Inkjet printed wireless biosensors on stretchable substrate

Wireless biosensors are suited for long-term monitoring of physiological signals. Inkjet printing wireless biosensors on stretchable substrate can be used to realize small and lightweight body monitoring devices. Current conductive inks have low processing temperature which makes it possible to use substrate materials with low temperature resistance. Conductive inks offer sufficient performance for realizing antennas and interconnections between components. In this work, we use thermoplastic polyurethane as the printing substrate. The inkjet printed conductors on polyurethane tolerate only moderate stretching. Despite the weak stretchability, polyurethane is a favorable option in realizing wireless biosensors since even modest stretching enables the biosensor to conform to the human body. The dielectric properties of the polyurethane were characterized using coplanar waveguides (CPW). The determined permittivity of the substrate is 3.2 and the loss tangent is 0.1. Due to high loss tangent CPW lines on polyurethane are much lossier at high frequencies than lines at PEN substrate. The performance of the sensor on PEN is studied with measurements and the antenna performance on polyurethane is investigated using simulations. The measured reading range of the sensor realized on PEN substrate at 300 µm distance from the body was 14 m. According to simulations changing to the lossy polyurethane substrate would not affect the performance of the sensors' antenna.