Biopolymer Membranes and Films: Health, Food, Environment, and Energy Applications [1 ed.] 0128181346, 9780128181348

Table of contents :
Cover
BIOPOLYMER
MEMBRANES
AND FILMS
Health, Food, Environment,
and Energy Applications
Copyright
Dedication
Contributors
Preface and acknowledgment
Acknowledgement
Part I: Fundamentals on biopolymers membranes and films
Fundamentals on biopolymers and global demand
Introduction
Polysaccharides and proteins: General characteristics
Polysaccharides
Alginate
Cellulose
Bacterial cellulose
Chitin and chitosan
Gellan gum
Hyaluronic acid
Carrageenan
Starch
Dextran
Agarose
Pectin
Proteins
Collagen and gelatin
Silk fibroin
Sericin
Soy protein
Applications of the membranes of biopolymers
Biomedical applications
Food
Environment
Energy
Global demand and market for biopolymers
Final remarks
Acknowledgments
References
Glossary
Fundamentals of two-dimensional films and membranes
General aspects of membranes and films
Processing
Dense membranes
Casting
Extrusion/film blowing
Coating
Spread coating
Spray coating
Dipcoating
Layer by layer
Porous membranes
Solvent casting and particulate leaching
Thermally induced phase separation
Electrospinning
Modification approaches
Plasticization
Cross-linking
Composites
Silicate layers
Carbon nanotubes
Cellulose nanocrystals
Blending
Ionic liquids
Final remarks
References
Characterization of biopolymer membranes and films: Physicochemical, mechanical, barrier, and biological prope ...
Introduction
Physicochemical characterization
Microscopy
Optical microscopy
Scanning electron microscopy
Transmission electron microscopy
Atomic force microscopy
Fourier transform infrared spectroscopy
Raman spectroscopy
Nuclear magnetic resonance
X-ray diffraction analysis
X-ray photoelectron spectroscopy
Thermal analysis
Thermogravimetry
Differential scanning calorimetry
Dynamic mechanical analysis
Physical characterization
Swelling degree
Degradation/erosion degree
Mechanical properties
Barrier properties
Volumetric methods
Gravimetric methods
Differential methods
Microbial penetration
Contact angle
Textural analysis
Biological characterization
Cytotoxicity, sensitization capacity, and irritation potential
Antimicrobial activity
Conclusions
Acknowledgment
References
Diffusion process through biodegradable polymer films
Introduction
Natural polymers
Polysaccharide based films
Proteins based films
Active films
Natural polymer diffusion mechanism
Mass transfer and diffusion
Mathematical modeling for the diffusive process in natural polymer films
Diffusive process in natural polymers
Conclusions
Acknowledgments
References
Separation processes with (bio)membranes: Overview and new phenomenological classification
Introduction
Fundamentals
General physical and structure of the membranes
Membrane feed flux: Frontal and tangential
Modules of membranes
Preliminary general description of the separation processes with membranes
Advantages and disadvantage of the separation processes with membranes
Advantages
Disadvantage
Usual classification of the separation processes with membranes: Dimensional
Some basic concepts
Flux and permeability
Driving forces
Selectivity
Description of separation processes with membranes
Differential pressure as driving force
Microfiltration
Ultrafiltration
Reverse osmosis
Nanofiltration
Difference of concentration as driving force
Dialysis
Difference of voltage as driving force
Electrodialysis
Difference of partial pressure as driving force
Gas permeation
Difference of vapor pressure as driving force
Pervaporation
New classification of separation processes with membranes
Conceptual conclusion
Mechanical unit operations
Diffusional unit operations
Consequence of proposed classification for experimentation
Biopolymers
Future trends
References
Part II: Applications of biopolymers membranes/films in health
Biopolymer membranes in tissue engineering
Introduction
Basic concepts of membrane characteristics envisioning tissue-engineering applications
Biomedical applications of membranes
Wound dressing
Drug delivery
Peripheral nerve regeneration
Cartilage regeneration
Guided bone tissue regeneration
Other tissues
Final remarks
Acknowledgments
References
Glossary
Biopolymer-based films and membranes as wound dressings
Introduction
Characteristics of skin lesions
Major requirements to be fulfilled by wound dressings
Types and properties of biopolymers used in wound dressings
Chitosan
Alginates
Xanthan gum
Bacterial cellulose
Hyaluronan
Collagen
Gelatin
Silk fibroin
Keratin
Blends, composites, and associations with other bioactive agents
Blends
Composites
Association with bioactive agents
Final remarks
References
Recent advances in biopolymer-based transdermal patches
Introduction
Transdermal drug delivery systems
Biopolymer-based transdermal films and patches
Chitosan based on transdermal films and patches
Pectin based on transdermal films and patches
Ethyl cellulose based on transdermal films and patches
Other biopolymers used in transdermal films and patches
Biopolymer-based microneedle patches
Hyaluronic acid/sodium hyaluronate based on dissolving microneedles
Carboxymethyl cellulose based on dissolving microneedles
Chitosan based on dissolving microneedles
Gelatin based on dissolving microneedles
Silk fibroin based on dissolving microneedles
Chondroitin sulfate based on dissolving microneedles
Miscellaneous biopolymer-based dissolving microneedles
Biopolymer-based nanofiber mats
Conclusion
References
Fundamentals and biomedical applications of biopolymer-based layer-by-layer films
Overview
Principles of LbL film assembly
Antimicrobial LbL films
Antimicrobial surface approach
Antibacterial and antifungal surfaces
LbL films for sensing applications
Tissue engineering applications via LbL technique
Surface functionalization and TE applications
Multilayered cell scaffolds for TE applications
LbL films for drug delivery applications
Gene delivery applications for LbL films
Combination of LbL with other molecular assemblies
Micelles
Liposomes
Conclusions and future challenges
Acknowledgments
References
Biopolymer membranes for dentistry applications
Introduction
Biopolymer membranes and films in dentistry
Natural and synthetic biopolymers
Collagen
Chitosan
Gelatin
Cellulose and derivatives
Hyaluronic acid
Poly(lactic-co-glycolic acid)
Other polymers
Blends, composites, and hybrid membranes
Blends
Silver nanoparticle composites
Hydroxyapatite composites
Calcium phosphate composites
Bioactive glass composites
Hybrid films
Treatments employing biopolymers
Endodontic therapy
Extractions, implants, and bone regeneration
Periodontology
Oral cancer treatment
Prosthetic dentistry
Caries prevention
Release of anesthetics
Drug delivery systems in the oral environment
Antimicrobial and antiinflammatory drugs
Anesthetics
Bone- and tissue-healing agents
Final considerations and future perspectives
References
Biopolymer-based coatings for cardiovascular applications
Introduction
Biopolymers used for cardiovascular applications
Chemical modification of chitosan: Sulfonation reaction
Hemocompatible properties of sulfated chitosan
Protein adsorption on raw and sulfated chitosan
Platelet adhesion on raw and sulfated chitosan
Sulfated chitosan-based coating on metal surfaces
Sulfated chitosan-based coating on stainless steel
Chitosan-heparin nanoparticle coating on NiTi alloys
Challenges using natural polymer coatings on metal surfaces
References
Orally disintegrating films of biopolymers for drug delivery
Introduction
Biopolymers used to produce orally disintegrating films
Production methods of orally disintegrating films
Innovative production methods of orally disintegrating films
Characterization of orally disintegrating films
Disintegration time
Mucoadhesion
Surface pH
Dissolution of orally disintegrating films
Conclusion
References
Skin rejuvenation: Biopolymers applied to UV sunscreens and sheet masks
Introduction
Physiological effects of UV exposure
Aging
Immune system
Skin structure and phototype
Sun exposure and sunscreen protection
Sheet masks and biopolymers
Conclusion
Acknowledgments
References
Part III: Application of biopolymers membranes/films in environment and energy
Heavy metal removal from industrial effluents using biopolymer membranes
Introduction
Chitosan
Obtaining chitosan membrane adsorbents
Chemistry of chitosan
Chitosan-based membranes
Blended chitosan membranes
Supported chitosan membranes
Composite membrane
Cellulose
Chemistry of cellulose
Cellulose modification
Cellulose-based membranes
Carrageenans
Chemistry of carrageenan
Carrageenan modification
Carrageenan-based membranes
Alginate
Chemistry of alginate
Alginate modification
Alginate-based membranes
Conclusions
References
Pesticide removal from industrial effluents using biopolymeric materials
Introduction
Overview of pesticides
Classification
Classification based on pesticide toxicological behavior
Classification based on pesticide biological targets
Classification based on pesticide chemical structure
Characteristics of pesticides and their environmental fates after application
Biopolymers used in the removal of pesticides
Conclusions and future perspectives
Acknowledgments
References
Dye removal from effluents using biopolymer membranes
The importance of dye removal from effluents
Dyes: Classification and uses
Environmental and public health risks
Guidelines for discharge of colored effluents
Methods for dye removal from effluents
Biopolymer membranes for dye removal
Biopolymers used for membrane preparation
Raw materials for biopolymer-based membrane preparation
Characterization techniques
Laboratory experiments using membranes for dye removal
Evaluation of membrane performance
Comparative analysis of several biopolymer membranes for dye removal
Real applications (real industrial effluents)
Perspectives and challenges
References
Pharmaceutical and synthetic hormone removal using biopolymer membranes
Pharmaceutical and synthetic hormone removal
Fabrication of membranes and biomembranes
Biopolymers used in membrane manufacturing
Biopolymers derived from bacterial fermentation products used for manufacturing membranes
Polylactic acid
Polyhydroxyalkanoates
Poly(butylene succinate)
Biopolymers derived from vegetable sources used for manufacturing membranes
Cellulose-based polymers
Alginate
Polyisoprene
Starch and cyclodextrins
Biopolymers derived from animal sources used for manufacturing membranes
Chitosan
Collagen
Silk
Biopolymer-metal-organic framework
Membranes and biomembranes produced by electrospinning process
Molecularly imprinted membranes and biomembranes
Theoretical factors considered for membrane separation
Analysis of the adsorption equilibrium
Adsorption kinetics
Removal pharmaceuticals and hormones
Conclusions and future outlook
References
Further reading
Biopolymer membranes in fuel cell applications
Introduction
Fuel cells
Alkaline fuel cells (AFCs)
Phosphoric acid fuel cells (PAFCs)
Solid oxide fuel cells (SOFCs)
Molten carbonate fuel cells (MCFCs)
Direct carbon fuel cells (DCFCs)
Proton exchange membrane fuel cells (PEMFCs)
Direct methanol fuel cells (DMFCs)
Biofuel cells (BFCs)
Biopolymer membranes for fuel cells
Chitosan (CS)
Self-cross-linked and salt-complexed chitosans
Chitosan-based polymer blends
Chitosan/inorganic filler composites
Chitosan/polymer composites
Cellulose (C)
Pure cellulose
Cellulose acetate
Cross-linked cellulose-based membranes
Cellulose-based graft copolymers
Cellulose-based materials doped with inorganic/organic compounds
Cellulose-based polymer composites
Alginate
Starch
Pectin
Agar
Gelatin
Summary and future perspectives
References
Biopolymer membranes for battery applications
Introduction
Ionic dopant/salt effects on biopolymers
Biopolymers incorporated with a plasticizer
Preparation of biopolymer membranes
Characterization of biopolymer membranes for batteries
Electrical impedance spectroscopy
Equivalent circuit model
Ionic conductivity
Dielectric properties
Ionic conduction mechanism
X-ray diffraction
Fourier transform infrared spectroscopy
Transference number measurement
Biopolymer membranes in the battery
Operating principle of a battery
Characterization of battery performance
Open circuit voltage
Discharge characteristics
Rechargeability
Conclusions and future prospects
References
Part IV: Applications of biopolymers membranes/films in food
Application of edible biopolymer coatings to extend the storage life of fresh fruits and vegetables
Introduction
Materials
Some biopolymers used in the production of edible coatings
Edible coatings and active edible coatings
Edible coatings
Active edible coatings
Considerations
References
Application of edible biopolymer coatings on meats, poultry, and seafood
Introduction
Meats, poultry, and seafood spoilage
Edible coatings and films
Materials used for edible coatings and films
Polysaccharide-based edible film
Alginate
Cellulose and derivatives
Chitosan
Pectin
Starch
Protein-based edible coating and films
Corn zein
Gelatin
Wheat gluten
Whey protein
Composite edible coatings and films
Active components incorporated in edible films and coatings to meats, poultry, and seafood
Effect of edible films and coatings on quality of meats, poultry, and seafood
Conclusion
Acknowledgments
References
Oxygen scavenging films and coating of biopolymers for food application
Introduction
Oxygen scavenging technology
Oxygen scavenging films
Oxygen scavenging agents
Iron and other metallic scavengers
Ascorbic acid and other natural OS agents
Enzymatic scavengers
Photosensitive dyes
Unsaturated hydrocarbons
Immobilization of microorganisms
Technologies for the preparation of oxygen scavenging films
Coating
Incorporation into packaging
Multilayer active films
Immobilization
Recent developments in oxygen scavenging films
Nanomaterials as oxygen scavenging systems
Future trends and conclusions
Acknowledgments
References
Biopolymers applied as ethylene-scavenging films and coatings
Introduction
Climacteric and nonclimacteric crops
Ethylene importance
Conventional methods for ethylene scavenging
1-Methylcyclopropene
Potassium permanganate
Adsorbent materials used in ethylene scavenging systems
Biopolymer films for ethylene control
Edible coating systems
Packaging film systems
Conclusion
Acknowledgment
References
Edible films and coatings made up of fruits and vegetables
Introduction
Fruit and vegetable-based films and coating technology
Current research and product development
Future trends
References
Probiotic-containing edible films and coatings of biopolymers
Introduction
Probiotics and prebiotics
Definitions
Beneficial effects on human health
Issues on the use of probiotics in food
Chemical, biochemical, and microbial activities of probiotics in polymeric matrices
The use of probiotics in bioactive packaging
Edible films and coating biopolymers containing probiotics
Polymeric matrices
Plant-derived biopolymers
Starch and derivatives
Cellulose and derivatives
Pectin
Alginate
Arabic gum
Animal-derived biopolymers
Chitosan
Gelatin
Casein
Whey protein
Microbial derived biopolymers
Dextran
Gellan
Xanthan
Microbial alginate
Methods of probiotic incorporation
Entrapment
Encapsulation
Microencapsulation
Nanoencapsulation
Probiotics release from polymeric matrices
Control of microbial growth
Microbial survival
Metabolic activity
Regulations on the use of probiotics in the food industry
Conclusion
Acknowledgments
References
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Back Cover

Citation preview

BIOPOLYMER MEMBRANES AND FILMS

BIOPOLYMER MEMBRANES AND FILMS Health, Food, Environment, and Energy Applications Edited by

PROF. MARIANA AGOSTINI PROF. CLASSIUS FERREIRA

DE

MORAES

DA

SILVA

PROF. RODRIGO SILVEIRA VIEIRA

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 © 2020 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. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818134-8 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Deans, Matthew (ELS-OXF) Acquisitions Editor: Payne, Edward (ELS-OXF) Editorial Project Manager: Kuhl, Mariana L. (ELS-SPA) Production Project Manager: Vijayaraj Purushothaman Cover Designer: Greg Harris Typeset by SPi Global, India

Dedication

To those who love us To those who inspired us To those who supported us To those who always believed on us

Contributors Vera Alejandra Alvarez Composite Materials Group (CoMP), Research Institute of Materials Science and Technology (INTEMA), National Scientific and Technical Research Council— Argentina (CONICET)—National University of Mar del Plata (UNMdP), Mar del Plata, Argentina Mariana Altenhofen da Silva Center of Agricultural Sciences, Federal University of Sa˜o Carlos, Araras, Sa˜o Paulo, Brazil Ioannis Anastopoulos Department of Chemistry, University of Cyprus, Nicosia, Cyprus Rog
erio Aparecido Bataglioli School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Marisa Masumi Beppu School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Andrea Cristiane Krause Bierhalz Department of Engineering, Federal University of Santa Catarina, Blumenau, Santa Catarina, Brazil Fernanda Carla Bombaldi de Souza Department of Materials Engineering and Bioprocess, School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil

Tito Roberto Sant’Anna Cadaval, Jr School of Chemistry and Food, Federal University of Rio Grande, FURG, Rio Grande, RS, Brazil Bruna Gregatti de Carvalho School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Rosemary Aparecida de Carvalho University of Sa˜o Paulo, Faculty of Animal Science and Food Engineering (FZEA-USP), Pirassununga, Sa˜o Paulo, Brazil Tecia Vieira Carvalho Nucleus of Studies and Research of the Northeast, NEPEN, Fortaleza, CE, Brazil Pascale Chevallier Laboratory for Biomaterials and Bioengineering, Department of MinMet-Materials Engineering, Laval University and University Hospital Research Center, Quebec City, QC, Canada Viktor Oswaldo Ca´rdenas Concha Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo— UNIFESP, Diadema, Sa˜o Paulo, Brazil Luana Roland Ferreira Contini Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo— UNIFESP, Diadema, Sa˜o Paulo, Brazil

Renata Francielle Bombaldi de Souza Department of Materials Engineering and Bioprocess, School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil

Rodrigo Cu
e-Sampedro School of Engineering and Sciences, Monterrey Institute of Technology, Monterrey, Mexico

Adria´n Bonilla-Petriciolet Department of Chemical Engineering, Aguascalientes Institute of Technology, Aguascalientes, Mexico

Joa˜o Dias-Ferreira Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Coimbra, Coimbra, Portugal

Josiane Gonc¸ alves Borges University of Sa˜o Paulo, Faculty of Animal Science and Food Engineering (FZEA-USP), Pirassununga, Sa˜o Paulo, Brazil

Guilherme Luiz Dotto Chemical Engineering Department, Federal University of Santa Maria–UFSM, Santa Maria, RS, Brazil

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xiv

Contributors

Meryem Sedef Erdal Department of Pharmaceutical Technology, Faculty of Pharmacy, Istanbul University, Istanbul, Turkey

Sevgi G€ ung€ or Department of Pharmaceutical Technology, Faculty of Pharmacy, Istanbul University, Istanbul, Turkey

Ju´lia Vaz Ernesto Department of Pharmaceutical Sciences, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo, Diadema, Brazil

Saartje Hernalsteens College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou, Jiangsu, China; Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo, Diadema, SP, Brazil

Ana R. Fernandes Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Coimbra, Coimbra, Portugal Emanuel M. Fernandes 3B’s Research Group, I3Bs—Research Institute on Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, Guimara˜es; ICVS/3B’s—PT Government Associate Laboratory, Braga/ Guimara˜es, Portugal Eduardo de Paulo Ferreira Chemical Engineering Department—Campus Santa M^ onica, Federal University of Uberl^andia, Uberl^andia, MG, Brazil Classius Ferreira da Silva Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil Ronaldo Ferreira do Nascimento Trace Analysis Laboratory (LAT), Department of Analytical and Physical Chemistry, Federal University of Ceara´—UFC, Fortaleza, CE, Brazil Leonardo Fernandes Fraceto Environmental Nanotechnology Lab, Science and Technology Institute of Sorocaba (ICTS), Sa˜o Paulo State University (UNESP), Sorocaba, Brazil Vitor Augusto dos Santos Garcia University of Sa˜o Paulo, Faculty of Animal Science and Food Engineering (FZEA-USP), Pirassununga, Sa˜o Paulo, Brazil Lucimara Gaziola de la Torre School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Sandy Danielle Lucindo Gomes Adsorption Separation Research Group, Department of Chemical Engineering, Federal University of Ceara, Fortaleza, CE, Brazil

Carmen Guadalupe Herna´ndez-Valencia Biotechnology Department, Laboratory of Biopolymers and Pilot Plant of Bioprocessing of Agro-Industrial and Food By-Products, Autonomous Metropolitan University, Mexico City, Mexico Ahmad Hosseini-Bandegharaei Department of Environmental Health Engineering, Sabzevar University of Medical Sciences, Tehran, Iran Mohd Ikmar Nizam Mohamad Isa Advanced Nano Materials (ANoMa) Research Group, Advanced Materials Team, Ionic State Analysis (ISA) Laboratory, Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu; Frontier Materials Research Group, Advanced Materials Team, Ionic & Kinetic Materials Research (IKMaR) Laboratory, Faculty of Science and Technology, Universiti Sains Islam Malaysia, Nilai, Negeri Sembilan, Malaysia Emine Kahraman Department of Pharmaceutical Technology, Faculty of Pharmacy, Istanbul University, Istanbul, Turkey Theo Guenter Kieckbusch School of Chemical Engineering, University of Campinas, Campinas, Sa˜o Paulo, Brazil Henryk Koroniak Faculty of Chemistry, Adam Mickiewicz University in Poznan, Poznan, Poland Ramo´n Dı´az de Leo´n Department of Polymerization Processes, Research Center of Applied Chemistry, Saltillo, Mexico
Eder Cla´udio Lima Institute of Chemistry, Federal University of Rio Grande do Sul, UFRGS, Porto Alegre, RS, Brazil

Contributors

Patricia Santos Lopes Department of Pharmaceutical Sciences, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo, Diadema, Brazil Vinı´cius Borges Vieira Maciel Department of Food Engineering, Faculty of Animal Science and Food Engineering, University of Sa˜o Paulo, Pirassununga, Sa˜o Paulo, Brazil Diego Mantovani Laboratory for Biomaterials and Bioengineering, Department of MinMet-Materials Engineering, Laval University and University Hospital Research Center, Quebec City, QC, Canada Gustavo Martı´nez-Castellanos Biochemical Engineering Department, Misantla Institute of Technology, Veracruz, Mexico Agnes Batista Meireles Biomaterials Evaluation and Development Center (BIOMAT)— Campus I, Federal University of Jequitinhonha and Mucuri Valleys, Diamantina, MG, Brazil Enayde de Almeida Melo Department of Consumer Science, Federal Rural University of Pernambuco, Recife, Brazil ˆ ngela Maria Moraes Department of Materials A Engineering and Bioprocess, School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Mariana Agostini de Moraes Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo— UNIFESP, Diadema, Sa˜o Paulo, Brazil Anafta´lia Felismino Morais Adsorption Separation Research Group, Department of Chemical Engineering, Federal University of Ceara, Fortaleza, CE, Brazil Nur Hafiza Mr Muhamaruesa Advanced Nano Materials (ANoMa) Research Group, Advanced Materials Team, Ionic State Analysis (ISA) Laboratory, Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia Roberto Nasser, Jr Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo, Diadema, SP, Brazil

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Beatriz C. Naveros Department of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, University of Granada, Granada; Nanoscience and Nanotechnology Institute (IN2UB), University of Barcelona, Barcelona, Spain Deise Ochi Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil Romina Paola Ollier Composite Materials Group (CoMP), Research Institute of Materials Science and Technology (INTEMA), National Scientific and Technical Research Council— Argentina (CONICET)—National University of Mar del Plata (UNMdP), Mar del Plata, Argentina € Yıldız Ozsoy Department of Pharmaceutical Technology, Faculty of Pharmacy, Istanbul University, Istanbul, Turkey Neith Pacheco Center for Research and Assistance in Technology and Design of the State of Jalisco, AC, CIATEJ, Southeast Unit, Merida, Mexico Anderson Espirito Santo Pereira Environmental Nanotechnology Lab, Science and Technology Institute of Sorocaba (ICTS), Sa˜o Paulo State University (UNESP), Sorocaba, Brazil Laura Oliveira P
eres Department of Chemistry, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo, Diadema, Brazil Ana Luiza Resende Pires Postgraduate Program in Biotechnology, Federal University of Espı´rito Santo, Vito´ria, ES, Brazil Rui L. Reis 3B’s Research Group, I3Bs— Research Institute on Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine; The Discoveries Centre for Regenerative and Precision Medicine, Headquarters at University of Minho, Guimara˜es; ICVS/3B’s—PT Government Associate Laboratory, Braga/Guimara˜es, Portugal

xvi

Contributors

Ansorena Marı´a Roberta Chemical Engineering Department, Food Engineering Group, Engineering Faculty, National University of Mar del Plata, Buenos Aires, Argentina; National Research Council (CONICET), Buenos Aires, Argentina Joa˜o Batista Maia Rocha Neto School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Luı´sa C. Rodrigues 3B’s Research Group, I3Bs—Research Institute on Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, Guimara˜es; ICVS/ 3B’s—PT Government Associate Laboratory, Braga/Guimara˜es, Portugal Laura Mabel Sanchez Composite Materials Group (CoMP), Research Institute of Materials Science and Technology (INTEMA), National Scientific and Technical Research Council— Argentina (CONICET)—National University of Mar del Plata (UNMdP), Mar del Plata, Argentina J.A. Sa´nchez-Ferna´ndez Department of Polymerization Processes, Research Center of Applied Chemistry, Saltillo, Mexico Andrelina Maria Pinheiro Santos Department of Chemical Engineering, Federal University of Pernambuco, Recife, Brazil Gilberto Dantas Saraiva Laboratory of Synthesis and Characterization of Materials—LASCAM, Department of Physics, State University of Ceara´ (UECE-FECLESC), Quixada´, CE, Brazil Patricia Severino Laboratory of Nanotechnology and Nanomedicine (LNMED), Institute of Technology and Research (ITP), University of Tiradentes, Industrial Biotechnology Program, Aracaju, Brazil Keiko Shirai Biotechnology Department, Laboratory of Biopolymers and Pilot Plant of Bioprocessing of Agro-Industrial and Food By-Products, Autonomous Metropolitan University, Mexico City, Mexico

Simone S. Silva 3B’s Research Group, I3Bs— Research Institute on Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, Guimara˜es; ICVS/3B’s—PT Government Associate Laboratory, Braga/ Guimara˜es, Portugal Mariangela de Fa´tima Silva Federal Institute of Education, Science and Technology of Mato Grosso do Sul (IFMS), Coxim, Mato Grosso do Sul, Brazil Jackson Wesley Silva dos Santos Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo— UNIFESP, Diadema, Sa˜o Paulo, Brazil Vicente de Oliveira Sousa Neto Laboratory of Study and Research in Pollutants Removal by Adsorption, LERPAD, Department of Chemistry, State University of Ceara´ (UECEFECLESC), Quixada´, CE, Brazil ˜ Joao Vinı´cios Wirbitzki da Silveira Institute of Science and Technology—Campus JK, Federal University of Jequitinhonha and Mucuri Valleys, Diamantina, MG, Brazil Jos
e L. Soriano Department of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, University of Granada, Granada, Spain Eliana B. Souto Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Coimbra, Coimbra; CEB—Centre of Biological Engineering, University of Minho, Braga, Portugal Clayton Campelo de Souza Laboratory for Biomaterials and Bioengineering, Department of Min-Met-Materials Engineering, Laval University and University Hospital Research Center, Quebec City, QC, Canada Thiago Bezerra Taketa School of Chemical Engineering, University of Campinas, Campinas, SP, Brazil Bruno Thorihara Tomoda Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil

Contributors

Fernanda Maria Vanin University of Sa˜o Paulo, Faculty of Animal Science and Food Engineering (FZEA-USP), Pirassununga, Sa˜o Paulo, Brazil Anna Cecilia Venturini Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil Rodrigo Silveira Vieira Adsorption Separation Research Group, Department of Chemical Engineering, Federal University of Ceara, Fortaleza, CE, Brazil

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Justyna Walkowiak-Kulikowska Faculty of Chemistry, Adam Mickiewicz University in Poznan, Poznan, Poland Joanna Wolska Faculty of Chemistry, Adam Mickiewicz University in Poznan, Poznan, Poland Patrı´cia Hissae Yassue-Cordeiro Federal University of Technology—Parana´, Londrina, Parana´, Brazil Cristiana Maria Pedroso Yoshida Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo— UNIFESP, Diadema, Sa˜o Paulo, Brazil

Preface and acknowledgment Biopolymers can be processed in several forms, such as micro/nanoparticles, films/ membranes, gels, and sponges. The book focuses on the use of membranes or films, for example, two-dimensional systems, covering a wide range of types of natural polymers and applications. The book is divided into chapters written by experts in the field of membrane applications in health, environment, energy, and food. This book can be used as a valuable reference for biopolymer scientists and researchers looking for cutting-edge applications of natural polymers. First of all, the topics presented in this book are avant-garde themes. One of the challenges of those working with two-dimensional polymer devices (membranes, films, and coatings) is to get out of the comfort zone and to know a little more about the applications that are not part of their expertise. Therefore, a broader view of the application horizon of these devices is still lacking. This book presents three parts with very different applications (Health, Environment and Energy, and Food); these applications were not chosen randomly; we intend to show the public that specific methods of preparation and characterizations of a given application can be perfectly adapted to another use. The researchers often find difficulties in the development of two-dimensional materials (membranes, films, and coatings); however, they do not realize that many publications focusing on other applications present similar problems that have already been solved. Even disastrous results in one application may represent a good result in

another field. As an example, researchers spend a lot of time developing a membrane for application as a wound dressing for burns treatment; however, sometimes they find that the material has inappropriate properties for dressings, but that presents excellent properties for packaging (they usually do not know that such developed material could be used for packaging! Wow! It does not work as a dressing but would be an excellent packaging!). In this sense, this book is intended to show a broad range of two-dimensional biopolymer applications, proving that good results in one application can guide good results in other uses as well. And why not say bad outcomes for one application can drive good results in other applications? The three key features and contents refer precisely to the fact that the proposed book focus on biopolymers, techniques for preparing membranes, and characterizing them. These contents are often convergent in different applications and may suggest new possibilities for novel applications and materials development. We are deeply grateful to all chapter authors for agreeing to be a part of this project. Authors used their experience from researching and teaching students to give chapters the relevance and appropriate content. We worked closely with them, proposing revisions to enhance harmony between the chapters; even with their full schedules, fortunately, the authors were very cordial and patient with us. We wish to thank the authors of books and articles, as well as the publishers for their permission to reproduce

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materials used here. We are also grateful to all those colleagues that even not being chapter authors devoted their precious time to help us to review the chapters. Thanks are also expressed to Elsevier Inc., especially Mr. Edward Payne, Mrs. Mariana K€ uhl Leme, and Mr. Vijayaraj Purushothaman for their support during this project. Finally, we have appreciated continued support and encouragement from our families, to whom we can never thank enough!

Mariana Agostini de Moraes, Classius Ferreira da Silva Institute of Environmental, Chemical and Pharmaceutical Science, Federal University of Sa˜o Paulo—UNIFESP, Diadema, SP, Brazil Rodrigo Silveira Vieira Department of Chemical Engineering, Federal University of Ceara´—UFC, Fortaleza, CE, Brazil

Acknowledgment to the chapters’ reviewers Prof. Andre´ Bezerra dos Santos Department of Hydraulic and Environmental Engineering Federal University of Ceara´ Fortaleza, CE, Brazil

Prof. Judith Pessoa de Andrade Feitosa Department of Organic and Inorganic Chemistry Federal University of Ceara´ Fortaleza, CE, Brazil

Prof. Andrea Cristiane Krause Bierhalz Department of Engineering Federal University of Santa Catarina— Campus Blumenau Blumenau, SC, Brazil

Prof. Julio Cesar Serafim Casini Department of Control and Automation Engineering Federal Institute of Sa˜o Paulo Sa˜o Jose´ dos Campos, SP, Brazil Prof. Melissa Gurgel Adeodato Vieira Department of Chemical Processes School of Chemical Engineering— UNICAMP Campinas, SP, Brazil

Dra. Daniele Farias Toulouse White Biotechnology (UMS INRA/INSA/CNRS) NAPA Center Bat B Ramonville-Saint-Agne, France Prof. Elizama Aguiar de Oliveira Department of Exact Science and Technology State University of Santa Cruz Ilhe´us, BA, Brazil

Prof. Odair P. Ferreira Laboratory of Advanced Functional Materials Department of Physics Federal University of Ceara´ Fortaleza, CE, Brazil

Prof. Gilcenara de Oliveira Nu´cleo de Pesquisa em Biologia Experimental Universidade de Fortaleza Fortaleza, CE, Brazil

Prof. Rinaldo Araujo Department of Chemistry and Environmental Federal Institute of Ceara´ Fortaleza, CE, Brazil

Prof. Joa˜o Vinı´cios Wirbitzki da Silveira Institute of Science and Technology Federal University of Jequitinhonha and Mucuri Valleys—Campus JK Diamantina, MG, Brazil

Prof. Roque Machado de Senna Institute of Environmental, Chemical and Pharmaceutical Sciences Federal University of Sa˜o Paulo Diadema, SP, Brazil

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C H A P T E R

1 Fundamentals on biopolymers and global demand Simone S. Silvaa,b, Luı´sa C. Rodriguesa,b, Emanuel M. Fernandesa,b, Rui L. Reisa,b,c a

3B’s Research Group, I3Bs—Research Institute on Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, Guimara˜es, Portugal bICVS/3B’s—PT Government Associate Laboratory, Braga/Guimara˜es, Portugal cThe Discoveries Centre for Regenerative and Precision Medicine, Headquarters at University of Minho, Guimara˜es, Portugal

Nomenclature AV

Aloe vera

BC CAGR CNCs 2D 3D EC ECM HA IL MB TA Tg SPEs SF SPI

bacterial cellulose compound annual growth rate cellulose nanocrystals two-dimensional three-dimensional ethylene carbonate extracellular matrix hyaluronic acid ionic liquid methylene blue ambiental temperature glass transition temperature dry solid polymer electrolytes silk fibroin soy protein isolate

Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00001-8

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# 2020 Elsevier Inc. All rights reserved.

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1. Fundamentals on biopolymers and global demand

1 Introduction The growing concerns for a sustainable environment and enhancement of the quality of people’s lives have been the starting points to evaluate the potential of natural polymers from renewable resources to create greener ways to address the problems of shortage of fossil fuel, health hazards, environmental issues, and solid waste management. Biopolymers can be classified as polysaccharides (e.g., chitin/chitosan, alginate, agarose, cellulose-based polymers, starch, and carrageenan) or proteins (e.g., soy protein, fibroin, sericin, gelatin, and collagen) and have been used alone or combined to produce membranes for multiple applications. However, large-scale commercialization biopolymer membranes are still a challenge. Therefore many processing techniques are available to produce membranes, and the choice of the appropriate one will depend not only on the features of the material itself but also on each particular application. Although the feasibility of the use of biopolymer membranes in different fields such as biomedical, food, energy, and the environment has been described in several studies, its use sometimes implies its modification and/or blending with other polymers (either natural or synthetic) to achieve adequate features for its application. This chapter addresses fundamental features in terms of intrinsic characteristics, main properties, and applications of biopolymers as membranes. Moreover, a look at the market trend is also discussed.

2 Polysaccharides and proteins: General characteristics 2.1 Polysaccharides Ranging from linear to highly branched structures, polysaccharides are, from a general point of view, polymeric carbohydrates composed of long monosaccharide units bounded by glycosidic linkages. As a consequence of their structure, they may assume different properties from their monosaccharide building blocks. Generally, polysaccharides from natural origins are simple carbohydrates with a unique monosaccharide repeat unit, which may be obtained or synthesized from a plethora of renewable resources. Natural polysaccharides are nontoxic and biodegradable, which increases their potential application. The most used biopolysaccharides summarized in Table 1 are obtained or synthesized from algae origin (e.g., alginate, agarose, carrageenan, fucoidan, and ulvan), plant origin (e.g., acemannan, cellulose, and starch), microbial origin (bacterial cellulose [BC], dextran, and gellan gum), and animal origin (e.g., chitin/chitosan, chondroitin sulfate, glycosaminoglycans, heparin, and hyaluronan). Moreover, these natural derivatives present a considerable number of reactive functional groups (e.g., hydroxyl, carboxyl, and amino groups), which significantly increase their applicability through chemical modification or physical blend. In the following sections a detailed description of selected polysaccharides’ physicochemical and biological properties is presented. 2.1.1 Alginate Alginic acid, also called alginate, is an anionic polysaccharide that is present in nature, mostly as one of the constituents of brown algae (Phaeophyceae) [88]. It is a linear unbranched I. Fundamentals on biopolymers membranes and films

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2 Polysaccharides and proteins: General characteristics

TABLE 1

General characteristics of the natural biomacromolecules.

Polymer

Source

Characteristic features

References

Agarose

Derived from agar and found in red seaweeds

• Composed of repeating units of (1 ! 3)-β-D-galactopyranose-(1 ! 4)3,6-anhydro-β-L-galactopyranose • Water-soluble and neutral polysaccharide • Low gelling temperature (32°C)

[1,2]

Alginate

Brown algae (Phaeophyceae)

• Gel formation in the presence of divalent cations (e.g., Ca2+, Mg2+) • Composed of β(1 ! 4)-linked D-mannuronic acid and α(1 ! 4)linked L-guluronic acid • Processed as hydrogels, membranes, capsules, fibers, or scaffolds

[3–9]

Bacterial cellulose

Gluconacetobacter xylinus or Acetobacter xylinum

• Glucan chains bound together by hydrogen bonds • Crystalline nanofibrillar structure • Conjugated with chitosan, alginate, gelatin, hyaluronic acid, and xyloglucan to improve performance

[10–16]

Carrageenan

Red algae

• Made up of repeating galactose units and 3,6-anhydrogalactose, both sulfated and nonsulfated • Three main types of carrageenan can be obtained: kappa (κ), iota (ι), and lambda (λ)

[17–19]

Cellulose

Woods

• High degree of crystallinity and rigid intra/intermolecular hydrogen bonds • Constituted by β-1,4glycosidic-linked D-glucose units • Insoluble in water and most organic solvents • Solubility and processability achieved by using ionic liquids • Undergoes chemical modification through esterification, graft copolymerization, or selective oxidation

[20–25]

Collagen

Cornea, blood vessels, skin, cartilage, bone, tendon, ligament, marine sponges, and fish skin

• Main component of the extracellular matrix and the most abundant protein present in mammalian tissues • Provides mechanical strength to tissues and stimulates cell adhesion and proliferation

[26–29]

Continued

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TABLE 1 General characteristics of the natural biomacromolecules—cont’d Polymer

Source

Characteristic features

References

• Can be chemically modified or combined with polysaccharides and/ or bioactive molecules to improve their physicochemical properties and biological performance Chitin

Cell walls of fungi, exoskeletons of arthropods such as crustaceans and insects, radulas of mollusks, and beaks of cephalopods

• • • •

Strong intermolecular bonding Insoluble in common solvents Solubilization using ionic liquids Molded into different forms (e.g., gels, beads, membranes, sponges, tubes, and fibers)

Chitosan

N-Deacetylated derivative of chitin

• Solubilization in aqueous acidic solutions • Designed as 2D- and 3D-based architectures such as fibers, particles, membranes, and composites at micro/nanolevel

[31,37,39,41, 49–68]

Dextran

Produced by several microorganisms

• Composed of (1 ! 6)-linked α-Dglucopyranosyl units • Soluble in water and organic solvents • Biocompatible and biodegradable

[27,46,69]

Gellan gum

Bacterium Sphingomonas elodea

• Anionic exopolysaccharide • Consists of a repeating unit of a tetrasaccharide: 1,3-linked-D-glucose, 1,4-linked-D-glucuronic acid, 1,4linked-D-glucose, and 1,4-linked-Lrhamnose • Thermally reversible gel with excellent stability and high gel strength • Forms gels in the presence of metal cations

[15,70–75]

Hyaluronic acid

Rooster combs or aqueous humors of cow’s eyes

• Composed of D-glucuronic acid and N-acetyl-D-glucosamine • Water-soluble polysaccharide • Low shape stability and poor mechanical properties

[18,19,49, 76–84]

Pectin

Structural material of all land-based plants

• Composed of poly-D-galacturonic acid molecules • Water-soluble biopolymer • Ability to form gels

[27,47,85]

Sericin

Waste during silk processing

• Soluble protein

[86]

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[9,30–48]

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2 Polysaccharides and proteins: General characteristics

TABLE 1 Polymer

General characteristics of the natural biomacromolecules—cont’d Source

Characteristic features

References

• Antioxidant, moisturizing ability, pH responsiveness, and mitogenic effect on mammalian cells Silk fibroin

Nonmulberry and mulberry silkworm Bombyx mori, insects, and spiders

• Beta sheet formation after metanol/ water solution treatment

[1,2]

Soy protein

Soy beans

• Low price, nonanimal origin, relatively long storage time, and stability

[87]

Starch

Corn, wheat, potato, and rice starch

• Composed of amylose and amylopectin • Starch can be transformed into thermoplastic materials or blended with synthetic polymers

[80–83]

polysaccharide composed of different amounts of β(1 ! 4)-linked D-mannuronic acid and α(1 ! 4)-linked L-guluronic acid [89]. One of the most relevant features of alginate is its ability to form gels in the presence of various divalent cations, e.g., Ca2+, Mg2+, due to the carboxylate groups of the guluronate cross-linking on the polymer backbone. Alginate is a biocompatible and antimicrobial polymer with immunogenicity, low toxicity, and stability in physiological conditions [3,4,90–92]. This polysaccharide may be processed in different forms as hydrogels, membranes, capsules, fibers, or scaffolds [5,6,93–96], which significantly enlarge the scope of application [6,7,76,77,91,93,94,97–100]. Furthermore, to optimize the physical properties of hydrogels, alginate has been covalently cross-linked and oxidized [8,101]; however, an associated drawback is the limited degradation of the covalently cross-linked alginate gels, since cells do not secrete the necessary enzymes for polymer cleavage. Nevertheless, alginate properties can be tailored or enhanced using other strategies based on conventional chemical modification such as graft polymerization, sulfation, or esterification [20,102,103]. 2.1.2 Cellulose Cellulose is the most abundant polysaccharide from natural origin in the world, and is mostly produced by plants. It is a polydisperse linear polysaccharide constituted by β-1,4glycosidic linked D-glucose units (so-called anhydroglucose units) [21] giving origin to a rigid straight chain due to the many inter- and intramolecular hydrogen bonds established among the many hydroxyl groups to form what is known as a cellulose microfibril, or simply fibril. This close packaging of the cellulose chains leads to areas of high crystallinity within the polymer and to high stability structures, which as a consequence promote considerable strength, remarkable inertness, and insolubility in water and common organic solvents. Significant efforts have been made to overcome these drawbacks, such as the chemical

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1. Fundamentals on biopolymers and global demand

modification of cellulose through esterification, graft copolymerization, or selective oxidation [104–107] to improve resistance to heat or abrasion [108–110], mechanical strength [111,112], water or oil repellency [113–115], or antibacterial activity [116]. Moreover, most of the chemical functionalization procedures are based on hydroxyl groups. To date, several studies have reported different designs, fabrications, and processing of cellulose. Grafted cellulose copolymers present a well-defined architecture, which promotes potential applicability in broader fields. Moreover, when stimuli-responsive side polymer chains were grafted into the polymeric chain [10,22,49,117], it gave foundation to cellulose-based smart materials [11,105]. A more recent approach allows a homogeneous modification and functionalization of cellulose through the use of ionic liquids (ILs), which may represent a green approach to cellulose processing, enlarging its range of applicability [12,118]. 2.1.3 Bacterial cellulose BC, similarly to plant-derived cellulose, is a natural polymer composed of microfibrils containing glucan chains bound together by hydrogen bonds. BC is produced extracellularly by Gram-negative bacterial cultures, most efficiently secreted from Gluconacetobacter xylinus (Acetobacter xylinum). It is a highly pure, biocompatible, and versatile material that can be utilized in several applications. Biosynthesized as a pellicle comprised of a random microfibrillar network of cellulose chains aligned in parallel, BC presents a large surface area capable of retaining a large amount of liquid [23]. However, like plant-derived cellulose, BC is quite inert, and its interaction (e.g., entrapment or grafting) with several bioactive compounds significant in tissue regeneration, such as drugs, polyelectrolytes, or proteins, is difficult. To overcome BC inertness, several functionalization techniques were performed as well as several conjugations with chitosan [13,119], alginate [14], gelatin [120], hyaluronic acid (HA) [15], and xyloglucan [78]. 2.1.4 Chitin and chitosan Chitin is a biopolymer of N-acetylglucosamine with some glucosamine, which is the main component of the cell walls of fungi, exoskeletons of arthropods such as crustaceans and insects, radulas of mollusks, and beaks of cephalopods; it is considered the second most abundant natural polymer after cellulose. Chemically, chitin is made of monomer units of 2-acetamido-2-deoxy-β-D-glucose connected through β(1 ! 4) linkages. Because of its highly crystalline structure and strong inter- and intramolecular bonds between the polymer chains, chitin is insoluble in common solvents. On its glucose ring, chitin has acetamido groups that undergo incomplete hydrolysis into primary amine groups, N-deacetylation of chitin, which leads to the formation of chitosan that can be easily dissolved in aqueous acidic solutions, which makes it suitable for various applications. Moreover, chitin is also widely used for controlled drug delivery systems, protein and enzyme carriers, and packaging material based on its natural antimicrobial activity [121]. Chitin and its derivatives (e.g., chitosan) have many useful properties that make them suitable for a wide variety of applications. Chitin and chitosan can be molded into different forms (e.g., gels, beads, membranes, sponges, tubes, and fibers). Also, their products are known to be antibacterial, antifungal, antiviral, nontoxic, and nonallergic [16,30–32,122–124]. Chitin structure modification and combination with other polymers and bioactive molecules are being studied to enhance its mechanical and chemical properties. I. Fundamentals on biopolymers membranes and films

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2 Polysaccharides and proteins: General characteristics

Addition and heating

Wash to remove moisture

Cast into molds

Ionic liquid Chitin-IL solution

Water coagulation

Chitin-IL film

Freeze drying

200 m

Chitin film

Chitin powder Raw materials

Solution

Membrane

Porous membrane

FIG. 1 Schematic diagram showing steps for chitin porous membrane preparation inspired by the methodology proposed in [127].

Furthermore, ILs, which are low-melting-point molten salts, are identified as good solvents for natural polysaccharides and accordingly research concerning the dissolution of chitin with proper ILs has attracted attention to fabricate new chitin-based functional materials [33,34,125,126]. Many ILs have been used to dissolve chitin; however, it has been shown that chitin dissolution requires a more basic anion, such as acetate, due to the increased number of hydrogen bond donors and acceptors [35,127]. However, little has been reported regarding the chemical modification of chitin in the ILs [36,125,128]. Chitin membranes were successfully prepared by using the IL 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) as solvent media [127], according to Fig. 1, which includes a schematic representation of the laboratory methodology employed. The study proved that the chitin/IL solution could be efficiently used to produce porous chitin membranes with tunable properties, acting as an environmentally friendly process to apply it as a biomaterial. Chitin membranes were prepared in a range of concentrations comprising between 2% and 3% with 2%, the tensile strength and elongation percentage increased with wt% chitin solution, which is supposed to be related to the porosity percentage of membranes that increase to lower concentrations [127]. The porosity and pore distribution pattern in the membrane also affected mechanical strength, enthalpy, and water absorption capacity. The study also reported the existence of a positive correlation between enthalpy and tensile strength for the chitin membranes. These chitin membranes had a porosity >80% and porous interconnectivity. Chitosan can be designed as 2D- and 3D-based architectures such as fibers, particles, membranes, and composites at the micro/nanolevel. Chitosan membranes developed using solvent casting methodology present swelling ability, cytocompatibility, oxygen permeability, moisture transmission, controlled release, antibacterial potential, epithelialization, and controlled water evaporation rate, enlarging the range of their application [50–53,129,130]. When combined with alginate to form a polyelectrolyte complex, the obtained membranes present improved tensile strength and adequate elongation at break, greater stability concerning changes in pH, and a more effective controlled release than that obtained for the individual polysaccharides [37,123,131]. Along this line, chitosan, together with other macromolecules, is being used in the development of various bionanocomposites [132–136]. 2.1.5 Gellan gum Gellan gum is an exopolysaccharide with an anionic nature that consists of a repeating unit of a tetrasaccharide: 1,3-linked-D-glucose, 1,4-linked-d-glucuronic acid, 1,4-linked-D-glucose, I. Fundamentals on biopolymers membranes and films

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1. Fundamentals on biopolymers and global demand

and 1,4-linked-L-rhamnose. Like alginate, gellan gum forms gels in the presence of metal cations and is easily processed into transparent gels that are resistant to heat [137]. Gellan gum gels, from a rheological point of view, are slightly soft, which is difficult for extrusion; however, this can be modulated through its combination with alginate or other polymers. Good results have already been reported using a 3% gellan gum/2% alginate blend, which was cross-linked with strontium ions after extrusion [70]. Despite the advantages of gellan gum, pure gellan gum film can be extremely brittle, and the incorporation of honey or virgin coconut oil appears to be an alternative pathway to reinforce their mechanical properties [71,72]. 2.1.6 Hyaluronic acid Hyaluronan (HA) (a disaccharide composed of D-glucuronic acid and N-acetyl-D-glucosamine) is an elastoviscous fluid containing hyaluronan derivatives. It is a naturally occurring complex sugar that forms a small part of the extracellular matrix (ECM) and is therefore nonallergic, which is an advantage of structural conservation regardless of the source. It is a readily water-soluble molecule, and thereof HA use in its native form in tissue engineering and drug delivery solutions is limited. Several cross-linking methodologies were employed to enlarge HA stability [73,74,138–140] as a covalent cross-linking technique, which provide the opportunity to combine HA with mechanically stronger polymers [11]. HA can produce gels with a lubricant and water-absorbing behavior, adding to their hygroscopic and homeostatic properties. 2.1.7 Carrageenan Carrageenans are linear polymers made up of repeating galactose units and 3,6anhydrogalactose, both sulfated and nonsulfated. Carrageenan can be obtained from some species of red algae [17,18]. Depending on the source and conditions from which carrageenan is extracted, three main types of carrageenan can be obtained: kappa (κ), iota (ι), and lambda (λ) that correspond to one, two, and three sulfate groups per disaccharide, respectively. Only ι- and λ-carrageenan can form physical gels, while κ-carrageenan hydrogels exhibit pH and temperature sensitiveness [19]. 2.1.8 Starch Starch is a carbohydrate polymer composed of two macromolecules, namely amylose, a linear polysaccharide, and amylopectin, a branching polysaccharide [79]. Amylose forms a colloidal dispersion in hot water, whereas amylopectin is completely insoluble. Starch can be obtained from many botanical species, e.g., corn, wheat, potato, and rice starch. Then, starch molecules produced by each plant species have specific structures and compositions; therefore the properties and mode of interactions of starch with other polymers differ depending on the source [80]. Moreover, starch may be chemically, enzymatically, or physically modified to enhance its properties and functionality such as solubility, viscosity, and thermal stability. Besides, starch has been transformed into a thermoplastic or blended with synthetic polymers to improve its properties [81–83].

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2.1.9 Dextran Dextran is a biodegradable neutral bacterial exopolysaccharide from the glucans family composed of (1 ! 6)-linked α-D-glucopyranosyl units. It is obtained by the polymerization of the α-D-glucopyranosyl moiety of sucrose in a reaction catalyzed by the enzyme dextransucrase [27]. Several microorganisms can produce it, presenting different molecular weights, ranging between 1000 and 40,000,000 Da), and structures varying from slightly to highly branched. Dextran is a biocompatible and biodegradable biopolymer that is soluble in water and organic solvents. Dextran chemically reactive hydroxyl groups are useful points for chemical conjugation or functionalization, which enable the preparation of different structures through blending dextran with bioactive agents or hydrophobic moieties forming, for example, dextran-based amphiphiles [69]. The development of novel functional scaffolds resulting from modification with different functional groups achieves spherical, tubular, and 3D network structures [69]. 2.1.10 Agarose Agarose is a natural polysaccharide derived from agar and found in red seaweeds [1,2]. It is a water-soluble, neutral polysaccharide composed of repeating units of (1 ! 3)-β-Dgalactopyranose-(1 ! 4)-3,6-anhydro-β-L-galactopyranose [2,141]. Agarose has been used as a bioink due to its low gelling temperature of 32°C, biocompatibility, and mechanical strength [1]. Agarose also has the advantage of enabling the printing of complex structures due to its unique gelling properties that do not require the presence of a cross-linker [141]. Numerous agarose derivatives have been prepared with different features and melting points, and from these the low-melting derivatives allow the formation of gels with thin fibers for enhanced sieving. 2.1.11 Pectin Pectin, also known as pectic polysaccharide, is a complex mixture of polysaccharides, mostly composed of poly-D-galacturonic acid molecules corresponding to an average molecular weight ranging from 50,000 to 180,000 Da, including mainly carboxyl groups in its chains [85]. Pectin is a water-soluble biopolymer found in all land-based plants as a structural material and in its utmost concentration is found in the central lamella cell wall. Pectin can form gels; however, it is dependent on its molecular size and degree of esterification.

2.2 Proteins Proteins are macromolecules existing in living systems, composed of structural units called amino acids that are attached in long chains. Containing mostly carbon, hydrogen, oxygen, nitrogen, and usually sulfur and phosphorus, proteins differ in the number and type of amino acids that assemble to form the polypeptide chain and in their 3D structure, which defines the proteins’ functional properties [142]. Proteins are generally self-assembled systems due to the presence of different intermolecular interactions such as hydrogen bonds, disulfide bridges, salt bridges, and hydrophobic and hydrophilic interactions. As so, proteins are large-sized molecules that, when dispersed in suitable solvents, form colloids, which is a property that distinguishes proteins from solutions containing small-sized molecules.

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Principal compounds in all cell proteins are of two types: bioactive or not bioactive. The bioactive proteins are also known as enzymes and are responsible for cell life cycle and the metabolism and synthesis of other compounds. The not bioactive ones, called storage proteins, are very stable, have excellent physical and chemical properties, and are used as biobased polymers for many applications [143]. Mainly due to their high availability in nature, low cost, biodegradability, and excellent biocompatibility, proteins have been used in the development of different architectures, including films, capsules, foams, composites, and gels [86,144]. In the following sections, some of the most frequently used proteins, namely silk fibroin (SF), sericin, collagen, gelatin, and soy proteins, are described. The main characteristics of the listed proteins are summarized in Table 1. 2.2.1 Collagen and gelatin Collagen is the main component of the ECM, and is the most abundant protein present in mammalian tissues (cornea, blood vessels, skin, cartilage, bone, tendon, and ligament), which provides mechanical strength to tissues and stimulates cell adhesion and proliferation [145]. Gelatin is a collagen derivative obtained by an incomplete denaturalization of collagen extracted from porcine skin and connective tissues; however, marine sources, e.g., marine sponges, fish skin, are also recognized as collagen sources [18]. Both collagen and gelatin have been employed to produce membranes, sponges, nanofibers, and microspheres for a wide range of applications [87,146,147]. Depending on the purpose, these proteins can be chemically modified or combined with polysaccharides and/or bioactive molecules to improve their physicochemical properties and biological performance. 2.2.2 Silk fibroin Silk is a class of proteins composed of fibroin, the structural protein of silk fibers, and sericin, the water-soluble glue-like protein that keeps the fibroin fibers together. SF is composed of glycine, alanine, and serine in different percentages. Silk proteins (fibroin and sericin) produced by silkworms are classified into nonmulberry and mulberry (Bombyx mori). Besides, SF can also be found in insects and spiders [148]. Given its favorable biocompatibility, elasticity, toughness, and mechanical properties, SF has been exploited in the production of matrices for different applications from tissue engineering and regenerative medicine to textiles or optoelectronics [149–151]. By controlling the protein secondary structure from α-helical chain arrangements into β-sheets via alcohol treatment or water vapor annealing, the biodegradation rate, mechanical properties, and release of bioactive molecules can be tuned [152]. Beyond that, at physiological pH, the silk is negatively charged, providing sites for initial electrostatic interactions with cationic macromolecules and small molecules. Those active molecules can be attached to SF, promoting the modulation of the SF proteins allowing adjustment to the requirements of the envisioned application [151]. 2.2.3 Sericin Sericin is a gumming protein that binds silk fibers together. It is composed of serine, glycine, glutamic acid, aspartic acid, threonine, and tyrosine. Sericin has attractive properties, namely antioxidant, moisturizing, collagen production, and pH responsiveness abilities [24].

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13

Sericin itself can form films, hydrogels, and sponges, but those matrices could have poor mechanical performance. Stable sericin-based matrices are produced using ethanol precipitation, cross-linking or blending with natural polymers and synthetic ones [25,28,29]. For instance, silk/agar blend membranes have good mechanical properties and antimicrobial ability [25]. 2.2.4 Soy protein Soy protein is a globular protein isolated from soybeans. Soy protein has advantages over the various types of natural proteins, namely low price, nonanimal origin, relatively long storage time, and stability. Besides, the combination of its properties with its reduced susceptibility to thermal degradation makes soy protein a plant-derived macromolecule of high interest [38–41,144]. Soy protein isolate (SPI) has film-forming ability; however, SPI membranes could have poor mechanical properties and relatively high moisture sensitivity, thus limiting their applications [42]. Therefore many efforts have been made to enhance the mechanical performance of soy protein-based membranes through the addition of a plasticizer, cross-linking agent, or even the association of soy protein with other biomacromolecules (e.g., chitosan, agar), which are methods that enhance the mechanical performance of the soy protein-based membranes, expanding their applications [42–45].

3 Applications of the membranes of biopolymers 3.1 Biomedical applications Biopolymer-based membranes made of natural polymers seem to be particularly attractive for biomedical applications due to their diversity and easy processing. In this research line, some progress has been made in extending their use in wound repair, bone guide regeneration, drug delivery, and others. As an example, chitin or chitosan acetate/formate polymer has been applied to produce chitin-reinforced nonwoven fabrics, used as artificial skin adhered to the body stimulating new skin formation, which accelerates the healing rate and reduces pain [124]. Besides, several studies on blended membranes derived from natural sources for biomedical applications have also been described in the literature [48,54–56], where their composition and processing techniques are important parameters to determine their features. In fact, the combination of polysaccharides and proteins is a method frequently used to design blended materials with improved performance regarding swelling, mechanical resistance, and biocompatibility, among other features. Plenty of attention has been focused on membranes based on chitosan blended with other biomacromolecules such as alginate, BC, cellulose, collagen, gelatin, keratin, sericin, and soy protein, among others. Recent studies also suggest a positive interaction between chitosan and plant extracts, e.g., Aloe vera (AV), a medicinal plant, can promote the improvement of antimicrobial action, water absorption, and biological performance of the blended membranes [45]; in other approaches, the interactions between chitosan and alginate [57,58] or gelatin [55,56,59,60] have also been employed in the preparation of polyelectrolyte complexes. These blended membranes have been mainly proposed

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as drug delivery systems and wound dressings, and for skin regeneration and guided bone regeneration. Besides the keen attention given to chitosan and chitosan-blended membrane applications for biomedical solutions, there are other natural polymers that are also advantageous alternatives. Alginate presents limitations regarding mechanical properties, degradation, and lack of cell recognition; however, alginate membranes have been widely used as cell carriers in tissue engineering and for wound-dressing/healing and protein/drug delivery devices [7,8,76,93,100,101]. Gellan gum is a noncytotoxic thermoreversible hydrogel [61] that can be injected into tissues and used for the encapsulation and in vitro culture of cells [62]. Nowadays, several medical and pharmaceutical applications of gellan gum have been reported such as “dual layer membranes” [63], bioink substrates for living cell printing [64], dressing materials [65], or as a vehicle for ophthalmic drugs [66]. From another source, we found cellulose and their enhanced derivatives. One of those is related to oxidized cellulose, which, because of its biodegradable, bactericidal, and hemostatic properties, has been used as a topical hemostatic wound dressing in a variety of surgical procedures and skin and subcutaneous tissue procedures [112]. Grafted cellulose copolymers have found applicability as micelles and drug carriers, due to their intrinsic, adsorbent, protein adsorption-resistant, and antibacterial properties. Moreover, when cellulose was grafted with stimuli-responsive side polymer chains [113–115], it gave foundation to cellulose-based smart materials that may find application in active packaging, biosensors, tissue engineering, antimicrobial surfaces, separation, and detection or smart clothing [20,116]. Currently, BC membranes are already commercially available as wound-dressing materials [10,22,117] based on their intrinsic properties such as high in vivo biocompatibility [11], optimal 3D cell attachment substrate, flexibility, high water retention, and gas exchange capabilities [12]. Additionally, BC membranes act as a physical barrier reducing pain, bacterial infection, and allowing drug transfer into the wounded region [22,118], displaying accelerated epithelialization and tissue regeneration rates in several wound-healing treatments, like diabetic foot wounds, chronic wounds, and burns [67,68]. The conjugation of BC with chitosan resulted in a combination of properties of the two biopolymers such as bioactivity, biocompatibility, biodegradability, creating an excellent dressing material capable of isolating the wound from the environment, and healing stimulation [121,122]. BC/chitosan wound dressing is considered to be an innovative solution considering their good antibacterial and barrier properties as well as adequate mechanical properties and, in the wet state, high moisture retention that may be applied as a dressing material for treating various kinds of wounds, burns, and ulcers [16]. BC/HA in the form of membranes demonstrated the potential application of these systems in tissue engineering and bone regeneration [68,75,84]. Research on BC in the form of membranes has been applied for guided bone regeneration in bone defects of critical and noncritical size [84]; in periodontal lesions [153]; and as a resorbable barrier membrane for preventing the invasion of fibroblast cells and fibrous connective tissue into bone defects [75]. HA has found applicability in a wide variety of medical fields from neurosurgery to cutaneous wound healing and even cosmetic practice where it is used as a dermal filler. HA gels found applicability in osteoarthritis treatment being used as a joint visco-supplement on

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osteoarthritis patients in which conventional therapies have failed. In the wound-healing field, an HA-derived material has been developed based on the bilaminar concept, Laserskin, a keratinocyte culture/transfer device. Clinical studies into Laserskin application with keratinocytes alone or cocultured with fibroblast demonstrated high rates of healing when cocultured systems were used [72,138]. In summary, HA profoundly impacted tissueengineering solutions due to its incorporation in biomaterials, yielding a new class of biocompatible, controllable, and readily degradable materials capable of promoting beneficial remodeling of engineered tissues, as well as preserving cell phenotypes [116]. Dextran is a water-soluble inexpensive and readily available polysaccharide that allows the production of biodegradable and biocompatible membranes by electrospinning. These have found application in various biomedical applications, such as a plasma volume expander and drug delivery systems [46,154,155]. Pectin presents a lack of toxicity and low production costs, which potentiate its use for the design of targeted-release dosage forms. Moreover, pectin’s physicochemical properties such as hydrophilicity make it attractive to study as wound-dressing materials allowing easy removal of exudates and maintenance of acidic pH acting as a barrier against bacteria or fungi [47]. SF-based membranes can promote the wound-healing process, angiogenesis induction, and osteogenic differentiation [17–19]. Therefore they have been suggested for different applications, namely as a wound dressing for skin regeneration, controlled release of antimicrobial drugs, and guided bone regeneration, among other uses [17,79,80].

3.2 Food Food products undergo regular alteration by a considerable number of physical, chemical, and microbial factors during storage. Therefore to maintain the quality of food products, food packaging has been used not only because it reduces food spoilage but also because it reinforces the quality and extends the shelf life of food products [156]. Many companies and researchers have been working on the development of new environmentally friendly packaging approaches using biopolymers such as edible films, coatings, and packaging materials. These food applications are related to biodegradability, adequate mechanical properties, and antioxidant and antimicrobial action of the natural polymers. They may also be used as carriers for the controlled release of drugs or fungicides. Fig. 2 summarizes edible coatings and film compositions. The main components are natural polymers from renewable sources such as alginate, chitosan, gelatin, and soy protein. They have been useful to enhance the shelf life of foods, improve their preservation, and protect them from oxidation and microbial spoilage. Alginate has been used in food and drink products. The US Food and Drug Administration classifies food-grade sodium alginate as a generally regarded as safe substance and lists its use as an emulsifier, stabilizer, thickener, and gelling agent. The unique gelling abilities of alginate at low temperatures, together with its good heat stability, make alginate ideal for use as a thickening agent [5,6]. Moreover, alginate-based edible coatings and films can maintain the quality and shelf life of prepared vegetables, fruits, seafood, and cheese by reducing dehydration, improving mechanical properties, and enhancing product appearance [6,7]. For instance, an alginate coating at 3% concentration maintained the quality of mango fruits, and

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Edible films and coatings

Main components

Function

Biopolymers - Polysaccharides (cellulose, starch, alginate, chitosan) - Proteins (soybean proteins, corn zein, gelatin, and keratin)

Additives

Solvents Water and ethanol

-plasticizers, emulsifiers, crosslinking agents

Structural protection to prevent biological, chemical and physical deterioration

-antioxidants, antimicrobials and nutraceuticals

- Lipids (waxes, free fatty acids and sucrose esters)

Barrier to gases, vapors, solutes and lipids - To improve or modify the functionality of the material - To improve the quality, stability, and safety of packaged foods

Vehicle of additives and active compounds

FIG. 2 Edible film and coating compositions and functions. Based on P.R. Salgado, C.M. Ortiz, Y.S. Musso, L. Di Giorgio, A.N. Mauri, Edible films and coatings containing bioactives, Curr. Opin. Food Sci. 5 (2015) 86–92.

prevented water loss and preserved the antioxidant properties, phenol, and flavonoid compounds of mango fruit during storage [6]. Also, antimicrobial agents such as thyme oil [3], carvacrol [92], sodium lactate, and sodium diacetate [4] have been incorporated into alginate-based edible coatings and films to decrease the microbial growth of coated food products. Chitosan has several applications in food and nutrition because of its high nutritional quality, strong antimicrobial properties against fungi, bacteria, and viruses, nontoxicity, oxygen and carbon dioxide barrier properties, and film-forming ability [16,30,122]. Based on these features, chitosan-based film can be manufactured for edible food packaging, where properties of chitosan films can be manipulated by degree of N-acetylation, molecular weight, and solvent evaporation. Different chitosan-based films have been produced and applied as food packaging materials or as food preservatives to fruits, vegetables, and meat [157]. Promising approaches involve the combination of chitosan with components such as active substances (e.g., plasticizers, antioxidant substances, and essential oils), and other polysaccharides, proteins, and lipids have been investigated aiming to adapt the final product, enhancing food quality lifespan and modulating the properties of the polymer. For instance, combinations of gum arabic with chitosan have fungicidal effects allowing the control of postharvest anthracnose in banana fruit [158]; also, the incorporation of green tea extract into chitosan film enhanced the antimicrobial and antioxidant activity of the film, maintained quality, and prolonged the shelf life of pork sausages [159]. Similar to alginate, investigations into the incorporation of essential oils (e.g., cinnamon oil, clove oil, eugenol, and ginger oil) in chitosan-based films have been demonstrated by prolonging food storage time without loss

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of texture, color, or overall acceptability and inhibition of the growth of microorganisms [160–162]. Besides, the addition of extracts from plants into chitosan also enhanced film activity, namely antioxidant action (citrus extract), barrier performance (thyme extract), and antimicrobial activity (honeysuckle flower extract) [163–165]. Despite promising findings, some limitations related to chitosan film production were still found. For example, chitosan incorporated with ginger and eugenol essential oils were fabricated for food packaging applications [162]; and the obtained film demonstrated great antioxidant action but water vapor permeability was still reasonably high. Therefore further development of edible antioxidant films to improve physical properties should be considered. Gelatin is a versatile substance as a component in foods, including desserts, gummy candies, and many yogurts, and provides viscoelastic and emulsifying properties as well as acts as an edible coating. Gelatin edible coatings and films have been applied by dipping, and used as an effective barrier against oxygen and to reduce moisture loss due to their high processability, adhesiveness, nontoxicity, and excellent film-forming abilities. Several studies have attempted to evaluate the effect of the addition of plasticizers, active substances, or other biopolymers such as soy protein [166], chitosan [167], cellulose derivative [168], and starch [169]. By using these approaches, the physicochemical properties and functionality of gelatin-based materials have been optimized. Despite that, the use of gelatin as a food package is limited due to poor mechanical properties. These drawbacks can also be altered by the incorporation of other food components, lipids, or oils (fatty acid, waxes) to protein to produce films with improved mechanical features [146]. Edible starch films and coatings are used for food protection to increase shelf life by retarding physiological processes, such as respiration, degradation of cell walls, and transpiration, and also restricting microbial action [170]. Films can be obtained by casting or thermal processing (thermoplastic starch) allowing the incorporation of a great number of components into the matrix, improving film properties [171–173]. At optimized conditions, starch films are transparent, odorless, tasteless, and colorless, with good mechanical barrier (impervious to oxygen), and optical properties. However, starch films have a highly hydrophilic nature that, conjugated with its retrogradation, limits their usefulness [171,174]. Nevertheless, these limits may be overcome by the incorporation of additives, such as lipids, other hydrocolloids, or reinforcement agents, promoting the achievement of more stable materials reducing the inherent weaknesses of natural starch [171,173]. As an example, chitosan/starch-blended films exhibited an improved barrier and mechanical properties as well as good antimicrobial effect [175]; they are also a good candidate for food packaging solutions. Soy protein (e.g., concentrates, isolates, and texturized forms) has been used for emulsifying and texturizing food formulations [176], and also in the manufacture of packaging materials [177] due to its good nutritional properties, functional features, and health effects [178]. In fact, soy proteins are rich in essential and nonessential amino acids; therefore they provide all of the nine essential amino acids and bioactive isoflavones. Soy protein coatings have been explored as an alternative edible coating for the preservation of fruits such as strawberry and apple as well as processed meats. In these studies, soy protein edible coating was found to be effective in reducing oxidative browning and moisture loss during the storage of cut apples, potatoes, carrots, and onions [39], and in delaying lipid oxidation and deterioration of meats during storage [41].

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Novel strategies involving 3D food printing technology using different types of food material with specific processing features have been explored as a way to personalize food design and small-scale food production. In this context, recent studies pointed out that 3D printing of soy protein mixtures with sodium alginate and gelatin can be well printed with stable geometry, suggesting their use in 3D food printing [40]. AV gel is an attractive material in food conservation, given its antimicrobial and antioxidant activity, and also as a source of functional foods, mainly in drinks and beverages [179]. It has also been evaluated as a potential component in edible coatings and films, since its low film-forming ability can restrict its use alone [180]. Additives or other coat-forming compounds (gelatin, chitosan, pectin) have been used to produce edible coatings for the preservation of different fruits and vegetables [181,182]. These approaches suggested that AV reduces respiration rate, ethylene production, weight loss, and softening while maintaining other features, e.g., color and firmness. For instance, the addition of AV gel to unripe banana starch/chitosan film-forming solutions had a cross-linking effect between the phenolic compounds in AV gel and starch molecules, leading to less rigid films and modulation of water vapor permeability [183].

3.3 Environment Currently, a significant concern is growing about the escalating pollution of natural resources. The deterioration of freshwater resources is a result of contamination by human activity such as the discharge of untreated or inadequately treated industrial effluents, or herbicides transported to the soil and aquatic systems. Proportional to population growth is the demand for vital resources, and consequently the challenge to achieve adequate and sustainable water supplies through new and inexpensive technologies for nontraditional water sources, such as brackish water and seawater purification [184]. Traditional methods in wastewater treatment, such as coagulation/flocculation, adsorption, ion exchange, membrane separation, evaporation, flotation, and electroprecipitation, may not be entirely capable of achieving the required extent of purification [185,186]. Therefore membrane technologies were selected as promising purification candidates because they are energy efficient and have a small footprint. Polymeric membranes control the rates of water and/or salt transport in different approaches such as reverse osmosis, forward osmosis, electrodialysis, reverse electrodialysis, and pressure-retarded osmosis [184,187–189]. Nowadays, membrane science is increasing the focus on natural raw materials to develop biopolymer membranes able to perform separation based on other driving forces like electrical charge, physicochemical interactions, and functionalization with groups capable of providing applications such as tunable water permeation and separation, toxic metal capture, toxic organic dechlorination, and biocatalysts [190,191]. Cellulose and cellulose derivatives are among the most intensively studied natural polymer sources for water purification membrane preparation [135,192–196]. As an example, a BC-based membrane with sodium alginate was developed for the separation of ethanol/water mixtures, which is impossible using conventional distillation due to the mixtures’ azeotropic nature [194]. Cellulose triacetate/silica composite membranes were developed according to sol-gel methodology for use in the retention of

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heavy ions (Cu2+, Ni2+, Cd2+, and Pb2+) from aqueous solutions [195]. The silica particles used as a filler retain the heavy ions [195]. Another approach was based on the antibacterial properties of cellulose derivatives, which were enhanced by the preparation of silver sulfadiazineloaded BC/sodium alginate composite membranes [196], which exhibited very good antibacterial properties for Escherichia coli and Staphylococcus aureus. Chitosan is used in the preparation of water purification membranes according to different preparation strategies [135]. For salt removal from water, chitosan was, for example, used as a coating layer for a poly(1,4-phenylene ether sulfone) membrane. The separation process values achieved for the proposed membranes were lower; however, this proposal may be preferred in the case of partial desalination processes [197]. Composite membranes of chitosan/ cellulose nanocrystals (CNCs) prepared by the freeze-drying technique followed by compacting were used to remove the organic dyes from water [198]. The membranes, after a contact time of 24 h, successfully removed more than 70% of positively charged dyes (Victoria Blue 2B, Methyl Violet 2B, and Rhodamine 6G). This outstanding performance is related to the individualizing effect of CNCs promoted by the freeze-drying process [198]. Composite membranes from biobased polymers prepared according to different approaches, blending the components (polymer-polymer composites) to interfacial polymerization (coating layer for a commercial membrane) and fillers from silver nanoparticles to carbon nanotubes, graphenes, or alginate, were successfully applied to water purification ultrafiltration and nanofiltration membrane processes because their compactness was suitable for the retention of inorganic or organic compounds [9,199,200]. Alginate-based composites have been extensively studied as they increase environmental compatibility and operational efficiency by acting as real alternatives for conventional activated carbon [200,201]. The interest devoted to alginate for the removal of heavy metals, industrial dyes, pesticides, antibiotics, and other pollutants in water and wastewater is due to its low cost and highly efficient absorption [9,200–202]. Mostly used in the form of beads [9], alginate composites were also studied in the form of sodium alginate/nanohydroxyapatite composite films for adsorption of Pb(II). The membranes were prepared by mechanical activation of natural hydroxyapatite nanoparticles compounded with sodium alginate as granules and films, which exhibited strong Pb(II) adsorption ability [203]. Also, a porous calcium alginate membrane prepared by freeze drying was studied as a methylene blue (MB) absorbent, showing that the adsorption of MB occurred on the surface of the membrane through monolayer adsorption following a pseudo-second-order reaction model [204].

3.4 Energy Society has shown a growing demand for improved and efficient sustainable energy technologies that are capable of reducing the global environmental footprint. Several alternative energy sources are being explored to reduce the dependence of fossil fuels, such as environmentally friendly fuel cells, batteries, supercapacitors, and dye-sensitized solar cells [205]. Dry solid polymer electrolytes (SPEs) represent safer alternatives to liquid electrolytes, as they are a class of ion-conducting materials in which the principal advantage is related to their

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physical state. Over the last few decades, the attention devoted to these materials has grown because they are thin films and can be incorporated into all solid-state electrochemical displays, allowing a significant reduction of the costs associated with device production, and because they enable the adoption of simplified architectures without the use of elaborate seals and present good mechanical strength and ionic conductivity [205–207]. The concept of dissolving inorganic salts in functional (polar) polymers goes back as far as the 1970s with the use of poly(ethylene oxide) [208], which is based on the interaction of metal ions with the polymer polar groups. These interactions, mainly derived from electrostatic forces and coordinating bonds, may be influenced by several factors such as polymer molecular weight, nature and distance of the functional groups, branching degree, polymer/salt ratio, nature and charge of metal cation, and counter ions [209]. These have an effective impact on cation mobility, are responsible for the ionic conductivity of the film, and result from weakly bonded cation transfer between coordinated sites along the polymer chain in the presence of an electrical field. Moreover, SPEs comprise both crystalline and amorphous regions, and crystalline phases can lower dc ionic conductivity greatly, as the ion transport occurs mainly in the amorphous area. Thus to overcome this disadvantage and improve SPE performance, several strategies were applied based on polymer blending, the addition of inorganic fillers, utilization of comb-branched copolymers, and the incorporation of plasticizers and doping agents [147,205,210,211]. Therefore new polar polymers with high amorphous contents and from different sources were involved in this study. Attempts at using biodegradable polymers to replace nonbiodegradable polymers are widely performed nowadays as they are nontoxic and environmentally friendly, minimizing environmental waste since they are reusable and recyclable [117]. Table 2 summarizes some natural polymers used in different energy approaches. One of the most studied polysaccharides is chitosan, which has found many applications in the energy field, for instance, as a proton exchange membrane in fuel cells [31,213], a dye sensitizer solar cells [214], and a polymer host in solid/composite polymer electrolytes for batteries and capacitors [32,136,209,215]. Besides several studies devoted to natural polymer-based film applications in SPE development, their exploitation is still underexplored and is considered almost limitless.

4 Global demand and market for biopolymers Current research in membrane science is now focusing more on biopolymers from natural raw materials with a well-defined structure to develop new membrane materials [221]. The use of biopolymers derived from bacterial fermentation products, mainly poly(butylene succinate), polylactic acid (PLA), polyhydroxyalkanoates, plant sources, such as cellulose, acemannan, alginate, starch, as well as those from animal sources, chitin, chitosan, collagen, sericin, and silk, have been gradually investigated for their suitability for several fields of applications. Traditional fossil-based polymeric materials used in membrane fabrications such as polyvinylidene fluoride, polypropylene, polyesters, and polyamide could be replaced by biopolymers depending on the target application. A recent report shows that the market size of bioplastics and biopolymers was $US6.00 billion in 2017 and is projected to reach US$14.92 billion by 2023, with the packaging industry I. Fundamentals on biopolymers membranes and films

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4 Global demand and market for biopolymers

TABLE 2

Natural polymer-based membranes applied to energy solutions.

Natural polymer

Tested additives

Characteristics

Tested applications

References

Hydroxyethyl cellulose

• Lithium salts • Glycerol

• Tg values around 70°C • Conductivity at 60°C, in order of 104 S cm1

• SPEs • Batteries

[106,107,117,212]

Chitosan

• Lithium salts • Glycerol • EC • Oleic acid

• Low crystallinity • TA conductivity 103 S cm1 • Tg values around 87°C

• Proton exchange membrane in fuel cells • Dye-sensitized solar cells • SPEs • Batteries • Capacitors • Electrochromic windows

[31,32,117,124,136, 213–218]

Agar

• Glycerol • Acetic acid • Triflate salts

• Highly transparent SPEs • Good mechanical and thermal properties • TA conductivity 104 S cm1

• SPEs • Batteries • Electrochromic windows

[117,218–220]

Gelatin

• Glycerol • Acetic acid • Lithium salts • Triflate salts

• Highly transparent SPEs • Good mechanical and thermal properties • TA conductivity doped in LiClO4 105 S cm1 • Conductivity at 80°C, 104 S cm1

• SPEs • Batteries • Electrochromic windows

[87,117,147,218]

Starch

• LiClO4 • Glycerol • EC

• TA conductivity doped in LiClO4 105 S cm1

• SPEs • Batteries

[172]

EC, ethylene carbonate; SPE, solid polymer electrolyte; TA, ambiental temperature.

being one of the major drivers for bioplastic and biopolymer consumption. The main players indicated in this market analysis include NatureWorks (USA), Braskem (Brazil), Novamont (Italy), BASF (Germany), Total Corbion PLA (Netherlands), Biome Bioplastics (UK), Bio-On (Italy), Toray Industries (Japan), Plantic Technologies (Australia), and Mitsubishi Chemical Corporation (Japan) [25]. This market is predicted to have a noticeable growth with a compound annual growth rate (CAGR) of 5.1%, and during the forecast period of 2019–25, the biopolymer market is estimated to grow with a CAGR of 19%. The categorization of biopolymers is largely dependent on its end-user industry, which includes the pharmaceutical, healthcare, food, and beverage industries. At the moment, in the medical industry, biodegradable polyesters are extremely useful in manufacturing surgical implants, and in the food and beverage industry, biopolymers are primarily used for manufacturing cellophane sheets I. Fundamentals on biopolymers membranes and films

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FIG. 3 Evolution of publications and citations on the last two decades for biopolymer membranes.

200 6000 Nº of publications Citations 4500

100

3000

50

Citations

Publications

150

1500

0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

0

that are massively utilized for food packaging [222]. With an increasing focus on sustainability and favorable government regulations for green procurement policies, it is expected that the biobased polymers market, which includes nonbiodegradable and biodegradable biopolymer grades, will have a significant opportunity in consumer goods applications. The production of membranes based on biopolymers is receiving more attention due to their unique features that can make them the selected solution in a wide range of applications. The current research in membrane science is more focused on the use of alternative polymers derived from natural raw materials for the development of new membranes [221,223], contributing to developing more sustainable materials. In a literature survey performed in October 2019, by considering the science and technology field over the last two decades, around 1900 documents listed on the Web of Science regarding biopolymers and membranes envisaging different uses were published. Looking at those results, Fig. 3 clearly indicates an increasing concern for the use of biopolymers in the development and study of new membrane systems, particularly in the last decade, representing an increase of 33% in publication results. Most results are manuscripts and review manuscripts; however, almost 11% are related to patent filing, most of them in the last decade, revealing an increase in innovation and higher concern for the protection of intellectual and industrial property in this field. This growing research and innovation during the last decade also supports increased awareness of the environmental impact of fossil-based plastics. In this sense, biopolymers are expected to complement or gradually replace modern plastic materials; however, some challenges such as the high price for biopolymers or mechanical performance limitation issues, need to be overcome.

5 Final remarks The exploitation of biopolymers in multiple fields such as biomedical, agricultural, food, energy, and the environment has demonstrated that the utilization of renewable feedstocks

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can address the problems of health hazards, solid waste management, and environmental issues involving plastics. In particular, the versatility of membranes obtained from natural polymers has been largely investigated; however, depending on the application, some concerns remained about their use. With certain chemical and biological cues, there are many possibilities for biopolymer membranes in biomedical applications. Nevertheless, only a few approaches reach in vivo and clinical needs. Therefore researchers and clinicians need to work closer together to develop practical and personalized biopolymer matrices. In the context of food, biopolymer membranes as edible coatings and films, as well as packaging materials, promote food preservation or modify the atmospheric packaging of highvalue products. However, appropriate studies on the interaction between food components and biopolymers during processing and storage are limited and should be investigated. Moreover, biopolymer edible films and coatings can also be used as active carriers, but for that purpose the mechanisms of deterioration of each food type and the mode of action of each package should be understood. Recent approaches on biopolymer membranes combine different technologies such as layer-by-layer and electrodeposition techniques as well as 3D food printing, and emphasize the need for the creation of multilayer coatings with multipreservation environments and personalized food design. These strategies could represent a step forward in the use of biopolymer films and coatings in the food field. SPEs based on natural sources are still an underexplored strategy with yet unsolved scientific problems like dissociation and transport numbers, which require new approaches and techniques. The principal drawback related to the use of natural solid polymer-based electrolytes in practical applications, such as electrochemical devices, is the presence of water, which dissolves lithium ions creating a new phase. In parallel are the low values of ionic conductivity achieved that may be overcome by the incorporation of plasticizers or ILs. In perspective, there is an unlimited plethora of materials that may be explored and tuned to modulate their properties. The use of natural source polymers for manufacturing membranes has become a major achievement to lower the human impact on the environment, as resources are practically unlimited, wastes can be easily reprocessed, and new uses can be found for them. Another advantage is the hydrophilicity of the most explored natural polymers, cellulose and chitosan, which assure higher fluxes for water, making them very efficient for separation processes. New methodologies and approaches are still underexplored in this area and will significantly increase the impact of natural polymers on water purification membranes. The use of surfactants for membrane synthesis will promote better filler dispersion into the membrane and will also allow manipulation of the shape and dimensions of polymeric membrane pores. Similarly, the incorporation of functionalized or derivatized nanofillers will selectively separate chemical species impacting the quality of human life. Targeting membrane product solutions designed in a way that allows them to be reusable or by taking part in a circular economy process, the use of biobased polymers that includes nonbiodegradable and biodegradable polymers will continually grow, contributing to the development of more sustainable materials. Thus the demand for biobased polymers combined with an increase in membrane systems will further drive the use of biopolymer materials in applications that are currently dominated by petrochemical-based polymers.

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Acknowledgments The authors especially acknowledge financial support from the Portuguese FCT (SFRH/BPD/93697/2013). This work is also financially supported by the FCT R&D&Iprojects with references PTDC/CTM-BIO/4706/2014(POCI-01-0145-FEDER-016716) and R&D&I Structured Projects with reference NORTE-01-0145-FDER-000021.

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Glossary Angiogenesis The physiological process of novel blood vessel formation from the preexisting vasculature under the control of angioinhibitory molecules. Bioactive Ability of a biomaterial to induce or modulate biological activity. Phenomenon by which a biomaterial elicits or modulates biological activity. Biocompatible Material or substance from natural or synthetic origin that when employed to treat, augment, or replace a tissue, organ, or body function does not promote a negative reaction from the surrounding tissue or host. Biodegradable Material able to be organically broken down by the host with no damage associated with residual waste formation, avoiding the need for second surgery for retrieval. Blend membrane A thin film resulting from a mixture of two or more polymers that have been blended together to create a new material with different physical properties. Composite Material composed of two or more physical and/or distinct chemical properties, suitably arranged or distributed to present different characteristics compared to the individual components, which remain separated within the final structure. Drug delivery Process of getting a drug into an appropriate body compartment with controllable release performance in time and availability, promoting pharmacological effects in a patient. Extracellular matrix Highly dynamic and complex extracellular part of tissue that comprises the noncellular aspect of tissues, usually providing structural support to the cells between other important functions. Growth factor Generally, a protein or steroid hormone capable of regulating many aspects of cellular processes, including survival, proliferation, migration, and differentiation. Those are naturally occurring signaling molecules in the body that control cell division, growth, and repair. Hydrogel Network of macromolecular polymers containing a large amount of water. Membrane Layer of material whose structure has lateral dimensions much greater than thickness; also known as thin film. It may act as a selective barrier between two phases and allow the selective transport of mass species, such as gases, liquids, or ions, when exposed to the action of a driving force. Natural biomaterial Material derived, harvested, or obtained from natural sources used for biological applications with the advantage of presenting similar features to the host tissues or cells.

