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Pregibon is the lead author of a paper in the March
9 (2007) issue of Science.
He and co-author Patrick Doyle, the Doherty
Associate Professor of Chemical Engineering, believe their particles
could become an effective and inexpensive way to perform medical
diagnostic tests at a patient's bedside.
Current testing methods are cost-prohibitive for
bedside use, Pregibon said. The MIT particles are inexpensive to
manufacture, and their results are as accurate, if not more so, than
the results from more expensive systems, he said.
The particles offer a new way to do "multiplexed
detection"-testing a single sample for multiple targets. In the
laboratory, a common (but expensive) multiplexing technique involves a
planar microarray-a flat surface with many spotted probes that each
test for different targets. The MIT researchers are taking this
approach away from planar surfaces onto free-floating particles.
With the tiny particles, it is much easier to
custom-design each biological test, said Doyle. "It's very easy to
tailor what you give a customer. You could have 100 types of particles
and mix them together," he said.
The researchers' particle fabrication method gives
them exquisite control over the particles' shape and chemical
characteristics.
As two streams of monomers (liquid precursors
loaded with fluorescent dye or molecular probe) flow side by side
through a microfluidic device, ultraviolet light repeatedly strikes
the streams. A chemical reaction initiated by the light causes the
liquid to solidify, forming a single particle with two distinct ends.
Each particle takes on the shape of a "mask" (similar to a
transparency film) through which the UV light is aimed.
One end of each particle is a fluorescent "dot-pattern"
barcode that reveals what the target molecule of the particle is, and
the other end is loaded with a probe and only turns fluorescent if the
target molecule is present. The particles can also be designed to each
test for multiple targets, by adding several unique regions.
"We can make the particles, encode them and add
functionality all in a single step," said Pregibon.
When a mixture of particles is added to a test
sample, target molecules (DNA, proteins, etc.) will bind to the region
of the particles containing the corresponding probe. This interaction
can be detected by fluorescence, which is brighter when more of the
target is present.
To rapidly "read" the particles, the researchers
designed a custom "flow cytometer" using a microfluidic device and
standard microscope. In this flow-through system, the oblong,
disk-like shape of the particles ensures that they are precisely
aligned for accurate scanning. Each time a particle flows past a
detector, its barcode is read and the corresponding target is
quantified.
The microparticles are inexpensive because they can
be produced efficiently in a single step. The design of the particles
also makes the scanning devices cheaper. With multiple distinct
regions, the barcode can be read and the target quantified using a
single fluorescent color, which greatly simplifies detection.
The particles are also unique in that they are made
of a spongy polymer "hydrogel" called poly(ethylene glycol). That
polymer enhances the sensitivity of the test because it is porous,
allowing the target molecules to diffuse into it.
For the Science paper, the researchers created
particles with DNA probes attached at one end. They demonstrated that
the particles could accurately and reproducibly detect the presence of
multiple target DNA sequences, and they anticipate similar results
with RNA, proteins and cytokines.
The researchers are focusing on bedside diagnostics
and "theranostics"-the emerging concept of providing personalized
diagnostic therapy. This method for tailoring therapies to each
patient could be a breakthrough for treating diseases like cancer and
cardiovascular disease. The particles could also be used to
genetically profile individual patients and screen for bioterrorism or
other hazardous environmental agents. |
