U.S. patent application number 15/414528 was filed with the patent office on 2017-07-13 for segmented scaffold designs.
The applicant listed for this patent is Abbott Cardiovascular Systems Inc.. Invention is credited to Chad Abunassar, Boris Anukhin, Syed Faiyaz Ahmed Hossainy, Michael Ngo, John E. Papp, Lewis B. Schwartz, Mikael Trollsas.
Application Number | 20170196718 15/414528 |
Document ID | / |
Family ID | 48087725 |
Filed Date | 2017-07-13 |
United States Patent
Application |
20170196718 |
Kind Code |
A1 |
Papp; John E. ; et
al. |
July 13, 2017 |
SEGMENTED SCAFFOLD DESIGNS
Abstract
Segmented scaffolds composed of disconnected scaffold segments
with overlapping end rings are disclosed. Scaffolds with at least
one discontinuous link are also disclosed.
Inventors: |
Papp; John E.; (Temecula,
CA) ; Hossainy; Syed Faiyaz Ahmed; (Hayward, CA)
; Ngo; Michael; (San Jose, CA) ; Abunassar;
Chad; (San Francisco, CA) ; Anukhin; Boris;
(Santa Cruz, CA) ; Trollsas; Mikael; (San Jose,
CA) ; Schwartz; Lewis B.; (Lake Forest, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Abbott Cardiovascular Systems Inc. |
Santa Clara |
CA |
US |
|
|
Family ID: |
48087725 |
Appl. No.: |
15/414528 |
Filed: |
January 24, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14333354 |
Jul 16, 2014 |
9585779 |
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15414528 |
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13584678 |
Aug 13, 2012 |
8834556 |
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14333354 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F 2002/826 20130101;
B29C 2035/0838 20130101; A61F 2/90 20130101; A61F 2240/001
20130101; A61F 2/915 20130101; B29L 2031/7534 20130101; A61F
2002/9155 20130101; B29K 2995/006 20130101; A61F 2/958 20130101;
A61F 2/89 20130101; A61F 2210/00 20130101; A61F 2230/0013 20130101;
A61F 2002/91591 20130101; A61F 2230/0054 20130101; A61F 2/86
20130101; B29C 66/1248 20130101; B29C 65/64 20130101; B29K 2067/046
20130101; B29C 35/0805 20130101; B29C 66/71 20130101 |
International
Class: |
A61F 2/915 20060101
A61F002/915; B29C 65/00 20060101 B29C065/00; B29C 35/08 20060101
B29C035/08; B29C 65/64 20060101 B29C065/64; A61F 2/958 20060101
A61F002/958; A61F 2/89 20060101 A61F002/89 |
Claims
1-26. (canceled)
27. A method, comprising: using a tube made from a biodegradable
polymer composition; making a scaffold from the tube comprising
laser cutting a scaffold pattern from the tube, wherein the
scaffold pattern comprises scaffold segments and linking elements
connecting adjacent scaffold segments; crimping or pre-crimping the
scaffold; and creating a discontinuity in at least one of the
linking elements of the crimped or pre-crimped scaffold.
28. The method of claim 27, wherein the discontinuity is created by
a laser cutting through the at least one of the linking
elements.
29. The method of claim 27, wherein the scaffold is pre-crimped to
a balloon or a mandrel.
30. The method of claim 27, wherein the creating the discontinuity
in the at least one of the linking elements is laser cutting the at
least one of the linking elements.
31. The method of claim 30, wherein the crimped or pre-crimped
scaffold is on a mandrel during the laser cutting the at least one
of the linking elements.
32. The method of claim 31, wherein the crimped or pre-crimped
scaffold is on a balloon during the laser cutting the at least one
of the linking elements.
33. The method of claim 27, wherein the biodegradable polymer
composition comprises polycaprolactone (PCL).
34. The method of claim 27, wherein a scaffold segment comprises a
plurality of diamond shape cells, a first end forming peaks and
troughs, and a second end forming peaks and troughs, and wherein a
peak at the first end is longitudinally aligned with a trough at
the second end, and a length of the segment is equal to a width of
two diamond shape cells, or a peak at the first end is
longitudinally aligned with a peak at the second end, and a length
of the segment is equal to a width of one diamond shape cell.
35. A method, comprising: using a tube made from a biodegradable
polymer composition; making a scaffold from the tube comprising
laser cutting a scaffold pattern from the tube, wherein the
scaffold pattern comprises adjacent ring elements interconnected by
linking elements; and after cutting the tube to make the scaffold
pattern, cutting a plurality of the linking elements.
36. The method of claim 35, wherein at least two linking elements
adjoin a first ring of the ring elements with a second ring of the
ring elements.
37. The method of claim 36, wherein the cutting creates
discontinuities in a portion of the links, such that a helical
pattern of discontinuous links are formed in the scaffold
pattern.
38. The method of claim 36, wherein one of the at least two linking
elements is cut and the other one of the at least two linking
elements is not cut.
39. The method of claim 35, wherein the polymer composition
comprises polycaprolactone (PCL).
40. The method of claim 35, further comprising: after making the
scaffold pattern, placing the scaffold on a mandrel, and cutting
the plurality of the linking elements while the scaffold is on the
mandrel.
41. The method of claim 40, wherein the scaffold is in a
pre-crimped configuration when on the mandrel.
42. The method of claim 40, further comprising, crimping the
scaffold to a balloon having a balloon diameter, wherein the
scaffold in the pre-crimp configuration has a pre-crimped diameter
that is between an initial diameter and the balloon diameter, and
wherein the scaffold is crimped form the pre-crimped diameter to a
diameter less than the balloon diameter.
43. A method, comprising: using a scaffold having a pattern made
from a biodegradable polymer, the scaffold pattern comprising
adjacent ring elements interconnected by linking elements; placing
the scaffold on a balloon or a mandrel; and laser cutting a
plurality of the linking elements.
44. The method of claim 43, wherein the scaffold is in a
pre-crimped configuration prior to the laser cutting.
45. The method of claim 43, wherein the laser cutting of the
plurality of linking elements includes one or both of: removing all
of a linking element connecting adjacent ring elements; or making a
linking element a discontinuous linking element, wherein a space
between free ends of the discontinuous linking element is between 2
to 5, 5 to 10 or 10 to 50 microns.
46. The method of claim 43, wherein the scaffold comprises a first
ring adjoined with a second ring by a first plurality of linking
elements, the second ring adjoined with a third ring by a second
plurality of linking elements, and the third ring adjoined with a
fourth ring by a third plurality of linking elements; the fourth
ring adjoined with a fifth ring by a fourth plurality of linking
elements; wherein at least one of the first plurality of linking
elements is cut, none of the second plurality of linking elements
is cut, none of the third plurality of linking elements is cut, and
at least one of the fourth plurality of linking elements is cut.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates to methods of treatment of blood
vessels with polymeric medical devices, in particular, stent
scaffolds.
Description of the State of the Art
[0002] This invention relates to radially expandable
endoprostheses, that are adapted to be implanted in a bodily lumen.
An "endoprosthesis" corresponds to an artificial device that is
placed inside the body. A "lumen" refers to a cavity of a tubular
organ such as a blood vessel. A stent is an example of such an
endoprosthesis. Stents are generally cylindrically shaped devices
that function to hold open and sometimes expand a segment of a
blood vessel or other anatomical lumen such as urinary tracts and
bile ducts. Stents are often used in the treatment of
atherosclerotic stenosis in blood vessels. "Stenosis" refers to a
narrowing or constriction of a bodily passage or orifice. In such
treatments, stents reinforce body vessels and prevent restenosis
following angioplasty in the vascular system. "Restenosis" refers
to the reoccurrence of stenosis in a blood vessel or heart valve
after it has been treated (as by balloon angioplasty, stenting, or
valvuloplasty) with apparent success.
[0003] Stents are typically composed of scaffolding that includes a
pattern or network of interconnecting structural elements or
struts, formed from wires, tubes, or sheets of material rolled into
a cylindrical shape. This scaffold or scaffolding gets its name
because it physically holds open and, if desired, expands the wall
of the passageway. Typically, stents are capable of being
compressed or crimped onto a catheter so that they can be delivered
to and deployed at a treatment site.
[0004] Delivery includes inserting the stent through small lumens
using a catheter and transporting it to the treatment site.
Deployment includes expanding the stent to a larger diameter once
it is at the desired location. Mechanical intervention with stents
has reduced the rate of acute closure and restenosis as compared to
balloon angioplasty.
[0005] Stents are used not only for mechanical intervention but
also as vehicles for providing biological therapy. Biological
therapy uses medicated stents to locally administer a therapeutic
substance. The therapeutic substance can also mitigate an adverse
biological response to the presence of the stent. A medicated stent
may be fabricated by coating the surface of either a metallic or
polymeric scaffold with a bioresorbable polymeric carrier that
includes an active or bioactive agent or drug. Polymeric
scaffolding may also serve as a carrier of an active agent or drug
by incorporating a drug throughout the scaffolding material.
[0006] The stent must be able to satisfy a number of mechanical
requirements. The stent must have sufficient radial strength so
that it is capable of withstanding the structural loads, namely
radial compressive forces, imposed on the stent as it supports the
walls of a vessel. This structural load will change as a function
of time as the vessel heals, undergoes positive remodeling, or
adapts to the presence of the stent. Once expanded, the stent must
adequately provide lumen support during a time required for
treatment in spite of the various forces that may come to bear on
it, including the cyclic loading induced by the beating heart. In
addition, the stent must possess sufficient flexibility with a
certain resistance to fracture.
[0007] Stents implanted in coronary arteries are primarily
subjected to radial loads, typically cyclic in nature, which are
due to the periodic contraction and expansion of vessels as blood
is pumped to and from a beating heart. Stents implanted in
peripheral blood vessels, or blood vessels outside the coronary
arteries, e.g., iliac, femoral, popliteal, renal and subclavian
arteries, however, can undergo significant nonpulsatile forces and
must be capable of sustaining both radial forces and crushing or
pinching loads. These stent types are implanted in vessels that are
closer to the surface of the body, and may be close to joints.
Because these stents are close to the surface of the body, they are
particularly vulnerable to crushing or pinching loads, which can
partially or completely collapse the stent and thereby block fluid
flow in the vessel.
