CNC-mediated functionalized MWCNT-reinforced double-network conductive hydrogels as smart, flexible strain and epidermic sensors for human motion monitoring

Soft, stretchable, and smart strain-sensing hydrogels have attracted significant attention due to their broad applicability in emerging fields. However, developing hydrogel-based strain-sensing materials with finely tuned mechanical and sensing properties remains challenging, primarily due to the inherent brittleness of traditionally fabricated hydrogels. In this study, a novel flexible strain- and epidermis-sensitive sensor was designed using a cellulose nanocrystal (CNC)-mediated acid functionalized multiwalled carbon nanotube (A-MWCNT)-reinforced double-network conductive hydrogel. This dual-network hydrogel system was fabricated by integrating a covalently crosslinked acrylamide (Amm) and [2-(acryloyloxy) ethyl] trimethyl-ammonium chloride (AETAC) with a physically crosslinked network of A-MWCNTs, which were uniformly dispersed via CNCs. Incorporating hydrogen bonding and strong electrostatic interactions within the physical network introduced reversible sacrificial bonds, significantly enhancing the hydrogel's mechanical strength. The hydrogel exhibited mechanical and sensing performance, including sufficient stretchability (431.6%), remarkable sensitivity, a gauge factor (GF) of 4.32 at 400% strain, toughness of 65.6 kJ m^-3, Young's modulus of 1.5 kPa, and rapid response and recovery times of 100 msec. Furthermore, it demonstrated excellent cycling stability over 100 cycles and effective sensing capabilities across a broad strain range, from small deformations (5%) to large strains (400%). The conductivity of 0.09 S m^-1, facilitated by the formation of conduction pathways through the AETAC and A-MWCNTs, further enhanced its performance. Moreover, the hydrogel exhibited practical applicability in detecting various large-scale and physiological human movements. Functioning as a wearable electronic skin, it represents a highly flexible and adaptable material suitable for applications in soft robotics, flexible sensors, and health monitoring devices.