WO2018102855A1

WO2018102855A1 - Bioamplifier and impedance measurement system

Info

Publication number: WO2018102855A1
Application number: PCT/AU2017/000263
Authority: WO
Inventors: Boris SHARFF; John Paul Mckeown; Aaron Russell
Original assignee: Visionsearch Pty Limited
Priority date: 2016-12-06
Filing date: 2017-12-06
Publication date: 2018-06-14

Abstract

A biological amplifier (10) for measuring an electrical activity of nerve cells of a physiological function in a subject (18) has a plurality of electrodes (24, 26) at least one of which is a sensor electrode (24), for measuring the electrical activity. The sensor electrode (24) is adapted to be placed on the skin of the subject above the nerve cells. The biological amplifier also has means (20, 28, 56) for measuring contact impedance between the skin of the subject and the sensor electrode (24) due to a voltage applied between the plurality of electrodes. The contact impedance measuring means operates continuously and concurrently with the measuring of the electrical activity of the nerve cells of the physiological function by the biological amplifier.

Description

BIOAMPLIFIER AND IMPEDANCE MEASUREMENT SYSTEM

TECHNICAL FIELD

The present invention relates to apparatus and methods for the continuous and concurrent, or real-time, measurement of the contact impedance between the skin of a subject and skin contacting electrodes for a biological amplifier used to detect, amplify and measure electrical activity of nerve cells of a physiological function, such as the subject's heart activity, general brain activity or optic nerve activity.

BACKGROUND ART

It is known to measure the electrical activity of nerve cells by placing electrodes connected to a biological amplifier on the skin of a subject above the neural structures (nerve cells) related to the physiological function of interest. The electrical signals detected by the electrodes are amplified, filtered and passed to an associated computer for signal processing and display of the readings of electrical activity. For heart activity, electrodes are placed on the skin in the region of the heart; for general brain activity, electrodes are placed at various locations on the scalp; for somatosensory activity, electrodes are placed on the limbs or head or over the spine; and for optic nerve activity, electrodes are placed on the scalp above the visual cortex. As the electrical signals generated by the nerve cells of each of the heart, brain and optic nerve are very small, it is vital that, in all situations, as efficient a contact as possible is achieved between the electrodes and the skin to obtain the best electrical signal which will provide the most accurate readings of electrical activity for the operator. The measurement of the efficiency of this contact is a measurement of what is called the contact impedance. Conventionally, operators measure the contact impedance when an electrode is initially placed in position on the skin above the nerve cells of interest. If the contact impedance is too high, then the electrode is reapplied as necessary with appropriate skin cleaning and conductive gel until the contact impedance is acceptably low. If the contact of the electrode to the skin subsequently deteriorates during the taking of the readings (the measurement), then the readings from the time of the deterioration are compromised because they do not take into account the change in the contact impedance. The operator may not be aware of this until the taking of the readings has ended, at which point the readings are usually discarded, the electrode is reapplied so that contact impedance is acceptably low, and fresh readings are taken. Or, worse still, the operator may not become aware of the deterioration of the contact even after the taking of the readings has ended, and so the operator subsequently relies upon those readings. These shortcomings of the prior art have been found by the inventors to be due to the failure of conventional biological amplifiers to measure the contact impedance between the skin and electrodes in a manner that is continuous and concurrent, or in real-time, with the measurement of the electrical activity of the nerve cells of the physiological function by the biological amplifier.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide the operator with a continuous and concurrent measurement of the contact impedance between the skin of a subject and skin contacting electrodes connected to a biological amplifier used to detect, amplify and measure electrical activity of the nerve cells related to a physiological function so that any deterioration of the contact between the electrodes and the skin while the readings of electrical activity are being taken can be identified and immediate corrective action taken.

According to the present invention, there is provided a biological amplifier for measuring an electrical activity of nerve cells of a physiological function in a subject, comprising:

(a) a plurality of electrodes, at least one of which is a sensor electrode for measuring the electrical activity, the or each sensor electrode adapted to be placed on the skin of the subject above the nerve cells, and

(b) means for measuring contact impedance between the skin of the subject and the or each sensor electrode due to a voltage applied between the plurality of electrodes, wherein the contact impedance measuring means operates continuously and concurrently with the measuring of the electrical activity of the nerve cells of the physiological function by the biological amplifier.

