Advances in Sustainable Polymers: Processing and Applications [1st ed. 2019] 978-981-32-9803-3, 978-981-32-9804-0

Table of contents :
Front Matter ....Pages i-xxxi
Front Matter ....Pages 1-1
Biodegradable Polymer-Based Nanohybrids for Controlled Drug Delivery and Implant Applications (Aparna Shukla, Pralay Maiti)....Pages 3-19
Biobased Nanohydrogels for Controlled Drug Delivery (Sarat K. Swain, Kalyani Prusty)....Pages 21-41
Biocompatible Polymer Based Nanofibers for Tissue Engineering (Gajanan K. Arbade, T. Umasankar Patro)....Pages 43-66
Bioactive Glasses: Prospects in Bone Tissue Engineering (Neha Mulchandani, Vimal Katiyar)....Pages 67-83
Biomaterials for Biomedical Devices and Implants (Devarshi Kashyap, Vaibhav Jaiswal, Subramani Kanagaraj)....Pages 85-109
Carbohydrate Therapeutics Based on Polymer-Grafted Glyconanoparticles: Synthetic Methods and Applications (Konda Reddy Kunduru, Tushar Jana)....Pages 111-130
Production of Polyhydroxyalkanoates and Its Potential Applications (Chethana Mudenur, Kona Mondal, Urvashi Singh, Vimal Katiyar)....Pages 131-164
Front Matter ....Pages 165-165
Chitosan-Based Edible Coating: A Customise Practice for Food Protection (Tabli Ghosh, Vimal Katiyar)....Pages 167-182
Polysaccharide-Based Films for Food Packaging Applications (K. Dharmalingam, R. Anandalakshmi)....Pages 183-207
Biopolymer Dispersed Poly Lactic Acid Composites and Blends for Food Packaging Applications (J. Bindu, K. Sathish Kumar, Satyen Kumar Panda, Vimal Katiyar)....Pages 209-235
Bacterial Cellulose Based Hydrogel Film for Sustainable Food Packaging (Nabanita Saha, Oyunchumeg Zaandra, Smarak Bandyopadhyay, Petr Saha)....Pages 237-245
Front Matter ....Pages 247-247
Green Composites Based on Aliphatic and Aromatic Polyester: Opportunities and Application (Tabli Ghosh, Shasanka Sekhar Borkotoky, Vimal Katiyar)....Pages 249-275
Advances in Bio-based Polymer Membranes for CO2 Separation (Babul Prasad, Rajashree Borgohain, Bishnupada Mandal)....Pages 277-307
Microbial Fuel Cell: A Synergistic Flow Approach for Energy Power Generation and Wastewater Treatment (Chin-Tsan Wang, Thangavel Sangeetha)....Pages 309-334
Sustainable Polymer-Based Microfluidic Fuel Cells for Low-Power Applications (Moumita Sardar, Ravi Kumar Arun, Ebenezer Olubunmi Ige, Preeti Singh, Gagan Kumar, Nripen Chanda et al.)....Pages 335-361
Sustainable Polymeric Nanocomposites for Multifaceted Advanced Applications (Rituparna Duarah, Deepshikha Hazarika, Aditi Saikia, Rajarshi Bayan, Tuhin Ghosh, Niranjan Karak)....Pages 363-395
Bio-based Polymeric Conductive Materials for Advanced Applications (Gourhari Chakraborty, Vimal Katiyar)....Pages 397-410
Superhydrophobic Interfaces for High-Performance/Advanced Application (Nirban Jana, Dibyangana Parbat, Uttam Manna)....Pages 411-457
Uses of Ceramic Membrane-Based Technology for the Clarification of Mosambi, Pineapple and Orange Juice (Murchana Changmai, Sriharsha Emani, Ramgopal Uppaluri, Mihir Kumar Purkait)....Pages 459-483

Citation preview

Materials Horizons: From Nature to Nanomaterials

Vimal Katiyar Raghvendra Gupta Tabli Ghosh Editors

Advances in Sustainable Polymers Processing and Applications

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research.

More information about this series at http://www.springer.com/series/16122

Vimal Katiyar Raghvendra Gupta Tabli Ghosh •

•

Editors

Advances in Sustainable Polymers Processing and Applications

123

Editors Vimal Katiyar Department of Chemical Engineering Indian Institute of Technology Guwahati Guwahati, Assam, India

Raghvendra Gupta Department of Chemical Engineering Indian Institute of Technology Guwahati Guwahati, Assam, India

Tabli Ghosh Department of Chemical Engineering Indian Institute of Technology Guwahati Guwahati, Assam, India

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-32-9803-3 ISBN 978-981-32-9804-0 (eBook) https://doi.org/10.1007/978-981-32-9804-0 © Springer Nature Singapore Pte Ltd. 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

This book is dedicated to Prof. Yoshiharu Kimura who is a constant source of inspiration

Preface

This book addresses the systematic overview of sustainable polymers targeting the recent advances in state-of-the-art applications. The book is focused on discussing the use of biopolymers over fossil-derived polymers in various facets of life. In line with various recent advances for the use of sustainable polymers, the book discusses various processing techniques and subsequent applications of sustainable polymers and associated nano-structured materials including biocomposites to be used in biomedical, food storage, energy sectors, and high-performance applications. More specifically, this collection consists of three parts, which are targeted to discuss biomedical, food packaging, and other recent advances including fuel cells, membrane, conductive materials, multifaceted application relating to the use of sustainable polymers in past, present, and future prospects. The book begins by discussing different aspects of biomedical and tissue engineering applications including the use of biomaterials, carbohydrate therapeutic-based nanohybrids, nanohydrogels, biocomposites, polymer-grafted nanobiomaterials for biomedical devices and implants, nanofibers, and others. In this book, Chap. 1 confers the impact of various biodegradable and biocompatible polymers as control drug delivery vehicles in tissue engineering. The application of biodegradable polymer nanohybrids in drug delivery is the major concern in this part. The applications of polymer nanohybrids are discussed in detail especially in biomedical fields and implant materials using various composites. The addition of fillers to the biodegradable polymeric matrices are discussed, and a comparison is made with pure polymer. The fabrication and characterization of various biobased nanohydrogels are discussed in Chap. 2 with the suitability of their applications as potential drug transporter. Moreover, the toxicity and stability of biobased nanohydrogels are discussed after the encapsulation of various drugs. The present part will also explore the possibilities of safe and controlled release of various therapeutic drugs with the help of different biobased nanohydrogels. In Chap. 3, a comprehensive review of the current trends and challenges for nanofiberbased scaffolds with and without drugs for tissue engineering applications has been made. Drug delivery of drug-incorporated scaffolds is also summarized vii

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with diverse traits for the fabrication and property evaluation of various polymers and their scaffolds such as poly(e-caprolactone)-chloramphenicol nanofiber scaffolds. Additionally, cell adhesion and growth of different biological tissues are also deliberated with forthcoming visions of tissue engineering. In Chap. 4, the prospects of bioactive glasses in bone tissue engineering have been explored with their various methods of synthesis and use. The chapter summarizes the past, present, and future scenarios of bioactive glasses along with commercially available ones with their specific composition. The chapter delivers the prospects of hybrid materials, bioactive glasses as a potential candidate in combination with ceramic and polymers for the focused area. Chapter 5 introduces the necessity of biomaterials for developing biomedical devices and implants in regard to available designed materials. The prospects for various biomaterials needed for developing biomedical devices and implants are discussed by highlighting the different categories of implants with their function including polymer-based polycentric knee joint, dynamic foot, custom-made ankle-foot orthosis, suction, and suspension-incorporated socket for lower limb amputees, and direct socket fabrication system. Polysaccharides can be chemically modified for being a potential candidate to develop nanoparticle-enabled conjugates, therapeutics, photosensitizers, and others. In Chap. 6, various aspects of polysaccharide nanoparticle systems, living polymerization techniques, and their potential use in drug delivery, imaging, and biomedical areas are discussed. Specifically, glyconanoparticles being surface decorated polymer chains consisting of carbohydrate perform a vital role in developing medicine to meet the clinically unmet needs. Further, in this part, recent examples of polysaccharide-based nanoparticulate systems from the literature, related work, and their potential uses in the biomedical area are detailed. In Chap. 7, the traits of polyhydroxyalkanoates (PHAs) as the emerging, biocompatible, non-toxic, sustainable biopolymers in the biomedical field have been discussed. The available fermentation technologies for developing PHAs from available sources have been explored in the subsequent chapter. Part II of this book discusses various processing techniques of biopolymers for the targeted development of food packaging materials. The part begins with addressing the use of chitosan as edible coating materials on various food products. Further, it details the polysaccharide-based materials including their composites and blends for food packaging application. This is followed by a detailed discussion of the poly(lactic acid)-based biocomposites for food packaging depicting various properties and shelf life studies. At the end of this part, the use of an outgrowth material form such as hydrogel from bacterial cellulose with an enormous capacity of absorbing water is explored for food packaging application. Chapter 8 covers the use of chitosan as an edible coating material for preserving food products. The chapter presents the use of chitosan as a tailor-made approach for an improved shelf life of food products. A detailed discussion on the fabrication of chitosan-based composites and blends has been made, which can offer tuned properties against individual use as edible coating materials. Moreover, chitosan as an edible coating

Preface

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can bring bioactive compounds to food products preserving food properties with better shelf life. Chapter 9 discusses the application of various polysaccharide-based materials such as starch, cellulose, and chitosan for food packaging application. Biopolymers possess some inherent properties and can be modified by following proper techniques, which are discussed elaborately in the present chapter. Chapter 10 covers various modification techniques of PLA for preserving food products. The chapter details the use of PLA-based blends and composites with clay, cellulose nanocrystals, thermoplastic starch, and chitosan as food packaging materials. Additionally, the developed biodegradable polymer composites and blends offer an improved shelf life of various perishable food products, which in turn reduces the use of fossil-based packaging materials. Chapter 11 mainly deals with the preparation of hydrogel using bacterial cellulose-based polysaccharide–polyol complex biomaterial modifying the existing polyvinylpyrrolidone–sodiumcarboxymethylcellulose hydrogel film. The details about ‘bacterial cellulose-based hydrogel film and its efficiency’ for extending the shelf life of soft and delicate fruits and vegetable are outlined. Moreover, a new generation bioplastic hydrogel film for sustainable food packaging for agro-bio products is elaborated in detail. Part III of this book includes Chaps. 12–19, which discuss the recent advances in sustainable polymers including green composites based on aliphatic and aromatic polyesters for a wide application, advances in biobased polymer membranes for CO2 separation, microbial fuel cell for wastewater treatment, sustainable microfluidic cell for low power generation, sustainable polymer composites for multifaceted advanced application, and biopolymer-based conductive materials for advanced application. In Chap. 18, various promising and durable superhydrophobic interfaces and its prospects in advance applications are discussed in detail. Further, a few elegant strategies involving mechanically durable, self-healing and three-dimensional wettability are introduced to fabricate highly durable superhydrophobic materials. In addition, Chap. 19 discusses the preparation and application of ceramic membranes for microfiltration application. The chapter elaborates on the fabrication of cost-effective ceramic membranes for fruit juice clarification including mosambi, pineapple, and orange juice providing less pectin content, higher clarity, good citric acid content, and others. Additionally, the chapters of the current book volume are contributed by various authors, and thus for further clarification, the reader of this book can contact the corresponding authors of a specific chapter. Guwahati, India

Vimal Katiyar Raghvendra Gupta Tabli Ghosh

About Fourth International Symposium on Advances in Sustainable Polymers (ASP-17): From 08–11 January 2018 Organized by IIT Guwahati

Centre of Excellence for Sustainable Polymers (CoE-SusPol), Department of Chemical Engineering, successfully organized the Fourth International Symposium on Advances in Sustainable Polymers (ASP-17) during 8–11 January 2018 with the aim of promoting various biodegradable plastic-based technologies in line with the global emphasis on environmental protection and sustainable growth. During this scientific gathering, many distinguished Professors and senior scientists, researchers, policymakers, from across the country, senior officials of various industries including the Reliance Industries Limited, Indian Oil Corporation Limited, Oil India Limited, Bharat Petroleum Corporation Limited, Numaligarh Refinery Limited, and Brahmaputra Chemicals and Petrochemicals Limited, etc. including delegates representing the USA, Canada, Japan, Taiwan, Czech Republic, Singapore, Nepal, Thailand, Germany, Australia, and other countries attended this symposium. Serving as one platform which brought together all the stakeholders including academia and industry, the four days of the symposium had four plenary sessions, three bilateral symposium sessions, and multiple technical sessions. The bilateral symposium sessions included: Indo-Japan Bilateral Symposium, Indo-Taiwan Bilateral Symposium, and Indo-Nepal Bilateral Symposium. On the sideline of the symposium, one-day technical session was also organized by IIT Guwahati and Kyoto Institute of Technology, Japan. Experts from more than fifteen countries delivered expert talk on different aspects of sustainable polymers and allied areas. Pre- and post-symposium workshops on Polymer Processing and 3D Printing and Molecular Modelling and Simulation of Sustainable Polymers for young scientists and Ph.D. scholars were also successfully conducted. The symposium began with the opening remarks of Prof. Ashok Misra, Former Director, IIT Bombay, who graced the occasion as the distinguished guest while Professor Emeritus Yoshiharu Kimura, Kyoto Institute of Technology, Japan, was the Chief Guest in the inaugural session. It concluded with a total of 111 paper presentations and 81 poster presentations from researchers covering a wide range of applications relating to advances made in the field of sustainable polymers. The symposium also witnessed the congregation xi

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About Fourth International Symposium on Advances in Sustainable Polymers…

of interactive dialogues and expert talks by eminent scientist from across the globe with Prof. Ramani Narayan, Michigan State University, USA, Prof. Kohei Oda, KIT, Japan, Prof. Amar K. Mohanty, University of Guelph, ON, Canada, Arup K. SenGupta, Lehigh University, Bethlehem, USA, Prof. S. Sivaram, Former Director, NCL, Pune, to name a few. The symposium provided ample opportunity for the researchers across the world in sharing and gathering knowledge on various aspects of sustainable polymers starting from overviewing the current research activities and global trends on sustainable polymers (biobased and biodegradable plastics) and biobased materials to promoting these activities in their countries. The highlight of the symposium being the signing of MoU between “IITG and Ming Chi University of Technology, Taiwan” and “IIT Guwahati and Tomas Bata University in Zlín, Czech Republic”. The session was chaired by Prof. Dr. Henry H. Chen, Counsellor and Director, Science and Technology Division (TECC) of Taiwan in India. Dr. Vimal Katiyar served as an organizing chair for ASP 17. The fifth edition of ASP series was organized in Japan in October 2019. IIT Guwahati is always thankful to the North Eastern Council (NEC), Government of India, for providing all the supports to accelerate this initiative. The conference was supported by the North Eastern Council, Government of India (NEC), Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India, Department of Chemicals and Petrochemicals (DCPC), Ministry of Chemicals and Fertilizers, Government of India, CSIR and the Oil India Limited, Reliance Industries Limited, Indian Oil Corporation Limited, etc., which were the major sponsors. Prof. Vimal Katiyar Chair, ASP 17, Coordinator, CoE-SusPol, IIT Guwahati [email protected]

Acknowledgements

This book has developed out of a series of talks delivered in the Fourth International Symposium on Advances in Sustainable Polymers (ASP-17) at the Indian Institute of Technology Guwahati, Assam, India. The editors are grateful to all the speakers from several countries for conveying their state of the work experience in the symposium focusing the sustainable development of polymers to reduce the environmental impact by non-degradable materials. This edited book is an outcome of ASP-17 symposium which offered a platform to the researchers who have significantly contributed in the area of sustainable polymers and allied areas. The editors would like to acknowledge all of them with gratitude for their contributions to this book by sharing their research outcome and overviews, which will certainly be beneficial to the targeted audience. The editors would like to express their heartfelt gratefulness towards Prof. Yoshiharu Kimura, Kyoto Institute of Technology, Japan, for his valuable contribution in the area of biobased polymers, biofunctional polymers, and high-performance biobased materials. The editors would like to dedicate this book to him for his esteemed research contribution in the area of sustainable polymers. Further, we are indebted to Swati Meherishi, TCA Avni, Ashok Kumar, and Radhakrishnan Madhavamani for their continuous coordination, help, and support in completing the book. We would like to express our sincere gratitude towards all the staff at Springer Nature, Scientific Publishing Services (P) Ltd., Taramani, Chennai, India, for their constant help and support in completing the book. Guwahati, Assam, India

Vimal Katiyar Raghvendra Gupta Tabli Ghosh

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Contents

Part I 1

Sustainable Polymers for Biomedical Applications

Biodegradable Polymer-Based Nanohybrids for Controlled Drug Delivery and Implant Applications . . . . . . . . . . . . . . . . . . . . . . . . . Aparna Shukla and Pralay Maiti

2

Biobased Nanohydrogels for Controlled Drug Delivery . . . . . . . . . Sarat K. Swain and Kalyani Prusty

3

Biocompatible Polymer Based Nanofibers for Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gajanan K. Arbade and T. Umasankar Patro

3 21

43

4

Bioactive Glasses: Prospects in Bone Tissue Engineering . . . . . . . . Neha Mulchandani and Vimal Katiyar

67

5

Biomaterials for Biomedical Devices and Implants . . . . . . . . . . . . . Devarshi Kashyap, Vaibhav Jaiswal and Subramani Kanagaraj

85

6

Carbohydrate Therapeutics Based on Polymer-Grafted Glyconanoparticles: Synthetic Methods and Applications . . . . . . . . 111 Konda Reddy Kunduru and Tushar Jana

7

Production of Polyhydroxyalkanoates and Its Potential Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Chethana Mudenur, Kona Mondal, Urvashi Singh and Vimal Katiyar

Part II

Sustainable Polymers for Food Packaging Applications

8

Chitosan-Based Edible Coating: A Customise Practice for Food Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Tabli Ghosh and Vimal Katiyar

9

Polysaccharide-Based Films for Food Packaging Applications . . . . 183 K. Dharmalingam and R. Anandalakshmi

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10 Biopolymer Dispersed Poly Lactic Acid Composites and Blends for Food Packaging Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 209 J. Bindu, K. Sathish Kumar, Satyen Kumar Panda and Vimal Katiyar 11 Bacterial Cellulose Based Hydrogel Film for Sustainable Food Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Nabanita Saha, Oyunchumeg Zaandra, Smarak Bandyopadhyay and Petr Saha Part III

Sustainable Polymers for Other Emerging Application

12 Green Composites Based on Aliphatic and Aromatic Polyester: Opportunities and Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Tabli Ghosh, Shasanka Sekhar Borkotoky and Vimal Katiyar 13 Advances in Bio-based Polymer Membranes for CO2 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Babul Prasad, Rajashree Borgohain and Bishnupada Mandal 14 Microbial Fuel Cell: A Synergistic Flow Approach for Energy Power Generation and Wastewater Treatment . . . . . . . . . . . . . . . . 309 Chin-Tsan Wang and Thangavel Sangeetha 15 Sustainable Polymer-Based Microfluidic Fuel Cells for Low-Power Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Moumita Sardar, Ravi Kumar Arun, Ebenezer Olubunmi Ige, Preeti Singh, Gagan Kumar, Nripen Chanda and Gautam Biswas 16 Sustainable Polymeric Nanocomposites for Multifaceted Advanced Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Rituparna Duarah, Deepshikha Hazarika, Aditi Saikia, Rajarshi Bayan, Tuhin Ghosh and Niranjan Karak 17 Bio-based Polymeric Conductive Materials for Advanced Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Gourhari Chakraborty and Vimal Katiyar 18 Superhydrophobic Interfaces for High-Performance/Advanced Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 Nirban Jana, Dibyangana Parbat and Uttam Manna 19 Uses of Ceramic Membrane-Based Technology for the Clarification of Mosambi, Pineapple and Orange Juice . . . . . . . . . 459 Murchana Changmai, Sriharsha Emani, Ramgopal Uppaluri and Mihir Kumar Purkait

Editors and Contributors

About the Editors Dr. Vimal Katiyar is currently working as a Professor in the Department of Chemical Engineering at Indian Institute Technology Guwahati, India. He received Ph.D. degree in Chemical Engineering from Indian Institute of Technology Bombay, India. His main area of research includes sustainable polymer development, its processing and their structure property relationship, rheological aspects, migration studies, toxicological effects, polymer degradation, polymer based nanomaterials, food packaging, clean and green energy technologies. Currently, he is a coordinator for three centers of excellence at IIT Guwahati including Centre of Excellence for Sustainable Polymers funded by Department of Chemicals and petrochemicals, Govt. of India, Centre of Excellence for Biofuels and Biocommodities funded by Department of Biotechnology, Govt. of India and NRL Centre of Excellence for Sustainable Material at IIT Guwahati. Prof. Katiyar is dedicated in developing the cost-effective, bio-based and biodegradable plastic products and its related technologies using various feedstock including bio-derived plastics and biopolymers. Currently, he is engaged in establishing India’s first heat stable biodegradable polymer production pilot plant. He is also a co-inventor of 22 granted/filled patents. He had published more than 100 peer reviewed research articles in highly reputed journals and more than 200 conference papers and 30 book chapters. Under his able guidance, 10 of his students have got their Ph.D. and placed across the reputed institutions in India and abroad. His research group has received multiple National and International innovation awards in the development of bio-based polymeric products, nano-biomaterials, and related technologies. Dr. Katiyar is currently working on more than fifteen projects in the area of sustainable biopolymers, agriculture, food processing and related technologies. He also had grant from Ministry of Food Processing industries, Govt. of India to work in the area of Food packaging, migration and its characteristics. He acted as a catalyst towards bringing the Joint Degree in Food Science & Technology program between IIT Guwahati and Gifu University.

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Raghvendra Gupta is an Assistant Professor in the Department of Chemical Engineering, IIT Guwahati. He has previously worked as a researcher in BITS Pilani (India), Institute of High Performance Computing, A*STAR (Singapore) and University of Sydney (Australia). His research interests are based around understanding transport processes in chemical and biomedical applications, and he is current research is on multiphase flows, microfluidics and interfacial phenomena. He has authored 18 research publications in reputed journals. Tabli Ghosh is a research scholar in the Department of Chemical Engineering, IIT Guwahati. Her work focuses on developing and evaluating the health impacts of edible medicinal nano-coatings for food products.

Contributors R. Anandalakshmi Advance Energy & Materials Systems Laboratory (AEMSL), Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Gajanan K. Arbade Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology, Pune, Maharashtra, India Ravi Kumar Arun Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India Smarak Bandyopadhyay Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic Rajarshi Bayan Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India J. Bindu ICAR-Central Institute of Fisheries Technology (CIFT), Cochin, India Gautam Biswas Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati, India Rajashree Borgohain Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India Shasanka Sekhar Borkotoky Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Gourhari Chakraborty Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Nripen Chanda Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India

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Murchana Changmai Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India K. Dharmalingam Advance Energy & Materials Systems Laboratory (AEMSL), Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Rituparna Duarah Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India Sriharsha Emani Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Tabli Ghosh Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Tuhin Ghosh Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India Deepshikha Hazarika Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India Ebenezer Olubunmi Ige Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India; Department of Mechanical and Mechatronic Engineering, Afe Babalola University, Ado-Ekiti, Nigeria Vaibhav Jaiswal Department of Mechanical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Nirban Jana Department of Chemistry, Indian Institute of Technology, Guwahati, India Tushar Jana School of Chemistry, University of Hyderabad, Hyderabad, India Subramani Kanagaraj Department of Mechanical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Niranjan Karak Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India Devarshi Kashyap Department of Mechanical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Vimal Katiyar Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India

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Editors and Contributors

Gagan Kumar Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India Konda Reddy Kunduru School of Chemistry, University of Hyderabad, Hyderabad, India Pralay Maiti School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi, India Bishnupada Mandal Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India Uttam Manna Department of Chemistry, Indian Institute of Technology, Guwahati, India Kona Mondal Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Chethana Mudenur Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Neha Mulchandani Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Satyen Kumar Panda ICAR-Central Institute of Fisheries Technology (CIFT), Cochin, India Dibyangana Parbat Department of Chemistry, Indian Institute of Technology, Guwahati, India T. Umasankar Patro Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology, Pune, Maharashtra, India Babul Prasad Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India Kalyani Prusty Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur, Odisha, India Mihir Kumar Purkait Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Nabanita Saha Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic Petr Saha Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic Aditi Saikia Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India

Editors and Contributors

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Thangavel Sangeetha Department of Energy and Refrigerating Air-Conditioning Engineering, Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, National Taipei University of Technology, Taipei, Taiwan Moumita Sardar Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India K. Sathish Kumar ICAR-Central Institute of Fisheries Technology (CIFT), Cochin, India Aparna Shukla School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi, India Preeti Singh Materials Processing and Microsystems Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India Urvashi Singh Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Sarat K. Swain Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur, Odisha, India Ramgopal Uppaluri Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam, India Chin-Tsan Wang Department of Mechanical and Engineering, National Ilan University, Yilan City, Taiwan

Electro-Mechanical

Oyunchumeg Zaandra Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, Czech Republic

Abbreviations

AAEM 5Acl AEM AF AFC AFM AFO AgNPs AIDS AIS AK Ama ASGP-Rs ATP ATR ATRP BC BCs BCNC BHT BK BMI BMSCs BPA BPEI BSA BSG BTCAD BUT b-CD

Alkaline anion-exchange membranes Dipentaerythritol pentaacrylate Anion-exchange membranes Abaca fibre Alkaline fuel cells Atomic force microscopy Ankle-foot orthosis Silver nanoparticles Acquired immunodeficiency syndrome Alcohol-insoluble solids Above knee Antimicrobial activity Asialoglycoprotein receptors Attapulgite Attenuated total reflectance Atom-transfer radical polymerization Bacillus cereus Bacterial cellulose Bacterial cellulose nanocrystals Butylated hydroxytoluene Below knee Bismaleimide Bone marrow stromal cells Bisphenol A Branched polyethylenimine Bovine serum albumin Brewer’s spent grain 1,2,3,4-Butanetetracarboxylic acid 1,4-Butanediol b-Cyclodextrin

xxiii

xxiv

CA CAP CD CDs CDW C-Dxt/pNIPAm CF CF4 CFCs CFD CFX CH CH4 CIP CJ CJo CJp CLC CMC CMCH CNC CNF CNS CNT CNWs CO2 COD Cp CPB CS-MNP CTGF CuO CUR CVD DA DABCO DAGP DBTDL DCC DCPD DDS DEA DEG DEX DGEBA

Abbreviations

Contact angle Chloramphenicol Cyclodextrin Carbon dot(s) Cell dry weight Crosslinked-Dextrin/ PNIPAm Cake filtration Tetrafluoromethane Chemical fuel cells Computerized flow diagram Ciprofloxacin Chitosan Methane Ciprofloxacin hydrochloride Centrifuged juice Centrifuged orange juice Centrifuged pineapple juice Chemical-looping combustion Carboxymethyl cellulose Carboxymethyl chitosan Cellulose nanocrystals Cellulose nanofiber Central nervous system Carbon nanotube Cellulose nanowhiskers Carbon dioxide Chemical Oxygen Demand Heat capacity Complete pore blocking Chitosan-coated magnetic nanoparticles Connective tissue growth factor Copper oxide Curcumin Chemical vapour deposition Diels–Alder Diaminobicyclooctane Dimer acid-glycerol modified polyol Dibutyltin dilaurate N,N′-dicyclohexylcarbodiimide Dicyclopentadiene Drug delivery systems Diethanolamine Diethyleneoxide glycol Dexamethasone Diglycidyl ether of bisphenol A

Abbreviations

DLA DMA DMAc DME DMF DMM DMSO DOP DOX DP-CFX DRS DSC DTMS EB EC ECH ECM EDC EDS EDX EFB EG EIS EMAP EMI EOF EPA ESAR ESR ETCJ ETCJo ETCJp ETS EVOH FAS FDM FD–POSS FESEM FFF FFV FI FJm FJo FJp FSC

xxv

D(-)lactic acid Dynamic Mechanical Analysis Dimethylacetamide Dimethyl ether Dimethylformamide Dimethoxymethane Dimethyl sulphoxide Dopamine Doxorubicin Dendronized polymer-ciprofloxacin Diffuse reflectance Differential scanning calorimetry Dodecyltrimethoxysilane Elongation at break Escherichia coli Epichlorohydrin Extracellular matrix 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide Energy-dispersive spectroscopy Energy-dispersive X-ray Empty fruit bunch fibres Ethylene glycol Electrochemical impedance spectroscopy Equilibrium modified atmosphere packaging Electromagnetic interference shielding Electro-osmotic flow Environmental protection agency Energy storing and return Equilibrium swelling ratio Enzyme-treated centrifuged juice Enzyme-treated centrifuged orange juice Enzyme-treated centrifuged pineapple juice Engelhard Corporation Titanosilicate Ethylene vinyl alcohol Fluorinated alkyl silane Fused deposition modelling Fluorinated-decyl polyhedral oligomeric silsesquioxane Field emission scanning electron microscopy Fused filament fabrication Fractional free volume Fouling index Fresh mosambi juice Fresh orange juice Fresh pineapple juice Fixed-site carrier

xxvi

FTIR FWO G GA GAMA GAOs GC GelMA GF GNS GO GO-COO-HP-b-CD GP GPr GPC GPTMS GRAS GTE HA HAc HABCD HBL 3HB 3HHx 3HO 3HV Hc HCA HCl HC-MFCs HDI HDPE HMF HOS HPEI HPLC HPMC HPS HPU HP-b-CD H-S HUVECs ICG

Abbreviations

Fourier-transform infrared spectroscopy Flynn–Wall–Ozawa Glycolide Gum Arabic 2-Gluconamidoethyl methacrylate Glycogen-accumulating organisms Gas chromatograph Gelatin methacrylate Glass fibre Graphene nanosheets Graphene oxide Hydroxypropyl-b-cyclodextrin conjugated with carboxylated graphene oxide Glycopolymer Glass powder Gel permeation chromatography Glycidoxypropyl-methyldiethoxysilane Generally recognized as safe Green tea extract Hydroxyapatite Hyaluronic acid Heptakis(6-amino-6-deoxy)-b-cyclodextrin Hydrodynamic boundary layer 3-Hydroxybutyrate 3-Hydroxyhexanoate 3-Hydroxyoctanoate 3-Hydroxyvalerate Heat of crystallization Hydroxyl carbon apatite Hydrochloric acid Honeycomb microbial fuel cells 1,6-Hexamethylene diisocyanate High-density polyethylene Hydroxymethylfurfural Hardleaf oatchestnut starch Hyperbranched polyethylenimine High-performance liquid chromatography/High-pressure liquid chromatography Hydroxypropyl methylcellulose Hydroxypropyl starch Hyperbranched polyurethane Hydroxypropyl-b-cyclodextrin conjugated Hestern-Shram Human umbilical vein endothelial cells International Commission On Glass

Abbreviations

ICP ICR IEM IPB IPDI IPN IR JF KAS KBr L LA LAMA LBL LCST LDPE LI LLA LLDPE LOC LPM MALDI MAP MBG MBS MC mcl-PHA MDEA MDI MDPE MEA MEMA MEMS MF MFCs MFI mgCNC MH MMMs MMT MODA MOFs MPEG MPT MS

xxvii

Inductively coupled plasma Instantaneous centre of rotation Ion-exchange membranes Intermediate pore blocking Isophorone diisocyanate Interpenetrating network Infrared Jute fibre Kissinger–Akahira–Sunose Potassium bromide Lactide Lactic acid 2-Lactobionamidoethylmethacrylate Layer-by-layer Lower critical solution temperature Low-density polyethylene Listeria Inuaba L(+) lactic acid Linear low-density polyethylene Lab-on-a-chip Litres per minute Matrix-assisted laser desorption/ionization Modified atmospheric packaging Mesoporous bioactive glass N, N-Methylenebisacrylamide Methyl cellulose Medium chain length polyhydroxyalkanoates Methyldiethanolamine Methylenediphenyl diisocyanate Medium-density polyethylene Membrane electrode assembly Mannosyloxyethyl methacrylate Micro-electro-mechanical systems Microfiltration Microbial fuel cells Melt Flow Index Magnetic CNC Metformin hydrochloride Mixed matrix membranes Montmorillonite Microbial oxidative degradation analyser Metal-organic frameworks Methyl poly(ethylene glycol) Metoprolol tartrate Mass spectrometer

xxviii

MSC MTX MWCNT NaMMT NBCA NDI NF NGC nHA NHS NIPAm NMP NMR NPs O2 OCV ODA OLLA OPC OTR P(3HB-co-3HHx) P(HB-co-HV) P3HB P(NIPAm-co-AA) PA PAA PAH PAm PAm/C PAm/D PAm/D@Ag PAMAM PAN PAOs PBAT PBG PBS PBSA PBST PBT PC PCL PCL-diol PCU pDA

Abbreviations

Mesenchymal stem cells Methotrexate Multi-walled carbon nanotubes Sodium montmorillonite N-Butyl cyanoacrylate Norbornene diisocyanate Nanofiltration Nerve guide conduit Nano-hydroxyapatite N-hydroxysuccinimide N-isopropyl acrylamide Nitroxide-mediated polymerization Nuclear magnetic resonance Nanoparticles Oxygen Open-circuit voltage Octadecylamine Lactic acid oligomer Oxygen permeability coefficients Oxygen transmission rate Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Poly(beta-hydroxybutyric-co-beta-hydroxyvaleric acid) Poly(3-hydroxybutyrate) Poly(N-isopropyl acrylamide-co-acrylic acid) Polyamide Poly(acrylic acid) Polyallylamine hydrochloride Polyacrylamide Polyacrylamide/chitosan Polyacrylamide/Dextran Polyacrylamide/Dextran@Ag Poly (amidoamine) Polyacrylonitrile Polyphosphate-accumulating organisms Poly(butylene adipate-co-terephthalate) Poly(butylene oxide) glycol Poly(butylene succinate) Polybutylene succinate-co-adipate Poly(butylene succinate-co-terephthalate) Polybutylene terephthalate Polycarbonate Poly(caprolactone)/Poly (Ɛ-caprolactone)/Polycaprolate Poly(e-caprolactone) diol Poly(carbonate-urea)urethane Polydopamin

Abbreviations

PDI PDLA PDLLA PDMAEMA PDMS PDO PE PEEK PEG PEG PEG-400 PEG-LA PEM PEMFC PEO PES PET PEVA PFA PG PGA PHA PHB PHBHHx PHBV PLA PLCG PLCL PLGA PLGA PLLA PMMA PNIPAAm PNIPAm PNS POM POSS POTS PP PPAd PPFEMA PPG PS

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Polydispersity index Poly(D-lactic acid) Poly(D, L-lactic acid) Poly(N,N-dimethylamino-2-ethyl methacrylate) Polydimethylsiloxane Poly(dioxanone) Polyethylene Poly(ether ether ketone) Poly(ethylene glycol) Poly(ethylene oxide)glycol Polyethylene glycol PEG-linked lactobionic acid Proton-exchange membrane Proton-exchange membrane fuel cells Polyethylene oxide Poly(ethylene succinate) Poly(ethylene terephthalate) Poly (ethylene-co-vinyl acetate) Perfluoroalkoxy Propylene glycol Polyglycolic acid/Polyglycolide Polyhydroxyalkanoates Polyhydroxybutyrate Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Poly(3-hydroxybutyrate-co-3-hydroxyvaleric acid)/ Poly(3-hydroxybutyric acid-co-3-hydroxyvalerate) Poly(lactic acid) Poly (L-lactide-co-e-caprolactone-co-glycolide) Poly (L-lactic acid-co-e-caprolactone) Poly(lactide-co-glycolide) Poly(lactic-co-glycolic acid) Poly(L-lactic acid) Poly(methyl methacrylate) Poly(N-isopropyl acrylamide) Poly N-isopropyl acrylamide Peripheral nervous system Polyoxymethylene Polyhedral oligomeric silsesquioxane 1H,1H,2H,2H-perfluorooctyl-triethoxysilane Polypropylene Polypropylene adipate Polymerized perfluoroalkyl ethyl methacrylate Poly(propylene oxide)glycol Polystyrene

xxx

PS-b-P (AcGalEMA-co-S) PSU PTB PTFE PTT PTX PU PVA PVAc PVAm PVC PVDC PVDF PVLA PVP PWf PWi Qt RAFT Re RED RGO RHF RMFCs RMS RMSE RO ROMP ROP SACH SBF scl-PHA SEM SET-LRP SF SFn SFEP SHG SHS SI-ATRP SILM SiNP

Abbreviations

Polystyrene-block-poly(2-(2′,3′,4′,6′-tetra-Oacetyl-b-d-galactosyloxy)ethyl methacrylate-co-styrene Polysulfone Patellar tendon bearing Polytetrafluoroethylene Polytrimethylene terephthalate Paclitaxel Polyurethane Polyvinyl alcohol Polyvinyl acetate Polyviny amine Polyvinyl chloride Polyvinylidene chloride Polyvinylidene fluoride or polyvinylidene difluoride Lactose-carrying polystyrene Polyvinylpyrrolidone Correspond to the pure water hydraulic permeability values for fresh membrane Correspond to the pure water hydraulic permeability values for cleaned membrane Quercetin Reversible addition-fragment chain transfer polymerization Reynolds number Reverse electrodialysis Reduced graphene oxide Rice husk flour Rumen Microbial fuel cells Root mean square (dimension less) Root mean square error (dimension less) Reverse osmosis Ring-opening metathesis polymerization Ring-opening polymerization Solid-ankle cushioned heel Simulated body fluid Short-chain-length polyhydroxyalkanoates Scanning electron microscopy Single-electron transfer living radical polymerization Shigella flexneri Silk fibroin Surfactant-free emulsion polymerization Second harmonic generation Superhydrophobic surface Surface-initiated ATRP Supported ionic liquid membrane Silica nanoparticles

Abbreviations

SMFCs SMP SNP SOFC SPB SWCNT Tc Tcc TE TEA TEM TEMPO TEOS TEP Tg TGA TiO2 XRD

xxxi

Sediment microbial fuel cells Shape memory polymer Sulphur nanoparticles Solid oxide fuel cell Standard pore blocking Single-walled carbon nanotube Crystallization temperature Cold crystallization temperature Tissue engineering Triethanolamine Transmission electron microscopy 2,2,6,6,-Tetramethylpiperidine-1-oxyl Tetraethyl orthosilicate Triethyl phosphate Glass transition temperature Thermogravimetric analysis Titanium dioxide X-ray Diffraction

Part I

Sustainable Polymers for Biomedical Applications

Chapter 1

Biodegradable Polymer-Based Nanohybrids for Controlled Drug Delivery and Implant Applications Aparna Shukla and Pralay Maiti

Abstract This chapter describes the importance and applications of different biodegradable and biocompatible polymers as a control drug delivery vehicles used in tissue engineering. Here, different types of biopolymers with their properties are described by taking various examples. The use of different types of fillers for the synthesis of polymer composites and their drug delivery has been discussed. The advantages of biodegradable polymer nanohybrids in drug delivery are the major concerns in this chapter. The addition of fillers in the biodegradable polymeric matrices is discussed, and a comparison is made with pure polymer. The applications of polymer nanohybrids in different fields are discussed in detail especially in biomedical fields. Further, drug delivery and implants materials are summarized for biomedical applications using various composites. Keywords Biodegradable polymer


Nanohybrids
Drug delivery
Implant

1 Introduction Synthetic polymers and their products are most commonly employed in every aspect of life. Fast progress in material science and technology has developed new plastic-based products with better mechanical design and durability. Numerous non-biodegradable polymers such as polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET), phenolic resin, melamine resin, polyester resin, and polycarbonate (PC) are the major ingredients used for the manufacturing of plastic products. The plastic product includes plastic bottles, food containers, food packaging, cosmetics containers, appliances, and microwavable packaging, which are tremendously used owing to their excellent performances [1]. However, applications of plastic products are A. Shukla
P. Maiti (&) School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi 221005, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_1

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limited to single use especially in medical and food industry. Since these plastics are non-biodegradable and they prevail in environment for years even after their expiry time, thereby leaves huge accumulation of plastic wastes in environment. Plastic goods production has significantly increased from 1.5 million tons in 1950 to 245 million tons in 2008 with 9% of annual growth rate [2]. According to the literature reports, PVC wastes produced in China, USA, and Japan are 1,686,000, 435,000, and 214,000 tons, respectively [3]. After the consumptions, plastics are furnished in environment by either of the processes dumping, landfills, animal ingestion or entrapment, and incineration. Consumption of plastics by aquatic organisms is a critical concern since ocean is the major sink for accumulating polymeric wastes [4], which lead to bio-magnification causing the death of fishes, birds, reptiles along with extinction of endangered species. Disposal of plastics through incineration leads to CO2 emission of 673–4605 g kg−1 which has the major influence on global warming through huge emission of greenhouse gases posing a threat to human environment [5]. Due to versatile properties, it is difficult to minimize the consumption of plastics, but their replacement with biodegradable polymers can be an efficient process.

1.1

Emergence of Biodegradable Polymers

Biodegradable polymers, i.e., biopolymers are most promising candidates to replace petroleum-based plastics. According to the definition by European Bioplastics Association, biopolymers are based on renewable resources and can be degraded. However, biodegradation basically stands for the degradation of materials by enzymatic activity. Biopolymers are classified into four different categories. • Produced by living organisms directly, e.g., cotton, silk, wool, lignin, starch, cellulose, natural rubber, and poly(hydroxyalkanoates) (PHAs). • Produced by polymerization of monomers obtained through nature or derived from natural materials, e.g., poly(lactic acid) (PLA), soy-based polyol, and their derivatives. • Contains a combination of monomers from renewable resources with petroleumderived monomers, e.g., soy-based polyurethane (PU). • Produced from blends of renewable resources and petroleum-based materials [2], e.g., blend of starch and poly(vinyl alcohol). Polyester-based biodegradable plastic has gained importance since the last few years due to the presence of ester bonds which can be easily hydrolyzed to their respective monomers under biological conditions. Thus, biodegradable plastics could satisfactorily replace conventional plastics with an advantage of degradation by microorganisms after their disposal. Their biological origin low immunogenicity foregrounds them as an ideal material for packaging and other consumable products. Biodegradability of polycaprolactone (PCL) and PLA polymers was analyzed in compost under a controlled condition and was measured using a microbial oxidative degradation analyzer (MODA) according

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to ISO 14855-2 size and shape, where the effect on biodegradability was also assessed [6]. Biodegradation is a bio-induced chemical phenomenon involving cleavage of chemical bonds by microorganisms or their products like enzymes and surfactants in a controlled manner. It is differentiated from the common degradation where the disintegration of polymers into respective fragments occurs roughly without the involvement of microorganisms, e.g., thermal degradation, photodegradation, weathering, and chemical degradation, etc. [7]. Bio-based polymers obtained from renewable energy resources bring down the emission of greenhouse gases, when they are burned after use, the carbon dioxide generated is converted to biomass by photosynthetic way maintaining the carbon neutrality [8].

1.2

Advantages and Challenges of Biodegradable Polymers

One of the bright features of biodegradable polymer is their biocompatible nature and biodegradability in the presence of bioactive agents like enzymes and microorganisms after their use [9]. They play important roles in environmental safety concern due to their progressive applications in fields like biomedical, packaging, and agricultural use. PLA is one of the most commercially used polymers due to its hydrolyzable ester linkage and its low production cost. Energy consumption for its production is 20–25% less than petroleum-based polymer which can be further minimized. PLA has gained much importance being a precursor of biodegradable plastics. Fermentation of PLA produces biologically active L(+) form or D(−) or DL lactic acid which is an advantage over chemical synthesis [10]. Though biodegradable polymers are economic in use, some of their properties such as brittle nature, low heat distortion temperature, low-melt viscosity, poor thermal and mechanical properties, restricted gas barrier properties, and slow degradation rate limit their use in the industrial sector. The above-mentioned drawbacks can be overcome by improving thermomechanical properties by copolymerization, making blends and adding fillers in the polymer matrix. Further, incorporation of nanoparticles into polymer has led to significant improvement in polymer properties. For example, only small fraction, i.e., 1–5 wt% layered silicate nanocomposites exhibit distinctly improved degradation behavior, thermal and mechanical properties, barrier, and flame retardance features in comparison with unfilled polymer matrix [11].

2 Biodegradable Polymers 2.1

Polysaccharides

Multifunctional polysaccharides like dextran, cyclodextrin (CD), chitosan (CH), alginate, and hyaluronic acid (HAc), etc. are commonly used in medical applications like tissue engineering and drug delivery. They are a highly hydrophilic

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material, biocompatible in nature and provide tuned biodegradability. They have well-defined structures, branching but certain limitations like broad and highmolecular-weight distribution and lack of mechanical strength restrict their functions. These disadvantages can be overcome by modification of polysaccharides with other synthetic polymers. CD. CDs are a group of cyclic oligosaccharides [12] comprised of a-1,4-linked glucopyranose units with a hydrophobic cavity and hydrophilic exterior end. CD can form inclusion complexes with a broad variety of substances, importantly drugs, which has drawn significant attention since their identification. They are classified as a, b, and y containing 6, 7, and 8 glucose units. The truncated cone structure possess inner space (4.7–8.3 Å diameter, 7.9 Å height) cavity in which less polar molecules can be totally or partially inserted [13]. New applications of CD are visualized including designing of CD for sustained release of drug when administered orally detains the release and offers site-specific delivery. CD has also been used as a carrier in gene therapy [14]. Supramolecular aggregates of CD as polyrotaxanes [15], polymer grafted CD, chemically crosslinked CD are designed to meet the biomedical demands and open new ways to empower drug delivery systems with modern performances [16]. CH. CH is a linear polysaccharide consisting of 1,4 linked N-acetylglucosamine units forming exoskeleton of crustaceans. Structurally, it is similar to the HAc showing similar ability in wound healing [17]. Insolubility of chitin in many common solvents limits its application in biomedical fields. Thus, to overcome this issue, chitin derivative, produced by chitin deacetylation, yields a polysaccharide with random repeating units of D-acetylglucosamine and N-acetylglucosamine. Degradation of CH is primarily caused by the lysozyme enzyme, and its degradation rate depends on the degree of acetylation and crystallinity [18]. Disruption of extensive hydrogen bonding in CH can be done by the inclusion of bulky side groups like isobutyl which accelerates the degradation rate [19]. CH is used extensively as a wound dressing material over last 20 years due to its water absorptivity, oxygen permeability, and hemostatic nature. Additionally, CH has also been explored as a drug delivery vehicle. Hydrophilicity of CH leads to the formation of crosslinked or blended with other polymers for controlling the drug release rates. Fabrication of CH into numerous nanoparticles for drug delivery application is also studied [20]. Easier conversion of CH into porous matrices makes it a promising material in tissue engineering scaffold. HAc. HAc is the linear anionic naturally occurring polysaccharide constituted by alternating units of D-glucuronic acid and N-acetyl-D-glucosamine making a member of glucosamine–glycan family [21]. It is water soluble and forms highly viscous solutions. Viscoelastic properties of synovial fluid and vitreous humor are caused by the presence of a huge amount of HAc. It possesses additional properties like biocompatibility, nontoxic, biodegradable, and non-immunogenic, thus used extensively in the fields like surgical, tissue engineering, and drug delivery [22]. Due to hydrophilic nature, HAc can absorb a huge quantity of water and expand up to 100 times of its volume forming hydrated network [23], and this property makes HAc-based materials as a suitable hydrogel for drug delivery.

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7

Polyesters

PCL. PCL is synthetic aliphatic polyester, a commonly available degradable polymer, used in drug delivery applications and is produced via ring-opening polymerization of Ɛ-caprolactone. Despite its synthetic origin, it is known to be biodegradable and biocompatible. Penicillium oxalicum degrades PCL in 4 days yielding water-soluble 6-hydroxyhexanoic acid suggesting chain scission by enzyme [24]. Due to its low Tg and good solubility, PCL is mostly used in tissue engineering as a scaffold material for drug delivery systems [25]. Its fast degradation rate limits its application in drug delivery which can be overcome through modification of PCL with other polymers either by synthetic route of block copolymers or by making blends with poly(lactide-co-glycolide) (PLGA) or PLA to improve degradation rate and reactivity [26]. Further, micelle formation by using any of the hydrophilic block segment like PEG, poly(acrylic acid) (PAA), poly(N-isopropylacrylamide) (PNIPAAm), or poly(N,N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) conjugated to the hydrophobic PCL segment is also explored [27]. PLA and its copolymers. PLA is aliphatic polyester produced from polycondensation of lactic acid, produced by animals, plants, and microorganisms through fermentation. The presence of methyl group imparts the hydrophobicity to the specified polymer [28]. PLA properties can be tuned by racemization of D- and L-isomers. Since PLA is a chiral molecule and it has four forms; poly(L-lactic acid) (PLLA), poly (D-lactic acid) (PDLA), poly(D,L-lactic acid) (PDLLA): a racemic mixture of PLLA and PDLA, and meso-poly(lactic acid). Only PLLA and PDLLA have been extensively studied and are being used in biomedical fields. PLLA is semi-crystalline, while PDLLA is amorphous which leads to a difference in degradation rates and mechanical properties. PLLA possess higher tensile strength, low elongation at break and higher modulus with Tg of *60 °C, and melting temperature around 175 °C [17]. PDLLA has slightly lower Tg (50–55 °C) and low mechanical strength, but have a higher degradation rate than PLLA [29]. Modifications of PLLA or blending or copolymerization with other polymers have been done to reduce the degradation time. Similarly, PDLLA is often conjugated with other degradable polymers like PLGA [30], PEG [31], and CH [32] for creating novel composites with improved properties. PLGA. It is a copolymer of lactide and glycolide formed by ring-opening polymerization which is obtained commercially in a variety of molecular weight depending on lactide (L) and glycolide (G) ratio. The degradation rate of PLGA increases with higher L:G ratio [17]. Higher weight ratios of hydrophilic PGA also increase the degradation rate of PLGA [33]. For example, the degradation time for PLGA with L:G of 50:50, 75:25, and 85:15 is found to be 1–2, 4–5, and 5–6 months in an aqueous environment [34]. It is the most studied polymer for biomedical applications and is commonly used in sutures, drug delivery systems, and tissue engineering due to its biocompatibility and controlled biodegradation behavior. Owing to controlled degradation behavior among polyesters, it is extensively utilized in drug delivery applications including chemotherapeutics, proteins [35], vaccines [36], antibiotics [37], analgesics [38], and anti-inflammatory drugs [39].

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Polyurethane

Polyurethanes, first synthesized in 1930, are used in the form of fiber and flexible foam due to its elastic and thermoplastic nature. PUs are composed of three components, a long-chain polyol, polyisocyanate, and chain extender and by varying the components one can design PU as per the requirements. The suitability of physical properties and their biocompatibility led to the assimilation in biomedical fields. Chemical properties of PU can easily be altered through modification with chemical components, and thus, a wide variety of PUs with different applications can be prepared. So far, different types of PUs are developed with widely varying properties such as thermoresponsive [40, 41], shape memory [42, 43], pH responsive [44], self-healing [45], and self-cleaning [46]. Segmented PU is composed of a hard and soft segment which is responsible for mechanical properties of the polymer. Biostability is the prime requirement for long-term implantable material, and therefore, the degree of crystallinity is important aspects in designing PUs. Further, hydrogen bonding in hard segment of PU chain contributes in forming stable urethane linkages which resist enzymatic hydrolysis [47]. PUs are extensively used as adhesives, coatings, construction materials, synthetic leather, wound dressing, cushion material, and other biological systems [48]. Efforts have been made for developing nanoscale drug delivery systems for cancer treatment using PUs [49]. Polymers have been used for the last few decades for drug encapsulation and their subsequent slow delivery [50]. Various drug delivery systems are so far used utilizing hydrogels, nanofibers, and nanoparticles as a drug carrier to improve therapeutic efficacy by reducing the dose, toxicity, and other side effects while enhancing the bioavailability of drugs [51].

