Abstract:
An implantable medical device lead having an integral biostable in-sutu grown oxide insulation and process for forming that includes a lead body extending from a proximal end to a distal end, and a plurality of conductor wires electrically coupling the proximal end and the distal end of the lead body, with one or more of the plurality of conductor wires being formed of a material having a chemically modifiable surface for producing an insulating oxide layer thereon. An insulation layer corresponding to the native oxide layer is formed about the one or more of the plurality of conductor wires.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to implantable medical devices for delivering electrical stimulation to a target area of a patient, and in particular, the present invention relates to a conductor material having integral oxide insulation utilized in implantable medical device leads of implantable medical devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    A wide assortment of implantable medical devices (IMDs) are presently known and in commercial use. Such devices include cardiac pacemakers, cardiac defibrillators, cardioverters, neurostimulators, and other devices for delivering electrical signals to a portion of the body and/or receiving signals from the body. Pacemakers, for example, are designed to operate so as to deliver appropriately timed electrical stimulation signals when needed, in order to cause the myocardium to contract or beat, and to sense naturally occurring conduction signals in the patient&#39;s heart.  
           [0003]    Devices such as pacemakers, whether implantable or temporary external type devices, are part of a system for interacting with the patient. In addition to the pacemaker device, which typically has some form of pulse generator, a pacing system includes one or more leads for delivering generated stimulation pulses to the heart and for sensing cardiac signals and delivering sensed signals from the heart back to the pacemaker. As is known, pacemakers can operate in either a unipolar or bipolar mode, and can pace the atria or the ventricles. Unipolar pacing requires a lead having only one distal electrode for positioning in the heart, and utilizes the case, or housing of the implanted device as the other electrode for the pacing and sensing operations. For bipolar pacing and sensing, the lead typically has two electrodes, a tip electrode disposed at the distal end of the lead, and a ring electrode spaced somewhat back from the distal end. Each electrode is electrically coupled to a conductive cable or coil, which carries the stimulating current or sensed cardiac signals between the electrodes and the implanted device via a connector electrically coupled to the device.  
           [0004]    In the field of cardiac stimulation and monitoring, endocardial leads are placed through a transvenous route to position one or more sensing and/or stimulation electrodes in a desired location within a heart chamber or interconnecting vasculature. During this type of procedure, a lead is passed through the subclavian, jugular, or cephalic vein, into the superior vena cava, and finally into a chamber of the heart or the associated vascular system. An active or passive fixation mechanism at the distal end of the endocardial lead may be deployed to maintain the distal end of the lead at a desired location.  
           [0005]    Routing an endocardial lead along a desired path to a target implant site can be difficult and is dependent upon the physical characteristics of the lead. At the same time, as will be readily appreciated by those skilled in the art, it is highly desirable that the implantable medical lead insulation possess high dielelectric properties, and exhibit durable and bio-stable mechanical and electrical properties, predictable and consistent flexibility, sufficient flex fatigue resistance, wear/abrasion resistance, and minimal thickness.  
           [0006]    In order to perform reliably, cardiac pacing leads need to be positioned and secured at a targeted cardiac tissue site in a stable manner. One common mechanism for securing an electrode position is the use of a rotatable fixation helix. The helix exits the distal end of the lead and can be screwed into the body tissue. The helix itself may serve as an electrode or it may serve as an anchoring mechanism to locate an electrode mounted to the lead body adjacent a targeted tissue site. The fixation helix may be coupled to a drive shaft that is further connected to a coiled conductor that extends through the lead body as generally described in U.S. Pat. No. 4,106,512 to Bisping et al. A physician rotates the coiled conductor at a proximal end to cause rotation of the fixation helix via the drive shaft. As the helix is rotated in one direction, the helix is secured in the cardiac tissue. Rotation in the opposite direction removes the helix from the tissue to allow for repositioning of the lead at another location.  
           [0007]    Combination devices are available for treating cardiac arrhythmias that are capable of delivering shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, uses coil electrodes for delivering high-voltage shock therapies. An implantable cardiac lead used in combination with an ICD may be a quadrapolar lead equipped with a tip electrode, a ring electrode, and two coil electrodes. A quadrapolar lead requires four conductors extending the length of the lead body in order to provide electrical connection to each electrode.  
