Dry electrodes for biopotential measurements

Wet gel electrodes are widely used for biopotential measurements such as electroencephalography (EEG) and electrocardiogram (ECG). However, such electrodes have several disadvantages such as skin irritation and decrease of signal quality due to gel drying. For EEG, setting up the gel electrodes and especially removal of the gel after EEG monitoring is a tedious and somewhat painful treatment. Sometimes the use of gels is avoided by replacing the electrodes with sponges soaked in a wet electrolyte. Set up and cleaning afterwards will become fast and easy, but electrode drying, poor signal quality on hairy skin, and corrosion of electrode holders due to the electrolyte might result.

Using dry electrodes can avoid these drawbacks; hence the development of such sensors gets a lot of attention. Commercially available 'dry' EEG headsets are typically equipped with metal dry electrodes, which are often hard, so subjects will feel pain after wearing the headsets for a while. A possible solution is the use of softer materials for the electrodes and further combining such electrodes with a spring-like system to avoid high-skin pressure.

At imec, polymer-based dry electrodes with comb-shape design are investigated and compared with the various wet/dry electrode-types available. For this work, a strong collaboration with a third party (a specialist in polymer fabrication) is established. Two kinds of electrodes are evaluated: (a) non-conductive polymer electrodes fabricated by stereo lithography and coated by a conductive metal, and (b) softer conductive polymer electrodes (polymer with conductive additives) fabricated using a molding process, see fig. 1. Impedance measurement on phantoms and human skin are performed for our dry electrodes and compared to various other types of electrodes, see fig. 2 for the experimental set-up). As an example, the results for polymer electrodes with carbon black as conductive additive are shown in fig. 3. The polymer electrode with the highest carbon (C) content has a rather low impedance, approaching the impedance of a wet gel electrode. For larger biopotentials, such as ECG, the rather low impedance of the high-C electrode is fine. For recording of weak biosignals (such as EEG), these impedance values will disturb the measurements, hence the use of these dry electrodes combined with a pre-amplifier (so called 'active electrode') is strongly preferred.n clinical settings, electrode impedance is also lowered by using various skin preparation techniques, such as the use of abrasive gel or skin moisturizers. Hence the polymer electrode impedance after use of these skin preparation techniques is evaluated (abrasive gel, wet-gel, moisturizers), see fig. 4 and 5. The tested skin preparation is indeed lowering the electrode-skin impedance, but the effect is only temporal, hence not interesting for long-term biopotential monitoring.

Figure 1

Figure 1: Polymer-based dry electrodes with comb-shape design.

Figure 2

Figure 2: Impedance measurement setup on phantoms and human skin.

Figure 3

Figure 3: Comparison of normalized impedance of electrodes versus the carbon black content, as measured using phantoms and human skin (various test persons). Impedance values are close to wet gel electrodes for the highest carbon content.

Figure 4

Figure 4: Impedance measurement of dry electrodes on human skin after use of Nuprep-abrasive gel (for conductive polymer electrodes). The abrasive-gel treatment will remove the upper part of the skin (stratum corneum), resulting in a decrease in impedance, however, this effect is only temporally.

Figure 5

Figure 5: Impedance measurement of dry electrode on human skin after hydration of skin by wet gel (for conductive polymer electrodes, tests on 3 subjects). Hydration of the skin (ie. by applying wet gel for 1 hour) will reduce the skin impedance, however, this effect is only temporally.

Finally, our dry electrode prototypes were applied on EEG and ECG systems to acquire biopotential signals. For ECG monitoring, the wearable imec-ECG system is equipped with our conductive polymer dry electrodes. Even without pre-amplification, very promising results are obtained, as shown in fig. 6.

Figure 6

Figure 6: Tests using imec's ECG system. (1) electrodes assembly (2) electrodes position on body; (3) ECG signals from wet-gel and conductive polymer dry electrodes. No pre-amplification is used. Even for the higher impedance polymer electrodes, the ECG signal is from good quality, with the PQRST peaks clearly visible.

Initial tests of EEG monitoring are performed, using imec's EEG headset with conductive polymer dry electrodes in combination with signal pre-amplification. The recordings are very promising, as is illustrated in fig. 7. The EEG signal with eyes closed, shows very clearly the presence of alpha waves. All test subjects considered the use of the headset with the polymer electrodes much more comfortable than using the headset with metal dry electrodes.

Figure 7

Figure 7: Initial tests of EEG monitoring using imec's EEG headset with conductive polymer dry electrodes in combination with signal pre-amplification: (left) EEG with eyes open, and (right) EEG with eyes closed, the alpha waves are clearly visible.