“Deep Eutectic Solvent-in-Water” Hydrogels Enable High-Performance Flexible Strain Sensors for Electronic Skin with a Wide Operating Temperature Range

Conductive hydrogels have become one of the hot topics in flexible strain sensors owing to excellent biocompatibility, attractive mechanical flexibilities, and conductive properties. However, the time-consuming preparation of hydrogels and their unsuitable properties limit their application in low-temperature environment and high temperatures. Here, a new class of "deep eutectic solvent-in-water" hydrogels (DIWHs) is reported for the first time through a one-step gelation process in situ without solvent displacement, fabricated by combining a hydrogel with deep eutectic solvent (DES). The DIWH is constructed using a dynamic oxidation and coordination system composed of sodium lignosulfonate (Ls) and Fe3+. The effect of DES and the optimal mass ratio of water and DES on the hydrogel properties was synthetically investigated. The addition of DES not only shortens the polymerization time to 8 s and enhances the mechanical properties of the hydrogel but also provides some unique properties. For example, the addition of DES gives the gels greater self-healing ability and antibacterial properties. When the mass ratio of water to DES was 1:3, excellent antifreezing and antidrying properties were imparted to the gel, and the elasticity of the hydrogel was maintained even at −80 °C or stored at 60 °C for 7 days. Furthermore, the hydrogel exhibited strong interfacial adhesion to natural and synthetic materials (up to 60 kPa on glass) due to the presence of Ls with a catechol structure. In conclusion, this work stimulates more interest in the sustainable and high-value utilization of DES and fully demonstrates the advantages of this new easy-to-prepare coacervation gel for sensing.