BODY IMPLANTABLE LEAD

Abstract: A body implantable lead (10) having an electrode (30) with a thin layer (60) of a biocompatible precious metal exposed to body tissue. The electrode structure (38) is of a relatively inexpensive base metal such as titanium, which is considered biocompatible for use as a cathode. A thin layer (50) of titanium is sputtered over the distal tip of the electrode assembly. A thin outer layer (60) of a precious metal from the plantinum group is sputtered over the sputtered titanium layer. The completed electrode assembly is used in the fabrication of a body implantable lead using standard assembly practices.

Description: BODY IMPLANTABLE LEAD BACKGROUND OF THE INVENTION 1. Field of the Invention - The present invention relates generally to body implantable electronic eauipment and more specifically relates to body implantable leads. 2. Description of the Prior Art - It is common to use platinum and platinum iridium electrodes in body implantable leads for cardiac pacing. This is desirable because the platinum and platinum iridium alloy (usually 90% platinum and 10% iridium) have a high resistance to corrosion within the implant environment. These materials are also desirable for the construction of electrodes because of the relatively low polarization associated with electrodes from this material. The major disadvantage to this technique, however, is that a solid platinum or platinum iridium electrode is quite expensive to fabricate because of the high raw material cost. A second approach to electrode fabrication is discussed by Williamson in U.S. Patent No. 3,749,101 issued on July 31, 1973. Williamson describes an electrode having a basic titanium structure with a platinum insert. Williamson teaches that by platinizing (i.e., plating platinum onto) the platinum insert to produce platinum black, a non-polarizing muscle stimulating electrode is fabricated. This technique, though not in and of itself popularly used, does teach the coating of a titanium electrode housing with a layer of a noble metal, in this case platinum. Notice, however, in the teaching of Williamson, the titanium is not totally covered and therefore is also available as a stimulation surface. This factor is brought out very clearly at column 4, lines 5-7 in which Williamson states that his lead "features a second electrode housing which is itself capable of carrying the stimulating current should there be any malfunction of the platinum. A further problem that is found in Williamson is the difficulty in attaching the platinum sleeve or coil reliably to the titanium electrode assembly. Williamson does not go into any great detail in this matter, but those of ordinary skill in the art will be able to see that there is some difficulty in establishing and maintaining reliable attachment over serveral years' use. It is well known in the scientific community that platinum black is very fragile such that in a cardiac electrode application, it would be susceptible to detachment simply by insertion into the heart. It is also established that platinum black is very susceptible to chemical contamination which would tend to negate its non-polarization properties. A topic not addressed by Williamson is that other metals within the platinum group (i.e., rhodium and iridium) may in fact be more desirable as electrode surfaces than is platinum or an alloy of 90% platinum and 10% iridium. Little experimentation has been conducted with these materials since they, in fact, are too hard to be easily manufactured into electrodes of implantable size using techniques conventionally used to manufacture electrodes. SUMMARY OF THE INVENTION The present invention teaches a body implantable lead having an electrode which overcomes these problems stated in the prior art. That is to say, the present invention utilizes a very tiny amount of precious metal to be placed in a very thin layer over the top of a base electrode assembly. This technique is very important in reducing the raw material cost of the electrode assembly. The second major advantage of the present invention is that the sputtering of the surface material over a layer of sputtered titanium, which is itself sputtered over the base metal, provides reliable adhesion of the noble metal electrode surface. A third important and interesting aspect of the present invention is that it permits use of materials for the electrode surface which are not currently useful in the prior art electrodes. This is because the basic shape and characteristic form of the electrode assembly are created by common manufacturing techniques applied to the base metal electrode assembly. The material used for the stimulating surface, because it is merely sputtered on, need not be machined or worked in any way to establish its basic form. The basic form is established by the base metal electrode assembly. For this reason, materials having very desirable corrosion resistance and low polarization properties such as rhodium and iridium may now be used on body implantable leads without the difficulties associated with machining and forming such materials. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a body implantable lead of the unipolar type incorporating the present invention. FIG. 2 is a cross sectional view of the electrode assembly employing the present invention incorporated within the body implantable lead of FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is described in relation to the preferred mode of its practice. This involves the construction of a unipolar tined lead for stimulation and sensing using a cardiac pacer within the right ventricle. Those of ordinary skill in the art however, will be readily able to apply the invention as taught herein to other types of body implantable leads. FIG. 1 is a plan view of a body implantable lead 10 incorporating the present invention. At the proximal end of the body implantable lead 10, a conductive terminal pin 16, which is attached to and used to conduct a stimulation pulse from an implantable pulse generator (not shown) to the body implantable lead, is attached to a length of coiled wire conductor 11. An insulator sheath 12 is formed about a portion of the pin 16 and the electrical conductor 11. O-rings 14 formed from the insulator 12 are used to establish a proper seal between connector sleeve 12 and the implantable pulse generator to prevent the ingress of body fluids. The conductor 11 within the main body of body implantable lead 10 is covered with a insulating sheath 20. Insulating sheath 20 is preferably of polyurethane material. However, other commonly used body compatible insulators such as silicone rubber may be used. Insulating sheath 20 is stretched or formed at its distal end to cover the entire electrode assembly at enlargement 22. Non-conducting tines 18 are employed to assist in acute fixation in the manner common in the art. The stimulating surface of body im#lantable lead 10 is at the very distal tip. The distal tip 30 directly contacts body stimulatable tissue during pacing. The lead as described is similar to that shown and described in the commonly assigned U.S. Patent No. 3,902,501. FIG. 2 is a side sectional view of the electrode employed in body implantable lead 10 of the present invention. Imaginary line 32 has been drawn to show the extent to which distal tip 30 is exposed. That is, that part of the electrode assembly which is proximal (to the right) of imaginary line 32 is covered by the stretched portion 22 of outer sheath 20 (shown in FIG. 1). That portion of the electrode assembly in FIG. 2 which is distal of dashed line 32 is the exposed distal tip 30. The electrode assembly has a longitudinal lumen 34 into which the conductor coil 11 of the body implantable lead is inserted. This body implantable lead coil 11 is attached, using methods common in the art in the manner shown, for example, in the above-referenced patents. Shoulders 36 are used to ensure that outer sheath 20 is properly gripped by the electrode assembly to ensure that the outer sheath 20 is reliably attached. The entire electrode assembly 38 is fabricated from a single piece of commercially pure grade titanium. A distal lumen 44 is used for the insertion of a round piece of insulating material. This material reduces the overall effective stimulating surface and thereby increases effective current density of the electrode. Transverse lumen 40 is used to attach the non-conducting material which is inserted into distal lumen 44. Layer 50 is a layer of reactive metal (e.g., titanium, niobium, etc.) which is sputtered over the distal tip of titanium electrode assembly 38. This layer is on the order of 1,000 to 10,000 A in thickness for the preferred titanium although thicknesses of as much as 50,000 A are readily achievable in relatively short sputtering cycles. This layer of sputtered titanium (or other reactive metal) coats surface 42 of titanium electrode assembly 38. The sputtered layer 50 adheres to surface 42 of titanium electrode assembly 38, because a material is chosen which exhibits high chemical reactivity. In the preferred mode, this coincidentally produces a titanium/titanium interface at surface 42. However, sputtered layer 50 may be niobium or other highly reactive metal. A layer of precious metal 60 is next sputtered over the sputtered titanium layer 50. The thickness of layer 60 may be as much as 50,000 A although a thickness of 1,000 A to 10,000 A is preferred. Because layer 60 is sputtered over layer 50, and layer 50 has been activated by the previous sputtering operation, excellent adhesion is established between precious metal layer 60 and reactive metal layer 50. Layer 60 may be selected from the group comprising platinum, rhodium, iridium, or alloys thereof. Of course, FIG. 2 shows layers 50 and 60 much larger than scale. However, they are shown in this manner for the purpose of visualizing the present invention. The sputtering process is accomplished using a Perkin-Elmer Model 2400 Sputtering System. This device has the capability for both AC and Magnetron modes. Experience has shown that the Magnetron mode produces a higher sputtering rate, being on the order of 10,000 to 100,000 A per hour because of the magnetic field present. However, the AC mode which has a slower sputtering rate is preferred because it produces much more uniform target ware. Although the controlling of pressure within the sputtering chamber controls porosity and therefore, the presence of pinholes, the resulting product will almost always contain some significant number of pinholes. It is therefore very important to use a material for electrode housing 38, which is of itself a body compatible cathode material such as titanium. Attempts were made to fabricate similar electrodes using vacuum deposition techniques. Because the target in the vacuum deposition process is not self-cleaning as it is in the sputtering process, results obtained were not nearly as satisfying. The vacuum deposition electrodes showed less adhesion because the target was not self-cleaning and because the technique inherently provides less control over the porosity of the surface. The present invention as taught herein is readily applied in the preferred mode to endocardial pacing electrodes. Those of ordinary skill in the art will be readily able to apply the teachings herein to body implantable leads of other configurations and applications. What is claimed is:

