Helical graft

Abstract

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 center 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.

Claims

1. A method of inhibiting flow instability in a graft for biomedical use, the graft comprising flow tubing which is for use in vivo to carry blood and which is made of biocompatible material, the flow tubing having a tubing portion with an internal diameter and defining a flow lumen, wherein the flow lumen of the tubing portion is of circular cross-section, the flow lumen of said tubing portion having a centre line and being substantially free of ribs or grooves, wherein the graft is thermally set such that it retains a twisted shape and the centre line of the flow lumen follows a helical path about a longitudinal axis so as to define a helical center line, the helical center line having a helix angle and a helix amplitude, the helix angle being less than or equal to 45, and the helix amplitude being less than or equal to one half of the internal diameter of the tubing portion, and wherein the flow tubing is flexible and is capable of adopting a configuration in which the longitudinal axis is curved and the helical center line of the tubing portion follows a helical path about the curved longitudinal axis.

2. A method as claimed in claim 1, wherein the amplitude of the helical center line divided by the internal diameter of the tubing is at least 0.05.

3. A method as claimed in claim 1, wherein the helix angle is less than or equal to 15.

4. A method as claimed in claim 1, wherein the centre line of the tubing portion follows a substantially helical path about an axis which is curved.

5. A method as claimed in claim 1, wherein the graft further comprises a pharmaceutical coating.

6. A method as claimed in claim 1, wherein the tubing portion comprises a tubular wall which resists reduction of the amplitude of the helical centre line.

7. A method as claimed in claim 1, wherein the graft is thermally shape set.

8. A method as claimed in claim 1, wherein the tubing portion has a wall comprising a helical winding to help maintain a circular cross-section of the flow lumen.

9. A method as claimed in claim 8, wherein the helix angle of the helical winding is larger than the helix angle of the helical centre line of the flow lumen.

10. A method as claimed in claim 1, wherein the graft comprises ePTFE.

11. A method of inhibiting flow instability in a graft for biomedical use, the graft comprising flow tubing which is for use in vivo to carry blood and which is made of biocompatible material, the flow tubing having a tubing portion with an internal diameter and defining a flow lumen, wherein the flow lumen of the tubing portion is of circular cross-section, wherein the graft is thermally set such that it retains a twisted shape and the center line of the flow lumen follows a helical path about a longitudinal axis so as to define a helical center line, the helical center line having a helix angle and, a helix amplitude, the helix angle being less than or equal to 45, the helix amplitude being less than or equal to one half of the internal diameter of the tubing portion, and the amplitude of the helical centre line divided by the internal diameter of the tubing portion is at least 0.05, and wherein the flow tubing is flexible and is capable of adopting a configuration in which the longitudinal axis is curved and the helical center line of the tubing portion follows a helical path about the curved longitudinal axis.

12. A method as claimed in claim 11, wherein the helix angle is less than or equal to 15.

13. A method as claimed in claim 11, wherein the center line of the tubing portion follows a substantially helical path about an axis which is curved.

14. A method as claimed in claim 11, wherein the graft further comprises a pharmaceutical coating.

15. A method as claimed in claim 11 wherein the tubing portion comprises a tubular wall which resists reduction of the amplitude of the helical center line.

16. A method as claimed in claim 11, wherein the graft is thermally shape set.

17. A method as claimed in claim 11, wherein the tubing portion has a wall comprising a helical winding to help maintain a circular cross-section of the flow lumen.

18. A method as claimed in claim 11, wherein the helix angle of the helical winding is larger than the helix angle of the helical center line of the flow lumen.

19. A method as claimed in claim 11, wherein the graft comprises ePTFE.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

(2) FIG. 1 is an elevation view of a tubing portion in accordance with the invention;

(3) FIG. 2 is a perspective view of a vascular graft;

(4) FIG. 3 is a perspective view of another vascular graft;

(5) FIG. 4 is a perspective view of a vascular graft;

(6) FIG. 5 is a view of an experimental balloon;

(7) FIG. 6 is a view of a vascular graft twisted with a flexible member during manufacture;

(8) FIG. 7 is a view of part of the vascular graft of FIG. 6, to an enlarged scale;

(9) FIG. 8 is a view of the vascular graft made by the method shown in FIGS. 6 and 7;

(10) FIG. 9 is a view of another vascular graft made by the same method;

(11) FIGS. 10a and 10b are views illustrating another method of manufacturing a graft;

(12) FIGS. 11a and 11b are views illustrating another method of manufacturing a graft;

(13) FIGS. 12a to 12e are views illustrating a method of manufacturing a graft;

(14) FIGS. 13a and 13b are views illustrating another method of manufacturing a graft;

(15) FIG. 14 shows elevation views of tubing portions used in experiments; and

(16) FIG. 15 shows elevation views of tubing portions used in further experiments.

