VASCULAR PROSTHESIS

Abstract: A vascular prosthesis (20) is adapted for use as access graft having a permanent set, kink-resistant U-bend section (21).

Description: VASCULAR PROSTHESIS This invention concerns a vascular prosthesis adapted for use as an access graft, for example, for use in dialysis patients. At present, to make a vascular access graft, straight pieces of conventional artificial vascular prostheses, having circular cross-sections, are placed, usually, in the forearm of patients during surgery, and simply bent into a loop in situ. However, a major problem is that the artificial vessel may kink at the point where the vessel bends, and, because the vessel is circular, the lumen will totally occlude thus interrupt¬ ing normal blood flow. This problem may be overcome to some extent by having access grafts with thick walls and small internal diameters which are more kink resistant than those with thinner walls and larger internal diameters. However if the walls of the graft are too thick and lumen too small, the graft will not only have a reduced compliance but also it will be very difficult to repeatedly introduce the large bore needle, i.e. 15 gauge, used for dialysis purposes. The present invention provides a vascular prosthesis for use as an- access graft which overcomes., to some extent, the problems aforesaid. According to the invention there is provided a vascular prosthesis adapted for use as an access graft which has a permanent set, kink-resistant U-bend section. The cross-section of the prosthesis may be circular or it may be non-circular. Examples of non- circular cross-sections for prostheses are tricuspid epitrochoid and deltoid. The vascular prosthesis may be arranged to be 'self sealing' after multiple needle puncture. According to the invention there is also provided a method of making a vascular prosthesis which comprises the steps: a) making a length of bio-compatible vascular prosthesis by coagulation casting and not allowing the casting to dry, b) forming a U-bend in the prosthesis without introducing a kink therein, c) drying the prosthesis while so formed. According to the invention there is also provided a method of making a vascular prosthesis which comprises the steps: a) making a length of bio-compatible vascular prosthesis by coagulation casting and allowing the casting to dry, then re-wetting the casting. b) forming a U-bend in the prosthesis without introducing a kink therein, c) drying the prosthesis while so formed. The prosthesis may be made of a heat degradable polymer and then drying is effected at a temperature which will not degrade the polymer. However, drying may clearly be carried out at temperatures up to 40°C (blood heat) as polymers which will degrade at such temperatures are obviously unsuitable for implantation. A solution of coagulatable polymer may be dissolved in an organic solvent and may be coagulation cast onto a mandrel to make the prosthesis. The mandrel may have a circular cross-section, or a non-circular cross-section. Examples of non- circular cross-sections are tricuspid epitrochoid and deltoid. The mandrel may be a pre-formed tube of PTFE. The mandrel may be dip coated in the polymer solution. The polymer solution may, however, be extruded through an extrusion head onto the mandrel to more easily provide a prosthesis of uniform wall thickness. The polymer solution may contain a pore-forming agent soluble in a coagulant to leave a porous cast. The polymer may comprise polyurethane, and may be a linear segmented poly(ether)urethane with a number average molecular weight in the region 20,000 to 60,000. The solvent may be aprotic, and may comprise N,N-Dimethylacetamide or N,N-Dimethylformamide. The concentration of the polymer in the solution may be between 10 and 30 grams/decilitre. The coagulant may comprise water. The invention will be further apparent from the following description with reference to the figures of the accompanying drawings, which show by way of example only, three forms of the vascular prosthesis embodying same. Of the drawings :- Figure 1 shows a. top view of the prosthesis of the invention; Figure 2 shows the cross-section of three types of the prosthesis shown in Figure 1; Figure 3 shows the cross-section of three types of mandrels to produce the prosthese ' s of Figure 2. Figure 1 shows a vascular prosthesis 20 adapted for use as an access graft having a permanent set, kink-resistant U-bend section 21. The cross-sectional shape of the prosthesis 20 is not limited. Figure 2 shows three cross-sections of the prosthesis 20, a circular cross-section (a); a tricuspid epitrochoid cross-section (b) and a deltoid cross-section (c). It may be arranged for the prosthesis 20 to have a non-circular cross-section, whereof the lumen 22 will not totally occlude, thus further increasing the kink-resistance of prosthesis 20, The prosthesis 20, is made from a bio-compatible material, such as an elastomer, or a combination of elastomer and polytetrafluoroethylene (PTFE). Preferred elastomers include polyurethane, polyurethaneurea, segmented polyurethanes and segmented polyurethaneureas. These materials give the prosthesis 20 the property of tending to be 'self-sealing' after needle_puncture, even after multiple needle puncture. The prosthesis 20 has a wall thickness and lumen size capable of withstanding multiple needle puncture by a large bore, i.e., 15 gauge needle used for dialysis treatment, while maintaining a compliance similar to that of natural vessels, overcoming the problem which at present prevents such thicker walled, small diameter conventional prostheses being used as access grafts. The prosthesis 20, having a permanent set, kink-resistant U-bend 21 is made from a length of vascular prosthesis by coagulation casting a solution of coagulatable heat degradable polymer dissolved in an organic solvent onto a mandrel. Before allowing the casting to dry, or after re-wetting the casting after drying, a U-bend is formed in the prosthesis without introducing any kink therein, and the prosthesis so formed is dried. The prosthesis 20 is dried in an oven at a temperature which will not degrade the polymer, usually up to 40°C The U-bend 21 is formed in the length of prosthesis, after the introduction of an internal flexible former to prevent collapse of the lumen of the prosthesis on bending, either by drying over a shaped former, or by simply bending the length of prosthesis, having locating means to maintain the U-bend in position until dried. Once dried the internal flexible former is removed. A solution of coagulatable polymer, such as a linear segmented polyetherurethane with a number average weight of 20,000 to 60,000, is dissolved in a solution comprising an organic solvent, such as N,N-Dimethyl- acetamide or N,N-Dimethylformamide, at a concentration of polymer in the solution of between 10 and 30 grams/ decilitre. In addition, the polymer solution contains a pore-forming agent, soluble in a coagulant to leave a porous cast, such as sodium hydrogen carbonate ground to an average particle size of 60 microns in an amount between 10 and 60 percent by weight. The polymer solution also contains a surfactant, such as sodium dodecyl sulphate, in an amount between 1 and 10 percent by weight. A length of prosthesis- is produced either by dip coating a mandrel in the polymer solution, or, to more easily produce a prosthesis having a uniform wall thickness, by extruding the polymer solution through an extrusion head as described in GB-A-2,204,873. The polymer-coated mandrel is then immersed iri a coagulant and allowed to coagulate. The preferred coagulant is water which is maintained at a constant temperature throughout the coagulation process, usually 40°C. The coagulation process normally takes 1 to 2 hours. The mandrel is not limited in its cross-section. Figure 3 shows three examples of mandrels. Mandrel 23 has a circular cross-section and is used to produce the prosthesis of Figure 2(a). Mandrel 24 has a tricuspid epitrochoid cross-section and is used to produce the prosthesis of Figure 2(b). Mandrel 25 has a deltoid cross-section and is used to produce the prosthesis of Figure 2(c) . It will be appreciated that it is not intended to limit the invention to the above example only, many variations, such as might readily occur to one skilled in the art, being possible, without departing from the scope thereof as defined by the appended claims.

