US 2004O176828A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2004/0176828A1 O’Brien (43) Pub. Date: Sep. 9, 2004

(54) LOW POLARIZATION COATINGS FOR Publication Classification IMPLANTABLE ELECTRODES (76) Inventor: Robert C. O’Brien, Sykesville, MD (51) Int. Cl." ...... A61N 1/05 (US) (52) U.S. Cl...... 607/119, 607/122 Correspondence Address: WILSON GREATBATCH TECHNOLOGIES, (57) ABSTRACT INC. 10,000 WEHRLE DRIVE CLARENCE, NY 14031 (US) Coatings for inplantable electrodes Such as pacing elec trodes, neurostimulator electrodes, and electroporating elec (21) Appl. No.: 10/792,315 trodes and Sensing electrodes are described. The coatings are highly biocompatible, having low polarization. They consist (22) Filed: Mar. 3, 2004 of a biocompatible, conductive Substrate, Such as of Sintered Related U.S. Application Data platinum/10% iridium; a thin film outer layer of biocompat ible, conductive carbon; and a biocompatible, conductive (60) Provisional application No. 60/451,423, filed on Mar. intermediate layer having a high Surface area. The interme 3, 2003. Provisional application No. 60/475,535, filed diate layer is preferably of Sputtered titanium nitride and on Jun. 4, 2003. increases the Surface area of the carbonaceous Outer layer.

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LOW POLARIZATION COATINGS FOR in many cases, a portion of the electrode working Surface IMPLANTABLE ELECTRODES must be dedicated to the medication administering function. 0009. The problem of encapsulation has also been CROSS-REFERENCE TO RELATED addressed by the use of carbon as an electrode material. APPLICATIONS Carbon is much more highly biocompatible than the previ 0001. The present application claims priority from U.S. ously used noble metals and valve metals. For example, U.S. provisional application Serial No. 60/451,423, filed Mar. 3, Pat. No. 4,033,357 to Helland et al. describes the use of 2003 and 60/475,535, filed Jun. 4, 2003. pyrolitic carbon for pacing electrodes, although no means of increasing the Surface area is discussed. In that regard, pacing electrode designs according to this patent meet the BACKGROUND OF THE INVENTION requirement of improved biocompatibility, but they do not 0002) 1. Field of the Invention have the high Specific Surface characteristics of currently 0003. This invention generally relates to coatings for used columnar titanium nitride. implantable electrodes Such as pacing electrodes, neuro 0010 Various methods of reducing the polarization of Stimulator electrodes, and electroporating electrodes and implantable carbon electrodes by increasing their Surface Sensing electrodes. The coatings are highly biocompatible, area have also been tried. U.S. Pat. No. 4,281,668 to Richter having low polarization. They consist of a biocompatible, et al. describes one method comprising heating glassy car conductive Substrate; a thin film Outer layer of biocompat bon parts in air to form a porous layer of increased Surface ible, conductive carbon; and a biocompatible, conductive area. Gluing the glassy carbon to the electrode Stem com intermediate layer having a high Surface area. The interme pleted the electrode assembly. Another method is described diate layer increases the Surface area of the carbonaceous in U.S. Pat. No. 4,612,100 to Edeling et al. in which high outer layer. Sputter power levels are used to induce porosity on the order of 20 angstroms in sputtered carbon films. U.S. Pat. No. 0004 2. Prior Art 4,609.508 to Edeling et al. also noted that microscopic 0005 Three overriding requirements for implantable porosity is induced in the course of the pyrolytic processing electrodes are biocompatibility, biostability, and low energy of vitreous carbon. Still more efforts to increase the Surface loSS. Broadly, the biocompatibility requirement is met if area of carbon thin films in pacing electrodes are described contact of the electrode with body tissue and blood results in in U.S. Pat. No. 5,074,313 to Dahl et al. This patent teaches little or no immune response from the body, especially Sputtering a carbon thin film over a layer of high Surface area thrombogenicity (clotting) and encapsulation of the elec Sputtered metallic aluminum. A similar method is described trode with fibrotic