Table 1.

Some of the main features of various crosslinking types of hydrogels. PBS phosphate buffer saline

	Classification	Exogenous dissolution agents	In situ formation or not	Expected dissolution time	Potential for wound treatment	Advantages	Disadvantages
Chemically cross-linked hydrogels	Stimuli-sensitive hydrogels	Nothing	Yes	Immediately	Better application in vivo	Hydrogel are pure and less toxic	Low mechanical strength, less crosslinking species, less selectivity of polymer, long gelation time
	Supramolecular self-assembly hydrogels	Mild chemical irrigant	Yes	Within 2 min	Better application in vivo	Hydrogels have better mechanical properties and less toxic effects	Self-assembly process is difficult to control
Physically cross-linked hydrogels	Thiol-thioester exchange	Thiolate	Yes	Within 25 min	Better application in vivo	A cheaper way for hydrogel dissolution	Dissolution times of hydrogel are too long, and toxicity of thiolate is unknown
	Thiol-disulfide exchange	Thiol-containing reducing agent	Yes	Within 10 min	Better application in vivo	Built-in redox-sensitivity as living cells	Cytotoxicity of hydrogels or dissolution agents are uncertainty
	Retro-Michael reaction	Glutamate, PBS (pH 7.4), or light	Yes	2 days (glutamate); 4 days (PBS) 4.5 min (light)	Further research is needed	Increased stability for sustained release under highly reducing conditions	Michael acceptors for retro Michael Reaction have been less studied, and the effect of hydrogel dissolution is poor with side reaction
	Retro-Diels-Alder reaction	Dimethy formamide	No	0.4 h (100 °C)	Further research is needed	Hydrogels are formed need no catalysts or initiators	The dissolution temperatures of hydrogel are too high