A highly stretchable, sensing durability, transparent, and environmentally stable ion conducting hydrogel strain sensor built by interpenetrating Ca2+-SA and glycerol-PVA double physically cross-linked networks

Ion conducting hydrogels are attracting increasing attention in wearable sensors, owing to their advantages of withstanding a large range of strains, good biocompatibility, and excellent sensing properties. However, simultaneous realization of the above advantages and other practical application requirements such as extreme environmental tolerance and optical transparency remains challenging. Herein, high-performance PVA/glycerol/sodium alginate(SA)/CaCl2 (PGSC) ionic hydrogel sensors with dual physically cross-linked network was fabricated to overcome these challenges. As the first cross-linked network, glycerol cross-linked PVA maintain the basic framework structure of the hydrogel. The second network was formed though ionic cross-linking between sodium alginate (SA) and multivalent cations (Ca2+). This ionic interaction can be regarded as sacrificial bonds to dissipate mechanical energy. Significantly, Ca2+ was introduced to the hydrogel by the water-glycerol mixed solvent displacement approach, not single solvent displacement approach reported by most literatures. Benefiting from the dual network structure, the hydrogel exhibited good mechanical properties (maximum strain 816%, maximum stress 2.29 MPa) as well as fast self-recovery ability after stretching. The introduction of a large number of ions imparted the hydrogel with high conductivity (2.08 × 10−2 S/cm), and high sensitivity over a relatively wide range (GF = 2.68 at 500% strain). The PGSC sensors exhibited good transparency (96.5% at 600 nm) and the ability to be used at extreme environments for a long time. Conductive hydrogels can also serve as monitoring devices to make accurate and stable electrical signal outputs to physiological signals emanating from various parts of the human body. The combination of these excellent capabilities highlights the great potential of PGSC hydrogel in wearable sensors and other flexible electronic device.Graphical abstract