Characterization of RF Differential Tissue-Coupled Powering (DTCP): An Emerging Sub-GHz Wireless Power Transfer Link for Bio-Implants

Among emerging wireless power transfer (WPT) modalities, RF differential tissue-coupled powering (RF-DTCP) is particularly compelling because it directly harnesses biological tissue as part of the power-delivery path while remaining compatible with ultrathin, thread-like implants. In this work, we present a comprehensive theoretical and experimental characterization of RF-DTCP as a wearable-to-implant WPT scheme. We develop a generalized analytical framework that models arbitrary four-electrode TX-RX configurations, derive port-level admittances from the underlying impedance network, and use these to express power transfer efficiency as functions of geometry, tissue properties, and frequency, including a closed-form optimum operating frequency in the capacitive-dominated regime. In ex vivo pork tissue, this first sub-GHz (0.5–0.6 GHz) demonstration of RF-DTCP sustains 10–25\% PTE $>$ 5mm depth, as implant length increases from 1–8 cm while preserving superior resilience to angular misalignment, thereby extending all previous body-coupled links that were confined to < 10 MHz.