Crosslinker length dictates step-growth hydrogel network formation dynamics and allows rapid on-chip photoencapsulation

Abstract Hydrogels formed via free radical-mediated thiol-ene step-growth photopolymerization have been developed for a broad range of tissue engineering and regenerative medicine applications. While the crosslinking mechanism of thiol-ene hydrogels has been well-described, there has been only limited work exploring the physical differences among gels arising from variations in crosslinker properties. Here, we show that the character of linear polyethylene glycol (PEG) dithiols used to crosslink multi-arm polyethylene glycol norbornene (PEGNB) can be used as a facile strategy to tune hydrogel formation kinetics, and therefore the equilibrium hydrogel network architecture. Specifically, we report the dramatic effect of crosslinker length on PEGNB hydrogel formation kinetics and the formed hydrogel properties. It is shown that the hydrogel formation kinetics and formed hydrogel properties can be tuned by solely varying the crosslinker length. It was hypothesized that under identical reaction conditions, a more accessible 3.5 k PEG dithiol crosslinker would improve network ideality relative to a shorter 1.5 k crosslinker. Longer linkers consequently promote significantly more rapid macromer crosslinking and therefore gelation. Accelerated gel formation satisfies an urgent unmet need for rapid polymerization in droplet microfluidics. Using long linkers, we demonstrate the ability to photopolymerize PEGNB microgels under flow on a microfluidic chip, with reliable control over microgel size and shape in a high-throughput manner. To further validate the potential of this platform to produce novel, microstructured cell carrier vehicles, 3T3 fibroblasts were successfully encapsulated and cultured over 14 days with excellent cell viability. This study demonstrates that PEGNB hydrogel dynamics could be readily customized to fulfill a variety of needs in tissue engineering, controlled cell delivery, or drug release applications.