[0008] The superficial femoral artery (SFA), in particular, can
subject a scaffold to various nonpulsatile forces, such as radial
compression, torsion, flexion, and axial extension and compression,
which place a high demand on the mechanical performance of
implants.
[0009] Thus, in addition to high radial strength, stents or
scaffolds for peripheral vessels such as the SFA, require a high
degree of crush recovery. The term "crush recovery" is used to
describe how the scaffold recovers from a pinch or crush load,
while the term "crush resistance" is used to describe the minimum
force required to resist a permanent deformation of a scaffold.
[0010] Stents made from biostable or non-bioerodible materials,
such as metals, have become the standard of care for percutaneous
coronary intervention (PCI) as well as in peripheral applications,
such as the superficial femoral artery (SFA), since such stents
have been shown to be capable of preventing early and late recoil
and restenosis. In the SFA, where the artery undergoes extensive
movement, self expanding stents made from materials such as Nitinol
are the standard of care.
[0011] However, in many treatment applications, the presence of a
stent in a body is necessary for a limited period of time until its
intended function of, for example, maintaining vascular patency
and/or drug delivery is accomplished. Moreover, it is believed that
biodegradable scaffolds allow for improved healing of the
anatomical lumen since they allow the vessel to return to its
natural state as compared to metal stents, which may lead to a
reduced incidence of late stage thrombosis. In these cases, there
is a desire to treat a vessel using a polymer scaffold, in
particular a bioerodible polymer scaffold, as opposed to a metal
stent, so that the prosthesis's presence in the vessel is for a
limited duration.
[0012] There are numerous challenges to overcome when developing a
polymer scaffold, particularly in peripheral blood vessels, or
blood vessels outside the coronary arteries in which a stent is
subjected to both radial forces and nonpulsatile forces. One way of
addressing the adverse effects of nonpulsatile forces is to implant
stents as a series of disconnected segments. In this way, the
transmission of nonpulsatile forces along the stent are reduced or
eliminated.
SUMMARY OF THE INVENTION
[0013] Embodiments of the present invention include a segmented
scaffold comprising: two or more radially expandable disconnected
scaffold segments arranged end to end, wherein each segment
includes two or more undulating cylindrical rings composed of
struts, and wherein rings at an end of each segment comprise peak
undulations projecting longitudinally outward from the end of the
segment and comprise valley undulations extending longitudinally
toward the segment, and wherein the peak and valley undulations of
adjacent rings overlap.
[0014] Embodiments of the present invention include a method of
delivering a scaffold: providing a segmented scaffold crimped over
a delivery balloon, the segmented scaffold comprising two or more
radially expandable disconnected scaffold segments arranged end to
end, wherein each end of the segments comprises undulating
cylindrical rings composed of struts and wherein undulations of
adjacent segments overlap; and expanding the scaffold segments to a
deployment diameter, wherein the undulations of the adjacent
segments overlap at the deployed diameter.
[0015] Embodiments of the present invention include a radially
expandable scaffold segment comprising: two or more connected
undulating cylindrical rings composed of struts, wherein the
undulating rings of each segment form a plurality of diamond-shaped
cells with two pairs of opposing vertices, one pair being
longitudinally aligned and one pair being circumferentially
aligned, and wherein alternating diamonds around at least one end
ring are omitted to form peak and valley undulations along the at
least one end ring with a longitudinal length that is a
longitudinal length of the diamond-shaped cells.
[0016] Embodiments of the present invention include a scaffold
comprising: a plurality of scaffold segments in a crimped reduced
configuration; and at least one discontinuous linking element
between adjacent segments comprising a discontinuity located
between the adjacent segments.
[0017] Embodiments of the present invention include a method of
modifying a scaffold comprising: providing a scaffold in a crimped
reduced configuration, wherein the scaffold comprises longitudinal
scaffold segments and linking elements connecting adjacent scaffold
segments; and creating a discontinuity in at least one linking
element between at least one set of adjacent segments.
INCORPORATION BY REFERENCE
[0018] All publications and patent applications mentioned in this
specification are herein incorporated by reference to the same
extent as if each individual publication or patent application was
specifically and individually indicated to be incorporated by
reference, and as if each said individual publication or patent
application was fully set forth, including any figures, herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 depicts an exemplary stent scaffold.
[0020] FIG. 2 depicts an exemplary scaffold pattern which shows
schematically the forces acting on the scaffold.
[0021] FIG. 3A depicts an exemplary scaffold segment of a segmented
scaffold.
[0022] FIG. 3B depicts a close-up view of a portion of the scaffold
segment in FIG. 3A illustrating various features.
[0023] FIG. 4 depicts a segmented scaffold composed of a plurality
of axial segments from FIG. 3A.
[0024] FIG. 5 depicts scaffold segments of a segmented scaffold
mounted over a balloon in a folded configuration.
[0025] FIG. 6 is a schematic illustration of a segmented scaffold
with segments illustrating the reduced diameter of unsupported
sections of the vessel wall between the segments.
[0026] FIG. 7 depicts a flattened view of an exemplary scaffold
segment 400 similar to the segment depicted in FIG. 3A.
[0027] FIG. 8 depicts an exemplary segment based on the segment of
FIG. 7 in which alternating diamonds are omitted one end and
in-line diamonds are removed on the other end.
[0028] FIG. 9 depicts an exemplary segment based on the segment of
FIG. 7 in which alternating diamonds are omitted one end and
off-set diamonds are removed on the other end.
[0029] FIG. 10 depicts two in-line segments of FIG. 8 that are
interlinked.
[0030] FIG. 11 depicts a three-dimensional view of two interlinked
segments.
[0031] FIG. 12 depicts interlinked segments in a crimped state with
about 50% ring overlap or engagement of the end rings.
[0032] FIG. 13 depicts interlinked segments in a crimped state with
about 100% ring overlap or engagement of the end rings.
[0033] FIG. 14 depicts an expanded view of the interlinked
scaffolds of FIG. 10.
[0034] FIG. 15 depicts an image of a deployed segmented scaffold
with large segment to segment gaps.
[0035] FIG. 16 depicts deployed segmented scaffolds of which the
left-most segment is rotated relative to the middle segment
resulting in a non-uniform gap.
[0036] FIG. 17 depicts a schematic of part of the interlink area of
the crimped segmented scaffold of FIG. 13.
[0037] FIG. 18A depicts an interlinked region of crimped
interlinked segments.
[0038] FIG. 18B depicts an expanded view of one of the peak
undulations.
[0039] FIG. 19 depicts a portion of an end ring of a segment with a
head portion and body portion.
[0040] FIG. 20 depicts a portion of an end ring of a segment with a
head portion and body portion.
[0041] FIG. 21 depicts a close-up view of a discontinuous linking
element between scaffold segments.
[0042] FIG. 22 depicts a two-dimensional projection of adjacent
scaffold segments that are all disconnected.
[0043] FIG. 23 depicts a two-dimensional projection of adjacent
scaffold segments that include both intact and discontinuous
linking elements.
[0044] FIG. 24 depicts a close-up view of a region between scaffold
segments, like those shown in FIG. 3A.
[0045] FIG. 25 depicts a close-up view of a portion of a scaffold
like the one shown in FIG. 1.
[0046] FIG. 26 depicts a pattern which is pattern from FIG. 1 with
one discontinuous linking element and one intact linking element
between each ring.
[0047] FIG. 27 depicts a pattern which is the pattern from FIG. 1
with one discontinuous linking element and one intact linking
element at every third segment gap.
[0048] FIG. 28 depicts a segmented scaffold, which is the scaffold
from FIG. 4 with two intact linking elements and two discontinuous
linking elements between each segment.
[0049] FIG. 29 is an image of interlinked segmented scaffold in a
deployed state from a bench test.
[0050] FIG. 30 depicts images of an interlinked segmented scaffold
composed of two segments.
[0051] FIG. 31 depicts images of interlinked segmented scaffolds
composed of three in-line segments.
[0052] FIG. 32 depicts images of an interlinked segmented scaffold
composed of three off-set segments.
[0053] FIG. 33 depicts images of an interlinked scaffold segmented
composed of six in-line segments.
DETAILED DESCRIPTION OF THE INVENTION
[0054] Coronary arteries refer generally to arteries that branch
off the aorta to supply the heart muscle with oxygenated blood.
Peripheral arteries refer generally to blood vessels outside the
heart. In both coronary artery disease and peripheral artery
disease, the arteries become hardened and narrowed or stenotic and
restrict blood flow. In the case of the coronary arteries, blood
flow is restricted to the heart, while in the peripheral arteries
blood flow is restricted leading to the kidneys, stomach, arms,
legs, feet, and brain. The narrowing is caused by the buildup of
cholesterol and other material, called plaque, on the inner walls
of the vessel. Such narrowed or stenotic portions are often
referred to as lesions. Arterial disease also includes the
reoccurrence of stenosis or restenosis that occurs after an
angioplasty treatment. Although there are probably several
mechanisms that lead to restenosis of arteries, an important one is
the inflammatory response, which induces tissue proliferation
around an angioplasty site. The inflammatory response can be caused
by the balloon expansion used to open the vessel, or if a stent is
placed, by the foreign material of the stent itself.
[0055] A stent, a stent scaffold, or scaffold includes a plurality
of cylindrical rings connected or coupled with linking elements.
When deployed in a section of a vessel, the cylindrical rings are
load bearing and support the vessel wall at an expanded diameter or
a diameter range due to cyclical forces in the vessel. Load bearing
refers to the supporting of the load imposed by radial inwardly
directed forces. Structural elements, such as the linking elements
or struts primarily serve to maintain stability and connectivity
between the rings. For example, a stent may include a scaffold
composed of a pattern or network of interconnecting structural
elements or struts.
