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Page 10 of 15 Li et al. Soft Sci 2023;3:22 https://dx.doi.org/10.20517/ss.2023.11

Table 1. Summary of the performance of different types of hairy-compatible EEG electrodes
Hair Specific Impedance
Materials Fabrication Softness 2 Connection Ref.
compatibility (kΩ·cm )
PMA sponge Dip coating Yes Yes 19.40 ± 3.60 FCA This work
(15 Hz, forehead)
65.54 ± 12.67
(15 Hz, hairy site)
Gelatin gel Chemical Yes Yes 5.46 ± 0.76 Stud Wang et al. [22]
polymerization (10 Hz, hairy site)
PEDOT hydrogel Chemical Yes Yes 20.7 Stud Hsieh et al. [9]
polymerization (10 Hz, hairy site)
[28]
PEDOT:PSS-coated Replica and coating Yes Yes -- -- Zhang et al.
PDMS pillars
[23]
Au-coated silicon pin Microfabrication and Yes Yes 7.5 -- Wang et al.
thermal deposition (10 Hz, hairy site)
[42]
Cellulose sponge -- No Yes 1171.3 (forehead); Stud Ko et al.
1089-1727.9
(hairy site)
Conductive fabric -- Yes No 18.1-28.3 Stud Kuang et al. [18]
and (15 Hz, forehead);
metal-coated sponge 53.1-91.5
(15 Hz, hairy site)
[29]
Ag NW/MA sponge Dip coating Yes Yes 0.6-1.2 Silver wire Lin et al.
(30 Hz, hairy site)
FCA: flexible connector array; MA: melamine; PMA: poly (3,4-ethylenedioxythiophene) polystyrene sulfate/melamine; PEDOT:PSS: poly(3,4-
ethylenedioxythiophene) polystyrene sulfonate

the Fp1 location, which had similar skin conditions to the neighboring Fp2 [Supplementary Figure 9A]. Our
sponge electrode demonstrated similar EEG features to the solid gel electrode, with a Pearson’s correlation
coefficient (r) of 0.977 between the EEG signals recorded by the two electrodes [Figure 4F]. On hairy sites,
our sponge electrode performed comparably to a commercial comb electrode located adjacent to it near Cz
[Supplementary Figure 9B], with a Pearson’s correlation coefficient (r) of 0.860 [Figure 4G]. It is worth
noting that the sponge electrode is compatible, soft, and skin-friendly, not causing any skin irritation and
not leaving visible markings after wearing for 1 hour, whereas the comb electrode is stiff and uncomfortable,
leaving noticeable marks on the skin [Supplementary Figure 10]. The ease of setup, high stability, and user-
friendliness of our sponge-electrode-integrated VR headset makes it a promising system for simultaneous
EEG recording and VR interaction.

Finally, we evaluated the performance of our sponge electrode in a VR-BCI system. To this end, we
designed a CNV task in a custom first-person perspective VR game for the “Go/No-Go” classification. The
electrode was placed on Cz, located over the central region of the cerebral cortex [Figure 5A], which is a
typical location of interest for detecting CNV potentials. CNV is a well-established slow cortical EEG
potential associated with anticipation and attention generated from sources in the prefrontal and central
regions of the cerebral cortex [44-46] . Our VR game was designed to resemble decision-making while driving,
which is a commonly used scenario and application for CNV-based BCIs [Figure 5B, Movie S1] [47-49] . The
subject was instructed to remember the direction of the middle symbol of the Flanker task (Stimulus 1)
[Figure 5C] and respond exactly 4.3 seconds later (as guided by the countdown on display) by pressing a
[50]
button on the VR controller the moment the diverging sign (Stimulus 2) appeared when the Stimulus 1 was
a “Go”. If Stimulus 1 was a “No-Go”, the subject was instructed to ignore Stimulus 2. The input was only
accepted within a 300 ms window after Stimulus 2, and the response time of the subject was displayed as
feedback during the “Go” trials to keep the subject alert to Stimulus 2 [Figure 5B]. Positive visual feedback
in the form of a gem object and an increasing score was delivered if the response of the subject was correct