U.S. patent application number 16/560393 was filed with the patent office on 2019-12-26 for stent having flexibly connected adjacent stent elements.
The applicant listed for this patent is W. L. Gore & Associates, Inc.. Invention is credited to Joseph R. Armstrong, Edward H. Cully, Mark Y. Hansen, Brian L. Johnson, Bret J. Kilgrow, Larry J. Kovach, James D. Silverman.
Application Number | 20190388252 16/560393 |
Document ID | / |
Family ID | 39417903 |
Filed Date | 2019-12-26 |
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United States Patent
Application |
20190388252 |
Kind Code |
A1 |
Armstrong; Joseph R. ; et
al. |
December 26, 2019 |
STENT HAVING FLEXIBLY CONNECTED ADJACENT STENT ELEMENTS
Abstract
An open stent (a stent having open space through its thickness
at locations between the ends of the stent), incorporating
flexible, preferably polymeric, connecting elements into the stent
wherein these elements connect adjacent, spaced-apart stent
elements. Preferably the spaced-apart adjacent stent elements are
the result of forming the stent from a helically wound serpentine
wire having space provided between adjacent windings. Other stent
forms such as multiple, individual spaced-apart ring-shaped or
interconnected stent elements may also be used. The connecting
elements are preferably longitudinally oriented.
Inventors: |
Armstrong; Joseph R.;
(Flagstaff, AZ) ; Cully; Edward H.; (Newark,
DE) ; Hansen; Mark Y.; (Flagstaff, AZ) ;
Johnson; Brian L.; (Flagstaff, AZ) ; Kilgrow; Bret
J.; (Flagstaff, AZ) ; Kovach; Larry J.;
(Flagstaff, AZ) ; Silverman; James D.; (Flagstaff,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
W. L. Gore & Associates, Inc. |
Newark |
DE |
US |
|
|
Family ID: |
39417903 |
Appl. No.: |
16/560393 |
Filed: |
September 4, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15454328 |
Mar 9, 2017 |
10456281 |
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16560393 |
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11560774 |
Nov 16, 2006 |
9622888 |
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15454328 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F 2002/825 20130101;
A61F 2/89 20130101; A61F 2002/075 20130101; A61F 2002/828
20130101 |
International
Class: |
A61F 2/89 20060101
A61F002/89 |
Claims
1. A diametrically expandable endoprosthesis comprising a
substantially open tubular stent and at least one flexible
non-metallic strip, said strip oriented substantially
longitudinally and connected to the stent to limit foreshortening
and elongation of the stent.
2. The diametrically expandable endoprosthesis of claim 1, wherein
the tubular stent is comprised of spaced-apart stent elements
connected by the at least one flexible non-metallic strip.
3. The diametrically expandable endoprosthesis of claim 1, wherein
the tubular stent is at least 80% open.
4. A diametrically expandable stent comprising at least two
circumferentially oriented spaced-apart stent elements and at least
one substantially longitudinally oriented flexible connecting strip
connecting adjacent spaced apart stent elements, said stent having
a length between ends of the stent and a deployed circumference,
wherein said stent has a tubular area comprising the stent length
multiplied by the deployed circumference and wherein at least 50%
of the tubular area is open.
5. The diametrically expandable stent according to claim 4 wherein
at least 60% of the tubular area is open.
6. The diametrically expandable stent according to claim 4, wherein
at least 80% of the tubular area is open.
7. The diametrically expandable stent according to claim 4, wherein
at least 90% of the tubular area is open.
8. The diametrically expandable stent according to claim 4, wherein
at least 93% of the tubular area is open.
9. The diametrically expandable stent according to claim 4, wherein
at least 80% of the tubular area is open and three connecting
strips are used.
10. The expandable prosthesis comprising at least two
longitudinally spaced-apart stent elements, said stent elements
comprising a serpentine form having apices separated by relatively
straight segments, wherein alternating apices point in opposing
directions, and wherein the at least two longitudinally
spaced-apart apices are connected by at least one substantially
longitudinally oriented flexible filament.
11. The expandable prosthesis according to claim 10, wherein said
filament includes at least one transversely oriented loop around at
least one apex.
12. The expandable prosthesis according to claim 10, wherein the
spaced-apart stent elements comprise a substantially open tubular
stent.
13. A diametrically expandable support structure comprising at
least two circumferentially oriented spaced-apart support elements
and at least one substantially longitudinally oriented flexible
connecting strip connecting adjacent spaced-apart support
elements.
14. The diametrically expandable support structure of claim 13,
wherein the support structure has a length between ends of the
structure and a deployed circumference, wherein the support
structure has a tubular area comprising the structure length
multiplied by the deployed circumference and wherein at least 50%
of the support structure is open.
15. The diametrically expandable stent according to claim 13
wherein at least 60% of the tubular area is open.
16. The diametrically expandable stent according to claim 13,
wherein at least 80% of the tubular area is open.
17. The diametrically expandable stent according to claim 13,
wherein at least 90% of the tubular area is open.
18. The diametrically expandable stent according to claim 13,
wherein at least 93% of the tubular area is open.
19. The diametrically expandable stent according to claim 13,
wherein at least 80% of the tubular area is open and three
connecting strips are used.
20. An expandable support structure comprising at least two
longitudinally spaced-apart support elements, the support elements
comprising a serpentine form having apices separated by relatively
straight segments, wherein alternating apices point in opposing
directions, and wherein the at least two longitudinally
spaced-apart apices are connected by at least one substantially
longitudinally oriented flexible filament.
21. The expandable prosthesis according to claim 20, wherein the
filament includes at least one transversely oriented loop around at
least one apex.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/454,328, filed Mar. 9, 2017, which is a divisional of
U.S. patent application Ser. No. 11,560,774, filed Nov. 16, 2006,
now U.S. Pat. No. 9,622,888, issued Apr. 18, 2017, both of which
are incorporated herein by reference in their entireties for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of implantable
stents having flexibly connected adjacent stent elements.
BACKGROUND OF THE INVENTION
[0003] The use of implantable stents in the vasculature and other
body conduits has become commonplace since first proposed by Dotter
in the 1960's. These devices were required to have a small,
compacted diameter for insertion into the intended body conduit and
transport, typically via a catheter, to a desired site for
deployment, at which site they were expanded to a larger diameter
as necessary to fit interferably with the luminal surface of the
body conduit. They developed into balloon expandable stents that
were expanded by plastically deforming the device with an
inflatable balloon on which the expandable stent was mounted in the
compacted state, the balloon being attached to the distal end of
the catheter and inflated via the catheter. Self-expanding stents
subsequently evolved, these devices being forcibly compacted to a
small diameter and restrained at that diameter by a sleeve or other
means. Following delivery to a desired site for deployment, they
are released from the restraint and spring open to meet the luminal
surface of the body conduit. These devices are typically made from
nitinol metal alloys and typically rely on the superelastic and
biocompatible character of this metal. Nitinol stents that rely on
the shape memory attributes of that material are also known.
[0004] The evolution of implantable stents included the use of a
tubular covering fitted to the stent, either to the outer or the
luminal surface (or both surfaces) of the stent. These covered
stents have generally come to be referred to as stent-grafts. The
coverings were generally of a polymeric biocompatible material such
as polyethylene terephthalate (PET) or polytetrafluoroethylene
(PTFE). See, for example, U.S. Pat. No. 4,776,337 to Palmaz. This
patent also describes that the covering may be optionally provided
with perforations if desired for particular applications. Because
of the open area provided by the perforations, such devices having
perforated coverings may be considered to be a sort of hybrid stent
and stent-graft, as are devices that include stent frame having
metallic stent elements and polymeric elements connecting, covering
or other otherwise being attached to the stent elements. The
presence of the polymeric elements reduces the otherwise open space
between the adjacent metallic stent elements, either very slightly
or very substantially depending on the intended application and
mechanical design.
