U.S. patent application number 13/779655 was filed with the patent office on 2013-07-04 for polymer stent with break-away links for enhanced stent retention.
This patent application is currently assigned to Abbott Cardiovascular Systems Inc.. The applicant listed for this patent is Abbott Cardiovascular Systems Inc.. Invention is credited to Timothy A. Limon, Stephen D. PACETTI, Yunbing Wang.
Application Number | 20130172982 13/779655 |
Document ID | / |
Family ID | 41063887 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130172982 |
Kind Code |
A1 |
PACETTI; Stephen D. ; et
al. |
July 4, 2013 |
POLYMER STENT WITH BREAK-AWAY LINKS FOR ENHANCED STENT
RETENTION
Abstract
Polymer stents with break-away links and methods of forming the
links for improved stent retention on an expandable member during
delivery are disclosed.
Inventors: |
PACETTI; Stephen D.; (San
Jose, CA) ; Limon; Timothy A.; (Cupertino, CA)
; Wang; Yunbing; (Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Abbott Cardiovascular Systems Inc.; |
Santa Clara |
CA |
US |
|
|
Assignee: |
Abbott Cardiovascular Systems
Inc.
Santa Clara
CA
|
Family ID: |
41063887 |
Appl. No.: |
13/779655 |
Filed: |
February 27, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12047040 |
Mar 12, 2008 |
8414638 |
|
|
13779655 |
|
|
|
|
Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2/915 20130101;
B29K 2033/20 20130101; B29C 66/53241 20130101; B29K 2001/00
20130101; B29K 2067/043 20130101; B29L 2023/007 20130101; A61F 2/82
20130101; B29C 65/4895 20130101; B29C 67/0014 20130101; B29K
2069/00 20130101; B29L 2031/7542 20130101; B29L 2031/7534 20130101;
B29C 66/612 20130101; A61F 2/91 20130101; B29C 66/8122 20130101;
B29K 2083/00 20130101; B29C 2035/0822 20130101; B29K 2075/00
20130101; B29C 65/48 20130101; B29K 2055/02 20130101; B29K 2059/00
20130101; B29K 2023/00 20130101; B29K 2067/046 20130101; B29C
2035/0827 20130101; B29K 2001/14 20130101; B29K 2023/083 20130101;
B29C 65/1667 20130101; B29K 2025/00 20130101; B29C 65/1645
20130101; B29K 2027/08 20130101; B29L 2028/00 20130101; A61F
2250/0071 20130101; B29L 2031/605 20130101; B29C 66/83221 20130101;
B29K 2027/06 20130101; A61F 2/9522 20200501; B29K 2029/00 20130101;
B29K 2077/00 20130101; B29C 66/69 20130101; B29C 66/73117 20130101;
B29K 2001/18 20130101; B29C 65/1664 20130101; B29C 65/76 20130101;
B29C 66/1122 20130101; B29K 2079/08 20130101; B29C 65/1606
20130101; B29K 2001/12 20130101; B29K 2071/00 20130101; B29K
2995/006 20130101; B29C 65/08 20130101; B29K 2023/086 20130101;
B29C 65/1612 20130101; B29C 66/71 20130101; B29C 66/73791 20130101;
B29C 66/8122 20130101; B29K 2905/00 20130101; B29C 66/71 20130101;
B29K 2083/00 20130101; B29C 66/71 20130101; B29K 2079/08 20130101;
B29C 66/71 20130101; B29K 2077/00 20130101; B29C 66/71 20130101;
B29K 2075/00 20130101; B29C 66/71 20130101; B29K 2071/00 20130101;
B29C 66/71 20130101; B29K 2069/00 20130101; B29C 66/71 20130101;
B29K 2067/046 20130101; B29C 66/71 20130101; B29K 2067/043
20130101; B29C 66/71 20130101; B29K 2067/00 20130101; B29C 66/71
20130101; B29K 2059/00 20130101; B29C 66/71 20130101; B29K 2055/02
20130101; B29C 66/71 20130101; B29K 2033/12 20130101; B29C 66/71
20130101; B29K 2033/08 20130101; B29C 66/71 20130101; B29K 2031/04
20130101; B29C 66/71 20130101; B29K 2027/16 20130101; B29C 66/71
20130101; B29K 2027/06 20130101; B29C 66/71 20130101; B29K 2025/08
20130101; B29C 66/71 20130101; B29K 2025/06 20130101; B29C 66/71
20130101; B29K 2023/22 20130101; B29C 66/71 20130101; B29K 2023/18
20130101; B29C 66/71 20130101; B29K 2023/086 20130101; B29C 66/71
20130101; B29K 2023/083 20130101; B29C 66/71 20130101; B29K 2023/08
20130101; B29C 66/71 20130101; B29K 2023/00 20130101; B29C 66/71
20130101; B29K 2009/06 20130101; B29C 66/71 20130101; B29K 2003/00
20130101; B29C 66/71 20130101; B29K 2001/00 20130101 |
Class at
Publication: |
623/1.15 |
International
Class: |
A61F 2/82 20060101
A61F002/82 |
Claims
1. A stent comprising: a scaffold formed of a plurality of
polymeric structural elements crimped to a reduced profile onto a
balloon catheter, wherein the structural elements are plastically
deformed when the scaffold is crimped to the reduced profile from a
larger diameter which results in a gripping force on the balloon
catheter by the crimped stent, wherein the gripping force is not
sufficient to prevent the stent from shifting position or
separating from the catheter prior to delivery; and at least one
breakable connecting element connecting two of the structural
elements, wherein the at least one breakable connecting element
facilitates retention of the stent at a reduced diameter so that
the stent does not shift position or separate from the catheter
prior to delivery, wherein upon expansion of the stent the at least
one breakable connecting element breaks to allow expansion of the
stent due to a radially outward force on the stent.
2. The method of claim 1, wherein the structural elements comprise
a biodegradable polymer.
3. The method of claim 1, wherein the structural elements comprise
a plurality of circumferential rings, the at least one connecting
element being formed between adjacent cylindrical rings.
4. The method of claim 1, wherein the structural elements comprise
a plurality of undulating circumferential rings including peaks and
valleys, the at least one connecting element connecting the peak of
one ring with the peak of an adjacent ring.
