TECHNICAL FIELD This invention relates generally to a method of forming a suture yarn for use in medical sutures and to the formed suture yarn itself. More particularly, the invention relates to a method of making an absorbable, biocompatible suture yarn from a bioabsorbable polymer.

BACKGROUND ART Sutures for use in the medical field are predominantly multi-filament structures formed by twisting together, braiding or otherwise combining a plurality of filaments. Typically speaking, a predetermined number of filaments are combined together to form a suture yarn, and a plurality of suture yarns are braided or twisted together to form the medical suture. In an alterative arrangement, a core suture yarn is wrapped in sheath yarns to form the medical suture by either braiding or twisting.

Although monofilament sutures are popular, multi-filament sutures have been found generally to provide better tensile strength, knotting strength and other beneficial handling characteristics. The individual filaments are generally fabricated from a polymeric resin, and are optimally formed by extrusion from a polymer melt.

A highly desirable polymer for use in forming the individual filaments is poly (glycolic acid) (PGA) . Filaments extruded from PGA are bioabsorbable and biocompatible, and therefore can safely be absorbed into the body after a relatively short time period, making it unnecessary for the sutures to be removed from the patient. One example of an absorbable

suture made of a PGA polymer can be found in U.S. Patent No. 4,621,638.

Producing multi-filament suture yarn is conventionally done using a two-step extrusion process. In the first step the melted polymer is extruded using known melt-spinning techniques to produce yarns having low orientation. These yarns are subsequently draw-twisted in a second, off-line step whereby the fibers are drawn to strengthen the filaments and then the filaments are twisted to form a cohesive suture yarn. U.S. Patent No. 5,102,419 shows one example of forming surgical sutures using this conventional two-step extrusion process. U.S. Patent No. 5,232,648 forms multi-filament, bioabsorbable sutures using a two-step extrusion process and discloses data of the melt-spinning conditions for forming the fibers and drawing conditions for drawing the fibers into surgical filaments.

U.S. Patent No. 5,294,395 discloses an extrusion process and a redrawing process to form a thermoplastic onofilament suture. However, because of their relatively large diameters, monofilament sutures are drawn at much slower speeds than multi-filament sutures. In addition, as a result of the larger size of the monofilament sutures, the extrusion process in U.S. Patent No. 5,294,395 uses a liquid quenching tank and heaters disposed between the drawing rolls. For at least these reasons, equipment for producing monofilament sutures is inappropriate for producing multi- filament suture yarns.

Although a continuous spin drawing process has been used for many years to produce industrial fibers, using such a one- step extrusion process with a PGA polymer is heretofore

unknown. In a conventional spin drawing process, a polyethylene terephthalate (PET) or other comparable resin is used to form synthetic multi-filament yarns for use in, for example, carpets or ropes. For a detailed discussion of this process, reference is made to U.S. Patent No.

3,803,282, or U.S. Patent No. 4,003,974. These patents, however, do not contemplate using an absorbable, biocompatible polymer resin such as PGA, and therefore fail to improve upon known methods for manufacturing absorbable medical sutures.

DISCLOSURE OF INVENTION It is a principal object of the present invention to provide an improved method of producing absorbable, biocompatible suture yarn.

Accordingly, one object of the invention is to provide a high output method of producing surgical yarn made of a bioabsorbable polymer such as poly (glycolic acid) (PGA) .

It is another object of the invention to melt extrude a bioabsorbable polymer into filaments in a one-step spin drawing process using a lubricant that will not breakdown the absorbable suture yarn.

It is another object of the invention to melt extrude a bioabsorbable polymer into filaments in a two-step spin drawing process using a finishing solution that will not breakdown the absorbable suture yarn.

It is still another object of the invention to use an air entanglement process for making a multi-filament suture yarn.

It is yet another object of the invention to form an absorbable, biocompatible suture yarn of a PGA material using a spin drawing process.

In accordance with one aspect of the invention, a method of forming an absorbable biocompatible suture yarn comprises the steps of melt-extruding a bioabsorbable polymer to form a plurality of surgical filaments in a first step, drawing the extruded filaments in a second step, and combining the drawn filaments into a parallel contiguous arrangement to form a yarn. The yarn is fed through an air jet entangler to entangle the filaments in the yarn.

