U.S. patent application number 11/141834 was filed with the patent office on 2006-11-30 for stent with flexible sections in high strain regions.
Invention is credited to Anthony J. Abbate, Svava Maria Atladottir, David C. Gale, Klaus Kleine, Stephen Pacetti.
Application Number | 20060271170 11/141834 |
Document ID | / |
Family ID | 36968883 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060271170 |
Kind Code |
A1 |
Gale; David C. ; et
al. |
November 30, 2006 |
Stent with flexible sections in high strain regions
Abstract
A stent for treating a bodily lumen with a flexible section in a
high strain region is disclosed. A flexible section may be
selectively positioned to reduce an amount of strain in the high
strain region when subjected to the applied stress during use to
inhibit or prevent fracturing in the high strain region.
Inventors: |
Gale; David C.; (Sunnyvale,
CA) ; Kleine; Klaus; (Los Gatos, CA) ; Abbate;
Anthony J.; (Santa Clara, CA) ; Atladottir; Svava
Maria; (San Francisco, CA) ; Pacetti; Stephen;
(San Jose, CA) |
Correspondence
Address: |
SQUIRE, SANDERS & DEMPSEY LLP
1 MARITIME PLAZA
SUITE 300
SAN FRANCISCO
CA
94111
US
|
Family ID: |
36968883 |
Appl. No.: |
11/141834 |
Filed: |
May 31, 2005 |
Current U.S.
Class: |
623/1.49 |
Current CPC
Class: |
A61F 2002/91533
20130101; A61F 2/91 20130101; A61F 2002/91575 20130101; A61F 2/915
20130101; A61F 2250/0029 20130101 |
Class at
Publication: |
623/001.49 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1. A structural element of a stent comprising: a flexible section
embedded within a high strain region of a structural element, the
flexible section extending through the high strain region from an
abluminal surface of the high strain region to a luminal surface of
the high strain region, the flexible section comprising a greater
flexibility than a majority of the structural element outside of
the flexible section; wherein the high strain region comprises a
higher strain as compared to other regions of the structural
element when the stent is subjected to an applied stress during
use, wherein the section is selectively positioned to increase
resistance to strain in the high strain region to inhibit or
prevent fracturing in the high strain region when subjected to the
applied stress during use.
2. The stent of claim 1, wherein the stent comprises a biostable
polymer and/or a bioabsorbable polymer.
3. The stent of claim 1, wherein the flexible section comprises a
biostable polymer and/or a bioabsorbable polymer.
4. The stent of claim 1, wherein the flexible section comprises a
lower modulus than a majority of the structural element outside of
the flexible section.
5. The stent of claim 1, wherein the flexible section comprises a
glass transition temperature at or below body temperature.
6. The stent of claim 1, wherein the high strain region comprises a
region of the stent configured to bend when the structural element
is under an applied stress during use.
7. The stent of claim 1, wherein the section is selectively
positioned proximate a center of a width of a bending portion of
the structural element.
8. The stent of claim 1, wherein the section is selectively
positioned proximate a neutral axis of a bending portion of the
structural element.
9. The stent of claim 1, wherein a cross-section of the section is
slot-shaped and positioned along at least a portion of a length of
the high strain region.
10. The stent of claim 1, wherein a width of the flexible section
varies along the length of the high strain region.
11. The stent of claim 1, wherein a width of the section along the
length is directly proportional to strain along a length of the
high strain region when the structural element is under stress.
12. The stent of claim 1, wherein a width of the section is widest
at a center of a bending portion of the high strain region and
decreases along a length of the high strain region.
13. A method of forming a stent comprising: forming a cavity within
a high strain region of a structural element, the cavity extending
through the high strain region from an abluminal surface of the
high strain region to a luminal surface of the high strain region,
wherein the high strain region comprises a higher strain as
compared to other regions of the structural element when the stent
is subjected to an applied stress during use; forming a flexible
section within all or substantially all of the cavity, the flexible
section comprising a greater flexibility than a majority of the
structural element outside of the flexible section, wherein the
section is selectively positioned to reduce an amount of strain in
the high strain region to inhibit or prevent fracturing in the high
strain region when subjected to the applied stress during use.
14. The method of claim 13, wherein the stent comprises a biostable
polymer and/or a bioabsorbable polymer.
15. The method of claim 13, wherein the flexible section comprises
a biostable polymer and/or a bioabsorbable polymer.
