Graphene e-tattoos for unobstructive ambulatory electrodermal activity sensing on the palm enabled by heterogeneous serpentine ribbons

Electrodermal activity (EDA), also known as galvanic skin response (GSR), is a widely used non-invasive quantitative index of mental stress, finding applications in diverse fields like psychiatry, neurology, and consumer assessments. The palms, with their high density of eccrine sweat glands, are the optimal sites for EDA monitoring. However, current commercial wearable EDA sensors employing gel electrodes and wires on the palm suffer from several limitations: they are obstructive and socially stigmatizing, prone to delamination, and their signal quality degrades over time due to electrode dehydration. Dry electrode sensors have been developed to address the issues of gel electrodes, but these often measure EDA from locations other than the palms (e.g., wrists, forearms), resulting in signal contamination from apocrine sweat glands. Existing dry, wireless hand sensors are also relatively thick and obstructive. Therefore, a need exists for an unobtrusive, robust, and high-fidelity palm sensor for ambulatory EDA monitoring. Ultrathin, skin-soft wearable electronics, known as e-tattoos, offer superior skin conformability, mechanical imperceptibility, and long-term stability for monitoring various physiological signals. Sub-micron-thin graphene e-tattoos (GETs), developed by the authors' group, are highly transparent, skin-conformable, and stretchable when patterned into serpentine shapes. GETs have shown the lowest electrode-to-skin impedance among ultrathin wearable electrodes and exhibit diverse sensing modalities. A significant challenge, however, remains in reliably interfacing these ultrathin, stretchable GETs with thicker, rigid printed circuit boards (PCBs) for reliable data acquisition (DAQ) in ambulatory settings where skin deformation is unpredictable. The sub-micron thinness of GET makes it vulnerable to rupture, making conventional electrical connections unsuitable. While silver paste and liquid metal have been used, they introduce stiffness mismatches and require complex encapsulation, leading to thicker interconnects. This lack of robust connection between ultrathin devices and rigid circuitry hinders the practical ambulatory use of many ultrathin wearable sensors.

The existing literature extensively covers the use of EDA as a physiological indicator of stress and its applications in various fields. Many studies highlight the limitations of traditional EDA sensing methods involving gel electrodes, emphasizing the drawbacks of discomfort, obstructiveness, and signal degradation. A significant body of research focuses on developing alternative dry electrode technologies to improve comfort and long-term stability. However, these often compromise the signal quality by measuring EDA from less ideal locations on the body. The use of graphene and other 2D materials in flexible and wearable sensors has shown considerable promise, with numerous publications demonstrating the feasibility of creating ultrathin, conformable devices for various bio-signal monitoring applications. However, a major gap in the literature is the development of robust and reliable interfaces between these ultrathin sensors and the rigid electronics needed for data acquisition in real-world, ambulatory settings. This study directly addresses this gap by proposing and demonstrating a novel approach to create a mechanically robust interface for ultrathin wearable sensors.

This study introduces a novel approach to create a mechanically robust electrical contact between sub-micron-thin GET and a millimeter-thick EDA wristband. This is achieved through the use of heterogeneous serpentine ribbons (HSPR) and a soft interlayer. HSPR consists of a GET serpentine partially overlapping with a sub-micron-thin gold on-polyimide (Au/PI) serpentine, both supported by the skin, relying on van der Waals forces for adhesion without any added adhesive. The Au/PI ribbon's other end interfaces with the rigid electrodes on the EDA wristband via a reusable soft interlayer with conductive vias. Finite element modeling (FEM) shows that HSPR leads to a 50-fold strain reduction compared to heterogeneous straight ribbons (HSTR), enhancing stretchability to 42% tensile strain before failure. An equivalent circuit model confirms the negligible Au-graphene contact resistance compared to GET-skin interface impedance, indicating current flow primarily through the GET-skin interface. The fabrication process involves a wet transfer of large-area CVD graphene onto a polyimide (PI) layer, followed by the lamination of a 750-nm-thin Au/PI bilayer with a strategic overlap creating the HSPR. Laser cutting is used to pattern the serpentine shape. The soft interlayer, composed of Ecoflex with conductive rubber disks, acts as a mechanical buffer between the Au nanomembranes and the rigid wristband electrodes. The mechanical properties of both HSPR and the soft interlayer were characterized using tensile testing and a dynamic mechanical analyzer (DMA). FEM simulations were used to model the strain distribution in HSPR under various loading conditions, validating the strain-reducing effect of the design. Electrode-to-skin interface impedance was characterized using an LCR meter. EDA measurements were performed using the GET-based sensor and a commercial gel-based reference sensor on the palms of human subjects during various activities. A custom algorithm was developed to detect and select SCR events for correlation analysis between the two sensor types, considering parameters like amplitude, peak time, response time, rise time, and recovery time. The wearability of the GET-based sensor was evaluated under various conditions, including movement, metal rubbing, and water exposure. Ambulatory long-term EDA sensing was also demonstrated over 15-hour periods.

