Biomedical Engineering Reference
In-Depth Information
Figure 10.13
Numerical calculation of trapping of particles in an octopole geometry (from [42]).
geometries where the electrodes themselves are shaped in three dimensions: indeed
the instabilities are much reduced in this geometry (see Section 10.2.7.2)).
There is, however, a way to avoid some of these problems by using the elec-
trodeless DEP and although it is not as flexible as the usual electrode-based meth-
ods, it offers enough advantages to be favorably considered in some circumstances
[43-45].
This technique is based on insulating channels having constrictions. If one ap-
plies an electric field between the entrance and the exit of the channel, the field lines
have to squeeze in the constrictions creating therefore high gradients. It is thus an
easy way to use the DEP effect only by structuring the channel. No electrochemistry
is involved and most of the instabilities disappear. This has a cost however: using
this geometry, there is only one electric field that controls the displacements of the
particles in the channel. Therefore, all the tasks have to be performed sequentially
in contrast with in situ electrodes-based DEP in which many traps can be controlled
independently in a parallel way (Figure 10.14).
10.2.4 Characterization of Particles
10.2.4.1 Real-World Particles
Of course, particles that need to be manipulated or separated particularly in bio-
micronano technology are generally not solid homogeneous spheres. They can actu-
ally be extremely diverse and go from DNA molecules [46] to cells [47] or viruses
[48]. They are usually modeled by an effective sphere although multishell models
may be more accurate to account for some of the particle characteristics, to the price
of a higher number of parameters. More than absolute quantitative measurements,
studies on complex particles are often relative between slightly different systems.
10.2.4.2 Collection Rate
Now that we know how to apply a force to a particle, and, actually design the
energy well in which this particle is immersed, we can use this knowledge to char-
