Dynamic Stiffening to Improve Vasculogenesis of hiPSC-Derived Endothelial Progenitors

Multiple groups have reported on the impact of hydrogel stiffness on vascular network formation in vitro, with overall findings indicating that less stiff hydrogels better support vasculogenesis. However, the majority of this research utilized hydrogels with static stiffness, even though vasculogenesis occurs in tandem with changes in extracellular matrix material properties. To that end, we hypothesize that dynamic modulation of hydrogel stiffness during the process of vasculogenesis, recapitulating changes observed during embryonic development, would improve vascular network formation. Using our Collagen I/Norbornene-modified hyaluronic acid hydrogel system, we swelled in additional crosslinker and photoinitiator and exposed the hydrogel to UV light, enabling hydrogel stiffening at user-defined time points with no significant effect on cell viability. We observed that in situ stiffening at early time points, prior to the onset of significant cell migration, resulted in more robust vascular network formation relative to unstiffened controls, while stiffening at later time points disrupted existing vascular networks. These trends continued in in vivo experiments in nude mice, with cell-laden hydrogels stiffened at early time points resulting in improved blood flow, while those stiffened at later time points had the opposite effect. We hypothesized that this was due to differential impacts of focal adhesion kinase (FAK) activation following in situ stiffening, as supplementation with a Rho kinase inhibitor, downstream of FAK, partially reversed the effects of in situ stiffening. Taken together, this research demonstrates the benefits of incorporating dynamic cues into hydrogel design to create more physiologically relevant vasculature.