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New Sensor Measures Stress Levels, Shows Promise in Deep-Space Missions

If individuals were asked to gauge their stress levels, what would they reply? A lot, or a little, or do not know?

A wireless sweat sensor that can detect a hormone related to stress. Image Credit: Caltech.

While those responses are all valid, they are not particularly helpful to medical professionals and researchers because they are not easily measured and are subjective. Nevertheless, instead of a more improved technique of quantifying stress, a standard method that was used for decades included a stress questionnaire.

A blood test, which was the main alternative to the questionnaire, can offer quantitative data but here a trained professional is required to draw the blood and the procedure itself can be quite stressful, where a large needle is often poked. All these procedures can alter the results of many people. However, something better may soon be available.

Wei Gao, assistant professor of medical engineering at Caltech has created a new wireless sweat sensor that can precisely identify the cortisol levels. Cortisol is a natural compound that is often believed to be the body’s stress hormone.

In the latest study that appeared in the Matter journal, Gao and his fellow scientists demonstrated a technique to design and develop the mass-producible device and the way it operates. They also demonstrated the effectiveness of this device at identifying the levels of cortisol in near real time.

The development of a precise and low-cost device for quantifying cortisol levels may enable easier and more extensive monitoring of stress as well as other conditions like depression, post-traumatic stress disorder, and anxiety—all of these are linked to variations in cortisol levels.

The sensor created by Gao is prepared by utilizing a similar kind of method as another sweat sensor that was newly developed again by Gao. This sweat sensor can quantify the uric acid level in the bloodstream, which is handy for tracking conditions like kidney disease, diabetes, or cardiovascular disease.

This sweat sensor as well as the latest one developed by Gao and his research team is made of graphene, which is a sheet-like form of carbon. With the help of a laser, a plastic sheet is etched to create a 3D graphene structure with very small pores in which sweat can be examined.

These tiny pores produce a considerable amount of surface area in the sensor, making it sufficiently sensitive to identify compounds that only exist in trace amounts in sweat. Those minute pores in the latest sensor are combined with an antibody—a kind of immune system molecule that is particularly susceptible to cortisol—and thus enable it to identify the compound.

Two different ways were used to test the sensor. In one test, the sweat of a volunteer was examined for a span of six days, and data that represents the levels of cortisol was obtained. Cortisol levels in a healthy individual both increase and decrease on a day-to-day cycle. These levels rise soon after a person wakes up every morning and drop all through the day, and that is precisely what the sensor identified.

According to Gao, this is the first-ever demonstration of a sensor that can track the day-to-day fluctuation of cortisol levels in a noninvasive manner. Tracking the daily cortisol cycle of patients may indicate the presence of their mental health conditions. Gao added.

Depression patients have a different circadian pattern of cortisol than healthy individuals do. With PTSD patients, it's another different one.

Wei Gao, Assistant Professor, Department of Medical Engineering, Caltech

In the other test, variations in the levels of cortisol were captured as they occurred in reaction to an acute stressor. This was achieved by performing a couple of experiments. Test subjects in the first experiment were asked to do aerobic exercises, because rigorous exercise can increase the cortisol levels considerably.

Similarly, test subjects in the second experiment were asked to immerse their hands in ice water—a stressor that can easily trigger the release of cortisol. In both sets of experiments, the sensors immediately identified the increasing cortisol levels.

Our analysis time could be only a few minutes. Typically, a blood test takes at least one to two hours and requires stress-inducing blood draw. For stress monitoring, time is very important.

Wei Gao, Assistant Professor, Department of Medical Engineering, Caltech

The sensor developed by Gao could be used in standard medical applications on the planet, but now the device is also being vetted for possible off-world applications. NASA announced in October that Gao is one of the six scientists who was chosen to take part in analyses of the health of humans on deep-space missions.

As part of the program, funding will be given to Gao to create the sensor technology into a system for tracking the anxiety and stress levels of astronauts. The Translational Research Institute for Space Health (TRISH) is administering this program.

We aim to develop a wearable system that can collect multimodal data, including both vital sign and molecular biomarker information, to obtain the accurate classification for deep space stress and anxiety.

Wei Gao, Assistant Professor, Department of Medical Engineering, Caltech

The study detailing the sensor and Gao’s findings is titled “Investigation of cortisol dynamics in human sweat using a graphene-based wireless mHealth system,” and appears online on February 26th, 2020, and in print in the April issue of the Matter journal.

Co-authors of the study include postdoctoral scholars in medical engineering, Rebeca Torrente Rodríguez, Minqiang Wang, and You Yu; medical engineering graduate students Jiaobing Tu, Yiran Yang (MS ’18), Jihong Min (MS ’19), and Changhao Xu.

Other co-authors include non-degree student Yu Song, visiting associate Cui Ye, and Waguih William Ishak, professor of psychiatry and behavioral neurosciences and vice-chair of education and research in the Department of Psychiatry and Behavioral Neurosciences at Cedars Sinai Medical Center.

The study was funded by the Rothenberg Innovation Initiative, the Caltech and City of Hope Biomedical Research Initiative, the Carver Mead New Adventures Fund, and the National Institutes of Health.


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