## 6 Modifications of Chitosan  
Although chitosan owns exceptional biological activity, such as biodegradability, hemostasis, antiseptic, antibacterial, biocompatibility and promoting wound treatment and so on, its poor water solubility and mechanical properties greatly limit its application in biomedicine. Improving the solubility of chitosan is the key factor for the rational utilization of its various uses. To improve the water-insoluble defects of chitosan, by hydrolysis of the main chain, it was degraded into chitosan oligosaccharide, which has good water solubility, is easy to disperse and absorb and has many unique physicochemical properties and biological activities. The most suitable hydrolytic enzyme of chitosan is chitosanase. Chitosanase from different microbial sources hydrolyzes different substrates. According to the action model of chitosanase, it can be divided into two types: endo- and exo-type. The endo chitosanase mainly releases a dimer, trimer or oligosaccharidesacharide, while the exo chitosanase produces a monosaccharide residue, namely glucosamine, from the non-reducing end of chitosan or chitosoligosaccharide [38]. Both chitosan and cellulose are polysaccharide compounds formed by D2 glucose linked and polymerized by glycosidic bonds. Due to the structural similarity, cellulase should also have a similar degradation effect on chitosan [39]. The acetylated amino group in chitosan can also be modified by chitinase, lysozyme, tyrosinase, lactose and peroxidase, which can effectively improve the biological properties of chitosan, such as antioxidant and antibacterial properties [40; 41; 42]. Chitosan can be modified by acylation, carboxylation and etherification to produce a series of chitosan derivatives with different properties, which can improve its water solubility, biological activity and mechanical properties, and expand the application of chitosan in various fields because the basic unit of chitosan is glucosamine, which contains a lot of active amino and hydroxyl groups. Modified chitosan derivatives have attracted more and more attention because they are superior to unmodified chitosan in chemical, biological and functional aspects such as solubility and gelation. As shown in Figure 4, previous studies also extensively reported the chemical modification of chitosan, such as acidylation, carboxylation, etherification, etc. [43; 44]. The present review only focuses on the study of graft copolymerization and the blending modification of chitosan in recent years. 1. Carboxylation: chitosan molecules contain more free -NH\({}_{2}\) and -OH, introducing carboxyl functional groups, among which carboxymethyl is the most common, that replace the side chain ammonium salt, can obtain water-soluble, alcohol soluble, organic solvent-soluble, surface-active and fibrous polymer derivatives. [PERSON] et al. [45] suspended the chitosan powder in isopropanol, added isopropanol chloroacetate solution to the solution, reacted at room temperature and continuously stirred with magnetic force to obtain carboxymethyl chitosan with a high degree of protonation. [PERSON] et al. [46] successfully prepared \(N\),\(O\)-carboxymethyl chitosan from chitosan and chloroacetic acid under alkaline condition. The obtained carboxymethyl chitosan not only retains the original superior properties, but also improves the solubility more effectively, and has the function of moisturizing. 2. Etherification: the hydroxyl of chitosan can react with methyl ether, ether, benzyl ether and other alkylating agents to form an ether. By the way of cellulose modification, hydroxylkyl chitin and carboxalkyl chitin can be obtained by the reaction of basic chitin and etherification reagent. 3. Crosslinking: chitosan can be crosslinked in or between molecules through -OH and -NH\({}_{2}\) with aldehydes, anhydrides or epoxides with two functional groups, and grafted to form network polymers to obtain crosslinked products with improved mechanical properties, providing conditions for further grafting modification. 4.