Advancements in Wearable EEG Technology:

Electrode Characterization and Signal Quality Assessment

Andrea Farabbi

1 a

, Andrea Costanzo Palmisciano

1 b

, Matteo Rossi

1 c

, Niccol

o Antonello

2 d

Diana Trojaniello

2 e

, Tommaso Ongarello

, Pietro Cerveri

1,3 f

and Luca Mainardi

1 g

Department of Electronics, Information and Bioengineering, Politecnico di Milano, Milan, Italy

EssilorLuxottica Smart Eyewear Lab, EssilorLuxottica, 20133 Milan, Italy

Department of Industrial and Information Engineering, University of Pavia, 27100 Pavia, Italy

Keywords:

Electroencephalography, Wearable Systems, Biomedical Signal Processing, P300, Alpha Activity, Electrodes

Characterization.

Abstract:

This research contributes to the advancement of practical, user-friendly EEG devices for both research and

real-world applications. The paper presents a comprehensive study on the development and characterization

of wearable electroencephalography (EEG) recording in non-traditional electrode locations. In particular, we

focus on optimizing electrode placement, material selection, and signal quality assessment. Our investigation

includes impedance testing of various electrode materials, comparative analysis of dry versus wet electrodes,

and validation through standard EEG protocols. Results demonstrate the feasibility of acquiring high-quality

EEG signals from over-the-ear locations where using gold-plated brush electrodes with retractile pins, show

superior impedance characteristics (10

Ω) compared to other tested materials. We also validate and compare

dry electrodes by means of an eyes-open/eyes-closed protocol, conﬁrming the ability to detect alpha rhythm

modulation in non-traditional electrode placements.

1 INTRODUCTION

Electroencephalography (EEG) has been a fundamen-

tal tool in neuroscience and clinical practice (Berger,

1929). This non-invasive technique measures the

electrical activity of the brain by recording voltage

ﬂuctuations resulting from ionic current ﬂows within

neurons (Teplan, 2002) and emerging to the scalp.

EEG provides excellent temporal resolution and has

a variety of application such as cognitive processes,

diagnosing neurological disorders, and developing

brain-computer interfaces (Lotte et al., 2018).

Traditional EEG systems typically utilize the in-

ternational 10-20 system for electrode placement

(Klem et al., 1999), which yields comprehensive spa-

tial information, but presents signiﬁcant limitations

for its use outside controlled laboratory or clinical en-

vironments. The complexity of setup, need for skin

https://orcid.org/0000-0001-5582-4654

https://orcid.org/0009-0007-2379-1465

https://orcid.org/0000-0003-2519-0720

https://orcid.org/0000-0002-0803-5385

https://orcid.org/0000-0001-8935-5593

https://orcid.org/0000-0003-3995-8673

https://orcid.org/0000-0002-6276-6314

preparation, and use of conductive gels make tradi-

tional EEG systems impractical for long-term or ev-

eryday use (Casson et al., 2010).

Recent advancements in miniaturized electronics,

sensor technologies, and signal processing algorithms

have sparked interest in developing more portable and

user-friendly EEG acquisition devices (Mihajlovic

et al., 2015). These wearable EEG systems aim to

enable continuous, high-quality brain monitoring in

real-world settings, potentially expanding the applica-

tions of EEG in ﬁelds such as personalized medicine,

cognitive monitoring, and human-computer interac-

tion (Lin et al., 2014).

The development of wearable EEG technology

faces several signiﬁcant challenges due to the dif-

ﬁculty in measuring high quality EEG signal using

small devices. These include optimizing electrode

placement for non-traditional locations, improving

signal quality from dry electrodes, ensuring user com-

fort and social acceptability, managing power con-

sumption for continuous recording, and developing

robust algorithms for real-time signal processing in

noisy environments (Mullen et al., 2015). Addition-

ally, wearable EEG systems often employ a reduced

number of channels compared to traditional setups,

which further complicates the issue of optimal elec-

716

Farabbi, A., Palmisciano, A. C., Rossi, M., Antonello, N., Trojaniello, D., Ongarello, T., Cerveri, P. and Mainardi, L.

Advancements in Wearable EEG Technology: Electrode Characterization and Signal Quality Assessment.

