An expandable intra-gastric balloon (1) for treating obesity. The balloon can be implanted in the stomach of a patient in order to reduce the volume of the stomach. The balloon (1) comprises at least one flexible bag (2) presenting an inside face (2A) and an opposite, outside face (2B). The inside and outside faces (2A, 2B) form the surface of the at least one flexible bag (2). At least a portion of the surface (2A, 2B) is covered by a coating (6, 6') comprising parylene.