Ionic Liquid-based Physical Sensors
Use of ionic liquids has grown in recent years, particularly for various chemical applications, because of their unique material properties. In recent decades, several wearable devices and flexible electronics have been developed, and ionic liquids have been directly incorporated to construct physical sensors within these devices due to their low volatility, thermal stability, mechanical flexibility and reversible deformability. Here, several representative ionic liquid-based physical sensors are reviewed based on their sensing schemes: capacitive and resistive sensing. For the capacitive physical sensors, a pressure sensor housing ionic liquid in a deformable chamber that can be integrated within a microfluidic device is demonstrated. In addition, a tactile sensor consisting of an array of ionic liquid nano-liter droplet capacitors is developed that exhibits high sensitivity and rapid response. For resistive sensors, a concept of electrofluidic circuit, ionic liquid-filled fluidic channels, is introduced. By arranging electrofluidic components, pressure sensors with great long-term and thermal stability, and the sensors with analog and digital operation functions, are demonstrated. Adopting the electrofluidic concept, a flexible and optically transparent keypad is also reviewed. The ionic liquid based physical sensors provide desired advantages for various applications and are useful for future development of new types of physical sensors.