Modified polyester composition and preparation process and use thereof

Abstract: A modified polyester composition comprising a mixture of a polyester having at least a glycolic acid unit and/or a lactic acid unit with a polyether modified silicone is suitable as material for implanting intravitally

Description: The present invention relates to a bioabsorbable modified polyester composition mainly used for medical materials, a preparation process thereof and use of the composition. The modified polyester of the invention is the result of mixing a polyester having at least a glycolic acid unit and/or a lactic acid unit with a polyether modified silicone. As used herein, the term "polyether modified silicone" means a polyorganosiloxane having as a substituent at least a hydroxyalkyl group or a polyoxyalkylene chain. The modification of the polyester composition can give enhanced flexibility and softness of a bioabsorbable polyester and extend its use for medical materials. Polyesters represented by polylactic acid (hereinafter abbreviated as PLA), polyglycolic acid (hereinafter abbreviated as PGA) and glycolic acid/lactic acid copolymers (hereinafter abbreviated as PGLA) are interesting bioabsorbable materials. These materials are nonenzymatically hydrolyzed in vivo. The decomposition products, glycolic acid and lactic acid, are finally converted to carbon dioxide and water through a metabolic pathway, and are excreted externally. For example, high molecular weight PGA is processed into a form of fiber and used for aseptic surgery materials such as sutures and gauzes. PLA and PGLA are also excellent in processability and solubility in solvents and hence have already been used as the matrix of slow release medicines for intravenous injection by being processed into, for example, microspheres. Japanese Patent Publication SHO 64-2383(1989) has disclosed that PGLA and PLA are processed into the form of film and used an adhesion inhibitor for curing diseases due to tissue adhesion. The preparation of polyesters such as PGA, PLA and PGLA is well documented in the literature as exemplified by Polymer, Vol. 20, p1459(1979). The preparation is generally carried out by the ring-opening polymerization of glycolide and/or lactide which are dehydrated cyclic dimers of glycolic acid and lactic acid. A process for preparing polyesters by the dehydrating polycondensation of glycolic acid and/or lactic acid is also disclosed in, for example, Japanese Patent Laid-Open Publication SHO 56-45920(1981). Polyesters such as PGA, PLA and PGLA are excellent in bioabsorbability and hence utilized in various medical materials. Recently, it has been further desired to provide flexibility and softness to these polyesters. For example, PLA and PGLA have a good film-forming ability and have been known to form a tough film by casting a solution of PLA and PGLA. However, these films are still and unsuitable for use in vivo, particularly for application to soft tissue. According to the information of the present inventors, it has been confirmed that even the film of PGLA which, is said to be the most flexible becomes stiff by completely removing the solvent remaining in the film after casting. Preferred embodiments of the present invention may provide a novel modified polyester composition which is flexible, bioabsorbable and useful for medical materials, a preparation process for the composition, and an intravitally implantable material prepared from the composition. As a result of an extensive investigation carried out in order to solve the above problems, the present inventors have unexpectedly found that, in the preparation of a bioabsorbable polyester composed of a glycolic acid unit and/or a lactic acid unit, a mere admixture of the above bioabsorbable polyester with a certain kind of silicone is effective for the modification of the bioabsorbable polyester, particularly for the improvement of flexibility. Thus the present invention has been completed. Aspects of the present invention include a modified polyester composition obtainable by mixing a polyester containing at least a glycolic acid unit and/or a lactic acid unit with a polyether modified silicone, and an intravitally implantable material composed of the composition. The present invention provides a novel modified polyester composition which is excellent in flexibility, solubility and surface smoothness as compared with conventional bioabsorbable polyesters. Additionally, the present invention provides a simple and reasonable process for preparing the above novel and modified polyester composition. In the preparation process, the properties of the resulting modified polyester composition, for example, hydrolyzability, solubility, and processability, can be controlled by adjusting the amount and molecular weight of the silicone. Further, the modified polyester composition which is provided by the invention is readily mixed with conventional PGA, PLA and PGLA, and hence can be utilized for the modification of these conventional polymers, particularly for the improvement of flexibility and solubility. The composition of the present invention is preferably obtained by