16. The method of claim 13, wherein the flexible section comprises
a lower modulus than a majority of the structural element outside
of the flexible section.
17. The method of claim 13, wherein the cavity is formed by laser
cutting the high strain region.
18. The method of claim 13, wherein the flexible section is formed
by depositing a polymer-solvent mixture into the cavity.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to implantable medical devices, such
as stents. In particular the invention relates to stents having
structural elements with flexible or deformable sections that
increase the crack resistance of the stent during use.
[0003] 2. Description of the State of the Art
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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 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.
[0008] The stent must be able to satisfy a number of mechanical
requirements. First, the stent must be capable of withstanding the
structural loads, namely radial compressive forces, imposed on the
stent as it supports the walls of a vessel. Therefore, a stent must
possess adequate radial strength. Radial strength, which is the
ability of a stent to resist radial compressive forces, is due to
strength and rigidity around a circumferential direction of the
stent. Radial strength and rigidity, therefore, may be also be
described as, hoop or circumferential strength and rigidity.
[0009] Additionally, the stent should also be longitudinally
flexible to allow it to be maneuvered through a tortuous vascular
path and to enable it to conform to a deployment site that may not
be linear or may be subject to flexure. The material from which the
stent is constructed must allow the stent to undergo expansion.
Once expanded, the stent must maintain its size and shape
throughout its service life despite the various forces that may
come to bear on it, including the cyclic loading induced by the
beating heart. Finally, the stent must be biocompatible so as not
to trigger any adverse vascular responses.
[0010] The structure of a stent is typically composed of
scaffolding that includes a pattern or network of interconnecting
structural elements or struts. 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 contracted (to allowed 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. Such movement typically results
in substantial deformation of localized portions of the stent's
structure.
[0011] Stents have been made of many materials such as metals and
polymers, including biodegradable polymeric materials. An advantage
of stents fabricated from polymers is that they can be designed to
have greater flexibility than metal stents. However, a potential
problem with polymeric stents is that their struts may be
susceptible to cracking during crimping and expansion especially
for brittle polymers. The localized portions of the stent pattern
subjected to substantial deformation tend to be the most vulnerable
to failure.
[0012] Therefore, it would be desirable reduce or eliminate the
susceptibility of cracking in a polymeric stent, particularly in
localized regions of high deformation. It is also desirable to
maintain or increase their flexibility while improving crack
resistance.
SUMMARY OF THE INVENTION
[0013] The present invention is directed to embodiments of a stent
having structural elements that may include a flexible or
deformable section embedded within a high strain region of a
structural element. The flexible section may extend through the
high strain region from an abluminal surface of the high strain
region to a luminal surface of the high strain region. The flexible
section may have a greater flexibility than a majority of the
structural element outside of the flexible section. The high strain
region may have a higher strain as compared to other regions of the
structural element when the stent is subjected to an applied stress
during use. The section may be selectively positioned to increase
resistance to strain in the high strain region to inhibit or
prevent fracturing in the high strain region when subjected to the
applied stress during use.
[0014] A further aspect of the invention is directed to embodiments
of a method of forming a stent. The method may include forming a
cavity within a high strain region of a structural element. The
cavity may extend through the high strain region from an abluminal
surface of the high strain region to a luminal surface of the high
strain region. The high strain region may include a higher strain
as compared to other regions of the structural element when the
stent is subjected to an applied stress during use. The method may
further include forming a flexible section within all or
substantially all of the cavity. The flexible section may have a
greater flexibility than a majority of the structural element
outside of the flexible section. The section may be selectively
positioned to reduce an amount of strain in the high strain region
to inhibit or prevent fracturing in the high strain region when
subjected to the applied stress during use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1A depicts a stent.
[0016] FIG. 1B depicts a three-dimensional view of a portion of a
stent.
[0017] FIG. 2A depicts a side view of a portion of a stent.
[0018] FIG. 2B depicts a side view of an expanded portion of a
stent.
[0019] FIG. 2C depicts a side view of a compressed portion of a
stent.
[0020] FIG. 3 depicts an expanded view of curved portion of a
stent.
[0021] FIG. 4 depicts a side view of a portion of a stent.
[0022] FIG. 5 depicts a side view of a portion of a stent.
[0023] FIG. 6 depicts a side view of a portion of a stent with a
flexible section.