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Polysaccharides Biodegradable carbohydrate polymers that are found in all living organisms composed of long chains of monosaccharide units joined by glycosidic linkages. Scaffold Artificial constructs that maintain their structural integrity once implanted; capable of supporting 3D tissue formation, cell attachment, migration, and proliferation as well as enabling vital product diffusion to cells as nutrients and oxygen. Solid polymer electrolytes Solid solutions of monovalent or divalent metal salts in polymers, with the ability of ion transfer under an electrical field. Synthetic biomaterial Manufactured or produced material from synthetic polymers with the ability to easily tailor its properties such as degradation rate and mechanical properties to suit biological applications. Tissue engineering Technology directed toward self-regeneration based on the use of biomaterials and/or biologically active molecules and cells to create tissue-engineered solutions capable of addressing the mechanical and structural requirements to restore normal function to damaged tissue and not just its biologic properties.

I. Fundamentals on biopolymers membranes and films

C H A P T E R

2 Fundamentals of two-dimensional films and membranes Andrea Cristiane Krause Bierhalza, Mariana Altenhofen da Silvab, Theo Guenter Kieckbuschc a

Department of Engineering, Federal University of Santa Catarina, Blumenau, Santa Catarina, Brazil bCenter of Agricultural Sciences, Federal University of Sa˜o Carlos, Araras, Sa˜o Paulo, Brazil c School of Chemical Engineering, University of Campinas, Campinas, Sa˜o Paulo, Brazil

1 General aspects of membranes and films In general, two-dimensional (2D) films and membranes can be characterized as flat structures having lateral dimensions considerably larger than their thickness. The use of the term film or membrane is mainly related to the field of application. The term membrane is very well established in the area of separation processes with various industrial applications such as filtration and purification. In this case membranes are associated to structures that separate two distinct media and act as a selective barrier, through which mass transfer may occur under a variety of driving forces. Despite the wide use of synthetic polymers as membranes in this area, biopolymer-based membranes have become increasingly important for applications that include removal of heavy metals, dyes, and pesticides from industrial effluents, biocatalysis, and toxic organic dechlorination [1]. The term membrane is also used in applications of fuel cell and batteries, where they separate anode from cathode and function as an ionic conductor. The term film is widely used in food packaging applications. In this area the terms edible films and edible coatings are sometimes referred to as synonyms, but they differ in the preparation form. Edible films are first molded as solid sheet and then applied on the product surface or between food components, whereas edible coatings are formed directly onto food surfaces after the contact of the product with the film-forming solution [2]. These films and coatings are often intended to provide additional barrier protection to extend shelf life of the food product. However, they can also be designed to act as carriers of antimicrobials, antioxidants, nutrients, flavors, and colorants, improving the overall quality of the food. Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00002-X

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2. Fundamentals of two-dimensional films and membranes

In the context of biomaterials, where biopolymers are of great importance due to their biodegradability and biocompatibility, both the terms films and membranes are usually used without distinction, in particular for reference to the free-standing devices for wound dressing applications. The terms coating and film coating are extensively used for applications involving bone implants and cardiovascular devices, where biopolymer-based coatings can aid in their process of integration with surrounding tissues. According to their morphology, structures of films and membranes may be dense (nonporous) or porous. Films are usually referred to dense structures, while the term membrane is used for both dense and porous structures. Pores inside a biopolymer membrane may be closed (isolated) and open (connected), which are often irregularly distributed along the pathway [3]. For food applications, homogeneous and dense structures impact on mechanical resistance, barrier properties, and control of the release of active agents. On the other hand, for separation processes, the permeate flow across the dense membrane can be quite low, impairing efficiency. Therefore pore size and pore size distribution determine the membrane separation performance. In biomaterials such as scaffolds for tissue engineering, the porosity and the interconnection between pores influence proliferation and cell attachment. In membranes for wound dressings, the porosity contributes to the permeability to water vapor and exudates absorption, which regulates the healing process. Membranes for separation processes can be also classified as symmetrical (isotropic) and asymmetrical (anisotropic) regarding their structure. Symmetric membranes refer to a uniform structure throughout the entire membrane thickness. Asymmetric membranes can assume two main configurations: phase separation membranes and thin film composite membranes. The phase-separated membranes, also called integrally skinned membranes, are heterogeneous in structure but homogeneous in chemical composition. These structures have a gradual change of pore size and porosity along the thickness forming a denser “skin” layer at one surface of the membrane, which can be porous or nonporous. Thin composite membranes are both structurally and chemically heterogeneous and consist of a porous substrate coated with a thin dense film of a different polymer [4]. In an attempt to improve the properties of biobased films and membranes, strategies involving modification of composition have received increasing attention, especially in the areas of food and biomaterials. The matrices can be obtained by blending the main biopolymer with one or more polymers, natural or synthetic, forming a single phase. Two or more elements such as particles, fibers, and platelets can also be combined with biopolymers resulting in a multiphase system. Several techniques are suitable to obtain dense and porous biobased films or membranes with different characteristics for a wide range of applications. Fundamentals of these main techniques (solvent casting, extrusion, dipcoating, layer by layer, and electrospinning) will be presented in this chapter as well as the more commonly modification used for biobased films and membranes.

2 Processing The characteristics of films and membranes based on biopolymers strongly depend on their composition and preparation procedures [5]. Different techniques have been I. Fundamentals on biopolymers membranes and films

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2 Processing

FIG. 1 Wet and dry methods for the production of biopolymer 2D films and membranes.

Processing methods

Dry

Wet

Dense

Porous

Dense

Casting Coating Layer-by-layer

Solvent casting and particulate leaching thermally induced phase separation electrospinning

Extrusion Film blowing

implemented to obtain materials with different arrangements to meet domestic and industrial application demands. There are two main principles involved in the production of biopolymer 2D films and membranes, polymer solution (wet methods) and polymer melt (dry methods). Polymer solution processes basically comprise the use of organic or inorganic solvents and subsequent evaporation causing further entanglement of the biopolymer chains as the concentration increases. When solvent evaporation is attained, the biopolymeric matrix is recovered in a solid format. The techniques based on this principle include solvent casting (with and without porogens), freeze drying, coating, and electrospinning. On the other hand, polymer melt processes, also called mechanical or thermoplastic methods, use heat to promote polymer flow and assemble as a homogeneous phase. A crystalline or semicrystalline biopolymer, at high temperature (above its melting point), will flow and turn into an amorphous structure. As temperature cools down, polymer chains will turn back into a solid realigned as a crystalline structure. Extrusion, compression molding, two-roll milling, and melt spinning are examples of methods based on this principle [6]. Despite the film-forming process, matrix components should form a spatially rearranged gel structure with all incorporated film-forming agents, such as biopolymers, plasticizers, other additives, and solvents in the case of polymer solution method. It is essential to understand the chemical properties and structure of biopolymers as well as additives, to tailor their properties [7, 8]. Solubility in water and other solvents is very important when selecting a solvent for wet method. Thermoplasticity of biopolymers, including phase transition, glass transition, and gelatinization characteristics, should be understood for dry method [9]. The dry and wet processing methods discussed in this chapter are presented in Fig. 1.

2.1 Dense membranes 2.1.1 Casting Casting, also known as wet casting method, is the most conventional methods to produce biopolymer-based films and membranes. By this method the polymer is dissolved in an appropriate solvent, which may include additives, and forms a homogeneous solution. I. Fundamentals on biopolymers membranes and films

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Biopolymer molar mass, type, concentration, and the kind of solvent used are the main parameters affecting viscosity of the biopolymeric solution. The polymeric solution is then poured and spread directly over a supporting slab (frame) to form a thin film. After solvent evaporation, which depending on the temperature and drying method can take several hours to days, the remaining solid film can be obtained by manually peeling it off the support. The frame can be made of different materials such as glass, polyethylene, polyester, metal, plexiglass, and Teflon [5, 6]. Casting methods can be classified, according to the nature of the solvent, in solvent casting and water-based casting. Solvent casting uses an organic solvent to solubilize the biopolymer. Depending on the application the use of organic solvents can impose a disadvantage, since a purifying step became necessary to eliminate solvent residues. On the other hand, in waterbased technique, instead of using organic chemicals, the main solvent is water. This process is suitable for making hydrophilic natural polymer films or membranes such as chitosan, starch, alginate, pectin, cellulose, and some proteins [8, 9]. The thicknesses of casted films and membranes range from 25 to 200 μm depending on solution concentrations, residual moisture content, and swelling properties of material. Most of casted films are solid films without any pore, and surface postprocessing is usually required to create or increase the number of pores in the film [6]. Films and membranes containing chitosan require, in addition to water as the main solvent, a small amount of acid, usually acetic acid, to increase the solubility of chitosan. Chitosan films can be easily prepared by wet casting of chitosan salt solution followed by drying [oven or infrared (IR) drying] [10]. The technique of casting has also been widely reported for starch-based films [11] and typically includes solution preparation, gelatinization, casting, and drying. For casting, starch and plasticizers are mixed together followed by solution heating. The gelatinized suspensions are then poured onto a flat plate and dried at predetermined time/temperature. Alginate films, as most polysaccharide-based films, are commonly prepared by means of a solvent-evaporation technique [12, 13]. In general, there is no standard casting method to prepare polysaccharide films with required functions and properties, and it depends on the type of polysaccharide, solvent, and additives. Alginate films prepared by casting exhibit poor water resistance because of their hydrophilic nature, but the ability of alginate to produce insoluble gels improves with the addition of calcium. However, gel formation of alginate with calcium ions is so instantaneous that it jeopardizes smooth film casting in most cases so that only small concentrations of calcium can be added to maintain a homogeneous and spreadable casting solution. Thus other strategies are necessary to solve this problem, such as immersing the alginate film in CaCl2 solution after casting. The drying temperature can vary significantly; da Silva et al. [12] dried alginate films at 30°C, 40°C, 50°C, and 60°C and reported that films obtained at higher temperatures, despite decreasing the drying time, were thinner, had lower moisture content, and were less flexible due to considerable glycerol evaporation. Bagheri et al. [14] also proved that drying alginate films at high temperatures highly influences the overall film characteristics. The structure of protein-based films and membranes depends on the amino acid composition, degree of protein denaturation, type of interactions among protein chains, and processing methods, which determine their final properties. These structures are typically produced by casting and drying of dilute (usually 5%–10% w/v) protein solutions in water

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or in aqueous ethanol, depending on the protein [15]. Soy protein isolate [16], whey protein [17], and fish myofibrillar protein [18] films usually use water as solvent. Other proteins with higher contents of nonpolar amino acids are dissolved in aqueous ethanol solutions, such as zein [19] and wheat gluten [20]. Other organic solvents, such as acetone, methanol, and isopropyl alcohol, can also be employed. The development of film structures during drying of cast protein solutions involves formation of hydrophobic, hydrogen, and ionic bonds. Covalent intramolecular and intermolecular disulfide bonds also play a role in film formation from sulfur-containing proteins (e.g., wheat gluten, soy protein isolate, whey protein, and egg albumen). The pH of protein film-forming solutions is usually adjusted away from the protein’s isoelectric point to avoid protein precipitation [15]. Temperature and relative humidity (RH) determine the drying rate of biopolymer solutions and therefore affect properties of protein films [21]. Alcantara et al. [22] reported that rapid drying at 95°C and 30% RH yields soy protein films stronger, thinner, stiffer, and less extendable than films dried more slowly at 21°C and 50% RH. Casting technique shows great versatility since it can be used with most of the biodegradable polymers and no special equipment set up is needed. In laboratory scale, it is the most widely used technique to produce films and membranes based on polysaccharides and proteins. However, it has two important disadvantages: the difficulty in scaling-up its production to larger size since film dimensions are usually no larger than 25–30 cm and the long drying times that can drastically limit industrial application [7, 23]. Increasing the drying temperature can effectively shorten the drying time and render the method more applicable, but as previously mentioned, film properties can be drastically affected. 2.1.2 Extrusion/film blowing The blown film extrusion process is evolving from manufacturing films made from conventional synthetic polymer to biodegradable polymers. Extrusion technology has become an attractive option for the production of biopolymer-based films and membranes owned to its higher productivity and lower space requirements when compared with the traditional casting method [24, 25]. Actually, extrusion is the most used process to obtain industrial plastics in large scale, since it can achieve high temperatures in short times and its operation is simple [26]. The blown film extrusion represents an efficient way to make films with thin thickness, and the polymer needs to possess relatively high melt strength [27]. Tubular film processing has unique characteristics that differentiate it from other extrusion techniques. In this process the melted biopolymer exits from the extruder head into the die. It then flows around the mandrel and emerges through a round opening (ring) to form a tube. Air is injected through the center of the die head into the ring, inflating the tube into a thin tubular bubble of desired diameter. This tube is then flattened at the nip rolls and then taken to the winder. In the balloon formation the films are stretched transversally (by injected air) and longitudinally (by elongation rolls) simultaneously with cooling, which results in biaxial orientation [28, 29]. By varying the blow-up ratio (BUR), defined as the ratio between bubble diameter and die diameter, screw configuration and speed, air pressure, feeding rate, material moisture during feeding and temperature profile, films of different thicknesses (10–150 μm), properties, and degree of orientation can be manufactured. The process is continuous, so the blown film forms long continuous sheets, which can be rolled for transport and storage [26].

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Despite the increased use of extrusion technology in the production of biodegradable films, a better understanding of its effects on the several biopolymers is required. The properties of the final product are influenced by a variety of factors like processing conditions and depend strongly on the inherent properties of the biopolymer being used [24]. Biopolymers are usually difficult to handle while forming the blown film as they are stiff and brittle, with poor elongation properties, resulting in low bubble strength and bubble tears. Biodegradable starch films can be produced by blown extrusion. The extrusion disrupts the starch granules, yielding a homogeneous and fluid material known as thermoplastic starch (TPS). This material expanded by blowing into a tubular matrix produces rigid films, which require the addition of plasticizers to decrease the melting temperature of the starch and increase flexibility [30]. However, poor melt tenacity, which is defined as the ability of the melt to deform without rupture, has been identified as one of the potential limitations when extruding TPS at elevated temperatures, and some strategies to improve the properties of blown TPS films have been proposed [31]. Several studies on the film blowing of starch-containing materials have introduced moderate quantities of starch in a synthetic polymeric system [32]; have used starch together with another biodegradable polymer, such as polylactic acid (PLA) [33], poly(vinyl alcohol) (PVOH) [34], poly(butylene adipate coterephthalate) (PBAT) [30, 35], polyhydroxyalkanoate (PHA) [36], and pectin [37]; and have used other plasticizers such as sorbitol and urea [38] as a complement to or as replacement for glycerol, the most common plasticizer for TPS. Moisture resistance can be chemically improved by chemically modifying starch by grafting or acetylation [39, 40]. PLA is a commercially available biopolymer with good mechanical and thermal properties and high optical transparency but lower barrier properties and thermal stability compared with those of conventional polymers. As most biopolymers, PLA is brittle and has low melt strength making blow extrusion a challenging process [41]. Blown films of PLA/clay nanocomposites have been successfully produced, resulting in films with improved mechanical and barrier properties [42]. In a recent study, Herrera et al. [41] investigated the properties of PLA/chitin nanocrystal (ChNCs) composites produced by blow extrusion. The authors showed that the addition of 1 wt% ChNCs increased the tear strength by 175% and the puncture strength by 300%. Chitin nanocrystals increased the glass transition temperature (Tg) by 4°C compared with the reference material and slightly enhanced the film degree of crystallinity. The chitin nanocomposite also had lower fungal activity and lower electrostatic attraction between the film surfaces. 2.1.3 Coating Coating is referred to the application of functional polymers on the surfaces of objects to achieve protection or confer special properties [43]. In food systems, for example, an alternative to direct application of antimicrobial solutions is to incorporate the antimicrobials into an edible coating that is then applied onto the food surface by various methods, each one with their own advantages and disadvantages. These methods include dipcoating, spread coating, and spray coating, and the selection of an appropriate coating method imparts the protection effect of the coating and also determines the production cost and process efficiency. The performance of edible coatings not only depends on the coating methods employed but also on the properties of the coating materials (type, amount, density, viscosity, and surface tension). Many natural materials, like proteins, polysaccharides, and lipids, have the capability to

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generate well-performing edible coatings [44]. Coating technology holds great potential in food preservation; antimicrobial, superhydrophobic, and self-cleaning surfaces for medical, textile, paper, and packaging industry; and agriculture applications [43–47]. 2.1.3.1 Spread coating

Spread coating is a very simple method often used to form films with the help of tools, like spreader, brush, rods, and spatula. It consists basically of direct spreading the biopolymer solution on the surface of the product (direct coating) or on the packaging material (indirect coating) [48]. Direct coating is an effective way to limit the growth of microorganisms and maintain the quality of food products [49]. After spreading the biopolymeric solution onto food to form even films, the coated material is dried under certain conditions, such as laminar-flow chamber and drying tunnel. The coatings can influence gas and water vapor permeability coefficient, and the antimicrobials can gradually migrate from the films onto the food surface [43]. Indirect coating improves packaging attributes of simple material. Chitosan/caseinate solutions were coated onto paper by brushing the surface, thus obtaining multiple-layer functional films for food packaging [50]. Though chitosan layer on coated paper did not have a significant influence on mechanical properties, it greatly reduced the water vapor permeability of the material. Multilayer coatings of microfibrillar cellulose (MFC) and shellac have been deposited on paper and paperboard. This multilayer system with MFC as a first layer and shellac as the top layer decreased both oxygen and water vapor transmission rates [51]. In paper-coating applications, difficulties raised by the use of most biopolymers due to hydrophilicity, crystallization behavior, brittleness, or melt instabilities still hinder a full expansion at industrial scale. However, a judicious combination of select materials (biopolymers, its blends, plasticizers, and nanofillers) and surface structuring has increased the chances to provide a fully protective biobased paper coating [52]. 2.1.3.2 Spray coating

Spray coating is usually achieved by means of compressed air-assisted sprayer and knapsack sprayer. Among the several coating methods, this technique offers as its main advantages such as uniform coating, reasonable thickness control, and the possibility of multilayer applications to produce single and multilayer films. Moreover, spraying systems do not contaminate the coating solution, allow coating solution temperature control, and can facilitate scale-up and automation of continuous production since spray systems are widely used in several industrial applications. A spray system increases the surface area of the liquid by the formation of droplets and distributes them over the coating surface area by means of a set of nozzles [53]. Different from other methods, spray coating can work with large surface areas, and since the coating is carried out contactless, a contour coat may be applied to uneven surfaces [47]. Coating solution properties like viscosity, density, and surface tension and the atomization pressure are critical parameters and must be controlled to obtain the desired effect on the product. To improve adherence and wettability between layers, several surface treatments such as electroplating, chemical process, anodic oxidation, and thermal spraying have been used. Corona treatment, often used in industrial applications in coating, printing, and laminating of plastics, cloth, and paper, consists of a high-voltage, high-frequency electrical discharge applied to a surface, increasing its energy and consequently the adhesion of the solid material

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surface. Corona discharge on the base substrate can enhance adhesion in spray coating method and improve barrier properties in packaging materials [54]. The effect of spray time, nanoclay content, and corona discharge on barrier and mechanical properties of Kraft paper spray coated with a mixture of cellulose nanofibrils (CNF) and nanoclay particles was evaluated. Results showed that corona discharge enhanced the wetting of paper surfaces, improving adherence between paper and CNF composite. Increased spray time improved tensile strength and barrier properties, while higher nanoclay content enhanced its barrier properties but reduced its tensile strength [47]. In food edible coating, factors like irregular surface, costs, and simplicity empowered dipping as the most used method, once it allows covering all the surface by immersion and the excess solution removed through draining or by a dryer. On the other hand, if the objective is to obtain a thinner and uniform coating, spraying is more effective. The efficiency of the process can be increased if electrostatic spraying is used [44]. This methodology is being adapted to the food industry and allows high efficiencies avoiding coating wastage, which is the major drawback of the dipping method [44, 55]. To enhance food protection from spoilage, combination of biopolymer spray coating with other treatments, such as gamma irradiation and modified atmosphere, tends to be more effective. The influence of coating biopolymer (chitosan, alginate, and soy protein) and coating application methods (dipping, enrobing, spraying, and electrostatic spraying) on coated mozzarella cheese properties was investigated by Zhong et al. [44]. Spraying methods led to thinner coatings and alginate-coated cheese showed the best overall physicochemical properties during storage. Another promising application of spray-coated biopolymer films is their use as mulching in agriculture as alternative to synthetic plastic mulching. The use of spray coatings is compatible with usual agronomical practices since the biopolymeric water-based solutions can be applied by means of airbrushes commonly used by farmers to apply fertilizers and other chemicals. The spraying technique is particularly suitable for crops grown in trays or pots where operating a spray gun is less labor demanding than plastic films that require manual operations such as cutting and laying out the plastic films on the trays or pots [45]. Immirzi et al. [56] developed a sodium alginate-based mulching spray able to create a film on soil resistant to weed stem penetration. The mechanical and radiometric properties and its biodegradation were assessed by means of laboratory measurements and strawberry cultivation field tests. The mechanical properties degraded when exposed to field conditions, but coating kept its mulching effect for 6 months. Biodegradation tests in soil showed a 65% mineralization after 6 months. Chitosan spray also tested as mulching during the growth of V. lucidum plants. Degradation started after 3 months and allowed some weeds to emerge in the containers, but, in general, the mulch performed better than the herbicide [45]. 2.1.3.3 Dipcoating

Dipcoating is a very simple technique for coating substrates with thin and uniform films. In this method the substrate is dipped into the polymer solution, withdrawn at a constant speed with subsequent solvent evaporation and/or thermal treatment [57]. The process of insertion and removal of the substrate from the solution must be carried out with controlled and constant speed and without any type of vibration or external interference, to ensure the deposition of a homogeneous film. Advantages of this method include easy operation, inexpensive

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setup, low processing temperature, uniformity of deposition, ability to coat complex shapes, and absence of waste [58]. The coating thickness can be controlled by adjusting the concentration of the suspension and the number of dips and varying the withdrawal speed. In general the thickness increases with the withdrawn speed and with the number of dips [58]. However, several other factors may influence the final characteristics of the films, such as temperature and pH of the polymer solution, and immersion time. Kaur et al. [59] studied the effect of the pH and temperature of cellulose acetate (CA) solution to coat ceramics by dipcoating. The authors observed that the pore size, porosity, and permeability of the polymeric layer decreased with increasing pH of the solution. The temperature influenced the pore size and the appearance of imperfections in the film. In this process, surfactants can be added to ensure uniform wetting of the substrate by reducing the surface tension of the solution below the surface tension of the substrate and also to minimize surface defects in the film and to act as an antifoaming agent during solution preparation [60]. In coating materials, such as alginate or pectins, gelation can be induced by a subsequent cross-linking step [43]. Dipcoating technique has been used with biopolymers in several areas. For ceramic or metallic implant coating, for example, the films formed function to prevent infections if the biobased matrices are incorporated with active agents and also facilitate interactions between the implant and the surrounding tissue [61, 62]. In foods, film coatings improve protection and limit damages caused by loss of moisture, temperature variations, and transportation, thereby significantly extending shelf life [43]. Other applications of dipcoating include the coating of ceramic support with biopolymers to prepare polymer-ceramic composite membranes for separation purposes [59]. 2.1.4 Layer by layer Layer by layer (LbL) technique represents a simple, versatile, and cost-effective strategy for obtaining multilayered films with controlled properties for a wide range of applications. Nanostructured thin films are formed by deposition of alternating sequential layers of oppositely charged polymers on a substrate such that one layer joins the previous layer via electrostatic interaction [63, 64], as illustrated in Fig. 2. LbL technique presents several advantages including operating under mild conditions, facile incorporation of functional compounds, and production of uniform films with controlled thickness and predetermined superficial properties. In addition, this process is cost-effective and environmentally friendly [64]. Many deposition technologies have been developed and resulted in materials with distinct structures and properties. They can be divided into five main categories: (i) immersive assembly, (ii) spin, (iii) spray, (iv) electromagnetically driven, and (v) fluidic assembly [65]. The immersive LbL assembly is the most used method for nanofilm production. It consists of immersing the substrate material in a diluted polyelectrolyte solution for a time span that allows adsorption. The system is then washed to remove the excess polyelectrolyte, and the procedure is repeated to form the next layer [63]. Film characteristics such as thickness and topography are influenced by several process parameters including temperature, time, and drying conditions and also by the concentration, ionic strength, and pH of the solutions used [64]. The ionic strength of the polyelectrolyte

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FIG. 2 Schematic representation of LbL process. Reproduced with permission from F. Croisier, C. J
er^ome, Chitosan-based biomaterials for tissue engineering, Eur. Polym. J. 49(4) (2013) 780–792, Elsevier.

solutions is a parameter directly related to the control of the thickness of deposited layers. An increase in ionic strength, within set limits, results in increased film thickness. In addition, the reduction of pH and an increase in number of layers favor an increase in thickness, surface roughness, and elastic modulus of the film [43]. Electrostatic interaction is the main and most used driving force for the development of nanostructured films. It is based on an excess of charge able to promote the neutralization and the charge at each new deposited layer. However, other mechanisms such as hydrophobic interactions, hydrogen bonding, charge-transfer interactions, biologically specific interactions, coordination chemistry, covalent bonding, stereocomplexation, and surface sol-gel processes can also conduct the process [63]. In most of the studies, the substrate onto which the polyelectrolytes are to be deposited is designed to interact via ionic bonding with the cationic or anionic polyelectrolyte to form the first layer. Therefore the self-assembled multilayer cannot be detached without a significantly collapse of the film [66]. Strategies for obtaining free-standing films comprise the use of sacrificial layers, which are solubilized in a solvent that only acts on these layers, keeping the multilayers of interest intact. Adverse chemical and physical changes in the films during the separation stage, however, can result in defects such as increased roughness and traits [63, 66]. Another potential approach for polysaccharide multilayered films is the choice of hydrophobic substrates such as Teflon and polypropylene, which allow easy removal of the film after drying. Larkin et al. [66] built detachable and free-standing polyelectrolyte multilayers of hyaluronic acid and chitosan assembled on hydrophobic polypropylene. According to the authors the weak forces between the first layer are composed of hyaluronic acid, and the substrate enabled the film detachment. The number of layers significantly influenced the detachment process. Films with 30 or more layers, for instance, were more prone to detach

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without causing ruptures. Substrates of polystyrene and polypropylene were also found suitable to deliver nanostructured films of chitosan and alginate [67]. The LbL self-assembly technique presents enormous potential for diverse biomedical applications and devices, including tissue engineering, drug delivery, medical implants, microbioreactors, and antibacterial coatings. The ease of incorporation of drugs and other active agents into multilayers turns this technique very attractive for controlled drug release systems [63, 68, 69]. In food packaging area, LbL edible coatings have been applied on fresh fruits and vegetables to provide prolonged storability, enhanced physiological quality, improved appearance, and antimicrobial protection. In addition, such edible coatings can be used for encapsulation of active compounds as antioxidants, nutraceuticals, flavorings, and colorants [70]. Chitosan, being the only positively charged polysaccharide, can be attached electrostatically to several negatively charged or neutral compounds, such as alginate, hyaluronic acid, heparin, xanthan gum, carboxymethylcellulose, and pectin, to form multilayered films for various applications [71].

2.2 Porous membranes 2.2.1 Solvent casting and particulate leaching In casting the polymer is initially dissolved in an organic solvent. Afterward the particles with specific dimensions consisting mainly of water-soluble salts, such as sodium chloride and sodium acetate, are added to the solution to induce porosity. The mixture is placed in molds, and the solvent is removed, resulting in a membrane composed with the polymer and particles. This membrane is then placed in a bath for enough time to solubilize only the particles, thus leaving a porous structure [3]. The resulting membranes present high porosity and uniform pore morphology, with the possibility to modify pore size and porosity independently, besides requiring simple setup [72]. This method is typically used for water-insoluble polymers, since hydrophilic biopolymers swell and deform when placed in water during the salt-leaching step. An alternative that has been employed to achieve porosity by this technique for water-soluble polymers is to perform matrix cross-linking prior to the leaching step. This strategy was carried out for gelatin [73] and for chitosan-gelatin scaffolds [72] using NaCl crystals as a porogen and acetone/water mixtures as cross-linker. 2.2.2 Thermally induced phase separation Thermally induced phase separation is also a simple method to prepare highly porous membranes with a well-interconnected porous structure. The technique consists in solubilizing the polymer of interest in a high boiling point and low-molecular weight solvent, at elevated temperature, to form a homogeneous solution. This solution is cast into flat sheets and then cooled at a controlled rate to induce solidification and separation into a polymer-rich phase and a polymer-poor phase. Finally the solvent is removed by extraction or freezedrying, forming a microporous structure [3, 74]. The macrostructure of the membranes is determined by polymer-solvent interactions. The cooling rate determines the pore size, which influences the mechanical integrity of the

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membranes. The advantages of this technique include the simplicity of the process, high reproducibility, high porosity, and low tendency to form imperfections [74]. 2.2.3 Electrospinning Electrospinning is one of the most used techniques for preparation of 2D porous membranes based on biopolymers. These membranes are composed of nanofibers, and their structure usually presents high porosity, high surface area, and pore interconnectivity. Electrospun membranes made from biodegradable polymers have gained increasing prominence in applications involving tissue engineering and regenerative medicine, since their structure is similar to the extracellular matrix of many tissues, such as skin, blood vessels, and bones. Other promising applications include filtration, sensor materials, lithium battery separators, drug delivery, wound dressings, and hemostatic devices [74, 75]. A typical electrospinning apparatus is composed of a metallic needle connected to a syringe, a high-voltage supplier, and a metallic collector (Fig. 3A). With the aid of a syringe pump, the polymer solubilized in a suitable solvent is injected through the needle at a controlled flow rate. When a high voltage (of the order of 5–30 kV) is applied to the needle through an electrode, the liquid drop on the tip of the needle experiences an increase on its surface charge density assuming a cone-like shape (Taylor cone). When the electrical potential exceeds the surface tension of the liquid, a filament at the tip of the Taylor cone is formed and ejected from the needle tip to the collector plate. During the trajectory from the nozzle to the collector, the filament elongates, and most of the solvent is evaporated, leaving behind distributed dry nanofibers, which form an electrospun membrane (Fig. 3B) [76]. The diameter of the nanofibers and, hence, the resulting membrane nanostructure can be controlled by several processing parameters including solution viscosity, surface tension, voltage, feed rate, solution conductivity, distance between the needle tip and the collector, and the size of the needle orifice in addition to the ambient temperature and RH. The fibers High voltage supply

Polymer solution

Seringe driver Fiber formation 10 µm

Taylor cone

(A)

Collector plate

(B)

FIG. 3 Schematic representation of the electrospinning setup (A) and SEM image of electrospun biobased membrane (B). Reproduced with permission from F. Croisier, C. J
er^ ome, Chitosan-based biomaterials for tissue engineering, Eur. Polym. J. 49(4) (2013) 780–792, Elsevier.

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may be arranged randomly or aligned. In both cases, they form a 2D fibrous membrane that generally has less than 1-mm thickness due to residual electrostatic repulsion, which prevents layering of fibers on top of each other [75]. A wide range of biodegradable polymers have been successfully electrospun into nanofibers, including natural proteins such as chitosan, collagen, gelatin, silk, hyaluronic acid, and alginate. To improve their processability, electrospinning is often performed by blending these natural polymers with biodegradable synthetic polymers, such as PVA, polyethylene oxide (PEO), polyethylene glycol (PEG), and poly(lactide-co-glycolide) (PLGA). An overview of the most frequent solvent and process parameters used for electrospinning of biopolymers is found in Ref. [77].

3 Modification approaches 3.1 Plasticization Biopolymers are rarely used on their own to produce 2D films and membranes. As in the plastic industry, several additives and modifiers can be blended to obtain the desired functional properties of the material. Plasticizers are an important class of low molar mass nonvolatile compounds that are widely used to improve or promote plasticity, flexibility, and processability of polymers by lowering their glass transition temperature (Tg). International Union of Pure and Applied Chemistry (IUPAC) defined plasticizer as “a substance incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability, or distensibility” [78, 79]. In polymer science, plasticizers can be classified as internal and external. Internal plasticizers are part of the polymer molecules, which are either copolymerized into the polymer structure or reacted with the original polymer. Shaikh et al. [80] prepared novel CA films through acetylation of cellulose from sugarcane bagasse and concluded that the residual hemicellulose content ( 5%) present in bagasse acted as an internal plasticizer for films that exhibit adequate mechanical properties without addition of an external plasticizer. External plasticizers are low volatility substances that are added to polymers. They do not chemically react but interact with polymer chains reducing inter- and intramolecular interactions. External plasticizers can be selected from a variety of compounds, to tune film properties as desired [81]. Plasticizers, for biopolymer-based films and membranes, are divided into water soluble (hydrophilic) and water insoluble (hydrophobic). Hydrophilic plasticizers dissolve in the aqueous medium when added to polymer dispersions and in high concentration can lead to an increase in water migration in the biopolymer. In contrast, hydrophobic plasticizers may close the microvoids in the film, decreasing water uptake. However, water-insoluble plasticizers can undergo phase separation, if not properly dispersed, causing flexibility losses or yet to the formation of heterogeneities during film drying [79]. Biopolymer films and membranes are usually brittle and rigid with low elongation values in the range of 2%–5%. So, plasticizers are systematically added to overcome their brittleness; to avoid film shrinkage, pores, and cracks; and to allow proper processing, handling, and storage. These substances, added in specific amounts, are able to interfere in chain to chain

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interactions, decreasing intermolecular forces, including hydrogen bonding, increasing free volume, and water molecule diffusivity inside the polymeric matrix [25, 82–84]. On the other hand, several studies report adverse effects of plasticizers on biopolymer-based film and membrane attributes. These effects include mainly increase in gas, solute and water vapor permeability, and water solubility and decrease in structure cohesion [19, 82, 83]. So the key issue is to find an equilibrium between cross-linking degree (necessary to reduce water solubility but usually induces brittleness) and plasticizer addition. However, there seems to be no way to characterize a plasticizer behavior in terms of certain fundamental properties, since its behavior greatly depends on the biopolymer to which it is added, especially on their functional groups [84]. It is not unusual for multiple plasticizers to be used in a single formulation, since each plasticizer will respond differently. In a recent work, Huo et al. [19] studied the synergistic plasticization effect of glycerol and a series of polyethers—PEs (ethylene glycol, polypropylene glycol, and polytetramethylene glycol) in the properties of zein films. Excellent elongation at break (245%–350%) was observed when suitable ratio of glycerol and PEs (1:1) was used at 0.45 g/g zein. The synergistic plasticizing effects on films were attributed to the structural change of zein molecules, that is, the disruption of inter-α-helix packing and the increase of β-sheet content. Santana and Kieckbusch [85] studied the influence of different polyols as plasticizers on calcium alginate film properties. Films plasticized with glycerol and xylitol were more hygroscopic and showed higher water vapor permeability than films with mannitol. From the results, these authors suggest the use of glycerol and mannitol mixtures to obtain optimized mechanical properties for alginate films. The selection of a plasticizer, for biopolymeric films, is usually based on the compatibility between components. The degree of plasticity is highly dependent on the chemical structure of the plasticizer, including chemical composition, molar mass, and functional groups. Above a critical concentration the plasticizer can exceed the compatibility limit with the biopolymer, and phase separation with plasticizer exclusion is usually observed [82]. Most plasticizers, used for polysaccharide- and protein-based films, are hygroscopic and hydrophilic in nature. Generally, their concentration into hydrocolloid film forming solutions varies between 10% and 60% by weight of the biopolymer. At low plasticizer concentrations, some studies have indicated a phenomenon of “antiplasticization.” Aguirre et al. [86] studied the effect of glycerol and sorbitol on properties of triticale protein films. Glycerol-plasticized films showed higher hygroscopicity and water vapor permeability compared with sorbitol. For both films, tensile strength and Young’s modulus decreased as the glycerol content increased (plasticizing effect). However, an unexpected increase in the tensile strength and Young’s modulus of the films containing low levels of sorbitol (20 g 100/g) was ascribed to the antiplasticization of triticale films. The same trend was reported by Mali et al. [87], who observed that small amounts of glycerol and sorbitol ( Cu > Cd > Ba > Sr > Ca > Co, Ni, Zn > Mn [119]. Since cross-linking depends on the ion ability to diffuse through the film and on their ionic size, barium ions (0.195 nm), for example, could fill a larger space between the alginate chains producing a tighter arrangement than with calcium ions (0.097 nm) [120]. Bierhalz et al. [118] compared the properties of alginate films containing the antimicrobial natamycin and cross-linked with calcium and barium ions by the two-step process described earlier. Film properties were affected by the type of ion used in the second stage, while natamycin release rate and the antimicrobial activity were influenced by the ion used in the first stage. Films cross-linked with barium in the second stage showed lower water uptake, moisture content, and water solubility but higher water vapor and oxygen permeabilities, a consequence of an inefficient cross-linking inside the film structure, which affected film morphology and, consequently, increased the opacity and roughness. Enzymatic cross-linking of biopolymer-based films is an attractive alternative considering enzymes’ high specificity and possibility to work under mild conditions. Also, due to possible toxic effects imposed by chemical cross-linking agents such as formaldehyde, glutaraldehyde, and glyoxal, enzymatic cross-linking could be a better alternative for protein-based films for food and medical applications [121]. Cross-linking can be a result of direct enzymatic catalysis of cross-link formation or can occur indirectly by enzymatic production of a crosslinking agent, such as H2O2, which will oxidize reactive structures with subsequent crosslinking formation [95]. Transglutaminase enzyme (TGs) is able to form isopeptide bonds between protein chains, enhancing protein-based film properties. Enzymatic cross-linking by transglutaminase has been performed for gelatin-casein films [122], starch/gelatin [123], and gelatin films [121, 124].

3.3 Composites The development of composites represents one of the most preferred strategies for modifying the properties of 2D biobased films and membranes. A composite is generally defined as the combination of two or more distinct constituents to obtain a material with properties

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that cannot be achieved by the individual components. The discontinuous phase that consists of strong load-carrying material, called reinforcement, is embedded in the weaker continuous phase, called matrix, and the two phases remain separated by an interface [125]. The composites can be categorized into three main classes: laminated composites (layers of materials held together by the matrix), fibrous composites (long or short fibers reinforce the matrix), and particulate composites (dispersed particles reinforce the matrix) [126]. Biobased composite films and membranes have been investigated for better structural strength, thermal stability, barrier and rheological properties, tunable biodegradability, sustained release of active agents, and even cost-efficiency. Moreover, some composite structures can also provide active properties to films and membranes, such as antimicrobial activity and oxygen scavenging ability, which is very attractive for food packaging and biomedical applications [127]. Composites consisting of a biopolymer matrix reinforced with particles having at least one of the dimensions in the nanometric range (1–100 nm) are called bionanocomposites [127]. These materials have received increasing attention due to the possibility of producing differentiated properties using very low loads, typically less than 5 wt%. This ability can be attributed to the very high aspect ratio and high surface area of the nanosized reinforcements that lead to ultralarge interfacial area between the constituents. This feature alters the molecular mobility, the relaxation behavior, and, consequently, the attributes of the resulting nanocomposites [128]. Films and membranes loaded with nanoparticles that have antimicrobial activity have been extensively investigated due to the nanofiller high surface-to-volume ratio and elevated surface reactivity that enable the membrane to inactivate microorganisms more efficiently than their micro- or macroscale equivalents [129]. Several types of materials have been explored for the manufacture of composite biopolymeric membranes for biomedical, food packaging, and environment protection applications (Fig. 5). Layered silicates (clays), cellulose whiskers and nanocrystals, metal and metal oxide particles, and, more recently, carbon-based particles are among the most widely used materials. Besides the type of material used for the composite formation, the properties of the resulting films are strongly influenced by factors such as shape, size, and size distribution of reinforcement; reinforcement percentage; distribution of the reinforcement in the matrix; and reinforcement-matrix bonding conditions [130]. Micro- and nanoparticles of metal and metal oxides are commonly employed to assign antimicrobial function for both biomedical and food packaging applications. This effect can be obtained with metallic ions such as silver, copper, gold, and platinum. Metal oxides such as TiO2, ZnO, and MgO are also widely used [127, 131]. In addition to antimicrobial activity against a broad spectrum of microorganisms, the inclusion of these nanoparticles in films and membranes can enhance mechanical and barrier properties and prevent photodegradation of the polymeric materials. 3.3.1 Silicate layers Silicate layers have demonstrated to be very attractive reinforcement for nanocomposite systems due to their availability, low cost, relatively simple processability, and effectiveness. The most widely studied type of clay fillers is montmorillonite (MMT), a hydrated aluminasilicate layered clay consisting of an edge-shared octahedral sheet of aluminum hydroxide between two silica tetrahedral layers [132]. Nanosheets of MMT clay have thickness of

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Polymer

Nanoparticle Nanoplatelets Nanotubes Nanofibers Nanowires

Dispersing at least one nanostructure in polymer

Nanocomposite

Straight diffusion path Tortuous diffusion path – Weak barrier in – Improved barrier in pristine polymer nanocomposite

Example: layered silicate nanocomposite

Exfoliated

Intercalated

Tactoids

FIG. 5 Schematic illustration of different materials structured as nanocomposites. Reproduced with permission from S.D.F. Mihindukulasuriya, L.-T. Lim, Nanotechnology development in food packaging: a review, Trends Food Sci. Technol. 40(2) (2014) 149–167, Elsevier.

approximately 1 nm and aspect ratio of 10–10,000, and the range of surface area is 750 m2/g. Their dispersion leads to interfacial interaction between the filler and the polymeric material, which invigorates stress transfer, strengthening the polymer composite [133]. Biopolymer/clay nanocomposites often promote significant increases in the mechanical and barrier properties of films, which is highly attractive for applications in food packaging. Mechanical properties of the nanocomposites are strongly dependent on the content of the filler and the interfacial interaction between the polymer matrix and the dispersed clay. Gas and water vapor barrier depend on the compatibility between clay and matrix, aspect ratio of clay platelets, and the structure of the nanocomposite. An increase in barrier properties can be attributed to the presence of ordered dispersed silicate layers with large aspect ratios inside the polymer matrix, which force the gases and water vapor molecules to follow a tortuous path, increasing the path length for diffusion and greatly slowing their permeation rate. There are three types of possible polymer-clay formations: tactoid, intercalated, and exfoliated (Fig. 5). Tactoid structures are formed because of the low affinity between clay and the polymer. In this case the polymer is unable to intercalate between the silicate sheets; therefore polymer and the clay tactoids remain immiscible, resulting in agglomeration of the clay in the

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matrix and poor macroscopic properties of the material. Intercalated structures are obtained from a moderate affinity between the polymer and clay. In this case, interlayer spaces expand slightly as polymer chains penetrate the basal spacing of clay, but the shape of the layered stack remains the same. High affinity between the polymer and the clay generates exfoliated structures. In these structures, clay clusters are well separated into single sheets within the continuous polymer phase [127]. Due to this high interaction between clay and polymer, exfoliated nanocomposites tend to exhibit the preferable properties [127, 132]. The creation of a particular structure depends on the nature of the components and the method of preparation. The preparation of composite films with layered silicates and water-soluble biopolymers can also be conducted with the solvent-casting procedure. By this method the clay is initially soaked in a solvent in which the polymer is soluble to swell and exfoliate into individual platelets. Biopolymer and clay are blended, and then the solvent is removed by evaporation or precipitation. The polymer adsorbs onto the laminated sheets, and after solvent removal, the sheets reassemble, sandwiching the polymer to form, in the best scenario, an ordered multilayer structure [134]. The main advantage of this method is that intercalated nanocomposite structures can be achieved even for low-polar or nonpolar polymers [135]. Direct melt intercalation is also a popular method to form nanocomposites with layered silicates. This technique comprises mixing clay and the polymer in a molten state above the softening temperature of the polymer, either statically or under high shear conditions using an extruder, mixer, or ultrasonicator. Under these conditions, if the surfaces are sufficiently compatible with the chosen polymer, polymeric chains migrate into galleries between silicate layers and form either an intercalated or an exfoliated nanocomposite. The melt intercalation approach is mostly used for thermoplastic polymer/clay nanocomposites in which the polymeric matrix is mechanically mixed by conventional techniques at elevated temperatures like extrusion or injection molding [134, 135]. TPS-clay nanocomposite films with intercalated and exfoliated structures have been successfully obtained by using direct melt intercalation with extrusion [136, 137]. An alternative method that bypasses the use of temperature and solvents is ball milling. It consists of a solid-state mixing at ambient temperature in which clay dispersion is promoted by the energy transfer between milling media (usually spheres) and polymer/clay mixture, grinding and mixing them intimately. Mangiacapra et al. [138] prepared pectin and MMT composite films by this method. After efficient mixing of the components, milled powders were cast by water. Another approach to prepare nanocomposites with silicate layers involves in situ polymerization, suitable for synthetic biodegradable polymers such as PLA and polycaprolactone. By this technique, monomers migrate to layered silicate galleries so that the polymerization occurs between the interleaved sheets. 3.3.2 Carbon nanotubes Considerable effort has been allocated to the use of carbon nanotubes (CNTs) in composite materials because of their high surface area, light weight, and outstanding mechanical and electrical properties. CNTs are carbon allotropes in the form of cylinders arranged by rolling graphite sheets. CNTs may consist of a one-atom thick single-wall nanotube or a number of concentric tubes called multiwalled nanotubes, having extraordinarily high aspect ratios and elastic moduli [139].

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Improvement in mechanical, thermal, and barrier properties is among the several advantages provided by biocomposites with CNTs. For instance, a biopolymer chitosan/ multiwalled CNT nanocomposite showed increased tensile modulus and strength of approximately 93% and 99%, respectively, with the incorporation of only 0.8 wt% of CNTs into the chitosan matrix [140]. Functions such as flame retardance can also be obtained [135]. Solution processing is the most common method for preparing polymer/CNT nanocomposites, which consists of physical mixing the polymer and CNTs in a solution using energetic agitation followed by solvent evaporation [135]. Usually, CNTs are initially soaked in a solvent, such as water, and the suspension is homogenized using ultrasonication [140]. In melting compounding technique, CNTs are mixed into molten/viscous polymer by shear mixing, and the final nanocomposite films can be manufactured by compression molding or extrusion. A critical issue to be considered in processing of nanocomposites with CNTs is the difficulty in achieving a uniform dispersion within the polymer matrix. This behavior is attributed to the high aspect ratio of CNTs and their strong van der Waals interactions, which result in poor dispersion in most solvents and reagglomeration within polymer matrices. The dispersion of CNTs can be improved with the use of surfactants, such as sodium dodecyl sulfate; physical treatments with sonochemical and plasma modification; or chemical functionalization [133, 135, 139]. Farahnaky et al. [141] compared pectin/CNC composite films prepared by chemical interaction to those prepared by physical mixing. Chemical bonding was produced by dispersing pectin and CNT powder in ethanol, followed by refluxing at 60°C. Results demonstrated that nanocomposites prepared with chemical modification showed improved properties than those prepared by physical mixture. These attributes include higher tensile strength and Young’s moduli, lower water vapor permeability, longer degradation time, and stronger structural integrity. 3.3.3 Cellulose nanocrystals Nanocellulose has also been highly investigated for production of composite films with biopolymers. Besides low density, high strength, and flexibility, nanocellulose is biodegradable, biocompatible, and chemically inert, and its surface can be chemically modified to introduce functional groups [142, 143]. The source and the processing conditions, in turn, determine the final dimensions and properties of nanocelluloses and divide them into two major groups: nanocrystals (CNCs) and cellulose nanofibrils (CNFs) [143]. The incorporation of nanocellulose into biopolymer-based matrices represents an excellent strategy to improve the properties of the films without affecting the biodegradability of the polymer. Starch-nanocellulose composite characteristics have been widely evaluated and, in general, are reported to have improved mechanical properties and water resistance with contents usually ranging from 1 wt% to 10% [143]. Changes in properties can be observed with very low loads, as reported in a study by Savadekar and Mhaske [144], where incorporation of only 0.4 wt% nanocellulose in TPS matrix increased the tensile strength in 46% and reduced the oxygen permeability in 93% as compared with control films. Matrices of other polysaccharides such as chitosan and alginate have also been successfully reinforced with nanocellulose. Good dispersion and interfacial interaction are often observed because of the similar chemical structures of polysaccharides and nanocellulose. Suitable

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results are obtained with proteins such as gelatin, soy protein and silk fibroin. Ning et al. [145] developed a composite of gelatin/glycerin and CNCs and reported favorable interfacial adhesion between the CNCs and the matrix. The distinguished adhesion was associated to the presence of a large number of hydroxyl groups in CNC, which disrupts the hydrogen bonds between the gelatin molecules and forms new and stronger bonding between all the molecules. Therefore improvements in mechanical resistance and dielectric constant of the films were observed [145]. Most of the nanocomposites of biopolymers with nanocellulose are obtained by solutioncasting technique. Melt compounding and solid-state ball milling are procedures also applied in starch-based nanocomposites [143]. In melt compounding process, however, the control of operation parameters is necessary to avoid nanocellulose degradation due to high shear stresses and high temperatures [142]. The polar surface of nanocellulose can hinder its dispersion in nonpolar polymers. In the case of nonwater soluble polymers, uniform dispersion of nanocellulose can be achieved by coating the surface of cellulose nanowhiskers with surfactants to reduce their surface energy and improve its compatibility with polymers. Another approach consists in grafting of hydrophobic chains on the surfaces of cellulose nanowhiskers [142].

3.4 Blending Blends composed of a mixture of two or more biopolymers and a mixture of biopolymers with synthetic polymers are widely employed in the attempts to modify the properties of films and membranes. Blending comprises a cost-effective technique in which the components can be combined in a molten state or can be solubilized in the same solvent to form a homogeneous material. If film preparation by solubilization blending is considered, the choice of a common solvent is the most important issue for the favorable outcome of the resulting blend. For example, in cellulose-chitosan blends, the polysaccharide blend may be formed in trifluoroacetic acid. Cellulose-chitin films were prepared from solutions in a mixture of dimethylacetamide-LiCl [146]. Miscibility of the components has a key impact on the properties of the blend. It is attributed to specific interactions between the components, such as hydrogen, ion, and dipole bonds. When the polymers forming the blend are compatible, the resulting films and membranes have a single glass transition temperature and show intermediate mechanical properties between those of the two constituents. Incompatibilities of the constituent polymers lead to heterogeneous structures, with phase separations due to differences in viscosity, molecular weight, and molecular structure. Different strategies to improve the compatibility between the polymers and consequently the performance of the final blend have been developed. Basically, compatibilization may be nonreactive or reactive. Strategies for nonreactive compatibilization include the incorporation of block copolymers, of amphiphilic low molecular weight compounds, or of a third polymer, miscible or at least partially miscible in both components. Reactive methods are more efficient and very specific for biopolymers because they make use of additives that inherently contain several reactive groups. A reactive compatibilization can be achieved, for example, by the

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addition of a substance miscible in one component of the blend and reactive toward the functional groups of the other one [147]. Another promising polymer modification technique is known as solid-phase process and combines high pressure and shear deformation applied to the solid components of the blends. Under these conditions a very effective mixing of the reagents takes place on a molecular scale, and several chemical reactions may occur with very low activation energies [146]. Some biopolymers, however, such as proteins, can be degraded and denatured when subjected to processes that combine high temperature and high pressure [148].

3.5 Ionic liquids The low solubility of some biopolymers in water and/or conventional organic solvents represents a challenge for the preparation of films and membranes seeking the expansion and specificities of the field of applications. This occurs particularly with well-known polysaccharides such as cellulose and chitin, which exhibit structural heterogeneities combining amorphous and crystalline regions with strong inter- and intramolecular interactions [149]. The conventional dissolution procedure for cellulose and chitin, for example, needs severe conditions with adverse circumstances related to toxicity, high cost, volatility, difficulty in solvent recovery, and the necessity of multiple pretreatments. In addition, the molecular mass of polymers can be significantly reduced during processing with organic solvents. Ionic liquids (ILs), known as green solvents, have emerged as a class of alternative solvents for biopolymer processing, offering opportunities not only for dissolution but also for regeneration, modification, usage as a reaction medium, and even for the formation of new hybrid materials, such as ionogels [150]. The possibility of cost-effective and easier processing of biopolymers associated with the application of green chemistry principles, which include reducing the use of toxic and hazardous solvents, is a steady growing field of research. ILs are a group of organic salts with low melting temperature ( 20 A) since the chitosan chains are blocking the zeolite pores. The specific area of zeolite-loaded films was about 24% higher than the chitosan films without zeolite.

4 Biological characterization The standardized methods for the biological characterization of biopolymer films are practically nonexistent, so in this section the characterizations are based on methods widely used

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FIG. 7 Risk management for biocompatibility evaluations. Based on Guidance for Industry and Food and Drug Administration Staff, 2016.

in biopolymers, or biomaterials in general, and therefore may be suitable for characterization of biopolymer films. All biopolymers used as biomaterials must be evaluated regarding their biocompatibility. Usually, the tests are based on ISO 10993 that describes the biological evaluation of medical devices in part 1 [99]. Nevertheless, other organizations such as the Food and Drug Administration [100], Organization for Economic Co-operation and Development (OECD), and nongovernmental organizations, composed of institutions of countries from all over the world, also proposed modification or even additional tests to support the biological applications [101]. The risk management process could be used as a tool for planning the tests applied to the biopolymers (Fig. 7). Usually, it begins with the concept of direct or indirect contact with the new device: surface device (intact skin, mucosal membrane, and breached compromised surface); external communicating device (blood path, indirect, tissue/bone/dentin, and circulating blood); implant device (tissue/bone and blood) [99]. Exposition time and cumulative effects are also described. For each subcategory there is the division according to the duration of contact, consisting of “limited” when the contact time is less than or equal to 24 h; “prolonged” when it is from 24 h to 30 days; or “permanent” when the contact time is longer than 30 days [99]. Specific biological tests can be requested for each particular type of medical device, which can be found in specific guides. For example, for application as a blood implant device, implantation and hemocompatibility should be tested, among others; these are parameters that are not required in the case of a surface device in contact with the intact skin surface [99].

4.1 Cytotoxicity, sensitization capacity, and irritation potential In terms of specific tests, ISO 10993-1 [102] reports that all materials should be tested regarding their cytotoxicity, sensitization capacity, and irritation potential. In addition to such tests, the annexes detail the evaluation and endpoints for consideration, a summary of biocompatibility documentation, new processing/sterilization [103, 104], and formulation changes, as well as sample preparation (ISO 10993-12, 2012 [105]). It is important to note that for combined materials, even if the general principles are applied, it must be necessary to run additional tests.

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For the use of material in extracts (Fig. 8), it is necessary to determine concentration and sample preparation. Determination of controls also depends on how the material will be clinically used, and the most common are natural rubber latex as positive controls and gauze as negative controls. Nowadays, the tests are mainly performed in vitro (Fig. 9), following the world recommendation to minimize in vivo tests. These first three biocompatibility tests may be performed using cell cultures, including primary or cell lines, to estimate the effect caused by the exposure of the biomaterial to the cells. As a first test and the one performed for all biomaterials, the cytotoxicity test uses BALB/c 3 T3 mouse fibroblasts or normal human epidermal keratinocytes and neutral red uptake as the cytotoxicity endpoint. Other vital dyes could also be employed, such as 3-(4,5dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) or 3-(4,5dimethyl-2-thiazolyl)-5-(3-carboximetoxiphenyl)-2-(4-sulfophenyl)-2H-tetrazolium [99, 106]. Sensitization is described as the toxicological endpoint associated with chemicals that have the intrinsic ability to cause skin allergy [107]. This adverse effect results from an overreaction of the adaptive immune system. Induction of sensitization on the first contact with allergy symptoms resulted from further contact [107–109]. Currently, an in vitro validated sensitization test is not yet available, so the employed animal method is the mouse local lymph node assay (LLNA) [110]. Nevertheless, if the biomaterial is topical applied, it is possible to evaluate it using the “adverse outcome pathway” (AOP) for skin sensitization. An AOP is a model that links exposure to a substance with a toxic effect by identifying the sequence of biochemical events required to produce the toxic effect [109]. The AOP for skin sensitization needs at least four tests:

FIG. 8 Example of planning a film extraction for use in biological safety trials regarding indirect cytotoxicity. Positive controls: films containing zinc diethyldithiocarbamate (SPU-ZDEC), dibutyldithiocarbamate (SPU-ZDBC), and/ or natural rubber latex. Negative control: gauze.

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FIG. 9 Schematic representation of in vitro tests using mammal cells as an alternative to in vivo tests.

OECD TG 442C [111, 112]—Direct Peptide Reactivity Assay addressing the first key event of skin sensitization; OECD TG 442D [113]—ARE-Nrf2 Luciferase Test Method addressing the second key event of skin sensitization; OECD TG 442E [113, 114]—In Vitro Skin Sensitization addressing the activation of dendritic cells typically assessed by expression of specific cell surface markers, chemokines, and cytokines; and OECD 442A [115]—Skin Sensitization addressing T-cell proliferation, which is indirectly evaluated in the murine LLNA. To estimate the irritation potential of the biomaterial, ISO 10993-1 [99] describes mainly the use of the skin irritation test, described by OECD TG 439, which involves the application of a chemical or mixture for up to 4 h on the RhE test system—comprised of nontransformed human-derived epidermal keratinocytes, which have been cultured to form a multilayered, highly differentiated model of the human epidermis and monitoring for reversible damage to the skin caused by a local inflammatory reaction (the innate [nonspecific] immune system). Cell viability is determined by quantitatively measuring the enzymatic conversion of the yellow tetrazolium dye MTT into a purple/blue formazan salt. Damage to the cells caused by the application of a test substance can be determined by a change in the amount of dye conversion in the tissues [111, 116]. For medical devices that are used as implants or external communicating devices, intradermal testing is more relevant in approaching the application, and so for the detection of irritation activity, intracutaneous testing shall be used as described in ISO 10993-10 [117]. Biopolymer membranes and films for biomaterial applications must be carefully assayed by several techniques before contact with biological interfaces or clinical usage for adequate biological characterization. Besides, it must be considered that the improved characteristics or changes in the biomaterial may result in an entirely different biological response emphasizing the importance of proper laboratory experimental designs.

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4.2 Antimicrobial activity Regarding antimicrobial activity, it is possible to determine the biostatic or biocide microbial properties of the films in solid or liquid medium. The method’s choice will depend on the film’s characteristics, as well as the microorganism that will be used. Antimicrobial activity could be performed by adapting the Clinical and Laboratory Standards Institute [118] guidelines for macrodilution and microdilution in broth. Macrodilution uses larger volumes and, consequently, more samples. Briefly, for both cases, the microorganism was added to the tubes (macrodilution) or 96 microplate wells (microdilution) in different concentrations, and the films were added regarding their proportion. Positive and negative controls were also included. The final concentration, using the pure medium as control, was defined as the lowest concentration that yielded no visible growth of bacteria after 24 h incubation at 37°C, depending on the tested microorganism. The biostatic level was determined by plating a 100 μL suspension collected from the test tubes without visible bacteria growth onto the agar plates and then incubating at 37°C for another 24 h. If there is growth, the films are considered to be biostatic, but if growth does not exist, the films are also considered biocides. On the other hand, antimicrobial activity could also be performed using the disk inhibition zone assay, which qualitatively evaluates the antimicrobial activity of the films according to the Bauer-Kirby disk diffusion test, with modifications [119, 120]. The films produced with and without (control) antimicrobial agents were aseptically cut into 10 mm discs and placed on plates containing Mueller-Hinton agar or another rich medium, which had been previously spread with 0.1 mL of inoculums, each containing 108 CFU mL1 of microorganism cultures, previously standardized. The plates were incubated at 37°C for 24 h, depending on the microorganism. The diameter of the growth inhibition zones around the discs was measured, and the growth under the film discs (area of contact with the agar surface) was visually examined and could be evaluated to determine the biostatic action, using another plate, as previously described.

5 Conclusions Several characterization techniques applied to biopolymer films and membranes were reported in this chapter. A broad search in the literature showed that mechanical characterization, FTIR spectroscopy, SEM, barrier properties, and XRD are the main techniques used to characterize biopolymer membranes and films. The choice of which characterization techniques will be used depends mainly on the application of the material, related mostly to the biomedical, food, and environmental fields.

Acknowledgment The authors would like to thank the financial support received from Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) under project numbers 2016/25120-7 and 2016/17555-3.

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[87] B. Jasse, A.M. Seuvre, M. Mathlouthi, Permeability and structure in polymeric packaging materials, in: Food Packaging and Preservation, Springer, Boston, MA, 1994, pp. 1–22, https://doi.org/10.1007/978-1-46152173-0_1. [88] ASTM E96/E96M:16, Standard Test Methods for Water Vapour Transmission of Materials, 2016. [89] P. Humbert, C. Courderot-Masuyer, S. Robin, D. Oster, R. Pegahi, Exudates absorption and proteases trapping in venous leg ulcers, J. Wound Care 26 (6) (2017) 346–348, https://doi.org/10.12968/jowc.2017.26.6.346. [90] S. Wittaya-areekul, C. Prahsarn, Development and in vitro evaluation of chitosan–polysaccharides composite wound dressings, Int. J. Pharm. 313 (1–2) (2006) 123–128, https://doi.org/10.1016/j.ijpharm.2006.01.027. [91] I.J.H. Barrientos, E. Paladino, P. Szabo´, S. Brozio, P.J. Hall, C.I. Oseghale, R. Zelko´, Electrospun collagen-based nanofibres: a sustainable material for improved antibiotic utilisation in tissue engineering applications, Int. J. Pharm. 531 (1) (2017) 67–79, https://doi.org/10.1016/j.ijpharm.2017.08.071. [92] J. Kalaiselvimary, M. Sundararajan, M.R. Prabhu, Preparation and characterization of chitosan-based nanocomposite hybrid polymer electrolyte membranes for fuel cell application, Ionics 24 (11) (2018) 3555–3571, https://doi.org/10.1007/s11581-018-2485-7. [93] P. Moazzam, H. Tavassoli, A. Razmjou, M.E. Warkiani, M. Asadnia, Mist harvesting using bioinspired polydopamine coating and microfabrication technology, Desalination 429 (2018) 111–118, https://doi.org/ 10.1016/j.desal.2017.12.023. [94] W. Shao, J. Wu, H. Liu, S. Ye, L. Jiang, X. Liu, Novel bioactive surface functionalization of bacterial cellulose membrane, Carbohydr. Polym. 178 (2017) 270–276, https://doi.org/10.1016/j.carbpol.2017.09.045. [95] L. Wang, R.J. Mu, Y. Li, L. Lin, Z. Lin, J. Pang, Characterization and antibacterial activity evaluation of curcumin loaded konjac glucomannan and zein nanofibril films, LWT 113 (2019) 108293, https://doi.org/10.1016/j. lwt.2019.108293. [96] A.A. Alshahrani, M.S. Algamdi, I.H. Alsohaimi, L.D. Nghiem, K.L. Tu, A.E. Al-Rawajfeh, in het Panhuis, M., The rejection of mono-and divalent ions from aquatic environment by MWNT/chitosan buckypaper composite membranes: Influences of chitosan concentrations, Sep. Purif. Technol. 234 (2020) 116088, https://doi.org/ 10.1016/j.seppur.2019.116088. [97] ASTM C373-88, Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity, and Apparent Specific Gravity of Fired Whiteware Products, ASTM International, West Conshohocken, PA, 2006. [98] P.H. Yassue-Cordeiro, C.H. Zandonai, C.F.D. Silva, N.R.C. Fernandes-Machado, Desenvolvimento e caracterizac¸a˜o de filmes compo´sitos de quitosana e zeo´litas com prata, Polı´meros 25 (5) (2015) 492–502, https://doi.org/10.1590/0104-1428.2059. [99] International Organization for Standardization (ISO), Biological Evaluation of Medical Devices – Part 5: Tests for In Vitro Cytotoxicity, ISO 10993-5, 2009. [100] Food and Drug Administration, Guidance for Industry and Food and Drug Administration Staff: Factors to Consider When Making Benefit-Risk Determinations in Medical Device Premarket Approval and De Novo Classifications, Department of Health and Human Services, Rockville, 2012. https://www.fda.gov/downloads/ MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM313368.pdf At 30.01.2019. [101] J.P. Oliveira, H.I. Melendez-Ortiz, E. Bucio, P.T. Alves, M.I. Lima, L.R. Goulart, A.B. Lugao, Current methods applied to biomaterials–characterization approaches, safety assessment and biological international standards, Curr. Top. Med. Chem. 18 (4) (2018) 256–274, https://doi.org/10.2174/1568026618666180410151518. [102] International Organization for Standardization (ISO), Biological Evaluation of Medical Devices—Part 1: Evaluation and Testing Within a Risk Management Process, 2016, ISO 10993-1. [103] International Organization for Standardization (ISO 11137-2), Sterilization of Health Care Products – Radiation – Part 2. Establishing the Sterilization Dose, (2013). [104] International Organization for Standardization (ISO 13409), Sterilization of Health Care Products – Radiation Sterilization – Substantiation of 25 kGy as a Sterilization Dose for Small of Infrequent Production Batches, (2002). [105] International Organization for Standardization (ISO), Biological Evaluation of Medical Devices—Part 12: Sample Preparation and Reference Materials, 2012, ISO 10993-12. [106] Organization for Economic Cooperation and Development (OECD), Guidelines for Testing of Chemicals Nº 129, Guidance Document on Using Toxicity Tests to Estimate Starting Doses for Acute Oral Systemic Toxicity Tests, OECD Publications, 2010. [107] S. Adler, D. Basketter, S. Creton, O. Pelkonen, J. Van Benthem, V. Zuang, E. Benfenati, Alternative (non-animal) methods for cosmetics testing: Current status and future prospects—2010, Arch. Toxicol. 85 (5) (2011) 367–485, https://doi.org/10.1007/s00204-011-0693-2.

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[108] ICCVAM – NTP, Evaluations of Non-Animal Skin Sensitization Test Methods and Testing Strategies, National Toxicology Program - U.S. Department of Health and Human Services, 2014, http://ntp.niehs.nih.gov/ pubhealth/evalatm/test-method-evaluations/immunotoxicity/nonanimal/index.html. [109] N. Kleinstreuer, J. Strickland, D. Allen, W. Casey, Predicting skin sensitization using ToxCast assays, in: Abstract# 1062C Presented at the Society of Toxicology 53rd Annual Meeting, 2014 http://ntp.niehs.nih. gov/iccvam/meetings/9wc/posters/kleinstreuer-tox21ss-wc9.pdf at November 2014. [110] OECD, Test No. 429: Skin Sensitisation: Local Lymph Node Assay, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2010, https://doi.org/10.1787/9789264071100-en. [111] OECD, Test No. 439: In Vitro Skin Irritation: Reconstructed Human Epidermis Test Method, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2015, https://doi.org/ 10.1787/9789264242845-en. [112] OECD, Test No. 442C: In Chemico Skin Sensitisation: Direct Peptide Reactivity Assay (DPRA), OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2015, https://doi.org/ 10.1787/9789264229709-en. [113] OECD, Test No. 442D: In Vitro Skin Sensitisation: ARE-Nrf2 Luciferase Test Method, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2018, https://doi.org/10.1787/9789264229822-en. [114] OECD, Test No. 442E: In Vitro Skin Sensitisation: In Vitro Skin Sensitisation Assays Addressing the Key Event on Activation of Dendritic Cells on the Adverse Outcome Pathway for Skin Sensitisation, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2018, https://doi.org/10.1787/9789264264359-en. [115] OECD, Test No. 442A: Skin Sensitization: Local Lymph Node Assay: DA, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris, 2010, https://doi.org/10.1787/9789264090972-en. [116] United Nations, Globally Harmonized System of Classification and Labelling of Chemicals (GHS), Second revised edition, http://www.unece.org/trans/danger/publi/ghs/ghs_rev02/02files_e.html, 2010. [117] International Organization for Standardization (ISO), Biological Evaluation of Medical Devices – Part 10: Tests for Irritation and Skin Sensitization, ISO 10993-10, 2010. [118] P.A. Wayne, Clinical and Laboratory Standards Institute (CLSI). Performance Standards for Antimicrobial Susceptibility Testing. Twenty-Second Informational Supplement M100-S21, Wayne, 2012. [119] W.K. Loke, S.K. Lau, L.L. Yong, E. Khor, C.K. Sum, Wound dressing with sustained anti-microbial capability, J. Biomed. Mater. Res. 53 (1) (2000) 8–17, https://doi.org/10.1002/(SICI)1097-4636(2000)53:1%3C8::AID-JBM2% 3E3.0.CO;2-3. [120] A.G. Ponce, R. Fritz, C. Del Valle, S.I. Roura, Antimicrobial activity of essential oils on the native microflora of organic Swiss chard, LWT- Food Sci. Technol. 36 (7) (2003) 679–684, https://doi.org/10.1016/S0023-6438(03) 00088-4.

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C H A P T E R

4 Diffusion process through biodegradable polymer films Jackson Wesley Silva dos Santosa, Mariangela de Fa´tima Silvab, Viktor Oswaldo Ca´rdenas Conchaa, Cristiana Maria Pedroso Yoshidaa a

Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil bFederal Institute of Education, Science and Technology of Mato Grosso do Sul (IFMS), Coxim, Mato Grosso do Sul, Brazil

1 Introduction Environmental awareness has been increasing as well as the interest of the use of materials with less impact that biodegrade in a shorter time and with properties that provide applicability in different areas [1]. Researches have been conducted by several groups, proving the potential of natural polymers, defined as macromolecules from renewable sources, which have film-forming capacity, such as polysaccharides (cellulose, chitosan, starch, and others), proteins (whey proteins, casein, collagen, gelatin, and others), and lipids (beeswax, carnauba wax, fatty acids, and paraffins) [2]. It is important to note that films from natural polymers have the biodegradation facilitated compared with those obtained by synthetic polymers. This reduces the environmental impact caused by the excessive disposal of materials derived from synthetic polymers that have an average biodegradation time of 100–400 years. To increase the functionality of biopolymeric films, some authors incorporated active substances into the three-dimensional matrix, such as antimicrobial compounds, antioxidants, absorbers, and antiinflammatories, forming active films. The application of these active substances protects the product surfaces by gradually releasing the active compounds. These active substances promote different applications, such as the incorporation of natural antimicrobial agents in active food packaging [3], addition of colorimetric indicator for rapid detection of hydrogen sulfide [4], and incorporation of therapeutic agents to act as wound dressings [5]. Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00004-3

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The incorporation of the active substances highlights the study of the diffusive process of the molecules incorporated into the natural polymeric films. The diffusion process will control the release of actives; water and gas vapor barrier property; inhibition of microorganisms; antiinflammatory action; and others [3, 6, 7]. Diffusion is defined as the process of mass transport from one part of the system to another, having as its driving force the difference in concentration. Distinct factors influence diffusive mechanisms, such as molecule size, pore size, and system geometry [8]. This chapter aims to present the literature researches about the diffusion mechanism of active substances through natural polymer films, such as polysaccharides and proteins with and without lipid incorporation in the filmogenic matrix. These active films were developed for application in different areas (active packaging, dressings, oral disintegrating film, and others), indicating the importance of active compound diffusivity in system efficiency. It will be considered biopolymers all those materials that have fast biodegradation and are also obtained from natural and renewable sources. In this chapter, in addition to the processes of diffusion through biopolymeric films, the increase in the number of scientific research for various film raw material and the mechanisms of diffusion in natural polymers will be discussed.

2 Natural polymers According to Leja and Lewandowicz [9], a biodegradable polymer is defined as one in which the first degradation mechanism is the metabolism of microorganisms. Biodegradation occurs through the action of a biological agent, which may be bacteria, fungi, and algae. Among the biodegradable polymers are synthetic and natural polymers, which undergo biodegradation but at different speeds. Each of these polymers has distinct sources of raw materials [10]. Modified synthetic polymers are formed by macromolecules consisting of the repetition of smaller units. The monomers have petroleum as the primary raw material source, classified as a nonrenewable source. However, these materials have a high total biodegradation time and can reach up to 500 years, which causes a major environmental problem. It is essential to highlight that in many cases, the useful life of this material is short, and after its disposal, its decomposition time is long [11]. The source of raw material from natural polymers is renewable, and just like synthetic polymers, the chemical structure is formed by macromolecules that are made up of monomers [10, 12]. The interest in natural polymers is growing around the world. From 2005 to 2009 the demand for this type of polymer doubled, boosting studies and research [13]. The main advantages of these materials are highlighted by several authors, including nontoxic degradation to the environment, biocompatibility, and stable shelf life and processing [13–15]. Several polymers from renewable sources have been studied in the formation of films, such as proteins and polysaccharides. Fig. 1 shows the source of the raw material of some materials used to form natural polymer films.

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Biopolymers

2 Natural polymers

Class

Biomass

Polylsaccharides: chitosan, cassava starch, cellulose. Proteins: collagen, gluten, gelatin

Bioderived monomers

Polylactate, lactic acid

Microorganism metabolism product

Polyhydroxyalkanoates

Obtaining source

Examples

FIG. 1 Source of natural polymer raw materials. From Authors.

Analyzing publications in journals/articles indexed in the Web of Science database, it can be noted that in the last 20 years, there has been a growing increase in works involving natural polymers, which reinforces their great interest and potential for use. In the search, tools were used to cover all the published works available in the database, such as the asterisk symbol (*) so that the search terms were considered plural and also singular (i.e., “film*” will find “film” and “films”). Fig. 2 shows the number of items published on this topic from 1999 to 2019. The search terms used were as follows: • • • •

Biopolymer* AND Film* Biopolymer* AND Membrane* Natural Polymer* AND Film* Natural Polymer* AND Membrane*

Fig. 3 shows the number of publications of some polysaccharides used in the formation of biopolymer films from 1999 to 2019. Fig. 4 shows the number of citations available in the last 20 years for some polysaccharides used in the making of films. The search terms used in the Web of Science database to search for polysaccharide-based natural polymers were as follows: • • • • • •

(Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR

Natural Natural Natural Natural Natural Natural

Polymer* Polymer* Polymer* Polymer* Polymer* Polymer*

AND AND AND AND AND AND

Film* Film* Film* Film* Film* Film*

OR OR OR OR OR OR

Membrane*) AND Alginate Membrane*) AND Starch Membrane*) AND Cellulose Membrane*) AND Gellan Gum Membrane*) AND Pectin Membrane*) AND Chitosan

I. Fundamentals on biopolymers membranes and films

4. Diffusion process through biodegradable polymer films 600

500

500 Number of publication

600

400 300 200

400 300 200 100

0

0 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19

100

19 9 20 9 0 20 0 0 20 1 0 20 2 0 20 3 0 20 4 0 20 5 0 20 6 0 20 7 0 20 8 0 20 9 1 20 0 1 20 1 1 20 2 1 20 3 1 20 4 1 20 5 16 20 1 20 7 1 20 8 19

Number of publication

100

(A)

(B)

Year

600

Year

500 Number of publication

Number of publication

450 500 400 300 200 100

400 350 300 250 200 150 100 50

(C)

Year

(D)

19 9 20 9 0 20 0 0 20 1 0 20 2 0 20 3 0 20 4 0 20 5 0 20 6 0 20 7 0 20 8 0 20 9 1 20 0 1 20 1 1 20 2 1 20 3 1 20 4 1 20 5 1 20 6 1 20 7 1 20 8 19

0

19 9 20 9 0 20 0 0 20 1 0 20 2 0 20 3 0 20 4 0 20 5 0 20 6 0 20 7 0 20 8 0 20 9 1 20 0 1 20 1 1 20 2 1 20 3 1 20 4 1 20 5 16 20 1 20 7 1 20 8 19

0

Year

FIG. 2

Number of scientific researches carried out on the subject considering different expressions found in the literature: (A) Biopolymer* and film*, (B) Biopolymer* and membrane*, (C) Natural Polymer* and film*, and (D) Natural Polymer* and membrane*. Based on data available on Web of Science.

Number of publications

2500 2000 1500 1000 500 0 Alginate

Starch

Cellulose Gellan gum

Pectin

Chitosan

Material

FIG. 3

Number of publications of scientific works indexed in Web of Science developed with some polysaccharides in the preparation of biopolymeric films from 1999 to 2019. Based on data available on Web of Science.

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101

2 Natural polymers

Number of citations

80,000 60,000 40,000 20,000 0

Alginate

Starch

Cellulose Gellan gum

Pectin

Chitosan

Material

FIG. 4

Number of citations obtained from scientific works indexed in Web of Science developed with some polysaccharides in the preparation of biopolymeric films from 1999 to 2019. Based on data available on Web of Science.

Fig. 5 shows the publication number of works indexed in Web of Science about films obtained from some available proteins, the search period is from 1999 to 2019, the number of citations obtained from scientific works on films made from some proteins is illustrated in Fig. 6, and the search terms used were as follows: • • • • • • •

(Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR (Biopolymer* OR

Natural Natural Natural Natural Natural Natural Natural

Polymer* Polymer* Polymer* Polymer* Polymer* Polymer* Polymer*

AND AND AND AND AND AND AND

Film* Film* Film* Film* Film* Film* Film*

OR OR OR OR OR OR OR

Membrane*) AND Albumin Membrane*) AND Casein Membrane*) AND Collagen Membrane*) AND Gelatine Membrane*) AND Gluten Membrane*) AND Soy Protein Membrane*) AND Whey Protein

Number of publications

700 600 500 400 300 200 100 0 Albumin

Casein

Collagen

Gelatin

Gluten

Soy protein

Whey protein

Material

FIG. 5 Number of publications of scientific works indexed in Web of Science developed with some proteins in the preparation of biopolymeric films from 1999 to 2019. Based on data available on Web of Science.

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4. Diffusion process through biodegradable polymer films

Number of citations

20,000 15,000 10,000 5000 0 Albumin

Casein

Collagen

Gelatin

Gluten

Soy protein

Whey protein

Material

FIG. 6 Number of citations obtained from scientific works indexed in Web of Science developed with some proteins in the preparation of biopolymeric films from 1999 to 2019. Based on data available on Web of Science.

The number of publications and the number of citations for polysaccharide films and protein films are shown in Figs. 3 and 4, respectively. The number of citations follows the publication number for both materials. In the case of films based on polysaccharides, the highlights are chitosan, cellulose, and gellan gum. However, gelatin, collagen, and albumin are highlights of protein films. On the other hand, films based on pectin, alginate, and starch (polysaccharides) and films based on proteins such as and gluten, casein, soy, and whey protein have less relevance in scientific publications (Figs. 5 and 6). Natural polymers are currently classified as green materials by their rapid degradation when compared with synthetic polymers [1]. Synthetic polymers have a higher degradation time as compared with polymers from renewable sources. The degradation rate of these materials can take years, for example, plastic that can take more than 500 years to degrade, which negatively impacts the environment. However, biopolymers have a higher degradation rate than synthetic polymers, which depending on the medium, may last for hours or days [16]. De Silva et al. [17] studied the degradation time of embedded chitosan-based films of buriti oil, which has antimicrobial property; these films after 4 months of testing in soil were utterly degraded. According to Freile-Pelegrı´n et al. [18], films developed based on agar exposed in tropical climate conditions for 90 days began to have their mechanical properties impacted by the degradation effect of the material after 30–45 days of the exposition. According to Elsabee and Abdou [19], natural polymer films and coatings have characteristics of the structure, solubility, and permeability, which offer the potential for application in various areas and can extend their functionality through the incorporation of additives and their release. Sectors such as food, health, and agriculture already use these materials in microbial combat, for example, since films can be enriched with agents that detect and fight microorganisms [20].

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3 Polysaccharide based films

3 Polysaccharide based films An affinity for water molecules characterizes polysaccharide films, exhibiting selective O2 and CO2 permeability and lipid migration resistance [21]. When the moisture loss is desired, the polysaccharide films could be used due to the water molecule’s affinity. In foods, for example, they can be used to prevent dehydration and surface darkening significantly. Table 1 summarizes the development of films based on some polysaccharides. TABLE 1

Development of films based on different polysaccharides.

Raw material

Other compounds

Starch

Preparation methodology

Drying condition

References

Glycerol (plasticizer), blueberry, distilled water

Filmogenic solution was mechanically stirred in a thermostatic bath at 90°C for 35 min

Forced air convection oven at 35°C for 24 h

Luchese et al. [22]

Chitosan

Chlorophyll

Chitosan was dispersed in aqueous acetic acid solution, and it was mechanically stirred for 45 min until complete solubilization

At room temperature (25°C) and subsequently in a forced air oven at 28°C for 24 h

Maciel et al. [23]

Cellulose

Ethanol, distilled water, plasticizing agent

The cellulose was dissolved in ethanol and water solution and homogenized for 5 min under stirring, then heated in a bath at 85°C (15 min). Finally the plasticizer was added

Drying at room temperature (25°C)

Park et al. [24]

Alginate

Glycerol, distilled water

Alginate was solubilized in water, stirred at room temperature for 18 h. Then glycerol was added, and the solution was stirred for 12 h

30°C for 48 h

Costa et al. [25]

Pectin

Glycerol, calcium chloride

Different concentrations (by mass) of pectin were dissolved in distilled water. Glycerol (plasticizer) was added to the filmogenic solution at different concentrations. The solution was heated to 70°C and poured in Petri dishes

Ventilated and conditioned chamber at 25
1°C with a relative humidity of 40
2% for 3 days

Galus et al. [26]

Gelana gum

Cassava starch, glycerol

Cassava starch was solubilized in water and heated until complete gelation. Gelana gum was added at different concentrations. Glycerol was used as a plasticizer. The mixture was filtered and again heated to 80°C

The films were dried under different temperature conditions: 40°C, 50°C, 60°C, 70°C, and 80°C

Xiao et al. [27]

From Authors.

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4. Diffusion process through biodegradable polymer films

4 Proteins based films Proteins in their natural state are present in animal tissue and vegetables [28]. The physicochemical properties of proteins depend on the organization of the amino acids along the polymeric chain. In general, protein denaturation occurs by external agents such as heat, acids, and bases, which, combined with solvents, contribute to the formation of protein-based films. Protein films have flexibility and permeability to gases, vapors, and liquids, and the main types of proteins used for the formation of films are gelatin, casein, zein (corn protein), gluten, and soy protein [29]. Protein films are generally more stable than polysaccharide films. Protein films are biodegradable, and their degradation does not negatively impact the environment [30]. Table 2 provides a summary of the development of protein films from different raw materials. TABLE 2 Development of films based on different proteins. Raw material

Other compounds

Preparation methodology

Drying condition

References

Soy protein

Plasticizing agent, sodium hydroxide

Plasticizer was solubilized in distilled water, then soy protein was slowly added, and the pH adjusted to make the medium basic. The solution was heated in a thermostatic bath at 70°C for 20 min

Room temperature, approximately 22°C with 35% relative humidity for 20 h

Wan et al. [31]

Whey protein

Glycerol, distilled water, stearic acid. Acetic acid and sodium hydroxide for pH adjustment

The whey protein was dispersed in glycerol-containing solution and heated to 90°C for 30 min. Then stearic acid was added, and the solution was homogenized by mechanical stirring for 10 min. The pH was neutralized

Room temperature until film formation

Yoshida et al. [32]

Gelatine

Glycerol (plasticizer), corn oil, sodium hydroxide (pH adjustment), distilled water

Gelatin was solubilized in distilled water, and plasticizer and corn oil were added. The pH of the solution was adjusted to 10.54. The solution was kept under heating at 80°C for 30 min, after which step it was homogenized

Room temperature 23°C for 24 h and 50% relative humidity

Nur Hanani et al. [33]

Collagen

Glycerol (plasticizer), glacial acetic acid (pH adjustment)

Collagen was solubilized in water, and the pH of the solution was adjusted to 3.0. Then the solution was heated to 90°C for 30 min. The solution was kept under magnetic stirring by adding the bioactive agent. Finally the plasticizer was incorporated into the solution

Air circulation oven at 30°C for 24 h

Makishi [34]

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105

5 Active films

TABLE 2 Raw material

Development of films based on different proteins—cont’d Other compounds

Preparation methodology

Drying condition

References

Gluten

Ethanol, distilled water, glycerol (plasticizer), ammonium hydroxide (pH adjustment)

Gluten was dispersed in ethanol and glycerol solution, stirring for 10 min. Distilled water and ammonium hydroxide were added, followed by heating (75–77°C)

Air circulation oven drying with at 32°C (15 h)

Gennadios et al. [35]

Albumin

Polyethylene glycol (plasticizer), sodium hydroxide

Dry albumin was solubilized in water, and polyethylene glycol was added. The pH of the filmogenic solution was adjusted and heated to 45°C for 20 min

Drying at 25°C with 50% relative humidity

Taqi et al. [36]

Casein

Polyvinyl alcohol, triethanolamine, glycerol

Polyvinyl alcohol was dissolved in water and heated to 80°C for 1 h under constant stirring. In parallel the casein and glycerol solution was dissolved in triethanolamine under magnetic stirring at 35°C for 1 h. The two solutions were mixed at room temperature for 15 min

Ambient conditions for 48 h

Ucpinar Durmaz and Aytac [37]

From Authors.

