Implantable medical device conductor insulation and process for forming

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10/407,653 (Attorney docket P10722.00) filed on Apr. 4, 2003 and entitled “IMPLANTABLE MEDICAL DEVICE CONDUCTOR INSULATION AND PROCESS FOR FORMING”, which claims priority and other benefits from U.S. Provisional Patent Application Ser. No. 60/371,995, filed Apr. 11, 2002, entitled “BIO-STABLE IMPLANTABLE MEDICAL DEVICE LEAD CONDUCTOR INSULATION AND PROCESS FOR FORMING”, both of which are incorporated herein by reference in their entireties.

[0002] The present invention relates generally to implantable medical device leads for delivering therapy, in the form of electrical stimulation, and in particular, the present invention relates to conductor coil insulation in implantable medical device leads.

[0003] Implantable medical electrical leads are well known in the fields of cardiac stimulation and monitoring, including neurological stimulation and cardiac pacing and cardioversion/defibrillation. In the field of cardiac stimulation and monitoring, endocardial leads are placed through a transvenous route to position one or more sensing and/or stimulation electrodes in a desired location within a heart chamber or interconnecting vasculature. During this type of procedure, a lead is passed through the subclavian, jugular, or cephalic vein, into the superior vena cava, and finally into a chamber of the heart or the associated vascular system. An active or passive fixation mechanism at the distal end of the endocardial lead may be deployed to maintain the distal end of the lead at a desired location.

[0004] It is highly desirable that implantable leads have the lowest possible profile while the insulation maintain sufficient integrity to electrically isolate one or more conductors of the leads over the life of the implanted lead.

[0005] The following drawings are illustrative of particular embodiments of the invention and therefore do not limit its scope, but are presented to assist in providing a proper understanding of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements, and:

[0006] FIG. 1 is a schematic diagram of an exemplary implantable medical device in accordance with one embodiment of the present invention;

[0007] FIG. 2 is a cross-sectional view of a lead of the exemplary device taken along cross-sectional lines II-II of FIG. 1;

[0008] FIG. 3 is a cross-sectional view of the lead of the exemplary device taken along cross-sectional lines II-III of FIG. 1;

[0009] FIG. 4 is a cross-sectional view of a coiled wire conductor forming a filar of a multi-filar conductor coil according to one embodiment of the present invention;

[0010] FIG. 5 is a cross-sectional view of a coiled wire conductor forming a filar of a multi-filar conductor coil according to another embodiment of the present invention; and

[0011] FIG. 6 is a cross-sectional view of an exemplary cabled wire conductor according to yet another embodiment of the present invention.

[0012] FIG. 1 is a schematic diagram of an exemplary implantable medical device in accordance with one embodiment of the present invention. As illustrated in FIG. 1, an implantable medical device 100 according to the present invention includes an implantable medical device lead 102 and an implantable medical device housing 104, such as an implantable cardioverter/defibrillator or pacemaker/cardioverter/defibrillator (PCD), for example, for processing cardiac data sensed through lead 102 and generating electrical signals in response to the sensed cardiac data for the provision of cardiac pacing, cardioversion and defibrillation therapies. A connector assembly 106 located at a proximal end 101 of lead 102 is insertable within a connector block 120 of housing 104 to electrically couple lead 102 with electronic circuitry (not shown) of housing 104.

[0013] Lead 102 includes an elongated lead body 122 that extends between proximal end 101 and a distal end 121 of lead 102. An outer insulative sheath 124 surrounds lead body 122 and is preferably fabricated of polyurethane, silicone rubber, a fluoropolymer or a combination thereof. Coiled wire conductors in accordance with one embodiment of the present invention are positioned within lead body 122, as will be described in detail below. Distal end 121 of lead 102 includes a proximal ring electrode 128 and a distal tip electrode 126, separated by an insulative sleeve 130. Proximal ring electrode 128 and distal tip electrode 126 are electrically coupled to connector assembly 106 by one or more coil conductors, or filars extending between distal end 121 and proximal end 101 of lead 102 in a manner shown, for example, in U.S. Pat. Nos. 4,922,607 and 5,007,435, incorporated herein by reference in their entireties.

