U.S. patent application number 11/430334 was filed with the patent office on 2006-11-16 for elastomerically impregnated eptfe to enhance stretch and recovery properties for vascular grafts and coverings.
This patent application is currently assigned to Scimed Life Systems, Inc.. Invention is credited to Ronald Rakos, Krzysztof Sowinski.
Application Number | 20060259133 11/430334 |
Document ID | / |
Family ID | 29999140 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060259133 |
Kind Code |
A1 |
Sowinski; Krzysztof ; et
al. |
November 16, 2006 |
Elastomerically impregnated ePTFE to enhance stretch and recovery
properties for vascular grafts and coverings
Abstract
An elastomerically recoverable PTFE material is provided
including a longitudinally compressed fibrils of ePTFE material
penetrated by elastomeric material within the pores defining the
elastomeric matrix. The elastomeric matrix and the compressed
fibris cooperatively expand and recover without plastic deformation
of the ePTFE material. The material may be used for various
prosthesis, such as a vascular a prosthesis like a patch, a graft
and an implantable tubular stent. Further, a method of producing
the elastomerically recoverable PTFE material is provided
herein.
Inventors: |
Sowinski; Krzysztof;
(Wallington, NJ) ; Rakos; Ronald; (Neshanic
Station, NJ) |
Correspondence
Address: |
HOFFMANN & BARON, LLP
6900 JERICHO TURNPIKE
SYOSSET
NY
11791
US
|
Assignee: |
Scimed Life Systems, Inc.
|
Family ID: |
29999140 |
Appl. No.: |
11/430334 |
Filed: |
May 9, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10179484 |
Jun 25, 2002 |
|
|
|
11430334 |
May 9, 2006 |
|
|
|
Current U.S.
Class: |
623/1.54 ;
264/257; 427/2.25; 623/926 |
Current CPC
Class: |
A61L 27/16 20130101;
A61L 27/34 20130101; A61L 27/507 20130101; A61L 27/16 20130101;
Y10T 428/249933 20150401; C08L 27/18 20130101 |
Class at
Publication: |
623/001.54 ;
623/926; 427/002.25; 264/257 |
International
Class: |
A61F 2/06 20060101
A61F002/06; A61F 2/02 20060101 A61F002/02; B29D 23/00 20060101
B29D023/00 |
Claims
1. An elastomerically recoverable PTFE material comprising: (a) an
ePTFE material defined by nodes and fibrils, said fibrils being in
longitudinally compressed state and defining pores of a size
sufficient to permit penetration of an elastomeric material; and
(b) an elastomeric matrix within said pores; said compressed
fibrils and elastomeric matrix cooperatively permitting
longitudinal expansion and elastomeric recovery without plastic
deformation of said ePTFE material, wherein said elastomeric matrix
is a polycarbonate urethane material having a shore hardness rating
between 70A and 75 D.
2. A vascular prosthesis of claim 1 wherein said elastomeric
recovery of PTFE matrix is a vascular prosthesis.
3. The vascular prosthesis of claim 2 wherein said vascular
prosthesis is a patch.
4. The vascular prosthesis of claim 2 wherein the vascular
prosthesis is a graft.
5. An implantable tubular stent graft comprising: (a) an ePTFE
material defined by nodes and fibrils, said fibrils being in
longitudinally compressed state and defining pores of a size
sufficient to permit penetration of an elastomeric material; (b) an
elastomeric matrix within said pores; said compressed fibrils and
elastomeric matrix cooperatively permitting longitudinal expansion
and elastomeric recovery without plastic deformation of said ePTFE
material, wherein said elastomeric matrix is a polycarbonate
urethane material having a shore hardness rating between 70A and 75
D; and (c) a longitudinally expandable stent.
6. A method of producing an elastomerically recoverable PTFE
structure comprising the steps of: (a) providing an ePTFE material
defined by nodes, fibrils and pores, wherein said pore size is a
space defined by the distance between said nodes and distance
between said fibrils; (b) providing an elastomeric material, said
elastomeric matrial is a polycarbonate urethane material having a
shore hardness rating between 70A and 75 D; (c) compressing said
fibrils longitudinally wherein said pore size is sufficient to
permit penetration of said elastomeric material; and (d) applying
said elastomeric material within said pores to provide a
structurally integral elastomerically recoverable PTFE
material.
7. The method according to claim 6 further comprising the step of
permitting the elastomeric material to dry within said pores while
said fibrils are still longitudinally compressed defining the
elastomeric matrix.
8. The method according to claim 6 wherein said ePTFE material is a
tube, having an internal diameter and an external diameter.
9. The method according to claim 8 wherein said compressing step
includes the steps of: (a) pulling the ePTFE tube over a mandrel
having an outer diameter of approximately the same dimensions as
the internal diameter of the ePTFE tube; and (b) compressing at
least a portion of the ePTFE tube along the longitudinal axis of
the tube while the tube is supported by the mandrel.
10. The method according to claim 6 wherein said applying step
includes the step of dip coating at least the compressed portion of
the ePTFE material into a container of elastomeric material.
11. The method according to claim 6 wherein said applying step
includes the step of spray coating at least the compressed portion
of the ePTFE material with the elastomeric material.
12. The method according to claim 6 wherein said applying step
includes the step of brushing the elastomeric material onto at
least the compressed portion of the ePTFE material.
13. The method according to claim 6 wherein said compressing step
includes the step of compressing the ePTFE material uniformly along
its entire length, and wherein said applying step includes the
applying elastomeric material over the entire longitudinally
compressed ePTFE material.
14-21. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional patent
application Ser. No. 10/179,484, filed on Jun. 25, 2002, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to an
elastomerically recoverable PTFE which is made from expanded,
porous polytetrafluoroethylene (ePTFE) and impregnated with
elastomer to be longitudinally compliant for allowing at least a
portion of the ePTFE structure to stretch and recover along the
longitudinal axis thereof.
BACKGROUND OF RELATED TECHNOLOGY
[0003] It is well known in the art that polymers, such as
polytetrafluoroethylene (PTFE), are used to form a prosthesis. A
tubular graft may be formed by stretching and expanding PTFE into a
structure referred to as expanded polytetrafluoroethylene (ePTFE).
