Biomedical Engineering Reference
In-Depth Information
results are expressed as the adherent fraction, which is the ratio of post-spun
to pre-spun cell numbers. This adhesion assay applies relative low detach-
ment forces (< 10
- 3
dynes/cell) and is therefore limited to short attachment
times (typically < 60 min). For longer adhesion times, cellular attachment
strength often exceeds the maximum centrifugal force, and the assay loses
sensitivity.
The major limitation of centrifugation assays is that each experiment is
limited to the application of only a single force, and multiple experiments
at different speeds are required to obtain mean adhesion strength values,
typically defi ned as the centrifugal force that produces 50% detachment.
However, for the study of integrin-mediated cell adhesion, it has been dem-
onstrated that short-term cell adhesion may be analyzed as a function of
ligand density at a single centrifugation speed as an alternative to conduct-
ing multiple runs at different speeds (Keselowsky
et al.
, 2003; Reyes and
Garcia, 2003). For a fi xed centrifugal force, the fraction of adherent cells
increases in a sigmoidal fashion with ligand density, and shifts in this adhe-
sion profi le refl ect differences in adhesion strength.
3.3.3 Hydrodynamic fl ow
Hydrodynamic fl ow encompasses a variety of devices that deliver laminar
fl ow over adherent cells to generate detachment shear forces. These systems
allow the application of a wide range of detachment forces to a large cell
population and generally provide the most reliable measurements of adhe-
sion strength. The adhesion strength is typically reported as the shear stress
(force/area) at the fl ow chamber wall (τ
w
) that produces 50% detachment.
While τ
w
is a useful measure of the detachment force, the net applied force
results from the hydrodynamic drag and torque on the adherent cells. It
must be stressed that these parameters are highly sensitive to cell shape
and the size and position of the cell-substrate contact points (Elineni and
Gallant, 2011; Gallant and Garcia, 2007; Gallant
et al.
, 2002, 2005; Hammer
and Lauffenburger, 1987; Xiao and Truskey, 1996). Therefore, these geo-
metrical effects must be accounted for when differences in cell spreading
or focal adhesion assembly are expected because τ
w
will not adequately
describe the actual detachment force applied to the cell.
Hydrodynamic fl ow systems require the use of specialized fl ow cells
that are categorized according to the fl ow confi guration. The basic geom-
etries are:
parallel plate
rotating disk
radial fl ow between parallel disks.