[0056] FIG. 1 illustrates a portion of an exemplary prior art stent
or scaffold pattern 100 shown in a flattened view. The pattern 100
of FIG. 1 represents a tubular scaffold structure so that an axis
A-A is parallel to the central or longitudinal axis of the
scaffold. FIG. 1 shows the scaffold in a state prior to crimping or
after deployment. Pattern 100 is composed of a plurality of ring
struts 102 and link struts 104. The ring struts 102 form a
plurality of cylindrical rings, for example, rings 106 and 108,
arranged about the cylindrical axis A-A. The rings have an
undulating or sinusoidal structure with alternating crests or peaks
116 and troughs or valleys 118. The rings are connected by the link
struts 104. The scaffold has an open framework of struts and links
that define a generally tubular body with gaps 110 in the body
defined by rings and struts. A cylindrical tube may be formed into
this open framework of struts and links by a laser cutting device
that cuts such a pattern into a thin-walled tube that may initially
have no gaps in the tube wall.
[0057] The structural pattern in FIG. 1 is merely exemplary and
serves to illustrate the basic structure and features of a stent or
scaffold pattern. A stent such as stent 100 may be fabricated from
a polymeric tube or a sheet by rolling and bonding the sheet to
form the tube. A tube or sheet can be formed by extrusion or
injection molding. A stent pattern, such as the one pictured in
FIG. 1, can be formed on a tube or sheet with a technique such as
laser cutting or chemical etching. The stent can then be crimped
onto a balloon or catheter for delivery into a bodily lumen.
[0058] The width and/or radial thickness of the struts in a
scaffold may be 80 to 400 microns, or more narrowly, 100 to 250
microns, 140 to 180 microns, 200 to 400 microns, 140 to 160
microns, or 300 to 350 microns. The thickness and width can be
different. For example, the width can be at or about 350 microns
(e.g., .+-.10 microns) and the thickness can be at or about 300
microns (e.g., .+-.10 microns).
[0059] Semicrystalline polymers such as poly(L-lactide) (PLLA) with
glass transition temperature (Tg) above human body temperature may
be suitable as materials for a totally bioresorbable scaffold since
they are relatively stiff and strong at the conditions of the human
body. However, they tend to be brittle at these conditions. These
polymer systems exhibit a brittle fracture mechanism in which there
is little plastic deformation prior to failure. As a result, a
stent fabricated from such polymers can be vulnerable to fracture
during fabrication and use of a scaffold, i.e., crimping, delivery,
deployment, and during a desired treatment period
post-implantation.
[0060] Embodiments of the present invention are applicable to
endovascular treatment of coronary and peripheral disease in
coronary arteries and various peripheral vessels including the
superficial femoral artery, the iliac artery, and carotid artery.
The embodiments are further applicable to various stent types, such
as self-expandable and balloon expandable stents. The embodiments
are further applicable to various stent designs including
scaffolding structures formed from tubes, wire structures, and
woven mesh structures. Embodiments also applicable to different
materials that are permanent implants such as polymers and metals
like Nitinol, Algeloy, stainless steel and cobolt chrome.
[0061] In general, the initial clinical need for a bioresorbable
scaffold is to provide mechanical/structural support to maintain
patency or keep a vessel open at or near the deployment diameter.
The scaffold is designed to have sufficient radial strength or
vessel wall support for a period of time. The vessel wall support
provided by the stent allows the stented segment of the vessel to
undergo healing and remodeling at the increased diameter.
Remodeling refers generally to structural changes in the vessel
wall that enhance its load-bearing ability.
[0062] A period of vessel wall support is required in order to
obtain permanent positive remodeling and vessel healing and hence
maintenance of vessel patency. As the polymer of the stent
degrades, the radial strength of the scaffold decreases and the
load of the vessel is gradually transferred from the scaffold to
the remodeled vessel wall. In addition to the decline in radial
strength, the degradation of the scaffold also causes a gradual
decline in the mechanical integrity, i.e., connectivity of struts
and the size and shape of the overall scaffold structure. The
struts gradually resorb and disappear from the vessel.
[0063] The amount of movement experienced by a peripheral scaffold
in the peripheral artery is greater than what a coronary scaffold
experiences in the coronary artery. A peripheral scaffold can be
subjected to a high degree of flexing, axial
elongation/compression, pinching, bending, and torsion after
implantation. Axial stresses on the scaffold can arise from the
axial compression and extension, flexural stresses are imposed by
lateral flexing, crushing forces are imparted by pinching, while
helical stress can arise from torsional forces.
[0064] Such stresses are propagated along the length of the
scaffold and can impart significant forces throughout the scaffold
structure. The forces can cause failure in ring struts, resulting
in a decrease or loss in vessel wall support provided by the
scaffold. Such forces can be transmitted along the length of the
scaffold by link struts that connect rings.
[0065] Link strut breakage is not inherently deleterious to either
performance or safety. Bench testing and animal study results
suggest that scaffold properties of radial strength, crush
recovery, and crush resistance are primarily attributable to the
mechanical/structural integrity of the rings in the scaffold and
not the links.
[0066] Strut breakage can lead to release of fragments in the blood
and tissue irritation from broken strut fragments. Fragment release
could result in thrombosis. Broken fragments can be mechanically
injurious to the vessel leading to tissue irritation or even vessel
dissection and perforation.
[0067] FIG. 2 depicts the exemplary scaffold pattern 100 which
shows schematically the forces acting on the peripheral scaffold.
Line A-A represents the cylindrical axis of the stent. The arrows
around the edges represent the forces acting on the scaffold during
delivery and after deployment. Arrows 111 represent bending, arrows
112 represent radial compression, and arrows 114 represent axial
compression. Bending occurs during delivery through torturous
anatomy and after deployment.
[0068] Radially compressive forces on the scaffold are caused by
the push back of the vessel walls on the scaffold. Axial
compressive forces in the SFA arise due to movement of a leg such
as during walking or bending of the leg. In the SFA, the axial
compressive forces can be considerable as the vessel is compressed
up to 7% or more and relaxed repeatedly up to one million
cycles/year.
[0069] Cracks in the scaffold occur when it is subjected to a
sufficiently high force such as resulting from bending during
delivery or repetitive forces after deployment that cause fatigue.
These cracks can cause a loss of radial strength or separation of
struts of the scaffold that drift downstream of the scaffold.
[0070] A crack in the ring strut may cause a reduction or loss of
radial strength, while a crack in the link is less damaging to the
scaffold in terms of radial strength, crush resistance, and crush
recovery. It is believed that if the axial forces on the scaffold
were reduced, the occurrence of ring cracks would be significantly
reduced. When axial forces that travel through the links to the
ring struts are reduced, then the potential for ring strut
fractures are also reduced.
[0071] The various embodiments of the present invention are
directed to peripheral scaffolds that are subjected to significant
nonpulsatile forces upon implantation. Embodiments are further
directed to methods and systems for delivering such peripheral
scaffolds. The embodiments of the scaffold designs are directed to
reducing or eliminating strut fracture and breakage during use of
the scaffold.
[0072] The embodiments are also directed to implanting such
scaffolds in areas or vessels where there is no significant vessel
movement such as coronary, iliac, renal etc.
[0073] The segmented scaffolds disclosed herein have advantages
over non-segmented scaffods. For example, segmented scaffolds offer
a substantial cost savings over conventional stent manufacturing by
reducing the number of lengths of scaffolds needed in a product. In
addition, the segmented scaffold segments with the disclosed
diamond pattern have substantially higher radial strength (more
than double that of a conventional stent) over conventional stent
patterns which provides improved vessel holding open ability. This
can be useful in highly calcified anatomy.
[0074] Various embodiments include a scaffold composed of axial
scaffold segments that are not connected by link struts.
Embodiments of such a scaffold include two or more radially
expandable axial scaffold segments arranged axially end to end. The
axial segments, in particular, axially adjacent segments are not
connected by any physical structure or material of the scaffold.
The axial segments, however, may be indirectly in contact through
another structure such as a support member or a sheath. The axial
segments may further be connected by structures not part of
structure from which the scaffold segments are formed, such as a
tube.
[0075] In general, upon deployment of the scaffold segments, forces
subjected on one axial segment cannot be transmitted to other axial
segments through linking struts as such forces are by linking
struts of a scaffold shown in FIG. 1. The axial segments may be
composed of a plurality of interconnected struts. Forces subjected
to a segment can be transmitted between struts within the segment,
but not between segments.
[0076] In some embodiments, the axial segments are composed of one
or more cylindrical rings of struts. A cylindrical ring may be
composed of undulating struts having crests and troughs. The
cylindrical rings of struts that are adjacent in a segment are
connected. The rings may be connected by link struts.
Alternatively, the rings may be directly connected to one another
without link struts. The number of rings in a segment may be one or
any number greater than one. In some embodiments, a segment can
have 1 or more, 2 or more, 1 to 6 rings, 1 to 3 rings, 2 to 6
rings, or 2 or 3 rings.
[0077] Upon deployment, the axial segments remain intact for a
period of time and maintain a ring shape at or near the deployed
diameter. Since the axial segments are not connected, they are
uncoupled which prevents transmission of axial compression between
segments. The decoupled axial segments retain sufficient radial
strength to support the vessel at or near the deployed diameter.
The decoupling of the axial segments reduces stress, for example,
from axial compression that causes fracture of ring struts. The
reduced ring strut fracture helps maintain the radial strength and
the crush recovery and resistance to broken off struts of the
scaffold floating down the vessel as emboli. The decoupling of
rings reduces or prevents propagation of fracture of rings due to
bending of the scaffold structure along its axis.
[0078] In some embodiments, a scaffold with disconnected axial
segments can be fabricated by forming the axial segments
separately. For example, a scaffold pattern can be cut into a
thin-walled tube having an axial length the same as the desired
axial segment. Alternatively, a scaffold can be fabricated by laser
cutting a tube and then axial segments can be formed cutting the
scaffold into disconnected axial segments by cutting link struts
between segments or cutting the link struts between segments off
entirely. Unless otherwise specified, scaffold segments or segments
refer to disconnected scaffold segments or segments.
[0079] The stability of an axial segment depends on the length of
the axial segment. The stability is inversely related to the length
of the axial section. The susceptibility to fracture from
nonpulsatile forces, however, is directly related to the length of
the axial section. The length of the axial segments should be large
enough so that it has a desired stability, while having reduced
fracture arising from nonpulsatile forces.
