Biomedical Engineering Reference
In the following, we will focus on techniques suitable for on-the-flow analyte
characterization. Concepts relying on specific cell capturing, for example, in
functionalized microfluidic devices with automated electrical or imaging detection
will not be addressed.
Over the past decade, many concepts have been developed to simplify and
miniaturize on-the-flow analyte characterization by utilizing microfluidic channels
that integrate fluidic handling (e.g., pumping, valves), on-chip sample preparation
(e.g., mixing), particle manipulation (e.g., flow focusing, on-chip sorting), and
miniaturized optics [ 7 , 8 ].
Several recent review articles give an excellent overview of microfluidic flow
cytometer technologies [ 9 - 11 ]. We will concentrate on their fluidic handling and
how this is relevant for POC flow cytometers.
Conventional flow cytometers use sophisticated flow cells which allow for
hydrodynamic flow focusing, in which large amounts of sheath flow are used
to confine the analyte particles into a narrow stream. Flow focusing guarantees
a constant speed for all particles and prevents sticking to the flow cell wall.
A major goal driving the development of microfluidic flow cytometers is to reduce
overall size of the instrument and the required amount of analyte and sheath
fluid. Microfabrication techniques can be used to realize low-cost and miniaturized
flow chips; however, in order to enable analyte focusing with little to no sheath
fluid, new concepts have to be implemented. Due to the parabolic flow profile in
microfluidic channels, it is essential to either confine the particle path in order
to ensure uniform particle velocity or to implement detection schemes which
can handle the large velocity distribution. Many concepts have been suggested to
achieve analyte focusing to align the particles in microfluidic channel. They include
inclusion of mechanical structures in flow channels [ 12 , 13 ] and the use ultrasound
effects [ 14 ], to confine and align the cell in microfluidic channel. A very interesting
approach uses the inertia of the fluid acting on particles in shaped microchannels
to enable precise cell positioning in the stream [ 15 - 19 ]. This technique allows for
sheathless positioning and can be used to concentrate particles to a focused position.
Moreover, it has been suggested that this approach also evenly spaces cells and
particles along the direction of flow and potentially minimizes coincident detection
[ 20 ]. Unfortunately, the inertial focusing depends on particle properties (e.g., size,
shape), dimensions of the channel, and process parameters (e.g., particle speed).
For instance, smaller particles need a longer distance to reach their stable positions
in microfluidic channels. Consequently, this concept does not represent a general
solution for sheathless analyte focusing in microfluidic-based flow cytometer. Even
though designed and optimized for a specific application, this approach might be
a very good solution. Using curved microfluidic channels combined with two-
dimensional sheath flow provides a solution for particle focusing which is less
elegant but also less critical [ 21 ]. The inclusion of chevron-shaped mechanical
structures at the channel wall is another very interesting concept for hydrodynamic
focusing in microfluidic channels [ 22 ]. The chevrons cause the sheath fluid provided
from one or two sides to embed the analyte stream from all sides.