3 Development of Controlled Drug Release Using Biodegradable Polymer Nanohybrids The main aim of drug delivery systems is to enhance the therapeutic outcome and to minimize the shortcomings of a pure drug such as toxicity administered which results in other severe side effects. This situation can be overcome by controlling the release of drug after intake [52]. An ideal drug delivery systems should carry the drug without any loss and then should only be accumulated on a diseased cell or tissue instead of normal cells, thus attaining better efficacy and minimizing other deleterious side effects [53]. The carriers for intracellular delivery inside the human body should essentially possess the following requirements. • Should be biocompatible • Should be able to improve the solubility of hydrophobic drugs • Should protect the drugs form other undesirable interactions and keep the drug stable during circulation time

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9

• Should be target-specific and improve patient compliance • Should be biodegrade into small fragments for renal elimination without accumulating in other body parts. Till now several nanosystems are developed including liposomes [54], micelles [55], dendrimers [56], and nanotubes for drug delivery. Nanoparticles that typically have one of the dimensions in nanoscale are able to overcome the barriers faced by the drugs, e.g., poor water solubility [57], burst release [58], high toxicity, and low stability [59]. The selection of polymer as a drug delivery agent requires knowledge of surface and bulk properties of polymers including their mechanical, chemical, and biological functions. Surface properties like hydrophilicity and surface energy usually control the biocompatible nature of polymer with biological tissues and blood [60]. The bulk properties that need to be taken care of generally include molecular weight, modulus, and its solubility [61]. Various polymers have been explored for drug delivery and can be classified as biodegradable and non-biodegradable polymers. Biodegradable polymers are further classified into two types; naturally occurring and synthetic polymers [62]. Polymers such as polysaccharides, CH, pullulan, dextran, HAc, and proteins are naturally occurring biodegradable polymers, and they possess excellent biocompatibility, good cell adhesion, and proliferation rate [63]. Their non-specified structure, uncontrolled degradability, and high aqueous solubility limit their biological applications. Natural synthetic polymers like polyesters and polypeptides, owing to their well-controlled structure with controlled degradation rate and drug release properties can be integrated with other polymers to improve the properties for therapeutic applications [53]. Controlled release of drugs from polymer-based agents is achieved through regulated biodegradation and diffusion of drug from polymeric matrix. Since last few decades, many of the diseases like cancer and diabetes are being treated using biodegradable polymer nanoparticles to overcome the issues related to stability, bioavailability, and targeted release of drugs. Polyesters are widely used as drug carriers due to their favorable biodegradation and biocompatible nature, and additionally, food and drug administration approved their clinical use. PLA nanoparticles embedded with novel anticancer drug quercetin (Qt) show sustained release revealing a vehicle for better cancer treatment. Slow diffusion and strong interaction between the drug and PLA cause this sustained release of Qt from PLA matrix (Fig. 1a). Cell culture studies demonstrate killing of 50% breast cancer cells within 2 days in drug-loaded system [64]. For minimizing inflammation and recording peripheral nerve signals, polyimide nerve cutoff electrode with controllable drug release has been developed. To control the release of dexamethasone (DEX) from the DEX-loaded PLLA and/or PLGA nanofiber deposition can be done onto functional nerve cutoff electrode through electrospinning technique. Further, drug release rates can be observed through high-performance liquid chromatography (HPLC). Considerably, the drug release rate is enhanced from 16 to 28% in PLLA-loaded nanofibers and from 68 to 87% in PLGA-loaded nanofibers due to increased diffusion rates of DEX after 24 h. This

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differential release pattern is due to lower molecular weight of PLGA and its fast degradation rate [65]. Various nanostructures of PCL using surface-modified layered silicates with improved and diverse mechanical, thermal, and surface properties have been explored as drug control drug delivery systems (Fig. 1b, c). Scaffolds of PCL and its nanohybrids with and without drug were prepared through electrospinning for maintaining fiber dimension to control the degradation rate. Biocompatibility of these nanohybrids was tested at gene level by subcellular localization of pro-apoptotic signaling cascade, HIPK2 in human epithelial cells, illustrating their proper harmony with biological system under study [66]. Oral drug delivery, being preferred route due to better patient compliance, is limited for delivery of most peptide/protein drugs as they are potentially not absorbable in the gastrointestinal tract because of poor enzymatic catalysis and low permeability across the biological membranes. For overcoming these barriers, polymer-based particulate drug delivery systems are suggested [67]. Nanoparticles prepared through the blending of biodegradable polymer, PCL and a polycationic non-biodegradable acrylic polymer have been used as an oral drug carrier for insulin delivery in diabetic rats. These polymer-insulin nanoparticles decrease the fasted glycemia in a dose-dependent manner and also increase serum insulin level with the improvement in glycemic response to oral glucose for an extended period of time [68]. Hydroxypropyl-b-cyclodextrin conjugated with carboxylated graphene oxide (GO-COO-HP-b-CD) is designed as a novel drug carrier for anticancerous drug (paclitaxel, PTX). PTX incorporated into GO-COO-HP-b-CD

Fig. 1 a In vitro release profiles of Qt from PLA nanoparticles with different formulations in Phosphate buffer solution at pH *7.4 [64]; b Sustained drug release profile of pure PCL and its prepared nanohybrid scaffolds [66]; c Schematic representation of drug diffusion from pure polymer and its nanohybrid [66]; d Schematic representation of drug-loaded GO-COO-HP-b-CD nanosphere formation [69]; e In vitro release profiles of PTX from GN/PTX nanospheres in Phosphate buffer solution at pH *7.4, 6.5, and 5.0 [69]

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nanospheres (Fig. 1d) is suitable with good blood biocompatibility for intravenous use. In vitro release profile (Fig. 1e) represents the initial burst release from the cavity of Hydroxypropyl-b-cyclodextrin conjugated (HP-b-CD) followed by slow release continuously over 150 h with 58.9% release at pH 7.4, while 70.2% release occurs at pH *5, suggesting more drug release in tumor tissues at lower pH. In vitro cytotoxicity assessment shows significant inhibition of HeLa cells growth suggesting the system as a novel nanocarrier for controlled and targeted delivery of hydrophobic drugs for cancer treatment [69]. Reports are available showing entrapment of drugs in CD matrix or modifying the surface of nanoparticles using CD with an aim to alleviate the delivery of drugs safely [70]. Correia et al. [71] have developed a nanocomposite consisting of undecylenic acid modified thermally with hydrocarobonized porous silicon nanoparticle (UnTHCPSi NPs) loaded with sorafenib, an anticancerous drug, and surface conjugated with 6-amino-6-deoxy-b-cyclodextrin showing the efficacy of polymeric surface fictionalization with drug-loaded nanoparticle in biomedical field. Heptakis(6-amino-6-deoxy)-b-cyclodextrin (HABCD) is a modified CD with enhanced hydrophilicity and physicochemical properties. The substitution of hydroxyl groups is favorable for drug release applications since they work as capping agent by covering the pores of the carrier and protecting the drug-loaded molecules from the external environment. Siriviriyanun et al. [72] explored the efficiency of surface functionalization of GO with CD and dendrimers for control release of therapeutic agents like doxorubicin and camptothecin. GOs are promising nanocarriers due to its two-dimensional structure and high drug loading capacity mainly due to covalent bonding, and electrostatic interactions. CD and poly(amidoamine) (PAMAM) dendrimers were coated on the GO surface. Earlier reports on GO conjugated with dendrimer loaded with anticancer drug, doxorubicin, exhibit control release, and efficient cellular uptake by Hela cells are also available. Series of nanohybrid composites of CH/dopamine (DOP) in TiO2 are prepared through sol–gel technique, and drug release is studied using electrochemical and UV absorbance method. The composites can be prepared by dissolving DOP in the CH aqueous solution. Further, the coating of these composites is done using different amounts of TiO2 (10–50 wt%). Incorporation of TiO2 in DOP/CH composite enhanced the drug entrapment and considerably sustained the release of drug (with 100% release in 16 h while DOP/CH composites without TiO2 coating released the whole drug just in 10 min at pH *7.4) (Fig. 2d). The coating of TiO2 increases drug passage time through its pores (whether in vitro or in vivo) and eliminates different stimuli such as temperature, pH, and ultrasonic irradiation to remove the coating on the surface of drug carrier system [73]. Addition of graphene nanoparticle to CH matrix increases the mechanical strength, and thermosensitivity of CH hydrogel and subsequently eliminates the burst release of drug. The feasibility of using thermosensitive CH-graphene nanohybrid hydrogel for control drug delivery of methotrexate (MTX) is also explored. CH-graphene nanohybrid hydrogels are biocompatible, and swelling behavior is improved by the inclusion of graphene in CH matrix. In vitro release of MTX decreased with the addition of graphene in CH hydrogels following slower and more controlled release behavior.

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Fig. 2 a Sustained drug release profile of PU and its indicated nanohybrids [75]; b Schematic model of drug release kinetics in pure and polymer nanohybrid [75]; c Sustained drug release profile of indicated PU and its indicated nanocomposites [76]; d cumulative DOP release (%) of B1–B4 at pH 7.4 by using UV–Vis method [73]

The anti-tumor effect of nanohybrid hydrogels on breast cancer cells proved that MTX-loaded nanohybrid hydrogels significantly inhibited the growth of MCF-7 breast cancer cells [74]. Sulfonation of GO followed its subsequent grafting with polyurethane chain enhances the property of polymer and modified graphene as a filler. Chemical tagging was confirmed through spectroscopic techniques and self-assembly in nanohybrids was observed as compared to pure polymer. Nanohybrids showed sustained drug release as compared to pure polymer where burst release was observed (Fig. 2a, b) (schematic model for drug release) and exhibited better biocompatible nature even with a lower percentage of filler suggesting these nanohybrids as a potential biomaterial for tissue engineering applications [75]. PU nanocomposites were prepared by dispersing 2-D nanoclay in polyol followed by prepolymer formation and its subsequent chain extension. Improvement in toughness and stiffness in the presence of nanoclay was observed. Controlled biodegradation in PU nanohybrids was observed in enzymatic media. Biocompatible nature of these nanohybrids was assessed through platelet adhesion, aggregation, and hemolytic assay. These biocompatible nanohybrids of PU exhibited sustained drug

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release by controlling crystallite size of polyurethane through alteration of chain length of extender or through the incorporation of disk like nanoclay, creating a tortuous path that results in slow diffusion (Fig. 2c). These PU nanohybrids are promising biomaterial for tissue engineering and drug delivery [76]. Drug release behavior from biodegradable polymers was investigated as the main tool for rate of drug release in two ways: (1) diffusion kinetics: slow, continuous, and sustained release for a prolonged period of time, (2) polymer erosion kinetics: fast or burst release of drugs due to erosion of polymeric surfaces. Polymer erosion can be either bulk or surface erosion, and bulk erosion is due to cleavage of chemical bonds after a certain time period. The initial drug release is due to fraction of drug present in the polymer surface, while the rest of the drug in polymer matrix is emancipated due to hydration of biodegradable polymer, i.e., a biphasic model of drug release.

4 Biodegradable Polymer Nanocomposites for Tissue Engineering A logical consequence has been the incorporation of organic or inorganic nanofiller into biodegradable polymers to produce nanohybrids based on hydroxyapatite, metal nanoparticles, or carbon nanostructures, in order to prepare new biomaterials with enhanced properties. Polymer nanocomposites are the result of the combination of polymers and inorganic/organic fillers at the nanometer scale [77]. Tissue engineering involves structure–function relationships in normal and pathological tissues and development of a biological substitute for restoring, maintaining, and improving tissue functions [78]. The use of engraft causes a series of problems mostly infections, poor biocompatibility, a higher grade of cytotoxicity, stress shielding effects, which are caused by differences in mechanical strength of implant and host organ. The materials used for tissue engineering must be biocompatible with favorable mechanical properties. For scaffold fabrication in the tissue engineering field, polymers are the primary used materials and many types of biodegradable polymeric materials are commonly utilized. PGA, PLA, and their copolymers, PLGA, are the family of linear aliphatic polyesters which are most frequently used in tissue engineering [79]. The lyophilized hydrogels of polyurethane brushes with CH backbone can be prepared through grafting technique, where surface modification of CH maintaining hydrophilic and hydrophobic balance is checked by swelling ratio and contact angle [82]. These brush copolymers exhibited sustained drug release following Fickian diffusion. The excellent cytocompatibility and good cell proliferation confirmed their biocompatible nature. Injectable hydrogel formation of these brush polymers was confirmed by injecting sol subcutaneously in rats [80]. Several biopolymers that are commonly exploited for vascular artificial vascular grafts include poly(tetrafluoroethylene), poly(ethyleneterepthalate), polyurethane, poly (lactide-co-glycolide), PCL, poly(ethylene glycol)-b-poly(L-lactide-co-caprolactone),

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and poly(depsipeptides), etc. Essential requirements for the biomaterial to be used as implants should be mechanically strong together with the preservation of cell attachment and proliferation [81]. Among the biomaterials mentioned above, PU is tremendously used due to their excellent mechanical properties that match well with the soft tissues. Polyether-based PUs are stable in vitro than polyester and PC-based PU, however, significantly degrade by enzymes, pH, and oxidative stress. Therefore, polyester- and PC-based PUs are widely used in drug delivery systems, scaffolds, and artificial vascular graft [82]. Poor in vivo biostability of PU is further reinforced by nanocomposite formation. Polyhedral oligomeric silsesquioxane (POSS) is covalently linked to poly(carbonate-urea) urethane (PCU) backbone forming POSS-PCU nanohybrid. The advantage of using POSS includes its excellent anti-thrombogenic property and mechanical strength, required the property for vascular graft. This developed POSS-PU nanohybrid is stable under oxidative environment and do not undergo hydrolytic cleavage [83]. Several non-resorbable products, as well as biomaterials, have been employed with an aim of reconstruction of soft tissue in the field of dentistry with varying success. The advantages of using PLA and its copolymers as an implant include low rigidity, processability, controlled biodegradation and drug encapsulation with subsequent drug delivery. In dentistry, for successful dental implant placement and its long-term functional success, maintaining the original dimensions of the alveolar ridge is crucial. Biodegradable space fillers can be employed for minimizing alveolar ridge resorption following a tooth extraction, membranes, and graft materials. Biocompatible PLA space fillers fabricated by fusing porous PLA particles loaded with drugs promote regeneration and maintain the original socket dimensions [84, 85].

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A. Shukla and P. Maiti nanofibers and hydrogel deposition. Sens Actuators B Chem 215:133–141. https://doi.org/10. 1016/j.snb.2015.03.036 Singh NK, Singh SK, Dash D, Purkayastha BPD, Roy JK, Maiti P (2012) Nanostructure controlled anti-cancer drug delivery using poly (e-caprolactone) based nanohybrids. J Mater Chem 22:17853–17863. https://doi.org/10.1039/C2JM32340K Carsten Christophersen P, Fano M, Saaby L, Yang M, Mørck Nielsen H, Mu H (2015) Characterization of particulate drug delivery systems for oral delivery of peptide and protein drugs. Curr Pharm Des 21:2611–2628. https://doi.org/10.2174/13816128216661504 16100943 Damgé C, Maincent P, Ubrich N (2007) Oral delivery of insulin associated to polymeric nanoparticles in diabetic rats. J Control Release 117:163–170. https://doi.org/10.1016/j. jconrel.2006.10.023 Tan J, Meng N, Fan Y, Su Y, Zhang M, Xiao Y, Zhou N (2016) Hydroxypropyl-bcyclodextrin–graphene oxide conjugates: carriers for anti-cancer drugs. Mater Sci Eng C 61:681–687. https://doi.org/10.1016/j.msec.2015.12.098 Claveria-Gimeno R, Vega S, Grazu V, de la Fuente JM, Lanas A, Velazquez-Campoy A, Abian O (2015) Rescuing compound bioactivity in a secondary cell-based screening by using c-cyclodextrin as a molecular carrier. Int J Nanomedicine 10:2249–2259. https://doi.org/10. 2147/IJN.S79480 Correia A, Shahbazi MA, Mäkilä E, Almeida S, Salonen J, Hirvonen J, Santos HA (2015) Cyclodextrin-modified porous silicon nanoparticles for efficient sustained drug delivery and proliferation inhibition of breast cancer cells. ACS Appl Mater Interfaces 7:23197–23204. https://doi.org/10.1021/acsami.5b07033 Siriviriyanun A, Popova M, Imae T, Kiew LV, Looi CY, Wong WF, Chung LY (2015) Preparation of graphene oxide/dendrimer hybrid carriers for delivery of doxorubicin. Chem Eng J 281:771–781. https://doi.org/10.1016/j.cej.2015.07.024 Safari M, Ghiaci M, Jafari-Asl M, Ensafi AA (2015) Nanohybrid organic–inorganic chitosan/ dopamine/TiO2 composites with controlled drug-delivery properties. Appl Surf Sci 342:26– 33. https://doi.org/10.1016/j.apsusc.2015.03.028 Leyla S, Li Y, Marcus B, Kim C, Ramazan A (2017) Structural and biological properties of thermosensitive chitosan–graphene hybrid hydrogels for sustained drug delivery applications. J Biomed Mater Res A 105:2381–2390. https://doi.org/10.1002/jbm.a.36096 Patel DK, Senapati S, Mourya P, Singh MM, Aswal VK, Ray B, Maiti P (2017) Functionalized graphene tagged polyurethanes for corrosion inhibitor and sustained drug delivery. ACS Biomater Sci Eng 3:3351–3363. https://doi.org/10.1021/acsbiomaterials. 7b00342 Mishra A, Singh SK, Dash D, Aswal VK, Maiti B, Misra M, Maiti P (2014) Self-assembled aliphatic chain extended polyurethane nanobiohybrids: emerging hemocompatible biomaterials for sustained drug delivery. Acta Biomater 10:2133–2146. https://doi.org/10.1016/j. actbio.2013.12.035 Gorrasi G, Vittoria V, Murariu M, Ferreira ADS, Alexandre M, Dubois P (2008) Effect of filler content and size on transport properties of water vapor in PLA/calcium sulfate composites. Biomacromolecules 9:984–990. https://doi.org/10.1021/bm700568n Shalak R, FOX C (1988) Tissue engineering proceedings: workshop held at Granlibakken. Lake Tahoe, California, February, pp 26–29 Bolland BJRF, Kanczler JM, Ginty PJ, Howdle SM, Shakesheff KM, Dunlop DG, Oreffo ROC (2008) The application of human bone marrow stromal cells and poly(DL-lactic acid) as a biological bone graft extender in impaction bone grafting. Biomaterials 29:3221– 3227. https://doi.org/10.1016/j.biomaterials.2008.04.017 Mahanta AK, Senapati S, Maiti P (2017) A polyurethane–chitosan brush as an injectable hydrogel for controlled drug delivery and tissue engineering. Polym Chem 8:6233–6249. https://doi.org/10.1039/C7PY01218G

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81. Dias RCM, Góes AM, Serakides R, Ayres E, Oréfice RL (2010) Porous biodegradable polyurethane nanocomposites: preparation, characterization, and biocompatibility tests. Mater Res 13:211–218. https://doi.org/10.1590/S1516-14392010000200015 82. Yang J, Lv J, Gao B, Zhang L, Yang D, Shi C, Feng Y (2014) Modification of polycarbonateurethane surface with poly (ethylene glycol) monoacrylate and phosphorylcholine glyceraldehyde for anti-platelet adhesion. Front Chem Sci Eng 8:188–196. https://doi. org/10.1007/s11705-014-1414-1 83. Kannan RY, Salacinski HJ, Ghanavi JE, Narula A, Odlyha M, Peirovi H, Seifalian AM (2007) Silsesquioxane nanocomposites as tissue implants. Plast Reconstr Surg 119:1653– 1662. https://doi.org/10.1097/01.prs.0000246404.53831.4c 84. Tyler B, Gullotti D, Mangraviti A, Utsuki T, Brem H (2016) Polylactic acid (PLA) controlled delivery carriers for biomedical applications. Adv Drug Deliv Rev 107:163–175. https://doi. org/10.1016/j.addr.2016.06.018 85. Thomas NG, Sanil GP, Rajmohan G, Prabhakaran JV, Panda AK (2011) Fabrication and anti-microbial evaluation of drug loaded polylactide space filler intended for ridge preservation following tooth extraction. J Indian Soc Periodontol 15:260. https://doi.org/10. 4103/0972-124X.85671

Chapter 2

Biobased Nanohydrogels for Controlled Drug Delivery Sarat K. Swain and Kalyani Prusty

Abstract Advances in biobased polymers have great considerable attention leading to the evolution of various novel drug delivery systems (DDS). The biocompatibility and biodegradability of these biopolymers, coupled to the large variety of chemical functionalities make them a promising carrier for DDS. The encapsulation of drugs using high molecular weight polymers can improve significantly both tumour targeting and therapeutic efficacy due to the improved permeability and water retention behaviour. However, polysaccharide-based nanohydrogels are prepared by reinforcing different nanomaterials in polysaccharides matrix for drug delivery applications. Further, the drugs release efficiency of biobased nanohydrogels can be enhanced, when tailor-made carbon quantum dot and specific nanostructured materials are reinforced to the specific materials. In this chapter, the preparation and characterization of different biobased nanohydrogels are discussed with suitability of their applications as potential drugs transporter. Moreover, the toxicity and stability of biobased nanohydrogels are studied after encapsulation of various drugs. The present chapter aims to explore the possibilities of safe and controlled release of various therapeutic drugs with the help of different biobased nanohydrogels. Keywords Biobased nanohydrogel delivery system


Therapeutic drugs
Encapsulation
Drug

1 Introduction In recent times, with rapidly evolving market demands, the pharmaceutical industry is constantly looking for advanced and novel methods to tackle technological challenges. The need for excipient has transformed from providing volumes, weight, flowability, etc. to advanced roles such as enhancing drug performances, drug release, stability, bioavailability, etc. [1, 2]. In modern scenario, natural, S. K. Swain (&)
K. Prusty Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur, Odisha 768018, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_2

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eco-friendly, and biodegradable materials such as natural polysaccharides as well as their derivatives are considered foundations for constructing/building advanced and effective drug delivery systems (DDS) in terms of its easy processability, less toxicity, stability, and sustainability [3–8]. The wide scope of chemical modifications of these natural polymers combined with biodegradability and biocompatibility nature makes them excellent candidates for effective DDS [9]. Hydrogels with enhanced features including alterable physical and chemical properties, high water retention properties, biodegradability, biocompatibility, tailored mechanical properties have started a revolution in biomedical and pharmaceutical industries [10–15]. Natural polymers and their other sources have several advantages such as low cost, noncytotoxicity, and easy availability and are a potential candidate for DDS for the effective drug-releasing behaviour [16–18]. However, they are having certain shortcomings like microbial contamination, uncontrolled rate of hydration, and fall in viscosity during storage, etc. have led to some further improvements in the conventional hydrogel systems [19]. In this regards, grafting of synthetic polymer chains on natural polymeric backbone in presence of an external crosslinker has created future opportunities to design advanced cross-linked hydrogels with enhanced features for sustainable DDS [20]. Nanohydrogels are three-dimensional hydrated porous nanosized hydrogel particles with many interesting properties such as high water uptake, and small particle sizes [21, 22]. Moreover, natural biopolymer-based nanohydrogel has great considerable attention as DDS due to its biocompatibility, biodegradability, and similarity to the macromolecular components of the extracellular matrix [23]. The deliberation of pharmaceutical effect to DDS is an amalgamation of bioconjugates chemistry, molecular biology, pharmaceutics and polymer science [24]. However, pharmaceutical agents are administered to the systemic circulation on the basis of controlling the non-specific toxicity, pharmacokinetics, pharmacodynamics, non-immunogenicity, and bio-recognition of target site [25, 26]. DDS outplay the traditional systems with a higher tendency of selective delivery of drugs to specific sites, maintaining desired drug level, eliminating side effects and better absorption within the targeted cell [27]. Nanoparticles (NPs) ranging from size 10 to 1000 nm in diameter behave as excellent drug delivery vehicles [28–30]. Effective encapsulation of anticancer drugs is achieved by several covalent and non-covalent interactions with the large available functional surface area [31, 32]. Polymeric nanocarriers systems, made from biodegradable and biocompatible polymers such as natural proteins like gelatin, albumin, collagen, cellulose, etc. and several synthetic polymers like poly (lactic acid) (PLA), polyglycolic acid (PGA), polyacrylamide (PAM), dendrimers, etc. can be used as effective DDS [33]. Basu et al. [34] designed green synthesis and swelling behaviour of Ag-nanocomposite semi-interpenetrating network (IPN) hydrogels for release of ciprofloxacin (CFX) antibiotic drugs, where 95% drug is released after 350 min. Aminabhavi et al. [35] prepared acrylamidegrafted-guar gum blended with chitosan IPN hydrogel for delivery of CFX, where 60% CFX drug is released within 12 h. Garcia et al. [36] prepared novel gel (Dendronized polymer-ciprofloxacin: DP-CFX) for delivery of CFX antibiotics drugs, where 47% drug is released within 24 h.

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A well regulated and controlled drug delivery is important to achieve: (1) sustained a constant concentration of therapeutically active compounds in the blood with minimum fluctuations; (2) preservation and encapsulation of bioactive compound having a less half-life; (3) foreseeable and reproducible and continuous drug release rates; (4) enhanced therapeutics and better patient compliance; (5) solution of problems related to drug stability; (6) removal of side effects, drug wastage and frequent dosing. Schematic illustrations of biobased nanohydrogels for controlled delivery of drugs are represented in Scheme 1. The literature for designing of biobased nanohydrogels with encapsulation of various drugs is summarized in Table 1.

Scheme 1 Schematic illustrations of biobased nanohydrogels for controlled delivery of drugs

Table 1 Drugs encapsulated nanohydrogels of biobased materials with reinforcement of metal/ polymers S. No.

Biobased matrix

1 2 3

Dextran Chitosan Tragacanthgum

4 5

Carboxymethyl chitosan Chitosan

6 7

Sodium alginate Dextrin

8 9

Chitosan Dextran

Reinforcement/ polymer Silver Vermiculite Poly(vinyl pyrrolidone) Glutaraldehyde

Hydrogel/ nanohydrogel Nanohydrogel Hydrogel Hydrogel

Drug name

References

Ornidazole Diclofenac CFX

[37] [38] [39]

Hydrogel

Nepafenac

[40]

Poly ethylene glycol GO PNIPAm

Hydrogel

Protein

[41]

Hydrogel Hydrogel

5-Fluorouracil Ornidazole & CFX CUR Dexamethasone

[42] [43]

PNIPAm Poly (glycolic acid) 10 Dextran Acrylamide Note CFX: Ciprofloxacin; GO: Graphene Oxide; Curcumin

Nanohydrogel Hydrogel

[44] [45]

Hydrogel Acyclovir [46] PNIPAm: Poly N-isopropyl acrylamide; CUR:

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Although a numerous number of articles regarding the preparation and characterization of nanohydrogels for the applications as a carrier for release of drugs are available in the literature, however, the concise articles related to the biobased nanohydrogels by reinforcement of different nanostructured materials are scanty for the review. This chapter involves the preparation and characterization of different nanohydrogels. The study of the important properties including swelling, water retention, rheological behaviour including antimicrobial activities are discussed. Moreover, in vitro and in vivo release of different cancer and antibiotics drugs are compared in the present chapter.

2 Techniques for Preparation of Biobased Nanohydrogels 2.1

In Situ Polymerization

‘Living’ radical polymerization or surface-initiated controlled polymerization technique provides a potential route to prepare organic and inorganic core-shell hybrid NPs possessing the ability to control the shell structure and thickness. The elimination of the core templates from the core-shell hybrid NPs lead to the generation of the required nanocapsule. This technique involves the use of silica NPs commonly as templates due to their customizable nature. Specifically, the hydroxyl moieties present on silica surface templates can be conveniently altered and are used in the polymerization process of specific monomers as an initiator. Synthesis of block polymers can be done by sequential addition of monomers using surface-initiated atom transfer free radical polymerization. The polymer shells which is prepared through in situ polymerization of the specific surface generally undergo stabilization by crosslinking before the removal of the core templates, which differentiates it from layer-by-layer self-assembly method. Mu et al. [47] designed a pH-sensitive nanocapsule employing surface-initiated atom transfers radical polymerization (ATRP) method. In this method, conjugation of the bromoacetamide groups onto the surface of the silica NPs. The ATRP of t-butyl acrylate and styrene is subsequently initiated on the surface of the NPs. The co-monomers upon changing into t-butyl acrylate, N-isopropyl acrylamide (NIPAm) and N,N-Methylenebisacrylamide (MBS), crosslinked nanocapsule having pH-sensitive and thermo-sensitive shells are prepared using a similar method.

2.2

Microemulsion Method

In microemulsion method, inorganic particles with a size in nanometer scale are obtained with minimum agglomeration [48]. Oxide and carbonate NPs are generated successfully using this method [49–52]. This microemulsion is thermodynamically stable and transparent. In W/O microemulsion, nanosized water droplets are

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distributed uniformly throughout the continuous hydrocarbon phase and surrounded by the surfactant monolayer [53, 54]. Using this method, hydroxide or oxalate precursor particles are found better. The desired oxide system is achieved after subsequent drying and calcination at an appropriate temperature. Thus, the microemulsion method successfully generates dispersed nanosize inorganic and organic particles with minimum agglomeration. In spite of certain limitations of this method such as requirement of a large amount of oil and surfactant phases with low production yield. Further, microemulsion method gives an alternate technique for synthesis of different varieties of organic as well as inorganic NPs [55, 56].

2.3

Precipitation Polymerization

Thermo-sensitive nanogels are conventionally prepared by precipitation polymerization method. When the polymerization temperature of the NIPAm based polymers supersedes the Lower Critical Solution Temperature (LCST), the growing chains of the NIPAm would collapse upon reaching the critical length. This method results in the formation of precursor particles [57]. This technique includes the origination of charges from the initiator fragments and incorporation of sufficient amount of water into the collapsed chains. These vary from the traditional emulsion polymerization method used for water-insoluble monomers, in which particles with compact structure are synthesized. The completion of polymerization process is followed by the fall in the temperature of the system below the LCST. As a result, the produced hydrogels swell by incorporating more water. Pelton et al. [58] formulated PNIPAm based nanogels using precipitation polymerization method. A chain of core-shell nanogels having pH-sensitive shell and thermo-sensitive core was designed by Li et al. using this method [59].

3 Characterization Methods 3.1

Spectroscopic Characterization

UV-Visible Spectroscopy. Figure 1 represents the UV–Visible absorption spectrum of chitosan/alginate and lysozyme loaded chitosan/alginate hydrogels. The pure lysozyme exhibits broadband at around 280 nm due to their essential amino acids [60]. The strong absorption peak of UV–Visible spectrum obtained at 278 cm−1 relates to the characteristic absorption for lysozyme, whereas chitosan/ alginate had no absorbance in between wavelength 250–800 cm−1. PAm/Chitosan (PAM/C) hydrogel gives UV–Visible peaks of 260 nm which is not visible in nanogold embedded PAm/C nanohydrogel. As a result, slight hypsochromic shift is observed at around 2 nm when lysozyme is added into the chitosan/alginate hydrogel, which established the subsistence of lysozyme in the composite hydrogel

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Fig. 1 Ultraviolet-visible spectra of chitosan/alginate and chitosan/alginate loaded with lysozyme hydrogels. Source Food Chemistry 240, p. 361. Reproduced with permission from Elsevier Science, Ltd. [61]

and also clearly indicates that the change of microenvironment adjacent to the lysozyme. Fourier Transforms Infrared Spectroscopy (FTIR). In order to better understand the structural characteristics of xylan, NIPAm, and xylan based poly N-isopropyl acrylamide-co-acrylic acid hydrogels which are obtained by the crosslinking co-polymerization method, the hydrogels are analyzed by FTIR spectroscopy (Fig. 2). The main characteristics absorption bands of xylan appear at 3442 (stretching of–OH groups on xylan), 2923 (C–H stretching vibration of alkane), 1619, 1250, 1160, 1040, 967, and 893 cm−1 (b-glucosidic bands between the xylose units) [62]. The broad peak appeared at 1040 cm−1 corresponding to C–O–C stretching of pyranoid ring in xylan [63]. The main bands associated to the gel 2 are the following: 3442, 1714 (C=O stretching), 1452 cm−1 (COO− symmetric stretching vibration), which clearly indicates that acrylic acid had been introduced onto the xylan structure successfully. The prominent peaks appeared at 3268 and 1540 cm−1 comes from N–H asymmetric stretching vibration and bending vibration band of carbonyl (C=O) in amide group in NIPAm, respectively [64, 65]. The prominent peaks appeared at 1158 cm−1 is assigned to C–C contraction vibration absorption peak in the isopropyl group of NIPAm [66, 67], but a new peak appeared at 796 cm−1 corresponding to N–H swing vibration absorption peak. X-ray Diffraction (XRD) Analysis. The nature of pristine magnetic NPs, chitosan, chitosan coated magnetic nanoparticles (CS-MNP), Curcumin (CUR) loaded CS-MNP grafted-polyacrylic acid grafted ethylenediamine-b-cyclodextrin (CUR(CS-MNP)-g-poly (AA)-g-en-b-CD), and CS-MNP grafted polyacrylic acid grafted ethylenediamine-b-cyclodextrin (CS-MNP-g-poly (AA)-g-en-b-CD) is represented in XRD as shown in Fig. 3. The broad x-ray diffraction peak of magnetic NPs showed the appearance of six main peaks at 2h values of 30.1°, 35.5°, 43.2°, 53.5°, 57.0° and 62.8°. The characteristics XRD peak of chitosan exhibit at 2h = 11.0°

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Fig. 2 Fourier transform infrared spectra of xylan and gel 2. Source Carbohydr Polym 151, p. 189. Reproduced with permission from Elsevier Science, Ltd. [68]

and 20.0°. However, the broad XRD peak of chitosan-coated magnetic NPs exhibits weak broad peaks of spinel Fe3O4 (2h = 30.1°, 35.5°, 43.2°, 53.5°, 57.0° and 62.0°), which is confirmed that chitosan is successfully coated onto the magnetic nanoparticle surface. The sizes of MNPs are observed to be 25 nm obtained by using Scherrer formula [69]. The main characteristic peaks of (CS-MNP)-g-poly (AA) hydrogels between 2h = 20.0° to 32.0° and 2h = 38.0° to 50.0° region indicated partial crystalline areas [70]. When grafting of ethylenediamine-bcyclodextrin onto the surface of CS-MNP-g-poly (AA)-g-en-b-CD held, the XRD peak exhibit amorphous behaviour, which is indicated by the broaden peak approximately at 2h = 21.0°. The main peak appeared at 12.5° is assigned to the depth of the b-cyclodextrin cavity. Further, grafting of ethylenediamine affects the crystalline structure of CS-MNP-g-poly (AA)-g-en-b-CD. In drug-loaded DDS, the intensity of broad peak of b-cyclodextrin at 2h = 12.5° is decreased, which confirmed that the encapsulation of drugs within b-cyclodextrin cavity held.

3.2

Microscopic Characterization

Scanning Electron Microscopy (SEM) Analysis. The morphological behaviour of pure konjac glucomannan, konjac glucomannan/sodium alginate-3 and konjac glucomannan/sodium alginate/GO-3 hydrogels are represented in Fig. 4. The surface of the pure konjac glucomannan hydrogel exhibits more than one pores, just

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Fig. 3 XRD pattern of magnetic NPs, chitosan, chitosan-coated magnetic NPs, chitosan-coated magnetic NPs-grafted-poly (acrylic acid)–grafted ethylenediamine-b-cyclodextrin, and curcumin loaded chitosan-coated magnetic NPs-grafted-poly (acrylic acid)–grafted ethylenediamineb-cyclodextrin hydrogels. Source Chemical Engineering Journal 284, P. 1259. Reproduced with permission from Elsevier Science, Ltd. [71]

like a net. However, the surface of konjac glucomannan/sodium alginate-3 hydrogel exhibits a multiplated structure with larger gaps among the leaf-like plates than the pores of the pure konjac glucomannan hydrogel. However, the surface of konjac glucomannan/sodium alginate/GO-3 shows the most compact structure among these hydrogels, just like a tight flower bud. These may be due to the interaction of GO with sodium alginate and konjac glucomannan [72]. When GO is incorporated, the dissociative COO groups of sodium alginate can be crosslinked with COO groups of GO by Ca2+ cations to create calcium carboxylate (–COO–Ca–OOC–) bonds. Further, the hydroxyl and epoxides functional groups present on the basal planes of GO may form strong hydrogen bonds and other types of interactions with sodium alginate and konjac glucomannan. Transmission Electron Microscopy (TEM) Analysis. The nanostructural behaviour of CS-MNP-grafted-poly (acrylic acid)–grafted ethylenediamine-bcyclodextrin ((CS-MNP)-g-poly (AA)-g-en-b-CD) is determined by transmission electron microscopy as represented in Fig. 5. From the figure, it is clearly indicated

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Fig. 4 Scanning electron microscopy of pure konjac glucomannan (a and d); konjac glucomannan/sodium alginate-3 (b and e); and konjac glucomannan/sodium alginate/GO-3 (c and f) hydrogels. Source Colloids and Surfaces B: Biointerfaces 113, p. 223. Reproduced with permission from Elsevier Science, Ltd. [42]

that the chitosan-coated magnetic NPs are specified by faint black coloured spots that are encompassed with the grey polymer layer. The mean particle size is found to be 45 nm from TEM analysis. The particle sizes are determined by dynamic light scattering analysis for CS-MNP-g-poly (AA)-g-en-b-CD), which is observed to be 115 nm. As a result, the increased particle size obtained from dynamic light scattering study confirmed that the presence of aggregated particles in aqueous medium [73].

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Fig. 5 Transmission electron microscopy of CS-MNP)g-poly (AA)-g-en-b-CD) hydrogels. Source Chemical Engineering Journal 284, P. 1259. Reproduced with permission from Elsevier Science, Ltd. [71]

3.3

Study of Toxicity

Cytotoxicity estimation of any drug delivery agent is one of the major challenges faced by the scientific community before animal and human trials. Cytotoxicity being one of the most essential properties of any biomaterial, several techniques such as MTT assays and others are employed for analyzing the biocompatibility of any new material. MTT assay is used to assess the cytocompatibility of xylan based hydrogels with the help of NIH3T3 cells [68]. It is clear from the cultivation results for 1 and 3 days at 37 °C (Fig. 6), the NIH3T3 cell viability values of gel-2 increases with increase in duration of cultivation as 113% and 132% for 24 h and 72 h, respectively. Further, the proliferation capacity of the cells after incubation for 72 h surpassed that for 24 h as well as, the cell viability values superseded 100% viability of control group for both time periods. The above analysis implies that the NIH3T3 cells are compactable with gel-2. Parallel results are obtained for xylan Fig. 6 NIH3T3 cell viability of gel-2 by MTT assay at 37 °C after incubation for 24 h and 72 h. Source Carbohydr Polym 151, p. 189. Reproduced with permission from Elsevier Science, Ltd. [68]

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Scheme 2 The diagram of drug delivery behaviours and cell proliferation of the xylan based P (NIPAm-g-AA) hydrogels copolymer networks. Source Carbohydr Polym 151, p. 189. Reproduced with permission from Elsevier Science, Ltd. [68]

based hydrogels which establishes evidence for it to have excellent properties to be used in drug delivery fields [74, 75]. The schematic illustration of drug delivery behaviours and cell proliferation of the xylan based NIPAm-g-AA hydrogels copolymer networks are represented in Scheme 2 [68].

3.4

Swelling Behaviours

The swelling behaviour of PAM-dextran (PAM-D) and silver NPs embedded PAm-D nanohydrogels in various solvents like H2O, NaOH (0.1M), NaCl (0.1M), and HCl (0.1M), basic buffer (9.14), acidic buffer (4.01) measurement is carried out by weight gain considering time as a function. The swelling property of PAm-D hydrogel and silver NPs embedded PAM/D nanohydrogels depends on the cross-linked density of the gel network. With an increase in crosslinking density equilibrium swelling goes on decreasing. It may be due to loose network structure for thermodynamic free volume for which, polymer network chains can accommodate large number of solvent molecules [76]. In this work, swelling percentage of silver NPs embedded PAM/D nanohydrogel is found to be higher than that of PAM-D hydrogel (Fig. 7a–f). In 72 h, the percentage of swelling of silver NPs embedded PAM/D hydrogel is 4276% at pH 4.01 (acidic buffer capsules) but is 5942% at pH 9.14 (basic buffer capsules). The PAM/D hydrogel containing hydrophilic moiety get protonated in acidic medium and causes intermolecular

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Fig. 7 Swelling percentage of PAM-D and silver NPs embedded PAM-D nanohydrogels in different solvent a H2O, b acidic buffer, c basic buffer, d HCl, e NaOH and f NaCl with 1.5% concentration of nanosilver. Source Materials Sci. and Engineering C 85, p. 130. Reproduced with permission from Elsevier Science, Ltd. [37]

hydrogen bonding. As a result, it lowers the percentage of swelling (Fig. 7f). In a basic medium, the percentage of swelling is higher due to the presence of hydrophilic groups in hydrogel network which remained unprotonated and cause the intermolecular hydrogen bonding with water molecules and results in more swelling percentage (Fig. 7e). Further, it may be due to the ‘charge screening effect’ of excess Na+ in the swelling media, which shields the sulfonate and carboxylate anions and prevents effective anion-anion repulsion [77]. Such type of explanation specified for improvement of swelling percentage in NaOH (Fig. 7b) than acid (Fig. 7d). Figure 7c represents the swelling percentage of PAM-D hydrogel and silver NPs embedded PAM-D nanohydrogels in NaCl. From Fig. 7c, it is clearly indicating that polyelectrolyte effect causes the decrease in swelling percentage in

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saline water. However, the percentage of swelling of both hydrogels and nanohydrogels are higher in double distilled water than that of saline water (Fig. 7a). These may be due to the fact that the PAM/D network having charged groups is more susceptible to enlarge its dimension for minimizing cohesion repulsion between light charges [78].

3.5

Antibacterial Activities

Recently, nanotechnologies have expanded its applications in biomedical field including the prevention of diseases using atomic-scale functional materials. Fascinatingly, silver NPs have been considered to increase the resistant strains of bacteria to the most potent antibiotics. In this study, Escherichia Coli (EC), Shigella Flexneri (SF), Bacillus Cereus (BC), and Listeria Inuaba (LI) are used as the test bacteria to observe the antibacterial behaviour of PAM/D and silver NPs embedded PAM-D (PAM/D@Ag) nanohydrogels with different percentages of AgNps as illustrated in Fig. 8. Herein, the % of antibacterial behaviour of the synthesized nanohydrogel shows among three bacteria, BC bacteria has more resisting power. The SF has lowest antibacterial property than other bacteria’s in all synthesized nanohydrogels. As shown in the Fig. 8, with an increase in silver NPs%, the antibacterial property gradually increases. The incorporation of bacteria in PAM/ D@Ag nanohydrogels restrict the growth of bacteria in nanohydrogels crosslinking due to the existence of silver NPs. This can cause due to hydrogen bonding interaction among amide group of acrylamide chains and silver NPs [79]. Again, addition of silver NPs increases the aspect ratio of the nanohydrogels which increases bacterial resistivity. Hence, PAM/D@Ag nanohydrogels are used as effective DDS. Similar explanations are also found, which are in accordance with another author [80]. Fig. 8 Antibacterial behaviour of different percentage of PAM/D@Ag nanohydrogel hybrid composites against B. Cereus (BC), S. Flexneri (SF), and L. Inuaba (LI), Escherichia Coli (EC). Source Materials Sci. and Engineering C 85, 130. Reproduced with permission from Elsevier Science, Ltd. [37]

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4 Controlled Release of Drugs with Encapsulation of Biobased Nanohydrogels 4.1

In Vitro Release of Drugs

Figure 9 represents the progressive drug (CFX and ornidazole) release rate of dextrin and crosslinked-Dextrin/PNIPAm 8 (c-Dxt/PNIPAm 8) in different pH conditions (1.2, 6.8 and 7.4). It is already established that the drug release rate is primarily influenced by the structure and % equilibrium swelling ratio (ESR) of the hydrogels. Higher values of the % ESR of hydrogels provide higher surface availability for the diffusion of the drug to the release media. The hydrogel-based tablet formulations enable the release of water-soluble drugs via dissolution process. Apart from that, erosion of the matrix also leads to the release of a small amount of drug. From Fig. 9, it is clear that the alkaline medium facilitates a higher release rate of ornidazole as compared to an acidic medium. This is due to the fact that the drug remains in collapsed state in the c-Dxt/PNIPAm 8 at pH 1.2 which results in less swelling. Similar observation is seen in case of CFX, which showed enhanced release rate at pH 7.4 than pH 1.2. Nevertheless, the hydrogels exhibited reduced swellings at neutral/alkaline medium providing a better and regulated release of the encapsulated drug than dextrin. Subsequently, the release rate of both CFX and ornidazole at elevated pH conditions are found to be higher than that of acidic pH. Among the other hydrogelators, c-Dxt/PNIPAm 8 showed a more

Fig. 9 In vitro release profile of ornidazole drug from dextrin and c-Dxt/PNIPAm 8 (a); and CFX drug from dextrin and c-Dxt/PNIPAm 8 (b). Source ACS Appl Mater Interf 7, p. 14338. Reprinted with permission from American Chemical Society [43]

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controlled delivery of the drug than dextrin. The erosion percentage of the hydrogel is retarded by the covalent linkages present in the cross-linked gel network in comparison to dextrin, which in turn influences the absolute releasing behaviour of both CFX and ornidazole.

4.2

In Vivo Drugs Delivery

The section a and b of Fig. 10 represent the plasma drug concentration that is meticulously measured in regular intervals of 12 and 24 h, respectively. Further, compartmental analysis is employed to compute the pharmacokinetic parameters. In oral suspension, the time required to obtain the Tmax (peak plasma concentration) is found to be greater than that for the continuous release from the tablet formulations for CFX and ornidazole. The above observation illustrates the effective releasing nature of the tablet formulations, which is due to the formation of cross-linked matrix of the hydrogel. For CFX, the peak plasma concentration, Cmax is found to be notably minimum in case of tablet dosage form in comparison to the oral suspension, that is the evidence of low rate of absorption, which is the primary target to be fulfilled in case of sustained release in dosage form. No markable difference in Cmax value is noticed between the tablet dosage and oral suspension formulations of ornidazole. The amount of absorption should be higher than that of the oral suspension, that is the key objective to be fulfilled in case of effective and

Fig. 10 In vivo release formulation of CFX (a) and ornidazole (b) drug from dextrin and c-Dxt/ PNIPAm 8 tablets. Source ACS Appl. Mater Interf 7, p. 14338 Reprinted with permission from American Chemical Society [43]

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sustained release formulation. The AUC value, given by the area under the curve, is the indication of the extent of absorption. AUC values of tablet form of both CFX and ornidazole are much higher than that for oral suspension, suggesting that the in vivo drug delivery is effectively controlled by the crosslinked matrix of the hydrogel. The percentage coefficient variation and high standard deviation may be diminished by increasing the number of animals per group, which is a regulatory constraint in our country.

5 Conclusion The biodegradable polymeric nanohydrogels have the properties of biocompatibility and non-toxicity along with good swelling properties and antibacterial behaviour. The materials are suitable as a drug transporter for controlled release of drugs. The present chapter reveals the advantage and disadvantages of different methods for the designing of potential nanohydrogels. Nanohydrogels can be identified by various techniques such as in situ, microemulsion, and precipitation. In vitro and in vivo methods are also discussed for the release of drugs by biocompatible nanohydrogels. This contribution may explore the ideas regarding the competency of biobased nanohydrogels for controlled drug delivery. Acknowledgements Ms. K. Prusty earnestly acknowledged the Department of Science and Technology, Government of India for awarding Inspire Fellowship to pursue doctoral degree.

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Chapter 3

Biocompatible Polymer Based Nanofibers for Tissue Engineering Gajanan K. Arbade and T. Umasankar Patro

Abstract In the past decade, various biocompatible polymer-based electrospun nanofiber scaffolds have been used extensively for tissue repair and regeneration. Variety of biomolecules, therapeutics, and other biologically active molecules have been embedded either into the scaffolds prior to electrospinning or functionalized on the surface of the nanofiber for drug delivery applications. Moreover, various electrospinning techniques have been developed in order to increase the porosity and surface area of nanofibers for higher bioavailability and improved cell growth and proliferation. The present chapter provides a brief and comprehensive review of the recent trends and challenges of the use of these nanofiber scaffolds with and without drugs for tissue engineering applications. Further, a few drug delivery studies of drug-incorporated scaffolds have been summarized in the present chapter. The chapter also briefly covers the drug delivery studies of the authors’ own work based on chloramphenicol-loaded poly(e-caprolactone) scaffolds. Lastly, the summary and future prospects of tissue engineering have been deliberated. Keywords Biocompatible polymer Tissue engineering


Nanofibers
Scaffolds
Drug delivery


1 Introduction Tissue engineering (TE) is a multidisciplinary research area, which comprises both engineering and life sciences. It takes continuous efforts for the development of biological replacements for tissue and organ. TE has evolved as an important branch of biotechnology due to its immense potential not only in curing

G. K. Arbade
T. U. Patro (&) Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology, Pune, Maharashtra 411025, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_3

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life-threatening diseases but also in organ regeneration. The TE approaches have been successfully demonstrated by various researchers both in vitro and in vivo for several therapeutic treatments from simple wound healing to cancer treatment. The beauty of this technology lies in the design and development of wide range of scaffolds, which supports for cell growth and when loaded with drugs, the scaffolds supply the drug to the focal point in an adequate amount within the stipulated time duration. In this regards, techniques like controlled or sustained drug release can avoid injecting drugs in large quantities, which otherwise may produce cytotoxicity in the body. Till date, numerous TE scaffolds have been prepared including all kinds of materials such as ceramics, polymers, metals, and composites. In fact, some of them have already been in use and are available in the market by several trade names for various biomedical applications [1–4]. Among these, polymerbased nanofiber scaffolds offer some distinct advantages over other traditional scaffolds these are high porosity, interconnecting 3D network-like structure, high surface area for cell growth and infiltration, elasticity, and flexibility for soft-TE, and tuneable biodegradable properties, achieved by innovative polymer chemistry. However, various antibacterial, antimicrobial and anticancerous drugs and nanoparticles, can also be incorporated in fabricating polymer-based nanofiber scaffolds for tailor-made properties. Based on this discussion, the present book chapter aims to give a concise overview of the fundamental understanding of the topic and the current trends on preparation, properties, and the applications of nanofiber scaffolds for TE, organ repair, and replacement.

2 Basic Requisite Properties of a TE Scaffold The ideal scaffold material for TE should have the following properties [5]: • Biocompatibility: The scaffold should provide support for cell attachment, proliferation with minimum host immune response. • Mechanical Stability: The scaffold should have enough mechanical strength and an appropriate structural framework. Because of this property, it will be stable while being handled during implantation [6]. • Architecture: The scaffold should provide sufficient surface area-to-volume ratio as it helps in cellular attachment, proliferation, and differentiation during the new tissue generation. The architecture with the interconnected pore structure is a requisite property, where porous nature helps in cell adhesion, penetration, and migration [7, 8]. • Biodegradability: The efficacy of a nanofiber scaffold is dependent on its ability to degrade chemically in the biological environment with time without producing cytotoxic substances. These by-products are generally excreted from the body in various ways. This also ensures the devoid of surgical procedures to remove the foreign body. The degradation rate of the polymer should match the rate of new tissue formation [9–12].

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• Bioactivity: During the post-implant reactions, the biomaterial and host tissue immunological reaction may occur. The scaffold may contain a growth factor or other biologically active components and may also act as a delivery vehicle [13, 14]. The biomaterial may include biological cues to enhance the cell adhesion, proliferation or may have physical cues such as topology which influences the cell morphology and alignment [15, 16]. • Surface Properties: Surface properties, such as wettability and porosity, strongly influence cell adhesion and growth. The scaffolds should have a comparable pore size that of tissue. This helps in cellular proliferation and infiltration [17–19]. • Sterilizability: The scaffolds often need to undergo a sterilization process during their use. The process ensures free of contamination, if any, in the scaffold during surgical operations to avoid infections [20]. As this process is carried out in various buffer solutions, the scaffold should not disintegrate mechanically.

3 Nanofiber Scaffolds Fibers having diameters in the range of few nanometres up to a micron scale refer to nanofibers. Nanofiber scaffolds can be made by various techniques such as drawing, self-assembly, template synthesis, phase-separation, and electrospinning [21, 22]. Out of these, electrospinning is perhaps the most promising technique due to its versatility and scalability. Using electrospinning, the nanofibers are spun onto a substrate (collector) in a “layer-by-layer” fashion resulting into a 3D interconnecting fibrous and porous structure, which is known as a “scaffold” [23]. Due to their unique structure, they exhibit high surface area-to-volume ratio and porosity. These properties make them suitable candidates for cell attachment and growth; particularly when the scaffold is made from a biocompatible polymer. In fact, the scaffold serves as a template for the cells to attach, grow, differentiate, and supply nutrients for cell functioning. Moreover, the nanofiber scaffolds often exhibit a strong resemblance to the extracellular matrix (ECM) present in living beings. Hence, they are also referred to as “biomimetic scaffolds”. Figure 1 presents a digital image of an electrospun nanofiber scaffold of poly(e-caprolactone) (PCL) and its scanning electron microscopy image, which illustrates the fibrous structure with interconnecting porosity. A considerable effort has been made to fabricate various nanofiber scaffolds for achieving improved biocompatibility by imparting high porosity and developing different architectures. Various strategies, such as bath electrospinning [24], co-electrospinning [25] and salt-leaching have been adopted to enhance porosity and thus to obtain tuned properties for bioavailability. By doing so, sometimes the mechanical properties get affected adversely [26–28]. Hence, while improving the bioavailability, a reasonable balance is often maintained to retain the desired mechanical strength of the scaffold [29].

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Fig. 1 a Digital image of Poly(e-caprolactone) (PCL) nanofiber scaffolds and b scanning electron microscopy image of PCL nanofibers

4 Synthesis of Nanofibers Scaffolds by Electrospinning The most promising pathway to produce these nanofibers is the electrospinning technique, wherein the fibers are prepared usually from a polymer solution or melt or using materials like nanoparticles and their composites. The technique is quite old; however, it has been rekindled in recent years due to their wide applications in the areas of biomedical [22, 30, 31], sensor [32, 33], filtration [34] and energy harvesting [35] areas. This is evident from the huge number of scientific papers being published in these areas using electrospinning. Over the years, electrospinning has evolved as a standard technique to produce nanofibers from a wide range of polymers with a wide range of architectures. There are quite a few excellent books [36, 37] and review papers [35, 38] published particularly on various electrospinning techniques. Readers are recommended to refer to those literature, wherein various aspects, including the basics and the on-going research in the field, and the applications of the technique have been described comprehensively. Nevertheless, in the following section, we briefly introduce the technique for the readers’ convenience. Electrospinning is a simple and yet versatile technique, which uses “electrostatic forces” to overcome the viscous forces and surface tension of the polymer droplet and as a result, the droplet initially forms a cone-like structure near the needle known as “Taylor cone” and subsequently elongates to make a fiber. The technique has gained widespread popularity in recent years for the synthesis of polymer-based nanofibers for various pharmaceutical, biomedical, and TE fields. The versatility of the technique can be realized by its distinct features such as (i) the technique is simple and economical, (ii) scaffolds with nanometre diameter fibers with huge surface area and porosity can be produced by adjusting the solution viscosity and spinning voltage, (iii) the technique provides flexibility of incorporation of a wide variety of nanomaterials and drugs in various polymer solutions. In the electrospinning process, a polymer solution is loaded in a syringe connected with a metallic needle and the flow of the solution is precisely controlled by

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a motor. A separate setup may be used for spinning of fibers from the polymer melt. A collector often a metallic plate or a rotating drum is placed in a certain distance. The collector acts as a counter electrode. A high potential difference in few kilovolts is applied between the needle and the collector. Due to the electrostatic forces, fibers are formed from the polymer droplet and deposited on the collector. During this process, as the solution is ejected away from the needle toward the collector and the polymer solution jet rapidly elongates and dries up as the solvent evaporates. The fiber diameter usually depends on the solution viscosity, electric voltage and the distance between the needle and the collector [39]. The typical electrospinning apparatus consists of four major components: a spinneret with a metallic needle, a syringe pump, a high-voltage power supply, and a grounded collector [40] as shown in Fig. 2. Various parameters, which influence the size and quality of nanofibers, are solution properties, process parameters, ambient temperature, and humidity, etc. [41]. The solution properties include the molecular weight of the polymer used for making polymer solution, the viscosity of the solution, surface tension, conductivity, dielectric constant, and solvent volatility [39]. While, process parameters comprise applied voltage, tip to collector distance, flow rate, and needle diameter [39]. Significant scientific work has been invested in standardizing and customizing the technique for producing a wide variety of fiber architectures and for up-scaling. Several electrospinning techniques have been developed for increasing the porosity of nanofibers for better cell proliferation and infiltration. For instance, a collection of electrospun nanofibers into a water-bath has been used to enhance the pore size of PCL and methyl poly(ethylene glycol) (MPEG) nanofibers [24]. The presence of water-soluble MPEG, in fact, facilitated the formation of bigger pores. Co-electrospinning technique using two polymers, where one polymer is used as a sacrificial component, has been used to increase the porosity of nanofibers [42]. A combination of micro- and nanofibers has been prepared to tailor and enhance the porosity of scaffolds so as to increase cell growth and infiltration [43, 44]. An

Fig. 2 Schematic representation of a typical electrospinning setup showing different components mentioned thereby

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advanced technique like 3D printing of PCL scaffold in combination with poly (lactic-co-glycolic acid) (PLGA), nanofiber coating provides mechanical support for tissue growth due to 3D-printed PCL and improved cell growth due to excellent biocompatibility of PLGA [5].