5 Active films The active natural polymeric films have potential applications in different areas, expanding the properties of the biopolymeric films. Incorporating active compounds the biopolymeric films add functionalities such as antimicrobial, antioxidant, nutraceutical, and probiotic activities in the pharmaceutical area they can act like bandages and in drug release control [38]. In films that control microbial growth, substances are incorporated into their structure, acting as antimicrobial agents, reducing or inhibiting the growth of the microorganism. Substances such as sodium benzoate, potassium sorbate, propionic acid, nisin, and lactic acid can be added to the matrix of films formed from chitosan, starch, alginate, casein, and cellulose, for example, creating active films that decrease the action of different pathogens such as Escherichia coli, Staphylococcus aureus, and Salmonella montevideo [39]. Antioxidant acting films contain substances with antioxidant properties. According to Pires et al. [40], essential oils of cloves, cinnamon, fennel, garlic, ginger, and thyme can perform this function. Incorporation of natural antioxidants is preferable when compared with synthetics, which may have a toxic effect. Films based on chitosan, methylcellulose, milk protein, and zein have been developed with the addition of antioxidants [41]. There are a growing number of studies that evaluated the addition of probiotic agents in films, which help in the development of human health. Films based on alginate, starch,

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gelatin, and agar, among others, had the incorporation of bacteria that act as probiotics such as Bifidobacterium lactis, Lactobacillus acidophilus, and Lactobacillus plantarum. In the preparation of these materials, plasticizing agents have also been incorporated into the film matrices to improve mechanical properties. In the studies performed the viability and stability of the films were evaluated, which indicated potential application in the food area, as in functional foods [42]. The use of natural polymeric films in drug release is also studied by several authors, who highlighted the advantages of using these systems, such as reduced toxicity, lower number of doses, safe administration, and targeting specific targets. The films for controlled drug release, which the drug is physically or chemically entrapped, will depend on the properties of the polymer used in the system [43].

6 Natural polymer diffusion mechanism 6.1 Mass transfer and diffusion Mass transfer is the phenomenon that describes the movement of matter, and the concentration gradient is the driving force. Diffusion is the process by which matter is transported from one part of the system to another as a result of molecular motion [8]. Studies have been carried out to evaluate the mass transfer process in different systems, being as active packaging, which in the composition of the packaging material can be added additives that give it specific characteristics. Active films to inhibit microbial growth, such as Ouattara et al. [44], who added acetic acid and propionic acid in chitosan films to act as antimicrobial agent. The addition of α-tocopherol in chitosan films was studied by Otero-Pazos et al. [45] as an additive with antioxidant action. The α-tocopherol can decrease the rate of lipid oxidation, which enhances its use in the composition of active packages for high-fat foods such as meat products. Flores-Martı´nez et al. [46] evaluated the diffusion behavior of Jamaica pepper essential oil in gelatin, Aloe vera, and glycerol films, indicating that the essential oil showed slow diffusion through the film, which may be desirable for its use in cheese and meat products, as Jamaica pepper essential oil acts as a preservative. Hydrogel carboxymethyl cellulose (CMC) films have been synthesized for the evaluation of active release, such as moxifloxacin hydrochloride, a drug used against infections and burn scars. Different concentrations of CMC and moxifloxacin hydrochloride were used to study the diffusive behavior of the drug [47]. Fernandes [48] developed films based on gelana and pectin, both polysaccharides, for the incorporation of triamcinolone acetonide, a drug that acts on the symptoms of oral inflammatory diseases. The diffusive character of the drug was studied to evaluate the controlled release of the active compound during its application. The use of protein-based films also presents the potential application for the diffusive mechanism for drug release. Vulcani [49] evaluated the release of progesterone—a hormone used to treat infertility, contraception, and ovarian problems—in collagen matrices. This system showed the ability to release the required amount of hormone in 10 days to synchronize the estrous cycle of bovine females.

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Diffusion is also related to permeability in natural polymeric films. Permeability is a product of solubility and diffusivity and is governed by the Fick and Henry laws [6]. In starch films, water vapor permeability is influenced by temperature, which is directly related to equilibrium (solubility) and kinetic (diffusivity) state. However, lipid films are responsible for forming a barrier to water vapors due to the hydrophobic character of its structure [6, 50].

6.2 Mathematical modeling for the diffusive process in natural polymer films Fick proposed the mathematical expression describing the diffusive process of species in 1855, which was based on the heat conduction equation established by Fourier in 1822. Fick proposed that the mass transfer rate is proportional to the gradient of concentration measured at a given position [8]. Eq. (1) describes this process, also known as Fick’s first law: F ¼ D

∂C ∂x

(1)

where F is the mass flow, D is the diffusion coefficient, C is the concentration of the chemical species, x is the distance, and ∂ C/∂x is the concentration gradient in the axial direction x. The diffusion coefficient represents the rate at which molecules move through a diffuser matrix. The matrix can be a gaseous, liquid, or solid medium. However, the concentration gradient may vary according to position and time (t), just as the diffusion coefficient varies as a function of time [8]. Eq. (2) presents this relationship for multidirectional systems in a plane in Cartesian coordinates:
2  ∂C ∂ C ∂2 C ∂2 C ¼ DðtÞ + + (2) ∂t ∂x2 ∂y2 ∂z2 By simplifying Eq. (2), considering a unidirectional system with a time-varying diffusion coefficient, it becomes ∂C ∂2 C ¼ DðtÞ 2 ∂t ∂x

(3)

Eq. (3) is also known as Fick’s second law. Describing the diffusive process in natural polymers, the authors used Fick’s second law as a starting point for the mathematical modeling of the process [51]. The natural polymeric films are considered as a flat plate. Fig. 7 represents the diffusion process in a natural polymer, in which diffusion will preferably occur in the x-direction because the film thickness is very thin, favoring diffusivity in this direction. It is possible to apply the modeling proposed by Crank [8] where the auxiliary variable T is introduced and the concentration can be expressed by   ∞ h i 4C0 X 1 ð2m + 1Þπx sen C¼ (4) exp ð2m + 1Þ2 π 2 T L π m¼0 ð2m + 1Þ where the thickness of the polymeric film L is considered constant. Eq. (4) is obtained by taking the following boundary conditions:

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4. Diffusion process through biodegradable polymer films

Material flow

x z

Natural polymer y

Material flow

FIG. 7 Schematic representation of natural polymeric films as a flat plate. From Authors.

C ¼ C0 ,0 < x < L, T ¼ 0 C ¼ 0,x ¼ 0,T > 0 C ¼ 0,x ¼ L, T > 0 Integrating Eq. (4) into space and time, the solution is ∞ h i Mt 8 X 1 ¼1 2 exp ð2m + 1Þ2 π 2 T 2 π m¼0 ð2m + 1Þ M∞

(5)

Assuming the uniform solute concentration at the initial time and constant film thickness and also considering that the diffusion coefficient is approximately constant [8], Eq. (5) is written as " # ∞ Mt 8 X 1 ð2m + 1Þ2 π 2 Dt ¼1 2 exp (6) π m¼0 ð2m + 1Þ2 L2 M∞ According to Crank [8], another solution of Eq. (6) can be used, where the error function (ierfc) is introduced; this new solution is applicable in the study of the diffusive process in shorter times and is presented in Eq. (7):  1=2 Mt Dt ¼4 2 (7) M∞ L The study of the diffusive process in polymers has shown different behaviors of this process, which were classified according to the diffusion rate and the relaxation rate of the polymeric matrix denominated: Fickian and non-Fickian diffusion. Eq. (8) expresses the mass transfer rate for this case: Mt ¼ ktn

(8)

where k is a constant and n is the parameter related to the diffusion mechanism. When n ¼ 1/2, the behavior of the diffusion process is denominated to be Fickian; when n ¼ 1, the mass transport is directly proportional to time, and when n has values between 0.5 and 1, it is designated that the behavior of matter transport is non-Fickian; this equation is used to describe diffusive processes in which the temperature is the primary influencer [52]. I. Fundamentals on biopolymers membranes and films

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6 Natural polymer diffusion mechanism

According to Ritger and Peppas [53], for Fickian and non-Fickian diffusion, situations can be applied to processes where time is considered short; Eq. (9) describes the mass transfer rate for short times: Mt ¼ ktn M∞

(9)

Materials that have a much lower diffusion rate than the relaxation rate (case I), or the opposite, a much higher diffusion rate (case II), are classified as Fickian diffusion. The relaxation rate is related to the ability of the matrix to adjust due to the presence of the penetrant in its structure. Non-Fickian diffusion occurs when diffusion and relaxation rates are comparable. Cases I and II are described in terms of a single parameter, diffusion. Non-Fickian behavior requires two or more parameters that describe the effects of relaxation and diffusion [8]. Fig. 8 shows the graphical representation of sorption and desorption in Fickian and non-Fickian systems. Fig. 8 shows the different behaviors of Fickian and non-Fickian diffusion. In Fickian diffusion (a), mass transfer depends on the concentration that is controlled by the diffusion coefficient. The other behaviors are related to the sorption and desorption that occurs in the material, and the sorption is related to two phenomena: absorption (when a matter is associated with the interior of another phase) and adsorption (which deals with the association of matter on the surface). The term sorption is also used when both phenomena cooccur, while desorption will be the removal of absorbed/adsorbed matter [54]. The pseudo-Fickian process (b) occurs when the sorption and desorption curves have the same shape; however, the

Sorption

Mt

Pseudo-Fickian

Mt

Fickian

Sorption

Desorption

(A)

t1/2

Desorption

(B)

Double Stage

Sorption

First stage

Desorption

(C)

t1/2

Second stage

Mt

Mt

Non-Fickian

t1/2

(D)

t1/2

FIG. 8

Graphical representation of sorption and desorption behavior of Fickian and non-Fickian systems: (A) Fickian; (B) Pseudo-Fickian; (C) Non-Fickian; and (D) Double-Stage. Modified from J. Crank, The Mathematics of Diffusion, Oxford University Press, 1975. I. Fundamentals on biopolymers membranes and films

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4. Diffusion process through biodegradable polymer films

TABLE 3 Mathematical models used to determine the diffusion coefficient. Model 2

D ¼ 0;049h t0, 5

D ¼ D0 exp

Ea RT 0

D(t) ¼ Dp

Mt M∞

  1 Def t 1=2 ¼4 ¼ kt2 πL2

Equation number

Condition

References

10

Half-life time method, a methodology that determines the theoretical D value to be compared by the experimentally obtained diffusion coefficient value

Ouattara et al. [44]

11

Used when temperature influences the diffusive process

Ouattara et al. [44] and Wang et al. [55]

12

Used due to the “swelling” effect that the polymer matrix suffers because of the solvent flow through it

Ozdemir and Floros [51]

13

Used for relatively short times

Ritger and Peppas [53], Yoshida et al. [56], and Remedio et al. [3]

D¼


2 kL π 4

14

Variation of Eq. (13)

Wang et al. [55]

D¼

π 2 α 16

15

Increased released/absorbed mass is linear over time

Batista [43]

From Authors.

initial portion remains for a short time. The non-Fickian behavior curve (c) occurs when the desorption rate is initially faster than the sorption rate but later becomes slower, and the two curves intersect. Double stage sorption (d) is the graphical representation of systems where a near equilibrium is rapidly reaching the polymer surface, being the first sorption stage. The second stage is related to an increase in surface concentration and occurs more slowly than diffusion, which makes it a limiting factor. In this phase the concentration is uniform across the surface, and its growth is independent of film thickness [8]. Table 3 illustrates the different models used for the determination of the diffusion coefficient through natural polymer films. Various methods for calculating the diffusion coefficient were studied as a function of time, where each author uses the most appropriate for the systems being analyzed. It is essential to highlight that the methods of determination of the coefficient D are based on the physical characteristics of the material, such as obstructive effects, hydrodynamic interactions, and free volume theory [52].

6.3 Diffusive process in natural polymers The study of the diffusion process in biopolymer films is vital to evaluate the applicability of these materials in different areas. It was searched in Web of Science base; the terms were as follows: (Biopolymer* OR Natural Polymer* AND Film* OR Membrane*) AND (Diffusivity OR Mass Transport OR Diffusion). Several publications about the diffusion process in natural polymers from 2001 to 2019 were found (Fig. 9). The diffusive process of molecules in natural polymer films is related to the structure of the filmogenic matrix, the size of the molecule, and the conditions of the system in which the film is submitted. Mali et al. [47] developed hydrogel-based carboxymethyl cellulose-based films I. Fundamentals on biopolymers membranes and films

111

6 Natural polymer diffusion mechanism

Number of publication

100 80 60 40 20 0

Year

FIG. 9 Publications about the diffusion process in natural polymers between 2001 and 2019. Based on data available on Web of Science.

for citric acid release and detected the release of acid under the action of Fickian and non-Fickian behavior. Non-Fickian behavior is characterized by matrix swelling due to the presence of the penetrant; generally, polymers in the vitreous state exhibit this behavior. The properties of these polymers are time dependent, and characteristics such as polymeric structure in solubility and diffusional mobility also influence non-Fickian performance. In Fickian behavior, present in polymers in the “malleable” state, i.e., that responds quickly to changes in their conditions; for example, when there is a penetrating presence, the polymer chains promptly adjust to the new condition. The structures of these polymers have finite sorption and desorption response rates of the molecules that diffuse in the polymeric matrix [8]. According to Flores et al. [57], it was observed that films based on tapioca starch with glycerol acting as plasticizer had a high amorphous degree, due to the elaboration of the film using the casting technique. This contributed to greater relaxation of the polymeric matrix, which influenced the release and diffusivity of potassium sorbate to act as an antimicrobial agent. Table 4 shows the potassium sorbate diffusion coefficient values obtained for films

TABLE 4

Diffusion coefficient values for different formation conditions of biopolymer films. Diffusion coefficient (×10211 m2 s21)

Method

pH 3

pH 4.5

Film solution: 1.6°C min for 25 min, dried at 50°C for 2 h with RH ¼ 22% Preconditioned at 25°C, RH ¼ 80%–90% for 7 days

1.42
0.12

2.86
0.29

Film solution: 1.8°C min1 for 30 min, dried at 50°C for 2 h with RH ¼ 22% Preconditioned: 25°C, RH ¼ 80%–90%, 7 days

0.90
0.11

1.99
0.15

Film solution: 1.8°C min1 for 30 min, dried at 50°C for 2 h with RH ¼ 22% Preconditioned: controlled environment (RH ¼ 0%, 2 days)

4.99
1.07

7.87
1.44

1

Data from S. Flores, A. Conte, C. Campos, L. Gerschenson, M. Del Nobile, Mass transport properties of tapioca-based active edible films, J. Food Eng. 81 (2007) 580–586.

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prepared in different conditions, indicating the mass transfer under different pH conditions. All the films were preconditioned at 25oC, RH ¼ 80%–90% for 7 days. Other conditions may also contribute to the diffusion process in natural polymers; according to Otero-Pazos et al. [45], in the study of chitosan films incorporated with α-tocopherol to act as antioxidant, it is emphasized that the migration of the active compound is influenced by the nature of the polymer (structural arrangement) and composition of the food in which the film is used, which includes fat content and alcohol content. This could be associated with the solubility of the active compound, which is higher in lipids and ethanol. The ethanol concentration contributes to the increase of the diffusion coefficient; 50% by volume of ethanol, the D value obtained was equal to 1.12  1016 m2 s1, and increasing the ethanol concentration to 95%, the D value was 2.24 1016 m2 s1. Temperature is a condition that also contributed to the migration of molecules in the system. Ouattara et al. [44] described that the temperature decrease influenced the behavior of the diffusion process of the release of acetic and propionic acids in chitosan matrices. The tests were performed at 24°C, 10°C, and 4°C. The temperature reduction caused a decrease in the values of the diffusion coefficient under the conditions of the Arrhenius model (Eq. 11), which described the system. The migration of molecules influenced by temperature is explained by thermodynamic characteristics, where the energy supplied for diffusion control is provided by the activation energy, and no morphological modification occurs in the film structure. Table 5 presents the diffusion values obtained in the tests performed by the author. Another factor that may influence the diffusion mechanisms is the addition of other compounds in the diffuser matrix, in addition to the substances that undergo the diffusive process. Ozdemir and Floros [51] pointed out that the presence of beeswax in milk protein films contributed to potassium sorbate diffusion. The tests were performed with a wax concentration ranging from 0% to 11.7% concerning the weight of dry solids present in the filmogenic solution. The low concentration of wax increased the diffusion of potassium sorbate, and the increase of beeswax concentration caused the decrease of sorbate diffusivity. This fact is explained by the nature of the interaction between the two compounds, being hydrophobic, after the homogenization of the wax. The filmogenic solution increases the tortuosity of the polymeric matrix, thus decreasing the movement velocity of potassium sorbate. Potassium sorbate diffusivity values at 25°C ranged from 5.4 to 9.8 1011 m2 s1. The size of diffuser molecules should also be evaluated in the study of diffusive processes in natural polymers. Remedio et al. [3] evaluating the diffusion of nisin and potassium sorbate in chitosan-based films indicated the size effect of these two substances in the diffusion results, which the molar mass of potassium sorbate was 154 g mol1, presenting a higher TABLE 5 Influence of temperature on the diffusion coefficient. Diffusion coefficient (×10216 m2 s21) Temperature (°C)

Propionic acid

Acetic acid

4

0.91

1.19

10

1.27

1.49

24

1.87

2.59

Data from B. Ouattara, R.E. Simard, G. Piette, A. Begin, R.A. Holley, Diffusion of acetic and propionic acids from chitosan-based antimicrobial packaging films, J. Food Sci. 65 (2000) 768–773.

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113

diffusion coefficient (1.91–2.29  1013 m2 s1), and a smaller molecule size, nisin had lower diffusivity (1.23–1.35  1013 m2 s1), considering the molar mass equal to 3354 g mol1. Different researches determined the diffusion coefficient value through natural biopolymer matrix films, indicating that the polymeric matrix affects the release process of the bioactive compound (Table 6).

TABLE 6 Diffusion coefficients calculated based on the release of the bioactive compound from natural polymer matrix films. Polymeric matrix film

Bioactive compound (w/w)

Diffusivity (m2 s21) 13

Potential application

References Remedio et al. [3]

Nisin (2.5%)

1.29710

Antimicrobial active packaging

Potassium sorbate (2.5%)

2.2921013

Antimicrobial active packaging

Chitosan (1 g 100 g1)

Vitamin C (5%) and annatto powder (1%)

2.281010

Antiaging mask treatment

Reacetylated chitosan (1 g 100 g1)

Vitamin C (5%) and annatto powder (1%)

0.141010

Antiaging mask treatment

Bilayer poly(lactic acid) PLA and zein (ZN)

Quercetin (5%)

1.5 1015

Food packaging

Vela´squez et al. [59]

Corn starch (10 g 100 g1), chitosan (1 g 100 g1)

Cinnamon essential oil (0.5%, 1.0%, 1.5%, 2.0%)

0.5% ! 2.811014 1.0% ! 2.98 1014 1.5% ! 3.36 1014 2.0% ! 3.22 1014

Packaging with fatty foods

Ke et al. [60]

Aloe vera mucilage and gelatin (2% w/w)

Pimento (Pimenta dioica) (0.5, 1.0. 1.5%)

0.5%!1.06
0.01 1014 1.0%!1.13
0.01 1014 1.5%!1.46
0.23 1013

Food packaging

FloresMartı´nez et al. [46]

Chitosan (1.25 g 100 g1)

Carvacrol (9.6 mg L1) Grape seed extract: gallic acid (684:6.5 mg L1)

1.8 1014 (5oC) 3.6 1014 (25oC) 1.9 1013 (45oC)

Water-based food products at different temperatures

Rubilar et al. [61]

Carvacrol (60 mg L1) Grape seed extract: gallic acid (400:3.8 mg L1)

2.0 1014 (5oC) 6.0 1014 (25oC) 5.4 1013 (45oC)

Chitosan (2 g 100 g1)

Afonso et al. [58]

Continued

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4. Diffusion process through biodegradable polymer films

TABLE 6 Diffusion coefficients calculated based on the release of the bioactive compound from natural polymer matrix films—cont’d Polymeric matrix film

Bioactive compound (w/w)

Diffusivity (m2 s21)

Zein (16 g 100 g )

Zataria multiflora essential oil (ZEO ¼ carvacrol and thymol) (10%)

Carvacrol 2.461015 (4oC) 48.951015 (37oC) Thymol 2.361015 (4oC) 38.221015 (37oC)

Chitosan (2% w/v) and fish gelatin (6% w/v)

Natural antioxidants (5%):

1

Potential application

References

Active food packaging systems

Kashiri et al. [62]

Active packaging for food preservation or active coating for pharmaceutical applications

Benbettaı¨eb et al. [63]

Edible film packages to meat

Chandra Mohan et al. [64]

Tyrosol

685.3
181.01015

Ferulic acid

2.0
0.1 1015

Caffeic acid

2.0
0.2 1015

Ferulic + caffeic acid

3.9
0.5 1015

Cinnamaldehyde from Cinnamon (Cinnamomum cassia) 20 mg mL1

8.1
0.081020 (10oC) 9.7
0.021020 (15oC)

Eugenol from Clove (Syzygium aromaticum) 10 mg mL1

1.07
0.051014 (10oC) 1.19
0.031014 (15oC)

Chitosan (2% w/v) and fish gelatin (6% w/v)

Coumarin (5%)

Irradiation doses: 40 kGy ¼ 1.90
0.5 1011 60 kGy ¼ 2.04
0.1 1011

Biobased active packaging film

Benbettaı¨eb et al. [63]

Chitosan:Alginate ¼ 1:1 (50% alginate fraction)

Nisin (30 mg/film)

8.03
3.411013

Antimicrobial packaging materials

Chandrasekar et al. [65]

Chitosan:Alginate ¼ 2:1 (33% alginate fraction)

Nisin (30 mg/film)

2.45
0.511013

Chitosan:Alginate ¼ 1:1 (66% alginate fraction)

Nisin (30 mg/film)

0.87
0.231013

Chitosan (0.5 g 100 g1) and pectin (0.5 g 100 g1)

Anthocyanin powder (grape) (0.25 g 100 g1)

8.40
0.451011 (pH ¼ 4.0) 0.26
0.041011 (pH ¼ 7.0) 1.20
0.31109 (pH ¼ 5.5)

pH indicator device in food packaging

Maciel et al. [66]

Tamarind seed starch (5 g 100g1) and xanthan gum (0.20 g 100 g1)

I. Fundamentals on biopolymers membranes and films

References

115

7 Conclusions The present work presented results that indicated the enormous potential of using natural polymers to form thin films, as vehicles of active substances, which diffuse through the filmogenic matrix. The applicability of natural polymer films increases with additives that provide some additional functionality such as antimicrobial, nutraceutical, antioxidant, antiinflammatory, cosmetic, drug incorporation, and dressings. Diffusion is a process that depends on system conditions, molecule size, matrix arrangement, and temperature. All of these factors must be considered in the development of new materials formed from renewable sources. The use of natural polymers as a film matrix could reduce the excessive volume of solid waste discarded, which severely impacts the environment and consequently on human health due to the synthetic compounds dumped in nature. Studies found in this work have shown the possibility of partial replacement of synthetic polymers by natural polymers. The use of natural polymers as active films, which present rapid degradation, can cover several areas, such as food, pharmaceutical, and veterinary. Different renewable raw materials were studied to obtain natural polymer films from macromolecule groups such as polysaccharides, lipids, and proteins. Starch, gelatin, chitosan, collagen, alginate, milk protein, and waxes, among others, have already been used in the development of films. Each raw material has its specificity that eventually allows the mixing of different raw materials for the development of new materials with improved characteristics. This study allowed us to gather information on the development of new materials that are characterized by sustainability, using resources from renewable sources, contributing to a better relationship between human needs and the environment.

Acknowledgments This work was financially supported by Sa˜o Paulo Research Foundation—FAPESP (grant 2016/21073-4) and the National Council for Scientific and Technological Development—CNPq.

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C H A P T E R

5 Separation processes with (bio) membranes: Overview and new phenomenological classification Roberto Nasser, Jr Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo, Diadema, SP, Brazil

1 Introduction The separation processes with membranes are unit operations using selective membranes for the separation of chemical species present in mixtures, suspensions, and solutions, liquid phase, and the gas phase, to purify those streams or obtain such components with high purity. Those processes are largely used in some processes such as water treatment and effluent conditioning but are considered relatively new in other areas when compared with other existing separation processes. They are very efficient in terms of separation, presenting low operating costs, mainly in energetic issues, as they do not involve phase change. The membranes act as a selective barrier allowing the permeation of the desired species presented at the solution making the partition very effective. A remarkable advantage of the separation processes with membranes is operating flexibility, as the membranes can be obtained of different materials, being assembled in modules, which can be removed and replaced in operation. As it is an alternative process to the conventional separation unit operations, with higher yields, the separation processes with membranes can be considered innovative. Before studying or initiating the usual experimentations to characterize the separation processes with membrane, it is important to discuss their classification, as the present method is very vague, using a dimensional method, relating the membraned cutoff and the size of the product to be retained, with no relation at all with other parameters of the process, making

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difficult the experimental step, as the present criteria does not indicate the process that should be studied. The previously exposed justifies the objective of developing a most comprehensive and phenomenological classification of the separation processes with membranes. This new classification method of the separation processes with membranes was developed at the industry and has been very useful to guide the experimental work for successfully implemented projects. This chapter aims to discuss whether the dimensional criteria for classifying the separation processes with membranes are correct. Evaluate the impact of this discussion on the experimentation, sizing, and specification of the systems using separation processes with membranes. Establish a more comprehensive classification method involving other process parameters, such as level of feed pressure, driving forces, membrane material, type of modules, and involved transport phenomena and application, supplying to the experimentation step more precise parameters, leading it to a more conclusive answer.

2 Fundamentals The separation processes with membranes consist of one physical separation of fluids resulting from the imposition of a driving force through a semipermeable membrane, built of inorganic or organic material, mainly polymeric materials, including the biopolymers. This driving force is responsible for the separation, obtaining two fractions, designated as permeate, which passes through the membrane, and retentate, or concentrate, which does not pass through the membrane [1]. The traditional definition criteria for separation processes with membranes relate the processes with the membrane cutoff, expressed in Dalton, that is, the minimal molar mass that can be retained by the membrane. Solute molecules with molar mass smaller than the cutoff permeate through the membrane and those with higher molar mass are retained [2]. The cutoff is then defined as the solute with lower molar mass presenting 95% retention for a given transmembrane pressure. The fundamentals of membrane are briefly described as follows: Section 2.1 giving a general overview of the membrane morphology and describing the different types of membranes briefly; Section 2.2 explaining the most important operating characteristic of the membrane system; Section 2.3 giving a summarized description of the available modulus of membrane; and Section 2.4 describing the preliminary general description of the separation process with membranes.

2.1 General physical and structure of the membranes In a separation or purification process, the membranes work as a barrier separating two phases restricting the transport, total or partial, of one or several chemical species presented at the phases [3].

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Synthetized from organic materials, generally polymers or biopolymers, and also ceramics with thickness varying between 1 and 10,000 microns, the membranes can be as follows: – Homogeneous and heterogeneous. The homogenous are obtained from polymeric films (PVA, biopolymers, silicone, etc.), dense, used in processes, where the transport phenomenon is the solution into the membrane, followed by diffusion through the thickness of the membrane, such as gas permeation and pervaporation. For the heterogeneous membranes, there is solution-diffusion, but a simple pass or not to pass through the porous membrane. – Symmetric or asymmetric. The asymmetric has relatively dense skin laying a porous structure, both obtained from polymers, used in processes where the transport mechanism is the solution diffusion, where the dense skin controls the fluxes, built from a single polymer or composite. The symmetric membranes do not only present solution-diffusion but also the simple pass and not to pass through the porous membrane. – Neutral or electric charged. Electric charged are swelled gels supported in microporous structures, where the separation mechanism is exclusion, obtained polymers resistant to temperature, hydrolysis, oxidation, polysulfone, polyethylene, and polytetrafluorethylene; recovered with styrene; and used in electrodialysis. – Solid or liquid. For the liquid membrane the selectivity occurs at the liquid phase, supported by a microporous structure, used in the dialysis process [2, 4].

2.2 Membrane feed flux: Frontal and tangential For the frontal flux the feed flux is perpendicular to the filtration media, resulting in the buildup of a filtration cake, which thickness gradually increases with the operating time, increasing the pressure drop and decreasing the filtration flux till stopping the process, due the complete clogging of the filtration media, requiring frequent shutdown of the system for cleaning or replacement of the filtration media. Frontal flux filtration is also designated as “dead-end filtration.” For the tangential flux the feed flux is tangential to the filtration media. The flux “sweeps” the material eventually retained at the filtration media, resulting in constant operating conditions with time, in terms of flux and pressure, without any buildup of the nonpermeating species, with no clogging, reducing shutdown for cleaning or replacement of the filtration media. Tangential flux filtration is designated as “cross-flow filtration,” and it is the characteristic for separation processes with membranes [1]. Fig. 1 illustrates this description.

2.3 Modules of membranes To optimize the processes using membranes, which generally require significant permeation area, it is necessary to maximize it by volume unit. This leads to the development of different types of modules [5]. The most used membrane modules are briefly described as follows: – Plate-and-frame modules: constituted by sandwiches of plane membranes supported by the frames, which also supported the spacers. The assembly is put together with other modules to increase the permeation area and for optimizing the operation in such a way that the

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Cake thickness Flux

Flux

Cake thickness

Time

Time

Feed

Feed

Permeate

Permeate

Dead end filtration

Crossflow or tangential filtration

FIG. 1 Dead-end and cross-flow filtrations. Author.

concentrate of one module feeds the next module up to the desired concentration is achieved. The plate and frame module is suitable for low processing volumes, due to its low packing density, 500 m2/m3 [6]. On the other hand, feed and permeate flows can be easily controlled, and the plates can be replaced with no need of exchange all the modules [2]. – Spiral wound modules: the membranes are wound around a central perforated pipe, which collects the permeate obtained at the membrane. The several layers of the membrane are put together in the modules with spacers, which keep the several wounds of the membrane apart and guarantee the turbulence, increasing the efficiency. The feed is done by the extremity of the module, being tangential to the membrane, guarantying the permeation through the membrane toward the perforated collector pipe [5]. The packing density of this module is 800 m2/m3 [6]. – Hollow fiber modules: the hollow fiber membranes have the shape of tubes or fibers of tiny diameters, varying between 20 and 1000 μm, obtained by extrusion through a single or multihole spinneret. The formed membrane can be dense or porous but usually have a thin skin outside or inside, which guarantees the membrane selectivity. This thin skin is supported by a dendritic structure, which gives the mechanical resistance to the ensemble. These modules are constituted by a bundle of membranes, obtained from different

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polymers, including biopolymers. The membrane bundle is placed in a pressurized cartridge. The described configuration is used for ultrafiltration, dialysis, gas permeation, and pervaporation [5]. The packing density of this module is 6000 m2/m3 [6].

2.4 Preliminary general description of the separation processes with membranes According to the previously described, the separation processes with membranes are incredibly effective, by the use of selective membranes, covering a wide range of products, in suspension or solution, from millimeters to ionic sizes. The separation processes with membranes are very economical, as no phase change is required, consuming much less energy than similar processes. To follow the trend of more economy, the membranes are assembled in modules, designed for maximizing the permeation by the unit of volume, always guaranteeing the tangential flux, which the most important characteristic of separation processes with membranes [1, 3, 5].

3 Advantages and disadvantage of the separation processes with membranes 3.1 Advantages – Lower costs Investment—as the units are in modules, easily scaled-up, simple, and compact. Operating costs—lower energetic consumption (no phase change) and less maintenance (no movable parts). – Better quality More effective—separation up to ionic level. Product integrity—as the processes with membranes do not require temperature, there is no harm to the product [2].

3.2 Disadvantage – Experimentation For new separation processes with membranes, the membrane specification requires experimentations in laboratory and in pilot units, which can be assembled in laboratory or directly at the plant. This step is crucial, but involves a lot of intern negotiation, but it allows a safe extrapolation, avoiding false interpretations [1, 3, 5, 7].

4 Usual classification of the separation processes with membranes: Dimensional This method classifies the different processes by the cutoff. See in Fig. 2 the conventional unit operations of separation compared with separation processes with membranes. As the dimensional criteria for classifying the separation processes with membranes considers the cutoff as the criteria of classification, pointing-out that cutoff is related to the

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FIG. 2 Comparison of conventional unit

Micron (mm) 10–4

10–3

10–2

10–1

1

10

Microfiltration

102

103

Filtration

Ultrafiltration Nanofiltration Reverse osmosis Dialysis

Gel chromat.

Electrodialysis Ion exchange

Electrophoresis

operations of separation and the separation processes with membranes using the dimensional method of classification. Based on A.C. Habert, R. Nobrega, C. Piaseck, Processo de Separac¸a˜o por Membranas, Laborato´rio de Membranas, Coordenac¸a˜o dos Programas de Po´s Graduac¸a˜o em Engenharia, Universidade Federal de Rio de Janeiro (UFRJ), Rio de Janeiro, E-papers, 2006.

Distillation Pervaporation Crystallization Gas permeation Extraction

1

10

Angstrom (Å)

Ultracentrifuge 2

10

10

3

Centrifuge

10

4

105

106

107

Ordinary separation processes Membrane separation processes

dimension of the molecule retained by the membrane, it is essential to explain that this dimensional criterion also considers that – All the membranes are porous, what is not valid, as, at present, the major part of the separation processes with membrane use dense membranes. – All the separation processes with membranes are mechanical, what is not valid, as the most separation processes with membranes are diffusional. – The transport phenomena involved for all the membrane processes is to pass or not through the membranes pour. – The common driving force for all the separation processes with membranes is the pressure drop, which is not valid, as the driving force varies according to the process. If the previously described were true, the classification of separation processes with membranes would be performed as follows: – Characterization of the contaminant, including its chemical nature. – Material selection and membrane cutoff. – Specify the peripherical items of the unit, defining the pressure drop to achieve the required permeate flux through the membrane.

5 Some basic concepts 5.1 Flux and permeability According to Schwartz et al. [8], the flux through the membrane depends on the permeability of the membrane for the component A. Both, flux and permeability depend on the I. Fundamentals on biopolymers membranes and films

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transport phenomena and the driving force. For the dense membranes the transport phenomena are explained by the model solution-diffusion. The component A needs to be soluble in the material of the membrane; the first step is the sorption of A in the skin of the membrane. The second step is diffusion through the thickness of the membrane. The third step is the exit of the permeate from the membrane, known as desorption. Then the flux of A through the membrane is given by NA ¼ ðPA =LÞ  ΔΦA

(1)

NA ¼ flux of A through the membrane PA ¼ permeability of A through the membrane L ¼ thickness of the separating layer of the membrane ΔΦA ¼ driving force across the membrane [1].

5.2 Driving forces As indicated in Eq. (1), ΔΦA is the driving force of the permeation of A through the membrane: – ΔP—an average of the difference of pressure between feed and permeate and the concentrate, known as transmembrane pressure. Usual for porous membranes, used in microfiltration and ultrafiltration processes. – ΔpA or Δpi—differential of vapor pressure or partial pressure, usual for processes using dense membranes, as pervaporation and gas permeation. – Δπ A—a difference of osmotic pressure, typical for asymmetric membranes with dense layer, simple or composite, as inverse osmosis and nanofiltration. – ΔV—a difference of electric potential, usual for asymmetric membranes with dense layer, simple or composite, such as electrodialysis. – ΔCA—difference of concentration between the feed and at the permeate. Typical for all the processes using semiporous and dense membranes. It is determinant for dialysis and also significant for reverse osmosis, nanofiltration, gas permeation, pervaporation, and electrodialysis. – ΔμA—difference of chemical affinity between A and the construction material of the membrane, typical for all the processes using semiporous or dense membranes, being very significant for dialysis, reverse osmosis, gas permeation, pervaporation, and electrodialysis.

5.3 Selectivity Eq. (1) can be used to compare the separating capability of a membrane for more species. Writing Eq. (1) for the species A and B: For A ! NA ¼ ðPA =LÞ  ΔΦA

(2)

For B ! NB ¼ ðPB =LÞ  ΔΦB

(3)

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Considering the same driving forces, the comparison of the fluxes can be done dividing Eq. (2) by Eq. (3), obtaining ðNA =NB Þ ¼ ðPA =PB Þ ¼ α

(4)

where α is the selectivity. The selectivity for processes with membranes is similar to relative volatility for separation processes governed by the vapor-liquid phase equilibrium, like distillation, absorption, and evaporation. For these processes the relative volatility is use to precise the number of equilibrium stages, usually high, what does not happen for the separations processes with membranes, where the separation is usually performed in on equilibrium stage, two stages in some cases and seldom in three equilibrium stage [1].

6 Description of separation processes with membranes This section gives a brief description of the separation processes with membranes, according to the major driving force governing the process.

6.1 Differential pressure as driving force 6.1.1 Microfiltration The microfiltration (MF) is very similar in terms of service to ordinary filtration. The sole difference is the tangential feed flow. It is used for suspensions, for retaining the material in suspension and permeating solvents and soluble materials, including cell and colloid separation, water and effluent treatments, food and pharmaceutical industry, and paint recovery (auto and textile industries). Microfiltration is mostly used in the effluent treatment system, in the version submerged hollow fiber cassettes, conjugate with a reactor characterizing the membrane bioreactor. The cassettes are placed in the bottom of the tank, where the turbulence is high. Usually the hollow fibers are in polyamide, porous at the outside shell. The involved pressure is very low or even vacuum, resulting in low energy consumption [9]. The membranes can be polymeric, biopolymeric, and ceramic, generally porous, symmetrically distributed, diameters varying between 0.1 and 10 μm. The usual operating differential pressure is 1 bar, maximal about 2 bar [3]. 6.1.2 Ultrafiltration Ultrafiltration is specific for dealing with macromolecules, being very similar to microfiltration. It is very used in fermentation, food industry, specifically in dairy, water, and effluent treatments. The membranes can be polymeric, biopolymeric, and ceramic, generally porous, symmetrically and asymmetrically distributed, diameters around 0.3 μm. The usual operating differential pressure varies between 1 and 7 bar [3].

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An interesting application performed at the industry is the quality improvement of the cooling tower by using a side filter provided with ceramic ultrafiltration membranes, processing about 10% of the recirculating cooling water flow rate. The impact over the thermic equipment is detectable but not easy to evaluate. However, the quality improvement of cooling water is evident by analyzing the turbidity, which for the 10 NTU at the feed of the membrane module and about 0.1 NTU at the permeate. For a complete 120 m3/h cooling tower, the require membrane module deals with 10 m3/h, permeating 1 m3/h [10]. 6.1.3 Reverse osmosis A process consisting of the solvent permeation, retaining low molar mass solutes, high operating pressures between 20 and 100 bar, which constitutes the main driving force. The membranes are usually asymmetric membranes with a porous structure under dense skin, with spiral wounded modules, permeation occurring by the solution-diffusion mechanism. This described configuration is commercially used for many decades. The dense skin is usually polyamide, which is generally used for the spiral wound configuration. Modules using hollow fiber bundles are also used for this service with a great economic advantage [3]. In this configuration, another polymer can be used such as cellulose. The main uses of reverses osmosis are seawater and salty nonpotable water desalinization, water recycling, ultrapure water for the semiconductor production, food and pharmaceutical industries, water conditioning for boilers, industrial water recycling, industrial effluents treatment, etc. Although reverse osmosis process consumes quite a lot of energy, the most exciting work to quote presents fascinating technical and social results, as it was implemented in a desertic area of Brazilian Northeast attending small communities, using solar energy [11]. The main driving force is pressure, or better, the osmotic pressure, mentioning that for seawater it ranges between 20 and 25 bar, and for the food industry, in the case of concentrating orange juice, at beginning the osmotic pressure varies between 20 and 25 bar and at the end of the process the osmotic of the concentrate is around 100 bar [5]. The mechanism of solution-diffusion is explained by the chemical affinity between the permeate and the membrane material. The permeate is dissolved at the membrane dense surface, followed by its diffusion along with the membrane thickness. The difference in concentration and the difference in chemical affinity constitute secondary driving forces [3, 5]. A different interpretation is that the driving force for diffusion is the difference of chemical affinity, and both concentration and pressure contribute to the driving force [4]. Reverse osmosis is mostly and successfully used providing an effective separation in molecular and ionic levels, specifically for monovalent ions. 6.1.4 Nanofiltration The nanofiltration process uses the same principle of the reverse osmosis, applicable to solutes of molar mass varying between 2000 and 3000, usually bivalents, with smaller osmotic pressure, requiring a pressure drop of 10–30 bar. In the same way, as described for reverse osmosis, the membranes are usually asymmetric with a porous structure under dense skin, with spiral wounded modules, permeation occurring by the solution-diffusion mechanism. This described configuration is commercially used for many decades. The dense skin usually is polyamide. Modules using hollow fiber bundles

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are also used for this service with a great economic advantage [3]. In this configuration, other polymers can be used such as cellulose acetate and biopolymers. Also the mechanism of solution-diffusion is the same as that described for reverse osmosis, meaning then that the main driving force is the pressure drop and the secondary driving forces are the difference of concentration and the difference in chemical affinity. The applications are very similar to those quoted for reverse osmosis, but due to the lower operating pressure of this process, it is possible to perform internal modification of the process with very positive environmental improvement an also a remarkable economic advantage. In the industry an inner change was achieved by replacing the acid hydrolysis, which introduces a significant amount of sulfuric acid and was responsible to adequate the feed for acetic acid recovery, by nanofiltration, establishing a very profitable project [12–14].

6.2 Difference of concentration as driving force 6.2.1 Dialysis Greater practical applications of separation process using membranes, where the driving force is the difference of concentration between the species to be separated. Its primary application is the hemodialysis, the artificial kidney. The membranes are usually asymmetric with a porous structure under a skin not wholly dense, using different polymers, including biopolymers, used mainly for the artificial kidney the most used configuration is hollow fiber modules. Although the main driving force is the difference in concentration, a secondary driving force is also required, pressure, varying between 2 and 4 to guarantee flux through the membrane [3]. Development to replace the actual polymer used took place in the industry, as follows. The objective was to develop an innovative process for producing cellulose acetate hollow fiber membranes. Cellulose acetate is a traditional filtration media, which readily forms a permselective surface by phase inversion. Using a multihole spinneret, very low diameter hollow fiber could be obtained. All steps were successfully performed, using the pilot spin machine existing at the industry. Cellulose acetate hollow fiber membrane costs 20% than that produced in polysulfone and 10% than that provided in polyimide. Although there are many other markets for hollow fiber membranes, the low costs for raw material fit very well for the cost reduction required artificial kidney. In Brazil the federal government spends 450 million €/y with this item, although only 40% of the population with kidney diseases is treated [15].

6.3 Difference of voltage as driving force 6.3.1 Electrodialysis In this process the membranes are electrically charged, acting selective barriers, removing compounds such as salts and acids from aqueous solutions. Electrodialysis occurs under the driving forces of electric potential and concentration gradients. I. Fundamentals on biopolymers membranes and films

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The plate and frame membranes are assembled in pairs, cathode and anode, constituting stacks of 200–400 pairs. The membranes for this process dense or composite, that is, selective layer, supported by the porous structure. The primary use of this process is desalination and also effluent treatment, recovering, in the case of sodium sulfate aqueous effluent, due to the presence of membranes cathode and anode, sulfuric acid, and sodium hydroxide. This process was industrialized in France and Brazil [5, 16, 17].

6.4 Difference of partial pressure as driving force 6.4.1 Gas permeation The main driving force of this process is the gradient of the partial pressure of the permeate. To guarantee the flux through the membrane, the outlet pressure must be lower than the partial pressure, allowing the effectiveness of a secondary driving force, the difference of concentrations, impacting over the mass transfer through the membrane. The difference of chemical affinity is another secondary driving force, which is in fact important, as the membranes for this process are usually dense, requiring difference of chemical affinity for allowing the separation [8]. This process occurs in dense or composite membranes, that is, defining composite as dense selective skin on a porous structure. All components are polymeric; usually PDMS (silicone) and the modules are spiral wound or hollow fibers. The commercial use of this process is the air fractionating, usually performed by hollow fiber modules, assembled in skids and installed by the supplier in facilities using nitrogen and oxygen intensively [18]. Another usual process is the separation or recovery of hydrogen from binary mixtures with carbon monoxide. Or even methane and nitrogen, using commercial membranes extremely selective to nitrogen, built in polysulfone or cellulose acetate. Another every use and very developed is the solvent recovery, in mixture with air, commonly generated by the converters industry (food packing), polymers processing, and textile industry [3].

6.5 Difference of vapor pressure as driving force 6.5.1 Pervaporation Membrane separation process is a process where the liquid feed is maintained in recirculation in contact with the membrane. The permeate is removed on the other side, under vacuum, as vapor. To guarantee the permeate flux, the pressure must be below the vapor pressure at feed temperature. The gradient of vapor pressure is the main driving force of this process. A vacuum pump is required to provide this low pressure, to keep the flux through the membrane and also to perform the condensation of the permeate vapors [3]. The pervaporation uses dense membranes built of polyvinyl alcohol (PVA), polysulfone, and other polymers, with thickness varying between 0.5 and 2 μm, assembled on a porous support, also polymeric, resulting in an asymmetric membrane, where the smaller porous of the support is close to the dense skin. The thickness of this set varies between 70 and 100 μm. The described set is placed on the nonwoven support with suitable thermic, chemical, and mechanical resistances, with a total thickness of around 100 μm [5]. I. Fundamentals on biopolymers membranes and films

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Most of the pervaporation membranes used for organic solvents dehydration are hydrophilic assemble in plate and frame modules. For solvent recovery from aqueous solutions, organophilic membranes are required, frequently using PDMS, assembled in a spiral wound or hollow fiber modules. The transport mechanism is identical to the gas permeation. The only difference is the main driving force, in this case, the gradient of vapor pressure, being the secondary driving force the chemical affinity of the permeate with the membrane material and the difference in concentration. Also, in the same way of gas permeation, considering the adequacy to the liquid feed, the mechanism is as follows: (i) sorption of the liquid components (ii) diffusion of the permeate component through the membrane (iii) desorption of the permeate leaving the membrane at the vapor phase Pervaporation is the only separation process with a membrane with phase change, concurring directly with distillation, being advantageous in energetic terms as, in distillation, the evaporation and condensation of the distillate and reflux must be considered. In pervaporation, only the condensation of the permeate is required. The most advantageous process for pervaporation occurs when the binary mixture presents an azeotrope, as the energy requirement is much lower than conventional [3]. The most commercial uses for pervaporation are in distillation [19]. But it is interesting to mention the excellent result of installing a pervaporation system integrated with a reactor where the hydrogenation reaction of acetone takes place. In industry a remarkable use of pervaporation took place in a unit producing methyl isobutyl ketone (MIBK) by installing a lateral filter with a pervaporation module to decrease in 1% the water concentration, displacing the reaction equilibrium and increasing the production and the profitability [20].

7 New classification of separation processes with membranes As described in item 5, the dimensional classification considers all the process as mechanical separations, considering as the only cutoff parameter and the transport phenomena through the porous diameter, not considering all the other processes, involving many other transport phenomena. This type of classification leads to misunderstandings for developing the experimental work. The proposition is to build a systematic method, according to the previously presented, considering the fact which is real when analyzing all the described separation processes with membranes: only the simple membrane processes using porous membranes are mechanical, and all the others are diffusional, as they involve many aspects as equilibrium, transport phenomena, and all the process aspects previously discussed in this chapter, such as the involved driving forces, indicating in the case where there is more than one, which is principal, and the secondaries, all of them coherent with the transport phenomena.

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Evaluate the type of the membrane suitable for the studied process and analyze the suitable membrane material for the proposed service and the type of geometry of the modules, concluding by the analysis and by listing the services already used by the analyzed process. This classification is a broad phenomenological process analysis, working with multiple parameters, allowing to begin the experimental study with a precise direction, much better than dimension only. In this way the process study with membranes renders broader and more complex, with usual practice and the concepts described in this study, making it possible to put together a system with a lot of with greater accuracy. Table 1 summarizes the characteristic of the described processes, indicating the driving forces and also including some examples. TABLE 1

Separation processes with membranes.

Process

Feed pressure

Driving forces (main and secondary)

Microfiltration

Low 1–2 bar

ΔP

Porous, symmetric, polymerics, ceramic/planes, spiral wound, hollow fiber

Exclusion by size

Separation of suspended cells and colloids; paint recovery (auto and textile industries)

Ultrafiltration

Low to medium 2–4 bar

ΔP

Porous symmetric. polymerics and ceramics Plate and frame, spiral wound, hollow fiber

Exclusion by size and equilibrium process

Water and effluent treatment biochemical processes, food industry; dairy

Dialysis

Low 2–4 bar

ΔC (Δμ, ΔP)

Porous, symmetric, spiral wound, hollow fiber

Concentration equilibrium solutiondiffusion

Hemodialysis (artificial kidney)

Electrodialysis

Independent

ΔE (ΔC)

Semiporous, asymmetric polymeric dense layer

Solutiondiffusion

Concentration of salty solutions; water purification

Nanofiltration

10–30 bar

ΔP (ΔC, Δμ)

Semiporous, asymmetric, dense layer/spiral wound, hollow fiber

Solutiondiffusion

Separation at molecular and ionic levels, for bivalent ions, optimization of chemical plants

Reverse osmosis

30–100 bar

ΔP (ΔC, Δμ)

Semiporous, asymmetric, dense layer/spiral wound, hollow fiber

Solutiondiffusion

Separation for monovalent ions Desalinization, juice concentration

Membrane type material and module

Transport phenomena

Services

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TABLE 1 Separation processes with membranes—cont’d

Process

Feed pressure

Driving forces (main and secondary)

Gas permeation

Vacuum (79.4% at 5 min E15: >79.4% at 5 min

Reddy and Ramana Murthy [48]

2.5 (w/v)

Fluconazole

Casting

23–28 s

48.7%–61.0% at 30 min

Renc¸ber et al. [6]

2.5 (w/v)

Cefuroxime axetil

Casting

100 min

Heinemann et al. [26]

Modified starch

63 at 73% (w/w)

Chlorpheniramine maleate

Hot-melt extrusion

6–11 s

Maltodextrin

3% (w/w)

Zingiber officiale extract

Casting

8
0.47

95% at 5 min

Pimparade et al. [51] Daud et al. [18]

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2.1 Production methods of orally disintegrating films Orally disintegrating films can be produced by different methods, with casting as one of the most used techniques, which consists of the evaporation of the solvent on plates [56]. The films are produced by the solubilization of different ingredients, forming the filmogenic solution, which can be composed of a polymer, plasticizer, active compound, and other ingredients such as saliva stimulating agent, flavor, and surfactant [57]. In this technique the film-forming solution is poured onto plates and dried in an oven, the thickness is controlled by the mass of the film-forming solution added onto the plate [58]. According to the literature, casting is the most used technique for the production of orally disintegrating films due to its ease of production and low cost, when compared with other processes [14, 51, 59]. Although the casting technique is widely used, it has some disadvantages, such as possible variation between batches due to the multiple steps used in the production [51]; bubble formation, which is inevitable during the preparation of the film solution; and long cycle time and unsatisfactory control of the film thickness, which may be related to possible varied rates of solvent evaporation in different parts of the film [59]. Considered an improvement in casting, in tape casting, the film-forming solution is spread with a spreader, which allows the film thickness to be controlled, and the drying can be performed faster in the same equipment by conduction of heat, convection, infrared radiation, or a combination of these mechanisms [60]. Among the existing extruders the screw extruder is the most used in the pharmaceutical industry for the production of films. A screw extruder provides more shear stress and a more intense blend when compared with a raw struder [61]. Hot melt extrusion is also used for the production of orally disintegrating films [62]. Some studies in the literature report the use of hot melt extrusion for the production of orally disintegrating films using modified starch as polymers [51] and maltodextrins [23], but as shown in Table 1, casting is the most studied technique.

2.2 Innovative production methods of orally disintegrating films In conventional production methods the active principle is incorporated into the formulation with the other ingredients, so all the stress applied during film production, transport, and storage can affect the stability and activity of the active compound [63]. In addition, depending on the concentration of the active compound that is incorporated in the formulation, it can modify the mechanical properties of the polymer matrix when obtained by the casting method [54]. The interest in customized dosages and methods, which facilitate the industrial scale production, has aroused the interest of researchers of the area. Among the new production techniques, it can be highlight the production of orally disintegrating films by ink jet [54, 64, 65], 3D [66], and flexographic printing techniques [22, 67]. In 3D printing, there is the possibility of three printing methods: drop on solid, fused deposition modeling, and fused filament fabrication, with the drop on solid as the most used [68]. Recently, Musazzi et al. [66] evaluated a new 3D printing technique, called “hot melt ram extrusion 3D printing”. The proposed method consists of mixing the active principle

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(paracetamol) with maltodextrins, glycerin, and other excipients; this mixture was loaded into the ram extruder, and the single dosage form was printed directly onto the packing foil [66]. Another way is the use of a printing technique for the incorporation of the active compound in a drug-free polymer matrix, which is currently the most studied technique. As the active compound is added only at the end of the process, the time required for the production of the polymer matrix can be optimized, for example, by using higher drying temperatures. The concentration of an active compound incorporated into the matrix exhibits increased accuracy, even for the production of drugs requiring a low dosage concentration. This concentration can be manipulated by modifying the concentration of the printing solution, changing the printing configurations or increasing the number of printed layers, so the material can be produced on demand (Fig. 1) according to the individual’s need [53, 69]. Another advantage of this technique is the possibility of incorporating multiple components, which can be in the same printing solution or in multiple layers in the case of immiscible components. This technique shows a wide range of new possibilities. With regard to large-scale manufacturing, it would only take one production line to make the base film, and the active compound could be manipulated and printed in a smaller space, or even printed out at any

FIG. 1 On-demand production of oral disintegration films by the printing technique taking into account the needs of different patients. Modified from M. Preis, J. Breitkreutz, N. Sandler, Perspective: concepts of printing technologies for oral film formulations, Int. J. Pharm. 494 (2015) 578–584, doi:10.1016/j.ijpharm.2015.02.032.

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time and on demand [22]. Due to the characteristics mentioned previously, there is increased interest in the printing technique because it allows to deposit, with high precision, the desired proportion of active compounds, besides allowing the single dosage or of multiple elements, depending on the technology that will be used for printing [69]. Eleftheriadis et al. [64] evaluated the printing of diclofenac sodium on commercial paper (edible sugar sheets), and these production methods showed good accuracy up to nine printed layers. The researchers observed correlation between the number of printed layers and the apparent permeability coefficient in vitro.

3 Characterization of orally disintegrating films 3.1 Disintegration time Disintegration time is one of the main characterizations made in films, because according to the time, it can be classified as films of fast or slow disintegration, in addition to trying to relate the time of release of the compound added; however there is no standardization concerning one method set to evaluate this time. For this reason, some studies available in the literature use more than one method to evaluate the disintegration time. Table 2 shows the different methods used to determine the disintegration time. One of the main problems encountered in in vitro experiments is the determination of the parameter defined as disintegration time, which generates a series of inconsistencies in the results [74], and as there is no standard in the determination, a large variation between the disintegration time is defined as ideal. In addition, it should be noted that there is also no standardization in relation to the solution used to determine the disintegration time, different solutions are reported, such as water [22, 71], phosphate buffer [56, 73], and artificial saliva [71]. The use of different solutions also complicates evaluating and comparing the ODFs produced with different polymers, since the characteristics (swelling, solubility, among others) of these solutions can vary greatly depending on the hydrophilic nature of most of the polymers used. Thus evaluating the disintegration time is essential for the commercialization of films [74]. The closer the conditions of the in vitro disintegration tests reflect the conditions of the oral cavity, the more accurate the disintegration time [55]. To characterize an orally disintegrating film, it is necessary to determine the time that the film takes to disintegrate, which is dependent on the method used and may vary according to the formulation [52], the type of polymer used, compounds added, and the plasticizer. In general, in vitro tests for evaluating disintegration time do not effectively simulate what happens in the oral cavity. Thus an alternative still little used is the evaluation of in vivo disintegration time, where it is possible to use a team to determine the disintegration time [4, 23, 25, 71, 75]. For this the team is trained to determine a fast and slow disintegration time. Dos Santos Garcia et al. [25] and Garcia et al. [4] used a unstructured hedonic scale, anchored with the words fast and slow, for the trained team to determine the disintegration time. According to the disintegration time, the film can be classified as fast (disintegrates in at most 60 s), mucoadhesive (disintegrates in a few minutes forming a gel), and mucoadhesive sustained (maximum of 8–10 h) [76]. II. Applications of biopolymers membranes/films in health

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TABLE 2

Different methods used to determine the disintegration time of orally disintegrating films.

Technique

Description

References

Slide frame

Sample of the film is fixed in a slide frame, the system is placed on a Petri dish, and then a drop of distilled water is deposited on the surface of the film. The disintegration time is the time required for the drop to dissolve the film and form a hole

Garsuch and Breitkreutz [70]

Petri dish

With the addition of distilled water (2 mL) in a petri dish, the film is deposited on the surface of the liquid. The time required for the film to completely dissolve is defined as the disintegration time

Garsuch and Breitkreutz [70]

Pharma test

The disintegration time was determined using a disintegration taster (Pharma Test, Frankfurt, Germany) with distilled water (37°C) and stirring. The disintegration time was taken as the time for ODF to disintegrate into tiny fragments

Liew et al. [71]

Slide frame and ball method

The film is fixed to a perforated plate (exposed area of 6 cm2). On the surface of the film is deposited distilled water (900 μL) at 37°C; then a stainless steel ball (d ¼ 10 mm and mass 4 g) is deposited on the surface of the film (to simulate the force of the tongue). Disintegration time is defined as the time required for the ball to pierce the film and fall into the lower part of the system

Steiner et al. [72]

Sponge surface

Initially the surface of a sponge (7 10 cm) is moistened with phosphate buffer, pH 6.4 (250 mL). Samples are deposited on the surface of the sponge. The complete disappearance of the sample will be considered as the disintegration time

Vuddanda et al. [73]

Sensor testing

They developed an optical pass-through confirmation sensor to automatically evaluate the disintegration time. For the evaluation distilled water or artificial saliva solution was used, kept at 37°C, in different volumes and times. The disintegration time was defined as the time at which the disintegration film and the test medium passed through the opening of the apparatus

Takeuchi et al. [55]

In addition, ingredients that accelerate the disintegration, generally used to increase the rate of saliva production, may be incorporated into the films, assisting to reduce the disintegration time [77]. Studies available in the literature generally report the use of citric acid as a salivary stimulating agent [78–80].

3.2 Mucoadhesion The mucoadhesiveness is related to the adhesion of the delivery system to the mucosa. In the case of orally disintegrating films, this property is very important because the release vehicle, together with the active principle, can be loaded due to mechanical movement of the buccal cavity and the natural process of salivation [81]. Thus the use of mucoadhesive biopolymers can prolong the residence time of the material, allowing its release and more effective action in the oral cavity [82]. II. Applications of biopolymers membranes/films in health

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The mechanism of mucoadhesion consists mainly of two steps: the contact between drug delivery system and the membrane and the penetration of the matrix into the membrane, which is the consolidation stage [81, 83, 84]. The mucoadhesion process is a complex phenomenon that is not yet well understood, but there are six classical theories: electronic, adsorption, wetting, diffusion, fracture, and mechanical theory [84]. In the case of orally disintegrating films, the process of how to wet and diffuse the water molecules in the film occur rapidly, which can lead to adhesion between the film and the mucosa. This property is mainly related to the properties of the polymer used to form the polymer matrix, which are influenced by molecular weight, functional group, degree of cross-linking, charge, conformation, and concentration [81]. In studies with HPMC (E50 and K100M), the authors observed that the higher the molecular mass of the biopolymer, the higher the mucoadhesiveness of ODFs [6]. For the analysis of mucoadhesion, there is no standardized method described. Several methods are described for the determination of mucoadhesiveness of films; however, there is great difficulty in correlating the in vivo and in vitro tests and finding a suitable material that simulates the oral mucosa [85]. In the methods described in the literature, the mucous membrane is generally brought into contact with the film, and mucoadhesiveness is measured as the force or work required to separate the film from the surface of the material being used to mimic the buccal mucosa (Fig. 2) [6, 17, 86, 87].

FIG. 2 Mucoahdesive testing system using a texture analyzer equipment and a tissue or animal mucosa to simulated human oral mucosa.

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TABLE 3 Parameters used for mucoadhesive testing using texture analyzer equipment: material to simulated human oral mucosa (tissue), probe withdrawal speed (speed), initial contact force (force), contact area (area), and contact time (time). Tissue Chicken pouch Porcine cheek mucosa

Speed 1.0 mm/s 0.1 mm/s

Force 1N 5N

Area 2

3.14 cm 2

1 cm

Porcine buccal mucosa

1 mm/min

0.2 N

400 mm

Porcine buccal mucosa

0.5 mm/s

0.05 N

–

Rabbits cheek pouch Rabbit cheek pouch Cow buccal mucosa

0.5 mm/s 0.5 mm/s 2.5 mm/min

1N 1N 6N

2

1 cm

2

1 cm

2

0.8 cm

2

Time

References

10 s

Peh and Wong [87]

30 s

Donnelly et al. [88]

180 s

Costa et al. [36]

60 s

Kraisit et al. [89]

60 s

Nair et al. [86]

60 s

Kumria et al. [90]

60 s

Renc¸ber et al. [6]

However, different parameters of analysis are used, such as applied force, withdrawal velocity, contact time, and material used to simulate the buccal mucosa, which may influence the determination of mucoadhesion strength and make it difficult to compare the materials by different groups (Table 3).

3.3 Surface pH Surface pH of orally disintegrating films is usually determined; however, different methodologies can be found. Patel and Poddar [91] proposed the pH measurements by using a previously prepared agar plate (2% m/V) using warmed isotonic phosphate buffer of pH 7.4. ODFs are put on prepared agar plate and left to swell for 2 h, and then a pH paper is placed in contact with the surface of the swollen film to measure the pH. Nafee et al. [92] also used a previously prepared agar plate and swelled the films for 2 h, however, using a phosphate buffer of pH 6.75. Shidhaye et al. [93] adapted this protocol using only 15 min to swell the film. Li et al. [37] also adapted the method proposed by Nafee et al. [92], by reducing the swell time to 1 h. Prabhu et al. [44], however, proposed surface pH measures using distilled water. Using a Petri dish and 0.5 mL of distilled water, films should be placed and kept for 30 s. The pH is measured after stabilization for 1 min. dos Santos Garcia et al. [25], Garcia et al. [4], and Tedesco et al. [21, 43] used an adaptation for Prabhu’s methodologies. Instead of using distilled water, those authors proposed using phosphate buffer (pH 6.75) to simulate oral mucosal pH. ODFs, independent of the biopolymer or principle active ingredient, generally presented surface pH close to the pH of the oral cavity (6.8), as shown in Table 4. However, this characteristic may vary with the incorporation of active compounds. In studies evaluating the addition of camu-camu powder, a fruit with pH close to 3.0, the authors verified that the film additives presented more acidic surface pH [4]. Similarly, Pimparade et al. [51] that produced films with modified starch incorporating Chlorpheniramine maleate observed surface pH close to 3.

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TABLE 4 Surface pH of orally disintegration films produced from different polymers. Biopolymer

Drug

Surface pH

References

Chitosan

Tramadol

6.4–6.82

Li et al. [37]

CMC

Flupentixol dihydrochloride

6.0–6.8

Abdelbary et al. [40]

Rasagiline


0.5 neutral

Prakasam and Bukka [41]

Gelatin

–

6.21–6.73

Tedesco et al. [43]

Gelatin

–

6.87
0.009

dos Santos Garcia et al. [25]

Gelatin

Camu-camu powder

5.7
0.3

Garcia et al. [4]

HPMC

Zingiber officiale extract

6.91
0.85

Daud et al. [18]

HPMC

Flupentixol dihydrochloride

5.7–6.9

Abdelbary et al. [40]

HPMC

Rasagiline

7.0
0.5

Prakasam and Bukka [41]

HPMC

–

6.21–6.73

Tedesco et al. [43]

HPMC

Orciprenaline sulfate

6.47–6.82

Prabhudessai et al. [16]

HPMC

Peanut skin extract

6.36–6.88

Tedesco et al. [21]

HPMC

Ezetimibe

6.74–6.85

Reddy and Ramana Murthy [48]

Pectin

Ezetimibe

6.75–6.80

Reddy and Ramana Murthy [48]

Pullulan

Ropinirole hydrochloride

6.60
0.10

Panchal et al. [94]

Pullulan

Ondansetron

6.1

Choudhary et al. [50]

Pregelatinized starch

Camu-camu powder

4.9
0.1

Garcia et al. [4]

Modified starch

Chlorpheniramine maleate

2.8–3.4

Pimparade et al. [51]

Maltodextrin

Zingiber officiale extract

7
0.25

Daud et al. [18]

3.4 Dissolution of orally disintegrating films The dissolution tests are an important tool to evaluate the release profile of compounds incorporated in the polymer matrix, which are carried out mainly to evaluate the concentration of the compound released over time. In the pharmaceutical industry, it is used to determine the release rate of a drug under standard conditions of liquid/solid interface, temperature, and solvent composition [28]. Since the release rate can be influenced by the components used in the formulation of the orally disintegrating films and hydrophilic or hydrophobic characteristics of all the components of the matrix, as well as the interaction between them, they can influence in a significant way the dissolution rate of the active principle. Generally the use of a hydrophilic polymer enables rapid dissolution upon contact with saliva [15], releasing the active compound efficiently. In the Brazilian [95], United States [96], European [97], and International [98] Pharmacopoeia, the dissolution test does not appear specifically for oral disintegration films. However, through the dissolution test presented, it is possible to determine the amount of active

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substance dissolved in the medium using specific analytical techniques according to the active compound of interest in samples collected at different times. Different compounds are evaluated in dissolution tests such as piroxicam [23], hydrochlorothiazide [99], theophylline [100], ibuprofen [101], duloxetine hydrochloride [102], and donepezil [103]. Studies in the literature also report the evaluation of the release of active compounds obtained from natural sources, such as phenolic compounds [17, 21]. According to the Pharmacopoeia, the dissolution system consists of three components, which are cylindrical open containers and hemispherical bottom; centralized stainless steel rods to promote agitation of the medium, which may be in the form of baskets or blades; and a system to control the speed of rotation of the stems. In general the test temperature is kept at 37
0.5°C and should be free of any source of vibration. Different solutions and volumes are used in the tests, such as 250 mL [102] and 900 mL [23, 101, 103], and solutions such as simulated saliva fluid with pH 6.8 [102], distilled water [100, 101], deionized water [23], 0.1 M HCl [103], phosphate-buffered saline [104], and the sample collection time according to the drug under evaluation. However, these devices present some disadvantages, such as high dissolution rates, depending on the stirring speed, the volume of solution, and the position of the films may influence the release system [28]. The dissolution methods described in the European Pharmacopoeia are not indicated for ODFs, because the ODF needs to be dissolved in a smaller volume and needs the force of the tongue [105]. Possibly because these dissolution tests are indicated to evaluate the release of solid oral dosages, such as tablets and capsules. Speer et al. [106] evaluated the dissolution of films from methods based on dissolution test for solid dosages: USP apparatus 1 (basket method), USP apparatus 2 (paddle and glass disk method), USP apparatus 3 (punch and filter), and Flow-through cell [107], indicating that all the methods could be used to evaluate the release of drugs in films. However, there are particularities for each type of film, for which the “punch and filter” method is more suitable for oral films, considering laminar flow and tongue strength [106]. This system has an additional device that consists of filter paper (2.6  3.6 cm) attached to a steel frame to create a flat interface between the filter and the frame at the bottom that separates the dissolution area from where the drug is detected. To simulate the mechanical strength of the tongue, a weight (14 g) is placed in the system. Adrover et al. [28] developed a release system for orally disintegrating films that has a continuous flow, which simulates the physiological conditions of the mouth, with flow rates of 1 mL/min and reduced retention volume, denominated as millifluidic dissolution device. Although other methods demonstrate good reproducibility and reliability to evaluate the dissolution of compounds in the orally disintegrating films, standard official basket or paddle apparatus is used to conduct dissolution studies in films [15].

4 Conclusion Different biopolymers can be used in the production of orally disintegrating films, since they have film-forming capacity. Their production is based on different techniques like casting, tape casting and extrusion, or more recently by the printing techniques. These films can

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be used as carriers of active compounds from natural or synthetic sources. The selection of biopolymers composition has significant influence on ODF properties such as disintegration time, release rate, mucoadhesiveness, and stability of the active compound. Depending on the ease of administration, orally disintegrating films as vehicles for active substances are commercially available. However, these products are insignificant freight to other forms of administration (tablets, pills, liquid forms, among others). It is important to highlight that the methodologies for characterization of these materials are not standardized, being several methodologies available in the literature that makes it difficult to compare important properties of disintegration films produced with different polymers.

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[78] P.M. Castro, F. Sousa, R. Magalha˜es, V.M.P. Ruiz-Henestrosa, A.M.R. Pilosof, A.R. Madureira, et al., Incorporation of beads into oral films for buccal and oral delivery of bioactive molecules, Carbohydr. Polym. 194 (2018) 411–421, https://doi.org/10.1016/j.carbpol.2018.04.032. [79] S. Raju, P. Sandeep Reddy, V. Anirudh Kumar, A. Deepthi, K. Reddy Sreeramulu, P.V. Madhava Reddy, Flash release oral films of metoclopramide hydrochloride for pediatric use: formulation and in-vitro evaluation, J. Chem. Pharm. Res. 3 (2011) 636–646. [80] V. Senthil, R.B. Khatwal, V. Rathi, S.T. Venkata, Levocetirizine dihydrochloride and ambroxol hydrochloride oral soluble films: design, optimization, and patient compliance study on healthy volunteers, Int. J. Health Allied Sci. 2 (2019) 246, https://doi.org/10.4103/2278-344x.126713. [81] A.B. Nair, R. Kumria, S. Harsha, M. Attimarad, B.E. Al-Dhubiab, I.A. Alhaider, In vitro techniques to evaluate buccal films, J. Control. Release 28 (2013) 10–21, https://doi.org/10.1016/j.jconrel.2012.11.019. [82] R.P. Chinna, K.S.C. Chaitanya, Y. Madhusudan Rao, A review on bioadhesive buccal drug delivery systems: current status of formulation and evaluation methods, Daru 19 (2011) 385–403. [83] F.C. Carvalho, M.L. Bruschi, R.C. Evangelista, M.P.D. Gremia˜o, Mucoadhesive drug delivery systems, Braz. J. Pharm. Sci. 1 (2010) 1–18, https://doi.org/10.1002/9781118747896.ch10. [84] P. Tangri, S. Khurana, S. Madhav, Mucoadhesive Drug Delivery: Mechanism and Methods of Evaluation, Retrieved from: www.ijpbs.net, 2011. [85] A.F. Borges, C. Silva, J.F.J. Coelho, S. Simo˜es, Oral films: current status and future perspectives: I-galenical development and quality attributes, J. Control. Release (2015), https://doi.org/10.1016/j.jconrel.2015.03.006. [86] A.B. Nair, B.E. Al-Dhubiab, J. Shah, P. Vimal, M. Attimarad, S. Harsha, Development and evaluation of palonosetron loaded mucoadhesive buccal films, J. Drug Delivery Sci. Technol. 47 (2018) 351–353, https:// doi.org/10.1016/j.jddst.2018.08.014. [87] K.K. Peh, C.F. Wong, Polymeric films as vehicle for buccal delivery: swelling, mechanical, and bioadhesive properties, J. Pharm. Pharm. Sci. 2 (1999) 53–61. [88] R.F. Donnelly, P.A. McCarron, M.M. Tunney, A. David Woolfson, Potential of photodynamic therapy in treatment of fungal infections of the mouth. Design and characterisation of a mucoadhesive patch containing toluidine blue O, J. Photochem. Photobiol. B Biol. 3 (2007) 59–69, https://doi.org/10.1016/j.jphotobiol.2006.07.011. [89] P. Kraisit, S. Limmatvapirat, M. Luangtana-Anan, P. Sriamornsak, Buccal administration of mucoadhesive blend films saturated with propranolol loaded nanoparticles, Asian J. Pharm. Sci. 13 (2018) 34–43, https:// doi.org/10.1016/j.ajps.2017.07.006. [90] R. Kumria, B.E. Al-Dhubiab, J. Shah, A.B. Nair, Formulation and evaluation of chitosan-based buccal bioadhesive films of zolmitriptan, J. Pharm. Innov. (2018), https://doi.org/10.1007/s12247-018-9312-6. [91] R. Patel, S. Poddar, Development and characterization of mucoadhesive buccal patches of salbutamol sulphate, Curr. Drug Deliv. 1 (2009) 140–144, https://doi.org/10.2174/156720109787048177. [92] N.A. Nafee, M.A. Boraie, F.A. Ismail, L.M. Mortada, Design and characterization of mucoadhesive buccal patches containing cetylpyridinium chloride, Acta Pharm. 53 (2003) 199–212. [93] S.S. Shidhaye, N.S. Saindane, S. Sutar, V. Kadam, Mucoadhesive bilayered patches for administration of sumatriptan succinate, AAPS PharmSciTech 9 (2008) 909–916, https://doi.org/10.1208/s12249-008-9125-x. [94] M.S. Panchal, H. Patel, A. Bagada, K.R. Vadalia, Formulation and evaluation of mouth dissolving film of ropinirole hydrochloride by using pullulan polymers, Int. J. Pharm. Res. Allied Sci. 1 (2012) 60–72. Retrieved from: www.ijpras.com. [95] Brazilian, Farmacopeia Brasileira, fifth ed., vol. 2, (2010). Retrieved from: www.anvisa.gov.br. [96] United States, USP Chapter 711—United States Pharmacopeia. Dissolution, 1, Chapter 711, 2011. [97] European, European Pharmacopoeia, 2.9.3. Dissolution test for solid dosage forms,01/2005:20903, 2014, pp. 228–230. [98] World Health Organization, The International Pharmacopoeia, Dissolution test for solid oral dosage forms, Working document QAS/18.756, 2018, pp. 1–8. [99] J.S. Boateng, K.H. Matthews, A.D. Auffret, M.J. Humphrey, G.M. Eccleston, H.N. Stevens, Comparison of the in vitro release characteristics of mucosal freeze-dried wafers and solvent-cast films containing an insoluble drug. Drug Dev. Ind. Pharm. 38 (2012) 47–54, https://doi.org/10.3109/03639045.2011.590496. [100] S. Puttipipatkhachorn, J. Nunthanid, K. Yamamoto, G.E. Peck, Drug physical state and drug-polymer interaction on drug release from chitosan matrix films, J. Control. Release 75 (2001) 143–153, https://doi.org/10.1016/ S0168-3659(01)00389-3.

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[101] D.G. Yu, X.X. Shen, C. Branford-White, K. White, L.M. Zhu, S.W. Annie Bligh, Oral fast-dissolving drug delivery membranes prepared from electrospun polyvinylpyrrolidone ultrafine fibers, Nanotechnology 20 (2009), https://doi.org/10.1088/0957-4484/20/5/055104. [102] A.M. El Sharawy, M.H. Shukr, A.H. Elshafeey, Formulation and optimization of duloxetine hydrochloride buccal films: in vitro and in vivo evaluation, Drug Deliv. 24 (2017) 1762–1769, https://doi.org/ 10.1080/10717544.2017.1402216. [103] K.B. Liew, Y.T.F. Tan, K.K. Peh, Characterization of oral disintegrating film containing donepezil for alzheimer disease, AAPS PharmSciTech 13 (2012) 134–142, https://doi.org/10.1208/s12249-011-9729-4. [104] C. Shen, B. Shen, H. Xu, J. Bai, L. Dai, Q. Lv, et al., Formulation and optimization of a novel oral fast dissolving film containing drug nanoparticles by Box–Behnken design–response surface methodology, Drug Dev. Ind. Pharm. 40 (2014) 649–656, https://doi.org/10.1016/j.ejps.2016.01.006. [105] R. Krampe, D. Sieber, M. Pein-Hackelbusch, J. Breitkreutz, A new biorelevant dissolution method for orodispersible films, Eur. J. Pharm. Biopharm. 98 (2016) 20–25, https://doi.org/10.1016/j.ejpb.2015.10.012. [106] I. Speer, M. Preis, J. Breitkreutz, Dissolution testing of oral film preparations: experimental comparison of compendial and non-compendial methods, Int. J. Pharm. 561 (2019) 124–134, https://doi.org/10.1016/j. ijpharm.2019.02.042. [107] I. Speer, M. Preis, J. Breitkreutz, Novel dissolution method for oral film preparations with modified release properties, AAPS PharmSciTech. 20 (2019) 1–12, https://doi.org/10.1208/s12249-018-1255-1.

II. Applications of biopolymers membranes/films in health

C H A P T E R

13 Skin rejuvenation: Biopolymers applied to UV sunscreens and sheet masks Joa˜o Dias-Ferreiraa, Ana R. Fernandesa, Jos
e L. Sorianob, Beatriz C. Naverosb,c, Patricia Severinod, Classius Ferreira da Silvae, Eliana B. Soutoa,f a

Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Coimbra, Coimbra, Portugal bDepartment of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, University of Granada, Granada, Spain cNanoscience and Nanotechnology Institute (IN2UB), University of Barcelona, Barcelona, Spain dLaboratory of Nanotechnology and Nanomedicine (LNMED), Institute of Technology and Research (ITP), University of Tiradentes, Industrial Biotechnology Program, Aracaju, Brazil eInstitute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo—UNIFESP, Diadema, Sa˜o Paulo, Brazil f CEB—Centre of Biological Engineering, University of Minho, Braga, Portugal

1 Introduction Ultraviolet (UV) rays are the most prominent kind of radiation hitting Earth. The UV radiation is fragmented in three main segments, namely, UVA (320–400 nm), UVB (280–320 nm), and UVC (100–280 nm), and despite its broad range, it does not penetrate the ozone layer. UVB, although more harmful than UVA, represents only up to 5% of the overall UV rays on the planet surface. Natural occurring phenomena of reflection, scattering, and absorption of light happen upon UV exposure. Reflection can be used for diagnostic purposes, while scattering relates to light dispersion being influenced by skin structures. Shorter-wavelength photons become more scattered, while longer wavelengths penetrate

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# 2020 Elsevier Inc. All rights reserved.

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deeper, thereby offering a key tool for the knowledge of light-required therapies, such as photodynamic therapy. Absorption is chemically structured dependent, being related to the absorption spectrum of each compound and its absorption peak. The interaction between photons and skin is mediated by structures—the so-called chromophores—present at the interface. The energy confined to photons is afterward translated to these structures, which change their degree of electronic excitation, becoming unstable. The skin milieu is complex in its composition, in which surrounding the chromophores an entire environment susceptible to this transitory condition exists. Thus the exceeding energy is responsible for the generation of reactive oxygen species (ROS)—as superoxide, peroxide, singlet oxygen, peroxynitrite—and, subsequently, the undergoing of several intracellular reactions responsible for skin damage. After ultraviolet (UV) exposure the cell experiences a complex multitude of reactions, which lead to tremendous changes in the physiology of the body. DNA is the principal structure affected by radiation as it absorbs light between the wavelengths 245 and 290 nm. Mammalia cells can fix damages occurring in the DNA structure. Subsequently, after a certain amount of accumulated mutations, cells become unstable, and cascade processes are induced by primary mechanisms of regulation, leading to cell death and preventing carcinogenesis. These processes include mitochondrial membrane disruption by cytochrome c exposure and complex Apaf-1/ caspase-9, BCL-2 and caspase-3, and caspase-8 recruitment [1]. Cell death is, however, not always the only outcome. When repair mechanisms fail, cells become resistant, which leads to cancer. Cancer cells do not have regulatory checkpoints, and, thus, they continuously divide and spread without restrictions, reaching other metastasized tissues and organs [2, 3]. Natural protection of the skin is granted by melanin and by vitamin D. Melanin synthesis occurs in melanosomes and in melanocytes, following a process known as melanogenesis. This pigment could act as an optical safeguard avoiding the light from penetrating the skin, being also capable of scattering or even absorbing the incident light. Melanin shows an absorption spectrum between 500 and 600 nm. It plays an additional role in ROS scavenger, acting through an intracellular antioxidant activity [4]. Suzukawa et al. confirmed that both melanin categories—eumelanin (more effective on UV blockage) and pheomelanin (less effective)—are gifted in allowing DNA rupture in the absence of light [4]. Eumelanin was even more injurious than pheomelanin. Such an event is strictly related to the structure of the molecule and to its capacity to react with intracellular building blocks. The mechanism is related to the ability to bind to the minor grooves of DNA molecule, assuring proximity, and eventually leading to strand breaks. The authors also showed that melanin and DNA interaction could compromise the activity of repairing enzymes in preventing further lesions, thus being the cause of DNA damage endurance. Vitamin D plays a critical role in the homeostasis of phosphorus, calcium, and bone metabolisms and in the modulation of the immune system. Its synthesis starts with a precursor molecule (7-dehydrocholesterol) that is present in the membranes of keratinocytes, followed by oxidation conversion mediated by UVB radiation into previtamin D3. This intermediate molecule is rapidly transformed into 25-vitamin D3 via hydroxylation in the liver by the cytochrome P450 isoenzyme 27A1. In kidneys, 1,25-dihydroxy vitamin D3 is produced as the active form of vitamin D [5]. Recent studies reveal that the human body requires an amount between 4000 and 10,000 IU of vitamin D to maintain the inner steadiness. Vitamin D synthesis acts as a

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regulatory process against exposure to UV radiation, through its absorption and energy quenching. These mechanisms are used to sustain required organic reactions and avoid additional harmful effects. To produce the required amount of vitamin D, 5000

–

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2 Overview of pesticides

363

TABLE 2 Pesticide classification based on the target organisms [34]. Type of pesticide

Target pest and type of action

Insecticides

Kill insects

Herbicides

Kill weeds and other unwanted plants

Rodenticides

Control rodents

Fungicides

Kill fungi

Acaricides

Kill mites that feed on plants and animals

Molluscicides

Kill mollusks

Bactericides

Kill bacteria

Avicides

Kill birds

Virucides

Act against viruses

Algicides

Control or kill algae

Larvicides

Inhibit the growth of larvae

Ovicides

Inhibit the growth of eggs of insects and mites

Piscicides

Act against fish

Silvicides

Act against woody vegetation

Termiticides

Kill termites

Nematicides

Kill nematodes that act as parasites

Some pesticides are also classified according to their function. For example: • growth regulators, which stimulate or retard the growth of pests; • defoliants, which cause plants to drop their leaves; • desiccants, which speed up the drying of plants for mechanical harvest, or cause insects to dry out and die; • repellents, which repel pests by taste or smell; • attractants, which attract pests, usually to a trap; • chemosterilants, which sterilize pests. There are several pesticides able to control more than one class of pest and may be considered in more than one pesticide class. A common example is 2,4-dichlorophenoxyacetic acid (widely known as 2,4-D), which is used as an herbicide for broadleaf weed control but is a plant growth regulator at low application rates [35]. 2.1.3 Classification based on pesticide chemical structure The chemical classification of pesticides is the most useful for researchers in the field of pesticides and environment. Compounds of different chemical groups have different toxicity mechanisms and act on pest organisms in different ways.

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FIG. 2 Chemical classification of pesticides. The pesticides can be classified as inorganic and organic, and in the case of organic, it is possible to subclassify in natural and synthetic.

The chemical classification of pesticides is rather complex due to the huge variety of compounds. In general, modern pesticides are organic chemicals, although some inorganic compounds (such as compounds based on sulfur and heavy metals) have been used. Organic pesticides can be further divided into natural or synthetic types, according to their origin, with the latter category being divided into many other classes of compounds, depending on the functional groups present in the molecules [36]. The general chemical classification of pesticides is shown schematically in Fig. 2. Besides, pesticides can be categorized as ionic or nonionic, according to their ionization characteristics [37].

2.2 Characteristics of pesticides and their environmental fates after application Although the well-known advantages of pesticides include enhanced economic potential in terms of increased production of food, with mitigation of vector-borne diseases, their disadvantages are related to serious unwanted side effects and health implications toward humans and the environment [38]. It is important to evaluate the fate of a pesticide once it has been applied to a crop, which is determined by its physical and chemical properties, the soil characteristics, and the environmental conditions. Therefore understanding the characteristics of both the site and the pesticide is the basis for assessment of its potential leaching to groundwater, which would affect human and environmental health. The impact of using these compounds must not only be analyzed near the application area, because they can migrate in different ways, consequently reaching other zones that are very distant [39]. The chemicals applied on crops may remain in the soil for a long time, reaching groundwater and being transported to watersheds and coastal lagoons, contaminating aquatic ecosystems. Hydrophobic pesticides may be absorbed and stored in the fatty tissues of animals, persisting in food chains for long periods of time [40]. Furthermore, these compounds may be rapidly metabolized and bioaccumulated in aquatic food chains, eventually reaching humans. As an example, endosulfan was found to be metabolized to endosulfan sulfate by bacteria and could persist as a toxic chemical in soils and aquatic sediments [41, 42].

