Abstract:
An implantable, substantially isodiametric, low resistance implantable lead having at least one electrode positioned in a stimulation/sensing portion of the lead as well as a method of manufacturing the same. At least the stimulation/sensing portion is unitized through partially surrounding and supporting insulation and conductive element(s) of the stimulation/sensing portion with a fused matrix of material having mechanical properties consistent with a body of the lead.

Description:
RELATED APPLICATIONS  
       [0001]    This is a division of U.S. patent application Ser. No. 09/760,437, filed Jan. 12, 2001, pending, which is a division of U.S. patent application Ser. No. 09/299,702, filed Apr. 26, 1999, issued as U.S. Pat. No. 6,216,045. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a lead, and in particular, to an implantable lead and a method of manufacturing such lead.  
         BACKGROUND OF THE INVENTION  
         [0003]    Implantable leads having ring electrodes can be used in a variety of applications, including delivery of electrical stimulation to surrounding tissue, neural or otherwise, as well as measuring electrical energy produced by such tissue. Whether serving in a stimulation capacity or a sensing capacity, such leads are commonly implanted along peripheral nerves, within the epidural or the intrathecal spaces of the spinal column, about the heart, and in the brain.  
           [0004]    Notwithstanding the application, the common requirements for such implantable leads include flexibility, strength, and durability. The extent of such qualities, of course, is dependent upon the nature of the use, for example, temporary or permanent implantation. While material selection and certain construction techniques can be tailored to assist in meeting these prescribed characteristics, an overriding consideration in the design of such leads is achieving at least an isodiametric stimulation/pacing portion thereof.  
           [0005]    The benefits of achieving desired levels of flexibility, strength, and durability are intuitive. The isodiametric characteristic is likely less obvious. Depending upon the application, an isodiametric lead can reduce the potential for damage to the lead during insertion (for example, when a lead is passed through an insertion needle to reach a patient epidural space) and/or placement, improve the ability of the lead to pass through tissue or a vascular system, and is more resistant to being immobilized by tissue growth at a permanent implantation site.  
           [0006]    Differing techniques have been used to produce isodiametric leads. One such technique concerns adhering a plurality of elements (i.e., conductive electrodes, conductive terminals, and spacing insulative tubing material) to produce a generally integral body. Tubing material separates a stimulation/sensing portion (i.e., alternating insulative tubing material and electrodes) from a terminal portion (i.e., alternating insulative tubing material and terminals). The electrodes, terminals, and tubing are independently formed but are intended to be isodiametric. Understandably, dimension variances in any one element can result in a lead having a varying diameter.  
           [0007]    Of further interest, to strengthen the plurality of element interfaces found in the stimulation/sensing portions and terminal portions of these leads, a composition, for example, medical grade epoxy, is injected within an interior of the leads in and about the stimulation/sensing portions and the terminal portions. While this technique does typically effect stabilization and strengthening of these critical regions, the end result can also be that these regions are too rigid and even brittle.  
           [0008]    Other techniques include applying a ring electrode(s) about an exterior surface of insulative tubing that forms the main body of the lead. The insulative tubing may be prepared to receive the electrode, for example, milled to remove an amount of material substantially equal to the material thickness of the ring electrode. Alternatively, the insulative tubing may be unprepared, for example, a ring electrode is simply “crimped” to a diameter substantially equal to the otherwise unadulterated diameter of the tubing.  
           [0009]    For all of the methods described above, a finished lead is still comprised of a plurality of independent components brought together in an effort to form an isodiametric cross-section. Element misalignment, inaccuracies in grinding, variances in electrode material thickness or individual element dimensions, or over/under-crimping could respectively result in at least undesirable variances in lead diameter.  
           [0010]    Accordingly, a need exists for a lead, as well as a method of fabricating such lead, that provides a requisite level of flexibility, strength, and durability, while further providing a true isodiametric body for at least the stimulation/sensing portion of the lead.  
