Design and fabrication of a flexible capacitive coplanar force sensor for biomedical applications

Abstract

Flexible capacitive sensors are acquiring more importance in nowadays applications. They are being used in a wide variety of fields, from fingerprints reading to force sensing, from cell growth evaluation to touch sensing. In the biomedical field, research is pushing towards micrometer scale capacitive sensors, requiring a complex fabrication process. This project aims at the design of an easy-to-use and low-cost capacitive force sensor for biomedical applications. For this purpose, a coplanar geometry is studied, developed and few prototypes were fabricated. In the current configuration, sensors' size is 4 mm thick, with a diameter of 8.5 mm but different sizes can be adopted as well. Furthermore, the capacitive sensor is made by a wafer of 3 different polymeric layers, in which is inserted a flexible polyimide PCB (130 μm thick). The prototypes were tested with a maximum force of 1 N, but wider range are permitted. In the current configuration, the tested sensors have a sensitivity of 172 fF/N, a resolution of 80 mN, a hysteresis of 20 fF and the percentage of nonlinearity is 1.087%. Its field of application could vary from sitting posture sensing, insole pressure sensor, tongue pressure monitoring, tactile pressure and many other biomedical applications in which a soft surface is needed.