U.S. patent application number 10/858589 was filed with the patent office on 2004-11-04 for stent covering formed of porous polytetraflouroethylene.
This patent application is currently assigned to Scimed Life Systems, Inc.. Invention is credited to Girton, Timothy Samuel.
Application Number | 20040220659 10/858589 |
Document ID | / |
Family ID | 24829763 |
Filed Date | 2004-11-04 |
United States Patent
Application |
20040220659 |
Kind Code |
A1 |
Girton, Timothy Samuel |
November 4, 2004 |
Stent covering formed of porous polytetraflouroethylene
Abstract
An endoprosthesis device and method of making it are disclosed.
More particularly, the endoprosthesis is a porous PTFE article used
in conjunction with a stent.
Inventors: |
Girton, Timothy Samuel;
(Maple Grove, MN) |
Correspondence
Address: |
HOFFMANN & BARON, LLP
6900 JERICHO TURNPIKE
SYOSSET
NY
11791
US
|
Assignee: |
Scimed Life Systems, Inc.
|
Family ID: |
24829763 |
Appl. No.: |
10/858589 |
Filed: |
June 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10858589 |
Jun 2, 2004 |
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09704494 |
Nov 2, 2000 |
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6770086 |
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Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61L 31/146 20130101;
A61L 31/048 20130101; B29C 67/202 20130101; A61F 2220/0058
20130101; A61F 2220/0066 20130101; A61F 2220/005 20130101; A61L
31/048 20130101; A61F 2220/0075 20130101; A61F 2/915 20130101; A61F
2002/91541 20130101; A61F 2/91 20130101; C08L 27/18 20130101 |
Class at
Publication: |
623/001.15 |
International
Class: |
A61F 002/06 |
Claims
1. An endoprosthesis device comprising: an elongate radially
expandable tubular stent having an interior surface and an exterior
surface extending along a longitudinal stent axis; and a stent
cover on said interior surface, exterior surface or both, said
stent cover being formed of a polytetrafluoroethylene having no
node and fibril structure; wherein said porous
polytetrafluoroethylene is formed by the steps of: providing an
interpenetrating network of siloxane/polytetrafluoroethylene;
removing said siloxane from said interpenetrating network leaving a
porous polytetrafluoroethylene structure without a node and fibril
structure.
2. The endoprosthesis device of claim 1 wherein said stent cover is
on said exterior surface and said interior surface of said
stent.
3. The endoprosthesis device of claim 1 wherein said stent cover is
expandable upon expansion of said stent.
4. The endoprosthesis device of claim 1 wherein said siloxane is
chemically extracted from said siloxane/polytetrafluoroethylene
interpenetrating network.
5. The endoprosthesis device of claim 4 wherein said siloxane is
chemically extracted by a compound selected from the group
consisting of toluene, heptane and chloroform.
6. The endoprosthesis device of claim 1 wherein said siloxane is
removed from said siloxane/polytetrafluoroethylene interpenetrating
network by heating said network to a temperature of at least about
300.degree. C.
7-12 (canceled).
13. An endoprosthesis device comprising: an elongate radially
expandable tubular stent having an interior surface and an exterior
surface extending along a longitudinal stent axis; and a stent
cover on said interior surface, exterior surface or both, which is
formed of a polytetrafluoroethylene having no node and fibril
structure; wherein said porous polytetrafluoroethylene comprises a
porous structure--having voids intermeshed between pockets of PTFE
without a node and fibril structure.
14 (canceled).
15. The endoprosthesis device of claim 13 wherein said stent cover
is on said exterior surface and said interior surface of said
stent.
16. The endoprosthesis device of claim 13 wherein said stent cover
is expandable upon expansion of said stent.
17. The endoprosthesis device of claim 1 wherein said siloxane is
polydimethylsiloxane.
18. The method of claim 7 wherein said siloxane is
polydimethylsiloxane.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an endoprosthesis device or
intraluminal device, in particular a stent, having a covering
comprising porous polytetrafluoroethylene formed by removing the
siloxane from an interpenetrating network of
polytetrafluoroethylene and siloxane, and to a method of making the
endoprosthesis device. The stent covering can be applied on the
exterior surface of the stent, on the interior surface of the
stent, or both, at a thickness of as low as about 15 microns.
BACKGROUND OF THE INVENTION
[0002] Endoprosthesis devices including stents, stent-grafts,
grafts, vena cava filters, balloon catheters, and so forth, are
placed or implanted within various body vessels for the treatment
of various diseases. One particular type of an endoprosthesis
device is the stent. A stent is implanted within a vessel for the
treatment of stenoses, strictures, or aneurysms in the blood
vessels. The devices are implanted within the vascular system to
reinforce diseased, partially occluded, weakened or abnormally
dilated sections of the blood vessel. Stents are often employed
after angioplasty to prevent restenosis of a diseased blood vessel.
