Scheme 1.

Synthetic strategies of chitosan based hydrogels. Strategy 1: generating aldehyde groups directly on polysaccharides through the oxidation of hydroxymethyl groups; Strategy 2: using linear polymers with two aldehyde groups at the ends of the polymer as the crosslinking agent; Strategy 3: using linear polymers with multiple aldehyde groups as the side chains of the polymer as the crosslinking agent; Strategy 4: using star polymers with aldehyde group at the end of each arm of the polymer as the crosslinking agent.