uniformly mixing the bioabsorbable polyester with a polyether modified silicone. The modified polyester composition of the present invention can provide medical materials which are soft and have good processability and high biocompatibility, for example, implanting materials such as artificial blood vessels, adhesion inhibiting materials, surgical sutures coating matrices for surgical sutures and matrices for slow release medicines. A detailed description will hereinafter be made of embodiments of the invention. The principal ingredient of the composition in the present invention is the bioabsorbable polyester which is obtainable by conducting the ring-opening polymerization of glycolide and/or lactide and has the glycolic acid unit and/or the lactic acid unit. The glycolide and the lactide for use in the preparation of these ingredients are cyclic dimers, each of which is readily prepared by the dehydrating polymerization of glycolic acid and lactic acid, and successive dehydration, respectively. Lactide includes D-lactide which is a cyclic dimer of D-lactic acid, L-lactide which is a cyclic dimer of L-lactic acid meso lactide which is a cyclic dimer of D- and L-lactic acid, and DL-lactide which is a racemic mixture of D- and L-lactic acid. Any type of lactide can be used, in the present invention. The polymerization in the invention can also be carried out in the presence of other lactones in addition to glycolide and/or lactide which are employed for the monomer. Exemplary lactones include β-propiolactone, β-butyrolactone, β-valerolactone, δ-valerolactone, ε-caprolactone, p-dioxanone and 3-methyl-1,4-dioxane-2,5-dione. The polyether-modified silicone is a polyorganosiloxane bonded to at least a hydroxyalkyl group or a polyoxyalkylene chain, so that it has hydrophilic properties. More generally the term "silicone" refers to a polyorganosiloxane having recurring structural units represented by the formula (VI) : wherein R is a hydrogen atom, halogen atom or an organic group; n is an integer of 1 to 4000; and each R may be the same or different. The group represented by R in the formula (VI) includes, for example, a hydroxyl group, alkyl group, hydroxyalkyl group, aryl group, haloaryl group and polyoxyalkylene group. Exemplary polyorganosiloxanes include polydimethyl siloxane, polydiethyl siloxane, polyphenylmethyl siloxane, polychlorophenyl siloxane, polyfluoro siloxane and polyether modified polydimethyl siloxane. Representative silicones have one or more terminal hydroxyl groups and are represented by the formula (I) : wherein R 1 and R 1 ' are a hydrogen, an alkyl group, a hydrogen terminated polyoxyalkylene group having a polymerization degree of 200 or less, a hydroxyl group, a hydroxyalkyl group or a hydroxy terminated polyoxyalkylene group having a polymerization degree of 200 or less, and at least one of R 1 and R 1 ' is a hydroxyl group or a hydroxy terminated group; R 2 and R 3 are alkyl groups, phenyl groups or halogen atoms, and may be the same or different; and n is an integer of 1 to 4000. Another type of silicone has hydroxyl groups on the side chain of the polysiloxane and is represented by the formula (IV) : wherein R 11 is a hydrogen atom, halogen atom, alkyl group, or a phenyl group and may be the same or different; R 12 is a hydroxyl group, hydroxyalkyl group, hydroxy terminated polyoxyalkylene group having a polymerization degree of 2 to 200, or an alkyl group wherein one of its hydrogen atoms is substituted with a hydroxy terminated polyoxyalkylene group; R 13 is a hydrogen atom, halogen atom, alkyl group, phenyl group or the same group as R 12 and may be the same or different; X is an integer of 4000 or less, 0 inclusive; y is an integer of 1 to 4000; and recurring unit (A) and recurring unit (B) are arranged in random or in block. No particular limitation is imposed upon the molecular weight of the silicone used in the present invention. The molecular weight(Mw) is preferably from 1,000 to 300,000 and more preferably from 10,000 to 100,000. With molecular weight less than 1,000, it is often difficult to give the polyester the desired flexibility. On the other hand, a molecular weight exceeding 300,000 leads to high viscosity of the silicone itself and difficulty in handling. The amounts of the above glycolide and lactide for use in the preparation of the copolymer and composition of the invention are determined depending upon the desired ratio of the lactic acid unit to the glycolic acid unit in the polyester. That is, the glycolide and lactide may be used singly or as a mixture in any arbitrary ratio. More particularly, the amount of glycolide and/or lactide may be varied according to the object for the utilization of resulting modified polyester composition. That is, for example, when a polyester configuration in the structural units of the modified polyester is polylactic acid or polyglycolic acid, lactide or glycolide, respectively, is used alone. When said polyester is a copolymer of lactic acid and glycolic acid, the amount of lactic acid and glycolic acid can be determined depending upon the properties required for the resulting modified polyester. Generally, known