[0024] FIG. 7 depicts a side view of an expanded portion of a stent
with a flexible section.
[0025] FIG. 8 depicts a side view of a compressed portion of a
stent with a flexible section.
[0026] FIG. 9 depicts a side view of a portion of a stent with
multiple flexible sections.
[0027] FIG. 10 depicts a three-dimensional side view of a portion
of a stent with a cavity.
DETAILED DESCRIPTION OF THE INVENTION
[0028] For the purposes of the present invention, the following
terms and definitions apply:
[0029] The "glass transition temperature," T.sub.g, 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 T.sub.g 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. T.sub.g 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
mobility.
[0030] "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 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.
[0031] "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.
[0032] Furthermore, a property of a material that quantifies a
degree of strain with applied stress is the modulus. "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, and the
strain rate or rate of deformation. For example, below its T.sub.g,
a polymer tends to be brittle with a high modulus. As the
temperature of a polymer is increased from below to above its
T.sub.g, its modulus decreases.
[0033] The "ultimate strength" or "strength" of a material refers
to the maximum stress that a material will withstand prior to
fracture. A material may have both a tensile and a compressive
strength. The ultimate strength may be calculated from the maximum
load applied during a test divided by the original cross-sectional
area.
[0034] The term "elastic deformation" refers to deformation of an
object in which the applied stress is small enough so that the
object moves towards its original dimensions or essentially its
original dimensions once the stress is released. However, an
elastically deformed polymer material may be prevented from
returning to an undeformed state if the material is below the
T.sub.g of the polymer. Below T.sub.g, energy barriers may inhibit
or prevent molecular movement that allows deformation or bulk
relaxation.
[0035] "Elastic limit" refers to the maximum stress that a material
will withstand without permanent deformation. The "yield point" is
the stress at the elastic limit and the "ultimate strain" is the
strain at the elastic limit. The term "plastic deformation" refers
to permanent deformation that occurs in a material under stress
after elastic limits have been exceeded.
[0036] "Neutral axis" refers to a line or plane in a structural
member subjected to a stress at which the strain is zero. For
example, a beam in flexure due to stress (e.g., at a top face) has
tension on one side (e.g., the bottom face) and compression on the
other (e.g., the top face). The neutral axis lies between the two
sides at a location or locations of zero strain. The neutral axis
may correspond to a surface. For a linear, symmetric, homogeneous
beam, the neutral axis is at the geometric centroid (center of
mass) of the beam. However, the neutral axis for a curved beam does
not coincide with the centroidal axis.
[0037] The strain increases in either direction away from the
neutral axis. The length and strain of material elements parallel
to the centroidal axis depend on both the distance of a material
element from that axis and the radius of curvature of the curved
beam.
[0038] In general, polymers can be biostable, bioabsorbable,
biodegradable, or bioerodable. Biostable refers to polymers that
are not biodegradable. The terms biodegradable, bioabsorbable, and
bioerodable, as well as degraded, eroded, and absorbed, are used
interchangeably and refer to polymers that are capable of being
completely eroded or absorbed when exposed to bodily fluids such as
blood and can be gradually resorbed, absorbed and/or eliminated by
the body.
[0039] A stent made from a biodegradable polymer 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. After the process of degradation,
erosion, absorption, and/or resorption has been completed, no
portion of the biodegradable stent, or a biodegradable portion of
the stent will remain. In some embodiments, very negligible traces
or residue may be left behind. The duration is typically in the
range of six to eighteen months.
[0040] In addition, a medicated stent may be fabricated by coating
the surface of a stent with a polymeric carrier. The coating may
include a bioactive agent. A "bioactive agent" is a moiety that is
mixed, blended, bonded or linked to a polymer coating, or to a
polymer from which a stent is made, and provides a therapeutic
effect, a prophylactic effect, both a therapeutic and a
prophylactic effect, or other biologically active effect upon
release from the stent.
