A Novel Strategy to Enhance the Bone Healing Efficacy of Composite Scaffolds via Induction of Cell Recruitment and Vascularization

This study devised a novel strategy to develop a functionally improved scaffold that enhances the healing of large bone defects via synergistic activation of vascularization and cell recruitment. To this end, we fabricated round and ring-shaped silk fibroin/<i>Broussonetia kazinoki</i> (SFBK) composite scaffolds. The round scaffolds had a diameter of 1.5 mm, and the ring-shaped scaffolds had a 6-mm diameter with a 1.5-mm hole in the center. All scaffolds had a 3-mm thickness. A portion of round SFBKs was cross-linked with stromal cell-derived factor 1 (SDF-1), and ring-shaped scaffolds underwent in vitro angiogenic stimulation, in vivo vascularization, or both. These scaffolds were assembled by fitting a round SFBK into the center of a vascularized SFBK scaffold before implantation into a rat model with critical-sized calvarial defects. Implantation with puzzle-fitted scaffolds promoted bone regeneration, and the scaffold that underwent both SDF-1 immobilization and vascularization processes showed the greatest efficacy in the healing of defects. The bone healing efficacy of puzzle-fitted scaffolds involved their ability to stimulate microvascular network formation, collagen synthesis, and stem cell recruitment at defects. <i>B. kazinoki</i>-released calcium ions also participated in synergistic bone regeneration. These results suggest that the strategy of fitting SDF-1-linked SFBK into a vascularized ring-SFBK scaffold is useful in recruiting multipotent stem cells via newly formed blood vessels toward the center of scaffolds. This induces balanced and uniform bone regeneration. Overall, this study highlights the needs of calcium release, neovascularization, and stem cell recruitment for synergistic enhancement of bone regeneration.