tissue. The biostability requirement in U.S. Pat. No. 5,118,400 to Wollam in which metallic means that all physical, electrical, and chemical properties titanium, aluminum, or Zirconium as high Surface area of the electrode/coating System remain constant and materials are Sputtered to a “hillocked' morphology. unchanged over the life of the patient. The low energy loSS 0011. In addition to increasing the surface area, U.S. Pat. requirement is met if electrode polarization is a minimum. No. 4,495,039 to Cerise et al. shows that polaraization of 0006. In the patent literature, U.S. Pat. No. 4,602,637 to pyrocarbon electrodes can be reduced by means of electro Elmqvist describes a commonly used pacing electrode hav chemical activation of the carbon Surfaces. This is done by ing Sputtered columnar titanium nitride as a coating mate immersing the carbonaceous material in concentrated acid rial. This form of titanium nitride has good conductivity and imposing an electrical current thereto. combined with a high Specific Surface area, resulting in 0012. Thus, there is still a need for an implantable favorable polarization and Sensing properties. The disadvan electrode construction having the requisite improved bio tage of titanium nitride, however, is that it degrades the compatibility along with high Specific Surface characteris electrical properties of Surrounding tissue after implantation. tics, Such as provided by columnar titanium nitride. The This occurs as the body tissue encapsulates the columnar present electrode fulfills this need in terms of both low titanium nitride in fibrotic tissue, which has a lower con polarization and minimum energy requirements for accept ductivity than normal tissue. able Sensing properties. 0007. In the case of a pacing electrode, fibrotic tissue raises the stimulation threshold. The stimulation threshold is SUMMARY OF THE INVENTION the minimum energy required to produce a cardiac contrac 0013 Electrode polarization is defined as the residual tion. A raised Stimulation threshold, in turn, impacts the Voltage at the end of a Voltage pulse acroSS the electrode/ battery life of the system so that the medical device must be tissue interface. FIGS. 1 and 2 show oscilloscope traces of explanted Sooner than desired. The encapsulation proceSS the Voltage response of pacing electrodes to a 10 msec also interferes with Sensing of intrinsic millivolt Signals Square wave pulse in a Saline electrolyte. In both cases, the required by pacemakers. In prior electrode designs, the electrode is coated with a Sintered platinum/iridium alloy. In fibrotic encapsulation problem has been addressed by incor the former graph, the electrode-sintered layer is not further porating a means of metering or eluting Steroidal medication coated and the polarization is >-400 mV. The platinum/ to the tissue contact Site over time. iridium alloy Sintered coating in the latter graph is further 0008 However, steroidal medication is not completely provided with a coating of porous titanium nitride having an effective in eliminating the Stimulation threshold rise due to outer layer of Sputtered amorphous carbon according to the encapsulation. Steroidal medication eluting arrangements present invention. The polarization reading is <-45 mV. have a relatively short duration of effectiveness, and also add 0014. The present electrode exhibits relatively low polar cost and complexity to the System, add risk of infection, and, ization because of the greatly increased Surface area US 2004/0176828A1 Sep. 9, 2004 imparted by the Sputtered carbon, which mimics the physical Supporting an integral tip or head 14. The electrode is of a Structure of conventional Sputter coated columnar titanium material Selected from tantalum, titanium, Zirconium, iri nitride. Furthermore, the present electrode is both biocom dium, platinum, and niobium. Preferably, the electrode is of patible and biostable because the Sputtered carbon coating is platinum/10% iridium. The electrode head 14 has a cylin Strongly adherent and chemically inert. In that light, the drically shaped portion 16 forming into a curved or dome excellent biocompatibility of Sputtered carbon lessens or shaped active surface 18. As shown in FIGS. 5 and 6, a eliminates the requirement for providing Steroidal medica biocompatible, electrically conductive, high Surface area tion to the tissue Surrounding the electrode. layer 20 is formed over the active substrate surface 18. The 0.015 These and other aspects of the present invention thickness of layer 20 is about 0.1 micrometers (um) to about will become increasingly apparent to those skilled in the art 20 tim, preferably about 5um. The high Surface area layer by reference to the following description and to the 20 increases the effective Surface area of the electrode head appended drawings. 