PUMPING CASSETTE

A membrane-based valve comprising a top plate (214), a bottom plate (216) and a midplate (204) located between the top plate and the bottom plate, with a flexible membrane (202) interposed between the top plate and midplate. The top plate includes a pneumatic port (208) configured to deliver pneumatic pressure to the membrane. The midplate includes an inlet (218) and an outlet (220) in fluid communication with the membrane. The valve further comprises a valve seat (210). The bottom plate further comprises fluid paths defining the inlet and the outlet. The membrane and top plate define a control gas chamber (212), and the membrane and midplate define a valving chamber (206), the inlet and outlet in communication with the valving chamber (206) via holes formed through the midplate (204). A thickened rim (222) of the membrane is configured to be held in a groove (224) in the midplate. A cross-sectional shape of a part of the membrane comprises a first region with a cross-sectional curvature in a first direction that is a direction toward the top plate and a second region located within the first region with a cross-sectional curvature in a second direction opposite the first direction, and the pneumatic pressure from the pneumatic port is configured to force the membrane against the valve seat to close the valve, or draw the membrane away from the valve seat to open the valve.