Stretchable and conductive hydrogels have been intensively studied as wearable electronics to monitor the physiological activities of human bodies. However, it remains a challenge to fabricate robust hydrogels as sensors with complex 3D structures. Here, we designed a 3D printable ink from cellulose nanocrystals (CNCs), deep eutectic solvents (DESs), and ionically cross-linked polyacrylic acid (PAA). DESs composed of choline chloride and ethylene glycol served as a nonvolatile medium with high ionic conductivity. The dispersion of CNCs in a mixture of DESs, acrylic acid, and Al3+ ions formed ionogels with a reversible physical network for 3D printing. After the printing process, the ionogel was solidified by the photopolymerization of acrylic acid in the presence of Al3+ ions to form a second ionically cross-linked network. The first physical network of CNCs provides an energy-dissipating mechanism to make a strong and highly stretchable nanocomposite ionogel. When compared to hydrogels, we found that the DES/CNC nanocomposite ionogel was more stable in the air because of the low volatility of DESs. We further used the DES/CNC ink to 3D print an auxetic sensor with negative Poisson's ratios so that the sensor provided a conformal contact with the skin during large deformation. In addition, the auxetic sensor could continuously monitor and identify different motions of the human body by the change in resistance. These results demonstrate a simple and rapid strategy to fabricate stable and sensitive strain sensors from cheap and renewable feedstock.
Keywords: 3D printing; auxetic materials; cellulose nanocrystals; deep eutectic solvents; nanocomposite ionogels; stretchable electronics.