Tubes formed of ePTFE exhibit certain beneficial properties as
compared with textile prostheses. The expanded PTFE tube has a
unique structure defined by nodes interconnected by fibrils. The
node and fibril structure defines pores that facilitate a desired
degree of tissue ingrowth while remaining substantially
fluid-tight. Tubes of ePTFE may be formed to be exceptionally thin
and yet exhibit the requisite strength necessary to serve in the
repair or replacement of a body lumen. The thinness of the ePTFE
tube facilitates ease of implantation and deployment with minimal
adverse impact on the body.
[0004] While exhibiting certain superior attributes, ePTFE material
is not without certain disadvantages. One disadvantage is the
porosity of the ePTFE structure which permits cellular ingrowth.
The ingrowth is undesirable if one uses the ePTFE material as a
temporary graft to bridge vessels and it is desired to have clear
access to the ePTFE graft for replacement or removal.
[0005] U.S. Pat. No. 5,665,114 to Weadock et al. discloses an
implantable prosthesis made of ePTFE wherein the pores are filled
with an in situ cross-linkable biocompatible and biodegradable
material. The bio-material may be applied to the ePTFE using force
to fill the pores with a dispersion or solution of the biomaterial,
which is subsequently insolubilized therein.
[0006] U.S. Pat. No. 5,152,782 to Kowligi et al. discloses a
non-porous elastomeric coating on a PTFE graft. The elastomeric
coating is made of polyurethanes or silicone rubber elastomers. The
elastomeric coating is applied to the graft by radially expanding
the PTFE graft, and dipping or spraying the graft with the
elastomeric coating. The radial expansion is controlled to ensure
that the polymer coating penetration is restricted to the outer
layers of the PTFE tube.
[0007] Another disadvantage is the ePTFE material has a tendency to
leak blood at suture holes and often propagate a tear line at the
point of entry of the suture. The suture holes in ePTFE do not
self-seal due to the inelasticity of ePTFE material. As a result,
numerous methods of orienting the node and fibril structure have
been developed to prevent tear propagation. These processes are
often complicated and require special machinery and/or materials to
achieve this end. Prior art suggests encapsilling the ePTFE
material with a liquid elastomer layer, the elastomer fills in and
seals that suture hole.
[0008] U.S. Pat. No. 5,192,310 to Herweck et al. discloses a
self-sealing PTFE or ePTFE vascular graft having a primary and
secondary lumen. The primary lumen is to accommodate blood flow.
The secondary lumen shares the outer wall as a common wall with the
primary lumen. The secondary lumen is filled with a
non-biodegradable elastomer material, such as silicone rubber,
polyurethane, polyethers or fluoropolymers.
[0009] U.S. Pat. No. 5,904,967 to Ezaki et al. discloses a puncture
resistant bio-compatible medical material for use on as grafts or
artificial blood vessels. The puncture resistant material is
sandwiched between two porous layers of the graft. The porous
layers may be made of polyester resin or polyethylene
terephthalate/polybuthylene terephthalate. The puncture resistant
layer may be a styrene and/or olefin elastomer or isoprene
derivatives. The layers are bonded by an adhesive or by fusion with
heat.
[0010] Another problem is that ePTFE material exhibits a relatively
low degree of longitudinal compliance. Expanded PTFE is generally
regarded as an inelastic material. It has little memory and
stretching results in deformation. In those instances when a
surgeon will misjudge the length of the graft that is required to
reach between the selected artery and vein, the surgeon may find
that the graft is too short to reach the targeted site once the
graft has been tunneled under the skin. Expanded PTFE vascular
grafts typically exhibit minimal longitudinal compliance, and hence
the graft does not stretch significantly along its longitudinal
axis. Accordingly, in such cases, the surgeon must then remove the
tunneled graft from below the skin and repeat the tunneling
procedure with a longer graft.
[0011] Elasticity of an ePTFE vascular graft is important when used
for bypass implants such as an axillofemoral bypass graft, wherein
the vascular graft extends between the femoral artery in the upper
leg to the axillary artery in the shoulder, as well as a
femoropopliteal bypass graft extending below the knee. Such bypass
grafts often place restrictions upon the freedom of movement of the
patient in order to avoid pulling the graft loose from its anchor
points. For example, in the case of the axillofemoral bypass graft,
sudden or extreme movements of the arm or shoulder must be entirely
avoided. Similarly, in the case of the femoropopliteal bypass
graft, bending the knee can place dangerous stress upon the graft.
The above-described restricted movement is due largely to the
inability of the ePTFE vascular graft to stretch along its
longitudinal axis when its associated anchor points are pulled
apart from one another. Such restrictive movement is especially
important during the early period of healing following implantation
when there is still little tissue incorporation into the graft and
it can move within the subcutaneous tunnel.
[0012] It is desirable to incorporate elastomeric properties into
the PTFE. This incorporation is difficult because PTFE is a
hydrophobic material making it difficult to wet with the
hydro-based elastomers, and the elastomers are hydrophilic making
them naturally attracted to other elastomeric molecules. When
elastomeric material is applied to PTFE the two materials repel
each other and the elastomer flow away from the nodes to a less
hydrophobic area, the pores. The pores between the fibrils, are too
small for the elastomeric material to penetrate. Thus, the
elastomer remains on the surface of the fibrils and coat the
exterior of PTFE.
[0013] Prior art suggests surface coating the ePTFE material with
elastomer by dipping, spraying, or adhesive bonding. One
disadvantage is that the coating may flake or separate from the
ePTFE material, as well as add to the thickness of the ePTFE
material.
[0014] U.S. Pat. No. 4,304,010 to Mano discloses a tubular
prosthesis which is made of PTFE with a porous elastomeric coating
on the outer surface. The elastomeric coating, which may be
cross-linked, is described as being fluorine rubber, silicone
rubber, urethane rubber, acrylic rubber or natural rubber, and may
be applied to the PTFE prosthesis by wrapping, dipping, spraying or
use of negative pressure.
[0015] U.S. Pat. No. 5,026,591 to Henn et al. discloses a coating
product which contains a substrate and scaffolding, such as PTFE or
ePTFE, where the pores are filled with a thermoplastic or
thermosetting resin. The substrate may be of a diverse selection;
i.e., woven, non-woven, fabric, paper, or porous polymer.
Application of the resin to the PTFE substrate uses rollers to
provide a controlled even coating.
