US20090036973A1 - Ptfe layers and methods of manufacturing - Google Patents

Abstract

Thin PTFE layers are described having little or no node and fibril microstructure and methods of manufacturing PTFE layers are disclosed that allow for controllable permeability and porosity of the layers. In some embodiments, the PTFE layers may act as a barrier layer in an endovascular graft or other medical device.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of application Ser. No. 11/106,150, filed Apr. 13, 2005, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• Polytetrafluoroethylene (PTFE) layers have been used for the manufacture of various types of intracorporeal devices, such as vascular grafts. Such vascular grafts may be used to replace, reinforce, or bypass a diseased or injured body lumen. One conventional method of manufacturing “expanded” PTFE layers is described in U.S. Pat. No. 3,953,566 by Gore. In the methods described therein, a PTFE paste is formed by combining a PTFE resin and a lubricant. The PTFE paste may be extruded. After the lubricant is removed from the extruded paste, the PTFE article is stretched to create a porous, high strength PTFE article. The expanded PTFE layer is characterized by a porous, open microstructure that has nodes interconnected by fibrils.
• Such an expansion process increases the volume of the PTFE layer by increasing the porosity, decreasing the density and increasing the internodal distance between adjacent nodes in the microstructure while not significantly affecting the thickness of the PTFE layer. As such, the conventional methods expand the PTFE layer and impart a porosity and permeability while only providing a negligible reduction in a thickness of the PTFE layer. In situations where a thin PTFE layer, and specifically, a thin PTFE layer having a low fluid permeability is needed, conventional PTFE layers are largely unsatisfactory due to the porosity and highly permeable nature of the expanded PTFE layer.
• Therefore, what has been needed is improved PTFE layers and improved methods for manufacturing the PTFE layers. In particular, it would be desirable to have thin PTFE layers that have a controllable permeability to fluids (gases, liquids or both). It may also be desirable to have such thin PTFE layers that have a high degree of limpness and suppleness to allow mechanical manipulation or strain of such a PTFE layer without significant recoil or spring back.
BRIEF SUMMARY OF THE INVENTION
• Embodiments of the present invention provide PTFE layers and films and methods of manufacturing the PTFE layers and films. Embodiments of the present invention may include one or more layers of a fluoropolymer, such as PTFE. Embodiments of PTFE layers may include at least a portion that does not have a significant or discernable node and fibril microstructure.
• In one embodiment, a method of processing PTFE includes providing a layer of PTFE, applying stretching agent to at least a portion of the layer of PTFE and stretching the layer of PTFE while the layer of PTFE is wet with stretching agent. In another embodiment, a method of processing PTFE includes providing a layer of PTFE, applying stretching agent to at least a portion of the layer until a saturated portion of the surface is saturated with stretching agent and stretching the layer of PTFE while the layer of PTFE is saturated with stretching agent. In another embodiment, a method of processing PTFE includes providing a stretched layer of PTFE that has been stretched in at least a first direction, applying stretching agent to at least a portion of the stretched layer and stretching the stretched layer of PTFE while the layer of PTFE is wet with stretching agent. Also, for some embodiments, the direction of the first direction and the direction of the second stretch may be substantially the same or different. For example, in one embodiment, the first direction is the machine direction and the second stretch is carried out or performed in the transverse direction. In another embodiment, the first direction is the machine direction and the second stretch is carried out in substantially the same machine direction. In other embodiments, the first direction may be a transverse direction. Also, for some embodiments, the stretch in the first direction may have been carried out with sufficiently low stretching agent content so as to produce a significant or discernable node and fibril microstructure during the stretch in the first direction. In other embodiments, the stretch in the first direction may have been carried out while the layer of PTFE was wet with stretching agent to the extent that little or no node and fibril microstructure was created during the stretch in the first direction.
• In another embodiment, a method of processing PTFE includes providing a layer of PTFE, applying stretching agent to at least a portion of the layer of PTFE, stretching the layer of PTFE while the layer of PTFE is wet with stretching agent, stretching the stretched layer of PTFE a second time and calendering the twice stretched layer of PTFE so as to densify, compress and further thin the material. Another embodiment is directed to a method of processing PTFE including providing a layer of PTFE, applying stretching agent to at least a portion of the layer until at least a portion of the layer is saturated with the stretching agent to form a saturated portion and stretching the layer of PTFE. Other embodiments include PTFE layers made by any combination of the methods discussed above.
• Regarding layer embodiments, one layer embodiment is directed to a thin PTFE layer having low porosity, low fluid permeability, substantially no node and fibril structure, and having a thickness of about 0.00005 inch to about 0.005 inch. Another embodiment is directed to a thin PTFE layer, having substantially low porosity, substantially low fluid permeability, substantially no node and fibril structure, and a high degree of limpness and suppleness so to allow mechanical manipulation or strain of the PTFE layer without significant recoil or spring back.
• In another embodiment, a PTFE composite film comprises a first layer including a stretched layer of PTFE that has a closed cell microstructure with a plurality of interconnected high density regions substantially free of node and fibril microstructure between the high density regions. The PTFE composite film also comprises a second layer of expanded PTFE which is secured to the first layer and which includes node and fibril microstructure. In another embodiment, a thin fluid-PTFE layer having low or substantially no fluid permeability is produced by providing a PTFE layer, adding a stretching agent to the PTFE layer and stretching the PTFE layer in at least one direction to reduce a thickness of the PTFE layer. In another embodiment, a thin layer of PTFE includes a stretched layer of PTFE that has a closed cell microstructure with a plurality of interconnected high density regions substantially free of node and fibril microstructure between the high density regions.
• Another embodiment is directed to a multi-layered vascular graft that includes a first tubular body having an outer surface and an inner surface that defines an inner lumen of the vascular graft and a second tubular body having an outer surface and an inner surface coupled to the outer surface of the first tubular body. In this embodiment, one of the first tubular body and the second tubular body includes a fluid-permeable PTFE layer, and the other tubular body comprises a fluid-PTFE layer having low or substantially no fluid permeability. In another embodiment, an inflatable endovascular graft includes a body portion having an inflatable channel that defines an inflatable space. The inflatable space of this embodiment is at least partially surrounded by a thin PTFE layer having low or substantially no fluid permeability.
• Another embodiment is directed to a stretched PTFE layer having low or substantially no fluid permeability that includes a closed cell microstructure having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having substantially no node and fibril microstructure. In another embodiment, a composite film includes a fluid-permeable, expanded PTFE layer secured to a surface of a thin stretched PTFE layer having a closed cell microstructure, having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having substantially no node and fibril microstructure.
• Another embodiment is directed to a tubular structure having a composite film with a fluid-permeable, expanded PTFE layer secured to a surface of a thin, stretched PTFE layer. The thin, stretched PTFE layer has a closed cell microstructure with high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and with substantially no node and fibril microstructure. In another embodiment, an endovascular graft includes a composite film with a fluid permeable, expanded PTFE layer secured to a surface of a thin stretched PTFE layer. The stretched PTFE layer has a closed cell microstructure with high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and with substantially no node and fibril microstructure.
• In another embodiment, a thin PTFE layer has substantially low porosity, low fluid permeability, substantially no node and fibril structure, and a high degree of limpness and suppleness so to allow mechanical manipulation or strain of the PTFE layer without significant recoil or spring back. In another embodiment, a thin layer of PTFE includes a stretched layer of PTFE that has a closed cell microstructure with a plurality of interconnected high density regions substantially free of node and fibril microstructure between the high density regions. In another embodiment, a method of controlling the porosity, density or both of a PTFE layer, includes stretching the PTFE layer at least one time at a preselected temperature and preselected stretching agent content for the at least one stretch.
• These features of embodiments will become more apparent from the following detailed description when taken in conjunction with the accompanying exemplary drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 illustrates a ram extruder extruding a PTFE ribbon that is being taken up on a spool.
• FIG. 2 illustrates a calendering process of the PTFE ribbon ofFIG. 1.
• FIGS. 3 and 4 illustrate a tentering process with stretching agent being applied to a PTFE layer during the stretching process.
• FIGS. 5 and 6 illustrate a machine direction stretching process of the stretched PTFE layer ofFIGS. 3 and 4.
• FIGS. 7 and 8 illustrate a final calendering or densification process performed on a stretched PTFE layer.
• FIG. 9 is a scanning electron microscope (SEM) image of a PTFE layer at a magnification of 20,000.
• FIG. 10 is a SEM image of the PTFE layer ofFIG. 9 at a magnification of 14,000.
• FIG. 11 is a SEM image of the PTFE layer ofFIG. 9 at a magnification of 7,000.
• FIG. 12 is a SEM image of the PTFE layer ofFIG. 9 at a magnification of 3,000.
• FIG. 13 is a SEM image of the PTFE layer ofFIG. 9 at a magnification of 500.
• FIG. 14 schematically illustrates a composite PTFE film that comprises a PTFE layer having low or substantially no fluid permeability and a porous PTFE layer.
• FIG. 15 schematically illustrates a simplified tubular structure that comprises an outer layer having low or substantially no fluid permeability and a fluid-permeable inner layer.
• FIG. 16 schematically illustrates a simplified tubular structure that comprises a layer having low or substantially no fluid permeability and a fluid-permeable outer layer.
• FIG. 17 illustrates an embodiment of an endovascular graft having a network of inflatable conduits.
• FIGS. 18 to 20 are transverse cross sectional views of an inflatable conduit of the graft ofFIG. 17.
