The discovery of tetrazine click‐induced secondary interactions is reported as a promising new tool for polymeric biomaterial synthesis. This phenomenon is first demonstrated as a tool for poly(ethylene glycol) (PEG) hydrogel assembly via purely non‐covalent interactions and is shown to yield robust gels with storage moduli one to two orders of magnitude higher than other non‐covalent crosslinking methods. In addition, tetrazine click‐induced secondary interactions also enhance the properties of covalently crosslinked hydrogels. A head‐to‐head comparison of PEG hydrogels crosslinked with tetrazine‐norbornene and thiol‐norbornene click chemistry reveals an approximately sixfold increase in storage modulus and unprecedented resistance to hydrolytic degradation in tetrazine click‐crosslinked gels without substantial differences in gel fraction. Molecular dynamic simulations attribute these differences to the presence of secondary interactions between the tetrazine‐norbornene cycloaddition products, which are absent in the thiol‐norbornene crosslinked gels.

Progressive stiffening of the extracellular matrix (ECM) is observed in tissue development as well as in pathologies such as cancer, cardiovascular disease, and fibrotic disease. However, methods to recapitulate this phenomenon in vitro face critical limitations. Here, we present a poly­(ethylene glycol)-based peptide-functionalized ECM-mimetic hydrogel platform capable of facile, user-controlled dynamic stiffening. This platform leverages supramolecular interactions between inverse-electron demand Diels–Alder tetrazine–norbornene click products (TNCP) to create pendant moieties that undergo non-covalent crosslinking, stiffening a pre-existing network formed via thiol–ene click chemistry over the course of 6 h. Pendant TNCP moieties have a concentration-dependent effect on gel stiffness while still being cytocompatible and permissive of cell-mediated gel degradation. The robustness of this approach as well as its simplicity and ease of translation give it broad potential utility.