| The emerging EEG-based brain-computer interfaces (BCIs) and wearable devices application have received intense global interest in recent years, aiming to real-world scenarios such as physiological monitoring, neuro-feedback training and neuro-marketing. However, despite all the recent technological advances in acquisition electronics and signal processing, more practical, convenient and reliable EEG electrodes, still remain a crucial technological challenge.State-of-the-art EEG electrodes are divided into two categories:wet electrodes and gel-free electrodes. The second category can be split further between dry electrodes and semi-dry electrodes. The classical wet electrodes have become "gold standard" in clinics and laboratories for EEG recording. However, the inconvenience and discomfort issues severely limit real-word applications. The dry electrodes demonstrate many merits such as quick setup, user friendliness and cleanliness due to the elimination of conductive gels, but they often suffer from high electrode-skin impedance and signal instability. The semi-dry electrodes don't require the application of gel and minimalizes the risk of dirtying the hair, while the impedance mismatch and signal instability are found due to uncontrolled and unexpected liquid release. In this study, we proposed a novel porous ceramic-based semi-dry electrode prototype aiming to overcome the problems of the existing electrodes technology. The detailed works are as follows.In chapter 2, a novel passive ceramic-based semi-dry electrode prototype have been proposed and the performance of the semi-dry electrode was evaluated systematically. With the help of capillary forces of the porous ceramics pillars, the semi-dry electrodes build a stable electrode/scalp interface by penetrating hair and releasing a small amount saline in a controlled and sustained manner. The semi-dry electrode/scalp impedances were low and stable (22.2 ± 8.5 k?, n= 10), and the variation between nine different positions was less 5 k?. The semi-dry electrodes have shown non-polarization characteristics and the maximum difference of equilibrium potential between eight electrodes was 579 ?V. The semi-dry electrodes impedance demonstrated long-term stability, and the impedance only increased by 10 k? within 8 h. EEG signals were simultaneously recorded using a 9-channel gel-based electrode and semi-dry electrode arrays setup on ten subjects. The average temporal cross-correlation between them in the eyes open/closed and the steady state visually evoked potentials (SSVEPs) paradigm were 0.938 ± 0.037 and 0.937 ± 0.027 respectively. Together with the advantages of quick setup, self-application and cleanliness, the result suggests the semi-dry electrode is suitable for emerging real-world EEG applications, such as brain-computer interfaces and wearable EEGs.In chapter 3, the performance of the semi-dry electrode was evaluated by simultaneous recording of the wet and semi-dry electrodes pairs in five classical BCI paradigms:eyes open/closed, motor imagery BCI, P300-speller, N200-speller and steady-state visually evoked potential-based BCI. The grand-averaged temporal cross-correlation was 0.95 ± 0.07 across the subjects and the nine recording positions, and these cross-correlations were stable throughout the whole experimental protocol. In the spectral domain, the semi-dry/wet coherence was greater than 0.80 at all frequencies and greater than 0.90 at frequencies above 10 Hz, with the exception of a dip around 50 Hz (i.e. the powerline noise). More importantly, the BCI classification accuracies were also comparable between the two types of electrodes. Overall, these results indicate that the semi-dry electrode can effectively capture the electrophysiological responses and is a feasible alternative to the conventional EEG electrode in BCI applications.In chapter 4, the electrode-skin impedances of three typical bioelectrodes (wet, semi-dry, and dry) have been studied systemically, concerning not only magnitude but stability. Various factors have been investigated including types of electrodes, skin locations, pressure, skin abrasion, and electrode contact area. The electrode-skin impedance always decreases at various skin locations in the following order:forearm, scalp, and forehead for all electrodes. The electrode/scalp impedance of the dry electrode, semi-dry electrodes, and wet electrode are 57.5-540.0 k?,10.3-38.4 k?, and 1.4-2.8 k? respectively for six subjects, while their specific impedances are 58.50 ± 64.16 k?·cm2,1.1±0.6 k?·cm2, and 1.1±0.3 k?·cm2 (n= 6) respectively. The impedance variation of semi-dry electrodes (1.2±0.9 k?) is minimal and it is even negligible for wet electrodes (0.1 ±0.1 k?) while that of dry electrodes is considerably large (31.2 ± 31.3 k?) during 10 min (n= 5). The impedance of dry electrodes is significantly high and unstable. As a result, not satisfactory EEG signals can be obtained. Moreover, the dry electrode impedances are lowered significantly under pressure or after skin abrasion. Accordingly, alpha rhythms from the dry electrodes appeared with the assistance of pressure or skin abrasion. These findings provide insights for the development of new gel-free electrodes to complement the emerging new EEG applications, such as brain-computer interfaces and wearable EEGs.In chapter 5, the electrochemical impedance spectroscopy was systematically analyzed by the equivalent electrical circuit fitting method, including three typical electrodes, and semi-dry or dry electrodes under pressure and after skin abrasion. The specific impedance decreases in the following order:dry electrodes, semi-dry electrodes, and wet electrodes. The impedance difference between the three typical electrodes mainly attribute to the interface contact resistance (Res) and the skin impedance (Rs, CPEes-T). From dry electrodes to wet electrodes, both Rs and Res decreases while CPEs-T increases sequentially. Moreover, a paralleled capacitance (CPEes-T) arises at the dry electrode-skin interface due to lack of enough electrolyte. Under pressure or after skin abrasion, the specific impedances of two types of electrodes decrease, but more significant for the dry electrode. The trend of EIS fitting parameters under pressure is similar to those after skin abrasion. Specifically, the skin impedance decrease with Rs decreasing and CPEs-T increasing, and the interface contact impedance also reduce with Res decreasing and CPEes-T rising for dry electrodes. Interestingly, whether the two types of electrodes are under pressure or after skin abrasion, their specific impedance variation lies between the contact resistance Res and the skin resistance Rs. The systematic study on the electrode-skin impedance spectroscopy is expected to provide theoretical support for the new development of gel-free electrodes. |
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