[0005] Generally, a fully covered stent-graft can be considered to
have a surface area (hereinafter A.sub.max) equal to the
circumference of the expanded stent multiplied by the length of the
stent. For a conventional, open frame stent (as opposed to a
stent-graft), the surface area represented by all of the stent
elements is only a small portion of the maximum surface area
A.sub.max. The actual surface area covered by the stent, meaning
the area covered by all components of the stent (including
connecting elements) in their deployed state, is A.sub.stent. The
porosity index, or P.I., describes the open area (the portion of
the maximum surface area not covered by all components of the stent
assembly) as a percentage of maximum surface area, wherein:
P.I.=(1-(A.sub.stent/A.sub.max)).times.100%.
[0006] The open area may be a continuous single space, such as the
space between windings of a single helically wound stent element.
Likewise the open area may be represented by the space between
multiple individual annular or ring-shaped stent elements. The open
area may also be represented by the total area of multiple
apertures provided by either a single stent element (e.g., as shown
by FIGS. 1B and 2B of U.S. Pat. No. 4,776,337) or by multiple stent
elements providing multiple apertures. If multiple apertures are
provided they may be of equal or unequal sizes. The use of a
perforated graft covering or of polymeric elements in addition to
metallic stent elements may also reduce the open area.
[0007] Stents having a porosity index of greater than 50% are
considered to be substantially open stents.
[0008] In addition to the porosity index, the size of any aperture
providing the open area must be considered if it is intended to
cover only a portion of a stent area for a specific stent
application. For multiple apertures, often the consideration must
be for the largest size of any individual aperture, particularly if
the apertures are to provide for a "filtering" effect whereby they
control or limit the passage of biologic materials from the luminal
wall into the flow space of the body conduit.
[0009] Various stent devices combining metallic stent elements with
polymeric connecting elements are known; see, for example U.S. Pat.
No. 5,507,767 to Maeda et al. Another is a stent provided with a
flexible knitted sleeve having small open apertures in the fashion
of chain link fencing, from InspireMD Ltd. (4 Derech Hashalom St.,
Tel Aviv 67892 Israel). Perforated stent-grafts are also known;
see, for example WO00/42949.
SUMMARY OF THE INVENTION
[0010] The present invention relates to several approaches to
creating an open stent, that is, a stent having open space through
its thickness at locations between the ends of the stent, by
incorporating flexible, preferably polymeric connecting elements
into the stent wherein these elements connect adjacent,
spaced-apart stent elements. Preferably the spaced-apart adjacent
stent elements are the result of forming the stent from a helically
wound serpentine wire having space provided between adjacent
windings. Other stent forms such as multiple, individual
spaced-apart ring-shaped stent elements may also be used as will be
described, but embodiments presented that utilize the helically
wound serpentine forms are preferred for many applications.
[0011] The adjacent, spaced-apart stent elements are substantially
circumferentially oriented, meaning that they have a general
direction of orientation perpendicular to the longitudinal axis of
the stent, when the stent is in a straight (unbent state) form,
plus or minus 45.degree..
[0012] The flexible, preferably polymeric connecting elements
provide a means for keeping the stent elements equally spaced and
allow the construction of a stent having good flexibility. These
flexible connecting elements are preferably substantially oriented
in a longitudinal direction with respect to the stent, meaning that
they are more longitudinally oriented than circumferentially
oriented. They may range in orientation from being perfectly
parallel to the longitudinal axis of the stent (when the stent is
in a straight, unbent form) up to an angle of 45.degree. from the
longitudinal axis. More particularly, they may be oriented at
angles of less than or equal to about 45.degree., 40.degree.,
35.degree., 30.degree., 25.degree., 20.degree., 15.degree.,
10.degree., 5.degree., 4.degree., 3.degree., 2.degree. or 1.degree.
from the longitudinal axis, or may be virtually parallel to the
longitudinal axis of the stent. Being parallel to the longitudinal
axis or close to parallel (e.g., +/-5.degree.) is preferred.
[0013] The described stents have very good porosity index values,
typically at least 50% and may be made to be at least 55%, 60%,
65%, 70%, 75%, 80%, 85%, 90%, 95% or 97%. These stents can be
considered to be substantially open.
[0014] The flexible connecting elements may optionally provide a
substrate for the delivery of therapeutic agents such as drugs,
which may be intended for any of a variety of medical purposes.
Coatings for other purposes (e.g., to render the surface
hydrophilic) may also be applied.
[0015] The stent may also be designed to allow it to be removed
after its primary therapeutic effect has occurred. The stent may be
removed by including elements that may be snared with an
intravascular snare.
[0016] While polymeric materials are preferred as the connecting
elements, other non-polymeric materials such as nitinol wire offer
good flexibility and may also be used.
[0017] A helically wound, undulating or serpentine metal wire stent
structure provides good flexibility and is preferred for providing
the necessary stent structure having spaced apart stent elements.
These structures and other stent structures may be used for
self-expanding or balloon expandable stents made respectively from
(for example) nitinol or stainless steel. While metal wire is
preferred for structures of this type, they may also be made from
metal tubing by various known machining methods.
[0018] While metal materials are preferred, the stent elements may
optionally be made of various polymeric materials that offer
suitable physical and mechanical properties and may offer other
unique properties desired for specific applications (such as, for
example, bioabsorbable polymers).
[0019] The first fundamental embodiment is a stent structure
wherein at least one substantially longitudinally oriented
flexible, preferably polymeric strip is used to maintain spacing of
adjacent stent elements, such as the adjacent windings of a helical
stent form. More than one such strip may be used, with multiple
strips (two or more) preferably being spaced apart equal
circumferential distances around the circumference of the stent.
While the orientation of the strip or strips is primarily
longitudinal, it may also have a helical component such that it is
not parallel to the longitudinal axis of the stent when the stent
is in a straight or `unbent` state. The strip is substantially
longitudinally oriented if it is parallel to the longitudinal axis
of the stent when it is in the straight or unbent state, plus or
minus 45.degree.. The use of the substantially longitudinally
oriented strip results in a flexible stent with good bending
properties. The strip limits elongation of the stent, particularly
during expansion of the stent from a compacted diameter to a fully
deployed diameter. It also limits foreshortening of the stent, also
particularly during expansion.
[0020] A second fundamental embodiment, which may also be based on
the helically wound serpentine wire form, uses a flexible,
preferably polymeric filament laced along the length of the stent
to locate the adjacent stent elements (e.g., adjacent helical
windings) with respect to each other. This filament is preferably
laced so that it includes a transversely oriented loop around one
half of a full sinusoid of the serpentine wire, with these loops
created around sequential sinusoids along the length of the stent
that are preferably axially aligned. One or more of these filament
lacings may be provided along the length of the stent. When more
than one lacing filament is used, they are preferably spaced apart
in equal circumferential amounts (e.g., if three lacing filaments
are used along the length of the stent, they are preferably spaced
120.degree. apart).