[0041] Representative examples of polymers that may be used to
fabricate embodiments of implantable medical devices disclosed
herein include, but are not limited to, poly(N-acetylglucosamine)
(Chitin), Chitosan, poly(3-hydroxyvalerate),
poly(lactide-co-glycolide), poly(3-hydroxybutyrate),
poly(4-hydroxybutyrate),
poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyorthoester,
polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lactic
acid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide),
poly(L-lactide-co-D,L-lactide), poly(caprolactone),
poly(L-lactide-co-caprolactone), poly(D,L-lactide-co-caprolactone),
poly(glycolide-co-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), 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. Additional
representative examples of polymers that may be especially well
suited for use in fabricating embodiments of implantable medical
devices disclosed herein 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-hexafluoropropene) (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, poly(vinyl
acetate), styrene-isobutylene-styrene triblock copolymers, and
polyethylene glycol.
[0042] The term "implantable medical device" is intended to
include, but is not limited to, balloon-expandable stents,
self-expandable stents, stent-grafts, and vascular grafts. In
general, implantable medical devices, such as stents, can have
virtually any structural pattern that is compatible with a bodily
lumen in which it is implanted. Typically, a stent is composed of a
pattern or network of circumferential and longitudinally extending
interconnecting structural elements or struts. In general, the
struts are arranged in patterns, which are designed to contact the
lumen walls of a vessel and to maintain vascular patency. A myriad
of strut patterns are known in the art for achieving particular
design goals. A few of the more important design characteristics of
stents are radial or hoop strength, expansion ratio or coverage
area, and longitudinal flexibility.
[0043] Polymer tubes used for fabricating stents may be formed by
various methods. These include, but are not limited to extrusion or
injection molding. Conventionally extruded tubes tend to possess no
or substantially no radial orientation or, equivalently, polymer
chain alignment in the circumferential direction.
[0044] A tube used for fabricating a stent may be cylindrical or
substantially cylindrical in shape. In some embodiments, the
diameter of the polymer tube prior to fabrication of an implantable
medical device may be between about 0.2 mm and about 10 mm, or more
narrowly, between about 1 mm and about 3 mm.
[0045] In general, the scaffolding of conventional stents is
designed so that the stent can be radially compressed (to allowed
crimping) and radially expanded (to allow deployment). A
conventional stent expands and contracts through movement of
individual structural elements of a pattern with respect to each
other.
[0046] Medical devices are typically subjected to stress during
use, both before and during treatment. "Use" includes
manufacturing, assembling (e.g., crimping a stent on a balloon),
delivery of a stent through a bodily lumen to a treatment site, and
deployment of a stent at a treatment site. The scaffolding or
substrate experiences stress that results in deformation or strain
in the scaffolding. For example, during deployment, the scaffolding
of a stent can be exposed to stress caused by the radial expansion
of the stent body. In addition, the scaffolding may be exposed to
stress when it is mounted on a catheter from crimping or
compression of the stent. During delivery, a stent can be exposed
to stress from the flexure of the stent body as it passes through
tortuous pathways of a vessel. In addition, after deployment a
stent is exposed to stress arising from changes in shape of a
vessel induced by the beating of the heart.
[0047] These stresses can cause the scaffolding to crack or
fracture. Failure of the mechanical integrity of the stent while
the stent is localized in a patient can lead to serious risks for a
patient. For example, there is a risk of embolization caused by a
piece of the polymeric scaffolding breaking off from the stent.
[0048] As indicated above, selected regions of a stent may be
subjected to relatively high stress and strain when the device is
under an applied stress during use. Thus, such regions may have
different mechanical requirements than regions that experience
relatively low stress and strain.
[0049] Generally, the stress and the strain in a stent are not
uniformly distributed throughout its structure. Some regions may
experience no or substantially no stress and strain, while other
portions may experience relatively high stress and strain. As
indicated above, certain localized portions of the stent's
structure undergo substantial deformation. It is these regions that
are the most susceptible to fracture and cracking. The
susceptibility to cracking may be reduced by making such regions
more resistant to strain.
[0050] Unfortunately, many polymers used for stent scaffoldings,
are relatively brittle or inelastic at biological conditions. This
is particularly true for polymers with a T.sub.g above a body
temperature. In this case, the polymer in the stent never reaches
its T.sub.g, and therefore, the polymer remains relatively
inelastic while in the body. Polymers, in general, and many
polymers used in scaffoldings for devices tend to have a relatively
high degree of inelasticity, and, hence have relatively low
strength compared to a metal. Polymers can have an ultimate strain
as low as 5% of plastic strain. In general, the ultimate strain for
a polymer is highly dependent on material properties including
percent crystallinity, orientation of polymer chains, and molecular
weight. Therefore, polymer-based scaffoldings are highly
susceptible to fracture at regions of a medical device subjected to
relatively high stress and strain.
