Abstract:
An implantable medical device system advantageously utilizes low frequency (e.g., about 1-100 kHz) transcutaneous energy transfer (TET) for supplying power from an external control module to an implantable medical device, avoiding power dissipation through eddy currents in a metallic case of an implant and/or in human tissue, thereby enabling smaller implants using a metallic case such as titanium and/or allowing TET signals of greater strength thereby allowing placement more deeply within a patient without excessive power transfer inefficiencies.

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:
         “TRANSCUTANEOUS ENERGY TRANSFER PRIMARY COIL WITH A HIGH ASPECT FERRITE CORE” to James Giordano, Daniel F. Dlugos, Jr. and William L. Hassler, Jr., Ser. No. 10/876,313;   “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, now abandoned;   “SPATIALLY DECOUPLED TWIN SECONDARY COILS FOR OPTIMIZING TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER CHARACTERISTICS” to Resha H. Desai, William L. Hassler, Jr., Ser. No. 10/876,057; and   “LOW FREQUENCY TRANSCUTANEOUS TELEMETRY TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Ser. No. 10/876,058.       

   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 above-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 TET is not required for powering the device, the long-term nature of these devices benefits from use of TET, allowing an implanted device of reduced size and complexity. Moreover, these devices may remain unpowered between adjustments, which provides additional advantages such as not requiring a battery. 
   It is known to surgically implant a medical device such as, for example, a cardiac pacemaker or an adjustable gastric band, under the surface of a patient&#39;s skin to achieve a number of beneficial results. In order to actively operate within a patient, these medical implants require a reliable, consistent power source. Currently, medical implants are powered by either non-rechargeable batteries, rechargeable batteries that use a TET system to recharge the batteries, or directly by a TET system. In order to transfer sufficient power to the secondary coil of the TET system to operate an implant, TET systems have typically operated at frequencies from 100 kHz to upwards of 30 MHz. At these higher frequency levels, the alternating electromagnetic field that the primary coil generates couples not only to the secondary coil, but also to any metallic objects near it, including a metallic implant case. This parasitic coupling produces eddy currents in the implant case. These eddy currents reduce the amount of effective power transferred to the secondary coil, thereby increasing the amount of power required from the primary coil to drive the implant. In addition, the eddy currents can cause heating of the metallic case. Heating a metallic implant case by more than 2° Celsius above normal body temperature can have derogatory effects on the implant recipient. The level of eddy currents produced in a metallic object is directly proportional to the alternating magnetic field frequency raised to the second power. Accordingly, the greater the frequency of the energy transfer signal, the greater the eddy currents and energy transfer losses. In addition, operating a TET system at frequencies above 100 kHz requires that the system conform to FCC regulations. 
   To reduce the problems associated with eddy currents and power transfer losses when using a TET system with an implant, it has traditionally been necessary to enclose the implant in a non-metallic material, such as a ceramic outer casing, or to place the secondary coil outside of the implant outer casing and connect the coil by a pair of leads extending into the casing. Alternatively, TET systems have been used as low energy trickle charge systems that operate continuously to recharge internal implant batteries. Each of these solutions to the eddy current problem, however, is either expensive, cumbersome, or increases the complexity of the implant device. Accordingly, in order to reduce the problem of eddy currents when powering an implant, and minimize the issue of FCC regulations, it is desirable to have an energy transfer system that operates at low frequencies. In particular, it is desirable to provide a high power, low frequency TET system in which the secondary coil may be encased within the implant without significant power losses or development of eddy currents. 
   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 dermal tissue, which thus reduces the effective amount of power transferred to the implant. 
   Consequently, a significant need exists for enhancing TET power transfer through the dermis of a patient and into a hermetically sealed case of an implanted medical device without significant power losses. 
   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 advantageously transmits between 1 to 100 kHz, thereby achieving an electromagnetic pattern that may more efficiently penetrate a physical boundary such as a metallic case of an implant or human tissue, without excessive power loss due to eddy currents, thereby avoiding heating. 
   In one aspect of the invention, the transcutaneous energy transfer (TET) system has an external primary power supply that energizes an external primary resonant circuit having a primary coil in electrical communication with a capacitance to form a resonant tank circuit having peak resonance within a range of 1 to 100 kHz. The TET power therefrom reaches an internal secondary resonant circuit including a secondary coil in electrical communication with a capacitance to form a resonant tank circuit having peak resonance within a range of 1 to 100 kHz to power an electrical load. 
