U.S. patent application number 11/106131 was filed with the patent office on 2006-10-19 for ptfe layers and methods of manufacturing.
This patent application is currently assigned to TriVascular, Inc.. Invention is credited to Joseph W. Humphrey, Jeffry B. Skiba.
Application Number | 20060233990 11/106131 |
Document ID | / |
Family ID | 36617032 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060233990 |
Kind Code |
A1 |
Humphrey; Joseph W. ; et
al. |
October 19, 2006 |
PTFE layers and methods of manufacturing
Abstract
Single, continuous PTFE layers having lateral zones of varied
characteristics are described. Some of the lateral zone embodiments
may include PTFE material having little or no nodal and fibril
microstructure. Methods of manufacturing PTFE layers allow for
controllable permeability and porosity of the layers, in addition
to other characteristics. The characteristics may vary from one
lateral zone of a PTFE layer to a second lateral zone of a PTFE
layer. In some embodiments, the PTFE layers may act as a barrier
layer in an endovascular graft or other medical device.
Inventors: |
Humphrey; Joseph W.; (Santa
Rosa, CA) ; Skiba; Jeffry B.; (Chandler, AZ) |
Correspondence
Address: |
HOFFMANN & BARON, LLP
6900 JERICHO TURNPIKE
SYOSSET
NY
11791
US
|
Assignee: |
TriVascular, Inc.
Santa Rosa
CA
|
Family ID: |
36617032 |
Appl. No.: |
11/106131 |
Filed: |
April 13, 2005 |
Current U.S.
Class: |
428/36.9 ;
428/35.7; 428/36.91 |
Current CPC
Class: |
B29C 48/022 20190201;
B29K 2027/18 20130101; B29C 55/005 20130101; Y10T 428/1352
20150115; A61L 27/16 20130101; B29C 48/08 20190201; C08L 27/18
20130101; B29C 43/24 20130101; Y10T 428/139 20150115; A61L 27/507
20130101; Y10T 428/1393 20150115; A61L 27/16 20130101; Y10T
428/24802 20150115; A61F 2/82 20130101; B29C 43/222 20130101; A61L
27/56 20130101 |
Class at
Publication: |
428/036.9 ;
428/035.7; 428/036.91 |
International
Class: |
B32B 1/08 20060101
B32B001/08 |
Claims
1. A method of processing PTFE, comprising: providing a layer of
PTFE; selectively applying a stretching agent to at least one
lateral zone of the layer of PTFE in a predetermined pattern; and
stretching the layer of PTFE.
2. The method of claim 1 wherein the layer of PTFE is stretched
while the at least one lateral zone is wet with the stretching
agent.
3. The method of claim 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 of claim 1 wherein the stretching of the layer of
PTFE comprises stretching in a machine direction.
5. The method of claim 1 wherein the stretching of the layer
comprises stretching the layer in a direction transverse to the
machine direction.
6. The method of claim 1 further comprising calendering the
stretched layer of PTFE to compress and densify the PTFE layer.
7. The method of claim 1 wherein the stretching agent comprises an
isoparaffin.
8. The method of claim 1 wherein the stretching agent is selected
from the group consisting of naptha, mineral sprits, alcohol, MEK,
toluene and alcohol.
9. The method of claim 1 wherein the stretching agent content of
the layer of PTFE prior to selective application of the stretching
agent is about 0 percent by weight to about 22 percent by
weight.
10. The method of claim 1 further comprising stretching the
stretched layer of PTFE a second time.
11. A method of processing PTFE, comprising: providing a layer of
PTFE having a stretching agent content level; selectively removing
stretching agent from at least one lateral zone of the portion of
the layer of PTFE in a predetermined pattern; and stretching the
layer of PTFE.
12. The method of claim 11 wherein the layer of PTFE is stretched
while at least a portion of the layer of PTFE is wet with
stretching agent.
13. The method of claim 11 wherein stretching the layer of PTFE
comprises stretching the layer of PTFE by a stretch ratio of about
2:1 to about 20:1.
14. The method of claim 11 wherein the stretching of the layer of
PTFE comprises stretching in a machine direction.
15. The method of claim 11 wherein the stretching of the layer
comprises stretching the layer in a direction transverse to the
machine direction.
16. The method of claim 11 further comprising calendering the
stretched layer of PTFE to compress and densify the PTFE layer.
17. The method of claim 11 wherein the stretching agent comprises
an isoparaffin.
18. The method of claim 11 wherein the stretching agent is selected
from the group consisting of naptha, mineral sprits, alcohol, MEK,
toluene and alcohol.
19. The method of claim 11 further comprising applying stretching
agent to the layer of PTFE prior to selective removal of the
stretching agent.
20. The method of claim 19 wherein the stretching agent content of
the layer of PTFE prior to application of the stretching agent is
about 3 percent by weight to about 22 percent by weight.
21. The method of claim 19 further comprising spreading the
stretching agent after application to the layer of PTFE with a
skimming member disposed adjacent the layer of PTFE.
22. The method of claim 11 further comprising stretching the
stretched layer of PTFE a second time.
23. A method of processing PTFE, comprising: providing a layer of
PTFE; applying a stretching agent to at least one lateral zone of a
surface of the layer in a predetermined pattern until the lateral
zone is saturated with the stretching agent; and stretching the
layer of PTFE while lateral zone of the layer of PTFE is saturated
with the stretching agent.
24. The method of claim 23 further comprising stretching the
stretched layer of PTFE a second time.
25. A PTFE layer comprising a layer made by providing a layer of
PTFE; selectively applying a stretching agent to at least one
lateral zone of the layer of PTFE in a predetermined pattern; and
stretching the layer of PTFE.
26. A PTFE layer comprising a layer made by providing a layer of
PTFE having a stretching agent content level; selectively removing
stretching agent from at least one lateral zone of the portion of
the layer of PTFE in a predetermined pattern; and stretching the
layer of PTFE.
27. A PTFE layer comprising a layer made by providing a layer of
PTFE; applying a stretching agent to at least one lateral zone of a
surface of the layer in a predetermined pattern until the lateral
zone is saturated with the stretching agent; and stretching the
layer of PTFE while lateral zone of the layer of PTFE is saturated
with the stretching agent.
28. 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 at least one of the first tubular
body and the second tubular body comprises a PTFE layer having a
first lateral zone with a substantially low porosity, a low fluid
permeability and no discernable node and fibril microstructure, and
a second lateral zone which is fluid-permeable and has substantial
node and fibril microstructure.
29. A tubular structure comprising a layer of PTFE having a first
lateral zone which is fluid-permeable and which has substantial
node and fibril microstructure and a second lateral zone with 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.
30. An endovascular graft comprising a PTFE layer having a first
lateral zone with a liquid-permeable, expanded PTFE layer adjacent
a second lateral zone having (a) a closed cell microstructure
having high density regions whose grain boundaries are directly
interconnected to grain boundaries of adjacent high density regions
and (b) substantially no node and fibril microstructure.
31. The endovascular graft of claim 30 wherein the endovascular
graft comprises an inflatable endovascular graft having at least
one inflatable channel and wherein the second lateral zone bounds
at least a portion of the inflatable channel.
32. A thin, continuous PTFE layer, comprising: a first lateral zone
with a substantially low porosity, a low fluid permeability, 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; and a
second lateral zone which is fluid-permeable and has a substantial
node and fibril microstructure.
33. A method of processing a layer of PTFE, comprising: providing a
layer of PTFE; stretching the layer of PTFE; and applying a
stretching agent to the PTFE layer during the stretching
process.
34. The method of claim 33 wherein the formation of a discernable
node and fibril microstructure is created during the stretching
process prior to application of the stretching agent to the PTFE
layer.
Description
BACKGROUND OF THE INVENTION
[0001] 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.
[0002] 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.
[0003] 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
[0004] 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 node
and fibril microstructure.
