Human\nMuscle Inspired Anisotropic and Dynamic Metal\nIon-Coordinated Mechanically Robust, Stretchable and Swelling-Resistant\nHydrogels for Underwater Motion Sensing and Flexible Supercapacitor\nApplication

Mechanically robust and anisotropic\nconductive hydrogels have emerged\nas crucial components in the field of flexible electronic devices,\nsince they possess high mechanical properties and intelligent sensing\ncapabilities. However, the hydrogels often swell on exposure to aqueous\nmedium because of their hydrophilicity, which compromises their mechanical\nproperties. Additionally, the hydrogels’ isotropic polymeric\nnetworks demonstrate isotropic ion transport, which significantly\ndiminishes the sensing capabilities of electrical devices based on\nhydrogels. These factors greatly limit their use in flexible and wearable\nsensors. In this study, we have developed poly(acrylamide-<i>co</i>-maleic acid-<i>co</i>-butyl acrylate) based\nanisotropic hydrogels by prestretching and drying, followed by ionic\ncross-linking to fix the alignment. The anisotropic arrangement of\nthe polymer network resulted in significant improvements in mechanical\nperformance and electrical conductivity along the prestretching direction.\nThis anisotropic hydrogel combines hydrophobic and metal ion-ligand\ninteractions, enhancing the maximum tensile strength up to 11 MPa\nalong the prestretching direction, about 3 times higher than in the\nperpendicular direction. The optimized 200% prestretched hydrogel\nexhibited high tensile strength (7 MPa), flexibility (fracture strain\n370%), high toughness (16 MJ m<sup>–3</sup>) and antiswelling\nbehavior in water (equilibrium swelling ratio 2% after 15 days). alongside\nhigher conductivity (3 times higher) and strain sensing ability (4\ntimes higher gauge factor) along the prestretching direction. The\nhydrogel demonstrated efficient and stable underwater sensing for\nunderwater communication and to monitor human limb position and movement.\nThe anisotropic hydrogel electrolyte-based flexible supercapacitor\nexhibited 117 Fg<sup>–1</sup> specific capacitance at 0.5 Ag<sup>–1</sup>, and maximum energy density 5.85 Whkg<sup>–1</sup>, significantly higher than the corresponding values for the isotropic\nhydrogel-based device (88 F g<sup>–1</sup> and 4.4 Whkg<sup>–1</sup>, respectively). This hydrogel mimics the structural\ndesign of unidirectionally oriented muscle fibers, showing better\ndirection dependent functional properties than the corresponding isotropic\nhydrogel. The anti-swelling ability and retention of mechanical and\nconductive properties of these hydrogels in aqueous environment suggest\nlong-term usage capability of these functional materials.