[0051] However, it is desirable for certain regions of a device to
be relatively stiff or inelastic (high modulus) and strong. Such
regions may experience relatively low stress and strain during use,
but act as support members that resist radial compressive forces
imposed on a deployed stent. Additionally, as discussed above,
longitudinal flexibility in a stent facilitates delivery of the
stent to a treatment location. Therefore, it is desirable to have a
stent that is resistant to cracking in localized regions of high
deformation, sufficiently rigid to maintain the structural
integrity of the sent, and longitudinally flexible during
delivery.
[0052] FIG. 1A illustrates a three-dimensional view of a
conventional stent 10 with a typical stent pattern. The stent may
have a pattern that includes a number of interconnecting elements
or struts 15. Variations of the structure of such patterns are
virtually unlimited. As shown in FIG. 1A, the geometry or shape of
stents vary throughout their structure. A pattern may include
portions of struts that are straight or relatively straight, an
example being a section 17. In addition, patterns may include
struts that include curved or bent portions as in a section 20.
Patterns may also include intersections of struts with curved or
bent portions as in sections 25 and 30. As shown in FIG. 1A, struts
15 of stent 10 include luminal faces 35, abluminal faces 40, and
side-wall faces 45.
[0053] FIG. 1B depicts a portion 54 of a strut depicting a
latitudinal axis 50 and a longitudinal axis 55 along a straight
section of portion 54. Portion 54 has an abluminal or luminal face
59 and a sidewall face 57. A longitudinal axis 51 on a curved
section of a strut may be defined as a tangent to a curvature at a
location on the curved section. A corresponding latitudinal axis 56
is perpendicular to longitudinal axis 51. In other embodiments, a
latitudinal cross-section may include any number of faces or be a
curved surface.
[0054] In some embodiments, a pattern of a stent such as that
pictured in FIG. 1A, may be formed from a tube by laser cutting the
pattern of struts in the tube. The stent may also be formed by
laser cutting a polymeric sheet, rolling the pattern into the shape
of the cylindrical stent, and providing a longitudinal weld to form
the stent. Other methods of forming stents are well known and
include chemically etching a polymeric sheet and rolling and then
welding it to form the stent. The stent may be formed by injection
molding of a thermoplastic or reaction injection molding of a
thermoset polymeric material.
[0055] As indicated above, crimping and expansion of a stent with a
typical pattern, such as in FIG. 1A, result in localized regions of
the pattern having high deformation or strain that are susceptible
to failure. The stent pattern depicted in FIG. 1A may be used to
illustrate such localized regions. Some portions of a stent pattern
may have no or relatively no strain, while others may have
relatively high strain. Straight or substantially straight sections
of struts such as section 17 of stent 10 in FIG. 1A experience no
or relatively no strain. These sections, however, do act as support
members that maintain the structural integrity of the stent. On the
other hand, sections 20, 25, and 30 may experience relatively high
strain when the stent is expanded or crimped.
[0056] For example, FIGS. 2A-C depict partial planar side views of
luminal or abluminal surfaces of a portion 60 from a stent in a
relaxed state (neither expanded or compressed) that includes
straight sections 65 and a curved section 70 with an angle 85. When
a stent undergoes radial expansion, portions of struts bend
resulting in an increase of angle 85 between straight sections 65,
as shown in FIG. 2B.
[0057] A bending plane of portion 60 corresponds to the plane
through which sections 65 sweep through as they bend. For example,
the bending plane may correspond to the plane of the sheet of
paper. Thus, the abluminal and luminal surfaces may be
substantially parallel to the bending plane of the bending portions
of structural elements such as portion 60.
[0058] FIGS. 2B-C depict portion 60 in a plane of bending. Radial
expansion of a stent causes substantially no strain in straight
sections 65. However, the bending of section 60 causes relatively
high stress and strain in curved section 70. As illustrated in FIG.
2B, when a stent is expanded a concave portion 75 of curved section
70 experiences relatively high tensile stress and strain and a
convex portion 80 of curved section 70 experiences relatively high
compressive stress and strain. As shown in FIG. 2C, when a stent is
crimped, angle 85 decreases and concave portion 75 experiences
relatively high compressive strain and convex portion 80
experiences relatively high tensile strain.
