CA2519226A1 - Endoluminal stent having mid-strut interconnecting members - Google Patents

Abstract

An endoluminal stent (10) composed of a plurality of circumferential expansion elements (12) arrayed to form the circumference of the stent (10) and extending along the longitudinal axis of the stent (10), and a plurality of interconnecting members (14) that interconnect adjacent pairs of circumferential expansion elements (12), the interconnecting members (14) joining struts (16) of adjacent pairs of interconnecting members at approximate mid-points of the struts (16).

Description

Title Endoluminal Stent Having Mid-Strut Interconnecting Members Background of the Invention The present invention relates generally to endaluminal scents, covered stems and stmt-grafts designed fox delivery into an anatomical passageway using minimally invasive techniques, such as percutaneous intravascular delivery using a delivery catheter passed over a guidewixe. More particularly, the present invention relates to endoluminal scents having a scaffold structure and structural geometry which is 1o particularly well-suited for providing physiologically acceptable radial or hoop strength and longitudinal flexibility, while also presenting a luminal surface thereof which presents less obstruction to longitudinal shear forces during fluid flow across the luminal surface of the inventive device while maximizing fatigue life and corrosion xesistance. Additionally, the inventive endoluminal stem is characterized by 15 a geometry that uniquely has a negative coefficient of longitudinal foreshortening upon radial expansion. Thus, a unique aspect of the inventive endoluminal scent isr that it elongates rzpon radial expansion.
Endoluminal scents are generally tubular scaffolds fabricated from impiantable biocompatible materials. Stems have a generally tubular geometry characterized by a 2o central lumen, a longitudinal axis, a circumferential axis and a radial axis.
Conventional endoluminal stems fall within three general classifications:
balloon expandable, self expanding and shape-memory. galloon expandable scents requixe mechanical intervention, such as by using a balloon catheter, to apply a positive pxessuxe radialhy outward from, a central lumen of the scent to mechanically deform the 25 stmt and urge it to a larger diameter. Self expanding stems utilize inherent nnaterial mechanical properties of the scent matexzal to expand the scent. Typically, self expanding stems are fabricated of materials that rebound when a positive pressure is exerted against the material. Self expanding scents are fabricated such that their zero-stress configuration conforms to the second larger diameter. The self expanding 30 scents are drawn down to the first smaller diameter and constrained within a delivery catheter fox endoluminal delivery. Removal of the constraint reieases the constraining pressure and the sel~ expanding stmt, under its own mechanical properties, rebounds to the second larger diameter. Finally, shape-memory scents rely upon unique alloys -I-that exhibit shape memory under certain thermal conditions. Conventional shape-memory scents are typically nickel-titanium alloys known generically as nitinol, which have a transition phase at or near normal body temperature, i.e., 3'7 degrees Centigrade.
The prior art is replete with vaxious scent designs across all scent classifications. One of the difficulties with many conventional stmt designs arises due to the conflicting criteria between the desired properties of circumferential ox hoop strength of the stunt, longitudinal or column strength, longitudinal flexibility, fish-scaling of individual structural members of the stmt, fatigue life, corrosion to resistance, corrosion fatigue, hemodynarnics, radioopacity and biocompatibility and the capability of passing the stmt through an already implanted stent.
Typically, stents that are designed to optimize for hoop strength typically sacrifice either column strength and/or longitudinal flexibility, while scents that are designed to optimize for column strength often compromise longitudinal flexibility andlox hoop strength.
1s Most conventional stems exhibit longitudinal foreshortening upon radial expansion of the stmt. Longitudinal foreshortening is a well-known property that results from the geometric deformation of the scent's structural members as the stmt radially expands from a contracted state to a diametrically expanded state.
Several prior art stems have been invented that claim a lack of appreciable foreshortening of 2o the stem as a novel feature of the stent. Heretofore, howevex, a scent that longitudinally elongates upon radial expansion from a contracted state to a diametrically expanded state is unknown in the art.
Tt has been found desirable to devise an endoluminal stmt which employs a sexies of first and interconnecting members arrayed in geometrical patterns which 2s achieve a balance between hoop strength, column strength and longitudinal flexibility of the endoluminal stmt. Many conventional scents employ a series of circumferential structural elements and longitudinal structural elements of varying configurations. A
large number of conventional stems utilize circumferential structural elements configured into a serpentine configuration or a zig-zag configuration. The reason 30 underlying this configuration is the need for radial expansion of the stmt.
