Abstract:
A vacuum mandrel for use in fabricating an implantable electrode comprises a hollow body member and a first groove provided radially on an outer surface of the hollow body member. The first groove is adapted to receive an implantable electrode and retain the electrode in place with a vacuum pressure during an elastomeric encapsulation of the electrode. The vacuum mandrel further comprises a vacuum port provided in the first groove.

Description:
BACKGROUND 
       [0001]    Many types of implantable medical devices, such as pacemakers, defibrillators, and vagus nerve stimulators, have leads connected to an electronics unit. The distal end of the lead typically comprises or is coupled to one or more conductive electrodes. Such electrodes are typically fragile and thus should be handled carefully by the implanting surgeon when attaching the electrode to the relevant tissue to be stimulated. Fabrication of such electrodes is often a painstaking, time-consuming process. 
       BRIEF SUMMARY 
       [0002]    A vacuum mandrel is disclosed that is used during the fabrication of an implantable conductive electrode. In accordance with at least one embodiment, the mandrel comprises a hollow body member having an axis, a first groove provided radially on an outer surface of the hollow body member, and a vacuum port provided in the groove. The first groove is adapted to receive the implantable electrode. In a particular embodiment, the first groove is a spiral groove around the outer periphery of the hollow body. By application of vacuum pressure through the hollow body member and the vacuum port, an electrode can be retained in place in the first groove while an elastomer, or another type of insulating material, is applied to the electrode. One or more second grooves, which are preferably longitudinal relative to the axis of the hollow body member, may be provided on opposing ends of the spiral groove to receive the elastomer and thereby form gripping members that an implantation surgeon can use when coupling the resulting electrode to a neural tissue such as a vagus nerve. 
         [0003]    In accordance with another embodiment, a method comprises locating a conductive electrode formed on end of a lead adjacent a first, preferably spiral, groove in a vacuum mandrel, retaining the conductive electrode in the first groove through vacuum pressure applied through the at least one port, and applying an insulator over the conductive electrode while the electrode is retained in place in the first groove. The first groove contains at least one port through which the vacuum is applied to retain the conductive electrode in place. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0005]      FIGS. 1   a - 1   c  show a vacuum mandrel in accordance with a preferred embodiment of the invention; 
           [0006]      FIG. 2  shows an enlarged portion of the vacuum mandrel; 
           [0007]      FIG. 3  shows a side of the vacuum mandrel opposite from that shown in  FIGS. 1 and 2 ; 
           [0008]      FIG. 4  shows a ribbon electrode provided on a lead; 
           [0009]      FIG. 5  shows a view of the vacuum mandrel with an electrode wrapped around a groove formed in the mandrel in accordance with a preferred embodiment of the invention; 
           [0010]      FIG. 6  illustrates the connection between the vacuum mandrel and a vacuum source; 
           [0011]      FIG. 7  shows a lead with coil electrode formed thereon and elastomer formed on a portion of the electrode in accordance with a preferred embodiment of the invention; 
           [0012]      FIG. 8  shows a completed electrode assembly in accordance with embodiments of the invention; and 
           [0013]      FIG. 9  shows an enlarged portion of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. Any numerical dimensions provided herein are merely exemplary and do not limit the scope of this disclosure or the claims that follow. 
         [0015]      FIG. 1   a  shows a plan view of a vacuum mandrel  10  in accordance with a preferred embodiment of the invention. The mandrel  10  comprises an elongate hollow body member formed from stainless steel or tool steel and has an exterior finish of nickel plating with Teflon®. As illustrated  FIG. 1   b,  the mandrel  10  is generally circular in cross section, although other cross sectional shapes are possible as well. As shown, the vacuum mandrel  10  comprises a plurality of segments  12 ,  14 , and  16 . Segment  12  has a diameter D 3  that, as is evident from  Figure 1   a,  is greater than the diameter D 1  of segment  16 . In accordance with at least one embodiment, D 1  is approximately 0.171 inches and D 3  is approximately 0.50 inches. The length L 1  of segment  12  preferably approximately is 2.094 inches and the length L 3  of segment  16  preferably is approximately 0.875 inches. Segment  12  includes a tip portion  11  provided at one end as shown in  Figure 1   a.  Tip portion  11  has a diameter D 2  of approximately 0.312 inches and a length L 4  of approximately 0.625 inches. Segment  14  has a generally frustoconical shape that transitions between segments  12  and  16 . The slope of segment  14  preferably is at an angle A 1  of approximately 60 degrees as shown in  Figure 1   a,  and the length L 2  of segment  14  is approximately 0.281 inches. Mandrel  10  may be formed as a unitary hollow body or in multiple pieces that are joined together in a suitable manner (e.g., by welding). 