In yet another aspect of the invention, surgical filaments used to form the surgical yarn are treated with a finishing solution having a lubricant and an anti-static agent in a non-aqueous-based carrier.

In accordance with another aspect of the invention, an absorbable, biocompatible suture yarn is formed by the step of melt-extruding a bioabsorbable polymer to form a plurality of surgical filaments in a first step, drawing th extruded filaments in a second step, and combining the draw filaments into a parallel contiguous arrangement to form a yarn. The yarn is fed through an air jet entangler to entangle the filaments.

These and other objects, aspects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Figure l is a schematic diagram of a spin-draw extruder used in accordance with a first embodiment of the present invention;

Figure 2 is a flow diagram of a method of forming PGA filaments in accordance with the first embodiment of the present invention;

Figure 3 is a side elevational view of a jet entanglement apparatus for processing the suture filaments in accordance with the present invention;

Figure 4 is a sectional plan view of part of the air jet entanglement apparatus in accordance with the present invention;

Figures 5A and 5B are perspective views of air jet entanglement chambers in accordance with the present invention;

Figure 6 is a flow diagram of a method of forming PGA filaments in accordance with a second embodiment of the present invention;

Figure 7 is a schematic diagram of a fiber extrusion system used in accordance with the second embodiment of the present invention; and

Figure 8 is a schematic diagram of a draw-twister used in accordance with the second embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The suture yarn produced by the disclosed method is formed from a bioabsorbable polymer . hile the embodiment disclosed below uses a poly (glycolic acid) (PGA) in forming the filaments, bioabsorbable polymers such as, for example, glycolide, lactide, caprolactone, p-dioxanone, trimethylene carbonate and other bioabsorbable poly (hydroxycarboxylic acid) and physical and chemical combinations thereof can be used without departing from the scope of the subject invention.

Extruded PGA polymer yields an absorbable, biocompatible filament ideally suited for use as a suture yarn. The PGA suture safely breaks down in the patient's body after a tim period sufficient for the sutured tissue (for example) to heal .

In a first embodiment, the PGA polymer is used in a spin drawing process and preferably has an inherent viscosity at 30°C of between 0.9 dl/g and 1.2 dl/g as measured in a solvent of hexafluoroacetone sesquihydrate, with a concentration of polymer in the solvent of 0.5% w/v. The moisture content of the polymer prior to extrusion is less than 50 ppm.

The method for forming the suture yarn from the PGA polymer in accordance with the first embodiment features three primary processing steps, namely:

1. Forming individual filaments

2. Combining the filaments to form a multi-filament suture yarn

3. Jet entanglement of the yarn.

1. Forming Individual Filaments

Individual filaments fabricated from the PGA polymer resin are preferably melt extruded using a spin draw process. Figure 1 shows a spin draw extrusion apparatus for forming the PGA filaments. With reference to Figure 1 and the flow chart in Figure 2, a dry PGA polymer, preferably having a moisture content no greater than 50 ppm, is fed into a hopper 10, extruded through a melt extruder 12 and formed into filaments by a spin head 14 containing filtration media and a spinneret. In one suitable example, a 3/4" (an approximately .29 centimeter) diameter, 20:1 1/d extruder is used to form the individual filaments, and is operated at a temperature of 250°C. The formed filaments are then passed through a hot collar 16 and are quenched in air as they drop from the hot collar .

2. Combining Filaments to Form a Multi-Filament Yarn

The number of filaments used to form a yarn depends on the overall denier of the suture yarn and whether the yarn is to be incorporated into a sheath or core.

After quenching, the individual filaments 19 are passed through a finish applicator 18 and then over a series of godets 20, 22 and 24 for drawing and relaxing the filaments. The finish applicator applies a spin finish solution to the filaments to assist in subsequent processing. The spin finish solution comprises a lubricating agent, e.g., mineral oil, an anti-static agent, such as sorbitan monolaurate, and a solvent, e.g., xylene. Xylene is a non-aqueous carrier and thus will not break down the PGA filaments. Of course, the spin finish is washed and rinsed or otherwise removed

from the suture yarns after processing is complete.

