U.S. patent application number 11/810755 was filed with the patent office on 2007-10-18 for self-sealing ptfe vascular graft and manufacturing methods.
This patent application is currently assigned to Boston Scientific Scimed, Inc.. Invention is credited to Edward Dormier, Jamie Henderson, David Lentz, Gary Loomis, Ronald Rakos, Krzysztof Sowinski, Richard Zdrahala.
Application Number | 20070244539 11/810755 |
Document ID | / |
Family ID | 24352275 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070244539 |
Kind Code |
A1 |
Lentz; David ; et
al. |
October 18, 2007 |
Self-sealing PTFE vascular graft and manufacturing methods
Abstract
An implantable microporous ePTFE tubular vascular graft exhibits
long term patency, superior radial tensile strength and suture hole
elongation resistance. The graft includes a first ePTFE tube and a
second ePTFE tube circumferentially disposed over the first tube.
The first ePTFE tube exhibits a porosity sufficient to promote cell
endothelization, tissue ingrowth and healing. The second ePTFE tube
exhibits enhanced radial strength in excess of the radial tensile
strength of the first tube.
Inventors: |
Lentz; David; (LaJolla,
CA) ; Henderson; Jamie; (Oakland, NJ) ;
Dormier; Edward; (Rockaway, NJ) ; Zdrahala;
Richard; (Eden Prairie, MN) ; Loomis; Gary;
(Morristown, NJ) ; Rakos; Ronald; (Neshanic
Station, NJ) ; Sowinski; Krzysztof; (Wallington,
NJ) |
Correspondence
Address: |
HOFFMANN & BARON, LLP
6900 JERICHO TURNPIKE
SYOSSET
NY
11791
US
|
Assignee: |
Boston Scientific Scimed,
Inc.
|
Family ID: |
24352275 |
Appl. No.: |
11/810755 |
Filed: |
June 7, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10822200 |
Apr 9, 2004 |
7244271 |
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11810755 |
Jun 7, 2007 |
|
|
|
10212609 |
Aug 5, 2002 |
6719783 |
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|
10822200 |
Apr 9, 2004 |
|
|
|
09525710 |
Mar 14, 2000 |
6428571 |
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|
10212609 |
Aug 5, 2002 |
|
|
|
09008265 |
Jan 16, 1998 |
6036724 |
|
|
09525710 |
Mar 14, 2000 |
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08588052 |
Jan 22, 1996 |
5800512 |
|
|
09008265 |
Jan 16, 1998 |
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Current U.S.
Class: |
623/1.4 ;
623/1.44 |
Current CPC
Class: |
A61F 2/06 20130101; A61L
27/56 20130101; Y10S 623/901 20130101; A61F 2002/0086 20130101;
C08L 27/18 20130101; A61F 2002/072 20130101; A61L 27/507 20130101;
A61L 27/16 20130101; A61L 27/16 20130101 |
Class at
Publication: |
623/001.4 ;
623/001.44 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1. A multi-layered ePTFE graft comprising: a first ePTFE tubular
structure having a first internodal distance; a second ePTFE
tubular structure having a second internodal distance different
than said first intemodal distance, said second ePTFE tubular
structure being disposed about said first ePTFE tubular structure;
and a self-sealing gel interposed between said first and second
ePTFE tubular structures, wherein said gel is selected from the
group consisting of gelatin, collagen, albumin, casein, algin,
carboxymethyl cellulose, carageenan, furcellan, agarose, guar,
locus bean gum, gum arabic, hydroxyethyl cellulose, hydroxypropyl
cellulose, methyl cellulose, hydroxyalkylmethyl cellulose, pectin,
partially deacetylated chitosan, starch and starch derivatives,
including amylase and amylopectin, xanthan, polylysine, hyaluronic
acid, and its derivatives, heparin, their salts, and mixtures
thereof.
2. A multi-layered graft according to claim 1, wherein said first
intemodal distance is greater than said second internodal
distance.
3. A multi-layered graft according to claim 1, wherein said second
ePTFE tubular structure is disposed externally about said first
ePTFE tubular structure.
4. A multi-layered graft according to claim 1, wherein said
self-sealing gel comprises a single layer having resealable
properties.
5. A multi-layered graft according to claim 1, wherein said
self-sealing gel is flowable.
6. A multi-layered ePTFE vascular graft useful for repeated
hemoaccess comprising: a first ePTFE tubular structure having a
first internodal distance; a second ePTFE tubular structure having
a second internodal distance different than said first internodal
distance, said second ePTFE tubular structure being disposed about
said first ePTFE tubular structure; and a self-sealing gel
interposed between said first and second ePTFE tubular structures,
wherein said gel is selected from the group consisting of gelatin,
collagen, albumin, casein, algin, carboxymethyl cellulose,
carageenan, furcellan, agarose, guar, locus bean gum, gum arabic,
hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose,
hydroxyalkylmethyl cellulose, pectin, partially deacetylated
chitosan, starch and starch derivatives, including amylase and
amylopectin, xanthan, polylysine, hyaluronic acid, and its
derivatives, heparin, their salts, and mixtures thereof.
7. A multi-layered graft according to claim 6, wherein said first
internodal distance is greater than said second internodal
distance.
8. A multi-layered graft according to claim 6, wherein said second
ePTFE tubular structure is disposed externally about said first
ePTFE tubular structure.
9. A multi-layered graft according to claim 6, wherein said
self-sealing gel comprises a single layer having resealable
properties.
