Bilayer oxidized sodium alginate–carboxymethyl chitosan hydrogel microspheres enable sustained BMP-2 release for enhanced bone regeneration

Bone morphogenetic protein-2 (BMP-2) is a highly potent osteoinductive factor that has received approval from the U.S. Food and Drug Administration due to its significant osteogenic properties. Nonetheless, its clinical utility is limited by adverse effects linked to supraphysiological dosing and its brief half-life. Consequently, there is a pressing need for a safe and effective delivery system to enable the sustained release of BMP-2. In this study, we have developed bilayer-structured oxidized sodium alginate-carboxymethyl chitosan (OAC) microspheres through the application of electrospraying and the Schiff reaction. The inner layer, composed of oxidized sodium alginate, electrostatically adsorbs BMP-2, while the porous polyelectrolyte membrane on the surface enhances adsorption, thereby effectively regulating the prolonged and controlled release of BMP-2. We assessed the minimal osteogenic induction concentration of BMP-2 on rat bone marrow mesenchymal stem cells (rBMSCs) to optimize the BMP-2 loading concentration within the microspheres.<i>In vitro</i>experiments demonstrated that the bilayer membrane structure of the hydrogel microspheres significantly delayed the release of BMP-2, facilitating a long-term, sustained release. Furthermore, the microspheres facilitated the proliferation, migration, and osteogenic differentiation of rBMSCs. The osteogenic-promoting efficacy of the BMP-2-encapsulated OAC microspheres was further corroborated<i>in vivo</i>through implantation alongside calcium phosphate cement into the dorsal region of nude mice. Collectively, the BMP-2-encapsulated OAC microspheres we developed constitute a promising clinical approach to augment scaffold degradation and osteogenesis for the repair of bone defects.