bioabsorbable polyesters obtained by the polymerization of glycolide and/or lactide are roughly divided into a lactic acid based polymer containing lactic acid as a principal component and a glycolic acid based polymer containing glycolic acid as a principal component. Glycolic acid based high molecular weight polymers are processed into fibers and used for sterilized surgery materials such as sutures and gauzes. Lactic acid-glycolic acid copolymers and lactic acid homopolymers are excellent in processability and solubility in solvents and are hence processed into pellets, needles, films and microspheres. These materials used for intravital implanting or as a matrix for slow release medicines for intravenous injection. High molecular weight lactic acid homopolymers, in particular, are processed into bars or plates and used for bioabsorbable bonesetting plates in the curing of fractures. The modified polyester compositions of the invention exhibit softness as compared with unmodified bioabsorbable polyesters, have further improved applicability and are suitable for intravitally implanting materials. This characteristic is attained with modified polyesters prepared both by glycolide or lactide alone and with the combined use of glycolide and lactide. Consequently, the modified polyester having softness can be obtained and employed for various medical materials having the above characteristics. As mentioned above, the amount of glycolide and lactide can be determined depending upon the object for use. Glycolide or lactide can be polymerized substantially in the absence of a catalyst. However, it is preferred to use a catalyst in order to obtain a high molecular weight polymer within a short time. Various compounds which exhibit catalytic activity on the reaction can be used as the catalyst. Exemplary known catalysts include stannous octoate, tin tetrachloride, zinc chloride, titanium tetrachloride iron chloride, either complex of boron tifluoride, aluminum chloride antimony trifluoride, lead oxide and other compounds containing multivalent metals. Tin compounds and zinc compounds are preferably used. A particularly preferred tin compound is stannous octotate. Polymerization may be carried out in the presence or absence of the catalyst. Bulk polymerization in a molten state is preferred. In the molten polymerization, the polymerization temperature may be in principle be above the melting point of the monomer, i.e., glycolide or lactide. When solvents such as chloroform and dichloroethane are used, polymerization can be carried out at temperatures lower than the melting point. A temperature exceeding 250°C is unfavourable because decomposition of the resulting polymer takes place. An exemplary modified polyester composition of the invention is a composition containing the specific silicone compound and the polyester composed of at least glycolic acid units and/or lactic acid units as basic structural units. The admixture of the silicone physically modifies the bioabsorbable polyester. No particular restriction is imposed upon the proportion of each component in the modified polyester composition of the invention. However, the amount of the unreacted monomer in particular is preferably 10% by weight or less in the composition in order to exhibit good mechanical strength of the modified polyester composition. The modified polyester composition of the invention may also be prepared by removing unnecessary components from the mixture obtained by the above preparation process. Separation of the unnecessary components from the reaction mixture is generally conducted by conventional fractional precipitation methods utilizing the difference in solubility. Unreacted glycolide or lactide may also be eliminated by heating under reduced pressure in the latter half of or after polymerization. Generally, different kinds of polymers have poor compatibility and are difficult to mix. Consequently, bioabsorbable polyesters such as PGA, PLA and PGLA are very difficult to give flexibility by the addition of common silicones. The modified polyester composition of the invention is a mixture of the polyester composed of basic structural units containing at least glycolic acid units and/or lactic acid units and a specific silicone compound, i.e., polyether modified silicone. A silicone having in the structure a polyether chain such as a polyoxyalkylene group, that is, a so-called polyether modified silicone exhibits good compatibility with the modified polyester composition containing the polyester-silicone copolymer of the invention and polyester-silicone copolymer itself. It has been unexpectedly found that the polyether modified silicone has also good compatibility with conventional polyesters such as PGA, PLA and PGLA and is hence effective for providing flexibility to the polyester. The polyether modified silicone to be mixed with the polyester having at least glycolic acid units and/or lactic acid units is a polyorganosiloxane having as a substituent at least a hydroxyalkyl group or a polyoxyalkylene chain. A particularly preferred substituent is a polyoxyethylene chain having a polymerization degree in the range of 2 to 200. No