Abstract:
A feedthrough terminal assembly for an active implantable medical device (AIMD) includes a conductive terminal pin or lead wire which extends through a conductive ground plane of the AIMD in non-conductive relation. A feedthrough capacitor associated with the terminal pin or lead wire has first and second sets of electrode plates coupled, respectively, to the conductive pin or lead wire and to the ground plane. A breathable electromechanical connection material conductively couples the capacitor&#39;s electrode plates to respective components of the AIMD, which allows helium gas to pass freely therethrough during a standard pressurized or vacuum pull helium leak detection test. A breathable washer may be disposed between an alumina insulator and a surface of the capacitor. An additional further breathable coating or conformal coating may be placed over a surface of the feedthrough capacitor disposed toward the interior of the AIMD.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to feedthrough terminal 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. More specifically, this invention relates to an improved feedthrough capacitor filtered terminal assembly of the type incorporating a hermetic seal to prevent passage or leakage of fluids through the terminal assembly, wherein breathable components are provided to accommodate and facilitate post manufacture and pre-usage testing of the hermetic seal. 
     Feedthrough terminal pin 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 active implantable medical devices such as cardiac pacemakers, defibrillators and the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage of electrical signals from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of patient body fluids into the medical device housing, where such body fluids could otherwise interfere with the operation of and/or cause damage to internal electronic components of the medical device. 
     In the past, two primary technologies have been employed to manufacture the hermetic seal. One technique involves the use of a ceramic insulator, typically alumina, which is sputtered to accept brazing material. This alumina insulator is brazed to the terminal pin or pins, and also to an outer metal ferrule of titanium or the like. The alumina insulator supports the terminal pin or pins in insulated spaced relation from the ferrule which is adapted for suitable mounting within an access opening formed in the housing of the medical device. In an alternative technique, the hermetic seal comprises a glass or glass-ceramic based seal forming a compression or matched glass seal for supporting the terminal pin or pins within an outer metal ferrule. 
     The feedthrough terminal pins are typically connected to one or more lead wires which, in the example of a cardiac pacemaker, sense signals from the patient&#39;s heart and also couple electronic pacing pulses from the medical device to the patient&#39;s heart. Unfortunately, these lead wires can act as an antenna to collect stray EMI signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. For example, it has been documented that stray EMI signals emanating from cellular telephones can inhibit pacemaker operation, resulting in asynchronous pacing, tracking and missed beats. To address this problem, hermetically sealed feedthrough filter terminal assemblies have been designed to include a filter capacitor for decoupling EMI signals in a manner preventing such unwanted signals from entering the housing of the implantable medical device. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,751,539; 5,905,627; 5,973,906; 6,008,980; and 6,566,978. 
     While feedthrough capacitor filter terminal assemblies have provided a significant advancement in the art, one potential area of concern is that the filter capacitor is often incorporated into the terminal pin assembly in a way that can mask a defective hermetic seal. More particularly, detection of a defective braze or a defective glass-based seal structure, which would permit undesirable leakage of patient body fluids when mounted on a medical device and implanted into a patient, can be obstructed by the mounting of the filter capacitor and its associated electromechanical connections. For example, with reference to the feedthrough filter capacitor shown in U.S. Pat. No. 4,424,551, a ceramic filter capacitor is bonded to a glass seal and then embedded in epoxy material. Typical post-manufacture leak testing is performed by mounting the feedthrough terminal assembly in a test fixture, and then subjecting one side of the feedthrough terminal assembly to a selected pressurized gas, such as helium. While the bulk permeability of the epoxy material is such that helium under pressure can penetrate therethrough in the presence of a defective hermetic seal, the duration of this pressure test (typically only a few seconds) is often insufficient to permit such penetration. Accordingly, the epoxy material can mask a defective hermetic seal. The thus-tested feedthrough terminal assembly can then mistakenly be incorporated into a medical device and implanted into a patient, wherein slow leakage of patient body fluids through the feedthrough assembly can cause the medical device to malfunction or fail. 
