## 7 Biological Properties  
Chitosan, because of the strong hydrogen bond between molecules, possesses a regular molecular chain, good crystallinity and other properties such as adsorption, moisture retention, a film-forming ability, etc. It is a naturally available alkaline polysaccharide, while others like pectin, dextran, cellulose, starch, agar-agar, carrageenanan, alginic acid, etc., are either acidic or neutral. It is a biocompatible substance that gradually breaks down to safe products like amino sugars, which absorbed completely in the body. The enzyme lysozyme hydrolyzed chitosan in vivo into oligomers that trigger macrophages to release \(N\)-acetyl-p-glucosamine for catalyzation of oligomers to produce substituted glucosamines and p-glucosamine [61]. Several studies reported that MW, DDA, active groups and the bonding mode of chitosan and its derivatives are vital factors for their marvelous biomedical properties. The biological properties and corresponding biomedical applications of chitosan are shown in Figure 9.  
1. Antibacterial. Chitosan is an only natural weak alkaline polysaccharide that easily dissolves in a dilute acid solvent. The dissolved chitosan contains an amino group (NH3+), which can inhibit bacteria by binding negative electrons and its antibacterial activity may enhance with concentration. Chitosan with different molecular weight can stop several bacteria growths, and has a strong inhibitory effect on _Escherichia coli_ and _Staphylococcus aureus_, showing similar characteristics with antibiotics [62]. The antibacterial mechanism of chitosan can be divided into two ways: One is that chitosan forms a layer of the polymer by adsorbing on the cell surface. The membrane prevents the transport of nutrients to the cells and plays the role of bacteriostasis and decontamination; the second is that chitosan penetrates the cells through osmosis, adsorbs the anionic cytoplasm in the cells and causes flocculation, which disrupts the normal physiological activities of cells and kills bacteria [63; 64; 65]. When the molecular weight of chitosan is different and the bacteria that act on it are different, the antibacterial mechanism of chitosan is different, but in essence, its antibacterial property comes from the antibacterial factor NH3+[66; 67]. Chitosan has amino and acetyl groups on its molecular chain, so it is amphoteric. Its isoelectric point is pH 6.2. When the pH of the solution is higher than this value, chitosan will not have a positive charge and no bacteriostatic effect. If it is lower than this value, it will have a positive charge. The degree of deacetylation of chitosan may also affect its antibacterial effect. The content of the free amino group and the bacteriostatic rate increased with the increase of the deacetylation degree [68]. The bacterial strains, pH, temperature, salinity, molecular weight, concentration  
Figure 8: Schematic illustration of epigallocatechin gallate (EGCG) incorporated carboxymethyl chitosan (CMCS)/konjac glucomannan (KGM) biocomposite films [58]. and degree of deacetylation of chitosan may be closely related to the antibacterial effect of chitosan. However, considering the high number of bacteria strains and the complexity of the bio environment generated, it is not clear the specific interaction between these factors [69; 70]. The latest studies have enforced an investigation of the inhibitory activity of biofilm production of chitosan and its derivatives as prominent agents and diminish virulence properties by several pathogenic bacteria. Conjugation of chitosan with other bioactive materials can further promote its antimicrobial activity [71]. Ionic gelation was used to produce chitosan nanoparticles (CNP) and a different concentration of CNP (0-20% _w/w_) was used to prepare starch-based nanocomposite films. An in vitro and in vivo study proved the antimicrobial characteristics of CNP-starch films.