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

adhesive layer is necessary to prevent the delamination of the coating layer under pillar deformation, which
adds complexity and cost to the fabrication process.

Soft conductive sponges offer a hair-friendly option for EEG recording, as they can make better contact
[29]
with the scalp when compressed despite hairs . These sponge electrodes can be integrated onto various
types of headwear such as caps and headbands , making them ideal for portable and wireless EEG
[30]
[31]
recording. Recent works reported that silver-nanowire-coated melamine (MA) sponges can record high-
fidelity steady-state visual evoked potentials (SSVEP) on hairy scalps [29,32] . However, silver nanowires require
a polymer coating for improved chemical stability and reduced toxicity. PEDOT:PSS is a conducting
polymer with high chemical stability, conductivity, and biocompatibility [33,34] . It has both ionic and
[35]
electronic conductivity, enabling lower contact impedance with the skin than that of gold . These
characteristics make PEDOT:PSS a popular choice for electrode materials in bioelectrical signal
recording [9,28,35,36] . In this paper, we fabricate soft PEDOT:PSS/MA (PMA) sponge electrodes that can be
easily integrated on a commercial VR headset for EEG recording during VR experiences. The PMA sponge
electrode is mechanically compliant and can conform to the hairy scalp upon being pressed by the VR
headset strap, resulting in high-fidelity multichannel EEG recordings. We developed a VR program to
demonstrate the PMA sponge electrode-based VR-BCI system. We have successfully detected the
contingent negative variation (CNV)-related potentials using our VR-EEG BCI system.

EXPERIMENTAL
Chemicals and materials
MA sponges were purchased from Amazon. PEDOT:PSS (Clevios PH1000, solid content 1.3%) was
obtained from Heraeus Precious Metals North America Daychem LLC (Vandalia, OH, USA). Dimethyl
sulfoxide (DMSO, 99.9%) was purchased from Beantown Chemical (Hudson, NH, USA). Copper films were
purchased from Nimrod Copper Company. Solid gel electrodes (Kendall ECG electrodes) and comb
electrodes were obtained from Kendall and OpenBCI (Brooklyn, NY, USA), respectively. All chemicals were
used as received without further purification.

Fabrication of PMA sponges
PMA sponges were prepared by a dipping method. In brief, MA sponge pieces were punched into cylinders
with a height of 10 mm and a diameter of 12 mm. A PEDOT:PSS solution was prepared by mixing 7 g of
Clevios PH1000 dispersion and 3 g of DMSO under stirring at room temperature for 30 minutes. The MA
sponge cylinders were then dipped into the PEDOT:PSS solution and squeezed four times to enhance
infiltration of the PEDOT:PSS solution. The PEDOT:PSS solution with MA sponges was then placed into a
desiccator and kept under vacuum for 15 minutes to further enhance infiltration of the PEDOT:PSS
solution. The sponges were then scooped up, and excess PEDOT:PSS solution was removed using filter
paper. The sponges were dried at 110 °C in an oven for 2 hours. After another dipping and drying process,
PMA composite sponges with a dark blue color were obtained.

Characterization
The morphology of both MA sponges and PMA sponges was characterized by scanning electron
microscopy (SEM, Scios 2HiVac, Thermal Fisher Scientific, Canada). The UV-Vis absorption of
PEDOT:PSS solution and washing solution was measured on a UV-Vis NIR spectrometer (Cary 5000,
Agilent, USA). The mechanical properties of the sponges were characterized using a dynamic mechanical
analyzer (RSA-G2, TA Instruments).