5. The method of claim 4, wherein all of the peaks of the one ring
are connected with the peaks of the adjacent ring around a
circumference of the stent.
6. The method of claim 1, wherein alternating peaks of one ring are
connected with the peaks of an adjacent ring around a circumference
of the stent.
7. The method of claim 1, wherein the at least one connecting
element connects portions of the two structural elements that move
apart when the stent expands from the reduced profile.
8. A stent comprising: a scaffold including a large diameter
configuration and a reduced profile configuration, wherein the
scaffold in the large diameter configuration comprises: a plurality
of polymeric structural elements which is configured to be crimped
to the reduced profile configuration onto a balloon catheter,
wherein the structural elements are plastically deformed when the
scaffold is crimped to the reduced profile configuration from the
large diameter configuration which results in a gripping force on
the balloon catheter by the crimped stent, wherein the gripping
force is not sufficient to prevent the stent from shifting position
or separating from the catheter prior to delivery, wherein the
scaffold in the reduced profile configuration is crimped onto the
balloon catheter and comprises: the plurality of polymeric
structural elements and at least one breakable connecting element
formed in the reduced profile configuration which connect two of
the structural elements, wherein the at least one breakable
connecting element facilitates retention of the stent at a reduced
diameter of the reduced profile configuration so that the stent
does not shift position or separate from the catheter prior to
delivery, wherein upon expansion of the stent the at least one
breakable connecting element breaks to allow expansion of the stent
due to a radially outward force on the stent.
9. The method of claim 8, wherein the structural elements comprise
a biodegradable polymer.
10. The method of claim 8, wherein the structural elements comprise
a plurality of circumferential rings, the at least one connecting
element being formed between adjacent cylindrical rings.
11. The method of claim 8, wherein the structural elements comprise
a plurality of undulating circumferential rings including peaks and
valleys, the at least one connecting element connecting the peak of
one ring with the peak of an adjacent ring.
12. The method of claim 11, wherein all of the peaks of the one
ring are connected with the peaks of the adjacent ring around a
circumference of the stent.
13. The method of claim 8, wherein alternating peaks of one ring
are connected with the peaks of an adjacent ring around a
circumference of the stent.
14. The method of claim 8, wherein the at least one connecting
element connects portions of the two structural elements that move
apart when the stent expands from the reduced profile.
Description
[0001] This application is a divisional application of application
Ser. No. 12/047,040 filed Mar. 12, 2008, which is incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to implantable medical devices, such
as stents.
[0004] 2. Description of the State of the Art
[0005] This invention relates to radially expandable
endoprostheses, which 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.
[0006] A stent is an example of such an endoprosthesis. Stents are
generally cylindrically shaped devices, which 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 the
diameter 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.
[0007] The treatment of a diseased site or lesion with a stent
involves both delivery and deployment of the stent. "Delivery"
refers to introducing and transporting the stent through a bodily
lumen to a region, such as a lesion, in a vessel that requires
treatment. "Deployment" corresponds to the expanding of the stent
within the lumen at the treatment region. Delivery and deployment
of a stent are accomplished by positioning the stent about one end
of a catheter, inserting the end of the catheter through the skin
into a bodily lumen, advancing the catheter in the bodily lumen to
a desired treatment location, expanding the stent at the treatment
location, and removing the catheter from the lumen.
[0008] In the case of a balloon expandable stent, the stent is
mounted about a balloon disposed on the catheter. Mounting the
stent typically involves compressing or crimping the stent onto the
balloon. The stent is then expanded by inflating the balloon. The
balloon may then be deflated and the catheter withdrawn. In the
case of a self-expanding stent, the stent may be secured to the
catheter via a constraining member such as a retractable sheath or
a sock. When the stent is in a desired bodily location, the sheath
may be withdrawn which allows the stent to self-expand.
[0009] The structure of a stent is typically composed of
scaffolding that includes a pattern or network of interconnecting
structural elements often referred to in the art as struts or bar
arms. The scaffolding can be formed from wires, tubes, or sheets of
material rolled into a cylindrical shape. The scaffolding is
designed so that the stent can be radially compressed (to allow
crimping) and radially expanded (to allow deployment). A
conventional stent is allowed to expand and contract through
movement of individual structural elements of a pattern with
respect to each other.
[0010] In advancing a stent through a body vessel to a deployment
site, the stent must be able to securely maintain its axial as well
as rotational position on the delivery catheter without
translocating proximally or distally, and especially without
becoming separated from the catheter. Stents that are not properly
secured or retained to the catheter may slip and either be lost or
be deployed in the wrong location. The stent must be "crimped" and
retained on the catheter in such a way as to minimize or prevent
distortion of the stent and to thereby prevent abrasion and/or
reduce trauma to the vessel walls.
[0011] Generally, stent crimping is the act of affixing the stent
to the delivery catheter or delivery balloon so that it remains
affixed to the catheter or balloon until the physician desires to
deliver the stent at the treatment site. Current stent crimping
technology is sophisticated. Examples of such 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.
[0012] Stents can be made of many materials such as metals and
polymers, including biodegradable polymer materials. Current
intra-arterial stents are composed of a metallic scaffolding or
backbone. As such, they are permanent implants. It is the function
of a stent to dilate a vessel, tack of dissections, and prevent
vasospasm. Once the vessel is fully healed, it is not clear whether
the stent serves a further function or purpose. With coronary
stents, particularly drug-delivery stents, the complication of
thrombosis, both sub-acute, and late is now a serious concern. A
completely biodegradable stent may be a solution to the late
thrombosis problem as it disappears. Also, in most patients,
atherosclerotic disease progresses through life. The biodegradable
stent provides patients with additional treatment options for the
stented regions.
[0013] Thus, biodegradable stents are desirable in many treatment
applications in which the presence of a stent in a body may be
necessary for a limited period of time until its intended function
of, for example, maintaining vascular patency and/or drug delivery
is accomplished. A biodegradable polymer stent can be crimped on to
a catheter in a manner similar to a metal stent. However, there are
problems with stent retention that are unique to polymeric
stents.
