stops the penetration of the stain. This difference is due to the difference in their cell-wall structure. In Gram-positive bacteria, the peptidoglycan layer is thick, which acts as a protective cover and these do not require any type of rigid cell wall. However, in Gramnegative bacteria the peptidoglycan layer is thin or insignificant; so, they require a rigid cell wall for protection and support. In decolorizing solution, Gram-positive bacteria retain the crystal blue or violet color. Between the above two types of bacteria, Gram-negative bacteria are more resistant to antibiotics due to the presence of an impermeable cell wall as compared to Gram-positive ones. Fig. 12 shows Gram-positive and Gram-negative bacteria. An antibacterial test of Au NPs with Escherichia coli, with Bacillus subtilis, and with Staphylococcus aureus has been depicted in Fig. 13A–C.

Fig. 12 Gram-positive and Gram-negative bacteria.

ANTI BACTERIAL TEST WITH DIFFERENT BACTERIA BY DISC METHOD WITH Au NPs SAMPLES PREPARED BY GREEN ROUTE

(a)

(b)

(c)

Fig. 13 Antibacterial test of Au NPs: (A) with Escherichia coli, (B) with Bacillus subtilis, and (C) with Staphylococcus aureus.

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6. Applications in MNPs in everyday life (A) Medicine (Behera et al., 2019): (i) Drug delivery (ii) Nanoparticles deliver chemotherapy drugs directly to cancer cells (iii) Drugs containing dendrimers for targeted delivery (B) Electronics: (i) Reduces density in memory chips (ii) Nanotelevision, nanocomputer, and nanoprojector (iii) Nanoelectronics reduces weight and space improving display in screens (C) Environment: (i) Cleaning of existing pollution—Iron nanoparticles can be effective in cleaning organic solvents that are polluting ground water) (ii) Silver nanoclustures used as catalysts that reduces polluting by-products generated in the process of manufacturing propylene oxide a starting material for the manufacture of various types of plastics (iii) Diesel fuel-containing cerium oxide to reduce emissions of carbon monoxide (iv) Epoxy-containing carbon nanotubes used to make wind mill blades (v) Silicon nanowires embedded in a polymer result in low-cost, high-efficiency solar cells (D) Consumer products: (i) Silver nanoparticles in fabrics that kill bacteria making clothes odor resistant (ii) Skin-care products that use nanoparticles to deliver vitamins deeper into the skin (iii) Lithium ion batteries containing nanoparticles-based electrodes provide high efficiency battery (iv) Flame retardant property by putting a coat over furnitures with carbon nanofibers (v) Certain food products, e.g., vegetable oils, containing nanodrops of components such as vitamins, minerals, and petrochemicals (E) Sports goods: (i) Increasing the strength of tennis racquets by adding nanotubes to the frames which increases control and power when they hit the ball (ii) Reducing the rate at which air leaks from tennis balls by carbon nanopowders

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7. Smart nanomaterials From the past decade on, nanomaterials have had a unique place in the field of science and technology. Recent advancements in nanomaterials lead to new materials with multiple functions. These new materials are prepared because nanosamples have large surface areas, magnetic and photo properties, molar extinction coefficients, variable reduced dimensions, increased absorption, quantum effects, and reactivity with plasmonic properties. The properties of smart nanomaterials are significantly affected by temperature, pressure, pH, moisture, applied field (electric or magnetic), etc. The smart nanomaterials show progress in the field of drug delivery, self-healing, coating applications, display technologies, etc. The most effective application of smart nanomaterials is that they have a great potential toward targeted drug delivery in the medical or biomedical fields. Smart nanomaterials are also utilized in the field of agriculture. These are used on the principle of targeted drug delivery for a particular infection in plants. MNPs are added to agricultural chemicals (agrochemicals), which reduce damage to plants by other plants and also reduces the release of hazardous chemicals to the environment. The effective result can be achieved by right penetration and transport of NPs into the plant body (Gonza´lezMelendi et al., 2008). Till date, smart nanomaterials have not been widely used in the different fields of applied sciences and technology, due to low targeting capacity, poor photo stabilities, side effects on other organs, insufficient cellular uptake, small blood retention, etc. Advanced research in the field of smart nanomaterials is carried out in different parts of the world due to their smart properties. The modern scientific community continuously searches for a smart nanomaterial, which has the property of reversing changes in its physical, chemical, or biological properties.

8. Summary Noble MNPs like NPs of gold, silver, copper, and platinum are extensively used as antibacterial agents, cosmetic ingredients, catalysts, sensors, photodynamic therapy agents, drug delivery agents, etc. Looking into the various advantages of biological synthesis, in particular, using plant extracts over the conventional chemical route, here we report phyto-synthesis and characterizations of gold NPs using an aqueous extract of palm fruits. The samples

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were characterized using UV-visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering, photoluminescence (PL) spectroscopy, and scanning electron microscopy (SEM). A characteristic surface plasmon resonance band found near 550 nm shows the formation of spherical gold NPs. Shifting in the FTIR spectrum shows that interaction occurs between gold NPs and phytochemicals present in the extract. Dynamic light scattering study shows that the hydrodynamic diameter of synthesized gold NPs have 50 nm and the zeta potential is () 15.0 mV. A negative zeta potential reveals that prepared NPs are surrounded by negative charges. Decrease in the emission intensity of plant extracts is a result of energy transfer from phytochemicals to gold NPs. A scanning electron micrograph shows the formation of nearly spherical NPs of 45 nm. Synthesis of gold NPs via a green route possesses excellent therapeutic activity. The synthesized MNPs have effective antibacterial properties against Grampositive bacteria (Staphylococcus aureus) and Gram-negative bacteria (E. coli, Bacillus subtilis, etc.).

References Behera, A., Mohapatra, S., & Verma, D. K. (2019). Nanomaterial: Fundamental principle and application. In Nanotechnology and nanomaterial applications in food, health and biomedical science (p. 343). CRC Press, ISBN:9781771887649. Cheng, Y., Wang, J., Rao, T., He, X., & Xu, T. (2008). Pharmaceutical applications of dendrimers: Promising nanocarriers for drug delivery. Frontiers in Bioscience, 13, 1447–1471. Fu, H. L., Cheng, S. X., Zhang, X. Z., & Zhuo, R. X. (2007). Dendrimers/DNA complexes encapsulated in a water soluble polymer and supported on fast degrading star poly (DLlactide) for localized gene delivery. Journal of Controlled Release, 124, 181–188. Gonza´lez-Melendi, P., Ferna´ndez-Pacheco, R., Coronado, M. J., Corredor, E., Testillano, P. S., Risuen˜o, M. C., … Perez-de-Luque, A. (2008). Nanoparticles as smart treatment-delivery systems in plants: Assessment of different techniques of microscopy for their visualization in plant tissues. Annals of Botany, 101(1), 187–195. Kohler, M. (2004). Nanotechnology: An introduction to nanostructuring techniques. Wiley-VCH. Li, Y., Cheng, Y., & Xu, T. (2007). Design, synthesis and potent pharmaceutical applications of glycodendrimers: A mini review. Current Drug Discovery Technologies, 4, 246–254. Pardhi, D. M., Karaman, D.Ş., Zhang, Q., Satija, S., Mehta, M., Charbe, N., Carron, P. M., Tambuwala, M., Bakshi, H. A., & Behera, A. (2020). Anti-bacterial activity of inorganic nanomaterials and their antimicrobial peptide conjugates against resistant and nonresistant pathogens. International Journal of Pharmaceutics, 586, 119531. https://doi.org/ 10.1016/j.ijpharm.2020.119531. Tomalia, D. A. (2004a). Birth of a new macromolecular architecture: Dendrimers as quantized building blocks for nanoscale synthetic organic chemistry. Aldrichimica Acta, 37(2), 39–57. Tomalia, D. A. (2004b). Dendrimer as quantized building blocks for nanoscale synthetic organic chemistry. Aldrichimica Acta, 37(2), 39–57. Wiener, E. C., Brechbiel, M. W., Brothers, H., Magin, R. L., Gansow, O. A., & Tomalia, D. A. (1994). Dendrimer-based metal chelates: A new class of magnetic resonance imaging contrast agents. Magnetic Resonance in Medicine, 31, 1–8.

CHAPTER 20

Bioactivity prospection, antimicrobial, nutraceutical, and pharmacological potentialities of Carica papaya Abdul Ghaffara, Bushra Munirb, Muhammad Jahangeerc, Mehvish Ashiqd, Sarmad Ahmad Qamare, and Bilal Ahmadf a

Department of Biochemistry, Government College University, Faisalabad, Pakistan Institute of Chemistry, University of Sargodha, Sargodha, Pakistan c Food and Biotechnology Research Center, PCSIR Laboratories Complex, Ferozpur Road, Lahore, Pakistan d Department of Chemistry, The Women University Multan, Multan, Pakistan e Department of Biochemistry, University of Agriculture, Faisalabad, Pakistan f Department of Horticulture, MNS-University of Agriculture, Multan, Pakistan b

1. Introduction Papaya is a tropical plant with melon-like fruit, having 3- to 5-inch diameter and a very smooth, thin epidermis, with a green color in unripe fruits and a vivid orange-yellow color in fully ripe fruit, which can range from pale (light yellow) to deep salmon-pink, and a pleasantly sweet flavor (Hussein et al., 2011). Many spherical black seeds are seen in the fruit’s center cavity. Hawaiian papayas, which are small-sized papayas, normally weighing between 1.1 and 2.2 pounds, and Mexican papayas, which are also referred to as large papayas and can be more than 10 pounds weight, are the two vital kinds of papayas. Papaya fruits that are ripened are abundant in enzymes, iron, calcium, amino acids, vitamins, and other nutrients. Papaya protein is very easily digested, and the fruit is normally eaten raw, but can also be processed into pickles, juice, preserves, sherbets, jellies, or cooked like cucurbits. Each civilization developed its own distinct native health practices for the treatment of different diseases. Traditional systems of medicine are considered the world’s oldest work, and represent a companion that has served long in the fight against diseases alongside living a healthy, fit lifestyle. Herbal medical systems have been proposed and constructed to attain good health and treat diseases and disorders since time immemorial via trial-and-error experimentation and experiential opinion (Karunamoorthi et al., 2012). Antiviral and Antimicrobial Smart Coatings https://doi.org/10.1016/B978-0-323-99291-6.00020-7

Copyright © 2023 Elsevier Inc. All rights reserved.

