The development of wearable healthcare technologies hinges on the ability to accurately and comfortably monitor electrophysiological signals from the human body. Traditional methods often rely on stiff, bulky electrodes requiring pre-gel application. These electrodes cause significant user discomfort, especially during extended use, and introduce motion artifacts that contaminate the measured signals, reducing the accuracy and reliability of the data. The need for a more comfortable and artifact-free system is paramount for long-term monitoring of vital signs and for applications such as human-machine interfaces. Existing dry electrodes often compromise on conductivity or flexibility, limiting their applicability. This research addresses this critical need by introducing a novel ultra-thin, conformable dry electrode design that minimizes discomfort and drastically reduces motion artifacts while maintaining high signal fidelity. The design leverages the synergistic properties of graphene and PEDOT:PSS to achieve unprecedented levels of conductivity and flexibility, opening up new avenues for advanced wearable health monitoring and human-computer interaction.

The literature extensively covers various approaches to improve the performance of skin electrodes. Researchers have explored different materials, such as carbon nanotubes [2] and metal-organic frameworks [1], to enhance conductivity and flexibility. Significant effort has been dedicated to creating stretchable conductive fibers [3] and bioresorbable sensors [4] for applications in various body regions. Artificial sensory neurons [5, 6] and intrinsically stretchable transistor arrays [7] showcase progress in creating sophisticated sensing systems. Graphene-based electronic tattoo sensors [8] represent a significant advance in creating wearable sensors. However, issues with noise from conventional Ag/AgCl electrodes [9, 10] remain a challenge. While ionic liquid gels [11] have been explored to improve long-term recordings, the need for gels introduces other complexities. The limitations of EEG monitoring [12] highlight the need for improved electrode designs for long-term and high-fidelity recordings. Previous work on thin-film electrodes [13] provides a foundation for this research, but there is a continued need for improvements in conductivity and flexibility. Printed electrodes on hydrogels [14] and hydrogel-based microelectronics [15, 17] are explored, but each has its own tradeoffs and limitations. The use of conductive polymers such as PEDOT:PSS has shown promise, but optimization is still needed to achieve desired conductivity, flexibility, and transparency.

The study focuses on the fabrication and characterization of a novel ultra-thin dry epidermal electrode based on Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and graphene. The researchers utilized chemical vapor deposition (CVD) to grow graphene on copper foil, followed by the spin-coating of PEDOT:PSS solution with added surfactants (Sodium dodecyl sulfate (SDS) and benzyl sodium sulfonate (BSL)) to enhance conductivity and uniformity. The copper was then etched away, leaving a thin layer of PEDOT:PSS transferred onto the graphene. The resulting structure is termed PEDOT:PSS-transferred CVD-grown graphene (PTG). This fabrication process allows for the creation of an ultra-thin (~100 nm), transparent, and highly conductive electrode. The electrical conductivity was enhanced by optimizing the concentration of SDS and BSL in the PEDOT:PSS solution. The researchers systematically investigated the effects of different concentrations of SDS and BSL on the sheet resistance and transmittance of the PTG films. The optimized concentration of SDS was 1.0 wt%, balancing conductivity and transmittance. To improve both stretchability and conductivity, BSL was added, and 5.0 wt% was found to give superior conductivity, reaching approximately 4142 S/cm. The mechano-electrical stability of PTG was assessed by bending and strain tests, demonstrating the electrode's ability to withstand deformation and maintain its conductivity. The mechanism behind the enhanced conductivity and stability was investigated by various techniques, including Raman spectroscopy, UV-vis-NIR spectroscopy, Electron Spin Resonance (ESR) spectroscopy, and Grazing-incidence wide-angle X-ray scattering (GIWAXS). The studies using these characterization techniques confirm the formation of a highly ordered structure, and strong π–π interaction between PEDOT:PSS and graphene contribute to the superior performance. To evaluate its biocompatibility and performance in electrophysiological measurements, the PTG electrodes were used to record electrooculogram (EOG), electrocardiogram (ECG), and surface electromyogram (sEMG) signals from human subjects. The impedance of the electrodes was measured and compared to that of traditional Ag/AgCl and pure PEDOT:PSS electrodes. The motion artifacts generated by the PTG electrodes were compared under static and dynamic conditions using an electromechanical vibrator to simulate movement, demonstrating minimal motion artifacts. Long-term EEG monitoring was also conducted over 12 hours, which included sleeping, exercising, and periods of rest. The quality of signals recorded during these varied activities was analyzed using Fast Fourier Transform (FFT) to identify distinct brainwave patterns.

