This page is part on a series on the Unicorn EEG system, see also the review of the Unicorn Hybrid Black, and the Unicorn Naked case and 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.