Biomedical Engineering Reference
In-Depth Information
Figure 4
. Schematic of field flow fractionation. DEP electrodes on the bottom microchannel
surface create a nonuniform electric field. The cells are levitated from negative DEP force. The
cells levitated in the center of the channel are advected faster than cells near the channel walls.
This provides a mechanism for separating cells based upon their electrical properties. Re-
printed with permission from Gascoyne and Vykoukal (5).
where
, F is the electrical permittivity, T is the electrical conductivity,
and X is the angular field frequency. In this way, the DEP force depends not
only on the dielectric properties of the particle and medium, but also on the fre-
quency of the applied field. For a sphere, the real part of
K
is bounded as -0.5 <
Re(
K
) < 1.0. Positive DEP occurs for Re(
K
) > 0, where the force is toward the
high electric field and the particles collect at electrode edges. The converse of
this is negative DEP, which occurs when Re(
K
) < 0, where the force is in the
direction of decreasing field strength, and particles are repelled from electrode
edges. Since the dielectrophoretic force scales with the cube of particle size, it is
effective for manipulating particles of the order of one micron or larger. DEP
has been used to separate blood cells and to capture DNA molecules (13,22).
DEP has limited effectiveness for manipulating proteins that are on the or-
der of 10-100 nm (3). However, for these small particles DEP force may be both
augmented and dominated by the particle's electrical double layer, particularly
for low-conductivity solutions (5).
DEP has been used to manipulate macromolecules and cells in microchan-
nels. For example, Miles et al. (13) used DEP to capture DNA molecules in a
microchannel flow. Gascoyne and Vykoukal (5) present a review of DEP with
emphasis on manipulation of bioparticles. An example of a cancer cell separa-
tion device is shown in Figure 4. Here, interdigitated DEP electrodes are fabri-
cated on the surface of a microchannel. Cells are transported through the
channel using pressure-driven flow. Negative DEP forces levitate the cells in the
microchannel at varying heights, depending on the electrical properties of the
cell. Since the velocity profile in the microchannel is parabolic, cells that are
levitated in the center of the channel advect downstream faster than cells near
j
=
1
