A paper published in Nature Communications posited that a graphene e-tattoo (GET) on the palm of the hand could monitor ambulatory electrodermal activity (EDA), which can be a proxy for mental stress.
Investigators from the University of Texas at Austin and Texas A&M University sought to develop an EDA sensor that does not suffer from the same flaws as extant sensors, such as obstructive and stigmatizing designs that are not compatible with real-world use. The investigators previously developed a sub-micron-thin, imperceptible GET but had difficulty connecting the GET with the rigid circuit boards needed to process and transmit the EDA data. In this study, a new version of the GET was developed and tested for its ability to report EDA data in real-world conditions.
The GET comprises 2 heterogeneous serpentine ribbons which are partially overlaid with gold and free from adhesive. The GET is accompanied by a wristband which houses the rigid circuit board and electrodes that connect to the 2 serpentine ribbons enabling wireless EDA monitoring in real-time.
In a proof-of-concept experiment, 5 participants wore the GET and a comparator gel-based sensor while watching a 13-minute-long video with 5 scenarios designed to elicit expectation, uncontrolled emotional, controlled emotional, and habituation responses.
During the trials, the GET-measured EDA responses tended to have fewer flections in skin conductance level and similar skin conductance response output compared with the gel sensor. However, no significant differences were observed between outputs (all P >.05), indicating that the GET-measured EDA output was robust when compared with an extant gel-based device.
To test the GET in real-world movement and use scenarios, the investigators attempted typical hand movements, such as clenching, bending at the wrist, grabbing a cellphone, and poking. During movements, the GET recorded small motion artifacts which differed from skin conductance response signals and could be easily identified and removed from the output.
To test for durability of the GET, 300 cycles of metal rubbing to simulate typing on a laptop plus 300 cycles of wood rubbing to simulate working at a desk, momentary water exposure, and environmental humidity experiments were conducted. The GET sensor survived both friction and water perturbations with comparable EDA outputs prior to and following experimental conditions.
One participant used the GET sensor for long periods of time. In three 15-hour long nonstop EDA monitoring sessions that included exercising, driving, and sleeping, the GET did not require replacement.
In situations where the GET sensor stopped reporting EDA output, no mechanical failures with the GET device were found. Instead, the gold-on-polyimide layer ruptured in the area where the gold component had maximum strain.
No adverse skin irritation effects were reported.
This GET sensor is the first stretchable interface that has the capability of monitoring ambulatory EDA in real-world, free-moving conditions.
We spoke with Nanshan Lu, PhD, of the Department of Aerospace Engineering and Engineering Mechanics at the University of Texas at Austin about the GET sensor and what this device could mean for the future.
Could you broadly explain your research interests?
Over the past 11 years, my lab has been engineering a variety of wearable non-invasive electronic tattoo stickers that can be attached to different areas and locations of the skin to measure different biometrics. The goal is to digitize the human body, just like we digitize a car or an aircraft, so we can know what’s going on with the human body in terms of its performance, emotions, and so on. Current wearables, such as smartwatches or smart rings, only have limited locations and modalities. But, actually, every inch of our skin is radiating data. Brain activities radiate EEG (electroencephalogram), heart beats ECG (electrocardiogram) and SCG (seismocardiogram), and muscles EMG (electromyogram). Those kinds of signals have to be measured right on top of the target tissue. So that’s why we want to have a distributed sensor network to simultaneously measure multimodal signals from multiple locations. But, commercial wearables are still based on rigid electronics that are usually quite bulky and not compatible with our soft and curvy skin surfaces. That’s why we’re building the e-tattoos which are ultra-thin, ultra-soft, basically, hair thin and skin soft, but electronically functional. They are temporary tattoo sticker-like wearables. They are not permanent, and we can incorporate sensors, processors, Bluetooth chips, and batteries on them.
What are the developments that you discussed in this paper?
Our palm is a special location for electrothermal activities, or EDA. That’s because our palm has the highest density of eccrine sweat glands, which are controlled by sympathetic nerves. When we have mental stresses or emotional swings, those kinds of sympathetic nerve responses cannot be controlled consciously because EDA is a peripheral response. It’s widely used already as an indicator of mental stress for patients with mental health issues or emotional responses during gaming or human-robot interactions. We developed the ultrathin GET EDA sensor to capture these signals, but the palm is difficult to design for because we use it all the time. Our sensor is thin and soft and it’s imperceptible, so [it’s] likely [it] won’t have a social stigma during use.
How do you perceive this technology being used in the future?
We’ve demonstrated ambulatory free living environment use. It has to work during exercise, sleep, work, study, while driving, and shopping. We hope that in the future, this could be an everyday sensor that can continuously monitor emotional stimulating events for patients, or it can be directly used for doctors or pilots during training to see how emotionally responsive they are to the machines they are dealing with.
In your publication you use the sensor in a proof-of-concept experiment to demonstrate that it showed response to stimulus. Do you have any future plans to collaborate with other parties in a more clinical setting to validate the instrument?
Yes, in fact we are looking for a good collaborator in the mental health space or psychiatry space to further advance this technology. Currently, we only have collaborators working in the human-robot interaction field who are interested in applying this. We are interested in finding a collaborator who would like to test the sensor for clinical use.
What are your future plans for this line of research?
We are also developing brain sensors for monitoring the central nervous system. We are very interested in looking at both central and peripheral nerve responses when people are exposed to different conditions or dealing with different machines. From the technology standpoint, these optically imperceptible tattoos could be used not only on the palm, but also on the face because people can’t see it. We can perform all kinds of electrical potential or skin conductance measurements. Previously, we applied GET sensors around the eyes to monitor ocular rotation. And we demonstrated using eye movements could control a drone. You looked left with a drone [and it] would fly left. So, this e-tattoo could also be a direct human-machine interface.
Jang H, Sel K, Kim E, et al. Graphene e-tattoos for unobstructive ambulatory electrodermal activity sensing on the palm enabled by heterogeneous serpentine ribbons. Nat Commun. 2022;13(1):6604. doi:10.1038/s41467-022-34406-2