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Applications of Dielectrophoresis
Dielectrophoresis can be used to manipulate, transport, separate and sort different types of particles. DEP is being applied in fields such as:


 * Bioterrorism
 * Medical diagnostics
 * Drug discovery
 * Cell therapeutics
 * Particle filtration

The most effort in studying DEP has been directed towards satisfying the unmet needs in the biomedical sciences.

As biological cells have dielectric properties dielectrophoresis has many medical applications. Prototypes that separate cancer cells from healthy cells have been made. . DEP has made it possible to characterize and manipulate biological particles like blood cells, stem cells, neurons, pancreatic β cells, DNA, chromosomes, proteins and viruses.

DEP can be used to separate particles with different sign polarizabilities as they move in different directions at a given frequency of the AC field applied. DEP has been applied for the separation of:


 * Live and dead cells, with the remaining live cells still viable after separation
 * Cancer cells from blood
 * Strains of bacteria and viruses
 * Red and white blood cells

DEP can also be used to detect apoptosis soon after drug induction measuring the changes in electrophysiological properties.

Modelling of dielectric data
To determine the dielectric properties of cells such as membrane permittivity and cytoplasmic conductivity mathematical models are used on the DEP spectra data.

The simplest theoretical model is that of a homogeneous sphere surrounded by a conducting dielectric medium.

This model ignores the fact that cells have a complex internal structure and are heterogeneous.

A multi-shell model in a low conducting medium can be used to obtain information of the membrane conductivity and the permittivity of the cytoplasm.

For a cell with a shell surrounding a homogeneous core with its surrounding medium considered as a layer, as seen in Figure 2, the overall dielectric response is obtained from a combination of the properties of the shell and core.

$$ \varepsilon_{1eff}^*(\omega)= \varepsilon_2^*\frac{(\frac{r_2}{r_1})^3+2\frac{\varepsilon_1^*-\varepsilon_2^*}{\varepsilon_1^*+2\varepsilon_2^*}}{(\frac{r_2}{r_1})^3-\frac{\varepsilon_1^*-\varepsilon_2^*}{\varepsilon_1^*+2\varepsilon_2^*}}$$

where 1 is the core, 2 is the shell, r1 is the radius from the centre of the sphere to the inside of the membrane and r2 is the radius from the centre of the sphere to the outside of the membrane.

DEP as a cell characterisation tool
DEP is mainly used for characterising cells measuring the changes in their electrical properties. To do this, many techniques are available to quantify the dielectrophoretic response, as it is no possible to directly measure the DEP force.

These techniques rely on indirect measures, obtaining a proportional response of the strength and direction of the force that needs to be scaled to the model spectrum. So most models only consider the Clausius-Mossotti factor of a particle.

The most used techniques are:


 * Collection rate measurements: this is the simplest and most used technique. Electrodes are submerged in a suspension with a known concentration of particles and the particles that collect at the electrode are counted.
 * Crossover measurements: the crossover frequency between positive and negative DEP is measured to characterise particles. This technique is used for smaller particles (e.g. viruses), that are difficult to count with the previous technique.
 * Particle velocity measurements: this technique measures the velocity and direction of the particles in an electric field gradient.
 * Measurement of the levitation height: the levitation height of a particle is proportional to the negative DEP force that is applied. Thus, this technique is good for characterising single particles and is mainly used for larger particles such as cells.
 * Impedance sensing: particles collecting at the electrode edge have an influence on the impedance of the electrodes. This change can be monitored to quantify DEP.

Electrode geometries
At the start, electrodes were made mainly from wires or metal sheets. Nowadays, the electric field in DEP is created by means of electrodes which minimize the magnitude of the voltage needed. This has been possible using fabrication techniques such as photolithography, laser ablation and electron beam patterning. These small electrodes allow the handling of small bioparticles.

DEP well electrodes
These electrodes were developed by Hoettges et al. and offer a rapid and low cost way to quantify DEP.

A glass slide is attached to the bottom of the well to contain the sample. Successive conductive layers of the laminate are connected to the two phases of an AC signal so that the walls of the “wells” have electrodes of alternate potential, while the field gradient formed along the walls moves cells by DEP [7, 51].

The advantages of this technique are mainly:
 * Rapid
 * Low cost
 * Different samples can be measured in different wells
 * Bigger amount of sample can be monitored

The dielectrophoretic properties of cells can be monitored by light absorption measurements: positive DEP attracts the cells to the wall of the well, thus when probed with a light beam the well becomes more transparent; vice versa for the negative DEP, in which the well becomes less transparent.