“Wow, you’re quite salty,” says Stefan van der Fluit, looking over my numbers. I could have told him that. I have just done a sweaty, 45-minute workout on an exercise bike and the salt is already starting to crystallise on my T-shirt. But van der Fluit knows exactly how salty. I have just sweated out 347 milligrams of sodium in 370 millilitres of water. That’s on the high side, sodium-wise, and I need to replenish.
Van der Fluit is the co-founder of a company called Flowbio, based in London, which specialises in sweat analysis for athletes. During my workout, I was wearing a sensor called the S1 on my upper arm, which collects sweat in a tiny channel, automatically measures its volume and sodium concentration, and transmits the data to a smartphone app. Using that data, the app calculates my total losses.
If I were an endurance athlete, that data would be extremely valuable, perhaps the difference between winning and losing. I’m not, but van der Fluit is. As a competitive cyclist, he has had long-standing problems with dehydration. But since he started using the sweat sensor, they have gone away, and his performance has improved.
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Flowbio’s S1 is one of a handful of wearable sweat sensors that have come onto the market in the past few years. They are mainly aimed at people who sweat a lot in the course of their jobs – athletes and manual workers – but they are also available to the general public, and, in the not-too-distant future, similar devices could be collecting all manner of health-related information from regular Joes like me.
The idea of using sweat to get a deeper understanding of our health is going to be groundbreaking
Although sweat is mostly water and salt, it is also dripping with biomolecules that can provide all sorts of useful insights about what is going on inside our bodies. “Sweat is data,” says Flowbio’s head of research and development, Roeland Mingels. And now the race is well under way to put that data to good use.
Sweat has long been recognised as a marker of health. For centuries, it was noted that children with especially salty sweat failed to thrive. In the 17th century, this was blamed on bewitchment, but it is now known to be a symptom of the congenital condition cystic fibrosis. In the Middle Ages, a mysterious illness called sudor anglicus (sudor is Latin for sweat) struck England. The main symptom was profuse sweating, closely followed by death. We sweat when feverish, and night sweats can be an indicator of serious underlying health problems, including cancer, autoimmune disease, brain injury and hormonal disturbances.
Complications with the sweating system can also be problematic. Excessive sweating, or hyperhidrosis, can cause dehydration and skin problems and can be socially embarrassing. The opposite, hypohidrosis, where people sweat very little, or even anhidrosis, in which people can’t sweat at all, can cause heat stroke – a potentially life-threatening condition.
Why do we sweat?
Anhidrosis notwithstanding, sweating is a fact of life. Most of us have between 2 million and 4 million functioning sweat glands all over our skin that leak constantly, a process called insensible sweating because we don’t notice it. But when our internal thermostat rises, either as a result of an increase in ambient temperature or due to heat generated by exercise, those glands put their hands to the pump and the sweat really starts to flow.
The function of this “eccrine” sweat is evaporative cooling, which keeps our body temperature from rising too high. But because sweat is derived from blood plasma and the interstitial fluid between cells, molecules dissolved in these also leak into sweat. The main non-watery components of sweat are electrolytes – sodium, potassium and chloride ions – but these are the tip of the sweatberg.
All told, more than 30,000 biomolecules have been detected in sweat, ranging from metabolites such as glucose and lactic acid to peptides, proteins, vitamins, inflammatory molecules, hormones and neurotransmitters. Substances we put into our bodies, such as pharmaceuticals and recreational and performance-enhancing drugs, also find their way into sweat. So do environmental pollutants such as heavy metals and chemicals found in plastics.
That presents a juicy opportunity. Because sweat reflects what is going on inside our bodies, it could be used as a “diagnostic biofluid”, says at Northwestern University in Evanston, Illinois.
There are other such fluids, of course – blood, urine and interstitial fluid are routinely sampled and analysed to provide biological insights. But they have drawbacks. All three generally require a trip to a clinic followed by laboratory analysis, and collecting blood and interstitial fluid is invasive. Blood and urine samples provide detailed snapshots, but cannot be used for continuous monitoring, which for certain compounds, such as glucose, is more useful. And while interstitial fluid can solve that problem, wearable monitors still have to penetrate the skin with a thin electrode. “I think for routine use, one has legitimate concerns around risks for pain and infection and dislodgement and other sorts of complications,” says Rogers.
These drawbacks led to a search for biofluids that can be sampled continuously and non-invasively. Tears, saliva and breath condensates have all been tried, but sweat trumps them all, he says.
You don’t even have to overheat or exercise to produce the stuff. A well-established procedure called iontophoresis, in which a sweat-inducing drug such as carbachol is applied to the skin along with a weak electrical current, reliably stimulates the sweat glands for extended periods. “This allows us to sample sweat at any given time,” says at the California Institute of Technology in Pasadena.
Over the past decade or so, this combination of accessibility and continuity has led to a wealth of research on the contents of sweat. “There are a lot of clinically relevant biomarkers in our sweat,” says Gao. “We could use this information for health monitoring, disease diagnosis and, importantly, this will give us early warning before we develop more severe health conditions.”
The diagnostic potential of sweat has been recognised since the early 20th century, when biochemists discovered that it is more than just salty water. Until now, however, sweat analysis has found only two medical uses, according to at the University of Cincinnati, Ohio. One is to diagnose cystic fibrosis in newborns and young children via high levels of chloride ions in their sweat, a result of the genetic change that causes the condition. The other is to map nerve damage, which can stop sweat glands from functioning.
The modest impact is partly because of historical difficulties in sampling and analysing sweat. Even though it can be collected on demand, quantities are limited and the concentrations of the molecules of interest can be vanishingly small. The 370 millilitres of water and 347 milligrams of sodium I sweated out at is an extrapolation of my whole body’s output; the sensor itself gathered a fraction of 1 per cent of that over 45 minutes of prolific perspiration. The cystic fibrosis test, developed in the 1950s, requires the collection of a large amount of sweat through iontophoresis followed by laboratory analysis.
