Multicellularity, Culture Duration, and Hydrogel Stiffness Guide Induced Pluripotent Stem Cell-Derived Endothelial Progenitor Cell Contractility.

Human induced pluripotent stem cells (hiPSCs) offer patient-specific and immune-evasive sources for generating diverse cell types; yet lack of vascularization in hiPSC-derived tissues remains a major limitation for both therapeutic applications and disease modeling. Elucidating the mechanisms underlying vascular network formation in hiPSC-derived cells is therefore imperative. We and others have previously demonstrated that hiPSC-derived endothelial progenitor cells (hiPSC-EPs) self-assemble into lumenized microvascular networks when cultured in 3D norbornene-functionalized hyaluronic acid-based hydrogels. Herein we investigated the early period of culturing to characterize contractility of hiPSC-EPs. We hypothesized that multi-cell cooperativity would increase over time and would be dependent on the original hydrogel storage modulus. To quantify cellular contractility either 4 or 7 days after encapsulation, 3D kinematic analysis was performed on single and small multi-cell clusters of hiPSC-EPs embedded in NorHA-based hydrogels. Contractile responses were significantly and non-linearly influenced by multicellularity, culture duration, and hydrogel stiffness. Novel to this study was the observation that NorHA hydrogels exhibited compressible behaviors, with greater compressibility occurring in NorHA hydrogels with lower stiffness. Hence, the kinematic analysis was modified to incorporate separate deviatoric and volumetric strain indices. At day 7, multicellularity synergistically increased both strain components. These findings indicated that hiPSC-EP contractility and mechanical interactions with the hydrogel are governed by culture duration, multicellularity, and hydrogel stiffness; providing mechanical insight on hiPSC-EP self-assembly into microvasculature networks, a critical step towards development of functional vascular tissues for regenerative medicine and disease models.