   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 block diagram illustrating an exemplary energy transfer system in accordance with the present invention; 
       FIG. 2  is a block diagram illustrating the low frequency TET system of the present invention; 
       FIG. 3  is a block diagram of a second embodiment for the secondary resonant circuit; and 
       FIG. 4  is a graphical representation of the gain verses frequency response of the primary and secondary resonant circuits. 
       FIG. 5  is a magnetic flux diagram of a prior art TET system having a primary coil and implanted secondary coil. 
       FIG. 6  is a cross section view of 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. 7  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. 
   

   DETAILED DESCRIPTION OF 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  20  for an implant device  22  in accordance with the present invention. As shown in  FIG. 1 , TET system  20  includes a primary circuit  24  comprising a power supply  26  located external to a physical boundary  28 . Boundary  28  may be the skin of a human or animal body, such as in the case of a medical implant, or may be any other type of inanimate material or tissue depending upon the particular application of TET system  20 . Primary circuit  24  also includes a primary resonant circuit  30  that is electrically coupled to power supply  26  to resonate at a designated power signal frequency. An alternating magnetic field  32  is generated in primary coil  30  in response to an electrical signal provided by power supply  26 . 
   TET system  20  also includes a secondary resonant circuit  34  in a spaced relationship from primary resonant circuit  30 . Secondary resonant circuit  34  is located on the opposite side of boundary  28  from primary resonant circuit  30  within implant  22 . Secondary resonant circuit  34  is electrically coupled to primary resonant circuit  30  via alternating magnetic field  32 , symbolically illustrated in the figures as arrows emanating from primary resonant circuit  30  and propagating towards secondary resonant circuit  34 . Secondary resonant circuit  34  generates an electrical signal  36  from field  32 . Signal  36  is rectified by a filter  40  and applied to an implant load  42  to operate the implant  22 . 
   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  provides a more detailed schematic of the energy transfer system  20  of the present invention. As shown in  FIG. 2 , primary resonant circuit  30  comprises a tuned tank circuit having a capacitance made up of one or more capacitors  44  connected in parallel with an inductive coil  46 . Capacitance  44  and coil  46  are selected to resonate at a particular frequency when connected to power supply  26 . In order to transfer power to secondary resonant circuit  34  without generating excessive eddy currents, primary resonant circuit  30  is designed to operate at low frequency levels. For purposes of this discussion, the terms “low frequency” and “low frequency level” refer to frequencies below 100 kilohertz (kHz). In order to transmit sufficient power to drive implant  22  at low frequency levels, capacitor  44  and coil  46  are selected to maximize the Q or quality factor of the circuit  30  and, thus, produce a high gain from resonant circuit  30 . In the embodiment described herein, capacitor  44  and coil  46  are selected to produce a Q factor exceeding 100. 
   To obtain a high Q factor at a low signal frequency level, capacitor  44  is selected so as to provide a high voltage at a minimum equivalent series resistance (ESR). An example of a suitable type of capacitor for obtaining high voltage/low ESR performance is a chip-on-glass (COG) dielectric capacitor. Additionally, to maximize the Q factor of primary resonant circuit  30 , coil  46  is formed so as to minimize the coil impedance and, thus, the power loss in the circuit. One method for minimizing coil impedance in the present invention is to form coil  46  from Litz wire. Litz wire is composed of individual film insulated wires that are braided together to form a single conductor. The Litz wire minimizes power losses in coil  46  due to the skin effect, or tendency of radio frequency current to be concentrated at the surface of the conductor. In addition to Litz wire, other types of high current, low power loss conductors may also be utilized for primary coil  46  in the present invention without departing from the scope of the invention. The combination of the high voltage capacitance with a high current/low power loss coil enables primary resonant circuit  30  to transfer sufficient power to drive an implant, such as, for example, 1 to 4 watts of power, by virtue of using a low transfer signal frequency. 
   As shown in  FIG. 2 , secondary resonant circuit  34  comprises a secondary coil  50  that is electrically connected in series with one or more capacitors  52  to form a series tuned tank circuit. Capacitor  52  may be any type of capacitor that enables the tank circuit to resonate in a frequency range that encompasses the resonant frequency of primary circuit  30 . Similarly, coil  50  may be any type of conductor that produces minimum impedance while effectively coupling with primary resonant circuit  30  to transmit sufficient power for operating load  42 . Secondary resonant circuit  34  is tuned to have a lower Q and broader bandwidth than primary resonant circuit  30  in order to couple with a broader range of resonant frequencies, and eliminate the need to individually tune the secondary resonant circuit to a particular primary resonant circuit, although it should be appreciated that a higher Q may be used. 
