U.S. patent application number 10/342748 was filed with the patent office on 2004-05-20 for polymeric endoprosthesis and method of manufacture.
Invention is credited to DeSimone, Joseph M., Glenn, Richard A., Holbrook, Kevin D., Smith, Jeffrey A., Williams, Michael S..
Application Number | 20040098090 10/342748 |
Document ID | / |
Family ID | 32303864 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040098090 |
Kind Code |
A1 |
Williams, Michael S. ; et
al. |
May 20, 2004 |
Polymeric endoprosthesis and method of manufacture
Abstract
Improved polymeric endoprostheses and methods of making
endoprostheses are disclosed. Said endoprostheses exhibit improved
overall compliance, selective regional compliance, and selective
radial strength without varying the geometries of selected regions.
Numerous other physical characteristics of said endoprostheses may
be selectively varied during manufacture. Some embodiments may
comprise one or more erodible material. Some embodiments may
comprise one or more therapeutics incorporated into said
endoprosthesis via a solvent in a supercritical state.
Inventors: |
Williams, Michael S.; (Santa
Rosa, CA) ; Holbrook, Kevin D.; (Windsor, CA)
; Glenn, Richard A.; (Santa Rosa, CA) ; Smith,
Jeffrey A.; (Santa Rosa, CA) ; DeSimone, Joseph
M.; (Chanel Hill, NC) |
Correspondence
Address: |
Deanna J. Shirley
3418 Baldwin Way
Santa Rosa
CA
95403
US
|
Family ID: |
32303864 |
Appl. No.: |
10/342748 |
Filed: |
January 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60426898 |
Nov 15, 2002 |
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60426737 |
Nov 15, 2002 |
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60426734 |
Nov 15, 2002 |
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60426126 |
Nov 14, 2002 |
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60426125 |
Nov 14, 2002 |
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Current U.S.
Class: |
623/1.13 ;
623/1.16 |
Current CPC
Class: |
A61F 2220/005 20130101;
A61F 2250/0067 20130101; A61F 2250/0068 20130101; A61F 2002/91541
20130101; A61F 2/915 20130101; A61F 2220/0058 20130101; A61F
2250/0014 20130101; A61F 2/91 20130101; A61F 2002/91558
20130101 |
Class at
Publication: |
623/001.13 ;
623/001.16 |
International
Class: |
A61F 002/06 |
Claims
We claim:
1. An endoprosthesis comprising one or more erodible materials, a
first region and a second region, wherein said first region
comprises a first degree of overall compliance and said second
region comprises a second degree of overall compliance, wherein
said first degree of overall compliance is greater than said second
degree, whereby when said endoprosthesis is disposed within a body
lumen comprising walls comprising irregular morphology, said first
region is substantially compliant with said walls.
2. The endoprosthesis of claim 1 wherein said endoprosthesis
comprises a first end and a second end, and wherein said first
region is proximate said first end.
3. The endoprosthesis of claim 1, wherein said endoprosthesis
comprises an endoprosthesis element and a lumen defined
therethrough, wherein said endoprosthesis element comprises a cross
section in said first region and in said second region, and wherein
said cross section in said first region is substantially similar to
said cross section in said second region.
4. The endoprosthesis of claim 1 wherein said endoprosthesis
comprises a plurality of endoprosthesis elements and a lumen
defined therethrough, wherein said endoprosthesis elements comprise
linear dimensions in said first region and in said second region,
and wherein said linear dimensions of said first region are
substantially similar to said linear dimensions of said second
region.
5. An endoprosthesis comprising one or more erodible materials, a
first region and a second region, wherein said first region
comprises a first degree of radial conformability and said second
region comprises a second degree of radial conformability, wherein
said first degree of radial conformability is greater than said
second degree of radial conformability.
6. The endoprosthesis of claim 5 wherein said endoprosthesis
comprises a first end and a second end, and wherein said first
region is proximate said first end.
7. The endoprosthesis of claim 5 wherein said endoprosthesis
comprises an endoprosthesis element and a lumen defined
therethrough, wherein said endoprosthesis element comprises a cross
section in said first region and in said second region, and wherein
said cross section in said first region is substantially similar to
said cross section in said second region.
8. The endoprosthesis of claim 5 wherein said endoprosthesis
comprises a plurality of endoprosthesis elements and a lumen
defined therethrough, wherein said endoprosthesis elements comprise
linear dimensions in said first region and in said second region,
and wherein said linear dimensions of said first region are
substantially similar to said linear dimensions of said second
region.
9. The endoprosthesis of claim 5 wherein said endoprosthesis
further comprises one or more connecting members, and said first
region is proximate said one or more connecting members.
10. An endoprosthesis comprising one or more erodible materials, a
first region and a second region, wherein said first region
comprises a first outward radial force and said second region
comprises a second outward radial force, wherein said second
outward radial is greater than said first outward radial force.
11. The endoprosthesis of claim 10 wherein said endoprosthesis
comprises a first end and a second end, and wherein said first
region is proximate said first end.
12. The endoprosthesis of claim 10 wherein said endoprosthesis
comprises an endoprosthesis element and a lumen defined
therethrough, wherein said endoprosthesis element comprises a cross
section in said first region and in said second region, and wherein
said cross section in said first region is substantially similar to
said cross section in said second region.
13. The endoprosthesis of claim 10 wherein said endoprosthesis
comprises a plurality of endoprosthesis elements and a lumen
defined theretbrough, wherein said endoprosthesis elements comprise
linear dimensions in said first region and in said second region,
and wherein said linear dimensions of said first region are
substantially similar to said linear dimensions of said second
region.
14. The endoprosthesis of claim 10 wherein said endoprosthesis
further comprises one or more connecting members, and said first
region is proximate said one or more connecting members.
15. An endoprosthesis comprising one or more erodible materials, a
first region and a second region, wherein said first region
comprises a first axial flexibility and said second region
comprises a second axial flexibility, wherein said first axial
flexibility is greater than said second axial flexibility.
16. The endoprosthesis of claim 15 wherein said endoprosthesis
comprises a first end and a second end, and wherein said first
region is proximate said first end.
17. The endoprosthesis of claim 15 wherein said endoprosthesis
comprises an endoprosthesis member and a lumen defined
therethrough, wherein said endoprosthesis member comprises a cross
section in said first region and in said second region, and wherein
said cross section in said first region is substantially similar to
said cross section in said second region.
18. The endoprosthesis of claim 15 wherein said endoprosthesis
comprises a plurality of endoprosthesis elements and a lumen
therethrough, wherein said endoprosthesis elements comprise linear
dimensions in said first region and in said second region, and
wherein said linear dimensions of said first region are
substantially similar to said linear dimensions of said second
region.
19. An endoprosthesis comprising at least one erodible polymer, a
first region and a second region, wherein said at least one
erodible polymer comprises a first density in said first region and
a second density in said second region that is greater than said
first density.
