Abstract: The invention relates to a valve stopper configured to be positioned inside a barrel of an injection device for injecting a composition through a distal, including: a membrane having a proximal and distal face, the membrane being configured to separate a, distal chamber of the barrel from a proximal chamber; a lateral wall extending from the membrane to define a distal cavity, the lateral wall having a circumferential sealing surface configured to sealingly engage the inner surface of the barrel; where the proximal face presents a concave shape and the membrane includes a notch extending through the membrane between the proximal and distal face, being configured to selectively create a fluid path through the membrane from the proximal face to the distal face depending on the pressure exerted by a composition onto the proximal face of the membrane for transferring fluid only from the second chamber to the first.