Composite bioabsorbable materials and surgical articles made therefrom

Abstract: The present invention relates to composite materials having a core portion 25 formed from a first bioabsorbable material and at least one shell portion 35, 38 of a second bioabsorbable material joined to the core portion. The first and second bioabsorbable materials have different rates of bioabsorption. In a preferred embodiment, the core portion is coextruded with the shell portion to form a composite filament or sheet. The materials of the present invention may be used as sutures or formed into medical devices or surgical articles for implantation within a living organism.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The invention relates to bioabsorbable materials and, more particularly, to filaments that are monofilament sutures. The filament includes a core of a first bioabsorbable material and at least one shell joined to the core, the shell being formed of a second bioabsorbable material. The first and second bioabsorbable materials have different absorption rates. 2. Description of the Related Art Bioabsorbable materials are known in the art for a variety of uses, particularly those uses in which the bioabsorbable material is implanted within a living organism for medical purposes. The term "bioabsorbable" is used to describe a material capable of being broken down into smaller constituents which can be metabolized and/or excreted by a living organism. Hydrolysis is one mechanism by which many bioabsorbable materials. are broken down following implantation within a living organism. Such materials are useful for temporarily holding tissues in a desired position during healing, and being absorbed by the organism after a period of time. Various bioabsorbable materials and medical devices made therefrom are known in the art. See, e.g. U.S. Patent Nos. 2,668,162; 2,703,316; 2,758,987; 3,225,766; 3,297,033; 3,422,181; 3,531,561; 3,565,077; 3,565,869; 3,620,218; 3,626,948; 3,636,956; 3,736,646; 3,772,420; 3,773,919; 3,792,010; 3,797,499; 3,839,297; 3,867,190; 3,878,284; 3,982,543; 4,047,533; 4,052,988; 4,060,089; 4,137,921; 4,157,437; 4,234,775; 4,237,920; 4,300,565; 4,429,080; 4,441,496; 4,523,591; 4,546,152; 4,559,945; 4,643,191; 4,646,741; 4,653,497; and, 4,741,337; U.K. Patent No. 779,291; D.K. Gilding et al., "Biodegradable polymers for use in surgery -- polyglycolide/poly(lactic acid) homo- and copolymers: 1", Polymer, Volume 20, pages 1459-1464 (1979), and D.F. Williams (ed.), Biocompatibility of Clinical Implant Materials, Vol. II, ch. 9: "Biodegradable Polymers" (1981). See also US-A- 5123912 for a discussion of synthetic bioabsorbable multifilament sutures, and natural and synthetic monofilament bioabsorbable sutures. Fiber-reinforced compositions used for surgical implants have been described in U.S. Patent Nos. 4,279,249; 4,550,449; 4,539,931; 4,743,257; 5,080,665. The fiber-reinforcements in these compositions provide increased strength and rigidity as may be required for bone repair, but render these compositions unsuitable where flexibility is required. The present invention provides an implantable composite having a bioabsorption rate which varies over time and which is suitable for use in making a variety of medical devices, including those in which flexibility is a desirable characteristic. Further, document NL-A-92 01 035 describes an implant made of a material that is re-absorbable inside the human or animal body that is manufactured from a first re-absorbable material and a second re-absorbable material. As such materials copolymers of lactide and glycolide are mentioned, wherein the glycolide component of the second material is higher than in the first material. Further, a poly(L-lactide) is mentioned as first material. Furthermore, document FR-A-26 35 966 describes implants which are made by first forming the implant with at least two different filaments made of at least two different materials and then heating these filaments to form an implant structure. The implant comprises a knitted or woven structure which, as a result of the substantially surface bonding of the individual filaments due to heating, causes the filaments to melt together at the location where the filaments contact each other. Therefore, the irregular distribution of the two materials in different regions of the implant leads to nonuniform bioabsorption. Summary of the invention Therefore starting from document NL-A-92 01 035 it is the object of the present invention to provide an implantable filament having a bioabsorption rate which is variable all the time and which imparts flexibility for the sutures made from this composite. This object is solved by a composite bioabsorbable monofilament suture which comprises the features of claim 1. Furthermore, the invention is directed to a method for making such a monofilament suture. The present invention relates to monofilament sutures having a core portion formed from a first bioabsorbable material and at least one shell portion formed from a second bioabsorbable material joined to the core portion. The first and second bioabsorbable materials have different rates of bioabsorption. In a preferred embodiment, the core portion is coextruded with the shell portion to form a composite filament. The materials of the present invention may be used as sutures or formed into medical devices or surgical articles for implantation within a living organism. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a filament having a core portion and a shell portion according to the present invention. Fig. 2 is a coextrusion die optionally used to form a composite filament according to the present invention. Fig. 3 schematically illustrates a spinning process used to form filaments according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the drawings, Fig. 1 shows a filament 10 having a core portion 20 surrounded by a shell portion 30. Shell portion 30 is joined to core portion 20 in such a manner that the two portions form a single, structural element, i.e., filament 10. As such, filament 10 is a composite material in which all of the constituent elements contribute to the properties of the overall structure. Thus, filament 10 differs from coated filaments in that the coatings of such filaments, even for very thick coatings, do not contribute substantially to the strength of the filament. A composite having a generally cylindrical core portion made from polydioxanone with a diameter in the range of about 0.01 mm to about 0.8 mm, with a shell portion made from polycaprolactone having a thickness of about 0.001 mm to about 0.5 mm will provide a filament having acceptable flexibility and strength. In particularly useful embodiments, the core portion and shell portion are substantially co-extensive. It is also contemplated, however, that composites wherein the shell portion only partially covers the core portion can be made in accordance with this invention. The materials used to form the core and shell portions of composite filament 10 are bioabsorbable materials. The core portion comprises polydioxane and the shell portion comprises polycaprolactone. Materials used to form the shell portion of the above composite material include bioabsorbable polymers, copolymers, or mixtures fabricated from polycaprolactone. Particularly preferred combinations for composite materials having a core with a higher rate of hydrolysis than the shell include a core of polydioxanone with a shell of polycaprolactone. The composite filaments of the present invention may be fabricated by a number of known polymer processing techniques. One preferred technique is coextrusion. Extrusion is a polymer processing technique in which a viscous melt is fed, under pressure, through a die in a continuous stream. In coextrusion, plural polymer melt streams feed through individual flow channels to be shaped as a single polymeric article. The polymer melts may be combined in several ways. Plural melts may be extruded separately from a single die, plural melts may be extruded partly separately followed by combined extrusion, or plural melts may be coextruded together. A coextrusion die which may be used to form a composite filament according to the present invention is illustrated schematically in Fig. 3. In Fig. 3, the polymer melt used to form core 20 of filament 10 passes through central flow channel 70 where it is shaped as it passes through channel exit 72. The polymer melt used to form the shell 30 of filament 10 passes through outer flow channel 80 where it is shaped as it passes through channel exit 82. The two concentric polymer melts are joined at die exit 90, forming a continuous polymer melt. The dies and methods described above with reference to Fig. 3 are exemplary of coextrusion techniques used to form the filaments of the present invention. Any coextrusion process may be used to form the bioabsorbable composite filaments and sheets. Examples of other coextrusion processes suitable for use in forming the materials of the invention as well as examples of suitable extrusion apparatus which may be used with the dies of Figs. 3 and 4 are found in Vols. 4 and 6 of the Encyclopedia of Polymer Science and Engineering, (John Wiley & Sons, New York), c. 1986, pp. 812-832 and 571-631, respectively. In another preferred embodiment of the present invention, the composite materials may be spun into fibers to be used as monofilament sutures. An exemplary spinning process is schematically illustrated in Fig. 3. To produce fibers having the core/shell structure of Fig. 1, the core and shell constituent materials are separately melted. The constituent materials are separately fed as polymer melts 213 to spinneret 215 and are combined in the manner shown in Fig. 2 just before the spinneret exit orifice. Although a single filament is shown for clarity in Fig.3, is understood that spinnerets having any desired number of exit orifices in a range of exit diameters may be used. Molten monofilament 216 exits spinneret 215, passes through air gap 214, and enters quench bath 217. Air gap 214 is the distance between the spinneret exit and bath 217. Preferably, the air gap distance ranges from 0.5 to 100 centimeters and, more preferably, from about 1 to 20 centimeters. As monofilament 216 passes through