Novel therapeutic core-shell hydrogel scaffolds with sequential delivery of cobalt and bone morphogenetic protein-2 for synergistic bone regeneration.

Enabling early angiogenesis is a crucial issue in the success of bone tissue engineering. Designing scaffolds with therapeutic potential to stimulate angiogenesis as well as osteogenesis is thus considered a promising strategy. Here, we propose a novel scaffold designed to deliver angiogenic and osteogenic factors in a sequential manner to synergize the bone regeneration event. Hydrogel fibrous scaffolds comprised of a collagen-based core and an alginate-based shell were constructed. Bone morphogenetic protein 2 (BMP2) was loaded in the core, while the shell incorporated Co ions, enabled by the alginate crosslinking in CoCl2/CaCl2 solution. The incorporation of Co ions was tunable by altering the concentration of Co ions in the crosslinking solution. The incorporated Co ions, that are known to play a role in angiogenesis, were released rapidly within a week, while the BMP2, acting as an osteogenic factor, was released in a highly sustainable manner over several weeks to months. The release of Co ions significantly up-regulated the in vitro angiogenic properties of cells, including the expression of angiogenic genes (CD31, VEGF, and HIF-1α), secretion of VEGF, and the formation of tubule-like networks. However, BMP2 did not activate the angiogenic processes. Osteogenesis was also significantly enhanced by the release of Co ions as well as BMP2, characterized by higher expression of osteogenic genes (OPN, ALP, BSP, and OCN), and OCN protein secretion. An in vivo study on the designed scaffolds implanted in rat calvarium defect demonstrated significantly enhanced bone formation, evidenced by new bone volume and bone density, due to the release of BMP2 and Co ions. This is the first study using Co ions as an angiogenic element together with the osteogenic factor BMP2 within scaffolds, and the results demonstrated the possible synergistic role of Co ions with BMP2 in the bone regeneration process, suggesting a novel potential therapeutic scaffold system. STATEMENT OF SIGNIFICANCE: This is the first report that utilizes Co ion as a pro-angiogenic factor in concert with osteogenic factor BMP-2 in the fine-tuned core-shell hydrogel fiber scaffolds, and ultimately achieves osteo/angiogenesis of MSCs and bone regeneration through the sequential delivery of both biofactors. This novel approach facilitates a new class of therapeutic scaffolds, aiming at successful bone regeneration with the help of angiogenesis.