[0005] In one embodiment, a method of processing PTFE includes
providing a layer of PTFE, selectively applying a stretching agent
to at least one lateral zone of the layer of PTFE in a
predetermined pattern and stretching the layer of PTFE. In another
embodiment, a method of processing PTFE includes providing a layer
of PTFE having a stretching agent content level and selectively
removing stretching agent from at least one lateral zone of the
layer of PTFE in a predetermined pattern and stretching the layer
of PTFE. In yet another embodiment of a method of processing PTFE,
a layer of PTFE is provided. Stretching agent is applied to at
least one lateral zone of a surface of the layer in a predetermined
pattern until the lateral zone is saturated with stretching agent.
Next, the layer of PTFE is stretched while the lateral zone of the
layer of PTFE is saturated with stretching agent.
[0006] In another embodiment, a PTFE layer includes a layer made by
providing a layer of PTFE, selectively applying a stretching agent
to at least one lateral zone of the layer of PTFE in a
predetermined pattern and stretching the layer of PTFE. In another
embodiment, a PTFE layer includes a layer made by providing a layer
of PTFE having a stretching agent content level, selectively
removing stretching agent from at least one lateral zone of the
portion of the layer of PTFE in a predetermined pattern and
stretching the layer of PTFE. In another embodiment, a PTFE layer
includes a layer made by providing a layer of PTFE, applying a
stretching agent to at least one lateral zone of a surface of the
layer in a predetermined pattern until the lateral zone is
saturated with stretching agent and stretching the layer of PTFE
while lateral zone of the layer of PTFE is saturated with
stretching agent.
[0007] An embodiment of a multi-layered vascular graft includes a
first tubular body having an outer surface and an inner surface
that defines an inner lumen of the vascular graft. A second tubular
body having an outer surface and an inner surface is coupled to the
outer surface of the first tubular body. At least one of the first
tubular body and the second tubular body includes a PTFE layer
having a first lateral zone with a substantially low porosity, a
low fluid permeability and no discernable node and fibril
structure, and a second lateral zone which is fluid-permeable and
has substantial node and fibril microstructure.
[0008] In another embodiment, a tubular structure includes a layer
of PTFE having a first lateral zone that is fluid-permeable and has
a substantial node and fibril microstructure and a second lateral
zone with 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. In another embodiment,
an endovascular graft includes a PTFE layer having a first lateral
zone that is fluid-permeable adjacent a second lateral zone with 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. In yet another embodiment, a PTFE layer
includes a first lateral zone with a substantially low porosity, a
low liquid permeability, 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. The PTFE layer also includes a second lateral zone
which is fluid-permeable and has a substantial node and fibril
microstructure.
[0009] 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
[0010] FIG. 1 illustrates a ram extruder extruding a PTFE ribbon
being taken up on a spool.
[0011] FIG. 2 illustrates a calendering process of the PTFE ribbon
of FIG. 1.
[0012] FIGS. 3 and 4 illustrate a tentering process with a
stretching agent being applied to a PTFE layer during the
stretching process and with portions of the tentering machine not
shown for purposes of clarity of illustration.
[0013] FIGS. 5 and 6 illustrate a stretching process in the machine
direction of the stretched PTFE layer of FIGS. 3 and 4.
[0014] FIGS. 7 and 8 illustrate a final calendering or
densification process performed on a stretched PTFE layer.
[0015] FIG. 8A illustrates a method of application of a stretching
agent in a preselected pattern during a transverse stretching
process in a direction that is substantially orthogonal to a
machine direction by a tentering machine in order to produce PTFE
layers having characteristics which may vary across the layer in a
desired pattern.
[0016] FIG. 8B illustrates as side view of the method of FIG. 8A
with portions of the tentering machine not shown for purposes of
clarity of illustration.
[0017] FIG. 8C is an enlarged view of an alternative embodiment of
a portion of the PTFE layer of FIG. 8A containing stretching agent
in a preselected pattern, taken within the encircled portion 8C of
FIG. 8A.
[0018] FIG. 8D is an enlarged view of an alternative embodiment of
a portion of the stretched PTFE layer of FIG. 8A having a pattern
of varied permeability, taken within encircled portion 8D of FIG.
8A.
[0019] FIG. 8E is an enlarged view of an alternative embodiment of
a portion of the PTFE layer of FIG. 8A containing stretching agent
in a preselected pattern, taken within the encircled portion 8C of
FIG. 8A.
[0020] FIG. 8F is an enlarged view of an alternative embodiment of
a portion of the stretched PTFE layer of FIG. 8A having a pattern
of varied fluid permeability, taken within encircled portion 8D of
FIG. 8A.
[0021] FIG. 8G is an enlarged view of an alternative embodiment of
a portion of the PTFE layer of FIG. 8A containing stretching agent
in a preselected pattern, taken within the encircled portion 8C of
FIG. 8A.
[0022] FIG. 8H is an enlarged view of an alternative embodiment of
a portion of the stretched PTFE layer of FIG. 8A having a pattern
of varied fluid permeability, taken within encircled portion 8D of
FIG. 8A.
[0023] FIG. 8I is an enlarged view of an alternative embodiment of
a portion of the PTFE layer of FIG. 8A containing stretching agent
in a preselected pattern, taken within the encircled portion 8C of
FIG. 8A.
[0024] FIG. 8J is an enlarged view of an alternative embodiment of
a portion of the stretched PTFE layer of FIG. 8A having a pattern
of varied fluid permeability, taken within encircled portion 8D of
FIG. 8A.
[0025] FIG. 8K is an enlarged view of an alternative embodiment of
a portion of the PTFE layer of FIG. 8A containing stretching agent
in a preselected pattern, taken within the encircled portion 8C of
FIG. 8A.
[0026] FIG. 8L is an enlarged view of an alternative embodiment of
a portion of the stretched PTFE layer of FIG. 8A having a pattern
of varied fluid permeability, taken within encircled portion 8D of
FIG. 8A.
[0027] FIG. 9 is a scanning electron microscope (SEM) image of a
PTFE layer at a magnification of 20,000.
[0028] FIG. 10 is a SEM image of the PTFE layer of FIG. 9 at a
magnification of 14,000.
[0029] FIG. 11 is a SEM image of the PTFE layer of FIG. 9 at a
magnification of 7,000.
[0030] FIG. 12 is a SEM image of the PTFE layer of FIG. 9 at a
magnification of 3,000.
[0031] FIG. 13 is a SEM image of the PTFE layer of FIG. 9 at a
magnification of 500.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] FIG. 17 illustrates an embodiment of an endovascular graft
having a network of inflatable conduits.
[0036] FIGS. 18 to 20 are transverse cross sectional views of an
inflatable conduit of the graft of FIG. 17.
[0037] FIG. 21 is a transverse cross sectional view of an
embodiment of a tubular inflatable conduit.
[0038] FIG. 22 is an elevational view that illustrates another
embodiment of an inflatable endovascular graft.
[0039] FIG. 23 illustrates an embodiment of an inflatable
bifurcated endovascular graft.
DETAILED DESCRIPTION OF THE INVENTION
[0040] 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 may be 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.times.106 seconds. Stretched PTFE
layers processed by embodiments of methods discussed herein having
no discemable 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.
[0041] FIGS. 1-8 illustrate processing of PTFE material to form a
thin, stretched PTFE layer having low or substantially no liquid
permeability for particular liquids, such as water based liquids.
Such an embodiment may be useful where it is desirable to exclude
water based fluids and other fluids, such as body fluids of a
patient. Some PTFE layer embodiments discussed herein may also be
substantially impermeable to air and other gases. As such,
embodiments of the stretched PTFE layers are not "expanded" in the
conventional sense as taught by Gore in 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.
[0042] Referring to FIG. 1, a fine PTFE resin powder is compounded
with an extrusion agent such as a liquid lubricant to form a PTFE
compound 10. 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.
[0043] 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.RTM. H, ISOPAR.RTM. K and ISOPAR.RTM. M all of which are
manufactured by ExxonMobil Corporation. Additional lubricants
include mineral spirits, naptha, 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.
