A membrane puncturing mechanism comprises a first element and a second element arranged with opposing surfaces. At least one opposing surface comprises a sloping region inclined relative to the other opposing surface. The first element is moveable towards and away from the second element. A pin for puncturing a membrane is coupled to the first element such that movement of one of the pin and first element causes movement of both. A spring is operable to bias the first element towards the second element. An actuator is removably positioned between the opposing surfaces, the actuator being withdrawable from between and moveable along the opposing surfaces such that, as the actuator is withdrawn, the actuator progressively moves the first and second elements apart against the bias of the spring so as to load the spring and such that the actuator being fully withdrawn from between the opposing surfaces.
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