IEEE Spectrum November, 2014 - 48

Top protective
textile

Paper
microfluidics
Sensor
Paper
microfluidics
Antenna

Adhesive
Epidermis

Sensor
Dermis
Chips

Sweat
duct
Sweat
glands

Flex circuit

Hypodermis
Sweat-porous
adhesive

PErSPiration DEtECtiVE: this patch, developed at the University of Cincinnati, uses paper microfluidics to wick sweat from the skin
through a membrane that selects for a specific ion, such as sodium. onboard circuitry calculates the ion concentration and sends the
data to a smartphone. the electronics within the patch are externally powered, as in an rFiD chip.

My group at the University of Cincinnati, working with
Joshua Hagen and other scientists at the U.S. Air Force
Research Laboratory, at Wright-Patterson Air Force Base,
in Ohio, began five years ago to look for a convenient way
to monitor an airman's response to disease, medication,
diet, injury, stress, and other physical changes during both
training and missions. In that quest, we developed patches
that stimulate and measure sweat and then wirelessly relay
data derived from it to a smartphone. In 2013 the Air Force
expanded on my group's work and that of our collaborators
by sponsoring the Nano-Bio-Manufacturing Consortium, in
San Jose, Calif., created to accelerate the commercialization
of biomonitoring devices such as sweat sensors.
My colleagues and I started by looking for something sweat
could reveal that would be useful to a large number of people. We settled on monitoring physical fatigue-in particular,
alerting athletes if they were about to "crash" because of overexertion or dehydration. This problem may sound mundane,
but it is hard to predict. Even million-dollar athletes regularly leave competitions because of cramping, and warning
of an approaching imbalance in electrolytes could prompt
an athlete to take in fluids to avoid such a mishap.
With the testing of athletes in mind, we started by measuring
the substances dissolved in sweat. You probably know, thanks
to decades of commercials for Gatorade, that sweat is rich with
electrolytes, electrically charged ions of elements like sodium,
chlorine, and potassium, with concentrations from ones to
tens of millimoles per liter. (In biological terms, that is actually a lot: Normally, blood has a 3.5 to 5.2 millimolar concen48

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tration of potassium. That is, it contains 3.5 to 5.2 millimoles
of potassium per liter.) Ideally, we wanted to figure out the
balance of electrolytes in sweat and how it correlates to the
balance of electrolytes in the blood, because it is an imbalance of electrolytes in the blood that causes severe symptoms
of dehydration like muscle cramping.
Measuring the saltiness of sweat doesn't turn out to be
particularly useful for monitoring athletes, because levels
of sodium and chloride in sweat don't correlate with any
particular changes in blood levels. That's because the cell
membranes lining the sweat gland act as "salt pumps." When
messages from the central nervous system trigger the membranes to push negatively charged chloride ions out, they
drag positively charged sodium ions with them, maintaining a neutral charge in the sweat duct. The insides of the cells
become less salty than their exteriors. This imbalance draws
water through the cell membranes into the sweat duct, until
the sodium and chloride concentrations again match; as a
result, the cells shrink until they can replenish themselves
by pulling in water and salt from adjacent cells. The process
repeats to create more and more sweat.
On the other hand, measuring levels of sodium and chloride
in sweat is essential in diagnosing cystic fibrosis. The cells lining
the upper portion of the sweat ducts normally reabsorb most
of the salt that is produced by the sweat creation. (The body is
smart; we need to retain those electrolytes.) But for patients
with cystic fibrosis, the cells that handle that reabsorption don't
work properly, and simple benchtop equipment in a doctor's
office can detect the presence of saltier-than-normal sweat.
illUStration bY

James Provost

http://SPectrUm.ieee.orG

Table of Contents for the Digital Edition of IEEE Spectrum November, 2014