Smart Hydrogel Turns Breathing and Light Into Bone-Repairing Signals

Every year, millions of people worldwide suffer bone defects caused by trauma, tumor removal, infection, or congenital malformation, and repairing large segmental gaps remains one of orthopedic surgery’s most stubborn challenges. The standard options all carry drawbacks: inert metal implants and bioceramics provide mechanical support but lack bioactivity and can loosen, become infected, or trigger stress-shielding bone resorption, while autologous bone grafts, though biologically active, are limited by donor-site morbidity and simple scarcity of supply. A team of researchers in China now reports a strikingly different approach: an injectable hydrogel that harvests the patient’s own breathing movements to generate electrical signals and, when illuminated with near-infrared light, produces gentle warmth that accelerates healing. In a rabbit model of critical-sized rib defects, the material drove rapid, vascularized bone regeneration in just four weeks, a process that conventionally takes around three months.

The new material, described in the Journal of Advanced Research, is built around black phosphorus nanosheets, a two-dimensional nanomaterial that has attracted intense interest in bone tissue engineering for a rare combination of properties. The nanosheets are intrinsically piezoelectric, meaning they convert mechanical stress into electrical signals, mimicking the endogenous electro-microenvironment that native bone generates under loading. They also convert near-infrared light into heat with high efficiency, enabling precise, non-invasive local hyperthermia. Crucially, they are biodegradable, and their degradation products are mainly phosphate ions, which serve directly as nutrient precursors for bone mineralization. The challenge has always been stability: black phosphorus reacts readily with oxygen and moisture, oxidizing and aggregating in aqueous environments, which undermines long-term biomedical performance.

To solve this, the researchers embedded the nanosheets in an injectable hydrogel made from recombinant humanized type I collagen that had been methacrylated to allow photo-crosslinking, a system they call BPNS@rhCOL1MA. The collagen, produced by microbial fermentation, shares high sequence homology with human collagen and provokes negligible immune responses, giving it an edge over hydrogels derived from decellularized animal matrices. The hydrogel’s three-dimensional network mimics the natural extracellular matrix, stabilizes the nanosheets against oxidation and aggregation, and provides a scaffold for cell adhesion, proliferation, and osteogenic differentiation. Because the material is injectable, it can minimally invasively fill irregular defect geometries, and its mechanical properties can be tuned by adjusting crosslinking density to match the surrounding bone.

Characterization confirmed the design at every scale. Transmission electron microscopy and atomic force microscopy revealed ultra-thin, few-layer nanosheets with preserved crystallinity, while zeta potential measurements showed a stable negative surface charge that prevented aggregation. Scanning electron microscopy of the composite hydrogel exposed an interconnected, biomimetic microporous architecture with the nanosheets uniformly distributed within the hydrogel walls, a porosity essential for nutrient diffusion and tissue ingrowth. Rheological testing showed that the storage modulus scaled with nanosheet concentration, giving the gel an adaptable viscoelastic profile suited to conforming to traumatic bone voids while resisting early physiological displacement. Infrared spectroscopy confirmed that the collagen’s characteristic amide bands remained intact, indicating the nanosheets act as a synergistic dopant rather than a disruptive agent.

The dual physical responses proved robust. Piezoresponse force microscopy demonstrated consistent electromechanical coupling across the scaffold, and the effective piezoelectric coefficient rose with nanosheet concentration to approximately 13.0 pC/N, far exceeding the 0.7 to 2.3 pC/N typical of native bone and comparable to leading engineered piezoelectric bone biomaterials. Under physiological-range pressures of 0.5 to 2.0 newtons, the scaffold produced stable, step-like voltage outputs, showing it can transduce ordinary biomechanical strain into bioelectric signals. On the thermal side, under 808 nm near-infrared irradiation at a moderate 1.0 W/cm2, the optimized formulation reached and held 40 to 42 degrees Celsius within five minutes, squarely within the mild-hyperthermia window known to accelerate bone healing, with a photothermal conversion efficiency of roughly 27.7 percent and stable performance over six successive on-off cycles.

In laboratory cultures of rabbit bone marrow mesenchymal stem cells, the nanosheets alone were biocompatible and mildly osteoinductive, but adding near-infrared irradiation transformed the picture. Live/dead staining and CCK-8 assays showed that photothermal activation significantly boosted cell proliferation and viability, while immunofluorescence and quantitative PCR revealed markedly stronger expression of the osteogenic markers RUNX2, ALP, OCN, and COL1 in the irradiated group than in either untreated controls or nanosheet-only cultures. The researchers propose that mild heat activates heat shock protein pathways and increases membrane fluidity and ion channel activity, which then couples with the piezoelectric signals to amplify downstream osteogenic signaling cascades such as MAPK/ERK and BMP/Smad.

