referring now to fig1 of the drawings , the stent illustrated therein is for occluding the ductus arteriosus and is formed of a wire of shape memory - effect material , preferably the near equi - atomic nickel and titanium alloy which is known per se . in an expanded , occluding state , the stent comprises an occluding anchor portion 10 formed by a series of turns of the wire so that a conical helix form is defined . the stent further comprises another anchor portion 12 formed by a scroll or spirally wound part of the wire . the anchor parts 10 and 12 are interconnected by a straight linking part 14 extending axially of the cone defined by the occluding anchor part 10 and perpendicularly with respect to the plane of the spiral anchor part 12 . as can be seen from fig1 , a single length of wire forms the whole device . when the shape memory effect alloy is in a martensitic condition , the stent is malleable and can be deformed into a straight wire w ( see fig2 ). typically , the wire has a diameter of 0 . 3 mm and is arranged to convert from martensite to austenite at or slightly above 37 degrees c . such conversion causes the substantially straight length of wire w as illustrated in fig2 to convert to the shape illustrated in fig1 after release by the delivery system at the desired placement site , as will be described hereinafter . referring now to fig2 to 4 , the delivery system illustrated therein comprises a catheter 20 having a distal end 22 and a proximal end 24 . at its distal end 22 , the catheter 20 is closed by a compliant elastomer guide / seal member 26 having a spherically rounded end with a central bore 28 therethrough receiving a distal end of the wire w . a proximal end of the wire w is a friction fit within a stainless steel connector bush ( see particularly fig4 ) within the catheter 20 . the connector bush 30 also receives a stainless steel pusher wire 32 extending out of the proximal end 24 of the catheter 20 . the catheter 20 is provided with cooling liquid inlet and outlet ports 34 and 36 , respectively . in use , with the wire w of the stent straightened and contained in the catheter 20 , the distal end 22 of the catheter is introduced into the vascular system via a suitable point , such as the femoral artery , and the stent is positioned within the ductus 40 ( see fig5 ) which interconnects the pulmonary artery 42 and the aorta 44 and which is required to be occluded . until being delivered to the ductus 40 , the wire w is maintained in its martensitic form by circulating coolant through the catheter 20 via the ports 34 and 36 . ejection of the stent from the catheter 20 is effected by holding the catheter 20 steady whilst pushing on the wire to cause the wire w to be ejected through the member 26 at the distal end 22 . as the stent is ejected , it is warmed by blood at 37 ° c ., causing the stent to expand so as adopt the structure illustrated in fig1 and 5 . disengagement of the wire 32 from the stent is effected simply by pulling on the wire 32 so that the bush 30 slides off the wire w which is now firmly anchored in the ductus 40 , the strength of the friction fit between the wire wand the bush 30 being sufficient to permit this to take place . the strength of the friction fit is such that it can exert a force on the stent which is greater than that required to withdraw a partially deployed stent but less than that required to withdraw a fully deployed stent . in this way , the stent can be controlled and repositioned as well as being finally ejected . referring now to fig6 and 7 , an alternative arrangement of bush 30 is formed of shape memory alloy material instead of stainless steel . in this case , the arrangement is such that , upon heating , that portion of the bush 30 which engages the wire w expands into its memorised condition , thereby releasing the wire w . the bush 32 is deformed in its low temperature condition by radially compressing or crimping it around the wire w so that it is held firmly . by careful design of these parts , the stent can still recovered even when 95 % of it has been ejected . the portion of the bush 30 which is engaged with the wire 32 does not have a memory of increased diameter and therefore remains securely attached to the latter . the wire 32 can be made from a number of metals that have kink resistance and suitable spring qualities . in a further embodiment ( not shown ), the stent is held in the end of a long plastics tube by an interference fit . a wire which is a sliding fit within the plastics tube is used to stiffen the tube and to eject the stent from the end of the plastics tube . in a further modification ( also not shown ), at least the tip of the catheter at the distal end thereof has a second lumen which allows it to be slipped over and to follow a previously - introduced guide wire . such a technique is per se known in the art . referring now to fig8 and 9 of the drawings , the stent illustrated therein consists of a wire formed of two spirally wound anchor parts 10 and 12 interconnected by link part 14 formed of a loop of the length of wire forming the anchor parts 10 and 12 . as can be seen from fig9 , the spiral windings of the anchor parts 10 and 12 are wound in the opposite sense and have their central axes slightly laterally displaced . in this way , there is an enhanced occluding effect . the wire is a thin shape - memory alloy wire where the resilience properties of the spiral coils forming the anchor parts 10 and 12 made from thinner wires means that they will conform to the shape of the vessel in which they are implanted and project as little as possible beyond the vessel . as can be seen from fig8 and 9 , the anchor parts 10 and 12 are planar and the link part 14 is a part turn or loop . alternately , it may be a whole turn or several turns of wire which join the centres of the spiral anchor parts 10 and 12 . the length of the link part 14 between the anchor parts 10 and 12 will generally be between 0 . 