Patent Description:
The present application generally relates to wires made of MP35N® (<NUM>%Co, <NUM>%Ni, <NUM>%Cr, <NUM>% Mo), which can be solid or have a core of a different electrically conductive metal clad with MP35N. More specifically, the present application relates to electropolishing solid and clad MP35N wires. Following electropolishing, the thusly treated MP35N wire is formed into an implantable lead.

MP35N is a registered trademark of SPS Technologies, Inc. , Jenkintown, Pennsylvania.

The present invention relates to a method for manufacturing an implantable medical lead according to claim <NUM>.

The use of MP35N in the manufacture of implantable leads in cardiac rhythm management and neurological electrical stimulation devices is well known. Over typical device lifetimes, a lead wire, regardless of its materials of construction, is subjected to stress cycling imposed by the heartbeat and is expected to survive <NUM> million stress cycles, or more. Premature fatigue fracture of an implanted lead is sometimes caused by imperfections in the wire from which the lead is constructed. The undesirable imperfections can result in concentration of stresses at a specific location on the wire surface. A vexing type of imperfection is a tiny surface fissure that is commonly referred to as a chevron.

The current state of wire forming is to draw a relatively large diameter wire through a series of progressively smaller dies to produce a wire of a final, lesser diameter. However, wire drawing processes inherently produce a distribution of tiny chevrons on the wire surface, so removing chevron imperfections helps reduce or eliminate premature fatigue failure of an implanted lead initiated by these features.

With relatively large diameter wires, mechanical methods such as shaving are commonly used to remove a surface layer from the wire. However, shaving is not feasible for wires, particularly wires of MP35N, that have the requisite relatively small diameters that are needed for making implantable medical leads, and the like. Implantable leads are typically made from wires having diameters that range from about <NUM> to about <NUM> (about <NUM> inches to about <NUM> inches).

<CIT> and <CIT> describe electropolishing wires being thereafter used in medical implants.

Another commonly used technique for removing chevrons and similar types of imperfections from the outer surface of a wire is to remove a surface layer using a plasma or by sputtering. However, these methods are relatively slow and difficult to accomplish on long spools of wire.

Therefore, there is a need in the industry for a technique that is commercially viable for removing surface imperfections such as chevrons from the outer surface of a wire and that is applicable to the range of wire diameters that are typically used for implantable leads.

Aspects and embodiments of the invention are set out in the appended claims.

These and other aspects of the present invention will become more apparent to those of ordinary skill in the art by reference to the following detailed description and the appended drawings.

Implantable electrical leads used with pacemakers, defibrillators, and neurostimulators are subjected to fatigue stresses in service. Many implantable leads are made from a clad wire containing an alloy of MP35N jacketing a core of a different electrically conductive metal, for example, silver, gold, tantalum, platinum, and titanium. Prior to forming a lead, the MP35N wire had previously been drawn through a series of progressively smaller dies until a wire of a desired diameter is achieved. However, the wire drawing process is known to introduce tiny surface imperfections referred to as chevrons into the wire surface. A chevron and like imperfections can be the site of fatigue failure when the wire is coiled or braided or otherwise formed into an implantable lead. Therefore, a primary focus of the present disclosure is to improve the fatigue life of an implantable lead, particularly a lead comprising MP35N, by removing surface imperfections from the wire before it is coiled or braided into a lead.

According to the present disclosure, electropolishing MP35N wires (solid or clad) prior to forming them into implantable leads meets this need.

According to the present disclosure, chevrons and other undesirable imperfections are substantially, if not completely, eliminated by electropolishing the MP35N wire to remove a thin layer from the outer surface of the wire. Electropolishing is a relatively rapid process that can be run fast enough to be commercially viable and is applicable to the range of wire diameters that are typically used for implantable leads. Electropolishing a wire that is intended to be manufactured into an implantable lead is performed continuously with the wire being fed from a first, payout spool, passing through the electropolish system including a tank containing an electropolishing solution, and then being wound up on a second, take-up spool. If desired, the equipment can be arranged to electropolish multiple spools of wire simultaneously.

In that respect, the present disclosure relates to an electropolishing operation that is performed after final wire drawing. This means that no upstream wire process changes are required. Regarding the degree or amount of surface removal, electropolishing is readily controllable using process parameters including, but not limited to, speed (time in the electropolishing solution), chemistry of the electropolishing solution, voltage/current of the power supply which establishes an electrical potential between the anodically-charge wire and cathode plates immersed in the electropolishing solution, and the temperature of the electropolishing solution. Additionally, the electropolishing process is relatively easy to monitor by measuring the final diameter of the wire using conventional equipment such as a laser micrometer. Furthermore, in addition to removing chevron imperfections, electropolishing produces a desirable smooth, bright surface finish.

