Abstract:
Terminal pins comprising an outer coating of palladium coating a core material other than of palladium for incorporated into feedthrough filter capacitor assemblies are described. The feedthrough filter capacitor assemblies are particularly useful for incorporation into implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from provisional application Ser. No. 60/749,456, filed Dec. 12, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to terminal pins comprising palladium or palladium alloys for incorporated into feedthrough filter capacitor assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The terminal pin feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device.  
         [0004]     2. Prior Art  
         [0005]     Feedthrough assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator, or neurostimulator, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for passage of electrical signals from the exterior to the interior of the medical device. The conductive terminals are fixed into place using a gold brazing process, which provides a hermetic seal between the pin and insulative material. Conventionally, the terminal pins have been composed of platinum or a combination of platinum and iridium. Platinum and platinum-iridium alloys are biocompatible and have good mechanical strength, which adds to the durability of the feedthrough. However platinum is a precious metal that creates a manufacturing cost barrier.  
         [0006]     The replacement of platinum and platinum alloys by palladium and its alloys offers several advantages. First, platinum has a density of 21.45 grams/cc. Palladium has a density of 12.02 grams/cc. These materials are priced by weight, but used by volume, which means that palladium has a significant cost advantage over platinum. Secondly, platinum and palladium have nearly equivalent mechanical properties. After high temperature brazing, there is no significant degradation in the mechanical properties of palladium, such as in strength and elongation, in comparison to platinum. Palladium also has comparable soldering and welding characteristics, and it has good radiopacity. Finally, previous research indicates that palladium is biocompatible under both soft tissue and bone studies. Palladium and additive materials that are typically combined with it to form alloys are regarded as chemically inactive.  
       SUMMARY OF THE INVENTION  
       [0007]     In a preferred form, a feedthrough filter capacitor assembly according to the present invention comprises an outer ferrule hermetically sealed to either an alumina insulator or fused glass dielectric material seated within the ferrule. The insulative material is also hermetically sealed to at least one terminal pin. That way, the feedthrough assembly prevents leakage of fluid, such as body fluid in a human implant application, past the hermetic seal at the insulator/ferrule and insulator/terminal pin interfaces.  
         [0008]     According to the invention, the terminal pin of a feedthrough assembly, and preferably of a feedthrough filter capacitor assembly, are composed of palladium. The terminal pin can be a uniform wire-type structure of palladium or an alloy thereof, or it can comprise an outer palladium coating over a core material. The core can be of platinum, tantalum, niobium or other electrically conductive materials commonly used in implantable medical devices. In that respect, palladium is an alternative corrosion resistant material that provides a considerably less expensive terminal pin than conventional platinum or platinum-iridium terminal pins while still achieving the same benefits of biocompatibility, good mechanical strength and a reliable hermetic feedthrough seal. Replacement of platinum and platinum-iridium terminal pins with a palladium-based material is done without employing complex and expensive manufacturing operations and, generally, without the addition of a secondary manufacturing process.  
         [0009]     These and other objects and advantages of the present invention will become increasingly more apparent by a reading of the following description in conjunction with the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective view of a feedthrough assembly embodying the novel features of the invention.  
         [0011]      FIG. 2  is an enlarged sectional view taken along line  2 - 2  of  FIG. 1 .  
         [0012]      FIG. 3  is a cross-sectional view of one embodiment of a terminal pin  16  comprising an outer layer of palladium  16 A coating an inner core  16 B of electrically conductive material.  
         [0013]      FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 2 .  
         [0014]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring now to the drawings,  FIGS. 1 and 2  show an internally grounded feedthrough capacitor assembly  10  comprising a feedthrough  12  supporting a filter discoidal capacitor  14 . The feedthrough filter assembly  10  is useful with medical devices, preferably implantable devices such as pacemakers, cardiac defibrillators, cardioverter defibrillators, cochlear implants, neurostimulators, internal drug pumps, deep brain stimulators, hearing assist devices, incontinence devices, obesity treatment devices, Parkinson&#39;s disease therapy devices, bone growth stimulators, and the like. The feedthrough  12  portion of the assembly  10  includes terminal pins  16  that provide for coupling, transmitting and receiving electrical signals to and from a patient&#39;s heart, while hermetically sealing the interior of the medical instrument against ingress of patient body fluids that could otherwise disrupt instrument operation or cause instrument malfunction. While not necessary for accomplishing these functions, it is desirable to attach the filter capacitor  14  to the feedthrough  12  for suppressing or decoupling undesirable EMI signals and noise transmission into the interior of the medical device.  
         [0016]     More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  18  defining an insulator-receiving bore surrounding an insulator  20 . Suitable electrically conductive materials for the ferrule  18  include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof, the former being preferred. The ferrule  18  may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding flange  22  extends from the ferrule  18  to facilitate attachment of the feedthrough  10  to the casing (not shown) of, for example, one of the previously described implantable medical devices. The method of attachment may be by laser welding or other suitable methods.  
         [0017]     The insulator  20  is of a ceramic material such as of alumina, zirconia, zirconia toughened alumina, aluminum nitride, boron nitride, silicon carbide, glass or combinations thereof. Preferably, the insulating material is alumina, which is highly purified aluminum oxide, and comprises a sidewall  24  extending to a first upper side  26  and a second lower side  28 . The insulator  20  is also provided with bores  30  that receive the terminal pins  16  passing there through. A layer of metal  32 , referred to as metallization, is applied to the insulator sidewall  24  and the sidewall of the terminal pin bores  30  to aid a braze material  34  in hermetically sealing between the ferrule  18  and the insulator  24  and between the terminal pins  16  and the insulator  24 , respectively.  