[0016] U.S. Pat. No. 5,653,747 to Dereume discloses a stent to
which a graft is attached. The graft component is produced by
extruding polymer in solution into fibers from a spinnerette onto a
rotating mandrel. A stent may be placed over the fibers while on
the mandrel and then an additional layer of fibers spun onto the
stent. The layer of layers of fibers may be bonded to the stent
and/or one another by heat or by adhesives. The porous coating may
be made from a polyurethane or polycarbonate urethane which may be
bonded by heat or by adhesion to the support.
[0017] U.S. Pat. No. 4,321,711 to Mano discloses a vascular
prosthesis of PTFE with an anti-coagulant coating and bonded to its
outer surface a porous elastomer coating containing a coagulant.
The elastomer is used in its crosslinked state and is made of
fluorine rubber, silicone rubber, urethane rubber, acrylic rubber
or natural rubber. The elastomeric coating is bonded to the PTFE by
dipping, spraying and/or applying negative pressure to inside wall
PTFE to pass elastomer through the wall.
[0018] U.S. Pat. No. 4,955,899 to Della Conna et al. discloses a
longitudinally compliant PTFE graft. The PTFE tube is
longitudinally compressed and the outer wall of the PTFE is coated
with a biocompatible material, such as polyurethanes or silicone
rubber elastomers. The coating is applied by compressing the PTFE
tube on a mandrel, and dipping or spraying the PTFE with the
elastomer. The elastomer coating is restricted to the outer layers
of the PTFE tube. The elastomer coated PTFE is dried while in the
compressed state.
[0019] Other prior art suggests bonding a separate layer of
elastomer to the ePTFE material to enhance the elasticity. One
disadvantage is the added thickness of the PTFE. Another
disadvantage, as stated above with the elastomer coatings, is the
layers will separate over time and can flake off the PTFE. Examples
of bonding layers of elastomer to PTFE is discussed below.
[0020] U.S. Pat. No. 4,816,339 to Tu et al. discloses a
bio-compatible material made from layers of PTFE and hydrophobic
PTFE fibers coated with an elastomer mixture. The bio-compatible
material disclosed is a PTFE layer, elastomer/PTFE mixed layer,
elastomer layer and hydrophilic monomer fibrous elastomer matrix
layer. The elastomer layer is made from polyurethane. The elastomer
is applied to the combined PTFE layer by heating and radially
expanding the combined PTFE layers and dipping or spraying the
combined PTFE layers with elastomer.
[0021] U.S. Pat. No. 5,628,782 to Myers et al. discloses a
biocompatible base material such as PTFE or ePTFE with an outer
deflectably secured outer covering. The preferably outer covering
is non-elastic porous film or fibers, preferable PTFE. The outer
covering is secured to the base by use of an adhesive.
[0022] U.S. Pat. No. 6,156,064 to Chournard discloses a braided
self-expandable stent-graft-membrane. This three layer invention
has an interior graft layer which is braided PET, PCV or PU fibers;
a middle layer which is the stent; and an exterior membrane layer
which is a silicone or polycarbonate methane. The membrane layer is
applied to the exterior of the stent layer by dipping, by braiding,
by spraying or by fusing; which includes use of adhesive, solvent
bonding or thermal and/or pressure bonding.
[0023] It is desirable to provide an ePTFE material that achieves
many of the above-stated benefits without the resultant
disadvantages associated therewith and disadvantages of similar
conventional products. It is also desirable to make this
elastomerically recoverable PTFE material available to be
manufactured in a variety of used such as an implantable
prosthesis, patch material, graft, or stent.
SUMMARY OF THE INVENTION
[0024] The present invention provides an elastomerically
recoverable PTFE material that was made of ePTFE material, defined
by nodes and fibrils, and an elastomeric matrix. The fibrils were
longitudinally compressed, defining a pore size that is a
sufficient size to permit penetration of an elastomeric material.
The elastomeric material penetrated the pores, which defined the
elastomeric matrix within the pores. The compressed fibrils and
elastomeric matrix cooperatively permitted longitudinal expansion
and elastomeric recovery without plastic deformation of the ePTFE
material.
[0025] The elastomerically recoverable PTFE material is used for a
variety of applications, such as implantable prosthesis. This
includes vascular prosthesis such as patches, grafts or implantable
tubular stent graft with a longitudinally expandable stent.
[0026] Another aspect of this invention was to provide a method of
producing an elastomerically recoverable PTFE material. The steps
to produce this material are discussed below.
[0027] First step was to provide the ePTFE material defined by the
nodes, fibrils and pores with the required dimensions and
specifications to produce the desired end product. The ePTFE
material varied in their dimensions and specifications such as an
ePTFE tube defined by an internal diameter and an external
diameter.
[0028] Next, the fibrils were compressed longitudinally, the pore
size was sufficiently large enough to permit the elastomeric
material to penetrate the pores. The compression step was performed
by a variety of techniques. For example, the ePTFE tube was pulled
over a mandrel with an outer diameter approximately the same size
as the internal diameter of the ePTFE tube. The entire ePTFE tube
or at least a portion of the ePTFE tube was compressed along the
longitudinal axis of the tube while the tube was supported by the
mandrel. The ePTFE tube was compressed uniformly along the entire
length or any portion thereof of the ePTFE material.
[0029] Then the elastomeric material was applied within the pores
to provide a structurally integral elastomerically recoverable PTFE
material. Elastomer was applied by a variety of techniques, such as
dipping, spraying or brushing techniques. The elastomeric material
was applied over the entire longitudinally compressed ePTFE
material or to any portion thereof.
[0030] One advantage of this method was that the compression step
and the application steps were interchangeable to produce an
elastomerically recoverable PTFE material with various properties,
such as different expansion ratios. The various properties of the
final product were produced by performing the compression step
prior to, between, and/or after the application of the elastomeric
material.
[0031] Finally, the elastomeric material was dried within the pores
while the fibrils were still longitudinally compressed which
defined the elastomeric matrix. An advantage of this method was
that the drying step was performed between applications of the
elastomeric material or after completion of all the elastomeric
material application depending on the desired end product use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a schematic representation of the microstructure
of ePTFE material defined by nodes 1, fibrils 2 and pores 3.
[0033] FIG. 2 and FIG. 3 are schematic representations of the
microstructure of longitudinally compressed ePTFE material defined
by nodes 1, longitudinally compressed fibrils 2 and pores 3.
[0034] FIG. 4 is a schematic representation of the microstructure
of the ePTFE material of the present invention, defined by nodes 1,
longitudinally compressed fibrils 2 and elastomeric matrix 4 within
the pores 3.