While stents are most notably used in blood vessels, they have also
been implanted in other bodily vessels including urinary tracts and
bile ducts to reinforce and prevent neoplastic growth.
[0003] Stents are typically longitudinal tubular devices formed of
biocompatible materials and come in a variety of construction
types, and are often expandable in nature. Many if not all of the
materials used for stents involve metal or carbon fiber materials
which are highly electro-positive and are bio-active. Since stents
tend to be used under conditions were they are counteracting
disease processes, supporting healing processes, or guarding
against stenosis of a passage, bio-activity, which may encourage
undesirable or poorly regulated growth processes, or lead to clot
formation, should be avoided.
[0004] Coating of the stent can keep the stent from directly
contacting surrounding tissue or fluids, and thus can theoretically
protect against unwanted electrochemically induced tissue
reactions.
[0005] In the field of expandable stents, a further problem arises
due to the fact that many stent constructions involve structures
that have numerous apertures or spaces between various strands or
structural elements of the stent such as those structures that are
filamentous, wire-like, or of a tubular nature in which various
openings have been cut or etched into the stent. With these
constructions, tissue may grow through the openings of the stent.
Furthermore, the stent itself may provoke a foreign body reaction
and be both a stimulus for and a framework supporting,
proliferative tissue growth, resulting, for example, in scar tissue
or restenosis of the very region it is placed to control.
[0006] One approach to this drawback is to provide a coating,
liner, cover or both, for the stent which prevents the healing or
diseased layer of tissue from directly contacting the stent, or
from passing through the stent in any way. Such liners may be
formed, for example, of porous polytetrafluoroethylene (PTFE) which
allows the passage of fluids and vital materials while serving as a
barrier to tissue growth. However, when applying such a
construction, a further difficulty which may arise is that the
layer or sleeve of polymer must be attached to the stent for
example, by staples or sutures at one end, or is prone to
developing loose pockets or folds which might accumulate organic
matter or lead to sepsis or unusual growth. Also, the necessarily
thin liner material may detach or degrade. The risk of loose or
unattached liner material is particularly great for constructions
which utilize poorly adherent polymers, such as PTFE, or structures
which seek to combine an expandable stent of stiff material, which
changes both its dimension and its shape, with a dissimilar liner
or shell.
[0007] One method for overcoming these problems is found in U.S.
Pat. No. 6,010,529 in which tube of polymeric material, e.g.
expanded polytetrafluoroethylene (PTFE), is passed through the
interior of a stent body and is turned back upon itself over the
stent to form a cuff. The assembly is then heated and the outer
layer contacts and coalesces with the inner layer, closely
surrounding the stent body within a folded envelope having a
continuous and seamless end. Porosity is imparted to the PTFE by
previous stretching or expansion the material.
[0008] Another type of covered stent which permits radial expansion
is shown in WO 96/00103. As shown and described therein, a metallic
expandable stent includes an outer covering of ePTFE. The ePTFE
cover exhibits suitable expansion capabilities so as to enable the
cover to expand upon expansion of the underlying stent. A
polytetrafluoroethylene/- lubricant blend may be extruded into a
tube and the tube heated to remove the lubricant. Then, in order to
impart the expandable characteristics to the ePTFE cover during
formation of the ePTFE cover material, the ePTFE must undergo
successive processing steps of expanding the material, sintering
the material, radially dilating the material and resintering the
dilated material, a procedure that is quite process intensive. The
device described therefore requires precise manufacturing
techniques and is extremely processing sensitive. Careful
processing of the material forming the cover is required in order
for the cover to exhibit sufficient expansion capabilities.
[0009] U.S. Pat. No. 5,824,046 describes a composite intraluminal
device, in particular an elongate radially expandable tubular stent
having an interior luminal surface and an opposed exterior surface
extending along a longitudinal stent axis. A stent cover is formed
of unsintered ePTFE which is expandable.
[0010] There remains a need in the art to provide a stent with a
cover material that is sufficiently expandable, has the requisite
barrier properties and yet allows the passage of fluids and vital
materials, without requiring extensive processing procedures and is
thus easily manufactured and applied to the stent.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a method of forming porous
polytetrafluoroethylene (PTFE) without having to stretch or expand
the material, and to a radially expandable endoprosthesis device
covered with the solid but expandable polymer covering comprising
the porous PTFE material obtained using the method of the present
invention. The porous PTFE covering physically isolates the
endoprosthesis from surrounding blood and tissue.
[0012] Specifically, the porous PTFE is prepared by extracting
siloxane from an interpenetrating network (IPN) of PTFE and
siloxane, leaving behind a porous PTFE structure without having to
expand and stretch the PTFE. Consequently, the PTFE material used
in the endoprosthesis device coverings of the present invention is
not expanded PTFE, but it is porous.
