Abstract: An inflatable, flexible device (10) for use as an expandable closure device comprises an inlet part (14) of a substantially tubular configuration, an expandable bag part (12) integrally connected to the inlet part and made from a highly flexible and highly elastic material such as natural rubber, synthetic rubber, plastics, polymer or elastomer materials or combinations thereof, optionally reinforced by fiber or mesh included in the material. The expandable bag is changeable from an unexpanded state to an expanded state by inflating the bag through the introduction of an inflation gas into the interior through the inlet part. The expandable bag defines in the expandable state a sphere, and includes in its unexpanded state a plurality of inwardly bent folds extending parallel with the inlet part and extending from positions juxtaposed the inlet part along a major part of the outer surface of the expandable bag part.