Most commercially available glucose sensors detect glucose levels in blood, which means that the patient has to go through the painful ordeal of pricking their finger several times a day and dabbing the sensor with blood. The new device, developed by a team led by Ali Javey of UC Berkeley, is completely non-invasive and can detect the levels of sodium, potassium and lactate (which is the same as the lactic acid produced by active muscles) as well as glucose in a person’s sweat. It can also measure skin temperature.

Sweat contains hundreds of different molecules ranging from simple ions like sodium and potassium to more complex protein molecules, as well as heavy metals such as cadmium and mercury. Sweat analysis is mainly carried out in medical laboratories today and most clinics are ill equipped to study the tiny volumes of liquid involved.

Javey’s team’s sensor can be worn directly on the skin and measures analytes in sweat as it appears on the skin’s surface. The device, which contains an array of five sensors on a flexible substrate, can identify a single type of ion or molecule among thousands of others in a sample depending on the electrical signals it produces.

“The more glucose or lactate in your sweat, for example, the more electrical current is generated at the sensor surface, and the more sodium and potassium, the larger the voltage,” explain team members Wei Gao and Sam Emaminejad at Berkeley. “But the current generated from glucose and lactate sensors are affected by temperature. When your skin temperature goes up, the higher temperature increases the signal from glucose sensor, making it look like you are releasing more glucose in your sweat than you actually are. As a result, it’s important to measure both temperature and molecules at the same time to calibrate the device.”

Wireless device transmits via Bluetooth

The device can wirelessly transmit information via Bluetooth and the researchers say that they have already developed an application to synchronize the data obtained from the sensor to mobile phones. Indeed, they have fitted the device onto “smart” wristbands and headbands. And the fact that the device is fabricated on a mechanically flexible polyethylene terephthalate (PET) substrate means that it can easily be contacted onto skin. There are two versions of the device: a completely flexible and disposable sensor array that binds to the skin like a temporary tattoo, and a flexible printed circuit board that is re-usable.

The team tested out its devices on dozens of volunteers as they exercised. The experiments lasted from a few minutes to over an hour.

Large-scale clinical studies possible

The sensor might easily be miniaturized further, say Gao and Emaminejad. “The number of biochemicals we target can also be ramped up so we can measure a lot of things at once. That makes large-scale clinical studies possible, which will help us better understand athletic performance and physiological responses to exercise.”

George Brooks, who is an exercise physiologist at Berkeley, and who collaborated in this work, says that although the device can be used to measure vital metabolites and electrolyte levels in the sweat of healthy individuals, it could also be adapted to monitor body fluids other than perspiration of patients suffering from illness or injury. It might even be used to detect the presence of illegal drugs in sweat, which would be useful in anti-doping tests for athletes.

The sensor is described in Nature doi:10.1038/nature16521.

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