Patent Application
20070265692
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.
The present invention relates to methods of forming a medical electrical lead having a porous surface electrode and a thin capacitive layer over the electrode.
Leads formed according to embodiments of the present invention may be particularly suitable for placement in a coronary vein in the left side of the heart. Additionally, leads formed according to embodiments of the present invention may have sufficient pacing thresholds for coronary vein placement.
According to one embodiment, as shown in
A connector 54 is operatively associated with the proximal end 42 of the conductive lead body 38. The connector 54 may be of any standard type, size or configuration. Connector 54 is electrically connected to the electrode 50 by way of a conductor coil 58 that extends through the interior lumen of lead body 38. Conductor coil 58 is generally helical in configuration and includes one or more conductive wires or filaments.
At least one electrode 50 is operatively associated with the distal end 46 of the conductive lead body 38. The electrode 50 can be formed from platinum, stainless steel, MP35N, titanium, a platinum-iridium alloy, or another similar conductive material. In one embodiment, the electrode 50 is disposed proximate to the distal end 46 of the lead 14. Alternatively, the electrode 50 can be located anywhere along the conductive lead body 38 between the proximal end 42 and the distal end 46. According to yet another embodiment of the present invention, the electrode 50 can be a tip electrode. A tip electrode is located at the very distal end 46 of the lead body 38 and is commonly employed in left ventricular leads. Multiple electrodes may also be utilized according to embodiments of the present invention.
According to exemplary embodiments of the present invention, as shown in
According to one embodiment of the present invention, the microporous surface 64 of the electrode 50 is formed by sintering microparticles of a generally conductive material onto the base material of the electrode 50. The microparticles may be made of platinum, platinum iridium, titanium, or another metal compatible with the base electrode material. Sintering is a conventional method for adhering particles to a substrate by heating the material below its melting point until its particles adhere to each other. In one embodiment, the microparticles may be placed on the electrode 50 by packing them around the electrode 50 in a mold. The mold is then heated for a desired period of time until the microparticles adhere to the electrode substrate. In another embodiment, the microparticles may be packed in the mold in the presence of a second material that melts away during the sintering process. The second material may be in a granular form and may include such materials such as polymethylmethacrylate (PMMA). The dissipation of the second material encourages pore formation during the sintering of the microparticles.
Other suitable materials for the particles may include platinum, titanium, and tantalum. In alternative embodiments the particles may be made of any material that is compatible with the base material of the electrode 50. In one embodiment, the particles are formed or supplied as microspheres, however, other shapes may also be suitable. Mircospheres made from a wide variety of materials are commercially available from a variety of sources.
In addition to sintering, other processes for forming the microporous surface 64 can include plasma spray coating, liquid metal coating, electrode burning, laser scribing, acid etching, mechanical abrasion, particle blasting, thermal spray coating, plasma etching, diamond coating, and powder metallurgy such as a casting or forming processes.
In a further embodiment of the present invention, the average pore size of the porous surface 64 may be optimized for placement in the coronary veins, which generally require smaller lead sizes. The average pore size may be further optimized based on the degree of tissue in-growth desired. An optimized average pore size for the coronary venous electrode may be between about 1 and 200 microns. In further embodiments, the average pore size may be between about 50 and about 100 microns.
In yet further embodiments of the present invention, various mechanical features may be incorporated into the porous surface 64 formed on the electrode 50. Such features may include ridges, holes, abrasion areas (such as formed via grit blasting or chemical etching), voids, steps, etc. Mechanical features formed in the porous surface 64 of the electrode 50 may contribute to electrode 50 retention and fixation stability. Such electrodes 50 may include a macro primary mechanical feature formed from the sintered spheres. A structure may be formed from a process such as highly energetic ion bombardment, for example, with argon in a plasma assisted process, such that surface features and porosity on the order of 100 nm to 10 μm are formed via bubbling or reformation of the metallic surface. In still further embodiments, tertiary structural features may be included through deposition of the capacitive coating 70 or a degradable protective coating 80. In addition, the electrode 50 as a whole may have a fractal morphology.
As shown in
According to one embodiment of the present invention, the capacitive coating 70 includes a biocompatible metal or metallic compound. Exemplary compounds include platinum, tantalum, platinum iridium, platinum oxide, titanium oxynitride, titanium oxide, tantalum oxide, tantalum nitride, titanium carbide, iridium oxide, and combinations thereof. Other biocompatible metallic compounds known to those in the art also can be used. According to alternative embodiments of the present invention, the capacitive coating 70 can be a capacitive polymer derived from, for example, poly(pyrrole), poly(naphthalene), poly(thiophene), PEDOT, Nafion, poly(ethylene) oxide or other suitable polymers.
