casting a flexible membrane into a first membrane through hole of the second rigid layer such that {TAIL ~ the flexible membrane} has a passageway surface that is adapted to be either substantially coplanar to the second surface of the first rigid layer when the valve is in an open position, or bulged with a central portion thereof being substantially coplanar to the plateau surface of the valve seat when the valve is in {HEAD ~ a closed position} 
casting a flexible membrane into a first membrane through hole of the second rigid layer such that { TAIL ~ the flexible membrane } has a passageway surface that is adapted to be either substantially coplanar to the second surface of the first rigid layer when the valve is in an open position , or bulged with a central portion thereof being substantially coplanar to the plateau surface of the valve seat when the valve is in { HEAD ~ a closed position }
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