In a preferred form, the contact impedance measuring means comprises a differential amplifier and the plurality of electrodes are connected to input terminals of the differential amplifier.

Preferably, the plurality of electrodes comprise a sensor electrode and a reference electrode.

Alternatively, the plurality of electrodes may comprise a pair of sensor electrodes connected to the input terminals of the differential amplifier, and a reference electrode connected to the ground of the differential amplifier.

In another preferred form, the plurality of electrodes comprise a plurality of sensor electrodes and a reference electrode connected to the input terminals of a plurality of differential amplifiers.

In yet another preferred form, the plurality of electrodes comprise a plurality of pairs of sensor electrodes connected to the input terminals of a plurality of differential amplifiers, and a reference electrode connected to the ground of the differential amplifiers.

Preferably, the biological amplifier further comprises an impedance voltage generator.

In all preferred forms, the output of the differential amplifier is connected to two different electronic circuits. One circuit uses the signal for the biological or physiological measurement of interest, the other circuit uses the signal to take the contact impedance measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a biological amplifier in accordance with a first preferred embodiment of the invention, and which is being used to measure the electrical activity of the nerve cells of the brain of a subject, and in which a sensor electrode placed on the scalp of the subject and a reference electrode are used to measure the contact impedance between the scalp and the sensor electrode.

Fig. 2 is a schematic representation of a biological amplifier in accordance with a second preferred embodiment of the invention, and which is being used for a similar purpose to the biological amplifier of Fig. 1 , and in which a pair of sensor electrodes are used to measure contact impedance.

Fig. 3 is a schematic representation of a biological amplifier in accordance with a third preferred embodiment of the invention, and which is being used for a similar purpose to the biological amplifier of Fig. 1 , and in which a plurality of sensor electrodes and a reference electrode and a plurality of differential amplifiers are used to measure contact impedances of each of the sensor electrodes. Fig. 4 is a schematic representation of a biological amplifier in accordance with a fourth preferred embodiment of the invention, and which is being used for a similar purpose to the biological amplifier of Fig. 2, and in which a plurality of pairs of sensor electrodes are used to measure contact impedance.

MODES FOR CARRYING OUT THE INVENTION

Each of the biological amplifiers 10, 12, 14, 16 shown in Figs. 1 to 4 receives signal inputs from electrodes attached to the skin of a subject 18, and has a contact impedance measuring means that operates to measure each electrode's contact impedance continuously and concurrently with the amplifier's function of detection, amplification and measurement of electrical activity of nerve cells of a physiological function.

This is achieved by applying a voltage of fixed stable amplitude from an impedance voltage generator 20 of the biological amplifier through an impedance 22 (herein termed Zs) to one or more sensor electrodes while those sensor electrodes are detecting the electrical activity of the nerve cells underlying the skin (i.e. a biological or physiological signal).

With reference to Fig. 1 , there is a sensor electrode 24 (also termed a unipolar electrode) and a reference electrode 26. The sensors 24, 26 are connected to inputs of a differential amplifier 28 of the contact impedance measuring means. The sensor electrode 24 is placed on the skin of the subject's scalp above the nerve cells of interest, and the reference electrode 26 is placed in contact with the subject's ear lobe.

The differential amplifier 28 amplifies the difference between two voltages. By connecting the voltage signal from sensor electrode 24 onto one input terminal 30 of the differential amplifier and connecting the voltage signal from reference electrode 26 onto the other input terminal 32 of the differential amplifier, the resultant output voltage will be proportional to the difference between the two input voltage signals. The differential amplifier 28 is provided with power by connection of its own ground 34 to the reference ground of the biological amplifier 10 and by connection of its supply voltage through supply terminal 36 to the internal power of the biological amplifier 10. This

measurement between a sensor electrode 24 and a reference electrode 26 is termed a unipolar measurement.