[0021] A third fundamental embodiment includes a stent provided
with a perforated covering of a flexible, preferably polymeric
graft material, in the form of a sheet of material rolled and
preferably seamed to form a tube, or alternatively as a seamless
integral tube, that is provided with a multiplicity of perforations
or apertures. The covering material is thus integral or monolithic
as opposed to being created from separate filaments, threads or
other assembled components with multiple crossover points that add
to thickness and may be vulnerable to breakage and consequent
unraveling of the covering. The perforated covers described are
used to connect adjacent spaced apart stent elements and may also
be used with a variety of stent forms including separate rings,
helically wound wires, machined metal tubes, etc. The perforated
covering is thin, strong and flexible and, in combination with the
stent elements, results in a flexible, thin and strong stent.
Perforation sizes may be as desired, with relatively small
perforations on the order of 0.10 mm (minimum aperture size) being
preferred for carotid applications where it is desirable to
minimize risk of introduction of emboli into a bloodstream. Larger
apertures (for example 1, 2 or even 5 mm) may be preferred for
larger diameter vessels (e.g., the thoracic arch) where it may be
desired to stent the arch (for example, in the treatment of a
dissection and/or containment of emboli) while allowing blood flow
to side branches through the apertures of the graft material. These
embodiments preferably have high porosity indices and preferably
incorporate hexagonal apertures, although other shapes are
possible.
[0022] It is also possible to combine the different stent element
connecting techniques described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows a perspective view of a preferred stent for use
with the present invention.
[0024] FIG. 1A shows a plan view of details of serpentine wire
forms described by FIG. 1.
[0025] FIG. 2 describes a perspective view of an open-frame stent
wherein spaced-apart, adjacent stent elements are interconnected by
a flexible strip of implantable material.
[0026] FIGS. 2A-2D are transverse cross sectional views that show
various relationships between connecting strips and stent
elements.
[0027] FIG. 2E is a perspective view showing that a connecting
strip may have a slight helical orientation while still being
substantially longitudinally oriented.
[0028] FIG. 2F shows a longitudinal cross section of a stent
provided with two longitudinally oriented strips placed 180.degree.
apart.
[0029] FIG. 3A is a schematic view plan view of serpentine wire
segments from portions of four adjacent helically wound stent
elements, showing a substantially longitudinally oriented filament
interconnecting adjacent stent elements.
[0030] FIGS. 3B and 3C are detail views of a portion of FIG. 3A
showing particulars of the filament relationship with the
serpentine wire stent elements.
[0031] FIG. 3D shows a schematic plan view of an alternative
serpentine wire form for a stent wherein the serpentine wire may be
provided with a circumferential rather than helical orientation,
and with stent elements interconnected by a substantially
longitudinally oriented filament.
[0032] FIG. 3E is a detail view of a portion of FIG. 3D.
[0033] FIG. 3F shows the same stent form as FIG. 3D with an
alternative arrangement of the interconnecting filament.
[0034] FIG. 3G shows an alternative stent form to that of FIG. 3F
with the same filament arrangement.
[0035] FIG. 3H shows an alternative arrangement of the
substantially longitudinally oriented interconnecting filament.
[0036] FIG. 3I is a detail view of a portion of FIG. 3H.
[0037] FIG. 3J shows an alternative arrangement of the
substantially longitudinally oriented interconnecting filament.
[0038] FIG. 3K is a detail view of a portion of FIG. 3J.
[0039] FIGS. 3L-3N show examples of filament arrangements that may
be used with opposing apices.
[0040] FIG. 4A is a plan view of a helically wound serpentine wire
stent provided with a hexagonally perforated graft covering, as it
would appear if the tubular form of the stent was cut along its
length and laid out flat.
[0041] FIG. 4B is a plan view of a series of individual serpentine
stent rings, arranged with the direction of the apices opposing
each other rather than aligned in the same direction as shown in
FIG. 4A, provided with a hexagonally perforated cover.
[0042] FIG. 4C is a transverse cross section of a stent 10 provided
with the perforated covering 40.
[0043] FIGS. 4D-4G describe plan views of alternative perforated
coverings having alternatively shaped perforations.
[0044] FIG. 4H is a perspective view of a stent provided with a
perforated cover shown as it would appear deployed in vivo, located
in a vessel at a site where the stent crosses a side vessel and
with a balloon introduced through a cover perforation and extending
into the side vessel.
[0045] FIGS. 4I and 4J show stents provided with perforated
coverings placed over thoracic aneurisms.
[0046] FIG. 4K shows a stent provided with a perforated cover used
to repair a dissection in a blood vessel.
[0047] FIG. 4L shows a stent provided with a perforated cover used
to hold plaque against a vessel wall while still allowing flow
through an adjacent side branch vessel.
[0048] FIG. 4M shows a stent provided with a perforated cover used
to hold plaque or emboli contained against the wall of a small
blood vessel.
[0049] FIG. 4N shows a stent provided with a perforated cover
located distally to a stent-graft located to repair an aneurysm
wherein the perforated cover stent aids in anchoring the
stent-graft near a branch vessel while allowing flow through the
branch vessel.
[0050] FIG. 4P is a schematic side view of a stent tensile test
configuration.
DETAILED DESCRIPTION OF THE DRAWINGS
[0051] Several open frame stent designs are presented, wherein
adjacent stent elements are connected by flexible connecting
elements. These flexible connecting elements are typically
polymeric and may take various forms such as strips, filaments or
perforated sheets. Typical stent forms are helically wound metallic
wire (e.g., nitinol or stainless steel) or multiple ring-shaped
metallic stent elements. The helically wound wire is preferably
serpentine wire as will be further described. The serpentine wire
may also be oriented circumferentially in the fashion of individual
rings or may alternatively be a single continuous wire arranged as
will be described. In addition to wire forms, various other stent
forms, typically metallic but not limited to metallic, may also be
created that lend themselves to the present invention. Among these
are machined tubular forms wherein individual ring-shapes, or
alternatively perforated tubes or other forms (e.g., helical) that
extend continuously between opposing ends of the stent.
[0052] FIG. 1 shows a perspective view of a stent 10 that is
preferred for use with the present invention. The stent 10 shown
comprises a helical winding of a length of serpentine wire 18.
Sequential windings of the helical wound serpentine wire 18 result
in spaced-apart adjacent stent elements 12. The ends 17 of wire 18
may be secured by any suitable method (e.g., welding) to the
adjacent helical winding. For clarity, stent 10 is shown with a
mandrel 16 extending through and beyond both ends of the stent
lumen, making the side closest to the viewer visually apparent
while blocking the view of the side of stent 10 furthest from the
viewer. Mandrel 16 is present only for clarity of visualization and
is not a part of stent 10.
[0053] The helically wound serpentine wire 18 extends continuously
between opposing ends of stent 10, wherein opposing apices 22a and
22b formed of wire bends of relatively small radii are
interconnected by straight or relatively straight wire segments 24.
The apices typically "point" in directions that are substantially
parallel to the longitudinal axis 19 of the tubular form of the
stent 10, with alternating apices 22a and 22b pointing in opposite
directions, that is, pointing to opposite ends of the stent. As
shown by FIG. 1, it is preferred that apices pointing in one
direction (e.g., apices 22a) are aligned along a first common line
while the apices pointing in the opposite direction (e.g., apices
22b) are aligned along a second common line.
[0054] FIG. 1A shows a plan view of details of serpentine wire
forms described by FIG. 1; dimensions relate to Example 1 described
below. Dimension 27 is considered as the height of adjacent
opposing apices while dimension 28 is the width of adjacent
opposing apices. Dimension 29 describes one full period of the
serpentine form.
[0055] FIG. 2 describes a perspective view of an open-frame stent
10 wherein spaced-apart, adjacent stent elements 12 are
interconnected by a flexible strip 14 of implantable material.
