Wide laser makes simple tweezers
Technology Research News
of medical diagnostics and biomedical research involves trapping, manipulating
and sorting individual cells and like-sized bits of matter.
One way to move cells is via laser tweezers, which work because
the light lasers emit has enough momentum to share objects that are as small
as cells. Laser tweezers consist of laser beams moved rapidly by mirrors
or laser beams spread into intricate patterns by computer-generated holograms.
They can move one or more microscopic objects with great precision,
but use relatively complicated optics, including rapidly adjustable mirrors
and computer-controlled liquid crystal displays.
Researchers from the Colorado School of Mines have demonstrated
a simple way of manipulating cells that promises to be less expensive than
The researchers use a diode laser bar that emits light in a 100-micron
wide by 1-micron thick laser beam. Diode laser bars contain arrays of tiny
lasers that are made from light-emitting semiconductor materials like gallium
arsenide. A micron is one thousandth of a millimeter.
The researchers demonstrated the technique by aiming this broad
beam of light perpendicularly through a tiny channel carrying fluids. The
light formed a 100-micron wide barrier to cells and particles within the
fluid because transparent spherical objects that bend light more than the
liquid around the objects are drawn to the focal point of an intense light
By angling the beam and/or blocking portions of it, the researchers
were able to trap and move cells at will. "We demonstrated that we could
move particles anywhere in the 100-[micron] line by simply allowing certain
parts of the beam through to the sample," said Robert Applegate, a researcher
at the Colorado School of Mines.
When the beam from the researchers' device is at an angle to a flow
containing cells, the force of the trap plus the force of the flowing fluid
drives the cells along the beam until they are pushed to the opposite side
of the channel. Blocking the beam from the area containing the cells releases
the cells, which makes it possible to return cells to the flow at any point
along the ribbon, he said.
The method can be used to sort cells by taking advantage of laminar
flow. In small fluid channels, particles moving within in a given portion
of the flow stay in that streamline until something forces them out of it.
Cells in fluid could be flowed into a microfluidic device that has multiple
output channels, and the researchers' optical trap could be used to direct
specific cells to particular channels. "Since we can control which streamline
each cell is in by where we release it from the trap, we can sort the cells,"
The researchers are working out the parameters they will use to
sort cells, said Applegate. "We could sort cells based on many different
parameters," he said. Cells are typically sorted by size or by the presence
of fluorescent marker chemicals.
The lasers and optics needed for the researchers' method would make
for more affordable cell-sorting machines than current cell sorters, said
Applegate. Cell sorter equipment ranges in price from tens to hundreds of
thousands of dollars. "The lasers we are using, as well as the optics, are
much [less] expensive," he said.
The researchers are working on methods of multiplexing, or splitting
the laser beam to make multiple optical traps. "Since we work well below
the power limits of the laser, we will be able to use a single laser to
run multiple cell sorters simultaneously," he said.
The method could be used practically in two to five years, said
Applegate's research colleagues were Jeff Squier, Tor Vestad, John
Oakley and David W. M. Marr. The work appeared in the September 20, 2004
issue of Optics Express. The research was funded by the National
Institutes of Health (NIH).
Timeline: 2-5 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Microfluidics and BioMEMS; Biotechnology
Story Type: News
Related Elements: Technical paper, "Optical Trapping, Manipulation,
and Sorting of Cells and Colloids in Microfluidic Systems the Diode Laser
Bars," Optics Express
October 20/27, 2004
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