Electrochromic double-network hydrogel for flexible strain sensing and on-board energy storage in wearable devices

The development of self-powered, integrated flexible electronics is essential for overcoming the challenges of portability and energy sustainability in next-generation wearable systems. In this work, we present a multifunctional hydrogel engineered by incorporating viologen derivatives into a carboxymethyl cellulose/polyacrylamide (CMC/PAM) double-network matrix. This design couples the redox electrochromism of viologens with supercapacitive energy storage, yielding a dual-functional material capable of real-time optical monitoring of charge states. The hydrogel exhibits an optical contrast of 53.1% and a coloration efficiency of 88.1 cm² C^-1, together with balanced mechanical robustness, deformability, and ionic conductivity, delivering a tensile strength of 63 kPa, an elongation at break of 240%, and a conductivity of 1.3 S m^-1. The EFH-assembled supercapacitor achieves an areal capacitance of 4.44 mF cm^-2 at 0.02 mA cm^-2, an energy density of 3.18 μWh cm^-2, and a power density of 22.68 μW cm^-2, while retaining 79.0% of its initial capacitance with a Coulombic efficiency of 92.9% after 1900 cycles. The integrated strain sensor exhibits a gauge factor of 2.73 and maintains stable sensing performance over 500 stretching cycles. A self-powered system that integrates an electrochromic supercapacitor with a strain sensor enables continuous, real-time monitoring of human motion, representing a significant step toward the seamless integration of energy storage, sensing, and feedback in flexible electronics.