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According to the definition given of the term nutraceutical by its instigator, Dr. Stephen DeFelice, it may be understood as “Food offers many health and medical advantages when we go through the treatment and preventive measures of diseases” (Brower et al., 1998). Plants have long been used as agents of phytotherapies for treatment and prevention of a variety of illnesses. The pharmaceutical industry, on the other hand, has thrived in the modern period by embracing current scientific procedures and revolutionary biotechnological tools, and has developed a number of lifesaving treatments that provide rapid recovery (Brower et al., 1998). In recent years, health-conscious individuals have become aware of the chronic side effects and long-term health risks associated with conventional drugs. This has prepared the way for a surge in interest in nutraceutical plants that has never been seen before (Zeisel, 1999). Nutraceuticals are immunity boosters and can be used as preventative measures. Papaya is a large fruit crop that has been used as a nutritious and therapeutic plant or herb from ancient times. Although Central America and Mexico are its origin, now, papaya cultivation has spread across the world, including tropical areas. According to prevailing research, people use papaya as a source of food and a phytotherapeutic agent by preparing it in a particular way and then consuming its portions. However, there may be a scarcity of protective and efficient information about Crica Papaya as a medical plant. This examination is an attempt to efficiently demonstrate papaya’s potential in phytotherapies by using data-mining approaches from previous scientific investigations in this regard (Fig. 1), and identifies the responsible active

Fig. 1 Medicinal properties of different parts of Carica papaya.

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components. The study also assesses current potential for upcoming development and manipulation of papaya-based phytotherapeutic compounds that are economical, accessible, and safe (Self et al., 2020).

2. Nutraceutical potentialities of Carica papaya Nutraceuticals, which, according to current rules, cannot be classified or defined as either food or medication, but may be interpreted as food supplements with therapeutic effects for health maintenance (Fig. 2), in particular, for specific pathologic disorders, are one alternative therapy. As a result, a treatment strategy focused on nutraceuticals for health maintenance has sparked a global “nutraceutical revolution.” Stephen DeFelice, MD, organizer and director of FIM—the Foundation for Innovation in Medicine— Cranford, NJ, in 1989 created the expression “nutraceutical,” consolidating the words nutrient and pharmaceutical. Nutraceuticals, as per DeFelice, are “sources of food (or food portions of various varieties) which give restorative or wellbeing benefits, including ailment counteraction or potentially treatment.” Notwithstanding this, there is no administrative definition for the word nutraceutical as it is normal utilized in advertising. Plants have been utilized as phytotherapeutic agents to forestall and treat an assortment of ailments since ancient times. Here, we need to know the difference between the essential terms “phytochemical” and “nutraceutical.” in essence, the term nutraceutical refers to a substance that is part of a food, or the food itself, which provides clinical and medical benefits in prevention or

Fig. 2 Contribution of nutritional and pharmacological sciences to nutraceutical development.

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treatment of sickness, whereas the term phytochemical refers to nonsupplement bioactive plant substances found in natural products, grains, vegetables, and other plant food sources that appear to have a connection with lower risk of major ongoing disease. The following are major nutraceutical attributes of Carica papaya.

2.1 Antimicrobial properties 2.1.1 Anthelmintic property There is a long tradition of utilizing plants that are therapeutic, and some of these plants have been exposed to experimentation so that their anthelmintic action can be demonstrated. Papaya is one of the plants that has been considered to be helpful against roundworm in humans (Starley et al., 1999). The product of the papaya is utilized to treat parasitosis and contaminated skin injuries in papaya-producing locales. Papaya seeds are used to treat intestinal parasites in people and livestock. Papaya concentrates used against various helminths and Caenorhabditis elegans seem to be powerful agents in vitro (Werner & Smith, 1992), and the in vivo viability of papaya latex in combating nematodes of the rat gastrointestinal tract—the Heligmosomoides polygyrus—has been illustrated. Papaya Latex got from natural resources was demonstrated to have anthelmintic action in combating the parasitic disease in chickens known as Ascaridia galli in a new trial examination. In infected mice, an investigation was done to evaluate the activity of papaya latex’s anthelmintic measures in combating Heligmosoraoides polygyrus. 2.1.2 Bacteriostatic property Antibacterial activities are found in the greenish unripe leaf extract of papaya, while the dry and brownish samples’ extracts have the best blood purifier and energizer qualities (2015). On account of its antibacterial properties, the green, unripe papaya offers restorative potential. It eliminates microbes from the digestive tract (Ayoola, 2010). The yellowish (ripening) papaya leaf can be used to combat aging diseases, while the brown leaf can be utilized to purge the body. Nasal blockage can be mitigated by taking the seeds of ripe papaya natural product as a dose for treatment (Awah et al., 2017). 2.1.3 Antitrichomonal activity Trichomonas vaginalis is a flagellated protozoan that parasitizes the human urethra, vagina, and prostate gland, causing trichomoniasis. Inflammatory

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illness, infertility, cervical distortion, epididymitis, prostatitis, urethritis, cervicitis, and vaginitis have all been connected to this parasitic disease. Vaginal or urethral discharge, dysuria, pruritus, stomach pain, and edema or erythema are the most well-known clinical signs of trichomoniasis (Childers et al., 2014). T. vaginalis may likewise assume a critical part in the spread and acquisition of HIV infection. Action antagonistic to trichomonas against T. vaginalis was tried utilizing methanolic extracts from 22 Mexican healing spices. C. papaya and Cocos nucifera had the finest activity against the trichomonas among the plants tested, having IC50 above 5.6 and 5.8 g/mL, respectively (Enard et al., 2009).

2.2 Pharmaceutical properties 2.2.1 Anticancer activity Individuals on the Gold Coast of Australia consume papaya leaf juice for its supposed disease-fighting properties, with various accounts of effective cases showing up in different media. Papaya leaf extracts have likewise been used as a native medication for treatment of diseases, including malignant growth and irresistible infections (Miyaki et al., 2010). Papaya leaf tea or concentrate has is famed as being a tumor-killer (O’Doherty et al., 2018). A study was carried out to determine the anticancer action of the aqueous concentrate of papaya leaves against different cancer cell lines, in addition to its potential immunomodulatory impacts, and to endeavor to distinguish the active constituents and establish the critical development of the inhibitory action of papaya extricate on tumor cell lines to research possible applications of papaya in disease treatment (Takagi, 1992). Substances recently found to have antitumor activity in papaya leaves include tocopherol, lycopene, flavonoid, and benzyl isothiocyanate (van Breemen & Pajkovic, 2008). As indicated by a report, no clinical or animal model studies have been established yet, with just seven cell cultures having undergone in vitro-based examination, producing the results that C. papaya concentrates may influence improvement against several sorts of disease related to these cell lines. Beside this, a few examinations have found that some compounds are present in minor quantity, having a synthetic nature and showing bioactivity, with anticancer potential. These discoveries plainly demonstrate the fact that papaya has antitumoral affects against an assortment of malignancies, suggesting that it very well may be valuable in disease treatment strategies (Basu et al., 2013).

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2.2.2 Antidiabetic activity The earliest-known disease throughout the world related with metabolism is diabetes mellitus, and a flood of interest in plant medicines for diabetic patients has started in recent years. The main challenge in diminishing the dangers associated with diabetes and its results is the problem of managing hyperglycemia. There have been many instances of Carica papaya leaf being utilized to treat dengue fever and intestinal sickness (Rene et al., 2014). As per examinations, ripe brown papaya parts significantly lower plasma glucose levels in individuals and type 2 diabetic patients (Yeboah et al., 2012). The application of Carica papaya chloroform concentrate to diabetic rodents brought about a decrease in blood glucose, fatty oils, and transaminases. The aqueous concentrate of Carica papaya significantly diminished blood glucose levels (Owoyele et al., 2008) in streptozotocin-induced diabetic rodents. It additionally brings down the degrees of cholesterol, triacylglycerol, and aminotransferases in the blood. Although in diabetic rodents, after treatment, their low level of insulin in plasma did not change, the level in nondiabetic rodents increased considerably (Pan
es et al., 2016). In nondiabetic treated animals, pancreatic cells of islet cells have a distinctive nature, while in diabetes-free rodents, the concentrates supported islet indication and recovery, as well as cell size preservation (Mun˜oz et al., 2010). Carica papaya decreased hepatocyte disturbance, in addition to the development of glycogen and lipids, in the livers of diabetic rodents. Taking everything into account, the aqueous concentrate of C. papaya affected diabetic rodents, as well as further developing their lipid profile. These discoveries support Carica papaya’s potential for reducing the occurrence of diabetes, however, more research with human subjects is required. In tests on lab animals, Carica papaya seed extracts were found to have antidiabetic properties. In Wistar rats, the hypoglycemic and hypolipidemic actions of Carica papaya Linn aqueous seed extract reduced FBS (fasting blood serum), serum fatty acid (triglyceride) (Sasidharan et al., 2011), TC (total cholesterol), LDL-C (low-density lipoprotein cholesterol), and, exceptionally, VLDL-C (very-low-density lipoprotein cholesterol) in a portionsubordinate form (Ahmad et al., 2011). Phytochemical examinations uncovered the existence of reducing sugars, anthocyanosides, anthraquinones, tannins, saponins, flavonoids, and alkaloids in concentrates (Owoyele et al., 2008).