DOI: 10.5220/0013153300003911

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 18th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2025) - Volume 1, pages 716-720

ISBN: 978-989-758-731-3; ISSN: 2184-4305

trode placement (Debener et al., 2015). The use of dry

electrode designs is also affected by the presence of

hair which can substantially increase the impedance

potentially comprommising the measured EEG signal

quality(Lopez-Gordo et al., 2014).

In this study we chose over-the-ear locations for

EEG electrodes as it is justiﬁed by different factors.

Firstly, this area offers a balance between signal qual-

ity and user comfort, as it is less obtrusive than tra-

ditional scalp placements (Bleichner and Debener,

2017). Secondly, the proximity to temporal and pari-

etal lobes provides access to relevant brain activity

(Mikkelsen et al., 2015) while maintaining a socially

acceptable form factor (Norton et al., 2015).

This paper presents a comprehensive study ad-

dressing several key aspects of electrodes employed

in EEG enabled devices, with a speciﬁc focus on over-

the-ear electrode placement. Our research tackles

three areas of interest: i) evaluating different electrode

materials and designs to enhance signal acquisition in

this unique anatomical region; ii) comparing dry and

wet electrodes for over-the-ear placement; and iii) as-

sessing the quality of EEG signals obtained from our

over-the-ear prototype system through standard EEG

protocols and comparing them with traditional setups.

By addressing these crucial aspects in the con-

text of over-the-ear placement, our study aims to con-

tribute to the advancement of practical, user-friendly

EEG devices suitable for both research and real-world

applications.

The following sections detail our methods for

electrode characterization and signal quality assess-

ment speciﬁc to over-the-ear placement, present our

ﬁndings, and discuss their implications for the future

of wearable EEG technology.

2 METHODS

In the next sections, three data acquisitions protocols

are described and then the resulting data are later anal-

ized.

All the data acquisition on subjects was performed

according to the descriptive rules reported in the ex-

perimental protocol (Opinion 46/2023, dated Decem-

ber 18

, 2023) that received approval from the Po-

litecnico di Milano Ethical Committee.

2.1 Dry Impedance Testing

The study involved a set of 6 gender-balanced healthy

subjects (age: 28 ± 1.73). Four different dryelec-

trode types were tested: conductive elastomer (CE),

PLA+carbon, TPU+carbon, and gold-plated with re-

tractile pins brush (GPR) electrodes (Brainbit Inc.,

New York, NY, USA). Each electrode material was

tested three times per subject.

The impedance was measured using a Palm-

Sens EmStat 4S potentiostat (PalmSens BV, Houten,

Netherlands) in a two-electrode conﬁguration. We

positioned the test electrode (serving as the working

electrode) behind the left ear, while a gel electrode,

acting as the reference and counter electrode, was

placed below the working electrode (see Figure 1 for

reference). This placement mimics the intended con-

ﬁguration for an around-the-ear wearable EEG sys-

tem.

Figure 1: Positioning of the electrodes in the skin-electrode

impedance test.

The potentiostat generated a sine wave with an

amplitude of 100mV

rms

and 0V bias, sweeping fre-

quencies between 0.5Hz and 10kHz.

2.2 Dry vs. Wet Electrode Comparison

A set of ﬁve healthy participants (2 females, age: 28.6

± 4.84,) were recruited for a comparative study of dry

and wet electrodes during baseline recordings. For

dry electrodes, we used both brush CE and GPR elec-

trodes in occipital area. For wet electrode recordings,

we utilized an EBNeuro BEPlus LTM ampliﬁer with

a 61-electrode cuff and gel was applied under the ex-

amined electrodes (i.e., the ones placed in the same

locations of the dry conﬁguration).

Each subject underwent 1 minute of baseline

recording in a resting state with eyes open for each

electrode conﬁguration.

2.3 Eyes Open/Closed Experiment

The study involved 10 healthy subjects (age: 27.2 ±

1.8, gender-balanced). We used GPR electrodes con-

nected to a MAX30001 evaluation kit (Analog De-

vices Inc., Wilmington, MA, USA), recording EEG

signals at 512Hz. Flat CE electrodes were used for

ground (GND) and reference, placed at the nasion.

