To date, various hydrogels have been manufactured for wearable sensors, but the simultaneous realization of optical transparency, remarkable mechanical properties, excellent sensing properties and anti-freezing features by using cheap raw materials and an easy synthesis process remains challenging. Here, high performance PVA/glycerol/NaCl (PGN) ionic hydrogel sensors were facilely manufactured to overcome these challenges. The proposed hydrogel sensors were fabricated by physically mixing the biocompatible poly(vinyl alcohol) and nontoxic glycerol in water as the gel matrix followed by soaking in saturated NaCl aqueous solution. The resulting PVA/glycerol/NaCl hydrogels exhibited good transparency (>88%), favorable stretchability (570.7%), outstanding mechanical strength (3.1 MPa), and high toughness (8.65 MJ m⁻³). Meanwhile, the PGN ionic hydrogel sensors displayed excellent stretching sensitivity with a gauge factor of 4.01, and it could repeatedly and stably detect accurate electrical signals under both large strains and subtle strains. Furthermore, our hydrogel sensors were endowed with anti-freezing features on account of the presence of both glycerol and NaCl, and the sensors could still maintain good mechanical and strain-sensitive performance at a low temperature (−20 °C). Based on the above characteristics, the biocompatible PGN hydrogel sensors could be employed as wearable devices to detect various human motions (e.g. finger, wrist, elbow, and knee joints) at room temperature and low temperature. Therefore, the as-prepared hydrogels show promising potential as high-performance wearable strain sensors for practical applications in biomimetic prostheses and human activity monitoring systems.

Flexible and conductive hydrogel sensors have great potential for applications in wearable and implantable devices, electronic skin and healthcare diagnosis. However, it remains a great challenge to develop an integrated tough hydrogel sensor combining stretchability, adhesiveness, self-healing ability and high sensitivity via a simple, fast and economical approach for multi-field applications. In this work, we designed a multifunctional composite self-adhesive conductive hydrogel by using chitosan (CS), tannic acid (TA) and polyacrylic acid (PAA), and ionic cross-linker Al³⁺. The synergistic multiple and reversible coordination bonds and hydrogen bonds in the matrix lead to a combination of high stretchability, rapid self-recovery, great anti-fatigue and rapid self-healing properties of the tough hydrogel. Additionally, the ionic gels displayed durable and repeatable adhesiveness ascribed to the presence of catechol groups from TA, which can adhere directly on human skin without residuals and inflammatory responses. Furthermore, our ionic hydrogel sensors exhibited a broad strain window (0–1400%) and demonstrated brilliant stretching sensitivity (gauge factor as high as 12.2), and it displayed stable sensing performance for repeated real-time monitoring of both large and subtle deformation (e.g. knee joint motions, breathing modes, and even the slight pulse changes in different motion states). This work provides a feasible method to construct self-adhesive and self-healing hydrogel sensors with high sensitivity and a large-range detection capacity and paves a way for versatile applications in electronic skin, healthcare monitoring and medical electrodes.