An inner cannula (20) for a tracheostomy tube is made by extruding tubing (41) and cutting into separate lengths or preforms (42). Each preform (42) is placed in a mould (50, 51, 52) and blow moulded to expand the preform into contact with an inner surface (53, 54) of the mould which is shaped to produce a pattern on the outside of the preform of two intersecting diagonal corrugations (211, 212) that act to strengthen the blow moulded preform against lateral forces. The inner surface of the mould may also be textured so that the blow moulded preform is given a texture (213) to reduce friction with the outer tube (1).