The key findings of this research demonstrate the successful development and validation of a novel, unobtrusive, and highly reliable ambulatory EDA sensing system using graphene e-tattoos. The study's significant contributions are: 1. **Heterogeneous Serpentine Ribbons (HSPR) for Strain Reduction:** The HSPR design drastically reduces strain concentration at the interface between the ultrathin GET and the rigid wristband, resulting in a 50-fold improvement in stretchability compared to the HSTR. This is crucial for maintaining the integrity and functionality of the sensor during normal hand movements and activities. 2. **Soft Interlayer for Strain Isolation:** The incorporation of a soft interlayer further mitigates strain transfer from the rigid wristband to the delicate Au/PI and GET, ensuring long-term sensor stability. FEM simulations showed a significant reduction in strain on the Au layer with the soft interlayer compared to without, preventing potential failure due to shearing forces. 3. **Superior Electrode-to-Skin Impedance:** The GET exhibited lower electrode-to-skin impedance than traditional gel electrodes, attributed to its superior conformability to the skin's micro-texture. This results in a higher fidelity signal, with less noise and better signal-to-noise ratio. 4. **Validated EDA Signal Quality:** A novel EDA event selection policy and rigorous correlation analysis confirmed that the EDA signals obtained from the GET-based sensor are highly comparable to those from the gold-standard gel electrode-based sensors in terms of SCR parameters, validating the accuracy of the proposed system. The unbiased selection process eliminated any bias that might have been introduced due to selecting only the 'good' signals. 5. **Successful Ambulatory Monitoring:** The GET-based sensor successfully performed continuous EDA monitoring for 15-hour periods across diverse activities, showcasing its suitability for long-term ambulatory use. The sensor demonstrated significantly better stability compared to the gel electrodes, which often delaminated during the same period. The motion artifacts introduced are considerably smaller and distinct from SCR signals, making them easily removable. 6. **Robustness to Environmental Factors:** The GET sensor demonstrated robustness to environmental factors like metal rubbing and brief water exposure, indicating its suitability for real-world conditions.

This work successfully addresses the long-standing challenge of creating a mechanically robust interface between ultrathin, stretchable wearable sensors and rigid electronics for ambulatory applications. The proposed HSPR design, combined with a soft interlayer, significantly improves the stretchability and longevity of the sensor, overcoming limitations associated with previous approaches. The validation of the EDA signal quality through rigorous correlation analysis confirms the system's accuracy and reliability. The successful demonstration of long-term ambulatory monitoring underscores the practical potential of this technology for continuous, unobtrusive health monitoring. The generalizability of the HSPR concept to other ultrathin skin-conformable electronics opens up possibilities for various applications beyond EDA sensing. This technology has the potential to transform the field of wearable sensing, enabling the development of comfortable, reliable, and long-term monitoring systems for various physiological parameters.

This study presents a significant advancement in the field of wearable sensors by introducing a novel heterogeneous serpentine ribbon (HSPR) design for creating robust interfaces between ultrathin devices and rigid electronics. The successful integration of this design with graphene e-tattoos enables long-term, unobtrusive ambulatory electrodermal activity (EDA) sensing. The superior performance compared to traditional methods is validated through rigorous testing and analysis. Future research could explore the application of this HSPR design to other types of ultrathin wearable sensors and investigate the integration of additional sensing modalities into the e-tattoo platform for a more comprehensive health monitoring system.

While the study demonstrates significant improvements in ambulatory EDA sensing, certain limitations should be acknowledged. The current design relies on van der Waals forces for adhesion between the GET and Au/PI; although robust in testing, variations in skin conditions or prolonged use might affect the long-term stability of this interface. The study primarily focused on uniaxial strain, and further investigation is needed to understand the sensor's performance under more complex loading conditions. The sample size for the human subject testing could be increased for greater statistical power. Lastly, while robustness to brief water exposure was demonstrated, prolonged or submersion in water might affect sensor performance.