SUMMARY OF THE INVENTION
[0014] Embodiments of the present invention include a stent
comprising: a scaffold formed of a plurality of polymeric
structural elements crimped to a reduced profile onto a balloon
catheter, wherein the structural elements are plastically deformed
when the scaffold is crimped to the reduced profile from a larger
diameter which results in a gripping force on the balloon catheter
by the crimped stent, wherein the gripping force is not sufficient
to prevent the stent from shifting position or separating from the
catheter prior to delivery; and at least one breakable connecting
element connecting two of the structural elements, wherein the at
least one breakable connecting element facilitates retention of the
stent at a reduced diameter so that the stent does not shift
position or separate from the catheter prior to delivery, wherein
upon expansion of the stent the at least one breakable connecting
element breaks to allow expansion of the stent due to a radially
outward force on the stent.
[0015] Embodiments of the present invention include a stent
comprising: a scaffold including a large diameter configuration and
a reduced profile configuration, wherein the scaffold in the large
diameter configuration comprises: a plurality of polymeric
structural elements which is configured to be crimped to the
reduced profile configuration onto a balloon catheter, wherein the
structural elements are plastically deformed when the scaffold is
crimped to the reduced profile configuration from the large
diameter configuration which results in a gripping force on the
balloon catheter by the crimped stent, wherein the gripping force
is not sufficient to prevent the stent from shifting position or
separating from the catheter prior to delivery, wherein the
scaffold in the reduced profile configuration is crimped onto the
balloon catheter and comprises: the plurality of polymeric
structural elements and at least one breakable connecting element
formed in the reduced profile configuration which connect two of
the structural elements, wherein the at least one breakable
connecting element facilitates retention of the stent at a reduced
diameter of the reduced profile configuration so that the stent
does not shift position or separate from the catheter prior to
delivery, wherein upon expansion of the stent the at least one
breakable connecting element breaks to allow expansion of the stent
due to a radially outward force on the stent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 depicts a stent.
[0017] FIG. 2A depicts an expandable member.
[0018] FIGS. 2B-C illustrate a crimping process of a stent onto an
expandable member.
[0019] FIG. 2D depicts a stent expanded or deployed due to
inflation of a balloon.
[0020] FIG. 3A depicts a stent pattern shown in a flattened
condition.
[0021] FIG. 3B depicts a close-up view of a curved structural
element of a stent.
[0022] FIGS. 4A-B depict connected structural elements.
[0023] FIGS. 5A-C depict connected structural elements.
[0024] FIG. 6A depicts a schematic illustration of a stent in a
crimped state.
[0025] FIG. 6B depicts a schematic illustration of a stent in a
crimped state.
[0026] FIG. 7A depicts structural elements treated with a solvent
to form a solvent bridge between the structural elements.
[0027] FIG. 7B depicts the structural elements of FIG. 7A with a
connecting element or link formed between the structural
elements.
[0028] FIG. 8A illustrates crimping and formation of connections
between structural elements in a step-wise fashion.
[0029] FIG. 8B depicts a close-up view of an axial section of a
stent positioned within a bore of a crimper and crimped to a
reduced profile.
[0030] FIG. 8C depicts a close-up view of the stent of FIGS. 8A-B
with a crimped axial section within the bore of the crimper.
[0031] FIG. 9 depicts a stent having a sheath disposed over and
retaining the stent at a reduced profile.
[0032] FIG. 10 depicts a stent formed from poly(L-lactide) having
connecting elements or links formed between structural
elements.
DETAILED DESCRIPTION OF THE INVENTION
[0033] Embodiments of the present invention relate to facilitating
retention of polymeric stents on a delivery catheter for delivery
of a stent at an implant site in a vascular lumen. These
embodiments also apply to stent-grafts and generally tubular
medical devices.
[0034] In some embodiments, embodiments of the present invention
are particularly directed to retention of biodegradable, polymeric
stents on a balloon of a catheter assembly. A biodegradable polymer
stent has many advantages over metal stents, including the ability
to be placed in the body only for the duration of time until the
intended function of the stent has been performed.
[0035] However, retention of a polymer stent on a delivery catheter
has been proven to be more challenging than that of a metallic
stent. Polymeric materials, in general, have lower yield stress and
higher elastic strain than metals. This results in the polymeric
stent gripping the balloon with less force after crimping. Polymer
stents can require wider struts than metal stents so as to provide
suitable mechanical properties, such as material strength, for the
stent. At the crimping stage, less space is provided between the
struts which can result in worse stent retention than a metallic
stent. Moreover, the use of high processing temperature during the
crimping process to enhance stent retention may not be possible
since a polymeric stent may have a glass transition temperature
(Tg) close to body temperature. Higher processing temperatures may
cause the polymeric stent to lose some of its preferred mechanical
properties.
[0036] In certain embodiments, a stent is a tubular device
constructed of a pattern of a plurality of interconnected
structural elements which are often also referred to as struts or
bar arms. In such embodiments, a stent can include circumferential
rings connected by linking struts.
[0037] Referring to FIG. 1, an exemplary stent 14 is illustrated.
Stent 14 can include a plurality of structural elements or struts
16 connected by linking struts 11, with interstitial spaces 18
located in between the struts. The plurality of struts 16 can be
configured in an annular fashion in discrete "rows" such that they
form a series of "circumferential rings 5" throughout the body of
stent 14.
[0038] A stent pattern such as that pictured in FIG.1 is configured
to be radially compressible and expandable to allow the stent to be
crimped to reduced profile during delivery and deployed at an
implant site, respectively. A stent can be positioned over a
delivery catheter and compressed or crimped to a delivery diameter.
"Delivery diameter" refers to a diameter at which a stent is
introduced and transported in a bodily lumen. The bending of the
structural elements of a stent pattern allow it to contract as the
stent is crimped or expanded as the stent is deployed. For example,
structural elements of stent 14 can bend inward to allow crimping
as shown by arrows 8 and outward as shown by arrow 9.
[0039] FIGS. 2A-C illustrate the crimping process. FIG. 2A depicts
an expandable member, such as a balloon 10, integrated at a distal
end of a catheter assembly 12. In some embodiments, balloon 10 is
intended to include any type of enclosed member such as an elastic
type member that is selectively inflatable to dilate from a
collapsed configuration to a desired and controlled expanded
configuration. The balloon 10 should also be capable of being
deflated to a reduced profile or back to its original collapsed
configuration. FIG. 2B illustrates a stent 14 in cross section
positioned over balloon 10. Stent 14 is illustrated to have struts
16 separated by gaps 18 (as can also be seen in FIG. 1). In some
embodiments, the diameter of stent 14 as positioned over the
collapsed balloon 10 is much larger than the collapsed diameter of
the balloon 10.
