Abstract:
Adjustable gastric band implants contain a hollow elastomeric balloon with fixed end points encircling a patient&#39;s stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In current bands, this saline solution must be injected into a subcutaneous port with a needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. As an alternative to using a percutaneously accessed injection port, a system for regulating the flow of saline that is totally implanted may rely upon bi-directionally pumping fluid from an implant device. This system instead transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implanted reservoir. A magnetically permeable rod centered within the primary coil increases power coupled to the secondary coil.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to four co-pending and commonly-owned applications filed on even date herewith, the disclosure of each being hereby incorporated by reference in their entirety, entitled respectively: 
   “MEDICAL IMPLANT HAVING CLOSED LOOP TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER REGULATION CIRCUITRY” to William L. Hassler, Jr., Ed Bloom, Ser. No. 10/876,038; 
   “SPATIALLY DECOUPLED TWIN SECONDARY COILS FOR OPTIMIZING TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER CHARACTERISTICS” to Reshai Desai, William L. Hassler, Jr., Ser. No. 10/876,057, now U.S. Pat. No. 7,191,007; 
   “LOW FREQUENCY TRANSCUTANEOUS TELEMETRY TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Ser. No. 10/876058; and 
   “LOW FREQUENCY TRANSCUTANEOUS ENERGY TRANSFER TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/876,307. 
   FIELD OF THE INVENTION 
   The present invention relates, in general, to medically implantable devices that receive transcutaneous energy transfer (TET), and more particularly, such implant devices that optimize power transfer. 
   BACKGROUND OF THE INVENTION 
   In a TET system, a power supply is electrically connected to a primary coil that is external to a physical boundary, such as the skin of the human body. A secondary coil is provided on the other side of the boundary, such as internal to the body. With a subcutaneous device, both the primary and secondary coils are generally placed proximate to the outer and inner layers of the skin. Energy is transferred from the primary coil to the secondary coil in the form of an alternating magnetic field. The secondary coil converts the transferred energy in the AC magnetic field to electrical power for the implant device, which acts as a load on the secondary coil. 
   In a TET system, the primary and secondary coils are placed on separate sides of the boundary or skin. This separation typically results in variations in the relative distance and spatial orientation between the coils. Variations in the spacing can cause changes in the AC magnetic field strength reaching the secondary coil, in turn causing power fluctuations and surges in the implant device. Implant devices, such as those used in medical applications, usually rely upon a microcontroller to perform various functions. These microcontrollers require a consistent, reliable power source. Variations in the supplied power, such as sudden changes in voltage or current levels, may cause the device to perform erratically or fail to function at all. Accordingly, one issue associated with conventional TET systems is that the physical displacement of either the primary or secondary coils from an optimum coupling position may cause an unacceptable effect on the output power supplied to the implanted device. 
   As an example of an implantable device that may benefit from use of TET is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient&#39;s stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient. 
   To that end, in the below-referenced co-pending applications, an implanted infuser device regulates the flow of saline without requiring injection into the subcutaneous port. This system instead transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implant connected to the gastric band within the abdomen. 
   Although such TET powering of an implant, such as to recharge batteries, is a generally known procedure, using TET for an artificial sphincter system, such as an adjustable gastric band, presents a number of challenges. Adjustable gastric bands are most beneficial to patients that are morbidly obese. Providing a secure location to subcutaneously attach an implant that presents a reduced incident of discomfort often means that the implant is under a thick layer of skin and adipose tissue. A major challenge in using TET thus is transferring magnetic energy between the primary and secondary coils through this thick layer of tissue, which thus reduces the effective amount of power transferred to the implant. 
   It is also generally known to include a magnetic shield across an external side of a primary coil used in TET powering of an artificial heart, such as described in U.S. Pat. No. 6,389,318. Such magnetic shields are generally a flat disk that overlays the top and sides of the primary coil for the purpose of shielding from other conductors in the external environment. Perforations are included for ventilation since such primary coils are continually positioned on the patient. To be conformal, a preferred material is silicon impregnated with ferrite powder so that its low magnetic loss serves as a back plane that reflects magnetic energy from the primary coil. While providing advantages for external sources of electromagnetic interference, such shields are not believed to substantially assist in directing the magnetic flux to the secondary coil of an implanted medical device. 