[0059] FIG. 3 depicts an expanded view of curved section 70 with a
neutral axis 90 indicated. As indicated above, the strain along the
neutral axis is zero. For a strut that is symmetric along its
longitudinal and latitudinal axis, the neutral axis may be a
surface perpendicular to a plane of bending (i.e., perpendicular to
the plane of the sheet of paper). Additionally, as in a curved
beam, a centroidal axis 95 of a curved section of a strut, such as
section 70, does not coincide with the neutral axis. Therefore, in
a small, narrow region of curved section 70 along neutral axis 90,
there is zero or relatively low strain.
[0060] Furthermore, the strain in a structural element or beam
increases with distance from the neutral axis. Therefore, the
strain in section 70 depends on distance from the neutral axis 90
and the change in curvature from an equilibrium or unstressed
curvature of section 70. For instance, a bending moment 100 that
tends to straighten section 70, as in FIG. 2B, results in strain
above and below the neutral axis. The magnitude of the strain as a
function of the distance from the neutral axis at a plane 105 is
illustrated by arrows 110 and 115. Above neutral axis 90, the strut
is in compression with a compressive strain that increases with
distance from neutral axis 90, as shown by arrows 110. Below
neutral axis 90 the strut is in tension with a tensile strain that
increases with distance from neutral axis 90, as shown by arrows
115. Similarly, a bending moment that tends to increase the bend in
section 70, as in FIG. 2C, will result in tension above the neutral
axis and compression below the neutral axis.
[0061] In general, the greater the change in curvature of a curved
portion of a strut due to an applied stress, the larger the
magnitude of the strain at a given distance away from the neutral
axis (See arrows 120 and 125 in FIG. 3). The maximum tensile and
compressive strain occur the edges of a structural element. In FIG.
3, the maximum tensile and compressive strain along plane 105 is at
120 and 125, respectively.
[0062] Furthermore, the magnitude of the strain in a bending
element or beam varies along the longitudinal axis of the element.
The maximum strain tends to occur at a center of symmetry of the
bending element. The strain tends to decrease along the
longitudinal axis with distance going away from the center of
symmetry of the bend. For example, if plane 105 is a center of
symmetry of bending, the strain decreases with distance away from
plane 105 along a longitudinal axis of section 70 at a fixed
distance from neutral axis 90.
[0063] FIG. 4 depicts a partial planar side view of a portion 150
from a stent in an unstressed state that includes curved section
155, straight sections 160 at an angle 165, and straight section
170. Radial expansion of the stent increases angle 165 and crimping
decreases angle 165. The stress and strain in straight sections 160
and 170 is relatively small. When a stent is expanded, section 155
has relatively high tensile stress and strain above a neutral axis
175 and relatively high compressive stress and strain below neutral
axis 175. Alternatively, when a stent is expanded, section 155 has
relatively high tensile stress and strain below neutral axis 175
and relatively high compressive stress and strain above neutral
axis 175.
[0064] FIG. 5 depicts a partial planar side view of a portion 200
from a stent in an unstressed state that includes a curved section
205, straight sections 225, 230, and 240. Straight sections 225 and
230 are at an angle 235 and straight sections 230 and 240 are at an
angle 250. Radial expansion of the stent increases angles 235 and
250 and crimping decreases angles 235 and 250. The stress and
strain in straight sections 225, 230, and 240 is relatively small.
When a stent is expanded, section 205 has relatively high tensile
stress and strain above a neutral axis 260 and relatively high
compressive stress and strain below neutral axis 260.
Alternatively, when a stent is compressed, section 205 has
relatively high tensile stress and strain below neutral axis 260
and relatively high compressive stress and strain above neutral
axis 260.
[0065] The description and analysis relating to stress and strain
distribution in a medical device is not limited to the structures
in FIGS. 2A-C, 3, and 4. Analysis of strain distribution in a
device, or generally any structure, subjected to applied stress may
be performed for a device or structure of virtually any
geometry
[0066] As indicated above, high strain regions of polymeric stents,
such as section 70 in FIGS. 2A-C, section 155 in FIG. 3, and
section 205 in FIG. 4, may be susceptible to cracking during
crimping or expansion of a stent. In general, the resistance to
strain of the high strain region may be increased by increasing the
flexibility of the region.
