GB2274396A

GB2274396A - An electrode assembly

Info

Publication number: GB2274396A
Application number: GB9301459A
Authority: GB
Inventors: Douglas D Russell; Gordon M Young; John C Wyatt
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1993-01-26
Filing date: 1993-01-26
Publication date: 1994-07-27
Anticipated expiration: 2013-01-26
Also published as: GB2274396B; GB9301459D0

Abstract

An electrode assembly 1 for detecting an electro potential in a subject or object includes a conductor 2, preferably consisting of parallel fibres. The conductor has a first face 3 which makes contact, via an electrolyte, with the subject. An electrode 4 is connected to another region 5 of the conductor to provide an output signal representative of the detected electro potential. A reservoir of electrolyte is connected to the conductor (eg impregnated between the fibres) and electrolyte is provided during use. Alternatively, the electrode may consist of a "Swiss roll" of metal film and electrolyte-impregnated fibre, or the electrolyte can be a solid stick. <IMAGE>

Description

AN ELECTRODE ASSEMBLY This invention relates to an electrode assembly for detecting an electro potential in a subject or object, which assembly is particularly, but not exclusively, suitable for detecting electro potential activity in a human brain.

Conventional electrodes for detecting electro potential activity in a human brain take the form of a silver chloride coated silver button which is attached directly to a shaven area of the subject human such as to the head thereof by sticking plaster, with or without the interposition of a spot of electrolyte between the electrode surface and human skin.

With such a form of electrode it is necessary to shave the subject contact area as the interposition of hair will prevent effective contact being established between the electrode and the skin surface. This need to shave is undesirable and considerably restricts the freedom and frequency with which an electrode may be applied to different sites on the human subject.

Additionally, the use of sticking plaster to attach the conventional electrode to the skin of a human subject is uncomfortable and permits relative movement to take place between the electrode, skin and underlying bone if the subject moves. This can give spurious results and leads to the production of output signals from the electrode which are not representative of the electro potential being monitored. Similar errors can be introduced by the loss of electrolyte from between the electrode and skin surface such as by drying up of the electrolyte.

Further errors can be introduced by the relatively high impedance displayed by such conventional electrodes which has the effect of masking a relatively weak electro potential being monitored.

There is thus a need for a generally improved electrode assembly which at least minimises the foregoing disadvantages inherent in conventional electrodes.

According to a first aspect of the present invention there is provided an electrode assembly for detecting an electro potential in a subject or object, including a conductor having a first face for making contact, via an electrolyte, with a surface of the subject or object in which an electro potential is to be detected, an electrode connected to a second face or region of the conductor other than said first face, which electrode is operable to provide an output signal representative of a detected electro potential, and electrolyte means forming part of or connected to said conductor whereby, in operation, electrolyte can be provided between the first face of the conductor and the surface of the subject or object.

Preferably the conductor is an elongate bundle of side-by-side, substantially parallel, fibres whose two ends provide said first and second faces, and more preferably the conductor fibres are made of polyester, are bound together with resin, and are impregnated with a substantially non-ionic surfactant.

Conveniently the electrode is a metal chloride coated metal wire and more preferably the metal wire is made of silver and the chloride coating is of silver chloride.

Advantageously the electrode chloride coating is in turn coated with a conducting gel layer preferably made from an aqueous solution of potassium chloride and gelatin or methyl cellulose.

Preferably the electrolyte means is at least in part formed by a blind bore in the conductor opening through said second face thereof into which bore the electrode wire projects, which bore forms a reservoir for containing an electrolyte which, in operation, flows therefrom along the conductor fibres to said first face thereof.

Conveniently the electrode is a sheet of silver foil coated on one side thereof with a layer of silver chloride and a sheet of fibre saturated with electrolyte, the two sheets being placed one upon the other with the fibre sheet in contact with the silver sheet on the side thereof opposite to the chloride coating, and rolled up together so that the chloride layer is outermost on the roll with the fibre sheet separating the chloride coating on one side of the silver sheet from the silver surface on the other side of the silver sheet.

