Dynamic tweezers sort cells

23 Jul 2002

Holographic optical tweezers create a 3D array of hundreds of individual optical traps.

Using computer-generated holograms, US scientists have transformed a single laser beam into hundreds of independent optical traps. The team from the University of Chicago believes its dynamic system, which can arbitrarily trap particles in three dimensions, will open up research activities in areas such as cell sorting and microfluidics. (Optics Communications 207 169)

"Our largest fully functional array comprises nearly 2000 traps in a plane, and more than 400 traps in a three-dimensional orthorhombic crystal" research leader David Grier told Optics.org. "We've also created dynamically reconfigured arrays of optical vortices for microfluidic applications."

To make these arrays, Grier's group uses a liquid-crystal spatial light modulator (SLM) to reshape the beam from a Nd:YVO₄ laser operating at 532 nm. A sequence of computer-designed phase holograms then reconfigures the beam into a pattern.

This pattern of beams is then relayed into an inverted microscope where 1 mW of power holds one particle in each trap. Increasing this to 10 mW per trap transports the trapped particles in 2 micron-steps at a speed of 10 microns per second.

"We are limited by the refresh rate of the SLM and its ability to encode the information required to make the traps," explained Grier. "We could take larger steps, but then we'd risk losing our trapped particle."

Using their system, the researchers trapped a variety of colloidal particles, such as latex and graphite, and biological specimens, such as red blood cell phantoms and chromosomes. "We believe we have trapped ten particles in ten different planes with a maximum separation of 80 microns," said Grier.

Now the team is turning its attention to sorting particles. According to Grier, any array of traps can be used to sort fluid-borne materials. "We call the resulting process optical fractionation," he said. "It allows us to sort particles, cells and other materials on the basis of their differing affinities for optical traps."

The group is also using this principle to study transport problems where particles are driven across a modulated potential energy landscape by external forces. "We can create arbitrary potential landscapes with light," explains Grier. "Our system may be the first in which we can continuously vary all the control parameters while monitoring the microscopic transport processes."

Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.