Implantable Lead

This invention relates to the field of implantable electrical leads for use with various implantable electrical devices such as cardiac pacers and defibrillators.

Conventional implantable leads for use with implantable electrical devices such as cardiac pacemakers and defibrillators are typically constructed of a helically wound conductor having an outer insulation layer of tubular form surrounding the wire helix. The tubular insulation is most commonly of an elastomeric material such as silicone or polyurethane. The combination of a helically wound conductor with elastomeric outer insulation provides these conventional constructions with a substantial amount of potential elastic deformation in the direction of the length of the lead.

The fundamental requirements of implantable leads are that they must have excellent mechanical integrity, electrical insulating properties and biocompatibility, and must be flexible with a long flex life to accommodate attachment to a beating heart.

Conventional implantable leads have several disadvantages. The silicone or polyurethane outer coverings are not ideally biocompatible and are frequently known to provoke adverse tissue reactions over time. Polyurethane outer coverings are known to crack under stress. Silicone outer coverings are vulnerable to abrasion over time. Additionally, these conventional leads are known to break during attempts to remove them from implanted patients by the application of a tensile force. In these cases the remaining portion must be abandoned within the patient's body or must be surgically removed.

Implantable lead wires using insulation materials other than the conventional silicones or polyurethanes have been described previously. U.S. Pat. No. 4,573,480 describes an implantable electrode lead in the form of a helically wound conductor having a tubular insulating layer surrounding the helically wound wire wherein the tubular insulating layer is porous polytetrafluoroethylene (hereinafter PTFE) having a pore size limited to a maximum size described as "being essentially impervious to body fluids to prevent tissue growth thereon." This pore size is described as being not larger than 4 microns. While pore sizes of this range and smaller are known to preclude cellular ingrowth, the material remains pervious to body fluids which will wet out such an insulating layer shortly after implantation. The result is that the effectiveness of the electrical insulation is destroyed. Alternatively, this patent teaches that the tubular porous PTFE insulating layer may be provided with an outer covering of smooth and impervious material. While this alternative construction prevents the wetting out of the porous PTFE layer by body fluids, it loses the biocompatible advantage provided by the tissue contacting outer surface of porous expanded PTFE.

The present invention is an implantable lead having improved tensile strength, excellent flexibility, excellent insulating characteristics, improved biocompatibility and controlled amounts of elongation during the application of tension to the lead. The amount of elongation may be varied over different portions of the length of the lead.

The implantable lead comprises a helically wound electrical conductor having a layer of elastomeric insulating tubing coaxially surrounding the helix formed by the helically wound conductor. The layer of elastomeric insulating tubing is in turn coaxially surrounded by a layer of porous PTFE tubing having a microstructure of nodes interconnected by fibrils. At least a portion of the length of the porous PTFE may be under longitudinal compression whereby the fibrils of the microstructure within that portion of the length have a substantially bent and wavy appearance. Consequently this portion of the length may be stretched or extended until the fibrils of the microstructure are straightened as a result of tension applied to that portion of the lead. The entire length of a lead may be provided with an exterior covering of longitudinally compressed porous PTFE tubing if desired.

The term coaxial is herein used in relationship to the longitudinal axis of the helix formed by the at least one helically wound conductor.

The porous PTFE outer covering provides the lead wire with excellent flexibility, biocompatibility and tensile strength while simultaneously providing the lead with a controlled amount of extensibility.

The fibril length of the porous PTFE must be adequate to provide the necessary amount of flexibility and extensibility for the intended application and preferably should be of adequate size to present an acceptable biocompatible surface to the blood chemistry to which the outer surface of the lead will be exposed. The preferred fibril lengths are greater than about 4 microns and most preferably greater than about 10 microns. Fibril length is measured as taught by U.S. Pat. No. 4,972,846.

The layer of tubular elastomeric insulation surrounding the helically wound conductor provides excellent insulation because these materials are substantially impervious to body fluids. They are disadvantageous as the exterior surface of a lead because they are not ideally biocompatible and are known in some patients to provoke adverse tissue reactions. The additional exterior covering of porous PTFE tubing of the lead of the present invention provides a superior biocompatible surface. While this porous exterior covering is likely to be wet out by body fluids and is consequently inadequate by itself as an effective electrical insulation material for purposes of implantation in living bodies, the combination of a porous PTFE exterior covering surrounding a layer of impervious elastomeric insulating tubing offers a superior biocompatible lead insulation.

The at least one helically wound conductor wire used with the present invention may be a single conductor or alternatively may be multiple-filar if more than one conductor is required for a desired application.

The porous PTFE having a microstructure of nodes interconnected by fibrils used for the exterior tubular covering of the inventive lead is made according to the teachings of U.S. Pat. No. 4,187,390 and 3,953,566. The tubular covering of porous PTFE may be provided with bent or wavy fibrils resulting in stretch characteristics in a similar manner to that taught by U.S. Pat. Nos. 4,877,661 and 5,026,513.

