Anisotropic Anti-Swelling Hydrogels with Hydrophobic Association and Metal–Ligand Cross-Links for Applications in Underwater Strain Sensing and Anisotropic Actuation

We report herein a strategy to design mechanically strong and anisotropic metal ion cross-linked conducting hydrogel materials and their possible application in anisotropic resistive strain sensing. A dynamic hydrophobic association was incorporated in a chemically cross-linked poly(acrylamide-co-methacrylic acid) hydrogel by incorporating a hydrophobic comonomer appended with a terpyridine ligand. After prestretching this hydrogel, Fe³⁺ ion-ligand cross-linking was established with carboxylic group of methacrylic acid and the terpyridine unit of the hydrophobic comonomer to lock the alignment of the polymer chains, which significantly enhanced the mechanical performance of the hydrogel. The anisotropic hydrogels achieved high mechanical strength of 1.6-2.7 MPa, breaking strain of 250-320%, toughness of 3-4 MJ m^-3, and elastic modulus of 1.7-2.5 MPa under optimized experimental condition. Significantly inferior mechanical performance was observed when the load was applied in the direction perpendicular to prestretching direction. High fracture energy of 0.99 ± 0.4 kJ m^-2 -similar to that of (∼1000 J m^-2) the biological load bearing tissue cartilage-could be achieved when the crack was introduced perpendicular to prestretch direction. The anisotropic alignment of the polymer chains in the hydrogel was confirmed by FESEM and SAXS experiments. The Fe³⁺ ions as well as the hydrophobe concentration played a vital role in altering the mechanical properties of these hydrogels. The presence of hydrophobic association contributes to enhancing the mechanical anisotropy of the hydrogels, whereas the methacrylic acid-Fe³⁺ cross-links contributed to enhancing the tensile strength and stiffness of these anisotropic hydrogels. The hydrogel materials demonstrated anisotropic resistive strain sensing and anisotropic actuation behavior. These anisotropic swelling-resistant hydrogels also showed capability for stable and repetitive underwater strain sensing, which may have potential applications in underwater human motion sensing and soft robotics application.