Adjacent stent elements 12 may be individual ring-shaped elements
or alternatively adjacent windings of a continuous helically wound
wire. The use of connecting strips 14 provides the resulting stent
with good flexibility and good axial strength while maintaining the
spacing of the adjacent stent elements 12.
[0056] Connecting strips 14 are preferably polymeric, and may be
made from a variety of implantable polymeric materials including
bioabsorbable polymers. Bioabsorbable polymers particularly lend
themselves to the construction of stents having flexibly connected
stent elements wherein the stent may be removed (particularly a
helically wound stent) after degradation of the bioabsorbable
connecting elements. ePTFE strips (porous expanded PTFE) made from
ePTFE films are particularly preferred for their strength,
flexibility and biocompatibility.
[0057] Strips may extend for less than the entire length of the
stent if appropriate. If more than one strip is used, the
individual strips may have different lengths or the same length. It
is generally preferred, however, that all strips extend for the
full length of the stent.
[0058] Strips 14 are joined to stent 10 at points where they are in
mutual contact. It is preferred that all such contact points are
attached. Preferably they are attached by the use of suitable
adhesives or by melt-bonding of the strip polymer. Various
biocompatible adhesives may be used, including melt-bondable
thermoplastics.
[0059] For ePTFE strips made from ePTFE films, a preferred adhesive
is a continuous coating of a thermoplastic fluoropolymer,
particularly fluorinated ethylene propylene (FEP). The FEP coating
may be applied to the ePTFE film by a process which comprises the
steps of: [0060] a) contacting one side of the ePTFE film with a
layer of FEP film (or another alternative thermoplastic polymer if
so desired); [0061] b) heating the composition obtained in step a)
to a temperature above the melting point of the thermoplastic
polymer; [0062] c) stretching the heated composition of step b)
while maintaining the temperature above the melting point of the
thermoplastic polymer; and [0063] d) cooling the product of step
c).
[0064] The thermoplastic film coating applied to the ePTFE film by
this method may be either continuous (non-porous) or discontinuous
(porous). If discontinuous, the process may be adjusted to achieve
the desired degree of porosity to include a coated film that is as
porous as the precursor ePTFE film. The coated film used for the
present invention is most preferably a continuously (non-porous or
substantially non-porous) coated film.
[0065] While a single strip 14 may be used along only one side of
the stent 10, oriented in a direction substantially parallel to the
longitudinal axis of the stent, multiple (at least two) such strips
are preferred, with the strips preferably being equally spaced
radially about the stent. Three connecting strips 14 are preferred
to achieve uniform bending properties, as shown by the transverse
cross section of FIG. 2A. Ideally these are spaced equidistant
radially (120.degree. apart) around the circumference of the stent,
although some variability of this spacing has not been seen to be
particularly detrimental to bending properties. Strips 14 are shown
attached to the outer surface of stent 10. Alternatively, strips 14
may be attached to the inner surface of stent 10 (FIG. 2B) or to
both the inner and outer stent surfaces (FIGS. 2C and 2D).
[0066] As shown by FIG. 2E, strip 14 may have a slight helical
orientation while still being substantially longitudinally
oriented, although orientations parallel to the longitudinal axis
are preferred.
[0067] FIG. 2F shows a longitudinal cross section of a stent 10
provided with two strips 14 placed 180.degree. apart, with the
section being taken through each of the two strips 14. This figure
shows how each strip 14 is placed on the inner surface of stent 10,
with ends of the strip 14 extending beyond the ends of the stent.
The strip 14 in this embodiment is in excess of twice the length of
the finished stent 10, allowing the ends of strip 14 to be wrapped
back over and attached to the outer surfaces of the stent elements
12, with the extreme ends of strip 14 overlapping and attached to
each other (reference 25). As well as strips 14 being attached
(bonded) to their points of contact with stent elements 12, they
are attached to each other between adjacent stent elements as shown
(reference no. 26). The ends of the stent 10 may optionally be
provided with a circumferentially applied tape covering 23 that
aids in securing the extreme wire ends of the wire stent.
[0068] A self-expanding stent was made using nitinol wire of round
cross section and of 0.15 mm diameter. The wire was wound into the
helically wound serpentine form (shown in FIG. 1) on a
manufacturing mandrel having a series of pins protruding from its
external surface at specific points. The resulting serpentine form
had apices of about 0.3 mm radius (measured from the center point
of the bend to the surface of the wire closest to the center point
of the bend). The height of adjacent opposing apices was about 1.6
mm (dimension 27 in FIG. 1A) while the width of adjacent opposing
apices was about 2.3 mm. The resulting stent incorporated eight
full periods of the serpentine pattern for each full revolution of
the serpentine wire about the stent circumference. The stent was of
about 6 mm inside diameter, having been manufactured to its fully
deployed diameter at which the connecting strips were to be
attached. The completed stent may subsequently be compacted to a
smaller diameter for insertion into a vasculature by various known
means including iris type compaction devices or by pulling the
stent through funnel devices.
[0069] Following forming of the nitinol wire into the helically
wound stent, the finished wire form, while still on the
manufacturing mandrel, was then placed into a convection oven set
at 450.degree. C. for 9 minutes for heat treating, and removed from
the oven and allowed to cool to ambient temperature. The stent was
then removed from the mandrel.
[0070] Connecting strips for this 5 cm long stent were 1.0 mm wide,
11 cm long strips of ePTFE film of about 0.012 mm thickness
subsequently provided with a continuous (substantially non-porous)
FEP coating. The ePTFE used to make these strips was of about 0.5
g/cc density and of about 50 micron average fibril length. The
coated strip was of about 0.05 mm thickness. Break strength for
these strips was 0.5 kg or greater.
[0071] Average fibril length of the ePTFE film was estimated from
scanning electron photomicrographs of the surface of the film. Film
thickness measurements are preferably made (including for the
determination of bulk volumes for density values) by placing a
sample between the pads of a Mitutoyo model no. 804-10 snap gauge
having a part no. 7300 frame and gently easing the pads into
contact with the opposing surfaces of the sample under the full
force of the spring-driven snap gauge pads.
[0072] Three strips were used to create the stent, spaced apart
radially. Each of the three strips was aligned longitudinally
(parallel to the longitudinal axis of the stent) with and covering
a row of wire apices pointing at the same end of the stent.
Arrangement of the strips was such that between two strips was one
row of uncovered apices pointing in the same direction as the rows
covered by the two strips. The other two spaces between the
connecting strips each had two rows of uncovered apices pointing in
the same direction as the covered apices.
[0073] The stent having substantially longitudinally oriented
connecting strips was manufactured using the helically wound wire
stent and FEP coated ePTFE connecting strips described above. The
manufacturing process involved fitting a sacrificial 5 mm inside
diameter, longitudinally extruded and expanded ePTFE tube onto a
porous metal 5 mm diameter stainless steel mandrel. Two
circumferential wraps of 6 mm wide FEP coated ePTFE film were
applied over the surface of the sacrificial tube with the FEP
facing away from the sacrificial tube. These two wraps were located
about 4.4 cm apart. Three of the 11 cm long FEP coated strips were
laid lengthwise along the surface of the sacrificial ePTFE tube
with their FEP coated surface facing away from the sacrificial
tube. These strips were spaced approximately 120.degree. apart
circumferentially. The ends of the strips were temporarily secured
to the mandrel with ePTFE tape. The helically wound 5 cm long
serpentine wire stent was then fitted over the assembly with the
stent ends centered on each circumferential wrap of 6 mm wide ePTFE
film. Care was taken to see that the adjacent helical stent
windings were spaced apart equal distances. After removing the
temporary ePTFE tape, the radial position of each of the three
ePTFE connecting strips was then adjusted with respect to the stent
so that each strip was located along a row of stent apices pointed
in the same direction for all three strips.