         [0016]    Segment  16  includes a first, preferably spiral (e.g., helical) groove  20  formed radially about the outer surface of segment  16  as shown in  FIG. 1   a.    FIG. 2  shows an enlarged view of segment  16  of the vacuum mandrel. In the embodiments of  FIGS. 1 and 2 , the first groove  20  comprises a spiral groove having about 2.5 revolutions around the outer surface of the mandrel. It will be appreciated by persons of skill in the art that non-spiral grooves can be used, and that where a spiral groove is employed the number of revolutions of the groove can be varied. The first groove  20  can have any dimensions that are suitable for the application described herein. In one embodiment, the groove  20  has a generally flat bottom surface  21  and flat side walls  23  formed generally perpendicular to the bottom surface  21 . In this embodiment, the groove has a rectangular cross-sectional shape, with a width W 1  of approximately 0.053 to 0.058 inches and a height H 1  of approximately 0.024 to 0.028 inches. 
         [0017]    As best shown in  FIG. 2 , a raised surface  25  is provided within groove  20 . The raised surface  25  preferably is formed integrally with the segment  12  although, in other embodiments, the raised surface can be a separately formed component that is then adhered in a suitable manner (e.g., welded, glued, etc.) to surface  21 . The raised surface  25  preferably has a height relative to the bottom surface  21  of approximately 0.002 to 0.003 inches and a width W 2  that is less than the width W 1  of the groove  20 . In one embodiment, the width W 2  of the raised surface  25  may range from approximately 0.028 to 0.032 inches. In such embodiments, therefore, the width W 2  of the raised surface is approximately 45% to 60% of the width W 1  of the first groove  20 . At least one vacuum port  22  is provided within the first groove  20 . In accordance with one embodiment, eight vacuum ports  22  are provided, although any number of ports sufficient to retain an electrode in place in the first groove may be employed. Preferably, the vacuum ports  22  comprise bore holes that extend through the raised surface  25  and into the hollow interior portion of segment  16 . In at least one embodiment, the eight radially extending vacuum ports  22  are spaced apart in increments of 30 degrees. 
         [0018]    Referring to  Figure 1   a , segment  12  of mandrel  10  comprises a lead groove  36  provided longitudinally along at least some or all of the length of segment  12 . Referring to  FIGS. 1   a  and  1   c , the lead groove  36  that is provided longitudinally along segment  12  relative to a plane that contains central axis  19  preferably is formed in the outer surface of the segment  12  at a location disposed on an angular measure A 2  from the plane. In one embodiment, the lead groove  36  preferably has a depth D 8  of approximately 0.089 inches and a width W 3  of approximately 0.063 inches. As will be explained above, an implantable lead resides in the lead groove  36  during fabrication of an electrode. Thus, the lead groove  36  has an engaging surface that engages the electrode during manufacturing. 
         [0019]    The interior of the hollow body member is shown in  FIG. 1   b.  In the embodiment depicted, the dimensions of the hollow interior of the body vary, although persons of skill in the art will appreciate that various hollow chamber designs may be employed. The hollow interior of segment  12  is shown with a diameter D 4  of approximately 0.19 inches. The hollow interior of segments  14  and  16  has a diameter D 5  of approximately 0.06 inches. The hollow interior of segment  16  extends to the end of, or just beyond the end of, the first groove  20 . The hollow interior may also extend throughout the hollow body and be sealed off via a plug fastened by various techniques such as welding, screw threads, or adhesive. 
