Capacitive medical electrode

Medical electrodes in embodiments of the teachings may include one or more of the following features: (a) a metallic conductor, (b) the metallic conductor sandwiched between a first dielectric layer adjacent a top surface of the metallic conductor and a second dielectric layer located on a bottom surface of the metallic conductor, (c) a conductive gel coating on at least one of the first and second dielectric layers, (e) the metallic conductor, the dielectric layers, and the conductive gel being wrapped to form a multi-tiered electrode having a plurality of conductive surfaces, (f) an adhesive adhering the metallic conductor with the dielectric layers, (g) a tab connector to provide a connection to electrical monitoring equipment and (h) an attachment connector to provide electrical connection with a patient.

TECHNICAL FIELD

The present teachings relate generally to medical electrical sensing and stimulation devices. More particularly the present teachings relate to a capacitance electrode for sensing and reproducing electric potentials at the surface of living tissue and introducing electrical potentials into the tissue.

BACKGROUND

The use of electrodes for sensing electrical activity at the surface of living tissue, such as during the performance of an electroencephalograph (EEG), an electromyograph (EMG), an electrocardiograph (EKG) or a galvanic skin response (GSR) procedure is well known. These electrodes and others are also used for stimulating living tissue, e.g., TENS (Transcutaneous Electric Nerve Stimulation), defibrillation, pacing (internal and external), or for transferring energy from electrical devices to the body as in electrocautery. These and other prior electrodes provide resistive coupling to the test subject, so as to facilitate the monitoring of electrical activity therein or contain a metallic conductor in chemical contact with an electrolytic medium.

Resistively coupled electrodes have proved to be generally suitable for their intended purposes, however, these electrodes do possess inherent deficiencies, which detract from their utility. For example, resistively coupled electrodes can consume a lot of power, which is undesirable for battery driven devices. Further, they can generate a substantial amount of heat, which can cause burns in defibrillation applications.

Additionally, there are limitations that may occur with both the sensing and stimulation applications using resistively coupled electrodes. Motion artifact, half-cell potential, and non-linearity or distortions of the signal at the electrode-electrolyte interface are some of the limitations that may occur with sensing applications. In stimulation applications, limitations also include non-uniform current density, spikes in amplitude at the onset of the signal, and resistive power loss. All of which are related to the electrode-patient interface.

The transmission of an electrical signal between an electrode and an ionic medium involves certain capacitive and chemical issues. Current exists in metal as a flow of electrons through the crystal lattice of the material. In contrast, current in an ionic solution requires the movement of cations and/or anions through the solution. The electrical interaction between metal and an ionic solution can occur as a capacitive process, an inductive process, or as a chemical reaction.

Typically, both capacitive and chemical interactions take place during electrical activity between a patient and an electrode. The volume of ionic solution on a metal is called the Helmholtz double layer and contains both the capacitive and chemical reactions. Generally all electrodes have a capacitive component except for silver/silver chloride electrodes, commonly used for ECG sensing, at small currents. Additionally, platinum or other inert metals can transmit signals in a purely capacitive mode, but also at small currents only.

The nature of the reaction for most electrodes depends on multiple factors. Generally, the metal composition of the electrode determines the threshold at which chemical reaction will occur, and what they will be, presuming a saline ionic solution. Most metals, including stainless steel, will produce hydrogen and chlorine gases as a byproduct of the chemical reaction of the metal with the ionic solution. This is undesirable because chlorine gas can possibly irritate the patient's tissue at the anode. Further, these gases can cause corrosion of the electrode itself.

Generally, all electrodes, except for silver/silver chloride electrodes and a few others, have a strong capacitive component. Silver/silver chloride avoids this capacitive component at small currents by “anodal chloridization of the electrode surface”. However, the silver/silver chloride electrodes create a capacitive interference with large currents. Electrochemical polarization of physiological electrodes is an undesirable but seemingly unavoidable phenomenon that detracts from the performance of implanted electronic prosthetic devices. In the case of noble metals, polarization causes a significant waste of stimulation energy at the electrode surface. With non-noble metals, the energy waste is even greater and may involve electrolytic corrosion reactions. Such corrosion may destroy the electrode and may possibly leave toxic residues in body tissues. The electrode-electrolyte interface presents to a cardiac pacemaker a highly capacitive load having multiple time constants of the same order of magnitude as the 1- or 2-millisecond (msec) duration of a pacemaking impulse. Thus, an applied square wave of current on the electrodes does not obey Ohm's law and does not elicit a square wave of voltage, nor is the voltage waveform a constant slope (ramp), as would be expected from a single lumped capacitor. Rather, the voltage rises in less than a microsecond to an initial value and then more slowly, in at least two different time constants, until the end of the pulse. This capacitive interference, complicates stimulation with this type of electrode.

