This page is part on a series on the Unicorn EEG system, see also the other posts for a review of the Unicorn Hybrid Black, and a 3D-printed case for the Unicorn Naked with connectors for EEG, EMG and ECG.

Together with the Unicorn Hybrid Black EEG system that I reported about in another post, I purchased a Unicorn Naked system. It is basically the bare PCB board of the Unicorn Hybrid Black amplifier, including connectors and electrodes, but without the housing and the cap. It comes with a LiPo battery, the cable bundle to connect the electrodes (including the LED strip), a set of 8 dry electrodes, a pack of 50 stick-on electrodes, and a Bluetooth USB dongle. It pairs with the computer just like the Unicorn Hybrid black and uses a unique device name; mine is UN-2022.01.10, which suggests that it includes the date of production and the serial number.

Although it comes with the same g.SAHARA Hybrid dry electrodes as the Unicorn Hybrid Black, the reason for me specifically getting the Naked version is that I want to attach other types of EEG electrodes and also use it for other biosignals like EMG and ECG. For EEG there are four types of electrodes that are common:

• electrode paste, often combined with cup electrodes
• electrode gel, usually applied with a syringe
• wet sponge-like electrodes with saline solution, i.e., salt water
• dry electrodes

The advantage of dry electrodes and wet sponge electrodes over the others is that you can put them on quickly, you can put them on on yourself, and they don’t leave any residue. Electrodes with gel or paste are more suited for a lab environment where a researcher or clinician applies the electrodes to the participant or patient.

However, they all share the same basic physical principles to pick up the potential differences on the scalp due to activity in the brain. Electric currents that flow through the head due to neuronal activity in the brain consist of ionic currents, i.e., these currents correspond to the displacement of positively charged Na+ and K+ and negatively charged Cl- ions. On the other hand, in the amplifier, the lead (wire) and the electrode the electric current is conducted using electrons. The electrode makes the contact with the nonmetallic part of the circuit, i.e., the scalp. The concept of the electrode as the interface between conductive and non-conductive materials has been long known and also applies to fields outside of neurophysiology; the Wikipedia lemma puts this nicely in perspective and provides links to the electrochemistry that happens at the interface.

On this page I present the details and design considerations for sponge electrodes that I constructed myself, based on Ag/AgCl ring electrodes that I had available. The design that I currently consider the most optimal due to its simplicity is electrode design 7. You can find more details further down on this page, including links to the sponge material.

Comparing different electrode principles

Dry electrodes rely on a little bit of sweat to “wet” the outer layer of the skin and to provide the ions required for conduction. Initially gold-plated contacts were used for dry electrodes, for example the original Gtec g.SAHARA electrodes, but also these high-density EMG electrodes. More recent is the use of conductive polymers, as for example in the Unicorn system and in the ANT Neuro waveguard™ touch electrodes. Note that conductive polymer electrodes are not only used in EEG, heart rate monitor bands from Polar and Garmin are also using conductive polymer electrodes mounted on the inside of a chest strap. Dry electrodes do require a decent amount of pressure for good electrode-skin contact and signal, which makes them less comfortable (and to some participants even painful).

Whereas dry electrodes are interesting for self-application and BCI use at home, clinical and research applications have other requirements and especially the signal quality weighs in more heavily. Electrodes used for clinical EEG applications and long-term monitoring often use sticky paste such as Ten20 and EleFix as the electrolyte, and this is also what we have so far mostly been using with the OpenBCI Ganglion and Cyton aplifiers for EEGsynth performances. In research lab settings, conductive gel is often used to make the contact between the electrode and the scalp. In this case, the electrode does not need direct contact with the gel but can sit “on” the hair, as the electrode gel forms a bridge between the electrode and the tissue. A third alternative to dry electrodes water-based sponge electrodes that use salt water as the electrolyte interface between the scalp and electrode. The advantage of using salt water is that the hair dries quickly after taking the cap off, and there is no residue. The higher electrode-skin impedance that both dry electrodes and water-based electrodes have compared to electrode paste and gel does impose additional requirements on the electronic design of the amplifier and ADC: the common-mode signal tends to be larger, more variable between electrodes, and movement artifacts result in larger potential fluctuations, which requires the amplifier to have a larger dynamic range and a better CMMR. Integrated devices such as the Texas Instruments ADS1299 and Analog Devices AD7768 nowadays make the design and implementation of suitable amplifiers easier.

For a long time, water-based sponge electrodes for EEG have been largely the exclusive domain of EGI (now part of MagStim) with their high-density geodesic nets, but Emotiv has also been using them in their Epoc headset, as well as TMSi and MindAffect. Dry electrode systems continue to get a lot of attention for consumer-oriented BCI applications, but it is interesting to see that water-based sponge electrodes are now gaining traction and multiple research-oriented companies have recently released new water-based high-density EEG electrode systems (BrainProducts, ANT Neuro, MBrainTrain, NeuroScan).

Designing my own wet “sponge” electrodes

For the applications in young children and infants that typically have less hair than adults, we think that water-based sponge electrodes are promising to provide a quick application of the electrode cap, a more comfortable experience for the participant, and still good enough signal quality. We don’t expect the signal quality to out-perform the gel-based active electrodes that we nowadays use in our research labs, but with the faster application of the cap in combination with it being wireless, we hope that the compliance of young kids and infants will increase and that in effect we might be able to capture longer and more useful EEG data. Although moving the experiments out of the lab is not a direct requirement (although it would be a nice tor have feature), a lightweight wireless EEG amplifier would further contribute to the comfort. These are main reasons for exploring the Unicorn Naked in combination with water-based sponge electrode designs.

