Researches On Implantable Antennas For Wireless Biomedical Applications

Wireless data telemetry and power transfer technologies for biomedical and healthcare applications have received a lot of attentions recently. The ageing population poses many challenges to healthcare systems, especially on chronic illness management. It is believed that the market potential is very huge in the design and development of wireless medical equipments and personal medical health care systems. From the state of arts, designing high-accuracy and robust wireless biomedical equipments still presents as a great challenge. This thesis adopts analytical, numerical calculation and experiment method to set up a simulation model for the human coupling effect, and the thesis targets designing high-efficiency wireless power transfer system and compact, high-accuracy wireless data transmission system including implantable antennas. We would analyze the coupling effect of the whole system and design the wireless power and data transmission sub-system intended for biomedical applications, and further implement the optimized design and conduct performance evaluation for the wireless biomedical and healthcare systems. The main contributions of this dissertation are listed as below:1. The transmission characteristic of the electromagnetic waves through biological tissues is analyzed. An elemental oscillating electric dipole was considered as a radiation source for analyzing the absorbed power in the near-field region and far-field region, respectively. Thus the received power in the near-field region or far-field region can be calculated based on the absorbed power in different regions. And this would help to further study the optimal frequency on far-field wireless power transfer.2. Based on the theory of multi-resonant frequency to achieve wideband impedance characteristic, a combined dual-band antenna with microstrip line antenna and planar inverted-F antenna is designed for biomedical applications. Dual-band operation is a good solution to extend the lifetime of the implantable devices. A miniaturized dual-band implantable antenna was designed based on the radiation characteristic of small antennas and the achievement of multi-resonant frequency points, the measured results demonstrated the priority of the proposed antenna. In order to further reduce the dimensions of implantable antenna, a hybrid patch/slot antenna for biomedical devices is analyzed and tested. To design a very small implantable antenna for neuro-microsystem, an implantable antenna integrated with the transmitter was designed and fabricated based on CMOS technology.3. A circularly polarized implantable antenna is designed under condiseration of human body effect. Circular polarized wave has shown good propriety in wireless communication since the reduction of multipath can be achieved with the use of circular polarization in hospitals. After this, the characteristic of an implantable microstrip patch antenna with a center square slot was studied. A compact capacitively loaded CP implantable patch antenna was designed with good impedance matching and size reduction. The effect of coaxial cable was discussed.4. A circularly polarized capsule antenna with multli-layer open loops is designed for ingestible capsule endoscope systems. A three-layer helical antenna, which consists of three open loops and connecting via to connect the adjacent loops, was studied. Compared with the publication relative works, the proposed capsule antenna has wider axial ratio bandwidth and smaller size. In order to further demonstrate the accuracy of the design, two circularly polarized antennas were embedded in the rat as in-vivo testing to investigate the effects of the live tissues on the antenna performances.5. Safety considerations such as electric and thermal effect of human body on the far-fiele wireless power transfer are systematically studied. And the received power is significantly enhanced by adding a parasitic patch on the human body. An implantable rectenna for far-field biomedical wireless power transfer. Far-field wireless power transfer is suitable for ultra-low-power sensor networks and other devices. Safety conditions are considered to establish the transmitter power, thus to establish the received power level of the implantable antenna. A method of adding a parasitic patch on the human body is used to enhance the directivity of the proposed implantable antenna. The power level received by the proposed implantable antenna can be estimated based on the safety considerations which including FCC rules of exposure limits, Specific Absorption Rate(SAR) limits and temperature increase limit. Finally, an integrated rectenna solution is presented. The entire conversion efficiency is enhanced with the method of adding the parasitic patch over the human body.