Depolarized Pre-Gelled Electrodes

The present invention relates generally to electrodes for detecting the potential of the electric field present on the tissue or skin of a living organism, and more particularly to a structure which operates to maintain electrodes in a depolarized state prior to use.

Recent advances in medical technology have often involved the measurement or recording of electrical activity which occurs as a function of underlying biological activity. This in turn has led to considerable research and development in the field of sensing electrodes adapted for use in a number of applications where biopotentials are measured on the skin or tissue of a living organism. Silver-silver chloride electrodes combined with an electroconductive paste or gel have been found to be particularly suitable for biopotential measurements, as illustrated by U.S. Pat. Nos. 4,328,809 to B. H. Hirschowitz, et al. and 4,270,543 to K. Tabuchi et al. Electrodes of this type have been previously assembled in matrix configurations for use in screening and mapping applications as illustrated by U.S. Pat. Nos. 4,416,288 to Freeman, 4,486,835 to Bai and 4,628,937 to Hess et al.

Many electrodes are packaged in a pre-gelled state wherein an electrolytic paste or gel is packaged as part of the electrode. The gel may be located in a central gel reservoir consisting of a molded cup, or it may be contained in a dye-cut hole in a foam which encapsulates a gel saturated open cell compressible foam column, such as shown by U.S. Pat. No. 3,868,946. In most instances, the pre-gelled electrodes are sold ready for use with an electrically conductive material such as metal or a metal chloride in contact with the electrolyte gel.

A pre-gelled electrode system is generally not a battery by itself, but forms a part of a battery-system consisting of two or more electrodes placed on the body. In such a system, a complex battery is formed consisting of many interactive components including the electrode material (frequently silver/silver chloride), the electrode gel, internal body chemistry and external skin conditions, skin preparation, temperature, air condition and chemistry, etc. Obviously, some of these factors are not subject to control, but in order to get the best data possible, especially in instances where DC biopotentials are of interest, artifacts, such as DC offsets, should be reduced to the lowest level. Clearly, pre-gelled electrodes can possibly represent such undesired DC voltage artifact which should be limited to the lowest voltage possible; ideally zero volts. Most pre-gelled electrodes when introduced in the battery system outlined above contribute some unwanted DC voltage (polarization effect) to biopotential measurements. It is important to lower the possibility of such DC artifacts occurring in a degree sufficient to have a substantial adverse effect on biopotential measurements.

It is not feasible, in situ, to measure only the DC contributed by the pre-gelled electrodes when they are placed in a battery system. Instead, manufacturers and researchers have attempted to establish the magnitude of these potentials by mounting two pre-gelled electrodes under test in a back to back fashion with the electrolyte gels of the two electrodes touching and the leads from the electrodes connected to a voltmeter. In fact, this is a fair measurement since the technique does create a battery by using two electrodes and eliminating the body. When pre-gelled electrodes are tested this way, using a high impedance digital voltmeter, DC voltages to approximately 10 millivolts are frequently found.

Many attempts have been made to reduce the polarization effect present in biopotential electrodes. This was first done by modifying the electrode structure in an attempt to avoid dissimilar junction effects as illustrated by U.S. Pat. No. 3,496,929 to F. J. Domingues. Subsequently, an oxidizing agent has been added to the electrolyte gel to reduce the metal on the surface of the electrode sensing element to a metal action which reacts with the anion of the electrolyte to produce an insoluble compound which is deposited on the sensing element to render it non-polarizable. This method of producing a non-polarizable electrode is illustrated by U.S. Pat. No. 4,377,170 to H. M. Carim.

Although the prior art structures for reducing the polarization effect present with biopotential electrodes accomplish this purpose to some extent, they require a basic chemical material change in the electrodes, and do not effectively eliminate offset potentials which can cause a significant error in a biopotential measurement taken with the electrode. Prior art structures do not provide a simple, removable method for depolarizing various types of pre-gelled electrodes. Also, these prior structures do not provide either an efficient or cost effective method for depolarizing a large number of electrodes combined in an electrode mapping or screening matrix.

It is the primary object of the present invention to provide a novel and improved assembly to effectively depolarize single or multiple pre-gelled biopotential electrodes.