10. A multi-layered ePTFE graft comprising: a first ePTFE tubular
structure having a first internodal distance; a second ePTFE
tubular structure having a second internodal distance different
than said first internodal distance, said second ePTFE tubular
structure being disposed about said first ePTFE tubular structure;
and a biodegradable material interposed between said first and
second ePTFE tubular structures.
11. A multi-layered graft according to claim 10, wherein the
biodegradable material is a gel.
12. A multi-layered ePTFE graft comprising: a first ePTFE tubular
structure; and a second ePTFE tubular structure, said second ePTFE
tubular structure being disposed about said first ePTFE tubular
structure; wherein the graft exhibits a radial tensile strength of
at least 0.48 kg/mm.sup.2.
13. The multi-layered graft of claim 12, wherein said first ePTFE
tubular structure has a first porosity and said second ePTFE
tubular structure has a second porosity.
14. The multi-layered graft of claim 13, wherein said second
porosity is different than sand first porosity.
15. A multi-layered ePTFE graft comprising: a first ePTFE tubular
structure; and a second ePTFE tubular structure, said second ePTFE
tubular structure being disposed about said first ePTFE tubular
structure; wherein the graft is capable of withstanding elongation
of at least 690% without breaking.
16. The multi-layered graft of claim 15, wherein said first ePTFE
tubular structure has a first porosity and said second ePTFE
structure has a second porosity.
17. The multi-layered graft of claim 16, wherein said second
porosity is different than said first porosity.
18. A multi-layered ePTFE graft comprising: a first ePTFE tubular
structure; and a second ePTFE tubular structure, said second ePTFE
tubular structure being disposed about said first ePTFE tubular
structure; and a self-sealing material interposed between said
first and second ePTFE tubular structures, wherein the graft
exhibits no or immeasurable leakage 30 seconds subsequent to
puncture with a water source.
19. The multi-layered graft of claim 18, wherein said first ePTFE
tubular structure has a first porosity and said second ePTFE
tubular structure has a second porosity.
20. The multi-layered graft of claim 19, wherein said second
porosity is different than said first porosity.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 10/822,200, filed Apr. 9, 2004, now allowed, which is a
continuation of U.S. application Ser. No. 10/212,609, filed on Aug.
5, 2002, now U.S. Pat. No. 6,719,783, which is a continuation of
U.S. application Ser. No. 09/525,710, filed on Mar. 14, 2000, now
U.S. Pat. No. 6,428,571, which is a continuation-in-part of U.S.
application Ser. No. 09/008,265, filed on Jan. 16, 1998, now U.S.
Pat. No. 6,036,724, which is a divisional of U.S. application Ser.
No. 08/588,052, filed on Jan. 22, 1996, now U.S. Pat. No.
5,800,512, the full contents of all of which are incorporated
herein by reference.
FIELD OF INVENTION
[0002] The present invention relates generally to a tubular
implantable prosthesis such as vascular grafts and endoprostheses
formed of porous polytetrafluoroethylene. More particularly, the
present invention relates to a multi-layered tubular self-sealing
graft or endoprosthesis formed from primarily expanded
polytetrafluoroethylene.
BACKGROUND OF THE INVENTION
[0003] It is well known to use extruded tubes of
polytetrafluoroethylene (PTFE) as implantable intraluminal
prostheses, particularly vascular grafts. PTFE is particularly
suitable as an implantable prosthesis as it exhibits superior
biocompatability. PTFE tubes may be used as vascular grafts in the
replacement or repair of a blood vessel as PTFE exhibits low
thrombogenicity. In vascular applications, the grafts are
manufactured from expanded polytetrafluoroethylene (ePTFE) tubes.
These tubes have a microporous structure which allows natural
tissue ingrowth and cell endothelization once implanted in the
vascular system. This contributes to long term healing and patency
of the graft.
[0004] Grafts formed of ePTFE have a fibrous state which is defined
by interspaced nodes interconnected by elongated fibrils. The
spaces between the node surfaces that is spanned by the fibrils is
defined as the intemodal distance (IND). A graft having a large IND
enhances tissue ingrowth and cell endothelization as the graft is
inherently more porous.
[0005] The art is replete with examples of microporous ePTFE tubes
useful as vascular grafts. The porosity of an ePTFE vascular graft
can be controlled by controlling the IND of the microporous
structure of the tube. An increase in the IND within a given
structure results in enhanced tissue ingrowth as well as cell
endothelization along the inner surface thereof. However, such
increase in the porosity of the tubular structure also results in
reducing the overall radial tensile strength of the tube as well as
reducing the ability for the graft to retain a suture placed
therein during implantation. Also, such microporous tubular
structures tend to exhibit low axial tear strength, so that a small
tear or nick will tend to propagate along the length of the
tube.
[0006] The art has seen attempts to increase the radial tensile and
axial tear strength of microporous ePTFE tubes. These attempts seek
to modify the structure of the extruded PTFE tubing during
formation so that the resulting expanded tube has
non-longitudinally aligned fibrils, thereby increasing both radial
tensile strength as well as axial tear strength. U.S. Pat. No.
4,743,480 shows one attempt to reorient the fibrils of a resultant
PTFE tube by modifying the extrusion process of the PTFE tube.
[0007] Other attempts to increase the radial tensile, as well as
axial tear strength of a microporous ePTFE tube include forming the
tubular graft of multiple layers placed over one another. Examples
of multi-layered ePTFE tubular structures useful as implantable
prostheses are shown in U.S. Pat. Nos. 4,816,338; 4,478,898 and
5,001,276. Other examples of multi-layered structures are shown in
Japanese Patent Publication Nos. 6-343,688 and 0-022,792.