[0035] FIG. 5 is a schematic representation of the ePTFE materials
of the present invention formed into an implantable tubular graft 5
defined by the microstructure having nodes 1, longitudinally
compressed fibrils 2 and elastomeric matrix 4 within the pores
3.
[0036] FIG. 6 is a schematic representation of the ePTFE material
of the present invention formed into a patch 6 defined by the
microstructure having nodes 1, longitudinally compressed fibrils 2
and elastomeric matrix 4 within the pores 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] The invention described herein provides an elastomerically
recoverable PTFE material which has a combination of high stretch
and elastomeric compression, high flexibility, and high strength
without deformation of the material. In this regard it not only
exceeds previously available PTFE products, but is unique among
elastomer coated plastic materials.
Expanded PTFE
[0038] The precursor for this invention is porous ePTFE which is
well known in the art and is described in detail, for example, in
U.S. Pat. Nos. 3,953,566 and 3,962,153, which is incorporated
herein by reference as shown in FIG. 1. Generally, paste-forming
techniques are used to convert the polymer in paste form to a
shaped article which is then expanded, after removing the
lubricant, by stretching it in one or more directions; and while it
is held in its stretched condition it is heated to at least
348.degree. C. after which it is cooled. The porosity that is
produced by the expansion is retained for there is little or no
coalescence or shrinking upon releasing the cooled, final
article.
[0039] Paste-forming of dispersion polymerized
poly(tetrafluoroethylene) is well known commercially. Extrusions of
various cross-sectional shapes such as tubes, rods and tapes are
commonly obtained from a variety of tetrafluoroethylene resins, and
other paste-forming operations such as calendering and molding are
practiced commercially. The steps in paste-forming processes
include mixing the resin with a lubricant such as odorless mineral
spirits and carrying out forming steps in which the resin is
subjected to shear, thus making the shaped articles cohesive. The
lubricant is removed from the extruded shape usually by drying.
[0040] The paste-formed, dried, unsintered shapes are expanded by
stretching them in one or more directions under certain conditions
so that they become substantially much more porous and stronger.
Expansion increases the strength of PTFE resin within preferred
ranges of rate of stretching and preferred ranges of temperature.
It has been found that techniques for increasing the crystallinity,
such as annealing at high temperatures just below the melt point,
improve the performance of the resin in the expansion process.
[0041] The porous microstructure of the ePTFE material is affected
by the temperature and the rate at which it is expanded. The
structure consists of nodes 1 interconnected by very small fibrils
2. In the case of uniaxial expansion the nodes 1 are elongated, the
longer axis of a mode being oriented perpendicular to the direction
of expansion. The fibrils 2 which interconnected the nodes 1 are
oriented parallel to the direction of expansion. These fibrils 2
appear to be characteristically wide and thin in cross-section, the
maximum width being equal to about 0.1 micron (1000 angstroms)
which is the diameter of the crystalline particles. The minimum
width may be one or two molecular diameters or in the range of 5 or
10 angstroms. The nodes 1 may vary in size from about 400 microns
to less than a micron, depending on the conditions used in the
expansion. Products which have expanded at high temperatures and
high rates have a more homogeneous structure, i.e., they have
smaller, more closely spaced nodes 1 and these nodes 1 are
interconnected with a greater number of fibrils 2.
[0042] When the ePTFE material is heated to above the lowest
crystalline melting point of the poly(tetrafluoroethylene),
disorder begins to occur in the geometric order of the crystallites
and the crystallinity decreases, with concomitant increase in the
amorphous content of the polymer, typically to 10% or more. These
amorphous regions within the crystalline structure appear to
greatly inhibit slippage along the crystalline axis of the
crystallite and appear to lock fibrils and crystallites so that
they resist slippage under stress. Therefore, the heat treatment
may be considered an amorphous locking process. The important
aspect of amorphous locking is that there be an increase in
amorphous content, regardless of the crystallinity of the starting
resins. Whatever the explanation, the heat treatment above
348.degree. C. causes a surprising increase in strength, often
doubling that of the unheated-treated material.
[0043] The preferred thickness of ePTFE material ranges from 0.025
millimeter to 2.0 millimeters; the preferred internodal distance
within such ePTFE material ranges from 20 micrometers to 200
micrometers. The longitudinal tensile strength of such ePTFE
material is preferably equal to or greater than 1,500 psi, and the
radial tensile strength of such ePTFE material is preferably equal
to or greater than 400 psi.
Elastomeric Material
[0044] The elastomeric material of this invention was biocompatible
elastomer such as polyurethanes, adhesive solutions and elastomeric
adhesive solutions. Suitable candidates for use as an elastomer
typically have a Shore hardness rating between 70A and 75D. Most of
the above-mentioned elastomers can be chemically or biologically
modified to improve biocompatability; such modified compounds are
also candidates for use in forming elastomeric material
impregnation.
[0045] Apart from biocompatability, other requirements of an
elastomer to be a suitable candidate for use as elastomeric
material impregnation are that the elastomer be sufficiently
elastic to maintain compressed portions of ePTFE material in the
compressed condition when it is not being stretched. The elastomer
should also be sufficiently elastic to effect closure of suture
holes formed by a suture needle. The amount of elastomeric material
needed is the amount to impregnate the ePTFE and provide the
desired elasticity for the end product use, without supersaturating
the ePTFE and creating an exterior outer coatings of elastomer on
the ePTFE. Yet another requirement of such elastomers is that they
be easily dissolvable in low boiling point organic solvents such as
tetrahydrofuran, methylene chloride, trichloromethane, dioxane,
dimethylformamide, and dimethylacetamide (DMAc) by way of example.
Finally, suitable elastomeric materials should lend themselves to
application by either the dip, brush or spray coating methods
described below.
[0046] The term elastomeric as used herein refers to a substance
having the characteristic that it tends to resume an original shape
after any deformation thereto, such as stretching, expanding or
compression. It also refers to a substance which has a non-rigid
structure, or flexible characteristics in that it is not brittle,
but rather has compliant characteristics contributing to its
non-rigid nature.
[0047] The polycarbonate urethane polymers particularly useful in
the present invention are more fully described in U.S. Pat. Nos.