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In general, these chemical compounds may undergo various chemical transformations and reach other ecosystems outside the area of application, consequently intoxicating other nontarget species [43]. Meanwhile, at the global scale, the most volatile compounds may be rapidly transported over long distances by atmospheric processes. The evaporation-condensation mechanism was first observed for organochlorine compounds and organophosphates, which were volatilized from applications on banana plantations in Central America, subsequently reaching the ice in the Arctic region [44]. There are several factors that affect the migration and leaching behavior of pesticides (and their transformation products) to groundwater [38]. Important factors related to the nature of the pesticide include the following: • Solubility. Pesticides and their transformation products can be grouped into (a) hydrophobic pesticides, which are persistent, bioaccumulable, and become strongly bound to soil, and (b) polar pesticides, which can move through soil by runoff and leaching, hence reaching groundwater and constituting a problem for the supply of drinking water to the population. • Volatilization. The higher the vapor pressure of a pesticide, the faster it is released to the atmosphere reducing the amount that remains in the soils for the leaching process. This does not necessarily mean, however, that pesticides with high vapor pressures pose no threat to groundwater. The lower the value of the Henry’s law constant (H), the greater the leaching potential of a pesticide. Volatility is also influenced by environmental conditions such as temperature, relative humidity, and air movement. High temperature and low humidity increase evaporation rates. Examples of pesticides with high H values and thus low leaching potentials include trifluralin, triallate, phorate, and dieldrin. • Soil adsorption. The tendency of a pesticide to leach also depends on how strongly it adsorbs to soil. Compounds that are strongly adsorbed onto soil are not likely to leach, regardless of their solubility. They are retained in the root zone, where they are taken up by plants or are eventually degraded. On the other hand, compounds that are weakly adsorbed will leach to varying degrees, depending on their solubility. The strength of sorption is in function of pesticide chemical properties, soil types, and the amount of organic matter. • Degradation. The longer the compound half-life is, the longer it is available to manage the target pest. However, the pesticide is also subject to leaching process over this period of time. Once applied, pesticides break down in the environment by means of a number of processes: exposure to light (photodegradation), chemical reactions in the soil (such as hydrolysis), and the action of soil microbes or other organisms (biodegradation). The compounds that result from pesticide degradation can remain in animals, plants, and water sources, and they can become more concentrated as they move through the food chain levels [45]. Environmental conditions such as temperature, moisture, and pH also affect the pesticide half-life. The rate of degradation is expressed in terms of the amount of time required to reduce the pesticide concentration up to 50%. Persistent pesticides with a half-life longer than 21 days pose the greatest threat to water quality. There are several other important factors, apart from the nature of the pesticide [46]: • Soil characteristics. The potential for pesticides to leach or to be transported in runoff also partially depends on the soil characteristics. The soil texture is governed by the proportions

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15. Pesticide removal from industrial effluents

of sand, silt, and clay. Soils with larger particle sizes present higher aeration, higher porosity, and greater risk of leaching. In addition, the organic matter influences the amount of water that is retained and how well pesticides are adsorbed. • Irrigation management. Irrigation increases the risk that pesticides will migrate to groundwater and surface waters. Irrigating saturated soils promotes pesticide flow; furthermore, irrigation that leads to the frequent downward movement of water, beyond the root zone of plants, also promotes the pesticide leaching process to groundwater. This is of particular concern in areas where frequent irrigation is necessary because the soils have a coarse texture. Proper irrigation management is crucial to minimize the risk of pesticide groundwater contamination. As a conclusion the pesticides most susceptible to leaching are those with high solubility in water, low adsorption to soil, and long-term persistence. The application of these pesticides to sites with sandy soils, shallow groundwater depth, and either a wet climate or extensive use of irrigation results in a high risk of groundwater contamination.

3 Biopolymers used in the removal of pesticides A biopolymer is a general term that can be applied to both biobased polymers and biodegradable polymers. The latter are materials that can be completely degraded by aerobic or anaerobic processes, whereas the biobased polymers may not necessarily be biodegradable. It is important to note that not all biodegradable polymers are biobased ones [47]. However, most of the available biopolymers can be obtained from natural sources, either being directly isolated or being produced by certain living organisms [47]. A general classification of the main biodegradable polymers is shown in Fig. 3. Biotechnology can be considered as one of the newest areas of science, covering genetics, proteomics, and enzyme engineering. It involves biochemical processes (bioprocesses) that Biodegradable polymers

Agropolymers

Proteins

Polysaccharides

Biopolyesters

From microorganism

Synthetic

Natural monomer

Synthetic monomer

FIG. 3

Classification of biodegradable polymers. In this picture the polymers are divided in agropolymers (such as proteins and polysaccharides) and biopolyesters that could be synthetic or from microorganisms.

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3 Biopolymers used in the removal of pesticides

can be conveniently employed to produce biopolymers, among other substances [48]. The main advantage of the use of biotechnology for this purpose is that high biopolymer production can be achieved in shorter times than those offered by nature itself [47]. Bendig et al. [49] provided a summary of the commonest biopolymers produced by living organisms, together with a comparison with their corresponding natural sources, as shown in Table 3. Biopolymers such as chitin, chitosan, starch, cyclodextrins, alginates, and cellulose are being widely studied for the production of novel and low-cost polymeric materials that could be useful in a variety of water treatments. Most of the aforementioned materials represent abundant, widely distributed, renewable, biodegradable, stable, and modifiable resources [7]. Chitin is an abundant mucopolysaccharide consisting of 2-acetamido-2-deoxy-β-D-glucose units connected by β-(1 ! 4) linkages [21]. It is synthesized by marine diatoms, crustaceans, some insects, algae, fungi, and yeasts, and it can be produced industrially by alkaline deproteinization [19]. Depending on the alkali concentration, the N-deacetylated product, called chitosan, can be obtained. Chitosan has several properties that provide it with medical, pharmaceutical, and agricultural applications [50]. Also, its adsorbent properties have been used for the development of membranes for the removal of pesticides in water and for the development of different kinds of composites [4, 51, 52]. The chemical structures of chitin and chitosan are shown in Fig. 4. Chitin and chitosan have been tested as adsorbents for the removal of 2,4-dichlorophenoxyacetate (2,4-D) from aqueous media [53]. Harmoudi and coworkers [54] conducted adsorption tests at different pH values using chitin and chitosan from shrimp shells. It was found that maximum herbicide removal was achieved at pH 3.7, which was attributed to electrostatic interaction between the protonated chitin/chitosan and the anionic pesticide (pKa ¼ 2.64). Using pH 3.7, removal values of 67% and 90% were obtained for chitin and chitosan, respectively, after approximately 60 min of treatment. Chitin and chitosan have also been studied for the adsorption of glyphosate present in an aqueous medium [6]. Rissouli et al. [55] used chitin and chitosan derived from shrimp shells TABLE 3 Biopolymers produced using living organisms, together with their corresponding natural sources [49]. Biopolymer

Monomer

Natural source

Microorganism to produce it

Cellulose

Glucose

Plants, microorganisms

Hyaluronic acid

N-Acetylglucosamine

γ-PGA

Biodegradable

Biocompatible

Acetobacter xylinum

✓

✓

Vertebrates, Streptococci

Streptococci, Bacillus subtilis

✓

✓

Glutamic acid

Bacillus spp., microorganisms

Bacillus spp.

✓

✓

Silk

Protein

Bombyx mori, spider, bee

Escherichia coli

✓

✓

Collagen

Protein

Vertebrates

Yeasts

✓

✓

Chitosan

N-Acetylglucosamine (partly deacylated)

Fungi

Yeasts, bacteria

✓

✓

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15. Pesticide removal from industrial effluents

CH2OH

CH2OH O

H

H

H OH

*

O

H O

H

OH

*

H

H H

O

H

H

NHCOCH3

NHCOCH3

n

(A) CH2OH

CH2OH O

H

H

H *

OH

O

H O

H

OH

H

H H

NH2

O

H

H

*

NH2

n

(B) FIG. 4 Chemical structures of (A) chitin and (B) chitosan.

and found that highest glyphosate adsorption occurred at pH between 3.76 and 5.04; this result was attributed due to electrostatic interaction between the pesticide and polymer. Besides the use of chitosan provided better removal results compared with chitin. Rissouli et al. [56] also used chitin and chitosan to remove the herbicide linuron, with chitosan presenting much higher adsorption capacity compared with chitin. In both cases the maximum adsorption capacity was observed at pH 5.75. Abdeen and Mohammad [57, 58] removed ethoprophos from aqueous solutions using chitosan that they prepared from shrimp shells, achieving good results for physical adsorption of the compound. Chitosan has also shown good activity for the removal of isoproturon [59]. The effect of the adsorbent type on oxadiazon removal was studied by Arvand et al. [60]. The adsorbents investigated were chitin, chitosan, bentonite, and activated carbon. The best results for the removal of the pesticide were obtained using chitosan and activated carbon, which showed maximum activities at pH values of 6 and 2, respectively. Chitosan membranes, one of them also containing alginate, were also employed for the removal of glyphosate [3]. The most common and commercially available alginate is extracted from brown algae by treatment with NaOH, producing a sodium alginate salt [61]. Alginate is a water-soluble anionic polysaccharide mainly composed of mannuronic (M) and guluronic III. Application of biopolymers membranes/films in environment and energy

369

3 Biopolymers used in the removal of pesticides

(G) acids, forming a linear copolymer containing different blocks: consecutive G residues, consecutive M residues, and alternating M and G residues (Fig. 5) [62]. Since alginates from different sources differ in their M and G contents and in the length of each of the possible blocks, this leads to over 200 different alginate structures [63]. Carneiro et al. [3] prepared chitosan, alginate, and chitosan/alginate membranes by spreading polymeric solutions onto Petri dishes and leaving them to dry at 50°C. The chitosan-containing membranes showed the best glyphosate removal efficiencies, while the alginate membranes were unable to adsorb glyphosate. Once again, these results could be explained in terms of electrostatic interactions: since alginate possesses neutral or negative charges, it is unable to remove a negatively charged pesticide. Agostini de Moraes et al. [64] investigated the use of pristine and multilayer alginate and chitosan membranes for the removal of diquat, difenzoquat, and clomazone. It was demonstrated that electrostatic interactions between the polymeric material and the herbicide were responsible for the adsorption process. The positively charged diquat and difenzoquat were successfully removed by the negatively charged alginate membranes, whereas they could not be removed by the pristine chitosan membrane. Furthermore the neutral compound clomazone was not removed by either negatively charged alginate or positively charged chitosan. The authors suggested that even though the multilayer alginate and chitosan membrane did not provide better results, compared with the simple alginate membrane, experiments should be performed to test the possibility of using the –

*

–

OH

OOC

O

OOC

OH

O

OH O

O

–

OH

OH

O

OOC

–

OH

OOC

n

(A) –

* O

OOC

–

HO

O O

HO

HO –

O

OOC

O

HO

*

O

O

OH

O

O

OH

OOC

HO

O O

HO

*

HO –

OOC

O

O

HO

n

(B) OH

–

OH –

* O

HO

OOC

O HO

O

O

OH

O –

(C)

HO

OOC

O HO

*

O

O

OH

O –

OOC

OOC

FIG. 5 Chemical structures of (A) G block, (B) M block, and (C) alternating M and G blocks in alginate. III. Application of biopolymers membranes/films in environment and energy

n

370

15. Pesticide removal from industrial effluents

multilayer material for the simultaneous adsorption of positively and negatively charged pesticides in the corresponding oppositely charged layers. Starch is a highly abundant biopolymer present in living plants, which is mainly composed of a mixture of two polyglucans (amylopectin and amylase), together with only a single type of carbohydrate (glucose) [7]. There are some starch derivatives that deserve special attention, such as cyclodextrins. These are torus-shaped oligosaccharides formed by 6–12 (α-1,4)-linked D-glucopyranose units [7]. The most common natural ones are α-, β-, and γ-cyclodextrins, which are composed of six, seven, and eight glucose units, respectively (Fig. 6) [26, 65].

OH

OH

O

HO

O

O

O

HO

O OH

OH

O

HO

HO OH

OH

O OH

O

HO

HO

HO

O

O

HO

OH

OH

OH

O

O

O OH

O

HO

O

OH

HO

OH

OH

OH O OH

O OH

O OH

OH O

OH

O

OH

O

OH

OH

OH

OH O OH

O HO

(A)

O

(B)

HO

OH

OH HO

O O

O OH

HO OH

O

HO

O

OH

OH

O

HO O

HO OH

OH

O

O OH

OH

O

OH OH OH

O

O HO

OH

O

OH

O

OH OH

O O OH

(C)

HO

FIG. 6 Chemical structures of (A) α-, (B) β-, and (C) γ-cyclodextrins.

III. Application of biopolymers membranes/films in environment and energy

O

O

371

3 Biopolymers used in the removal of pesticides

In addition to offering the advantages of being nontoxic and biodegradable, cyclodextrins can be produced on a large scale. Their macrocyclic structures, in the form of a truncated cone shape with hydrophobicity in the central cavity and hydrophilicity on the surface, enable these molecules to encapsulate nonpolar compounds in the cavity and adsorb polar molecules on the surface [66, 67]. Cyclodextrins are water-soluble materials that can be polymerized to generate insoluble cyclodextrin polymers that present high swelling capacity in water. Liu et al. [68] prepared a set of seven different cyclodextrin polymers from four cyclodextrins, producing both single and composite polymeric materials. Epichlorohydrin was used as the cross-linking agent, while the dispersant was a mixture of Span 80 and Tween 20 (75:25 by mass). The polymer removal efficiencies were tested using 10 different pesticides: fomesafen, bromacil, simazine, atrazine, fenamiphos, fipronil, benalaxyl, butene-fipronil, pretilachlor, and butachlor. The results obtained for the remediation of water containing this biopolymer reduced the pesticide concentrations at environmentally security levels. Alsbaiee and coworkers [69] prepared a porous β-cyclodextrin polymer from the corresponding cyclodextrin and aromatic groups, using nucleophilic aromatic substitution. The prepared materials were tested for the removal of metolachlor, and the results showed that a fast uptake of the target molecule was achieved, with an adsorption rate higher than obtained using nonporous β-cyclodextrin and some activated carbons. The material could be reused without significant loss of activity. Cellulose is one of the most abundant polymers present in nature [70]. This carbohydrate can be obtained from plants such as ramie, flax, hemp, jute, and cotton, as well as from wood [71], which makes this material inexpensive and interesting for the development of water treatment systems [72]. The chemical structure of cellulose comprises two glucose rings joined by a β-D-1,4-glycosidic bond (Fig. 7) [15]. Microfibrils are found in plants as a result of the aggregation of cellulose chains, and they can be deconstructed to generate cellulose nanofibers [71]. Additionally, cellulose may be chemically modified by anchoring different functional groups, according to the desired application [73]. Alila and Boufi [74] modified cellulose fibers to increase the adsorption capacity toward three different pesticides (alachlor, linuron, and atrazine), obtaining promising results. A year later, Alila and coworkers [75] presented another study in which they investigated the removal of several herbicides (alachlor, linuron, and metalaxyl) by cellulose fibers modified by anchoring hydrocarbon moieties onto them. Experiments were carried out under both batch and column conditions, and it was found that the modified cellulose could be easily regenerated and reused in multiple treatment cycles. FIG. 7 Chemical structure of cellulose. OH HO

OH

O

* O

* O

HO OH

O OH

III. Application of biopolymers membranes/films in environment and energy

n

372

15. Pesticide removal from industrial effluents

Nanocellulose (nanocrystals derived from cellulosic materials) has been used in the treatment of water to remove chlorpyrifos, an organophosphate insecticide. Moradeeya and collaborators [76] conducted batch adsorption studies using nanocellulose to adsorb chlorpyrifos from a solution prepared using methanol-water (60:40), obtaining excellent removal results. Guar gum is a very abundant natural polysaccharide that can be isolated from guar seeds, where it is present in the endosperm [77]. The chemical structure of guar gum (Fig. 8) consists of a β-D-1,4-mannose backbone and an α-D-1,6 galactose side chain, with a mannose/galactose ratio near 2:1 [78, 79]. Kee et al. [80] employed guar gum to treat farm effluents containing phenol,2,4-bis(1,1dimethylethyl) phthalate and bis(2-ethylhexyl) phthalate by coagulation and flocculation. It was found that the removal occurred by chemical interactions between the organic pollutants and the coagulant. It was concluded that this treatment constitutes an efficient and lowcost way to treat farm effluents. Several composite materials containing biopolymers have been tested in water treatments for pesticide removal. For example, chitosan-montmorillonite (MMT)-CuO composites were employed for the removal of dichlorvos by adsorption. In comparison with the activity with other MMT-CuO composites containing gum ghatti and polylactic acid, they showed that the best removal of dichlorvos was obtained with the chitosan-based composite material [81, 82]. The same three composite materials were also tested for monocrotophos adsorption, with the best results achieved using the (MMT)-CuO composites containing polylactic acid [81, 83]. For preparation of the MMT-CuO, an aqueous suspension of the MMT was first treated with copper chloride, followed by an ammonia solution until the formation of a dark blue color. Finally the MMT-CuO was left overnight to adsorb the corresponding biopolymer from each polymeric solution. Chitosan was also used to prepare composite chitosan beads modified with ZnO nanoparticles for the adsorption of permethrin [51]. Briefly, for preparation of the composite material, commercial ZnO was dissolved in acidic aqueous medium. The chitosan was then

OH OH

OH OH O

O

HO

HO OH

OH O OH O

HO *

O

O

OH O

OH O

HO

O HO

OH

O

O

OH O

HO

O

*

OH

n

FIG. 8 Chemical structure of guar gum.

III. Application of biopolymers membranes/films in environment and energy

3 Biopolymers used in the removal of pesticides

373

incorporated, and the system was sonicated for 30 min. Finally a NaOH solution was added drop by drop until pH 10 was reached. The mixture was kept at 60°C during 3 h, followed by being filtered, washed, and dried. Badawy and coworkers [84] prepared chitosan-CuO and chitosan-ZnO nanoparticles by cross-linking with glutaraldehyde and then with epichlorohydrin. The selected model pesticides were fenamiphos, lambda-cyhalothrin, abamectin, methyl thiophanate, diazinon, imidacloprid, and methomyl. The preparation of the composite materials was achieved by first dissolving the selected metal oxide, followed by its incorporation in a chitosan solution (in aqueous acetic acid), and finally the addition of glutaraldehyde and epichlorohydrin as the first and second cross-linking agents, respectively. The chitosan-metal oxide nanoparticles were used to develop solid-phase extraction cartridges that offered rapid and simple extraction of the target molecules. The Zn composite materials were more active than the Cu ones. Copper-coated chitosan nanocomposites were prepared by Jaiswal et al. [85], with the aim of removing malathion from agricultural runoff. The nanoparticles were prepared by incorporating copper sulfate in a chitosan solution (in 5% acetic acid), followed by dropwise addition of a solution of NaOH. Batch sorption studies were first performed with the model pesticide solution, after which real polluted water samples were used, collected from a canal in Bithoor (Kanpur). Good results were obtained in all cases, and it was additionally found that other organophosphorus pesticides (methyl parathion and parathion) could also be removed. The removal of methyl parathion from wastewater using gold nanoparticle/chitosan composite beads was studied by Dwivedi et al. [86]. The gold nanoparticles were first prepared from HAuCl4 in a medium containing this reactant, ascorbic acid, and chitosan. The beads were then produced by simple dripping of the chitosan solution into an alkaline solution. Good results were achieved for removal of the pesticide. Saifuddin et al. [87] used microwave irradiation to prepare cross-linked chitosan/silver nanoparticle composite beads, which showed promising results in the removal of atrazine [43, 87, 88]. Similarly, but using a different biopolymer, Pal et al. [89] prepared and studied alginate beads containing silver nanoparticles for the removal of atrazine. The composite beads were prepared using a typical methodology involving the dripping of an aqueous alginate solution into a CaCl2 solution. To include the silver nanoparticles in the composite materials, they were previously prepared from AgNO3 (which was included in the alginate solution) by means of a microwave irradiation technique. Good results were obtained for atrazine removal, and it was found that the composite adsorbent could be reused in at least 26 treatment cycles, with convenient regeneration using HNO3. Other alginate composite beads that have been tested for the removal of pesticides are the montmorillonite-alginate beads prepared by Etcheverry et al. [90] for the removal of cationic pesticides such as paraquat. Several clay composites were prepared by Narayanan et al. [91] from carboxymethyl cellulose and organoclays (nanobentonite modified with dimethyl dialkyl amine, octadecylamine, and aminopropyltriethoxysilane) for use in the removal of pesticides during water treatments. Evaluation was made of the ability of the composite materials to adsorb atrazine, butachlor, carbendazim, carbofuran, imidacloprid, isoproturon, pendimethalin, methyl thiophanate, and thiamethoxam. The best removal efficiency was toward methyl thiophanate (99.7%), while the lowest was found for atrazine (45.5%). One year later,

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15. Pesticide removal from industrial effluents

Narayanan et al. [92] presented another study in which a composite material developed from carboxymethyl cellulose and an organomodified clay (nanomontmorillonite modified with dimethyl dialkyl amine) was used for the removal of several pesticides. In such case the target molecules were imidacloprid, thiamethoxam, and atrazine. The best adsorption was observed for imidacloprid, followed by thiamethoxam and atrazine. Sawicki and Mercier [93] prepared cyclodextrin-functionalized mesoporous silica adsorbents and investigated their abilities to remove 14 different pesticides from aqueous systems. The compounds belonged to the hexachlorocyclohexane-based, hexachlorobicycloheptenebased, and p,p0 -substituted biphenyl-based pesticide families. The greatest affinity was found to be toward the latter class mentioned. Additionally, the best adsorption activities were found for the materials containing low to intermediate amounts of cyclodextrin groups. Since β-cyclodextrins have a shape and size that allow them to suitably accommodate benzyl compounds, Salazar and coworkers [94] decided to prepare β-cyclodextrin polymers decorated with Fe3O4 nanoparticles. For this, magnetic nanoparticles were prepared by the coprecipitation method. Then the β-cyclodextrin polymers (nanosponges) were prepared from the corresponding cyclodextrin and diphenylcarbonate under heating in an ultrasonic bath. To generate the decoration the nanosponges were suspended on the nanoparticles. The prepared composite materials were evaluated for the removal of 4-chlorophenoxyacetic acid and 2,3,4,6-tetrachlorophenol, obtaining good results. After each treatment the adsorbent was easily recovered by the use of an external magnetic field. El Ghali and coworkers [95] modified cotton fibers and loaded them with copper(II) ions to evaluate the ability to remove linuron. First, cotton fibers were treated to obtain 6-chlorodeoxycellulose. The cellulose derivative was then aminated and loaded with the copper ions by preparing a suspension containing the two components, aiming at achieving saturation of the ions on the support at neutral pH. The results showed the removal of the selected pesticide, but not the desorption process. A summary of the biopolymeric materials presented for pesticide removal is shown in Table 4. TABLE 4

Biopolymeric materials presented for pesticide removal.

Biopolymeric material

Pollutant

References

Chitin

2,4-Dichlorophenoxyacetic acid (2,4-D)

[53]

Glyphosate

[55]

Linuron

[56]

Oxadiazon

[60]

2,4-Dichlorophenoxyacetic acid (2,4-D)

[53]

Glyphosate

[55]

Chitosan

[54]

[54]

[6] [3]

III. Application of biopolymers membranes/films in environment and energy

375

3 Biopolymers used in the removal of pesticides

TABLE 4 Biopolymeric materials presented for pesticide removal—cont’d Biopolymeric material

Pollutant

References

Linuron

[56]

Ethoprophos

Alginate

Alginate/chitosan

Cyclodextrin polymer

Cellulose

[57]

Oxadiazon

[60]

Diquat

[64]

Difenzoquat

[64]

Clomazone

[64]

Isoproturon

[59]

Glyphosate

[3]

Diquat

[64]

Difenzoquat

[64]

Clomazone

[64]

Glyphosate

[3]

Diquat

[64]

Difenzoquat

[64]

Clomazone

[64]

Fomesafen

[68]

Bromacil

[68]

Simazine

[68]

Atrazine

[68]

Fenamiphos

[68]

Fipronil

[68]

Benalaxyl

[68]

Butene-fipronil

[68]

Pretilachlor

[68]

Butachlor

[68]

Metolachlor

[69]

Alachlor

[75] [74]

Linuron

[75] [74] Continued

III. Application of biopolymers membranes/films in environment and energy

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15. Pesticide removal from industrial effluents

TABLE 4

Biopolymeric materials presented for pesticide removal—cont’d

Biopolymeric material

Pollutant

References

Metalaxyl

[75]

Atrazine

[74]

Nanocellulose

Chlorpyrifos

[76]

Guar gum

Phenol,2,4-bis(1,1-dimethylethyl)

[80]

Bis(2-ethylhexyl) phthalate

[80]

Dichlorvos

[81, 82]

Monocrotophos

[81, 83]

Permethrin

[51]

Fenamiphos

[84]

Lambda-cyhalothrin

[84]

Abamectin

[84]

Methyl thiophanate

[84]

Diazinon

[84]

Imidacloprid

[84]

Methomyl

[84]

Fenamiphos

[84]

Lambda-cyhalothrin

[84]

Abamectin

[84]

Methyl thiophanate

[84]

Diazinon

[84]

Imidacloprid

[84]

CuO/chitosan

Methomyl

[84]

Cu-chitosan

Malathion

[85]

Methyl parathion

[85]

Parathion

[85]

Ag/chitosan

Atrazine

[87]

Au/chitosan

Methyl parathion

[86]

Ag/alginate

Atrazine

[89]

Montmorillonite-alginate

Paraquat

[90]

Montmorillonite-CuO-chitosan/ gum ghatti/polylactic acid ZnO/chitosan

CuO/chitosan

III. Application of biopolymers membranes/films in environment and energy

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3 Biopolymers used in the removal of pesticides

TABLE 4 Biopolymeric materials presented for pesticide removal—cont’d Biopolymeric material

Pollutant

References

Carboxymethyl celluloseorganoclays

Atrazine

[91] [92]

Butachlor

[91]

Carbendazim

[91]

Carbofuran

[91]

Imidacloprid

[91] [92]

Carboxymethyl celluloseorganoclays

Isoproturon

[91]

Pendimethalin

[91]

Methyl thiophanate

[91]

Thiamethoxam

[91] [92]

Cu/cellulose

Linuron

[95]

Cyclodextrin-functionalized mesoporous silica

α-Hexachlorocyclohexane

[93]

β-Hexachlorocyclohexane

[93]

γ-Hexachlorocyclohexane

[93]

δ-Hexachlorocyclohexane

[93]

Heptachlor

[93]

Heptachlor epoxide

[93]

Endosulfan sulfate

[93]

Endosulfan

[93]

Aldrin

[93]

Dieldrin

[93]

Dichlorodiphenyldichloroethylene

[93]

Dichlorodiphenyldichloroethane

[93]

Dichlorodiphenyltrichloroethane

[93]

Methoxychlor

[93]

4-Chlorophenoxyacetic acid

[94]

2,3,4,6-Tetrachlorophenol

[94]

β-Cyclodextrin polymers-Fe3O4 nanoparticles

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15. Pesticide removal from industrial effluents

4 Conclusions and future perspectives This chapter provides an overview of pesticides and their fates in the environment, considering the potential routes of contamination. The need of developing alternative techniques using new organic compounds and not necessarily involving conventional processes has led to the emergence of biopolymers. Among the currently available biopolymers, we highlight as the main ones those that are most abundant in nature, describing their chemical characteristics and providing examples of their potential for the pesticide removal from wastewaters. Also, in this chapter, we bring up the application of new technologies such as the production of biopolymeric membranes with nanomaterials, which can be applied to enhance the pesticide absorption. Most of the published studies have found good removal rates and good turnover values for pesticide remediation. However, many of the techniques are still at the stage of development by research groups in universities and research centers. Further progress will be needed to address issues such as scale-up and production costs, with analyses of the economic viability of these technologies, where necessary. Therefore progress in this field will require the industrial scaling-up, with the development of pilot trials to evaluate the potential of biopolymer-based systems at larger scales. It should be strongly emphasized that, due their characteristics as nontoxic biocompatibility and biodegradability, these materials have excellent potential for commercial applications, being an ecofriendly alternative for wastewater treatment systems.

Acknowledgments R.P.O., L.M.S., and V.A.A. would like to thank CONICET, UNMdP, and ANPCyT for financial support. L.F.F. and A. E.S.P. would like to thank the Sa˜o Paulo State Research Foundation (FAPESP, grant #2017/21004-5).

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C H A P T E R

16 Dye removal from effluents using biopolymer membranes
Guilherme Luiz Dottoa, Eder Cla´udio Limab, Tito Roberto Sant’Anna Cadaval, Jrc, Adria´n Bonilla-Petricioletd, Ioannis Anastopoulose, Ahmad Hosseini-Bandegharaeif a

Chemical Engineering Department, Federal University of Santa Maria–UFSM, Santa Maria, RS, Brazil bInstitute of Chemistry, Federal University of Rio Grande do Sul, UFRGS, Porto Alegre, RS, Brazil cSchool of Chemistry and Food, Federal University of Rio Grande, FURG, Rio Grande, RS, Brazil dDepartment of Chemical Engineering, Aguascalientes Institute of Technology, Aguascalientes, Mexico eDepartment of Chemistry, University of Cyprus, Nicosia, Cyprus f Department of Environmental Health Engineering, Sabzevar University of Medical Sciences, Tehran, Iran

1 The importance of dye removal from effluents The report “Water for People Water for Life” from United Nations World Water Development [1] discusses the world’s water crisis. According to this report, the demand for fresh water has increased severely due to consumption by the agricultural, industrial, and domestic sectors. Industrial activities are responsible for 22% of freshwater consumption, generating a large volume of effluents, which contain different types of contaminants. Among the several contaminants present in industrial waters and effluents, dyes are one of the most harmful [2]. Dyes are used in different industrial sectors, including paints, foods, textiles, pharmaceuticals, cosmetics, plastics, rubber, paper, printing, and dye manufacturing [3]. The correct number and quantity of produced dyes is difficult to estimate, but it has been reported that there are more than 100,000 dyes, with production at 106 tons per year. During the different processing methods, around 15% are lost from the total quantity of dyes, which are harmful industrial effluents [4]. The textile industry is mainly responsible for the generation of colored effluents, since, in some cases, 3000 m3

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FIG. 1

Inadequate release of colored effluents. http://www.philozon.com.br/ noticias/uso-de-ozonio-para-remocao-decor-em-efluentes-texteis/.

of water are used to produce 1 ton of tissue [5]. If untreated or incorrectly treated, these effluents represent risks to human beings and the environment [2, 4]. Fig. 1 shows the consequences caused by the incorrect discharge of colored effluents. Due to these risks, legislation has become more stringent [6] and many methods have been developed to treat colored effluents using different approaches [2–4, 7–9].

1.1 Dyes: Classification and uses Synthetic dyes account for the largest amount used industrially. From a structural viewpoint, these dyes are formed by two main groups: chromophores and auxochromes. The first is responsible for providing color and the second enhances the affinity of the products [10]. A great number of synthetic dyes are commercially available, and therefore their organization and identification are problematic. The majority of synthetic dyes are identified by their Color Index, which is a five-digit number (see https://color-index.com/) [11]. These dyes have a complex chemical structure, with aromatic rings and several chemical functions. Also, they are applied in several fields and have different properties. Because of this, synthetic dyes can be classified in different forms [4, 10]. Classifications can be based on chemical structure (azo, indigoid, xanthene, nitro, etc.), nuclear structure (anionic, cationic, or nonionic), or industrial application (reactive, disperse, vat, acid, etc.) [12]. Table 1 presents the classification of dyes regarding their industrial application. This table also provides the main chemical structures present in each class and the uses of these dyes.

1.2 Environmental and public health risks As can be seen in Table 1, dyes are used in a series of industrial and research areas. In most of these areas, dyes are responsible for providing beneficial characteristics to the products. It is then clear that synthetic dyes are commercially and industrially important. Among all the sectors that use dyes in their processes, the textile industry is responsible for the largest

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TABLE 1

Classification of dyes regarding industrial application.

Category

Applications

Disperse dyes

Polyester, nylon, cellulose, Benzodifuranone, azo, anthraquinone, Disperse Red 1 cellulose acetate, and acrylic fibers nitro, and styryl Disperse Orange 3 Disperse Blue 35 Disperse Red 11

Acid dyes

Nylon, wool, silk, acrylics, paper, leather, food, and cosmetics

Anthraquinone, xanthene, azo, nitro, and triphenylmethane

Acid Red 18 Acid Yellow 23 Acid Black 48 Acid Red 359

Basic dyes

Paper, polyacrylonitrile, nylons, polyesters, medicine, silk, wool, and cotton

Hemicyanine, azo, cyanine, diazahemicyanine, azine, diphenylmethane, xanthene, triarylmethane, acridine, anthraquinone, and oxazine

Basic Red 46 Basic Black 1 Brilliant Green Basic Brown 1

Direct dyes

Cotton, rayon, paper, leather, and nylon

Phthalocyanine, azo, oxazine, and stilbene

Direct Orange 34 Direct Black 38 Direct Red 23 Direct Brown 1

Vat dyes

Cotton, cellulosic fibers, rayon, and Indigoids and anthraquinone wool

Vat Blue 6 Vat Green 1 Vat Blue 1 Vat Orange 11

Reactive dyes

Cotton, other cellulosics, wool, and Anthraquinone, formazan, nylon phthalocyanine, azo, and oxazine

Reactive Black 5 Reactive Blue 4 Reactive Red 120 Reactive Red 2

Sulfur dyes

Cotton and rayon

Indeterminate structures

Sulfur Orange 1 Sulfur Brown 21 Sulfur Green 12 Sulfur Black 1

Mordant dyes

Wool, leather, and natural fibers

Azo and anthraquinone

Natural Black 1 Mordant Blue 3 Mordant Blue 14 Mordant Red 3

Solvent dyes

Fuels, waxes, lubricants, plastics, wood, stains, varnishes, lacquers, butter, margarine, and resins

Azo, anthraquinone, phthalocyanine, and triarylmethane

Solvent Red 24 Solvent Red 26 Solvent Yellow 124 Solvent Blue 35

Fluorescent dyes

Biology, soaps, detergents, fibers, oils, paints, and plastics

Stilbene, pyrazoles, coumarin, and naphthalimides

Sulforhodamine B Kiton Red 620 Rhodamine Perchlorate HB7

Chemical structures

Examples

Adapted from V.K. Gupta, Suhas, Application of low-cost adsorbents for dye removal – a review, J. Environ. Manage. 90 (2009) 2313–2342; M.A.M. Salleh, D.K. Mahmoud, W.A.W.A. Karim, A. Idris, Cationic and anionic dye adsorption by agricultural solid wastes: a comprehensive review, Desalination 280 (2011), 1–13; S. Nikfar, M. Jaberidoost, Dyes and Colorants, Reference Module in Biomedical Sciences, Elsevier, 2014, pp. 252–261; M.T. Yagub, T.K. Sen, S. Afroze, H. M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interf. Sci. 209 (2014) 172–184.

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generation of colored effluents [13]. The exact volume of textile effluents generated worldwide is difficult to measure and is modified each year. However, it is estimated that 3
103 m3 of effluent is produced after processing 20 ton of textiles per day [14]. The textile effluents are then generated in large volumes and contain several contaminants, mainly synthetic dyes. For these reasons, dye-containing effluents from textile industries are extremely dangerous, representing risks for the environment and human health [2]. The environmental risks caused by inadequate disposal of colored effluents are countless and have already been discussed. Here, only a brief explanation is presented. Textile effluents, even with very low dye concentrations (1 mg L1), provoke color changes (Fig. 1) in water bodies, leading to a reduction in sunlight transmission. As a consequence, the metabolism of aquatic plants and animals is affected [10]. Dyes are bioaccumulative and can cause eutrophication [2]. The exposition, inhalation, and/or ingestion of these dyes or their degradation products (aromatic amines, for example) can cause severe effects to animals and plants, since most dyes are mutagenic, carcinogenic, teratogenic, and can also cause microtoxicity [13]. A literature review [15] revealed that synthetic dyes induce various toxic, cytotoxic, genotoxic, mutagenic, and carcinogenic effects on different organisms when exposed to such compounds. The risks of dye exposure, ingestion, or inhalation to human health are dependent of several factors, including exposure time, amount ingested, and others [16]. Until now, several adverse effects to human health caused by the dyes have been proven, e.g., attention-deficit hyperactivity disorder, bronchial asthma, allergic reactions, skin disorders, dysfunction of the kidneys, and dysfunction in the reproductive system, liver, brain, and central nervous system [12, 15, 17, 18].

1.3 Guidelines for discharge of colored effluents Considering the adverse effects caused by dyes, legislation has become more stringent regarding the standards for the discharge of colored effluents [6, 18]. Dyes are applied in several industrial sectors, including paints, foods, textiles, pharmaceuticals, and others [3]. Depending of the industrial sector, the use and release of dyes can be controlled by different organizations. For example, the use of food and pharmaceutical dyes is controlled by the European Commission in the European Union or Food and Drug Administration in the United States [17]. In the last few years, the release of colored effluents in the textile sector was controlled by organizations in each country, presenting specific requirements in each case [6]. Fortunately, an international standard to release colored effluents in water bodies has been established [18, 19]: the International Dye Industry Wastewater Discharge Quality Standards (TIWDQS), based on the Zero Discharge of Hazardous Chemicals Programme [18]. According to this international standard, the effluent to be released should have the following characteristics: biological oxygen demand below 30 mg L1, chemical oxygen demand below 30 mg L1, color below 1 mg L1, pH between 6 and 9, suspended solids below 20 mg L1, temperature below 42°C, and no presence of toxic pollutants [19]. In our opinion, this legislation facilitates the groups that work in the area of dye removal from effluents. The target is now improvement of the currently available technologies for dye removal to attain the TIWDQS standard, coupling eco-friendly practices and low cost.

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1.4 Methods for dye removal from effluents Aqueous effluents containing dyes must be treated before disposing to avoid their negative impact on the environment. However, it is difficult to decide on a single technique that resolves this problem. Existing methods of dye removal can be separated into three categories: biological, chemical, and physical treatments [20]. Biological methods: Biological dye removal methods incorporate some form of living organism in their process. Usually, a combination of aerobic and anaerobic conditions can be implanted [21]. Its large application is due to its cheap and easy operation; however, this treatment is insufficient to completely remove molecules such as synthetic dyes from water [22]. Besides this, other biological dye removal methods are algae degradation, enzyme degradation, and microbial and fungal cultures. Biological dye removal methods have a removal percentage ranging from 75% to 90%, being the enzyme degradation method that is most adequate and reliable. This method is cheap, efficient, nontoxic, and allows material reuse. Since this method deals with living organisms, its growth rate can be a limitation. Thus system instability is common in biological processes and the reaction mechanisms can be difficult to control [18]. Chemical methods: There are many chemical methods for dye removal such as electrochemical degradation, advanced oxidation process, Fenton reaction, and photochemical, ozonation, and ultraviolet irradiation. However, usually, chemical removal methods are costly compared to biological and physical methods, and are unattractive because they require specific structures and high electrical energy to power equipment or reactors. In addition, reagent consumption on a large scale is an issue commonly reported [18, 23]. Another undesirable characteristic of this method is the generation of secondary contaminants due to the reaction of the dyes, presenting an additional disposal problem. Sometimes, the secondary contaminants can present greater toxicity in relation to the precursor dye molecule [24]. Physical dye removal methods: Physical removal methods are usually straightforward and commonly accomplished using the mass transfer mechanism as their basis. Thus, among the cited methods (biological, chemical, and physical), physical dye removal is the most extensively used industrially [18]. Conventional physical dye removal methods are adsorption, coagulation/flocculation, ion exchange, membrane filtration, nanofiltration, ultrafiltration, and reverse osmosis. These methods are often chosen for their simplicity, easy operation, and efficiency. Since these methods require the least amount of chemicals compared to biological or chemical dye removal methods and do not deal with living organisms they are considered more predictable than the other two methods [25].

2 Biopolymer membranes for dye removal Membrane-based techniques are currently viewed as technological and economical options for effluent treatment. The textile and food industries are the main beneficiaries of membranes to remove dyes in wastewater treatment [18, 26]. Membrane application is normally hailed as a clean and environmentally friendly technology. Simplicity of use and the application of a standard design to handle large feed volumes are some of the advantageous aspects of membrane-based treatment techniques. In addition, the use of membranes in

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wastewater treatment occurs with negligible use of additives, in moderate temperature conditions, and without phase change, demanding a low energy supply [18, 26].

2.1 Biopolymers used for membrane preparation The development of membranes has increased significantly in the last 40 years. The market value of membranes used in water treatment reached US$10.8 billion at the end of 2019. However, currently, membrane fabrication uses fossil-based polymers for precursor material [27]. Thus membrane production is also recognized as having low sustainability, since the environmental impact of plastic wastes represents a global problem, and the recycling technologies are limited. This environmental restriction has been stimulating the research of potential biopolymers able to substitute the fossil material in membrane production. In this context, polymers derived from biomaterials have been the focus of studies worldwide to obtain membranes. Natural polymers from animal sources such as chitin, chitosan, collagen, and polylactic acid or from vegetable sources like cellulose, lignin, alginate, and starch are currently being extensively researched. Though the use of biopolymers for the preparation of membranes is very well documented in the literature, their application on a larger scale is still a challenge for technological development [28–30].

2.2 Raw materials for biopolymer-based membrane preparation The biopolymer-based membrane can be produced from petroleum-based biodegradable polymers, renewable resource-based polymers, and polymers from mixed sources [27, 31, 32]. The different ways of producing bioplastics are presented in Fig. 2 [31, 32].

Raw materials

Petroleum-based biodegradable polymers

Renewable resource based polymers Polymers from mixed sources

Aliphatic polyesters Aliphatic-aromatic polyesters Poly (vinyl alcohol) Polyesters Thermosets

Starch plastics Cellulosics Proteinous plastics Poly (lactic acid) Polyhydroxyalkanoates

FIG. 2 Raw materials used to produce membranes. Adapted from R.A. Sheldon, Green and sustainable manufacture of chemicals from biomass: state of the art, Green Chem. 16 (2014) 950–963; M.M. Reddy, S. Vivekanandhan, M. Misra, S.K. Bhatia, A.K. Mohanty, Biobased plastics and bionanocomposites: current status and future opportunities, Prog. Polym. Sci. 38 (2013), 1653–1689.

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Petroleum-based biodegradable polymers: Bioplastics or biodegradable polymers can be obtained from fossil sources (petroleum or coal). The best-known biodegradable polymers from petroleum are polycaprolactones, polyesteramides, aliphatic copolyesters, and aromatic copolyesters. Renewable resource-based polymers: These are polymers obtained from renewable resources such as plants or animals. In this group, it is possible to find polymers traditionally produced from petrochemicals like polyethylene and polypropylene. However, these polymers are synthesized from biological materials. Although they come from biological sources, these polymers are not biodegradable. Some biopolymers such as bio-polyethylene terephthalate have the same polymeric structure as the petrochemical source and can be recycled [27]. Polymers from mixed sources: These polymers are the materials obtained from the combination of monomers from petroleum and biobased sources. These types of membranes are partially biodegradable and seek to merge the characteristics of the two materials [27, 31, 32].

2.3 Characterization techniques After the synthesis of polymeric membranes, it is necessary to obtain information in relation to the characteristics of this material. These characteristics provide information about mechanical properties, chemical structure, morphologic design, and permeability, for example [33–37]. Mechanical properties: The main mechanical characteristics of polymeric membranes are Young’s modulus, elongation at break, and tensile strength. Young’s modulus describes tensile elasticity or the tendency of the membrane to deform along an axis when opposing forces are applied along that axis. It is defined as the ratio of tensile stress to tensile strain and may be referred to simply as the elastic modulus [35]: σ E¼ (1) ε where E is Young’s modulus in pascal, σ is the uniaxial stress or uniaxial force per unit surface in pascal, and ε is the strain or proportional deformation (change in length divided by original length) (nondimensional). Fig. 3 presents an example of the stress-strain curve for different membranes from propionate lignin and cellulose for water purification. Morphologic design: To obtain the morphological characteristics of membrane surfaces, techniques such as scanning electron microscopy, transmission electron microscopy, and atomic force microscopy are used. These microscopic techniques make it possible to evaluate characteristics such as measurement of the surface roughness of membranes, porous structure, microcompounds inserted in the membrane materials, and homogeneity. It is possible to analyze membrane structures in the nonmetric scale, demonstrating little differences between polymeric matrixes used to produce the membranes [38, 39]. Chemical structure: The chemical characteristics of membranes can be accompanied by several spectroscopic techniques such as infrared (Fourier transform infrared), ultravioletvisible, X-ray photoelectron spectroscopy, X-ray diffraction, and nuclear magnetic resonance. During the production process or even during use, these techniques identify reactions that have altered the chemical composition of the membranes. Many chemical changes are proposed to alter mechanical or morphological characteristics of the precursor materials of III. Application of biopolymers membranes/films in environment and energy

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1.4´108 Normalstress MPKL CTA MHL MPHL MKL MOL MPOL

Normal stress (MPa)

1.2´108 1´108 8´107 6´107 4´107 2´107 0.0

0.0

2.0

4.0

6.0

8.0 e (%)

10.0

12.0

14.0

16.0

FIG. 3 Stress-strain curve for different membranes from propionate lignin and cellulose [35].

membrane production, and these reactions can be confirmed using spectroscopic techniques. In addition, it is possible that during use, reactions occur between the material retained in the membrane and functional groups that make up the polymer matrix [40–43]. Pure water flux and permeability: Pure water flux and permeability through the membrane are important parameters to be evaluated before application in dye removal [43–45]. The pure water flux through a porous medium can be described by Darcy’s law: J¼

KΔP μΔx

(2)

where K is the permeability constant, μ is the dynamic viscosity, ΔP is the pressure difference across the membrane, and Δx is the membrane thickness. To determine the permeability of a membrane, the flux of water through the membrane at different outlet pressures is measured. The permeability of the membrane can be determined using the slope of the flux versus the pressure change.

2.4 Laboratory experiments using membranes for dye removal The apparatus used for experimental membrane dye removal is illustrated in Fig. 4 [43]. In the schematic diagram, 1 is the permeate tank; 2 is the filtration membrane module; 3 is the pressure regulating valve; 4 is the heat exchanger; 5 is the pressure gauge; 6 is the flow meter; 7 is the feed pump; 8 is the feed tank; and 9 is the thermometer.

2.5 Evaluation of membrane performance The performance of membranes in filtration experiments can be characterized by measuring dye rejection rates, fouling measurement, and selectivity [34, 36, 43].

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FIG. 4 Illustration of apparatus used for dye removal using membranes [43].

Rejection rates: Dye removal (the rejection rates) can be determined using Eq. (3):
 Cp Rð%Þ ¼ 1  100 Cf

(3)

where Cp and Cf are the concentrations of a constituent of permeate and feed solution, respectively. Antifouling property: The pure water flux of membranes before and after dye removal must be measured to investigate the fouling resistance. Flux recovery ratio (FRR), which indicates the antifouling property of the membranes, can be determined using the following equation:
 Jw2 FRRð%Þ ¼ 100 (4) Jw1 where Jw2 and Jw1 are the flux after regeneration and before the dye removal experiment, respectively. Selectivity: The ability of a polymeric membrane to separate the substances A and B is characterized by ideal selectivity, which can be defined as: 0 1 CpA B C C 100%  R B fA C A Selectivity ð%Þ ¼ B (5) C¼ @ CpB A 100%  RB CfB where Cp and Cf are the concentration of permeate and feed solution, respectively.

2.6 Comparative analysis of several biopolymer membranes for dye removal Table 2 lists the research work carried out on several biopolymer membranes for dye removal from water. Analysis of this table shows that membranes composed of biopolymers are interesting materials for the removal of dyes using continuous systems. Materials like cellulose, alginate, chitosan, and polyvinyl alcohol are successfully used for this target. The membranes are able to separate different types of dyes like methylene blue, Congo red, and red 23, among others, with dye rejection from 70% to 100%. The permeate flux can attain 4000 L m2 h1. III. Application of biopolymers membranes/films in environment and energy

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TABLE 2 Comparative analysis of several biopolymer membranes for dye removal. Membrane specification

Effluents

Component(s) removed

Permeate flux 2

h

1

References [26]

Freestanding calcium alginate/ polyacrylamide hydrogel nanofiltration membrane

Brilliant Blue 100% dye rejection G250

27 L m

Cellulose composite membrane

Congo Red and NaCl

98% dye rejection

45 L m2 h1

[34]

Carboxy methyl cellulose sodium salt

Methylene blue and Evans blue

99%–92% dye rejection

22–36 L m2 h1

[36]

Carboxy methyl cellulose sodium salt

Methylene blue and Evans blue

99%–86% dye rejection

13–29 L m2 h1

[36]

Nanofibrous chitosan/poly(vinyl alcohol)/SiO2

Red 23

98% dye rejection

2500–4000 L m2 h1

[40]

Carboxymethyl chitosan-coated Fe3O4 nanoparticles

Direct Red 16 99% dye rejection

36 L m2 h1

[38]

Nanoclay/chitosan on polyvinylidene difluoride microfiltration support

Methylene blue

260 L m2 h1

[41]

Polyhydroxybutyrate–calcium alginate/carbon nanotube composite nanofibrous filtration membrane

Brilliant blue 98% dye rejection

32.95 L m2 h1

[45]

21.4 L m2 h1

[46]

Poly(ether sulfone) and poly(vinyl alcohol) Reactive dye composite

97% dye rejection

70%–95% dye rejection

2.7 Real applications (real industrial effluents) In the last few years, the demand for industrial water has increased due to the growth of this economic sector. In parallel, membrane utilization has been hailed as a clean and environmentally friendly technology. The simplicity, provision of modular design for handling large industrial-scale feed volumes, operation under moderate temperature conditions with no phase change, and negligible use of additives are some of the advantageous aspects of this technology. Additionally, retention efficiencies and stability characterizing most of the membrane-based processes under varying experimental conditions enable easy scaleup of these techniques. Membrane-based processes are highly propitious for the treatment of complex effluents such as textile and food effluents, which are highly deleterious and heterogeneous due to the presence of complex and recalcitrant constituents, such as dyes, salts, and other chemicals. However, to ensure environmental compliance, industries have called for process intensification through the introduction of hybrid treatment techniques. The effectiveness of these hybrid processes can be reached by a permutation of treatment techniques that complement each other. Many of these processes are focused on the combined physicochemical/biological

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References

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and membrane separation operations. Thus the application of progressively evolving membrane technology in the treatment of effluents containing dyes can successfully bring about process intensification. It should be noted, however that the applicability of any process in the industrial field should be consolidated only after analyzing the pragmatism of the economic perspective. The evaluated parameters may include capital and operating costs, revenues, benefit/cost ratios, and payback times. Such an arrangement should propose to offset the initial capital cost through conservation and extensive reuse of energy, materials, and process water. The judicious use of membrane-based treatment techniques in industries thereby infuses an element of sustainability in the process and can be viewed as a promising operation of environmental focus and interesting cost effectiveness.

2.8 Perspectives and challenges Research in developing materials such as membranes for water purification has shown promising results in recent years. This can be noticed by the numerous published works using several biopolymer matrices for water treatment containing not only dyes but also toxic metals and other molecules that can cause damage to the environment. However, advances in the development of membranes from biopolymers are not yet sufficient for these materials to replace or compete for space in the market of membranes with polymers of fossil origin. Although advances in the deep knowledge of chemical and physical manipulation of biopolymers have allowed the production of several types of biomembranes, what can be noticed is that the advances are prompt. However, it is hard to develop a biomembrane that encompasses all the necessary characteristics in terms of efficiency in its use and especially its cost. Thus the challenges in the development of biomembranes are to obtain routes for biopolymer-based membrane synthesis, which promote materials with good mechanical and morphologic design associated with high permeability and rejection rates. In addition, it is of fundamental importance that the materials have durability in their use at the large scale, with antifouling properties and selectivity when a real effluent is utilized. Finally, these materials must have all these characteristics and an attractive price for the consumer, so that they can establish themselves as the dominant technology that can be produced on a large scale.

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erez-Ibarbia, T. Majdanski, S. Schubert, N. Windhab, U.S. Schubert, Safety and regulatory review of dyes commonly used as excipients in pharmaceutical and nutraceutical applications, Eur. J. Pharm. Sci. 93 (2016) 264–273. [18] V. Katheresan, J. Kansedo, S.Y. Lau, Efficiency of various recent wastewater dye removal methods: a review, J. Environ. Chem. Eng. 6 (2018) 4676–4697. [19] Textile industry wastewater discharge quality standards, zero discharge of hazardous chemicals program, https://www.textilepact.net/pdf/publications/reports-and-award/textile-industry-wastewater-qualityguideline-landscape-report.pdf, 2015. [20] L. Tang, J. Yu, Y. Pang, G. Zeng, Y. Deng, J. Wang, X. Ren, S. Ye, B. Peng, H. Feng, Sustainable efficient adsorbent: alkali–acid modified magnetic biochar derived from sewage sludge for aqueous organic contaminant removal, Chem. Eng. J. 336 (2018) 160–169. [21] M.A.M. Al-Alwani, N.A. Ludin, A.B. Mohamad, A.A.H. Kadhum, A. Mukhlus, Application of dyes extracted from Alternanthera Dentata leaves and Musa Acuminata Bracts as natural sensitizers for dye-sensitized solar cells, Spectrochim. Acta A Mol. Biomol. Spectrosc. 192 (2018) 487–498. [22] Y. Pan, Y. Wang, A. Zhou, A. Wang, Z. Wu, L. Lv, X. Li, K. Zhang, T. Zhu, Removal of azo dye in an up-flow membrane–less bioelectrochemical system integrated with bio-contact oxidation reactor, Chem. Eng. J. 326 (2017) 454–461. [23] A.B. Santos, F.J. Cervantes, J.B. van Lier, Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology, Bioresour. Technol. 98 (2007) 2369–2385. [24] J. Wang, X. Wang, G. Zhao, G. Song, D. Chen, H. Chen, J. Xie, T. Hayat, A. Alsaedi, X. Wang, Polyvinylpyrrolidone and polyacrylamide intercalated molybdenum disulfide as adsorbents for enhanced removal of chromium (VI) from aqueous solutions, Chem. Eng. J. 334 (2018) 569–578. [25] N.A. Khan, B.N. Bhadra, S.H. Jhung, Heteropoly acid loaded ionic liquid@metal–organic frameworks: effective and reusable adsorbents for the desulfurization of a liquid model fuel, Chem. Eng. J. 334 (2018) 2215–2221. [26] J. Dasgupta, J. Sikder, S. Chakraborty, S. Curcio, E. Drioli, Remediation of textile effluents by membrane based treatment techniques: a state of the art review, J. Environ. Manage. 147 (2015) 55–72. [27] F. Galiano, K. Bricen˜o, T. Marino, A. Molino, K.V. Christensen, A. Figoli, Advances in biopolymer-based membrane preparation and applications, J. Membr. Sci. 564 (2018) 562–586.

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[28] C. Shi, C. Lv, L. Wu, X. Hou, Porous chitosan/hydroxyapatite composite membrane for dyes static and dynamic removal from aqueous solution, J. Hazard. Mater. 338 (2017) 241–249. [29] Y. Ma, P. Qi, J. Ju, Q. Wang, L. Hao, R. Wang, K. Sui, Y. Tan, Gelatin/alginate composite nanofiber membranes for effective and even adsorption of cationic dyes, Compos. B Eng. 162 (2019) 671–677. [30] R. Sabarish, G. Unnikrishnan, Polyvinyl alcohol/carboxymethyl cellulose/ZSM-5 zeolite biocomposite membranes for dye adsorption applications, Carbohydr. Polym. 199 (2018) 129–140. [31] R.A. Sheldon, Green and sustainable manufacture of chemicals from biomass: state of the art, Green Chem. 16 (2014) 950–963. [32] M.M. Reddy, S. Vivekanandhan, M. Misra, S.K. Bhatia, A.K. Mohanty, Biobased plastics and bionanocomposites: current status and future opportunities, Prog. Polym. Sci. 38 (2013) 1653–1689. [33] X. Zhang, B. Lin, K. Zhao, J. Wei, J. Guo, W. Cui, S. Jiang, D. Liu, J. Li, A free-standing calcium alginate/polyacrylamide hydrogel nanofiltration membrane with high anti-fouling performance: preparation and characterization, Desalination 365 (2015) 234–241. [34] T. Puspasari, K. Peinemann, Application of thin film cellulose composite membrane for dye wastewater reuse, J. Water Process Eng. 13 (2016) 176–182. [35] L.M. Neva´rez, L.B. Casarrubias, O.S. Canto, A. Celzard, V. Fierro, R.I. Go´mez, G.G. Sa´nchez, Biopolymers-based nanocomposites: membranes from propionated lignin and cellulose for water purification, Carbohydr. Polym. 86 (2011) 732–741. [36] M. Wasim, M. Shafiq, R.U. Khan, A. Sabir, Crosslinked integrally skinned asymmetric composite membranes for dye rejection, Appl. Surf. Sci. 478 (2019) 514–521. € Tekinalp, S.A. Altinkaya, Development of high flux nanofiltration membranes through single bilayer [37] O. polyethyleneimine/alginate deposition, J. Colloid Interface Sci. 537 (2019) 215–227. [38] S. Zinadini, A.A. Zinatizadeh, M. Rahimi, V. Vatanpour, H. Zangeneh, M. Beygzadeh, Novel high flux antifouling nanofiltration membranes for dye removal containing carboxymethyl chitosan coated Fe3O4 nanoparticles, Desalination 349 (2014) 145–154. [39] Z. Karim, A.P. Mathew, M. Grahn, J. Mouzon, K. Oksman, Nanoporous membranes with cellulose nanocrystals as functional entity in chitosan: removal of dyes from water, Carbohydr. Polym. 112 (2014) 668–676. [40] S.A. Hosseini, M. Vossoughi, N.M. Mahmoodi, M. Sadrzadeh, Efficient dye removal from aqueous solution by high-performance electrospun nanofibrous membranes through incorporation of SiO2 nanoparticles, J. Clean. Prod. 183 (2018) 1197–1206. [41] P. Daraei, S.S. Madaeni, E. Salehi, N. Ghaemi, H.S. Ghari, M.A. Khadivi, E. Rostami, Novel thin film composite membrane fabricated by mixed matrix nanoclay/chitosan on PVDF microfiltration support: preparation, characterization and performance in dye removal, J. Membr. Sci. 436 (2013) 97–108. [42] H. Yang, J. Gong, G. Zeng, P. Zhang, J. Zhang, H. Liu, S. Huan, Polyurethane foam membranes filled with humic acid–chitosan crosslinked gels for selective and simultaneous removal of dyes, J. Colloid Interface Sci. 505 (2017) 67–78. [43] Y. He, G. Li, H. Wang, Z. Jiang, J. Zhao, H. Su, Q. Huang, Experimental study on the rejection of salt and dye with cellulose acetate nanofiltration membrane, J. Taiwan Inst. Chem. Eng. 40 (2009) 289–295. [44] V.C. Souza, M.G.N. Quadri, Organic–inorganic hybrid membranes in separation processes: a 10-year review, Braz. J. Chem. Eng. 30 (2013) 683–700. [45] J. Guo, Q. Zhang, Z. Cai, K. Zhao, Preparation and dye filtration property of electrospun polyhydroxybutyrate– calcium alginate/carbon nanotubes composite nanofibrous filtration membrane, Sep. Purif. Technol. 161 (2016) 69–79. [46] J. Babu, Z.V.P. Murthy, Treatment of textile dyes containing wastewaters with PES/PVA thin film composite nanofiltration membranes, Sep. Purif. Technol. 183 (2017) 66–72.

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C H A P T E R

17 Pharmaceutical and synthetic hormone removal using biopolymer membranes J.A. Sa´nchez-Ferna´ndeza, Ramo´n Dı´az de Leo´na, Rodrigo Cu
e-Sampedrob a

Department of Polymerization Processes, Research Center of Applied Chemistry, Saltillo, Mexico b School of Engineering and Sciences, Monterrey Institute of Technology, Monterrey, Mexico

1 Pharmaceutical and synthetic hormone removal Innovations in synthetic methods have changed the way scientists think about designing and building molecules, enabling access to more expansive chemical space and to molecules possessing the essential biological activity needed in future investigational drugs. In addition, the ability of the pharmaceutical industry to discover molecules to treat unmet medical needs and deliver them to patients efficiently in the face of an increasingly challenging regulatory landscape is dependent on continued invention of transformative, synthetic methodologies [1]. A wide range of micropollutants (MPs), sometimes named potential emerging pollutants (PEPs), composed mostly of pharmaceuticals, personal care products, and pesticides, occur in very low concentrations of a few nanograms to micrograms per liter in the water cycle [2, 3]. These pollutants cover a large array of chemicals, many of which are endocrine disruptors. Common endocrine disruptors are hormones, antidepressants, painkillers, personal care products, pesticides, plasticizers, and flame retardants [4–8]. These contaminants have been known to be present in the environment for decades, from sources such as wastewater treatment plant (WWTP) effluent [9–12]. Disinfectants are often highly complex products of active substances. Animal husbandry in veterinary used as growth promoters and used in agriculture also discharges drugs and their metabolites and disinfectants into the environment through liquid manure and wastewater; the substances may finally enter groundwater via soil [9, 13–16]. Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00017-1

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# 2020 Elsevier Inc. All rights reserved.

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An ideal purification technique should have a single-step purification procedure, with no or minimal use of solvent and minimal use of energy that is stable over a wide range of physicochemical parameters [17, 18]. A membrane acts as a barrier between two phases that allows substances to be selectively transported from one side to the other [19]. Membranes are classified as porous or dense, depending on their structure [20]. Transport properties and selectivity of a membrane are strongly dependent on its pore structure [21]. The applications of membrane-based processes of separation and purification include the following [18, 22–25]. 1. Osmotic membrane processes for production of drinking water for example. a. reverse osmosis (RO) b. forward osmosis (FO) c. pressure-enhanced osmosis (PEO) d. pressure-retarded osmosis (PRO) 2. Nanofiltration (NF) membranes used in the pharmaceutical industry for the advanced purification of active pharmaceutical ingredients (API). 3. Ultrafiltration (UF) membranes applied for colloidal particles and macromolecules and the removal of pathogens from dairy products. 4. Microfiltration (MF) membranes that allow the permeation of mono- and multivalent ions and of proteins and sugars. 5. Ion-exchange membranes used in electrodialysis (ED) to concentrate or remove salt ions and in practical applications in industry to produce cooling tower water, the desalination of seawater. 6. Membranes used in gas-liquid processes such as vapor permeation (VP) or pervaporation (PV) for the purification or removal of organic solvents, the dehydration of (bio)fuels, the breaking of azeotropes, and the recovery or removal of gases (H2, O2, CO2). 7. Membrane-Coated UiO-66 MOF for application in gas storage, gas/liquid separation, CO2 capture, catalytic reaction, and the removal of harmful substances. Membrane affinity is developed to permit the purification of molecules based on differences in physical/chemical properties or biological functions rather than in molecular weight/size [26, 27]. Rather than operate purely on the sieving mechanism, membrane affinity is based on the separation of the membrane selectivity to capture molecules, by immobilizing specific ligands onto the membrane surface [28]. The diffusivity of the substrate (target molecule), affinity, can also be understood as molecular recognition between the membrane and the substrate and can theoretically range from zero to infinity [18]. Molecular level control is very much attained through Langmuir-Blodgett (LB) and self-assembled monolayers [22].

2 Fabrication of membranes and biomembranes 2.1 Biopolymers used in membrane manufacturing The membranes act as a semipermeable barrier that regulates the transport of substances between two adjacent phases. The structure of the membrane that is related to the fabrication

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process will determine the effectiveness of the separation. Based on volume and concentration into a medium, membranes have important role in drug separation, the wastewater treatment, and the food, beverage, and pharmaceutical industries. Natural polymers with multilength-scale hierarchical structures are renewable, biocompatible, biodegradable, and low cost. Membranes derived from diverse natural polymers have marvelous mechanical properties, tunable optical properties, and so on. They have been applied in many fields such as energy generation and storage [29, 30], environmental engineering [31], tissue engineering [32], and green electronics [33]. Biopolymers can be produced from green and sustainable sources such as microorganisms, plants, and animals. 2.1.1 Biopolymers derived from bacterial fermentation products used for manufacturing membranes 2.1.1.1 Polylactic acid

Lactic acid is a chiral molecule that exists as L-lactic acid and D-lactic acid. Their copolymerization will result in amorphous or crystalline structures that will impact the final properties of polylactic acid (PLA). PLA can be obtained through two different pathways: by direct polycondensation of hydroxyl acid or by ring opening polymerization of cyclic lactide monomer. The stability of PLA membranes was also investigated in wet conditions at different temperatures by evaluating the change in their mechanical properties. The membranes were found to be stable at 25°C but degradable at 60°C (even without any microorganism). A novel preparation methodology for PLA membranes was recently proposed by Chinyerenwa et al. [34]. The filtration efficiency of porous poly(L-lactic acid) (PLLA) fibrous membranes was examined against aerosol particles from 30 to 100 nm [35]. As an aliphatic polyester, PLLA can be completely decomposed under the reaction of microorganisms, water, acid, and alkali in nature and finally produces carbon dioxide and water. 2.1.1.2 Polyhydroxyalkanoates

Many companies are involved in the production of polyhydroxyalkanoates (PHA). The market is still very small, but the fact that there are over 150 different types of PHAs that can be synthesized by employing different bacterial species opens new possibilities to this material, and it can be expected to be well presented in the near future. The solubility of PHA homopolymers is in general very low. Among the biopolymers, naturally produced by bacteria, the polyester belonging to the group of PHA, the poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV), has received growing attention due to its benign ecological and recycle properties [36]. PHBV has been used in combination with PLA to form novel blended membranes. By mixing these two polymers, the resulting membranes had improved strength and ductility compared with pure PHBV membranes [37]. Keawsupsak et al. [38] prepared biodegradable membranes for water treatment using PLA blended with poly(butylene succinate) (PBS), poly(butylene adipate-co-terephthalate) (PBAT) or PHBV via nonsolvent-induced phase separation using N-methyl-2-pyrrolidone (NMP).

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2.1.1.3 Poly(butylene succinate)

PBS is a semicrystalline biopolymer obtained by the polymerization of butanediol and succinic acid. The synthesis of PBS goes through a first esterification reaction of succinic acid and butanediol followed by a polycondensation of the obtained oligomers. The use of PBS for the fabrication of polymeric membranes has been limited due to its weak strength and poor mechanical resistance. To overcome this limitation, PBS is often blended to other polymers such as polyethersulfone (PES) [39] and cellulose acetate (CA) [40].

2.1.2 Biopolymers derived from vegetable sources used for manufacturing membranes 2.1.2.1 Cellulose-based polymers

Cellulose is a polysaccharide produced by plants. It is one of the main polymers present in nature consisting of long macromolecular chains of β-D-glucose units. Cellulose mainly derives from lignocellulosic biomass composed also of lignin and hemicellulose. Cellulose is as one of the most important natural polymers on the market. Cellulose ethers are used in different applications and can be processed when their propensity to absorb water is considered. Using CA membranes through the phase inversion approach, by incorporating Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposites, has increased the removal of pharmaceutical contaminants from wastewater. The hydrodynamic properties and adsorption capacity were evaluated with aqueous solutions of diclofenac sodium (DS) and tetracycline (TC), and the nanocomposite membranes showed an improved permeability compared with neat cellulose acetate. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux compared with the pure polymer one (529 vs 36 L m
2 h
1) and a 10-fold increase in adsorption capacity for DS [41, 42].

2.1.2.2 Alginate

Alginate is a natural polysaccharide extracted from algae, widely studied and employed for food and biomedical applications. Alginate consists in linear chain structures of 1–4 linked mannuronic acid and L-guluronic acid. It is generally extracted from brown algae (Phaeophyceae). Alginates that are rich in L-guluronate form strong but brittle gels, and those rich in D-mannuronate are weaker but more flexible [43]. Alginate has obtained much interest because of its strong hydration and antifouling behavior to oils. A research from Zhu exhibited that some kinds of solid surfaces coated with alginate had antiviscous oil-fouling behavior [44]. Novel separation composite membrane derived from porous Ca2+/alginate hydrogel thin film on commercial filter paper supporting substrate (alginate@FP) was fabricated via freezedrying and subsequent in situ gelation. The obtained alginate@FP composite has demonstrated superhydrophilic/underwater and superoleophobic characteristics, high permeation flux, high separate efficiency for emulsions and outstanding antioil-fouling performance. Also the alginate@FP could also remove organic pollutants and heavy metal ions from water [45].

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2.1.2.3 Polyisoprene

Polyisoprene rubber membranes have been produced and studied by Dobre et al. in nonstretched and stretched form [46]. They investigated the gas diffusion of hydrogen, oxygen, nitrogen, methane, and carbon dioxide through the membranes. From the data obtained, it was observed that the gas permeability was affected by the stretching intensity in the membrane. CO2 molecules, for instance, presented a permeability of 3187 barrer in nonstretched membranes. This increased up to 8290 barrer at the maximum evaluated membrane stretching. For nonpolar gases, however, the permeability was found to decrease as molecular size increased: hydrogen, 5131 barrer; oxygen, 1180 barrer; nitrogen, 971 barrer; and methane, 510 barrer in nonstretched membrane. Increasing stretching only slightly increased the permeability. Of course, that is dependent of the dimensions of film in the study. 2.1.2.4 Starch and cyclodextrins

One such material is starch, a naturally abundant material with oxygen-rich functional sites. Starch is also significantly hydrophilic however is not charged in solution and therefore of limited use in charged nanofiltration membranes. Starch is a natural polymer regenerated from carbon dioxide and water via photosynthesis in plants. It contains about 30% amylase, 70% amylopectin, and less than 1% lipids and proteins from plants. Starch-based biodegradable polymers have found attractive applications in food industry, as edible films and food packaging, but until now, only a few studies have been conducted with the aim of preparing polymeric membranes. Zarei and Ghaffarian [47] reported the fabrication of CA-starch biodegradable membranes via phase inversion, using glycerol as plasticizer additive. The presence of starch strongly was responsible of the membrane biodegradability, while hydrophilicitywas influenced by total polymer and starch content. Almasi et al. used carboxymethyl cellulose (CMC) and montmorillonite (MMT) as reinforcing filler in a starch matrix for the preparation of biocomposite films [48]. Starch and graphene oxide to synthesize thin-film composite nanofiltration membranes have been used. Graphene includes several moieties that acquire charge; therefore graphene oxide-starch composite can increase both hydrophilicity and charge. Graphene oxide-starch composites were synthesized by esterification. Functionalization of graphene oxide with starch adds to the functional groups present on edges of graphene oxide (GO) sheets creating a hyperbranched structure in the polyamide membrane [49]. Cyclodextrins (CDs) are cyclic oligosaccharides obtained by the enzymatic degradation of starch that can form inclusion complexes with many substances. The inclusion of guest molecules in CD cavities is mainly dependent on steric compatibility and polarity criteria. The forces governing complexation include high water repulsion energy in the CD cavity, van der Waals interactions, hydrophobic forces, and hydrogen bonds. 2.1.3 Biopolymers derived from animal sources used for manufacturing membranes 2.1.3.1 Chitosan

As a polysaccharide, chitosan (CS) can have a large density of reactive groups and a wide range of molecular weights. CS is a linear heteropolymer of N-acetyl-D-glucosamine and d-glucosamine linked by β-(1-4)glycosidic bonds. It is obtained by partial deacetylation of

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chitin, the second largest and most abundant polysaccharide in nature after cellulose. The degree of acetylation (DA) is an essential characteristic of CS. It represents the fraction of N-acetyl-D-glucosamine relative to the total number of units. Thanks to its hydrophilicity and good chemical resistance, CS has also been investigated for the preparation of dense membranes used in gas separation. However, pure CS membranes can present an excessive swelling degree in aqueous solutions making it necessary to cross-link or blend them with other polymers [50, 51]. Zhao et al. developed cross-linked N,O-carboxymethyl amphoteric CS on PES membrane with glutaraldehyde for BSA adsorption [52]. Moreover, Wang et al. used O-carboxymethyl CS with graphene oxide to enhance desalting properties [53]. Padaki et al. coated CS on the polypropylene membrane for desalination [54]. 2.1.3.2 Collagen

Collagen is the most abundant protein of animal connective tissue present in the skin, tendons, cartilage, and bones. The pervaporative performances of these membranes were evaluated for the dehydration of ethanol, isopropanol, ethylene glycol, and acetone. One of the major advantages of these membranes is the possibility to have high flux even at low temperatures. However, because of its protein nature, the membranes are sensitive to extreme pH, high temperatures, and microorganisms. Owing to its special triple-helical structure, which contributes to its unique biological, physical, and chemical properties and biodegradability, collagen has been widely used in food, cosmetic, biomedical, and pharmaceutical industries. Collagen molecules with a length and width of about 300 and 1.5 nm, respectively, are first assembled longitudinally, transversely, and horizontally through parallel staggering process into collagen microfibrils or fibrils with a crystal-like structure containing a repeated periodical D-banding of approximately 65 nm [55]. Three categories of collagens with different hierarchical architectures, including collagen molecules (Col), collagen microfibrils (F-col), and collagen fiber bundles (Agcol), were systematically biofabricated based on the biosynthesis pathway of natural collagen [56]. The Col, F-col, and Ag-col membrane materials were prepared by casting drying method. It was further found out that collagens with higher hierarchical structures had more superior thermal stability, mechanical properties, and biodegradability. The static mechanical properties of Ag-col, F-col, and Col had the tensile strengths of about 3.39, 1.55, and 1.42 MPa, respectively. The tensile strength of Ag-col was significantly higher than that of Col, suggesting that the obvious improved tensile strength of Ag-col is controlled not only by the microstructure of collagen material without the addition of adjuvants but also by the cross-linking agent. 2.1.3.3 Silk

Silkworm silk, a low-density structural protein, possesses a high content of β sheet or α helices with a core domain of random coils making it more crystalline and robust in nature. Silk fibers (e.g., Bombyx mori) are one of the strongest elastomeric natural biomaterials with exceptionally strong, extensible, and tough properties. Their typically nanofibrous structure, which is composed of nano-β-crystallites connected by amorphous chains, endows silk with these extraordinary performance values. Sericin is a macromolecular protein (17–18 types of amino acids) that is the major component of silk. For a long time, sericin, was considered as a

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waste by-product formed during the silk degumming process. Amino acid sequences of silk vary from species to species imparting unique physical and chemical properties to the protein, which substantiates its use as a biomaterial in numerous biomedical fields such as tissue engineering, in vitro models, and drug delivery. It has been developed as a high-efficient and scalable liquid exfoliation method to directly extract silk nanofibrils (SNFs) with controllable diameters from  20 nm (individual SNFs) to 100 nm and contour lengths of 0.3–10 μm from silk fibers using protein denaturant deep eutectic solvent (PD-DES) urea/guanidine hydrochloride [57].

2.2 Biopolymer-metal-organic framework The interface between a porous solid and a polymer has been explored in the mixed-matrix membranes (MMMs) literature. A wide variety of polymers for MMMs have been examined, including polysulfone, poly(vinyl acetate), poly(ether imide), polyimide (Matrimid), and polybenzimidazole. Although different types of metal-organic framework (MOF)-polymer combinations for MMMs have been discussed, application of these types of polymers to encapsulate a MOF pellet with a membrane coating has not been explored [58, 59]. The use of MOFs is growing. Their electrostatic, acid-base, π-complexation, H-bonding, and hydrophobic interactions and coordination to metal sites and framework functions play important roles in pharmaceutical adsorption. MOFs are produced as crystal powders, generally with submicron and micron size regime with low packing densities. This does not ensure their efficient processing and integration and therefore limit their deployment opportunities. Moreover, most of MOFs are produced from expensive and synthetically derived petrochemical precursors. Their preparation often implies time- and energy-consuming multistep synthetic procedures and requires toxic solvents and drastic conditions [60]. Polymer-MOF combination allows mitigating many shortcomings, encountered in MOFs, giving rise to stable and multifunctional composites. However, the use of synthetic polymers derived from petrochemical products along with the toxicity of most of them raises many environmental and health issues. Hence a step toward more innocuous materials is the incorporation of biomolecules within the framework of the crystalline MOFs or the use of biopolymers instead of synthetic motifs during their synthesis or as postmodifying reagents for the preformed MOFs [61]. Several types of MOFs have been used as precursors to yield porous carbons for a wide range of applications, such as gas adsorption [58], catalysis [62], rejecting of toxic heavy metal and ions from water and wastewater [59, 63, 64], and electrochemical supercapacitors [65]. In addition to these applications, MOF-derived porous carbons, due to their remarkable high surface area, thermal and chemical stability, and their tendency to interact with aromatic compounds, have acquired a great significance as sorbents for the removal of chemical pollutants [66]. UiO-66, with many predominant advantages such as tailorable structure, a high surface area, and superstability, is the most representative Zr-metal-organic framework. UiO-66 consists of octahedral [Zr6O4(OH)4] clusters with 1,4-benzenedicarboxylic acid. Owing to the electrostatic attraction and the π-π interaction between the benzene ring of the organic ligand of UiO-66 and the benzene ring of phenolic compounds, UiO-66 also showed high adsorption onto adsorbent for removal of phenolic compounds. To further expand the application of

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UiO-66 as a sorbent for removal of pollutants from water, it is necessary to study the adsorption behaviors of emerging pollutants, such as pharmaceutical and personal care products (PPCPs) [67]. Recently, some of the amino derivatives of MOFs were used for the adsorption of chemicals in different media; as a result, it has been found that -NH2 can promote the electrostatic interaction between MOFs and a substrate [68]. UiO-66 and UiO-66-NH2 were observed to be water-stable MOFs exhibiting high hydrothermal stability up to 773 K [69, 70]. Several types of biopolymers originated from natural resources are used for boosting the efficiency of MOF. The most ubiquitous are the polysaccharide family that includes CD, cellulose, chitin, CS, alginate, agarose, and polypeptides such as gelatin silk that are also used in this field as well as the PLA [61]. For example, Cu-BTC (benzene-1,3,5-tricarboxylic acid) was successfully bonded with cotton fabrics through interaction between Cu and cellulose functional groups. Adsorption of ethion as organophosphorus insecticide onto Cu-BTC@cotton composite was studied. Binding sites of composite represented in cellulose functional groups and Cu of MOF were both linked with ethion via sulfur [71]. Further the interactions would add to the chitin-copper interactions of pure chitin resulting in predominant nucleation of the MOF crystallites inside the hollow fibers [72]. Aluminum-based MOF/sodium alginatechitosan (Al-MOF/SA-CS) composite was employed for the removal of bisphenol A in industrial wastewater [73]. It is even a study based on diffusive gradients in thin films (DGT) with a high capacity binding gel containing porous carbon material (PCM) derived from zeolitic imidazolate framework and for in situ measurement of sulfonamides (SAs), fluoroquinolones (FQs), and trimethoprim, frequently detected antibiotics in environmental water. The diffusive coefficients of antibiotics were measured simultaneously by the diaphragm diffusion cell method. The cell contained source and receptor solutions, which were separated by an agarose diffusive gel of 0.80-mm thickness and a PES membrane of 0.13-mm thickness [74]. Lo´pez-Maya et al. deposited UiO-66 on silk fibers. Both obtained a loading of approximately 50% (w/w) on silk fibers [75]. The Carlos Martı´-Gastaldo group has reported the use of a chiral Cu(II) three-dimensional MOF based on the tripeptide Gly-L-His-Gly (GHG) for the enantioselective separation of methamphetamine and ephedrine [76]. A biocatalytic membrane with immobilized laccase for micropollutants removal has been reported by Ren et al. [77]. The performance of polymer-MOF membranes depends massively on the particle size of the MOF crystals and their uniform coating on the material surface. These two parameters need to be balanced to prevent defect when the crystals are bigger and pore clogging of the support if the crystals are too small. The use of CS as an interfacial compatibilizer or a binding agent circumvents some of these drawbacks and accounts for the successful formation of ternary membrane-based composites. Other example is mixing cellulose and MOF particles and then processing such hybrid composite and forming pellets and membranes/films through compression and extrusion [71].

2.3 Membranes and biomembranes produced by electrospinning process The electrostatic spinning or electrospinning is a technique where submicron fibers can be obtained by applying an electrostatic field to a polymer melt or solution. It is a highly versatile technique in that the surface topography, fiber morphology, and orientation are largely

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dictated by solution properties and operating conditions. Since the rheology of the polymer solution is vital to the fiber formation process, solution properties such as polymer molecular weight and concentration directly affect fiber properties [21]. Membrane-based technologies have gained popularity over the last few decades due to their high separation efficiencies, relatively low costs, and ease of operation. These increasing interests from researchers and private industries come from the numerous applications that nanofibers have on tissue engineering, water and air filtration, controlled drug delivery, protective clothing, and carbon microelectromechanical system. Besides the application, another attractive feature about electrospinning is its relatively low cost and straightforward process compared with other techniques for fiber fabrication. Cellulose acetate (CA) nonwoven mesh with fiber diameter ranging from 200 nm to 1 mm was prepared by electrospinning technique. The use of electrospinning improved membrane affinity that was developed to permit the purification of molecules based on physical/chemical properties or biological functions rather than molecular weight/size. Rather than operate purely on the sieving mechanism, membrane affinity is based on its separation within the selectivity of the membrane to immobilize specific ligands onto the membrane surface. In consequence a commonly used membrane material, polysulfone (PSU), was electrospun into nonwoven fiber mesh, then heat treated, and surface modified [26, 28]. The electrospun nanofibers can also be potentially applied in areas such as nanofiltration, membrane distillation, geothermal water desalination, and capacitive deionization applications. A high flux thin-film nanofibrous composite (TFNC) membrane based on polyacrylonitrile (PAN) nanofibers, coupled with a thin layer barrier of cross-linked polyvinyl alcohol. With a middle-layer PAN scaffold with porosity of 85% and a cross-linked PVA layer barrier with an approximate thickness of 0.5 μm, the TFNC membrane system was tested for ultrafiltration (UF) applications. A hot-pressing step is necessary to improve the integrity of the nanofiber membranes and the nanofiber/backing layer in the case of TFNC membranes in NF application [78]. Several studies have reported the incorporation of CD by blending in electrospinning polymer solutions or functionalization of the surface of electrospun nanofibers for adsorption of organic molecules from water and wastewater applications [5], a series of polyacrylonitrile (PAN)/activated biochar nanofibrous membranes (NFMs) [79], with different loadings of biochar (0%–2%, w/w) were fabricated using electrospinning for the removal Chlortetracycline (CTC) is a broad-spectrum antimicrobial agent, commonly used as veterinary medicine for poultry, swine, and livestock from water.

2.4 Molecularly imprinted membranes and biomembranes Molecularly imprinted polymers (MIPs), based on the molecular imprinting technology, generate specific recognition sites and have been increasingly applied in many fields, including solid-phase extraction, chromatographic separation [80], and chemical sensor development [81]. Molecular imprinting is a technique used to induce molecular recognition properties in synthetic polymers in response to the presence of a target molecule acting as a template during the formation of the three-dimensional structure of the polymer [82, 83]. Two basic approaches can be distinguished based on the interactions used in the imprinting step:

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1. The covalent approach, in which functional monomers and templates are bound to each other by covalent linkage prior to polymerization. 2. The noncovalent approach, in which the prepolymerization arrangement of the template and the functional monomers is formed by noncovalent interactions such as ionic interactions or hydrogen bonding. Following polymerization the template can be removed by solvent extraction. Noncovalent molecular imprinting has become increasingly relevant, and many chemists have constructed molecularly imprinted polymeric materials using this method since Arshady and Mosbach reported what they called a simple template strategy for the synthesis of substrate-specific polymers, which is based on complementary interactions [84]. The noncovalent molecular imprinting approach is based on forces such as hydrogen bonding, van der Waals forces, hydrophobic interactions, and/or π-π stacking that are involved in the print molecule-matrix clusters and further in the molecular recognition process [18]. The recognition of a target molecule by molecular recognition sites in molecularly imprinted materials corresponds to the incorporation of a permeant into the membrane in membrane transport. If the membrane diffusion process within the membrane and dissociation of a target molecule from a molecular recognition site or release of it from the membrane are accompanied by a molecular recognition process, then membrane transport with permselectivity would be attained. Localization of molecular recognition sites on the membrane surface is expected to give high membrane performance. Toward this end the photofunctionalization of microfiltration membranes was applied to obtain MIMs [85]. Molecular imprinting works similarly to a lock with a single compatible key configuration. It also resembles some biological systems, such as antibodies and antigens, enzymes and substrates, or hormones and receptors. Membrane separation technique (MST) is a comparatively ideal method that is widely used in many fields, such as solid-liquid extraction, drug purification, water treatment, oil refinement, and gas separation. Atom transfer radical polymerization (ATRP) is one of the most versatile and robust techniques to synthesize well-defined polymers. Compared with traditional polymerization process, ATRP offers some advantages such as an initiator being anchored on the membrane surface in advance, enabling the polymerization process to only occur on the surface and avoiding embedding of the template molecules [86]. Another method that can be included in the subdomain of alternative molecular imprinting is the phase-inversion method, which involves the coagulation of a polymeric solution in a nonsolvent. In the case of MIP preparation, the polymeric solution contains the dissolved matrix-print molecule complex. As an application of alternative molecular imprinting to membrane separation, chiral separation has been intensively studied. In terms of enantioselectivity, when a concentration difference is used as the driving force for membrane transport, the transport of the enantiomer was delayed compared with that of its antipode [87]. The fact that the permselectivity remains opposite to the adsorption selectivity can be attributed to the suppression of the diffusivity of the preferentially adsorbed enantiomer because of its relatively high affinity to the membrane compared with the affinity between the antipode and the membrane. The compromise is given by the fact that enhancement of the flux through the membrane usually leads to a simultaneous reduction in permselectivity and vice versa. Hence, it is

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3 Theoretical factors considered for membrane separation

important and indispensable to enhance both key factors so that MIMs will be applicable in various industries. MIMs are required to have high porosity to ensure a high surface area so that they can provide a higher flux and permselectivity. The recognition of a target molecule by a nanofiber fabric bearing molecular recognition sites corresponds to the uptake or incorporation of a permeant into a membrane during a membrane transport process. Nanofiber membranes bearing molecular recognition sites have also been fabricated by the simultaneous application of alternative molecular imprinting and electrospray deposition. Such prepared nanofiber materials are called molecularly imprinted nanofiber membranes (MINFMs). The setup for the fabrication of nanofiber materials is simple, and the preparation is relatively easy under ambient conditions.