[0074] For each strip, both ends were laid back over the outer
surface of the stent in line with the portion of the strip beneath
the stent, with about 0.5 cm of the very ends of both strips
overlapping as shown by reference no. 25 in FIG. 2F. The strips
were secured in this position with a temporary wrapping of 0.012 mm
thick polyimide film. Each stent end was then given a
circumferential wrapping with a narrow gold marker band (e.g.,
0.0625 mm by 0.025 mm) and an outer circumferential wrapping of
another layer of 6 mm wide FEP coated ePTFE film, this time with
the FEP facing inward. The entire assembly was wrapped in polyimide
film and placed into a convection oven set at 320.degree. C. for 10
minutes with a vacuum applied to the porous metal mandrel. After
removal from the oven and being allowed to cool to about ambient
temperature, the polyimide film was removed. Any of the
circumferentially wrapped ePTFE film covering the stent ends that
protruded beyond the ends of the wire stent was trimmed off with a
scalpel.
[0075] The three ePTFE connecting strips were well adhered to the
stent and to themselves between the stent elements (reference no.
26, FIG. 2F). The resulting stent had a porosity index of about
82%. It demonstrated good flexibility and kink resistance in
bending. The stent was able to be bent to an inside bend radius of
3 mm without kinking.
[0076] The same stent made with a single connecting strip of the
same dimensions would have a porosity index of 93%
[0077] A second fundamental embodiment uses a flexible, preferably
polymeric filament laced along the length of the stent to locate
the adjacent stent elements (e.g., adjacent helical windings) with
respect to each other. Stent elements include serpentine wire or
machined elements having repeating sinusoids with apices connecting
relatively straight segments. The stent elements may be individual
rings or may be adjacent windings of a helically wound serpentine
wire, or may be any other stent form having spaced apart elements
that lend themselves to being connected by a filament. Alternate
apices of the serpentine form point in opposite directions, i.e.,
toward opposite ends of the stent. The filament may be of a variety
of polymers including PET, polyurethane, PTFE, etc. Porous expanded
PTFE fibers are preferred because of their strength, flexibility
and biocompatibility. The filaments may optionally be provided with
coatings of adhesives (including heat-activated adhesives) to allow
bonding to stent elements. The filaments may also be a
bioabsorbable polymer such as PGA, PLA, PGA/PLA, PGA/TMC, etc. The
use of a bioabsorbable filament is possible in that, once deployed
at a desired site in a body conduit, the interference of the
deployed stent elements with the luminal surface of the body
conduit will hold the stent in place with the elements properly
spaced apart for most implant applications.
[0078] The filament is substantially longitudinally oriented (along
the length of the stent) as it connects adjacent, spaced-apart
stent elements, meaning that the predominant orientation of the
filament, represented by a straight line (neglecting the curvature
of the exterior surface of the stent, i.e., as if the stent form
were considered in a flattened, plan view) laid over the filament
between the ends of the stent, is substantially longitudinally
oriented. This line is considered substantially longitudinally
oriented if it is parallel to the longitudinal axis of the stent,
plus or minus 45.degree.. Most preferred orientations are parallel
to the longitudinal axis, or very close to parallel (plus or minus
5.degree.). As shown by the schematic plan view of FIG. 3A, the
filament is preferably laced so that it includes a transversely
oriented loop around one half of a full period of the serpentine
wire, with these loops created around sequential periods of the
serpentine form that lie along the length of the stent in axial
alignment. One or more of these filament lacings may be provided
along the length of the stent. When more than one lacing filament
is used, they are spaced apart in equal radial amounts (e.g., if
three lacing filaments are used along the length of the stent, they
are spaced 120.degree. apart). The use of three filaments offers a
particularly good combination of stent flexibility and
strength.
[0079] FIG. 3A is a schematic view plan view of serpentine wire
segments from portions of four adjacent helically wound stent
elements 12 incorporating opposing apices 22a and 22b connected by
straight segments 24. These spaced-apart stent elements 12 are
interconnected by filament 30 extending between both ends (not
shown) of stent 10. Filament 30 is laced through stent elements in
the fashion shown so that it includes longitudinal segments 301 and
transverse loop segments 30t. This form of lacing provides very
good stent flexibility and axial strength while allowing a degree
of length variability in that the application of tension to the
length of the stent results in a slight amount of length extension.
The pattern of the lacing shown is best followed by examination of
the close-up details of FIGS. 3B and 3C, with arrows 30a, 30b and
30c showing the lacing accomplished in that respective
sequence.
[0080] FIG. 3D shows a schematic plan view of an alternative to the
helically wound wire stent of (for example) FIGS. 1 and 3A. The
sinusoids shown in FIG. 3D include opposing apices 22a and 22b
connected by straight segments 24 in the fashion shown previously
for the helically wound constructions. The stent portion shown in
FIG. 3D differs in that, once per revolution, straight segment 24
is replaced by a longer straight segment 32 that connects one
winding (or stent element) 12 to the adjacent winding (or stent
element) 12. This use of the longer straight segments 32 allows the
individual windings 12 to be circumferentially oriented
(perpendicular to longitudinal axis 19) rather than helically
oriented wherein the windings have a pitch that is less than
perpendicular to the longitudinal axis 19. This stent arrangement
may be used with connecting strips 14 in a fashion similar to that
shown in FIG. 2, and for other embodiments as will be subsequently
described. FIG. 3D and the detail of FIG. 3E show the transverse
segment of the lacing filament 30t to extend around the adjacent
air of longer straight segments 32 as well as one adjacent straight
segment 24 of conventional length. Any other filaments extending
between ends of the stent (e.g., two other filaments if a total of
three are used) are laced in the fashion shown by FIG. 3A-3C.
[0081] Alternatively, as shown by FIG. 3F, the transverse loop 30t
may be fitted only around one longer straight segment 32 and the
adjacent straight segment 24 of conventional length, thereby
passing between the adjacent long straight segments 32.
[0082] FIG. 3G shows a similar stent structure to that of Figure F,
differing primarily in that the long straight segments 32 are
offset by one period of the serpentine form per each
circumferential revolution. The filament lacing pattern is the
same.
[0083] An additional advantage of the lacing patterns shown in
FIGS. 3A-3G is that manufacture of these patterns may be easily
automated.
[0084] Other lacing techniques are also possible, such as those
shown by FIG. 3H and associated detail FIG. 3I, and FIG. 3J and
associated detail FIG. 3K. FIGS. 3L-3N show examples of how the
lacing techniques may be applied to stents having opposing apices.
FIG. 3N shows an embodiment wherein two or more filaments are
twisted together, capturing the stent element as the twisting
proceeds.
[0085] It is apparent that a variety of substantially
longitudinally oriented filament lacing methods may be used with
various stent forms having spaced-apart stent elements, each method
having advantages. For example, substantially longitudinally
oriented patterns that also incorporate transverse filament lacing
aspects (for example, of the type described by FIG. 3A) can be used
to affect torsional properties of the stent.
