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Twin-beam optical trapping spins cells

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Lynn M. Savage

Detecting the causes of cancer at its earliest stages would be of great help in fighting the disease. However, the sooner one tries to see the beginnings of tumor development, the less change is visible in a cell’s components and in their arrangement within the protoplasm.

Various forms of spectroscopy help identify the proteins and other materials inside a cell and could help researchers uncover the roles that cellular components play in turning a healthy cell into a cancerous one. To do so, spectroscopic techniques must have access to all of the molecules throughout the cell, not just those scannable in a top-down view.

Now investigators at the Takamatsu and Kita-gun campuses of Kagawa University in Japan have proposed a technique that enables 3-D spectroscopic characterization of cells. Led by Ichirou Ishimaru of the university’s faculty of engineering, they used an optical trapping method comprising two laser beams — one positioned above a cell, one below it — to precisely spin the cell on two axes. They spectrographically scanned trapped liver or breast tumor cells in two dimensions, spun individual cells along the axis perpendicular to the beam or oblique to it (or both at once), then acquired another 2-D scan.

Researchers used a pair of laser beams to optically trap — and spin in place — living breast cancer cells. Courtesy of Hiroaki Kobayashi.

The researchers used a visible-wavelength laser made by Coherent Inc. of Palo Alto, Calif., to generate twin beams that apply light pressure on opposite sides of the cell’s center of gravity, about 10 μm apart. The amount of light pressure that is generated depends on the beam’s intensity; therefore, changing the laser’s power proportionally alters the rotational speed of the cell. The group used ~10 mW of power, which is low enough to be safe for the cell.

The pressure is a function of the light absorption by the protoplasm or of the reflection and refraction from the organelles. According to group member Hiroaki Kobayashi, now at the Kagawa Prefectural Industrial Technology Center in Takamatsu, each beam provides a different amount of force because the distribution of the refractive indices with the cell is not uniform.

The investigators used a custom-built microscope, employing an objective lens with a numerical aperture of 0.3 to maintain a slight trapping force while still enabling spinning, as well as a galvanometer to change the focal point of each beam, imparting control over the spin direction.

They continued to spin and scan the cell until all points within the cell were scanned and their spectra acquired — then mathematically transformed the 2-D data into a 3-D map of the cell’s proteins. According to Kobayashi, the method enables higher-resolution scanning than such cell-imaging techniques as optical coherence tomography or confocal fluorescence microscopy.

The researchers are developing a spectroscopic technique — called variable contrast-phase spectrometry — that works with the cell-rotation process to provide a 3-D spectrographic map of a cell’s contents.

Applied Physics Letters, March 27, 2006, 131103.

Jun 2006

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