• FIG. 21 is a transverse cross sectional view of an embodiment of a tubular inflatable conduit.
• FIG. 22 is an elevational view that illustrates another embodiment of an inflatable endovascular graft.
• FIG. 23 illustrates an embodiment of an inflatable bifurcated endovascular graft.
DETAILED DESCRIPTION OF THE INVENTION
• Embodiments of the present invention relate generally to thin PTFE layers, PTFE films, composite films having two or more PTFE layers and methods of manufacturing the PTFE layers, films and composite films. Some particular embodiments are directed to thin PTFE layers having low or substantially no fluid permeability with a microstructure that does not include significant fibril and nodal structure as is common with expanded PTFE layers. It may also be desirable for some embodiments of such thin PTFE layers that have a high degree of limpness and suppleness so to allow mechanical manipulation or strain of such a PTFE layer without significant recoil or spring back. Such PTFE layers may be manufactured and used for construction of endovascular grafts or other medical devices. For some applications, embodiments of PTFE films may include one or more discrete layers of PTFE that are secured together to form a composite film. As used herein, the term “composite film” generally refers to a sheet of two or more PTFE layers that have surfaces in contact with each other, and in some embodiments, may be secured to each other such that the PTFE layers are not easily separated. The individual PTFE layers used in some of the PTFE composite film embodiments herein may have the thinness and low fluid permeability characteristics discussed above in combination with other layers having the same or different properties Some PTFE layer embodiments have a low fluid permeability while other PTFE layer embodiments have no or substantially no fluid permeability. A PTFE layer having a low fluid permeability may, for some embodiments, be distinguished from the permeability of a standard layer of expanded PTFE by comparing fluid permeability based on Gurley test results in the form of a Gurley Number or “Gurley Seconds”. The Gurley Seconds is determined by measuring the time necessary for a given volume of air, typically, 25 cc, 100 cc or 300 cc, to flow through a standard 1 square inch of material or film under a standard pressure, such as 12.4 cm column of water. Such testing maybe carried out with a Gurley Densometer, made by Gurley Precision Instruments, Troy, N.Y. A standard porous fluid permeable layer of expanded PTFE may have a Gurley Number of less than about 15 seconds, specifically, less than about 10 seconds, where the volume of air used is about 100 cc. In contrast, embodiments of layers of PTFE discussed herein having low fluid permeability may have a Gurley Number of greater than about 1500 seconds where 100 cc of air is used in the test. An embodiment of a PTFE layer discussed herein having no or substantially no fluid permeability may have a Gurley Number of greater than about 12 hours, or up to a Gurley Number that is essentially infinite, or too high to measure, indicating no measurable fluid permeability. Some PTFE layer embodiments having substantially no fluid permeability may have a Gurley Number at 100 cc of air of greater than about 1×106 seconds. Stretched PTFE layers processed by embodiments of methods discussed herein having no discernable node or fibril microstructure may initially have substantially no fluid permeability. However, such PTFE layer embodiments may subsequently be stretched during a manufacturing process, such as the manufacture of an inflatable endovascular graft, during which process the PTFE layer may become more fluid permeable and achieve a level of low permeability as discussed above.
• FIGS. 1-8 illustrate processing of PTFE material to form a thin, stretched PTFE layer having low or substantially no fluid permeability for particular fluids. As such, embodiments of the stretched PTFE layers are not “expanded” in the conventional sense as taught by Gore in, e.g., U.S. Pat. No. 3,953,566. For example, the stretched PTFE layers may be substantially thinned during stretching whereas prior art “expansion” processes typically leave the thickness of the expanded material somewhat unchanged but generate distinct nodal and fibril microstructure along with increased porosity and permeability in order to accommodate the expansion of the layer in plane of the layer.
• Referring toFIG. 1, a fine PTFE resin powder is compounded with an extrusion agent such as a liquid lubricant to form aPTFE compound10. A variety of different PTFE resins may be used such as the lower extrusion ratio, higher molecular weight fine powder coagulated dispersion resins (available from 3M Corporation, Ausimont Corporation, Daikin Corporation, DuPont and ICI Corporation) The PTFE molecules used in these resins typically have an average molecular weight of from about 20 million to about 50 million or more. Optionally, an additive, such as powdered or liquid color pigment or other resin additive may be added to the PTFE resin and lubricant to change the properties of the final PTFE layer. For example, a fluorinated copolymer may be added (such as perfluoropropylvinylether-modified PTFE) to improve the bondability of the PTFE layer. Additive is typically provided in a mass amount that is less than 2% of the mass of the PTFE resin, but it may be provided in any amount that produces a desired result. Additive may be combined with the PTFE resin before the lubricant is added so as to ensure homogenous mixing of the additive throughout the PTFE resin.
• A variety of different types of extrusion and stretching agents, or lubricants, may be compounded with the PTFE powder resin. Some examples of lubricants that may be mixed with the PTFE resin include, but are not limited to, isoparaffin lubricants such as ISOPAR® H, ISOPAR® K and ISOPAR® M all of which are manufactured by ExxonMobil Corporation. Additional lubricants include mineral spirits, naphtha, MEK, toluene, alcohols such as isopropyl alcohol, and any other chemical that is capable of saturating the PTFE resin. In addition, two or more lubricants may be blended together for some lubricant embodiments. The amount of lubricant added to the PTFE resin may vary depending on the type of lubricant used as well as the desired properties of a final PTFE layer. Typically, however, the percent mass of lubricant for some compound embodiments may vary from about 15% to about 25% of the compound mass, specifically, from about 17% to about 22% of the compound mass, and more specifically from about 18% to about 20% of the compound mass.
• The PTFE resin and lubricant may be mixed until a substantiallyhomogenous PTFE compound10 is formed. Compounding of the PTFE resin and lubricant is typically carried out at a temperature below the glass transition temperature of the PTFE resin which is typically from about 55° F. to about 76° F. Compounding of the PTFE resin may be carried out at a temperature below about 50° F., and specifically, at a temperature of from about 40° F. to about 50° F., so as to reduce shearing of the fine PTFE particles. Once mixed, the PTFE compound may be stored at a temperature of above approximately 100° F., and typically from about 110° F. to about 120° F. for a time period that ensures that the lubricant has absorbed through the PTFE resin particles. The storage time period typically may be greater than about six hours, and may vary depending on the resin and lubricant used.
• Once the compounded PTFE resin andlubricant10 have been suitably prepared, thecompound10 may be placed in an extruder, such as theram extruder12 shown inFIG. 1. Theram extruder12 includes abarrel13 and apiston14 that is configured to slide within a chamber of thebarrel13 and form a seal against an inner cylindrical surface of thebarrel13. Thecompound10 is placed in the chamber of theextruder12 between the distal end of thepiston14 and an extruder die16 sealed to theoutput end18 of theextruder12. Theram extruder12 may also includeheat elements20 disposed about theoutput end18 of thebarrel13 which are configured to uniformly heat theoutput end18 of theextruder12. In some methods, theoutput end18 of the extruder is heated before the compoundedPTFE resin10 is loaded into the chamber. An embodiment of aram extruder12 may include a Phillips Scientific Corporation vertical three inch hydraulic ram extruder.
• Once the PTFE resin compound is loaded, thepiston14 is advanced towards theoutput end18 of theextruder12, as indicated byarrow21, which increases the chamber pressure and forces thePTFE compound10 to be extruded through anorifice22 of the die16 to form anextrudate24. Theextrudate24 may be in the form of a ribbon or tape that is then wound onto a take upspool26 as indicated by the arrow adjacent the take up spool inFIG. 1. The ram extrusion process represents a mechanical working of thecompound10 and introduces shear forces and pressure on thecompound10. This working of the compound results in a more cohesive material in the form of extrudate ribbon ortape24.
• Processing conditions may be chosen to minimize the amount of lubricant that is evaporated from thePTFE extrudate ribbon24. For example, thePTFE compound10 may be extruded at a temperature that is above the glass transition temperature, and typically above about 90° F. ThePTFE extrudate ribbon24 is generally fully densified, non-porous and typically has approximately 100% of its original amount of lubricant remaining upon extrusion from thedie16. The die16 may also be configured to produce anextrudate24 having other configurations, such as a tubular configuration. Also, for some methods, thePTFE compound10 may be processed to form a preform billet before it is placed in theextruder12. In addition, a de-ionizing air curtain optionally may be used to reduce static electricity in the area of theextruder12. In one example, theram extruder12 has abarrel13 with a chamber having an inside transverse diameter of about 1 inch to about 6 inches in diameter. Embodiments of the die16 may haveorifices22 configured to produce an extrudate ribbon ortape24 having a width of about 1 inch to about 24 inches and a thickness of about 0.020 inch to about 0.040 inch, specifically, about 0.025 inch to about 0.035 inch.
• After extrusion, the wetPTFE extrudate ribbon24 may be calendered in a first direction or machine direction, as indicated byarrow27, to reduce the thickness of thePTFE extrudate ribbon24 into aPTFE layer28 as shown inFIG. 2. During the calendering process, the width of thePTFE extrudate ribbon24 and calenderedPTFE layer28 changes little while thePTFE extrudate ribbon24 is lengthened in the machine direction. In one embodiment, thePTFE extrudate ribbon24 and calenderedPTFE layer28 may be about 6 inches to about 10 inches in width. The calendering process both lengthens and reduces the thickness of thePTFE ribbon24 to formPTFE layer28 that is taken up byspool32. During calendering, thePTFE extrudate ribbon24 may be calendered between adjustableheated rollers30 to mechanically compress and reduce the thickness of thePTFE ribbon24. As such, the calendering process also encompasses a second mechanical working of thecompound10. Suitable equipment for the calendering process includes acustom 12 inch vertical calender machine manufactured by IMC Corporation, Birmingham, Ala.
• While it may be possible to store thePTFE extrudate ribbon24 for an extended period of time after extrusion, lubricant in thePTFE extrudate ribbon24 will evaporate from theribbon24 during the storage period. As such, it may be desirable in some instances to calender thePTFE extrudate ribbon24 almost immediately after extrusion so as to better control the lubricant level in thePTFE extrudate ribbon24. For some embodiments, thePTFE ribbon24 will have a lubricant content of about 15% to about 25% immediately prior to calendering.
