13.56 MHz Near Field magnetic coupling efficiency evaluation for IMDs powering

This paper focuses on the wireless powering of implanted medical devices (IMDs) thanks to the widely used technology NFC (Near Field Communications). This technology uses a magnetic coupling at 13.56 MHz. In this work, the IMD is represented by a titanium case with a coil wound around it. CST simulations and VNA measurements are performed with two titanium samples (grade 2, G2, and grade 5, G5) for evaluating the impact of the titanium case on the electrical parameters, mainly the impedance Z parameters matrix. The human body is modeled in simulation and represented in measurements thanks to a phantom. Evaluation of the Z parameters in simulation and measurements are shown in a first step and used in a second one for defining the efficiency reachable in our case of study. The efficiency is computed with simulated and measured results and allows to quantify the ability to consider the NFC link for powering. As the distance between transmitting and receiving coils is in the range of 50 mm, including a distance between reader and human tissue of 30 mm, the efficiency (custom) is in the range of 10 to 20% depending on the load impedance of the IMDs. This work shows a fruitful agreement between simulation and measurements for designing NFC link in the context of IMDs powering. Conclusions are given for improving this physical link by modifying the structure of the reader, and the definition of the efficiency given can be used as a fruitful figure of merit.