Surface Modification of Biopolymers

By Vijay Kumar Thakur

This book addresses surface modification techniques, which are critical for tailoring and broadening the applications of naturally occurring biopolymers. Biopolymers represent a sustainable solution to the need for new materials in the auto, waste removal, biomedical device, building material, defense, and paper industries.

Features:

• First comprehensive summary of biopolymer modification methods to enhance compatibility, flexibility, enhanced physicochemical properties, thermal stability, impact response, and rigidity, among others
• Address of a green, eco-friendly materials that is increasing in use, underscoring the roles of material scientists in the future of new "green" bioolymer material use
• Coverage applications in automotive development, hazardous waste removal, biomedical engineering, pulp and paper industries, development of new building materials, and defense-related technologies
• Facilitation of technology transfer

• Language: English
• Publisher: Wiley
• Release date: May 5, 2015
• ISBN: 9781119044918

Book preview

PREFACE

Polymers have been playing a key role in the economy of the most of the countries of modern world since last century and have become indispensable part of everyday’s life. Polymers have been frequently classified into natural and synthetic polymers. Although natural polymers were frequently used by the people of earlier civilization for a number of applications directly/indirectly, usage of the synthetic polymers has dominated the modern world especially in the past few decades. However, very recently due to the rising environmental awareness and various other issues, compared to the traditional petroleum-based polymers, the usage of materials that can be procured form our nature is rising very rapidly in a number of applications. Indeed, the sustainable development through the use of environmental friendly biobased polymeric materials has become the hottest topic of conversation as well as research all around the globe during the past few years. Biobased polymeric materials obtained from the nature are emerging as a potential viable alternative to traditional synthetic materials. Different kinds of biobased biopolymers represent a renewable feedstock of materials for different usage. Some of the various biopolymer materials that are used in a number of applications are natural cellulosic fibers, starch, agar, chitosan, poly(3-hydroxyalkanoates), and so on. The renewable feedstock of biopolymers extensively depends on the availability of biobased resources in the different regions of the world, the new developments in the use of these materials, and the agricultural production as most of the biopolymers are directly/indirectly related to the field of agriculture. Applications of any biopolymer material in a particular application stresses on the specific physical, chemical, thermal, mechanical, economic, and degradation properties so as to offer significant advantages over their synthetic counterpart. In addition to these requirements, the easy availability of these biopolymeric materials is one of the most significant parameters in their commercialization as it is directly related to the final cost of the material in the market. Different kinds of biopolymers depending on their compositions can be used in a number of applications such as in biomedical (e.g., stent and drug delivery vehicles), in food packaging, as polymer composites for structural applications, as electrolyte for energy storage in super capacitor/battery, as adhesives, in cosmetic industries, and most frequently in textile industries.

The employment of biopolymers provided the opportunity to explore beyond the conventional strategies for numerous applications. The main hurdles in their use include the lack of desired physicochemical/mechanical and biological properties. The only way to utilize them effectively is to alter their properties by surface modification techniques. By utilizing different surface modifications, most of the times the specific application properties of the different biopolymers can be easily obtained. For a particular application, the analysis of the structure–property relationship of a biopolymer under investigation is of utmost importance. At present, a number of biopolymers are being tested for their commercial applications and some of the thrust areas include biomedical, packaging, food production, and automotive. Some biopolymers can directly replace synthetically derived materials in traditional applications, whereas others possess unique properties that could open up a good range of new commercial opportunities. In this book, best efforts have been made to incorporate sufficient information on different surface modification techniques to alter their specific properties for targeted applications. The ultimate objective of this book is to give an extensive overview about the surface modification and applications of biopolymers for multifunctional applications.

This book consists of 15 chapters and gives an overview on different kinds of biopolymers, their surface modification, and successful utilization for different applications. It also summarizes the developments made in the area of surface functionalization of biopolymers. A number of critical issues and suggestions for future work are discussed in a number of chapters, underscoring the roles of researchers for the efficient development of new techniques for surface modification through value addition to enhance their use.

