Cellulose nanocrystalline hydrogel based on a choline chloride deep eutectic solvent as wearable strain sensor for human motion

Owning to the viscoelastic properties, good biocompatibility and high strain sensitivity, choline chloride-based deep eutectic solvent (DES) hydrogels have been considered to be promising wearable strain sensors for human motion. However, traditional hydrogels are far away from the wearable strain sensor applications caused by their unsatisfied conductivity and weak mechanical properties. Herein, the strategy for functional ionic inorganic/organic interpenetrating (IPN) hydrogels preparation by cyclic freezing/thawing method was successfully developed. Polyvinyl alcohol (PVA) was proposed to dissolve in choline chlorede-based DES as hydrogel matrix for the first time. Encouragingly, the obtained DES/PVA/CNCs/g-C3N4 hydrogel (choline chloride with glucose) exhibits excellent mechanical properties, included excellent tensile strength (≈ 2.55 MPa), high elongation (≈1200 %) and satisfactory tensile modulus (≈3.65 MPa). Interestingly, the thermal diffusivity (the maximum value was 0.675 W/mK) and conductivity (the maximum value was 0.18 mm2/s) of the DES-hydrogels were successfully achieved through adding graphitic-like nitride nanosheet (g-C3N4) and sustainable cellulose nanocrystalline (CNCs). These enhancements were attributed to the synergistic interactions of powerful hydrogen bonding among DES, CNCs, g-C3N4 and PVA chains. More importantly, the as-prepared hydrogels have the potential as a human motion sensor to accurately in-situ detect human activities on the fingers, wrists, elbows and knee joints. Those hydrogel-type strain sensors with flexibility, excellent mechanical properties, self-recovery, good heat transfer, and electrical conductivity have broad application prospects in the fields of intelligent robot, bionic prostheses, and human care areas.