DETAILED DESCRIPTION

(17) The tubing portion 1 shown in FIG. 1 has a circular cross-section, an external diameter D.sub.E, an internal diameter D.sub.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 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.

(18) It will be seen that the amplitude A is less than the tubing internal diameter D.sub.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.

(19) FIG. 2 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 2a and outlet 3a 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.

(20) 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.

(21) It will be seen that the prosthesis 10 in FIG. 2 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.

(22) The arrangement of FIG. 3 is similar to that of FIG. 2, 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.

(23) Part of the envelope 20 within which the tubing portion 1 is defined is shown in FIG. 3. 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.

(24) The vascular graft 10 shown in FIG. 4 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 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.

(25) 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.

(26) The tubing 1 has an inlet 2 at one end and an outlet 3 at the other end. The tubing has inlet 2a and outlet 3a 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.

(27) 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 FIG. 3, 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.

(28) 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.

(29) FIG. 5 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 FIG. 5.

(30) 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 FIG. 5.

(31) 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 FIG. 5.

(32) The balloon of FIG. 5 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.

(33) 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.

(34) A tubing having a wall defining a longitudinally extending cavity having a centre line following a substantially helical path was manufactured as follows.

(35) 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.

(36) 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.

(37) 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.

(38) Referring to FIGS. 6 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 FIG. 8. 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.

(39) 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 meters (roughly equal to blood pressure). It was observed that the helical geometry was maintained after 24 hours.

(40) FIG. 9 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.

(41) In an alternative manufacturing method, only one tube, rather than two, is used. The method is described with reference to FIGS. 10a and 10b. An elongate member, in the form of a thread 101, is helically wound round an initially cylindrical tube 1. As seen in FIG. 10a, 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.

(42) 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.

(43) 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 FIGS. 11a and 11b. 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 FIG. 11b. The tubing adopts the helical geometry in the same manner as the balloon shown in FIG. 5. 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.

(44) 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.

(45) Another method of making a graft is described with reference to FIGS. 12a to 12e. This method involves the use of a helical mandrel.

(46) FIG. 12a 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.

(47) In order to form a helical portion, a length of straight flexible tube 1, whose external diameter D.sub.E is greater than the internal diameter D.sub.M of the core of the mandrel, is fed generally along the core of the mandrel, as shown in FIG. 12b. 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.

(48) After being treated to make it retain its helical shape, e.g. by thermosetting, the tube is removed from the mandrel, as shown in FIGS. 12c and 12d.

(49) 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.

(50) 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).

(51) 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.

(52) FIG. 12e 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.

(53) 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.

(54) Another method of making a graft is described with reference to FIGS. 13a and 13b. FIG. 13a 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.

(55) 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.

EXAMPLE 1

(56) Experiments were carried out using polyvinyl chloride tubing with a circular cross-section. Referring to the parameters shown in FIG. 1 the tubing had an external diameter D.sub.E of 12 mm, an internal diameter D.sub.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 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.sub.E from the swept width W:
2A=WD.sub.E
So:

(57) $A=\frac{W-D_E}{2}$

(58) In this example the swept width W was 14 mm, so:

(59) $A=\frac{W-D_E}{2}=\frac{14-12}{2}=1\mathrm{mm}$

(60) As discussed earlier, relative amplitude A.sub.R is defined as:

(61) $A_R=\frac{A}{D_r}$

(62) In the case of this Example, therefore:

(63) $A_R=\frac{A}{D_r}=\frac{1}{8}=0.125$

(64) 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.

(65) FIG. 14 shows the results of three experiments, at Reynolds numbers R.sub.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.

EXAMPLE 2

(66) 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. FIG. 15 shows the results of two experiments with near-wall ink release, with Reynolds numbers R.sub.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.

(67) It will be appreciated that this invention, in its first aspect, is concerned with values of relative amplitude A.sub.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.sub.R equal zero, as there is no helix. Therefore, with values of relative amplitude A.sub.R approaching zero, the ability of the tubing portion to induce swirl will reduce. The lowest workable value of relative amplitude A.sub.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.

(68) 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.