Research On Ultra-weak Coupled Near-field Communication For Tiny Implantable Medical Devices

Implantable medical devices have become an important tool used by doctors and bioscience researchers to diagnose and treat diseases.To reduce the risk and difficulty of implant surgery,miniaturization is the research trend of advanced implantable medical devices in the future.However,a key technical challenge in the miniaturization of implantable medical devices is how to achieve reliable NFC(near field communication)and wireless power transmission through an inductive link with ultra-low coupling(when the coupling coefficient is as low as 0.01).This dissertation aims to design a near field communication system that can coexist with wireless energy transmission on ultra-low coupled inductive links.The main innovations of this dissertation include the following aspects.(1)In order to realize simultaneous transmission of reliable information and energy under ultra-low coupling,this dissertation proposes an NFC scheme based on dual carriers.This scheme using dual carriers shares the inductive link with wireless power transmission,instead of using the conventional load modulation technique.An uplink carrier,whose frequency is different from that of energy carrier,is selected for the uplink signal from inside to outside of the body,so that the uplink(body-to-vitro)recerver is able to filter out the energy carrier interference in the frequency domain.Finally,the demodulation design of the uplink receiver is feasible under ultra-low coupling.(2)To minimize the interference from the high power energy carrier to uplink receiver,this research proposes an magnetic balanced inductive link for uplink receiver.By adding an in-vitro communication coil which overlaps with the in-vitro power supply coil,in-vivo signal can be coupled and picked up,and the energy carrier interference can be maximally ballanced-out.By optimizing the relative position parameters of the coils,the in-vitro power supply coil and the in-vitro communication coil are balanced in the magnetic field of each other.The magnetical-balanced inductive link significantly reduces the interference from wireless power transmission to NFC.The signal to interference ratio is improved by 65.72 d B compared to that of single coil pair inductive link.(3)In order to approach the limit of reliable NFC and stable power supply under ultralow coupling,the link budget analysis is conducted towards the goal of the receiver sensitivity physical limits.Following the budget,the matching circuit parameters of inductive links are optimized.(4)Crystal device cannot be used for oscillator in some implantable devices because of the size limitation.The research of the dissertation thus designed a low-power crystalless clock source circuit based on an all-digital frequency-locked loop.By tracking the frequency of unmodulated energy carrier,frequency-locked loop could avoid frequency errors caused by process,voltage,and temperature,and output a stable clock.(5)Low power demodulating of binary phase shift keying(BPSK)signals is a challenge for implanted medical devices.This research designed an improved 1-bit all-digital BPSK demodulator.This demodulator uses 1-bit analog-to-digital converter to sample the received signal and then directly demodulates the sampled signal,so that the demodulator avoids using complex analog circuits such as filters,amplifiers,and mixers,and eliminates the need of a phase-locked loop for bit-synch.The simplification significantly reduces the power consumption of the BPSK demodulator yet keeps the demodulation quality.Based on the listed innovations and improvements,the research for the dissertation designed and implemented a NFC system prototype for an implantable glaucoma instruments.Two prototypes support our research.The one prototype using a single-coil pair inductive link achieved reliable data transmission when the coupling coefficient was as low as 0.008.The one using a magnetic balanced inductive link realized reliable data transmission when the coupling coefficient was as low as 0.005.This research demonstrated the first NFC system achieving communication and energy transmission with inductive link whose coupling coefficient is less than 0.01(till March 2018).Implemented using TSMC 0.18 ?m CMOS integrate circuit process,the power consumption of the in-vivo receiver's BPSK demodulator is 0.1 m W(the lowest power consumption of coherent BPSK demodulator in published works of implantable medical devices till March 2018).The crystalless internal clock generator consumes only 0.51 m W with frequency offset less than ±0.025%.The total power consumption(including transmitter power)of the in-vivo communication circuit is only 0.71 m W.The prototyped results show that the system,methods and techniques of the NFC proposed in this dissertation are suitable for the future implantable medical devices with the demand of both small size and low power consumption,and also suitable for Internet of Things applications which require simultaneous transmission of reliable information and energy under ultra-low coupling.