20. The endoprosthesis of claim 19 wherein said endoprosthesis
comprises a first end and a second end, and wherein said first
region is proximate said first end.
21. The endoprosthesis of claim 19 wherein said endoprosthesis
further comprises one or more connecting members, and said first
region is proximate said one or more connecting members.
22. An endoprosthesis comprising a first region and a second
region, wherein said first region comprises a first diffusion
coefficient and said second region comprises a second diffusion
coefficient that is greater than said first diffusion
coefficient.
23. The endoprosthesis of claim 22 wherein said endoprosthesis
comprises a luminal surface and a vascular surface, and said first
region is disposed on said luminal surface, and said second region
is disposed on said vascular surface.
24. An endoprosthesis comprising one or more polymeric materials, a
first and a second region, wherein said one or more polymeric
materials comprise a first degree of crystallinity in said first
region and a second degree of crystallinity in said second region
that is greater than the first degree of crystallinity.
25. The endoprosthesis of claim 24 wherein said endoprosthesis
further comprises one or more connecting members, and said first
region is proximate said one or more connecting members.
26. An endoprosthesis comprising one or more erodible materials and
one or more endoprosthesis elements, wherein said one or more
endoprosthesis elements comprises a trapezoidal cross-section.
27. The endoprosthesis of claim 26 wherein said endoprosthesis
comprises a vascular surface area, and said surface area is 10% or
more greater than the surface area of an endoprosthesis comprising
equivalent non-trapezoidal endoprosthesis elements.
28. An expandable endoprosthesis comprising one or more
endoprosthesis elements, said endoprosthesis elements comprising a
plurality of apices alternating with a plurality of straight
sections, said apices comprising a first width and said straight
sections comprising a second width, wherein the second width is
greater than said first width.
29. The endoprosthesis of claim 28 wherein upon expansion, said
endoprosthesis elements bend preferentially at said apices.
30. The endoprosthesis of claim 29 wherein following expansion,
said apices comprise an angle of between 40 and 65 degrees.
31. The endoprosthesis of claim 30 wherein said endoprosthesis
comprises a first material, wherein said first material undergoes
strain induced crystallization upon expansion.
32. An expandable endoprosthesis comprising means for limiting
expansion of said endoprosthesis.
33. The endoprosthesis of claim 32 wherein said endoprosthesis
comprises an endoprosthesis element comprising a plurality of
apices, said apices comprising one or more stop portions, wherein
said stop portions are separate from one another prior to
expansion, and abut one another upon expansion of said
endoprosthesis.
34. The endoprosthesis of claim 32 comprising one or more stop
elements, wherein said one or more stop elements comprise a curved
configuration prior to expansion and a linear configuration
following expansion, wherein said linear configuration prevents the
further expansion of said endoprosthesis.
35. An endoprosthesis comprising one or more erodible materials,
wherein said endoprosthesis comprises one or more endoprosthesis
elements and one or more reinforcing elements.
36. The endoprosthesis of claim 35 wherein said reinforcing
elements comprise a biocompatibly corrosive metal.
37. The endoprosthesis of claim 35 wherein said reinforcing element
is encapsulated by said one or more endoprosthesis elements.
38. The endoprosthesis of claim 35 wherein said endoprosthesis
comprises a luminal surface and a vascular surface, and wherein
said reinforcing element is disposed on said luminal surface.
39. The endoprosthesis of claim 35 wherein said reinforcing element
encapsulates said one or more endoprosthesis elements.
40. An expandable endoprosthesis comprising poly-lactic acid and
polycaprolactone in a ratio of between 80:20 and 95:5.
41. The endoprosthesis of claim 40 wherein said endoprosthesis is
annealed at a temperature of between 50 and 200 degrees C. for a
duration of between one half and 24 hours.
42. The endoprosthesis of claim 41 wherein said endoprosthesis
undergoes strain induced crystallization upon expansion.
43. The endoprosthesis of claim 42 wherein said endoprosthesis
comprises an endoprosthesis element comprising a plurality of
apices alternating with a plurality of straight sections wherein
said endoprosthesis undergoes strain induced crystallization upon
expansion proximate the apices.
44. The endoprosthesis of claim 43 wherein said apices define an
included angle between straight sections comprise of between 40 and
90 degrees following expansion of the endoprosthesis.
45. An endoprosthesis comprising one or more polymeric materials
that undergo plastic deformation between 3 and 20 ksi.
46. The endoprosthesis according to claim 45 wherein said one or
more polymeric materials continues to strengthen following plastic
deformation until the point of material failure.
47. The endoprosthesis of claim 1 further comprising one or more
therapeutic substances incorporated into the endoprosthesis using a
solvent in a supercritical state.
48. The endoprosthesis of claim 47 wherein said endoprosthesis is
formed from one or more curable materials using a first set of
first set of parameters to achieve said first set of physical
properties and using a second set of parameters to achieve said
second set of physical properties.
49. A method of manufacture of an endoprosthesis comprising:
providing a mold; placing a first material into said mold; placing
a second material into said mold; apply heat and pressure to mold
to form film; removing said film from said mold; and forming a
cylinder from said film to define said endoprosthesis.
50. The method of claim 49 wherein said mold comprises a first
region and a second region, and wherein said first material is
placed in said first region and said second material is placed in
said second region.
51. The method of claim 50 wherein said first material comprises a
first set of properties and said second material comprises a second
set of properties.
52. The method of claim 51 wherein said properties comprise
material density, modulus of elasticity, rate of erosion,
extensibility, compressibility, mechanical strength, tensile
strength, degree of crystallinity, diffusion coefficient, and
permeability.
53. A method of manufacture of an endoprosthesis comprising the
steps of: coextruding a first material and a second material to
form a generally tubular structure; and selectively removing
portions of said first material from said tube.
54. The method of claim 53 comprising the additional step of
selectively removing portions of said second material.
55. The method of claim 54 wherein said first material comprises a
first set of properties and said second material comprises a second
set of properties.
56. The method of claim 55 wherein said properties comprise
material density, modulus of elasticity, rate of erosion,
extensibility, compressibility, mechanical strength, tensile
strength, degree of crystallinity, diffusion coefficient, and
permeability.
57. The method of claim 49 wherein the method further comprises the
step of immersing said endoprosthesis and a hydrophilic therapeutic
agent in water beneath a blanket of carbon dioxide in its
supercritical state, whereby said hydrophilic therapeutic agent is
incorporated into said endoprosthesis.
58. The method of claim 49 wherein the method further comprises the
step of immersing said endoprosthesis and a hydrophobic therapeutic
agent in carbon dioxide in its supercritical state, whereby said
hydrophobic therapeutic agent is incorporated into said
endoprosthesis.