         SUMMARY OF THE INVENTION  
         [0011]    One aspect of the present invention is directed to an implantable lead including a lead body, having a distal end and a proximal end, whereas the lead body is formed of a material having prescribed mechanical properties. Extending from the distal end of the lead body, a first region includes a plurality of electrodes. A first insulative material, having mechanical properties consistent with the material of the lead body, separates adjacent electrodes. Extending from the proximal end of the lead body, a second region includes at least one terminal. A second insulative material, having mechanical properties consistent with the material of the lead body, separates adjacent terminals. A conductor couples each terminal to at least one corresponding electrode of the plurality of electrodes, wherein the conductor(s) extends along an interior passage defined by the lead body, first region, and second region. In addition to the at least one conductor, the interior passage of the first region is substantially filled with a third insulative material having mechanical properties consistent with the material of the lead body:  
           [0012]    Another aspect of the present invention concerns a method of forming a substantially isodiametric lead. Specifically, such lead has a prescribed diameter and includes at least one electrode separated from at least one terminal by a lead body, wherein the at least one electrode is electrically coupled to the at least one terminal by a conductor passing through a passage defined by at least the lead body. The forming steps include assembling the at least one electrode and the at least one terminal relative to the lead body to form an assembly, including connecting the at least one electrode to the at least one terminal via the conductor. The assembly is subjected to an over-molding process that over molds the assembly with a first material to form an intermediate assembly. This first material is compatible with and has mechanical properties consistent with a material of the lead body. Ultimately, the intermediate assembly is processed to remove all material of the intermediate assembly in excess of the prescribed diameter.  
           [0013]    An object of the present invention is to avoid the shortcomings of known leads and manufacturing techniques for the same.  
           [0014]    Another object of the present invention is to provide a method of forming a lead having a true isodiametric body for at least the stimulation/sensing portion of the lead.  
           [0015]    Another object of the present invention is to provide a lead having a true isodiametric body for at least the stimulation/sensing portion of the lead.  
           [0016]    Another object of the present invention is to provide a lead having a low resistance from a terminal to a coupled electrode to reduce energy consumption during system operation.  
           [0017]    Other aspects, objects, and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following Specification together with the provided drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    In reference to the following figures, like reference numerals and letters indicate corresponding elements:  
         [0019]    [0019]FIG. 1 is a perspective view of a multi-electrode lead in accordance with the present invention;  
         [0020]    [0020]FIG. 2 is a plan view of another embodiment of a multi-electrode lead in accordance with: the present invention;  
         [0021]    [0021]FIG. 3 is a sectional view of the lead of FIG. 2, taken along line III-III;  
         [0022]    [0022]FIG. 4 is a perspective view of a preferred conductor;  
         [0023]    [0023]FIG. 5 is a plan view of an assembly of elements on a mandrel used to form a lead in accordance with the present invention;  
         [0024]    [0024]FIG. 6 is a sectional view of a transitional element;  
         [0025]    [0025]FIG. 7 is a perspective view of an electrode spacer;  
         [0026]    [0026]FIG. 8 is a perspective view of a terminal spacer;  
         [0027]    [0027]FIG. 9 is a sectional view of a stylet guide;  
         [0028]    [0028]FIG. 10 is a sectional view of a cap electrode; and  
         [0029]    [0029]FIG. 11 is a schematic representation of one embodiment of an assembly fixture used to assemble a lead in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Various embodiments, including preferred embodiments, will now be described in detail below with reference to the drawings:  
         [0031]    [0031]FIG. 1 illustrates a preferred embodiment of multi-electrode lead  10 . While the leads illustrated and generally discussed here have eight electrodes, lead  10  of the present invention may be constructed having any number of electrodes (i.e., one or more):  
         [0032]    Lead  10  includes a proximal end  12  and a distal end  14 . The proximal end  12  includes a plurality of electrically conductive terminals  16 , and the distal′ end  14  includes a plurality of electrically conductive electrodes  18 . While typically each terminal  16  is electrically connected to a single electrode  18  via a conductor  20  (FIG. 3), a terminal  16  can be connected to two or more electrodes  18 .  
         [0033]    Terminals  16  and electrodes  18  are preferably formed of a non-corrosive, highly conductive material. Examples of such material include stainless steel, MP35N, platinum, and platinum alloys. In a preferred embodiment, terminals  16  and electrodes  18  are formed of a platinum-iridium alloy.  
         [0034]    Spanning between electrodes  18  of the distal end  14  and terminals  16  of the proximal end  12 , body  22  is formed from a medical grade, substantially inert material, for example, polyurethane, silicone, or the like. While the specific material used for body  22  is not critical to the present invention, body  22  must be non-reactive to the environment of the human body, provide a flexible and durable (i.e., fatigue resistant) exterior structure for the components of lead  10 , and insulate adjacent terminals  16  and/or electrodes  18 .  
