In the past two decades, remarkable progress has been made in the development of surgical techniques for bone reconstruction, significantly improving clinical outcomes. However, major reconstruction after trauma or cancer is still limited by the paucity of autologous material and donor site morbidity. Recent advances in the field of tissue engineering have generated new approaches for restoring bone defects. In spite of this progress, the necessity of suitable blood supply to ensure cell function is a major challenge in the development of more complex and functional grafts. Many investigators have successfully demonstrated the use of different strategies including growth factor delivery and in vitro coculture of ECs and MSCs to develop vascular structures. MSCs have the ability to secrete a wide range of bioactive cytokines and growth factors that can influence nearby cells via paracrine signaling. This crosstalk between ECs and MSCs is mutually beneficial, as ECs enhance osteogenic differentiation of hMSCs through direct cell-cell contact and paracrine signaling. In the native environment of cortical bone, both cell populations, osteogenic and vasculogenic, follow a unique well-defined pattern, called osteons.

The goal of this proposed study was to develop a novel bio-inspired and vascularized bone construct, harvesting the synergistic effects of pro-angiogenic growth factor delivery and coculture of ECs and MSCs. To address this goal, we first developed mesoporous calcium deficient hydroxyapatite apatite microparticles, with biological properties closer to bone than commercially available hydroxyapatite, and capable of efficiently loading and sustainably releasing pro-angiogenic growth factors. We then demonstrated the successful fabrication of a novel bio-inspired 3DP fibrin-PCL composite scaffold, with mechanical strength comparable to bone. The utilization of these scaffolds in constructing osteons for bone regeneration demonstrated the promising capacity of the construct to improve neovascularization. In light of these results, we hypothesized that cell placement or patterning could play a critical role in neovascularization. Which lead us to investigate the role of distance between cell populations, introduced via 3D printing, in ECs/MSCs crosstalk. Our results suggested that controlling the distance between ECs and MSCs in coculture, using 3D printing, could influence angiogenesis.