3 Theoretical factors considered for membrane separation The flux (J) of membrane is calculated by J ¼ V=ðA  ΔtÞ

(1)

where V is the volume of the permeate (L), A is the membrane area (m ), and Δt is the sampling time interval. The transport process of substrate through the membrane can be placed in two categories corresponding to the two main membrane types: transport through a nonporous (dense) membrane and transport through a porous membrane [18]. The membrane-transport phenomena for the former case can be explained by a solution-diffusion theory. Thus the permeability coefficient can be represented as 2

Pi ¼ Di Si

(2)

where Di and Si refer to the diffusion coefficient and the solubility coefficient, respectively. Membrane transport through a porous membrane; the permeability coefficient can be written as Pi ¼ Di Ki

(3)

where Ki is the partition coefficient and the ratio between the equilibrium concentration of substrate i in the membrane and the concentration in the solution is equilibrated with the membrane Si ¼ Cm =Co or Ki ¼ Cm =Co

(4)

where the concentration in the equilibrium is Cm and Co is the initial concentration. The solubility and partition coefficients are parameters that are determined thermodynamically, whereas the diffusion coefficient is determined kinetically. The temperature is important factor to determine the process of adsorption. It may influence the adsorption rate behavior in the kinetic process and the maximum adsorption capacity at equilibrium process. Moreover the dependence of the adsorption on temperature provides thermodynamic information, which can be used to evaluate the nature of the adsorption system.

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3.1 Analysis of the adsorption equilibrium The most frequently used equations, Langmuir and Freundlich isotherm models, are used to analyze the isotherm data for the purpose of optimizing the design of an adsorption system [88]. It is also an important step to establish the suitable correlation for equilibrium conditions. The Langmuir isotherm (Eq. 5) and the maximum adsorption capacity determinate using the linearized form (Eq. 6) are empirical models based on the following assumptions involving homogeneous adsorption. First the sorption takes place at specific homogeneous sites within the adsorbent. Second, no further sorption can take place. Third the adsorption capacity of the adsorbent is finite. Fourth the size and shape of all sites are identical and energetically equivalent [89]. The Freundlich model (Eq. 7) whose linearized form is expressed by Eq. (8) is suitable for a highly heterogeneous surface composed of different classes of adsorption sites. This model mainly has two assumptions: First, with the increase of surface coverage of adsorbent, the binding strength gradually decreases. Second the adsorption energies of active sites on the surface of adsorbent are different [90] qe ¼

qm KL Ce 1 + KL Ce

(5)

Ce Ce 1 ¼ + qe qm qm KL

(6)

qe ¼ KF Ce 1=n

(7)

log ðqe Þ ¼ log ðKF Þ + 1=n log ðCe Þ

(8)


1

where qm is the maximum adsorption capacity (mg mL ), KL is the Langmuir equilibrium constant related to the free energy of adsorption (L mg
1), Co is the initial concentration of adsorbate (mg L
1), KF is the Freundlich constant or the value of qe at Ce ¼ 1 mg L
1 (mg mL
1 (L mg
1)1/n), and n is heterogeneous factor related to intensity of adsorption. Another common isotherm model, the Dubinin-Radushkevich (D-R) model, is always used to test the equilibrium data. The model is given as the following equations:
(9) qe ¼ qD
R exp
KD
R ε2   1 ε ¼ RT ln 1 + (10) Ce where qD-R is the maximum adsorption capacity (mg mL
1), KD-R is the D-R constant (mol2 J
2) related to the sorption energy, ε is the Polanyi potential (J mol
1), R is the gas constant (8.314 J mol
1 K
1), and T is the absolute temperature (K).

3.2 Adsorption kinetics The use of kinetic models is extremely important in removal studies, since they enable elucidation of the adsorption mechanism and the mass transport behavior in the solution. Various kinetic models can be used to describe these processes. To investigate the adsorption

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mechanisms of pharmaceutical, various kinetic models have been applied, pseudo-first order (Eq. 11) and pseudo-second order (Eq. 12):
qðtÞ ¼ qe 1
e
K1 t (11) qðtÞ ¼

K2 qe 2t 1 + K 2 qe t

(12)

where q(t) is the amount of sorbent adsorbed at any time t (mg g
1), qe is the amount of solute adsorbed onto the adsorbent at equilibrium (mg g
1), K1 is the rate constant of pseudo-first order model (min
1), t is the time (min), and K2 is the rate constant of the pseudo-second order model. The pseudo-first order model assumes that the adsorption rate is directly proportional to the difference between the amounts of hormone removed at a given time and at equilibrium, while the pseudo-second order model assumes that the adsorption rate is proportional to the square of the difference between the amounts of hormone adsorbed at a given time and at equilibrium. Since the adsorption sites are relatively uniform, the description of adsorption by the pseudo-second order kinetic model indicates that more than one active site is being used by the adsorbate [79].

4 Removal pharmaceuticals and hormones Pharmaceuticals are designed to be chemically stable; however, they undergo physicochemical and biotic transformations. Thus understanding pharmaceutical biodegradability, conjugation and deconjugation, metabolic pathways, persistence, and sorption is required to predict their environmental fate. Many pharmaceuticals are ionizable in water, existing as charged or neutral species depending on solution pH. Ionic interactions play an important role in sorption; thus pH governs the sorption for many of these compounds. Ciprofloxacin sorption efficiency is an example [91, 92]. In general, mass transfer through a membrane can be understood as two successive processes: the solute’s molecules are first adsorbed into the membrane’s pores and then transported by convection (diffusion and/or advection) through the membrane. In this transport process the membrane separation performance depends on the incorporation of substrate into the membrane due to either solubility or partition. Moreover the diffusion of a target molecule within a membrane is governed by molecular size, shape, and electronic charge. The fate, transport, and biological impact of endocrine-disrupting compounds (EDCs) in surface waters are controlled by several attenuation processes, of which photolysis, sorption, and biodegradation are believed to be the most significant [92–95]. Writer et al. studied the biodegradation and attenuation of model EDCs by stream biofilm, sediment, and water matrices to better understand EDC fate in surface waters [96]. Model EDCs included the steroidal hormones, 17β-estradiol, estrone, 17α-ethynylestradiol, and the alkylphenol and alkylphenolpolyethoxylate compounds, 4-nonylphenol, 4-nonylphenolmonoethoxylate, and 4-nonylphenoldiethoxylate. The microporous composite membrane polysulfone was used for ultrafiltration membrane production for macrolide antibiotics, which were coated

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with polyvinyl alcohol (PVA) to improve the hydrophilicity and smoothness of the membrane surface to decrease fouling [97]. On the other hand the interaction of ibuprofen and sulfamethoxazole using bilayer membrane of polyelectrolytes CS/polystyrene sulfonate on polyamide microfiltration membrane (MF) has also been studied; here the phosphate interferes negatively with the removal efficiency [22]. CS composed of β-1,4-oligoglucopyranosides has desirable membrane-forming properties, excellent biocompatibility, and good hydrophilicity, finding wide applications in membrane-mediated separation process [98]. CD, a molecule composed of cyclic α-1,4-oligoglucopyranosides, is host molecule possessing hydrophobic cavity and a hydrophilic external surface. CS is diffusion selective, while CD is sorption selective during membrane-mediated resolution process. Semiinterpenetrating networks of structured CS/β-CD composite membrane were prepared and studied in respect to permeselectivity toward tryptophan enantiomers. This was designed to study the effect of the CD presence on the CS membrane in enantiomer separation [99, 100]. The CS-modified membranes showed higher water flux and increase in water flux with increasing temperature as compared with the analog TFC RO membranes without losing their salt rejection capability. This is promising in connection with solar-powered RO, where low-grade thermal energy is used to heat the feedwater. Egusa et al. studied the effect of using nanofibrillation on the CS properties [101]. Nanofibrillated CS was used to prepare nanofiber sheets under neutral operation conditions. On the other hand, Kamrani et al. enhanced the quality of membrane surface by CS to eliminate diphenhydramine and mebeverine from effluents [102]. This membrane had the best performance for diphenhydramine (97%) and mebeverine ( 98%) at pH ¼ 3 with the least fouling (22.6 L m
2 h
1). Modification of membrane surface was applied by reacting amine functional groups in CS chains with unreacted acyl chloride. Thermal resistance and mechanical properties can be a problem for the fabrication of PLA membranes; however, strategies such as blending PLA with other polymers and compounding PLA with various fillers or composites had great impact. PLA is widely used in health and medical science to prepare tissue engineering scaffolds for human cell growth and supports for controlled drug release; however, it has only recently been considered as a material for manufacturing membranes for water treatment applications [103]. Hydrogels are moderately cross-linked polymers that can swell significantly but cannot be dissolved in water. Hydrogels have been shown to provide a dynamic and permissive microenvironment, supportive of native-like extracellular matrix functions [104] also exhibiting nonhemolytic behavior and biocompatibility toward animal cells in numerous studies [105]. The most recent achievements relating to the application of hydrogel nanocomposites for the detection and removal of contaminants are of recent origin [106]. As a polysaccharide, alginic acid can have a large density of reactive groups and a wide range of molecular weights. Alginic acid is a linear heteropolymer of L-guluronate that provide brittle gels and D-mannuronate that is the flexible part of its structure linked by (1,4)-linked β-D-mannuronic (M) and α-L-guluronic (G). Alginate presents high biocompatibility, low toxicity, and ease of gelation by addition of divalent cations such as Ca+2. In membrane production, alginate has been particularly exploited for its hydrophilic nature in connection with filtration of aqueous solutions and for the dehydration of solvents. Alginate is a natural bioinspired material, environmentally friendly, and harsh environment tolerant.

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De Moraes et al. studied the preparation of blend alginate-CS membranes for herbicide adsorption [107]. The obtained membranes exposed on their surface binding sites for positive/ negative charged pesticide molecules. De Moraes et al. described the preparation of bilayer membranes to adsorb Paraquat, Diquat, Difenzoquat, and Clomazone herbicides from contaminated water. In particular the blend biopolymeric membranes showed the capability to adsorb positively charged compounds on the alginate layer and, at the same time, negatively charged herbicides in the CS layer [107]. The effect of amending micropollutants during upstart of an MBR system has been examined by the ability to mineralize selected micropollutants. The study explored the evolution and stability of the involved microbial communities and their correlation with the removal ibuprofen, naproxen, and diclofenac [108, 109]. Additionally, the analysis of these pharmaceutical products has been analyzed by liquid chromatography with hybrid linear ion-trap mass spectrometer equipped with a polar reversed-phase column to achieve good separation and minimize matrix [110] and UPLC-tQ-MS methods [111]. Direct contact membrane distillation (DCMD) and forward osmosis (FO) were investigated for wastewater treatment in space. Retention of natural steroid hormones, estrone, 17α-ethinylestradiol, and 17β-estradiol, by these two processes was evaluated [90, 112–116]. Adsorption together with size exclusion and charge attraction/repulsion must be taken into account when considering removal of pharmaceuticals as emerging contaminants from water by RO and NF using the commercial membranes XLE, LFC-1, CPA3, SWC1, NF90, and NF270 for the removal of glucocorticosteroids, dexamethasone, and the anesthetics procaine and lidocaine [25, 117]. Moreover, the feasibility of formation of inclusion complexes between estrone and estradiol with β-CDs was investigated. A study conducted with the CD polymer showed that the material can affect the rapid removal of organic micropollutants such as pesticides and pharmaceuticals in an aqueous medium [118]. A research group [119] has studied the ability of MT to form complexes with free β-cyclodextrin (bCD) revealing high affinity of the steroid for the hydrophobic CD cavity, with an association constant (ka) of 7540 L mol
1. The organic-inorganic hybrid composite was produced by the functionalization of silica with bCD using citric acid as a binder for adsorption of the hormone 17α-methyltestosterone steroid. The adsorption was observed at acid pH, with a capacity of 11 mg g
1. The kinetic and isotherm models indicated that the adsorption occurred by a physical mechanism at independent sites with the steroid molecule possibly captured by two bCDs [79]. Because the important fibrillar structure of natural silks was preserved, free-standing SNF membranes fabricated by using the vacuum filtration of SNF dispersions exhibited nanoporous structure, solvent insolubility, excellent mechanical properties, and thermal stability. SNF membranes showed size selectivity and adsorption performance for ions, dyes, and protein molecules when used as protein-based filtration membranes. SNF membranes with a unique structural features and reinforced properties have potential applications in energy storage devices and environmental engineering. Sericin, a by-product of the silk degumming process, has found its application in biomedical science, regenerative medicine, textile industry, and membrane fabrication. Sericin exhibits inherent properties like hydrophilicity, and it is known to be amphoteric, antioxidant, antifouling, antimicrobial, and nontoxic in nature [120–124]. These physicochemical properties make it very suitable as a biosorbent to enhance membrane surface properties for the selective removal ibuprofen from aqueous environment.

III. Application of biopolymers membranes/films in environment and energy

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17. Pharmaceutical and synthetic hormone removal

Using surface adsorption as a parameter, Semia˜o and Sch€afer studied the adsorption and retention of estrone and estradiol using polysulfone, polyester, and polyamide membranes [125]. Among all the membranes tested, polyamide NF membranes showed the highest hormone adsorption. The size of the pores in association with steric exclusion and pH of the medium were also crucial for surface adsorption of hormones at neutral pH and high solute, and membrane interaction was attributed to electrostatic repulsive effect of the solute from the membrane surface [125]. Several researchers have developed tighter and more charged membranes for creating cellulose acetate (CA) NF membranes that incorporate charged surface-modifying macromolecules (CSMM). These membranes were evaluated using ultrapure water spiked with parts per million levels of carbamazepine (CBZP), ibuprofen (IBUP), and sulfamethazine (SFMZ) [126]. CBZP is an anticonvulsant and mood-stabilizing drug used mainly in the treatment of epilepsy, bipolar disorder, and trigeminal neuralgia. IBUP is widely used to treat pain, fever, and inflammation in humans. SFMZ is a widely used antimicrobial agent added to the feed of meat-producing animals to treat infections. Cellulose acetate membranes are usually fabricated by phase inversion to obtain a flat sheet polymer membrane. Due to highly crystalline and hydrophobic character of the fibers, cellulose can be used as a reinforcement increasing the tensile strength and Young’s modulus without altering the elongation at the break point [127]. Other researchers describe the preparation of a multifunctional membrane, which can separate, adsorb, and catalyze the pollutants simultaneously, and studied its potential application on the removal of bisphenol A (BPA), an endocrine disruptor widely found in aquatic environments, which has an irreversible detrimental effect on human. Laccase, a ligninolytic, was chosen as the enzyme of interest due to its activity toward various micropollutants including BPA [128, 129]. Suitable NF membranes, like polyamides and poly(pyrazinamide) [130] and polyacrylonitrile-biochar composite nanofibrous membrane [79], were selected based on their physical properties (e.g., permeability, rejection, and adsorption capacity) and their enzyme immobilization efficiency (e.g., enzyme loading and activity) and covalent bonding [131, 132]. A combined electrochemical technique with HPLC studies in order to obtain information about the dynamics of possible complexation estrone and estradiol with CD, cyclic compounds that have been widely used in the pharmaceutical industry due to their ability to form inclusion complexes [118]. Ejhed et al. studied the physical and chemical properties of several micropollutants and the effect of hydraulic retention time (HRT) [119]. Although a longer HRT would be expected to have a positive effect on the removal of micropollutants, this has not been shown in any full-scale test of different types of on-site wastewater treatment facilities (OWTF). Table 1 exhibits the removal of different pharmaceuticals and the membrane used. A longer relaxation time was used to maintain a stable HRT and avoid excessive membrane fouling [126]. Further study investigated the occurrence of sulfonamides, macrolides, and trimethoprim in municipal wastewater treatment plants [120–122], as well as sulfonamide antibiotics [123]. The removal efficiency for micropollutants was studied at the scale of a municipal wastewater treatment plant (WWTP) upgraded with postozonation [124]. Compounds more resistant to oxidation by ozone such as atenolol and benzotriazole were increasingly eliminated with increasing ozone doses [125].

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4 Removal pharmaceuticals and hormones

TABLE 1

Removal of different pharmaceuticals and the membrane used.

Membrane

Pharmaceutical

Refs.

Hollow fiber and hollow sheet

Naproxen, ibuprofen, diclofenac, ketoprofen, and gemfibrozil

Arriaga et al. [108]

Polypropylene and cellulose triacetate

Estrone and 17β-estradiol

Cartinella et al. [112]

Sheet membranes

Aceclofenac, carbamazepine, diclofenac, enalapril, and trimethoprim

Celiz et al. [110]

Acrylonitrile copolymer with acrylic acid Diosgenin and stigmasterol

Dima et al. [83]

Polyamide

Hydrocortisone, dexamethasone, procaine, and lidocaine

Do´lar et al. [25]

Cellulose triacetate and polyamide

Carbamazepine, diclofenac, ibuprofen, and naproxen

Jin et al. [33]

Polyamide

Erythromycin, estriol, naproxen, and others

Kim et al. [113]

Polyamide

Tetracyclines, sulfonamides, and hormones

Koyuncu et al. [133]

Polyvinylidene fluoride

Virus

Lee et al. [134]

Polyamide thin-film composite

Carbamazepine, triclosan, ibuprofen, sulfadiazine, sulfamethoxazole and sulfamethazine

Lin et al. [114]

Cellulose nanofiber

Bovine serum albumin

Ma et al. [26]

Hollow-fiber membrane

Pharmaceuticals, endocrine disruptors, personal Monsalvo et al. [135] care products and pesticides

Regenerated cellulose UF membranes with a polypropylene

Estrone, 17b-estradiol, progesterone, and testosterone

Neale et al. [136]

Thin polyamide skin layer on top of a microporous polysulfone support

Sulfamethoxazole, carbamazepine, and ibuprofen

Nghiem et al. [137]

Poly(ether sulfone) nanofibers impregnated with β-cyclodextrin

Estradiol and the pesticide chlorpyrifos

Sch€afer et al. [5]

Polyamide active layer with polysulfone and polyester

Estrone and 17β-estradiol

Semia˜o et al. [138]

Polyamide, polyethylene terephthalate, Estrone and 17β-estradiol polyethylene naphthalate and polysulfone

Sch€afer et al. [5]

Polyamide

Endocrine disruptors, pharmaceuticals, and personal care products

Snyder et al. [115]

Polyamide

Perfluorohexanoic acid

Soriano et al. [12]

Composite polyamide

Nitrosoalklyamines

Steinle-Darling et al. [123]

Polyacrylonitrile/biochar

Chlortetracycline, carbamazepine, and diclofenac

Taheran et al. [131] Continued

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414

17. Pharmaceutical and synthetic hormone removal

TABLE 1 Removal of different pharmaceuticals and the membrane used—cont’d Membrane

Pharmaceutical

Refs.

Chitosan/polystyrene sulfonate

Ibuprofen, sulfamethoxazole, carbamazepine, Thomas et al. [22] estradiol, lidocaine, bisphenol A, and bisphenol S

Thin-film composite membranes with a Pharmaceutically active compounds cross-linked aromatic polyamide top layer

Verliefde et al. [117]

Silk sericin

Ibuprofen

Verma and Subbiah [124]

Chitosan/β-cyclodextrin

Enantiomer separation of tryptophan racemate

Wang et al. [99]

Stream biofilm (epilithon)

Steroidal hormones and alkylphenols

Writer et al. [96]

Chitosan

Artemisinin

Zhang et al. [86]

Reverse osmosis and micro-, nano-, and ultrafiltration have been used for pharmaceutical elimination. These are filtration processes requiring a selective semipermeable membrane [127]. The pressure difference between filtrate and feed sites of the separation membrane is the driving force during reverse osmosis. Micro- and ultrafiltration successfully eliminate a significant level of pharmaceuticals, but nanofiltration and reverse osmosis achieve higher levels of removal. Therefore the removal efficiencies of different compounds vary widely. Removal depends upon the pharmaceutical’s physicochemical properties, molecular size, charge, hydrophobicity, polarity, diffusivity, and solubility; the membrane properties, surface charges, hydrophobicity, pore size, and permeability; and operating conditions, flux rate, transmembrane pressure, feedwater quality, and the rejection/recovery potential of the membrane [128, 129]. An experimental study using regenerated cellulose UF membranes with a polypropylene support layer for obtaining organic matter-water partition coefficients (KOM) was applied to quantify and elucidate the influence of solute-solute interactions for steroidal hormone removal by ultrafiltration. The results indicated that the removal of all hormones increased in the presence of organic matter, and this was related to hormone-organic matter interactions [139]. For all membrane materials, it is critical to consider factors other than permeability and selectivity in material design [130]. Advanced filtration processes such as nano-, micro-, and ultrafiltration effectively remove pharmaceuticals from wastewater. But membrane fouling, high operational cost, and high energy demands could limit their applications [131].

5 Conclusions and future outlook Synthetic chemistry has historically been a powerful force in the discovery of new medicines and has an even greater impact to accelerate the pace of drug discovery and expand the reach of synthetic chemistry. However, pharmaceuticals have been widely detected in effluents from wastewater treatment plants and river water. Because of their biological activity, concerns about their potential risks to aquatic organisms have been raised. Steroidal hormones are also considered emerging contaminants in wastewater worldwide, due to their

III. Application of biopolymers membranes/films in environment and energy

References

415

widespread presence and adverse effects on both aquatic organisms and humans. On the other hand, several reports have been produced on the treatment of pharmaceutical compounds and endocrine-disrupting chemicals in recent decades. Most of the treatment technologies deal with the treatment of wastewaters from chemical and fermentation processes. Use of hybrid technologies has been made for the treatment of certain compounds, which are not completely eradicated by the single-stage treatment. The use of hybrid technologies mainly removes the pollutant almost completely or within safe discharge limits. This shows the promising nature of these kinds of systems in the removal of emerging contaminants from aqueous environmental streams. However, further research is needed to increase the adsorption capacity of fabricated membranes to compete with commercial adsorbents. It will require a step change in knowledge to understand the link between disruption of multiple interacting pathways and their consequent effects on wildlife and human health. In doing so, it has laid the groundwork for an environmentally friendly culture in the chemical discipline. And of course, further steps need to be taken toward sustainability. The current research in membrane science is more and more focused on the possibility of using alternative polymers derived from natural raw materials for the development of new membranes.

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Further reading M. Patel, et al., Pharmaceuticals of emerging concern in aquatic systems: chemistry, occurrence, effects, and removal methods, Chem. Rev. 119 (6) (2019) 3510–3673. C. Yan˜ez, J. Basualdo, P. Jara-Ulloa, J.A. Squella, Inclusion complexes of estrone and estradiol with b-cyclodextrin: voltammetric and HPLC studies, J. Phys. Org. Chem. 20 (7) (2007) 499–505. A. G€ obel, et al., Occurrence and sorption behavior of sulfonamides, macrolides, and trimethoprim in activated sludge treatment, Environ. Sci. Technol. 39 (11) (2005) 3981–3989. E.C. Lima, Removal of emerging contaminants from the environment by adsorption, Ecotoxicol. Environ. Saf. 150 (2018) 1–17. D.J. Larsson, C. de Pedro, N. Paxeus, Effluent from drug manufactures contains extremely high levels of pharmaceuticals, J. Hazard. Mater. 148 (3) (2007) 751–755. N. Le-Minh, R.M. Stuetz, S.J. Khan, Determination of six sulfonamide antibiotics, two metabolites and trimethoprim in wastewater by isotope dilution liquid chromatography/tandem mass spectrometry, Talanta 89 (2012) 407–416. N. Nakada, et al., Removal of selected pharmaceuticals and personal care products (PPCPs) and endocrine-disrupting chemicals (EDCs) during sand filtration and ozonation at a municipal sewage treatment plant, Water Res. 41 (19) (2007) 4373–4382. J. Hollender, et al., Elimination of organic micropollutants in a municipal wastewater treatment plant upgraded with a full-scale post-ozonation followed by sand filtration, Environ. Sci. Technol. 43 (20) (2009) 7862–7869. K.C. Wijekoon, et al., The fate of pharmaceuticals, steroid hormones, phytoestrogens, UV-filters and pesticides during MBR treatment, Bioresour. Technol. 144 (2013) 247–254. P. Verlicchi, A. Galletti, M. Petrovic, D. Barcelo´, Hospital effluents as a source of emerging pollutants: an overview of micropollutants and sustainable treatment options, J. Hydrol. 389 (3–4) (2010) 416–428. H.B. Park, et al., Maximizing the right stuff: the trade-off between membrane permeability and selectivity, Science 356 (6343) (2017) eaab0530. M.B. Ahmed, et al., Progress in the biological and chemical treatment technologies for emerging contaminant removal from wastewater: a critical review, J. Hazard. Mater. 323 (2017) 274–298.

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C H A P T E R

18 Biopolymer membranes in fuel cell applications Justyna Walkowiak-Kulikowska, Joanna Wolska, Henryk Koroniak Faculty of Chemistry, Adam Mickiewicz University in Poznan, Poznan, Poland

1 Introduction The constantly growing demand for energy driven by technological progress, increasing global population and depletion of fossil fuel resources, combined with the growing need to limit or eliminate negative impacts on the natural environment, have recently lead to a clear intensification of research and development into alternative and renewable sources of energy. Many countries and industrial sectors considered the Kyoto Protocol a milestone and a universal guidepost for their efforts to prevent global warming [1,2]. That resulted in accelerating the process, and it seems that there are promising new technological solutions available. This includes fuel cells. Developing compact, reliable fuel cell systems supports more efficient use of fossil fuels and conversion to new sources of energy that limit carbon dioxide emissions, leading to a reduction in environmental pollution. Commercializing the currently available technologies is still limited by such factors as the high cost of producing platinum-based electrode catalysts and Nafion membranes, the insufficient stability of membrane-electrode assemblies (MEA), unsatisfactory lifetime of cells in operating environments, and complexity of water handling in the systems. One of the difficulties could be resolved by making membranes from natural materials or their derivatives that are readily available and easy to recycle and have well-controlled structure and the desired conductivity. The necessary conductivity, thermal stability, and mechanical durability as well as the ease of physical processing is a feature of polysaccharides and, to an extent, proteins and nucleic acids.

Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00018-3

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# 2020 Elsevier Inc. All rights reserved.

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18. Biopolymer membranes in fuel cell applications

2 Fuel cells Fuel cells are capable of revolutionizing the fuel market and make us independent of fossil resources because they offer very efficient systems, not limited by the Carnot cycle, generating energy from hydrogen or methanol, and so carbon-neutral materials. In essence, fuel cells are electrochemical devices for the direct conversion of energy from the fuel’s chemical bonds to electricity, which is accompanied by the release of water and heat [3]. The sole requirement for ensuring the continuity of the process is supplying reactants and removing the waste products. At the heart of most fuel cells lies a membrane-electrode assembly (MEA) consisting of a cathode and anode separated by an ion-conducting electrolyte membrane. Electrodes are made of porous carbon covered by precious metal catalyst nanoparticles, usually platinum. The oxygen and hydrogen needed for reaction are supplied through gas diffusion layers (GDL). Single fuel cells can be connected in series into assemblies called stacks. A stack’s power output can be regulated by the number of the cells. PEFCs are classified by the type of electrolyte used, the fuel, or operating temperature. So far, eight main groups of devices have been identified: alkaline fuel cells (AFC), solid oxide fuel cells (SOFC), molten carbonate fuel cells (MCFC), direct carbon fuel cells (DCFC), phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), and bioelectrochemical fuel cells (BFC). Simplified operation diagrams of these are presented at Fig. 1, and the breakdown of their features and applications is presented in Table 1. The main advantages and disadvantages of specific solutions as well as their recommended use are briefly discussed in the following sections of this subchapter.

2.1 Alkaline fuel cells (AFCs) The first fuel cell developed for practical and operational use was the alkaline fuel cell. The electrolyte applied in AFCs is an aqueous solution of sodium or potassium hydroxide [13]. The electrochemical process for generating electricity starts at the anode, where gaseous H2 reacts with hydroxyl anions to give water molecules and release electrons. The electrons are passed through an external circuit to the cathode and recombine with water and oxygen to produce OH
ions that migrate through the electrolyte to reach the anode. In general, AFCs operate within a temperature range of 60–200°C. They are also characterized by advantageous high power density (approximately 1 kW/m3), high efficiency, quick start, and the use of relatively inexpensive catalysts for electrode reactions. On the other hand a problem is the sensitivity of AFCs to carbon dioxide, present in the supplied gases (H2 and O2), that reacts readily with KOH or NaOH to form the corresponding insoluble carbonate (M2CO3). Such salts destroy the porous structure of the electrode and eventually crush the basic reactions of the cell. Furthermore the formation of sodium or potassium carbonates adversely affect hydroxyl ion concentration in the electrolyte causing a decrease in ion conductivity and a lowering of the cell’s performance [13]. The aforementioned deficiencies result in the necessity of using the purest possible H2 and O2, for example, prevents the possibility of using atmospheric air as a source of oxygen, which in turn significantly increases the operating costs

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2 Fuel cells

e–

e– Electrolyte

Cathode

Anode

AFC

2H2 + 4OH-  4H2O + 4e -

H2 H2O

SOFC

2H2 + 2O2-  2H2O + 4e -

H2

MCFC

2H2 + 2CO32-  2H2O + 2CO2 + 4e -

H2 H2O, CO2

C + 4OH-  CO + 2H O + 4e -

OHO2

O2 + 2H2O + 4e -  4OH-

O2

O2 + 4e -  2O2-

O2 CO2

O2 + 2CO2 + 4e -  2CO32-

O2

O2 + 2H2O + 4e -  4OH-

O2 CO2

O2 + 2CO2 + 4e -  2CO32-

O2

O2 + 4e -  2O2-

H+

O2 H2O

O2 + 4H+ + 4e -  2H2O

O2CO32-

2 2 Molten 2OH- + CO  CO 2-+ H2O hydroxide C + 6OH-  CO 22- + 3H 3O + 4e 3 2

C H2O

OH-

C + 2CO32-  3CO2 + 4e C + CO32-  CO + CO2 + 2e 2C + 2CO32-  3CO + 2e -

C CO2

CO32-

C + 2O2-  CO2 + 4e C + O2-  CO + 2e C + CO2  2CO CO + 2O2-  CO2 + 2e -

C CO2, CO

PAFC PEMFC

2H2  4H+ + 4e -

H2

DMFC

2CH3OH + 2H2O  2CO2 + 12H+ + 12e-

CH3OH CO2

H+

O2 H2O

3O2 + 12H+ + 12e -  6H2O

C6H12O6 CO2

H+

O2 H2O

6O2 + 24H+ + 24e -  12H2O

DCFC

Molten carbonate Solid oxide

BFC

C6H12O6 + 6H2O  6CO2 + 24H+ + 24e -

O2-

Fuel

Oxygen/air

FIG. 1 Schematic diagram of eight types of fuel cells with anodic and cathodic reactions, fuel used (red arrow), anodic product (violet arrow), ions conducted (yellow arrow), oxidant (green arrow), and cathodic substrate or product (blue arrow). ACF, alkaline fuel cell; SOFC, solid oxide fuel cell; MCFC, molten carbonate fuel cell; DCFC, direct carbon fuel cell; PAFC, phosphoric acid fuel cell; PEMFC, polymer electrolyte membrane fuel cell; DMFC, direct methanol fuel cell; BFC, bioelectrochemical fuel cell.

of the cell. Therefore alkaline fuel cells are rarely used for commercial purposes, though they have found application, for example, in Apollo mission.

2.2 Phosphoric acid fuel cells (PAFCs) Phosphoric acid fuel cells are the first type of cell produced in commercial quantities, currently widely used in the United States, Europe, and Japan. The electrolyte in this type of cell is relatively stable orthophosphoric acid (H3PO4) in the form of a gel, deposited on porous Teflon silicon carbide (SiC). The ion conductivity of H3PO4 is low at low temperatures; therefore PAFCs operate at temperatures about 150–250°C, reaching current densities in the range

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TABLE 1 Comparison of different fuel cell systems’ electrolytes, fuels, operating temperatures, and applications. Fuel cell type

Common electrolyte

Fuel

Operating temperature (°C)

Main applications

References

Transportation, space, military, energy storage system

[4–6]

AFC

Solution of potassium hydroxide

H2

60–200

PEMFC

Solid polymeric membrane

H2

50–200

[4,6,7]

DMFC

Solid polymer membrane

CH3OH

60–200

[4,6,8]

SOFC

Solid ceramic inorganic oxide

H2, CO, CH4, other

600–1000

MCFC

Molten potassium or lithium carbonate

H2, CO, CH4, other

600–700

DCFC

Molten salts and hydroxide

Carbon source

400–700

PAFC

Phosphoric acid

H2

150–250

Combined heat and power for decentralized stationary power systems

[4–6]

BFC

Solid polymer membrane

Organic compounds

35–40

Wastewater treatment, recovery of pure materials, water softening

[5,12]

Combined heat and power for stationary decentralized systems and for transportation

[4–6] [4,6]

[9–11]

of 100–400 mA/cm2 at a voltage of 0.6–0.8 V. At the anode the hydrogen molecule splits into the electrons and positively charged hydrogen ions. Both species reach the cathode, the electrons via an external electrical circuit and protons through the electrolyte, and react with the oxygen molecule to form water. The water vapor created at the cathode is discharged with the excess of oxidant (O2 or air) and does not dilute the electrolyte [13]. Contrary to ACFs and PEMFCs, PAFCs exhibit higher tolerance to gases (H2 and O2) contaminated with CO2, high thermal and electrochemical stability, long life (up to 40,000 operating hours), and a capacity to produce and separate electricity and heat at the same time. Cogeneration, that is, combined heat and power (CHP) generation, makes it possible to capture waste heat and use it in most industrial and commercial applications. The main disadvantage of PAFCs is their high costs of manufacture caused by the need to construct the cell with acid-resistant materials, since the electrolyte is highly aggressive and corrosive, and to coat the electrodes with an accordingly dispersed platinum catalyst [14]. Moreover, low tolerance of the electrodes to sulfur-containing compounds or carbon monoxide (platinum catalyst poisoning) and relatively slow oxygen reduction at the cathode may substantially reduce the cell’s performance [15,16]. Despite these drawbacks, PAFCs are successfully

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2 Fuel cells

427

applied in combined systems with a capacity of 50–1000 kW, supplying electricity and heat for households and large stationary power systems with a capacity of 5–20 MW.

2.3 Solid oxide fuel cells (SOFCs) Solid oxide fuel cells (also termed ceramic fuel cells) are high-temperature fuel cells where liquid electrolyte has been replaced by a ceramic membrane exhibiting good conductivity of oxygen ions. The most commonly used electrolyte for SOFCs is a ceramic layer of so-called dense yttria-stabilized zirconia composed of zirconium dioxide (ZrO2) stabilized with 8–10 mol% of yttrium oxide (Y2O3). High thermal and chemical stability together with very good ion conductivity allows the ceramic fuel cells to operate in the range of 600–1000°C. The cell structure only enables the transport through the electrolyte of oxygen ions O2
resulting from ionization at the cathode. When O2
ions reach the anode, they react with the gaseous fuel. The product of the reaction is water (in the case of hydrogen fuel) and electrons that can flow in the outer perimeter, giving an electric current. An extremely important aspect is the possibility of using different fuels in SOFC such as hydrogen, CO, or more complex gases, such as methane, propane, or ethane [17]. The specific operating conditions of SOFCs, besides long-term thermal and chemical stability of electrode materials and interconnectors, also require high-temperature corrosion resistance under redox conditions that significantly increases the costs of manufacture, due to component prices; limits their efficiency; and consequently shortens operating time. The high working temperature, as well as cell performance in combination with some unavoidable differences in thermal expansion coefficients of individual cell components, causes the cell to be exposed to significant thermal stresses, which can easily lead to the destruction of its ceramic elements. On the other hand the lack of a liquid phase in the cell allows for a fairly free shaping of its geometry. Thus single SOFC cells are manufactured in a variety of shapes (tubular, flat monolithic, and single chamber), which means they are often used in the power industry, since unlike other FCs they can provide the highest possible power to obtain. In addition, the high temperature of the residual gases allows SOFCs to be used in a gas turbine coupled with an electrogenerator; hence, CHP operation increases the overall efficiency of the system up to 80%. Although SOFCs are easy to build, relatively cheap to operate, and resistant to damage during operation and have low emission regarding air pollution and greenhouse gasses (GHG), their use is limited due to their high costs of manufacture, sensitivity to sulfur and other contaminants, and complex, time-consuming start-up and cooling-down procedures. SOFCs are mainly used for medium and large stationary power systems, including CHP, with a capacity of a few kilowatt to tens of megawatt [14,18].

2.4 Molten carbonate fuel cells (MCFCs) Like SOFCs, molten carbonate fuel cells are high-temperature cells working at approximately 600–700°C. The electrolyte in MCFCs is molten at high temperatures (650–750°C) consisting of mixtures of lithium and potassium (62 mol% Li2CO3–38 mol% K2CO3) or lithium and sodium (50 mol% Li2CO3–50 mol% Na2CO3) carbonate suspended in a highly inert, porous, and ceramic beta-alumina solid electrolyte (BASE) matrix consisting of isomorphic

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aluminum oxide combined with a mobile lithium or sodium or potassium ions [19]. The anodic reaction of hydrogen gas with oxidant CO2
3 delivered from the electrolyte gives carbon dioxide and water as products, and electrons released are transferred through an external circuit to the cathode. The cathodic reaction of carbon dioxide and oxygen result in the formation of carbonate ions that migrate to the electrolyte. The very high operating temperatures of MCFCs significantly improve their efficiency (50%–60%) due to the enhanced kinetics of the cathode processes, compared with PEMFCs and PAFCs, that eliminate the need for applying expensive precious metals as catalysts, as well as purifying of fuel of carbon monoxide or dioxide, which can interfere with the work of the cathode. Additionally, various hydrocarbons, biogas, LPG, methanol, and hydrogen can be used as a fuel, since fuel reforming can occur inside the cell [20]. In addition, MCFCs can operate in cogeneration, driving a gas turbine and steam turbine and providing heat. Their main drawback is a mobile, aggressive, and corrosive electrolyte that has a significant negative impact on the electrodes and the cell components, so the hardware requires the use of nickel and high-grade stainless steel, respectively. The electrolyte also has low tolerance to changes in operating temperatures, which affects the mechanical stability of the fuel cell. MCFCs have found application in various marine and stationary systems with capacities in the range of 100 kW–5 MW, where its weight and size, in combination with long start-up time are of less importance.

2.5 Direct carbon fuel cells (DCFCs) The direct carbon fuel cell is the only fuel cell capable of converting the chemical energy of solid carbon fuels directly into electricity [21]. The fuel can be any substance rich in carbon such as coal, lignite, coke, biomass, and organic waste, while the oxidant is oxygen supplied to the device either in its pure form or with atmospheric air [22–24]. The conversion of energy occurs through direct electrochemical oxidation avoiding conventional combustion, which considerably increases the efficiency of the process. There are three main types of electrolyte used in DCFCs, namely, molten hydroxides [9,25,26], molten carbonates [27,28], and solid oxides [29–32]. The electrochemical reactions at the anode are complex and strongly depend on system structure. In general, carbon reacts with various species such as hydroxyl OH
, 2
carbonate CO2
ions, giving as products carbon dioxide, carbonate ions 3 , and oxygen O and water (hydroxide-DCFCs), carbon dioxide, or/and carbon monoxide (carbonate-DCFCs and oxide-DCFCs). Released in electrochemical process electrons pass through the external circuit to the cathode where they recombine with oxygen and water, carbon dioxide, or ox2
ygen alone to form OH
, CO2
ions, respectively [21]. 3 , or O The main advantages of DCFCs are their significantly higher efficiency compared with other FCs (up to 80%), the suitability of cheap catalysts to ensure sufficient electrochemical activity, and the possibility of using different forms of solid carbon fuels and generally accessible raw materials. A fuel cell powered by coal fuel produces electricity at competitive costs. The cost of 1 kW delivered by DCFC-based power plant can be smaller than in conventional power plants with the same capacity. In addition, the generation of electricity in the cell consumes twice as little fuel as efficient coal-fired power plants, thereby reducing the emission of carbon dioxide per unit of generated energy. Compared with a conventional combustion

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power plant, DCFCs supplied with coal emit a 10-fold smaller volume of exhaust gases, since carbon oxidizes electrochemically. While generating electricity, DCFCs produce carbon dioxide usually as a separate fraction in the form of an almost pure gas stream. Therefore, unlike in conventional combustion, there is no need to use energy-intensive, expensive, and often complicated installations to capture CO2 from the exhaust gas stream. The purity of the CO2 stream generated in DCFCs makes the capture more cost effective and efficient than its sequestration from the exhaust stream in conventional thermal power plants. The drawbacks of DCFCs are poor power densities, short-term durability, and purity of the fuel; impurities in the long-term operation of the cell may considerably affect energy dissipation, high degradation of cell components in operating conditions, fuel supply system, or upscaling of the technology to kilowatt and larger size systems. Therefore DCFCs have not yet been commercialized, and a lot needs to be done before using DCFCs on an industrial scale [33].

2.6 Proton exchange membrane fuel cells (PEMFCs) Among other fuel cells, PEMFCs are the most efficient and promising power generation alternative, one that is gaining growing worldwide attention, since they can potentially reduce consumption of energy, emission of pollutants, and dependence on fossil fuels. Proton exchange membrane fuel cells, also called polymer electrolyte membrane fuel cells, commonly abbreviated as PEMFCs, use as an electrolyte, a solid polymer membrane capable of a selective ion transfer. The use of a solid electrolyte eliminates the need for liquid electrolytes, including corrosive acids and alkalis. In a PEM fuel cell, hydrogen molecules penetrate into the anode’s porous structure to come into contact with the catalyst layer (platinum), where they split into protons and eject electrons. The latter pass through an external circuit generating electric current, whereas the protons migrate through the membrane. When both species reach the cathode and are in contact with the platinum catalyst, they combine with oxygen, forming water that is removed from the cell as a liquid or gas. An individual PEM fuel cell typically consists of the following components: (i) two outer bipolar plates (BP) with gas channels, (ii) two gas diffusion electrodes (GDE) formed by two gas diffusion layers (GDL) and two catalyst layers (CL), and (iii) a superionic membrane solid electrolyte (Fig. 2) [34]. A combined GDE and ion-conducting membrane constitute a membrane electrode assembly (MEA) regarded as the “heart” of a single fuel cell [35]. The MEA is responsible for exchange of gaseous reactant and product between CL and gas channels, electron flow from the current collector to the CL via the GDL, and selective proton transfer through the membrane to the CL [36]. The electrodes are usually porous, graphited carbon fiber or sheets, Tefloned at the CL-GDL interface, and covered with a platinum or platinum alloy catalyst at the electrode-membrane interface [37–39]. CL is in direct contact with the solid electrolyte and GDL and, being an active layer, is the center where electrochemical redox reactions occur [36]. The gas diffusion layers are typically made of porous carbon cloth or paper with a thickness of 0.1–0.3 mm [34]. GDLs provide an effective diffusion of gases to the catalyst, heat transfer during the cell’s operation, proper mechanical strength of the MEA, and adequate hydration of the system, that is, the water is stored at the surface to maintain membrane

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e

e H2O

FIG. 2

Gas channel

Gas channel

H

O2

H2

Schematic diagram of PEMFC and transport of electrons, protons, reactants, and products within the MEA.

ion-conducting properties at certain level, and the excess of water is removed from the catalyst layer to prevent flooding [40]. The MEA separates two graphite bipolar plates (BPs) with gas channels and a current collector. The main functions of the BPs within the cell are fuel (usually H2) and oxidant (O2) distribution, water and heat management, and carrying away the current. They are also responsible for separation of individual cells in the stack [41]. The proton exchange membranes, an essential component of a MEA, is a semipermeable membrane made from an ionomeric polymer. Common themes critical to proton exchange membranes used as electrolytes include high ionic (ca. 10 S/m) and low electron conductivities; adequate mechanical properties, allowing use of a membrane as thin as possible (thickness approximately 10–250 μm); low fuel and oxidant permeability; thermal, hydrolytic, and oxidative stability; long lifetime; and low cost [42]. A milestone achievement in the development of electrolytic materials employed in PEMFCs was invented in the late 1960s by DuPont, the first membrane based on polytetrafluoroethylene and sulfonated perfluorovinyl ether, known under the tradename Nafion [43]. Proton exchange fuel cells are compact light weight units, suitable for continuous operation at two temperatures ranges, that is, 50–80°C and above 100–200°C for low-temperature (LTPEMFCs) [44] and high-temperature (HT-PEMFCs) systems [45], respectively. LT-PEMFC performance is superior to HT-PEMFC but only under pure hydrogen conditions [46]. The use of hydrogen gas obtained by hydrocarbon conversion without any prior purification results in poisoning of the catalyst through CO adsorption [47]. Moreover, water management in LT-PEMFCs is a real challenge, whereas HT-PEMFCs can operate in dry conditions, and no humidifiers are needed [48]. The important advantages of PEMFCs over other FC technologies are their higher power density; longer lifetime; easier start-up, measured in seconds; and lower manufacturing costs. Higher efficiency can be provided by increasing working temperature due to the higher reaction rate. For LT-PEMFCs, a working temperature above 100°C may result in a decrease in conductivity induced by water evaporation, which plays an important role in proton transport in that type of PEM fuel cell. Better efficiency can be achieved by using released heat in a

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cogeneration system [49,50]. The main disadvantages of PEMFCs are their low tolerance to carbon monoxide CO, high costs of manufacture (catalyst and membrane materials), and low durability under operating conditions. Various characteristics make the PEMFCs one of the most promising technologies for a wide range of power applications, from stationary [51–53], to automotive [54–57], to portable power units [58–60]. However, there are still many commercialization problems that need to be resolved, especially the cost of manufacture. Nevertheless, PEMFCs are expected to be fully commercialized in the coming two decades [7].

2.7 Direct methanol fuel cells (DMFCs) In comparison with other FCs, direct methanol fuel cells are a relatively new technology. Actually, DMFCs are a subcategory of PEMFCs, since the solid polymer membrane is used as an electrolyte and its construction is similar to that of a classical PEMFC type cell [61–63]. The system is fueled with methanol (CH3OH), a liquid that can be easily stored and transported due to its relatively wide temperatures range under standard conditions. Methanol, supplied as an aqueous solution to the anode, undergoes electrochemical reaction at the catalyst surface (Pt and Ru particles) producing protons and carbon dioxide and releasing electrons. The protons migrate through the ion-conducting membrane toward the cathode, where they combine with electrons and oxygen, forming pure water. The main advantages of DMFC cells are their fast and easy start-up, low operating temperature, safe operation due to usage of solid polymer electrolyte, cheap, easy to handle, and with fuel available from many sources. However, the poor performance of DFMCs limits their application to low power portable devices with a capacity range from 1 W to 1 kW, such as personal notebook computers, mobile phones, or cameras [8,64,65].

2.8 Biofuel cells (BFCs) Another distinct subcategory of proton exchange membrane fuel cells is the group of bioelectrochemical fuel cells, also termed biofuel cells, converting the energy from the fuel directly into electricity with the help of biocatalysts [66–69]. Biocatalysts, such as microorganisms, selected enzymes, or even cell organelles, can generate large loads of energy utilizing various organic substrates [68]. In general, bioelectrochemical fuel cells can be divided into two main types depending on the biocatalyst used, that is, microbial fuel cells (MFC) and enzymatic fuel cells (EFC), since oxidation reactions of the fuel can be catalyzed by either living cells or selected enzymes, respectively. The main advantages of using the biocatalysts result from their high catalytic activity, selectivity, and specificity, as well as their relatively low cost of preparation. However, research on the development of appropriate fuel cell units and the full assessment of their properties require a diverse range of knowledge due to the complexity of the processes occurring in microorganisms and the sensitivity of these bioelectrochemical systems to various factors. Like other FCs the MFC electrode structures (anode and cathode) should be characterized by a specific porosity to allow efficient transport of the fuel to the catalyst surface [68]. Furthermore the electrodes are usually separated by either a salt bridge, a polymeric ion exchange, a porous membrane, or a ceramic membrane [68,69].

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Enzymatic fuel cells (EFCs) use cheap enzymes in an isolated form, thus making it possible to obtain high specificity of redox reactions and preventing the cross-reactions typical of living cells from occurring. This may help in physical electrode separation or even eliminate the need of using a membrane separator [68,69]. Although EFCs exhibit higher power densities (still lower than in conventional FCs), their main drawbacks are their short life span due to the enzyme impermanence and the lack of a natural barrier, such as a biological cell membrane, that leads to its relatively fast degradation, and low efficiency, due to incomplete oxidation of the fuel. For example, in living cells, oxidation of glucose, a typically employed substrate, produces carbon dioxide and releases 24 e
, whereas in EFCs, isolated glucose dehydrogenase oxidizes the substrate to gluconic acid only, leaving many electrons present in the organic compound unexploited [69]. These deficiencies can be overcome by effective immobilizing of the enzyme system [70], applying of multicomponent enzyme cascades or whole cell organelles [71], and optimizing the surface area of the anode using materials with multidirectional and multidimensional pore structures [72] or using the mediators that may support anode charge transfer mechanisms and balance proton migration with electron transfer [66,73]. The microbial fuel cell (MCF) is another type of bioelectrochemical fuel cell, using microorganisms like bacteria as biocatalysts for conversion of organic matter into electricity. Any biodegradable source seems to be suitable for energy generation, due to the microbes’ ability to derive organic substrates of diverse origins, starting from single small molecules (glucose, ethanol, or acetate), through biopolymers (cellulose, lignocellulose, chitin, starch, etc.), and ending with mixtures of organic compounds in wastewater (brewery, synthetic, starch processing, dye, landfill leachates, etc.) [74–76]. Generally, MFC systems, either as two chamber or single chamber, consist of two anodic and cathodic compartments (Fig. 3). Organic substrates undergo oxidation by microorganism cells in the anode part, to give carbon dioxide and protons to the solution with simultaneous release of electrons to the nearby electrode. The reaction proceeds under anaerobic conditions. The electrons then pass toward the cathode through the external electric circuit, which in the situation of a potential difference between the anode and the cathode creates a current. At the same time the protons formed on the anode migrate through an ion-conducting membrane into the oxygenated cathode. A chemical or microbiological reduction process occurs at the cathode, where protons in combination with electrons and oxygen form water [75,77]. Unlike EFCs, MFCs can be characterized by a long lifetime (up to 5 years) and the complete and effective oxidation of organic matter to carbon dioxide [78–80]. However, due to slow transport across cellular membranes, they provide low power densities [48]. The use of mixed culture microorganisms can improve the MFC system and results in higher power outputs [69]. Despite the potential benefits of wastewater treatment, bioelectrochemical fuel cell technology is still in the early stages of development and requires many improvements before it can be used extensively. However, the pace and scope of research will probably soon result in the construction of an efficiently working cell, which gives hope for the future and confirms the usefulness of this technology, not only in power generation from biowaste but also in the field of remote sensors, hydrogen production, desalination, heavy metal removal, bioremediation, and other unexplored applications.

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FIG. 3 (A) Single-chamber MFC with open air cathode. (B) Simplified view of a two-chamber MFC with possible modes of electron transfer is shown. (1) Direct electron transfer (via outer membrane cytochromes), (2) electron transfer through mediators, and (3) electron transfer through nanowires [76].

3 Biopolymer membranes for fuel cells Three of the eight types of cell specified earlier are polymer electrolyte fuel cells (PEFC) with membranes for selective proton conductivity. The low-temperature units have three main applications: as stationary systems (e.g., Ene-Farm in Japan), for powering vehicles (e.g., the Toyota MIRAI), and in mobile equipment (such as the Upp charger by Intelligent

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Energy). Their popularity results from their high power density, short start-up times, and flexibility in load-changing situations. Supply and storage of gas hydrogen still remain an aspect that requires improvement. The situation is different in the case of direct methanol fuel cells (DMFC)—they are easy to refill, and fuel transport is relatively simple. In both types of fuel cells, the ion-exchange membrane is most often made of Nafion, a durable material with good proton conductivity and chemical stability. Disadvantages of the fluoropolymer include the high cost of production, difficult recycling process, conductivity dependent on humidity level, and relatively high methanol permeability. Recent R&D work shows that Nafion could be substituted by natural polymers. First of all, their cost of production could be significantly lower due to their abundance in nature, and the process of isolating the substance does not require time-consuming and frequently expensive technologies. Furthermore, they are a part of the natural organic carbon cycle, unlike synthetic polymers that are difficult to recycle and store. Moreover, synthetic materials have a long degradation time, and that also negatively impacts the environment. The chart in Fig. 4 shows the dynamics of growing interest in biopolymers to be used in PEMs expressed by the increasing number of research projects and publications in the relevant scientific literature. Many of the articles deal with ways of modifying polysaccharide and protein biopolymers. These are characterized by insolubility or negligible solubility in water (biopolymers form colloidal arrangements), which prevents easy processing and the production of flexible and stable films. The methods of processing the raw material can be divided into two major groups: chemical and physical. The first one covers etherification, esterification, grafting, and cross-linking, while the second consists mostly in plasticization or forming composites and mixes [81]. Modification reactions aim at replacing or adding functional groups to polymer chains. This improves such properties of the biomaterial as gelatinization or retrogradation and makes the resultant film more mechanically stable and easier to apply to metal or glass surfaces. In terms of modifying natural polymers using such physical processes as plasticization or grafting, adding the plasticizer supports formation of noncovalent interactions with the polymer chain, for instance, van der Waals forces or ion and hydrogen bonds that can help separate polymer chains, increasing the material’s ion permeability.

120

Scopus

Science Direct

Web of Science

100 80 60 40 20 0

Before 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019* 2009

FIG. 4 Number of publications found in three scientific databases (Scopus, Science Direct, and Web of Science). The search keywords were “chitosan,” “cellulose,” “alginate,” “starch,” “pectin,” “agar,” or “gelatin proton exchange membranes” in all fields (∗ until April 2019).

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Grotthuss mechanism (proton hopping)

Vehicle mechanism

435

FIG. 5 Scheme of proton transfer mechanisms; top, Grotthuss mechanism where the protons are passed through hydrogen bonding; bottom, vehicle mechanism where proton transport occurs with the aid of a moving “vehicle,” for example, H2O and [Im]. Reprinted with permission from T. Ueki, M. Watanabe, Macromolecules in ionic liquids: progress, challenges, and opportunities, Macromolecules 41 (2008) 3739–3749. https://dx.doi. org/10.1021/ma800171k. Copyright 2008 the American Chemical Society.

Most often, polymer membranes with the desired properties are made of biopolymers from polysaccharides, such as cellulose, chitosan, alginate, starch, pectin, or agar and their derivatives, and from proteins, for example, gelatin. So far, biopolymers and their derivatives have been used for preparing solid polymer membranes in PEMFC, DFMST, and BFC fuel cells. The important thing in transferring protons through the membrane is that it impacts the efficiency and speed of the whole fuel cell’s operation. Proton transportation through the membrane can take place in two ways: by the Grotthuss [82,83] or vehicle mechanism [84,85] (Fig. 5). The first case assumes the presence of hydrogen bond networks that carry protons with simultaneous forming of OH bonds in one place and breaking them in another. For that reason, it is also called the hopping mechanism, chain mechanism, or structure diffusion. Nonetheless, it cannot explain all unconventional proton transfer systems. In the second mechanism, proposed by Krauer et al., protons are carried by small molecules, for example, H2O or NH3, as H3O+ or NH+4 ions. This is accompanied by simultaneous movement of discharged “vehicles” in the opposite direction, and here the hydrogen bonds sequence is not necessary. In this subchapter, we will focus on presenting the selected main types of natural materials, including chitosan, cellulose, alginate, starch, pectin, agar, or gelatin, that have proven suitable for production of biopolymer electrolyte membranes for fuel cells. Their detailed properties and possible approaches to the synthesis are described in the following sections.

3.1 Chitosan (CS) In the past few years, chitosan (CS) being a natural low-cost, nontoxic, biodegradable, and biocompatible polysaccharide has been extensively investigated as a solid biopolymer electrolyte in low and intermediate temperature polymer electrolyte-based fuel cells, such as the proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), and alkaline fuel cell (AFC). Nowadays, CS is one of the most promising membrane materials, because it

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has hydrophilic character and can be used at high temperature, combined with low humidity environments, and importantly, it has low-methanol permeability. The use of a chitosan biopolymer for fuel cell technologies is novel and challenging where biological products are usually considered as waste, nonhazardous, low cost, and environmentally benign [68]. Moreover, both the solubility in acidic solutions and aggregation with polyanions impart chitosan with excellent gel forming properties. In an acidic medium, CS is transformed into a typical polyelectrolyte. The pH of such acids is less than 6.5 [86], since at higher pH levels, the biopolymer loses its charges and may precipitate out of the solution due to deprotonation of dNH2 groups. Chitosan has cationic nature due to the protonation of amino groups on the polymer backbone and becomes a cationic polyelectrolyte upon dissolution in acidic solvents. Interestingly, besides polyelectrolyte gels’ forming properties, CS can serve also as a matrix material for ion-solvating polymer composite membranes. In addition, chitosan has promising application in electrodes of polymer electrolyte-based fuel cells and also in biofuels. However, use of chitosan has been delayed, probably due to the earlier mentioned difficulty with dissolution in general organic solvents. This is due to their rigid crystalline structure caused by the establishment of hydrogen bonding between their dNH2 and dOH groups within a single CS structural unit and also between units in a biopolymer chain. Moreover, chitosan exhibits low electrical conductivity, since the three hydrogen atoms in the hydroxyl groups present in the CS monomer are strongly bonded to the structure. Thus the biopolymer cannot migrate H+ under electric field to behave as a proton conductor [87,88]. Pure chitosan generally undergoes a sharp swelling change from a hydrophobic state to a swollen state as the pH reduces into the acidic range, for example, in fuel cell operation state. It is, however, assumed that if CS is dissolved in acetic acid and the resulting solution is made into a thin film, then the H+ or H3O+ and CH3COO
ions in the acetylated CS film will be dispersed in the immobilized CS solvent and that these ions can be mobilized by the influence of an electric field. If the H+ or H3O+ ions are more mobile than the CH3COO
ions, the film becomes a proton conductor. Such swelling behavior has been attributed to the changes in chitosan structure in acidic medium brought about by the protonation of the dNH2 groups that leads to the dissociation of the hydrogen bonding between amino and hydroxyl functional groups in the polymer network [89]. Another drawback of CS is its mechanical properties that are not sufficiently good to meet industrial requirements. Chitosan is also very brittle due to a very high glass transition temperature (Tg ¼ 203°C [90]). These disadvantages can be overcome by introducing various chemical and physical modification to the chitosan. Over the years a great number of studies on the chemical modification of CS have been reported [91–93]. Such methods provide a way for developing new derivatives with promising physiochemical properties. As shown in Fig. 6, fully deacetylated chitosan contains three main types of reactive functional groups, an amino group and both primary and secondary hydroxyl groups at the C-2, C-6 and C-3 positions, respectively, which give it a considerable chance of chemical modification. The dOH and dNH2 functional groups in CS enable various chemical modification to tailor it for specific applications. Generally, modification of chitosan is categorized into four different groups, namely, (i) sulfonation, (ii) phosphorylation, (iii) quaternization, and (iv) chemical cross-linking processes. Most researchers have focused on the incorporation of sulfate/sulfonate/sulfamate [94–99] or phosphate/phosphonate [100–104] groups onto

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Primary hydroxyl group

S P

FIG. 6 Functional groups in chitosan.

CS structure (Fig. 7). Thus, the sulfonation and phosphorylation reactions may lead to sulfonated, sulfated, sulfamated, phosphorylated, or phosphonated CS derivatives, respectively, which are jointly abbreviated SCS and PCS when they cannot be distinguished. Such modification reactions allow for obtaining water-soluble SCS and PCS with a high degree of substitution. CS-based membrane electrolytes are being studied as an alternative candidate for PEMFC applications, potentially to produce economical fuel cells. Generally, there are four groups of chitosan-based membranes: (i) self-cross-linked and salt-complexed chitosans, (ii) chitosan-based polymer blends, (iii) chitosan/inorganic filler composites, and (iv) chitosan/polymer composites. 3.1.1 Self-cross-linked and salt-complexed chitosans A pure chitosan membrane is an insulator, but it can host ionic conductivity when solvated with lithium nitrate (Li+-complexes) [105] or proton supplying salts (H+-complexes), such as ammonium salts (chloride [106], acetate [107], sulfate [107], nitrate [108,109], or triflate [110]). The addition of salts also destroys the crystalline region of chitosan chains and promotes the amorphous nature of CS the electrolyte, which can result in better ionic conductivity. Moreover the conductivity of the CS film is generally related to the number of ions and the mobility of conductive species in the polymer complexes [108] and also coulombic interaction between salts and functional groups of CS [107]. To increase the amorphous phase content in the CS matrix, plasticization is widely performed [109]. Addition of plasticizers is a simple, low cost, and effective way to improve the conductivity of solid polymer electrolytes. Among numerous of plasticizers the most commonly used include ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate [111,112]. Ng and Mohamad demonstrated that the conductivity of chitosan acetate-NH4NO3 was successfully increased by a magnitude of two from 10
2 to 1 mS cm
1, by adding ethylene carbonate as a plasticizer [111]. Ionic and covalent cross-linking processes have been proposed to reduce the crystallinity of the CS membrane. It is believed that by diminishing the crystallinity of the CS membrane, it will exhibit better water absorption, and so an optimized membrane structure for ionic transport will be obtained [113]. The proton conductivity of chitosan membranes is enhanced by crosslinking and doping with sulfuric acid, since the ion exchange capacity of CS membranes increased from 2.5 meq g
1 for non-cross-linked membranes to 5.2–5.7 meq g
1 for sulfuric acid

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S

I

O S

C

P

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cross-linked CS membranes [105]. However, an atomistic molecular modeling and simulation study suggested that the ionic conductivity of glutaraldehyde cross-linked CS was lower than that of sulfuric acid cross-linked CS [113]. The simulation study of the polymer-electrolyte system consisting of CS, H2O, H3O+, OH
, and SO2
ions inside of the simulation cell 4 suggested that sulfates anchored in the amino groups of the CS membrane led the mobility of the charge carrier ionic species, and the percentage of water and sulfates could improve the conductivity in CS membranes to a value of 20 mS cm
1 [114]. Chitosan membranes, crosslinked in sulfuric acid, were evaluated for methanol permeability at high to medium methanol concentrations and compared with the methanol permeability in Nafion 117 membranes [88]. Methanol permeability through CS membranes of medium molecular weight was found to be 8.0  0.5  10
7 cm2 s
1. This is almost three times lower than the permeability found for Nafion 117 of 2.3  0.2  10
6 cm2 s
1 at the same methanol concentration and temperature [88]. Moreover an increased methanol concentration resulted in a decreased methanol permeability for chitosan, compared with increased methanol permeability at the same temperature for Nafion 117 [88]. Mukoma et al. presented studies using cross-linked chitosan membranes for application as alternative PEMFCs. The results indicated that although chitosan membranes took up more water than Nafion 117 at room temperature, their use in fuel cells at higher temperature levels was not possible because of their instability. Only fuel cell operations at low temperatures could be sustained [115,116]. A promising binary cross-linking agent, consisting of sulfosuccinic acid and glutaraldehyde as ionic and covalent cross-linkers, respectively, was employed in the preparation of cross-linked CS membranes [117]. Introducing sulfosuccinic acid in addition to glutaraldehyde was found to improve proton conductivity. At the same time an increase of methanol permeability as compared with using glutaraldehyde alone was observed. 3.1.2 Chitosan-based polymer blends Polymer blends are macroscopically homogeneous physical mixtures of two or more polymers with/without any chemical bonding between them [118]. The aim of forming polymer blends is generally to combine the respective properties of each partner within a nearhomogeneous material while overcoming some of their weaknesses, although the latter goal is not always reached [119]. Blending technology also provides attractive opportunities for reuse and recycling of polymer wastes [120]. CS is hydrophilic and thus has a high degree of swelling. An excessively high level of water uptake increases the fragility of the membrane and makes it less durable under a fuel cell operating conditions. To overcome the disadvantage of loss in mechanical strength in the wet state, CS is blended with polymers mainly such as poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(vinyl pyrrolidone) (PVP), nylon, alginate, or cellulose [120]. A series of blend chitosan sulfate membranes have been developed by grafting the chitosan monomers with sulfonic groups and then cross-linking reaction of the polymers between the sulfonic groups in the chitosan sulfate and the amino groups in the pure chitosan monomers [87]. The good phase compatibility of the blend polymer after cross-linking reaction was attributed to the similar chemical structure of the chitosan ionomer and chitosan sulfate. The cross-linked membrane with chitosan sulfate with the content of 9 wt% of SCS showed the highest proton conductivity of 31 mS cm
1 at 80°C. This is mainly because high temperatures are propitious to proton movement. Like other PEM materials the proton conductivity

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of such CS membranes was enhanced with increasing temperature. However, when the mass ratio of chitosan sulfates exceeded 15 wt%, a decreasing tendency of membranes conductivity was noticed [87]. There are numerous studies on chitosan blended with PVA due to the compatibility of these polymers [121] and its good miscible properties [122]. The miscibility of chitosan and PVA blends has been confirmed by Lewandowska et al. using dynamic mechanical analysis (DMA) [123]. Furthermore the existence of strong hydrogen bonding between the hydroxyl groups in CS and the hydroxyl groups in PVA as a result of blending also provides good mechanical properties [121]. In addition to the noted improved mechanical stability, it was found that blending of either chitosan with PVA or quaternized chitosan (Q-CS) with quaternized polyvinyl alcohol (Q-PVA) promotes enhanced proton conductivity and lower methanol crossover [124,125]. Liao et al. synthesized Q-CS nanoparticles and incorporated them into a Q-PVA matrix to fabricate a composite polymer membrane [126]. The crosslinking between Q-CS and Q-PVA may help maintain the dimensional stability, minimize the interfacial resistance, and improve ion transport. In addition, Xiong et al. synthesized cross-linked composite membrane from Q-PVA and Q-CS using glutaraldehyde as the cross-linking reagent [127]. Chitosan was also blended with PVA with formaldehyde as a cross-linker [128]. More recently, Gopi et al. developed polyvinyl alcohol/chitosan blend anion exchange membrane with quaternizing agents such as hexadecyltrimethylammonium bromide and 2,3,5-triphenyltetrazolium chloride [129]. An increase in ionic conductivity was observed after blending PVA into NH4I [130] or NH4NO3 [122] doped CS membrane, possibly because blending CS with PVA provides more salt complexation sites. The introduction of a phosphonic acid group into the CS matrix may serve to improve its ion exchange capacity. A blend of phosphorylated CS and PVA membranes exhibited high ion exchange capacity combined with good water uptake and water retention capability. Interestingly the results were comparable, in a few cases, to Nafion 117 membrane parameters [131]. CS-PVA cross-linked with sulfosuccinic acid (SSA) and modified with sulfonated polyethersulfone mixed-matrix membranes was found to be a methanol-barrier electrolyte in DMFCs [132]. However, the durability and performance of these membranes needed improvement. Another tough hydrophilic polymer that gets dispersed in the CS matrix when doped into it is poly(vinyl pyrrolidone) (PVP) [133]. PVP, upon blending with chitosan followed by cross-linking with glutaraldehyde, forms a semiinterpenetrating network whose schematic representation is shown in Fig. 8 [134]. The cross-linking of the obtained polymer blend with sulfuric acid significantly reduces methanol permeability and enhances the ionic conductivity [134]. Polyelectrolyte chitosan-based membranes were also prepared with poly(acrylic acid) (PAA) [135]. PAA is typical an anionic polyelectrolyte that possesses high charge density based on the dissociated carboxyl group and is therefore compatible with the cationic CS polyelectrolyte. Among the blends synthesized, the membrane blend with 50 wt% CS and 50 wt% PAA was identified as ideal for DMFC applications as it exhibited low methanol permeability (3.9  10
8 cm2 s
1), excellent physicomechanical properties, and comparable high proton conductivity (38 mS cm
1). Authors proved that the complex membrane with other proportions had lower conductivity as compared with pure CS and the weight ratio of polycation and polyanion is an important parameter to achieve optimal proton conductivity [135]. Moreover the addition of salts such as sodium chloride, ammonium chloride, or

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B

W

C G

C S

FIG. 8

Schematic semiinterpenetrating network of CS and PVP blend with cross-linking agents proposed by Smitha et al. [134].

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18. Biopolymer membranes in fuel cell applications

magnesium chloride improved the ionic conductivity of the CS/PAA membrane. The use of additional salt increased the number of charge carriers, which effectively facilitated conduction [136]. Another polyanionic electrolyte, a commercially available copolymer, (P(AA-AMPS)) composed of acrylic acid (AA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) with a high density of carboxyl and sulfonic groups, was employed in CS-based membrane preparation [137]. The results indicated that the methanol permeability was reduced, while proton conductivity was increased with the increase of the of P(AA-AMPS) content in the chitosan blend. The highest proton conductivity of 36 mS cm
1 and the lowest methanol permeability of 2.4  10
7 cm2 s
1 were observed when the P(AA-AMPS) content was 41 wt% [137]. Cross-linked membranes were prepared by blending poly(4-styrenesulfonic acid-comaleic acid) (PSSA-MA) and CS, which was based on an esterification reaction between dOH of CS and dCOOH of PSSA-MA and the complex formation of NH+3 of CS and SO
3 of PSSA-MA. PSSA-MA contains strong (dSO3) and weak (dCOOH) acidic groups, which can be used for ionic cross-linking and also proton conduction. Properties of blend membranes, such as mechanical parameters and ionic conductivity, were found to be impacted by PSSA-MA content [138]. 3.1.3 Chitosan/inorganic filler composites Properties of CS-based membranes can be improved by incorporating a vast variety of inorganic components to produce a composite membrane [139]. Fabricating chitosan-inorganic filler composites is a simple and effective method that has the potential to provide a unique combination of organic and inorganic properties. In other words the organic CS matrix can provide the film-forming property and be chemically functionalized, while the inorganic component may enhance the mechanical strength and thermal stability [140]. The properties of such composite membranes made of interacting components strongly depend not only on the nature of the polymeric and inorganic fillers used but also on the amount, homogeneous dispersion, size, and orientation of the solid particles dispersed in the polymeric matrix [141,142]. The interfacial morphology of hybrid membranes is tightly associated with different preparation techniques. So far, two strategies for incorporating inorganic species into a polymer matrix have been proposed: (i) in situ formation of inorganic particles within the polymer matrix through sol-gel reaction or crystallization and (ii) physical mixing of organic solutions with inorganic fillers followed by simple casting [143]. In the first technique the nanosized inorganic particles and uniform dispersion can be achieved, and in some cases the covalent bonds formed between organic and inorganic components may allow delicate tailoring of the interfacial properties [144]. However, the difficulties in controlling the hydrolysis and condensation reaction in sol-gel processes complicate membrane preparation and limit polymer selection. In the latter strategy, organic and inorganic components usually interact through weak hydrogen bonding, van der Waals forces, and/or electrostatic interactions [145]. The interphase in the membranes, which is a domain extending from the inorganic filler surface to the organic bulk, dramatically impacts the overall membrane properties [143]. The objectives for chitosan-inorganic composite membrane preparation generally falls into the following categories: (i) balancing the hydrophilic-hydrophobic nature of CS, (ii) reducing fuel crossover, (iii) enhancing mechanical and thermal properties, and (iv) improving the proton conductivity by the introduction of solid inorganic proton

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conductors [146]. Embedding nonporous or porous inorganic fillers with the proper structure and pore size within the membrane plays an important role in reduction of methanol crossover because they can interfere with polymer chain packing and create a more winding diffusion path. Hygroscopic oxides such as (SiO2)n, ZrO2, TiO2, or Al2O3 have been used to increase the water retention capacity of a polymer composite at relatively low humidity conditions, although they alone do not have significant intrinsic proton conductivity [141,147]. Among different metal oxides, hydrophilic silica has been considered as an attractive material for the preparation of proton exchange membranes owing to its unique ability to impart reinforcement and thermal stability. Sagheer et al. reported an improvement in thermal and mechanical stability of chitosan-SiO2 hybrid composites [148], whereas Nikje et al. prepared hybrid membranes based on chitosan and organically modified nanosilica with better thermal stability and dispersion with 3 wt% filler [149]. It is believed that hybridizing inorganic silica with proton-conducting groups can improve structural/thermal stability and proton conductivity at higher temperatures. Moreover, CS-oxide composite membranes with different oxides such as MgO, CaO, SiO2, or Al2O3 were found to possess conductivities between 1 and 10 mS cm
1 in wet state [150]. However, the proton conductivity decreases with the increase in filler content, due to the relatively low proton conductivity of the fillers themselves and their considerable diluting effect on the proton exchange groups present in the original polymer matrix. Wang et al. demonstrated that the proton conductivity of a CS membrane decreased with the incorporation of silica particles, from 21 mS cm
1 in case of CS membrane to 16 mS cm
1 for a CS-silica composite membrane [151]. For this reason, inorganic fillers are often functionalized before being embedded into a CS membrane, to reduce methanol permeability while maintaining or improving the proton conductivity. A number of surface functionalized silica submicrospheres bearing different groups such as sulfonic, carboxylic, or quaternary groups were embedded into a CS matrix. Such incorporation of sulfonated and carboxylated silica led to enhanced proton conductivity of the resulting CS-oxide composite membranes compared with the conductivity of a pure CS membrane [151]. A functionalized organic-inorganic nanostructured N-p-carboxy benzyl CS-silica-PVA hybrid polyelectrolyte complex as PEM for DMFC applications was prepared in two steps using the sol-gel method and cross-linking with formaldehyde and sulfuric acid, followed by oxidation of the thiol group into the sulfonic acid group, as shown in Fig. 9 [152]. The strong acidic dSO3H groups were grafted onto the inorganic silica-based segments (less swellable), while weak acidic dCOOH groups were grafted on the organic CS-based segments (highly swellable) to achieve a highly proton conductive and stable polyelectrolyte material. The hydrophobic nature of the aromatic rings that are introduced into the CS chain alleviated the problem of excess swelling associated with the CS membrane. Among the inorganic fillers, TiO2 has been the subject of considerable studies due to its superior stability, availability, hydrophilicity, UV resistance, excellent transparency for visible light, and promising applications in fuel cells. For a long time, TiO2 fillers were available only in granular form. Now, it has been reported that the addition of titanate nanotubes (TNTs) as the inorganic filler can improve the thermal and mechanical properties of organic membranes and reduce their methanol crossover [153]. Compared with their bulk counterpart, TNTs have unique physical properties arising from their large surface area, open ends, and tubular and layered wall structure [154]. Due to the relatively low proton conductivity of

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FIG. 9 Schematic reaction pathway for the preparation of sulfonated silica/N-pcarboxyl benzyl CS/PVA hybrid membrane, proposed by Tripathi et al. [152]. MPDMS, mercaptopropylmethyldimethoxysilane.

such CS/TNT hybrid membranes, the chitosan membranes filled with TNTs were modified by phosphoric acid [155]. The incorporation of phosphorylated titanate nanotubes not only increased proton conductivity by facilitating proton transfer by constructing continuous conductive channels with the aid of P-OH groups and adsorbed H2O molecules but also improved the methanol barrier property. Another common inorganic filler is zeolites. They represent a group of aluminosilicates with symmetrically stacked alumina and tetrahedral silica, which results in an open and stable three-dimensional honeycomb structure with negative charge. This net negative charge makes zeolite possess high cation exchange capacity. Zeolite particle pore size, its content, and hydrophilic/hydrophobic nature have a significant impact on CS/zeolite membrane properties [156]. Wu et al. reported that appropriate zeolite content enhanced the mechanical strength of composites [157]. On the other hand, Wang et al. demonstrated that excess zeolite may cause a reduction in the mechanical strength of composite membranes due to the formation of too many interfacial voids at the interface of CS and the zeolite [156]. Incorporation of hydrophobic zeolites increased the diffusion resistance of methanol and consequently decreased the methanol permeability, whereas the opposite trend was observed using hydrophilic zeolites [156,158]. Interfacial morphology of CS/zeolite membranes could be improved by incorporation of sorbitol as a plasticizer, by appropriate control of membrane formation temperature [143], and also by specific functionalization of zeolite particles (e.g., with silane

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coupling agent), which causes suppression of methanol crossover [158,159]. Despite the advantage of some reported CS/zeolite membranes in methanol resistance, they had less proton conductivity as compared with a pure CS membrane, possibly because incorporation of zeolite reduces water uptake [159]. Ways to improve the proton conductivity of CS/zeolite membranes are sulfonation [159] or phosphorylation [160] modifications. Among the phosphate salt family, hydroxyapatite with the chemical composition of Ca10(PO4)6(OH)2 has been demonstrated as an attractive inorganic filler for fuel cells, due to its prominent conductive properties for protons [161]. The biomimetic mineralized hydroxyapatite (BMHA) particles are incorporated into the CS matrix to prepare CS/BMHA hybrid membranes. The hybrid membranes exhibited at most 127% higher proton conductivity than the pure CS membrane because of the intrinsic high proton conductivity of the BMHA particles, their strong hydrophilicity, and the large free volume of the membranes. Moreover, due to the tortuous crossover pathways and higher hydrophilicity, the hybrid membranes exhibited higher methanol resistance than the pure CS membrane [161]. 3.1.4 Chitosan/polymer composites Many synthetic polymers, such as poly(aryl ether ketone) (PAEK), polysulfone (PSF), or polybenzimidazole (PBI) are being investigated as alternatives to Nafion, due to their desirable durability. These polymers can be converted to sulfonated polymers by a sulfonation process, which improves their proton conductivity [6]. These synthetic membranes are used in combination with the CS membrane to make polymer composite membranes with improved properties. Sulfonated poly(aryl ether ketone)s (SPAEKs) are a promising candidate as an alternative to a Nafion membrane because they are cheap and possess good mechanical properties and high thermal stability [162]. However, the highly sulfonated membrane tends to swell excessively under humid conditions and loses its dimensional stability. To overcome these problems, attempts to cross-link SPEEK with CS were proposed. A sulfonated poly(ether ether ketone) (SPEEK) substrate was coated with a cross-linked CS barrier layer to form the two-layer composite membranes [163]. The cross-linked SPEEK/CS membrane showed good thermal stability (above 240°C) and significantly stronger methanol barrier property. Interestingly the thickness of this CS layer also had no obvious effect on water uptake. The results indicated that introducing a cross-linked CS layer onto the SPEEK surface was an effective method for improving the performance of the SPEEK membrane, especially by reducing methanol crossover. Muthumeenal et al. prepared CS-based composite membranes using phthaloylated CS and sulfonated polyethersulfone (SPES) that were suitable for DMFC applications [164]. The introduction of the phthaloyl group into the CS matrix increased its solubility in organic solvent, film formability, flexibility, resistance to methanol crossover, and suitable ion conductivity. A cross-linked composite membrane having a thin CS layer on microporous sulfonated polysulfone (SPSF) was synthesized [165]. The SPSF/CS composite exhibited higher proton conductivity than Nafion 117 at temperatures above 100°C. The membrane also showed adequate thermal stability and can therefore be considered as a potential alternative fuel cell membrane, especially for high-temperature operations. CS-based polymer and Nafion have been used in combination for the purpose of enhancing the methanol resistance of Nafion and ionic conductivity of CS [166]. The novel doublelayer PEM comprising a layer of structurally modified CS as a methanol barrier layer, coated

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on Nafion 112, was prepared and assessed for DMFC applications [166]. Proton conductivity and methanol permeability measurements showed improved transport properties of the designed CS-based membrane compared with Nafion 117. Moreover, DMFC performance tests revealed a higher open circuit voltage and power density, as well as overall fuel cell efficiency for the double-layer membrane in comparison with Nafion 117, especially at elevated methanol solution feed. The obtained results clearly indicated that the CS-based double-layer proton exchange membrane is a promising PEM for high-performance DMFC applications. Moreover a novel and promising triple layer of PEM containing two layers of structurally modified CS, as methanol barrier layers, both sides coated with Nafion 105 was prepared and tested for high-performance DMFC applications [167]. Proton conductivity and methanol permeability measurements showed improved transport properties for such multilayer membrane compared with Nafion 117 with approximately the same thickness. Additionally, DMFC tests revealed higher open circuit voltage, power density output, and overall fuel cell efficiency for the triple-layer membrane than Nafion 117, especially at concentrated methanol solutions.

3.2 Cellulose (C) Cellulose, just after chitosan, is the second most investigated natural polymer in terms of potential applications in preparing selective solid electrolyte membranes for fuel cell technology. Cellulose, the most abundant organic polymer on Earth, is a stable, no-toxic, renewable, biodegradable, hydrophilic material, insoluble in water and common organic solvents that possesses high tensile strength. Its poor solubility, high hydrophilicity, and good mechanical properties are due to the strong intra- and intermolecular hydrogen bonding network formed by the interaction of hydroxyl groups (C-2, C-3, and C-6) at equatorial positions of a chair-like conformation pyranose ring. On an industrial scale, cellulose is obtained from wood and its waste products by chemical treatment with strong alkalis and acids to separate the cellulose from lignin, pectin, and hemicellulose. However, growing demand for plant-origin cellulose and thus wood consumption as a raw material may cause deforestation and consequently have a significant negative impact on the environment [168,169]. Although higher plants are the major source of cellulose, various bacteria are capable to produce it as an alternative source. Bacteria of the genus Gluconacetobacter (formerly Acetobacter sp.), in particular Gluconacetobacter xylinus and G. hansenii are the most widely investigated systems for producing cellulose [170–174]. In fact, in 1886, Brown was the first to report the potential of using these microorganisms in the synthesis of this biopolymer [175]. Bacterial cellulose (BC) is a highly crystalline form of cellulose [176] that exhibits excellent properties such as unique nanostructure [177], high degree of polymerization [178], high water holding capacity [179], excellent thermomechanical stability, and low hydrogen permeation [180,181]. Therefore BC and its derivatives feature exceptional capabilities and provide a promising future in various fields such as the food industry, biomedicine, electronics, and fuel cell technology [182,183]. Mechanical processing methods of any cellulose source that apply high shear forces such as high-pressure or ultrasonic homogenization, grinding, microfluidization, or cryocrushing allow preparation of nanocellulose fibrils also called cellulose nanofibers (CNFs) [184–187]. CNFs that feature both

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crystalline and amorphous cellulose phases can be characterized by low density, high aspect ratio, high toughness and tensile strength, high optical transparency and low optical haze, low porosity, and low expansion coefficient [188–192]. Therefore CNFs have been investigated as potential materials for various applications such as nanocellulose paper for solar cells, a separating membrane for flexible lithium ion batteries, composite gas barrier film for coatings and packaging, and composite nanofiber paper for electronics [193–197]. Chemical treatment of CNFs with sulfuric or hydrochloric acids leads to the formation of another cellulose form, that is, nanocrystal cellulose (NCC) or cellulose nanocrystals (CNCs). The process consists in hydrolysis of the amorphous part of CNFs, while shorter chain length crystalline regions remain untouched. Moreover the reaction with sulfuric acid result in sulfate ester formation; thus the CNC surface is negatively charged, and the CNCs themselves can be easily dispersed in water [198–201]. Over the years a large amount of researches on the modification of various forms of cellulose has been reported. Such methods have provided strategies for developing new materials with promising physiochemical properties suitable for further application as membranes for fuel cells. In general, properties of various cellulose forms have been modified including noncovalent and covalent bonding. Cellulose is highly hydrophilic and insoluble in water and other organic solvents and therefore incompatible with other hydrophobic polymer matrices. However, the presence of reactive secondary and primary hydroxyl groups at carbons C-2, C-3 and C-6 of dehydroglucose units assist in modifying the surface of the cellulose chains and modulating the properties accordingly to the needs of application. Esterification studies have shown how to establish the relative reactivities of the hydroxyl groups in the following decreasing order: C-6 OH >>C-2 OH > C-3 OH [202]. The common chemical modifications of cellulose can be divided into three groups, namely, (i) simple modification reactions of OH groups such as esterification, etherification, oxidation, or salt formation (ii); cross-linking with small organic compounds; and (iii) a grafting approach with either polymeric substrate using a coupling agent or monomer and an initiator agent to induce the polymerization of the monomer from the nanoparticle surface and combining two different polymers, one modified with an azide group and the other with alkyne function using “click” linking chemistry. The composition and structure of the material in the membrane are the crucial factor for any membrane-based technology. Cellulose-based membranes performance can also be optimized using physical modification strategies such as (i) doping cellulose matrices with various acidic or amphoteric components, (ii) plasticizing cellulose derivatives with low–molecular weight substances, and (iii) preparing of biopolymer multicomponent composite materials with cellulose matrices. 3.2.1 Pure cellulose As previously mentioned, pure cellulose-based materials in the form of nanofibers (CNFs) or nanocrystals (CNCs) can be obtained in acid- or alkali-mediated chemical processes. However, application of an enzymatic catalyst in hydrolysis reactions results in the formation of pure CNFs and CNCs with the absence of acidic groups or other heteroatoms such as sulfur, nitrogen, or chlorine (chlorates). Pure unadulterated microcrystalline cellulose or nanocellulose membranes have scarcely been investigated for fuel cell applications. Only Smolarkiewicz et al. [203] and Bayer et al. [198] reported on the proton conductivity of PEMs prepared from cellulose

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FIG. 10 Photographs of (A) cellulose nanofiber (CNF) slurry, (B) cellulose nanocrystal (CNC) slurry, (C) conventional cellulose-based paper, CNF paper, and CNC paper of similar thickness, ca. 90 μm. Reprinted with permission from T. Bayer, B. V. Cunning, R. Selyanchyn, M. Nishihara, S. Fujikawa, K. Sasaki, S.M. Lyth, High temperature proton conduction in nanocellulose membranes: Paper fuel cells, Chem. Mater. 28 (2016) 4805–4814. Copyright 2016 American Chemical Society.

microcrystal (CMCs) and nanofiber (CNFs) or nanocrystal (CNCs) electrolyte membranes, respectively (Fig. 10). CMC films developed by compressing pellets at room temperature under 10 MPa pressure exhibited low conductivity equal to 2  10
6 mS cm
1 at 70°C (no humidity determined) [203]. On the other hand, membranes based on CNFs and CNCs prepared in the form of thin paper, using either filtration under vacuum followed by hot pressing for 20 min at 110°C and 1.1 MPa, or only vacuum filtration, achieved maximum proton conductivity of 5  10
2 mS cm
1 at 100°C and 4.6 mS cm
1 120°C with 100% relative humidity, respectively [198]. Conduction of protons was suggested to follow a Grotthuss-type water-mediated mechanism. The higher conductivity in CNC even at 120°C was considered a result of the increased number of charge carriers and hydrophilicity/acidity of the sulfuric acid groups introduced during acid hydrolysis. The membranes also showed hydrogen permeability approximately three orders of magnitude lower than in Nafion, with slightly better gas barrier properties of CNCs than CNFs due to the densely packed microstructure and more crystalline nature of CNC. Moreover, nanocellulose fuel cells were stable under operating conditions, such as a temperature of 80°C and 95% relative humidity and those with embedded CNC paper membranes displayed better performance (17 mW cm
2) than with CNF (0.8 mW cm
2) due to lower cell resistance [198].

