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

The core of the new platform is a hollow bimetallic nanoparticle, or HBNP, produced through a well-established trick of nanochemistry known as galvanic replacement. The researchers began with silver nanoparticles acting as sacrificial templates. When these silver particles are exposed to a gold salt solution, a spontaneous electrochemical exchange takes place: gold atoms deposit onto the silver surface while silver atoms dissolve away. The result is a porous, hollow shell of mixed gold and silver that inherits the outer dimensions of the original template but carries a cavernous interior. That hollow cavity is far more than an aesthetic curiosity. It dramatically boosts the particle’s interaction with light and provides substantial internal volume for carrying drug molecules, effectively turning each nanoparticle into a microscopic cargo hold surrounded by a light-absorbing metal wall.

Why hollow and why bimetallic? The answer lies in the physics of surface plasmons, the collective oscillations of conduction electrons that metal nanoparticles exhibit when struck by light. Solid gold spheres absorb strongly in the green part of the visible spectrum, which is poorly suited to biomedical use because visible light barely penetrates tissue. Hollow structures, however, shift this absorption dramatically toward the near-infrared region, a spectral window often called the biological window, where light travels through skin and blood with comparatively little loss. The gold-silver combination adds further advantages: silver contributes to tuning the plasmon resonance and lowering material cost, while gold lends chemical stability and biocompatibility. Earlier work by some of the same authors had already shown that hollow gold-silver nanoparticles can sustain high photothermal stability, making them natural candidates for repeated heating cycles.

Onto these hollow shells the team grafted poly(N-isopropylacrylamide), or PNIPAM, a polymer with a famous party trick. Below roughly 32 degrees Celsius, PNIPAM chains in water are hydrated and extended, forming an open, swollen network that allows small molecules to diffuse through freely. Warm the polymer past its lower critical solution temperature and it abruptly collapses, expelling water and shrinking into a dense, hydrophobic globule. This sharp, reversible transition has made PNIPAM a workhorse of thermoresponsive materials science for decades. In the new nanocomposite, the polymer acts as a programmable gatekeeper around the hollow metal core: open and permeable at body temperature or below, but capable of being squeezed shut, or squeezed open, depending on how heat is applied to the system.

The drug delivery experiments revealed a nuanced picture of how these gates behave. The nanocomposites efficiently loaded doxorubicin, a widely used chemotherapy agent that intercalates into DNA and halts cell division. When the loaded particles were placed in acidic conditions mimicking the environment inside tumors and cellular endosomes, pH 5.5, they released 84 percent of their cargo within 24 hours. Surprisingly, at physiological pH 7.4 the particles still released 76 percent over the same period, indicating that acidity alone is not the dominant switch in this design. Near-infrared irradiation, meanwhile, did not act as the primary trigger for release, but it significantly accelerated the kinetics: under light, the total released fraction climbed to 90 percent within the same 24-hour window. The mechanism is photothermally induced collapse of the PNIPAM shell. As the plasmonic core absorbs the light and converts it to heat, the polymer network contracts and relaxes in cycles, pumping drug molecules out of the hollow interior far faster than passive diffusion alone would allow.

That distinction between trigger and accelerator matters for how the platform would actually be used in a clinical setting. Because the polymer shell modulates the rate rather than the total extent of release, a clinician could in principle control how quickly chemotherapy is delivered to a tumor simply by adjusting the timing and intensity of light exposure, while the acidic tumor microenvironment provides a baseline bias toward unloading at the target site. The reversible nature of the PNIPAM transition also means the gate can open and close repeatedly as the light is pulsed, a feature that earlier opto-mechanical nanoshell-polymer composites first hinted at more than two decades ago and that this work now implements in a bimetallic, drug-loaded format.

