Abstract:
An electrical lead body for an implantable electronic medical device has multiple layers of insulating material encapsulating a conductor that is wound in a spiral manner along the length of the lead. The layered structure provides resistance to fracture from mechanical stresses. A manufacturing process for producing this electrical lead is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/199,097 filed on Aug. 19, 2011, and claims benefit of U.S. provisional patent application No. 61/401,867 filed on Aug. 20, 2010 and U.S. provisional patent application No. 61/469,167 filed on Mar. 30, 2011, the disclosures in which are incorporated herein by reference as if set forth in their entirety herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to the structure and manufacturing process for fracture resistant, implantable electrical lead bodies for use in conjunction with implantable electronic medical devices, such as cardiac pacemakers and defibrillators, that monitor and/or stimulate a tissue of an animal for therapeutic purposes. 
         [0005]    2. Description of the Related Art 
         [0006]    Numerous medical conditions, such as cardiac and neurological dysfunctions, are treated by implanted electronic devices which provide monitoring and/or electrical stimulation to the affected tissues of an animal. These devices are of various types and constructions, and typically attach to the animal tissue via implanted leads. These leads may be partially or entirely intra-vascular. 
         [0007]    Failures in the bodies of such leads may result in compromise of some or all of the functional intent of the implanted electronic device. Lead body failure modes include partial or complete insulation break, insulation perforation, partial or complete conductor coil fracture, EMI pickup by the lead body, and lead body maturation or dislodgement. For Implanted Cardiac Defibrillators (ICDs), lead body failures may manifest as over sensing, under sensing, loss of capture or non-capture, loss of output, Pacemaker Mediated Tachycardia (PMT). See Chakri Yarlagadda, MD, FACC, FASNC, FSCAI, Director of Non-Invasive Cardiology, St Joseph Health Center; Invasive Cardiologist, Ohio Heart Institute: “Pacemaker Malfunction”, Feb. 18, 2009,  eMedicine from WebMD , (http://emedicine.medscape.com/article/156583-overview). 
         [0008]    The above described lead body failures often result from mechanical stresses introduced by surgical sutures, post-surgery flexure, or lead body conductor coil mechanical resonances. The coiled helixes of the lead body conductors form a natural spring with very little damping, and can easily resonate in response to mechanical inputs from body motion. Left unchecked, such resonance will eventually result in mechanical abrasion and weakening of the surrounding lead body insulation, as well as lead body conductor coil breakage due to local metal fatigue. The above lead body failures can also be promoted by pinching of the lead body structure from suturing or pinching between skeletal structures, such as the upper ribs and clavicle. A closely wound conductor helix can be kinked by such, thus predisposing the electrical conductor to fracture. 
         [0009]    With the definition of a Lead Defect being that which requires surgery to correct a fracture or sensing flaw, recent studies have shown that approximately 15% of ICD patients experience a lead defect within five years of implantation, that 40% of ICD patients experience a lead defect within eight years of implantation, and that the annual failure rate levels off at 20%/year beyond ten years. See Thomas Kleemann, MD: Herzzentrum Ludwigshafen, Germany, “Increasing Rates of ICD Lead Failure Noted During Long Term Follow-Up”,  Heartwise, Apr.  30, 2007, (http://www.theheart.org/article/787831.do), repeated in Medscape May 4, 2007. 
       Consequences of Lead Body Failure: 
       [0010]    Lead body fractures in conjunction with ICDs may result in misinterpretation of conductor fracture induced noise as fibrillation, leading to subsequent inappropriate shocks to the patient. These shocks are often repetitive due to the structural problem in the lead body and can be traumatic to the patient. Most importantly, shocks due to lead body fractures are no longer synchronized to the patient&#39;s intrinsic heart beat in the normal fashion, and have therefore been known to induce ventricular fibrillation. In such cases, because the lead body is damaged, adequate energy for defibrillation may not be delivered and result in death to the patient. Lead body fractures in the case of pacemakers can cause over sensing and/or failure to capture which can result in the patient fainting. 
