Abstract:
An electrical feedthrough for an implantable medical device (IMD) is provided that employs a feedthrough conductor having a non-platinum based inner core and one or more layers of a conductive coating to control oxide growth on the surface of the conductor. The coating permits soldering the feedthrough conductor to IMD electronics. The resulting feedthrough provides a substantial cost savings over feedthroughs employing a solid platinum or platinum-iridium conductor

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to feedthroughs for use in implantable medical devices. More specifically, the invention relates to feedthrough designs having multi-layer feedthrough conductors.  
       BACKGROUND  
       [0002]     Electrical feedthroughs are used in implantable medical devices (IMDs) such as cardiac rhythm management devices (e.g., pacemakers and implantable cardioverter/defibrillators) to electrically connect electronic circuitry contained in a hermetically-sealed housing to external components such as, for example, therapy leads. Such feedthroughs include one or more conductive elements that extend into the hermetically-sealed interior of the device housing where they are electrically connected to the IMD electronic components (e.g., control circuitry or battery), through an insulating material, and outside the IMD, where they are electrically connected to therapy lead terminals. Electrical feedthroughs for implantable medical devices may also incorporate filters for filtering electromagnetic interference (EMI) that could impair the performance of the other IMD electronics.  
         [0003]     The feedthrough may contact body fluids after implantation. Accordingly, feedthroughs are typically constructed of biocompatible materials. Platinum and platinum-iridium alloys are commonly-used as feedthrough conductor materials because they are biocompatible. Because of the high cost of platinum, however, it is desirable to identify alternative feedthrough conductor materials and configurations.  
         [0004]     Other biocompatible conductive materials such as tantalum and niobium and their alloys are susceptible to surface oxide growth, which is encouraged by various high temperature processes (e.g., brazing) the conductor undergoes during fabrication of the feedthrough. The oxide layer impairs the electrical connections between the feedthrough conductor and the IMD electronic components, including the EMI filters when used, to which it is connected. Furthermore, the oxide layer limits the available methods of establishing that electrical connection. In particular, this oxide layer hinders electrically connecting the feedthrough and the IMD electronic component by soldering the feedthrough conductor to a terminal or port on the electronic component. In some cases, however, it may be especially desirable, from a manufacturing standpoint, to solder the feedthrough conductor to the IMD electronic components.  
         [0005]     U.S. Pat. No. 5,531,003, issued to Selfried et. al., teaches a feedthrough utilizing a tantalum or niobium terminal pin coated with a thin film of a conductive metal, e.g., platinum, to reduce the insulating effect of the oxide layer on the tantalum or niobium pin. As taught therein, the coating “must not be too thick” so as to prevent the glass insulating material used to seal the terminal pin into the feedthrough from “seeing” the tantalum or niobium terminal pin and not just the coating. The &#39;003 patent specifically teaches that a coating thickness of 10,000 angstroms (1 micron) or less is satisfactory. It has been found, however, that with coatings this thin on tantalum or niobium terminal pins, the resulting feedthrough conductor cannot be readily soldered to provide a robust electrical connection to the IMD internal electronic devices after the aforementioned high temperature processes are performed. One possible explanation for this is that with such thin coatings, the tantalum or niobium terminal pin material migrates to the surface of the coating during the high-temperature processes such as brazing. As a result of this migration, an oxide layer may form on the surface of the coating, thus inhibiting the solderability of the terminal pin.  
         [0006]     Accordingly, there is a need in the art for an implantable medical device feedthrough utilizing a conductor design that is inexpensive, but which also permits the conductor to be soldered to the IMD electronic circuitry.  
       SUMMARY  
       [0007]     The present invention, according to one embodiment, is an implantable medical device for delivering a therapy. The device includes a hermetically-sealed housing enclosing an electronic component, a header coupled to the housing and adapted to receive a terminal pin of a therapy lead, and a feedthrough coupled to the housing. The feedthrough includes a ferrule that mates with an opening in the housing, an insulating material, and a conductor coupled to the electronic component inside the housing by a soldered joint. The conductor includes a conductive metal core and an oxide-resistant coating.  
         [0008]     The present invention, according to another embodiment, is a feedthrough for use in an implantable medical device having a hermetically-sealed housing, a header coupled to the housing and adapted to receive a therapy lead, and a printed circuit board located within the housing. The feedthrough includes an annular ferrule adapted to mate with an opening in the housing, an insulating material disposed within and coupled to the ferrule, and a conductor extending through the insulating material. The conductor has a proximal portion disposed within the header and a distal portion within the housing, and includes a conductive inner core and an oxide-resistant cladding with a thickness of at least about 2.0 microns.  