[0008] Artificial bypass grafts are often used to divert blood flow
around damaged regions to restore blood flow. Vascular prostheses
may also be used for creating a bypass shunt between an artery and
vein. These bypass shunts are often used for multiple needle
access, such as is required for hemodialysis treatments. These
artificial shunts are preferable to using the body's veins, mainly
because veins may either collapse along a puncture track or become
aneurysmal, leaky or clotted, causing significant risk of pulmonary
embolization.
[0009] While it is known to use ePTFE as a vascular prosthesis, and
these vascular prostheses have been used for many years for
vascular access during hemodialysis, there remain several problems
with these implantable ePTFE vascular access grafts. One major
drawback in using ePTFE vascular grafts as access shunts for
hemodialysis is that because of ePTFE's node-fibril structure, it
is difficult to elicit natural occlusion of suture holes in the
vascular prosthesis made from ePTFE tubing. As a result, blood
cannot typically be withdrawn from an ePTFE vascular graft until
the graft has become assimilated with fibrotic tissue. This
generally takes 2 to 3 weeks after surgery. Furthermore, ePTFE's
propensity for axial tears make it undesirable as a vascular access
graft, as punctures, tears, and other attempts to access the blood
stream may cause tears which propagate axially with the grain of
the node fibril structure.
[0010] Providing a suitable vascular access graft has also been
attempted in the prior art. Schanzer in U.S. Pat. No. 4,619,641
describes a two-piece coaxial double lumen arteriovenous graft. The
Schanzer graft consists of an outer tube positioned over an inner
tube, the space between being filled with a self-sealing adhesive.
The configuration of this coaxial tube greatly increases the girth
of the graft, and limits the flexibility of the lumen which
conducts blood flow. Herweck et al., in U.S. Pat. No. 5,192,310
describes a self-sealing vascular graft of unitary construction
comprising a primary lumen for blood flow, and a secondary lumen
sharing a common sidewall with the primary lumen. A
non-biodegradable self-sealing elastomeric material is disposed
between the primary and secondary lumen.
[0011] While each of the above-referenced patents disclose
self-sealing vascular grafts, none disclose a tubular access graft
structure exhibiting enhanced radial tensile strength, as well as
enhanced resistance to axial tear strength. Furthermore, the
multi-layered ePTFE tubular structures and vascular access grafts
of the prior art exhibit smaller microporous structure overall, and
accordingly a reduction in ability of the graft to promote
endothelization along the inner surface. Furthermore, Schanzer does
not provide a self-sustaining resealable layer, but rather an
elastomeric layer which "fills" the area between the two tubes.
[0012] It is therefore desirable to provide a self-sealing ePTFE
graft for use in a human body which exhibits increased porosity
especially at the inner surface thereof while retaining a high
degree of radial strength at the external surface thereof. The
graft may preferably be used as a vascular access graft.
[0013] It is further desirous to produce an ePTFE vascular access
graft which exhibits increased porosity at the outer surface
thereof while retaining a high degree of radial tensile and suture
retention strengths.
[0014] It is still further desirous to provide a self-sealing graft
with increased resistance to axially propagating tears.
SUMMARY OF THE INVENTION
[0015] It is an advantage of the present invention to provide a
self-sealing ePTFE graft with increased resistance to axially
propagating tears.
[0016] It is a further advantage of the present invention to
provide a self-sealing ePTFE graft providing superior assimilation
capabilities and resealable properties.
[0017] It is a further advantage of the present invention to
provide a self-sealing ePTFE vascular graft exhibiting an enhanced
microporous structure while retaining superior radial strength.
[0018] It is a still further advantage of the present invention to
provide an ePTFE tubular structure having an inner portion
exhibiting enhanced porosity and an outer portion exhibiting
enhanced radial tensile strength, suture retention, and suture-hole
elongation characteristics.
[0019] It is yet another advantage of the present invention to
provide a multi-layered ePTFE tubular vascular graft having an
inner layer which has a porosity sufficient to promote cell
endothelization and an outer layer having a high degree of radial
tensile strength.
[0020] It is an additional advantage of the present invention to
provide a multi-layered ePTFE tubular vascular access graft having
an outer layer whose porosity is sufficient to promote enhanced
cell growth and tissue incorporation, hence more rapid healing, and
an inner layer having a high degree of strength.
[0021] In the efficient attainment of these and other advantages,
the present invention provides a self-sealing ePTFE graft
comprising a first expanded polytetrafluoroethylene (ePTFE) tubular
structure having a first porosity, a second ePTFE tubular structure
having a second porosity less than said first porosity, said second
ePTFE tubular structure being disposed externally about said first
ePTFE tubular structure to define a distinct porosity change
between said first and second tubular structures, and a resealable
polymer layer interposed between said first and second ePTFE
tubular structures.
[0022] In another embodiment, the present invention provides an
ePTFE self-sealing graft, the graft formed of a first EPTFE tubular
structure, a second ePTFE tubular structure disposed externally
about said first EPTFE tubular structure, and further including a
self-sustained resealable polymer layer interposed between the
first and second ePTFE tubular structures.
[0023] The ePTFE self-sealing graft preferably may be used as a
vascular access graft. As more particularly described by way of the
preferred embodiment herein, the first and second ePTFE tubular
structures are formed of expanded polytetrafluoroethylene (ePTFE).
