Omni-Directional Ultrasonic Powering via Platonic Solid Receiver for mm-Scale Implantable Devices

Abstract

Despite the recent advancements in implantable microdevices, providing sufficient electrical power to deeply seated microsystems has remained challenging due to the small dimension of the system, limiting the total storable energy. Ultrasound powering, where a portion of the externally induced ultrasonic wave is converted to electrical power by a small receiver, has been explored as an attractive source of power, especially for deeply seated implantable microdevices. While all other components have been advanced and miniaturized, the ultrasonic receiver is still a slab of bulk piezoelectric materials, e.g., diced PZT (lead zirconate titanate). Such a rectangular or disc shape is not an ideal form factor for wireless ultrasonic power transfer due to many challenges, particularly angular sensitivity with respect to an incoming ultrasonic wave. In this paper, we present the first demonstration of omnidirectional ultrasonic powering enabled by the high geometrical symmetry of three-dimensional polyhedral shapes. Based on our 3D printing technique of lead-free piezoelectric barium titanate ceramic, we designed highly symmetric, miniaturized, regular polyhedra, known as Platonic solids (i.e., cube, octahedron, dodecahedron), as well as a sphere. For each geometry, we investigate the effect of axial and radial piezoelectric poling, output power levels, efficiency, and angular sensitivity while the surface areas are the same. Across all the geometric shapes, radially poled Platonic solid receivers produce at least 1 order of magnitude larger electrical power density compared to diced PZT. Further, we observed that the higher the order of Platonic solid, the more excellent power transfer efficiency and omnidirectionality. The 3D printability of the Platonic solid also allows for customizable packaging, which we implemented as an implantable light source. Overall, the proposed ultrasonic powering scheme warrants a revolutionary solution for implantable biomedical devices.