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Biomed. Eng.-Biomed. Tech. 2017; aop

Paulo Pedrosaa, Patrique Fiedlera, Vanessa Pestana, Beatriz Vasconcelos, Hugo Gaspar, Maria H. Amaral, Diamantino Freitas, Jens Haueisen, João M. Nóbrega and Carlos Fonseca*

In-service characterization of a polymer wick-based quasi-dry electrode for rapid pasteless electroencephalographyDOI 10.1515/bmt-2016-0193Received October 3, 2016; accepted March 28, 2017

Abstract: A novel quasi-dry electrode prototype, based on a polymer wick structure filled with a specially designed hydrating solution is proposed for electroencephalogra-phy (EEG) applications. The new electrode does not require the use of a conventional electrolyte paste to achieve a wet, low-impedance scalp contact. When compared to stand-ard commercial Ag/AgCl sensors, the proposed wick elec-trodes exhibit similar electrochemical noise and potential drift values. Lower impedances are observed when tested in human volunteers due to more effective electrode/skin contact. Furthermore, the electrodes exhibit an excellent autonomy, displaying an average interfacial impedance of 37 ± 11 kΩ cm2 for 7 h of skin contact. After performing bipolar EEG trials in human volunteers, no substantial

differences are evident in terms of shape, amplitude and spectral characteristics between signals of wick and com-mercial wet electrodes. Thus, the wick electrodes can be considered suitable to be used for rapid EEG applications (electrodes can be prepared without the presence of the patient) without the traditional electrolyte paste. The main advantages of these novel electrodes over the Ag/AgCl system are their low and stable impedance (obtained without conventional paste), long autonomy, comfort, lack of dirtying or damaging of the hair and because only a minimal cleaning procedure is required after the exam.

Keywords: bioelectric sensors; dry electrode; EEG; hydra-ting solution; in-vivo impedance; polymer wick.

IntroductionFor past decades, modern medicine has relied on non-invasive monitoring techniques for bioelectric potentials produced by the human body, such as electroencepha-lography (EEG, brain activity), electrocardiography (ECG, heart activity) and electromyography (EMG, muscular activity), in order to accurately comprehend the func-tional physiology and pathologies of humans. In particu-lar, the human EEG is, due to its high temporal resolution, a powerful tool for detecting acute brain disorders such as epileptic seizures, acute encephalitis, head trauma, coma and stroke [14, 16, 21, 28, 34]. More recently, EEG has also been applied to non-medical fields such as brain-com-puter interfaces (BCI) [18].

The conventional bioelectric potential acquisition setup relies on the use of the standard silver/silver chlo-ride (Ag/AgCl) wet electrodes [20, 32, 33]. These are consid-ered the gold standard of electrodes [1, 6, 10], as they are non-polarizable with excellent reliability, achieving low and almost frequency independent electrode/skin contact impedance values on the order of a few tens of kΩ cm2 [20, 32]. However, a preliminary skin preparation and a paste application are needed before the exam in order to lower the electrode/skin impedance. This preparation is

aPaulo Pedrosa and Patrique Fiedler: These authors contributed equally to this work.*Corresponding author: Carlos Fonseca, CEMUC – Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal; and Universidade do Porto, Faculdade de Engenharia, 4200-465 Porto, Portugal, Phone: +351-225081995, Fax: +351-22-041-49-00, E-mail: [email protected] Pedrosa: CEMUC – Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal; Universidade do Porto, Faculdade de Engenharia, 4200-465 Porto, Portugal; and Centro de Física, Universidade do Minho, 4710-057 Braga, PortugalPatrique Fiedler: Institute of Biomedical Engineering and Informatics, Technische Universität Ilmenau, 98693 Ilmenau, GermanyVanessa Pestana, Beatriz Vasconcelos, Hugo Gaspar and Diamantino Freitas: Universidade do Porto, Faculdade de Engenharia, 4200-465 Porto, PortugalMaria H. Amaral: Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Porto, 4050-313 Porto, PortugalJens Haueisen: Institute of Biomedical Engineering and Informatics, Technische Universität Ilmenau, 98693 Ilmenau, Germany; and Biomagnetic Center, Dept. of Neurology, University Hospital Jena, Friedrich Schiller University Jena, 07747 Jena, GermanyJoão M. Nóbrega: Institute for Polymers and Composites/I3N, University of Minho, 4800-058 Guimarães, Portugal

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2 P. Pedrosa et al.: Polymer wick quasi-dry electrode for rapid pasteless EEG

time consuming, uncomfortable for the patient, requires trained staff and extensive cleaning after the exam. Fur-thermore, some patients have developed severe allergic reactions to the commonly used gels or pastes [32] and the risk of short-circuiting adjacent electrodes due to paste running increases with electrode density. Susceptibility to motion artifacts has also been widely reported [20, 32]. Nevertheless, with proper skin preparation and correct conductive paste application, the wet Ag/AgCl electrodes exhibit excellent EEG signal quality [18].