         [0035]    Serving as a sheath, body  22  substantially provides the exterior structure that contains the internalized elements of lead  10 . Specifically, body  22  provides an enclosure for each conductor  20  that connects a terminal  16  with one or more electrodes  18 . Each conductor  20  is formed of a conductive material that exhibits the desired mechanical properties of low resistance, corrosion resistance, flexibility, and strength. For consideration, however, it should be appreciated that in the context of a multiple electrode lead  10 , a plurality of conductors  20  are required to fit within the interior of body  22 . Accordingly, the cross-sectional area of each conductor  20  is restricted. As but one example, for a lead in accordance with the present invention that has an outer diameter of approximately 0.055 inches, conductor  20  could be on the order of approximately 0.0065 inches.  
         [0036]    While stranded bundles of stainless steel, MP35N, platinum, platinum-iridium alloy, drawn-brazed silver (DBS) or the like can be used, the preferred embodiment of conductors  20  utilizes wires formed of multi-strands of drawn-filled tubes (DFT), as illustrated in FIG. 4. Each strand is formed of a low resistance material  20   a  and is encased in a high strength material  20   b  (preferably, metal). A selected number of strands (seven, for this example) are wound and coated with an insulative material  20   c . With regard to the operating environment of the present invention, insulative material  20   c  protects the individual conductors  20  if body  22  were breached during use. Wire formed of multi-strands of drawn-filled tubes to form conductors  20 , as discussed here, is available from Temp-Flex Cable, Inc. (City, State).  
         [0037]    In addition to providing the requisite strength, flexibility, and resistance to fatigue, conductors  20  formed of multi-strands of drawn-filled tubes, in accordance with the preferred embodiment, provide a low resistance alternative to other conventional materials. Specifically, a stranded wire, or even coiled wire, of approximately 60 cm and formed of MP35N or stainless steel or the like would have a measured resistance in excess of 30 ohms. In contrast, for the same length, a wire formed of multi-strands of drawn-filled tubes, as illustrated in FIG. 4, could have a resistance less than 4 ohms. Accordingly, in a preferred embodiment, each conductor  20 , having a length equal to or less than 60 cm, has a resistance of less than 25 ohms.  
         [0038]    In a more preferred embodiment, each conductor  20 , having a length equal to or less than 60 cm, has a resistance equal to or less than 10 ohms. In a most preferred embodiment, each conductor  20 , having a length equal to or less than 60 cm, has a resistance of less than 4 ohms.  
         [0039]    As an alternative embodiment, body  22  can further encompass stylet tubing  24  (FIG. 3). Stylet tubing  24  extends from the proximal end  12  to a point within a distal portion of lead  10 ; however, in a preferred embodiment, stylet tubing  24  extends to cap electrode  34 . In cooperative reference to FIG. 2, stylet tubing  24  operatively receives stylet  100  for purposes of allowing better control over lead  10  during placement.  
         [0040]    Lead Assembly  
         [0041]    While the following discussion provides but one example of a sequence of steps to form a lead similar to that illustrated in FIGS. 2 and 3. One having ordinary skill in this art shall appreciate that the following steps may be performed in a differing order or otherwise inconsequentially modified to still yield the present invention. Consequently, such minor variations are still regarded as being within the scope of the present invention and should be construed in such manner.  
         [0042]    Furthermore, for purposes of illustration, the following example includes certain physical dimensions to illustrate the relationship between elements as well as effects of differing processes. Accordingly, the provided physical dimensions are used merely for example and shall not restrict the scope of the present invention.  
         [0043]    The following illustrative example concerns the construction of an eight electrode, epidural lead that accommodates a stylet. One skilled in the art shall appreciate, however, that a lead in accordance with the present invention may have more than or less than eight electrodes and/or have a larger or smaller diameter than the following example and remain within the scope of this disclosure.  
         [0044]    In reference to FIG. 5, stylet tubing  24  is positioned over mandrel  150 . Stylet tubing  24  has an outer diameter of approximately 0.02 inches.  
         [0045]    Depending on the quantity of conductors  20  required (e.g., for this illustration, eight) and the size (i.e., diameter) of such conductors  20 , arranging and securing conductors  20  can be problematic when they are being arranged and secured about an element having the dimensions of stylet tubing  24 .  