1 and 5 mm . instead of being spiral , one or more of the anchor parts 10 and 12 may be cycloidal ( fig1 ) or spiral - cycloidal ( fig1 ). when implanted , the planar anchor portions 10 and 12 will be distorted by the walls of the vessels in which they are implanted , yielding an implant which has either two conical anchor portions , two flat anchor portions , or one flat and one conical anchor portion . in all cases , the implant will have been stretched longitudinally and its elastic recoil will ensure that the implant has adopted the minimum length possible within the anatomy and that it therefore projects as little as possible into the vessels on either side of the implant . the above typical dimensions may be varied by at least ± 50 % depending upon the extent of biological variation of patients to be fitted with the stent implant . in some cases , it may be desirable for the minimum diameter of one or both of the spiral anchor parts 10 and 12 to be less than 2 mm , although this figure is limited by the diameter of the wire employed . generally , nickel - titanium shape memory alloy wire cannot be completely straightened when it has been formed into a coil of a diameter less than 10 times the diameter of the constituent wire . this limitation may be overcome by constructing the stent implant from thin wire chosen to be approximately one tenth the diameter of the minimum diameter of the spiral anchor part 10 or 12 . this results in a simple implant of reduced mechanical strength . alternatively , a multi - stranded wire may be employed made from a number of finer wires twisted or braided together to form the stent implant . this results in a more complex implant of greater mechanical strength . the manufacture of wire spirals is very common using mandrel - based winding techniques or centreless , roller - based forming processes . however , the manufacture of paired spirals interconnected at their centres and nominally flat or asymmetric forms is more complex . a three - stage procedure for achieving these latter forms when manufactured from shape memory alloy will now be described with reference to fig1 a and 12 b and 13 a and 13 b . this procedure involves winding the shape - memory alloy wire onto a double frusto - conical mandrel 50 ( fig1 a ) or a conical - cylindrical mandrel 52 ( fig1 b ). in each case , the wire is wound into a deep groove thread 54 of the appropriate shape cut into the periphery of the mandrel 50 or 52 . normally , this shape would be impossible to remove from the mandrel . however , although the wire shape may be destroyed when removing it from the mandrel , the form can be recovered when the material is warmed above its trigger temperature . the shape of the mandrel 50 or 52 defines the number of wire turns in the stent and the diameters of those turns . once wound onto the mandrel 50 or 52 , the wire is fixed at its ends by appropriate clamps or attachment means ( not shown ). then , the entire assembly of wire and mandrel is heat - treated so that the wire will adopt the shape imparted by the mandrel when its memory is recovered . this temperature is usually in the region of 350 ° c . to 550 ° c . after cooling , the wire is removed from the mandrel 50 or 52 and heated gently to make it re - adopt the shape of the mandrel , typically a long spiral whose diameter varies along its length . this is then inserted into a flattening clamp assembly 60 ( see fig1 a and 13 b ). the flattening clamp 60 includes a fixed clamp member 62 from which guide rods 64 extend . a movable clamp member 66 and a divider 68 are slidably mounted on the rods 64 and slidable on the rods 64 from an unclamped position as illustrated in fig1 a to a clamped position as illustrated in fig1 b . the clamp member 62 carries a series of hooks or pegs 69 which are used to trap and retain one or more turns of the spiral in a position which is laterally offset from the axis of the spiral . when six or seven hooks or pegs 69 are used to trap individual coils of a parallel - sided cylindrical spiral , the cycloidal pattern illustrated in fig1 can be produced . similarly , a conically sided spiral used with the same number of hooks or pegs 69 can produce the less - symmetrical spiral - cycloidal pattern illustrated in fig1 . if only one hook or peg 69 is used , the entire axis of the trapped spiral can be offset from the untrapped part , resulting in the pattern illustrated in fig9 . such hooks or pegs 69 may be provided on the clamp member 62 or on the divider 68 . pockets 70 are provided in both clamp members 62 and 66 . these define the overall length of the sections of the spiral contained within them . alternatively or additionally , one or more of the pockets 70 may be provided in the appropriate surface or surfaces of the divider 68 . in a further embodiment , the divider 68 may be omitted completely , or more than one divider 68 may be provided . when no dividers 68 are used , a flat spiral of zero longitudinal pitch is produced . the divider or dividers can be introduced to define the longitudinal pitch of particular turns of the spiral . when the appropriate divider ( s ) 68 and hook ( s ) or peg ( s ) 69 have been fitted to impose the required secondary structure to the spiral , the various parts are compressed together as illustrated in fig1 b and locked in place , followed by further heat treatment . such further heat treatment is carried out in two steps , namely annealing at high temperature to remove all “ memory ” of the shape retained from the mandrel , and treatment at a lower temperature to “ memorise ” the shape in which the wire is held in the flattening clamp . the final shape of the stent is produced when it has been released from the flattening clamp . referring now to fig1 and 15 , the catheter illustrated therein comprises outer and inner walls 80 and 82 which are maintained in spaced apart relationship by a pair of internal ribs 84 which divide the area between the walls 80 and 82 longitudinally into feed and return passages 86 and 88 for cooling liquid . these ribs are removed at the distal end 22 of the catheter 20 so as to allow the liquid from the feed passage 86 to return via the passage 88 .