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

Abstract

ABSTRACT 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.