59. The method of claim 53 wherein the method further comprises the
step of immersing said endoprosthesis and a hydrophilic therapeutic
agent in water beneath a blanket of carbon dioxide in its
supercritical state, whereby said hydrophilic therapeutic agent is
incorporated into said endoprosthesis.
60. The method of claim 53 wherein the method further comprises the
step of immersing said endoprosthesis and a hydrophobic therapeutic
agent in carbon dioxide in its supercritical state, whereby said
hydrophobic therapeutic agent is incorporated into said
endoprosthesis.
Description
RELATED APPLICATIONS
[0001] This application is related to Provisional U.S. Patent
Application Serial No. 60/426,898 entitled "Polymeric
Endoprostheses and Methods of Manufacture", to Williams, et al.,
Provisional U.S. Patent Application Serial No. 60/426,737 entitled
"Improved Endoprostheses and Methods of Manufacture", to Williams,
et al., Provisional U.S. Patent Application Serial No. 60/426,734,
entitled "Photocurable Endoprostheses and Methods of Manufacture",
to Williams et al., Provisional U.S. Patent Application Serial No.
60/426,126 entitled "Carbon Dioxide-Assisted Methods of Providing
Biocompatible Intraluminal Prostheses", to Williams, et al., and
Provisional U.S. Patent Application Serial No. 60/426,125 entitled
"Intraluminal Prostheses and Carbon Dioxide-Assisted Methods of
Impregnating Same with Pharmacological Agents" to Williams, et
al.". The above applications are commonly owned. All of the above
applications are hereby incorporated by reference, each in its
entirety.
FIELD OF THE INVENTION
[0002] The invention herein relates generally to medical devices
and the manufacture thereof, and to improved endoprostheses for use
in the treatment of strictures in lumens of the body. More
particularly, the invention is directed to polymeric endoprostheses
and addresses the shortcomings of the prior art, especially, but
not limited to, material limitations including radial strength and
elastic recoil.
BACKGROUND OF THE INVENTION
[0003] Ischemic heart disease is the major cause of death in
industrialized countries. Ischemic heart disease, which often
results in myocardial infarction, is a consequence of coronary
atherosclerosis. Atherosclerosis is a complex chronic inflammatory
disease and involves focal accumulation of lipids and inflammatory
cells, smooth muscle cell proliferation and migration, and the
synthesis of extracellular matrix. Nature 1993;362:801-809. These
complex cellular processes result in the formation of atheromatous
plaque, which consists of a lipid-rich core covered with a
collagen-rich fibrous cap, varying widely in thickness. Further,
plaque disruption is associated with varying degrees of internal
hemorrhage and luminal thrombosis because the lipid core and
exposed collagen are thrombogenic. J. Am Coll Cardiol.
1994;23:1562-1569 Acute coronary syndrome usually occurs as a
consequence of such disruption or ulceration of a so called
"vulnerable plaque". Arterioscler Thromb Vasc Biol. Volume 22, No.
6, June 2002, p. 1002.
[0004] In addition to coronary bypass surgery, a current treatment
strategy to alleviate vascular occlusion includes percutaneous
transluminal coronary angioplasty, expanding the internal lumen of
the coronary artery with a balloon. Roughly 800,000 angioplasty
procedures are performed in the U.S. each year (Arteriosclerosis,
Thrombosis, and Vascular Biology Volume 22, No. 6, June 2002, p.
884). However, 30% to 50% of angioplasty patients soon develop
significant restenosis, a narrowing of the artery through migration
and growth of smooth muscle cells.
[0005] In response to the significant restenosis rate following
angioplasty, percutaneously placed endoprostheses have been
extensively developed to support the vessel wall and to maintain
fluid flow through a diseased coronary artery. Such endoprostheses,
or stents, which have been traditionally fabricated using metal
alloys, include self-expanding or balloon-expanded devices that are
"tracked" through the vasculature and deployed proximate one or
more lesions. Stents considerably enhance the long-term benefits of
angioplasty, but 10% to 50% of patients receiving stents still
develop restenosis. (J Am Coll Cardiol. 2002; 39:183-193.
Consequently, a significant portion of the relevant patient
population undergoes continued monitoring and, in many cases,
additional treatment.
[0006] Continued improvements in stent technology aim at producing
easily tracked, easily visualized and readily deployed stents,
which exhibit the requisite radial strength without sacrificing a
small delivery profile and sufficient flexibility to traverse the
diseased human vasculature. Further, numerous therapies directed to
the cellular mechanisms of accumulation of inflammatory cells,
smooth muscle cell proliferation and migration show tremendous
promise for the successful long-term treatment of ischemic heart
disease. Consequently, advances in coupling delivery of such
therapies to the mechanical support of vascular endoprostheses,
delivered proximate the site of disease, offer great hope to the
numerous individuals suffering heart disease.
[0007] While advances in the understanding of ischemic heart
disease as a complex chronic inflammatory process take place,
traditional diagnostic techniques such as coronary angiography
yield to next generation imaging modalities. In fact, coronary
angiography may not be at all useful in identifying inflamed
atherosclerotic plaques that are prone to producing clinical
events. Imaging based upon temperature differences, for example,
are undergoing examination for use in detecting coronary disease.
Magnetic resonance imaging (MRI) is currently emerging as the state
of the art diagnostic for arterial imaging, enhancing the
detection, diagnosis and monitoring of the formation of vulnerable
plaques. Transluminal intervention guided by MRI is expected to
follow. However, metals produce distortion and artifacts in MR
images, rendering use of the traditionally metallic stents in
coronary, biliary, esophageal, ureteral, and other body lumens
incompatible with the use of MRI.
[0008] Consequently, an emerging clinical need for interventional
devices that are compatible with and complementary to new imaging
modalities is evident. Further, devices that exhibit improved
trackability to previously undetectable disease within remote
regions of the body, especially the coronary vasculature are
needed. And finally, devices that both exhibit improved mechanical
support and are readily compatible with adjunct therapies in order
to lower or eliminate the incidence of restenosis are needed.
SUMMARY OF THE INVENTION
[0009] An endoprosthesis is provided comprising one or more
erodible materials, a first region and a second region, wherein
said first region comprises a first degree of overall compliance
and said second region comprises a second degree of overall
compliance, wherein said first degree of overall compliance is
greater than said second degree, whereby when said endoprosthesis
is disposed within a body lumen comprising walls comprising
irregular morphology, said first region is substantially compliant
with said walls. In some embodiments, the greater compliance is
proximate one or both ends of the endoprosthesis. Alternatively,
the connecting members of an endoprosthesis may be more compliant
according to the invention. The improved compliance can be attained
without altering the cross section or geometry of the
endoprosthesis. Radial conformability, axial flexibility, linear
extensibility, outward radial force, density, crystallinity,
permeability and diffusion coefficient can all be altered according
to the invention. In some embodiments according to the invention,
the endoprosthesis elements comprise a trapezoidal cross section,
narrowed apices, a metal reinforcing element, one or more
therapeutic agents. Some embodiments according to the invention
comprise an expandable endoprosthesis comprising poly-lactic acid
and polycaprolactone in a ratio of between 80:20 and 95:5. The
endoprosthesis may further be is annealed at a temperature of
between 50 and 200 degrees C. for a duration of between one half
and 24 hours, and may additionally undergo strain induced
crystallization upon expansion.