As the treated filaments are passed over the godets 20 22 and 24, the filaments are combined in a parallel contiguous arrangement to form a yarn 21. A tension is applied to the yarn as it passes over the godets to draw th yarn to the desired draw ratio. In one example, godet 20 takes up 2000 feet/minute (fp ) (approximately 60.95 decameters/minute (dpm))of yarn and is not heated, godet 22 is heated to 58°C and operated to take up 2005 feet (approximately 61.10 decameters) of yarn per minute, and godet 24 is heated to 110°C and takes up 9000 fpm (approximately 274.30 decameters) of yarn (per minute) . A draw ratio of approximately 4.5X is achieved by operating the godets at these speeds, although the preferred range of draw ratio can be from 3 to 5X.

Filaments extruded for use in sheath or core construction preferably have a denier of between 0.2 to 6.0 denier, more preferably between 1.5 to 3.0 denier and ideally 2.2 denier. Sheath filaments desirably have a low denier to maintain smooth surface characteristics. Core filaments, on the other hand, may have a higher denier, especially for suture with a high overall denier .

The preferable range of denier of the yarn is shown in Table 1.

TABLE 1

Denier Range

Filaments Minimum Maximum

7 14.5 16 . 3 16 33.1 37 . 3 21 43.4 49 . 0 28 57.9 65 . 3 35 72.4 81 , . 6

The resulting yarn suitable for use to manufacture PGA braid preferably has a tenacity of at least 6.0 gpd and a breaking elongation between 15% and 40%.

3. Jet Entanglement of the Multi-Filament Yarn

As the yarn leaves the last godet 24 it proceeds to a jet entanglement apparatus 30 shown in block outline in Figure 1 and in detail in Figures 3, 4, 5A and 5B.

In jet entanglement, a fluid is forced at elevated pressure into a chamber through which the multi-filament yarn is passed. The fluid is preferably air or some other gas. The turbulence of the gas causes the filament to entangle or intermingle in the area impinged by the jet. The movement of the yarn and the size of the chamber interact to create turbulent pulsations which entangle the filaments together . Therefore, even with a constant pressure air supply, the yarn can exit the chamber with discrete regularly spaced

apart areas of entanglement alternating with non-entangled areas. The entangled portions are retained by the yarn through subsequent processing steps. As will be appreciated, jet entanglement of the yarn achieves many of the same features of twisting the yarn but at a much higher speed and with a simpler process, thus reducing the costs associated with combining the individual filaments.

With reference to Figures 3, 4, 5A and 5B, the air jet entanglement apparatus 30 comprises a slub catcher 32, eyelets 34, 36 and 38, support frame 40, and quick release connector 42. The entangler also includes an air supply line 44 and a mounting plate 46 for supporting the entanglement device. The mounting plate includes an adjustment slot 48 with adjustment wing nut 50, support box 52, roller 54 and entanglement body 56.

The entanglement body 56 is preferably fabricated as an integral single piece from a hard, durable material such as tungsten carbide, ceramic coated steel, or solid ceramic. As best seen in Figures 5A and 5B, the entanglement body is formed to have a chamber 58 extending lengthwise along the moving path of the threadline in which the entanglement takes place, and a threading slot 60 to facilitate easy threading of the suture yarn within the chamber . The chamber 58 can be triangular or rectangular in cross-sectio to facilitate turbulence.

The height H of the triangular chamber 58 in Figure 5A, i.e., the distance from the base to the apex of the triangular cross-section, is preferably from about 2.0 millimeters to about 5.0 millimeters, and more preferably between 2.3 and 4.3 millimeters. The rectangular chamber 5

in Figure 5B preferably has a height H of approximately 3.1 millimeters and a width W of approximately 1.1 millimeter. The preferred length of the chamber shown in Figure 5B is approximately 25 millimeters. Air or other suitable gas is introduced into the chamber through an orifice 62, which may be oriented at an angle y from a line perpendicular to the longitudinal path of the threadline. Angle y can be from about 0° to about 15°. The diameter of the orifice can range from about 1 to 3 millimeters, and more preferably from about 1 to 1.5 millimeters. The air is introduced into the chamber at a preferred pressure of about 80 to 100 psi (5.6 to 7.0 kilograms per square centimeter), although a range of about 10 to 100 psi (.70 to 7.0 kilograms per square centimeter) is acceptable.