[0040] Additionally, as illustrated in FIG. 2B, the balloon 10 and
the stent 14 are positioned in a crimping device 20. Stent 14 can
be positioned in device 20 and held in place by application of
pressure from crimping device 20. The device may also optionally
heat the stent during crimping. The crimping device 20 can be any
device used in the art or in this disclosed herein. Crimping device
20 then applies inward radial pressure to the stent 14 on the
balloon 10, as shown by arrows 21. Stent 14 positioned over the
balloon 10 is crimped to a reduced stent configuration (reduced
crimped configuration), as illustrated in FIG. 2C.
[0041] FIG. 2D depicts stent 14 expanded or deployed due to
inflation of balloon 10. An inflation fluid such as a gas or liquid
is pumped into the balloon to inflate it. The radially outward
force from the inflated balloon expands the stent to a delivery
diameter.
[0042] As the structural elements bend, they plastically deform
which results in a gripping force on the catheter by the stent.
Metal stents deform plastically with a relatively small strain to
yield which facilitates stent retention. However, polymers have a
lower flexural and tensile moduli than metals and have relatively
larger strains to yield than metals. As a result, the gripping
force of polymer stent is lower than a metal stent. In addition,
polymer stents are susceptible to creep which can also reduce stent
retention. Stents can be exposed to elevated temperatures during
sterilization, either from methods based on radiation or ethylene
oxide, which further increases creep. If the gripping force is not
sufficient, a stent may have a tendency to shift position or
separate from the catheter prior to delivery.
[0043] Embodiments of the present invention include connecting,
linking, or fusing structural elements to facilitate retention of a
stent on a catheter. In such embodiments, breakable linking
connecting elements are employed to facilitate retention of a
polymeric stent on a delivery catheter. These breakable linking
elements maintain a stress, gripping the balloon, that would not be
present in a crimped stent pattern without breakable links. In
these embodiments, a stent is crimped to a reduced profile over a
support element or catheter and a portion of the structural
elements are connected, linked, or fused. At least one breakable
connecting element is formed that connects or links two structural
elements to retain or facilitate retention of the stent on a
support element such as a delivery catheter or balloon. The
breakable connecting element(s) retain or facilitate retention of
the stent at the crimped diameter and on the support element.
[0044] As described below, portions of structural elements can be
connected or linked through formation of connecting elements using
various methods including thermal welding, solvent welding, laser
welding, application of polymer solution, application of adhesives,
or ultrasonic welding. In these embodiments, the stent may be
crimped to a reduced profile so that the portions of structural
elements to be joined are in contact or close enough to allow
linking and forming the connecting elements between such
portions.
[0045] As indicated above, a stent contracts and expands through
movement of structural elements with respect to one another. In
these embodiments, parts, portions, or sections of structural
elements that tend to move apart upon expansion of the stent are
fused or connected, thus reducing or preventing expansion. The
movement that is reduced or prevented can be circumferential or
longitudinal. For example, the movement of structural elements
depicted by arrows 8 and 9 of FIG. 1 is roughly
circumferential.
[0046] Embodiments of the present invention can including forming
connections between portions of any such structural elements that
are in contact or closely spaced upon crimping the stent. The
connecting elements formed between such structural elements reduce
or prevent movement of such structural elements with respect to one
another, thus retaining the stent at the crimped configuration.
[0047] In some embodiments, connecting elements are formed between
structural elements on neighboring cylindrical rings that tend to
move apart to allow expansion of a stent from a crimped profile. In
other embodiments, portions of structural elements can be connected
that bend inward to allow crimping or outward to allow expansion,
as shown by arrows 8 and arrow 9 of FIG. 1, respectively.
[0048] FIG. 3A depicts a stent pattern 40 shown in a flattened
condition so that the pattern can be clearly viewed. When the
flattened portion of stent pattern 40 is in a cylindrical form, it
forms a radially expandable stent. Stent pattern 40 includes a
plurality of cylindrical rings 42 of diamonds with each ring
including a plurality of diamond-shaped cells 44. Embodiments of
stent pattern 40 may have any number of rings 42 depending on the
desired stent length. For reference, line A-A represents the
longitudinal axis of a stent using the pattern depicted in FIG. 3A.
Diamond shaped cells 44 are made up of struts 46 and 48 that form a
curved element and struts 50 and 52 that form an opposing curved
element.
[0049] Pattern 40 further includes linking struts 56 that connect
adjacent cylindrical rings. Linking struts 56 are parallel to line
A-A and connect adjacent rings between intersections 58 of
cylindrically adjacent diamond-shaped elements 44 of one ring and
intersection 58 of cylindrically adjacent diamond shaped elements
44 of an adjacent ring. As shown, linking elements connect every
other intersection along the circumference. Rings 42 can also be
described as being formed by two opposing undulating rings 60 and
62 with opposing peaks 64 and 66.
[0050] As shown in FIG. 3A, stent pattern 40 is in a crimped or
reduced profile state. Radial expansion from the reduced profile
state is allowed through the outward bending of the curved elements
formed by struts 46 and 48 and struts 50 and 52, as shown by arrows
70 and 72. As the curved elements bend, they move apart as shown by
arrows 74 and 76.
[0051] As shown in FIG. 3A, curved elements on adjacent rings are
connected or fused. Stent pattern 40 has connecting elements 80
that connect the peaks of the curved elements formed by struts 46
and 48 and struts 50 and 52. Connecting elements 80 prevent the
curved elements from moving away and bending. Connecting elements
80 reduce or prevent radial expansion of stent pattern 40 and are
under tension due to a tendency for stent pattern 40 is expand. The
tension of connecting elements 80 results in a radially directed
gripping force that facilitates retention of the stent on a
supporting element.
[0052] The connecting elements can be selectively distributed
throughout the pattern to achieve a desired retention of the stent
on the supporting element. In one embodiment shown in FIG. 3A,
connecting elements connect all of the peaks of adjacent of
adjacent rings around a circumference of the stent. In other
embodiments, the connecting elements connect some, but not all of
the peaks of adjacent rings around a circumference of a stent. For
example, alternating peaks of adjacent rings may be connected.
