Promotion and control of angiogenic activity in poly(ethylene glycol) diacrylate hydrogels for tissue engineering applications

Promotion and control of angiogenic activity have been achieved by covalently linking an angiogenic-signaling protein or peptide to poly(ethylene glycol) diacrylate hydrogels, thereby yielding a system which can be used to develop vascularized, functional engineered tissues. The field of tissue engineering has the capability of providing greatly needed biological organs and tissues for transplantation therapy. In living tissues of physiologically relevant size, metabolically active cells far from a nutrient source undergo necrosis when starved of oxygen and vital nutrients, but the incorporation of microvasculature would allow the production of functional tissues larger than 200 µm, the diffusion limit of oxygen through tissue. By combining a supporting matrix material, signaling factors, and cells, an environment has been created to support the formation and control of endothelial tubes. Poly(ethylene glycol) (PEG)-based hydrogels, which are hydrophilic and resistant to protein adsorption and subsequent non-specific cell adhesion, were modified to contain cell-adhesive ligands and growth factors to support cell and tissue function. Human endothelial cells were cultured in these systems in vitro. Covalent immobilization of vascular endothelial growth factor (VEGF) was shown to promote endothelial cell angiogenic activity, including migration, cell-cell contact formation, and tubule formation, in PEG hydrogels in two and three dimensions. Furthermore, patterning the micron-scale spatial presentation of cell-adhesive ligands and VEGF controlled and accelerated tubule formation. Endothelial tubules formed on restricted patterns less than 70 µm wide showed increased expression of angiogenic receptors VEGFR1, VEGFR2, and EphA7, in addition to increased production and secretion of laminin, a tubule-associated extracellular matrix protein. The incorporation of QK, a synthetic angiogenic peptide, was also shown to promote endothelial cell proliferation and tubulogenesis in two and three dimensions in vitro at levels comparable to those achieved through signaling by VEGF. This work improves upon previous research on angiogenic growth factor release from tissue engineering matrices by showing that localized, covalently-bound signaling can promote angiogenesis, thereby providing an engineered, predictable response in a local environment without systemic effects. Additionally, these findings provide a valuable method to incorporate capillary networks throughout tissue engineering matrices to support the continued development of functional tissue-engineered products for clinical use