POLYMER-METAL FOIL STRUCTURE FOR NEURAL STIMULATING ELECTRODES

Abstract: A neural stimulating electrode structure includes a sheet of metal foil (18) having portions defining holes (19). The holes form patterns allowing the metal foil to flex without buckling when subjected to compressive forces. The holes also define discrete electrodes on the metal foil when the foil is cut. A first polymeric base layer (14) covers a first face of the metal foil. A second polymeric base layer (14) covers a second face of the metal foil. The first and second polymeric base layers fill the holes in the metal foil. A third polymeric base layer covers both the first and second polymeric base layers. The third polymeric base layer has portions defining pairs of holes (28). Each pair of holes is associated with one of the discrete electrodes. Each hole exposes a different portion of the discrete electrode for contacting tissue, passing current and/or measuring voltage.

Description: POLYMER-METAL FOIL STRUCTURE FOR NEURAL STIMULATING ELECTRODES Background of the Invention The present invention relates to the biomedical arts. It finds particular application in conjunction with cuff electrodes for stimulating nerves and will be described with particular reference thereto. It will be appreciated, however, that the present invention is also applicable to other types of implanted electrodes and biomedical devices. Many types of nerve tissue damage do not heal. Such injuries leave a patient permanently without an appropriate nerve path for electrical signals or action potentials which travel from the brain to muscles or other biological tissue to cause a biological response. Similarly, such a discontinuity prevents action potentials from carrying sensory information or other biological feedback from the tissues to the brain. Moreover, there is also a tendency for action potentials to commence propagating naturally from below the injury site to the biological tissue causing an unconscious and unwanted biological response. Analogously, action potentials can propagate from above the injury site to the brain causing pain and erroneous sensory feedback. Electrical potentials can be applied to nerve trunks and fibers to block the propagation of action potentials and for controllably initiating the propagation of action potentials in an upstream direction, a downstream direction, or both. Neural electrodes, such as illustrated in U.S. Pat. No. 4,602,624 to Naples, Sweeney, and Mortimer, and U.S. Pat. No. 5,324,322 to Grill, Jr., Creasey, Ksienski, Veraart and Mortimer controllably initiate and/or block action potentials in the nerves. Although the prior art neural electrodes have proven effective, they do have drawbacks. Initially, certain types of prior art neural electrodes are labor intensive to manufacture, requiring the hand of a skilled fabricator to weld conducting wires to foil and fitting these foil conductors to silicone rubber coverings. Hand fabrication results in various problems, including the tendency of the foil conductors buckling when the metal-silicone structure is flexed. The flexure causes the foil to work harden and ultimately become a source for mechanical failure, stress corrosion, cracking and/or subsequent conduction failure. Also, for neural electrodes whose patterns are formed with a metal deposition on a flexible substrate (e.g., silicone rubber or polyamide) there is a tendency for the metal to crack. The cracks on the surface appear as "cracked mud" at a microscopic level. Such cracks have the tendency to fill with uncured polymer when pressure is applied to a metal-polymer structure. This causes the uncontrolled formation of metal "islands," separated by the nonconducting polymer. Nonconducting polymer between "islands" eliminates electrical conduction along a trace by causing multiple open circuits. The present invention provides a new and improved neural electrode and method of manufacture which overcomes the above-referenced problems and others. Summary of the Invention A neural stimulating electrode comprises a sheet of metal foil having holes formed at predetermined locations. The holes are positioned to form patterns which allow the metal foil to flex without buckling when subjected to compressive forces. The holes also define a plurality of discrete electrodes on the metal foil when the foil is cut. A first polymeric base layer covers a first face of the metal foil. A second polymeric base layer covers a second face of the metal foil. The first and second polymeric base layers fill the holes in the metal foil. A third polymeric base layer covers both the first and second polymeric base layers. The third polymeric base layer has portions defining pairs of holes. Each pair of holes is associated with one of the discrete electrode structures . Each hole in each of the pairs of holes exposes a different portion of the discrete electrode structure for contacting tissue, passing current and/or measuring voltage. In accordance with one aspect of the invention, the three polymeric base layers are laser cutable. In accordance with another aspect of the invention, the three polymeric base layers are silicone rubber. In accordance with another aspect of the invention, the neural stimulating electrode forms a curving surface for conforming to an outside surface of a nerve. A principal advantage of the present invention is that voids formed