Control of thiol-maleimide reaction kinetics in PEG hydrogel networks.

Michael-type addition reactions are widely used to polymerize biocompatible hydrogels. The thiol-maleimide modality achieves the highest macromer coupling efficiency of the reported Michael-type pairs， but the resulting hydrogel networks are heterogeneous because polymerization is faster than the individual components can be manually mixed. The reactivity of the thiol dictates the overall reaction speed， which can be slowed in organic solvents and acidic buffers. Since these modifications also reduce the biocompatibility of resulting hydrogels， we investigated a series of biocompatible buffers and crosslinkers to decelerate gelation while maintaining high cell viability. We found that lowering the polymer weight percentage (wt%)， buffer concentration， and pH slowed gelation kinetics， but crosslinking with an electronegative peptide was optimal for both kinetics and cell viability. Including a high glucose medium supplement in the polymer solvent buffer improved the viability of the cells being encapsulated without impacting gelation time. Slowing the speed of polymerization resulted in more uniform hydrogels， both in terms of visual inspection and the diffusion of small molecules through the network. However， reactions that were too slow resulted in non-uniform particle dispersion due to settling， thus there is a trade-off in hydrogel network uniformity versus cell distribution in the hydrogels when using these networks in cell applications.