Vacuum Lamination of a Stretchable Sensor System in PolyPropylene

The explosive growth of the so-called Internet-of-Things, where more and more everyday objects are becoming `smart' and connected, demands for reliable integration technologies for electronics in all kinds of materials. If we want these electronics to be as least intrusive as possible, they preferably conform to the shape of the contemplated object. In this article we want to present a technique that allows the integration of a smart sensor system in a thermoplastic material (polypropylene, PP) by vacuum lamination. This laminated stack can then be thermoformed from a flat sheet into the desired 3D shape. The sensor system in question is a sensor bus incorporating three inertial movement sensors. Each sensor is placed on a separate small, thin (200μm) FR4 PCB together with some necessary peripheral components. These smart sensor nodes are then placed on a flexible, stretchable circuit, which is then laminated between two 2mm-thick PP sheets. These sheets can subsequently be heated and thermoformed. In this particular case, the PP was used to create a smart ankle-foot orthosis (AFO). Although long-term reliability needs to be improved, we can show that the discussed technology allows for a successful lamination and thermoforming. This paper will elaborate on the sensor system, stretchable bus system and lamination technique, together with the encountered problems and implemented mitigations. While the current application is the aforementioned AFO, the sensor system can easily be expanded to other types of sensors and the thermoforming process allows for a wide range of possible applications.