5,133,742 and 5,229,431 which are incorporated in their entirety
herein by reference. These polymers are particularly resistant to
degradation in the body over time and exhibit exceptional
resistance to cracking in vivo. These polymers are segmented
polyurethanes which employ a combination of hard and soft segments
to achieve their durability, biostability, flexibility and
elastomeric properties.
[0048] The polycarbonate urethanes useful in the present invention
are prepared from the reaction of an aliphatic or aromatic
polycarbonate macroglycol and a diisocyanate n the presence of a
chain extender. Aliphatic polycarbonate macroglycols such as
polyhexane carbonate macroglycols and aromatic diisocyanates such
as methylene diisocyanate are most desired due to the increased
biostability, higher intramolecular bond strength, better heat
stability and flex fatigue life, as compared to other
materials.
[0049] The polycarbonate urethanes particularly useful in the
present invention are the reaction products of a macroglycol, a
diisocyanate and a chain extender.
[0050] A polycarbonate component is characterized by repeating
##STR1##
[0051] units, and a general formula for a polycarbonate macroglycol
is as follows: ##STR2##
[0052] wherein x is from 2 to 35, y is 0, 1 or 2, R either is
cycloaliphatic, aromatic or aliphatic having from about 4 to about
40 carbon atoms or is alkoxy having from about 2 to about 20 carbon
atoms, and wherein R' has from about 2 to about 4 linear carbon
atoms with or without additional pendant carbon groups.
[0053] Examples of typical aromatic polycarbonate macroglycols
include those derived from phosgene and bisphenol A or by ester
exchange between bisphenol A and diphenyl carbonate such as
(4,4'-dihydroxy-diphenyl-2,2'-propane) shown below, wherein n is
between about 1 and about 12. ##STR3## Typical aliphatic
polycarbonates are formed by reacting cycloaliphatic or aliphatic
diols with alkylene carbonates as shown by the general reaction
below: ##STR4##
[0054] wherein R is cyclic or linear and has between about 1 and
about 40 carbon atoms and wherein R.sup.1 is linear and has between
about 1 and about 4 carbon atoms.
[0055] Typical examples of aliphatic polycarbonate diols include
the reaction products of 1,6-hexanediol with ethylene carbonate,
1,4-butanediol with propylene carbonate, 1,5-pentanediol with
ethylene carbonate, cyclohexanedimethanol with ethylene carbonate
and the like and mixtures of above such as diethyleneglycol and
cyclohexanedimethanol with ethylene carbonate.
[0056] When desired, polycarbonates such as these can be
copolymerized with components such as hindered polyesters, for
example phthalic acid, in order to form carbonate/ester copolymer
macroglycols. Copolymers formed in this manner can be entirely
aliphatic, entirely aromatic, or mixed aliphatic and aromatic. The
polycarbonate macroglycols typically have a molecular weight of
between about 200 and about 4000 Daltons.
[0057] Diisocyanate reactants according to this invention have the
general structure OCN--R'--NCO, wherein R' is a hydrocarbon that
may include aromatic or nonaromatic structures, including aliphatic
and cycloaliphatic structures. Exemplary isocyanates include the
preferred methylene diisocyanate (MDI), or 4,4-methylene bisphenyl
isocyanate, or 4,4'-diphenylmethane diisocyanate and hydrogenated
methylene diisocyanate (HMDI). Other exemplary isocyanates include
hexamethylene diisocyanate and other toluene diisocyanates such as
2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4'
tolidine diisocyanate, m-phenylene diisocyanate,
4-chloro-1,3-phenylene diisocyanate, 4,4-tetramethylene
diisocyanate, 1,6-hexamethylene diisocyanate, 1,10-decamethylene
diisocyanate, 1,4-cyclohexylene diisocyanate, 4,4'-methylene bis
(cyclohexylisocyanate), 1,4-isophorone diisocyanate,
3,3'-dimethyl-4,4'-diphenylmethane diisocyanate,
1,5-tetrahydronaphthalene diisocyanate, and mixtures of such
diisocyanates. Also included among the isocyanates applicable to
this invention are specialty isocyanates containing sulfonated
groups for improved hemocompatibility and the like.
[0058] Suitable chain extenders included in this polymerization of
the polycarbonate urethanes should have a functionality that is
equal to or greater than two. A preferred and well-recognized chain
extender is 1,4-butanediol. Generally speaking, most diols or
diamines are suitable, including the ethylenediols, the
propylenediols, ethylenediamine, 1,4-butanediamine methylene
dianiline heteromolecules such as ethanolamine, reaction products
of said diisocyanates with water and combinations of the above.
[0059] The polycarbonate urethane polymers according to the present
invention should be substantially devoid of any significant ether
linkages (i.e., when y is 0, 1 or 2 as represented in the general
formula hereinabove for a polycarbonate macroglycol), and it is
believed that ether linkages should not be present at levels in
excess of impurity or side reaction concentrations. While not
wishing to be bound by any specific theory, it is presently
believed that ether linkages account for much of the degradation
that is experienced by polymers not in accordance with the present
invention due to enzymes that are typically encountered in vivo, or
otherwise, attack the ether linkage via oxidation. Live cells
probably catalyze degradation of polymers containing linkages. The
polycarbonate urethanes useful in the present invention avoid this
problem.
[0060] Because minimal quantities of ether linkages are unavoidable
in the polycarbonate producing reaction, and because these ether
linkages are suspect in the biodegradation of polyurethanes, the
quantity of macroglycol should be minimized to thereby reduce the
number of ether linkages in the polycarbonate urethane. In order to
maintain the total number of equivalents of hydroxyl terminal
groups approximately equal to the total number of equivalents of
isocyanate terminal groups, minimizing the polycarbonate soft
segment necessitates proportionally increasing the chain extender
hard segment in the three component polyurethane system. Therefore,
the ratio of equivalents of chain extender to macroglycol should be
as high as possible. A consequence of increasing this ratio (i.e.,
increasing the amount of chain extender with respect to
macroglycol) is an increase in hardness of the polyurethane.
Typically, polycarbonate urethanes of hardnesses, measured on the
Shore scale, less than 70A show small amounts of biodegradation.
Polycarbonate urethanes of Shore 75A and greater show virtually no
biodegradation.
[0061] The ratio of equivalents of chain extender to polycarbonate
and the resultant hardness is a complex function that includes the
chemical nature of the components of the urethane system and their
relative proportions. However, in general, the hardness is a
function of the molecular weight of both chain extender segment and
polycarbonate segment and the ratio of equivalents thereof.