• Depending on the calendering speed and roller positioning, thePTFE ribbon24 may be calendered down to produce aPTFE layer28 of any suitable thickness. The reduction ratio of an embodiment of the calendering process, which is a ratio of the thickness of thePTFE extrudate ribbon24 to the thickness of the calenderedPTFE layer28, maybe from about 3:1 to about 75:1, and specifically from about 7.5:1 to about 15:1. In one particular embodiment, for aPTFE extrudate ribbon24 having a thickness of about 0.030 inch, calendering may reduce its thickness to about 0.001 inch to about 0.006 inch, specifically, from about 0.002 inch to about 0.004 inch. In some instances, thePTFE ribbon24 may be calendered to aPTFE layer28 which has a thickness that is slightly greater than a final desired thickness, so that the final stretch of thePTFE ribbon24 causes thefinal PTFE layer28 to have its desired thickness.
• The calendering temperatures and processing parameters may be chosen so that the calenderedPTFE layer28 still has a significant amount of residual lubricant after the calendering process. For this embodiment, theadjustable rollers30 may be heated to a temperature from about 100° F. to about 200° F., and specifically from about 120° F. to about 160° F. during the calendering process. After calendering, a residual amount of lubricant will remain in thePTFE layer28 which typically may be from about 10% to about 22% lubricant by weight remaining, specifically about 15% to about 20% lubricant by weight.
• Once thePTFE ribbon24 has been calendered to producePTFE layer28,PTFE layer28 then may be mechanically stretched transversely (also called the cross machine direction), in the longitudinal direction (also called the machine direction), in both of these directions or any other suitable direction or combination of directions, in order to thin thePTFE layer28, generate a suitable microstructure and mechanically work the PTFE. It should be noted that although this specification describes a process whereby a PTFE layer is stretched transversely, then stretched longitudinally and then densified, the order these steps are performed in may be changed. For example, a PTFE layer may be first stretched longitudinally then stretched transversely. Such a layer optionally may then be densified as discussed below. For the transverse stretching process shown inFIGS. 3 and 4, atentering machine34 may be used to mechanically stretch the calenderedPTFE layer28 into a stretchedPTFE layer36. One embodiment of asuitable tentering machine34 includes a 60 inch wide by 28 foot long tenter having a T-6 10 horsepower drive unit, manufactured by Gessner Industries, Concord, N.C.
• For some embodiments, in order to produce the desired combination of any of thickness, porosity, fluid permeability as well as mechanical properties, process parameters such as temperature, stretch ratios and material lubricant content ofPTFE layer28 may be controlled before and during the stretching process of the PTFE layer. As such, for some embodiments, a stretching agent orlubricant40 optionally may be applied to the calenderedPTFE layer28 during the stretching process as shown inFIGS. 3 and 4. Applying the stretchingagent40 to thePTFE layer28 prior to or during the stretching process of thePTFE layer28 may be used to control the lubricant content of the stretchedPTFE layer36. This technique may be used to impart particular characteristics to the stretchedPTFE layer36 such as thinness, low porosity and low or substantially no fluid permeability. This method embodiment also allows for the stretchedPTFE layer36 to have a high degree of limpness and suppleness so to allow mechanical manipulation or strain of such a PTFE layer without significant recoil or spring back which may be particularly useful for some applications. If a high density, liquid-impermeable and gas-impermeable PTFE layer28 having low or substantially no fluid permeability is desired, thePTFE layer28 may be saturated throughout the thickness of thePTFE layer28 with one or more stretchingagents40 during stretching. If a moreporous PTFE layer28 is desired, a lesser amount of stretchingagent40 will be applied onto thePTFE layer28. Stretching thePTFE layer28 may be carried out for some embodiments at a temperature of about 80° F. to about 100° F., specifically, about 85° F. to about 95° F.
• The stretchingagent40 may be the same lubricant used to form thePTFE compound10 or it may be a different lubricant or combination of lubricants. In some embodiments, the stretching agent may be applied in sufficient quantities to thePTFE layer28 to saturate thePTFE layer28 during the stretching process. The stretching agent maybe applied by a variety of methods to a surface, such as theupper surface38, of thePTFE layer28 during the stretching process. For example, the stretchingagent40 may be sprayed over theentire layer28 or only on selected portions of thePTFE layer28 by, e.g., a method such as by aspray mechanism42 to theupper surface38 of thePTFE layer28. In such an embodiment, the stretchingagent40 is applied to thePTFE layer28 after thePTFE layer28 unwinds fromspool32 and passes under thespray mechanism42. The stretchingagent40 may be applied uniformly over one or both sides of thePTFE layer28, on only one side of thePTFE layer28, or only on selected portions of thePTFE layer28 at a temperature of typically about 70° F. to about 135° F., specifically, about 105° F. to about 125° F., and more specifically, about 110° F. to about 120° F.
• If a PTFE layer having low or substantially no fluid permeability is desired, thePTFE layer28 may be stretched in one or more directions while fully saturated until the desired thickness is achieved. It should be noted that as thePTFE layer28 is stretched, the capacity of the resulting stretchedPTFE layer36 to absorb stretchingagent40 increases. As such, if it is desirable to maintain a saturated status of thePTFE layer28 and stretchedPTFE layer36, it may be necessary to add stretching agent multiple times or over a large area in order to maintain that saturated state of thePTFE layer36 and the effect of lubricant temperature for a period of time.
• FIG. 4 illustrates the stretching agent orlubricant40 being applied toupper surface38 of thePTFE layer28 byspray mechanism42 as thePTFE layer28 is being stretched transversely. For saturated stretching embodiments, it maybe necessary to apply sufficient stretching agent so as to pool or puddle the stretching agent on theupper surface38 of thePTFE layer28. The pooled or puddled stretching agent may be spread over theupper surface38 of thePTFE layer28 by a skimmingmember44 that has asmooth contact edge46 adjacent theupper surface38 of thePTFE layer28. While not shown, multiple skimming members may be used with some or all having a smooth contact edge or alternatively a grooved/patterned contact edge. The skimmingmember44 is disposed adjacent thespray mechanism42 displaced from the spray mechanism in the machine direction of thePTFE layer28 such that the stretchingagent40 applied by thespray mechanism42 runs into the skimmingmember44 and is spread by the motion of the stretchingagent40 andPTFE layer28 relative to the skimmingmember44. The skimmingmember44 may be in contact with theupper surface38 of thePTFE layer28 or also may be disposed slightly above theupper surface38, depending on the desired configuration of the set up, the type of stretching agent being used as well as other factors.
• Embodiments of methods discussed herein may be useful to reduce a thickness of thePTFE layer28 to a stretchedPTFE layer36 of any thickness down to about 0.00005 inch; typically from about 0.00005 inch and 0.005 inch. Typical transverse stretch ratios may be from about 3:1 to about 20:1. In one embodiment, a calenderedPTFE layer28 having a width of about 3 inches to about 6 inches, may be transversely stretched, as shown inFIGS. 3 and 4, into a stretchedPTFE layer36 having a width of about 20 inches to about 60 inches. This represents a stretch ratio of about 3:1 to about 12:1. In another embodiment, a calenderedPTFE layer28 having a width of about 3.5 inches to about 4.5 inches may be transversely stretched, as shown inFIGS. 3 and 4, into a stretchedPTFE layer36 having a width of about 20 inches to about 60 inches. This represents a stretch ratio of about 7.8:1 to about 13:1.
• As discussed above, the thickness, fluid permeability, porosity and average pore size of the PTFE layers36 may be effected by the amount and temperature of stretchingagent40 applied to thelayer36 prior to or during stretching, the temperature of the layer, the stretching agent that is applied to the PTFE layer, or both, prior to stretching and the stretch rate. By adjusting these parameters, these characteristics may be optimized in order to produce a PTFE layer that is suited to a particular application. For example, if thePTFE layer36 is used as a moisture barrier for clothing, the parameters may be adjusted to produce an average pore size of less than about 6 microns. Alternatively, if thePTFE layer36 is used in an endovascular graft that benefits from tissue in-growth, the average pore size is adjusted to be greater than 6.0 microns. In other embodiments, where thePTFE layer36 is a barrier layer for use in an endovascular graft, the pore size may be smaller, such as from about 0.01 micron to about 5.0 microns. In addition, embodiments of the stretchedPTFE layer36 are fusible and deformable and easily may be fused with or secured to other PTFE layers having different properties. At any point after thePTFE layer28 is stretched, the stretchedPTFE layer36 may be sintered to amorphously lock the microstructure of thePTFE layer36. Sintering may be performed to combine the stretchedPTFE layer36 with other layers of PTFE to form multi-layer composite films, such as those used for endovascular grafts and the like discussed below.
• The stretched PTFE layer optionally may be subjected to a second stretching process, as shown inFIGS. 3,4,5 and6, wherein the stretchedPTFE layer36 is formed into a twice-stretchedPTFE layer46. Once again, as discussed above, it is important to note that although the method embodiments discussed herein are directed to a first transverse stretch and subsequently to a longitudinal or machine direction stretch, the order of the stretch directions may be reversed and other combinations of stretch directions and numbers are also contemplated. For example,PTFE layer28 may be stretched twice in the machine or longitudinal direction without any transverse stretching.PTFE layer28 may be stretched first in a longitudinal or machine direction and then in a transverse direction. In addition, aPTFE layer28 may be stretched three or more times. Some or all of the speeds, stretch ratios, temperatures, lubricant parameters and the like discussed herein may be the same but need not necessarily and typically will not be the same for any of these various stretching steps regardless of the order the stretching steps.
• This optional second stretching process subjects thePTFE layer36 to yet another mechanical working. The second stretching process shown inFIGS. 5 and 6 is being carried out in the machine direction; however, the second stretching process may also be carried out in any other suitable direction, such as transversely. The twice-stretchedPTFE layer46 is wound ontospool48 after undergoing the second stretching process.Additional stretching agent40 optionally may be applied to a surface of the stretchedPTFE layer36 as thelayer36 is being stretched a second time. If higher porosity and fluid permeability are desired, the second stretch may be performed with the stretchedlayer36 in a dry state without the addition of lubricant during the second stretch. If the stretchedPTFE layer36 has residual lubricant without additional lubricant added, the second stretching process will generate a microstructure having significant nodes connected by fibrils. The second stretching process may be carried out at a temperature of about 85° F. to about 95° F. for some embodiments. The stretch ratio for the second stretch maybe up to about 20:1, specifically, about 6:1 to about 10:1.
