Engineers create wearable sensor that flexes with skin

Engineers at the University of Illinois Chicago have created a wearable wireless sensor that sits on the skin and can measure skin temperature along with glucose, ammonium, sodium, potassium and pH levels in sweat.

The sensor, which is made of a flexible, antimicrobial materials and does not have a battery, is highly sensitive and accurate. The technology could be applied to devices that monitor things like hyperglycemia in diabetic patients or hydration during exercise, the researchers report in Science Advances.

Compared to the wearable wireless sensing devices currently available, the new device can measure multiple variables with a very small footprint, said study senior author Pai-Yen Chen.

"In traditional wearable devices, we cannot simultaneously detect many physiological parameters," said Chen, professor of electrical and computer engineering in the College of Engineering at UIC.

"We wanted to build something that can wirelessly monitor multiple parameters, and we wanted to make the wearable device very, very compact."

The resulting invention sits on the skin like a tattoo and is smaller than a playing card, about half the size of conventional systems, Chen said. It's made of a liquid metal injected into a porous, plastic-like material developed by collaborators at Caltech and University of Missouri-Columbia. The material is shaped into coil antennas and sensors, and flattened onto a soft surface that can flex with the body.

"Because this is a wearable, it must be flexible, stretchable, twistable, foldable and bendable. You cannot just use a traditional, conventional metal," said Chen.

The researchers also injected an antimicrobial molecule into the porous, flexible material, which makes it 99.9% resistant to common skin pathogens including MRSA, a type of staph infection.

The sensor has no battery or wires. Instead, it sends data wirelessly to a portable reader through inductive coupling: the transfer of energy and information from one antenna to another through magnetic flux.

"We set up this wireless sensing system inspired by quantum mechanics," said Chen.

The separate reader and the wireless sensor follow the concept of parity-time symmetry, where a system remains unchanged if mirrored or reversed in space and time. When biomarkers, like electrolyte levels in a person's sweat, change, they alter the sensors' impedance to flow of alternating current. The reader then detects the resonance frequency shifts predicted by a mathematical formula and displays the corresponding health data to the wearer.

In experiments, the researchers tested the sensor on people exercising and measured skin temperature and the electrolytes in sweat. They also tested the sensor on people eating high-carbohydrate or high-fiber meals, then exercising and going about their daily activities. The sensor could accurately test the influence of diet on these wearers' sweat glucose and ammonium levels.

"These two are the most important factors for diabetes," said Chen. Diabetes interferes with production of insulin, which helps process glucose, and with the liver, which processes ammonium.

Now, Chen and his colleagues are working on developing the sensor's ability to monitor for more biomarkers that could expand its usefulness to other health care contexts.

"The next step we are trying to do is something more complicated. We're still working with University of Missouri and Caltech on this to monitor uric acid and lactose levels," he said. "We'll try to expand this, and try to apply this technique to other biomarkers."

Other UIC authors of the study are Zhilu Ye, Minye Yang and Yichong Ren.