particular limitation is placed upon the molecular weight. However, a molecular weight exceeding 3000 leads to high viscosity and it becomes difficult to obtain a uniform mixture. High molecular compounds such as polyorganosiloxane which have Si-O bonds in its principal chain have a high bending ability of the chain and a weak intermolecular force, and hence have a low glass transition temperature and are very flexible. However, as mentioned above, common polyorganosiloxane represented by polydimethylsiloxane, i.e., common silicone containing no polyoxyalkylene substituent is hydrophobic, almost insoluble in water and has poor compatibility with polyesters such as PGA, PLA and PGLA. When a common silicone is used for the plasticizer of the polyesters, phase separation takes place and expected flexibility cannot be obtained. On the other hand, the polyether modified silicone used in the present invention is a polyorganosiloxane bonded with at least a hydroxyalkyl group or a polyoxyalkylene chain and hence exhibits hydrophilic properties. For example, polyether modified silicone having a structure in which about a half of methyl groups in polydimethylsiloxane is replaced with polyoxyethylene chain is completely soluble in water. Further, the hydrophilic silicone has good compatibility with polyesters such as PGA, PLA and PGLA, and provides a uniformly blended mixture. Consequently, the resulting polyester composition has increased flexibility and surface smoothness as compared with the original polyester. On preparing the composition of the invention, an admixture of the polyester and the polyether modified silicone may be carried out in the presence or absence of solvents. PLA and PGLA are soluble in an organic solvent such as chloroform and thus mixing can be readily carried out by previously dissolving one or both of the silicone and the polyester in the solvent. PGA is difficultly soluble in organic solvents except a certain kind of solvent such as hexafluoroisopropanol. Consequently, it is favorable to mix by heat-melting above the melting point of about 225°C. However, heating above 250°C is unfavorable because of decomposition of PGA. The content of the polyether modified silicone is preferably in the range of 0.1 to 30% by weight in the composition. Further, another aspect of the present invention is medical materials constituted with the modified polyester composition. More particularly, the medical materials include intervitally implantable materials such as artificial blood vessels and artificial skin, adhesion inhibiting materials, surgical sutures and coating materials thereof. Silicone has already been used more than 30 years as a material for replacing soft tissue and is known to have biocompatibility. For example, silicone rubber obtained by curing silicone is utilized for artificial heart valves, artifical mamma, implanting material for retinal detachment surgery, artificial skin, artificial nose, artificial ear and artificial esophagus. The silicone used in the invention may be an oil or gel at room temperature and physiologically almost inactive. For example, polydimethylsiloxane in particular has a LD 50 value, i.e., 50% lethal dose of 35 g/kg or more and corresponds to nontoxic in the toxicity classification. Consequently, the medical materials provided by the invention and constituted by the modified polyester composition are suitable for biocompatible medical materials. The modified polyester composition of the invention can be readily processed into fibers, films, plates, bars and other various shapes. These processed materials are flexible and have increased solubility in organic solvents as compared with conventional polyesters such as PGA, PLA and PGLA. These effects result from the low glass transition temperature, small intermolecular forces and high flexibility of the silicone chain in the polyester composition. The materials provided by the invention can be used in a variety of applications by changing the composition of the silicone. That is, in the case where the silicone is very hydrophilic or the amount of the silicone is small although the silicone is slightly hydrophilic, the material containing said silicone can be used for bioabsorbable materials because the polyester chain can be hydrolyzed in vivo. For example, the material can be utilized for surgical sutures in the form of fibers, adhesion inhibiting materials after processing into films or sheets, bonesetting plates in the form of plates or bars and matrices of slow release medicines after mixing with medicines and successively processing into microspheres. Since the hydrophilic property of the material can be controlled by the variation of silicone composition, the rate of hydrolysis can be adjusted, which characteristic is very convenient for matrices of slow release medicines. On the other hand, in the case where the silicone is slightly hydrophilic and in a large amount, the hydrophilic property of the material itself is decreased and bioabsorbability is lost. Consequently, such material can be used for a non-absorbable and biocompatible material, for example, an implanting material for blood vessels. The