Low-Power Implanted Sensor for Orthodontic Bond Failure Diagnosis and Detection

This paper presents a wireless low-power capacitive humidity sensor for the detection of the bond failure between the tooth and orthodontic braces. The system comprises of a wireless power transfer circuitry, a humidity sensor, and a rectifier. A humidity sensor consists of two branches of inverters having reference and sensing capacitance connected to their outputs. Change in humidity levels will change the capacitance of the sensing element, creating a phase difference between the outputs of inverters. Taking into account the mismatch and process variation errors a minimum threshold of 50 fF change in the sensing capacitance is chosen for detection. The chip was designed and fabricated using 180-nm RFCMOS Global Foundries process. The capacitive sensing element is implemented using top metal for postprocessing. The implantable microchip is powered up by a specifically designed wireless power transfer system. It considers the mouth complex environment and variable loading. A good 13.9% of coupling efficiency was achieved with a 1-cm separation between the internal chip and external powering inductive coils. To the best of our knowledge, it is believed that this is the first wireless electronic solution to be developed for orthodontic diagnosis.