Thermally Induced Gelation of Cellulose Nanocrystals in Deep Eutectic Solvents for 3D Printable and Self-Healable Ionogels

Abstract

Soft ionotronics with self-healing capability and tough mechanical properties are highly desirable for wearable sensors that monitor physiological signals. Rapid prototyping by 3D printing further expands design spaces to enhance the performance of wearable sensors. However, it remains a challenge to achieve printability, mechanical toughness, and self-healing capability simultaneously. Here, we demonstrated a simple route to 3D printable ionogels by thermally induced gelation of cellulose nanocrystals (CNCs) in a deep eutectic solvent (DES). Although DES has been used as a nonvolatile and ionic conductive medium for ionogels, a large quantity of CNCs has to be dispersed in DES to achieve the ideal rheological behavior required for the direct ink writing process. Our strategy significantly reduced the concentration of CNCs needed to prepare printable inks with strong physical networks in DES by triggering the desulfation of CNCs at high temperatures. Further, photopolymerization of acrylic acid and acrylamide with Al3+ ions in the composite inks led to ionogels that contained multiple types of dynamic bonding. Compared with common hydrogels, our DES ionogels exhibited high mechanical toughness and self-healing capability to extend the lifetime of ionotronics. The ionogels were then printed as triangular lattice structures to increase the sensitivity of wearable sensors. In short, we demonstrated a strategy to fully utilize renewable nanomaterials, CNCs, for robust wearable ionotronics.