Crack-resistant, ultra-stretchable ionic hydrogel for high-performance wearable physiological sensors

Microcracking is a critical issue plaguing wearable electronic devices. Traditional ionic conductive hydrogels tend to develop expanding cracks under repeated stretching and bending, which shortens device lifespan and compromises sensing stability. Developing stretchable, crack-resistant hydrogels with steady conductivity has long been a major challenge in flexible bioelectronics.

Researchers from Yanshan University have created a new interface-engineered ionically conductive hydrogel to solve the above problems. This material features ultra-high stretchability, excellent crack resistance and strong self-adhesion, and can stably collect physiological signals, making it a promising candidate for wearable sensing devices.

This research, titled Interface-engineered Ionically Conductive Polyoxometalate-based Hydrogels with High Stretchability and Notch-insensitivity for Wearable Sensors, has been published in the Chinese Journal of Polymer Science. The hydrogel is notch-insensitive, meaning it resists crack growth even with pre-existing cuts. The project was led by Professor Tifeng Jiao and Associate Professor Zhihui Qin, and Xiaojiao Shi is the first author.

The team blended L-arginine-modified silicotungstic acid nanocomplexes (L-arg@SIW) into a poly (acrylic acid, PAA) network. Electrostatic force and hydrogen bonds form dynamic connections inside the material. These connections redistribute mechanical stress and help suppress crack propagation during deformation when the hydrogel is stretched or bent.

Mechanical tests show the hydrogel delivers remarkable performance. Its fracture strain reaches 1572%, toughness 1483 kJ/m³ and fracture energy 6.82 kJ/m². Notably, even with pre-made cuts, it can stretch over 1000% without observable crack advancement, proving its superb anti-crack capability under extreme deformation.

The hydrogel boasts strong adhesion to various surfaces such as paper, skin-like biological tissue, metal, glass, PET, PMMA and silicone, and no extra glue is needed. Besides, polyoxometalate nanocomplexes build continuous ion transport channels inside the material. It achieves an ionic conductivity of 0.15 S/m while retaining good flexibility.

Made into wearable strain sensors, the hydrogel features wide working range, high sensitivity, fast response and stable cycling performance. Its maximum gauge factor (GF) is 8.06, with a response time of 149 ms and a recovery time of 380 ms.

The sensor can accurately capture both large body movements and tiny physiological actions. It delivers steady electrical signals when detecting movements of fingers, wrists, elbows and knees. It also works stably in repeated tests for subtle actions like cheek bulging, swallowing and touching.

Apart from motion detection, the hydrogel can serve as a flexible bioelectrode. Compared with traditional Ag/AgCl electrodes, it has lower skin contact impedance and captures clear ECG signals with well-resolved PQRST waveforms. Its signal-to-noise ratio hits 21.70, higher than the 17.27 of commercial electrodes. It can also steadily collect electromyogram (EMG) signals under different gripping forces, showing great potential for wearable electrophysiological monitoring.

Integrating high stretchability, anti-crack performance, strong adhesion, good ion conductivity and stable signal monitoring, this new hydrogel offers a promising material solution for next-generation wearable electronics, smart rehabilitation equipment and human-machine interaction systems.