Wireless Communication with Medical Implants: Antennas and Propagation

With the increased sophistication of medical implants, there is a growing need for flexible high-speed communication with the implant from outside the body. Today the communication is done by an inductive link between the implant and an external coil at a low carrier frequency. Extended range and communication speed are possible to achieve by increasing the carrier frequency and the bandwidth. One frequency band that is available for this application is the newly standardized 400 MHz MICS band, which has the benefit of being reserved mainly for medical and metrological applications. In addition, the 2.45 GHz ISM band is a possibility, but has the drawback of being heavily used by other applications, such as wireless computer networks and microwave ovens. In order to assess the usability of wireless communication with medical implants, we have investigated the design of implantable antennas to be used in the body. Both theoretical limits and practical designs of the antennas are described. The SAR levels of the implanted antennas have been calculated and have been found to be at a safe level. We have investigated the wave-propagation from the implanted antenna to the outside, and its dependence on the position of the patient’s limbs and the size of the body. Full wave 3D-simulations of the wave propagation are feasible, as the radio link between the patient and a base station placed in the same room is very short in terms of wavelengths in the MICS band. We have simulated the wave propagation in a furnished room and compared the results with measurements of the same room. The results from these investigations are evaluated in terms of their impact on the link budget for a prototype MICS system. From these calculations conclusions on the necessary complexity of the transceivers are drawn, such as the need for both spatial and polarization diversity to fully exploit the potential of the communication link.

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