Reza Erfani

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 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 m thick Kapton layer and aligned around a 3 mm tissue at 20 MHz.

This brief proposes and demonstrates a reliable power and data transfer scheme for biomedical implants through a resonant capacitive link. Flexible, conformable, and biocompatible patches are adopted with an automatic resonant frequency calibration technique for the first time. A voltage feedback network, a Class-D driver, and a custom algorithm have been developed for the low overhead calibration. An ex vivo chicken skin tissue-based experiments with 4 cm2 flexible, conformal capacitive patches demonstrated the method. Experimental results show that the power delivered to the load (PDL) of the link extends up to 150 mW and maximum power transfer efficiency (PTE) of the link is 54%. A simultaneous data transfer with a rate of up to 170 kbps alongside power transfer using 1.5 to 2 mm-thick biological skin tissue has been achieved.

This paper presents a 1.3mA current stimulator integrated circuit (IC) with closed-loop, active charge balancing for nerve interfacing applications. The stimulator employs two current-based digital-to-analog converters (DACs), one each for anodic and cathodic phases, with binary-weighted transistors and 2b-programmable least-significant bit (LSB) currents. The biphasic, cathodic-leading, stimulus current pulse amplitude is 5b-programmable in four ranges of ~ 12-341μA, 23-682μA, 32-992μA, and 42-1,300μA. A low-power (~10μW), active charge balancer monitors the site voltage after each stimulus cycle and, if necessary, injects positive or negative current pulses via two 3b auxiliary DACs to bring the site voltage back within a userset safety window. All digital circuitry operate from 3.3V, whereas the stimulator output stage operates from 5V with a stimulus site common-mode voltage of 3.3V for better use of the available voltage headroom in cathodic-leading, single-supply stimulation. A prototype chip is fabricated in 0.5μm 2P/3M CMOS, and measured results from benchtop electrical characterization of the chip are presented.