It can be easy to forget to stay hydrated, particularly for people like athletes or soldiers. That's why a number of groups have been developing wearable devices that measure the user's hydration levels for them, letting them know when they're getting low. One of the latest comes to us from North Carolina State University, and it can take the form of either a wrist strap or an adhesive chest patch.

The device incorporates two electrodes made of conductive silver nanowires, which are embedded in a matrix of a flexible, stretchable polymer known as polydimethylsiloxane. Also on board are a button cell battery and a low-power microprocessor with Bluetooth.

Those electrodes (seen below) measure the electrical impedance of the skin, which varies at a known rate in accordance with skin hydration levels. The hydration of the skin, in turn, reflects the hydration of the body as a whole.

Readings from the device are wirelessly transmitted to a nearby computer or smartphone, where they're analyzed by an app. If the software determines that the wearer is in danger of becoming dehydrated, it provides an alert.

The app-user could be the wearer themselves, or it could be an athlete's coach, a soldier's commanding officer, or even a hospital patient's doctor. With the latter scenario in mind, the chest patch version also incorporates three electrocardiography electrodes, so that it could additionally be used to monitor heart rate.

Prototypes have been tested in the lab, on artificial skin samples of varying hydration levels. The devices were found to produce readings just as accurate as those obtained by a large commercial system which is based on a similar principle, but that utilizes rigid probes instead of flexible electrodes. That system also costs around US$8,000, whereas it is estimated that the patch/strap could be manufactured for around the same cost as a fitness tracking device such as a Fitbit.

The lab tests additionally revealed that ambient humidity didn't affect the device's readings.

As mentioned earlier, we have recently seen other hydration-tracking wearables, although they've taken different approaches to the task. There's been a patch that does the job by measuring skin temperature, for instance, along with a wrist band that analyzes sweat, a mouthpiece that analyzes saliva, and a bracelet that uses an optical sensor to analyze the color of blood.

The North Carolina research was led by Yong Zhu and John Muth, and is described in a paper that was published in the journal Advanced Healthcare Materials.

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