Ac electrokinetics is concerned with the study of the movement and behaviour of particles in suspension when they are subjected to ac electrical fields. The development of new microfabricated electrode structures has meant that particles down to the size of macromolecules have been manipulated, but on this scale forces other than electrokinetic affect particles behaviour. The high electrical fields, which are required to produce sufficient force to move a particle, result in heat dissipation in the medium. This in turn produces thermal gradients, which may give rise to fluid motion through buoyancy, and electrothermal forces. In this paper, the frequency dependency and magnitude of electrothermally induced fluid flow are discussed. A new type of fluid flow is identified for low frequencies (up to 500 kHz). Our preliminary observations indicate that it has its origin in the action of a tangential electrical field on the diffuse double layer of the microfabricated electrodes. The effects of Brownian motion, diffusion and the buoyancy force are discussed in the context of the controlled manipulation of sub-micrometre particles. The orders of magnitude of the various forces experienced by a sub-micrometre latex particle in a model electrode structure are calculated. The results are compared with experiment and the relative influence of each type of force on the overall behaviour of particles is described.

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Ac electrokinetics: a review of forces in microelectrode structures

A review is presented of the present status of the theory, the developed technology and the current applications of dielectrophoresis (DEP). Over the past 10 years around 2000 publications have addressed these three aspects, and current trends suggest that the theory and technology have matured sufficiently for most effort to now be directed towards applying DEP to unmet needs in such areas as biosensors, cell therapeutics, drug discovery, medical diagnostics, microfluidics, nanoassembly, and particle filtration. The dipole approximation to describe the DEP force acting on a particle subjected to a nonuniform electric field has evolved to include multipole contributions, the perturbing effects arising from interactions with other cells and boundary surfaces, and the influence of electrical double-layer polarizations that must be considered for nanoparticles. Theoretical modelling of the electric field gradients generated by different electrode designs has also reached an advanced state. Advances in the technology include the development of sophisticated electrode designs, along with the introduction of new materials (e.g., silicone polymers, dry film resist) and methods for fabricating the electrodes and microfluidics of DEP devices (photo and electron beam lithography, laser ablation, thin film techniques, CMOS technology). Around three-quarters of the 300 or so scientific publications now being published each year on DEP are directed towards practical applications, and this is matched with an increasing number of patent applications. A summary of the US patents granted since January 2005 is given, along with an outline of the small number of perceived industrial applications (e.g., mineral separation, micropolishing, manipulation and dispensing of fluid droplets, manipulation and assembly of micro components). The technology has also advanced sufficiently for DEP to be used as a tool to manipulate nanoparticles (e.g., carbon nanotubes, nano wires, gold and metal oxide nanoparticles) for the fabrication of devices and sensors. Most efforts are now being directed towards biomedical applications, such as the spatial manipulation and selective separation/enrichment of target cells or bacteria, high-throughput molecular screening, biosensors, immunoassays, and the artificial engineering of three-dimensional cell constructs. DEP is able to manipulate and sort cells without the need for biochemical labels or other bioengineered tags, and without contact to any surfaces. This opens up potentially important applications of DEP as a tool to address an unmet need in stem cell research and therapy.

/pdf/review-article-dielectrophoresis-status-of-the-theory-99vmv2147w.pdf

Review Article—Dielectrophoresis: Status of the theory, technology, and applications

https://www.cell.com/biophysj/pdf/S0006-3495(99)76908-0.pdf

Separation of Submicron Bioparticles by Dielectrophoresis

Insulator-based (electrodeless) dielectrophoresis (iDEP) is an innovative approach in which the nonuniform electric field needed to drive DEP is produced by insulators, avoiding problems associated with the use of electrodes. Live and dead Escherichia coli were concentrated and selectively released by applying stepped DC voltages across a microchannel containing an array of insulating posts etched in glass. The only electrodes present were two platinum wires placed in the inlet and outlet reservoirs, producing mean electric fields of up to 200 V/mm across the insulators. The cells were labeled with Syto 9 and propidium iodide and imaged through a fluorescent microscope. Cell trapping and release were controlled by modifying the relative responses of electrokinesis and DEP by adjusting the magnitude of the applied voltage. Dead cells were observed to have significantly lower dielectrophoretic mobility than live cells, whereas the electrokinetic mobilities of live and dead cells were indistinguishable. The ...

http://ca.sandia.gov/microfluidics/research/pdfs/DielectroPre.pdf

Dielectrophoretic Concentration and Separation of Live and Dead Bacteria in an Array of Insulators

Dielectrophoretic platforms for bio-microfluidic systems.

Dielectrophoretic separation of bacteria using a conductivity gradient

Dielectrophoresis, the lateral motion induced on particles by non-uniform electric fields, is a sensitive function of the electrical conductivity of the particle suspending medium. This dependence is exploited in a new technique for separating bioparticles from suspended mixtures. The bioparticles are first immobilised by positive dielectrophoresis at electrodes in a separation chamber, and the conductivity of the liquid flowing through the chamber is then gradually and continuously increased so as to produce a conductivity gradient with time. The bioparticles are released from the electrodes according to their own dielectric properties and as a function of flow rate and medium conductivity. This is demonstrated for pure suspensions and mixtures of the bacteria Bacillus subtilis, Escherichia coli and Micrococcus luteus.

Penelope A. Dyda

Papers

Dielectrophoretic separation of bacteria using a conductivity gradient

Dielectrophoretic separation of bacteria gradient