Table 1.

Some important aspects of different degradation mechanisms.

	Enzymatic degradation	Hydrolytic degradation	Photolytic degradation
Sensitive moieties	MMP-, plasmin-, and elastase-sensitive peptides	Ester, hydrazine, and acetal linkages	Azobenzene, o-nitrobenzyl, and coumarin
Degradation times	Hours to days	Days to weeks	Seconds to minutes
Influence factors	Sequences of enzyme-sensitive peptides, concentration and activity of enzymes	Molecular weight of monomer, hydrogel concentration and water content, solution pH, ratio of hydrolysable linkages in network backbone	Light wavelength, intensity, irradiation time, and the site of photosensitive moieties in network, the ratio of host-guest inclusion complex
Advantages	Predictable cell-mediated degradation	Mild reaction conditions without involving any trigger molecules, biocompatible byproducts	High controllability on spatiotemporal degradation, deep tissue regulation of hydrogel degradation
Disadvantages	Limited controllability on spatiotemporal degradation	Difficult to predict degradation kinetics, limited controllability on spatiotemporal degradation	Light-induced harmful effects to cells, potential toxic byproducts