As the editors of Surface Modification of Biopolymers, we have enjoyed working with the individual authors and appreciate their diligence and patience. We do hope that this book will contribute significantly to the basic knowledge of students and researchers all around the globe working in the field of biopolymers.

We would like to thank Anita Lekhwani (Senior Acquisitions Editor) and Cecilia Tsai (Senior Editorial Assistant) along with publisher (John Wiley & Sons, Inc.) for their invaluable help in the organization of the editing process.

Dr. Vijay Kumar Thakur, Ph.D., MRSC

Washington State University, U.S.A.

Dr. Amar Singh Singha, Ph.D.

National Institute of Technology, India

1

SURFACE MODIFICATION OF BIOPOLYMERS: AN OVERVIEW

Manju Kumari Thakur¹, Ashvinder Kumar Rana², Yang Liping³, Amar Singh Singha⁴, and Vijay Kumar Thakur⁵

¹ Division of Chemistry, Govt. Degree College Sarkaghat, Himachal Pradesh University, Shimla, Himachal Pradesh, India

² Department of Chemistry, Sri Sai University, Palampur, Himachal Pradesh, India

³ Polymer Engineering and Catalysis, Institute of Chemical and Engineering Sciences, Singapore, Singapore

⁴ Department of Chemistry, National Institute of Technology, Hamirpur, Himachal Pradesh, India

⁵ School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA

1.1 INTRODUCTION

Among various materials used in our everyday life, polymers play the most imperative role along with their use in a number of industries for versatile applications [1–3]. Polymers have been known to play a key role in the economy of the most of the countries of modern world since last century [4, 5]. Polymers have been frequently classified into natural and synthetic polymers [6, 7]. Although natural polymers were frequently used by the people of earlier civilization for a number of applications directly/indirectly, usage of synthetic polymers has dominated the modern world especially the last few decades [8–11]. Synthetic polymers have even replaced some of the commonly used metallic materials due to their enormous advantages such as light weight, chemical/water resistant, versatility, decent mechanical/thermal properties, and easy tailor ability [12–15]. Both natural and synthetic polymers can be easily distinguished depending upon their inherent properties and their structural property relationship [16]. However, during the last few years, sustainable development through the use of environmentally friendly materials has become the hottest topic of conversation as well as research all around the globe [17, 18]. In this direction, the usage of materials that can be procured from our nature is rising rapidly for a number of applications [19, 20]. In fact the materials obtained from the nature are becoming a potentially viable alternative to a number of traditional synthetic materials that are generally synthesized from petroleum-based resources [21–23]. The effective utilization of the materials obtained from nature offers a number of challenges for their successful usage as well as offers new opportunities from the economic and environmental point of view [24, 25]. The past few decades has seen a pronounced advancement in the development of new eco-friendly materials that are procured from bio-based biopolymers for vast applications [26–28]. Different kinds of bio-based biopolymers represent a renewable feedstock of materials for different usage [29, 30]. The renewable feedstock of biopolymers extensively depends upon the availability of bio-based resources in different regions of the world, the new developments in the use of these materials, and the agricultural production as most of the biopolymers are directly/indirectly related to the field of agriculture [31, 32]. Applications of any biopolymer material in a particular application stresses on the specific physical, chemical, thermal, mechanical, economic, and degradation properties so as to offer significant advantages over their synthetic counterpart [33, 34]. In addition to these requirements, the easy availability of these biopolymeric materials is one of the most significant parameters in their commercialization as it is directly related to the final cost of the material in the market [35, 36]. Different kinds of biopolymer-based materials found in the nature can play one of the key roles in the modern industries to make the final product green [37, 38]. The use of biopolymer-based materials ranges from house hold applications to advanced applications in the defense [39, 40]. Different kinds of biopolymers depending upon their compositions can be used in a number of applications as follows: biomedical (e.g., stent, drug-delivery vehicles), food packaging, polymer composites for structural applications, as electrolyte for energy storage in super capacitor/battery, adhesives, cosmetic industries, and most frequently in textile industries [41, 42].