[0010] An endoprosthesis according to the invention may comprise
and endoprosthesis element comprising a plurality of apices
alternating with a plurality of straight sections wherein said
endoprosthesis undergoes strain induced crystallization upon
expansion proximate the apices. Methods of manufacturing
endoprostheses according to the invention are also disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a plan view of the distal end of a conventional
balloon catheter having a stent according to the invention mounted
thereon.
[0012] FIG. 2 shows the embodiment of FIG. 1 in its deployed
configuration.
[0013] FIGS. 3A-C illustrate a method of manufacture according to
the invention.
[0014] FIG. 4 is a plan view of a component used in a method
according to the invention.
[0015] FIGS. 5A-C illustrate an alternative method according to the
invention.
[0016] FIG. 6 depicts an alternative embodiment according to the
invention.
[0017] FIG. 7 illustrates yet another embodiment according to the
invention.
[0018] FIG. 8 illustrates an additional embodiment according to the
invention.
[0019] FIG. 9 is a plan view of an embodiment according to the
invention.
[0020] FIG. 10A is an end view of a cross section of an embodiment
according to the invention.
[0021] FIG. 10B is an end view of a cross section of an
endoprosthesis of the prior art.
[0022] FIG. 11 is a plan view of an alternative embodiment
according to the invention.
[0023] FIG. 12 is a plan view of an alternative embodiment
according to the invention.
[0024] FIG. 13A is a plan view of another alternative embodiment
according to the invention. FIG. 13B is a plan view of a portion of
the element of FIG. 13A illustrating the reconfiguration of the
element when in its deployed configuration.
[0025] FIG. 14A is a plan view of yet another alternative
embodiment according to the invention. FIG. 14B is a plan view of a
portion of the element of FIG. 14A illustrating the reconfiguration
of the element when in its deployed configuration.
[0026] FIG. 15 is an end view of a cross section of yet another
embodiment according to the invention.
[0027] FIG. 16 is an end view of a cross section of yet another
embodiment according to the invention.
[0028] FIG. 17 is an end view of a cross section of yet another
embodiment according to the invention.
[0029] FIG. 18 is a graph illustrating the modulus of elasticity of
prior art materials and materials according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] Although the invention herein is not limited as such, some
embodiments of the invention comprise materials that are
bioerodible. "Erodible" refers to the ability of a material to
maintain its structural integrity for a desired period of time, and
thereafter gradually undergo any of numerous processes whereby the
material substantially loses tensile strength and mass. Examples of
such processes comprise hydrolysis, enzymatic and non-enzymatic
degradation, oxidation, enzymatically-assisted oxidation, and
others, thus including bioresorption, dissolution, and mechanical
degradation upon interaction with a physiological environment into
components that the patient's tissue can absorb, metabolize,
respire, and/or excrete. Polymer chains are cleaved by hydrolysis
and are eliminated from the body through the Krebs cycle, primarily
as carbon dioxide and in urine. "Erodible" and "degradable" are
intended to be used interchangeably herein.
[0031] The term "endoprosthesis" refers to any prosthetic device
placed within a body lumen or duct to in order to therapeutically
treat the body lumen or duct, including but not limited to the
objective of restoring or enhancing flow of fluids through a body
lumen or duct.
[0032] A "self-expanding" endoprosthesis has the ability to revert
readily from a reduced profile configuration to a larger profile
configuration in the absence of a restraint upon the device that
maintains the device in the reduced profile configuration.
[0033] "Balloon expandable" refers to a device that comprises a
reduced profile configuration and an expanded profile
configuration, and undergoes a transition from the reduced
configuration to the expanded configuration via the outward radial
force of a balloon expanded by any suitable inflation medium.
[0034] The term "balloon assisted" refers to a self-expanding
device the final deployment of which is facilitated by an expanded
balloon.
[0035] The term "fiber" refers to any generally elongate member
fabricated from any suitable material, whether polymeric, metal or
metal alloy, natural or synthetic.
[0036] The phrase "points of intersection", when used in relation
to fiber(s), refers to any point at which a portion of a fiber or
two or more fibers cross, overlap, wrap, pass tangentially, pass
through one another, or come near to or in actual contact with one
another.
[0037] As used herein, a device is "implanted" if it is placed
within the body to remain for any length of time following the
conclusion of the procedure to place the device within the
body.
[0038] The term "diffusion coefficient" refers to the rate by which
a substance elutes, or is released either passively or actively
from a substrate.
[0039] As used herein, the term "braid" refers to any braid or mesh
or similar woven structure produced from between 1 and several
hundred longitudinal and/or transverse elongate elements woven,
braided, knitted, helically wound, or intertwined by any manner, at
angles between 0 and 180 degrees and usually between 45 and 105
degrees, depending upon the overall geometry and dimensions
desired.
[0040] Unless specified, suitable means of attachment may include
by thermal melt, chemical bond, adhesive, sintering, welding, or
any means known in the art.
[0041] "Shape memory" refers to the ability of a material to
undergo structural phase transformation such that the material may
define a first configuration under particular physical and/or
chemical conditions, and to revert to an alternate configuration
upon a change in those conditions. Shape memory materials may be
metal alloys including but not limited to nickel titanium, or may
be polymeric. A polymer is a shape memory polymer if the original
shape of the polymer is recovered by heating it above a shape
recovering temperature (defined as the transition temperature of a
soft segment) even if the original molded shape of the polymer is
destroyed mechanically at a lower temperature than the shape
recovering temperature, or if the memorized shape is recoverable by
application of another stimulus. Such other stimulus may include
but is not limited to pH, salinity, hydration, and others.
[0042] As used herein, the term "segment" refers to a block or
sequence of polymer forming part of the shape memory polymer. The
terms hard segment and soft segment are relative terms, relating to
the transition temperature of the segments. Generally speaking,
hard segments have a higher glass transition temperature than soft
segments, but there are exceptions. Natural polymer segments or
polymers include but are not limited to proteins such as casein,
gelatin, gluten, zein, modified zein, serum albumin, and collagen,
and polysaccharides such as alginate, chitin, celluloses, dextrans,
pullulane, and polyhyaluronic acid; poly(3-hydroxyalkanoate)s,
especially poly(.beta.-hydroxybutyrate), poly(3-hydroxyoctanoate)
and poly(3-hydroxyfatty acids).
