This method allows quick and inexpensive electrode manufacturing and opens the possibility of creating electrodes that are customized for individual usercases, allowing a user to go from a CAD design to a functional prototype in less than 90 min only using a desktop 3D printer.
The electrodes are fingered for easy penetration through hair, and it is possible to change the tip diameter, tip profile and finger length to best penetrate through different types of hair on an individual-by-individual basis in near real time.
The electrodes can measure EEG signals and can be used for BCI (brain–computer interface) applications, which do not require a high SNR (Signal-to-noise ratio).
Electroencephalography (EEG) is the monitoring of a person’s brainwaves by placing small metal electrodes on the scalp. Traditional EEG systems are significantly limited by the requirement to have a wet gel added to the electrodes, which is used to make a good contact between the electrode and the scalp through hair.
This water-based gel, with an added ionic compound, such as sodium chloride, takes a long time to apply; leaves a mess; dries out over time; and is highly unpopular with patients, users, researchers and clinicians. In recent years, a significant number of approaches towards dry EEG electrodes have been proposed, and several are now available commercially (Figure 1).
However, while there is much interest in dry electrode technology and many of the attempts to develop such electrodes show promising results, dry electrodes are still not in widespread practical use due to a number of factors. These range from poor contact noise, to difficulties in keeping the electrodes attached, to the substantial costs required to purchase new electrodes.
We investigated 3D printing of EEG electrodes because it offers substantially quicker and more accessible manufacturing than traditional moulding or milling techniques. As a rapid manufacturing technique, it gives flexibility, so that many different design iterations can be developed, tested and improved.
For the purpose of EEG electrodes, this also means that different electrodes with various parameters, such as the number of fingers and tip sizes/shapes, can be tailored to each different person on demand. In particular, we have considered only desktop-grade 3D printers in order to allow inexpensive and easy access to the electrode manufacturing process.
A consumer-grade Ultimaker 2 printer was used with a 20-μm layer resolution and position precision on the X/Y/Z axes of 12.5/12.5/5 μm. A downside of this 3D printer is that it uses a Bowden-type feeder, which was problematic when attempting to print particularly brittle materials.
Mechanical Design and Printing
Our fabrication process first prints the mechanical structure of an electrode using standard, non-conductive, 3D printer plastics, and second, we coat these by hand with a suitable conductive material to make the final EEG electrode.
The design starting point is thus a mechanical model of a fingered electrode, which has long and thin prongs, which penetrate the hair and achieve contact with the skin. Multiple versions of a basic fingered design were considered (Figure 3a) in order find suitable settings that produced high quality mechanical devices after printing.
Five different design iterations are presented in Figure 3a together with a UK £1 coin for comparison. The electrode diameter of Versions 1.0–1.3 is 10 mm, and the length of the prongs is 4 mm, 7 mm, 7 mm and 10 mm, respectively.
State-of-the-art EEG electrodes have either a silver base with AgCl coating or have a sintered Ag/AgCl coating. Applying a sintered Ag/AgCl coating requires access to a specialist laboratory and chemical processes and, so, is not suitable for our aim of low cost easy to manufacture electrodes. It is possible to purchase Ag/AgCl ink (see Table 2) and use it similar to our approach below, but this is expensive and again not in-line with our current aim. Instead, to make the electrodes conductive and suitable for electro-physiological recordings, we coated them with silver. This gives a reduction in performance compared to Ag/AgCl, but with a much easier manufacturing process.
Table 2: Comparison of different silver coatings available.
There are several solutions for silver conductive coatings on the market, summarized in Table 2. Most options are either expensive or difficult to apply with the exception of pure silver paint, which costs £10 for 3 g and can be applied by hand simply using a paint brush.
Before the silver paint was applied to the electrodes, the safety datasheet was carefully examined in order to determine any potential hazards when applying the paint and also to determine its bio-compatibility.
The following materials were listed:
- Silver: Metallic element often used in biomedical applications, which has no safety implications to humans.
- 1-Ethoxypropan-2-ol: Commonly-used organic solvent labelled R10 and R67 under the European Union Regulation No. 1272/2008 for hazardous materials. R10 means that the element is flammable, and R67 means that the vapours may cause drowsiness and dizziness.
- Ethanol: Often used as an antiseptic or solvent. Labelled R11 meaning that it is highly flammable.
- Acetone: Organic solvent often used in the cosmetics industry. Listed as R11 (highly flammable), R36 (can cause eye irritation) and R67 (vapours may cause drowsiness and dizziness). Repeated exposure may cause skin dryness or cracking (R66).
- Ethyl acetate: Solvent that has the same hazardous labels as acetone.
Figure 4: Example of a 3D printed EEG electrode coated with silver paint. (a) Underside showing fingers for penetrating the hair; (b) Top side showing 4 mm snap connector.
To perform the coating, the silver paint is applied using a very fine paint brush and left to dry for one hour. Approximately, 1.5 g of silver paint are sufficient for the coating of 10 electrodes, and just one layer of coating provides good conductivity levels. The surface area of one of our electrodes is on average 957 mm2, giving an approximate coating thickness of 30 μm. An example of a final printed and coated electrode is shown in Figure 4.
To confirm the coating, the DC resistance between the prongs and the snap connector was measured to be approximately 0.4 Ω, which compares favourably with measurements of a commercial dry EEG electrode (approximately 0.3 Ω) and for a standard Ag/AgCl electrode (approximately 0.4 Ω).
- The tips of the prongs have a diameter of 2 mm, and the total height is 10 mm.
- The electrode base diameter is 15 mm in order to allow a future active circuit to be fitted on top.
- The total printing time varied depending on the settings used and was approximately 20 mins.
- Each iteration was 3D printed using standard PLA (Polylactic Acid) plastic.
- To make the electrodes suitable for electro-physiological recordings they were coated with silver.
Overview of the material chemistry that informs the manufacturing and electrode design. Presented is the electrode design and the manufacturing process. A detailed performance characterization is given using an EEG head phantom, and the results are discussed, which also overviews the progress toward