Biomedical Engineering Reference
In-Depth Information
may overlap. An ideal system would have 10 to 20 particles passing through the various
portions of the fringe pattern at any instant in time (e.g., you do not want all of the parti-
cles at the center of the pattern, but distributed throughout the pattern). Also, the use of
forward scattering can collect the most data from the system, but this may not always be
attainable based on the geometry of the device of interest. Back- or side-scattered fringe
patterns typically lose most of the information from the pattern because the intensity of
these scatterings is not as large as forward scattering.
An improvement over laser Doppler systems is a dynamic laser Doppler light-scattering
velocimetry technique. Here, the total intensity of the scattered light is measured as well
as the fringe pattern distortion. The intensity is typically averaged over time so that some
of the time-dependent fluctuations induced by differences in particle size, location, or scat-
tering properties can be removed. Without going through all of the mathematical equa-
tions that are required to smooth the intensity signals of a distorted fringe pattern, this
technique can provide a direct measurement of fluid velocities based on time-average
values of scattered intensities. This is a better collection method than laser Doppler veloci-
metry, but it is more difficult to implement because of the cross-correlation that is needed
between particles.
14.3 FLOW CHAMBERS: PARALLEL
PLATE/CONE-AND-PLATE VISCOMETRY
Many cells that we have discussed so far in this textbook are subjected to shear stress in
their natural environment (endothelial cells, epithelial cells, and all blood cells, among
others). Therefore, there is a need for an accurate experimental technique that can be used
to expose cells to precise physiological shear stresses. There are two types of flow cham-
bers that are typically used in labs, and they are the parallel plate-based flow chambers
and the viscometer-based flow chambers.
Parallel plate-based systems use a pressure gradient to drive the fluid motion through
the system. The most commonly used is termed the parallel plate flow chamber
( Figure 14.3 ), which uses a driving force to push fluid through a rectangular channel.
Cells can be cultured on the bottom surface of the flow chamber or can be seeded within
the fluid itself. Typically,
these devices are designed so that
the velocity is fully
FIGURE 14.3
Fluid flow
Fluid flow
Schematic of a parallel plate
and a radial parallel plate system. In both of
these systems, cells (shown as black ovals) can
be adhered to the bottom of the plate or can be
seeded within the fluid. The shear stress can be
modulated based on the fluid properties and
the channel dimensions, and therefore, many
different physiological
conditions
can
be
investigated.
Parallel plate
Radial parallel plate
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