Abstract:
Pacemakers and other active implantable devices require power, and a battery is likely to limit device lifetime and add bulk. In principle, wireless power transfer (WPT) could resolve both issues. However, existing WPT technologies have not been able to deliver energy to miniaturised and deeply implanted devices, such as leadless cardiac pacemakers (LCPs). There are safety standards that restrict the strength of electromagnetic fields in the body which limit power transfer. Also, most implantable devices use a titanium case to provide a hermetically sealed enclosure for the electronics, and this acts as a barrier to wireless power reception. There is increasing pressure to reduce the size of implants as this makes their deployment easier and less invasive (such as delivery through the venous network). The purpose of this thesis is to design and develop an effective method of powering small, deeply implanted devices.
An alternative to using magnetic fields for power transfer is to use a quasi-static electric field which can supply power deep inside the body. Recent developments have shown the use of the technology in shallow subcutaneous power transfer. This thesis investigates the impedance behaviour of living tissue as seen by the transmitter electrodes of an electric field power transfer system to overcome the challenges of providing power to deeply implanted devices. A simple resistor dominated model was developed and shown to be appropriate for frequencies up to 50 MHz. This supports the hypothesis that power transfer in tissue is dominated by volume conduction (e.g., real current flow rather than displacement current) that can be generated and received by either capacitive or conductive electrodes. This thesis expands on this finding to design, model and develop a novel method to use electric field-based technology to power deeply implanted devices based on volume conduction – conductive transcutaneous energy transfer or cTET. Alternatively, where the connection to the skin is capacitive (insulated electrodes), the technology is called CcTET (capacitively coupled cTET). The first cTET prototype based on capacitive coupling (CcTET) can deliver more than 10 mW of power independent of implantation depth while meeting the specific absorption rate (SAR) limit of 2 W/kg.
Wireless charging may overcome the problem of battery size; however, the method of wireless charging can also have a substantial impact on device size. Inductive power transfer relies on a pick-up coil which takes up significant space. For the first time, this thesis introduces a technique to repurpose the commonly used metallic casing as the receiver electrode. This avoids the need for additional bulky components. The idea was implemented
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as part of a leadless pacemaker, designed to resemble the Micra™ pacemaker from Medtronic. In-vivo experiments showed more than 10 mW power transfer to the heart. During the experiment, the subcutaneous temperature was maintained within the IEC60601-1 limit. Power delivery did not impact the animal’s vital signs and simultaneous pacing was uninterrupted.
The outcome of this thesis is a new power supply platform suitable for deeply implanted and miniaturised biomedical devices. This is an important contribution towards multi-chamber pacing using LCPs, which was not feasible prior to cTET.