           [0008]    Pacemaker systems, as well as other medical devices such as those mentioned above, can utilize a wide variety of lead designs. Many considerations are taken into account when optimizing the design of a lead. For example, minimizing lead size is important since a smaller device is more readily implanted within the cardiac structures or coronary vessels of a patient. Electrical insulation between multiple conductors and their associated electrodes is crucial to providing the desired therapeutic effect of electrical stimulation. Ideally, this insulation must be biostable or have predictable performance for the life of the lead, with minimal degradation of mechanical and electrical properties. With the increased number of insulated conductors required in quadrapolar and other multipolar leads, the diameter of the lead body is increased. It is desirable, however, to minimize the lead body diameter while maintaining proper insulation and the structural integrity of the lead.  
           [0009]    Moreover, providing features that make a lead easier to implant and extract allows the clinician to complete the associated surgical procedure more safely and in less time. Finally, an optimized lead design is ideally manufactured using techniques that are relatively simple and easy to verify. The resulting product should be easy to test so that manufacturing defects can be detected prior to the implant of the device within a patient. What is needed, therefore, is an improved lead design that takes all of the foregoing factors into account, thereby providing a miniaturized implantable medical device configuration with improved long-term survival within the patient and providing suitable current carrying capacity for conducting pacing pulse or cardioversion/defibrillation shock therapy.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention relates to an implantable medical device lead that includes a lead body extending from a proximal end to a distal end, and a plurality of conductor wires electrically coupling the proximal end and the distal end of the lead body, with one or more of the plurality of conductor wires being formed of a material having a chemically modifiable surface for producing an insulating oxide layer thereon. An insulation layer corresponding to the native oxide layer is formed about the one or more of the plurality of conductor wires.  
           [0011]    In another embodiment of the present invention, an implantable medical device includes a housing, having a connector block, generating electrical signals for delivering cardiac therapy, and a lead having a lead body extending from a proximal end to a distal end, the proximal end of the lead being insertable within the connector block and electrically coupling the housing and the lead. A plurality of conductor wires electrically couple the proximal end and the distal end of the lead body, with one or more of the plurality of conductor wires being formed of a material having a chemically modifiable surface for forming an insulating oxide layer thereon. An insulation layer, corresponding to the insulating oxide layer, is formed about the one or more of the plurality of conductor wires.  
           [0012]    According to a preferred embodiment of the present invention, one or more of the plurality of conductor wires are formed of one of a refractory metal, an alloy of two or more refractory metals, and a semiconductor material. For example, the one or more of the plurality of conductor wires are formed of one of a tantalum metal and a tantalum-tungsten alloy, and the first insulation layer is an oxide of one of tantalum, tantalum-tungsten and other refractory metal or refractory metal alloy formed about the surface of the plurality of conductor wires. Or still further, the one or more of the plurality of conductor wires may be formed of one of a silicon-silicon dioxide, carbon fibers having a surface modified to diamond layers, a nitride, a carbon alloy, and a zirconium-zirconia composite.  
           [0013]    According to another preferred embodiment of the present invention, the one or more of the plurality of conductor wires includes an inner layer having a surface, and an outer layer clad to the surface of the inner layer, with the inner layer being a high strength material having increased fatigue resistance and the outer layer corresponds to the chemically modifiable surface. The inner layer is preferably one of a refractory metal alloy and a semiconductor material, and the outer layer is a pure refractory metal. For example, the inner layer is tantalum-tungsten and the outer layer is one of tantalum and niobium.  
           [0014]    According to yet another preferred embodiment of the present invention, the one or more of the plurality of conductor wires includes a core formed of a high conductivity material, such as gold, silver or molybdenum, for example.  
           [0015]    According to yet another preferred embodiment of the present invention, the one or more of the plurality of conductor wires includes an inner layer having a surface, and an outer layer clad to the surface of the inner layer, with the inner layer being a high strength material having increased fatigue resistance and the outer layer corresponds to the chemically modifiable surface. In addition, the one or more of the plurality of-conductor wires further includes a core formed of a high conductivity material such as gold, silver or molybdenum, for example. The inner layer is preferably one of a refractory metal alloy and a semiconductor material, and the outer layer is a pure refractory metal. For example, the inner layer is tantalum-tungsten and the outer layer is one of tantalum and niobium.  