         [0046]    While any number of techniques may be used to achieve such arrangement of conductors  20  relative to stylet tubing  24 , FIG. 11 illustrates an example of a fixture  200  that can assist in this task. Specifically, fixture  200  includes first rotary clamp  202 , iris  204 , iris  206 , second rotary clamp  208 , and clamp  210 . Rotary clamps  202  and  208  each include a corresponding plurality of conductor clamps  203 . While not required, it is preferred that the plurality of conductor clamps  203  of each rotary claim  202  and  208  be positioned within an arbitrary perimeter  205 , whereas perimeter  205  should be equal to or greater than a fully-opened inner diameter of either iris  204  or  206 :  
         [0047]    As illustrated, mandrel  150 , including stylet tubing  24 , passes through irises  204  and  206  and second rotary clamp  208  and is secured between clamps  202  and  210 . Each conductor  20  similarly passes through irises  204  and  206  and is secured between respective clamps  203  of rotary clamps  202  and  208 .  
         [0048]    Conductors  20  secured within fixture  200  are prepared for assembly in that a prescribed amount of insulative material  20   c  is removed at or about the proximal and distal ends of each conductor  20  to expose conductive material  20   a  and  20   b . As will be discussed later, this exposed conductive material  20   a  and  20   b  of the proximal and distal ends of each conductor  20  is eventually joined to an electrode  18  and a terminal  16 . Accordingly, the exposed conductive material  20   a  and  20   b  is arranged at differing positions relative to stylet tubing  24  to accommodate the serial arrangement of terminals  16  and electrodes  18 .  
         [0049]    The rotational nature of rotary clamps  202  and  208  provides unobstructed access to the in-process lead  10 . Specifically, upon securing a single conductor  20  between opposing (or non-opposing) clamps  203 , the rotary clamps  202  and  210  are simply rotated to allow access to unoccupied clamps  203 .  
         [0050]    When all of the conductors  20  are strung between claims  202  and  208 , irises  204  and  206  are actuated to close and draw conductor(s)  20  closely about the outer diameter of stylet tubing  24 . When conductor(s)  20  are resting against the outer diameter of stylet tubing  24 , conductor(s)  20  are secured in place. Conductor(s)  20  may be secured using adhesive and/or subjected to a force applied through use of a temporary or permanent restraint, for example, one or more crimped collars.  
         [0051]    While the illustration of FIG. 11 shows but one embodiment of fixture  200 , one skilled in the art should appreciate that other techniques/structures may be employed to position conductors  20  adjacent an exterior surface of stylet tubing  24 . Specifically, clamps  203  of each rotary clamp  202  and  208  could be moveable along respective radial paths (not shown) that would allow strung conductors  20  to be moved from a first position to a second position adjacent the exterior surface of stylet tubing  24 . Alternatively, conductors  20  could initially be secured to one end of stylet tubing  24  and only a single iris could be used to draw the unsecured portions of conductors  20  toward stylet tubing  24 . As yet another alternative, while the various alternatives offered provide some mechanism to control the rate of movement and relative positioning of conductors  20 , an operator could simply manipulate the conductor(s)  20  to manually position and secure them relative to stylet tubing  24 .  
         [0052]    Once all conductors  20  are secured to stylet tubing  24 , transitional element  26 , electrode(s)  18 , electrode spacer(s)  28 , outer tubing  23 , terminal spacer(s)  30 , terminal(s)  16 , and stylet guide  32  are positioned over, and concentrically arranged with, stylet tubing  24 . The arrangement of these elements is in accordance with that illustrated in FIG. 5.  
         [0053]    Transitional element  26  is illustrated in FIG. 6. As will be discussed later, transitional element  26  provides a platform to receive cap electrode  34  (FIG. 10). Transitional element  26  further provides a durable guide  26   a  to direct a distal end (not shown) of stylet  100  to cap electrode  34  via passage  26   b . Transitional element  26  is preferably formed of a conductive material, for example, the same material used to form electrodes  18 .  
