the tubing portion 1 shown in fig1 has a circular cross - section , an external diameter d e , an internal diameter d i and a wall thickness t . the tubing is coiled into a helix of constant amplitude a ( as measured from mean to extreme ), constant pitch p , constant helix angle θ and a swept width w . the tubing portion 1 is contained in an imaginary envelope 20 which extends longitudinally and has a width equal to the swept width w of the helix . the envelope 20 may be regarded as having a central longitudinal axis 30 , which may also be referred to as an axis of helical rotation . the illustrated tubing portion 1 has a straight axis 30 , but it will be appreciated that in alternative designs the central axis may be curved . the tubing portion has a centre line 40 which follows a helical path about the central longitudinal axis 30 . it will be seen that the amplitude a is less than the tubing internal diameter d i . by keeping the amplitude below this size , the space occupied by the tubing portion can be kept relatively small , whilst at the same time the helical configuration of the tubing portion promotes swirl flow of fluid along the tubing portion . fig2 shows a prosthesis 10 comprising a length of hollow tubing having an inlet 2 at one end and an outlet 3 at the other end . a generally helical tubing portion 1 is provided at the outlet 3 thereof . the prosthesis has inlet 2 a and outlet 3 a flaps at its ends which have been surgically fastened by suturing to regions of an artery remote from a blockage 7 in the artery , the prosthesis thus acting as an arterial bypass graft . it could also be surgically connected between an artery and a vein so as a vascular access graft for e . g . renal dialysis . blood from the circulatory system can flow from the inlet 2 to the outlet 3 along a hollow interior or lumen 4 . the helically formed tubing portion 1 is disposed adjacent to the outlet 3 . its non - planar curvature induces a swirl to the flow to improve circulation by rendering the distribution of wall shear stress relatively uniform and suppressing flow separation and flow instability , and as a result inhibiting the development of vessel pathology . the swirl flow may also resist the build up of intimal hyperplasia at the join and downstream of the join with the vein or artery . the tubing can be made of suitable bio - compatible material and such materials are commercially available and known to those skilled in the art . in order to maintain the tubing open and prevent collapse or kinking it is possible to use a stent or other structural support of plastic , metal or other material internally , externally or integral to the wall of the tubing . it will be seen that the prosthesis 10 in fig2 is generally arch shaped . this arch may itself be provided in a single plane . if the arch is non - planar then this will also tend to induce swirl flow and it will be desirable to ensure that the swirl flow induced by the non - planar arch is in the same direction as that induced by the helical tubing portion 1 . the arrangement of fig3 is similar to that of fig2 , except that the helically formed tubing portion 1 extends substantially the full length of the prosthesis 10 . this type of arrangement may simplify manufacture as the tubing could be made in a continuous length which simply has to be cut to appropriate shorter lengths to form prostheses . part of the envelope 20 within which the tubing portion 1 is defined is shown in fig3 . the swept width w defines the width of the envelope . the longitudinal axis 30 of the envelope is curved , the tubular portion being arch shaped . the centre line 40 follows a helical path about the axis 30 . the vascular graft 10 shown in fig4 has a substantially circular cross - section . the tubing is coiled into a helix of constant amplitude a ( as measured from mean to extreme ), constant pitch p , constant helix angle θ and a swept width w . the tubing 1 is contained in an imaginary envelope 20 which extends longitudinally and has a width equal to the swept width w of the helix . the envelope 20 may be regarded as having a central longitudinal axis 30 , which may also be referred to as an axis of helical rotation . the illustrated tubing 1 has a curved axis 30 . the tubing has a centre line 40 which follows a helical path about the central longitudinal axis 30 . the tubing 1 has a helical portion 6 extending longitudinally and circumferentially with the same pitch as pitch p of the helical centre line 40 . the helical portion 6 consists of a strip of material secured to the wall 62 of the tubing 1 . the tubing 1 has an inlet 2 at one end and an outlet 3 at the other end . the tubing has inlet 2 a and outlet 3 a flaps at its ends which have been surgically fastened by suturing to regions of an artery 8 remote from a blockage 7 in the artery , the graft 10 thus acting as an arterial bypass graft . it could also be surgically connected between an artery and a vein so as to serve as a vascular access graft for e . g . renal dialysis . blood from the circulatory system can flow from the inlet 2 to the outlet 3 along a hollow interior or lumen 4 of the graft 10 . it operates in a manner similar to the graft of fig3 , having a non - planar curvature and resist the development of pathology . the swirl flow may also resist the build up of intimal hyperplasia at the join and downstream of the join with the vein or artery . the tubing 