5 Biocompatible Polymers as TE Scaffolds The most important requisite property of nanofiber scaffold for TE is that the polymer to be used in a biological environment should not produce any cytotoxicity or should have a minimal immune response. This refers to the term “biocompatibility”. A wide range of biocompatible polymers including both natural and synthetic varieties are available now, which are extensively used in various TE scaffolds. New synthetic polymers and derivatives of the existing polymers have also been developed in order to tailor and manipulate to obtain the desired properties. The selection criteria for different biological scaffolds are based on their biocompatibility, biodegradability, solubility, and processability of the polymer used, adequate surface area to support and grow living cells, mechanical strength for handling and reabsorption of the degradation products, etc. Further, the use of nanofibers as scaffolds requires the properties mentioned earlier. A list of often used selected biocompatible polymers is summarized in Table 1. These polymers are studied widely and used as nanofiber scaffolds in various TE applications.

6 Nanofiber Scaffolds for TE Applications Nanofiber scaffolds have been used successfully in various tissue repairs and regenerations. The polymer is selected based on its biocompatibility, spinnability as nanofibers and biodegradability. Some of the important TE applications in the human body where the nanofibers have been demonstrated successfully either in vitro or in vivo are shown in Fig. 3.

6.1

Skin Tissue

Skin is the largest organ of the body and most vulnerable to injuries due to its exposure to the external world. Scaffolds such as films, hydrogels, nano/micron fibers, and membranes have been demonstrated to potential skin substitutes. Among the various scaffolds, nanofibers are the promising candidates for skin substitutes as they mimic the native ECM. Collagen/polyvinylpyrrolidone (PVP) nanofibers consisting of collagen core and PVP shell were used for growth and proliferation of human umbilical vein

Chemical structure

Agarose

Alginate

Chitosan

Silk fibrin

(-Glycine-Serine-Glycine-alanine-Glycine-Alanine-)n

Natural polymers Collagen

Polymer

Table 1 List of commonly used biocompatible polymers used in TE

Non-toxic Biocompatible Biodegradable

Biocompatible Mechanical stability Low immunogenicity Biocompatible Biodegradable Antibacterial Wound healing Mucoadhesive

Biocompatible Good mechanical strength Low antigenicity

Properties

[59, 60]

Wound dressing and drug release

(continued)

[56–58]

[32, 53–55]

[50–52]

[45–49]

Reference(s)

Bone and cartilage TE

Drug delivery, wound dressing, and skin TE

Bone, cartilage, and skin TE

Structural repair of bone, cartilage, and vascular TE

Applications

3 Biocompatible Polymer Based Nanofibers for Tissue Engineering 49

Chemical structure

Drug delivery and wound healing

Biocompatible Mechanical stability Chemical stability Biodegradable Biocompatible Mechanical strength

PU

PLLA

Cartilage and vascular TE

Wound dressing and corneal TE

Drug delivery, wound healing, and Bone TE

Skin, bone, cartilage, vascular, nerve TE, wound healing, and drug delivery

Applications

Biocompatible Good mechanical properties

Biodegradable Biocompatible Good mechanical properties Structural flexibility Biocompatible Hydrophilic

Properties

PVA

PEG

Synthetic polymers PCL

Polymer

Table 1 (continued)

(continued)

[49, 75–78]

[72–74]

[68–71]

[66, 67]

[61–65]

Reference(s)

50 G. K. Arbade and T. U. Patro

Reference(s)

[82, 83]

[79–81]

Note PEG: Poly(ethylene glycol); PVA: Poly(vinyl alcohol); PU: Polyurethane; PLLA: Poly(L-lactic acid); PLGA: Poly(lactic-co-glycolic acid); PLCG: Poly (L-lactide-co-e-caprolactone-co-glycolide)

Drug delivery and bone tissue

Biocompatible Wettability Biodegradable

PLCG

Applications Drug delivery, bone, skin tissue and neural regeneration

Properties Biocompatible Good mechanical strength

Chemical structure

PLGA

Polymer

Table 1 (continued)

3 Biocompatible Polymer Based Nanofibers for Tissue Engineering 51

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Fig. 3 Different domains of TE

endothelial cells (HUVECs), and it was also found that the cells had grown along the orientation of nanofibers [84]. Collagen together with PCL scaffolds has been extensively used in skin TE [85]. Chitosan-PVA [66] and poly(ethylene glycol terephthalate)-poly(butylene terephthalate)-based scaffolds [67] have also been developed for dermal substitution [86, 87]. Porous scaffold constructs of PLGA help in cell proliferation and infiltration of human skin keratinocytes [80, 88]. Silk-based scaffold has also been used as a skin substitute due to its outstanding biocompatibility, mechanical property, and controlled degradation [51]. The ideal scaffold material required for wound dressing should protect the wound from bacterial infections as well as promote the healing process [89]. Chitosan, a biopolymer, facilitates wound healing, and its nanofiber scaffolds have been used as a wound dressing patch [55, 90]. The nanofiber scaffolds prepared from PU loaded with ciprofloxacin have been demonstrated as wound-dressing mats by Unnithan et al. [91]. PCL/gelatin biocomposite matrices support the ECM remodeling and help in wound healing [9, 92]. Ottosson et al. [93] have shown that the differently organized PCL nanofibers (random and aligned) accelerate the wound closure in in vitro models. The natural plant extracts blended with biocompatible polymers like PCL/gum tragacanth and and henna leaves extract with chitosan are found to be potential wound dressing materials in TE [95, 96].

6.2

Bone and Cartilage Tissue

Bone is an important organ of the body, and it provides structural support and flexibility to the body. It works as a structural framework, stores minerals, maintains homeostasis, and is a blood pH regulator. Bone disorders (osteoporosis,

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arthritis) are of significant concerns due to their increased number of cases. Accidental fractures and trauma also do contribute to bone problems [96]. In extreme cases, bone transplant is generally carried out for bone treatment. The techniques like autograft and allograft have achieved tremendous success in treating bone defects. Autograft has limitations like restricted availability donor site morbidity, prolonged rehabilitation and increased risk of deep infection [97]. The second one has limitations like the risk of transmissible diseases, viral infection, efficacy, and immunological complications [98]. Stainless steel, Co–Cr alloys, Ti and Ti–Al alloy have been used conventionally for a bone replacement, but these materials are prone to corrosion and release cytotoxic ions [99]. A shape memory alloy of nickel and titanium, like “Nitinol”, has been used as bone TE scaffold due to its biocompatibility, plasticity, damping properties [100]. Ceramics like bioactive glasses and calcium phosphate have been used for bone repair due to their biocompatibility, good mechanical properties, and low cost. Due to their chemical stability and processing advantages, nanofiber scaffolds have been used for bone repair and regeneration. The methods used for bone repair include the growth of bone tissue using stem cells, gene therapy, osteogenic growth factors, and osteoinductive cells. 3D bio-printing has also been developed for manufacturing polymer-based scaffolds for bone regeneration [101, 102]. Electrospun PCL based porous nanofiber scaffolds were produced by varying the concentration of dichloromethane and acetone as a solvent mixture [104]. These scaffolds were coated with calcium phosphate in stimulated body fluids. As a result, they showed improved wettability, cell adhesion, and proliferation properties and could potentially be used in bone TE. As bone constitutes *90% collagen, it is one of the most suitable scaffold materials for bone repair among the natural polymers. Moreover, collagen is biocompatible and available in abundance. Collagen type I which promotes proliferation and differentiation of human mesenchymal stem cells (MSC) into the osteoblasts in vitro and osteogenesis in vivo [104]. The composite scaffolds of collagen-apatite are known to support the bone repair mechanism [46]. Natural polymer like chitosan stimulates the osteoprogenitor cell differentiation, but composite scaffolds of chitosan-gelatin show better bioactivity and cellular behavior [32, 105]. Natural polymers functionalized with synthetic ones, e.g., cellulose functionalized with PCL is widely used for scaffold bone TE. These scaffolds provide porosity and mechanical strength for the osteogenic differentiation [106]. PLA, PLA-hydroxyapatite (HA) and PLA-HA-graphene oxide scaffolds have shown osteoblast growth and proliferation on their surface [107]. Adult cartilage tissue has been shown to have limited self-repair capacity in abnormalities, trauma, or aging-related degeneration like osteoarthritis [108]. Adult MSCs have the inherent property of differentiating cells of different lineages such as bone and cartilage. This property is utilized during the culture of MSCs with silk-based nanofiber scaffolds and used for the cartilage repair in vitro [108]. Gelatin/PLA superabsorbent 3D nanofiber scaffolds significantly repair the cartilage defect in rabbits [109].

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Vascular Tissue

Vascular tissue is a part of the circulatory system of the body, which transports nutrients, oxygen, and biological waste through various means. Vascular tissue is prone to damage due to accidental trauma. Often, natural polymers have been used to fabricate scaffolds for vascular grafts due to their biocompatibility, mechanical stability, and less immunogenicity. Collagen and elastin-based electrospun nanofibrous scaffolds have been used for vascular grafting, as these are the key structural constituents of the vascular ECM [107]. Fu et al. [110] have fabricated electrospun gelatin/PCL and collagen/poly(L-lactic acid-co-e-caprolactone) (PLCL) scaffolds for the regeneration of many vascular tissues. Tillman et al. [111] have developed stable PCL/collagen nanofiber scaffolds for vascular reconstruction.

6.4

Nerve Tissue

The nervous system is the most important system in the body. Any damage to this system affects sensory as well as motor functions. The human nervous system consists of the central nervous system (CNS) and peripheral nervous system (PNS). The major complications in CNS are associated with the neurodegenerative disorders and destruction in the brain region and spinal cord due to trauma. The neurons of CNS do not have property to regenerate under normal conditions. In PNS damage, gold standard routine treatments are autografts and allografts. But, these are limited due to their poor availability, requirement of the second surgery, infectious diseases, immunological complications and functional loss at the donor site [40, 112]. The recent advancements in TE offer a great promise in the treatment of nerve damage since the diversity (sensory and motor neurons) of the nervous system requires various strategies in different locations. The damaged nerve tissue can be replaced by nanofiber-based artificial nerve guide conduit (NGC). Kim et al. [113] have developed NGCs based on aligned nanofibers, which show selective permeability to neurotransmitters and signalling compounds using materials like scaffolds of PLGA and PU. An artificial NGC made by layered nanofiber scaffolds has been used as nerve regenerator. The NGC comprises of two layers, where the inner layer was made up of aligned PCL nanofibers to enhance transport of signalling molecules and the outer layer made by randomly-oriented nanofibers to increase the mechanical strength [114]. The surface porosity allows oxygen and nutrient transport, flexibility to avoid nerve compression; while the nerve growth factors boost the regeneration process. Biodegradability of the scaffold reduces the post-surgical problems and improves nerve restoration. PCL-chitosan [1], silk fibroin [115], PCL-PLGA [116], PCL-gelatin [117], PCL-collagen, and plasma-treated PCL [118] electrospun nanofiber scaffolds have been developed for peripheral nerve injury repair. Another approach of TE, i.e., scaffolds seeded with cells itself can be used for neural TE.

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Chitosan and silk fibroin nanofiber scaffolds seeded with adipose-derived stem cells have shown improvement in nerve regeneration [119]. The scaffolds have been successfully implemented in rat models [101].

7 Drug Delivery Using Nanofiber Scaffolds Drug delivery is a generic term referred to a supply of the drug to a damaged body part or an infection in order to heal it therapeutically. Drug delivery should be done in a controlled or desired way to prevent toxicity and to mitigate the side effects caused by overdosage [120, 121]. Several methods have been used to release the drug in a controlled way; such as the traditional use of capsules and by directly injecting the drug into the body, etc. Relatively, a new way of supplying the drug into the focal point of the body is by using scaffolds. Among the scaffolds, electrospun-based nanofiber scaffolds offer a versatile pathway for incorporating drugs into nanofibers and release them in the desired manner. The drug release in such scaffolds can be controlled by tailoring the interaction between the drug molecules and the carrier polymer used for electrospinning. Often, a slow release is desirable to cure chronic diseases. Prevention of sudden or “burst release” still remains a challenge in many drug delivery systems. Burst release may cause a high immune response and toxicity. Several strategies have been adopted to minimize the burst release of drug from nanofibers [67, 122, 123]. Considerable work has been reported both in vitro and in vivo drug release studies from the drug-containing vehicle often a capsule or a nanofiber using a wide range of antibacterial and anti-cancerous drugs [62, 70, 94, 124, 125]. The efficacy of the drug depends on the interaction between the drug molecule and cells in a biological environment. Drug release from a nanofiber scaffold in a physiological condition is mainly governed by two mechanisms; diffusion and biodegradation of the polymer used. The steady-state diffusion is given by the Fick’s 1st law of diffusion, which can be described as J ¼ D

@c @x

where J is the diffusion flux (the concentration of drug that passes through a unit area per unit time), c is the position-dependent drug concentration in the scaffold, D is the coefficient of drug diffusion, and x is the distance traveled by the drug molecule. The negative sign indicates the drug concentration gradient. The second mechanism is due to degradation of the polymer used to prepare the nanofiber. Polymer degradation generally begins on the nanofiber surface and slowly penetrates into the bulk. Further, polymer degradation is known to occur by two mechanisms: bulk degradation and surface degradation. Bulk degradation referred to be a homogeneous process, occurs uniformly throughout the polymer matrix; whereas surface degradation is a heterogeneous process in which degradation is restricted to a thin surface layer [126].

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The affinity of the drug toward the polymer plays an important role in the release of the drug. Drugs with a weaker bond with the polymer release faster than the one having a stronger bond. The rate of drug release should match the rate of polymer degradation. Hence, the selection criterion for the polymer is done based on this phenomenon. Here, we present a typical method of drug delivery with a few examples.

7.1

In Vitro Drug Release from Nanofiber Scaffolds

A typical method of in vitro drug release study from a drug-loaded electrospun nanofiber scaffold is carried out in the phosphate buffer solution at various pH values. The scaffolds are immersed in Phosphate buffer solution and mechanically agitated in an orbital rotator at required temperatures. After specific time intervals, the measured volume of the sample solution was periodically withdrawn and replaced by the same volume of fresh buffer. The specific absorbance peak intensity of the drug recorded and the values are compared with the standard absorption curve of the same drug in order to quantify the drug release. Here, we present a few literature examples of drug-release behavior of some standard drugs embedded in nanofiber scaffolds in order to give a basic idea of drug delivery to the reader. Example 1 The use of nanofiber scaffolds as a drug-release carrier has been carried out by Kenawy et al. [127] for the first time. The group demonstrated release property of an antibacterial drug, tetracycline from poly(lactic acid) (PLA), poly (ethylene-co-vinyl acetate) (PEVA), and PLA-PEVA nanofibers scaffolds. Here 14 % PEVA and the blend of PLA and PEVA, (50:50) polymer concentrations were used for electrospinning. The polymer solutions were prepared in chloroform. As tetracycline is insoluble in chloroform, it was solubilized in methanol and added to the polymer solution. During the electrospinning, a positive voltage of 15kV with a flow rate of 21 mL/h was maintained and the fibers were collected on a rotating drum-type collector. The release studies showed initial burst release for a few hours. The PEVA-tetracycline (5%) nanofibers showed a higher rate of drug release than that of PLA with a saturation period in 5 days. The blend with 50:50 polymer concentration has shown relatively smooth release over a period of 5 days. Following this study, the use of electrospun nanofibers [14, 121, 128–131]. Example 2 Hu et al. [122] used emulsion electrospinning technique for the preparation of PCL and poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) nanofiber scaffolds with different drugs like metformin hydrochloride (MH) and metoprolol tartrate (MPT). The drug to polymer ratio used in nanofibers scaffolds was kept at 1:50. The scaffolds were prepared using both pure polymer and with drugs. In vitro, drug release studies of MH and MPT from PCL and PHBV were carried out in Phosphate buffer solution of pH 7.4. In this study, initial high burst release was seen and maximum release of drug was found to be *86 %. The drug release was sustained up to 21 days. The antibacterial drug rifamycin

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embedded in PCL scaffolds showed small initial burst release and sustained drug release up to 8 days as studied by another group [132]. Example 3 Poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA) loaded with ciprofloxacin electrospun blends nanofiber scaffolds have been fabricated by Zupancic et al. [131]. They studied the release behavior of ciprofloxacin hydrochloride (CIP) from various monolithic (PCL and PMMA) and blend nanofibers (PMMA-PVA, PMMA-PEO, and PMMA-chitosan). The CIP release from PCL nanofibers was found to be 46%; while from PMMA nanofibers, it was just 1.5% in 40 days. PMMA blended with 10% PEO, PVA, and chitosan separately was used to prepare electrospun nanofibers. These nanofiber scaffolds exhibited different drug-release profiles, such as PEO containing nanofiber mats revealed high burst release; chitosan containing mats demonstrated slow and gradual release, and PVA containing mats showed small burst release followed by sustained release. The drug release can be further tuned for desired time periods using a blending technique (e.g., a blend of PMMA and PVA or chitosan). The nanofiber scaffolds have been demonstrated as drug carriers for the treatment of several medical conditions; such as skin, bone, and joint infections [14, 131]. Example 4 Zhang et al. [121] have developed a simple blended electrospun membrane loaded with CIP using poly(D, L-lactic-coglycolide) (PLGA), poly(glycolic acid) (PGA), and poly(dioxanone) (PDO). A linear drug release profile and a sustained antibacterial activity against both Staphylococcus aureus and Escherichia coli were observed during the study. The addition of PDO changed the stack structure of PLGA, which in turn influenced the fiber swelling and created drug diffusion channels. These scaffolds could be good candidates for reducing postoperative infection or be associated with another implant to resist biofilm formation. The PLGA/PDA/CIP blend scaffolds showed higher drug-release capacity (98%) as compared to that of PLGA/PGA/CIP nanofibers (18–20%). Example 5 Arbade et al. [62] studied the release of chloramphenicol (CAP), a broad-spectrum antibacterial drug embedded in PCL from PCL-CAP nanofiber scaffolds. The PCL and CAP both were dissolved in a mixture of tetrahydrofuran (THF) and methanol (3:1). 10 wt% PCL and different drug concentrations like 5, 10, and 20 wt% of CAP were used in this study. The fiber diameter was found to increase with an increase in CAP content. The in vitro CAP release studies were performed in Phosphate buffer solution of pH 7.4 in glass vials. PCL with 5 wt% CAP (CAP 5) showed almost linear and higher drug release compared to the higher weight fraction CAP nanofibers (CAP 10 and CAP 20). This was presumably due to a smaller fiber diameter of CAP 5 nanofibers. The different nanofiber scaffolds showed different release profiles. A sustained release of CAP in 20 days with an initial burst release was observed. Figure 4 presents the percent release of CAP as a function of time from PCL-CAP nanofiber scaffolds.

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Fig. 4 Release profile of CAP from PCL-CAP nanofiber scaffolds in phosphate buffer solution [62]. The image was reproduced with the copyright permission from IOP

8 Conclusions and Future Prospects TE is an exciting interdisciplinary research area comprising of life sciences and engineering. Nanofiber-based scaffolds have been successfully demonstrated in tissue repair and regeneration. Electrospinning technique has been emerged as an efficient and versatile technique to prepare a wide range of polymer nanofibers with different architectures. The polymers are often incorporated with various antibacterial agents and drugs in order to induce various biological functions. Both synthetic and natural polymers have been used to prepare TE scaffolds. The selection of polymer used for nanofiber scaffold is based on its biocompatibility, biodegradability, resorbability of bi-products, and spinnability, i.e., the ability to form fibers by electrospinning. The latter property is a function of the polymer’s solubility, molecular weight, and polarity, etc. Significant efforts have been devoted to enhancing the bioavailability and increased porosity in the nanofibers as so to have better cell growth, infiltration, and nutrient supply. Toward achieving the above properties, the electrospinning technique has been modified and customized suitably. The physicochemical and mechanical properties of scaffolds, which play an important role in their successful implementation as a TE scaffold, have been studied. Both in vitro and in vivo studies on cell adhesion, proliferation, infiltration, differentiation, and nutrient transport using nanofiber scaffolds are highly crucial for TE applications. Another area of application of nanofiber scaffolds is the drug delivery, wherein various drugs and antibacterial agents are embedded in polymers and their release properties have been standardized. It has been observed that the nanofiber scaffolds give better control of drug release than the conventional methods. The drug release depends on polymer–drug interaction and the degradation of the polymer in a physiological environment. We envisage significant importance of this area of research, i.e., TE using nanofiber scaffolds, considering the advantages of these nanofibers over their

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conventional counterparts. Moreover, they have been proven for their efficient biological cell and tissue growth. Therefore, the future research may be focused on deployment of these scaffolds for organ regeneration and the challenges, like toxicity, biological functions, and polymer degradation involved in it, in view of the increasing number of patients with organ failures in recent times. Organ regeneration using nanofiber scaffold is expected to offer a less invasive and cost-effective method for medical treatment.

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Chapter 4

Bioactive Glasses: Prospects in Bone Tissue Engineering Neha Mulchandani and Vimal Katiyar

Abstract Bioactive glasses, the most fascinating materials, have gained significant attention in bone tissue engineering since their discovery. These materials have the ability to bind to the bone and become an integral part of it, thereby allowing its regeneration. However, different methods have been utilized for the synthesis of bioactive glasses with specific compositions. The current chapter summarizes the various methods adopted for the synthesis of glasses along with their use and their applicability in bone tissue engineering. Further, the mechanism of bone formation has been presented highlighting the specific stages thereof. The commercially available bioactive glasses have been summarized along with their specific composition. Furthermore, the concept of hybrid materials has been focused with several case studies highlighting the potential of bioactive glasses in bone regeneration supported by in vitro and in vivo biocompatibility tests. The chapter thus provides a brief summary of the prospects of bioactive glasses as potential ceramic materials for targeted bone tissue engineering applications. Keywords Bioactive glass


Bone
Tissue engineering
Ceramics
Polymers

1 Introduction The conventional methods of repairing the damaged tissues include autografts, allografts, and xenografts which suffer from several drawbacks such as the scarcity of donors, disease transmission and low clinical success. Tissue engineering has emerged as one of the promising alternatives for the repair/regeneration of damaged tissues. In order to cure the diseased tissues, various methods have been developed N. Mulchandani
V. Katiyar (&) Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati 781039, Assam, India e-mail: [email protected] N. Mulchandani e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_4

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incorporating the use of polymers, ceramics, metals, etc., and the materials thereof are regarded as biomaterials. The first-generation biomaterials were intended to be bio-inert in order to minimize the formation of scar tissue. Bioactive glasses emerged as the second-generation materials providing interfacial bonding between the implant and the host tissue. The third generation of biomaterials involved tissue regeneration and repair by combining the resorbable and bioactive characteristics of the biomaterial, such as the bioactive glasses with gene activation properties [1]. The revolution in biomaterials from tissue replacement to tissue regeneration is shown in Fig. 1. Bioactive glasses gained enormous attention in the field of tissue engineering due to their extraordinary characteristics such as bioactivity, biocompatibility, osteoconductivity, and biodegradability. The remarkable discovery of bioactive glass by Prof. Larry Hench in 1969 (University of Florida, USA) was the result of the conversation between Larry Hench and Colonel Clinker (who had returned from his duty in Vietnam as Supply Officer with Army Medical Corps) during their bus ride to the US Army Materials Research Conference (1967). Prof. Hench talked to the Colonel about his recent investigations on the radiation-resistant semiconductors using vanadium phosphate (V2O5-P2O5) glasses that had the ability to resist the radiation damage. During this, Colonel Clinker described his amputations witnessed in Vietnam and asked Larry Hench if he was able to develop the materials that would survive the exposure to human body so that body would not reject the implants unlike those of metallic and plastic and would be able to save the limbs of the diseased persons [2]. The proposal regarding the development of such materials was submitted in 1968 to the US Army Medical R&D Command which was funded for one year for testing the hypothesis. Using the phase diagrams, the glass composition of 45% SiO2, 24.5% CaO, 24.5% Na2O, and 6% P2O5 was found to be close to a ternary eutectic which was melted, cast, and tested as an implant in rats, and the implants were found to be bonded with the bone. The in vitro and in vivo tests resulted in the formation of hydroxyapatite (HA) layer which was bonded to the collagen. A strong bonding between the collagen and HA was observed and thus was the most fascinating material discovered, which was recognized as “45S5 Bioglass®” [3]. Since its discovery, it has been widely used by the researchers for testing its widespread applications. One of the most important criteria for the biomaterials to serve as an ideal scaffold material is to mimic the extracellular matrix (ECM) onto which the cells would adhere, multiply, and function. The ability of a biomaterial to form a bond with the living tissues is termed as “bioactivity.” The major compositional characteristics of bioactive glasses that distinguish it from the traditional soda–lime glasses are: (a) SiO2 content less than 60 mol%, (b) high CaO and Na2O content, and (c) higher ratio of CaO:P2O5 [4]. Bioactive glasses suffer from several disadvantages such as their brittle and stiff nature which restricts them to be molded into complex shapes and often leads to the fracture under mechanical loads. Additionally, the trauma and infections also lead to the critical size bone defects and their treatment using conventional methods is often encountered by immune rejection, high cost, limited availability, scar tissue formation, painful secondary surgeries, little or no integration with the host tissue, etc. Bioactive glass, on the other hand, being bioactive in nature is considered to be the most suitable alternative to allografts with a great potential in

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Fig. 1 Revolution in biomaterials: Tissue replacement to tissue regeneration

orthopedic applications. Bioactive glass undergoes dissolution inducing an apatite layer which is similar to that of natural bone, thereby stimulating osteogenesis and angiogenesis which in turn allow the interfacial bonding between the bone and the implant leading to osteointegration. However, it is essential for the implant to mimic the structure of the bone in order to promote the regeneration of the bone. Bioactive glass has the ability to bind to both, the bone and soft tissues; however, it has been mainly studied in orthopedic applications as it promotes the formation of bone-like apatite layer. Bioactive glass has been reported for the adhesion and proliferation of osteoblast cells like MC3T3-E1 and MLO-A5. Furthermore, bioglass undergoes dissolution creating ions such as Si, P, Ca, Cu which are involved in enhancing the proliferation of osteoblast cells and in turn the formation of bone [5]. The bioactive behavior of HA may be increased on adding Si during its synthesis. In case of bioactive glass, the silanol groups work as catalysts leading to the nucleation of apatite phase in order to form apatite layers on the surface.

2 Methods of Synthesis 2.1

Melt-Quench Synthesis

Melt-quenching technique involves the fusion of a mixture of precursors (usually oxides) followed by quenching. The generalized scheme for the melt-quench synthesis of bioactive glass is shown in Fig. 2. Typically, the precursors are mixed in

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appropriate ratios followed by grinding in order to break the agglomerates so as to obtain fine powder mixture and further subjected to ball milling in the presence of media to reduce the particle size. The resulting fine powder is melted at high temperature (1100–1650 °C) usually in a furnace. The mixture is usually kept for an hour at the melting temperature in order to get the homogenous material. The molten material is usually poured into the preheated specific molds followed by quenching with cold water, liquid N2, etc. [6]. The simplistic melt-quench method was adopted by Vitale-Brovarone et al. [7] for the development of bioactive glass-ceramic scaffolds having the mechanical strength comparable to that of cancellous bone (2– 12 MPa). The molar composition of the glass-ceramics made by them was: 45% SiO2, 26% CaO, 15% Na2O, 7% MgO, 4% K2O, and 3% P2O5. They mixed the precursors and melted them in a platinum crucible at 1400 °C for 1 h followed by quenching in cold water which resulted in a frit. It was then subjected to ball milling followed by sieving, and the final grain size obtained was below 30 µm. The glass transition temperature of the glass-ceramic was found to be 550 °C, and it had two crystallization temperatures, i.e., 600 and 800 °C. They used the bioactive glass-ceramic to develop 3D scaffolds using sponge-impregnation (polyurethane open-cell sponge was chosen as a template) method followed by its thermal treatment. The resulting scaffolds were found to have a trabecular structure along with highly interconnected pores. The scaffolds were found to have a pore content of 50 vol% along with mechanical strength as 4 MPa and good bioactivity. They were also able to achieve the scaffolds with low porosity and high mechanical strength (5 MPa) which would be suitable for bone grafting applications. The developed materials were suggested to be effective materials for bone substitution. Melt-derived technique has been widely used for the synthesis of bioactive glass which may be obtained in bulk, granular, or fiber form. Bioactive glass fibers made using

Fig. 2 Generalized scheme for melt-quench synthesis of bioactive glass

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melt-derived approach are often in the range of hundreds to tens of micrometers (diameter) and have the mechanical strength superior to the glasses in bulk form. The drawback associated with the preparation of fibers using melt spinning is the inability to form it into nanoscale level due to the limitations of the process.

2.2

Sol–Gel Method

Sol–gel method has several advantages over melt-derived, including the bioactivity and ability to process the material to nanoscale level. The bioactive glasses in the nano-range can be exploited for various biomedical applications including biomembranes, biocatalysis, biosensors along with having an impact on the regeneration of the diseased tissue when used as a reinforcement. In line to this, Kim et al. [8] have synthesized bioactive glass nanofibers using sol–gel method via electrospinning. They prepared a solution of bioactive glass with composition 70SiO2.25CaO.5P2O5 which was electrospun and thermally treated. The concentration of the sol affected the fiber diameter; i.e., the diameter of the fiber was found to reduce with the concentration of sol, and it was thus possible to control the diameter of the fiber using electrospinning in order to make the fibers with specific pore size and diameter for the targeted needs. The nanofibers (220-nm average diameter) were tested for their biocompatibility using bone marrow-derived stem cells and found to be bioactive along with osteogenic potential which may be considered as the most promising candidates for bone regeneration applications. Ravarian et al. [9] had synthesized bioactive glass using sol–gel method with composition 64 mol% SiO2, 31 mol% CaO, and 5 mol% P2O5. The typical method for synthesizing bioactive glass involved the preparation of tetraethyl orthosilicate (TEOS) solution in nitric acid and allowing for acid hydrolysis of TEOS. This was followed by the addition of triethyl phosphate and calcium nitrate tetrahydrate, thereby casting the prepared solution in cylindrical containers. They kept the solution at room temperature for 10 days in order to form the gel and heated it to 70 °C for 3 days followed by heating at 120 °C for 3 days and then at 700 °C for 24 h. Later, the bioactive glass (wt%) was mixed with HA in different ratios (wt%). The mixture was ball milled, followed by suspending in distilled water and stirring to obtain a slurry into which the polymeric foams were soaked and dried at ambient temperature. The samples were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, and scanning electron microscopy (SEM) after soaking the samples in simulated body fluid (SBF) to determine the primary phases of bioactive glass, functional groups, and formation of the apatite layer, respectively. They found that the presence of bioactive glass had an influence on the structure of HA and led to the formation of silicate HA with improved bioactivity. The formation of bioactive glass usually requires the utilization of sodium and calcium nitrate along with nitric acid which are highly soluble, decompose easily by application of heat, and are available at low cost [10]. The sintering is usually done above 600 °C which is essential to remove the by-products which are toxic to

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the respiring cells [11]. Such high temperature usually leads to the formation of bioactive glass with a crystalline phase as the crystallization temperature of 45S5 Bioglass® is in the range of 610–800 °C. The crystallization of bioactive glasses leads to a reduction in their bioactivity, and in order to prevent this, Rezabeigi et al. [12] proposed an organic, nitrate-free method to synthesize 45S5 Bioglass® by sol– gel process. The precursor TEOS was dissolved in ethanol followed by adding into dilute lactic acid and stirred until a clear solution was obtained. This solution was added to the mixture of triethyl phosphate (TEP) and lactic acid which in turn was added to the mixture of calcium lactate pentahydrate and lactic acid. The obtained solution was further added to the mixture of sodium lactate and lactic acid. The resulting transparent solution was left at room temperature for 45 days. The pH of the sol was maintained at 1–2 by controlling the amount of lactic acid. The obtained gel was dried at 170 °C for 4 days under humid conditions. The dried gel was subjected to thermal analysis which revealed the removal of residual organics and water below 550 °C and crystallization temperature at 614 °C. They selected the temperature as 550 °C for the stabilization of bioactive glass and the heating rate of 0.4 °C/min. Thus, they adopted a straightforward and inexpensive route for the synthesis of 45S5 Bioglass® which was completely amorphous in nature. The process is flexible which yields the glass with submicron size and homogenous nature along with high surface area (11.75 m2/g). The formulation of 45S5 Bioglass® is found to exhibit high bioactivity, and the sol–gel technique often leads to the development of glasses with higher rate of HA formation. Thus, the synthesized glass may be considered as a promising candidate for the scaffolds for regenerating the diseased tissues. The generalized scheme for sol–gel synthesis of bioactive glass is shown in Fig. 3.

Fig. 3 Generalized scheme for sol–gel synthesis of bioactive glass

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73

Microwave-Assisted Synthesis

The conventional melt-quench method is suitable for the massive production of bioactive glasses; however, the volatile component (P2O5) tends to escape at the elevated temperature. Bioactive glasses can also be synthesized using microwave energy irradiation. The sol–gel method offers the advantage of lower reaction temperature, homogenous composition of particles, suitable for hard tissue regeneration. However, the precursor materials used for its synthesis are usually expensive. Therefore, the fabrication of bioactive glass using microwave-assisted synthesis route was executed by Sarkar and Byong [13] in order to reduce the cost of the process/method. The precursors used by them were calcium nitrate tetrahydrate, sodium silicate solution, and diammonium hydrogen phosphate for the Ca, Si, and P sources, respectively. They dissolved the precursors in deionized water followed by ultrasonication. This was followed by microwave-assisted synthesis for 15–25 min and washing the resulting powder with deionized water and filtering. The synthesized powder was then subjected to drying at 80 °C for 24 h followed by calcination at 700 °C. The powders were characterized to determine its structure and morphology along with in vitro studies to determine the apatite layer formation. The synthesized powder was found to be amorphous in nature which retained the glass phase after calcination. Further, immersing the bioactive glass in the SBF led to the formation of apatite after 3 days which confirmed the biocompatibility of the glasses.

3 Bioactive Glasses in Bone Tissue Engineering 3.1

Mechanism of Bone Formation

Bioactive glass after implantation to the site reacts with the physiological fluids and leads to the formation of hydroxyl carbon apatite (HCA) layer which is similar to the mineral component present in the natural bone. The mechanism of bone formation is shown in Fig. 4 with different stages thereof. The bond formation begins with the exchange of alkali ions of bioactive glass with the hydrogen ions from the physiological fluids leading to the hydrolysis of silica groups. This is followed by the dissolution of silica upon increase in the concentration of hydroxyl groups at the surface of bioactive glass. This leads to the formation of silanol groups (Si–OH) that condense to form a layer of silica gel at the surface of bioactive glass [14]. The calcium and phosphate accumulate from the physiological fluid as well as the bulk of bioglass and form a layer of CaO–P2O5 on the surface of silica which is further accumulated by hydroxyl and carbonate ions leading to the formation of HCA [15, 16]. Bioactive glass being structurally and chemically similar to HA leads to the adsorption of growth factors which, in turn, activate the M2 macrophages [17]. This in turn triggers the osteoprogenitor cells and mesenchymal stem cells to migrate and attach to mineral layer and differentiate to form osteoblasts. The osteoblasts then allow the generation and deposition of ECM followed by bone mineralization.

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Fig. 4 Mechanism of bone formation

The bioactive glass thus dissolves and leads to the formation and growth of bone by depositing new ECM.

3.2

Types of Bioactive Glasses

Various types of bioactive glasses can be fabricated for different applications which have varied composition. The parameters affecting the rate of dissolution of bioactive glasses and their ability to form apatite layer are: 1. Composition of bioactive glasses 2. Morphology 3. Technique used to fabricate the glass.

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Table 1 Commercially available bioactive glasses with their composition Bioactive glasses (commercial)

Composition SiO2 Na2O

CaO

P2O5

SrO

MgO

CaF2

K2O

Origin

NovaBone

46.1

24.4

26.9

2.6

–

–

–

–

BonAlive Cerabone

53.8 34.0

22.7 –

21.9 44.7

1.7 16.2

– –

– 4.6

– 0.5

– –

45S5

46.1

24.4

26.9

2.6

–

–

–

–

13-93

54.6

6.0

22.1

1.7

–

–

–

–

StonBone

44.5

27.2

21.5

4.4

2.4

7.7

–

7.9

TheraGlass

70

–

30

–

–

–

–

–

NovaBone, LLC, Jacksonville, Florida Turku, Finland Straumann Group, Switzerland Mo-Sci, Rolla, Missouri Mo-Sci, Rolla, Missouri RepRegen Ltd, London, UK MedCell Bioscience, Cambridge, UK

It may be imperative to choose the bioactive glasses based on the ability of apatite formation which is highly dependent on the above-stated parameters. The commercially available bioactive glasses with their composition are shown in Table 1. The patients (over 1 million) have been implanted with NovaBone™ which is known for its use as an osseous defect filler and the capability of inducing new bone formation. In order to determine the apatite forming ability of implants, an ISO standard (ISO/FDIS 23317) has been put forward. However, the standard has some limitations; i.e., the samples are compared wherein the surface area-to-volume ratio is fixed and the test is static. The test is ideal when comparing the samples having different compositions, but it becomes difficult to compare the samples that have the same composition but differ in their morphology (fibers, particles, granules). The comparison of glasses made from sol–gel and melt-derived techniques would not be possible as the bioactive glasses made from these techniques have highly different surface areas. Furthermore, the in vitro conditions do not exactly mimic the in vivo environment and thus the in vitro studies may also not be regarded as the ideal tests. In this regard, Maçon et al. [18] developed a facile test by making modifications in the existing ISO standard in order to compare the rate of dissolution along with apatite formation. The test was named as TC04 method after being accepted by Technical Committee 4 (TC04) of the International Commission on Glass (ICG). In order to validate the test and ensure its reproducibility, it was conducted in eight different laboratories in seven countries. They used the commercial bioactive glass samples as mentioned in Table 1. The particle sizes for all the samples were in the range of 45–90 µm except for NovaBone. They conducted the bioactivity tests in the

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SBF which was prepared by adopting Kokubo’s method. The tests were performed according to TC04 method as well as ISO/FDIS 23317. The limitation of ISO/FDIS 23317 method was its restriction to regular, geometric shaped samples including disks and tiles. Conversely, the TC04 method used the fixed mass of bioactive glasses in SBF and a constant surface area was thus utilized. Typically, the bioactive glass powders were immersed in SBF (75 mg/50 ml) and agitated in an orbital incubating shaker at 120 rpm at 37 °C. The samples were incubated for different time points, and after each time point, the samples were filtered, washed, and collected to determine the concentration of ions using inductively coupled plasma (ICP) along with SBF as a control. The formation of HCA was found to be different for varied compositions of bioactive glasses which was dependent on their network connectivity. The increased content of silica led to the increased network connectivity. They concluded the proposed method was laid appropriate for the bioactive glass particles and those with high surface area. It is essential for the implants to form an interfacial bond with the living tissue in order to function as an ideal material for bone repair/growth. Hench and Ethridge expressed the criterion for biomaterials for bone fixing which described the materials to be equivalent to the host tissue being able to form a stable interfacial bond between the host tissue and itself and its ability to respond to the external stimuli (physical) in the same manner as that of the host tissue. Additionally, the mechanical and physical properties of biomaterials are of prime importance apart from its bioactivity. For bone repair and regeneration, the major problem often accounted is that of stress shielding which is mainly due to the higher Young’s modulus of the implant carrying all its load (or) leading to the improper distribution of load. Thus, it is important to design materials that would bind with the natural tissues and withstand the mechanical loads when implanted into the body.

4 Applications: Case Studies The cells communicate with the biomaterials at the nanoscale, and various properties such as the chemical composition, surface area, hydrophilicity, and topography affect the interaction of cells with the biomaterials. In this regard, Leite et al. [19] have synthesized bioactive glass nanoparticles using sol–gel method and further incorporated strontium with the aim of improving their biological outcome. They used endothelial (HUVECs) and osteoblast-like cells (SaOS-2) which are known to affect the physiological bone regeneration for conducting the biocompatibility studies. The nanoparticles were found to be bioactive in nature, and the bone mineralization studies showed the formation of apatite layer upon immersion in SBF which was confirmed by energy dispersive spectroscopy (EDS). The bioactive glass nanoparticles incorporated with strontium were found to promote the angiogenic phenotype of HUVECs. Both the nanoparticle formulations exhibited an adequate support for osteogenic differentiation.

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4.1

77

Bioactive Glass Hybrids

While fabricating materials for bone tissue engineering applications, it is important to design the scaffolds that are biodegradable and their rate of degradation is similar to the rate of new bone where the newly formed bone would eventually replace the scaffold. The mechanical integrity of the scaffold must be sufficient to support the bone regeneration process. However, it is difficult to combine all these properties in a single material, and therefore the composites are often considered to be advantageous while fabricating scaffolds. The conventional composite materials consist of phases that are distinct at macroscale and thus are not suitable as scaffold materials that require the homogenous phases at molecular level along with uniform mechanical, physical, chemical, and biological properties. In line to this, Mondal et al. [20] demonstrated the possibility of using hybrid biomaterials consisting of organic and inorganic components for bone tissue engineering applications due to the molecular-level interactions among them leading to the formation of single-phase materials. They prepared class II hybrid biomaterials consisting of tertiary bioactive glasses as inorganic phase covalently cross-linked to organic polymers. Firstly, they synthesized the copolymers of vinylpyrrolidone and triethoxysilane in varying molar ratios so as to vary the amount of silane groups in the copolymer. Thereafter, tertiary bioglass was mixed with the copolymer in order to allow for the hydrolysis and polycondensation leading to the formation of Si–O–Si and Si–O–P networks between the two phases. They found that the covalent bonding between copolymer and bioglass was increased with the increasing number of functional groups, whereas the degradation was decreased along with the decrease in the dissolution products of bioglass. Incorporation in SBF led to the formation of apatite layer on the surface of materials which was found to be dependent on the weight ratios of organic and inorganic materials. Further, they reported that the increased content of bioglass led to the improved deposition of apatite layer along with the improved biocompatibility of the fabricated hybrid materials. Furthermore, Allo et al. [21] chose the combination of biodegradable polymers, i.e., poly(e-caprolactone) (PCL) and bioactive glasses in order to make hybrid biomaterials with the aim of tailoring the physical, mechanical, biological, and degradation behavior of the resulting materials. The combination of inorganic and organic materials in different ratios was synthesized using the sol–gel process. In a typical sol–gel process, the hydroxyl groups are formed by the hydrolysis of metal alkoxides which is followed by the formation of a three-dimensional network due to the polycondensation reaction. The stiffness and the brittleness of bioactive glasses restrict their formation into the complex shapes and promote their fracture under loads. On the other hand, the flexibility of PCL along with its biodegradable and biocompatible nature allows its easy processing into complex three-dimensional structures. The properties of the two materials being complementary to each other were speculated to be favorable in developing materials with desired toughness, biocompatibility, bioactivity, and predictable degradation and thus were explored for their potential as three-dimensional scaffolds for bone regeneration which were fabricated by electrospinning. The polymer chains

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(dissolved in “methyl ethyl ketone” which is water miscible and has the ability to dissolve inorganic materials) were introduced into the sol during the formation of inorganic network in order to allow for the chemical interactions between the polymer chains and bioactive glass in order to form homogenous hybrid materials. The intermolecular interaction between the organic polymer chains and the inorganic bioactive glass was determined by FTIR which confirmed the molecular interaction of the two phases by H bonding between the silanol hydroxyl groups of glass and the carbonyl group of PCL. The developed hybrids were found to have immense potential in bone tissue engineering applications.

4.2

Hyperthermia Treatment

The elemental composition and their amount in the bioactive glass affect the behavior as well as the bioactivity of the glasses. The properties of the bioactive glasses can be controlled by varying the elements present in its structure. Several researchers have made an effort to fabricate bioactive glasses with tailored characteristics such as angiogenesis capacity, mitochondrial activity, anti-inflammatory activity, and magnetic bioactivity for various applications including chronic wound healing, reconstruction of bone defects, and tumor treatment. In a similar fashion, Koohkan et al. [22] have synthesized mesoporous bioactive glasses (MBG) by sol–gel method for antibacterial applications and hyperthermia treatment by incorporating Fe and copper oxide (CuO) into the glasses. They subjected the synthesized bioactive glasses to various characterizations in order to identify their structure, morphology, bioactivity, and biocompatibility. The glasses were found to have a good surface area with magnetic properties and nanoparticle morphology. The development of alkaline semi-apatite layer was confirmed by alkaline phosphatase assay. Further, MTT assay confirmed their biocompatible nature. Bioactive glass incorporated with iron and Cu was found to have superparamagnetic property along with no angiogenesis property. Further, the in vitro studies indicated the potential of the magnetic bioglass incorporated with copper in hyperthermia treatment of bone defects.

4.3

Large Bone Defects

Although tissue engineering has made its impact in treating the several bone-related diseases and disorders, the treatment of large bone disorders is still a challenge for the clinicians. The major causes of such disorders could be trauma, infections, accidents, tumors, etc., and the major issues often witnessed in their treatment are the accelerating bone formation and bone healing. Furthermore, it is important to track the degradation products from the scaffolds that would possibly accumulate in the tissues and organs in order to identify the degradation mechanism of the biomaterials. In this context, Wu et al. [23] developed the biofunctional materials for

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biolabeling by incorporating europium (Eu) into the bioactive glass scaffolds (in situ) using polyurethane sponge co-templates. Eu is also known for its ability to bind with HA [24] along with pro-angiogenic properties [25, 26]. The scaffolds thus fabricated were found to have luminescence property that allowed the labeling of bone marrow stromal cells (BMSCs) in vitro as the degradation of scaffolds occurred by releasing the ionic products which influenced the luminescence change of the scaffolds. Further, in vivo studies were conducted by using the ovariectomized rat models (OVX) for the reconstruction of the femoral defects and inducing bone defects (3.5-mm diameter) in the distal region of femur. The bone defects were filled randomly with MBG along with Eu-incorporated MBG (2 and 5 mol%), and the rats were sacrificed after 4 and 8 weeks, observed under micro-CT, and evaluated for histological analysis. The bone formation was seen to be higher along with higher amount of bone formation area for the bioactive glasses with Eu content as compared to MBG alone. Thus, the scaffolds with Eu showed the improved bone formation by promoting the osteogenic differentiation of BMSCs suggesting their potential for biolabeling and regeneration of bone.

4.4

Bioactive Glass Hydrogels

Bioactive glasses are often used for the hard tissue regeneration due to their brittle nature and poor water uptake capacity. In order to tailor the properties of bioactive glasses to exploit their potential in soft tissue regeneration, Shirazi et al. [27] fabricated the bioactive glass by cross-linking with gelatin methacrylate (GelMA) during the sol–gel synthesis. With the speculation that the polymer cross-linking to the bioactive glass network would control the condensation of silica network and in turn reduce the brittleness of the glass, they functionalized GelMA with glycidoxypropyl-methyldiethoxysilane (GPTMS) before forming the polymer–glass hybrid. The concentration of organic and inorganic compounds was optimized in order to tailor the physicochemical, mechanical, and degradation behavior of the hybrids. The homogenously distributed bioactive glass in the polymer network led to the formation of hydrogels with superelastic behavior with high strength, high water uptake, biocompatibility, and osteoconductivity. The osteoblast proliferation along with the bone-specific enzyme secretion was enhanced by the precipitation of Ca–P particles on to the surface of hybrids. These properties make the organic–inorganic hybrids as potential candidates for engineering the damaged tissues and the complex tissue interfaces.

4.5

Osteosarcoma Treatment

Osteosarcoma is a type of bone cancer that leads to the formation of immature bone and has affected a large number of people. The treatment methods such as radiation

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or chemotherapy are ineffective in curing the disease, and it is often difficult for the patients to survive. With the aim of developing safe and effective biomaterials for the clinical cure of osteosarcoma, Rana et al. [28] developed gallium-doped bioactive glasses. Gallium, which is known for its effective use against cancer [29, 30], was doped during the melt-quench process in different ratios followed by producing glass rods which were cut into disks. The synthesized materials were characterized to identify their structure, morphology, biocompatibility, and formation of apatite layer. The human osteosarcoma cells (Saos-2) were used to determine the viability of the cells in the glasses in the presence and the absence of gallium. It was observed that the viability of the tumor (osteosarcoma) cells was reduced to *50% in the presence of gallium containing bioactive glasses which was further dependent on the dosage. Additionally, no adverse effects were observed on the cell growth in case of normal human osteoblast cells. The HA formation was observed for all the glasses (those containing gallium and control). Furthermore, the glasses doped with gallium preferentially led to the localized delivery of gallium ions at the targeted site and thus have a substantial potential in the treatment of bone cancer.

4.6

Electrospun Scaffolds

The composition of the bioactive glass along with its structure and hierarchy are the parameters affecting the bone regeneration. Therefore, the scaffolds that mimic the host tissue structure would be able to recruit cells followed by their attachment, proliferation, and differentiation. The electrospun scaffolds mimic the ECM of the bone, thereby providing the bioactive cues for the bone repair/regeneration. The angiogenesis and osteogenesis may be improved by doping the trace elements. In line to this, Weng et al. [31] doped Sr and Cu into the bioactive glass in order to enhance the osteoblastic phenotype and improve the bone vascularization, respectively. They prepared the silica sol mixture of bioactive glass doped with Sr and Cu along with poly(vinylpyrrolidone) solution and mixed the two solutions in 1:1 ratio prior to electrospinning, and they further coated the mandrel with PCL in order to allow for the easy removal of glass fibers. The polymers were then removed from the electrospun fibers by sintering the fibers at 600 °C for 5 h. The apatite layer formation was higher for nanofibers doped with Sr as compared to that with Cu. Furthermore, the release of Cu2+ and Sr2+ ions was seen over a period of 30 days which could be tuned by the individual contents of Cu and Sr. The in vitro studies (determined from multiple cell lines) revealed that the nanofibers doped with Sr promoted the osteogenesis along with suppressing osteoclastogenesis (osteoclasts are responsible for the resorption of bone), whereas the angiogenesis was enhanced by the nanofibers doped with Cu. The developed nanofibers doped with Sr and Cu were found to be promising materials for bone tissue regeneration.

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Surface Functionalization

Another important way of tailoring the properties of bioactive glass is by surface functionalization in order to improve the contact of the implants with that of bone and promote the bone regeneration process. A combination of cellulose nanocrsytals (CNC) and 45S5 bioactive glass was used by Chen et al. [32] in order to fabricate hybrid coating which was electrophoretically co-deposited onto 316L stainless steel plates. The bioactive glass was found to be wrapped with CNC (nanoporous) layer. CNC acted as a template (in situ) for tuning the morphology and mineralization of HA. Incubation in SBF led to the formation of needlelike structures on the coating. In vitro studies were conducted using MC3T3-E1 cell line which was found to adhere, proliferate, and differentiate onto the surface of hybrid coating. The electrophoretic deposition was found to be an effective method to develop configurations of coating with tailored characteristics (mechanical and ion release) for smart bone-contacting materials.

5 Summary The current chapter highlights the discovery of bioactive glasses followed by different fabrication techniques for its synthesis such as melt-quenching, sol–gel, and microwave-assisted synthesis. The mechanism of bone formation has been presented which explains the steps involved in the formation of apatite layer on the surface of bioactive glass upon reaction with the physiological fluids. Various types of commercially available bioactive glasses have been remarked followed by the testing method of glasses made from different routes. Furthermore, several case studies involving the utilization of bioactive glasses in hyperthermia treatment, electrospun scaffolds in bone regeneration, bioactive glass hybrids and hydrogels for bone tissue engineering, osteosarcoma treatment, etc., have been explained with a significant outcome. The potential of bioactive glasses in bone tissue engineering is thus commendable with significant scope in engineering various tissues along with the treatment of bone-related diseases.

