Electrode For Transcutaneous Stimulation

Abstract

An electrode for transcutaneous stimulation capable of operating within an optimum power density range. The electrode is provided with a current diffusing screen which maintains a physiologically acceptable power density by increasing the effective stimulation surface area. In addition, a second screen is employed to increase the surface area of the input electrical connection to inhibit electrolysis as well as to lower the overall electrode impedance. The entire electrode structure is flexible so that it is easily conformable to any surface of the body.

Description

BACKGROUND OF THE INVENTION

Man has long transmitted electrical signals to and from the body by way of electrical contact with the skin. Early examples of a device which was applied to the skin to accomplish an electrical "treatment" are found in the U.S. Pat. Nos. 1,059,090 and 1,305,725. The electrodes used with these devices have been found to be poor for chronic stimulation (24 to 48 hours continuous stimulation) because of their rigidity and inability to be readily conformed to many parts of the body. These electrodes are extremely bulky because of the very thick sponge which they use to avoid "hot spots". In addition, these electrodes use a rigid metal plate which prevents adequate current diffusion across the surface of the sponge because the plate does not allow the surface of the sponge to readily conform to the body. Later developments are typified by the electrodes used in conjunction with the well-known electrocardiograph (EKG).

A typical EKG electrode generally consists of a rigid or semi-rigid metal plate covered by a sponge-like material. A conductive paste permeates the sponge to limit motion artifacts so as to provide an ionic conductive path which avoids high concentration gradients of metallic ions between the plate and skin. The function of such an electrode is merely to make an electrical contact with the skin for monitoring purposes only. EKG electrodes are generally not appropriate for stimulation purposes because they are so thick that they do not readily conform to all parts of the body and create hot spots due to high power density because of their relatively small surface area.

When I first explored the viability of specific transcutaneous nerve stimulation, an electrode similar to the EKG electrode and having an increased surface area was employed. I found that this electrode had hot spots due to its rigidity and lack of current diffusion and operated with excessive power densities due to high interface impedance. As indicated in The Skin Senses, edited by Donald R. Kenshalo, published by Charles C. Thomas Company (1968), a power density of 250 millicalories/cm.sup.2 will result in skin tissue damage when the current exceeds 100 milliamps and the duty cycle (ratio of on time to off time) exceeds 20 percent. That current level and duty cycle are generally considered as maximums for skin stimulation because above those values excessive muscle stimulation may take place. In addition, it has been determined that human tissue resistance decreases with an increase in current. The practical consequence of this is that transcutaneous stimulation should be accomplished with constant current stimulators. Also, constant current stimulation has a long term continuous effect which is not present with constant voltage stimulation and, constant current stimulation is independent of tissue and electrode impedance variations from one patent to another. With the constant current restraint, it was discovered that electrode power densities can best be optimized by operating upon the electrode impedance and the effective stimulation surface area.

SUMMARY OF THE INVENTION

The present invention provides a disposable electrode which is especially adapted for chronic transcutaneous stimulation. It is designed such that its impedance lies within an optimum range and incorporates a conductive member with substantially uniformly diffuses the current over a contact area greater than that in the prior art devices thus avoiding excessive power densities. Through these features, the power density of the electrode of the present invention is maintained within an optimum range which is below the damaging value stated above. In addition, the entire electrode structure may be made flexible so that it will conform to any surface of the body.

The present invention provides a body conformable electrode which comprises an electrical connection means, a skin interface means, and a flexible means for diffusing current flow from said electrical connection means with substantial uniformity across substantially the entire surface area of said skin interface means.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIG. 1 is an exploded view of a preferred embodiment of the present invention.

FIG. 2 is a cross-section of the embodiment of FIG. 1 is an assembled state .

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in exploded form a chronic disposable transcutaneous stimulator electrode in a preferred embodiment. Specifically there is shown an adhesive member 10 having a single adhesive surface 11. On the adhesive surface 11 of the member 10, there is positioned a snap button mount 12, having a head 13 which passes through a hole 18 in the member 10. Snap button 12 provides a means for electrically attaching the electrode via a lead to a stimulator (not shown). The snap button 12 may be of a type commonly employed on EKG electrodes, it being understood that the particular attachment may take various forms such as, for example, a screw-on type of attachment. Positioned on the adhesive side 11 of the member 10 and in contact with the button 12, is a low impedance screen 14. A snap ring 15 provides an electrical connection for the electrode by being inserted through the screen 14 and into electrical contact and mechanical locking engagement with snap button 12. Snap ring 15 also serves to hold screen 14 in intimate electrical contact with the snap button 12. Alternatively, the snap ring 15 could be eliminated and snap button 13 could be spot welded in position on the screen 14. Although shown as a screen, the element 14 may, within the restriction that it be flexible, be a solid metal foil, or a conductive plastic or rubber.