[0053] In these embodiments, the connecting elements are capable of
breaking to allow expansion of the stent during stent implantation.
In such embodiments, the stent can be mounted over an expandable
member which applies a radially outward force. The force is high
enough to cause the connecting elements to fracture, split, or
break to allow expansion of the stent. FIG. 3B depicts a close-up
view of a curved element as it expands due to the outward radial
force from an expandable member. The curved elements defined by
struts 46 and 48 and struts 50 and 52 bend outward as shown by
arrows 70 and 72. The outward radial force is strong enough to
break connecting element 80 to allow the expansion, forming pieces
80A and 80B. In other embodiments, a connecting element can break
off of one of the curved elements leaving the entire connecting
element on one of the curved elements. In additional embodiments,
the connecting element can break off of both of the curved
elements. However, generation of particulates is generally
undesirable as they can constitute an embolic hazard. Consequently,
the connecting element can be designed so that it parts in a
specified location without shedding any particles.
[0054] Another exemplary embodiment is shown in FIG. 4A which
depicts a section 81 of stent 14 of FIG. 1 in a crimped state. FIG.
4A depicts a bending element 17 made up of struts 16A and 16B which
move apart as shown by arrow 9. Line B-B represents the
circumferential direction of stent 14. Struts 16A and 16B are
connected by a connecting element 80A which reduces or prevents
outward bending of bending element 17. Additionally, struts 16B and
11A are connected by a connecting element 80B which reduces or
prevent outward bending of struts 16B and 11A. Such connecting
elements can be distributed throughout the stent pattern of stent
14 to reduce or prevent expansion of the stent from the crimped
state.
[0055] FIG. 4B depicts another exemplary embodiment depicting a
section 82 of stent 14 of FIG. 1 in a crimped state. FIG. 4B
depicts a bending element 17A made up of struts 16C and 16D which
move apart. Line B-B represents the circumferential direction of
stent 14. Struts 16C and 16D are connected by a connecting element
80C which reduces or prevents outward bending of bending element
17.
[0056] In certain embodiments, the dimensions or mass of connecting
elements or links can be adjusted to allow breaking of the links
upon application of a radial force on the stent by an expandable
member. A pressure applied by an expandable member can be in the
range of 2 to 16 atm. For example, a thickness or average
thickness, tc, of connecting element 80 in FIG. 5A can be adjusted.
Increasing tc increases both the mass of the connecting elements
and the surface area of the portions of the structural elements
that are fused or connected, thus increasing the strength of the
connection. Additionally, a ratio of a length, lc, of the
connecting elements to tc can be one, greater than one (FIG. 5A),
or less than one (FIG. 5B). In some embodiments, connecting
elements can be separated by a point or region of contact between
connected structural elements. FIG. 5C illustrates a point of
contact 83 between connecting elements 80A and 80B.
[0057] In some embodiments, structural elements can be connected or
fused by thermal welding, or more generally, by applying heat to
soften or melt polymer material of the structural element or a
bonding or connecting polymer. In such embodiments, portions of
structural elements can be heated to soften or melt the portions of
structural elements to be connected. In one embodiment, the
portions are in contact and upon cooling, the portions are
connected or fused. In another embodiment, the softened or melted
portions are closely spaced. The closely-spaced softened or melted
portions can be then be brought into contact by further radial
compression of the stent. Upon cooling the portions are fused or
connected. In an additional embodiment, upon softening or melting,
a bridge of polymer material is formed between the portions and
upon cooling the portions are connected or fused. In further
embodiments, a melted polymer material can be disposed between
contacted or closely spaced portions of the structural element and
allowed to cool to connect or fuse the structural elements.
[0058] In another embodiment, a melted polymer composition can be
applied between or on the portions of the structural elements to be
joined to form the connecting element. The polymer composition can
be the same or different from the polymer material that makes up
the structural elements. In one embodiment, formation of the
connecting element from the polymer composition does not melt
polymer material of the structural elements. The melted material
adheres to the structural elements and solidifies to form the
connecting elements. In one embodiment, the melting temperature
(Tm) of the connecting element composition is less than the polymer
composition of the coating on the structural element, the
scaffolding polymer of the structural element, or both.
[0059] In certain embodiments, a connecting element polymer
material can be selected to have different mechanical properties
from the scaffolding polymer of the structural elements. In
particular, a material for the connecting element can be selected
that has a propensity to fracture or break when a radial force is
applied the stent during deployment. In one embodiment, the
connecting element polymer material is a brittle or glassy polymer
at human body temperature, which is approximately 37.degree. C.
Such a material has a Tg above human body temperature. In some
embodiments, the connecting element polymer has a lower strength,
lower elongation at break, or both than the scaffolding polymer. In
exemplary embodiments, a connecting element polymer includes
poly(L-lactide) (PLLA), poly(L-lactide-co-D,L-lactide),
poly(D,L-lactide), poly(D,L-lactide-co-glycolide), poly(glycolide)
and poly(L-lactide-co-glycolide) (PLGA).
[0060] In some embodiments, the connecting element may be composed
of biodegradable or biosoluble material. The connecting element may
also be composed of a bioabsorbable metal or metallic alloy. The
connecting element may be exposed to bodily fluid during delivery
prior to deployment, resulting in degradation of the mechanical
properties of the connecting element. The degradation in mechanical
properties can facilitate the breaking of the connecting element
upon deployment of the stent. In such embodiments, the connecting
element material can have a higher degradation rate than the
polymer of the structural element to further facilitate the
breaking of the connecting element upon deployment. In an exemplary
embodiment, the scaffolding of the stent can be formed from PLLA
and the connecting element material can be a faster degrading
material such as polyglycolide (PGA) or PLGA.
[0061] In certain embodiments, heat can be applied to fuse or
connect portions of structural elements with a heating element,
such as a heated metal tip. FIG. 6A depicts a schematic
illustration of a stent 90 in a crimped state. Heating elements 92
with heated tips 94 are positioned adjacent or in contact with
structural elements to be joined. A polymer composition 96 is shown
melted by heated tips 94. Stent 90 can be disposed over a rotatable
support which allows rotation of stent 90, as shown by an arrow 98
so that connecting elements can be formed around at selected
locations around the circumference of stent 90.