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2.2.3 Antioxidant activity The oxidation of oxidizable compounds such as fat can easily be stopped or slowed by molecules known as antioxidants, and the function of these antioxidants is reliant on their ability to scavenge free radicals that are reactive in food (Kadiri et al., 2016). The ethyl acetate and n-butanol fractions in papaya seed cause them to be high in total phenolics and total flavonoids, which is why they have antioxidant properties. Annona squamosa seed’s antioxidant activity is, when compared to organic extract of seeds of C. papaya, shown to have efficient antioxidant properties with maximum content of phenol (Sriram, 2012). The hydroxyl free radical scavenging capabilities and DPPH of the ethyl acetate fraction were found to be the strongest, surpassing those of sodium benzoate and ascorbic acid, respectively. The ABTS and radical scavenging activity of n-butanol was found to be highest. The ethyl acetate and n-butanol fractions had stronger antioxidant activity than the ether, water, petroleum, and ethyl acetate fractions (Kadiri & Olawoye, 2016). Not only did the ethyl acetate and n-butanol fractions have stronger antioxidant activities than the ethanol fractions, water, ether and petroleum, but they similarly had maximum superoxide anion and hydrogen peroxide radical scavenging activities as compared to the other extract portions. The antioxidant properties of the ethyl acetate and n-butanol fractions are attributable to their high levels of total phenolics and total flavonoids. The portion of ethyl acetate was subjected to column chromatography, which produced two phenolic compounds: p-hydroxybenzoic acid and vanillic acid, both of which have effective antioxidant properties. The researchers concluded that papaya seeds and their chemicals could be effective as naturally occurring antioxidants. These findings were consistent with those of following investigations. Total phenolic content (TPC), total antioxidant activity (TAA), and total flavonoid content (TFC) was measured of several papaya tree components, including young leaves, seeds, unripe fruits, and ripe fruits. Unripe fruit had the maximum antioxidant action (90.67
0.29%) in the crocin bleaching assay, followed by its seeds, leaves, and ripe fruits, in that order. The young leaves had significantly stronger scavenging effect as compared to other plant segments, and the dose necessary to reduce DPPH control solution absorbance by 50% (EC50) mathematically was 1.0
0.08 mg/mL. For unripe fruit, ripe fruit, and seeds, EC50 standards were 4.3
0.01 mg/mL, 6.5
0.01 mg/mL, and 7.8
0.06 mg/mL, respectively. Remarkably, it’s observed that the young

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leaves contain the peak antioxidant concentration (424.89
0.22 mg GAE per 100 gram dry weight and 333.14
1.03 mg). There was no evidence of a link between TPC and TFC or carotene bleaching action. In conclusion, antioxidants were highly notable in the order of young leaves>undeveloped fruit>ripe fruit>seed, and more research was recommended for the segregation and documentation of phytoconstituents responsible for antioxidant action, taking into account all of the parameters calculated (Maisarah et al., 2013). The best source of antioxidants are C. papaya seeds, with medicinal implications and the potential to be used in the food industry and pharmaceutical industry (Li et al., 2020). Polyphenols are abundant in the leaves, which are thought to be responsible for C. papaya’s antioxidant qualities. In a study, the polyphenols were extracted in an augmented process of extraction from leaves. Water-to-leaf ratio, temperature, and extraction time all had an impact on the phenol quantity that was extracted, influencing the scavenging and overall antioxidant activity (Maisarah et al., 2013). The best extraction parameters for this system were 70°C for 20 min and a water-to-leaf ratio of 100:7.5 mL per gram. Water was declared safe for extracting components of antioxidants and was recommended for extracting the polyphenols from the leaves. In addition, a scheme for making crude powder from the leaves was discovered. 2.2.4 Rejuvenation activity The body’s normal process for reestablishing dermal and epidermal tissue is wound mending, otherwise called wound fix or restoration. (Singh et al., 2014). Concentrates from green papaya epicarp, which were utilized in this research, were discovered to be viable in wound mending. It is found that applying the unripe natural product papaya to ongoing skin ulcers improves desloughing, granulation, and recuperating. Anuar et al. (2008) discovered that Skin treatment with papaya mush mash containing papain and chymopapain demonstrated utility in desloughing necrotic tissue and preventing contamination. 2.2.5 Contraceptive activity The oral administration of a fluid suspension of papaya seed powder causes sterility in 40% of treated male rodents while having no impact on genital organ weight, spermatogenesis, or spermatozoa motility (Das & Suganthan, 2010). Albino rodents and mice treated with fluid concentrate of Carica papaya seeds at 0.5 mg and 5 mg per kilogram body weight per day for 7 days lost their fertility totally. In test animals, the papaya seed extract gets a range

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of reactions depending upon the amount, time, and method of administration. Dry papaya seed powder, suspended in refined water and administered orally to hares, brought about absolute infertility in the test subjects (Cardamone & Puri, 1992). Oral organization of roughly developed papaya seeds at 100 mg per kilogram body weight for about two months brought about debasement of the germinal epithelium and germ cells, a loss in quantity of Leydig cells, and the occurrence of vacuoles in tubules, as per a significant report (Goyal et al., 2010). Another investigation determined that oral administration of the aqueous extract of C. Papaya (Linn.) seeds at doses including 50 mg per kilogram, 100 mg per kilogram, and 800 mg per kilogram body weight leaves juice in patients with dengue fever, in a randomized controlled preliminary of 228 patients, affirmed a huge expansion in dengue fever side effects. The ordinary progression of the estrous cycle was disturbed, yet ovulation and the measure of ova shed were unaffected. In adult male hares, the prophylactic adequacy and reversibility of a chloroform concentrate of papaya seeds were investigated. Following 75 days of treatment, sperm count started to drop, (Loftus et al., 2005) arriving at extreme oligospermia (under 20 million/ mL) and uniform azoospermia after 120 days. After 45 days, sperm motility and viability were considerably reduced, and after 75 days, they were reduced to less than 1%. Spermatogenesis had halted past the degree of spermatocytes, as indicated by histology of the testis. The hematology and serum natural chemistry boundaries revealed no indications of damage. Impacts were comparable between both dosage schedules (Groups II and III) and returned to normal within 45 days of treatment being halted. Papaya seeds are said to have emmenagogue, abortifacient, and antifertility impacts. Day by day oral doses of benzene and alcohol extracts (20 mg per kilogram BW for 30 days) had no impact on body or regenerative organ loads, nor did they weaken liver or kidney function in rodents. Infertility and unusual estrous cycles were brought about by aqueous concentrates (1 mg per kilogram BW for seven and fifteen days) and benzene extracts administered orally to female rodents. The sperm motility of male rodents administered ethanol seed extract orally (10 or 50 mg/day for 30, 60, or 90 days) or intramuscularly (0.1 and 1.0 mg per day for 15 and 30 days) showed a decline. Testis mass and sperm count were similarly decreased by the oral dosages. Male rodents’ fertility was comparatively decreased in investigations utilizing aqueous seed extracts. After the treatment was stopped, both male and female rodents’ fertility returned to normal within 60 days (Cornel et al., 2012).

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There is no danger related to eating fresh, ripe papaya products when pregnant. Unripe and semiripe papaya, however, might be risky to consume during pregnancy, as this may cause unnatural birth cycle in pregnant women who are vulnerable. When contrasted with ripe papaya, the unripe natural product contains significantly more latex. Raw papaya latex incorporates papain and chymopapain, as per Vij and Prashar (2015). The papain appears to breakdown a protein or proteins that are necessary for joining the recently formed egg to the uterine wall (Windhausen et al., 2009). 2.2.6 Renal activity The nephroprotective role of ethanolic concentrates of papaya seed and pumpkin seed has been tested, and the outcomes affirmed that ethanolic concentrates of the two seeds showed protection against cisplatin-instigated nephrotoxicity. In cell reinforcement studies, for example, nitric oxide scavenging action and lipid peroxidation in the kidney likewise confirmed the nephroprotective role of ethanolic concentrates of the two seeds. Research was carried out to determine the defensive impact of aqueous seed concentrate of C. Papaya L. on gentamicin-instigated hepatotoxicity and nephrotoxicity, and the outcomes revealed that administration of the aqueous concentrate before gentamicin exposure forestalled extreme biochemical changes and liver and kidney disturbances (Zhao et al., 2010). An intriguing examination showed the diuretic impacts from root concentrates of C. Papaya administered orally to rodents at a dose of 10 mg/kg, which showed a 74% increment in urine formation, which was tantamount to the effect of a comparable dose of hydrochlorothiazide. Tea is produced using dried and crushed leaves of Carica papaya, and the leaf decoction is utilized as a laxative for ponies and to treat genitourinary system issues (Zhao et al., 2010). Ethanolic extract of Carica papaya leaves was effective in removing the bad effects of mercuric chloride, which causes nephrotoxicity in the kidney, by reducing the oxidative stress. Flavonoids and alkaloids present in Papaya leaves extract show nephroprotective activity. The most specific biomarkers are semaphorin 3A, KIM 1, and NGAL for renal damage (Folegatti et al., 2020). 2.2.7 Effect on the gastrointestinal track Among the essential drivers of stomach ulcers and related issues are an assortment of substances including dietary components, microorganisms, and medications. Plant compounds have an antiulcer impact, although the specific mechanism isn’t completely understood. The counter-ulcerogenic

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properties of C. Papaya extricate were tested on headache medicine that instigated ulcers in rodents, and the outcomes uncovered that C. Papaya might apply its gastroprotective impacts by free radical scavenging activity (Somanah et al., 2012). When an investigation was directed to assess the gastroprotective impacts of aqueous Carica papaya seed extract on ethanolactuated gastric ulcer in male rodents, the outcomes uncovered that the concentrate shielded the gastric mucosa from the ethanol impact and fundamentally diminished gastric juice volume and acidity in a dose-dependent way when contrasted with the control group. Papain extract is utilized to treat an assortment of gastrointestinal and intestinal issues. In areas where papaya trees grow, their natural product is utilized as a traditional remedy for gastrointestinal issues. Be that as it may, there has been almost no evidence found with respect to its physiological impact in humans, or evidence of viability. In accordance with these discoveries, Hughes et al. (2013) utilized a double blind, placebo-controlled preliminary treatment plan to examine clinical impacts of the papaya preparation Caricol, and found that it adds to the protection of stomach-related physiology. It mitigates an assortment of significant issues, including blockage, indigestion, and irritable bowel syndrome (IBS). The green papaya leaf extract helps in the therapy of conditions like ongoing acid reflux, overweight and weight, arteriosclerosis, hypertension, and heart weakness (Ayoola, 2010). The antiulcerogenic and cell-reinforcement actions of fluid concentrate of C. papaya seed action against indomethacin-incited peptic ulcer in male rodents researched, and the outcomes revealed that C. papaya seed concentrate altogether improved gastric pH and levels of ulcer hindrance when contrasted with indomethacin-actuated ulcer rodents (Ghoti et al., 2011). 2.2.8 Immunity booster C. papaya leaves extract is ordinarily used to treat intestinal sickness and splenomegaly, and commonly used to treat anemia, which can equally be brought about by jungle fever. With an IC50 of 15.19 g/mL, an oil-ether concentrate of C. papaya seed skin showed huge antimalarial action. This suggests that the papaya seeds contain antiplasmodial compounds that are especially dynamic. Furthermore, Titanji et al. (2008) found that C. papaya leaves and seeds have antiplasmodial action with an IC50 of around 60 g/mL. The members of one community in Ethiopia, as reported by Karunamoorthi and Tsehaye (2012), consume papaya seed powder blended with nectar orally as an antimalarial drug (Azikiwe et al., 2009).

