33.6 A Millimetric Batteryless Biosensing and Stimulating Implant with Magnetoelectric Power Transfer and 0.9pJ/b PWM Backscatter

Bioelectronic implants transform clinical therapies by offering unprecedented tools for precise sensing and intervention inside the human body. Wireless, battery-free, and miniaturized (mm-scale) bio-implants are highly desirable to enhance safety, simplify surgery, minimize behavioral disruptions, and boost long-term stability over conventional bulky implants [1–3]. Developing such devices faces two crucial challenges: safe and reliable wireless power transfer (WPT) and efficient bidirectional telemetry. Magnetoelectric (ME) WPT, which converts low-frequency magnetic fields to electrical energy via acoustic coupling (Fig. 33.6.1, top), is an emerging WPT modality specialized for mm-sized implants. It offers strong penetration through various mediums without absorption or reflection issues, leading to superior WPT efficiency, power level under safety limits, and misalignment tolerance, all of which have been demonstrated in large animal models [1]. However, uplink telemetry in ME-powered implants, essential for realtime sensing and closed-loop physiological control, remains challenging. Integrating a second antenna, such as an inductive coil, is a straightforward solution to add uplink to ME implants [4], but it increases the implant size and complicates integration. Sharing a single transducer for power and data is highly desirable for device miniaturization, as demonstrated in inductive coupling and ultrasound [2]. Two recent studies [5, 6] have explored the converse ME effects to realize ME uplink (Fig. 33.6.1, middle). The feasibility of directly driving an ME transducer with AC voltages for uplink is reported in [5], but due to the kΩ-impedance of ME transducers, this method consumes significant power for a batteryless implant. On the other hand, [6] demonstrates low-power ME backscatter using load shift keying (LSK) based on an observation that changing the passive load to an ME transducer during its ringdown modulates its vibration frequency. The high-Q ME transducer vibrates for more than 30 cycles after turning off the excitation magnetic field. Despite the exciting progress, LSK ME backscatter has limited SNR and data rate, due to the ME transducer’s high Q. Specifically, SNR is limited by a direct trade-off between frequency shift and signal strength, while the data rate is limited by the relatively long excitation and ringdown periods.