On the Non-idealities of a Capacitive Link for Wireless Power Transfer to Biomedical Implants

This paper studies the performance of a resonant capacitive wireless power transfer (C-WPT) link for biomedical implants in the presence of non-idealities. The study emphasizes on finding an accurate electrical model of a practical C-WPT link, which can be used to investigate the performance of the link under different practical/non-ideal scenarios. A sound knowledge about these non-idealities is crucial for device optimization. For the first time, a circuit model has been presented and analyzed, which is applicable to a practical C-WPT link undergoing plate mismatch, flexion, tissue contraction, and stretching. Our model considers the finite conductivity of the body tissue and fringe fields formed by capacitor plates. Analytical and HFSS^TM simulation results have been presented for different non-idealities, and are in good agreement. Additionally, we show a procedure to interpolate non-ideal case results. The study shows that plate misalignment (causing reduction in parallel plate overlap area) and skin tissue contraction (while muscle grows) are the most detrimental individual factors to the link performance. We recorded ∼32% and ∼14% power transfer efficiency decrease due to these two worst-case scenarios, respectively for a C-WPT link comprising of two pairs of 400 mm² parallel plates (12 cm edge-to-edge separation) coated with 63.5 µm thick Kapton layer and aligned around a 3 mm tissue at 20 MHz.