Highly Flexible Multi-Modal Capacitive Pressure Sensor Responsive to Tactile and Wide Pressure Range

Abstract

Flexible pressure sensors have been attracting great amount of attention as they are key elements in realizing artificial skin, friendly 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 tiny stimuli such as tactile or blood pulse, but be operable over a wide pressure range as well to catch up with the performance of human skin. In addition, they need to be thin enough as it can be applied to high flexible products. For this reason, film-type pressure sensors using a variety of materials and structures have been studied. 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 relation between dynamic range and thickness of the pressure sensors. Furthermore, the problem becomes more difficult considering 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 ultra-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 can 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 distinguished response to low and high pressure regions, and thus it has multimodal sensing capability for tactile and pressure signals by adjusting high sensitivity in low pressure region, 1.1 kPa-1 in the region below 700 Pa.

By using 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 ca. 20 μm, thus finally the sensor has high flexibility, e.g. foldability or wrinkability. Together with multi-modal sensing capability with high sensitivity and large 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 be a key role to realize artificial skin for prosthetic bodies and smart healthcare.