A wireless passive pH sensor for real-time in vivo milk quality monitoring

We present a wireless passive sensor for remote in vivo milk pH measurement. The sensor consists of a planar spiral inductor connected in parallel to a varactor forming a LC resonant circuit. A pH combination electrode made of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode, is connected in parallel to the varactor. As the milk pH changes during spoilage, the voltage across the electrodes varies, shifting the resonant frequency of the sensor. For in vivo monitoring the sensor is sandwiched between dielectric spacers for encapsulation, and to reduce parasitic capacitive coupling and eddy current loss. The resonant frequency of the sensor is tracked remotely by an interrogator coil coupled to the sensor inductor. In vivo measurement shows that milk pH can be monitored with 100 kHz/pH sensitivity and 0.12 pH accuracy over 6.8-4.4 pH range.