         [0009]     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic cutaway view of an implantable medical device employing a feedthrough according to one embodiment of the present invention.  
         [0011]      FIG. 2  is a perspective view of an exemplary feedthrough for use in an implantable medical device according to one embodiment of the present invention.  
         [0012]      FIG. 3  is a partial cross-sectional view of an exemplary feedthrough, taken along the line  3 - 3  in  FIG. 2 , according to one embodiment of the present invention.  
         [0013]      FIG. 4  is a cross-sectional view of an exemplary feedthrough conductor with multiple-layer coating over a solid conductor wire, for use in the feedthrough according to one embodiment of the present invention.  
         [0014]      FIG. 5  is a partial cross-sectional view of an exemplary feedthrough employing an EMI filter capacitor, according to one embodiment of the present invention. 
     
    
       [0015]     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1  shows a schematic cutaway view of an implantable medical device  10  incorporating a feedthrough according to one embodiment of the present invention. As shown in  FIG. 1 , the IMD  10  includes a header  12 , a housing  14 , one or more electronic components such as a printed circuit board (PCB)  16 , one or more external components such as a therapy lead  17 , and a feedthrough  20 . The PCB  16  is disposed within the hermetically sealed interior of the housing  14 . The feedthrough  20  is coupled to the housing  14  and extends partially within the housing  14  and partially outside the housing  14 .  
         [0017]     The header  12  encapsulates the portion of the feedthrough  14  that extends externally to the housing  14 , and it operates to operatively couple a terminal block on a distal end of the therapy lead  17  to the feedthrough  20  (for simplicity, this coupling is not shown). The PCB  16  is adapted to electrically couple with the portion of the feedthrough  20  that extends within the housing  14 . Thus, during operation of the IMD  10 , the PCB  16  can communicate electrically with an electrode (not shown) at the distal end of the therapy lead  17 , by way of the feedthrough  20 .  
         [0018]      FIG. 2  shows a perspective view of a feedthrough  20  according to one embodiment of the present invention. As shown in  FIG. 2 , the feedthrough  20  includes a ferrule  22 , one or more feedthrough conductors  24 , and an insulator  26 .  FIG. 2  depicts an embodiment having a plurality of conductors  24 , although other embodiments may employ more or fewer conductors. In one embodiment, the feedthrough  20  includes a single conductor  24 . The ferrule  22  has a size and shape adapted to mate with an opening in the housing  14  (see  FIG. 1 ). The conductors  24  extend through the insulator  26 , which is disposed within the ferrule  22 , from inside the housing  14  to outside the housing  14 . The insulator  26  operates to electrically isolate the feedthrough conductor  24  from the ferrule  22 .  
         [0019]      FIG. 3  shows a partial cross-sectional view of the feedthrough  20  electrically coupled to the PCB  16 , according to one embodiment of the present invention. As shown in  FIG. 3 , the ferrule  22  is generally annular with an interior opening  27  and an interior wall  28 , and may be constructed of an electrically conductive, biocompatible material, for example, titanium. The ferrule  22  is adapted to mate with a wall  30  of the housing  14 . The ferrule  22  may be hermetically attached to the housing by a weld joint  31 . The insulator  26  is partially disposed within the ferrule interior opening  27  and hermetically attached (e.g., by brazing) to the ferrule  22 . In one embodiment, the insulator  26  consists of a metallized ceramic.  
         [0020]     As depicted in  FIG. 3 , in one embodiment, the conductor  24  passes through an aperture  32  in the insulator  26 . In one embodiment, the conductor  24  is sealed into the insulator  26  by a brazing operation as is known in the art. As depicted in  FIG. 3 , the braze material  35 , which may consist of gold, hermetically seals the interior of the IMD housing. Inside the housing, the conductor  24  mates with the PCB  16 . In one embodiment, the PCB  16  includes one or more ports  33 . The port  33  is adapted to receive the conductor  24  and is electrically connected to corresponding PCB electronic circuitry. The electrical connection between the conductor  24  and IMD electrical device may be completed by effecting a solder joint  34  (e.g., a standard formulation of tin/lead solder) between the conductor  24  to an electrical trace surrounding the port  33 . Alternatively, the conductor  24  may be soldered or welded to a conductive pad or terminal on the PCB  16 .  
         [0021]     As further shown in  FIG. 3 , the conductor  24  includes an inner wire or core  36  and an oxide-resistant coating  38 . The core  36  may be made from any good electrically-conductive material, including, but not limited to tantalum, niobium, titanium, molybdenum, copper, or alloys of any of these metals. Although not a requirement, it may be beneficial to make the conductor  24  from biocompatible materials. According to such embodiments of the invention, the core  36  may consist of tantalum, niobium, titanium, or alloys of these metals. In one embodiment, the core  36  is made from a combination of tungsten and tantalum or a combination of zirconium and niobium, which can improve the mechanical fatigue characteristics of the core  36 . The coating  38  can be applied by cladding or other coating processes known in the art, including electroplating and physical vapor deposition processes such as sputtering.  
