A Varactor-Based, Inductively Coupled Wireless pH Sensor

An inductively coupled, wireless sensor was fabricated for remote measurement of pH. The sensor consisted of a planar spiral inductor connected to a surface mount varactor, which was a voltage controlled capacitor, forming an inductive-capacitive (LC) resonant circuit. The pH electrodes, made of a thick-film antimony/antimony oxide sensing electrode and a thick-film silver/silver chloride reference electrode, were connected in parallel to the varactor. A voltage change across the electrodes due to the pH variation in the test medium would change the capacitance of the varactor, shifting the resonant frequency of the LC circuit. By inductively coupling the spiral inductor with a detection coil, the resonant frequency of the LC circuit was remotely monitored, allowing measurement of the pH. The advantage of the described pH sensor is its wireless and passive nature, which allows for long-term pH monitoring in inaccessible area. The sensor will be useful for remote monitoring of pH during industrial or food processes. When miniaturized, the sensors can also be used for biomedical applications such as remote tracking of gastric or esophageal pH on patients suffering from gastroesophageal reflux disease.