The entanglement body 56 is secured to the support frame 40 by a locking plate 64 and locking screw 66. Quick release connector 42 for the air supply line 44 is secured to the support block 52 and, in turn, supports the support frame 40 and allows for consistent and precise positioning of the entanglement body between eyelets 36 and 38. Of course, the entanglement body can be of any of the various configurations and dimensions suitable for the suture entangling process of the present invention.

With reference back again to Figures 1, 3 and 4, after suture yarn 21 passes over godet 24, it is then passed through eyelet 34, slot 33 in the slub catcher 32, through eyelet 36 and into chamber 56 at a preferred speed of up to about 9000 fpm (274.31 decameters per minute) . Air is injected from orifice 62 at approximately 80 to 100 psi (5.6 to 7.0 kilograms per square centimete ) and preferably at an angle y of about 5°. The turbulence is characterized by

pulsations, i.e., discrete impingements of air, on the suture yarn to produce impingements about, for example, every l cm to 4 cm. A slight tension of between .05 to .10 grams per denier (gpd) is applied to the yarn as it is draw through the entangler. After exiting from chamber 56, the entangled yarn passes through eyelet 38, around roller 54, and onto a take-up spool 68 on winder 69 (see Figure 1) .

Subsequent processing of the entangled yarn may entail combining it with other entangled yarns and then twisting o braiding the combined yarns. Sutures can be formed with yarns comprising a separately constructed core around which sheath yarns are braided. In one embodiment, 3 to 7 air entangled yarns are individually fed into a center of a structure as it is being braided to compose the core. The to 7 yarns can also be plied or twisted in a separate operation to form a core yarn. Alternatively, the need to ply multiple yarns can be eliminated by spinning large denier yarns having hundreds of filaments. The large denie yarns, on the order of from about 100 to about 1000 denier, are jet entangled in accordance with the invention.

On the other hand, sheath yarns do not need to be plied. For example, about 4 to 36 sheath yarns may be braided around a constructed core to form the finished suture. Alternatively, the finished suture may be braided with sheath yarns only and thus without a core.

Spin drawing a PGA polymer to form surgical filaments and subsequently processing the filament yarn using an air entangler offers many advantages over conventional medical suture forming methods. For example, spin drawing a PGA polymer significantly increases production over the

conventional two-step extrusion method of forming suture filaments. Moreover, using a jet entangler to entangle the yarn eliminates the need for a twisting operation of the individual filaments and the accompanying capital costs of twisting equipment and facilities, resulting in lower production costs.

Another advantage of jet entanglement is that yarn processability is improved and breakage of filaments is reduced. Even if the filament breaks it can only strip back as far as the next closest impingement or entangled area, which is generally no more than about 1 to 4 centimeters. In addition, air entanglement tends to imbed broken ends of filaments within the suture yarn, reducing the likelihood that the broken filaments will accumulate during further processing. These results are especially important when the individual filaments possess a low denier, e.g., a denier less than 2. Therefore, another related advantage of employing jet entanglement is that it enables the suture to be made from filaments of low denier at a low defect rate. Use of lower denier filaments, as discussed above, is desirable because lower denier filaments result in a smoother suture.

In a second embodiment, PGA polymer is used in a two step lag-drawing process that includes two primary processing steps, namely:

1. Forming as-spun filaments

2. Draw-twisting the as-spun filaments to form a coherent multi-filament suture yarn.

As in the first embodiment, the PGA polymer used in the lag-

drawing process preferably has an inherent viscosity at 30° of between 0.9 dl/g and 1.2 dl/g as measured in a solvent o hexafluoroacetone sesquihydrate, with a concentration of polymer in solvent of 0.5% w/v. The moisture content of th polymer prior to extrusion is less than 50 ppm.

Figure 6 is a flow diagram of a method of forming PGA filaments in accordance with the second embodiment of the present invention.

