Patent Publication Number: US-2010125321-A1

Title: Eptfe fill of coil filar gaps

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C §119 of U.S. Provisional Application No. 61/114,577, filed on Nov. 14, 2008, entitled “EPTFE FILL OF COIL FILAR GAPS,” which is herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a medical electrical lead including one or more coiled electrodes. More particularly, the present invention relates to a coiled electrode including an electrically transparent cover. 
     BACKGROUND 
     Medical electrical leads such as pacemakers and defibrillators may include a lead body having a coiled electrode that is implanted at an appropriate location within a patient&#39;s heart. An implantable defibrillator, for example, includes a lead assembly having at least one defibrillation electrode, such as a defibrillation coil. Some lead assemblies include a cover such as a polytetrafluoroethylene (PTFE) cover that extends over at least a portion of the outer surface of the coiled electrode. Such covers are used, for example, to prevent tissue ingrowth and to facilitate removal of the lead from the vessel in which it has been implanted. One challenge with such covers is that they may move during insertion of the lead through an introducer, potentially leaving a portion of the electrode exposed. This challenge may be heightened when the electrode coil is formed with spaces between turns of the coil to increase electrode flexibility, because the spaces tend to reduce the contact area between the electrode surface and the cover. 
     SUMMARY 
     One embodiment of the present invention is a medical electrical lead including a lead body, at least one conductor, and at least one coiled electrode located on the lead body. The lead body includes a proximal end and a distal end. A terminal connector for connecting to a pulse generator or the like is located at the proximal end of the lead body. The conductor is coupled to the terminal connector and extends within the lead body from the proximal to the distal end. The coiled electrode is operatively coupled to the conductor extending within the lead body. The coiled electrode includes at least one wound conductive filar that defines an outer electrode surface including a plurality of gaps in the wound conductive filar. A polymeric filling including non-expanded polytetrafluoroethylene is disposed in and substantially fills at least some of the gaps. A polymeric cover including expanded polytetrafluoroethylene is disposed over the outer surface of the coiled electrode and is bonded to the polymeric filling provided in the gaps. 
     Another embodiment of the present invention is a method of forming an electrode. The method includes forming a coiled electrode including at least one conductive filar wound to define, in longitudinal cross-section, a plurality of turns and a gap between each turn. Additionally, the method includes filling at least a portion of the gaps with a polymeric filling comprising a non-expanded polytetrafluoroethylene; wrapping a cover comprising one or more layers of a thin polymeric film comprising expanded polytetrafluoroethylene over the outer surface of the electrode; and bonding the cover to the filling. In some embodiments, the polymeric filling includes one or more layers of a thin polymeric film comprising polytetrafluoroethylene. In other embodiments, the polymeric filling includes a filar comprising polytetrafluoroethylene. The cover can be sintered to the fillings disposed in the gaps. 
     According to another embodiment, a medical electrical lead includes an insulative lead body including a lumen through which a conductor extends and at least one coiled electrode located on the lead body and operatively coupled to the conductor. The coiled electrode includes at least one wound conductive filar that defines, along its longitudinal cross-section, a plurality of turns and a plurality of gaps disposed between the turns. A polymeric filling comprising polytetrafluoroethylene is disposed in and substantially fills at least some of the gaps. The filled gaps have a width of between about 0.0002 inches and about 0.0020 inches. A polymeric cover comprising expanded polytetrafluoroethylene is disposed over an outer surface of the coiled electrode and is bonded to the polymeric filling. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a medical electrical lead including at least one coiled electrode according to various embodiments of the present invention. 
         FIGS. 2A-2D  are longitudinal cross-sectional views of a portion of a lead body including a coiled electrode according to various embodiments of the present invention. 
         FIGS. 3A and 3B  are longitudinal cross-sectional views of a coiled electrode including a polymeric filling according to various embodiments of the present invention. 
         FIGS. 4A and 4B  are partial schematic views of a coiled electrode including a polymeric cover according to various embodiments of the present invention. 
         FIG. 5  is a flow chart of a method according to an embodiment of the present invention. 
         FIG. 6  is a flow chart of a method according to another embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. 
     Embodiments of the present invention are directed to a medical electrical lead. According to some embodiments, the medical electrical lead can be configured for implantation within a patient&#39;s heart. According to other embodiments, the medical electrical lead is configured for implantation within a patient&#39;s neurovascular regions. 