FIG. 1 describes a cross section of the implantable lead of the present invention wherein a helically wound electrical conductor is coaxially covered by a layer of tubular elastomeric insulation which is in turn coaxially covered by an exterior tubular layer of porous PTFE having a microstructure of nodes interconnected by fibrils.

FIG. 2 describes a cross section of the implantable lead of one embodiment of the present invention wherein the exterior tubular layer of porous PTFE is shown under longitudinal compression wherein the fibrils of the microstructure have a bent and wavy appearance.

FIG. 3 describes a cross section of the implantable lead of one embodiment of the present invention wherein the exterior tubular layer of porous PTFE is shown under tension wherein the fibrils of the microstructure are substantially straight.

FIG. 1 describes a cross section of the implantable lead 10 of the present invention wherein a helically wound electrical conductor 12 is coaxially covered by a tubular layer of an elastomeric electrically insulating material 14 such as silicone or polyurethane. The lead is further provided with an exterior coaxial tubular covering of porous PTFE 16 having a microstructure of nodes interconnected by fibrils.

Elastomeric materials are herein defined as polymeric materials which at room temperature can be stretched under low stress to at least twice their original length and, immediately upon release of the stress, will recover with force to their approximate original length.

FIGS. 2 and 3 describe cross sections of a preferred embodiment wherein FIG. 2 shows the lead in a relaxed state and FIG. 3 shows the lead under longitudinal tension. As shown by FIG. 2, if the exterior covering of porous PTFE tubing is fitted around the tubular elastomeric insulating layer so that the porous PTFE tubing is under some degree of longitudinal compression, the fibrils 22 that interconnect the nodes 24 within the microstructure of the porous PTFE 10 tubing 16 will assume a bent and wavy appearance due to the longitudinal compression applied to the porous PTFE tubing 16 when it is fitted over the elastomeric tubing 14 during construction of the lead.

When longitudinal tension is applied to the lead as shown by FIG. 3, the spring characteristic of the helically wound conductor 12 and the elastic characteristic of the elastomeric tubing 14 allow those components of the lead construction to extend longitudinally. Additionally, the bent fibrils 22 of the porous PTFE tubing 16 as shown previously in FIG. 2, are able to extend until they become straight and taut fibrils 26 as a result of the applied tension, as shown by FIG. 3. It is apparent that the amount of extension of the porous PTFE tubing is a function of the amount of longitudinal compression applied to the porous PTFE tubing during construction of the lead and the resulting amount of bending that is applied to the fibrils.

The exterior tubular covering of porous PTFE 16 is fitted over the exterior surface of the elastomeric tubing 14. The relationship between the inside diameter of the porous PTFE tubing and the outside diameter of the elastomeric tubing should be such that there is at most only a small amount of interference when these respective diameters are measured with the tubes in relaxed states with no tension applied. Preferably there is no interference between these respective diameters.

The porous PTFE tubing 16 is most easily fitted over the elastomeric tubing 14 prior to inserting the helically wound conductor into the bore of the elastomeric tubing. This avoids any risk to the conductor. For constructions involving a slight interference fit between the respective diameters of the two tubes, the elastomeric tubing 14 may be placed under tension adequate to cause a significant reduction in its outside diameter before fitting it into the exterior porous PTFE tubing. This tension is preferably applied with a pull-wire attached to the ends of the bore of the elastomeric tubing, thereby not interfering with access to the exterior surface of the elastomeric tubing. The exterior tubular covering of porous PTFE is then fitted over the reduced diameter of the tensioned elastomeric tube.

After the exterior tubular covering of porous PTFE has been fitted over the tensioned elastomeric tubing, the tension on the elastomeric tubing is released. The diametrical interference between the porous PTFE tubing and the elastomeric tubing results in the securing of the two tubes together. Alternatively, a biocompatible adhesive such as a silicone adhesive may be used during the construction process.

The porous PTFE tube is longitudinally compressed according to the amount of extensibility, if any, that is desired of the completed implantable lead. The longitudinal compression can be uniformly applied over the chosen length of porous PTFE tubing or alternatively the compression can be applied non-uniformly if it is desired to have different extensibility characteristics over different parts of the lead.

The helically wound conductor is preferably fitted into the bore of the elastomeric tubing after completion of the insulation construction. The at least one helically wound conductor wire used with the present invention may be a single conductor or alternatively may be multiple-filar if more than one conductor is required for a desired application. Multiple conductors will require that the individual conductors be separately insulated from each other, preferably by a layer of insulation covering the surface of each individual conductor. In the case of a single helically wound conductor, the conductor may optionally be separately insulated within the two-layer coaxially oriented insulation of the present invention. Insulation covering the surface of helically wound conductors is preferably applied to each conductor surface prior to helical winding of the conductors.

For cardiac pacemaker and defibrillator lead applications, the conductor is preferably a MP35N nickel alloy stainless steel material.