Typically, the 4,4'-methylene bisphenyl diisocyanate (MDI) based
systems, a 1,4-butanediol chain extender of molecular weight 90 and
a polycarbonate urethane of molecular weight of approximately 2000
will require a ratio of equivalents of at least about 1.5 to 1 and
no greater than about 12 to 1 to provide non-biodegrading polymers.
Preferably, the ratio should be at least about 2 to 1 and less than
about 6 to 1. For a similar system using a polycarbonate glycol
segment of molecular weight of about 1000, the preferred ration
should be at least about 1 to 1 and no greater than about 3 to 1. A
polycarbonate glycol having a molecular weight of about 500 would
require a ratio in the range of about 1.2 to about 1.5:1.
[0062] The lower range of the preferred ratio of chain extender to
macroglycol typically yields polyurethanes of Shore 80A hardness.
The upper range of ratios typically yields polycarbonate urethanes
on the order of Shore 75D. The preferred elastomeric and biostable
polycarbonate urethanes for most medical devices would have a Shore
hardness of approximately 85A.
[0063] Generally speaking, it is desirable to control somewhat the
cross-linking that occurs during polymerization of the
polycarbonate urethane polymer. A polymerized molecular weight of
between about 80,000 and about 200,000 Daltons, for example on the
order of about 120,000 Daltons (such molecular weights being
determined by measurement according to the polystyrene standard),
is desired so that the resultant polymer will have a viscosity at a
solids content of 43% of between about 900,000 and about 1,800,000
centipoise, typically on the order of about 1,000,000 centipoise.
Cross-linking can be controlled by avoiding an isocyanate-rich
situation. Of course, the general relationship between the
isocyanate groups and the total hydroxyl (and/or amine) groups of
the reactants should be on the order of approximately 1 to 1.
Cross-linking can be controlled by controlling the reaction
temperatures and shading the molar ratios in a direction to be
certain that the reactant charge is not isocyanate-rich;
alternatively a termination reactant such as ethanol can be
included in order to block excess isocyanate groups which could
result in cross-linking which is greater than desired.
[0064] Concerning the preparation of the polycarbonate urethane
polymers, they can be reacted in a single-stage reactant charge, or
they can be reacted in multiple states, preferably in two stages,
with or without a catalyst and heat. Other components such as
antioxidants, extrusion agents and the like can be included,
although typically there would be a tendency and preference to
exclude such additional components when a medical-grade polymer is
being prepared.
[0065] Additionally, the polycarbonate urethane polymers can be
polymerized in suitable solvents, typically polar organic solvents
in order to ensure a complete and homogeneous reaction. Solvents
include dimethylacetamide, dimethylformamide, dimethylsulfoxide
toluene, xylene, m-pyrrol, tetrahydrofuran, cyclohexanone,
2-pyrrolidone, and the like, or combinations thereof. These
solvents can also be used to delivery the polymers to the ePTFE
layer of the present invention.
[0066] A particularly desirable polycarbonate urethane is the
reaction product of polyhexamethylenecarbonate diol, with methylene
bisphenyl diisocyanate and the chain extender 1,4-butanediol.
[0067] The solvents used in the present invention must be capable
of wetting the membrane surface and penetrating the pores. In the
case of ePTFE membranes, wettability of the surface is difficult to
accomplish due to the surface tension properties of the
fluoropolymeric structure. Many solvents will not readily wet the
surface of ePTFE sufficiently to penetrate the pores. Thus, the
choice of elastomeric material and solvent must be made with these
properties in mind. The elastomeric material must be sufficiently
dissolvable or softened at the interface to flow and penetrate into
the membrane pores.
[0068] Progressive wetting of the membrane permits the elastomer to
enter the pores of the ePTFE material and thus contribute to
achieving the advantages of enhanced stretch and recoverability of
the present invention. Membranes formed of a hydrophobic material
such as ePTFE are difficult to wet. The type of solvent employed
must be both capable of dissolving the elastomeric material and of
wetting the surface of the membrane. Suitable solvent materials,
which have been found to be useful with polyurethane elastomeric
materials and ePTFE membranes include, without limitation,
dimethylacetamide, tetrahydrofuran, ethers, methylene chloride,
chloroform, toluene and mixtures thereof. The mixture of solvent
and elastomeric material provides a balance of wetting and solvent
properties which are particularly effective at causing penetration
and entrapment of the elastomeric material within the pores of the
ePTFE.
[0069] Other solvents may be employed provided they are capable of
wetting the membrane, i.e., ePTFE surface, i.e., reducing surface
tension such that the elastomeric material will flow into the
porous microstructure, and are capable of sufficiently dissolving
the elastomeric material to cause flow and penetration into the
membrane. The solvents chosen should have little or no effect on
the membrane and serve only as a means to infiltrate the
microstructure and carry the elastomeric material therewith. The
solvents are then removed by evaporation and the elastomeric
material is permitted to dry and resolidify within the porous
structure.
[0070] Examples of a suitable elastomeric material were sold under
the trademark name "BIONATE" by Polymer Technology Group of
Berkley, Calif. and "CORETHANE" by Corvita Corporation of Miami,
Fla. Such elastomeric materials were designed to be dissolved in
various solvents for use in solution casting, extruding or for
coating of medical products. The polycarbonate urethane was
dissolved in the solvent known as DMAc.
[0071] The method of formulating the liquefied elastomeric material
was the same, as known in the art. This solution was prepared by
dissolving polyurethane pellets in the above-described DMAc solvent
in a heated glass reactor equipped with a cold water condenser held
at 60 C. Such polyurethane pellets may also be dissolved in the
solvent at room temperature through continuous stirring. The use of
the heated reactor was preferred, as it dissolved the polyurethane
pellets in a few hours, whereas the method of stirring the solution
at room temperature took approximately two days.
[0072] The preferred solids content for "Corethane 2.5W30" was 7.5%
by weight; however, the solids content have ranged up to 15% by
weight, depending upon the specific polymer composition, the dip,
brush, spray technique parameters, and the intended end uses.
Various grades of Corethane solution are useful depending on the
intended end use. Where multiple applications were employed, the
composition of the elastomeric material were varied between the
applied layers.
Method of Producing Elastomerically Expandable EPTFE Material
[0073] In practicing the preferred method, the ePTFE starting
material is initially in the form of a cylindrical tube having an
inside diameter up to 50 millimeters. The length may vary depending
on the intended end use.