   While the shield described in U.S. Pat. No. 6,389,318 provided some shaping of the magnetic flux from the primary coil, one undesirable characteristic thereof was that the magnetic flux was flattened, providing less efficient power coupling to deeply embedded implantable devices. In the application described for artificial hearts, the secondary coil was near to the surface of the patient&#39;s skin and thus this apparently did not pose a problem. 
   In U.S. Pat. No. 5,715,837, enhancing the effectiveness of TET was addressed by increasing the magnetic permeability of the flux path through the dermis of the patient by implanting soft iron pellets therein. It would be undesirable to implant metal pellets for a number of reasons. First, in the morbidly obese patient, it may require a significant amount of pellets to seed the flux path. Second, the patient may object to this permanent implantation. Third, being ferrous objects, tissue damage or discomfort may result if the patient were in the presence of a strong magnetic field typical of a Magnetic Resonance Imaging (MRI) machine. Fourth, these ferrous objects would create artifacts that would hamper diagnostic imaging such as MRI and CT. Fifth, the chemical or physical properties of these pellets may have a deleterious effect on the dermis. 
   It is further inconvenient to shape the magnetic flux as described in U.S. Pat. No. 5,715,837 with opposing horseshoe shaped ferrite cores insofar as it is desirable to eliminate such mass from an implanted device to make it smaller. In addition, it is further desirable to eliminate materials that respond to strong magnetic fields, as mentioned with regard to soft iron pellets or a partially exposed, implanted ferrite core. 
   In U.S. Pat. No. 5,279,292, a charging system for an implantable hearing aid or tinnitus masker included a receiving coil that is implanted under the skin in mastoid. The receiving coil included a ferrite core that projected outward through the skin. Thus, a transmitting coil is placed over the exposed end of the ferrite core, mechanically aligning the primary coil and enhancing magnet coupling to the receiving core. Due to the relatively small amount of power transferred, the ferrite core is described as being small and unobtrusive and being hidden behind the external ear. However, it is undesirable to have an exposed implant that tends to allow infections. In addition, use of a transformer instead of TET as in this application also makes the implant not compatible with MRI machines. 
   Consequently, a significant need exists for enhancing TET power transfer from a primary coil through the dermis of a patient to an implanted device that contains a secondary coil. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention overcomes the above-noted and other deficiencies of the prior art by providing a transcutaneous energy transfer (TET) system that includes a primary coil having a magnetic flux shaping member centered within its circular diameter, thereby forming a toroidal magnetic flux field with an implanted secondary coil in a patient that is more elliptical in cross section. Thereby, greater power coupling efficiency is achieved, enabling medical implants to be placed at greater depths. Thus, applications such as remotely controllable adjustable gastric bands for the morbidly obese may be used without having to resort to inconveniently high power levels nor to having to embed separate secondary coils near the surface of the dermal layer. 
   In one aspect of the invention, an external TET device includes a circular power coil that is energized at a resonant frequency to provide magnetic flux to a secondary coil of an implanted medical device. A very highly magnetically permeable member is centered within the primary coil to shape a resulting magnetic field into a more elliptical toroidal shape with respect to its longitudinal axis. 
   These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. 
       FIG. 1  is a circuit block diagram of a transcutaneous energy transfer (TET) system coupled to a power supply. 
       FIG. 2  is a magnetic flux diagram of a prior art TET system having a primary coil and implanted secondary coil. 
       FIG. 3  is a magnetic flux diagram of a TET system having a magnetic flux conducting core centered within the primary coil to shape a resultant magnetic flux. 
       FIG. 4  is a plot of power induced in a secondary coil by various lengths of a flux shaping core in the primary coil and different depths of separation between primary and secondary coils. 
       FIG. 5  is a an alternative magnetic flux shaping core formed from a slender steel rod divided into sixteen sections electrically isolated along their length, each having a “pie-shaped” cross section. 
