Ultrasound-activated piezoelectric Silk-PVDF hydrogel reprograms the osteoimmune microenvironment via NRF2 signaling for accelerated bone regeneration

Bone defects pose significant clinical challenges due to the limited regenerative capacity of adult bone and the shortcomings of existing biomaterials, which lack dynamic electromechanical signaling crucial for repair. Here, we present an injectable, ultrasound-responsive piezoelectric hydrogel engineered to synergize silk fibroin’s (SF) structural adaptability with polyvinylidene fluoride’s (PVDF) bioelectrical activity. Methacrylated silk fibroin (SM) enables rapid UV-triggered crosslinking via a cost-effective photoinitiator system, while electrospun PVDF nanofibers, cryosectioned into microscale units, confer dynamic piezoelectric responsiveness. Under ultrasound stimulation, PVDF generates localized electrical cues that transiently elevate reactive oxygen species (ROS), activating the NRF2 antioxidant pathway to resolve oxidative stress, which polarizes macrophages toward pro-regenerative M2 phenotypes, enhances osteogenic differentiation of adipose-derived mesenchymal stem cells (ADSCs) and promotes angiogenesis in vitro . Additionally, it was also confirmed that implantation of critical-sized femoral defects in rats could facilitate bone regeneration by micro-CT and histological analysis in vivo . This platform transcends beyond passive scaffolding by recapitulating bone’s electromechanical-immune axis offers a paradigm shift toward smart biomaterials for complex skeletal defects. The integration of PVDF’s ultrasound-triggered piezoelectricity with SM’s bioactivity establishes a multifunctional system that dynamically regulates redox homeostasis, immune modulation, and tissue remodeling, addressing unmet needs in bone tissue engineering. Structured abstract Background Critical-sized bone defects face limited regeneration due to inadequate electromechanical signaling in conventional biomaterials. Purpose To develop an injectable ultrasound-responsive piezoelectric hydrogel that dynamically regulates osteoimmune microenvironment for accelerated bone repair. Methods SM was crosslinked using Irgacure 2959/TEA/NVP system. Cryosectioned PVDF nanofibers were incorporated for piezoelectricity. In vitro studies assessed osteogenesis (ADSCs), angiogenesis (HUVECs), and macrophage polarization (RAW264.7) under ultrasound stimulation through the skin simulator (1 MHz, 100 mW/cm 2 , pulsed wave, 10 min/day). Critical-sized femoral defects (3 mm diameter) in rats were implanted with hydrogels and stimulated accordingly,then evaluated via micro-CT/histology at 4 weeks. Results PVDF generated localized electrical cues under ultrasound, activating NRF2 pathway via transient ROS elevation; Enhanced osteogenic differentiation and angiogenesis; Macrophage polarization to pro-regenerative M2 phenotype; Significant bone regeneration (BV/TV↑) with mature lamellar bone formation in rat defects. Conclusions The SM/PVDF hydrogel transcends passive scaffolding by: Recapitulating bone's native piezoelectricity through ultrasound-triggered electromechanical stimulation; Reprogramming osteoimmune microenvironment via ROS-NRF2 axis; Enabling injectable, dynamic regulation of bone healing cascades.