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Chapter 5

Biomaterials for Biomedical Devices and Implants Devarshi Kashyap, Vaibhav Jaiswal and Subramani Kanagaraj

Abstract India is currently importing different types of medical implants and devices, which cost about 0.65 million USD. Most of the imported implants and devices are found to be expensive and not suitable for Indian patients because of their design limitations. Thus, it is very much essential to develop indigenous devices at an affordable cost without compromising their functional activities. The chapter will discuss the design and development of patient-specific biomedical devices and implants as per ISO/ASTM standards in order to meet their individual requirements. In the specified area, the products being developed by our research team have been kept under two different categories namely implants and biomedical devices. Under the implant category, the products such as (i) ultrahigh molecular weight polyethylene-based acetabular cup, (ii) 3D printed shape memory polyurethane-based aneurysm coil, and (iii) different types of cerium-based anti-scavenging materials to absorb the excess reactive oxygen species (ROS) to preserve the residual hearing after cochlear implant fixation are being developed and tested. In case of biomedical devices, prosthetic and orthotic devices such as (i) polymer-based polycentric knee joint, (ii) dynamic foot, (iii) custom-made ankle-foot orthosis, (iv) suction and suspension incorporated socket for lower limb amputees, and (v) direct socket fabrication system are being developed and trialled. A discussion on the different materials including polymers, ceramic, and composites which are used to improve the performance of the above-discussed biomedical devices and implants will be deliberated in details. Keywords Shape-memory polymer Orthosis Knee joint





Aneurysm
Lower limb
Prosthesis


D. Kashyap
V. Jaiswal
S. Kanagaraj (&) Department of Mechanical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_5

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1 Introduction The global medical device market has been consistently growing due to the rapidly increasing population and consumer awareness about various medical procedures and preventive cares associated with a better and longer life. As per World Health Organization (WHO) data for the year 2015–16, more than 74% of required medical devices in India worth 2.87 billion USD are being imported. The medical device field is a technology and research-intensive field which is governed by stringent approval regulations creating a hindrance in the introduction of new implantable medical devices to the market. However, the introduction of more promising substitutes to traditionally used materials or improving the design of the presently established device is viable and easier to bring the devices to the market. Further, the indigenously produced devices will not only be cheaper but also better suit the needs of our countrymen and improve the economy of our country. The Biomedical devices and Biomaterials laboratory, IIT Guwahati is currently working on improving the devices associated with the presently used biomedical devices and implants after receiving guidance and feedback from the medical fraternity. In this direction, the product being developed by our research team has been kept under two different categories namely implants and biomedical devices. Under the implant category, the products such as (i) ultrahigh molecular weight polyethylenebased acetabular cup, (ii) 3D printed shape memory polyurethane-based embolization implants, and (iii) different types of cerium-based anti-scavenging materials to absorb the excess ROS to preserve the residual hearing after cochlear implant fixation are being developed and tested. In case of biomedical devices, prosthetic and orthotic devices such as (i) polymer-based polycentric knee joint, (ii) dynamic foot, (iii) custom-made ankle-foot orthosis (AFO), (iv) suction and suspension incorporated socket for lower limb amputees, and (v) direct socket fabrication system are being developed and trialled. This chapter gives a detailed overview of the endovascular implants and lower limb prosthesis that are being developed in our laboratory. Biomaterial is defined as a substance that has been engineered to take a form which, alone or as part of a complex system, is used to direct and control the interactions with components of living systems, the course of any therapeutic or diagnostic procedure. An implant material should be biocompatible and should not produce any harmful or toxic effect in the living tissue or body, as such, there are a very limited number of materials which can be used as implants. Since the early 1960s, ultrahigh molecular weight polyethylene (UHMWPE) has been used as a major bearing surface material in total hip and total knee joint prostheses. However, wear debris produced from UHMWPE causes adverse tissue biological response such as gradual bone absorption known as osteolysis, which results in loosening of the prosthesis and discomfort to the patient, eventually leading to revision surgery. Therefore, to ensure the long-term success of the prosthesis and prevent revision surgery, the wear resistance of the alternative materials under in vivo condition must be improved. Wang et al. [1] reported that the use of carbon fiber-reinforced polyether ether ketone (PEEK) composite acetabular in articulation with a zirconia

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ceramic head, which was found to be an excellent bearing combination. Zoo et al. [2] reported that the hardness and wear resistance of UHMWPE were increased by 12% and 80%, respectively, at 0.5 wt% of multi-walled carbon nanotubes (MWCNT). Wang et al. [3] reported the reduction of 50% wear rate for the UHMWPE/Ti composite cup against 316 L stainless steel in simulated body fluid. Liu et al. [4] prepared UHMWPE/bovine bone hydroxyapatite (BHA) composites and tested for their biomedical applications. Ge et al. [5] studied the wear behavior of acetabular cup made of UHMWPE/Natural coral against CoCrMo balls and reported about 70% reduction in wear loss. Plumlee et al. [6] tested UHMWPE/Zr composites and reported up to 36% of wear volume reduction in comparison to that of unfilled UHMWPE [6]. Borruto [7] explored an alternative bearing surface made of PEEK/ carbon nanofibers, where low quantity of wear debris was generated irrespective of lubricants used. Wood et al. [8] reported that the wear rate of UHMWPE was decreased by 57% with the reinforcement of carbon nanofibers. Tai et al. [9] developed UHMWPE-graphene oxide composites, where the yield strength of composite is reported to be increased by 20% at 0.5 wt% of graphene. The reinforcements as discussed above can effectively bear load to protect the polymer matrix and thus improve the wear resistance of virgin UHMWPE, which can be used as an alternative material for the development of a next-generation acetabular cup. The trauma caused during cochlear implantation triggers the formation of ROS, which is essential to fight against inflammation. However, when produced in excess, it causes damage to the hair cells resulting in loss of residual hearing. To counter this loss, antioxidants are introduced at the site of insertion during surgery. Conventional antioxidants such as vitamin E, vitamin C, amifostine, Trolox, and curcumin have not been found 100% effective due to their short degradation time, poor uptake by cells, high dose requirement, and limited scavenging of only one type of ROS [10– 12]. On the other hand, ceria nanoparticles have been found to suppress the excess generation of ROS and associated damages nine times better than Trolox [13]. In addition, ceria nanoparticles can scavenge more than one type of radicals and are believed to provide long-term protection owing to its ability to regenerate. Radical scavenging capabilities of nanoceria are due to its ability to switch between Ce3+ and Ce4+ states, which can be improved either by reducing its size or doping with elements of different valency. A fourfold increase in ROS scavenging has been reported by ceria–zirconia (Ce0.7Zr0.3O2) solid solution compared to pure ceria [14]. Gadolinium infused nanoceria, Gd-CeNPs, has been found to show radical scavenging capabilities as well as an MRI contrast enhancer [15].

2 3D Printed Embolic Agent for Endovascular Embolization A vast majority of the world population (4.8%) suffers from vascular malformations like aneurysm [16]. Aneurysms are ballooning or bulging of the blood vessel due to its weakening of the wall. The primary principle behind the treatment of these

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conditions is to prevent the flow of blood to the diseased part so as to avoid bursting of the vasculature and thus, prevent hemorrhage. These conditions are treated either by open surgery, where the surgeon clips the vasculature to prevent flow of blood or by interventional radiology techniques, where an embolic agent is sent to the diseased part through the vasculature with the help of a catheter which is monitored by the help of fluoroscopy method such as X-ray. The advantages of interventional techniques are decreased mortality rate, risk, pain, and hospital stay for the patients [17]. A number of embolic agents such as platinum coils, N-butyl cyanoacrylate (NBCA), Onyx® which is ethylene vinyl alcohol (EVOH) copolymer dissolved in dimethyl sulfoxide (DMSO), polyvinyl alcohol (PVA) microspheres, and tris-acryl gelatin microspheres are presently being used for endovascular embolization. However, the above-mentioned agents have many advantages as well as limitations [18]. Further, the sustainability of the above implants has not yet been studied. PVA has been identified as sustainable polymeric material because of its biodegradability. EVOH is a copolymer of ethylene and vinyl alcohol and is used in packaging worldwide for its superior barrier properties which have been approved by the FDA. EVOH is one of the dominant materials that conform to strict international emission standards for fuel systems in vehicles to prevent the harmful fuel vapor permeation from fuel tanks and fuel pipes from escaping to the environment. Further, it is recyclable and when burnt, primarily releases carbon dioxide and water and is much more sustainable than other alternatives such as polyvinylidene chloride (PVDC). Calcium alginate is a new material that has shown great potential as an embolic agent. Calcium alginate is produced from alginic acid which is a natural polysaccharide gel derived from brown algae. Alginic acid can be ionically cross-linked with non-toxic divalent cation solution such as calcium chloride to form a stable alginate gel. However, further studies are going on to investigate the suitability of calcium alginate as an endovascular embolization implant. Additive manufacturing has the potential for being a sustainable manufacturing process for a sector like medical implants where customized single or small batches of goods are to be fabricated. The flexibility in product design, increasing material and energy efficiency by reducing wastage, and decreasing energy consumption, repairability of the product and recyclability of material puts a strong case for the sustainable nature of the process [19]. As such, the Biomedical Devices and Biomaterials Laboratory at IIT Guwahati has been working on developing a 3D printed porous radiopaque shape memory polyurethane-based implant as an embolic agent for endovascular embolization, which can overcome the problems associated with the presently used products. The shape memory polymer (SMP) can be compressed and sent with a smaller footprint through a catheter to the desired location, and it will recover to its original dimension upon reaching the desired site. The porous nature of the embolic agent is preferred to increase the volumetric expansion of the SMP and to increase the surface area for physiological responses and cell proliferation which would have better healing and embolization. The advantage of using 3D printing technology includes the patient-specific embolic agent, which can be fabricated in a shorter time at an affordable cost. The patient-specific digital 3D model of the aneurysm or the diseased vasculature can be generated from the X-rays, CT scans, MRI, etc.

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SMPs are smart materials which can remain in temporary shape under certain conditions and recover to their original shape when an external stimulus is applied. In our case, a thermally responsive SMP having a switching temperature of 35 °C is used. In this regards, SMP MM3520, which has a switching temperature nearer to normal human body temperature, is used. The SMP can recover to its original shape from the body temperature and avoid the need of an external stimulus. The 3D printing technology-based fused filament fabrication (FFF) is also known as the fused deposition modeling (FDM), which can be used for various applications because of its low cost, high reliability, and ease of operation. In this technique, a thermoplastic polymer filament is extruded through a nozzle to get the desired shape. As the production of porous structure below 250 µm is very difficult with FFF technique, salt leaching technique can be incorporated along with FFF. Sodium chloride (NaCl) is used as a porogen to generate porous structure. On the other hand, tungsten particles are added to the SMP to increase its radiopacity and explore their potential applications in endovascular embolization techniques. The salt and tungsten are incorporated in the SMP during the filament fabrication for the FFF process. The NaCl particles (diameter 55% PHA [92]. These kinds of nutrient limitations cause filamentous and non-filamentous bulking. The merits associated with the

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Fig. 3 Methods of PHA production from lignocellulosic and non-lignocellulosic biomass

cultivation of the mixed cultures are its economic feasibility, process simplicity, without monostatic processing, and efficient use of microbial flora naturally occurring waste sludge [86, 87]. Polyphosphate-accumulating organisms (PAOs) and the glycogen-accumulating organisms (GAOs) can be grown in the mixed culture conditions [90]. The GAOs can compete with PAOs and are substrate dependent which stores the products inside the cell during the fermentation process. These are capable of converting the stored glycogen to PHB [93]. In anaerobicaerobic processing, PHAs plays a key role in the ecophysiology when there is a separation in the electron donor and acceptor availability in PAOs [94]. The factors which can affect the PHA production are carbon source, pH (6.5–8), C/N, and the temperature. The increase in temperature decreases the accumulation. The uptake of acetate by microorganisms can produce the copolymer of hydroxybutyrate and hydroxyvalerate with dominating hydroxybutyrate content of *60 to 100% [94]. The general process of PHAs production is shown in Fig. 3.

2.5

Characteristics of PHAs

PHAs are thermoplastic in nature and vary in their property depending on the chemical compositions. They are resistant to moisture and possess aroma barrier property. They are also brittle and stiff, which are insoluble in water and soluble in chloroform, resistant to hydrolytic degradation, good ultraviolet resistance, poor resistant to acid and bases, comparatively less sticky than traditional polymers, most importantly they are biocompatible and non-toxic, and therefore are potential candidates in medical industries. Though the characteristics of the PHAs produced by the pure cultures are studied widely, there is less data available for the PHAs produced by the mixed cultures. The homopolymer of PHB is having a crystallinity *55 to 80% with low impact strength and high brittleness [95]. These are having the glass transition temperature (Tg), melting temperature (Tm), and cold

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crystallization temperature (Tcc) from −1 to 10 °C [96, 97], 175 °C, and 63.4 °C, respectively. The elongation to break is *5% [98]. In contrast, the industrial copolymers such as poly (3HB-co-3HV) are having the better properties such as the improved mechanical properties because of the improved toughness, impact strength, and flexibility changes occurred due to the incorporation of the hydroxyvalerate units in the hydroxybutyrate polymeric chain [99]; similarly, the crystallinity of the poly(3HB-co-3HP) decreases with increasing the fraction of the 3HP units [100]. The weight average molecular weight of the PHAs ranges from 253 to 696 kDa produced using the whey and glycerol liquid phases, respectively, with the polydispersity index (PDI) of 2.2 and 2.7 [97], and the molecular weight can go up to *1000 kDa. The crystallinity of scl-PHAs and mcl-PHAs is 40–80% and 20–40%, respectively [101]. With these production strategies and the characteristic properties, the subsequent section deals with the detailed description of the applications of PHAs in different fields of medicine and the food industry.

3 Applications of PHAs 3.1

Articular Cartilage Repair

Articular Cartilage. Articular cartilage is a tough layer of white smooth connective tissue, which covers the ends of bones. The white color is due to the absence of blood vessels. It serves as a self-lubricating body due to its high fluid content which in sense reduces the internal frictions that damage the bones. Cartilage tissue is composed of 1–5% of chondrocytes with a large amount of specific matrix components such as fibrins, polysaccharides, and collagen. These components are responsible for biomechanical properties [102]. Articular Cartilage Damage. Articular cartilage damage can occur because of a traumatic accident and previous knee injuries and also because of wear tear actions for a long time. The materials used in tissue engineering serve as physical and mechanical support and an adhesive substrate for the cell adhesion after seeding. Among the available substrates, polymers pose an effective and suitable substrate to enable cell proliferation and differentiation. There exists a number of biodegradable polymer scaffolds for the repair of cartilages [103]. Articular cartilage damage repair is not spontaneous because of the inability of chondrocyte to regenerate itself. It is inactive in vivo so there is a need to develop unique technologies and products in tissue engineering to solve the problems associated with cartilage tissue engineering, in which the chondrocytes are considered as the seed cells for their harmless and simple collection methods, for the rapid in vitro proliferation, and for the safer autologous transplantation [98, 99]. Chondrocytes are important cells, mostly used in clinical researches and therapies [104]. One of the main problems associated with the cartilage tissue engineering is the degradation of chondrocytes

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with reduced secretion of matrix proteins [101, 102] leading to the loss of biomechanical properties. Articular Damage Repair Using PHAs. PHAs are a group of biopolyesters which are synthesized by various genera of bacteria under stressed conditions. The property of PHAs that makes compatible with living cells is its biodegradability. The studies have demonstrated the biodegradation of P(3HB), poly (3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV), and poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) P(3HB-co-3HHx) with the help of lipase and other hydrolytic enzymes. This degradation will form the degradation products such as oligomers of hydroxyl acids [105]. The class of PHAs is having special features as biomaterials and is listed as follows: 1. 2. 3. 4. 5. 6. 7.

Biodegradability Cytotoxicity Biocompatibility Non-carcinogenicity Adjustable chemical and physical properties Tunable properties Easy surface modification

The problem associated with the degradation of chondrocytes can be overcome using biopolymers such as PHAs. There are many polymers which are in use for synthetic biopolymer scaffolds [106]. Studies conducted by using the PHB blend with poly (hydroxybutyrate-co-hydroxyhexanoate) on rabbit articular cartilage chondrocytes showed that there is a positive effect on the extracellular matrix formation, which could be identified using second harmonic generation (SHG) imaging technique in combination with confocal fluorescence microscopy, which in turn confirms the anchoring properties of the polymer for type II collagen fibers and the penetration into internal layers of the scaffolds, and it was also found that there is an increase in the glycosaminoglycan of extracellular matrix [107]. Use of biomaterial scaffolds in the field of cartilage tissue engineering can contribute the better biomechanical properties along with the cell growth for articular cartilage development [108]. The combination of transcriptional therapy and scaffolds made up of biomaterials along with the induction system can overcome the degradation of chondrocyte in vivo and improve the protein secretion by matrix. This combination is useful in the formation of better autologous articular cartilage for the implantation purpose. The studies used tetracycline expression system in combination with poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) polymer membranes [106]. There are many animal models used for such testing, and Fig. 4 shows the existing animal models used for the articular damage repair studies and the treatment sites [104]. Articular cartilage is having the lowest volumetric density of cells compared to any tissue in the body. It is having very low self-healing capacity once damaged. This leads to the osteoarthritis and functionality loss in bone joints. Traditional

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Fig. 4 Various animal models used for the articular cartilage damage repair studies

treatments such as osteoscopic abrasion and microfracture techniques are not efficient in damage repair [109]. The evaluation of biochemical environment for the cell growth and proliferation can be made by detecting the extracellular matrix produced during cell proliferation. These cells can grow in vitro on the surface of PHBV, PHB, PHBHHx, or on its blends [110]. Some studies suggest that the blends of PHAs are better than using a single polymer [110]. The strength and elastic properties of PHBHHx can be tailored to meet the needs for developing soft tissues and bone tissues [111]. The blend of 60 wt% PHBHHx with PHB can induce and improve the cell growth and proliferation of chondrocytes [103].

3.2

Cardiovascular Patch Grafting

Cardiovascular disease is an important, dominant cause of mortality and morbidity worldwide. The cardiovascular graft is also called as a cardiovascular bypass. It is a surgical procedure performed to redirect the blood flow from one part to another part of an organ by reconnecting the blood vessels. The grafting is carried out to treat the condition such as insufficient/inadequate blood flow (ischemia) caused by atherosclerosis. For such conditions, the organ transplantation is necessary.

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The generally preferred graft is an autograft such as person’s own veins and arteries. But the recent advancements focused on the use of allografts and polymers such as polyethylene terephthalate (PET), polytetrafluoroethylene, and PHAs such as PHB in the cardiovascular patch grafting. The common bypass sites are heart, especially the coronary artery, and legs for the treatment of peripheral vascular diseases. Owing to the beneficial properties for the choice as a biomaterial, PHAs are applicable for resolving numerous medical problems in vivo and in vitro. A wide variety of implants which include medical sutures and valves are used for guiding in tissue repair and adhesion barriers, and the devices used for regeneration of organs such as cardiovascular patches, articular cartilage repair scaffolds, and substitutes for the bone grafts can be functionalized with the use of various forms of the PHAs. Use of PHA in tissue engineering has an advantage over the other available methods with respect to the flexible properties associated with various combinations and the building blocks with the various methods of fabrication [105]. Researchers constructed the biodegradable and biocompatible trileaflet heart valve scaffolds by using the porous PHAs with a pore size of 180–240 µm. The method involved in developing such biomaterials was harvesting of the vascular cells originating from ovine carotid arteries by in vitro expansion and seeding into the heart valve scaffold by using cardiopulmonary bypass. It can be found that the tissue-engineered heart valve scaffolds fabricated from PHAs can be used in implanting the pulmonary position in lambs with respective functions [112]. The blended polymers of PHAs with other polymers are widely used, such as the blend of P(3HHx-co-3HO) with polyglycolide (PGA), and it can be observed that the non-progressive heart valve regurgitation occurs at six months after implanting the material. The study also demonstrated the small intestinal submucosal lining inside a pulmonary valved stent which can prevent the artificial stent-related problems [113]. Also, the PHAs can be made surface active for various applications, one among that is the surface modification of random copolyester of 3-hydroxybutyrate and 3-hydroxyhexanoates by ammonia plasma treatment. This leads to the better growth of endothelial cells of human umbilical veins on the surface of ammonia plasma-treated PHBHHx coated with fibrionectin than on PHBHHx coated with only the fibronectin [114]. Most widely used clinical heart valve replacement therapies include the mechanical and xenografts or homografts which are used as implants, though they are unable to grow or repair. However, the donor organs are rare and common prostheses, which are having many disadvantages, as an alternative solution and to resolve the problems associated with the cardiovascular diseases, the tissue engineering is one such field under discussion, and the other is the development of biobased materials to solve such problems [115]. Also, several other materials such as decellularized extracellular matrix [116], PGA [117] and poly (lactic acid) (PLA) and PHA can be used to provide the suitable environments for the growth of cells, their differentiation and regeneration. Compared to PLA and PGA, medium-chain-length PHA (mcl-PHA) poses more flexibility and are suitable for the applications such as leaflets inside the trileaflet valves. Studies also showed the implantation of whole tissue leaflet heart valve developed with tissue engineering by using the copolyester of 3-hydroxyhexanoate and 3-hydroxyoctanoate [118].

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145

Meniscus Repair Devices

Meniscus Injury. Meniscus is a cartilage tissue that acts as a cushion between femur (thighbone) and the tibia (shinbone), which also stabilizes the knee joint and disseminates the forces between surfaces [119]. There exist two menisci in every knee joint. Knee meniscus acts as a shock absorber and helps in stress transmission. Meniscal tears are known as knee injuries. Meniscal injuries cause the hyperextension of knee, or it could be a risk factor in the younger and elder patients, respectively. They can be damaged or injured during the rotating activities under high pressure. It is characterized by its complex structure and biochemical composition. The outer third region is vascularized and composed of a dense fibrous matrix with fibroblast-kinds of cells; middle region is composed of fibrocartilaginous tissue consisting of both the fibroblasts and chondrocytes, while the inner third avascular region consists of chondrocyte-kind of cells. Clinically, more than one million patients undergo meniscectomy each year [114, 115]. The meniscus injury tear and the normal bone structure are shown in Fig. 5. The outer third region injuries can be repaired by surgical suturing, while the tears/injuries occurred in the inner avascular region cannot be repaired due to the inefficiency in the intrinsic healing leading to the meniscus deterioration [120]. Partial or total meniscectomy can be performed to reduce the symptoms of meniscus injuries. However, the meniscectomy increases the possibilities of osteoarthritis [121]. Thus the controlled applications of connective tissues growth factor (CTGF) and transforming growth factor beta 3(TGFb3) can be used for seamless healing of avascular meniscus injuries by stepwise differentiation of synovial mesenchymal

Fig. 5 Meniscus tear condition and the normal condition

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progenitor/stem cells [122]. Another way is to develop the artificial meniscal implants for the replacement of severely injured meniscus to restore the normal functioning of the knee joint. 3D elemental analysis is the best method to evaluate variations in the material stiffness [119]. The meniscus repair can also be done using the meniscus fibrochondrocytes by using the tissue engineering technique. In these cases, the assessment techniques used are gene expression, histological, biological, and immunofluorescent assays [123]. The polymer composites can be used that replicate the structure and function of musculoskeletal tissues [124]. PRP-enriched PGA-hyaluronan scaffolds and commercially available Hyaff-11 (hyaluronan based scaffold by Fida Advanced Biopolymers, Italy), BioSeed-C, a synthetic polymer scaffold made up of fibrin, polyglycolic/PLA and polydioxanone (by BioTissue, Switzerland) can be used for the treatment of the chondral defects [125]. PHAs are suitable for soft tissue repair, as a viscosupplements and in augmentation [126]. Poly(4-hydroxybutyrate-co-hydroxyalkanoate) copolymer with the molecular weight of 10,000–10,000,000 Da are used for the development of meniscus regeneration devices, cell encapsulation devices, controlled release devices, etc. [127]. The PHAs do not show acute or chronic health effects when used in vivo; also these are renewable and sustainable resource to reduce the landfill [128]. The whole menisci can be regenerated using poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) scaffold. The study conducted on the scaffolds loaded with meniscal cells in rabbits undergoes the total meniscectomy and its effects to protect the cartilage degeneration. The PHBV implants can form the neomenisci after 18 weeks of transplantation. This development can be analyzed by using the hematoxylin and eosin staining of neomenisci, by detecting the fibrocartilage regeneration. This can also be confirmed with the presence of type I collagen in neomenisci by immunohistochemical analysis by staining with Sirius scarlet trinitrophenol. The PHBV scaffolds compatible with the allogenic meniscal cells can also decrease the cartilage degeneration [129].

3.4

Molded Products: Disposable Needles, Syringes, Sutures, Surgical Gloves, Gowns, and Others

There are many types of molding techniques in the plastic processing. These include injection, rotational, compression, and blow molding. Injection Molding. In this process, the melted plastic is forced into the cavity of a mold, and after cooling, the molded product can be obtained from the mold. This is used in the mass production of the products. Rotational Molding. The process involves the packing of powdered plastic into the hollow molds and is secured to pipe-like spokes extending from the central hub. The rotation, swinging, and the temperature of the furnace melts the plastic and

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sticks to walls, during the rotation, and it produces the products. It is of 2 types, structural foam molding and thermoforming. Compression Molding. The process involves the pressing of slug of plastic in between two heated mold halves. The samples formed are then cooled in air; these are further classified into film and gas-assisted molding. Blow Molding. It is similar to injection molding, except the pouring of hot liquid plastic vertically out of barrel in a molten tube, and after cooling, the sample can be obtained [130]. Syringe and needle industry are important among the industries producing medical devices. It drives a strong trend in the research and development industry. Other important factor that drives the production of such products is the need for the reduction of cost of samples, growth of the field, increase of the commodity prices, attraction toward the private labeled brands, and also the increasing competition between the producers [131]. There are two types of syringes and needles based on the usability of the products: 1. Disposable 2. Non-disposable The properties of PHAs are similar to that of the petroleum-based plastic polypropylene (PP); it distinguished by its physical properties and can produce the transparent film at the melting point, which is more than 130 °C. In pharmaceuticals, hospitals, life science, and health industries, the attention is increasing regarding the problems associated with the non-disposal syringes and needles, due to the contemporary sensitivity of exposure to the dreadful diseases such as acquired immunodeficiency syndrome (AIDS), hepatitis, and other blood originated and transferred diseases. Currently, the disposable syringes and needles are available; however, they are made up of petroleum-based plastics such as PP, which takes years to degrade and decompose and are not environmentally safe and are not sustainable in nature. So it would be right way to provide the bioplastic syringes with needle safety shields, caps, multidose syringes used in the case of diabetes, specimen tubes, scalpels, lancets, suction canisters, sharp containers, etc. Along with PHA, there are also other biopolymers used in the production of the disposable products, which includes poly(e-caprolactone) (PCL), cellulosebased plastics, polybutylene adipate terephthalate, and PLA. [132]. Poly (3-hydroxybutyrate-co-3-hydroxyhexanoates) (P(3HB-co-3HHx)) can also be used in the manufacturing of syringes [133]. Surgeons currently use a wide variety of surgical sutures, with different raw material compositions such as natural silk, PGA, and PLA. These are used in the form of monofilament, multifilament, and twisted surgical structures. However, the strength of suture is also an important parameter to consider. According to the European pharmacopedia, detection of the strength can be done by breaking the suture load tied in a single knot which is expressed in newtons, and thickness can be expressed by metric size. However, properties such as elasticity, pliability, less

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capillarity, antibacterial action, and biocompatibility and biodegradability are also important parameters. They are convenient to use as sutures because of its reliability and the sorption of wound components. The surgical sutures can be modified with a core and shell structure having the particular thickness and porosity of the PHB as a shell or can be formed by mixing with the other polymers and additives. The change of polymer content in the fiber below 15% cannot affect the metric size and strength [134]. The monofilaments’ surgical sutures made up of two types of polymers such as PHB and polyhydroxyvalerate can give the strength required for the healing and can cure the muscle–fascial wounds. The studies conducted on the silk and catgut suggests that there was no acute vascular reaction at the implantation site or no inflammation, necrosis, malignant tumor, or calcification [135]. Surgical gloves, gown, aprons, masks, surgical eye goggles, respiratory masks, and face visors are also considered as the personal protective equipment. These should be worn in healthcare provision. These create the barrier between the people working in the health care and an infectious agent from the patient to reduce the risk of transmitting the microorganisms from workers to the patients [136]. Mitsubishi Gas Chemical Co., Inc. Japan, produces the gloves in the name of Biogreen using P3HB, and Tepha Inc produces the monofilament sutures and absorbable surgical films using poly(4-hydroxybutyrate).

3.5

Possible Application of PHA in Packaging Sector

Food Packaging. Food safety and security are the most important concerns in the food industry and also to individuals. Packaging is the only aspect to enhance the shelf life of food product. Food packaging is mainly used to preserve the quality and safety of food products during the storage and transportation. It is also most important to extend their shelf life by preventing unfavorable conditions like spoilage by microorganisms, light, contaminants, oxygen, and moisture [137]. It also acts as a barrier toward permeation of water vapor, oxygen, carbon dioxide, and other volatile compounds. Plastic is one of the basic materials used in packaging sector. Based on the convenience of use, easy processability, low cost, and exceptional physicochemical properties seek interest toward conventional plastic earlier. On the contrary, conventional plastics are not biodegradable and generate serious environmental hazard. Also, these plastics are responsible for generating plastic waste disposal problem along with consuming the non-renewable and finite resources which affect the depletion of carbon footprint [138]. The most commonly used polymers in the sector of food packaging are polyolefin such as PP, PET, polyethylene (PE), and polystyrene (PS). Nowadays, the importance of developing biodegradable materials is focused more due to environmental concern to remit plastic waste disposal problems. Moreover, biopolymers are one of the most appealing alternatives of pristine petro-based polymers due to their environmentally friendly, biodegradable, and renewable nature. On the other hand, the challenging task of using biopolymer is to

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control their oxygen and water vapor permeability as per the nature of food product [139]. Further, several interrelated factors influence the characteristics of barrier properties of the polymeric film. The responsible factors are structural organization of polymer chains, chemical affinity between the permeant and the matrix, hydrogen bonding, degree of cross-linking, processing technique, and degree of crystallinity [140], respectively. The growing concerns on the environment and depletion of fossil fuels caused by non-biodegradable packaging materials have exaggerated global attention on the application of biodegradable and biobased plastic as packaging materials, namely PLA, PGA, poly(butylene succinate) (PBS), PCL, poly(butylene adipate-co-terephthalate) (PBAT), and PHA [137, 141]. Besides the environmental concerns, fluctuating prices of fossil feedstock have aided to increase the trend toward bioplastic. A future-oriented strategy is developing to impede the dependence of global industry on fossil feedstock by implementing bioplastics for bulk applications such as food packaging and also to minimize current environmental issues arising from plastic disposal [142]. Biobased polymers can be directly extracted from biomass (polysaccharides, proteins, and lipids) and can also be synthesized from bioderived monomers (PLA or other polyesters), which are produced directly by microorganisms (PHA, bacterial cellulose, xanthan, pullulan). Biobased polymers are usually coated on papers and cardboards, applied with different coating techniques (solutions coating, surface sizing, curtain coating, or compression molding) [143]. PHAs become one of the principal biopolymers due to their numerous structural variations, resulting in multiple different properties, and, subsequently, different fields of application. This is the only class of bioplastics with a complete green life cycle, renewable resources, which act as feedstock of the production (biobased), both synthesis of their monomeric building blocks, and their subsequent polymerization is biocatalyzed by living cells (biosynthesized). Along with these multifarious properties, they also have biocompatible and biodegradable properties. In this context, PHAs do not cause any adverse effect on the environment and undergo degradation by the action of living organisms [139, 140]. However, PHB is the most common class of biopolymer, which is better established for using as a composite material over hundreds of PHA [144], and its application in the area of food packaging is summarized in Table 2. Additionally, owing to their identical chemical and physical characteristics, PHAs would be considered as a potential alternative for conventional polymers [142, 143]. Perishable Food. Perishable food has high levels of nutrients and free water molecules which aid to create an ideal environment for microbial growth. These are more prone to spoil, decay, or become unsafe to consume if not kept properly under refrigerated condition. Raw meat, cured meat, meat salad are the examples of such class of food materials. Further, the difficulty of handling perishable food supply chains is due to their short lifetime, susceptible to spoilage, and uncertainty of demand [147, 148]. The European retail stores have found the maximum food spoilage rate of 15% [149–151]. Therefore, it is important to produce packaging

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Table 2 Application of generally used PHB and its composites in the area of food packaging Matrix polymer

Reinforced material

Observations

Area of use

References

PHB

Vanillin

Active food packaging

[145]

PHB

PHB

Paper

Active food packaging Active food packaging Food packaging

[146]

PHB

Eugenol and pediocin Sporolipid

PHB

Cellulose

Food packaging

[149]

PHB

–

Food packaging

[150]

PHBV

–

• Good release of vanillin from the film into the food simulants for fatty food which showed antimicrobial action • This antimicrobial film is more effective against fungi than bacteria • It has shown reduction in maximum thermal decomposition and the tensile strength • This film showed antimold and antibacterial activity that will help to preserve food • Control release of sporolipid was obtained from this matrix • Can be used as food preservatives • Coating of PHB over fiber-based packaging showed better performance • Enhanced barrier to gases and vapors properties with appropriate thickness • PHB coating with minimum thickness showed high barrier to moisture and aroma • Double-layer biocomposite of PHB over cellulose paper showed increased water and moisture barrier properties and less surface roughness • Elastic modulus and mechanical strength of the composite increased with PHB content • PHB jars showed good physical, dimensional, mechanical, and sensorial properties for storage of fat-rich food products • It is suitable for freezers and microwave ovens • It is a good substitute of PP • Good mechanical and barrier properties in comparison with traditional thermoplastic used for packaging • Better water vapor transmission rate compared to conventional thermoplastics

Food packaging

[151]

[147]

[148]

material, which will help to expand shelf life and reduce the food-based waste materials. Moreover, biodegradable packaging material can be a useful alternative for the reduction of municipal solid waste. Most concerning thing is that a wide range of

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microorganisms is responsible for the production of PHAs’ family which are biodegradable in nature. Through the help of fermentation process, this polymer is produced in the microbial cells, and solvent extraction technique is mostly used to harvest the polymer using solvents including chloroform, methylene chloride, or propylene chloride [152]. The oxygen barrier property of a food packaging vessel for perishable food products (e.g., meat and follow-up products, salad, various convenience foods) displays a decisive role for its conservation. Furthermore, oxidative degradation has poor influence over color, flavor, and microbial stability of several food products. The ability of packaging film is to act as a barrier to overcome the driving force or concentration gradient of the difference in oxygen partial pressures (0–2%) inside and outside the package (21%) which helps to maintain product quality. The homopolyesters PHB, poly(4HB), and scl-PHA copolyesters (PHBV) exhibit high oxygen barrier (or low oxygen transmission rate (OTR)), which is essential to inhibit the growth of aerobic bacteria and the oxidative spoilage of unsaturated fatty acids [153]. Several authors have reported that PHA and its blends with improved thermal, physical, and barrier properties are used in food packaging application. Further, blends of PHB/PCL with activated nisin have shown its effectiveness as shelf life enhancer for vacuum-packed sliced cooked ham. Also, this nisin-activated PHB/PCL blend and PHB/PCL/organo-clay-based nanocomposites can be used as active food packaging material [154]. Based on the current research, PHA/ PHB-based films could be a possible alternative for conventional non-biodegradable films. Levkane et al. [155] have investigated and compared the effect of pasteurization on conventional (PE, PP) and biobased packaging (PLA, PHB) for storage of meat salad packed, and PHB films had shown fruitful outcome for this particular food product. [144]. Fresh Fruits. The presence of high amount of water makes fruit highly perishable (contains 80–90% water by weight). They have a very less shelf life due to the evaporation of water if left without cuticle. Major losses have occurred between harvesting and consumption of fresh fruits and vegetables in quality and quantity. During the harvesting of fruits, a change of the gaseous balance has occurred between the consumption of oxygen and the production of carbon dioxide. At this stage, maturation and senescence have occurred as gas transfer rates become increased which is also responsible for metabolic loss [156]. Previously, equilibrium modified atmosphere packaging (EMAP) has proved to be an efficient way to delay senescence and spoilage and thus extend the shelf life of product, without using chemical preservatives [152, 153]. Like any other conventional food packaging, the main disadvantage of EMAP is the extensive use of plastic films resulting in huge quantities of domestic plastic wastes [157]. During the last decade, recycling of these materials increased, but only a small part of the generated amount of plastic is finally recycled. Therefore, sustainable biobased biodegradable plastics concept is growing much with good environmental impact as a replacement of non-degradable conventional plastics based on fossil oil.

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The water vapor barrier properties for the fresh fruits and packaged products, whose physical or chemical deterioration is related to their equilibrium moisture content, are of major importance for maintaining or extending the product shelf life. Requirements for high or low water vapor barriers are dependent on the type of the packaged food; for fresh food products, it is important to avoid desiccation while, for example, for the bakery products that are easily subjected to fungal infections by molds, it is essential to avoid water inflow [155, 156]. High barrier for flavoring compounds is also an important factor in order to preserve the flavor of the food. PHA polyesters and copolyesters have water vapor, CO2, and flavor barrier properties. In comparison to other biopolymers from natural origin such as starch, it has shown considerable hydrophobicity [153]. Thus, it would be a good packaging material for packaging of fresh fruits. Researchers have proved the effective use of PHB films commercially after getting the similar outcome of orange juice simulant and dressing packed in PHB and high-density polyethylene (HDPE) with similar quality variations. This positive outbreak helps to establish the usage of PHB film for commercial juices and other acidic beverages and dressings or other fatty foods [157–159]. Dairy Product Packaging. Dairy products are highly susceptible to spoilage as it contains a high percentage of fat. Thus, packaging material used should be selected in such a way that it possesses good grease resistance and barrier properties against oxygen and moisture. The packaging of fat-rich products like mayonnaise, margarine, and cream cheese can be packed using PHB as a replacement of PP after checking physical, mechanical (dynamic compression and impact resistance), sensorial, and dimensional (dimensions, volumetric capacity, weight, and thickness) parameters [145, 160]. Similarly, Muizniece-Brasava and Dukalska [161] have stated that PHB materials are suitable materials for the storage of sour cream. Medicine Packaging. PHAs have many applications in medical sector due to its biocompatibility, biodegradability, and green credentials. Additionally, tissue engineering, bioimplant patches, drug delivery, and surgical applications are most common of them. Different types of PHAs and its derivatives are experimented as potential scaffold materials such as PHB, PHBV, PHBHHX, and many more [160, 162]. PHAs are used for drug delivery in many animals including mice, sheep, dog, rabbit, cattle, and even in humans for the treatment of gingivitis [163]. PHB/ hydroxyapatite composite has been used for bone tissue regeneration and scaffolding [164, 165]. Also, nanohydroxyapatite with PHB composite has been developed for scaffold fabrication with improved strength and modulus properties [166]. PHAs can be used for repairing damaged nerves as they are piezoelectric [167] and also used for wound dressing [168]. PHB can be used as microcapsules in therapy or as materials for cell and tablet packaging [169]. Fragile Item Packaging. PHAs and its derivative can also be applicable for the development of biodegradable foams for brittle item packaging [170]. Foams are generally made by polyurethane (PU) which is synthetic in nature. It is also rigid as well as non-biodegradable; thus, to overcome this obstacle, PHB has been added with PU to improve Tg and biodegradability [171].

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Fig. 6 Various applications of PHA

Others. PHA and its derivatives are most commonly applicable in packaging and medical sectors especially in drug delivery, scaffolding, tissue implants, and many more. Other than this, it could be applicable in many sectors (Fig. 6) [46]. It can be applicable in cosmetics section to make containers, shampoo bottles, cups, etc. It is also applicable in agriculture sector as a biodegradable carrier for long-term dosage of insecticide or fertilizers. A promising application of PHAs could be as solid substrates for denitrification of water and wastewater. It helps to reduce the power for denitrification and also favorable for development of microbial films. In this regard, there is no potential risk of releasing dissolved organic carbon with the effluent in comparison to conventional process. PHB has proved already for removing liquid-soluble organic pollutants from water [172]. PHAs are also useful in controlling bacterial pathogens in certain aquaculture applications [173]. PHB can also be used as a microbial agent in aquaculture for fish diet, and its effect on growth of fishes depends on both concentration and species [174]. The long-side-chain hydroxyacids along with PHAs have tested in pressure-sensitive adhesive formulations. Other than food packaging, PHAs can be used to produce dairy cream substitutes or flavor delivery agents in foods. Additionally, fiber materials, such as non-woven fabrics, have been made by PHAs [175], and it is considered as a potential source for the synthesis of chiral compounds and raw materials for paints. PHA monomers can be converted to other attractive molecules such as b-hydroxy acids, b-hydroxyalkanols, b-acyllactones, b-amino acids, and b-hydroxyacid esters [46, 172]. Moreover, b-hydroxyacid esters have potential to replace existing solvent with biodegradable solvents [171, 173]. Migration. Migration refers to the release of a substance from one medium to another. In case of food packaging, it is generally considered as the transfer of additive materials, which remain in direct or indirect contact with food products.

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The migration materials generally include nanofillers, plasticizers, thermal stabilizers, monomers, oligomers, antioxidants, light stabilizers, contaminants, and metal components present in packaging material. However, the report on the investigation of the migration of specific substances from PHA packaging has not been studied completely. Further, the migration of components from PHB films has been studied using different food simulants such as ethanol, acetic acid, and n-heptane [150]. The total migration showed by the film is within the limit of standard value of 8.0 mg dm−2 or 50 mg kg−1, indicating the safety of PHAs film in food packaging applications.

4 Conclusion and Future Scope Biodegradable polymer or bioplastic is gaining much interest and becomes a promising candidate for a wide range of applications, and use of this bioplastic will address minimum three issues: reduction in petroleum-based feedstock for plastic materials, reduction of CO2 emissions, and environmental protection. Nowadays, the tendency to reduce the use of petroleum-based plastics in different applications has focused interest on their substitution by biodegradable plastics. PHAs and its derivatives are a good replacement of petroleum-based polymers in comparison to other bioplastics. One of the important properties of PHB is its compatibility with blood and tissues of mammals. Due to this property, PHB could be applicable for medicinal sector such as drug delivery, surgical implants, as seam threads for healing of wounds and blood vessels and in scaffolding. It can also be used in food industry as a replacement of traditional non-biodegradable packaging material and applicable to the textile industry. Due to these interesting properties, PHB is expected to be a better substitute for PP, PE, PET, and polyvinyl alcohol (PVA) in different applications [176]. Incorporation of additives for improving the properties of PHAs will also be helpful in a broader way to establish the market throughout the world.

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Part II

Sustainable Polymers for Food Packaging Applications

Chapter 8

Chitosan-Based Edible Coating: A Customise Practice for Food Protection Tabli Ghosh and Vimal Katiyar

Abstract This chapter demonstrates the use of chitosan as an edible coating material for improved storage life of food products especially perishable food products. Chitosan is a versatile biopolymer derived from renewable resources with several befitting properties. Additionally, chitosan is a substantial biodegradable, biocompatible and non-toxic material extensively used for enormous applications. Among existing applications, chitosan and its derivatives are constant materials for edible coating. The edible coating is considered a tailor-made customised technique for food protection including fruits and vegetables, meat and meat products and others. The specified packaging sectors aid in maintaining food integrity without changing or degrading nutritional quality, which possibly a problem in other post-harvest techniques. Further, chitosan has various medicinal properties such as antimicrobial, antibacterial, antidiabetic and others, which make it a promising agent in day-to-day life. However, the poor mechanical and barrier properties of chitosan may restrict its use in food packaging sections. In this regards, the fabrication of chitosan-based composites and blends can offer tuned properties against individual use as edible coating materials. Moreover, chitosan as an edible coating can deliver bioactive compounds to food products maintaining food properties with an improved shelf life. Keywords Chitosan Application


Edible coating
Food properties
Food protection


T. Ghosh
V. Katiyar (&) Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India e-mail: [email protected] T. Ghosh e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_8

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1 Introduction From the past few years, edible food packaging has gained considerable attention in delivering tailor-made packaging materials to protect and preserve food products. Edible food packaging generally involves the use of sustainable renewable materials, which are widely available and easy to fabricate. Due to the limited resources available for developing fossil-based packaging, the use of sustainable materials is intensively increasing for food packaging. Further, the use of fossil-based packaging laid to increase in carbon footprint and global warming unlike sustainable polymers [1]. Interestingly, the specific sector of edible food packaging reduces plastic-based waste in terms of reduced use of fossil materials. The globalisation for edible food packaging-based market is growing day by day. According to the report by Allied Market Research entitled “Edible Packaging Market by Material type and End user: Global Opportunity Analysis and Industry Forecast, 2017–2023”, the market for edible food packaging was marked at $697 million in 2016, which is supposed to reach $1097 million by 2023 [2]. The use of edible food packaging is increasing remarkably due to the increase in consumption of processed food and increase in hygiene concern, packaging waste, etc. Moreover, the inclusion of edible food packaging to preserve food products generally follows two techniques such as edible coatings and films. The edible coating includes the application of edible materials on food products via dip or spray coating [3]. On the other hand, edible films include the application of a thin layer of edible material to wrap food products or used between food products as a layer [4]. The edible packaging in various regions requires many manufacturing regulations and high manufacturing cost, etc., which restrict the use of edible food packaging. Edible food packaging has many advantages including edible, biocompatible, non-toxicity, health benefits and others. Moreover, edible food packaging improves the aesthetic property of food products and reduces oxygen and water vapour permeability. Thus, the specific sector is growing attention to be used extensively for the enormous beneficial properties. Moreover, perishable food products including fruits and vegetables, fish and fish products, meat and meat products, milk and milk products easily get degraded by environmental factors such as gaseous condition, temperature, light, microbial growth, etc. Fresh fruits and vegetables have a short life span for their perishable nature, where respiration rate is a factor which is responsible for biochemical and physiological process of fruits and vegetables [5, 6]. However, the respiration rate of fresh process is a dependent variable of temperature, gaseous concentration and storage time, which can be controlled for improved shelf life of fresh produces [5, 6]. On the other hand, fresh fish is generally degraded due to various chemical, enzymatic reactions and microbial attack, which can be preserved with proper preservation techniques [7, 8]. Further, meat and meat products are degraded by lipid, protein degradation, due to oxidation, which can be prevented with the use of antioxidant agents [9, 10]. Milk and milk products degrade by microbial attack, light, temperature and others. Besides, semi-perishable food products also require various treatments to maintain food property during storage. In this regards, food

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preservation technique plays an important role in preserving various food products, among which application of edible films and coatings is a focused technique to preserve food products. Edible films and coatings are generally developed using biopolymers such as cellulose, chitosan, starch, agar, gelatine, silk, collagen, etc. Among available biopolymers, chitosan has been extensively used in the development of edible food packaging for improved shelf life of food products. The use of chitosan coating on fresh strawberry improves quality and storability, where chitosan coating can reduce respiration rate of strawberries by reducing degradation [11]. Further, use of chitosan and gelatine films incorporated with essential oils can preserve fish by inhibiting microbial spoilage [12]. As mentioned, the widely used edible materials for edible coatings and edible films generally include various biopolymers especially polysaccharides such as cellulose, chitosan, starch, pectin and others for their renewable, non-toxic and easy availability (Fig. 1). Polysaccharide-based edible coatings generally include cellulose and its derivatives [13, 14], chitosan and its derivatives [15, 16], starch [17, 18], gum Arabic [19, 20] and others. Several biopolymers have obtained extensive attention in the field of food packaging in various forms such as edible coating and films, biodegradable films, hydrogel and others [1, 20]. Chitosan is the second most used polysaccharides after cellulose. In addition, the combined use of biopolymers for edible food packaging can provide additional benefits for extending shelf life of food products in terms of improved mechanical properties, thermal properties, barrier properties, etc. Further, Chitosan-based packaging has achieved extensive use in food packaging for their biodegradable, renewable nature and antimicrobial

Fig. 1 Biopolymers and intended modification techniques used in edible coating

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properties. Based on this discussion, the present chapter discusses the use of chitosan-based edible coating material for various food products. Moreover, a detail discussion for various properties of edible-coated food products has been discussed for improved product life.

2 Properties of Chitosan and Its Derivatives Chitosan is one of the polysaccharides derived from shells of crustaceans, cell wall of fungi, invertebrates, insects and others. Noticeably, chitosan is a deacetylated product of chitin, which is composed of N-acetyl-D-glucosamine (Fig. 2). Further, chitin can be converted to chitosan by a deacetylation process, with the aid of alkali solution. Chitosan is a potential candidate for various area of life including food industry, agricultural sector, cosmetic industry, water treatment, paper industry, medical devices, pharmaceutical industry, etc. However, the present chapter is focused on the use of chitosan in edible food packaging sections. Chitosan has the properties of non-toxicity, biocompatibility, biodegradability, and antimicrobial activity, which increase the use of this specified material in comparison to existing biopolymers [1]. However, the biopolymers due to their unique traits have attained a great application in multifaceted area [1, 21]. Further, chitosan provides many health beneficial properties including antioxidant property [22], antidiabetic property [23], weight reducing activity [24], anticancer activity [25], cholesterollowering activity [26] and others, which make it a promising agent for edible films and coatings with added benefits. Moreover, chitosan has antibrowning [27],

Fig. 2 Focused properties of chitosan for food packaging application

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antimicrobial activity [28] to food products, which makes it an ideal candidate for edible food packaging. Dutta et al. [29] reported the use of chitosan-based antimicrobial films, which provides food safety and enhances the shelf life of food products. Further, Coma et al. [30] use chitosan as bioactive packaging materials for analysing dairy product preservation. The properties of chitosan can be improved when used with other bioactive agents [31]. However, chitosan is used to prepare blends and composites with other polymers for improved packaging properties such as antimicrobial property. Further, functionalized chitosan provides added advantages to be used in food packaging application. Thus, a discussion has been made based on improved properties of chitosan-based edible food packaging.

3 Strategies for Tailored Properties of Chitosan-Based Edible Coating The chemically unmodified form of chitosan offers varied application, where available functional groups in chitosan including primary amine, primary and secondary hydroxyl group can deliver tuned properties. However, chitosan can be chemically modified, where the functional groups can form intramolecular/ intermolecular hydrogen bonding, grafting, complex cross-linking, hybrid cross-links, composite-based networks and others. Moreover, the biopolymer is sensitive towards pH, electro-responses, molecular weight, temperature, humidity, moisture content, which impart a great effect on its usage. Thus, addition of other available biopolymers and other agents can help to improve stability of chitosan for acting as an edible food packaging material. The generally accessible ways for improved chitosan are fabrication of chitosan-based biocomposites and blends. Further, the activity of chitosan is modified, when bioactive agents are added including essential oils, plant extract, fruit extract, etc. Composites are defined as a multi-phase system, where fillers are reinforced in a matrix system, which results in improved property if incorporated adequately. Composites have generally two phases such as continuous phase (matrix materials) and dispersed phase (filler or reinforcement). The inclusion of filler materials should be optimum for formulations of composites; otherwise, there may form agglomeration in discontinues phase resulting in poor properties. Further, fabrication of polymer composites has many beneficial properties such as load transferring activity, design flexibility, improved application, high advanced application, etc. In this regards, formulation of chitosan-based biocomposites mainly involves as filler and matrix material to form composites. The use of chitosan as packaging materials is restricted due to poor mechanical property, water resistance due to hydrophilicity properties, which can be improved by forming chitosan composite [32]. The development of starch (25% amylose)-incorporated chitosan composites can help to improve the mechanical property in terms of tensile strength and elongation at break [32]. Further, the addition of chitosan/starch composites can help in reducing water vapour permeability in comparison to neat chitosan [32]. The use of

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plasticizer and gelatine also affect chitosan composite for tuned mechanical and barrier properties [33]. The addition of nanocellulose and plasticizer as reinforcement to chitosan matrix also improves mechanical and water vapour barrier properties [34]. Moreover, chitosan-based composite with 15% cellulose nanofibers and 18% glycerol has comparable strength and stiffness in comparison with many synthetic polymers [34]. The development of chitosan-based coatings reinforced with gum Arabic can enhance the shelf life of a banana [35]. The gum Arabic reinforced chitosan-based edible coating offers improved shelf life of banana till 33 days [35]. Chitosan composites and three-layer coating (beeswax-chitosanbeeswax) with beeswax filler can maintain strawberry quality; however, three-layer coating provides more improved quality for strawberry during storage [16]. In this way, chitosan-based composites are used widely for edible food packaging materials with improved properties. Polymer blends commonly include mixing of two polymers with different properties which blend together to form a new material with different physical and chemical property. The polymer blends generally involve immiscible, compatible and miscible types, whose constitutes are varied based on the composition. Chitosan blends with other biopolymers are widely used as edible films and coatings of food products. The fabrication of blends resulted in improved packaging property in terms of barrier, thermal and mechanical property. The available natural resources including starch [36], gelatine [33], quinoa protein [37], soy protein [38] are widely used for developing blends with chitosan for food packaging application. Further, chitosan is also used to make blends with other biodegradable and synthetic polymers. Chitosan blends with polylactic acid, polyhydroxylbutyrate and polybutylene succinate are widely used for improved properties. However, chitosan having ability to form chemical cross-linking can help in improved polymer properties for wide application. Chitosan is also used for the preparation of blends with synthetic polymers for improved biodegradability. Considerably, chitosanbased blends are considered as a great replacement for conventional polymers in various areas of research. However, various compatibilizers, plasticizers, crosslinking agents are used to improve properties of chitosan blends. The general interactions between the two phases of chitosan blends generally include hydrogen bonding, ionic bonding, dipole bonds, etc. Moreover, chitosan-based edible coatings act as a better carrier to transfer active ingredients to food products. The nanoparticles of chitosan are also widely used for developing nanocomposites. Edible food packaging acts as a better carrier for bioactive agents for providing antioxidant effect, antidiabetic effect and anticarcinogenic effect. The bioactive agents are generally extracted from various plants, spices, fruits and vegetables with the aid of solvent extraction, ultrasonic extraction, microwave extraction, supercritical fluid extraction and others. The inclusion of bioactive agents in edible coating helps in improving the quality of food products. Chitosan nanoparticles in the form of nanofibers and nanocrystals are extensively utilised to transfer various kinds of bioactive agents in human diet through the application of edible coating. The bioactive compound-rich spices include cinnamon, cloves, ginger, garlic, turmeric, onion, fenugreek, etc. The major bioactive

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compounds in various spices include gingerol, curcumin, allicin, cinnamaldehyde, flavonoids, eugenol, hydroxybenzoic acid, hydroxycinnamic acids, fructan, etc. Further, fruits and vegetables, tea and coffee are also rich sources of bioactive compounds. Tea contains polyphenols, caffeine, flavonols, phenolic acid, theaflavin, thearubigen, etc. Coffee contains bioactive compounds such as chlorogenic acid, caffeine, trigonelline, diterpenes, caffeine, caffeic acid, etc. In addition, there are other available sources of bioactive compounds, which are widely used for enhancing food quality.