[0080] The radial strength and radial stiffness of a scaffold or
scaffold segment increases with the degree of connectivity of a
scaffold. The degree of connectivity refers in part to the number
of link struts between rings and the length of the link struts:
more link struts and shorter link struts tend to increase strength
and stiffness. The increase in strength and stiffness from increase
in link struts has practical limitations for the FIG. 1 type
designs. As the number of links is increased, the width of each
ring strut is reduced to accommodate for the extra link when in the
crimped state.
[0081] The stiffer the scaffold, the more susceptible the scaffold
is to fracture. In the present embodiments, since compressive
forces are not transmitted along an entire scaffold length, the
scaffold segments can be made with a higher connectivity than a
scaffold that does not have disconnected axial segments.
[0082] In the scaffolds such as the one depicted in FIG. 1, the
crests of the axial rings are axially aligned or approximately
axially aligned. The stiffness of the axial segments of such a
scaffold can be increased by increasing the number of link struts
between axially adjacent peaks of adjacent rings. Every pair of
aligned peaks between adjacent rings can be connected, every other
pair of aligned peaks can be connected, or every third pair of
aligned peaks can be connected by a link strut.
[0083] In some embodiments, the axial segments may be composed of
rings arranged such that the crests in one ring are axially aligned
or almost axially aligned with the troughs in an adjacent ring. The
rings are connected by at least one link strut between an aligned
crest and trough. Stiffness is greatest with a link strut between
each aligned crest and trough. Greater flexibility is introduced by
having fewer than every aligned crest and trough connected by a
link strut. For example, only every other aligned crest and trough
can be connected, or only every third aligned crest and trough can
be connected by a link strut. Additionally, the length of the link
struts in the axial segments can be adjusted to modify the
stiffness of the axial segment. Decreasing the length of the links
increases both the radial strength and radial stiffness of the
axial segment since the number of rings per segment length is
maximized. Alternatively the ring crests on one ring can be aligned
with the ring crests of adjacent rings. In this case, when the
scaffold is crimped, the link does not occupy space between ring
struts. This allows for the maximizing of ring strut width which
results in higher radial strength. Such a pattern may also be
described as a plurality of rings composed of diamond-shaped
elements formed of struts. The elements of the rings are connected
at circumferentially aligned vertices of the diamond-shaped
elements. Axially adjacent rings are connected at axially aligned
vertices either by a short link strut or at the intersection of
vertices of elements of adjacent rings.
[0084] FIG. 3A depicts an exemplary axial segment 320 viewed in a
flattened configuration composed of a plurality of rings of
undulating struts with crests and troughs. Line A-A is the
longitudinal axis of the axial segment. An exemplary ring 322 has
crests 324 and troughs 326. As shown in FIG. 3A, every crest in
ring 322 is connected to every trough in adjacent ring 328 by a
short link strut 330. The arrangement of rings 322 and rings 328
forms a plurality of rings 329 of diamond-shaped elements 331
formed of struts. The diamond-shaped elements 331 of the rings are
connected at circumferentially aligned vertices of the
diamond-shaped elements.
[0085] Ls is the length of the axial segment. Ls may be 3 to 6 mm,
6 to 8 mm, 8 to 10 mm, 10 to 12 mm, or greater than 12 mm in an as
cut or as fabricated configuration. Ls increases when the segment
is crimped to a decreased diameter and then decreases when expanded
from a crimped configuration. Length change is affected by the
number of peaks in a ring and the width of the diamonds. The length
change (increases or decreases) with the number of peaks and
(increases or decreases) with the width of the diamonds.
[0086] FIG. 3B depicts a close-up view of a portion 339 of axial
segment 320 illustrating various features. As shown in FIG. 3B, Lr
is the length of a ring strut, for example, strut 332 between a
crest and trough in a ring and Wrs is the width of the ring strut.
L.sub.1 is the length of short link strut 330 that connects crests
and troughs of adjacent rings and Wls is the width of the link
strut. .theta. is the angle at the longitudinal vertex of the
diamond shaped cells, i.e., between struts 332 and 334 in a ring
that intersects at a crest or trough. .phi. is the angle between
struts 332 and 336 which are joined by short link strut 330 and a
diamond-shaped cell. Hc is the height of the diamond-shaped cell
and We is the length of the diamond-shaped cell.
[0087] .theta. may be 90 degrees, 90 to 95 degrees, 95 to 100
degrees, 100 to 110 degrees, or greater than 110 degrees. .theta.
may be 90 degrees, 85 to 90 degrees, 80 to 85 degrees, 70 to 80
degrees, or less than 70 degrees. .phi. may be 90 degrees, 85 to 90
degrees, 80 to 85 degrees, 70 to 80 degrees, or less than 70
degrees. .phi. may be 90 degrees, 90 to 95 degrees, 95 to 100
degrees, 100 to 110 degrees, or greater than 110 degrees.
[0088] Exemplary values for .theta. and .phi. are about 70 and 110
degrees, respectively. Values in this range tend to reduce segment
shortening from crimping to deployment. Other exemplary values for
.theta. and .phi. are about 110 and 70 degrees, respectively.
Values in this range tend to increase segment's radial strength and
crush resistance. Another variable that affects the angles above is
the lased tube diameter and the final deployed diameter. Generally,
for polymers, the lased tube diameter is slightly larger than the
final deployed diameter.
[0089] The segments can include radiopaque marker embedded within
holes in the scaffold segment to aid in visualization of the
implanted scaffold. In some embodiments, the markers are embedded
in holes in the short link struts 330 of FIG. 3A. In other
embodiments, the markers are embedded in holes in ring struts 332
of FIG. 3B.
[0090] When a scaffold segment is crimped, the Ls increases which
is caused by bending at the vertices of the diamond-shaped
elements. Specifically, when the scaffold segment is crimped,
.theta. decreases and .phi. increases. When a scaffold segment is
deployed, the Ls shortens which is caused by bending at the
vertices of the diamond-shaped elements corresponding to an
increase in .theta. and a decrease in .phi..
[0091] The segment properties of radial strength and stiffness can
be modified through adjustment of the as-cut geometrical parameters
of the diamond-shaped elements. For example, radial strength and
stiffness is increased by increasing Hc which results in a decrease
in We and also corresponds to a decrease in .phi. and an increase
in .theta..
[0092] In some segment design embodiments, the diamond-shaped
elements are square-shape or approximately square-shaped in the
as-cut condition. In such embodiments, .phi. is the same or
approximately the same as .theta.. For example, ABS(.phi.-.theta.)
may be 2 or about 2 degrees or less than 2 degrees.
[0093] In other segment design embodiments, the diamond-shaped
elements can be taller or greater in the circumferential direction
or, Hc>We and .phi.>.theta.. In such embodiments, the
.theta.-.phi. may be greater than 2 degrees, 2 to 4 degrees, 4 to 8
degrees, greater than 8, about 3 degrees, about 4 degrees, or about
5 degrees.
[0094] L.sub.1 may be less than 10% or 10% to 20%, 20% to 30%, 30
to 40%, or greater than 40% of a ring strut length between a crest
and a trough. Exemplary link struts may have a length of less than
0.01 in, 0.01 to 0.02 in, 0.02 to 0.04 in, 0.04 to 0.06 in, or
greater than 0.06 in. In some embodiments, adjacent rings are
connected at an intersection of the opposing crests and troughs
such that a length of the link strut is effectively the width of
the intersection and L.sub.1 is zero.
[0095] FIG. 4 depicts a segmented scaffold 340 composed of a
plurality of axial segments 341 to 347, from FIG. 3A. The delivery
of a scaffold composed of decoupled or disconnected axial segments
can be achieved by disposing the axial segments over a catheter
delivery balloon. The axial segments can be arranged end to end and
spaced apart on a single balloon or multiple balloons arrange end
to end. The axial segments may be crimped over the balloon to a
reduced diameter configuration to allow for delivery of a vascular
system to a treatment site.
[0096] Generally, stent crimping is the act of affixing a radially
expandable scaffold or stent to a delivery catheter or delivery
balloon so that it remains affixed to the catheter or delivery
balloon until the physician desires to deliver the stent at the
treatment site. Delivery balloons may be compliant, semi-compliant,
or noncompliant and are made from PEBAX, nylon, or other type of
common balloon material. Examples of such crimping technology which
are known by one of ordinary skill in the art include a roll
crimper; a collet crimper; and an iris or sliding-wedge crimper. In
the sliding wedge or iris crimper, for example, adjacent
pie-piece-shaped sections move inward and twist toward a scaffold
in a cavity formed by the sections, much like the leaves in a
camera aperture.
[0097] FIG. 5 depicts a projection of axial segments 351, 352, and
353 disposed over a balloon 350 in a deflated configuration. Axial
segments are crimped tightly over the balloon in a reduced diameter
configuration. A crimped configuration generally may correspond to
the inner surface of the segments in contact with the outer surface
of a balloon. The axial segments are spaced apart by a distance,
size, or width Lg, which is the gap between segments. Lg can change
during inflation and deployment of the segments to a deployed
diameter due to movement of the segments and axial contraction or
shortening of the segments. Lg at deployment should be large enough
to avoid interference or contact of the segment ends during bodily
movements. Lg at deployment should be large enough so that there is
axial stability and the support of the vessel is continuous. In
exemplary embodiments, the segments when deployed are spaced apart
by 0.5 to 2 mm, or more narrowly, 0.5 to 1 mm, 1 to 2 mm, 2 to 3
mm. The required Lg is determined by the anatomy that the segmented
scaffold will be deployed in to, i.e., in the SFA it will need to
be greater than for the Iliac where vessel compression and bending
are virtually zero. In general, Lg is higher for anatomies with
higher vessel compression and bending.
[0098] Factors that influence a desired Lg at deployment include
the axial compression in the vessel, bending of the vessel, and
stability in presence of side branches coming off of a segment of
the vessel where the scaffold is implanted.
[0099] When compressive loads are placed on the scaffold the axial
compression may occur predominantly between segments. Generally, it
is important to allow for the decrease in the spacing of the
segments during compression and loading. Therefore, Lg at
deployment should be large enough so that the segments do not
contact or interfere with each other during axial compression. The
Lg at deployment can be selected to allow for an axial compression
of zero, below 7%, or 7 to 15%, or for example, about 13%.