         [0011]    Potential complications during lead-change surgery include vascular injury, venous thrombosis, cardiac tapenade, hemothorax, pneumothorax, perforation of heart, avulsion of right ventricle, bleeding, and infection. See Chakri Yarlagadda, supra. 
         [0012]    Considering: a) the potential for inappropriate shocks to the patient; b) the severity of potential lead-replacement surgical complications; c) the existence of a few million ICD and pacemaker patients worldwide with hundreds of thousands of new implants being added yearly, and d) the potential for lead failure rates noted above, there is therefore a need for implanted medical leads to attain improved robustness and resistance to fracture from mechanical stresses. An alternative method for design and manufacturing of a lead body with improved performance is therefore needed. 
         [0013]    The present invention pertains to a geometry and manufacturing process to create an implantable medical lead using a reflow process employing heat shrinkable tubing along the entire length of the lead. That medical lead comprises a polymer sandwich of varying polymer hardnesses surrounding the lead coils. Bottomley in U.S. Published Patent Application No. 2008/0243218 employs PET (poly(ethylene terephthalate) for various purposes including overmold insulation, spiral wrap insulation, lead-end terminations, and as spot heat shrink to aid in manufacturing steps. But Bottomley teaches neither using a sandwich of polymers of different hardnesses which are reflowed into a sandwich around the lead coils, nor the reflow technique of the present invention wherein heat shrinkable tubing is employed along the entire length of the lead and then heated to effect a uniform polymer reflow. Bottomley instead teaches a drawdown reflow process using a heated die. The Kampa, et al. U.S. Pat. No. 7,112,298 describes the use of a polymer to form the diameter of a catheter lumen, and manufacturing the balance of the catheter in layers around this interior coating. Drawdown through heated dies is mentioned as a method of forming outer layers of the catheter. Neither the full-length heat shrinkable tubing process of the present invention, nor implantable lead manufacture, is mentioned. The Snow U.S. Published Patent Application No. 2001/0010247 teaches manufacturing of reinforced thin walled cannula with a relatively large lumen, in which a coated elongate member is wound in a helical manner around a mandrel. Heat shrinkable tubing compresses the elongate prior to heating to aid in the elongate member sealing its own interlocking edges. Other layers may be added on top of the elongate, with heat used to fuse these subsequent layers together. No mention is made of implantable lead manufacture via a sandwich of polymers of different hardnesses which are reflowed into a sandwich around lead coils. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention defines an alternative design and associated manufacturing process which together produce implantable medical lead bodies with improved robustness and resistance to fracture from mechanical stresses, resulting in a decrease in the presently experienced lead failure rates detailed above. They are intended as an alternative to prior design and manufacturing techniques, attempting to overcome recognized limitations of the prior art. 
         [0015]    In the present implantable medical lead body a lead body conductor coil or coils is embedded in a sandwich of a polymer (such as for example polytetrafluoroethylene, silicone, or polyurethane) which has degrees of hardness in such a way as to allow for lead body flexure in both the radial and longitudinal directions. That polymer sandwich provides a supporting structure for the lead body conductor coils. 
         [0016]    Another aspect of the current invention is attainment of intimate contact between the polymer sandwich material and the lead body conductor coils by means of a reflow process. 
         [0017]    A further aspect is providing the ability to vary the lead body flexibility and handling characteristics by selecting different combinations of polymer hardness and thickness. 
         [0018]    Yet another aspect of the invention is use of the softest polymer layer directly over the lead body conductor coils to encapsulate them and to provide flexibility as well as mechanical damping. 
         [0019]    Another aspect of the current invention is elimination of mechanical resonances in the conductor coils by the polymer sandwich. 
