Industrial-grade bending insensitive, transparent nano-force touch sensor

Abstract

Force touch sensors that recognize the location and pressure of external stimulus has attracted attention for application in commercialized smartphones, but they also have huge potential for future electronics, such as wearable health monitoring devices, flexible displays, and humanoid robots. To realize high-performance force touch sensors, enormous amounts of research have been performed, concentrating on high sensitivity and/or functionalities, such as flexibility and transparency. However, it is still challenging to demonstrate a force touch sensor that can be practically used in dynamically flexible applications because of inevitably induced mechanical stress in sensor materials during bending and/or releasing. In this dissertation, we newly report a nano-force touch sensor, which detects mechanical stimulus not only with high sensitivity and transparency, but also without performance degradation in dynamically flexible (bending) situations. A key is the development of a hierarchical nano-composite (HNC) film that consists of a nano-structured plastic base, coplanar electrodes, and a compressible polymer dielectric possessing metal nano-particles. Through theoretical and experimental investigation of the proposed HNC film under external stimulus, we confirmed that the external stimulus can generate maximized internal stress due to the unique nano-structures to enhance sensitivity. Also, stress occurs near the structural neutral plane of the HNC film, resulting in bending-insensitive operation of the device. Since the developed sensor relies on a conventional semiconductor fabrication process, a large-area (7-inch) nano-force touch sensor shows industrial-grade performance in terms of uniformity and reproducibility. Using the proposed method, the 7-inch nano-force touch sensor was successfully integrated into a commercial smartphone, and it recognized pressure as well as the touch position of a human finger on the smartphone. Also, since the bending-insensitivity of the nano-force touch sensor, the health monitoring devices successfully detected human pulse and foot pressure as well as maintaining constant pressure sensing performance in a variety of bending situations.