[0086] As noted above, the chosen filaments may be provided with
adhesives if desired for attachment of contact points of the
filaments to the stent elements. While this may inhibit stent
flexibility, it may be desired for other reasons such as precisely
limiting the length of an expanded stent. ePTFE filaments may be
provided with melt-bondable coatings of polymers having lower melt
temperatures than PTFE; fluorinated ethylene propylene (FEP) is an
example. Such a coating may be applied by various methods including
extrusion over the filament, powder coating of the filament with
powdered FEP that is subsequently melted to flow over the filament
surface, or running the filament through a bath of molten FEP
optionally followed by pulling the filament through a die to
achieve uniformity of the coating. Alternatively, the stent may be
provided with a coating of adhesive such as by powder coating with
FEP. ePTFE filaments may be made by rolling ePTFE films (see, for
example, U.S. Pat. No. 5,288,552 to Hollenbaugh et al.). The films
used to create filaments may be FEP coated films of the type
described previously.
[0087] An example of a 6 mm expanded diameter nitinol wire stent
having adjacent elements connected by filaments was created using
the same type of helically wound serpentine wire stent as created
for Example 1. The filament chosen was an ePTFE filament provided
with a coating of FEP melt-bondable adhesive. The filament had a
diameter of 0.1 mm, a tensile strength of 620 grams and a weight
per unit length of 0.018 grams/meter. Eight filaments were laced
into the stent, connecting the adjacent stent elements as shown by
FIGS. 3A-3C, with one filament laced into each row of apices that
pointed at one end of the stent. The resulting stent exhibited
excellent flexibility, being able to be bent to an inside bend
radius of almost zero without kinking. Porosity index for this
stent was 97%.
[0088] A third fundamental embodiment is a stent provided with a
perforated covering of a thin, strong and flexible, preferably
polymeric graft material, in the form of a sheet of material rolled
and preferably seamed to form a tube, or alternatively as a
seamless integral tube, that is provided with a multiplicity of
perforations or apertures. The graft material is thus integral or
monolithic as opposed to being created from separate filaments or
threads with multiple crossover points that add to thickness and
may be vulnerable to breakage and consequent unraveling of the
covering. Various implantable polymeric materials may be used for
the perforated cover including PTFE, PET, polyurethane, silicone,
fluoroelastomers and bioabsorbable polymers.
[0089] FIG. 4A is a plan view of a helically wound serpentine wire
stent 10 provided with a perforated graft covering 40, as it would
appear if the tubular form of the stent was cut along its length
(parallel to the longitudinal axis) and laid out flat. The stent is
the same type as shown in FIG. 1. The hexagonally perforated
covering 40 is a preferred covering offering good flexibility and
strength when made as described below.
[0090] FIG. 4B is a similar plan view of a series of individual
serpentine stent rings 12 wherein two rings per row are connected
by stent elements, arranged with the direction of the apices
opposing each other rather than aligned in the same direction as
shown in FIG. 4A; the stent of FIG. 4B uses the same hexagonally
perforated graft covering 40 as the stent of FIG. 4A.
[0091] FIG. 4C is a transverse cross section of a stent 10 provided
with the perforated covering 40. The stent elements 12 shown are of
wire having a rectangular cross section. Stent elements 12 are
provided with a coating of an adhesive 41 such as a melt-bondable
FEP applied to the stent elements 12 by powder coating.
[0092] FIGS. 4D-4G describe plan views of alternative perforated
coverings having different hole patterns. While hexagonal
perforations are preferred, the other perforation patterns such as
rectangular, circular, triangular and square may be desirable for
certain applications. It is also apparent that, for perforation
patterns that are not perfectly symmetrical within the plane of the
covering, that the covering sheet may be oriented as desired with
respect to the length of the stent. Perforations may be made to
desired sizes, with aperture size being defined herein as the
diameter of the largest inscribed circle that may be fitted into
the opening. It is further apparent that different size openings
may be provided within the same covering. The total opening area
and the amount of web material between openings may be selected to
provide the desired porosity index.
[0093] The rectangular patterns of FIG. 4D may also be made by
using slits through the material. Either the rectangular or slit
pattern may be usefully oriented with the long dimensions of the
slits or rectangles parallel to the longitudinal axis of the stent.
Openings of this type may allow the covering to be attached to a
stent in a partially or fully compacted state, wherein, upon
expansion of the stent, the slit or rectangle deforms into a larger
generally hexagonally shaped aperture.
[0094] FIG. 4H is a perspective view of a stent 10 provided with a
perforated cover 40 shown as it would appear deployed in vivo,
located in a vessel at a site where the stent 10 crosses a side
vessel (not shown). A guidewire 42 is introduced through a
perforation aligned with the entrance to the side vessel, after
which a balloon catheter is introduced over the guidewire directing
the balloon 44 through the same perforation. The balloon 44 may
subsequently be inflated to rupture the perforation and create an
enlarged transmural opening through the perforated graft cover
aligned with the entrance to the side vessel.
[0095] FIGS. 4I and 4J show stents 10 provided with perforated
coverings 40 implanted into blood vessels 41 over thoracic
aneurisms 46. The aneurisms 46 may be filled with a suitable gel or
other biocompatible material that is held in place by perforated
cover 40 while blood flow is provided through the lumen of the
stent 10. Blood flow is also maintained into side branch vessels 48
through the perforated cover 40.
[0096] FIG. 4K shows how a stent 10 provided with a perforated
cover 40 may be used to repair a dissection 49 in a blood vessel
41.
[0097] FIG. 4L shows a stent 10 provided with a perforated cover 40
used to hold plaque 52 against a vessel wall while still allowing
flow through an adjacent side branch vessel.
[0098] FIG. 4M shows a stent 10 provided with a perforated cover 40
used to hold plaque or emboli 52 contained against the wall of a
small blood vessel 48.
[0099] FIG. 4N shows a stent 10 provided with a perforated cover 40
located distally to a stent-graft 54 located to repair an aneurysm
wherein the perforated cover stent aids in anchoring the
stent-graft near a branch vessel while allowing flow through the
branch vessel. The aneurisms may optionally be filled with coils, a
suitable gel or other biocompatible material.
[0100] FIG. 4P is a schematic side view of a stent tensile test
configuration.
[0101] Perforated covers were created by initially wrapping several
layers of an ePTFE film that includes a discontinuous (porous)
layer of FEP. Films made as taught by U.S. Pat. No. 5,476,589 to
Bacino are suitable for FEP coating and use in this application.
The film used ranged from 2.5 to 5 microns in thickness and had a
density range of about 0.5 to 1.0 g/cc. The film was wrapped
circumferentially, with the FEP side oriented outwards, onto a
glass mandrel approximately 1 mm diameter larger than the outside
stent diameter. Other materials, including biocompatible polymers
and metals could be used for the perforated cover structure, with
process parameters adjusted accordingly. Twelve layers of the film
were wrapped around the mandrel surface, with a range of 2 to 100
layers considered desirable. The wrapped mandrel was placed in a
convection oven set at 320.degree. C. for 12 minutes, and then
allowed to cool to about ambient temperature.
[0102] While the perforations may be formed by various methods
including the use of, for example, mechanical punches, laser
cutting is preferred for speed and precision.
[0103] For cutting the perforations, the wrapped mandrel was set up
on a computer controlled laser cutting tool that utilizes a beam
with a wavelength of 10.6 .mu.m (Keyence ML-G9310, Woodcliff Lake
N.J.). Shorter wavelengths lasers have been tried (e.g., 157 nm
wavelength laser) with the cut quality being higher (straighter
cuts with less thermally effected zone as evidenced by less
material retraction when visually inspecting scanning electron
microscope images). The laser was programmed to cut hexagonal
apertures with side length of 0.15 mm. Adjacent hexagons were
offset in honeycomb fashion to minimize the amount of material
between the resulting apertures and to provide relatively uniform
web widths between adjacent apertures. Accounting for the laser
beam width of 50 microns, the side length of the resulting
hexagonal aperture is about 0.2 mm. Depending on the intended
application of the stent, the length of the cut side of the
hexagons may range from 0.025 to 5 mm, with 0.1 mm to 1 mm being
preferable. Other aperture shapes of widely ranging sizes may also
be cut. The perforations may be made to be of uniform shape, or
not. It is also anticipated that the perforations may be cut after
attaching the cover to the stent.