[0044] The PTFE resin and lubricant may be mixed until a
substantially homogenous PTFE compound 10 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.degree. F. to about
76.degree. F. Compounding of the PTFE resin may be carried out at a
temperature below about 50.degree. F., and specifically, at a
temperature of from about 40.degree. F. to about 50.degree. 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.degree. F., and typically from about 110.degree. F. to about
120.degree. 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.
[0045] Once the compounded PTFE resin and lubricant 10 have been
suitably prepared, the compound 10 may be placed in an extruder,
such as the ram extruder 12 shown in FIG. 1. The ram extruder 12
includes a barrel 13 and a piston 14 that is configured to slide
within a chamber of the barrel 13 and form a seal against an inner
cylindrical surface of the barrel 13. The compound 10 is placed in
the chamber of the extruder 12 between the distal end of the piston
14 and an extruder die 16 sealed to the output end 18 of the
extruder 12. The ram extruder 12 may also include heat elements 20
disposed about the output end 18 of the barrel 13 which are
configured to uniformly heat the output end 18 of the extruder 12.
In some methods, the output end 18 of the extruder is heated before
the compounded PTFE resin 10 is loaded into the chamber. An
embodiment of a ram extruder 12 may include a Phillips Scientific
Corporation vertical three inch hydraulic ram extruder.
[0046] Once the PTFE resin compound is loaded, the piston 14 is
advanced towards the output end 18 of the extruder 12, as indicated
by arrow 21 which increases the chamber pressure and forces the
PTFE compound 10 to be extruded through an orifice 22 of the die 16
to form an extrudate 24. The extrudate 24 may be in the form of a
ribbon or tape that is then wound onto a take up spool 26 as
indicated by the arrow adjacent the take up spool in FIG. 1. The
ram extrusion process represents a mechanical working of the
compound 10, and introduces shear forces and pressure on the
compound 10. This working of the compound results in a more
cohesive material in the form of an extrudate ribbon or tape
24.
[0047] Processing conditions may be chosen to minimize the amount
of lubricant that is evaporated from the PTFE extrudate ribbon 24.
For example, the PTFE compound 10 may be extruded at a temperature
that is above the glass transition temperature, and typically above
90.degree. F. The PTFE extrudate ribbon 24 is generally fully
densified, non-porous and typically has approximately 100% of its
original amount of lubricant upon extrusion from the die 16. The
die 16 may also be configured to produce an extrudate 24 having
other configurations, such as a tubular configuration. Also, for
some methods, the PTFE compound 10 may be processed to form a
preform billet before it is placed in the extruder 12. In addition,
a de-ionizing air curtain may optionally be used to reduce static
electricity in the area of the extruder 12. In one example, the ram
extruder 12 has a barrel 13 with a chamber having an inside
transverse diameter of about 1 inch to about 6 inches in diameter.
Embodiments of the die 16 may have orifices 22 configured to
produce an extrudate ribbon or tape 24 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.
[0048] After extrusion, the wet PTFE extrudate ribbon 24 may be
calendered in a first direction or machine direction, as indicated
by arrow 27, to reduce the thickness of the PTFE extrudate ribbon
24 into a PTFE layer 28 as shown in FIG. 2. During the calendering
process, the width of the PTFE extrudate ribbon 24 and calendered
PTFE layer 28 changes little while the PTFE extrudate ribbon 24 is
lengthened in the machine direction. In one embodiment, the PTFE
extrudate ribbon 24 and calendered PTFE layer 28 may be about 6
inches to about 10 inches in width. The calendering process both
lengthens and reduces the thickness of the PTFE ribbon 24 to form
PTFE layer 28 which is taken up by spool 32. During calendering,
the PTFE extrudate ribbon 24 may be calendered between adjustable
heated rollers 30 to mechanically compress and reduce the thickness
of the PTFE ribbon 24. As such, the calendering process also
encompasses a second mechanical working of the compound 10.
Suitable equipment for the calendering process includes a custom 12
inch vertical calendar machine manufactured by IMC Corporation,
Birmingham, Ala.
[0049] While it may be possible to store the PTFE extrudate ribbon
24 for an extended period of time after extrusion, lubricant in the
PTFE extrudate ribbon 24 will evaporate from the ribbon 24 during
the storage period. As such, it may be desirable in some instances
to calender the PTFE extrudate ribbon 24 almost immediately after
extrusion so as to better control the lubricant level in the PTFE
extrudate ribbon 24. For some embodiments, the PTFE ribbon 24 will
have a lubricant content of about 15% to about 25% immediately
prior to calendering.
[0050] Depending on the calendering speed and roller positioning,
the PTFE ribbon 24 may be calendered down to produce a PTFE layer
28 of any suitable thickness. The reduction ratio of an embodiment
of the calendering process, which is a ratio of the thickness of
the PTFE extrudate ribbon 24 to the thickness of the calendered
PTFE layer 28, may be between about 3:1 to about 75:1, and
specifically between about 7.5:1 to about 15:1. In one particular
embodiment, for a PTFE extrudate ribbon 24 having a thickness of
about 0.030 inches, calendering may reduce the thickness to about
0.001 inch to about 0.006 inch, specifically, between about 0.002
inch to about 0.004 inch. In some instances, the PTFE ribbon 24 may
be calendered to a PTFE layer 28 which has a thickness that is
slightly greater than a final desired thickness, so that the final
stretch of the PTFE ribbon 24 causes the final PTFE layer 28 to
have its desired thickness.
[0051] The calendering temperatures and processing parameters may
be chosen so that the calendered PTFE layer 28 still has a
significant amount of residual lubricant after the calendering
process. For this embodiment, the adjustable rollers 30 may be
heated to a temperature between about 100.degree. F. and about
200.degree. F., and specifically between about 120.degree. F. and
about 160.degree. F. during the calendering process. After
calendering, a residual amount of lubricant will remain in the PTFE
layer 28 which may typically be between about 10% to about 22%
lubricant by weight remaining, specifically about 15% to about 20%
lubricant by weight.
[0052] Once the PTFE ribbon 24 has been calendered to produce PTFE
layer 28, PTFE layer 28 may then be mechanically stretched
transversely (also called the cross machine direction), in the
longitudinal direction (also called the machine direction), both of
these directions or any other suitable direction or combination of
directions, in order to thin the PTFE layer 28, 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 may
optionally then be densified, as discussed below. For the
transverse stretching process shown in FIGS. 3 and 4, a tentering
machine 34 may be used to mechanically stretch the calendered PTFE
layer 28 into a stretched PTFE layer 36. One embodiment of a
suitable tentering machine 34 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.
[0053] For some embodiments, in order to produce desired thickness,
porosity, permeability as well as mechanical properties, process
parameters such as temperature, stretch ratios and material
lubricant content of PTFE layer 28, may be controlled before and
during the stretching process. As such, for some embodiments, a
stretching agent or lubricant 40 may optionally be applied to the
calendered PTFE layer 28 during the stretching process as shown in
FIGS. 3 and 4. Applying the stretching agent 40 to the PTFE layer
28 prior to or during the stretching process of the PTFE layer 28
may be used to control the lubricant content of the stretched PTFE
layer 36. This technique may be used to provide characteristics to
the stretched PTFE layer 36 such as thinness, low porosity and low
or substantially no permeability. This method embodiment also
allows for the stretched PTFE layer 36 to 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
which may be particularly useful for some applications. If a high
density, liquid and gas-impermeable PTFE layer 28 is desired, the
PTFE layer 28 may be saturated throughout the thickness of the PTFE
layer 28 with one or more stretching agents 40 during stretching.
If a more porous PTFE layer 28 is desired, a lesser amount of
stretching agent 40 will be applied onto the PTFE layer 28.
Stretching the PTFE layer 28 may be carried out for some
embodiments at a temperature of about 80.degree. F. to about
100.degree. F., specifically, about 85.degree. F. to about
95.degree. F.