[0062] In additional embodiments, laser energy can be used to form
connecting elements between structural elements. In such
embodiments, laser energy can be focused on structural elements to
be joined to melt the structural elements to form a connecting
element. Exemplary lasers include excimer, ultraviolet, infra-red,
visible and ultra-fast pulse lasers. Infra-red lasers heat
polymeric materials effectively although ultraviolet and excimer
lasers can work as well. FIG. 6B depicts a schematic illustration
of a stent 100 in a crimped state. Laser beam sources 102 are
positioned to direct laser energy onto closely spaced structural
elements or structural elements in contact. Laser energy is
directed as shown by arrows 104 to melt a portion of structural
elements to form connecting elements between them. In other
embodiments, a connecting element polymer composition is positioned
adjacent the structural elements and melted with laser energy to
form connecting elements. As shown in FIG. 6B, stent 100 can be
rotated, as shown by an arrow 106 so that connecting elements can
be formed at selected locations around the circumference of stent
100.
[0063] In further embodiments, structural elements can be connected
or fused through the use of solvent bonding. In general, solvent
bonding refers to the process of joining structural elements made
of polymers by applying a solvent capable of dissolving, swelling,
or softening the surfaces to be joined, and contacting the surfaces
treated by the solvent. Upon removal of the solvent through
evaporation, the surfaces are fused or connected. In these
embodiments, a solvent can be applied to closely spaced or
structural elements in contact. The solvent is capable of
dissolving or swelling the polymer of the scaffolding.
Representative examples of solvents may include, but are not
limited to, chloroform, acetone, tetrahydrofuran, chlorobenzene,
ethyl acetate, 1,4-dioxane, ethylene dichloride, 2-ethyhexanol, and
combinations thereof.
[0064] In some embodiments, an amount of solvent is applied to
closely spaced portions of structural elements to form a connection
or bridge of a polymer solution between the structural elements.
The solution includes dissolved polymer from the structural
element. The solvent from the solution may then be removed through
evaporation, leaving a connecting element composed of the polymer
of the structural element. FIG. 7A depicts structural elements 110
and 112 that have been treated with a solvent that has formed a
bridge 114 between the structural elements. As shown in FIG. 7B,
upon removal of the solvent, a connecting element 116, is formed
between structural elements 110 and 112. The solvent can be applied
by various methods such as by means of a micro-syringe, injet type
nozzle, or piezoelectric dipenser. The solvent can be allowed to
evaporate at room temperature. Alternatively, a room temperature or
heated gas can be blown on the stent to facilitate evaporation of
the solvent.
[0065] In another embodiment, solvent is applied to portions of the
structural elements to be connected to form a polymer solution on a
surface of the treated portions. The treated portions are then
brought in to contact, for example, by additional radial
compression or crimping of the stent. Removal of the solvent
results in the formation of a connecting element between the
structural elements made from the polymer of the structural
element.
[0066] In additional embodiments, a connecting element between
structural elements may be formed using a polymer solution
containing a polymer which is the same or different from a polymer
of the structural element. The polymer solution may be applied in a
manner similar to that described in solvent bonding. For instance,
a polymer solution may be applied to a surface of closely spaced
structural elements. In one embodiment, a bridge of solution is
formed between the structural elements. Alternatively or
additionally, the treated portions of the structural elements can
be brought into contact, for example, by further crimping or radial
compression of the stent. The solvent in the solution may then be
removed from the solution as discussed above, leaving a connecting
element composed of the polymer in the solution. In some
embodiments, the polymer in the solution is different from the
polymer of the structural element and the solvent of the solution
may be a nonsolvent or weak solvent for the polymers of the
structural element.
[0067] In further embodiments, various types of biostable and
biodegradable adhesives may be used to form connecting elements.
Exemplary adhesives include, but are not limited to, thermosets
such as, for example, epoxies, polyesters and phenolics;
thermoplastics such as, for example, polyamides, cyanoacrylates,
polyesters and ethyl vinyl acetate (EVA) copolymers; and elastomers
such as, for example, natural rubber, styrene-isoprene-styrene
block copolymers, and polyisobutylene. Other adhesives include, but
are not limited to, proteins; cellulose; albumin; starch;
poly(ethylene glycol); fibrin glue; and derivatives and
combinations thereof.
[0068] Various methods may be used to selectively apply materials
discussed in the above embodiments to a stent, i.e., polymer melts,
solvents, polymer solutions, and adhesives. Such methods include
ink jet-type coating, electrostatic coating, roll coating, thermal
deposition with masking, plasma polymerization with masking, direct
application of polymer/solvent solution by micro-syringe, direct
polymer melt application, and spray coating with photomasking. In
an exemplary embodiment, a controlled deposition system ink
jet-type coating method can be used that applies various substances
only to certain targeted portions of a stent. A representative
example of such a system, and a method of using the same, is
described in U.S. Pat. No.
[0069] 6,395,326 to Castro et al. A controlled deposition system
can be capable of depositing a substance on a stent having a
complex geometry, and otherwise apply the substance so that
substance is limited to particular portions of the stent, such
surfaces of structural elements that are to be joined or connected.
The system can have a dispenser and a holder that supports the
stent. The dispenser and/or holder can be capable of moving in very
small intervals, for example, less than about 0.001 inch.
Furthermore, the dispenser and/or holder can be capable of moving
in the x-, y-, or z-direction, and be capable of rotating about a
single point.
[0070] The controlled deposition system can include a dispenser
assembly. The dispenser assembly can be a simple device including a
reservoir, which holds a substance such as a polymer melt, solvent,
polymer solution, adhesive, prior to delivery, and a nozzle having
an orifice through which the substance is delivered. One exemplary
type of dispenser assembly can be an assembly that includes an
ink-jet-type printhead. Another exemplary type of a dispenser
assembly can be a microinjector capable of injecting small volumes
ranging from about 2 to about 70 nL, such as NanoLiter 2000
available from World Precision Instruments or Pneumatic PicoPumps
PV830 with Micropipette available from Cell Technology System. Such
microinjection syringes may be employed in conjunction with a
microscope of suitable design.
[0071] Furthermore, selective application of substances to surfaces
of a stent may be performed using photomasking techniques.
