A field-flow dielectrophoretic separation method in a 3D filtering chip has been developed in this work. The separation method was possible due to the special configuration of the DEP filtering chip, which has a structure similar to a classical capacitor with two parallel plate electrodes (realized by using a stainless steel mesh) and a dielectric medium (defined by a suspension of 100 µm diameter silica beads in buffer solution). The dielectrophoretic phenomenon is generated by the non-uniformities of the dielectric media, which produce a gradient of the electric field and, as a consequence, a DEP force. If a suspension medium with cells flows through the filter, the DEP force can trap these cells around the contact points between the silica beads (if the cells exhibit positive DEP) or they are repelled into the space between the beads (if the cells exhibit negative DEP). It is shown that for two different cell populations, the frequency of the electric field and permittivity of the media can be tuned in such a way that one population will exhibit positive DEP and the other one negative DEP. The population that expresses negative DEP can be easily flushed out due to the hydrodynamic force which is larger at the center point between the beads. In such a way two cell populations can be separated. The working principle was verified with both live and dead yeast cells. Best results for the separation of viable and nonviable cell populations were achieved at an applied voltage of 150 V in a frequency range between 10 kHz and 20 kHz for flow rates of 0.1 ml min −1 and 0.2 ml min −1 . With a few of these devices cascaded in series, higher efficiency could be achieved. As a result, this device and the associated proposed separation method can be very useful tools for bio-pharmaceutical industries since continuous flow separation at relatively high flow rates is both time and cost saving.

The paper presents a 3-D filter chip employing both mechanical and dielectrophoretic (DEP) filtration, and its corresponding microfabrication techniques. The device structure is similar to a classical capacitor: two planar electrodes, made from a stainless steel mesh, and bonded on both sides of a glass frame filled with round silica beads. The solution with the suspension of particles flows through both the mesh-electrodes and silica beads filter. The top stainless steel mesh (with openings of 60 mum and wires of 30 mum-thickness) provides the first stage of filtration based on mechanical trapping. A second level of filtration is based on DEP by using the nonuniformities of the electric field generated in the capacitor due to the nonuniformities of the dielectric medium. The filter can work also with DC and AC electric fields. The device was tested with yeast cells (Saccharomyces cerevisae) and achieved a maximal trapping efficiency of 75% at an applied AC voltage of 200 V and a flow rate of 0.1 mL/min, from an initial concentration of cells of 5 x 10(5) cells/mL. When the applied frequency was varieted in the range between 20 and 200 kHz, a minimal value of capture efficiency (3%) was notticed at 50 kHz, when yeast cells exhibit negative DEP and the cells are repelled in the space between the beads.

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