14. 0028 FIG. 7 shows an outer layer 22 consisting of BRIEF DESCRIPTION OF THE DRAWINGS amorphous carbon having been coated over the high Surface 0016 FIG. 1 is an oscilloscope graph of the voltage area intermediate layer 20. The amorphous carbon outer response of a pacing electrode only provided with a sintered layer 22 is preferably applied by physical vapor deposition platinum/iridium alloy powder coating and no other. to a thickness of about 10 nanometers (nm) to about 1.0 um, and preferably about 0.2 um thick. Depositing by a physical 0017 FIG. 2 is an oscilloscope graph of the voltage Vapor means takes advantage of the intrinsic low polariza response of a pacing electrode provided with a sintered tion and high biocompatibility of amorphous carbon while platinum/iridium alloy powder coating in turn provided with exploiting the ability of a thin film of this material to a titanium nitride intermediate layer and an Outer layer of conform to the roughness of the high Surface area active Sputtered amorphous carbon. layer 20. Conforming to the underlying intermediate layer 0018 FIG. 3 is a side elevational view of a machined 20 increases the Surface area of the amorphous carbon outer electrode 10. layer 22. This means that rather than forming a continuous, pore free film over a Substrate, the present invention takes 0019) FIG. 4 is an enlarged view of the indicated area in advantage of the fact that physical vapor deposited thin films FIG 3. can be made to preserve any very fine Scale porosity present 0020 FIG. 5 is an enlarged, cross sectional view of the in the Substrate, including porosity having dimensions con electrode in FIGS. 3 and 4 provided with an electrically siderably less than the film thickness. conductive, biocompatible, high Surface area layer 20. 0029. The intermediate active layer 20 can be any mate 0021 FIG. 6 is an enlarged view of FIG. 5. rial that has high biocompatibility, biostability, and electrical conductivity. Examples include carbon, boron, platinum, 0022 FIG. 7 is an enlarged view of the electrically iridium, gold, titanium, tantalum, niobium, ruthenium, Zir conductive, biocompatible, high surface area layer of FIGS. conium, and alloys thereof. In addition, electrically or ioni 5 and 6 provided with an amorphous carbon outer layer 22. cally conductive compounds of these metals, and alloys may 0023 FIG. 8 is a photograph showing the surface of a be used, including iridium oxide, iridium nitride, titanium Single particle in a Sintered platinum/10% iridium electrode nitride, titanium carbide, titanium carbonitride, tantalum coating with 200 nm thin film Sputtered carbon coated nitride, tantalum carbide, tantalum carbonitride, niobium directly on to the surface thereof. The width of field shown carbide, niobium nitride, niobium carbonitride, ruthenium is 5.5 um. oxide, ruthenium nitride, Zirconium oxide, Zirconium nitride, Zirconium carbide, and mixtures thereof. In cases 0024 FIG. 9 is a photograph showing the surface of a where the compounds of the intermediate active layer 20 are platinum/10% iridium Sintered particle, as shown Schemati not electrically conductive, they can be made So by doping cally shown in FIG. 7, provided with a 200 nm thin film, with Small amounts of extraneous elements. For example, nitrogen-doped carbon Overlayer Sputtered onto a titanium titanium dioxide, a dielectric in its pure State, is made nitride intermediate layer. The width of field shown is 5.5 conductive by doping with niobium. plm. 0030 All of these intermediate active layer 20 materials 0.025 FIG. 10 is a cross-sectional photograph of the can be applied to the machined electrode Surface in Such a coating