Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-05-10
2001-12-04
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06327504
ABSTRACT:
TECHNICAL FIELD
The invention relates to the transfer of electromagnetic energy between a primary coil located outside the skin and a secondary coil located beneath the skin. More specifically, the invention concerns the circuitry used in connection with the secondary coil to condition the electromagnetic energy into a useful form.
BACKGROUND
Many implanted medical devices are powered by electricity. Some devices, such as artificial pacemakers, may be battery-powered because their power requirements are comparatively low. Other devices require considerably more power and cannot be adequately supplied by a battery on a long-term basis. An implantable blood pump, for example, requires considerably more power than an ordinary pacemaker. A pacemaker rhythmically generates electrical impulses to stimulate the heart muscle, but the pacemaker does not perform any mechanical pumping action. An implantable blood pump, by contrast, mechanically assists the heart muscle to pump blood, and for this reason has a considerably greater power requirement.
Power requirements for a typical pacemaker may be on the order of 10 milliwatts, as compared with an implantable blood pump, which may have a power requirement one thousand times greater. Implantable electric storage batteries are unable to provide such power for long periods of time, and the frequent invasive surgery that would be required to replace the batteries makes this option undesirable. If the implantable batteries are rechargeable, then the batteries must be recharged from time to time, and power must in some way be delivered to recharge the batteries. Once again, use of invasive surgery to deliver the recharge is not a desirable alternative.
Ordinarily the power source for a high-power device must be external to the body. To deliver the power to the device, the power must somehow transit through the skin. Power may be delivered through the skin by a percutaneous wire, but this method has drawbacks. A wire penetrating the skin provides a source for infection. Moreover, there is a risk that a wire penetrating the skin may accidentally be torn out, which may cause loss of power to the device and trauma to the patient.
Another way to deliver power through the skin is by way of induction. Two coils of electrically conductive wire, a primary coil external to the skin and a secondary coil implanted within the patient, may be inductively coupled. By energizing the primary coil with a time-varying current, a time-varying magnetic flux is produced by the primary coil. If the secondary coil is in proximity to the primary coil and is appropriately oriented, the time-varying magnetic flux will induce a time-varying current within the secondary coil, according to the principle of mutual induction. Power may be delivered through the skin using mutual induction. Systems delivering energy or power in this way are often called transcutaneous energy transfer, or TET, systems, sometimes referred to as TETS.
Mutual induction is a consequence of Faraday's Law of Induction. Faraday's Law holds that the electromotive force (emf) in a conducting coil of N turns and the rate of change of magnetic flux through the coil are related. Faraday's Law is embodied within the equation E=−N(d&PHgr;/dt), where E is the induced emf, N is the number of turns in the coil, and (d&PHgr;/dt) represents the change in magnetic flux with respect to time. The negative sign is a matter of convention, and is indicative of the direction of the induced emf.
Both the current in the primary coil and the current induced in the secondary coil are time-varying. Direct current (dc) will not result in a time-varying magnetic flux, and therefore will not create mutual induction. If the device being powered operates using dc, the current or voltage in the secondary coil must be conditioned for use by the device. Ordinarily, conditioning includes such functions as rectifying the current or voltage, filtering it to remove high-frequency components, and regulating it to provide substantially constant amounts of current or voltage.
SUMMARY
The present invention is directed to a transcutaneous energy transfer system having a secondary coil with integrated power conditioning circuitry. The transcutaneous energy transfer system may be particularly useful with an implantable blood pump having a pump control module. The pump control module ordinarily is implanted in the abdomen. Integration of the power conditioning circuitry with the secondary coil permits immediate conversion of energy induced at the secondary coil to dc current, instead of at the pump control module. Consequently, thinner leads can be used to couple the secondary coil to the pump control module, thus enhancing system versatility by expanding sites available for module implantation.
Importantly, the secondary coil and power conditioning circuitry can be arranged to provide a relatively low profile for implantation, while also limiting the generation of eddy currents in the power conditioning circuitry. Eddy currents in the power conditioning circuitry may be created when the circuitry is in proximity to the time-varying magnetic flux generated by the primary coil. A reduction in the generation of eddy currents helps avoid the creation of excessive heat in the area of the secondary coil, and resulting inflammation of nearby tissue. In particular, the power conditioning circuitry can be mounted within the aperture defined by the secondary coil with a structure and orientation designed to avoid coincidence with magnetic flux lines extending substantially perpendicular to the major plant defined by the secondary coil.
Ordinarily, power-conditioning circuitry is constructed as a substantially planar package or module. The circuitry may be placed on one or more circuit boards mounted within the aperture defined by the secondary coil and may be electrically connected to the secondary coil such that time-varying currents induced in the secondary coil are delivered to the conditioning circuitry. Although the conditioning circuitry may be encased in a non-conductive medium, the conditioning circuitry and circuit board may consist of conductors and semiconductors. To reduce the profile of the secondary coil, the planar circuitry is oriented to reside within the coil aperture and occupy the major plane defined by the coil.
Placement of the conditioning circuitry inside the secondary coil may have significant space-saving advantages, but may pose practical difficulties. The time-varying magnetic flux generated by the primary coil ordinarily will induce some time-varying eddy currents, in addition to the current induced within in the secondary coil. These induced eddy currents may appear in conductors and semiconductors in the power conditioning circuitry near the secondary coil. Eddy currents generate heat, due to the natural and inherent resistivity of the conductors.
The present invention provides a way for the circuitry to be mounted within the secondary coil yet minimize eddy currents and thereby reduce the risk of excessive heating. TET systems may be in many forms, and the orientation of the lines of magnetic flux may be estimated. Where the primary and secondary coils are substantially circular and are axially aligned, for example, the location lines of magnetic flux can be reasonably estimated, especially along the diameters and near the centers of the coils.
The present invention provides that the circuitry attendant to the secondary coil may be substantially planar, and may be placed so that the plane of the circuitry is substantially tangent to the lines of magnetic flux. In general, the lines of magnetic flux may pass through the major plane of the secondary coil at near-right angles, and magnetic flux lines near the center of the secondary coil's aperture may be substantially parallel to the secondary coil's axis.
When the planar circuitry is oriented with its plane substantially tangent to the lines of magnetic flux, eddy currents within the conductors and semiconductors in the circuitry are
Dolgin Alexander
Rintoul Thomas C.
Fish & Richardson P.C. P.A.
Getzow Scott M.
Thoratec Corporation
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