Twin-beam optical trapping spins cells
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
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
Applied Physics Letters, March 27, 2006, 131103.
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