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TECHNOLOGY REVIEW MARCH /APRIL
chopping it up, and chemically
extracting the plutonium---a dirty,
expensive process that is also a
major step toward building an
atomic bomb. The traveling-wave
reactor produces plutonium and
uses it at once, eliminating the
possibility of its being diverted
for weapons. An active region less
than a meter thick moves along
the reactor core, breeding new
plutonium in front of it.
The traveling-wave idea dates
to the early 1990s. However,
Gilleland's team is the first to
develop a practical design. Intel-
lectual Ventures has patented the
technology; the company says it
is in licensing discussions with
reactor manufacturers but won't
name them. Although there are
still some basic design issues
to be worked out---for instance,
precise models of how the reac-
tor would behave under accident
conditions---Gilleland thinks a
commercial unit could be run-
ning by the early 2020s.
While Intellectual Ventures has
caught the attention of academics,
the commercial industry---hoping
to stimulate interest in an energy
source that doesn't contribute to
global warming---is focused on
selling its first reactors in the U.S.
in 30 years. The designs it's pro-
posing, however, are essentially
updates on the models operat-
ing today. Intellectual Ventures
thinks that the traveling-wave
design will have more appeal a
bit further down the road, when a
nuclear renaissance is fully under
way and fuel supplies look tight.
"We need a little excitement in
the nuclear field," says Forsberg.
"We have too many people work-
ing on 1/10th of 1 percent change."
--- Matthew L. Wald
Diagnostic tools that are cheap to
make, simple to use, and rugged
enough for rural areas could save
thousands of lives in poor parts of the
world. To make such devices, Harvard
University professor George Whitesides
is coupling advanced microfluidics with
one of humankind's oldest technologies:
paper. The result is a versatile, disposable
test that can check a tiny amount of urine
or blood for evidence of infectious dis-
eases or chronic conditions.
The finished devices are squares of
paper roughly the size of postage stamps.
The edge of a square is dipped into a urine
sample or pressed against a drop of blood,
and the liquid moves through channels
into testing wells. Depending on the
chemicals present, di erent reactions
occur in the wells, turning the paper blue,
red, yellow, or green. A reference key is
used to interpret the results.
The squares take advantage of paper's
natural ability to rapidly soak up liquids,
thus circumventing the need for pumps
and other mechanical components com-
mon in microfluidic devices. The first
step in building the devices is to create
tiny channels, about a millimeter in
width, that direct the fluid to the test
wells. Whitesides and his coworkers soak
the paper with a light-sensitive photo-
resist; ultraviolet light causes polymers in
the photoresist to cross-link and harden,
creating long, waterproof walls wherever
the light hits it. The researchers can even
create the desired channels and wells by
simply drawing on the paper with a black
marker and laying it in sunlight. "What
we do is structure the flow of fluid in a
sheet, taking advantage of the fact that
if the paper is the right kind, fluid wicks
and hence pulls itself along the chan-
nels," says Whitesides. Each well is then
brushed with a di erent solution that
reacts with specific molecules in blood or
urine to trigger a color change.
Paper is easily incinerated, making
it easy to safely dispose of used tests.
And while paper-based diagnostics
(such as pregnancy tests) already exist,
Whitesides's device has an important
advantage: a single square can perform
many reactions, giving it the potential to
diagnose a range of conditions. Mean-
while, its small size means that blood
tests require only a tiny sample, allowing
a user to simply prick a finger.
Currently, Whitesides is developing
a test to diagnose liver failure, which is
indicated by elevated levels of certain
enzymes in blood. In countries with
advanced health care, people who take
certain medications undergo regular
blood tests to screen for liver problems
that the drugs can cause. But people with-
out consistent access to health care do
not have that luxury; a paper-based test
could give them the same safety margin.
Paper Diagnostic Tests
George Whitesides has created a cheap, easy-to-use
diagnostic test out of paper.
COLOR CHANGE Paper tests, such as those
shown here, could make it possible to diag-
nose a range of diseases quickly and cheaply.
A small drop of liquid, such as blood or urine,
wicks in through the corner or back of the paper
and passes through channels to special testing
zones. Substances in these zones react with spe-
cific chemicals in the sample to indicate different
conditions; results show up as varying colors.
These tests are small, simple, and inexpensive.
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