3D-Printed Flexible Conductive Hydrogel as a Wearable Platform for Dual Functions of Strain and Colorimetric Lactate Sensors

Strain sensors have received considerable attention in personal healthcare due to their ability to monitor real-time human movement. However, the lack of chemical sensing capabilities in existing strain sensors limits their utility for continuous biometric monitoring. Although the development of dual wearable sensors capable of simultaneously monitoring human motion and biometric data presents significant challenges, the ability to fabricate these sensors with geometries tailored to individual users is highly desirable. Herein, we report three-dimensional (3D)-printed flexible conductive hydrogels designed to serve dual functions as both strain and colorimetric sensors for sweat lactate detection. Poly(ethylene glycol)-bisurethane methacrylate with varying molecular weights was synthesized and utilized as a cross-linker in photopolymerizable resins. These resins were employed to fabricate various 3D complex architectures via a liquid crystal display 3D printer, resulting in hydrogels with remarkable mechanical properties, including excellent stretchability, toughness, elastic recovery, and fatigue resistance. Postfabrication treatment with NaOH enhances the ionic conductivity of the hydrogel, enabling its use as a strain sensor capable of detecting compression, bending, and stretching through real-time resistance changes. In addition, a lactate assay was immobilized onto the hydrogel, allowing it to function as a colorimetric sensor for sweat lactate with a detection range of 0 to 25 mM, encompassing the physiological threshold associated with muscle fatigue. These dual-function sensors facilitate comprehensive, real-time monitoring of both human physiological activities and lactate levels within a single device, highlighting their potential for the on-demand fabrication of customizable smart health-monitoring systems.