Synthetic material apt to stably adsorb high quantities of heparin and process for the production thereof.

0001The present invention relates to a new synthetic material apt to stably adsorb high quantities of heparin. Said material is therefore particularly suited for manufacturing protheses or other medical equipment, to which it confers remarkable non-thrombogenic properties when placed in contact with blood.

0002The invention also concerns a preferred process for the production of the aforementioned material, and the use of such material in manufacturing protheses and other medical equipment designed to come in contact with blood.

0003It is known that blood, in contact with the surface of prosthetic elements, generally determines the formation of blood clots. This at present constitutes the most serious factor limiting the use of synthetic materials (and particularly polymeric materials) in the field of protheses and artificial organs. The formation of blood clots may be chemically avoided by the systemic administration of anti-coagulants. The inadequacy of such a step is evident in the case of protheses designed to remain permanently in the organism, since, in this case, the entire circulatory system of the patient must be maintained in a modified condition, often for life. The above perhaps constitutes the main reason whereby it has not yet been possible to introduce an artificial heart in medical practice.

0004In order to reduce blood clot formation caused by polymeric material, a number of methods have been suggested; among these, the superficial heparinization of the above materials is undoubtedly one of the most important. Heparin is in fact endowed with marked anti-coagulant properties and is commonly used, in aqueous solution, as an anti-coagulant. It consists of a high charge-density acid mucopolysaccharide containing sulphonic, sulphamic and carboxylic groups. The aqueous solution behaves like a strong acid, and at a physiological pH, the substance is completely ionized. As mentioned above, it is kncwn that heparin, bonded or in any case adsorbed by any type of material, is apt to confer to this-latter non-thrombogenic properties.

0005The U.S. Patent No. 3.865.723 hence describes basic polymers, obtained by polyaddition of bis-acrylamides with secondary diamines, apt to form stable complexes with heparin, but having mechanical properties of high rigidity and scarce moldability, which make them in practice unsuitable for use in the biomedical field.

0006Other similar polymeric materials have been described in Experientia 1973, 29, 93; Polymer 1977, 18, 378; J. Polym. Science 1977, 15, 2151; Polymer 1978, 19, 1063; Biomaterials 1983, 4, 218; Italian Patent application No. 20968 A/84 filed on May 17, 1984.

0007Of particular interest are polymers based on poly(amidoamine)s (PAA), which are apt to form stable complexes with heparin in an aqueous medium; such a property is maintained also when the PAA are inserted as segments in block copolymers with other types of polymers - e.g. polyethylene and polystyrene - or are grafted onto the surface of various materials, as glass, Dacron, etc.

0008Recent studies have finally been made on the possibility of associating PAA with polyurethanes. Said association in fact appears to be particularly advantageous, as it would allow to combine the easy heparinization properties of PAA with the satisfactory mechanical characteristics of polyurethanes, which have placed these materials among those most frequently used as components for equipment designed to come in contact with blood; e.g. vascular protheses, blood filters, catheters, cardiac valves, etc.

0009A first proposed method for obtaining said association consists in reacting first macrodioles with an excess of diisocyanate, thereby obtaining a polyurethane with reactive diisocyanate branches, onto which the PAA are subsequently grafted. A second method consists instead in carrying out a chemical etching directly onto a polyurethane prothesis, grafting thereon first the diisocyanate and subsequently the PAA.

0010Nevertheless, neither of said methods have given satisfactory results. In fact, in the first case, the reaction of copolymerization is not easy to control, whereby the product obtained does not provide sufficiently constant and reproducible chemical-physical characteristics, to allow a practical application thereof. The second method does not have this drawback, but the chemical etching of the grafting onto the polyurethane prosthetic element deteriorates, at times even considerably, its mechanical characteristics. In either case, there is anyhow a relative incompatibility with blood, which is essentially determined by the presence of a still insufficient number of active centres capable of stably bonding heparin.

0011The object of the present invention is therefore to supply a new synthetic material, providing the same satisfactory mechanical characteristics of polyurethane and being furthermore heparinizable to a considerable extent, and thus endowed with marked non-thrombogenic properties when placed in contact with blood.

0012A further object of the invention is to supply a particularly simple and efficient process for producing the aforementioned material.

