Highly Flexible Capacitive Pressure Sensor Responsive in Wide Pressure Range with High Sensitivity

Abstract

Flexible pressure sensors have been attracting a great amount of interest as they have a potential to serve as core elements in realizing artificial skin, human-machine interface, and wearable healthcare products. To apply pressure sensors to the aforementioned applications, they should not only be highly sensitive in low pressure region to detect small stimuli such as tactile or blood pulse, but also be operable over a wide pressure range to catch up with the wide dynamic range of human skin. In addition, they need to be thin enough as they can be applied to high flexible products. For this reason, film-type pressure sensors using a variety of materials and structures have been widely studied in recent years. However, it has been challenging to realize a wide pressure sensing range and high flexibility at the same time, because there is a trade-off relationship between the operation range and the thickness of pressure sensors. Furthermore, it is particularly challenging to realize a multi-modal sensor having high sensitivity in low pressure region as well. In this study, we propose a flexible capacitive pressure sensor using a structured ionic gel film as a dielectric layer whose capacitance is responsive to applied pressure. The ionic gel film is largely beneficial for high sensitivity attributed to high capacitance based on electric double layers, and it has suitable mechanical properties as a pressure sensing material because it has low Young’s modulus similar to that of rubber. By structuring the surface of the ionic gel film in several micro-meter scale to utilize confined air between the ionic gel and an electrode, linear response to large pressure range of over 100 kPa was secured. In addition, the complete pressure sensor was shown to be highly flexible because the thickness of the ionic gel film can be reduced near or even below 10 μm. By using an appropriate surface structure of ionic gel film, the sensor shows distinctive response to low and high pressure regions so that it has a multi-modal sensing capability for tactile with a high sensitivity of 1.1 kPa-1 at a low pressure of 470 Pa and a uniform linearity signals up to a high pressure of 100 kPa . With several μm-thick plastic substrates at both bottom and top sides of the ionic gel film, the overall thickness of the pressure sensor can be as small as ca. 20 μm so that the sensor has high degree of flexibility, e.g. foldability or wrinkability. Together with multi-modal sensing capability with high sensitivity and wide pressure range, we believe the proposed pressure sensor can be used for the applications requiring both high flexibility and high performance such as wearable or body-attachable devices, and thus will play a key role to realize artificial skin for prosthetic bodies and smart healthcare.