1.2 STRUCTURES OF SOME COMMERCIALLY IMPORTANT BIOPOLYMERS

Among the various biopolymer materials, a few materials such as natural cellulosic fibers, starch, agar, chitosan, and poly(3-hydroxyalkanoates) (PHAs), are being used in a number of applications [24, 43–49] . In the following section, we briefly describe some of the commercially important biopolymers, as their detailed introduction along with their modification/applications has been given in the upcoming chapters.

1.2.1 Natural Fibers

Among the various fibers available naturally/synthetically, natural cellulosic fibers are of much importance due to their intrinsic properties [48–50]. These fibers have been reported to be used by human beings for thousands of years ago starting from early civilization in the formation of bridges for on-foot passage as well as in naval ships to biomedical in the present time [48–50]. Depending on their extraction as well as on the part of the plant from which they are taken, their properties vary considerably [50]. Figure 1.1 shows the schematic representation of natural fibers [51].

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FIGURE 1.1 Structure of lignocell ulosic natural fiber.

Reproduced with permission from Ref. [51]. © 2013 Elsevier.

Natural cellulosic fibers primarily contain cellulose, hemicellulose, and lignin as their primary constituent and have been well researched as well as documented in the existing literature [48]. Figure 1.2 shows the structure of cellulose found in natural fibers. Cellulose (a nonbranched polysaccharide) is the prime constituent of all lignocellulosic natural fibers and has been found to exist in two crystalline forms, namely, cellulose I and II [48–51]. Cellulose is a linear condensation polysaccharide that comprises a d-anhydro glucopyranose units joined by β-1,4-glycosidic bonds. On the other hand, hemicelluloses are composed of a combination of 5- and 6-ring carbon ring sugars and have been found to remain associated with cellulose even after the removal of lignin [49, 50]. As opposed to the structure of cellulose, hemicelluloses exhibit a branched structure and consist of mixtures of polysaccharides with much lower molecular weight compared to cellulose [48–50].

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FIGURE 1.2 Structure of cellulose.

Reproduced with permission from Refs. [48–50]. © Elsevier.

Among different constituents of natural cellulosic fibers, lignin is one of the highly branched components. It is a complex chemical compound present in huge quantities in the cell walls of plants. It is the main binding agent for components of the plants and serves as a matrix to the embedded cellulose fibers along with hemicellulose. The structure of lignin is highly branched that consists of phenyl propane units. These units are organized in a complex three-dimensional structure linked together through numerous types of carbon–carbon and ether bonds.

1.2.2 Chitosan

Chitosan is another most significant biopolymer that is derived from chitin (produced by many living organisms) [1, 3, 52] . Chitin is the second most abundant natural polymer available on earth after cellulose and is found in a number of organisms from crustaceans such as lobsters, crabs, shrimp, and prawns along with insects to some types of fungi [31, 34, 52]. Chitin is a nitrogen-rich polysaccharide and a high-molecular-weight linear polymer composed of N-acetyl-d-glucosamine (N-acetyl-2-amino-2-deoxy-d-glucopyranose) units linked by β-d-(l → 4) bonds. Figure 1.3 shows the comparative chemical structure of chitin, chitosan, and cellulose [49, 52].

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FIGURE 1.3 Structure of chitosan, chitin, and cellulose.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

Chitosan is most frequently produced from chitin by deacetylation process [31, 34, 49, 52]. Figure 1.4 shows the scheme for the extraction of chitosan from chitin.

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FIGURE 1.4 Deacetylation of chitin to chitosan.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

Chitosan has been found to exhibit a basic character and is one of the highly basic polysaccharides compared to other natural polysaccharides that are acidic such as cellulose, agar, pectin, dextrin, and agarose [24, 29–53]. The degree of deacetylation and the charge neutralization of ─NH2 groups along with ionic strength have been found to control the intrinsic pKa value of chitosan. Figure 1.5 shows the schematic illustration of the versatility of chitosan.

c1-fig-0005

FIGURE 1.5 Schematic illustration of chitosan’s versatility. At high pH (above 6.5), chitosan’s amine groups are deprotonated and reactive. At low pH (<6.7), chitosan’s amines are protonated, confirming the polycationic behavior of chitosan.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

Along with physical modification, chitosan is most commonly modified by a number of chemical techniques including traditional chemical modification techniques such as photochemical, enzymatic, radiation, and plasma-induced graft copolymerization. Figure 1.6 summarizes some of the commercially used modification techniques for surface modification of chitosans.

c1-fig-0006

FIGURE 1.6 Chemical modification of chitosan for different applications: (a) methylation, (b) thiolation, (c) azylation, (d) copolymerization, and (e) N-succinylation.