         [0054]    Electrode spacer  28  is illustrated in FIG. 7. Similarly, terminal spacer  30  is illustrated in FIG. 8. Functionally, electrode spacer  28  and terminal spacer  30  accurately defines a space between adjacent electrodes  18  and terminals  16 , respectively. Electrode spacer  28  and terminal spacer  30  are preferably formed of the same material as outer tubing  23 . However, spacers  28  and  30  may be formed of a material that differs from that of outer tubing  23 ; provided however, any differing material used for electrode spacer  28  and/or terminal spacer  30  must be compatible with and possess largely the same mechanical properties (e.g., non-reactive to the environment of the human body, flexible and durable) as outer tubing  23 . At least for purposes of this example, spacers  28 , and  30  are formed of a polyurethane material, for example, Bionate 75D (Polymer Tech. Group, City, State). As is noted in FIG. 5, spacers  28  and  30  should have an outer diameter greater than lead  10 .  
         [0055]    Outer tubing  23  separates electrodes  18  from terminals  16 . In a preferred embodiment, outer tubing  23  has a diameter substantially equal to a diameter of lead  10 . Alternatively, outer tubing  23  may have a diameter less than lead  10 , or a diameter greater than lead  10 . In regard to the latter alternative, outer tubing  23  must have a wall thickness greater than a differential between a radius of lead  10  and a radius (to the outer diameter) of outer tubing  23 . For this particular example, outer tubing  23  has a nominal outer diameter of approximate 0.055 inches.  
         [0056]    Stylet guide  32  is illustrated in FIG. 9. Stylet guide  32  provides an inlet to stylet tubing  24 . Stylet guide  32  is preferably formed of conductive material, for example, the same material used to form electrodes  18 . Stylet guide  32 , as well as terminals  16 , electrodes  18 , and transitional element  26 , preferably each have an outer diameter equal to or greater than a nominal diameter of lead  10 . In a more preferred embodiment, these elements each have an outer diameter greater than a nominal diameter of lead  10 .  
         [0057]    Following the assembly of each of the elements described above, terminals  16  and electrodes  18  are joined to their respective conductors  20 . Generally, each terminal  16  (and each electrode  18 ) is positioned relative to exposed conductive material  20   a  and  20   b  of a conductor  20  and is joined in a manner that facilitates a transfer of electrical energy, for example, resistance weld or laser weld. Once all terminals  16  and electrodes  18  are secured, stylet guide  32  is secured to a proximal-most terminal  16 , and transitional element  26  is secured to a distal-most electrode  18 . Provided transitional element  26  and stylet guide  32  are formed a conductive material, these elements may be secured using a process consistent with that used to join terminals  16  and electrodes  18  with conductors  20 . Otherwise, transitional element  26  and stylet guide  32  can be joined using an adhesive, cement or the like.  
         [0058]    The completed assembly (FIG. 5) is then over-molded, using well known injection molding techniques, using a material having mechanical properties consistent with a material(s) used to form outer tubing  23 , electrode spacer  28 , and terminal spacer  30 . In a preferred embodiment, the over-molding material and the material of outer tubing  23 , electrode spacer  28 , and terminal  28  are the same.  
         [0059]    This process has the beneficial effect of unitizing the element assembly to form lead  10 . Moreover, electrode spacers  28  and terminal spacers  30  are placed in a state of flow, which, at least in part, results in a filling of regions between terminals  16 /electrodes  18  and stylet guide  24 . Consequently, terminals  16  and electrodes  18  are partially surrounded (i.e., along an interior surface) and supported by a fused matrix of; material. Importantly, as electrode spacers  28  and terminal spacers  30  are formed of a material mechanically equivalent to that of body  22 /outer tubing  23 , the stimulation/sensing portion and terminal portion of lead  10  are stabilized and strengthened while also retaining their flexible properties.  
         [0060]    The over-molded assembly (not shown) is then subjected to a grinding process to remove all excess material. In a preferred process, the over-molded assembly is subject to centerless grinding, wherein excessive material, including over-molded material, electrode material, terminal material, and the like, is removed. Pursuant to the described over-molding and grinding of the entire lead assembly, an isodiametric lead is obtained, which is further free of any gaps or voids between insulative material and conductive material that may otherwise exist in conventional devices.  
         [0061]    Following the grinding process, cap electrode  34  is affixed to transitional element  26  using conventional means, for example, resistance welding, laser welding, or the like.  
         [0062]    While addressed in part above, as the invention has been described herein relative to a number of particularized embodiments, it is understood that modifications of, and alternatives to, these embodiments, such modifications and alternatives realizing the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein, and it is intended that the scope of this invention claimed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.