Biomedical Engineering Reference
In-Depth Information
10.2.7 DEP-Based Separations
10.2.7.1 Combining DEP and a Hydrodynamic Flow
In contrast with electrophoresis, and except for traveling-wave DEP, DEP is thus a
trapping and an analytical tool. Therefore, to achieve any kind of separation with
this concept, another flow should be added. In that sense, DEP based separations
are very similar to affinity chromatography where a mixture is brought in contact
with a matrix by a flow in a column. Some of the components of this mixture hav-
ing more affinity to the matrix are then trapped on it while the other components
can flow with the solvent. The trapped species is then eluted in a second time by
changing the solvent (for instance, its pH) or by adding a molecule having an even
higher affinity for the matrix.
DEP is the equivalent of a “smart” matrix in the sense that the affinity of par-
ticles for the traps can be tuned externally by changing the electric field characteris-
tics. However, it still needs an eluant to flow the particles on the electrodes.
The simplest idea that comes to mind is to flow the particles on the energized
electrodes. By adjusting the characteristics of the electric field, one component of
the mixture is trapped on the electrodes while the others flow with the liquid. There
are quite a few examples of this approach that use different geometries for the elec-
trodes or/and sequences of flow in one or two directions [58-60].
Another way of achieving a good separation of particles is to combine DEP and
gravity in the field flow fractionation. This is the coupling between DEP levitation
and a Poiseuille flow well-controlled by a difference of pressure in a microfluidic
channel [61, 62]. The height at which the particles levitate is given by (10.27). As
the Poiseuille flow is characterized by a parabolic velocity profile, the fluid velocity
is different for each height and particles of different dielectrophoretic characteristics
(different Re(
f
CM
)) will travel at different speeds which results ultimately in differ-
ent elution times.
10.2.7.2 Dielectrophoretic Ratchets
A good illustration of the use of DEP is the dielectrophoretic ratchet. This experi-
ment can be declined in two versions (the Brownian ratchet or the shifted ratchets)
that we briefly describe here.
Brownian Ratchet
The theoretical models on which these particular experiments are based are de-
scribed in [63]. Reference [64] provides a very complete review on these “force-
free” motion phenomena. Let us imagine Brownian particles in a potential similar
to the one described in Figure 10.19. They are trapped in the minima of this po-
tential and, if the potential barriers are large enough, which is supposed here, their
concentration profiles are very narrow and centered close to these minima (Figure
10.19(b)). Now, let us switch off this potential (i.e., we now impose to the particles
a flat potential), the particles are going to freely diffuse and, as a consequence, the
concentration peaks will broaden (Figure 10.19(c)). After a time
t
off
, we cycle back
to the saw-tooth potential (Figure 10.19(d)). The particles are again trapped in
the minima of the potential. If
t
off
is sufficiently large a nonnegligible fraction of