It has been found that platinum electrodes can avoid toxicity since they produce only a small amount of chlorine. However, approximately 60% of the current through a platinum pacemaker electrode occurs through capacitance. Thus existing stimulation electrodes mostly include capacitive effects, however, the capacitance is complex and extremely variable. This capacitance is undesirable for several reasons. The capacitance varies in a nonlinear fashion with a myriad of parameters including temperature and rate of change of the electrical signal coming from the patient. This capacitance degrades the electrical signal coming from the patient and is impossible to model for filtering purposes. Further, the capacitance's resistive component also degrades the electrical signal. There are at least two ways the chemical reactions occurring at the electrode surface affect electrical signals. First, is the formation of gas bubbles, which act as a physical barrier to current passage. Second, the half-cell potential changes with small perturbations in the physical environment, creating electrical noise.

Purely capacitive electrodes solve this problem since they avoid chemical reactions all together, but existing technology limits their applications. An example of a purely capacitive electrode is dispersive electrodes used in electrocautery. These electrodes consist of a sheet of metal and a non-conductive adhesive gel in contact with the skin. The adhesive gel has low conductivity but a high dielectric constant. The metal foil forms one plate of the capacitor and the skin forms the other. The capacitance of these electrodes typically ranges in the Pico farad range. Because the electrocautery unit operates in the 400-kilohertz range, the reactance is low.

Dispersive electrodes also require a low impedance interface. Resistive dispersive electrodes can monitor the adequacy of the contact between the electrode and the patient's body by contact quality monitoring (“CQM”) circuitry in an electrosurgical generator. Current generator systems have safety circuits, which can detect when a resistive electrode does not have good attachment to the body. If something has caused the electrode to be applied without adequate initial contact with the body or some event during surgery has caused the adequate initial contact to become inadequate, these safety circuits will detect that problem and terminate the current being applied.

While existing capacitive electrodes do not have the edge effect (electrical fields on the edge of the electrode) of concern for resistive type dispersive electrodes and the current transfer is much more uniform across the surface of the electrode compared to resistive types, they are not compatible with the above described CQM circuits, and thus when used do not have this protection against inadvertent misapplication of electrocautery units used during electrosurgery. Lossy dielectric designs, such as the design described in U.S. Pat. No. 5,836,942, overcome this problem, but the design's resistive component adds to unwanted heat generation. Problems faced by designers of medical electrodes include minimizing overall heat generation and maximizing uniformity of the current density.

Another disadvantage associated with traditional stimulating electrodes, is they often cause an initial uncomfortable shock before attaining a stable sensation.

In view of the foregoing, it is desirable to provide an electrode suitable for use in EEG, EMG, EKG, and GSR procedures and the like overcoming the disadvantages of the prior art by manipulating the electrode-electrolyte interface of a medical electrode in contact with a biological system and providing a large capacitance in a standard sized electrode. It is desirable to have a substantially capacitive electrode to avoid chemical reactions. Additionally it is desirable to have an electrode with a constant predictable capacitance and that can avoid a half-cell potential.

SUMMARY

A method of manufacturing a medical electrode in embodiments of the teachings may include one or more of the following features: (a) coating layers of dielectric film having a metallic conductor sandwiched between said layers with a conductive gel, (b) wrapping the metallic conductor sandwiched between the layers of dielectric to form a multi-tiered electrode having a plurality of conductive surfaces, (c) placing the multi-tiered electrode into a plastic case, (d) gluing the metallic conductor to the dielectric layers, (e) wherein the dielectric layers is a capacitive grade Mylar, (f) wherein the metallic conductor is conductive ink, (g) wherein the metallic conductor is silver, (h) wherein the plurality of conductive surfaces can be capacitively coupled to a patient, (i) wherein there is no chemical reaction between the metallic conductor and the conductive gel, and (j) wherein there is no galvanic contact between the metallic conductor and the conductive gel.

