Abstract:
A method of manufacturing an implantable electrical lead body MRI used in such applications as cardiac pacing, electrical nerve stimulation and intracardiac defibrillation applications that is biocompatible upon implantation in an animal and compatible with a magnetic resonance imaging scanner for the purpose of diagnostic quality imaging is disclosed. The method involves a relatively rigid first substrate layer, a conductive coil layer being precisely placed over the first substrate layer, a relatively soft second substrate layer over the conductive coil layer and a relatively rigid third substrate layer over the second substrate layer.

Description:
FIELD OF THE INVENTION 
     The invention relates to methods of manufacturing an implantable electrically conductive lead body used in such applications as cardiac pacing, intracardiac defibrillation and electrical nerve stimulation that is biocompatible upon implantation in an animal and compatible with a magnetic resonance imaging scanner for the purpose of diagnostic quality imaging. 
     BACKGROUND 
     Magnetic Resonance Imaging (MRI) is commonly used to view the internal organs of medical patients. To create an image, the patient is placed into very strong static and varying magnetic and radio frequency (RF) fields. For this reason, MRI is generally prohibited for patients with implanted ferromagnetic and/or electrically conductive objects, such as pacemakers, implantable defibrillators and nerve stimulators. Although it is feasible to minimize and even eliminate the use of ferromagnetic materials in implanted devices, these types of devices still require electrically conductive components that are affected by the fields produced by an MRI scanner. 
     U.S. Pat. No. 7,917,213, authored by the inventors of the present invention and incorporated herein by reference describes in detail the electrical and dimensional parameters of an MRI compatible lead body which minimizes the induced voltages and currents that can cause localized heating and/or distortion of an MRI image. This design requires that the diameter and pitch of the conductive coil within the lead body be closely controlled over its entire length. 
     Current methods of producing implantable lead bodies utilize various methods of polymer deposition such as spraying, dip coating, and extruding, however, these methods do not provide axial and diametric control of the conductive coil within the required tolerances and are thus unsuitable for producing MRI compatible lead bodies. 
     The invention as described and claimed herein details a process for manufacturing MRI compatible lead bodies which maintains close control of the helix pitch as well as the position of the coil in relation to the center line of the lead body, both of which relate to achieving the target RF performance. 
     SUMMARY 
     In one embodiment the invention discloses a method of manufacturing an MRI compatible conductive lead body. The method includes providing a mandrel defining a first end, a second end and an outer diameter and then applying a first substrate layer over the mandrel, with the first substrate layer defining an outer surface. The first substrate layer is next reflowed to conform closely to the mandrel. A conductive coil layer is wound around the outer surface of the first substrate layer and then secured to the mandrel at least at the first end and the second end. A second substrate layer is applied over the outer surface of the first substrate layer and reflowed to fuse with the first substrate layer and the conductive coil layer, permanently securing the conductive coil layer to the lead body. A third substrate layer is applied over the outer surface of the second substrate layer and reflowed causing the third substrate layer to fuse with the second substrate layer. The lead body is removed from the mandrel and trimmed to expose the conductive coil layer, allowing the lead body to be capable of electrical communication. 
     In another embodiment the invention discloses a method of manufacturing an MRI compatible conductive lead body. The method includes providing a mandrel defining a first end and a second end and an outer dimension substantially conforming to a desired inner dimension of a lumen defined by the lead body, with the mandrel coated with a non-stick material. A first set of blockers is placed at the first and second end of the mandrel, with the first set of blockers serving to prevent the migration of subsequently applied layers during the manufacturing process. A first substrate layer is applied between the first set of blockers, followed by the application of a first length of heat shrink material over the first substrate layer. The first length of heat shrink material is exposed to a sufficient amount of heat for a sufficient length of time to cause the first substrate layer to reflow, resulting in the first substrate layer conforming to the coated mandrel. Following reflowing of the first substrate layer, the first length of heat shrink material is removed. A conductive coil layer is wound over the outer surface of the first substrate layer and secured to the mandrel. A second set of blockers is placed at the first and second end of the lead body followed by applying a second substrate layer between the second set of blockers. A second length of heat shrink material is placed over the second substrate layer and exposed to a sufficient amount of heat for a sufficient length of time to cause the second substrate layer to reflow, resulting in the second substrate layer encapsulating the conductive coil layer and fusing with the first substrate layer. The second length of heat shrink material is removed and discarded. A third substrate material is placed over the second substrate layer. A third length of heat shrink material is applied over the third substrate and exposed to a sufficient amount of heat for a sufficient length of time to cause the third substrate layer to reflow and fuse to the second substrate layer. The third length of heat shrink material is removed and discarded. The first, second and third set of blockers are loosened from the mandrel, allowing the lead body to be removed from the mandrel. Upon removing the lead body from the mandrel the blockers are removed to expose the conductive coil layer, allowing the lead body to be capable of electrical communication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross section of a non-stick coated mandrel with a first set of blockers attached. 
         FIG. 2  is a longitudinal cross section of the lead body shown in  FIG. 1  with a first substrate applied following reflowing of the first substrate. 
         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. 
         FIG. 4  is a longitudinal cross section of the lead body shown in  FIG. 3  following the application of a second substrate prior to reflowing the second substrate. 
         FIG. 5  is a longitudinal cross section of the lead body shown in  FIG. 4  following reflowing the second substrate. 
         FIG. 6  is a longitudinal cross section of the lead body shown in  FIG. 5  following the application of a third substrate prior to reflowing the third substrate. 
         FIG. 7  is a longitudinal cross section of the lead body following reflowing of the third substrate. 
         FIG. 8  is a longitudinal cross section of the lead body following removal of the mandrel. 
         FIG. 9  is a longitudinal cross section of the completed lead body. 
         FIG. 9A  is a lateral cross section of the lead body taken through the lines  9 A- 9 A of  FIG. 9 . 
         FIG. 10  is a flow chart illustrating the steps of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     Nomenclature 
     
