IEEE Spectrum November, 2014 - 49

With sodium and chloride off the table, we looked at a number of other substances in the blood whose levels increase
when the body gets dehydrated and that diffuse into sweat in
a more orderly way, meaning that when they appear in high
concentration in sweat, they must be at a high concentration
in the blood. Like sodium and chloride, these are small ionic
solutes in sweat-ones that I can't specify here, unfortunately,
because of confidentiality agreements.
Although we couldn't use them directly to gauge dehydration,
we weren't quite done with sodium and chloride. We found that
the faster the sweating, the saltier the sweat (because there is
less time for the body to reabsorb the sodium and chloride).
Correlating the levels of electrolytes in sweat with their levels
in blood isn't exactly straightforward. That's because their diffusion from blood into sweat is slow. So as the rate of sweating
increased, the telltale substances we were tracking in the sweat
became more diluted. By monitoring sodium and chloride levels,
too, we could correct our sweat measurements accordingly.
Detecting sodium and chloride ions requires two things:
an electrode coated with an ion-selective membrane, and a reference electrode, typically made of silver chloride. The coating
for the ion-selective membrane is a standard polymer-like the
plastic used to make plumbing pipes-through which ions have
great difficulty penetrating, along with a special ionophore
molecule that allows the passage of only one type of ion. If the
ionophore is for sodium, sodium is able to easily penetrate into
the polymer coating, and because
sodium is a positively charged ion,
a
voltage of several millivolts builds
What'S in
up. Because the voltage of the reference electrode does not change, you
can measure the total voltage of a
Biomarkers contained
circuit by connecting the two elecin sweat can give
trodes with a meter, calculating the
indications about
the physical state
voltage induced by the ion-selective
of the body. they
membrane, and from that calculatinclude electrolytes,
ing the ion concentration. As sodium
metabolites, proteins,
and amino acids.
and chloride generation by sweat are
here's a sampling:
interrelated, you also obtain a simple
measurement of chloride.
Electrolytes
* Sodium
That's how we can find out how
* Chloride
much
salt is in sweat. Trickier is cap* Potassium
turing the sweat quickly, getting it to
* Calcium
the sensors, and then disposing of it,
Metabolites
because
you don't want to hang on
* Lactate
to old sweat and mix it with new. We
* Creatinine
* Glucose
decided to use paper microfluidics,
* uric acid
the lowest-cost form of plumbing
we could find that would move fluid
Small Molecules
* Amino acids
along the patch. Pregnancy-test sticks
* DheA
use
paper microfluidics in this way.
* Cortisol
In our patch, the paper wicks sweat
Proteins
in a tree-root pattern, maximizing the
* Interleukins
collection
area while minimizing the
* tumor necrosis factor
volume of paper. To keep the sweat
* Neuropeptides

Sweat?

pumping along after it passes through the sensors, these microfluidic channels direct the sweat to a superabsorbent hydrogel,
such as the filler used in diapers, which pulls the sweat out of
the paper and stores it. The patch can pull sweat along for several hours with the hydrogel swelling only 2 to 3 millimeters,
enlarging it to hundreds of times its original volume.
We built a sodium sensor, the voltage meter, a communications antenna, the microfluidics, and a controller chip onto
a patch that's externally powered (like an RFID chip) by a
smartphone. We printed it onto a flexible substrate and, with
the help of researchers at the 3M Co., coated it with a sweatporous adhesive so that it could stick to the skin. In tests, this
patch performed as well as the benchtop electrolyte-sensing
systems used by doctors to test for cystic fibrosis. We have
had a couple of people in our research group wearing the
patches for as long as a week.
Right now our industry partners are preparing to use standard flexible-electronic manufacturing processes to produce
several hundred patches for more extensive human trials,
which are expected to start before the end of the year. We're
also adding about a half dozen other sensors that will detect
additional ions besides sodium and chloride and use them to
predict things like exertion level and muscle injury or damage. The initial results look promising, and if the upcoming
human trials go well, it's not a far stretch to imagine using
the patch in conjunction with the RFID-reading mats that
already record marathoners' split times to also identify runners at risk of a dangerous electrolyte imbalance.

this kind of passive patch should work great for athletes,

who are usually pumping out plenty of sweat. But my colleagues
and I also wanted to measure sedentary people-for example
cystic fibrosis patients, who normally don't sweat much.
The solution is to use an electrical process, called iontophoresis, which stimulates skin to produce sweat. Iontophoresis works by placing an electrically charged medication on
the skin and using an electrode and a low current-less than
1 milliampere per square centimeter-to draw the medication into the skin.
Doctors have used iontophoresis for years to push antiinflammatory drugs through the skin to reach injured tissue.
And they have used it in a cystic fibrosis test for newborns
to infuse pilocarpine, a medication that stimulates sweat
glands, into the skin.
We've built the components needed to add this same capability to our patch. By carefully controlling the current that
drives the iontophoresis, and therefore the absorption of
pilocarpine, we can keep sweat flowing on as big or as small
a spot as we want for hours, and possibly even days, at a time.
Electrolytes are by far the easiest component of sweat to
measure. But metabolites-like lactate, creatinine, and glucose-
shouldn't be too much harder.
The lactate level is a great indicator of a person's ability to
cope during rigorous exercise or while on life support. Lactate,
or lactic acid, is a by-product of burning glucose without oxySPectrUm.ieee.orG

north american

nov 2014

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