Shape reprogramming of 3D printed ionogels by solvent exchange with deep eutectic solvents

Extrusion-based 3D printing, such as direct ink writing (DIW), is an emerging technique used to fabricate soft materials for advanced applications. For example, ionogels with sufficient stretchabilities and long-term stabilities have been printed for use as wearable sensors to monitor human activities. However, DIW printing of geometrically complex structures is challenging without supporting materials. Additionally, most ionogels exhibit insufficient mechanical toughness. We present a simple strategy with which to prepare 3D printable and tough ionogels that can be adjusted to any topography. The ink for 3D printing is a dispersion of cellulose nanocrystals (CNCs) in an aqueous solution of polyvinyl alcohol (PVA). After printing, the ink underwent a freeze‒thaw process to form soft but stretchable hydrogels via crystallization of the PVA. Hydrogels with planar geometries were then reconfigured into the desired 3D shapes. Finally, the water in the hydrogels was replaced by a deep eutectic solvent (DES) composed of choline chloride (ChCl) and glycerol. The solvent exchange process induced PVA crystallization to form ionogels with higher mechanical toughnesses than common hydrogels. We then implemented this simple method to prepare wearable sensors that adapted to the irregular surfaces of human bodies. This work provides a shape-reprogramming strategy for expanding the design space of wearable sensors.