Transparent, high-strength, stretchable, sensitive and anti-freezing poly(vinyl alcohol) ionic hydrogel strain sensors for human motion monitoring

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.