Hierarchically Oriented Anisotropic Nanoscale Hydrogels with Enhanced Strength and Conductivity via Metal–Ligand Interactions and Salt-Soaking Strategy for Sensing and Energy-Storage Applications

Polymeric hydrogels are crucial for flexible electronics and biomedical engineering but often lack strength and unidirectional response due to isotropic polymer chain orientation. This work presents a bioinspired, scalable strategy to fabricate anisotropic double-network hydrogels based on a physically cross-linked chitosan network and a hybrid (chemically and physically) cross-linked poly(acrylamide-co-maleic acid) network. Prestretching-induced chain alignment, Fe3+-mediated metal ion-ligand coordination for locking the aligned structure, and kosmotropic/chaotropic ion-induced interaction furnish anisotropic hydrogels with a hierarchical architecture spanning from the macroscale to the nanoscale and molecular scale, which closely mimics the structural hierarchy of skeletal muscles. The observations─macroscale organization reflected in mechanical anisotropy, microstructural pore alignment evidenced from FESEM analysis, nanoscale chain orientation indicated by SAXS experiments, and molecular scale alignment suggested from angular dependence of birefringence in polarized optical microscopy─demonstrated the presence of organization in multiple length scales. The hydrogels showed high tensile strength (up to 11 MPa) and toughness (15 MJ m–3), unidirectional strain sensing, antiswelling, and ionic conductivity. The applications of these hydrogel materials as a flexible, remotely operated underwater strain sensor as well as in bionic skins, touch panels, and communication aids for individuals with hearing or speech impairments are demonstrated. The hydrogel electrolyte-based high-performance flexible supercapacitor (specific capacitance ∼ 222 Fg–1 and energy density ∼ 11 Whkg–1) retained a high specific capacitance (∼ 195 Fg–1) even at subzero temperature (−15 °C). These strategically designed anisotropic hydrogels hold great potential for advanced applications in flexible and robust electronics and energy-storage platforms.