Optoelectronic Tweezers Manipulate Microparticles

In the July 21 issue of Nature, researchers from the University of California, Berkeley, report the development of a novel technique for the manipulation of microscopic objects by electrokinetic forces using a photoconductive sample cell. The approach enables the dynamic control of single objects or of multiple objects in parallel, and it may have applications in cell sorting or in the growth of materials from colloidal suspensions.

The micromanipulation system features a sample cell defined by an upper ITO-coated glass electrode and a lower photosensitive surface constructed of layers of doped and undoped amorphous silicon and silicon nitride atop ITO-coated glass. The cell is biased with an AC signal. Exposing the photoconductive surface to 625-nm light creates a "virtual" electrode and attracts or repels particles depending on their properties. Because an optical intensity of only 10 nW/µm² is required to create a virtual electrode, the output of a simple, incoherent source -- a Lumileds LED in the experiments -- is sufficient. The addition of a 1024 × 768-element Texas Instruments micromirror-based spatial light modulator and an objective lens to the setup enables the creation of complex, high-resolution patterns of micron-scale virtual electrodes.

To demonstrate the potential applications of the technique, the scientists sorted live and dead cells, manipulated 4.5-µm polystyrene beads in a 15,000-trap array and produced a virtual sorting machine that separated 10- and 24-µm beads.