Advancements in Wearable EEG Technology: Electrode Characterization and Signal Quality Assessment

717

The EOEC serves as an excellent initial test for

new EEG systems due to its simplicity and reliabil-

ity (Barry et al., 2007). The primary objectives were

to validate the ability of our chosen electrode conﬁg-

urations to capture meaningful EEG signals in both

traditional and non-traditional scalp locations, to as-

sess the sensitivity of our setup in detecting the well-

known alpha rhythm (i.e, activity in the 8 − 12Hz fre-

quency band) modulation associated with eye closure,

and to evaluate the overall signal quality and reliabil-

ity of EEG recordings obtained from our prototype

system.

Each subject participated in two recording ses-

sions:

1. GPR electrode placed in the occipital zone (serv-

ing as a ground truth for eyes-closed alpha activ-

ity)

2. GPR electrode placed over the ear (our prototype

conﬁguration)

In each session, subjects followed a protocol alter-

nating between eyes open and eyes closed states. The

protocol consisted of two minutes with eyes open, fol-

lowed by two minutes with eyes closed. This cycle

was repeated twice, resulting in a total recording dura-

tion of eight minutes per session. The recorded signal

was then ﬁltered between 1-30 Hz to mitigate artifacts

and line interference effects.

3 RESULTS

3.1 Impedance Testing

The impedance tests revealed signiﬁcant differences

among the tested electrodes. GPR electrodes demon-

strated substantially lower impedance in the band

of interest (1 − 100 Hz), approximately 10

Ω, com-

pared to the other electrode types, which exhibited

impedance around 10

Ω. See Figure 2 for details on

impedance response of the materials tested.

3.2 Dry vs. Wet Electrode Comparison

In the comparison of dry and wet electrodes, we fo-

cused on the high-frequency components (above 55

Hz) of the Power Spectral Density (PSD), where noise

components are dominant. CE electrodes exhibited

the highest PSD values in this high-frequency range

(see Figure 3 for reference), indicating greater suscep-

tibility to noise. The GPR electrodes showed interme-

diate PSD values, while wet electrodes demonstrated

the lowest PSD values, conﬁrming their superior per-

formance in noise rejection.

Figure 2: Electrode-skin impedance for the tested materials.

To further assess the signal quality, we computed

the signal prevalence for each electrode type as the

ratio of the power in each traditional brain activity

bands (delta, theta, alpha, and beta) to the power in the

high-frequency band (> 55 Hz), which we consider

representative of noise. Table 1 presents these ratios,

reported as median ± interquantile range (IQR).

Table 1: Median and IQR of brainwave band power ratios

normalized by the power in the > 55 Hz band (associated

with noise) for wet, CE, and GPR electrode types. Values

represent the relative strength of each brainwave band com-

pared to the high-frequency noise component.

Band WET GOLD DRY

delta (1-4Hz) 30.02 ± 17.07 8.49 ± 3.51 6.04 ± 5.59

theta (4-8Hz) 7.03 ± 12.33 1.52 ± 2.59 0.83 ± 1.80

alpha (8-12Hz) 2.87 ± 1.96 1.00 ± 1.29 0.36 ± 0.18

beta (12-30Hz) 2.18 ± 1.86 0.85 ± 0.99 0.61 ± 0.27

Figure 3: Comparison of Power Spectral Density (PSD) for

frequencies above 55Hz between CE, GPR, and wet elec-

trodes. PSD is reported on a logarithmic scale, with median

IQR shown for each electrode type.

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In this analysis, higher values correspond to more

reliable and robust signals. Consistent with the PSD

results, wet electrodes achieved the highest ratios

across all frequency bands, indicating the best sig-

nal quality. The GPR electrodes showed intermedi-

ate performance, while the CE electrodes yielded the

lowest ratios, suggesting they are more susceptible to

noise interference. Due to the superior impedance

characteristics demonstrated by GPR electrodes, the

subsequent EOEC experimental results will focus ex-

clusively on this electrode type

3.3 Eyes Open/Closed Experiment

Both electrode placements (occipital and over-ear)

captured clear modulation of alpha activity corre-

sponding to the eyes open/closed cycles. The spectro-

grams (see Figures 4 and 5) showed increased power

in the alpha frequency band (8-12 Hz) during eyes-

closed periods as expected (Barry et al., 2007). This

effect was more pronounced in the occipital electrode

placement (as expected from the literature), but was

also clearly visible in the over-the-ear conﬁguration.

The alpha ratio (power in alpha normalized

by broadband power) analysis revealed signiﬁcantly

higher values during the eyes-closed phases in both

recording setups (see Table 2).