         [0020]    Still another aspect is a lead body conductor coil pattern that leaves some space (approximately one-fourth to two times the conductor width) between adjacent turns of the coil conductors (or between adjacent filars of the electrical conductor) to allow movement without coil-to-coil interference. 
         [0021]    A further aspect of the current invention is incorporation of a lumen core at the center of the lead body. 
         [0022]    Another aspect of the current invention is production of an implantable medical lead body with improved robustness and resistance to fracture from the suture and flexing introduced mechanical stresses commonly experienced by implantable leads. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a longitudinal cross section of a non-stick coated mandrel with a first set of blockers attached. 
           [0024]      FIG. 2  is a longitudinal cross section of the lead body shown in  FIG. 1  with a first insulating layer applied following reflowing of the first insulating layer. 
           [0025]      FIG. 3  is a longitudinal cross section of the lead body shown in  FIG. 2  following the winding of a conductive coil layer and an attached second set of blockers. 
           [0026]      FIG. 4  is a longitudinal cross section of the lead body shown in  FIG. 3  following the application of a second insulating layer prior to reflowing the second insulating layer. 
           [0027]      FIG. 5  is a longitudinal cross section of the lead body shown in  FIG. 4  following reflowing the second insulating layer. 
           [0028]      FIG. 6  is a longitudinal cross section of the lead body shown in  FIG. 5  following the application of a third insulating layer prior to reflowing the third insulating layer. 
           [0029]      FIG. 7  is a longitudinal cross section of the lead body following reflowing of the third insulating layer. 
           [0030]      FIG. 8  is a longitudinal cross section of the lead body following removal of the mandrel. 
           [0031]      FIG. 9  is a longitudinal cross section of the completed lead body. 
           [0032]      FIG. 9A  is a lateral cross section of the lead body taken through the lines  9 A- 9 A of  FIG. 9 . 
           [0033]      FIG. 10  is a flow chart illustrating the steps of the method of the invention. 
           [0034]      FIGS. 11A and 11B  are side and end views, respectively, of only an electrical conductor coil that has been wound in the conventional manner; 
           [0035]      FIGS. 12A and 12B  are side and end views, respectively, of only an electrical conductor coil wound according to the present invention; 
           [0036]      FIG. 13  is a side view of a dual filar coil assembly that alternatively may be used in the electrical lead in place of the coil in  FIG. 12 ; and 
           [0037]      FIG. 14  is a side view of a double dual filar coil assembly that alternatively may be used in the electrical lead in place of the coil in  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The lead body configuration and associated manufacturing process encompassed by the current invention together provide improved robustness and resistance to fracture from the suture, flexing, and vibration introduced mechanical stresses commonly experienced by implantable leads. 
         [0039]    The current invention spans the areas of geometric configuration design, material selection, manufacturing techniques, and manufacturing steps. 
       DEFINITIONS 
       [0040]    “Filar” means the number of separate conductive strands wound onto the lead body. 
         [0041]    “Reflow” means applying sufficient pressure and temperature to a polymeric material to cause it to change configuration. 
         [0042]    “Teflon®” is used here in its generic sense and includes PTFE, ETFE, FEP and other non-stick coatings. 
         [0043]    The manufacturing process creates the layers of the electrical lead body  100  in a step-by-step fashion from the inside-out as follows. 
