An expandable tube for deployment within a blood vessel is disclosed. In one arrangement, the tube comprises an elongate frame that is reversibly switchable from a radially expanded and longitudinally contracted state to a radially contracted and longitudinally expanded state. The frame comprises a plurality of longitudinally deformable elements for providing longitudinal expansion and contraction of the frame and a plurality of circumferentially deformable elements for providing radial expansion and contraction of the frame. The longitudinally deformable elements can be expanded or contracted longitudinally substantially without any change in the shape of the circumferentially deformable elements. The plurality of circumferentially deformable elements comprises a plurality of sets of circumferentially deformable elements. Each set of circumferentially deformable elements forms a closed ring around an axis of elongation of the frame. Each closed ring consisting exclusively of the circumferentially deformable elements. At least two of the closed rings occupy overlapping ranges of longitudinal positions when the frame is in the radially expanded and longitudinally contracted state and occupy non-overlapping ranges of longitudinal positions when the frame is in the radially contracted and longitudinally expanded state.