Development of Ingestible Thermometer with Built-in Coil Antenna Charged by Gastric Acid Battery and Demonstration of Long-Time in Vivo Telemetry

A safe and affordable ingestible thermometer measuring core body temperature has the potential to become a future healthcare device for versatile applications in daily life. In this study, we developed an ingestible thermometer charged by a gastric acid battery. The device can operate in bowels by using the charged energy in multilayer ceramic capacitors as a storage capacitor. Adopting this strategy for energy storage solves the issues related to a conventional button battery: risk of injury to the digestive tract, bad disposability, and degradation. Additionally, to make it easy to assemble a coil antenna and electrical circuits in the device automatically, we developed a fabrication process based on a vertical stacking process of printed circuit boards with coil patterns. The dimensions of the prototyped device were smaller than those of existing ingestible thermometers. In an experiment involving a dog, we successfully recorded the temperature in the digestive tract for 24 h in cycles of approximately 10 or 20 min, using a rectal thermometer and an existing ingestible thermometer as references. The temperature variations in time among our device, rectal thermometer, and existing ingestible thermometer were almost parallel. The recording ability of the core body temperature using our device has the potential to measure basal body temperature during sleep, the circadian rhythm, and fever type easily and robustly in daily life. Our ingestible thermometer is a step toward the development of sensors that can be swallowed for preventive medicine and health promotion.