         [0020]      FIGS. 1   a  and  2  also show one or more second grooves  30 ,  32  formed at, or near, opposing ends of first groove  20 . Second grooves  30  and  32  may advantageously have a different cross-sectional shape than groove  20 . Each second groove  30 ,  32  preferably is curved and extends circumferentially preferably for less than one complete revolution around segment  16  and, in some embodiments, extends for three-fourths of one revolution. As better shown in  FIG. 3 , each second groove  30  and  32  ends in a longitudinal groove portion  31  and  33 , respectively. Each second groove  30 ,  32  may have a pitch of approximately from 0.65 to 0.70, a radius of curvature of approximately 0.020 inches and a depth D 9  (shown in  FIG. 2 ) of approximately 0.018 inches. Each of the longitudinal groove portions  31  and  33  may have a length D 7  ( FIG. 3 ) of approximately 0.074 inches. Further, each second groove  30 ,  32  begins at or near an end of the first groove  20 . For example, second groove  32  begins at end  37  ( FIG. 2 ) of first groove  20 . The beginning of each second groove  30 ,  32  preferably is centered within first groove  20  thereby forming a continuous groove within the body segment  16 . In one embodiment, each second groove  30  and  32  preferably comprises a spiral curved groove that extends for three-fourths of a revolution and the central flat-bottom groove  20  (having bottom surface  21 ) extends for 2.5 revolutions. Thus, the combination of the two spiral curved second grooves  30  and  32  and the central flat first groove  20  form a groove that extends for, in a particular embodiment, four total revolutions around the body segment  16 . 
         [0021]    The vacuum mandrel  10  may be used during a manufacturing process for an electrode. The electrode preferably comprises an electrode such as may be used in conjunction with an implantable medical device such as a vagus nerve stimulator. 
         [0022]      FIG. 4  shows an electrode  52  provided on an end of lead  50 . The electrode  52  preferably is an electrically conductive ribbon electrode that, when further processed as described below, can be attached to a nerve or nerve bundle such as a cranial nerve (e.g., a vagus nerve). The electrode  52  preferably comprises a conductive ribbon electrode formed from platinum, platinum-iridium, or other suitable material. In a particular embodiment, the electrode  52  is approximately 0.040 inches wide by approximately 0.500 inches long by approximately 0.0005 inches thick. The electrode  52  is coupled (e.g., welded) to the lead  50  at approximately the mid-point of the electrode. The lead  50  comprises an electrical conductor that is covered by an insulator and that electrically couples the implanted device (not shown) to electrode  52  provided on the end of the lead. When implanted, the electrode  52  is placed in contact with the tissue to be stimulated. Through the lead  50  and electrode  52 , the implanted device is able to deliver electrical current to the tissue to be stimulated. 
         [0023]      FIG. 5  shows the body segment  16  of the vacuum mandrel  10  with the lead  50  and electrode  52  disposed thereon. The electrode  52  is placed on the mandrel and wrapped around and located within at least a portion of the first groove  20 . By residing in the first groove  20 , the ribbon electrode covers at least one or more, and preferably all, of the vacuum ports  22 .  FIG. 5  also shows the lead  50  extending down the length of the segment  16 . The remaining length of the lead rests in the longitudinal lead groove  36  formed in segment  12  ( FIG. 1   a ). 
         [0024]      FIG. 6  illustrates the vacuum mandrel  10  coupled to a vacuum tube  62  that, in turn, connects to a vacuum source  60 . The vacuum tube  62  preferably comprises a flexible hose of rubber or other suitable material. The tip  11  of the vacuum mandrel is inserted into the vacuum tube  62 . Once the vacuum source  60  is turned on, the vacuum pressure thereby created through the mandrel  10  and vacuum ports  22  will cause the ribbon electrode to be retained in place during the next part of the manufacturing process. The raised surface  25  provides an engaging surface for the electrode in groove  20  and enables the electrode edges to be encapsulated by the elastomer/insulator. In one embodiment, the vacuum pressure is approximately 28 inches Hg, although the pressure can be varied as desired. 