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An investigation was carried out on Plasmodium berghei-contaminated mice to test the antimalarial viability of methanol concentrate of C. papaya seeds. The seed concentrate of C. papaya suppressed jungle fever parasitaemia in an effective manner (P 0.05). These effects are dose-dependent and comparable to those of chloroquine phosphate. These results support the local farmers’ utilization of C. papaya seeds in malaria (Kini et al., 2009). Since this research was carried out on animals, clinical trials in humans are necessary, particularly when C. papaya seeds are not unsafe or harmful (Ayoola & Adeyeye, 2010). When contrasted with the action of ethyl acetic acid and different concentrates of C. papaya leaves, a higher antiplasmodial action was demonstarted with an IC50 of 2.96
0.14 g/mL. Be that as it may, the presence of alkaloids in papaya leaves might clarify why it is often utilized as an antimalarial medicine. Nonetheless, there is yet to be a detailed report on papaya’s phytochemistry and method of activity relating to its antiplasmodial action. Consequently, more pharmacological and phytochemical studies on papaya are required to deal with the questions about its viability and safety issues. 2.2.9 Antimalarial activity Almost 50% of the global population is believed to be in danger of suffering intestinal illness, with sub-Saharan Africa being the most hazardous district. Jungle fever is often highlighted as both an ailment and a cause of poverty. Jungle fever is pervasive in nature in the most deprived parts of world, which is understandable (Karunamoorthi, 2014). It tremendously affects ill-health and mortality all through the world’s tropical and subtropical environments, especially in Asian and sub-Saharan African nations. Pregnant women and infants younger than 5 years are often the primary victims. Moreover, on the grounds that the main disease transmission season coincides with planting and harvesting seasons, it is a considerable obstruction to economic development. A study reported artemisinin-based combination therapies (ACTs) for the current intestinal disease control system (Karunamoorthi et al., 2012). In any case, the prognosis of vector-borne diseases, including jungle fever, has been categorized by a dangerous atmospheric deviation’s simultaneous impacts, pesticide opposition, and therapeutic obstruction. Since intestinal sickness is a significant general medical condition in assetpoor regions with unsound healthcare systems, it often proves impossible to meet the healthcare requirements of poor people. Existing effective antimalarial ACTs (Karunamoorthi & Bekele, 2009) are, much of the time,

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costly and unavailable, and, furthermore, ongoing research highlights the rise of multidrug-resistant strains against ACTs. 2.2.10 Antidengue agent Dengue fever, often called break-bone fever, is one of the most contagious diseases caused by infections worldwide. Four connected viruses (Flaviviridae) are basically the major cause of this disease and it is spread by Aedes aegypti, Aedes albopictus, and mosquitoes by their infectious bite. Dengue fever is regarded as a serious public health problem across the world, with more than 2.5 billion people—representing about 40% of the world’s population—thought to be at risk of contracting the disease. Every year, the World Health Organization calculates that there may be 50–100 million cases worldwide. Only nine nations are known to have suffered significant dengue epidemics prior to 1970. As a result, it is obvious that dengue fever is developing and resurging as a worldwide health hazard in the current changing environment, which is cause for serious concern and necessitates the development of creative solutions and techniques in terms of vector control, surveillance, and case management. Dengue fever is mostly controlled via a mix of vector control, personal protection, and drug-assisted treatment for sickness. However, there are currently no specific medications or a potentially effective vaccination against the dengue virus (DENV). As a result, identifying an economical and effective agent to be used in antidengue medication is essential. According to topical research, C. papaya leaf juice may be effective in the treatment of dengue virus infections. As a result, Yeh et al. (2012) used an in vitro hemolytic experiment to examine the membrane stabilization capacity of papaya leaf extracts. Two milliliters of blood were taken and used in the assays from healthy volunteers and patients confirmed by serological section to have an existing dengue infection. Fresh papaya leaves were rinsed in distilled water, then crushed, and the extract taken out with 10 mL of cold purified water at three different maturity stages (mature, immature, and partly matured). Heat- and hypotonicity-induced hemolysis experiments were used to explore membrane stabilizing properties. Heat-induced hemolysis was significantly reduced in extractions of papaya plants from all three maturity stages when linked to controls (p 0.05). In vitro, papaya leaf extracts of all three maturity stages inhibited greater than 25% of dengue disease processes, causing biological membrane destabilization at concentration of 37.5 g/mL, suggesting that they may have a therapeutic consequence on dengue disease

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progressions triggering biological membrane weakening. Ahmad et al. (2011) did a trial in Pakistan to see if C. papaya leaf extracts were effective in fighting dengue fever virus (DFV) in patients of 45 years of age. For 5 days, 25 mL of liquid extract was administered twice a day, in the morning and evening. Neutrophils (NEUT), platelet counts (PLT), and white blood cells (WBC) were all measured in the patients’ blood samples prior to the extract administration. Following administration, re-examination of blood sample was performed, and it was discovered that PLT, NEUT, and WBC levels had all significantly increased. The solvent extract of C. papaya leaves shows a strong probable efficacy in fight against dengue fever virus, according to blood sample analysis. However, it is crucial to highlight that this is only a preliminary analysis, and extra research is required to isolate and identify the accountable bioactive chemicals, as well as their method of action, in order to effectively battle dengue fever and other viral infections in the future (Ahmad et al., 2011). 2.2.11 Obesity control Antiobesity effects are confirmed in a wide range of plants and their products, including C. papaya. A study was conducted to evaluate the antiobesity potential of solvent fruit extracts of C. papaya L. on obese mice that were fed a (HFD) high-fat cafeteria diet, and the results revealed that the BMI, body weight, and organ weight of the kidney, liver, and spleen were significantly lower in treated groups as compared to HFD control animals. Furthermore, the study’s findings confirmed that total cholesterol, serum glucose, VLDLcholesterol, LDL-cholesterol, and triglycerides were significantly lower in the treated groups, whereas HDL-cholesterol was significantly higher among treated groups in a dose-dependent manner when compared to the HFD group (Fig. 3) (Islam et al., 2015). 2.2.12 Sickle cell anemia Papaya is said to be used in folk medicine as an herbal cure for sickle cell anemia. It has been stated in previous studies that papaya plant was used in traditional medicine as an agent against sickle cell anemia, and it has been proved by experimentation that papaya’s antioxidant composition is indeed a strong agent against sickle cell anemia (Imaga et al., 2010). Ethyl acetate is responsible for the protection of hemoglobin from sickling and, in recent research, it has been concluded that there is a huge amount of this compound in unripe papaya extracts (Akubor, 2011).

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Lipoprotein lipase

Fig. 3 Diagrammatic representation of the mechanism of action of the antiobesity property of Carica papaya as observed in HFD-fed rats (SOD: superoxide dismutase, MDA: malondialdehyde, HDL: High density lipoprotein, GR: glutathione reductase, TNF-α is abbreviated as tumor necrosis factor-α, and LDL is abbreviated as lowdensity lipoprotein, TC: total cholesterol, MG: monoglyceride, TG: triglyceride, FA: fatty acid, HF: high fat, IDL: intermediate-density lipoprotein). Reprinted from Od Ek, P., Deenin, W., Malakul, W., Phoungpetchara, I., Tunsophon, S. (2020). Anti obesity effect of Carica papaya in high fat diet fed rats. Biomedical Reports 13(4), 1-1. Open access article distributed under creative common license.

2.2.13 Heart diseases Many carbohydrates, such as glucose and starch, are abundantly present in unripe papaya pulp (Imaga et al., 2010), as well as medicinal cardenolides and saponins, which are used to treat congestive heart failure (Athmaselvi et al., 2014). 2.2.14 Hepatoprotective activity The liver is the largest solid internal organ in the human body and liver disease is a significant cause of mortality all over the world. The use of solvent leaf extract of Carica papaya as medicine to treat several diseases of the liver is common in the southwestern zone of Nigeria and other regions of Africa. Several studies have lately proved the effective use of these plants’ leaves and seeds in the treatment of liver disease and in oral health at safe levels. Carica papaya leaf extracts were found to reduce drug-induced hepatotoxicity in investigations by Ji et al. (2013). In both experimental models, hepatoprotective efficacy of C. papaya leaves were demonstrated by significant reductions in all blood indicators. These extracts significantly reduced the total

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protein level, superoxide dismutase, and glutathione, showing a corresponding reduction in the level of thiobarbituric acid-reactive material and enhancements in liver histology. According to test results, Carica papaya leaves exhibit hepatoprotective effects, which may be due in part to their antioxidant capabilities. A study has been conducted to find the hepatoprotective aspects of C. Papaya and vitamin E against carbon tetrachloride (CCL 4)-induced hepatotoxicity, and the results verified that both C. Papaya and vitamin E demonstrated considerable hepatoprotection against CCL4-generated hepatotoxicity (Mohammed & Alipour, 2014). Pretreatment with medium and high doses of C. Papaya caused hepatotoxicity, according to other findings, but C. Papaya caused more prominent alterations in ALP levels than that of vitamin E. The goal of the study was to see if C. papaya leaves could protect the liver from ethanol and antitubercular drug-induced liver damage. The analysis showed that hepatoprotective action was observed from the reduced levels of all serum indicators in both of the experimental animal models fed with plant extracts (Adeneye & Olagunju, 2009).