The present disclosure further relates to taking the thusly electropolished wire, for example, an MP35N wire, and building it into a wound filar implantable medical lead. Over the typical lifetime of an implanted medical lead, the lead wires or filars are subjected to stress cycling imposed by the heartbeat, and the lead is expected to survive <NUM> million stress cycles, or more. Leads that are built from electrically conductive MP35N wires or filars that have been surface treated or electropolished according to the present disclosure are better suited to withstand this rigor without device failure than similar MP35N wires that have not been so treated.

As used in herein, the term MP35N is defined according to ASTM F562 as a wrought <NUM>% cobalt, <NUM>% nickel, <NUM>% chromium, <NUM>% molybdenum, by weight, alloy for surgical implant applications.

Further, as used herein, when an MP35N wire is discussed, it is understood that the wire can be a solid MP35N wire or have a clad construction with an MP35N sheath or jacket cladding or covering a core of a different electrically conductive metal. Suitable core materials include silver, gold, tantalum, platinum, and titanium.

Turning now to the drawings, <FIG> is a schematic view of an electropolishing system or assembly <NUM> according to the present disclosure. The electropolishing system <NUM> has an open-ended tank or container <NUM> comprising a surrounding sidewall <NUM> supported on and extending upwardly from a bottom wall or base <NUM>. An electropolishing solution <NUM> is contained in the tank <NUM>. For cobalt-chromium alloys such as MP35N, suitable electropolishing solutions typically contain sulfuric acid mixed with water and/or a glycol compound. Suitable electropolishing solutions are described in <CIT> to Faust (from about <NUM>% to about <NUM>% sulfuric acid and from about <NUM>% to about <NUM>% glycerol, by weight, the balance being water). An exemplary solution is a mixture of about <NUM>% to about <NUM>% sulfuric acid in ethylene glycol. The temperature of the electropolishing solution <NUM> typically ranges from about <NUM> deg. C to about <NUM> deg. C (about <NUM>°F to about <NUM>°F).

Other suitable electropolishing solutions are described in <CIT> (from about <NUM>% to about <NUM>% sulfuric acid and from about <NUM>% to about <NUM>% ortho phosphoric acid, the combined acid content being at least <NUM>% but not over <NUM>% by weight of the solution with the balance being water), <CIT> (from about <NUM>% to about <NUM>% sulfuric acid, from about <NUM>% to about <NUM>% phosphoric acid, and from about <NUM>% to about <NUM>% chromic acid, the combined acid concentration being above <NUM>% but not over <NUM>% by weight, the balance being water), <CIT>. , <CIT>, and <CIT>, and <CIT>.

A pair of upper and lower cathode plates <NUM> and <NUM> is positioned lengthwise in the tank <NUM>, immersed in the electropolishing solution <NUM>. A gap <NUM> resides between the cathode plates <NUM>, <NUM>. In an alternate embodiment, only one of the cathode plates is used. Anodically charged first and second or upstream and downstream brushes or pulleys <NUM> and <NUM> reside at opposed ends of the cathode plates <NUM>, <NUM>, also immersed in the electropolishing solution <NUM>. Suitable materials for the cathode plates <NUM>, <NUM> and the anode brushes or pulleys include materials that are resistant to degradation in the electropolishing solution <NUM> and that exhibit good conductivity, for example, titanium, zirconium, stainless steel, and copper.

The cathode side <NUM> of an electrical power supply <NUM> is electrically connected to the cathode plates <NUM>, <NUM> immersed in the electropolishing solution <NUM>. The anode side <NUM> of the electrical power supply <NUM> is electrically connected to the upstream and downstream anodically-charged pulleys <NUM>, <NUM> immersed in the electropolishing solution <NUM>. The power supply <NUM> typically delivers a direct current with voltages ranging from about <NUM> volts to about <NUM> volts. In an alternate embodiment, the electrical power supply <NUM> delivers an alternating current to the cathode plates <NUM>, <NUM> and the upstream and downstream anodically-charged pulleys <NUM>, <NUM>.

