Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1998-07-06
2001-11-27
Schaetzle, Kennedy (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06324430
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to transcutaneous energy transfer (TET) devices and more particularly to an improved primary coil for such device which reduces sensitivity to metal objects in the proximity of the coil and which substantially strengthens the magnetic field output from the coil for a given energy input.
BACKGROUND OF THE INVENTION
Many medical devices are now designed to be implantable, including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochler implants, neuromuscular simulators, biosensors, and the like. Since almost all of the active devices (i.e., those that perform work) and many of the passive devices (i.e., those that do not perform work) require a source of power, inductively coupled transcutaneous energy transfer (TET) and information transmission systems for such devices are coming into increasing use.
These systems consist of an external primary coil and an implanted secondary coil, separated by an intervening layer of tissue. This design generally results in a loosely-coupled transformer with no magnetic shielding. Therefore, transformer parameters, such as mutual and self-inductance values, and the effective series resistance of each coil, can be altered by the presence of conductive objects, for example a metal plate, in the vicinity of the coil. Such parameter changes can result in undesired, and in some cases potentially catastrophic, variations in power delivered to the implanted device. Further, an unshielded primary coil generates a magnetic field which is directed in substantially equal parts toward the secondary coil, where it performs useful work, and way from the secondary coil where the magnetic field energy is substantially wasted. If a higher percentage of the magnetic field from the primary coil could be directed to the implanted secondary coil, the energy required to drive the TET device could be reduced. This could result in the device being driveable from a lower energy, and thus a smaller, lighter and less expensive source, or less drainage on an existing source, facilitating longer battery life between replacement or recharging.
A need therefore exists for an improved primary coil construction for a TET device which both reduces sensitivity of the device to conducting objects in the vicinity of the coils and which increases the percentage of magnetic field generated by the primary coil which reaches the secondary coil, thereby significantly enhancing the energy transfer efficiency of the TET device.
SUMMARY OF THE INTENTION
In accordance with the above, this invention provides a transcutaneous energy transfer device having an external primary coil to which energy to be transferred is applied and an implanted secondary coil inductively coupled to the primary coil and connected to apply energy to a subcutaneous utilization device, the invention being characterized by the inclusion of a magnetic shield covering the primary winding. The shape of the shield is generally substantially the same as that of the primary coil, but the size of the shield should be greater than that of the primary coil. More paiticularly, to fully reflect magnetic field toward the secondary coil, the shield should overlap the primary coil on all sides by at least the thickness (t) of the shield. Where the primary coil has a generally circular shape with a diameter d, the shield has a generally circular shape with a diameter D, where D>d and preferably D≧d+2t. The thickness of the shield for a circular shield is preferably much greater than D/&mgr; where &mgr; is the magnetic permeability relative to free space of the shield material, or more generally, t>>X/&mgr;, where X is a major dimension of the shield.
The shield normally has a plurality of ventilation perforations formed therein which perforations are preferably formed parallel to the magnetic field direction so that the path taken through the material of the shield is as short as possible. For embodiments where the primary coil is circular, the perforations are a plurality of radial slots, which slots are slightly wedge-shaped for an illustrative embodiment. To assure adequate ventilation, the perforations should make up between approximately 25% and 75% of the shield area. Since the perforations reduce &mgr; of the shield material, for the shield thickness to continue to satisfy t>>X/&mgr;, t needs to increase proportionally (i.e., if the shield is 50% perforated, shield thickness t
p
=2t). Perforation size should also be small compared the smallest coil in the TET device.
The shield should also be flexible so as to be able to conform to the contours of a patient's body. To achieve this flexibility, for one embodiment of the invention the shield is formed of a low loss magnetic material in a flexible polymer matrix, the shield being formed of a ferrite powder in a silicon rubber for an illustrative embodiment. For another embodiment, the shield is formed of a plurality of segments of a very high permeability material connected by a porous, flexible material. To the extent there are spacings between adjacent segments in a direction substantially perpendicular to the primary coil magnetic field in order to enhance flexibility, such spacings are much smaller than spacings in a direction parallel to the magnetic field.
The shield is also dimensioned and formed of a material which reflects most of the magnetic field directed away from the secondary coil back toward the secondary coil. This significantly enhances the efficiency of energy transfer across the skin boundary by the TET device.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.
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Hart Robert M.
Keville Stephen J.
Zarinetchi Farhad
Abiomed, Inc.
Nutter & McClennen & Fish LLP
Schaetzle Kennedy
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