Waterproof Wearable Can Analyze Sweat, Even While You're Swimming

Athletes at the 2018 Ironman World Championship triathlon were among the first to test a new sweat sensor under development by engineers at Northwestern University. As the finest triathletes in the world swam 2.4 miles, biked 100 miles, and ran a marathon, this tiny wearable adhered directly to their skin like a translucent band-aid, measuring the unique chemistry of each and every drop of sweat — conveying vital measurements that can make or break an intense endurance race.

When it comes to wearable sensors in sports, we traditionally think of monitoring metrics like power output or heart rate that can help an athlete manage their performance. But the wearable field is expanding, and scientists are looking to measure a wider range of factors that can influence athletic performance. The sweat sensor is a member of this new class of wearable tech.

It was developed by John Rogers, Ph.D., an engineering professor at Northwestern and chairman of the board of directors at Epicore Biosystems — a company that specializes in flexible skin devices that monitor fluids. He tells Inverse that the device’s early tests at the Ironman World Championship were successful and that they’ll look to launch a commercial version of the device within the year. His Science Advances paper, released Friday, provides some details as to how the patch actually works.

“We were interested in seeing whether the devices would stay on, frankly,” Rogers says of the device’s trial at the Ironman World Championship. “They’re running around, they’re swimming, they’re biking. The devices performed well the whole time, they were collecting sweat and doing the readout as we expected. The athletes found the devices were providing valuable information — they weren’t disruptive in any way or irritating in any way.”

Measuring Sweat Loss and Sweat Chemistry

Rogers’s sweat sensor, which is so far unnamed, isn’t the first sweat sensor around. He believes, however, that his design and robustness set it apart. The new paper indicates that the sensor is completely electronics-free and is based on the design of the flexible adhesive patch, which sticks on to the skin like a band-aid.

“We’re collecting essentially one or two droplets of sweat in total,” Rogers says. “The sweat as it’s emerging from the skin immediately goes into fully enclosed channels and reservoirs where we can read out sweat rate, sweat loss, and sweat chemistry.”

From there the wearable actually functions like a thermometer, Rogers says. As the sweat fills the channels, it either turns red or blue. The clockwise distance between those two red or blue marks indicates how much sweat has been lost — though the two marks, when placed on human skin, are fairly faint:

The only aspect of sweat chemistry this device can measure, at least as it currently exists, is electrolyte levels. That’s notable for two reasons. Firstly, some of the authors of this paper are employed by the Gatorade Sports Science Institute, so electrolyte loss is definitely in their wheelhouse. Still, maintaining electrolyte balance — particularly sodium— can be important over the course of long-duration endurance events. Most people have more than enough sodium in their diets to start with, but for athletes running marathons and ultramarathons, there are guidelines for sodium intake (especially for “salty sweaters”).

This wearable uses a combination of chemicals that react with the molecules lost in sweat. Once electrolyte loss is high, the sweat trapped in the patch will change color if things are getting dire:

“We have a chemical reagent that changes in color from light pink to dark red depending on electrolyte levels,” Rogers says. “Just by eye you can look and see ‘Oh, my sweat electrolyte level is very high, or it’s medium, or it’s low.’ You can use that information without any electronics at all.”

Tests on Actual Athletes

The paper is mostly concerned with the construciton of the devices, but it does note that the patch successfully filtered out sweat from saltwater in swim sessions for the Ironman athletes. It also adhered to the skin for over two hours of “underwater operations.” He calls the Ironman trial a “pilot study” — though he notes that they’ve tested it on “about 250 athletes at this point,” including tennis players and basketball players

"“We were happy to see the devices are working pretty well."

As it stands, the device doesn’t transmit information to, say, a mobile app. Right now the devices would be disposable — you would use a new one for each race or training session. He also indicated that there will be a commercial launch within the year and that he’s working with a “large” industry partner.

“We’re pretty deep into testing these devices, though I would say the Ironman event in Kona was the most challenging, given all the variety of circumstances as you move through the race,” he adds. “We were happy to see the devices are working pretty well.”

> Abstract: Noninvasive, in situ biochemical monitoring of physiological status, via the use of sweat, could enable new forms of health care diagnostics and personalized hydration strategies. Recent advances in sweat collection and sensing technologies offer powerful capabilities, but they are not effective for use in extreme situations such as aquatic or arid environments, because of unique challenges in eliminating interference/contamination from surrounding water, maintaining robust adhesion in the presence of viscous drag forces and/or vigorous motion, and preventing evaporation of collected sweat. This paper introduces materials and designs for waterproof, epidermal, microfluidic and electronic systems that adhere to the skin to enable capture, storage, and analysis of sweat, even while fully underwater. Field trials demonstrate the ability of these devices to collect quantitative in situ measurements of local sweat chloride concentration, local sweat loss (and sweat rate), and skin temperature during vigorous physical activity in controlled, indoor conditions and in open-ocean swimming.