Another object of the present invention is to provide a novel and improved removable electrical short circuiting system which effectively depolarizes one or more pre-gelled biopotential electrodes during shipment and storage without causing degradation of the gel.

Yet another object of the present invention is to provide a novel and improved removable electrical short circuiting system which effectively depolarizes one or more pre-gelled biopotential electrodes, bringing each electrode to a stable, near zero DC offset level, until use. This is done by shipping and storing the electrode system electrically shorted before use, at which time the short circuiting system is removed.

Yet still another object of the present invention is to provide a novel and improved pre-gelled biopotential electrode or electrodes having a removable depolarizing structure bridging the electrolyte gel and an electrode terminal or leads. The depolarizing structure is operable to depolarize single electrodes, groups of paired electrodes, and multiple electrodes which are arranged in an electrode matrix.

A further object of the present invention is to provide a novel and improved packaging assembly wherein a single or a plurality of pre-gelled biopotential electrodes are formed in a package with a removable electrical short circuiting package element which effectively depolarizes the electrode or electrodes during shipment and storage, and until use.

Still further object of the present invention is to provide a novel and improved electrode matrix assembly which is formed with a removable depolarizing layer bridging the area between the electrode leads for the assembly and the electrolyte gel. This depolarization layer closes the open end of an insulating cup containing the gel and then extends into contact with the electrode leads or terminals.

These and other objects of the present invention are accomplished by forming a single biopotential electrode or a matrix of such electrodes which is adapted to be stored and shipped in a pre-gelled state with an electrolyte. The electrode or electrode matrix is formed in a manner wherein the electrolyte gel and the contact or contacts for the electrode may be bridged by a shorting element which is removable prior to the use of the electrode or electrode matrix. Preferably, the shorting element consists of electroconductive material, such as wire, strap or electroconductive packaging material, which is removed before the electrode is used. This shorting element can close the open end of a gel-containing opening for an electrode, or may close a plurality of gel containing openings in an insulating face sheet for an electrode matrix.

FIG. 1 is an exploded view of an electrode matrix formed in accordance with the present invention;

FIG. 2 is a sectional view of the assembled electrode matrix of FIG. 1 provided with electrolyte gel;

FIG. 3 is a sectional view of an electrode packaging assembly for the electrode matrix of FIG. 2;

FIG. 4 is a sectional view of a second embodiment of an electrode depolarizing assembly of the present invention; and

FIG. 5 is a sectional view of a third embodiment of an electrode depolarizing assembly of the present invention.

Referring now to FIGS. 1 and 2, a pre-gelled biopotential electrode matrix indicated generally at 10 is formed in a manner which particularly adapts it for use in an electrode matrix depolarizing assembly formed in accordance with the present invention. The electrode matrix 10 includes a central substrate 12 which consists of a sheet of Mylar or other flexible, electrical insulating material, with electrodes 14 and electrical conductors 16 for each electrode applied as overlying layers on one surface of the substrate. The electrodes 14 may be positioned on the substrate in any desired number or configuration required for a specific biopotential mapping or screening use. These electrodes may be formed of any suitable conductive metal commonly used for biopotential electrodes but for purposes of description herein, the electrodes will be described as silver/silver chloride electrodes. The electrodes 14 are formed by first depositing a layer of silver onto the substrate 12, and at the same time, the leads 16, each of which contacts one of these silver layers, are deposited to provide conductors for the electrodes. These leads may also be formed of silver, although other electrically conductive materials such as copper may also be employed. A silver chloride layer is then deposited over the silver layer at each electrode location to form the basic silver/silver chloride layers for the electrode.

Although electroplating or deposition is one method for forming the electrodes 14 and leads 16 on the substrate 12, other methods commonly known to the printed circuit art may be employed. For example, the necessary compositions to form the electrodes and electrode leads may be coated on the substrate 12 by a brush coating or screen printing process as an alternative to electrode deposition. In all cases, the result will be a plurality of biopotential electrodes arranged in a pattern on a flexible, insulating substrate with conductive leads extending outwardly to one edge of the substrate.

