Polymer-based Dry Electrodes for Biopotential Measurements

Monitoring biopotential signals such as electrocardiography (ECG) and electroencephalography (EEG) provides important information about certain health-related conditions which open up a broad range of applications. The electrical and mechanical properties of electrodes used for the measurements have a great impact on the quality of the signals. Conventional wet electrodes, which contain conductive gel to hydrate the skin and lower the electrode/skin impedance, are widely used. Furthermore, the flexible gel stays in good contact with the skin during movement, hence the impedance also remains low when the subject is moving. However, the gel’s various drawbacks are also well-known. For example, before an EEG monitoring, the setup of wet electrodes is not only often time-consuming but also ought to be done by an expert. During EEG monitoring, signal degradation occurs due to the gel drying out, and skin irritation due to the glue used to fix the electrodes on the scalp are often observed. After EEG recording, removing the glue to detach the electrode is difficult and can mess up the hairstyle.

To avoid these drawbacks of the wet electrodes, various types of dry electrodes have been introduced. In this thesis, dry electrodes fabricated from a flexible conductive polymer are presented. The elastic properties -especially the flexibility- of the conductive polymer ensure higher user comfort than obtained by rigid metal electrodes. The conductivity of the polymer does away with the need for any extra conductive coating layer, which often flakes off during use. Electrodes with various pin-shapes (length, width, density) are fabricated for different applications. Various additives have also been added to the polymer, and additive type and quantity are optimized to obtain low contact impedance and good mechanical properties for better comfort and skin contact.

Impedance measurements on phantoms and human skin are carried out for impedance characterization. The electrodes with optimal material composition, i.e. polymer containing 43% of carbon, show only ~10 times higher electrode/skin impedance than that of wet electrodes. Applying abrasive gel or lotion with the polymer dry electrodes and producing sweat before the measurements reduce the impedance of polymer electrodes. However, the impedance does not remain low because the agents reducing the impedance (gel, lotion or sweat) disappear with time. The influences of these skin pretreatments and sweat processes on the electrode/skin impedance with time are investigated by modelling the recorded impedance with the equivalent electrical circuits.

To investigate the mechanical properties of the polymer dry electrodes, compression tests are carried out for elastic modulus and compliance characterization. The elastic modulus of polymer materials increases with growing carbon content and electrodes made from polymer with a higher carbon content show to be less compliant. Also the electrode shape is influencing the mechanical properties: the electrodes with more pins, thicker pins and shorter pins are less compliant. The compliance of the pin-shaped electrodes can be predicted rather well using the elastic moduli of the polymer materials and the shapes of the electrodes.

Polymer electrodes with optimal material composition are directly capable of recording strong biopotential signals such as ECG. The ECG signals recorded using polymer dry electrodes show a high similarity to those recorded by the neighboring wet electrodes when the subjects sit still and avoid movements. Pin-shaped polymer electrodes are sensitive to motion artifacts when applied on non-hairy skin. This proves that the shape of the electrode should depend on the applications.

To record low amplitude signals such as EEG, polymer electrodes need to be coupled with active circuits. In this thesis, our own active circuits are fabricated, and their compatibility with commercial EEG recording systems is investigated. The EEG signals recorded by polymer dry electrodes are compared with those of their adjacent wet electrodes. Polymer dry electrodes are capable of recording good quality EEG signals however they are more sensitive to pick up power line interferences (50 Hz signals) and they show more baseline drift (low frequency signals). Both of the 50 Hz signals and the baseline drift can easily be filtered out and the baseline drift reduced while polymer electrodes are becoming stabilized. The signals containing baseline drift still have sufficient good quality for interpretation by an experienced medical staff. However, it has to be noted that the baseline drift in the raw signals might result in misinterpretation of EEG signals distributed at low frequency (< 2 Hz). Besides using the commercial EEG recording systems, the signals of polymer dry electrodes are compared with those of wet electrodes and AgCl-coated polymer electrodes using imec’s EEG recording system. Polymer electrodes show higher impedance and slightly lower signal quality than the other 2 types of electrodes, which might be improved after they are stabilized. A stabilized electrode shows fewer impedance variation and baseline drift resulting in better signals. 

Besides the electrical and mechanical properties of the polymer electrodes, biocompatibility and user comfort are also important criteria of good electrodes. The polymer material used in this thesis is non-cytotoxic. No skin irritation is found after applying the polymer electrodes up to a few days. In addition, no participants report any skin discomfort during and after measurements. As to the user comfort of EEG monitoring, only a few participants with a larger head-size experience local pressure from the one-size electrode fixation device. Furthermore, all participants report that the application and removal of polymer dry electrodes are much more convenient than conventional wet electrodes.

To conclude, our polymer dry electrodes show high user comfort and are able to record good quality ECG and EEG signals. Therefore, combining polymer electrodes with the proper fixation devices may be seen as promising alternatives to either the commercially available devices equipped with dry electrodes or the conventional wet electrodes.