shown in FIG. 9. The width of field shown is 6.8 um way that the resulting coatings have high Surface areas with FIG. 11 is a photograph of an electrode Substrate consisting very fine Scale roughneSS and porosity. When these high of sintered particles of platinum/10% iridium coated with a Surface area coatings are then coated with the conformal thin fine sintered diamond powder single layer. The width of field film carbon outer layer 22, their biocompatibility, biostabil shown is 5.5 tim. ity, and polarization characteristics are improved. It is also 0.026 FIG. 12 is a photograph detailing the diamond within the Scope of this invention that doping the amorphous powder coating on the platinum/10% iridium electrode body carbon outer layer 22 with nitrogen provides a further shown in FIG. 11. The width of field shown is 5.5um. improvement to the electrode polarization. Nitrogen doping is preferably at a concentration of about 1 ppm to about 57 DETAILED DESCRIPTION OF THE atomic percent. PREFERRED EMBODIMENTS 0031 Techniques useful for this purpose include sinter 0027. Referring now to the drawings, FIGS. 3 and 4 processing, micromachining, grit blasting, chemical etching, illustrate a machined electrode 10 comprising a stem 12 and the like. For example, Some forms of carbon can be used US 2004/0176828A1 Sep. 9, 2004

to provide high Surface area intermediate layers. These EXAMPLE I include Sintered diamond doped with boron or, carbon nanotubes or fullerenes. Carbon nanotubes or fullerenes Platinum/20% Iridium Machined Electrode increase the conductivity of the machined electrode active Provided with a Sintered Platinum/10% Iridium Surface 18 and can be grown in Situ or attached to the Coating Having a Titanium Nitride Intermediate electrode Surface. Further disclosure regarding carbon nano Layer and a Sputtered Carbon Overlayer tubes and fullerenes can be found in U.S. application Ser. 0037. A number of machined electrodes comprising plati No. 10/719,632, filed Nov. 21, 2003, which is assigned to the num/20% iridium provided with a sintered platinum/10% assignee of the present invention and incorporated herein by iridium coating were Sputter coated with columnar titanium reference. nitride (TiN) according to the previously discussed U.S. Pat. No. 4,602,637 to Elmquist et al. The sputter deposited 0032. Additionally, the machined electrode active surface titanium nitride had a high Specific Surface area due to its 18 to which the intermediate high surface area active layer columnar structure with crystallite diameters of about 100 to 20 is applied may itself have had its Surface area increased. about 500 nanometers 0038 Acarbon overlayer was then applied to the titanium 0.033 AS previously discussed, the preferred method of nitride coating by Sputtering in argon using a carbon target. applying the thin film amorphous carbon outer layer 22 to This consisted of placing the thusly processed electrodes in the intermediate high Surface area active layer 20 is by a Denton Vacuum LLC “Discovery” 18-deposition chamber. physical vapor deposition processes Such as Sputtering The chamber was evacuated to about 8x107 torr prior to a (deposition by plasma activation) or evaporation (deposition 3.5 millitorr process gas being introduced therein. The by thermally activated vaporization). However, the carbon process gas consisted of pure argon or 93% argon, 7% overlayer 22 may also be applied in the form of pyrolytic nitrogen, both at 105 SCCM. The platinum/20% iridium carbon (thin film carbon thermally deposited by decompos machined electrodes provided with the Sintered platinum/ ing a liquid carbonaceous precursor), or by chemical vapor 10% iridium coating and having a 8 um thick titanium deposition (thin film carbon thermally deposited by decom nitride intermediate layer were then Supported on an posing a gaseous carbonaceous precursor) methods. The unheated, rotating table about four to five inches from a three-inch diameter piece of carbon (graphite) as a target. resulting carbon outerlayer may then range in atomic struc The magnetron cathode was set at 250 W DC and Sputtering ture from glassy (a completely amorphous, random structure continued for 30 minutes. The thickness of the resulting with no covalent