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Li et al. Soft Sci 2023;3:22 https://dx.doi.org/10.20517/ss.2023.11 Page 9 of 15

hairy site is similar to that on the hairless site, except that the hair creates an additional barrier between the
electrode and the skin. The electrode-hairy skin interface is modeled as an RC circuit of Rch and Cch, where
Rch and Cch are the contact resistance and the contact capacitance, respectively [Figure 3D]. The full
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frequency spectrum shows that the dry sponge electrodes have a high specific impedance of 452.71 kΩ·cm
at 10 Hz due to the separating effect of the hair. By contrast, when wet, the specific impedance of the sponge
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decreased to 45.50 kΩ·cm at 10 Hz, which is approximately ten times lower than that of the dry sponge
[Figure 3E]. Impedance measurement by the Brain Vision Recorder software showed that the dry sponge on
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the hairy site has a specific impedance of 464.71 ± 72.24 kΩ·cm at 15 Hz, while the wet sponge has a much
lower specific impedance of 65.54 ± 12.67kΩ·cm at 15 Hz [Figure 3F]. These results further highlight the
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importance of the softness of sponges in establishing a good interface with the hairy scalp and reducing
impedance. In addition, since no skin preparation was done on the hairy site, the stratum corneum on the
hairy scalp is thicker, resulting in a more significant increase in conductivity compared to the forehead.

The mechanical stability of the sponge under repeated compression is essential for reusability. As shown in
Figure 3G, the specific impedance of the sponge remains unchanged after 100 cycles of squeezing at 50%
compressive strain, demonstrating that the sponge electrodes can be used multiple times with the VR
headset. The electrochemical stability of the electrode over a prolonged period is crucial for long-term EEG
signal recording. Therefore, we measured the stability of electrode impedance in different locations for 60
minutes. The contact impedance varied depending on the location of the electrode, with the dry sponge on
the forehead exhibiting lower impedance compared to the wet sponges on the hairy scalp [Figure 3H]. Most
of the electrodes exhibited stable impedance over time. Furthermore, we investigated the reusability of the
PMA sponge. As shown in Supplementary Figure 7, the electrode exhibited similar impedance after putting
on and taking off the VR headset ten times, indicating that our sponge electrode can be used for at least ten
uses. Finally, we compared our sponge electrodes with other types of electrodes in the literature, including
wet gel, textile, comb electrodes, and soft pillar electrodes. We can see that our sponge electrode
outperforms those electrodes with softness, low impedance, good compatibility with hair, ease of
integrability, and reusability [Figure 3I, Table 1]. In comparison to other sponge electrodes [Table 1] [17,29,42] ,
our PMA sponges were simply but reliably integrated onto the VR headset through an ultrathin FCA,
making the whole system easy to assemble and also soft and comfortable to wear.

Next, we studied the capability of the sponge electrodes to record EEG signals during eye-close and eye-
open conditions, using four participants in our study. The purpose of this experiment is to demonstrate that
sponge electrodes can successfully record increased alpha rhythm, which is a characteristic of EEG that is
present during the absence of visual stimuli, such as the eyes being closed. The sponge electrodes were
integrated onto the VR headset using a flexible FCA at Fp2 as a hairless site and Cz and Oz as hairy sites
[Figure 4A]. The end of the FCA can be directly connected to the EEG data acquisition system without
requiring additional fixation [Figure 4B]. As shown in Figure 4C, the filtered EEG signals from Cz and Oz
demonstrated a clear increase in amplitude in the alpha band (8-12 Hz) during the eye-close period
compared to the eye-open period, resulting in increased power. Time-frequency analysis and power spectra
density (PSD) results confirmed this increase in alpha power in the range of 8-12 Hz during the eye-closed
period compared to the eye-open period [Figure 4D and E]. Thus, the EEG data recorded with the sponge
electrode exhibited the characteristic feature of increased alpha oscillation power during the absence of
visual stimulation . The stable integration of the sponge electrodes on the VR headset enables consistent
[43]
EEG recording over a typical VR session of 60 minutes [Supplementary Figure 8], which is sufficient
considering that typical VR applications do not exceed this duration to avoid issues such as VR
cybersickness and discomfort . We also compared the performance of our sponge electrodes with
[12]
commercial electrodes on both hairless and hairy sites. For hairless sites, we attached a solid gel electrode at