Orthogonally Crosslinked Gelatin-Norbornene hydrogels for Biomedical Applications.

The thiol-norbornene photo-click reaction has exceptionally fast crosslinking efficiency compared with chain-growth polymerization at equivalent macromer contents. The orthogonal reactivity between norbornene and thiol/tetrazine permits crosslinking of synthetic and naturally derived macromolecules with modularity, including PEG-norbornene (PEGNB), gelatin-norbornene (GelNB), among others. For example, collagen-derived gelatin contains both cell adhesive motifs (e.g., RGD) and protease-labile sequences, making it an ideal macromer for forming cell-laden hydrogels. First reported in 2014, GelNB has increasingly been used in orthogonal crosslinking of biomimetic matrices in various applications. GelNB can be crosslinked into hydrogels using multi-functional thiol linkers (e.g., dithiothreitol (DTT) or PEG-tetra-thiol (PEG4SH) via visible light or longwave UV light step-growth thiol-norbornene reaction or through an enzyme-mediated crosslinking (i.e., horseradish peroxidase, HRP). GelNB-based hydrogels can also be modularly crosslinked with tetrazine-bearing macromers via inverse electron-demand Diels-Alder (iEDDA) click reaction. In this review, we survey the various methods for preparing GelNB macromers, the crosslinking mechanisms of GelNB-based hydrogels, and their applications in cell and tissue engineering, including crosslinking of dynamic matrices, disease modeling and tissue regeneration, delivery of therapeutics, as well as bioprinting and biofabrication. This article is protected by copyright. All rights reserved.