[0067] Certain embodiments of a structural element of a stent may
include a flexible section embedded within a high strain region of
the structural element. As noted above, a high strain region (e.g.,
section 70 in FIG. 2A) may have a higher strain as compared to
other regions of the structural element when the stent is subjected
to an applied stress during use.
[0068] In an embodiment, the flexible section may extend through
the high strain region from an abluminal surface of the high strain
region to a luminal surface of the high strain region. Generally,
the abluminal and luminal surfaces of a stent tend to be in or
substantially in a bending plane or surface of a structural element
of a stent.
[0069] The flexible section may have a greater flexibility than a
majority of the structural element outside of the flexible section.
In particular, the flexible section may be less brittle (or not
brittle at all) and may have a lower modulus than a majority of the
structural element outside of the flexible section. The flexible
section may also have a T.sub.g at or below body temperature. Thus,
a more flexible material in the flexible section may tend to be
more resistant to the high strain in the high strain region during
use of a stent.
[0070] As indicated above, applied stresses during use such as
crimping, delivery and deployment cause deformation or strain in
the structural elements of a stent. High strain regions may
correspond to regions of structural elements that are configured to
bend when stress is applied to a stent during use. Such stresses
may arise from crimping, delivery, deployment, and beating of the
heart. Examples of such high strain regions are sections 20, 25,
and 30 in FIG. 1A.
[0071] In some embodiments, the flexible section may be selectively
positioned to inhibit or prevent fracturing in the high strain
region when subjected to the applied stress during use. One
embodiment may include the flexible section selectively positioned
proximate a center of a width of an abluminal surface and a luminal
surface in the high strain region of the structural element. As
indicated above, (See FIG. 3) a neutral axis of a high strain
region of a stent may be at or proximate a center of a width of an
abluminal and luminal surface. Therefore, in some embodiments the
flexible section may be selectively positioned proximate a neutral
axis of the high strain region of the structural element. In order
to adequately increase the resistance to strain of the high strain
region, it is advantageous for the flexible section go all the way
through the high strain region from the abluminal to the luminal
surface. However, in some embodiments, the section may go partially
through the thickness of the high strain region, either from the
abluminal or the luminal surface.
[0072] A cross-section of the flexible section in a plane of the
abluminal/luminal surface may be a variety of shapes, e.g., long
and narrow; circular; oval; etc. In one embodiment, the flexible
section may be slot-shaped or long and narrow, and positioned along
at least a portion of a length of the high strain region. In some
embodiments, the cross section of the flexible section can vary
through the thickness of the stent.
[0073] FIG. 6 depicts a portion 300 of a structural element that
has a flexible section 305 in a high strain region 310 of portion
300. Region 310 has a neutral axis 315. Angle 320 is the angle
between an arm 325 and an arm 330 of portion 300.
[0074] The flexible section may separate at least a portion of the
high strain region into three sections: flexible section 305, rigid
section 335, and rigid section 340. The separation of a portion of
the high strain region into three sections may facilitate, allow,
and/or increase independent movement of arms 325 and 330. The
flexible section may also tend allow arms 325 and 330 to bend with
greater flexibility.
[0075] FIG. 7 depicts portion 300 of FIG. 6 when it is expanded.
Similarly, FIG. 8 depicts portion 300 when it is compressed. As
illustrated in FIGS. 7 and 8, it is expected that the area of
flexible section 305 changes as portion 300 bends, i.e., changes as
a function of angle 320 between arms 325 and 330. The change in the
area is due to the difference in stress-strain behavior of flexible
section 305 and sections of portion 300 surrounding flexible
section 305, i.e., sections 335 and 340, and the rest of the
portion 300. Flexible section 305 has a lower modulus and is less
brittle than sections 335 and 340 and the rest of portion 300.
[0076] In some embodiments, the width of the flexible section may
be relatively constant along the length of the high strain region.
Alternatively, it may be desirable for the width of the flexible
section to vary along the length of the high strain region. As
depicted in FIG. 6, the width of flexible section 305 when portion
300 is unstressed is widest at a plane of bending symmetry 317 and
decreases with distance from plane 317.
[0077] It may be advantageous for the width of the flexible section
along the length of the high strain region to be directly
proportional to a magnitude of the strain along the high strain
region when the structural element is under stress. Therefore, a
flexible section may be widest at or proximate to the center of a
bending portion of the high strain region and decrease in either
direction along a length of the high strain region.