0013According to the present invention, said objects are reached through a synthetic material consisting of a polyurethan resin, cross-linked with copolymeric chains in correspondence of at least part of its own urethane groups <chemistry id="chem0001" num="0001"><img file="EP0230281A2_D0001.tif" /></chemistry>, as per the following general formula: <chemistry id="chem0002" num="0002"><img file="EP0230281A2_D0002.tif" /></chemistry>wherein, <ul id="ul0001" list-style="none"><li>PU--- is a polyurethanic polymeric chain;</li><li>R¹ is an alkyl or aralkyl, containing functional groups inert to isocyanates;</li><li>R² is an alkylene;</li><li>R³ is an alkylene, arylene or other bivalent organic radical; and</li><li>_Rⁿ is a polyamidoaminic radical.</li></ul>

0014The aforespecified material is preferably produced with a process comprising the following steps: <ul id="ul0002" list-style="none"><li>a) Preparing a solution of a poly(amidoamine) of general formula: <chemistry id="chem0003" num="0003"><img file="EP0230281A2_D0003.tif" /></chemistry>wherein, R¹ is an alkyl or aralkyl, containing functional groups inert to isocyanates; R² is an alkylene; and Rⁿis a polyamidoaminic radical with a diisocyanate of general formula: <chemistry id="chem0004" num="0004"><img file="EP0230281A2_D0004.tif" /></chemistry>wherein, R³ is an alkylene, arylene, or other bivalent organic radical in a first anhydrous solvent;</li><li>b) Preparing a solution of a polyurethane in a second anhydrous solvent;</li><li>c) Thoroughly mixing the solutions obtained in a) and b)_;</li><li>d) Desiccating the solution obtained in c).</li></ul>

0015It has in fact been found that, by mixing <ul id="ul0003" list-style="none"><li>a) a solution of PAA with secondary amino end groups - easily obtainable with methods known in literature - and of suitable diisocyanates (e.g. hexamethylenediisocyanate, toluenediisocyanate, and the like), in anhydrous solvents such as CHC1₃, CH₂Cl₂, DMSO; with</li><li>b) a polyurethane solution in anhydrous dimethylformamide (or dimethylacetamide, or hexamethylphosphoramide),</li></ul> one obtains solutions which, by elimination of the solvent, give rise to new materials having the characteristics of cross-linked block copolymers - in which the polyurethanic chains are interconnected with poly(amido- amine) chains - endowed with excellent mechanical properties similar to those of the starting polyurethane, and capable, at the same time, of stably adsorbing quantities of heparin even hundred times superior to those obtained with known materials, thereby assuming excellent non-thrombogenic properties.

0016The reactions which occur, operating in accordance with the invention, can be so represented: <chemistry id="chem0005" num="0005"><img file="EP0230281A2_D0005.tif" /></chemistry> wherein, <ul id="ul0004" list-style="none"><li>PU--- represents a generic polyurethanic chain, of which only one functional group is evidenced;</li><li>R¹ is an alkyl or aralkyl, containing functional groups inert to isocyanates;</li><li>_R² is an alkylene;</li><li>R³ is an alkylene, arylene or other bivalent or_lanic radical; and</li><li>Rⁿ is a polyamidoaminic radical.</li></ul>

0017The reciprocal quantities of the two main components of the system, polyurethane and poly(amidoamine), are easily adjustable by varying their proportion in the reaction mixture, and the cross-linking of the end-product may be graded by varying the quantity of diisocyanate.

0018The preferred values of the molar ratio between polyurethane and poly(amidoamine) are included between 100:1 and 1:1, while those of diisocyanate in respect of PAA are between 0.75:1 and 5:1. If the quantity of isocyanic groups is greatly in excess in respect of the quantity of amine groups of the PAA, even direct cross-linking between polyurethanic chains could be obtained. After elimination of the solvents and desiccation, the new product appears as an elastic and flexible material. At this point, it is totally insoluble in any type of solvent, and particularly in water.

0019Heparin adsorption tests have been carried out on samples of the materials obtained. The quantity of heparin which can be adsorbed superficially, and is not dilutable except with drastic methods, as for example with NaOH 0.1M, is very high, i.e. from 10 to 100 times higher than that which can be adsorbed with all the other known materials apt to be heparinized. The invention will anyhow be better explained in the following examples, which should however not be considered as limiting the same.

EXAMPLE 1

00200.65 g of hexamethylenediisocyanate were added to 10 ml of a 6% by weight solution in CHCl ₃ (anhydrous solvent) of a PAA having the following formula: <chemistry id="chem0006" num="0006"><img file="EP0230281A2_D0006.tif" /></chemistry>wherein, <chemistry id="chem0007" num="0007"><img file="EP0230281A2_D0007.tif" /></chemistry>and, R¹= -CH₃; R₂ = -(CH₂)₄ having amino end groups and an average numeric molecular weight equal to about 15,000, obtained according to the descriptions in literature (P. Ferruti et al., J. Polym. Sci. Polym. Chem. Edn. 1977, 15, 2151; Polymer 1977, 18, 387; E. Martuscelli et al., Polymer 1978, 19, 1329).

