Anisotropic Hydrogels with Multiscale Hierarchy Based on Ionic Conductivity for Flexible Sensors

Nature materials (skin, tissue, plant fiber, and nacre) exhibit super strength and toughness attributed to their unique multiscale hierarchical anisotropic structure. Hydrogels with excellent flexibility, adjustability, and good biocompatibility have become the exciting frontiers in soft electronics, while anisotropic hydrogels that mimic natural materials still face challenges. Herein, we fabricated an anisotropic poly(vinyl alcohol) hydrogel (DS-PVA) with a multiscale hierarchical structure (micro, submicron, and nano) by directional freezing versus salting out, which differs from isotropic hydrogel mechanical performance and force stimuli electrical response behavior. The PVA hydrogel parallel to the freezing direction (DS-PVA(parallel to)) showed excellent strength 2.35 MPa (about five times DS-PVA(perpendicular to)), toughness 1712 KJ/M-3 (9.4 times DS-PVA(perpendicular to)), stability, and antifatigue characteristics, while the DS-PVA(perpendicular to) indicated a higher sensitivity (GF = 4.4, epsilon = 5%) and a faster response/recovery time (127 ms/63 ms). These hydrogels, similar to natural materials with anisotropic mechanical properties, asymmetric sensing properties, and directional recognition, will provide a strategy for flexible sensor applications.