The present invention provides a tissue expander (10) comprising a self-inflating core (12) having a non-inflated state and an inflated state and a coating (14) surrounding said core (12), in which said core (12) comprises a compacted material having a central portion (16) of a first, higher, (average) density D and a peripheral portion (18) of a second, lower, (average) density d, a border (19) between said central portion (16) and said peripheral portion (18) and wherein said coating (14) includes a plurality of first and/or second apertures (20, 22) through said coating (14). It also provides a method of manufacturing the tissue expander (10). The arrangement is such as to allow for the controlled ingress of water into the expandable core (12) which allows for the control of the delay before expansion and the rate of expansion once initiated.