Synergistic Strategy of the Freeze–Thaw Process with Prestretching and Metal Ion Cross-Linking to Access Biotissue-Inspired Mechanically Robust Hydrogels with Anisotropic Hierarchy

Integrating high mechanical strength, excellent conductivity, and directional sensing capabilities into polymeric hydrogels remains challenging due to their inherently random network orientation. Inspired by hierarchically anisotropic biological tissues (cartilage, tendon, muscle, and silk), we have employed a synergistic combination of prestretching and a freeze-thaw strategy to fabricate mechanically robust and flexible double-network hydrogels with multiscale hierarchical anisotropy. We prepared a prestretched poly(vinyl alcohol)/poly(acrylamide-co-maleic acid) anisotropic hydrogel network stabilized by Fe³⁺ ion cross-links. The repeated freeze-thaw process induced poly(vinyl alcohol) microcrystalline domain formation within this anisotropic network, significantly enhancing the mechanical properties compared to the isotropic hydrogel: 27-fold higher tensile strength (∼9 MPa), 10-fold higher stiffness (∼1.8 MPa), and 9-fold higher toughness (∼11 MJ m^-3). The hydrogel demonstrated ∼7 times higher conductivity and a ∼2 times higher gauge factor along the prestretching direction, which enabled application in directional sensing. A high output of the current along the parallel direction to prestretching compared to the perpendicular direction enabled the demonstration of "AND" and "OR" logic gates for soft material-based binary computing. The robust antiswelling capability of the hydrogel (an equilibrium swelling ratio of ∼50% was reached in 1 day, and no further swelling after immersion in water was seen for 15 days) was utilized to demonstrate efficient underwater strain sensing and information communication, with a potential to locate the position and receive communication from the distressed swimmer. A flexible supercapacitor device constructed with the anisotropic hydrogel-based electrolyte exhibited a significantly enhanced performance (a specific capacitance of ∼154 vs ∼122 Fg^-1 at 0.5 Ag^-1 and a maximum energy density of ∼7.7 vs ∼6.1 Wh kg^-1) compared to the isotropic hydrogel-based device. By mimicking the highly integrated structural composition of biological tissues, our approach offers a pathway toward advanced bioinspired materials for next-generation soft electronics, binary computation, and integrated energy storage systems.