Deposition and removal of a mask can be used to selectively apply
substances to surfaces of substrates. Masking deposition is known
to one having ordinary skill in the art.
[0072] Additionally, the substances of the present invention can
also be selectively deposited by an electrostatic deposition
process. Such a process can produce an electrically charged or
ionized substance. The electric charge causes the substance to be
differentially attracted to the stent, thereby resulting in higher
transfer efficiency. The electrically charged substance can be
deposited onto selected regions of the stent by causing different
regions of the device to have different electrical potentials.
[0073] In further embodiments, ultrasonic welding may be used to
connect or fuse structural elements. Ultrasonic welding refers to a
technique in which high-frequency ultrasonic acoustic vibrations
are used to weld objects together, typically plastics. In
ultrasonic welding an element for emitting ultrasonic vibrations, a
sonotrode or horn is connected to a transducer. The sonotrode is
positioned against the portions to be welded and a very rapid
(.about.20,000 KHz), low-amplitude acoustic vibration is applied to
the portions. Typically, in ultrasonic welding, the portions to be
welded are sandwiched between an ultrasonic transducer and a rigid
support. The transducer and support may be configured as pincers to
clamp the postions together before applying the ultrasonic energy.
The acoustic energy is converted into heat by friction so that
portions to be welded are ultimately connected or fused by the heat
generated.
[0074] As discussed above, fusing, bonding, or forming a connection
between portions of structural elements using the methods described
above requires the stent to be in a sufficiently reduced profile or
crimped state so that such portions to be sufficiently close or in
contact. In some embodiments, the connecting elements are formed
after crimping in the absence a radial compressive force. In such
embodiments, the portions of the structural elements are close
enough or in contact that to allow formation of the connecting
elements. The stent can be removed from the crimper and then one of
the above techniques is applied to form the connecting
elements.
[0075] In other embodiments, sufficiently close spacing or contact
of the portions of the structural element may require inward radial
compression during the fusing or connecting process. In some
embodiments, the connecting elements can be formed during crimping.
In one such embodiment, an axial section of a stent is crimped to a
reduced profile and a connection is formed between structural
elements of the axial section. In an exemplary embodiment, an axial
segment of a stent can be disposed in a short bore crimper. The
crimper reduces the profile of axial segment within the bore and an
axial portion extending out from the proximal side of the bore. One
or more connections may be formed between structural elements in
the axial portion extending out from the proximal side of the
crimper bore. An uncrimped axial segment proximal to the crimped
section is then positioned within the crimper bore. The axial
segment is crimped, followed by the formation of connections to
structural elements, as described above. The process is repeated in
a step-wise fashion until the entire length of the stent is crimped
and connections formed along the its length.
[0076] FIG. 8A illustrates crimping and formation of connections
between structural elements in a step-wise fashion. FIG. 8A depicts
a short bore crimper 120 and a catheter 122 positioned through a
bore 124 of short bore crimper 120. An axial thickness T of short
bore thickness is not long enough to crimp an entire axial length
of a stent 126 disposed over catheter 122 and positioned just
outside of bore 124. To crimp an axial segment 126A of stent 126,
it is positioned within bore 124 in the direction of an arrow 125
followed by radial compression of the axial segment. Laser beam
sources 128 are positioned to direct laser energy onto a crimped
portion of stent 126 to form connections between structural
elements.
[0077] FIG. 8B depicts a close-up view of an axial section 130 of
stent 126 positioned within bore 124 and crimped to a reduced
profile. A proximal axial section 132 extending out of bore 124
also has a reduced profile and axial section 134 is uncrimped.
Laser beam sources 128 are then used to form connections between
structural elements in axial section 132. The method of forming the
connections is not limited to laser welding. For example, a heated
tip, an ultrasonic welder, or a dispenser for dispensing solvent,
polymer solution, polymer melt, or adhesive can be used. After
crimping and forming the connections, the stent is moved axially in
the direction of arrow 125 and another axial section is crimped
followed by formation of connections between structural
elements.
[0078] In an alternate embodiment, connections between structural
elements can be formed in a crimped axial section of a stent
extending out from distal side of a crimper bore. FIG. 8C depicts a
close-up view of stent 126 with a crimped axial section 140 within
bore 124 of crimper 120. A crimped proximal axial section 142
extends out of the distal side of bore 124. Axial section 142 has
already been crimped by crimper 120. Laser beam sources 128 are
positioned on the distal side of bore 124 to form connections
between structural elements in axial section 142. Stent 126 is
crimped and connections are formed in a step-wise fashion until the
entire length of stent 126 is crimped.
[0079] In additional embodiments, a method of forming connections
between structural elements can include crimping a stent to a
reduced profile followed by positioning a cylindrical sheath over
the crimped stent to maintain the reduced profile. At least some
structural elements of the stent in the reduced profile state are
in contact or sufficiently close to form connections between them.
The sheath may then be slid axially off of the stent to expose an
axial section of the stent. Connections may then be formed on the
exposed axial section. Connections may be formed in further axial
sections of the stent in a step-wise fashion. FIG. 9 depicts a
stent 150 having a sheath 152 disposed over and retaining stent 150
is a reduced profile or crimped configuration. Sheath 152 has been
slid as shown by arrows 154 exposing axial section 156 of stent
150. Laser beam sources 158 are positioned to form connections
between portions of structural elements in axial section 156.
Sheath 152 is slid in a step-wise fashion in the direction of
arrows 154 to exposed additional axial sections in which
connections are formed.
[0080] In yet another embodiment, the full length of a polymer
stent is crimped to a reduced profile. This brings structural
elements into proximity which are to be joined to form breakable
links. Next, all of the links are formed via application of heat to
melt and fuse the elements together to form the links.
[0081] Generally, a stent may be formed, for example, from a tube
or a sheet rolled into a tube. The sheet or tube, for example, may
be formed by various methods known in the art such as extrusion or
injection molding. A pattern may then be cut into the polymeric
tube by laser cutting or chemical etching to form the stent.
[0082] Additionally, as mentioned above, a stent fabricated from
embodiments of the stent described herein can be medicated with an
active agent. A medicated stent may be fabricated by coating the
surface of the polymeric scaffolding with a polymeric carrier that
includes an active or bioactive agent or drug.
