Human sweat is about to have a whole new meaning for people living with diabetes. A recent study shows that you can measure blood sugar levels in your own sweat. Researchers at Penn State have developed a prototype of a wearable, noninvasive glucose sensor that can measure glucose in human sweat with the touch of a fingertip. The touch-based test means far fewer finger pricks, which are often quite painful. The investigational, touch-based test, measures blood sugar in sweat and applies a personalized algorithm that correlates it with glucose in blood. According to a recent proof-of-concept study, it’s more than 95% accurate at predicting blood glucose levels before and after meals.

Glucose Monitoring is a daily routine for many

Most persons with diabetes must check their blood glucose levels by performing multiple and painful finger-prick blood tests, or via subdermal implanted sensors. Researchers have long sought ways to make this testing routine less taxing, starting with determining blood levels without taking blood. 

In previous blogs we covered various methods to monitor glucose levels, including continuous glucose monitoring devices but also emerging technologies like using saliva as a pain-free and cheaper alternative to blood for monitoring diabetes, measuring glucose in your tears and using a smart-patch. Using human sweat is another novel and promising way of monitoring blood glucose levels.  

Correlation between glucose in sweat and blood

The new wearable, non-invasive monitoring device developed at Penn State is based on research that has shown a strong correlation between glucose levels in sweat and blood. According to Prof. Huanyu “Larry” Cheng, in Penn State’s engineering science and mechanics department, the concentration of glucose in human sweat is about 100 times less than the concentration in blood. The team’s prototype device is sensitive enough to accurately measure the glucose in sweat and reflect the concentration in blood using a personalized algorithm. 

But there are multiple challenges. Because levels of the sugar are much lower than in blood, they can vary with a person’s sweat rate and skin properties, so the glucose level in their sweat usually doesn’t accurately reflect the value in their blood. The good news is that some newly developed devices could do the job while simply using diabetic patient perspiration or even adhered to the user’s skin. 

A Personalized Blood Glucose Sweat Sensor 

The study shows that the system developed by the Penn State team collected sweat from a fingertip, measured glucose, and then corrected for individual variability. They made a touch-based sweat glucose sensor with a polyvinyl alcohol hydrogel on top of an electrochemical sensor, which was screen-printed onto a flexible plastic strip. The hydrogel absorbed tiny amounts of sweat. Inside the sensor, glucose in the sweat underwent an enzymatic reaction that resulted in a small electrical current that was detected by a hand-held device. The researchers also measured the volunteers’ blood sugar with a standard finger-prick test, and they developed a personalized algorithm that could translate each person’s sweat glucose to their blood glucose levels. In tests, the algorithm was more than 95% accurate in predicting blood glucose levels before and after meals. To calibrate the device, a person with diabetes would need a finger prick only once or twice per month. But before the sweat diagnostic can be used to manage diabetes, a large-scale study must be conducted. 

What does it look like? 

The device, which is about the size of a coin, will be on a patch applied to the skin near sweat glands. It consists of a small vial containing multiple chambers that has a hydrophobic — water repelling — valve near the opening made of silicone rubber. The channel has a hydrophilic — water attracting — coating for easy collection of the sweat. Unlike other devices that require two openings, the single opening reduces the amount of evaporation, leading to longer storage time for later analysis.

How does the sweat sensor work?

The sensor incorporates a foam electrode, made up of laser-induced graphene coated with a nickel/gold alloy. Although graphene is very strong, chemically stable, and electrically conductive, it isn’t glucose-sensitive on its own. That’s why the scientists have used nickel, which is very glucose-sensitive, in the electrode. The gold is added to reduce the risk of an allergic reaction to the nickel. 

Via capillary action, the device draws sweat in through a small inlet, and carries it into a microfluidic chamber filled with an alkaline solution. The system keeps that solution from coming into direct contact with the wearer’s body, an important consideration, since alkaline solutions can damage the skin. 

Glucose molecules present within the sweat react with the solution, creating a compound that is channeled into the foam electrode. That compound reacts with the nickel, producing an electrical signal. Using either an external or a built-in device to measure the strength of that signal, it’s possible to ascertain the glucose level in the sweat, and in the bloodstream. 

On-the-spot analysis can be done using a colorimetric approach in which a color-coded analyte is preplaced in the various chambers. This sensitive chemical responds to the pH or glucose level and can be read by the naked eye or a photo taken with a smartphone. Also, the researchers can analyze the sweat at different time points using different chambers — called chrono-sampling. 

In a test of the technology, the sensor was placed on a volunteer’s arm using a skin-safe adhesive. Readings were taken both one and three hours after they had consumed a meal – right before those readings, the person worked out briefly to produce a little sweat. The sensor indicated that their blood glucose dropped between the two readings, reporting levels in line with those obtained using a commercially available glucose monitor. 

Conclusion

The device will be of great interest to the healthcare industry. It will for example be useful for monitoring overheating or to adjust exercise levels for optimum performance in athletics. The researchers are also collaborating with a team at Penn State Hershey Medical School on disease monitoring as the device can have one chamber color-coded for pH, a second for glucose and a third for sodium, all of which are disease markers.