GB2130521A - A synthetic tubular structure - Google Patents

Abstract

Production of a synthetic tubular structure for use as an arterial prosthesis comprising a cellular polyurethane tube (10) preformed by coagulation casting onto a former. A water soluble coating (11) having a smooth surface is separately applied to another former (12), and a polyurethane film (13) is solvent cast onto the coating (11). The preformed tube (10) is subsequently shrunk onto the film (13) and the former (12) is removed by dissolving the coating (11) in water. The materials are bio-compatible and compliant to pulsatile flow, and the inner film (13) is smooth to within a tolerance of 10 microns. <IMAGE>

Description

SPECIFICATION A synthetic tubular structure THIS INVENTION concerns the production of a synthetic tubular structure, particularly though not exclusively for use as a prosthesis for the replacement of human or animal blood vessels and especially arterial tube The invention relies with particular emphasis on the need to produce an arterial prosthesis having an internal diameter less than 6 millimetres.

Larger bore artery may be replaced synthetically with less regard for the mechanical or hydraulic properties of the artificial tubing since flow characteristics are more readily maintained in a larger conduit. In small bore arterial prosthesis, on the other hand where the volume of blood flow is low, the properties of the wall of the tubing have a much greater affect on the flow characteristics and are therefore much more critical.

Matching of the mechanical properties of the prosthesis to those of the host artery is essential if efficient transport of pulsatile flow through their junction is to be achieved. Incompatibility of mechanical parameters can cause disruption in the flow characteristics at the junction leading to partial reflection of the incident pulse and a consequent reduction in flow through the junction. Pulsatile flow in the peripheral arteries supplying the arterioles is essential for satisfactory tissue perfusion. This matching of the mechanical properties, which is of vital importance for a true prosthesis, has been largely ignored in the production of previous synthetic arterial replacements.

If blood is to be successfully transported through the arterial system to the capillaries, energy losses during flow must be minimised.

A healthy arterial system will achieve this naturally but when a prosthesis is introduced energy losses due to incompliance of the material of the prosthesis and frictional resistance from the inner surface thereof can be considerable. Transmission of pulsatile flow with minimal energy loss requires the use of a tube of high compliance to radial extension, typically for the superficial fumoral artery in the thigh for at least the first 7% of such extension. At greater radial extensions it is desirable for the compliance of the tube to decrease and resist further distention thus protecting against bursting and aneurysm formation, features analogous to the behaviour of the healthy arteries.

A further problem experienced by any prosthesis once implanted is fibrous encapsulation. This growth of tough fibrous tissue at and around the site of the implant, with a compliant vascular prosthesis can cause a severe restriction of pulsatile flow if the prosthesis relied only upon external dilation for pulse transmission. However, this problem may be circumvented if the prosthesis also has a degree of wall compressibility.

Frictional energy losses at the wall of the tube can be minimised by ensuring as smooth a surface as possible. A smooth surface will also inhibit pooling of procoagulents and the adhesion of platelets which would otherwise ultimately give rise to thrombosis. Many previous synthetic prostheses have overlooked this aspect of performance, preferring instead an open structure at the wall to assist tissue ingrowth or pseudointima formation, although preudointimal proliferation has been found to be the most usual cause of blockages. It is therefore an object of this invention to prepare a tubular prosthesis which has a smooth interior surface with surface defects being maintained at less than 10 microns.

Therefore the design requirements of a successful arterial prosthesis are that it must have a similar mechanical response to pulsatile flow when compared with natural material; it must be highly compliant to initial internal pressure but increasing in stiffness as the pressure increases; some of the compliant pulsatile extensibility must be accommodated without external dilation of the tube, because fibrosis might prevent this; and the inner surface must be as smooth as possible. All of these requirements are of course additional to the necessity for the material of the construction to be stable and bio-compatible thus not to provoke rejection symptoms.

Polyurethanes are materials ideally suited to the construction of arterial replacements.