• If thePTFE layer28 is stretched in two or more directions, the rate of stretching in the two directions; e.g., the machine direction and the off-axis or transverse direction, may have different or the same stretch rates. For example, when thePTFE layer28 is being stretched in the machine direction (e.g., first direction), the rate of stretching is typically in the range from about two percent to about 100 percent per second; specifically, from about four percent to about 20 percent per second, and more specifically about five percent to about ten percent per second. In contrast, when stretching in the cross machine or transverse direction, the rate of stretching may be in the range from about one percent to about 300 percent per second, specifically from about ten percent to about 100 percent per second, and more specifically about 15 percent to about 25 percent per second.
• Stretching in the different directions may be carried out at the same temperatures or at different temperatures. For example, stretching in the machine direction is generally carried out at a temperature below about 572° F., and for some embodiments, below about 239° F. In contrast, stretching in the transverse direction is typically carried out at a temperature above the glass transition temperature, and usually from about 80° F. to about 100° F. Stretching PTFE layers28 at lower temperatures will reduce stretchingagent40 evaporation and retain the stretchingagent40 in thePTFE layer28 for a longer period of time during processing.
• Either the stretchedPTFE layer36 or the twice-stretchedPTFE layer46 optionally may be calendered in order to further thin and density the material. The twice-stretchedPTFE layer46 is shown being calendered inFIGS. 7 and 8. In this example, the twice-stretchedPTFE layer46 is unwound fromspool48, passed throughcalender rollers50 and52, formed into a densifiedlayer54, then taken up onspool54. The calender machine may be the same machine or a different machine as that indicated inFIG. 2 and discussed above. This final calendering or densification ofPTFE layer46 generally produces a highly densifiedPTFE layer54 that has no discernable microstructure features, such as pores, and has low or substantially no fluid permeability. The methods of compressing and stretching PTFE layers may both be used to control thinning of the PTFE layer and the microstructure that results from the thinning process. The densifiedPTFE layer54 may also lack the suppleness and limpness mechanical properties of the stretched PTFE layers36 and46 discussed above. Therollers50 and52 may be adjusted to have any suitable separation to produce aPTFE layer54 having a thickness of about 0.00005 inch to about 0.005 inch. Therollers50 and52 may also be heated during the calendering process, with typical temperatures being from about 90° F. to about 250° F.; specifically, from about 120° F. to about 160° F.; more specifically, from about 130° F. to about 150° F.
• The following example describes specific methods of manufacturing of the stretched PTFE layers36. In this embodiment, 1000 grams of resin are compounded with an isoparaffin based lubricant; specifically, ISOPAR® M, in a mass ratio of lubricant-to-PTFE compound from about 15% to about 25%. Compounding of the PTFE resin and lubricant is carried out at a temperature below 50° F., which is well below the glass transition temperature of the PTFE resin of between about 57° F. to about 75° F.
• ThePTFE compound10 maybe formed into a billet and stored at a temperature of about 105° F. to about 125° F. for six or more hours to ensure that the lubricant substantially has penetrated and absorbed through the resin. Thereafter, thePTFE compound10 is placed in anextruder12, as shown inFIG. 1. ThePTFE compound10 may then be paste extruded from theorifice22 of thedie16 of theextruder12 at a temperature above the resin glass transition temperature. In one embodiment, the paste is extruded at a temperature from about 80° F. to 120° F. A reduction ratio, e.g., a ratio of a cross sectional area of thePTFE compound10 before extrusion to the cross section area of thePTFE extrudate24 after extrusion, may be from about 10:1 to about 400:1, and specifically may be from about 80:1 to about 120:1. Theextruder12 maybe a horizontal extruder or a vertical extruder. Theorifice22 of the extrusion die16 determines the final cross sectional configuration of theextruded PTFE ribbon24. Theorifice22 shape or configuration of the extrusion die16 may be tubular, square, rectangular or any other suitable profile. It may be desirable to preform the PTFE compound (resin and lubricant) into a billet.
• ThePTFE extrudate ribbon24 is then calendered, as shown inFIG. 2, at a temperature from about 100° F. to about 160° F. to reduce a thickness of thePTFE ribbon24 and form a PTFE layer orfilm28. The temperature at calendering may be controlled by controlling the temperature of therollers30 of the calender machine. The PTFE layer may be calendered down to a thickness from about 0.001 inch to about 0.006 inch, and specifically, down to a thickness of about 0.002 inch to about 0.003 inch. At the end of the calendering, the calenderedPTFE layer28 may have a lubricant content of about 10% by weight to about 20% by weight.
• Referring again toFIGS. 3 and 4, after calendering, one side or both sides of the calenderedPTFE layer28 are sprayed with an isoparaffin-based stretchingagent40 at a prescribed temperature so that the PTFE film orlayer28 is flooded and fully saturated through the thickness of thePTFE layer28. The saturated, calendered PTFE layer may then be stretched in a direction that is substantially orthogonal to the calendering direction by atentering machine34 to reduce a thickness of thePTFE layer28 and form a stretchedPTFE layer36. The stretchedPTFE layer36 may have a thickness of about 0.00005 inch to about 0.005 inch; specifically, the stretchedPTFE layer36 may have a thickness of about 0.0002 inch to about 0.002 inch. ThePTFE layer28 typically is tentered or stretched at an elevated temperature above the glass transition temperature, specifically, from about 80° F. to about 100° F., more specifically, about 85° F. to about 95° F.
• Wet tentering with the stretchingagent40 allows thePTFE layer28 to be thinned without creating substantial porosity and fluid permeability in the stretchedPTFE layer36. While the stretchedPTFE layer36 will have a porosity, its porosity and pore size typically will not be large enough to be permeable to liquids, and often will be small enough to have substantially no fluid permeability. In addition, the stretchedPTFE layer embodiment36 does not have the conventional node and fibril microstructure but instead has a closed cell microstructure in which boundaries of adjacent nodes are directly connected with each other. The fluid-impermeable stretched PTFE film orlayer36 typically may have a density from about 0.5 g/cm3 to about 1.5 g/cm3, but it may have a larger or smaller density for some embodiments. In addition, with regard to all of the methods of processing layers of PTFE discussed above, any of the PTFE layers produced by these methods may also be sintered at any point in the above processes in order to substantially fix the microstructure of the PTFE layer. A typical sintering process may be to expose the PTFE layer to a temperature of about 350° C. to about 380° C. for several minutes; specifically, about 2 minutes to about 5 minutes.
• The various methods discussed above may be used to produce PTFE layers having a variety of desirable properties. The scanning electron microscope (SEM) images shown inFIGS. 9 to 13 illustrate different magnifications of a microstructure of a PTFE film orlayer110 made in accordance with embodiments of the present invention.PTFE layer110 has a generallyclosed cell microstructure112 that is substantially free of the conventional node and fibril microstructure commonly seen in expanded PTFE layers. Embodiments of thePTFE film110 may have low fluid-permeability, or no or substantially no fluid-permeability. One or more ofPTFE layer110 may be used as a barrier layer to prevent a fluid such as a liquid or gas from permeating or escaping therethrough.
• At a magnification of 20,000, as seen inFIG. 9, the microstructure of the stretchedPTFE layer110 resembles a pocked-like structure that comprises interconnectedhigh density regions114 and pockets orpores116 between some of thehigh density regions114. ThePTFE film110 may be considered to have a closed cell network structure with interconnected strands connectinghigh density regions114 in which a high density region grain boundary is directly connected to a grain boundary of an adjacent high density region. Unlike conventional expanded PTFE (“Eptfe”) which typically has a substantial node and fibril microstructure that is discernable when viewed at a SEM magnification of 20,000,PTFE layer110 lacks the distinct, parallel fibrils that interconnect adjacent nodes of ePTFE and has no discernable node and fibril microstructure when viewed at a SEM magnification of 20,000, as shown inFIG. 9. The closed cell microstructure of thePTFE layer110 provides a layer having low or substantially no fluid permeability that may be used as “a barrier layer” to prevent liquid from passing from one side of the PTFE layer to the opposite side.
• Though PTFE film orlayer110 is configured to have low or substantially no fluid permeability,PTFE layer110 nonetheless has a porosity. ThePTFE layer110 typically has an average porosity from about 20% to about 80%, and specifically from about 30% and about 70%. In one embodiment, aPTFE film110 has a porosity of about 30% to about 40%. In another embodiment, aPTFE layer110 has a porosity of about 60% to about 70%. Porosity as described in these figures is meant to indicate the volume of solid PTFE material as a percentage of the total volume of thePTFE film110. An average pore size in thePTFE layer110 is may be less than about 20 microns, and specifically less than about 0.5 micron. In one embodiment, aPTFE layer110 has an average pore size of from about 0.01 micron to about 0.5 micron. As can be appreciated, if tissue ingrowth is desired, thePTFE film110 may have an average pore size of greater than about 6.0 microns. As described below, depending on the desired properties of theresultant PTFE layer110, embodiments of methods may be modified so as to vary the average porosity and average pore size of thePTFE film110 in a continuum from 10 microns to 50 microns down to substantially less than about 0.1 micron.
• PTFE layer110 may have a density from about 0.5 g/cm³to about 1.5 g/cm³, and specifically from about 0.6 g/cm³to about 1.5 g/cm³. While the density of thePTFE film110 is typically less than a density for a fully densified PTFE layer (e.g., 2.1 g/cm³), if desired, the density of thePTFE layer110 may be densified to a higher density level so that the density of thePTFE layer110 is comparable to a fully densified PTFE layer.FIGS. 9 to 13 illustrate aPTFE film110 having a closed microstructural network and that is substantially impermeable to liquid and gas; other embodiments of PTFE layers may be manufactured using the methods discussed herein to have other suitable permeability values and pore sizes.
• PTFE film110 may have an average thickness that is less than about 0.005 inch, specifically from about 0.00005 inch to about 0.005 inch, and more specifically from about 0.0001 inch to about 0.002 inch.
