A cartridge for manipulating samples in liquid droplets on a working film with an electrode array when the working film is placed on said electrode array, the cartridge comprising: a) a body (2,2',2") that comprises an upper surface, a lower surface, and a number of wells configured to hold therein reagents or samples, each well comprising a top opening, and a bottom opening for releasing a liquid from the well; b) a piercable bottom structure impermeable to liquids and configured to seal at least one of the bottom openings of the wells; c) a working film located below the lower surface of the body (2,2',2"), the working film being impermeable to liquids and comprising a hydrophobic upper surface; d) a peripheral spacer located below the lower surface of the body (2,2',2") and connecting the working film to the body (2,2',2"); and e) a gap between the lower surface of the body (2,2',2") and the hydrophobic upper surface of the working film, the gap being defined by the peripheral spacer; characterized in that the cartridge further comprises f) at least one top piercing system, each located within at least one of the wells for releasing a reagent or sample (6,6') from said at least one well into the gap; and g) wherein each top piercing system comprises a piston with a piercing element, the piston being configured to be movable within said well and the piston comprising at least one gasket that is sealingly abutting the inner wall of the well for providing a seal between the piston and the inner wall of the well, and the piercing element being configured as a thorn located at a front side of the piston and being configured to pierce the piercable bottom structure for releasing a reagent or sample (6,6') from said at least one well into the gap upon moving the piston within the well toward its bottom opening. a) a body (2,2',2") that comprises an upper surface, a lower surface, and a number of wells configured to hold therein reagents or samples, each well comprising a top opening, and a bottom opening for releasing a liquid from the well; b) a piercable bottom structure impermeable to liquids and configured to seal at least one of the bottom openings of the wells; c) a working film located below the lower surface of the body (2,2',2"), the working film being impermeable to liquids and comprising a hydrophobic upper surface; d) a peripheral spacer located below the lower surface of the body (2,2',2") and connecting the working film to the body (2,2',2"); and e) a gap between the lower surface of the body (2,2',2") and the hydrophobic upper surface of the working film, the gap being defined by the peripheral spacer; f) at least one top piercing system, each located within at least one of the wells for releasing a reagent or sample (6,6') from said at least one well into the gap; and g) wherein each top piercing system comprises a piston with a piercing element, the piston being configured to be movable within said well and the piston comprising at least one gasket that is sealingly abutting the inner wall of the well for providing a seal between the piston and the inner wall of the well, and the piercing element being configured as a thorn located at a front side of the piston and being configured to pierce the piercable bottom structure for releasing a reagent or sample (6,6') from said at least one well into the gap upon moving the piston within the well toward its bottom opening.