Remotely-Powered Batteryless Implantable Local Temperature Monitoring System for Freely Moving Mice

Brown adipose fat tissue of a mouse dissipates the animal’s energy in the form of heat. Chronically recording the temperature of this tissue helps biologists to study the metabolism of the animal and gain new insights about the obesity in humans. Such temperature recordings have never been carried out previously. To obtain accurate data during the in vivo tests, the stress introduced on the animal should be minimal. This requires a light-weight, and small sized implant that wirelessly transmits the sensors data to the base-station and allows the mouse to freely move inside the cage. Local temperature measurements also need miniature sensors that can be inserted in the brown fat tissue. The existing implantable systems are not suitable for the specific requirements of this study. In this thesis, a local temperature monitoring system with a light-weight, small sized, remotely- powered, and batteryless implantable unit is developed. The system consists of the base- station which supplies power to the implantable unit via magnetic coupling. The power transfer coils are designed to cover the entire area of a standard size cage. The base-station also receives the sensor data and sends it to a PC for further analysis. The implantable unit performs three main functions. It harvests the energy from the magnetic fields and creates a stable supply for other blocks of the chip. The sensor readout digitizes the sensor response. Finally the data is sent to the base-station by a transmitter. Simple digital circuits are also implemented to control operation of different blocks and generate data packets for communication. The thesis mainly focuses on low-power implementation of the remotely powered chip. Several circuit and system level techniques have been proposed to achieve this goal. A semi-passive rectifier is proposed to efficiently convert the induced AC voltage to a DC voltage for the rectifier loads below 100 μW. High sensitivity thermistors are used to measure the temperature. An ultra low-power time-domain sensor readout circuit is also proposed. This SAR algorithm based circuit directly digitizes the resistance and does not need any low-noise front-end amplifier for operation. A free running oscillator is employed as the transmitter. The communication data-rate is increased by implementing a wide band non-coherent receiver for on-off keying modulated signals. In the system level design, implantable chips sequentially perform different operations in time. The data averaging is also performed at the base-station to achieve higher accuracy. Two prototype implantable chips have been fabricated in 0.18 μm CMOS technology. The first system is continuously powered and needs at least 29 μWto start operation. It records data of one thermistor with accuracy of ±0.09 ◦C. To avoid the interference between the power carrier and data communication, the second system time interleaves power transfer, sensor readout, and data communication. This chip also includes a feedback loop that adjusts the duration of power transfer frame to control the rectified voltage level. This chip dissipates 30 μW while recording two thermistors with accuracy of ±0.05 ◦C. The second fabricated chip has been mounted on an implantable PCB with dimensions of 25 by 14mm2 together with the power reception coil, transmitter antenna and two temperature sensors. This PCB is covered with bio-compatible silicone and weights less than 1.2 g. It has been implanted in a laboratory mouse with weight of about 35 g. First implanted experiment shows vulnerability of data communication to changes in antenna’s environment. Further in vivo tests are provisioned to prove the capability of the developed system on recording real-time high accuracy temperature data.