Optimal Frequency for Wireless Power Transmission Into the Body: Efficiency Versus Received Power

Wireless power delivery to implantable devices has been traditionally achieved through low-frequency (<;10 MHz) inductive coupling. More recent work has moved toward the UHF band (300 MHz-3 GHz), with models showing maximal efficiency is achieved at these frequencies. However, for many applications, maximizing efficiency is a secondary goal to maximizing received power at the implant, and it remains unclear whether the optimal frequency for efficiency is different than the optimal frequency for maximizing received power. Furthermore, previous models generally do not include a ferrite core in the implant, which could shift the optimal frequency. To address these questions, we developed a theoretical model with frequency-dependent ferrite permeability and dispersive tissue. We show that when a ferrite core is included, the optimal frequency for efficiency remains in the UHF band. Conversely, when maximizing received power rather than efficiency, both high (>300 MHz) and low (<;10 MHz) frequencies yield similar levels of received power, and this remains true with or without a ferrite core. For shallow implants, heating of the core can reach unsafe levels when operated in the UHF band at field levels approaching the specific absorption rate (SAR) limit. These results have implications for a wide range of medical applications, including sub-millimeter-sized implants and fast recharge of implantable batteries.