Elastic and conductive gellan gum/polyvinyl alcohol physical hydrogels with low swelling potential for sustainable flexible electronics

Conductive hydrogels are considered to be promising materials for flexible electronics, but those nondegradable are bound to produce substantial undesirable electronic wastes once widely applied, inconsistent with the long-term sustainable development goals. To develop biodegradable conductive hydrogels for sustainable flexible electronics, a green chemical reaction-free strategy was developed to manufacture a novel type of composite hydrogels, exploiting biodegradable gellan gum and polyvinyl alcohol as the matrices and natural-derived tannic acid as the crosslinking agent. The technique involves successive cool-induced sol−gel transition, freeze-thawing and Na2SO4 solution treatment. A universal electronic testing machine, scanning electronic microscope, X-ray diffractometer, thermogravimetric analyzer, and electrochemical work station were employed to study the mechanics, structure and morphology, swelling and conductivity of the hydrogels. The results indicate strong, elastic and tough hydrogels were achieved owing to the multi-crosslinked multiple-network structure. The maximum compressive stress (90% strain), tensile strength, breakage elongation and fracture energy reach 10.7 MPa, 1.2 MPa, 890% and 480 kJ/m3 respectively. The stress recovery rate still remains 86% after 10 consecutive loading-unloading under 200% strain. The highest conductivity reaches 3.27 S/m, which along with the excellent mechanics confers them good strain-sensing behavior. Moreover, the final hydrogels show low swelling in both water (the equilibrium swelling ratio (ESR) < 300%) and physiological saline (ESR < 90%). The excellent mechanical performance, high conductivity and good strain sensing, low swelling and inherent biodegradability as well as the ecofriendly manufacturing process imply potential prospect in sustainable flexible electronics.