Tannic acid-coated cellulose nanocrystal-reinforced transparent multifunctional hydrogels with UV-filtering for wearable flexible sensors

Transparent multifunctional cellulose-based conductive hydrogel for wearable strain sensors and arrays

Conductive hydrogels as promising candidate materials for flexible strain sensors have gained considerable attentions. However, it is still a great challenge to construct hydrogel with multifunctional performance via natural polymer. Herein, a novel multifunctional conductive hydrogel based on methylcellulose and cellulose nanocrystal was prepared via a facile and low-cost strategy. Methylcellulose (MC) was introduced to not only guarantee the stability of tannic acid coated cellulose nanocrystal (TA@CNCs) in LiCl solution, but also improve anti-freezing ability. The obtained hydrogel exhibited high transparency (98 % at 800 nm), good stretchability (663.1 %), low temperature tolerance (-23.9 °C), superior conductivity (2.89 S/m) and excellent UV shielding behavior. Flexible strain sensor assembled by the prepared hydrogels can be used to detect human body motions include subtle and large motions, and exhibited good sensitivity and stability over a wide temperature range. Multiple flexible hydrogels can also be assembled into a 3D sensor array to detect the distribution and magnitude of spatial pressure. Therefore, the hydrogels prepared via natural polymers will have broad application prospects in wearable devices, electronic skin and multifunctional sensor components.

Highly stretchable anti-freeze hydrogel based on aloe polysaccharides with high ionic conductivity for multifunctional wearable sensors

Conductive hydrogels have limitations such as non-degradability, loss of electrical conductivity at sub-zero temperatures, and single functionality, which limit their applicability as materials for wearable sensors. To overcome these limitations, this study proposes a bio-based hydrogel using aloe polysaccharides as the matrix and degradable polyvinyl alcohol as a reinforcing material. The hydrogel was crosslinked with borax in a glycerol-water binary solvent system, producing good toughness and compressive strength. Furthermore, the hydrogel was developed as a sensor that could detect both small and large deformations with a low detection limit of 1 % and high stretchability of up to 300 %. Moreover, the sensor exhibited excellent frost resistance at temperatures above -50 °C, and the gauge factor of the hydrogel was 2.86 at 20 °C and 2.12 at -20 °C. The Aloe-polysaccharide-based conductive hydrogels also functioned effectively as a wearable sensor; it detected a wide range of humidities (0-98 % relative humidity) and exhibited fast response and recovery times (1.1 and 0.9 s) while detecting normal human breathing. The polysaccharide hydrogel was also temperature sensitive (1.737 % °C-1) and allowed for information sensing during handwriting.