Reproduced with permission from Ref. [52]. © 2013 Elsevier.

1.2.3 Agar

Agar has been commonly recognized as a hydrophilic colloid that is most frequently extracted from certain marine algae and is accumulated in the cell walls [2] . Due to its significant biological activities such as antioxidative, anticancer, anticoagulant, antiviral, and immunomodulating activities, it is extensively used as a gelling/stabilizing agent in a number of industries starting from food to pharmaceutical. Figure 1.7 shows the structural motifs of agar polysaccharides showing carbon numbering (C1─C6).

c1-fig-0007

FIGURE 1.7 Structural motifs of agar polysaccharides showing carbon numbering (C1─C6).

Reproduced with permission from Ref. [53]. © 2013 Elsevier.

Agar is represented by the structural formula (C12H18O9)n and is built on a disaccharide-repeating unit of 3-linked β-d-galactose (G) and 4-linked 3,6-anhydro-α-l-galactose (AG) residues. Depending on the source of the agar, there is possible occurrence of different substituents such as sulfate, methoxyl, and/or pyruvate at various positions in the polysaccharide chain. Agar is extracted from macroalgae by a number of conventional techniques, and recently great efforts are being made to extract it rapidly using a microwave-assisted methodology [32, 43, 44, 53]. Depending on the targeted applications, agar is surface modified accordingly [43, 44]. For example, agar is extracted after the alkali treatment to be used as catalysts supports and templates for the preparation of metal oxides using the same methodology and has been characterized by a number of techniques such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nuclear magnetic resonance spectroscopy (CP-MAS ¹³C NMR and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)) [53]. Figure 1.8 shows the SEM images of the alkali treated and untreated agar. These images clearly illustrate the structural changes taking place as a result of alkali treatment.

c1-fig-0008

FIGURE 1.8 SEM micrographs of nontreated (a) and alkali-treated (b) mesoporous agar materials.

Reproduced with permission from Ref. [53]. © 2013 Elsevier.

The results obtained from the SEM images were further confirmed by DRIFTS and NMR. Figure 1.9 shows the presence of different functional groups on both the treated and untreated agar.

c1-fig-0009

FIGURE 1.9 Attenuated total reflectance infrared (FTIR-ATR) spectra of nontreated (a) and alkali-treated (b) agar extracted from Gracilaria gracilis.

Reproduced with permission from Ref. [53]. © 2013 Elsevier.

Figure 1.10 also confirms the agarose structure of the extracted material after the alkali treatment. The structures of both the raw and alkali-treated agar show significant changes in their peaks.

c1-fig-0010

FIGURE 1.10 CP-MAS ¹³C NMR spectra of native agar (a) extracted at 100°C and alkali-treated agar (b) extracted at 140°C from Gracilaria. Top structure depicts the various carbons (C1─C6 from G and AG) associated with the different NMR peaks.

Reproduced with permission from Ref. [53]. © 2013 Elsevier.

1.4 POLY(3-HYDROXYALKANOATES)

PHAs are one of the unique families of polymers that are gaining significant attention during the past few years [49, 54, 55] . PHAs are linear, biodegradable polyesters that are synthesized by a wide variety of bacteria through the fermentation of sugars, lipids, among others [49, 54, 55]. Due to their biodegradable, renewable, and biocompatible nature, PHAs are emerging as one of the most promising materials for biomedical applications [49, 54, 55]. The structure and different applications of PHAs have been discussed in some of the recent reviews. Similar to other previously discussed polymers, PHAs also suffer from few drawbacks. Figure 1.11 shows the structural representation for different kinds of PHAs [54]. Some of the disadvantages of the PHAs include the brittleness and poor mechanical properties that restrict its successful applications [49, 54, 55]. Therefore to get the desired properties, it is most of the time modified with suitable materials/techniques.

c1-fig-0011

FIGURE 1.11 Chemical structure of PHAs.

