PEG-Collagen Interpenetrating Networks Support Enhanced Vasculogenic Self-Assembly and Impact Cell-Mediated Remodeling.

The biophysical cues of natural and synthetic hydrogels, including stiffness and the rate of cell-mediated degradation, are often tuned to better understand how to form vessel networks in tissue constructs. Interpenetrating networks (IPN) combine the bioactivity and fibrillar architecture of naturally derived hydrogels and the tunability of synthetic hydrogels. We developed a poly(ethylene glycol) (PEG)-collagen (type I) IPN to investigate the interactive effects of stiffness, the rate of proteolytic degradation, and a fibrillar collagen network on the formation of microvascular networks and cell-mediated hydrogel remodeling. Endothelial cells and fibroblasts were encapsulated in the PEG-collagen IPN, wherein the initial stiffness and rate of degradation were controlled by matrix metalloproteinase-sensitive peptide cross-linker concentration and identity, respectively. We found increased vascular network assembly in PEG-collagen IPN hydrogels that were stiff and slowly degrading and decreased cell-mediated stiffening in hydrogels that were soft and more rapidly degrading compared to PEG hydrogels. Collagen in the IPN was rapidly remodeled by the cells. In both PEG-only and IPN conditions, we found that the cells made the hydrogels more viscoelastic over the course of the experiment. To test if these results were due to the bioactivity or fibrillar architecture of collagen, we evaluated materials where collagen was not fully cross-linked or was added as dry-spun fibers. Unlike the IPN, both materials were less supportive of vasculogenic assembly and did not lead to a reduction in cell-mediated stiffening, suggesting that collagen's fibrillar network is important for increasing vasculogenic potential. Taken together, these results highlight the important interactions of matrix stiffness, degradability, and fibrillar architecture in the design of hydrogels to support vascularization.