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3.2.2 Cellulose acetate The presence of numerous reactive hydroxyl groups on the surface of cellulose makes it possible to introduce various surface modifications. In many cases these methods are meant to disturb the crystallinity order or alter the crystal modification by reducing intermolecular hydrogen bonding and thus increasing dispersibility within organic solvent or polymer resin, the latter to improve mechanical properties. Generally the cellulose surface has been either modified by small organic and inorganic molecules to provide sulfate esters, acetates, succinic acid monoesters, aliphatic ethers, silylated ethers, carboxymethylated, and urethane linkages or oxidized by TEMPO-mediated treatment with hypochlorite to give carboxylic acid moiety [81,185,199]. Cellulose sulfate or acetate is typical products formed during the extraction procedure used to prepare nanoparticles from the native cellulosic substrate (Fig. 11) [185]. However, less common acetylation method using acetic acid mixed with hydrochloric acid was employed to prepare functionalized cellulose in a single step [204]. Fischer esterification of hydroxyl groups induced simultaneous hydrolysis of the amorphous parts of cellulose chains and isolation of functionalized nanoparticles with an acetylated surface. Lam et al. [205] used cellulose acetate (CA) to fabricate a single electrode that supported a direct methanol fuel cell (DMFC). This novel architecture combines the elimination of the polymer electrolyte membrane (PEM) and the integration of the anode and cathode into one component. CA was chosen as the coating material for its ease of application onto an electrode surface and its hydrophilic and electrically insulating properties. The CA film provided an effective coating to prevent short circuiting between the anode and cathode catalyst layers. The single electrode supported DMFC had a total thickness of 3.88  10
2 cm and showed a 104% improvement in volumetric specific power density over a two electrode DMFC configuration under passive conditions at 25°C and atmospheric pressure [205]. Moreover the studies on cellulose acetate-based material for fuel cell application were also reported by Tang et al. [206]. The authors used cellulose acetate microfiltration membrane or

FIG. 11 Examples of surface simple chemical modifications of cellulose nanostructures.

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Nafion as a separator of anode and cathode while constructing dual chamber microbial fuel cell. CA material is cost-effective and can effectively separate the bacteria from the cathode and reduce oxygen diffusion to the anode while permitting ion transport. The comparison of performances of the CA microfiltration membrane system with Nafion MFC showed similarity in internal resistance (263 Ω for CA, 267 Ω for Nafion) and achieved maximum power density (0.831  0.016 W/m2 and 0.872  0.021 W/m2) and reduction of pH gradient across the membrane. However, due to the oxygen and substrate diffusion, coulombic efficiency was noticeably lower for CA-MFC (38.5  3.5% for CA and 74.7  4.6% for Nafion) [206]. Most biopolymer films have very low electrical conductivity at ambient temperature in their native state. By incorporating a plasticizer the sufficient mobility of ions and the salt solvating power necessary for ionic conduction can be acquired [207]. Preparation of a solid polymer electrolyte based on cellulose acetate (CA) as host polymer complexed with ammonium tetrafluoroborate (NH4BF4) as doping salt and polyethylene glycol (PEG600) as a plasticizer using the solution casting method has been reported by Harun et al. [208]. The highest proton conductivity obtained for CA doped with NH4BF4 film was 2.18  10
4 mS cm
1, whereas for material enhanced by the addition of 30 wt%, PEG600 increased for almost two order of magnitude to 1.41  10
2 mS cm
1. 3.2.3 Cross-linked cellulose-based membranes The second type of chemical modification that has been applied in the preparation of cellulose-based materials is cross-linking with small organic molecules. Usually, such reactions involve simple conversion of reactive dOH groups into ester or ether derivatives using bifunctionalized agents, for example, dicarboxylic acids or telechelic linear compounds with epoxide moieties at both ends. Introduction of the cross-linkage between the cellulosic chains may result in decreasing water uptake exceeding a certain percentage of the added crosslinking agent and degree of crystallinity [209]. Cross-linking strategy has been employed in preparation of PEM by blending cellulose (C) with sulfosuccinic acid (SSA). Seo et al. presented an elaboration of a membrane by casting a solution of commercially available C with various concentrations of SA followed by thorough drying and subsequent annealing to induce the cross-linking process (Fig. 12) [209]. The detailed characterization of the resulting polymer electrolyte membranes demonstrated that with an increase of cross-linking agent wt% incorporated in the polymer structure, thermal stability (Td10 above 250°C), ion exchange capacity (max. 0.53 meq g
1 for SSA 30 wt%,) and proton conductivity (23 and 42 mS cm
1 at 25°C and 80°C, respectively, for SSA 30 wt%) improved, and a decrease in crystallinity occurred. Moreover, water uptake experiments revealed that the SSA content higher than 25 wt% prevents the membrane to swell excessively. On the other hand, maximum tensile strength at break and Young’s modulus of 5.65 MPa and 5.92 MPa, respectively, achieved for SSA 15 wt% indicated that with higher SSA content, some loss of mechanical properties is observed due to increasing amount of sulfonic groups and that thus higher hydrophilicity [209]. Kasai et al. also investigated the cellulose sulfate (CS) electrolyte membranes cross-linked with ethylene glycol diglycidyl ether (EDGE) as potential alternative material for DMFC application [210]. CS was prepared by sulfating regenerated cellulose followed by basemediated epoxide ring opening with EDGE and subsequent hydrolysis in aqueous hydrochloric acid (Fig. 13).

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FIG. 12

Schematic procedure for the preparation of cross-linked C-SSA membranes.

C

The resulting membranes showed proton conductivity of 81 mS cm
1 at room temperature, comparable with that of Nafion 112, while methanol crossover was almost twice as low. 3.2.4 Cellulose-based graft copolymers Grafting methods are the third type of chemical modifications recently employed in developing cellulose-based polymers. Two different strategies can be applied to graft a polymer on surface: grafting to and grafting from [211,212]. The first approach consists in combining cellulose with another polymer using coupling agent, whereas in the latter, one the polymer is constructed on cellulose matrix via polymerization of suitable monomer in the presence of initiator. The main advantage of the grafting-to strategy is the high level of control of the resulting product’s properties, because base polymers can be fully characterized prior to grafting. However, the reaction medium shows high viscosity since macromolecular species are involved, and there are low densities of grafting due to steric hindrance. The grafting-from strategy appears to be effective in acquiring high densities of grafting onto the surface due to low medium viscosity and reduced of steric hindrance. However, the lack of control and precise determination of grafted polymer molecular weight are its main disadvantages. In most of reports describing cellulose material modifications using grafting methods, the grafting-from approach was employed [212]. Lin et al. [213] presented investigations on the transport and sorption properties of solid electrolyte membranes for DMFC. The electrolytic material was prepared by modifying bacterial cellulose (BC) membranes with different concentrations of 2-acrylamido-2-methyl-1propanesulfonic acid (AMPS) using a UV-light induced grafting-from polymerization technique (Fig. 14). Benzophenone (BP) was used as a photoinitiator.

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FIG. 13

Schematic procedure for the preparation of cross-linked CS-EDGE membranes.

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FIG. 14 Schematic representation of UV-induced grafting-from polymerization process of AMPS monomers onto the BC membrane [213].

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The resulting AMPS-g-BC membranes showed a maximum ion exchange capacity and proton conductivity of 1.79 mmol g
1 and 29 mS cm
1, which is similar to that of Nafion 115, whereas methanol permeability (5.64  10
7 cm2 s
1) was more than twice as low. MEA systems of DMFC fabricated with AMPS-g-BC gave a power density of 16 mW cm
2. These results indicated that the AMPS-g-BC membrane is an effective methanol barrier and a potential candidate for use in DMFC [213]. 3.2.5 Cellulose-based materials doped with inorganic/organic compounds Biopolymer-based materials doped with inorganic or organic compounds are composite materials that have also been investigated as proton-conducting electrolytes with potential application in fuel cell technology. In general, dopants such as inorganic phosphoric acid (H3PO4) [214] and ammonium salts (NH4Cl, NH4F, and NH4Br) [215–217] or organic phytic acid (PA) [214], glycolic acid (GA) [218], oleic acid (OA) [219], and imidazole (Im) [203,220,221] have been applied in the preparation of natural-based materials mainly to increase proton conductivity both in the hydrated and dehydrated state (up to 100 mS cm
1). However, they can also influence the dominant type of proton transport mechanism (vehicle or Grotthuss) and the mechanical properties and thermal stability of the resulting materials. Jiang et al. [214] reported on a novel BC-based proton-conducting electrolyte doped with phosphoric acid (H3PO4/BC) and phytic acid (PA/BC). The H3PO4/BC and PA/BC materials were prepared by immersing the BC membranes directly into a water solution of H3PO4 and PA, respectively. The H3PO4/BC and PA/BC membranes showed maximum ionic conductivities up to 80 mS cm
1 at 20°C or 110 mS cm
1 at 80°C (cH3PO4 ¼ 6.0 mol L
1) and 50 mS cm
1 at 20°C and 90 mS cm
1 at 60°C (cPA ¼ 1.6 mol L
1), respectively. They also shared good mechanical properties, flexibility, and thermal stability. The H3PO4/BC and PA/BC membranes incorporated in the membrane electrode assembly reached initial power densities of 17.9 mW cm
2 and 23.0 mW cm
2. Achieving such high power densities by applying the cellulose membranes in PEMFCs was reported for the first time [214]. More recently, another dopants used in the preparation of proton-conducting polymers based on different forms of cellulose is imidazole and its derivatives [203,220,221]. Such heterocyclic electroactive molecules containing nitrogen are attractive media to substitute water in PEMs. Their unique physical and chemical properties including high thermal stability, amphoteric character, and ability to form a network of hydrogen bonds also observed in water and high degree of molecular autoionization can be favorable for efficient proton transport [222]. Replacing the water in polymer electrolytes with heterocyclic compounds as proton solvents gives an opportunity to improve proton conduction behavior at operating temperatures above 100°C based on a Grotthuss-type mechanism [223,224]. To exploit the advantages of cellulose structure, novel composite materials containing microcrystalline cellulose (MC) doped with imidazole (Im) have been developed by Smolarkiewicz et al. [203,220]. The preparation procedure involved the use of chloroform as a solvent, in which cellulose, unlike imidazole, is insoluble and forms a suspension. As a result the N-heterocyclic molecules were attached only to the surface biopolymer chains of the resulting materials. The MC-Im composites were thermally stable and, under anhydrous conditions, achieved maximum conductivity up to 2  10
3 mS cm
1 at 160°C that was almost four orders of magnitude higher than that of the pure cellulose sample [203]. On the other hand, Tritt-Goc et al. reported on the preparation of nanocomposite materials based on cellulose nanocrystals (CNC) as a matrix

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support, doped with imidazole (Im) using various concentration rates of dopants [221]. The CNC-Im film with the highest concentration of imidazole exhibited maximum conductivity of 2.7  10
1 mS cm
1 at 140°C and so five orders of magnitude higher than that of pure CNC paper and high thermostability with temperature of a 10% weight loss (Td10) at around 180°C. 3.2.6 Cellulose-based polymer composites Most cellulose (C)-based polymer composites can be prepared using the latex blending method, due to the insolubility of the biopolymer in water or commonly used organic solvents. In some C-based composite materials, cellulose was applied as a nanofiller to improve the properties of Nafion [225,226]. Jiang et al. [225] presented studies on the preparation of nanocomposite membranes based on bacterial cellulose (BC) pulp and Nafion (N), their mechanical and thermal properties, and the performance of selected materials in MEAs for PEMFCs and DMFCs. The membranes were prepared by mixing (latex blending) different ratios of BC and Nafion in an alcohol solution that led to homogeneous suspensions, casting them onto glass plates, immersing them in deionized water and either drying at room temperature or subsequent annealing at 110°C for 1 h. The annealed BC/N nanocomposite membranes exhibited higher mechanical strength, thermal stability, dense structure, and selectivity; lower water uptake, methanol crossover, and ratio of volume swelling; and enhanced performance at RT. They also showed the maximum proton conductivity of 71 mS cm
1 at 30°C and 100% relative humidity and improved power densities of 106 mW cm
2 when applied in PEMFCs [225]. Likewise, Gadim et al. [226] reported on the thermal and mechanical properties and proton conductivity of Nafion/BC-based nanocomposite membranes and compared them with Nafion. However, the membranes were prepared by diffusing a Nafion dispersion into a 3-D network of nanofibrillar bacterial cellulose material—a speedy and more straightforward method. The resulting Nafion/BC nanocomposites were transparent and showed good viscoelastic and thermal properties, lower proton conductivity than pure Nafion, and a maximum power density of 40 mS cm
1 at 40°C and 98% RH and 16 mW cm
2 when assembled in MEA for PEMFC application. The reduced protonic conductivity was due to the lower concentration of the ion-conducting acidic species [226]. For high-performance fuel cell application, the microstructure of Nafion membranes have also been effectively modified by acid-hydrolyzed cellulose nanocrystals (CNCs) [227]. Due to improved water uptake, Nafion/CNC nanocomposites exhibited higher conductivity even at elevated temperatures (>100°C), almost one order of magnitude lower methanol permeability and maximum power density of 91 mW cm
2 [227], which makes them promising candidates for commercial DMFC applications. Also, other synthetic and natural polymers such as poly(4-styrenesulfonic acid) (PSSA) [181,228] and chitosan (CS) [229] or natural rubber (NR) [230] have been used in the modification of cellulose materials to obtain composite membranes with unique properties and potential applications in fuel cell technology. Gadim et al. [181,228] reported on the preparation of composite proton-conducting membranes based on bacterial cellulose (BC) supported with PSSA, produced by in situ radical polymerization of sodium 4-styrenesulfonate accompanied by cross-linking with poly(ethylene glycol) diacrylate (PEGDA) and subsequent conversion into acidic form. The resulting BC/PSSA/PEGDA materials exhibited excellent mechanical properties and ion exchange capacity (2.25 mmol g
1). Additionally, the best composite

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membrane showed a maximum proton conductivity comparable or even higher than Nafion of ca. 100 mS cm
1 (94°C, 98% RH) and was highly dependent on hydration level, since that decreased to 42 mS cm
1 at 60% RH [228]. Such a biobased polymer electrolyte elaborated in MEA achieved a maximum power density of 40 mW cm
2 [181]. On the other hand, Rahman et al. [229] and Ladhar et al. [230] reported on preparation of biocomposite membranes such as CS/C cross-linked with sulfosuccinic acid (SSA) and NR/cellulose nanocrystals (CNC) via casting approach. The resulting modified materials exhibited maximum proton conductivity of 6.6  10
3 mS cm
1 and 4.7  10
2 mS cm
1, respectively.

3.3 Alginate Not only alginate (ALG) is the term used for the salts of alginic acid, but also it refers to all derivatives of alginic acid and alginic acid itself [231]. The ALGs have many attractive properties such as excellent biocompatibility, nontoxicity, nonimmunogenicity, biodegradability, and low cost and can be easily gelled with divalent cations such as Ca2+ [232,233]. Moreover, ALGs can be easily cross-linked with glutaraldehyde, 1,6-hexane diamine, and other bifunctional organic compounds. It is known that it can also dissolve in a solution of multivalent ions [234]. However, like CS, the alginate has some limitations in its applications due to several weaknesses, which include high solubility in water and low mechanical strength [235]. The six-membered ring structure of the ALG suggests that it is difficult to increase the rigidity or compaction of the biopolymer backbone. Therefore the ALG structure creates large void volumes and allows the absorption of water molecules. Unfortunately, excessive absorption of H2O may cause membrane swelling and decrease the membrane’s selectivity. The challenge is to balance membrane permeability and selectivity to water or gas [236]. To solve this problem the alginate has been modified using various methods that can be classified as covalent cross-linking, ionic cross-linking, and nonbond interactions [237]. It has been shown that covalent cross-linking with other polymers that possess polar groups in the backbone increases the stability of a membrane, due to such polar groups reducing the hydrophilic nature of the polymer. Moreover the ionic cross-linking provides better results, due to the polymer electrolyte complex produced, which is both robust and hydrophilic [238]. In recent decades the alginate-based membranes have been studied as an alternative candidate for fuel cell applications. Smitha et al. prepared the ionically cross-linked blend membranes consisting of CS and sodium alginate. The resulting polyelectrolyte membranes were found to be suitable for DMFC applications due to their low methanol permeability and excellent mechanical stability combined with relatively high proton conductivity (42 mS cm
1) [239]. Such membranes did not provide significant advantages compared with Nafion 117, but their quite simple manufacturing procedure and use of inexpensive biopolymers rendered the blend a potential candidate for fuel cells. Furthermore, both ionic and covalent cross-linked CS/ALG polyelectrolyte membranes were fabricated with different molar ratios by Eldin et al. [240] (Fig. 15). In the latter technique the activation process of ALG was achieved using glutaraldehyde as a covalent cross-linker agent. The ion exchange capacity of the covalent cross-linked membranes increased (up to 5.96 meq g
1) with increasing CS content. Moreover the membranes developed have low methanol permeability (2.2  10
9–2.5  10
10 cm2 s
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A

FIG. 15 Ionic and covalent cross-linking of CS and ALG membranes proposed by Eldin et al. [240].

Nafion 117 membranes (1.1  10
9 cm2 s
1). These results also suggested that covalent crosslinked CS/ALG membranes could be suitable candidates as low-cost polyelectrolyte membranes for DMFC applications. More recently a new membrane was synthesized containing pure ALG, calcium chloride (CaCl2) as cross-linking agent, and glycerol as plasticizer [241]. The best fabricated membrane III. Application of biopolymers membranes/films in environment and energy

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18. Biopolymer membranes in fuel cell applications

presented high proton conductivity (10.1 mS cm
1) and very low methanol permeability (2.0  10
7 cm2 s
1). Moreover, glycerol and CaCl2 clearly enhanced the poor mechanical properties of the ALG biopolymer. On the other hand, alginate-based polyelectrolyte blend membranes were prepared from different compositions of ALG and κ-carrageenan [242]. The films were cross-linked with glutaraldehyde and subsequently sulfonated with BZ2S. The methanol permeability (4.9  10
6 cm2 s
1) and proton conductivity (32 mS cm
1) of ALG/κ-carrageenan membranes increased with increasing content of the latter component. Another novel polyion complex membrane was synthesized for DMFC application through the blending of ALG with the synthetic polymer Pebax (poly(ether-block-amide)) [243]. The blend was covalently cross-linked with glutaraldehyde and sulfonated with sulfuric acid. An advantageously low methanol permeability of 9.3  10
8 cm2 s
1 combined with high proton conductivity of 70 mS cm
1 was obtained compared with corresponding values of 1.8  10
6 cm2 s
1 and 80 mS cm
1 reported for the Nafion 117 membrane. The alginate-based electrolyte membranes with PVA have been prepared using combined blending and chemical cross-linking procedures [244]. First the homogeneous, nonporous PVA/ALG membranes were prepared and then successfully cross-linked with glutaraldehyde. The results indicated that both ionic conductivity and methanol permeability through the PVA/ALG membranes increased with increasing ALG content in the PVA/ALG membranes. Moreover the ionic conductivity measured at 25°C for PVA/ALG membrane with a weight ratio of 60/40, increased from 30 to 90 mS cm
1 compared with the value of the PVA membrane. To overcome the low mechanical strength of PVA/ALG membranes, the heteropolyacids (HPAs) such as phosphomolybdic acid, phosphotungstic acid, or silicotungstic acid as inorganic fillers were added [245]. It was demonstrated that the proton conductivity of PVA/ ALG/HPA mixed membranes was higher than the conductivity of the PVA/ALG blend. Additionally, the methanol crossover rates for PVA/ALG blend membrane and PVA/ALG/ HPA membranes were lower compared with Nafion 117. This was attributed to the higher hydrophilicity of PVA/ALG blend and PVA/ALG/HPA mixed-matrix membranes that favored the selective sorption of H2O from the methanol-water solution. Also, graphene oxide (GO) was used in the preparation of nanocomposite films of sodium alginate [246]. The results reveal that hydrogen bonding and high interfacial adhesion between the GO filler and the ALG matrix significantly changed the thermal stability and mechanical properties of the resulting film. Moreover the thermogravimetric analysis confirmed that the thermal stability of the ALG/GO composite was better than that of neat ALG film.

3.4 Starch Pure starch has a capability to form biodegradable films that show physical characteristics similar to synthetic polymers such as transparency, resistance to oxygen passage, and semipermeability to carbon dioxide. However, it is water sensitive due to its high hydrophilicity and exhibits poor mechanical and ion-conducting properties, which makes it unsuitable for certain applications [247]. To improve the physical and functional properties of starch, various chemical and physical strategies have been developed. Among chemical methods

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allowing for conversion of reactive hydroxyl groups, starch has been modified by (i) simple etherifications or esterifications with inorganic and organic species, (ii) oxidations, and (iii) cross-linking and grafting reactions with bifunctional compounds (Fig. 16). Also physical modifications such as (i) plasticization with low-molecular weight compounds or (ii) blending with other polymers or hydrophobic substances have been employed to obtain starch-based materials with desired properties. Although starch and some of its derivatives have found application in the textile, pharmaceutical, cosmetics, paper, textile, and adhesive industries, relatively little has been reported on their application in PEM technology. Dragunski et al. [248] and Pawlicka et al. [249] presented studies on the preparation and characterization of amylopectin-rich starch (S) materials grafted with poly(propylene oxide)-1,4-diisocyanate-2-toluene (PPODICT) and doped with lithium perchlorate (LiClO4). The lithium salt was added to give the material ion-conducting properties. The resulting S-g-PPODICT films possessing urethane connections exhibited good mechanical strength and maximum ion conductivity of 1.5 mS cm
1 [249]. The same authors reported on solid polymer electrolytes based on amylopectin rich, native corn, modified corn, and cassava starches plasticized with glycerol or ethylene glycol, containing different concentrations of lithium perchlorate, LiClO4 [250–252]. The addition of plasticizers decreases the biopolymer crystallinity, dissociates ion aggregates, and lowers the glass transition temperature, which has a substantial influence on the resulting material’s ion-conducting properties. Glycerol, ethylene carbonate (EC), and poly(ethylene glycol) are commonly used plasticizers, since they have the ability to interact with biopolymer chains through hydrogen bonding and van der Waals forces forming polymeric networks. Such noncovalent interactions may result in an increase in the mechanical strength and toughness of plasticized starch-based materials [253]. The transparent films obtained by plasticization of amylopectin-rich starch with 30 wt% of glycerol or ethylene glycol and the subsequent addition of LiClO4 showed ion conductivity of 5.1  10
2 mS cm
1 at 30°C and 7.2 mS cm
1 at 80°C and 7.9  10
2 mS cm
1 at 30°C and 3.3  10
1 mS cm
1 at 80°C for samples plasticized with glycerol and ethylene glycol, respectively [250]. Not only temperature but also relative humidity have a significant impact on polymer electrolyte conductivity as demonstrated by Rodrigues et al. [251]. The authors reported that ionic conductivity of plasticized starch materials doped with different values of lithium salt increased almost 30 times (from 3.9  10
2 mS cm
1 to 9.5  10
1 mS cm
1) when RH changed from 22% to 55%. The increasing amount of water incorporated in the polymer network facilitated transport of ions by the higher diffusion coefficient or promotion of plasticization effect in the polymeric chains [254,255]. Moreover the source of starch and type of modification introduced may affect the conducting properties of the resulting materials. Oxidized and acetylated starch-based polymers plasticized with 30 wt% glycerol and containing two different LiClO4 concentrations were synthesized and characterized by Pawlicka et al. [252]. The values of ionic conductivity were 1.8  10
1 mS cm
1 at 32°C and 1.5 mS cm
1 at 85°C or 4.5  10
1 mS cm
1 at room temperature and 2.4 mS cm
1 at 80°C for oxidized or acetylated starch-based materials. Although all the starch polymeric electrolytes presented earlier were not fabricated into the membrane electrode assembly, their good ionic conducting properties and thermal stability show that they are promising materials for application in electrochemical devices.

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18. Biopolymer membranes in fuel cell applications

FIG. 16

Chemical modifications of starch: (A) etherification and esterification reactions, (B) oxidation, and (C) cross-linking and grafting reactions.

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More recently, Obasi et al. [256,257] developed proton exchange membranes based on cassava starch and studied its performance in mediatorless dual chamber microbial fuel cells. Three types of PEMs were prepared with different degree of modifications, that is, (i) gelatinized pure starch (GS), (ii) gelatinized starch doped with 5.9% of sodium chloride (GS/NaCl), and (iii) gelatinized starch blended with sodium alginate, calcium chloride, sodium hydroxide, and activated carbon (GS/ALG/aC) [256]. The maximum power densities from the constructed MFCs were 945.7 mW m
2, 1068.5 mW m
2, and 570.8 mW m
2 for GS, GS/NaCl, and GS/ALG/aC, respectively. Additionally, GS/ALG/aC composite material showed mechanical strength and stability as well as improved PEM performance and extended cell durability due to the beneficial physical properties of ALG [257]. The prolonged cell operating lifetime was due to the capability of ALG to immobilize enzymes by inclusion and encapsulation, thereby stopping bacteria from eating up the starch. CaCl2 mixed with the alginate caused rapid gelatinization by electrostatic cross-linking and hence improved the ionic conductivity of the solid medium (starch) [257]. Although maximum power output was less than that already reported in the literature for other membrane materials (3600 mW cm
1), the feasibility of successful MFC operation using relatively low-cost cassava starch-based materials is attractive that would significantly increase the commercial potential of MFC application.

3.5 Pectin Compared with chitosan, cellulose, or alginate, pectin (PC) has been far less examined in terms of its application in PEMFC and, in particular, DMFC. The term pectin covers a very complex, heterogeneous family of polysaccharides that can be found in walls of plant cells, playing an important role in the processes of growing, differentiating, and defending plants. As a natural, biodegradable, cost-effective, hydrophilic, and anionic biopolymer, PC exhibits excellent gelling and stabilizing properties, capability to form films, and low methanol permeability. The characteristics encouraged studies on utilizing the material to fabricate electrolyte membranes for fuel cell applications. Mishra et al. [258,259] reported for the first time on amidated pectin (AP)-based membranes cross-linked with glutaraldehyde (GA) as ion-conducting films for DMFCs. The ethanolamine- [258] and diethanolamine [259]-modified AP/GA and DAP/GA materials were prepared using the solvent casting method. In both cases the authors observed a considerable increase in crystallinity caused by the introduction of amide functions. The resulting AP/GA and DAP/GA membranes exhibited sufficient mechanical strength and had, respectively, the maximum methanol permeability of 1.98  10
6 cm2 s
1 and 1.88  10
6 cm2 s
1 (comparable to Nafion) and the maximum ionic conductivity of 1.1 mS cm
1 and 25 mS cm
1 at room temperature. Strong affinity toward water of residual carboxylic, amine, and hydroxyl groups led to the formation of hydrophilic regions in the polymer network that absorbed water, which enabled easy proton transfer and, consequently, ensured reasonably good ionic conductivity. Polymers blending method is one of the most promising approaches since the resultant blends retain the desired features of individual components and may ensure achieving

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18. Biopolymer membranes in fuel cell applications

optimum membrane capacity and performance. Pasini Cabello et al. [260] developed composite membranes based on PC and chitosan (CS) and investigated their key transport and mechanical properties as well as water holding and ion exchange capacities. These CS/PC films containing different amounts of biopolymers were prepared by a solution casting technique involving plasticization with glycerol and cross-link with GA followed by modification with sodium 4-formyl-1,3-benzenidisulfonate and subsequent HCl hydrolysis [260]. The latter step was aimed at improving ion-conducting properties by introducing sulfonic acid groups. Durability of resultant films strongly depended on PC content. The higher the amount of PC in composite membranes, the better was the durability. Although pure PC and CS samples showed higher ion exchange capacity (IEC), water uptake and proton conductivity, for CS/PC composite membrane with 50% (v/v) of pectin incorporated, methanol permeability decreased notably from 4.24  10
6 cm2 s
1 for pure CS membrane to 1.51  10
6 cm2 s
1. Proton conductivities had the same tendency, since pure CS conductivity was 2.44 mS cm
1 at 30°C and decreased of about one order of magnitude for a membrane with 50/50 (v/v) CS/PC composition [260]. Incorporation of inorganic fillers into polymer matrix was found to provide the desired properties of biopolymer-based membranes such as mechanical strength and thermal stability. Sulfonic acid-modified titanium oxide nanoparticles (S-TiO2) are an example of a nanofiller that can promote ion-conducting properties and limit methanol permeability when dispersed into the polymer blend. Subramanian and Raj [261] developed PC/CS-based nanocomposite membranes containing S-TiO2 and studied their key properties in terms of applicability as a DMFC solid electrolyte. The PC/CS/S-TiO2 membranes were prepared by a solution casting technique that involved mixing the solutions of CS and PC followed by addition of dispersed S-TiO2 and subsequent binary cross-linking with sulfosuccinic acid (SSA) and glutaraldehyde (GA). The membranes differed in the amount of S-TiO2 introduced, including a reference PC/CS sample without titanium nanoparticles. The resultant nanocomposite PC/CS/S-TiO2 membranes showed relevant mechanical and thermal properties, as well as high charge-density per unit volume. Incorporation of S-TiO2 (3 wt% optimum content) into the PC/CS polymer network improved the membranes’ water uptake, IEC, and ion conductivity and decreased methanol permeability. Thus they are promising candidates for DMFC application. A similar approach was reported by Mohanapriya et al. [262], who investigated nanocomposite membranes based on pectin and polyvinyl alcohol (PVA) flexible polymeric network. The hybrid PC/PVA/ S-TiO2 membrane with methanol crossover is restricted, and proton conductivity improved by the presence of S-TiO2 nanoparticles and exhibited a maximum power density of 27 mW cm
2 at 70°C in DMFCs [262].

3.6 Agar Agar, another candidate for biopolymer-based electrolytes, is a low-cost, biodegradable, nontoxic, hydrophilic, and chemically stable natural polymer obtained from certain species of red algae or seaweeds. The presence of two types of cation coordinating oxygen sites, that is, hydroxyl (dOH) and ether (dCdOdCd) in the structure and high chains flexibility, gives agar the ability to capture a large quantity of ions and allow their migration [263]. Thus it has been recently investigated as a potential biopolymer suitable for preparation of

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membrane electrolytes for electrochemical devices. Raphael et al. [264] reported on transparent film membranes based on agar cross-linked with formaldehyde, plasticized with glycerol, and doped with different amounts of acetic acids. Their ion conductivity depended on acetic acid concentration, relative humidity, and operation temperatures. Highest values were obtained for a sample with 50 wt% of acetic acids and varied from 1.1  10
1 mS cm
1 at room temperature to 9.6  10
1 mS cm
1 at 80°C. Furthermore, Shahi et al. [265] presented studies on BFCs using agar salt bridge doped with potassium chloride. The maximum voltage produced by an agar-based biofuel cell was 0.145 V with the current density of 0.05 A m
2. These values were lower than in the case of Nafion-based BFC (0.504 V, 0.1 A m
2) in the same operating conditions, due to the low resistance of the latter membrane for proton transport as compared with the agar salt bridge. More recently, Boopathi et al. have investigated on PEM fuel cell solid membranes based on agar doped with ammonium nitrate (NH4NO3) alone [266] or plasticized with glycerol [267]. These electrolyte films were prepared by the solution casting technique. The maximum ion conductivities of 6.57  10
1 mS cm
1 at room temperature and 1.09 mS cm
1 at 70°C was achieved for an agar/NH4NO3 sample containing of 60 wt % of ionic dopant [266]. The addition of glycerol as plasticizer (up to 40 wt% to 40 agar/60 NH4NO3) improved the ion conduction properties to reach the maximum ion conductivity of 1.44 mS cm
1 at room temperature and 2.41 mS cm
1 at 70°C.

3.7 Gelatin Gelatin is a translucent, colorless, and flavorless biopolymer prepared by the partial hydrolysis of collagens that are generally extracted from the boiled bones and connective tissues of animals [268,269]. In the past two decades, gelatin-based membrane electrolytes have been studied as an alternative candidate for FC application to possibly produce economical fuel cells. It is known that a gelatin electrolyte shows conductivity of the order of 5  10
2 mS cm
1 at room temperature, good electrochemical reversibility, and very high transparency [270,271]. The proton-conducting polymer systems based on commercial gelatin cross-linked with formaldehyde and plasticized with glycerol, containing hydrochloric [272] or acetic [273] acid, were also developed. The use of inorganic acid caused that all the electrolytes obtained were amorphous and showed good thermal and chemical stability, with the ionic conductivity values of 4  10
2 mS cm
1 [272]. The conductivity of the gelatin-based electrolyte system with the use of organic acid was slightly lower (2  10
2 mS cm
1) [273] but still promising as a novel alternative electrolyte for fuel cell applications. Moreover the gelatin seem to be an attractive soft material that can be 3-D printed and can be used as a feedstock for MFC operation [274].

4 Summary and future perspectives This chapter presented selected biopolymers that could be used as ion-exchange membranes in fuel cells. For each of the materials, the analysis covered the properties desired in fuel cells, methods of modifying the biopolymer to enhance or provide the property, and possible applications in specific cell types. Currently the most popular solid electrolyte is Nafion, although successful research and development works on biopolymers could contribute to their quick commercialization because the natural materials offer a range of new

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18. Biopolymer membranes in fuel cell applications

benefits and eliminate some of the limitations that we face in the case of Nafion. The most significant of those is the high cost of processing the fluoropolymer, both in production and in disposal, that often requires advanced neutralization procedures due to the presence of fluorine. Biopolymers not only are easier to isolate because of their abundance in nature but also biodegrade, remaining an integral part of the organic carbon circulation cycle. Furthermore, they have such properties as lower methanol permeability or higher hydrophilicity that Nafion lacks, inhibiting its wider use. The results of research and development works already produced on the use of natural polymers in fuel cells are more than promising, although the range of difficulties that still require resolving remains relatively broad. Overcoming these obstacles would make it possible to commercialize fuel cells and help reduce the negative impact of satisfying global power demand on environment and, in time, maybe even end reliance on fossil fuels. That would mean an industrial and economic revolution, greatly contributing to more sustainable use of natural resources by modern societies.

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C H A P T E R

19 Biopolymer membranes for battery applications Nur Hafiza Mr Muhamaruesaa, Mohd Ikmar Nizam Mohamad Isaa,b a

Advanced Nano Materials (ANoMa) Research Group, Advanced Materials Team, Ionic State Analysis (ISA) Laboratory, Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia bFrontier Materials Research Group, Advanced Materials Team, Ionic & Kinetic Materials Research (IKMaR) Laboratory, Faculty of Science and Technology, Universiti Sains Islam Malaysia, Nilai, Negeri Sembilan, Malaysia

1 Introduction With ever-increasing modern technologies, a portable power supply such as the rechargeable battery is a necessity. The use of rechargeable batteries is more sustainable and economical as compared to a one-time-use battery; in addition, it can also reduce hazardous waste caused by battery disposal. Currently, rechargeable batteries have been used in the majority of electronic devices such as wearable gadgets, laptops, smartphones, and also electric/hybrid cars. The lithium-ion battery is among the most popular rechargeable battery used in the market today because of its high energy density, stable cyclability, and light weight [1]. However, the high reactivity of heavy metals has become the main problem in the fabrication of a safe battery and leads researchers to find alternatives [2]. A battery based on a proton (H+ ion)-conducting membrane has been reported as a promising alternative to lithium-ion batteries. This is because the small radius of an H+ ion makes it easy to intercalate onto electrode surfaces, which in turn results in better electrochemical cell performance [3]. More importantly, there are no safety issues and the cost is low [2]. Research on polymer membranes began in the 1970s after Wright and coworkers discovered ionic conductivity in a poly(ethylene oxide) (PEO)-alkali metal salt complex, and Armand and coworkers proposed to assemble the polymer membrane in a rechargeable battery [4]. Since then, numerous studies have been actively conducted in an attempt to make them suitable for

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FIG. 1 Percentage of publications related to polymer membranes for battery applications published from 1995 to 2018. Data from ScienceDirect Online databases (accessed October 2018).

rechargeable battery applications. This research has also continued to increase every year, as evidenced by the increase in the percentage of papers published each year regarding this field (Fig. 1). A polymer membrane or so-called electrolyte is the most important component in a battery. It plays the role of a chemical medium that allows the transferring of an electrically charged ion between electrodes during the charge/discharge process. The desired properties for a polymer membrane to be utilized in a rechargeable battery, which is also essential in overall battery performance, are as follows [5]: (i) High ionic conductivity, σ
 104 S cm1, at room temperature. This is the most critical parameter in the preparation of the polymer membrane. For liquid electrolyte, ionic conductivity is generally higher compared to that of the polymer membrane, in which the magnitude order of 102 S cm1 is considered normal, while 104 S cm1 would be sufficient for the membrane. However, higher ionic conductivity would be more beneficial for battery applications. (ii) High ionic transference, tion  1, at room temperature. It is necessary to have a higher ionic transference number to reduce the concentration effect. A higher concentration gradient would lead to battery failure as a result of higher internal resistance. (iii) Wide electrochemical stability window. The required operating voltage for each battery is different depending on the active material used in the electrode component. For a rechargeable proton battery, the standard operating voltage is within 1 V. Therefore the electrochemical window for the electrolyte material must be within the potential range. A lower electrochemical stability window than desired can contribute to poor battery performance. (iv) Good mechanical strength. Good mechanical properties of the polymer membrane are essential to accomplish large-scale battery production. Mechanical stability is often related to the flexibility and elasticity of the membrane, which means that the material

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should not be brittle when stress arises during the battery assembly and fabrication process. This feature is also necessary especially for rechargeable battery types to avoid membrane damage and capacity loss during multiple charge/discharge cycles in which both can eventually result in battery failure. (v) Excellent chemical and thermal stability. In a solid-state battery, the polymer membrane is assembled between two electrodes (anode and cathode). Therefore the compatibility of these three components is essential to prevent the occurrence of undesirable chemical reactions at the electrode/electrolyte interfaces. Moreover, the membrane should be thermally stable as battery performance (i.e., discharge capacity), cycle life, and safety can be damaged by heat. The excellent thermal properties of the membrane can prevent thermal runaway (thermal abuse/effect) due to short circuit and overcharge of the battery, and also ensure the safe use of the battery at the high operating temperature range. Generally, in the polymer membrane, the polymer will act as the host material to allow the dissociation of ionic dopant/salt into the polymer matrix to produce ionic conductivity [6]. The selection of polymer depends on two factors: the first is that the material is a polar group polymer which has sufficient electron donors for cation coordination, the second is that the material should have high amorphous content to provide a flexible polymer chain for ion transport [7]. Pioneering work and most studies have been focused on PEO in membrane preparation because of its high solvation and coordination ability [8]. However, due to the depletion of fossil fuel, which is the raw material for synthetic polymer, as well as increasing environmental regulation to achieve fewer polluting products, this has motivated researchers to shift to natural polymers [4]. Natural polymers, which are also known as biopolymers or biodegradable polymers, are extracted from natural resources such as plants and animals. This material can provide a sustainable platform, and the most importantly it can diminish environmental problems due to toxicity and nonbiodegradable waste. Besides, it also has a comparable ionic conductivity value to the synthetic polymer when developed as a membrane, which is in the range of 1011 to 106 S cm1 [9–12]. Examples of natural polymers that are widely studied include cellulose and chitosan. Even so, the ionic conductivity value is still insufficient for membrane material and may require many improvements.

1.1 Ionic dopant/salt effects on biopolymers Ionic dopant/salt is often used to increase ionic conductivity as well as the mechanical and chemical stability of the biopolymer membrane. Generally, the role of ionic dopant/salt is to provide ions for the conduction to occur in the biopolymer matrix. The addition of ionic dopant/salt increases ionic conductivity in two ways: (1) it increases the amorphous phase of the biopolymer membrane and (2) it increases the complexation between ionic dopant/salt and the biopolymer material. For the optimum effect, the selection of ionic dopant/salt must consider the following criteria [8, 13]: 1. Small cation radii 2. Large anion radii

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Low lattice energy Good thermal stability Nontoxicity Less sensitivity to ambient moisture Good solubility

Various types of ionic dopant/salt have been previously studied, but the most prevalent used in biopolymer membrane preparation is ammonium salt (inorganic salt) as it can provide ions—proton (H+ ion) species—for the conduction process. Nevertheless, it should be noted that the ion transport occurs only in amorphous material. A report by Moniha et al. [14] studied the effect of ammonium salt on the amorphous nature and ionic conductivity of the biopolymer membrane. They showed that peak intensity due to the crystalline nature of iota-carrageenan disappeared and the hump peak became broader after the addition of salt, which implies an increase in both amorphous content and ionic conductivity value. Similar behavior was observed for agar-NH4I and pectin-NH4SCN membranes [15, 16]. Pandi et al. [17] and Koduru et al. [18] stated that the increase in amorphous content improved ion mobility due to reduction of the energy barrier, which therefore increased the ionic conductivity. Ammonium salt is also considered to be a good proton donor, it could support proton conduction through the biopolymer matrix as well as promoting ionic conductivity as high as 103 S cm1 [10, 19–26]. Ahmad and Isa [25] reported the interaction between carboxymethyl cellulose (CMC) and ammonium chloride salt. They showed that the shifting of the peak is due to protonation of H+ ions, which dissociated from ammonium salt onto the coordinating site of the polymer host. This protonation process increased the number of salt dissociations and also ionic conductivity. Other studies done by Sohaimy and Isa [27], Mejenom et al. [28], and Noor and Isa [29] have also reported similar results. The values of the transference number, tion, in most biopolymer-ammonium salt complexes were reported to be higher than 0.5, which further proved that the mobile ionic species is mainly due to proton (H+ ions) [30, 31].

1.2 Biopolymers incorporated with a plasticizer The ionic conductivity of the biopolymer membrane can be further enhanced by a plasticizing method. The plasticizer can penetrate and disrupt the cohesive force between the biopolymer chains due to small molecule size, which increases the segmental motion and creates more free volume for ion transport [13]. The modified biopolymer chains improve the amorphous phase and ease the movement of free ions through the polymer backbone, this results in overall ionic conductivity enhancement. Isa and Noor [20] reported that ionic conductivity of CMC-ammonium thiocyanate was enhanced to about three orders of magnitude after plasticized with ethylene carbonate. Similar results were obtained for plasticized CMCammonium chloride with propylene carbonate [25] and plasticized CMC-oleic acid with glycerol [31]. Nevertheless, the use of a plasticizer may sometimes reduce the mechanical stability of the biopolymer membrane, depending on the type and amount of plasticizer used [32]. Therefore, the miscibility of the plasticizer with a biopolymer host and ionic dopant/salt is essential to produce a very flexible and high ionic conductivity biopolymer membrane. Various types of plasticizers have been explored and the respective physical properties are listed

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1 Introduction

TABLE 1 Structure and physical properties of the common plasticizers used in biopolymer membrane preparation. Melting point (°C)

Boiling point (°C)

Donor number

Dielectric constant (εr)

EC

36.4–38.0

244.0–248.0

16.4

89.9

PC

55.0 to 48.8

240.0–242.0

15.1

64.6–66.1

EG

12.9

197.3

N/A

N/A

PEG

N/A

N/A

N/A

19.9

DMC

2.4

90.0

17.2

3.1

Glycerol

17.8

290.0

19.0

N/A

DEC

43.0

126.0

16.0

2.8

Plasticizer

Structure

DEC, diethyl carbonate; DMC, dimethyl carbonate; EC, ethylene carbonate; EG, ethylene glycol; N/A, not available; PC, propylene carbonate; PEG, polyethylene glycol.

in Table 1. The choice of plasticizer depends on high dielectric constant, high donor number, and high boiling temperature to assist more salt dissociation, this later on increases the mobility of free ions through the biopolymer matrix [33]. In addition to that, the introducing of a plasticizer into a biopolymer-salt complex has been reported to increase the amorphous content. Plasticization of poly(ε-caprolactone)-NH4SCN complex [34] and CMC-NH4Br complex [35] with ethylene carbonate was found to be more amorphous compared to that of unplasticized complexes. Instead of modifying the structure of the polymer chain, the presence of plasticizer was reported to provide a

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new pathway/coordinating site for ion transport, which promotes the dissociation of H+ ions for the protonation process and therefore enhances the ionic conductivity of the plasticized system. From these reviews, it can be concluded that the addition of ionic dopant/salt and plasticizer material has modified the structural properties of the biopolymer chain and promoted the protonation of H+ ions through the system, and does increase the ionic conductivity.

2 Preparation of biopolymer membranes Solution casting is the most favored method for the preparation of flexible, free-standing, and transparent biopolymer membranes. Traditionally, the solution casting method refers to a process of solidification of the biopolymer membrane/thin film by evaporating the solution. In this method, a biopolymer is dissolved in a suitable solvent depending on its solubility at either room or specific temperature. The solution is stirred using a magnetic stirrer until it turns into a homogeneous solution with no phase separation. The chosen ionic dopant (inorganic salt or weak acid) and plasticizer material are also added into the same solution and stirred well for uniform mixing. The homogeneous solution is then cast into Petri dishes (Teflon disc) and left to dry in a controlled condition to allow the solvent to evaporate. A simple oven-drying technique can also be adopted to minimize the drying period. Prior to characterization, all the membranes must be kept in a desiccator to prevent moisture contamination. This method is simple, cost effective, and more importantly, it can be cast into a desired size and shape in the mold. A schematic diagram of the biopolymer membrane preparation method is depicted in Fig. 2.

3 Characterization of biopolymer membranes for batteries Four common techniques can be employed to investigate the properties of the biopolymer membrane, including electrical impedance spectroscopy (EIS), X-ray diffraction (XRD),

FIG. 2 Biopolymer membrane preparation by using the solution casting method.

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Fourier transform infrared (FTIR) spectroscopy, and transference number measurement (TNM). Details of each technique are discussed as follows.

3.1 Electrical impedance spectroscopy EIS is a useful technique for characterizing the electrical properties of the biopolymer membrane. The EIS data can be presented in many types of plot, but the most commonly used is in Cole-Cole plot form or so-called impedance plot, in which the imaginary part of impedance, Zi, is plotted against the real part of impedance, Zr. Each plot obtained corresponds to the impedance behavior at a particular frequency and is also represented by an appropriate equivalent circuit model. The analysis of these data is briefly explained in the following section. 3.1.1 Equivalent circuit model The typical Cole-Cole plot of the biopolymer membrane and its corresponding equivalent circuit are presented in Fig. 3. The Cole-Cole plot is not only used to investigate the electrical behavior of materials but also helps to distinguish the effect of bulk material, grain boundary, and electrode/electrolyte interfaces on ion conduction. Therefore, a different semicircular shape observed from the Cole-Cole plot can explain a different phenomena occurring in the biopolymer membrane. From Fig. 3A, the plot shows a combination of an incomplete semicircle with an inclined spike. The incomplete semicircle in the higher frequency region corresponds to the bulk effect of the biopolymer membrane, it is equivalent to a parallel combination of bulk resistance (Rb) with constant phase element (CPE1) or some known bulk capacitor (Cb). The incline spike FIG. 3 (A) Cole-Cole plot (Nyquist impedance plot) of a biopolymer membrane with (B) its equivalent circuit.

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FIG. 4 Schematic diagram of ion migration and ion accumulation in a biopolymer membrane in the presence of SS blocking electrodes. Reproduced with permission from N.K. Zainuddin, A.S. Samsudin, Investigation on the effect of NH4Br at transport properties in k-carrageenan based biopolymer electrolytes via structural and electrical analysis, Mater. Today Commun. 14 (2018) 199–209. Copyright (2018), Elsevier.

in the lower frequency region refers to the polarization effect at the electrode/electrolyte interfaces, it is represented by CPE2 or a double-layer capacitor, Cd. These two processes are illustrated in Fig. 4. Under the polarization condition (Fig. 4), the charge carriers in the biopolymer membrane are not able to move out to the external circuit due to the stainless steel (SS) blocking electrode. The opposite charge carrier, which is attracted to the electrode surface charge, causes the formation of another monolayer charge at the interfaces or so-called electric double layer [36]. The overall equivalent circuit for this Cole-Cole plot type can be fitted as in Fig. 3B. For an ideal plot, the spike in the lower frequency region should exhibit a vertical straight line (90 degrees) with the imaginary axis in order to show ideal capacitive behavior [37]. However, this is impossible to see in a practical condition, and as a result, CPE is often used in the model circuit to represent nonideal capacitance. From the Cole-Cole plot in Fig. 3A, Rb refers to the bulk response due to ion migration in the biopolymer membrane, while CPE1 represents the immobile polymer chain. Another CPE2, which is connected in series, refers to the accumulation of double-layer charge at the interface of the biopolymer membrane and blocking electrodes [7]. In some contexts, the semicircle may not be visible in the plot, this indicates that only the resistive component of the biopolymer would prevail due to mobile ions inside the membranes [35]. 3.1.2 Ionic conductivity As in any electrolyte or biopolymer membrane system, ionic conductivity is the benchmark for practical application purposes. In general, ionic conductivity is a result of ion conduction from one site to another through a biopolymer matrix. Two factors, in particular, can influence ionic conductivity: (1) fraction of amorphous phases and (2) salt dissociation. For that reason,

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the biopolymer membrane is commonly measured as a function of salt/plasticizer composition and temperature. The calculation of ionic conductivity, σ, can be made using Eq. (1): σ¼

t Rb A

(1)

where A (cm2) is the contact area between electrode/electrolyte and t (cm) is the thickness of the studied membrane. Bulk resistance, Rb, can be determined from the interception between the semicircle and the inclined straight line at the real part impedance of the Cole-Cole plot (Fig. 3). Based on Eq. (1), Rb is related to the ionic conductivity of the biopolymer membrane, in which the smaller Rb value will result in higher ionic conductivity. The influence of temperature on the ionic conductivity of the biopolymer membrane can be observed through the study of the temperature dependence of ionic conductivity (log σ against 1000/T) and interpreted in terms of the Arrhenius or Vogel-Tammann-Fulcher (VTF) model. Besides, it can be used to identify the nature of the studied membrane of either the amorphous or crystalline phase, as will be shown by XRD analysis. If the conductivitytemperature relationship exhibits a linear plot with a regression value approximately unity (R2  1), then the membrane follows Arrhenius behavior (Fig. 5A and B) [38, 39]. In a such model, the biopolymer membrane is substantially amorphous and the mechanism of ion transport is due to the hop of the ionic charge species between two coordinating sites [7]. As the temperature increases (Arrhenius model), the polymer backbone and side chain experience a higher vibrational dynamic, which causes the coordinating sides to move closer between each other. This is expected to assist more ions hopping to the nearest empty side due to less energy required and hence improve ion transport and ionic conductivity simultaneously. In contrast to the VTF model, ion transport is governed by both polymer segmental motion and ionic motion due to the strong interrelation between them [40]. These generate curvature of an ionic temperature plot as can be seen in Fig. 5C and D. In a such model, the biopolymer membrane is commonly presented in both the crystalline and amorphous phases [7]. For VTF behavior, the increase in ionic conductivity at higher temperatures can be related to the freevolume theory, where the polymer will expand and create a larger volume for the ions and polymer segmental mobility [7]. This will lead to the significant ion transport and consequently enhanced ionic conductivity. The Arrhenius and VTF relation of ionic conductivity is given by Eqs. (2), (3), respectively: σ ¼ σ o exp 

σ ¼ AT

1=2

Ea kB T

B exp  kb ðT  To Þ

(2)  (3)

where, σ o is the conductivity preexponential factor, Ea is the activation energy, kB is the Boltzmann constant, T is the absolute temperature in kelvin, A is the preexponential factor of the charge ion number, B is the pseudoactivation energy of polymer segmental motion, and To is the Vogel temperature corresponding to the equilibrium glass transition temperature, Tg, where To ¼Tg  50 K. The value of activation energy for both biopolymer membrane types can be evaluated from the slope of log σ against 1000/T.

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FIG. 5

Some examples of (A) and (B) Arrhenius plot and (C) and (D) Vogel-Tammann-Fulcher plot. Reproduced with permission from (A) S. Monisha, T. Mathavan, S. Selvasekarapandian, A. Milton Franklin Benial, G. Aristatil, N. Mani, M. Premalatha, D. Vinoth Pandi, Investigation of bio polymer electrolyte based on cellulose acetateammonium nitrate for potential use in electrochemical devices, Carbohydr. Polym. 157 (2017) 38–47. Copyright (2017), Elsevier; (B) M. Premalatha, T. Mathavan, S. Selvasekarapandian, S. Selvalakshmi, S. Monisha, Incorporation of NH4Br in tamarind seed polysaccharide biopolymer and its potential use in electrochemical energy storage devices, Org. Electron. 50 (2017) 418–425. Copyright (2017), Elsevier; (C) R. Baskaran, S. Selvasekarapandian, N. Kuwata, J. Kawamura, T. Hattori, AC impedance, DSC and FT-IR investigations on (x)PVAc–(1x)PVdF blends with LiClO4, Mater. Chem. Phys. 98 (2006) 55–61. Copyright (2006), Elsevier; (D) R. Alves, J.P. Donoso, C.J. Magon, I.D.A. Silva, A. Pawlicka, M.M. Silva, Solid polymer electrolytes based on chitosan and europium triflate, J. Non-Cryst. Solids 432 (2016) 307–312. Copyright (2016), Elsevier.

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3.1.3 Dielectric properties Dielectric study provides information about the polarization that occurred within the frequency study and it can also be used to verify the ionic conductivity behavior. Dielectric constant refers to the amount of charge stored in each cycle of the applied electric field and the amount of dipole alignment inside the membrane [41]. Dielectric loss represents the component of energy loss of a mobile charge or dipole alignment during each cycle of the applied electric field [41]. The value of the dielectric constant, εr, and dielectric loss, εi, are calculated using Eqs. (4), (5), respectively: εr ð ω Þ ¼

Zr
ωCo Z2r + Z2i

(4)

ε i ð ωÞ ¼

Zi
ωCo Z2r + Z2i

(5)

From the equation, Co ¼ εoA/t (εo is the permittivity of free space), ω ¼ 2πf (f is the frequency measured in Hz), Zr is the real impedance, and Zi is the imaginary impedance. In order to detect the relaxation peak in the high-frequency region, the values of real Mr and imaginary Mi part of the electrical modulus are calculated based on Eqs. (6), (7): εr Mr ¼ 2 2
(6) εr + εi Mi ¼

ε2r

εi
+ ε2i

(7)

If both dielectric and modulus formalism do not show any relaxation peak in the plot, loss tangent can be adopted. Loss tangent is the measure of the ratio of dielectric loss, εi, to dielectric constant, εr, or imaginary electrical modulus, Mi, to real electric modulus, Mr. It is given by: tan δ ¼

εi Mi ¼ εr M r

(8)

3.1.4 Ionic conduction mechanism Further study on the ionic conduction mechanism helps in explaining the method of ion hopping with the influence of temperature. This mechanism is evaluated at a minimum or no polarization effect occurred by using Jonscher’s universal power law, as proposed by Jonscher in 1977 [20, 42]. The power law relation is expressed as: σ ðωÞ ¼ σ dc + σ ac

(9)

σ ac ¼ Aωs

(10)

σ ac ¼ εo εi ω

(11)

The σ ac can be given by

and also

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Consolidating Eqs. (10), (11) gives the following equations: εr εi ω ¼ Aωs

(12)

A s1 ω εo

(13)

εi ¼

By applying the natural logarithm rule to Eq. (13), the final relation is written as: lnεi ¼ ln

A + ðs  1Þ lnω εo

(14)

where σ ac is ac conductivity, σ dc is dc conductivity, ω is the angular frequency (ω ¼ 2πf), εo is the permittivity of free space, εi is the dielectric loss, A is a parameter dependent on temperature, and s is the power law exponent in the range of 0 < s < 1. The value of exponent s can be calculated from the slope, m, of ln εi against ln ω, where m ¼ s  1. Another way to determine the value of exponent s is by applying the natural logarithm rule to Eq. (10), the final relation is written as in Eq. (15). For this method, the value of exponent s is retrieved from the slope, m, of ln σ ac against ln ω, where m ¼ s: ln σ ac ¼ ln A + s ln ω

(15)

In order to study the mechanism of ion hopping in the biopolymer membrane, the value of exponent s is compared to four different theoretical conduction models through the plot of s against T, as illustrated in Fig. 6 [27]. In a small polaron hopping model, the exponent s is dependent on the temperature where the value is increased with increasing temperature. Whereas in an overlapping large polaron tunneling model, the exponent s is first decreased to a minimum value and starts to rise with further increasing temperature. In the quantum mechanical tunneling model, the exponent s is independent of temperature with the value at FIG. 6 Theoretical conduction model for (A) SPH, (B) OLPT, (C) QMT, and (D) CBH mechanism.

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about 0.8, however some studies show a lower value than mentioned [10, 27, 43]. The correlated barrier hopping model is also temperature independent with the exponent s ! 1 when T ! 0 K.

3.2 X-ray diffraction XRD is a powerful technique to identify the phase changes in the biopolymer membrane— it is available in crystalline, amorphous, or both regions. Amorphous material exhibits a broad peak/hump; crystalline materials give only a sharp peak, while semicrystalline materials consist of both sharp and broad peaks (Fig. 7). In the crystalline phase, arrangement of the biopolymer chain is ordered with immobile ions and the amorphous phase possessing no ordered arrangement with mobile ions. Pandi et al. [17] and Koduru et al. [18] stated that ion transport in solid material only occurred in the amorphous phase by way of hopping mechanism. A higher degree of amorphous phases can promote further ion mobility due to the reduction of energy barrier and consequently increased ionic conductivity [44]. Therefore reducing the crystalline region in the biopolymer matrix is essential for ionic conductivity enhancement.

FIG. 7 Typical XRD pattern and the respective structure for (A) amorphous, (B) crystalline, and (C) semicrystalline material. Reproduced with permission from M.A. Ramlli, M.I.N. Isa, Structural and ionic transport properties of protonic conducting solid biopolymer electrolytes based on carboxymethyl cellulose doped with ammonium fluoride, J. Phys. Chem. B 120 (2016) 11567–11573. Copyright (2016) American Chemical Society.

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FIG. 8 Example of XRD deconvolution for a biopolymer membrane.

The reduction in the crystalline region can be determined using two different methods. The first method is the XRD deconvolution method. This method is used to segregate the crystalline and amorphous region of the membrane as shown in Fig. 8. In general, the degree of crystallinity is inversely proportional to the amorphous region, where the reduction in the degree of crystallinity gives the higher amorphous region. The degree of crystallinity, χ c(%), is calculated using Eq. (16), where Ac is the area representing the total crystalline region and Aa is the area representing the total amorphous region. χ c ð%Þ ¼

Ac Ac + Aa

(16)

Another method is called the full width at half maximum (FWHM)-Scherer method. In this method, the increase of the amorphous region can be interpreted by correlating between the broadness of the XRD spectrum peak (FWHM) and the crystallite size, which can be calculated using Scherer’s formula as shown in Eq. (17). The membrane that possesses the highest amorphous region gives the widest XRD peak width (FWHM) and the smallest crystallite size [45]: L¼

0:94λ FWHM cos θ

(17)

˚ ), FWHM is the full width at half From the equation, λ is the X-ray wavelength (1.5406 A maximum, and θ is the Bragg diffraction angle of the studied membrane.

3.3 Fourier transform infrared spectroscopy FTIR spectroscopy is an excellent method for the identification of various kinds of functional groups/vibrational bands. When the membrane is exposed to infrared radiation, the molecules

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absorb the radiation at a particular wavelength region and cause changes in dipole moment to produce the final FTIR spectrum. The results are generally presented in two forms: (1) absorbance versus wavelength and (2) transmittance versus wavelength. Apart from that, FTIR spectroscopy can also predict the complexation that occurs in the biopolymer membrane. The ion is considered to be complexed/interacted with the coordinating site of the polymer chain when there is any peak shifting to either lower or higher wavenumber, peak disappearance, and/or emergence of the new peak found in the spectrum [44]. This complexation can also contribute to ionic conductivity, similar to that reported by previous researchers who investigated the correlation between ionic conductivity and complexation of biopolymer membranes [24, 25, 28]. However, it should be noted that this method is not suitable for diatomic molecules because there is no change in dipole moments such as Br2, O2, and N2. Fig. 9 shows some common vibrational modes in the region between 4000 and 600 cm1. As ionic conductivity generally depends on the number of free charge species and their mobility in the biopolymer membrane as given in Eq. (18), it is essential to evaluate the transport parameter involved. Several techniques have been proposed to calculate the transport parameters. One of the most common methods used is the Rice and Roth method. This method is calculated based on the EIS result (in Section 3.1). Since the equation in the Rice and Roth method is related to the Arrhenius equation, this method is valid only for the Arrhenius-type sample [46]. From Eq. (18), σ is the ionic conductivity, η is the number density of free charge species, q is the ionic charge, and μ is the mobility of free charge species. σ ¼ Σμqη

(18)

The FTIR deconvolution technique is another established method for determining the qualitative parameters of ion transport. FTIR deconvolution is a process that distinguishes

FIG. 9 Some common vibrational modes in the region between 4000 and 600 cm1.

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FIG. 10 Example of FTIR deconvolution for a biopolymer membrane.

or separates the overlapped peak that exists in the spectrum. Through this method, the value of free ions and contact ions can be determined. In general, free ion refers to the number of ions supplied due to the ionic dopant/salt (donor agent) dissociation process inside the membrane, and the contact ion represents the formation of ion pairs and ion aggregation. Free ion enhances the ionic conductivity of the membrane as it improves ion migration through the biopolymer backbone, whereas contact ion reduces the amount of free ion due to its larger size and lower mobility [47]. An example of the deconvoluted plot is presented in Fig. 10. The percentages of free ion (FI %) and contact ion (CI %) are expressed as below: FI ð%Þ ¼

Af  100% Af + Ac

(19)

CI ð%Þ ¼

Ac  100% Af + Ac

(20)

where Af is the area of free ion and Ac is the area of contact ion. By obtaining the percentage of free ion, the transport parameters (number density, η, ionic mobility, μ, and diffusion coefficient, D) of free ions can be calculated. These parameters are given by Eqs. (21)–(23), respectively. η¼

M  NA  FI ð%Þ Vtotal

(21)

σ ηe

(22)

μ¼

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D¼

μkB T e

493 (23)

From the equations, M is the number of moles of material used, NA is the Avogadro’s number, σ is the ionic conductivity, e is the electric charge, kB is the Boltzmann constant and T is the absolute temperature in kelvin. Vtotal is the total volume of the studied membrane, where V is equal to mass, m, divided by density, ρ, of each material used.

3.4 Transference number measurement In the biopolymer membrane, the mobility of both ion and electron species may contribute to overall ionic conductivity. However, the information regarding these species could not be obtained through the ionic conductivity data. Therefore analysis of transport or transference number is essential as it can differentiate between which species, as either ion or electron, is more pronounced in the conduction process. Generally, the transference number is the fraction of the current carried by a given conducting species [37]. Different conducting species may possess different mobilities due to the different current they may carry, and hence result in a different reading of the transference number [48]. Several techniques can be applied for TNM analysis such as the galvanostatic polarization method, potentiostatic polarization method, pulsed field gradient-nuclear magnetic resonance method, electromotive force method, and Tubandt method [49]. Among these, the direct current polarization method is the most common method used to determine the transference number value [27, 30, 31]. This method employs a membrane with two SS blocking electrodes as depicted in Fig. 11. When the voltage (1.5 V) is applied to the circuit, the initial current, Ii, will decrease until steady-state current, Iss, is achieved. At this state, all mobile ions are depleted. The residual of the current flow is due to electronic migration inside the membrane. A typical plot of normalized current against time is presented in Fig. 12. From the plot in Fig. 12, the value of the transference number for the particular charge species can be determined. The transference number of ions, tion, and electrons, te, is expressed as Eqs. (24) and (25), respectively, where Iss is the saturated current and Ii is the initial current of the biopolymer membrane. FIG. 11 Experimental setup for TNM measurement via the dc polarization method.

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FIG. 12

A typical plot of normalized current against time. Reproduced with permission from N.A.M. Noor, M.I.N. Isa, Investigation on transport and thermal studies of solid polymer electrolyte based on carboxymethyl cellulose doped ammonium thiocyanate for potential application in electrochemical devices, Int. J. Hydrog. Energy 44 (2019) 8298–8306. Copyright (2019), Elsevier.

tion ¼

Ii  Iss Ii

te ¼

Iss Ii

(24) (25)

The majority of studies have reported that only mobile ions (cation and anion) could contribute to the transport number [21, 27]. If the transference number is higher than 0.5 (tion > 0.5), it is assumed that the conducting species in the biopolymer membrane has mainly resulted from ions [50]. Otherwise, ionic conductivity is largely due to electron mobility [50]. Ideally, the transference number for the biopolymer membrane should be approximately 1 (tion  1) so that only ion mobility occurs (purely ionic), and no concentration gradient would form [27, 37]. A high concentration gradient will contribute to poor electrochemical cell performance once the membrane is utilized in a practical work [27]. The ions are basically composed of both cation and anion. Since this work is related to the proton (H+ ion) conduction system, the membrane must be predominantly due to cationic species, which can be further determined through ionic mobility and diffusion coefficient data. The calculation for the ionic mobility, μ, of cation and anion can be done using the following relation: σ (26) μ ¼ μ + + μ ¼ nq tion ¼

μ μ + + μ

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(27)

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The diffusion coefficient of cation, D+, and anion, D-, is given by: D ¼ D + + D ¼ tion ¼

kTσ ne2

D D + + D

(28) (29)

If the value of cation for both parameters is higher than anion, it can be concluded that the majority of ionic species in a biopolymer membrane are cation. Otherwise, it is mainly due to anionic species.

4 Biopolymer membranes in the battery The battery is composed of one or more electrochemical cell(s) that can generate electricity by a chemical reaction. The battery can be categorized into primary and secondary types. A primary battery (disposal battery) is a single-use battery and cannot be recharged after usage. Typical examples of primary batteries are alkaline battery, lithium battery, zinc-air battery, silver oxide battery, mercury battery, zinc-carbon battery, and manganese dry battery. A secondary battery is a rechargeable battery. This battery consists of a reversible electrochemical cell reaction that allows the battery to be discharged and recharged multiple times. Some examples of secondary batteries available in the market are lithium-ion battery, ironnickel battery, lead-acid battery, nickel-cadmium battery, nickel-metal hydrate battery, nickel-zinc battery, and sodium-sulfur battery. Because this chapter is focused only on the rechargeable proton battery, its operating principle and characterization technique are reviewed and discussed in a subsequent section.

4.1 Operating principle of a battery A battery consists of two electrodes (anode and cathode) separated by the electrolyte (also known as biopolymer membrane). Both anode and cathode are composite materials—it is composed of active material, a conductive material, and a binder. The active material stores ions for the electrochemical reaction in the electrode, the binder is used to bond the active material with a conductive agent, and the conductive material acts as the additive component which is used to improve the electronic conductivity of the electrodes [51]. The different components used in the electrode play important roles on the performance of a battery as it is depends on the efficiency of the ions and electrons transported between the electrodes [52]. The key in developing of a good rechargeable proton battery is that the anode must be able to supply the H+ ion into the system, while the cathode should possess good reversible properties [53, 54]. The utilization of a biopolymer membrane with a high ionic conductivity value as mentioned earlier is also needed to allow fast diffusion of ion transport inside the battery. Several types of rechargeable proton battery have been previously studied, the most commonly used is the Zn + ZnSO47H2O/MnO2 type. This type of battery uses zinc (Zn) metal

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19. Biopolymer membranes for battery applications

FIG. 13

Schematic illustration during (A) discharging and (B) charging processes in a Zn + ZnSO47H2O/biopolymer membrane/MnO2 proton battery.

with zinc sulfate heptahydrate (ZnSO47H2O) as the anode and manganese dioxide (MnO2) as the cathode. The basic principle during discharging and charging in this battery is illustrated in Fig. 13. The operating principle in a proton battery is similar to all rechargeable battery types, in which a reversible chemical reaction causes the ions to move between electrodes. As shown in Fig. 13, the discharging and charging mechanism of this battery is based on the movement of proton (H+ ion) species inside the battery. When the battery is discharged (Fig. 13A), Zn in the anode is chemically oxidized and each Zn atom releases two electrons to form Zn2+ ions. The electron produced is then moved out through the external circuit and reaches the cathode. Meanwhile, the proton (H+ ion), which resulted from the hydrated zinc sulfate, is simultaneously transferred and diffused through the biopolymer membrane to form a balanced ionic charge between the electrodes. All the electrons received by the cathode reduce the MnO2 to Mn2+ ions. Upon completion of the discharging process, the Zn anode is completely oxidized and all H+ ions are considered to be accumulated at the cathode side. The reverse chemical reaction occurs during the charging process (Fig. 13B). When the battery is connected to the external power supply, the electron deposited at the cathode is pulled back to the anode side, while the H+ ion migrates back through the biopolymer membrane to maintain the ionic charge balance. Deintercalation of the electron into the anode material reduces the Zn2+ ion to Zn metal and the Mn2+ ion is oxidized to MnO2. Finally, when all the ions and electrons are moved back to the anode, the battery is considered completely charged and ready to be discharged again. It can be deduced that the electrochemical reaction in the battery involves the redox (reduction and oxidation) reaction at both anode and cathode

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4 Biopolymer membranes in the battery

TABLE 2 Possible reaction occurring at both anode and cathode in a Zn + ZnSO47H2O/ MnO2 rechargeable proton battery. Electrode

Possible reaction

Anode

Zn Ð Zn2+ + 2e

E0 (V) 0.76 

ZnSO47H2O Ð ZnSO4 + 7H + 7OH +

Cathode Overall



0.82

MnO2 + 2e + 4H Ð Mn + 2H2O +

2+



1.22 

Zn + ZnSO47H2O + MnO2 + 2e + 4H Ð Zn + 2e + ZnSO4 + 7H + 7OH + Mn2+ + 2H2O +

2+

+

1.28

Reproduced with permission from M.F. Shukur, M.F.Z. Kadir, Hydrogen ion conducting starch-chitosan blend based electrolyte for application in electrochemical devices, Electrochim. Acta 158 (2015) 152–165. Copyright (2015), Elsevier.

materials and the possible chemical reactions are depicted in Table 2. The overall reaction in the rechargeable proton battery can be calculated in accordance with the standard potential of each electrode, where E0ox + E0red ¼ E0cell [55].

4.2 Characterization of battery performance The performance of the battery is usually evaluated using a battery test bench, also known as a battery tester. The following section highlights the essential parameters that can be adopted to characterize the battery. 4.2.1 Open circuit voltage Open-circuit voltage (OCV) is performed to measure the stable voltage of each fabricated battery without connecting to any load. The OCV data are commonly presented in the form of voltage (V) against time (h). For this measurement, the battery is left in an open-circuit condition for a few hours until a stable voltage is achieved. The battery is considered stable if the voltage remains constant for about 24h. Based on the overall battery reaction stated in Table 2 in Section 4.1, the theoretical voltage is 1.28V and therefore the experimental OCV value for the Zn + ZnSO47H2O/ MnO2 battery can be estimated within the stated range. However, the value of OCV obtained in the experiment may vary for each battery system depending on the properties of the biopolymer membrane used and their chemical compatibility with the electrode materials. 4.2.2 Discharge characteristics The graph in Fig. 14 shows the typical discharge curve for a rechargeable battery. Note that the characteristic of the discharge curve might be different for each battery; some batteries exhibit a flat discharge curve (plateau), while others exhibit a slope discharge curve (no plateau). For a typical discharge characteristic (Fig. 14), there is a small voltage drop at the beginning of the discharging process, which then becomes relatively constant (plateau region) until the battery is almost completely discharged (exhausted). After this period, the voltage drops sharply. Samsudin et al. [21] considered the initial voltage drop to be due to the activation polarization effect, it is arises from the sluggish electrode kinetic. The plateau region indicates a constant voltage output. This condition allows the battery to provide constant

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FIG. 14

19. Biopolymer membranes for battery applications

Schematic of a typical discharge curve for a rechargeable battery.

power during the discharge period [56]. In addition, it is also referred to as constant energy stored in a battery; therefore a wide plateau region is advantageous as it reflects a large energy storage capacity. The immediate voltage drop at the end of the discharge curve is due to battery exhaustion. At this stage, all the H+ ions are depleted and the intercalation process will not occur. As in any battery system, electrical current also plays an important role in the discharge characteristics and is commonly expressed in amperes (A). Previous researchers have studied the effect of different currents on the characteristics of the discharge curve in their proton battery systems and showed a rapid discharge rate at a higher current drain. At higher discharge current (I
0.1 mA), the ion concentration changes very quickly and causes uneven H+ ion distribution at the cathode side, which therefore results in a decrease in cell capacity [4]. However, at the lower current drain (I < 0.1 mA), the rapid discharge rate can be related to the higher internal resistance, which restricts the ion intercalation process inside the cell and also reduces the cell capacity [57]. From the discharge curve analysis, the performance of a battery can be investigated. Generally, the performance of the battery is specified based on three critical parameters: battery capacity, energy density and power density. These parameters are described next [57–59]. (i) Capacity, Q (ah), refers to the ability of a battery to store or deliver electrical energy during the charging or discharging processes, while specific capacity, Qs (Ah kg1), is the capacity of a battery with respect to the mass of active material contained in the electrode. Both capacity, Q and specific capacity, Qs are given in Eqs. (30) and (31), respectively, where I is the amount of current drain, t is the discharging or charging time, and m is the mass of active material.

III. Application of biopolymers membranes/films in environment and energy

5 Conclusions and future prospects

499

Q¼It

(30)

It m

(31)

Qs ¼

(ii) Power density, P, is the measure of power output per unit weight or size of a battery. In other words, it refers to the capability of a battery to deliver energy. The higher the power density, the more significant amount of energy can be delivered to the device. The unit for power density is W kg1. Power density is expressed as Eq. (32), where I is the amount of current drain and V is the voltage. P¼IV ¼

E t

(32)

(iii) Energy density, E (Wh kg1), is the amount of energy that can be stored by a battery in a given weight or size. A battery that possesses high energy density can power a device for a longer time. Energy density can be calculated using the following equation: E¼QV ¼Pt

(33)

where I is the amount of current drain, t is the discharging or charging time, Q is capacity, and V is the voltage. 4.2.3 Rechargeability Rechargeability is another essential characteristic that needs to be considered in developing a rechargeable battery type, it is refers to the ability of a cell to complete the discharge/ charge cycle below the maximum capacity loss ( 50%) [48]. Lei et al. [49] developed composite films based on pectin and konjac glucomannan incorporated with tea polyphenol aiming to generate active food packaging. They verified that appropriate tea polyphenol levels increased the mechanical and resistant properties of the films significantly. 4.1.5 Starch Starch is a natural polymer composed of anhydroglucose residues consisting of amylose (linear-chain polysaccharide) and amylopectin (branched-chain structure formed by glucose) [7]. Starch can be obtained from different renewable sources such as corn, cassava, and potato. Edible coatings and films performed from starch are widely employed due to their characteristics of odorless, tasteless, transparency, and biodegradability [50]. Starch coatings and films present good oxygen and carbon dioxide barriers [9]. However, the starch coatings and films are soluble in water and offer a weak water vapor barrier due to the high hydrophilicity. Usually, starch films present excellent mechanical properties [51]. Nouraddini et al. [52] developed and characterized edible films based on eggplant flour and corn starch films and observed a decrease in mechanical properties when compared with the pure corn starch films. However, the incorporation of eggplant improved the film biodegradability compared with the only starch.

4.2 Protein-based edible coating and films The fibrous proteins are insoluble in water and consist of the primary structural components of the animal tissues. The globular proteins present solubility in water and aqueous solutions involving acids, bases, or salts and are responsible for developing several functions in living systems [53]. Different types of globular proteins such as soy protein, wheat gluten, whey protein, and corn zein have been studied due to their properties and capacity to form edible coatings and films. Different solvents have been used to synthesize coatings and films from protein solutions, such as ethanol, water, or ethanol-water combinations [35]. Protein-based coatings and films typically present an efficient oxygen barrier, inclusive at low relative humidity conditions [7]. In this section, we exhibit some different types of protein (corn zein, gelatin, wheat gluten, and whey protein) have been employed to prepare edible coatings and films that comprise.

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521

4.2.1 Corn zein Corn is the principal source of zein, comprising 45%–50% of the total corn protein [54]. Zein is a prolamin protein with high solubility in 70%–80% ethanol [55], presenting hydrophobic characteristics related to the high content of nonpolar amino acids [54]. Zein biodegradable edible coatings and films have an excellent film-forming capacity [7], forming final characteristics such as tough, glossy, and hydrophobic grease proof [54]. Zein edible materials should be added with plasticizers aiming to reach flexibility since this kind of protein is highly brittle [56]. Edible coatings and films obtained from zein present outstanding moisture blockers [23]. Cross-linking agents and fatty acids could be incorporated to enhance the water vapor barrier characteristics of zein coatings and films [57]. 4.2.2 Gelatin Gelatin is produced from the hydrolysis process of fibrous hydrophobic protein, the collagen. Collagen is the natural occurrence as the principal component of bone, skin, and connective tissue [7]. Gelatin is a protein that comprises a high film-forming ability with excellent mucoadhesive properties. It is considered nontoxic, nonirritant, and food-grade material [58]. Gelatin edible coatings and films are readily available and exhibit good transparency and mechanical and barrier properties [59, 60]. Musso et al. [61] developed edible films based on gelatin and curcumin aiming to apply as a potential smart food packaging since they could inform consumers if the product was suitable for consumption through their capacity to sense pH changes. Soo and Sarbon [62] incorporating rice flour into edible gelatin films noted an improvement on tensile strength and elongation at break, besides that an increase in the water vapor permeability while decreasing film solubility. 4.2.3 Wheat gluten Wheat gluten is a hydrophobic protein obtained from wheat flour. The process of the edible coatings and film formation is facilitated by own properties of gluten, such as cohesiveness and elasticity [7]. Two components are found in wheat gluten—prolamine and glutelin— usually named as gliadin and glutenin, respectively. Gliadin is soluble in ethanol 70%, while glutenin is not soluble in this same condition [63]. Wheat gluten can be solubilized in aqueous-based solutions at low or high pH, considering low ionic strength [7, 64]. Edible coatings and films performed with wheat gluten can be added with plasticizers (glycerin) to improve its flexibility [63]. The formation of wheat gluten film and its stabilization are a crucial function for the processing of foods such as dough and batter-based systems. Gluten proteins generate a viscoelastic film network to entrap gas, along with starch and other ingredients [65]. Zhang and Mittal [66] stated that wheat gluten films present very high-water sensitivity and permeability, which could represent the significant and potential commercial application. 4.2.4 Whey protein Whey proteins are obtained from cheese and casein manufacture, which consist of a different amount of proteins (α-lactalbumin, β-lactoglobulin, immunoglobulins, and others). It is

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21. Edible biopolymer coatings on meats, poultry, and seafood

available as whey protein concentrate (20%–80% of protein content) or whey protein isolate (higher than 90% of protein content) [19]. Whey proteins exhibit an excellent capacity to form films, presenting as principal advantages unique gas barrier properties at low relative humidity, volatile aromas, and lipids [67]. Furthermore, whey protein films are characterized by excellent mechanical properties [68]. Whey proteins are characterized as hydrophilic, and consequently the films have some limitations to moisture affinity [7].

4.3 Composite edible coatings and films The most studies involving edible materials have been focused on composite or multicomponent films aiming to discover new complementary advantages of each component and to reduce the disadvantages [68, 69]. Composite coatings or films are elaborated associating a hydrophilic structural matrix and a hydrophobic compound (i.e., lipids) that result in a better functionality when compared with the pure hydrocolloid films, particularly concerning their properties of moisture barrier. According to Hassan et al. [7], the main objective of manufacturing composite (heterogeneous) coatings and films is to improve the mechanical properties and permeability. Composite materials can be obtained either in successive layers (bilayers), emulsions, or dispersions [23]. Polysaccharides and proteins usually present the capacity to form films with excellent mechanical properties, however, with weak moisture barriers due to their hydrophilic nature. The addition of lipids to produce composite coatings and films provides suitable moisture barriers [23, 70]. The emulsified components are achieved during one casting and one drying process. According to Fabra et al. [71], the functional properties of the emulsified components are related to the method of preparation, type, and quantity of components (hydrocolloid and lipid) and their compatibility. Emulsion-based coatings and films were prepared by Kamper and Fennema [72] using fatty acids and methylcellulose to enhance the water vapor barrier properties. Younis and Zhao [73] developed edible films based on blends of pectin and chitosan and observed that synergistic effect related to the mechanical properties was observed for the blends when compared with the pure polysaccharides. Dou et al. [74] prepared active edible films incorporating tea polyphenols into gelatin-sodium alginate solution and observed that the addition of bioactive was an effective method to improve the physical properties and antioxidant activity of the films. Tsai and Weng [75] prepared a new edible composite film based on whey protein isolate and zein and found that the increase of whey protein isolate content promoted maximum elongation and break elongation of the composite films.

5 Active components incorporated in edible films and coatings to meats, poultry, and seafood Edible film or edible coating could carry food additives such as antioxidants, antimicrobials, colorants, fortified nutrients, and flavors. Incorporation of additives into coating material has the advantage of allowing the slower release from the film to the surface product, remaining at high concentrations for extended periods [9].

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6 Effect of edible films and coatings on quality of meats, poultry, and seafood

523

Natural antioxidants have been studied to apply in meat and meat products to inhibit or retard lipid oxidation. Viji et al. [76] observed that natural extracts or powder was effective in inhibiting lipid oxidation in different frozen, frozen, and processed fish species such as hamburgers, meatballs, minced fish, and marinated cuts. Among the natural antioxidants present in fruits and vegetables, there are three main groups: ascorbic acid and phenolics as hydrophilic antioxidants and carotenoids as lipophilic antioxidants. Some vitamins (ascorbic acid and alpha-tocopherol), herbs and spices (rosemary, thyme, oregano, sage, basil, pepper, clove, cinnamon, and nutmeg), and plant extracts (green tea and grape seed) have also been widely used to delay lipid oxidation in various products derived from pork, beef, and poultry [77]. Natural phenolic antioxidants, as well as synthetics, can effectively sequester free radicals, absorb light in the ultraviolet region (100–400 nm) and chelate transition metals and thus delay autoxidation and the production of undesirable odors and flavors in meat products [78]. Fang et al. [79] studied the incorporation of gallic acid into a chitosan-based coating for pork steaks. They observed that adding higher concentration gallic acid (0.4%) resulted in higher protein oxidation compared with the addition of lower concentration (0.2%), suggesting that the antioxidant and antimicrobial agent level should be optimized in food coating to avoid possible undesirable prooxidation effects. A natural antioxidant, rosemary ethanolic extract (10%), was incorporated into active packaging material providing a delay in the lipid oxidation in minced chicken breast and thigh patties [80]. Essential oils are natural compounds derived from herbs and spices that exhibit antioxidant and antimicrobial activity and therefore attract interest as additives in the food industry. Essential oils (EO) are generally recognized as safe (GRAS), containing active compounds [81]. To overcome their intense flavor, essential oils can be added into edible coatings (alginate-based coating) to improve food safety and quality, but use and choice of essential oils should consider the consumer sensory acceptability of the final product [15]. In recent years the interest in the development of edible films and coatings by incorporating EO as nanoemulsions in the polymeric matrix has increased [82–85]. Nanoemulsion is a type of emulsion with a droplet size between 20 and 200 nm and has shown increased biological activity of the encapsulated lipophilic active compound into edible coating (banana starchbased coating) [86]. Oregano essential oil and green tea extract were incorporated into an active packaging system promoting a preservative effect against microbial spoilage and lipid and protein oxidation and consequently preserving the color and sensorial properties in foil meat stored under refrigeration [87].

