Researchers have used electrical fields to manipulate nucleic acids and proteins for more than 50 years, but similar systems have only recently begun to emerge for working with whole cells. Dielectrophoresis is nevertheless quickly gaining appeal as a basis for microfluidic cell-sorting.
Hyongsok (Tom) Soh and Patrick Daugherty at the University of California, Santa Barbara, recently demonstrated the feasibility of dielectrophoresis-activated cell sorting, or ‘DACS’, with a microfluidic chip that uses dielectrophoretic forces to steer bacteria tagged with polystyrene beads into a collection channel (X. Hu et al. Proc. Natl Acad. Sci. USA 102, 15757–15761; 2005). Initial experiments showed that one round of DACS could achieve more than 200-fold enrichment of a rare subpopulation of cells at rates of 10,000 bacterial cells per second. They initially tested a single-stage, single-channel device, but Soh believes DACS is ideal for parallel operations. “It is relatively straightforward to design cascaded, sequential sorting stages that operate in parallel,” he says. “This allows high purity and cell recovery without sacrificing throughput.”
Soh is quick to point out that DACS is in no position to usurp fluorescence-activated cell sorting, because of the binary nature of its sorting mechanism; but it shows great promise for high-throughput screening, he says. “We just completed screening the first molecular library and performed epitope mapping with DACS,” Soh explains, “and we've shown that it can be faster, cheaper and simpler than commercial assays.”
Evotec Technologies in Hamburg, Germany, is also taking advantage of dielectrophoresis for its Cytocon 400 system. The key to this is the CellProcessor microfluidic chip, which contains a three-dimensional array of electrodes that allow users to design and control electrical-field configurations for cell manipulation.
“We developed the CellProcessor platforms for precise and fully automated sorting in a microfluidic environment,” says Gabriele Gradl, Evotec's vice-president of cell handling and analysis. “The underlying technology makes cell analysis and isolation reproducible and predictable down to the single-cell level.” The resulting platform allows for the delicate manipulation of small numbers of cells, in which the gentle handling provided by combining dielectrophoresis with hydrodynamic flow can be useful.