SPECIFICATIONProduction of cellular polyurethaneThis invention relates to the manufacture of poromeric polymeric products by solvent exchange and more particularly, but not exclusively, the production of polymeric products suitable for use as replacement for human tissue.
It is well known that synthetic replacement human tissue must be compatible with the natural tissue and environment in which it is to be implanted. At first it was though that the essential requirement must be biological compatibility. However, the failure of synthetic tissue which was biologically compatible led to the realisation that it was necessary for synthetic tissue to be mechanically compatible also. In other words synthetic tissue must have about the same mechanical properties as natural tissue.
This requirement for mechanical compatibility is of particular importance for some types of vacular prostheses. Circulation of the blood around the body occurs as a succession surges consequent upon the pumping action of the heart. These surges are accommodated by expansion of the wall of a blood vessel followed by contraction after the surge has passed. If the mechanical properties of the vascular prosthesis will not permit such expansion and contraction then the flow of blood may be disturbed and the junction between the prosthesis and the natural tissue may be damaged.
It has been proposed to produce synthetic tissue having machanical properties which match the properties of natural tissue by forming a polyurethane product with voids therein. Specifically a vascular prothesis can be made by dipping a rod shaped mandrel into a solution of a polyurethane. The viscosity of the solution is such that a coating of solution will remain on the mandrel as it is withdrawn from the solution. The coated mandrel is then dipped in a non-solvent for the polyurethane, e.g.
water, to bring about solvent displacement of the polyurethane solvent thereby forming voids within the polyurethane and at the same time precipitating the polyurethane. After drying to remove the water or other residual solvent the procedure is repeated a number of times until the coating on the madrel has reached the desired thickness. The resultant product after removal from the mandrel comprises a tube of sponge-like polyurethane.
It has been found that it is difficult to make artificial tissue of the ideally desirable density by the above described "dipping" procedure. That is because in order to form a coating on the mandrel the viscosity of the polyurethane solution must be carefully controlled which in practice places a limit on the maximum concentration of polyurethane in the solution. At the concentrations consistent with the appropriate viscosity for dipping the porous or cellular polyurethane formed during solvent displacement is not sufficiently strong to form a 'self-sustaining" product so that it collapses thereby eliminating may of the voids and forming a denser than might otherwise be expected. If the concentration is raised to meet this problem then the viscosity increases so that a uniform coating of adequate thickness cannot be readily obtained by the "dipping" method.
The present invention has been made from a consideration of this problem.
According to the invention there is provided a process for the manufacture of cellular polyurethane products comprising forming a mould for the product from a material which includes a releasable solvent, then depositing a polyurethane solution in said mould, whereby the releaseable solvent migrates from the mould into the polyurethane solution causing displacement of the polyurethane solvent, and thereby precipitating the polyurethane in cellular or porous form. The releaseable solvent must be completely miscible with the polyurethane solvent ad be a non-solvent for the polyurethane.
With the present invention the polyurethane solution can be of a much higher concentration than in the dipping method; for example concentrations up to 40% be weight have been used successfully. Even high concentrations can be used if the solution is warmed. As a result of using such high concentrations the cellular product is formed on solvent displacement is "self-sustaining" and does not collapse. Thus a product of the desirable density can be formed readily. In addition since the product is formed by moulding in one operation, rather than being built up be successive dipping operations, it avoids the danger of creating boundary layers of different porosity.
The material from which the mould is formed is preferably a water based gel such as gelatine or agar. Other material can be used if desired such as ice. Once important feature of the mould material is that it should include a liquid that will be released for solvent displacement; in the examples given above the liquid is water. The mould should also be rigid enough not to be deformed during filling and solvent exchange processes, and should not suffer excessive syneresis on standing.
The term mould as used herein is intended to include not only a body defining the shape of the product to be formed, but other kinds of support surfaces. For example it is possible to form sheets of cellular polyurethane products by the present invention by merely depositing a coating of polyurethane solution on the flat surface of a gel.
The present invention is particularly suited to the production of vascular protheses. Such products can be formed by first preparing a gel mould in the form of a hollow elongate cylinder of the apropriate cross-section. A core of circular cross-section made of glass or other suitable material is located concentrically in the mould and the annular space thereby defined is filled with polyurethane solution. Water is released from the gel into the polyurethane solution to cause displacement of the polyurethane solvent and precipitation of the polyurethane. When the solvent displacement has been substantially concluded the product in the form of a cellular polyuretane tube is withdrawn from the mould, washed to remove residual solvent, and dried.
The polyurethane material used in the present invention is chosen in accordance with the intended use of the product obtained therefrom. Where the product is intended for use as human tissue the polyurethane must be biocompatible. There is a wide range of polyurethane materials which are appropriate for such use.
The mechanism of solvent displacement is well known and, therefore, not described here in detail. It should be noted however, that the nature of the product produced by the invention will depend, inter alia, on the rate of such displacement. In addition to poromeric structure of the product can be modified by the inclusion of additives such as are usually included for example surfactants, salts, nucleating agents and the like.
Generally a certain amount of contraction takes place upon solvent displacement so that if the polymer adhered to the mould surface an uneven or broken surface would be formed on the product. It is therefore an integral feature of the invention that the skin formed at the gel-polyurethane interface does not adhere to the gel, either because the polyurethane solution is substantially unable to penetrate the gel structure, or because of the formation of a thin liquid film of the mixed solvents at the interface, or from a combination of these reasons. A similiar argument applies in the case of a mould made of ice. The products obtained by the invention consequently have a smooth outer surface.Tubes can be given a non-poromer.c inner surface if desired by precoating the mandrel with a suitable film forming polymer, which may or may not be the polymer used to form the bulk of the tube.
CLAIMS (Filed on 29 July 1982)1. A process for the manufacture of cellular or porous polyurethane products comprising forming a mould for the product from a material which includes a releasable solvent, then depositing a polyurethane solution in said mould whereby the releaseable solvent migrates from the mould into the polyurethane solution causing displacement of the polyurethane solvent and thereby precipitating the polyurethane in cellular or porous form, said releaseable solvent being miscible with the polyurethane solvent and a non-solvent for the polyurethane.
2. A process a claimed in Claim 1, wherein the polyurethane solution contains up to 40% by weight polyurethane.
3. A process a claimed in Claim 1 or Claim 2, wherein the releasable solvent is water.
4. A process as claimed in any preceding claim, wherein the mould forming material includes a gel or ice.
5. A process as claimed in Claim 4, wherein the gel is water based.
6. A process as claimed in any preceding claim, wherein the mould defines a cylinder.
7. A process as claimed in Claim 6, wherein a core is located in the cylinder to define a mould for a tube.
8. A process as claimed in Claim 7, wherein the core is of a circular cross-section.
9. A process as claimed in Claim 7 or Claim 8 wherein the core is made of a rigid material.
10. A process as claimed in any of Claims 7 to 9 wherein the core is coated with a film forming polymer.
11. A cellular or porous polyurethane product produced by the process as claimed in any preced