1 may be made of various materials . suitable bio - compatible materials are commercially available and known to those skilled in the art . one suitable material is polyester . a knitted polyester yarn such as polyethylene terephthalate , known as dacron ( trade mark ) is a particular example . the helical portion may be made of the same material or another material , such as polypropylene . the helical portion , rather than being a separate strip secured to the wall 62 of the tubing 1 , may be an integral part thereof , for example by being knitted or stitched in to the wall . fig5 shows the result of an experiment carried out on a toy balloon 55 . the balloon was of the elongated type . it was supported , without being inflated , on a cylindrical rod and a plastic strip 51 cut from another balloon was glued onto the outside of the supported balloon to form a longitudinally and circumferentially extending helical strip 6 . a straight line 50 was drawn along the balloon . after the glue had set , the balloon was inflated and the inflated balloon is shown in fig5 . it will be seen that the inflated balloon 55 has a helical lumen . as with the tubing for fluid flow , it has a helical centre line 40 , which follows a helical path about a longitudinal axis 30 . the longitudinal axis is at the centre of an imaginary cylindrical envelope 20 within which the balloon is contained . the amplitude a of the helix is shown in fig5 . it will be noted that after inflation the straight line 50 adopts a wave shape which remains consistently along the same side of the balloon , so that the entire line 50 remains visible in the elevation view of fig5 . the balloon of fig5 starts as a cylindrical membrane with a helical portion which is of greater ( in this case double ) wall thickness than the rest of the balloon . during inflation the thicker helical portion will tend to resist extension in all directions , including circumferential and longitudinal directions , thereby influencing the shape of the expanded balloon . instead of adopting the normal cylindrical shape , the balloon forms a shape with a helical centre line 40 . the balloon is internally pressurised in a manner to some extent analogous with the internal pressurisation of the tubing of the preferred embodiments of the invention . the helical portion causes what would otherwise be a cylindrical shape to adopt and maintain helical geometry . a similar effect is obtained by the helical portion of the tubing for body fluid flow , wherein the helical portion tends to help the tubing maintain its helical longitudinal cavity , i . e . to resist “ straightening out ”. a tubing having a wall defining a longitudinally extending cavity having a centre line following a substantially helical path was manufactured as follows . a pair of flexible cylindrical tubes made from polyester were internally supported by insertion of respective closely fitting coiled springs . the two supported tubes were then positioned adjacent to each other and twisted around each other . the pair of tubes were thermoset in the twisted configuration by immersion in hot water followed by removal and cooling . the tubes were separated and the coil springs removed . the internal geometry of each tube so formed consisted of a longitudinally extending cavity having a centre line following a substantially helical path . one of the tubes was subjected to internal pressurisation by insertion of a cylindrical balloon which was then gently inflated . because of the flexible nature of the material forming the tube , the effect of the internal pressurisation was to straighten out the helix , in that the pitch was increased and the amplitude decreased . such a straightening out effect is however resisted by the use of a helical portion applied to the tube , as described herein . the helical portion is applied to each of the tubes before they are deformed and thermoset as described above . during the step of twisting the two tubes around each other , they are positioned so that their respective helical portions lie in contact with each other . a similar method was used to manufacture another tubing having a wall defining a longitudinally extending cavity with a centre line following a substantially helical path . in this case , the tubing was made of expanded polytetrafluoroethylene ( eptfe ). biocompatible tubing of this type is available for use as vascular prostheses , for example from vascutek limited or boston scientific corporation . referring to fig6 and 7 , a length of eptfe tubing 1 was internally supported by insertion of a length of silicone rubber tubing 70 . a length of polyvinyl chloride ( pvc ) tubing 71 was internally supported by insertion of a closely fitting coiled spring . the two supported tubes were positioned adjacent to each other and twisted around each other . the support tube 70 was clamped at each end by respective clamps 73 , these clamps also serving to clamp the ends of the pvc tube 71 . the internally supported , twisted and clamped tubes were placed in an oven at 180 ° c . for 5 minutes and then cooled by immersion in water at room temperature . the tubes were separated and the support tube 70 was removed from the tubing 1 . the tubing was thermoset in a twisted configuration , as seen in fig8 . although the amplitude of the helix was reduced compared to the amplitude during the heating step , the tubing had the desired longitudinally extended cavity with a centre line following a substantially helical path . a test was carried out on the tubing 1 to investigate its ability to maintain its helical geometry . one end was clamped and the other end was connected to a water supply at a pressure head of 1 . 