17.1 Omnidirectional Magnetoelectric Power Transfer for Miniaturized Biomedical Implants via Active Echo

Wireless, batteryless, and miniaturized implants promise transformative therapies for various neurological, psychiatric, and cardiac disorders. Beyond conventional battery-powered bulky implants with leads, miniaturized implants promise drastically reduced infection risk and surgical complexity, and improved patient acceptance and precision. Despite significant efforts on wireless power transfer (WPT) for implants, most demonstrations are not sufficiently reliable to sustain practical usage, when misalignment between the external hub and the implant persists due to motions, respiration, and heart beating, as well as the restricted device orientation during implantation (e.g. in spinal cords or vessels). The power transfer efficiency (PTE) of WPT degrade drastically under misalignment, rendering the system unstable and unusable (Fig. 17.1.1). Transmitting higher power is a simple way to power a misaligned implant but is restricted by safety limits, heating, and battery lifetime of the external device. It can be improved by introducing closed-loop regulation that adjusts the driver’s power based on the implant’s feedback [1, 2]. However, this approach is only effective for smaller misalignment without addressing the fundamental efficiency drop. Recently proposed magnetoelectric (ME) WPT improves misalignment tolerance over existing inductive and ultrasound methods due to magnetic flux concentration effects [3]. Additionally, ME WPT at acoustic resonance (100s of kHz), is not subject to tissue absorption, reflection, and/or scattering issues met by RF, ultrasound, and optical WPT methods, allowing a miniaturized implant to receive high power at large depths reliably and safely [4]. However, ME WPT still cannot tolerate large rotations and lateral offset. On the other hand, multi-coil inductive WPT has made significant progress towards omnidirectional charging [5]. But existing multi-coil WPT are designed for high power applications (W to kW), with large TX/RX coils and high coupling coefficients k>10%). Adopting these methods in miniature implants is nontrivial because the critical coupling sensing at the TX side cannot work with the tiny mutual inductance of mm-scale coils. Moreover, mutual inductance (M) among the TX coils degrades overall PTE. A 3D coil array cancels M effectively [6], but its spherical structure is bulky and incompatible with a wearable device for powering implants.