To support the substrate 12 and to permit the electrode matrix to be shipped and stored in a pre-gelled state, the substrate is mounted between two thin sheets of foam or other insulating material 18 and 20. The sheet 20 is a solid backing sheet, while the sheet 18 is a face sheet which is formed with a plurality of punched or die-cut holes 22 extending therethrough. The holes 22 are positioned to correspond with the electrodes 14 when the face sheet 18 is placed into position over the substrate 12. It will be noted that the face sheet 18 and the backing sheet 20 are cut to be shorter than the substrate 12 so that an end 24 of the substrate where the electrical leads 16 terminate projects outwardly from the electrical insulating foam face and backing sheets as shown in FIG. 2. In some instances, it is possible for only one of the sheets on the side of the substrate bearing the electrical leads to be cut short, and in this case, it would be the sheet 18.

Once the face and backing sheets 18 and 20 are in position, the holes 22 are filled with electrolyte gel 26 to provide a pre-gelled electrode matrix. Thus, the silver chloride layer of each of the electrodes 14 is in contact with the electrolyte gel, and the unit is now in condition for depolarization.

To depolarize the pre-gelled electrode matrix 10 in accordance with the present invention, the electrical lead 16 for each electrode must be electrically connected with the electrolyte gel 26 which covers that specific electrode. This can be accomplished with separate electrically conductive strips extending between each individual electrode lead and the gel covering the electrode, but ideally, all electrodes in a matrix may be depolarized by a single conductive sheet, such as the sheet 28 in FIG. 3, which contacts all of the leads 16 and then extends into contact with the electrolyte gel covering each electrode. It is important for the conductive, depolarizing element, whether it is a conductive sheet or a single strip, to be an inert material which does not cause the electrolyte gel to degrade. This is especially true when the depolarizing element is a sheet which constitutes part of a packaging assembly, such as that shown in FIG. 3, for in this form the depolarizing element can be in contact with the gel for extensive periods while the pre-gelled electrode matrix 10 is shipped and stored prior to use.

In the formation of the package of FIG. 3, an electrically conductive sheet 28 is engaged with the surface of the face plate 18 so as to close the holes 22 and contact the gel 26 in each of these holes. This same conductive sheet extends out over the end of the foam sheet 18 and then down into contact with the projecting ends of the electrical leads 16 on the end 24 of the substrate. Thus, the electrolyte gel is connected by a direct electrical connection to the leads 16 to thereby provide a direct short circuit for the potentials generated by the battery effect provided by the pre-gelled electrode structure. In effect, the electrically conductive sheet 28 shorts out the opposed terminals of the battery thereby depolarizing the electrode matrix. This electrically conductive sheet can be a conductive Mylar.TM. sheet or any suitable electrically conductive sheet which will provide a direct conductor between the electrolyte gel and the electrical leads 16 without causing degradation of the electrolyte. Thus, the sheet 28 is preferably an inert conductive sheet, such as a carbon impregnated Mylar.TM. sheet, which will not cause either chemical or electrical degradation of the gel. Once this conductive sheet is in place, the electrode may be packaged in any suitable, non-conductive outer wrap or packaging configuration 30 to protect the electrode matrix in storage and shipment.

The electrode matrix packaging assembly of FIG. 3 effectively depolarizes the biopotential electrodes in the matrix so that a depolarized matrix is available at the time of use. At this time, the packaging material 30 and the electroconductive sheet 28 will be removed and the electrode will be used immediately.

As illustrated by FIG. 4, the present invention may be used with conventional pre-gelled biopotential electrodes in either individual or matrix form to depolarize the electrode prior to use. The invention is not limited to the combination of the depolarizing element with a packaging assembly, although this is an effective method for constructing the element. In FIG. 4, a single electrode indicated generally at 32 is mounted upon a support member 34 which may be a ring of plastic foam material or other material which forms an insulating cup. The electrode may include a silver layer 36 and a silver chloride layer 38 which are mounted in an insulating cup 40 formed by the support member 34. An electrolyte gel 42 is placed in contact with the silver chloride layer 38 to provide a pre-gelled electrode.

