Highly stretchable and conductive liquid metal-reinforced interpenetrating polymer network hydrogel for wearable devices

Abstract

Wearable devices enjoy high convenience and accessibility and thus are especially suitable for everyday health and fitness monitoring and motion detection. Therefore, the device-tissue interface requires bio-compatible, soft, stretchable, and conductive material. For this purpose, we fabricated conductive hydrogel (GPPgel) via in-situ interpenetrating polymerization of polyacrylic acid and polypyrrole, with dispersed gallium-indium-tin liquid metal droplets (GaInSnDs) and iron ions. In GPPgel, the interpenetrating polymer chains are crosslinked via dynamic metal coordinate bonds and hydrogen bonds in addition to π-π bonds. Thus, environmental stress would be dissipated through an energy dissipation mechanism via reversible dissociation of dynamic bonds. As a result, GPPgel displays high ductility (stretching up to 1740 %), high toughness (10.8 MJm⁻³, G'≈ 400 kPa), high tensile strength (1.85 MPa), low swelling, sufficient adhesiveness, as well as self-healing capability with electrical properties recovering 95.5 % after self-healing, and elasticity with frequency-independent performance. As polypyrrole is conductive on its own, with the help of iron ions and GaInSnDs, the conductivity of GPPgel reaches 2.74 ± 0.2 S/m and linearly changes following the bending of the hydrogel, offering the capability to detect the movement of the wearer. Moreover, it features a rapid strain response time of 104 ms, and maintains stable electrical performance. These performances comprehensively meet the needs of most wearable devices, besides, all the components in GPPgel show good bio-compatibility at the applied level in the hydrogel. Collectively, GPPgel displayed broad-frequency mechanical sensing and biological adaptation characteristics and the strategy of applying liquid metal in hydrogels with an interpenetrating network holds promise for further development.