     One method to resolve this issue is depicted in  FIGS. 1 and 2 , which are similar to FIGS. 1 and 2 of U.S. Pat. No. 6,566,978, the contents of which are incorporated herein. These figures disclose a quadpolar feedthrough capacitor  20  mounted on a quadpolar feedthrough terminal assembly  22  and affixed to the ferrule  24  by means of non-porous electromechanical connection  26 . The electromechanical connection material  26  around the entire perimeter of the capacitor  20  has bulk permeability insufficient to permit passage of helium gas (or other testing medium) during a leak detection test of standard duration. In order to facilitate leak detection testing in this prior art device, a gap  28  is formed between the ceramic capacitor  20  and the alumina hermetic seal insulator  30 . The purpose of this gap  28  is to allow for ready passage of leak detection gases from the hermetic terminal areas or along lead wire  32  through the insulator  30  to flow to a leak detection vent hole  34 . However, providing such a gap  28  between the ceramic capacitor  20  and the insulator  30  surface can result in the tendency to trap contaminants, cleaning solvents or the like into this enclosed space. Conductive polyimides or conductive epoxies or solders are typically used to form the electrical connection between the lead wire  32  and the inside diameter metallization  36  of the ceramic capacitor  20 . Conductive polyimides or epoxies are also typically used to form the connection between the capacitor  20  outside diameter metallization  38  and the ferrule  24 . 
     After curing, these conductive polyimide or epoxy materials are typically cleaned using a grit blasting system with sodium bicarbonate as the blasting medium. Sodium bicarbonate, otherwise known as baking soda, is highly soluble in water. Accordingly, de-ionized water rinses are used to ensure that no baking soda is left on the part as the sodium bicarbonate dissolves readily into the water cleaning solvent. After drying out, trace elements of the sodium bicarbonate can be left inside any cavity or air gap, for example, the gap  28  formed between the ceramic capacitor  20  and the alumina insulator  30 . The sodium bicarbonate residue is hygroscopic. That is, it will tend to absorb moisture from the surrounding air which can degrade the electrical insulation properties of the quadpolar feedthrough terminal assembly  22 . 
     For medical implant applications, it is typical that the insulation resistance requirement be 10 Gigaohms or even higher. In order to consistently achieve an insulation resistance greater than 10 Gigaohms, it is essential that all surfaces be extremely clean. Accordingly, any trace element of sodium bicarbonate or other contaminant left behind leads to rejection of the devices due to lowering of the insulation resistance below 10 Gigaohms. 
     Another issue associated with the gap  28  between the ceramic capacitor  20  and the insulator  30  is associated with the high voltage requirements of an implantable cardioverter defibrillator (ICD). Even low voltage devices like pacemakers are sometimes subjected to high voltage pulses. This is typical during an external defibrillation event. There has been a proliferation of automatic external defibrillators (AEDs) in the marketplace. One can now find AEDs in airplanes, hotels, sporting places and many other public venues. Accordingly, pacemaker wearers are being subjected to an ever-increasing number of high voltage shocks in the patient environment. One can see that the gap  28  is an area where electric field enhancement can occur. That is, when a high voltage is applied to the lead wires  32 , there could be a tendency for a high electric field stress to occur in the air gap  28 . These high electric field stresses can lead to ionization of the air gap  28 , a resulting plasma, and a catastrophic high voltage breakdown of the assembly  20 . This so called avalanche breakdown would cause an implantable medical device to not function, which would of course be life threatening for a pacemaker or defibrillator dependent patient. 
     Another method to resolve the issue of leak detection testing is depicted in  FIGS. 3 and 4 , which are similar to FIGS. 5 and 6 of U.S. Pat. No. 6,765,779, the contents of which are incorporated herein.  FIGS. 3 and 4  disclose a unipolar feedthrough capacitor  40  mounted on a hermetic terminal assembly  42 . The feedthrough capacitor  40  incorporates outer diameter metallization  44 . An electrical attachment  46  is made from the capacitor outside diameter metallization  44  to the ferrule  48 . This connection  46  is typically formed with a high temperature thermosetting conductive polymer such as a conductive polyimide. There are gaps left around the circumference of connection material  46  to provide helium leak detection pathways. This is generally described in U.S. Pat. No. 6,765,779 in column 2, lines 24 through 67 and in column 3 lines 1 through 33. There is also an axial gap  50  formed between the feedthrough capacitor  40  and the surface of the hermetic terminal  42 . The purpose of this axial gap  50  is so that if there was a defective gold braze  52 ,  54 , helium atoms could readily penetrate the annular space between the lead wire  56  and the inside diameter of the insulator  58 . Accordingly, said helium atoms could then pass readily through the axial gap  50  and out through the spaces left in the circumferential conductive polyimide attachment material  46 . As previously mentioned, leaving an axial gap  50  can trap contaminants between the capacitor  40  and the insulator  58  of the terminal assembly  42  and also has the tendency to enhance (squeeze or compress) electric fields during the application of a high voltage to the device. 