[0054] The stretching agent 40 may be the same lubricant used to
form the PTFE compound 10 or it may be a different lubricant or
combination of lubricants. In some embodiments, the stretching
agent may be applied in sufficient quantities to the PTFE layer 28
to saturate the PTFE layer 28 during the stretching process. The
stretching agent may be applied by a variety of methods to a
surface, such as the upper surface 38, of the PTFE layer 28 during
the stretching process. For example, the stretching agent 40 may be
sprayed over the entire layer 28, or only on selected portions of
the PTFE layer 28 by a spray mechanism 42 to the upper surface 38
of the PTFE layer 28. The stretching agent 40 is applied to the
PTFE layer 28 after the PTFE layer 28 unwinds from spool 32 and
passes under the spray mechanism 42. The stretching agent 40 may be
applied uniformly over one or both sides of the PTFE layer 28, on
only one side of the PTFE layer 28, or only on selected portions of
the PTFE layer 28 at a temperature of typically about 70.degree. F.
to about 135.degree. F., specifically, about 105.degree. F. to
about 125.degree. F., and more specifically, about 110.degree. F.
to about 120.degree. F.
[0055] If a PTFE layer having low or substantially no fluid
permeability is desired, the PTFE layer 28 will be stretched in one
or more directions while fully saturated until the desired
thickness is achieved. It should be noted that as the PTFE layer 28
is stretched, the capacity of the resulting stretched PTFE layer 36
to absorb stretching agent 40 increases. As such, if it is
desirable to maintain a saturated status of the PTFE layer 28 and
stretched PTFE layer 36, it may be necessary to add stretching
agent multiple times or over a large area in order to maintain that
saturated state of the PTFE layer 36 and the effect of the
stretching agent 40 temperature (about 110.degree. F. to about
120.degree. F.) for a period of time. As such, stretching agent 40
may be added prior to the initiation of the stretching process or
at any time during the stretching process. A method whereby
stretching agent 40 is applied to the PTFE layer during the
stretching process may allow for the formation of discemable node
and fibril microstructure creation during the stretching process
prior to application of the stretching agent 40 to the PTFE layer;
however, thinning of the PTFE layer will still take place once the
stretching agent 40 has been applied and stretching continues.
[0056] FIG. 4 illustrates the stretching agent 40 being applied to
upper surface 38 of the PTFE layer 28 by spray mechanism 42 as the
PTFE layer 28 is being stretched transversely. For saturated
stretching embodiments, it may be necessary to apply sufficient
stretching agent so as to pool or puddle the stretching agent on
the upper surface 38 of the PTFE layer 28. In such a case, the
pooled or puddled stretching agent may be spread over the upper
surface 38 of the PTFE layer 28 by a skimming member 44 that has a
smooth contact edge 46 adjacent the upper surface 38 of the PTFE
layer 28. The skimming member 44 is disposed adjacent the spray
mechanism 42 displaced from the spray mechanism in the machine
direction of the PTFE layer 28 such that the stretching agent 40
applied by the spray mechanism 42 runs into the skimming member 44
and is spread by the motion of the stretching agent 40 and PTFE
layer 28 relative to the skimming member 44. The skimming member 44
may be in contact with the upper surface 38 of the PTFE layer 28 or
may also be disposed above the upper surface 38, depending on the
desired configuration of the set up, the type of stretching agent
being used as well as other factors. Multiple skimming members may
be used with some or all of the skimming members having a smooth
contact edge or alternatively a grooved/patterned contact edge.
[0057] Embodiments of methods discussed herein may be useful to
reduce a thickness of the PTFE layer 28 to a stretched PTFE layer
36 of any thickness down to about 0.00005 inch, but typically from
about 0.0005 inch and 0.005 inch. Typical transverse stretch ratios
may be from about 3:1 to about 20:1. In one embodiment, a
calendered PTFE layer 28 having a width of about 3 inches to about
6 inches, may be transversely stretched, as shown in FIGS. 3 and 4,
into a stretched PTFE layer 36 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 calendered PTFE layer 28
having a width of about 3.5 inches to about 4.5 inches, may be
transversely stretched, as shown in FIGS. 3 and 4, into a stretched
PTFE layer 36 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.
[0058] As discussed above, the thickness, porosity, average pore
size and fluid permeability of the PTFE layers 36 may be affected
by the amount and temperature of stretching agent 40 applied to the
layer 36 prior to or during stretching. In addition, the
temperature of the PTFE layer, the type of stretching agent that is
applied to the PTFE layer, and the stretch rate may also affect the
thickness, porosity, average pore size and fluid permeability of
the PTFE layer 36. 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 the
PTFE layer 36 is used as a moisture barrier for clothing, the
parameters may be adjusted to produce an average pore size of less
than about 6.0 microns. Alternatively, if the PTFE layer 36 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 the PTFE layer 36 is a barrier layer for
use in an endovascular graft, the pore size may be smaller, such as
between about 0.01 microns and about 5.0 microns. In addition,
embodiments of the stretched PTFE layer 36 are fusible and
deformable and may easily be fused with other PTFE layers having
different properties. At any point after the PTFE layer 28 is
stretched, the stretched PTFE layer 36 may be sintered to
amorphously lock the microstructure of the PTFE layer 36. Sintering
may be performed to combine the stretched PTFE layer 36 with other
layers of PTFE to form multi-layer films, such as those used for
endovascular grafts and the like discussed below.
[0059] The stretched PTFE layer optionally may be subjected to a
second stretching process, as shown in FIGS. 3, 4, 5 and 6, wherein
the stretched PTFE layer 36 is formed into a twice-stretched PTFE
layer 46. 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 direction steps
may be reversed and other combinations of stretch directions and
numbers are also contemplated. For example, PTFE layer 28 may be
stretched twice in the machine or longitudinal direction without
any transverse stretching. PTFE layer 28 may be stretched first in
a longitudinal or machine direction and then in a transverse
direction. In addition, a PTFE layer 28 may be stretched three or
more times. Some or all of the speeds, stretch ratios,
temperatures, lubricant parameters and the like as discussed herein
may be the same or similar to those previously described, but need
not necessarily be so. Moreover, these parameters typically will
not be the same for any of these various stretching steps,
regardless of the order in which they are undertaken.
[0060] This optional second stretching process subjects the PTFE
layer 36 to yet another mechanical working. The second stretching
process shown in FIGS. 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-stretched PTFE layer 46 is wound onto spool 48 after
undergoing the second stretching process. Additional stretching
agent 40 optionally may be applied to a surface of the stretched
PTFE layer 36 as the layer 36 is being stretched a second time. If
higher porosity and fluid permeability are desired, the second
stretch may be performed with the stretched layer 36 in a dry state
without the addition of lubricant during the second stretch. If the
stretched PTFE layer 36 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.degree. F. to about 95.degree. F. for some embodiments.
The stretch ratio for the second stretch may be up to about 20:1,
specifically, about 6:1 to about 10:1.
[0061] If the PTFE layer 28 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 the PTFE
layer 28 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.
[0062] 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.degree. F., and for some embodiments,
below about 239.degree. 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.degree. F. to about 100.degree. F. Stretching PTFE layers 28 at
lower temperatures will reduce stretching agent 40 evaporation and
retain the stretching agent 40 in the PTFE layer 28 for a longer
period of time during processing.
[0063] Either the stretched PTFE layer 36 or the twice-stretched
PTFE layer 46 optionally may be calendered in order to further thin
and densify the material. The twice-stretched PTFE layer 46 is
shown being calendered in FIGS. 7 and 8. In this example, the
twice-stretched PTFE layer 46 is unwound from spool 48, passed
through calender rollers 50 and 52, formed into a densified layer
54, then taken up on spool 54. The calender machine may be the same
machine or a different machine as that indicated in FIG. 2 and
discussed above. This final calendering or densification of PTFE
layer 46 generally produces a highly densified PTFE layer 54 that
has no discemable 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 densified PTFE layer 54 may also lack the
suppleness and limpness mechanical properties of the stretched PTFE
layers 36 and 46 discussed above. The rollers 50 and 52 may be
adjusted to have any suitable separation to produce a PTFE layer 54
having a thickness of about 0.00005 inch to about 0.005 inch. The
rollers 50 and 52 may also be heated during the calendering
process, with typical temperatures being from about 90.degree. F.
to about 250.degree. F.; specifically, from about 120.degree. F. to
about 160.degree. F.; more specifically, from about 130.degree. F.
to about 150.degree. F.