Advantageously the electrolyte means includes the conductor and the electrode, with one end of the electrode being connected to the second face of the conductor and with the fibre sheet and conductor together forming a reservoir for electrolyte which, in operation, flows from the fibre sheet, along the conductor to the first face thereof.

Preferably the fibre sheet has a substantially non-conducive, substantially non-reactive, substantially open matrix, and more preferably is made from cellulose material.

Conveniently the electrode assembly includes a housing around at least the junction between the electrode and conductor forming, with the electrode and conductor, a reservoir for containing electrolyte to be passed along the conductor to the first face thereof.

Advantageously the conductor is in the form of a elongate stick of hard electrolyte gel which also constitutes all or part of the electrolyte means.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic longitudinal cross sectional view through an electrode assembly according to a first embodiment of the present invention, Figure 2 is a diagrammatic longitudinal side view of an electrode assembly according to a second embodiment of the present invention, and Figure 3 is an end view of an electrode portion of the assembly of Figure 2.

An electrode assembly according to the present invention is intended for use to detect electro potentials in a subject or object and for the sake of convenience will be described hereafter in terms of use with a human being for detecting electro potential activity in a human brain. As shown in the accompanying drawings an electrode assembly 1 according to a first embodiment of the invention as illustrated in Figure 1 includes a conductor 2 having a first face 3 for making contact, via an electrolyte (not shown), with a surface of the subject or object in which an electro potential is to be detected.

The assembly 1 also includes an electrode 4 connected to a second face or region 5 of the conductor 2 other than the first face 3. The electrode 4 is operable to provide an output signal representative of a detected electro potential.

The assembly 1 also include electrolyte means forming part of or connected to the conductor 2 whereby, in operation, electrolyte can be provided between the first face 3 of the conductor and the surface of the subject or object.

In the assembly 1 shown in Figure 1 of the accompanying drawings the conductor 2 is an elongate bundle of side-by-side substantially parallel, fibres whose two ends provide the first and second faces 3 and 5. Any suitable fibres can be used such as polyester fibres, bound together with resin and impregnated with a substantially non-ionic surfactant. A suitable conductor 2 is in the form of a coherent bundle of parallel fibres 3mm square or in diameter and lOmm in length.

The electrode 2 in the Figure 1 embodiment is a metal chloride coated metal wire, preferably a silver wire of 26 Standard Wire Gauge coated over a length of approximately 30mm with a coating of silver chloride.

The electrolyte means is at least in part formed by a blind bore 6 in the conductor 3 opening through the second face or region 5 thereof. Conveniently the bore 6 is formed with a lmm diameter drill to a depth of about 7mm. The silver chloride coated end of the wire 4 is a push fit into the bore 6 which forms a reservoir for containing an electrolyte which in operation flows therefrom along the fibres of the conductor 2 to the first face 3 thereof. Any suitable medical electrolyte can be used preferably a Normal aqueous solution of potassium chloride to which is added a standard electrolyte gel.The gel addition has the effect of delaying drying out of the electrolyte at the junction between the face 3 of the conductor 2 and the subject surface and the amount added should be less than that needed to increase the viscosity of the electrolyte to an undesirable level such as to prevent adequate flow.

The bundle of fibres constituting the conductor 2 may be soaked in electrolyte for a period of time preferably of the order of 12 hours to improve the supply of electrolyte to the conductor face 3. Further to improve the supply of electrolyte to the face 3 of the conductor 2 the conductor 2 is contained at least partially in a housing 7, preferably made of plastics material, which surrounds at least the junction between the electrode 4 and the conductor 2. The conductor 2 is secured in the housing 7 in any convenient manner such as by projecting fingers or flanges 7a. The housing 7 has an open ended bore 8 therethrough of a diameter larger than the cross sectional dimensions of the conductor 2 and electrolyte can be introduced into the end of the bore 8 through which the wire 4 extends. The free end of this wire 4 is connected directly to the input of an amplifier.The construction of the housing 7 is such that the electrolyte comes into contact only with the chloride layer on the wire to avoid unwanted electrolytic effects and consequent noise generation.