Claims: II. CLAIMS 1. A body implantable lead (10) for delivery of stimula tion energy to a desired body site of the type having an exposed electrode (30) carried by the lead, cha racterized by said electrode comprising an electrode assembly (38) of a first material and a second ma terial (60) covering the exposed portion of said first material, said second material being a noble metal or an alloy thereof. 2. A body implantable lead according to claim 1 wherein said second material (80) has a thickness no greater 0 than 50,000 A 3. A body implantable lead according to claim 2 wherein said second material (60) has a thickness of approxi 0 mately 1,000 to 10,000 A 4. A body implantable lead according to claim 1, 2 or 3 wherein said first material is titanium. 5. A body implantable lead according to any one of claims 1 to 4 wherein said second material (60) is selected from the group consisting of platinum, iridium, rhodium and alloys thereof. 6. A body implantable lead according to any one of claims 1 to 5 further comprising: a layer (50) of a reactive metal intermediate said first material and said second material (60). 7. A body implantable lead according to claim 6 where in said layer (50) of reactive metal is 1,000 AO to 10,000 AO in thickness. 8. A body implantable lead according to claim 6 or 7 wherein said layer (50) of reactive metal is titanium

Inventors: KRASKA, ROBERT C., LESSAR, JOSEPH F.
Applicants: MEDTRONIC, INC.
Application year: 1982
Main classification: A61N1
IPC codes: A61N1