[0064] The following example describes specific methods of
manufacturing of the stretched PTFE layers 36. In this embodiment,
1000 grams of resin are compounded with an isoparaffin based
lubricant; specifically, ISOPAR.RTM. 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.degree. F., which is well below the glass transition
temperature of the PTFE resin of between about 57.degree. F. to
about 75.degree. F.
[0065] The PTFE compound 10 may be formed into a billet and stored
at a temperature of about 105.degree. F. to about 125.degree. F.
for six or more hours to ensure that the lubricant substantially
has penetrated and absorbed through the resin. Thereafter, the PTFE
compound 10 is placed in an extruder 12, as shown in FIG. 1. The
PTFE compound 10 may then be paste extruded from the orifice 22 of
the die 16 of the extruder 12 at a temperature above the resin
glass transition temperature. In one embodiment, the paste is
extruded at a temperature from about 80.degree. F. to 120.degree.
F. A reduction ratio, e.g., a ratio of a cross sectional area of
the PTFE compound 10 before extrusion to the cross section area of
the PTFE extrudate 24 after extrusion, may be from about 10:1 to
about 400:1, and specifically may be from about 80:1 to about
120:1. The extruder 12 may be a horizontal extruder or a vertical
extruder. The orifice 22 of the extrusion die 16 determines the
final cross sectional configuration of the extruded PTFE ribbon 24.
The orifice 22 shape or configuration of the extrusion die 16 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.
[0066] The PTFE extrudate ribbon 24 is then calendered, as shown in
FIG. 2, at a temperature from about 100.degree. F. to about
160.degree. F. to reduce a thickness of the PTFE ribbon 24 and form
a PTFE layer or film 28. The temperature at calendering may be
controlled by controlling the temperature of the rollers 30 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 calendered PTFE
layer 28 may have a lubricant content of about 10% by weight to
about 20% by weight.
[0067] Referring again to FIGS. 3 and 4, after calendering, one
side or both sides of the calendered PTFE layer 28 are sprayed with
an isoparaffin-based stretching agent 40 at a prescribed
temperature so that the PTFE film or layer 28 is flooded and fully
saturated through the thickness of the PTFE layer 28. The
saturated, calendered PTFE layer may then be stretched in a
direction that is substantially orthogonal to the calendering
direction by a tentering machine 34 to reduce a thickness of the
PTFE layer 28 and form a stretched PTFE layer 36. The stretched
PTFE layer 36 may have a thickness of about 0.00005 inch to about
0.005 inch; specifically, the stretched PTFE layer 36 may have a
thickness of about 0.0002 inch to about 0.002 inch. The PTFE layer
28 typically is tentered or stretched at an elevated temperature
above the glass transition temperature, specifically, from about
80.degree. F. to about 100.degree. F., more specifically, about
85.degree. F. to about 95.degree. F.
[0068] Wet tentering with the stretching agent 40 allows the PTFE
layer 28 to be thinned without creating substantial porosity and
fluid permeability in the stretched PTFE layer 36. While the
stretched PTFE layer 36 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 stretched PTFE layer embodiment 36
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 or layer 36 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.degree. C. to about 380.degree. C. for
several minutes; specifically, about 2 minutes to about 5
minutes.
[0069] In another aspect of the methods and PTFE layers discussed
herein, the PTFE layer 28 may be selectively lubricated in a
predetermined horizontal or lateral spatial pattern with a
stretching agent 40. The predetermined horizontal spatial pattern
may be formed from various lateral zones or sections which may each
have varying levels of stretching agent 40 content or stretching
agent 40 content gradients within and/or between lateral zones.
Lateral zones of a PTFE layer can extend in any direction across a
layer of PTFE, including transversely, longitudinally or any
direction in between these directions. Lateral zones of a layer of
PTFE are distinguishable from a thickness gradient of stretching
agent 40 content whereby the content of stretching agent 40 varies
stepwise or continuously through the thickness of a layer of PTFE.
Selective application of the stretching agent 40 by spray mechanism
42 to a surface of a layer of PTFE may be carried out using the
methods described herein or using other conventional methods. The
levels of stretching agent 40 contained within the various lateral
zones of the PTFE layer 28 may vary from about 0 percent stretching
agent content by weight to a level of substantial saturation of
stretching agent 40.
[0070] PTFE layer 28 having a predetermined pattern of stretching
agent 40 may be stretched in at least one direction such that the
lateral zones of the stretched PTFE layer 36 that contained more
stretching agent 40 during stretching will have a lower
permeability (e.g., substantially impermeable), while the lateral
zones of the stretched PTFE layer 36 that contained less stretching
agent 40 during stretching will have a higher permeability.
Typically, the lateral zones of the stretched layer 36 that
contained more stretching agent 40 may also have a reduced
thickness relative to the lateral zones that contained less
stretching agent 40 during the stretching process. In some
embodiments, it may be desirable to have lateral zones or regions
of the PTFE layer 28 that are substantially saturated with
stretching agent 40 adjacent lateral zones or regions of PTFE layer
28 that have a low enough stretching agent content to allow
expansion of the PTFE layer so as to produce a substantial node and
fibril microstructure with relatively high fluid permeability.
Stretching may be carried out in the machine direction, in a
direction that is substantially transverse or orthogonal to the
machine direction, or any direction in between. Alternatively, it
may be possible to stretch the PTFE layer 28 radially about a
point.
[0071] Referring to FIGS. 8A-8D, PTFE layer 28 may have stretching
agent 40 applied in a predetermined pattern to the PTFE layer 28,
such as the exemplary checker board pattern shown in FIG. 8C (which
shows an enlarged view of a portion of the PTFE layer 28). This
checker board pattern includes rectangular lateral zones 60 which
are substantially saturated with stretching agent 40 throughout the
thickness of the lateral zones 60 and which are visible on the
surface of the layer of PTFE. The checker board pattern also
includes rectangular lateral zones 62 which have a significantly
lower stretching agent 40 content throughout the thickness of the
zones 62. Upon stretching, lateral zones 60 and 62 transform into
lateral zones 64 and 66, respectively, of stretched PTFE layer 36,
as shown in FIG. 8D. Lateral zones 64 and 66 are elongated in the
transverse direction relative to the length of zones 60 and 62
because of expansion of PTFE layer 28 in the transverse direction
during the tentering process shown in FIGS. 8A and 8B. Lateral
zones 60-66 represent areas on the PTFE layer 28 that extend across
the surface of the PTFE layer 28 in any direction or in any shape
or configuration. The checker board pattern of FIG. 8C is provided
for exemplary purposes only. In general, the stretching agent
content level may be substantially constant throughout a thickness
of the PTFE layer 28 at any point on the PTFE layer 28 or within a
particular lateral zone; however, a stretching agent content level
gradient may also be present across the thickness of the PTFE layer
28 if desired. In addition, while the lateral zones 60 and 62 are
described and shown in FIGS. 8C and 8D as being defined by
substantially discrete stretching agent content levels, other
embodiments of lateral zones could include areas of the PTFE layer
28 which include a stretching agent content level gradient in any
desired direction or pattern.
