Abstract:
A transcutaneous recharging system for providing power to an implantable medical device comprises a primary side circuit for transmitting power in the form of magnetic flux; and a secondary side circuit integral to the implantable medical device for receiving the power transmitted from the primary side circuit and for providing the received power to recharge a battery in the implantable medical device, wherein the primary and secondary side circuits are not physically coupled. A variety of attachment configurations are disclosed for attaching and shielding the secondary circuit directly onto the housing of the implantable medical device, inclusive of flexible printed circuit coils and wire coils recessed into helical notches.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation-in-part of U.S. Application Serial No. 949612, filed Sep. 12, 2001, which in turn derives priority from Korean Application Serial No. KR 2001-28347 filed May 23, 2001. The aforesaid applications are commonly owned by the named inventors. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to implantable medical devices such as pacemakers and defibrillators and, more particularly, to an improved rechargeable power supply configuration including a remote primary circuit for contactless charging, and a housing design for the implantable medical device that incorporates a non-contact secondary circuit for charging by the remote primary circuit.  
         [0004]     2. Description of the Background  
         [0005]     It is forecast that the US market for implantable medical devices will grow 10.9% per year through 2007, to nearly $24.4 billion. The growth leaders are anticipated to be cardiac resynchronization devices, implantable cardioverter defibrillators (ICDs), drugeluting stents, bioengineered tissue implants, neurological stimulators, cochlear implants and retinal implants. Much of this growth is due to technological advances in the devices themselves which make them less obtrusive and more reliable. Also, based on increasing clinical evidence of therapeutic effectiveness and lifesaving benefits, third-party insurance concerns are covering an expanding number of heart patients for pacemakers, implantable cardioverter defibrillators and coronary stents. These devices are enabling persons afflicted with cardiac rhythm disorders and heart failure to live a more normal life without dependence on complex drug regimens. The most pressing need for further technological advances lies in the size and weight of implanted devices, and this remains the major challenge for many researchers. The size of an implanted device directly affects the comfort of the patient. Particularly, if an implant is large it will require that much large opening in the living body either to insert or remove it, possibly causing an excessive bleeding and increasing vulnerability to infection during the implantation.  
         [0006]     A battery occupies 50 to 80% of volume in most of implanted medical devices. However, batteries have a limited lifespan and must be replaced periodically. The replacement also requires a surgical operation to make an opening in the body, which is very inconvenient to and can be dangerous for some patients. For this reason, transcutaneous power transmission has been tired as a form of non-contact power transmission.  
         [0007]     For example, a prior art charger for implanted medical device is disclosed in U.S. Pat. No. 4,143,661, which shows a very large coil implanted in a human body so as to surround a leg or the waist to use it as the secondary coil. Implanting such a large coil adversely affects the patient&#39;s condition. In addition, a large coil inserted into a human body could cause damages to the body.  
         [0008]     Another prior art charger is disclosed in U.S. Pat. No. 5,358,514. The charger disclosed therein includes a secondary transformer, a battery and other supplemental circuitry. For magnetic flux supplied from outside of a human body to reach the charger, the charger cannot be enclosed in a metal case, which imposes restrictions on the design of the implanted device. Since ferromagnetic core surrounded by a coil is used as a component of a secondary transformer, it is bulky and vulnerable to impact from outside.  
         [0009]     Yet another prior art charger is disclosed in U.S. Pat. No. 6,505,077 to Kast et al., which shows a recharging coil  54  carried on the housing exterior surface  64  of a medical device  20 . The recharging coil  54  is manufactured from copper wire, copper magnet wire, copper litz woven wire, gold alloy and the like, and is coupled to recharging feedthroughs  68  with an electrical connection  56 .  
         [0010]     None of the foregoing nor any known contactless battery charging systems are well-adapted for incorporation directly in/on the housings of existing implantable medical devices, rather than at remote locations. This is because existing designs are too bulky and unsuitable for implantation, are too prone to oxidation once implanted (and to poisoning the patient), are too inefficient for practical charging, or are simply incompatible with the materials of most implantable medical devices. For example, for magnetic flux supplied from outside of a human body to reach a charger, the charger cannot be enclosed in a metal case.  
         [0011]     Consequently, it would be greatly advantageous to provide a completely sealed and safe contactless battery charging system with secondary coils that can be incorporated directly in/on the housings of most existing implantable medical devices, so as to minimize space.  