Their counterparts, the so-called dry electrodes, do not require any previous skin preparation or paste appli-cation and are commonly based on inert-like materials, either metallic-like or insulator-coated metals [1, 15, 32]. However, these electrodes still present important drawbacks such as incorrect and/or uncomfortable skin contact due to their intrinsically stiff nature, which does not allow them to adapt to the individual human body curvature [5, 7], high electrode/skin impedances [32] and higher sensitivity to movement artifacts [3, 5, 25, 32].

Recently, Peng et al. [26] proposed a conceptually different EEG electrode that combines the advantages of the wet and dry sensor systems, based on a titanium micro porous transducer connected to an electrolyte res-ervoir. The electrolyte is delivered through the porous titanium to the skin by pressing the reservoir, keeping the electrode/skin contact wet during signal acquisition. Although this is an interesting and innovative concept, microporous titanium is expensive and quite delicate to process, as commented by the authors [26]. In addition, the electrode shape is not optimal to penetrate the hair layer and the non-continuous delivery of the electrolyte (reservoir pressing is required) may lead to a variation of the electrode-skin impedances, with negative effects on signal stability. While making use of the same princi-ple, Li et al. [17] proposed a more functional device in the form of a five ceramic pin structure assembled in a plastic body that also works as a liquid electrolyte reservoir. The reported results clearly validate the concept but this is a complex, seven parts device and the pins are very stiff and may become uncomfortable for the patient. Furthermore, porous ceramics fabrication is expensive and the conduc-tive fluid (sodium chloride) has limited hydration ability.

In this work, the authors propose a different electrode approach where the core material is a single pin-shaped polymer wick whose mechanical properties and porosity were optimized for bio-electrode application, by using a simple and cost effective technique [24]. A sponge soaked with a specifically prepared hydrating solution (instead of a liquid reservoir with sodium chloride) located at the back of the wick works as the fluid reservoir and signal

transduction relies on a chlorided silver wire inserted in the sponge. This technique avoids eventual liquid spills and the hydrating solution ensures a more effective skin hydration. More importantly, no continuous pressing of the reservoir is needed, since the hydrating solution is continuously delivered to the skin due to capillary action. Finally, the wick material and the developed manufactur-ing approach enable a low-cost production [19], particu-larly when compared with the aforementioned porous titanium approach.

In a previous work, the authors focused on a first application-specific proof of principle of the wick-elec-trode concept in the context of auditory event related activity using an oddball paradigm [19]. The results were analyzed by assessing the signal quality of the wick elec-trodes using electrode setups and signal parameters common in psychophysiological fields of applications. In contrast, the validation study presented in the paper at hand provides an in-depth technical presentation of the electrode concept and the assessment of the mechanical stability and the electrochemical electrode-electrolyte and electrode-skin interfacial characteristics, respectively, under laboratory and in-service conditions. In addition, the electrode-skin interfacial impedance was recorded over a period of 7 h in order to demonstrate the long-term stability of the wick electrode in combination with the specifically designed hydrating solution. Furthermore, the paper presents an extended EEG validation, analyz-ing multiple spontaneous and visual event-related activ-ity. The applied bipolar electrode setup enables a direct and objective comparison of the signal quality acquired with novel wick and conventional paste-based electrodes, while minimizing further influencing factors.

MethodsWick electrode fabrication

The electrode prototype is composed of three main parts, as shown in Figure 1. The upper part of the electrode, formed by a polyurethane sponge, works as the reservoir that supplies the hydrating solution to the lower wick structure. This sponge is inserted into the cavity of a pin-shaped polymer wick (second component), which establishes the skin contact and promotes a continuous delivery of a controlled amount of hydrating solution to the electrode tip/skin contact point. The third component, a chlorided silver wire, is inserted into the sponge and works as the signal transduction element, while the hydrating solution establishes the electrical bridge between the skin and the Ag/AgCl wire via the wick structure. The structure is inserted into a silicone sleeve. In order to be properly placed in the EEG cap and to promote an effective contact to the scalp, a fixation mecha-nism also had to be designed, Figure 1C.



P. Pedrosa et al.: Polymer wick quasi-dry electrode for rapid pasteless EEG 3

To produce the polymer wick, a polycarbonate polymer is ground and sieved to obtain a powder with a particle size below 500 μm. A mold is then filled with the powder and inserted in a cus-tom built sintering system. The sintering process takes place with an applied pressure of 56 kPa for 5 min at a constant temperature of 165°C. The detailed production process and parameters were described and optimized in a previous work [19].

