Iontronic Pressure Sensor Based on Modulation of Electrical-Double-Layer

An iontronic pressure sensor that uses modulation of electrical charges in the electrical-double-layer(EDL) due to pressure was demonstrated in this study.<br/>In recent years, tactile sensors using various pressure-sensitive materials such as piezoresistive, capacitive, piezoelectric, and friction types have been developed to detect a wide range of pressures for applications in artificial joints, robots, and wearable devices. Among pressure sensors, the capacitive type is being commercialized at the product level as touch screens and biometric authentication in smartphones and PCs, taking advantage of its features such as high sensitivity, low power consumption, and fast response time. In particular, capacitive pressure sensors using EDLs of iontronic thin films are capable of excellent sensing over a wide range and with high resolution. Ionic liquids (ILs), which are commonly used as materials for iontronic thin films, have high electrical conductivity, electrochemical stability, and thermal stability. Gelation of ILs in the liquid state to form ionotropic thin films provides structural stability and reduces the risk of ILs leakage. The principle of conventional iontronics based pressure sensors is that the capacitance of the EDL is directly proportional to the contact area between electrodes and surfaces of the iontronic thin film. In other words, it is necessary for external pressure to cause mechanical deformation of the iontronic thin film in order for sensing. Therefore, to use iontronic thin films as pressure sensitive materials, microfabrication and fine patterning processes are required. Even as a pressure sensor, having a fine pattern remains a challenge in terms of structural stability.<br/>In this study, we confirm that EDLs at the interface between iontronic thin films composed of ionic liquids and polymers and metal electrodes are modulated by the physical addition of pressure. This phenomenon is not due to mechanical deformation of the ionic liquid thin film, but to an increase in the charge of the EDL due to the application of pressure. Using this mechanism, IL-based iontronic thin films can function as deformation-independent pressure sensors. In this study, IL gel thin films fabricated without patterning were used as iontronic thin films. In addition, the signal was amplified by an electrical-double-layer transistor (EDLT). Iontronic thin film was applied as the gate insulating layer of the EDLT and the EDL modulation due to pressure was output as modulation of current value. Organic semiconductor material was selected as the channel material of the EDLT, and at low voltage the channel can be controlled by EDLs generated by IL gels. The use of a transistor as a signal amplifier also facilitated the use of a pressure sensor matrix. The pressure sensor matrix is important for pressure mapping, and this device is useful for applications in artificial joints, robots, and wearable devices.<br/>This research uses the new concept of ion modulation in EDLs, which expands the possibilities of devices using EDLs.