           [0016]    According to a still further preferred embodiment of the present invention, a conductor insulation is formed within a medical device lead of an implantable medical device by immersing a first portion of a conductor wire formed of a material having a chemically modifiable surface for producing an insulating oxide layer thereon within an anodization solution corresponding to the producing of the insulating oxide layer. A bias is applied corresponding to a predetermined current limit and a corresponding voltage limit to the conductor wire, and a determination is made as to whether a predetermined cell anodization voltage has been reached. The predetermined cell anodization voltage is maintained for a predetermined time period, and the bias is removed and the conductor wire is advanced through a water rinse so that a next portion of the conductor wire is immersed within the anodization solution. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a side plan view of an implantable medical device lead making use of the conductor insulation of the present invention.  
         [0018]    [0018]FIG. 2 is a cross-sectional view of a lead of an implantable medical device according to the present invention, taken along cross-sectional lines II-II of FIG. 1.  
         [0019]    [0019]FIG. 3 is a cross-sectional view of a lead of an implantable medical device according to the present invention, taken along cross-sectional lines III-III of FIG. 1.  
         [0020]    [0020]FIG. 4 is a cross-sectional view of an individual coiled wire conductor forming a multi-filar conductor coil according to the present invention.  
         [0021]    [0021]FIG. 5 is a cross-sectional view of a lead of an implantable medical device according to an alternate embodiment of the present invention, taken along cross-sectional lines II-II of FIG. 1.  
         [0022]    [0022]FIG. 6 is a cross-sectional view of a lead of an implantable medical device according to an alternate embodiment of the present invention, taken along cross-sectional lines III-III of FIG. 1.  
         [0023]    [0023]FIG. 7 is a cross-sectional view of an individual coiled wire conductor forming a multi-filar conductor coil according to the present invention.  
         [0024]    [0024]FIG. 8 is a schematic diagram of a conductor wire anodization system for forming a conductor insulation according to the present invention.  
         [0025]    [0025]FIG. 9 is a flowchart of a method for forming conductor insulation in a medical device lead of an implantable medical device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    [0026]FIG. 1 is a schematic diagram of an exemplary implantable medical device in accordance with the present invention. As illustrated in FIG. 1, an implantable medical device  100  according to the present invention includes an implantable medical device lead  102  and an implantable medical device housing  104 , such as an implantable cardioverter/defibrillator or pacemaker/cardioverter/defibrillator (PCD), for processing cardiac data sensed through lead  102  and generating electrical signals in response to the sensed cardiac data for the provision of cardiac pacing, cardioversion and defibrillation therapies. A connector assembly  106  located at a proximal end  101  of lead  102  includes a conductive connector pin  108 , a conductive ring element  110 , and two insulative segments  112  and  114 . Insulative segments  112  and  114  are each provided with a plurality of sealing rings  116  for sealing the proximally disposed connector pin  108  and ring element  110  within an elongated socket  118  located in a connector block  120  of housing  104  when connector assembly  106  is inserted within elongated socket  118  of conductor block  120  to electrically couple lead  102  with electronic circuitry (not shown) of housing  104 .  
         [0027]    Lead  102  includes an elongated lead body  122  that extends between proximal end  101  and a distal end  121  of lead  102 . An outer insulative sheath  124  surrounds lead body  122  and is preferably fabricated of polyurethane, silicone rubber, or an ethylene tetrafluoroethylene (ETFE) or a polytetrafluoroethylene (PTFE) type coating layer. Coiled wire conductors in accordance with the present invention are positioned within lead body  122 , as will be described in detail below. Distal end  121  of lead  102  includes a proximal ring electrode  126  and a distal tip electrode  128 , separated by an insulative sleeve  130 . Proximal ring electrode  126  and distal tip electrode  128  are electrically coupled to conductive ring element  110  and conductive connector pin  108 , respectively, by at least one coiled and/or cabled wire conductor in a manner shown, for example, in U.S. Pat. Nos. 4,922,607 and 5,007,435, incorporated herein by reference in their entireties.  
