Table 2.

Crosslinking mechanisms of hydrogel.

Type	Crosslinking Mechanism	Crosslinking Method	References
Physical crosslinking	Thermally-induced entanglement of polymer chains	Through the interaction between molecules, the crosslinked structure is formed in the hydrogel.	[31,32]
	Molecular self-assembly		[33,34]
	Ionic gelation		[35]
	Electrostatic interaction		[36,37]
Chemical crosslinking	Free radical polymerization	Using chemical reactions and multi-functional monomers, the monomer compound is polymerized into a polymer to form a hydrogel.	[38]
	Photopolymerization	Under the action of specific light sources and photo initiators, hydrogels are formed by covalent bond crosslinking.	[39]
	Radiation polymerization	The water molecules in the polymer solution are irradiated by high-energy rays to produce a variety of active groups that can react with the polymer to generate polymer free radicals, and the polymer-free radicals are then crosslinked to form a network structure polymer.	[40]