One downside to medical sensors that test human sweat: You have to sweat. Sweating from exertion or a stifling room temperature can be impractical for some patients and unsafe for others. And unless they are on the second leg of the Tour de France, it’s unlikely patients will want to sweat all day for the benefit of a sensor reading.
But researchers at the University of Cincinnati have come up with a novel way to stimulate sweat glands on a small, isolated patch of skin so subjects can stay cool and comfortable and go about their daily routine without spending hours on a treadmill.
UC professor Jason Heikenfeld and UC graduate Zachary Sonner came up with a device the size of a Band-Aid that uses a chemical stimulant to produce sweat, even when the patient is relaxed and cool. The sensors also can predict how much patients sweat, an important factor in understanding the hormones or chemicals the biosensors measure.
"Doctors would love to know if chemical concentrations are increasing or decreasing over time," Heikenfeld said. "What was your baseline before you got sick? Then by measuring the change in concentrations, we know even more about how sick you are or how quickly you are getting better."
Blood analysis is considered the gold standard for biometric analysis. But biometric testing with blood is invasive and often requires the use of a lab. It is far more difficult for doctors to perform continuous monitoring of blood over hours or days.
Sweat provides a non-invasive alternative, with chemical markers that are more useful in monitoring health than saliva or tears, Heikenfeld said.
“People for a long time ignored sweat because, although it can be a higher-quality fluid for biomarkers, you can’t rely on having access to it,” Heikenfeld said. “Our goal was to achieve methods to stimulate sweat whenever needed — or for days.”
Scientists say sweat provides much of the same useful information about patients as blood. The problem has always been getting the same consistent sample as is possible with a standard blood draw, he said.
For the study, the researchers applied sensors and a gel containing carbachol, a chemical used in eyedrops, to their subject’s forearm for 2.5 minutes.
They used three methods to obtain sensor data: the gel and sensors alone and in combination with memory foam padding (to provide better contact between the sensor and the skin) and iontophoresis, an electrical current at 0.2 milliamps that drives a tiny amount of carbachol into the upper layer of the skin and locally stimulates sweat glands but causes no physical sensation or discomfort.
Then they recorded data obtained from the subject’s sweat for 30 minutes using sensors that measured concentrations of sweat electrolytes. Carbachol was effective at inducing sweating under the sensor for as long as five hours. Heikenfeld said a subsequent study successful generated sensor results for several days using this process to stimulate sweat.
They used a pH-sensitive dye to observe the results. The orange dye turned blue when it reacted with sweat. This demonstrated that the sweat glands were stimulated evenly across the sensor area.
“This work represents a significant leap forward in sweat-sensing technology,” the study concluded.
University of Cincinnati