In operation the electrode assembly of Figure 1 can be applied to a surface of a human subject without the requirement for use of adhesive fixing to the subject surface.

This may be done by mounting the electrode assembly in a head piece which conveniently takes the form of straps fitting over the subject head at a spacing therefrom. To this end the housing 7 may have a flange 9 through which the conductor 2 projects. The assembly 1 is attached by means of this flange 9 to the head straps at a convenient distance from the subject surface so that the projecting end of the conductor 2 makes contact with the surface via the face 3.

This form of electrode assembly according to the invention avoids the requirement to shave hair from the subject surface as the fibres of the conductor 2 will readily make contact through the hair with the subject surface. The use of the fibre conductor 2 avoids direct contact between the chlorided silver electrode surface and the surface of the subject which with conventional button electrodes considerably reduces the useful life of the chloride layer. Adequate electrolyte, in use, flows down the fibres of the conductor 2 to the face 3 to maintain an adequate interface of electrolyte between the conductor 2 and subject surface to maintain good electrical contact.

The electrode assembly of Figures 2 and 3 according to a further embodiment of the present invention is basically similar in construction to the embodiment of Figure 1 and like parts have been given like reference numbers and will not be further described in detail. The major difference in construction with the Figures 2 and 3 embodiment is that the electrode is of two part construction. In this embodiment the silver wire 4 is connected at one end 4a to a spiral electrode 10 which directly abuts at its other end the face 5 of the conductor 2. The spiral electrode 10 is shown in end view in Figure 3.

This spiral electrode 10 comprises a sheet of silver foil 11 coated on one side lla thereof with a layer of silver chloride, and a sheet 12 of fibre saturated with electrolyte.

The two sheets are placed one upon the other with the fibre sheet 12 in contact with the silver sheet 11 on the side llb thereof opposite to the chloride coating side lla so that the chloride coated side or layer lla is outermost on the roll with the fibre sheet 12 separating the chloride coating or layer on the side lla from the uncoated silver surface on the other side llb of the silver sheet 11. This "Swiss roll" form of construction can be seen in Figure 3 of the accompanying drawings. The fibre sheet 12 has a substantially non-conductive, substantially non-reactive, substantially open matrix, and may be made from cellulose. A suitable material is filter paper with a mass of 92 gm/m2, a thickness of 0.22mn, an ash content of 0.06% and a retention value of 20pin.

In this embodiment the electrolyte means comprising the conductor 2 and the electrode 10 so that the fibre sheet 12 and conductor 2 together form a reservoir for electrolyte which in operation flows from the fibre sheet 12 along the conductor 2 to the first face 3 thereof. This reservoir may be bounded at least in part, and augmented by, the location of the electrode 10 in the housing 7 so that the bore 8 thereof provides additional reservoir capacity for electrolyte.

The layered construction of the electrode 10 in the second embodiment of the present invention also has the effect of desirably reducing the impedance of the electrode assembly.

The relatively high impedance of standard button electrodes gives spurious results and can mask small electro-potentials being detected. The impedance is reduced in the electrode assemblies according to the present invention by effectively increasing the surface area of the electrode 4 or 10. This is done in the first embodiment by the depth to which the chloride coated surface of the wire projects into the conductor 2.

By use of an electrode assembly according to the present invention which has lower impedance than conventional medical button electrodes and a smaller contact area at the face 3 with the subject surface, together with the ability to mount such electrode assemblies in contact with the subject surface without direct attachment by adhesive thereto, it is possible to avoid errors inherent in the conventional button electrode.

