Powering smart orthopedic implants using acoustic waves: An ex vivo study

This work experimentally investigates the feasibility of acoustic power transfer for battery-free full-metal smart orthopedic implants. Two receiver placements were evaluated on both knees of a human cadaver: a stem placement, where the acoustic path includes cancellous bone, and a tray placement, where the acoustic path avoids bone but integration constraints are more stringent. Receivers with resonance frequencies between 250 kHz and 2 MHz were designed using finite element simulations, analytical modelling, and phantom experiments. Measurements show that increasing frequency significantly reduces transmitted power due to higher attenuation in cancellous bone. It was shown that up to 29 mW and 20 mW of power could be transferred with the stem and tray placement, respectively, with an acoustic intensity of 720 mW/cm² and mechanical index below 1.9, hence respecting exposure limits. Volumetric power densities of 91 mW/cm³ and 1400 mW/cm³ were reached with the stem and tray placement, respectively, which are comparable or higher than those reached with electromagnetic techniques (from 1.16 to 194 mW/cm³) while compliance with electromagnetic exposure limits is not always clearly addressed in the studies. This work demonstrates the feasibility of milliwatt-level acoustic powering of smart orthopedic implants while highlighting challenges related to transmitter alignment and narrow operating bandwidth.