     Accordingly, there is a need for an EMI filtered hermetic feedthrough terminal assembly suitable for human implant that will avoid the issues associated with an air gap and/or leak detection pathways, but at the same time provide for a helium leak detection path. The present invention fulfills this need by providing an improved feedthrough terminal assembly suitable for use in an implantable medical device or the like, wherein the feedthrough assembly includes breathable electromechanical connections, breathable washers and breathable coatings and conformal coatings for accommodating and facilitating post-manufacture hermetic seal leak testing. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an improved feedthrough terminal assembly for use in an active implantable medical device (AIMD) is provided. The AIMD may comprise a cardiac pacemaker, an implantable defibrillator, a congestive heart failure device, a hearing implant, a cochlear implant, a neurostimulator, a drug pump, a ventricular assist device, an insulin pump, a spinal cord stimulator, an implantable sensing system, a deep brain stimulator, an artificial heart, an incontinence device, a nerve stimulator, a bone growth stimulator, a gastric pacemaker, a prosthetic device, or the like. 
     The medical device comprises a conductive terminal pin or lead wire extending through a conductive ground plane of the AIMD in non-conductive relation. The ground plane may comprise a ferrule or a housing for the AIMD. A feedthrough capacitor is associated with the conductive terminal pin or lead wire, and has first and second sets of electrode plates. Means are provided for conductively coupling the first and second electrode plates to the terminal pin or lead wire. Means are also provided for conductively coupling the second set of electrode plates to the ground plane. 
     In one embodiment, at least one of the conductive coupling means comprises a breathable electromechanical connection material which allows a leak detection gas to pass therethrough during a standard pressurized or vacuum pull leak detection test. 
     For the present invention, breathable means that the component in question has sufficient bulk permeability so as to pass a leak detection gas therethrough, while preventing liquids and solid materials to pass therethrough. More particularly, such components are considered to be breathable if they freely pass helium gas during a standard pressurized or vacuum pull helium leak detection test, which typically is performed in less than one minute. 
     The assembly may include a hermetic insulator adjacent to a face of the capacitor and disposed between the terminal pin or lead wire and the conductive ground plane. Typically, a washer is disposed between the hermetic insulator and the capacitor. 
     In an embodiment of the present invention, the washer comprises a breathable washer which allows a leak detection gas to pass therethrough during a standard pressurized or vacuum pull leak detection test. The washer may include an adhesive layer comprising a breathable material which allows a leak detection gas to pass therethrough during a standard pressurized or vacuum pull leak detection test. 
     A coating or conformal coating may be disposed over the capacitor on a surface disposed toward the inside of the AIMD. The coating or conformal coating may comprise a breathable material which allows a leak detection gas to pass therethrough during a standard pressurized or vacuum pull leak detection test. 
     The present invention also resides in a process for assembling a feedthrough terminal assembly for an active implantable medical device (AIMD), comprising the steps of providing a feedthrough capacitor having first and second sets of electrode plates. The first set of electrode plates are conductively coupled to a conductive terminal pin or lead wire extending through the feedthrough capacitor. The second set of electrode plates are conductively coupled to a ground plane for the AIMD, which may comprise a ferrule or a housing for the AIMD. 
     In one embodiment, during at least one of the coupling steps, the associated components are electromechanically joined with a breathable material which allows a leak detection gas to pass through the coupling during a standard pressurized or vacuum pull leak detection test. Alternatively, or in addition, a washer of breathable material may be provided adjacent to a surface of the capacitor. Typically, the washer is disposed between the capacitor and a hermetic insulator. Alternatively, or in addition, a surface of the capacitor may be coated or conformally coated with a breathable material. 
     The washer may comprise a compressible gas permeable gasket or membrane. The washer may also comprise a fiber washer, a double corrugated washer, a perforated washer, or a post casting stretched membrane washer. The material is treated to create sufficient permeability to allow the leak detection gas to pass therethrough, while excluding movement of liquids, dissolved solids and/or solid contaminants therethrough, thus making it breathable. For example, the treating process may comprise the steps of incorporating into a molten thermoplastic a surfactant in a sufficient gas-stabilizing amount. The dispersion is pressurized to form a hot solution of the gas in the molten thermoplastic. The hot solution is dispensed under lower pressure, whereby the gas is released from the solution to form a hot foamed material. The hot foam is compressed between two substrates, such as the capacitor and the insulator, to force the gas from the foam. 
     Another treatment process to create the desired permeability includes the steps of polymerization-induced phase separation of a thermoplastic at a moderate temperature, followed by a high temperature treatment to cause oxidative thermal degradation of the thermoplastic. Typically, in such a treatment process, the material comprises an epoxy-amine cross-linked matrix having a diamine monomer and a polyvinyl, polymethyl, and/or polyether additive. 
     Yet another treatment process includes the steps of selectively removing the hyperbranched polymer phase of a cured epoxy/hyperbranched polymer blend using a solvent. 