At the Donders Centre for Cognitive Neuroimaging we have been long-time users of BrainProducts amplifiers and caps. Nowadays we mainly use the actiCaps that have active electrodes, but some of our research applications do not allow active electrodes (e.g., in the MEG and MRI scanner) so we still occasionally use EEG caps using gel-based passive ring-electrodes. These are also the electrodes and caps that we used in the past, so we have a box full of old and worn caps and electrodes. I decided to recycle these old caps and electrodes for the development to minimize investments.

Here you can see a photo of the old electrodes mounted on a cap.

And here a close-up of the electrode ring and holder.

Electrode design 1

I came up with the first design prior to coming up with the plan to recycle the existing ring electrodes. I considered getting Ag/AgCl pellets (or see here) and making the electrode completely from scratch. However, these pellets are quite expensive. Furthermore, it would be a lot of work.

It is after this initial design that I came up with the idea of reusing the existing sintered ring electrodes that we used in our EEG labs in the past (prior to switching to active electrodes) and that we still use occasionally. See above in the introduction; these are also still available from Medcat or BrainProducts.

Electrode design 2

For the 2nd design we decided to reusing the existing sintered ring electrodes and that we wanted the electrode wires to run underneath the cap, so that they would all come out through one hole at the back, close to the head-mounted EEG amplifier. Having the wires under the cap would reduce the chances of infants getting hold of them and pulling the wires.

I printed the electrode in two parts using transparent PETG on my Prusa i3 MK3S+.

Here is a photo on the outside of the EEG cap:

and on the inside of the cap:

In this design we make use of 10 mm cotton rolls that are used in dentistry and they come in different diameters (8, 10, and 12 mm). These are easy to cut to the desired length.

I tried the “design 2” electrodes out on myself and on a colleague, using a pinch of salt in a cup of water to wet the sponges. They worked very well, the signal was good, as well as the comfort.

Electrode design 3

Having the wires under the cap – as with design 2 – is fine if they are attached to the cap and don’t wander around (in which case they might get stuck under an actual electrode), but the consequence is that for each individual cap size you need a set of electrodes (as they are fixed to the cap). The subsequent design therefore puts the electrode and wire on the outside of the cap, other than that it is very similar to design 2.

I actually did not yet print a compete set of 10 of these, but expect them to work just as well as design 2; the only difference might be that design 2 has a longer “stem” under the cap, and therefore probably slightly more pressure to get the cotton between the hair onto the skin.

Here is a photo on the outside of the EEG cap:

and on the inside:

Electrode design 4

With design 2 and 3 we realized that for infants and young children the challenge to get through the hair is less and that the narrow 10 mm cotton roll might be slightly uncomfortable. Therefore we started thinking about making a much flatter one.

This is a very flat design with the electrode ring on the inside of the cap; it would directly be covered with a round piece of felt.

When working on this design, I found out that the round top surface of the ring electrodes is actually not conductive but some sort of grey epoxy that is used to glue the lead wire into place. The inside and the flat bottom side are actually the Ag/AgCl material. It is actually surprising that our design 2 – which only touches the round surface – worked so well.

Electrode design 5

With design 5 I returned to having the ring electrode on the outside of the cap and added a small v-shaped clip. Both the bottom and the v-shaped clip are covered with felt.

Here is a photo on the outside of the EEG cap:

and on the inside:

I constructed a complete set of 10 of these. Actually I made a few more, schicht was also needed because the v-shape is a bit fragile when inserting through the hole; it broke on a few of the electrodes.

With 8 of these on the anterior part of my head up to my vertex (where my hair is regretfully getting rather thin) and with two behind my left and right ear, we again did an experiment now also using an EEG electrode impedance meter. Upon initial wetting with salt water and application on my head, all impedances were above 200 kOhm, which is the upper range of our meter. Additional salt water and some extra pressure using Elastofix brought the impedances down to a measurable range; they ranged from 35 kOhm to 120 kOhm, but some of them stayed above 200 kOhm.

The EEG signals were OK on those electrodes with an impedance below 200 kOhm, but the ones with a higher impedance remained very noisy. The signal was not as good as with the earlier design 2. It is interesting to see that the electrode impedance was such a good predictor of signal quality.

We think that the v-shape – which bridges the salt water from the bottom felt disk to the inside of the electrode ring – is simply too small and not conductive enough. Also, the electrodes were not really designed for my head, as I am not completely bald.

Electrode design 6

When testing version 5 with the v-shape, we found that the electrode skin impedance at best was 35 kOhm, but that for quite some electrodes (especially the ones with little downward pressure) it was >200 kOhm. The suspicion is that the small v-shaped strip of felt has too little contact. Consequently we came up with a design that uses a bigger vertical plug of felt.

The conductivity to the electrode ring is here implemented with a felt plug that needs to be compatible with the 6 mm inner diameter of the electrode ring, hence we could not use the dental cotton rolls any more. Instead we found wool felt plugs that are designed to clean the barrel of guns. Convenient with these is that they come in many different sizes, as there are guns with many different diameters.

Electrode design 7

This is a further simplification of design 2. The ring electrode is placed in the original holder, and an 8 mm cotton plug is used to bridge the contact between the skin and the flat bottom side of the ring electrode.

The only part that needs to be printed for this is the tube that holds the cotton plug in place, other than that the electrode is mounted in the old standard holders.

Here is a photo on the outside of the EEG cap:

and on the inside:

Summary

This is still an ongoing project, not all electrode designs have been constructed and/or tested yet – especially not with the EEG impedance meter. I will keep this page updated with new developments.