Flexible Capacitive Pressure Sensor with Optimized Interdigital AgNWs Electrodes for Wide-Range Pressure Monitoring

Flexible capacitive pressure sensors are widely used in many applications due to their simple structure and low cost. However, high sensitivity and wide linear range are often incompatible. To solve this problem, five kinds of capacitance structures based on interdigital microneedle array electrode (IMAE) with different needle spacing and needle height are simulated and fabricated in this work. The two face-to-face bonded microneedle arrays (MNAs) in the IMAE are fabricated by template casting with PDMS as substrate. Silver nanowires (AgNWs) dispersion liquids with different concentrations are spin-coated on the MNA to serve as conductive electrodes. PDMS doped with AgNWs is used as a dielectric adhesive layer to increase the dielectric constant. The AgNWs-coated IMAE can increase the effective electrode area and deformation capacity dramatically when compared to the flat metal electrode. The optimized IMAE sensor with a needle height of 600 μm and needle spacing of 800 μm makes a maximal sensitivity of 0.059 kPa^-1 from 0 to 60 kPa (linearity of 6.57%). The linear measuring range of the IMAE sensor is much wider than that of the sensor based on the flat electrode. By introducing NaHCO₃-induced air gap into the dielectric layer, the maximum sensitivity can be improved to 0.203 kPa^-1 from 0 to 3 kPa. The IMAE sensor has a fast response time (125/250 ms) and can maintain performance without degradation through 2500 test cycles. It shows good performance in wide-range pressure monitoring such as micromass (160 mg) detection, pulse monitoring, twist, and gesture measurement.