Should I Use Wet Or Dry EEG Electrodes? [5 Things To Consider]

Deciding whether to use wet EEG electrodes or dry EEG electrodes for your research can be difficult. Here is what to bear in mind before making a decision.

Mobile EEG drives demand for dry electrodes

Until a few years ago, electroencephalography (EEG) research could only take place in labs or clinics. This is because traditional EEG equipment is so large and stationary that to obtain a clean brain signal, the wearer has to sit still during experiments

In recent years, however, mobile EEG devices have matured. They now match today’s consumer electronics in portability, size and weight. They allow researchers to glimpse brain activity in completely new environments. Participants can move around within a laboratory, and even move outside.

Because of these new possibilities, demand for flexible, mobile EEG solutions continues to increase. And with the demand for mobile EEG, comes a demand for a mobile and flexible electrode: the dry electrode.

Conventional EEG records with so-called “wet” electrodes, which use a layer of conductive gel or paste to increase conductivity between electrode and skin. Unfortunately, the gel can leave residue in a participant’s hair and takes time to apply.

In fact, with studies that require high spatial resolution (using up to 128 or 256 electrodes) experiments that use wet electrodes can take hours to set-up.

Dry electrodes, meanwhile, do not require conductive gel and are much faster to set-up. This convenience, however, is at the cost of a noisier signal that is prone motion artifacts.

Dry electrodes are trading increased convenience for signal stability and quality.

Data quality comparison

So then, are dry electrodes as good as wet electrodes? Or at least, do dry electrodes deliver valid results?

Julia Kam, a Psychology Professor at the University of Calgary, conducted a study to find out (Kam et al., 2019).

Kam et al. (2019) compared the signals produced by wet and dry electrodes recorded on 27 individuals. They looked at: the spectral power in low frequency bands; event-related potential components (P3b); and a single trial classification task.

The results showed that both systems performed well, but wet electrodes had a slightly improved signal quality. 

The conclusion was that dry electrodes can record EEG data as well as wet electrodes in a stationary setting.

This conclusion corroborates with similar studies that considered the signal-to-noise ratio, overall signal quality, and wearing comfort of dry and wet electrodes (Hinrichs et al., 2020; Di Flumeri et al., 2019; Leach et al., 2020).

Cleaning motion artifacts

One key advantage of mobile EEG is its ability to monitor neural activity in “real world” environments, where participants are not stationary.

Recording EEG data while a participant moves, as in gait research, is increasingly common. However, under such conditions, electrodes can shift and cable movement can cause noise, or “motion artifacts”.

There are different approaches to reduce EEG motion artifacts. They include: finding a balance between adhesion pressure and comfort; using software to clean noisy data; and dual-layer setups, in which a second EEG system detects motion artifacts and subtracts them from the main signal.

Combinations of different techniques can significantly improve signal quality, and the ideal approach largely depends on the individual experiment.

There are no best practices for dry electrode signal processing yet, aside from filtering data that lies outside the spectrum of interest, however we expect best practices will emerge soon, as there is substantial research in this field.

Take a look at our article to find out more about cleaning noisy EEG signals.

Choosing wet or dry EEG electrodes

Whether you should choose wet or dry electrodes for your experiment depends on your experimental set-up. It is a trade-off between convenience and data quality.

While dry electrodes provide more ease of use, their signal quality is usually lower than that of wet electrodes.

However, dry electrodes often deliver data that is good enough for many experimental paradigms, and are increasingly used with the emergence of signal cleaning techniques. 

What should I consider when choosing EEG electrodes?

There are a few factors to consider when making your electrode choice.

Is my experiment stationary or mobile?

For stationary applications, dry electrodes can provide good quality data recordings without the discomfort of gel.

For mobile applications, wet electrodes provide better signal quality and are less prone to motion artifacts. This is because they have a low electrode-skin impedance, and additional adhesion provided by the conductive gel itself.

How long am I measuring for?

At the beginning of an experiment, the impedance values of wet electrodes improves over time. This is because the gel dissipates into a more even layer.

However, at some point the gel will begin to dry and evaporate, leading to a reduction in signal quality and an increase in motion artifacts.

How long you can obtain a stable signal is largely dependent on the type and quality of gel you use. In sum, shorter recordings are more likely to benefit from the increased data quality of wet electrodes. Longer studies: not so much.

If you care more about signal stability than signal detail, dry electrodes may be the better option, as you won’t need to readjust them.

How many EEG channels am I using?

Setting up an EEG with wet electrodes can be labor intensive and time consuming, especially with high density systems (64 channels or more). In fact, you could end up spending more time on set-up and cleaning than on data acquisition.

What is more, gel pools can form if you are using too many electrodes, which will cause neighboring electrodes to form bridges over the gel. This can skew the measurements as channels merge together.

The future of EEG electrodes

Not only are engineers developing ever more sophisticated data processing and cleaning techniques for EEG signals, but new electrode types are emerging that promise to reduce noise in mobile applications.

C-shaped electrodes that attach behind the ear (Sterr et al., 2018), printable thin-layer electrodes (Li et al., 2020), and in-ear electrode arrays (Athavipach et al., 2019), have all passed proof-of-concept, and are becoming mature, feasible alternatives to conventional electrodes.

Mentalab has the right electrode for you

Because mobile EEG research is so varied, Mentalab offers a variety of electrodes. We have two types of wet electrodes and two types of dry electrodes that work with the same amplifier: Mentalab Explore.

At only 27 grams and 4 x 4 x 2 cm, Mentalab Explore provides a truly mobile solution for EEG research. It collects high quality bio-signal data but weighs about as much as a matchbox.