         [0028]    Fixation of distal tip electrode  128  within the patient is assisted by an active or a passive fixation mechanism, which may or may not constitute a distal electrode, to maintain contact of distal tip electrode  128  with tissue to ensure adequate stimulation or sensing at distal end  121  of lead  102 . For example, such fixation mechanism includes active, retractable and extendable helical coils (not shown) adapted to be extended and screwed into tissue at the desired site, or passive, soft pliant tines  131 , as described, for example, in U.S. Pat. No. 3,902,501, typically formed of silicone rubber or polyurethane.  
         [0029]    Such pace/sense distal tip electrodes and fixation mechanisms are also currently used in conjunction with large surface area cardioversion/defibrillation electrodes extending proximally along the length of lead body  122  for either right atrial or ventricular placement. Separate electrical conductors and connectors are employed to connect the cardioversion/defibrillation electrodes with connector block  120  of housing  104  for applying cardioversion/defibrillation shock energy to the respective heart chamber. In this variation, electrode  126  may be considered to represent an elongated cardioversion/defibrillation electrode of any of the well-known types.  
         [0030]    [0030]FIG. 2 is a cross-sectional view of a lead of an implantable medical device according to the present invention, taken along cross-sectional lines II-II of FIG. 1. As illustrated in FIG. 2, lead  102  of implantable medical device  100  includes a quadrifilar conductor coil  200  including four individual coiled wire conductors  202 A,  202 B,  202 C and  202 D extending within insulative sheath  124  of lead body  122 . Coiled wire conductors  202 A- 202 D electrically couple proximal ring electrode  126  and distal tip electrode  128  with conductive ring element  110  and conductive connector pin  108 , respectively, of connector assembly  106 .  
         [0031]    [0031]FIG. 3 is a cross-sectional view of a lead of an implantable medical device according to the present invention, taken along cross-sectional lines III-III of FIG. 1. As illustrated in FIGS. 2 and 3, each of the individual coiled wire conductors  202 A,  202 B,  202 C and  202 D are parallel-wound in an interlaced manner to have a common outer and inner coil diameter. As a result, conductor coil  200  forms an internal lumen  204 , which allows for passage of a stylet or guide wire (not shown) within lead  102  to direct insertion of lead  102  within the patient.  
         [0032]    Alternately, lumen  204  may house an insulative fiber, such as ultrahigh molecular weight polyethylene (UHMWPE), liquid crystal polymer (LCP) and so forth, or an insulated cable in order to allow incorporation of an additional conductive circuit and/or structural member to aid in chronic removal of lead  102  using traction forces. Such an alternate embodiment would require insertion and delivery of lead  102  to a final implant location using alternate means, such as a catheter, for example. Lumen  204  may also include an insulative liner (not shown), such as a fluoropolymer, polyimide, PEEK, for example, to prevent damage caused from insertion of a style/guidewire (not shown) through lumen  204 .  
         [0033]    [0033]FIG. 4A is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to a preferred embodiment of the present invention. As illustrated in FIG. 4A, one or more of the individual coiled wire conductors  202 A,  202 B,  202 C and  202 D includes a conductor wire  210  surrounded by an insulative layer  212 . According to the present invention, conductor wire  210  is a material having a surface that can be modified chemically (i.e., nanoengineered, anodized, etc.) or physically (chemical vapor deposition, physical vapor deposition, ion implantation, thermal oxidation, electric arc processing, or other process) to produce a native oxide, such as tantulum, tantalum-tungsten, or other refractory metal or alloy of refractory metals, or a semiconductor material, such as silicon-silicon dioxide, carbon fibers with a surface modified to diamond like or diamond layers, nitrides, carbide alloys, zirconium-zirconia composites, for example, so that insulative layer  212  is formed about conductor wire  210  using a process, described in detail below, in which a thin, flexible high dielectric oxide film is formed along a surface  211  of conductor wire  210 .  
