Sorting Matter with Tiny Fingers of Light

EMBARGOED FOR RELEASE AT 1:30 P.M. CST MARCH 5

University of Chicago physicists can use multiple beams of light to selectively sort microscopic particles, biological cells and large molecules, they will report Wednesday, March 5, at the American Physical Society meeting in Austin, Texas.

Manipulating these beams of light has led to one of the newest techniques in microfluidics, the science of transporting fluids through networks of miniature channels. University of Chicago Physics Professor David Grier calls the new technique "optical fractionation," because it involves using light to sort one fraction of objects from another.

"It's a new approach based on very old principles," Grier said. "If you've seen the tiles on a Spanish roof that direct rainwater, in some respects this operates on the same principle."

The potential applications of optical fractionation include routine medical testing, pharmaceutical research and the development of entirely new biotechnological markets. The principles underlying optical fractionation were discovered by Grier and Pamela Korda, who received her Ph.D. in Physics at Chicago in 2002, with assistance from Michael Taylor, a participant in the University's Research Experiences for Undergraduates program. The technique now is being developed in collaboration with Physics graduate student Kosta Ladavac and fourth-year Physics concentrator Karen Kasza with support from the National Science Foundation.

Optical fractionation is based on holographic optical tweezers technology. HOT technology uses forces exerted by strongly focused, computer-generated holograms to create very large arrays of discrete optical traps. Each trap is capable of suspending a microscopic object motionless in three dimensions.

Using HOT technology is like having microscopic hands. "You can really reach in, move things relative to each other, orient them, bring them together, take them apart, and, to an increasing degree, transform them," Grier said.

The technology is based on the principle that dielectric particles-those that do not conduct electric current- experience forces that draw them to where the light is brightest. Grier and his group so far have received two patents on the technology. His team continues to extend the technology's applications and now has 20 patents pending both domestically and abroad. HOT technology led to the founding of Arryx Inc. and its development of the BioRyx(tm) 200 system, which R&D Magazine selected as one of the 100 most technologically significant products of 2002.

With optical fractionation, forces exerted by flowing fluid drive the objects of interest through an array of optical traps created by the HOT technology. Depending on the competition between the driving force and the forces exerted by the optical traps, the objects can either continue in their original direction or get deflected into a new direction that is dictated by the asymmetry of the array.

Optical fractionation is complementary to and more flexible than existing techniques such as gel electrophoresis, Grier said. Almost all of the existing techniques involve a competition between two forces that act in opposite directions along the same axis. In the case of gel electrophoresis, an applied electric field drives objects such as DNA in one direction, while the viscous drag of the gel opposes that motion.

"You put your sample down, you apply the force, the sample spreads out along the direction of the force and eventually you have to stop and collect your sorted products," Grier said. Not so with optical fractionation, which deflects the selected fraction away from the direction of the optical force and so operates continuously.

One also can adjust the laser wavelength and power and the trap geometry, allowing the traps to instantly sort objects ranging in size from less than 100 nanometers (the size of the Human Immunodeficiency Virus), to near 100 micrometers (the diameter of a human hair).

"All you change is the latticework of light, which results from a computer-generated hologram," Grier said. "What you're changing is one line of software."

Arryx, the Chicago company based on Grier's HOT technology, already has conducted an analysis of the existing cell fractionation market, said Ken Bradley, a founder and Chief Operating Officer of Arryx.

"There are existing companies using different techniques that sell tools for cell sorting, and those markets are in excess of a billion dollars a year," Bradley said. "You would not displace that entire market, but those are the sorts of numbers you get."

Optical fractionation also is one element needed to transform microfluidics from a plumbing system to a factory, Bradley said. One example of the latter would be using HOT technology to produce labs-on-a-chip.

The idea is to make medical testing faster and less expensive than using a full-scale laboratory with human technicians.

With a lab-on-a-chip, Grier said, "you would just squirt some sample in, push a button and the chip would do all the work. It would shuttle all the chemicals around, do all the testing, then report the results through some sort of human interface. Optical tweezers are one way to create structures that actually do the analysis."