in the metal foil filled with silicone rubber form islands of insulating material, which along with regions of conducting material create a continuous composite structure with a conducting surface that accommodates compression (i.e., bending) without buckling. Another advantage of the present invention is that it reduces manual labor and manufacturing time. Still another advantage of the present invention is that it facilitates automated, mass production techniques by using laser cutable materials (e.g., platinum or silicone rubber) along with chemical etchable materials. Yet, another advantage of the present invention is that it facilitates implantation and interconnection with electrical leads. Still yet another advantage of the present invention is that it prolongs electrode life. A further advantage of the present invention is that the electrode is electrically insulated and more flexible than solid foil sheets sandwiched between two layers of polymer. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. Brief Description of the Drawings The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. FIGURE 1 is a top elevational view of an electrode construction in accordance with a first embodiment the present invention; FIGURE 2 is a flow chart for manufacturing an electrode in accordance with the present invention; FIGURE 3a is a top elevational view of sheet of metal foil; FIGURE 3b is a top elevational view of a sheet of metal containing voids; FIGURE 3c is a top elevational view of a partially constructed electrode consisting of a metal sandwiched between two layers of a polymer; FIGURE 4 is a cross-sectional side view of the polymer-metal-polymer structure shown in FIGURE 3c; FIGURE 5 is a top elevational view of a completed electrode according to the first embodiment; FIGURE 6a is a graphic representation of an electrode structure which has been prestressed to form the structure with a curved surface; FIGURE 6b is an enlarged elevational view of a portion of FIGURE 1 with compression forces; FIGURE 7 is a top elevational view of the metal sheet having holes forming a mesh; FIGURE 8 is a top elevational view of a partially constructed electrode consisting of a metal sandwiched between two layers of a polymer; FIGURE 9 is a cross-sectional side view of the polymer-metal-polymer structure shown in FIGURE 8; FIGURE 10 is a top elevational view of a completed electrode according to the second embodiment; FIGURE 11 is a top elevational view of a portion of FIGURE 8 illustrating that longitudinal loads may be accommodated by the planar structure while avoiding buckling; and, FIGURE 12 is a cross-sectional side view of a two- layer electrode structure. Detailed Description of the Preferred Embodiments FIGURE 1 sets forth a cut-out trace of an electrode structure 24 (FIGURE 5) . The cut-out trace 24 includes a plurality of electrodes, each composed of a conductive path 16 connected to an electrical lead pad 10 at one end and an electrical connection portion 12 at a second end. A polymer 14 provides mechanical interconnections between the lead pads 10 and connection portions 12. It is also noted that in the preferred embodiment, the polymer is a laser cutable polymer. The construction of the electrode structure begins with a sheet of metal foil 18 (FIGURE 3a) . In the preferred embodiment, the thickness of the foil 18 is preferably between about lOμm and about 75μm. With reference to the flow chart of FIGURE 2 and the illustration of FIGURE 3b, holes 19 are cut or etched into the foil 18 in a cutting or etching step A. The holes 19 are cut to form patterns for creating the multiple electrodes within the foil sheet 18. If desired, residual internal stresses are then removed from the foil 18 in an annealing step B. Next, the laser cutable polymer 14 is introduced into the holes 19 of the metal foil 18 in a filling step C. The laser cutable polymer 14 is preferably silicone rubber. However, other materials with similar qualities are also contemplated. The resulting metal-polymer combination is then sandwiched between two layers of the polymer 14 in a covering step D to form a polymer-metal- polymer structure 22 (see FIGURES 3c and 4) . It is appreciated that fill step C can be accomplished as part of course step D, and that when fill step C and cover step D are distinct steps, it is possible to use different polymers in steps C and D. The electrodes are then defined within the polymer-metal-polymer structure 22 in a tracing step E. The cut-out trace 24, having lead pads 10 and connection portions 12, is then created from the tracing in a cutting step F (see FIGURE 1) . To complete the construction of the electrode structure 26, the cut-out trace 24 is laminated with an additional polymer layer or sheet of the laser cutable polymer 14 in step G to form a laminated sheet of electrodes 26 (see FIGURE 5) . Either one or both surfaces of cut-out trace 24 can be laminated. Alternatively, the cut-out trace 24 itself may be used or the cut-out trace 24 may be associated in some other manner to a non-conducive surface. Lead pads 10 and connection portions 12 are created in the electrode structure 26 in a create openings step H. Preferably, a laser cutting device is used to create the openings. However, the openings may also be created mechanically. The beam properties (e.g., intensity, wavelength, etc.) of the laser are adjusted so that the laser