bonding) to turbostratic (microcrystalline, carbon thin film was about 200 nm to about 300 nm. Short range order) to graphitic (crystalline with covalent bonding in two dimensions) to diamond (crystalline with 0039 The thusly-manufactured electrodes were then Sub jected to polarization testing and the results are Set forth covalent bonding in three dimensions). below: 0034 AS previously discussed, the thickness of the amor phous carbon outer layer 22 may vary from a few atomic layers to a micrometer, and more. The nature of the Sputter Sintered Pt/10% Ironly: -440 mV deposition mechanism in particular allows the carbon over Sintered Pt/10% Ir with sputtered carbon coating: -139 mV Sintered Pt/10% Ir with sputtered TIN and -75 mV layer film thickness to exceed the pore size of the high a sputtered carbon overlayer, surface area active layer 20 by a factor of three or more argon process gas: while Still preserving the high porosity, high Surface area Sintered Pt/10% Ir with sputtered TIN and -45 mV a sputtered carbon overlayer, characteristic of this Substrate. Moreover, a manufacturabil argonf7% nitrogen process gas: ity advantage results from the ability to Sequentially Sputter the intermediate layer 20 and the outer amorphous carbon layer 22 with no additional handling of the electrodes 0040. The photographs in FIGS. 8 to 10 show the micro between Sputter process StepS. Structures associated with the above polarization results. In particular, FIG. 8 shows the surface of a single particle in a 0035. The effect is to provide an electrode coating 20, 22, sintered platinum/10% iridium electrode coating with a 200 which is biocompatible, and biostable, and which has advan nm thin film Sputtered carbon coated directly on to the tageously low polarization because of the greatly increased surface thereof. This electrode had a polarization of -139 Surface area imparted to the carbon overlayer 22 by the mV. FIG. 9 shows the surface of a single particle in a sintered platinum/10% iridium electrode with a 200 nm thin intermediate high Specific Surface active layer 20. The film Sputtered nitrogen-doped carbon overlayer coated onto excellent biocompatibility of the amorphous carbon coating a high Specific Surface area Sputtered TiN intermediate layer. has the advantage of providing a consistently low and Stable This electrode had a polarization of -43 mV. FIG. 10 is a Stimulation threshold, which helps lessen or eliminate the cross section (fracture) of the coating shown in FIG. 9 requirement for providing Steroidal medication to the tissue where each TiN column is clearly tipped with nitrogen surrounding the implanted electrode 10. doped amorphous carbon. 0.036 The following examples describe the manner and 0041 Accordingly, increasing the surface area of the process of a coated electrode according to the present Sputtered carbon layer by means of an intermediate layer of invention, and set forth the best mode contemplated by the high Surface area columnar titanium nitride significantly inventors of carrying out the invention. reduces the polarization of an implantable electrode. Doping US 2004/0176828A1 Sep. 9, 2004 the amorphous carbon with nitrogen, by adding nitrogen to 5. The electrode of claim 1 wherein the carbon-containing the Sputter process gas, further reduces polarization. coating is amorphous carbon or amorphous carbon doped with nitrogen. EXAMPLE II 6. The electrode of claim 5 wherein the nitrogen is provided in the carbon-containing coating at a concentration Electrode with Sintered Platinum/10% Iridium of about 1 ppm to about 57 atomic percent. Coating Having a Sintered Diamond Intermediate 7. The electrode of claim 1 wherein the carbon-containing Layer and a Sputtered Carbon Overlayer coating is characterized as having been deposited by at least 0.042 A porous sintered single layer of doped diamond one of the group consisting of Sputtering, evaporation, a powder having a particle Size of about 3 um was sintered to pyrolytic process, and chemical vapor deposition. a Substrate consisting of porous Sintered platinum/10% 8. The electrode of claim 1 wherein the Substrate is iridium particles. The platinum/10% iridium particles them characterized as having its Surface area increased by at least Selves