US 3500823 A
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March 17, 1970 P. c. RICHARDSON ETAL 3,500,823
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ATTORNEY United States Patent 3,500,823 Patented Mar. 17, 1970 3,500,823 ELECTROCARDIOGRAPHIC AND BIOELECTRIC CAPACITIVE ELECTRODE Philip C. Richardson and Alfredo Lopez, In, San Antonio, Tex., assignors to the United States of America as represented by the Secretary of the Air Force Filed Nov. 20, 1967, Ser. No. 684,574 Int. Cl. A61b /04 US. Cl. 128-206 3 Claims ABSTRACT OF THE DISCLOSURE BRIEF SUMMARY OF THE INVENTION In the electrode of the present invention, anodized aluminum disks which have a transistorized circuit built directly on the electrodes are used to record the electrocardiogram and other bioelectric parameters when placed directly on the skin.
A high input impedance field effect transistor used in the circuitry permits capacitive coupling between the high impedance electrode and the biological source. The circuit is used to match this high impedance electrode to the lower impedance of a subsequent differential amplifier. The electrodes are placed directly on the unprepared skin of the patient and are held in place by an easily operated attachment device.
BACKGROUND OF THE INVENTION The present invention pertains to an electrode apparatus for use in electrocardiographic or bioelectric recording of internal conditions of a human body.
Electrodes are commonly used in the medical field for detecting electrical signals produced in a living body. Present day electrodes often do not provide a true output when recording low level signals from the skin surface of a living body. For example, present day electrodes are very sensitive to any mechanical disturbances resulting from the motion of the electrode on the skin. Movement of the skin relative to the electrode results in a mechanical disturbance of the electrolyte material commonly used in the electrode at the electrode metal-electrolyte interface to increase conductivity and, hence, results in un- Wanted signals being produced. Such signals are usually known in the medical profession as motion artifacts. Further disadvantages of some of the commonly used electrodes are that they are quite large and difficult to secure to the skin surface and are also impractical for remaining on a skin surface for any length of time. Some electrode assemblies which are essentially free from motion artifacts and are very small, have the disadvantage of being extremely diflicult to properly apply to a body and of having only a small area of skin contact and hence, have high source impedance. The high source impedance can result in interferences being picked up from the leads connected to electrode pairs and noise gen erated at the input of the amplifier to which they are connected.
It is, therefore, the principal object of the present invention to provide an electrode assembly for detecting low level electrical signals from the skin of a living body which has a small area of skin contact, and thus, high source impedance, yet has no noise interference picked up or generated.
A further object of this invention is a novel electrode which produces no polarization effects in the body tissues because there is an intentional discontinuity in the direct current pathway at the electrode contact area and thus there is no net flow of charge from skin to electrode.
BRIEF DESCRIPTION OF THE DRAWING The features of the invention are better understood from the following description of the preferred embodiment taken in conjunction with the accompanying drawings in which:
FIGURE 1 shows the electrode with its related electronic circuitry shown in schematic diagram form.
FIGURE 2 shows an enlarged view of the electrode of the present invention.
FIGURES 3a and 3b show electrocardigraphs produced in actual experiments.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURES l and 2, a typical embodiment of an insulated electrocardiographic and bioelectric electrode is shown. The electrode 10 may conveniently be made from a conductive material such as copper, aluminum, or stainless steel having an insulation on its outer or skin contacting surface, and may be of any convenient size. A suitable electrode has been made from one and one-half inch diameter disk of soft aluminum cut from a sheet one-quarter inch thick. The disk may be conveniently milled to a thickness of one-sixteenth inch, leaving a central stub 12, three-sixteenths inch high and one-eighth inch diameter on its reverse side. The stub 12 may be drilled and tapped for electrical and mechanical connection. The disk is then treated to provide an insulating film on its surface.
