Patch captures sweat to track body’s response to exercise

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A new soft, flexible microfluidic device sticks on forearm or back skin to measure sweat and show how the body is responding to exercise.

A little larger than a quarter and about the same thickness, the simple, low-cost device analyzes key biomarkers to help a person decide quickly if the user needs to drink more water, replenish electrolytes, or even seek medical help.

Designed for one-time use of a few hours, the device even detects the presence of a biomarker for cystic fibrosis. In the future, it may more broadly useful for disease diagnosis.

“The intimate skin interface created by this wearable, skin-like microfluidic system enables new measurement capabilities not possible with the kinds of absorbent pads and sponges currently used in sweat collection,” says study leader John A. Rogers, professor of materials science and engineering, biomedical engineering, and neurological surgery in the McCormick School of Engineering and Northwestern University Feinberg School of Medicine.

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“Sweat is a rich, chemical broth containing a number of important chemical compounds with physiological health information. By expanding our previously developed ‘epidermal’ electronics platform to include a complex network of microfluidic channels and storage reservoirs, we now can perform biochemical analysis of this important biofluid,” he says.

Sweat analysis

“We already know how to put electronics on the skin in a natural manner—here our challenge was dealing with fluid flow and the collection, storage, and analysis of sweat in a thin, soft, and flexible device,” says collaborator Yonggang Huang, who worked on the device’s design and optimization. Huang is a professor of civil and environmental engineering and mechanical engineering at the McCormick School.

“The sweat analysis platform we developed will allow people to monitor their health on the spot without the need for a blood sampling and with integrated electronics that do not require a battery but still enable wireless connection to a smartphone,” he says.

Exercise bikes and a bicycle race

In a study of accuracy and durability, researchers tested the device on two different groups of athletes: one cycling indoors in a fitness center under controlled conditions and the other participating in the El Tour de Tucson, a long-distance bicycle race in arid and complex conditions. The researchers placed the device on the arms and backs of the athletes to capture sweat.

During moderate or vigorous exercise, sweat winds through the tiny microscopic channels of the device and into four different small, circular compartments. In the compartments, reactions with chemical reagents result in visible color changes in ways that quantitatively relate to pH and concentrations of glucose, chloride, and lactate.

When a smartphone gets close to the device, the wireless electronics trigger an app that captures a photo of the device and analyzes the image to yield data on the biomarker concentrations.

“We chose these four biomarkers because they provide a characteristic profile that’s relevant for health status determination,” says Rogers, director of Northwestern’s Center for Bio-Integrated Electronics. “The device also can determine sweat rate and loss, and it can store samples for subsequent laboratory analysis, if necessary.”

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In the group that cycled indoors, the researchers compared the new device’s biomarker readouts to conventional laboratory analysis of the same sweat and found the two sets of results agreed with each other. (Conventional methods include capturing sweat with absorbent patches taped to the skin and analyzing them off-site.)

With the long-distance cyclists, the researchers tested the durability of the device in the complex and unpredictable conditions of the desert. They found the devices to be robust: They stayed adhered to the athletes’ skin, did not leak, and provided the kind of quality information the researchers sought.

Details appear in the journal Science Translational Medicine.

Other investigators on the team are from the Sarver Heart Center of the University of Arizona, the University of Illinois at Urbana-Champaign; Ajou University, Korea; Hanyang University, Korea; Zhejiang University, China; and MC10, Inc., and L’Oréal.

L’Oréal, the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign, the National Research Foundation of Korea, and the National Institutes of Health supported the research.

Source: Northwestern University