A microfluidic device (1, 1', 1") for concentrating target cells, characterised in that the device comprises: a first capture area (2, 2', 2") through which a sample can be flowed, said first capture area (2, 2', 2") comprising a plurality of electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis when activated, and to release said target cells when de-activated, said electrodes (5, 5', 5") arranged such that a sample flowing through the area will flow over them; a waste area downstream of the first capture area (2, 2', 2") and in fluid communication with said first capture area (2, 2', 2"); a second capture area (6, 6', 6") downstream of the first capture area (2, 2', 2") and in fluid communication with said first area (2, 2', 2"), said second capture area (6, 6', 6") being of a smaller volume than the first capture area (2, 2', 2") and comprising a plurality of electrodes (5a, 5a', 5a") tuned to trap target cells using di-electrophoresis, said electrodes (5a, 5a', 5a") arranged such that a sample flowing through the second area (6, 6', 6") will flow over them; a plurality of flow regulators adapted to ensure that the volumetric flow rate of sample travelling through the second capture area (6, 6', 6") is lower than the volumetric flow rate of sample travelling through the first capture area (2, 2', 2"); the device arranged such that when the electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis in the first capture area (2, 2', 2") are activated sample flows out from the first capture area (2, 2', 2") to the waste area, and when the electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis in the first capture area (2, 2', 2") are de-activated sample flows out from the first capture area (2, 2', 2") to the second capture area (6, 6', 6"). a first capture area (2, 2', 2") through which a sample can be flowed, said first capture area (2, 2', 2") comprising a plurality of electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis when activated, and to release said target cells when de-activated, said electrodes (5, 5', 5") arranged such that a sample flowing through the area will flow over them; a waste area downstream of the first capture area (2, 2', 2") and in fluid communication with said first capture area (2, 2', 2"); a second capture area (6, 6', 6") downstream of the first capture area (2, 2', 2") and in fluid communication with said first area (2, 2', 2"), said second capture area (6, 6', 6") being of a smaller volume than the first capture area (2, 2', 2") and comprising a plurality of electrodes (5a, 5a', 5a") tuned to trap target cells using di-electrophoresis, said electrodes (5a, 5a', 5a") arranged such that a sample flowing through the second area (6, 6', 6") will flow over them; a plurality of flow regulators adapted to ensure that the volumetric flow rate of sample travelling through the second capture area (6, 6', 6") is lower than the volumetric flow rate of sample travelling through the first capture area (2, 2', 2"); the device arranged such that when the electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis in the first capture area (2, 2', 2") are activated sample flows out from the first capture area (2, 2', 2") to the waste area, and when the electrodes (5, 5', 5") tuned to trap said target cells using di-electrophoresis in the first capture area (2, 2', 2") are de-activated sample flows out from the first capture area (2, 2', 2") to the second capture area (6, 6', 6").