Force-measuring touch sensors are widely used in the fields of automotive, industrial, and medical applications. Recently, capacitive-type touch sensors have attracted considerable attention in these fields because these sensors are well known to have low temperature sensitivity, low power consumption, and a robust structure. Recently, there have been many researches on flexible capacitive-type touch sensors for curved surfaces while having these advantages. However, the main target is to apply it to a rigid curved surface which is not deformed. Most of these sensors have calibration problems because their initial values and sensitivities change depending on the curvature of the surface. In recent years, it is also necessary to apply force to surfaces that are fluidly actuated in the field of soft robotics or well-deformed surface (e.g., biological surface, artificial skin). In this case, the ideal touch sensor measures the force up to tens of kPa without changing the initial value and sensitivity according to the applied surface curvature. However, there is not much research on bending-insensitive touch sensors at present, and studies on existing bending-insensitive touch sensors have small force measurement ranges (~ 2 kPa).
In this paper, studies were conducted to develop a bending-insensitive capacitive-type touch sensor capable of measuring a relatively wide range of forces (up to tens of kPa). The sensor consists of top and bottom electrode layers, and there is a dielectric layer in between. The bottom electrode is made of patterned FPCB and the top electrode is made of stretchable electrode, so it can be easily extended to bending. The study contents are as follows: 1) We fabricated two types of stretchable top electrodes applicable to our devices by using AgNW and AgNW/PEDOT: PSS hybrid materials, which are typical materials for stretchable electrodes, to fabricate electrodes with little change in resistance according to the stretch. The thickness of the electrodes is determined by coated number using mayer rod method. Through resistance change analysis according to stretch, we can confirm that fabricated electrodes using AgNW/PEDOT:PSS hybrid materials have very small initial sheet resistance (~0.8 Ω/sq) and gauge factor of about 3. 2) We analyzed the effect of resistance change caused by stretching of the top electrode on capacitance change of the sensor while maintaining the distance between top and bottom electrodes constant. 3) Finally, depending on the geometry of the structures forming the dielectric layer (e.g., height and shape of the structures, and distance between structures), the distance between the top and the bottom electrodes is determined during bending, which causes a change in the capacitance of the sensor. Through the bending tests, we propose a dielectric layer structure that has the smallest change.