Many of this class of materials are known to be bio-compatible and have already been approved for internal prosthetic use. They are stable materials with a wide range of mechanical performance ranging from those of a soft elastomer to those which are substantially rigid.

According to the present invention there is provided a method of producing a synthetic tubular structure for use as an arterial prosthesis comprising the steps of applying to a former in a dust-free environment, a soluble coating having an outer surface in which any defects are no greater than 10 microns, casting in intimate contact over said coating a film of a material which will thus provide a smooth inner surface for said structure, forming a cellular polyurethane tube by coagulation casting, shrinking the latter onto said film to form a bond therebetween and dissolving said coating to enable the finished structure to be removed from said former.

Incorporated within the inventive step herein defined and explained is a tubular structure produced by the aforesaid method, and especially a prosthetic tubular material for use in the replacemént of animal or human artery.

Details of one embodiment of the method and the tube produced thereby will now be given, by way of example only, with reference to the accompanying drawings, in which:~ Figure 1 is a sectional view of the tubular structure during production; Figures 2a and 2b illustrate, in cross-section, the intended performance of a tube produced by this method; and Figure 3 illustrates schematically one form of apparatus for producing the structure.

Referring now to the drawings, the tubular structure is produced by coagulation casting a cellular polyurethane material to produce the tubular body 10. This is performed as will be described on a cylindrical mandrel or former.

Separately, a water soluble coating 11, for example of polyvinyl-alcohol, is applied to a mandrel or former 12 in a dust-free environment such that the surface of the coating is substantially free from defects the largest of which are maintained within the tolerance of 10 microns in size. Applied to the outer surface of the coating 11 by solvent casting is a polyurethane film 1 3 approximately 1 5 microns thick the inner surface of which corresponds with the smooth outer surface of the coating 11. Once the film 1 3 is fully cured the preformed foam tube 10 is applied to the film 13 to form a bond therewith. This can be effected by heat shrinking or by initial swelling or stretching, then relaxing the material utilising its natural resilience, or by any other step which ensures intimate contact at the interface.The final process step involves soaking the whole structure in water to dissolve the polyvinyl-alcohol coating 11 thus enabling the mandrel 12 to be removed. The resultant product consists of the outer cellular tube 10 and the polyurethane film or skin 13 which has an internal surface complying with the required characteristics.

With reference to Figs. 2a and 2b, the produce is shown in use as an arterial prosthesis which following a period after being implanted in the body is surrounded by a fibrous capsule 14. With pulsatile blood flow along the prosthesis the outer foam tube 10 is able to compress usefully to absorb a substantial proportion of the internal pressure thus reducing the need for dilation of the tube as a whole. The cellular polyurethane is wholly resilient thus to compress and expand in direct response to the pulsatile flow.

Referring now to Fig. 4, an apparatus for producing the cellular tube 10 by the technique of coagulation casting comprises a carriage 20 capable of being driven in both directions along a fixed track 21 by means of a chain or belt drive member 22 drivingly connected to a motor 23. In an experimental version of the apparatus the track 21 is approximately 140 centimetres in length and the motor 23 is designed to drive the carriage 20 at such a speed that it travels the length of the track 21 in one direction in approximately 3 3 minutes at a constant speed. Both the length of the track 21 and the speed at which the carriage is driven may be varied to suit the requirements of the process.An extruder head 24 connected to the carriage 20 has an inlet port partway along its length to which a pipe 25 is connected for the supply of a polymer solution to the extruder head 24.

Slidable within the head 24 is a sleeve 26 of a material such as polytetrafluroethylene, and the sleeve 26 surrounds a mandrel or former 27 in fixed linear disposition with respect to the track 21 but rotatably mounted in supporting brackets 28 one at each end of the former. The former 27 and the extruder head 24 are, in use, fully submerged in a bath S containing coagulating liquid. Polymer solution is stored in a tank 29 and circulated by a pump 30 to a valve 31 on the carriage 20.

