Stephen Graham Shirley

Trapping of micrometre and sub-micrometre particles by high-frequency electric fields and hydrodynamic forces

We demonstrate that micrometre and sub-micrometre particles can be trapped, aggregated and concentrated in planar quadrupole electrode configurations by positive and negative dielectrophoresis. For particles less than in diameter, concentration is driven by thermal gradients, hydrodynamic effects and sedimentation forces. Liquid streaming is induced by the AC field itself via local heating and results, under special conditions, in vortices which improve the trapping efficiency. Microstructures were fabricated by electron-beam lithography and modified by UV laser ablation. They had typical gap dimensions between 500 nm and several micrometres. The theoretical and experimental results illustrate the basic principles of particle behaviour in ultra-miniaturized field traps filled with aqueous solutions. The smallest single particle that we could stably trap was a Latex bead of 650 nm. The smallest particles which were concentrated in the central part of the field trap were 14 nm in diameter. At high frequencies (in the megahertz range), field strengths up to 56 MV can be applied in the narrow gaps of 500 nm. Further perspectives for microparticle and macromolecular trapping are discussed.

Paired microelectrode system: dielectrophoretic particle sorting and force calibration

Abstract We describe a closed microsystem used for free-flow dielectrophoretic separation of particles and cells. Particles have been separated on the basis of size and cells separated from artificial particles. Dielectrophoretic forces were calculated numerically and compared with experimental and analytic results. While the influence of local heating is of minor importance under our experimental conditions, the consideration of liquid-channel interfaces gives rise to an remarkable increase of dielectrophoretic forces.

Dielectrophoretic manipulation of suspended submicron particles

Planar and three‐dimensional multi‐electrode systems with dimensions of 2 — 40 μm were fabricated by IC technology and used for trapping and aggregation of microparticles. To achieve negative dielectrophoresis (repelling forces) in aqueous solution, radiofrequency (RF) electric fields were used. Experimentally, particles down to 100 nm in diameter were enriched and trapped as aggregates in field cages and dielectrophoretic microfilters and observed using confocal fluorimetry. Theoretically, single particles with an effective diameter down to about 35 nm should be trappable in micron field cages. Due to the unavoidable Ohmic heating, RF electric fields can induce liquid streaming in extremely small channels (12 μm in height). This can be used for pumping and particle enrichment but it enhances Brownian motion and counteracts dielectrophoretic trapping. Combining Brownian motion with ratchet‐like dielectrophoretic forces enables the creation of Brownian pumps that could be used as sensitive separation devices for submicron particles if liquid pumping is avoided in smaller structures.

Trapping of viruses in high-frequency electric field cages

High-frequency electric field cages can stably trap cells and microparticles in aqueous media [1, 2]. Such cages can be made by semiconductor fabricat ion techniques and are not to be confused with electromagnetic field devices used for t rapping atomic and elementary particles [3]. The behavior of various dielectric microparticles in uniform and nonuniform a.c. electric fields was investigated by Pohl in the 1970s and discussed in his monograph [4]. The mot ion of individual cells in nonuniform a.c. fields, termed dielectrophoresis (DP), was studied in subsequent decades [5]. The force, F, acting on a spherical dielectric particle of radius, r, in a timeperiodic electric field, f , can be expressed as a dipole approximat ion by

MICRODEVICE FOR CELL AND PARTICLE SEPARATION USING DIELECTROPHORETIC FIELD-FLOW FRACTIONATION

We describe the use of a micro system for dielectrophoretic field flow fractionation (DFFF) of particles and cells. Micro-objects are separated on the basis of differences in size and/or passive dielectric properties, respectively. Bow-like strip electrode pairs have been proven to be the most effective separation systems.

Growth of anchorage-dependent mammalian cells on microstructures and microperforated silicon membranes

Anchorage-dependent cells (mouse fibroblasts L929 and 3T3) were cultivated on microstructures made by semiconductor technology. Both cell lines showed normal growth on silicon surfaces covered with microelectrode arrays as well as on microperforated silicon membranes with square pores made by anisotropic etching (5, 10 or 20 μm edge length at the top and 1.2, 6.2 or 16.2 μm at the bottom). The cells spread over the 5 and 10 μm pores, but mostly failed to cover the 20 μm ones. They were able to cross the silicon membrane through the pores and to grow and spread on the under side of the membrane. Small pores (about 1 μm2) impeded but did not prevent cells crossing the membrane. Medium and large pores were freely crossed. Negative dielectrophoresis was used to achieve accurate positioning of cells above pores or to repel them from the chip surface (a.c., square wave, 2.5 V peak-to-peak, 5 MHz). The results are discussed with respect to their microtool applications for single-cell technologies.

High-frequency electric-field trap for micron and submicron particles

SummaryFour e-beam-processed, planar electrodes with gaps between 0.5 and 4 μm were used to create quadrupole electric-field trap. The electrodes were immersed in an aqueous particle suspension and driven by kHz to MHz signals of several volts amplitude. Micron and submicron particles could be stably trapped by negative dielectrophoresis. Latex beads of 1000, 600, 100 and 14 nm diameter could be concentrated between the electrodes (positive dielectrophoresis) or levitated as condensed cloud (negative dielectrophoresis). The results are surprising since polarisation forces depend on the volume of the particle and, up to now, it was expected that thermal forces would dominate the behaviour of particles with diameters <100 nm. However, micron-scaled electrode configurations allow the application of extremely strong fields (up to 20 MV/m) and open up new perspectives for microparticle handling and macromolecule trapping.

New Micro Devices for Single Cell Analysis, Cell Sorting and Cloning-on-a-Chip: The CytoconTM Instrument

We present the CytoconTM, an instrument for kinetic analysis of single cells, cell separation, cell sorting and sample preparation for single cell techniques. 3D systems with micro-electrodes and channels of 40 µm height and 400 µm width were assembled from glass chips. The quality of the micro systems is suited for highly sensitive optical detection techniques and the cells can be analyzed at subcellular resolution. The time course of loading trapped Jurkat T-lymphoma cells with a membrane permeable dye was observed in a “Loader” micro system. Reagent changes for cell loading and washing could be achieved within seconds. In the “Separator” micro system, cell separation is based on differences in size and passive dielectrophoretic properties in combination with field flow fractionation. The “Sorter” micro system was used for sorting small cell populations and for depositing single Jurkat cells into separate wells of a Terasaki plate after optical analysis. The viability of cells processed in these micro-electrode systems is routinely greater than 95%.