         [0034]    For example, according to a preferred embodiment of the present invention, in order to maximize the quality of the oxide layer formed on conductor wire  210  that is produced to form insulative layer  212 , conductor wire  210  is formed of tantalum and insulative layer  212  is a thin, flexible, high dielectric tantalum oxide film that is formed on tantalum conductive wire  210  using the process described below. In this way, by utilizing a material having a surface that can be modified chemically to produce a native oxide, the present invention reduces the thickness of lead  102 , and improves metal ion induced oxidation resistance and radi-opacity of lead  102 , compared with leads using less radi-opaque conductor materials, such as MP35N for example. Although conductor wire  210  is described in FIG. 4A as being formed of tantalum, it is understood that other materials having a surface that can be modified chemically to produce a native oxide, such as niobium, for example, may be utilized, and that therefore the present invention is not intended to be limited to the use of tantalum when maximizing the quality of the oxide layer formed on conductor wire  210  that is produced to form insulative layer  212 .  
         [0035]    In addition, in instances where maximizing the quality of the oxide layer formed on conductor wire  210  that forms insulative layer  212  is not a primary goal, it is understood that, according to the present invention, conductor wire  210  may be formed by a refractory metal alloy or semiconductor material, such as tantalum-tungsten, for example, so that insulative layer  212  is a thin, flexible, high dielectric oxide film that is formed on conductive wire  210  using the process described below.  
         [0036]    [0036]FIG. 4B is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to a preferred embodiment of the present invention. As illustrated in FIG. 4B, one or more of coiled wire conductors  202 A- 202 D of FIG. 4A, described above, may include a redundant second insulative layer  214  positioned over conductor wire  210  and insulative layer  212 , so that both insulative layer  212  and insulative layer  214  correspondingly electrically isolate each of coiled wire conductors  202 A,  202 B,  202 C and  202 D. Insulative layer  214  is preferably an ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layer having high dielectric resistance, polyimide, or a combination of polyimide and ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layers.  
         [0037]    [0037]FIG. 5A is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 5A, according to an alternate embodiment of the present invention, in order to increase the mechanical strength and the flex fatigue performance of lead  102 , one or more of coiled wire conductors  202 A,  202 B,  202 C or  202 D is formed with conductor wire  210  being formed by an inner layer or layers  215  that is a refractory metal alloy or semiconductor material, such as tantalum tungsten, for example. A surface  219  of inner layer or layers  215  is clad with an outer layer  213  that is a material having optimal oxide formation, for instance pure tantalum or other pure refractory metal, such as niobium, for example. In addition, similar to the embodiment described in FIG. 4A, insulative layer  212  is a thin, flexible, high dielectric oxide film that is formed on outer layer  213 , so that, for example, if out layer  213  is chosen to be tantalum, insulative layer  212  is a thin, flexible, high dielectric tantalum oxide film that is formed on outer layer  213  using the process described below.  
         [0038]    In this way, by including inner layer or layers  215  that are composed of higher strength materials with increased flex fatigue resistance, the alternate embodiment of FIG. 5A enables the mechanical strength and flex fatigue performance of lead  201  to be increased when compared to lead  102  having conductor wire  210  as described above in reference to FIG. 4A. In addition, by including outer layer  213  with optimal oxide formation qualities, the oxide quality for forming the oxide layer resulting in insulative layer  212  is maintained.  
         [0039]    [0039]FIG. 5B is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 5B, one or more coiled wire conductors  202 A- 202 D of FIG. 5A, described above, may include redundant second insulative layer  214  positioned over outer layer  213  and inner layer  215 , which form conductor wire  210 , and insulative layer  212 , so that both insulative layer  212  and insulative layer  214  correspondingly electrically isolate each of coiled wire conductors  202 A,  202 B,  202 C and  202 D. As described above, insulative layer  214  is preferably an ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layer having high dielectric resistance, polyimide, or a combination of polyimide and ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layers.  
         [0040]    [0040]FIG. 6A is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 6A, according to an alternate embodiment of the present invention, in order to provide lead  102  according to the embodiment described above in FIG. 4A with increased conductive characteristics, one or more of the individual coiled wire conductors  202 A- 202 D is formed as described above in reference to FIG. 4A and further includes a core  217  formed of a high conductivity material and centrally located within conductor wire  210 , with conductor wire  210  being surrounded by insulative layer  212 , as described above in reference to FIG. 4A above. According to the present invention, core  217  is formed of a material having high conductivity characteristics, such as gold, silver or molybdenum, for example.  