Further, the second ePTFE tubular structure is adheringly supported
over the first ePTFE tubular structure to form a composite tubular
graft. The strength of this adhesion can be varied as desired to
control the characteristics exhibited by the resultant composite
structure.
[0024] In its method aspect, the present invention provides a
method of forming a self-sealing ePTFE graft. The method includes
the steps of providing a first ePTFE tubular structure having a
desired porosity and strength combination. A second ePTFE tubular
structure is provided, also having the desired porosity and
strength combination. The second ePTFE structure is disposed over
the first ePTFE so as to define a composite vascular graft.
[0025] The method of the present invention also provides for the
positioning of an intermediate structure between the first and
second ePTFE tubular structures. Examples of such structures
include an additional ePTFE layer and fibers or thin films of PTFE
or other suitable polymers. This intermediate structure also
contributes to the resultant porosity and strength of the vascular
graft. This intermediate structure can also preferably be a
resealable polymer layer interposed between the first and second
ePTFE tubular structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic longitudinal cross-section of a
multi-layer ePTFE vascular graft of the present invention.
[0027] FIG. 2 is a longitudinal cross-section of an alternate
embodiment of the present invention producing a multi-layer ePTFE
vascular graft.
[0028] FIG. 3 is a scanning electron micrograph showing a
cross-sectional view of a vascular graft produced using the present
invention.
[0029] FIG. 4 is a perspective showing of one of the tubular
structures of the graft of FIG. 1 over-wrapped with a layer of
ePTFE tape.
[0030] FIG. 5 is a cross-sectional showing of an alternate
embodiment of the ePTFE vascular graft of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] The prosthesis of the preferred embodiments of the present
invention is a multi-layered tubular structure which is
particularly suited for use as a vascular access graft. The
prosthesis is formed of extruded polytetrafluoroethylene (PTFE) as
PTFE exhibits superior biocompatability. In the present invention,
a first ePTFE tubular structure having a first porosity is placed
circumferentially interior to a second ePTFE tubular structure.
Further, a resealable polymer layer is interposed as an
intermediate structure between said first and second ePTFE tubular
structures.
[0032] PTFE is particularly suitable for vascular applications as
it exhibits low thrombogenicity. Tubes formed of extruded PTFE may
be expanded to form ePTFE tubes where the ePTFE tubes have a
fibrous state which is defined by elongate fibrils interconnected
by spaced apart nodes. Such tubes are said to have a microporous
structure, the porosity of which is determined by the distance
between the surfaces of the nodes, referred to as the intemodal
distance (IND). Tubes having a large IND (greater than 40 microns)
generally exhibit long term patency as the larger pores promote
cell endothelization along the inner blood contacting surface.
Tubes having lower IND (less than 40 microns) exhibit inferior
healing characteristics, however they offer superior radial tensile
and suture retention strengths desirable in a vascular graft. The
present invention provides a composite tubular structure which
promotes long term patency of the graft by providing for enhanced
cell endothelization along the inner surface while exhibiting
enhanced strength due to the presence of the outer layer.
[0033] Referring to FIGS. 1 and 2 of the drawings, composite graft
10 of the present invention is shown. Graft 10 is an elongate
tubular structure formed of PTFE. Graft 10 includes a pair of
coaxially disposed ePTFE tubes 12 and 14, tube 12 being the outer
tube and tube 14 being the inner tube. A central lumen 15 extends
through composite graft 10, defined further by the inner wall 14a
of inner tube 14, which permits the passage of blood through graft
10 once the graft is properly implanted in the vascular system.
[0034] Each tube 12 and 14 may be formed in a separate extrusion
process. The process for the paste extrusion of PTFE tubes is well
known in the extrusion art. Once extruded, the tubes are expanded
to form ePTFE tube. As will be described hereinbelow, the tubes are
expanded using differing process parameters (rates, deformation
levels, temperatures, etc.) to develop the desired microporous
structures. The specifically designed structure of the resulting
composite tube has defined properties of strength and porosity
which yield a graft 10 having long term patency and good healing
characteristics as well as superior strength characteristics. It is
also contemplated within the present invention to use PTFE which
was extruded as sheets, expanded, and subsequently wrapped to form
tubes. An ePTFE tape or ribbon helically wrapped into a tubular
structure is also contemplated within the present invention.
[0035] The present invention is designed to produce grafts with
substantially different node/fibril structures with respect to the
internal and external portions of the graft which are adjacent to
the internal and external graft surfaces. As an example, the inner
tube 14 is designed to have relatively high IND while the outer
tube 12 is designed to have a lower IND. Further, a distinct
porosity change is clearly defined at the interface 13 between
tubes 12 and 14. The inner tube 14 having a higher IND to allow
enhanced cell endothelization, while the outer tube 12 having a
lower IND provides superior strength to the overall composite.
[0036] An electron micrograph of such a structure produced
according to the present invention is shown in FIG. 3. The
disparate IND's between the inner tube 14 and outer tube 12 are
clearly evident, along with the step change in IND at the interface
13 between the inner tube 14 and outer tube 12. In this example,
the strength of the interface 13 has been established by the
processing conditions described below to fully adhere the inner
tube 14 and outer tube together, hence preventing relative motion
and providing enhanced strength.
[0037] Graft 10 of the present invention may be formed by expanding
a thin wall inner tube 14 at a relatively high degree of
elongation, on the order of approximately between 400 and 2000%
elongation preferably from about between 700% and 900%. Tube 14 is
expanded over a cylindrical mandrel (not shown), such as a
stainless steel mandrel at a temperature of between room
temperature and 645.degree. F., preferably about 500.degree. F.