(i) Longitudinal Compression Process
[0074] The compression process of the ePTFE took place either prior
to, during or after the applications of the elastomeric material.
The porous wall structure of the ePTFE easily allowed compression
back to the starting length of the unexpanded PTFE tube structure
before it was expanded during the manufacturing process. The
fibrils 2 of the uncompressed ePTFE were longitudinally compressed
which change the shape of the pore 3, and elastomer is permitted to
penetrate into the polymeric matrix of the ePTFE. Nodes 1 that are
forced closer together to allow the fibrils 2 to bow out or
wrinkle, which may increase the distance between the fibrils 2, and
change the pore 3 shape. The pore 3 which permit the elastomer to
enter into this space defines the sufficient size of the pore. This
sufficient size of the pore 3 is the range between the starting
state and the overly compressed state. The starting state is where
the node 1 is farthest apart from the opposite node 1 and the
connecting fibrils 2 are taut between the two nodes 1. The overly
compressed state is where the opposite nodes 1 are pushed together,
creating two smaller pores 3, and the fibrils 2 are crushed as
well. In this case the pore 3 space is too small to permit the
elastomer to enter within this space. One way to compress the ePTFE
was to pull the ePTFE tube over a cylindrical supporting mandrel
with an outer diameter that about equal to the internal diameter of
the ePTFE tube. The ePTFE was compressed along the longitudinal
axis of the ePTFE. The compression procedure was accomplished by
mechanical or thermal procedures. The mechanical procedure included
manually squeezing the ePTFE from both of its ends until a
predetermined final compressed length is reached. The thermal
procedure included evenly heating the portion of the ePTFE that is
desired to be compressed. The compression step included uniformly
compressing the PTFE tube along its entire length to produce a tube
that stretched along its entire length up to 90% compression, or
localized compression to satisfy the intended end use. The percent
compression is defined as the ratio of the final compressed length
to the initial uncompressed length. The desired percent compression
depends upon the ePTFE manufactured expansion ratios, and depending
upon the intended use of the final product. Visually the compressed
ePTFE material appears to be denser because the internodal distance
has been decreased, as a result of the nodes 1 being forced closer
together. FIGS. 2 and 3 show the decreased distance between the
nodes 1 limits the space available for the fibrils 2, resulting in
the fibrils 2 crinkling, wrinkling or possibly folding. Visually
the compressed ePTFE material appears to be wrinkled, crinkled or
folded, the larger the compression ratio the more wrinkling was
seen.
[0075] Once the whole or any section of the ePTFE material has been
uniformly compressed along its length, as shown in FIGS. 2 and 3,
it is secured on the mandrel by mechanical means such as Teflon
tape or clamps about the ends of the compressed ePTFE tube.
(ii) Application of Elastomeric Material Process
[0076] Once the pore 3 is a sufficient size to permit penetration
of the elastomeric material, the elastomer may be applied. The
elastomeric material flowed into the pores 3 between the fibrils 2,
the point of least repelling force, to escape the hydrophobic
forces from the ePTFE material, nodes 1 and fibrils 2. The
elastomeric material became entrapped between the pores 3 and
embedded the internal fibrils 2 and nodes 1. The embedded fibrils 2
and nodes 1 acted as an internal structural support for the
elastomeric webbing or matrix 4. As mentioned above, the
longitudinal compression process was performed before, during or
after the applications of the elastomeric material depending on the
desired properties for the end-use product. The application process
entailed initially dissolving the elastomer in a suitable solution
as discussed above, defining the elastomeric material. The
elastomeric material was then applied to the compressed and
uncompressed ePTFE by various techniques including dip coating,
brushing, spraying and the like
[0077] For example, the elastomeric material was applied to the
ePTFE by the method of dip coating which is known in the art by use
of a dip coating machine. Attention must be placed on the
parameters of the machine and length of the ePTFE to prevent an
uneven application. The dip coating machine method of application
consisted of the mandrel extended vertically downward from a motor
which continuously rotated the mandrel and ePTFE material secured
thereto (compressed or uncompressed ePTFE). Motor is, in turn,
supported by a bracket adapted to travel vertically upward and
downward. Bracket included a smooth bushing through which a smooth
vertical support rod passes. Bushing was adapted to slide upwardly
and downwardly along support rod. Bracket further included a
threaded collar through which a threaded rotatable drive rod
passes. The lowermost end of drive rod is secured to the drive
shaft of a second motor which rotated in a first rotational
direction to raise mandrel and which rotated in an opposing
rotational direction to lower mandrel. Both motor and support rod
were supported at their lower ends by a base. The upper end of
support rod was fixedly secured to bracket which rotatably
supported the upper end of drive rod. Motor of dip coating machine
was initially operated to raise mandrel to its uppermost position.
A tall, slender container containing the above-described solution
was placed upon base immediately below mandrel. Motor was then
operated in the reverse rotational direction to lower mandrel, and
ePTFE material section secured thereto, into the solution. The
variables controlled by dip coating machine include the speed at
which mandrel was immersed and withdrawn and the rotational speed
of mandrel. These parameters were controlled to ensure that the
solution penetrates the ePTFE to allow for the impregnation of the
elastomeric material.
[0078] Another example of applying the elastomeric material to the
ePTFE material involved the use of spraying which was preformed by
a spray coating machine. The elastomeric material to be sprayed is
first prepared in the same manner as described above for the dip
coating process. The elastomeric material was inserted within
cylinder of a pump for delivery through a plastic tube to a spray
nozzle. An inert gas, such as nitrogen, was also supplied to spray
nozzle through connecting tube from supply tank. An inert gas was
preferably used to minimize reactions which elastomeric material
can undergo upon exposure to air and oxygen. The mandrel with the
ePTFE was supported for rotation about a horizontal axis. One end
of mandrel was coupled to the drive shaft of a first motor within
motor housing, while the opposite end of mandrel was rotatably
supported by bracket. Both motor housing and bracket were supported
upon the base. The aforementioned first motor continuously rotated
the mandrel at speeds of up to 500 rotations per minute. The spray
nozzle was supported for reciprocal movement above and along
mandrel. The spray nozzle was secured to support rod which included
at its lowermost end a carriage. A threaded drive rod was coupled
at a first end to the drive shaft of a second motor within motor
housing for being rotated thereby. The opposite end of threaded
drive rod was supported by and freely rotated within bracket.