         [0041]    In this way, by including core  217  within conductor wire  210 , that is composed of a material having high conductivity characteristics, the alternate embodiment of FIG. 6A enables the conductive characteristics of lead  201  to be increased when compared to lead  102  that includes conductor wire  210  as described above in reference to FIG. 4A, while maintaining the oxide quality of the oxide layer resulting in insulative layer  212 .  
         [0042]    [0042]FIG. 6B is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 6B, one or more coiled wire conductors  202 A- 202 D of FIG. 6A, described above, may include redundant second insulative layer  214  positioned over outer layer  213  and inner layer  215 , which form conductor wire  210 , and insulative layer  212 , so that both insulative layer  212  and insulative layer  214  correspondingly electrically isolate each of coiled wire conductors  202 A,  202 B,  202 C and  202 D. As described above, insulative layer  214  is preferably an ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layer having high dielectric resistance, polyimide, or a combination of polyimide and ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layers.  
         [0043]    [0043]FIG. 7A is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 7A, according to an alternate embodiment of the present invention, in order to provide lead  102  according to the embodiment described above in FIG. 5A with increased conductive characteristics, one or more of the individual coiled wire conductors  202 A- 202 D is formed as described above in reference to FIG. 5A and further includes core  217  formed of a high conductivity material and centrally located within conductor wire  210 , with conductor wire  210  being surrounded by insulative layer  212 , as described above in reference to FIG. 4A above. As previously described above, core  217  is formed of a material having high conductivity characteristics, such as gold, silver or molybdenum, for example.  
         [0044]    In this way, by including inner layer or layers  215  that are composed of higher strength materials with increased flex fatigue resistance, along with outer layer  213  having with optimal oxide formation qualities, and core  217  having high conductivity characteristics, the alternate embodiment of FIG. 7A increases both the mechanical strength and flex fatigue performance of lead  102  and the conductive characteristics of lead  102  when compared to lead  102  that includes conductor wire  210  as described above in reference to FIG. 4A, while maintaining the oxide quality of the oxide layer resulting in insulative layer  212 .  
         [0045]    [0045]FIG. 7B is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to an alternate embodiment of the present invention. As illustrated in FIG. 7B, one or more coiled wire conductors  202 A- 202 D of FIG. 7A, described above, may include redundant second insulative layer  214  positioned over outer layer  213 , inner layer  215  and core  217 , which form conductor wire  210 , and insulative layer  212 , so that both insulative layer  212  and insulative layer  214  correspondingly electrically isolate each of coiled wire conductors  202 A,  202 B,  202 C and  202 D. As described above, insulative layer  214  is preferably an ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layer having high dielectric resistance, polyimide, or a combination of polyimide and ethylene tetrafluoroethylene (ETFE) or other fluoropolymer-type coating layers.  
         [0046]    [0046]FIG. 8 is a schematic diagram of a conductor wire anodization system for forming a conductor insulation according to the present invention. As illustrated in FIG. 8, a conductor wire anodization system  300  for forming insulative layer  212  on conductor wire  210 , as described above, includes an anodization cylinder  302  and a stainless steel mesh cathode cylinder  304  inserted within anodization cylinder  302 , along with an anodization solution  306 , corresponding to the desired insulative layer  212 , located within anodization cylinder  302  and cathode cylinder  304 . Anodization solution  306  may be dilute sulfuric acid, phosphoric acid, or ammonium tartrate, for example. In a preferred embodiment of the present invention, anodization solution  306  is a dilute sulfuric acid at  1 % volume. It is understood that anodization cylinder  302  and cathode cylinder  304  are shown cross-sectionally in FIG. 8. According to the present invention, cathode cylinder  304  extends a height H from a bottom portion  312  to a top portion  313 , and anodization cylinder  302  is initially filled and maintained with anodization solution  306  so that anodization solution  306  remains at height H within anodization cylinder  302 . As a result, anodization solution  306  remains at the same height H within anodization cylinder  302  as cathode cylinder  304  throughout the anodization of conductor wire  210  to form insulative layer  212 , according to the present invention. Height H of cathode cylinder  304  is not critical and can be chosen to correspond to any particularly relevant value. According to a preferred embodiment of the present invention, anodization solution  306  is dilute sulfuric acid at approximately 1% volume, however, it is understood that, according to the present invention, anodization solution  306  is not intended to be limited to dilute sulfuric acid at 1% volume, but rather could include other volumes and/or solutions, such as phosphoric acid, or ammonium tartrate, for example.  