The chlorided wire is fabricated from an Ag wire (φ = 250 μm, 99.99%, Goodfellow, Cambridge, UK) by dipping it for 2 min in a 50 mm FeCl3 solution. Afterwards, the Ag/AgCl electrode wire is thoroughly rinsed with distilled water and inserted into a cylindrical poly urethane sponge, i.e. in its turn, inserted in the polymer wick cavity, see Figure 1A and B. Finally, the wick material and sponge are encased in a silicone sleeve for increased mechanical stability and to minimize hydrating solution evaporation.

Hydrating solution

An important requirement for a surface electrode to accurately moni-tor an electrophysiological signal is that the electrode/skin imped-ance should be as low as possible, which implies either to remove the outer skin layer or to hydrate it, so that a conductive bridge can be established. Since a simple water-saline solution is unable to effi-ciently promote skin hydration, a specific hydrating solution was developed to be used with the wick electrode, fulfilling multiple requirements: (i) to display an appropriate electric conductivity, (ii) to possess surface energy and viscosity values low enough to provide a capillarity effect in contact with the polymer wick, (iii) to contain efficient and innocuous skin hydration agents (iv) to be promptly absorbed by the skin without spreading, dirtying or damaging the hair and (v) to not interfere with the charge transfer kinetics of the Ag/AgCl system.

The selected hydrating solution was previously developed and tested and contains propylene glycol (Acofarma S.A., Madrid, Spain), Tween 80 (Acofarma S.A.), sodium chloride (VWR International, USA) and Kathon CG (Acofarma S.A.) dissolved in water, with the concentrations reported in Table 1.

Propylene glycol (PG) and Tween 80 were added to increase skin permeation. The excellent skin permeation properties of PG are related to keratin solvation within the skin’s stratum corneum,

Figure 1: Wick electrode concept: (A) 3D model overview, (B) photo of the polycarbonate polymer wick electrode prototype and (C) photo of the wick electrode + cap fixation accessory, adapted from the bridge electrode holding system.

Table 1: Composition of the hydrating solution (in de-ionized water) used in the wick electrodes.

(NaCl) m/v (Tween 80) v/v (PG) m/v (Kathon CG) v/v

0.9% 3% 10% 0.1%

where PG competes with water for hydrogen bonding sites and also to intercalation in the polar head groups of the lipid bilayer [27]. On the other hand, Tween 80 is a well-known non-ionic surfactant, presenting an excellent balance between skin tolerance and pen-etration enhancement effects, being often used as a mild skin pen-etration enhancer [37]. Tween 80 has also the effect of decreasing the surface tension of water and greatly improving the wettability of the PC wick by the hydrating solution. Both components are GRAS (generally recognized as safe) by the food and drug administra-tion (FDA). Sodium chloride (Sigma-Aldrich, St. Louis, USA) was added for electric conductivity and the chosen isotonic concentra-tion (0.9%) minimizes ionic concentration gradients between the hydrating solution and skin and consequent skin irritation reac-tions. Kathon CG (Sigma-Aldrich, St. Louis, USA) was added to prevent microbial contamination whenever the solution had to be stored [10].

Mechanical tests

Compressive strength is an important mechanical parameter to allow the application of a specific electrode/skin adduction so that proper electrode/skin contact is achieved. The pin-shaped electrodes were tested for compressive strength with a EZ-2X (Shimadzu Corporation, Kyoto, Japan) testing machine equipped with a 500 N cell, operat-ing at a 1 mm/min strain rate. The acquired force and displacement values were post-processed using a custom MATLAB (Mathworks, Natick, MA, USA) algorithm by application of a low-pass filter in order to reduce quantization and measurement noise. Subsequently, the first derivative of the force-displacement curve was calculated. The initial physical damage of the samples was identified as a strong deviation from stress-strain behavior, showing a strong peak in the evolution of the derivative.




Electrochemical studies

The electrochemical studies were performed using a Gamry G300 potentiostat (Gamry Instruments Inc., Warminster, PA, USA). All elec-trochemical characterization of the wick electrodes was performed using the hydrating solution as electrolyte (Table 1), while a commer-cial paste (ECI Electro-Gel, Electro-Cap International Inc., Eaton, OH, USA) was used to test the Ag/AgCl commercial electrodes (B10, EASY-CAP GmbH, Herrsching am Ammersee, Germany), for comparison. The sintered Ag/AgCl commercial electrode was also used as refer-ence electrode in electrochemistry studies. A Faraday cage was used in all the tests in order to reduce ambient noise.

