Recent advances in chitosan-based materials; The synthesis, modifications and biomedical applications
The attention to polymer-based biomaterials, for instance, chitosan and its derivatives, as well as the techniques for using them in numerous scientific domains, is continuously rising. Chitosan is a decomposable naturally occurring polymeric material that is mostly obtained from seafood waste. Because of its special ecofriendly, biocompatible, non- toxic nature as well as antimicrobial properties, chitosan-based materials have received a lot of interest in the field of biomedical applications. The reactivity of chitosan is mainly because of the amino and hydroxyl groups in its composition, which makes it further fascinating for various uses, including biosensing, textile finishing, antimicrobial wound dressing, tissue engineering, bioimaging, gene, DNA and drug delivery and as a coating material for medical implants. This study is an overview of the different types of chitosan-based materials which now a days have been fabricated by applying different techniques and modifications that include etherification, esterification, crosslinking, graft copolymerization and o-acetylation etc. for hydroxyl groups’ processes and acetylation, quaternization, Schiff’s base reaction, and grafting for amino groups’ reactions. Furthermore, this overview summarizes the literature from recent years related to the important applications of chitosan-based materials (i.e., thin films, nanocomposites or nanoparticles, sponges and hydrogels) in different biomedical applications.
Introduction
Recently, extensive development has been made in the field of biomedical by using polymers, which is not only cost effective, but also results in promising outcomes. On the basis of sources, these types of polymers can be divided into two types, naturally occurring polymers and man-made synthetic polymers (Tian, Tang, Zhuang, Chen, & Jing, 2012). These two major classes of polymers are distinguished on the basis of the structural distinction present in them (Molavi, Barzegar-Jalali, & Hamishehkar, 2020). Typically, many naturally occurring polymers (i.e., polysaccharides and proteins) instinctively bend into squeezed shapes in sophisticated modes that confide in their prime structures and many of their biological benefits are interlinked with their structures. On the other hand, almost every synthetic polymer (i.e., polyesters and polycarbonates) possesses simple as well as more arbitrary structures. Furthermore, naturally occurring biopolymers are usually decomposable as well as interacting with living systems in a more precise manner. Nevertheless, they also possess some drawbacks that can hinder their applications. These types of polymers possess lesser mechanical strength, unrestrained degradation rate and conceivable immunogenicity, which significantly bound their applications in some fields (Stahl, Emberger-Klein, & Menrad, 2021).
In contrast to these limitations, biopolymers have gotten much attention from researchers and resulted in some promising benefits for humanity. The recent developments of these polymers include efficient drug delivery to targeted areas, identification, and detection of diseased influenced areas, biosensing, recovery of medicine and cure of specific diseases (Feng et al., 2021). For instance, polymeric material-based carriers have been developed which provide utmost perfections in the effective delivery of the bioactive as well as therapeutic drugs at targeted areas of the body, which in turn result in progressing treatment of ailments, like malignancies, grafting of specific organs as well as toxicities (Hao et al., 2021). In the meantime, the coherent structures of intelligent polymeric material-based carriers can offer enhanced ability of response towards a specific stimulus either from exterior or inherent signal in explicit abrasions, in this manner attaining precise and directed localization at abrasion locates and activated discharge of consignments at the diseased areas for better therapeutic efficiency (Zhang et al., 2021; Zhang et al., 2021; Zhang et al., 2021; Zhang et al., 2021).
Naturally derived polymeric materials or biopolymers (secluded from natural sources) mostly occur in long chain macromolecules (Deb, Al-Attraqchi, Chandrasekaran, Paradkar, & Tekade, 2019). Among these macromolecules’ polysaccharides, proteins, and polyesters are some common polymeric materials obtained from flora and fauna kingdoms (Aravamudhan, Ramos, Nada, & Kumbar, 2014). These constituents possess certain benefits above man-made polymers, for instance, lesser cytotoxicity and exceptional biocompatibility. There are vast numbers of methods which employ these materials for analysis purposes. Among these described biopolymers, chitosan is the most widely applied natural polymer afterwards cellulose, alginate, pectin, starch, and lignin (Arshad, Zubair, & Ullah, 2020; Zubair & Ullah, 2021).