         [0047]    According to the present invention, conductor wire  210  is a material having a surface that is chemically modifiable to produce a native oxide, such as tantalum and tantalum alloys, niobium and niobium alloys, and other refractory metals and refractory metal alloys and their oxides. According to the present invention, as described above, conductor wire  210  may include inner layer or layers  215  and outer layer  213  and/or core  211  materials with different, more desirable properties, such as silver, gold, etc. for lower electrical resistivity, and/or TaW or other materials for higher strength and/or flex fatigue resistance. In this way, by utilizing a pure material as outermost surface  213 , the present invention provides improved produceability and an optimized oxide growth to produce insulative layer  212 .  
         [0048]    A silicone plug  308  is positioned about conductor wire  210  to maintain a good moving seal between conductor wire  210  and plug  308 . Once moving seal between plug  308  and conductor  212  is formed, plug is press-fit within a hole  310  formed at a bottom portion  312  of anodization cylinder  302  to form a press-fit seal between plug  308  and bottom portion  312  of anodization cylinder  302 , and conductor wire  210  is routed from a spool  313  through conductor wire anodization system  300  over pulleys  314 - 318  and onto a take-up spool  320 .  
         [0049]    Pulley  314  is positioned over anodization cylinder  302 , conductor wire  210  is positioned within plug  308 , and hole  310  is centrally positioned along bottom portion  312  of anodization cylinder  302  so that conductor wire  210  is centrally located within anodization cylinder  302  and cathode cylinder  304  so that all sides of conductor wire  210  are equally spaced from an inner wall  305  of cathode cylinder  304 .  
         [0050]    A continuous water rinse  322  is applied to conductor wire  210  as conductor wire  210  passes between pulley  316  and  318  and excess water from water rinse  322  is collected in a water collection bin  324  and discarded through a drain  326 .  
         [0051]    Alternatively, the system features described in the present invention could be incorporated into a continuous spool-to-spool system; in this case, rather than anodizing discrete lengths of wire, a steady-state process could be used, in which anodization of wire would be done continuously as wire moves through the system spool-to-spool. If separate steps, which require different anodization parameters are necessary, multiple separate stages could be incorporated in this type of continuous process, with each stage accomplishing the various different process functions.  
         [0052]    [0052]FIG. 9 is a flowchart of a method for forming conductor insulation in a medical device lead of an implantable medical device according to the present invention. As illustrated in FIGS. 8 and 9, according to the present invention, insulative layer  212  is formed on conductor wire  210  using conductor wire anodization system  300 . Conductor wire  210  is inserted in conductor wire anodization system  300 , Step  400 , by first forming a moving seal between plug  308  and conductor wire  210 . For example, silicone plug  308  is glued onto conductor wire  210 , and subsequently cured broken free from conductor wire  210  to maintain a good moving seal between plug  308  and conductive wire  210 . Plug  308  is then press-fit within hole  310  to form a press-fit seal between plug  308  and bottom portion  312  of anodization cylinder  302 . Alternately, an o-ring seal mechanism, similar to that used in hemostasis valves, could be used to seal conductor wire  210 . Conductor wire  210  is then routed from spool  313  and through conductor wire anodization system  300  by being advanced through plug  308 , over pulleys  314 - 318  and onto take-up spool  320 .  
         [0053]    Once conductor wire  210  is routed from spool  313  to take-up spool  320 , anodization cylinder  302  is then filled with anodization solution  306  and water rinse  322  is turned on, Step  402 . Once a first end of conductor wire  210  along spool  313  and a second end of conductor wire  210  along take-up spool  320  are connected to a positive terminal of a power supply, and cathode cylinder  304  is connected to a negative terminal of the power supply, Step  404 , anodization parameters, such as a desired current and voltage limitation, along with a corresponding time limit for maintaining a desired anodization voltage to form insulative layer  212 , are entered into the power supply, Step  406 . For example, according to a preferred embodiment of the present invention, it may be desirable to have a constant current limit within a range of approximately 0.5 to 2.5 mA/cm 2  and allow the cell voltage to reach approximately 50 volts to 200 volts for optimum properties, and to hold the maximum cell voltage, when reached, for between approximately 2-35 minutes, for example.  
