A hydrogel engineered with cell-like particles mimicking skin tissue achieves 99.9% compressibility and ultralow energy loss while generating its own voltage to sense strain without batteries.
(Nanowerk Spotlight) Hydrogels should be the ideal material for wearable sensors. These polymer networks, swollen with water to comprise up to 90% of their mass, can stretch, conduct ions, and interface seamlessly with biological systems. Yet a persistent problem has blocked their path to widespread use: the mechanical properties needed for real-world applications work against each other.
Toughness requires sacrificial chemical bonds that snap under stress and reform afterward. But this breaking and reforming creates internal friction, which manifests as hysteresis: energy that goes into deforming the material but never comes back out. A hydrogel with high hysteresis recovers slowly, drifts in its measurements, and wears out quickly.
Engineers have tried layering polymer networks, embedding nanoparticles, and introducing crystalline domains. Each strategy improved one property while degrading others. Compressibility proved especially difficult; most hydrogels fail when squeezed beyond 90% of their original thickness.
Epithelial tissue offers a clue to solving this problem. In skin and other organs, epithelial cells pack tightly together and deform collectively under mechanical stress. Each cell absorbs force, changes shape and recovers without losing energy to friction. If a synthetic material could replicate this cellular architecture, it might escape the trade-offs that plague conventional hydrogels.
Preparation process and characterization of epithelium-like structure hydrogel (PLTAV). (a) Schematic illustrations of epithelial tissue, hydrogel preparation process, mechanical energy dissipation pathway, triple sensing modes (self-powered, visual, underwater). In situ CLSM images of (b) emulsion and (c) PLTAV before equilibrium (Yellow fluorescence stems from hydrophilic Rhodamine B). OM images of (d) PLTAV before equilibrium and (e) PLTAV hydrogel. AFM height images of PLTAV hydrogel at (f) low magnification and (g) high magnification. (h) FTIR and (i) XPS spectra of emulsion and PLTAV hydrogel. (Image: Reproduced from DOI:10.1002/advs.202510444, CC BY) (click on image to enlarge)
The key innovation involves creating what the researchers call “cell-like particles” within the hydrogel structure. The team synthesized these microscopic particles using a water-in-oil high internal phase emulsion, a technique that disperses tiny water droplets containing dissolved monomers (small molecules that link together to form polymers) throughout an oil phase, creating a template for particle formation when the monomers polymerize. These resulting particles function analogously to biological epithelial cells. They deform reversibly under mechanical stress, divide into smaller units, and reaggregate when the stress lifts, all without losing significant energy to friction or permanent damage.
The resulting material, which the researchers named PLTAV after its polymer components, contains 90.4 wt% water yet demonstrates strong mechanical performance. It stretches to 1368% of its original length before breaking and withstands toughness values of 2.64 MJ·m⁻³. Most notably, it exhibits a hysteresis of just 4.7% at 300% strain, meaning it returns nearly all the energy put into deforming it.
The hydrogel also compresses to 99.9% of its original thickness and still recovers its shape, far exceeding the 90% limit of previous materials. It maintained these properties through 600 loading-unloading cycles at 1300% strain, demonstrating exceptional durability.
The researchers then developed an enhanced version called PLTAV-SC by soaking the original hydrogel in a solution of sorbitol, a common sugar alcohol, and choline chloride, a nutrient compound. This modification exploits two complementary effects. First, a hydration effect disrupts ice crystal formation, enabling the material to remain flexible at temperatures as low as -45 °C. Second, a salting-out effect causes polymer chains to aggregate and stiffen, boosting mechanical strength.
The PLTAV-SC hydrogel achieved even higher stretchability of 2021% and toughness of 6.10 MJ·m⁻³. It maintained low hysteresis and extreme compressibility while gaining freeze resistance that previous hydrogels lacked.
What makes these materials particularly valuable for sensing applications is their unique electrical behavior. The cell-like particles contain ionic polymer segments with positively charged groups and free bromide anions. When the hydrogel stretches or compresses, these ions migrate at different rates. The large polymer-bound ions move slowly while the small bromide ions move quickly. This difference generates a measurable voltage through what physicists call the piezoionic effect, allowing the hydrogel to function as a self-powered strain sensor without requiring an external battery.
The PLTAV sensor detects tensile strain with a sensitivity of 3.6 mV per 100% strain and compressive strain up to 25 mV per 100% strain. The sensors maintained stable voltage output across multiple cycles and different compression speeds. At -45 °C, the PLTAV-SC sensor’s maximum sensitivity increased to 40 mV per 100% strain because the cold slowed the larger ions even further, amplifying the mobility difference.
Beyond self-powered sensing, the hydrogels incorporate a fluorescent molecule that glows blue under ultraviolet light. In the unstretched hydrogel, these molecules cluster together in aggregates that emit bright fluorescence. As the material stretches, the polymer chains pull apart and scatter these clusters, causing the glow to dim proportionally with strain. This creates a visual indicator of deformation useful for monitoring structural integrity.
For conventional resistance-based sensing, the PLTAV sensor achieved a linear response across a strain range of 0.5% to 1300%, with response and recovery times of 38 and 40 milliseconds. These speeds significantly exceed those of many previously reported strain sensors.
The researchers also demonstrated underwater applications by encapsulating the sensors in a water-repellent coating. These waterproofed sensors detected human motion, water flow velocity up to 9.6 L/min, and water depth down to 6 meters. The team showed practical applications including the transmission of Morse code messages through compression-generated voltages and the detection of subtle physiological signals such as wrist pulses underwater.
This work establishes that biological design principles can resolve long-standing engineering trade-offs in hydrogel development. By creating materials that combine extreme mechanical properties with multiple sensing modalities and freeze resistance, the research points toward a new generation of wearable devices, underwater monitoring systems, and human-machine interfaces capable of operating reliably in environments where current sensors fail.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
