Wide laser makes simple tweezers

By Eric Smalley, Technology Research News

Much 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 laser tweezers.

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 beam.

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," said Applegate.

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.

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
Funding:   Government
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

Page One

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Cooler material boosts fuel cells

Pen stroke cuts PDA Web clutter

Wide laser makes simple tweezers

Biochip levitates droplets
Nanotubes form transparent film
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Mechanical valve design goes nano
Angles increase optical storage
Crystal links ultraviolet photons


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