Impact of Transcutaneous Energy Transfer on the electric field and specific absorption rate in the human tissue

Inductive power transfer technology has proven to be a promising solution for powering implantable heart pumps such as left ventricular assist devices, eliminating the need for a percutaneous driveline and reducing the risk of severe infections significantly. However, the required high power transfer capability of a Transcutaneous Energy Transfer (TET) system raises questions about human safety regarding the exposure to electric and magnetic fields. The focus of this paper is on the internal electric fields and the Specific Absorption Rate (SAR) caused by a prototype TET system designed to transfer 30 W across the skin at 800 kHz and 35 V output voltage. Numerical simulations show that the internal electric field and the SAR can locally attain high values within the fat tissue due to the large voltage potential at the implanted coil terminals. It is further shown that the parasitic capacitances of the energy transmission coils and the power electronic circuit of the implant can cause common-mode voltages at the energy receiving coil terminals, which increase the internal electric field strength additionally. Hence, the power electronic circuit and the grounding scheme of the system need to be adapted in order to eliminate common-mode voltages. As an additional countermeasure, an electric shielding based on carbon conductive compounds is presented in this paper, which is able to reduce the maximum internal electric field strength from 224 V/m to 77V/m and the maximum SAR and from 1.21 W/kg to 0.25 W/kg with only 1 % of additional power loss.