Abstract:
Ferrules made of nano-titanium for incorporation 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. Nano-titanium experiences significantly less grain growth after high temperature brazing in comparison to commercially pure (CP) titanium and the titanium alloy Ti-6Al-4V. For that reason, nano-titanium is an ideal material for use in implantable medical applications where high strength, structural integrity even after heating and corrosion resistance are desired.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from provisional application Ser. No. 60/765,928, filed Feb. 7, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    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 the manufacture of biocompatible metallic ferrules from nano-titanium. Preferably, the nano-titanium ferrules are 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 feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device. 
       SUMMARY OF THE INVENTION 
       [0003]    A feedthrough filter capacitor assembly comprises an outer ferrule of titanium 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. A gold braze typically accomplishes these hermetic seals. 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. In a preferred form, a filter capacitor is mounted on the insulator and electrically connected to the terminal pins and to the ferrule to prevent unwanted EMI signals from traveling along the terminal pins and entering the interior of the medical device. 
         [0004]    Titanium is used for the outer ferrule because it is chemically and biologically compatible with human fluids and tissue. However, during the high temperature gold braze process the grain structure of titanium can grow significantly. At room temperature, commercially pure (CP) titanium has a grain size of about 10 μm. After gold brazing, the grain size is typically from about 200 μm to about 600 μm. This magnitude of change can result in feedthrough geometry changes, secondary operations failure and degradation of ultimate and yield strength properties. 
         [0005]    Therefore, there is a need for a new form of titanium that is useful in manufacturing ferrules for implantable medical devices, and the like. This need is predicated on the desirable biocompatibility of titanium along with its relatively light weight and corrosion resistance. The new form of titanium must have all of these attributes while maintaining its structural strength and dimensional integrity, even after high temperatures braze processing. The use of nano-titanium in the manufacture of implantable ferrules, and the like, fulfills these requirements. 
         [0006]    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 
         [0007]      FIG. 1  is a perspective view of a feedthrough assembly embodying the novel features of the invention. 
           [0008]      FIG. 2  is an enlarged sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0009]      FIGS. 3A to 3C  are microphotographs of the grain size of as received commercially pure titanium in comparison to the same material after having been subjected to various braze heating profiles. 
           [0010]      FIGS. 4A to 4C  are microphotographs of the grain size of as received nano-titanium in comparison to the same material after having been subjected to various braze heating profiles. 
           [0011]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 2 . 
           [0012]      FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring now to the drawings,  FIGS. 1 and 2  show an internally grounded feedthrough filter capacitor assembly  10  comprising a feedthrough  12  supporting a filter discoidal capacitor  14 . The feedthrough filter capacitor 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 device against ingress of patient body fluids that could otherwise disrupt device 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 along the terminal pins  16 . 
         [0014]    More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  18  defining a bore surrounding an insulator  20 . 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. 
         [0015]    The terminal pins  16  consist of niobium, tantalum, nickel-titanium (NITINOL®), titanium, particularly beta titanium, titanium alloys, stainless steel, molybdenum, tungsten, platinum, platinum-iridium, palladium, palladium alloys, and combinations thereof. 
         [0016]    The insulator  20  is of a ceramic material such as of alumina, zirconia, zircon a 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. 
         [0017]    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. 
         [0018]    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. 
         [0019]    Titanium is an electrically conductive material that is preferred for the ferrule  18 . More particularly, commercially pure (CP) titanium is a desirable ferrule material because it is lightweight and chemically and biologically more compatible with human tissues than a commonly used titanium alloy designated Ti-6Al-4V. However, because commercially pure titanium experiences significant grain growth after high temperature brazing, its degraded mechanical properties and deformation behavior prevent it from being the ideal ferrule material. 
         [0020]    Furthermore, titanium grain growth cannot be significantly reduced by changing the gold braze profile. A standard profile frequently used to gold braze a ferrule of commercially pure titanium to an alumina insulator is as follows: (1) 35° C./min to 1,055° C. for 10 minutes, (2) 5° C./min to 1,065° C. for 10 minutes, (3) 75° C./min to 1,090° C. for 18 seconds, (4) power off and cool to 500° C., (5) liquid nitrogen purge cool to room temperature. An alternate method for brazing CP titanium to an alumina insulator is referred to as the “without soak braze profile” and is as follows: (1) 35° C./min to 1,055° C. for 0.1 minutes, (2) 35° C./min to 1,065° C. for 0.1 minutes, (3) 75° C./min to 1,090° C. for 18 seconds, (4) power off and cool to 500° C., (5) liquid nitrogen purge cool to room temperature. This latter braze profile does not result in significantly reduced titanium grain growth. 
         [0021]    On the other hand, although the titanium alloy Ti-6Al-4V is generally considered to be chemically inert, biocompatible with human tissue, and corrosion resistant to human body fluids and other corrosive environments, vanadium and aluminum potentially release alloy elements into the body. 
         [0022]    Nano-titanium, defined as titanium having a grain size of less than about 1 μm, and preferably from about 0.10 μm to about 0.50 μm, is stronger than both commercially pure titanium and the titanium alloy Ti-6Al-4V. Table 1 compares typical mechanical properties of commercially pure titanium, titanium alloy Ti-6Al-4V and nano-titanium. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Mechanical Properties of Titanium 
               
             
          
           
               
