Since the concept of wireless power transfer has been proposed and implemented by Tesla over a hundred years ago, it has obtained lots of breakthroughs and applied in many potential fields especially in implantable biomedical devices [1-2]. The implantable biomedical devices are surgically introduces into the human body to rebuild body function, achieve a better quality of life, and expand longevity. With the development of microelectronics, biotechnology, and materials, the industry of implantable biomedical devices grows up quickly. The researches show that in America 8% to 10% of the people have experienced an implantable medical device, while in industrialized countries 5% to 6% of population have consumption demand in implantable medical devices. How to provide a constant power supply, for implantable medical devices, becomes a restricted problem. In order to guarantee the implanted medical devices to work normally, the energy storage or harvesting components should be included in these devices. In the traditional implantable medical devices, there are two ways to provide power for the devices. The common method is the implantable batteries are used to be as a power supply, and the other method is to transfer the power in vitro to the devices with the lead wires through the skin. For implantable batteries, their lifetime, size and toxic composition will lead to potential hazard to the patients. The transcutaneous wires, as well, will bring infection and reliability problems. In order to address above problems, the technology of wireless power transfer is used in the biomedical devices. The system has two parts in which one part including power supply and the transmitter coil is placed outside the patients' body, and the other part including the receiver coil is placed in the patients' body. For this system, the receiver coil should be limited as small as possible and the power transfer efficiency should be as high as possible. Different from the traditional two-coil wireless power transfer system, a novel three-coil system is proposed in this paper. In the novel system, there are two rectangular transmitter coils and one circular receiver coil. To obtain the maximum power transfer efficiency, the system is optimal designed according to the following step as shown in Fig. 1. Firstly, the size and the geometry of the receiver coil are determined. A circular coil with a radius of 10mm is chosen as the receiver coil. Secondly, the distribution of the magnetic field generated by the rectangular coil is analyzed based on the Biot-Savart's law. Then the relationship between the distance and the magnetic field intensity will be obtained and expressed as a formula according to the analysis result. Finally, the above formula is chosen as the optimization constrains and the power transfer efficiency is chosen as the optimization objective function. After the optimal designing, the optimal parameters of the transmitter coils are obtained and verified through the experimental system as shown in Fig. 2. The power transfer distance is about 30cm, and the power received by the receiver coil can achieve about 5W. The detailed results will be given in the full paper.
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