Biomedical Engineering Reference
In-Depth Information
Figure 10.11
Polarizable particles attracted toward high electric field region in positive DEP
regime.
to these areas forcing the particles to be repelled from them): it is the nega-
tive DEP case where the particles appear to be driven towards the low elec-
tric field areas.
·
Another interesting point is that
F
~
ÑE
2
. This dependence with the square
of the amplitude of the electric field implies that DEP can be used with dc or
ac fields. In the first case however, electrophoresis will compete with DEP in
the motion of the particles. The use of high-frequency ac fields is particularly
interesting because it suppresses electrolysis or, more generally, electrochem-
istry at the surface of the electrodes.
·
The force depends on the gradient of the electric field intensity. Therefore,
to transport particles over large distances, it would be necessary to maintain
a large gradient over such distances, which requires large electric fields (and
thus high voltages) in the macroscopic world. On the other hand, large lo-
cal gradients are more easily created in microstructures (small scales). This
explains why, with the development of microfabrication and its use in life
science, this effect has been effectively rediscovered recently to manipulate,
characterize, or sort particles. However, to transport them over large dis-
tances remains a challenge. As we will see later in the text, a traveling wave
or a succession of elementary displacements triggered by gradients over
small scales can overcome this difficulty.
One last word before detailing the consequences of these expressions. The DEP
traps are formed and conditioned by the geometry of the electrodes that are
in the
solution
(except for the electrodeless DEP detailed further in this chapter). This is
markedly different from electrophoresis where the electrodes are physically placed
out of the channel (although in electrical contact with it). Working with high fre-
quencies minimizes the electrochemistry at the surface, however, we will see in
particular in Section 10.2.6.2 that it cannot avoid electrohydrodynamic flows or a
direct contact of metal electrodes with the objects to be manipulated. This last point
