Foldable, Eco-Friendly and Low-Cost Microfluidic Paper-Based Capacitive Droplet Sensor

Conventional electronic components are non-foldable and non-flexible with complex and expensive fabrication processes. Considering the recent interest in foldable electronics, we report a foldable, eco-friendly, low-cost, highly sensitive, and stable microfluidic paper-based capacitive droplet $(\mu$ PCD) sensor. The sensor is fabricated using readily available materials (filter paper and copper tape) through a simplistic, easy-go- designed, and low-cost bench-top fabrication process. The design of the capacitive sensor is based on a parallel plate capacitor attained by direct deposition of copper tape on filter paper. The filter paper serves as a dielectric. Very small quantities $(\mu$ Ls) of liquid can easily flow through filter paper due to micro-pores in it. The $10 \times 10$ mm sensor can detect water droplets as small as $10 \mu \mathrm{L}$ with sensitivity $3 \pm 0.5$ pF$/ 1 \mu \mathrm{L}$. The sensor response is insensitive to bending as tested by measuring the capacitance response at different bending radius from 0 to 7mm. The sensor responds accurately and repeatedly for different volumes of water droplets at different bending radius. Foldability of the sensor makes it suitable for the environment with different geometries, shapes, and dimensions like industry, body parts, and objects for liquid detection.