had a pore size of about 15 lum. The diamond layer one method Selected from the group consisting of Sinter was sintered by heating to a temperature of about 800° C. to processing, micromachining, grit blasting, and chemical about 1,100 C. at atmospheric pressure in a carbon-con etching. taining atmosphere Such as a hydrocarbon partially com 9. The electrode of claim 1 wherein the intermediate busted in air. Time at temperature was on the order of one coating has a first thickness of about 0.1 um to about 20 um. minute. This conductive Sintered diamond intermediate 10. The electrode of claim 1 wherein the carbon-contain layer was then Sputter coated with an outer layer of amor ing coating has a Second thickness of about 10 nm to about phous carbon, thereby improving the polarization of the 1.0 lim. resulting Surface according to the present invention. 11. An implantable electrode, which comprises: 0.043 FIG. 11 is a photograph of the sintered platinum/ a) a Substrate comprising platinum and iridium; 10% iridium electrode. The sintered particles serve as a b) a biocompatible and electrically conductive interme Substrate for the fine Sintered diamond powder intermediate diate coating comprising titanium nitride or diamond layer. The enlarged photograph of FIG. 12 shows the doped with boron Supported on the Substrate; and diamond powder coating on the platinum/10% iridium elec trode body. Robust necks between the diamond particles and c) an amorphous carbon-containing coating adhered to the the platinum/iridium Surface indicate good adhesion of the intermediate coating. 12. The electrode of claim 11 wherein the carbon-con Sintered layer. taining coating is doped with nitrogen. 0044) It is appreciated that various modifications to the 13. A method for providing an implantable electrode, inventive concepts described herein may be apparent to comprising the Steps of: those of ordinary skill in the art without departing from the Spirit and Scope of the present invention as defined by the a) providing a Substrate comprising platinum and iridium; appended claims. b) coating an intermediate layer Selected from the group What is claimed is: consisting of carbon, boron, platinum, iridium, gold, 1. An implantable electrode, which comprises: titanium, tantalum, niobium, ruthenium, Zirconium, iri dium oxide, iridium nitride, titanium nitride, titanium a) a Substrate; carbide, titanium carbonitride, tantalum nitride, tanta b) a biocompatible and electrically conductive interme lum carbide, tantalum carbonitride, niobium carbide, diate coating Supported on the Substrate; and niobium nitride, niobium carbonitride, ruthenium Oxide, ruthenium nitride, Zirconium oxide, Zirconium c) a carbon-containing coating adhered to the intermediate nitride, Zirconium carbide, titanium dioxide doped with coating. niobium, diamond doped with boron, and mixtures 2. The electrode of claim 1 wherein the Substrate is thereof on the Substrate; Selected from the group consisting of tantalum, titanium, Zirconium, iridium, platinum, and niobium. c) sputtering a carbon-containing coating to the interme 3. The electrode of claim 1 wherein the Substrate is diate layer; and Sintered platinum/iridium. d) utilizing the coated Substrate as an implantable elec 4. The electrode of claim 1 wherein the intermediate trode. coating is Selected from the group consisting of carbon, 14. The method of claim 13 including providing the boron, platinum, iridium, gold, titanium, tantalum, niobium, carbon-containing coating comprising amorphous carbon. ruthenium, Zirconium, iridium oxide, iridium nitride, tita 15. The method of claim 14 including doping the amor nium nitride, titanium carbide, titanium carbonitride, tanta phous carbon with nitrogen. lum nitride, tantalum carbide, tantalum carbonitride, nio 16. The method of claim 13 including providing the bium carbide, niobium nitride, niobium carbonitride, intermediate coating having a first thickness of about 0.1 um ruthenium oxide, ruthenium nitride, Zirconium oxide, Zirco to about 20 um and the carbon-containing coating having a nium nitride, Zirconium carbide, titanium dioxide doped second thickness of about 10 nm to about 1.0 um. with niobium, diamond doped with boron, and mixtures thereof. k k k k k