The main objective of this study was to evaluate two types of dry EEG electrode. In the paper, we describe the comparison of two comb electrodes. The first was an electrode based Ag-AgCl alloy and the second was electrode based on a flexible conductive polymer. Testing of these electrodes was performed based on the need to increase convenience when measuring EEG signals while maintaining the same signal characteristics.
Most brain-computer interfaces (BCI) are based on brain
wave recording using electroencephalography (EEG).
EEG technology is a non-invasive method for recording
signals derived from brain activity. EEG uses electrodes
deposited on the human scalp to capture the signal that
passes through the skull. This signal is considerably
weakened compared to the original, so the EEG device must be
suitably designed to capture it [
In general, there are several types of EEG electrodes. Disc electrodes used in medicine require the use of conductive electrode for optimal impedance and data quality. However, the use of a gel has serious disadvantages and problems which are particularly noticeable when capturing EEG signals in real-life conditions by using EEG device by laymen. Therefore, so-called dry electrodes that do not require the presence of any additive are used for BCI applications. The use of dry electrodes that do not require gel is often very advantageous as it provides a quick setting of the device without time-consuming preparation, but often brings new problems such as comfort and signal quality. Recent studies focus on the use of EEG signals in mobile BCI applications. Such applications should be based on the use of such EEG devices that should be as comfortable as possible and should be easy to use for the individual.
Dry electrodes have the ability to make EEG
technology available for mobile applications. Mobile EEG
applications and EEG devices can make life easier or more
comfortable for many people. There are several areas of
use of Brain to computer interfaces (BCI) such as games
or assisting people. BCI-based assistance may include
applications to improve the nervous system or restore nerve
bonds in the case of paralysis [
The EEG device, which would allow ordinary people to use the BCI interface in everyday life, has several conditions. These conditions are suitable device design, ease of use in non-clinical settings, comfort, painlessness, and cleanliness. Medical EEGs use conductive gel electrodes and are made in the form of an elastic cap. The mobile EEG should be usable without the need for shaving the head and comfortably enough, so dry EEG electrodes should be used. For these reasons, we have performed the experiment of the using and properties of dry electrodes. 2
In the experiment, two types of electrodes were compared. As the first step, the resistance of the electrodes was evaluated using the Volt-Ampere method. In the second part, the quality of the signals measured with these electrodes was compared. 2.1
The OpenBCI headset was used to measure the EEG signal. The headset is designed as a plastic 3D printed construction, that allows place electrodes up to 35 different positions by standard 10/20 configuration system. In the experiment, the brain signal from 10 locations of frontal and temporal lobes was measured.
The headset works wirelessly. The measured data is sent
via the Bluetooth 4.0 wireless communication protocol to
the USB dongle receiver. The headset can also store data
directly on a microSD card when the device is not
connected to any wireless receiver. The entire system is
powered by batteries, providing greater patient electrical safety
and portability [
The input impedance of the amplifiers in the headset is
500MW. The lead resistance of the electrode is negligible
compared to the large impedance at the amplifier input [
The first dry EEG electrode TDE-200 was used. The
electrode is also known as Post electrode, shown in Figure 1.
It is a dry electrode, made of Silver-silver chloride
(AgAgCl) alloy with a diameter of 10 mm. This electrode
is specific in that it contains 12 pins to improve contact
through fur or hair. The pins are 2 mm long to provide
good contact with the skin surface through the hair. They
provide accurate and clear transmission of surface
biopotentials. The electrodes connection is provided by a screw
to which the conductive cable is then attached [
The advantage of the electrodes is the low resistance due to the metal composition. The use of electrodes does not require the addition of a moisturizing gel to improve conductivity or to remove hair from the measured head area. Thanks to the small pins, the electrode can reach the skin surface.
The disadvantage is the painful setting of the electrodes before the measurement and the occasional subjective pain even during longer measurements. Pins often push too much on the skin, but leave no injuries. 2.3
The second tested sample was dry Brush electrode by Datwyler, shown in Figure 2. The electrode is designed for better comfort. The electrodes are based on a flexible conductive, elastic main body with a conductive coating covering the contact area, ensuring comfort during monitoring and setting of the headset. Brush electrode also has small pins, but these are soft and movable due to that they are made of conductive polymer. There are 15 pins with a length of 5 mm and the contact area has a diameter of 12 mm. They are attached to standard snap lead cabling.
The advantage of the Brush electrodes is that they are
designed for dry signal acquisition, so they do not need
moisturizing gels or hair removal for using. Thanks to
longer pins they can work through the hair. Elastic
material ensures an easier setting and painless measurement
on the skin [
The amount of voltage and the current passing through the electrodes was measured. Because of the small size and irregular shape of the electrode electrodes, we used the copper plate on which we placed the electrode to measure. Then we measured the current passing through the plate and the end of the electrode and the voltage between these points. As the first was measure the Post electrode. The Post electrode is made of Silver-silver chloride (Ag-AgCl), therefore, lower resistance was assumed. This assumption was confirmed by measurement.