       FIG. 6  is a further alternative magnetic flux shaping core depicted as a ferrite pot core shaped as a disk with a ring groove formed in a bottom surface to receive a primary coil. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings in detail, wherein like numerals indicate the same elements throughout the views,  FIG. 1  depicts the relationship between a transcutaneous energy transfer (TET) system  10  that has an external device  12  and an implanted device  14 . The external device  12  includes a primary coil  16  that is external to a patient. The implanted device  14  includes a secondary coil  18  that inductively receives power from the primary coil  16  transcutaneously through a dermal layer  20  of the patient, as depicted by alternating current (AC) magnetic flux lines  22 . The primary coil  16  is connected in parallel with capacitance  24  to form a resonant parallel tank circuit  26 . The AC magnetic flux  22  generated by the resonant tank circuit  26  is collected by secondary coil  18 , which is connected in series with a secondary capacitance  28  to form a secondary resonant series tuned tank circuit  30 , which delivers power to implant circuitry  32 . 
   As an example of an implanted device  14  that would benefit from TET is an infuser device, described in greater detail in the referenced applications, that regulates the amount of fluid dispensed bi-directionally into an expandable gastric band following implantation of the band during weight reduction surgery. The TET system  10  may consist of the primary coil  16  occasionally placed outside a patient&#39;s abdomen when adjustment of the gastric band is desired and the secondary coil  18  within an infuser implanted device  14  that has been anchored subcutaneously on a layer of muscular fascia within the patient. The illustrative primary coil  16 , having an outer diameter (OD) of about five (5) inches (13 cm) and consisting of one hundred two (102) turns of litz wire made up of one hundred (100) individually insulated thirty (30)-gauge magnet wires, is connected in parallel with 9.2 microfarads of capacitance, creating a parallel tuned resonant tank circuit with a very high Q. The secondary coil  18  is connected in series with a capacitor  28  forming a series tuned resonant tank circuit and is activated by receiving AC magnetic flux energy from the primary coil  16 . The two tuned tank circuits  24 ,  30  are tuned to the same frequency for optimal power transfer. 
   Implantable, bi-directional infusing devices that would benefit from enhanced TET powering and telemetry are disclosed in four co-pending and co-owned patent applications filed on May 28, 2004, the disclosure of which are hereby incorporated by reference in their entirety, entitled (1) “PIEZO ELECTRICALLY DRIVEN BELLOWS INFUSER FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Ser. No. 10/857,762; (2) “METAL BELLOWS POSITION FEED BACK FOR HYDRAULIC CONTROL OF AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Rocco Crivelli, Ser. No. 10/856,971; (3) “THERMODYNAMICALLY DRIVEN REVERSIBLE INFUSER PUMP FOR USE AS A REMOTELY CONTROLLED GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,315; and (4) “BI-DIRECTIONAL INFUSER PUMP WITH VOLUME BRAKING FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,763. 
     FIG. 2  shows a generally known TET device  40  that achieves a magnetic field, depicted as shallow flux lines  42  between parallel primary and secondary TET coils  44 ,  46 . Primary coil  44  transfers magnetic flux  42  through an abdominal wall  48  to the secondary coil  46 . Due to losses and the shape of the magnetic field  42 , the secondary coil  46  is constrained to be placed relatively close to the exterior of the abdominal wall  48  since the magnetic field  42  has a circular toroidal shape that does not achieve optimal energy transfer between the two coils  44 ,  46 . 
     FIG. 3  depicts the TET system  10  of  FIG. 1  that advantageously shapes a TET magnetic field  52  into an elliptical shape that more efficiently operates through an abdominal wall  58  of a patient. Thus, at an implanted depth equivalent to the prior art secondary coil  18 , more power is transferred. Alternatively, a secondary coil  18 ′ may be placed at a greater depth for more secure attachment and enhanced patient comfort yet be able to receive sufficient power. In particular, a ferrite rod  62  aligned at a circular center of an external primary coil  16  extends along a center axis A-A of the primary coil, shaping the magnetic flux  52  into an elliptical toroidal shape, and causing an increase in flux density within the secondary coil  18 . 
   This enhanced power transfer is depicted in  FIG. 4 , showing the difference in energy transfer efficiency before and after placement of the ferrite cores  62  of different lengths into the primary coil  16 . It was shown that a benefit existed for additional power received in the secondary circuit for separation distances of 1.5 to 5.5 inches by the inclusion of a core of lengths between 1 to 4 inches. Extrapolating from the results indicates that some benefit would be appreciated by a shorter length of a core, if constrained by available clearance considerations. In addition, longer lengths of a core may be used to obtain additional power coupling efficiencies. 
