New sweat sensor measures cortisol levels to detect stress

Scientists have successfully trialed wearable sweat sensors that can accurately measure cortisol levels in the body. The cortisol levels in sweat have been found to be representative of the levels found in the blood.

Essential reading: Top fitness trackers and health gadgets

Sweat is often viewed as one of the next frontiers in wearable technology. Every drop of this liquid tells a story about what is happening inside the body. Sweat provides information on dehydration, stress, muscle cramping, high cholesterol, depression and even blood glucose.

It can also be used to measure cortisol, often called the “stress hormone.” High levels of the substance for too long can cause health issues such as high blood pressure, sleep disturbances, type 2 diabetes and more. This is why its important to keep your stress levels low.

There are fitness trackers and smartwatches out there that quantify the levels of stress. They do this by measuring heart rate variability (HRV) which is the variation in the time interval between your heart beats. The higher the value the lower the stress. Unfortunately fatigue from intense exercise effects HRV in the same way so there is no way to distinguish the cause.

But something better may be around the corner.

Wearables that measure cortisol

We wrote in early 2020 about research from Caltech. They have developed a sweat sensor made of graphene that directly measures cortisol levels in a person’s body. Because of its small molecular size, cortisol diffuses and can be found in sweat. Concentration levels in in the liquid closely reflect its circulating levels in the body.

The tech contains a plastic sheet at the end with tiny laser generated pores which collect sweat. This goes through an antibody (a type of immune system molecule), that can detect cortisol as it is very sensitive to the substance.

Assistant professor of medical engineering at Caltech Wei Gao was the main author of the paper called “Investigation of cortisol dynamics in human sweat using a graphene-based wireless mHealth system“. It can be found in the April 2020 issue of Matter, the content studio owned by Twitter founder Evan Williams.

The technology works in real-time and is mass producible. It is inexpensive, non-invasive and accurate giving it the potential for widespread use not just for measuring stress but also for detecting anxiety post-traumatic stress disorder and depression. All of these are correlated to elevated cortisol levels.

“Depression patients have a different circadian pattern of cortisol than healthy individuals do,” Gao says.

“With PTSD patients, it’s another different one.”

Plus the analysis is done very quickly. Unlike typical blood tests the results are available in 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,” he said.

“We aim to develop a wearable system that can collect multimodal data, including both vital sign and molecular biomarker information,” he says.

The technology was tested in two different ways. The first involved monitoring a person for six days around the clock. Cortisol levels rise and fall on a daily cycle, which higher values typically experienced in the morning. Data dished out by the sweat sensor correlated with this natural cycle. The second approach involved stressing the participant with high levels of exercise in one test and ice water in another. Both are correlated with high levels of cortisol, with the results showing exactly this.

This is not the only successful trial. Similar tech was covered in the January 2022 issue of Science. It is about a cortisol tracking smarwatch that was developed by researchers at UCLA (University of California, Los Angeles).

We’re not sure if this is entirely new tech or an advanced version of the 2020 device seeing as all this research comes from universities in California. The wearable tracks cortisol non-invasevily, accurately and in real-time as shown in the picture below by analyzing sweat.

The team has developed a flexible field-effect transistor (FET) biosensor array that exploits a previously unreported cortisol aptamer. You can read about it in full on this link – a word of warning as it is quite techy.

This wearable has the advantage of having “label-free detection, the sensing system is autonomous and wireless, the cortisol detection limits are ultralow (1 pM), and they validated sweat cortisol as a stress biomarker in a large clinical study.” Researchers connected the sweat sensor to an Android app to test out the results.

They add that for wearable applications, monitoring relative changes in biomarkers in an individual over time is more important than single readings. That’s because baseline cortisol levels differ from person to person. This might change further down the line once enough data is collected allowing for more thorough research. For now, think of it the way Fitbit tracks skin temperature. The device measures variations rather than absolute values.

Our takeaway

Will this type of technology make it into your next Garmin or Apple Watch? The answer to this would be no – at least not in 2022. But it is only a matter of time before it does. This type of technology has the potential to transform healthcare.

The science of sweat is still in the early stages although clinical uses trace back to the 1950’s. At least for now, it remains a relatively underexplored biofluid.

We posted recently our review of Gatorade’s effort on this front. Their sweat patch does not measure cortisol. Instead, the product analyzes differences in color caused by chemical reactions to monitor both the level of dehydration and the concentration of chloride in the body.

One day we will get smartwatches that can provide this type of info. But that day is still a few years off. 

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