[0072] FIGS. 8E-8L illustrate alternative embodiments of the effect
of applying predetermined patterns and amounts of stretching agent
40 on a layer of PTFE before and after a stretching process,
wherein lateral zones 60 are substantially saturated with
stretching agent 40 and lateral zones 62 has a relatively low level
of (or substantially no) stretching agent 40. After stretching,
lateral zones 60 and 62 transform into lateral zones 64 and 66,
respectively. In other alternative embodiments, FIGS. 8E and 8F
show circles of relatively high or substantially saturated
stretching agent 40 content and the elliptical shape the circles
may assume after stretching. FIGS. 8G and 8H show, in another
example, a pattern in which the rectangular cells of relatively
high or substantially saturated stretching agent 40 content become
square in shape after stretching. FIGS. 8I and 8J show elliptical
patterns of relatively high or substantially saturated stretching
agent 40 content that become circular after stretching. Finally,
FIGS. 8K and 8L illustrate a bull's eye pattern that is stretched
into an elliptical shape during stretching.
[0073] As discussed above, the stretching agent 40 content of
lateral zones 62 may be chosen such that standard expansion takes
place for the PTFE material within lateral zones 62 upon
stretching. Standard expansion of PTFE may produce expanded PTFE
(ePTFE) within lateral zones 64 after stretching, which typically
has a substantial node and fibril microstructure that is
discernable when viewed in by SEM. Lateral zones 60 may be
substantially saturated with stretching agent 40 such that
expansion of the PTFE layer 28 within lateral zones 60 produces
PTFE material of lateral zones 64 which is thinner and less
permeable than the material of lateral zones 66. For some
embodiments, the PTFE material of lateral zones 64 may be
substantially impermeable and may have a closed cell
microstructure. The closed cell microstructure may have a plurality
of interconnected nodes but is substantially free of fibrils
between the nodes (when viewed at a SEM magnification of 20,000
such as shown in FIG. 9). Put another way, the material of lateral
zones 64, which may be the same as or similar to the material shown
in FIG. 9, shows no discernable node and fibril microstructure when
viewed by SEM at a magnification of 20,000. In addition, the
stretching agent content of lateral zones 60 and 62 may be chosen
such that lateral zones 64 and 66 of the stretched PTFE layer 36
may vary with respect to density, thickness, and/or porosity.
[0074] The stretching agent 40 may be applied in any preselected
lateral spatial pattern and in any desired amount or concentration
level within the lateral zones of that pattern. For some
embodiments, the stretching agent 40 may be selectively applied to
the PTFE layer 28 by a spray mechanism 42 that may be controlled
through computer or manual control. The spray mechanism 42 shown in
FIGS. 8A and 8B may be configured to be controllable to a
substantial degree of spatial resolution so that fine preselected
patterns of stretching agent 40 may be applied to the PTFE layer
28. In some embodiments, the spray mechanism 42 may include an
inkjet head, such as is commonly used on an inkjet printer device.
Other embodiments of applying the stretching agent 40 may include,
but are not limited to, a contact roller which may be smooth,
textured or grooved. Droplets or stream application may be used
with an optional skimming member or blade that may also be smooth,
textured or grooved. A squeegee that is smooth, textured or grooved
may also be used to spread stretching agent delivered by droplets
or stream spray. Also, a sponge that is smooth, textured or grooved
may be used as well as a rotating drum having a pattern disposed
thereon. Silk screen type of methods and the like may also be used
to apply the stretching agent 40.
[0075] For other embodiments of methods of producing PTFE layers
having lateral zones of varied fluid permeability as well as other
characteristics, selective removal or reduction of stretching agent
40 content from the PTFE layer 28 in a predetermined pattern may be
used as opposed to selective addition of stretching agent 40. In
such a method, a PTFE layer 28 could be produced having a high
level of stretching agent 40 content, up to a saturated level, with
subsequent removal of some of the stretching agent by the selective
application of heat or other energy in a predetermined lateral
spatial pattern. The selective application of heat selectively
evaporates or boils the stretching agent from the PTFE layer 28. In
such an embodiment, an array of LED lasers 60, or the like, could
be disposed adjacent the PTFE layer 28, as shown in FIGS. 3 and 4.
The LED laser array 60 could be controlled manually, by a computer
or any other suitable means so as to apply laser energy to the PTFE
layer 28 as it passes by the laser array 60. As such, stretching
agent 40 is uniformly applied to the PTFE layer 28 by the spray
mechanism 42 and optional skimming member 44 as shown in FIGS. 3
and 4 so as to produce a PTFE layer 28 having a substantially
uniform stretching agent 40 content level. Then, as the PTFE layer
28 passes adjacent the LED laser array 60, the individual LED
lasers of the array are selectively activated to as to produce a
pattern of lateral zones, such as the lateral zones shown in FIGS.
8C-8L. In addition to laser energy, any other suitable form of
energy that can be spatially controlled could also be used. For
example, radiofrequency energy, ultrasound energy and the like
could also be used for selective removal or reduction of stretching
agent 40 from the PTFE layer 28. In addition, air jets or nozzles
dispensing air or other gases at a specified temperature, pressure
and direction may be used to selectively remove the stretching
agent 40 from the PTFE layer by spraying the gas from the air jet
onto the PTFE layer to either blow the stretching agent from the
PTFE layer or through the PTFE layer. Also, the gas expelled from
such air jets could be heated to facilitate evaporation of the
stretching agent from the point of impact of the compressed or high
velocity gas from the air jets.
[0076] As discussed above, the PTFE layers 36 may or may not
include a discernable node and fibril microstructure. If the
stretched PTFE layers 36 or lateral zones of the stretched PTFE
layers include a discernable node and fibril microstructure, the
stretched PTFE layers 36 or lateral zones thereof may have a
uniaxial fibril orientation, a biaxial fibril orientation, or a
multi-axial fibril orientation. The stretched PTFE layers 36 or
twice-stretched PTFE layers 46 within a multi-layer PTFE film may
be positioned in any configuration, such that the fibrils in one
PTFE layer (if any) are parallel, perpendicular, or at other angles
relative to the fibrils of an adjacent PTFE layer. The stretched
PTFE layers with lateral zones can be used on any of the stent
graft embodiments discussed below. In some embodiments, it may be
desirable to use a stretched PTFE layer with impermeable lateral
zones disposed about or bordering inflatable channels and permeable
lateral zones in other areas of the stent graft.
[0077] 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 in FIGS. 9 to 13 illustrate
different magnifications of a microstructure of a PTFE film or
layer 110 made in accordance with embodiments of the present
invention. PTFE layer 110 has a generally closed cell
microstructure 112 that is substantially free of the conventional
node and fibril microstructure commonly seen in expanded PTFE
layers. Embodiments of the PTFE film 110 may have low
fluid-permeability, or no or substantially no fluid-permeability.
One or more of PTFE layer 110 may be used as a barrier layer to
prevent a fluid such as a liquid or gas from permeating or escaping
therethrough.
[0078] At a magnification of 20,000, as seen in FIG. 9, the
microstructure of the stretched PTFE layer 110 resembles a
pocked-like structure that comprises interconnected high density
regions 114 and pockets or pores 116 between some of the high
density regions 114. The PTFE film 110 may be considered to have a
closed cell network structure with interconnected strands
connecting high density regions 114 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
which typically has a substantial node and fibril microstructure
that is discernable when viewed at a SEM magnification of 20,000,
PTFE layer 110 lacks the distinct, parallel fibrils that
interconnect adjacent nodes of ePTFE and has no discemable node and
fibril microstructure when viewed at a SEM magnification of 20,000,
as shown in FIG. 9. The closed cell microstructure of the PTFE
layer 110 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.
[0079] Though PTFE film or layer 110 is configured to have low or
substantially no fluid permeability, PTFE layer 110 nonetheless has
a porosity. The PTFE layer 110 typically has an average porosity
from about 20% to about 80%, and specifically from about 30% and
about 70%. In one embodiment, a PTFE film 110 has a porosity of
about 30% to about 40%. In another embodiment, a PTFE layer 110 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 the PTFE film 110.
