Miniature 2-D flow cytometer
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
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
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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