bath 21, it solidifies. In a preferred embodiment, quench bath 217 is a water bath. Within the quench bath, filament 216 is driven by drive roller 218. As it emerges from the bath, a wiper may optionally remove excess water. Monofilament 216 wraps around a first godet 221 and is prevented from slipping by nip roll 222. Godets 301, 302, 303, and 304 engage and drive the filament prior to a stretching operation. The number of godets is not critical; any suitable godet system may be employed in the spinning process of the present invention. During stretching, monofilament 216 is drawn through hot water bath 223 by godets 224, 305, 306, 307, and 308 or any other suitable arrangement. This second godet system rotates at a higher speed than that of godet 104 to provide a desired stretch ratio. Following stretching, monofilament 216 may be annealed within hot air oven chamber 225, driven by godets 226, 309, 310, 311, or 312, or any other suitable drawing system. Take-up reel 313 winds the finished monofilament product. Other suitable fiber manufacture processes, including other spinning processes, which may be used to form the composite materials of the present invention are described in Volume 6 of the Encyclopedia of Polymer Science and Engineering, (John Wiley & Sons, New York), c. 1986, pp. 802-839. Other polymer processing techniques may be used to form the composite filaments of the present invention. These include, but are not limited to, casting, injection molding, and blow molding. In short, any polymer processing technique capable of creating composite materials having shell portions adequately joined with core portions are contemplated to form the composite bioabsorbable materials of the present invention. In general, the formation of a composite depends on both materials being heated to a temperature at which fusion of at least the surface of the materials begins. Where coextrusion is the method used to from the composite, both materials are heated to a temperature above their respective melting points. Where pressing is used to from the composites, the combination of temperature and pressure should be such that at least the surface of the core portion and the surface of the shell portion in contact therewith begin to fuse so that the two portions are joined into a unitary structure. As will be appreciated by those skilled in the art, a certain amount of mixing of the core and shell materials will occur at the interface of those two portions. The amount of mixing may be only at the molecular level and thus, as a practical matter undetectable. However, in some instances the amount of mixing may be sufficient to produce a micro-shell layer at the interface between the shell and core portions. This micro-shell layer is made from a mixture of the core and shell materials and has its own bioabsorption profile. The composite filaments may be used as monofilament sutures. The filaments may be woven to form sheets which may be used to form various prosthetic devices such as vascular grafts, muscle grafts, bone fillers, etc. Alternatively the filaments can be formed into non-woven fabrics by lamination with or without additional polymeric matrix. Such non-woven fabrics may be used for the same purposes as the woven fabrics listed above. It is within the scope of this invention to coat or impregnate the bioabsorbable composite materials of the present invention as well as implants and prosthetic devices made therefrom with one or more materials which enhance its functionality, e.g., surgically useful substances, such as those which accelerate or beneficially modify the healing process when the material is implanted within a living organism. Thus, for example, antimicrobial agents such as broad spectrum antibiotics (gentamicin sulphate, erythromycin or derivatized glycopeptides) which are slowly released into the tissue can be incorporated to aid in combating clinical and sub-clinical infections in a surgical or trauma wound site. To promote wound repair and/or tissue growth one or several substances can be introduced into the composite bioabsorbable materials of the present invention or impregnated into fabrics or prostheses made from the composite bioabsorbable materials. Exemplary substances include polypeptides such as human growth factors. The term "human growth factor" or "HGF" embraces those materials, known in the literature, which are referred to as such and includes their biologically active, closely related derivatives. The HGFs can be derived from naturally occurring sources and are preferably produced by recombinant DNA techniques. Specifically, any of the HGFs which are mitogenically active and as such effective in stimulating, accelerating, potentiating or otherwise enhancing the wound healing process are useful herein. Growth factors contemplated for use in the materials of the present invention include hEGF (urogastrone), TGF-beta, IGF, PDGF, FGF, etc. These growth factors, methods by which they can be obtained and methods and compositions featuring their use to enhance wound healing are variously disclosed, inter alia, in U.S. Patent Nos. 3,883,497; 3,917,824; 3,948,875; 