[0043] Representative natural erodible polymer segments or polymers
include polysaccharides such as alginate, dextran, cellulose,
collagen, and chemical derivatives thereof (substitutions,
additions of chemical groups, for example, alkyl, alkylene,
hydroxylations, oxidations, and other modifications routinely made
by those skilled in the art), and proteins such as albumin, zein
and copolymers and blends thereof, alone or in combination with
synthetic polymers.
[0044] Suitable synthetic polymer blocks include polyphosphazenes,
poly(vinyl alcohols), polyamides, polyester amides, poly(amino
acid)s, synthetic poly(amino acids), polyanhydrides,
polycarbonates, polyacrylates, polyalkylenes, polyacrylamides,
polyalkylene glycols, polyalkylene oxides, polyalkylene
terephthalates, polyortho esters, polyvinyl ethers, polyvinyl
esters, polyvinyl halides, polyvinylpyrrolidone, polyesters,
polylactides, polyglycolides, polysiloxanes, polyurethanes and
copolymers thereof.
[0045] Examples of suitable polyacrylates include poly(methyl
methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate),
poly(isobutyl methacrylate), poly(hexyl methacrylate),
poly(isodecyl methacrylate), poly(lauryl methacrylate), poly(phenyl
methacrylate), poly(methyl acrylate), poly(isopropyl acrylate),
poly(isobutyl acrylate) and poly(octadecyl acrylate).
[0046] Synthetically modified natural polymers include cellulose
derivatives such as alkyl celluloses, hydroxyalkyl celluloses,
cellulose ethers, cellulose esters, nitrocelluloses, and chitosan.
Examples of suitable cellulose derivatives include methyl
cellulose, ethyl cellulose, hydroxypropyl cellulose, hydroxypropyl
methyl cellulose, hydroxybutyl methyl cellulose, cellulose acetate,
cellulose propionate, cellulose acetate butyrate, cellulose acetate
phthalate, arboxymethyl cellulose, cellulose triacetate and
cellulose sulfate sodium salt. These are collectively referred to
herein as "celluloses".
[0047] Examples of synthetic degradable polymer segments or
polymers include polyhydroxy acids, polylactides, polyglycolides
and copolymers thereof, poly(ethylene terephthalate),
poly(hydroxybutyric acid), poly(hydroxyvaleric acid),
poly[lactide-co-(epsilon-caprolactone)],
poly[glycolide-co-(epsilon-caprolactone)], polycarbonates,
poly-(epsilon caprolactone) poly(pseudo amino acids), poly(amino
acids), poly(hydroxyalkanoate)s, polyanhydrides, polyortho esters,
and blends and copolymers thereof.
[0048] The degree of crystallinity of the polymer or polymeric
block(s) is between 3 and 80%, more often between 3 and 65%. The
tensile modulus of the polymers below the transition temperature is
typically between 50 MPa and 2 GPa (gigapascals), whereas the
tensile modulus of the polymers above the transition temperature is
typically between 1 and 500 MPa.
[0049] The melting point and glass transition temperature of the
hard segment are generally at least 10 degrees C., and preferably
20 degrees C., higher than the transition temperature of the soft
segment. The transition temperature of the hard segment is
preferably between -60 and 270 degrees C., and more often between
30 and 150 degrees C. The ratio by weight of the hard segment to
soft segments is between about 5:95 and 95:5, and most often
between 20:80 and 80:20. The polymers contain at least one physical
crosslink (physical interaction of the hard segment) or contain
covalent crosslinks instead of a hard segment. Polymers can also be
interpenetrating networks or semi-interpenetrating networks.
[0050] Rapidly erodible polymers such as
poly(lactide-co-glycolide)s, polyanhydrides, and polyorthoesters,
which have carboxylic groups exposed on the external surface as the
smooth surface of the polymer erodes, also can be used. In
addition, polymers containing labile bonds, such as polyanhydrides
and polyesters, are well known for their hydrolytic reactivity.
Their hydrolytic degradation rates can generally be altered by
simple changes in the polymer backbone and their sequence
structure.
[0051] Examples of suitable hydrophilic polymers include but are
not limited to poly(ethylene oxide), polyvinyl pyrrolidone,
polyvinyl alcohol, poly(ethylene glycol), polyacrylamide
poly(hydroxy alkyl methacrylates), poly(hydroxy ethyl
methacrylate), hydrophilic polyurethanes, HYPAN, oriented HYPAN,
poly(hydroxy ethyl acrylate), hydroxy ethyl cellulose, hydroxy
propyl cellulose, methoxylated pectin gels, agar, starches,
modified starches, alginates, hydroxy ethyl carbohydrates and
mixtures and copolymers thereof.
[0052] Hydrogels can be formed from polyethylene glycol
polyethylene oxide, polyvinyl alcohol, polyvinyl pyrrolidone,
polyacrylates, poly (ethylene terephthalate), poly(vinyl acetate),
and copolymers and blends thereof. Several polymeric segments, for
example, acrylic acid, are elastomeric only when the polymer is
hydrated and hydrogels are formed. Other polymeric segments, for
example, methacrylic acid, are crystalline and capable of melting
even when the polymers are not hydrated. Either type of polymeric
block can be used, depending on the desired application and
conditions of use.
[0053] The use of polymeric materials in the fabrication of
endoprostheses confers the advantages of improved flexibility,
compliance and conformability, permitting treatment in body lumens
not accessible by more conventional endoprostheses. Such advantages
over a more conventional metal alloy are most readily apparent in
an endoprosthesis comprising longitudinal connecting members, for
example. Such connecting members, when fabricated from one or more
polymeric materials, allow compression of the connecting member
under compression loads, or, alternatively, stretching under
tension, while maintaining axial stability. In addition, more
connecting members at more points on the endoprosthesis can be
utilized, stabilizing the device without rendering the device
overly rigid.
[0054] Fabrication of an endoprosthesis according to the invention
allows for the use of different materials in different regions of
the prosthesis to achieve different physical properties as desired
for a selected region. A material selected for its ability to allow
elongation of longitudinal connecting members on the outer radius
of a curve in a lumen, and compression on the inner radius of a
curve in a vessel allows improved tracking of a device through a
diseased lumen. A distinct material may be selected for support
elements in order that the support elements exhibit sufficient
radial strength. Further, the use of polymeric materials readily
allows for the fabrication of endoprostheses comprising
transitional end portions with greater compliance than the
remainder of the prosthesis, thereby minimizing any compliance
mismatch between the endoprosthesis and diseased lumen. Further, a
polymeric material can uniformly be processed to fabricate a device
exhibiting better overall compliance with a pulsating vessel,
which, especially when diseased, typically has irregular and often
rigid morphology. Trauma to the vasculature, for example, is
thereby minimized, reducing the incidence of restenosis that
commonly results from vessel trauma.
