Tough Trilayer Composite Hydrogel Inspired by Crocodile Skin Structure for Flexible Sensors

As a high-performance polymer material, conductive hydrogels are widely employed in the fields of motion monitoring, electronic skin, and energy storage devices, which rely on flexible materials, including hydrogel, elastomer, and composite hydrogel. However, preparing a composite hydrogel with excellent mechanical properties is a great challenge. Inspired by the structure of crocodile skin, a trilayer structure conductive composite hydrogel was prepared. The three layers were Ecoflex elastomer, poly(acrylamide-2-hydroxyethyl methacrylate) (PAAm-HEMA) hydrogel, and graphene/2-hydroxyethyl methacrylate (G/PHEMA) hydrogel, respectively. Covalent bonds were generated by a photochemical reaction between elastomer Eco and the P(AAm-HEMA) hydrogel. Covalent bonds were also formed between the P(AAm-HEMA) hydrogel and G/PHEMA hydrogel by the chemical reaction of N,N′-methylenebis(2-propenamide), which worked as cross-linking agent; hydrogen bonding between these two hydrogels also formed. These physical and chemical interactions provided firm bonding between the layers and prevented interlayer slippage under an external force. The G/PHEMA-P(AAm-HEMA)-Eco composite hydrogel possessed high fracture stress and elongation at break of up to 2.1 MPa and 1305%, respectively. The conductivity of 0.028 S/m was attributed to the incorporation of graphene in the network of the G/PHEMA hydrogel. Based on the excellent mechanical properties and electrical conductivity, this composite hydrogel was applied as a flexible sensor to detect human motion signals. These results indicate that the trilayer G/PHEMA-P(AAm-HEMA)-Eco composite hydrogel represents a promising material, paving the way for innovative applications in next-generation flexible electronic devices.