   To achieve the greatest energy transfer efficiency, a highly magnetically permeable ferrite core  62  has been placed within the primary coil  16 . As stated previously, we determined that the optimum core  62  is of a long, skinny design. Testing indicates that a ferrite core rod  62  with a length of about 3 inches and a width of about 0.75 inches is the optimal size for the given primary coil  16  at which energy transfer is at its most efficient without going into magnetic saturation or wasting energy in the form of eddy current losses within the core  62 . 
   With the long and slender core design, most of the magnetic flux is drawn toward the ferrite core  62 , causing the field to collapse radially into the core  62  and changing the shape of the field  52  from circular to elliptical. This effect leads to an increase in the flux density within the secondary coil  18 . In an exemplary version, a ferrite core of 3 inches length and 0.75 inches diameter was placed within the center of a 5 inch diameter primary coil  16  of the transcutaneous energy transfer (TET) system  10 . With the addition of this core  62 , the power coupling efficiency to the secondary TET coil was increased by up to 55%. 
   Laminated Steel Core. 
   In  FIG. 5 , as an alternative to a ferrite rod core, a cylindrical core  80  was made of a Carpenter Steel 430 FR stainless steel. The core was 1.25 inches in diameter, and 3.0 inches long. This core was longitudinally segmented into 16 different “pie” shaped  82  radial portions that were electrically isolated from each other by a high temperature epoxy (Duralco 4525)  84 . This laminating process was done in order to minimize the eddy current losses in the core  80 , while trying to maximize the magnetic flux carrying capability of the core. The magnetic permeability of the steel is actually half of the ferrite materials, but the saturation flux density is around four times higher, allowing for much more magnetic flux to pass through the same sized core. 
   “Pot” Core. 
   In  FIG. 6 , as a further alternative to a ferrite rod core, a ferrite pot core  90  was made to reshape the magnetic field so as to increase the range, and/or power coupling of the TET system  10 . The pot core  90  has a disk portion  92  that covers an exteriorly facing side of the primary coil  16 . A cylindrical flange  94  is inwardly directed toward the patient attached to the circumference of the disk portion  92 , thereby assisting electromagnetic interference shielding of the primary coil  16  that resides within a circular groove  96 , which also defines a central rod  98 . The circular groove  96  opens toward an inward face of the pot core  90  that would be placed toward the patient. The central rod  98  shapes the magnetic field into a more elliptical toroidal shape as previously discussed. The pot core  90  may be visualized by taking an E shape and revolving it about the centerline of the center prong of the E. The material thickness of the pot core may be minimized in order to reduce the eddy current losses within the core without causing the core to go into magnetic saturation. 
   By virtue of the foregoing, the elliptical shape of the magnetic field increases the coupling efficiency between the primary and secondary TET coils. Increased coupling efficiency between the coils reduces the amount of power required from the primary coil and also increases the range at which the external primary coil can be separated from the subcutaneous secondary coil, an important consideration in facilitating weight reduction surgery where thick abdominal walls are typically encountered. 
   While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications may readily appear to those skilled in the art. 
   For example, it will become readily apparent to those skilled in the art that the above invention has equal applicability to other types of implantable bands. For example, bands are used for the treatment of fecal incontinence. One such band is described in U.S. Pat. No. 6,461,292, which is hereby incorporated herein by reference. Bands can also be used to treat urinary incontinence. One such band is described in U.S. patent application 2003/0105385, which is hereby incorporated herein by reference. Bands can also be used to treat heartburn and/or acid reflux. One such band is described in U.S. Pat. No. 6,470,892, which is hereby incorporated herein by reference. Bands can also be used to treat impotence. One such band is described in U.S. Patent Application Publ. No. 2003/0114729, which is hereby incorporated herein by reference. 
   As another example, it should be appreciated that a shaping magnetic flux with a highly magnetically permeable member centered within the primary coil enhances transcutaneous energy transfer used for telemetry in addition to or in the alternative to TET for powering an implanted device. For instance, a primary coil may be used intermittently for telemetry sending or receiving or include a primary telemetry coil in addition to a primary power coil. 
   As yet a further example, it should be appreciated that in applications that require a relatively low amount of power transfer and/or only occasional periods of time for TET, materials chosen for the core may include those subject to eddy currents and heating. Alternatively, thermally insulating materials may be included to protect the skin of the patient from discomfort or injury.