5 metres ( roughly equal to blood pressure ). it was observed that the helical geometry was maintained after 24 hours . fig9 shows another length of eptfe tubing manufactured using the above method . in this case the tubing 1 used at the start was of the armoured type , having an external helical winding 74 with a large helix angle ( close to 90 °). this type of tubing is used in prostheses subject to external bending forces , for example going across joints such as the knee , and the helical winding serves to help maintain a circular cross - section . it will be noted that such armoured tubing was also successfully modified to have a longitudinally extending cavity with a centre line following a substantially helical path . in an alternative manufacturing method , only one tube , rather than two , is used . the method is described with reference to fig1 a and 10 b . an elongate member , in the form of a thread 101 , is helically wound round an initially cylindrical tube 1 . as seen in fig1 a , the thread 101 is arranged helically along the tubing so as to extend longitudinally and circumferentially thereof . the thread is tensioned and causes the tube to distort helically , such that its longitudinally extending cavity has a centre line following a substantially helical path . the pitch is dictated by the pitch of the winding of the thread . the amplitude is dictated by the tension on the thread . the tension , and hence the helical deformation , is maintained by securing the ends of the thread , for example to a suitable rig . the deformed tube is then heated so as to thermoset and so as to soften the thread sufficiently for it to bond to the tube . the thread therefore serves the purposes first of creating the helical geometry during the tensioning step , and later of helping to retain that geometry when the tube is used and internally pressurised by e . g . arterial pressure . as with other methods described herein , the tubing may be externally or internally supported during this process . in a preferred method a knitted polyester yarn such as polyethylene terephthalate , known as dacron ( trade mark ), is a suitable material for the tube , whilst the elongate member may be polypropylene . the tube may be externally supported with helically wound ( with a very large helix angle , close to 90 °) polypropylene . with these materials the heating step is carried out by heating the tube and tensioned thread in an oven at 140 ° c . in another alternative manufacturing method using only one tube , the tube is initially cylindrical , with a helical portion extending along its wall . the method is described with reference to fig1 a and 11 b . in this method , tubing 1 is provided with a reinforcing strip 51 adhered to its outside surface so as to extend longitudinally and circumferentially of the tubing . an inflatable device 55 is located inside the tubing . the inflatable device is inflated in order to expand the tubing . during this process the helically arranged strip 51 causes the tubing to expand to a shape having a longitudinal , helical cavity , as seen in fig1 b . the tubing adopts the helical geometry in the same manner as the balloon shown in fig5 . the tubing is thermoset in this condition and allowed to cool , in order to retain the desired helical shape . the material of the inflatable device 55 is chosen to withstand the elevated temperature required to thermoset the tubing . the helical portion , in the form of strip 51 , thus serves the purposes first of creating the helical geometry during the inflation step , and later of helping to retain that geometry when the tube is used and internally pressurised by e . g . arterial pressure . another method of making a graft is described with reference to fig1 a to 12 e . this method involves the use of a helical mandrel . fig1 a is a schematic illustration of a helical mandrel for use in this method . the mandrel consists of a rigid rod 300 , shaped into a helix . the mandrel extends longitudinally and circumferentially around a cylindrical space which defines a core 301 of the mandrel . in the embodiment shown , the pitch and the amplitude of the helix are constant along the length of the mandrel , but they may vary if desired . in order to form a helical portion , a length of straight flexible tube 1 , whose external diameter d e is greater than the internal diameter d m of the core of the mandrel , is fed generally along the core of the mandrel , as shown in fig1 b . because the tube is wider than the space inside the mandrel , it is forced to adopt a helical form . the tube may be externally or internally supported to retain its cross - sectional shape during this process . after being treated to make it retain its helical shape , e . g . by thermosetting , the tube is removed from the mandrel , as shown in fig1 c and 12 d . as can be seen , the pitch of the helical portion is the same as the pitch of the mandrel , subject to some possible relaxation of the tube when removed from the mandrel . the amplitude of the helical portion will be determined by the external diameter of the tube and the internal diameter of the core of the