Reproduced with permission from Ref. [54]. © 2013 Elsevier.

The effect of microstructure and composition of PHAs, along with the influence of compositional distribution and blending on mechanical properties, has been reported in detail by a number of researchers [49, 54, 55]. Figure 1.12 schematically shows the biochemistry and crystallinity of the PHAs, as these control most of the properties of the resulting material [55].

c1-fig-0012

FIGURE 1.12 Overview of PHA synthesis: schematic depiction of (a) chain polymerization catalyzed by enzymes, (b) a PHA granule with granule-associated proteins, (c) different forms of the PHB polymer chain, and (d) semicrystalline polymer structure. (e) AFM image of PHBV film; (f) final plastic products.

Reproduced with permission from Ref. [55]. © 2013 Elsevier.

A number of mechanisms have been proposed for the synthesis of PHAs from different precursors (Figure 1.13; [55]).

c1-fig-0013

FIGURE 1.13 Proposed polymerization mechanism for the synthesis of PHA.

Reproduced with permission from Ref. [55]. © 2013 Elsevier.

As it is evident from the existing literature that PHAs alone cannot satisfy the strict requirements for certain applications, most of the time it is blended with other materials. It has been reported that such blends exhibit a number of properties ranging from complete cocrystallization through partial segregation. Figure 1.14 shows the variation in phase structure of some of the blends.

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FIGURE 1.14 Variation of phase structures in PHB/PHB-HV blends.

Reproduced with permission from Ref. [55]. © 2013 Elsevier.

Although these polymers have some disadvantages, they are promising candidates as novel material for petroleum-based materials with the advantages of being biodegradable.

1.5 STARCH

Among various polysaccharides, starch is an industrially important biopolymer frequently used in food industry as well as in other industries. Depending on the targeted applications, starch has been used as a colloid stabilizer, thickener, and bulking/gelling agent [56] . Due to its enormous advantages, nearly 60 million tons of production has been reported all over the world [56]. Figure 1.15 shows the common structure sketches of starch granules. Starch has been found to be composed of different kinds of glucoses (primarily amylose (AM) and amylopectin (AP)) and occurs in the form of granules.

c1-fig-0015

FIGURE 1.15 Structure sketches of starch granules.

Reprinted with permission from Ref. [56]. © 2010 Wiley-VCH.

Starch is classified into normal, wx, and high AM depending upon the amount/ratio of amylose/amylopectin. Although starch is available in large quantity and offers numerous advantages, it has also some shortcomings similar to other biopolymers and has limited applications in its native form. A number of surface modification techniques are being used to alter the surface properties of starch and has been described in detail in the extensive reviews by Pei-Ling as well as in Chapter 7 in this book. Among these, one of the most promising is high hydrostatic pressure (HHP), which is a well-established nonthermal processing technology. Figure 1.16 shows the A- and B-type polymorphic structure of amylase as a result of HHP processing [56].

c1-fig-0016

FIGURE 1.16 A- and B-type polymorphs of amylase.

Reprinted with permission from Ref. [56]. © 2010 Wiley-VCH.

As a result of HHP processing, the structure of starch has also been found to be affected. Figure 1.17 shows the morphological images of the starch processed under high pressure. It is obvious from the micrographs that the outer surface of the starch granule is quite resistant to the HHP, and at the same time, the inner part is filled with structure having the shape of gel.

c1-fig-0017

FIGURE 1.17 SEM microstructure of potato starch with details of outer and inner part of starch structure: native (a); treated with high pressure at 600 MPa 3 min (b–d).

Reprinted with permission from Ref. [56]. © 2010 Wiley-VCH.

