Stefan Fiedler

Three-dimensional electric field traps for manipulation of cells--calculation and experimental verification.

The forces acting on dielectric particles and living cells exposed to alternating and rotating fields generated by three-dimensional multi-electrode arrangements are investigated. Numerical procedures are described for the calculation of the electric field distribution and forces. The physical treatment considers electrodes of any shape and dielectric particles of complex structure. Particle and cell trapping are based on negative dielectrophoretic forces produced by high-frequency a.c. or rotating electric fields up to 400 MHz. Various multi-electrode systems were realised in commercially fabricated microelectrode systems, and tested for their ability to move and assemble microparticles or living cells without contact with the electrodes. The field distribution and accuracy of phase-controlled power application was tested using individual artificial particles trapped in the electric field cage. Position and trajectories of particle motion were measured. The paper gives an overview of electrode and field cage design in the microscale range.

Radio-frequency microtools for particle and live cell manipulation

Single particles can be manipulated by applying high frequencies to ultramicro electrode arrays fabricated on planar structures. Heat production can be reduced to the extent that intense electric fields can be applied even to unmodified cell culture media. Animal cells grow normally in the high field (up to 100 kV/m) between such continuously energized multielectrodes. As with laser tweezers [1-3], this technique can capture particles and cells in field traps, generate linear movement, and permit cell cultivation. It can also produce micropatterns of pH gradients, field-cast objects, and control cell adhesion. These microtools may be combined to develop cell separators, microsensors, and controlled-biocompatibility surfaces.

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

High frequency electric fields for trapping of viruses

Combining dielectrophoretic and hydrodynamic forces in micro electrode structures allows enrichment and stable trapping of viruses in aqueous solutions. Fluorescently labelled Influenza and Sendai viruses were collected from solutions of 2*105 – 2*108 viruses/μl within a few seconds. In the central part of the trap a virus aggregate of about 2–9 μm in diameter was formed. This corresponds to a local enrichment of viruses up to a factor of about 1400.

Particle micromanipulator consisting of two orthogonal channels with travelling-wave electrode structures.

Abstract In the paper experimental and numerical results for a simple particle micromanipulator fabricated in silicon technology are presented. It consists of three orthogonal liquid filled channels above meander-shaped planar electrode strips. By applying appropriate alternating, rotating or travelling electric fields in the chamber dielectric forces acting on particles suspended in the liquid are induced allowing trapping, movement and separation of them. The efficient manipulation of small particles with typical dimensions of several micrometers using electrogradient techniques (EGT) requires electrode structures of the same size. Due to the complex electrode geometry a numerical procedure is used for calculation of particle trajectories and optimising the design. In the micro range small fabrication defects are likely to cause large changes of the properties of the manipulator. Therefore, a test procedure based on electrode processes in aqueous media and the pH-dependent fluorescence intensity of a marker solution which enabled us to visualise the working states, surface coatings and fabrication defects of microstructured electrodes via a microscope is introduced.

Electrocasting - formation and structuring of suspended microbodies using a.c. generated field cages

Hybrid microelectrode chip structures were used to generate three dimensional field cages. The a.c. field generated negative dielectrophoretic forces and caused suspended particles to aggregate. The shape and size of the aggregate depends on the material, suspending liquid, electrode arrangement, electrode driven and size. Particle-aggregates can be stabilised by chemical or physical means. Photopolymerization was used to solidify latexes into artificial microbodies in the micrometer range and antibody-mediated agglutination was also used. Single particles or aggregates can be covered by one or more layers of different materials and geometries. We show a glass sphere covered by a structured multivesicular coating and a gas bubble covered by a latex particle layer.The microbody casting technology can find application in materials science, pharmaceutical formulation and biotechnology.