       FIG. 1  illustrates a defibrillation lead  10 , which includes an elongated, insulative lead body  12  extending from a proximal end  16  to a distal end  20 . The proximal end  16  is configured to be operatively connected to a pulse generator via a connector  24 . A conductor  32  extends within the lead body  12  from the connector  24  to at least one coiled electrode  28  located on the lead body  12 . The lead body  12  can also include one or more fixation members for securing and stabilizing the lead body  12  including the one or more electrodes  28  at a target site within a patient&#39;s body. The fixation member(s) can be active or passive. 
       FIGS. 2A-2D  are longitudinal cross sectional views of a portion of the lead body  12  including the coiled electrode  28  according to various embodiments of the present invention. The coiled electrode  28  includes an outer surface  36  and extends from a first end  40  to a second end  44 . According to various embodiments of the present invention, the coiled electrode  28  is formed from at least one conductive filar  46 . In some embodiments, the coiled electrode  28  is formed from a plurality of conductive filars  46 . 
     According to one exemplary embodiment of the present invention, as shown in  FIGS. 2A-2D , the coiled electrode  28  is formed from two conductive filars  46 ,  47  wound in parallel to define a plurality of turns  48  and a gap  52  between each turn. A polymeric filling  60  is disposed in at least some of the gaps  52  existing between each of the turns  48 . The coiled electrode  28  also includes a polymeric covering  64  disposed over the outer surface  36  of the electrode and extending from the first end  40  to the second end  44  of the electrode  28 . The polymeric covering  64  is bonded to the polymeric filling  60  disposed in the gaps  52 . According to other embodiments, the polymeric cover  64  may extend beyond the electrode  28  and cover at least a portion of the lead body  12  in addition to the electrode  28 . 
     The polymeric filling  60  can be disposed in some or all of the gaps between each of the turns  48  of the coiled electrode  28 . The gaps  52  between the conductive filars  46 ,  47  are sufficiently wide so as to receive the polymeric filling  60  disposed therein. Additionally, the gaps  52  are sufficiently wide to maintain flexibility of the electrode  28 . Flexibility is an important feature of coiled defibrillation electrodes. In general, the gap width can be represented by the following mathematical expression: 
       gap width=wire pitch−(no. of filars×filar diameter) 
     The wire pitch is the distance in the longitudinal direction that a single filar covers in one rotational wind. According to one embodiment, a width of the gaps  52  ranges from about 0.0002 to about 0.020 inches. According to another embodiment, the gap width is about 0.0010 inches. 
     In some embodiments, as shown in  FIG. 2A , the polymeric filling  60  is disposed in substantially all of the gaps  52  extending from the first end  40  to the second end  44  of the electrode  28 . In other embodiments, as shown in  FIGS. 2B and 2C , the polymeric filling  60  is disposed in a portion of the gaps  52  located at either the first end  40  ( FIG. 2B ) or the second end  44  ( FIG. 2C ) of the electrode  28 . In still other embodiments, as shown in  FIG. 2D , the polymeric filling  60  is disposed in a portion of the gaps  52  located at both the first end  40  and the second end  44  of the electrode  28 . In this particular example, the polymeric filling  60  is not disposed in the gaps  52  in the middle portion  62  of the electrode  28 . 
     According to some embodiments of the present invention, the polymeric filling  60  includes one or more layers of a polymeric film  66 .  FIG. 3A  is a longitudinal cross-sectional view of a bifilar coiled electrode  28  including multiple layers of a polymeric film  66  disposed in the gaps  52  between the conductive filars  46 ,  47 . The polymeric film  66  has a width equal to or less than the width of the gap  52  between each of the turns  48  of the coiled electrode  28 . The polymeric film  66  is wound into the desired gaps  52  of the coiled electrode  28  such that the gaps  52  are substantially filled with the polymeric film  66 . As shown in  FIG. 3A , the filling  60  extends in the gap  52  between each conductive filar (or group of filars) from essentially a top surface  68  to a bottom surface  70  of the coiled electrode  28 . Multiple layers of the polymeric film  66  may be necessary to substantially fill the desired gaps  52 . The film  66  can be wrapped about the electrode  28  such that at least some of the gaps  52  are filled, as shown in  FIGS. 2A-2D . 