                   
                 Ultimate 
                 Yield 
                   
               
               
                   
                 Strength 
                 Strength 
                 Elongation 
               
               
                   
                 (ksi) 
                 (ksi) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 CP Ti 
                 50–90 
                 30–70 
                 16–27 
               
               
                   
                 Ti—6Al—4V 
                 125 
                 115 
                 12 
               
               
                   
                 Nano-Ti 
                 125–160 
                 115–152 
                  6–12 
               
               
                   
                   
               
             
          
         
       
     
         [0023]      FIGS. 3A to 3C  show CP grade  2  titanium microstructures before and after brazing. The grain size is about 10 μm before brazing and from about 200 μm to about 600 μm after brazing. In particular,  FIG. 3A  is a SEM photograph of as received CP titanium,  FIG. 3B  is a photograph of the same grade of titanium after having been subjected to the previously described standard braze profile and  FIG. 3C  is a photograph of CP grade  2  titanium after having been subjected to a “without soak braze profile.” The significant growth in grain size is readily apparent regardless of the braze profile. 
         [0024]      FIGS. 4A to 4C  show nano-titanium microstructures before and after brazing. The grain size is about 0.26 μm before, and from about 80 μm to about 200 μm after brazing. In particular,  FIG. 4A  is a SEM photograph of as received nano-titanium,  FIG. 4B  is a photograph of the same grade of titanium after having been subjected to the previously described standard braze profile and  FIG. 4C  is a photograph of nano-titanium after having been subjected to a “without soak braze profile.” While the grain size of the samples that were subjected to the respective braze profiles has increased from that of the as received sample, the magnitude of grain growth is significantly reduced from that of the CP grade  2  titanium shown in  FIGS. 3B and 3C . 
         [0025]    In that respect, the combination of biocompatibility, high strength, and lack of toxic alloying elements makes nano-titanium an attractive alternative to commercially pure titanium and the titanium alloy Ti-6Al-4V, particularly as a material for manufacturing ferrules for hermetically sealed feedthrough filter capacitor assemblies. Nano-titanium used for fabricating ferrules for medical implantable hermetic seal feedthroughs may be made by one of the following non-limiting processes: equal channel angular pressing (ECAP), cold rolling, cold extrusion, severe plastic deformation (SPD), and the like. The thusly processed nano-titanium body is then subjected to a machining process to provide the product ferrule. 
         [0026]    Equal channel angular pressing (ECAP) is a processing procedure in which titanium is subjected to intense plastic straining but without the introduction of any change in the cross-sectional dimensions of the sample. This straining is achieved by simple shear by pressing the titanium through a die containing two channels, equal in cross-section, intersecting at an angle of Φ. An important characteristic of ECA pressing is that it provides the potential for refining the grain size of titanium down to a nanometer range of about 0.10 μm to about 0.5 μm. Homogeneous microstructures of ultrafine titanium grains may be achieved most readily when using a die with an internal angle of Φ=90°. 
         [0027]    A laser pyrolysis machine as described in U.S. Pat. No. 6,193,936 to Gardner et al, entitled to “Reactant Delivery Apparatuses,” which is incorporated herein by reference, can also be used to produce nano-titanium according to the present invention. 
         [0028]    As further shown in  FIGS. 1 ,  2 ,  5  and  6 , a preferred form of the feedthrough filter capacitor assembly  10  includes a 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 . 
         [0029]    A method of making or forming a ferrule for a feedthrough by metal injection molding is also described herein. In general, the metal injection molding process begins by designing and making a suitable mold. Next, titanium particles generally having spherical shapes with a nominal diameter of from about 0.10 μm to about 0.50 μm are mixed with a binder which may be a blend of polymers, wax and other materials. A thermal mechanical process is used to mix the combination of about 40% binder and about 60% titanium particles, by weight. The mixture is then pelletized and injected into a mold. This produces a “green part” which is typically about 19% to about 25% larger than the finished product. The green part is then subjected to a debinding process where about 90% of the polymer binding material is removed through thermal, solvent, or catalytic reactions. The resulting “brown part” is then sintered by heating it to about 96% of the melting point for titanium. Sintering shrinks the brown part by about 17% to about 22% to nearly full density. The ferrule is then complete with no further annealing steps being required. 
         [0030]    In one illustrative method of the present invention, a ferrule  18  is produced through the process of metal injection molding, where the mold produces the surrounding flange  22  completely integral with main body of the ferrule and provides for any other design features as desired. In the preferred method, the pellatized titanium is mixed with a binder in a debinding system called catalytic debinding which yields ferrules that are particularly dimensionally stable. The debinding system is available from Phillips Origen Powder Metal Molding (Menomonie, Wis.). The molding is performed on a conventional molding press used for injection molding plastics, but with an altered profile on the screw. The mold is run hot to increase the flow rate of the material. Hot oil is used to heat the mold. The mold is preferably equipped with pressure transducers to indicate the pressure in the mold cavity and to thereby adjust molding parameters accordingly. The “green” ferrule undergoes debinding in a gas-tight oven at elevated temperature. The “brown” ferrule is then sintered in an atmosphere controlled high temperature oven. 
         [0031]    While a preferred metal injection molding process has been described, it should be readily apparent that various metal injection molding processes may be used to produce a ferrule according to the present invention. The process described above is for illustrative purposes only. 
         [0032]    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.