Ultrasonic thermal dust: A method to monitor deep tissue temperature profiles.

We demonstrate a tetherless, sub-millimeter implantable temperature sensing system employing ultrasonic powering and ultrasonic backscatter modulation assembled using commercially available components. We demonstrate two sizes of sensors based on available components with volumes of 1.45 mm3 and 0.118 mm3 . Individual sensors are able to resolve ±0.5°C changes in temperature, suitable for medical diagnostic and monitoring purposes. We verify less than 0.3°C drift in temperature readings over 14 days in physiological conditions. This approach is compatible with more sophisticated temperature sensors such as classic proportional to absolute temperature (PTAT) integrated circuits, as well as digital backscatter approaches. Our goal is to solve a long-standing issue: chronic and tetherlessly monitoring deep tissue temperature with high spatial accuracy.