Chitosan (CS), a naturally occurring polysaccharide-based biopolymer obtained from the chitin’s deacetylation reaction (Fig. 1), is the most abundant polysaccharide-based material after the cellulose (Saha, Zubair, Khosa, Song, & Ullah, 2019; Zubair, Arshad, Pradhan, & Ullah, 2020). This polymer extensively originated from flora and fauna territories, together with yeast, algae, insects, and crustaceans. Chitin is a constituent of the crustacean’s skeleton, the fungus’s cell wall of the fungus as well as the cuticle of particular species of insects. Besides this, it is also present in exoskeletons, tendons, inside layers of excretory, respiratory and digestive tracks and eyes of arthropods (Deng, Wang, Chen, & Liu, 2020). Even though this polymer has prevalent origins, the crucial and chief source is the marine or ocean. The predictable annual report of chitin production in the ocean is approximately billions of tons. On the commercial scale, most chitin and chitosan are acquired as a waste of shellfish, for instance, crabs, crayfish, shrimps, cuttlefish, prawns, and krills. The waste obtained from shellfish entail different percentage of these polymers, i.e., crabs and shrimps entail 13–16 % and 14–27 % of chitosan or chitin, correspondingly (Varun et al., 2017).
CS is made up of two copolymers (d-glucosamine and N-acetyl-d-glucosamine) resulting from β-1, 4-linkage between them, which makes it polycationic heteropolysaccharide as well as a high molecular weight structure (Zhao, Fan, Liu, & Li, 2022). Generally, this polymeric material is insoluble in water. Still, it turns out to be soluble in such media which is acidic in nature (pH < 6), for instance, phosphoric acid, hydrochloric acid and lactic acid (Moura, Figueiredo, & Gil, 2007). The polycationic feature of CS is peculiarity gained due to amino group’s protonation ability. Lysozyme, in many cases is used to hydrolyze the alkaline polysaccharide (in vivo) to generate oligomers, which encourage macrophages to release the N-acetyl-d-glycosaminidase enzyme, which in turn is pursued by the synthesis of N-acetyl glucosamine, d-glucosamine and substituted glucosamine. These generated products can be utilized entirely by our body with quite an ease (Bakshi, Selvakumar, Kadirvelu, & Kumar, 2020).
CS’s non-toxic nature, excellent biodegradability, biocompatibility, and antibacterial ability make it an appealing material for daily life applications such as the food industry, biomedical and pharmaceutical applications (Zubair, Arshad, & Ullah, 2020). Fig. 2 depicts the summary of the applications of the CS-based materials. On the other hand, some structural issues limit its applications, such as the insoluble nature of this polymer in neutral media and amino groups weaker positive charge. Lately, derivatives and polymer complexes of CS have been developed, resulting in better properties. For the manufacturing of chemically modified CS, there are different functional groups (C2-amino, C3-primary, and C6-secondary hydroxyl groups) which are advantageous (Mutreja, Thakur, & Goyal, 2020). It also has been noted that derivatives of this polymer maintain the unique properties of CS and result in enhanced features through the accumulation of different modified functional groups. In addition, structural modifications have correspondingly been executed to enhance the characteristic antimicrobial ability of CS. It also has been reported that the antimicrobial ability of this polymer can be enhanced by simply intermixing it with other polymers (natural and synthetic polymers) (Azmy, Hashem, Mohamed, & Negm, 2019; Zubair, Zahara, Roopesh, & Ullah, 2023).
Presently, two approaches are most frequently applied to acquire CS through chitin acetylation reaction up to different degrees. The first process uses the heterogeneous approach of deacetylation in solid phase system, while the second one intricate homogeneous approach of deacetylation of chitin in an aqueous system in pre-swollen phase as well as under vacuum atmospheres (Methacanon, Prasitsilp, Pothsree, & Pattaraarchachai, 2003). The deacetylation reaction has to be processed for a considerable time in both techniques while being exposed to concentrated alkali solution. Both techniques (homogeneous and heterogeneous conditions) might influence the processing time for the deacetylation reaction of chitin, which can range from 1 to 80 h. On the other hand, several other synthesis techniques have also been explored to reduce the high alkaline solution concentration and reaction time, which was significantly higher in other methods. These techniques engaged thermo-mechanical procedures via cascade reactors operated at low alkali concentrations (Pillai, Paul, & Sharma, 2011), consecutive alkali analyses using thiophenol in dimethyl sulfoxide (Kaur & Dhillon, 2015), microwave dielectric heating system (Lertwattanaseri, Ichikawa, Mizoguchi, Tanaka, & Chirachanchai, 2009), flashy treatment with the help of saturated steam (Khan & Alamry, 2021), and repeated washing by using a cascade of distilled water (Caligiani et al., 2018). Step by step procedure of CS synthesis is shown in Fig. 3. The literature study also recommended various cutting-edge techniques for isolating CS using non-conventional energy sources, which were capable of transferring energy quickly and directly into the substrate and increasing reaction effectiveness (Rahman & Rashid, 2013). Another technique involving enzymatic processing can also fabricate CS instead of alkaline solution processing at elevated temperatures (Cai et al., 2006).