     According to other embodiments of the present invention, the polymeric filling  60  includes a non-conductive, non-porous polymeric filar  72 .  FIG. 3B  is a longitudinal cross-section of a bifilar coiled electrode  28  including two conductive filars  46 ,  47  and a non-conductive filar  68 . In one embodiment, the polymeric filar  72  is co-wound with the conductive filars  46 ,  47  during fabrication of the coiled electrode  28 . In another embodiment, the polymeric filar  72  is wound into the desired gaps  52 , between the conductive filars  46 , as shown in  FIGS. 2A-2D , after the electrode  28  has been fabricated. The polymeric filar  72  substantially fills the desired gaps  52  between the conductive filars  46 . As best shown in  FIG. 3B , the polymeric filar  72  is wound such that a longitudinal cross-section of the coiled electrode  28  shows a repeating pattern of a first conductive filar  46   a  directly adjacent to the second conductive filar  46   b . The polymeric filar  72  is directly adjacent to the second conductive filar  46   b.    
     As shown in  FIGS. 2A-2D , the polymeric cover  64  is disposed over the outer surface  36  of the coiled electrode  28  from substantially the first end  40  to the second end  44  of the electrode  28 . In certain embodiments, the polymeric cover  64  may extend beyond one or both ends  40 ,  44  of the electrode  28  and over at least a portion of the lead body  12 . According to various embodiments, the polymeric cover  64  may include one or more layers of a thin polymeric film. In some embodiments, the polymeric cover  64  can include as many as 120 layers of a thin polymeric film  74 . The resulting polymeric cover  64  can have a thickness ranging from about 1 to about 25 microns. 
     The polymeric film  74  may be wrapped about the outer surface  36  of the electrode  28  to form the polymeric cover  64  according to a variety of methods. An exemplary film wrapping process is shown and described in U.S. Pat. No. 7,020,529 entitled “Defibrillation Electrode Cover” the description of which is incorporated herein by reference.  FIG. 4A  is a partial schematic view of a lead body  12  including a coiled electrode  28  having a helically wrapped polymeric cover  64   a .  FIG. 4B  is a partial schematic view of a portion of a lead body  12  including a coiled electrode  28  having a cylindrically wrapped polymeric cover  64   b.    
     According to various embodiments of the present invention, the polymeric filling  60  and the polymeric cover  64  can be fabricated from structurally similar polymers having different material properties. According to various embodiments, the polymeric filling  60  is formed from a first polymeric material having a first set of material properties and the polymeric cover  64  is formed from a second polymeric material having a second set of material properties. The first polymeric material used to fabricate the filling  60  may differ in dielectric strength, porosity, and/or linear strength from the second polymeric material used to form the polymeric cover  64 . In some embodiments, for example, the polymer filling  60  includes a polymer of a higher dielectric strength than the polymer used to form the polymer cover  64 . In other embodiments, the polymer filling includes an essentially non-porous polymeric material or a polymeric material having a low degree of porosity and the polymer cover includes a porous polymeric material. In certain embodiments, the porous polymeric material has sufficient porosity to promote conductivity. 
     According to other embodiments of the present invention, the polymer filling  60  includes a non-expanded polymer and the polymer cover  64  includes an expanded polymer. The non-expanded polymer used to form the filling  60  has a higher dielectric strength than the expanded version of the same polymer. Additionally, the non-expanded polymer is essentially non-porous or has a lower porosity than the expanded polymer. The non-porous characteristics of the non-expanded polymer makes it unable to support conductivity. In contrast to the non-expanded polymer, the expanded polymer has a degree of porosity that is large enough to support conductivity when wetted with an appropriate ionic fluid, but small enough to prevent tissue ingrowth. 
     According to one embodiment, the polymer filling includes a non-expanded version of the same polymer used to make the polymer cover. Varying forms of the same polymer, or two polymers with structurally similar chemical backbones bond well to one another. A polymeric cover  64  that is strongly bonded to the polymeric filling  60  may be less likely to shift during implantation of the electrode. Thus, the potential for a portion of the electrode becoming exposed during implantation can be minimized. Minimizing exposure of the coiled electrode prevents tissue ingrowth. The prevention of tissue ingrowth into the coiled electrode is an important factor in facilitating removal of the lead from the implanted location. 
     Suitable biocompatible polymers that can be used to fabricate the polymeric filling  60  and the polymeric cover  64  include non-expanded and expanded versions of the following exemplary polymers, included but limited to, the following: polyethylene (PE), polypropylene (PP), fluorinated ethylene propylene (FEP), ethylene-tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), or suitable biocompatible polymers known to those of skill in the art. 
     According to one embodiment, the polymeric filling  60  is fabricated from PTFE and the polymeric cover  64  is formed from expanded polytetrafluoroethylene (ePTFE). The PTFE used to form the filling can be essentially non-porous and thus serves as an insulator in the gaps between the coil turns. The ePTFE used to form the cover  64  can be fabricated such that is has a degree of porosity sufficient to support conductivity, but small enough to prevent tissue ingrowth. 
