In this regard, aments of scientific literature showed that natural and abundant chitosan with biocompatibility, biodegradability and non-immunogenic properties could be the smart polymers  
Figure 1: Overview of production and structure of chitosan. for numerous biomedical purposes such as drugs and gene delivery vehicles, permeable membranes, tissue engineering scaffolds and so on [14]. Chitosan can be biodegradable in the human body, so it is not accumulated in the body, and there is no toxicity, irritation and antigen immunity. It is generally considered to have good biocompatibility and plays a very important role in human physiological activities. Although chitosan has superior biological activity, biocompatibility, biodegradability, antibacterial, antispecific, hemostatic and wound healing and other special functions, its application is limited to a certain extent by its poor water solubility and mechanical properties [15]. In a practical application, due to the molecular chain of chitosan containing a large amount of amino and hydroxyl groups, the chemical modification of chitosan, such as acylation, carboxylation and etherification, especially graft copolymerization and blending modification, can change the molecular structure of chitosan, generating a series of chitosan derivatives, which can improve its water solubility, biological activity and mechanical properties, and endow on it some special properties, extending the application of chitosan in various fields. A variety of polymer materials can be grafted on the primary, secondary hydroxyl and amino groups of chitosan. The graft copolymer of chitosan not only has the original biocompatibility and degradability of chitosan but also improves its solubility and endows it with other special properties. However, due to the small reactivity difference between the three functional groups, it is difficult to introduce a side chain at a fixed point and quantitatively, and it is difficult to separate and purify the intermediate product or the final product of the reaction, which limits the study on the chitosan grafting reaction to some extent. The research focuses on finding the appropriate reaction reagent, mild experimental conditions and efficient separation methods. At present, the application of the chitosan graft copolymer in medicine is still under continuous exploration. With the development of the research, the application of the chitosan graft copolymer in pharmacy will become more extensive. Blending modification is a common method of polymer modification. It is easy to give full play to the advantages of two or more kinds of polymers and effectively expand the application range of polymer materials. When the compatibility of components is good, a thermodynamic stable system can be formed to achieve the synergetic effect. Poor compatibility results in the separation of components. To avoid this disadvantage, a proper amount of cross-linking agents can be added to make the components cross-linked or the blends cross-linked through a hydrogen bond. The blend of chitosan with other natural or synthetic polymer materials can synthesize the excellent properties of each component to produce functional materials suitable for various fields. A large number of studies have proved that the unique biomedical characteristics of chitosan and its derivatives are closely related to their structure. In this paper, the molecular structure and properties of chitosan are reviewed, and the modification of chitosan and its application as biomedical materials in recent years are summarized.