September 13, 2022 – Even as you read this, your body is working to maintain balance – and not just in the “not falling” sense. Hordes of chemical reactions are happening inside of you, producing energy, processing waste, and keeping you healthy. Along the way, your body sends signals about your well-being.
Wearable Technology may reveal some of these signals, such as heart rate or sleep rounds. Many other important clues to your health are evident in the blood. The problem: most people don’t like being stuck with a needle. (Just ask anyone with diabetes who has had to prick their finger a dozen times a day.)
But there may be an alternative. Sweat comes from the water in our blood, which means sweat “is like a window into the blood,” says Sarah Everts, science journalist and author of The joy of sweat: the strange science of perspiration.
Since sweat is easier to reach than blood, researchers are investigating whether this could be a painless way for us to get a better sense of our health.
What’s really in our sweat?
Sweating has puzzled scientists for centuries. As early as the second century AD, Galen – a prominent Greek physician in the Roman Empire – explored whether people could sweat body fat from their pores or detoxify their blood by sweatsaid Everts.
While fatty tissue does not ooze out of your pores, other substances will. The sweat is 99% water but contains small amounts of sodium, chloride, lactate, glucose, cortisol, ammonia, urea, ethanol and small proteins.
Sweat may also contain traces of chemicals and toxinssuch as heavy metals and bisphenol A (BPA), but only if they were present in the blood. (Everts reported a rare case where a nurse’s sweat turned red after eating huge amounts of chips with red dye.)
For normal, healthy people, the liver and kidneys handle most of your body’s efforts to rid itself of toxins – and do it very well without the need for a sauna.
How is sweat monitoring used today?
There are several ways medicine — and law enforcement — are already using sweat monitoring.
A high level of chloride in sweat is a symptom of cystic fibrosis, an inherited disease that makes children sick by disrupting the normal functioning of cells in the lungs. At the end of 1950 sweat chloride test has become part of the diagnosis of infants with cystic fibrosis and is now considered the gold standard.
But that involves sticking probes on a infantthe patient’s skin and triggering the patient’s sweating by sending a slight electrical impulse. Sweat is collected in a rolled up plastic tube and evaluated for chloride.
Sweat chloride tests ‘are done regularly, but it’s clunky’, says John Rogers, PhD, professor at the McCormick School of Engineering at Northwestern University. That’s why he and his team developed anti-sweat stickers. The color-changing stickers have tiny channels, valves, and reservoirs that, when stuck to the surface of the skin, can capture and store sweat as it emerges, making it easier to collect and analyze. In a recent studyRogers and his team have shown how well this device works in diagnosing cystic fibrosis in children.
“The vision is a sweat test that can be mailed to people and done at home, to make this screening test accessible to people who may not have access to these kinds of facilities,” explains Rogers. “You wouldn’t need skilled staff or expensive lab instruments.”
There is a strong link between the level of alcohol in your blood and the amount found in your sweat.
Since 2003, what is called SCRAM CAMERAs (meaning Continuous alcohol monitoring SCRAM) were created to help the police and the courts continuously monitor the alcohol consumption of high-risk offenders and domestic violence cases.
It’s like having a breathalyzer strapped to your ankle, always looking for alcohol in your sweat.
What else could sweat monitoring do?
In a world with more advanced wearable sweat monitoring devices, a person could theoretically:
- Measure stress through cortisol production. A study has shown that it is possible to detect cortisol with a wearable patch. But the work is still in its early stages and has not been used for any meaningful clinical evaluation.
- Let drinkers know it’s time to take them home. Research has shown that flexible patches (those that are probably much more comfortable than a SCRAM CAM) can detect ethanol in the bloodstream. So imagine wearing a little patch that sends push notifications to your phone if you’ve had a little too much to drink at happy hour.
- Tell a coach that an athlete needs a break. Imagine an absorbent patch on the skin that collects information about lactate levelsthen instantly sends the results to the coach’s computer screen on the sidelines, letting him know it’s time to substitute a player.
- Save people with diabetes from so many finger pricks. Other previous studies show that non-invasive, bandage-like wearable technologies could potentially measure glucose through sweat. Recently, researchers at Ohio State University created a “smart collar” which can monitor the glucose levels of the person wearing it. The results suggest the sensor “will work to monitor other important chemicals in sweat,” according to a Press release.
But the science and technology to do these things is not there yet. There is also conflicting evidence to prove whether sweat is a reliable way to track all the things that might interest us.
Another problem: while sweating can offer insight into what might be going on inside the body, it doesn’t always accurately reflect reality. For example, talking about athletes and exercise, blood lactate levels show how hard the muscles are working. But sweating itself too lactate product.
This means that someone who trains hard can sweat more and produce higher levels of lactate in their sweat. But this extra lactate may not accurately show muscle fatigue or effort.
While it’s great to get information about your sweat chemistry during a workout, the data may not be very helpful if you have a high sweat rate.
What’s holding back sweat monitoring?
There are two main obstacles to learning sweat chemistry – and until recently they were stuck in a kind of “chicken or egg” impasse.
First, there is the act of capturing the data. Advances in biomonitoring patches, such as Rogers sweat stickers and others portable devicesmake sweat data capture more feasible.
But challenge number two is understanding if the captured data is meaningful.
“There are many different biomarkers in sweat, and it hasn’t been studied very carefully in the past because there wasn’t a clean, reproducible way to collect sweat,” says Rogers.
This is where Rogers thinks microfluidic devices, like the sweat sticker, will become even more valuable — helping researchers get more and better sweat data.
What could be even more useful than sweat monitoring?
Although sweat contains information that could be useful, “the body has evolved to keep internal information in and external information out, so accessing [biomarkers] slapping something on the skin is not easy – that’s why we take blood samples, they take a part of the body,” explains Jason Heikenfeld, Ph.D.professor at the University of Cincinnati.
Heikenfeld is a researcher and developer of portable and flexible electronics. He also understands why many see potential in sweat monitoring, but isn’t so sure it’s practical.
“We spent a lot of time sweating because it was the Holy Grail, [offering] continuous non-invasive access to things in the body,” he says. But “the set of things you can measure is limited. And we found the sweat to be much harder [to monitor accurately]. Whole blood is well buffered; its pH does not change. Sweat salinity and pH change everywhere based on sweat rate, and it confuses the diagnostics in the sensors like crazy.
This is why Heikenfeld believes that for most measurements, the future of wearable chemical monitoring devices is not in perspiration monitoring but rather in interstitial fluid (ISF) detection.
Interstitial fluid exists under the skin, between each cell. It contains elements that leak out blood, which means it looks even more like blood than sweat.
ISF detection only requires micro-needle type patches or wired sensors. This technology is already available for some biomarkers, such as continuous blood glucose monitoring worn on the back of the arm with a sensor that penetrates the skin.
“The big future, and where we are 100% active these days, is pore fluid detection,” says Heikenfeld. “Most of the things you would want to measure in blood, you are able to do in interstitial fluid.”
He says his team is almost ready to release a review that supports this claim.
Still, that doesn’t mean sweat won’t have a place, says Heikenfeld. He sees opportunities to use sweat to track levels of hormones (such as those that regulate stresssex and sleep) and to monitor the levels of a drug in the body and track the rate at which it is broken down.
But for now, monitoring interstitial fluid and sweat needs a lot more research before any mass-market use becomes available.