Anisotropic cellulose nanofibril piezoionic organohydrogel fabricated by directional freezing for flexible strain...

Flexible self-powered strain sensors based on piezoionic hydrogels demonstrate great promise for wearable devices, e-skins, and health monitoring. However, existing piezoionic hydrogels typically suffer from limited piezoionic output performance, falling short of the demands of high-performance, self-powered wearable sensors. This study addresses this challenge by constructing directional ion transport channels using a cellulose nanofibril (CNF)-assisted directional freezing technique. CNF acts as nanofillers that enhance ion selectivity in the channels while improving the mechanical properties of polyvinyl alcohol (PVA)/CNF organohydrogels, enabling efficient directional ion transport with K₃[Fe(CN)]₆ as an ion source. The resulting organohydrogel exhibits high extensibility (479.61 %), ultimate stress (3.63 MPa), and transparency (90 %). Under a pressure of 5 N, it generates an output voltage of up to 199.45 mV within 150 ms, demonstrating a strong linear relationship between stress and output voltage in the range of 2-5 N (R² = 0.99). After directional freezing, the output voltage and current increased by 2.49 and 1.94 times, respectively. Additionally, the selective enhancement of ion mobility by CNF has improved the piezoionic output, resulting in a 2.02-fold increase in piezoionic voltage. These high-sensitivity piezoionic organohydrogels, with their efficient electromechanical conversion capabilities, indicate a promising future for flexible strain sensors.