4 Properties of Chitosan-Based Edible-Coated Food Products The edible-coated food products are analysed for various properties to measure the effectiveness of edible coating on the quality of food product. Fruits and vegetables are generally characterised by physical, chemical and antioxidant properties. Further, microbial count of stored fruits and vegetables during the storage is also observed for good quality maintenance. The sensory properties of the stored products are also observed for their customer acceptability by trained and semi-trained panel. The fruits and vegetables are generally degraded due to continuing respiring, which are greatly affected by storage condition. Thus, respiration rate of produces is an important factor to be considered for improved shelf life. A harvester determines the time of harvesting by observing the physical properties of fruits and vegetables. Fruits are generally harvested at their mature stage but unripe, which continue ripening after harvesting. The climacteric fruits and vegetables should be harvested at the proper stage; otherwise, they continue ripening very fast which degrade their property.

4.1

Physical Property

The physical properties of food products include the measurement of weight, volume, size, shape, surface area, density (true and bulk density), sphericity, colour and others. The application of edible coating can reduce the rate of ripening of fresh commodities, thus measurement of physical properties helps to know the ripening stages. The weight of food products is measured using the weighing balance. The volume of fruits is measured by (1) water displacement method and (2) measuring the dimension of any food products. The shape and size of food products can be related to sphericity, which can be measured by calculating major diameter, intermediate diameter and minor diameter of selected produce using Vernier calliper. Density of commodities is generally measured by measuring the ratio of weight by volume. Further, aspect ratio relates to width to height of a particular commodity. The colour factors (L, a*, b* values) of food products are generally

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measured using colorimeter, where L defines about brightness; a* defines about redness (positive values) to greenness (negative values); b* defines about blueness (negative values) to yellowness (positive values).

4.2

Chemical Property

The chemical properties of food products generally include pH, total soluble solids, titratable acidity, reducing sugar and others. The pH of food products can be measured using pH metre, where pH of food products generally increases with ripening. Further, total soluble solids are generally measured using digital refractometer in terms of °Brix. The other chemical properties of food products include nutritional compositions such as moisture content, carbohydrate, protein, fat, fibre and others. Further, application of edible coating can greatly influence the antioxidant property of antioxidant-rich food products, which can be determined in terms of total phenolic content, total flavonoid content and others. The use of proper edible coating generally helps to preserve nutritional and antioxidant properties of food products.

4.3

Texture Property

The texture of food products is one of the significant properties for quality evaluation and customer acceptance of food products. The texture of food products is a considerable factor which describes the inner condition (micro and macrostructure) of food products. The texture properties are generally determined using texture analyser, where firmness is considered an important parameter for quality measurement of food products. Further, other parameters of texture include adhesiveness, cohesiveness, consistency, gumminess, stiffness, stringiness, chewiness, etc. Among all, hardness (soft, firm, hard), cohesiveness, elasticity, adhesiveness include primary texture properties, brittleness, chewiness, gumminess include secondary texture properties. The hardness/firmness is the most significant property to be measured for quality analysis of fruits and vegetables.

4.4

Respiration Rate

The respiration rate of fruits and vegetables is generally measured by using gas analyser. The fruits and vegetables are generally stored in closed chamber at specific temperature and headspace gas concentrations are measured in terms of oxygen and carbon dioxide using gas analyser. Fruits and vegetables continue respiring after harvesting also. The respiration rate of commodities generally depends on the storage temperature, gas concentration, etc. The respiration rate

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increases with increasing storage temperature. Further, modification of gas concentration inside package can help to improve shelf life. In this regards, various applied packaging technology such as modified atmospheric packaging, controlled atmospheric packaging can further help to enhance the shelf life of fruits and vegetables. The respiration rate (ml kg−1 h−1) of fruits and vegetables is measured using Eqs. (1) and (2) [6].

Respiration rate ðO2 Þ;

RRO2 ¼

Respiration rate ðCO2 Þ;

RRCO2 ¼

  GiO2  GOf 2  Vf 100  W  ðDtÞ   f GiCO2  GCO  Vf 2 100  W  Dt

ð1Þ

ð2Þ

The linear forms of Eqs. (1) and (2) are presented in Eqs. (3) and (4). GOf 2 ¼ GiO2 

RRO2 :W:Dt  100 Vf

ð3Þ

GOf 2 ¼ GiO2 

RRO2 :W:Dt  100 Vf

ð4Þ

where Gi and Gf are the initial and final gas concentration, respectively; W is the weight of fruits taken, Vf is the free volume available and Dt is the storage time. The packaging technologies such as modified atmospheric packaging (MAP) and controlled atmospheric packaging can also be utilised in addition to chitosan-based edible coating food products for long storage. The MAP generally includes the modification of atmosphere inside package by passively and actively. The modification has been done by replacing O2 and CO2 by inert gasses such as N2. The modification in gaseous condition inside package reduces the respiration rate of fruits and vegetables which reduce the ripening of produces. Further, the reduction of O2 gas inside the package reduces the growth of aerobic microbial count. The technology is useful for improving the shelf life of perishable food products such as perishable fruits and vegetables, meat, fish, poultry, milk products, etc.

4.5

Sensory Property

Sensory properties of edible-coated food products are generally evaluated for colour, flavour, taste, texture, overall acceptability using hedonic scale chart. The sensory analysis of food products is generally measured by trained, semi-trained, and untrained panellist. The use hedonic scale is generally based on nine-scale or five-scale chart.

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Microbiological Property

The microbiological study for edible-coated fruit products can be done in terms of mesophiles and psychrotrophs. The microbial analysis is generally followed in accordance with ISO 4833:2003 and ISO 7954:1987. The serial decimal diluted samples are generally pour plated on plate count agar and incubated at 35 °C for two days and 10 °C for 10 days to count mesophilic and psychrotrophs microbial count, respectively.

5 Application of Chitosan in Edible Coating Traditional plastic packaging material causes an enormous unfavourable effect on the world because of its non-degradable nature and generating harmful elements. Nevertheless, these plastic materials have many advantageous properties as packaging materials over others. The adverse issues of traditional petroleum-based plastic materials consequence in the use of partial and fully eco-friendly/renewable resource materials for developing plastic food packaging materials. In fact, the developed environment-friendly packaging materials include the use of bio-based nanofillers, nanocomposites, biodegradable plastics and bioplastic, having better properties as that of conventional plastic packaging materials. For example, use of starch materials with polyvinyl alcohol (PVOH) in combination can reduce the use of petrochemical-based resource and the plasticization agents used with starch and PVOH includes glycerol, sorbitol, urea and citric acid. In addition to all, from past day’s use of tamper-resistant wide mouth package, multilayer packages and indicators were also found for packaging systems. Furthermore, another development of polymer nanotechnology takes account of design and development of polymer materials filled with materials or devices having a nanosize range as described above such as use of nanofillers, nanocomposites and nanosensors. Involvement of nanotechnology provides better safety with improved barrier properties of packaging materials. Besides this, nanoparticles can be added as antimicrobial agents as coating bioactive agent, dispersing sachets (packets of materials). Inclusion of nanofillers in polymers can be in the form of antimicrobial agents, nanosensors and bioactive compounds which increases the functionality of the package. Incorporation of nanofillers helps to improve mechanical properties, antimicrobial properties and moreover, use of nanosensor inside the package helps to trace and monitor the package conditions. Nanocomposites include use of composite materials for packaging such as nanocomposites of biodegradable starch/clay for food packaging. The stability of use of nanotechnology in packaging materials can be studied by transmission electron microscope (TEM), atomic force microscopy (AFM) and wide-angle x-ray diffraction (WAXD). The application of bioactive coating using functionalized chitosan (acylation with the aid of palmitoyl chloride) and bioactive agents such as limonene, oregano,

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red thyme, peppermint, and lemongrass can effectively improve shelf life of fresh produce strawberry [39]. However, the functionalized chitosan with limonene and tween®80 provides beneficial property in comparison to others. The applied bioactive coating materials help to preserve strawberry for 14 days at 4 °C due to the antimicrobial agents; however, the functionalization of chitosan helps in adhesion property of chitosan. The application of various chitosan-based edible coating on food products is summarised in Table 1. As discussed earlier, chitosan is

Table 1 Summary of chitosan-based edible coating for various food products Coating materials

Food products

Technique

Discussion

References

Chitosan (0.5, 1, 1.5% for 5 min at 20 °C)

Strawberry storage condition: 5, 10 °C Fresh cut carrot

Dip coating

Extend shelf life Maintain quality Control decay

[40]

Vacuum impregnation

Water vapour transmission reduces colour preservation Better mechanical response during cold storage Reduce acceptability with chitosan and vitamin E No change in consumer acceptance on flavour, sweetness and firmness Extend commercial life of bananas Sugar spots delayed due to coating

[41]

High molecular weight chitosan Methylcellulose Oleic acid

Chitosan Vitamin E

Strawberry storage condition: 2 °C, 88–89% RH for 1 week

Dip coating

Chitosan 1-Methylcyclopropene

Bananas storage condition: 8 days 22 °C, 85% RH Whiteleg shrimp storage condition: 0 ± 1 °C, 10 days Strawberry storage condition: Perforated PET boxes at 4 ± 1 °C for 10 days Bream storage condition: 4 ± 1 °C for 21 days

Dip coating

Chitosan carboxymethyl chitosan

Chitosan Oleic acid

Chitosan Vitamin C Tea polyphenols

[42]

[43]

Dip coating

Good antimicrobial agents Improved melanosis inhibitors

[44]

Dip coating

Enhance antimicrobial activity Improve water vapour resistance

[45]

Dip coating

Reduce chemical spoilage Retard decay of fish

[46]

(continued)

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Table 1 (continued) Coating materials

Food products

Technique

Discussion

References

Chitosan Chitooligosaccharides Glutathione

White shrimp storage condition: −3 °C for 30 days Tuna fillets storage condition: 4 °C for 12 days Newhall navel orange Storage condition: 5 ± 0.5 °C, 85 −90% RH 120 days Whiteleg shrimp Storage condition: 0 ± 1 °C, 10 days Strawberry Storage condition: Perforated PET boxes at 4 ± 1 °C for 10 days Bream Storage condition: 4 ± 1 °C for 21 days White shrimp Storage condition: −3 °C for 30 days Tuna fillets Storage condition: 4 °C for 12 days

Dip coating

Prevent metamorphism of white shrimp Extend shelf life

[47]

Dip coating

Better antimicrobial properties Extend shelf life

[48]

Dip coating

Preserve fruit property Improved quality

[49]

Dip coating

Good antimicrobial agents Improved melanosis inhibitors

[44]

Dip coating

Enhance antimicrobial activity Improve water vapour resistance

[45]

Dip coating

Reduce chemical spoilage Retard decay of fish

[46]

Dip coating

Prevent metamorphism of white shrimp Extend shelf life

[47]

Dip coating

Better antimicrobial properties Extend shelf life

[48]

Chitosan Gelatine Clove oil

Chitosan Hairy fig fruit extract

Chitosan Carboxymethyl chitosan

Chitosan Oleic acid

Chitosan Vitamin C Tea polyphenols

Chitosan Chitooligosaccharides Glutathione

Chitosan Gelatine Clove oil

(continued)

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Table 1 (continued) Coating materials

Food products

Technique

Discussion

References

Chitosan Hairy fig fruit extract

Newhall navel orange Storage condition: 5 ± 0.5 °C, 85%−90% RH 120 days Cut pineapple Storage condition: Ambient air condition

Dip coating

Preserve fruit property Improved quality

[49]

Dip coating

Improved texture Improved mechanical and thermal properties

[50]

Chitosan Cellulose nanofibers Magnetic cellulose nanofibres

used with other biopolymers, bioactive agents on perishable food products for enhanced properties. The shelf life of food products is also affected by molecular weight of chitosan, percentage of deacetylation and others. Based on this, some of the detailed application of chitosan-based edible coating on food products is discussed below. The use of chitosan also helps in reducing antimicrobial property for being acted as an antimicrobial agent against mesophilic and psychrophilic microorganism.

6 Conclusion The fossil-based plastics are widely used for food packaging application for enormous packaging properties. However, these classes of plastic waste create worldwide problem, which acts as a driving agent to the use of biopolymers for edible packaging. The use of edible coating is an alternative method to the use of fossil fuels as packaging materials. The commercial use of chitosan and its derivatives as edible coating needs to be explored in near future for the beneficial facts. Moreover, the combined use of primary and secondary packaging is more beneficial for improving the product life of food materials.

References 1. Ghosh T, Katiyar V (2018) Cellulose-based hydrogel films for food packaging. In: Mondal M (ed) Cellulose-based superabsorbent hydrogels. Polymers and polymeric composites: a reference series. Springer, Cham, pp 1–25. https://doi.org/10.1007/978-3-319-77830-3_35 2. https://www.alliedmarketresearch.com/edible-packaging-market

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Chapter 9

Polysaccharide-Based Films for Food Packaging Applications K. Dharmalingam and R. Anandalakshmi

Abstract The existing food packaging materials made up of fossil fuel-based polymers pose a serious threat to the environment. This is the motivation behind the extensive research on biopolymer sources including polysaccharides, proteins and lipids so as to produce biodegradable food packaging materials. Amongst the existing biopolymer sources, commendable attention has been diverted to polysaccharide materials due to their abundancy, film-forming abilities and good gas barrier properties. Despite their desirable properties, polysaccharide-based films demonstrate a poor water barrier and mechanical properties. Further, they are expensive in comparison with conventional plastic materials which restrict the commercialisation. In this regards, an extensive research effort has been made to improve the inherent properties exhibited by the biopolymer-based films by fabricating composites, nanocomposites, blends and addition of cross-linking agents. Amongst available, starch is a kind of polysaccharides consisting of different ratios of amylose and amylopectin, which determines its property. Modified starch with other polymers/nanofillers exhibits improved film properties. In addition, cellulosic derivatives as ionic binders are of a good choice in controlling the moisture and also enhance the mechanical properties of food packaging films. Moreover, chitosan like polysaccharide exhibits an antibacterial activity which is an important property to produce films of higher shelf life and to maintain product integrity. The quest for producing low-cost biodegradable food packaging films derived from polysaccharides with better water barrier and mechanical properties is a never-ending process and demands a multidisciplinary approach to accomplish this goal. The present chapter mainly focuses on recent research accomplishments on polysaccharide-based films for food packaging applications.

K. Dharmalingam
R. Anandalakshmi (&) Advance Energy & Materials Systems Laboratory (AEMSL), Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati 781039, Assam, India e-mail: [email protected] K. Dharmalingam e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_9

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Keywords Food packaging films Packaging films

K. Dharmalingam and R. Anandalakshmi


Starch
Cellulose
Chitosan
Shelf life


1 Introduction The issue of environmental pollution is a global concern in the past few decades. The petrochemical-based plastics are primarily employed in the food packaging sectors, which tend to damage the environment for increasing carbon footprint and plastic-based waste. Plastic food packaging materials decompose over a long period ranging from hundreds to thousands of years [1]. A viable substitute for this problem is to increase the usage of biodegradable food packaging materials as they decompose or break down in a shorter period on compositing. Biodegradable food packaging materials instantly mix with the soil, are degraded by the microbes and bacteria present in the soil, which aids in soil fertility. The biodegradable food packaging materials are obtained from various renewable resources such as animal, microbial and agriculture feedstock. The main property that differentiates biodegradable food packaging materials from conventional packaging materials is accounted to sustainability and biodegradability for the environment-friendly packaging materials [2]. Polysaccharides are natural biodegradable polymers, widely used to prepare food packaging materials. Polysaccharides commonly block the permeation of oxygen due to their hydrogen-bonded network [3]. Polysaccharides are used to develop food packaging films for various applications because of their wide availability in nature, renewability, biocompatibility, film-forming ability and low cost [4, 5]. Amongst various polysaccharides, starch and cellulosic materials are derived from plants. Starch possesses a major disadvantage due to their strong hydrophilic nature resulting in poor water barrier and mechanical properties in comparison with petrochemical-based plastic films [6]. Another major polysaccharide material, chitosan is an amino-polysaccharide molecule and is derived from fungal cell wall, arthropods and crustaceans. The inherent antibacterial property of chitosan promotes its applicability in the biodegradable food packaging films. Films prepared from a single polymer do not often satisfy the requirements for food packaging applications. Hence, significant research has been done in improving the inferior properties of a single-polymer-based film by producing composites, nanocomposites and using cross-linking agents (functionalization). A composite film is made up of two or more different materials with distinct physical or chemical properties. The resultant composite films exhibit superior mechanical and barrier properties. In nanocomposites, at least a material of nanoscale (80% of relative humidity (RH). The RH (%) where the second phase exponential starts is called as the critical RH (Fig. 4). At RH regions ( 10) which due to molecular diffusion across the liquid-liquid interface results in a concentration gradient in the direction of flow. Here, the Peclet number (Pe) indicates the convection to diffusion flow. Two different parallel streams travel down the same channel at Re <  2100, for dominating the viscous effects over the inertial effects [22, 23]. The length of the fuel cell should be limited to mixing of the two streams for preventing the oxidation and reduction reactions, so the separation of fuel and oxidant is required [21]. The microfluidic fuel cells has been designed with several different architectural “T”, “Y”, and “F” shape configurations [13]. Due to the high surface-to-volume ratio, the microfluidic fuel cells generate power from surface reactions [16]. The major issue about microfluidic fuel cells is its reactant fuels and energy efficiency. The controlling factors are the reactant’s solubility and mass transport that greatly affect the energy densities [13]. The membrane removal from microfluidic fuel cells eliminates the membrane fouling, water management, reactant crossover and also significantly reduces the material cost [1, 9, 24]. From the early 1950s, microfluidics was applied in basics ink-jet technology using a liquid in the nano- and subnanoliter ranges. But, from other sources it is found that the first microfluidic device was developed in the 1970s. In the year 1979, a silicon wafer miniaturized gas chromatograph (GC) was discovered. In the 1980s, the first micropumps and microvalves had been introduced [19]. The huge developments of microfluidics are done since 2002 [16]. After that, microfluidic fuel cells technology has grown rapidly, and therefore as a strong candidate, it is considered for the next generation as portable micropower sources [25].

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Development of Miniaturized Fuel Cells

The continuously growing social demand for portable electronic devices stimulates the rapid development of small but high power sources of energy with lightweight, high durability, and long-time operation [9, 26, 27]. In fuel cell, due to the thermodynamic limitation the highest possible open circuit voltage is 1.23 V and it requires large size, high temperature, and high cost. Therefore, fabricating the microfluidic fuel cells not only downscales the cell size for improving energy densities but also establishes new materials with novel conceptual designs at low temperature and pressure [2]. Because of the complicated system structures of conventional fuel cell, it is difficult to miniaturize components and fluid volumes. So, novel designs of microfluidic fuel cells are being established to overcome all the limitations. The miniaturization of fuel cells increases the surface-to-volume ratio and leads to further improvements in energy density [28]. Miniature fuel cells can be a good alternative due to their advantages of simplicity, durability, and nature friendliness than batteries with high energy-conversion efficiency and capability of producing electricity as long as fuel and oxidant are supplied to the electrodes [26, 29]. Since microstructures are available in microfuel cells, the impact of miniaturization can be derived from the so-called scaling law [30]. Assuming a scaling factor of S, the ratio of the surface area to the volume scales is given as: Surface S2 / ¼ S1 Volume S3

ð1Þ

Here, with miniaturization (S < 1 and S1 > 1), surface effects will become dominant in microfuel cells. Similarly, for a constant flow rate, the pressure drop scales as Dp / S3

ð2Þ

The Peclet number describing the ratio of convection to molecular diffusion scales is given as Pe ¼

UL /S D

ð3Þ

where the mean velocity U and the diffusion coefficient D are assumed to be constant. The same law applies to Reynolds number, which describes the ratio of inertial force to viscous force: Re ¼

UDh /S v

ð4Þ

where Dh is the hydraulic diameter of the channel and v is the kinematic viscosity.

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Theoretical and Hydrodynamic Model

An overview of numerical modeling of the microfluidic transport phenomena in respect to microfluidic fuel cells is available in several reports. The velocity field  u for incompressible Newtonian fluids due to laminar nature can be expressed by solving the Navier–Stokes equations for momentum conservation in 3D. According to the continuum and laminar nature through a microchannel, the following assumptions are: (1) the density and the velocity of the fluid are constant; (2) the body forces such as the fluid weight are dominated by viscous shear stress. Using the Navier–Stokes and continuity equations and applying the mass and momentum conservation, the velocity profile can be expressed as [15, 31]:
 @u q þ u
ru ¼ rp þ lr2  u þ f ð5Þ @t Here, q and l represent the density and viscosity. u and p denote the velocity and pressure, respectively. The nonlinear convective terms are neglected at very low Reynolds numbers, resulting in linear and predictable Stokes flow:
 @u q u þ f ð6Þ ¼ rp þ lr2  @t The mass conservation for fluid flow obeys the continuity equation and the incompressibility condition with the assumptions: @p þ r
ðquÞ ¼ 0 @t

ð7Þ

 ¼ 0. A parabolic velocity For a fluid with constant density, this leads to: r
u profile is obtained for pressure-driven flow, whereas for electro-osmotic flows, the velocity profile is flat and uniform across the channel. For increasing the parasitic load to drive the flow, the channel size should be reduced and ultimately the net power generation would diminish. The pressure drop for pressure-driven laminar flow in a length of the channel L with mean velocity U and hydraulic diameter Dh is given by: Dq ¼

32lLU D2h

ð8Þ

The pumping power W is obtained by multiplying the pressure drop by the flow rate Q: W ¼ Dp  Q ¼

32lLUQ D2h

ð9Þ

Based on fully developed flow through the straight channel neglecting the contributions from inlet and outlet feed sections and other minor losses, the

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pumping power is calculated from Eq. (9). In microfluidic fuel cell, as the fluid flow is laminar when the microchannel size is decreased to micrometer. The Peclet number is defined as: Pe ¼

UDh D

ð10Þ

where Dh is a characteristic length perpendicular to the direction of the flow, U is the average flow velocity, and D is the diffusion coefficient of the particle or molecule, which has a positive correlation with the width of channel. The calculated width of the diffusion mixing region is in agreement with the theoretically predicted and experimentally trend represented by the form:
dx /

DHl U

1=3 ð11Þ

where l is the length of mixing zone for a normal microchannel and dx is the diffusion mixing region. For normal-shaped microchannel, “l” is equal to the length of the fluid flows downstream. While for a ladder-shaped microchannel, the parameter “l” is the

Fig. 3 Schematics of fuel chamber in a microchannel device. a Top views of the ladder-shaped and normal-shaped. b Microchannel structure and geometric parameters employed in the simulation. Reprinted with permission [31] (copyright 2013). International Journal of Electrochemical Science

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effective length of the widths of the gaps, which is much smaller than its actual length. Thus, the diffusion transverse to the direction of flow could be minimized by decreasing effective l. This geometric parameter would be an important consideration in the design of fuel cell systems (see Fig. 3).

3 Fabrication of Microfluidic Fuel Cell 3.1

Fabrication Technique for Microfluidic Fuel Cells

By adapting new materials and geometries, the microfluidic device architecture can be modified to more successful reaction conditions quickly at low price. Fabrication follows the forming of channels by solid substrate, before drilling holes into the substrate, and finally bonding it to seal the channels by using another plate [16]. Microfluidic devices can be fabricated using inexpensive and compatible well-established microfabrication and micromachining techniques such as photolithography and laser etching with high volume manufacturing. A variety of methods are available for fabrication of microfluidic devices, including

Fig. 4 Different fabrication methods for microfluidic devices. Reprinted with permission [33] (copyright 2014). Sensors and Actuators B: Chemical

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conventional machining, photolithography, soft lithography, wet etching, reactive ion etching, hot embossing, injection molding, in situ construction, laser ablation, and plasma etching, etc. [32] (see Fig. 4). For producing microchannel, the micromachining technology includes photolithography, thermal growth of silicon oxide, chemical, electrochemical, and ion etchings, chemical and physical vapor depositions, and anodic bonding. Lithography and etching are used for the fabrication of glass-based microfluidics. New techniques, such as powder blasting, are also developed [34]. Many other materials with different fabrication methods and combinations of materials are also used. Another new microfabrication technique is laser micromachining. But, the disadvantages of laser micromachining are its equipment cost and very slow process [35]. Three key materials are reviewed for the fabrication of microfluidic fuel cells, namely (i) base materials that compose the devices, (ii) ion exchange membranes (IEMs), and (iii) catalytic electrodes.

3.2

Base Material for Microfluidic Fuel Cells

This section will discuss the materials that are developed for microfluidic fuel cells device. Initially, silicon and glass materials are used for microfluidic devices using micromachining techniques with high resolution and can be integrated easily into clean room environment [36]. Micromachining techniques of silicon and glass materials involve the use of photolithography, wet and dry etching, electron beam lithography, and a variety of other techniques in clean room facilities [32]. The combination of photolithography and wet etching allows creating complicated channel geometries [35]. The silicon and glass materials show an excellent chemical and physical stability, solvent compatibility with optical and surface properties. But, silicon is expensive and unsuitable for optical detection. On the other hand, glass is difficult to etch with vertical sidewalls as it is an amorphous material [34]. Due to their chemical inertness and optical transparency, Pyrex and quartz are also used as base materials [35]. So, the traditional materials such as silicon, glass, or ceramics are unfavorable in microscaled implementations [16]. Alternatively, the polymer microfabrication method becomes more attractive, due to their wide range of physical properties and low cost [34]. The most common chip materials are elastomer, polydimethylsiloxane (PDMS), etc., which is well suited to rapid prototyping [16, 35].

3.3

Membranes for Ionic Transport

The microfluidic fuel cells used a physical barrier such as a proton exchange membrane (PEM) to separate the fuels from the oxidant in the anodic and cathodic compartments. But, in some cases there are still some issues for applying the PEM in a wide range of applications. The polymeric membrane must be the first preference as an inseparable and

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most valuable part of PEMFCs. Some bottlenecks could be due to the fuel crossover through the membrane, where a mixed potential at cathode lowers the fuel cell performance [22]. Also the proton conduction and the blocking of fuel permeation are the big issues for PEMFC. In this problem, Nafion (which is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer) fulfills proton conduction due to its sulfonate groups, but need improvement for fuel diffusion blockage [37, 38]. Brushett et al. [4] improved the performance of alkaline anion exchange membranes (AAEMs) by using Nafion-based membranes in acidic H2/O2 fuel cells. Chitosan-like hydrogel is natural polymer as well synthetic polymer which is reputed as an excellent candidate in membrane processes because of its ability to trap water with highly charged surface [39]. This feature makes the system compatible in high temperatures by delaying dehydration. At the same time, these positive charges give a good interaction between chitosan and Nafion [38]. Mitrovski et al. [40] introduced a Nafion membrane sealed by a hydrophobic polymer such as PDMS that eliminates the instability problems associated with the dehydration at higher temperatures as a single pore membrane. Using an elastomeric membranes, the overall output power is improved. However, two challenges are often encountered when relying on single pore membrane models. First, very few methods exist for convenient fabrication on nanometer scale. Second, very small amount of materials can be transported through a single nanopore [41].

3.4

Catalytic Electrode Materials

The catalytic electrode materials are located side-by-side on the glass bottom wall of the microfluidic fuel cells devices where the electrode is coated by lift-off and etch, two different approaches supported by lithography [15, 42]. Due to their lightweight, high electronic conductivity, and low cost, the carbon-based electrode materials are employed, including carbon fibers, graphite particles, graphite felt, carbon paper, carbon cloth, and foams [43, 44]. For high power density, carbon cloth, Ag/AgCl, Toray carbon paper, sat. KCl, CE PEM, and thin-film Pt are also used in many applications [45]. Using small Pd and Pt nanoparticles on microporous electrodes optimizes the cell performance by improving (i) particle size, (ii) particle shape, (iii) electrode active surface area, and (iv) flow-through porous electrode [46]. Many other catalysts that are comprised of Pt or Pd metal, such as Pt/C, PtRu/C, Pd/C, and Pd/MWCNT, are used to enhance the electrochemical reactions in the fuel cells (see Fig. 5). The power density with MWCNTs is higher due to activity, stability, selectivity, and higher active area [42, 47]. The carbon fiber electrodes are modified by enhancing their electrochemical properties, including thermal treatment, chemical modification, hydrothermal ammoniated treatment electrochemical modification, metal compound coating, metal nanoparticle coating, and so on which are not favorable to practical applications. The use of expensive novel materials with high energy consumption in the modification processes and the possible occurrence of unwanted secondary reaction such as the hydrogen evolution could drastically affect the cell performance [48].

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Fig. 5 SEM images of various catalytic electrode materials at different magnifications. a Pd nanotubes, b Pt/C on carbon paper. Reprinted with permission Arjona [46] (copyright 2015). ECS Electrochemistry Letters

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4 Polymers in Microfluidic Fuel Cells 4.1

Polymers in the Fabrication of Microfluidic Fuel Cell Design

Polymeric materials are widely used in microelectronics and various microsystem applications such as MEMS, microsensors, and microfluidics. Basically, polymers are high molecular weight macromolecular substrates [34]. It has various advantages over other materials such as low cost, inexpensive processing, flexibility, and ease of fabrication in various shapes and sizes compared to other microfluidic substrates such as silicon or glass. For microfluidic applications, polymers can be the perfect substrate at low cost and fluid-tight sealing with the devices and systems are required [49]. Clean room may not be necessary for using polymer, and infinite numbers of device with different types of channels can be made quickly for different applications. Depending on the number of devices and the design change, fabrication of polymer-based microfluidics can be classified into replication and direct fabrication [34]. Many other aspects like size, weight, physical resistance, and lifetime are also considered. Microfluidic components such as channels, diaphragms, and valves can be made using various materials such as elastomers, PMMA, PDMS, and solution gels [11, 15]. In the presence of external force, elastomer can be extended, but returned to the original state when the force vanishes [34]. So their deformation at high degree makes them very attractive for applications where high flexibility is required [49]. Lightweight materials such as polymethyl methacrylate (PMMA) and polydimethylsiloxane (PDMS or Sylgard 18) are preferable. Moreover, UV-sensitive resists, SU-8, are powerful material for its flexibility and capability to be sealed to each other by a hot-pressing technique [30, 50, 51]. Polydimethylsiloxane (PDMS) is inexpensive but an excellent candidate, offering several advantages such as cheap, stable, flexible, nonflammable, optically transparent, glass fusible, good optical transparency, biocompatible, and impermeable [19, 52]. SU-8 is highly functional and easy integratable negative photoresist with excellent mechanical properties, high bond strength, etch resistance, low process temperature, high aspect ratio with faster drying and are chemically stable [31, 32, 36, 53]. The thermoplastic polymeric substrate PMMA can be reheated and reshaped in many times before hardening. It also provides other excellent properties such as high chemical resistance, low frictional coefficient, and good electrical insulation that make PMMA a good substrate for microfluidic devices involved in numerous chemical applications. PMMA involves the fabrication method such as X-ray exposure, hot embossing, and laser machining [8].

4.2

Polymer Materials as Proton Exchange Membrane

Recent development of polymeric micromachining makes polymers an attractive strong candidate for microfluidic fuel cells device [34]. There are numerous number

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Fig. 6 a SU-8 membrane with a 4 inch diameter with perforated 3-lm pores and 3.6 lm thickness; b SEM images of the membrane after attaching the membrane on a dummy Si wafer; SEM images of c top and d bottom view of the SU-8 membrane with 3-lm pores. Nanoscale black dots after Au coating appear on the top surface. Reprinted with permission [54] (copyright 2010). Journal of Micromechanics and Microengineering

of polymeric materials fabricated by ion track etching in polycarbonate and anodization of aluminum, which produce randomly hexagonal arrays of nanopores, respectively [54] (see Fig. 6). Among all, Nafion and chitosan are widely used for ionic transport through the membrane [38]. Ideally, ionic transport through the membrane must be fast and highly selective, but in Nafion-based membrane, which is an ionomer permits on the exchange of proton across the membrane [37]. Choi et al. [54] reported a sacrificial layer technique to produce membrane in thin polystyrene (PS) supported on a Si substrate and Kjeang et al. [55] used Nafion membrane with V2+ and VO2+ solutions as anolyte and catholyte, respectively. Instead of several technical issues, Nafion membrane is found very attractive in direct methanol fuel cells for avoiding the cathode flooding, anode dry-out, and fuel crossovers [4, 56]. Nafion membrane sealed by a hydrophobic polymer such as PDMS eliminates the instability problems associated with the dehydration of the ionomer at higher temperatures in PEMFC systems. The overall output power can be improved by using elastomer of higher reactant solubility [40].

4.3

Polymers as the Electrode Materials

Polymers are known as electrical and thermal insulators, but when filled with appropriate conductive materials, they can be used as heat sinks, electrical

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conductors, and even magnets [57]. If elastomeric material is added with conductive fillers in sufficient quantity, the composite conductive material advances its properties such as deformation, force, sensitivity to temperature and moisture [58]. Thermoplastic materials are categorized in large-scale materials, which help in searching the appropriate polymer for every application. Polymers for high temperatures (250 °C) such as polyetheretherketone (PEEK), chemicals (alkaline solutions, acids, and solvents) resist polymer such as perfluoralkoxy (PFA), molded microstructures for soft and elastic such as polyoxymethylene (POM) or hard and brittle such as polysulfone (PSU). Optically transparent materials such as (cyclo-olefin copolymer) and opaque ones such as polyamide (PA) also can be used with graphite materials. PVDF has piezo-electrical effect for using catalytic electrode. Hence, for micromolding of thermoplastic polymers in microcomponents has been considered [57]. In many researches for creating micromachined flexible devices, carbon black in polyurethane (PU) elastomer has been used [58]. The carbon nanotube/polyaniline carbon paper (CNT/PANI carbon paper) electrodes contain the carbon nanotubes (CNT) for enhancing the electron transfer ability [43]. Engel et al. [58] prepared conductive elastomers (both PU and PDMS) using multi-walled carbon nanotube (MWCNT) filler and compared their physical and performance properties to the carbon black composites. Other variants of carbon

Fig. 7 Typical microfabrication process for the microfluidic fuel cell. a Electrode fabrication. b Microchannel fabrication. c Integrating the microchannels with the electrodes by PDMS-glass bonding (sectional view). Reprinted with permission [31] (copyright 2013). International Journal of Electrochemical Science

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family such as exfoliated graphite could be suitable electrode in microfluidic fuel cells [59]. Varcoe et al. [60] investigated the performance of a H2/O2 fuel cell using polytetrafluorethylene (PTFE)-bonded Pt/C, Ag/C, and Au/C cathodes in their researches. Brushett et al. [4] also used to characterize and optimize GDEs with PTFE-bonded Ag/C and Pt/C catalysts in alkaline media (see Fig. 7).

5 Paper-Based Polymer in Flexible Microfluidic Fuel Cells 5.1

Paper Microfluidics

Paper is produced from moist fibers of cellulose pulp of wood and grass by pressing and drying them into thin flexible sheets. Among many of its benefits and uses in our daily life, it was first used as litmus paper for analytical purpose in the 1800s. Since then, filter paper has been used in laboratories for metal spot testing, chromatography, and filtration. In paper, cellulose fibers are arranged in random manner, thus creating a porous structure. This porosity and hydrophilic nature of paper assists in the transport of a fluid which is known as capillary motion or wicking. Its use as a substrate material in microfluidics was first demonstrated in 2007 for portable bioassays [61] (see Fig. 8). The microfluidic channel can be fabricated by using a hydrophobic photoresist to create walls to drive the fluid/reagent at a particular location on paper surface. Over the years, researchers have explored paper microfluidics for several applications to develop portable, miniaturized, and

Fig. 8 Fabrication steps for microfluidic paper-based analytical devices. (a, b) Photolithography. (c) Embossing of the parafilm (d) by clamping and heating sandwich is fastened (e, f). Reproduced with the permission of [62]

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ultra-low-cost devices, focusing largely on chemistry, biology, environment, and health diagnostics. A much wider coverage of paper application in the development of modern self-sustaining devices depends on the advantages associated with paper which could be briefly listed as follows: – – – – –

An inexpensive and renewable resource Lightweight and thin Combustible, biodegradable, and suitable for disposable devices Flexible and compatible for biochemical assays Easy manipulation depending on application and modification for mass production – White color is best for visual and colorimetric analysis. These benefits of paper have been used by researchers for microfluidic applications and development of numerous Lab-on-Chip devices and microfluidic paper-based analytical devices (lPADs). The following section describes some of the best-known techniques used for fabrication of paper-based devices.

5.2

Theoretical Background and Flow Control in Paper Substrate

The transport of a fluid such as water through paper occurs through the passive wicking mechanism. This wicking-type flow in a capillary is of laminar fashion due to fiber length scales and pore size (typically less than 20 lm) resulting in very low Reynolds numbers flow [63]. Capillary-based flow of a liquid has been observed in many areas such as paper chromatography, printing, textile, to name a few. The capillary force-based model first reported by Lucas and Washburn in the early nineteenth century is presented below [64, 65]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi crt cos h x¼ 2l where c is surface tension of the fluid having viscosity l imbibing a distance x in time t, r is capillary radius, and h is the contact angle between the fluid and capillary wall. This implies that fluid penetration distance (x) increases with increasing effective capillary radius (r). Interesting to note that lateral flow model of Washburn holds only as long as x  z where z is the height of fluid in a vertical column when the negative force of gravity is equal to the positive capillary force [66]. Overall, the equation is a first-order approximation for fluid transport and overestimates lateral wicking speed of fluid penetration distance. The factors such as swelling of fibers during wetting, increase in hydrodynamic resistance, and non-uniform flow of fluid have been neglected.

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Capillary flow is a result of pressure gradient across the fluid meniscus in a capillary due to a nonzero surface tension force caused by the cohesive interactions within the fluid molecules which are present at the interface [67]. This capillary pressure on a fluid can be described using the Young–Laplace equation P¼

2c cos h r

where c is surface tension, h is contact angle, and r is capillary radius. The values of liquid surface tension and its contact angle with paper surface must be known to apply Young–Laplace equation. Since the capillary is open at both ends and negligible gravitational forces are considered, atmospheric and hydrostatic pressures values become negligible (Washburn 1921). Thus, viscosity, surface tension of the liquid, its contact angle with the paper fiber material, and the capillary size are the key parameters that influence fluid flow in paper. However, the application of this model to paper microfluidics is hindered by the uncertainty of the capillary size. Therefore, Washburn equation is applicable in paper microfluidics to determine the channel length and time taken for the sample to reach a particular location. To slow fluid movement, the channel width can be increased to increase the viscous resistance [64, 68]. Flow in a complex fibrous network structure considered as porous media can be explained by using well-known linear force–flux relation known as Darcy’s law [63]. Q¼

kwh DP ll

where Q is volumetric flow rate, k is permeability of the paper, l is the length of the paper channel, w is the width, h is the height, µ is the dynamic viscosity, and DP is the pressure gradient. In 1956, the law was first determined empirically for water flow-through sand-dirt system (Bobeck). Very recently, Elizalde and co-workers theoretically investigated the capillary imbibition of fluid in paper-like substrates with macroscopic geometry [69]. A fluid dynamic model was derived using Darcy’s law for homogeneous porous substrates with arbitrary cross-sectional shapes, which allows to determine the cross-sectional profile for a prescribed fluid  required  velocity or mass transport rate (w ¼ w0 exp  lx0 ). Walji and MacDonald [70]

emphasized the influence of parameters associated with larger fluid volumes and quantified their impact. They investigated experimentally the impacts of several properties such as larger width and length, surrounding conditions (humidity and temperature) during imbibitions in paper along with abundant fluid reservoirs. It was found that fluid flow velocity in paper varies with temperature and width but is independent with length of the paper channel and humidity. Porosity and permeability are important factors in determining fluidic behavior. Therefore, a group reported a theoretical analysis of a pressed paper and observed that a pressed paper

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can be a useful experimental tool for studying porosity and permeability. Moreover, the relationship in pressed paper is expected to be useful in the design of paper-based microfluidic devices that include several pressed regions [71, 72]. In paper microfluidics, fluid transport depends on the structure of the paper and material used for its fabrication. The one-way capillary-based fluid transport in paper microfluidic devices described as self-pumping or passive pumping faces could pose difficulty in precise manipulation and control of the flow in the chip [73]. Being a passive system, the flow rate of fluid cannot be altered once the microfluidic device is fabricated. Basically, there are two factors that could characterize the capillary flow in a microfluidic system—first, physico-chemical properties of the different interfaces involved, and second, interdependence of flow domain in both the microscale and macroscale on geometrical characteristics. Combining these capillary flow determinants could encourage investigations on the influence of various aspects of paper parameters such as fiber source and size variations, type of paper, chemical treatment for flow manipulation [74, 75]. Paper with different shapes and sizes such as rectangular, circular, trapezoidal, sector type have been studied to observe fluid dynamics. These reports first consider a substrate and design and then try to predict the instantaneous position of the meniscus in paper as well as other porous materials. Very recently, Elizalde et al. [69] proposed an inverse calculation method which allows to determine the cross-sectional profile required for a fluid velocity/mass transport rate to have rational designs for paper-based microfluidics. The photocatalytic property of TiO2 on paper surface was studied in creating channels [76]. The approach is beneficial in two ways: first, the liquid flows over paper surface instead of paper matrix. This eliminates problems such as fiber swelling which further induces complexity in fluid flow and liquid retention, cellular transport, interaction between biological samples and paper fibers [77–80]. Second, a roll-to-roll process is adopted to deposit TiO2 over paper surface (on-time reagent delivery, high-speed, large-scale manufacturing-oriented method which is critical in fabrication of paper-based assays). Similarly, fluid delivery in paper-based devices could be manipulated in several ways. For example, a paper channel can be connected and disconnected manually using a switch. To achieve this, a method is proposed to pattern the paper surface using plasma treatment. The hydrophobic paper with well-defined hydrophilic channels could be made [81]. The time of fluid arrival by varying length and width of the channel was also studied [63, 82]. Similarly, studies about the application of quantity-based dissolvable sugar over paper surface to induce the time delay [75] and water-soluble pullulan film to cut off the fluid flow from the sensing area acting as shut-off value [83] were reported. Likewise, use of paraffin wax as surfactant to modify the surface characteristics of paper and change the fluid dynamics has been studied in the past few years [75, 77, 82, 84, 85]. However, these findings are associated with one or more limitations such as user expertise, reagent volume, complex fabrication, and processing steps. On the other hand, fluid flow control by active methods has been investigated as well. Hwang et al. [86] placed a paper channel on a disk and demonstrated fluid control by using centrifugal force. Fluid control could be done in both forward and reverse directions

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by varying the strength of the centrifugal force and capillary force. Application of electric field on a paper surface to control the movement of an electrolyte solution was demonstrated by using pencil-made graphite electrodes [87]. Facile fluid flow rate control method is proposed using wax printing technique to vary the permeability of the paper. Brightness and length of wax patterns were found to critically affect the fluid movement [88]. Very recently, Jafry et al. [89] compared the flow profile in paper-based channel in terms of their geometry using photolithography and wax printing techniques It was observed that the two fluids (water and oleic acid) followed the Lucas–Washburn equation only when the width of the paper channel is large enough. This method could facilitate simultaneous addition of chemicals without varying the length and width of the channel. Overall, the paper-based microfluidics still require improved techniques for rapid and controlled manipulation of fluid depending on various applications. In addition, it must be noticed that paper-based microfluidic channels are open to the environment which restrict its potential application in many other areas.

5.3

Paper-Based Miniaturized Fuel Cells

The last decade has seen rapid development of paper microfluidic devices popularly known as lPADs. This advancement has encouraged researchers to focus on the development of new energy-generating devices which could be flexible, miniaturized, affordable, and well integrated to power them along with delivering high power performance. In this direction, several types of paper-based devices have been reported for power generation such as fuel cells, lithium-ion batteries, supercapacitors, to name a few. Recently, Chang et al. [90] proposed an environment-friendly reverse electrodialysis (RED)-based energy harvesting method using chemical energy conversion from salinity gradients. REDs does not discharge any pollutants like CO2 and convert Gibbs free energy into electricity by using an ion-selective membrane. Moreover, REDs can be an integral part of the disposable lPADs and thus eliminate the integration issues. However, REDs involve several components or layers to be integrated and deposit over paper surface leading to complex design and fabrication steps along with the use of environmentally unsafe chemicals and hazardous reagents. Paper-based energy devices are still facing limitations in terms of their longer working life or durability, shelf-life, environmental induced external conditions, repeatability issues. It is hopeful that the involvement of network fibre structures on paper substrates in a crisscrossed manner would drive novel fabrication methods which can push for mass production of such devices and guide the industries to take them up for commercialization and application of new materials to enhance their energy generation capacity. On the need to ease device fabrication, process optimization via simple electrode preparation and cost-effective paper-microfluidic fuel cells. Arun et al. [91] showed a novel technique using graphite-based electrode over a paper-strip device operating on self-pumping mechanism to generate power

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estimated at respective power and current density of 650 mA cm−2. A modification of this device by deposition paper substrate utilizing fast reversible redox reaction yielded generation capacity in terms of current density approximated able as a low device cost micro-energy device [92].

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32 mW cm−2 and graphene oxide on a significant energy 325 mA cm−2 suit-

6 Conclusion In this chapter, highlights on the principles of microfluidic fuel cells have been presented without ambiguities. We delineated important physical concepts of transport and reaction dynamics in microfluidic fuel cells using established theoretical framework. The utilization of low-cost polymeric elastomers is emphasized because of the ease of fabrication of polymer materials, flexibility and suitability as base materials in microfluidic fuel cell. Common polymeric materials used in the design of microfluidic fuel cell-like PDMS and PMMA have been discussed. Of the several polymeric materials used as ion barrier and ion exchange membrane, Nafion has been favored as proton exchange membrane which could be improved by careful choice of reactant-resistant elastomers. Also, the suitability of polymer electrodes was discussed. Conductivity can be imparted onto polymeric elastomers at matrix level which could advance its usefulness as electrode materials in fuel cell application. Since power density of requirement of fuel cells is expected to be maintained at optimized level, the involvement of catalyst in microfluidic fuel cells could enhance system performance. Interestingly, MWCNT has been identified as a candidate catalyst suitable for high-performance microfluidic fuel cells systems owing to its unique features such as high activity, stability, selectivity, and high surface area. In a facile approach, paper-based microfluidic fuel cells though restricted due to its requirement of exposure ambient condition to initiate capillary actions could be applicable where intricate flexibility is required for one-time energy generation. Overall, the concept of sustainability and polymer utilization in fabrication and functionality of fuel cell on microfluidic platform has been discussed based on flexibility, fuel oxidation performance, and rapid energy generation capacity. There are propositions in favor of microfluidic fuel cells as the future of electrochemical power generators owing to its capability for high energy density performance and ease of performance and portability cum suitability of power delivery in microscale. This is obvious since product miniaturization and compact technologies would define the spectrum of next-generation devices, hence the challenge for high energy density power generators suitable for components in microconfinements. When there is a demand for on-chip power generation and storage, microfluidic fuel cells would become a choice approach to achieving this sort of flexibility and exactitude of power delivery. Some arguments have opined that solutions developed for other microfluidic devices could also be employed as fuel in microfluidic fuel cells redox flow battery as a sustainable approach to energy generation. The wide range of chemical fuels suitable for microfluidic fuel cells

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operation is a reason for its choice as the power generator of the future. However, there are propositions that vapor-feed microfluidic fuel cells possesses the potential for achieving high energy density compared to traditional liquid-feed microfluidic fuel cells. This would require the use of modified polymeric elastomers with tailored functionality to conserve the transport of gas fuel and energy generation with the micro-environment. Certainly, the prospect of this emerging usefulness of microfluidic fuel cells is largely dependent on the response of polymer materials as conduits, ion exchange membranes and electrodes to bioelectrochemical activities, biocompatibility and energy generation potentials. Further matrix level modifications on polymeric materials used in microfluidic fuel cells would include corrosion-resistant properties for polymer electrolytes and polymer-based electrodes. This could be achieved by cross-linking of electrolytic alloys and conductive polymeric elastomers. Inclusion of graphene into pre-polymer mix could impart anti-corrosion capacity in microfluidic fuel cells electrodes. One important aspect of polymeric ion exchange membranes in microfluidic fuel cells is durability, and creating polymer structures that will withstand degradation under operational redox processes would be milestone in microscale fuel cell technology. Specially tailoring porosity, ion selectivity, mechanical resilience, and matrix integrity are essential aspects that will define sustainability of polymeric materials in microfluidic fuel cells. Emerging microscale techniques such as chemically mediated surface patterning of polymeric membrane materials could be employed to address rapidity of fuel oxidation process and energy release in microfluidic fuel cells. Significant knowledge gap still exists in order to explore and upscale the usefulness of microfluidic fuel cell from laboratory scale to industrial operations using advance polymeric materials as the backbone of the power generation device. Acknowledgements The authors thank Prof. Harish Hirani, Director, and Dr. Nilrudra Mandal, Head, Material Processing and Microsystems Laboratory, CSIR-CMERI, Durgapur, for their encouragements. Support from DST-SERB grant under project no. GAP-221112 is gratefully acknowledged.

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Chapter 16

Sustainable Polymeric Nanocomposites for Multifaceted Advanced Applications Rituparna Duarah, Deepshikha Hazarika, Aditi Saikia, Rajarshi Bayan, Tuhin Ghosh and Niranjan Karak

Abstract In recent times, bio-based hyperbranched polymers have attracted tremendous interest in industrial and scientific research, owing to their promising and unique attributes over their synthetic conventional analogs. Industrially important bio-based hyperbranched polymers such as polyurethane, polyester, poly (ester-amide), and epoxy have been developed over the last two decades from Advanced Polymer and Nanomaterial Laboratory of Tezpur University. They are synthesized by the Ax + By (x, y  2) approach with or without solvent, following the dictates of “Green Chemistry.” Again, it is a well-known fact that “virginity is not virtue” in case of polymers and the conventional filled composite systems are inappropriate to improve the performance of such bio-based polymers and hence unable to meet the service demands of advanced applications. Thus, nanotechnology, in recent times, is adopted to develop a variety of nanocomposites of the above sustainable polymers with different types of nanomaterials. The developed nanocomposites showed significant improvement of desired properties including mechanical, thermal, chemical, biological, optical, etc., along with special properties like shape memory, self-healing, self-cleaning, biocompatibility, etc. A brief overview of such sustainable materials including their applications from advanced air cleaning paints to injectable bone tissue scaffold including smart materials is discussed in this chapter.




Keywords Bio-based polymer Hyperbranched structure Polyester Epoxy Nanocomposites Application











Polyurethane


R. Duarah
D. Hazarika
A. Saikia
R. Bayan
T. Ghosh
N. Karak (&) Advanced Polymer & Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, 784028 Tezpur, Assam, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_16

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1 Introduction In recent times, different types of polymers, especially polyurethane(s) (PU), epoxy, and polyester, have gained significant research interest in various fields owing to their various attributes like physical and chemical stability, versatile properties, ready availability, etc. [1, 2]. They are widely used for diverse applications including surface coating, laminates, paints, adhesives, inks, biomaterials, smart materials, etc. [2]. In this milieu, syntheses of such polymers using bio-based resources have gained tremendous attention for the last few decades due to the depletion of fossil fuel-based resources which were conventionally used as the monomers for the synthesis of such polymers [3]. Additionally, the high rate of consumption of the synthetic polymers causes a detrimental effect on the environment and health. Henceforth, bio-derived monomers are used in place of petroleum-based monomers for the synthesis of conventional polymers in order to reduce the hazardous impact of the synthetic polymers on the environment [2]. In addition, biodegradability is a serious issue of synthetic polymers because they occupy large space in landfills and release toxic gaseous products upon thermal degradation. Therefore, development of biodegradable polymers using bio-derived raw materials is an apt choice. Prior art literature advocates successful preparation of bio-based polymers of PU, epoxy, and polyester using different types of renewable resources such as vegetable oils (sunflower oil, cashew oil, castor oil, palm oil, linseed oil, soya bean oil, coconut oil, etc.), polysaccharides (starch, chitin, sucrose, etc.), agricultural by-products (rice husk), cashew nut seed oil [4–8]. However, the applications of such bio-based polymers in many advanced fields are limited as they possess low hardness, poor mechanical strength, and weak alkali resistance [9, 10]. Thus, nanotechnology, the most attractive technique in recent times, is adopted to develop a variety of nanocomposites of the above sustainable polymers with almost all types of nanomaterials from zero to two including one-dimensional [9]. The developed polymer nanocomposites exhibited improvement in desired attributes such as mechanical, thermal, chemical, biological, optical, electrical, catalytic, etc., along with special properties like antimicrobial, antistatic, fluorescent, shape memory, self-healing, self-cleaning, biocompatibility, etc., depending on the nature, state of dispersion, and interfacial interactions of the nanomaterials in such systems [11–13]. A general idea of all such materials including their applications from advanced air cleaning paints to injectable bone tissue scaffold including smart materials is discussed in the chapter. The sustainability issue of the materials in the context of current demands is also addressed.

