Implantable Spiral Wound Stimulation Electrodes

Abstract

Electrical stimulation electrodes for implantation within a living body for transmission of stimuli to excitable neural or contractile cells. The electrodes are characterized by flexibility to permit following of contour variations and maintenance of electrical contact with the stimulation region. The electrodes include a pair of parallel spaced apart helically wound conductors maintained in this configuration. When the electrode is implanted, undesired excitation of nearby structures can be avoided.

Description

The invention described herein was made in the course of work under a grant or award from the Department of Health, Education and Welfare.

This invention relates to implantable stimulation electrodes for the application in a living body of a current of sufficient magnitude in proximity of any desired excitable neural or contractile cells to activate those neural or contractile cells in the immediate vicinity of the electrode while avoiding excitation of nearby structures. Although not limited thereto, the electrodes are useful in bladder stimulation systems, of which those disclosed in the patents of co-inventor William E. Bradley, U.S. Pat. No. 3,236,240 and No. 3,543,761, are exemplary.

Previous stimulation electrodes have included single or concentric disc electrodes which are characterized by a lack of mechanical flexibility which often causes them to lose electrical contact with the stimulation region. Grid or mesh electrodes have electrical characteristics which require the use of a backing sheet of insulative material which cause the electrode to give unidirectional stimulation. Such electrodes have the further disadvantage that any large sheet covering a contractile organ in the body causes a massive fibrotic reaction that ultimately impairs the organ's contractile ability. Prior art electrodes have had the further disadvantage of induction of wide current fields which stimulate contiguous structures in an undesirable manner.

The electrode according to the present invention is flexible such that it can follow contour variations in an organ without interfering with the organ's normal contractile function. The electrode may be embedded in the wall of the organ so that only neural and contractile cells located within this wall will be excited when electrical stimuli are applied. The electrode is characterized by two parallel conducting wires extending in a helical fashion and maintained in this configuration.

The invention is illustrated in the accompanying drawings in which:

FIG. 1 is a diagrammatic illustration of one form of spiral wound electrode with insulating core material;

FIG. 2 is a similar diagrammatic illustration showing the approximate volume of tissue which is stimulated by a stimulus applied in the use of the electrode;

FIGS. 3 and 4 are diagrammatic illustrations of the steps of an imbrication technique for embedding the electrode in tissue to be electrically stimulated; and

FIG. 5 is an illustration of an alternative form of electrode in which conductor wires are interwoven with insulative spacing and supporting strands.

Referring to FIG. 1, the electrode, indicated generally at 10, is comprised of two parallel conducting wires 11 and 12 wrapped helically around a flexible cylindrical insulative core 13. Current flow between conductors 11 and 12 is provided through the tissue in which the electrodes are embedded. The lead-in conductors 15 and 16 are insulated by flexible insulative tubes 17 and 18, respectively, between the source of electrical stimuli and the body situs to be stimulated by the electrode. By using a small diameter core, between about 1 to 3 millimeters, and varying the spacing between the two wires, the field confining properties of the electrode can be varied.

The spatial relationship between conductors 11 and 12 may be maintained by providing core 13 with shallow spiral channels or grooves in which the conductors are wound; or the conductors may be secured by means of adhesive material, such as medical grade Silastic adhesive; or the conductor may be produced by printed circuit techniques.

The flexibility of the electrode configuration is determined by the mechanical properties of the core and of the wires. The wire can be any implantable electrical current conductor, such as stainless steel, platinum or one of its alloys, such as platinum-irridium, carbon impregnated polyester (Dacron), or the like. The insulating core is typically made from implantable grade silicone rubber, nylon, silk, or other implantable insulating material. The insulating tubes 17 and 18 for the lead-in wires are typically implantable silastic or polytetrafluoroethylene (Teflon) tubing.

As seen in broken lines in FIG. 2, experimentation has shown that stimulus is effective within a volume described by a cylinder 20 concentric to the electrode 10, the diameter of the cylinder being equal to the diameter D of the electrode plus twice the spacing S between the helical wires (D + 2S) and with a height or length equal to the electrode length L plus the cylinder diameter (L + D + 2S). As a general rule, the best confining properties of the electrode are achieved when the spacing S between parallel wires 11 and 12 approximately equals the diameter D of the electrode. For example, to stimulate the nerve net supplying the intestine or urinary bladder, the electrodes were made with the spacing between the conductors and diameter equal, and between about 1 and 3 millimeters. Stimuli were completely confined to the intestinal or bladder wall during chronic implants of two months duration. The tissue remained excitable during this time when stimulated at rates between 10 and 40 pulses per second at pulse amplitudes below 50 volts. Intravesical pressure rises in excess of 60 centimeters H.sub.2 O were typically obtained and complete bladder evacuation was induced.