[0014] FIG. 2 is a cross-sectional view of a lead of the exemplary device taken along cross-sectional lines II-II of FIG. 1. As illustrated in FIG. 2, lead 102 of implantable medical device 100 includes a quadrafilar conductor coil 200 including four individual filars, or coiled wire conductors 202A, 202B, 202C and 202 extending within insulative sheath 124 of lead body 122. Coiled wire conductors 202A-202D electrically couple proximal ring electrode 128 and distal tip electrode 126 with connector assembly 106. It is understood that although the present invention is described throughout in the context of a quadrafilar conductor coil, having each of two electrodes electrically coupled to a connector assembly via two of the four individual coiled wire conductors, the present invention is not intended to be limit to application in a quadrafilar conductor coil. Rather, the lead conductor insulator of the present invention can be utilized in any conductor configuration, including the use of any number of conductor coils depending upon the number of desired electrodes, and would include the use of a single filar electrically coupling the electrode to the connector. Furthermore, as illustrated in FIG. 6, a lead conductor according to an alternate embodiment of the present invention may be in the form of a cable 630 including a plurality of bundled wire strands 632-638.

[0015] FIG. 3 is a cross-sectional view of the lead of the exemplary device taken along cross-sectional lines III-III of FIG. 1. As illustrated in FIGS. 2 and 3, each of the individual filars or coiled wire conductors 202A, 202B, 202C and 202D are parallel-wound in an interlaced manner to have a common outer and inner coil diameter. As a result, conductor coil 200 forms an internal lumen 204, which allows for passage of a stylet or guide wire (not shown) within lead 102 to direct insertion of lead 102 within the patient.

[0016] Alternately, lumen 204 may house an insulative fiber, such as ultrahigh molecular weight polyethylene (UHMWPE), liquid crystal polymer (LCP), polyester and so forth, or an insulated cable (i.e. cable 630 illustrated in FIG. 6) in order to allow incorporation of an additional conductive circuit and/or structural member to aid in chronic removal of lead 102 using traction forces. Such an alternate embodiment would require insertion and delivery of lead 102 to a final implant location using alternate means, such as a catheter, for example. Lumen 204 may also include an insulative liner (not shown), such as a fluoropolymer, polyimide, PEEK, for example, to prevent damage caused from insertion of a style/guidewire (not shown) through lumen 204.

[0017] FIG. 4 is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to some embodiments of the present invention. As illustrated in FIG. 4, one or more of the individual coiled wire conductors 202A, 202B, 202C and 202D includes a conductor wire 210 surrounded by an insulative layer 212. According to the present invention, insulative layer 212 is formed of a hydrolytically stable polyimide, such as a Soluble Imide (SI) polyimide material, for example, (formerly known as Genymer, Genymer SI, and LaRC™ SI) as described in U.S. Pat. No. 5,639,850, issued to Bryant, and incorporated herein by reference in its entirety, to insulate conductor coils in implantable medical device leads. Such SI polyimide material is currently commercially available through license from NASA, for example. The thickness of the insulative layer 212 ranges from approximately 0.0001 inches up to approximately 0.0050 inches, forming a corresponding wall thickness W of the insulative layer 212. By utilizing the hydrolytically stable polyimide material as an insulative layer 212, the present invention provides an improved electrically insulating material that is hydrolytically stable in implantable (in vivo) applications.

[0018] According to one embodiment of the present invention, the insulative layer 212 is applied onto the conductor wire 210 in multiple coats, that is, layer 212 is comprised of multiple layers of a hydrolytically stable polyimide resulting in a desired wall thickness W. The coating is applied in such a way to provide a ductile, robust insulative layer that enables a single filar, i.e., coiled wire conductor, or multiple filar, i.e., coiled wire conductors, to be wound into a single wound conductor coil 200 of sizes ranging from an outer diameter D (FIG. 3) of 0.010 inches to 0.110 inches. For example, the coating process includes a solvent dip followed by an oven cure cycle to drive off the solvents and column 7, line 63 to column 8, line 14 of U.S. Pat. No. 4,056,651, which is incorporated herein by reference, describes a coating procedure which may be employed to manufacture embodiments of the present invention. According to an exemplary embodiment, wire 210, having a diameter between approximately 0.003 inch and approximately 0.005 inch, after being cleaned with an alkaline solution, undergoes 32 coating passes resulting in wall thickness W of approximately 0.0005 inch. For this embodiment the inventors have found that, in order to assure an adequate toughness and flexibility of each imidized coating layer, that is to prevent cracking upon subsequent processing of the coated wire, each layer should be exposed to a high enough temperature, for example an oven temperature between approximately 650° F. and approximately 850° F., for a sufficient time to drive off residual solvent. Thus, multiple coating passes forming insulative layer 212 on conductor wire 210 provides the ductility that is needed to make the coated conductor wire 210 into a conductor coil 200 that can withstand the long term flex requirements of an implantable lead. However, according to an alternate embodiment, one or more wire filars may be wound into a coiled configuration prior to applying a layer or layers of a hydrolytically stable polyimide. The inventors further contemplate spraying processes and extrusion processes known to those skilled in the art may also be employed to manufacture embodiments of the present invention.

