Electric field sensor

Details Electric field sensor Electric field sensor

2002-09-30

2004-10-19

Cohen, Lee S. (Department: 3739)

Surgery

Diagnostic testing

Structure of body-contacting electrode or electrode inserted...

C600S395000, C600S523000, C128S902000

Reexamination Certificate

active

06807438

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electric field sensors in the medical field for the detection of alternating electrical fields originating from within the body to produce electro-cardiograms (ECGs) and electro-encephalograms (EEGs) and the like, as well as heart rate monitoring. It also relates to other applications for sensing external electric fields.
BACKGROUND TO THE INVENTION
The detection of electrical potentials occurring on the human body is the basis for ECG/EEG diagnostic procedures used to assess heart conditions and brain functions (hereafter “ECG”). An extensive science has been established on the basis of coupling conductive electrodes to the human body to sense the low-level electrical signals that the body is able to generate.
A feature of this technology in the past has been to focus on reducing electrical resistance at the skin/electrode interface. For this purpose ECG electrodes are often used in conjunction with conductive gels and suction cup attachment mechanisms. These arrangements are uncomfortable for the user, restrict mobility, and have limited useful life.
Dry Electrodes—Prior Art Approach
Investigations have been made into using capacitive pickups to detect electrostatic potentials on the skin of a patient. Examples in the literature include the text “Introduction to Bio-Electrodes” by Clifford D. Ferris, published by Plenum Press in 1974. In this text the author discusses experiments with insulated, capacitive electrodes based upon the configuration (page 184):
“Body surface (skin)/Dielectric/metal/FET”.
A shielded single electrode and a two-electrode circuit based on a capacitive electrode are depicted on page 185. Electrode capacitance is reported as 14 uF/cm
2
at page 187.
The text “Electrodes and Measurement of Bio-Electric Events” by L. A. Geddes, published in 1972 by Wiley-Interscience discusses “dry electrodes” at pages 98-103. A single electrode circuit based on a insulated anodized electrode and FET transistor is depicted at page 100. A value for electrode capacitance is reported at page 102 as being 3200 picoFarads Capacitance ranges of 5000-20000 picoFarads/cm
2
are referenced at page 102. In particular, this reference reports (page 102):
“At present there are attempts to provide ultra thin films of insulating materials having high dielectric constants and strengths so that a high electrode-to-subject capacitance will be attained . . . ”.
This statement recites that obtaining a high level of capacitive coupling is an objective and necessarily presumes that such electrodes will be placed in intimate contact with the body of the subject being measured.
In the text “Principles of Applied Biomedical Instrumentation” 2nd edition, L. A. Geddes, L. E. Bater published by Wiley Interscience, 1975, the author observes (at page 217):
“To obtain an electrode-subject impedance that is as low as possible, every effort is made to obtain a high capacitance by using a very thin dielectric having a high dielectric constant.”
Capacitance values from 5,000 pF/cm
2
to 20,000 pF/cm
2
are cited.
A Technical Note entitled “New Technologies for In-Flight, Pasteless Bioelectrodes” by D. Prutchiand A. M. Sagi-Dolev, published in Aviation, Space and Environmental Medicine, June 1993 (page 552) describes a capacitive, dry bioelectrode for obtaining EEG and ECG signals obtained through a plate anodized with aluminum oxide. Coating thicknesses of 50 um and 170 um are referenced. Allowing for a dielectric value of 10 (for aluminum oxide) this thickness would provide an electrode with the ability to develop a capacitance of about 50 pF/cm
2
to 180 pF/cm
2
, if intimately presented to a conducting surface.
Accordingly, the prior art has addressed the problem of capacitive dry electrodes in terms of developing high capacitive values for insulated electrodes placed in intimate contact with the surface being monitored. These prior investigative efforts have been focused on maximizing the coupling between the electrode and the skin surface carrying the potential to be detected. This has led to electrodes that employ thin dielectric surfaces that are capable of providing capacitive values from about 50-1000 picoFarads/cm
2
and higher. It is a necessary adjunct to establishing high capacitive coupling to a body that the electrodes be pressed intimately against the surface being sensed, and that the surface be smooth and free of defects.
True Effective Capacitance
It is believed that all of the capacitive values cited in the prior art references are based on the premise that cited capacitance values are for the maximum capacitance that an insulated electrode can develop when pressed against a conductive surface.
A capacitive pickup electrode for an ECG system may be designed to have a capacitive value of several hundred picoFarads per square centimeters when its insulated plate surface is laid over a smooth, highly conductive counter-electrode surface, such as a sheet of copper. This is the condition for maximum capacitance. However, when placed proximate to the human skin, the dead layer of the skin acts effectively as an insulating spacer, removing the plate of the pickup electrode further from the source of the electric field being sensed. In such a configuration, the effective value of the capacitive coupling between a typical, high capacitance pickup electrode e.g. 100
+
pF/cm
2
and the field source within the human body may be on the order of 1-100 picoFarads/cm
2
depending on the intimacy of contact with the body and the presence of sweat or hair on the skin. The prior art has endeavoured to maximize this capacitance value.
Difficulties of Intimate Coupling
The results of prior art endeavours have been only moderately successful. One problem that has arisen is the extensive sensitivity of these capacitive electrodes of prior design to variations in the gap or intimacy of contact between the electrode and the skin. When intimate contact is the objective, even the presence of hair or sweat can cause variations in the value of capacitive coupling being established. The procedure of pressing dry electrodes against the body has presented similar inconveniences to those arising in the use of conductive electrodes, e.g., discomfort and limited mobility due to intimate contact protocols. In particular, prior art systems have never been reported as operating through clothing fabric. No proposal has been made to obtain alternating electrical signals of the ECG, EEG type, etc. through use of dry capacitive electrodes that are not positioned at fixed locations on the skin surface of a subject.
Further difficulties associated with the use of dry electrodes pressed into intimate contact with the skin of a person are tribo-electric effects—electrical charges created by sliding friction and pressure. Tribo-electric effects deliver large, essentially static charges, to the pickup electrode.
Such charges impose a near DC or very low frequency drift in the background level over which the more relevant, higher frequency signals are imposed. To discharge the amplifier input and pickup electrode of such capacitively acquired charge, the input resistive impedance of the high impedance first stage amplifier should be carefully selected.
Thus a particular concern when sensing alternating signals is the band-pass capabilities of the sensing system. Ideally, the pickup electrode should drive an amplifier with a complementary input impedance which, in the case of ECGs is able to process low level, e.g. milli-volt, signals in the range 0.05 H
z
to 150 H
z
. The lower cut-off frequency should be stable in order to restore the bias value of the driven amplifiers to its normal value in cases where the circuit is over-driven by a very low frequency or DC offset signal.
To minimize the disruptions caused by very low frequency or DC over-driven off-sets, the capacitive coupling to the body (C) should be matched to the input impedance of the amplifier sensor (R) via a preferred, tuned RC-relation. This allows the sensor to have a stable band pass. U.S. Pat. No. 3,744,482 a

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