         [0044]    As best shown in  FIG. 1 , the method begins at step  50  with the procurement of a mandrel  10 , which can be stainless steel, Teflon® or other materials able to withstand the temperatures and pressures of the method of the present invention. The mandrel  10  defines an outer dimension which will eventually correspond to the inner dimension of the lumen  30  of the eventually completed electrical lead body  100 . The mandrel  10  also defines a tapered end  10   a  and a non-tapered end  10   b . The tapered end  10   a  serves to facilitate easier loading of tubular first  16 , second  22  and third  26  insulating layers onto the mandrel  10  as well as the heat shrink tubing (not shown) used to reflow the first  16 , second  22  and third  26  insulating layers that form the substrate of the lead body. In this embodiment, the mandrel  10  is coated with a layer of non-stick coating  12  such as Teflon® or another compound characterized by chemical inertness as well as possessing significant non-stick characteristics. In one embodiment, the mandrel comprises a stainless steel wire with a sheet of Teflon® applied to it. A first set of blockers  14  at step  52  is placed over the Teflon® coated  12  mandrel  10  and serves to assist in preventing the migration of subsequently applied layers during the manufacturing process. In one embodiment the first set of blockers  14  comprise tubing of heat shrink material that is heated following application causing the blockers  14  to decrease in size and closely conform to the outer contours of the mandrel  10 . The first set of blockers  14  can be made of PET (polyethylene terephthalate) heat shrink material, however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. 
         [0045]      FIG. 2  shows the lead body following the application of a first insulating layer  16  between the first set of blockers  14  at step  54  which serves to create a uniform outer diameter as well as acting to add structural strength to the eventually completed lead body  100 . In one embodiment, the first insulating layer is made of a 55D polyurethane material such as Pellethane, made by Dow Chemical, which is relatively rigid and adds strength and integrity to the eventually completed lead body  100 . In other embodiments, the first insulating layer  16  can also be made of other urethane, silicone or other polymeric materials able to withstand the temperature and pressure requirements necessary to reflow and provide the necessary biocompatibility. The first insulating layer  16  can be made by an extrusion process. 
         [0046]    Then, the first insulating layer  16  is applied to the mandrel  10  as a tube which is slid over the tapered end  10   a  of mandrel  10  followed at step  56  by sliding a first length of heat shrink tubing (not shown) also over the tapered end  10   a , over the not yet reflowed first insulating layer  16 . The first length of heat shrink tubing material (not shown) is then exposed at step  58  to heat for a period of time sufficient to cause the first heat shrink material (not shown) to decrease diametrically in size and to reflow the first insulating layer  16 . In one embodiment, suitable heat shrink materials include FEP (fluorinated ethylene polypropylene), however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. Due to variables such as the pitch of the spiral wound electrical conductor  20  and the thickness of the first, second and third insulating layers  16 ,  22 ,  26  it is difficult to characterize the heat treatment necessary to cause the first, second and third insulating layers  16 ,  22 ,  26  to reflow. In one embodiment, a vertical reflow system is used (not shown), which is well known to those skilled in the art. A vertical reflow system comprises a cylindrical chamber which is provided with a heat source through which the lead body is sequentially passed. It has been found that the first, second and third insulating layer  16 ,  22 ,  26  successfully reflow at a temperature of 450 degrees C., plus or minus 25 degrees C. when passed through a vertical reflow system at a speed of 0.1 to 0.3 centimeters per second. Following reflowing of the first insulating layer  16  the first length of heat shrink tubing (not shown) is removed and discarded at step  60 . 
         [0047]    The process of gradual heating, with compression applied by the heat-shrinkable-tubing, results in a relatively uniform thickness layer of polymer being deposited on the mandrel, forming first insulating layer  16 . 
         [0048]      FIG. 3  illustrates step  62  and the placement of a conductive coil layer  15  formed by winding the electrical conductor  20  in a longitudinal spiral around the outer surface of the first insulating layer  16 . With additional reference to  FIG. 12 , the coils or turns  200  of the electrical conductor  20  are wound helically along the length of the electrical lead. That winding process creates a space  42  between adjacent turns on at least one side of the longitudinal axis  40  of the electrical lead body  100 . The distance of each space  42  is approximately one-fourth to two times the diameter of the electrical conductor  20 . The spaces  42  allow the conductor to deflect when stressed radially and also allows for an individual coil turns to rotate sideways slightly. 
