Dual-Functionalization Cellulose Nanofiller Use to Simultaneously Enhance Hydrogel Sensors’ Mechanical and Electrical Properties

Hydrogel sensors have experienced rapid advancements due to their sensitivity in detecting motion signals and the versatility of their synthesis strategies. However, to enhance their intrinsic performance or to provide additional functionalities necessary for complex application requirements, hydrogel sensors typically require the incorporation of various nanofillers. As demand grows for sensors that are skin-friendly, wearable, and mechanically stable, cellulose nanofibers (CNF) as a natural polymer have emerged as indispensable nanofillers. Nevertheless, despite being a widely utilized natural biomass nanofiller, CNF exhibit poor conductivity, and the second cross-linked network they form often results in the improved performance of hydrogel sensors at the cost of their electrical properties. In this study, surface-active carboxyl functional groups of CNF, which were prepared through nanofibrillation and modification with deep eutectic solvent (DES), were utilized to load the organic conductor PEDOT/PSS via electrostatic attraction. This resulted in the formation of functionalized nanofillers designated as PEDOT/PSS@OA-CNF. Subsequently, PAA/PEDOT/PSS@OA-CNF hydrogel sensors were synthesized through rapid in situ polymerization using an aqueous solution of acrylic acid (AA), photoinitiator, and polymerization under UV-light. The incorporation of PEDOT/PSS enables OA-CNF to function as a nanofiller, simultaneously enhancing both its mechanical properties (stress–strain: from 2.91 MPa and 321.23% to 4.69 MPa and 512.73%) and electrical conductivity (0.12 to 0.58 S/m), and the dual-functionalization cellulose nanofillers with improved mechanical and electrical properties of hydrogel sensors were achieved. Notably, the PAA/PEDOT/PSS@OA-CNF hydrogel sensor exhibits a high sensitivity to motion detection and long-term stability. Moreover, its compatibility with 3D printing allows for the fabrication of complex shapes, making it adaptable to various practical applications. It can precisely monitor both subtle (drinking and coughing) and large-scale (joint movement) human motions, demonstrating potential applications in human motion monitoring, flexible sensors, and stretchable electronic devices.