But in the past decade or so, advances in microfluidics, materials science, diagnostics, batteries and data transmission have made real-time sweat analysis possible using wearable patches. Such wearables have transformed the prospects for sweat diagnostics, says Rogers.
How do sweat trackers work?
The first applications to make good on that promise are sensors like Flowbio’s, which gather the low-hanging fruit of water and electrolytes. Three other companies have similar sensors on the market aimed at athletes and gym fanatics, to inform them about their rehydration needs during and after a workout.
One of the companies is in Cambridge, Massachusetts, which Rogers co-founded. In 2021, it released a product called the Gx Sweat Patch in partnership with design company Smart Design and the sports drinks company Gatorade. It is a flexible, disposable, stick-on wearable that is attached to the upper arm before a workout.
The patch has two microfluidic channels that gradually fill with sweat. One measures the overall volume and the other the concentration of electrolytes using a colour-changing dye. To get their results, users photograph the patch at the end of their session and send the image to Epicore, which calculates whole-body losses and recommends which electrolyte-filled Gatorade product to drink and in what quantity in order to rehydrate. Around 3 million of these patches have been sold, according to Rogers.
How to stay hydrated
Epicore has since rolled out a similar product, Connected Hydration, which is designed to protect workers in physically draining industries such as oil and gas, mining and agriculture. Dehydration and heat exhaustion are a growing problem for such industries as the climate warms, says Epicore CEO , and companies increasingly recognise the danger – not least to themselves, as the cost of insuring against heat-related industrial injuries is skyrocketing.
Connected Hydration transmits real-time data via Bluetooth and a smartphone to employers, who can warn workers of dehydration or heat exhaustion. When employees have lost 2 per cent of their body weight in sweat – a validated point at which dehydration affects physical and cognitive performance – the device vibrates, prompting them to take on fluid and electrolytes. “It hits the nail well, to provide a novel solution for a major public health threat,” says at ETH Zurich in Switzerland.
A non-invasive anti-doping tool offers a lot of promise
Analysing sweat loss, sweat rate and electrolyte loss is relatively simple, but it is just the first block of what Rogers and his colleagues ultimately hope to build into a mighty edifice of diagnostic tools.
The first step for Rogers is adding a pH monitor to his company’s sweat patches. Since pH is affected by lactic acid buildup in the muscles, which increases as you work out, it acts a useful proxy for exercise intensity.
Beyond water, salt and pH lies a largely unexplored ocean of biological information with many potential applications. Earlier this year, Epicore teamed up with the US Anti-Doping Agency to develop a patch to detect performance-enhancing drugs in sweat. “As an athlete, anti-doping sucks,” says Ghaffari. “Nobody likes urine, nobody likes blood. A lot of athletes are afraid of needles. So, a potential non-invasive tool offers a lot of promise.”
Sweat analysis could also be used to monitor recreational drugs in people who are in recovery or who are required to abstain as a condition of their bail. Most of the major ones – alcohol, opioids, cocaine, amphetamines and cannabis – come out in sweat and can be detected, says Brasier.
The microfluidic pipeline is rich with other potential applications. For instance, in collaboration with the University of California, Los Angeles, Gao is planning a clinical trial of an early-warning system for gout, an agonising type of arthritis caused by excessive levels of uric acid in the bloodstream. Uric acid levels in sweat closely mirror those in the blood, so a wearable sweat patch could alert users to an impending flare-up, allowing them to adjust their drugs or change their diet to avoid it.
High uric acid levels are also a risk factor for cardiovascular disease, diabetes and kidney disease, making monitoring even more valuable. Similarly, monitoring of the inflammatory molecule C-reactive protein could provide useful information on a range of inflammatory conditions.
Conception cues
Gao’s work even extends to our hormonal system. Working with NASA and the US military, he is developing a sweat patch that monitors cortisol, a hormone associated with stress. Likewise, he has his sights set on monitoring the female reproductive hormone oestrogen, which surges just before ovulation and can be used as a cue to attempt conception (or avoid it).
“There are large gaps in scientific knowledge about the role of sex hormones in women’s health,” says Pauline Maki at the University of Illinois College of Medicine, who has consulted for Gao’s spin-off company Persperity. “These hormones fluctuate naturally on a day-to-day basis in younger women, and current methods require a blood draw to quantify these fluctuations. The new sweat-based technology provides a highly innovative method for quantifying these fluctuations without the burden of a blood draw and a lab visit. Moreover, the sensors can measure other biomarkers, such as those associated with pain or mood.”
The list of potential uses goes on – other groups are investigating using sweat to monitor everything from heart failure to circadian rhythms and biological age. “I think we’re going to establish a pretty powerful pipeline of applications that will be commercialised over time,” says Rogers.
Nevertheless, sweat does have its limitations. Though the concentrations of many of the compounds it contains correlate nicely with those in blood, many others don’t – glucose, for instance – so they cannot be used as a proxy. Heikenfeld also cautions that sweat biosensing is likely to be far more complex than the sensing involved in any other commercial wearable device. As a result, monitoring interstitial fluid may be more popular, despite its invasiveness, he says.
Nonetheless, Heikenfeld believes that sweat is better than other non-invasive biofluids, and that the challenges of collecting and analysing it can be overcome.
That means perspiration pioneers aren’t sweating just yet. “I think the idea of sweat sensing to get a deeper understanding of our biomolecular health is going to be quite groundbreaking,” says van der Fluit. Success, after all, is 1 per cent inspiration, 99 per cent perspiration.