0021To the solution were added 10 ml of a 10% solution of a commercial polyurethane (Pellethane 2363r80A) sold by Upjohn Polymers of Hertogen- bosh Holland, in DMF. After homogenizing, the solution was subjected to vacuum in a room temperature desiccator, until the solvent was completely eliminated.

0022The resulting product appeared in the form of a resistent and flexible material, with properties similar to those of the starting polyurethane.

0023The quantity of basic nitrogen atoms corresponded exactly to that foreseen on the basis of the PAA quantity used. These were determined by immersion in an excess titrated acid solution and subsequently by acidometric titration of the residual acid. During this process, no passage of soluble material in solution was observed. The absence of free NCO groups was observed.

EXAMPLE 2

0024The same procedure was followed as in the previous example; however, Pellethane 23b3-80A was substituted by a polyurethane especially prepared according to the teachings of D.J. Lyman et al. (International Journal Polymer Materials 1977, 5, 211).

0025The resulting product presented similar analytical data for what concerns the PAA content, the absence of extractable material and the absence of free NCO groups. Its mechanical properties were also similar to those of the starting polyurethane.

EXAMPLES 3 - 7

0026Using the same polyurethane of Example 1 (10% solution in DMF), the same PAA (6% solution in CHC13 anhydro.us solvent) and hexamethylenediisocyanate in variable proportions, and proceeding as described above, one obtained the materials indicated in Table 1, wherein: <ul id="ul0005" list-style="none"><li>- the second column (PU) indicates the quantity of the polyurethane solution (in ml);</li><li>- the third column (PAA) indicates the quantity of the PAA solution (in ml);</li><li>- the fourth column (DIISO) indicates the quantity of the diisocyanate (in g).<img file="EP0230281A2_D0008.tif" /></li></ul>

0027In each case, the tests revealed a quantity of amino groups corresponding to the PAA quantity used, the absence of free NCO groups and the absence of extractable material.

EXAMPLES 8 - 10

0028The procedure was the same as in the previous Examples, using however the synthetic polyurethane mentioned in Example 2 (10% solution in DMF), again the _PAA of the previous Examples 16% solution in CHCL₃ anhydrous solvent), and hexamethylenediisocyanate. The materials indicated in the following Table 2. were obtained. (The indications correspond to those of Examples 3- 7). <tables id="tabl0001" num="0001"><img file="EP0230281A2_D0009.tif" /></tables>

0029Again, in this case, the tests revealed a quantity of amino groups corresponding to that of the PAA used, and the absence of free NCO groups and of extractable material.

EXAMPLE 11

0030A film of the product obtained according to Example 1, having a thickness of 0.5 mm and a surface of 5 x 5 cm, was immersed in a 0.5% solution of heparin in H₂O/C₂H₅OH/CH₃COOH, 48.5/48.5/5.3 in volume, for 24 hours. After this time, the film was repeatedly washed in a phosphate buffer solution (pH 7.4) until no heparin remained in the water used for washing. The heparin stably adsorbed in the material was then extracted using a NaOH solution 0.1M, and was determined quantitatively by means of standard biological tests. The heparin extracted with soda amounted to 25 mg, corresponding to 500.10^-3 mg/cm².

0031Identical results were obtained with the materials mentioned in Examples 2 - 10.

0032By way of comparison, the quantities of heparin adsorbed by known heparinizable materials are reported hereunder: <tables id="tabl0002" num="0002"><img file="EP0230281A2_D0010.tif" /></tables>

EXAMPLES 12 - 15

0033Still with the same starting products of Example 1, further materials according to the invention were prepared by varying this time merely the quantity of polyurethane, while the quantities of PAA and of hexamethyle- nediisocianate remained constant. The materials indicated in the following Table 3 were obtained. For these materials, in addition to the tests carried out for the materials of Examples I to 10 which confirmed the previous results, also the basic nitrogen content and the heparin content (Table 4), as well as some more interesting mechanical properties (Table 5), were determined. By way of comparison, we are also reporting hereunder (Example 15) the data of a polyurethane grafted by chemical etching, first with diisocyanate and subsequently with PAA, according to one of the known methods discussed further above. <tables id="tabl0003" num="0003"><img file="EP0230281A2_D0011.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0230281A2_D0012.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0230281A2_D0013.tif" /></tables>