A piston (25) comprises a sealing element (30) which is in contact with inner circumferential surface (20) of an ampoule structure (10). The cross section of structure interior space (15) is designed such that a compression state is set in sealing element when a contact zone is in guide region of structure, and another compression state is set in sealing element, when the contact zone is in sealing region of structure. A blocking device prevents contact zone movement from sealing region into guide region.
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