         [0049]    The electrical conductor  20  in one embodiment is MP35N drawn fused tubing sold under the name DFI® but could also be any non-ferromagnetic material having sufficient conductivity to deliver electrical energy through the lead body  100 . The MP35N drawn fused tubing is an insulated conductor which could be insulated by such bio-compatible materials such as Teflon®, polyimide, urethanes or other materials. The conductive coil layer  15  may be initially secured in place using a variety of methods (e.g., crimping, swaging, heat shrink, others) (not shown). It is understood that the winding pattern for the conductive coil layer  15  shown herein is for purposes of illustration only and therefore does not limit the scope of the invention. As an example, the winding pattern as illustrated is monofilar, however, the invention is also compatible with multifilar applications. It is also understood that while a single conductive coil layer is shown in the drawings, this is for purposes of illustration only and therefore additional embodiments utilizing multiple conductive coil layers are also compatible with the method of this invention and therefore within its scope. 
         [0050]    In one embodiment the second set of blockers  18  comprises a heat shrink material, where at step  64  the heat shrink material is placed over the coil between the second set of blockers  18  and serves to prevent the migration of the subsequent (i.e., second  22  and third  26 ) insulating layers. In one embodiment, suitable heat shrink materials include PET (polyethylene terephthalate) heat shrink material, however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. Placement of the second set of blockers  18  is followed by the application of heat to cause the heat shrink material to shrink in size. 
         [0051]      FIG. 4  shows the application at step  66  of a second insulating layer  22  over the uncompleted lead body. In one embodiment the second insulating layer  22  comprises a polyurethane material which is a softer material than 55D polyurethane and functions as a dampener or shock absorber. Thus the degree of hardness of the second insulating layer  22  is less than the degree of hardness of the first insulating layer  16 . Additionally, the second insulating layer  22  serves to precisely bind the conductive coil layer  15  to the first insulating layer  16  thus ensuring the accuracy of the intended diameter and pitch of the conductive coil layer  15  which maintains the desired electrical performance characteristics. The second insulating layer  22  is applied to the lead body as a tube which is slid over the tapered end  10   a  of the mandrel  10  and uncompleted lead body. 
         [0052]      FIG. 5  shows the lead body following reflowing of the second insulating layer  22 . Reflowing is accomplished at step  68  by sliding a second length of heat shrink tubing (not shown) over the second insulating layer  22  which at step  70  is then exposed to a sufficient amount of heat for a period of time sufficient to cause the heat shrink tubing (not shown) to decrease in size and to reflow the second insulating layer  22 . In one embodiment, suitable heat shrink materials include an FEP (fluorinated ethylene polypropylene) heat shrink material, however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. The pressure exerted on the second insulating layer  22  by the decreasing size of the heat shrink tubing (not shown), in combination with the exposure to heat energy causes the material of the second insulating layer  22  to reflow, results in the second insulating layer  22  being uniformly molded around the uncompleted lead body. Thus the second insulating layer material flows between the turns of the electrical conductor  20  and into contact with the first insulating layer  16 . This results in the electrical conductor  20  being permanently secured in place. Reflowing of the second insulating layer  22  also results in the second insulating layer  22  fusing with the first insulating layer  16 , while still maintaining separate layers. Following reflowing of the second insulating layer  22 , the heat tubing (not shown) is removed and discarded at step  72 . 
         [0053]    As shown in  FIG. 6 , a third insulating layer  26  is applied at step  76  as a tube that is slid over the lead body. In one embodiment the third insulating layer  26  comprises a 55D urethane material which is a relatively firm material, which primarily serves to add strength and an additional degree of integrity to the completed lead body  100 . Also shown in  FIG. 6  is the addition of a third set of blockers  28  which can be heat shrink material placed towards the outer ends (unnumbered) of the uncompleted lead body. It should be noted that in some embodiments, the third set of blockers  28  may not be used, depending on the thicknesses of the insulating layers. Placement of the third set of blockers  28  is followed by the application of heat to cause the heat shrink material to reduce in size, thereby securing the third set of blockers at the desired position on the lead body. When used, the third set of blockers  28  functions to prevent the reflowed third insulating layer  26  from flowing beyond the third set of blockers  28 . The third set of blockers  28  can be made of PET (polyethylene terephthalate) heat shrink material, however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. 
