4D Biofabrication of Magnetically Augmented Callus Assembloid Implants Enables Rapid Endochondral Ossification via Activation of Mechanosensitive Pathways

This study introduces a 4D biofabrication approach combining osteogenic callus assembloids with magnetic nanoparticle (MNP) augmentation to accelerate endochondral ossification. Scaffold-free cartilage–bone assembloids containing hypertrophic chondrocytes and osteoprogenitors were dynamically stimulated within a magnetically responsive extracellular matrix. Remote magnetic actuation induced cyclic strain and calcium flux, activating TRPV4- and Piezo1-dependent mechanosensitive signaling and promoting VEGF secretion, vascular invasion, and mineralized matrix formation. In murine segmental-defect models, magnetically actuated implants achieved near-complete bone bridging within four weeks, outperforming non-magnetized controls. The findings establish magnetically augmented callus assembloids as smart, stimulus-responsive implants capable of autonomous remodeling and vascularized bone regeneration.