Dual-stimuli-responsive, anti-freezing, and conductive ionic hydrogels for smart wearable and outdoor UV radiation monitoring devices

Many studies have shown that a certain amount of UV radiation can help the human body synthesize vitamin D. However, prolonged exposure of the skin can cause erythema, inflammation, and skin aging. In recent years, UV monitoring equipment has attracted extensive interest from researchers, and accurate monitoring of UV intensity is a core element of UV monitoring equipment. It is important to display outdoor UV intensity in a simple, convenient, and effective way. However, the development of UV/stress dual-responsive ion-conductive hydrogels with excellent conditioning capabilities for wearable devices remains a major challenge. In this work, we successfully fabricated a gelatin-glycerol-ammonium sulfate (GGS) ionic conductive hydrogel with excellent mechanical properties: high tensile strength of 330 kPa (elongation at break of 225%), high transparency of 90%, good frost resistance (-50 degree celsius), heavy plasticity, and good conductivity. This GGS organic hydrogel has dual responsiveness to UV light and strain. Moreover, the GGS hydrogel has stable electrical signal output in different joint movements, which suggests that it can be used to monitor physiological signals and human movement. Photochromic-GGS hydrogel can achieve reversible color change in response to UV stimulus, and can be used as an outdoor UV intensity indication device. Other than that, the GGS hydrogel also exhibits good re-molding, recyclability, and reusability. Low-cost photochromic-GGS hydrogel's simultaneous stability and environmental friendliness allows GGS hydrogel to be used as a green and sustainable smart material. To the best of our knowledge, these combined features cannot be realized by conventional strategies. Thus, our design strategy for GGS ion-conducting hydrogels promises to be a new generation of "smart" multi-stimuli-responsive hydrogels for visualization of UV-responsive devices, interactive wearables, and visual display devices.