Pipe 32 carries the solution from the pump 30 to the valve 31, and the solution may be recirculated by a pipe 33 to the storage tank 29. A pipe 34 delivers the solution from the tank 29 to the pump 30. The valve 31 can be actuated by means of a handle 35 to direct the polymer solution back to the storage tank 2.9 for continuous circulation therethrough or alternatively and proportionally to pipe 25 and thus extruder head 24. At the start of an operating cycle the carriage 20 is preferably at the right-hand end of track 21 when viewed in Fig. 4. A fixed metal sleeve 36 advances the PTFE sleeve 26 to the left within the extruder head 24 so that polymer solution may flow out of the right-hand end of the extruder head 24 as the carriage 20 progresses to the left along track 21.At the start of the production run, the handle 35 on valve 31 is turned to permit the polymer solution to flow into pipe 25 and extruder head 24. Also, the former 27 is continuously rotated via a motor 37 and gears 38.

When the carriage reaches the left-hand end of the track with the polymer solution extruded out fully along the length of the former, the PTFE sleeve 26 engages a further metal sleeve 39 at the left-hand of the former 27, and is caused to move to the right within the extruder head 24. The length of the sleeve 39 is such as to cause sleeve 26 to seal the inlet from pipe 25 into head 24 and prevents the coagulating liquid within the bath S from entering the extrusion head.

Finally, the handle 35 is returned to the position in which it continuously circulates the polymer solution through the tank 29. Rotation of the former 27 continues until the formed polymer tube is stable. The extrusion rate, and therefore the thickness of the tube produced during the process can be controlled by adjusting the rate of feed of the pump 30 and/or by varying the drive speed from motor.

23. Additionally, the flow from pump 30 can be varied selectively during travel thus to produce a tube having varying thickness and thus resilience along its length. Alternatively, the profile of the former 27 can be other than uniform.

The apparatus just described operates to produce the cellular polyurethane tube by a technique known as coagulation casting.

Many polymers are manufactured in this manner, and selection of their characteristics such as pore size, elasticity and other properties is well known in the art. It is believed that this technique is novel, however, in the production of a tubular structure. The polymer used is a segmented polyether urethane and to this is preferably added a low specific gravity filler such as sucrose and a diluent such as acetone.

In the solvent casting of the film 13 the forme 11 is first coated with a material which in a dust free environment will produce a coating of uniform thickness having a smooth outer surface, i.e. with surface defects maintained to within 10 microns in size, and which is readily soluble after the product is formed, and is inert to the extent that it does not interact with the polyurethane film 13 to be cast thereon. An example of a suitable material is polyvinyl-alcohol since once formed it will resist most polyurethane solvents and is readily applied to the former 11 by dip coating. The material is allowed to dry in a dust free environment in an oven at 80 C for approximately one hour. The polyurethane film 13 is solvent cast onto the coating 12 by one of a number of suitable methods such as dip coating, spray coating, brushing or electrostatic deposition.The finished film may be up to 50 microns in thickness and is preferably within the range of from 10 to 20 microns, and should have elastic properties which will correspond with those of the cellular foam tube to which it will be bonded.

Before attaching the cellular tube 10 it may be necessary to apply to the film 13 a bonding agent or adhesion enhancer.

The cellular tube 10 has an internal diameter approximately equal to the diameter of the former 11 and is initially slid over a thin walled tube (not shown). The tube has an internal diameter larger than that of the former 11 and the film 13 thereon. This tube is then placed over the film 13 and withdrawn permitting the cellular tube 10 to relax by resilience into contact with the outer surface of the film 13. The whole assembly is then placed in an oven at a temperature which will encourage bonding of the cellular material to the film surface. If polyurethane is the material used for the tube 10 and the film 13 then the oven is typically at approximately 1 00 C and the residence time, about 1 hour.