         [0054]    Conversely, the insulative layer  212  is produced by anodizing conductor wire  210  at a constant voltage rather than a constant current. Alternate embodiments can include a combination of constant current, constant voltage, pulse trains and any other suitable electrochemical method of forming an oxide.  
         [0055]    Once the anodization parameters have been entered, the corresponding bias is applied to conductor wire  210  and cathode cylinder  304 , Step  408 , initiating the formation of an oxide layer along a portion  330  of conductor wire  210  extending within anodization solution  306  between plug  308  and top portion  313  of cathode cylinder  304 .  
         [0056]    Once the bias is applied to conductor wire  212 , Step  408 , a determination is made as to whether the desired anodization cell voltage has been reached, i.e., 50 volts for example, Step  410 . In this way, by applying the current bias to conductor wire  210  and cathode cylinder  304 , the anodization voltage is increased while a steady current is maintained until the predetermined anodization voltage limit is reached. Once the predetermined anodization voltage limit is reached, YES in Step  410 , a determination is made as to whether the anodization voltage has been held steady for a predetermined time period, i.e., approximately 15 to 30 seconds, Step  412 . In this way, once the predetermined anodization voltage limit has been reached and the anodization voltage has been applied for the predetermined time period, YES in Step  412 , the bias is removed and conductor wire  210  is advanced from spool  313 , along pulleys  314 - 318 , through water rinse  322  and onto take-up spool  320 , Step  414 , so that a new portion  330  of conductor wire  210  extends within anodization solution  306  between plug  308  and top portion  313  of cathode cylinder  304 .  
         [0057]    Conductor wire  210  is advanced in Step  414  so that portion  313  of conductor wire  210  within anodization solution  306  between plug  308  and top portion  313  of cathode cylinder  304  is removed from cathode cylinder  304 , and a new portion of conductor wire  210  is inserted within anodization solution  306  between plug  308  and top portion  313  of cathode cylinder  304 . It is desirable to overlap portion  313  of conductor wire  210  that is removed from anodization solution  306  with the new portion of conductor wire  210  is inserted within anodization solution  306  to prevent the formation of gaps in insulative layer  212 .  
         [0058]    Once conductor wire  210  is advanced, a determination is made as to whether the end of conductor wire  210  within spool  313  has been reached, Step  416 . For example, the end of conductor wire  210  is determined to be reached in Step  416  by monitoring the tension on conductor wire  210  and determining that the end of conductor wire  210  has been reached in response to increased tension, or in response to decreased tension, indicating conductor wire  210  has left spool  313 . In addition, this monitoring of conductor wire  210  tension could also be utilized to detect other problems, such as conductor wire  210  sticking to adjacent wire-wraps on spool  313  (increased tension), or loss of seal provided by silicone sealings on conductor wire  210  (decreased tension). In the alternative, for example, the end of conductor wire  210  is determined to be reached by including an indicator tape or a wire length counter based on rotation of spool  313 .  
         [0059]    If the end of conductor wire  210  within spool  313  has not been reached, NO in Step  416 , the bias is applied to conductor wire  212 , Step  408 , and Steps  408 - 412  are repeated for the new portion of conductor wire  210  advanced within. Once it is determined that the end of conductor wire  210  within spool  313  has been reached, YES in Step  416 , the power supply is disconnected from conductor wire  210  and cathode cylinder  304 , conductor wire  210  is advanced within take-up spool  320  so that all of conductor wire  210  is advanced through water rinse  322  and received within take-up spool  320 , Step  418 .  
         [0060]    While a particular embodiment of the present invention has been shown and described, modifications may be made. For example, conductor wire  210  may be anodized to form insulative layer  212 , covered with a much thinner polymer coating (which typically contain microdefects) and coiled or made into a cable. The resulting advantage is that since tantalum is corrosion resistance, it will be immune to pinhole corrosion. Another benefit of tantalum according to the present invention is that it will self heal if current leaks from any imperfection caused by handling of coiling, provided that the wire is in the positive polarity. It is therefore intended in the appended claims to cover all such changes and modifications, which fall within the true spirit and scope of the invention.