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Miniature 2-D flow cytometer

May 2006
Raquel Harper

Conventional flow cytometers are large and expensive and usually provide one-dimensional data, but two-dimensional data is needed for detecting subtle differences in the morphology of cells. Researchers from the University of Alberta in Edmonton, Canada, are developing a miniaturized wide-angle 2-D flow cytometer that they believe will be less expensive than current lab-based systems for medical diagnostics and that will perform better.

Two-dimensional data can reveal small changes that one-dimensional data cannot. For example, certain diseases cause the size of mitochondria to change, and 2-D information can show changes in these organelles, which are smaller than the wavelength of light. However, to get 2-D, high-resolution data from a traditional flow cytometer requires a photodetector mounted on a movable arm or a number of photodetectors mounted at different positions, and it usually takes a long time to move from one angle to another to detect all the essential scattering information.

Instead, the researchers created an optical-waveguide-based cytometer using a glass prism, a microfluidic chip, a glass hemispherical lens from Edmund Industrial Optics of Barrington, N.J., and a digital 2-D CCD camera from Nikon (Figure 1).

Figure 1.
The experimental setup of the miniature cytometer shows that the waveguide is excited in the microchannel by prism coupling, and the scattered light, which exits via the hemispherical lens, is imaged by a digital camera. Images reprinted with permission of Cytometry.

To test their system, they used latex beads as cell mimics. The chip consisted of two microscope slides separated by a distance large enough to allow the beads to flow freely. They used a prism to couple light from a HeNe laser from JDSU of Mantaca, Calif., into the channels, which also acted as liquid-core waveguides. The optically leaky nature of the waveguide allows excitation by prism coupling.

When light hit a bead flowing in the channel, it scattered, some of it upward. The hemispherical lens collected the light in two dimensions across a wide angular range, and the CCD camera imaged it. The investigators found that their lens provided light scatter between 140° and 180° for 9.6- and 4.0-μm-diameter latex beads (Figure 2).

Figure 2.
Two-dimensional scatter is shown here from a 9.6-μm latex bead.

Although they found that their cytometer could collect light over a large angular range in 2-D, they would eventually like it to collect scattered light from 0 to 180°, and thus obtain all the information on cell morphology that is contained in scattering data.

Group leader Kirat Singh explained that, because the microfluidic channel and the waveguide are one and the same, the researchers can use lasers with much lower intensity than a conventional cytometer would require and get the same signal intensity. The intensity of light is very high in the channel, so they can excite a very high level of scattering from the cells using a lower-power laser.

“In fact, the waveguide acts as an intensity amplifier so that the intensity within the liquid-waveguide core may be higher than the intensity of the wavelength itself,” Singh said. He explained that the low-power lasers help make the miniature system less expensive than current flow cytometers.

The researchers are working on making the system more powerful and capable of collecting scattered light across an even larger angular range. They also hope to come up with computer software that will analyze the scattering data and provide them with information about the cell’s size, shape and contents.

Cytometry, April 2006, pp. 307-315.

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