cutting device cuts through the laminated layer and polymer layer of step D while leaving the metal foil 18 intact. Therefore, the foil 18 is exposed to form connection portions 12 which provide metal to tissue contact for passage of current or measurement of voltage. The openings 28 expose the lead pads 10 and connection portions 12. The lead pads 10 are connected to respective connection portions 12 via covered paths 16. A prestressing step I optionally forms the electrode structure 26 into a curved surface as shown in FIGURE 6a. A curved surface is useful for conforming the electrode structure to an outside surface of a nerve or other similar structure 27. FIGURE 6b shows a portion of the connection path 16. Arrows in FIGURE 6b represent compressive forces experienced by the foil 18 when it is flexed. The holes cut in the metal foil 18, in conjunction with the polymer filling 14, allow the foil to flex without buckling when subjected to these compressive forces. A second embodiment of the invention will be described with reference to FIGURES 2 and 7-11. The construction of the electrode structure begins with a sheet of metal foil 18. With reference to FIGURE 2, holes are cut or etched into the foil 18 during the cutting step A to form a mesh pattern (see FIGURE 7) . The holes could be cut into rectangular, or other geometric circular shapes. After the holes are cut, residual internal stresses may optionally be removed from the foil 18 during the annealing step B. Next, the laser cutable polymer 14 is introduced into the holes of the metal foil 18 in the filling step C. The laser cutable polymer 14 is preferably silicone rubber. However, other materials with similar qualities are also contemplated. The resulting metal-polymer combination is then sandwiched between two layers of the polymer 14 in the covering step D to form the polymer- metal-polymer structure 22 (see FIGURES 8 and 9) . The metal layer of the resulting polymer-metal-polymer structure 22 is electrically conductive in both the X and Y planes . The tracing of desired electrodes is then formed from the polymer-metal-polymer structure 22 in the tracing step E (see FIGURE 10) . The cutting step F includes cutting holes through the tracing to create isolated regions of electrical conduction. The cutting is preferably done using a laser device. To complete the construction of the electrode structure, the cut-out trace 24 is laminated in coating step G. The lead pads 10 and electrical connection portions 12 are created in the coated tracing in the create opening step H. Preferably, a laser cutting device is used to create the openings. A beam intensity of the laser is adjusted so that the laser cutting device cuts through the laminated layer and polymer layer while leaving the metal foil 18 intact. The foil 18 is exposed at the lead pads 10 and connection portions 12 to provide metal to tissue contact for passage of current or measurement of voltage . In FIGURE 10, lead pad 10-1 and connector 12-1 are electrically connected to each other and electrically isolated from all other lead pads and connectors on the electrode structure. Similar connections exist between lead pads 10-2, 10-3 and connectors 12-2, 12-3, respectively. FIGURE 11 shows a portion of the foil mesh 18. Arrows in FIGURE 11 represent compressive forces experienced by the foil mesh 18 when it is flexed. The holes cut in the foil mesh 18, in conjunction with the polymer filling 14, allow the foil to flex without buckling when subjected to these compressive forces. FIGURE 12 illustrates a two-layer electrode structure created by stacking two polymer-mesh-polymer structures on top of one another. Other multiple-layer electrode structures are also contemplated. The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims: CLAIMS: - 12 " Having thus described the preferred embodiment, the invention is now claimed to be: 1. A neural stimulating electrode structure comprising: a sheet of metal foil having a first face and a second face, with holes located within the metal foil, the holes formed in predetermined patterns allowing the metal foil to flex without buckling when subjected to compressive forces, the holes also defining a plurality of discrete electrodes on the metal foil when the foil is cut; a first polymeric base layer covering the first face of the metal foil; a second polymeric base layer covering the second face of the metal foil, the first and second polymeric base layers further filling the holes in the metal foil; a third polymeric base layer covering both the first and second polymeric base layers; and external connection points, formed at locations where portions of the third polymeric base layer and one of the first and second polymeric base layers have been removed, such that portions of the metal foil are exposed, the exposed metal foil portions corresponding to pairs of holes, each pair of holes being associated with one of the discrete electrodes, and each hole in each of the pairs of holes exposing a different portion of the discrete electrode for at least one of contacting tissue, passing current and measuring voltage. 2. The neural stimulating electrode structure as set forth in claim 1 wherein: the three polymeric base layers are silicone rubber. 3. The neural stimulating electrode structure as set forth in claim 1 wherein: the three polymeric base layers are laser cutable. 