[0104] After cutting, the wrapped mandrel was heated in a
convection oven set at 370.degree. C. for 5 minutes. This
post-cutting heating step has the benefit of both improving the cut
quality (i.e., smoothing the edge) and minimizing the width of the
membrane between the cut hexagons. This heating process encourages
retraction of polymer thereby narrowing the membrane width, and may
also result in a thickness increase. For example, the difference
between pre-heated to post-heated web width has been measured to
change from approximately 0.20 mm to 0.075 mm.
[0105] After the heating step, the resulting perforated stent cover
was stripped from the glass mandrel and inverted so the FEP that
was on the outer surface became the inner surface. Optionally,
radiopacity enhancements could be added to the perforated cover
such as attaching gold foil segments to the cover by, for example,
the use of a suitable adhesive or by locally melting the FEP (for
example, with a heated soldering iron).
[0106] Following manufacture of the perforated cover, a suitable
stent is obtained. The stent is preferably made of nitinol, but can
be fabricated of a material such as stainless steel, cobalt
chromium, or bioabsorbable materials (e.g., polyglycolic acid, or
other). The stent may optionally be provided with radiopaque
enhancements such as gold or platinum/iridium markers crimped,
embossed, or otherwise attached to the stent frame. Various stents
forms were attached to perforated covers made as described. 6 mm
stents of the type used to make the strip connected and the
filament connected stents were used, differing only in being of 3
cm length. 37 mm helically wound wire stents were made with eight
full periods of the serpentine wire forms per circumference
(reference no. 29, FIG. 1A), and the width of adjacent opposing
apices (reference no. 28, FIG. 1) being equal to about 6.7 mm. The
height of adjacent opposing apices (reference no. 27, FIG. 1A)
being equal to about 9.5 mm. 37 mm stents were heat treated on
their manufacturing mandrels in a convection oven set at
470.degree. C. for 20 minutes. After being removed form the oven
and allowed to cool to ambient temperature, the resulting stent was
removed from the mandrel.
[0107] The obtained stent was powder coated with a thin layer of
FEP powder. This was done by using FEP powder (Dupont FEP
Fluoropolymer Resin, Product Type 5101) in a table top blender
within which the stent is suspended. Other melt processable
polymers could be used, included fluoroelastomers, drug eluting
polymers, or other polymers. The stent was placed within the
blender with FEP powder and the blender activated. The powder
dispersed into the volume of the blender chamber and powder coated
the stent. After approximately 3 seconds, the stent was removed,
and next placed into a convection oven set at 320.degree. C. for 5
minutes. After this time, the stent was removed and allowed to air
cool.
[0108] The stent was then placed on a mandrel having an outer
diameter approximately equal to the inner diameter of the stent.
The mandrel was covered on its outer diameter with polyimide film.
To temporarily fix the stent to the mandrel, the stent was placed
in a convection oven set at 320.degree. C. for 4 minutes.
[0109] After removal from the oven and cooling of the stent and
mandrel assembly, the perforated cover structure was coaxially
positioned over the stent. The perforated cover was axially
tensioned over the stent, causing it to decrease in diameter and
come in full contact with the outer diameter of the stent. The
cover ends were temporarily fixed to length on the mandrel by ePTFE
tape. A temporary layer of ePTFE film was then tightly wrapped
around the assembly. The perforated cover was then placed within a
convection oven set at 320.degree. C. oven for 12 minutes. After
removal from the oven and being allowed to cool to ambient
temperature, the temporary film wrapping was removed and the stent
and perforated cover assembly removed from the mandrel. The
perforated cover was then trimmed flush with the end of the
stent.
[0110] The perforated cover structure may also be attached by
mechanical means such as fiber or discrete mechanical attachment
points (e.g., clips, etc). The perforated cover may be on the
outside of the stent elements, or it may be on the inside of the
stent elements, or it may be on both.
[0111] The resulting assembly should then be inspected to ensure
good adherence of the stent to the perforated cover. This final
assembly can then be cooled below its martensitic temperature,
crimped and loaded within a catheter delivery system for
implantation into a body conduit following sterilization.
[0112] Assembling a covered stent in the preceding manner has a
number of advantages. First, the radial strength of the device can
be optimized independent of device flexibility and perforation
size. By attaching the perforated cover structure, the axial
distance separation of stent rows on the outer radius of curvature
is minimized and bending is accomplished by reducing the space
between the adjacent stent elements on the inner radius of
curvature. This allows stents to be constructed that give more
uniform support to a curved vessel.
[0113] The above-described perforated cover structure minimizes the
amount of material covering the vessel lumen. This minimal material
is anticipated to allow the tissue to heal faster around and over
the stent. The porosity index of the stent and perforated cover may
be quite high, comparable to that of conventional stents alone.
This can be accomplished by minimizing the amount of material with
the structural portion of the stent. Material can be removed
because the traditional metal portion of the stent does not need to
be optimized for vessel scaffolding, bending uniformity, or other
conventionally considered attributes. The coverage of the vessel
luminal surface (including the perforated cover and metal stent) is
preferably less than 50%, with less than 40%, 30%, or even 20%
being possible and usually preferred. These numbers correspond to
porosity indices of 50%, 60%, 70%, and 80% respectively.
[0114] For the stent provided with the perforated cover, the
luminal area covered by the combined stent and cover may be
determined using a stent inspection system (e.g., Visicon Finescan,
Napa Calif.). Using the inspection system, a 1.0 cm length of the
cylindrical device is imaged as a 360 degree flat pattern. The
contrast of the image should be sufficient to allow full
visualization of the perforated structure and stent. From the flat
pattern image, the area of coverage of the stent and perforated
cover can then be determined. The percentage of porosity is then
determined using the porosity index equation presented above.
[0115] The perforated cover provides a favorable substrate to
deliver drugs or other therapeutic agents to the vessel. Because
the perforated cover has uniformly sized perforations, the elution
of the drug into the vessel can be controlled more precisely and
uniformly.
[0116] The perforated cover structure creates uniform support of
the vessel. The opening size in traditional stents or stents
covered with a woven material can vary depending on how the stent
is bent, on the diameter it achieves within a stenotic vessel, or
other factors.
[0117] The structure is also highly reliable. For stents
constructed with a woven covering, a single broken fiber may
unravel and protrude into the vessel lumen. With the perforated
cover structures described herein, all web locations are supported
by multiple additional webs in close proximity, minimizing the
potential for a broken web to protrude into the lumen of the
vessel. In addition, the web is fully attached to the stent
minimizing the risk of material protruding into the lumen.
[0118] Another desirable aspect of the perforated cover is it
allows continued access to side branch vessels. A 0.35 mm diameter
guide wire can be easily threaded through the wall of the stent
with a perforated cover pattern with perforations on the order of
about 0.5 mm smallest transverse dimension. This perforation can
then be dilated up using an interventional dilator to 1 millimeter
diameter. Exchanging a balloon for the dilator, and positioning the
balloon through the perforated cover over the guide wire, the
perforated cover can be dilated to the desired size. This process
opens a transmural hole that would allow passage of other devices
(e.g., balloons, stents, etc) into the side branch and minimizes
the disruption of the blood flow to the side branch artery.