4,338,397; 4,418,691; 4,528,186, 4,621,052; 4,743,679 and 4,717,717; European Patent Applications 0 046 039; 0128 733; 0131 868; 0136 490; 0147178; 0150 572; 0177 915 and 0 267 015; PCT International Applications WO 83/04030; WO 85/00369; WO 85/01284 and WO 86/02271 and UK Patent Applications GB 2 092 155 A; 2,162,851 A and GB 2 172 890 A. When incorporating wound healing substances such as those discussed above, it may be advantageous to use composite materials having at least one shell layer are formed from a bioabsorbable material having a relatively high rate of bioabsorption. By incorporating wound healing substances in a high rate bioabsorption layer, the substance will be more quickly absorbed while the remaining composite material will still retain sufficiently good mechanical properties to perform its medical or surgical function. To decrease abrasion, increase lubricity, etc., the bioabsorbable composite materials can be coated with various substances. Copolymers of glycolide and lactide and polyethylene oxide, calcium salts such as calcium stearate, compounds of the pluronic class, copolymers of caprolactone, caprolactone with PEO, polyHEMA, etc. may be employed. Table I below sets forth typical conditions for extruding, stretching composite monofilament sutures in accordance with this invention: CONDITIONS OF MANUFACTURING COMPOSITE MONOFILAMENT OF THE PRESENT INVENTION EXAMPLE Suture Size 3/0 Material Polycaprolactone Polydioxanone Process Conditions Extrusion Operation Extruder Extruder 1 (shell) Extruder 2 (core) extruder screw, rpm 0.6 1.3 pump rpm 4.4 5.8 barrel temp., °C, zone A 80 52 barrel temp., °C, zone B 120 135 barrel temp., °C, zone C 120 135 clamp temp., °C 120 135 adapter temp., °C 120 130 Process Conditions Extrusion Operation pump temp., °C 120 135 barrel melt temp, °C 117 149 pump melt temp., °C 117 150 spinneret melt temp., °C 118 118 barrel pressure, psi 760 1250 pump pressure, psi 500 1000 spinneret pressure, psi 840 1980 pump size, cc per revolution 0.16 0.16 outside diameter of spinneret orifices, mm 0.28 -- inside diameter of spinneret orifices, mm -- 0.25 quench bath temp., °C 18 18 Stretching (Orienting) Operation EXAMPLE draw bath temp., °C 50 first godet, mpm 4.2 second godet, mpm 19.7 second oven temp., °C 70 third godet, mpm 24.6 draw ratio 5.9 : 1 Annealing Operation EXAMPLE oven temp., °C 50 time (hrs.) 18 The physical properties of the sutures and the procedures employed for their measurement are set forth in Table II as follows: PROCEDURES FOR MEASURING PHYSICAL PROPERTIES OF COMPOSITE MONOFILAMENT SUTURES OF THE PRESENT INVENTION Physical Property Test Procedure knot-pull strength, kg U.S.P. XXI, tensile strength, sutures (881) straight-pull strength, kg ASTM D-2256, Instron Corporation elongation, % ATSM D-2256 tensile strength, kg/mm 2 ASTM D-2256, Instron Corporation Series IX Automated Materials Testing System 1.03A Table III below sets forth the physical properties of the composite monofilament suture of the present invention. Physical Property diameter (mm) 0.295 shell thickness (mm) 0.036 core thickness (mm) 0.223 knot-pull strength (kg) 2.2 straight-pull strength (kg) 3.0 elongation (%) 26 tensile strength (kg/mm 2 ) 47

Claims: A composite bioabsorbable monofilament implantable within a living organism comprising: a core portion formed from a first bioabsorbable material; a shell portion joined to said core portion, said shell portion formed from a second bioabsorbable material; said first and second bioabsorbable materials having different rates of bioabsorption, wherein said first bioabsorbable material comprises polydioxanone and said second bioabsorbable material comprises polycaprolactone, and the monofilament is a monofilament suture. A bioabsorbable filament as recited in claim 1, wherein said bioabsorbable filament is formed by coextruding said core portion and said shell portion. A method for making a monofilament suture from a composite bioabsorbable material comprising: forming a first melt of a first bioabsorbable material; forming a second melt of a second bioabsorbable material, said second bioabsorbable material having a different rate of bioabsorption than said first bioabsorbable material; wherein said first bioabsorbable material comprises polydioxanone and said second bioabsorbable material comprises polycaprolactone; coextruding said first and second material melts to form a composite bioabsorbable material having a core portion and a shell portion integral with said core portion such that said first melt forms said core portion and said second melt forms said shell portion, and wherein said coextrusion is performed through a spinneret to form a filament, said filament then being stretched to form said monofilament suture.

Inventors: BENNETT STEVEN L, JIANG YING, LIU CHENG-KUNG, PIETRANGELI JEFFREY P, ROBY MARK S, STEVENSON RICHARD P, BENNETT, STEVEN L., JIANG, YING, LIU, CHENG-KUNG, PIETRANGELI, JEFFREY P., ROBY, MARK S., STEVENSON, RICHARD P.
Applicants: UNITED STATES SURGICAL CORP, UNITED STATES SURGICAL CORPORATION
Application year: 1995
Main classification: A61L31
IPC codes: A61L31, A61L15, A61L17, A61L27