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Inexpensive, portable devices
that can rapidly screen cells for leukemia or HIV may soon be possible thanks
to a chip that can produce 3D focusing of a stream of cells, according to
researchers.
“HIV is diagnosed based on
counting CD4 cells,” said Tony Jun Huang, associate professor of
engineering science and mechanics at Penn
State University.
“Ninety percent of the diagnoses are done using flow cytometry.”
Huang and his colleagues designed
a mass-producible device that can focus particles or cells in a single stream
and performs three different optical assessments for each cell. They believe
the device represents a major step toward low-cost flow cytometry chips for
clinical diagnosis in hospitals, clinics, and in the field.
“The full potential of flow
cytometry as a clinical diagnostic tool has yet to be realized and is still in
a process of continuous and rapid development,” the team said in Biomicrofluidics. “Its current high
cost, bulky size, mechanical complexity, and need for highly trained personnel
have limited the utility of this technique.”
Flow cytometry typically looks at
cells in three ways using optical sensors. Flow cytometers use a tightly
focused laser light to illuminate focused cells and to produce three optical
signals from each cell. These signals are fluorescence from antibodies bound to
cells, which reveals the biochemical characteristics of cells; forward
scattering, which provides the cell size and its refractive index; and side
scattering, which provides cellular granularity. Processing these signals
allows diagnosticians to identify individual cells in a mixed cell population,
identify fluorescent markers and count cells and other analysis to diagnose and
track the progression of HIV, cancer, and other diseases.
“Current machines are very
expensive, costing $100,000,” said Huang. “Using our innovations, we
can develop a small one that could cost about $1,000.”
One reason the current machines
are so large and expensive is the method used to channel cells into single file
and the necessary alignment of lasers and multiple sensors with the single-file
cell stream. Currently, cells are guided into single file using a delicate 3D
flow cell that is difficult to manufacture. More problematic is that these current
machines need multiple lenses and mirrors for optical alignment.
“Our approach needs only a simple
one-layer, 2D flow cell and no optical alignment is required,” said Huang.
Huang and his team used a
proprietary technology named microfluidic drifting to create a focused stream
of particles. Using a curved microchannel, the researchers took advantage of
the same forces that try to move passengers in a car to the outside of a curve
when driving. The microfluidic chip’s channel begins as a main channel that
contains the flow of carrier liquid and a second channel that comes in
perpendicularly that carries the particles or cells. Immediately after these
two channels join, the channel curves 90 degrees, which moves all the cells
into a horizontal line. After the curve, liquid comes into the channel on both
sides, forcing the horizontal line of cells into single file. The cells then
pass through a microlaser beam.
An advantage of this microfluidic
flow cytometry chip is that it can be mass-produced by molding and standard
lithographic processes. The fibers for the optical-fiber delivered laser beams
and optical signals already exist.
“The optical fibers are
automatically aligned once inserted into the chip, therefore requiring no bulky
lenses and mirrors for optical alignment,” said Huang. “Our machine
is small enough it can be operated by battery, which makes it usable in Africa and other remote locations.”
The researchers tested the device
using commercially available, cell-sized fluorescent beads. They are now
testing the device with actual cells.