4. The neural stimulating electrode structure as set forth in claim 1 wherein: a second neural stimulating electrode structure is stacked on the neural stimulating electrode structure. 5. The neural stimulating electrode structure as set forth in claim 4 wherein: the metal foil is annealed for removing residual internal stresses. 6. The neural stimulating electrode structure as set forth in claim 1 wherein: the metal foil has a thickness between about lOμm and about 75μm. 7. The neural stimulating electrode structure as set forth in claim 1 wherein: the neural stimulating electrode structure forms a curving surface for conforming to an outside surface of a nerve. 8. A neural stimulating electrode structure comprising: a sheet of metal foil having a first face and a second face, with holes forming a mesh, the mesh allowing the metal foil to flex without buckling when subjected to compressive forces, the mesh also enabling electrical conduction in an x-direction and a y- direction across the metal foil; a first polymeric base layer covering the first face of the metal foil; a second polymeric base layer covering the second face of the metal foil, the first and second polymeric base layers filling the holes defined by the mesh; a third polymeric base layer covering both the first and second polymeric base layers; and external connection points, formed at locations where portions of the third polymeric base layer and one of the first and second polymeric base layers have been removed, such that portions of the metal foil are exposed, the exposed metal foil portions corresponding to pairs of holes, each pair of holes being associated with an electrically isolated region on the metal foil . 9. The neural stimulating electrode structure as set forth in claim 8 wherein: the three polymeric base layers are silicone rubber. 10. The neural stimulating electrode structure as set forth in claim 8 wherein: the three polymeric base layers are laser cutable. 11. The neural stimulating electrode structure as set forth in claim 8 wherein: a second neural stimulating electrode structure is stacked on the neural stimulating electrode structure. 12. The neural stimulating electrode structure as set forth in claim 8 wherein: the metal foil is platinum foil having a thickness between about lOμ and about 75μm. 13. The neural stimulating electrode structure as set forth in claim 8 wherein: the metal foil is annealed for removing residual internal stresses . 14. An electrode structure comprising: a plurality of electrodes formed on a common structure, each electrode defined to include a conductive path connected between a first lead pad and a second lead pad, the conductive path configured to include a plurality of voids to increase the flexibility of the conductive path; and, a non-conductive material located in relationship to the electrodes so that (i) for each electrode there is a conductive capability between the first lead pad, the conductive path, and the second lead pad, (ii) each of the electrodes are isolated from each other; and (iii) each of the conductive paths are covered and voids therein are filled. 15. A method of manufacturing a neural stimulating electrode structure, the method comprising: etching holes into a metal foil sheet, the holes forming patterns allowing the metal foil to flex without buckling when subjected to compressive forces, the holes also defining a plurality of discrete electrodes on the metal foil; filling the holes with a laser cutable polymer, the polymer accommodating the compressive forces; sandwiching the metal foil between two layers of the polymer; cutting the sandwiched metal foil to form at least two of the at least two electrodes within the metal foil, each electrode being electrically isolated from other electrodes on the metal foil; covering the at least two electrodes with a second coating of the polymer; and, creating one pair of openings for each of the at least two covered electrodes, one opening in each of the pairs of openings exposing a different portion of the metal foil, the exposed metal foil being used for interfacing the electrode with one of tissue and an electrical lead. 16. The method of manufacturing a neural stimulating electrode structure according to claim 15, further including: after the etching step and before the filling step, annealing the metal foil sheet for removing residual internal stresses. 17. The method of manufacturing a neural stimulating electrode structure according to claim 15, wherein the sandwiching step includes: applying the polymer to a first face of the metal foil; and, applying the polymer to a second face of the metal foil. 18. The method of manufacturing a neural stimulating electrode structure according to claim 17, wherein after the step of cutting and before the step of creating, the method further includes: forming the at least two electrodes into a curved surface for conforming to an outside surface of a nerve. 19. The method of manufacturing a neural stimulating electrode structure according to claim 15, further including: creating a multiple layer electrode structure by stacking at least one other neural stimulating electrode on the neural stimulating electrode.

Inventors: MORTIMER J THOMAS, MORTIMER, J., THOMAS
Applicants: AXON ENGINEERING INC, MORTIMER J THOMAS, AXON ENGINEERING, INC., MORTIMER, J., THOMAS
Application year: 1997
Main classification: A61N1
IPC codes: A61N1