Tube 14 is preferably but not necessarily fully sintered after
expansion. Sintering is typically accomplished at a temperature of
between 645.degree. F. and 800.degree. F. preferably at about
660.degree. F. and for a time of between about 5 minutes to 30
minutes, preferably about 15 minutes. The combination of the ePTFE
tube 14 over the mandrel is then employed as a second mandrel, over
which outer tube 12 is expanded. The ID of the outer tube 12 is
selected so that it may be easily but tightly disposed over the OD
of inner tube 14. The composite structure 10 is then sintered at
preferably similar parameters. The level of elongation of outer
tube 12 is lower than that of inner tube 14, approximately between
200% and 500% elongation preferably about 400%. The expansion and
sintering of outer tube 12 over the inner tube 14 serves to
adheringly bond the interface 13 between the two tubes, resulting
in a single composite structure 10.
[0038] In an alternate embodiment the outer tube ID may be less
than the inner tube OD. In this embodiment, the outer tube is
thermally or mechanically radially dilated to fit over the inner
tube. The composite structure may then be sintered at about
660.degree. F. Construction in this manner provides a snug fit of
the tubes, and enhances the bonding interface between tubes, and
also may augment recoil properties of an elastomeric intermediate
layer.
[0039] As shown in FIG. 3, the resulting composite structure has an
inner surface defined by inner tube 14 which exhibits an IND of
between 40 and 100 microns, spanned by a moderate number of
fibrils. Such microporous structure is sufficiently large so as to
promote enhanced cell-endothelization once blood flow is
established through graft 10. Such cell-endothelization enhances
the long term patency of the graft.
[0040] The outer structure, defined by outer tube 12, has a smaller
microporous structure, with IND of about 15-35 microns and a
substantial fibril density. Such outer structure results in an
increase in the strength of the outer tube, and hence of the
composite structure. Importantly, the outer surface defined by the
outer tube 12 exhibits enhanced suture retention due to the smaller
IND.
[0041] Furthermore, the resulting composite structure exhibits a
sharp porosity change between the outer tube 12 and inner tube 14.
This sharp porosity transition is achieved by providing an inner
tube 14 having generally a given uniform porosity therealong and
then providing a separate outer tube 14 having a resultant
different porosity uniformly therealong. Thus a distinct porosity
change is exhibited on either side of the interface 13 defined
between inner tube 14 and outer tube 12.
[0042] In addition, the forming process described above results in
a bonded interface between the inner tube 14 and outer tube 12. The
interface exhibits sufficient interfacial strength resulting from
the direct sintering of the outer tube 12 over inner tube 14 so as
to assure complete bonding of the two tubes. The strength of the
interface between the two tubes may be independently varied through
selection of processing conditions and relative dimensions of
precursor extruded tubes 12 and 14 as desired to yield a range of
performance.
[0043] Referring now to FIGS. 4 and 5, a further embodiment of the
present invention is shown. Tubular graft 20 is a composite
structure similar to graft 10 described above. Graft 20 includes an
outer tube 22 and an inner tube 24 formed generally in the manner
described above. In order to further control the porosity and
strength of the graft 20, especially at the interface between outer
tube 22 and inner tube 24, an additional layer may be employed in
combination with outer tube 22 and inner tube 24.
[0044] As specifically shown in FIGS. 4 and 5, an additional layer
26 may be employed between inner tube 24 and outer tube 22. Layer
26 may include a helical wrap of ePTFE tape 27 placed over inner
tube 24. The additional layer 26, however, may also exist as a
sheet, film, yarn, monofilament or multi filament wrap, or
additional tube. The additional layer 26 may consist of PTFE, FEP,
or other suitable polymer composition to obtain the desired
performance characteristics. Layer 26 may be used to impart
enhanced properties of porosity and/or strength to the composite
graft 20. For example, an additional layer 26 of ePTFE tape 27
having a low IND and wrapped orthogonally to the length direction
of graft 20 would increase the radial strength of the resultant
composite graft. Similarly, a layer of ePTFE having a high IND
would increase the porosity of the composite structure thereby
further promoting cell endothelization and/or tissue ingrowth.
[0045] In a preferred embodiment of the present invention, the
intermediate layer may be a resealable polymer layer, employed in
order to create a self-sealing graft. The self-sealing graft may be
preferably used as a vascular access device. The graft is also
preferably implantable. When used as an access device, the graft
allows repeated access to the blood stream through punctures, which
close after removal of the penetrating member (such as, e.g., a
hypodermic needle or cannula) which provided the access.
[0046] The intermediate, or resealable polymer layer may be
additionally augmented with a pre-sintered PTFE (or FEP) bead wrap,
or wire support coil. The pre-sintered PTFE bead wrap is a
cylindrically extruded solid tube of PTFE that is sintered, then
helically wrapped around the desired layer (inner, intermediate, or
outer). A graft of this embodiment shows enhanced strength and
handling characteristics, i.e., crush resistance, kink resistance,
etc.
[0047] In another preferred embodiment, a self-sustained resealable
polymer layer may be interposed between first and second ePTFE
tubular structures. For the purpose of this specification, the term
self-sustained refers to a tubular structure which possesses enough
structural stability to be formed and subsequently stand alone
without the use of additional tubular layers, or any other
"molding" type formation, i.e., not a resinous polymer which is
injected, or fills a space between an outer and inner tube. Some
examples include the elastomeric layers employed in the present
invention, as well as the resealable intermediate layers shown in
Examples 3 and 4 of the present invention.
