Vascular induction and cell infiltration into peptide-modified bioactive silk fibroin hydrogels

In hydrogel-based soft tissue engineering, vascular induction into a hydrogel as well as long-term volume retention is essential to maintain tissue shape and function without causing necrosis in the deeper part of the hydrogel. A silk fibroin (SF) hydrogel shows a sufficiently high mechanical strength to maintain its shape during implantation for a month, but it has not been well evaluated whether it has vascular-inducing bioactivity to achieve its replacement by vascularized tissues. Here, we produced a vascular-inducing peptide (VIP) containing an endothelial cell (EC)-adhesive REDV and vascular endothelial growth factor-mimicking QK peptides to modify the SF hydrogel. In vitro experiments showed that the modification of the SF hydrogel with VIP changed only biological properties of the hydrogel due to the bioactivity of VIP. Subcutaneous implantation of SF hydrogels in rats revealed isotropic EC migration into the hydrogels, which was followed by infiltration of macrophages and fibroblasts. Since these macrophages and fibroblasts appeared to degrade the SF network and to produce collagen, respectively, SF hydrogels were replaced gradually by regenerated tissues. VIP accelerated cell infiltration and doubled the formation of blood vessels in the regenerated tissue. These results suggest the potential of the VIP-modified SF hydrogel as a material for soft tissue engineering applications.