A tissue expander 10, for use in medical, veterinary and dental surgery, comprises (i) a self-inflating core 12 (having a non-inflated state and an inflated state, for example formed from a hydrophilic self-inflating polymer) with a top surface 14, bottom surface 16 and a side surface 18 and having a circumference C and a longitudinal axis X and (ii) a coating (e.g. a flexible or stretchable material) which covers the said surfaces, surrounding the core. The coating comprises a top portion 22, a bottom portion 24 and a side portion 26, the side portion having a concertinaed structure of one or more axially spaced circumferentially extending interconnected rings (28, Fig 4), each ring both extending around the axis X and being spaced apart from each other along the axis X. In one embodiment a plurality of adjacent axially spaced circumferentially extended interconnected rings is provided, whereas in another embodiment the rings are formed from a single ring with a spiral form (see Fig 3).