The electrode is so designed that it will follow contour variations in an organ stimulated by it without interfering with the organ's normal contractile function. Thus, by embedding the electrode in the wall of an organ, only neural and contractile cells located within this wall will be excited when electrical stimuli are applied between the two wires in the electrode so that contraction of only the stimulated organ will be accomplished. The electrode can be surgically embedded by incision of the outer layers of the organ to be stimulated, placing the electrode entirely in the wound produced by the incision and suturing over it.

Alternatively, as shown in FIGS. 3 and 4, the electrode 10 may be positioned on the surface 21 of the organ by means of sutures 22. The sutures 22 are passed into and out of the tissue 23 of the organ on one side of the electrode, passed over the electrode and then into and out of the tissue on the opposite side, as shown in FIG. 3. Then, as shown in FIG. 4, the electrode 10 is imbricated in the tissue by pulling the opposite ends of the suture 22 together around and over the electrode, and the ends of the suture are tied. This, of course, is repeated as required along the length of the electrode.

An alternative embodiment of the stimulation electrode according to the present invention is shown in FIG. 5. According to this embodiment, the conductor wires 11A and 12A, here shown as solid black lines, are interwoven with six strands of small diameter insulative strands such as braided nylon suture, silk, polyester, or the like. Conductors 11A and 12A are wrapped helically in parallel spaced relation, with the spacing between conductors approximately equaling the diameter of the electrode, as already described. In order to maintain the conductors spaced from one another, a pair of insulator spacer strands 25 and 26 are wrapped in the same helical fashion spaced between the adjacent winds of the conductors. These insulative strands 25 and 26 are shown as being stippled, for greater clarity.

The conductor wires 11A and 11B and alternating spacers 25 and 26 are maintained in the described relationship by means of four insulative strands 27-30 helically wound in the opposite direction from the conductor and spacer strands with substantially the same spacing and interwoven with the conductors and spacers. Thus it will be seen that one supporting strand is interwoven so as to pass under each conductor wire and over each spacer strand. The next adjacent supporting strand is interwoven to pass over each conductor wire and under each spacer strand and so on for the length of the electrode. The result is a symmetrical tubular configuration in which the helical spatial relationship of the conductor wires is maintained and separation of the wires is assured. A flexible insulative core may be within the electrode or not, as desired. For convenience the woven electrode is shown as formed on an elongated cylindrical core or mandrel which may be removed before the electrode is put into use. Conductors are provided with insulated lead-ins for connection to a source of electrical stimuli. This form of electrode may be imbricated in the manner already described.

It is apparent that many modifications and variations of this invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of example only and the invention is limited only by the terms of the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrical stimulation electrode for implantation in a living body, said electrode consisting essentially of:

A. a pair of elongated parallel spaced apart helically wound electrical conductors, said conductors being conductive filaments,

B. means for supporting and maintaining said conductors in parallel spaced apart helically wound configuration, said means comprising:

1. a pair of elongated parallel spaced apart helically wound insulative spacer strands, said spacer strands being alternated with said conductors and wound in the same direction, and

2.

2. two pairs of elongated parallel spaced apart helically wound insulative supporting strands, said supporting strands being wound in the opposite direction from said conductors and spacer strands and interwoven therewith, and

C. insulated conductor means for connecting said helically wound conductors

to a source of electrical stimuli. 2. An electrical stimulation electrode according to claim 1 further characterized in that said conductors are spaced apart a distance about equal to the diameter of the helical configuration.

3. An electrical stimulation electrode according to claim 2 further characterized in that the spacing between said conductors and the diameter of said helaical configuration are between about 1 and 3 mm.

4. An electrical stimulation electrode according to claim 1 further characterized in that said insulative strands are formed from nylon.

5. An electrical stimulation electrode according to claim 1 further characterized in that said insulative strands are formed from silk.

6. An electrical stimulation electrode according to claim 1 further characterized in that said insulative strands are formed from polyester.

7. An electrical stimulation electrode according to claim 1 further characterized in that said conductors are formed from stainless steel wire.

8. An electrical stimulation electrode according to claim 1 further characterized in that said conductors are formed from platinum wire.

9. An electrical stimulation electrode according to claim 1 further characterized in that said conductors are formed from platinum alloy wire.

10. An electrical stimulation electrode according to claim 1 further characterized in that said conductors are formed from carbon impregnated polyester filament.