Wet, dry, active and passive electrodes. What are they, and what to choose?

Wet, dry, active and passive electrodes. What are they, and what to choose?

In some cases, the number of decisions to be made can be overwhelming when a new experiment with electroencephalography (EEG) is started. Especially when we must define and justify the equipment that will be used. Although we talk about the physical differences between wet, dry, active or passive electrodes; There is not much information about how the data are affected when using one or the other in a research experiment, practical convenience, and other advantages or disadvantages of using different types of electrodes.

First of all, we will define what each one of these types of electrodes consists of:

• Wet electrodes are electrodes generally made of silver/silver chloride material (Ag / AgCl). They use an electrolytic gel material as a conductor between the skin and the electrode.

• Dry electrodes consist of a single metal that acts as a conductor between the skin and the electrode. This material is usually stainless steel.

• The active electrodes have a pre-amplification module immediately after the conductive material between the skin and the electrode. This allows the signal to be amplified before additional noise is added between the electrode and the system that would capture, process or amplify the signal.

• Passive electrodes do NOT have a pre-amplification module, as is the case with active electrodes. Instead, it simply extends the connection from the conductive material to the equipment for capturing, processing or amplifying the signal.

To read more: A brief introduction to EEG and the types of electrodes

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Wet electrodes

According to researches[2,3,4,5,6], the impedance present between the electrode and the skin is directly related to the performance of the electrode. In this case, it is very helpful to use active electrodes, since it stabilizes the performance of the electrode, reducing the dependence on the conductive gel.

Researchers from the Institute of Neurosciences and Mental Health in Canada [1], conducted a study in which 8 people participated. The EEG signal was taken using a V-Amp from Brain Products of each of the participants while emitting auditory stimuli. This allowed to obtain the Event Related Potentials (ERPs).

As can be seen in the following image, the active electrodes showed a more immediate response according to the potentials (ERPs) detected after having emitted the auditory stimulus. And in addition, it also showed less error (voltage difference) between the reference signal and the measurement taken with the electrodes.

Signals obtained by EEG readings. A) Each of the graphs shows the reference signal and the signal measured with each of the electrodes (wet passive, active wet and dry passive, respectively). B) Cranial topography of the voltage difference between the reference signals and the electrodes. C) Graphically shows the level of error obtained in each type of electrode.

The virtues found in wet electrodes have a cost, depending on the needs of the experiment, this cost can be very high. For the application of dry electrodes, it is necessary to carry out an additional step. This step is the application of the gel for each electrode. If the electrode is in a sensitive area, this may not be the best solution, since the gel can irritate the area (For example, if you are taking signals near the eyes). If we put for example a LiveCap on, it would involve the application of gel to 64 electrodes, one application for each active channel in the cap.

If it is intended to be used for extended periods of time, it should also be considered that the gel can be dehydrated [6,7,8], so a re-application and interruption of signal monitoring may be necessary. In addition, the removal of the electrodes requires a little more time for cleaning the electrodes, as well as the skin where they were placed. Finally, we must talk about the sequels that the electrodes can leave dry after monitoring. Although cases are rare, there are concerns about the toxicity of gels used as conductors [9]; the most frequent cases are dermatitis [10, 11, 12].

Dry electrodes

Active electrodes vs. passive electrodes

The type of information that is intended to be measured can significantly influence the decision between using active or passive electrodes.

Research has shown that the speed of changes in voltage during measurement with passive electrodes can significantly influence the amount of noise introduced into the signal [13]. It is also recommended to use active electrodes in case the individual in question is in motion since the movements can generate deformations in the signal. This is the same case when working in areas with considerable electromagnetic noise in the environment, or if the distance between the electrode and the system of capture, processing or amplification of the signal.

On the other hand, active electrodes generally have a higher price than passive electrodes. In addition, they are heavier and require more space, so there is less freedom of movement with the active electrodes than with the passive ones.

Conclusion

Like everything in life, there is no perfect choice of electrodes for all cases. Instead, the needs of the research and experiment should be analyzed in detail, in order to carefully define the set of electrodes that will be used.

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References

[1] Mathewson, K. E., Harrison, T. J. L., & Kizuk, S. A. D. (2016). High and dry? Comparing active dry EEG electrodes to active and passive wet electrodes. Psychophysiology, 54(1), 74–82.doi:10.1111/psyp.12536

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[8] Lagow C H, Sladek K J and Richardson P C 1971 Anodic insulated tantalum oxide electrocardiograph electrodes

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[9] Cochran R J and Rosen T 1980 Contact dermatitis caused by ECG electrode paste Southern Med. J. 73 1667–8

[10] Uter W and Schwanitz H J 1996 Contact dermatitis from propylene glycol in ECG electrode gel Contact Dermatitis

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[11] Dwyer CM, Chapman R S and Forsyth A 1994 Allergic contact dermatitis from TENS gel Contact Dermatitis 30 305

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[12] Coskey R J 1977 Contact dermatitis caused by ECG electrode jelly Arch. Dermatol. 113 839–40

[13] Laszlo, S., Ruiz-Blondet, M., Khalifian, N., Chu, F., & Jin, Z. (2014). A direct comparison of active and passive amplification electrodes in the  same amplifier system. Journal of Neuroscience Methods, 235, 298–307. doi: 10.1016/j.jneumeth.2014.05.012

Autor: Edith Granados @Noronha edition