       SUMMARY OF THE INVENTION  
       [0012]     It is, therefore, an object of the present invention to provide a transcutaneous power transmission apparatus for use in an implantable medical device.  
         [0013]     It is another object to provide a transcutaneous power transmission apparatus for use in an implantable medical device that is small and compact, and can be implanted with the medical device, thereby minimizing surgery and subsequent treatments.  
         [0014]     It is another object to provide a transcutaneous power transmission apparatus for use in an implantable medical device that optimizes the transcutaneous magnetic coupling to minimize charging time.  
         [0015]     According to the present invention, the above-described and other objects are accomplished by providing an apparatus for providing power to an implantable medical device comprising a primary side circuit for transmitting power in the form of magnetic flux; and a secondary side circuit integral to the implantable medical device for receiving the power transmitted from the primary side circuit and for providing the received power to recharge a battery in the implantable medical device, wherein the primary and secondary side circuits are not physically coupled. A variety of attachment configurations are disclosed for attaching and shielding the secondary circuit directly onto the housing of the implantable medical device, inclusive of flexible printed circuit coils and wire coils recessed into helical notches. The system can be utilized for various implantable medical devices that requires electrical power, such as an artificial heart, a pacemaker, an implantable cardiverter defibrillator, a neurostimulator, a GI stimulator, an implantable drug infusion pump, a bone growth stimulation device, and many other devices. The system improves the power transmission coupling such that sufficient electric power can be transmitted to the medical device repeatedly without having to take the implanted medical device out of the human body. Further, since charging is more efficient and the secondary coils are integral to the implant housing the size of the battery can be reduced, thereby reducing the overall size of the implanted medical device. Moreover, the secondary coil(s) conform to the implant housing and are hermetically sealed to be non-obtrusive, non-corrosive and medically safe. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof when taken together with the accompanying drawings in which:  
         [0017]      FIG. 1  is a side cut-away view of the primary recharging unit  4  used in the present invention.  
         [0018]      FIG. 2  is a front view of the primary recharging unit  4  as in  FIG. 1 .  
         [0019]      FIG. 3  is a side cut-away view of the contactless power transfer housing  6  used in the present invention.  
         [0020]      FIG. 5  illustrates an exemplary circuit schematic that is suitable for the present invention.  
         [0021]      FIG. 6  depicts waveforms of the control signals s 1 , s 2 , s 3 , and s 4  applied to the circuit of  FIG. 6 .  
         [0022]      FIGS. 7 and 8  (A&amp;B) illustrate the operation of the contactless power transfer system, inclusive of primary recharging unit  4  located outside the human body and contactless power transfer housing  6  which is part and parcel to the implantable medical device implanted inside the human body.  
         [0023]      FIGS. 9-12  illustrate alternative configurations of the secondary side coil(s)  36 .  
         [0024]      FIG. 13  illustrates two alternative form-fitting embodiments of the secondary unit  6 .  
         [0025]      FIG. 14  illustrates alternative placements of secondary coils  36 .  
         [0026]      FIGS. 15 and 16  illustrate the leakage flux paths imparted by the present device.  
         [0027]      FIG. 17  is a perspective drawing of one embodiment of a shoulder strap  50  designed to be worn to suspend the primary charging unit  4  at the correct position on the body.  
         [0028]      FIG. 18  is a perspective drawing of another embodiment which is a vest  60   
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     The present invention is a contactless power transfer system for an implantable medical device, which includes a primary recharging unit located outside the human body and a contactless power transfer housing forming a portion of the implantable medical device that is implanted inside the human body. A number of embodiments of the present invention will now be described in details with reference to the accompanying drawings.  
