Ultrasound‐Mediated OFF‐to‐ON Switching of Bone Regeneration via a Bioorthogonal Cell‐Capture Hydrogel, Enabling Spatiotemporal Control and Monitoring

Repairing critical-sized bone defects remains a formidable challenge in orthopedics, primarily due to the limitations of current stem cell-based therapies, including poor engraftment, limited real-time monitoring of cell fate, and a lack of spatiotemporal control over therapeutic activity. To address these challenges, in this study, we present a bioorthogonal hydrogel platform engineered for precise stem cell capture and ultrasound-triggered bone regeneration. Bone marrow mesenchymal stem cells (BMSCs) were dual-functionalized with trans-cyclooctene (TCO) and biogenic gas vesicles (GVs) via a metabolic glycoengineering approach. The resulting BMSCs-GV-TCO were efficiently and covalently captured within tetrazine-functionalized hydrogels at the defect site through a rapid inverse electron-demand Diels-Alder reaction. This strategy ensured targeted cell homing and retention. The GVs served as a potent acoustic contrast agent, enabling noninvasive and real-time ultrasound monitoring of cell delivery and localization. Furthermore, the unique ultrasound-responsive property of GVs enabled their on-demand collapse, acting as a remote-controlled switch to precisely upregulate osteogenic transcription factors and enhance bone regeneration, as validated in a rat critical-sized calvarial defect model. This study presents a novel, efficient, and traceable strategy for stem cell-based therapy, with broad potential to advance the treatment of bone defects and the clinical translation of regenerative medicine.