[0100] The bending of a vessel with implanted segments results in a
decrease in the Lg at the concave or inner side of the bend with
the gap widening toward the convex or outer side of the bend. The
segments at the inner side of the bend can interfere or make
contact with each other if the initial gap is not wide enough. The
Lg at deployment can be selected to allow for bending of 20 to 30
degrees or less than 30 degrees, or about 30 degrees. In this case,
a 3 mm gap reduces to 0.8 mm at the inner side of the gap.
[0101] The scaffold segments may be deployed in a vessel that
includes a side branch and a gap between segments that overlap this
side branch. In this case, Lg can be the width of the side branch
or greater or less than the width of the side branch. To maintain
axial stability of a segment of a segmented scaffold over a side
branch, the length of a segment needs to be longer that the side
branch so that the radially supported length of the segment is
typically 1.5 times the segment diameter when deployed. This
diameter:length ratio can be less than a 1:1 ratio, a 1:1 ratio, a
1:1.5 ratio or a 1:2 ratio or greater. The ratio is dependent among
other things on the size of the nonpulsatile forces at the delivery
site. For example, the Lg at deployment can be less than 2 or 3
mm.
[0102] The diamond pattern disclosed herein tends to maximize the
relative friction between the vessel wall and the segments. With
this and the high radial and axial rigidity of the diamond pattern,
endothelialization of the segments may be sped up and vessel
irritation may be reduced. With quick endothelialization, the
scaffold/vessel wall becomes a composite structure which in itself
enhances the radial strength and hence crush resistance of the
vessel/scaffold composite. With most, if not all of the movement
transferred to the gaps between the segments, the design utilizes
the natural flexibility of the vessel walls to handle any
compression, bending and torsional movements.
[0103] In some embodiments, a single high radial strength and stiff
scaffold segment, such as described above, may be implanted at an
implant site. Implanting a single segment without additional
segments may be useful in treatments involving vessels that do not
undergo axial compression, torsion, or bending. Examples include
the Iliac and Renal artery.
[0104] During deployment at a lesion site of a conventional balloon
expandable stent or scaffold, the balloons generally start to
expand at the proximal and distal ends first, producing a dog bone
shape. As pressure is increased, the balloon expands in the center,
expanding the scaffold in the center also.
[0105] With the segmented scaffold which can include several short
scaffolds on a single balloon, the balloon can expand in a similar
manner, i.e., expanding at the proximal and distal ends first,
followed by expansion of a center section. Expansion at the ends
first has the tendency to push the segments axially towards the
center of the balloon which decreases the segment to segment gap.
The gap may be decreased to the point that the segments collide
with each other. This movement of the individual segments axially
along the balloon during deployment, therefore, can change the
segment to segment gap to an undesirably small size which can
result in interference of the segments. Additionally, the segment
to segment spacing will not necessarily be the same between all
segments. A reduced gap or zero gap may be acceptable where
nonpulsatile forces are virtually zero.
[0106] In pre-clinical animal studies, bioabsorbable polymer
disconnected segmented scaffolds have been shown to have high
radial strength and fracture resistance. The sections of the artery
along the segments are held open at a desired diameter. However, in
some cases, the sections of the artery at the gaps between the
segments are not held open to the same degree as along the
segments. There appears to be "sagging" or focal restenosis of the
vessel wall inward into the artery lumen at the gaps between
segments. For example, in a case where the gaps between the
segments were on the order of 5 mm, sagging or focal restenosis was
observed.
[0107] FIG. 6 is an image of bench tested segmented scaffold
showing segment 352 and a portion of segments 351 and 353. The
sections of vessel wall 372 along the segments are supported at a
diameter of the segments. Sections 374 of the vessel wall along
gaps 360 between the segments sag inward toward the lumen.
[0108] Embodiments of the present invention include segmented
scaffolds and delivery thereof that reduce or prevent the vessel
sagging between the segments while maintaining high radial strength
and fracture resistance.
[0109] Embodiments of the present invention include deploying a
segmented scaffold in a manner that the ends of the adjacent
scaffolds segments overlap or are interlinked. The segments that
are overlapped or interlinked are disconnected and are not in
contact. The segment ends overlap. Therefore, there no gap between
segments that is a strip or band with no support that completely
encircles the vessel wall. Equivalently, there is no longitudinal
position without support from a segment between the ends of
adjacent segments that extends completely around the circumference
of the vessel wall or scaffold.
[0110] Embodiments also include segmented scaffold segments in a
crimped reduced state with ends of the adjacent scaffolds segments
that overlap or are interlinked. The crimped scaffold segments can
be crimped over a delivery balloon to allow balloon assisted
delivery of the segments to a deployed state in a vessel. The
scaffold segments are interlinked in a manner that upon expansion
of the segments to a deployed state, the deployed segments are
interlinked as described.
[0111] Although specific embodiments are described herein, the
embodiments generally apply to segmented scaffold made up of
segments composed of struts forming a plurality of circumferential
undulating rings, the undulations include peaks and valleys, as
exemplified above. Undulating can refer to, but is not limited to,
to a wave-like appearance or form. The wave-like appearance can be
smooth, such as sinusoidal from, or jagged, such as a zigzag form.
The ends of the segments, therefore, include an undulating ring
also with peaks and valleys. A peak or valley undulation refers
generally to the portion of an undulation or wave on either side of
a peak or valley. The peak undulations project longitudinally
outward or away from the end of the segment and the valley
undulations extend longitudinally inward or toward the segment.
[0112] The interlinking of two adjacent scaffold segments with the
above general structure is described with respect to the peak
undulation and valley undulation of neighboring end rings of
adjacent segments. The peak undulations of a first ring overlap or
extend into the valley undulation of an adjacent ring. Likewise,
the peak undulations of the adjacent ring overlap or extend into
the valleys of the first ring. The degree of overlap or
interlinking can be described in terms degree of extension of the
peak undulations into the valley undulations.
[0113] The peak and valley undulations in the crimped state are
compressed close to one another relative to the expanded or
deployed state. The segments described are provided in the crimped
state with the interlinking of the neighboring rings of adjacent
segments. The degree of overlap may be greater in the crimped state
than the deployed state since the degree of overlap may decrease as
the segments are expanded.
[0114] Interlinked segmented scaffolds can be formed using the
scaffolds segments described, for example, in FIGS. 3A-B and 4.
FIG. 7 depicts a flattened view of another exemplary scaffold
segment 400, like segment 320 depicted in FIG. 3A. Line A-A
represents the longitudinal axis of the segment. Segment 400 has an
end ring 401 of diamond cells made up of two undulating rings
connected at peaks, one of which is undulating end ring 402
composed of peaks 404 and valleys 406. Peak undulations 408 are
composed of struts 408A and 408B which extend from two adjacent
valleys and meet at a peak. Valley undulations 410 are composed of
struts 410A and 410B which extend from two adjacent peaks and meat
at a valley. Peak undulations project longitudinally outward from
the segment and valley undulations extend longitudinally inward
into the segment. The height or length of the valley and peak
undulations is one half the longitudinal length of a diamond,
Wc/2.
[0115] A segmented scaffold can be provided in a deployed state
with scaffold segments such as segment 400 that are arranged with
interlinking of adjacent end rings. The peak undulations 408 can
overlap or interlink with the valley undulations 410.
[0116] However, for the segment 400 such a deployed configuration
may be difficult or impossible to achieve in practice.
Specifically, the peak undulations may not fit into the valley
undulations in a desired crimped state since the angle at the
valley is very small in the crimped state. Additionally, even if
the peak undulation overlaps the valley in a crimped state, the
degree of overlap at crimping is small compared to the length
decrease of the segment from the crimped to deployed state. When
the segments are deployed, from the crimped to deployed state, the
degree of overlap will decrease and may disappear when the segments
are deployed.
[0117] Embodiments further include segmented scaffolds with
segments that are modified to have valley undulations with a
greater length or height which allow overlap in the crimped state
and overlap when the segments are deployed. The degree of potential
overlap is higher relative to the length change of a segment from
the crimped state to a deployed state. In such embodiments, the
length of the potential overlap of ends of segments can be at or
about the longitudinal length of cells of the pattern. Specifically
with respect to the exemplary segment 400, the length of the
potential overlap is at or about the longitudinal length of a
diamond cell of the segment.
[0118] Embodiments include segments which are a modification of the
segments as described with an end ring of diamonds and undulating
end ring of struts. The modification includes omitting diamonds of
the diamond end ring at one or both ends. In particular,
alternating diamonds may be omitted from one or both ends of a
segment. Omitting diamonds refers to removing the struts forming
every other peak undulation. For example, struts 408A and 408B in
FIG. 7 may be omitted.
[0119] The resulting segment has an end ring with peak and valley
undulation that provides a greater potential overlap. The degree of
shortening upon expansion is unchanged. Therefore, when the
modified scaffold segments that are interlinked in the crimped
state are expanded to the deployed state, there is significant
overlap remain in the deployed state.
[0120] FIG. 8 depicts an exemplary segment 420 based on segment 400
of FIG. 7 in which alternating diamonds are omitted at each end.
Equivalently, every other peak undulation is omitted on both ends
of the segment. Specifically, every other pair of struts 408A and
408B are omitted. In addition, the diamonds that are omitted at
opposite ends are longitudinally aligned or opposite from one
another or "in-line" diamonds are omitted. Thus, the embodiment in
FIG. 8 will be referred to as an "in-line segment." The modified
segment, therefore, has an end ring with an undulating, zigzag
structure in which the length of a "zig" and "zag" or from a valley
to a peak is twice the length of a side of a diamond of a diamond
cell. The longitudinal length of a peak or valley undulation is the
longitudinal length of a diamond cell.
[0121] As shown in FIG. 8, segment 420 has an undulating end ring
421 composed of peaks 424 and valleys 426. Peak undulations 428 are
composed of pairs of in-line struts, (428A, 428B) and (428C, 428D)
which extend from two adjacent valleys and meet at a peak. Valley
undulations 430 are composed of pairs of in-line struts (430A,
430B) and (430C, 430D) which extend from two adjacent peaks to a
valley. Each of the two inline struts is the length of a side of
the diamond cells.
[0122] Peak undulations project longitudinally outward from the
segment and valley undulations extend longitudinally inward into
the segment. As is shown below, the maximum potential length of
overlap is one half the longitudinal length of a diamond, 1/2 Wc.