Electrochemical impedance spectroscopy (EIS) studies were performed at the open circuit potential (OCP) for frequencies ranging from 0.1 Hz to 10 kHz, with a 7 mV (rms) AC probe signal, using the EIS300 software from Gamry. Three different wick electrodes were used, to assess fabrication reproducibility.

The electrochemical noise data were acquired with pairs of wick electrodes immersed in the hydrating solution, by using the Gamry ESA410 software (Gamry Instruments Inc., Warminster, PA, USA). Three pairs of wick electrodes and one pair of Ag/AgCl commercial electrodes were tested. Data were acquired using a 1000 Hz sampling rate for a period of 12 min and then analyzed using a custom MatLab (The Mathworks Inc., Natick, MA, USA) algorithm. The electrochemi-cal noise analysis was performed by first applying a 20th order Butter-worth bandpass filter with cut-off frequencies at 0.5 Hz and 100 Hz. The first 10 s of each filtered dataset were neglected to avoid includ-ing considerable filter artifacts in subsequent evaluations. Then, suc-cessive segments of 30 s were considered for the calculation of the root mean square (RMS) values of noise and drift rate, over the total acquisition time. The power spectral density (Welch estimation) was calculated for 12 min segments, representative of the Ag/AgCl refer-ence and wick electrode pair recordings.

In-vivo impedance studies were performed in three healthy vol-unteers (two males and one female) using the already mentioned

frequency range and probe signal. In this case, a custom-made elec-tronic safety circuit was used that cuts the current flowing through the body if it eventually exceeds a maximum allowed value of 100 μA. For each subject, a conventional Ag/AgCl electrode and a wick elec-trode were positioned close to each other (distance of ~2 cm) at the O2 position according to the international 10–20 system of electrode placement [33] and an additional Ag/AgCl commercial electrode was placed at Fp2. The Ag/AgCl electrodes were applied in combination with conventional electrolyte paste and fixed using an adjustable stripes cap (Nihon Kohden, Tokyo, Japan). The same cap was used to fix the wick electrodes to the scalp by using a custom-made fixation accessory (cp. Figure 1C). Before electrode application, the skin at the electrode positions was cleaned with alcohol pads to ensure similar skin condition for all volunteers.

In order to assess the long-term applicability of the new elec-trodes and their autonomy from manual rehydration, the wick elec-trode/scalp impedance was measured hourly during 7 h, with a wick electrode positioned at the Cz position and a counter-electrode (Ag/AgCl) positioned at the Fp2 position of the head of the volun-teer. The measurement frequency was 10 Hz. The electrolyte paste at the conventional Ag/AgCl counter electrode was renewed every 2 h, while no re-filling was performed on the wick electrode reservoir.

EEG monitoring

All EEG trials were performed on five healthy adult male volunteers (informed consents were obtained from all subjects) using a bipolar measurement setup as shown in Figure 2. The setup included four conventional Ag/AgCl ring electrodes (EASYCAP GmbH, Herrsching am Ammersee, Germany) and two wick electrode assemblies, con-nected to four independent bipolar inputs of a commercial biosignal amplifier (RefaExt, ANT B.V., Enschede, Netherlands). In each case, the reference inputs of the two bipolar inputs where connected to the same reference electrode, while the test inputs were either connected

Figure 2: Scheme of the (A) overall measurement setup of the EEG tests applying a double bipolar measurement configuration for direct comparison of commercial Ag/AgCl reference electrodes and wick electrodes.Arrows indicate that the potential difference is measured between the end-arrow and begin-arrow (reference) electrodes. Photo of the (B) electrode montage at the O2 position, showing the wick electrode in the center between two Ag/AgCl electrodes.




to a wick electrode or a conventional Ag/AgCl ring electrode. The electrodes were placed close to each other (~2 cm center-to-center) at positions Fp2 and O2 according to the international 10–20 system [33]. Thus, the setup enabled simultaneous parallel EEG acquisition from frontal-to-occipital and from occipital-to-frontal electrodes, see Figure 2. The conventional Ag/AgCl ring electrodes were applied with the commercial electrolyte paste, while the wick electrodes were used in combination with the proposed hydrating solution. Prior to elec-trode application, all electrode positions were thoroughly cleaned by rubbing with alcohol pads to guarantee a similar skin condition for all volunteers.

During each EEG acquisition, four independent recordings were sequentially performed: resting state EEG (open eyes), α activ-ity (closed eyes), eye blinking artifacts, as well as a pattern reversal visual evoked potential (VEP), in accordance with the International Society for Clinical Electrophysiology of Vision (ISCEV) 2010 stand-ard [22].

