Cross-Linking and Degradation of Step-Growth Hydrogels Formed by Thiol–Ene Photoclick Chemistry
2012
Shi, Han | Lin, Chien-Chi
Thiol–ene photoclick hydrogels have been used for a variety of tissue engineering and controlled release applications. In this step-growth photopolymerization scheme, four-arm poly(ethylene glycol) norbornene (PEG4NB) was cross-linked with dithiol containing cross-linkers to form chemically cross-linked hydrogels. While the mechanism of thiol–ene gelation was well described in the literature, its network ideality and degradation behaviors are not well-characterized. Here, we compared the network cross-linking of thiol–ene hydrogels to Michael-type addition hydrogels and found thiol–ene hydrogels formed with faster gel points and higher degree of cross-linking. However, thiol–ene hydrogels still contained significant network nonideality, demonstrated by a high dependency of hydrogel swelling on macromer contents. In addition, the presence of ester bonds within the PEG-norbornene macromer rendered thiol–ene hydrogels hydrolytically degradable. Through validating model predictions with experimental results, we found that the hydrolytic degradation of thiol–ene hydrogels was not only governed by ester bond hydrolysis, but also affected by the degree of network cross-linking. In an attempt to manipulate network cross-linking and degradation of thiol–ene hydrogels, we incorporated peptide cross-linkers with different sequences and characterized the hydrolytic degradation of these PEG-peptide hydrogels. In addition, we incorporated a chymotrypsin-sensitive peptide as part of the cross-linkers to tune the mode of gel degradation from bulk degradation to surface erosion.
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