Optical Peristalsis

Part of the digestion process consists of the massaging movement of powerful esophageal muscles urging food particles along the alimentary track. The same sort of "peristalsis" can also be carried out at the nanoscopic level with small objects in the grip of cleverly crafted light pulses. David Grier and Brian Koss at the University of Chicago use the optical tweezer method of controlling particles with multiple laser beams, but instead of a static array of beams, they use computer-generated holograms to convert a single beam of light into large numbers of optical traps. Each hologram may be considered to be a specialized diffraction grating, producing intricately articulated networks of hundreds of optical traps. Objects can fall into these light traps and then the traps can be moved, thus transporting the objects.

The aim is to move and position sub-micron things in 3D space. Applications include inserting the object into a microscopic reservoir and pulling it back (parallelism is one of the technique's strengths), or centering or rotating a biological cell in a microscope's field of view. Grier's work has led to a commercial version of this holographic optical tweezers, one in which a pattern of 200 optical traps can be refreshed or modified at a rate of 100 times per second. (By the way, how forefront research is turned into saleable products is an interesting story by itself. For example, the company Grier started, Arryx, Inc.---http://arryx.com---has a scientific advisory board (SAB) with notable scientists from Princeton, NIH, the Whitehead Institute, Harvard, and Northwestern.) In the "peristalsis" mode of operation, particles are deliberately handed off from one optical trap to another, as in a bucket brigade. In a separate "thermal ratchet" mode of operation, the transfer from trap to trap might involve intervals of free diffusion; this mode should be useful for fractionating DNA molecules (see previous Update) as part of the process of sequencing a gene.

Speaking as a physicist, Grier says the most important aspect of his group's holographically generated tweezer patterns is the ability to implement time-varying potential energy landscapes for moving tiny objects in a "force-free" way. Speaking as a biophysicist, Grier points to the ability to reach into a microscopic environment and to position samples just where you want them. (Koss and Grier, Applied Physics Letters, 2 June 2003; 773-702-9176, lab website)