Improved Sweat Artifact Tolerance of Screen-Printed EEG Electrodes by Material Selection-Comparison of Electrochemical Properties in Artificial Sweat

Abstract

The accurate diagnosis of sleep disorders requires an electroencephalography (EEG) recording. However, in-home EEG-based sleep monitoring has been hindered by the present cumbersome electrode systems which are impractical for patient self-application. To overcome this shortcoming, we recently introduced a self-applicable facial electrode set with screen-printed EEG electrodes. However, one remaining challenge in overnight EEG measurements is the presence of a low-frequency sweat artifact commonly occurring in sleep recordings. Here, the purpose was to improve the electrochemical properties of the electrodes so that they are more sweat tolerant. We selected three commercial Ag and Ag/AgCl inks (manufactured by Dupont, PPG, and ECM) for the fabrication of screen-printed electrode sets with differently shaped ink layers and compared their electrochemical properties. The open circuit potential and electrical impedance spectroscopy measurements were conducted in a custom-made electrochemical cell filled first with physiological saline and then with artificial sweat. Based on our measurements, the Ag/AgCl layer on top (i.e. in contact with the solution) mainly determines the electrochemical characteristics whereas the Ag layer underneath plays only a minor role. In artificial sweat, the ECM electrodes possessed the most satisfactory properties being very stable with the most reproducible impedance characteristics (standard deviations were significantly (p<0.05) lower compared to the PPG and DUPONT electrodes). This study demonstrates that the screen-printing technique is suitable for the production of highly reproducible EEG electrodes with high stability and adequate impedance characteristics. However, careful selection of ink materials is essential due to the observed differences in their electrochemical stabilities in artificial sweat.

Effect of Sweating on Electrode-Skin Contact Impedances and Artifacts in EEG Recordings With Various Screen-Printed Ag/Agcl Electrodes

“…comparison of the average band power from 0.5 to 2 Hz before and after sweating), the lowest increase of the average band power due to sweating was invariably achieved with the ECM electrodes. No systematic differences in signal quality between A, B, and C electrode types were found as was expected based on the results of the previous in vitro study [16].…”

“…According to the previous in vitro measurements conducted in an electrochemical cell, the PPG electrodes were the most unstable after immersion in artificial sweat whereas the ECM electrodes performed in the most stable manner [16]. We hypothesized that the differences between the inks might be due to their chemical compositions.…”

“…Sweat contains substances such as sodium chloride and lactic acid [15] which may interfere with the electrochemical properties of the electrode-electrolyte interface [14] and possibly change the V HC ( Fig.1). Furthermore, our previous in vitro electrochemical investigations revealed that the electrochemical stability of screen-printed electrodes decreases in artificial sweat, but this reduction can be minimized by a careful selection of Ag/AgCl ink [16]. Therefore, we assume that the impedance mismatch might not be the only reason for sweat artifacts.…”

“…Based on the results obtained in our previous in vitro study [16], we selected six out of nine different screenprinted electrodes for the present in vivo study with 11 healthy volunteers. The present study compares two commercial Ag and Ag/AgCl ink materials manufactured by either Engineered Conductive Materials ECM LCC or PPG Industries Inc. and three differently constructed ink layers implemented by Screentec Oy (Oulu, Finland).…”

“…The screen-printed electrode sets (Fig. 2) for in vivo measurements were manufactured by Screentec Oy (Oulu, Finland) in a similar technological process as previously described in detail [6], [16]. The electrodes were printed in a flat-bed sheet silk screen printing unit on a flexible polyethylene terephtha- Table 1).…”