         [0030]      FIG. 1  is a side cut-away view, and  FIG. 2  is a front view of the primary recharging unit  4 , which generally comprises a toroid-shaped housing  27  with charging coils  15  on one side, and circuit components  23  on the other side that are connectable by power cable  22  to a controller (not shown) for controlled application of recharging power. The controller can be located either inside or outside of the primary recharging unit. The advantage of including the controller inside is minimizing the unit. Furthermore, the primary recharging unit can include some battery unit along with the controller. (All-in-one structure) The housing  27  is preferably filled with an isolation composite  19  such as ferrite, Molypermalloy powder, or Kool Mu®. The recharging power derived from the controller is regulated by the on-board circuit components  23  resident on a printed circuit board  21 , and is then applied to the charging coils  15 . The circuit components  23  on printed circuit board  21  are contained within an enclosed metal case  24 , case  24  being recessed and seated inside housing  27 . The charging coils  15  are isolated from the printed circuit board  21  by a layer of isolation material  20 , that may be any good electrical insulation material, and which is sandwiched between the circuit board  21  and the back wall of metal case  24 . Additionally, the charging coils  15  are isolated from the printed circuit board  21  by a layer of heat insulation material  25 , that may be any good heat insulation material, and which is sandwiched between the back wall of metal case  24  and ferrite core  18  (to be described). Charging coils  15  are connected to the circuit components  23  via a power cable  26 . The circuit components  23  of printed circuit board  21  generate an AC power transfer signal in a frequency range of from 1-300 kHz. While a variety of circuit designs will suffice for this purpose,  FIG. 5  (described later) illustrates one exemplary circuit schematic that is suitable for present purposes. The power transfer signal is transmitted to secondary coils  36  of the medical device that is implanted inside the human body (see  FIG. 2  to be described), where it is inductively picked up and converted to a DC recharging signal that is used to charge the battery power source of the implanted medical device.  
         [0031]     The charging coils  15  are wound onto a bobbin  17  for stability and ease of assembly, and the bobbin  17  is inserted into a toroid ferrite core  18  that is formed with a circular recess for receiving the bobbin  17 . The ferrite core  18  provides EMI shielding capabilities against outside interference and, due to the open-face toroid configuration, directionalizes the transmission to maximize power transmission to the implantable medical device. Ferrite core  18  is preferably an efficient magnetic material such as Alnico (an alloy composed of iron, cobalt, nickel, aluminum, and copper) or Ferrite, but may be may be any other suitable core material such as iron, etc. The primary charging coils  15  are enclosed inside the ferrite core  18  by an isolation composite cover  14 , which is a disc of smaller diameter than the toroid-shaped housing  27  and which protrudes slightly beyond the plane of housing  27 . The isolation composite cover  14  seals the charging coils  15 , bobbin  17  and ferrite core  18  inside the toroid-shaped housing  27 , and also positions a flux sensor  16  centrally over the ferrite core  18 . Moreover, as seen later the isolation composite cover  14  serves as a skin depressant during use to maximize the magnetic coupling between the primary recharging unit  4  and the secondary. The flux sensor  16  may be a conventional Hall Effect sensor element as used in magnetic field variation meters and the like. The flux sensor  16  may be integrally molded in composite cover  14  such that it is positioned within the air gap of the ferrite core  18 , and this is coupled back to the controller to ensure that the correct flux field will be set up within the core  18  material.  
         [0032]      FIG. 3  is a side cut-away view, and  FIG. 4  is a front cut-away view of the contactless power transfer housing  6  according to the present invention which forms a portion of an implantable medical device that is implanted inside the human body. The contactless power transfer housing  6  remains integral to the implantable medical device once it has been implanted inside the human body, in contradistinction to prior art contactless charging systems which place secondary coils remotely from the actual implanted device. The contactless power transfer housing  6  generally comprises an enclosed housing  30  formed of conventional implant material such as titanium. The housing  30  contains a rechargeable battery  44  powering any of a variety of implantable medical devices  46 , such as an artificial heart, a pacemaker, an implantable cardiverter defibrillator, a neurostimulator, a GI stimulator, an implantable drug infusion pump, a bone growth stimulation device or other electronic devices. It is preferable to use a small and stable battery  44  in the medical device. Lithium-ion and lithium-polymer batteries are examples of small and thin batteries. Although the lithium-ion battery is more efficient, the lithium-polymer battery is preferable because it is more stable.  
