An apparatus for use in a fluid conduit comprises a plurality of segments arranged along a longitudinal axis, each segment comprising an extendable structure configured for extension in a lateral direction relative to the longitudinal axis, wherein the apparatus is configured for the sequential initiation of lateral extension of the extendable structures of at least two segments. At least two segments may be configured to define different maximum extended dimensions in the lateral direction and may include adjacent support surfaces which provide mutual support for one another when the at least two segments are laterally extended. The apparatus may be configured to form a seal, an anchor, a supporting structure and/or a flow restriction in a fluid conduit.