Biomedical Engineering Reference
In-Depth Information
Parallel plate
Among the various devices, the parallel plate fl ow chamber has been the
most extensively used to examine cellular phenomena, including cell adhe-
sion (Doroszewski
et al.
, 1977; Lawrence
et al.
, 1987; Truskey and Pirone, 1990;
van Kooten
et al.
, 1992). As described above, when combined with a micro-
scope, this confi guration allows direct observation of the attachment process,
and the fl ow conditions can be readily validated. Variations of this fl ow device
have also been used to characterize cell detachment and adhesion strength.
The wall shear stress depends on the fl uid viscosity, volumetric fl ow
rate and device dimensions. Since a constant shear stress is generated for
a given fl ow rate in a standard fl ow chamber, several experiments must
be conducted at different fl ow rates to fully characterize cell adhesion as
a function of applied force. In addition, the maximum detachment forces
generated under well-characterized fl ow regimes are often insuffi cient to
detach well-spread cells.
To address these limitations, minor modifi cations have been made to the
standard parallel plate geometry to produce a range of shear stresses that
varies over the length of the chamber. This has been achieved by construct-
ing devices with variable dimensions. For example, a chamber has been
fabricated in which the height varies linearly along the length (Burmeister
et al.
, 1996; Xiao and Truskey, 1996). Another device produces a linear gradi-
ent in shear stress by varying the chamber width inversely with the distance
from the inlet (Powers
et al.
, 1997; Usami
et al.
, 1993).
Rotating disk
Rotating disk devices include the single spinning disk and the small-gap
parallel disk viscometer. These specialized fl ow chambers apply forces that
vary linearly with radial distance, allowing the application of a range of
detachment forces to a large cell population in a single experiment (Garcia
et al.
, 1997; Horbett
et al.
, 1988; Mohandas
et al.
, 1974; Pratt
et al.
, 1988; Weiss,
1961). In addition to the linear dependence on radial position, the shear
stress is also a function of the fl uid density, fl uid viscosity, and the angular
velocity of the disk. This confi guration is unique in that the substrate is put
in motion to generate the detachment force.
In a typical experiment, a substrate containing uniformly seeded cells
is spun at a constant speed and adherent cells are counted at specifi c
radial positions corresponding to a known range of shear stress values. As
expected from a simple probabilistic model, the fraction of adherent cells
decreases non-linearly with shear stress and this profi le is used to calculate
the shear stress for 50% detachment (τ
50
), which represents the mean adhe-
sion strength. This system has been used to develop quantitative analyses of