         [0033]     The front surface of the housing  30  is defined by a circular recess that is covered by a ferromagnetic composite sheet  32  for protection. Sheet  32  may be any thin ferromagnetic sheet material to prevent magnetic flux generated from nearby electronic devices from affecting the medical device, such as a polymer or resin sheet containing iron particles, which may be laminated or coated onto the entire front surface of the housing  30  and across the circular recess. Ferrite compounds in liquid phase, film shape, or solid phase can be utilized as the shield layer  32 . The ferrite compounds in liquid phase include a shielding paint that is a mixture of paint and ferrite powder for absorbing electromagnetic flux, such as SMF series products that are produced by Samhwa Electrics. Film type ferrite material includes ferrite polymer compound film supplied by Siemens of Germany. Secondary side coils  36  are contained within the circular recess. When a current is supplied to a coil, magnetic flux is produced in the coaxial direction. Thus, power transmission efficiency is enhanced by placing the flat secondary coil  36  inside the living body oriented directly outward toward the skin such that the primary coils  15  of the recharging unit  4  can be brought into frontal parallel alignment. The secondary side coils  36  are contained within an isolation layer  34 . In accordance with the present invention, the secondary side coils  36  are a flat and thin single-layer windings so that they fit flush within the circular recess without disrupting the exterior surface profile of the otherwise small and implantable medical device. A preferred method of forming the secondary side coils  36  integrally with isolation layer  34  is by conventional flex-PCB methods, laminating the coils  36  between opposing polyamide sheets, the plastic then serving as isolation layer  34 . Alternatively, the coils  36  may be electronically printed directly onto a polymer substrate, and preferably sealed therein by overlaying a second polymer sheet. A “Flexible PCB” is a term of art in the electronics industry, meaning flexible polyamide film with conductive traces thereon. Flexible printed circuits are thin, lightweight, flexible, durable, and meet a wide range of temperature and environmental extremes such as those encountered in the human body. Flexible printed circuits are well-suited for applications requiring fine line traces (such as coils), and are much better suited for dynamic applications such as human implantation. Moreover, flex PCBs flex and can conform to the exterior housing of most implantable medical devices, taking no additional space. The ability to layer a flexible PCB coil  36  into a recess on the housing  30  greatly reduces manufacturing costs, and the flush configuration also reduces the incision needed to implant the system and avoids complications. Most importantly, the flat concentric coil-to-coil inductive coupling that results gives an efficient transcutaneous power transfer. However, one skilled in the art should understand that the present invention is not confined to flex-PCB methods, as other method exist (and will be described) for arranging a substantially flat single-layer coil  36  onto the surface of an implanted medical device.  
         [0034]     The gauge, number of turns, and length of single-layer coil  36  will depend on factors such as desired power transmission, distance from the primary coil outside the living body and battery charging time and may be determined empirically.  
         [0035]     A flux sensor  38  is positioned within the air gap of coils  36 . As above, the flux sensor  36  may be a conventional Hall Effect sensor element integrally formed in isolation layer  34 , and this indicates proper alignment with the Hall Effect sensor  16  on the primary recharging unit  4 , which is coupled back to the controller to ensure that the optimum flux field is attained when the primary coils  15  are aligned with secondary coils  36 .  
         [0036]     The secondary side coils  36 , isolation layer  34 , and flux sensor  38  are set into a composite material  42  which fills the recess in housing  30  and hermetically seals those components therein. The filler composite  42  is a medically-safe material such silicon or latex which prevents corrosion to the coils  36  and also prevents a possible release of foreign materials from the device inside a living body.  
         [0037]      FIG. 5  shows an exemplary circuit schematic of the charging unit  4  and contactless power transfer housing  6  that is suitable for present purposes. In operation, a current is provided to the charging unit  4  from an external power source  505 , and switches  515 ,  517 ,  520 , and  522  are controlled by control signals s 1 , s 2 , s 3 , and s 4 . The control signals s 1 , s 2 , s 3 , and s 4  are generated by the controller of  FIG. 1  and correspond to waveforms  120  to  123 , respectively, as shown in  FIG. 6 . When AC current i 1  flows in the primary coil  15  by the operation of the switches and a capacitor  525 , current i 2  is induced in the inductively-coupled secondary coil  36 , having substantially the same waveform of current i 1 . The AC current i 2  is rectified to a direct current by diodes  542 ,  544 ,  546  and  548 . The resultant direct current is provided to charge rechargeable battery  44  of the medical device  6 .  
         [0038]      FIG. 6  depicts waveforms of the control signals s 1 , s 2 , s 3 , and s 4  as well as the currents in the primary and secondary windings  15 ,  36 . Any known circuits for charging a rechargeable battery may be used. Examples of such circuits are MAX745, MAX1679, MAX1736, MAX1879 provided by MAXIM and LTC1732-4/LTC1732-4.2 and LT1571 series provided by Linear technology.  