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2 Materials 2.1

PU

PU is a special class of polymers containing hard and soft segments in a single polymeric backbone and obtained through the step-growth reaction between diols and diisocyanates [14, 15]. Again three-dimensional, globular, highly functionalized non-entangled architecture of hyperbranched PU (HPU) added significant advantages in the field of PU due to their low cost, easy one-pot preparation, high solubility, low melt, and solution viscosity. Generally, the starting materials used for the PU synthesis are classified into the following categories. A concise description of the components used during PU preparation is given below. Di/poly Isocyanate. Isocyanates are one of the essential and reactive components for the preparation of PU containing cumulated double bond (R–N=C=O), and its reactivity is governed by the positive charge density on the carbon atom which is more susceptible toward nucleophilic attack [16–18]. Prior art literatures advocate the use of di/polyfunctional isocyanates as reactive building blocks which are also considered as the part of the hard segment of the polymeric backbone [19]. Henceforth, isocyanates exhibited a crucial role to accomplish the desired properties of PU. Structures of some diisocyanates with their possible applications used in PU synthesis are enlisted in Table 1. Among all the available diisocyanates, MDI and TDI are extensively used for the production of commercialized PU goods because of its high reactivity and low cost compared to other diisocyanates. As per the report of 2015, global production of TDI crossed the mark of 5 billion pounds. Even though aromatic diisocyanates are highly used for PU preparation, some severe drawbacks like low UV resistance and low stability minimize their application and broadening the scope of aliphatic diisocyanates. Macroglycol. Like isocyanates, macroglycol is another essential component that consists of di- or poly-hydroxyl groups with a molecular weight 400–5000 g mol−1 [20]. These are considered as a soft segment in PU backbone. Nowadays PU is prepared using different types of macroglycols like polyester polyols, hydrocarbon-based polyols, polyether-based polyols, polydimethylsiloxane-based polyols, etc. Among different types of available macroglycols, poly(ethylene oxide) glycol (PEG), poly(propylene oxide)glycol (PPG), poly(butylene oxide)glycol (PBG), poly(e-caprolactone) diol (PCL-diol), bis(hydroxyalkyl)-polydimethylsiloxane, poly(ethylene adipate), poly(ethylene glutarate), etc., are regularly used for the production of PU. However, recent literatures cite the synthesis of different types of bio-derived macroglycols such as castor, soybean, grape seed, sunflower, olive oil-based polyols, glucose, and different fatty acid-based polyols which were successfully used for the preparation of PU with more biodegradability and biocompatibility compared to the conventional petroleum-based macroglycols [21, 22].

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Table 1 Types of diisocyanates in PU synthesis Type

Name

Major applications

Aliphatic diisocyanates

1,6-hexamethylene diisocyanate (HDI)

Dental materials, automobile coatings, UV-resistant coatings, contact lenses Biodegradable scaffolds, foams, UV-resistant coatings Integral–skin foams, UV-resistant coatings Scaffolds, fibers, UV-resistant coatings Elastic PU

1,4-diisocyanatobutane

2,2,4-trimethylhexamethylene diisocyanate (TMDI)

Isophorone diisocyanate (IPDI) Norbornane diisocyanate (NDI)

Aromatic diisocyanate

4,4’-methylenebis (cyclohexyl diisocyanate) 2,4/2,6-toluene diisocyanates

Methylenediphenyl diisocyanate (MDI)

Xylene diisocyanate (XDI)

Naphthalene diisocyanate

UV-resistant PU materials Coatings, elastomers, sealants, adhesives, thermoplastic foams Rigid foams, high resilience, tires, wheels, dental restorative material, etc. Sports equipments, thermoplastic foams, and elastomers Bioabsorbable surgical composition, high tear-resistant PU foams, etc.

Structure

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Chain Extender. Chain extenders are generally categorized as diols, diamines, and hydroxylamines with a molecular weight less than 400 g mol−1 and generally used for the extension of polymeric chain to obtain a high molecular weight PU. Among all the chain extenders, 1,4-butanediol, ethylene glycol, 1,6-hexanediol, glycerol, 1,4-cyclohexanedimethanol, triethanolamine (TEA), ethanolamine, diethanolamine (DEA), ethylene diamine, diethylene triamine, isophorone diamine, etc. [14, 23], are used regularly for the preparation of PU. Recently, bio-based chain extenders like esterified monoglyceride, fatty amide, etc., of vegetable oils are routinely used for the synthesis of PU. Catalyst. However, to alter the reaction kinetics and to modify the reaction conditions like time, temperature, some catalysts are commonly used during the production of PU. In general, there is no specific need of catalysts in aromatic diisocyanate-based PU reactions because of their high reactivity. However, in case of aliphatic diisocyanates, catalysts are quite essential. Literature reports advocate the use of a number of amine-containing catalysts (e.g., dimethylcyclohexylamine, diaminobicyclooctane (DABCO), dimethylethanolamine, triethylamine), organometallic catalysts (e.g., dibutyltin diacetate, dibutyltin dilaurate (DBTDL), tin octoate (TO), iron acetylacetonate), etc., for the PU preparation [14, 23]. Even though the use of catalysts is very helpful, toxicity and costly separation technique minimize its demand for PU production. Recent literatures based on PU mostly reported the synthesis of PU without catalysts. Additives. To obtain the desired set of properties for different applications, additives are used during PU preparation along with macroglycols and diisocyanates. On the basis of the function of the additives, they are classified into following categories like crosslinkers, plasticizers, fillers, curing agents, blowing agents, moisture scavengers, stabilizers, colorants, flame retardants, etc. [24]. Additives are very useful to attain desired properties, but the toxicity of most of the additives, leaching and migrating tendency from the matrix, and incompatibility with PU matrix limits their usefulness. Therefore, a huge research interest was projected to develop bio-based additives which not only reduces the toxicity, but also increases the compatibility and biodegradability of the PU matrix.

2.2

Polyester

Polyester is used as the matrix for the fabrication of polyester nanocomposites. It is generally synthesized by direct esterification of carboxylic groups of polybasic acids with hydroxyl groups of polyhydric alcohol through polycondensation reaction, though other kinds of esterification reactions are also employed [25]. Thus, for the synthesis of different polyesters, a huge number of polybasic acids and polyhydric alcohols are used and some of them are listed in Table 2. Monobasic acids such as acrylic acid, lactic acid, glycolic acid are also used along with the polybasic acid to terminate the polymeric chain. Further, some anhydrides (maleic anhydride,

D-mannitol

Sorbitol

Trimethylolpropane 1,3-propane diol

Xylitol

Pentaerythritol

Erythritol

Itaconic acid

Adipic acid

Glutaric acid

Aspartic acid

Sebacic acid

Succinic acid

Tartaric acid

Polyhydric alcohol Glycerol

Structure

Citric acid

Polybasic acids

Table 2 Different types of polybasic acids and polyhydric alcohols Structure

(continued)

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Polybasic acids 1,4-butane diol

Isosorbide

Ethylene glycol

Fumaric acid

2,2-bis(hydroxymethyl) propionic acid

Ricinoleic acid

Structure

Table 2 (continued) Polyhydric alcohol

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trimellitic anhydride, dimethyl terephthalate, pyromellitic anhydride, glutaric anhydride, succinic anhydride, etc.) and polybasic acids (isophthalic acid, terephthalic acid, etc.) are also used for the synthesis of different types of polyesters. Further, different types of vegetable oil like castor oil, sunflower oil, rubber seed oil, soybean oil, safflower oil, tung oil, etc., are also used for the synthesis of polyesters. Furthermore, reactants with polar functional groups (–COOH, –NH2, –OH, etc.) such as 2-bis(hydroxymethylpropionic) acid, poly(ethylene glycol) are essential to achieve water solubility/dispersibility of polyesters [26, 27]. In order to obtain the thermosetting systems, they are cured with different crosslinking agents. The mixture of epoxy resins and hardener is one of the most commonly used curing systems for the formulation of thermosetting polyesters [28].

2.3

Epoxy

The preparation of epoxy resins involves different types of materials which are both renewable and non-renewable in nature. These materials are briefly presented below. Bisphenol A (BPA) and Epichlorohydrin (ECH). BPA and ECH are organic synthetic compounds with the chemical formulae: (CH3)2C(C6H4OH)2 and C3H5ClO, respectively. BPA is synthesized by acetone condensation with two equivalents phenol in the presence of a strong acid or sulfonated polystyrene resin [29]. Similarly, ECH is produced from the substitution reaction of glycerol with hydrogen chloride in the presence of a carboxylic acid catalyst followed by base treatment [30]. Both of these two reactants—BPA and ECH—are chemically reacted together in the presence of sodium hydroxide (NaOH) at elevated temperature to form diglycidyl ether of BPA (DGEBA) [31]. Vegetable Oil. Vegetable oils are the most commonly used renewable resource-based starting material with a wide selection of structural and functional variations. These are mainly extracted from seeds or stems of the plants which are versatile and easily available. The most commonly used oils are castor oil, linseed oil, soyabean oil, etc., whose –C=C– bond was epoxidized using molecular oxygen or organic per acids. Also, it can be converted into monoglyceride by glycerolysis with glycerol or polyol through a condensation reaction. The linear aliphatic chain of vegetable oil provides flexibility and plasticizing effect to the resins [32]. Polysaccharide and Polyol. Polysaccharides are naturally occurring polymeric carbohydrates which are linked together by glycosidic linkages. The most commonly used polysaccharides are starch, cellulose, and chitin. Similarly, polyols are the aliphatic alcohols or triglyceride molecules that are extracted from plant or animal source, e.g., glycerol, sorbitol, etc. [8, 33, 34]. These polyols or polysaccharides can be used as the starting materials for branching epoxies.

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Nanomaterials

The term “nano” signifies a dimension which is one-billionth of a meter (m), i.e., one nanometer (nm) in order. Thus, a “nanomaterial” is a matter with at least one of their dimensions in the nanoscale. According to the International Standard Organization, nanomaterials are material with any external dimension in the nanoscale or having internal structure or surface structure in the nanoscale [35]. These nanomaterials having one, two, or three extra dimensions in the nanoscale region are engineered, manufactured, or incidental nanoparticles, nanofibers, nanorods, nanosheets, nanoribbons, nanotubes, nanocubes, core-shell nanoparticles, etc. [36]. Various definitions can be found in state-of-the-art literature, albeit retaining the essence of the nanodimension. Nanomaterials can be prepared by a host of physical and chemical techniques, including ball-milling, cutting, extruding, chipping molecular assembling, and many more bottom-up approaches as well as top-down approaches [37]. In order to appreciate the diversity of nanomaterials, they are classified mainly on the basis of their origin, morphology, and dimensions. Depending upon the elemental origins, nanomaterials can be classified into three classes: (a) organic, (b) inorganic, and (c) hybrid (containing both organic and inorganic constituents). Again, nanomaterials can be differentiated in terms of morphology, where their main characteristic includes sphericity, flatness, and aspect ratio. Small aspect ratio morphologies usually attain the form of sphere, oval, cubic, helical, or rod, while high aspect ratio morphologies are found in the shape of zigzag, helices, and belts [38]. Lastly, based on dimensions, nanomaterials are classified into three classes: (a) zero dimension (0-D), (b) one dimension (1-D), and (c) two dimension (2-D). These nanomaterials possess their own unique physical and chemical properties that set them apart from one another [39]. All of these nanomaterials are utilized for the preparation of polymer nanocomposites. The choice of nanomaterial mostly depends upon the desired properties and targeted applications. Some of the commonly employed nanomaterials in PU nanocomposites are briefly described below. Carbonaceous Nanomaterials. Carbonaceous nanomaterials are basically nanomaterials with carbon as the main element in nature and are found in 0-D, 1-D, and 2-D (Fig. 1). Due to their abundance and facile preparation, they are the most significant nanomaterials in recent times, with tunable physical, chemical, electrical, optical, thermal, and biological properties. Graphitic nanostructures such as carbon nanotube (CNT), graphene oxide (GO), reduced graphene oxide (RGO), Carbon dot (s) CDs constitute the most frequently used nanomaterials for preparation of polymer nanocomposites. Graphene is a single atom thick layer (2-D) of sp2hybridized carbon atoms, arranged in a honeycomb structure. Graphene possesses unique properties like high elastic modulus, large theoretical specific surface area, excellent strength, etc. [40]. These qualities endow graphene to be an apt choice for the fabrication of polymer nanocomposites. CNT is a cylindrical (1-D)-type layer of sp2-hybridized carbon atoms and possesses unique length-to-diameter ratio, which may extend up to 132,000,000:1 [41]. Multi-walled CNT (MWCNT) or

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Fig. 1 Commonly used carbon-based nanomaterials in polymer nanocomposites

single-walled CNT (SWCNT) exhibits extraordinary mechanical and electrical properties as well as thermal conductivity, due to which it is extensively used for nanocomposite fabrication. In stark contrast, CDs is the newest member in the carbon nanomaterial family with a near-graphitic core of dimension less than 10 nm (0-D). CDs exhibits outstanding nanostate aqueous solubility, exciting optical properties, and profound biocompatibility and hence largely used in many polymer nanocomposites [42]. In recent times, CDs has been used for the development of polymer nanocomposites for optical, biological, and photocatalytic applications. Besides these, polymer nanocomposites have been also manufactured using fullerene C60, cellulose nanofiber, polyaniline (PANI) nanofiber, etc., for a host of advanced applications. Bora et al. [43] fabricated polyester nanocomposites by dispersing GO in tetrahydrofuran by ultrasonication and mixing with polyester matrix with the help of mechanical shearing force and ultrasonication. Further, thermosetting nanocomposite was obtained by curing with methyl ethyl ketone peroxide hardener [43]. Hazarika and Karak [44] incorporated functionalized GO into citric acid-based waterborne hyperbranched polyester matrix through in situ polymerization technique in the absence of any solvent and compatibilizing agent. Further, Naebe et al. [45] reported epoxy nanocomposites modified with well-dispersed thermally reduced graphene nanoplatelets.

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Nanomaterials of Metal, Non-metal, and Their Derivatives. The nanomaterials of metal, non-metal, and their derivatives are some of the commonly found nanomaterials in nature and come in various sizes and morphology. They include different metals, metal oxides, metal chalcogenides, inorganic minerals, nanoclays, and so on. Reduction of metals like silver (Ag), gold (Au), platinum (Pt), palladium (Pd), etc., in the presence of suitable capping agents provides a facile way of generating such metal nanomaterials [46]. Nanometal oxides of iron (Fe3O4), copper (CuO/CuO2), nickel (NiO), zinc (ZnO), etc., and metal ferrites (MFe3O4, M=Fe, Cu, Ni, Co, etc.) are prepared by variety of techniques like solvolysis, wet chemical, sol-gel, or hydrolysis method using organometallic precursors. Inorganic minerals like SiO2 and hydroxyapatite are some of the most abundant minerals on the earth surface. Various polymorphs of SiO2 are used for polymer nanocomposite fabrications to introduce mechanical and thermal stability [47]. Moreover, Ag-SiO2based poly(butylene adipate-co-terephthalate) nanocomposites are also prepared using in situ sol-gel process [48]. In this regard, Zhao et al. [49] prepared acrylated 2-(2-mercapto-acetoxy)-ethyacrylated 2-(2-mercapto-acetoxy)-ethyl ester-capped colloidal ZnS nanoparticle by the reaction of thioacetamide with zinc acetate in N, N-dimethylformamide with narrow size distribution. Further, they incorporated this modified ZnS into acrylated hyperbranched polyester through UV radiation-initiated free-radical polymerization and spin coating [49]. On the other hand, nanoclays are hydrous aluminum phyllosilicate thin platelets or sheets having layered structure. Polymer nanocomposites of nanoclays like montmorillonite (MMT) are found to enhance mechanical and thermal properties, tailored for commercial applications such as coatings, adhesives, fibers, thermoplastic elastomers, and foams [50]. Kong et al. [51] studied the exfoliation behavior of epoxy clay nanocomposites varying the electronegativities of the aromatic diamine curing agent and curing temperatures. Hybrid Nanomaterials. Hybrid nanomaterials are an emerging class of nanomaterials that have garnered copious amount of attention in the last decade. In such nanomaterials, two or more different types of nanomaterials interact with each other by a specific mechanism in the same system [52]. They might also contain both inorganic and organic components and hold exceptional advantages over the individual ones. On the basis of the interactions between the nanocomponents, various types of morphology can be achieved, such as embedded nanohybrid, decorated nanohybrid, etc. The advantage of employing such hybrid systems lay on the fact that a different range of attributes can be imparted in a particular nanostructure. Examples of such hybrid nanomaterials include Fe3O4/graphene that shows excellent magnetic property, CQD/TiO2 with outstanding photocatalytic activity, HAp/graphene that shows osteogenetic activity, and many more [53]. Hazarika and Karak [54] prepared CDs@TiO2 nanohybrid through a facile and greener one-pot hydrothermal protocol and incorporated into waterborne hyperbranched polyester through in situ polymerization technique without using any solvent and compatibilizing agent. Fe3O4/CNT (electromagnetic shielding material) and Pt/CeO2/graphene (electrochemical application) are some of the recent examples [55].

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3 Methods of Preparation of Polymer Nanocomposites In a typical nanocomposite system, the polymer and nanomaterial coexist by means of various physico-chemical interactions and maintain a synergistic relationship between the macro- and nanostructures. There are many diverse preparative methods for the fabrication of polymer nanocomposites. Some of the most commonly encountered techniques for the fabrication of polymer nanocomposites are briefly discussed below.

3.1

Solution Mixing Technique

In the solution mixing technique, the nanomaterial is swollen and dispersed in an appropriate solvent or mixture of solvents by means of mechanical force and ultrasonication. The nanomaterial in the dispersed phase is then mixed with a solution of polymer by means of shearing mechanical force and further ultrasonication. The nanocomposite is finally obtained by evaporating the solvent. In this technique, the level of dispersion of the nanomaterial in the polymer matrix depends upon the interactions among polymer, nanomaterial, and solvent [56]. When nanomaterial–solvent interactions become greater, the nanomaterial disintegrates in the solvent itself. In contrast, when polymer–nanomaterial interactions become greater, the polymer chains penetrate into the nanomaterial, resulting in the formation of an exfoliated nanocomposite. Chen et al. [57] prepared PU/functionalized RGO nanocomposite by using DMF as a solvent and ultrasound treatment for 30 min. Further, Wu et al. [58] also prepared PU/modified RGO nanocomposite by using this technique by preparing PU and the nanomaterial solutions in DMF. Then, these solutions were mixed for 30 min and stirred at 70 °C to obtain the nanocomposite [58]. Clay-based polyester nanocomposites were fabricated by Konwar et al. by mixing the dispersed clay in xylene with acrylate-modified polyester resin with the help of mechanical stirring and ultrasonication [59]. Deckar et al. [60] fabricated polyester nanocomposites by combining 2, 2-bis-hydroxymethyl propionic acid-based second and fourth generation of polyester with a pentaerythritol ethoxylate core with unmodified sodium MMT clay (0– 100%) in water through solution intercalation method.

3.2

In Situ Polymerization Technique

The in situ polymerization technique involves swelling and dispersion of the nanomaterial in a suitable pre-polymer or monomer, followed by polymerization reaction in order to attain the required nanocomposite. In this technique, the level of dispersion of the nanomaterial in the polymer matrix is better compared to the

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solution mixing technique. This is because, as the nanomaterial is added before the initiation of the polymerization reaction, hence it can participate in the reaction or crosslinking process. In situ polymerization technique often leads to the formation of exfoliated nanocomposite, as the being-formed polymer chains can easily penetrate and adhere to the surface of nanomaterial, resulting in strong polymer– nanomaterial interactions [56]. Hazarika and Karak [54] reported incorporation of functionalized GO into citric acid-based waterborne hyperbranched polyester matrix through in situ polymerization technique in the absence of any solvent and compatibilizing agent. Waterborne polyester nanocomposites were fabricated using CDs through facile and greener in situ polymerization technique in the absence of solvent and compatibilizing agent. CDs used in this case was obtained from citric acid, glycerol, and cow urine through one-pot hydrothermal method [61]. Duarah et al. [62] synthesized starch-modified HPU nanocomposites using CDs-Ag nanohybrid by in situ technique. Wang et al. [63] prepared PU/RGO nanocomposite by in situ polycondensation of poly(tetramethylene glycol) and MDI using ethylene glycol as the chain extender in the presence of RGO sheets. Moreover, Gao et al. [64] fabricated epoxy/silver nanocomposites by in situ formations of silver nanoparticles within the epoxy matrix.

3.3

Melt Mixing Technique

The melt mixing technique involves the direct mixing of nanomaterial with the polymer at molten temperature, either statically or under shear. Suitable mixing and processing equipments like twin-screw mixers, injection molding, extrusion molding, compression molding, etc., are used to obtain the fabricated nanocomposites. In this technique, amorphous polymers are generally processed above their glass transition temperature (Tg), while semi-crystalline polymers are processed above their melting temperature. Compared to in situ polymerization technique, the penetration or intercalation of the polymer in the nanomaterial is poor, due to high melt viscosity of the polymer. Thus, mostly partially exfoliated nanocomposites are obtained from this technique. This technique also allows the use of polymers which are not suitable for in situ polymerization or solution mixing technique and is compatible with current industrial processes. In addition to these, other techniques such as sol-gel process, coagulation spinning, latex fabrication, template synthesis, plasma treatment are also employed for the fabrication of polymer nanocomposites [65]. Chun et al. [66] also reported PU/MMT nanocomposite using twin-screw extruder together with a compatibilizer to improve the dispersion of MMT by melt mixing technique. The nanocomposite of biodegradable aliphatic polyester with nanoclay was fabricated by Lee et al. through melt mixing of polyester with two different kinds of clay, namely Cloisite 30 B and Cloisite 10A [67]. Poly(ethylene 2,6-napthalate)-based aromatic polyester nanocomposites with CNT were reported by Kim et al. [68] through melt blending process. For this, CNT was modified through hydrogen bonding formation in order to introduce some carboxylic acid

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onto it which helped to enhance the intermolecular interactions between the polyester matrix and CNT.

4 Characterization Techniques Characterization of polymer nanocomposites is a vital aspect for understanding their composition, morphology, and properties. Since polymer nanocomposites include both polymer matrix and nanomaterial, therefore unlike fine chemicals and materials, complexities may arise due to their combined presence. Polymer usually adapts a macromolecular assembly of coiled and entangled chains, while nanomaterials adjust and orient themselves in the polymer matrix by virtue of different physico-chemical interactions. Hence, characterization of such polymer nanocomposites involves sophisticated analytical techniques and their understanding due to complexities and diversity in their structures. These sophisticated analytical techniques are briefly discussed in the following sections.

4.1

Spectroscopic Techniques

Infrared (IR) Spectroscopy. IR spectroscopy works upon the principle of interaction of IR electromagnetic radiation with the molecules of the polymer nanocomposites. This interaction results in absorption of radiation of certain wavelengths, which corresponds to the energy transition between the vibrational and rotational states of the atoms or group present in the nanocomposite. This spectroscopy records the absorbance or transmittance intensity (in percentage) as a function of frequency (in wavenumber, cm−1). As each specific atom or group, whose vibrational and rotational energies lie in the IR region (400–4000 cm−1), exhibits characteristic absorption, IR spectroscopy is used to determine the functional groups present in the structures of the polymer nanocomposites. However, as polymer nanocomposites have complex structures than fine chemicals, some sophisticated variations like Fourier transform infrared spectroscopy (FTIR) and attenuated total reflection (ATR) IR spectroscopy are employed. Bayan and Karak [3] reported the presence of bands at 3441–3450 cm−1, 1735 cm−1, 1637 cm−1, 1031 cm−1 due to overlapping of –NH and –OH, ester –C=O, amide –C=O, and – O–C=O stretching vibrations, respectively, in the structure of PU using FTIR spectroscopy. UV–Visible Spectroscopy. UV–Visible spectroscopy functions on the principle of interaction of UV or visible electromagnetic radiation with the molecules in the polymer chains. On absorption of UV (wavelength of 200–400 nm) or Vis (wavelength of 400–800 nm) by the specific light-absorbing group in the polymer chains called chromophore, electronic excitation occurs as per theory of

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quantization of energy. This spectroscopy records a plot of wavelength (in nm) versus molar absorptivity (e, L mol−1 cm−1) or optical density or absorbance (A) of a unit concentration of polymer nanocomposite sample within unit traverse length of the sample. The values of molar absorptivity and their corresponding wavelengths are utilized to assign the nature of electronic transitions and the presence of chromophoric groups present in the polymeric chains. Gogoi et al. [2] reported the formation of CDs using UV–Vis spectroscopy (absorption peak at wavelength 265 nm). Nuclear Magnetic Resonance (NMR) Spectroscopy. NMR spectroscopy represents an important technique for the elucidation of structure of the polymer nanocomposites. NMR generally observes and records the local magnetic fields present around atomic nuclei. Apart from chemical structure, other parameters like physical state, isomerism, stereochemistry, yield, kinetics of polymerization, degree of branching, etc., can be evaluated with the help of NMR spectroscopy. Generally, 1 H and 13C NMR spectroscopic techniques are employed to determine the presence and orientation of 1H nuclei and 13C nuclei in the nanocomposite, even though other nuclei like 19F, 29Si, and 31P are also utilized, if present. Even though NMR presents a simple, rapid, and convenient technique, structure determination for polymer nanocomposites is a tedious exercise due to its macromolecular assembly, as compared to fine chemicals. The NMR spectrum is recorded as a plot of intensity (in percentage) versus chemical shift (ppm), with different kinds of 1H/13C environment assigned accordingly. Further, Ghosh and Karak [4] confirmed the chemical structure of DAGP (dimer acid-glycerol-modified polyol) using 1H and 13 C NMR spectral analyses.

4.2

Microscopic Techniques

Scanning Electron Microscopy (SEM). SEM acts as a significant microscopic technique for studying the surface morphology of the polymer nanocomposites. SEM involves imaging of the surface of a material with the help of a focused beam of electrons. The electron beams interact with the surface of the specimen and reflected off producing various signals that can be used to obtain information about the surface topography and composition. Duarah and Karak [34] studied the surface morphology of biodegraded polymeric films after 6 weeks of bacterial degradation. Transmission Electron Microscopy (TEM). TEM represents another highly sophisticated microscopic technique for elucidating the bulk morphology of polymer nanocomposites. TEM involves imaging of the inside of the material with the help of focused beam of electrons. The beam of electrons is transmitted through the inside of the specimen generating signals that are used to acquire features such as morphology, crystalline structure, and even elemental information. Gogoi et al. [2] determined the size of CDs by TEM analysis.

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Atomic Force Microscopy (AFM). AFM is a type of high-resolution scanning probe microscopic technique for evaluating the surface topology of nanocomposites. AFM functions by scanning over a small area of the specimen, generating the surface topology and simultaneously measuring local properties like height/ thickness, friction, magnetism, etc. Guo et al. [5] characterized the morphology and the functionalization of the alumina nanoparticles using AFM images.

4.3

Other Techniques

Elemental Analysis. Elemental analysis is the most classical method to obtain information about the elemental composition of polymer nanocomposites. Some of the common techniques used for characterization of polymer nanocomposites, under the guise of elemental analysis include CHN (carbon, hydrogen, and nitrogen), heteroatom (halogens, sulfur, phosphorus, etc.), atomic absorption spectrometry (metals, metalloids, halogen, sulfur, phosphorus, etc.), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), etc. Gogoi et al. [1] reported the elemental analysis of CDs which showed the presence of C and O in a weight ratio of 61.47:36.53. X-ray Diffraction (XRD). XRD generally involves scanning of powdered or thin sheets of polymer nanocomposites by X-ray beam of a specific wavelength (CuKa, wavelength of 1.54 nm) over a range of incident angle (h). If the nanocomposite has some crystalline regions, then the interaction of X-ray results in a sharp peak seen in the diffractogram, i.e., plot of peak intensity versus scattering angle (2h). By this technique, the degree of crystallinity or amorphousness of the nanocomposite can be predicted. Also, the order of diffraction (n) and interplanar distance (d) between the successive crystalline planes can be evaluated using Bragg’s law, nk = 2d sinh. Ghosh and Karak [4] measured the degree of crystallinity of thermoplastic HPU from XRD. Mass Spectrometry. Mass spectrometry is a straightforward analytical technique for obtaining the mass of polymer nanocomposites. In this technique, molecules are bombarded with high-energy electron beam and the abundance or intensity of the ionized chemical species of the nanocomposites is recorded based on their mass-to-charge ratio (m/z). The fragmentation pattern of the ionized species can be analyzed to obtain the molecular weight of the nanocomposite. Sometimes, a liquid chromatography (such as high-pressure liquid chromatography (HPLC)) is coupled to a mass spectrometer (MS) (HPLC-MS) to get more accurate results as this technique separates the different fractions first and then analyzed by mass spectrometry subsequently. Jordan et al. [36] performed the ex situ kinetic studies of the polymerization of 2-phenyl-2-oxazoline using FTIR spectroscopy and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass

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spectrometry which resulted in a linear relationship between the reaction time and degree of polymerization of the polymer. Gel Permeation Chromatography (GPC). GPC is the most popular technique to determine the molecular weight and distribution, i.e., number average, molecular weight, weight average molecular weight, etc., of polymers. As polymers can be mixture of different sizes of segments, as chain lengths may vary, separation is based on difference in hydrodynamic volumes of the polymer molecules of varying molecular weights. In this technique, the chromatographic column is packed by semi-rigid polystyrene bead crosslinked with divinylbenzene and the polymer sample solution is eluted through the column. The large-sized polymer chains will elute out initially, followed by small-sized polymer chains. The chromatogram depicts the refractive index or UV absorption intensity as function of elution volume, from which molecular weights can be determined compared to the standard polymer of known molecular weight. Dutta and Karak [5] determined the number average molecular weight and weight average molecular weight of a bio-based waterborne polyester using GPC. Viscometry. Viscosity is a vital property of polymer, in terms of their processing and ultimate application. The solution viscosity of polymers can be determined by capillary viscometers like Oswald and Ubbelohde. However, Ubbelohde viscometer is mostly used for this purpose. This viscometer is a suspended level viscometer, and the flow time of the medium is not dependent on the volume. So the flow time can be measured for a series of different concentration (maximum 0.5 dL g−1) by successive dilution in the viscometer by addition of pure solvent only. The viscosity of concentrated polymer solution or melt viscosity can be measured by Brookfield viscometer, Red hood viscometer, Mooney viscometer, etc. Duarah et al. [34] reported the shear viscosity of hyperbranched epoxy resins under a constant shear stress of 100 Pas. Thermogravimetric Analysis (TGA). TGA is a very helpful analytical technique for determining the thermal stability of polymer nanocomposites. TGA determines the weight loss of polymeric material under air or under inert atmosphere and is recorded as function of change of weight (weight loss or weight residue percentage) of the sample versus temperature or time. In addition, the amount of moisture or any other volatiles, plasticizers, fillers, etc., present in the material can be obtained from a TG thermogram. Gogoi et al. [1] evaluated the thermal stability of thermosetting waterborne HPU nanocomposites using TGA. Differential Scanning Calorimetry (DSC). DSC is a thermal analytical technique that is used to study the change in the heat changes in a polymeric nanocomposite with temperature. A polymer sample of known mass is heated or cooled, and the changes in its heat capacity are tracked as changes in the heat flow, with respect to a reference sample. The DSC thermogram is a plot of change of heat energy or enthalpy versus temperature. DSC gives much vital information about polymeric materials such as specific heat, Tg, crystalline melting point, amount of endothermic/exothermic energy. Hazarika et al. [6] determined the Tg of polyester thermosets from DSC studies.

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5 Properties The main objective of fabricating polymer nanocomposites lies in knowing and understanding their properties, ultimately utilizing them for appropriate applications. Polymeric nanocomposites, due to their versatile properties, are being used for a variety of commercial products. These properties are divided into physical, mechanical, chemical, thermal, optical, biological, electrical, and magnetic. These salient features of these properties are discussed below.

5.1

Physical Properties

The physical properties are the inherent characteristic of polymeric nanocomposites, including solubility, viscosity, crystallinity, etc. Solubility. The solubility or dissolution of polymer nanocomposites in a suitable solvent is slow, due to their high molecular weight, and coiled and entangled structure. For solubilization of polymer nanocomposite in any solvent, the polymer–solvent interaction must be greater than solvent–solvent interactions and interactions within the nanocomposite matrix. In this process, the chains of polymer matrix are unfolded and solvated by penetration of solvent molecules in between polymer chains. This state of dissolution is called swelling, and most of the higher molecular weight polymers undergo this process instead of forming a true solution. In case the solvated polymer molecules diffuse out from the swollen state and homogenize with the solvent, then a true solution is formed. Bio-based HPU showed better solubility in different solvents than the linear analog due to unique geometry of the former. Duarah et al. [15] reported starch-modified HPUs which were found to be highly soluble in polar aprotic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF). Viscosity. The viscosity of polymer nanocomposites is the opposition/resistance to the flow under any applied external conditions. Both melt viscosity and solution viscosity of such nanocomposites are different, due to high molecular weight, coiled and entangled structure, presence of secondary forces. The viscosity of polymer nanocomposite is dependent upon the inherent nature like structure, molecular weight, concentration of solutions, temperature, and external applied force. Ghosh and Karak [4] reported the solution viscosity of bio-based multifunctional macroglycol containing smart thermoplastic HPU elastomer. Crystallinity. Crystallinity occurs when there is a long-range order in the molecular arrangement of polymer chain segments. Polymers with simple and highly regular structural units may have the tendency to crystallize. This crystallinity is further influenced by the presence of nanomaterials that serves as nucleating centers, from which crystallization may originate. Duarah et al. [15] reported the

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crystallinity of the soft segments of HPU by measuring the heat of crystallization (Hc) from DSC.

5.2

Mechanical Properties

Mechanical properties are the most crucial part of the polymer nanocomposites, as most of their future applications are dependent on the durability of the material. The mechanical properties of polymeric materials include tensile strength, tensile modulus, elongation-at-break, scratch hardness, Shore A hardness, pencil hardness, impact resistance, etc. These mechanical properties of pristine polymers are generally poor compared to other materials like metals or ceramics. As a result of which, pristine polymers do not serve as suitable end products for advanced applications. Consequently, polymer nanocomposites have been fabricated using nanomaterials of organic and inorganic origin with virgin polymers. These nanocomposites not only compensate for the poor mechanical properties of polymers but also impart stability and new interesting set of properties to the sustainable material. The mechanical properties of the nanocomposites mostly depend upon their molecular weight, arrangement of polymer chains, physico-chemical interactions, and dispersion of nanomaterial in the polymer matrix. Yadav et al. [12] also found that mechanical properties of PU were greatly enhanced after incorporation of 2 wt% of functionalized graphene nanoplatelets. Wu et al. [58] also reported PU nanocomposites with hyperbranched aromatic polyamide-functionalized RGO with improved tensile modulus and tensile strength. De et al. [69] prepared bio-based hyperbranched epoxy resin from castor oil-based hyperbranched polyester polyol, which exhibited high tensile strength, elongation, scratch and impact resistance properties. SiO2-based polyester nanocomposites showed enrichment in tensile strength at yield point, and maximum enhancement of 19% was observed for the nanocomposite with 5 wt% of SiO2. This result depicts the adhesion between the polyester matrix and the nanomaterials [70]. Fu et al. [71] prepared hyperbranched poly(trimellitic anhydride-triethylene glycol) ester epoxy and DGEBA interpenetrating networks which exhibited improved impact strength up to 7 times compared to the unmodified system.

5.3

Chemical Properties

Polymer nanocomposites may show certain reactivity toward chemicals like acid, base, salt, etc., depending upon the nature of the polymer matrix and the nanomaterial. This reactivity is mainly dependent on chemical composition and the presence of free reactive functionalities in the polymer structure. The chemical reactivity of polymers is definitely helpful for their modification and tailoring to

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obtain newer properties. However, in the hindsight, such reactivity is undesirable for their long-term application, as it will alter the properties and state of the material. Generally, polymer nanocomposite exhibits better resistance to chemical reactivity under different media, in comparison with pristine polymers. This can be attributed to the reinforcing ability of the nanomaterial by various physico-chemical interactions that strengths the polymer matrix and makes it chemically resistant. Barua et al. [8] demonstrated that the hyperbranched glycerol-based epoxy showed better chemical resistance properties compared to the DGEBA epoxy thermosets.

5.4

Thermal Properties

Thermal stability of pristine polymers is a major challenge considering their low thermostability and high flammability. This is because polymers are composed of mainly covalent bonds between the molecules, and covalent bonds are comparatively weaker than other types of bonds. The significant features responsible for the thermal behavior of polymers are chemical linkages, molecular weight, and their distribution, secondary forces, crosslinking, crystallinity, etc. In order to improve the thermal behavior of polymers, polymer nanocomposites provide a useful way by incorporation of nanomaterials of organic/inorganic origin. The study of thermal behaviors of polymer nanocomposites reveals not only thermal degradation patterns of the polymeric material but also the mechanism of degradation. Other properties such as flame retardancy, Tg, heat capacity, specific heat are also obtained from the thermal study of nanocomposites. Thermal properties of the nanocomposites are important from the point of view of their processing and service life. Scognamillo et al. [72] reported that Tg of PU increased in the presence of very low amounts of graphene. Polyester nanocomposites with 0.75 wt% of ZnO improved the thermal stability of the polyester, whereas the decrease in degradation temperature was observed with 1.0 wt% of ZnO owing to aggregation of it [73]. Duarah and Karak [34] synthesized hyperbranched epoxy resins from starch and found that the thermoset showed better thermal stability compared to the linear epoxy-based thermoset.

5.5

Optical Properties

Optical properties of polymer nanocomposites depend upon their interaction with light. Polymer nanocomposites show optical activities like refractive index, transparency, haziness, color, transmittance, reflectance, gloss, etc., upon interaction with light [74]. This interaction is dependent upon the nature of the polymer matrix as well as the nanomaterial. Polyester nanocomposites show fascinating optical properties such as fluorescence, phosphorescence, transparency, refractive index in the presence of optically active functional moieties and fluorescent nanomaterials

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(quantum dots, metal nanoparticles, etc.). Previous literature reports the synthesis of fully bio-based aliphatic hyperbranched polyesters which showed wavelength dependent photoluminescence properties [75]. Further, waterborne polyester nanocomposites with CDs and CDs@TiO2 emitted different colors on exposure of UV light and exhibited wavelength dependent up- and down-conversion photoluminescent properties. They also showed excellent transparency (>95%) under visible light [54, 62].

5.6

Biological Properties

Polymer nanocomposites may show biological responses like biodegradation, antimicrobial activity under different biological environments in the presence of microorganisms like bacteria, fungi, etc. Most of the synthetic polymers do not show biological responses. On the other hand, almost all naturally occurring polymers such as cellulose, starch, vegetable oil-based polymers, natural rubber attract microorganisms and are biodegradable. Most of these polymers either inherently contain hydrolyzable linkages such as esters, amides or generate such groups which are broken down by microorganism. In general, biodegradability of such polymeric materials is dependent on degree of hydrophilicity. Polymer nanocomposites containing metals/metalloids like Cu, Ag, Zn, Sn, Sb, etc., show antimicrobial properties. Although the actual mechanism of their action is still debatable, it is believed that such materials are absorbed by a microorganism which then alters their metabolism, making them disintegrate or dormant. Jena et al. [76] reported antibacterial and antifungal HPU/ZnO nanocomposites. The antimicrobial activity improved with ZnO concentration in HPU. Yadav et al. [77] also reported HPU/MWCNT nanocomposite with good antimicrobial activity against Escherichia coli. A significant enhancement in biodegradation behavior against Pseudomonas aeruginosa and Bacillus subtilis bacterial strains was observed in Mesua ferrea L. seed oil-based highly branched polyester by Konwar et al. [7]. The rate of biodegradation further increased with increase in the loadings of the nanoclay. Barua et al. [78] fabricated epoxy/silver–RGO-curcumin nanocomposite and studied their biocompatibility and antimicrobial properties.

5.7

Magnetic Properties

Certain polymer nanocomposites may show magnetic behavior, depending upon the nature of polymer matrix or nanomaterial. This magnetic behavior occurs when the magnetic dipoles of the material are activated by an external magnetic field. However, most of the polymers are significantly unresponsive toward magnetic

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behavior, except those with unpaired electrons in their structures or magnetic materials like Fe, Co, Ni in the nanocomposite matrix. Hence, such polymer nanocomposites are exploited for magnetic property in many applications. Magnetite (Fe3O4) is one of the strongest known magnetic particles, which induces magnetism when dispersed in the epoxy matrix. Several studies were performed by different groups on modified or functionalized Fe3O4 such as silane-modified Fe3O4, PANI-modified Fe3O4, polypropylene-functionalized Fe3O4, Fe@Fe3O4, graphene/Fe@Fe3O4 which showed low wear rates, good magnetic properties, and similar hysteresis results compared to unmodified Fe3O4 when incorporated in the epoxy matrix [78]. Moreover, the magnetic and electromagnetic wave absorption behavior of PU nanocomposite reinforced with flexible iron particle was reported [79].

5.8

Shape Memory Properties

Shape recovery, shape fixity, and recovery time illustrate shape memory effects of a polymeric material. In response to temperature changes, significant deformation of the shape memory polymer (SMP) occurs. The material can change from a glassy state to a rubbery state across the Tg. The material becomes more flexible on increase in temperature and therefore can be easily deformed or molded. On the other hand, increase in temperature hardens the polymer sustaining the new shape. This process can be repeated without material fatigue. Mesua ferrea L. seed oil-based HPU exhibited solvent-induced shape memory behavior, as reported by Kalita et al. [80]. Lee and Yu [81] fabricated a PU/SWCNT nanocomposite with good electroactive shape memory effect. Graphene-covered vapor-grown carbon nanofiber-based bio-based polyester nanocomposite fabricated by Tang et al. showed electrostatic shape recovery behavior. The shape recovery ratio of 97% was attained within 90 s at 20 V [82]. However, the development of epoxy-based materials as shape memory materials has remained sparse due to their high moduli and relatively low failure strains, both being unattractive properties for medical applications even though not unfavorable (low strain) or even necessary (high strength) to more challenging applications such as structural applications [83]. Duarah et al. [15] demonstrated the shape memory behavior of starch-based HPU with good shape recovery and shape fixity at 40 °C.

6 Applications The sustainable polymeric nanocomposites have a number of applications in different fields. Some of them are summarized in Table 3. The important some of them are also elaborated below.

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Table 3 Applications of different polymer nanocomposites Applications

Polymer nanocomposites

Applications/properties

References

Surface coatings and paints

4,4-bis (4-hydroxyphenyl) pentanoic acid-based hyperbranched polyester Castor oil-based waterborne alkyd cured with butylated formaldehyde Sunflower oil-based hyperbranched alkyd resin/ZnO nanocomposite PU/MWCNT nanocomposite PU/ZnO nanocomposite Polyester/MWCNT nanocomposite Polyester nanocomposite with 20 wt% MWCNT Graphene-covered vapor-grown bio-based polyester nanocomposite Functionalized GO-based waterborne polyester nanocomposite

UV-curable coating with cationic initiator

[84]

Anticorrosive coating

[85]

Anticorrosive material

[86]

Excellent wear resistance

[87]

Antibacterial coating EMI efficiency increased from 30 MHz to 1.5 GHz EMI was increased to 18 GHz

[88] [89]

Electromagnetic interference shielding (EMI) Shape memory

Catalytic

Miscellaneous

CDs@TiO2-based waterborne polyester nanocomposite CNT-based polyester nanocomposite

PU/MWCNT nanocomposite PU-titania nanocomposites Clay-based epoxy nanocomposites

6.1

[90]

Electroactive shape recovery behavior

[91]

Heterogeneous catalyst for Aza-Michael addition reaction with good reusability and recyclability Photocatalyst for reduction of 4 nitrophenol to 4-aminophenol

[44]

Supercapacitor with fine specific capacitance of 15.67 cmF/cm2 with 90% stability (after 1000 cycles) Electroactive shape memory behavior Excellent piezoelectric behavior with tunable dielectric properties Structural materials with improved mechanical properties

[92]

[54]

[93] [94] [95]

Self-healing Material

Self-healing polymers are those polymers which can recover itself from damage in the presence of suitable stimuli and regain its original set of properties [96]. In this milieu, PUs have gained significant attention due to its inborn structural

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inhomogeneity in a single polymeric backbone and tunable properties by simple variation of compositions [23]. Several techniques were used to incorporate such healing tendency in PU such as microencapsulation, microvascular network formation, covalent and non-covalent bond formation which are discussed in the chapter to give a clear overview to the reader in the self-healing field [96, 97]. Scientific reports on microcapsule-based healing were first reported in 2001 by White et al. in which dicyclopentadiene (DCPD)-containing microcapsules and Grubb’s catalysts were homogeneously dispersed in an epoxy matrix [98]. Upon fracture, these capsules released the DCPD monomer and underwent ring-opening metathesis polymerization (ROMP) on contact with the Grubb’s catalyst to repair the microcracks [98]. Du et al. [99] reported the synthesis of self-healable PU with 80% healing efficiency using thermoresponsive Diels–Alder (DA) reaction through the incorporation of furan and bismaleimide (BMI) groups into the polymeric chain. Using the thermoreversible chemistry of DA bond, Feng et al. [100] synthesized a self-healable thermoplastic PU with 71% healing efficiency. Zhao et al. [101] reported the synthesis of poly(siloxane-urethane) elastomer in which DA and retro DA reaction containing groups were incorporated for healing purpose. Using temperature-initiated reshuffling capability of S-S bonds and H-bonding, Jian et al. [102] synthesized self-healing PU in which disulfide-containing molecules were used as a structural component. Yuan et al. [103] prepared epoxy-TPU pre-polymer coating systems which showed extended shelf-life in comparison with the reported epoxy-PCL system. In 2013, Luo et al. [104] proposed a system incorporating electrospun thermoplastic poly(e-caprolactone) (PCL) into a commercial epoxy (DGEBA) coating. This system displayed both shape memory ability (recovery of the matrix to bring the crack surfaces in spatial proximity) and self-healing capacity.

6.2

Self-cleaning Material

Self-cleaning polymers are an emerging class of materials, inspired by nature. As the name suggests, such types of polymers are specialized in self-decontamination or purification by a cleaning medium like liquid (primarily water). The self-cleaning phenomenon is also related to the surface contact angle. Surface contact angle (h) is the angle formed at the three-phase boundary (solid/liquid/gas) between the surfaces of the liquid drop to the surface of the solid (Fig. 2). In general, if the contact angle is 90°, the surface is defined as a hydrophobic surface. Similarly, a surface with a water contact angle approaching Fig. 2 A schematic representation of hydrophilic, hydrophobic, and ultra-hydrophobic surfaces

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Fig. 3 Self-cleaning of a superhydrophobic surface by water droplet with very high contact angle and b photocatalytic hydrophilic surface by a water droplet with the help of sunlight

zero is classified as ultra (super)-hydrophilic and a surface with a contact angle >150° is usually categorized as ultra (super)-hydrophobic [105]. These self-cleaning polymers are primarily categorized into: hydrophobic and hydrophilic, and both these types clean the surface by different mechanisms (Fig. 3). The hydrophobic technique makes the water droplets to slide and roll over the surfaces, thereby carrying the dirt away with them, while the hydrophilic technique uses appropriate metal oxides to seat the water that removes the dirt from the surface. In addition, hydrophilic polymers using metal oxides have the additional property of chemically breaking down complex dirt deposits by photocatalytic cleaning mechanism [106]. Nanocomposites of PU with photoactive materials like TiO2, Ag, ZnO, CdS, CDs, graphene, CNT, etc., have been found to demonstrate self-cleaning behavior through photocatalysis. Nano-TiO2/PU composites were fabricated by grafting technique, which were found to degrade organic dirt. Nano-TiO2 shows photoripening activity under visible radiation and acts as a photoactive component. Nano-titania grafted-PU surface was found to degrade stearic acid by 24 h of exposure under solar radiation [107]. Metal-based nanohybrids also show good photoharvesting property that can be applied to photocatalytic self-cleaning applications. In some of such instances, bio-based HPU nanocomposites of RGO-silver-reduced carbon nanodots were exploited for self-cleaning behavior by degradation of common organic pollutants under solar-assisted mechanism [108]. Again, photocatalytic degradation of formaldehyde and methylene blue were reported using CDs-based waterborne biodegradable polyester nanocomposite [61].

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Biomedical Application

Polymer nanocomposites have attracted considerable interest in the field of biomedical [109, 110]. Polymeric soft materials have the distinct advantage of being conformable to living tissue, thus being uniquely suitable for biomedical applications of almost all kinds. In addition, many polymers can be tailored with specific degradation and mechanical properties that make them very advantageous when deployed in contact with biological tissues. In this context, PU with extensive structure/property is one of the most bio- and blood-compatible materials known today. These materials played a major role in the development of many medical devices ranging from catheters to a total artificial heart. Properties such as durability, elasticity, elastomer-like character, fatigue resistance, compliance, and acceptance or tolerance in the body during the healing became often associated with PU [108]. Again, nanomaterials such as GO, RGO, MWCNT, CDs are reportedly used to fabricate PU nanocomposites as biomaterials. For instance, incorporation of ZnO, Ag, TiO2, etc., imparts antibacterial activity to the host polymer [111, 112]. Das et al. [113] prepared PU nanocomposites with peptide-functionalized MWCNT to accelerate in vivo bone tissue regeneration. Duarah et al. [62] fabricated bio-based HPU nanocomposite with CDs-Ag nanohybrid for potential application as a self-expandable antimicrobial stent. The same group also developed a bio-based HPU nanocomposite reinforced with reduced CDs as a self-tightening suture for prospective application in endoscopic surgeries [114]. Furthermore, HAp microparticle-based polyester elastomer was reported for employment as a porous scaffold in tissue engineering [115]. Meng et al. [116] reported CNT-based biodegradable poly(1,8-octanediol-citrate) polyester nanocomposite which exhibited enhanced in vivo biocompatibility and bovine serum albumin adsorption hydrophilicity of the nanocomposite. In 1999, Hu et al. [117] developed a solution containing epoxy compound for crosslinking biological tissues and bio-prosthetic materials. It was used to crosslink collagenous biological materials.

7 Conclusion Thus, the chapter tried to provide an overview of sustainable polymer nanocomposites obtained from bio-based components, as much as possible. These nanocomposites showed excellent performance and the significant improvements in properties by incorporation of the appropriate amount and nature of nanomaterials. A wide spectrum of potential applications is envisaged for such materials. However, a number of aspects still remained unexplored which include 100% bio-based renewable resources as the raw materials, life cycle assessment of the products, etc. In nutshell, these partially bio-based polymer nanocomposites have great potential as sustainable materials for various advanced applications required for today’s material society.

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Chapter 17

Bio-based Polymeric Conductive Materials for Advanced Applications Gourhari Chakraborty and Vimal Katiyar

Abstract Polymer-based materials have become the obvious alternative to conventional materials in all the areas. Polymers are utilized for extensive applications, where the conductive material application is considered as one of the potential areas. Bio-based polymers are environment friendly degradable polymers which not only reduce the solid disposal from electrical and electronics wastes but also maintains the property required for targeted application in a better way. The conductive application of bio-based polymers includes insulators, capacitor, electrochemical sensor, biosensor, biomedical, energy storage etc., where the properties of neat biopolymers need to be improved via developing bio-based polymeric composites and blends. In this regards, the composite fabrication with conductive fillers or blending with conductive polymers such as polyaniline or polypyrrole can provide conductive nature. Biodegradable polymers including poly (lactic acid), polycaprolactone, cellulose, and chitosan are utilized for different targeted applications. This present chapter discusses the different current researches based on biodegradable polymers targeting conductive material application. Moreover, the different modification techniques and possible application area of the bio-based polymers are also detailed.




Keywords Bio-based polymers Conductive application composites Conductive blends





Conductive

G. Chakraborty
V. Katiyar (&) Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India e-mail: [email protected] G. Chakraborty e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_17

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1 Introduction The processability and versatility of polymer make itself economical and more profitable which stands as a suitable alternative for the existing metal based or ceramic goods. The modified combination of various polymers depending upon the targeted application via developing blends or composites of polymer in combination with inorganic reinforcement makes polymer more lucrative to the researchers and put a great step towards industrial application [1–3]. However, scarcity of fossil fuel-based feedstock further leads considerable attention towards developing bio-based polymers, where bio-based polymers are degradable in nature which simultaneously addresses the sustainability issue of the environment [4–7]. Apart from packaging, application of bio-based polymers are extended towards biomedical [8], conductive field [9, 10], etc. In this following chapter, a discussion has been made regarding the conductive applications of biopolymers which are biodegradable, biocompatible, and non-toxic in nature. Most of the bio-based polymers such as poly lactic acid (PLA), chitosan, cellulose, poly hydroxyl butyrate (PHB) are insulators in nature [11]. So, in order to utilize these polymers in conductive materials, suitable modifications are required based on the targeted application. Moreover, thermoplasticity and mechanical strength of some available bio-based polymers make them a remarkable candidate for high temperature application. The conductive material-reinforced bio-based nanocomposites can be utilized for developing capacitor, electrochemical sensor, biosensor, battery, semiconductor applications. Considerably, the utilization of bio-based and biodegradable polymer provide reduced electronic hazards.

2 Bio-based Polymers for Conductive Application Bio-based polymers have some comparable properties similar to conventional polymers; however, the biodegradable nature of biopolymer makes itself a lucrative agent for market. Considerably, most of the bio-based polymers are insulator in nature, where proper processability of these polymers with different conductive polymers and conductive reinforcements may create a different prospect in the field of electrical and electronics application. In some of the cases, the thermal stability, mechanical strength, and comparable solubility parameter of modified biopolymers enhance their use towards electrochemical and biosensor application. Biopolymers such as PLA, PHB, cellulose, chitosan deliver the required properties that are applicable to be used as a matrix for conductive materials. PLA is aliphatic polyester having good solubility in organic solvents such as chloroform, 1-4 dioxane, acetonitrile, etc. and offer good melt processability, which makes it a good processable material in both solution and melt processing [12]. Further, hydrophilic polymers including chitosan and cellulose also have good compatibility with modified fillers such as carbon nanotubes (CNT), graphene, metal oxides which makes these polymers suitable for selective conductive applications [13–16].

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3 Modification Techniques The conductive application of bio-based polymers needs modification, which may enhance the electron mobility of such polymer. The modification techniques for developing conductive materials can be broadly classified as conductive composite and conductive blend in addition to suitable modifiers.

