first and second surfaces positioned opposite to each other, wherein the first surface comprises a groove disposed around a center of the first surface and an outermost peripheral edge of the septum defines a total area bounded by the outermost peripheral edge wherein the second surface is a single planar surface that fills the entirety of the total area bounded by the outermost peripheral edge;

a varying distance between the first surface and the second surface configured such that when a higher pressure is applied to the first surface than the second surface, a component of a force exerted on the first surface acts towards an axis (A) which intersects the center of the first surface and a center of the second surface; and

a housing defining a compartment for a liquid drug wherein the septum is mounted on the housing in an unpierced condition with the first surface of the septum facing the compartment.