From the above discussion, it is quite clear that each biopolymer has certain limitation that restricts its use for a number of applications. The main obstacles include the lack of desired physicochemical/mechanical and biological properties. The only way to effectively utilize them is to alter their properties by surface modification techniques. By utilizing different surface modifications, most of the times, the specific application properties of different biopolymers can be easily obtained. For a particular application, the analysis of the structure–property relationship of a biopolymer under investigation is of utmost importance. At present, a number of biopolymers are being tested for their commercial applications and some of the thrust areas include biomedical, packaging, food production, and automotive. Some biopolymers can directly replace synthetically derived materials in traditional applications, whereas others possess unique properties that could open up a good range of new commercial opportunities. In this book, best efforts have been made to incorporate sufficient information on different surface modification techniques to alter their specific properties for targeted applications.

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2

SURFACE MODIFICATION OF CHITOSAN AND ITS IMPLICATIONS IN TISSUE ENGINEERING AND DRUG DELIVERY

Dilip Depan¹ and Raj Pal Singh²

¹ Biomaterials and Biomedical Engineering Research Laboratory, Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, LA, USA

² Advanced Research Centre in Pharmaceutical Sciences and Applied Chemistry, Bharati Vidyapeeth University, Pune, India

2.1 INTRODUCTION: BIOMATERIALS

A biomaterial is essentially a material that is used and adapted for a medical application [1]. Biomaterials can have a benign function, such as being used for a heart valve, or may be bioactive and used for a more interactive purpose such as hydroxyapatite (HA)-coated hip implants [2–5]. Biomaterials are also used every day in dental applications, surgery, and drug delivery (a construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time) [6–10]. The type of material used is also dependent on the anticipated mode of applications, the need for surface functionalization, and the need of the cell types of interest in terms of porosity.

2.1.1 Biomaterials: Evolution and Properties

Recently, tissue-engineering strategies using engineered biomaterials that support and promote tissue growth have been proposed for reconstructive surgeries [11, 12] . A biomaterial is required to employ functional cells that constitute the target tissue, a matrix or scaffold supporting those cells, bioactive molecules regulating cellular behavior, and the compatible integration of this composite in the damaged tissue [13]. The success of the approach depends on the understanding of the impact of the following criterion:

Biocompatibility: A biomaterial must be immune acceptable. The host immune system should recognize the implant as part of the host and not reject it. Furthermore, a biomaterial should minimize an inflammatory response [14].

Biodegradability: The implanted foreign material should be degraded and expelled from the host’s physiological system. The implants are required to provide mechanical support and the degradation rate must be engineered to comply with the growth rate of new tissue [15].

Biomechanical properties: Our primary purpose about biomaterials is that they exists to repair, correct, or improve a physiological defect or deformation by their implantation. In most cases, implanted biomaterials must have certain mechanical strength to maintain tissue growth and integration [16, 17].

Surface chemistry: The surface chemistry is the main factor to cellular adhesion [18], as the cell-material interface is mediated by proteins adsorbed from the surrounding medium onto the substratum [19, 20]. However, a highly adhesive surface allowing for a strong cell adhesion may also result in cell immobilization [14]. Although the common synthetic polymeric scaffolds have great advantages in terms of their degradability and other properties, they lack desirable surface properties. One approach to providing them is by modifying their surfaces, by physical adsorption of compounds, or by chemical modification.

Micro topography: On smooth surfaces, the cells are able to spread, perhaps forming greater number of hemi-desmosomes as anchors to the substrate. In contrast, on rougher surfaces, the cells appear to form local contacts that allow the cells to span across the space between surfaces [21].

Porosity, pore size: The microstructure of scaffolding material should be highly porous with interconnected pore network. In such scaffolds, the large void volume facilitates anchorage-dependent cell seeding; maximize attachment, migration and growth, extracellular matrix production, fluid circulation, and vascularization within the pore space throughout the scaffold structure [22].

