A Long-Term Stable and Low Interfacial Impedance Hydrogel Electrode for Wearable Brain-Computer Interfaces.

Introduction:: Brain-computer interfaces (BCIs) have become integral to many medical applications, including the rehabilitation of stroke patients. However, the long-term use of EEG electrodes in wearable BCI devices outside the rehabilitation center is limited by the lack of long-term stability and preparation-free recording capability of the gold standard silver/silver-chloride electrodes. To address this limitation, we have designed a conductive-polymer-based hydrogel EEG electrode (POLiTAG) that exhibits low electrode-skin interfacial impedance and maintains a lower impedance value than gel-based electrodes for 29 days. Our designed electrode offers a promising solution for realizing long-term and wearable BCIs based on EEG technology. To illustrate this, we applied the designed electrode to a wireless single-channel EEG device that detects changes in alpha rhythms in eye-open/eye-close conditions.

Furthermore, we validated that the designed electrode could capture alpha rhythms using motor imagery-based BCIs and low-frequency time-locked event-related potentials from healthy subjects with similar or better performance than gel-based electrodes. Finally, we demonstrated the use of the designed electrode in online BCI-based functional electrical stimulation, which can potentially be used for post-stroke rehabilitation. Our results show the potential for our electrode to enable long-term EEG-based BCI applications outside of the rehabilitation center, thus facilitating new possibilities of extending the treatment to home for patients with neurological disorders.


Materials and Methods:: The study developed a conductive polymer-hydrogel-based electrode for long-term, preparation-free EEG recordings in wearable BCIs for post-stroke rehabilitation. The authors fabricated the electrodes by blending poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with 2-Acrylamido-2-methylpropane sulfonic acid (AMPS) hydrogel monomer, glycerol, Triton X-100, and LiCl. The electrode's interfacial impedance and electrical stability were characterized using electrochemical impedance spectroscopy (EIS). Thermogravimetric analysis (TGA) and continuous weight monitoring quantified the electrode's water-retaining capability. The performance of the proposed electrode was compared with commercially available electrodes in terms of impedance, stability, and EEG recording signal quality.

The EEG signal recording capability was evaluated by measuring alpha rhythms during an eye-open/eye-close paradigm and by capturing oscillatory rhythms during motor imagery protocols and event-related potentials in healthy subjects. The proposed electrode was integrated into a wireless single-channel EEG acquisition device and tested on human subjects for an eye-open/close paradigm to evaluate the performance of capturing the alpha rhythm as a wireless wearable device. Finally, the use of the electrode in online BCI-based functional electrical stimulation in healthy subjects is demonstrated.


Results, Conclusions, and Discussions:: Results and Discussion:
The results of the study showed that POLiTAG electrodes demonstrated significantly lower impedance values compared to other similar studies and commercially available semidry electrodes while also exhibiting good long-term stability for up to 30 days. The POLiTAG electrodes also showed excellent EEG signal quality, with comparable performance to Ag/AgCl gel electrodes for detecting alpha rhythms during eye-open/eye-close conditions, as well as for capturing motor imagery and event-related potentials. The practical use of the POLiTAG electrodes in a wireless EEG device for real-time signal recording was demonstrated, showing reliable performance for capturing EEG signals and for use in an online EEG-based brain-computer interface (BCI) task. The BCI task involved using the POLiTAG electrode to detect an event-related desynchronization (ERD) in the alpha rhythm, which is an indicator of the motor attempt of the subject, to apply functional electrical stimulation to the upper limbs. The subject could deliver correct BCI commands with an accuracy of 74.29%. This compares well with the gel-based electrode case in which the same subject had an accuracy of 60%. The study suggests that the POLiTAG electrode is a promising alternative to traditional Ag/AgCl gel electrodes for EEG recording, with potential applications in long-term wearable, rehabilitation, and assistive EEG-based BCI technologies.
Conclusions
We designed POLiTAG electrodes with low impedance and long-term stability. They exhibited the lowest impedance compared to other dry and commercial electrodes under the same conditions. Due to their water-maintaining capability, the POLiTAG electrodes demonstrated impressive stability for at least four weeks. We used a wireless EEG acquisition device with an integrated POLiTAG electrode to demonstrate proof-of-concept for EEG monitoring in wearable electronics. The electrode achieved similar or better performance than gel-based electrodes for EEG recordings and BCI applications. It captured alpha oscillatory rhythms and low-frequency time-locked event-related potentials from healthy subjects, and we demonstrated its use in BCI-based functional electrical stimulation for motor rehabilitation. The performance of POLiTAG could be further improved by developing an electrode grid that spans a larger area. Ultimately, this electrode enables long-term EEG monitoring, such as wearable EEG-based BCIs coupled with functional electrical stimulation for post-stroke rehabilitation.

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