The measurements showed electrode voltage values of 2.6 to 4.3mV and current values were measured in the range of 101-102mA. The electrode resistance was then calculated by the Volt-Ampere method. The resistance values were from 26 to 43mW. Brush electrode measurement was performed to compare the values of resistance. In the experiment, the voltage between the ends of the electrode was 2,6V . The Brush electrode current values were in the range of 70-78mA, which depended on the Brush electrode being pressed to the copper plate. The average current value was 74mA. Resistance was calculated from the mean values by the volt-ampere method. Its values were calculated as 35W.
Since the electrode is on the entire surface of the polymer, its resistance values are relatively high. Approximate resistance values could also be measured directly by the multimeter.
The next step was to measure the EEG signals. An OpenBCI headset has been used to record signals. Signals were measured to compare the values and properties of signals obtained from two different electrodes.
Post and Brush electrodes were placed in the EEG headset at the same time to observe signal similarities under the same conditions. Two session signals were recorded during the experiment.
Based on the 10-20 configuration system, in the first
session Brush electrodes were placed on the right
hemisphere of the head and Post electrodes placed on the left
hemisphere. In the second session, the electrode positions
were reversed. Figure 4 shows the electrode positions of
the experiment in 10-20 configuration system [
The brain waves of a subject in the state of relaxation
were recorded. The subject sat still in a comfortable
position while playing the video. The subject watched the
video was with a light concentration. Each session lasted
60 seconds. In the state of relaxation and light
concentration, in the brain are created so-called alpha waves [
Figure 5 shows skin irritation. It can be seen that the Brush electrode leaves the skin reaction longer than the Post electrode. The pictures were captured after 10 minutes of EEG recording. After 30 minutes almost no reaction was seen in the Post electrode application area. In contrast, the Brush electrode left a visible sign after 30 minutes. 5
In the Matlab programming environment, the signals were
modified from the default format in which they are saved,
to matrix for ease of evaluation. Then the alpha frequency
bandwidth was filtered out of the signal. The alpha wave
frequency is 8-12Hz. Alpha waves are created in a state of
wakefulness when a person feels relaxed. By filtering this
bandwidth some artefacts have been removed and only the
clear brain signal from the subject’s relaxation state has
remained [
Individual hemispheres and brain parts are responsible for the various processes. Nevertheless, we can compare the values of signals in opposite positions. Artefacts or some brain activity with stronger pulses can also be occurred on opposite sides of the hemisphere simultaneously. In the Matlab programming environment, signals obtained from individual electrodes were compared. In the first step, the voltage levels on the both electrodes were compared. The comparison was always performed between the electrodes at the same positions on the opposite hemispheres.
The best comparison can be made on the forehead on the electrodes Fp1 and Fp2. This part transmits an electrical signal coming from visually evoked potentials and from eye movement. Therefore, comparing these signals is appropriate for the case in our experiment where the subject watched the video while recording the EEG.
The correlation of the raw signals from the first session was 0.92 on the Fp1 and Fp2 electrodes and after filtering the alpha bandpass, the correlation was 0.87. In the second session, the correlation between Fp1 and Fp2 signals was 0.98 and 0.97 on the alpha frequency band.
Figures 6 and 7, show the simultaneous signal plot of the Alpha wave EEG signal at electrodes Fp1 and Fp2 during the first and second session. From the EEG measurements, we can observe the signal sequence similarity on the electrodes Fp1 and Fp2.
In Figures 8 and 9, show the comparison of the frequency spectres. The Brush electrode signal is displayed in blue and the Post electrode signal is displayed in red. The Figures 8 represents frequency spectre of signal from the first session, where at position Fp1 there was the Brush electrode and at Fp2 was Post electrode. The Figure 9 represent frequency spectre from the second session, where electrode positions were replaced.
The increase in frequency spectrum power on Brush electrodes was on average 20% in the first session. In the second, the increase with the use of Brush electrodes was on average 10%. 6
Measurements of the electrodes electrical properties showed significantly higher Brush electrode resistance values. From the comparison of EEG signals, it can be stated that although the Brush electrodes is made of a conductive polymer and has a higher resistance, the signal values obtained by it are comparable to those of the signal obtained by the metal Post electrode. By connecting the electrodes to the OpenBCI hardware it has been shown that the use of a higher resistance electrode does not affect the measurement of the brain signal. The use of a suitable amplifier in the EEG headset smoothes the difference between electrode resistances.
The research presented in this paper was supported by the Ministry of Education, Science, Research and Sport of the Slovak Republic under the research project VEGA 1/0511/17, by the Cultural and Educational Grant Agency of the Slovak Republic under grant No. 009TUKE-4/2019 and by the Slovak Research and Development Agency project No. APVV-SK-TW-2017-0005.