material provided by the invention can provide lubricity to the material surface as a result of the property of the silicone. Hence the material can enhance the ligating property (ease of knotting) of sutures when the material is employed for the coating of surgical sutures. The present invention will hereinafter be illustrated further in detail with reference to examples. Physical and chemical properties in the examples were measured by the following methods. Polymerization rate; After completing the polymerization reaction, the reaction mixture was dissolved in hexafluoroisopropanol(HFIP) or methylene chloride to make a solution having a known concentration. The amount of residual monomer was determined by gas chromatography and the polymerization rate was calculated. Average molecular weight; Polymer was dissolved in HFIP and weight average molecular weight (M w ) and number average molecular weight (Mn) were measured by gel permeation chromatography (GPC). Solution viscosity; Polymer was dissolved in chloroform to prepare a solution having a concentration of 0.5 g/dl. Viscosity was measured at 25 ± 0.5°C with an Ubbelohde viscosimeter. Solution viscosity η was calculated from the following equation. η = log e (T 1 /T o )/C wherein T o = Blank test time T 1 = Test time C = Concentration of solution (0.5 g/100 ml) Tensile strength test; A film test piece having a width of 10 mm and a length of 50 mm was prepared. Tensile strength was measured with a chuck distance of 20mm at a chuck separation rate of 50 mm/min. Composition of copolymer; Sample was dissolved in a deuterated chloroform/HFIP mixture (1/9) and H-NMR spectrum was measured with a 90 MHz NMR equipment. The mole ratio in the copolymer composition was calculated from the peak strength ratio based on methyl hydrogen in dimethylsiloxane, methylene hydrogen in glycolic acid unit, and methyl hydrogen in lactic acid unit, respectively. Glass transition temperature (Tg); Tg was measured with a differential scanning calorimeter at a temperature rise rate of 10 °C/min. Hydrolysis test; A film specimen was immersed in a phosphoric acid buffer solution (pH 7.3) at 37°C for a week. Evaluation was made by weight loss rate and Mw decrease rate of the film after immersion. Examples 1-3 In a glass polymerization tube, 6.11 g of lactide, 4.36 g of glycolide and 0.40% of a 90% lactic acid solution were charged. As a catalyst, 0.1 ml of stannous octoate solution in toluene (64.8 mg/10 ml) was added. The polymerization tube was fitted with a degassing cock, dried by deairing for several hours and melt-sealed under vacuum. Reaction was conducted in an oil bath at 180°C for 18 hours. The polymerization rates of glycolide and lactide were 99.4% and 98.9%, respectively. Mw calculated from GPC was 52000. A 10% methylene chloride solution of the copolymer PGLA thus obtained was mixed with silicone having hydroxyethyl groups at both ends (t-ROH type) or silicone having polyoxyethylene chains in the side chains (b-ROH type) in an amount of 20 parts by weight or 10 parts by weight, respectively, per 100 parts of the polyester. The mixtures were individually cast on a Teflon plate, and the solvent was evaporated at room temperature to form films. Residual solvent in the films was completely removed by drying at 50°C for 24 hours under reduced pressure of 3mm Hg. Table 1 illutrates results on the tensile test of films. Each film had very low Young's modulus and high elongation and was soft. These films were hydrolyzed under an environment similar to in vivo, and decrease in weight and molecular weight was observed. These films were suitable for applying to soft tissue as implanting materials or adhesion inhibiting materials.

Claims: A modified polyester composition comprising a mixture of a polyester having at least a glycolic acid unit and/or a lactic acid unit with a polyether modified silicone which is a polyorganosiloxane having as a substituent at least a hydroxyalkyl group or a polyoxyalkylene chain. A composition according to claim 1 wherein the modified silicone is a polyorganosiloxane having as a substituent a polyoxyalkylene chain. A composition according to claim 1 wherein the modified silicone is a polyorganosiloxane having as a substituent a polyoxyethylene chain having a polymerisation degree of 2-200. A composition according to claim 1 wherein the modified silicone is a polyorganosiloxane having as a substituent a hydroxyalkyl group. A composition according to any preceding claim wherein the composition contains 0.1-30% by weight of the modified silicone. An intravitally implanting material comprising a modified polyester composition according to any preceding claim. An artificial blood vessel, adhesion inhibiting material for medical use, surgical suture, coating matrix for surgical suture or a matrix for a slow release medicine composed of the material of claim 6.

Inventors: IIMURO SHIGERU, OHTAGURO MASAMI, SHINODA HOSEI, IIMURO, SHIGERU, OHTAGURO, MASAMI, SHINODA, HOSEI, OHTAGURO, MASAMI, 54 AZA SANTAKANE
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1990
Main classification: C08L67
IPC codes: A61L17, C08L83, C08L67, C08G63, A61L27, A61L31