All data were post-processed by application of a bandpass fil-ter (Butterworth, 30th order, 1–40 Hz) followed by manual selection of characteristic signal sequences of 10 s. The VEP post-processing included manual exclusion of artifact-afflicted trials and subsequent averaging of the remaining trials. In order to objectively compare the recordings, the root mean square deviation (RMSD), the Pearson cor-relation coefficient (CORR), and the power spectral densities (PSD, Welch estimation) were calculated, in accordance with our previous publications [3, 25]. In addition, we calculated the magnitude squared coherence [13] to access the compared signal conformity across fre-quencies. Therefore, we used analysis windows of 512 samples length with 50% overlap, investigating frequencies from 1 to 40 Hz with 0.1 Hz steps. Using the same window parameters and frequency steps, we also performed a time-frequency-analysis for the α activity signal episodes using the short-time Fourier transformation [23].

Results

Mechanical tests

The polycarbonate wick electrodes display an average compressive strength of 5 ± 2 N. Typical adduction forces used during the in-vivo impedance and EEG tests were between 1.5 and 2 N. Consequently, it can be concluded that the PC wick structures fabricated by the sintering method are able to easily withstand the typical forces present in routine applications without irreversibly deforming or damaging the porous wick structure.

Electrochemical evaluation of the wick electrodes

Electrochemical Impedance Spectroscopy (EIS)

The Bode-like amplitude plots of the wick and commer-cial electrodes/skin interface impedances are shown in Figure 3, together with the values for the chlorided silver

Figure 3: Impedance amplitude (top) and phase (bottom) plots of the wick and reference electrode pairs.

wire electrodes, similar to those that were used as com-ponents of the wick electrodes (Section “Wick electrode fabrication” and Figure 1). All impedance values were normalized with respect to the geometrical area of the electrodes (silver wire area in the case of the wick elec-trodes). Since this study focuses on the comparison of two different electrode concepts, the EIS studies of the wick electrodes were executed using the proposed hydrating solution (Section “Wick electrode fabrication”), while the standard commercial paste (ECI Electro-Gel) was used with the commercial Ag/AgCl electrodes.

The commercial electrode exhibits impedance values on the order of ~10–40 Ω cm2, while the wick elec-trodes display higher impedance values, ranging from ~900 Ω cm2 to ~1300 Ω cm2, as can be seen in Figure 3. It is important to note the excellent reproducibility among the impedance values for the three wick electrode samples.

When the frequency decreases, both the commercial Ag/AgCl and wick electrodes display a smooth imped-ance increase, accompanied by a negative phase shift, indicating an increasing contribution of the capacitive component to the total impedance, as expected from a




Randles-like behavior depicted on the proposed equiva-lent circuit shown in Figure 4. Within the frequency band of standard EEG, at 10 Hz, the impedance of the Ag/AgCl with the paste and polymer wick electrodes with the hydrating solution are 15 Ω cm2 and 1000 Ω cm2, respec-tively. Note that even though the wick electrodes display higher impedance values than the commercial ones, those impedances are still considerably below the gener-ally observed in-service electrode/skin impedance values (usually >10 kΩ cm2, at 10 Hz [9, 30]), thus not compromis-ing the in-vivo performance of the wick electrodes.

Building on the promising results of the impedance bench studies, the authors decided to implement a more application-driven impedance characterization by per-forming in-vivo trials in human patients [2, 36], whose results are shown in Figure 5. Moreover, it is important to note that the wick electrodes present more reproducible impedance values than the commercial ones, with elec-trode/skin impedances varying from 3–5 kΩ cm2 while the commercial electrodes exhibit a higher variation of the

Figure 4: Equivalent Randles circuit proposed to describe the elec-trical behavior of the electrode/electrolyte interface.

Figure 5: In-service impedance characteristics of wick and com-mercial electrode when placed in the O2 position for three different subjects at a distance of approximately 2 cm.Measurements were performed either between pairs of commercial and wick electrode or two commercial electrodes.

1 2

Impe

danc

e (k

Ω c

m2 )

3 4 5 6 7 8Time (h)

102

101

Figure 6: Wick electrode/scalp impedance vs. time.The wick electrode was positioned at the Cz position and a com-mercial Ag/AgCl electrode was positioned at Fp2. The measurement frequency is 10 Hz.

values, displaying both the lowest (~3 kΩ cm2) and highest (30 kΩ cm2) low frequency (1 Hz) impedance values.

One of the key features of the wick electrode concept is the ability to keep a wet electrode/scalp contact over a long period, thus achieving low interfacial impedance values. Therefore, one of the main questions to address when assessing the wick electrode performance is how long the contact hydration can be maintained. In order to evaluate the wick electrode’s autonomy, the electrode/scalp impedances measured hourly during 7 h are shown in Figure 6. The experiment was not continued for longer times due to limitations in subject availability, which was not related to the electrode function and/or comfort.