         [0039]      FIGS. 7 and 8  (A&amp;B) illustrate the operation of the contactless power transfer system, inclusive of primary recharging unit  4  located outside the human body and contactless power transfer housing  6  which is part and parcel to the implantable medical device implanted inside the human body. When the internal battery  44  ( FIG. 3 ) is in need of recharging, the noncontact recharging unit  4  is brought into facing proximity to the contactless power transfer housing  6  of the present invention, until as described below with regard to  FIG. 8  the flux sensors  16 ,  36  indicate alignment. By virtue of the isolation composite cover  14  being of smaller diameter and prootruding past the toroid-shaped housing  27 , the composite cover  14  serves as a skin depressant as shown, slightly depressing a circular area of skin to maximize the transcutaneous magnetic coupling between the primary recharging unit  4  and the secondary  6 .  
         [0040]     Power is then applied through the primary recharging unit  4 , which delivers the charging signal through the secondary coil  36  to battery  44 . The two coils, acting as primary and secondary windings, form a transformer such that power from an external source connected to the primary coil  15  is inductively transferred to the battery  44  coupled to the secondary coil  36 .  
         [0041]     As seen in  FIG. 8A , the primary recharging unit  6  may not initially be perfectly aligned with the contactless power transfer housing  4 , especially since the latter is subcutaneous. This is readily apparent from feedback given by flux sensors  38  and  16 . With imperfect alignment there will be an uneven flux distribution through the two flux sensors  38 ,  16 . However, as seen in  FIG. 8B  as the primary recharging unit  6  is better aligned a more even flux distribution occurs through the two flux sensors  16 ,  38 , until the flux distribution is equal. At this point an optimum flux field has been obtained and the primary coils  15  are aligned with secondary coils  36 .  
         [0042]     One skilled in the art should understand that certain changes may be made without departing from the scope and spirit of the invention. For example, the ferromagnetic composite sheet  32  may cover just the recess at the front of housing  30 , but not the entire front of housing  30 .  
         [0043]      FIGS. 9-12  illustrate alternative configurations of the secondary side coil(s)  36 . In  FIG. 9 , the secondary side coil(s)  36  are formed integrally on the contactless power transfer housing  30  in a coreless configuration. This is accomplished by forming the housing  30  with a helical groove for seating the coil  36 . The coil  36  is completely recessed within the groove, and is sealed therein by silicon epoxy or the like.  
         [0044]     In this embodiment, the equivalent of the ferromagnetic composite sheet  32  (described in  FIG. 3 ) is implemented by coating a ferrite compound on the housing  30 , followed by printing or inlaying the coil windings  36 , and then coating the entire outer surface of with a silicon sealant material. It is also possible to eliminate the coating by incorporating ferromagnetic particles in the housing  30  itself, such as by molding the housing  30  with iron particles. Again, the contactless power transfer housing  6  remains integral to the implantable medical device once it has been implanted inside the human body, and in this case the coil  36  is firmly recessed and sealed within the groove. The embodiment of  FIG. 10  is similar to that of  FIG. 9  except that the secondary side coil(s)  36  are equipped with a core  40  formed integrally in the contactless power transfer housing  30 . The core  40  is a simple disc seated centrally in the coil  36  which helps to channel the magnetic flux, thereby ensuring a proper magnetic path and maximum power coupling when transferring power from the primary  4  to the secondary  6 . The core  40  should be in contact with the underlying ferromagnetic composite sheet  32  ( FIG. 3 ) or ferromagnetic particles in the housing  30 . The material of core  40  may be simple iron, or magnetic materials such as Alnico, Ferrite, etc., which magnetic materials have more efficiency than simple iron.  
         [0045]      FIG. 11  is an enlarged drawing illustrating coil  36  completely recessed within the groove, a strip of ferromagnetic composite  32  behind the coil  36  for insulation, and a coating of silicon epoxy  34  sealant over the outer surface.  
         [0046]      FIG. 12  illustrates a number of alternative multi-coil embodiments in which multiple secondary side coils  36  are formed as adjacent flat and thin single-layer windings, still capable of fitting flush within the housing as described above and not disrupting the exterior surface profile of the otherwise small and implantable medical device. Any number of adjacent secondary side coils  36  may be incorporated as a matter of design choice, three being shown. Each may be equipped with an air core as at (A), or a ferromagnetic core as at (B) to provide a flux path.  
