Table 1.

Bio-orthogonal chemistries to crosslink hydrogels.

Gelation Chemistry	Reactive Group #1	Reactive Group #2	Reaction Product	Notes	References
Copper-catalyzed azide-alkyne cycloaddition (CuAAC)				Potential Cu2+ toxicity may necessitate use of chelating ligands	[24]
				X: H (Slow reaction kinetics) X: F (Improved kinetics)	[25, 27–30, 123, 124]
Strain-promoted azide-alkyne cycloaddition (SPAAC)					[31–33]
					[34–37, 162]
Inverse electron demand Diels-Alder (IED-DA)					[51–54, 132]
				Extremely rapid gelation kinetics	[56]
Diels-Alder (DA)				Maleimides can cross-react with thiols	[58–68, 70, 71]
				Reversible under physiological conditions	[74]
Staudinger ligation				Slow reaction kinetics	[35, 75, 76]
Nitrile Oxide Cycloaddition				Highly reactive nitrile oxide must be generated in situ	[78, 79]