[0055] An additional advantage of polymers includes the ability to
control and modify properties of the polymers through the use a
variety of techniques. According to the invention, optimal ratios
of combined polymers, and optimal processing have been found to
achieve highly desired properties not typically found in polymers.
Polymers such as poly-l-lactic acid and poly-caprolactone, combined
in ratios of between 80:20 and 95:5 respectively, form materials
exhibiting a desirable modulus of elasticity. Further, the
annealing process (comprising heating of the materials according
chosen parameters including time and temperature) increases polymer
chain crystallization, thereby increasing the strength of the
material. Consequently, according to the invention, the desired
material properties can be achieved by using the appropriate ratio
of materials and by annealing the materials.
[0056] Additionally, the properties of polymers can be enhanced and
differentiated by controlling the degree to which the material
crystallizes through strain-induced crystallization. Means for
imparting strain-induced crystallization arc enhanced during
deployment of an endoprosthesis according to the invention. Upon
expansion of an endoprosthesis according to the invention, focal
regions of plastic deformation undergo strain-induced
crystallization, further enhancing the desired mechanical
properties of the device, such as further increasing radial
strength. The strength is optimized when the endoprosthesis is
induced to bend preferentially at desired points, and the included
angle of the endoprosthesis member is between 40 and 70
degrees.
[0057] Curable materials employed in the fabrication of some of the
embodiments herein include any material capable of being able to
transform from a fluent or soft material to a harder material, by
cross-linking, polymerization, or other suitable process. Materials
may be cured over time, thermally, chemically, or by exposure to
radiation. For those materials that are cured by exposure to
radiation, many types of radiation may be used, depending upon the
material. Wavelengths in the spectral range of about 100-1300 nm
may be used. The material should absorb fight within a wavelength
range that is not readily absorbed by tissue, blood elements,
physiological fluids, or water. Ultraviolet radiation having a
wavelength ranging from about 100-400 nm may be used, as well as
visible, infrared and thermal radiation. The following materials
are examples of curable materials: urethanes, polyurethane oligomer
mixtures, acrylate monomers, aliphatic urethane acrylate oligomers,
acrylamides, UV polyanhydrides, UV curable epoxies, and other UV
curable monomers. Alternatively, the curable material can be a
material capable of being chemically cured, such as silicone based
compounds which undergo room temperature vulcanization.
[0058] Some embodiments according to the invention comprise
materials that are cured in a desired pattern. Such materials may
be cured by any of the foregoing means. Further, for those
materials that are photocurable, such a pattern may be created by
coating the material in a negative image of the desired pattern
with a masking material using standard photoresist technology.
Absorption of both direct and incident radiation is thereby
prevented in the masked regions, curing the device in the desired
pattern. A variety of biocompatibly eroding coating materials may
be used, including but not limited to gold, magnesium, aluminum,
silver, copper, platinum, inconel, chrome, titanium indium, indium
tin oxide. Projection optical photolithography systems that utilize
the vacuum ultraviolet wavelengths of light below 240 nm provide
benefits in terms of achieving smaller feature dimensions. Such
systems that utilize ultraviolet wavelengths in the 193 nm region
or 157 nm wavelength region have the potential of improving
precision masking devices having smaller feature sizes.
[0059] An endoprosthesis comprising polymeric materials has the
additional advantage of compatibility with magnetic resonance
imaging, potentially a long term clinical benefit. Further, if the
more conventional diagnostic tools employing angiography continue
as the technique of choice for delivery and monitoring, radiopacity
can be readily conferred upon polymeric materials.
[0060] Though not limited thereto, some embodiments according to
the invention comprise one or more therapeutic substances that will
elute from the surface or the structure or prosthesis independently
or as the prosthesis erodes. The cross section of an endoprosthesis
member may be modified according to the invention in order to
maximize the surface area available for delivery of a therapeutic
from the vascular surface of the device. A trapezoidal geometry
will yield a 20% increase in surface area over a rectangular
geometry of the same cross-sectional area. In addition, the
diffusion coefficient and/or direction of diffusion of various
regions of an endoprosthesis, surface, may be varied according to
the desired diffusion coefficient of a particular surface.
Permeability of the luminal surface, for example, may be minimized,
and diffusion from the vascular surface maximized, for example, by
altering the degree of crystallinity of the respective
surfaces.
[0061] According to the invention, such surface treatment and/or
incorporation of therapeutic substances may be performed utilizing
one or more of numerous processes that utilize carbon dioxide
fluid, e.g., carbon dioxide in a liquid or supercritical state. A
supercritical fluid is a substance above its critical temperature
and critical pressure (or "critical point"). Compressing a gas
normally causes a phase separation and the appearance of a separate
liquid phase. However, all gases have a critical temperature above
which the gas cannot be liquefied by increasing pressure, and a
critical pressure or pressure which is necessary to liquefy the gas
at the critical temperature. For example, carbon dioxide in its
supercritical state exists as a form of matter in which its liquid
and gaseous states are indistinguishable from one another. For
carbon dioxide, the critical temperature is about 31 degrees C. (88
degrees D) and the critical pressure is about 73 atmospheres or
about 1070 psi.
[0062] The term "supercritical carbon dioxide" as used herein
refers to carbon dioxide at a temperature greater than about 31
degrees C. and a pressure greater than about 1070 psi. Liquid
carbon dioxide may be obtained at temperatures of from about -15
degrees C. to about 55 degrees C. and pressures of from about 77
psi to about 335 psi. One or more solvents and blends thereof may
optionally be included in the carbon dioxide. Illustrative solvents
include, but are not limited to, tetrafluoroisopropanol,
chloroform, tetrahydrofuran, cyclohexane, and methylene chloride.
Such solvents are typically included in an amount, by weight, of up
to about 20%.
[0063] In general, carbon dioxide may be used to effectively lower
the glass transition temperature of a polymeric material to
facilitate the infusion of pharmacological agent(s) into the
polymeric material. Such agents include but are not limited to
hydrophobic agents, hydrophilic agents and agents in particulate
form. For example, following fabrication, an endoprosthesis and a
hydrophobic pharmacological agent may be immersed in supercritical
carbon dioxide. The supercritical carbon dioxide "plasticizes" the
polymeric material, that is, it allows the polymeric material to
soften at a lower temperature, and facilitates the infusion of the
pharmacological agent into the polymeric endoprosthesis or
polymeric coating of a stent at a temperature that is less likely
to alter and/or damage the pharmacological agent.
[0064] As an additional example, an endoprosthesis and a
hydrophilic pharmacological agent can be immersed in water with an
overlying carbon dioxide "blanket". The hydrophilic pharmacological
agent enters solution in the water, and the carbon dioxide
"plasticizes" the polymeric material, as described above, and
thereby facilitates the infusion of the pharmacological agent into
a polymeric endoprosthesis or a polymeric coating of an
endoprosthesis.
