Julian P.H. Burt

Positive and negative dielectrophoretic collection of colloidal particles using interdigitated castellated microelectrodes

An interdigitated castellated microelectrode geometry has been found to facilitate particle collection arising from both positive and negative dielectrophoretic effects. These two forms of particle collection differ significantly as functions of the frequency of the applied non-uniform electric field and of the conductivity of the suspending medium. This behaviour is demonstrated for yeast cells and a theoretical explanation is presented in terms of the electric field patterns generated by the electrodes, as well as through consideration of the associated phenomenon of electrorotation.

Selective dielectrophoretic confinement of bioparticles in potential energy wells

The potential energy surfaces generated by microelectrodes of polynomial and interdigitated castellated geometry have been calculated for particles experiencing both positive and negative dielectrophoretic forces. The resulting forms of particle collection at these electrodes are governed by the locations of the potential energy wells, and the theoretical predictions of the modes of collection for particles experiencing positive and negative dielectrophoretic forces are verified using mixtures of viable and non-viable yeast cells, as well as of bacteria and blood cells. An important result for the interdigitated electrodes is the finding that particles trapped in potential energy wells under the action of negative dielectrophoresis can be more easily removed from the electrode structure (e.g. by fluid flow or gravitational forces) than those trapped under positive dielectrophoresis. This was verified for mixtures of bacteria and blood cells, viable and non-viable yeast cells.

Manipulation of herpes simplex virus type 1 by dielectrophoresis

The frequency-dependent dielectrophoretic behaviour of an enveloped mammalian virus, herpes simplex virus type 1 is described. It is demonstrated that over the range 10 kHz-20 MHz, these viral particles, when suspended in an aqueous medium of conductivity 5 mS m(-1), can be manipulated by both positive and negative dielectrophoresis using microfabricated electrode arrays. The observed transition from positive to negative dielectrophoresis at frequencies around 4.5 MHz is in qualitative agreement with a simple model of the virus as a conducting particle surrounded by an insulating membrane.

Development of biofactory-on-a-chip technology using excimer laser micromachining

Multilayer microelectrode structures, with 10 m feature sizes, have been fabricated using excimer laser ablation techniques. These structures will be incorporated into devices for the electro-manipulation and characterization of cells, microorganisms and other particles. The AC electrokinetic phenomena of dielectrophoresis, electrorotation and travelling electric field effects are utilized, all of which are dependent on the dielectric properties of the bioparticles. Examples are presented of a travelling wave junction and selective particle trap to be incorporated into a prototype biofactory-on-a-chip device. A technique for profiling the edges of via-holes has been developed to facilitate robust electrical connections through polyimide films in these multilayer devices.

Membrane changes accompanying the induced differentiation of Friend murine erythroleukemia cells studied by dielectrophoresis

Dielectrophoresis measurements obtained using an image processing technique are reported over the frequency range 1 Hz to 100 kHz for the Friend murine erythroleukemia cell lines DS19 and R1 before and after treatment with hexamethylene bisacetamide and dimethylsulfoxide, agents that induce terminal differentiation in DS19 but not in R1 cells. Data are analyzed according to the single shell dielectric model of the cell. The membrane capacitance was found to fall by 30% and membrane conductivity by a factor of at least 5 when DS19 cells were induced to differentiate. R1 cells showed no such response. While the theoretical model was found to be useful for comparing differences in data for the different cell lines, several significant discrepancies between its predictions and the experimental data were observed, including positive dielectrophoretic collection at frequencies below 20 Hz and a smaller than predicted response to the membrane permeabilizing agents saponin and valinomycin. Factors that may have accounted for these discrepancies include surface charge effecgs, conduction parallel to the plasma membrane surface, and intracellular compartments.

Applications of a new optical technique for measuring the dielectrophoretic behaviour of micro-organisms.

It is shown that the dielectrophoretic behaviour (motion in non-uniform a.c. electric fields) of micro-organisms can conveniently and reproducibly be measured by monitoring the decrease in optical absorbance of a cell suspension as the cells are collected at a micro-electrode array. The dielectrophoretic behaviour, as a function of the frequency of the applied electric field and conductivity of the supporting solution, can be determined more quantitatively and rapidly than by methods so far described in the literature. Results are presented for Micrococcus lysodeikticus, Bacillus subtilis and Escherichia coli for the frequency range 20 Hz to 4 MHz and theoretical considerations are presented for the effect of solution conductivity. A value of 0.2 S/m has been derived for the effective conductivity of M. lysodeikticus.

Theoretical and experimental investigations of the interdependence of the dielectric, dielectrophoretic and electrorotational behaviour of colloidal particles

New theoretical relationships are derived to link the dielectric properties of a suspension of colloidal particles to both the dielectrophoretic (DEP) and electrorotation (ROT) behaviour exhibited by a single suspended particle. It is found that the relaxation frequencies that characterize the dielectric spectrum of a colloidal suspension are close to, but different from, those that characterize the DEP and ROT responses. The extent of this difference is dependent on particle volume fraction and the intrinsic dielectric properties of both particle and suspending medium. Experimental results obtained for yeast cells in the frequency range from 1 kHz to 10 MHz provide confirmation of the theory.

Dielectrophoretic characterisation of Friend murine erythroleukaemic cells as a measure of induced differentiation

Dielectrophoresis measurements, the study of the motion of particles in non-uniform a.c. electrical fields, have been made on three cell lines (DS19, R1 and DR1) of Friend murine erythroleukaemia cells as a function of hexamethylene bisacetamide (HMBA) treatment. The effects of saponin treatment on R1 cells and neuraminidase on human red blood cells were also studied. It is shown that the dielectrophoretic behaviour can be interpreted in terms of cell surface charge and cell membrane conductivity effects. HMBA reduces the cell surface charge on all three cell lines, and in lines DS19 and DR10, where the cells are induced to differentiate, there is an increase in effective cell conductivity. This gain in conductivity is concluded to be associated with either an enhanced lateral electrophoretic motion of delocalised ions or of the polarisability of dipoles at the membrane surface.

A combined travelling wave dielectrophoresis and electrorotation device: applied to the concentration and viability determination of Cryptosporidium

We describe a microelectrode device, fabricated using photolithography and laser ablation, that combines the electrokinetic effects of travelling wave dielectrophoresis and electrorotation. Here it has been used to concentrate and then assay the viability of Cryptosporidium parvum oocysts.

An optical dielectrophoresis spectrometer for low-frequency measurements on colloidal suspensions

An optical system is described for measuring the response of suspended particles to imposed non-uniform electric fields in the frequency range 1 Hz to 4 MHz. Such dielectrophoretic measurements can provide details of the dielectric and surface charge properties of animate and inanimate particles. This simple, low-cost and rapid technique extends these measurements down to lower frequencies than previously reported, and has revealed new effects associated with particle surface charge and surface conductivity. At the lower frequencies corrections are required to take account of the electrode polarisation effects, which modify the electric field distribution in the colloidal suspension between the electrodes. Measurements are reported for silicon powder, yeast and the bacteria Micrococcus lysodeikticus and are compared with theoretical expectations. Biotechnological applications include cell separation, cell characterisation, cell-culture quality control and biomass determination.