         
           10  Mandrel 
           10   a  Tapered End of Mandrel 
           10   b  Non-Tapered End of Mandrel 
           12  Coating 
           14  First Blocker 
           16  First Substrate Layer 
           18  Second Blocker 
           20  Conductive Coil Layer 
           22  Second Substrate Layer 
           26  Third Substrate Layer 
           28  Third Blocker 
           30  Lumen 
           50  Step of Providing Non-Stick Coated Mandrel 
           52  Step of Placing 1 st  Set of Blockers on Mandrel 
           54  Step of Applying 1 st  Substrate Between 1 st  Set of Blockers 
           56  Step of Applying 1 st  Length of Heat Shrink Material Over 1 st  Substrate Layer 
           58  Step of Exposing 1 st  Length of Heat Shrink Material to Sufficient Heat to Reflow 1 st  Substrate Layer 
           60  Step of Removing 1 st  Length of Heat Shrink Material 
           62  Step of Winding Conductive Coil Layer Over 1 st  Substrate Layer 
           64  Step of Placing 2 nd  Set of Blockers Over Conductive Coil Layer and 1 st  Substrate Layer 
           66  Step of Applying 2 nd  Substrate Layer Over Conductive Coil Layer Between 2 nd  Set of Blockers 
           68  Step of Applying 2 nd  Length of Heat Shrink Material Over 2 nd  Substrate Layer 
           70  Step of Exposing 2 nd  Length of Heat Shrink Material to Sufficient Heat to Reflow 2 nd  Substrate Layer 
           72  Step of Removing 2 nd  Length of Heat Shrink Material 
           74  Step of Applying 3 rd  Set of Blockers 
           76  Step of Applying 3 rd  Substrate Layer Over Reflowed 2 nd  Substrate Layer, Between 3 rd  Set of Blockers 
           78  Step of Applying 3 rd  Length of Heat Shrink Material Over 3 rd  Substrate Layer 
           80  Step of Exposing 3 rd  Length of Heat Shrink Material to Sufficient Heat to Reflow 3 rd  Substrate Layer 
           82  Step of Removing 3 rd  Length of Heat Shrink Material 
           84  Step of Loosening 1 st , 2 nd  3 rd  Blockers from Mandrel 
           86  Step of Removing Lead Body from Mandrel 
           88  Step of Trimming Away Blockers to Expose the Conductive Winding Layer at Both Ends of Lead Body 
           100  Conductive Lead body
 
Definitions
 
       
    
