The invention relates to a device for applying an electric field to a suspension of cells, comprising at least one chamber which comprises at least one internal space 40 for holding the suspension, the internal space 40 comprising at least two segments 41, 42, wherein each segment 41, 42 comprises at least one electrode 43, 44 and wherein neighboring electrodes 43, 44 are separated from each other by at least one gap 47 which is at least partially filled with an insulating material 46, and wherein the edges of the electrodes 43, 44 facing each other within the internal space 40 are rounded. Rounding the electrodes' edges facing a neighboring electrode results in a significant reduction of field gradients and thus even of the risk of arcing. The invention further concerns a method, wherein voltage is applied to at least one active electrode 43, 44 while the electrodes 43, 44, 45 or electrode segments next and/or opposite to the active electrode 43, 44 are set to ground potential. Setting neighboring electrodes that surround the active electrode to ground potential results in decreased scattering of the electric field within the internal space so that the electrically active area is locally limited and the field lines are focused near the active electrode and thus control of the process is enhanced.
