. 2022 Jan 27;8(2):379–405. doi: 10.1021/acsbiomaterials.1c01145

Table 1. Summary of Common Hydrogels, Their Cross-Linking Methods, and Main Properties Used for SMTE.

hydrogel	natural/synthetic	thermal gelation	cross-linking agents	main properties	ref
collagen	natural	T = 37 °C	genipin, microbial transglutaminase	cell adhesives sites
collagen	natural	T = 37 °C	genipin, microbial transglutaminase	highly biodegradability	(32, 33, 36)
gelatin	natural	T < 30–35 °C	genipin, microbial transglutaminase	cell adhesive sites	(34−36)
				low immunogenicity compared to collagen
				highly biodegradability
fibrinogen	natural		thrombin/CaCl₂	cell adhesive sites	(5, 38, 39)
				pro-angiogenic properties
				binding sites for myogenic factors (bFGF-2, IGF-1, VEGF)
dECM hydrogel	natural	T = 37 °C	cross-linking methods depending on the hydrogel functionalization (e.g., UV light irradiation in case of methacrylation process)	recapitulation of biological and physical properties of tissue-specific native ECM	(41)
dECM hydrogel	natural	T = 37 °C		highly biodegradability	(41)
alginate	natural		CaCl_2, MgCl_2, SrCl_2, BaCl₂	instantaneous cross-linking in mild condition	(42−44)
				functionalization with RGD-motifs
				nonbiodegradable
PEG	synthetic		cross-linking methods depending on the hydrogel functionalization (e.g., UV light irradiation in case of diacrylation process)	easily functionalized with the addition of groups acrylate, thiol, vinyl sulfone, amine, and carboxyl	(36, 47)
PEG	synthetic			functionalization with proteins and peptides	(36, 47)
PAAm	synthetic		metal ions (Al³⁺, Cr³⁺), organic systems (phenol-formaldehyde), N-methylenebis(acrylamide)	functionalization with proteins and peptides	(48, 50, 58)
PAAm	synthetic			tunable mechanical properties by changing the ratio of acrylamide to bis-acrylamide components in the copolymer	(48, 50, 58)
PEGDA	synthetic		UV light irradiation in the presence of photoinitiator	functionalization with proteins and peptides	(51−53)
				low degradation rate and immunogenicity
				tunable mechanical properties by changing molecular weight/concentration of the polymer
GelMA	semisynthetic	T < 30–35 °C	UV light irradiation in the presence of photoinitiator	retainment of mostly cell adhesive sites from gelatin	(54−56)
GelMA	semisynthetic	T < 30–35 °C	UV light irradiation in the presence of photoinitiator	tunable mechanical properties and porosity according to the degree of methacrylation and cross-linking condition	(54−56)