System-Level Design and Closed-Loop Control of Inductively Powered Active Implantable Medical Devices: A Review

Wireless power transfer (WPT) has improved the development of active implantable medical devices (AIMDs) by enabling them to operate wirelessly, allowing uninterrupted treatment to patients without direct intervention. Inductive power transfer (IPT) is the most established technology, composed of magnetically coupled coils to form inductive links. However, delivering efficient, stable, and safe power to an implant is not trivial. This paper presents a methodological review on the design of IPT systems applied to AIMDs. The fundamental concepts of IPT are presented, including inductive link modeling, compensation networks, winding methods and the influence of frequency. Design and optimization techniques at the system level are discussed, detailing the most common topologies and incorporating electromagnetic (EM) safety and biocompatibility aspects. The main control methods for IPT systems to achieve stable output power are detailed and categorized based on their implementation in the primary, secondary, or through external communication. The advantages and disadvantages of each method are discussed. Finally, the literature findings are presented for various applications, such as pacemakers, neurostimulators, cochlear, or retinal implants. Both commercial industry solutions and research works are detailed. These works are compared quantitatively, including a metric to evaluate the quality of the proposed IPT systems.