2.2 CHITOSAN AS BIOMATERIAL: STRUCTURE–PROPERTY–FUNCTION RELATIONSHIP

Chitosan (CS) is a unique polysaccharide derived from partial deacetylation of chitin, which is, after cellulose, the most abundant natural polysaccharide [23] . Found in arthropod exoskeletons, each year several million tons of chitin is harvested worldwide from the shell of shrimp, lobster, crab, or krill [24]. With its chemical nature and biological properties, CS biomaterial is highly versatile [25]. Furthermore, CS has reactive amino (─NH2) and hydroxyl (─OH) groups that provide many possibilities for covalent and ionic modifications (Fig. 2.1). They can be easily modified with a large variety of groups that can be chosen to modify specific functionality such as biological and physical properties, aiming at particular applications [26].

c2-fig-0001

FIGURE 2.1 Chemical structure of chitosan (CS) illustrating the primary amine (─NH2) and primary and secondary hydroxyl (─OH) functional groups used for surface modification of chitosan.

The advantages of CS are as follows: (i) CS intrinsically possesses strong biological activity; (ii) it is biocompatible, biodegradable, bioresorbable and has a hydrophilic surface, which facilitates cell adhesion, proliferation, and differentiation [27]; (iii) due to its cationic nature in physiological pH, CS mediates nonspecific binding interactions with various proteins. Soluble proteins, most of which are negatively charged, may also be expected to have varying binding affinities to CS-based material [28]; and (iv) CS is made of glucosamine and N-acetyl-d-glucosamine units linked by one to four glycosidic bonds.

The latter moiety is a structural molecule found in glycosaminoglycans [29], which is a polysaccharide occurring ubiquitously within extra cellular matrix (ECM), including the one in bone and cartilage [30]. Glycosaminoglycans are known to be involved in several cell–cell/cell–matrix interactions, including specific bindings to growth factor receptors and adhesion proteins, and thus modulate cell morphology, motility, differentiation, synthesis, and function [31].

With its hydrophilic and cationic nature, and its structure analogous to glycosaminoglycans, CS is expected to be endowed with related biological activity [32–34]. In fact, CS exhibits interactions with ECM components, immune cells, and growth factors such as the fibroblasts growth factors [35]. CS has a number of other desirable properties for a tissue scaffold: it has anticoagulant properties and antibacterial and antifungal action [36].

Moreover, CS has an excellent ability to be processed into porous structures and smooth films for use in cell transplantation and in tissue regeneration [37]. CS properties have only been thoroughly studied in the last few decades [38], starting approximately when the scientific principles for use of the monomer N-acetylglucosamine in enhancing wound healing process were reported in 1960 [39]. In the 1970s, the role of CS in potentiating wound healing was documented for various animal models [40].

Since then, CS material has been widely investigated in a number of biomedical applications [35, 41–46], from wound dressings, drug or gene delivery systems, and nerve regeneration to space filling implant. The utility of CS as a scaffolding material to support cell growth and proliferation has also been reported [47–49]. As biomaterial, CS is an exceptional polysaccharide, the most promising of this class of materials. It has excellent potential for engineering numerous tissue systems, including bone tissue, by serving as a structural base material on which normal tissue architecture is organized. Figure 2.2 explains the various applications of CS.

c2-fig-0002

FIGURE 2.2 Various applications of chitosan (CS).

Reproduced with permission from Ref. [25]; © Elsevier.

However, although pure CS has very attractive properties, it lacks bioactivity and is mechanically weak [50]. These drawbacks limit its biomedical applications. For these reasons, it is highly desirable to develop a hybrid material made of CS and appropriate inorganic filler, hoping that it can combine the favorable properties of the materials, and further enhance tissue regenerative efficacy. CS is much easier to process than chitin, but the stability of CS materials is generally lower, owing to their more hydrophilic character and, especially, pH sensitivity. To control both their mechanical and chemical properties, various techniques are used.

The advantage of modifying CS is not only to improve its biodegradability and its antibacterial activity but also the hydrophilicity introduced by addition of the polar groups able to form secondary interactions (─OH and ─NH2 groups involved in H bonds with other polymers). The most promising developments at present are in pharmaceutical and tissue engineering areas.

For the recent breakthroughs in tissue engineering applications of CS, an attempt is made in this chapter to consolidate some of the recent findings to modify CS to introduce various functional groups.

