Multi-coil High Efficiency Wireless Charger System for Hermetically Sealed Biomedical Implants

Biomedical inductively-coupled transcutaneous implants with internal batteries typically rely on a two-coil charging system which places fundamental and practical limits such as requiring short coil-to-coil distance for useful Wireless Power Transfer (WPT) efficiency. In case of hermetic metal enclosures equipped with finite size dielectric window for magnetic flux penetration, two-coil configurations can induce substantial additional loss due to eddy currents generated on e.g. Ti metal surface by fringing B-fields. Dissipative losses are unwelcome as they increase the temperature of the implant. We focus here on high-performance implants with internal electronic circuits and power source which require frequent, rapid battery recharging. The case example is a wireless broadband neural recording device capable of high data rate transmission (> 40 Mbps). We describe a compact planar four-coil configuration to achieve efficient Wireless Power Transfer (WPT) across tissue layers exceeding 1 cm. For the device geometry discussed here, our system transfers up to 73 % and 46 % of RF energy across 16 mm-separated source-to-load coil, in absence or presence of a Ti-enclosure which embeds the energy harvesting coil pair respectively. Thin sheets of ferrites are integrated to enhance local B-fields. We are able to charge a 200 mAh medical-grade battery to useful 84% of its full charge capacity (near current saturation) within 1 hour through a sapphire window integrated with the hermetic Ti- enclosure. The measured temperature increase is 2.1 °C with Ti-can immersed in still saline, slightly above FDA requirements or more recent ISO standards. From physiological models, we expect that active cooling by body tissue surrounding the implant (such as microvasculature perfusion) will provide for a safe and efficient WPT method.