[0048] The ePTFE self-sealing graft can be used for any medical
technique in which repeated hemoaccess is required, for example,
but without intending to limit the possible applications,
intravenous drug administration, chronic insulin injections,
chemotherapy, frequent blood samples, connection to artificial
lungs, and hyperalimentation. The self-sealing ePTFE graft is
ideally suited for use in chronic hemodialysis access, e.g., in a
looped forearm graft fistula, straight forearm graft fistula, an
axillary graft fistula, or any other AV fistula application. The
self-sealing capabilities of the graft are preferred to provide a
graft with greater suture retention, and also to prevent excessive
bleeding from a graft after puncture (whether in venous access or
otherwise).
[0049] The graft is made self-sealing with the use of a resealable
polymer layer interposed between said first and second polymer
layer. The resealable layer functions by primarily two different
mechanisms. In one embodiment, the resealable polymer layer
comprises an elastomeric component. The term elastomeric as used
herein refers to a substance which is capable of essentially
rebounding to near its initial form or state after deformation. In
another embodiment, the resealable polymer layer comprises a
flowable material layer. The term flowable as used herein refers to
an amorphous material which fills a void created by a deformation
or puncture.
[0050] It is further contemplated within the present invention to
provide a composite vascular graft with an intermediate resealable
layer, and multiple interior or exterior layers of ePTFE.
Furthermore, the use of multiple intermediate layers possessing
resealable properties is also contemplated within the present
invention.
[0051] A number of different materials may be employed to provide
an elastomeric polymer component as contemplated in the present
invention. Furthermore, the elastomeric properties of the
intermediate layer may be imparted thereto as a result of an
inherent property of the material used, or as a result of the
particular method of constructing such a layer. The elastomeric
component may also be adhered to the first and second ePTFE tubular
structures. The adhesion may take place by mechanical means,
chemical means (use of an adhesive), either, or both. Some
polymers, particularly thermoplastic elastomers, become
sufficiently tacky through heating to adhere to the ePTFE tubular
structures. The elastomeric component may also exert a force in the
direction of the puncture, which if adhered to the first and/or
second ePTFE tubular structures may provide for either layer to
seal the puncture site. Some materials which may be used as an
elastomeric component in various forms include, but are not limited
to, polymers and copolymers, including thermoplastic elastomers and
certain silicones, silicone rubbers, synthetic rubbers,
polyurethanes, polyethers, polyesters, polyamides and various
fluoropolymers, including, but not limited to PTFE, ePTFE, FEP
(fluorinated ethylene propylene copolymer), and PFA
(polyfluorinated alkanoate). The materials may be utilized as the
elastomeric polymer layer in a number of different forms which
would impart the desired elastomeric characteristics to the layer.
In one embodiment, an extruded polymeric ribbon or tape wrap may be
wrapped helically into a tubular shape under tension.
Alternatively, a sheet, fiber, thread, or yarn may also be wrapped
under tension to impart an elastomeric layer.
[0052] In another preferred embodiment, a polymeric layer may be
applied from solution. The polymer may be dissolved or partially
dissolved in a solvent, and upon evaporation of the solvent, the
polymer is deposited as an elastomeric layer. The solvents used in
this system must be capable of wetting the ePTFE tubular surfaces.
Upon evaporation of the solvent, an elastomeric layer is deposited
which may penetrate into the pores of the adjacent ePTFE layer to
provide an anchoring effect for the polymeric layer. Upon
evaporation of the solvent, the elastomeric layer may also shrink
to provide the desired elastomeric characteristics.
[0053] In another embodiment of the elastomeric layer of the
present invention, a solvent spun polyurethane as disclosed in U.S.
Pat. Nos. 4,810,749; 4,738,740; 4,743,252 and 5,229,431, herein
incorporated by reference, may be employed. From such elastomeric
fibers may be formed a woven or non-woven textile-like layer with
sufficient fiber density to form a sealing layer while allowing a
puncturing member such as a hypodermic needle or cannula to
penetrate between the individual fibers. Furthermore, the
elastomeric fibers of said textile-like structure may be employed
under tension or compression to facilitate the recovery of the
fibers displaced by the penetrating member to their original
position after removal by the penetrating member.
[0054] Furthermore the elastomeric layer of the present invention
may additionally be impregnated with a gel to provide enhanced
sealing capabilities. Examples of such gels are hydrogels formed
from natural materials including, but not limited to, gelatin,
collagen, albumin, casein, algin, carboxy methyl cellulose,
carageenan, furcellaran, agarose, guar, locust bean gum, gum
arabic, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl
cellulose, hydroxyalkylmethyl cellulose, pectin, partially
deacetylated chitosan, starch and starch derivatives, including
amylose and amylopectin, xanthan, polylysine, hyaluronic acid, and
its derivatives, heparin, their salts, and mixtures thereof.
[0055] A number of different flowable polymer layers may be
interposed between said first and second tubular structures to
provide a self-sealing graft. The flowable polymer layer seals the
graft by possessing an amorphous quality which fills in any space
left open subsequent to puncture of the graft. It may simply fill
in the space left open in the interposed middle layer, or it may
additionally penetrate into the first and/or second ePTFE tubular
structures to fill any void left form puncture of either layer.