     Still another treatment process includes the steps of adding porous fillers to the material. The porous fillers are selected from the group consisting of porous insulative spheres, porous insulative nanotubes, open cell microspheres, microtublets, talc, porous filaments, nanofoams, and porous fibers. 
     Adhesive material may be selectively applied to surfaces of the washer without compromising permeability and leak testability of the surrounding breathable medium. Such adhesive material may be desirable for adhering the washer to an adjacent structure, preventing undesirable flow of a conductive polyimide, or to prevent arcing or high-voltage flashover. 
     Additional objects and advantages of the invention will be set forth in part in the drawings which follow, and in part will be obvious from the description or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following drawings are exemplary and explanatory only and are not restrictive of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the invention. In such drawings: 
         FIG. 1  is an isometric view of a prior art quadpolar feedthrough capacitor mounted to a quadpolar hermetic terminal assembly; 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is an isometric view of a prior art unipolar feedthrough capacitor mounted to a unipolar hermetic terminal assembly; 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is an isometric view of a unipolar feedthrough capacitor mounted to a unipolar hermetic terminal; 
         FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is an enlarged view taken of the area indicated by the number  7  in  FIG. 6 ; 
         FIG. 8  is an isometric view of a unipolar feedthrough capacitor mounted to a unipolar hermetic terminal assembly embodying the present invention; 
         FIG. 9  is a cross-sectional view taken along line  9 - 9  of  FIG. 8 ; 
         FIG. 10  is an enlarged view taken of the area indicated by the number  10  in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view similar to  FIG. 9 , illustrating another unipolar terminal assembly embodying the present invention; 
         FIG. 12  is an enlarged view taken of the area indicated by the number  12  in  FIG. 11 ; 
         FIG. 13  is an enlarged view taken of the area indicated by the number  13  in  FIG. 12 ; 
         FIG. 14  is a perspective view of upper surface of the breathable washer shown in  FIGS. 9 and 10 , illustrated with a selective area of adhesive surrounding the center hole; 
         FIG. 15  is a perspective view of an opposite or bottom surface of the breathable washer shown in  FIG. 14 ; 
         FIG. 16  is a perspective view similar to  FIG. 15 , illustrating an alternative configuration for applying adhesive to the bottom surface of the breathable washer; 
         FIG. 17  is a top perspective view of a breathable washer adapted for use with a hexpolar feedthrough capacitor application; and 
         FIG. 18  is a bottom perspective view of the breathable washer shown in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An improved feedthrough capacitor filter terminal assembly is provided for use in active implantable medical devices (AIMDs) and the like, such as in a cardiac pacemaker, an implantable defibrillator, a congestive heart failure device, a hearing implant, a cochlear implant, a neurostimulator, a drug pump, a ventricular assist device, an insulin pump, a spinal cord stimulator, an implantable sensing system, a deep brain stimulator, an artificial heart, an incontinence device, a nerve stimulator, a bone growth stimulator, a gastric pacemaker, or a prosthetic device. 
     A prior method of resolving the issues of leak testing is depicted in  FIGS. 5 ,  6  and  7 , taken from co-pending U.S. patent application Ser. No. 11/161,198, the contents of which are incorporated herein. These figures depict a unipolar feedthrough terminal assembly  60  similar to that shown in  FIGS. 3 and 4 . However, a non-conductive and non-adhesive washer  62  has been added and is pressed firmly against the top surface of the alumina insulator  64 , and one or more adhesive layers  66  have been placed on top of the washer  62 . The capacitor  68  is then seated into place on top of the adhesive layers  66 . The top surface of the adhesive layer  66  laminates to the bottom of the capacitor  68  and the bottom surface of the adhesive layer  66  laminates to the top of the washer  62 . The bottom surface of the washer  62  is not laminated to the top surface of the insulator  64 . This construction leaves a laminar delamination gap  70  that is a very thin gap which is sufficient to readily allow helium atoms to pass during a helium leak detection test without permitting the passage of larger molecules or compounds, i.e., water, sodium bicarbonate, etc. Electromechanical connection material  72  is spaced around the perimeter of the capacitor  68  similar to the gaps left around the circumference of connection material  46  in  FIGS. 3 and 4 . These gaps provide helium leak detection pathways to permit helium (or similar) atoms to pass during a leak detection test. However, the presence of the thin air gap  70  can still lead to high voltage field enhancement and arcing. 
     In accordance with the present invention, and as shown in  FIGS. 8-18 , there are three categories of breathable materials utilized in feedthrough terminal assemblies embodying the present invention: 
     1) The first category is breathable electromechanical connections. These electrical-mechanical connections, in general, connect between the capacitor active electrode plate set and one or more lead wires. The electromechanical connections can also be used to connect between the capacitor ground electrode plate set and the overall electromagnetic ground plane shield and/or ferrule of the hermetic terminal. 
