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09/13/2022 – Even as you read this, your body is working to keep its balance – and not just in the sense of “not falling over”. Countless chemical reactions take place inside you, producing energy, processing waste and keeping you healthy. Incidentally, your body sends out signals about your well-being.
Wearable technology can display some of these signals, like heart rate or sleep cycles. Many other important indications of your health can be found in the blood. The problem: Most people don’t like being pricked by a needle. (Just ask anyone with diabetes who had to prick their finger a dozen times a day.)
But maybe there is 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 Weird Science of Sweat.
Because sweat is easier to reach than blood, researchers are investigating whether it could be a painless way for us to gain greater insight into our health.
What’s really in our sweat?
Sweat has fascinated scientists for centuries. As early as the second century AD, Galen — a prominent Greek physician in the Roman Empire — was researching whether people could sweat body fat out of their pores or detoxify their blood through sweating, says Everts.
While fatty tissue doesn’t ooze out of your pores, other substances will. Sweat is 99% water but contains small amounts of sodium, chloride, lactate, glucose, cortisol, ammonia, urea, ethanol and small proteins.
Sweat can also contain traces of chemicals and toxins like heavy metals and bisphenol A (BPA), but only when they are present in the blood. (Everts once reported a rare case where a nurse’s sweat turned red after eating enormous amounts of red-dyed chips.)
In normal, healthy people, the liver and kidneys do most of your body’s efforts to get rid of toxins—and without the need for a sauna.
How is sweat monitoring used today?
There are a few ways medicine — and law enforcement — are already using sweat monitoring.
cystic fibrosis
High levels of chloride in sweat is a symptom of cystic fibrosis, an inherited disease that makes children ill by disrupting the normal functioning of the cells in the lungs. In the late 1950s, the sweat chloride test became part of the diagnosis of infants with CF and is now considered the gold standard.
To do this, however, probes are stuck to an infant’s skin and the patient is made to sweat by sending a small electrical impulse. Sweat is collected in a rolled up plastic tube and analyzed for chloride.
Sweat chloride tests “are routinely performed but are clumsy,” says John Rogers, PhD, a professor in the McCormick School of Engineering at Northwestern University. That’s why he and his team developed sweat stickers. The color-changing stickers have tiny channels, valves, and reservoirs that, when stuck to the skin’s surface, can capture and store escaping sweat, making it easier to collect and analyze. In a recent study, Rogers and his team showed how well this device works for diagnosing CF in children.
“The Vision is a sweat test that can be mailed to people and done at home to make this screening test available to people who may not have access to such facilities,” says Rogers. “You wouldn’t need the trained staff or the expensive benchtop equipment for the lab.”
alcohol monitoring
There is a strong correlation between the amount of alcohol in your blood and the amount in your sweat.
Beginning in 2003, SCRAM CAMs (which stands for SCRAM Continuous Alcohol Monitoring) were developed to assist police and courts in continuous alcohol monitoring of high-risk DUI offenders and domestic violence cases.
It’s like having a breathalyzer on your ankle, always checking for alcohol in your sweat.
What else could sweat monitoring do?
In a world of more advanced sweat monitoring wearables, a person could theoretically:
- measure stress through cortisol production. A study showed that it is possible to detect cortisol through a wearable patch. But the work is still at a very early stage and has not yet been used for meaningful clinical evaluation.
- Let drinkers know it’s time to go home. Research has shown that flexible patches (which are probably much more convenient than a SCRAM CAM) can detect ethanol in the bloodstream. So imagine wearing a little patch that sends push notifications to your phone when you’ve had a few too many at happy hour.
- Tell a coach an athlete needs a break. Imagine an absorbent patch on the skin that collects information about lactate levels and immediately sends the results to the coach’s computer screen on the touchline, letting him know it’s time for a player change.
- Save people with diabetes from so much finger pricking. Other early studies show that non-invasive, bandage-like wearable technologies could potentially measure glucose through sweat. Recently, Ohio State University researchers developed a “smart necklace” that can monitor the glucose levels of the person wearing it. The results suggest the sensor will “monitor other important chemicals in sweat,” according to a press release.
But the science and technology to do these things isn’t there yet. There’s also conflicting evidence as to whether sweat is a reliable way to keep track of all the things we might be curious about.
Another problem: while sweat can provide a glimpse of what might be happening inside the body, it doesn’t always perfectly reflect reality. For example, when we talk about athletes and exercise, blood lactate levels show how hard the muscles are working. But sweating itself also produces lactate.
This means that someone who is training hard may sweat more and produce higher levels of lactate in their sweat. But that extra lactate may not accurately indicate muscle fatigue or exertion.
While it would be cool to get feedback on your sweat chemistry during exercise, the data might not be as helpful if you have a high sweat rate.
What is holding back the sweat monitor?
There are two main obstacles to learning from welding chemistry – and until recently they were stuck in a bit of a “chicken or egg” impasse.
First there is the collection of data. Advances in biomonitoring patches, such as B. Rogers sweat stickers and other wearable devices, make the collection of sweat data more practical.
However, challenge number two is to understand whether the data collected 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 and reproducible way to collect sweat,” explains Rogers.
This is where Rogers thinks microfluidic devices like the sweat sticker will become even more valuable — by helping researchers get more and better data about sweat.
What could be more useful than sweat monitoring?
Although sweat contains information that might be useful, “the body has evolved to keep inside information in and outside information out, so access it [biomarkers] hitting something on the skin isn’t easy — that’s why we draw blood, they take part of the body,” says Jason Heikenfeld, PhD, a professor at the University of Cincinnati.
Heikenfeld is a researcher and developer of wearable and flexible electronics. He also understands why many see potential in sweat monitoring, but he’s not so sure if it’s practical.
“We spent a lot of time sweating because it was the holy grail [offering] non-invasive continuous access to things in the body,” he says. But “there is a limit to the set of things you can measure. And we found that sweat was a lot tougher [to monitor accurately]. Whole blood is well buffered; its pH does not change. Sweat salt and pH changes everywhere based on sweat rate and that confuses diagnostics in sensors like crazy.”
For this reason, Heikenfeld believes that the future of chemistry monitoring wearables for most interventions lies not in sweat monitoring but in interstitial fluid (ISF) measurement.
Interstitial fluid exists under the skin, between every cell. It contains things that ooze out of blood, meaning it’s even more like blood than sweat.
ISF detection only requires microneedle-like patches or wire-based sensors. This technology is already available for some biomarkers, e.g. B. Continuous 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 today, is interstitial fluid measurement,” says Heikenfeld. “Most of the things you want to measure in blood, you can do in interstitial fluid.”
He says his team is almost ready to release a review backing that claim.
But that doesn’t mean there’s no place for sweat, says Heikenfeld. He sees opportunities to use sweat to track hormone levels (e.g. to regulate stress, sex and sleep) and to monitor the level of a drug in the body and how quickly it breaks down.
But for now, both interstitial fluid and sweat monitoring require much more research before mass-market applications become available.
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