[0056] An example of a flowable polymer which may be used in the
present invention is an uncured or partially cured polymer. The
polymer may be cured by a number of activating means which would
activate curing subsequent to puncture of the graft, thereby
sealing with the curing of the polymer. Examples of materials for
such a flowable layer include, but are not limited to, uncured
elastomers such as natural or synthetic rubbers, and natural gums
such as gum arabic. Materials that are particularly useful in a
flowable layer include non-crosslinked polyisobutylene which is
also known as uncured butyl rubber.
[0057] Another flowable polymer layer which may be employed in the
present invention a gel. Gels are generally suspensions or
emulsions of polymers which have properties intermediate the liquid
and solid states. A hydrogel may also be used in the present
invention, and refers to polymeric material which swells in water
without dissolving, and which retains a significant amount of water
in its structure. The gels and hydrogels employed in the present
invention may be biodegradable, or non-biodegradable. They also
further may have polymeric beads (not to be confused with the
pre-sintered PTFE bead-wrap, which imparts structural stability)
suspended within the gel to effectuate sealing of the prosthesis.
Some examples of gels which may be used in the present invention
include, but are not limited to, silicone gels, gum arabic, and low
molecular weight ethylene/vinyl acetate polymers.
[0058] The following examples serve to provide further appreciation
of the invention but are not meant in any way to restrict the scope
of the invention.
EXAMPLE I
[0059] A thin extruded tube having wall thickness of 0.41 mm and an
inner diameter of 6.2 mm was expanded over a stainless steel
mandrel at 500.degree. F. to 900% elongation. The ePTFE tube was
then sintered at 660.degree. F. for 14 minutes, cooled, and removed
from the oven. A second thin extruded tube having wall thickness of
0.45 mm and an inner diameter of 6.9 mm was expanded over the first
tube/mandrel combination at 500.degree. F. and 400% elongation. The
composite was then sintered at 660.degree. F. for 14 minutes,
cooled and removed from the oven. The resultant composite tube had
a wall thickness of 0.65 mm and ID of 5.8 mm.
EXAMPLE 2
[0060] A thin extruded tube having wall thickness of 0.41 mm and an
inner diameter of 6.2 mm was expanded over a stainless steel
mandrel at 500.degree. F. to 700% elongation. The ePTFE tube was
then sintered at 660.degree. F. for 14 minutes, cooled, and removed
from the oven. A second thin extruded tube having wall thickness of
0.45 mm and an inner diameter of 6.9 mm was expanded over the first
tube at 500.degree. F. and 400% elongation. The composite was
sintered at 660.degree. F. for 14 minutes, cooled, and removed from
the oven. The resultant composite tube had a wall thickness of 0.67
mm and an inner diameter of 5.8 mm.
[0061] Table I presents physical property data for a vascular graft
of the type depicted in Example I described above. The composite
graft was removed from the mandrel and subjected to standard
testing of radial tensile strength and suture hole elongation. The
radial strength of the 900%/400% composite graft is equivalent to a
single layer 400% elongation graft and substantially stronger than
a single layer 900% elongation graft, despite an overall thinner
wall dimension. Additionally, the superior strength of the
composite graft is demonstrated by the higher elongation capable of
being borne by the graft prior to failure. The lower suture hole
elongation, indicative of a smaller tear being caused by suturing
and tensioning at a fixed value of 100 grams is clearly
demonstrated for the graft prepared by the method of the current
invention. TABLE-US-00001 TABLE 1 400% 900%/ Elongation 400% 900%
Single Elongation Elongation Physical Property Layer Composite
Single Layer Measurement Graft Graft Graft Radial Tensile Strength
(kg/mm.sup.2) 0.48 0.48 0.2 Radial Strain at Break (%) 550 690 531
Suture Hole Elongation (%) 87 81 158 Wall Thickness 0.72 0.65
0.73
EXAMPLE 3
[0062] Three composite grafts were constructed and their ability to
reseal after a puncture was tested. Graft No. 1 is an ePTFE
helically tape-wrapped graft with no resealable layer, and graft
Nos. two (2) and three (3) were constructed with a resealable
intermediate layer in the below-described procedure.
[0063] Graft No. 1 is an ePTFE graft used as the control in the
following experiment. Graft 2 was constructed by first placing an
ePTFE tubular structure with an inner diameter of 5 millimeters on
a mandrel. A thermoplastic elastomer tubing was then slid over the
ePTFE tube. Because of the tackiness of the thermoplastic
elastomeric tubing, it was necessary to manipulate the tubing by
rolling and stretching it in order to maneuver it over the first
ePTFE tubular structure to lie evenly thereon. A second ePTFE
tubular structure with an inner diameter of five (5) millimeters
was then radially stretched, or expanded to yield an ePTFE tubular
structure with an inner diameter of 8 millimeters. A metal sleeve
was then placed over the thermoplastic elastomer-covered tubular
structure. The second 8 mm inner diameter ePTFE tubular structure
was then placed onto the metal sleeve. The metal sleeve was then
retracted allowing the second tubular structure to come into
contact with the thermoplastic elastomer.
[0064] Both grafts Nos. 2 and 3 were prepared using this procedure.
Graft 2 was then placed in an oven and heated at 350.degree. F. for
10 minutes. There appeared to be little bonding between the layers,
and the graft was then heated for an additional 10 minutes at
450.degree. F.
[0065] Graft 3 was heated for five minutes at 400.degree. F. The
material did not appear to bond together, so the graft was then
heated at 425.degree. F. for an additional five minutes.