     2) The second category is breathable washers. These washers may be preformed or dispensed. They can be thermosetting, B-staged, compressible or rigid with adhesive coatings. Such breathable washers lay intermediate between the ceramic feedthrough capacitor and its underlying mounting surface. In general, for an EMI filtered hermetic terminal for an AIMD, these washers are intermediate between the ceramic capacitor and the alumina or glass hermetic seal. 
     3) The third category is breathable coatings and breathable conformal coatings that are placed on top of the ceramic capacitor (the surface disposed toward the inside of the AIMD housing). These breathable coatings provide mechanical strength, improve the cosmetic appearance and also improve the high voltage characteristics of the capacitor. 
     For the present invention, breathable means “free to pass detection gas, such as helium, during a standard pressurized or vacuum pull leak detection test”, which typically is performed in less than one minute. 
     Breathable electromechanical connections mainly consist of a thermosetting conductive material, such as a conductive epoxy, conductive polyimide, conductive silicone or the like. In general, these dispersible materials are conductive because they have been loaded with metallic flakes or spheres, such as silver or the like. In order to make these electromechanical connection materials porous in accordance with the present invention, porous fillers are added. This includes the entire category of breathable fillers, examples of which include porous carbon spheres, carbon nanotubes, open cell microspheres, microtublets, talc, porous filaments, carbon nanofoams, porous fibers or the like. For example, carbon nanofoams are electrically conductive, have high capacitance, and have continuous porosity and nanometer-scale dimensions. 
     Alternatively, pores can be formed in the thermosetting material using a variety of methods. In one such method, pores with either submicrometer-sized closed-cell structure or a bicontinuous percolating structure (“canallike”) can be formed in an electrically-conductive epoxy-amine cross-linked matrix utilizing the polymer additives poly-(vinyl methyl ether) (PVME) and a diamine monomer (such as polyoxyproylene diamine or polyoxyprolene triamine), and a two-step temperature cure: a polymerization-induced phase separation at a moderate temperature (100-120° C.) of a thermoplastic, . . . PVME . . . , in an epoxy-amine matrix followed by a high-temperature treatment (above 200° C.), as disclosed in Loera et al.  Porous Epoxy Thermosets Obtained by a Polymerization - Induced Phase Separation Process of a Degradable Thermoplastic Polymer . Macromolecules 2002, 35, 6291-6297, the contents of which are incorporated herein by reference. The two-step temperature cure will not only allow the formation of pores within the epoxy, but also provides an additional safeguard to prevent sodium bicarbonate contamination, in that cleaning can be performed after bonding to the ceramic at the moderate temperature (before the pores are formed), after which the material can be treated at the higher temperature, allowing pore formation. 
     A different technique for forming porous epoxy thermoset, in which a solvent (THF) is used to selectively remove the hyperbranched polymer phase of a cured electrically-conductive epoxy/hyperbranched polymer blend, is also known, as disclosed in Guo et al.  Phase Separation, Porous Structure, and Cure Kinetics in Aliphatic Epoxy Resin Containing Hyperbranched Polyester . Journal of Polymer Science: Part B: Polymer Physics 2006, Vol. 44, 889-899, the contents of which are incorporated herein by reference. 
     Breathable washers include materials that can be dispensed, can be set in place as a solid object, or compressed. Certain breathable washer materials include the same group as previously described for electromechanical connections except that in this case they are insulative (not filled with silver spheres or flake). This would include thermosetting epoxies, polyimides, plastics, silicones and the like. To make them breathable, they are filled with one or more porous fillers, including breathable fillers, porous insulative spheres, porous insulative nanotubes, open cell microspheres, microtublets, talc, porous filaments, porous fibers, silica powder, silica aerogel, or the like. 
     Alternatively, pores can be formed in the thermosetting material using a variety of methods. In one such method, pores with either submicrometer-sized closed-cell structure or a bicontinuous percolating structure (“canallike”) can be formed in an epoxy-amine cross-linked matrix utilizing the polymer additives poly-(vinyl methyl ether) (PVME) and a diamine monomer (such as polyoxyproylene diamine or polyoxyprolene triamine), and a two-step temperature cure: a polymerization-induced phase separation at a moderate temperature (100-120° C.) of a thermoplastic, . . . PVME . . . , in an epoxy-amine matrix followed by a high-temperature treatment (above 200° C.) (Loera et al. Macromolecules 2002, 35, 6291-6297). The two-step temperature cure will not only allow the formation of pores within the epoxy, but also provides an additional safeguard to prevent sodium bicarbonate contamination, in that cleaning can be performed after bonding to the ceramic at the moderate temperature (before the pores are formed), after which the material can be treated at the higher temperature, allowing pore formation. A different technique for forming porous epoxy thermoset, in which a solvent (THF) is used to selectively remove the hyperbranched polymer phase of a cured epoxy/hyperbranched polymer blend, is also known (Guo et al. Journal of Polymer Science: Part B: Polymer Physics 2006, Vol. 44, 889-899). 