Transcriptomic sequencing of stem cells cultured on the composite hydrogel with irradiation for seven days deepened the mechanistic picture. Differential expression analysis identified 3,948 upregulated and 2,755 downregulated genes relative to the plain collagen hydrogel, with enrichment in pathways governing osteogenesis, angiogenesis, and cellular responses to physical stimuli, including VEGF signaling, HIF-1 signaling, calcium ion transport, and the cellular response to heat. Western blotting validated the sequence at the protein level: elevated Piezo1 confirmed mechanosensitive signal transduction, increased HSP70 verified the thermal response, phosphorylated AKT and upregulated beta-catenin showed co-activation of the PI3K/AKT and Wnt/beta-catenin pathways, and elevated Runx2 and osteocalcin confirmed the osteogenic outcome, tracing a coherent axis from stimulus perception to bone formation.

The decisive test came in vivo. The team created a critical-sized, roughly two-centimeter segmental defect in rabbit ribs and compared four groups: untreated defects, defects filled with the plain collagen hydrogel, defects filled with the nanosheet-loaded hydrogel, and the nanosheet hydrogel plus daily transcutaneous near-infrared irradiation for one week. Because the piezoelectric effect is activated endogenously by respiratory strain, comparing the second and third groups isolated the piezoelectric contribution, while comparing the third and fourth isolated the photothermal one. After four weeks, micro-CT showed that untreated and plain-hydrogel defects had discontinuous bone connections, whereas both nanosheet groups formed robust bridging, with the irradiated group showing the highest bone mineral density, trabecular number, and superior tensile modulus, compressive modulus, and flexural strength.

Histology and molecular profiling revealed that the material accelerates endochondral ossification, the developmental route in which a cartilage template forms and is progressively replaced by vascularized bone. Untreated defects retained the most cartilage and produced the least bone, while the dual-stimulus group displayed abundant, mature, remodeled bone tissue. Chondrogenic markers SOX9 and COL2A1 declined fastest in the dual-stimulus group, indicating rapid cartilage regression and transition to bone, while MMP13 expression traced the dynamic progression from cartilage remodeling to mature bone deposition. Immunohistochemistry for CD31 confirmed enhanced angiogenesis, and collagen content rose stepwise across the groups. Safety assessments, including organ histology, serum liver and kidney markers, and blood counts, showed no systemic toxicity.

The study’s broader significance lies in showing that a single injectable platform can couple endogenous mechanical energy, harvested from something as involuntary as breathing, with externally controlled light-triggered warmth to orchestrate cellular fate, without external ultrasound or electrical devices. The authors note that among black phosphorus, piezoelectric, and photothermal bone regeneration systems reported to date, this is one of the first validated in an orthotopic, load-bearing defect with piezoelectric output activated by physiological loading alone. Future work will focus on precisely coordinating the chondrogenic and osteogenic phases under dual stimulation and advancing the material toward clinical translation for large-segment and load-bearing bone reconstruction, where a material that heals bone in weeks rather than months could transform reconstructive surgery.

Subject of Research: A piezoelectric-photothermal black phosphorus collagen hydrogel for endochondral bone regeneration

Article Title: A dual-bioeffect nanoengineered piezoelectric-photothermal hydrogel induces endochondral ossification and promotes critical-sized bone regeneration

Article References: Xu, C., Tian, B., Luo, J., Chen, L., Bai, B., Song, X., wang, X., Hu, C., Zhou, Q., Diao, Y., Zhou, G., Lei, D., Gong, L., Tang, J., & Hou, M. (2026). A dual-bioeffect nanoengineered piezoelectric-photothermal hydrogel induces endochondral ossification and promotes critical-sized bone regeneration. Journal of Advanced Research. https://doi.org/10.1016/j.jare.2026.10.025

Image Credits: AI Generated

DOI: 10.1016/j.jare.2026.10.025

Keywords: black phosphorus nanosheets, piezoelectric, photothermal, hydrogel, bone tissue engineering, endochondral ossification, recombinant human collagen, near-infrared irradiation, bone regeneration, mesenchymal stem cells, critical-sized defect, mild hyperthermia