After bonding the assembly is suspended in a circulating bath of water typically at 80 C, so that the water will dissolve the polyvinylalcohol within approximately half an hour, and thereafter the former 11 can be removed from the tubular product whereupon the latter is again washed several times to remove all possible traces of polyvinyl-alcohol. If required, the outer surface of the tube 10 may be sealed with a suitable material spray coated thereon. This will help prevent tissue growth into the foam when the product is used for prosthesis. The inner surface of film 13 can be coated or treated with known substances to improve bio-compatibility.

Recently available techniques for the measurement of the behaviour of human artery material have provided the information enabling a tubular structure as described herein to be produced which will perform as closely as possible to its natural counterpart, and the important requirements of compliance, biocompatibility, and smooth interior surface can be achieved by production methods which are not excessively expensive when compared with techniques which have already been tried for this purpose. More particularly, the product may be used in small bore arterial prosthesis having an internal diameter of 6mm or less.

It is envisaged that the tubular structure may be produced by a semi-continuous process, in which the film is cast onto a former precoated with a soluble coating, and the outer cellular polyurethane tube is then coagulation cast directly onto said film, the latter being perhaps cross-linked to enable a firm bond with the cellular material.

Claims (16)

1. A method of producing a synthetic tubular structure for use as an arterial prosthesis comprising the steps of applying to a former in a dust-free environment, a soluble coating having an outer surface in which any defects are not greater than 10 microns, casting in intimate contact over said coating a film of a material which will thus provide a smooth inner surface for said structure, forming a cellular polyurethane tube by coagulation casting, shrinking the latter onto said film to form a bond therebetween and dissolving said coating to enable the finished structure to be removed from said former.

2. A method according to Claim 1, wherein said coagulation casting is carried out by progressively extruding said polyurethane material from a travelling head onto a former which is submerged in a bath of coagulating liquid, said former being rotated during said extrusion and until the extruded polyurethane has cured to a stable state, whereupon the tube is removed from said former.

3. A method according to Claim 1, wherein the material of said tube is a segmented polyether urethane, and a low specific gravity filler is added thereto prior to casting.

4. A method according to Claim 1, wherein said coating is water-soluble and wherein said film is polyurethane which is solvent-cast onto said water-soluble coating and subsequently allowed to cure.

5. A method according to Claim 4, wherein said coating is polyvinyl-alcohol.

6. A method according to Claim 4, where said polyurethane film is between 10 and 20 microns in thickness.

7. A method according to Claim 4, wherein said cellular polyurethane tube is heat shrunk onto said polyurethane film.

8. A method according to Claim 1, wherein a bonding agent is applied to said film prior to the application thereto of said polyurethane tube.

9. A method according to Claim 4, wherein said polyurethane tube is permitted to relax by resilience into contact with the outer surface of said film, the structure being thereafter subjected to elevated temperature to encourage bonding of the cellular material to the film surface.

10. A method according to Claim 9, wherein said elevated temperature is maintained at or close to 1 00 C and for a period of 1 hour.

11. A method according to Claim 4, wherein the finished structure is water-washed repeatedly to remove all traces of said watersoluble coating.

12. A method according to Claim 1, wherein the outer surface of the finished structure is spray coated with a sealant.

13. A method according to Claim 1, wherein said structure is of circular crosssection.

14. A synthetic tubular structure for use as an arterial prosthesis, and made in accordance with any one of Claims 1 to 13, comprising a cellular polyurethane tube having intimately attached to the inner surface thereof a polyurethane film, any defects on the inner surface of said film being no greater than 10 microns.

15. A synthetic tubular structure according to Claim 14, wherein the outer surface of said cellular polyurethane tube is coated with a sealant.

16. A synthetic tubular structure according to Claim 14 or Claim 15, whos interior surface is bio-compatible and wherein said cellular polyurethane tube is responsive to pulsatile flow being highly compliant to a predetermined initial internal pressure without substantial external dilation, but increases in stiffness beyond said predetermined pressure.