Hierarchically Reinforcing Hydrogel for Multidimensional Bone Homeostasis Modulation to Regenerate Osteoporotic Defect

To balance the requirement of high mechanical properties and convenient injectability for the hydrogel design of osteoporotic bone defects (OBD) repair, a hierarchical hydrogel stiffening strategy is proposed through the synergetic utilizing of rapidly formed dynamic hydrazone crosslinking, high-strength double-bond chemical crosslinking, and contraction of activated shape-memory short fibers (SMSFs). In detail, the Schiff Base reaction mediated first network enabled stable precursor retention at the injection site without compromising injectability. Subsequent radical polymerization by photocrosslinking enhanced the hydrogel stiffness from 9.27 ± 0.77 kPa to 26.49 ± 3.48 kPa. Lastly, the shape memory effect (SME) induced contraction of the thermally activated SMSFs further densified the hydrogel network, ultimately elevating the stiffness to 53.40 ± 3.40 kPa. In vitro and in vivo studies confirm that this hierarchically stiffened hydrogel exhibited excellent biocompatibility and promoted osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) through mechanotransduction related Ras homologgene family, member A (RhoA)/a Rho-associated coiled coil-forming protein kinase (ROCK) pathway. The simultaneous releasing of alendronate (ALN) further enhanced osteogenesis in BMSCs. Notably, osteogenically differentiated BMSCs also effectively suppressed the osteoclastogenesis of macrophages. Thus, this injectable hydrogel, integrating hierarchical stiffening and ALN delivery, presents a promising therapeutic strategy for restoring bone homeostasis to accelerate regeneration in osteoporotic defects.