2.3 CHEMICAL MODIFICATION OF CS: AN OVERVIEW

Among the many reports on the derivatives of CS in the literature [51–53] , one can differentiate specific reactions involving the ─NH2 group at the C-2 position or nonspecific reactions of ─OH groups at the C-3 and C-6 positions (especially esterification and etherification) [54]. It is important to note that the ─NH2 in the C-2 position is the important point of difference between CS and cellulose, in which cellulose contains three ─OH groups of nearly equal reactivity. The main modification reaction involved the C-2 position, where quaternization of the amino group or a reaction in which an aldehydic function reacts with ─NH2 by reductive amination. This method has been proposed to introduce different functional groups on CS using acryl reagents in an aqueous medium; introduction of N-cyano-ethyl groups is said to produce some cross-linking through a reaction between the nitrile group and the amine group [55]. Furthermore, it is important to note that more regular and reproducible derivatives should be obtained from highly deacetylated chitin [56]—assuring control of the quality of the initial material that is essential before modification, especially when biological applications are to be explored.

CS is grafted with poly(ethylene glycol) (PEG), which is the most explored derivative of CS. Depending upon the extent of grafting, high molecular weight derivatives are more soluble [57]. Similarly, PEG can also be grafted by reductive amination of CS using PEG-aldehyde [58]. On the other hand, polypeptides have been grafted by reaction with N-carboxyanhydrides of amino acids with the purpose of developing new biomaterials [59]. Figure 2.3 gives an overview of the various chemical modification reactions of CS.

c2-fig-0003

FIGURE 2.3 Chemical reactions showing various routes to modify the surface of chitosan. .

Reproduced with permission from Ref. [41]; © Taylor and Francis

2.3.1 Graft Copolymerization with CS

Owing to its outstanding biological, chemical, and pharmaceutical applications, a variety of CS-graft copolymers have been synthesized to extend the utilization of CS in different areas such as drug delivery systems, hydrogels, and porous scaffolds. Although, scarcely explored and under-commercialized, the graft copolymerization of CS is an advancing field. The main advantage of graft copolymerization of CS is that it introduces side chains to design various molecular patterns, thus creating novel types of tailored hybrid materials composed of natural polysaccharides and synthetic polymers [60] . The process is not only cost-effective but also the polysaccharide portion of product is biodegradable. Another advantage is that the graft copolymers may be controlled by molecular structure, extent of grafting, and number of grafted side chains, making it one of the most attractive approaches toward constructing versatile molecular designs.

2.3.2 Grafting onto CS

The main advantage of grafting onto CS is the high degree of functionality of CS as the molecular backbone consists of one primary amine and two hydroxyl groups, per polysaccharide repeat unit. Several reports of grafting onto CS consist of grafting of vinyl monomers, such as acrylonitrile, vinyl acetate, and methacrylate, using 2,2-azo-bisisobutyronitrile in aqueous acidic solution [61–63]. Ceric ions can also be used as an initiator to graft copolymers of polyacrylamide and poly(acrylic acid) onto CS [64]. Using these techniques, various CS materials can be synthesized with specific functionality.

Modification of CS for tissue engineering applications has been investigated by grafting homopolymers and copolymers of lactic acid. Since, lactic acid–grafted CS can act as hydrogel, it can be used in gastrointestinal tract–based drug-delivery systems. Qu and coworkers have fabricated pH-sensitive CS and lactic acid–based hydrogels [65]. Since, CS is alkaline in nature, by combining it (as graft copolymer or blend) with the biodegradable polymer like polylactic acid, which generates acidic by-products, the local toxicity at the implant site can be reduced [66]. In a similar approach, CS has been modified by grafting lactic acid and subsequent intercalation into montmorillonite to synthesize CS–clay nanohybrids. The prepared nanohybrids offer promising applications due to their favorable swelling behavior and biocompatibility [67].

Furthermore, this kind of polymer intercalation is proven useful to load drug molecules, such as ibuprofen for controlled drug-delivery applications [68]. The incorporation of clay layers was observed to control the initial release of drug. It was concluded that a combination of grafting of biodegradable polymeric chains and clay reinforcement can be applied to achieve the desired combination of properties (mechanical, swelling, and controlled) of materials used for biomedical applications, as shown in Figure 2.4.

c2-fig-0004