Publication
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 f
Sandro Carrara, Diego Ghezzi, Gian Luca Barbruni