     Additionally, foamed hot-melt adhesives, formed by first stabilizing a dispersion of gas bubbles in a molten thermoplastic by incorporating in the molten thermoplastic a surfactant in a sufficient gas-stabilizing amount, subsequently pressurizing said dispersion to form a hot solution of the gas in the molten thermoplastic, dispensing said hot solution under lower pressure whereby said gas is released from said solution to form a hot foamed material, and compressing the hot foam between two substrates to force the gas from the foam and to form a bond between the substrates, can be used, as disclosed in U.S. Pat. No. 4,259,402, the contents of which are incorporated herein by reference. 
     There is another group of washers that includes the group of compressible gas permeable gaskets and membranes. For example, a thick layer of Gore-Tex (a gas permeable membrane) could be compressed between the ceramic capacitor and an alumina ceramic insulator. This would require that the capacitor be seated under pressure while the electromechanical connections are made between the capacitor outer diameter and the ferrule and/or the capacitor inner diameter hole(s) and lead wire(s). This would keep the capacitor firmly seated against the compressible porous membrane in a way that water and other contaminants are precluded from entering between the surfaces of the ceramic capacitor and the compressed membrane. However, in this case, the membrane would be semi-permeable or breathable. This would allow helium to freely pass while at the same time firmly seating the ceramic capacitor. Firmly seating the ceramic capacitor against these compressible surfaces eliminates small air gaps which cause high voltage stress risers in implantable defibrillator applications or in the case where external defibrillation is applied to a patient. 
     Breathable washers also include the group of breathable thermoplastics and breathable membranes. These breathable thermoplastics and breathable membranes can be coated with very thin layers of thermosetting adhesives. This allows these washers to be bonded both to the ceramic feedthrough capacitor and to the insulator bonding surfaces thereby preventing the formation of thin air gaps as mentioned above. As long as the thermosetting adhesive layer is kept very thin, this will still allow the leak detection gas, typically helium atoms, to pass quickly through. 
     Another category of breathable washers includes structurally formed washers, including hot or heat-seal installation and fabrication methods. This includes fiber washers, double corrugated washers, perforated washers and post casting stretched membranes which create mircoperforations. 
     In accordance with the invention, breathable coatings and breathable conformal coatings are generally used to coat the top surface of the capacitor. This is the surface that is pointed towards the inside of the AMID. The purpose of these coatings or conformal coatings is three fold: that is, to provide mechanical protection against handling for solvent cleaning during manufacturing of the AIMD, to provide a nice cosmetic finished appearance, and to improve the high voltage characteristics of the ceramic capacitor. In general, it is preferred that these coatings be very thin because they are mismatched in thermal coefficient of expansion with the underlying ceramic capacitor. A thin layer does not induce microfractures in or pull away from the relatively brittle ceramic capacitor materials. Conformal coatings are highly desirable in high voltage applications. This is because of the relatively high dielectric constant or permeability of the ceramic capacitor itself. The ceramic capacitor, for example, can have a dielectric constant (k) in excess of 2500. Air only has a dielectric constant of 1. When high voltage fields make a transition from such a high k material to a relatively low k material, field enhancements occur which can lead to arc-overs, surface cracks or even catastrophic high voltage avalanche breakdown of the ceramic capacitor. By adding a material of intermediate dielectric constant, such as an epoxy, polyimide, plastic or silicone material, the high voltage field is relaxed thereby creating much less tendency for arcing or high voltage breakdown to occur across the capacitor surface. In accordance with the present invention, these coatings and conformal coatings are breathable. 
     In order to make them breathable such that helium can pass through readily, they are in general loaded with a breathable filler, including the group of porous insulative spheres, carbon nanotubes, shortcut aligned fibers or whiskers that are porous, open cell microspheres, microtublets, talc, porous filaments, porous fibers, silica powder, silica aerogel, or the like. 