         [0025]    With the electrode held in place in first groove  20  by vacuum pressure, the next step in the manufacturing process is to apply an insulator such as an elastomer to all, or substantially all, of the lengths of first groove  20 , and second grooves  30  and  32 , thereby covering the ribbon electrode with the insulator. The ribbon electrode  52  preferably does not extend throughout the combined lengths of first groove  20  and second grooves  30  and  32 , and as such a portion of the insulator fills the grooves beyond the reach of the ribbon electrode. The insulator is applied by spraying or pouring by methods well known in the art. In a particular embodiment, the insulator comprises a silicone elastomer. However, persons of skill in the art will appreciate that other elastomers, and other insulators may be used. 
         [0026]    The insulator is then permitted to cure. Once cured, vacuum source is turned off and the lead  50  and insulator-covered electrode  52  assembly can be removed from the vacuum mandrel. Examples of the completed electrodes  52  are shown in  FIGS. 7 and 8 .  FIG. 7  shows an electrode with a cut away portion to better illustrate the elastomer  70  covering the ribbon electrode  52 . Because the elastomer cured while the electrode  52  was still wrapped in the spiral first groove  20  of the vacuum mandrel, the resulting electrode generally retains the shape of the first groove  20 . Other shapes are, of course, possible depending upon the needs of the particular application in which the electrode will be used. Further, because the elastomer covered the exposed electrode  52  and was not able to penetrate between the electrode  52  and the raised surface  25  of the first groove  20 , one side of the electrode is not covered with elastomer, i.e., the interior surface  71  of the spirally formed electrode. This interior surface is the surface that will be in contact with the body tissue (e.g., a vagus nerve) being stimulated. The elastomer generally is an electrical insulator and thus the surface of the electrode opposite the body tissue is electrically insulated from other body tissues while the surface of the electrode touching the nerve is in electrical contact with the nerve. 
         [0027]    The raised surface  25  on which the ribbon electrode rests while the elastomer is applied causes elastomer to fill the sides of the first groove  20  adjacent the electrode. As a result, some of the elastomer, such as that shown at reference numeral  90 , covers the side edges of the ribbon electrode and thereby covers any sharp edges that might otherwise cut into the nerve to which the electrode is attached. The relationship between the elastomer and the edges of the electrode are better shown in the enlarged view of  FIG. 9 . 
         [0028]    Reference numeral  80  in  FIG. 7  shows the elastomer that was applied to the spiral second grooves  30  and  32 . The width of second grooves  30  and  32  preferably is less than the width of first groove  20  as measured in the direction parallel to axis  19 . As such, as shown in  FIG. 7 , the two elastomer end portions  80  of the electrode assembly are narrower than the central portion that contains the ribbon electrode. 
         [0029]    The longitudinal groove portions  31  and  33  ( FIG. 3 ) are also filled with elastomer. After the elastomer cures and the electrode is removed from the mandrel, the elastomer that filled the longitudinal groove portions  31  and  33  form gripping portions  74  and  76 , respectively. The gripping portions  74  and  76  are used during implantation to attach the electrode to the nerve. More specifically, the gripping portions  74  and  76  are pulled in opposite directions using, for example, forceps. Pulling the gripping surfaces  74  and  76  apart in this manner stretches the spiral electrode so that it can be wrapped around the nerve. Once wrapped around the nerve, the gripping portions  74  and  76  are released and the spring-like nature of the spiral electrode  52  causes the electrode naturally to attach itself to the nerve. 
         [0030]    Another prior type of spiral electrode included a thread suture embedded in the elastomer. The ends of the suture protruded from the electrode and functioned as gripping mechanisms for the implantation surgeon. Unfortunately, embedding a thread suture in a spiral electrode adds complexity and time to the manufacturing process of such an electrode. Gripping portions  74  and  76  obviate the need for such a thread suture, although one could be included if desired. Without such a thread suture, the manufacturing of the disclosed electrode is made easier and less time consuming. 
         [0031]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.