parts paint picture
Technology Research News
Probes that go to the depths of Earth's
oceans and to planets like Mars beam back information about the topography
and geography of places we can't visit in person. Researchers at the University
of Washington have come up with probes hundreds of times smaller than
a blood cell that can explore the nooks and crannies of objects so small
they are difficult to image.
The probes, made from a pair of biological materials that form networks
in living cells, provide higher-resolution images than conventional scanning
microscopes, according to Henry Hess, a research assistant professor of
bioengineering at the University of Washington.
In cells, the protein kinesin travels on microtubule conveyor belts to
regularly shunt biological loads like chromosomes and vesicles from the
center of a cell to its edge. Vesicles carry waste materials.
Scientists have already tapped microtubules and motor proteins to transport
tiny payloads. To use them as probes, however, the researchers reversed
the proteins' roles, unleashing a horde of about 600 microtubules on a
kinesin-coated surface. The microtubules, doped with a fluorescent substance
that makes them easy to image, propelled themselves over the kinesin to
map out terrain. Each microtubule is 1,500 nanometers long and 24 nanometers
in diameter, or about three thousand times narrower than a human hair.
The microtubules travel in straight lines and they can easily cross over
each other, said Hess. One key to their utility, however, is in a limitation
-- the proteins cannot go everywhere. They can descend into troughs on
the surface, but their structure prevents them from bending enough to
climb peaks. Where there are little hillocks, or bumps, the microtubules
cannot tread. Bald spots on microtubule maps, therefore, correlate to
bumps on the terrain.
Conventional probe microscopes gain topographical information by scanning
a surface with a tiny tip. Narrow cavities in cells or MEMS devices, however,
are difficult for scanning tunneling and atomic force microscopes to access.
These could be characterized by the protein probes, said Hess.
Instead of scanning a fine tip along straight lines, the researchers'
probes are like "hundreds of critters running across the surface [sending]
you very simple information, which you integrate to get a complete picture,"
"Imagine an ant crawling across a surface," said Hess. A single ant moves
randomly across a surface and covers only a small area. If there are hundreds
of ants crawling around, however, "nothing on the surface will escape
their attention," he said. "If there is a puddle, the ants will not go
in there, so all the paths will go around the puddle."
A time-lapse recording of all ant movement would show an accurate picture
of where the ants went as well as what areas they avoided, like the puddle.
"In our case the ant is a microtubule [moving] across a surface coated
with kinesin," said Hess.
The crisscrossing microtubules travel at 250 nanometers per second and
are imaged every 5 seconds. The composite view becomes more detailed with
time and a complete picture emerges after about 40 minutes. The researchers
found the best image when they used every other frame, comparing the position
of the microtubules every ten seconds. "The microtubules need at least
6 seconds to get to a completely new spot, otherwise the tail [of the
protein] is where the tip has been before," said Hess.
Microtubules could also be used to image biological or radioactive contamination,
"which you donít want to touch with a tip," said Hess. They could be made
to sense more specific details, such as the magnetism of a surface or
its pH level, by coating it with, for instance, a pH sensitive dye, according
They next plan to make the microtubules maps more detailed, including
information on the height of the hillocks, said Hess.
Hess's research colleagues were John Clemmens, Jonathon Howard, and Viola
Vogel. They published the research in the February 13, 2001 issue of the
journal Nano Letters. The research was funded by NASA.
Timeline: 5 years<
TRN Categories: Biotechnology; Materials Science and Engineering;
Story Type: News
Related Elements: Technical paper, "Surface Imaging by Self-Propelled
Nanoscale Probes," Nano Letters, February 13, 2002.
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