Valve stopper for a medical injection device and medical injection device for injecting at least one composition

A valve stopper configured to be positioned inside a barrel of an injection device for injecting at least one composition through a distal end of the barrel, the valve stopper comprising:

• a membrane comprising a proximal face having a concave shape and a distal face, the membrane being configured to separate a first, distal chamber of the barrel from a second, proximal chamber of the barrel,
• a lateral wall extending distally, or both distally and proximally, from the membrane defining at least a distal cavity, the lateral wall comprising a circumferential sealing surface configured to sealingly engage an inner surface of the barrel,
• wherein the membrane comprises a notch extending through at least a part of a thickness of the membrane between the proximal face and the distal face, the notch being configured to transition between a closed position and an open position defining a fluid path through the membrane from the proximal face to the distal face depending on a pressure exerted by a composition onto the proximal face of the membrane, the notch maintained in the open position depending on the pressure exerted, wherein the notch extends only in a part of the thickness of the membrane, thereby delimiting a tear part in the thickness of the membrane that is not traversed by the notch, said tear part being configured to tear along the notch under a determined pressure exerted by the composition to create the fluid path, the lateral wall extending further distally than the distal face in the closed position and the open position, and
• wherein the membrane permits transfer of fluid only from the second proximal chamber to the first distal chamber through the cavity.