Spatiotemporalized Hydrogel Microspheres Promote Vascularized Osteogenesis via Ultrasound Oxygen Delivery

Disturbance of spatiotemporal oxygen balance is the main cause of delayed healing or nonhealing of large bone defects. The accurate administration of oxygen to regulate disruptions in the spatiotemporal oxygen equilibrium during 9 h of hypoxia is imperative for bone tissue regeneration. Herein, oxygen-loaded nanobubbles prepared by double emulsification are successfully embedded in GelMA/HepMA microsphere macromolecular meshwork by microfluidic techniques, and a spatiotemporalized hydrogel microsphere is constructed by noncovalently binding bone morphogenetic protein 2 (BMP-2). The spatiotemporalized hydrogel microspheres precisely “remote control” oxygen release by ultrasound in vitro 9 h after bone injury to regulate spatiotemporal oxygen homeostasis disorder, maintain a high level of vascular endothelial growth factor (VEGF) expression, and accelerate bone repair. The spatiotemporalized hydrogel microspheres possess good oxygen-carrying capacity and ultrasonic responsiveness, and the oxygen concentration increases to 1.63, 1.95, 2.11, and 2.29 times under the ultrasound action at different intensities of 1, 2, 3, and 4 W, respectively, providing the conditions for the precise regulation of spatiotemporal oxygen balance disorder by ultrasound. In the in vitro hypoxia model and in vivo rat femoral defect model, the spatiotemporal hydrogel microspheres show good vascularization and osteogenesis capabilities, which provide a new strategy for the clinical treatment of large bone defects.