There are many processes that can be used to grow or deposit capacitive coatings, such as electrodeposition, electroactivation, thermal deposition, and sputtering. Additional processes that can be used include plasma polymerization, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition (PECVD). All of these processes involve a myriad of inputs that each affect the eventual composition and morphology of the coating. The coating composition and its morphology, in turn, establish the electrically active surface area of the electrode (as opposed to the geometric surface area, based on the physical dimensions of the electrode).
The surface morphology of the capacitive coating may affect cellular attachment and cellular proliferation which may provide fixation of the electrode in vivo. For example, see Webster et al., “Osteoblast adhesion on nanophase ceramics,” Biomaterials, vol. 20, pp. 1221-1227, 1999; Chehroudi et al., “Effects of a grooved titanium-coated implant surface on epithelial cell behaviour in vitro and in vivo,” J. Biomed. Mat. Res., vol. 23, pp. 1067-1085, 1989; and Campbell et al., “Microtopography and soft tissue response,” J. Investigative Surg., vol. 2, pp. 51-74, 1989.
According to exemplary embodiments of the present invention, various sputtering deposition techniques are used to deposit the capacitive coating 70 on the microporous surface 64. Sputtering is a process by which thin films of uniform thicknesses may be deposited onto a substrate by accelerating ions, typically via glow discharge plasma, toward a “target” material such that atoms from the target are ejected and deposited on the substrate surface. If the plasma gas is chosen such that the sputtered atoms react with the ions in the plasma to form a new compound, which is then deposited on the surface of the substrate, the process is called “reactive” sputtering. In the case of a pacing electrode, the electrode serves as the substrate.
A variety of commercially available sputtering system designs may be used to accomplish the desired coating deposition. An example of one system is a cylindrical magnetron sputter deposition system. In such a system, a “target” is situated against the wall of a cylindrical vacuum chamber. The target serves as the source of metallic ions in the chamber once a low pressure glow discharge plasma is formed, which bombards the target surface with energetic ions that cause ejection of metallic target ions. The target may be made up of any number of metals, such as iridium, titanium, tantalum, etc. The sputtered target ions travel in random directions through the vacuum chamber, and may combine with a reactive gas that is present in the plasma to form a compound. The reactive gas is most usually oxygen or nitrogen, which is present typically with a portion of argon, which serves as an inert gas for sputtering.
The compound is often formed on the substrate, which sits at the center of the chamber, and may be biased with a voltage to attract sputtered ions and gas ions to form a compound coating on the surface. According to one embodiment, the present invention utilizes pulsed voltages to assist in uniformly depositing the capacitive coating 70. According to one embodiment of the present invention, the pulsed voltage applied to the substrate ranges from about −100 V to about −300 V. In further embodiments of the present invention, the pulsed voltages are induced in a square wave pattern with frequencies less than about 100 kHz.
Various processing parameters present in the sputtering system control the thickness and/or surface morphology of the compound coating on the substrate, as well as the composition of constituent materials (i.e. target metal and reactive gas atoms). Such parameters include the input power to the magnetron, the ratio of reactive gas to inert gas in the chamber, processing time, and chamber pressure.
According to one exemplary embodiment of the present invention the magnetron input power can range from about 100 W to about 300 W. In one particular embodiment, the magnetron power ranges from about 175 W to about 225 W.
The ratio of reactive gas (e.g. oxygen or nitrogen) to inert gas (e.g. argon) can range from about 15% (oxygen) to about 45% (oxygen). According to another embodiment of the present invention, the ratio of reactive gas to inert gas ranges from about 20% (oxygen) to about 30% (oxygen). In yet another exemplary embodiment of the present invention, the ratio of reactive gas to inert gas ranges from about 35% (oxygen) to about 45% (oxygen).
The processing time can range from about 30 seconds to about 4 minutes. According to another embodiment of the present invention, the processing time ranges from about 1 minute to about 2 minutes. According to yet another embodiment of the present invention, the processing time ranges from about 2.5 minutes to about 3.5 minutes.
The total chamber pressure can range from about 0.1 mTorr to about 15 mTorr. According to another embodiment of the present invention, the total chamber pressure ranges from about 3 mTorr to about 5 mTorr. According to yet another embodiment of the present invention, the total chamber pressure ranges from about 9 mTorr to about 11 mTorr.
In one embodiment, the capacitive coating 70 is formed under processing parameters that produce an average thickness at the porous electrode surface 64 that is much less than the average pore size or characteristic feature size of the porous surface, such that the underlying surface structure of the microporous surface 64 is substantially preserved. According to one embodiment of the present invention, an average thickness of the capacitive coating is less than about 500 nm. According to another exemplary embodiment, the average thickness of the capacitive coating ranges from about 10 nm to about 300 nm. According to a further embodiment of the present invention, the thickness of the capacitive coating ranges from about 150 nm to 300 nm. According to yet a further embodiment of the present invention, the thickness of the capacitive coating ranges from about 20 nm to about 60 nm. The resultant capacitive coating 70 is more complete and uniform across the porous surface 64 and provides better protection for the electrode 50 from the surrounding cardiac tissue.
