Sweat sensor finds stress quickly

Caltech researchers have produced a wireless sweat sensor that can accurately detect levels of cortisol, a natural compound that is commonly thought of as the body's stress hormone. In a paper appearing in the journal Matter, a team led by Wei Gao, assistant professor of medical engineering at Caltech, show how they designed and made the mass-producible device and demonstrated its effectiveness at detecting cortisol levels in near real time.

The scientists believe that developing an inexpensive, accurate device for measuring cortisol could allow for more widespread and easier monitoring of stress, as well as other conditions including anxiety, post-traumatic stress disorder, and depression—all of which are correlated with changes in cortisol levels.

The sensor Gao developed is prepared using an approach similar to another sweat sensor he recently created that can measure the level of uric acid in the bloodstream, which is useful for monitoring conditions like cardiovascular disease, gout, diabetes or kidney disease. Both that sweat sensor and the new one Gao’s team have created are both made of graphene, a sheet-like form of carbon. A plastic sheet is etched with a laser to generate a 3D graphene structure with tiny pores in which sweat can be analyzed. Those pores create a large amount of surface area in the sensor, which makes it sensitive enough to detect compounds that are only present in very small amounts in sweat.

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In the new sensor, those tiny pores are coupled with an antibody, a type of immune system molecule, specifically sensitive to cortisol, thus allowing it to detect the compound.

The researchers tested the sensor in two different ways. In one test, a volunteer's sweat was analyzed over a period of six days, and data representing cortisol levels were collected. In a healthy individual, cortisol levels rise and fall on a daily cycle.

In the other test, changes in cortisol levels were recorded as they occurred in response to an acute stressor. The researcher asked test subjects were asked to perform aerobic exercises, because intense exercise is known to cause a strong increase in cortisol. Test subjects were also asked to submerge their hands in ice water, a stressor sufficient to elicit cortisol release. In both experiments, the sensors detected rising cortisol levels right away.

Though Gao's sensor may find many uses in typical medical applications here on Earth, it is also targeted to potential off-world applications. In October, NASA announced that Gao is one of six researchers selected to participate in studies of the health of humans on deep-space missions. Gao will receive funding to develop the sensor technology into a system for monitoring the stress and anxiety of astronauts as part of the program, which is being administered by the Translational Research Institute for Space Health (TRISH).