Hierarchically Oriented Carbon Nanotubes/Poly(vinyl Alcohol) Composite Conductive Hydrogels for Flexible Strain Sensors

Hydrogels have been widely used in flexible electronics due to their water content similar to that of muscles and other biological tissues. However, it was a challenge to significantly improve mechanical robustness and electrical conductivity owing to their trade-off. To address this, a synergistic method of freeze casting and hot pressing was proposed to create a multiscale hierarchical structure in the hydrogel. In detail, the mixture of poly(vinyl alcohol) (PVA) and cellulose nanofibers (CNFs) with dispersed carbon nanotubes (CNTs) was freeze-cast and freeze-dried, then hot-pressed to enhance the crystallinity, and swollen to form an oriented PVA/CNT@CNF hydrogels. The resulting hydrogel (CNT:PVA mass ratio of 2:10) exhibited good comprehensive performances, with an electrical conductivity of 0.124 S m^-1, a tensile strength of 6.22 MPa, and a crystallinity of 13.98 wt %. Notably, the fracture energy displayed pronounced anisotropy, reaching 16.41 kJ m^-2 along the frozen direction, 2.47 times that in the perpendicular direction. Owing to its high conductivity and structural stability, the assembled flexible sensor can reliably monitor human motions (e.g., finger and wrist bending) and functions effectively as a touch-sensitive keypad for rapid and accurate human-machine interaction. This work offered a new strategy to prepare conductive hydrogels with high crystallinity for wearable sensing, health monitoring, and soft robotics.