 For most of us, sweat is that salty, sticky, and sometimes embarrassing fluid that signals everything from exertion to anxiety. But for doctors and biochemists, sweat offers more than a hint at people's emotional state. It can also give scientists information about a person's muscle fatigue or hydration level. Unfortunately, getting useful information from sweat hasn't always been easy because its composition changes over time. That's why researchers have been working on ways to measure the components of sweat in real time. Now, scientists say they've come up with a device that can do just that, and it's small enough to fit in the sweatband. The device works thanks to five sensors located on a flexible circuit board. They measure sodium, lactate, glucose, and potassium in sweat, as well as a person's body temperature. At the other end of the prototype are a bunch of chips that take data from the sensor and generate a detailed sweat profile of whoever's wearing the device during a workout. That information can then be sent wirelessly to a person's phone. The researchers tested this device on a dozen people as they exercised on stationary bikes or ran outdoors. They found that results were comparable to those generated by conventional machines. Putting all that together in a wearable platform that provides this continuous monitoring capability I think is qualitatively more powerful than these episodic single time point measurements that have been done in the past. The scientists say that they'd like to see the technology used by regular people, not just athletes or astronauts. The idea here is that looking at a real time sweat profile could help users figure out how well their body is responding to a specific exercise or to their workout as a whole. This kind of platform with other emerging devices that can measure clinical quality, ECG for example, blood flow, blood pressure, all these kinds of parameters will be possible in wearables in the future and I think it will take us into a whole new regime of management of human health care. The sensors can be mass produced for low costs, the researchers say. So a scaled down version of a device like this could end up in a fitness band or a smartwatch in the near future. But Rogers thinks it might make more sense to develop a wearable akin to a temporary tattoo, because smartwatches don't always get the right amount of skin contact. Until then though, we're probably going to keep thinking of sweat as the price you pay for a good workout.