     “Filar” means the number of separate conductive strands wound onto the lead body. 
     “Reflow” means applying sufficient pressure and temperature to a polymeric material to cause it to change configuration. 
     “Teflon®” is used here in its generic sense and includes PTFE, ETFE, FEP and other non-stick coatings. 
     Construction 
     As best shown in  FIG. 1 , the method begins with step  50  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 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 the first  16 , second  22  and third  26  substrate layers onto the mandrel  10  as well as the heat shrink material (not shown) used to reflow the first  16 , second  22  and third  26  substrate layers. 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 a 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. 
       FIG. 2  shows the lead body following step  54  and the application of a first substrate layer  16  between the first set of blockers  14  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 substrate layer is made of a 55 D 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 substrate 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 substrate 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 tube of a first length of heat shrink material (not shown) also over the tapered end  10   a , over the not yet reflowed first substrate layer  16 . The first length of heat shrink material (not shown) is then at step  58  exposed to heat for a period of time sufficient to cause the first length of heat shrink material (not shown) to decrease in size and to reflow the first substrate 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 conductive winding  20  and the thickness of the first, second and third substrate layers  16 ,  22 ,  26  it is difficult to characterize the heat treatment necessary to cause the first, second and third substrate 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 substrate 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 substrate layer  16  the first length of heat shrink material (not shown) is removed and discarded at step  60 . 
       FIG. 3  illustrates step  62  and placement of a conductive coil layer  20  over the outer surface of the first substrate layer  16 . The conductive coil layer  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  while maintaining MRI compatibility. 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  20  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  20  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. 
     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 ) substrate 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. 
       FIG. 4  shows the application at step  66  of a second substrate layer  22  over the uncompleted lead body. In one embodiment the second substrate layer  22  comprises an 80 A polyurethane material which is a softer material than 55 D polyurethane and functions as a dampener or shock absorber. Additionally, the second substrate layer  22  serves to precisely bind the winding layer  20  to the first substrate layer  16  thus ensuring the accuracy of the intended diameter and pitch of the conductive coil layer  20  which maintains the electrical performance characteristics necessary for MRI compatibility. The second substrate 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. 
       FIG. 5  shows the lead body following reflowing of the second substrate  22 . Reflowing is accomplished at step  68  by sliding a second length of heat shrink material (not shown) over the second substrate  22  which is then at step  70  exposed to a sufficient amount of heat for a period of time sufficient to cause the heat shrink material (not shown) to decrease in size and to reflow the second substrate 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 substrate layer  22  by the decreasing size of the heat shrink material (not shown), in combination with the exposure to heat energy causes the second substrate material  22  to reflow, resulting in the second substrate layer  22  being uniformly molded around the uncompleted lead body, resulting in the conductive winding  20  being permanently secured in place. Reflowing of the second substrate layer  22  also results in the second substrate layer  22  fusing with the first substrate layer  16 , while still maintaining separate layers. Following reflowing of the second substrate layer  22  the heat shrink material (not shown) is removed and discarded at step  72 . 
     As shown in  FIG. 6 , a third substrate layer  26  is applied at step  76  by sliding a tube over the lead body. In one embodiment the third substrate layer  26  comprises a 55 D 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 substrate 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 substrate 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. 
       FIG. 7  shows reflowing the third substrate  26  which is accomplished at step  78  by sliding a third length of heat shrink material (not shown) over the third substrate layer  26  which is then at step  80  exposed to a sufficient amount of heat for a period of time sufficient to cause the heat shrink material (not shown) to decrease in size and reflow the third substrate 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 substrate layer  26  the heat shrink material (not shown) is removed and discarded at step  82 . The pressure exerted on the third substrate layer  26  by the decreasing size of the heat shrink material, in combination with the exposure to heat energy causes the third substrate material  26  to reflow, resulting in the third substrate layer  26  being uniformly molded around the lead body. Reflowing of the third substrate layer  26  also results in the third substrate layer  26  fusing with the second substrate layer  22 , while still maintaining separate layers. 
       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  100  without damaging the lead body  100 . The function of the first, second and third sets of blockers  14 ,  18 ,  28  is to ensure that the first, second and third reflowed substrate 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  100  is trimmed (not shown) at step  88  to expose the conductive coil layer  20 , allowing later attached electrodes and connectors to be in electrical communication with various devices. 
       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 . 
       FIG. 10  is a flow chart illustrating the various steps of the manufacturing process, including reflowing of the first, second and third substrate layers  16 ,  22 ,  26 .