Anti-freezing, tough, and stretchable ionic conductive hydrogel with multi-crosslinked double-network for a flexible strain sensor

Ionic conductive hydrogels are an ideal material for use in flexible strain sensors due to their electrical conductivity, accurate data retrieval, and extensibility. However, conventional ionic conductive hydrogels including salt/organics or salt-based hydrogels with high ionic content exhibit modest antifreeze and limited mechanical properties, hampering their practical use across diverse sectors. The preparation process is tedious due to the incorporation of toxic photoinitiator and chemical crosslinking agents. Herein, through a simple, low-cost, and environmentally friendly strategy, we have developed a multi-crosslinked double network ionic conductive hydrogel by using polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ) and sodium alginate (SA) via UV irradiation, followed by immersion in a mixed solution of ferric chloride (FeCl3) and glycerol (Gly)/water. This synergistic effect of FeCl3 and Gly has endowed the PVA-SbQ/SA/FeCl3/Gly hydrogel with exceptional mechanical properties (tensile strength of 2.48 ± 0.45 MPa and elongation at break of 1452 ± 101 %), anti-freezing ability (-42.3 °C), and electrical conductivity of 0.38 S/m and 0.32 S/m at room temperature and −30 °C, respectively. Moreover, the hydrogel exhibits remarkable strain sensitivity (GF = 2.49), allowing it to be used as a strain sensor under both room temperature and subzero conditions. Consequently, the prepared hydrogel sensor can monitor various human motions, such as joint bending, running, and speaking. Therefore, our work offers a promising strategy for the preparation of PVA-based hydrogels with outstanding mechanical properties, electrical conductivity, and resistance to freezing, rendering them a practical option for flexible sensors. In addition, our anti-freezing method has significant potential in enhancing the accessibility of ionic conductive hydrogels.