For the in-line segment, the peaks (and valleys) of the end rings
are longitudinally aligned. As shown in FIG. 8, the minimum width
of segment 420 is the longitudinal length of a diamond cell,
Wc.
[0123] FIG. 9 depicts an exemplary segment 450 based on segment 400
of FIG. 7 in which alternating diamonds are omitted at both ends of
the segment. Specifically, every other pair of struts 408A and 408B
is omitted. Segment 450 differs from segment 420 of FIG. 8 in that
omitted diamonds at one end are not longitudinally aligned with
omitted diamonds at the other end. The diamonds omitted at one end
are circumferentially off-set by one diamond cell. The embodiment
in FIG. 9 will be referred to as an "off-set segment." An alternate
embodiment is omitted diamonds every third or every fourth diamond
around the circumference of the end ring.
[0124] As shown in FIG. 9, segment 450 has an undulating end ring
451 composed of peaks 454 and valleys 456. Peak undulations 458 are
composed of two pairs of in-line struts, as described in FIG. 8,
which extend from two adjacent valleys and meet at a peak. Valley
undulations 460 are composed of pairs of inline struts, as
described in FIG. 8, which extend from adjacent peaks to a valley.
Peak undulations project longitudinally outward from the segment
and valley undulations extend longitudinally inward into the
segment. As shown below, the potential overlap is one half the
longitudinal length of a diamond, 1/2 Wc. As shown in FIG. 9, the
minimum width of segment 450 is twice the longitudinal length of a
diamond cell, 2.times.Wc. Also, as shown in FIG. 9, the peaks at
one end are longitudinally aligned with the valleys at the other
end, for example, peak 454 is longitudinally aligned with valley
462.
[0125] FIG. 10 depicts two in-line segments 470 and 474 which are
interlinked. End ring 472 of segment 470 is interlinked with end
ring 476 of segment 474. For example, peak undulation 478 of
segment 470 projects into valley undulation 480 of segment 474.
Each peak of the interlinking peak undulation (e.g., peak 479) is
longitudinally aligned with the each valley of the interlinked
valley undulation (e.g., valley 481). FIG. 10 shows the first
segment 470 and second segment 474. Each of these first and second
segments has undulating cylindrical rings composed of struts and
forming a plurality of diamond-shaped cells. For each segment a
first ring at a first end comprises alternating peak and valley
undulations, a second ring at a second end comprises alternating
peak and valley undulations, the peaks at the first and second ends
extend in opposite directions to each other and the peaks are
longitudinally aligned with the peaks of the second ring.
Additionally, the first ring of the second segment is arranged with
the second ring of the first segment, such that the peaks of the
first ring of the second segment overlap the valleys of the second
ring of the first segment.
[0126] There are various ways of defining the degree of overlap or
interlinking of the segments. For example, the percent overlap of
interlinking of peak undulation 478 with valley undulation 480 can
be calculated from, L, the one half length of a diamond cell and
the length of the overlap of the peak undulation 478 with the
valley undulation 480, L': % overlap=L'/L.times.100%. The degree of
overlap at deployment may be 5 to 70%, or more narrowly, 5 to 20%,
20 to 30%, 30 to 40%, 40 to 50%, 50 to 60%, and 60 to 70%.
[0127] As shown in FIG. 10, there is a gap 482 between end rings
472 and 476 of segments 470 and 474, respectively. However, the gap
has an undulating profile that follows the interlinking profile of
end rings 472 and 476. As a result, there is no longitudinal
position completely around a vessel wall that is not supported.
FIG. 11 depicts a three-dimensional view of two interlinked
segments.
[0128] In order for deployed segments to have overlap, the segments
are provided in a crimped state with a degree of overlap. The
degree of overlap is selected so that upon expansion or deployment
to a target diameter, the deployed segments have a desired amount
of overlap. The degree of overlap at crimping may be 50 to 100%, or
more narrowly, 50 to 60%, 60 to 70%, 70 to 80%, 80 to 90%, or 90 to
100%. This overlap may be such that at the gap 482, the axial space
between ring struts from a first segment to the ring struts of a
second segment is the same as the ring strut spacing within the
segments. This will provide a continuous uniform vessel support at
the segments and across the segment gap also. In addition, gap 482
may be less than the length of a diamond, the same as the length of
a diamond, or more than the length of a diamond.
[0129] FIG. 12 depicts a side view of interlinked segments 490 and
494 with omitted end ring diamonds in a crimped state with about
50% ring overlap or engagement of the end rings. For example, a
peak undulation with a peak 492 is shown to be overlapping or
engaged within the valley undulation with a valley 496. The opening
of the diamond cell 498 is shown to have a deformed shape due to
the crimped state.
[0130] FIG. 13 depicts interlinked segments 500 and 504 with
omitted end ring diamonds in a crimped state with about 100% ring
overlap or engagement of the end rings. For example, a peak
undulation with a peak 502 is shown to be overlapping or engaged
within the valley undulation with a valley 506. The opening of the
diamond cell 508 is shown to be reduced significantly due to the
crimped state.
[0131] The crimped interlinked segments of FIGS. 12 and 13
demonstrate how the undulations of the modified rings allow both
crimping to a reduced profile and interlinking at crimped and
deployed states. FIG. 14 depicts an expanded view of the
interlinked scaffolds of FIG. 10. Peak undulation 520 is composed
of a first portion made up of pairs of struts 524A and 524B and a
second portion composed of pairs of struts 522A and 522B. Likewise,
valley undulation 526 is composed of a first portion made up of
pairs of struts 530A and 530B and a second portion composed of
pairs of struts 528A and 528B.
[0132] When the segment is crimped, the struts that make up the
peak and valley undulations bend inward. However, as shown in FIGS.
12 and 13, the first portion 524 and the second portion 522 of the
peak undulation 520 bend inward to different degrees at pivot
points 523A and 523B. The second portion 522 of the peak undulation
bends inward to a greater degree than the first portion 524, which
is within the valley undulation.
[0133] Likewise, the first portion 530 and the second portions 528
of the valley undulation 526 bend inward to different degrees at
points 529A and 529B, as shown in FIGS. 12 and 13. The first
portion 530 of the valley bends inward to a greater degree than the
second portion 528. The closely spaced struts of the first portion
cannot accommodate overlap of the peak undulation of the adjacent
segment, but allows for reduction of the segments to a low profile
or low diameter configuration. The struts of the second portion 528
of the valley undulation are spaced apart sufficiently to allow
overlap of the first portion 524 of the peak undulation of the
adjacent segment. In another embodiment, pivot points 523A and 523B
may be closer together. This would result in less room needed for
the interlink in the crimped state. Alternatively, the diamonds at
the interlink and several rows in from the interlink could be
adjusted in shape to accomplish a looser or tighter interlink fit
in the crimped state. In addition the diamonds could be axially
shorter, axially the same or axially longer near the end or at the
end of the segments.
[0134] The modified segmented scaffolds disclosed with alternating
diamonds removed could be deployed in configurations that are not
optimum or undesirable. These configurations may result develop
during deployment from the crimped state. A non-optimum
configuration includes segments with excessive segment to segment
gaps in the deployed state so that the vessel is not supported
adequately in the segment gaps. In such a configuration there is no
overlap of the end rings or the degree of overlap of the rings is
low, for example, less than 20 or 30%. FIG. 15 depicts an image of
a deployed segmented scaffold with large segment to segment
gaps.
[0135] In other non-optimum configurations segments may collide as
the vessel is axially compressed as would happen in the Superficial
Femoral Artery. Collisions can result from a configuration in which
the peaks and valleys of adjacent segments are not longitudinally
aligned, in contrast to segments 470 and 474 in FIG. 10. Such a
configuration can result from rotation of a segment during
deployment. FIG. 16 depicts deployed segmented scaffolds of which
the left-most segment is rotated relative to the middle segment
resulting in a non-uniform gap at "X". As a result, the segment gap
is nonuniform circumferentially.
[0136] Therefore, there is a need for a way to insure that the
segment gap is consistent between all segments, not excessive, and
also uniform circumferentially.
[0137] FIG. 17 depicts a schematic of part of the interlink area of
the crimped segmented scaffold of FIG. 13. A peak undulation 554 of
segment 550 is disposed in between peak undulations 556 and 558 of
segment 552 such that there is an overlap of approximately 100%
between the two segments. Peak undulation 554 is made up of struts
560 and 562, the sides of a diamond cell, which extend from
opposing vertices of the cell to meet at peak 564. The outer side
wall surface or profile of the struts 560 and 562 is straight and
smooth so that when the segments are deployed there is no
interaction between the surfaces of the adjacent scaffolds that
influences the relative positions of the adjacent segments.
[0138] FIGS. 18A-B and FIGS. 19-20 depict embodiments of segments
that are modified to maintain a consistent segment gap which is not
excessive and also reduce or prevent rotation during deployment
which results in a nonuniform gap circumferentially.
[0139] FIG. 18A depicts an interlinked region of crimped
interlinked segments 570 and 572. FIG. 18B depicts an expanded view
of one of the peak undulations. The peak undulations of the end
rings of segments 570 and 572 include head portion 578 and body
portion 580. Head portion 578 in the region of peak 574 has an
overhang 580 on either side of peak 574 along struts 582 and 584
that extend from peak 574 to vertices 576 and 577, respectively.
The overhangs 580 are situated between peak 574 and the vertices
576 and 577. The side wall surface between peak 574 and the
vertices extends inward at the overhang 580 on either side of the
peak undulation to form interlocking surfaces 586A and 586B.
[0140] A shown in FIG. 18A, peak undulation 590 is disposed between
peak undulations 592 and 594. The overhangs of the head portion of
peak undulation 590 are disposed past the overhangs of peak
undulations 592 and 594 such that the interlocking surfaces of peak
undulation 590 are engaged or in contact or can engage or make
contact when the segments are expanded.
[0141] When crimped, the adjacent segments are mechanically held in
a constant or fixed relationship to each other both
circumferentially and longitudinally. The adjacent segments are
held through engagement of the interlocking surfaces of the head
portions of the end rings. The adjacent rings overlap
longitudinally a consistent amount. Additionally, the segments are
radially locked into place by the mechanical engagement of the head
portions.
