Volume and concentration identification by using an electrowetting on dielectric device

An ultra-sensitive electrowetting on dielectric (EWOD) capacitance measurement system is proposed in this study. A 24-bit integrated circuit (IC) capacitance-to-digital (CDC) sensor is used to convert the capacitance changes caused by variations in droplet volume and concentration to digital data. A 2.3 mm by 2.3 mm printed circuit board (PCB) based electrode pair is used to sense the analog capacitance change. The capacitance of pure water and NaCl solutions are tested by the CDC system. Desktop-drawing software is used to fix the position of the droplet to obtain a small capacitance deviation of 10 IF. Result shows the CDC system can resolve the capacitance changes caused by adding 0.1 micro-liter droplets or by increasing the NaCl concentration by 1%. The digital output of the sensor is interface-friendly to microcontrollers.