The resultant voltage between the sensor electrode 24 and the reference electrode 26 is measured to provide the unipolar measurement. The amplitude of this resultant voltage is a measurement that is proportional to the sum of the contact impedance of the sensor electrode 24 (herein termed Ze) plus the contact impedance of its associated reference electrode 26 (herein termed Zr) plus the impedance through the subject's body between the two electrodes 24, 26 (herein termed Zb). The impedance Zs in series with the total impedance Ze+Zr+Zb act as a voltage divider through which the measuring current flows. The impedance Zb is small in comparison to other impedances and can be neglected for the purposes of this invention.

The biological amplifier 14 shown in Fig. 3 has a plurality of sensor electrodes 24, 38 and a reference electrode 26 and a plurality of differential amplifiers 28, 40 which are used to measure contact impedances of each of the sensor electrodes. The resultant voltage between each sensor electrode 24, 38 and the reference electrode 26 is measured to provide a plurality of unipolar measurements.

As shown with reference to Fig. 2, a second sensor electrode 42 may be connected to the differential amplifier 28 when a differential measurement (i.e. bipolar measurement) is being taken. The reference electrode 26 in this case is connected to the reference ground 48 of the differential amplifier 28. This is termed a single bipolar measurement.

The biological amplifier 16 shown in Fig. 4 has a plurality of pairs of sensor electrodes 24, 42 and 50, 44 and a reference electrode 26 and a plurality of differential amplifiers 28, 52 which are used to measure contact impedances of each of the sensor electrodes. This is termed a plurality of bipolar measurements.

In all of the above described arrangements of electrodes, the output of the or each differential amplifier is connected to two different electronic circuits 54, 56. Circuit 56 uses the signal for the biological or physiological

measurement of interest, and circuit 54 uses the signal to take the contact impedance measurement.

The impedance voltage is separated from the biological signal voltage by frequency. Biological signals of interest typically have frequencies below 100Hz, and a useful impedance signal has a frequency of many multiples of typical biological signal frequencies, and so the two measurements (of impedance and of nerve cell electrical activity) can be separated by frequency filters 58.

The or each impedance voltage generator 20 provides a voltage that is at a frequency significantly above the biological signal frequency of interest, typically several octaves above that frequency, and is several tenths of a volt in amplitude. This voltage source is joined to the sensor electrode of interest through an impedance Zs 22 that is of a value between one and several times the maximum acceptable contact impedance that is to be measured, typically from 5 kQ to 30kQ. When the impedance of each of a plurality of sensor electrodes is being measured, each sensor electrode is referenced to a single common negative reference electrode 26 (see Fig. 3). This negative reference electrode 26 is typically placed on the forehead or on the ear lobe (via an ear clip) for measurements on the head. For measurements on the body below the head, the reference electrode can be a set of electrodes that are meant to represent "zero potential" on the body below the head. The reference point could be derived by summing voltages from electrodes on limbs (the arms or legs) and the central terminal ("CT") on the sternum. These are typically connected by 5ΚΩ resistors to provide the summing. This also provides a reference point to which other electrodes are referred.

At the frequency of the impedance voltage generator, the resultant voltage in a unipolar measurement between the sensor electrode and the reference electrode is proportional to the total contact impedance of these two electrodes to the skin. Similarly, in bipolar measurements, the resultant voltage at the frequency of the impedance voltage generator between two sensor electrodes is a direct measurement of the total contact impedance of these two electrodes to the skin.

For unipolar measurements, the reference electrode has a large area and is attached to a clean, dry and exposed area of skin so that its contact impedance Zr is low, and the impedance Zb of the body is known to be small, and so the major component of the measured contact impedance is taken to be the contact impedance Ze between the unipolar sensor electrode and the skin.

For bipolar measurements, the impedance that is measured is the sum of the contact impedance of both of the two electrodes. If an unacceptably high impedance is measured, then both electrodes will require attention as it is not known which one has, or if both have, a faulty contact.