• While embodiments of methods discussed herein are directed to manufacturing PTFE layers, it should be appreciated that the methods discussed may also be useful in the manufacture of other fluoropolymer-based films having substantial, low or substantially no fluid permeability. As such, the methods discussed herein are not limited to the processing of PTFE materials. For example, the processing of other fluoropolymer resin-based materials, such as copolymers of tetrafluororethylene and other monomers, is also contemplated.
• The PTFE layers and PTFE films may be used in a variety of ways. For example, the PTFE layer and PTFE film embodiments of the present invention may be used for prosthetic devices such as a vascular graft, breast implants and the like. Other applications include tubing, protective clothing, insulation, sports equipment, filters, membranes, fuel cells, ionic exchange barriers, gaskets as well as others. Referring now toFIG. 14,PTFE layer110 maybe combined with, bonded to, or otherwise coupled, affixed or attached, partially or completely, to at least oneadditional layer118 to form acomposite film120. Depending on the use ofcomposite film120,layer118 may be chosen to have properties that combine with the properties oflayer110 to give the desired properties incomposite film120. Theadditional layer118 may include a porous PTFE layer, a substantially non-porous PTFE layer, an air or liquid permeable PTFE layer, an air- or liquid-impermeable layer, an ePTFE layer, a non-expanded PTFE layer, a fluoropolymer layer, a non-fluoropolymer layer, or any combination thereof. In one embodiment,layer118 is a porous, fluid permeable, expanded PTFE layer having a conventional node and fibril microstructure. If desired, one or more reinforcing layers (not shown) optionally may be coupled to thecomposite PTFE film120. The reinforcing layer may be disposed betweenlayers110 or118, or the reinforcing layer(s) may be coupled to an exposed surface ofPTFE layer110,PTFE layer118, or both.PTFE layer110 andlayer118 may be combined, bonded to, or otherwise coupled, affixed or attached, partially or completely, to one another using any suitable method known in the art. For example, an adhesive may be used to selectively bond at least a portion oflayers110 and118 to each other. Alternatively, heat fusion, pressure bonding, sintering, and the like may be used to bond at least a portion oflayers110 and118 to each other.
• FIGS. 15 and 16 are transverse cross-sectional views of two compositetubular structures130 and140, respectively.Tubular structures130 and140 may be a portion or section of an endovascular graft or the like. As shown inFIG. 15,tubular structure130 includes an innertubular body132 that comprises aninner surface134 and anouter surface136.Tubular body132 may comprise one or more layers of fluid-permeable PTFE. Such a fluid-permeable layer of PTFE may have a Gurley measurement of less than about 10 Gurley seconds.Tubular structure130 further comprises an outer tubular body138 that comprises aninner surface137 and anouter surface139.Inner surface137 of outer tubular body138 is coupled to theouter surface136 of the innertubular body132. Tubular body138 may comprise one or more PTFE layers having low fluid-permeability or substantially no fluid-permeability. In this configuration,inner surface134 of thetubular body132 defines aninner lumen135 oftubular structure130 and theouter surface139 of the tubular body138 defines anouter surface139 of thetubular structure130. Tubular body138 may be combined, bonded to, or otherwise coupled, affixed or attached, partially or completely, to thetubular body132 through any suitable method known in the art. For example, an adhesive may be used to selectively bond at least a portion of tubular body138 andtubular body132 to each other. Alternatively, heat fusion, pressure bonding, sintering, and the like, or any combination thereof, may be used to bond at least a portion of tubular body138 andtubular body132 to each other.
• As shown inFIG. 16,tubular structure140 includes an innertubular body142 that comprises aninner surface144 and anouter surface146.Tubular body142 may comprise one or more layers of PTFE having low or substantially no fluid permeability.Tubular structure140 further comprises an outertubular body148 that comprises aninner surface147 and anouter surface149.Inner surface147 of outertubular body148 is coupled to theouter surface146 of the innertubular body142. Outertubular body148 may comprise one or more layers of fluid-permeable PTFE. Embodiments of fluid-permeable layers of PTFE may have a Gurley measurement of less than about 10 Gurley seconds. In this configuration,inner surface144 of the innertubular body142 defines aninner lumen145 oftubular structure140 and theouter surface149 of the outertubular body148 defines anouter surface149 of thetubular structure140.Tubular body148 may be combined, bonded to, or otherwise coupled, affixed or attached, partially or completely, to thetubular body142 through any suitable method known in the art. For example, an adhesive may be used to selectively bond at least a portion oftubular body148 andtubular body132 to each other. Alternatively, heat fusion, pressure bonding, sintering, and the like, or any combination thereof, may be used to bond at least a portion oftubular body148 andtubular body142 to each other.
• Tubular structures130 or140 may define an inner diameter ID which is the diameter of the inner surface, which may define the area of flow throughtubular structure130 or140. An outer diameter OD, which is the diameter of theouter surface139 or149 of the outertubular layer138 or148. The inner diameter ID and outer diameter OD may be any desired diameter. For use in an endovascular graft, the inner diameter ID but is typically from about 10 mm to about 40 mm and the outer diameter OD is typically from about 12 mm to about 42 mm. The tubular layers may have any suitable thickness, however, fluid-impermeable PTFE layers138 and142 have a thickness from about 0.0005 inch and about 0.01 inch thick, and specifically from about 0.0002 inch to about 0.001 inch. Similarly, fluid-permeable PTFE layers132 or148 may also be any thickness desired, but typically have a thickness from about 0.0001 inch and about 0.01 inch, and specifically from about 0.0002 inch to about 0.001 inch. As can be appreciated, the thicknesses and diameters of thetubular structures130 or140 will vary depending on the use of the tubular structures.
• Tubular structures130 or140 may be formed as tubes through conventional tubular extrusion processes. Typically, however,tubular structures130 or140 may be formed fromPTFE layers110 or118, as shown inFIG. 14, that are folded on a shape forming mandrel over each other so that ends of the layers are overlapped and bonded (not shown). As another alternative, PTFE layers110 or118 may be helically wound about the shape forming mandrel to form the tubular structure. Some exemplary methods of forming a tubular PTFE structure is described in commonly owned, copending U.S. patent application Ser. Nos. 10/029,557 and entitled “Methods and Apparatus for Manufacturing an Endovascular Graft Section”, 10/029,584 and entitled “Endovascular Graft Joint and Method of Manufacture”, both filed on Dec. 20, 2001 to Chobotov et al., and U.S. Pat. No. 6,776,604 to Chobotov et al., the complete disclosures of which are incorporated herein by reference.
• The films and layers discussed herein are not limited to a singleporous PTFE layer118 and a single PTFE layer orfilm110 having low or substantially no fluid permeability. Thecomposite films120 andtubular structures130 or140 may include a plurality of porous fluid permeable PTFE layers (having the same or different node and fibril size and orientation, porosity, pore size, and the like), one or more non-porous, densified PTFE layers, and/or one or more PTFE layers110 having low or substantially no fluid permeability. For example,PTFE layer110 having low or substantially no fluid permeability may be disposed between an inner and outer porous PTFE film or layer. The inner and outer porous PTFE layers may have varying porosities or the same porosities. In such embodiments, thePTFE layer110 may have a reduced thickness relative to the porous PTFE layers. In other embodiments, however, thePTFE layer110 may have the same thickness or larger thickness than the porous PTFE layers. As an alternative embodiment toFIGS. 15 and 16,tubular structures130 or140 may comprise inner and outer tubular bodies that both have low or substantially no fluid permeability.
• Referring now toFIG. 17, a tubular structure that is in the form of an inflatableendovascular graft50 is shown. For the purposes of this application, with reference to endovascular graft devices, the term “proximal” describes the end of the graft that will be oriented towards the oncoming flow of bodily fluid, typically blood, when the device is deployed within a body passageway. The term “distal” therefore describes the graft end opposite the proximal end.Graft150 has aproximal end151 and adistal end152 and includes a generally tubular structure orgraft body section153 comprised of one or more layers of fusible material, including such materials as PTFE and ePTFE. The inner surface of the tubular structure defines an inner diameter and acts as a luminal surface for flow of fluids therethrough. The outer surface of the tubular structure defines an abluminal surface that is adapted to be positioned adjacent the body lumen wall, within the weakened portion of the body lumen, or both. Note that althoughFIG. 17 shows an inflatable endovascular graft, the layers and films of the present invention may be used in non-inflatable endovascular grafts as well, in addition to other medical and non-medical applications.
• A proximalinflatable cuff156 may be disposed at or near aproximal end151 ofgraft body section153 and a distalinflatable cuff157 may be disposed at or near a graft body sectiondistal end152.Graft body section153 forms a longitudinal lumen that is configured to confine a flow of fluid, such as blood, therethrough.Graft150 may be manufactured to have any desired length and internal and external diameter but typically ranges in length from about 5 cm to about 30 cm; specifically from about 10 cm to about 30 cm. If desired, astent159 may be attached at theproximal end151 and/or thedistal end152 of thegraft150. Depending on the construction of thecuffs156 and157 andgraft body section153, inflation ofcuffs156 and157, when not constrained (such as, e.g., by a vessel or other body lumen), may cause thecuffs156 and157 to assume a generally annular or toroidal shape with a generally semicircular longitudinal cross-section.Inflatable cuffs156 and157 may be designed to generally, however, conform to the shape of the vessel within which it is deployed. When fully inflated, cuffs156 and157 may have an outside diameter ranging from about 10 mm to about 45 mm; specifically from about 16 mm to about 42 mm.
• At least oneinflatable channel158 may be disposed between and in fluid communication with proximalinflatable cuff156 and optional distalinflatable cuff157.Inflatable channel158 in theFIG. 17 example has a helical configuration and provides structural support to graftbody section153 when inflated to contain an inflation medium.Inflatable channel158 further prevents kinking and twisting of the tubular structure or graft body section when it is deployed within angled or tortuous anatomies as well as during remodeling of body passageways, such as the aorta and iliac arteries, within whichgraft150 may be deployed. Together with proximal anddistal cuffs156 and157,inflatable channel158 forms an inflatable network over the length of thebody153. Depending on the desired characteristics of theendovascular graft150, at least one layer of the graft may be a PTFE layer having low or substantially no fluid permeability such as PTFE layer orfilm110. The PTFE layer may be one of the layers that forms theinflatable channels158, or the PTFE layer may surround or be underneath theinflatable channel158 andcuffs156 and157.
