Hydroxyapatite-coated gelatin microribbon scaffolds induce rapid endogenous cranial bone regeneration in vivo

Hydroxyapatite (HA) has a composition similar to mineral bone and has been used for coating macroporous scaffolds to enhance bone formation. However, previous macroporous scaffolds did not support minimally invasive delivery. Our lab has reported on gelatin-based microribbon (μRB) shaped hydrogels, which combine injectability with macroporosity and support cranial bone formation in an immunocompromised mouse model. However, gelatin alone was not sufficient to support cranial bone formation in immunocompetent animals. To overcome this challenge, here we evaluated two methods to incorporate HA into gelatin μRB scaffolds using either modified simulated body fluid (mSBF) or commercially available HA nanoparticles (HAnp). HA incorporation and distribution were characterized using scanning electron microscopy and energy-dispersive X-ray spectroscopy. While both methods enhanced MSC osteogenesis and mineralization, the mSBF method led to undesirable reduction in mechanical properties. HAnp-coated μRB scaffolds were further evaluated in an immunocompetent mouse cranial defect model. Acellular HAnp-coated gelatin μRB scaffolds induced rapid and robust endogenous cranial bone regeneration as shown by MicroCT imaging and histology. Co-delivery with exogenous MSCs led to later bone resorption accompanied by increased osteoclast activity. In summary, our results demonstrate the promise of gelatin μRBs with HAnps as a promising therapy for cranial bone regeneration without the need for exogenous cells or growth factors.