An implantable device includes a first sealed flexible shell configured for implantation within a breast of a human subject, an elastic filler material contained within the first sealed flexible shell, and a second sealed flexible, inelastic shell, which is disposed within the elastic filler material inside the first sealed flexible shell and is inflated with a volume of gas. The second shell includes a material selected such that an amount of the gas escaping from the second shell does not exceed 10−；8 Torr-liter/second when the gas pressure inside the second shell is 250 mbar higher than the gas pressure outside the second shell.
