Lignin-containing nanofiber-reinforced flexible strain sensors with excellent mechanical properties and ionic conductivity for human motion detection.

A multifunctional hydrogel with outstanding mechanical properties and excellent ionic conductivity holds immense potential for applications in various fields, such as healthcare monitoring, and various devices, such as wearable devices and flexible electronics. However, developing hydrogels that combine high mechanical strength with efficient electrical conductivity remains a considerable challenge. Herein, an ion-conductive hydrogel with excellent mechanical properties and ionic conductivity is successfully created. This hydrogel integrates sensing capabilities, freeze tolerance, and long-term solvent retention through a synergistic combination of lignin-containing cellulose nanofibers (LCNF), polyvinyl alcohol chains, ethylene glycol, and aluminum chloride. The resulting hydrogel demonstrates exceptional mechanical performance in terms of various factors, including tensile strength (1.28 MPa), strain capacity (794.94 %), toughness (6.32 MJ/m), and fatigue resistance. In addition, the incorporation of enhanced LCNF fillers harmonizes the mechanical properties and ionic conductivity of the ion-conductive hydrogel, effectively addressing the inherent trade-off between these two attributes-a common challenge associated with ionic hydrogels. Moreover, the ion-conductive hydrogel exhibits exceptional sensing stability (300 cycles at 80 % strain), ionic conductivity (0.82 S/m), and sensitivity along with near real-time response (300 ms), freeze tolerance (-24 °C), and prolonged solvent retention (180 d). This multifunctional ion-conductive hydrogel opens new pathways for designing advanced wearable sensors.