From PhysOrg: "Regenerative medicine therapies often require the growth of functional, stable blood vessels at the site of an injury. Using synthetic polymers called hydrogels, researchers [have] been able to induce significant vasculature growth in areas of damaged tissue. ... Because hydrogels are very compatible with biological tissues, they are a promising therapeutic delivery vehicle to improve treatment of peripheral artery disease, ischemic heart disease, and survival of cell and tissue transplants. ... the researchers incorporated specific chemical cross-links into the gels so that they would maintain their structural integrity and only degrade in the presence of enzymes called matrix metalloproteinases that are typically expressed by invading cells. They also incorporated into the matrices a protein, vascular endothelial growth factor (VEGF), which stimulates the growth of blood vessels. ... Incorporating these cross-links controlled the release of VEGF from the matrix so that VEGF was only released as the matrix was digested by invading cells ... With the degradable implant that included growth factors, after two weeks we saw that new vessels were growing into and around the implant."