North AmEricAN
52 | FEB 2013 | North AmEricAN | SPEctrUm.iEEE.orG
channelIng lIquIds: These basic microfluidic sensors [top] are built on filter paper.
Straight channels lead to circular sensor regions that turn red in the presence of nitrogen
dioxide. A single sample can be tested in multiple ways as it moves through a 3-D microfluidic
device, like this one [bottom], which was built through folding.
clockwise from toP: Xu li/wei shen/monash university; aleX wang/university of teXas at austin (2)
types resist water, they eventually did
absorb the liquid. Then we were con-
tacted by the Boston-based Sappi
Fine Paper North America, which
had created a polymer-coated pa-
per with an average surface rough-
ness of a couple of nanometers, just
a little higher than that of glass. That
seemed to be just the thing. With that
material, we were able to make reliable
pixels with switching times as short as
10 milliseconds—nearly suitable for
video. We are now working to develop
paper-based displays using electro-
wetting. We think this approach might
be ideal for smart labels on packages
that could, for example, show videos
about how the product is to be used, or
for displays—containing vital informa-
tion for soldiers in the field—that can be
rapidly destroyed if necessary.
For both displays and back-end
electronics, fabrication still remains
a problem. The fastest, cheapest way to build
paper electronics is to use a roll-to-roll printer.
But the state-of-the-art resolution of these ma-
chines is currently about 10micrometers. So flex-
ible electronics fabricated with these machines
would have feature sizes about the same as those
of silicon-based chips in 1971, when micro-
processors had about 2000 transistors. Improv-
ing this resolution without sacrificing printing
speed will take years and significant investment.
That being said, size isn’t everything. Displays—
particularly if they can be constructed economi-
cally—can still be quite readable and attractive
even if they’re constructed from components that
are much larger than those needed for advanced
integrated circuits. (After all, today’s state-of-
the-art tablets and e-readers boast pixel sizes
of about 100 µm, which is about 10000times
as large as the minimum feature sizes needed
to build state-of-the-art memory chips.) Some
circuit components need larger features to
function well. Radio-frequency ID tags, for ex-
ample, need relatively large antennas to be able
to pick up and transmit electromag netic waves
with radio wavelengths. Even the smallest RFID
chips reported to date are about 50 µm on a side.
And high-voltage power electronics tend to per-
form better when they’re made bigger, because
spreading a load over a large area reduces the
chance of electrical breakdown. This is a realm
where paper has already been used for many
years, as an insulator in transformers.
Microfluidics
those of us building paper-based electronic devices and displays are, to a cer-
tain extent, working against paper’s intrinsic properties. But there is one potential
application area where paper is clearly a natural fit: microfluidics.
Microfluidic devices work by transporting liquids
from one spot to another. In the realm of biomedical
technology, they’re particularly useful because they
allow you to perform tests like DNA analysis or toxin
detection on small liquid volumes, which cuts down on
costly chemicals and reagents and greatly reduces the
amount of bodily fluids that must be extracted from patients. To date, most micro-
fluidic units have been high-precision affairs that rely on plastic feed tubes and
externally powered pumps, which can take up a fair amount of counter space. If pat-
terned correctly, paper could be used to perform similar tests without these external
accessories. The narrow channels between fibers in paper excel at drawing in water
and other fluids automatically by capillary action.
Some companies have already taken advantage of this liquid-wicking capability
to create disposable pregnancy and blood-sugar tests. But recently the emphasis
has shifted to a “bottom up” approach. Instead of making inexpensive versions of
specific tests, researchers are now trying to develop a general class of paper-based
microfluidic systems that can then be adapted to make a variety of different tests,
for such tasks as monitoring liver function or diagnosing tuberculosis. If done right,
these tests could be compact, self-contained— and cheap. They could also be used
without a great deal of training, at home or out in the field,
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