Table 2. Click reactions for hydrogel for hydrogel formation. Comparison of important click reactions typically used for formation of cell compatible hydrogels.

Click reactions	Reacting functional groups	Reaction conditions221	Key features	Applications
CuAAC	Azide and alkyne	pH 4–12, reaction time <1 h, Cu catalyst required	– Bioorthogonal	Cell encapsulation and delivery,217 drug delivery,223,224 2D cell culture225
			– Reversible
			– Difficulties with complete removal of cytotoxic Cu
SPAAC	Cyclooctyne and azide	pH 7.4, reaction time <1 h	– No catalyst required	Cell encapsulation,230,231 3D cell culture216,218
Diels–Alder	Conjugated diene and substituted alkene	pH 5.5–6.5, reaction time <8 h	– No catalyst required	Cell encapsulation and release,234 controlled cargo delivery235
			– Longer reaction time than most of the other click reactions
Inverse electron demand Diels–Alder	Dienophile and diene	pH 7.4, reaction time <5 min	– Faster rate of reaction than many other Cu-free click reactions	Live cell imaging,238 drug targeting,239 cell surface protein labeling240
			– No catalyst required no catalyst required
Thiol–ene	Thiol and unsaturated functional group (radical mediated)	pH 6–8, reaction time <1 h	– Spatiotemporal control possible with select chemistries and using a photoinitiator	Cell encapsulation,149,150 degradable 3D cell culture147,246
Michael addition	Thiol and α,β-unsaturated carbonyl group	pH 6–8, reaction time <30 min	– No catalyst required	Cell encapsulation,157,160,250 controlled cargo delivery29,248
			– Reversible
Oxime	Aminooxy and aldehyde/ketone	pH 6–8, reaction time <30 min	– No catalyst required	Cell encapsulation,251 protein immobilization253