The contact impedance of many sensor electrodes may be

simultaneously undertaken. If all these sensor electrodes are connected as bipolar connected pairs to a respective differential amplifier and one or both of those electrodes in a bipolar connected pair has a high contact impedance, then the preferred circuit of the impedance current from that bipolar connected pair of sensor electrodes will be via an electrode of another bipolar connected pair of sensor electrodes with lower contact impedance instead of being via the one or both electrodes of the bipolar connected pair of sensor electrodes with high contact impedance which are being measured. This will give a false measurement of total contact impedance. This same difficulty arises with a plurality of sensor electrodes that are unipolar connected. To overcome this for both unipolar and bipolar connection of sensor electrodes, the biological amplifier of the present invention applies a contact impedance measuring signal to each bipolar connected pair of sensor electrodes in quick succession (i.e. multiplexed between each bipolar connected pair of sensor electrodes by means of a multiplexer frequency generator 64 and a multiplexer 66) and the contact impedance is correspondingly measured for each pair in turn.

The multiplexing ideally occurs at a frequency significantly above that of the contact impedance measurement frequency, such as from 10 to 20 times above, but not at a frequency that is an integer multiple above that of the contact impedance measurement in order to avoid aliasing. This multiplexing provides a contact impedance measurement of a plurality of pairs of sensor electrodes in the same time as it takes to provide a contact impedance measurement on non-multiplexed sensor electrodes (e.g. a single pair of sensor electrodes connected as a bipolar pair or a single sensor electrode connected as unipolar).

The contact impedance measuring means may be functionally activated or deactivated by external controls from, for instance, an associated computer.

An immediate visual indication of an unacceptable level of contact impedance can be provided by an indicator light 66 for the or each sensor electrode, or by the software within the associated computer, or by both. Use of indicator lights does not require the operator to request the software to take an impedance reading, and nor does it require that the operator even look at a control screen of the associated computer, as the indicator lights can be conveniently placed adjacent to the sensor electrodes themselves.

The present inventors have also developed an improved method of identifying a faulty electrode of a bipolar connected pair of sensor electrodes which are being used to measure the combined contact impedance of the bipolar connected pair. If the contact impedance of one sensor electrode of the bipolar connected pair is too high, it has previously been impossible to identify which one of the pair was responsible for the high measurement and was thus faulty. The improved method involves applying a contact impedance

measuring signal to one sensor electrode of the pair with a reference electrode, as with unipolar measurement, and then applying a contact impedance measuring signal to the other sensor electrode of the pair with the same reference electrode, and then measuring the contact impedance between the two sensor electrodes of the bipolar connected pair in the usual manner. The three measurements of contact impedance obtained by the above method enable the operator to identify which sensor electrode of the bipolar connected pair is faulty, and yet still know the combined contact impedance of the bipolar connected pair of sensor electrodes.

It will be readily appreciated by persons skilled in the art that various modifications may be made in details of design, construction and operation of the biological amplifier described above without departing from the scope or ambit of the invention. INDUSTRIAL APPLICABILITY

The present invention has industrial application in the field of biological amplifiers for detecting, amplifying and measuring the electrical activity of nerve cells involved in the functioning of a physiological activity.

Claims

CLAIMS:

1 . A biological amplifier for measuring an electrical activity of nerve cells of a physiological function in a subject, comprising:

2. The biological amplifier of claim 1 wherein the contact impedance measuring means comprises a differential amplifier and the plurality of electrodes are connected to input terminals of the differential amplifier.

3. The biological amplifier of claim 2 wherein the plurality of electrodes comprise a sensor electrode and a reference electrode.

4. The biological amplifier of claim 2 wherein the plurality of electrodes comprise a pair of sensor electrodes connected to the input terminals of the differential amplifier, and a reference electrode connected to the ground of the differential amplifier.

5. The biological amplifier of claim 2 wherein the plurality of electrodes comprise a plurality of sensor electrodes and a reference electrode connected to the input terminals of a plurality of differential amplifiers.

6. The biological amplifier of claim 2 wherein the plurality of electrodes comprise a plurality of pairs of sensor electrodes connected to the input terminals of a plurality of differential amplifiers, and a reference electrode connected to the ground of the differential amplifiers.

7. The biological amplifier of claim 1 further comprising an impedance voltage generator.

8. The biological amplifier of claim 2 wherein output of the differential amplifier is connected to two different electronic circuits.

9. The biological amplifier of claim 8 wherein a first one of the circuits uses a signal for the physiological function.

10. The biological amplifier of claim 8 wherein a second one of the circuits uses a signal to take a contact impedance measurement.