According to one particular embodiment of the present invention, a capacitive coating 70 having a thickness ranging from about 25 nm to about 50 nm and a surface morphology that has an “orange peel” like appearance, as shown in
According to another particular embodiment of the present invention, a capacitive coating 70 having a thickness ranging from about 285 nm to about 295 nm and a surface morphology that has a “rice grain” like appearance, as shown in
Post processing procedures, such as annealing or electrochemical activation, may also be performed to further optimize the capacitive coating 70 surface morphology.
In still further embodiments, as illustrated in
According to one such embodiment of the present invention, the degradable protective coating 80 includes a hydrogel. The hydrogel coating may be constructed of a biocompatible material such as poly(ethylene glycol) (PEG). Hydrogel protective coatings may aid or mediate tissue in-growth before it is completely degraded. In addition, the degradable, hydrogel protective coating may serve to protect the porous electrode from blood coagulation during placement of the lead such that a preliminary thrombus does not form until after the electrode is in the desired position. Many hydrogels, such as those formed from PEG, may also display lubricious characteristics and may therefore further aid in the insertion of the porous electrode into the coronary vein.
The PEG polymer may be in a variety of forms such as a copolymer, a cross-linked network of polymers, a graft polymer, or a polymer blend. PEG comes in a variety of high or low molecular weights which can be selected depending on the desired degradation properties. PEG can be deposited on the porous surface electrode 50 by dip coating the porous surface electrode. In further embodiments of the present invention, a solution containing the polyethylene glycol may be applied to the electrode 50 via a syringe.
In still further embodiments of the present invention, the surface of the electrode 50 may be protected via other degradable materials such as mannitol, or other degradable polymers, such as polylactic-co-glycolic acid (PLGA) or any polymer. The polymer is selected such that its surface or bulk degradation properties can be adjusted so as to dissolve over time to protect the porous surface 64 from thrombus formation or other undesirable biological processes until the electrode is in its final implant position.
The drug eluting properties of certain materials that can be used as the degradable protective coating 80 may also provide a site specific vehicle for delivery of drugs and other biologically active agents. Such agents may help to further reduce blood clotting and thrombi formation. According to one exemplary embodiment, one drug that may be desirable to load directly into or on the degradable protective coating 80, or the porous surface 64, includes dexamethazone acetate. Dexamethazone acetate is a anti-inflammatory agent and may contribute to the reduction of fibrous capsule growth and decrease the pacing threshold.
In further embodiments of the present invention, degradable protective coating materials 80 may be formed as a copolymer or a blend with other polymers to achieve desired degradation or drug eluting characteristics. Other drugs may be loaded into the pores, doped into or onto the degradable polymer and/or applied to the capacitive polymer coating. Further drugs may include, for example, clobetesol, everoliums, sirolimus, or dexamethazone phosphate.
A cylindrical DC magnetron sputtering system (ion Tech, Inc., Fort Collins, Colo.) was used to reactively sputter an iridium oxide capacitive coating onto a stainless steel test strip. The sputtering target was iridium (99.9% purity). Iridium targets are commercially available from a variety of sources. The sputtering parameters used to form the thin film are summarized below in Table 1.
A cylindrical DC magnetron sputtering system (ion Tech, Inc., Fort Collins, Colo.) was used to reactively sputter an iridium oxide capacitive coating onto a stainless steel test strip. The sputtering target was iridium (99.9% purity). Iridium targets are commercially available from a variety of sources. The sputtering parameters used to form the thin film are summarized below in Table 2.
By controlling the sputtering parameters, the thickness and/or surface morphology of the capacitive coating 70 were controlled. Altering the ratio of argon to oxygen in the total flow rate appears to most significantly impact the surface morphology of the capacitive layer. The chamber pressure also affects the surface morphology. Increasing the processing time of the sample increases the thickness of the capacitive layer.
The scope of the invention is not meant to be limited in application only to leads for implantation in coronary veins. Application of the disclosed embodiments may also be made to right sided bradycardia or tachycardia leads, or epicardial leads. For coronary venous applications, the disclosed embodiment may also be utilized on a non-electrode metallic portion of the lead body strictly for tissue integration and lead stabilization.
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.
This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/747,237, filed on May 15, 2006, entitled “Porous Surface Electrode for Coronary Venous Applications,” which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60747237 | May 2006 | US |