A medical electrode according to the present teachings may include one or more of the following features: (a) a metallic conductor, (b) a first dielectric layer adjacent a top surface of the metallic conductor, (c) a second dielectric layer located on a bottom surface of the metallic conductor, (d) a conductive gel coating on at least one of the first and second dielectric layers, (e) wherein the metallic conductor has a plurality of conductive sections, (f) wherein the conductive sections are in capacitive communication with adjacent sections, (g) wherein the dielectric layers are a capacitive grade Mylar, (h) wherein the metallic conductor is conductive ink, (i) wherein the metallic conductor is silver, (j) wherein the plurality of conductive sections can be capacitively coupled to a patient, (k) wherein there is no chemical reaction between the metallic conductor and the conductive gel, and (l) wherein there is no galvanic contact between the metallic conductor and the conductive gel.

A medical electrode according to the present teachings may include one or more of the following features: (a) a metallic conductor, the metallic conductor sandwiched between a first dielectric layer adjacent a top surface of the metallic conductor and a second dielectric layer located on a bottom surface of the metallic conductor, (b) a conductive gel coating on at least one of the first and second dielectric layers, the metallic conductor, the dielectric layers, and the conductive gel being wrapped to form a multi-tiered electrode having a plurality of conductive surfaces, (c) an adhesive adhering the metallic conductor with the dielectric layers, (d) a tab connector to provide a connection to electrical monitoring equipment, (e) an attachment connector to provide electrical connection with a patient, (f) wherein the metallic conductor is electrochemically isolated from the patient so that there is no galvanic interaction between them, and (g) wherein the dielectric layers allow the transfer of electrical signals and energy to and from the metallic conductor and the patient.

A medical electrode according to the present teachings may include one or more of the following features: (a) a plurality of metallic conductors, the metallic conductors sandwiched between a first dielectric layer adjacent a top surface of the metallic conductors and a second dielectric layer located on a bottom surface of the metallic conductors; and (b) a conductive gel coating on at least one of the first and second dielectric layers, the metallic conductors, the dielectric layers, and the conductive gel being layered form a multi-tiered electrode having a plurality of conductive surfaces.

A medical electrode according to the present teachings may include one or more of the following features: (a) a plastic rim, (b) a plurality of wire strands wrapped around the plastic rim, the wire strands spaced apart to form a plurality of conductive surfaces, and a high dielectric material isolating the rim and wire strands from a patient.

DETAILED DESCRIPTION

With reference toFIG. 1, a side profile view of a medical electrode in an embodiment according to the present teachings is shown. Electrode10can include an upper dielectric layer12and a lower dielectric layer14and a conductive metal16sandwiched between layers12and14. Layers12and14can be held together with a thin layer of adhesive18. Dielectric layers12and14can be made of a capacitive grade Mylar, however it is contemplated that layers12and14could be made of most any dielectric material such as cellophane, cellulose, acetate resin, Neoprene, or polyvinylchloride, for example without departing from the spirit of the teachings. Further, conductive metal16can be comprised of silver ink, however, conductive metal16can be comprised of most any conductive material such as carbon, gold, platinum, copper, or stainless steel, for example without departing from the spirit of the teachings. As illustrated adhesive18can be a pressure sensitive biocompatible glue, however, most any type of adhesive could be used such as polyurethane, hot melt, or aqueous adhesive emulsion, for example without departing from the spirit of the teachings. As illustrated, electrode10can be comprised of layers12and14having a thickness of about 0.5 mil, conductor16with a thickness of about 0.30 Mil, and adhesive layer18with a thickness or about 0.43 mil.