6 Effect of edible films and coatings on quality of meats, poultry, and seafood The use of edible coatings directly applied by dipping or spraying onto the food to increase the consumer acceptability and enhance the quality of shelf life of meats, poultry, and seafood is a promising technology [88]. The exploration and application of safe preservatives from the natural origin for the formation of films and coatings, which have high antimicrobial, antioxidant, and quality-enhancing properties, are highly desirable [89], particularly about the meat, poultry, and fish industries. Chemistry and nature of these films and coatings range

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21. Edible biopolymer coatings on meats, poultry, and seafood

from hydrophilic to hydrophobic boundaries to cover the whole range of food products [7]. Limited research has been performed using edible coatings in red meats; instead, their ability to maintain the quality of fish and chicken has been well demonstrated [15]. A wide variety of materials recently are used to produce edible coatings to prevent changes associated with seafood (Tables 1 and 2), meats (Table 3), and poultry (Table 4) products. These materials are TABLE 1 Application of different types of coatings to seafood. Product

Biopolymer

Fresh fillets of Atlantic cod (Gadus morhua) and herring (Clupea harengus)

Chitosan

Bream (Megalobrama amblycephala)

Functional additives

Results

References

No functional additives

Reduced lipid oxidation, chemical spoilage (total volatile basic nitrogen, trimethylamine, and hypoxanthine) and total plate count

[12]

Calcium alginate

Vitamin C and tea polyphenols

Coating inhibited the growth of bacteria, reduced the degree of chemical spoilage, retarded water loss, and enhanced the overall sensory values compared with uncoated bream. Vitamin C enhances the growth of total viable counts

[90]

Indian oil sardine (Sardinella longiceps)

Chitosan

No functional additives

The shelf life of 8, 10, and 5 days was observed, respectively, for 1% chitosantreated samples, 2% chitosantreated samples, and untreated sardine

[91]

Skinless pink salmon fillets (Oncorhynchus gorbuscha) fillets

Chitosan, egg albumin, soy protein concentrate, pink salmon protein powder, and arrowtooth flounder protein powder

No functional additives

Chitosan and soy protein concentrate-coated fillets delayed lipid oxidation in pink salmon fillets during frozen storage

[92]

Rainbow trout (Oncorhynchus mykiss) fillets

Carrageenan

Essential lemon oil

Good antimicrobial activity and limited lipid oxidation of fresh trout fillets stored at 4°C for 15 days

[93]

Golden pomfret (Trachinotus blochii)

Chitosan and fish gelatin

No functional additives

Chitosan/gelatin-based coating with the highest concentration of gelatin presented the best effect on preserving fillet’s quality during storage, via inhibiting myofibril degradation

[94]

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6 Effect of edible films and coatings on quality of meats, poultry, and seafood

TABLE 2

Application of different types of coatings to seafood (continuation).

Product

Biopolymer

Functional additives

Golden pomfret (Trachinotus blochii)

Fish gelatin

Rainbow trout fillets

Results

References

Tea polyphenol

The coating showed antimicrobial and antioxidant activities, reduced the weight loss, lower the pH and microbial growth, slowed down the myofibril degradation, and prevented the production of spoilage markers

[82]

Gelatin

Cinnamon oil

Gelatin coating with 1.5% and 2% v/v cinnamon oil inhibited the growth of bacteria and reduced the degree of chemical spoilage such as TVB-N and lipid oxidation

[95]

Rainbow trout fillets

Chitosan

Lactoperoxidase

Chitosan and chitosan/lactoperoxidase coating extended the shelf life of trout fillets by at least 4 days as compared with the control samples

[96]

Bighead carp (Aristichthys nobilis) fillets

Alginate

Horsemint (Mentha longifolia) essential oil

Antioxidant and antibacterial effects of sodium alginate coating and horsemint were more pronounced when a horsemint was used at 1% concentration

[97]

Nile tilapia (Oreochromis niloticus) fillets

Chitosan

Pomegranate peel extract (PPE)

The coating could retard the chemical spoilage parameters increasing, that is, nitrogen volatile base (TVB-N), peroxide value (PV), and reactive substances of thiobarbituric acid (TBARS). The increased concentrations from PPE strengthened coating film antimicrobial activity

[89]

Salmon (Salmo salar) fillets

Chitosan

Pink pepper residue extract

Bacterial counts were significantly lower with chitosan coating and pink pepper, contributing to the significant reduction of trimethylamine. Despite being similar to the control, CPP showed the lowest off-odor score

[98]

Beluga sturgeon fillets (Huso huso)

Jujube gumbased nanoemulsions

Nettle essential oil

The coating containing 3.5% nettle essential and 12% jujube gum was the best formulation in preserving the critical quality attributes for 15-day cold storage

[83]

used to prevent deterioration due to dehydration, oxidation, volatile flavor loss and/or gathering foreign odor, and microbial growth [105]. Among the most commonly used materials for producing films and edible coatings are polysaccharides such as chitosan, seaweed extracts (alginates, carrageenan, and agar), gums, cellulose, starch, and pectin [6]. These polysaccharides can form films and coatings with excellent barrier properties against the transport of gases such as oxygen and carbon dioxide [106]. Although some polysaccharides have a shallow water vapor barrier such as alginate and carrageenan, they are highly hygroscopic

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21. Edible biopolymer coatings on meats, poultry, and seafood

TABLE 3 Application of different types of coatings to meats and meat products. Product

Biopolymer

Functional additives

Beef steaks (Semitendinosus)

Chitosan and gelatin

Beef steaks (Longissimus dorsi)

Results

References

No functional additives

Weight loss, lipid oxidation, and discoloration of coated steaks decreased during 5 days of retail display

[13]

Alginate

Rosemary essential oil and oregano essential oil

Weight loss and lipid oxidation decreased. Coated meat was redder and tenderer, as well a more intense chroma. Oregano showed higher antioxidant activity and higher consumer acceptance

[15]

Ground beef patties

Apple peel powder/ carboxymethyl cellulose

Tartaric acid

Inhibited microbial growth against mesophilic aerobic bacteria, mold, and yeast and Salmonella enterica. As well as the lipid oxidation. Acceptability improved

[10]

Pastirma (dry cured meat)

Chitosan

No functional additives

Improvement in the sensory attributes with a distinct antioxidant effect and unusual antibacterial activities

[99]

Beef steaks (Longissimus thoracis)

Alginate

Rosemary essential oil and oregano essential oil

Beef with the edible coating with 0.1% of oregano essential oil was the most preferred

[81]

Lamb steaks (Semimembranosus and Biceps femoris)

Chitosan

Satureja khuzestanica essential oil free or nanoencapsulated

Encapsulated oil showed the best effect on oxidative stability and enabled control release of the antimicrobial agents that resulted in an extended antimicrobial activity during the storage compared with free essential oil

[85]

Sliced dry fermented sausage

Chitosan

No functional additives

Chitosan reduced the bacterial counts, while oxygen scavenger contributed more to color stability

[100]

Pork steaks (Longissimus thoracis et lumborum)

Chitosan

Bamboo vinegar powder

Microbial growth and lipid oxidation were inhibited. Odor, color, and overall acceptability were improved

[101]

and can be applied to the food surface to temporarily prevent dehydration [90, 93], which leads to texture, flavor, and color changes and also reduces saleable weight [5]. Chitosan is among the biopolymers with the highest potential of application in the development of films and bioactive coatings in fresh, frozen, and processed meat, poultry, fish, and seafood, due to proven antibacterial, antifungal, and antioxidant activities, the latter related to its ability to form complexes with divalent metals [13]. Chitosan is the polymeric material most commonly IV. Applications of biopolymers membranes/films in food

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6 Effect of edible films and coatings on quality of meats, poultry, and seafood

TABLE 4

Application of different types of coatings to poultry.

Product

Biopolymer

Functional additives

Results

References

Fresh chicken breast

Alginate

Thyme essential oil and propionic acid (individually and in combination)

During the storage time, the pH of the meat surface, color, and sensorial showed behavior between coatings. On the other hand, weight loss and microbiological deterioration had different responses. The alginatepropionic acid coating presented the best results

[102]

Fresh chicken breast fillets

Caseinate

Ginger essential oil (GEO) free or nanoencapsulated

The nanoemulsion-based edible coating containing GEO nanoemulsion increased the shelf life of the chicken breast by a significant decrease of total aerobic psychrophilic bacteria, molds and yeasts and achieved the highest total acceptance during storage

[84]

Fresh chicken breast fillets

Alginate

Cinnamon and rosemary essential oils (individually and in combination) and Nisin

Alginate solution containing both cinnamon and rosemary essential oil had a stronger effect in preserving the microbial quality and extend the shelf life of chicken breast meat about 6 days compared with the control

[103]

Fresh chicken breast fillets

Chitosan

Propolis extract

Bacteria growth was inhibited for 12 days at 4°C. The increase of total volatile nitrogen, TBARS, and peroxide value of coated samples was less than the control group

[104]

used recently as a coating for reduced lipid oxidation and chemical spoilage and inhibited the growth of deteriorating or potentially pathogenic microorganisms. Fan et al. [107] reported inhibition of microbial deterioration in silver carp coated with chitosan combined with grape seed extract and tea polyphenols. These authors indicated that the antimicrobial activity of the bioactive film incorporated with natural extracts could be significantly influenced by the biomolecule included in the active film. It is widely accepted that the binding of positively charged NH3 + groups of chitosan to the negatively charged bacterial surface, such as the outer lipopolysaccharide membrane of gram-negative bacteria, is the crucial factor for its antimicrobial efficacy at pH 20

Sneddon et al. [58] and Yang [59]

Activated Al2O3

Almost exclusively composed of aluminum oxide obtained from aluminum hydroxide by the hydroxylation process

50–500

60–150

Leyva-Ramos et al. [60] and Srivastava and Eames [61]

Activated carbon

Composed basically of carbon material derived from pyrolysis of materials such as wood, coconut shells, and bovine bones, among others

300–5000

10–25

Ben-Mansour et al. [62] and Sneddon et al. [58]

Clays (vermiculite)

2:1 phyllosilicates are composed of tetrahedral (T) and octahedral (O) sheets. The tetrahedral is [T4O10]4, where T ¼ Si4+, Al3+, Fe3+. The octahedral sheet consists of two planes of O2 and OH anions of octahedral with the central cations Mg2+ or Al3+

1.4–720

1.49–1.53

Maqueda et al. [63] and Vala´sˇkova´ and Martynkova´ [64]

adsorbents are available on the Japanese market: Neupalon (Sekisui Jushi) sachets that have no active phase impregnated; Sendomate (Mitsubishi Chemical), which is palladium-loaded activated carbon sachets; and the Hotafresh System (Honshu Paper) paper bag impregnated with bromide [80]. The products based on KMnO4 are not in direct contact with food due to their toxicity. Because of the physical shape, these adsorbers must be placed inside sachets made of gaspermeable materials, filters, or films, concentrating all adsorption areas to a limited space [53, 81]. Commercially, the best-selling form is that of small sachets that are added inside cartons or wooden boxes along with fruits or vegetables to modify the atmosphere by removing the formed ethylene. An alternative, widely used by DeltaTrackInc., Molecular Products, and BioXTEND, is the commercialized form of small tubes filled with adsorbent and oxidizing agents. It is noteworthy that some companies such as Stayproct, OzeanoUrdina, SensitechInc., Befresh Technology, Bioconservacio´n, and AgraCo Technologies offer ethylene removal filters that must be installed in refrigeration chambers for use in road, land, and sea containers. Although the vast majority of commercial products are based on KMnO4, the Sendomate and Hotafresh System products are based on palladium and bromide. The activated carbon adsorbs ethylene, while the catalytic palladium degrades it.

IV. Applications of biopolymers membranes/films in food

TABLE 2 Main ethylene adsorbents found in commercial products. Adsorber Activated aluminum

Activated alumina

Natural clays

Active phase (%)

Commercial name

Fill (granules)

Commercial form

References

NS

Air Repair Ethylene Gas Absorber (DeltaTrackInc.)

Shape: spherical pellets

Minipackets, minitube, tube, miniblanket, and blanket

DeltaTrackInc. [65]

NS

Stayproct (Stayproct Inc.)

Shape: spherical pellets

Filter and sachet

Stayproct [66]

7.5%

Granules OzeanoETH (OzeanoUrdina S.L.)

Shape: spherical pellets Granule size: 3–5 mm

Filter and sachet

OzeanoUrdina [67]

3.5%–5%

Ethysorb Tube (Molecular Products)

Shape: spherical pellets Granule size: 2.5–5.0 mm

Bulk beads/tube/blanket

Molecular Products [68]

4%–4.5%

BioX 4.0 (BioXTEND)

Shape: spherical pellets Granule size: 3.0 mm

Bulk granules/tube/sachet/module

BioXTEND [69]

8%–8.5%

BioX 8.0 (BioXTEND)

Shape: spherical pellets Granule size: 3.0 mm

Bulk granules/tube/sachet/module

BioXTEND [69]

8%–10%

BioX X (BioXTEND)

Shape: spherical pellets Granule size: 3.0 mm

Bulk granules/tube/sachet/module

BioXTEND [69]

NS

Greenkeeper (NPD Global Suppliers)

Shape: spherical pellets

Sachets

NPD Global Suppliers[70]

NS

Ryan Ethylene Absorption (Sensitech Inc.)

NS

Filters and sachets

Sensitech [71]

NS

Befresh (Befresh Technology)

NS

Filters and sachets

Befresh Technology [72]

TABLE 2 Main ethylene adsorbents found in commercial products—cont’d Adsorber

Active phase (%)

Commercial name

Fill (granules)

Commercial form

References

12%

Bi-On R12 (Bioconservacio´n S.A.)

Shape: cylindrical pellet Granule size: 2.3–4.0 mm

Filters and sachets

Bio-On Product of Bioconservacion AS [73]

Clay mineral

NS

It’s Fresh (It’s Fresh Inc.)

NS

Sachets

It’s Fresh [74]

Clays

NS

Retarder (Retarder)

Shape: cylindrical pellets

Sachet/tube

Retarder [75]

Zeolite

4%–6%

Super Fresh Media (Ethylene Control)

NS

Sachets and filters

Ethylene Control [76]

8%

Ethylene Removal Filters by Extend-a-Life (AgraCo Technologies International)

NS

Filters

AgraCo Technologies [77]

NS

Keep-It-Fresh (Keep-ItFresh)

Shape: cylindrical pellets

Sachets/bags

Keep-It-Fresh [78]

NS

Dry Pak’s line Drypak Industries

Shape: cylindrical pellets

Sachets/filters/films

Dry Pak [79]

NS, not specified.

564

23. Ethylene-scavenging films and coatings of biopolymers

Table 2 shows that the most commonly used commercial adsorbent is activated alumina, as it has high adsorption capacity and thermal stability, is cheap, and nontoxic [82]. Spricigo et al. [83] reported that in a system containing KMnO4-impregnated alumina (AlO3) and silica (SiO4) beads, surface area of contact is the most important variable to remove ethylene in comparison to relative humidity, time of exposition, temperature, and KMnO4 concentration. In comparison with microparticles of the same material, nanoparticles were more active, suggesting the influence of the material properties on the scrubbing process. These authors also correlated removal efficiency with discoloration of KMnO4 from purple to brown as an indicator of the oxidation reaction. Concerning the parameters that affect the oxidation reaction, Wills and Warton [55] evaluated KMnO4 supported on activated alumina produced by dipping the carrier into KMnO4 solution. It was reported that a relative humidity (RH) increase caused a negative response on ethylene absorption efficiency. At 20°C and RH 90% efficiency was 50% lower than at RH 70% (4% w/w KMnO4/alumina system), which is quite relevant since RH is generally at least 90% in agricultural packages. The estimated time to remove 90% of C2H4 in a closed system presented a substantial increase, according to the RH. High RH seems to hinder access to inner parts of the bead, limiting oxidation to KMnO4 located on the outer surface. These authors also observed that initial removal could only be maintained for approximately 24 h, reducing to 10% after 14 days, not only because of a decrease in the amount of KMnO4 caused by the reaction process (44% of oxidant remained in beads), but also because inaccessibility to oxidant was caused by the RH. It requires frequent replacement (for example, every new transportation journey, or several times during storage period), and the amount is in accordance with C2H4 production by the horticultural product [54]. Thus Wills and Warton [55] questioned KMnO4 system viability for larger packaging and C2H4 concentrations due to high quantities of absorbent required, suggesting that materials that promote increased accessibility to the inner parts of beads might diminish this problem. Table 3 presents examples of studies performed with fruits and vegetables in which the authors monitored the reduction in the amount of ethylene using adsorbents containing KMnO4. Although activated alumina is the most widely used commercial adsorbent (Table 2), it is observed that zeolite is also a high potential adsorbent (Table 3) and may be soon commercially employed similar to activated alumina. In all studies, adsorbents generally simulate storage conditions (controlled temperature and humidity) for long periods. Thus many studies simulate storage and transport conditions for fruits and vegetables for export [84, 90, 91].

5 Biopolymer films for ethylene control It was mentioned in the introduction section that there are few papers concerning the application of biopolymer membranes or films for ethylene capture. However, an accurate reading of these papers showed that the number is even lower if we consider those that quantify ethylene during shelf-life studies and therefore we can affirm that the reduction of ethylene levels is the action of ethylene scavengers. Most papers do not even measure gas concentrations such as carbon dioxide, and then make assumptions about ethylene levels based on the

IV. Applications of biopolymers membranes/films in food

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5 Biopolymer films for ethylene control

TABLE 3 Examples of adsorbents used to remove ethylene from fruits and vegetables—production and storage conditions. Adsorber

Fruit

Produce packaging, quantity, or content of active phase

Stock conditions

Zeolite

Peach and nectarine

10 kg KMnO4 and zeolite-based nanomolecular filter granules

0°C for 36 days

Emadpour et al. [84]

Cut apple

Sachets containing nanozeolites and KMnO4

Room temperature (laboratory) for 10 days

Khosravi et al. [85]

Tomato

Three doses of zeolite (0.5%, 1.0%, and 1.5%) and three concentrations of KMnO4 (0.5%, 1%, and 1.5%) based on fresh weight

18°C and 85% RH for 28 days

Salamanca et al. [86]

Apples

10 g of KMnO4-coated zeolite nanoparticle sachets

0°C and 90% RH for 5 months

Sardabi et al. [87]

Zeolite (bentonite)

Tomato fruit

1%, 2.5%, and 5% of palladium/ nanozeolite and 1%, 2.5%, and 5% of KMnO4/nanozeolite

7  0.5°C and 90  2% RH for 35 days

Mansourbahmani et al. [22]

Vermiculite

Cantaloupe melon

KMnO4-loaded vermiculite

3  2°C and 85  2% RH for 14 days

Sa´ et al. [88]

Strawberries

Packed column (135  6.5 cm diameter) or 10 g adsorbent packed in sachets

0°C and 20°C for 6 days

Wills and Kim [18]

Alumina nanofibers

Avocados

Membranes composed of alumina nanofibers incorporating alumina nanoparticles and KMnO4

5 days

Tirgar et al. [81]

Activated alumina

Lettuces

Waterproof and gas-permeable film sachets

0°C and 20°C for 45 days

Kim and Wills [89]

Aluminum oxide

Japanese pears

150 g of an adsorbent were included in some bags

0°C for 36 weeks

Giraldo et al. [90]

Natural clays

Kiwifruit

Sachets containing natural clays and KMnO4

0°C and 85%– 95% RH for 200 days

Bal and Celik [91]

References

RH, relative humidity.

ripeness of the fruit. In this section, only those papers that performed measurements of ethylene concentration or production of the systems in question are presented and discussed. The literature search also showed that biopolymers had been used to control ethylene in two different ways: (1) packaging film systems and (2) edible coating of fruits and vegetables. Conventional methods of ethylene control are often associated with the use of biopolymers to improve system performance. The following sections present different ways to control ethylene.

IV. Applications of biopolymers membranes/films in food

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23. Ethylene-scavenging films and coatings of biopolymers

5.1 Edible coating systems Gardesh et al. [92] used the methodology proposed by Mohammadi et al. [93] to prepare chitosan nanoparticles. The nanoparticles were prepared by ionotropic gelation with sodium tripolyphosphate, a classic method for making chitosan nanoparticles. The nanoparticles, after their formation, were centrifuged, resuspended in distilled water, and lyophilized. The lyophilized particles were redispersed in water (0.2% and 0.5%) and used for the coating of apples (cv. Golabkohanz). The best results were obtained with a nanochitosan coating with 0.5% chitosan; this coating eliminated the climacteric respiration peak and decreased the maximum ethylene production to almost 33%. Moreover, the coating enhanced the color quality of the fruit, decelerated fruit softening, and reduced weight loss during 9 weeks of storage. Qiuping and Wenshui [94] evaluated the performance of 1-MCP and chitosan coating on the storage of Indian jujube (Ziziphus mauritiana, cv. “Cuimi”). The fruits were submitted to three treatments (1-MCP, 1-MCP/chitosan, and chitosan), in addition to a control group (uncoated and without 1-MCP). The fruits treated with 1-MCP, chitosan, or their combination had the senescence inhibited when compared with the control group. The storage life was increased by 7, 5, and 8 days, respectively, for fruits treated with the 1-MCP/chitosan coating, 1-MCP, and chitosan coating. The combination 1-MCP/chitosan coating enhanced ascorbic acid and fruit firmness, decreased weight loss, showed better retention of chlorophyll content, delayed climacteric ethylene evolution and respiration rate, and reduced stem-end rot incidence compared to other treatments. Coating alone reduced stem-end rot incidence and weight loss, delayed the onset of climacteric ethylene production and respiration rate, but did not affect the peak levels of ethylene and respiration rates. They concluded that the 1-MCP/chitosan coating upgraded the storage life extension and quality of Indian jujube fruit at room temperature storage. Muy Rangel et al. [95] performed similar work on the storage of mango (Mangifera indica L.) treated with 1-MCP, chitosan coating, and their combination. The chitosan solution was sprinkled to produce coated fruits. Although all treatments were not effective in controlling fruit weight loss, all treated fruits showed reduced respiration rate as compared to the control; moreover, the mangoes treated with 1-MCP/chitosan coating showed a delay in their climacteric peak in 3 days. Respiration decreased in all treated fruits as compared to controls; 1-MCP + edible coating fruits delayed their climacteric peaks by 3 days. Deng et al. [96] used cellulose nanocrystals (CNC) to reinforce chitosan coating for improving the storability of postharvest pears (Pyrus communis L.). Three CNC concentrations were evaluated: 0%, 5%, and 10% w/w, in chitosan, dry basis. Beyond the nanocrystals, two surfactants (Tween 80 and Span 80) were also added to improve the wettability of the fruit’s hydrophobic surfaces. Each formulation was spray coated on each fruit using an air spray gun (15 mL/fruit) to achieve uniform surface coatings. After drying at ambient temperature under forced airflow for 1 h, the coated fruits were then stored at ambient conditions without packaging. The results of the ethylene analysis showed that the coating promotes a significant reduction in ethylene production compared to the control group (without coating). After 1 day of storage, ethylene production was 52.1, 12.0, 1.90, and 0 μL kg1 h1, respectively, for the control, 0%, 5%, and 10% CNC. Moreover, the 5% CNC-reinforced chitosan coating significantly prevented internal browning, reduced senescence, postponed the green chlorophyll

IV. Applications of biopolymers membranes/films in food

5 Biopolymer films for ethylene control

567

degradation of pear peels, and enhanced retained fruit firmness. They concluded that the chitosan coating was effective in delaying ripening and improving the storability of postharvest pears.

5.2 Packaging film systems Biopolymer film for ethylene control proves to be a potential, innovative, and sustainable material that can be a new option for fruit and vegetable storing/transporting with longer shelf life. Although Topic 4.2 presents several commercially available KMnO4 adsorbents and several types of research on fruit and vegetables, there is very little research on biopolymer films for ethylene control. Soares [97] developed a new and sustainable active packaging material to adsorb the ethylene phytohormone, based on ethylene gas adsorbent incorporation into chitosan-coated Kraft paper. Zeolite type Y impregnated with 10% silver was incorporated into chitosan filmogenic solution and applied as a Kraft coating surface. Four treatments were evaluated in the shelf-time test of cherry tomatoes: (1) boxes of Kraft paper containing the commercial adsorbent sachets, the positive control; (2) boxes of Kraft paper coated with chitosan; (3) uncoated Kraft paper boxes, the negative control; and (4) boxes of Kraft paper coated with chitosan and silver-impregnated zeolite. The results showed that even boxes of paper coated only with chitosan showed a reduction in ethylene concentration when compared to the negative control. However, the best results were observed for boxes of Kraft paper coated with chitosan/zeolite/Ag (10%), and these results were even better than the positive control group of the commercial sachet. The tomatoes conditioned in active coated Kraft paper (chitosan/ zeolite/Ag) presented a reduction in weight loss and maintenance of the texture, indicating the characteristic of the adsorption capacity of ethylene gas. In a very similar study by the same research group, Ochi [98] used zeolite type Y impregnated with 5% (w/w) palladium to coat Kraft paper with chitosan solution. Ethylene scavenging property was best for the material coated with the highest concentration of chitosan (0.6% w/w) and 1.2% (w/w) adsorbent. However, due to the high viscosity of the film-forming solution, a lower concentration of chitosan (0.2% w/w) was preferred despite lower adsorption capacity. Cherry tomatoes were packed in boxes made of the Kraft paper/chitosan/zeolite/Pd material for 30 days. Boxes prepared with this material presented the lowest ethylene levels (2.7 ppm s/g tomato) on their interiors, while uncoated Kraft paper boxes showed the highest level of ethylene, detected as a sudden peak (208 ppm s/g tomato). Titanium dioxide nanoparticles were added to the chitosan solution to produce chitosan/ TiO2 nanocomposite films by the casting method. In such a study, cherry tomatoes (Solanum lycopersicum L.) were blanketed with chitosan film and chitosan/TiO2 nanocomposite film and then inserted in low-density polyethylene (LDPE) bags under atmospheric air. A control group was also evaluated, putting the tomatoes inside the LDPE bags without any chitosan films. Every day, the samples were exposed to ultraviolet (UV) light for 180 min, and the tomato quality was checked twice a week. Tomatoes packaged in the chitosan/TiO2 nanocomposite films presented better quality than those in the chitosan film and control. Moreover, chitosan/TiO2 nanocomposite films showed a delay in the ripening process and changes in the quality of the tomatoes, which was attributed to the ethylene

IV. Applications of biopolymers membranes/films in food

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23. Ethylene-scavenging films and coatings of biopolymers

photodegradation activity when exposed to UV light, once the tomatoes had significantly lower ethylene concentrations than those in chitosan film and control [99]. KMnO4 powder is generally used in the form of sachets within climacteric fruit packages. KMnO4 is not allowed to be in contact with food due to its toxicity and purple color. However, it is possible to incorporate it into packaging material [3]. The study of the incorporation of ethylene absorbers in the matrix of polymeric films is still quite new. Warsiki [100] developed an ethylene gas-absorbing packaging material, incorporating KMnO4 into chitosan matrix film produced from wrapped tomatoes. Tomatoes were wrapped and conditioned at room temperature, and the active KMnO4/chitosan film could slightly inhibit fruit ripening. The author suggested increasing the KMnO4 concentration (higher than 7 g). Further studies were conducted using zeolite polymer films to remove ethylene produced by fruit. However, in these works, no biopolymers were used. Tukada [101] evaluated the use of commercial LDPE-based films with the incorporation of adsorbents. Shelf-life tests were performed to evaluate the effectiveness of using zeolite as an ethylene adsorbent by monitoring the gas composition inside a nectarine package. The addition of zeolites was not found to increase ethylene adsorption significantly. The presence of zeolite without an oxidizing agent (e.g., permanganate) only increases the ethylene permeation rate of the film, due solely to the formation of microchannels between the zeolite particles and the polymeric material. Fernandes [102] synthesized LDPE/LDPE-active films and also incorporated zeolites without oxidative agents. The author verified that the zeolite polymeric films presented good permeation to water vapor, oxygen, and ethylene reduction. However, the author did not conclude that the decline in ethylene content was due to zeolite adsorption or ethylene permeation loss by the film, as observed by Tukada [101].

6 Conclusion Conventional methods to control ethylene levels can be used in association with biopolymer membranes or films to potentiate them. Although the biopolymer coating can affect the ripening of fruits and vegetables by changing their respiration rate, they do not usually act as ethylene scavengers. On the other hand, biopolymer membranes or films can be used as packaging systems (active packaging) if the film has some device that can capture ethylene (i.e., silica, zeolite) or even decompose it (TiO2).

Acknowledgment The authors would like to thank the financial support received from Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo under project number 2016/25120-7.

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C H A P T E R

24 Edible films and coatings made up of fruits and vegetables Saartje Hernalsteens College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou, Jiangsu, China Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sa˜o Paulo, Diadema, SP, Brazil

1 Introduction When considering coatings and films, with edible and/or biodegradable properties, we can observe that each one of these products has different production methods, applications, and purposes. For example, most coatings are formulated as liquid solutions to be applied to the surface of a product as a moisture/gas barrier or even as a way to add solutes with interesting characteristics, improving oxidation stability, microbiological control, and gaseous exchange. Usually, it is desirable that the coating should be edible, so the coated food could be directly eaten, but this is not a definite requirement. One of the most well-known coating procedures is the wax coating on fruits and vegetables, adopted to keep the product fresh, avoiding overripening and moisture loss. It is important to remember that several other processed foods also undergo a coating process, for example, frozen multicomponent foods usually receive a coating solution between the different components, e.g., ready-to-eat ice cream cones or pie crusts coated with fat or chocolate-like components. Another example of a widely applied edible coating is egg wash and its substitute glazing emulsions used to give the desired appearance of baked bread. On the other hand, when we talk about films, we usually consider the actual packaging; thus most of the research and development is related to biodegradable films, although there are also edible films being developed. Nowadays, most of our packaging is made from paper and petroleum-based polymers, but there is still some use for traditional materials such as cotton, wood, silk, wool, and leather, which we have been using for centuries. Petroleumbased polymers have dominated the packaging market, thanks to their low price, mechanical properties, and processing adaptability (heat sealability, shape versatility, multilayers, etc.),

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but the accumulated waste generated by petroleum-derived polymers has raised considerable concerns over their harmful effects on the environment. In addition to environmental issues, we have observed an increased interest in the components that could be leaching into the food itself, such as phthalate. This results in an increasing demand to replace conventional plastics with renewable/biodegradable/edible polymers, not only because of governments policies but also because of consumers, who are becoming increasingly conscious of environmental and health issues. Therefore we may notice an increase in research related to the use of biopolymers, including the use of food chain by-products. This is not a new idea, as some biodegradable and/or edible films have been in use for a long time such as edible starchbased paper (rice paper), biodegradable cellulose casings, edible collagen and natural casings for meat products, and even husks and plant leaves from traditional food (Fig. 1). Usually, edible films and coatings are supposed not to affect the food sensory properties; nevertheless, there are several applications where films could also be used to promote flavor enhancements, for example, sushi/sandwich wraps can also be considered edible films as can pouches/films that melt when cooked. If we consider that the primary use of edible/biodegradable films and coatings is to extend the shelf life and quality of foods by preventing changes in aroma, taste, texture, appearance, or handling characteristics, it is possible to say that this is accomplished by inhibiting the migration of moisture, oxygen, carbon dioxide, flavors, and lipids. If we observe all these coating

(A)

(B)

(C)

(D)

(E)

(F)

FIG. 1 Edible and natural coating/films: (A) glazed bakery products; (B) fruits with wax coating; (C) collagen casing; (D) nori (seaweed)-wrapped rice dish; (E) Zongzi (traditional Chinese rice dish) wrapped in bamboo leaves; (F) Pamonha (traditional Brazilian corn dish) wrapped in corn husks.

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and film formulations, we will notice the presence of gums (pectin, alginates), polysaccharides (starches, cellulose, chitosan), proteins (gelatin, collagens, and vegetable proteins), and lipids. Hence, if we consider vegetable and fruit composition (starch, cellulose and hemicellulose, pectic substances, protein, and lipids), they can be regarded as good candidates for making film and coating solutions providing sensory, nutritional, or technological appeal. Due to all these attributes, together with health, nutritional, and environmental concerns related to the increase in the use and consumption of fruits and vegetables, we can observe since the year 2000 an increasing amount of scientific publications on this subject. Using the SciFinder database with the topic “edible film” (accessed on March 27, 2019), more than 5000 results were listed. Over 60% of the results were published in the last 10 years (2009–19), although only 3% of these results (and less than 2% of the patents) describe the work as being based on fruits or vegetables or their components, showing that academia and research centers are exploring such coatings and films. However, there is still a long way to go and a huge opportunity for developing edible films and coatings based on fruits and vegetables.

2 Fruit and vegetable-based films and coating technology The standard composition of edible films includes four major types of materials: lipids, resins/gums, polysaccharides, and protein. Their barrier properties depend on the characteristics of their source, e.g., films based on proteins and polysaccharides are very efficient oxygen and carbon dioxide barriers, whereas their resistance to water vapor transmission is limited. Multicomponent films have also been made in an effort to combine the advantages of the individual film-forming materials. The use of fruits and vegetables can also help mitigate the substantial postharvest loss of fruits and vegetables due to defects or an inadequate ripening stage. Pectic and cellulosic substances are the primary polysaccharides in fruit; consequently, the matrix of fruit puree edible films is comprised mainly of those components. The variety of sugars in fruit purees function as plasticizing agents in edible films, but the composition of such purees varies, depending on fruit cultivar and maturity, as well as agricultural and environmental conditions. Therefore each case and application requires a specific formulation. Many people, when reading about a fruit or vegetable film, will think about something like fruit leather as a film or food coating; however, fruit leather’s technical, mechanical, and physical properties should be changed to adapt to use as a film. Following this thinking, a fruit leather (or pestil) can be described and applied as an edible film or edible coating solution. In some countries, fruit leather made from grapes, peach, pear, apple, and apricot is a very well-known traditional food (Fig. 2), eaten as a snack or also used as an edible coating for nuts. Different dried fruits have a long tradition of use and processing, and are prized because of their sweet taste, nutritive value, and long shelf life. Fruit leather is one of the forms of dried fruit made from fruits with a few added ingredients (i.e., pectin, citric acid, starch, and

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(A)

(B)

FIG. 2 Fruit leather: (A) hawthorn, plum, and strawberry (commercial); (B) apple (homemade).

sugar/syrup/honey) and provides desirable properties such as the ability to be peeled off from molds/trays, stickiness, texture, and sweetness. Film/coating solutions can be produced using a single compound type (starch films) or several components (proteins, carbohydrates, lipids) according to final use. An attempt to obtain materials featuring unique sensory and nutritional properties to produce edible films, including fruits and vegetables (components, extracts, juices, purees, and processing wastes), has also been observed. In fact, more than 35 plant species have already been used to obtain edible films [1]. Apple production was around 80 million ton in 2017 (http://www.fao.org/faostat/) and the amount of waste generated after commercialization was very high considering both losses and processing wastes. Something between 20% and 30% of the crop is lost after harvesting, and when considering just apple juice processing, at least 10% is wasted as peels and pomaces. Keeping in mind that the United States alone processed over 700,000 ton of apples in 2018 (https://www.ams.usda.gov/) and that all those residues are mostly used as animal feed or fertilizer, or discarded into soil or landfill, this fruit alone already represents a huge opportunity for improving waste/by-product use. Although fruit and vegetable pomace/peel waste does not appeal to the senses, it is still a potential source of biomass and compounds with nutritional value, and if it is produced by the food processing industry, it can be easily designed as a food-grade material. This kind of material (beverage industry waste/by-product) is in fact rich in pectin, dietary fibers, and phytochemicals and can be used in different products. For example, pomace originated from the production of an isotonic drink, comprising orange, passion fruit, watermelon, lettuce, courgette, carrot, spinach, mint, taro, cucumber, and rocket, and was used to produce edible films; therefore an alternate process can also be applied to extract some interesting/high-valued components [2]. Summarizing the film and coating production process, films are stand-alone structures preformed separately and then applied to the food surface, between food components, or even sealed into edible pouches. Coatings, in turn, are formed directly onto the food surface by dipping, spraying, or panning. If we consider each ingredient of the film/coating formulation containing fruit/vegetable ingredients, we could describe them briefly as:

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• Fruits and vegetables (purees, residues, extracts): these consist of water, pectin, dietary fibers, lipids, phytochemicals, pigments, and phenolic compounds. • Plasticizers: sugar, glycerol, and sorbitol are the three most used ingredients able to act as a plasticizer. But usually, plasticizer content must be controlled, as it promotes reduced brittleness and stiffness, increased flexibility, stretchability, and toughness, which are all desirable, but it also impairs some film barrier properties. • Binding agents: edible hydrocolloids are added to improve some properties such as consistency, mechanical strength, and barrier properties. • Fillers (binding agents and polysaccharide nanostructures): as most biopolymers commonly used as binding agents in edible films exhibit weak mechanical resistance and barrier and thermal properties, polymer nanocomposites (mixtures of polymers with nanoparticles) can be applied. But success depends on several characteristics such as particle size and dispersion, because uniform nanoparticle dispersion within a polymer matrix changes the molecular mobility, relaxation behavior, and resulting thermal and tensile properties of the material. Cellulose nanostructures have been presented as a good option for nanofillers, providing high strength as well as good reinforcement effects [3]. • Functional additives: edible films can also be developed to provide different or improved characteristics/properties such as sensory, nutritional/health, and microbiological, and antimicrobial compounds can be applied, playing an essential role in active packaging. • Other additives: other components are needed to overcome technical problems occurring during the process. Enzymatic browning, for example, is very common on fruits and vegetables rich in phenolic components, and is undesirable due to sensory changes and decreased nutritional value. But this can be prevented by the addition of browning inhibitors such as ascorbic and citric acids. After formulation and adequate processing (milling, blending, homogenization), the second step usually applied is degassing, an essential step to remove air microbubbles, which tend to remain entrapped within the dried film, causing structural defects and thus mechanical failures. Most of the published documents cite vacuum degassing, but intensity, duration, and temperature are adjusted according to the properties of the formulation. The last step is to obtain the finished product. If a coating is being produced, it should pass through necessary steps to provide the desired shelf life and keep the food safety, which usually includes heat treatment (pasteurization or ultrahigh temperature) and packaging. If a stand-alone film is the final goal, several film-forming procedures, such as slit-die extrusion, blown film extrusion, calendaring, and casting, can be applied, but casting is the most used when producing edible films due to the usual presence of thermosensitive components. The process most described within the scientific community is bench casting, but there are also reports of other types of casting (Fig. 3).

3 Current research and product development It is not the aim of this chapter to provide a complete review of the possibilities of using fruits and vegetables in film/coating solutions but to give readers an overview of the opportunities of these technologies and applications. Otoni and collaborators [1] did an extensive

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FIG. 3 Schematic representation of the production of coatings and films.

review with an emphasis on the role that each film component plays in the resulting materials, showing that, since the 1990s, at least 36 different fruits and vegetables have been applied to produce edible or biodegradable coatings and films, in which 13 other polymers like starch, cellulose derivatives, gelatin, soy protein, chitosan and pectin, alginate, and carrageenan have been applied using plasticizers such as glycerol, sorbitol, sucrose, inverted sugar, and syrup. In considering established technologies, it is interesting to note that, in some countries, fruit leathers made from grapes, peach, pear, apple, and apricot are a very well-known and traditional food, eaten as a snack or used as an edible coating for nuts. Fruit leather is made by drying a layer of fruit puree until a leathery consistency is achieved and as it is mainly composed of low weight carbohydrates, which have low glass transition temperature and are highly hygroscopic, the product becomes sticky when stored at ambient relative humidity (RH). Although some studies on fruit leather are more focused on the drying technology, it is still possible to obtain important information related to the composition, film-forming characteristics, and properties of this fruit’s blends. For example, to develop new methods for pomegranate fruit leather production, the effects of various formulations and drying techniques over several aspects were evaluated. The results showed that the hydrocolloid used (locust bean gum and pregelatinized starch) resulted in superior physicochemical properties, and while microwave-assisted drying provided higher phenolic content and promoted nonenzymatic browning reactions, refractance window drying provided higher ascorbic acid and anthocyanin content [4]. In the published literature, there is still a lack of information on the effect of edible coatings/films on sensory characteristics of the food product, but the development of new products must be supported by a sensory study to link product formulations, storage conditions, and process parameters to consumer responses. Therefore taking fruit leather as an

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example, as it usually has intermediate water activity, low moisture content, and low pH, it has good microbiological stability during and after production, but due to its composition, the texture of fruit leathers is influenced by the environmental RH. Some works conclude that the texture was the attribute that most influenced consumer choice. As an example, mango leathers were produced using the cast-tape drying process and conditioned at different RH values. Starch addition to mango pulp resulted in fruit leathers with higher thicknesses, but lower tensile strength in both directions. However, mango leathers conditioned at 22.5% RH were very crisp and preferred by consumers, independent of the starch addition [5, 6]. Maltodextrin addition decreased the hygroscopicity of apple leather, and at low moisture content the leather becomes crispy. The addition of maltodextrin changed the morphology of the leather and led to some particles with granular form and increased the puncture force, puncture deformation, and maximum amplitude. Although mechanical properties were characterized by a brittle-to-ductile transition upon sample hydration, there was a region (between 11% and 33% RH) where, as the water activity increased, the puncture force also increased. This effect was interpreted by an antiplasticizing mechanism, where the hydration facilitates a molecular rearrangement, responsible for interactions between water and matrix macromolecules. In that work, it was possible to produce fruit leather with different textures and sensations when bitten and chewed by adding maltodextrin and changing the moisture content of the product [7]. The research using fruit/vegetable purees as edible coatings and films has put a lot of effort into characterizing the mechanical properties of the films and their mass transfer barriers. For example, peach puree film’s water vapor and oxygen permeabilities were evaluated at different RH values and temperatures. As usual for edible films, peach puree films were good oxygen barriers, but increasing the RH resulted in exponential increases in oxygen permeability. However, calcium addition and lower temperatures increased water vapor permeability values; hence, it was concluded that fruit barrier films are good oxygen barriers for low to intermediate moisture food systems such as nuts, confections, and baked goods [8]. Numerous studies focus on pathogenic microorganism growth inhibition using plant essential oils and extracts, where starch, pectin, and chitosan are widely used as a matrix to produce biodegradable and environmentally friendly edible films. Considering this and the aim to use whole fruits, fruit wastes, or fruit surpluses, it was demonstrated that the elaboration of composite cinnamaldehyde nanoemulsion edible films based on pectin and papaya puree is feasible [9]. In that work, both low methoxy pectin (LMP) and high methoxy pectin (HMP), papaya puree, and oil-in-water emulsions made of cinnamaldehyde, Tween 80, and water were used, and it was found that the addition of papaya puree significantly reduced tensile strength and elastic modulus of both HMP and LMP films, while increasing their elongation at break, indicating the plasticizing effect of the fruit puree, attributable to its high sugar content. It was observed that incorporation of the cinnamaldehyde and Tween 80 emulsion led to a reinforcement of pectin/papaya puree films. Although the presence of fruit puree increased the water vapor permeation, nanoemulsion improved the water barrier properties, minimizing the negative effect of the formulation containing fruit purees over the water vapor permeability test. In summary, it was demonstrated that the elaboration of composite cinnamaldehyde nanoemulsion-edible films based on pectin and papaya puree is feasible and some of the properties of the film could be controlled by the formulation and droplet sizes of these nanoemulsions [9].

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24. Edible films and coatings made up of fruits and vegetables

It is also essential to evaluate the different possibilities when formulating a film containing fruit purees, for example, films based on mango seed kernel (a waste from mango processing—puree, juice, nectar) and/or soy protein isolate (SPI) or fish gelatin (FG) were formulated. What is interesting about the use of mango seed kernel is that its composition is different from the fruit itself, and it contains a high amount of carbohydrate and lipids and a tiny amount of protein. The two protein sources were chosen due to the possibility of studying the effect of adding the mango kernel seed extract into two different structures, whereas gelatin film presents a highly organized structure and the SPI film presents disorganized structures. As a consequence, FG films were much thicker than SPI films, mainly because the highly organized structure disallows oil droplets to take position between structures (in the case of the gelatin film), while in SPI the extract droplets fill the gaps between the disorganized structures, thus limiting the increase in thickness. This kind of behavior was also observed in other properties of the film, and the use of the fruit kernel decreased water vapor permeation in SPI films but increased this value in FG films [10]. In addition, the film containing mango seed kernel possessed higher antioxidant activities; consequently, it was possible to use it as an edible and biodegradable film with exciting characteristics, although it still needs further development to achieve the proper formulation, focusing on sensory evaluation and technical/processing improvement. Due to the importance of the mango processing industry and its high amount of wastes, biodegradable coatings and films were produced using mango peel and antioxidant extracts of its seed kernel. Edible films formulated with mango peel showed good barrier properties with water vapor permeability (around 10
10 g m
1 s
1 Pa
1), where the addition of antioxidant extract did not show a significant effect on optical properties but increased the antioxidant activity and polyphenol content. Coating peaches with a solution made from mango peel (1.09%), antioxidant extract of mango seed kernel (0.078 g L
1), and glycerol (0.33%) showed that coated fruits presented less ethylene and CO2 production, and less O2 consumption (respectively 64%, 29%, and 39%) when compared with peaches without a coating [11]. Another issue that should be evaluated is the final goal of the material under development. For example, edible coatings were developed using flour made from ripe “Prata” banana (Musa spp.) peels, cornstarch, and glycerol, prepared by the casting technique, varying the cornstarch concentration and the heating time. That study chose banana peels due to the amount of discarded banana peels (about 40% of the total weight of the fresh fruit), and its composition: low tannins and high fiber content. That work concluded that if a film based on banana peel flour is to be used as an edible film with low water vapor permeability and high flexibility, it should have reduced levels of cornstarch, whereas if it is necessary to have high values of mechanical resistance such as for packaging applications, a higher cornstarch content would lead to better results [12]. Obviously, when compared with fresh fruits and vegetables, the utilization of agricultural by-products like fruit peels to prepare edible films seems much more profitable from the perspective of resource recycling and environmental protection, and needs further study; however, it could only be considered edible if it is possible to follow all the food safety standards and regulations. One example is the use of pomelo (Citrus grandis, or Chinese grapefruit) peels in edible film formulations. These fruit peels comprise approximately 50% of the total weight of the fruit and contain large amounts of physiologically active ingredients and nutrients such as pectin, flavonoids, essential oils, natural pigments, limonoids, and dietary fiber. In one

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work [13], films were prepared according to the casting technique, using pomelo peel flour, sodium alginate, glycerol, and tea polyphenols in water, assessing several properties such as mechanical performance, water vapor permeability, and antioxidant and antimicrobial properties, including a preliminary application study of this film on food storage (soybean oil). It was found that the high content of polysaccharides in the pomelo peel allows it to accept a wide range of phenolic compound additives, since enhanced interface binding among those hydroxyl groups could occur. The results showed that composite films with 10% tea polyphenols had relatively excellent combination properties and increased the soybean oil shelf life (delaying oil oxidation during storage) due to the stronger intermolecular interactions and more compact microstructure, showing potential application as a green alternative to bioactive packaging materials for oil products [13]. Blackberry (Rubus fruticosus cv. Tupy) pulp transferred bioactive compounds, antioxidant capacity, and color to arrowroot starch films. Increasing the concentration of blackberry pulp (from 0% to 40%, mass/mass of dry starch) in the film resulted in increased thickness (from 0.065 to 0.133 mm), increased elongation (from 3.18% to 13.59%), decreased tensile strength (from 22.71 to 3.97 MPa), increased water vapor permeability (from 3.62 to 4.60 g mm m
2 day
1 kPa
1), and solubility in water (from 14.18% to 25.46%). It was observed that films from arrowroot starch and blackberry pulp had good appearance and flexibility in handling, no fractures or grooves were evident, and that increasing incorporation of blackberry pulp into the film-forming solution caused a reddish color in resulting films, visible to the naked eye, which may be attractive for the preparation of food packaging. Usually, the color displayed by anthocyanins and its stability depends on several factors such as high sensitivity to degradation reactions when exposed to high temperature, light, oxygen, and especially pH, and the presence of copigments, which are the most determining factors of anthocyanin stability and color. Hence, the incorporation of blackberry pulp into the film could also be an alternative way to promote the stability of anthocyanins [14]. Not only fruits but also other vegetables were evaluated in coating/film formulation. For example, an edible film based on carrot puree, carboxymethyl cellulose (CMC), corn starch, and gelatin was developed. CMC and gelatin contents did not significantly affect film elongation, oxygen permeability, and water vapor permeability, but enhanced considerably film tensile strength. Tensile strength was also increased by corn starch, although the starch enhanced the water vapor permeation. The increase in the level of plasticizer (glycerol) decreased film tensile strength, but increased elongation and the permeabilities [15]. Ncama and collaborators published a review on plant-based edible coatings [16]. In that review, the application of coatings, immediately after harvesting, as one way to alleviate water loss of fresh produce was emphasized. Successful cases where the coating not only reduced water loss and delayed senescence but also increased the antimicrobial properties of the coated product were shown. Thus the application of edible coatings based on plant extracts from horticultural produce can provide extensive relief to consumers, but there are still some gaps to be fulfilled and properties not well evaluated, such as adhesion to a product surface (both ability and the required time), especially for products destined for distant markets or long-term storage. There is also a need to make a proper comparison with commercially used coatings and to diversify applications to focus on benefits such as inhibiting physiological disorders while holding onto the already obtained quality preservation and antimicrobial properties [16].

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Knowledge of antimicrobial and physical properties of new packaging materials is fundamental for future food preservation applications, but less attention is applied to the film microstructure. For example, films with acai berry (Euterpe oleracea fruit) puree and HMP as the main constituents were incorporated with apple skin polyphenols (ASP) and thyme essential oil (TEO), resulting in yellowish/reddish films. In that study, the significant influence of both compounds on the mechanical resistance and color parameters of acai berry/pectin-based edible films was observed, while the water vapor permeability was not significantly influenced by the antimicrobials incorporated. Scanning electron microscopy analyses allowed the study of the microstructure of the films and evaluated the microstructural changes after antimicrobial incorporation. The films incorporated with apple skin polyphenols at the highest tested concentration (6.07%, w/w) were brittle but there was little evidence of clustering and the thickness of the film was much more uniform compared to the control film. However, pits or voids of all sizes were apparent in cross-section, which could be linked to the brittleness of this film. On the other hand, films incorporated with the highest tested concentration of thyme essential oil (6.07%, w/w) had clusters and crater-like pits in their surface, and the cross-section of these films had a more cracked structure when compared to the control film. Thus it could be concluded that ASP improved film mechanical resistance, while TEO had a plasticizing effect [17]. Trying to better understand the effect of different components on films’ thermal, functional, and structural properties, edible films produced from various blends of papaya puree, gelatin, and defatted soy protein were cast at room temperature. The results indicated that the addition of gelatin to the papaya puree resulted in films with a significant increase in color properties, tensile strength, and seal strength. However, with the addition of defatted soy protein along with gelatin to the papaya puree, the films had shown a significant increase in elongation, water permeability, and water contact angle, and a decrease in water solubility. In addition, differential scanning calorimetric analysis indicated higher transition temperature (Tg), melting temperature (Tm), and enthalpy (ΔH) in papaya puree-blended films (papaya puree/defatted soy protein/gelatin, 1:4:3) (Tg ¼ 121.81  0.20°C, Tm ¼ 174.59  0.52° C, and ΔH ¼ 220.76  0.36 J g
1). The analysis of blended papaya films by Fourier transform infrared spectrometry revealed significant compatibility of papaya puree, gelatin, and defatted soy protein and their influence on the optical, mechanical, barrier, and structural properties. Thus, based on these observations, the blended papaya films could be effectively used as primary packaging material in the future [18]. Different studies were performed with different goals; some studies, for example, focused on delaying lipid oxidation, and in turn maintaining sensory quality of food, thus in this case, the effect of coating on food product shelf life and its sensory characteristics is the focus. One example of such application is the effect of Indian gooseberry puree/methylcellulose composite films on roasted cashew nuts. The composite films were formulated using response surface methodology and the efficacy of the films on roasted cashew nut quality during 90-day and 27-day storage at 27°C and 37°C, respectively, was investigated in terms of physical, chemical, and sensory attributes. The films containing the gooseberry puree showed higher antioxidant activities, and although all preference scores of the nuts packaged tended to decrease during storage time, independently of the film, gradually, films containing 0.5% w/w of the fruit puree, 1.80% w/v polyethylene glycol (PEG400), and 4.10% w/v

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methylcellulose were highly efficient against nut quality deterioration, and therefore it could be a promising application to extend the shelf life of the roasted cashew nuts [19]. After researching the subject, the tendency of exploring local plants that possess interesting properties, mainly due to their antimicrobial or antioxidative properties and underuse, was noticed. Guabiroba (Campomanesia xanthocarpa), for example, is a food plant that belongs to the Myrtaceae family, which is native to Brazil and rich in phenolic compounds and ascorbic acid. Thus the use of such additive in the production of a film could be interesting because, besides producing biodegradable matrices, it also has antioxidant properties, which can be an alternative to decreasing the lipid oxidation of some products. The production of an active biodegradable film based on blends of gelatin, corn starch, and guabiroba puree for application as a package for extra-virgin olive oil as a sachet was studied [20]. A film containing 10% of the puree was selected for the preparation of the sachet filled with olive oil due to low water vapor permeability (3.88  0.35 g mm m
2 day
1 kPa
1), good handling, and low solubility (19.78%). The results indicated that after 15 days of storage, the values of the acidity index and peroxide index of the extra-virgin olive oil were still within limits allowed by the current legislation, concluding that it is a promising alternative for oil packaging. It is important to remember that even if they are detachable from the casting surface, edible and/or biodegradable films often present weaker mechanical and water barrier properties than those based on petroleum-derived polymers, requiring the addition of another component to act as a reinforcement filler. One goal could be the obtaining of an optimum formulation to try to mimic some properties of the films used nowadays: ethylene-vinyl acetate, ethylene-vinyl alcohol (EVOH), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyamide (nylon), polybutylene adipate-co-terephthalate, polyethylene, polyethylene terephthalate, polylactic acid, polypropylene, polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), and polyvinyl alcohol [21, 22]. In trying to overcome this issue, cellulose fibers, hydroxypropyl methylcellulose, and carrot minimal processing waste (freeze dried and milled) were combined into biocomposite films, targeting the production of films that would result in a tensile strength higher than those of poly(ε-caprolactone) (PCL) (4 MPa) and LDPE (8–10 MPa) and similar to those of EVOH (6–19 MPa) and HDPE (19 MPa); a Young’s modulus similar to those of PS (2.8 GPa) and PVC (2.7–3.5 GPa) and higher than those of LDPE (150–340 MPa), PCL (386 MPa), and PVDC (200–600 MPa); and an elongation at break comparable to that of PS (2%–3%) [23]. Since that study intended to develop a biocomposite film comprising as much food processing waste as possible, but still featuring physical-mechanical properties that allow its commercial applicability as a packaging material, the optimized formulation contained 33 wt% of carrot waste and led to biodegradable biocomposites featuring properties that are suitable for food packaging applications (ca. 30 MPa of tensile strength, 3% elongation at break, and 2 GPa of Young’s modulus). Scaling up the production of such materials was shown to be feasible, and was successfully scaled up through a continuous casting approach, allowing the production of 1.56 m2 h
1. However, it was concluded that there may be mechanical depreciation depending on the processing parameters, requiring further studies on strategies to increase productivity while still maintaining physical-mechanical properties [23].

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24. Edible films and coatings made up of fruits and vegetables

4 Future trends Edible and biodegradable films, no matter the source of their components, have been under intensive research due to prospective applications and possible claims on health/nutrition and environmental benefits. However, these films, particularly those obtained from fruit and vegetable purees, still need further research, development, and improvement. On the one hand, we could think about delivering new flavors and textures for different kinds of food, following something similar to fruit leathers, and on the other hand, there is an urge to use agroindustry and food processing by-products and wastes, decreasing their weight on environmental issues and the food chain. Usually, edible films have excellent oxygen/gas barrier properties, but the moisture barrier is feeble, and most of the films based on fruits and vegetables are in fact water soluble, which could be interesting if we consider the application of edible coatings (which could melt during food preparation or in the consumer’s mouth). Nonetheless, its use is impaired in food with high moisture content, and even low moisture food would require very efficient moisture barrier external packaging to make it feasible to commercialize. Another problem that is often described is the poor mechanical properties of the films when compared to petroleumbased polymers, but as observed in the literature, a formulation can be optimized with the intent to achieve specific mechanical and barrier properties. Edible films based on fruits and vegetables could also incorporate other components to increase product shelf life or health-related components, like phytochemicals, vitamins, and gut-health-promoting components (probiotics/prebiotics). Thus it is not surprising that we will see more research exploring the use of fruits and vegetables, both whole or some of their components, on film and coating formulations. Although this could also be a good commercial output for local vegetables/fruits, whose distribution is still impaired due to technical and economic aspects, and despite the tremendous possible health and technological claims, more research focusing on application, sensory analysis, and scalability is still required. It is vital to make it clear that edible and biodegradable films and coatings are not expected to replace conventional packaging materials completely, but they could reduce the need for petroleum-based polymers and cellulose, as well as the environmental benefits of biodegradability. In addition, films containing fruits/vegetables (extracts, purees, juices) have been in evidence because they can extend food stability and reduce the exchange of moisture, gases, lipids, and volatiles between the food and the surrounding environment, and prevent surface contamination, helping to improve the efficiency of food packaging, and thus offering an supplementary effect over reducing requirements for petroleum-derived polymers. For now, edible and biodegradable films are still considered a high-cost packaging material and are applied in a niche market focusing on consumers with high environmental or food safety concerns. Nevertheless, the presence of fruits and vegetables on films could be a new appeal related to its sensory and nutritional characteristics, attracting also consumers who are health conscious and always seeking ways to have a better diet (even considering “on-the-go” products). On the other hand, if we consider the use of wastes and by-products, which have a much lower cost and noticeable effect on environmental issues by reducing both the need for petroleum-based polymers and reducing food wastes, the result may not necessarily be sensory/health appeal. A significant advance has been made since

IV. Applications of biopolymers membranes/films in food

References

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the 1990s, which shows the feasibility of these films, but future research and development should focus not only on determining the effect of the composition of the films on their properties, but also give further insight into sensory characteristics, stability under storage and transportation, scalability, and costs. In addition, the final objective of the research should be defined very clearly, because it is evident that the technologies and costs involved in a film/coating solution aimed at health/sensory aspects is somehow different to the one aimed at films targeting food waste reduction and environmental benefits of a biodegradable biocomposite.

References [1] C.G. Otoni, R.J. Avena-Bustillos, H.M.C. Azeredo, M.V. Lorevice, M.R. Moura, L.H.C. Mattoso, T.H. McHugh, Recent advances on edible films based on fruits and vegetables—a review, Compr. Rev. Food Sci. Food Saf. 16 (2017) 1151–1169. [2] R.M.S. Andrade, M.S.L. Ferreira, E.C.B.A. Gonc¸alves, Development and characterization of edible films based on fruit and vegetable residues, J. Food Sci. 81 (2) (2016) E412–E418. [3] H.M.C. Azeredo, M.F. Rosa, L.H.C. Mattoso, Nanocellulose in bio-based food packaging applications, Ind. Crop. Prod. 97 (2017) 664–671. [4] I. Tontul, A. Topuz, Effects of different drying methods on the physicochemical properties of pomegranate leather (pestil), LWT Food Sci. Technol. 80 (2017) 294–303. [5] R.S. Sima˜o, J.O. Moraes, P.G. Souza, B.A.M. Carciofi, J.B. Laurindo, Production of mango leathers by cast-tape drying: product characteristics and sensory evaluation, LWT Food Sci. Technol. 99 (2019) 445–452. [6] C.A. Torres, L.A. Romero, R.I. Diaz, Quality and sensory attributes of apple and quince leathers made without preservatives and with enhanced antioxidant activity, LWT Food Sci. Technol. 62 (2) (2015) 996–1003. [7] C. Valenzuela, J.M. Aguilera, Effects of maltodextrin on hygroscopicity and crispness of apple leathers, J. Food Eng. 144 (2015) 1–9. [8] T.H. Mchugh, C.C. Huxsoll, J.M. Krochta, Permeability properties of fruit puree edible films, J. Food Sci. 61 (1) (1996) 88–91. [9] C.G. Otoni, M.R. Moura, F.A. Aouada, G.P. Camilloto, R.S. Cruz, M.V. Loverice, N.F.F. Soares, L.H.C. Mattoso, Antimicrobial and physical-mechanical properties of pectin/papaya puree/cinnamaldehyde nanoemulsion edible composite films, Food Hydrocoll. 41 (2014) 188–194. [10] Z.A.M. Adilah, B. Jamilah, Z.A. Nur Hanani, Functional and antioxidant properties of protein-based films incorporated with mango kernel extract for active packaging, Food Hydrocoll. 74 (2018) 207–218. [11] C. Torres-Leo´n, A.A. Vicente, M.L. Flores-Lo´pez, R. Rojas, L. Serna-Cock, O.B. Alvarez-Perez, C.N. Aguilar, Edible films and coatings based on mango (var. Ataulfo) by-products to improve gas transfer rate of peach, LWT Food Sci. Technol. 97 (2018) 624–631. [12] P.B.F. Arquelau, V.D.M. Silva, M.A.V.T. Garcia, R.L.B. de Araujo, C.A. Fante, Characterization of edible coatings based on ripe “Prata” banana peel flour, Food Hydrocoll. 89 (2019) 570–578. [13] H. Wu, Y. Lei, R. Zhu, M. Zhao, J. Lu, D. Xiao, C. Jiao, Z. Zhang, G. Shen, S. Li, Preparation and characterization of bioactive edible packaging films based on pomelo peel flours incorporating tea polyphenol, Food Hydrocoll. 90 (2019) 41–49. [14] G.F. Nogueira, C.T. Soares, R. Cavasini, F.M. Fakhouri, R.A. Oliveira, Bioactive films of arrowroot starch and blackberry pulp: physical, mechanical and barrier properties and stability to pH and sterilization, Food Chem. 275 (2019) 417–425. [15] X. Wang, X. Sun, H. Liu, M. Li, Z. Ma, Barrier and mechanical properties of carrot puree films, Food Bioprod. Process. 89 (2011) 149–156. [16] K. Ncama, L.S. Magwaza, A. Mditshwa, S.Z. Tesfay, Plant-based edible coatings for managing postharvest quality of fresh horticultural produce: a review, Food Packag. Shelf Life 16 (2018) 157–167. [17] P.J.P. Espitia, R.J. Avena-Bustillos, W.-X. Du, R.F. Teofilo, N.F.F. Soares, T.H. McHugh, Optimal antimicrobial formulation and physical-mechanical properties of edible films based on acaı´ and pectin for food preservation, Food Packag. Shelf Life 2 (2014) 38–49.

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[18] S. Tulamandi, V. Rangarajan, S.S.H. Rizvi, R.S. Singhal, S.K. Chattopadhyay, N.C. Saha, A biodegradable and edible packaging film based on papaya puree, gelatin, and defatted soy protein, Food Packag. Shelf Life 10 (2016) 60–71. [19] P. Suppakul, R. Boonlert, W. Buaphet, P. Sonkaew, V. Luckanatinvong, Efficacy of superior antioxidant Indian gooseberry extract incorporated edible Indian gooseberry puree/methylcellulose composite films on enhancing the shelf life of roasted cashew nut, Food Control 69 (2016) 51–60. [20] N.M. Malherbi, A.C. Schmitz, R.C. Grando, A.P. Bilck, F. Yamashita, L. Tormen, F.M. Fakhouri, J.I. Velasco, L. C. Bertan, Corn starch and gelatin-based films added with guabiroba pulp for application in food packaging, Food Packag. Shelf Life 19 (2019) 140–146. [21] L. Bastarrachea, S. Dhawan, S.S. Sablani, Engineering properties of polymeric-based antimicrobial films for food packaging: a review, Food Eng. Rev. 3 (2) (2011) 79–93. [22] G. Li, P. Sarazin, W.J. Orts, S.H. Imam, B.D. Favis, Biodegradation of thermoplastic starch and its blends with poly(lactic acid) and polyethylene: influence of morphology, Macromol. Chem. Phys. 212 (2011) 1147–1154. [23] C.G. Otoni, B.D. Lodi, M.V. Lorevice, R.C. Leita˜o, M.D. Ferreira, M.R. de Moura, L.H.C. Mattoso, Optimized and scaled-up production of cellulose-reinforced biodegradable composite films made up of carrot processing waste, Ind. Crops Prod. 121 (2018) 66–72.

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C H A P T E R

25 Probiotic-containing edible films and coatings of biopolymers Carmen Guadalupe Herna´ndez-Valenciaa, Neith Pachecob, Gustavo Martı´nez-Castellanosc, Keiko Shiraia a

Biotechnology Department, Laboratory of Biopolymers and Pilot Plant of Bioprocessing of AgroIndustrial and Food By-Products, Autonomous Metropolitan University, Mexico City, Mexico b Center for Research and Assistance in Technology and Design of the State of Jalisco, AC, CIATEJ, Southeast Unit, Merida, Mexico cBiochemical Engineering Department, Misantla Institute of Technology, Veracruz, Mexico

1 Introduction The diet has been studied as a therapeutic agent, in addition to nourishing, owing to its ability to modulate the microbiome by regulation and interaction with gut bacteria; thus diet alters the microbial composition. In this regard the inclusion of probiotics in the diet helps to maintain the gut microbial balance since it prevents the colonization of pathogens, in addition to other beneficial effects as the improvement of lactose intolerance, diarrhea, and constipation. Probiotics also act as an aid of immune regulation and cancer prevention and to protect the mucosal barrier integrity in both animals and humans [1]. Thus probiotics are regarded as alternatives for treatment and/or alleviating the severity of infectious diseases, including gastroenteritis caused by rotavirus infections [2]. According to a recent analysis of data from preclinical and clinical studies about the efficacy of commercially available probiotic products on functional bowel disorders, the results were inconsistent and heterogeneous, and only a few of the probiotic products were specifically investigated in large, well-designed clinical trials [3]. Besides, strain specificity, dose, and measured health benefits should be considered when using probiotics. Despite these limitations, some probiotics produce a beneficial effect on functional bowel disorders [3]. The interest in research and development of edible films and coatings has been increasing in the last years as an alternative to food preservation techniques for the consumers. These materials can improve food quality and safety, owing to their ability to form a semipermeable barrier to water vapor and gases. They are also environmentally friendly because they are Biopolymer Membranes and Films https://doi.org/10.1016/B978-0-12-818134-8.00025-0

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# 2020 Elsevier Inc. All rights reserved.

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25. Probiotic-containing edible films and coatings of biopolymers

biodegradable and generally come from a renewable source. Edible films and coatings consist typically of polysaccharides, proteins, and lipids, or a combination of these compounds. These components are filmogenic; nevertheless, they presented poor mechanical properties. Therefore the addition of plasticizers, grafting, and cross-linking are usually carried out to improve their properties [4]. The incorporation of active compounds into edible films and coatings has been studied to increase shelf-life and maintain or even improve the quality of fruits and vegetables in postharvest. In this regard, the introduction of antioxidant compounds has been proposed, such as the preparation of coatings by grafting of quercetin to chitosan applied on Opuntia ficus-indica, to inhibit the enzymatic oxidation [5]. Active compounds have also been incorporated in polymeric matrices to delay the microbial decay in pitaya fruits [6]. Moreover, edible films and coating can protect active compounds from premature degradation and enhance their controlled release on the food surface [7]. Another approach is the inclusion of living microorganisms in these polymeric matrices based either on the properties of the film and coating material or on their ability to carry and support viable bacteria in food products. These properties were considered in a study carried out by Martı´nez-Castellanos et al. [8], the application of chitosan-based coatings, and the Lactobacillus plantarum on rambutan fruits (Nephelium lappaceum) successfully maintained the quality and retained the color. The inclusion of living microorganisms in polymeric matrices for the development of edible materials is challenging since microbes must remain viable and in high counts to exert probiotic effects or antimicrobial activities with neither altering barrier and mechanical properties of the film or coating nor changing the sensory properties of the food product.

2 Probiotics and prebiotics 2.1 Definitions Probiotics are currently defined as live microorganisms, either bacteria or yeasts, that, when consumed in suitable quantities, can lead to several health benefits on the host [9, 10]. The bacteria considered to be probiotics include Bifidobacterium [3, 10, 11], Propionibacterium [3], Leuconostoc [10], Pediococcus [10], Streptococcus [1, 3, 10], Lactobacillus [1, 3, 10, 11], Bacillus in vegetative and spore forms [10], Lactococcus [10], and Enterococcus [10, 11], as well as yeast belonging to genera Saccharomyces [1], Debaryomyces, Torulaspora, Kluyveromyces, and Yarrowia [10]. Furthermore the interest in the application of yeast as a probiotic has increased because of its high number of proteins, vitamin B, traces minerals, and numerous immune-stimulant compounds, such as proteases, β-glucans and mannan oligosaccharides [10]. Along with the consumption of probiotics, there is a class of compounds known as prebiotics, which are defined as a substrate that is selectively utilized by host microorganisms conferring health benefits [12]. Fructooligosaccharides and inulin, galactooligosaccharides, mannanoligosaccharide, and xylooligosaccharides are nondigestible fermentable oligosaccharides recognized as prebiotics for the enrichment of Lactobacillus and/or Bifidobacterium spp. Nowadays the scope of polyunsaturated fatty acids and polyphenols are also considered prebiotics since they are also selectively utilized by the host microbiome and have shown potential health benefits for the target host [12].

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2.2 Beneficial effects on human health Probiotics are associated with several beneficial effects on human health, as several studies have established that probiotics can help to reduce the symptoms of depression and anxiety. They also help with the prevention and improvement of diseases related to (1) cardiovascular health, as it decreases the cholesterol serum level [10]; (2) the immune system because probiotics supplements help to reduce upper respiratory illness symptoms, such as coughing, congestion, and sore throat [13]; (3) type 2 diabetes, as probiotics can help to subdue body weight gain and insulin resistance, and (4) clinical and subclinical vitamin deficiencies, since some studies have reported that probiotics produce B-group vitamins, such as biotin, riboflavin, nicotinic acid, folates, cobalamin, pyridoxine, thiamine, pantothenic acid, and vitamin K [9, 10]. All of these beneficial health effects have made probiotics to be one of the fastest-growing groups of dietary supplements globally [14]. Additionally, it is important to mention that probiotics are considered safe to use for the majority of the population. However, according to the guidelines for probiotic evaluation from the Food and Agriculture Organization and the World Health Organization, there might some risks for vulnerable populations. This is because probiotics are associated mainly with four side effects, “(1) systemic infections, (2) deleterious metabolic activities, (3) excessive immune stimulation in susceptible individuals, and (4) gene transfer” ([15], p. 5), and there is a correlation between having an underlying medical conditions and the appearance of these side effects [9, 15].

2.3 Issues on the use of probiotics in food 2.3.1 Chemical, biochemical, and microbial activities of probiotics in polymeric matrices The determination of the capacity of lactic acid bacteria (LAB) or other microorganisms trapped in films to metabolize substrates, food, or the films themselves has not been determined in a very specific way since most of the studies are based on viability analysis or acidification of the films or foods to which these technologies are applied. It is not far from the fact that if microorganisms are in constant growth (they increase their number or acidify their media), then they must have some metabolic activity in the films or growth from the coatings. Another important metabolic effect of the probiotic films and coatings are the competitive effects against microorganisms that spoil or contaminate food [16]. Evidently, antagonistic activities are not governed by a single mechanism or by the production of a single metabolite, so it can be established that the cells trapped in the polymeric matrices of the films are actively producing the antagonistic effects observed. For lactic acid bacteria embedded in edible materials, the objective, in addition to the antagonistic effects, is that the benefits that these probiotic microorganisms give to health are generated through their growth and reproduction activity, increasing their concentration or viability in films or coatings [16]. Some authors have reported that trapping probiotics in films with different polymer matrices can protect microorganisms from degradation during processing and allow a controlled release from the matrices [17–19]. In this sense, Singh et al. [20] evaluated the ability of different sodium carboxymethyl cellulose and hydroxyethyl cellulose films to maintain the stability and viability of La. rhamnosus GG, and they observed that when films were subjected to different pH conditions (2.4 and IV. Applications of biopolymers membranes/films in food

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7.4), the films could efficiently release trapped bacteria. In consequence, they function as a suitable vehicle for the release of viable and active microorganisms into a food. Studies carried out by Sa´nchez-Gonza´lez et al. [21] on sodium caseinate, pea protein, methylcellulose (MC), and hydroxypropyl methylcellulose (HPMC) films with La. plantarum showed the production and secretion of bacteriocins with activity against Listeria innocua, with those based on proteins having higher viability but lower production of bacteriocins. On the other hand the films elaborated with MC and HPMC were more effective in the inhibition of the pathogen evaluated, which was probably due to a faster adaptation to the polymer, and the consequent production of bacteriocins during 8 days of storage at 5°C. This is a clear indication that microorganisms are not only viable but also metabolically active in producing or releasing metabolites to the microenvironment in which they are, even if they are still trapped in film matrices or coatings, which is a clear limitation for mobility and the exchange of substrates with the environment. Studies about microenvironment stress on cell viability in films have been carried out by Sa´nchez-Gonza´lez et al. [22], who tested the resistance of La. acidophilus and La. reuteri in sodium caseinate and methylcellulose films. In that study, La. acidophilus showed better viability in both polymeric matrices than La. reuteri, which presented a significant reduction of its initial population during the first week of storage, probably due to the low adaptation of the microorganism to this type of polymeric matrix, which did not result in an adequate microenvironment for the metabolic activities of the microorganism. Besides, such conditions produced metabolic stress with the consequent reduction of the viable count. Studies about cellular immobilization of La. acidophilus in alginate-based coats have shown protective and similar effects to the previous research since this type of matrices can support viability count without significant changes while in storage at low temperatures during 8 days on the strawberry surface, which is related to the adaptive capacity of microorganism to different microenvironments [23]. Additionally, in this study, there was an increase in acidity and a decrease in pH, which is clearly an indicator of the metabolic activity of microorganisms in coatings because there was no change in the viability count, but there was a secretion of metabolites to the surface of the selected fruits. Evidently, there is a need to further analyze and investigate the biochemical activities of probiotics in coatings or films, as most studies only focus on viability, acidity, and antimicrobial activity in the matrices. This is interesting since microorganisms are limited in space and mobility because they are embedded in polymeric matrices, so the mechanism by which they diffuse to the food surface or grow from films and the way of secretion of compounds with antimicrobial activity (such as lactic acid and bacteriocins) must be established. 2.3.2 The use of probiotics in bioactive packaging In recent years the use of films and coatings has been expanded in the food sector as vehicles for bioactive compounds (vitamins, antioxidants, and probiotics) in different food systems [24, 25]. This opens up a new way of distributing and protecting food employing packages, which in addition to safeguarding it from mechanical damages provides modified atmospheres and protection against microbial contamination, and it can give additional characteristics that have been called “active.” The concept of active packaging has generally been used to define packages that, besides having the protective functions already mentioned, include biological and chemical elements that can improve interactions, protective functions, and extended stability of these materials in IV. Applications of biopolymers membranes/films in food

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foods, such as providing antimicrobial characteristics to improve the prevention, reduction, or inhibition of pathogenic and spoilage microorganisms growth [26]. Another extended concept applied to foods is packaging or bioactive system, which, in addition to traditional characteristics, contributes to the improvement of health benefits of food in consumers [27]. This approach comes from functional foods, but it is applied to coatings and films as vehicles for functional compounds to improve the nutritional characteristics of the food itself through the controlled or prolonged release of these elements into food product matrices [27]. In this same sense, probiotic microorganisms, among them lactic acid bacteria, largely comply with food safety regulations and requirements, and with the growing demand for probiotics, it has been proposed to embed them in films and coatings to transport food in a viable way [28, 29]. In addition, these types of bioactive edible coatings can improve the consumer health benefits by carrying probiotics, and it can even improve food stability by controlling the proliferation of spoilage microorganisms through competition and inhibition [16, 29]. Incorporating probiotics into films or coatings and maintaining their viability in polymer matrices to transport them to the digestive system can improve the modulation of the immune response, strengthen the intestinal barrier, and antagonize pathogenic microorganisms [16, 30]. There are many techniques for incorporating probiotic microorganisms into different polymer matrices to propose bioactive containers with greater viability and microbial count including exposure to sublethal heat or cold stress, osmotic stress, low pH, reduced redox potential, and modified atmosphere packaging [31]; however, microencapsulation of microorganisms is by far the most successful method. Thus the combination of this technique with edible films and coatings is one of the strategies that could preserve viability and efficiently release microorganisms to food [16]. Some authors have even proposed the substitution of antimicrobial compounds with probiotic microencapsulation to release biological substances from films and coatings to food matrices in a prolonged and gradual manner [32, 33]. For example, the use of the microencapsulation of La. acidophilus in starch-based coatings on the surface of baking bread showed that the microorganism remained stable after baking with counts of 107 CFU/bread. This is important because it showed that not only does microencapsulation improve the viability of La. acidophilus but also increases its stability against aggressive processing conditions on bread surfaces by generating antimicrobial protection and additional health benefits provided by probiotics [34]. Many are the approaches that have been addressed; however, not only it is important to evaluate the viability of probiotics in bioactive films and coatings, but also further research is needed to investigate the biological limitations of microorganisms concerning the different matrices and processing conditions. All of this is to maximize microbial stability in films, limit the changes that confer on food as bioactive elements, decrease or inhibit the growth of pathogenic and spoilage microorganisms, improve food safety, increase food stability, and maximize the health benefits to consumers.

3 Edible films and coating biopolymers containing probiotics 3.1 Polymeric matrices The increased interest of consumers in health, nutrition, food safety, and environmental issues has led to the improvement of the research on film-forming properties of edible films IV. Applications of biopolymers membranes/films in food

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25. Probiotic-containing edible films and coatings of biopolymers

and coatings for food packaging based on biopolymers [35, 36]. There are several studies related to the use of biopolymers to carry natural antimicrobial or antioxidant compounds, such as essential oils, organic acids, enzymes, and bacteriocins on active and bioactive packaging [16]. Nevertheless, the research on the development of films and coatings incorporated with probiotics as active food packaging is still emerging. Diver biopolymers with different structures and properties, such as starches, proteins, gums, cellulose, chitosan among others, have been studied for the development of novel edible films, coatings, and protection structures with probiotic cultures [16, 37–40]. (A) Natural produced biopolymers: This classification contains the biopolymers obtained by extractions from biomass and natural production by microorganisms. The principal materials extracted from biomass are polysaccharides (starch, cellulose, gums, chitin, and chitosan), proteins and polypeptides (collagen, gelatin, whey, albumin, casein, wheat gluten, soy, and zein), and lipids (wax, oils, and free fatty acids). The compounds produced naturally by microorganisms are bacterial cellulose, microbial polyesters, xanthan, and pullulan (Fig. 1). (B) Synthetic produced biopolymers: This classification involves the products obtained by biotechnology production or conventional synthesis from bioderived monomers. This material includes polylactides and polyglycolides (polylactic acid, polyglycolic acid, and poly(lactide-co-glycolide)) (Fig. 1). As biopolymers used for production of edible films, coatings and protection structures of bioactive compounds and probiotics need to be competitive against conventional polymers; not only biodegradability is taken into account for the elaboration of specific matrices, but also functionality, safety, durability, mechanical, and chemical resistance have an important influence [16]. The use of biopolymers depends on several features, including cost, availability, functional attributes, mechanical properties (strength and flexibility), optical quality (gloss and opacity), barrier requisites (water vapor, O2 and CO2 permeability), structure resistance to water, and sensorial acceptance. All of these characteristics are influenced by parameters such as manufacturing conditions, type, and source of the material [41]. Their effective application in films and coatings is derived from the interfacial interactions via functional groups among the biopolymeric matrix related to its origin [42]. As was described previously, for probiotics to play the intended role in human health, it is essential that both viability and the metabolic activity are maintained throughout product processing and supply chain, such as the gastrointestinal tract. In this sense, microencapsulation and active edible packaging concepts are promising strategies for protecting and delivering probiotic species [16]. Several commercially approved biopolymers are available to produce different systems to protect and deliver probiotic species [43]. In this sense, biopolymers as basic constituents of probiotics containing edible films, coatings, and protection structures (microcapsules) according to their source of production can be organized into three principal categories (Fig. 2): (A) Plant source (including marine plants): In this classification, there are polysaccharides, proteins, and lipids obtained from plants, including marine products. The most abundant materials that have been used as a basic biopolymer for probiotics containing edible films and coatings are presented in this category, such as starch [16, 44–47], cellulose [16, 44–47], pectin [16, 43–45], alginate [16, 44, 45, 47], and gums [43, 45].

IV. Applications of biopolymers membranes/films in food

Biopolymers in general Natural production

Synthetic production Natural produced by microoganisms

Extracted from biomass

Polysaccharides Starch

Proteins/polypeptides Animal proteins

Plant proteins

Cellulose

Collagen

Wheat gluten

Gums

Gelatin

Soy protein

Chitin chitosan

Whey Albumin

Bacterial cellulose

Lipids

Microbial polyesters

Wax (beeswax, carnauba wax) Oils Free fatty acids

Zein

Casein

FIG. 1

General classification or biopolymers based on their way of production [16, 38–40].

Biotechnology products or conventional synthesis from bio-derived monomers Xanthan pullulan

Polyhydroxyalkanoates (PHA) Poly(hydroxybutyrate) (PHB) Poly(hydroxybutyrateco-valerate) (PHB)

Polylactides and polyglycolides

Poly(lactic acid) (PLA) Poly(glycolic acid) (PGA) Poly(lactide-coglycolide) (PLGA)

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Biopolymers as basic constituents of probiotics containing edible films and coatings

Animal

Plant

Starch and derivatives Cellulose and derivatives

•

Plant exudates: Arabic gum

• •

Karaya gum Mesquite gum

• •

Plant extracts: Galactomannans Soluble soybean

Maltodextrin

Gluten (corn)

Pectin • •

Protein isolates: Pea Soy

Gelatin

Gellan

Casein

Dextran

Collagen

Bacterial cellulose

Whey proteins

Xanthan

Beeswax Pullulan Chitin

Waxes

Fatty acids

Microbial

•

Marin plants: Carrageenan

•

Alginate

Chitosan and derivatives

Phospholipids

Glycerides

FIG. 2 Classification or biopolymers as basic constituents of probiotics containing edible films and coatings based on their production source [16, 43].

(B) Animal source (including marine animals): As in the plant classification, biopolymers from animal origin are polysaccharides, proteins, and lipids. In this sense the most common material used as a primary constituent of probiotic-containing edible films is chitosan, although gelatin, casein, and whey proteins have also been studied [43]. (C) Microbial source: Recently, there has been an increase in the usage of biopolymers from this classification as basic constituents of edible films and coatings. The most used products are gellan, dextran, xanthan, pullulan, and bacterial cellulose, with some studies focused on gellan and dextran [36, 38, 43, 45, 48, 49].

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3.1.1 Plant-derived biopolymers 3.1.1.1 Starch and derivatives

Starch is a natural polymer found in a variety of plants, including (but not limited to) wheat, corn, rice, beans, and potatoes, that is extensively studied because of its abundance, biodegradability, low cost, and film-forming properties [45, 46]. This polysaccharide constitutes more than 60% of cereal kernels, and it is relatively easy to purify it. Depending on the source the granules of starch vary in structure, chemical composition, and shapes [45]. Amylose and amylopectin are the two main polysaccharides that essentially composed the starch granules with other components, such as lipids and proteins [45]. Amylose, the linear chain formed by α-1,4 anhydroglucose units, comprises approximately 20% (w/w) of starch, and it is responsible for its film-forming properties, while amylopectin, the branched polymer of short α-1,4 chains linked by α-1,6 glycosidic branching points occurring every 25–30 glucose units, constitutes the remainder [45, 46]. The differences of structure and the molar mass between amylose and amylopectin provide the variations of molecular and film-forming properties [45]. Starch and its derivatives are used in the food industry to retain and protect volatile compounds by acting as carriers to encapsulate compounds. This material has also been used for films and coatings [43].