Biological testing in two cancer cell lines, HeLa cervical cancer cells and MDA-MB-231 breast cancer cells, delivered encouraging results on several fronts. The bare nanocomposites showed low intrinsic cytotoxicity, meaning the particles themselves are relatively benign to cells, an essential property for any delivery vehicle. The cells also took up the nanoparticles efficiently, a prerequisite for any intracellular therapy. Most strikingly, the doxorubicin-loaded particles killed cancer cells more effectively than the same dose of free drug, and that cytotoxicity was amplified further when the cultures were irradiated with near-infrared light. The authors attribute this synergy to the combined effects of localized hyperthermia, which stresses and sensitizes the tumor cells, and accelerated intracellular drug release, which floods the cell with chemotherapy precisely when heat has weakened its defenses.

This chemo-photothermal synergy addresses one of the central frustrations of conventional chemotherapy. Free doxorubicin circulates through the entire body, damaging heart tissue and other healthy organs along with the tumor, and its effectiveness is often blunted by the emergence of drug resistance. Nanocarriers promise to concentrate the drug where it is needed, and light-triggered systems add a second layer of control, ensuring the payload is unleashed only at the illuminated site. The photothermal heating itself is therapeutic: temperatures reached by plasmonic nanoparticles under near-infrared illumination can ablate tumor cells directly, and clinical pilot studies with gold nanoshells have already demonstrated the feasibility of nanoparticle-mediated photothermal ablation in human prostate cancer. Combining that ablative heat with synchronized chemotherapy in a single particle, as the new platform does, could allow lower drug doses and shorter treatment courses.

The design also builds thoughtfully on a decade of related work. Thermosensitive lipid-coated hollow gold nanoshells have been explored for pancreatic cancer, PNIPAM-coated gold nanorods mediated by thiolated chitosan have shown thermo-pH responsiveness, and iron oxide nanoparticles coated with PNIPAM have been developed for dual-responsive doxorubicin delivery alongside magnetic resonance imaging. What distinguishes the new system is the integration of a hollow bimetallic core, whose galvanic synthesis is scalable and well understood, with a directly grafted thermoresponsive shell whose collapse is driven by the core’s own photothermal conversion. The particle is simultaneously the heater, the cargo hold, and the thermostat for its own release mechanism, an elegant division of labor packed into a structure only tens of nanometers across.

Significant hurdles remain before such particles reach patients, including long-term biodistribution and clearance studies, immunological profiling, and the challenge of delivering sufficient near-infrared light to deep-seated tumors. The authors note that no external datasets were generated or analyzed in the study, and the work remains at the in vitro stage. Yet the platform’s modularity is its strength: the galvanic replacement synthesis can in principle accommodate different metal combinations and template sizes, tuning the plasmon resonance across the near-infrared window, while the polymer shell chemistry can be adapted to carry other drugs or targeting ligands. As a demonstration that a single nanocomposite can heat on command, gate its own drug release thermally, and outperform free chemotherapy in cancer cells, the PNIPAM-coated hollow bimetallic nanoparticle marks a compelling step toward combination therapies that are precisely timed, locally confined, and light-controlled.

Subject of Research: Thermoresponsive PNIPAM-coated hollow bimetallic nanoparticles for photothermal-enhanced drug delivery and combined chemo-photothermal cancer therapy

Article Title: Design of thermoresponsive PNIPAM-coated hollow bimetallic nanocomposites for photothermal-enhanced drug delivery and chemo-photothermal therapy

Article References: Tovar, S., Velasco, B., Cambón, A., Carrillo-Torres, R. C., & Taboada, P. (2026). Design of thermoresponsive PNIPAM-coated hollow bimetallic nanocomposites for photothermal-enhanced drug delivery and chemo-photothermal therapy. Journal of Nanoparticle Research, 28(10), Article 256. https://doi.org/10.1007/s11051-026-06779-8

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06779-8

Keywords: hollow bimetallic nanoparticles, PNIPAM, photothermal therapy, doxorubicin, drug delivery, galvanic replacement, near-infrared irradiation, thermoresponsive polymers, chemo-photothermal therapy, HeLa cells, MDA-MB-231 cells, nanomedicine