With reference toFIG. 2, a top profile view of the medical electrode shown inFIG. 1in an embodiment according to the present teachings is shown. When layers12and14are adhered together with adhesive18sandwiching conductor16, electrode10appears in the form of strip20. Strip20may then be coated with a conductive gel22(FIG. 3). Coating gel22can be comprised of a conducting hydrogel, however, conductive gel22can be most any type of conductive substance, such as standard medical electrode gels, for example without departing from the spirit of the teachings. Gel22can be applied to both layer12and14. Strip20can be lengthened to achieve a higher capacitance, shortened to achieve a lower capacitance, and manufactured to a specific length to achieve a desired capacitance. Sections40are chosen for their size in that they have approximately the same surface area as a standard medical electrode. Further, sections40are chosen by the size of capacitance they will have. For example, each section40can be 1.5 inches in length and can have a capacitance of 7 nanofarads. Therefore a strip of ten sections40would achieve a total capacitance of approximately 70 nanofarads, which has been found to perform well. By knowing the capacitance of each section40of strip20, it is relatively easy to custom manufacture electrodes based upon the need of the user. Further, it is not necessary for section40to have a square shape. As shown inFIG. 2A, electrode10can have sections40with a circular shape.FIG. 2Bshows an electrode10having sections40with a rectangular shape. The shapes can be chosen depending on the application electrode10will be used for and/or which shape provides the desired capacitance. Conductive metal16can be solid with holes32, however, conductive metal can also be a mesh, wire frame, or segmented without departing from the spirit of the invention.

With reference toFIG. 3, a side profile view of a multi-tiered medical electrode in an embodiment according to the present teachings is shown. After conductive gel22is applied, strip20can be rolled or folded section40over section40from distal end28to proximal end30into a three-dimensional structure24to insure all surfaces of strip20are in electrical communication with a biological system34, such as a patient as will be described in more detail below. After strip20is rolled into multi-tiered structure24, multi-tiered structure24can be placed into plastic case38, which holds electrode10into a tightly wound or folded multi-tiered structure24. A tab connector26can be provided at proximal end30to provide a connection to electrical monitoring equipment (not shown), such as an EEG, an EMG, or an ECG, for example. Adhesive18and gel22are not applied to tab26so an external connector (not shown) can be applied to conductor16. Holes32can be located in electrode10to facilitate conduction of an electrical signal to and from skin or tissue34. Holes32allow for conduction of an electrical signal from the upper layers of structure24through gel22to the patient. Each hole32can extend completely through strip20. There is approximately a 2 mm strip around hole32where there is no silver ink coated on strip20. Holes32allow the current to travel down through structure24. Without holes32all of the current would travel along the outer edge of electrode10except for the portion of strip20closest to the patient's skin. Thus holes32shorten the current path to the patient considerably and thus decrease the resistance of electrode10. It's also of note that conducting gel22overlaps the outer edge of electrode10providing another path to the patient. As illustrated, holes32are placed at regular intervals. By having holes32at regular intervals the conductive path from the upper layers to the skin can be shortened. However, it is fully contemplated holes32can be at irregular intervals and randomly placed without departing from the spirit of the present teachings.

The present teachings overcome the disadvantages associated with some prior art systems by manipulating the electrode-electrolyte interface of medical electrode10in contact with biological system34. As discussed above, one embodiment of the present teachings provides conductor16, which can be isolated from gel22so there is no galvanic contact between gel22and conductor16, though not excluding galvanic contact with other components of the system, such as layer14. The dielectric properties of layers12and14not only prevent any galvanic contact between gel22and conductor16, but also allows for the transfer of electrical signals and energy to and from conductor16. Further, layers12and14eliminate any chemical interactions between gel22and electrode10. The folded or rolled structure24also maximizes the surface area of electrode10through its multi-tiered structure24. The function of structure24is to allow a large capacitance in a standard sized electrode through multi-tiered conductor24, which provides a large capacitance when connected to an A/C power source.

As stated above, the variables of note in an electrode design are the uniformity of the current density, the impedance, and (for internal electrodes) the toxicity. Purely capacitive electrode10provides a more biocompatible surface and also eliminates any oxidation-reduction reactions at interface36. Oxidation or reduction reactions at the electrode-electrolyte interface36set up an electrical potential, which can be measured, called a half-cell potential. Half-cell potential is sensitive to physical perturbations in the environment. The fluctuations in half-cell potential constitute an alternating current that is transmitted through patient electrode interface36creating a noisy signal. By eliminating half-cell potential, one source of motion artifact can be eliminated. It is of note that even in platinum electrodes monitoring small signals, the half-cell potential produces noise, even though the system is perfectly polarized. Since layers12and14are sandwiched between conductor16and gel22, there is no chemical reaction to affect the capacitance of electrode10. Therefore, since the capacitance is fixed in electrode10and does not vary with surface and signal characteristics, there are minimal motion artifact results. Further, electrode10and skin34are in series with the resistance of whatever monitor is connected to electrode10. This constitutes an RC circuit, which can be tuned to a particular frequency band. Thus the electrical signals of a patient can be more efficiently monitored without distortion or loss.

