Low-density foam as an adhesion facilitator between silicone rubber and copper sheet for use in flexible microwave antennas

An effective solution is presented to address the issue of poor adhesion of metal to silicone for the realization of flexible antennas for wearable applications. The low surface energy of silicone creates difficulties in adhering metal layers to it. Low-density polyurethane (PU) foam is used as an adhesion facilitator for interlocking silicone and a good adhesion strength with the copper layers is obtained via a commercially available polyurethane (PU) binder. The developed multi-layered substrate illustrates a high elongation at break of around 375% and very low water absorbance. The microwave characterizations show that the substrate exhibits a low permittivity value of 2.12 and a loss tangent of 0.02 at 10 GHz. The application of the substrate for wearable purpose is demonstrated by fabricating a simple microstrip antenna and testing its performance in both flat and bending profiles. The antenna characteristics exhibit a gain greater than 7 dB. The gain and bandwidth are almost consistent for all the bending profiles.