Noninvasive glucose monitoring devices are not currently commercially available in the
United States, so people with diabetes must collect blood samples or use sensors embedded
under the skin to measure their blood sugar levels. Now, with a new wearable device created
by Penn State researchers, less intrusive glucose monitoring could become the norm.
The researchers published the details of the noninvasive, low-cost sensor that can detect
glucose in sweat in the journal Biosensors and Bioelectronics.
The researchers constructed the device first with laser-induced graphene (LIG), a material
consisting of atom-thick carbon layers in various shapes. With high electrical conductivity
and a convenient fabrication time of just seconds, LIG appeared to be an ideal framework
for the sensing device — but there was a significant caveat.
The challenge is that LIG is not sensitive to glucose at all. So, the researchers needed
to deposit a glucose-sensitive material onto the LIG.
The team chose nickel because of its robust glucose sensitivity, and combined it with
gold to lower potential risks of an allergic reaction. The researchers hypothesized that
the LIG outfitted with the nickel-gold alloy would be able to detect low concentrations
of glucose in sweat on the skin’s surface.
A material with high glucose sensitivity was a priority. Sweat exhibits remarkably low
glucose concentrations compared to blood — but, there is a strong correlation between glucose
levels in sweat and blood. While the concentration of glucose in sweat is about 100 times less
than the concentration in blood, the team’s device is sensitive enough to accurately measure
the glucose in sweat and reflect the concentration in blood.
The nickel-gold alloy’s sensitivity allowed the team to exclude enzymes, which are often
used to measure glucose in more invasive, commercially available devices or in noninvasive
monitors proposed by other researchers. These enzymes, however, can degrade quickly with
time and changing temperatures.
Nonenzymatic sensors require alkaline solution, which can damage the skin and typically limits
device wearability. To curb this issue, the team attached a microfluidic chamber to
the LIG alloy. This chamber is smaller than previously developed configurations to promote
wearability and porous to allow for a range of movement, such as stretching or crushing.
It is connected to a collection inlet that passes sweat into the solution without allowing
the solution to touch the skin. The basic solution interacts with the glucose molecules
to produce a compound that reacts with the alloy. This reaction triggers an electrical
signal, indicating the concentration of glucose in the sweat.
With a smaller alkaline solution chamber, the entire device is roughly the size of a
quarter and is flexible enough to maintain a secure attachment to the human body.
In a proof-of-concept test, the researchers used a skin-safe adhesive to attach the reusable
device to a person’s arm one hour and three hours after a meal. The subject performed
a brief workout — just enough to produce sweat — right before each measurement time.
A few minutes after collecting the sweat, the researchers found that the detected glucose
concentration dropped from the first measurement to the next. The glucose measurements from
the device were verified by measurements made with a commercially available glucose monitor.
The team plan to improve upon their prototype for future applications, including addressing
how patients or clinicians may use the sensor for incremental glucose measurements or continuous
monitoring to determine treatment actions, such as administering insulin. They also intend
to refine and expand this platform for more comfortable monitoring of other biomarkers
that can be found in the sweat or interstitial fluids that fill the space between cells in
the body.