• Graft body153 may be formed of two or more layers or strips of PTFE that are selectively fused or otherwise adhered together as described herein, to form theinflatable cuffs156 and157 andinflatable channel158 therebetween. A detailed description of some methods of manufacturing a multi-layered graft are described in co-pending and commonly owned U.S. patent application Ser. Nos. 10/029,557 (which published as US 20030116260 A1), 10/029,584, U.S. patent application Ser. No. 10/168,053, filed Jun. 14, 2002 and entitled “Inflatable Intraluminal Graft” to Murch, and U.S. Pat. No. 6,776,604 to Chobotov et al., the complete disclosures of which are incorporated herein by reference.
• FIGS. 18 to 21 illustrate transverse cross sectional views of different embodiments ofinflatable channel158. As can be appreciated, the embodiments ofFIGS. 18 to 21 may also be applicable to the proximal anddistal cuffs156 and157.Inflatable channel158 defines aninflatable space162 that is created between aninner layer164 andouter layer166. If desired aninflation medium167 may be delivered into thespace162 to inflateinflatable space162.Inflation medium167 optionally may include adeliverable agent168 as shown inFIGS. 18 to 21, such as atherapeutic agent168 that may be configured to be diffused in a controlled manner or otherwise transmitted through pores (not shown) ininner layer164,outer layer166 or both. The embodiments shown inFIGS. 18-21 are merely exemplary, as it may be desirable to have preferential diffusion of thedeliverable agent168 throughlayer164 orlayer166. In addition, bothlayers164 and166 may be configured to allow a significant amount of diffusion ofdeliverable agent168, but with one of the two layers having a greater permeability to thedeliverable agent168 than the other layer. Whileinner layer164 andlayer166 are shown as having only a single layer of material, it should be appreciated that each oflayers164 or166 may include one or more layers to form a composite film of fluid-permeable PTFE, PTFE having low fluid permeability, PTFE having substantially no fluid permeability or any combination thereof. A more complete description of methods and devices for the delivery of a therapeutic agent can be found in copending and commonly owned U.S. patent application Ser. No. 10/769,532 (which published as US 20050171593 A1), filed Jan. 30, 2004 and entitled “Inflatable Porous Implants and Methods for Drug Delivery” to Whirley et al., the complete disclosure of which is incorporated herein by reference. A description of exemplary inflation medium materials can be found in copending and commonly owned U.S. patent application Ser. No. 11/097,467, filed Apr. 1, 2005 and entitled “A Non-Degradable, Low Swelling, Water Soluble, Radiopaque Hydrogel” to Askari et al., the complete disclosure of which is incorporated herein by reference.
• In the embodiment shown inFIG. 18,outer layer166 is permeable to fluids so as to allow thetherapeutic agent168, which may be a liquid, to diffuse over time in the direction ofarrow169 throughouter layer166. In such embodiments,inner layer164 typically has a low or substantially no fluid permeability, and could therefore be considered a “barrier layer.” Because the inner “barrier”layer164 has low or substantially no fluid permeability andouter layer166 is fluid permeable, the therapeutic agent will preferentially diffuse fromspace162 in the direction ofarrow169. The use of one (or more) porous fluid permeable outer PTFE layers and aninner layer164 having low or substantially no fluid permeability provides for improved release of a therapeutic agent through liquid permeableouter layer166. Varying the porosity or pore size across at least a portion ofouter layer166 may provide even more localized delivery of thetherapeutic agent168 throughouter layer166.
• In an alternative configuration shown inFIG. 19,inner layer164 may be substantially fluid-permeable to allow thetherapeutic agent168 to selectively diffuse in the direction ofarrow169 throughinner layer164 and into the lumen of the tubular structure (e.g.,lumen135,145 ofFIGS. 15 and 16). In such embodiments,outer layer166 typically has no or substantially no fluid-permeability and acts as a “barrier layer.” As such, the therapeutic agent will preferentially diffuse fromspace162 in the direction ofarrow169. The use of porous fluid permeable PTFE layers andouter layer166 having low or substantially no fluid permeability provides for improved release of a therapeutic agent into the inner lumen through fluid permeableinner layer164. Varying the permeability and/or porosity or pore size across at least a portion ofinner layer164 may provide even more localized delivery of thetherapeutic agent168 throughlayer164.
• As shown inFIG. 20, if it is desired to prevent theinflation medium167 from escaping frominflatable space162, both theinner layer164 andouter layer166 may comprise a “barrier” layer having low or substantially no fluid permeability. In such embodiments, the inner andouter layers164 and166 have low or substantially no fluid permeability. In such embodiments,inflation material167 typically will not contain a therapeutic agent. Referring toFIG. 21, the inflatable channel may be a substantiallytubular channel170 that is fused or otherwise adhered to layer164 that defines a portion of the graft. If delivery of a therapeutic agent is desired,tubular channel170 will be liquid-permeable and will allow diffusion of thetherapeutic agent168 through pores intubular channel170. In some embodiments, by varying the permeability and/or porosity or pore size across at least a portion ofchannel170 may provide a localized delivery of thetherapeutic agent168 selected portions ofchannel170. If however, it is desired to prevent theinflation fluid167 from escaping frominflatable space162, thentubular channel170 will act as a barrier layer and may comprise at least one layer of PTFE having low or substantially no fluid permeability.
• Referring now toFIGS. 22 and 23, therespective graft embodiments150 and180 shown include aninflatable channel158 has portions with a circumferential configuration as opposed to the helical configuration of theinflatable channel158 shown inFIG. 17. The circumferential configuration of portions of theinflatable channel158 may be particularly effective in providing the needed kink resistance for endovascular graft for effectively treating diseased body passageways such as a thoracic aortic aneurysm (TAA), abdominal aortic aneurysm (AAA), in which highly angled and tortuous anatomies are frequently found. In alternative embodiments, other cuff and channel configurations are possible.Inflatable channel158 may be configured circumferentially as shown inFIGS. 22 and 23.
• In addition to the substantially tubular grafts ofFIG. 22, bifurcated endovascular grafts as shown inFIG. 23, are also contemplated. The bifurcatedendovascular graft180 may be utilized to repair a diseased lumen at or near a bifurcation within the vessel, such as, for example, in the case of an abdominal aortic aneurysm in which the aneurysm to be treated may extend into the anatomical bifurcation or even into one or both of the iliac arteries distal to the bifurcation. In the following discussion, the various features of the graft embodiments previously discussed may be used as necessary in the bifurcated graft80 embodiment unless specifically mentioned otherwise.
• Graft180 comprises a firstbifurcated portion182, a secondbifurcated portion184 andmain body portion186. The size and angular orientation of thebifurcated portions182 and184 may vary to accommodate graft delivery system requirements and various clinical demands. The size and angular orientation may vary even betweenportion182 and184. For instance, each bifurcated portion or leg is shown inFIG. 23 to optionally have a different length. First and secondbifurcated portions182 and184 are generally configured to have an outer inflated diameter that is compatible with the inner diameter of a patient's iliac arteries. First and secondbifurcated portions182 and184 may also be formed in a curved shape to better accommodate curved and even tortuous anatomies in some applications. Together,main body portion186 and first and secondbifurcated portions182 and184 form a continuous bifurcated lumen, similar to the inner lumens ofFIG. 22, which is configured to confine a flow of fluid therethrough. A complete description of some desirable sizes and spacing of inflatable channels may be found in commonly owned, copending U.S. patent application Ser. No. 10/384,103 (which published as US 20040176836 A1), entitled “Kink-Resistant Endovascular Graft” and filed Mar. 6, 2003 to Kari et al., the complete disclosure of which is incorporated herein by reference.
• While not shown, it should be appreciated, that instead of circumferential channels and longitudinal channels, thebifurcated graft180 may comprise a helicalinflatable channel158, similar to that of the graft embodiment shown inFIG. 17 (or other channel geometries to achieve desired results), or a combination of helical and circumferential channels. A complete description of some embodiments of endovascular grafts that have helical and cylindrical channel configurations may be found in co-pending and commonly owned U.S. patent application Ser. No. 10/384,103 (which published as US 20040176836 A1). Other endovascular grafts that the liquid-impermeable PTFE film may be used with are described in U.S. Pat. Nos. 6,395,019 to Chobotov, 6,132,457 to Chobotov, 6,331,191 to Chobotov, and U.S. patent application Ser. Nos. 10/327,711 (which published as US 20030125797 A1), entitled “Advanced Endovascular Graft” to Chobotov et al. and filed Dec. 20, 2002, 10/168,053, the complete disclosures of which are incorporated herein by reference.
• As can be appreciated, the inflatable portions of thegraft180 optionally may be configured to have varying levels of fluid permeability and/or porosity, either within or between particular cuffs, channels or cuff/channel segments, so as to provide for controlled drug delivery, programmed drug delivery or both, into the vessel wall or lumen of the graft via elution of the agent from pores in the layers. For example, any desired portion of thegraft180 may include PTFE layers having low or substantially no fluid permeability. Such a configuration would be useful in applications in which the drug delivery rate and other properties of the graft or stent-graft (e.g. mechanical properties) may be selected for the particular clinical needs and indication that is contemplated for that device. In addition, the fluid permeability and/or porosity may be uniform within a particular cuff or channel but different between any given channel and/or cuffs. In addition to improved drug delivery, the variable porosity of the outer surface of the graft may also be beneficial for promoting tissue in-growth into the graft. It may be possible to make portions of the graft that are in direct contact with the body lumen to have a higher porosity and/or larger pore size so as to promote tissue in-growth. In particular, tissue in-growth may be beneficial adjacent to the proximal and distal ends of the graft.
• With regard to the above detailed description, like reference numerals used therein refer to like elements that may have the same or similar dimensions, materials and configurations. While particular forms of embodiments have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the embodiments of the invention. Accordingly, it is not intended that the invention be limited by the forgoing detailed description.

