Dielectrophoresis is a chemical effect that can be used to separate cells, viruses or other large biological particles from similar particles. The effect can reliably separate with such accuracy that dead or cancerous cells can be separated from their living counterparts.
The effect works by applying a large ~100v, high frequency ~100kHz electrical signal. This causes charged elements within the particle to move rapidly backwards and forwards. The effect of this is to create a dipole across the particle, making it act as a charged particle, and get moved by the electrical field. Separation occurs because the nature of the dipole is highly dependant on the composition of the molecule.
The effect was discovered in the 1980s and has not been seriously exploited yet. There is research being done though, with hopes that it may one day be used for purposes such as rapid food testing, cancer treatment, chemical testing and blood analysis.
Dielectrophoresis is not to be confused with electrophoresis, which works at lower frequencies and relies on the net charge of the particle to create motion.