[0142] In addition, the crimped segments can move around tight
bends in a vessel during delivery to the lesion site. Bending is
accommodated at each segment to segment connection by a
longitudinal space 596 between the head of each peak undulation and
the side walls 598 of the end ring of the adjacent segment.
[0143] During balloon inflation, the diamond pattern will open up,
for example, as shown in FIG. 30, which shows an exemplary
deployment of a segmented scaffold. In FIG. 30, each segment
shortens and opens independently of other segments. This is in
contrast to deployment of segments 570 and 572 of FIG. 18A. During
the initial stages of inflation the several heads at the ends of
each segment will stay mechanically engaged behind the heads of the
adjacent segment, thus holding the segment to segment relationship.
As the segments are expanded further, the heads will move further
apart circumferentially, as shown by arrows 599 in FIG. 18A, until
they finally pass by each other with no more engagement near the
fully deployed diameter. As a result, the natural longitudinal
shortening of the diamonds in the segments and also the balloon
lengthening which tends to increase the segment to segment gap
during deployment have less of a contribution to the final segment
to segment gap.
[0144] Therefore, the mechanical restraint during deployment
provides several advantages. The segment to segment longitudinal
relationship is maintained intact for a longer period of time, for
example, for a longer period of time during deployment. This
results in controlled and consistent final segment to segment gaps.
In addition, the segment to segment circumferential relationship is
maintained intact for a longer period of time. This results in less
circumferential rotation of individual segments and thus a
reduction in segment to segment collisions during vessel
longitudinal compression. Reduction of collisions results in a
reduced risk of vessel irritation, strut fracture, and emboli
production.
[0145] FIGS. 19 and 20 depict alternative head designs. FIG. 19
depicts a portion 600 of an end ring of a segment with a head
portion 602 and body portion 604. Line A-A is the longitudinal axis
of the segment and line B-B is the circumferential direction.
Overhangs 606 extend outward from body 604 with an angle A.sub.H
with respect to line B-B, rotated toward the segment. This
alternate head design enhances the longitudinal mechanical wedging
interlocking effect of adjacent segments.
[0146] FIG. 20 depicts a portion 610 of an end ring of a segment
with a head portion 612 and body portion 614. Line A-A is the
longitudinal axis of the segment and line B-B is the
circumferential direction. Overhangs 616 extend outward from body
portion 614 with an angle B.sub.H with respect to line B-B, rotated
away from the segment. The edges of the overhang at the head and
body have radii of curvature R.sub.1 and R.sub.2, respectively. R1
will always be slightly less that R2 so that in the crimped state
the R2 of one segment will have clearance at R1 to the struts of
the adjacent segment. R2 may be 1%, 10%, 20%, 50%, 100% or greater
than 100% of the height of one side of the head from strut 614.
[0147] The alternate head design in FIG. 20 changes the mechanical
characteristics of the head to head interaction. As R.sub.1 and
R.sub.2 are varied, the diameter at which the interlocked heads
separate is changed. This results in changes in segment spacing at
final deployment.
[0148] The scaffold segments may be crimped tightly on a delivery
balloon using a crimping apparatus such as an iris crimper. The
crimping process may include two stages, a pre-crimping process and
a final crimp process. In the pre-crimp process, the diameter of
the scaffold segments are reduced to a diameter between the initial
diameter and the balloon diameter prior to loading the scaffold
segments on the balloon. The diameter of the segments can be
reduced to the balloon diameter or 1 to 5% greater than the balloon
diameter. For example, the pre-crimping process can crimp segments
from a diameter of about 0.3 in to about 0.06 in.
[0149] The reason for the pre-crimp processes is to reduce the size
of the scaffold segments to allow greater accuracy of loading the
segments on the balloon with the desired degree of overlap or
interlinking. A detailed discussion of a pre-crimping process for
segmented scaffolds can be found in U.S. patent application Ser.
No. 13/441,756.
[0150] In the pre-crimping process, the scaffold segments in an
as-fabricated condition are placed over a mandrel and arranged end
to end. The scaffold segments are spaced apart axially. The
distance between the segments may be such that when the segments
are reduced to the pre-crimp diameter the segments do not make
contact with each other. For example, the scaffold segments are
placed over a stepped mandrel. The mandrel with the scaffold
segments is loaded into the pre-crimper, for example, an iris
crimper and crimped to the pre-crimp diameter. The pre-crimped
scaffold segments may further be placed inside a protective sheath
disposed in an outer surface of the each scaffold segment.
[0151] The pre-crimped segments may then be loaded onto a balloon
in a deflated state. The segments are placed over the balloon and
arranged so that adjacent segments have a certain degree of
overlap, for example, between 50 and 100%, or more narrowly, 50 to
60%, 60 to 70%, 70 to 80%, 80 to 90%, or 90 to 100%. The segments
and balloon are then crimped down with pressure. The pressure may
be applied at multiple steps with a dwell period between steps to
achieve segment retention on the balloon. The balloon may be
removed from the crimper one or more times, the removed segments
pushed together to obtain a desired overlap and placed back into
the crimper. Pressure may be applied to the balloon during the
final stages of the crimp process to enhance the scaffold retention
to the balloon in the crimped state. When the catheter is removed
from the crimper a protective sheath may be placed over the
scaffold segments.
[0152] Further embodiments of the present invention reduce or
eliminate torsional or extension-compression forces on the rings of
a scaffold and additionally address the sagging of vessel walls
between scaffold segments. The embodiments can include scaffolds
that are composed of segments that are not connected by linking
elements or are connected by some linking elements.
[0153] In these embodiments, a scaffold having a plurality of
segments in a crimped reduced configuration has at least one
discontinuous linking element between adjacent segments. The
scaffold can be crimped over a delivery balloon. The discontinuous
linking elements extend from end of adjacent segments, however, do
not connect the adjacent segments due to a discontinuity in the
linking element located between the adjacent segments. Since the
discontinuous linking elements do not connect the adjacent
elements, the propagation of forces between the adjacent rings is
reduced or eliminated. As a result, the scaffold is more fatigue
and fracture resistant. Additionally, since the discontinuous link
is structurally intact except for the discontinuity, the link can
help support the lumen wall between segments once the scaffold is
deployed in a vessel.
[0154] FIG. 21 depicts a close-up view of a discontinuous linking
element 700 between scaffold segment 702 and scaffold segment 704.
Discontinuous linking segment 700 includes a portion 706 connected
to segment 702 and a portion 708 that is connected to segment 704.
Linking strut 700 has a discontinuity, gap, or space 710 with a
length L.sub.disc between the free ends of portion 706 and 708.
L.sub.disc may be very small, for example, between 1 and microns.
In general, the width may be 2 to 5, 5 to 10, 10 50 microns, or
greater than 50 microns.
[0155] In some embodiments, the scaffold has no linking elements
between adjacent segments that connect adjacent segments with only
discontinuous linking elements between segments so that all
segments are disconnected. Adjacent segments may have 1, 2, 3, 4, 5
or more discontinuous linking elements between adjacent segments.
FIG. 22 depicts a two-dimensional projection of adjacent scaffold
segments that are disconnected. As shown in FIG. 22, scaffold
segments 710, 712, and 714 are arranged end to end. Discontinuous
linking elements 716 and 718 are between segments 710 and 712.
Discontinuous linking elements 720 and 722 are between segments 712
and 714.
[0156] In other embodiments, the scaffold includes adjacent
segments that are connected by at least one intact linking element.
Therefore, adjacent scaffold segments are connected by at least one
intact linking element and also include at least one discontinuous
linking element.
[0157] FIG. 23 depicts a two-dimensional projection of adjacent
scaffold segments that include both intact and discontinuous
linking elements. As shown in FIG. 23, scaffold segments 730, 732,
and 734 are arranged end to end. Scaffold segments 730 and 732 are
connected by intact linking element 736 and scaffold segments 732
and 734 are connected by intact linking element 742. Discontinuous
linking element 738 is between scaffold segments 730 and 732.
Discontinuous linking element 740 is between scaffold segments 732
and 734.
[0158] In some embodiments, the intact linking elements can be
frangible or designed to fail. Frangible linking elements have
weakened portions that facilitate fracture or breaking of the
linking element after the scaffold is deployed. Prior to fracture,
the frangible linking element provides stability to the scaffold
during crimping and for a time after deployment. However, at some
time after deployment the frangible linking elements fracture or
break at the weakened portion, disconnecting scaffold segments
which then prevents transfer of forces between segments. Features
that facilitate fracture include a narrowed portion of the linking
element, such as notch, or holes through a linking element.
Scaffolds disclosing frangible linking elements with various types
of weakened portions are disclosed in US2011/0066225 and
US2012/0065722.
[0159] The scaffold segments can have any structure or pattern. For
example, the scaffold segments can have the structure of a
plurality of rings composed of diamond-shaped elements formed of
struts such as the exemplary scaffold segment depicted in FIG. 3A.
Additionally, the segments can have a structure composed of
cylindrical undulating or sinusoidal rings with alternating crests
or peaks with the rings connected by linking elements, as depicted
in FIG. 1.
[0160] FIG. 24 depicts a close-up view of a region between segments
750 and 752, like those shown in FIG. 3A. The segments are shown in
an expanded configuration rather than in a crimped configuration
for ease of illustration. As shown, two discontinuous linking
elements 754 and two intact linking segments 756 are disposed
between segments 750 and 752. The number and arrangement of the
intact and discontinuous linking elements is exemplary and any
number and arrangement of linking elements between the ends of the
segments is possible. In the example shown, one end of a linking
element is connected to a trough of segment 750 and the other end
is connected to a peak of segment 752. Alternatively, one end of a
linking element can be connected to a peak of segment 750 and the
other end may be connected to a trough of segment 752. In another
alternative, segments 750 and 752 can be rotated relative to one
another by one cell and one end of a linking element can be
connected to the peak (or trough) of one segment and the other end
of the linking element can be connected to the peak (or trough) of
the other segment.
[0161] FIG. 25 depicts a close-up view of a portion 760 of a
scaffold like the one shown in FIG. 1. As shown in FIG. 25, two
linking elements are between ring 762 and 764, a discontinuous
linking element 766 and an intact linking element 768.