[0078] In certain embodiments, a high strain region may have two or
more flexible sections. In one embodiment, the two or more flexible
sections may be positioned at or proximate a center of a width of
an abluminal surface and luminal surface of the structural element.
The two or more flexible sections may be proximate a neutral axis
of the high strain region of the structural element. Multiple
cavities may allow reduction in strain in the high strain region
with less reduction in strength of the structural element. The
flexible sections may be the of similar shape and depth or vary in
shape and/or depth.
[0079] As an illustration, FIG. 9 depicts a portion 400 of a
structural element that has flexible sections 415, 420, and 425 in
a high strain region 410 of portion 400 that has a neutral axis
430.
[0080] In certain embodiments, a method of forming a flexible
section in a structural element of a stent may include forming a
cavity within a high strain region of a structural element. The
cavity may extend through the high strain region from an abluminal
surface of the high strain region to a luminal surface of the high
strain region. For example, the cavity may be formed by laser
cutting the cavity in the high strain region. As an illustration,
FIG. 10 depicts a portion 300 of a structural element with a cavity
355. FIG. 10 shows an interior surface 345 of section 335 and
exterior surface 350 of section 340.
[0081] Additionally, the method may further include forming a
flexible section within all or substantially all of the cavity. The
flexible section may be formed by depositing a polymer or a
polymer-solvent mixture into the cavity. A polymer-solvent mixture
may be deposited, for example, by dipping, spraying, or controlled
deposition.
[0082] A controlled deposition system can be used that applies
various substances only to certain targeted portions of an
implantable medical device such as a stent. A representative
example of such a system, and a method of using the same, is
described in U.S. Pat. No. 6,395,326 to Castro et al. A controlled
deposition system can be capable of depositing a substance in or on
an implantable medical device having a complex geometry, and
otherwise apply the substance so that coating is limited to
particular portions of the device. The system can have a dispenser
and a holder that supports the medical substrate. 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.
[0083] The controlled deposition system can include a dispenser
assembly. The dispenser assembly can be a simple device including a
reservoir which holds a composition prior to delivery, and a nozzle
having an orifice through which the composition is delivered. One
exemplary type of a 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.
[0084] A polymer-solvent mixture can also be selectively deposited
by an electrostatic deposition process. Such a process can produce
an electrically charged or ionized coating substance. The electric
charge causes the coating substance to be differentially attracted
to the device, thereby resulting in higher transfer efficiency. The
electrically charged coating substance can be deposited into or
onto selected regions of the device by causing different regions of
the device to have different electrical potentials.
[0085] Furthermore, selective deposition of an implantable medical
device may be performed using photomasking techniques. Deposition
and removal of a mask can be used to selectively deposit substances
into or onto surfaces of substrates. Masking deposition is known to
one having ordinary skill in the art.
[0086] Furthermore, some embodiments may include enhancing adhesion
between the flexible section and adjacent sections of the
structural element to inhibit separation or delamination. One
embodiment of improving adhesion may include allowing at least a
portion of a flexible section to mix with at least a portion of
adjacent sections. For example, a polymeric material for the
flexible section may be selected to be miscible with the polymeric
material of adjacent sections within a particular temperature
range.
[0087] In an embodiment, the temperature during or after deposition
of polymer-solvent mixture into a cavity in a high strain region
may be selected so that mixing of polymers occurs at and near an
interface of the flexible section and adjacent sections. As a
result of the mixing of polymers in an interfacial region, there
may be a gradual transition in composition, and hence, in
mechanical and other properties normal to the interfacial region
between layers. A gradual transition in properties in an
interfacial region may inhibit or prevent separation or
delamination of the flexible section from a structural element.
EXAMPLE
[0088] Some embodiments of the present invention are illustrated by
the following examples. The examples are being given by way of
illustration only and not by way of limitation. The parameters are
not to be construed to unduly limit the scope of the embodiments of
the invention.
[0089] A structural element may be composed of a high modulus
polymer such as poly(L-lactide), poly(D,L-lactide), or
poly(.epsilon.-caprolactone), mixtures thereof, or copolymers
thereof. A flexible section may composed of flexible polymers such
as poly(D,L-lactide)-polyethylene glycol-poly(D,L-lactide) triblock
copolymer, 70/30 poly(L-lactide-co-trimethylene carbonate), or
polyanhydrides.
[0090] 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.
* * * * *