3.1.1.2 Cellulose and derivatives

Cellulose constitutes the most abundant polysaccharide in nature, and it has been widely reported as a raw material for biodegradable films and coatings, mainly because of its renewability, low-cost, nontoxicity, biocompatibility, biodegradability, and chemical stability [45, 50]. This polysaccharide can be isolated from wood, cotton, hemp, and other plant-based materials [45]. Cellulose is a linear chain with two anhydroglucose rings ((C6H10O5)n), covalently linked through oxygen in a β-1,4 glycosidic bond. The number of repeated units per chain depends on the source [47]. Cellulose is insoluble in polar solvents but soluble in solvents with no similar chemical properties [47]. Due to cellulose being insoluble in water, derivatives produced by the partial or complete reaction of the hydroxyl groups have been studied and required for specific food applications. In this sense, MC, HPMC, and ethylcellulose have been produced with different solubility parameters [43].

3.1.1.3 Pectin

Pectin (one of the main components of the plant cell) is a polysaccharide constituted by poly α-1,4-galacturonic acid, and it is composed of at least three polysaccharide domains: homogalacturonan (the major component of pectin), rhamnogalacturonan-I, and rhamnogalacturonan-II. The carboxyl groups of the galacturonic acid units are esterified with methanol and sometimes partially with acetyl esterified. According to its degree of esterification, pectin can be classified as high (>50% esterified carboxyl groups) or low (6 log cycles CFU/g

Lactobacillus sakei

Sodium caseinate

Lactobacillus acidophilus and Bifidobacterium lactis

Probiotic bacteria

Food

Reference

10 days at 2°C

Fresh-cut apples and papayas

[56]

>6 log cycles CFU/cm

21 days at 4°C

Fresh beef

[59]

Calcium alginate

>6 log CFU/g

8 days at 5°C

Strawberries

[23]

Carnobacterium maltaromaticum

Alginate

>7 log cycles CFU/cm

28 days at 4°C

Smoked salmon

[57]

Lactobacillus acidophilus and Bifidobacterium bifidum

Gelatin

>8 log cycles CFU/g

6 days at 2°C

Hake fish

[24]

Lactobacillus paracasei Bifidobacterium lactis

Agar

>6 log cycles CFU/cm2

15 days at 4°C

Hake fillets

[61]

Lactobacillus rhamnosus

Alginate/whey protein concentrate

>6 log CFU/g crust 6.55–6.91 log CFU/portion

7 days at room temperature after digestion conditions

Coatings on bread

[54]

Lactobacillus plantarum and Kluyveromyces marxianus

Kefiran

Lactobacillus plantarum viability decreased less than 1.3 logarithmic cycles and Kluyveromyces marxianus decreased 0.7 logarithmic cycles at the end of storage

35 days at 20°C

ND

[60]

Lactobacillus plantarum

Methylcellulose

106 CFU/g

90 days at 20°C and 60% RH

Apple snacks

[55]

Lactobacillus acidophilus/ Lactobacillus casei

Sodium caseinate

5.5 log CFU/cm2

12 days at 4°C

ND

[30]

Continued

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TABLE 1 Viability of bacteria in polymeric films with different storage conditions—cont’d Probiotic bacteria

Film

Viability

Lactobacillus acidophilus, La. casei, La. rhamnosus, and B. bifidum

Carboxymethyl cellulose

Lactobacillus acidophilus and La. rhamnosus has been 107 CFU/g

Lactobacillus rhamnosus

Sodium carboxymethyl cellulose/ hydroxyethyl cellulose

>10 log CFU/mL at pH 7.4 >7 log CFU/mL at pH 2.4

Condition (time and temperature)

Food

Reference

42 days of storage at 4°C

ND

[58]

After preparation and released in PBS

ND

[40]

ND, not determined.

3.2.2 Encapsulation Edible coatings based on polymers can be utilized as encapsulating matrices to a probiotic microorganism to provide protection against low pH values, bile salts, and other constituent products that they may encounter during the gastrointestinal transit [62]. The encapsulation process is based on the embedding effect of an edible coating, which creates a microenvironment in the structural matrix that can control the interactions between the internal and the external part [63]. In this context, there are different types of encapsulation systems of which capsules or drops are obtained in micro with a diameter range of 1–1000 μm [64] or at the nanometric scale of diameter < 580 nm [65]. 3.2.2.1 Microencapsulation

The most common methods for obtaining microcapsules are described in the succeeding text. Also, Table 2 shows the studies carried out on the encapsulation of probiotics in polymer matrices. Extrusion is a process that consists of extruding a liquid mixture of the bioactive ingredient with its encapsulating matrix through an orifice and subsequently gelling the obtained droplets, usually by dripping them onto a gelling bath [76]. As shown in Table 2, this is the most used methodology. In general a cell suspension is mixed with a sodium alginate solution, and then the mixture drips into a solution containing multivalent cations (usually Ca2+ in the form of CaCl2). The droplets form gel spheres instantaneously, entrapping the cells in a threedimensional structure. This is because a polymer cross-linking occurs following the exchange of sodium ions from the guluronic acids with divalent cations (Ca2+, Sr2+, or Ba2+). These results regarding chain-chain association constitute the so-called egg-box model [77]. The size of the drop is affected by the characteristics of the alginate, extruder orifice diameter, and the distance between the syringe and the calcium bath [78]. On the other hand the emulsification process consists of the mix of two immiscible liquids, with one of the liquids being dispersed as small spherical droplets into the other. When the system consists of oil droplets dispersed in an aqueous phase, it is called an oil-in-water or O/W emulsion; however, if the system consists of water droplets dispersed in an oil phase, IV. Applications of biopolymers membranes/films in food

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TABLE 2

Viability of microencapsulated bacteria by different methods.

Probiotic bacteria Bifidobacterium lactis/ Lactobacillus acidophilus

Condition (time and temperature)

Encapsulation method

2.54  0.3 CFU/g (B. lactis) 6.24  0.12 CFU/g (L. acidophilus)

120 days at 37°C

Coacervation/ spray drying

[66]

4.26  0.13 CFU/g (B. lactis) 7.57  0.04 CFU/g (L. acidophilus)

120 days at 7°C

Matrix

Viability

Casein/pectin

Reference

Lactobacillus paracasei/ Bifidobacterium lactis

Sodium caseinate

>2 log CFU/mL (L. paracasei) >0.6 log CFU/mL (B. lactis)

After 90 min conditions of gastric

Emulsification/ gelation catalyzed by transglutaminase

[67]

Lactobacillus sp./ Bifidobacterium sp./Lactococcus lactis

Alginate/ gelatinized starches/ lecithin

>6 log CFU/mL

12 weeks at 23°C

Extrusion

[68]

Lactobacillus gasseri

Alginate/ chitosan

7 log CFU/mL

After 120 min conditions of gastric

Extrusion

[69]

Lactobacillus casei

Alginate/ chitosan

7.38 log CFU/mL

After 120 min conditions of gastric

Extrusion

[70]

Alginate/ chitosan/ carboxymethyl chitosan

7.91 log CFU/mL

Lactobacillus acidophilus

Starch

107 CFU/bread

After baking

Spray drying

[34]

Lactobacillus acidophilus

Alginate

22% survival rate

After 120 min conditions of gastric

Emulsification/ internal gelation by CaCO3

[71]

Lactobacillus rhamnosus

Alginate/ carrageenan

2.56 log CFU/g

15 days at 4°C

Emulsification

[72]

Lactobacillus casei

Alginate/ linseed or okra mucilages

>6 log CFU g/L

15 days at 5°C

Extrusion

[73]

>6 log CFU g/L

After 2 h at conditions of gastric

9.62 log CFU/mL

After nanoencapsulation

Electrospinning

[65]

Lactobacillus rhamnosus

Poly(vinyl alcohol)/ sodium alginate

Continued IV. Applications of biopolymers membranes/films in food

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TABLE 2 Viability of microencapsulated bacteria by different methods—cont’d Probiotic bacteria

Matrix

Viability

Condition (time and temperature)

Encapsulation method

Reference

Lactobacillus plantarum

Alginate/ arabinoxylan

7.05 log CFU/mL

Intestinal conditions

Cogelation

[74]

Bacillus sp.

Alginate/ chitosan

2  108 CFU/g

2 months at 4°C

Extrusion

[64]

Bifidobacterium longum

Soy protein isolate/ carrageenan

>4 log CFU/mL

30 days at 4°C

Coacervation

[75]

>1 log CFU/mL

After 120 min conditions of gastric

it is called a water-in-oil or W/O emulsion [79]. This process, compared with extrusion, is more practical as it allows size control of the drop through the speed of homogenization and the superficial activity of the encapsulated polymers [6, 78]. Some studies report that after the emulsification process, microencapsulation can be obtained after a gelation process [67, 71] as this links the macromolecular chains that give place to the formation of a branched polymer structure with a solubility that depends on the chemical nature of the starting materials [80]. For example, microcapsules obtained by emulsification can be catalyzed by transglutaminase [67] or be induced by Ca2+ because alginate is not stable under alkaline conditions [71]. Wu and Zhang [74] proposed another derivative process called cogelation, which is a process that uses alginate and arabinoxylan for the encapsulation of La. plantarum with a survival rate of 7.05 log CFU/mL in intestinal conditions. Coacervation is another method of microencapsulation that usually involves two or more polymers with opposite charges, forming insoluble complexes that are the result of electrostatic attractions; this process generally does not require high temperature or organic solvents, and it can retain higher stability of the encapsulated materials [81]. Oliveira et al. [66] prepared by complex coacervation using casein/pectin as the wall material, followed by spray drying to obtain microcapsules containing Bifidobacterium lactis and La. acidophilus. The microencapsulated microorganisms were more resistant to acidic conditions than free ones, and La. acidophilus maintained its viability for 120days at 7°C and 37°C with 6.24 0.12 and 7.57 0.04CFU/g, respectively. Spray drying involves the dispersion of the core material, followed by the homogenization of the liquid, and then the atomization of the mixture into a drying chamber. This leads to the evaporation of the solvent. The advantage of this process is that it can be operated continuously. The disadvantage is that the high temperature (85–90°C) used in the process might affect the viability of the probiotics [77]. 3.2.2.2 Nanoencapsulation

The formulation of nanometric materials has gained importance because at this scale, and they have more significant contact with the surfaces. These can be formulated by

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electrospinning, which is a methodology that combines electrospray and spinning. A high electric field is applied to a fluid, which can be a melt or a solution that comes out from the tip of a die, which acts as one of the electrodes. This leads to the droplet deformation, and finally to the ejection of a charged jet from the tip toward the counter electrode, leading to the formation of continuous fibers [77]. Ceylan et al. [65] nanoencapsulated La. rhamnosus into poly(vinyl alcohol) and sodium alginate-based nanofibers by electrospinning, and they determined that after the process, there was a survival rate of 83% (9.62 log CFU/mL) on fish fillets.

3.3 Probiotics release from polymeric matrices A colony count technique determines the viability of the probiotic microorganisms (Tables 1 and 2); however, in the encapsulation, there are reports on the viability under digestive conditions, simulating the gastric fluid (pH 2.5) or intestinal conditions (pH 6.8) at 37°C. 3.3.1 Control of microbial growth Many researchers have successfully tested the inclusion of LAB in different polymeric matrices and evaluated their viability and release, as well as their activation. Nevertheless, few works have studied growth control in matrices or from polymeric matrices. This is important because the growth or activity of microorganisms embedded in coatings and films matrices can compromise the polymeric materials by degrading them and diminishing their capacity due to a modified atmosphere, water vapor barrier, protection against contamination, etc. One of the most successful ways to transport probiotics in a viable, stable and even with prebiotics to food is microencapsulation [82–87]. Soukoulis et al. [54] showed that bread coated with sodium alginate polymers combined with whey protein concentrates (WPC) and probiotic bacteria (L. rhamnosus GG) can maintain adequate viability and survival under in vitro digestion conditions up to cell concentrations of 6.55–6.91 log CFU/portion, thus complying with WHO standards on the number of viable cells in a food for humans. Structural analysis of the crust of probiotic bread showed that the films containing WPC significantly improved the viability of microorganisms through different processing conditions. This growth initially decreased in all treatments during the first 24 h, but it was recovered in the final days of storage (sixth and seventh day), which demonstrates that microorganisms are protected and can be released or grow from films, fulfilling the objective of maintaining viability and release to food. Although most studies do not investigate the control of microbial growth from films and coatings, some authors report initial decreases in microbial counts, probably because of the stress conditions to which microorganisms are subjected; however, microbial counts are later recovered almost to their initial values [16, 20, 30, 54, 88]. Probably the recovery of the microbial count (viability) is due to an effect of the microbial reproduction in the microenvironments between the films or coatings and the food surface. It is essential to mention that although this microenvironment has the necessary nutritional characteristics for reproduction, it has neither sufficient space nor the necessary atmosphere nor the freedom of mobility for the normal growth of microorganisms since microorganisms are generally embedded in

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polymer matrices, which is a controlling or limiting factor for microbial growth. On the other hand, the fact that there is growth from films and coatings initially inoculated with probiotics is a promising way to bring probiotics stably into food, as they can function as an integral vehicle in food products, improving human health benefits. 3.3.2 Microbial survival Not much has been studied concerning the growth of microorganisms in edible film matrices; however, survival can be determined by the standard account of the microorganisms and its identification. The main objective of different films and edible coatings with microencapsulated or nonencapsulated probiotics is to bring microorganisms in a viable and stable way to food and even to the gastrointestinal tract [28]. For example, it has been shown that incorporating probiotics into edible Na-Alginate films can successfully protect and release probiotics at levels >106 CFU/g, which are within the suggested ranges for probiotic foods (106–109 viable cells), and even the use of this technology does not add a significant cost to the food, but it does bring a great benefit to the health of consumers [88]. Of course, it should be noted that the reported growths must be a product of the release and growth from the supports into the food, which could be improved with the inclusion of prebiotics, as they can further improve stability, increase growth and release of probiotics as well as improve host health and well-being [89]. Romano et al. [90] demonstrated that the inclusion of fructooligosaccharides to methylcellulose films with La. delbrueckii subsp. bulgaricus CIDCA 333 and La. plantarum CIDCA 83114 strains improved the integrity and stability of the bacteria, increasing the survival period in the film matrix. This must be generated in addition to the protective effect of the prebiotic for its use as a substrate by microorganisms. Other types of polymers have been evaluated to improve the viability and stability of microorganisms, among them are rice and corn starch, bovine gelatine, sodium caseinate, and soy proteins, which have been successfully combined with La. rhamnosus GG at refrigeration and ambient temperatures, notably improving the survival (viability) and stability of microorganisms [91]. The immobilization of B. bifidum and La. acidophilus in edible gelatine films has been observed to protect the preservation of viability and stability of microorganisms during 6 days of storage at 2°C [24]. Similarly, edible starch films combined with pullulan can change the viability of strains of La. reuteri ATCC 55730, La. plantarum GG ATCC 53103, and La. acidophilus DSM 20079 depending on the ratio between starch and pullulan and the storage temperature [25]. Another type of treatment that was tested to improve the viability and microbial load of probiotics in films and coatings was the intercalated application of starch-based coatings and microencapsulated probiotic bacteria on the surface of part-baked breads, which demonstrated that the surface of the breads could retain concentrations of (2.4–3.05 107 log/CFU g) even after a short baking heat treatment and with minimum losses of 1.0–1.4 log CFU/g after 24 h of storage at room temperature [92]. The application of films and coatings on foods in commercial systems implies more significant challenges for researchers, like food processing conditions, sensibly decrease the microbial count and viability of any probiotic, at least initially. Conditions, such as temperature and

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convective drying, will induce significant losses in the initial probiotic count due to alterations in the lipid bilayer that can lead to the loss of cell membrane integrity [93]. It has been reported that the use of some materials for the manufacture of films and coatings, such as pectins, cellulose, and some alginates, may decrease the viability of bacterial cells during the drying processing and storage, due to lesions in cell membranes caused by osmotic stress [87, 94, 95]. Studies conducted by Soukoulis et al. [54] showed that the initial decrease in viability of La. rhamnosus GG in alginate combined treatments with whey protein concentrates at two temperatures (60°C and 80°C) in coated bread had similar decreasing patterns during the first days of storage at room temperature. Despite the initial decrease, La. rhamnosus GG achieves recovery of its total viable count (log CFU/g) at the end of storage (sixth and seventh day), which implies the survival and growth of microorganisms in the coatings. 3.3.3 Metabolic activity The metabolic activity of microorganisms is closely related to the generation of primary and secondary metabolites that can generally be secreted to the environment where they are found. Probiotic bacteria can generate substances, such as organic acids, aldehydes, exopolysaccharides, some proteins, bacteriocins, and some peptides. Bioactive compounds of antimicrobial nature can be mixed directly with polymers in films and coatings to focus on the functional effect of these components on the food surface. Combination of bioactive compounds with coatings or packaging may have different advantages such as (a) maintain a high concentration without excessive migration toward the food, (b) generally the bioactive compound will not be a direct food additive, and (c) these agents have a partial effect on the surface flora [96]. One of the main advantages of probiotic microorganisms in food is the generation of metabolites for the maintenance of organoleptic characteristics and the reduction or inhibition of growth of pathogenic microorganisms; however, instead of producing metabolites independently and mixing them with food, it has been proposed to include microorganisms with films and coatings so that they interact with the food surface where most of the microbial flora is found. Many examples demonstrate the successful combination of films and coatings with probiotic microorganisms, and most refer to the metabolic activity as the generation or interaction with metabolites. Burgain et al. [97] analyzed the in vitro production of exopolysaccharides (EPS) by La. rhamnosus GG and their adhesion capacity and interaction with milk proteins (micellar casein, native or denatured whey proteins), and they observed improvements in the viability of the microorganisms in dairy products. The incorporation of La. acidophilus and La. casei to sodium caseinate films has shown that both microorganisms can remain viable at different concentrations during 12 days of storage and recover their growth to 5.5 log CFU/cm at the end of storage [30]. These variations were attributed to LAB adaptation to the new substrate (sodium caseinate) and the temperatures used [24]. Another important factor regarding the metabolic activity of LAB was the antimicrobial activity against Li. monocytogenes during the first 6 days of storage at 4°C concerning the control (P  .05). Additionally, in this study, the incorporation of bacteria to the films did not change the tensile strength (P > .05), and it also improved the appearance of the films significantly. These data are a clear indication that lactic acid bacteria can survive and remain

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metabolically active in films, preventing other microorganisms’ growth, through the production of either lactic acid, bacteriocins, diacetyl, or hydrogen peroxide [30]. Another similar study developed by Concha-Meyer et al. [57] presented the combination of alginate matrices with Carnobacterium maltaromaticum to preserve smoked salmon at low temperatures, obtaining bacteriostatic effects on Li. monocytogenes in concentrations of 104 CFU/cm for 28 days at 4°C and showing sustained metabolic activity throughout storage in the evaluated conditions. Some antioxidants have been combined into agar-based films with La. paracasei L26 and B. lactis B94 and green tea extracts in hake fillets, which has resulted in the migration and growth of probiotics on food surface, a decreased chemical spoilage indicators (total volatile bases and trimethylamine nitrogen, and pH changes), and a reduction in the growth of Shewanella putrefaciens and Photobacterium phosphoreum previously inoculated in food [61]. Lactic acid as a biological compound has advantages for food application since it is an effective antimicrobial agent; some foods can contain it and can be combined with different packaging. Pavli et al. [88] showed that the application of three strains of LAB in edible Na-alginate films on ham surfaces increased their population and acidified the medium due to activity and migration of LAB involved in the treatments. An effect that was modified by the different conditions of temperature and pressure used that affected the metabolism of microorganisms. Several authors have reported the acidification of films and coatings containing probiotic bacteria and LAB that are generally related to the production of lactic acid, which is produced by homo- and heterofermentative pathways. These results demonstrate that films and coatings with probiotic microorganisms cells can be used as efficient packaging methodologies to improve food safety, since microorganisms are generally metabolically active, which is a fact that is reflected in (a) high viability of microorganisms, (b) growth from films in food, (c) production of different metabolites, (d) control of pathogenic and spoilage microorganism, and (e) change in the organoleptic characteristics of food.

4 Regulations on the use of probiotics in the food industry The regulatory aspects that need to be considered for probiotics are efficacy, safety, quality control of manufacturing, and regulation of the health claims that can be made for individual products [98]. There is not a universally agreed framework regulation of probiotics between countries. In the United States, most probiotic products are classified as foods or dietary supplements regulated by the Food and Drug Administration (FDA). In the European Union, probiotics and food supplements are regulated under the Food Products Directive and Regulation (Regulation 178/2002/EC; directive 2000/13/EU). The EFSA must authorize all health claims for probiotics, and it is also responsible for assessing health claims made for probiotic products [98]. In the United States the regulatory requirements differ according to the proposed use of a probiotic, whether as a drug or a dietary supplement. Although virtually in all product categories regulated by the FDA, there is still no pathway to deal specifically with probiotics [16, 99]. The FDA defines a drug as an article intended for use in the diagnosis, cure, mitigation,

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treatment, or prevention of disease. The Dietary Supplement Health and Education Act (DSHEA) of 1994 defines a dietary supplement as a product taken orally that includes dietary ingredients, such as vitamins, minerals, herbs, and amino acids, that have the purpose of supplementing the diet [99]. If the probiotic is intended to be used as a drug, it must undergo a regulatory process authorized by the FDA before it is used on humans. The probiotic drug has to be determined to be efficient and harmless before undergoing the marketing process. If the probiotic is intended for use as a dietary supplement, it is regulated as a food by the FDA’s center [99]. When the probiotic falls in the food or food additive category, only a premarket notification is required [16]. Contrary to drugs, dietary supplements do not need FDA approval before being marketed. According to DSHEA the manufacturer must determine that all the manufactured and distributed dietary supplements are safe and that all claims or representations have sufficient evidence to demonstrate their truthfulness and accuracy. Additionally, the FDA Report [100] published a guide for good manufacturing practices in packaging, labeling, and holding operations for dietary supplements. The regulation allows that in addition to nutrient content claims, the manufacturer of dietary supplements may make structure/function or health claims for their products. For these claims the FDA requires that the manufacturer’s substantiation is accepted by an expert in the field and that all allegations are accurate and true. The Joint Food and Agriculture Organization of the United Nations/World Health Organization Expert Consultation on Evaluation of Health and Nutritional Properties of Probiotics in 2001 created guidelines for the evaluation of probiotics in food that could get the substantial of health claims, all of this to standardize the requirements to make health claims concerning probiotic agents. These guidelines advise (1) genus and species identification of the probiotic strain by utilizing a combination of phenotypic and genotypic analysis as clinical evidence that might indicate that the health benefits of probiotics might be strain specific, (2) in vitro experimentation to define the mechanism of probiotic effect, and (3) validation of the clinical health benefits of probiotic agents with substantiation of the clinical health benefits of probiotic agents with human trails. Safety assessment of the probiotic strain should at least dictate (i) the antimicrobial drug resistance patterns, (ii) probiotic metabolic activities, (iii) side effects presented in humans throughout the clinical trials and after marketing, (iv) production of toxins and hemolytic potential if there is any information about those properties of the probiotic strain, and (v) absence of infectivity in studies performed in animals [99]. Most probiotic microorganisms have a longlasting history of safe use as a food component, so safety evaluation does not denote a hurdle to be overcome. Among them, numerous microorganisms have been qualified with the presumption of safety (QPS) status for food applications by the European Food Safety Authority (EFSA) and have been classified as generally regarded as safe (GRAS) by the US Food and Drug Administration [16]. European legislation is more conservative as the term “probiotic” is not regulated, since the designation implies a beneficial health effect and should be considered a health claim by itself. Then many microorganisms currently used in food fermentation have a long history of safe use in the European Union; nevertheless, foods containing microorganisms that have not had a traditional use in food production in Europe before 1997 are considered a novel food, whose legislation is currently under regulation (EC9 No 2015/2283). In this legislation a novel food should be subjected to an in-depth characterization and safety assessment before commercialization on the European Market. Additionally the communication of any health claim

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may only be made after authorization by the European Commission, which requires a favorable opinion from the EFSA [16]. Even though technical regulation of bioactive substances and probiotics isolates with alleged functional or health properties is described in the European Union, the United States, and Latin American countries (such as Brazil), a specific law for edible material containing probiotic microorganisms is not provided [16]. In Mexico, since 1996, the Ministry of Commerce and Industrial Development, through the Directorate General of Standards, established in the Official Mexican Standard [101] the general labeling specifications of national manufactured and foreign prepackaged foods and nonalcoholic beverages. The producers of goods containing probiotics must focus on the compliance of the standards and the general specifications established by the Office of the Federal Prosecutor for the Consumer (known in Spanish as PROFECO). At present the Official Mexican Standard project [102] is intended to describe the use of the specifications of health properties declared on food and beverages that include probiotics.

5 Conclusion The research on the development of films and coatings with incorporated probiotics as active food packaging is still emerging. Nevertheless, biopolymers with different structures and properties, such as starches, proteins, gums, cellulose, and chitosan, have been studied as protection structures with novel and important beneficial characteristics. Probiotics are incorporated into the polymer matrices that serve as protection against low pH values, bile salts during the gastrointestinal transit, and growth-stimulating effects. This also improves viability through storage. The addition of probiotics to matrices should not affect mechanical properties, appearance, and permeability. However, the films or coatings may allow the probiotics to acidify and inhibit the growth of spoilage and pathogenic microorganisms. Therefore, it is necessary to carry out future studies about mechanisms that enhance the growth of probiotics by the use of prebiotics and available substrates and to determine the release control in the food. Technical regulation of bioactive substances and probiotics with alleged functional or health properties should be established for edible films of coatings containing probiotics, prebiotics, and active compounds.

Acknowledgments The authors would like to thank Consejo Nacional de Ciencia y Tecnologı´a (CONACyT). Miss Claudia H. Barrera is greatly acknowledged for the diligent editing and proofreading of this paper.

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IV. Applications of biopolymers membranes/films in food

Index Note: Page numbers followed by f indicate figures and t indicate tables.

A 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), 352, 451, 453f, 454 Activated carbon, 368, 560–561 Active edible coatings, 509–510 Active edible packaging, 594–597 Active pharmaceutical ingredients (API), 315, 398 Adhesive proteins, 274 Adsorbent materials, in ethylene scavenging systems, 559–564, 561–563t, 565t Adsorption, 360, 407, 411–412 equilibrium, 408 of glyphosate, 367–368 kinetics, 408–409 Pb(II) adsorption, SA/nanohydroxyapatite films for, 19, 353 permethrin, 372–373 protein adsorption, 279–281, 281f soil adsorption, 365 Advanced platelet-rich fibrin (A-PRF), 252 Adverse outcome pathway (AOP), 87–88 AFCs. See Alkaline fuel cells (AFCs) AFM. See Atomic force microscopy (AFM) Agar, 21t, 134, 299, 462–463, 608 Agarose, 5–7t, 11 Aldehydes, 51 Alginate (ALG), 134, 350, 353, 368–369, 508 biomedical applications, 14 casting, 38 characteristics of, 4–7, 5–7t chemistry of, 350 composites, for environmental applications, 19 edible coatings and films, 15–16, 518–519 fuel cells, 456–458 for green organic solvent nanofiltration, 353 heavy metal removal, 350–353 membrane reactor, 353, 354f modification, 350–352 nanocellulose, 56–57 physical/ionic cross-linking, 51–52 plant-derived biopolymers, 598 properties, 7

vegetable sources, 400 wound dressings, 173–174 Alginate-chitosan PEC membranes, 179 Alginate-disodium ethylenediaminetetraacetate dihydrate hybrid aerogel (Alg-EDTA), 351–352 Alginate-pectin blends, 179 Aliphatic polyester, 399 Alkaline fuel cells (AFCs), 424–425 1-Alkyl-imidazoles, 344, 345t Aloe vera (AV), 13–14, 18, 106, 151, 174, 182t α-cyclodextrins, 370–371, 370f American Society for Testing and Materials (ASTM), 80 Amino-functionalized multiwalled carbon nanotubes (MWCNTs-NH2), 341, 342f Ammonium salt, 437–439, 454, 480 Amniotic membranes (AM), 157 Ampicillin (AMP), 154 Ampoule sheet masks, 320 AMPS. See 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) Anesthetics, 259–261 Angioplasty, 273 Animal-derived biopolymers casein, 598–599 chitosan, 598 gelatin, 598 whey proteins, 599 Animal sources, membrane manufacturing chitosan (CS), 401–402 collagen, 402 silk, 402–403 Antimicrobial activity, of biopolymer films, 89 Antimicrobial LbL films antibacterial and antifungal surfaces, 223–225 antimicrobial surface approach, 223 Antimicrobial surface approach, 223 Antiplasticization, 48 Apple skin polyphenols (ASP), 584 Arrhenius equation, 112, 485, 486f, 491 Ascorbic acid, 519, 523, 539–540 Ascorbyl palmitate-β-cyclodextrin inclusion complex, 540

617

618

Index

Aspirin, 262 Asymmetric membranes, 36, 121, 125, 127, 129, 262 Atomic force microscopy (AFM), 72, 283 Atom transfer radical polymerization (ATRP), 406 Avogadro’s number, 493

B Bacterial cellulose (BC) membranes, 247–248 biomedical applications, 14 characteristics of, 5–7t, 8 nanocomposite membranes, 446–447, 451, 454–456 wound dressings, 174–175 Bacterial fermentation products poly(butylene succinate) (PBS), 400 polyhydroxyalkanoates (PHA), 399 polylactic acid (PLA), 399 Ball milling method, 55 Banana starch-chitosan edible films, AV gel, 18 Barrier properties, biopolymer membranes/films, 80–81 differential methods, 83 gas permeability coefficient, 81 gravimetric method, 82–83, 82f microbial penetration, 83 volumetric methods, 81–82 X permeability coefficient (XPC), 81 X transmission rate (XTR), 81 Battery applications, biopolymer electrolytes battery performance, characterization of discharge characteristics, 497–499 open-circuit voltage (OCV), 497 rechargeability, 499 charge/discharge process, 478–479 electrical impedance spectroscopy Cole-Cole plot, 483–484, 483f dielectric properties, 487 equivalent circuit model, 483–484 ionic conduction mechanism, 487–489 ionic conductivity, 484–486 electrochemical cell(s), 495 Fourier transform infrared (FTIR) spectroscopy, 490–493, 492f ionic dopant/salt effects, 479–480 natural polymers, 479 operating principle, 495–497 plasticizer, 480–482, 481t poly(ethylene oxide) (PEO)-alkali metal salt complex, 477–478 preparation, biopolymer membranes, 482, 482f primary battery, 495 properties, 478–479 proton (H+ ion)-conducting membrane, 477–478 rechargeable batteries, 477 secondary battery, 495

transference number measurement (TNMs), 493–495 X-ray diffraction (XRD), 489–490 Battery test bench, 497 Bauer-Kirby disk diffusion test, 89 β-cyclodextrins (bCD), 370–371, 370f, 374, 411 BFCs. See Biofuel cells (BFCs) Bifidobacterium spp., 542, 590, 601–602t Bioactive glass composites, 251–252 Biocomposite films, 401, 585 Biofuel cells (BFCs), 431–432 Bioink, 253 Biological characterization, of biopolymer films, 68–69t antimicrobial activity, 89 cytotoxicity, sensitization capacity, and irritation potential, 86–88 risk management process, 86, 86f Biological dye removal methods, 387 Bioplastics, 20–22 Biopolymer membranes and films, 18–19, 59–60, 133–135, 142, 143f biological characterization, 68–69t antimicrobial activity, 89 cytotoxicity, sensitization capacity, and irritation potential, 86–88 risk management process, 86, 86f chemical characteristics, 389–390 drawbacks, 59–60 dye removal (see Dye removal, biopolymer membranes) edible coatings and films (see Edible coatings and films) energy applications, 19–20, 21t environmental applications, 18–19 in fuel cell applications (see Fuel cells, biopolymer membranes) heavy metal removal, from industrial effluents (see Heavy metal removal) market for, 20–22 pesticides removal, 366–377, 374–377t physical characterization barrier properties (see Barrier properties, biopolymer membranes/films) contact angle, 83–84, 84f degradation/erosion degree, 79 mechanical properties, 79–80 publications, number of, 67–68, 68–69t swelling degree, 78–79 textural analysis, 85 physicochemical characterization (see Physicochemical characterization, biopolymer membranes/films) properties, 274 publications and citations on

Index

evolution of, 22, 22f polysaccharide films, 99–102, 100–101f protein films, 101–102, 101–102f rechargeable proton battery applications (see Battery applications, biopolymer electrolytes) 2D films and membranes (see Two-dimensional (2D) films and membranes) wound dressings (see Wound dressings, biopolymer membranes/films) Biosensing applications, LbL technique for, 225–227 Bipolar plates (BP), 429–430 Blended chitosan membranes, 337, 338f, 339t Blends biomedical applications, 13–14 dentistry applications, 250 wound dressings, 178–180 chitosan (CS), 439–442 ionic liquids (ILs), 58 polymer modification, 57–58 Blood-contact device surfaces, coatings, 274 stents, 274–277, 276t sulfated chitosan challenges, 284–285 chemical modification, 278–279, 279–280f NiTi alloys, chitosan-HEP nanoparticle coating on, 284 platelet adhesion, 281–282 protein adsorption, 279–281, 281f stainless steel, 282–284, 283f Blown film extrusion process, 37f, 39–40 Blow-up ratio (BUR), 39 Boltzmann constant, 485, 493 Bovine serum albumin (BSA), 279–281, 281f Bragg’s law, 74 Brown algae (Phaeophyceae), 4–7, 368–369, 400 Bubble point method, 85 Bubble sheet masks, 320 1-Butyl-3-methylimidazolium chloride ([BMIM]Cl), 59

C Calcium, 38, 51–52 Calcium alginate, 19, 151–153, 171–172t, 351, 524t Calcium phosphate composites, 251 Capsosomes, 234 Carbohydrates, 274, 275f Carbon dots, 182 Carbon nanotubes (CNTs), 55–56, 341 Carboxymethyl cellulose (CMC), 106, 179, 204–207t, 208–209, 401, 480 Caries prevention, 258 Carrageenans, 347, 508 characteristics of, 5–7t, 10 chemistry of, 347

619

heavy metal removal, 347–349 membranes, 349 modification, 347–349 Cartilage regeneration, 155–156 Casein, 598–599 Casting technique, 111–112 biopolymer 2D films and membranes, production of, 37–39, 37f orally disintegrating films (ODFs), 290, 291–293t, 294 Catalase, 310–311, 540–541, 546 Catalyst layers (CL), 429–430 Cellulose (C), 14, 342–347, 348t, 371 bacterial cellulose (BC) (see Bacterial cellulose (BC) membranes) cellulose acetate (CA), 449–450 cellulose-based graft copolymers, 451–454 cellulose nanocrystals (CNCs), 446–447, 455 cellulose nanofibers (CNFs), 446–447 characteristics of, 5–7t, 7–8 chemical structure of, 371, 371f chemistry of, 343, 343f coulombic efficiency, 449–450 cross-linked cellulose-based membranes, 450–451 dentistry applications, 247–248 and derivatives plant-derived biopolymers, 597 polysaccharide-based edible film, 519 edible film, 519 Gluconacetobacter hansenii, 446–447 Gluconacetobacter xylinus, 446–447 heavy metal removal, 342–347 inorganic/organic compounds, 454–455 modification, 343–344 chemical modifications, 447 physical modification strategies, 447 Nafion (N), 455 nanocrystal cellulose (NCC), 446–447 plant-origin cellulose, 446–447 polymer composites, 455–456 pure cellulose, 447–448 selective solid electrolyte membranes, 446 water purification membrane preparation, 18 Cellulose acetate (CA), 405, 449–450 Cellulose acetate nanofiber (CANF) membrane, 345–346, 346f Cellulose microcrystal (CMCs), 447–448 Cellulose nanocrystals (CNCs), 56–57, 447–448, 566–567 Cellulose nanofibers (CNFs), 447–448 Cellulose nanofibrils (CNFs), 41–42, 56 Cellulose triacetate/silica composite membranes, 18–19 Chemical cross-linking, 49–51, 50f Chemical dye removal methods, 387

620

Index

Chitin, 134, 367–368 biomedical applications, 13, 144, 145–151t characteristics of, 5–7t, 8–9 chemical structures of, 367, 368f linuron removal, 368 PLA/chitin nanocrystal (ChNCs) composites, 40 porous membrane preparation, 9, 9f Chitosan (CS), 13, 134, 144, 145–151t, 335, 367–368, 372–373, 507, 509, 539–540 animal-derived biopolymers, 598 blended chitosan membranes, 337, 338f, 339t BSA and FIB adsorption, 279–280 characteristics of, 5–7t, 8–9 chemical structure of, 277–278, 277f, 367, 368f chemistry of, 335–337 coating endovascular stent, 276 magnesium alloys, 277 NiTi alloys, chitosan-HEP nanoparticle coating on, 284 NiTi shape memory alloy surface, 277 composite membrane, 341 dentistry applications, 246–247 dissolving microneedle patches, 204–207t, 209 edible coatings and films, 16–17, 519 energy applications, 20, 21t ethoprophos removal, 368 film chitosan/Ag/ZnO blend films, 181 diffractogram of, 75, 75f wet casting, 38 zeolite-loaded chitosan films, textural analysis of, 85 zeolite NaA and zeolite NaA/chitosan film, diffractogram of, 75–76, 76f fuel cells, biopolymer membranes biological products, 435–436 chemical modification, 436–437 functional groups, 436, 437f inorganic filler composites, 442–445 ion-solvating polymer composite membranes, 435–436 polyanions, 435–436 polyelectrolyte, 435–436 polymer blends, 439–442 polymer composites, 445–446 SCS and PCS derivatives, 436–437, 438f self-cross-linked and salt-complexed chitosans, 437–439 swelling behavior, 436 uses of, 435–436 guided bone regeneration (GBR), 156 heavy metal removal, 335–341 linuron removal, 368

nanocellulose, 56–57 peripheral nerve regeneration (PNR), 155 platelet adhesion, 281–282 sunscreens, applications in, 316 chitosan-coated ZnO nanoparticles, 317 chitosan-hydroxyapatite gel, antibacterial activity of, 316 chitosan/TiO2 particles, 317 p-acetamide benzoylate chitosan, 317 supported chitosan membranes, 337–341 transdermal films and patches, 198–200t, 200–201 water purification membrane preparation, 19 wound dressings, 170–173 Chitosan and Aloe vera (CAV), 145–151t, 151 Chitosan-carbon dot devices, 182 Chitosan-hydroxyapatite sunscreen gel, 316 Chitosan-montmorillonite (MMT)-CuO composites, 372 Chlorhexidine (CHX), 233, 256, 258 Chlorosulfonic acid (CS-SC) BSA and FIB adsorption reduction, 280–281, 281f chemical modification, of chitosan, 279, 280f platelet adhesion, reduction in, 282 Chondroitin sulfate, 204–207t, 210 Chondroitin 6-sulfate (ChS), 275–276, 276t Ciprofloxacin-loaded calcium alginate membranes, 151–153 Citric acid, 49, 51 Clomazone, 369–370, 411 CMC. See Carboxymethyl cellulose (CMC) CMCs. See Cellulose microcrystal (CMCs) CNCs. See Cellulose nanocrystals (CNCs) CNFs. See Cellulose nanofibers (CNFs) CNTs. See Carbon nanotubes (CNTs) Coacervation method, 604 Coating techniques, 37f, 40–41 cardiovascular applications challenges, 284–285 NiTi alloys, chitosan-HEP nanoparticle coating on, 284 stainless steel surfaces, sulfated chitosan-based coating on, 282–284, 283f stents, 274–277, 276t dipcoating, 42–43 edible coatings (see Edible coatings and films) oxygen scavenging films preparation, 544–545 spray coating, 41–42 spread coating, 41 Cogelation, 604 Cole-Cole plot, 483–484, 483f Collagen, 402, 521 characteristics of, 5–7t, 12 dentistry applications, 246 guided bone regeneration (GBR), 156

Index

sunscreens, applications in, 316 wound dressings, 175–176 Collagen fiber bundles (Agcol), 402 Collagen microfibrils (F-col), 402 Collagen molecules (Col), 402 Combined heat and power (CHP) generation, 426–427 Composites, 341 cellulose (C), 455–456 definition, 52–53 dentistry applications bioactive glass composites, 251–252 calcium phosphate composites, 251 hydroxyapatite composites, 250–251 silver nanoparticle composites, 250 edible coatings and films, 522 fibrous composites, 52–53 laminated composites, 52–53 particulate composites, 52–53 2D biobased films and membranes bionanocomposites, 53 carbon nanotubes (CNTs), 55–56 cellulose nanocrystals, 56–57 silicate layers, 53–55, 54f wound dressings, 180–182 Compound annual growth rate (CAGR), 20–22 Contact angle, 83–84, 84f Copper-coated chitosan nanocomposites, 373 Copper nanoparticles, 181 Corneal wound-healing, 157 Corn zein, 521 Corona treatment, spray coating, 41–42 Correlated barrier hopping (CBH) model, 488–489 Covalent grafting process, 282–283 Cross-flow filtration, 121, 122f Cross-linking methods, 49–50 chemical cross-linking, 49–51, 50f enzymatic cross-linking, 50f, 52 physical/ionic cross-linking, 50f, 51–52 Cyclodextrins (CDs), 371, 401

D DCFCs. See Direct carbon fuel cells (DCFCs) Dead-end filtration, 121, 122f Degradation/erosion degree, biopolymers, 79 Degree of acetylation (DA), 401–402 Degree of deacetylation (DD), 277–278 Degree of esterification (DE), 520 Dental implants, 255–256 Dentistry, biopolymer membranes and films in, 245, 253f blends, 250 composites bioactive glass composites, 251–252 calcium phosphate composites, 251

621

hydroxyapatite composites, 250–251 silver nanoparticle composites, 250 drug delivery systems, 259–264, 262–263t anesthetics, 260–261 antimicrobial and antiinflammatory drugs, 259–260 bone- and tissue-healing agents, 261–264 hybrid films, 252–253 natural and synthetic biopolymers, 249 cellulose and derivatives, 247–248 chitosan, 246–247 collagen, 246 copolymers, 249 gelatin, 247 hyaluronic acid (HA), 248–249 poly(lactic-co-glycolic acid) (PLGA), 249 poly(lactide-co-caprolactone), 249 treatments, 253–254, 253f anesthetics, release of, 259 caries prevention, 258 endodontic therapy, 254–255 extractions, implants, and bone regeneration, 255–256 oral cancer treatment, 258 periodontology, 256–257 prosthetic dentistry, 258 Dermis, 167–168, 168f, 312–313 Desorption, 124–125 Dexamethasone (DEX), 233 Dextran biomedical applications, 15 characteristics of, 5–7t, 11 dissolving microneedle patches, 210 microbial derived biopolymers, 599 Dialysis, 128 Diclofenac sodium (DS), 400 Dietary supplements, 591, 608–609 Difenzoquat, 369–370, 411 Differential scanning calorimetry (DSC), 77–78 Diffusion process definition, 98 natural polymer films, 98 diffusion coefficients, 111–113, 111t, 113–114t mass transfer, 106–107 mathematical modeling, 107–110 number of publication, 110, 111f permeability, 107 temperature, influence of, 112, 112t Diffusive gradients in thin films (DGT), 403–404 Dimethyl sulfoxide (DMSO), 250 Dip-coating layer-by-layer technique, 256 Dipcoating technique, 40–43 Dipping method, 222 Diquat, 369–370, 411

622

Index

Direct carbon fuel cells (DCFCs), 428–429 Direct melt intercalation, 55 Direct methanol fuel cells (DMFCs), 431, 433–434, 449, 451, 454 Disinfectants, 397 Disintegration time, of ODFs, 296–297, 297t DMA. See Dynamic mechanical analysis (DMA) Dopamine, 282–284 Doxorubicin (DOX), 208–209, 233–234 Drug delivery systems, biopolymers, 153–155 for dental applications, 262–263t anesthetics, 260–261 antimicrobial and antiinflammatory drugs, 259–260 bone- and tissue-healing agents, 261–264 layer-by-layer (LbL) technique, 229–230 transdermal drug delivery system (see Transdermal drug delivery, biopolymers) DSC. See Differential scanning calorimetry (DSC) Dubinin-Radushkevich (D-R) model, 408 Dye removal, biopolymer membranes applications, 392–393 characterization techniques, 389–390 chemical structure, 389–390 mechanical properties, 389, 390f morphologic design, 389 pure water flux and permeability, 390 comparative analysis of, 391 from effluents, 383–387 biological methods, 387 chemical methods, 387 colored effluents discharge, guidelines for, 386 environmental and public health risks, 384–386 physical dye removal methods, 387 laboratory experiments, 390, 391f membrane performance, evaluation of antifouling property, 391 rejection rates, 391 selectivity, 391 membrane preparation, raw materials for, 388–389 perspectives and challenges, 393 Dynamic mechanical analysis (DMA), 78

E Echinomycin, 276–277 Edible coatings and films, 23, 35, 507, 509 and active edible coatings, 509–510 alginate, 15–16, 508 Aloe vera (AV) gel, 18 bacteriocins, 515–516 barrier properties, 518 biodegradability and renewability, 518 biodegradable edible coatings, 515–516 carrageenans, 508

chitosan, 16–17, 507 coating materials, properties of, 40–41 composite edible coatings and films, 522 compositions and functions, 15, 16f fruits and vegetables (see Fruits and vegetables, edible films and coatings) gelatin, 17 LbL edible coatings, 45 lipid oxidation, 516–517 materials, 506–507 meats, poultry and seafood, 516–517, 526–527t active components, 522–523 quality, effect on, 523–528 mechanical properties, 518 polysaccharide-based edible film, 518–520 probiotics, 596f animal-derived biopolymers, 598–599 encapsulation, 602–605 entrapment, 600–601, 601–602t metabolic activity, 607–608 microbial derived biopolymers, 599–600 microbial growth, control of, 605–606 microbial survival, 606–607 plant-derived biopolymers, 597–598 protein-based edible coating and films, 520–522 radical scavenging activity, 516–517 soy protein, 17 spray coating, 42 starch, 17 synthetic antioxidants, 516–517 waxes, 507 EFC. See Enzymatic fuel cells (EFC) Egg-box model, 51–52, 602 Electrical impedance spectroscopy Cole-Cole plot, 483–484, 483f dielectric properties, 487 equivalent circuit model, 483–484 ionic conduction mechanism, 487–489 ionic conductivity, 484–486 Electric charged membranes, 121 Electrodialysis (ED), 128–129, 398 Electromotive force method, 493 Electron beam irradiation, 248 Electron spectroscopy. See X-ray photoelectron spectroscopy (XPS) Electropolishing, 283 Electrospinning, 37f, 46–47, 46f, 166, 166f, 250, 253 Emulsification process, 602–604 Emulsion-based coatings and films, 522 Enamel deremineralization, chitosan, 247 Endocrine-disrupting compounds (EDCs), 409–410 Endocrine disruptors, 397 Endodontic therapy, 254–255

Index

Energy applications, natural polymers, 20, 21t, 23 Energy density, 499 Energy-dispersive X-ray spectroscopy (EDS), 71 Enzymatic biosensors, 226 Enzymatic cross-linking, 50f, 52 Enzymatic fuel cells (EFC), 431–432 Enzymatic scavengers, 540–541 Epichlorohydrin, 371 Epidermis, 167–168, 168f, 312–313 Epigallocatechin gallate, 258 Equivalent circuit model, 483–484 Erosion-controlled systems, 229 Escherichia coli, 223–225, 224t Essential oils (EO), 183, 523 Ethanolamine (EA), 49 Ethoprophos removal, 368 Ethyl cellulose, transdermal films and patches, 198–200t, 202 Ethylene biopolymer films, for ethylene control edible coating systems, 566–567 packaging film systems, 567–568 biosynthesis route of, 556 climacteric and nonclimacteric crops, 554–555 ethylene removal filters, 561, 561t exogenous ethylene, 556 fruits and vegetables, 554–555, 565t nonclimacteric horticulture, 556 plant phytohormone gas, 554 postharvesting, 554, 558 ripening, 554–556 S-adenosyl-L-methionine (AdoMet), 556 scavenging, conventional methods for adsorbent materials used in, 559–564, 561–563t, 565t 1-methylcyclopropene (1-MCP), 557–558, 566 potassium permanganate (KMnO4), 559, 564, 568 senescence, 554–556, 566 storage management, 554 texture, 556 Ethylene carbonate (EC), 437–439, 480–481 Ethylene glycol diglycidyl ether (EDGE), 450 Eumelanin, 310–311 Eumelanin-pheomelanin ratio (EPR), 313–314 European Food Safety Authority (EFSA), 609 European Standard BS EN 13726-1:2002 method, 82–83 Expanded poly(tetrafluorethylene) (ePTFE), 249 Extrusion technology, 37f, 39–40, 602

F Fibrinogen (FIB), 274, 279–281, 281f Fibroblast growth factor (FGF), 183 Fickian release model, 230 Fish gelatin (FG) film, 539, 547, 582

623

Fitzpatrick’s scale, 312–313, 313t Fluid handling capacity (FHC), 82–83 Foil sheet masks, 320 Food and Agriculture Organization, 591, 608–609 Food and Drug Administration (FDA), 314–315 5-Formyl-2-furansulfonic acid sodium salt (FFSA-SC) BSA and FIB adsorption reduction, 280–281, 281f chemical modification, of chitosan, 279, 280f platelet adhesion, reduction in, 282 Forward osmosis (FO), 398 Fourier transform infrared (FTIR) spectroscopy, 72–73, 72t, 490–493, 492f Fracture analysis, SEM technique, 71 Free ion (FI), 491–492 Freeze-drying technique, 71 Freundlich isotherm model, 408 Frontal flux filtration, 121, 122f Fruit puree edible films, 577 Fruits and vegetables, edible films and coatings barrier properties, 577 biodegradable films, 575–576, 586–587 cinnamaldehyde nanoemulsion edible films, 581 drying technology, 580 egg wash, 575 enzymatic browning, 579 fresh fruits and vegetables storage life, 505–506 frozen multicomponent foods, 575 fruit leather, 577–578, 578f, 580 individual film-forming materials, 577 liquid solutions, 575 oxygen/gas barrier properties, 586 paper and petroleum-based polymers, 575–576 pathogenic microorganism growth inhibition, 581 pectic and cellulosic substances, 577 petroleum-based polymers, 586–587 plasticizing agents, 577, 579 pomace/peel waste, 578 production process, 578–579 relative humidity (RH), 580–581 research and product development anthocyanins, 583 antiplasticizing mechanism, 581 brittle-to-ductile transition, 581 cast-tape drying process, 580–581 composite films, 582–585 differential scanning calorimetric analysis, 584 fish gelatin (FG), 582 hydrocolloid, 580 maltodextrin, 581 mechanical properties, 581 mechanical resistance, 582 microwave-assisted drying, 580 papaya puree, 581, 584

624 Fruits and vegetables, edible films and coatings (Continued) peach puree films, 581 plant-based edible coatings, 583 plasticizers, 579–580 resource recycling and environmental protection, 582–583 soy protein isolate (SPI), 582 tensile strength, 583 SciFinder database, 576–577 shelf life and quality of foods, 576–577 wax coating, 575 Fuel cells, biopolymer membranes agar, 462–463 alginate (ALG), 456–458 alkaline fuel cells (AFCs), 424–425 biofuel cells (BFCs), 431–432 cellulose (see Cellulose (C)) chitosan (CS) biological products, 435–436 chemical modification, 436–437 functional groups, 436, 437f inorganic filler composites, 442–445 ion-solvating polymer composite membranes, 435–436 polyanions, 435–436 polyelectrolyte, 435–436 polymer blends, 439–442 polymer composites, 445–446 SCS and PCS derivatives, 436–437, 438f self-cross-linked and salt-complexed chitosans, 437–439 swelling behavior, 436 uses of, 435–436 direct carbon fuel cells (DCFCs), 428–429 direct methanol fuel cells (DMFCs), 431, 433–434 electrochemical devices, 424 fluoropolymer, disadvantages of, 433–434 gas diffusion layers (GDL), 424 gas hydrogen, supply and storage of, 433–434 gelatin, 463 Grotthuss/vehicle mechanism, 435 ion-conducting electrolyte membrane, 424 Kyoto Protocol, 423 low-temperature units, 433–434 membrane-electrode assembly (MEA), 424 modifying polysaccharide and protein biopolymers, 434 molten carbonate fuel cells (MCFCs), 427–428 Nafion ion-exchange membrane, 433–434 natural polymers, 434 pectin (PC), 461–462 phosphoric acid fuel cells (PAFCs), 425–427 polymer electrolyte fuel cells (PEFC), 424, 433–434

Index

proton exchange membrane fuel cells (PEMFCs), 429–431 proton transfer mechanisms, 435f proton transportation, 435 selective proton conductivity, 433–434 solid oxide fuel cells (SOFCs), 427 stacks, 424 starch, 458–461, 460f Full width at half maximum (FWHM)-Scherer method, 490 Fumigation technique, 558

G Gallic acid (GA), 540, 545 Galvanostatic polarization method, 493 γ-aminopropyltriethoxysilane (γ-APS)-grafted stainless steel, 276 γ-cyclodextrins, 370–371, 370f Garcinia mangostana extract, 258 Gas diffusion layers (GDL), 429–430 Gas permeation, 129 Gelatin, 12 animal-derived biopolymers, 598 cellulose nanocrystals (CNCs), 56–57 dentistry applications, 247 dissolving microneedle patches, 204–207t, 209 edible coating and films, 17, 521 energy applications, 21t fuel cells, biopolymer membranes, 463 Jamaica pepper essential oil, diffusion behavior of, 106 wound dressings, 176–177 Gelatinized pure starch (GS), 461 Gellan gum, 599 biomedical applications, 14 characteristics of, 5–7t, 9–10 Gene delivery, LbL films, 231–232 Generally recognized as safe (GRAS), 523, 609 Gentamycin (GM), 154 Glass transition temperature (Tg), 78 Glucose oxidase (GOx), 226, 546 Glutaraldehyde cross-linked chitosan, 341, 341f Glutaraldehyde-cross-linked materials, 51 Glutathione (GSH) peroxidase, 310–311 Gluten proteins, 521 Glycerol, 47–49, 457–459, 462–463 Gold (Au)-dimercaptosuccinic acid (DMSA) thiolized cardiovascular stainless steel stent, 275–276 Gold nanoparticles, 373 Grafting-from strategy, 451, 460f Grafting-to strategy, 451, 460f Graphene oxide (GO), 401, 458 Gravimetric method, 82–83, 82f Green organic solvent nanofiltration, 353 “Green” oxidant, 559 Grotthuss-type mechanism, 454–455

Index

Groundwater, 359 Guar gum, 372, 372f Guided bone regeneration (GBR), 156, 245 chitosan, 247 gelatin, 247 Guided tissue regeneration (GTR) membranes chitosan, 247 gelatin, 247 Guluronate block (G-block), 508 Gum arabic, 16–17, 540, 598

H Heavy metal removal, 333–335 alginate, 350–353 carrageenans, 347–349 cellulose, 342–347 chitosan, 335–341 Heparin (HEP), 275–277, 276t, 284 Heterogeneous membranes, 121 High methoxy pectin (HMP) film, 581 Higuchi model, 230 Hixson-Crowell equation, 230 Hollow fiber membranes, 122 Homogenous membranes, 121 Honey, 183 Hot melt extrusion, 291–293t, 294 Hot melt ram extrusion 3D printing, 294–295 Hyalomatrix, 175 Hyaluronic acid (HA) biomedical applications, 14–15 characteristics of, 5–7t, 10 dentistry applications, 248–249 dissolving microneedle patches, 203–208, 204–207t wound dressings, 175 Hydrogel masks, 319 Hydrolyzed polyacrylonitrile (h-PAN), 349 Hydrophilic plasticizers, 47 Hydrophobic coatings, 506, 508 Hydrophobic pesticides, 364–365 Hydrophobic plasticizers, 47 Hydroxyapatite composites, 250–251 Hydroxyethyl cellulose, 543 Hydroxypropyl methylcellulose (HPMC), 316–317, 592 Hypodermis, 167–168, 168f

625

Ionic conduction mechanism, 487–489 Ionic dopant/salt, 479–480 Ionic liquids (ILs), 58–59 Iron-based oxygen scavengers, 537–539 Irrigation management, 366

J Jamaica pepper essential oil, 106 Jonscher’s universal power law, 487

K Keratin, 178, 506–507 KMnO4. See Potassium permanganate (KMnO4) Knit cotton masks, 320 Korsmeyer-Peppas model, 229–230

L Laccase, 412, 541 Lactic acid, 399 Lactic acid bacteria (LAB), 591 Lactobacillus acidophilus, 542, 592–593 Langmuir-Blodgett (LB) technique, 225–226 Langmuir isotherm model, 408 Laserskin, 14–15 Layer-by-layer (LbL) films, 234 biomedical applications, 219–220, 220f antibacterial and antifungal surfaces, 223–225 antimicrobial surface approach, 223 drug delivery devices, 229–230 gene delivery, 231–232 liposomes, 233–234 micelles, 232–233 sensing applications, 225–227 tissue engineering applications, 227–229 feature of, 219–221 principles of, 220–222 Layered membranes, wound dressings, 183 Levodopa-loaded transdermal patches, 198–200t, 202–203 Lidocaine, 261 Liposomes, LbL films, 233–234 Lithium perchlorate (LiClO4), 459 Low-density polyethylene (LDPE), 567–568 Low methoxy pectin (LMP) film, 581

I

M

Imidazole (Im), 454–455 Immersion method, 85 Industrial applications, biopolymer membranes, 35 In situ polymerization, 55 In-stent restenosis, 273–274 Integrally skinned membranes. See Phase separation membranes Ion-exchange membranes, 398 Ion hopping mechanism, 488–489

Macrodilution, 89 Magnetic nanoparticles (MNPs), 154 Mannuronate and guluronate block (MG-block), 508 Mannuronate block (M-block), 508 Mass transfer and diffusion, 106–107 Mathematical modeling, for diffusive process, 107–110 MCFCs. See Molten carbonate fuel cells (MCFCs) MEA. See Membrane electrode assembly (MEA)

626 Meats and meat products edible coatings and films active components, 522–523 quality, effect on, 523–528 spoilage, 516–517, 526–527t Mechanical properties, biopolymer membranes/films, 68–69t, 69, 79–80 Medihoney, 183 Melanin, 310–311, 313–314 Melanogenesis, 310–311 Melt compounding process, 57 Melt intercalation approach, 55 Membrane-coated UiO-66 MOF, 398 Membrane electrode assembly (MEA), 429–430, 430f Membrane filtration technologies, 333 Membrane manufacturing, biopolymers animal sources, 401–403 bacterial fermentation products, 399–400 electrospinning process, 404–405 metal-organic framework (MOF), 403–404 molecularly imprinted polymers (MIPs), 405–407 natural polymers, 399 vegetable sources, 400–401 Membrane reactor, 353, 354f Membrane separation process adsorption equilibrium, 408 adsorption kinetics, 408–409 advantage, 119, 123 asymmetric membranes, 36, 121 biopolymers, 133–135 chemical species, separation of, 119 definition criteria for, 120 diffusional unit operations, 132–133 diffusion coefficient, 407 dimensional classification method, 119–120, 123–124 disadvantage, 123 driving forces, 125 dialysis, 128 electrodialysis, 128–129 gas permeation, 129 microfiltration (MF), 126 nanofiltration, 127–128 pervaporation, 129–130 reverse osmosis, 127 ultrafiltration, 126–127 effluent conditioning, 119 flux and permeability, 124–125 frontal flux, feed flux, 121 heterogeneous membranes, 121 hollow fiber modules, 122 homogenous membranes, 121 mechanical unit operations, 132–133 neutral/electric charged membranes, 121

Index

permeability coefficient, 407 phenomenological classification method, 120, 130–133, 131–132t plate-and-frame modules, 121 selectivity, 125–126 solid/liquid membranes, 121 solution-diffusion theory, 407 spiral wound modules, 122 symmetric membranes, 36, 121 tangential flux, feed flux, 121 water treatment, 119 Membrane separation technique (MST), 406 Mercury porosimetry method, 85 Metal-chitosan complexes, 335, 336f Metal oxides, 53 Methylcellulose (MC), 519, 592 1-Methylcyclopropene (1-MCP), 557–558, 566 Methyl parathion removal, from wastewater, 373 Metolachlor removal, 371 Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposites, 400 Micelles, LbL films, 232–233 Microbial alginate, 600 Microbial derived biopolymers dextran, 599 gellan, 599 microbial alginate, 600 water-soluble polymers, 599 xanthan, 600 Microbial fuel cells (MFC), 431–432 Microbial penetration, in biopolymer films, 83, 83f Microcrystalline cellulose (MC), 454–455 Microdilution, 89 Microencapsulation, 594–597, 602–604, 603–604t Microfibrillar cellulose (MFC), multilayer coatings of, 41 Microfibrils, 371 Microfiltration (MF), 126, 398 Microfluidic LbL patterning, 228–229 Microfluidization, 528 Microneedle patches, biopolymers, 196 advantages, 203 dissolving microneedles, 203 advantages, 203 carboxymethyl cellulose (CMC), 204–207t, 208–209 chitosan, 204–207t, 209 chondroitin sulfate, 204–207t, 210 dextran, 210 gelatin, 204–207t, 209 hyaluronic acid/sodium hyaluronate (HA/SH), 203–208, 204–207t pectin, 204–207t, 210 silk fibroin, 204–207t, 209–210

Index

sodium alginate, 204–207t, 210 drug-coated microneedles, 203 hollow microneedles, 203 solid microneedles, 203 Micropollutants (MPs), 397 Microscopy, 69–70 atomic force microscopy (AFM), 72 excitation sources, 70 optical microscopy (OM), 70–71, 70t scanning electron microscopy (SEM), 70t, 71 transmission electron microscopy (TEM), 70t, 71–72 Microwave irradiation, 373 Minimal erythematous dose (MED), 313–314 Mitoxantrone (MTX), 233–234 Mixed-matrix membranes (MMMs), 403 Modified synthetic polymers, 98 Molecularly imprinted nanofiber membranes (MINFMs), 407 Molten carbonate fuel cells (MCFCs), 427–428 Montmorillonite (MMT), 53–54, 181, 401 Mora de Castilla, 509 Mucoadhesiveness, ODFs, 297–299, 298f, 299t Mulching, spray coating, 42 Multilayer active films, 544f, 545 MXene/alginate composites, 351

N Nafion (N), 455 ion-exchange membrane, 433–434 natural polymers, 434 Nanocellulose, 56–57, 372 Nanoemulsion, 523 Nanoencapsulation, 604–605 Nanofiber mats, 196, 210, 211t Nanofiber membranes, 407 Nanofiltration (NF), 127–128, 398 Nanomaterials, as oxygen scavenging systems, 546–547 Natural antioxidants, 528 Natural polymers advantages of, 98 classes, 274 definition, 97 degradation time, 102 energy applications, 20, 21t films active films, 105–106 characteristics, 102 diffusion mechanism (see Diffusion process, natural polymer films) in drug release, 106 publications in journals/articles, 99, 100f raw materials, source of, 98, 99f Neupalon (Sekisui Jushi) sachets, 560–561

627

Nitinol (NiTi), 276–277, 284 Nitrogen adsorption/desorption method, 85 N-methyl-2-pyrrolidone (NMP), 399 N,O-carboxymethylchitosan (NOCC), 179–180 Nonclimacteric respiration pattern, 554–555 Noncovalent molecular imprinting, 405 Nuclear magnetic resonance (NMR) spectroscopy, 74

O Oil-in-water (o/w) emulsions, 316–318, 318f Olefin compounds, 557 On-site wastewater treatment facilities (OWTF), 412 Open-circuit voltage (OCV), 497 Optical microscopy (OM), 70–71, 70t Oral cancer treatment, 258 Orally disintegrating films (ODFs), biopolymers, 290–296, 291–293t advantages, 289 characterization of disintegration time, 296–297, 297t dissolution tests, 300–301 mucoadhesiveness, 297–299, 298f, 299t surface pH, 299, 300t production techniques casting, 290, 291–293t, 294 hot melt extrusion, 291–293t, 294 printing technique, 294–296, 295f tape casting, 291–293t, 294 Organic pesticides, 364 Overlapping large polaron tunneling (OLPT) model, 488–489 Oxygen scavenging agents ascorbic acid and other natural OS agents, 539–540 enzymatic scavengers, 540–541 iron and other metallic scavengers, 537–539 microorganisms, immobilization of, 543 photosensitive dyes, 542 unsaturated hydrocarbons, 542–543 Oxygen scavenging films, 537 nanomaterials as, 546–547 preparation technologies, 543–546 coating, 544–545 immobilization, 546 incorporation into packaging, 544f, 545 multilayer active films, 545

P Packaging film systems, ethylene control, 567–568 Paddington cup, 82–83 PAFCs. See Phosphoric acid fuel cells (PAFCs) Paper-coating applications, biopolymers, 41

628 Pectin (PC) biomedical applications, 15 characteristics of, 5–7t, 11 dissolving microneedle patches, 204–207t, 210 fuel cells, 461–462 physical/ionic cross-linking, 51–52 plant-derived biopolymers, 597 polysaccharide-based edible film, 520 transdermal films and patches, 198–200t, 201–202 PEM. See Polyelectrolyte multilayers (PEM) Periodontal disease, 256 Periodontology, 256–257 Peripheral nerve injuries (PNIs), 155 Peripheral nerve regeneration (PNR), 155 Permeability coefficient, 81 Pervaporation (PV), 129–130, 398 Pesticides advantages of, 364 characteristics of, 364–366 classification, 362 based on biological targets, 362–363 based on chemical structure, 363–364 based on function, 363 based on target organisms, 362, 363t based on toxicological behavior, 362, 362t environmental fates after application, 364–366 factors degradation, 365 irrigation management, 366 soil adsorption, 365 soil characteristics, 365 solubility, 365 volatilization, 365 removal, biopolymers in, 366–377, 374–377t specific risk, in water pollution, 360–361, 361f Petroleum-based biodegradable polymers, 389 PEUF. See Polymer-enhanced ultrafiltration (PEUF) Pharmaceutical adsorption, 403 Pharmaceutical and personal care products (PPCPs), 403–404 Pharmaceuticals and hormones, removal of, 413–414t alginate, 410–411 β-cyclodextrin (bCD), 411 bisphenol A (BPA), 412 carbamazepine (CBZP), 412 cellulose acetate (CA) membranes, 412 ciprofloxacin sorption efficiency, 409–410 diphenhydramine, 410 direct contact membrane distillation (DCMD), 411 emerging contaminants, 411 endocrine-disrupting compounds (EDCs), 409–410 forward osmosis (FO), 411 free-standing SNF membranes, 411

Index

hydraulic retention time (HRT), 412 hydrogels, 410 ibuprofen (IBUP), 412 ionic interactions, 409 mebeverine, 410 microfiltration membrane (MF), 409–410 micro-, nano-, and ultrafiltration, 414 micropollutants, 411 nanofibrillation, 409–410 organic matter-water partition coefficients, 414 physicochemical and biotic transformations, 409 PLA membranes, 410 semiinterpenetrating networks, 409–410 sericin, 411 solute-solute interactions, 414 steroidal hormones, Emerging, 409–410, 414 sulfamethazine (SFMZ), 412 tryptophan enantiomers, 409–410 Phase inversion approach, 400, 406 Phase separation membranes, 36 PHB/PdNP nanocomposites, 547 Pheomelanin, 310–311 Phosphoric acid fuel cells (PAFCs), 425–427 Photocatalytic oxygen scavenging films, 538 Photodynamic therapy, 309–310 Photoinitiators, 543 Photosensitive dyes, 542 Physical dye removal methods, 387 Physical/ionic cross-linking, 50f, 51–52 Physicochemical characterization, biopolymer membranes/films Fourier transform infrared spectroscopy (FTIR), 72–73, 72t microscopy, 69–70 atomic force microscopy (AFM), 72 excitation sources, 70 optical microscopy (OM), 70–71, 70t scanning electron microscopy (SEM), 70t, 71 transmission electron microscopy (TEM), 70t, 71–72 nuclear magnetic resonance (NMR) spectroscopy, 74 publications, number of, 67–68, 68–69t Raman spectroscopy, 73–74 thermal analysis definition, 77 differential scanning calorimetry (DSC), 77–78 dynamic mechanical analysis (DMA), 78 thermogravimetric analysis (TGA), 77 X-ray diffraction (XRD) analysis, 74–76 X-ray photoelectron spectroscopy (XPS), 76–77 Plant-derived biopolymers alginate, 598 Arabic gum, 598 cellulose and derivatives, 597

Index

pectin, 597 starch and derivatives, 597 Plasmid DNA (pDNA), 231–232 Plasticizers battery applications, 480–482, 481t biopolymer films and membranes adverse effects on, 47–48 antiplasticizing effects, 48 glycerol and polyethers, synergistic plasticizing effects on, 47–48 hydrophilic and hydrophobic plasticizers, 47 noncharged vs. charged polymer films, 49 polyols, 48–49 starch-based films and membranes, 49 definition, 47 internal and external plasticizers, 47 Platelet-derived growth factor (PDGF), 183 Platelet-rich plasma (PRP), 281–282 PNR. See Peripheral nerve regeneration (PNR) Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), 399 Poly(β-amino ester) (PBAE), 230–232 Poly(butyl methacrylate), 244 Poly (butylene adipate-co-terephthalate) (PBAT), 399 Poly(butylene succinate) (PBS), 399 Poly(ethyl methacrylate), 244 Poly(ethylene glycol) (PEG), 244 Poly(L-lactic acid) (PLLA), 399 Poly(lactic-co-glycolic acid) (PLGA), 249 Polyacrylonitrile (PAN) nanofibers, 405 Polycarboxylic acids, 51 Poly(propylene oxide)-1,4-diisocyanate-2-toluene (PPODICT), 459 Polydimethylsiloxane (PDMS) microneedles, 231–232 Polyelectrolyte complex (PEC), 179 Polyelectrolyte multilayers (PEM), 220–222, 231–232 Polyethers (PEs), 47–48 Polyethyleneimine (PEI), 231 Polyethylene oxide (PEO), 250 Polyisoprene rubber membranes, 401 Polylactic acid (PLA), 40, 538 Polymer blending technique, 337 Polymer electrolyte membrane (PEM), 449 Polymer-enhanced ultrafiltration (PEUF), 337, 340–341 Polymer melt processes, 37, 37f Polymer solution processes, 37, 37f dense membranes casting, 37–39 coating, 40–43 layer by layer (LbL) technique, 43–45, 44f porous membranes electrospinning, 46–47, 46f solvent casting and particulate leaching, 45

629

thermally induced phase separation, 45–46 Poly(lactide-co-glycolide) (PLGA) microneedle arrays, 231–232 Polyols, 48–49 Poly(acrylic acid)/poly(ethylene glycol) (PAA/PEG) multilayers, 232 Poly(methyl methacrylate) (PMMA) resins, 244 Polysaccharides, 152–153t, 274, 335, 507 advantages of, 354 agarose, 5–7t, 11 alginate (see Alginate (ALG)) bacterial cellulose (BC) (see Bacterial cellulose (BC) membranes) carrageenans (see Carrageenans) cellulose (see Cellulose (C)) characteristics, 4–11, 5–7t chitosan (see Chitosan (CS)) cross-linking agents, 50–51 dextran biomedical applications, 15 characteristics of, 5–7t, 11 dissolving microneedle patches, 210 microbial derived biopolymers, 599 films development of, 103, 103t publications and citations on, 99–102, 100–101f gellan gum, 599 biomedical applications, 14 characteristics of, 5–7t, 9–10 guar gum, 372, 372f hyaluronic acid (see Hyaluronic acid (HA)) ionic liquids (ILs), 58 pectin (see Pectin (PC)) in pesticide removal, 360 starch (see Starch) wound dressings (see Wound dressings, biopolymer membranes/films) Polyvinyl alcohol (PVA), 154–155, 543 Polyvinyl chloride film (PVC) packaging, 516 Porous carbon material (PCM), 403–404 Potassium permanganate (KMnO4), 559, 564, 568 Potential emerging pollutants (PEPs), 397 Potentiostatic polarization method, 493 Poultry edible coatings and films active components, 522–523 quality, effect on, 523–528 spoilage, 516–517, 526–527t Power density, 499 Pressure-enhanced osmosis (PEO), 398 Pressure-retarded osmosis (PRO), 398 Printing techniques, for ODF production, 294–296, 295f

630 Probiotics active compounds, 589–590 antioxidant compounds, 589–590 diet, 589 edible films and coating biopolymers, 596f metabolic activity, 607–608 microbial growth, control of, 605–606 microbial survival, 606–607 polymeric matrices, 593–600 probiotic incorporation, 600–605 food industry, regulations on, 608–610 food preservation techniques, 589–590 functional bowel disorders, 589 infectious diseases, 589 living microorganisms inclusion, in polymeric matrices, 589–590 microbial composition, 589 and prebiotics bioactive packaging, probiotics uses in, 592–593 chemical, biochemical and microbial activities, in polymeric matrices, 591–592 definition, 590 human health, beneficial effects on, 591 Prostaglandin E2 (PGE2), 310–311 Prosthetic dentistry, 258 Protein, 507 adhesive proteins, blood-contact devices, 274 bioactive/not bioactive proteins, 12 biomedical applications, 152–153t BSA and FIB adsorption, on raw and sulfated chitosan, 279–281, 281f characteristics, 5–7t, 11–13 collagen (see Collagen) cross-linking agents, 50 edible coating and films, 520–522 corn zein, 521 fibrous proteins, 520 gelatin, 521 globular proteins, 520 wheat gluten, 521 whey proteins, 521–522 films aldehydes, 51 casting, 38–39 development of, 104, 104–105t diffusive mechanism, for drug release, 106 publications and citations on, 101–102, 101–102f ionic liquids (ILs), 58 natural sources, from animals and plants, 274, 275f sericin, 402–403 characteristics of, 5–7t, 12–13 properties, 12–13 soy protein (see Soy protein) whey proteins, 521–522, 599

Index

wound dressings (see Wound dressings, biopolymer membranes/films) Protein denaturant deep eutectic solvent (PD-DES), 403 Protonation process, 480 Proton exchange membrane fuel cells (PEMFCs), 429–431 Pseudo-first order model, 408–409 Pseudo-second order model, 408–409 Pulp masks, 320 Pulsed field gradient-nuclear magnetic resonance method, 493

Q Qualified with the presumption of safety (QPS), 609 Quantum dots, 182 Quantum mechanical tunneling (QMT) model, 488–489

R Raman spectroscopy, 73–74 Reactive oxygen species (ROS), 309–310 Rechargeability, 499 Regenerative endodontic therapy, 254–255 Relative humidity (RH), 38–39, 564 Respiration rate, climacteric pattern of, 554–555 Reverse osmosis (RO), 127, 398 Reversible addition-fragmentation chain transfer (RAFT), 320 Rice and Roth method, 491 Root canal treatment. See Endodontic therapy

S Sargassum glaucescens, 353 Scanning electron microscopy (SEM), 70t, 71 Seafood edible coatings and films active components, 522–523 quality, effect on, 523–528 spoilage, 516–517, 526–527t Selenium, 310–311 Self-reacting OS type, 537 Sericin, 402–403 characteristics of, 5–7t, 12–13 properties, 12–13 Sheet masks, for skin rejuvenation advantages, 318–319 ampoule sheet masks, 320 biocellulose masks, 319 biopolymers, 320, 321–322t, 322–326 bubble sheet masks, 320 composition, 318–319 foil sheet masks, 320

Index

hydrogel masks, 319 knit cotton masks, 320 marketed products, facial sheet mask technology, 320, 323–326t properties, 318–319 publications per scientific areas, number of, 318, 319f pulp masks, 320 Shelf-life tests, 568 Shellac, multilayer coatings of, 41 Sieving mechanism, 405 Silk fibroin (SF), 402–403 biomedical applications, 15 characteristics of, 5–7t, 12 corneal wound-healing, 157 dissolving microneedle patches, 204–207t, 209–210 properties, 12 wound dressings, 177–178 Silk nanofibrils (SNFs), 403 Silver nanoparticles, 180–181, 250 Silver sulfadiazine-loaded BC/sodium alginate composite membranes, 18–19 Simulated exudate fluid (SEF), 82–83 Sirolimus, 275–276, 276t Skin lesions, 167–169 phototype, classification of, 312–313, 313t structure, 167–168, 168f, 312–313 ultraviolet (UV) radiation exposure aging, 312 DNA damage, 310 immune system, disturbances in, 312 natural protection, 310–311 nonmelanoma and melanoma skin cancers, 314 sheet masks (see Sheet masks, for skin rejuvenation) sunburn, 313–314 sunscreen protection (see Sunscreens) tanning, 313–314 wound dressings, for skin regeneration (see Wound dressings, biopolymer membranes/films) Small interfering RNA (siRNA), 231–232 Small polaron hopping (SPH) model, 488–489 Sodium alginate (SA), 18–19, 350, 456, 518–519 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), 352 dissolving microneedle patches, 204–207t, 210 hydrogels, 351, 352f molecular structure, 351f transdermal films and patches, 198–200t, 202 Sodium alginate/nanohydroxyapatite composite films, 19 Sodium carboxymethyl cellulose, 316–317 Sodium hyaluronate (SH), 203–208, 204–207t Soil

adsorption, 365 characteristics, 365 Solid oxide fuel cells (SOFCs), 427 Solid-phase process, 58 Solid polymer electrolytes (SPEs), 19–20, 21t, 23 Solution casting method, 482, 482f Solution-diffusion process, 121, 127–128, 134 Solvent casting, 37f, 166, 166f dense membranes, 38 layered silicates, composite film preparation, 55 porous membranes, 45 Solvent-evaporation technique, 38 Soy edible films, 528 Soy protein advantages, 13 characteristics of, 5–7t, 13 edible coatings and films, 17 properties, 13 3D food printing, 18 Soy protein isolate (SPI), 13 Specific capacity, 498 SPEs. See Solid polymer electrolytes (SPEs) SPF. See Sun protection factor (SPF) Spray coating method, 40–42 Spread coating method, 40–41 Stainless steel, coating stents, 274–276, 276t sulfated chitosan-based coating, 282–284, 283f Staphylococcus sp., 224–225, 224t Starch, 370–371 biodegradable polymers, 401 characteristics of, 5–7t, 10 edible coatings and films, 17, 520 energy applications, 21t fuel cells, 458–461, 460f nanocellulose composite characteristics, 56 plant-derived biopolymers, 597 plasticizer, 49 thermoplastic starch (TPS) films, 40 transdermal films and patches, 198–200t, 202 Stents, coatings, 274–277, 276t Streptococcus mutans, 258 Sulfated chitosan chemical modification, 278–279, 279–280f coating, on metal surfaces challenges, 284–285 NiTi alloys, chitosan-HEP nanoparticle coating on, 284 stainless steel, 282–284, 283f hemocompatible properties platelet adhesion, 281–282 protein adsorption, 279–281, 281f monomers, chemical structure of, 277f, 278

631

632 Sulfonic acid-modified titanium oxide nanoparticles (S-TiO2), 462 Sulfosuccinic acid (SSA), 450, 451f Sunburn, 313–314 Sun protection factor (SPF), 314–316 Sunscreens, 322–326 application pressure, 315–316 biopolymers chitosan (see Chitosan (CS)) hydrolyzed collagen, 316 hydroxypropyl methylcellulose, 316–317 o/w emulsions, 317–318, 318f sodium carboxymethyl cellulose, 316–317 thickening agent, in sunscreen gel, 317–318, 318f UV filter delivery systems, 317–318, 317t, 318f xanthan gum, 316 classification of, 314–315, 315t dosage forms, 316 inorganic sunscreens, 314 organic sunscreens, 314 spreading time on mean thickness (Smean), 315–316 sun protection factor (SPF), 314–316 Superoxide dismutase (SOD), 310–311 Surface functionalization, LbL technique, 227–228 Surface pH, of ODFs, 299, 300t Swelling degree, biopolymers, 78–79 Symmetric membranes, 36, 121 Synthetic dyes, 384, 385t

T Tangential flux filtration, 121, 122f Tanning, 313–314 Tape casting, 291–293t, 294 TEM. See Transmission electron microscopy (TEM) Tensile test, 79–80, 80f Tetracycline (TC), 400 Textural analysis, biopolymer membranes/films, 85 Texture analyzer equipment, mucoadhesive testing, 298–299, 298f, 299t Thermal analysis biopolymer membranes/films differential scanning calorimetry (DSC), 77–78 dynamic mechanical analysis (DMA), 78 thermogravimetry (TGA), 77 definition, 77 Thermally induced phase separation, 37f, 45–46 Thermogravimetric analysis (TGA), 77 Thermoplastic starch (TPS), 40, 49, 59 Thin composite membranes, 36 Thin-film nanofibrous composite (TFNC), 405 3D food printing, soy protein, 18 Three-dimensional (3-D) scaffolds, 227–228 3D printing, 252–253

Index

Thyme essential oil (TEO), 584 Tissue engineering (TE) alginate membranes, 14 dentin/pulp tissue engineering, 254–255 electrospun membranes, 46 hyaluronic acid (HA), 10, 14–15 layer-by-layer (LbL) technique multilayered cell scaffolds, 228–229 surface functionalization, 227–228 membrane characteristics, 143–144 periodontal tissue engineering, hybrid films, 252 regenerative endodontic procedures, 254 silk fibroin, 12, 257 wound dressings, for skin regeneration (see Wound dressings, biopolymer membranes/films) Titanium dioxide (TiO2) nanoparticles, 181, 567–568 Titanium oxide nanotubes (TONT), 538 TNMs. See Transference number measurement (TNMs) Toxicological behavior, pesticide based on, 362, 362t Transdermal drug delivery, biopolymers, 154–155, 195–196, 197f, 211–212 advantages, 211–212 microneedles (see Microneedle patches, biopolymers) nanofiber mats, 196, 210, 211t transdermal films and patches adhesive-type transdermal patches, 196–197, 197f chitosan, 198–200t, 200–201 ethyl cellulose, 198–200t, 202 limitations, 198 matrix-type transdermal patches, 196–197, 197f pectin, 198–200t, 201–202 reservoir-type transdermal patches, 196–197, 197f sodium alginate, 198–200t, 202 starch, 198–200t, 202 xanthan gum, 198–200t, 202–203 Transference number measurement (TNMs), 493–495 Transglutaminase enzyme (TGs), 52 Transmembrane pressure, 125 Transmission electron microscopy (TEM), 70t, 71–72 Tricalcium phosphate (TCP), 251 Tubandt method, 493 Tubular film processing, 39 Two-dimensional (2D) films and membranes, 35 dense membranes, 36–37, 37f casting, 37–39 coating, 40–43 extrusion/film blowing, 39–40 layer by layer (LbL) technique, 43–45, 44f modification approaches blending, 57–58 composites, 52–57

Index

cross-linking, 49–52 ionic liquids, 58–59 plasticization, 47–49 porous membranes, 36, 37f electrospinning, 46–47, 46f solvent casting and particulate leaching, 45 thermally induced phase separation, 45–46

U UiO-66, 403–404 Ultrafiltration (UF), 126–127, 398, 405 Ultraviolet (UV) irradiation, 49–50, 546 Ultraviolet (UV) radiation, 309–310 exposure, effects of aging, 312 DNA damage, 310 immune system, disturbances in, 312 nonmelanoma and melanoma skin cancers, 314 sunburn, 313–314 tanning, 313–314 natural protection, of skin, 310–311 segments, 309–310 sheet masks (see Sheet masks, for skin rejuvenation) sunscreen protection (see Sunscreens) Unsaturated hydrocarbons, 542–543 Urocanic acid (UCA), 310–311

V Vacuum and modified atmosphere packaging, 516 Vapor permeation (VP), 398 Variable pressure method, 81 Variable volume method, 82 Vascular endothelial growth factor (VEGF), 183 Vegetable sources, membrane manufacturing alginate, 400 cellulose-based polymers, 400 polyisoprene, 401 starch and cyclodextrins, 401 Vitamin C, 310–311 Vitamin D, 310–311 Vitamin E, 310–311 Vogel-Tammann-Fulche (VTF) model, 485, 486f

W Wastewater treatment plant (WWTP), 397 Water-based casting, 38 Water hydration effect, 281 Water-in-oil (w/o) emulsions, 316 Water vapor transmission rate (WVTR), 169

633

Waxes, 507 Wet casting method. See Casting technique Wheat gluten, 521 Whey proteins, 521–522, 599 World Health Organization, 591 Wound dressings, biopolymer membranes/films, 151–153, 165–166 bioactive agents, association with, 182–183, 182t bioactive compounds, incorporation of, 166–167, 167f blends, 178–180 commercially available wound dressings, 170, 171–172t composites, 180–182 dense and porous structures, 166, 166f disadvantages, 166 fibrous structure, 166, 166f requirements, 169–170 role of, 170, 171t skin lesions, characteristics of, 167–169 sources, 166 types and properties of, 185–186t alginates, 173–174 bacterial cellulose, 174–175 chitosan, 170–173 collagen, 175–176 gelatin, 176–177 hyaluronan, 175 keratin, 178 silk fibroin, 177–178 xanthan gum, 174 Wound healing phases, 168

X Xanthan gum microbial derived biopolymers, 600 sunscreens, applications in, 316 transdermal films and patches, 198–200t, 202–203 wound dressings, 174 X permeability coefficient (XPC), 81 X-ray diffraction (XRD), 74–76, 489–490 X-ray photoelectron spectroscopy (XPS), 76–77

Y Young’s modulus, 48, 59, 79–80, 389, 450, 585

Z Zeolite, 559–560, 560f, 568 Zeolite-loaded chitosan films, 85 Zero2 oxygen scavenger, 542 Zinc oxide, 181