On top of, and in electrical contact with the screen 14 and snap ring 15 is a diffuser screen 16. The diffuser screen 16 is a highly flexible screen which acts as a low impedance current diffuser. Above the diffuser screen 16 is a skin interface pad 17 whose function is to prevent direct contact between the skin and the diffuser screen 16.

The electrode shown in FIG. 2 is assembled by placing the various elements into contact with each other and the adhesive surface 11. First, the head 13 of snap button 12 is forced through the hole 18 of the member 10. The low impedance screen 14 is then adhered to the surface 11 and the snap ring 15 is placed through it and into electrical contact and mechanical locking engagement with the snap button 12. The diffuser screen 16, which is larger than the low impedance screen 14, is then placed over the screen 14. Because of the size difference, the outer border of the diffuser screen 16 will be in direct contact with the adhesive surface 11. The interface pad 17, which is larger than the diffuser screen 16, is placed over the diffuser screen 16 into border contact with the adhesive surface 11. The interface pad 17 may be a foam material as commonly applied to EKG electrodes or, in the alternative, may be a cloth material. Because of the use of the thin flexible screens 14 and 16, the interface pad 17 may be much thinner than the sponge material used with the devices of U.S. Pat. Nos. 1,095,090 and 1,305,725. Both the low impedance screen 14 and the current diffuser screen 16 may be made of 316 stainless steel. All of the elements of the electrode with the possible exclusion of the snap ring 15 and snap button 12, are highly flexible which makes it possible to conform the electrode to any surface of the body.

In use, an electrically conductive paste, Saline or any other suitable substance is applied to the assembled electrode which is then adhered to the body by the remaining adhesive surface 11. The diffuser screen 16 allows the paste to permeate through to the low impedance screen 14. The diffuser screen 16 may alternatively be a flexible solid member having an interrupted surface to allow the suitable substance to flow through to the low impedance screen 14. In those instances where the low impedance screen 14 is a metallic substance, its surface area is sufficient to inhibit excessive polarization and electrolysis which might otherwise develop at both the snap button and snap ring due to excessive current densities. In any event, the surface area of the low impedance screen 14 when added to the surface area of the snap ring 15, is an aid to lowering the overall impedance of the electrode, without regard to the material used. Since the characteristics of the human skin-stimulator interface dictate a constant current stimulator, power density control can best be achieved by controlling the effective electrode contact area and electrode impedance. To eliminate hot spots, it is imperative that the current be diffused in a substantially uniform manner over the entire electrode contact area.

All of the above objects are accomplished in the electrode of the present invention. The impedance is held at a minimum through the combined effects of the low impedance screen 14 and current diffuser 16. In addition the current diffuser 16 provides a low impedance conformal contact to the skin interface pad 17 which increases the possible effective electrode contact area while eliminating hot spots. That is, the current is diffused in a substantially uniform manner over substantially the entire electrode contact area.

It has been determined that the optimum average power density range is 7.14 microwatts/cm.sup.2 /second to 357 microwatts/cm.sup.2 /second. The power density is within these limits when the effective electrode surface area is within the range of 3.5 cm.sup.2 to 50 cm.sup.2, and when the stimulator parameters of current and duty cycle are within the limits stated above. The most effective electrode surface area has been found to be approximately 12 cm.sup.2.

Obviously, many modifications and variations of the present invention are possible in view of the above teachings. One example of such a modification is the combination of the low impedance screen and diffuser screen into a single screen folded over on itself. Therefore, it is to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. A body conformable electrode which comprises:

electrical connection means including

means for increasing the electrical contact area of said electrical connection means comprising a substantially flat electrical conductor of a size greater than said electrical connection means;

skin interface means; and

electrically conductive flexible means positioned between said electrical connection means and said contact area increasing means for diffusing current flow from said electrical connection means with substantial uniformity across substantially the entire surface of said skin interface means.

2. The electrode of claim 1 wherein said flexible means has an interrupted surface.

3. The electrode of claim 1 wherein said flexible means comprises a conductive screen.

4. The electrode of claim 1 wherein said flexible means comprises a metal screen.

5. The electrode of claim 1 wherein said skin interface means has an effective skin contact surface area of at least 3.5 cm.sup.2 but no greater than 50 cm.sup.2.

6. The electrode of claim 5 wherein said skin interface means has an effective skin contact surface area of approximately 12 cm.sup.2.

7. The electrode of claim 1 wherein said contact area increasing means is metal and has a surface area sufficient to inhibit electrolysis and polarization.

8. The electrode of claim 7 wherein said contact area increasing means comprises a metal screen.

9. The electrode of claim 8 wherein said flexible means has an interrupted surface.

10. The electrode of claim 8 wherein said flexible means comprises a conductive screen.

11. The electrode of claim 8 wherein said flexible means comprises a metal screen.

12. The electrode of claim 8 wherein said skin interface means has an effective skin contact surface area of at least 3.5 cm.sup.2 but no greater than 50 cm.sup.2.

13. The electrode of claim 12 wherein said skin interface means has an effective skin contact surface area of approximately 12 cm.sup.2.