FRESH Bioprinting of Dynamic Hydrazone-Cross-Linked Synthetic Hydrogels

Dynamic covalent chemistry is an attractive cross-linking strategy for hydrogel bioinks due to its ability to mimic the dynamic interactions that are natively present in the extracellular matrix. However, the inherent challenges in mixing the reactive precursor polymers during printing and the tendency of the soft printed hydrogels to collapse during free-form printing have limited the use of such chemistry in 3D bioprinting cell scaffolds. Herein, we demonstrate 3D printing of hydrazone-cross-linked poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels using the freeform reversible embedding of suspended hydrogels (FRESH) technique coupled with a customized low-cost extrusion bioprinter. The dynamic nature and reversibility of hydrazone cross-links enables reconfiguration of the initially more heterogeneous gel structure to form a more homogeneous internal gel structure, even for more highly cross-linked hydrogels, over a relatively short time (<3 days) while preserving the degradability of the scaffold over longer time frames. POEGMA hydrogels can successfully print NIH/3T3 fibroblasts and human umbilical vein endothelial cells while maintaining high cell viability (>80%) and supporting F-actin-mediated adhesion to the scaffold over a 14-day in vitro incubation period, demonstrating their potential use in practical tissue engineering applications.