[0019] The use of the hydrolytically stable polyimide insulative layer 212 according to embodiments of the present invention offers an exceptional dielectric strength for electrical insulation. Through flex studies on conductor coils coated with the SI polyimide, the inventors have found that the insulative layer 212 also has high flex properties in regards to stimulating lead conductor coil flex testing. The SI coating in various wall thicknesses will remain intact on the coil filar until the coil filar fractures as seen in conventional conductor coil flex studies (reference 10 million to 400 million flex cycles at various 90 degree radius bends).

[0020] Conductor coils 200 (FIG. 2) according to the present invention can include a single filar or multiple filars, with each filar being an individual circuit that could be associated with a tip electrode, a ring electrode, a sensor, and so forth. The present invention enables the use of multiple circuits in a single conductor coil, resulting in a downsizing of the implantable medical device. For example, there is approximately a 40 to 50 percent reduction in lead size between known bipolar designs, which traditionally utilized an inner coil and inner insulation, outer coil and outer insulation, to a lead design having multiple circuits in a single conductor coil having the insulative layer 212 according to the present invention.

[0021] Hydrolytically stable polyimides do not show a notable decrease in mechanical performance over time when immersed in an aqueous environment, such as an implant environment. Examples of polyimides considered to be hydrolytically stable may have the following general recurring structure:
Wherein AR is either AR1 or AR2 that represent different dianhydrides and wherein either AR1 or AR2 is represented by the following general formula including isomeric variations thereof:
Wherein X can be represented by CH₂, CH₃—C—CH₃, O (Oxygen), C═O (carbonyl), S (sulfide), SO₂ (sulfonyl), CF₃—C—CF₃ (hexafluoropropane derivative), or no element (e.g., 3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA)) and wherein AR3 is a diamine and can be represented by the following formula:
including, as shown below, isomeric variations thereof:
Wherein Y and Z can be represented by CH₂, CH₃—C—CH₃, O, C═O, S, SO₂, or CF₃—C—CF₃. Similar to the dianhydride (e.g., AR1, AR2), the polyimide may be composed of one or more diamines (AR3) or combinations of the above structures. The resultant polyimides may be endcapped by a number of chemicals know to the industry (e.g., phthalic anhydride) and the polyimide or the polyamic acid precursor may be supplied in a variety of solvents known to those in the industry (e.g., N,N dimethylacetamide (DMAc), dimethyl foramide (DMF), N-methylpyrrolidinone (NMP)). The hydrolytically stable polyimide may utilize mole ratios of the dianhydrides and may possess offsets (excess of diamine to dianhydride) similar to those known to the industry. The polyimides may also be further modified by incorporating specialized constituents such as crosslinking agents (e.g., nadic groups), fluorine containing groups (e.g., CF₃, SF₅, hexafluoropropane), or processing aids commonly known to those in the industry. Examples of hydrolytically stable polyimides suitable for embodiments of the present invention are:

[0026] 1. LaRC™ SI, wherein:

• • [00027] AR1: X is O (4,4′-oxydiphthalic anhydride, ODPA)
  • [00028] AR2: X is no element (3,4,3′,4′-biphenyltetracarboxylic dianhydride, BPDA)
  • [00029] AR3: Z is O (3,4′-oxydianiline, ODA)

[0030] 2. A polyimide described in NASA technical report #NAS 1.71:LAR-15109-1 (published Aug. 31, 1994), wherein:

• • [00031] AR1: X is O (4,4′-oxydiphthalic anhydride, ODPA)
  • [00032] AR2: X is C═O (3,3′,4,4′- benzophenonetetracarboxylic dianhydride, BTDA)
  • [00033] AR3: Z is O (3,4′-oxydianiline, ODA)

[0034] 3. A polyimide described in U.S. Pat. No. 5,171,828, wherein:

• • [00035] AR1: X is O (4,4′-oxydiphthalic anhydride, ODPA)
  • [00036] AR2: X is no element (3,4,3′,4′-biphenyltetracarboxylic dianhydride, BPDA)
  • [00037] AR3: Y is O (4,4′-oxydianiline, ODA)

[0038] 4. LaRC™ TPI, wherein:

[0039] AR: X is C═O (3,3′,4,4′- benzophenonetetracarboxylic dianhydride, BTDA)

• • [00040] AR3: Z is C═O (m-BDA)

[0041] FIG. 5 is a cross-sectional view of a coiled wire conductor forming a multi-filar conductor coil according to another embodiment of the present invention. The insulative layer 212 of hydrolytically stable polyimide according to embodiments of the present invention can be utilized as a stand-alone insulation on a filer or as an initial layer of insulation followed by an additional outer layer as redundant insulation to enhance reliability. For example, according to an embodiment of the present invention illustrated in FIG. 5, in addition to conductor wire 210 and insulative layer 212, one or more of the individual coiled wire conductors 202A, 202B, 202C and 202D includes an additional outer insulative layer 214, formed of known insulative materials, such as ETFE, for example, to enhance reliability of the lead. According to the present invention, insulative layer 214 generally has a thickness T between approximately 0.0005 and 0.0025 inches, for example, although other thickness ranges are contemplated by the present invention. Since the outermost insulative layer, i.e., insulative layer 214, experiences more displacement during flex of lead 102 than insulative layer 212, it is desirable for insulative layer 214 to be formed of a lower flex modulus material than insulative layer 212, such as ETFE.

[0042] By utilizing the insulative layer 212 of the present invention, the stimulating lead is reduced in diameter, and is more robust in regards to mechanical flex and electrical insulation. The insulative layer 212 provides an extremely long-term flex-life performance associated with the ductility of the hydrolytically stable polyimide coating over conductor wires such as MP35N, used on conductor coils. These improved properties are related to the unique process of the multiple pass application of the hydrolytically stable polyimide.

[0043] The resulting insulative layer 212 provides a highly reliable insulating and mechanically robust coating over implantable stimulating leads.

[0044] While an insulative layer formed only of ETFE tends to be susceptible to creep, insulative layer 212 of the present invention, which is formed of hydrolytically stable polyimide, is mechanically more robust, hydrolytically stable and possesses exceptionally dielectric properties, making the hydrolytically stable polyimide desirable for long-term implant applications. The use of a thin layer of hydrolytically stable polyimide coating on conventional MP35N alloy coil filars may also act as a protective barrier to reduce the incidence of metal induced oxidation seen on some polyurethane medical device insulations.

[0045] FIG. 6 is a radial cross-section of an exemplary cabled wire conductor according to yet another embodiment of the present invention. FIG. 6 illustrates cable 630 including bundled wire strands 632-637 formed about a core wire strand 638, any or all of which strands may be formed from a Co—Ni—Cr—Mo alloy, MP35N, or any other conductive corrosion-resistant and biocompatible material of sufficient strength and toughness for incorporation into a medical electrical lead; a diameter of each wire strand in various embodiments is between approximately 0.0005 inch and 0.005 inch. Using a conventional stranding machine, wire strands 632-638 are each tightly bundled in a cable-like fashion; a lay or pitch of stranding is typically between 0.3 inch and 0.6 inch. As is further illustrated in FIG. 6, cable 630 includes an insulating layer 639 surrounding bundled wire strands 632-638, which is formed from a hydrolytically stable polyimide, examples of which have been previously described. It should be noted that, although FIG. 6 illustrates insulating layer 639 surrounding the plurality of wire strands as bundled, according to an alternate embodiment, one or more of each of the individual wire strands include an insulating layer of a hydrolytically stable polyimide, for example as illustrated in FIG. 4, and layer 639 may or may not be included. Another type of cable configuration, which may include a hydrolytically stable polyimide insulating layer, is described in U.S. Pat. No. 5,760,341, issued to Laske et al., the teachings of which are incorporated herein.

[0046] According to one embodiment, layer 639 may be applied to the bundled wire strands 632-638 by passing them through a polyamic acid solution and then heating the strands to a temperature sufficient to fully imidize the polyimide; likewise layer 212 may be applied to conductor 210 in a similar manner. As previously described, multiple coating passes may form layers 630 and 212. According to an alternate embodiment an extrusion process may be used to apply layer 639 or layer 212; the type of polyimide described by Example 4, above, may be particularly suitable for extrusion. According to yet another embodiment a second layer of another, insulative material is formed over layer 639, for example a layer of ETFE as described in conjunction with FIG. 5.

[0047] While a particular embodiment of the present invention has been shown and described, modifications may be made. It is therefore intended in the appended claims to cover all such changes and modifications, which fall within the true spirit and scope of the invention.