Enzymatic Cross-Linking of Dynamic Thiol-Norbornene Click Hydrogels

Enzyme-mediated in situ forming hydrogels are attractive for many biomedical applications because gelation afforded by the enzymatic reactions can be readily controlled not only by tuning macromer compositions, but also by adjusting enzyme kinetics. For example, horseradish peroxidase (HRP) has been used extensively for in situ crosslinking of macromers containing hydroxyl-phenol groups. The use of HRP on initiating thiol-allylether polymerization has also been reported, yet no prior study has demonstrated enzymatic initiation of thiol-norbornene gelation. In this study, we discovered that HRP can generate thiyl radicals needed for initiating thiol-norbornene hydrogelation, which has only been demonstrated previously using photopolymerization. Enzymatic thiol-norbornene gelation not only overcomes light attenuation issue commonly observed in photopolymerized hydrogels, but also preserves modularity of the crosslinking. In particular, we prepared modular hydrogels from two sets of norbornene-modified macromers, 8-arm poly(ethylene glycol)-norbornene (PEG8NB) and gelatin-norbornene (GelNB). Bis-cysteine-containing peptides or PEG-tetra-thiol (PEG4SH) were used as crosslinkers for forming enzymatically and orthogonally polymerized hydrogels. For HRP-initiated PEG-peptide hydrogel crosslinking, gelation efficiency was significantly improved via adding tyrosine residues on the peptide crosslinkers. Interestingly, these additional tyrosine residues did not form permanent dityrosine crosslinks following HRP-induced gelation. As a result, they remained available for tyrosinase-mediated secondary crosslinking, which dynamically increases hydrogel stiffness. In addition to material characterizations, we also found that both PEG- and gelatin-based hydrogels provide excellent cytocompatibility for dynamic 3D cell culture. The enzymatic thiol-norbornene gelation scheme presented here offers a new crosslinking mechanism for preparing modularly and dynamically crosslinked hydrogels.