1. Forming Multi-Filament Yarns

Multi-filament yarns fabricated from the PGA polymer resin are melt extruded using an extrusion system a shown in Figure 7. The step is substantially similar to ho the individual filaments are formed in step 1 of the first embodiment, except that two spin heads 14 are used. In Figure 7, a dry PGA polymer, preferably having a moisture content no greater than 50 ppm, is fed into a hopper 10, extruded to a melt extruder 12 and formed into filaments by spin heads 14 containing filtration media and a spinneret. In one suitable example, a 3/4" (an approximately .29 centimeter) diameter, 20:1 1/d extruder is used to form the individual filaments, and is operated at a temperature of 250°C. The formed filaments are then passed through hot collar 16 and are quenched in air as they drop from the hot collar. After quenching, the individual filaments are passed through a finish applicator 18 and then over a serie of godets 26 which are rotated at substantially the same speed of, for example, 1800 fpm (54.86 decameter per minute) and used to direct the as-spun filaments to low speed winders 80.

The finish applicator applies a spin finish solution to the filaments to assist in subsequent processing. The spin finish solution comprises a lubricating agent, e.g., mineral oil, an anti-static agent, such as sorbitan monolaurate, and a solvent, e.g., xylene. Xylene is a non-aqueous carrier and thus will not break down the PGA filaments. The spin finish is washed and rinsed or otherwise removed from the suture yarns after processing is complete. As will be appreciated, the lubricating agent is chosen to have certain characteristics such that it is biocompatible with mammals, has sufficient lubricity to minimize filament to filament friction and filament breakage, is liquid at the temperature of application, is hydrophobic, and has a viscosity at the temperature of application of between l and 100 cp. The desired lubricant is also essentially free of water or moisture so as to be chemically and physically nonreactive with the filament composition.

2. Draw Twisting the Multi-Filament Yarn

With reference to Figure 8, the as-spun yarn which has been collected is fed from spools 82 to a commercially available draw-twisting apparatus 84. As shown in the figure, each draw twisting apparatus includes a feed roll 86, draw roll 88 and a heat set roll 90 for drawing the multi-filament yarn. In addition, a take-up spindle 92 is used to wind the drawn filaments and, at the same time, impart a twisting motion to the filaments to provide about, for example, 1 to 2 twists per inch (approximately 2.564 centimeters) to the drawn filaments.

The draw ratio of the yarn as it is processed by

the draw twisting apparatus can be, for example, in the range of 2 to 20, and more preferably in the range of 4 to 5.

As shown in the flow chart of Figure 6, the draw-twisted yarn is tested and then used for subsequent processing, suc as braiding, to form sutures as discussed above with respec to the first embodiment.

As will be appreciated, both first and second embodiments use a lubricant and a finishing solution that is, among other characteristics, hydrophobic. In this manner, the PG filaments formed in the suture will not be broken down prematurely.

In the lag draw process disclosed in the second embodiment of the invention, the drawing step can be performed a relatively long period of time after the filaments are spun for example, a day or two later. However, the subject invention also contemplates performing the drawing step substantially immediately after the filaments are spun and lubricated to prevent aging of the filaments.

The second embodiment also contemplates using an air jet entangler, as disclosed above with respect to the first embodiment, to entangle the multi -filament yarn produced in the spinning process of step 1. In this modified process, the air jet entangler would replace the draw-twisting apparatus. The as-spun filaments are subsequently drawn over a series of godets rotating at sequentially increasing speeds to be drawn at a ratio in, for example, the range of 2 and 20, and more preferably in the range of 4 to 5.

The drawing step can be performed a relatively long period of time after the filaments are spun, for example, a day or two later. However, the modified process also contemplates performing the drawing step substantially immediately after the filaments are spun and lubricated to prevent aging of the filaments. After the drawing step, the filaments, which are combined in a parallel contiguous arrangement to form a yarn, are sent through the air jet entangler to be entangled as discussed above in step 3 of the first embodiment. The entangled filaments are then wound up conventionally, i.e., without twisting.

Although specific embodiments of the present invention have been described above in detail, it will be understood that this description is merely for purposes of illustration. Various modifications of and equivalent structures corresponding to the disclosed aspects of the preferred embodiments in addition to those described above may be made by those skilled in the art without departing from the spirit of the present invention which is defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.