Ultrasonic wireless powering offers a superior energy-conversion efficiency at millimeter scale dimensions and deeper penetration depth compared to the traditional inductive powering method. This makes ultrasound an attractive candidate for powering deep-seated implantable medical devices especially when the size restriction makes the incorporation of battery or inductive receiver impractical. Another advantage of the ultrasonic powering is the omni-directionality; to some extent due to the symmetric receiver geometry and to a larger extent due to the reflection at the air/body interfaces. Once the wave is launched into the body by the transmitter, it reflects off of the body-air interfaces resulting in waves coming in from all angles. In this paper, we investigate ultrasonic powering of mm-scale devices with specific emphasize on the output power levels, efficiency, range, and omni-directionality. Piezoelectric receivers 1×5×1 mm3, 2×2×2 mm3 and 2×4×2 mm3 in size are able to generate 2.48, 8.7, and 12.0 mW of electrical power at 20 cm powering distance, while irradiated at 1.15 and 2.3 MHz within FDA limits for medical imaging (peak acoustic intensity of 720 mW/cm2). The receivers have corresponding efficiencies of 0.4%, 1.7%, and 2.7%, respectively, at 20 cm powering distance. Due to the form factor and reflections from air boundaries, the output power stays constant to within 92% when the angular positions of the transmitter and receiver are varied around a cylindrical shell. In addition, a thin polymer coating results in marginal effect in the power output and can potentially help dealing with the associated risk of leaching lead from the PZT.
We investigate ultrasonic powering of mm-scale devices with specific emphasize on the output power levels, efficiency, range, and omni-directionality. Omni-directionality is to some extent due to the symmetric receiver geometry and to a larger extent due to the reflection at different boundaries. The PZT receivers with different geometries are examined to verify the omni-directionality and the realistic power level under the FDA limit of acoustic intensity. The receivers are able to generate a few mW of electrical power with the matched load at 20 cm away without showing any significant misalignment sensitivity.