## 8 Application of Chitosan Biomaterials
In skin tissue engineering, the rigid structure of chitosan fibers can enhance the mechanical resistance of the dermal matrix and prolong the degradation of the dermal matrix by wound cell collagenase [107]. In cartilage tissue engineering, chitosan sustained-release microspheres have good drug loading and drug-releasing properties. Microsphere scaffolds can well maintain the phenotype of chondrocytes, promote their adhesion and proliferation, and have a good application prospect in the construction of cartilage and repair of cartilage damage as a carrier of chondrocytes. Chitosan and its derivatives have been extensively applied in the study of artificial nerves because of their excellent biodegradability and biocompatibility. During the construction of artificial nerves, the function of normal peripheral nerves was not affected, which could promote nerve regeneration and provide conditions for the attachment, migration and proliferation of Schwann cells to play their normal functions. Gels, nanoparticles, films, compressed tablets, beads and microspheres are currently used as potential drug delivery systems [108; 109; 110]. Chitosan has excellent biological activities as mentioned before and has been widely used in the study of the drug carrier systems as drug conjugates, hydrogel systems and biodegradable release systems [111; 112; 113]. It is mainly used in gene therapy, biological imaging, delivery of proteins/peptides, anti-inflammatory drugs, growth factors, antibiotics and vaccines. Drug delivery routes include oral administration, nose, eye and percutaneous administration [114; 115]. The ionic interaction between the negatively charged sialic acid substructure in the mucus and the positively charged primary amino group of chitosan polymer could offer adhesion and permeability properties of chitosan. Self-assembled nanospheres were prepared by chemically-linked active amino groups on the chitosan backbone, which can circulate in the blood for a long time without being engulfed and can be transplanted to the target ligand, which is easy to deposit in the designated lesion site for treatment. [PERSON] [116] used carbodiimide to connect the bile acid to the glycolytic chitosan skeleton so that the chitosan had strong hydrophilicity, and the nano-microspheres circulated in the blood for a long time and could be loaded with doxorubicin, paclitaxel, doxycycline and other anticancer drugs to effectively treat tumors. Chitosan can attach nucleic acids via electrostatic bonding and also could be used to create non-viral gene delivery vectors, which enter into the cells without alienation of the DNA-chitosan complex [117; 118; 119; 120]. It shows the main part in both lysosomal escape and membrane adhesion of the encapsulated DNA for effective cell transfection. [PERSON] et al. [121] prepared siRNA/folate polychitosan lactate nanoparticles by ionic gelation, showing the potential of effective gene therapy for ovarian cancer. [PERSON] [122] investigated the antitumor activities of chitosan with a molecular weight of 3 K, 65 K and 600 kda and the zero-valent selenium (Se\({}^{0}\)) nanoparticles stabilized by oligosaccharides. High molecular weight chitosan stabilized nanoparticles are easier to release selenium than low molecular weight chitosan, and to be absorbed by HepG cells through electrostatic action. Additionally, they are more effective in inhibiting the activity of HepG2 cells. These nanoparticles could produce highly toxic Se\({}^{4+}\) from the less toxic Se\({}^{0}\) and release selenium upon high ROS production by cancer cells. This high toxic Se\({}^{4+}\) causes apoptosis and mitochondrial dysfunction via consuming antioxidant enzymes. Chitosan and its derivatives can regulate the immune system through molecular mediation, enhance the body's resistance to various pathogenic microorganisms, and show antitumor activity. The antitumor activity of chitosan varies with the molecular weight and the substituted functional groups.