[0013] In one embodiment the end of the prosthesis device of the
present invention includes an elongate radially expandable tubular
stent having an interior surface and in exterior surface extending
along a longitudinal stent access. The expandable tubular stent has
a stent cover on said interior surface, exterior surface or both,
the cover being formed of a porous polytetrafluoroethylene. The
porous polytetraflouroethylene cover is a non-stretched porous
structure, the non-stretched structure lacking note and viable
structure.
[0014] In particular, the present invention relates to a radially
expandable stent for use in treating stenoses wherein the stent is
covered with an expandable polymer covering comprising the porous
PTFE prepared according to the present invention that physically
isolates the stent from surrounding blood and tissue.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a perspective view of one type of intraluminal
device that may be used in the present invention.
[0016] FIG. 2 is a perspective view of a different intraluminal
device which may be used in the present invention.
[0017] FIG. 3 is a perspective view of the intraluminal device of
FIG. 1 illustrating the device having a polytetrafluoroethylene
cover on both the inner and outer surface of the device.
[0018] FIG. 4 is a cross-sectional view of the same intraluminal
device shown in FIG. 3.
[0019] FIG. 5 is the same intraluminal device as in FIG. 3
illustrating only the outer surface cover.
[0020] FIG. 6 is the same intraluminal device as in FIG. 3 with the
exception that only a liner or inner surface cover is shown.
[0021] FIG. 7 is a cross-section of the porous PTFE material of the
present invention.
[0022] FIG. 8 is a schematic representation of ePTFE prior art.
DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0023] The present invention provides a covered stent which may be
implanted intraluminally within a body vessel and disposed adjacent
an occluded, weakened or otherwise damaged portion of the vessel so
as to hold the vessel open. The covered stent is typically
delivered intraluminally via a balloon catheter. The device is
delivered in a compressed condition and once properly positioned
may be deployed by radial expansion. The most common form of
deploying the intraluminal device is by balloon expansion, however,
the present invention may also be deployed by use of a
self-expanding stent.
[0024] FIG. 1 illustrates an intraluminal device in the form of a
stent 12. FIG. 2 illustrates an intraluminal device in the form of
a stent 5 having a different construction than that shown in FIG.
1.
[0025] FIG. 3 illustrates generally at 10 an intraluminal device in
the form of a stent 12 as shown in FIG. 1 having a cover 14 on the
outer surface 12 and liner 16 on the inner surface, both of which
may be of the porous structure shown below in FIG. 7. The stent may
optionally have only a cover 14 as shown in FIG. 5, or only a liner
16 as shown in FIG. 6, or both as shown in FIG. 3. In a preferred
embodiment, the stent has both a cover 14 and a liner 16. The
liner, cover, or both, will be referred to hereinafter collectively
as a cover or covering. The cover provides an effective barrier
about the stent 12 preventing excessive cell or tissue ingrowth or
thrombus formation through the expanded wall of the stent 12.
[0026] FIG. 4 is a cross-sectional view of the same device as shown
in FIG. 3 with a cover 14 and a liner 16 around stent 12.
[0027] FIG. 1 is a more detailed illustration of stent 10 and shows
generally an elongate tube. The body of stent 12 defines an opposed
interior surface 11 and an exterior surface 13 and is formed of a
generally open configuration having a plurality of openings or
passages provided for longitudinal flexibility of the stent as well
as permitting the stent to be radially expanded once deployed in
the body lumen. Both the interior surface 11 and the exterior
surface 13 may have the porous PTFE covering of the present
invention. On the interior surface the covering is referred to as
the liner 12 as shown in FIG. 1 and on the exterior surface it is
referred to as a cover 14 as shown in FIG. 1.
[0028] While the figures illustrate a particular construction of
stent 10, one of skill in the art would recognize that the porous
PTFE covering material as described by the present invention would
find utility in any stent configuration, and in particular the open
stent configurations.
[0029] Stent 12 may be employed in combination with a cover 14 or
liner 16 but is preferably employed with both. The cover 14 may be
applied over the tubular stent 12 so as to fully circumferentially
surround the stent 12, while the liner 16 is applied inside and
through the stent 12 so that the stent 12 fully circumferentially
surrounds the liner 16.
[0030] The porous polytetrafluoroethylene (PTFE) material useful
herein is first obtained in the form of an interpenetrating network
of PTFE and siloxane, in particular, polydimethylsiloxane. The
silicone is then extracted from the IPN using either thermal or
chemical means. The removal of the silicone leaves behind a porous
PTFE structure. A particular material for use herein is Silon.RTM.,
an interpenetrating polymer network (IPN) of
polytetrafluoroethylene (PTFE) and polydimethylsiloxane (silicone)
supplied by Bio Med Sciences, Inc. located in Bethlehem, PA. Such
IPN polymer networks are described in U.S. Pat. No. 6,022,902
incorporated by reference herein in its entirety. In this patent,
Silon.RTM. is described as a breathable, hydrophobic polysiloxane
membrane reinforced with poly(tetrafluoroethylene).
