Chances are, unless it's a super-hot day or you're just about to deliver a speech in front of hundreds of VIPs, you don't think much about your sweat. It's an inconvenience at best, an embarrassment at worse. But it's time to stop being sniffy about sweat: thanks to a new generation of sensor technologies, sweat is helping to unlock the secrets of human health and sports performance.
Tech companies and biochemistry researchers have long been aware of the possibilities of sweat, and have been working on producing sensors that can monitor and interpret the fluid for uses including diagnosing disease and helping athletes improve their performance. Sensors that can analyse human sweat have been around for over a decade, but recent developments in sensor design and technology mean that systems are fast approaching commercialisation.
Apart from water, sweat contains all manner of other substances secreted from the body (including the waste products that bacteria eat to cause body odour). It's those substances that can help open a window into the body's internal workings, and whether it's healthy or otherwise.
"Among many bodily fluids, sweat provides a significant amount of information about a patient's health status and is readily accessible, making it suitable for wearable, noninvasive biosensing. Sweat contains important electrolytes, metabolites, amino acids, proteins, and hormones, which allows monitoring of metabolic diseases, physiological conditions, or a person's intoxication level," Stanford University researchers wrote in a paper on sweat sensors published in Science Advances.
Nextflex and GE Research have been working on a sensor that monitors dehydration by tracking the volume and make up of sweat. "If we're able to track the rate at which we're sweating, and perhaps track the concentrations in the sweat at the same time, we can really start to get a measure of what the body's state of hydration is," says Scott Miller, Nextflex's director of strategic programs.
The system has been tested with Air Force cadets to track their hydration while working. "Every time we interacted with the end-users in their field of operation, there was a ton of excitement and new learnings. In this particular case, we received an extremely positive reaction from the cadet who noted the comfort and ease of use of the device. We also gathered valuable information that has allowed us to improve the performance of new prototypes that we have made since the trials," Azar Alizadeh, principal scientist at GE Research, said.
Sweat sensors may have a role beyond physical health too. A team at Stanford University has already built a sensor that can measure cortisol, sometimes known as the 'stress hormone', in sweat. While such sensors could be potentially useful for monitoring stress in future, in the short term, they're more likely to be used for tracking disorders where levels of cortisol are too high or too low, such as Addison's disease or Cushing syndrome.
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The medical applications of sweat sensing are already being investigated. Take glucose, for example - a type of sugar that people with diabetes can struggle to process. Diabetics often have to measure the glucose levels in their blood by pricking their finger, squeezing out a drop of blood, and then using a tiny monitor to test their sugar levels it contains. Replacing that invasive testing with background sensing would ease the self-testing burden on diabetics; sweat monitoring is being explored as one alternative. A team in Washington State University, for example, has created a 3D-printed flexible glucose biosensor that could be incorporated in wearable systems to monitor sugar levels in sweat.
As well as monitoring response to treatment in certain diseases, sweat sensors could eventually be used to improve diagnosis of conditions, such as cystic fibrosis. Cystic fibrosis, a genetic condition that causes reduced life expectancy, is already diagnosed through a sweat test, but it's a clunky procedure that takes time to conduct, and takes days for the results to come back. Standard sweat tests measure concentrations of chloride ions in the sweat as evidence of disease; a sensor-based test would use the same principle, but would be far easier to administer and could give instantaneous results.
Analysing blood has been medicine's default way of measuring levels of various substances in the human body, but there are limits to what blood can tell you: analysing blood gives you a snapshot of the body's workings, and is time consuming and painful to gather. Sweat sensing would allow medics to gather data over a longer time span - how electrolyte levels change in a day, for example - rather than the one-and-done information a blood draw can give. As the electronics needed to analyse sweat are steadily shrinking, data analysis could be done on the wrist in real-time.
So why isn't sweat analysis more popular in medicine?
Collecting sweat for analysis isn't that easy. Several methods have been used in the past, including a whole body washdown (just what it sounds like), absorbent pads and macroducts, which stick to the skin and collect sweat into tiny tubes. Problems with accuracy, resources, how to get humans to sweat on demand, and leakage of sweat from the collection system (no, really) mean the time is ripe for a new generation of sensor-driven systems.
Because gathering and analysing sweat has traditionally been time-consuming and clunky, it's an area that's been relatively under-researched. Thanks to the advent of sweat sensors and the data they generate, it may be time for medicine to take a closer look at the fluid.
"I think nobody's really taken a look, because it hasn't in the past been very convenient to collect precise, pristine volumes of sweat from the body," says, John Rogers, professor of Materials Science and Engineering, Biomedical Engineering and Neurological Surgery at Northwestern University, who has been working on stretchable sweat sensors.
"A lot of the uncertainty is at the level of the biology, the devices can make these measurements very nicely, there's no question about that. We can quantify these concentrations across the physiologically relevant range in sweat. I think the biggest question is how does sweat chemistry correlates to blood chemistry," he says.
As yet, there's no consensus on the ideal setup for sweat sensors: some are based on biochemical analysis, others on bioelectrical. The former measure the electrical conductivity of sweat to work out electrolyte levels, while the latter measures the components of sweat directly. The information is either analysed in situ by microcontrollers or passed back to external systems for analysis, using wireless standards such as NFC or Bluetooth. Other systems use sensors that are scanned by smartphone apps to measure electrolyte levels according to how the sensors change colour when exposed to sweat.
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There are other problems with analysing sweat: the composition of sweat varies from person to person, and even for an individual it can change depending on factors including what they eat, so systems will need to be attuned to individuals' unique sweat patterns.
There are also technical problems that need addressing before sweat sensors can be used by either medics or consumers. In the longer term, sweat sensing systems will need to be as flexible and stretchable as possible to make sure they can be used for every type of skin.
"A flexible device could wrap around a simple curvature -- for example, around your wrist -- but if you want devices that match the mechanical geometries of various parts of body, you need something that not only bends, but is soft and stretchable. It's a critical mechanical attribute to allow a stable and imperceptible interface between monitoring biosensor technologies and the human body, the skin in particular," Northwestern's Rogers says.
The electronics and materials science behind sweat sensors may need some refining yet, but it does offer a tantalising prospect of a future where some medical tests can be performed from home using only a wearable patch. What's more, it may allow consumers as well as pro athletes to level up their performance based on a whole new source of data, making sweat sensor technology something worth getting hot and bothered about.
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