From the plot, it is apparent that the impedance remains in the range 20–50 kΩ cm2 over the complete meas-urement period, with an average value of 37 ± 11 kΩ cm2. The presented values represent the sum of both interfa-cial impedances at the frontal conventional (5–12 kΩ cm2 typical impedance) and the occipital wick electrode.

Electrochemical noise and drift analysis

The power spectral density (PSD) of noise of the wick elec-trodes is shown in Figure 7 in comparison to commercial electrodes. The depicted PSDs represent the noise gener-ated by three couples of wick electrodes, compared with a couple of commercial Ag/AgCl electrodes. The noise char-acteristics of both wick and commercial electrodes follow the 1/(freq.)α law, [11]. The noise power increases for low frequencies but at 0.5 Hz both the commercial and wick electrodes exhibit similar noise values, ranging from ~0.2 to 0.4 μV2/Hz, well in the range of typical amplifier noise [12].

Fast open circuit potential (OCP) stabilization times and low drift rates are required in order to avoid unwanted




masking of the low frequency and small amplitude EEG signals as well as to ensure compatibility with the limited dynamic input range of biosignal amplifiers. The OCP drift rates of the wick and commercial electrodes are plotted in Figure 8.

Once more, the results are similar between the two electrode concepts. The absolute value of the mean of the commercial electrodes is increased, while for the wick electrodes the variation is slightly higher. However, all electrodes (wick and commercial) exhibit drift rates that lie within the [−1; 1] μV/s range throughout the complete measurement interval of 12 min. As the amplitude of EEG signals is considerably larger than the drift values, they will not negatively influence the EEG signal quality.

EEG trials

As the last step of the electrode performance characteri-zation the wick electrodes were applied during several in-vivo tests in parallel with commercial Ag/AgCl paste electrodes. Figure 9 and Table 2 provide the comparison results of EEG trials in time domain. In Figure 9 exemplary overlay plots of EEG containing α activity (5 s, frontal refer-ence), eye blinking artifacts (10 s, occipital reference), and

Figure 7: Noise power spectral density of the wick electrode pairs.Commercial electrode data is shown as comparison.

Figure 8: OCP drift rate of the wick and commercial electrode pairs.

Figure 9: Time domain overlay plots of simultaneously recorded EEG signals using conventional EEG electrodes (red, solid) and wick electrodes (black, dotted): (A) 5 s of EEG containing α activity recorded using a frontal reference and occipital test electrodes; (B) 10 s of EEG containing eye blink artifacts using an occipital reference and frontal test electrodes; (C) VEP results showing 100 ms pre-stimulus to 400 ms post-stimulus.

VEP (500 ms) are presented. Visually, the compared con-ventional Ag/AgCl electrode and wick electrode signals resemble each other without considerable differences, and this result can be confirmed from the calculated CORR, RMSD and coherence values presented in Table 2.

In Figure 10A the PSD of EEGs containing α activity (frontal reference) and resting state EEG (occipital refer-ence) is shown as mean over all volunteers. Similar to the time domain evaluation, the recorded signals exhibit no considerable differences in the frequency domain. For the whole investigated frequency range of 1–40 Hz (standard EEG) the spectra are similar. The α activity peak is clearly pronounced in the frequency band from 8 to 12 Hz in the average PSD across all subjects. In addition, Figure 10B–D show an exemplary result of the time-frequency-analysis of the α activity recording of one volunteer for conventional reference electrodes (Figure 10B), the wick electrodes (Figure 10C) and the difference between both (Figure 10C). Again, the α activity is clearly visible in the corre-sponding frequency band. No considerable difference is visible between both electrode types. Similar results were obtained for the α recordings of the remaining volunteers.




Discussion

Mechanical tests

Owing to the wick electrode working principle, some mechanical force is essential to ensure a reliable inter-facial electrode/skin contact. However, due to the small pin top diameter of the electrodes, such forces may poten-tially translate into electrode adduction pressures able to

compromise both the material structure of the electrode as well as patient comfort. Compressive strength values of 5 ± 2 N were obtained during the mechanical tests of the wick electrode structures. The high standard devia-tion obtained for the compressive strength is due to the maximum polycarbonate particle size (0.5 mm) vs. elec-trode pin top diameter (1 mm), meaning that the mechani-cal properties of the structure may be compromised in the electrodes where a low number of (large) particles are bonded at the pin tip. However, the adduction forces used

Table 2: Quantitative EEG comparison results.

