Synthetically Tractable Click Hydrogels for Three-Dimensional Cell Culture Formed Using Tetrazine–Norbornene Chemistry

The implementation of bio-orthogonal click chemistries is a topic of growing importance in the field of biomaterials, as it is enabling the development of increasingly complex hydrogel materials capable of providing dynamic, cell-instructive microenvironments. Here, we introduce the tetrazine-norbornene inverse electron demand Diels-Alder reaction as a new cross-linking chemistry for the formation of cell laden hydrogels. The fast reaction rate and irreversible nature of this click reaction allowed for hydrogel formation within minutes when a multifunctional PEG-tetrazine macromer was reacted with a dinorbornene peptide. In addition, the cytocompatibility of the polymerization led to high postencapsulation viability of human mesenchymal stem cells, and the specificity of the tetrazine-norbornene reaction was exploited for sequential modification of the network via thiol-ene photochemistry. These advantages, combined with the synthetic accessibility of the tetrazine molecule compared to other bio-orthogonal click reagents, make this cross-linking chemistry an interesting and powerful new tool for the development of cell-instructive hydrogels for tissue engineering applications.