[0066] Grafts Nos. 1-3 were then tested for quality control. Each
graft was placed in a water entry pressure measuring device to
provide a constant pressure of water within the graft. The water
pressure was maintained at three pounds per square inch. The grafts
were then punctured with a 20 gauge needle. The needle was then
removed and any water leaving the puncture site was collected in a
beaker and measured over the time of collection. This procedure was
followed for a number of test runs. The results are shown in table
two below. TABLE-US-00002 TABLE 2 Graft No. 1 Graft No. 2 Graft No.
3 Run (grams water/30 (grams water/30 (grams water/30 Number
seconds) seconds) seconds) 1 21.7 0.2 1.2 2 20.0 1.3 0.5 3 23.1 0.2
-- 4 20.0 1.4 -- 5 21.9 -- -- Average Of 21.34 0.775 0.85 Trial
Runs
[0067] Graft No. 1 leaked steadily, as the puncture did not reseal.
While graft numbers 2 and 3 showed minimal leakage. In both cases,
the water leaked in a small trickle in the first few seconds (2-5
seconds), then stopped or slowed to an immeasurable seepage.
EXAMPLE 4
[0068] Three additional grafts were constructed with an
intermediate resealable layer and tested to determine their ability
to seal after puncture.
[0069] Graft No.4 was made using an ePTFE graft with an initial 5
millimeter inner diameter as the inner tubular structure. The 5 mm
ePTFE tube was extruded using a die insert of 0.257 inches, and a
mandrel of 0.226 inches. The inner tube was then stretched
longitudinally to 500% its original length, and sintered. The inner
tube was then stretched radially by placing it on a 5.95 mm
mandrel. A pre-sintered PTFE bead-wrap (0.014 inch .+-.0.002 inch
diameter, length approximately 30 cm) was then helically wrapped
around the exterior of the inner tube at 650 revolutions per minute
(RPM), and with a mandrel speed traversely at 800 RPMs. The wrap
helically repeated on the tube at approximately every 3.5 cm, and
at an angle of approximately 10.degree.-50.degree. with respect to
a radial axis. After wrapping, the coil wrapped tube was then
heated in an oven at 663.degree. F. for 10 minutes to sinter the
beads to the grafts.
[0070] A second ePTFE tube was then added exterior the beaded coil
and inner tube. The second ePTFE tube also had a 5 millimeter inner
diameter, and was a tube extruded using a die insert of 0.271
inches and mandrel size 0.257 inches, and was stretched to
longitudinally 500% its original length and sintered. The second
ePTFE tube was then radially stretched over a tapered 6-10 mm
mandrel to a graft of inner diameter of 10 mms. The second graft
was then transferred to a 10 mm hollow mandrel within which the
inner bead-wrapped graft was placed. The hollow mandrel was then
slid out to leave the second ePTFE tube exteriorly placed on the
bead covered graft. The composite device was then placed in an oven
for 20 minutes and heated at 663.degree. F. Three 3 cm long tubular
rings of thermoplastic elastomer (C-FLEX.RTM.) were then placed
over the composite stent graft. The rings had an inner diameter of
6.5 mms. A third outer ePTFE tubular structure was then placed over
the C-FLEX rings. The third outer tubular structure was a 3 mm
inner diameter ePTFE extruded tube was made using a die insert of
0.189 inches, and was stretched longitudinally to 700% its original
length and sintered. The third outer structure was then expanded
radially by stretching the tube over a 4-7 mm continuously tapered
mandrel, then further radially expanded over a 6-10 mm continuously
tapered mandrel. It was then placed on a 10 mm hollow mandrel with
the C-FLEX covered composite graft placed within the mandrel, and
the outer tube then covers the C-FLEX.RTM. upon removal of the
hollow mandrel. Graft 4 was then heated for 5 minutes at
435.degree. F.
[0071] Graft No. 5 was constructed using a 5 mm inner diameter
ePTFE extruded tube as the inner tubular structure. The 5 mm tube
was then radially stretched over a 5.95 mm diameter mandrel. A
corethane intermediate layer was then spun into a tube of 6 mm
diameter with 250 passes, and subsequently loaded onto the inner
ePTFE tube. A second outer ePTFE tubular structure (originally a 5
mm inner diameter) was then stretched to 10 mm over a 6-10 mm
continuously tapered mandrel. Graft No. 5 was then heated for 15
minutes at 400.degree. F.
[0072] Graft No. 6 was made using a 5 mm inner diameter ePTFE graft
as the inner tubular structure. The inner tube was then stretched
over a 6.2 mm hollow expansion mandrel. An ePTFE yarn was then
wrapped in two helical directions over the inner tube. A corethane
layer was then spun over the yarn wrapping as the third tubular
layer at a mandrel speed of 1,000 RPMs and a wrap angle of 50
degrees. The corethane covered graft was then heated at 230.degree.
F. for 10 minutes. An outer ePTFE tube with a 5 mm inner diameter
was then stretched to 10 mm over a 6-10 mm continuously tapered
mandrel. It was then mounted on the corethane layer with the use of
the hollow mandrel. Graft 6 was then heated for 30 minutes at
340.degree. F.
[0073] Grafts 4-6 were all tested by puncturing them with a
constant pressure water source attached to them as done with Grafts
Nos. 1-3. The results are shown in Table 3 below. TABLE-US-00003
TABLE 3 Graft 5 Graft 6 Graft 4 (grams (grams water/ Run No. (grams
water/seconds) water/seconds) seconds) 1 0.2 0 0 2 0 0 0 3 0 0.7
0
[0074] Various changes to the foregoing described and shown
structures would now be evident to those skilled in the art.
Accordingly, the particularly disclosed scope of the invention is
set forth in the following claims.
* * * * *