     Alternatively, pores can be formed in the thermosetting material using a variety of methods. In one such method, pores with either submicrometer-sized closed-cell structure or a bicontinuous percolating structure (“canallike”) can be formed in an epoxy-amine cross-linked matrix utilizing the polymer additives poly-(vinyl methyl ether) (PVME) and a diamine monomer (such as polyoxyproylene diamine or polyoxyprolene triamine), and a two-step temperature cure: a polymerization-induced phase separation at a moderate temperature (100-120° C.) of a thermoplastic, . . . PVME . . . , in an epoxy-amine matrix followed by a high-temperature treatment (above 200° C.) (Loera et al. Macromolecules 2002, 35, 6291-6297). The two-step temperature cure will not only allow the formation of pores within the epoxy, but also provides an additional safeguard to prevent sodium bicarbonate contamination, in that cleaning can be performed after bonding to the ceramic at the moderate temperature (before the pores are formed), after which the material can be treated at the higher temperature, allowing pore formation. 
     A different technique for forming porous epoxy thermoset, in which a solvent (THF) is used to selectively remove the hyperbranched polymer phase of a cured epoxy/hyperbranched polymer blend, is also known (Guo et al., Journal of Polymer Science Part B: Polymer Physics 2006, Vol. 44, 889-899). 
     Also, breathable conformal coatings for the purpose of aiding hermeticity testing can be formed by a composition/formulation containing a rubber, siloxane or urethane oligomer modified epoxy and an organic hardener, and optionally an organic diluent and a curing catalyst or a composition/formulation containing a silicone elastomer or gel and a metal chelate catalyst, and optionally a silica filler, a diluent and an adhesion promoter or coupling agent, as disclosed in U.S. Pat. No. 6,989,433, the contents of which are incorporated herein by reference. 
     With specific reference to a first preferred embodiment illustrated in  FIGS. 8-10 , a unipolar feedthrough terminal assembly  74  is shown which is similar to the terminal assemblies  22 ,  42  and  60  described above. However, there are a number of important improvements. Where electrical connection material  46  and  72  had previously been spaced with discontinuous gaps around the perimeter of the capacitor  44  and  68 , it can now be a continuous uninterrupted ring of breathable conductive electromechanical connection material  76 . Additionally, a breathable washer  82  is provided between the insulator  80  and the capacitor  78  to provide a pathway for helium atoms during a leak detection test. The main difference between the present invention and the prior art is that the breathable electromechanical connection material  76  and the breathable washer  82  each include porous fillers or membranes, as described above, that create a high bulk permeability such that the leak detection gas, typically helium atoms, can readily pass through. The breathable washer  82  is either bonded to or firmly pressed against the bottom of the capacitor  78  and the top of the hermetic seal insulator  80  such that there is no gap formed that could trap contaminants or lead to high voltage field enhancement. 
     It will be appreciated that the passageways through the breathable electromechanical connections  76  and the breathable washer  82  are generally too small to allow water, sodium bicarbonate, solids or other contaminants to pass to the interior of the assembly  74 . The passageways create a relatively high bulk permeability of the breathable electromechanical connection material  76  and breathable washer material  82  to leak detection gasses, such as helium. Such breathable materials  76  and/or  82  can be used in any type of feedthrough terminal assembly to permit passage of helium atoms under pressure during a leak detection test. 
     Referring back to  FIG. 9 , breathable electromechanical connection material  76   a  may also be used to connect the lead wire  83  to the first set of electrode plates in the capacitor  78 . In this manner, helium atoms may permeate through additional seals to provide great opportunity for detecting leaks. 
       FIGS. 11 ,  12  and  13  are similar to FIGS. 28, 29 and 30 of U.S. Pat. No. 6,765,779 (the contents of which are incorporated herein), and illustrate another embodiment of the present invention. This embodiment includes a protective and cosmetic non-conductive breathable coating or breathable conformal coating  84  in accordance with the present invention dispensed over the top of the capacitor  86 . A non-conductive breathable washer  88  in accordance with the present invention is placed or dispensed onto the insulator  90  into which the capacitor  86  is seated into place. Both the top and bottom surfaces of the breathable washer  88  laminate both to the bottom of the ceramic capacitor  86  and to the top of the hermetic insulator  90  during curing. In accordance with the present invention, this breathable washer  88  has sufficient permeability to readily allow helium atoms to pass during a helium leak detection test. Breathable electromechanical connection materials  92  and  94  also permit helium atoms to permeate through the materials  92  and  94  during a leak detection test. One can see that if either gold braze hermetic connection  96  or  98  is defective, then helium atoms will readily pass through the breathable materials  88 ,  92  and/or  94  and through the breathable coating or conformal coating  84 , wherein they would be readily detected and the device would fail the leak test. 
