We report here a synthetically simple yet highly tunable and diverse visible light mediated thiol-
vinyl gelation system for fabricating cell-instructive hydrogels. Gelation was achieved via a
mixed-mode step-and-chain-growth photopolymerization using functionalized 4-arm
poly(ethylene glycol) as backbone macromer, eosin-Y as photosensitizer, and di-thiol containing
molecule as dual purpose co-initiator/cross-linker. N-vinylpyrrolidone (NVP) was used to
accelerate gelation kinetics and to adjust the stiffness of the hydrogels. Visible light (wavelength:
400–700nm) was used to initiate rapid gelation (gel points: ~20 seconds) that reached completion
within a few minutes. The major differences between current thiol-vinyl gelation and prior visible
light mediated photopolymerization are that: (1) the co-initiator triethanolamine (TEOA) used in
the previous systems was replaced with multifunctional thiols and (2) mixed-mode polymerized
gels contain less network heterogeneity. The gelation kinetics and gel properties at the same PEG
macromer concentration could be tuned by changing the identity of vinyl groups and di-thiol
cross-linkers, as well as concentration of cross-linker and NVP. Specifically, acrylate-modified
PEG afforded the fastest gelation rate, followed by acrylamide and methacrylate-functionalized
PEG. Increasing NVP concentration also accelerated gelation and led to a higher network cross-
linking density. Further, increasing di-thiol peptide concentration in the gel formulation increased
hydrogel swelling and decreased gel stiffness. Due to the formation of thiol-ether-ester bonds
following thiol-acrylate reaction, the gels degraded hydrolytically following a pseudo first order
degradation kinetics. Degradation rate was controlled by adjusting thiol or NVP content in the
polymer precursor solution. The cytocompatibility and utility of this hydrogel system were
evaluated using in situ encapsulation of human mesenchymal stem cells (hMSC). Encapsulated
hMSCs remained alive (>90%) throughout the duration of the study and the cells were
differentiated down osteogenic lineage with varying degrees by controlling the rate and mode of
gel degradation