Threaded drive rod threadedly engaged a threaded collar within
carriage. Accordingly, rotation of drive rod caused the carriage,
and hence spray nozzle, to move in the directions designated by
dual headed arrow, depending upon the direction of rotation of
drive rod. A pair of micro switches which were periodically engaged
by carriage and which, when actuated, reverse the direction of
rotation of threaded drive rod in a manner which caused spray
nozzle to reciprocate back and forth along mandrel. Spray nozzle
made several passes along mandrel, repetitively spraying ePTFE
material as it rotated. Spray nozzle was caused to travel at a
linear speed of up to 50 centimeters per minute. The amount of
elastomeric material resulted from this spraying process was
determined by the speed of rotation of mandrel, the linear speed of
spray nozzle, the concentration of the elastomeric material, as
well as the rates of delivery of the elastomeric material. These
rates of delivery ranged up to 5 milliliters per minute for the
solution, and up to 5 liters per minute for the nitrogen gas. The
spray application was repeated as needed to reach the desired
properties and amount of elasticity for the end product use.
[0079] A final example of applying the elastomeric material to the
ePTFE material involved the use of a brushing technique which was
preformed by the same manner as the spraying machine by securing
the mandrel and providing a means of rotation for even application.
But instead of spraying the elastomeric material, it was evenly
brushed onto the compressed or uncompressed ePTFE.
[0080] The number of elastomeric material applications ranged
between 1 and 100 times, depending upon the concentration of the
elastomeric material used in the application process, depending
upon the application technique chosen and parameters of that
technique, and depending upon the intended use of the end
product.
[0081] While the application of elastomeric material by dipping,
brushing and spraying methods described above were directed to the
application of entire ePTFE material, those skilled in the art will
appreciate that such application may be used on only portions of
the compressed or uncompressed ePTFE material as well.
(iii) Drying Process
[0082] The drying process was performed upon the completion of the
application of the elastomeric material, or between applications of
the elastomeric material. The drying process solidified the
elastomeric material within the pores 3, defining, the elastomeric
matrix 4, by evaporating the solvent and completing the
impregnation of the elastomeric material within the pores 3 of the
ePTFE as shown in FIG. 4. The drying process depended on the
solvent used, which can include placing the mandrel with the ePTFE
into the oven or allowing the ePTFE to dry at ambient conditions
over an extended period of time. The drying process evaporated some
of the solvent if used between elastomer applications, or
evaporated all of the solvent at the completion of the final
product. Once the drying process was complete, the elastomerically
recoverable ePTFE was removed from the mandrel. While the above
describes the drying process and elastomeric application as
separate steps, one can appreciate that the steps may be
simultaneously occurring. For example, certain solvent
concentrations used with the spray technique can evaporate at
ambient temperatures upon application of the elastomer.
[0083] The elastomer matrix 4 serves two vital purposes; as a
bonding agent basically holding the ePTFE in the compressed state,
and as a recovery agent where after the material is longitudinally
stretched the elastomer matrix recovers the material back to the
compressed state without deformation.
[0084] The use of an elastomerically recoverable ePTFE tube as a
vascular graft 5 implanted within a patient provided an
axillofemoral bypass graft. The lower end of vascular graft was
anchored to femoral artery, while the upper end of vascular graft
was anchored to axillary artery. When conventional PTFE grafts were
used to perform such a bypass, raising of the arm placed tension on
graft, and placed stress upon the sutured ends of graft, sometimes
caused such ends to pull loose from the points at which they were
anastomosed to the aforementioned arteries. In contrast, the use of
elastomerically recoverable ePTFE vascular graft 5 in such
applications permitted the graft to be stretched without imparting
undue stress upon the anchored ends and the graft recovers to the
original size, thereby permitting the patient greater freedom of
movement, more comfort and no need to replace due to flaking or
deformation.
[0085] The above-described elastomerically recoverable ePFTE
material or patch 6 was implanted in the same manner as was
currently used to implant conventional ePTFE tubes, patches,
grafts, or tubular stent graft, and the like as shown in FIGS. 5
and 6. Moreover, the elastomeric material minimized suture hole
bleeding at the time of implantation, increased suture retention
strength, reduced serious weepage, and inhibited tissue ingrowth
because of its recovery and compression properties. While this
invention was described with reference to preferred embodiments
thereof, the description was for illustrative purposes only and was
not to be construed as limiting the scope of the invention. Various
modifications and changes may be made by those skilled in the art
without departing from the true spirit and scope of the invention
as defined by the appended claims.
EXAMPLES
Example 1
[0086] Placed 6 mm diameter ePTFE excel soft tube with 1500%
expansion over a mandrel of equal diameter. Manually longitudinally
compressed the tube about 50% and secured the ends of the tube with
Teflon tape. While in the compressed state, applied 7.5% Corethane
2.5W30 in DMAc by brushing technique. Placed tube into oven at
110.degree. F. to dry for 10 min. Remove from oven. Repeated the
application of elastomeric material by the brushing technique 2
more times. Then, removed tube and dried after each application
from mandrel. The elastomerically recoverable PTFE tube was
longitudinally stretch up to 90% its original length. Once the
stretching force was removed the tube recovered to its original
length without deformation. The stretching and recovery was
repeated multiple times without deforming the elastomerically
recoverable PTFE tube.
Example 2
[0087] Placed 6 mm diameter ePTFE tube of 800% expansion, expansion
velocity of 35 cm/sec, over a mandrel with similar diameter. Evenly
applied the 7.5% Corethane 2.5W30 in DMAc by the spraying
technique. Placed tube into oven at 110.degree. C. for 10 min. and
slightly dried the Corethane. Then, removed tube from oven, and
repeated elastomeric application by the spray technique and again
dried the 2.sup.nd application. Then, removed from oven and
longitudinally compressed ePTFE penetrated with elastomeric
material about 50% while on the mandrel, and secured with Teflon
tape. A third application of Corethane was applied by the spray
technique while the tube was in the compressed state. The tube was
placed in oven for 10 min. The tube was removed from the oven, and
then removed from the mandrel. The tube longitudinally stretched
about 300%. Upon release of stretching force the elastomerically
recoverable PTFE tube recovered to original size. The stretching
and recovery was repeated multiple times without deforming the
material.
* * * * *