“…With the amino (–NH 2 ) and hydroxyl (–OH) groups, their functions enable CS to conjugate with several molecules, such as ethylene glycol (PEG) [ 19 , 20 , 21 ], gallic acid [ 22 ], deoxycholic acid [ 23 ], and bombesin peptide [ 24 ]. According to the functional groups, CS also has metal chelation and reduction reaction capacities [ 25 , 26 ], antioxidant/antimicrobial activities [ 27 ], and pH-responsive ability [ 28 ]. With its unique characteristic, CS has been applied as a main natural polymer for the development of several biomaterials, such as pH-responsive hydrogels for delivering 5-fluoro uracil [ 29 , 30 ] and nanocarriers for delivering genes, proteins, DNA, siRNA, and anticancer drugs [ 31 ].…”