A Transparent, Freezing-Tolerant, and Mechanically Robust Ion-Conductive Hydrogel for Strain-Sensing Applications

Ion-conductive hydrogels have attracted tremendous attention and are considered promising for ionic skin. However, the simultaneous incorporation of excellent mechanical strength, low-temperature tolerance, and high conductivity and transparency remains a great challenge, which will restrict their scope of applications. Here, a transparent, freezing-tolerant, and mechanically robust ion-conductive hydrogel based on a double-network structure (k-carrageenan/poly(acrylic acid)) in a binary solvent system (ionic liquid/water) is proposed for strain-sensing application. The ionic liquid ([EMIM]Cl) is introduced into the double network by a simple one-pot polymerization method, followed by the subsequent drying treatment. Benefiting from the presence of the ionic liquid, the prepared transparent (>90% transmittance) ion-conductive hydrogel demonstrates excellent mechanical properties, including high fracture stretchability (>3000% strain), high tensile strength (>0.45 MPa), and low Young's modulus (65 kPa). Meanwhile, due to the reduction of water in the system, the ion-conductive hydrogels exhibit good freezing tolerance (<−25 °C), while maintaining a high conductivity of up to 0.25 S/m at room temperature. In addition, benefiting from the excellent properties, the prepared ion-conductive hydrogel-based flexible strain sensors are studied and demonstrated various strain-sensing applications toward tensile deformation to detect human motion signals, such as finger bending and walking. This work can provide a strategy to construct mechanically robust and transparent ion-conductive hydrogels with environmental stability for potential applications in wearable sensors.