3.1

Conductive Composite

The incorporation of conductive fillers into bio-based polymers offers conductive nature in a nonconductive polymer matrix. This is considered as one of the principal routes for enhanced electrical conduction of polymers. Further, various carbon allotropes such as graphene, CNT, carbon fiber, and different metal oxides are used as filler for various bio-based polymeric matrixes (PLA, PHB, cellulose, chitosan, etc.) [17]. Araujo et al. [18] have fabricated PHB-based conductive composite material by incorporating polyaniline fiber of diameter 70–100 nm. A significant improvement in conductivity was observed from 0.046 to 1.1 S/m on the incorporation of 28% polyaniline [18]. Sabzi et al. [11] used two different grades of graphene (based on aspect ratio) for fabrication of PLA-based composite in different vol% combination, where a significant improvement in electrical conductivity was achieved after percolation threshold loading [11]. Shen et al. [17] prepared graphene oxide (GO) and used different reducing reagents like glucose, PVP to improve the conductivity of GO. Further, this reduced GO was incorporated into PLA to get conductive PLA composite. It was observed that at 1.25 vol% of glucose reduced GO, and the conductivity reached 2.2 S/m [17]. Chakraborty et al. [19] utilized exfoliated graphene (GR) and PLA for fabrication of conductive nanocomposites. The significant decrement in impedance of PLA was also observed by incorporating graphene (Fig. 1a). The effect of PLA chain length on the impedance of the composite was investigated by effective polymer length approach, where the effective polymer length of PLA was found to be decreased with the GR content (Fig. 1b). The optimized composite also subjected for ethanol sensing [19]. Similarly, other allotropes of carbon are also used by several researchers for fabrication of conductive biodegradable polymeric composite. Potschke et al. [20] fabricated PLA-based conductive composite for liquid sensing incorporating MWCNT. In 2 wt% loading of MWCNT, a significant decrement in resistivity (50 kX) was observed. The melt spun composite was further subjected to different solvents for sensitivity study [20].

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Fig. 1 a Impedance analysis of the films of length 5 cm and width 5 mm, b change of effective length of PLA responsible for the resistance of composite with respect to GR loading [19]

3.2

Conductive Blends

Conductive polymers such as polyaniline, polypyrrole, polythiophene, poly (vinyl pyrrolidone) are used to prepare conductive polymeric blend in combination with biodegradable polymers [20, 21]. Rajeswari et al. [22] prepared poly (vinyl alcohol) and poly (vinyl pyrrolidone)-based blend in 70–30 ratio and different wt% of LiNO3 was added into it by solution processing technique. The conductivity value was found to be 6.82  10−4 S/cm at 25 wt% LiNO3 loading [22]. Huang et al. [23] prepared electroactive scaffold by using modified PLA and modified aniline pentamer. The copolymer film was solution cast using chloroform as solvent. It showed electroactivity in I–V response curve in HCl solution (reduction peak at 0.34 V) [23]. McKeon et al. [8] mixed PDLA and polyaniline (PANI) in different weight proportions and electrospun to enhance cell proliferation. The calorimetric investigation indicated good blending between the polymers. PDLA/PANI of 75/25 blended system conducted current (5 mA) with conductivity 0.0437 S/cm [8]. Polypyrrole-based blends with biodegradable polymer have good conductivity. Different bio-based biodegradable polymers like PCL, PLA, etc. are used as one of the components for blending. Durgam et al. [24] synthesized polypyrrole and PCL block copolymer and achieved conductivity of 32 S/cm for the system. The cell proliferation study showed significant improvement as shown by MTS assay [24]. Xu et al. [25] fabricated conductive conduit using PPY and PDLLA blends of different composition. The conductivity of the blend system increases with increasing loading of PPy and at 15% loading of PPy, the maximum conductivity is 15.56 mS/cm [25] (Fig. 2).

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Fig. 2 Schematic of biodegradable polymer-based conductive blend

4 Bio-based Conductive Polymeric Substance Applications Biopolymers have a variety of applications starting from packaging to biomedical. These are mostly insulating in nature. As mentioned, the incorporation of conductive property in biodegradable polymeric system extends its application in fields like a capacitor, electrochemical sensor, biosensor, battery and biomedical applications such as tissue engineering (Fig. 3). Moreover, the biodegradable polymers require good processability, high temperature stability, and flame-retardant property to be used in the field of insulator materials as a thermal insulator, electrical insulator, etc. The fabrication of insulating device needs different types of mechanical processing or chemical treatment which limits the probable application of biodegradable polymers in insulating purpose [9]. However, incorporation of elements like graphene, CNT enhances the

Fig. 3 Schematic of different application areas of conductive bio-based material

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thermal stability of this kind of polymers and incorporation of flame-retardant materials can further improve the flame retardant property. The inherent degradable nature and presence of oxygen moiety make this type of polymer more susceptible to thermal degradation, which may put a limit for the specified field. The mechanical machining of these materials into sheet or fiber or switch demands regularity and consistency for production which is a limiting factor for these type of polymer providing less mechanical strength. Thus, biodegradable polymer application in the area of insulators is very limited. Still, research is going on how ABS can be replaced by PLA or butadiene can be replaced by Poly (butylene succinate) (PBS) for insulator application. Besides, the development of insulator via polymer composites, bio-based conductive materials also has a versatile application which is described as follows.

4.1

Capacitor

Dielctricity of any material depends on the polarizability of the material in presence of electric field. It is required for the applications like capacitor and field effect transistors which demand a different range of dielectric value of material. Sudhakar et al. used blends of starch and chitosan and incorporated lithium perchlorate into it in different concentrations. These films were sandwiched between two activated carbon-coated stainless steel electrodes under cryobath attachment. Incorporation of plasticizer has increased the conductivity value to *10−3 S/cm from *10−7 S/cm. The material found to have less activation energy with increase in conductivity. Capacitance value was found to be increasing and a maximum value of 133 F/g was achieved [13] (Fig. 4). Liew et al. [26] used bio-based polymer polyvinyl alcohol for fabrication of capacitor and achieved a maximum ionic conductivity of 7.31 ± 0.01 mS/cm. The specific capacitance obtained from galvalostatic charge-discharge test and the value reported is 28.36 F/g with better cyclic voltammetry response for the system [26]. In most of the cases, bio-based composite systems were utilized as electrolyte between two electrode surfaces. Ion mobility inside these films increased the capacitance values. In some of the cases, multiple ionic systems also introduced Fig. 4 Parallel plate capacitor [26]

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inside biodegradable matrix to enhance the performance [10]. Other bio-based polymers such as cellulose acetate [27], and PCL [28] are also utilized as a matrix component for double layered capacitor system. Thus, it can be said that bio-based polymers has huge scope of application in this electronic area. Mobility of ions can be higher for the polymers which are having low glass transition temperature (Tg). PCL, PBS can be more suitable material for fabrication of capacitor. However, commercialization of these materials needs more application-oriented investigations which are carrying out by different research groups across the world.

4.2

Electrochemical Sensor

The journey of electrochemical sensor was started long back in 1950 and was used for monitoring oxygen. Presently, many electrochemical sensors are used for sensing liquids, vapours, and gases in both stationary and portable device forms. These types of devices are used in monitoring environment and detecting small molecules. A good electrochemical sensor should have the following characteristics like fast sensing, high selectivity, high sensitivity, multiple time usability, and a well-defined usable range. In addition to the above, if the material also possesses degradable nature and consisting of component/components which are bio-based in nature, then it also addresses both the performance and waste disposal problems [29]. Two commonly used sensor set up are utilized as direct sensing element between a two-probe system and as a working electrode of three electrode system. Qi et al. [30] fabricated cellulose, CNT-based sensor element for the detection of water. 3 wt% CNT-loaded cellulose composite was used for detecting water. It is reported that within 10 s the change in response is 300%. The sensor found to have concentration detection ability in different concentrations of water in water–ethanol mixture (0–100%) [30]. This investigation indicates that hydrophilic bio-based polymers can be utilized for detection of aqua-based components. It is based on diffusion of water inside the matrix which is followed by alternative networks formed by CNT in the matrix. Considering diffusion principle, some hydrophobic bio-based polymers like PLA was used in combination with graphene, MWCNT for electrochemical sensing of alcohols or organic vapours. Kobashi et al. [31] found good sensing property of PLA/CNT nanocomposites for different organic solvents like n-hexane, toluene, dichloromethane, chloroform, and ethanol. Sensitivity of the material depends on the difference in solubility parameters between the matrix and liquid. Response time reported to be 30 s for good solvents [31]. Similarly, Chakraboty et al. utilized exfoliated graphene (GR) and PLA system for ethanol sensing [19], Mai et al. fabricated PLA/CNT conductive composite for degradation sensing [32], and Kumar et al. used PLA/MWCNT composite for volatile organic vapour detection [33] (Figs. 5 and 6).

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Fig. 5 Different types of bio-based electrochemical sensor

Fig. 6 PLA/exfoliated graphene composite for ethanol detection for different concentrations [19]

4.3

Biosensor

Detection of biomolecules likes proteins, glucose, amino acids, antibodies, antigens etc. using enzyme or targeted binder as sensor comes under the category of biosensor. Bio-based polymers are utilized nowadays for detection of biomolecules in combination with metallic or organic binder cite embedded onto it. Khalid et al. [16] used enzyme-immobilized cellulose nanocrystal for detection of urea in the concentration range of 0.001–1 mM [16], where the oxidation peak found to increase with urea concentration. Wu et al. [34] fabricated biocomposite

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Fig. 7 Mechanism of biosensing

incorporating Pt, graphene, and chitosan as sensor elements, and glucose oxidase was incorporated to the system to improve selectivity toward the detection of glucose. Current- voltage responses (I–V) of modified electrodes indicate more responsive nature towards stimuli compared to base substances. One amphoteric measurement technique was used to analyse the performance of the sensor towards glucose detection. The detection limit of the sensor was reported as 0.6 µM, and application range was µM to 0.5 mM glucose concentration [34]. Different enzymatic bio-based combination detection techniques were also carried out by researches for glucose [35], cholesterol [36] etc. In biosensor area, one of the techniques is the detection of biomolecules by non-enzymatic route. Han et al. [37] prepared Pd and Pt modified graphene oxide grafted PLA composite for detection of serotonin. The modified electrode material found to have detection range of 0.1– 100 µM with detection limit of 8  10−8 M and response time of 10 s. Serotonin with interfering components like dopamine, uric acid, ascorbic acid is also tested for sensing, which is found to have similar response [37] (Fig. 7).

4.4

Battery

Bio-based conductive material applications are also extended in the area of fuel cell and microbial fuel cell. In fuel cell, proton conductivity of membrane helps in the generation of current in both the types of cells. As well as electrical conductivity of the bio-based composites can make themselves useable for electrode purpose. Mohanapriya et al. [38] used modified pectin-based PVA composite for permeation of methanol, where proton diffusion found to be increased by incorporating TiO2 modified pectin [38]. One of the mostly used bio-degradable polymers for proton exchange membrane preparation in the fuel cell is chitosan. In the case of alkaline fuel cell, different cross-linked chitosan and blended chitosan are used [39] Chitosan

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is also used as electrode material in some of the fuel cells. Different aldehyde modifications of chitosan can replace nafion for electrode coating for better conduction of ion [40]. Similarly, in some of the cases, cellulose also utilized along with chitosan for fuel cell application. In recent times, paper-based battery also opened up new horizon in the fuel cell technology especially for flexible and portable electronics. In the case of biobatteries, high pore size and large surface area enhance the performances of biobatteries. PAA and PPDT are the polymers which are utilized for making biobatteries in some of the cases [41]. In some of the cases, people are also trying to replace the electrode using biodegradable polymer-based conductive composite by incorporating graphene, CNT, carbon fiber, etc. into the matrix.

4.5

Conductive Biomedical Application

Biomedical applications such as tissue engineering, nerve regeneration requires biocompatible as well as conductive nature of materials. Biodegradable materials are extensively used materials in the field of biomedical. Further, PCL and PGA are utilized for the fabrication of different types of suture and further modified biopolymers in various forms such as hydrogel are also used for drug delivery. Nowadays, for neurological research, some of the biodegradable polymers in combination with conductive polymer are also investigated via various modification techniques. McKeon et al. [8] has fabricated electrospun of PDLA/PANI blends in different weight percentage, where 75/25 mixture of PDLA/PANI showed the highest conductivity of 0.0437 S/cm. Moreover, rat cell proliferation study showed 75/25 PDLA/PANI has a significant positive result. The blend also found to be degradable hydrolytically [8]. Similarly, a different blended polymeric system based on PPy or PANI is utilized in the tissue engineering application [21, 42, 43]. Durgam et al. [24] fabricated PPy- and PCL-based block copolymer for nerve regeneration. Using this material, they fabricated nerve guidance channels (NGC) and studied in vitro cell proliferation and conductivity analysis, where the block copolymer found to have zero toxicity after 24 h study. Interestingly, simulative nerve regeneration investigation indicated the scope of this type of material for future application [24]. The conductive nature of fiber simulates cell growth, which can open a new era in the biomedical field. Some other applications are also possible for replacement of mussels in heart. Biodegradable polymer-based conductive substance sometimes is also utilized in the case of biomedical imaging.

5 Shortcomings and Future Scope The utilization of conductive biomaterial for different applications are relatively a new area in comparison with the available conventional polymeric. Further, biodegradability, biocompatibility, and conductivity are considered as the major

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properties of bio-based polymeric composite and blends, which enhance the application domain of the specified materials from packaging to conductive applications. Further, the conductive application involves specially electronics, semiconductor, fuel cell, and advanced biomedical applications such as nerve regeneration, tissue engineering, etc. Even though the usability and effectiveness of these materials are still a subject of investigation, but it emerges as new sustainable material in the respective field from both the perspectives, property and after-use degradability. Bio-based materials also became lucrative because of abundant raw material resource. This above-mentioned positive features of bio-based material rather than bio-based conductive material make it a automatic choice as an alternative to the existing. However, real-field application of these materials still requires improvement in characteristics like application stability, achievement of required functionality of the material. Besides, the major targeted property, some other auxiliary properties also act as an influencive factor for the overall performances of a particular material in a selective application. As an example, chitosan and modified chitosan are very good conductors of proton and it makes it suitable to replace conventional nafion membrane. However, the swelling nature, chemical resistivity, and hydrolytic degradation confine its application to a particular pH range. Similarly, bio-based conductive nanocomposites possess dispersion problem, which results in variation in network during each time. It is independent of manual error. Thick film application towards sensing analysis has lots of obstacles to generate reproducible minimum error response like crystallinity of polymer, solubility of the polymer, consistency of the composite. The brittleness of polymers (e.g. PLA) originate defects and fractures on the topography, which provide detrimental effects in case of conductive suture application. In some of the conductive applications environmental stability like towards microorganisms and stability towards thermal increment is required based on the nature of application and form of the material. In this regard, most of the biodegradable polymer lacks its performance. Biodegradable polymer materials are the future materials to be used in various fields and, among which, conductive material preparation is considered as one of the essential sectors. Further, the present researches focus on the development of thin film sensor, paper sensor, paper-based biobatteries, which are the emerging areas in terms of science and technology. In this regards, conductivity and biocompatibility make this type of material more useful in comparison with conventional materials in the development of various materials, including suture, nerve regeneration, tissue growth studies, etc. Moreover, in vivo and in vitro investigations of developed materials are required to make useful commercial product for various objectives. Bio-based polymers has attained much interest in fuel cell application, and further, different investigations are going on the membrane, flexible electrode, over all set up made up of bio-based degradable polymer. So, it can be concluded that potential and property of biodegradable polymers in combination of conductive nature of conductive polymer and conductive reinforcements can lead to sustainable green devices for different conductive applications (Fig. 8).

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Fig. 8 Summary of shortcomings and future scope of bio-based conductive material

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Chapter 18

Superhydrophobic Interfaces for High-Performance/Advanced Application Nirban Jana, Dibyangana Parbat and Uttam Manna

Abstract Nature is full of wonders and scientific excellence. We can acquire a great deal of knowledge by studying it, and such studies help us to ameliorate our understanding to find solutions for the conundrums we face in our everyday life. In this context, bio-inspired (lotus leaf), non-adhesive superhydrophobicity is one such phenomenon which has received wide recognition for its exceptional ability to repel water. Superhydrophobicity has drawn great attention because of its various applications like oil–water separation, smart microfluidics, extended drug release, etc. The coexistence of appropriate chemistry and topography confers this special wettability to the reported materials in the literature. Any sort of perturbation in essential chemistry or topography in the artificial superhydrophobic materials results in loss of high repellency to water. Generally, the hierarchical and rough topography decorated with inert and low surface energy molecules provides the essential metastable trapped air which plays a pivotal role in achieving bio-inspired superhydrophobicity. The micro-/nanofeatures that are made out of mostly hydrophilic ingredients and are topped with low surface energy molecules are vulnerable to physical as well as chemical properties. Such limitations appeared as Achilles’ heels, which is the widely practiced synthetic approaches restricting these materials from prospective applications in practical scenarios. To overcome such practical obstacles, many strategies have been adopted to fabricate highly durable superhydrophobic materials. In this regard, the book chapter is focused to discuss different promising and durable superhydrophobic interfaces and their prospective advance applications.













Keywords Superhydrophobic Contact angle Lotus leaf Plasma treatment Template Spin coating Electrospinning Spraying Solvothermal Photolithography Layer-by-layer Responsive surface Self-healing Shape memory Bulk superhydrophobic Oil–water separation Drug delivery Drag reduction Water harvesting Self-cleaning Chemically reactive Bulk wettability
















































N. Jana
D. Parbat
U. Manna (&) Department of Chemistry, Indian Institute of Technology, Guwahati, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_18

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1 Introduction The great physicist Albert Einstein once said, ‘look deep into nature, and then you will understand everything better,’ which is still very relevant and explains the foundation of various disciplines of science. Thus, paying attention to natural phenomena can add new dimensions to our understanding of science and enable us to use them for various purposes [1–10]. Superhydrophobicity is one such phenomenon which can be seen at many biological interfaces [1–5, 7, 11], for example, lotus leaf, butterfly wings, rice leaf, water strider, to name a few (Fig. 1) [1, 2, 3, 5, 10–12]. These surfaces possess some unique features which help them stand out. In the last two decades, a boom in the frequency of publication on superhydrophobicity has happened as a result of realizing the potential of such anti-wetting surfaces which can be used as an effective tool to solve our daily life problems. Such property can be utilized to develop microfluidic devices, robotics, oil–water separation, drug delivery, water harvesting, self-cleaning, etc. [13–31]. There are several other extremes of wettability such as superhydrophilicity [32–35], superoleophobicity [36–40], omniphobicity [41, 42], underwater superoleophobicity [7, 8, 9, 43–46]. Among these various different liquid wettabilities, lotus leaf-inspired superhydrophobicity has gained the lion’s share of attention due to its innumerable (more formal) applications. In this regard, plenty of reported materials could not deliver a product because of the associated downsides. But there are some promising designs which can perform in practically relevant scenarios for a prolonged duration. The term ‘superhydrophobicity’ was first coined by Reick [47] in 1976 to describe a hydrophobic fumed silicon dioxide-based coating, where the force of adhesion was insignificant and the beaded water droplet barely underwent any change of its spherical shape. Major current challenges of superhydrophobic surfaces (SHSs) are related to durability in severe physical and chemical settings

Fig. 1 Digital photographs and scanning electron microscope (SEM) images of the surface morphology of various naturally existing superhydrophobic surfaces (SHSs). a Photograph (inset) and SEM image of the surface of lotus leaf. The random size distribution of the micropapillaes is further designed with fine branch-like nanostructures [11]; b photograph of a red rose (inset) and SEM image of the surface of rose petal [2]; c photograph (inset) and SEM image of the fine lamella-stacking nanostripes on the surface of butterfly wings [3]. Reproduced from Refs. [2, 3, 11]. Copyright ACS, RSC, and Springer, respectively

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[48–54]. In order to achieve superhydrophobicity, two criteria (hierarchical topography and low surface energy) need to be fulfilled and will be discussed elaborately later. The hierarchical or micro-/nanofeatures experience more load (or pressure) because of their low surface area and therefore get easily damaged upon any mechanical stress. Such damage of the topography results in loss of the anti-wetting property which impedes its application in practical scenarios [55–60]. However, for real-life application, the material should be able to cope with various forms of physical and chemical insults [51–54]. Different applications require a different set of characteristics and a certain degree of resilience. Suppose there is a need to use superhydrophobic mesh for oil–water separation, then the extreme water-repellent property of the mesh must remain intact—even after creasing, twisting, rolling which are some common physical manipulations the material will have to face during the separation and collection of the desired liquid phase. The materials which were developed in the early stage of development in the 1990s were mostly very delicate and were not able to sustain their anti-wetting property once the top surface is damaged [11, 23, 61–63]. Some strategies were later developed to combat these challenging issues—through adopting facile and scalable approaches. These issues will be discussed later in more detail; before that, the understanding of the fundamental basis behind this extreme water repellency is very important.

1.1

Different Models for Liquid Wettability on Solid Interfaces

Before going into the reasons why such property arises, we need to understand some fundamental models of wettability (Fig. 2). Thomas Young proposed the first model of wettability way back in 1804 and introduced the concept of contact angle in 1805 (Fig. 2a) [64]. He explained how liquid phase spreads on a solid surface and reaches equilibrium when various components of interfacial tension balance each other. The Young’s equation (Eq. 1) consists of three interfacial tensions that are acting between solid–liquid (csl ), liquid–gas (clv ), and solid–gas (csv ), and the hY is contact angle beaded liquid on solid interface as noted in Eq. 1. Cos hY ¼ ðcsv  csl Þ=clv

ð1Þ

But, Young’s consideration of the smooth surface was a non-ideal case because defect-free and completely smooth interfaces are rare in reality. Although this model was not a practical one, it led the path to the surface science. Then in 1936, a more realistic model was introduced in the literature by Wenzel (Fig. 2b) and the proposed model is widely recognized as Wenzel’s model [66], where he has taken the surface roughness factor into account. The roughness factor (r) is expressed as the ratio of the actual area of a rough surface to the geometrically projected area in his theoretical wettability model. The mathematical representation of the model is given in Eq. 2.

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Fig. 2 Schematic representation of different wettabilities (homogeneous and heterogeneous) of water droplet on different solid surfaces (smooth and rough surfaces), and the wetting behaviors are further explained by three models: a Young’s model (homogeneous wetting on smooth surface), b Wenzel’s model (homogeneous wetting on rough surface), and c Cassie’s model (heterogeneous wetting on rough surface). Reproduced from Ref. [65]. Copyright 2015 The Royal Society of Chemistry

Cos hW ¼ r Cos hY

ð2Þ

Both the models proposed by Young and Wenzel are considered as homogeneous wettability as the contact between the beaded liquid and the solid interface is continuous as shown in Fig. 2. However, this homogenous wettability of liquid on rough interface failed to explain the extreme wettability of water that is found on a lotus leaf. Later in 1944, Cassie and Baxter (Fig. 2c) considered that the trapped air in the micro- and nanostructures of a rough surface plays a pivotal role [67] in minimizing the contact between the beaded liquid and the solid interface and allowed only heterogeneous wettability, and eventually, the entrapped third external phase conferred the extreme liquid wettability on solid interface as illustrated in Fig. 2. The total fraction of contact area between the beaded liquid (f1) and solid (f2) interface is one. Then, the expression for the Cassie and Baxter wettability is formulated as; Cos hCB ¼ f1 Cos h1 þ f2 Cos h2

ð3Þ

Although both the theories can qualitatively predict the contact angle of a rough surface, there is no clear guideline which describes a more appropriate model in any given scenario. But, since the modeling can help in providing direction to design novel SHSs, many attempts have been made to come up with more useful models. One such report was published by Lundgren et al. [68]. They have studied different wettabilities of the top surface and walls of the pillars using molecular dynamics modeling, where these pillars are serving a crucial role for contact angle determination. When pillar heights are low, the system is in the Wenzel regime, and a transformation to the Cassie–Baxter regime is observed with an increase in the pillar height. The contact angles (CA) appear to be unaffected when pillar height is more than 15 Å. This Cassie–Baxter wettability model provided the basis to understand the fundamental behind the extreme water wettability—that commonly observed in various naturally occurring interfaces—that are lotus leaf, rice leaf,

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poplar leaf, wings of butterflies, compound eye of mosquitoes, legs of water strider, etc. [1, 2, 3, 5, 10–12]. Such water wettability is widely recognized as superhydrophobicity and formally defined based on the water contact angle (WCA). The angle between the tangent (at air–water interface) and solid interface at tri-phase contact line are measured as WCA, and the WCA estimated during the process of initial contact between the beaded water and solid interface is noted as advancing WCA. Another important parameter is the roll-off angle which defines as the minimum angle of tilting of the solid surface to start the rolling of the beaded water droplet. The solid interfaces which can show advancing CA of above 150° and roll-off angle (which is equivalent to contact angle hysteresis (CAH), the difference between the WCA during advancing and receding processes) below 10° are considered as superhydrophobic. Such surfaces are ubiquitous in nature. Electron microscopic investigation on the naturally occurring superhydrophobic interfaces revealed the existence of a dual degree of roughness. The hierarchical interfaces consist of micro-/nanodomains [69]. Such micro-/nanofeatures that are inherently decorated with waxy material reduce the solid–liquid contact area significantly. The presence of waxy materials onto these surfaces also indicates the necessity of low surface energy coating. Later on, experiments are designed to understand the essential physical and chemical parameters to develop artificial superhydrophobic interfaces. On that note, in 2002, Feng et al. [12] first provided a complete overview on the importance of both the optimum combination of the micro-/nanofeatures and the influence of appropriate arrangement of the microscopic features on the water droplet movement to achieve the artificial superhydrophobic material by investigating the naturally existing SHS like lotus leaf and rice leaf with a significantly high contact angle (>150˚) and low roll-off angle (150° (a–c) through LBL deposition technique by exploiting Michael addition reactions. Reproduced with permission from Ref. [95]. Copyright 2017 The Royal Society of Chemistry

Fig. 11 Fabrication of a photoreversibly tunable SHS which is designed with essential rough morphology produced by ten nanoparticle deposition cycles [100]. Illustrating the structural change in different SHS that can undergo photo-isomerization under UV and visible irradiation [101, 102]. Reproduced with permission from Refs. [100–102]. Copyright RSC and ACS

[103, 104]. Solvent from the extruded polymer fiber evaporates rapidly resulting in the formation of a long thread with diameter as small as tens of nanometers. The fiber diameter can be tuned by changing the applied voltage and viscosity of the solution. The porosity of nanofiber mats can be easily adjusted by controlling the processing parameters. Besides surface roughness, the film properties can also be modulated by changing chemical functionalities, such as the incorporation of fluorine [105]. As traditional electrospinning is very slow and takes hours to produce a free-standing film, needleless electrospinning has been developed for mass production of nanofibers.

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Fig. 12 Electrospinning of PCL mat before (a, c) and after (b, d) CVD of PPFEMA. Reproduced with permission from Ref. [106]. Copyright 2005 American Chemical Society

Surface morphology plays a crucial role in determining the wettability of the material. Surfaces containing beads exhibit more hydrophobic property than those with only nanofibers. According to these observations, higher WCAs are achieved by using low molecular weight polymers because yielding of bead formation is much more for lower molecular weight polymers than that of high molecular weight polymers. Ma et al. [106] showed the combination of electrospinning of poly (caprolactone) (PCL) with chemical vapor deposition (CVD) of polymerized perfluoroalkyl ethyl methacrylate (PPFEMA) (Fig. 12). The hierarchical surface roughness inherent to the PCL electrospun films and the extremely low surface free energy of the coating layer owing to fluorine molecules yields stable superhydrophobicity with a contactsuperhydrophobic materials from the literature angle of 175° and a threshold sliding angle less than 2.5° for a 20 mg water droplet. Jiang et al. [107] prepared a superhydrophobic PS film consisting of porous microspheres and nanofibers using electrospinning method. When concentrated solution (20– 30 wt%) of polymer is spun, the high viscosity and polymer content lead to rapid drying of the solution jet and nanofibers are formed. In case of dilute solutions (3– 5 wt%), the jet cannot withstand the elongation due to low viscosity and collapses to form droplets which eventually transforms into microparticles. The microparticle film is more hydrophobic than nanofiber film which is attributed to greater roughness. But, the microparticle film is not robust, and the particles disintegrate easily with water. So, they have used a solution of 6–10 wt%, in which one part will shrink to microspheres and the other part will solidify to nanofibers. The resulting film displayed stable superhydrophobicity because it has structural stability of nanofibers and the roughness of microspheres.

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3 Durability Issue and Some Approaches to Overcome It Over the last decade, many expensive and complicated methods were followed to prepare superhydrophobic coatings, and most of the surfaces failed to retain its property after mechanical abrasion and/or other harsh practical conditions as shown in Fig. 13 [108]. The surfaces in the early stage of development lost their property even on slight friction [109]. The low surface energy of the surface is also another criterion to achieve this property. Mostly, fluorinated or long hydrocarbon chain-containing moieties are used for this purpose. But, in most of the cases, they are associated through weak interactions like metal ion–sulfur interaction and electrostatic interaction and therefore susceptible to damage in different chemical environments [110, 111]. The covalently bonded groups possess the advantage of being more durable in various chemical [112]. Researchers have been also attempting to improve the durability of SHS upon physical abrasions because the hierarchical structures get easily damaged by mechanical forces. Verho et al. [51] discussed the issues regarding the lifetime of SHS in detail. Water droplets can easily roll off on SHS, but physically damaged SH surfaces are unable to show such rolling off because droplets adhere to them. Abrasion can destroy the dual degree of roughness and may increase surface energy by exposing the inner hydrophilic parts, resulting in compromised anti-wettability performance [113, 114]. Another important challenge is the use of chemicals that have an adverse effect on environment. Fluorine-based materials are used widely for anti-stain applications like masonry, leather, paper products, and textiles due to its inherent low surface energy leading to hydrophobicity and oleophobicity. Though fluorine is highly useful to achieve anti-wetting property, it poses environmental threats. Environmental protection agency (EPA) has issued stringent regulation on the use of fluoropolymers which could release highly toxic materials like perfluoroalkyl carboxylates or perfluorosulfonates into the environment [115–

Fig. 13 The effect of physical abrasion on superhydrophobic coating. Reproduced with permission from Ref. [108]. Copyright 2015 Royal Society of Chemistry

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117]. The shorter (4–6) carbon perfluoroalkyl chain products were identified as non-bioaccumulative. However, performances of fluoropolymer are compromised significantly when fluorinated chains become shorter. The use of non-fluorinated products can avoid environmental and health hazards and so are of increasingly commercial as well as scientific interests. In the recent past, few fundamentally different strategies are developed for improving the durability of artificial superhydrophobic interfaces; in that context, the methods like covalently cross-linking the porous functional coatings, creating multiscaled roughness with elastomeric polymers, optimizing essential chemistry on top of hierarchical features through covalent bonding, introducing a bio-inspired self-healing of essential low surface energy coating, auto-regenerative hierarchical interfaces, etc., are of special interests. Some of these emerging approaches for developing durable superhydrophobicity are discussed in the following sections.

3.1

Self-healing of Anti-wetting Property by Recovering Chemistry

As mentioned before, most of the super-liquid-repellent surfaces are susceptible to compromise the embedded extreme water repellency after incurring perturbation in essential chemical and physical [118–120]. Super-repellency of surfaces may be damaged completely or partially due to chemical oxidation from exposure to air, harsh chemical environment, strong light, or physical rubbing. It is very important to improve the durability for practical applications. In case of plants like lotus, whenever the epicuticular wax layer is damaged by external means, the plant restores the property by regenerating the wax. Inspired by such self-healing property of living plants, Li et al. [55] reported a method to artificially fabricate self-healing superhydrophobic coatings. They have used LBL method to achieve the micro- and nanofeatures using polyelectrolyte complexes of PAH and sulfonated PEEK with PAA. CVD of 1H,1H,2H,2H-perfluorooctyl-triethoxysilane (POTS) was performed at 120 °C for 3 h to obtain superhydrophobicity. POTS reacts with free carboxylic acid, amine groups, and neighboring POTS in the polyelectrolyte coating, and also the high porosity of the coating allows deep diffusion of low surface energy molecule POTS. O2 plasma treatment damages the surface and makes the surface superhydrophilic because of the generation of new hydrophilic groups. But, in ambient humid environment, the POTS molecules migrate to the surface and the superhydrophobicity is restored as shown in Fig. 14. Such self-healing function can extend the longevity of SH materials for practical applications [121]. Another such design was introduced by Wang et al. [56] where they had investigated the wettability property of fabrics coated with a hydrolysis product from a fluorinated alkyl silane (FAS) and fluorinated-decyl polyhedral oligomeric silsesquioxane (FD–POSS). The fabric can be coated by wet chemical coating

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Fig. 14 a Fabrication of beeswax—multiwalled CNT—PDMS block co-polymer-based SHS by spray coating, b sunlight-induced self-healing mechanism of the superhydrophobic material through the migration of beeswax. Reproduced with permission from Ref. [121]. Copyright 2018 The Royal Society of Chemistry

techniques such as padding, spraying, or dip coating. They observed that the surface can self-heal the embedded superhydrophobic and superoleophobic property after incurring chemical abrasion. Upon damaging the surface chemistry with plasma treatment, the surface free energy increased due to generation of polar groups on the surface, and as a result, hydrophobicity compromised significantly. The exposure of this damaged interface at elevated temperature allowed movement of the FD–POSS molecules and the top interface is again optimized with fluorinated alkyl chains. The temperature assisted restoration of low surface energy at the top of the regain of interface. The coating was capable of withstanding 6000 cycles Martindale abrasion and 200 cycles of wash without any significant change in its superamphiphobicity. Later, Liu et al. [122] prepared a fluorine-free self-healing hydrophobic surface, where mesoporous silica was used as a reservoir for low surface energy coating agents—that is octadecylamine (ODA) which can migrate at elevated relative humid environment and provided a facile basis for healing the damaged interface with essential chemistry and that eventually allowed to regain the lost anti-wetting property as demonstrated in Fig. 15. In 2013, Manna et al. [60] introduced another design, where they have coated commercially available polyolefin-based shrink-wrap with PEI/PVDMA multilayers by LBL method. The unreacted azlactone groups were further modified with decylamine to increase the hydrophobicity. But, there was a slight change in WCA, from 85° (uncoated shrink-wrap) to 115° in the case of 7 bilayers. The very thin layer of polymeric coating did not provide the necessary hierarchical features to

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Fig. 15 The reversible self-healing of pDA@silica—ODA superhydrophobic material. The self-healing property of the coated material is achieved by plasma treatment and pDA treatment simultaneously. Reproduced with permission from Ref. [122]. Copyright 2012 American Chemical Society

exhibit superhydrophobicity. Upon heating, the shrink-wrap shrunk and SEM images revealed that the wrinkles were studded with granular features which explain the significant increase in hydrophobicity. After heat-induced shrinking, WCA on 7-bilayer thick coating was found to be 154° with a sliding angle of 4°. Such superhydrophobic coating on a shrinkable substrate provided a facile basis to heal physical abrasion under elevated temperature.

3.2

Self-healing of Anti-wetting Property by Recovering Topography

In 2012, Puretskiy et al. [123] introduced an approach for self-repairing ultrahydrophobic properties by rearranging colloidal particle in the waxy matrix, where the material was fabricated by strategic use of perfluorinated wax (1-iodo1H,1H,2H,2H-perfluorodecane) and colloidal particles (3-aminopropyltriethoxysilane-modified silica particle) as shown in Fig. 16. The particles spontaneously segregate at the top interface and provided appropriate hierarchical features. Upon scratching, the surface loses its ultrahydrophobic property as the essential topography is significantly affected during this physical abrasion process. However, at elevated temperature, the waxy matrix melted and facilitated the reorganization of the colloidal particles at the top interface and eventually provided a newly formed hierarchical interface. Thus, the superhydrophobic property is recovered in the material. In 2017, Avijit [124] and his coworkers fabricated a graphene oxide-based superhydrophobic material with self-healing ability without application of any external stimuli. Application of external pressure on the soft polymeric material damaged the hierarchical topography, and the polymeric coating compromised its non-adhesive superhydrophobicity. However, the damaged topography was self-restored under ambient condition and the material displays non-adhesive superhydrophobicity. The rate of recovery of physical deformation and water

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Fig. 16 Representation of ultrahydrophobic materials that is prepared from perfluorinated wax and colloidal particles with self-repairing ability. (a, b). Melting of the wax results in migration of particles from bulk to topmost layer after mechanical damage and the property is restored (c). Reproduced with permission from Ref. [123]. Copyright 2012 American Chemical Society

Fig. 17 Digital images (a, b; scale bar: 5 mm) and advancing (c, e)/receding (d, f) CA images of the self-healing behavior of aminographine-doped polymer coating after the physical damage (a, c, d) and after self-healing (b, e, f) of the damage. The adhesive/non-adhesive interaction of beaded water droplets on tilted (3°) polymeric coating before (g, h) and after (i, j) self-healing of pressure-induced physical damage during repetitive (40 cycles) damage/self-healing process (l). k The plot shows the rate of self-healing of physical damage (red) and anti-wettability (black) of the polymeric coating with time. Reproduced with permission from Ref. [124]. Copyright 2017 American Chemical Society

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wettability was controlled by a change in the doping concentration of amino graphene oxide in the polymeric material. The recovery of water wettability was successfully achieved for more than 35 cycles, without any significant change in the adhesive property at the end of the recovery process as shown in Fig. 17. Such materials are highly capable of sustaining other severe physical and chemical challenges. Recently, Wu et al. [57] fabricated a conductive superhydrophobic film by depositing a layer of Ag nanoparticles and Ag nanowires on a thermally healable PCL/poly(vinyl alcohol) (PVA) composite film with subsequent deposition of 1H,1H,2H,2H-perfluorodecanethiol. The AgNPs–AgNWs layer provided the hierarchical micro- and nanofeatures to achieve superhydrophobicity and also served as a photothermal or electrothermal heater to enable healing of the underneath PCL/ PVA film with the assistance of a low-power NIR light irradiation or a low applied voltage. Because of such healing, the superhydrophobicity is restored even after cuts of several hundreds of micrometers wide. Lv et al. [125] prepared a shape memory polymer by polymerizing a mixture of diglycidyl ether of bisphenol A type epoxy resin (DGEBA), n-octylamine (OA), and m-xylylenediamine (MXDA) with a certain molar ratio in a structured template as illustrated in Fig. 18. The contact angle and sliding angle of a water droplet on the as-synthesized surface are about 153° and 3°, respectively, implying that the surface is non-adhesive superhydrophobic. The surface was pressed under external load to destroy the micro- and nanofeatures, and this collapsed structures caused a significant change in water wettability. However, the same damaged interface restored back the lost superhydrophobicity after heating the damaged polymeric material at 85 °C for 2 min. They also treated the surface with O2 plasma which is known to make surfaces hydrophilic by introducing oxygen-containing functionalities. Here, the O2 treatment made the surface superhydrophilic, but the surface was able to regain its anti-wetting property due to the burial of polar groups in the bulk during the heating process. Such design is inherently capable of sustaining both physical and chemical insults. Another self-healing superhydrophobic interface was prepared recently by replicating a hierarchically structured template with epoxy-based shape memory polymer [126]. The micro-/nanostructures were crushed by heating followed by pressing, and the micropillars showed uniform deformation and orientation. Heating at 120 °C for 30 min caused the recovery of the micro-/nanostructures. The change in wettability is mainly attributed to the change of hierarchical structures. They had also demonstrated its suitability in designing rewritable chips for droplet storage. Wang et al. [127] prepared metamorphic superomniphobic surfaces by combining hexagonal arrays of re-entrant textured mushroom-like pillars of a thiol-ene/ acrylate-based thermo-responsive SMP. The pillars were fabricated by photolithography and reactive ion etching, followed by treatment with fluorinated silane to impart low surface energy of less than 10 mN m−1. The morphology could be reversibly switched between collapsed structure and mushroom-like pillar structures by strategic use of elevated temperature, and this material was further extended for demonstrating rewritable superhydrophobic–hydrophilic patterned interface.

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Fig. 18 Artificial shape memory SHS with micro-/nanostructured arrays that can be applied in rewritable chip for droplet storage. Reproduced with permission from Ref. [125]. Copyright 2016 American Chemical Society

3.3

Bulk SHS

The success of the self-healable superhydrophobic materials in keeping the anti-wetting property intact mainly depends on their self-healing ability which underperforms after multiple cycles in most cases. Moreover, many of the existing designs of self-healing superhydrophobic demand application of appropriate external stimuli for the recovery process and the optimization of essential stimuli at the site of application in the practically relevant diverse and practical settings are always challenging. In the recent past, the concept of bulk superhydrophobicity [18, 59, 83, 128–133] has been introduced in the literature with few examples, where such materials are with essential topography and appropriate chemistry three dimensionally—including top surface and the interiors for displaying extreme water repellency, even after physical erosion of the top portion of the coatings as shown in Fig. 19. In a seminal report, Levkin et al. [128] prepared a superhydrophobic material by phase separation method utilizing photo-initiated isomerization of butyl methacrylate and ethylene dimethacrylate with porogen (cyclohexanol and 1-decanol). The use of divinyl monomers in the polymerization mixture resulted in highly cross-linked, insoluble materials that were mechanically more stable and more rigid as compared to the non-cross-linked polymers with a similar porous structure, and this material was inherently embedded with bulk superhydrophobicity. The synthesized material had continued to display extreme water repellency even after the removal of top interface of the polymeric material. Later, Yohe et al.

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Fig. 19 Few examples of bulk superhydrophobic materials from the literature. Reproduced with permission from Refs. [18, 130, 132]. Copyright ACS, RSC, and Science Direct, respectively

[130] reported a one-step fabrication method for durable 3D bulk superhydrophobic coatings via an electrospraying process, where PCL was used as a base polymer for electrospinning and hydrophobically modified polymer poly(glycerol monostearate-co-e-caprolactone) (PGC-C18) was used as dopant for optimizing the essential low surface energy environment, three dimensionally. Li et al. [131] developed a cost-effective approach for fabricating durable SHS by spray coating of polymerized organosilane/attapulgite clay nanocomposites. These nanocomposites were prepared by a modified Stober method where hexadecyltriethoxysilane and tetraethoxysilane were condensed together in the presence of attapulgite. The coating showed good stability during various solvent immersions, sand abrasion, extremes of temperature, and UV exposure. The coating displayed very high WCA (*160°) and low sliding angle (*2°). Polyurethane, PTFE, paper, textile, and aluminum foil could be easily coated by following this spray-based method. The APT crystals were responsible for the two-tier topography which trapped the air beneath water droplets. They also observed only the rod-like APT, which can provide the necessary morphology. Recently, Rather et al. [133] reported a bulk superhydrophobic material by forming a polymeric aerogel through 1,4-Michael addition reaction between dipentaerythritol pentaacrylate and BPEI followed by functionalization with long-chain hydrocarbon amines (i.e., decylamine and ODA) as illustrated in Fig. 20. The synthesis procedure was very easy, and the anti-wetting property was present throughout the material. The ‘reactive’ nature of the gel was attributed to the unreacted amine and acrylate groups which can be exploited further to modify it with a wide range of functionalities to tune the wettability. As reaction was advancing, the polymeric complexes kept on growing and upon keeping it without disturbances the nanocomplexes got covalently cross-linked with each other, resulting in a chemically reactive porous polymeric material. Further, a covalent association of essential low surface energy molecules provided durable and bulk superhydrophobicity. Being inspired by the facile chemistry between the polymer and cross-linker, Das et al. [15] prepared a material by incorporating amino graphene oxide (AGO). The AGO enhanced the mechanical robustness significantly. The AGO was also covalently linked to the polymeric network. The synthesized material that was

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Fig. 20 Covalently cross-linked 1,4-Michael addition reaction between amines and acrylates followed by appropriate hydrophobization with hydrophobic amine-containing small molecules to fabricate SHS. Reproduced with permission from Ref. [133]. Copyright 2016 American Chemical Society

loaded with appropriate amount of AGO provided flexible and bulk superhydrophobic coatings on various substrates as shown in Fig. 21. The bulk property of the material was confirmed by carrying out different physical abrasions across the coating, and superhydrophobic property was found to be unaltered even after performing harsh abrasion tests like sand paper abrasion, adhesive tape test, rolling, bending, etc.

Fig. 21 Aminographine-doped flexible polymeric coating embedded with three-dimensional superhydrophobic property. Reproduced with permission from Ref. [15]. Copyright 2017 Royal Society of Chemistry

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Maji et al. [30] recently reported a ‘reactive’ bulk coating which was prepared by spraying a mixture of BPEI (polymer) and 5Acl (cross-linker) in a suitable alcoholic solvent. The mixture was shaken for some time to ensure that the molecules reacted with each others and formed nanocomplex which provided the necessary topography for superhydrophobicity. The evaporation rate of a solvent plays a crucial role in film forming process, and that is why a coherent defect-free coating was obtained in pentanol. Owing to the reactive nature of the polymeric coating, its wettability can be tuned by modifying it with primary amines with different chain length. Computational studies were performed to investigate the effect of solvent on the addition reaction as well as on the morphology of the material. It was observed that the reaction progresses at a faster rate on increasing the hydrocarbon chain length of the alcoholic solvent. The fast reaction in pentanol was taken up to prepare a coherent film for spray coating. This approach is really promising because it can be used to coat large objects.

3.4

Stretchable and Compressible Superhydrophobicity

Fabrication of SHS with high mechanical durability is a great issue that limits its potential applications in practical scenarios. Most of the cases, superhydrophobicity of the material is compromised while exposing to large-strain compression or stretching. Therefore, to develop a mechanically durable SHS is a real challenge, and the demonstration of such kinds of surfaces is very rare in the literature. For example, in 2013, Cho and his coworkers [134] fabricated a rubber-like stretchable fibrous membrane that could exhibit superhydrophobic property. The stretchable membrane consisted of a coating of polyaniline and polytetrafluoroethylene on polyurethane fibrous matrix that could withstand  300% strain. The antiwettability remained intact for 1000 stretching cycles with advancing WCA *160° and CAH of *10°. Inspired by the bio-adhesive property of marine mussels, Liu et al. [135] demonstrated a stretchable superhydrophobic coating. The compressible and stretchable coating was developed on a commercially available polyurethane (PU) sponge by using LBL deposition of polydopamine (pDA) films and Ag nanoparticles. The coating was able to withstand 6000-cycle compressions and 2000-cycle tensile strains without compromising superhydrophobicity property. The strong interactions among the pDA interlayers, Ag nanoparticles, and sponge skeleton were claimed to be the reason behind the high compressibility and stretchability of the coating. Recently, some approaches are introduced on stretchable and compressible SHS that can extend its applications in developing flexible microfluidics, ultra-flexible electronics, gas sensors, flexible textiles, wearable devices, etc. In 2017, Rather et al. [136] had introduced a stretchable superhydrophobic material by strategic incorporation of the reactive nanocomplex of dipentaerythitol pentaacrylate and BPEI in a polyurethane (PU) fabric that can withstand 150% of physical deformation and also highly durable under several harsh physical and chemical conditions. The stretchable superhydrophobic matrix

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was further explored in gravity-driven filtration of oil–water mixture even under 150% of strain with very high efficiency of 99%. Later, Liu et al. [135] demonstrated an SHS with stretchability and compressibility that can withstand up to 70% compressive strain and 40% tensile strain. The material was fabricated by integrating pDA films and Ag nanoparticles on the commercial polyurethane (PU) sponge through LBL fashion. This finding stands out as a new tool for anti-icing application (at −15 °C). Besides, Das et al. [137] also utilized the impeccable stretchability of PDMS substrate to develop stretchable bulk superhydrophobic interface by controlled construction of aminographine-doped polymeric multilayers. After successful engineering of multilayers on the PDMS substrate and post-chemical modification of those multilayers with both ODA and glucamine, the coating was exhibiting both superhydrophobic and underwater superoleophobic property, respectively. FESEM images confirmed the progressive rupture on the surface (Fig. 22b) upon applying 100% strain which was eventually healed after releasing the strain (Fig. 22c). In addition to that, the coating was able to sustain up to 100% of tensile strain with exceptionally high durability toward severe harsh physical and chemically complex situations and UV radiations.

Fig. 22 a–c FESEM images of the stretchable multilayers of aminographine-doped 5Acl/BPEI nanocomplex at different conditions: a no strain, b 100% strain, and (c) released strain. d–g The restrained anti-wettability of the multilayers was further confirmed by the contact angle (e, g) and digital images (d, f) of the droplet bedded on the surface before (d, e) and after (f, g) applying 100% strain. Reproduced with permission from Ref. [137]. Copyright 2018 The Royal Society of Chemistry

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4 Applications 4.1

Absorption-Based Oil–Water Separation

Frequent oil spills and industrial oil-contaminated discharges are severely affecting the aquatic ecosystem. Aftermath of Exxon-Valdez or Gulf of Mexico oil spills acted as wake-up calls to realize their catastrophic effect [138, 139]. There are various traditional and less effective methods available to tackle such problems like booms and skimmers, sorbents, oil-eating bacteria, etc., but they all have downsides too. For example, bacteria take months to years to break down the spilled oil which is not a suitable option for marine spill because it requires quick action to avoid affecting a larger area, whereas sorbents are likely to lead another secondary water pollution and other traditional approaches are either less energy efficient or highly laborious. In the recent past, biomimicked extreme liquid wettability has been rationally utilized for separating oil and water mixture following energy-efficient and eco-friendly processes [140–143]. CNT sponge, PU sponge, stainless mesh, copper mesh, etc., were modified to make them superhydrophobic. Because of SH property, these materials repel water phase and allow the oil phase to pass through. PU or melamine sponges possess very high absorption capacity because of their high porosity and therefore are good candidates for absorption-based separation. On the other hand, the SH meshes selectively allow the oil phase to pass through. Teflon-coated stainless steel meshes were fabricated by Feng et al. [13] using spray and dry method. These superwetting surfaces were able to successfully separate diesel and water mixture. For example, a simple and facile 1, 4 conjugate addition reactions were exploited by Manna’s research group [15, 16] to fabricate a graphene-based flexible robust monolith and a polymer-based durable coating on fibrous cotton, and at the end, the synthesized materials are embedded with superhydrophobicity as shown in Fig. 23. Due to the flexible behavior of the monolith and compressible behavior of the coated cotton, both are highly mechanically durable in nature and both can withstand more than 65% compressive strain, without affecting the embedded superhydrophobicity. Both the superhydrophobic monolith and the coated cotton can efficiently separate different forms of oil–water mixtures under practically relevant conditions, and such demonstrations are rare in the literature and useful for practical applications. Furthermore, Gui et al. [17] synthesized a hydrophobic–oleophilic three-dimensionally interconnected CNT sponges by CVD and these sponges exhibited high porosity (>90%), structural flexibility, robustness, excellent selectivity, recyclability, and good absorption capacity.

4.2

Gravity-Driven Filtration-Based Oil–Water Separation

Superhydrophobic interfaces inherently repel water extremely—but at the same time, it soaks oil/oily phase instantly. So, the strategic association of

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Fig. 23 Artificially (flexible robust monoliths and durable polymeric coatings on commercially available absorbent cotton) fabricated bio-inspired superhydrophobic property that is further exploited in both absorption and filtration-based oil–water separation. Reproduced with permission from Refs. [15, 16]. Copyright ACS and RSC, respectively

Fig. 24 a Fabrication of cotton fabric decorated with TiO2 particles and post-modified with PTES to develop a SHS. b-d The fabric was explored in gravity-driven filtration-based oil–water separation for 1,2-dichloroethane. Reproduced with permission from Ref. [145]. Copyright 2015 The Royal Society of Chemistry

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superhydrophobicity with appropriately selected two-dimensional porous substrates (e.g., stainless steel mesh, fabric, polymeric membranes, etc.) provided a facile basis for separating oil–water mixtures through gravity-driven selective filtration technique without any external interventions as shown in Fig. 24, where fibrous fabric that was decorated with TiO2 particles and low surface energy molecules for appropriate optimization of essential chemistry and topography was used for gravity-driven selective filtration of oil phase from oil–water mixture [144–146]. For example, Liu et al. [144] have introduced a superhydrophobic–superoleophilic textile by functionalizing a commercially available textile substrate with magnesium—aluminum-layered double hydroxide (LDH) microcrystals along with a low surface energy coating. The textile was finally exploited in gravity-driven filtration of different oil–water mixtures like n-hexane, toluene, chloroform, diesel oil, etc., with different volume ratios. The separation efficiency was found to be above 95% for all the cases. Further, Li et al. [146] prepared a candle soot and silica-coated stainless steel mesh which exhibited superhydrophobic property with a WCA as high as *160° and roll-off angle *5°, whereas oil contact angle was found to be 0°. Oil and water mixtures were completely separated by the coated mesh with a separation efficiency greater than 98% for different oils irrespective of their viscosity through a gravity-driven filtration-based approach.

