An Effective Transcutaneous Energy Transfer (TET) System for Artificial Hearts

Abstract

Smart implantable medical devices such as cochlear implants, cardiac pacemakers and artificial heart pumps, consume electrical energy for the lifetime of the patient. The method of power delivery in these devices is principally dependent on their power requirements. Low power implantable devices, such as cardiac pacemakers requiring power in the range of 30 - 100 μW and can be operated from an internal battery for up to 10 years before exhaustion. However, high power implantable medical devices such as artificial hearts can require up to 30 W which is not suitable to be supplied from a battery. These devices are currently powered by an external power supply consisting of a wire passing through the skin and directly connecting to the artificial heart. However this method of power delivery introduces a substantial risk of infection which contributes to 40% of serious adverse events for Left Ventricular Assist Devices (LVADs). This thesis focuses on developing a wireless power supply solution for artificial hearts and LVADs. Transcutaneous Energy Transfer (TET) systems use magnetic fields to transfer power across the skin without direct electrical connectivity. This offers the prospect of lifetime operation and overcomes the infection risk associated with wires passing through the skin. The major issues relating to TET are the heating of tissue and the variable nature of the alignment between a primary coil located near the skin surface and a secondary coil located under the skin. The work carried out in this thesis uses a novel frequency control approach that allows for wide tolerance in the alignment between coils with separations and lateral displacements of 10 mm to 20 mm, with relatively small size (50 mm diameter) and enables the transfer of up to 25 W of power. Thermal and alignment performance was verified using a sheep experimental model. The secondary coil was implanted under the skin in six sheep and the system was operated to deliver a stable power output to a 15 W load continuously over four weeks. The maximum surface temperature of the secondary coil increased by a mean value of 3.4 ± 0.4°C (Standard Error of Mean). The highest absolute mean temperature was 38.3°C. The mean temperature rise at 2 cm from the secondary coil was 0.8 ± 0.1°C. The efficiency of the system exceeded 80% across a wide range of coil orientations. Histological analysis revealed no evidence of ii tissue necrosis or damage after four weeks of operation. In a bench-top experiment, the TET system was used to power the MicroMed's DeBakey HA5 LVAD. The contributions from this thesis include the development of an efficient TET system with frequency based power regulation and the implementation of the system in a non-compliant animal to carry out a full thermal and coupling study. It is concluded that the developed TET system demonstrates all key performance criteria as required for a power delivery system suitable for an implantable heart pump and eliminates the risk of driveline infections.