Table 1.

Comparison of GelNB hydrogel crosslinking mechanisms.

Crosslinking mechanism	Gelation speed	Catalyst or Initiator	Spatial-temporal control	Potential Challenges
Longwave UV light-Initiated thiol-norbornene polymerizations	Fast	Photo-initiator	High	• Cytotoxicity of radical species • UV light exposure & light attenuation
Visible light-initiated thiol-norbornene polymerizations	Fast to moderate	Photo-initiator	High	• Cytotoxicity of radical and initiator species
Enzyme-catalyzed thiol-norbornene click reactions	Moderate	Enzyme & cofactor	Low	• Require hydrogen peroxide as a cofactor
Tetrazine-norbornene iEDDA click reactions	Moderate	None	Low	• Produce nitrogen gas