Biophysical Modeling of Capacitive Electro-Quasistatic Human Body Powering

The increasing demand for wearables necessitates efficient energy harvesting and wireless power transfer solutions. Capacitive Electro-Quasistatic Human Body Powering (EQS-HBP) is a promising technology for wirelessly powering on-body devices, offering enhanced received power ($P_{rx}$) with full-body coverage. Unlike EQS Human Body Communication (EQS-HBC), which optimizes channel capacity, EQS-HBP focuses on maximizing ${\rm P_{rx}}$, requiring a distinct biophysical model tailored to lower termination impedance ranges where ${\rm P_{rx}}$ peaks. This paper presents comprehensive simulations-finite element method (FEM), distributed circuit modeling-and in-vivo experiments to characterize the body channel as a finite impedance wire, with impedance determined by body dimensions. Contact impedance between the body and receiver, inversely related to contact area, significantly affects ${\rm P_{rx}}$, necessitating careful design for devices with small contact areas. Furthermore, the body cross-sectional area influences voltage recovery after the point of load, with smaller cross-sections yielding reduced recovery. A lumped circuit model is developed to encapsulate these findings with circuit techniques to maximize ${\rm P_{rx}}$, demonstrating that series resonance in a ground-floated receiver reduces input impedance by over 65x and improves ${\rm P_{rx}}$ by more than 25× over parallel resonance. We also propose a method to approximate optimal loading impedance for various receiver configurations and analyze the impact of inductor Q factor. We prove that neither series nor parallel resonance can mitigate the transmitter return path capacitance. These insights enable the development of a much higher on-body wireless power transfer method, advancing wearable device technology for applications in healthcare, fitness, and beyond.