An average pore size in the PTFE layer 110 is may be less than
about 20 microns, and specifically less than about 0.5 micron. In
one embodiment, a PTFE layer 110 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, the PTFE film 110 may have an average
pore size of greater than about 6.0 microns. As described below,
depending on the desired properties of the resultant PTFE layer
110, embodiments of methods may be modified so as to vary the
average porosity and average pore size of the PTFE film 110 in a
continuum from 10 microns to 50 microns down to substantially less
than about 0.1 micron.
[0080] PTFE layer 110 may have a density from about 0.5 g/cm3 to
about 1.5 g/cm3, and specifically from about 0.6 g/cm3 to about 1.5
g/cm3. While the density of the PTFE film 110 is typically less
than a density for a fully densified PTFE layer (e.g., 2.1 g/cm3),
if desired, the density of the PTFE layer 110 may be densified to a
higher density level so that the density of the PTFE layer 110 is
comparable to a fully densified PTFE layer. FIGS. 9 to 13
illustrate a PTFE film 110 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.
[0081] PTFE film 110 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 tetraflurorethylene and other
monomers, is also contemplated.
[0082] The PTFE layer and PTFE films may be used in a variety of
ways. For example, the PTFE layers and PTFE films 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. For some of these applications, it may be desirable to
include PTFE layers that have variable characteristics with respect
to lateral zones of the PTFE layers, which may be produced by the
methods discussed above. Specifically, some applications may
require PTFE layers that have a high permeability in one lateral
zone and low or substantially no permeability in an adjacent
lateral zone. PTFE films having at least two PTFE layers combined
may overlap the predetermined patterns of the lateral zones of the
PTFE layers to achieve a more complex patterns of varied
characteristics. As such, any of the embodiments discussed below
may incorporate PTFE layers, stretched PTFE layers or PTFE films
that have lateral zones of varied characteristics as discussed
above.
[0083] Referring now to FIG. 14, PTFE layer 110 may be combined
with, bonded to, or otherwise coupled, affixed or attached,
partially or completely, to at least one additional layer 118 to
form a composite film 120. Depending on the use of composite film
120, layer 118 may be chosen to have properties that combine with
the properties of layer 110 to give the desired properties in
composite film 120. The additional layer 118 may include a porous
PTFE layer, a substantially non-porous PTFE layer, a gas- or
liquid-permeable PTFE layer, a gas- 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, layer 118 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 the composite PTFE film 120. The reinforcing layer
may be disposed between layers 110 or 118, or the reinforcing
layer(s) may be coupled to an exposed surface of PTFE layer 110,
PTFE layer 118, or both. PTFE layer 110 and layer 118 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 of layers 110 and 118 to each
other. Alternatively, heat fusion, pressure bonding, sintering, and
the like may be used to bond at least a portion of layers 110 and
118 to each other.
[0084] FIGS. 15 and 16 are transverse cross-sectional views of two
composite tubular structures 130 and 140, respectively. Tubular
structures 130 and 140 may be a portion or section of an
endovascular graft or the like. As shown in FIG. 15, tubular
structure 130 includes an inner tubular body 132 that comprises an
inner surface 134 and an outer surface 136. Tubular body 132 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 structure 130 further
comprises an outer tubular body 138 that comprises an inner surface
137 and an outer surface 139. Inner surface 137 of outer tubular
body 138 is coupled to the outer surface 136 of the inner tubular
body 132. Tubular body 138 may comprise one or more PTFE layers
having low fluid-permeability or substantially no
fluid-permeability. In this configuration, inner surface 134 of the
tubular body 132 defines an inner lumen 135 of tubular structure
130 and the outer surface 139 of the tubular body 138 defines an
outer surface 139 of the tubular structure 130. Tubular body 138
may be combined, bonded to, or otherwise coupled, affixed or
attached, partially or completely, to the tubular body 132 through
any suitable method known in the art. For example, an adhesive may
be used to selectively bond at least a portion of tubular body 138
and tubular body 132 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 body 138
and tubular body 132 to each other.
[0085] As shown in FIG. 16, tubular structure 140 includes an inner
tubular body 142 that comprises an inner surface 144 and an outer
surface 146. Tubular body 142 may comprise one or more layers of
PTFE having low or substantially no fluid permeability. Tubular
structure 140 further comprises an outer tubular body 148 that
comprises an inner surface 147 and an outer surface 149. Inner
surface 147 of outer tubular body 148 is coupled to the outer
surface 146 of the inner tubular body 142. Outer tubular body 148
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 surface 144 of the inner tubular body 142
defines an inner lumen 145 of tubular structure 140 and the outer
surface 149 of the outer tubular body 148 defines an outer surface
149 of the tubular structure 140. Tubular body 148 may be combined,
bonded to, or otherwise coupled, affixed or attached, partially or
completely, to the tubular body 142 through any suitable method
known in the art. For example, an adhesive may be used to
selectively bond at least a portion of tubular body 148 and tubular
body 132 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 body 148 and tubular
body 142 to each other.
[0086] Tubular structures 130 or 140 may define an inner diameter
ID which is the diameter of the inner surface, which may define the
area of flow through tubular structure 130 or 140. An outer
diameter OD, which is the diameter of the outer surface 139 or 149
of the outer tubular layer 138 or 148. 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 layers 138 and
142 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 layers 132 or 148 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 the tubular structures 130 or 140 will vary depending
on the use of the tubular structures.
[0087] Tubular structures 130 or 140 may be formed as tubes through
conventional tubular extrusion processes. Typically, however,
tubular structures 130 or 140 may be formed from PTFE layers 110 or
118, as shown in FIG. 14, that are folded on a shape forming
mandrel over each other so that ends of the layers are overlapped
and bonded. As another alternative, PTFE layers 110 or 118 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. No. 10/029,557 and entitled "Methods and Apparatus
for Manufacturing an Endovascular Graft Section", Ser. No.
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.
[0088] The films and layers discussed herein are not limited to a
single porous PTFE layer 118 and a single PTFE layer or film 110
having low or substantially no fluid permeability. The composite
films 120 and tubular structures 130 or 140 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 layers 110 having low or substantially no fluid
permeability. For example, PTFE layer 110 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, the PTFE layer 110 may have a
reduced thickness relative to the porous PTFE layers. In other
embodiments, however, the PTFE layer 110 may have the same
thickness or larger thickness than the porous PTFE layers. As an
alternative embodiment to FIGS. 15 and 16, tubular structures 130
or 140 may comprise inner and outer tubular bodies that both have
low or substantially no fluid permeability.
[0089] Referring now to FIG. 17, a tubular structure that is in the
form of an inflatable endovascular graft 50 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. Graft 150 has a proximal end 151 and a
distal end 152 and includes a generally tubular structure or graft
body section 153 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 although
FIG. 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.
[0090] A proximal inflatable cuff 156 may be disposed at or near a
proximal end 151 of graft body section 153 and a distal inflatable
cuff 157 may be disposed at or near a graft body section distal end
152. Graft body section 153 forms a longitudinal lumen that is
configured to confine a flow of fluid, such as blood, therethrough.
Graft 150 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, a stent 159 may be attached at the proximal end
151 and/or the distal end 152 of the graft 150. Depending on the
construction of the cuffs 156 and 157 and graft body section 153,
inflation of cuffs 156 and 157, when not constrained (such as,
e.g., by a vessel or other body lumen), may cause the cuffs 156 and
157 to assume a generally annular or torodial shape with a
generally semicircular longitudinal cross-section. Inflatable cuffs
156 and 157 may be designed to generally, however, conform to the
shape of the vessel within which it is deployed. When fully
inflated, cuffs 156 and 157 may have an outside diameter ranging
from about 10 mm to about 45 mm; specifically from about 16 mm to
about 42 mm.
