Abstract: An expansion device for temporary implantation beneath the skin and subcutaneous layer, there to be enlarged whereby to increase the surface area of tissue which overlays it, to provide a flap for use in plastic and reconstructive surgery. The device comprises an envelope with a substantially non-extensible base and a cover. Together the base and cover form an envelope. The cover is flexible, and when empty it is slack. The volume of the envelope is adjustably variable as a function of the amount of fluid it contains. Means is provided for admitting fluid to the envelope without puncturing the base or the cover.