     A significant advantage of this is that the gaps around the perimeters of the capacitors  40  and  68  in the connection material  46  and  72  previously described above in connection with  FIGS. 3 through 7  has been eliminated in the preferred embodiments. By providing breathable conductive electromechanical connection materials  76 ,  92  and/or  94 , or breathable non-conductive washer materials  82 ,  84 ,  88 , ( FIGS. 8-13 ), water has been generally precluded from entering the space between the breathable washer  82 ,  88  and the insulator  80 ,  90 ; however, helium is free to flow. This is also very important during water cleaning after sodium bicarbonate blasting. Additionally, since water molecules do not readily penetrate the porous fillers of the breathable electromechanical connection materials, this also means that the sodium bicarbonate, salts, or other solids which may be contained in the water, will not pass through. 
     Referring to the electrically conductive breathable connection materials such as those illustrated in  FIG. 11  as materials  92  and  94 , they can be made breathable by any of the aforementioned techniques of making the membranes breathable. They can also be made breathable by the addition of carbon monofilaments or carbon fibers that are porous in nature. Carbon is both conductive but can be made porous to allow helium to readily pass. 
     The geometries and shapes of the unipolar assemblies, as illustrated in  FIGS. 8-13 , can be extended to a variety of geometries and shapes including quadpolar, dual inline octapolar, inline nine polar and the like. In other words, the principles described herein can literally be applied to any feedthrough capacitor terminal assembly for human implant. 
       FIG. 14  illustrates the breathable washer  82  of  FIGS. 9 and 10 , and/or the breathable washer  88  in  FIGS. 11-13 , which have a selective area adhesive  100 . The selective area of adhesive  100  surrounds the center hole  101  through which the lead wire  83  will pass. Referring back to  FIG. 9 , one can see that there is a conductive electrical attachment material  76 A, which can be a thermal setting conductive adhesive such as a conductive polyimide, a solder or the like. During manufacturing, this thermal setting conductive material is typically centrifuged in place which puts great force on it. In the alternative case, during soldering, there may be fluxes or solder that would tend to flow down in the gap underneath the feedthrough capacitor. It is undesirable to have electrical conductive materials  76 A disposed in places where it doesn&#39;t belong (for example, underneath the capacitor where it could create short circuits). Additionally, it is relatively undesirable to adhesively coat the entire surface of the washer  82 . The reason for this is the adhesive coating material  100  would tend to flow into and block some of the pores or porosity of the breathable washer itself. Accordingly, it is a feature of the present invention to selectively place adhesives  100  in such a way that they allow helium to pass but they also selectively block the flow of the electrical conductive materials  76 A and keep them in their proper locations.  FIG. 14  is one such embodiment where the adhesive has been selectively placed in a circumferential manner to block said flow. 
       FIG. 15  shows the reverse side of the washer depicted in  FIG. 14 , illustrating the placement of selective adhesive dots  102 . These, of course, can be of any shape and any location. The purpose of these dots  102  is to mechanically attach the breathable washer  82  to its substrate mounting surface, for example, the alumina ceramic insulator  80  using selectively placed dots  102 , this prevents the entire surface of the breathable washer  82  from becoming sealed or saturated such that most of its pores will remain open. In other words, there are still plenty of areas where helium is free to pass. 
       FIG. 16  is an alternative embodiment relative to  FIG. 15 , showing stripes  104  instead of dots  102 . It will be obvious to those skilled in the art that these stripes  104  could be squiggles or any other patterns that could be made with adhesive attachments. 
       FIG. 17  is an adaptation of a breathable washer  106  to a multi-polar feedthrough capacitor. Multi-polar feedthrough capacitors are well known in the art and include bipolar, quadpolar, hexipolar and like configurations. In  FIG. 17 , squiggle patters of adhesive material  108  have been placed in such a way so that there is a laminated barrier between the adjacent apertures  101 . This is very important in high voltage implantable defibrillator applications in order to prevent high voltage flashover between adjacent high voltage pins. 
       FIG. 18  shows the reverse side of the washer  106  of  FIG. 17 . This washer is designed to be laminated and sandwiched between the feedthrough capacitor and its mounting surface which is typically the alumina ceramic insulator. One can see that there is a series of circumferential areas of adhesive  100  which would be laminated against the bottom of the feedthrough capacitor. This would have the same purpose as previously described in connection with  FIG. 14 , thereby preventing the flow of electrical conductive materials  76 A to areas where it is not desired. 
     In summary, adhesives can be selectively distributed as to simulate porosity enabling the filter to washer attachment to be made without compromising permeability and leak testability of the surrounding breathable medium. 
     Although several embodiments of the invention have been described in detail for purposes of illustration, various modifications of each may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited, except by the appended claims.