Abstract: A filter has a conduit having an opening for flow of fluid into the conduit, a pump for pumping fluid into the conduit through the opening, electrodes spaced apart from each other across the opening such that fluid flowing through the opening passes between the electrodes; and an AC electrical source for the electrodes, the source of AC electrical energy having a frequency and voltage such that an electric field created by the AC electrical energy in the area around the electrodes imposes a negative dielectrophoretic force on target particles carried by the fluid, the negative dielectrophoretic force being opposed to the direction of fluid flow through the opening and having sufficient strength to prevent the target particles from passing between the electrodes into the conduit. The filter is mounted in a bioreactor for filtering viable cells and retaining them in culture medium.