The electrode is given an insulating coating, desirably such as is obtained through an anodizing process. Although most metal oxides are suitable, aluminum oxide is preferred because the film deposited on aluminum is free from pores or grain structure when prepared by a suitable anodic treatment. A suitable anodizing process has been preformed by immersing the electrode in a known standard sulphuric acid anodizing bath, for 1 .1 hours wherein the voltage is brought up to 18 volts DC at amperes per surface square foot. Upon completion of the anodization, the electrode is removed from the bath and dyed using a colored dye. Although any color would be suitable, black is preferred to facilitate detection of any flaws in the oxide coating. The electrode is then placed in hot water for the purpose of oxide sealing.
Typical electrical characteristics of an electrode made from the above described process are: (1) resistance of greater than 4,000 megohms at 50 volts, (2) capacitance of 5,000 pico-farads at 30 Hz. The thickness of the dielectric oxide film made in accordance with the present invention may be calculated from the formula:
Where T is the film thickness in centimeters, k is the dielectric constant, A is the area in square centimeters, and C is the capacitance in microfarads. Assuming a dielectric constant of 9, the film can be calculated to be 0.7 mil thick.
The ohmic connection to the electrode is made at stub 12 and is connected to the input of the source follower circuit of FIGURE 1. Due to the high impedance of the electrode, the input of the source follower must be of similarly high impedance in order to have optimum signal transfer from the electrode to the source follower circuit. To achieve this requirement, field effect transistor 21 is utilized which achieves a high input impedance and also a lower output impedance to match the input impedance of subsequent circuitry.
The source follower circuit of FIGURE 1 is preferable mounted directly on the electrode to reduce noise pickup which would be expected at the very high impedance present at the electrode stub connection 12 and the gate 31 of the field effect transistor. The capacitor 27 is used as an output coupling capacitor and the resistor 29 is used in biasing the field effect transistor and as an output load resistance. The diodes 23 and 25 are set back to back, as shown in FIGURE 1, to stabilize the gate 31, but still maintain the high impedance required between gate electrode 31 and source electrode 33.
The outputs of two electrodes may be applied to any typical 1,000 gain differential amplifier with 60 db of common mode rejection in a manner well known in the art.
Each electrode and its associated components may be potted using Dow Corning Silastic RTV Silicone Rubber 588. A metal cap 40 may be placed over the RTV and is shown diagrammatically in FIGURE 1. Metal cap 40 is used for two purposes. The first is to shield the circuitry and probe from any induced or electrostatic electricity generated in the surrounding air. The second use of cap 40 is that of a low impedance ground return circuit for the skin.
Typical electrocardiograph recordings utilizing insulated electrodes according to the present invention are shown in FIGURES 3a and 3b.
From the above description, it can be appreciated that change in ohmic contact between the electrode and the skin, which is responsible for artifact with conventional electrodes, is impossible with the electrode of the current invention since there is no ohmic contact with the skin. A high impedance capacitive coupling is set up between the skin and the electrode. This high impedance coupling is presented to high impedance gate electrode 31 of the field effect transistor 21. The transistor circuit is a source follower circuit and changes the high impedance of the electrode to a low impedance to match input circuitry of a typical amplifier.
It is to be understood that the invention is not l mited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.
1. An electrocardiographic and bioelectric electrode comprising:
(a) an insulated disk means having a front face for contacting a-skin surface and a back face;
(b) said back face including a stub connecting means;
(0) first conductor means attached to said stub;
(d) conducting shield means surrounding said insulated disk means so that when said insulated frontface and said shield means contact a skin surface, capacitive coupling between the disk and the skin surface is attained, said first conductor means and said shield constituting a high impedance circuit.
2. The electrode defined by claim 1 further including an electronic circuit having an input connected to said first conductor means and said shield and having a low impedance output.
3. The electrode defined by claim 2 wherein said electronic circuit is attached to said disk.
References Cited UNITED STATES PATENTS 2,590,876 4/ 1952 Landaver 128-4l7 3,052,233 9/1962 Veling 1282.1 3,144,018 8/1964 Head 1282.1 3,253,595 5/1966 Murphy et a1. 128-405 WILLIAM E. KAMM, Primary Examiner