         [0022]     The oxide-resistant conductive coating  38  is applied to the core  36 , as depicted in  FIG. 3 , to control oxide growth on the surface of the core  36 . Conductors  24  consisting of only a tantalum- or niobium-based wire, without the coating  38 , may not be readily soldered to the port  33  of the PCB  16 , because the oxide layer would prevent the formation of an acceptable electrical connection. In various embodiments, the coating  38  may consist of oxide-resistant, electrically-conductive materials, including, but not limited to, gold, platinum, iridium, palladium, rhodium, ruthenium, titanium, and alloys thereof.  
         [0023]     High temperature processes performed on the feedthrough during fabrication can adversely impact the solderability of the coated feedthrough conductor  24 . One such exemplary process is a brazing process, which typically involves heating a portion of the feedthrough to an elevated temperature which in turn causes an increase in the temperature of the conductor  24 . Such high temperature processes can adversely affect the solderability of the conductor  24 , particularly where niobium, tantalum, or their alloys are used for the core  36 , and where the coating  38  is not applied to a sufficient thickness. If the coating  38  has a sufficient thickness, however, the resulting conductor  24  can be readily soldered to the IMD electronic components. Thus, in one embodiment, the coating  38  is applied to a sufficient thickness to provide a solderable surface after the conductor  24  is sealed into the insulator  26  by brazing. In one embodiment, the coating  38  has a thickness, t, of at least about 2 microns, which results in a conductor  24  that can be effectively soldered. In another embodiment, the thickness, t, is from about 2 microns to about 50 microns. In another embodiment, the thickness, t, is about 20 microns.  
         [0024]     Thus, exemplary embodiments of the feedthrough conductor  24  include a platinum or platinum-iridium clad coating  38  over a niobium or niobium-zirconium core  36 . Other embodiments may include a platinum or platinum-iridium clad coating  38  over a tantalum or tantalum-tungsten core  36 .  
         [0025]     In other exemplary embodiments, the coating  38  includes platinum or platinum-iridium deposited on a tantalum, tantalum-tungsten, niobium, or niobium-zirconium core  36  by sputtering.  
         [0026]      FIG. 4  shows another embodiment of a feedthrough conductor  24  for use in the feedthrough  20  of the present invention. As shown in  FIG. 4 , the conductor  24  includes an inner core  52 , an intermediate coating layer  54 , and an oxide-resistant outer coating layer  56 . The outer coating layer  56  may consist of an electrically-conductive, oxide-resistant material such as gold, platinum, iridium, palladium, rhodium, ruthenium, and alloys thereof. The intermediate coating layer  54  provides a barrier layer between the outer coating layer  56  and intermetallics which may form at the interface between the core  52  and coating during the aforementioned high temperature processing, particularly when the core  52  is made from niobium or its alloys. The intermediate layer  54  deters these intermetallics from adversely impacting the fatigue strength of the conductor  24  and/or the hermeticity of the seal between the conductor  24  and the insulator  26 . In one embodiment, the intermediate layer  54  is made from a relatively inexpensive, conductive material such as molybdenum. According to another embodiment, the conductor  24  consists of a niobium or niobium-zirconium core  52 , a tantalum or tantalum-tungsten intermediate cladding layer  54 , and a platinum or platinum-iridium outer cladding layer  56 .  
         [0027]      FIG. 5  shows partial cross-sectional view of an alternative embodiment of the feedthrough according to the current invention employing the coated conductor  24  and an EMI filter  60 , which may consist of a capacitive structure as is well known in the art. The EMI filter  60  filters electromagnetic interference that could otherwise inhibit the performance of the IMD electronics inside the housing  14 . As depicted in  FIG. 5 , in one embodiment, the conductor  24  extends through an aperture  62  in the EMI filter  60 , which is disposed coaxially with the aperture  32  in the insulator  26 . The conductor  24  is electrically connected to the EMI filter  60 . In one embodiment, the electrical connection between the conductor  24  and the EMI filter  60  may be made by forming a solder joint  66 , as shown in  FIG. 5 . In such an embodiment, the oxide-resistant coating  38 , when applied to a sufficient thickness as discussed above, promotes a robust soldered electrical connection between the EMI filter  60  and the conductor  24 . Alternatively, the EMI filter  60  and the conductor  24  may be electrically connected by other means, for example, by applying a metallized epoxy.  
         [0028]     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.