[0091] At least one inflatable channel 158 may be disposed between
and in fluid communication with proximal inflatable cuff 156 and
optional distal inflatable cuff 157. Inflatable channel 158 in the
FIG. 17 example has a helical configuration and provides structural
support to graft body section 153 when inflated to contain an
inflation medium. Inflatable channel 158 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 which graft 150 may be deployed. Together with
proximal and distal cuffs 156 and 157, inflatable channel 158 forms
an inflatable network over the length of the body 153. Depending on
the desired characteristics of the endovascular graft 150, at least
one layer of the graft may be a PTFE layer having low or
substantially no fluid permeability such as PTFE layer or film 110.
The PTFE layer may be one of the layers that forms the inflatable
channels 158, or the PTFE layer may surround or be underneath the
inflatable channel 158 and cuffs 156 and 157.
[0092] In addition, it may be desirable for some embodiments to
include a PTFE layer 110 that has varied permeability across
lateral zones of the PTFE layer 110 whereby the characteristic of a
lateral zone of the PTFE layer 110 corresponds to the function of
the PTFE layer in the lateral zone. For example, instead of
including a single PTFE layer 110 of PTFE material that has
substantially no fluid permeability, the layer may include lateral
zones of PTFE material that has substantially no fluid permeability
that correspond to the portion of the PTFE layer 110 that will be
adjacent the inflatable channel 158 and cuffs 156 and 157. In this
way, the inflatable channel 158 and inflatable cuffs 156 and 157
can be made resistant to fluid loss, while the graft body retains
its fluid-permeable character adjacent the inflatable channel 158
and inflatable cuffs 156 and 157. This type of arrangement could be
included in any of the graft embodiments discussed herein.
[0093] Graft body 153 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 the inflatable cuffs 156 and 157 and
inflatable channel 158 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, 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.
[0094] FIGS. 18 to 21 illustrate transverse cross sectional views
of different embodiments of inflatable channel 158. As can be
appreciated, the embodiments of FIGS. 18 to 21 may also be
applicable to the proximal and distal cuffs 156 and 157. Inflatable
channel 158 defines an inflatable space 162 that is created between
an inner layer 164 and outer layer 166. If desired an inflation
medium 167 may be delivered into the space 162 to inflate
inflatable space 162. Inflation medium 167 optionally may include a
deliverable agent 168 as shown in FIGS. 18 to 21, such as a
therapeutic agent 168 that may be configured to be diffused in a
controlled manner or otherwise transmitted through pores (not
shown) in inner layer 164, outer layer 166 or both. The embodiments
shown in FIGS. 18-21 are merely exemplary, as it may be desirable
to have preferential diffusion of the deliverable agent 168 through
layer 164 or layer 166. In addition, both layers 164 and 166 may be
configured to allow a significant amount of diffusion of
deliverable agent 168, but with one of the two layers having a
greater permeability to the deliverable agent 168 than the other
layer. While inner layer 164 and layer 166 are shown as having only
a single layer of material, it should be appreciated that each of
layers 164 or 166 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, 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.
_________, 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.
[0095] In the embodiment shown in FIG. 18, outer layer 166 is
permeable to fluids so as to allow the therapeutic agent 168, which
may be a liquid, to diffuse over time in the direction of arrow 169
through outer layer 166. In such embodiments, inner layer 164
typically has a low or substantially no fluid permeability, and
could therefore be considered a "barrier layer." Because the inner
"barrier" layer 164 has low or substantially no fluid permeability
and outer layer 166 is fluid-permeable, the therapeutic agent will
preferentially diffuse from space 162 in the direction of arrow
169. The use of one (or more) porous fluid-permeable outer PTFE
layers and an inner layer 164 having low or substantially no fluid
permeability provides for improved release of a therapeutic agent
through liquid-permeable outer layer 166. Varying the porosity or
pore size across at least a portion of outer layer 166 may provide
even more localized delivery of the therapeutic agent 168 through
outer layer 166.
[0096] In an alternative configuration shown in FIG. 19, inner
layer 164 may be substantially fluid-permeable to allow the
therapeutic agent 168 to selectively diffuse in the direction of
arrow 169 through inner layer 164 and into the lumen of the tubular
structure (e.g., lumen 135, 145 of FIGS. 15 and 16). In such
embodiments, outer layer 166 typically has no or substantially no
fluid-permeability and acts as a "barrier layer." As such, the
therapeutic agent will preferentially diffuse from space 162 in the
direction of arrow 169. The use of porous fluid-permeable PTFE
layers and outer layer 166 having low or substantially no fluid
permeability provides for improved release of a therapeutic agent
into the inner lumen through fluid-permeable inner layer 164.
Varying the permeability and/or porosity or pore size across at
least a portion of inner layer 164 may provide even more localized
delivery of the therapeutic agent 168 through layer 164.
[0097] As shown in FIG. 20, if it is desired to prevent the
inflation medium 167 from escaping from inflatable space 162, both
the inner layer 164 and outer layer 166 may comprise a "barrier"
layer having low or substantially no fluid permeability. In such
embodiments, the inner and outer layers 164 and 166 have low or
substantially no fluid permeability. In such embodiments, inflation
material 167 typically will not contain a therapeutic agent.
Referring to FIG. 21, the inflatable channel may be a substantially
tubular channel 170 that is fused or otherwise adhered to layer 164
that defines an inner lumen of the graft. If delivery of a
therapeutic agent 168 is desired, tubular channel 170 will be
liquid-permeable and will allow diffusion of the therapeutic agent
168 through pores in tubular channel 170. If however, it is desired
to prevent the inflation fluid 167 from escaping from inflatable
space 162, then tubular channel 170 will act as a barrier layer and
may comprise at least one layer of PTFE having low or substantially
no fluid permeability.
[0098] Referring now to FIGS. 22 and 23, the respective graft
embodiments 150 and 180 shown include an inflatable channel 158 has
portions with a circumferential configuration as opposed to the
helical configuration of the inflatable channel 158 shown in FIG.
17. The circumferential configuration of portions of the inflatable
channel 158 maybe 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 channel 158 may be configured circumferentially as shown
in FIGS. 22 and 23.
[0099] In addition to the substantially tubular grafts of FIG. 22,
bifurcated endovascular grafts as shown in FIG. 23, are also
contemplated. The bifurcated endovascular graft 180 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 graft 80 embodiment
unless specifically mentioned otherwise.
[0100] Graft 180 comprises a first bifurcated portion 182, a second
bifurcated portion 184 and main body portion 186. The size and
angular orientation of the bifurcated portions 182 and 184 may vary
to accommodate graft delivery system requirements and various
clinical demands. The size and angular orientation may vary even
between portion 182 and 184. For instance, each bifurcated portion
or leg is shown in FIG. 23 to optionally have a different length.
First and second bifurcated portions 182 and 184 are generally
configured to have an outer inflated diameter that is compatible
with the inner diameter of a patient's iliac arteries. First and
second bifurcated portions 182 and 184 may also be formed in a
curved shape to better accommodate curved and even tortuous
anatomies in some applications. Together, main body portion 186 and
first and second bifurcated portions 182 and 184 form a continuous
bifurcated lumen, similar to the inner lumens of FIG. 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, entitled "Kink-Resistant
Endovascular Graft" and filed Mar. 6, 2003 to Kari et al., the
complete disclosure of which is incorporated herein by
reference.
[0101] While not shown, it should be appreciated, that instead of
circumferential channels and longitudinal channels, the bifurcated
graft 180 may comprise a helical inflatable channel 158, similar to
that of the graft embodiment shown in FIG. 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. Other
endovascular grafts that the liquid-impermeable PTFE film may be
used with are described in U.S. Pat. No. 6,395,019 to Chobotov,
U.S. Pat. No. 6,132,457 to Chobotov, U.S. Pat. No. 6,331,191 to
Chobotov, and U.S. patent application Ser. No. 10/327,711, entitled
"Advanced Endovascular Graft" to Chobotov et al. and filed Dec. 20,
2002, Ser. No. 10/168,053, the complete disclosures of which are
incorporated herein by reference.
[0102] As can be appreciated, the inflatable portions of the graft
180 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 the graft
180 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.
[0103] 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.
* * * * *