         [0054]      FIG. 7  shows reflowing the third insulating layer  26  which is accomplished at step  78  by sliding a third length of heat shrink tubing (not shown) over the third insulating layer  26  which at step  80  is then exposed to a sufficient amount of heat for a period of time sufficient to cause the heat shrink tubing (not shown) to decrease in size and reflow the third insulating layer  26 . In one embodiment, suitable heat shrink materials include an FEP (fluorinated ethylene polypropylene) heat shrink material, however, it is noted that other materials possessing similar characteristics would also work, thus the invention is not considered to be so limited. Following reflowing of the third insulating layer  26  the heat shrink tubing (not shown) is removed and discarded at step  82 . The pressure exerted on the third insulating layer  26  by the decreasing size of the heat tubing material, in combination with the exposure to heat energy causes the third insulating layer material to reflow, resulting in the third insulating layer  26  being uniformly molded around the lead body. Reflowing of the third insulating layer  26  also results in the third insulating layer  26  fusing with the second insulating layer  22 , while still maintaining separate layers. 
         [0055]      FIG. 8  shows the lead body  100  following removal of the mandrel  10 . It is noted that a lumen  30  is formed where the mandrel  10  had previously been in place. Removal of the mandrel  10  at step  84  first requires loosening of the first, second and third sets of blockers  14 ,  18 ,  28 , which frees the mandrel  10  from the lead body  100 , allowing the mandrel  10  at step  86  to be withdrawn from the lead body without damaging the lead body. The function of the first, second and third sets of blockers  14 ,  18 ,  28  is to ensure that the first, second and third reflowed insulating layers  16 ,  22 ,  26  end at the same point. In one embodiment they would be perfectly aligned, but perfect alignment is not absolutely required. Following removal of the lead body  100  from the mandrel  10 , the lead body is trimmed at step  88  to expose the conductive coil layer  15 , allowing later attached electrodes and connectors to be in electrical communication with various devices. 
         [0056]      FIG. 9A  is a lateral cross section taken through the lines  9 A- 9 A of the completed lead body  100  ( FIG. 9 ) and shows the various layers built up during the manufacturing process and the lumen  30 . 
         [0057]      FIG. 10  is a flow chart illustrating the various steps of the manufacturing process, including reflowing of the first, second and third insulating layers  16 ,  22 ,  26 . 
       Geometric Configuration 
       [0058]      FIGS. 9 and 9A  illustrate the geometry of the present electrical lead body  100 . A lumen  30  is at the center of the body, surrounded by a first insulating layer  16  of a polymer, for example. Thus, the first insulating layer provides a cylindrical lumen core of the electrical lead body. Next is the conductive coil layer  15  comprising an electrical conductor  20  spirally wound in a coil extending longitudinally along the length of the lead electrical lead body  100 . The electrical conductor  20  has a metal wire  32  enclosed in a covering  34  of an electrically insulating material. The electrical conductor  20  in the coil layer  15  is wound from wire commonly used for implanted lead applications (stainless steel or a nickel-cobalt base alloy such as MP35, as examples). Surrounding the coil layer  15  is a second insulating layer  22  of a material (such as a polymer selected from silicone, polyurethane, or polytetrafluoroethylene, for example) that provides the necessary biocompatibility for the electrical lead body  100 . 
         [0059]    All the insulating layers by be made of the same general type of material or different materials, however, in either case the layers have a particular relationship in respect of their degrees of hardness. Specifically, the second insulating layer  22  is softer than the first and third insulating layers  16  and  26 . That is, the second insulating layer  22  has a lower degree of hardness that both the first and third insulating layers  16  and  26 . The first and third insulating layers  16  and  26  may have the same or different degrees of hardness. The relative hardness and thickness of the insulating layers  16 ,  22 , and  26  may be varied to affect the desired lead body flexibility and handling characteristics, provided that the second insulating layer  22  is softer than the first and third insulating layers  16  and  26 . The polymers of layers  16 ,  22 , and  26  provide inherent biocompatibility with the animal into which the electrical lead body  100  will be implanted. 
