Safely Powering Inductively Coupled Deeply Implantable Millimetre-Sized Devices

Abstract

This thesis presents a design methodology for wireless power transfer (WPT) frequency optimisation which incorporates specific absorption rate (SAR) limitations that account for adverse tissue heating effects, for near-field loosely-coupled inductive links used in deeply implanted millimetre-sized biomedical devices. Current approaches to WPT frequency optimisation do not consider the SAR within their design optimisation process, but instead verify their link meets SAR limits after optimising to other parameters. A figure-of-merit which incorporates SAR into the optimisation process is proposed, using a peak spatial-average SAR limit of 2 W/kg. Using the measured implant coil single-turn equivalent resistance, and the permissible SAR constrained magnetic field found in-silico, the optimal operating frequency and receiving coil design based on geometric constraints are calculated such that power delivered to load (PDL) is maximised, whilst remaining within SAR limits. The findings indicate that when adhering to SAR limits, at an implant depth of 100 mm, operating frequencies in the sub MHz to low MHz range, results in greater PDL in midbrain tissue. The main factor influencing PDL is the ratio of the tissue resistivity to coil resistance. Maximising this ratio leads to the highest achievable power transfer when SAR is the limiting factor. The implant coil resistance increases with frequency due to skin and proximity effects. Additionally, tissue resistivity decreases with frequency, and thus a lower operating frequency implies a greater PDL, irrespective of implanted coil geometry. Consequently, there is no optimal frequency when only considering SAR, but only a trend where lower frequencies are preferred, as the ratio of tissue resistivity to pickup coil resistance is inversely proportional to frequency. In light of these findings, application of induced electric field limits, which account for harmful effects due to electrostimulation, were applied to obtain a lower bound, and thus an optimum. In accordance with IEEE C95.1-2005 and ICNIRP 2010 human electromagnetic exposure guidelines, the optimal WPT frequency due to SAR and induced electric field limits is found to be 500 kHz and 700 kHz, respectively. Considering a 38AWG coil and an anatomical in-silico phantom: at 500 kHz, a PDL of 14.25 mW was calculated; and at 700 kHz, a PDL of 14.11 mW was calculated.