An RF-Ultrasound Relay for Adaptive Wireless Powering Across Tissue Interfaces

Single modality wireless power transfer has limited depth for mm-sized implants across air / tissue or skull / tissue interfaces because they either suffer from high loss in tissue (RF, Optical) or high reflection at the medium interface (Ultrasound (US)). This paper proposes an RF-US relay chip at the media interface avoiding the reflection at the boundary, and enabling efficient wireless powering to mm-sized deep implants across multiple media. The relay chip rectifies the incoming RF power through an 85.5% efficient RF inductive link (across air) using a multi-output regulating rectifier (MORR) with 81% power conversion efficiency (PCE) at 186 mW load, and transmits ultrasound using adiabatic power amplifiers (PAs) to the implant in order to minimize cascaded power loss. To adapt the US focus to implant movement or placement, beamforming was implemented using 6 channels of US PAs with 2-bit phase control (0, 90, 180, and 270°) and 3 different amplitudes (6-29, 4.5, and 1.8 V) from the MORR. The adiabatic PA contributes a 30-40% increase in efficiency over class-D and beamforming increases the efficiency by 251% at 2.5 cm over fixed focusing. The proof-of-concept powering system for a retinal implant, from an external PA on a pair of glasses to a hydrophone with 1.2 cm (air) + 2.9 cm (agar eyeball phantom in mineral oil) separation distance, had a power delivered to the load (PDL) of 946 μW. The 2.3 × 2 mm² relay chip was fabricated in a 180 nm high-voltage (HV) BCD process.