A wire supply or payout spool <NUM> residing outside the electropolishing tank <NUM> carries a length of untreated wire <NUM> ranging in diameter from about <NUM> to about <NUM> (about <NUM> inches to about <NUM> inches). that is intended to be treated in the electropolishing solution, for example, MP35N wire. In an electropolishing operation, untreated MP35N wire leaves the payout spool <NUM> and travels in a counterclockwise direction upwardly and over a first or upstream tensioning pulley <NUM> residing outside the electropolishing tank <NUM> where the wire <NUM> moves through an arc of about <NUM>° to descend into the tank containing the electropolishing solution <NUM> and then to the upstream anodically-charged pulley <NUM>. The upstream anodically-charged pulley <NUM> is positioned so that the unpolished wire <NUM> leaving the anode pulley <NUM> enters the gap <NUM> as an anodically-charge wire <NUM>, aligned substantially parallel with the opposed major faces of the cathode plates <NUM>, <NUM>. An electrical potential of about <NUM> amps to about <NUM> amps per square metre (about <NUM> amps to about <NUM> amps per square foot) of wire surface in the electropolishing solution <NUM> is applied between the anodically-charged wire <NUM> and the cathode plates <NUM> and <NUM>, and this potential forces a surface layer ranging from about <NUM> to about <NUM> (about <NUM> inches to about <NUM> inches) to dissolve from the wire <NUM> into the electropolishing solution. For example, with a wire having a diameter of <NUM> (<NUM>") and with there being <NUM>,<NUM> (one foot) of that wire in the electropolishing solution, there is <NUM><NUM> (<NUM> inch<NUM> or <NUM> ft<NUM>) of wire being polished. Accordingly, the appropriate current is from about <NUM> amps to about <NUM> amps.

The downstream anodically-charged pulley <NUM> is positioned so that after the wire <NUM> has travelled through the gap <NUM> between the cathode plates <NUM>, <NUM>, the thusly electropolished wire 38A contacts the downstream anodic pulley <NUM> to then travel through an arc of approximately <NUM>° upwardly, out of the electropolishing solution <NUM> and to a downstream tensioning pulley <NUM>. The electropolished wire 38A travels over the downstream tensioning pulley <NUM> through an arc of approximately <NUM>° and onto a take-up spool <NUM>. The take-up spool <NUM> is motor driven and controls the speed at which the untreated wire <NUM> is pulled from the payout spool <NUM> and through the electropolishing solution <NUM> in the tank <NUM>.

While the payout and take-up spools <NUM>, <NUM> are shown rotating in a counterclockwise direction, that is not required to practice the present disclosure. Depending on their positioning with respect to the electropolishing tank <NUM>, the payout and take-up spools <NUM> and <NUM> can simultaneously rotate in a clockwise direction or one of them can rotate in a counterclockwise direction while the other rotates in a counterclockwise direction. What is important is that the take-up spool <NUM> is rotating at about the same speed as the payout spool <NUM>. That way, the length of the untreated wire <NUM> leaving the payout spool <NUM> is substantially the same as the treated wire 38A being wound onto the take-up spool <NUM>.

Moreover, the upstream and downstream tensioning pulleys <NUM> and <NUM> are configured to compensate for any change in relative speed with respect to the payout and take-up spools <NUM> and <NUM> so that the untreated wire <NUM> travels through the electropolishing solution <NUM> in the tank <NUM> at an even, regulated speed. Regulating the speed with which the wire <NUM> moves through the electropolishing solution <NUM> is important so that the thickness of material removed from the outer surface of the wire is held within a close tolerance. The goal is to only remove as much thickness as is necessary to significantly reduce, if not eliminate, all surface imperfections and fissures such as chevrons from the treated wire 38A without removing more material than is necessary.

As the treated or electropolished wire 38A passes out of the electropolishing solution <NUM> in the tank <NUM>, its diameter is preferably monitored using a laser micrometer (not shown). If desired, an upstream laser micrometer (not shown) can be used to monitor the diameter of the untreated wire <NUM> entering the tank <NUM>. That way, the thickness of material removed from the treated wire 38A is closely monitored and recorded. Thus, the amount of surface removal from the wire and its surface finish is controlled through various parameters including wire speed through the electropolishing solution <NUM>, composition and temperature of the electropolishing solution, voltage and current from the electric power supply <NUM> delivered to the cathode plates <NUM>, <NUM> and the anodically-charged pulleys <NUM>, <NUM>, cathode plate arrangement, and agitation of the electropolishing solution.

<FIG> and <FIG> are photographs at x250, x1,<NUM> and x1,<NUM> magnifications, respectively, of an MP35N wire that has been subjected to a progressive wire drawing process prior to being wound into a coil. Surface fissures are particularly apparent in <FIG> and <FIG> as chevrons gouged into the outer surface of the wire by the drawing process.

In contrast, <FIG> and <FIG> are photographs at x1,<NUM> magnification of a section of an electropolished MP35N wire cut from the as-drawn wire used to make the coils shown in <FIG> and <FIG>. It is apparent that the surface fissures shown in the photographs of <FIG> and <FIG> have been significantly reduced, if not eliminated.

