A Figure-of-Merit for Design and Optimization of Inductive Power Transmission Links for Millimeter-Sized Biomedical Implants

Power transmission efficiency (PTE) has been the key parameter for wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions. It has been suggested that for mm-sized implants increasing the power carrier frequency (f_p) of the WPT link to hundreds of MHz improves PTE. However, increasing f_p significantly reduces the maximum allowable power that can be transmitted under the specific absorption rate (SAR) constraints. This paper presents a new figure-of-merit (FoM) and a design methodology for optimal WPT to mm-sized implants via inductive coupling by striking a balance between PTE and maximum delivered power under SAR constraints (P_L,SAR). First, the optimal mm-sized receiver (Rx) coil geometry is identified for a wide range of f_p to maximize the Rx coil quality factor (Q). Secondly, the optimal transmitter (Tx) coil geometry and f_p are found to maximize the proposed FoM under a low-loss Rx matched-load condition. Finally, proper Tx coil and tissue spacing is identified based on FoM at the optimal f_p. We demonstrate that f_p in order of tens of MHz still offer higher P_L,SAR and FoM, which is key in applications that demand high power such as optogenetics. An inductive link to power a 1 mm³ implant was designed based on our FoM and verified through full-wave electromagnetic field simulations and measurements using de-embedding method. In our measurements, an Rx coil with 1 mm diameter, located 10 mm inside the tissue, achieved PTE and P_L,SAR of 1.4% and 2.2 mW at f_p of 20 MHz, respectively.