Claims (58)

1. A method of processing PTFE, comprising:

providing a layer of PTFE;

applying a stretching agent to at least a portion of the layer of PTFE; and

stretching the layer of PTFE while the layer of PTFE is wet with the stretching agent to form a stretched layer of PTFE.

2. The method ofclaim 1 wherein the stretching agent is applied to substantially all of the layer of PTFE prior to stretching.

3. The method ofclaim 1 wherein stretching the layer of PTFE comprises stretching the layer of PTFE by a stretch ratio of about 2:1 to about 20:1.

4. The method ofclaim 1 wherein the stretching of the layer of PTFE comprises stretching in a machine direction.

5. The method ofclaim 1 wherein the stretching of the layer comprises stretching the layer of PTFE in a direction transverse to the machine direction.

6. The method ofclaim 1 further comprising calendering the stretched layer of PTFE to densify and compress the layer of PTFE.

7. The method ofclaim 1 wherein the stretching agent comprises an isoparaffin.

8. The method ofclaim 1 wherein the stretching agent is selected from the group consisting of naphtha, mineral spirits, alcohol, MEK, toluene and alcohol.

9. The method ofclaim 1 wherein a lubricant content of the layer of PTFE prior to application of the stretching agent is about 0 percent by weight to about 22 percent by weight.

10. The method ofclaim 1 further comprising spreading the stretching agent after application to the layer of PTFE with a skimming member disposed adjacent the layer of PTFE.

11. The method ofclaim 1 wherein stretching of the layer of PTFE is performed at a temperature of about 80° F. to about 100° F.

12. The method ofclaim 1 wherein stretching of the layer of PTFE is performed at a temperature of about 85° F. to about 95° F.

