High-strength anisotropic hydrogels as adhesive strain sensors for dual-environment applications†

Hydrogels exhibit tremendous potential for applications in electronic skin sensing, yet their interfacial hydration severely limits their sensing applications underwater. Herein, inspired by the underwater adhesion mechanism of mussels, we robustly anchor a poly(dopamine methacrylamide-2-methoxyethyl methacrylate) (P(DMA-MEMA)) copolymer onto the surface of anisotropic poly(acrylamide-acrylic acid/Fe³⁺) (P(AAm-AAc/Fe³⁺)) hydrogels. The adhesive hydrogels capable of adhering both in air and underwater are constructed by leveraging the hydrophobic properties of the adhesive polymer. The prepared adhesive hydrogel exhibits superior anisotropic mechanical properties, with stress along the pre-stretched direction varying from 4.40 MPa to 6.99 MPa, significantly higher than that of isotropic and vertically oriented hydrogels. Furthermore, the adhesive hydrogel demonstrates high adhesion performance in air and underwater on various substrate surfaces (glass, wood, ceramic, metal, and porcine skin). Additionally, the adhesive hydrogel as a strain sensor can be used for monitoring human motion information, underwater distress detection (“SOS”), and intelligent alarming. It shows anisotropic sensing properties, with conductivity and sensitivity along the pre-stretched direction of 260 mS m⁻¹ and 3.65, respectively, superior to the sensing performance in the vertical direction of the hydrogel. This study provides an effective method for preparing multifunctional anisotropic underwater adhesive hydrogels with high-strength tunability, offering potential prospects in underwater applications such as flexible sensors and soft robotics.