CS can be modified to suit better the characteristics needed for biomedical usage. CS is mainly subjected to chemical modifications for its enhanced solubility in aqueous media, rheological characteristics, stability towards heat and temperature, and oxidation resistance (Azmy et al., 2019). The active groups in CS’s chemical structure account for amino and hydroxyl groups at the C3 and C6 positions (Marín et al., 2019). Generally, the primary hydroxyl group is always more reactive than the secondary hydroxyl group (C3-OH). In comparison among amino and hydroxyl groups, the NH2– the amino group is more reactive than the primary hydroxyl group (owing to the free rotation around the bond). To create N-, O-, and N,O-modified derivatives of this biopolymer could be modified through chemical reactions on NH2, OH, or both of these groups (Dimassi, Tabary, Chai, Blanchemain, & Martel, 2018; Sahariah & Másson, 2017). Fig. 4 shows the structural representation of CS and some of its derivatives. Based on reaction conditions, on hydroxyl groups’ processes such as etherification, esterification, crosslinking, graft copolymerization and O-acetylation are performed, whereas on amino groups, reactions like acetylation, quaternization, Schiff’s base reaction, and grafting occur (Wang & Liu, 2014). Various modification techniques of CS are presented in Fig. 5. Table 1 summarizes different chemicals involved in CS modifications used for different applications.
CS becomes an innovative biomaterial for treating diseased cells, tissue engineering, and gene therapy after these different modifications under different conditions (Shi et al., 2006). In addition, different types of CS-based biomedical materials, such as CS hydrogels and networks, have been fabricated, resulting from aggregation and complexation processes (Berger, Reist, Mayer, Felt, & Gurny, 2004). These types of CS-based materials have been developed after modifications which are efficient for drug delivery applications (Bernkop-Schnürch & Dünnhaupt, 2012). CS is also frequently applied in cosmetic formulations as a rheology modifier and film-forming ingredient. Several cosmetic items on the marketplace are comprised of CS-based ingredients. For the absorption of water and/or bioactive components, hydrogels with regulated network topologies are frequently utilized. Additional applications of these materials include drug release through diffusion and pH control conditionings (Mitra & Dey, 2011). The type of crosslinkers used play a pivotal role in determining the swelling sensitivity of these hydrogels towards pH changes; for instance, hydrogels fabricated because of the ionic crosslinking show an enhanced swelling sensitivity towards pH changes as compared to covalently cross-linked hydrogels. This finding broadens the ionically cross-linked CS-based material’s prospective applications compared to covalently cross-linked materials (Pellá et al., 2018). Skin, bone, cartilage, liver, nerves, and blood vessels are just a few examples of tissues that may be engineered using CS and its derivatives (Sultankulov, Berillo, Sultankulova, Tokay, & Saparov, 2019). CS hydrogels that may be injected have been suggested for the treatment of cancer (Ahsan, Farooq, & Parveen, 2020). The details of CS modifications reported in literature are described below.
It is mainly identified reaction which involves Schiff base reactions may well be applied to modify CS biopolymers. CS can easily interact with most aliphatic and aromatic carbonyl compounds producing Schiff bases imines. The Schiff base generated by the reaction between CS and carbonyl compounds might be reduced by using sodium borohydride to produce N-derivatives of the CS (Baran, 2017; Tapdiqov et al., 2018). In the aqueous phase, these derivatives could bind with transition metal ions to create metal chelates that are insoluble in aqueous media that could be easily isolated. This reaction is highly beneficial for the usage of CS in the elimination of hazardous metals.
