Development of synthetic, biomimetic extracellular matrices to direct bone tissue regeneration.

Abstract

The prevalence of bone loss due to injury, disease and birth defects is a significant problem in the U.S., with costs of musculoskeletal conditions representing 12.7% of annual health care expenditures ($120 billion), and over 600,000 inpatient fracture reduction procedures performed annually. Contemporary bone replacement therapies are severely limited and invoke novel approaches. This thesis focuses on creation of biomimetic systems to direct regeneration of natural bone tissue, a promising new approach to bone tissue replacement. A bioinspired method for synthesis of a bone-like mineral film on biodegradable polymers has been developed and mechanistically characterized. Results indicate that heterogeneous mineral growth is driven by calcium binding to surface functional groups in the presence of phosphate counterions. Calcium binding is independently regulated by surface carboxyl/hydroxyl groups and phosphate ions in solution, and the effect of these regulators is similar in magnitude. These results suggest that the mechanism for mineral growth is more complex than simple electrostatic interactions, highlighting the inherent complexity of natural biomineralization processes. Mineral growth was then accomplished within a biodegradable, macroporous, three-dimensional scaffold to create a synthetic, biodegradable analog of the natural bone ECM for bone regeneration. Mineralized poly(lactide-co-glycolide) scaffolds displayed increased mechanical properties and enhanced osteoconductivity, consistent with previous studies using mineralized orthopaedic implant materials and bioactive glasses. In addition, the presence of a bone-like mineral film led to a 53% increase in bone regeneration in a rat cranial defect model. Mineralized scaffolds were also engineered to release vascular endothelial growth factor (VEGF) in a sustained manner to direct ingrowth of vascular tissue in concert with bone tissue regeneration. Sustained VEGF release induced rapid angiogenesis in both a rat cranial defect, and a subcutaneous pocket in SCID mice. Sustained VEGF release also led to an enhancement in mineralized tissue regeneration in the rat cranial defect, indicating that mineral presence and induced angiogenesis concertedly enhance bone tissue regeneration. Further, VEGF delivery led to enhanced differentiation of human mesenchymal stem cells implanted subcutaneously, suggesting that VEGF is involved in early osteogenic differentiation of bone precursor cells.