     According to various embodiments of the present invention, the polymeric cover  64  is bonded to the polymeric filling  60  disposed in the gaps  52  between the turns  48  of the coiled electrode  28 . In some embodiments, the polymeric cover is covalently bonded to the polymeric filling. The cover  64  may be bonded to the polymer filling  60  using a variety of methods including heat bonding, solvent bonding, or laser sintering. According to one embodiment, the cover  64  is sintered to the polymeric filling  60  using a laser, infrared (IR) gun, heat gun, or cover. 
     PTFE and ePTFE can be made to covalently bond to one another using surface modification techniques followed by using an adhesive tie-layer to covalently bond the two materials. In one embodiment, heat fusion can also be used to bond the ePTFE material to the PTFE material. In another embodiment, the surface of the conductive filar can be treated using plasma treatment techniques to provide a fluorocarbon containing coating. A fluorocarbon containing coating allows the fluoropolymer to flex in the same manner as the conductive filar. Exemplary fluorocarbon plasmas used to treat the surface of the conductive filar include, but are not limited to: fluoro ethylene propylene, perfluoropropane, and octafluorocyclobutane. The fluorocarbon containing coating provided on the surface of the conductive filar can be made to fuse with the fluorocarbon filling (e.g. PTFE, ePTFE, or another similar material) via heat fusion causing the polymer chains to physically interlock via Van der Waals interactions. This will enhance the adhesion between the plasma coated filar and the polymer filling. In yet another embodiment, the surface of the polymer filling is treated using chemical stripping using, for example, sodium naphthalene/argon or plasma etching to remove the fluorine groups from the polymeric material followed by applying a medical adhesive to a surface of the conductive filar and polymeric filling to bond the two materials together. 
       FIG. 5  is a flow chart of a method ( 100 ) used to fabricate a coiled electrode according to an embodiment of the present invention. First, a coiled electrode is formed by winding one or more conductive filars to form a coil (Block  110 ). The coiled electrode includes a plurality of turns and a gap existing between each turn. Each turn can include a single filar or a group of filars. In one embodiment, the coiled electrode is a bifilar coiled electrode with a gap existing between every two conductive filars. Next, according to one embodiment, at least a portion of the gaps are filled with a polymeric filling (Block  120 ). In certain embodiments, the polymeric filling includes a non-expanded polymer. In one embodiment, the non-expanded polymer is polytetrafluoroethylene. According to one embodiment, the gaps can be filled by wrapping a thin polymeric film, including a non-expanded polymer, into the gaps until the gaps are substantially filled. Multiple passes with a thin polymeric film may be required to substantially fill the gaps. According to another embodiment, a polymeric filar may be wound into the gaps. The polymeric filar should be sufficiently wide so as to substantially fill the gaps. After the gaps have been filled, a polymeric cover including one or more layers of a thin polymeric material, including an expanded polymer is wrapped about the outer surface of the coiled electrode (Block  130 ). In certain embodiments, the expanded polymer is expanded polytetrafluoroethylene (ePTFE). A helical wrap or a cylindrical wrap may be employed. Multiple layers of the polymeric film may be wrapped about the outer surface of the electrode to achieve a desired thickness. The cover is then bonded to the polymeric filling disposed in the gaps (Block  140 ). In certain embodiments, the cover is laser sintered to the polymeric filling disposed in the gaps. 
       FIG. 6  is a flow chart of a method ( 200 ) according to another embodiment of the present invention. According to this embodiment, a coiled electrode is formed including at least one conductive filar and a polymeric filar including non-expanded polymer. In certain embodiments, the coiled electrode includes two conductive filars. The conductive filar(s) and the polymeric filar are wound together simultaneously such that a longitudinal cross-section of the coiled electrode reveals a repeating pattern of a first conductive filar directly adjacent to a second conductive filar, directly adjacent to the polymeric filar (Block  210 ). After the electrode has been formed including the polymeric filar, a polymeric cover including one or more layers of a thin polymeric film including an expanded polymer is wrapped about the outer surface of the coiled electrode (Block  220 ). A helical wrap or a cylindrical wrap may be employed. Multiple layers of the polymeric film may be wrapped about the outer surface of the electrode to achieve a desired thickness. The cover is then bonded to the polymeric filling disposed in the gaps (Block  230 ). In certain embodiments, the cover is laser sintered to the polymeric filling disposed in the gaps. 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.