All water-soluble or hydrophilic polymers can form hydrogels through certain chemical cross-linking or physical cross-linking. These macromolecules can be divided into natural and synthetic categories according to their sources. Natural hydrophilic polymers include polysaccharides (starch, cellulose, alginic acid, hyaluronic acid, chitosan, etc.) and peptides (collagen, poly-L-lysine, poly-L-glutamine, etc.) . Synthetic hydrophilic polymers include alcohols, acrylic acid and derivatives thereof (polyacrylic acid, polymethacrylic acid, polyacrylamide, poly N-polyacrylamide, etc.).