A method for the separation of biological cells and bacteria based on traveling wave dielectrophoresis (twDEP) is presented. The microfluidic chip consists of a structured PDMS layer on glass. Parallel electrodes with increasing width and gap size, positioned along the micro fluidic channel, were used to expose cells to a twDEP force perpendicular to a pressure driven flow. With this gradient electrode structure we show that successful separations of mixtures of bacteria contaminated cells into subpopulations of viable cells and bacteria are feasible. (C) 2011 Published by Elsevier Ltd.