EEG test Reference electrode Mean RMSD ± STD in μV Mean correlation ± STD in % Mean coherence ± STD in %

α activity Fp2 2.5 ± 1.8 98.2 ± 2.1 97.3 ± 3.0Resting state Fp2 2.0 ± 1.9 98.4 ± 1.5 97.0 ± 3.3VEP Fp2 0.4 ± 0.2 99.1 ± 0.7 n.a.Eyeblink artifacts O2 7.9 ± 1.8 99.6 ± 0.0 93.6 ± 0.1

Figure 10: Frequency domain comparison of EEG acquired with both sets of electrodes: (A) Grand average overlay plot of the simultaneously recorded EEG signals using conventional EEG electrodes (red) and wick electrodes (black): resting state EEG (dotted lines) and EEG contain-ing α activity (solid lines); (B) time-frequency analysis of an exemplary α activity recording of one volunteer using conventional and wick electrodes, respectively; (C) difference of the time-frequency analysis results of both electrode types.




during the in-vivo electrode applications performed in this work were always in the range of 1.5–2 N and no problems were observed regarding the mechanical properties or electrode stability.

Electrochemical evaluation of the wick electrodes

Electrochemical impedance spectroscopy (EIS)

The impedance studies were performed taking two differ-ent approaches. While the first one is a pure laboratory study, the second approach aims at a more practical “in-service” characterization of the electrochemical imped-ance behavior.

The EIS study under lab conditions enables objec-tive comparison without biological influences. In order to better understand the displayed EIS results (Figure 3) the Randles electrical equivalent circuit shown in Figure 4 was proposed to describe, in a qualitative way, the electrical behavior of the electrodes in contact with the hydrating solution. In this circuit, Rs stands for the electrolyte resist-ance and Ci, Ri stand for the interfacial capacitance and charge transfer resistance associated with the Ag/AgCl redox processes, respectively [20]. The quasi-constant impedance and close to zero phase values in the high fre-quency regime (at ~1–10 kHz) observed for both Ag/AgCl commercial and wick electrodes suggest an essentially resistive behavior of the interface in this region, meaning that the capacitor of the equivalent circuit is essentially short-circuited. Consequently, either the hydrating solu-tion or electrolyte paste impedances are strongly dominat-ing the overall impedance characteristics.

Regarding the behavior of the Ag/AgCl commercial elec-trode and wire in the paste, the lower impedance displayed by the commercial Ag/AgCl electrode can be ascribed to the porous sintered nature of this electrode that displays a much higher specific area not taken into account in the area calculations (only the geometric area was considered). From the high frequency impedance of chlorided Ag wires in the electrolyte paste and hydrating solution it can be con-cluded that the respective solution impedances are about 20 Ω cm2 and 200 Ω cm2, respectively. This difference is probably related to the lower concentration of sodium chlo-ride (0.9% vs. 7.6% in the paste [6]) and to the presence of PG and Tween 80 in the hydrating solution. On the other hand, the wick electrodes exhibit the highest impedances in the high frequency regime (about 1000 Ω cm2) when com-pared to the chlorided Ag wire in the hydrating solution. This should be ascribed to the resistance of the hydrating

solution inside the pores of the wick polymer. Therefore, care should be taken when selecting a wick material for the electrode: the pore diameter should be low enough to enable capillary pressure induced fluid motion through the wick, but not so small that electric impedance becomes too high. The wick polymer used in this work has an average pore diameter of 12 μm and a porosity of 45%, as measured in a previous work [19].

The results obtained on human volunteers do not show the relatively constant high frequency impedance of the laboratory results because of the presence of skin, whose capacitance values are significantly lower than those of the electrode/liquid interface [20], pushing the constant impedance region to higher frequency values. The higher variation of the results obtained with commercial electrodes is caused by the difficulty to correctly apply the paste in patients with thick hair layers, especially at the O2 position, which was the case for one of the volunteers in this study. On the other hand, the wick electrodes were designed with a pin shape, which explains why they easily overcome the hair layer, establishing a pressure-assisted scalp contact that translates into a more reliable contact and reduced var-iation of the electrode/skin contact impedance values [3, 4]. Regarding the electrode autonomy, no interfacial imped-ance evolution was perceived with increasing contact time. This means that, on one hand, hydration takes place quickly and on the other hand, the electrode autonomy is longer than the 7 h defined for the test. It is important to note, that our observed electrode-skin interfacial impedance values didn’t show the increasing trend with application time that was observed in the work of Li et al. [17] for ceramic based wick electrodes. We attribute this fact to the specifically designed hydrating solution and the corresponding sponge and reservoir design presented in this work. Furthermore, there were no complaints regarding electrode comfort, not even for the 7 h test.

