Page 139 - Read Online

Page 2 of 15 Li et al. Soft Sci 2023;3:22 https://dx.doi.org/10.20517/ss.2023.11

and hairy sites. In another demonstration, we developed a VR task to evoke the contingent negative variation
potential and achieved a classification accuracy of 0.66 ± 0.07, represented by the cross-validated area under the
receiver operating characteristic curve. Our sponge-electrode-integrated VR headset is user-friendly and easy to
set up, marking a step toward future reliable, comfortable, and reusable VR-EEG technology.

Keywords: PEDOT:PSS, soft electrode, electroencephalography, virtual reality, brain-computer interface

INTRODUCTION
Electroencephalography (EEG) is a non-invasive technique that records neural activity from the scalp,
[1-3]
offering high temporal resolution, affordability, and versatility compared to alternative modalities such as
functional magnetic resonance imaging (fMRI) and invasive electrocorticography (ECoG). EEG has been
[4]
widely used for various applications, including sleep monitoring , clinical diagnosis and treatment of
neurological disorders such as epilepsy and stroke , and as a primary signal modality for both clinical and
[6]
[5]
non-clinical brain-computer interfaces (BCI) [1,7-9] . In recent years, virtual reality (VR) technology has
emerged as a new tool in the fields of cognition assessment , rehabilitation , pain relief , and BCI . VR
[11]
[10]
[13]
[12]
[14]
can create controlled environments that integrate intuitive, immersive, and interactive elements with
innovative input methods such as gaze direction and hand gestures, potentially replacing conventional
visual stimulation and feedback techniques. Therefore, the integration of EEG and VR represents a
promising opportunity for the improvement of both EEG and VR systems [15,16] . EEG can provide a real-time
stream of brain activity and cognitive state information that can be utilized by the VR application, while VR
can provide a unique environment for evoking and studying brain activity in realistic and immersive
simulations.

Current EEG and VR systems are implemented separately in hardware, resulting in cumbersome and
complicated systems when they are simply combined . Emerging research has demonstrated the viability
[16]
of integrating EEG electrodes directly on VR headsets for simultaneous EEG recording and VR
stimulation [17,18] , but these studies primarily target hairless areas such as the forehead. However, regions of
interest for brain activity analysis and many BCI applications are often located underneath the hairy parts of
the scalp such as the motor cortex and visual cortex, which are hard to access by existing VR-EEG systems.
To overcome hair interference in conventional EEG, the conductive liquid gel is commonly used, but it is
time-consuming to set up, limited in operating time, uncomfortable, and requires trained personnel .
[19]
Additionally, some ingredients in the gel electrodes, such as propylparaben, have been found to be harmful
[20]
to the skin . To address these issues, paintable gel electrodes with a fast liquid-to-solid transition speed
have been developed, simplifying the scalp preparation and electrode application process and reducing the
required time compared to commercial gel electrodes [21,22] . However, these electrodes require removal by
washing off after use, limiting their integration with other systems for multiple uses.

Dry electrodes have gained attention for EEG due to their ease of use, setup, removal, and integration with
wearable systems compared to gel electrodes . Various dry electrode designs, including microneedle ,
[3]
[23]
nanotube , and pillar [25-27] electrodes, have been explored over the past decade. These electrodes can
[24]
penetrate hair bundles to make direct contact with the scalp, providing reasonable EEG recordings.
However, their fabrication processes are often complex microfabrication processes, which are high-cost and
time-consuming [23,24] . Furthermore, transdermal microneedles can cause pain and potential skin
infections [23,27] . Recent studies have found that soft electrodes made of elastomer pillars coated with
conductive materials, such as gold and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
[23]
[28]
(PEDOT:PSS) , have greatly reduced stiffness and improved skin compatibility. However, an additional