         [0018]     Suitable metallization materials  32  include titanium, titanium nitride, titanium carbide, iridium, iridium oxide, niobium, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, and combinations thereof. The metallization layer may be applied by various means including, but not limited to, sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, and aerosol spray deposition, and thin cladding. Parylene, alumina, silicone, fluoropolymers, and mixtures thereof are also useful metallization materials.  
         [0019]     Non-limiting examples of braze materials include gold, gold alloys, and silver. Then, if the feedthrough  10  is used where it will contact bodily fluids, the resulting brazes do not need to be covered with a biocompatible coating material. In other embodiments, if the brazes are not biocompatible, for example, if they contain copper, they are coated with a layer/coating of biocompatible/biostable material. Broadly, the biocompatibility requirement is met if contact of the braze/coating with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that the braze/coating remains physically, electrically, and chemically constant and unchanged over the life of the patient.  
         [0020]     According to one embodiment of the invention, the terminal pins  16  consist of palladium and its alloys. Non-limiting examples include pure palladium and alloys comprising from about 50% to about 99% palladium along with other elements including those from the platinum group such as ruthenium, rhenium, and iridium, or refractory metals such as molybdenum, and boron, and combinations thereof.  
         [0021]     Mechanical properties of the terminal pin  16  can be tailored to a desired mechanical performance by adjusting the amounts of the elemental additions in the palladium alloy. For example, age hardening can be improved by increasing the amount of ruthenium. Other additions to the palladium alloy such as platinum, gold, copper, and zinc, for example increase the alloy&#39;s ability to be cold worked to achieve a higher tensile strength or to allow the alloy to be annealed and to increase its elongation characteristics.  
         [0022]     In another embodiment of the present invention, the terminal pins  16  comprise an exterior outer coating  16 A of palladium and palladium alloys applied as a coating to a core  16 B of a second, electrically conductive material other than palladium ( FIG. 3 ). Preferably, the core material  16 B is selected from the group consisting of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, platinum, and combinations thereof. The means of coating may include sputtering, cladding, and or plating. The coating may be applied through a process of sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding.  
         [0023]     For example, it is known that niobium readily oxidizes. This means that when it is used as a terminal pin material secondary operations are necessary in order to effect a hermetic braze with low equivalent series resistance (ESR). Providing a palladium outer coating  16 A over a niobium core  16 B in an evacuated atmosphere prior to formation of niobium oxide means that the thusly constructed terminal pin can be directly brazed into the insulator  20   
         [0024]     Although the terminal pin  16  is shown having a circular cross-section that is not necessary. The terminal pin  16  can have other cross-sectional shapes including square, triangular, rectangular, and hexagonal, among others. Nonetheless, the core  16 B has a diameter of from about 0.002 inches to about 0.020 inches and the outer coating  16 A has a thickness of from about 0.5 μm inches to about 0.002 inches.  
         [0025]     Up to now, terminal pins for feedthrough assemblies used in implantable medical devices, and the like, have generally consisted of platinum. However, replacement of platinum and platinum alloys by palladium and its alloys offers several advantages. For one, the density of platinum is 21.45 g/cc in comparison to palladium at 12.02 g/cc. Both of these materials are priced by weight, but used by volume. Therefore palladium has significant cost advantage over platinum. Secondly, palladium has comparable electrical conductivity to platinum (platinum=94.34 l/mohm-cm, palladium=94.8 l/mohm-cm and gold=446.4 l/mohm-cm). Thirdly, palladium and platinum have significantly equivalent mechanical properties. After high temperature brazing, there is no significant degradation of mechanical properties such as strength and elongation. Fourthly, palladium is both solderable and weldable. Fifthly, palladium has good radiopacity characteristics. This is an important consideration for viewing the terminal pin during diagnostic scans such as fluoroscopy. Lastly, but every bit as important, palladium is biocompatibility. Previous research indicates a variety of positive biocompatibility studies (both soft tissue and bone) for all elements used. Palladium and its alloy additives are regarded as chemically inactive.  
         [0026]     As further shown in  FIGS. 2, 4  and  5 , the feedthrough filter capacitor  10  includes the filter capacitor  14  that provides for filtering undesirable EMI signals before they can enter the device housing via the terminal pins  16 . The filter capacitor  14  comprises a ceramic or ceramic-based dielectric monolith  36  having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric  36  preferably has a circular cross-section matching the cross-section of the ferrule  18  and supports a plurality of spaced-apart layers of first or “active” electrode plates  38  in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates  40 . The filter capacitor  14  is preferably joined to the feedthrough  12  adjacent to the insulator side  26  by an annular bead  42  of conductive material, such as a solder or braze ring, or a thermal-setting conductive adhesive, and the like. The dielectric  36  includes lead bores  44  provided with an inner surface metallization layer. The terminal pins  16  pass there through and are conductively coupled to the active plates  38  by a conductive braze material  46  contacting between the terminal pins  16  and the bore metallization. In a similar manner, the ground plates  40  are electrically connected through an outer surface metallization  48  and the conductive material  42  to the ferrule  18 .  
         [0027]     It is appreciated that various modifications to the invention concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.