To summarize, the impedance values of the proposed wick electrode and hydrating solution combination can be considered suitable to be used in EEG applications, since they are lower and less dependent on the patient than the typical impedances of the standard Ag/AgCl electrode/scalp interface. Furthermore, the wick electrodes are suit-able for EEG exams with durations of up to 7 h.

Electrochemical noise and drift analysis

An increase of the noise power for low frequencies is common in electrochemical systems and is related to slow potential drift and ionic diffusion processes. Due to the fact that both electrode types show similar noise values




from 0.5 Hz, it is possible to conclude that the proposed electrodes are suitable to be used in EEG applications. The noise contribution of the combination of wick electrode and hydrating solution is of the same order of magnitude of that observed for commercial electrodes and consider-ably lower than the EEG signal amplitude, thus enabling a clean signal transfer from the body to the EEG amplifier [8, 29, 31, 35], from the point of view of the electrode/fluid interface.

As for the noise and impedance results, the OCP values emphasize the wick electrodes’ suitability to be used as EEG electrodes since they perform similar to the standard commercial Ag/AgCl electrodes. No influence of the electrode type on the low frequency EEG components or the dynamic range requirements of the biosignal ampli-fier is expected.

EEG trials

The order of magnitude of the quantitative values (mean over all volunteers) suggests that the small differences in time domain must be addressed to ambient noise sources and the spatial distance of the compared elec-trode positions [25]. In summary, the EEG tests comple-ment well the electrochemical characterization results and demonstrate that the proposed combination of wick electrode and hydrating solution provides EEG results comparable to conventional paste-based electrodes. It is also important to note that the wick electrodes could be used for about 2 h without noticeable increase of elec-trode/skin impedance, suggesting a longer autonomy than the classic Ag/AgCl electrodes. The autonomy of the wick electrodes will be further investigated in a future work. Furthermore, easier and more effective electrode/skin coupling were observed. Finally, it is important to note here that, unlike with conventional Ag/AgCl paste electrodes, no washing of the hair was needed after the acquisition and the volunteers considered the new elec-trodes comfortable to wear.

Further research is now under way to improve the wick electrode design and develop a cap system, in order to provide a comprehensive EEG electrode and cap system that will keep the advantages of the classic Ag/AgCl paste electrodes while avoiding the use of the gel paste. More-over, the novel wick electrode technology will enable the full preparation of the EEG exam without the presence of the patient. This is a major advantage over the classic gel and paste-based cap systems, especially when multichan-nel setups are needed and/or for uncooperative patient populations like children or psychiatric patients.

ConclusionsIn the present work, a new concept of quasi-dry EEG elec-trode was investigated and its performance compared with that of the standard Ag/AgCl sensors. This concept is based on a polycarbonate wick coupled with a polyu-rethane sponge saturated with a specifically designed hydrating solution and a chlorided Ag wire for signal transduction.

Main results show that in the bench tests, the imped-ance behavior of the wick electrodes is highly reproduc-ible, despite presenting higher values than those of the reference Ag/AgCl electrodes. However, the in-service tests showed lower impedances for the novel electrodes, due to their more effective electrode/skin contact. This was ascribed to the fact that the pin-shaped wick elec-trodes are able to permeate the hair layer and establish a more efficient (pressure assisted) scalp coupling than flat Ag/AgCl electrodes, despite the use of the conduc-tive paste. Electrode autonomy proved to be superior to 7 h. No considerable differences were found in the elec-trochemical noise and drift rate behavior, as well as in terms of shape, amplitude and spectral characteristics of the signals when compared to commercial paste-based (wet) electrodes.

Low and stable impedance, long autonomy, comfort-able, appropriate for rapid EEG (no need for the presence of the patient during electrode preparation) and no dirty-ing or damaging of the hair after the exam are the major advantages of this novel sensor over the conventional Ag/AgCl systems. It is then possible to conclude that the pro-posed quasi-dry wick electrode is a promising substitute for the commercial electrodes for EEG applications.

Acknowledgements: P. Fiedler and J. Haueisen acknowl-edge financial support by the German Federal Ministry of Education and Research (03IPT605A) and the Free State of Thuringia by funds of the European Social Fund (2015FGR0085). P. Fiedler, P. Pedrosa, H. Gaspar, B. Vasconcelos, C. Fonseca, and J. Haueisen acknowledge financial support by the German Academic Exchange Service (D/57036536). C. Fonseca acknowledges funds from FCT - Portuguese Foundation for Science and Technology, project PTDC/SAU-ENB/116850/2010, and J.M. Nóbrega acknowledges FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under project UID/CTM/50025/2013. P. Pedrosa, P. Fiedler, B. Vasconcelos, J. Haueisen and C. Fonseca acknowledge financial support by the Sev-enth Framework Programme, (Grant / Award Number: ‘IAPP-610950’).




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