Super stable, highly ion-conductive and transparent eutecto-/hydro-gel promotes wearable electronic and visual strain sensing

In recent years, hydrogels (HGs) have been extensively utilized as potential substrates for flexible wearable strain sensors, due to their excellent biocompatibility, stretchability and ease to incorporate various ionic /electronic conductive materials. However, water evaporation (i.e., dehydration) and low temperature intolerance severely restrain their sensing performance and lifetime. Eutectogels (EGs) as new emerging substrates, exhibit impressive low temperature tolerance and sound ionic conductivity, but they are prone to absorb water (i.e., hydration) in air and thus instable in morphology and ionic conductivity. Herein, we propose a novel concept of eutecto-/hydro-gel (EHG) with super stability, high ionic conductivity and transparency. As a proof-of-concept, we demonstrate the fabrication and strain sensing performance of a new EHG containing N-acryloyl glycinamide (NAGA), choline chloride (ChCl), glycerol (Gly) and water. EHG can maintain the dynamic balance between hydration and dehydration, and thus show much better long-term sensing performance. More importantly, we further show that flexible wearable visual and electronic strain sensor can be constructed by integrating photonic crystals into EHG. Due to the remarkable difference in sensing principles, the two sensing modes in this strain sensor can not only work properly without mutual interference, but also supplement each other to improve the sensing accuracy. The EHG material system and the fabrication strategy for dual-mode flexible wearable strain sensor provide a new solution for the development of high-performance flexible wearable devices.