Abstract: An expansion device is disclosed which, when mounted on an appropriate support, can make controlled advancement relative to the support in response to activation from a remote site. The device comprises first (6) and second (8) elements coupled for relative movement along an axis defined on a support 2. An expandable chamber is defined between the elements, in which chamber is a flexible and elastic membrane (24). A resilient mechanism urges the two elements towards each other to define a minimum volume for the chamber. The chamber can be expanded to separate the elements, and hold mechanisms typically in the form of latches, are provided for selectively restricting movement on the respective elements relative to the support.