3. Challenges and concluding remarks Papaya is, in fact, a traditional herbal medicinal plant. Its nutritional and therapeutic properties are well known all over the world. As an ethnomedicine, it has been taken as a nutraceutical to treat a range of disorders/illnesses, including cancer. It has also been used for different kinds of purposes in the past, including as a meat tenderizer, contraceptive, acne medicine, menstrual pain reliever, and hunger stimulant. The results of this investigation show that papaya plant components have potent antitrichochramal, antiplasmodial, anticancer, and antidengue effects. Papaya also has antibacterial, antiparasitic, antiinflammatory, antidiabetic, and contraceptive properties, as well as the ability to treat sickle cell anemia, heart disease, indigestion, and gastrointestinal problems. However, because the mechanisms of action aren’t well understood, further clinical research is needed to figure out what’s going on. This examination also points to papaya as one of the best nutraceutical plants. As a result, massive public health awareness efforts regarding the new link between food and optimal health must be launched through print and electronic media. There have been few investigations on the possible toxicity of papaya products, despite their benefits. The current investigation revealed that, except for infertility, the literature evaluated showed no evidence of any

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negative effects from consuming papaya extracts, fruit, or latex. Papaya leaves and roots, on the other hand, contain cyanogenic glucosides, which produce cyanide. Tannins can also be found in the leaves. Both chemicals have the potential to cause unpleasant responses at high concentrations. Breathing in papaya powder (which comprises enzymes papain and chymopapain) can also cause allergic reactions (Windhausen et al., 2009). Public health specialists and nutritionists can use social media to promote the advantages of papaya, both as a food and as a drug for treatment of different diseases. Furthermore, the protection and long-term use of edible plants and their biodiversity for food manufacturing and nutraceuticals at this time is considered a major concern. As a result, adequate preventative measures must be adopted to ensure papaya preservation and cultivation, which is also an excellent socioeconomic strategy. The authors expect that it will be tremendously beneficial to the advancement of traditional medicine in undeveloped countries as well.

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Index Note: Page numbers followed by f indicate figures and t indicate tables.

A Active packaging systems, 456–457 Adhesin adherence, 74–75 afimbrial adhesin, 75–76 colonization, 74 fimbrial adhesin, 75 intermolecular linkages, 75 internalization, bacteria, 76 polysaccharide adhesin, 76 protein adhesin, 75–76 Tir effectors remodeling, 76 Antiadhesive surfaces, 562, 563f electrostatic repulsion, 241–242 hydrogels advantage, 243–244 chitosan-based hydrogel, 243 nanoparticle-incorporated hydrogels, 243 mechanism of action, 241–242, 242f passive action polymer, 242–243 steric hindrance, 241–242 superhydrophobic surfaces, 241–242 zwitterionic polymers, 244 Antibacterial coatings applications, 41 chitosan modified 3-bromo-N,N, N-trimethyl propan-1-aminium bromide, 45–46 dual-responsive nanoparticles, 46, 49f HApN (poly(N-isopropylacrylamide) hyaluronan), 45 hydroxyapatite (HA), 41 poly(β-amino ester)-guanidine-phenyl boronic acid (PBAE-G-B) polymer, 46 polydopamine ferrocene-functionalized TiO2 molecules, 46–47 polydopamine nanoparticles (PDANPs), 41–45 zeolitic imidazolate framework-8 (ZIF-8), 46

Antibiotic susceptibility tests (ASTs), 113–114 Antifouling, natural cleaner, 268–269 Antifouling coatings, 228–229 Antifungal coatings, 48 Antigraffiti coatings, 227–228, 228f Antiicing surfaces, 224–225 Antimicrobial agent incorporated polymers active agents, 319 adhesive coating, 323–324 antibacterial coating, 325 antimicrobial composite coating, 320–322 antimicrobial film, 327–328 antioxidant and antimicrobial coating, 328 bifunctional (antifouling and antimicrobial) polymer brushes, 320, 321–322f coating on leather surface, 327 dual functional coating, 324–325 dual-layer antimicrobial coating, 326–327 edible paper coating, 319–320 environmentally benign antimicrobial coating, 324 hybrid antimicrobial coating, 326 switchable, smart antimicrobial, and antifouling coating, 322–323 Antimicrobial coatings antimicrobial surfaces, 230–231 applications, 230–231 fouling, 230–231 functionalized surface, 231–232 patterned antimicrobial surface, 231, 231f superwetting surface, 232 Antimicrobial lipids (AML), 193 Antimicrobial peptides (AMPs), 191–193 Antimicrobial polymer coatings applications, 310–311 biomedical industry AMPTMA, PEGDMA, and Q-PEI-MA catheter, 344

607

608

Index

Antimicrobial polymer coatings (Continued) antiadhesive and biocidal properties, 340–341 dual-functional polymers, 347–348 hybrid antimicrobial coating, 348 PDMS catheter, 344, 345f PDPA-b-PMPC copolymer brushes, 346 PEI-coated catheter, 340–341 PEI-PGMA and zwitterion monomer SBMA, 342–343 protein-engineered polymers, 341–342 QAS-based coating, 343–344, 345f QBEst and QBAm coating, 342 SS-TA-p(MPC)/p(Lys) coating, 341–342 supramolecular assembled lysozyme protein material, 346–347 categories, 310 composition, 309–310 defense mechanism, 310–311 factors to be considered, 310 food industry CS-GO films, 333–334 MC/HPMC film, 334–335 microbial growth, 329 natural polymer film, 331–333 PBAT and chitosan nanofibers (CS-NF), 329–330 pH-responsive MPN@DBS-NP and MPN@SNC particles, 332f, 335–336 PP-PEI-SMA-PEI polymer coating, 330–331, 332f PS-b-PTTBM film, 330–331 PSS lysine conjugate, 329 sodium alginate, carvacrol, and methyl cinnamate, 333 zein film, 332f, 334 limitations, 310–311 representative examples, 349–350t synthesis and design, 310 antimicrobial agent incorporated polymers, 319–328, 321–322f functionalized polymers, 311 polymerization techniques, 311

structurally modified polymers, 311–319, 318f textile industry gallic acid stabilized silver nanoparticles (GA@AgNPs), 339–340 Jeffamine-ED-2003, 336–337 multilayered cationic and anionic homopolymer (CHP/AHP)5, 337–339, 338f N-halamine moiety, 336 p(VBCHAM-co-AAx) and p(SSAmC16-co-GMAx) copolymer, 339 synthetic and natural polymers, 336 vinyl acrylic copolymer emulsion, 340 Antiviral coatings anti-COVID mask, 47–48 bacterial infections, 47 copper-based coatings, 47 HCoV-229E, 47 photocatalyst-coated TiO2, 48 polycaprolactone (PCL) spun masks, 47–48, 50f quaternary ammonium compounds (QACs)-based coatings, 47 SARS-CoV-2, 47–48 Automated identification technique, 114 MALDI-TOF, 121–128, 128f MicroScan, 130 Phoenix, 129–130 VITEK 2, 128–129 Auto-responsive coatings. See Smart coatings

B Biocidal release mechanism, 246–247 Bioengineered metal-based antimicrobial nanomaterials advantages, 490–492 antibiotics resistance, 490–492, 497–500 applied methods, 490–492 biological method, 490–492 bottom-up and top-down approach, 490–492 chemical method, 490–492 vs. conventional antimicrobials, 492–493

Index

copper nanomaterials, 509–511 future research, 523–524 gold nanomaterials antibacterial and antifungal activity, 506–507 biosynthesis, 506–507 hydrogen ion extrusion, 507–508 Kirby-Bauer technique, 506–507 microbial enzymes interaction, 507–508 Gram-negative and Gram-positive bacteria, 497–500 green nanotechnology advantages, 493–494 eco-friendly production, 493–494 microorganisms, 493–494 using algae, 497 using bacteria, 496–497 using fungi, 495–496 using plants, 494–495 mechanisms of action, 497–500 metallic nanoparticles (MNPs), 489–490 metal oxides nanoparticles, 490–492 microbe inhibition assessment, 497–500 nanostructures, 489 physical method, 490–492 quantum dots, 490–492 selenium nanomaterials, 508–509 silver nanomaterials antibacterial capabilities, 500–501 antifungal properties, 505–506 coated craft papers, 516 mechanisms for antibacterial action, 501–503 oligodynamic impact, 505–506 phyto-silver nanoparticle synthesis, 503–505 protein interaction, 501 toxicity, 500–501 surface characteristics, 490 for surface coatings capping/binding and reducing agents, 523 coated craft papers, 516 CuONP coated fabrics, 517, 517f, 522, 522f

609

gold nanoparticles coated cotton fabrics, 519–522, 521f herbal silver nanoparticles coated cotton fabrics, 517–519, 520f soluble soybean polysaccharides (SSPS), 516 surfactants, 516 textiles, nanoengineered functional, 515–516 tissue paper, 522–523 synthesis, 490–492 zinc nanomaterials, 511–515 Biofilm adhesion to biotic and abiotic surfaces, 87–88 anti-microbial coatings, 88–90 bacterial attachment, 102–103 beneficial process, 142 biofilm-dependent friction, 88–90 biofouling, 87–88 biological control methods antimicrobial lipids (AML), 193 antimicrobial peptides (AMPs), 191–193 enzymes, 181–184 nanoparticles, 188–191 phages, 184–185 QS system inhibitors, 185–188 cell density, 141–142 chemical control methods hydrogen peroxide, 180 mechanism of action, 179 ozone, 180 peracetic acid, 180–181 sodium hypochlorite, 180 components, 16 diseases, 14–16 ensnarling behavior, 88–90 extracellular polymeric substances (EPS), 141 food processing industry control of biofilm formation, 166 dairy industries, 164 factors influencing biofilm, 161–164 film forming properties, 160 fish processing industry, 164–165 foodborne pathogens, 161

