Vancomycin- and Strontium-Loaded Microspheres with Multifunctional Activities against Bacteria, in Angiogenesis, and in Osteogenesis for Enhancing Infected Bone Regeneration

Biomaterials that have capacities to simultaneously induce bone regeneration and kill bacteria are in demand because bone defects face risks of severe infection in clinical therapy. To meet the demand, multifunctional biodegradable microspheres are fabricated, which contain vancomycin to provide antibacterial activity and strontium-doped apatite to provide osteocompatibility. Moreover, the strontium component shows activity in promoting angiogenesis, which further favors osteogenesis. For producing the microspheres, vancomycin is loaded into mesoporous silica and embedded in polylactide-based microspheres via the double emulsion technique and the strontium-doped apatite is deposited onto the microspheres via biomineralization in strontium-containing simulated body fluid. Sustained release behaviors of both vancomycin and Sr²⁺ ions are achieved. The microspheres exhibit strong antibacterial effect against Staphylococcus aureus, while demonstrating excellent cell/tissue compatibility. Studies of differentiation confirm that the introduction of strontium element strengthens the angiogenic and osteogenic expressions of mesenchymal stromal cells. Subcutaneous injection of the microspheres into rabbit's back confirms their effectiveness in inducing neovascularization and ectopic osteogenesis. Finally, an infected rabbit femoral condyle defect model is created with S. aureus infection and the multifunctional microspheres are injected, which display significant antibacterial activity in vivo and achieve efficient new bone formation in comparison with biomineralized microspheres without vancomycin loading. The vancomycin- and strontium-loaded microspheres, being biomineralized, injectable, and biodegradable, are attractive because of their flexibility in integrating multiple functions into one design, whose potentials in treating infected bone defects are highly expected.