[0031] The removal of the siloxane from the IPN leaves behind a
porous PTFE structure without having to go through the added steps
of stretching or expanding the PTFE in order to obtain the porous
structure. Quite obviously, this simplifies the manufacturing
process by decreasing the number of steps required, and also
increases efficiency. Typically, porous PTFE requires the expanding
and stretching steps in order to achieve the porous structure. FIG.
7 illustrates generally at 20 a porous PTFE structure after removal
of the siloxane. The removal of the siloxane leaves behind the
porous structure wherein voids or pockets of air 25, are found
intermeshed in between pockets of PTFE 30.
[0032] The novel porous PTFE structure produced by the present
inventive process is quite different from the porous structure
produced by PTFE which has been stretched, or expanded. Typically,
PTFE which has been stretched, or ePTFE has a node and fibril
structure as seen in FIG. 8. After stretching, the ePTFE possesses
nodes 32 connected to fibrils 34. In between the nodes and fibrils
are pores 36.
[0033] Removing the siloxane from the IPN of siloxane/PTFE through
the use of heat involves heating the IPN structure to temperatures
of between about 300.degree. C. and about 390.degree. C. Chemical
removal of the siloxane may be accomplished using a compound
selected from the group consisting of toluene, heptane,
chloroform.
[0034] Sintering is typically accomplished at or above the
crystalline melting point of PTFE. Sintering is synonymous with
recrystallization. It refers to the bonding of particles in a mass
by molecular (or atomic) attraction in the solid state through the
application of heat below the melting point of the polymer.
Sintering causes the strengthening of the powder mass and normally
results in densification and often recrystallization.
[0035] A PTFE tube may be extruded as a tube from an extrusion
device, or extruded as a film and subsequently wrapped into a tube.
Extrusion techniques of PTFE are well known in the art.
[0036] As discussed above, the stent may be covered on the interior
surface 11 of the stent 10, the exterior surface 13 of the stent
10, or both. Preferably, the stent 10 is covered on both the
interior 11 and the exterior 13 surfaces of the stent 10. Having
the entire surface of the stent 10 covered with the porous PTFE of
the present invention provides an effective barrier about the stent
10 preventing excessive cell or tissue growth, or thrombus
formation through the expanded wall of a tubular stent 10.
[0037] In order for the covering of porous PTFE to function
effectively in combination with an expandable stent, the material
must exhibit sufficient expansion characteristics so as to enable
the stent cover to expand along with the radial expansion of the
stent 10. If the covering material does not effectively expand with
the stent, several problems can arise. The covering material may
tear, and may even detach from the surface of the stent if improper
or dissimilar expansion of the covering material occurs with the
expansion of the stent.
[0038] In order to improve the adhesion, and further prevent
detachment of the PTFE covering from the stent, the PTFE may be
fused or welded around or to the metal stent. This may be
accomplished either through a heating process and/or bonding
process. If heating is utilized, typically the PTFE will be heated
above its sintering temperature.
[0039] If an adhesive is utilized, preferably a biocompatible
adhesive is used. Such adhesives are known to one of skill in the
art and include, for example, polyurethanes, epoxies,
cyanoacrylates, polyamides, polyimides, silicones, and so forth.
Dispersions of PTFE or FEP (fluoroethylpropylene) may also be
utilized. This list is not exclusive and is intended for
illustrative purposes only, and is in no way intended as a
limitation on the scope of the present invention. There is a vast
number of adhesives that can be used for such applications, limited
by their biocompatibility, and by their ability to bond to
polymeric materials (e.g. PTFE) and metals, particularly in aqueous
environments.
[0040] The covering material may also be assembled to the
intraluminal device in more than one piece. Such a combination
would require overlapping of sorts of the PTFE material, and
subsequent fusion or adhesive bonding of the porous PTFE material
to itself.
[0041] It is preferable, however, to utilize the porous PTFE
covering in a continuous form such as a membrane or thin film. The
porous PTFE (after removal of the siloxane), in the form of a
membrane or a thin film, thus, preferably completely wraps the
metal stent, thereby providing a barrier that physically isolates
the stent from surrounding blood and tissue. This barrier further
helps prevent healing or diseased layers of tissue from directly
contacting the stent, or from passing through the stent in any way.
The porous PTFE allows the passage of fluids and vital materials,
however, while still serving as a barrier to tissue growth.
* * * * *