Dielectrophoresis (DEP) is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field.
[8] Furthermore, a study of the change in DEP force as a function of frequency can allow the electrical (or electrophysiological in the case of cells) properties of the particle to be elucidated.
Although the phenomenon we now call dielectrophoresis was described in passing as far back as the early 20th century, it was only subject to serious study, named and first understood by Herbert Pohl in the 1950s.
[9][10] Recently, dielectrophoresis has been revived due to its potential in the manipulation of microparticles,[2][4][5][11] nanoparticles and cells.
The orientation of the dipole is dependent on the relative polarizability of the particle and medium, in accordance with Maxwell–Wagner–Sillars polarization.
The simplest theoretical model is that of a homogeneous sphere surrounded by a conducting dielectric medium.
Where the particle consists of nested spheres – the most common example of which is the approximation of a spherical cell composed of an inner part (the cytoplasm) surrounded by an outer layer (the cell membrane) – then this can be represented by nested expressions for the shells and the way in which they interact, allowing the properties to be elucidated where there are sufficient parameters related to the number of unknowns being sought.
[2][4][5] This expression has been useful for approximating the dielectrophoretic behavior of particles such as red blood cells (as oblate spheroids) or long thin tubes (as prolate ellipsoids) allowing the approximation of the dielectrophoretic response of carbon nanotubes or tobacco mosaic viruses in suspension.
These equations are accurate for particles when the electric field gradients are not very large (e.g., close to electrode edges) or when the particle is not moving along an axis in which the field gradient is zero (such as at the center of an axisymmetric electrode array), as the equations only take into account the dipole formed and not higher order polarization.
To be precise, the time-dependent equation only applies to lossless particles, because loss creates a lag between the field and the induced dipole.
An equivalent time-averaged equation can be easily obtained by replacing E with Erms, or, for sinusoidal voltages by dividing the right hand side by 2.
DEP is being applied in fields such as medical diagnostics, drug discovery, cell therapeutics, and particle filtration.
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.
[33] In order to study larger populations of cells, the properties can be obtained by analysing the dielectrophoretic spectra.
Nowadays, the electric field in DEP is created by means of electrodes which minimize the magnitude of the voltage needed.
Polynomial is a new geometry producing well defined differences in regions of high and low forces and so particles could be collected by positive and negative DEP.
[35] Interdigitated geometry comprises alternating electrode fingers of opposing polarities and is mainly used for dielectrophoretic trapping and analysis.
Rather than use photolithographic methods or other microengineering approaches, DEP-well electrodes are constructed from stacking successive conductive and insulating layers in a laminate, after which multiple "wells" are drilled through the structure.
When alternating conducting layers are connected to the two phases of an AC signal, a field gradient formed along the walls moves cells by DEP.
Alternatively, the approach can be used to build a separator, where mixtures of cells are forced through large numbers (>100) of wells in parallel; those experiencing positive DEP are trapped in the device whilst the rest are flushed.
The highly parallel nature of the approach means that the chip can sort cells at much higher speeds, comparable to those used by MACS and FACS.
This approach offers many advantages over conventional, photolithography-based devices but reducing cost, increasing the amount of sample which can be analysed simultaneously, and the simplicity of cell motion reduced to one dimension (where cells can only move radially towards or away from the centre of the well).
Particles experiencing repulsive and weak attractive DEP forces are eluted by fluid flow, whereas particles experiencing strong attractive DEP forces are trapped at electrode edges against flow drag.
[41] Dielectrophoresis field-flow fractionation (DEP-FFF), introduced by Davis and Giddings,[42] is a family of chromatographic-like separation methods.
In DEP-FFF, DEP forces are combined with drag flow to fractionate a sample of different types of particles.
The use of photoconductive materials (for example, in lab-on-chip devices) allows for localized inducement of dielectrophoretic forces through the application of light.
In addition, one can project an image to induce forces in a patterned illumination area, allowing for some complex manipulations.