4.3

Controlled Drug Release of Bioactive Small Molecules

Recently, few reports proved that superhydrophobic materials can also be an efficient avenue for extended drug release. Yohe et al. [18] prepared a bulk superhydrophobic material by electrospinning the appropriately selected polymers. The presence of stearic moiety was responsible for the low surface energy of the material, and polymeric fibers provided appropriate topography for adopting metastable trapped air—that eventually provided extreme heterogeneous wettability. The trapped air layer acted as a temporary barrier for liquid water and controlled the sluggish penetration of aqueous phase into the bulk of the material as shown in Fig. 25. Eventually, this phenomenon delays the diffusion of cargo molecules from the bulk material which acted as a reservoir of those cargo molecules. They demonstrated the long-term release of preloaded anticancer agent SN-38 in Phosphate buffer solution, and the released drug remained highly bioactive. Thus, Yohe et al. [19] showed the advantages of 3D superhydrophobic (SH) materials over 2D SHS. In case of two-dimensional SH surfaces, the trapped air layer is very thin and can be displaced completely by water after a while. On the other hand, the 3D SH materials contain air layer throughout the material and so the drug release is prolonged significantly. However, the preloading of bioactive small molecules during electrospinning process can significantly restrict this biomimicked approach for widespread biomedical application as limited drug can be loaded following preloading process from specific solvent mixture of selected hydrophobic and hydrophilic polymers. Later, Manna et al. [93] reported an approach for designing superhydrophobic polymeric

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multilayers by using LBL assembly of PEI and poly(2viny-4,4-dimethylazlactone). The process led to a covalently cross-linked multilayer which can be functionalized with n-decylamine to make it superhydrophobic. Organic solvents were used for post-loading of the water-soluble cargos (TMR and 2-ABI) into the material, and release of small molecules was performed over 1 year in Phosphate buffer solution. Further, Grinstaff et al. [18] extended this water wettability approach in tunable drug release process through controlled displacement of metastable trapped air from the synthesized material. 3D superhydrophobic PCL electrospun meshes containing poly-(glycerol monostearate-co-e-caprolactone) as a hydrophobic polymer dopant were fabricated. The water wettability was controlled by changing the amount of this hydrophobic polymer dopant in the polymeric mesh. In 2017, Adil and his coworkers [21] introduced a facile approach for the controlled tailoring of water wettability on abundant fibrous cotton by exploiting a ‘reactive’ nanocomplex of a branched polymeric amine. The chemically reactive coated fibrous cotton was appropriately post-modified with various functional groups for adopting a range of water wettability— including adhesive and non-adhesive superhydrophobicity. Such principle allowed us for controlled and sustained release of post-loaded small molecules (model drug rhodamine 6G) for variable duration from few hours to more than 100 days in Phosphate buffer solution at 37 °C as shown in Fig. 26, and such material can be further used for wound management, preventing bacterial infections, etc. Wang et al. [147] introduced a tension-induced drug release system composed of superhydrophobic polymer composites of hydrophobic PCL and poly(glycerol monostearate carbonate-co-caprolactone) coated on a hydrophilic cellulose/polyester core through electrospraying method. The coating consisted of a hydrophilic native mesh core homogeneously loaded with hydrophilic dye molecules and hydrophobic polymer composites, and the microparticle-coated mesh was finally exhibiting adhesive superhydrophobicity with an advancing contact angle of 170°. The stain-dependent controlled release of entrapped anticancer agent cisplatin and 7-ethyl-10-hydroxycamptothecin to esophageal cancer cells (OE33) in vitro was observed by gradual water infiltration in the multilayered device through crack propagation mechanism depending on the magnitude of the applied strain. Recently, Rather et al. [148] extended further this biomimicked approach for developing dual and sustained drug delivery system, where two distinct bioactive drugs (i.e., tetracycline and doxorubicin) are post-loaded in a single superhydrophobic material and both the drug molecules were released in the Phosphate buffer solution over 180 days as shown in Fig. 27.

4.4

Anti-biofouling Coatings

Biofilm formation due to bacterial adhesion has an adverse effect in several conditions like pipeline corrosion, water contamination, hospital-associated infections, etc., which leads to severe damage in human life [149–152]. Myriad of researches are performed in designing anti-biofouling materials to prevent the bacterial biofilm formation [153, 154]. In this regard, SHS is very well known to exhibit

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Fig. 25 Proposed mechanism of drug release from a 3D superhydrophobic material and its comparison with 2D superhydrophobic material. Reproduced with permission from Ref. [18]. Copyright 2012 American Chemical Society

Fig. 26 Controlled release mechanism of small molecules of interest (red dots) by exploiting polymer-based superhydrophobic naturally abundant fibrous cotton in Phosphate buffer solution at 37 °C due to gradual infusion of liquid water into the interior of the material. Reproduced with permission from Ref. [21]. Copyright 2017 The Royal Society of Chemistry

anti-biofouling property. However, the reason behind the anti-biofouling behavior of SHS is mainly due to the entrapped air layer in between the hierarchical features which are demonstrated by Hwang et al. as shown in Fig. 28 [155]. An SHS was prepared by ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticle paint by following the fabrication process demonstrated by Lu et al. [156]. Upon prolonged immersion of the SHS under water, the gradual infiltration of water in replacement of the entrapped air layer was taken place and after complete replacement of air by water phase led to significant bacterial adhesion within 24 hours. Even, due to the coexistence of both micro-/nanofeatures, SHS promoted the bacterial adhesion to a large extent when compared to other control materials like glass slide, polyurethane, white polystyrene (PS) sheet, etc. Fadeeva et al. [157] fabricated a lotus leaf-mimicked titanium surface by using femtosecond laser ablation. After laser ablation, hydrophilic titanium (WCA *73°) possessed superhydrophobicity with a WCA of *166°. The material was further explored in investigating the selective retention pattern for two pathogenic bacteria. Highly

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Fig. 27 a, b The bulk superhydrophobic material prepared by gelation of ‘reactive’ nanocomplexes of 5Acl and BPEI. c, d Fluorescence microscopic images of tetracycline (green) and doxorubicin (red) drug-loaded material. f The sustained release profile of the anti-bacterial drug tetracycline and anticancer drug doxorubicin for 180 days. Reproduced with permission from Ref. [148]. Copyright 2019 The Royal Society of Chemistry

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Fig. 28 a Progressive reduction of WCA of the SHS over the time upon bacterial exposure. b Gradual replacement of the entrapped air layer across the hierarchical surface by water upon

selective anti-biofouling behavior of the superhydrophobic titanium surface was observed for gram-negative P. aeruginosa bacteria, while the gram-positive bacteria S. aureus were colonized on the same surface. Further, Privett et al. [22] reported surface with superhydrophobic as well as anti-bacterial properties by using fluorinated silica colloid. The adhesion of highly pathogenic Staphylococcus aureus and Pseudomonas aeruginosa was greatly reduced because of the combined effect of hierarchical features from silica colloids and a low surface energy fluorinated silane chemistry. The implication of this demonstration is that such surfaces possess potential in developing advanced medical devices.

4.5

Drag Reduction

Reducing drag is an important factor in microfluidic devices, ships, aircrafts, and submarines. The superhydrophobicity can be adopted as an efficient approach to reduce drag [158–162]. The trapped air layer at the solid–liquid interface acts as a lubricating layer and thereby drastically reduces the interaction of the surface with surrounding liquid phase. SHS is more advantageous than the existing bubble injection method because it is easy to implement as well as energy efficient. Leiden frost effect can also be demonstrated to achieve hydrodynamic drag reduction. But, the drawback of this method is it requires heating and therefore its energy efficiency is poor. In the year of 1999, Watanabe et al. [23] were first to discover laminar drag reduction for Newtonian fluids when water was allowed to pass through a pipe with a highly water-repellent coating on the inside. The coating was made up of fluorine-alkane-modified acrylic resin with hydrophobic silica to provide the required topography. Theoretical calculations implied that drag reduction can be

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Fig. 29 a–d Restoration behavior of superhydrophobicity for surfaces with varying pitch and height. a has smooth bottom surface and b–d are having microposts with nanostructured surface, e illustrating the successful gas film formation due to high stability of air in the nanostructures, f SEM images of a test surface where gold microstructures were deposited on nanostructured silicon surface. Reproduced with permission from Ref. [25]. Copyright 2011 APS Physics

achieved by a superhydrophobic coating not only in laminar flows but also in a turbulent flow with Reynolds numbers of practical interest. Even though SHSs have shown promising results in reducing drag, the air gradually gets displaced under liquid pressure and the surfaces become wet. Some strategies are also developed to maintain a stable air layer. Dong et al. [24] demonstrated the drag-reducing effect on a macroscopic ship by coating the curved surface of the ship with superhydrophobic coating. The SHS was able to reduce drag up to 49.1% as compared to a normal surface of the same wetting area. Despite showing promising results, the superhydrophobic coating was not suitable for practical applications due to poor durability under mechanical stress. Lee et al. [25] prepared a superhydrophobic coating with micro- and nanostructures. The microstructures become wetted before nanostructures because of the high stability of water in the nanogrooves. They used a line pattern of thin-film gold on the bottom surface as the cathode and a copper wire in water to act as an anode. When water penetrated and wetted the microstructures, the cathode came in contact with water and therefore electrolytic cycle closes, resulting in gas generation as illustrated in Fig. 29. So, this way the dewetting of the wetted surface can be achieved, and the surface can retain a stable gas layer even under high liquid pressure. Recently, Du et al. [26] combined the air injection and hydrophobicity adjustment method to maintain a layer of air on SHS. The air layer gave the desired slip effect and drag-reduction effect without being destroyed by shear flow. In 2014, Zhang et al. [27] demonstrated that an underwater drag-reducing effect can be seen when a submarine model was coated with superhydrophobic coating. The coating was fabricated by immobilizing hydrophobic copper particles onto PDMS surface.

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Water Harvesting

Water is the foundation of life and is an irreplaceable factor for sustainable development, but natural and man-made causes are leading to water scarcity all over the world. About 780 million people do not have access to clean and safe water worldwide according to the report of WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation in 2012. Different species like cactus Opuntia microdasys, cribellate spider, Australian desert lizards, Stenocara beetle, and Cotula fallax plant evolved with effective abilities to collect water from their environment. Such natural surfaces can be a source of inspiration for developing water-collecting materials. Four and half billion years of evolution has helped nature to achieve scientific excellence and bestow different living beings with astonishing attributes which help them to survive in hostile environments and thrive. Namib Desert of Africa is well known for its extreme low rainfall (less than 1 cm annually) and almost barren nature which makes life difficult for all living beings. But, to our surprise, some beetles are found there with the ability to collect water from fog-laden wind. The superhydrophobic–superhydrophilic pattern on its back is responsible for this astonishing phenomenon. Water condenses on the superhydrophilic bumps and the water slowly reaches a threshold, at which it slides to the hydrophobic part and then rolls to the beetle’s mouth [163]. Inspired by the water harvesting attribute of the Namib Desert beetle, Zhai et al. [91] developed hydrophilic patterns on SHS through strategic use of multilayers of polyelectrolytes. Later, Garrod et al. [28] prepared plasmachemical patterned superhydrophobic–superhydrophilic surfaces and have mimicked the Stenocara beetle’s back. Such synthesized patterned interfaces were utilized for demonstration of

Fig. 30 a Schematic of the hydrophilic plasma polymeric micropattern onto the SHS prepared by either of tetrafluoromethane (CF4) plasma fluorination of polybutadiene film, or oxygen plasma treatment on PTFE polymer film. b The patterned surface was exploited in water harvesting through microcondensation of aqueous phase. Reproduced with permission from Ref. [28]. Copyright 2007 American Chemical Society

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water harvesting from water mist as shown in Fig. 30. The role of dimension and arrangement of the hydrophilic region on the water harvesting efficiency was also examined in detail. In another report, some researchers prepared polymer bilayer films consisting of a bottom layer of PS and a top layer of poly (4-vinyl pyridine). During annealing at 160 °C which is higher than the glass transition temperature (Tg) of both the polymers, the top layer underwent morphological transformation due to dewetting. This micropatterned surface was studied for its water capture ability. Further, Sarkar et al. [29] developed a grassland-like morphology by electrospray ionization-induced creation and deposition of Ag nanoparticles. The nanowires were modified with ESD to a hydrophobic–hydrophilic pattern.

4.7

Self-cleaning

Self-cleaning coating on various substrates has a huge market and that explains why plethora of research has been carried out to develop such coatings. [11, 30, 164–167]. For example, dust deposition on solar cell reduces its efficiency. TiO2 was widely used in superhydrophilicity-induced self-cleaning surfaces due to its photo-induced superhydrophilicity. The self-cleaning ability of lotus leaf is well known [1, 7, 11]. Water droplets adopt a spherical shape on the leaf and slide off

Fig. 31 Self-cleaning property (f, g) of superhydrophobic material (a) by exploiting ‘reactive’ bulk polymeric coating which is designed by granular features (d, e) and low surface energy coating (a). Reproduced with permission from Ref. [30]. Copyright 2018 Royal Society of Chemistry

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easily carrying all the dust particles along with it. This phenomenon incited the thought that the superhydrophobicity can also be used to achieve such cleaning. To decorate any surface with this effect, the surface must display a very high CA and low CAH like a lotus leaf. In this context, in 2018, Maji et al. [30] reported a substrate-independent bulk superhydrophobic spray coating by ‘reactive’ nanocomplex of BPEI and that coating can be further exploited to prevent the contamination by dust particles (Fig. 31f-h). Similarly, another example of self-cleaning coating was introduced by Park et al. [31]. They fabricated a perfectly ordered microshell array on a flexible and transparent PDMS surface. The microshell PDMS showed CA of above 150° and CAH below 20° without any low surface energy coating owing to the intrinsic hydrophobicity of PDMS. Its cleaning performance was better than that of a flat PDMS surface. The reduction in solar cell efficiency can be prevented by such coating. Rather et al. [133] have developed some superhydrophobic bulk polymeric materials and demonstrated their self-cleaning ability by cleaning soiled surfaces with water without any trace of dust/sand particles. Another self-cleaning surface was introduced by Zhang et al. [168] where a fluorine-free approach was adopted for tailoring SHS. The fabrication procedure of the material was mainly focused on spray coating consisted of dodecyltrimethoxysilane (DTMS)-modified dispersion of epoxy resin and silica nanoparticles on glass or stainless steel mesh. Self-cleaning of the material was observed both in air and under oil. In 2014, Xu et al. [169] have fabricated superhydrophobic poly(ethylene terephthalate) (PET) textile by treating the microstructures of the fabric with alkali followed by PDMS modification. The PDMS coating rendered the essential low surface energy to the textile which was the main reason for showing the superhydrophobicity and self-cleaning property.

4.8

Anti-corrosive Performance

Corrosion of metals is a serious problem globally, and the annual cost of corrosion is equivalent to almost 3% of world GDP. Besides financial loss, plenty of unwanted situations can also arise. Corroded structures may lead to catastrophes like bridge collapse, failed pipelines causing huge loss of life and resources. Various metal ions (chromium, lead, etc.) are used to provide corrosion resistance, but their toxicity demands some innocuous solution for this problem. A strategic development of SHS by electrodepositing Mg–Mn−Ce magnesium plate in ethanol solution containing cerium nitrate hexahydrate and myristic acid for 1 min was demonstrated by Liu et al. as shown in Fig. 32 [170]. The as-prepared material was exhibiting the WCA of 159.8° and was providing a good corrosion resistance of magnesium alloy in 3.5 wt% aqueous solutions of NaCl, Na2SO4, NaClO3, and NaNO3. Another report was published by Xu et al. [171] where they demonstrated the high chemical durability; anti-corrosion activity in particular, of a superhydrophobic aluminum surface prepared through sol–gel method by

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Fig. 32 a Schematic representation of the fabrication of corrosion resistance magnesium alloy embedded with superhydrophobic property through electrodeposition process. b The model that described the role of superhydrophobic property in anti-corrosion activity. Reproduced with permission from Ref. [170]. Copyright 2015 American Chemical Society

using silica particle and PS spheres included as a template. Monomolecular perfluoroalkylsilane (FAS) was deposited on the coated dry material through CVD for essential low surface energy in the synthesized coating. Recently, Li et al. [172] prepared a 3D superhydrophobic miniature box (with CA = 154 ± 1° and SA = 4 ±1°) by using a PS-coated mesh fabricated by electrospinning. The box was used for gravity-driven filtration of various oil–water mixtures. Corrosion current density and corrosion potential which are indicators of the propensity of any surface to undergo corrosion were measured on untreated and superhydrophobic aluminum surface in 3.5 wt% solution. The results showed that superhydrophobic one is superior in terms of corrosion performance.

5 Conclusion This chapter summarizes the basis of SHS followed by fabrication techniques and real-time applications of such surfaces. The fabrication procedures predominantly focus on developing both the optimum chemistry and topography which are the prerequisites to achieve superhydrophobicity. A wide variety of top-down and bottom-up approaches that have been adopted for developing the essential micro-/ nanofeatures are described in this chapter. Further, these hierarchically featured interfaces are strategically decorated with low surface energy molecules for adopting artificial biomimicked superhydrophobicity. Initially, the ‘surface’ superhydrophobicity was demonstrated by investigating the naturally existing superhydrophobic lotus leaf surface. However, the surface with thin superhydrophobic coatings is mostly vulnerable to friction-induced stress and harsh chemical exposures. To combat such durability challenges, few important designs are introduced in the literature. For example, the self-healing ability of the superhydrophobic material by both gradual migration of the low surface energy molecules from nano-reservoirs and regeneration of topography was quite potentially convincing for practical applications and is not yet appropriate for long-term use

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owing to the inherent limitations of both the cases. With the growing interest in this field, immense research is eventuated on superhydrophobic property, and meanwhile, bulk superhydrophobic materials were introduced which consists of both hierarchical topography and chemistry on the surface as well as in the interior of the material providing both the chemical and physical durabilities to the material. Later on, the research was more significantly progressed in developing mechanically durable superhydrophobic material with impeccable permanence to both compressive and tensile strain. The property has been recognized for various potential applications in several fields like oil–water separation, controlled drug release, anti-biofouling, drag reduction, water harvesting, self-cleaning, anti-corrosion, etc. Myriads of prospective applications in real-life scenarios are catalyzing the evolution of materials with this special bio-inspired anti-wetting property to emerge as highly durable materials which are environment-friendly, industrially viable, and can retain their anti-wettability in various harsh conditions.

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Chapter 19

Uses of Ceramic Membrane-Based Technology for the Clarification of Mosambi, Pineapple and Orange Juice Murchana Changmai, Sriharsha Emani, Ramgopal Uppaluri and Mihir Kumar Purkait

Abstract This chapter discusses the preparation and characterization of low-cost ceramic membranes for microfiltration (MF) application. The objective of this chapter is to elaborate the development of inorganic membranes which are low in cost and have a definite pore size and porosity to clarify mosambi, pineapple and orange juice and thereby provide a good juice quality. The clarified juice will thus have very less pectin content, higher clarity and good citric acid content and lesser reduction in oBrix. Hence, this microfiltration operation will be responsible for retaining sugars and flavoured components along with the elimination of substance that contributes towards the deterioration of fruit juice quality such as different colloidal substances. The subsequent sections of this chapter intricate the various experimental studies that are carried out to determine the membrane morphology and the subsequent microfiltration of the different fruit juices. Keywords Fruit juice


Microfiltration
Membrane
Characterization
Quality

1 Introduction Membrane technology has become one of the most amicable separation technologies over the past few decades and is one of the most competitive replacements for conventional techniques. In simple words, a membrane is a barrier between two homogenous phases that prevent an intimate contact but allows a favoured transit of certain selected species across its structure. The main reason that membrane technology has attained so much attraction in recent years is because of the fact that it can operate without the addition of extra chemicals, with low energy usage, low M. Changmai
S. Emani
R. Uppaluri
M. K. Purkait (&) Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 V. Katiyar et al. (eds.), Advances in Sustainable Polymers, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-32-9804-0_19

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cost, maintenance and ease of usage. The membrane has the ability to aid in the transfer of certain species more easily than others owing to certain chemical or physical properties existing between the membrane used and the permeating species. The simple principle that a membrane functions as a filter should allow the passage of water through it while retaining suspended solids and other substances during the filtration process. In order to ensure the penetration of these specific substances, various methods have been identified. Examples of such methods include the concentration gradients maintained across a membrane, introduction of high pressure, maintaining a temperature gradient across the membrane surfaces and the existence of an electrical potential. Each of these methods corresponds to the respective driving force that aids in the separation process. Thus, the driving force (pressure, concentration, electrical potential and/or temperature) helps in separating a particular component through a membrane. The liquid that is able to pass through the membrane is known as “permeate”, and the liquid that is retained back is known as “retentate”. Membrane separation process is of a wide variety based on the configuration, driving force acted on the separation process, mechanism of transport of various components through the membrane and many other such specific features. The process of membrane separation mainly involves the separation of specific components from a liquid or a gaseous stream under the application of a pressure. Species smaller than the membrane pore size pass through the membrane, while the larger species are retained. Hence, large volumes can be treated with remarkable energy efficiency. For a pressure-driven process, the pore size of the membrane plays a major role. Reverse osmosis (RO) is one of the most complex techniques in membrane separation which concentrate low molecular weight organic materials and salts while allowing water and solvents to pass. Hence, it is extensively utilized in the removal of ionic species including the desalination of seawater and reclaiming of brackish well water. Nanofiltration (NF) also referred to as “loose RO” separates liquid in a region between RO and ultrafiltration (UF) displaying an excellent rejection of divalent ions. Nanofiltration applications generally include dye removal, desalting and demineralization. UF is basically the application of low pressure for the separation of selective components across a membrane. UF is mainly utilized for the purpose of clarification, concentration and diafiltration. The UF membrane is more porous compared to RO process. Hence, colloids, suspended solids and organic molecules like bovine serum albumin (BSA) are retained back by the UF membrane. The porous nature of the UF membranes aids in the operation of the membranes at high fluxes and low pressures. Microfiltration (MF) mainly separates materials in the range of 0.05– 2.0 lm. Common materials separated by MF are fat, bacteria, starch, moulds, etc., thereby providing new process opportunities in various industrial applications [1–4]. Mosambi, pineapple and orange juice are rich in carbohydrates (sugar, dietary fibre), fat, protein, vitamins, etc. Consumption of these fruits may be in various forms including salads, processed food, pickles, juice, etc. Since they are acidic in nature, preserving them in home by canning has become extremely easy [5–15]. However, of all the varieties of food products available, the juice of mosambi,

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pineapple and orange juice is the most popular product. Juice is usually obtained by peeling of the skin, crushing, and removing the seeds and passing through a sieve. Due to the presence of constituents such as protein, hemicellulose and cellulose, the juice tends to become highly dense and viscous, thereby creating hindrance in the clarification processes [16–29]. Clarification is one of the most essential steps for the recovery of natural colour substance and concentration by membrane technology. As these suspended solids are removed from the juice, the storage period of the clarified juice increases [30–35]. Few methods used in the pretreatment of fruit juice include treatment by pectinase enzyme, centrifugation of fruit juice in the presence of a fining agent, enzymes or both. However, these processes are costly and the main aim is to develop a process which would give us products equivalent to these processes but at much lower costs. Membrane processes, including MF and UF, are active substitutes for the existing methods keeping in mind their low capital cost and energy cost; these processes are non-thermal separation methods without a change in the phase and negligible cost of proteins, vitamins, sugars and salts [36–40]. MF and UF are preferred for the clarification of juice like pomegranate [41], apple [42, 43], melon [44], carrot [39], tomato [45, 46], pear [47, 48], pineapple [49], umbu [50], blood orange [51] and black currant [52]. MF membranes are characterized by pore sizes ranging from 0.1 to 10 µm and require very low operating pressure (1–3 bar) to carry out separation of suspended solids. The juice obtained from fruits usually has suspended solids and other removable contents in the micrometre range (30–80 µm). Hence, the utilization of microporous ceramic membranes has been utilized in the clarification of mosambi, pineapple and orange fruit juices for the removal of such suspended solids at very low applicable pressure.

2 Fabrication of Ceramic Membrane Polyvinyl alcohol (PVA) was used along with six different ceramic precursors; namely, kaolin, quartz, calcium carbonate, sodium carbonate, boric acid and sodium metasilicate can be used to fabricate the low-cost porous ceramic disc-shaped membranes. The present chapter discusses the preparation, characterization and application of membranes using the composition on a dry basis (41% kaolin, 15% quartz, 25% calcium carbonate, 10% sodium carbonate, 4.5% boric acid and 4.55% sodium metasilicate). The fabrication of the ceramic membrane begins with the preparation of a membrane mould using a hydraulic press (make: Velan Engineering, Tamil Nadu, India) at three different fabrication pressures, namely 29, 39 and 49 MPa for M1, M2 and M3, respectively. The prepared membranes (56.3 mm diameter and 5.1 mm thickness) are dried at 150 °C for 24 h to remove the moisture that has been adsorbed followed by sintering at 900 °C for 4 h at the rate of 2 °C/min in a muffle furnace. The sintered membranes were then allowed to cool in the furnace to 25 °C after shutting down the power. The final steps of the

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fabrication process were to polishing the membrane using C – 220 SiC abrasive paper and then cleaned in an ultrasonic cleaning bath (make: Elma, India, Model: T460) for 15 min.

3 Preparation of Mosambi, Pineapple and Orange Juice Fresh mosambi juice (FJm) is obtained by depulping mosambi fruit (sweet orange, Citrus sinensis (L) Osbeck) with manually operated extractor (screw type). The centrifuged juice (CJ) is obtained using a centrifuge (make: SORVALL, RC 5C PLUS), and enzyme-treated centrifuged juice (ETCJ) is obtained by enzymatic pretreatment of the juice and centrifugation at 4000 rpm for 20 min. The pretreatment includes the heating of mosambi juice with pectinase enzyme at 0.0004 w/v % at 42 °C for 100 min. This was followed by heating at 90 °C for 5 min for deactivating the leftover enzyme and then finally cooling the juice to ambient temperature (25 °C) for centrifugation [34]. Similar process is followed for obtaining fresh pineapple juice (FJp), centrifuged pineapple juice (CJp), enzyme-treated centrifuged pineapple juice (ETCJp), fresh orange juice (FJo), centrifuged orange juice (CJo) and enzyme-treated centrifuged orange juice (ETCJo). For the membranes M1, M2 and M3, MF is carried out using CF and enzyme-treated juice of mosambi, pineapple and orange at transmembrane pressure difference (DP) of 68.9, 137.8 and 206.7 kPa. The permeation was carried out through a membrane with a membrane area of 1.45  10–3 m2.

4 MF of Mosambi, Pineapple and Orange Juice Figure 1 illustrates the experimental set-up utilized for the MF of centrifuged and enzyme-treated centrifuged mosambi, pineapple and orange juice samples. The variation in transmembrane pressure differentials is brought forward by using an adjustable valve located after the rotameter, and the average pressure is evaluated using inlet and outlet pressure values for the flux decline analysis. The temperature of the juice in the feed tank can be controlled at 25 ± 1 °C during an MF run using the microprocessor-controlled chillier unit. An auxiliary electronic weighing balance is used to measure the membrane flux as a function of the weight of the juice samples collected at various time periods during a typical cross-flow MF run. During MF runs, different circulation rates (1 and 4 LPM) are achieved using variable frequency controller and needle valves located in the set-up. The MF experimental investigations are carried out for both M1 and M3 compacted membranes using CJ and ETCJ at various combinations of DP (69 and 207 kPa) and circulation rates (1 and 4 LPM). The feed and permeate samples after each MF run have been analysed using various analytical techniques summarized below.

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Fig. 1 Schematic of experimental set-up for the cross-flow fruit juice MF using low-cost ceramic membranes

5 Measurement of Juice Quality The analytical methods are applied to estimate various physicochemical parameters associated with CJ and ETCJ of mosambi, pineapple and orange juice feed and permeate samples. Determination of particle size distribution was done using a laser particle size analyser (LPSA, make: Malvern Mastersizer 2000, UK); total soluble solids (TSS) using Digital Refractometer (make: Atago; Model: DR-A1, India); the absorbance and transmittance using UV–Vis spectrophotometer (make: Perkin Elmer Precisel; Model: Lambda 35, USA); pH with a soil analysis kit (make: VSI Electronics, Model: VSI-06D1, India); viscosity by a rheometer (make: Thermo Electron Corporation; Model: HAAKE Rheostress 1) and density using Pycnometer. Acidity and alcohol insoluble solids (AISs) were measured using

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procedures described in the literature. In addition, the vitamin C (ascorbic acid) content of the juice samples is evaluated with volumetric method [53] using 2,6-dichlorophenol indophenol dye in oxalic acid titrating medium. The central objective of juice MF is to reduce AIS and retain Brix and vitamin C content in the permeate samples. Thus, analytical methods deploy will be relevant to evaluate the efficacy of low-cost ceramic membranes for pineapple juice MF.

6 Ceramic Membrane Characterization Figure 2 shows the changes in weight loss with an increase in temperature for starting powder for ceramic membrane fabrication. Major weight losses are observed below 800 °C mainly because of moisture loss and decarbonization of carbonate salts, whereas from 900 to 1000 °C the weight loss is insignificant. Hence, the maximum sintering temperature of 900 °C is used for the fabrication of the membranes. Figure 3 presents the X-ray diffraction (XRD) patterns for both unsintered raw material mixture and M3 membrane sintered at 900 °C. The kaolin-related peaks found in the unsintered powder disappeared in the membrane samples sintered at 900 °C suggesting the conversion of kaolinite and meta-kaolinite at 900 °C. Figure 4 presents the field emission scanning electron microscopy (FESEM) images for membranes M1, M2 and M3. The images suggest a highly porous structure without any deformation and cracks with a pore size of 2– 4 µm. Figure 5 shows the pure water flux profile for the three membranes. The pure water flux values changed from 4.62 to 18.63  10−4, 5.43 to 21.37  10−4 and 1.31 to 5.19  10−4 m3/m2s with a variation in applied pressures from 68.9, 137.8, 206.7 and 276 kPa for M1, M2 and M3 membranes, respectively. When the fabrication pressure is increased from 29 to 49 MPa, the membrane porosity increased from 35.4 to 39.4 which is due to the closer arrangement of the particles with an Fig. 2 Thermogravimetric analysis of the raw material mixture

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Fig. 3 X-ray diffraction patterns of unsintered and sintered powder mixture (membrane M1–M3) at 900°C

Fig. 4 FESEM images of a M1, b M2 and c M3 membranes

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Fig. 5 Pure water flux vs applied pressure for M1–M3 membranes

Fig. 6 Average pore size and porosity variation with changing fabrication pressure for M1–M3 membranes

increase in the applied pressure during membrane fabrication, which enabled higher surface area at lower pore sizes. The average pore sizes were found to be 1.84, 1.79 and 0.88 µm for membranes M1, M2 and M3, respectively (Fig. 6).

7 Membrane Flux and Fouling Mechanism For all the membranes (M1, M2 and M3), the permeate flux (J, m3/m2s) is evaluated using the expression as given below: J¼

V A  Dt

ð1Þ

where A (m2) is the membrane area involved in filtration, V (m3) is the permeate volume, and Δt (s) is the filtration time.

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The fouling mechanism existing during the MF of the fruit juices, four models were studied as proposed by Hermia, namely cake filtration (CF), intermediate pore blocking (IPB), standard pore blocking (SPB) and complete pore blocking (CPB) models [54]. During CF, the retained particles with a size greater than the pore size of the membrane resulted in the formation of a layer of deposited particles over the membrane surface called the cake layer which hinders the liquid permeation. During CPB, the particles lesser than the pore diameter enter the pore channel and blocking the pores bringing about a huge reduction in the size of the open pores responsible for the passage of the liquid. During SPB, the permeating particles stick to the membrane pore walls. IPB is a case where the particles may stick to the membrane surface or ultimately block the pores. The linearized flux decline models for CF, CPB, SPB and IPB phenomena are expressed as:

CPB: ln J 1 ¼ ln Jo1 þ kb t

ð2Þ

SPB: J 0:5 ¼ Jo0:5 þ ks t

ð3Þ

IPB: J 1 ¼ Jo1 þ ki t

ð4Þ

CF: J 2 ¼ Jo2 þ kc t:

ð5Þ

8 Fitness of Fouling Models Various fouling models (CPB, SPB, IPB and CF) have been evaluated for their fitness with the measured cross-flow MF data for both mosambi (ETCJm) and orange juice (ETCJo) feed samples. Among all models, CF model provides the best fitness. Corresponding fitness plots for DP effect are presented in Fig. 7a–f and cross-flow velocity effect in Fig. 8a–f. It can be observed that for cases corresponding to Fig. 7a, b and d, the deviations are significant during the early stages of MF operation. This indicates that the early stages of MF for both mosambi and orange juice are difficult to predict pore blocking phenomena. However, it has been also evaluated that the MF data obtained during the initial stages of operation can be modelled separately. Thus, it is apparent that the initial flux decline might be characterized with variant fouling model parameters and the later part of the flux decline could be represented by CF model [30, 31, 53, 55–61]. For all cases, corresponding to the DP effect (at u = 1.92 m/s), CF model can be observed to provide good fitness for the MF data measured between 10 and 30 min of the MF operation. The CF model fitness plots illustrating the effect of cross-flow velocity at DP = 207 kPa are presented in Fig. 8a–f. For all cases, it can be observed that the CF model failed to provide good fitness for the MF data measured during the initial 10-min MF operation. Thus, it is apparent that the initial

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Fig. 7 Fitness plots for CF model to represent MF flux data obtained with ETCJ at u = 1.92 m/s: a M1 membrane, mosambi juice, b M1 membrane, orange juice, c M2 membrane, mosambi juice, d M2 membrane, orange juice, e M3 membrane, mosambi juice, f M3 membrane, orange juice

membrane flux decline cannot be represented with the CF model parameters obtained for the later stages of the MF operation. Once again, it is evident that the pore blocking phenomena during the initial stages of the MF are complex and combination fouling model can better represent the experimentally obtained MF flux decline data. The regression coefficient value (R2) for the fitness of various pore blocking models to represent the cross-flow MF data of ETCJ mosambi for the M3 membrane is presented in Table 1. It can be observed that for very few cases R2 value is close to 1 and in several cases poor fitness with R2 value is in the range of 0.78–0.91 have been obtained.

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(f)

Fig. 8 Fitness plots for CF model to represent MF flux data obtained with ETCJ for various membranes at ΔP = 207 kPa: a M1 membrane, mosambi juice, b M1 membrane, orange juice, c M2 membrane, mosambi juice, d M2 membrane, orange juice, e M3 membrane, mosambi juice, f M3 membrane, orange juice

This is due to the inability of the CF model to represent the initial flux decline (1–10 min flux data). Similar R2 values have been obtained for the fitness of pore blocking model to represent cross-flow MF data of orange juice. These are summarized in Table 2. The root mean square (RMS) error for the fitness of CF model to represent cross-flow MF data obtained for M3 membrane and mosambi juice varied from 1.19 to 14.47%. Significantly, higher RMS values are due to the inability of the CF model to represent the initial flux decline data (1–10 min).

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Table 1 Regression coefficient (R2) values for the fitness of various pore blocking models to represent cross-flow MF of ETCJm (mosambi juice) using M3 membrane. [CPB: complete pore blocking; SPB: standard pore blocking; IPB: intermediate pore blocking; CF: cake filtration] ΔP (kPa)

Cross-flow velocity (m/s) R2

Complete pore blocking

Standard pore blocking

69 1.92 0.919 138 1.92 0.912 207 1.92 0.846 207 3.84 0.861 207 5.76 0.784 0.832 MF microfiltration; ETCJm enzyme-treated centrifuged mosambi

Intermediate pore blocking 0.987 0.979 0.927 0.939 0.873 juice

Cake filtration 0.996 0.999 0.976 0.983 0.987

Table 2 Regression coefficient (R2) values for the fitness of pore blocking models to represent MF flux data obtained for ETCJo (orange) juice and M1–M3 membranes Membrane name

ΔP (kPa)

Cross-flow velocity (m/s)

Complete pore blocking R2

Standard pore blocking

Intermediate pore blocking

Cake filtration

M1

69 207 69 207 69 207 69 207 69 207 69 207

1.92 1.92 5.76 5.76 1.92 1.92 5.76 5.76 1.92 1.92 5.76 5.76

0.784 0.733 0.810 0.807 0.883 0.832 0.892 0.867 0.921 0.869 0.900 0.842

0.822 0.781 0.842 0.843 0.922 0.867 0.923 0.898 0.955 0.909 0.932 0.880

0.856 0.825 0.870 0.875 0.952 0.897 0.948 0.925 0.979 0.941 0.958 0.913

0.912 0.895 0.915 0.926 0.988 0.944 0.983 0.964 0.998 0.982 0.989 0.961

M2

M3

Similarly, high RMS error values ranging from 5.5 to 7.5% have been obtained for the fitness of CF model for the membrane orange juice cross-flow MF data of M3 membrane. For the M3 membrane, Table 3a, b, respectively, presents the CF model parameters for ETCJ mosambi and orange juice. It can be observed that for the mosambi case, the CF constant reduced (10−3  107 s m−2) and Jo2 increased (0.2 −2  108) with an increase DP from 69 to 207 kPa at u = 1.92 m/s. This is due to the enhancement in flux with DP. Also, it can be analysed that, at a DP of 207 kPa, kc reduced (3−1  107 s m−2) and Jo2 reduced (2−1  108) with an increase in u from 1.92 to 5.76 m/s. This is due to an increase in flux with increasing cross-flow velocity.

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Table 3 M3 membrane CF model parameters for the representation of flux obtained during cross-flow MF of (a) mosambi (ETCJm) and (b) orange juice (ETCJo) feed samples (a) ΔP (kPa)

Cross-flow velocity (m/s)

kc  10−7 s m−2

Jo2  10−8

69 138 207 207 207 (b) ΔP (kPa)

1.92 1.92 1.92 3.84 5.76

10.00 5.00 3.00 2.00 1.00

0.20 1.00 2.00 1.00 1.00

Cross-flow velocity (m/s)

kc  10−7 s m−2

Jo2  10−8

69 207 69 207

1.92 1.92 5.76 5.76

5.00 3.00 2.00 1.00

1.00 1.00 1.00 1.00

Similarly, for orange juice ETCJ feed samples, at u = 1.92 m/s, the CF constant reduced (5−3  107 s m−2) and Jo2 remained constant (1  108) for a variation in DP from 69 to 207 kPa. This is due to the enhancement in flux with DP. Also, it can be analysed that, at a DP of 207 kPa, for ETCJ orange juice feed samples, the CF constant reduced (3−1  107 s. m−2) and Jo2 increased (1−2  108) for an increase in u from 1.92 to 5.76 m/s. Similar variations in these parameters can be observed for variations in membrane morphology in Table 4 at a DP of 207 kPa and u of 1.92 m/s. It can be observed that for mosambi ETCJ feed juice, kc increased (0.8−3  107 s m−2) along with an increase in Jo2 (1−2  108) for a reduction in average membrane pore size from 1.69 (M1) to 0.72 m (M3). However, for orange juice ETCJ feed samples, the corresponding variation in these

Table 4 CF model parameters for M1–M3 membranes at DP = 207 kPa and u = 1.92 m/s for (a) mosambi (ETCJm) and (b) orange juice (ETCJo) feed samples

(a) Membrane name

Cake filtration R2 Jo2  10−8

kc  10−7 s m−2

M1 M2 M3 (b) Membrane name

0.941 0.913 0.976

0.80 1.00 3.00

Cake filtration R2 Jo2  10−8

kc  10−7 s m−2

M1 M2 M3

0.895 0.944 0.961

0.60 1.00 3.00

1.00 2.00 2.00

0.90 0.90 1.00

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parameters refers to an increase in kc (0.8−3  107 s m−2) along with an increase in Jo2 (0.6−3  108). Figure 9a–f presents the parity plots for the fitness of CF model to represent the entire range of cross-flow MF flux data. Except for Fig. 9e, f, a significant deviation in parity exists for all other cases (Fig. 9a–d). The significant deviation in the parity for these cases is due to the inability of the CF model to predict the initial flux decline (1–10 min). It is opined that combination pore blocking models to represent initial (1–10 min) and later (11–30 min) flux decline data would give better parity.

Fig. 9 Parity plots for the fitness of CF model to represent cross-flow MF data obtained for ETCJ feed samples: a M1 membrane, mosambi juice, b M1 membrane, orange juice, c M2 membrane, mosambi juice, d M2 membrane, orange juice, e M3 membrane, mosambi juice, f M3 membrane, orange juice

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The parity fitness of CF model for membranes M1, M2 and M3 and ETCJ (mosambi) is about 6.46, 7.07 and 4.54%, respectively, in terms of RMS error. Corresponding parity fitness for orange juice cross-flow data is about 7.57, 3.84 and 3.72% in terms of the RMS error. The overall RMS error for the entire data obtained for mosambi and orange juice MF is 6.12 and 4.77%, respectively. For the cross-flow MF case, similar experimental trends are obtained for various cases of cross-flow velocities and DP. Figure 10a–d presents the fitness of CF model at a cross-flow velocity of 1.92 m/s for both M1 and M3 membranes. The corresponding coefficient of correlation (R2), slope and intercept parameters obtained from the fitness plots for various fouling models during the cross-flow MF of CJ and ETCJ for both M1 and M3 membranes is presented in Table 5. As shown in Fig. 10a–d, minor variations from the CF model exist either at the start or during the intermediate time periods of the MF runs. A critical comparison between the dead-end and cross-flow MF CF constants indicates that for CJ case, kc varied from 0.2−0.4  107 s.m−2 for dead-end MF and 0.1−0.2  107 s m−2 for cross-flow MF 8 with the M3 membrane. Corresponding J2 0 varied from 2−6  10 for dead-end 8 MF and 5−7  10 for cross-flow MF. Similarly, for ETCJ case, kc varied from 1 −5  107 s m−2 for dead-end MF and 0.9−3  107 s m−2 for cross-flow MF for the M3 membrane.

Fig. 10 Fitness of CF model to represent pineapple juice cross-flow MF flux data at a cross-flow velocity of 1.92 m/s: a M1 membrane and CJ; b M1 membrane and ETCJ; c M3 membrane and CJ; d M3 membrane and ETCJ

ETCJ

M1

CJ

Juice type CJ

Membrane name

207

69

207

69

207

Pressure (kPa) 69

0.92 0.94 0.97

SPB IPB CF

0.99

CF 0.90

0.97 0.98

SPB IPB CPB

0.97

0.87

CF CPB

0.84 0.85

SPB IPB

0.96

CF 0.82

0.95 0.96

SPB IPB CPB

0.95

0.99

CF CPB

0.96 0.97

SPB IPB

0.98

CF 0.94

0.94 0.96

SPB IPB CPB

Regression coefficient (R2) 0.92

Model CPB −0.5

−0.5

s

−0.5

−0.5

−0.5

−0.5

s

−0.5

s

−0.5

kc = 0.10  107 s m−2

ks = 2.889 m ki = 1162 m−1

−0.5

kb = 0.028 s−1

kc = 0.20 107 s m−2

ks = 3.402 m ki = 1591 m−1

−0.5

kb = 0.029 s−1

kc = 0.50  107 s m−2

s ks = 1.104 m ki = 233.7 m−1

−0.5

kb = 0.020 s−1

kc = 1.00  107 s m−2

s ks = 1.518 m ki = 400.0 m−1

kb = 0.023 s

−1

kc = 4.00  107 s m−2

ks = 1.896 m ki = 640 m−1

kb = 0.022 s

−1

kc = 7.00  107 s.m−2

s ks = 2.568 m ki = 953.9 m−1

−0.5

Slope kb = 0.027 s−1

(continued)

J02 ¼ 5:00  108

J01 ¼ 25502

ln ðJ01 Þ ¼ 10:22 J−0.5 = 163.3

J02 ¼ 7:00  108

J01 ¼ 33055

ln ðJ01 Þ ¼ 10:5 J−0.5 = 186.8

J02 ¼ 0:70  108

ln ðJ01 Þ ¼ 9:07 J−0.5 = 92.62 J01 ¼ 8445

J02 ¼ 1:00  108

J01 ¼ 12066

ln ðJ01 Þ ¼ 9:449 J−0.5 = 111.4

J02 ¼ 2:00  108

ln ðJ01 Þ ¼ 9:874 J−0.5 = 137.2 J01 ¼ 18052

J02 ¼ 3:00  108

ln ðJ01 Þ ¼ 10:06 J−0.5 = 150.5 J01 ¼ 21644

Intercept

Table 5 A summary of fouling model fitness parameters (coefficient of correlation, slope and intercept) to represent cross-flow pineapple juice MF flux data (measured at a cross-flow velocity of 1.92 m/s)

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Membrane name M3

Table 5 (continued)

Juice type ETCJ

207

Pressure (kPa) 69

0.92 0.93 0.95

SPB IPB CF

0.98

CF 0.91

0.96 0.97

SPB IPB CPB

Regression coefficient (R2) 0.95

Model CPB −0.5

−0.5

7

kc = 0.90  10 s m

−2

−0.5

s ks = 0.988 m ki = 256.2 m−1

kb = 0.015 s

−1

kc = 3.00  107 s m−2

s ks = 1.704 m ki = 545.2 m−1

−0.5

Slope kb = 0.021 s−1

J02 ¼ 2:00  108

J01 ¼ 13432

ln ðJ01 Þ ¼ 9:524 J−0.5 = 116.5

J02 ¼ 3:00  108

ln ðJ01 Þ ¼ 9:856 J−0.5 = 136.8 J01 ¼ 18270

Intercept

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Fig. 11 Parity plots for best-fit pore blocking models to represent CJ and ETCJ (pineapple juice) MF flux data for a dead-end and b cross-flow MF 8 8 Corresponding J2 0 is 1  10 for dead-end MF and is 2−3  10 for cross-flow MF. Thus, it is apparent that significant deviations exist between the CF constants obtained for both dead-end and cross-flow MF and an assumption to relate both dead-end and cross-flow MF is not valid. Also, it can be observed that for cross-flow MF, the Jo values for CJ are significantly lower (38−45  106 m3/m2 s) than those obtained for the ETCJ (58 −71  106 m3/m2 s), thus indicating that instantaneous fouling of the M3 membrane is significantly high for the CJ case due to greater quantity of colloidal and pectin material in the feed juice. Also, the CF constant for CJ is significantly lower (0.1−0.2  107 s m−2) than that obtained for the ETCJ (0.9−3  107 s m−2). This also indicates that flux enhancement with time is significant for the case of ETCJ when compared to the CJ. These analytical observations are in good agreement with the experimental trends presented in the earlier subsections. Figure 11a, b presents the parity plots for the predicted and measured experimental flux data in the dead-end and cross-flow MF, respectively. Both CJ and ETCJ cases have been included in each plot. It can be observed that for both cases of dead-end and cross-flow MF, the fitness of CF is good but not excellent. This is possibly due to the variation of dynamic CF layer in due course of time period for the MF runs. Thus, to achieve a more accurate prediction, combination models need to be considered in the modelling approach. The RMS error for the experimental data for dead-end and cross-flow MF is 9.92 and 6.32%, respectively.

9 Physicochemical Properties of Feed and Permeate Samples Table 6a, b presents the physicochemical properties of mosambi and orange ETCJ feed samples, respectively. It has been evaluated that for mosambi ETCJ feed samples, colour, clarity, TSS, citric acid, pH, density, viscosity, AIS and vitamin C

0.378–0.398 0.405–0.415 0.447–0.455

M1 M2 M3

11.8 11.8 11.8

TSS (oBrix)

Clarity (% T660)

Colour (A420)

61.65–61.85 63.45–64.75 68.08–68.38

9.2 9.2 9.1

TSS (oBrix)

66.41–66.86 71.605–71.655 75.620–75.695

Clarity (% T660)

0.551–0.576 0.582–0.608 0.604–0.638

Colour (A420)

M1 M2 M3 (b) Membrane name

(a) Membrane name

0.52 0.51 0.49

Citric acid (wt%)

0.85 0.84 0.83

Citric acid (wt%)

4.05 4.05 4.05

pH

3.89 3.93 3.96

pH

1.05 1.05 1.05

Density (g/cm3)

1.08 1.08 1.07–1.08

Density (g/cm3)

1.48–1.50 1.46–1.47 1.44–1.45

Viscosity (mPa. S)

1.54–1.58 1.52–1.56 1.50–1.54

Viscosity (mPa. S)

0.20–0.21 0.19–0.20 0.17–0.18

AIS (wt%)

0.23–0.24 0.22–0.23 0.21–0.22

AIS (wt%)

Table 6 Physicochemical properties of (a) mosambi and (b) orange juice (ETCJ) feed samples considered for cross-flow MF

41.48–41.85 41.42–41.78 41.32–41.65

Vitamin C (mg/100 ml)

23.31–23.65 23.21–23.59 23.16–23.54

Vitamin C (mg/100 ml)

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CJ (cross-flow MF) ETCJ (cross-flow MF)

CJ (cross-flow MF) ETCJ (cross-flow MF) (b) Juice type

(a) Juice type

85.75–92.12 92.14–97.57

13.4–13.5 13.4–13.5

TSS (oBrix)

Clarity (T660)

Colour (A420)

0.190–0.201 0.035–0.055

13.56 13.57

TSS (oBrix)

72.22–79.25 86.88–91.67

Clarity (T660)

0.74–0.86 0.54–0.65

Colour (A420)

0.77–0.79 0.76–0.78

Citric acid (wt%)

0.79 0.79

Citric acid (wt%)

3.54–3.58 3.55–3.6

pH

3.53–3.57 3.55–3.59

pH

1.02–1.03 1.02–1.03

Density (g/cm3)

1.02–1.03 1.02–1.03

Density (g/cm3)

2.031–2.161 1.891–2.051

Viscosity (mPa.S)

2.14–2.30 2.1–2.15

Viscosity (mPa.S)

0.15–0.17 Nil

AIS (wt%)

0.25–0.28 0.13–0.15

AIS (wt%)

25.27–25.56 22.94–23.53

Vitamin C (mg/100 ml)

25.39–25.62 23.44–23.65

Vitamin C (mg/100 ml)

Table 7 a: Physicochemical properties of CJ and ETCJ (pineapple juice) feed samples. b: Physicochemical properties of CJ and ETCJ permeate samples obtained for M1–M3 membranes during pineapple juice clarification

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are about 0.551–0.638, 66.41–75.695, 9.1–9.2, 0.83–0.85, 3.89–3.96, 1.07–1.08, 1.50–1.58, 0.21–0.24 and 23.16–23.65, respectively. Similarly, for orange ETCJ feed samples, properties such as colour, clarity, TSS, citric acid, pH, density, viscosity, AIS and vitamin C are about 0.378–0.455, 61.65–68.38, 11.8, 0.49–0.52, 4.05, 1.05, 1.44–1.50, 0.17–0.21 and 41.32–41.85. Thus, mosambi ETCJ feed samples have lower TSS, slightly lower pH and lower vitamin C content in comparison with the orange ETCJ feed samples. The objective of the MF operation is to minimize colour, clarity and AIS and achieve minimum variation in TSS, citric acid, pH, density, viscosity and vitamin C. Most important issue of the MF operation is to achieve negligible AIS content in the permeate and maximum citric acid and vitamin C content as these respectively represent pectin and nutrition content of the juice [31, 62–66]. The physicochemical properties of permeate samples for mosambi and orange ETCJ samples are presented in Table 6a, b, respectively. The membranes provided desired combinations of physicochemical properties for all cases. For mosambi and orange juice ETCJ permeate samples, colour reduced to 0.0278–0.0395 and 0.054– 0.138, respectively. Corresponding variation in clarity refers to an increase in clarity to 97.654–99.385 and 91.58–99.27, respectively. The TSS, citric acid, pH, density, viscosity and vitamin C content remained fairly constant with respect to corresponding physicochemical properties of the feed samples. The vitamin C and citric acid content of the permeate samples varied from 22.11 to 22.65 (mosambi), 35.36 to 35.85 (orange), 0.78 to 0.82 (mosambi) and 0.48 to 0.50 (orange) for permeate samples. Also, it can be noted that the AIS content in the permeate samples is negligible for all cases, which indicates negligible pectin content in the permeate samples and their long storage capability. Table 7a, b presents the physicochemical properties (such as colour, clarity, TSS, citric acid, pH, density, viscosity, AIS and vitamin C) of CJ, ETCJ feed samples and permeate samples of pineapple juice. Thus, it is apparent that the MF operation achieved the desired objective to retain nutrition content and sugars in the permeate samples and minimize or omit pectin (AIS) compounds. Further, it is interesting to note that only when ETCJ feed samples are considered, the pectin content in the permeate samples reduced to a negligible value. Thus, the MF of ETCJ feed samples provided high-quality clarified pineapple juice that can be subjected to long-term storage without further deterioration.

10

Summary

This work dealt with the preparation, characterization and applications of inexpensive ceramic membranes. Morphological characterization studies including FESEM, XRD, liquid permeation studies, porosity measurement and finally acid– base corrosion resistance have indicated that the prepared membranes possessed desired properties for MF applications. The prepared ceramic membranes are used for the treatment of three types of juice clarification. The prepared ceramic

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membranes provide higher membrane fluxes, adequate product quality and lower fouling. 1. The cross-flow MF study using ETCJ mosambi, pineapple and orange juice feed samples provided significant insights with respect to the effect of membrane and operating parameters on flux and separation factors. 2. All membranes provided excellent separation factors with respect to the complete removal of AIS (pectin), minimization of colour, and clarity and maximum retention of TSS, citric acid, pH, density, viscosity and vitamin C content. 3. The vitamin C and citric acid content of the permeate samples varied from 22.11 to 22.65 (mosambi), 22.94 to 23.53 (pineapple), 35.36 to 35.85 (orange), 0.78 to 0.82 (mosambi), 0.76 to 0.78 (pineapple) and 0.48 to 0.50 (orange) for permeate samples. Also, it can be noted that the AIS content in the permeate samples is negligible for all cases, which indicates negligible pectin content in the permeate samples and their long storage capability. 4. The membranes provided desired combinations of physicochemical properties for all cases. For mosambi, pineapple and orange juice ETCJ permeate samples, colour reduced to 0.0278–0.0395, 0.0356–0.0557 and 0.054–0.138, respectively. Corresponding variation in clarity refers to an increase in clarity to 97.654–99.385, 92.14–97.57 and 91.58–99.27, respectively. The TSS, citric acid, pH, density, viscosity and vitamin C content remained fairly constant with respect to corresponding physicochemical properties of the feed samples. 5. In summary, the membranes fabricated provided excellent performance characteristics towards the clarification of mosambi, pineapple and orange juices. While the comparative performance with respect to dead-end clarification is not significantly promising, the lower fouling indices obtained and the impact of operating parameters have been extremely useful to obtain good insights with respect to the MF operation. The obtained results are promising for the large-scale MF-based clarification of enzyme-treated centrifuged citrus fruit juices. 6. Finally, it can be concluded that the optimal performance of M3 membrane refers to higher transmembrane flux values and similar separation characteristics. In conclusion, prepared low-cost ceramic membranes provide promising opportunities for fruit juice MF.

References 1. Mulder M (1991) Basic principles of membrane technology. Kluwer Academic Publishers, Dordrecht 2. Cheryan M (1998) Ultrafiltration and microfiltration handbook. Technomic Publishing Co. Inc., Lancaster 3. Nunes SP, Pinemann KV (eds) (2001) Membrane technology in the chemical industry. Wiley-VCH Verlag Gmbh, Weinheim 4. Pearce G (2000) Introduction to membranes: membrane separation processes. CRC Press, Boca Raton, FL

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