610

Index

Biofilm (Continued) food spoilage and deterioration, 160–161 locations for biofilm formation, 160 meat industry, 165 poultry industry, 165 ready-to-eat (RTE) food industry, 165–166 formation, 3–4, 14–15 dispersion, 146 irreversible adherence, 144 maturation, 145–146 microcolony formation, 144–145 process, 142 representation, 147f reversible adherence, 143–144 health-care setting, 88 human health chronic wound infection, 157 cystic fibrosis (CF), 156 device- and nondevice-associated infections, 154–155 endocarditis, 156 immunocompromised patients, 155 medical device-related biofilm infections, 158–160 microbial adherence to biomaterial, 155 osteomyelitis, 157 pathogenicity vs. biofilm formation, 155 periodontitis, 157 rhinosinusitis, 157 surface properties, 156 lifestyle, 142 mechanism of attachment action, 102–103 mechanism of biofilm formation, 15–16, 15f microbial adhesion to solid surfaces, 87–88 microbially induced corrosion (MIC) acid-producing bacteria (APB), 170–171 anaerobic and aerobic corrosion, 168 bacteria, archaea, and fungi, 168 definition, 167–168

EPS production, 167 iron-oxidizing bacteria (IOB), 170 iron-reducing bacteria (IRB), 171 metal-oxidizing bacteria (MOB), 169–170 methanogens, 171–172 mitigation, 174–178 oil and gas industry, 168 oxygen-free MIC mechanisms, 172–173 in presence of oxygen, 173–174 process initiation, 167 sessile bacteria, 167 slime-forming bacteria, 170 sulfate-reducing bacteria, 168–169 sulfur-oxidizing bacteria, 168 microbial species, 141–142 nosocomial infection, 88–90 physical control methods, 178–179 resistance antibiotics failure, 148–149 biofilm-mediated antibiotic tolerance, 146–148 efflux pumps and membrane protein, 154 extracellular DNA (eDNA), 152 genetic material exchange, 151–152 growth rate, 149–150 innate and induced resistance, 146–148 mutation, 152–153 oxygen gradients, 149 persister cells, 150–151 planktonic bacteria, 146–148 quorum sensing (QS), 153–154 representation, 147f stress, 152 surface adhesion, 87–88 surface characteristics aqueous medium milieu, 99 biofilm structure, 100–102 cell properties, 99–100 conditioning film, 97–98 hydrodynamics, 98–99 solid-liquid interface initiation, 97 substratum compo sum, 97 surface charge, 90–92, 91f surface roughness, 92–94, 93f

Index

surface stiffness, 93f, 94–97 surface topography, 93f, 94 surface wettability, 92, 93f surface modification, 88–90 ultrasonication, 179 uses, 19–20 viral adsorption, 104 virulence activity, 88–90 viruses growth, 104 Biofire Film Array, 133–134 Bioinspired coatings. See Biomimetics Biomaterials and biomimetics biomimetic synthesis, 24 common biomaterials, 23, 26f musculoskeletal tissue engineering, 23, 26f recent advances active packaging, 54–55 antimicrobial peptides (AMPs), 52, 54–55, 57–58 antimicrobial-resistant (AMR) bacterial infections, 58–59 bacteriostatic and bactericidal surfaces, 54, 56f biocide coatings, 48–49 calcium phosphate coating, 56 copper ions, 53, 58–59 gentamicin, 51 glycerol monooleate (GMO), 58 halloysite nanotubes (HNTs), 53–54 lignin-based polyurethane coatings, 55 lignin with poly(methyl vinyl ether-comaleic acid) and poly(ethylene glycol), 50 nanoporous titanium oxide, 57–58 N-halamine polymers, 51 oil-containing films, 54 phytosynthesized nanoparticles, 54 PMMA/PDDA nanoparticle, 50–51 poly(3,4-ethylenedioxytiophene) (PEDOT)-based matrix, 59 poly(dimethylsiloxane) with silver nanoparticles (PDMS/AgNPs), 53 polyelectrolyte-copper nanocomposite films, 52 polyethylene terephthalate (PET) film, 52–53

611

quaternized ammonium copolymers, 48–49 SARSCoV-2, 57 silk protection, 52 silver nanoparticles, 55–56, 59 silver zeolite, 59 sodium alginate (SA), 51 sol-gel precursor/film, 56–57 styrene-maleic anhydride (SMA), 51–52 UV-A light-exposed hydrogel coatings, 49 zein films, 54 smart coating antibacterial coatings, 41–46, 49f antifungal coatings, 48 antiviral coatings, 47–48, 50f concept, 39–41 microbial infections, 39–41 techniques electrophoretic deposition (EPD), 24–28, 27f, 29f electrospinning technique, 38–39, 40f, 42f hydrothermal deposition, 35–36, 38f plasma electrolytic oxidation (PEO), 28–31, 32f pulsed laser deposition (PLD), 32–34, 35f, 37f Biomimetics antifouling, natural cleaner, 268–269 bioactive coatings, 265–267 bioinspiration engineering, 263 blue butterfly wings, 267–268 chameleon skin, 274–275 cicada wings, 273–274 color-changing film, 274–275 definition, 263 dermal denticles, 265–266 diamond-resembling carbon coatings (DLCs), 275–276 drag reduction, 265–267 gecko feet, 272–273 honeycomb structured materials, 270–272 lotus effect, 264–265 moth eye-inspired mold coating, 269–270

612

Index

Biomimetics (Continued) optically active surface coating, 269–270 pitcher plant-inspired coating, 268–269 reversible adhesive coatings, 272–273 self-cleaning and antibacterial surfaces, 273–274 shark skin-type coating, 265–267 slippery liquid-infused porous coating (SLIPS), 276–277 structural color coating, 267–268 superhydrophobicity dust repellant, 264–265 and robust coatings, 270–272

C Carica papaya antimicrobial properties anthelmintic property, 590 antitrichomonal activity, 590–591 bacteriostatic property, 590 challenges, 602–603 medicinal properties, 588–589, 588f nutraceuticals, 588 definition, 589–590 food supplements, 589 phytochemical, 589–590 therapeutic effects, 589, 589f papaya protein, 587 pharmaceutical properties anticancer activity, 591 antidengue agent, 599–600 antidiabetic activity, 592 antimalarial activity, 598–599 antioxidant activity, 593–594 contraceptive activity, 594–595 gastrointestinal track, 596–597 heart diseases, 601 hepatoprotective activity, 601–602 immunity booster, 597–598 obesity control, 600, 601f rejuvenation activity, 594 renal activity, 596 sickle cell anemia, 600 phytotherapies agents, 588–590 treatment, 587–588 Cassie-Baxter model, 293–294, 293f Chemical etching, 299 Chemical fixation, 115

Chemical vapor deposition (CVD), 374–375, 375f Chemo-responsive coating, 413–414 Chromic textiles, 225–227, 226f Collagenase, 79 Color-changing film, 274–275 Contact killing approach active action polymers, 244–246 antimicrobial peptides (AMPs), 245 drawback, 246 quaternary ammonium compounds (QACs), 244 quorum-sensing inhibition, 245 Copper nanomaterials, 509–511 Corneal infection, 17, 18f Corrosion-resistant coatings, 220–221, 220f Corrosion sensing coatings, 227 Culturing and identification techniques antibiotic susceptibility tests (ASTs), 113–114 automated identification technique, 114 MALDI-TOF, 121–128, 128f MicroScan, 130 Phoenix, 129–130 VITEK 2, 128–129 biochemical tests, 121, 126–127t complex media, 120 conventional culture method, 113–114 culture media, 120, 122–125t culture method lawn/carpet culture, 119 liquid culture, 119 pour-plate method, 119, 120f spread-plate method, 120 stab culture, 119 streak culture, 118, 118f stroke culture, 119 defined media, 120–121 microbial antibiotic sensitivity, 113 molecular methods amplification and nonamplificationbased methods, 131 Biofire Film Array, 133–134 flow cytometry, 134–135 loop-mediated isothermal amplification, 134 PCR, 131–133, 132f phenotypic approach, 114

Index

staining techniques chemical fixation, 115 differential staining, 116–117, 116f heat fixation, 115 negative staining, 115 simple staining, 115 structural properties, bacteria, 114–115

D Dental plaque, 16–17, 17f Diamond-resembling carbon coatings (DLCs), 275–276 Differential staining acid-fast staining, 117 Albert staining, 117 Gram staining, 116–117, 116f stains, 116 Dip-coating technique, 298 Drag reduction, 265–267 Dust repellant, 264–265

E Edible coatings and films antimicrobial/antiviral agents in, 455–456, 455f classification, 456, 456f food packaging material, 453–454 food quality, 453 future research, 475 lipid-based coatings free fatty acids, 472–474, 473f hydrophobic materials, 472 moisture barrier ability, 472 resins, 474–475, 474f wax, 472 materials, 456, 456f need for, 454–455 plant extract-based antibacterial coating, 484 polysaccharide-based films alginates, 469–470, 470f cellulose and derivatives, 461–464, 463–464f chitosan and its derivatives, 467–469, 467f, 469f hydrophilic polysaccharides, 461 pectin, 470, 471f

613

preparation, 461, 462f starch and derivatives, 464–467, 465–466f processed food, 453 protein-based edible films advantages and disadvantages, 456–457 casein milk proteins, 457–458, 458f cross-linking proteins, 456–457 gelatin-based films, 460, 461f important proteins, 457f whey protein, 458–460, 459f Edible coatings-on postharvest loss, 381–382, 383t Electrophoretic deposition (EPD) chitosan-silver composite coating, 24–25 diagrammatic representation, 24, 27f ferulic acid (FA)-loaded bioactive glass (BG)-chitosan (CS) composite coating, 25 gentamicin (GS)-loaded Zn halloysite nanotubes (HNTs)-chitosan coatings, 25–26, 29f graphitic carbon nitride, 28 iron oxide-chitosan-hydroxyapatite (HA) composite coatings, 26–27 minerals incorporated calcium silicate, 27–28 Electrospinning technique, 38–39, 40f, 42f, 298–299 Enzymes bacteria biofilm removal, 181 combined enzymes, 184 deoxyribonuclease I (DNase I), 182–183 limitation with enzymatic eradication of biofilm, 184 lyase, 183 lysostaphin, 183 polysaccharide-hydrolyzing enzymes, 182 protease, 181–182 Exotoxin, 80