     FIG. 3  illustrates an alternative version for TET system  20  in which secondary coil  50  is replaced with a pair of inductive coils  54 ,  56  connected in parallel. Coils  54 , 56  are connected in series with capacitor  52  to form a series tuned tank circuit  58 , which couples with primary resonant circuit  30  to transfer power to load  42 . Replacing secondary coil  50  with parallel coils  54 ,  56  increases the amount of magnetic flux  32  intercepted by the secondary resonant circuit, and the amount of power supplied to load  42 . 
     FIG. 4  provides a graphical representation of the gain verses frequency response of primary resonant circuit  30  and secondary resonant circuit  34 . As shown in  FIG. 4 , primary resonant circuit  30  is very frequency selective, as indicated by the steep curve  60 , thereby producing a high Q and power level at a narrow range of frequencies centered on the resonant frequency  62 . Conversely, secondary resonant circuit  34  has a significantly lower Q than primary resonant circuit  30  and is less frequency selective as indicated by the more rounded curve  64 . The broader bandwidth of secondary resonant circuit  34  desensitizes the circuit to shifts in the resonant frequency of the primary circuit  30 , thus enabling the secondary resonant circuit  34  to couple with one or more different primary circuits without having to be specifically tuned to the primary circuit resonant frequency  62 . 
   In an exemplary embodiment of the present invention, a TET system was experimentally produced having a resonant frequency range of between 1.6 and 1.7 kilohertz and a Q factor greater than 100. In this experimental circuit, primary coil  46  having an outer diameter of 5.25 inches was comprised of one hundred two ( 102 ) turns of Litz wire. The Litz wire was comprised of  100  strands of individually insulated thirty (30)-gauge magnet wire. The primary coil was placed in parallel with 9.4 microFarads of capacitance. The capacitance was a high voltage, high current, low ESR, COG dielectric capacitor. In addition, a ferrite core was incorporated with the primary coil  46  as described in the application incorporated by reference above, entitled “TRANSCUTANEOUS ENERGY TRANSFER PRIMARY COIL WITH A HIGH ASPECT FERRITE CORE” 
   The secondary resonant circuit was comprised of two coils connected in parallel. Each of the coils consisted of  325  turns of thirty-four (34)-gauge magnet wire. The coils each had an outer diameter of 2.4 inches. The parallel secondary coils were connected in series with a capacitance of 1.1 microFarads to create a series tuned tank circuit having a significantly lower Q than the primary resonant circuit. In the experimental circuit, the secondary circuit had a Q in the range of approximately ten (10) to fifteen (15). The experimental TET system transmitted approximately one watt of power between the primary and secondary circuits. The lower Q factor of the secondary resonant circuit enabled the circuit to couple with the primary resonant circuit without being specifically tuned and matched to the primary circuit. This exemplary circuit illustrates one configuration through which the present invention may be implemented. Additional circuit configurations and elements that maximize the Q factor of the primary resonant circuit may also be utilized to achieve low frequency TET power transfer in accordance with the present invention without departing from the scope of the invention. 
     FIG. 5  shows a generally known prior art TET device  140  that achieves a magnetic field, depicted as shallow flux lines  142  between parallel primary and secondary TET coils  144 ,  146 . Primary coil  144  transfers magnetic flux  142  through an abdominal wall  48  to the secondary coil  146 . Due to losses and the shape of the magnetic field  142 , the secondary coil  146  is constrained to be placed relatively close to the exterior of the abdominal wall  148  since the magnetic field  142  has a circular toroidal shape that does not achieve optimal energy transfer between the two coils  144 ,  146 . 
     FIG. 6  depicts a TET system  110  having a ferrite core that advantageously shapes a TET magnetic field  152  into an elliptical shape that more efficiently operates through an abdominal wall  158  of a patient. Thus, at an implanted depth equivalent to the prior art secondary coil  118 , more power is transferred. Alternatively, a secondary coil  118 ′ 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  162  aligned at a circular center of an external primary coil  116 , shaping the magnetic flux  152  formed an elliptical toroidal shape, causing an increase in flux density within the secondary coil  18 . 
   This enhanced power transfer is depicted in  FIG. 7 , 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  162  has been placed within the primary coil  116 . The optimum core  162  is of a long, skinny design. Testing indicates that a ferrite core rod  162  with a length of about 3 inches and a width of about 0.75 inches is the optimal size for the given primary coil  116  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  162 . 
   With the long and slender core design, most of the magnetic flux is drawn toward the ferrite core  162 , causing the field to collapse radially into the core  162  and changing the shape of the field  152  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  116  of the transcutaneous energy transfer (TET) system  10 . With the addition of this core  162 , the power coupling efficiency to the secondary TET coil was increased by up to 55%. 
   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.