Flexible Touch Sensors Made of Two Layers of Printed Conductive Flexible Adhesives

Touch sensors are crucial in controlling robotic manipulation when a robot interacts with environmental objects. In this study, multilayer flexible touch sensors in the form of an array were developed. The sensors use ink-type conductive flexible adhesives as electrodes which were printed on polyethylene terephthalate (PET) films in a parallel equidistance stripe pattern. Between the two printed layers, a double-sided adhesive film was used to combine each layer and was perforated at the junctions of the top and bottom electrodes with different-sized circles. These holes represent switching mechanisms between the top and bottom electrodes, and their sizes make the sensor respond to different levels of external pressure. We showed the durability of the fabricated sensor with 1 mm diameter holes by repeated experiments of exerting normal pressure ranging from 0 to 159.15 kPa for 1000 cycles. In case of 1 mm diameter holes, the state of each sensor node was reliably determined by the threshold pressures of 127.3 kPa for increasing pressure and 111.4 kPa for decreasing pressure. On the other hand, decreasing the hole size from 3 to 0.5 mm caused an increase in the threshold pressure from 1.41 to 214 kPa. The relation between the hole size and the threshold pressure was analyzed by a mechanical model. The sensor performance was also verified on curved surfaces up to 60 mm radius of curvatures. Additionally, we fabricated a sensor with three levels of sensitivity with a conventional method which was a thermal evaporation to show the extendibility of the idea.