[0083] Embodiments of the present invention described herein may be
applied to devices including, but not limited to, balloon
expandable stents, self-expanding stents, and stent-grafts.
[0084] In the case of a self-expanding stent, the stent can be
crimped over a support, such as a catheter. The stent is used to
open a lumen within an organ in a mammal, maintain lumen patency,
or reduce the likelihood of narrowing of a lumen.
[0085] The method according to the invention can be used to
increase retention in both polymeric and metallic stents. In one
embodiment, the polymer for use in forming the stent scaffolding
and/or the stent coating may be configured to degrade after
implantation by fabricating the stent either partially or
completely from biodegradable polymers.
[0086] In general, polymers for use in fabricating a substrate of a
stent or a coating for a stent can be biostable, bioabsorbable,
biodegradable or bioerodable. Biostable refers to polymers that are
not biodegradable. The terms biodegradable, bioabsorbable, and
bioerodable are used interchangeably and refer to polymers that are
capable of being completely degraded and/or eroded when exposed to
bodily fluids such as blood and can be gradually resorbed,
absorbed, and/or eliminated by the body. The processes of breaking
down and eventual absorption and elimination of the polymer can be
caused by, for example, hydrolysis, metabolic processes,
enzymolysis, oxidation, bulk or surface erosion, and the like.
[0087] It is understood that after the process of degradation,
erosion, absorption, and/or resorption has been completed, no part
of the stent will remain, or in the case of coating applications on
a biostable scaffolding, no polymer will remain on the device. In
some embodiments, very negligible traces or residue may be left
behind. For stents made from a biodegradable polymer, the stent is
intended to remain in the body for a duration of time until its
intended function of, for example, maintaining vascular patency
and/or drug delivery is accomplished.
[0088] Representative examples of polymers that may be used to
fabricate or coat a stent include, but are not limited to,
poly(N-acetylglucosamine) (Chitin), Chitosan,
poly(hydroxyvalerate), poly(L-lactide) (PLLA),
poly(L-lactide-co-D,L-lactide), poly(D,L-lactide),
poly(D,L-lactide-co-glycolide), poly(glycolide),
poly(L-lactide-co-glycolide) (PLGA), poly(hydroxybutyrate),
poly(hydroxybutyrate-co-valerate), polyorthoester, polyanhydride,
poly(glycolic acid), poly(L-lactic acid), poly(D,L-lactic acid),
poly(caprolactone), poly(trimethylene carbonate), polyester amide,
poly(glycolic acid-co-trimethylene carbonate),
co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes,
biomolecules (such as fibrin, fibrinogen, cellulose, starch,
collagen and hyaluronic acid), polyurethanes, silicones,
polyesters, polyolefins, polyisobutylene and ethylene-alphaolefin
copolymers, acrylic polymers and copolymers other than
polyacrylates, vinyl halide polymers and copolymers (such as
polyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl
ether), polyvinylidene halides (such as polyvinylidene chloride),
polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such as
polystyrene), poly(styrene-b-isobutylene-b-styrene), polyvinyl
esters (such as polyvinyl acetate), acrylonitrile-styrene
copolymers, ABS resins, polyamides (such as Nylon 66 and
polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,
polyethers, polyurethanes, rayon, rayon-triacetate, cellulose,
cellulose acetate, cellulose butyrate, cellulose acetate butyrate,
cellophane, cellulose nitrate, cellulose propionate, cellulose
ethers, and carboxymethyl cellulose. Another type of polymer based
on poly(lactic acid) that can be used includes graft copolymers,
and block copolymers, such as AB block-copolymers
("diblock-copolymers") or ABA block-copolymers
("triblock-copolymers"), or mixtures thereof.
[0089] Additional representative examples of polymers that may be
especially well suited for use in fabricating or coating a stent
include ethylene vinyl alcohol copolymer (commonly known by the
generic name EVOH or by the trade name EVAL), poly(butyl
methacrylate), poly(vinylidene fluoride-co-hexafluororpropene)
(e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare,
N.J.), polyvinylidene fluoride (otherwise known as KYNAR, available
from ATOFINA Chemicals, Philadelphia, Pa.), ethylene-vinyl acetate
copolymers, and polyethylene glycol.
[0090] The "glass transition temperature," Tg, is the temperature
at which the amorphous domains of a polymer change from a brittle
vitreous state to a solid deformable or ductile state at
atmospheric pressure. In other words, the Tg corresponds to the
temperature where the onset of segmental motion in the chains of
the polymer occurs. When an amorphous or semicrystalline polymer is
exposed to an increasing temperature, the coefficient of expansion
and the heat capacity of the polymer both increase as the
temperature is raised, indicating increased molecular motion. As
the temperature is raised the actual molecular volume in the sample
remains constant, and so a higher coefficient of expansion points
to an increase in free volume associated with the system and
therefore increased freedom for the molecules to move. The
increasing heat capacity corresponds to an increase in heat
dissipation through movement. Tg of a given polymer can be
dependent on the heating rate and can be influenced by the thermal
history of the polymer. Furthermore, the chemical structure of the
polymer heavily influences the glass transition by affecting chain
mobility.
[0091] "Strength" refers to the maximum stress along an axis which
a material will withstand prior to fracture. The ultimate strength
is calculated from the maximum load applied during the test divided
by the original cross-sectional area.
[0092] "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 results from the applied
force. For example, a material has both a tensile and a compressive
modulus. A material with a relatively high modulus tends to be
stiff or rigid. Conversely, a material with a relatively low
modulus tends to be flexible. The modulus of a material depends on
the molecular composition and structure, temperature of the
material, amount of deformation, and the strain rate or rate of
deformation. For example, below its Tg, a polymer tends to be
brittle with a high modulus. As the temperature of a polymer is
increased from below to above its Tg, its modulus decreases.
[0093] "Strain" refers to the amount of elongation or compression
that occurs in a material at a given stress or load.
[0094] "Elongation" may be defined as the increase in length in a
material which occurs when subjected to stress. It is typically
expressed as a percentage of the original length.
EXAMPLE
[0095] FIG. 10 depicts a stent formed from poly(L-lactide) having
connecting elements or links 200 formed between structural elements
202.
[0096] 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.
* * * * *