This process is used to modify CS biopolymers by using a free amino functional group on this biopolymer. It entails adding quaternary ammonium groups or might the salts be containing (small molecule) quaternary ammonium groups to the structure of CS (i.e., at amino group). These groups possess an enhanced hydration capacity and a higher steric hindrance. The reaction of quaternized CS results in improved water solubility and antimicrobial activities which are beneficial for biomedical or pharmaceutical applications (Ling, Li, Zhou, Wang, & Sun, 2015; Liu, Shen, Luo, Wang, & Sun, 2013).
These approaches are quite useful for modifying CS biopolymers with the help of halogen containing hydrocarbons, anhydrides, and acid halides as acylating substituents in a specific reaction media (Kurita, Ikeda, Yoshida, Shimojoh, & Harata, 2002; Piegat et al., 2019). These fabricated compounds cause breakdown of hydrogen bonds among CS molecules which alter the original crystal arrangement and considerably enhance CS solubility and application range in biomedical fields (Tapdiqov et al., 2018).
These approaches are applied for the insertion of acidic groups into the core chain structure of CS which results in the increment of solubility, film-forming capabilities, and moisturizing abilities of CS-based materials. Carboxymethylation of chitosan (CMCS) biopolymer can take place at the hydroxyl as well as amino groups, resulting in the fabrication of O-carboxymethyl and N-carboxymethyl products correspondingly. This polymer’s carboxymethylated derivative which became water-soluble (anionic polymer) was specifically modified to fabricate an anti-tumor drug carrier (Alves & Mano, 2008; Rinaudo, 2008). It has also been stated as a potential targeted drug carrier to the liver owing to its favorably positioned and prolonged retention in the liver and spleen following circulatory inoculation (Onishi, Nagai, & Machida, 2020). In addition, modification of this polymer with sugars on NH2 groups permits the introduction of sugars predictable by cells, viruses, and bacteria through carriers of definite drugs or antibodies (Khan & Ahmad, 2013; Liu, Willför, & Xu, 2015).
This approach is quite useful which not only improves the solubility but also results in the enhanced biological activity of CS-based materials (Liu, Pu, Liu, Kan, & Jin, 2017). The derivatives prepared because of this polymer are nowadays applied for therapeutic as well as pharmaceutical usages as orthopedic, wound healing, engineering of specific tissues (Silva et al., 2013; Silva et al., 2014) and precise drug or gene delivery to a particular body part (Ito, Yoshida, & Murakami, 2013; Kumar et al., 2012; Lu, Xu, Zhang, Cheng, & Zhuo, 2008; Sun, Xu, Liu, Xue, & Xie, 2003). CS might be employed for the absorption of prototype molecules resembling with natural recognition ingredients for instance, antibodies for diagnosis which becomes possible as a result of modification as well as molecular imprinting techniques (Kyzas, Lazaridis, & Bikiaris, 2013). In recent past, a lot of work has been performed in the fabrication of composites of this biopolymer with several other polymers like, polyethylene glycol, polypyrrole, collagen and starch as well as with inorganic constituent’s bioactive glasses and ceramic materials for various biomedical applications with quite an ease (Dan et al., 2013; Jayakumar et al., 2011; Kumar, Muzzarelli, Muzzarelli, Sashiwa, & Domb, 2004; Nakamatsu, Torres, Troncoso, Min-Lin, & Boccaccini, 2006).
Through cross-linking, CS may be modified to produce derivatives that retain stable chemical characteristics, are insoluble in acidic and basic media, and can be utilized as a carrier for the retention of substances like immobilized enzymes, drugs and toxic metal adsorbents. Before and after cross-linking, researchers studied the chemical composition and structures of metal complexes created by a reaction when CS was combined with a few heavy metal ions. They observed different absorption capacities of CS complexes after modification (Trimukhe & Varma, 2008). In addition to these types of modification approaches, there are other methods which could be applied. For instance, esterification, hydroxyalkylation and sulfonation are a few performed, and activities analyzed (Han et al., 2020; Hu et al., 2019). To create biomaterials for diverse uses, CS is being physically modified by mechanical grinding, ionization by radiation, and ultrasonic treatment (Mutreja et al., 2020).