With reference toFIGS. 4 & 4A, a side and top profile view of a multi-tiered medical electrode in an embodiment according to the present teachings is shown respectively. Electrode10can be oriented so that a central axis50of electrode10is perpendicular to skin34in a “jellyroll” configuration. This orientation provides a more uniform current density since all layers contribute equally. As can be seen fromFIG. 4A, strip20is rolled in contrast to strip20shown inFIG. 3, which is folded. After strip20is rolled it can be placed in a case52and then attached to a base plate54, which is made of an insulation material. Base plate54is utilized to stabilize electrode10as it sits upon the patient's skin34. Without base plate54, electrode10would have trouble remaining upright and would easily fall over and roll off of the patient.

With reference toFIGS. 5 & 5A, a top and side profile view of a TENS medical electrode in an embodiment according to the present teachings is shown respectively. A TENS electrode60consists of wire mesh62wrapped around plastic rim66and insulated with a high dielectric material64which is biocompatible and inert such as Formvar enamel. This wire mesh design provides for a robust electrode lending itself to use as an implantable electrode, which would be in direct contact with body fluids instead of a conducting gel. As illustrated, dielectric material64is made of Formvar enamel, however it is contemplated material64could be made of most any dielectric material such as glass or polyvinylchloride, for example without departing from the spirit of the teachings. In this application, wire mesh62provides the purely capacitive multi-tiered component. The purely capacitive component is created by the multiple capacitances in-between each strand68of wire mesh62. Wire mesh62can be made of copper, however, wire mesh62can be made of most any conductive metal such as gold, platinum, silver, or stainless steel, for example, without departing from the spirit of the teachings. Similar to multi-tiered structure24discussed above, electrode60is tiered horizontally to a patient's skin to provide the capacitive component similar to the electrode structure ofFIGS. 4 and 4A. The distance between wire lengths determines the electrical communication with an electrolytic medium and determines the capacitive value of electrode60. It's helpful if strands68don't come in contact with each other thus possibly decreasing the effective surface area. A randomly wound structure could be utilized, however, the inventors have discovered this structure performed inferiorly when compared to the structure shown inFIG. 5due to the wire insulation possibly cracking when randomly compressed. The structure of the electrode varies with the application. While the basic design is a wire mesh, the shape of the rim and the number of layers can be varied, for example, without departing from the spirit of the invention. As illustrated inFIG. 7, one variation without a rim provides flexibility for insertion. Other variations include a braided structure, such as a “rope” electrode.

In contrast to prior electrodes, the present teaching discloses electrode60has even lower impedance than previous capacitive electrodes. In the present teachings, the relatively high capacitance minimizes or lowers the significance of the reactance. Present TENS electrodes sometimes use high resistance materials such as carbonized rubber in order to achieve uniform current density. This resistance is undesirable as discussed above. A capacitive electrode with low impedance would provide a uniform current density unlike a resistive electrode. The lower impedance, which occurs as reactance, results in lower power consumption than resistive electrodes discussed above. This proves to be especially useful when electrode60is being used in applications utilizing batteries by greatly prolonging battery life.

FIGS. 6 and 6Ais a side and top profile view of a dispersive medical electrode in embodiments according to the present teachings is shown respectively. The structure of dispersive electrode70minimizes overall heat generation and maximizes uniformity of the current density. Electrode70provides very low impedance, a large part of which is reactance. Electrode70can have a first grid72and a second grid74which are isolated from each other. Grids72&74rest upon and are attached to insulated backing76comprised of Latex material, however, most any type of insulative material could be utilized without departing from the spirit of the teachings. A gel78, such as discussed above is then applied to the grids72and74. Grids72&74are comprised of copper, however, grids72&74can be comprised of most any conductive metal such as gold, platinum, silver, or stainless steel, for example, without departing from the spirit of the teachings. While electrode70capacitive electrode, it allows for use of the safety mechanisms of electrocautery units, such as the contact quality monitoring (CQM) system. This system monitors contact with a patient's skin by comparing the impedance between grids72&74. If the electrode contact is compromised, the CQM disables the electrocautery, thus preventing burns. One could also employ Mylar strip electrodes in a similar manner.

