| dc.description.abstract | Herein, the discovery and application of a novel class of supramolecular secondary interactions between the cycloaddition products of the inverse electron-demand Diels Alder (IEDDA) tetrazine-norbornene click reaction is described. First, a direct comparison of the bulk properties of poly(ethylene glycol) (PEG) hydrogels crosslinked with either the radical-mediated thiol-norbornene reaction or the tetrazine-norbornene reaction revealed significant and unexpected differences in the two gels. These differences in the storage moduli, swelling, and susceptibility to hydrolytic degradation of the two gel formulations could not be attributed to differences in crosslink density or crosslinking reaction efficiency. However, molecular dynamics simulations suggested the existence of supramolecular interactions between the tetrazine-norbornene click products (TNCP) which could provide additional physical crosslinking of the network. This was confirmed by the gelation of multi-arm PEG macromers end-functionalized with TNCP in the absence of covalent crosslinking. Unlike other supramolecular crosslinking moieties, TNCP can be synthesized in situ in physiologic conditions with the bio-orthogonal IEDDA reaction without the need for exogenous initiators or adjustment in pH, temperature, or ion concentrations. In situ TNCP installation was then leveraged as a mechanism for bio-orthogonal, controlled gel stiffening in a pre-existing and enzymatically-degradable covalent PEG-peptide network. Pendant TNCP groups demonstrated a concentration-dependent effect on bulk gel modulus, and TNCP installation yielded an approximately 2 kPa increase in shear storage modulus over the course of 4-6 hours. This approach had no effect on the viability of encapsulated cells and the increase in gel stiffness was long-lasting in culture conditions. Finally, TNCP-functionalized gelatin was used to create a shear-thinning, extrudable hydrogel. As in PEG-peptide hydrogels, the installation of pendant TNCP groups in covalently crosslinked gelatin networks increased the bulk shear storage modulus of the gel. Additionally, supramolecular gelatin gels were formed via installation of TNCP along norbornene-functionalized gelatin molecules. These supramolecular gel-TNCP hydrogels exhibited fitness for extrusion, demonstrating both shear thinning and rapid recovery of gel stiffness after shear. In short, this work presents the use of the bio-orthogonal IEDDA tetrazine reaction to synthesize supramolecular interacting domains in situ in hydrogel biomaterials. Future studies will serve to expand the remarkably broad potential applicability of this chemistry. | |