13. The method ofclaim 1 wherein the stretching agent is applied to the layer of PTFE at a temperature of about 110° F. to about 130° F.

14. The method ofclaim 13 wherein the stretching agent is applied to the layer of PTFE at a temperature of about 115° F. to about 125° F.

15. The method ofclaim 1 wherein stretching the layer of PTFE is performed at a temperature that is just above the glass transition temperature of the PTFE layer material.

16. The method ofclaim 1 wherein the provided layer of PTFE is produced by extruding a compounded PTFE resin through an extruder to form a PTFE ribbon extrudate.

17. The method ofclaim 16 wherein the compounded resin is extruded to an extrudate having a ribbon configuration and having a thickness of about 0.020 inch to about 0.040 inch.

18. The method ofclaim 16 wherein the PTFE ribbon extrudate is calendered to a reduced thickness of about 0.001 inch to about 0.005 inch prior to stretching.

19. The method ofclaim 1 wherein the stretching agent is applied to the layer of PTFE in sufficient quantity such that at least a portion of the layer of PTFE is saturated with stretching agent at the time of stretching.

20. The method ofclaim 1 further comprising stretching the stretched layer of PTFE a second time with no stretching agent added to the stretched layer of PTFE during the second stretch.

21. The method ofclaim 20 wherein the stretched layer of PTFE has a sufficiently low stretching agent content so to form a discernable node and fibril microstructure in the stretched layer of PTFE during the second stretch.

22. The method ofclaim 1 further comprising sintering the stretched layer of PTFE.

23. A method of processing PTFE, comprising:

providing a layer of PTFE;

applying a stretching agent to at least a portion of the layer of PTFE until a portion of the layer of PTFE is saturated with the stretching agent to form a saturated portion; and

stretching the layer of PTFE while the saturated portion of the layer of PTFE is saturated with the stretching agent.

24. The method ofclaim 23 wherein substantially all of the layer of PTFE is saturated with stretching agent to form a saturated portion prior to stretching.

25. The method ofclaim 24 further comprising sintering the layer of PTFE.

26. A method of processing PTFE, comprising:

providing a stretched layer of PTFE that has been stretched in at least a first direction;

applying a stretching agent to at least a portion of the stretched layer of PTFE; and

stretching the stretched layer of PTFE while the stretched layer of PTFE is wet with the stretching agent.

27. The method ofclaim 26 wherein the stretching agent is applied to substantially all of the stretched layer of PTFE prior to the stretching of the stretched layer.

28. The method ofclaim 26 wherein the stretched layer of PTFE has a lubricant content of below about 3% by weight prior to application of the stretching agent.

29. The method ofclaim 26 wherein the stretched layer of PTFE has been stretched in the first direction with a sufficiently low stretching agent content so to form a discernable node and fibril microstructure in the stretched layer of PTFE during the stretch in the first direction.

30. The method ofclaim 26 wherein the stretching of the stretched layer is performed in a direction different than the first direction.

31. The method ofclaim 30 wherein the first direction is a machine direction and the stretching of the stretched layer is performed in a transverse direction.

32. The method ofclaim 30 wherein the stretching of the stretched layer of PTFE material is performed in a transverse direction by a stretch ratio of about 2:1 to about 30:1.

33. The method ofclaim 26 wherein the stretching of the stretched layer is performed in substantially the same direction as the first direction.

34. The method ofclaim 33 wherein the first direction is a machine direction and the stretching of the stretched layer is performed in the machine direction.

35. The method ofclaim 26 wherein the stretched layer of PTFE has been stretched in the first direction while the layer of PTFE was wet with stretching agent applied to the layer of PTFE.

36. The method ofclaim 26 further comprising sintering the layer of PTFE.

37. A method of processing PTFE, comprising:

providing a layer of PTFE;

applying a stretching agent to at least a portion of the layer;

stretching the layer of PTFE while the layer of PTFE is wet with the stretching agent to form a stretched layer of PTFE;

stretching the stretched layer of PTFE a second time; and

calendering the twice-stretched layer of PTFE so as to densify and further thin the twice-stretched layer of PTFE.

38. The method ofclaim 37 wherein the stretching agent is applied to substantially all of the surface of the layer prior to stretching.

39. A PTFE layer comprising a layer made by providing a layer of PTFE;

applying a stretching agent to a surface of the layer; and

stretching the layer of PTFE while the layer of PTFE is wet with the stretching agent.

40. A PTFE layer comprising a layer made by providing a layer of PTFE;

applying a stretching agent to a surface of the layer;

stretching the layer of PTFE while the layer of PTFE is wet with the stretching agent; and

stretching the stretched layer of PTFE a second time.

41. A PTFE layer comprising a layer made by providing a layer of PTFE;

applying a stretching agent to a surface of the layer;

stretching the layer of PTFE while the layer of PTFE is wet with the stretching agent;

stretching the stretched layer of PTFE a second time; and

calendering the twice-stretched layer of PTFE so as to densify and further thin the twice-stretched layer of PTFE.

42. A thin PTFE layer comprising substantially low porosity, low permeability, no discernable node and fibril structure, and having a thickness of about 0.00005 inch to about 0.005 inch.

43. A composite PTFE film comprising:

a first layer comprising a stretched layer of PTFE that has a closed cell microstructure with a plurality of interconnected high density regions having no discernable node and fibril microstructure between the high density regions; and

a second layer of expanded PTFE which is secured to the first layer and which includes a substantial node and fibril microstructure.

44. A thin, substantially liquid-impermeable PTFE layer produced by:

providing a PTFE layer;

adding a stretching agent to the PTFE layer; and

stretching the PTFE layer in at least one direction to reduce a thickness of the PTFE layer without substantially creating a liquid permeability in the stretched PTFE layer.

45. The thin layer ofclaim 41 wherein the stretched PTFE layer comprises a closed cell microstructure that comprises a plurality of interconnected high density regions with no discernable node and fibril microstructure.

46. A multi-layered vascular graft comprising:

a first tubular body having an outer surface and an inner surface that defines an inner lumen of the vascular graft; and

a second tubular body having an outer surface and an inner surface coupled to the outer surface of the first tubular body,

wherein one of the first tubular body and the second tubular body comprises a fluid-permeable PTFE layer and the other tubular body comprises a PTFE layer having low fluid permeability.

47. The multi-layered vascular graft ofclaim 46 wherein the PTFE layer having low fluid permeability comprises a closed cell microstructure that comprises a plurality of interconnected high density regions wherein the closed cell microstructure has no discernable node and fibril microstructure.

48. The multi-layered vascular graft ofclaim 46 wherein the PTFE layer having low fluid permeability comprises a thin PTFE layer having substantially low porosity, no discernable node and fibril structure, and a high degree of limpness and suppleness so to allow mechanical manipulation or strain of the PTFE layer without significant recoil or spring back.

49. An inflatable endovascular graft comprising a body portion having an inflatable channel that defines an inflatable space, wherein the inflatable space is at least partially surrounded by a thin, PTFE layer having substantially no fluid permeability.

50. The inflatable endovascular graft ofclaim 49 wherein the PTFE layer having substantially no fluid permeability comprises a closed cell microstructure that comprises a plurality of interconnected high density regions and wherein the closed cell microstructure has no discernable node and fibril microstructure.

51. The inflatable endovascular graft ofclaim 49 wherein the PTFE layer having substantially no fluid permeability comprises a thin PTFE layer having substantially low porosity, no discernable node and fibril structure, and a high degree of limpness and suppleness to allow mechanical manipulation or strain of the PTFE layer without significant recoil or spring back.

52. A stretched, PTFE layer that comprises a closed cell microstructure having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having no discernable node and fibril microstructure and having substantially no fluid permeability.

53. A composite film comprising a fluid-permeable, expanded PTFE layer secured to a surface of a thin stretched PTFE layer having a closed cell microstructure having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having no discernable node and fibril microstructure.

54. A tubular structure comprising a composite film comprising a fluid-permeable, expanded PTFE layer secured to a surface of a thin, stretched PTFE layer having a closed cell microstructure having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having no discernable node and fibril microstructure.

55. An endovascular graft comprising a composite film with a fluid-permeable, expanded PTFE layer secured to a surface of a thin stretched PTFE layer having a closed cell microstructure having high density regions whose grain boundaries are directly interconnected to grain boundaries of adjacent high density regions and having no discernable node and fibril microstructure.

56. A thin PTFE layer, comprising substantially low porosity, low liquid permeability, no discernable node and fibril structure, and a high degree of limpness and suppleness so to allow mechanical manipulation or strain of the PTFE layer without significant recoil or spring back.

57. A thin layer of PTFE comprising a stretched layer of PTFE that has a closed cell microstructure with a plurality of interconnected high density regions having no discernable node and fibril microstructure between the high density regions.

58. A method of controlling the porosity, density or both of a PTFE layer, comprising:

stretching the PTFE layer at least one time at a preselected temperature while using a preselected stretching agent content for the at least one stretch.

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