F Fimbriae, 75 Flow cytometry, 134–135 Food packaging, 453–454. See also Edible coatings and films

614

Index

G Gold nanomaterials antibacterial and antifungal activity, 506–507 biosynthesis, 506–507 hydrogen ion extrusion, 507–508 Kirby-Bauer technique, 506–507 microbial enzymes interaction, 507–508 surface coated cotton fabrics, 519–522, 521f Granular activated carbon (GAC) with biofilm, 20 Green antibacterial and antifungal smart coating antiadhesive coatings, 562, 563f antifungal properties, 544–546, 544f bacterial contamination, 541–542 biofouling, 541 coating industries, 542–544 food packaging, 560–562 grafted biocide, 547–549 green metal nanoparticles, 542–544 health care and light activated antibacterial surfaces, 557–559, 558f hospital-acquired infections (HAIs), 541–542 kill and release coating, 547–550 light-responsive materials, 550–552, 551f, 557–559, 558f medical devices challenges, 556–557 device-/implant-associated infections, 552–553 hybrid PLGA nanoparticles, 552–553, 553f Schiff base-containing hydrogels, 553–554, 554f self-assembled LbL films, 555 thermoresponsive polymers, 555–556, 556f MF-LVO paint, 544–545, 544f multiple killing mechanism, 549 pH-responsive multilayered coatings, 547–549, 548f self-defensive coatings, 549 smart antibacterial coatings, 547–549, 548f

textiles, 559–560 types, 549–552, 549f, 551f vegetable oil-based polyurethanes, 545–546 water-based interior paints, 545–546 Green nanotechnology advantages, 493–494 eco-friendly production, 493–494 microorganisms, 493–494 using algae, 497 using bacteria, 496–497 using fungi, 495–496 using plants, 494–495 Green synthesis method, 490–492

H Heat fixation, 115 Hospital-acquired infections (HAIs), 541–542. See also Green antibacterial and antifungal smart coating Host invasion antibiotic resistance, 77 immunity, 77 infective dose (ID), 79 invasive bacteria, 77–78 listeriolysin O (LLO), 78 mucous membranes, 76–77 phospholipase A, 78 zipper and trigger mechanisms, 78–79 Hyaluronidase, 79 Hybrid antibacterial, antifungal, and antiviral smart coatings antibacterial coatings cellulose with titania/chitosan/AgNPs, 439 dendritic fibrous nanosilica (DFNS), 436–437, 437f dentine discs with silver nanoparticles, 435 layer-by-layer niosome coating, 437–438 nanocoated poly(lactic) acid (PLA) films, 435–436 norfloxacin (NFX) and poly(lactic-coglycolic acid) (PLGA), 438–439 silver nanocoated fabrics, 432–435

Index

vancomycin-loaded niosomes, 437–438 zinc-doped copper oxide nanoparticle contact lenses, 439–440 zinc oxide nanoparticle friction-reducing coating, 440 antifungal coatings acrylic paint formulations with titanium dioxide microspheres, 442–444 AgNP-coated nystatin (NY) and clotrimazol (CM) drugs, 440–441 cyclodextrin-based emulsions, 442, 443f graphene nanocoating on titanium, 442 nanocomposite hydrogels, 441–442 antiviral coatings laponite (LAP) and Cu2+ ions, 445 metal nanoparticle coatings, 444–445 nanoworm surfaces, 445–447, 446f tea-cinnamaldehyde-metal hybrid nanocoating, 447–448 waterborne spray-on nanocoating, 445–447, 446f biocidal activities, 431 biofilm formation, 431 biologically active nanocoatings, 432, 433–434t future research, 448–449 Hydrophilic and hydrophobic antifouling, 228–229 Hydrophobic surfaces. See Self-cleaning surfaces (SCSs) Hydrothermal deposition, 35–36, 38f

I Identification techniques. See Culturing and identification techniques Intelligent coatings. See Smart coatings Intumescent coatings, 229–230, 229f

L Layer-by-layer (LbL) technique, 379–381, 380f Lipid-based coatings free fatty acids, 472–474, 473f hydrophobic materials, 472

615

moisture barrier ability, 472 resins, 474–475, 474f wax, 472 Loop-mediated isothermal amplification, 134 Lotus effect and superhydrophobicity, 264–265

M MALDI-TOF, 121–128, 128f Metal-based nanomaterials. See Bioengineered metal-based antimicrobial nanomaterials Metal nanoparticles (MNPs) antibacterial test, 581, 582f applications, 583 characterization FTIR spectra, 579f hydrodynamic diameter, 580f photoluminescence (PL) spectra, 580f SEM micrograph, 579f UV-Visible spectra, 579f zeta potential, 580f classification based on their composition, 572, 572t based on their dimension, 572 dendrimers, 574 fullerenes, 573–574 one-dimensional nanoparticles, 572–573 quantum dots (QDs), 574 three-dimensional nanoparticles, 573 two-dimensional nanoparticles, 573 Gram stain test, 581, 582f preparation approaches, 575, 575f bottom-up method, 575–576 color, 577, 578f dimensions of nanomaterials, 574–575, 575f green synthesis, 572, 576–577 metal salt reduction, 577, 578f noble MNPs, 576–577, 577–578f physical and chemical methods, 571–572, 576 phytochemicals, 572, 576–577, 577f

616

Index

Metal nanoparticles (MNPs) (Continued) surface plasmon resonance (SPR), 577, 578f top-down method, 575 properties, 571 route of synthesis, 571–572 sizes, 571 smart nanomaterials, 584 Microbes. See also Microorganism anti-biofilm compounds, 4t biofilm, 3–4 characteristics, 3–4 classification, 8–9, 9f corneal infections, 17, 18f dental plaque, 16–17, 17f Germ Theory of disease, 3–4 history, 5–8, 7–8f host-pathogen interaction biofilm (see Biofilm) infection cycle, 14 strategy, 14 Koch postulation, 7–8, 8f medical implant diseases, 18–19 Pasteur swan-neck flask experiment, 6–7, 7f species, 4t spontaneous generation/abiogenesis, 6 strain development bacterial strains, 9 biochemical tests, 13–14 novel antibiotics, 12–13 primary screening, 10–11, 10f quality testing, 12–13 secondary screening, 11–12 urinary tract infection, 18, 19f Microbially induced corrosion (MIC) anaerobic and aerobic corrosion, 168 bacteria, archaea, and fungi, 168 common bacteria acid-producing bacteria (APB), 170–171 iron-oxidizing bacteria (IOB), 170 iron-reducing bacteria (IRB), 171 metal-oxidizing bacteria (MOB), 169–170 methanogens, 171–172 slime-forming bacteria, 170

sulfate-reducing bacteria, 168–169 sulfur-oxidizing bacteria, 168 definition, 167–168 EPS production, 167 mitigation bacteriophage treatment, 178 biocide treatment, 174–176 chelators, 177–178 D-amino acids, 176–177 norspermidine, 177 oil and gas industry, 168 oxygen-free MIC mechanisms biodegradation-MIC (BD-MIC), 173 extracellular electron transfer-MIC (EET-MIC), 172 metabolite-MIC (M-MIC), 172–173 in presence of oxygen corrosive metabolites, 174 metal-oxidizing bacteria, 173–174 oxygen concentration cell, 174 process initiation, 167 sessile bacteria, 167 Microorganism. See also Microbes adhesin adherence, 74–75 afimbrial adhesin, 75–76 colonization, 74 fimbrial adhesin, 75 intermolecular linkages, 75 internalization, 76 polysaccharide adhesin, 76 protein adhesin, 75–76 Tir effectors remodeling, 76 beneficial activities, 72, 73f collagenase, 79 detrimental agents, 72–73 geochemical activities, 71 host invasion antibiotic resistance, 77 immunity, 77 infective dose (ID), 79 invasive bacteria, 77–78 listeriolysin O (LLO), 78 mucous membranes, 76–77 phospholipase A, 78 zipper and trigger mechanisms, 78–79 immune system, 80

Index

impact on life, 71–73, 73f pandemics history, 74f pathogenic and nonpathogenic microorganisms, 73 quorum sensing (QS), 81–82 spreading factor, 79 toxins, 80–81 MicroScan, 130 Molecular method amplification and nonamplification-based methods, 131 Biofire Film Array, 133–134 flow cytometry, 134–135 loop-mediated isothermal amplification, 134 PCR, 131–133, 132f Multiplex PCR, 133

N Nanomaterials antimicrobial coatings mechanism of action, 417, 418f strategies, 414–415, 415f antimicrobial-resistant microbes, 413–414 bacterial surface attachment and colonization, 413–414 chemo-responsive coating, 413–414 future research, 425–426 microbial contamination, 413–414 physically responsive coating, 413–414 smart antimicrobial coatings chitosan, 422–423 design, 416, 416f diamond-like carbon coatings, 423 graphene, 423 induction stimulus, 424–425t silica-based nanomaterials, 424 silver nanoparticles, 419–420 and stimuli, 417–419 titanium dioxide/titania, 421–422 zinc nanoparticles, 421 transmission of infectious diseases, 413–414 Nanoparticles antibacterial activity, 252–254 cell components interaction, 253–254