CS prepared by thiolation can be synthesized by applying the derivatization process of the functional amino group with that of species or reagents containing the thiol group (Bernkop-Schnürch, Hornof, & Guggi, 2004; Liu, Li, Zhang, Wang, & Feng, 2021). The amino group at the second position of CS glucosamine is the leading site where thiol group can easily be immobilized. The thiol entailing solvent binds with CS by developing an amide linkage with the primary amino group of this biopolymer. The percentage of these substituent anions reflect these groups’ activity potential is lower at pH <5 during the reaction, which restricts the development of disulfide bonds (Khan & Alamry, 2021). This results in epidermal growth, which triggers intracellular tyrosine kinase stimulation, improving penetrability and membrane absorption (Zhang et al., 2018). In addition, this modified form of CS possesses enhanced adhesive potential, and inhibitory impact against phosphorylated glycoproteins. The thio-chitosan-based material in place of coating material for the modification of stainless steel having maleimido in combination with tannic acid resulted in the reduction of E. coli bacteria by approximately 70 % and adhesion up-to 90 % (Xu, Pranantyo, Neoh, Kang, & Fu, 2016).
The widespread applications of phosphorylated CS and its derivatives in the biomedical field such as, tissue engineering, as a carrier for drug delivery and tissue regeneration, are because of some of the characteristics of these materials which mainly include enhanced water solubility and metal chelation (Han et al., 2020). This form of phosphorylated CS was mostly obtained by the reaction of phosphorus pentoxide and CS. This process is carried out at a lower temperature and in the presence of methane sulfonic acid which behaves as a catalyst. The monophosphate grafted CS based materials possess enhanced antibacterial potential, swelling index, and ionic conductivity characteristics (Ardean et al., 2021; Jayakumar, Selvamurugan, Nair, Tokura, & Tamura, 2008). de Souza et al. (2020) developed phosphorylated chitosan–xanthan gum-based scaffoldings with improved osteoinduction for periosteal tissue engineering by introducing phosphate groups in the main structure of CS biopolymer. These scaffolds were prepared in the presence and absence of additives Silpuran® 2130 A/B and Kolliphor® P 188 which were employed as periosteal substitutes. These developed scaffolds were proven effective materials for promoting osseointegration.
These types of CS-based materials are synthesized by applying diverse sulfating components which mostly include concentrated sulfuric acid (Nagasawa, Tohira, Inoue, & Tanoura, 1971), oleum (Vikhoreva et al., 2005), SO3, SO3 in addition to different reagents like pyridine, trimethyl amine, sulfur dioxide, chlorosulfuric acid. The chemical approach of this type of modification is attractive because this results in the enhancement of the structural resemblance of CS salt with that of heparin (Mansour et al., 2017). On the other hand, the anticoagulant potential of these types of CS-based material is created as a result of the interaction of –SO4−2 in addition to positively charged peptide fragments (Cao, Wang, Hou, Xing, & Liu, 2014). CS sulfate-based substances have been found to possess additional biological possessions, like anti-sclerotic, antioxidant, antibacterial, anti-HIV, antiviral, and enzyme inhibition (Xing et al., 2005).
This method can be used to create hydroxyalkyl CS. In this process, glycidol and epoxides interact with CS. The process can result in O-hydroxyalkyl or N-hydroxyalkyl CS or a mixture of the two at any of the alcohol or amino group. Like some other modifications, this also results in enhanced water solubility (Aranaz, Harris, & Heras, 2010). This was additionally demonstrated by the fabrication of hydroxybutyl chitosan (HBCS), which results by the etherification of the hydroxybutyl group onto the CS molecule. The HBSC-derived polymeric substance that was created demonstrated exceptional hygroscopicity as well as moisture retention, encouraged immunocompetence potential, and had a superior antimicrobial effect (Li et al., 2019; Li, Wei, Zhang, Gu, & Guo, 2019).
The primary benefit of this modification is to alter the superficial of CS nanoparticles with ethylene glycol groups. This kind of modification is performed because ethylene glycol groups can provide improved stability and solubility to the nanocarriers in human bodily serum, as well as increased effectiveness of CS nanoparticle absorption in targeted cells via clathrin-mediated endocytosis, caveolae, and micro-pinocytosis paths (Cheng, Shiu, Chiu, Ballantyne, & Liu, 2021). Other characteristics of this kind of materials are the prolonged therapeutic supply and preventing multi-drug confrontation or resistance (Zou et al., 2021).
https://doi.org/10.1016/j.carbpol.2023.121318.
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