(324f) Simulation of Near-Size-Independent iDEP Separation Using Multiple Electric Fields

Dielectrophoresis
(DEP) is a non-linear electrokinetic force that depends on the magnitude of
gradients in the squared electric field and differences between electrical
properties of a particle and its surrounding media. For a particle immersed in
a semi-infinite media with a non-uniform electric field that is
well-approximated by a truncated multipole expansion (i.e., not too
nonlinear), the DEP force is:

where is the particle radius, is the ÒrealÓ operator, and is the so-called complex
Clausius-Mossotti factor which is dependent on the electrical properties of a
particle and its surrounding fluid. Owing to this dependence, DEP-based
techniques have shown great potential to isolate, separate, enrich, or otherwise
manipulate biological particles (e.g., cells, vesicles, DNA). Sensitivity to
particle size arises in two locations: the Clausius-Mossotti factor (for
particles with complex internal structures) and in the preceding term. The cubic dependence on particle
radius means that biological variability in size will likely dominate any
subtler Ð through much more relevant and interesting Ð electrical differences
indicative of a particular phenotype or composition of interest.

We have designed
a new dielectrophoresis separation that can reduce this primary dependence on particle
size. The technique combines
multiple DEP fields at different frequencies with an electrically insulating
constriction in channel depth. When an electric potential is applied across an insulating
constriction, the electric field is concentrated, forming a local gradient in
the electric field where the channel cross-sectional area changes. This
gradient drives DEP forces and has been used to separate particles in a number
of different systems and is typically referred to as ÒinsulatorÓ-based or
ÒiDEPÓ. The device layout is shown in Figure 1A and composed of a wide straight
microchannel with an angled constriction across its width. A pressure driven
flow is used to carry particles through the device. Electrodes are embedded
along the sidewalls on either side of the constriction, and are energized with
different electric potentials at different frequencies. The two field
frequencies are chosen based on analytical and empirical data: one where the
DEP response is sensitive to changes in parameters of interest (e.g.,
increasing cytoplasmic permittivity) and one that is opposite and insensitive
to changes in parameters of interest. The latter is typically low frequency
such that depends on membrane conductivity and
drives nDEP; the former is determined by multishell modeling or empirical data.
The ÒinsensitiveÓ or nDEP field is applied uniformly along the sidewall and
exerts a uniform nDEP force that exceeds the fluid drag force. The resulting
particle motion is parallel to the constriction and at an angle to the
direction of pressure driven flow. The ÒsensitiveÓ or pDEP field is applied to
the other electrode and Ð critically Ð increases along electrode length.
The separation depends on the sum of three forces: the uniform nDEP force, the increasing
pDEP force, and the fluid drag force. As the pDEP force increases, the sum of
the pDEP and nDEP forces decreases until the nDEP force no longer exceeds the
drag force and the cell is transported with the fluid out of the device. By
driving the magnitude and spatial variation in nDEP and pDEP forces, it is
possible to reduce the separations dependence on the drag force relative to the
DEP force. In preliminary
work, we have shown that, using a multishell model, it is possible to use this
technique to distinguish bacterial cells with membrane permittivities differing
only by a factor of two (Figure 1B).    

Key benefits of
this design are: continuous flow separation, variable sensitivity based on the
chosen electric potentials, and reduced sensitivity to particle size (Figure
4). One feature of the proposed technique is its ability to tune separation
sensitivity by changing the applied electric potentials, the fluid volumetric
flow rate, and the frequency of applied signals. Compared to other systems,
this device can operate on a wider range of cell samples and operates in
continuous flow.