[0065] As yet another example, carbon dioxide may be used to
"tackify", or render more fluent and adherent a polymeric
endoprosthesis or a polymeric coating on an endoprosthesis to
facilitate the application of a pharmacological agent thereto in a
dry, micronized form. A membrane-forming polymer, selected for its
ability to allow the diffusion of the pharmacological agent
therethrough, may then applied in a layer over the endoprosthesis.
Following curing by suitable means, a membrane that permits
diffusion of the pharmacological agent over a predetermined time
period forms.
[0066] Objectives of therapeutics substances incorporated into
materials forming or coating an endoprosthesis according to the
invention include reducing the adhesion and aggregation of
platelets at the site of arterial injury, block the expression of
growth factors and their receptors; develop competitive antagonists
of growth factors, interfere with the receptor signaling in the
responsive cell, promote an inhibitor of smooth muscle
proliferation. Anitplatelets, anticoagulants, antineoplastics,
antifibrins, enzymes and enzyme inhibitors, antimitotics,
antimetabolites, anti-inflammatories, antithrombins,
antiproliferatives, antibiotics, and others may be suitable. More
specific examples of the foregoing examples are set forth in
related Provisional Patent Application Serial No. 60/426,125, and
are incorporated herein.
[0067] Details of the invention can be better understood from the
following descriptions of specific embodiments according to the
invention. As an example, in FIG. 1, distal end 3 of standard
delivery catheter 1 is shown, bearing endoprosthesis 10. Although
an endoprosthesis according to the invention may be self-expanding,
endoprosthesis 10 mounted on distal end 3 is balloon-expandable.
Accordingly, endoprosthesis 10 is deployed via delivery catheter 1,
which comprises balloon 5 at distal end 3. Endoprosthesis 10 may be
fabricated from one or more of the foregoing conventional or shape
memory materials, polymers, or other suitable materials selected
for molecular weight, chemical composition and other properties,
manufactured to achieve any desired geometries and processed to
achieve sterilization, desired geometries and in vivo lifetime.
Endoprosthesis 10 is "crimped" down upon balloon 5 into its
low-profile delivery configuration. Endoprosthesis 10 can then be
tracked to a lesion site within a lumen of the body where
endoprosthesis 10 can be deployed. In order to deploy
endoprosthesis 10, balloon 5 is inflated via inflation medium
through catheter 1. The outward radial force of expanding balloon 5
expands endoprosthesis 10 to its deployed configuration, and
permanently plastically deforms endoprosthesis 10 to exert an
outward radial force upon the diseased lumen.
[0068] FIG. 2 illustrates endoprosthesis 10. Accordingly,
endoprosthesis 10 may be between 0.5 mm and 10.0 mm at its deployed
diameter, depending upon the size of the lumen of the patient (not
pictured). Endoprosthesis 10 comprises support elements 12 and one
or more connecting elements 14.
[0069] The manufacture of an endoprosthesis according to the
invention can be better understood from a discussion of FIGS. 3A-C.
FIG. 3A represents an end view of mold 20. As a first step in
preparing an endoprosthesis according to the invention, a blend of
poly-1-lactide and poly-caprolactone in a ratio of between 80:20
and 95:5 is attained. Raw material is placed onto mold 20, heated
and pressurized to produce flat cast film 25. Flat cast film 25 is
removed from mold 20, as shown in FIG. 3B, and rolled to form
endoprosthesis 30, shown in a plan view in FIG. 3C. Endoprosthesis
30, which is balloon-expandable, comprises thin film portion 32 and
one or more ribs 34. Alternatively, thin film portion 32 can be
removed at all but portions left to connect ribs to one another.
Also, in an alternative embodiment, one or more therapeutic agents
can be added to polymer mixture such that the resulting
endoprosthesis elutes one or more therapeutic agents in situ.
[0070] An alternative embodiment according to the invention may be
described in relation to FIGS. 4A-C. FIG. 4A is a plan view
depicting mold 40, etched onto flat plate 42. Mold 40 comprises
relief for endoprosthesis elements 44, and connecting members 46.
As a first step in fabricating an endoprosthesis using mold 40,
polymers having desired properties are placed onto mold 40, heated
and pressurized to form flat cast film 48, shown in FIG. 4B. Flat
cast film 48 is removed from mold 40, trimmed of excess via laser
technology known in the art, including but not limited to excimer
laser at a wavelength between 150 nm and 250 nm, or carbon dioxide
laser, and rolled to form endoprosthesis 50, shown in FIG. 4C.
Although a self-expanding alternative is possible, endoprosthesis
50 is balloon expandable. An endoprosthesis according to the
invention may alternatively be fabricated using injection molding,
compression molding, or by laser cutting a tube, or chemically
etching a tube.
[0071] Yet another alternative embodiment according to the
invention is illustrated in FIGS. 5A-C. Mold 60 of FIG. 5A
comprises relief for endoprosthesis elements 62 and connecting
elements 64. In a first step, suitable "masking" material 65 is
placed over etchings for connecting elements 64 before a desired
selection of endoprosthesis materials, chosen to confer desired
physical properties upon the resulting endoprosthesis elements, are
placed onto mold 60, heated and pressurized, preventing the
formation of connecting elements during the first step. Following
the formation of endoprosthesis elements 62, masking material 65 is
removed, leaving endoprosthesis elements 62 covered in a first-thin
film 63, as shown in FIG. 5B. A second selection of desired
endoprosthesis materials, chosen to confer desired physical
properties to be conferred upon the resulting connecting elements,
is then placed onto mold 60, heated and pressurized, to form
composite flat film 68, shown in FIG. 5C. In the alternative, a
masking material may be placed over endoprosthesis elements 62.
Following forming, composite flat film 66 is removed from mold 60,
trimmed of excess and rolled to form composite endoprosthesis 68,
shown in FIG. 51).
[0072] Alternatively, other regions of the endoprosthesis, for
example, the end regions, may be formed selectively from yet a
third polymeric composition in order to confer desired physical
properties on the resulting end regions. The luminal surface of the
endoluminal prosthesis is another example of a region of an
endoprosthesis may be selectively formed from a particular
polymeric composition. Physical properties that can be controlled
according to the invention include but are not limited to density,
modulus of elasticity, degree of crystallinity, permeability and
diffusion coefficient.
[0073] Turning now to FIG. 6, another embodiment according to the
invention is provided. Endoprosthesis 70 comprises highly compliant
tubular member 72 enveloping a rigid thin fiber 74. One or more
plastically deformable bonds 76 is formed at the intersections of
rigid thin fibers 74. Endoprosthesis 70 may be self-expanding,
balloon assisted, or balloon expandable.
[0074] An additional embodiment is illustrated in FIG. 7.