[0119] The stent device shall have sufficient strength to allow,
after deploying a partial length of the stent device, the stent
device to be repositioned without suffering damage. These forces
shall also be greater than the minimum forces required to break the
delivery catheter, which may be as defined in ISO 10555-1 as 3N,
5N, 10N, or 15N (depending on the diameter of the catheter).
Correspondingly, for the 5 holes per circumference samples with 5
intermediate webs between the holes (Sample B1 and B2 described
below), the minimum web tensile strength would be the ISO 10555
values divided by 5 webs, or 0.6N, 1N, 2N, or 3N.
[0120] Device tensile strengths were measured in a tensile testing
machine. 6 mm and 37 mm diameter stents made as described above
were clamped in semi-circular jaw inserts, as shown in FIG. 4P. The
jaw inserts were made from stainless steel and had a thickness of
approximately 3 mm. The width of the semi-circular cut in the jaw
inserts 56 was 5 to 15% larger than half the stent device's
circumference; for example, for the 6 mm diameter stent, a 10 mm
semi-circular cut jaw insert width was used while for the 37 mm
diameter stent, a 65 mm semi-circular jaw insert width was used.
This sizing allowed the ends of the device to be flattened and
easily fit within the semi-circular feature of the jaw insert 56.
This type of jaw insert configuration was selected to maximize the
potential for the sample to break away from the clamped jaw insert
56. Data from breaks that occur immediately adjacent to a jaw edge
are to be discarded. Gauge length 59 was measured from the top of
the semi-circular cut in the top jay insert to the bottom of the
semi-circular cut in the bottom jaw insert, as shown in FIG. 4P. To
minimize the risk of jaw slippage, the jaw inserts 56 were lined on
their inward face with 400 grit sandpaper using double-faced
transparent tape. The edge of the semi-circle was also provided
with a 1.5 mm radius. These semi-circular jaw inserts 56 were
placed within the tensile tester's standard serrated jaws 58, as
shown in FIG. 4P. The air pressure to the jaws was set to 0.62 MPa.
The tensile tester (Instron model number 5564, Instron Corp.,
Norwood Mass.) was programmed for a rate of 100% gauge length per
minute (rate consistent with ISO 7198 requirements). Using this
requirement, the 6 mm diameter stents were tested using a gauge
length of 15 mm with a rate of 15 mm/min, while the 37 mm diameter
stents were tested using a 40 mm gauge length with a rate of 40
mm/min. All testing occurred at room temperature (24.degree. C.).
Device tensile strength was determined at the peak force from
initial breakage (away from the jaw inserts) of the perforated
cover.
[0121] Tensile strength of the web between a pair of adjacent
perforations was measured using a fiber was threaded through each
of the perforations and around the intervening single web of the
perforated cover, as follows. On the bottom jaw of the tensile
tester, a rod smaller than the diameter of the stent (i.e., 4.6 mm
diameter stainless steel rod) was clamped in the tensile tester's
bottom jaw in a horizontal orientation. The stent to be tested was
slid onto the rod and against the tensile tester's jaw. A length of
fishing line (of about 0.35 mm diameter) was threaded around a
single web of the perforated cover by passing the line through two
perforations adjacent to the web, in one direction for the first
perforation and the opposite direction for the second perforation.
Both ends of the fishing line were clamped to the upper jaw of the
tensile tester with the air pressure supplied to operate the jaws
set at 0.062 MPa. Any smooth fiber or line with a tensile strength
of at least 25N and with a diameter smaller than the perforation
diameter could be used; smooth-surface fibers are necessary in
order to break rather than cut the web between the adjacent
perforations. When looping the fiber through a single web of the
device, care was taken to assure that only the web, and not any
section the metal portion of the stent, was looped. The tensile
tester was then run at a jaw separation rate of 100 mm/min
(consistent with ISO 7198 requirements for suture retention
strength test). Testing was performed at room temperature
(24.degree. C.). Web tensile strength was determined from the peak
force required to cause the single section of web to break.
[0122] Porosity index was also determined for stents provided with
the perforated cover. This can be determined using the stent
inspection system as described above or by analyzing each component
(stent and cover) separately. This is accomplished on a uniformly
perforated structure by measuring the area of the nominal
perforation and multiplying by the total number of perforations.
This is then divided by the total area of the cover, subtracting
this value from 100%, resulting in the cover area percentage. The
stent area ratio can be determined by using techniques commonly
know in the industry (e.g., ASTM F2081).
[0123] Bend radius was determined using the method defined in ISO
7198 (1998, section 8.9).
[0124] All perforated cover-based device samples were built as
described above with the following exceptions. For Samples A1 and
A2, the perforated cover was sized to fit on a 6 mm stent where
perforated cover aperture sizes were approximately 0.40 to 0.45 mm
with about 40 holes per circumference. For Samples B1 and B3, the
perforated cover was sized to fit on a 6 mm stent where perforated
cover aperture sizes were approximately 4.0-4.2 mm with 5 holes per
circumference. For Samples C1 and C3, the perforated cover was
sized to fit on a 37 mm stent where perforated cover aperture sizes
were approximately 0.40 to 0.45 mm with 220 holes per
circumference. For Samples D1 and D3, the perforated cover was
sized to fit on a 37 mm stent where perforated cover aperture sizes
were 5.5 to 5.6 mm with 20 holes per circumference. For Sample E,
the perforated cover was constructed using the same method as for
Samples D1 and D2, but the perforated cover was not attached to a
stent. All samples had an approximate thickness of 40 microns, as
determined by a calibrated snap gauge (Mitutoyo Model ID-C112EB on
base Mitutoyo Code 7004).
[0125] Samples were tested as described above. Sample A had
porosity index of 52% and a web tensile strength of 2.8N. The
Sample A2 was bent around a calibrated pin with a diameter of 5.5
mm without kinking. Samples A1 and A2 had a web width of about 60
to 100 microns. Sample B1 had a device tensile strength of 7.0N
while Sample B2 had a web tensile strength of 4.4N. Samples B1 and
B2 had a porosity index of 78%. The Sample B3 was bent around a
calibrated pin with a diameter of 1.5 mm without kinking. Samples
B1-B3 had a web width of about 100 to 120 microns. Sample C1 had a
device tensile strength of 95.8N while Sample C2 had a web tensile
strength of 2.0N. Samples C1 and C2 had a porosity index of 53%.
The Sample C3 was bent around a calibrated pin with a diameter of
12.8 mm without kinking. Samples C1-C3 had a web width of about 80
to 90 microns. Sample D1 had a device tensile strength of 20.9N
while Sample D2 had a web tensile strength of 4.7N. The Sample D3
was bent around a calibrated pin with a diameter of 6.4 mm without
kinking. Samples D1-D3 had a web width of about 120 to 200 microns.
Sample E (perforated cover only) had a device tensile strength of
21.0N. Sample D had a porosity index of 82%.
[0126] Note that when testing the perforated cover alone (Sample
E), the cover's tensile strength for this construct of 20
holes/circumference, corresponding to 20 webs/circumference, was
21N. Other constructs with 10, 5, or 2 holes may be desirable
(corresponding to 10, 5, and 2 webs/circumference respectively),
with estimated corresponding tensile strengths of 10N, 5N, and 2N
respectively.
[0127] While particular embodiments of the present invention have
been illustrated and described herein, the present invention should
not be limited to such illustrations and descriptions. It should be
apparent that changes and modifications may be incorporated and
embodied as part of the present invention within the scope of the
following claims
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