Ionic Flexible Sensors: Mechanisms, Materials, Structures, and Applications

Abstract

Over the past few decades, flexible sensors have been developed from the "electronic" level to the "iontronic" level, and gradually to the "ionic" level. Ionic flexible sensors (IFS) are one kind of advanced sensors that are based on the concept of ion migration. Compared to conventional electronic sensors, IFS can not only replicate the topological structures of human skin, but also are capable of achieving tactile perception functions similar to that of human skin, which provide effective tools and methods for narrowing the gap between conventional electronics and biological interfaces. In this review, the latest research and developments on several typical sensing mechanisms, compositions, structural design, and applications of IFS are comprehensively reviewed. Particularly, the development of novel ionic materials, structural designs, and biomimetic approaches has resulted in the development of a wide range of novel and exciting IFS, which can effectively sense pressure, strain, and humidity with high sensitivity and reliability, and exhibit self-powered, self-healing, biodegradability, and other properties of the human skin. Furthermore, the typical applications of IFS in artificial skin, human-interactive technologies, wearable health monitors, and other related fields are reviewed. Finally, the perspectives on the current challenges and future directions of IFS are presented.

All-Nanofibrous Ionic Capacitive Pressure Sensor for Wearable Applications

“…The pressure response of IPS(80) under different humidities (33–95% RH) is shown in Figure S12. The adsorption of water molecules can also promote the dissociation of the IL in IL-TPU NFMs, which results in the fact that the response of the sensor increases with humidity . Owing to the hydrophobicity of TPU and [EMIM] + [TFSI] − , the pressure response of IPS(80) does not exhibit obvious drift in the range of 33–75% RH.…”

Fatigue-free artificial ionic skin toughened by self-healable elastic nanomesh

“…In terms of the critical requirements for long service life, the material should not only heal itself upon damage but also resist crack propagation during fatigue loads. However, striking a balance between self-healability and fatigue resistance has proved to be a challenging task for stretchable ionic conductors (also known as ionic skins)-the most important artificial analogs of human skin with similar moduli and ion-conducting nature 5 – 7 . Noteworthily, most self-healable ionic skins are produced by incorporating dynamic covalent or physical crosslinks in the ion-conducting network which reconfigures through chain re-arrangement 8 – 10 .…”

Biological Hair-Inspired AgNWs@Au-Embedded Nafion Electrodes with High Stability for Self-Powered Ionic Flexible Sensors

“…Compared to conventional rigid sensors, IFS exhibit distinct properties such as deformability, superhydrophilicity, and biocompatibility and have shown broad application prospects in fields such as medical monitoring, human-interactive technologies, and electronic skins. As one of the key elements in the IFS for collecting ion charges and transmitting signals, FEs are facing a new challenge, that is, being flexible and stable. − Standard FEs are typically prepared by sputtering indium tin oxide (ITO) thin films with excellent electrical conductivity. However, researchers are looking for alternatives to ITO because of the facts that ITO films are too brittle to meet the demands for next-generation flexible electronics.…”