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"In the same way that miniaturization led to a revolution in computing,
the idea is that miniature laboratories of fluid being pumped from one
channel to another, with reactions going on here and there, can
revolutionize biology and chemistry," says Martin Bazant, associate
professor of applied mathematics and leader of the research team.
Within the lab on a chip, biological fluids such as blood are pumped
through channels about 10 microns, or millionths of a meter, wide. (A
red blood cell is about 8 microns in diameter.) Each channel has its
own pumps, which direct the fluids to certain areas of the chip so
they can be tested for the presence of specific molecules.
Until now, scientists have been limited to two approaches to designing
labs on a chip, neither of which offer portability. The first is to
mechanically force fluid through microchannels, but this requires
bulky external plumbing and scales poorly with miniaturization.
The second approach is capillary electro-osmosis, where flow is driven
by an electric field across the chip. Current electro-osmotic pumps
require more than 100 volts of electricity, but the MIT researchers
have now developed a micropump which requires only battery power (a
few volts) to achieve similar flow speeds and also provides a greater
degree of flow control.
The key to boosting energy efficiency is altering the electric field
in the channel, Bazant said. Instead of placing electrodes at each end
of the channel, as in capillary electro-osmosis, the voltage can be
lowered substantially with alternating current (AC) applied at closely
spaced microelectrode arrays on the channel floor. Existing AC
electro-osmotic pumps, however, are too slow for many applications,
with velocities below 100 microns per second.
In the new system, known as a three-dimensional AC electro-osmotic
pump, tiny electrodes with raised steps generate opposing slip
velocities at different heights, which combine to push the fluid in
one direction, like a conveyor belt. Simulations predict a dramatic
improvement in flow rate, by almost a factor of twenty, so that fast
(mm/sec) flows, comparable to pressure-driven systems, can be attained
with battery voltages. Experiments in the lab of Todd Thorsen,
assistant professor of mechanical engineering, have recently
demonstrated the effectiveness of the design.
"It's just a huge improvement with a very simple idea," said Bazant.
Thorsen's group is working toward integrating the pumps into a
portable blood analysis device, which soldiers could carry onto the
battlefield. If exposure to chemical or biological weapons were
suspected, the device could automatically and rapidly test a miniscule
blood sample, rather than sending a large sample to a lab and waiting
for the results. The chips are so small and cheap to make that they
could be designed to be disposable, Bazant said, or they could be made
implantable.
Potential applications are not limited to military use - imagine going
to a doctor's office and getting test results immediately. The
technology could also be useful for first responders. If emergency
personnel knew immediately whether a person had suffered a heart
attack or a stroke, they could start the appropriate treatment right
away.
Labs on a chip can also be used in traditional chemistry or biology
labs to speed up processes such as DNA testing or screening for the
presence of certain antigens. Only tiny amounts of reactants would be
needed, and experiments could be done more rapidly and efficiently.
"Instead of a thousand people pouring test tube A into test tube B in
different laboratories, you've got a tiny little chip with thousands
of experiments all going on at once," Bazant said.
Bazant and former MIT postdoctoral associate Yuxing Ben published an
article on the theoretical work in the online edition of the journal
Lab on a Chip, and a related experimental paper will appear in an
upcoming edition of Applied Physics Letters. Co-authors on that paper
with Bazant and Thorsen are graduate student J.P. Urbanski and
postdoctoral associate Jeremy Levitan.
The research was funded by the U.S. Army through the Institute for
Soldier Nanotechnologies.
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