Ultrasound-Responsive Core-Shell Microspheres for On-Demand Release of Bioengineered rhBMP-2 Enhances Bone Regeneration

The clinical application of recombinant human bone morphogenetic protein-2 (rhBMP-2) is hampered by its high cost, nonspecific diffusion, and severe side effects resulting from uncontrolled burst release from carriers. To overcome these challenges, we developed an intelligent bone repair system that integrates on-demand delivery with acoustic activation. We first biosynthesized highly active rhBMP-2 cost-effectively using a yeast expression system and then encapsulated it into ultrasound-responsive chitosan/tripolyphosphate core-shell microspheres (UCC-rhBMP-2). These microspheres exhibited exceptional kinetic stability under physiological conditions (>80% retention over 21 days in vitro), yet enabled rapid, ultrasound-triggered "burst" release of the payload (>80% within 15 minutes). Furthermore, the combination of COMSOL simulation and experimental verification elucidated the underlying mechanism: ultrasound, via cavitation, can reversibly modulate the pore structure within the microspheres, thereby achieving an “on-off” controlled drug release. This programmed release effectively induces the differentiation of bone marrow mesenchymal stem cells into osteoblasts. In a rat calvarial defect model, this synergistic therapy demonstrated unprecedented bone regeneration efficacy. Micro-CT quantification revealed that the “UCC-rhBMP-2 + US” group achieved a 50.3% reduction in bone void volume and a 1.9-fold increase in new bone area at 4 weeks, significantly outperforming all control groups (p < 0.01). This study not only provides an economical and efficient strategy for rhBMP-2 delivery but also pioneers a novel paradigm for bone regeneration by actively modulating the tissue microenvironment via physical stimulation. Schematic: Schematic illustration of the Ultrasound-Responsive Microsphere Hydrogel for Bone Defect Treatment. Fabrication of rhBMP-2 protein and the ultrasound-responsive microsphere hydrogel. Application and potential mechanism of the ultrasound-responsive microporous hydrogel in repairing bone defects in a rat model.