  The wireless power transfer efficiency to implanted bioelectronic devices is
constrained by several frequency-dependent physical mechanisms. Recent works
have developed several mathematical formulations to understand these mechanisms
and predict the optimal operating conditions. However, existing approaches rely
on simplified body models, which are unable to capture important aspects of
wireless power transfer. Therefore, this paper proposes the efficiency analysis
approach in anatomical models that can provide insightful information on
achieving the optimum operation conditions. First, this approach is validated
with a theoretical spherical wave expansion analysis, and the results for a
simplified spherical model and a human pectoral model are compared. The results
show that although a magnetic receiver outperforms an electric one for
near-field operation and both sources could be equally employed in far-field
range, it is in mid-field that the maximum efficiency is achieved with an
optimum frequency between 1-5 GHz depending on the implantation depth. The
receiver orientation is another factor that affects the efficiency, with a
maximum difference between the best and worst-case scenarios around five times
for the electric source and over 13 times for the magnetic one. This approach
is used to analyze the case of a deep-implanted pacemaker wirelessly powered by
an on-body transmitter and subjected to stochastic misalignments. We evaluate
the efficiency and exposure, and we demonstrate how a buffered transmitter can
be tailored to achieve maximum powering efficiency. Finally, design guidelines
that lead to optimal implantable wireless power transfer systems are
established from the results obtained with the proposed approach.