Supplementary operations may be applied to the wire before and after electropolishing. For example, it may be advantageous for the untreated wire <NUM> to be cleaned in a commercial alkaline detergent before electropolishing and rinsed and dried after electropolishing. Such supplementary operations are performed in-line with the electropolishing operation.

In the art of lead manufacturing, an electrical conductor, for example, the electropolished MP35N wire 38A, is often referred to as a "filar".

The electropolished wire 38A is now acceptable for incorporation into an implantable medical lead <NUM> (<FIG>) according to the present disclosure. That is, the electropolished wire 38A that has had an outer surface layer ranging from about <NUM> to about <NUM> (about <NUM> inches to about <NUM> inches) removed from the untreated wire through treatment in the electropolishing system <NUM> shown in <FIG> so that surface fissures such as chevrons, and the like, have been significantly reduced, if not eliminated, is manufactured into an implantable lead. An implantable lead <NUM> is expected to survive <NUM> million stress cycles, or more. Leads that are built from electrically conductive MP35N wires or filars 38A that have been surface treated according to the present disclosure are believed to be better suited to withstand the rigors of millions of stress cycles without device failure than similar MP35N wires that have not been treated. That is because lead failure due to chevron-type surface fissures, and the like, have been greatly reduced, if not eliminated.

<FIG> illustrates that one, two, three, four or more of the electropolished wires or filars 38A have been interwound into an exemplary elongate and flexible cylindrically-shaped coil <NUM> extending to a proximal end 52A and a distal end 52B. In the illustrated embodiment, the coil <NUM> defines a lumen <NUM>. While not shown in the drawing, the proximal end 52A of the coil is configured for secure connection to an electrical contact or connector that is connectable to an implantable medical device such as a cardiac pacemaker, cardiac defibrillator, neurostimulator, and the like. A pacing/sensing electrode <NUM> that is configured for physical contact with body tissue, for example, myocardial tissue is electrically secured to the distal end 52B of the multi-filar coil <NUM>.

Preferably, the filars comprising the multifilar coiled lead <NUM> are provided with an insulative coating <NUM>. A suitable insulative coating <NUM> is an insulative polyimide or fluroimide coating. A suitable insulative polyimide coating is described in <CIT>, titled "Medical Devices with Aromatic Polyimide Coatings". The insulative coating <NUM> has a thickness of about <NUM> (about <NUM> inches) and helps prevent electrical "shorts" between side-by-side filars in a multifilar lead construction where one of the filars is electrically connected to a first electrode and a second filar is electrically connected to a second electrode. The insulative coating also helps prevent electrical conductivity to other conductive pathways within the body.

One skilled in this art will recognize that the number of filars 38A and their coupling to the pacing/sensing electrode <NUM> has many possible coil and electrode/contact combinations. Moreover, different combinations of the number of filars coupled to an electrode can be used. For example, in a quadrifilar construction, two filars could be coupled to one electrode or contact and two filars could be coupled to a second electrode or contact. Alternatively, three of the four filars could be coupled to a first electrode or contact and the fourth filar coupled to a second electrode or contact.

Thus, the present disclosure relates to improvements in the construction of implantable medical leads. MP35N, which is a material that is well known for use in building medical leads, is often subjected to a progressive die drawing process to obtain a desired wire diameter prior to lead manufacture. However, it is known that drawing a wire through a series of progressively smaller dies is prone to introduce surface fissures such as chevrons into the wire surface. These surface imperfections can be the situs or place of fatigue failure, especially as a lead is subjected to over <NUM> million stress cycles during its useful life in a cardiac pacing application. A novel method for removing surface imperfections before the wire is coiled into a lead is to move the wire through an electropolishing bath to remove a thin surface layer containing the surface imperfections and fissures to thereby provide the thusly treated wire. According to the present disclosure, electropolishing a wire, particularly a MP35N wire (solid or clad), is an advancement in the art as it is believed to result in a coiled lead that is better capable of being subject to millions of stress cycles without failing than a similar wire that has not been electropolished.

Claim 1:
A method for manufacturing an implantable medical lead, comprising the steps of:
a) providing an untreated wire comprising MP35N having a first diameter, where MP35N is an alloy containing <NUM>% Co, <NUM>% Ni, <NUM>% Cr and <NUM>% Mo, by weight;
b) electropolishing the untreated wire to remove a surface layer ranging from about <NUM> to about <NUM> (about <NUM> inches to about <NUM> inches) from the wire to thereby provide a treated wire having a second diameter, less than the first diameter; and
c) winding the treated wire into an implantable medical lead.