Mechanical characterization of soft substrates for wearable and washable electronic systems

Integration of complex electronics into our everyday outfit is a key challenge for the tomorrow-wearable technologies. When performing data processing and communications, these circuits are an essential part of sensing and actuating smart systems. The choice of an appropriate electronic substrate is essential for robust and reliable, yet imperceptible systems. In this paper, we study and mechanically improve the conventional substrates of the flexible circuit board (FCB) industry for highly flexible and washable textile-integrated circuits. First, we study the mechanical limits of polymeric substrates to better apprehend how permanent damages occur during washing cycle. Measurements of plasticization threshold of our films under bending showed that bending radii below 1 mm occur during washing, and that 13 μm-thick polyethylene terephthalate (PET) film can be elastically deformed down to 0.4 mm of bending radius. Second, we focus on the electrical and mechanical behavior of a stack composed of a PET substrate, aluminum conductive tracks, and polydimethylsiloxane encapsulation. An analytical model of stacks under bending is correlated with experimental results to demonstrate the impact of the thicknesses of the different layers and to improve the reliability under washing. Using the same model, we investigate different materials for encapsulation to further optimize the mechanical reliability of FCB while decreasing its overall thickness