Claims: CLAIMS 1. A vascular prosthesis adapted for use as access graft having a permanent set, kink-resistant U-bend section. 2. A vascular prosthesis according to claim 1, wherein the cross-section of the prosthesis is circular. 3. A vascular prosthesis according to claim 1, wherein the cross-section of the prosthesis is non-circular. 4. A vascular prosthesis according to claim 2, wherein the cross-section is tricuspid epitrochoid. 5. A vascular prosthesis according to claim 3, wherein the cross-section is deltoid. 6. A vascular prosthesis according to any preceding claim, comprising a bio-compatible material. 7. A vascular prosthesis according to any preceding claim wherein the prosthesis is self sealing after multiple needle puncture. 8. A method of making a vascular prosthesis having a permanent set, kink-resistant U-bend section, comprising the steps: a) making a length of bio-compatible-vascular prosthesis by coagulation casting and not allowing the casting to dry, b) forming a U-bend in the prosthesis without introducing a kink therein, c) drying the prosthesis while so formed. 9. A method of making a vascular prosthesis which comprises the steps: a) making a length of bio-compatible vascular prosthesis by coagulation casting and allowing the casting to dry, then re-wetting the casting, b) forming a U-bend in the prosthesis without introducing a kink therein, c) drying the prosthesis while so formed. 10. A method according to any one of claims 9 or 10, wherein the prosthesis is dried at a temperature which will not degrade the bio-compatible material. 11. A method according to claim 8,' wherein the drying temperature is less than 40°C 12. A method according to any one of claims 8 to 11, wherein a solution of coagulatable polymer dissolved in a solution comprising an organic solvent is coagulation cast onto a mandrel. 13. A method according to claim 12, wherein the mandrel has a circular cross-section 14. A method according to claim 12, wherein the mandrel has a non-circular cross-section. 15. A method according to claim 14, wherein the mandrel has a tricuspid epitrochoid cross-section. 16. A method according to claim 14, wherein the mandrel has a deltoid cross section. 17. A method according to any one of claims 12 to 16, wherein the mandrel is a pre-formed tube of PTFE. 18. A method according to any one of claims 8 to 17, wherein the mandrel is dip coated in the polymer solution to provide a prosthesis of uniform wall thickness. 19. A method according to any one of claims 8 to 18, wherein the polymer solution is extruded through an extrusion head onto a mandrel to provide a prosthesis of uniform wall thickness. 20. A method according to any one of claims 8 to 19, wherein the polymer solution contains a pore-forming agent soluble in a coagulant to leave a porous cast. 21. A method according to any one of claims 8 to 20, wherein the polymer comprises polyurethane. 22. A method according to claim 21, wherein the polyurethane is a linear segmented poly(ether)urethane with a number average molecular weight in the region 20,000 to 60,000. 23. A method according to any one claims 8 to 22, wherein the solvent is aprotic. 24. A method according to claim 23, wherein the solvent comprises N,N-Dimethylacetamide or N,N-Dimethyl- formamide. 25. A method according to any one of claims 8 to 24, wherein the concentration of the polymer in the solution is between 10 and 30 grams/decilitre. 26. A method according to any one of claims 8 to 25, wherein the coagulant comprises water.

Inventors: UNDERWOOD CHRISTOPHER JOHN, UNDERWOOD, CHRISTOPHER, JOHN
Applicants: NEWTEC VASCULAR PRODUCTS LIMIT, NEWTEC VASCULAR PRODUCTS LIMITED
Application year: 1992
Main classification: A61L27
IPC codes: A61F2, A61L27, B29C47