With reference toFIG. 7, an overhead profile view of an implanted pacing/defibrillation medical electrode in an embodiment according to the present teachings is shown. Pacing/defibrillation electrode80consists of a length of steel wire82, formed in a spiral. Electrode80can be formed of steel wire82; however, electrode80can be formed of most any conductive metal such as gold, platinum, copper, or stainless steel, for example, without departing from the spirit of the teachings. Electrode80would, similar to the other electrodes discussed above, be coated with a material, which has a good biocompatibility and a high dielectric constant. The biocompatibility would allow for insertion into a patient and the dielectric would allow for electrical contact between the patient and wire82while preventing any chemical reaction between wire82and the patient. Electrode80could be straightened and threaded into a hollow catheter for insertion. Once the tip of the catheter was attached to the endocardium, the catheter would be removed and the electrode would assume a predetermined shape such as a spiral.

Pacing/defibrillation electrode80provides another embodiment where the technology of the present teachings would improve existing electrodes. Present pacing/defibrillation leads sacrifice uniformity of current density for low impedance. Capacitive leads have an inherently better current uniformity as discussed above.

With reference toFIG. 8, a frequency response curve representation taken from a medical electrode in an embodiment according to the present teachings is shown. One skilled in the art can readily see the electrodes of the present teachings function as an RC circuit when applied to the patient and connected to a monitor.FIG. 8, shows conductivity versus frequency for a wire electrode. As can be seen there is no shift in phase angle over the selected range of an electrode pair according to the present teachings. There is a frequency range where there is little loss of signal, but a drop off at both the high and low ends. The plot ofFIG. 8shows that the capacitance of the electrode of the present teachings is predictable.

With reference toFIG. 9, a circuit diagram model of a medical electrode in an embodiment according to the present teachings is shown. Experimental data verifies the circuit diagram inFIG. 9as a model for a medical electrode in accordance with the present teachings. Using this circuit diagram allows calculation of the parameters required for a given frequency range. The lower cutoff is determined by the capacitance; the higher the capacitance, the lower the cutoff frequency. Likewise, the resistance of the monitor determines the upper limit, the higher the viewing resister, and the higher the cutoff. With a standard 10M-ohm resister, the frequency is very high. This can be adjusted, however.

With reference toFIGS. 10 and 11, profile views of a medical electrode in embodiments according to the present teachings are shown. The inventors have found that by adding inductance to capacitive electrode100, fine tuning of electrode100can be achieved. A purely capacitive electrode will transmit a square wave as a ramp, with a subsequent decay to baseline similar to a saw tooth waveform. Inductance can balance this tendency and can allow the transmission of a square wave into the tissue essentially unchanged. The added inductance can be achieved by providing concentric rings102of ferrous in a layer of gel104, which is in contact with the skin. Rings102are insulated from the electrolyte solution of gel104by an inert material106. Alternatively a number of tiny rings could be randomly imbedded the gel with the rings perpendicular to the flow of current.

With reference toFIG. 12, a side profile of a medical electrode in some embodiments according to the present teachings is shown. By adding tiny spheres202of ferric material, which have been coated with insulation, into electrode200inductance can affect both the current in the silver ink and the current in gel204. Alternatively, an inductance circuit206can be placed in series with electrode200to provide inductance for fine tuning. Circuit206could be placed at the tab on electrode200or in the circuitry of source208.

With reference toFIG. 13, a top profile view of a medical electrode in an embodiment according to the present teachings is shown. In this embodiment, a layer300is added between electrode10and the patient that would focus or direct an electric signal. Layer300would consist of an insulating sheet with a hole302. Insulating sheet300would have adhesive to attach to the skin. Current would be forced through hole302. This would allow the signal to be more localized. The current by necessity transverses only the aperture, rather than the entire contact surface as it would without the added layer.

One skilled in the art will appreciate the present teachings can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present teachings are limited only by the claims follow.