Ultra-thin sensor array for 3D curvature sensing

Flexible and stretchable systems-in-foil allow easy attachment to bodies with non-planar shapes and therefore offer a very attractive approach to new forms of sensing dynamic 3D shape changes. They may find applications in structural health monitoring or in the medical fields. This paper describes the design and fabrication of a novel 3D flexible curvature sensor-array on a plastic foil substrate which could be used for respiration monitoring of newborns. Each sensor element consists of four strain gages in a Wheatstone bridge configuration. To suppress sensor response on foil stretching and to increase sensitivity to foil bending the strain gages are located on opposite foil surfaces. Two resistors of the Wheatstone bridge are placed on the top and the two others, which are orthogonal to the top side resistors, on the bottom of the foil substrate. Thereby, an output signal can be achieved, which is at least 100% higher when compared with a one-sided sensor design. To characterize the sensor, bending experiments have been performed of both the double-sided and one-sided sensor designs. As the carrier foil material, we used a SU-8 photoresist additionally encapsulated with Polyimide from both sides to protect the sensing elements. The resistors are made of gold and are fabricated by a sputter process with subsequent photolithography. The advantage of our process sequence is that the complete double sided sensor with a thickness below 50 mu m can be fabricated without the need to flip over the substrate in between. A key challenge in the fabrication process is the interconnection between the top and the bottom resistors. The interconnect vias are made in a photo definable interlayer and can withstand the bending experiments without disruption.