A crystalline reinforced dual-network hydrogel for wearable sensing devices

The rapid development of wearable sensing interfaces urgently requires flexible electronics with high strength, high stretchability, and stable conductivity. This study proposes a multi-stage fabrication strategy to prepare dual-network (DN) hydrogel electrodes. A covalent crosslinked network of poly(acrylic acid-co-sodium acrylate) (P(AA-co-NaAA)) was obtained via digital light processing (DLP) three-dimensional (3D) printing. A crystalline crosslinked network of poly(vinyl alcohol) (PVA) was formed via freeze-thaw cycles and an annealing process. Then, sodium citrate (NaCit) was used as a regulating ion to achieve segment rearrangement and structural reconstruction. By adjusting the PVA content, AA/NaAA molar ratio, and NaCit concentration, the strength and stretchability of the DN hydrogels were significantly enhanced, breaking the traditional trade-off between strength and stretchability in hydrogels. Characterization showed that the multi-stage fabrication continuously increases the content and regularity of the crystalline PVA, and the structure evolves from a loose to dense porous framework. Given the comprehensive mechanical and conductive properties of the resulting material, the optimized formulation, with a PVA content of 9 wt%, AA/NaAA molar ratio of 4 : 1, and NaCit concentration of 0.5 mol L^-1, was selected. The resulting hydrogel electrodes exhibited a continuous resistance response over a wide strain window and maintained a stable signal output after 2000 cycles at 50% strain. When integrated into a wearable glove, the hydrogel electrodes transformed finger bending deformation into an electrical signal to drive synchronous movements of a robotic hand, thus demonstrating the application potential of the hydrogel electrodes as wearable sensing interfaces for human-machine interaction.