Table 2: Distribution of Alpha ratio recorded in the occipital

and over-the-ear locations in the different phases of the pro-

tocols (EO: eyes open, EC: eyes closed) along the examined

population (median ± IQR).

Location EO 1 EC 1 EO 2 EC 2

Occipital 0.76 ± 0.21 3.88 ± 0.74 0.81 ± 0.17 4.21 ± 0.60

Over-the-ear 0.62 ± 0.13 3.31 ± 0.67 0.92 ± 0.20 3.54 ± 0.59

The results show that both electrode placements

successfully captured the expected alpha rhythm

Figure 4: Spectrogram (top) and alpha ratio (bottom) for the

occipital electrode placement during eyes open/closed cy-

cles. The spectrogram show power across frequencies over

time, while the alpha ratio plot displays the ratio of alpha

band power to total power. EO = Eye-Open period; EC =

Eye-Closed period.

Figure 5: Spectrogram (top) and alpha ratio (bottom) for

the over-ear electrode placement during eyes open/closed

cycles. The spectrogram shows power across frequencies

over time, while the alpha ratio plot displays the ratio of

alpha band power to total power. EO = Eye-Open period;

EC = Eye-Closed period.

modulation between eyes-open and eyes-closed con-

ditions. As anticipated, the occipital placement

showed the strongest effect, with the highest alpha

ratios during eyes-closed periods. The over-the-ear

placement demonstrated a robust ability to detect the

alpha rhythm modulation, with alpha ratios compara-

ble to those observed in the occipital placement.

These ﬁndings suggest that the over-the-ear elec-

trode placement, using GPR electrodes, is capable of

reliably detecting fundamental EEG phenomena such

as alpha rhythm modulation. This supports the po-

tential of this conﬁguration for use in wearable EEG

devices.

4 DISCUSSION

Our study on wearable over-the-ear EEG technol-

ogy revealed several signiﬁcant ﬁndings. GPR elec-

trodes demonstrated superior impedance characteris-

tics (10

Ω compared to 10

Ω for other types), sug-

gesting better electrode-skin interface performance.

While wet electrodes showed the best signal quality,

GPR electrodes provided a practical alternative bal-

ancing signal quality and user convenience.

The EOEC experiments validated the feasibility

of acquiring meaningful EEG signals from over-the-

ear locations, with alpha activity modulation compa-

rable to traditional occipital placements. This sug-

gests over-the-ear placement could be viable for cer-

tain EEG applications, offering advantages in user

comfort and social acceptability. The ability to cap-

ture meaningful EEG data from this location is par-

ticularly signiﬁcant for developing wearable devices

that could be worn for extended periods in everyday

settings.

Advancements in Wearable EEG Technology: Electrode Characterization and Signal Quality Assessment

719

The combination of over-the-ear electrode place-

ment and GPR electrodes offers a promising conﬁg-

uration for wearable EEG devices, providing an opti-

mal balance of signal quality, potential user comfort,

and practical applicability. This addresses key chal-

lenges in developing wearable EEG technology for

everyday use. However, while our results are promis-

ing for short-term recordings, the long-term stability

and comfort of the proposed conﬁgurations require

further investigation. Additionally, performance dur-

ing physical activity or in noisy environments needs

to be assessed.

Future work should focus on increasing sample

size, assessing long-term stability and comfort, inves-

tigating performance during complex cognitive tasks

beyond the EOEC paradigm, and developing spe-

cialized signal processing algorithms for over-the-ear

recordings. These investigations will provide a more

comprehensive understanding of the capabilities and

limitations of over-the-ear EEG recordings.

5 CONCLUSION

Our research demonstrated that GPR electrodes

achieve superior impedance characteristics for over-

the-ear EEG signal acquisition. The successful de-

tection of alpha rhythm modulation in over-the-ear

locations, comparable to traditional occipital place-

ment, validates this approach for EEG recording. The

combination of over-the-ear placement and GPR elec-

trodes provides a promising conﬁguration for wear-

able EEG devices, effectively balancing signal quality

and user comfort. Future studies investigating more

complex cognitive tasks beyond EOEC paradigms

would further validate and strengthen these ﬁndings,

potentially expanding the applications of this wear-

able EEG technology.

ACKNOWLEDGEMENTS

This work was carried out in the EssilorLuxottica

Smart Eyewear Lab, a Joint Research Center between

EssilorLuxottica and Politecnico di Milano.

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