Abstract: A stent for placing in a tubular organ so as to support its diameter to remain above a critical diameter. The stent comprises a spiral coil or cylinder made of a two-way shape memory alloy and has a super-elastic state in which the stent's diameter is at least about the critical diameter or more but within a physiological range tolerated by the organ and having a soft state in which the stent's diameter, after changing to this state from the super-elastic state, which is less than the critical diameter. The shape memory alloy has two transition temperatures being within a range that will not damage biological tissue, of which a first transition temperature is a temperature in which it changes from the soft state to the super-elastic state and of which a second transition temperature is a temperature in which it changes from the super-elastic state to the soft state. The arrangement being such that after changing into one of the two states, the stent remains in that state at body temperature.