Flexible sensor based on self-assembly of halloysite nanotubes in ionic liquids/thermoplastic polyurethane composites

Ionogels are gradually entering the flexible sensor field due to their excellent ionic conductivity, thermal stability and wide electrochemical window. It is critical to balance the mechanical and conductivity properties of ionogels for further application. Herein, inspired by vapor-induced phase separation strategy and self-assembled strategy, a flexible strain-sensor was developed by adding ionic liquid (IL) decorated halloysite nanotube (HNT) into thermoplastic polyurethane (TPU) matrix. As a phase separation material, the rigid regions of TPU offered strength and toughness for ionogels, while the soft regions offered high stretchability. Due to electrostatic interaction, the cations and anions of IL self-assembled onto the inner and outer surface of HNT. The HNT obtained orderly arrangement structure after shear process, which provides effective ionic transport channels for IL. As a strain sensor, the as-prepared TPU@IL@HNT sensor exhibited highly gauge factor, fast response speed (400 ms), and excellent cycle stability (5625 cycles) at a strain of 40 %. A linear relation between strain and resistance change was found for TPU@IL@HNT sensor, which is an important criterion for their application on strain sensors. As a whole, the TPU@IL@HNT sensor prepared in this work could serve as an excellent sensing unit for wearable devices.