Superelastic Hydrogels for Mechano-Electric Sensing

Hydrogels are materials that can retain large amounts of water, making them broadly applicable to many areas, including wound care, tissue engineering, and soft electronics. Conventional hydrogels usually possess limited mechanical strength. The current concept of double network hydrogels provides largely increased mechanical stability, thus allowing also mechano-electrical sensing. However, the chemical and structural complexity of many double network hydrogels complicate understanding and tailoring their mechanical and electrical properties over a broad range.<br/><br/>We synthesized a new class of elastic and mechanically stable hydrogels based on alternating amphiphilic copolymers that in water spontaneously form physical networks that constitute the hydrogels, with no crosslinking chemistry required. The hydrophobic copolymer blocks consist of alkyl chains that form well-defined nanodomains acting as dynamic crosslinks. The mechanical and dynamic properties of the hydrogels can be directly controlled by the lengths of the alkyl chains and the hydrophilic repeat units. The highly elastic hydrogels allow elongations of more than 10000%, with tensile moduli in the kPa-range to MPa-range, adjustable to the range of moduli and deformation of skin at joints. We demonstrate <i>in situ</i> X-ray diffraction under tensile stretching and compression, together with conductivity measurements to relate the nanodomain structure directly to the mechano-electrical properties.<br/><br/>The intrinsic ion conductivity of the hydrogels allows mechano-electrical sensing of stretching and compression from very small to large deformations for thin films, over a broad range of frequencies, suitable to sense joint motion and speech.