         [0047]      FIG. 13  illustrates two form-fitting embodiments similar to that of  FIG. 9  but better suited for use with implantable medical devices that do not have a housing with a flat surface. Their surface may be convex or concave. In either case, the secondary coils  36  can be made to conform by forming the housing  30  by seating them in grooves that are graduated so that they conform to the surface profile, such that when the coils  36  are inlayed they are either convex outward (as at A) or convex inward (as at B). By patterning the grooves in housing  30  and laying the coils  36  in the patterned grooves the coils  36  can be made to conform to devices with irregular surfaces. Still, the coil(s)  36  are completely recessed within the groove, and may be sealed therein by silicon epoxy or the like.  
         [0048]      FIG. 14  illustrates alternative placements of secondary coils  36  which may be placed on the inside front surface of the housing  30  (as at A) or, alternatively, on the outside front surface (as at B). The inside mounting (A) is possible with non-metallic housings such as plastic or composite, and avoids the need to seal the patterned grooves with silicon or the like. In either case, the secondary coils  36  reside flat against the housing  30  by seating them in grooves that conform to the surface profile.  
         [0049]      FIG. 15  illustrates the magnetic flux coupling path imparted by the present device, and  FIG. 16  illustrate the leakage flux paths imparted by the present device. One of the biggest challenges in medical electronics is controlling electromagnetic interference (EMI) while maintaining the low leakage currents necessary for maximum power transmission. In most electronic devices, EMI is controlled in a known manner by integrating filters, such as Y-capacitor-type filters, to protect against common-mode interference. However, since common-mode interference occurs primarily because of parasitic coupling paths, it is important to keep such paths to a minimum in the design. Leakage flux has the effect of adding inductance that produces a voltage drop when current is present. Leakage can be controlled by the shape of the core and by the arrangement of the windings. In the present design, the core is as compact as possible and the windings close together in order to minimize leakage flux. It also helps to reduce leakage and EMI if a physician ensure that the primary charging unit  4  is optimally aligned with the implanted secondary recharging unit  6  during use. This requires placement of the primary charging unit  4  on the human body as close as possible to the secondary unit  6  for efficient power transmission. For this purpose, the present invention may include a belt or vest that is worn by the patient and that suspends the primary charging unit  4  at the correct position on the body. Given an array of pockets, the primary unit  4  can be disposed at various points on the belt/vest.  
         [0050]      FIG. 17  is a perspective drawing of one embodiment of a shoulder strap  50  designed to be worn to suspend the primary charging unit  4  at the correct position on the body. Again, given one or more (an array) of pockets along the inside of shoulder strap  50 , the primary unit  4  can be disposed at any one of various points on the strap  50 , thereby ensuring pinpoint positioning of the primary charging unit  4  relative to the secondary unit  6 .  
         [0051]      FIG. 18  is a perspective drawing of another embodiment which is a vest  60 , again designed to be worn to suspend one or more primary charging units  4  at the correct positions on the body. Again, given one or more (an array) of pockets along the inside of vest strap  50 , a plurality of primary units  4  can be disposed at a plurality of points on the vest  60 , thereby ensuring pinpoint positioning.  
         [0052]     It should now be apparent that the foregoing transcutaneous power transmission system for use in an implantable medical device offers is extremely small and compact and minimizes surgery and subsequent treatments. The specific configuration of the primary unit  4  and secondary unit  6  optimizes the transcutaneous magnetic coupling to minimize charging time. The system can be utilized for various implantable medical devices that requires electrical power, such as an artificial heart, a pacemaker, an implantable cardiverter defibrillator, a neurostimulator, a GI stimulator, an implantable drug infusion pump, a bone growth stimulation device, and many other devices. Sufficient electric power can be transmitted to the medical device repeatedly without having to take the implanted medical device out of the human body. Further, since charging is more convenient the size of the battery  44  can be reduced, thereby reducing the overall size of the implanted medical device. Since the secondary coil(s)  36  can be formed in a variety of shapes in or on the housing  30 , it is easy to design medical devices that conform to the inside of a living body.  
         [0053]     Having now set forth the preferred embodiments and certain modifications of the concepts underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth in the appended claims.