Alternatively, both rings can be discontinuous linking elements so
that the two rings are disconnected.
[0162] The scaffold of FIG. 1 is not designed specifically for use
as a segmented scaffold, i.e., there are no pre-defined segments as
in the segments of FIG. 3A. However, sets of rings can be
identified as segments, where a set is one or more rings, e.g.,
rings 106 and 108 in FIG. 1. Discontinuous linking elements can be
between every ring, every other ring, every third ring, etc., to
form segments of one ring, two rings, three rings, etc. Segments
can be connected by including at least one intact linking element.
Segments can be disconnected by having no intact linking elements.
Segments can be disconnected on both ends.
[0163] FIG. 26 depicts pattern 770 which is pattern 100 from FIG. 1
with one discontinuous linking element and one intact linking
element between each ring. FIG. 27 depicts pattern 772 which is
pattern 100 from FIG. 1 with one discontinuous linking element and
one intact linking element at every third segment gap, as shown by
the arrows. The segments consist of three rings.
[0164] In some embodiments, the disconnected linking elements may
be arranged in a pattern along the length of the scaffold. Between
the first end and second end of the scaffold, the disconnected
linking elements may be offset circumferentially from one segment
gap to the next. For example, the discontinuous linking elements
can form a helical pattern. Offsetting the discontinuous linking
elements tends to make the scaffold more stable once implanted.
FIG. 28 depicts segmented scaffold 780, which is scaffold 340 from
FIG. 4 with two intact linking elements and two discontinuous
linking elements between each segment. Discontinuous linking
elements 781-786 are offset circumferentially from gap to gap to
form a helical pattern.
[0165] A scaffold with discontinuous linking elements can be formed
from a scaffold with intact linking elements in a crimped reduced
configuration. Scaffolds such as pattern or scaffold 100 in FIG. 1,
pattern or scaffold 780 of FIG. 28 (with all linking elements
intact) can be formed by laser machining a tube in an expanded
configuration. The scaffold may then be crimped or pre-crimped to a
reduced configuration. The discontinuous linking elements can be
formed by laser cutting the selected linking elements.
[0166] The laser cutting can be performed with the scaffold crimped
over a delivery balloon. Alternatively, the laser cutting can be
performed with the scaffold crimped over a mandrel or some other
support to prevent damage of the balloon by the laser. The scaffold
may then be removed from the support and crimped over a delivery
balloon. In another alternative, the scaffold may be crimped over
delivery balloon with a protective sheath over the balloon to
protect the balloon from the laser cutting. After laser cutting to
create the discontinuous links, the protective sheath can be
removed from the balloon by allowing a slight recoil in the
scaffold and pulling off the sheath.
[0167] A further aspect of the present invention is variation of
width of linking elements along the longitudinal axis. The
variation in linking elements can include variation of width of
intact linking elements and discontinuous linking elements. Certain
sections of scaffold may be more susceptible to fracture from
radial compression, torsion, flexion, and axial extension and
compression. It is expected that the susceptibility to strut
fracture depends on strut width. Therefore, the width of linking
struts can account for the difference in the forces along the axis
of the scaffold.
[0168] In these embodiments, the width of linking elements at
segment gaps at the ends of a scaffold can be greater or less than
the widths in a middle section. In an exemplary embodiment, the
width of the linking elements at gaps at the two ends, gaps 790 and
791 and gaps 794 and 795 can be greater than the widths of the
linking elements in gaps 792 and 793. The larger strut widths can
be 10 to 100% larger, or more narrowly, 10 to 30%, 20 to 50%, or 40
to 80% larger.
[0169] The scaffold segments of the present invention can be made
from variety of biodegradable polymers including, but not limited
to, poly(L-lactide) (PLLA), polymandelide (PM), poly(DL-lactide)
(PDLLA), polyglycolide (PGA), polycaprolactone (PCL),
poly(trimethylene carbonate) (PTMC), polydioxanone (PDO),
poly(4-hydroxy butyrate) (PHB), and poly(butylene succinate) (PBS).
The scaffold segments can also be made from random and block
copolymers of the above polymers, in particular,
poly(L-lactide-co-glycolide) (PLGA) and
poly(L-Lactide-co-caprolactone) PLGA-PCL. The scaffold can also be
made of a physical blending of the above polymers. The scaffold
segments can be made from PLGA including any molar ratio of
L-lactide (LLA) to glycolide (GA). In particular, the stent can be
made from PLGA with a molar ratio of (LA:GA) including 85:15 (or a
range of 82:18 to 88:12), 95:5 (or a range of 93:7 to 97:3), or
commercially available PLGA products identified as having these
molar ratios. High strength, semicrystalline polymers with a Tg
above body temperature include PLLA, PGA, and PLGA.
[0170] "Radial strength" is the ability of a stent to resist radial
compressive forces, relates to a stent's radial yield strength and
radial stiffness around a circumferential direction of the stent. A
stent's "radial yield strength" or "radial strength" (for purposes
of this application) may be understood as the compressive loading,
which if exceeded, creates a yield stress condition resulting in
the stent diameter not returning to its unloaded diameter, i.e.,
there is irrecoverable deformation of the stent. When the radial
yield strength is exceeded the stent is expected to yield more
severely as only minimal additional force is required to cause
major deformation. "Stress" refers to force per unit area, as in
the force acting through a small area within a plane. Stress can be
divided into components, normal and parallel to the plane, called
normal stress and shear stress, respectively. Tensile stress, for
example, is a normal component of stress applied that leads to
expansion (increase in length). In addition, compressive stress is
a normal component of stress applied to materials resulting in
their compaction (decrease in length). Stress may result in
deformation of a material, which refers to a change in length.
"Expansion" or "compression" may be defined as the increase or
decrease in length of a sample of material when the sample is
subjected to stress.
[0171] As used herein, the terms "axial" and "longitudinal" are
used interchangeably and refer to a direction, orientation, or line
that is parallel or substantially parallel to the central axis of a
stent or the central axis of a tubular construct. The term
"circumferential" refers to the direction along a circumference of
the stent or tubular construct. The term "radial" refers to a
direction, orientation, or line that is perpendicular or
substantially perpendicular to the central axis of the stent or the
central axis of a tubular construct and is sometimes used to
describe a circumferential property, i.e., radial strength.
[0172] "Strain" refers to the amount of expansion or compression
that occurs in a material at a given stress or load. Strain may be
expressed as a fraction or percentage of the original length, i.e.,
the change in length divided by the original length. Strain,
therefore, is positive for expansion and negative for
compression.
[0173] "Strength" refers to the maximum stress along an axis which
a material will withstand prior to plastic deformation and then
fracture. The ultimate strength is calculated from the maximum load
applied during the test divided by the original cross-sectional
area.
[0174] "Modulus" may be defined as the ratio of a component of
stress or force per unit area applied to a material divided by the
strain along an axis of applied force that result from the applied
force. For example, a material has both a tensile and a compressive
modulus.
[0175] The underlying structure or substrate of an implantable
medical device, such as a stent can be completely or at least in
part made from a biodegradable polymer or combination of
biodegradable polymers, a biostable polymer or combination of
biostable polymers, or a combination of biodegradable and biostable
polymers. Additionally, a polymer-based coating for a surface of a
device can be a biodegradable polymer or combination of
biodegradable polymers, a biostable polymer or combination of
biostable polymers, or a combination of biodegradable and biostable
polymers.
EXAMPLES
[0176] FIG. 29 is an image of an interlinked segmented scaffold in
a deployed state from a bench test. The middle segment is an
off-set segment. As shown from the figure, there is no axial
section all the way around the vessel wall that is not supported by
a segment. Additionally, there is no sagging of the vessel wall
inward the lumen in between the segments as is shown for the
segmented scaffolds in FIG. 6. The vessel wall in FIG. 29 appears
to be supported uniformly at the scaffold diameter along the entire
length of the segmented scaffold.
[0177] FIG. 30 depicts images of an interlinked segmented scaffold
composed of two segments. Diamonds are omitted only from one end of
each segment. The uppermost image depicts the interlinked segments
in a fully crimped state over a delivery balloon. The middle image
depicts the interlinked segments in a semi-expanded state. The
nonuniform or uneven expansion at the ends is due to the
characteristic behavior of delivery balloons of inflating first at
the ends. The degree of engagement of the segments increases from
the crimped to the semi-expanded state. The uneven expansion causes
sliding of the segments together which increases the engagement.
The bottom image is the fully expanded scaffold showing the
interlinking of the segments.
[0178] FIG. 31 depicts images of interlinked segmented scaffolds
composed of three in-line segments. The uppermost image depicts the
three interlinked segments in a fully crimped state over a delivery
balloon. The next two images depict the three interlinked segments
undergoing uneven expansion. The next two images depict the three
interlinked segments close to full expansion. The bottom image is
the fully expanded scaffold showing the interlinking of the middle
segment with the end segments. The end segments were observed not
so slide on the balloon during the uneven expansion.
[0179] FIG. 32 depicts images of an interlinked segmented scaffold
composed of three off-set segments. The uppermost image depicts the
three interlinked segments in a fully crimped state over a delivery
balloon. The next two images depict the three interlinked segments
undergoing uneven expansion. The next two images depict the three
interlinked segments close to full expansion. The bottom image is
the fully expanded scaffold showing interlinking of the middle
segment with the end segments. The end segments were observed not
so slide on the balloon during the uneven expansion. The segments
end tips were in-line at full expansion.
[0180] FIG. 33 depicts images of an interlinked scaffold segmented
composed of six in-line segments. The uppermost image depicts the
five interlinked segments in a fully crimped state over a delivery
balloon. The next two images depict the five interlinked segments
undergoing uneven expansion. The fourth image depicts the five
interlinked scaffolds close to full expansion. The bottom image is
the fully expanded scaffold showing the interlinking adjacent
segments. The segments were observed not so slide on the balloon
during the uneven expansion.
[0181] While particular embodiments of the present invention have
been shown and described, it will be obvious to those skilled in
the art that changes and modifications can be made without
departing from this invention in its broader aspects. Therefore,
the appended claims are to encompass within their scope all such
changes and modifications as fall within the true spirit and scope
of this invention.
* * * * *