Follow the herd. A light-projected pattern of shrinking concentric rings guides microparticles to the center.

Sifting with Light

A team of researchers has invented a versatile and powerful technique that uses light and electrical force to pick up and sort thousands of delicate microscopic objects at once. Such a tool has multiple biological and physical applications, such as fetal screening or assembling nanocrystals.

The new technique is actually a hybrid of two existing methods used to sort tiny objects: optical "tweezers" and dielectrophoresis, or DEP. Optical tweezers resemble a Star Trek–type tractor beam: A focused beam of light creates an optical trap that attracts and holds microscopic objects as small as a single atom. DEP achieves similar control over tiny particles by inducing a charge in them with a magnetic field and then collecting them, like a magnet picking up iron filings.

Both techniques have drawbacks, however. Optical tweezers, though versatile, can only move objects within a small area at one time, while DEP requires an extensive setup and a high-powered laser that could damage delicate cells. But by combining elements of each method, electrical engineer Ming Wu and his colleagues at the University of California, Berkeley, have invented a device that needs a much less intense light source and can be easily reconfigured for various uses.

Rather than a tractor beam or a magnet, Wu's "optoelectronic tweezers" work more like a photocopier. The light source projects an optical pattern onto a photoconductive layer, turning electrodes in the layer "on" so that they generate a DEP field. The field then draws particles or cells suspended in liquid above the slab into the desired patterns. Such high-resolution control means the device can create numerous "traps" and move thousands of particles at once. The team describes its method in the 21 July issue of Nature.

"I think this is fantastic," says David Grier, a physicist at New York University who works on holographic optical trapping. He says the technique has potential to improve the efficiency of such delicate, difficult procedures as cell sorting and the precise tailoring of nanomaterials. There's also great potential for marrying this new method with existing optical techniques that have other capabilities, such as manipulating objects in three dimensions, he adds.

Related sites
Integrated Optical movie
20um Conveyer movie
Ming Wu's page at UC Berkeley
David Grier's holographic optical trapping page