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.

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Lysozymes in human urine have crucial clinical significance as an indicator of renal tubular and glomerular diseases. Most lysozyme detection methods rely on the enzyme-linked immunosorbent assay (ELISA), which is usually a tedious procedure. Meanwhile, aptamer sensors and fluorescence-based techniques for lysozyme detection have emerged in recent studies. However, these methods are time-consuming and highly complex in operation, and some even require exorbitant reagents and instruments, which restricts real-time clinical monitoring as diagnostic approaches. Therefore, a rapid and low-cost lysozyme detection method with facile preparation is still in demand for modern precision medicine. Herein, we propose a magnetoelastic (ME) immunosensor for lysozyme detection by detecting changes in resonance frequency under a magnetostrictive effect. The detection system is composed of a magnetoelastic chip with an immobilized lysozyme antibody, a solenoid coil, and a vector network analyzer. Since the ME sensor is ultrasensitive to mass change, the frequency offset caused by mass change can be utilized to detect the content of lysozyme. The immunosensor is evaluated to possess superior sensitivity of 138 Hz/μg mL–1 in terms of the resonance frequency shift (RFS). In addition, our sensor displays an outstanding performance in specificity experiments and shows a relatively lower detection limit (1.26 ng/mL) than other conventional lysozyme detection methods (such as ELISA, chemiluminescence assay, fluorescence, and aptamer biosensors).

Ionic flexible sensors (IFS) usually consist of an ionomer matrix and two conductive electrodes, the failure of which mostly originates from interfacial debonding between matrix and electrode layers. To improve electrode's adhesion and impedance matching with matrix, polymer binder or plasmonic heating technology is used to enhance the adhesion of electrodes, but there are technical challenges such as high resistance and harsh conditions. Herein, inspired by biological hair, we proposed a reliable and facile method to form AgNWs@Au-embedded Nafion flexible electrodes (AN FEs) for IFS without rigorous temperature and harsh conditions. Through integrating the spraying and electrodepositing Au method, we achieved that the AgNWs are partly embedded in the matrix layer for forming the embedded layer, similar to the root of biological hair, which is used to fix the FEs and collect the ion charges. The other parts of AgNWs exposed on the surface form the conductive mesh layer for transmitting the signal, analogous to the tip of biological hair. Compared with other AgNWs FEs, AN FEs exhibit high adhesion (∼358 kPa) and low sheet resistance (∼ 3.7 Ω/□), and high stabilities after 100 washing cycles, 200 s H 2 O 2 corrosion or 1500s HCl corrosion. A self-powered IFS prepared by AN FEs can achieve dual sensing of mechanical strain and ambient humidity and still has promising sensing performance after being exposed to air for 2 months, which further indicates potential applications of the prepared FEs in next-generation multifunctional flexible electronic devices.