         [0060]    A cushioning, vibration damping sandwich is formed by surrounding the conductive coil layer  15  by polymer layers  16 ,  22  and  26 . This sandwich structure of multiple coaxial insulating layers reduces mechanical resonances within the coiled electrical conductor  20 , thereby minimizing such resonances as a potential cause of conductor fatigue which could eventually result in lead body failure. The relative softness of second insulating layer  22  relative to the first and third insulating layers  16  and  26  is essential to achieve this vibration dampening. 
         [0061]    With reference to  FIGS. 9 and 12A , the coils or turns  200  of the electrical conductor  20  are wound helically in a spiral around the first insulating layer  16  along the electrical lead. That winding process creates a space  210  between adjacent turns  200  on at least one side of the longitudinal axis  40  of the electrical lead body  100 . The distance “S” of each space  210  is approximately one-fourth to two times the diameter “D” of the electrical conductor  20 . The spaces  210  allow the electrical conductor to deflect when stressed radially. In conjunction with the relatively softer polymer second insulating layer  22 , these spaces  210  also allow for the individual coil turns to rotate sideways slightly, thereby reducing the potential for kinking cause by suturing or crushing. Such a kink  150 , as occurred in previous leads  130  (see  FIG. 11B ), cause a weakness in the electrical conductor  140  at that point, potentially leading to conductor material fatigue and eventually breakage over time in the presence of repeated mechanical stress. The turns  160  of the electrical conductor  140  in such previous leads were closely spaced, typically adjacent turns touched each other. 
         [0062]    The new electrical lead body  100  depicted in  FIGS. 9 and 12  has a single filar electrical conductor structure. Alternatively, a multiple filar conductor structure may be used. With reference to  FIG. 13 , the electrical conductor  300  has two filars  302  and  304  in close proximity to each other, preferably abutting, tangentially along their lengths and wound in a longitudinal spiral lengthwise along the electrical lead body, thereby forming a filar pair  305 . Each filar  302  and  304  has a conductive wire encased in a outer layer of insulation. As with the single filar, electrical conductor  20  described previously herein, each turn  306  of the filar pair  305  has a space  308  there between. Each space  308  is approximately one-fourth to two times the diameter of the electrical conductor of each filar. Alternatively, more the two filars be grouped together like filars  302  and  304  and that group wound as a spaced apart spiral along the length of the electrical lead body. 
         [0063]      FIG. 14  illustrates the structure of another multiple filar electrical conductor  330  that as two groups of filars wound in spaced apart interleaved helixes. This electrical conductor  330  has first and second filar groups  332  and  334 . The first filar group  332  has first and second filars  336  and  338  in close proximity to each other, preferably abutting, tangentially along their lengths, and the second filar group  334  has third and fourth filars  340  and  342  also in close proximity to each other, preferably abutting, tangentially along their lengths. Each filar  336 ,  338 ,  340 , and  342  has a conductive wire encased in an outer layer of insulation. The two filar groups  332  and  334  are wound in a longitudinal spiral along the length of the electrical lead body. The first and second filar groups  332  and  334  are continuously spaced apart by a distance approximately one-fourth to two times the diameter of the electrical conductor in each filar. Alternatively, there may be more than two filar groups wound as a spaced apart spiral along the length of the electrical lead body, and each group may have more than two filars. 
         [0064]    The foregoing description was primarily directed to a certain embodiments of the industrial vehicle. Although some attention was given to various alternatives, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from the disclosure of these embodiments. Accordingly, the scope of the coverage should be determined from the following claims and not limited by the above disclosure.