Could chemicals in our sweat be the next thing activity trackers start to measure? Researchers are ramping up efforts to refine this technology so they can.

Many of us already wear some form of activity tracker or smartwatch, but these devices are still fairly limited as to how much they can really tell us about our health.

“The Fitbit, the wearable, the smartwatch have hit a roadblock,” says Sameer Sonkusale, a professor of electrical and computer engineering at Tufts University School of Engineering, who is developing a sweat sensor with his team.

“They can definitely give you a lot of information. But it does not provide you a real window on how you’re doing internally or biochemically.”

If researchers can get it to work effectively, this kind of technology has a lot of potential for monitoring athletes, or people in active careers such as the army, to optimize performance and make sure they stay healthy. It could also help assess how well medication, or diet and nutrition, is working to combat health issues in some people.

How healthy is your sweat?

Sonkusale and colleagues are working to develop an accurate, economic and convenient sweat sensor that can be used to track people’s health.

“Sweat is a useful fluid for heath monitoring since it is easily accessible and can be collected non-invasively,” said Trupti Terse-Thakoor, who worked with Sonkusale on the project when she was a post-doctoral scholar at Tufts.

“The markers we can pick up in sweat also correlate well with blood plasma levels which makes it an excellent surrogate diagnostic fluid.”

The team at Tufts have developed a flexible sensor the size of a postage stamp that can be attached to the skin using a fabric bandaid to measure changes in sweat in real time.

Using a wireless connection to the sensor the researchers tested how well it could measure chemicals in the sweat of volunteers during exercise. The first prototype of the sensor can measure sodium ions – which show how hydrated someone is; and ammonia ions, lactate and acidity – which indicate how tired a person is and how exhausted their muscles are. The researchers also took sweat samples from the volunteers so they could check the accuracy of their sensor.

They found that their device was able to accurately measure the chemicals they were testing and could sense changes in concentration within 30 seconds.

The testing is still at an early stage, but Sonkusale and colleagues think their sensor could also be used to measure other chemicals such as chloride ions, which can be a sign of health status in people with the genetic lung disease cystic fibrosis, or levels of the stress hormone cortisol, which is implicated in a lot of health problems.

Overcoming challenges

There has been interest in using sweat for diagnostics for a while, but there are challenges involved in collecting a high enough volume of sweat and many substances are quite diluted in sweat compared to the blood. Sweat composition also varies from person to person.

“With this panel of biochemicals you will have a baseline and then you would see how it varies. It’s perfect for longitudinal, continuous real time monitoring, but not really as a single diagnostic test,” says Sonkusale.

He admits that creating an effective sweat sensor has not been easy. “The way our patch is made is we have a way for the sweat to continuously evaporate naturally on the other side. So that provides it with a continuous sampling of sweat. The sensors have to be integrated precisely in that path of flow, so that you can monitor sweat rate so there are a lot of engineering challenges is to get this to work.”

Because everyone sweats slightly differently and has a different composition of chemicals in their sweat, it’s also important to calibrate the sensor each time it is used.

Another challenge that the team at Tufts has overcome is to make the sensor as flexible as possible by making their sensor out of metallic threads coated with conductive inks.

“You can essentially integrate them in a very complex sort of environment, which is not possible with any other flexible kind of sensing platforms,” explained Sonkusale.

“Flexible devices woven into fabric and acting directly on the skin means that we can track health and performance not only non-invasively, but completely unobtrusively – the wearer may not even feel it or notice it.”

Although their device is the only one that uses flexible threads, Sonkusale and the team at Tuft’s are not the only ones working on devices to measure chemicals in sweat. There are several groups trying to develop successful sensors, for example at Northwestern University in Illinois, or at California Institute of Technology.

Similarly, last month scientists from Penn State University reported that they had designed a sweat sensing device that can change color depending on the amounts of glucose or sodium in the sweat and also in response to changes in acidity.