617

cell membrane interaction, 252 copper oxide nanoparticles, 190 dendrimers, 191 gold nanoparticles, 190 liposomes, 191 metal ion release, 253 nonoxidative mechanism, 254 organic nanoparticles, 190–191 oxidative stress, 253 penetration mechanism, 252 schematic representation, 251–252, 251f silver nanoparticles, 189 titanium oxide nanoparticles, 189–190 truncated triangular silver nanoplates, 188–189 zinc oxide nanoparticles, 189 Nanotechnology antibacterial coating in therapeutic and health-care bacterial infection, 369–370 bio ceramics and bioactive glasses, 367 chemical vapor deposition (CVD), 374–375, 375f electrophoretic deposition of coatings, 367–368 implants, 366–367 magnesium metal and alloy, 368 metal and metal oxide nanoparticles, 370 physical vapor deposition (PVD) coating, 370–372, 372f plasma-assisted antibacterial coating, 372–373 polymer-based nanoparticles, 370 silica-based nanomaterials, 367 smart antibacterial coatings, 376–377, 378f sol-gel process, 375–376, 377f thermal evaporation (TE), 373–374, 374f titanium and its alloy, 368–369 antibacterial material coating, 358 antibiotic resistance, 359 antifungal coating in therapeutic and health-care

618

Index

Nanotechnology (Continued) chlorhexidine coating with nanoparticle additives, 377–379 edible coatings-on postharvest loss, 381–382, 383t fungal infections, 377–379 layer-by-layer (LbL) technique, 379–381, 380f long-term antimicrobial device coatings, 385 silver nanoparticles, 379 tablet coating, 382–385 antiviral coating applications, 389, 390–391t, 392, 392f challenges, 385–386 functional nanomaterials, 399–400, 400f graphene oxide, 388–389, 388f metal nanoparticle-based antiviral strategies, 387 metal oxides/metal ions, 393–399, 400f mode of action, 386 polymer, 393, 399f vector-borne viral transmission, 386 viral entry inhibitors, 387 viricidal and antiviral properties, 392–393, 393f, 394–398t applications, 361f classical laws, 360–361 definition, 360 drug delivery, 359 fungal biofilms, 358 future research, 401 implant infections, 358 material properties, 361–362 nanomedicine, 360 nanovaccines, 359 pathogenic bacteria, 357–358 in smart coatings COVID-19, 358–360, 363 global smart coating market, 363 medicine, 365–366 nanocoating, 362, 364 nanoscale silver particles, 362–363 self-assembly, 363 virology, 360 Negative staining, 115 Nested PCR, 133

P PCR, 131–133, 132f Phoenix method, 129–130 Photocatalysis-assisted self-cleaning surface, 223–224 Physically responsive coating, 413–414 Physical vapor deposition (PVD) coating process, 370–371 principle and components, 371, 372f system boundary, 371–372, 372f Plant extract-based antibacterial coating antibacterial/antimicrobial properties, 482–483 applications, 481 aromatic plants, 482 curcumin, 482–483 edible coating, 484 extraction method, 484 food and pharmaceutical industry, 481–482 incorporation route, 485 mangosteen extract, 482–483 metal protection, 481 tannic acid, 482–483 Plasma-assisted antibacterial coating, 372–373 Plasma electrolytic oxidation (PEO) bioactive glass, 30 Ca and P on titanium dental implants, 30–31 Ca/P/Al coating, 31–32 diagrammatic representation, 32f porous silver-doped TiO2 coatings, 31 process, 28–29 Ti-6Al-4V, 29–30 TiO2 coatings codoped with nitrogen and bismuth, 31 ZrO2/TiO2 coatings, 31 Polysaccharide-based films alginates, 469–470, 470f cellulose and derivatives bactericidal action, 463–464, 464f chemical structure, 461–462, 463f hydroxypropyl methyl cellulose (HPMC), 462–463, 463f methylcellulose (MC), 462–463, 463f chitosan and its derivatives antibacterial properties, 467–469, 469f

Index

chemical modification, 467, 467f hydrophilic polysaccharides, 461 pectin, 470, 471f preparation, 461, 462f starch and derivatives antibacterial properties, 465–467, 466f chemical structure, 464–465, 465f Proteinaceous toxins, 80 Protein-based edible films advantages and disadvantages, 456–457 casein milk proteins, 457–458, 458f cross-linking proteins, 456–457 gelatin-based films, 460, 461f important proteins, 457f whey protein, 458–460, 459f Pulsed laser deposition (PLD) bovine hydroxyapatite-aluminaclinoptilolite composite coatings, 33–34 chitosan/polyvinyl alcohol-embedded gold nanoparticle films, 33 experimental setup, 32, 35f fish-bone-derived biphasic calcium phosphate coatings, 33 fluor-carbonated hydroxyapatite coating, 34 fluoride-incorporated apatite (FCP) coating, 34, 37f process, 32–33

Q Quorum sensing (QS) system, 81–82 AHL signal molecules biodegradation/alteration, 187 biosynthesis, 186–187 alarmone scheme, 188 inhibition, 185–186 interfering via analog compounds, 187–188

R Reversible adhesive coatings, 272–273 RT-PCR, 133

S SARS-associated coronavirus, 113 Selenium nanomaterials, 508–509

619

Self-cleaning surfaces (SCSs) and antibacterial surfaces, 273–274 applications antibacterial coatings, 301 anticorrosion, 301–302 antiicing protection, 302–303 antireflective and transparent coatings, 303 blood repellent, 299–300 fabrics and textiles, 300–301 medical industry, 302 oil water sorption and separation, 303–304 solar cell and water harvesting, 300 approaches chemical etching, 299 dip-coating technique, 298 electrospray/electrospinning coating, 298–299 basics, 289 from nature, 290, 291f overview, 290, 291f photocatalysis-assisted self-cleaning surface, 223–224 superhydrophobic coatings, 222–223, 222f from cellulose and its derivatives, 297–298 from fatty acids, 297 from natural waxes, 295–296 from proteins, 296 wettability Cassie-Baxter model, 293–294, 293f definition, 290 Wenzel model, 293f, 294–295 Young’s equation, 291–293, 292f Self-defensive antibacterial coatings bacteria-secreted substance-responsive self-defensive antibacterial coatings, 250 pH-responsive self-defensive antibacterial coatings, 248–249 response of, 248, 249f Silver nanomaterials antibacterial capabilities, 500–501 antifungal properties, 505–506 coated craft papers, 516

620

Index

Silver nanomaterials (Continued) mechanisms for antibacterial action, 501–503 oligodynamic impact, 505–506 phyto-silver nanoparticle synthesis, 503–505 protein interaction, 501 tissue paper, 522–523 toxicity, 500–501 Simple staining, 115 Slippery liquid-infused porous surfaces (SLIPS), 268–269, 276–277 Smart coatings antifouling coatings, 228–229 antigraffiti coatings, 227–228, 228f antiicing surfaces, 224–225 antimicrobial coatings, 230–232, 231f applications, 219 chromic-based smart textiles, 225–227, 226f coating composition, 219 corrosion-resistant coatings, 220–221, 220f corrosion sensing coatings, 227 external disturbances, 219 intumescent coatings, 229–230, 229f self-cleaning coatings, 222–224, 222f smart window coatings, 232–233 stimulus, 219 Smart kill and release antibacterial coatings, 250–251 Smart window coatings, 232–233 Sol-gel process, 375–376, 377f Spreading factor, 79 Staining techniques chemical fixation, 115 differential staining, 116–117, 116f heat fixation, 115 negative staining, 115 simple staining, 115 structural properties, bacteria, 114–115 Structural color coating, 267–268 Structurally modified polymers cationic block copolymer film, 314–315 classification, 311–312 glucomannan, 316–317 Jeffamine-ED-2003, 313–314

N-halamine moiety, 312 N-methyl polyethyleneimine (PEIs), 312 nonleachable antibiofilm and antibacterial coating, 317–319 poly glycidyl methacrylate (GMA)/ poly sulfobetaine methacrylate (SBMA) and polyethyleneimine (PEI), 314 PP-PEI-SMA-PEI system, 315 properties, 311–312 quaternary ammonium methacrylate compounds (QAC-1, QAC-2), 316 quaternary ammonium silane (QAS), 315–316 quaternary benzophenone-based ester (QBEst) and quaternary benzophenone-based amide (QBAm), 313 silicone contact lenses, 314 synthesis, 317–319, 318f Superhydrophobic coatings, 264–265 Surface characteristics, biofilm aqueous medium milieu, 99 biofilm structure, 100–102 cell properties, 99–100 conditioning film, 97–98 hydrodynamics, 98–99 solid-liquid interface initiation, 97 substratum compo sum, 97 surface charge, 90–92, 91f surface roughness, 92–94, 93f surface stiffness, 93f biofilm formation, 96–97 elastic modules, 94–95 microbial persistence, 95–96 Young’s modules, 94–95 surface topography, 93f, 94 surface wettability, 92, 93f Surface charge, 90–92, 91f Synergistic antibacterial coatings, 250

T Tablet coating, 382–385 Teichoic acid, 100–101 Thermal evaporation (TE), 373–374, 374f Topographic modification, 247 Toxin, 80–81

Index

U

nonoxidative mechanism, 254 oxidative stress, 253 penetration mechanism, 252 schematic representation, 251–252, 251f need for smart coating, 241 physical strategies, 247 polymers, 254–255 smart and synergistic antimicrobial coatings advantages, 248 self-defensive antibacterial coatings, 248–250, 249f smart kill and release antibacterial coatings, 250–251 synergistic antibacterial coatings, 250 vs. traditional antibacterial coatings, 248 topographic modifications, 247

Urinary tract infection, 18, 19f

V Viricidal and antiviral properties, 392–393, 393f, 394–398t VITEK 2, 128–129

W Wenzel model, 293f, 294–295 Whipple’s disease, 113 Working principles of smart coating applications of smart coating, 256–257 chemical strategies antiadhesive surfaces, 241–244, 242f biocidal release mechanism, 246–247 contact killing approach, 244–246 concept, 240 future research, 257 metal ion and oxide-based antimicrobial coatings, 255–256, 256f nanoparticles antibacterial activity, 252–254 cell components interaction, 253–254 cell membrane interaction, 252 metal ion release, 253

621

Y Young’s equation, 291–293, 292f

Z Zinc nanomaterials, 511–515