The present invention provides a tissue expander (10) comprising: a self-inflating core (12), having a non-inflated state and an inflated state, a top surface (14), a bottom surface (16) and a side surface (18)； a circumference (C) and having a longitudinal axis X anda coating (20) covering said surfaces (14, 16, 18) and surrounding said core (12), in which said coating (20) comprises a top portion (22), a bottom portion (24) and a side portion (26)； wherein, said side portion (26) comprises a concertinaed structure comprising one or more axially spaced circumferentially extending interconnected rings (28) each extending around said axis X and each being spaced from each other along said axis X.