The key findings of this study demonstrate the superior performance of the ultra-conformal PTG electrodes compared to traditional Ag/AgCl and pure PEDOT:PSS electrodes. The PTG electrodes exhibited significantly improved conductivity (4142 S/cm), reaching a record high value and exceeding previously reported values for PEDOT:PSS-only films. The ultra-thin nature (~100nm) of the PTG electrode ensured excellent conformability to the skin, minimizing motion artifacts. The enhanced optoelectronic performance was attributed to the synergistic effect between graphene and PEDOT:PSS, leading to a high degree of molecular ordering in PEDOT and efficient charge transfer, confirmed by Raman, UV-Vis-NIR, ESR and GIWAXS characterization. This synergy resulted in a much lower sheet resistance (24 Ω/sq) compared to pure PEDOT:PSS films. The mechano-electrical stability tests demonstrated a remarkable resistance to bending and stretching, with minimal resistance change even at high strain levels. The PTG electrodes showed low interfacial impedance comparable to Ag/AgCl gel electrodes, facilitating high-quality electrophysiological recordings. Importantly, the electrodes consistently exhibited significantly lower motion artifacts than both Ag/AgCl and pure PEDOT:PSS electrodes, as demonstrated in EOG, ECG, and sEMG recordings. The successful long-term monitoring of EEG signals (12 hours) without significant degradation, despite changes in the subject's activity level (sleeping, exercising, resting) and sweat, further underscores the practicality and effectiveness of the PTG electrodes. The application of PTG electrodes in facial sEMG demonstrated superior performance compared to Ag/AgCl electrodes, showing more stable signals and allowing simultaneous monitoring of sEMG and laser speckle contrast imaging (LSCI). The ability to use PTG electrodes to successfully control a robotic hand via both facial and finger sEMG signals demonstrated the potential of the technology for human-machine interfaces.

The results of this study address the critical need for improved skin electrodes for wearable healthcare and human-machine interfaces. The developed ultra-conformal PTG electrodes offer significant advantages over traditional methods by combining high conductivity, flexibility, and transparency with minimal motion artifacts. The synergistic effect between graphene and PEDOT:PSS, leading to a highly ordered structure and efficient charge transfer, is a key factor contributing to the superior performance. The ability to perform long-term monitoring of EEG and other biosignals without noticeable degradation demonstrates the practical applicability of these electrodes in real-world scenarios. The findings have implications for various fields, including clinical diagnosis, rehabilitation engineering, and human-computer interaction. The use of the PTG electrode for facial sEMG monitoring, combined with LSCI, opens up new possibilities for non-invasive and comprehensive evaluation of facial nerve function. The successful control of a robotic hand using sEMG signals acquired from finger and facial muscles highlights the potential of PTG electrodes for developing advanced prosthetic devices and other assistive technologies.

This research presents a significant advancement in the field of epidermal electronics. The novel ultra-conformal PTG electrodes offer a compelling solution for long-term, high-fidelity electrophysiological monitoring with minimal motion artifacts. Their superior conductivity, flexibility, and transparency, combined with their biocompatibility, make them an ideal candidate for a wide range of applications, including wearable healthcare, human-machine interfaces, and advanced prosthetics. Future research could focus on exploring the scalability of the fabrication process for mass production and investigating the potential of integrating other functionalities into the electrode design, such as sensors for temperature or other physiological parameters.

While the study demonstrates the superior performance of PTG electrodes in various applications, some limitations should be acknowledged. The long-term stability of the electrodes beyond 12 hours needs further investigation. The sample size for the human subject studies was relatively small, and larger studies are needed to confirm the generalizability of the findings. The cost-effectiveness of the fabrication process relative to existing technologies needs to be assessed for widespread adoption. Further research is required to explore the full biocompatibility and potential long-term effects of prolonged skin contact with the PTG electrodes.