An Approximate Electromagnetic Model for Optimizing Wireless Charging of Biomedical Implants

Computational modeling is increasingly used to design charging systems for implanted medical devices. The design of these systems must often satisfy conflicting criteria, and fast electromagnetic solvers are pivotal for enabling multi-criteria optimization. In this paper, we look at wireless power transfer for implantable devices and the specific absorption rate and induced currents related to the implanted side of the design. We present an analytical model based on the quasi-static approximation as a fast, yet sufficiently accurate, alternative for full wave electromagnetic modeling. The analytic model was benchmarked against full-wave simulations to validate accuracy and improvement in computation time. Our analysis shows that the analytic model allows for feasible complete optimization of coil shapes, as the analytic model takes only 11 seconds to compute a single iteration, while the full-wave model takes 5 hours to compute the same case. The maximum difference with full-wave simulations was less than 25\% and the mean difference less than 2.3%. Adding a novel figure of merit into the multi-criterion optimization resulted in a 16% higher charging speed. The specific absorption rate and coupling factor were both experimentally verified to show that the measured results are within a 5~mm coil offset margin, which validates the simulation results.