When you're in the midst of a workout, it can be hard to know when you need to drink more water or replenish electrolytes. That's part of the reason why an international team of scientists has created a skin patch that tells you. Unlike some similar devices, it can store and analyze sweat in place on the skin, in real time.

The coin-shaped soft adhesive patch is designed for a single use of a few hours, and is placed on the skin of the forearm or back.

As the wearer engages in physical activity, their sweat makes its way through microfluidic channels in the device, ending up in four separate circular reservoirs. Each of these compartments in turn contains a different reagent, which reacts with a specific biomarker within the sweat – these being glucose, chloride, lactate, and pH.

The reactions cause the outside of each reservoir to change color. Utilizing the camera on the user's smartphone, a dedicated app analyzes those color changes to determine the levels of the corresponding biomarkers. A readout on the phone's screen displays the data, which also includes sweat rate and total sweat loss. The patch itself doesn't require a power source.

In its current form, the low-cost device additionally measures concentrations of a biomarker associated with cystic fibrosis. Down the road, it could conceivably be equipped with reagents that detect other diseases, too.

The research is being led by Prof. John A. Rogers of Northwestern University. A paper on the study was published today in the journal Science Translational Medicine.

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