The electrode 32 includes a projecting snap button cable connection 44 which is in electrical contact with the silver layer 36 and which is designed to snap onto an electrical lead to a measuring and indicator unit. This is obviously a conventional pre-gelled electrode structure and is used as illustrative of a number of conventional pre-gelled biopotential electrodes with which the depolarizing structure of the present invention can be effectively employed.

To depolarize a single biopotential electrode of the type shown in FIG. 4, an electrically conductive material 46 is provided which is of sufficient size to contact the cable connector or terminal 44 and then to pass around one end of the support 34 across the opposite side of the assembly into contact with the electrolyte gel 42. This conductive material, which may be the same as that forming the sheet 28 in FIG. 3, provides an electrical path between the terminal 44 and the electrolyte gel for the biopotential electrode to provide a short circuit path therebetween. For example, the conductive material 46 may be formed by a Mylar outer layer 48 which supports an inert conductive coating layer 50. A pressure releasable adhesive is provided between the layer 50 and the support member 34 to hold the conductive material 46 in place but to permit its removal by means of a pull tab 52.

In FIG. 4, electrical contact is made between the terminal 44 and the conductive material 50, by an aperture 54 in the conductive material 46 which snaps over and receives the terminal. Once the conductive material is in place, suitable outer packaging material, such as the packaging material 30, can be placed over and around both ends of the unit to completely enclose the unit. This packaging material is a non-electrically conductive material which encloses the conductive material and the electrode.

With a single electrode, the conductive material 46 may be in the form of a elongated strip extending between the electrolyte and the terminal. Preferably, the strip is of sufficient width to cover the cup 40 to both enclose the gel and to provide maximum contact therewith. For purposes of illustration, the electrodes herein have been described as silver/silver chloride electrodes, and the electrolyte gel used might be sodium or potassium chloride. However, any suitable metal and electrolyte combination commonly used to form biopotential electrodes could be employed with the present invention.

FIG. 5 illustrates a very desirable structure for shorting pre-gelled electrodes during shipment and storage prior to use. This structure, indicated generally at 56 includes two pre-gelled electrodes 58a and 58b mounted back to back and held together by a nonconductive carrier 60 of Mylar or similar material. The Mylar carrier 60 extends between the electrodes 58a and 58b and permits subsequent separation of the electrodes. Holes 61 formed in the Mylar permit full contact between the electrolyte gel in the electrode 58a and the gel in the opposing electrode 58b. The components of each electrode will be given the same reference numbers differentiated by the letters "a" and "b".

The electrodes include a support member 62 which forms an insulating cup 64 open at one end 66 and which receives electrolyte gel 68. The carrier 60 extends across the open ends of the support members for each electrode and is provided with the openings 61 which permit the gel 68 in each electrode to make electrical contact with the gel in the opposing electrode. These openings should be as large as possible to permit full contact between the electrolyte gels 68a and 68b while still facilitating the subsequent separation of the electrodes 58a and 58b.

In contact with the electrolyte gel in each insulating cup is an electrode element 72 having a projecting snap button cable connector or terminal 74 for connection to an external cable. Each electrode element 72 may include a silver layer 76 and a silver chloride layer 78 or other conventional electrode element forming materials.

The structure 56, due to its configuration, constitutes a true battery and this battery is electrically short circuited during shipment and storage by a conductive wire or strip 80 electrically connected between the terminals 74a & b. This conductive wire or strip shorts the potential battery and brings the electrode elements to a stable, near zero volt DC level. Prior to use, the conductive wire or strip 80 is removed and the electrodes 58, now at near a zero DC level, are separated from the carrier 60 and immediately placed on a patient for bioelectric measurement purposes. After the wire or strip is removed, the electrodes should be promptly used, for if they are left unshorted they may once again develop unwanted DC offset potentials.

The electrode depolarizing structures of the present invention operate effectively to permit shipment and storage of biopotential electrodes in a pre-gelled condition while negating the battery effect normally present with such electrodes. The electrodes are effectively short circuited by an electrical conductor which may constitute part of the packaging assembly for the pre-gelled electrode and which is easily removed prior to electrode use. Once the conductor is removed, an unpolarized electrode structure is provided which will sense biopotentials.