mandrel . the above description concerns a batch processing method for forming the helical tubing , but this method also lends itself to continuous operation . a continuous length of flexible tube can be drawn through a comparatively short length of mandrel , and can be treated to retain its shape as it is drawn through ( for example , by heating and then cooling a tube formed from a thermosetting resin ). experiment has shown that the tube rotates relative to the mandrel when it is drawn through in this way . thus , some form of lubrication may be required to enable smooth functioning of the process . fig1 e is a schematic cross - section through the tube and the mandrel as the tube is drawn . it will be seen that the mandrel contacts the outside of the tube , and so the mandrel can be supported from below ( at 320 ) without interfering with the drawing process . the mandrel can be formed in any suitable manner , and the method of forming the mandrel will depend to a large extent on the size of the tubes being treated . the mandrel could be formed by winding a rod around a member with a circular cross - section , or it may be made by machining , for example using a cnc milling machine . another method of making a graft is described with reference to fig1 a and 13 b . fig1 a shows a straight steel rod 110 held in tension between two clamps ( not shown ). a soft steel wire 112 has been wound on to the steel rod in a helical manner , i . e . to extend longitudinally and circumferentially of the rod . the wire 112 is secured in place by silver solder . the wire 112 forms a guide showing where a tubing 1 is to be wound around the rod 110 , which acts as a mandrel . by using the wire 112 as a guide , the pitch ( or helix angle ) of the tubing when wound onto the rod is predetermined . the tubing is then heated and cooled in order to thermoset it . it is separated from the rod and when it separates it “ relaxes ” whereby its helical amplitude reduces . in this example , the tubing is made of eptfe . experiments were carried out using polyvinyl chloride tubing with a circular cross - section . referring to the parameters shown in fig1 the tubing had an external diameter d e of 12 mm , an internal diameter d i of 8 mm and a wall thickness t of 2 mm . the tubing was coiled into a helix with a pitch p of 45 mm and a helix angle θ of 8 °. the amplitude a was established by resting the tubing between two straight edges and measuring the space between the straight edges . the amplitude was determined by subtracting the external diameter d e from the swept width w : in this example the swept width w was 14 mm , so : as discussed earlier , “ relative amplitude ” a r is defined as : water was passed along the tube . in order to observe the flow characteristics , two needles 80 and 82 passing radially through the tube wall were used to inject visible dye into the flow . the injection sites were near to the central axis 30 , i . e . at the “ core ” of the flow . one needle 80 injected red ink and the other needle 82 blue ink . fig1 shows the results of three experiments , at reynolds numbers r e of 500 , 250 and 100 respectively . it will be seen in all cases that the ink filaments 84 and 86 intertwine , indicating that in the core there is swirl flow , i . e . flow which is generally rotating . the parameters for this example were the same as in example 1 , except that the needles 80 and 82 were arranged to release the ink filaments 84 and 86 near to the wall of the tubing . fig1 shows the results of two experiments with near - wall ink release , with reynolds numbers r e of 500 and 250 respectively . it will be seen that in both cases the ink filaments follow the helical tubing geometry , indicating near - wall swirl . furthermore , mixing of the ink filaments with the water is promoted . it will be appreciated that this invention , in its first aspect , is concerned with values of relative amplitude a r less than or equal to 0 . 5 , i . e . small relative amplitudes . in a straight tubing portion both the amplitude a and the relative amplitude a r equal zero , as there is no helix . therefore , with values of relative amplitude a r approaching zero , the ability of the tubing portion to induce swirl will reduce . the lowest workable value of relative amplitude a r for any given situation will depend on the speed of flow and the viscosity and density of the fluid ( i . e . reynolds number ) and on the pitch ( helix angle ) and the particular use of the tubing portion . relative amplitudes of at least 0 . 05 , 0 . 10 , 0 . 15 , 0 . 20 , 0 . 25 , 0 . 30 , 0 . 35 , 0 . 40 or 0 . 45 may be preferred . the various manufacturing methods described herein are not limited to the manufacture of tubing with a relative amplitude equal to or less than 0 . 5 , unless otherwise specified . the methods are considered to be of independent patentable significance and are applicable to the manufacture of tubing with larger amplitudes , whilst also being particularly useful for making tubing of small relative amplitudes .
a graft includes a flow tubing having a tubing portion defining a flow lumen . the flow lumen of the tubing portion is substantially free of ribs or grooves . a centre line of the flow lumen follows a substantially helical path with a helix angle less than or equal to 65 °. the amplitude of the helix is less than or equal to one half of the internal diameter of the tubing portion .