Additionally because of the way in which the electrode assemblies can be mounted on frame straps passing over and around the subject head, it is possible to use a greater number of electrode assemblies per unit area, with increased sensitivity, and by use of the electrolyte reservoir avoid tracking of the electrolyte between the electrode assembly conductors on a subject surface. Additionally there is no requirement to shave the subject surface such as a scalp.

Further improvements can be achieved with the assemblies illustrated, for example by coating the chloride coating on the wire electrode 4 of the Figure 1 embodiment with a conducting gel layer which reduces the risk of damaging the chloride layer. The conducting gel layer may be an aqueous solution of potassium chloride gelled with gelatine or methyl cellulose, Alternatively the conductor 2 need not be a bundle of fibres as illustrated but could be in the form of an elongate stick of hard electrolyte gel which also constitutes all or part of the electrolyte means. In other words the conductor 2 is made self lubricating.

Claims

1. An electrode assembly for detecting an electro potential in a subject or object, including a conductor having a first face for making contact, via an electrolyte, with a surface of the subject or object in which an electro potential is to be detected, an electrode connected to a second face or region of the conductor other than said first face, which electrode is operable to provide an output signal representative of a detected electro potential, and electrolyte means forming part of or connected to said conductor whereby, in operation, electrolyte can be provided between the first face of the conductor and the surface of the subject or object.

2. An assembly according to claim 1, wherein the conductor is an elongate bundle of side-by-side,- substantially parallel fibres, whose two ends provide said first and second faces.

3. An assembly according to claim 2, wherein the conductor fibres are made of poylester are bound together with resin, and are impregnated with a substantially non-ionic surfactant.

4. An assembly according to claims 1 to 3, wherein the electrode is a metal chloride coated metal wire.

5. An assembly according to claim 4, wherein the metal wire is made of silver and the chloride coating is of silver chloride.

6. An assembly according to claim 4 or claim 5, wherein the electrode chloride coating is in turn coated with a conducting gel layer.

7. An assembly according to claim 6, wherein the conducting gel layer is an aqueous solution of potassium chloride and gelatine or methyl cellulose.

8. An assembly according to claim 4 when appended to claim 2 or claim 3, wherein the electrolyte means is at least in part formed by a blind bore in the conductor opening through said second face thereof into which bore the electrode wire projects, which bore forms a reservoir for containing an electrolyte which, in operation, flows therefrom along the conductor fibres to said first face thereof.

9. An assembly according to claim 1 or claim 2, wherein the electrode is a sheet of silver foil coated on one side thereof with a layer of silver chloride and a sheet of fibre saturated with electrolyte, the two sheets being placed one upon the other with the fibre sheet in contact with the silver sheet on the side thereof opposite to the chloride coating, and rolled up together so that the chloride layer is outermost on the roll with the fibre sheet separating the chloride coating on one side of the silver sheet from the silver surface on the other side of the silver sheet.

10. An assembly according to claim 9, wherein the electrolyte means includes the conductor and the electrode, with one end of the electrode being connected to the second face of the conductor and with the fibre sheet and conductor together forming a reservoir for electrolyte which, in operation, flows from the fibre sheet along the conductor to the first face thereof.

11. An assembly according to claim 9 or claim 10. wherein the fibre sheet has a substantially non-conductive, substantially non-reactive, substantially open matrix.

12. An assembly according to claim 11, wherein the fibre sheet is made from cellulose material.

13. An assembly according to claim 8 or claim 10, including a housing around at least the junction between the electrode and conductor forming with the electrode and conductor, a reservoir for containing- electrolyte to be passed along the conductor to the first face thereof.

14. An assembly according to claim 1 or claim 9, wherein the conductor is in the form of an elongate stick of hard electrolyte gel which also constitutes all or part of the electrolyte means.

15. An electrode assembly for detecting an electro potential in a subject or object, substantially as hereinbefore described and as illustrated in Figure 1 or Figures 2 and 3 of the accompanying drawings.