Endoprosthesis 80 comprises a generally tubular member 82 that
further encapsulates cavity 84. Cavity 84 is filled with a suitable
curable material 86. Following deployment by balloon expansion,
curable material 86 cures to impart rigidity to endoprosthesis
80.
[0075] FIG. 8 illustrates an end view of alternative embodiment of
the invention comprising layer 110 into which .alpha.-hydrophilic
therapeutic agent has been incorporated. Following fabrication of
endoprosthesis 115 according to any of the methods described herein
from any of suitable material, endoprosthesis 115 is immersed in a
solution of polymer, water and hydrophilic therapeutic agent,
underlying a "blanket" of supercritical carbon dioxide. The carbon
dioxide renders the polymer more receptive to the incorporation of
therapeutic agent. The polymer comprising the therapeutic agent
forms layer 110 on the surface of endoprosthesis 115 for elution in
situ.
[0076] Turning now to FIG. 9, a portion of an element of an
endoprosthesis according to the invention is illustrated as a flat
section. Endoprosthesis elements 120 are generally serpentine, and
between 0.008 and 0.010 inches wide. Two opposed connecting members
125 are disposed between endoprosthesis elements and are spaced
spirally at 45 degrees. FIG. 10A represents an end view of a
cross-section taken along the longitudinal axis of endoprosthesis
126 according to the invention. Endoprosthesis elements 127
comprise trapezoidal cross-sections, oriented such that the
broadest side of the trapezoid is disposed at the outer diameter,
or vascular surface of endoprosthesis 126. Such a cross section
maximizes the vascular surface area of endoprosthesis 126 by over
20% as compared to an equivalent cross sectional area, while
allowing endoprosthesis 126 to be crimped down to a minimal profile
for tracking and delivery through the vasculature. Endoprosthesis
126 may be excimer laser cut from a cylinder, and endoprosthesis
elements 127 can accordingly be cut to exhibit a trapezoidal
cross-section. FIG. 10B illustrates an end view of a cross section
of a prior art endoprosthesis comprising elements 128 having
generally rectangular cross-sections.
[0077] In FIG. 11, endoprosthesis element 130 is generally
elliptical or ovular in shape. Connecting members 135 adjoin each
adjacent endoprosthesis element 130 generally at the midsections
131 and ends 132 of endoprosthesis elements 130. Endoprosthesis
elements 130 may be fabricated from a first material exhibiting a
high modulus of elasticity- and strength, while connecting members
135 may be fabricated from a second, more flexible material, such
as an elastomer.
[0078] FIG. 12 depicts a portion of an element to be used in the
fabrication of an alternative embodiment according to the invention
in a partially expanded or deployed configuration. Endoprosthesis
members 140 comprise a thinner cross-section at the inner apex 145
to allow for preferential bending at inner apex 145 upon expansion.
Such preferential bending enhances uniform deployment of an
endoprosthesis. Included angle 146 is between 40 and 65 degrees.
Upon expansion, strain induced crystallization is induced in the
polymer at the bending site, increasing the degree of
crystallization, and consequently the strength of the material, at
the bending site.
[0079] FIG. 13A illustrates a portion of an alternative embodiment
according to the invention wherein generally serpentine
endoprosthesis elements 150 comprise deployment stops 151 at one or
more apex 152. As illustrated in FIG. 13B, once expansion of the
endoprosthesis reaches a certain point, the edges of deployment
stops 151 touch one another and prevent further expansion of that
element and force expansion of the next element, thus ensuring
uniform expansion.
[0080] FIG. 14A illustrates yet another embodiment according to the
invention prior to expansion. FIG. 14B illustrates a portion of the
embodiment of FIG. 14A after expansion. Endoprosthesis elements 155
comprise deployment stops 156 inside each crown element 157. Upon
reaching a linear shape as shown in FIG. 14B, deployment stops 156
prevent further expansion of that element and force expansion of
the next element, thus ensuring uniform expansion.
[0081] An alternative embodiment according to the invention is
illustrated in a cross section of an endoprosthesis 160 shown in
FIG. 15. Endoprosthesis elements 165, of a trapezoidal shape,
comprise metal reinforcement elements 166. Metal reinforcement
element 166 may be fabricated from any suitable biocompatibly
corrosive metal, such as, for example, Magnesium. This composite
can greatly enhance the mechanical performance of the device.
[0082] FIG. 16 depicts a cross section of endoprosthesis 170.
Endoprosthesis elements 175 comprise metal reinforcement layer 176
disposed on luminal surface 177 of endoprosthesis 170. Similar to
the metal reinforcement elements 166 depicted in FIG. 15, metal
reinforcement layer 176 may comprise any suitable biocompatibly
corrosive metal. FIG. 17 illustrates a cross section of
endoprosthesis 180. Endoprosthesis elements 181 are encapsulated by
metal reinforcement layer 182, which may comprise any suitable
biocompatibly corrosive metal. This encapsulation may be
spray-coated, dipped, electrostatically coated, ion beam deposited
or coated by any means known by those skilled in the art.
[0083] Turning now to FIG. 18, the stress-strain curve exhibited by
materials according to the invention is curve A. The engineering
tensile stress strain curve was obtained by static loading of the
material, that is, by applying the load slowly enough that all
parts of the material are in equilibrium at any instant. For most
engineering materials, the curve will have an initial linear region
in which deformation is reversible and time independent. The slope
in this region is Young's modulus. The proportional elastic limit
is the point where the curve starts to deviate from a straight
line. The elastic limit is the point on the curve beyond which
plastic deformation is present after release of the load. If the
stress is increased further, the stress strain curve departs more
and more from the straight line. In FIG. 18, the curve for a
brittle material is indicated at B. A typical copolymer trend is
expressed in curve C, and for a low modulus material in curve D.
Curve A closely resembles the stress-strain curve of a stainless
steel alloy, radically surpassing the performance of know polymers
under stress.
[0084] According to the invention, a poly-l-lactide blend with
poly-caprolactone in a ratio of between 80:20 and 95:5 is
preferred. A material prepared comprising the foregoing ratio of
polymers consistently achieves the modulus of elasticity
illustrated as curve A in FIG. 18. The shape of this curve mirrors
that obtained by biometals such as 316L, stainless steel, a
material commonly used in vascular stents. Further, if the mixture
is annealed at roughly 100 degrees C. in an inert, moisture-free
environment for between 1 and 24 hours, and most desirably between
1 and 3 hours, polymer chain crystallization is enhanced, and
consequently the point at which plastic deformation occurs is
increased. Still further, upon deployment, strain induced
crystallization is initiated, further raising the point on the
curve at which plastic deformation occurs.
[0085] While particular forms of the invention have been
illustrated and described above, the foregoing descriptions are
intended as examples, and to one skilled in the art will it will be
apparent that various modifications can be made without departing
from the spirit and scope of the invention.
* * * * *