Of these conventional scents which employ serpentine or zig-zag circumferential structural elements, many also employ longitudinal structural elements which join adjacent circumferential structural elements and provide a modicum of longitudinal ar column strength while retaining longitudinal flexibility of the device. Additionally, many conventional stems require welds to join mating surfaces of the stem, Heretofore, however, the art has not devised a unibody stmt structural element geometry which achieves a balance between hoop strength, column strength and longitudinal flexibility, degree of longitudinal foreshortening, circumferential strength or hoop strength of the stmt, longitudinal strength or column strength, longitudinal flexibility, fish-scaling of individual structural membexs of the stmt, fatigue life, corrosion resistance, corrosion fatigue, hemodynamics, xadioopacity, biocompatibility and the capability of passing the stem through an already implanted stmt. The team 1o "fish-scaling" is used in the art and herein to describe a condition where some stent structural elements extend beyond the circumferential plane of the stmt during either radial expansion, implantation or while passing the stmt through a bend in the vasculature. Those of ordinary skill in the art understand that fish-scaling of stmt structural elements may cause the stent to impinge or snag upon the anatomical tissue either during endoluminal delivery or after implantation. The term "unibody"
as used herein is intended to mean a scent that is fabricated without the use of welds and as an integral body of material.
The inventive endoluminal stmt may be, but is not necessarily, fabricated by vapor deposition techniques. Vapor deposition fabrication of the inventive scents 2o offers many advantages, including, without limitation, the ability to fabricate stems of complex geometries, the ability to control fatigue life, corrosion resistance, corrosion fatigue, bulk and surface material properties, and the ability to vary the transverse profiles, Z-axis thickness and X-Y-axis surface area of the stmt's structural elements in manners that affect the longitudinal flexibility, hoop strength of the stunt and radial 2s expansion profiles.
Summary of the Invexition Endoluminal scent, covered stmt and scent-graft design inherently attempts to optimize the functional aspects of radial expandability, i. e., the ratio of delivery 3o diameter to expanded diameter, hoop strength, longitudinal flexibility, longitudinal foreshortening characteristics, column strength, fish-scaling of individual structural members of the scent, fatigue life, corrosion resistance, corrosion fatigue, heznodynamics, biocompatibility and the capability of stmt-through-scent delivery.
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Conventional stenfi designs have had to compromise one or more functional features of a stmt in order to maximize a particular functionality, e.g., Iongifudinal flexibility is minimized in order to achieve desirable column strength or high hoop strengths are achieved at the expense of small ratios of radial expandability. It is an objective of the s present invention to provide designs fox endoluminal unibody stems that achieve balances between the ratio of radial expandability, hoop strength, longitudinal flexibility and column strength, with biocompatibility, hemodynamics, xadioopacity, minimal or no fish-scaling and increased capacity for endothelialization.
In accordance with a preferred embodiment of the present invention, the 1o inventive endoluminal scent is formed of a single piece of biocompatible metal or pseudometal and having a plurality of circumferential expansion members co-axially aligned along a longitudinal axis of the stent and a plurality of interconnecting members interconnecting adjacent pairs of circumferential expansion members.
Each of the plurality of circumferential expansion members comprises a generally 15 sinusoidal ring structure having successive peaks and valleys interconnected by scent strut members. Each of the interconnecting members interconnects adjacent pairs of circumferential expansion members at approximate mid-points of stem strut members on the adjacent pairs of circumferential expansion members. In order to enhance longitudinal flexibility of the inventive scent, it has been found desirable to include 2o minor terminal regions of each interconnecting member that are narrower in width than a major intermediate region of the interconnecting member. The minor terminal regions are positioned at both the proximal and distal end of each interconnecting member and are narrower in width to enhance flexion at the junction region between the scent strut member and the intexcoxmecting member, Additionally, it has been 25 found desirable to form each of the minor terminal regions of the interconnecting members in the form of generally C-shaped sections extending proximally ox distally from the intermediate region of each interconnecting member.
In accordance with all embodiments of the present invention, each of the plurality of circumferential expansion members and the plurality of interconnecting 3o members may be fabricated of like biocampatible materials, preferably, biocompatible metals or metal alloys. In this manner, both the plurality of circumferential expansion elements and the plurality of interconnecting members have like physical material properties, e.g., tensile strength, modulus of elasticity, plastic deformability, spring _q._ bias, shape memory or super-elastic properties. Alfiernatively, the plurality of circumferential expansion members and interconnecting members may be fabricated of biocompatible materials, preferably, bioeompatible metals or rrzetal alloys which exhibit different physical or material properties. In this latter case, the plurality of cixcumferential expansion elements may, fox example, be fabricated of a plastically deformable material, such as stainless steel, while the plurality of interconnecting members are fabricated of a shape memory or super-elastic material, such as nickel-titanium alloys, or of a spring biased material, such as stainless steel.
Heretofore, joints between discrete sections of endoluminal stems required 1o welds in order to join sections of the stmt. One particular advantage ofthe present invention is that by forming the scent using vapor deposition techniques, not only axe discrete sections atomically joined without the use of welds, but different materials may be employed in different and discrete sections of the stmt in order to impart distinct material properties and, therefore, functionality, to the discrete sections.
15 Finally, the present invention also includes a self supporting endoluminal graft. As used herein the term "graft" is intended to indicate any type of tzzbular member that exhibits integral columnar and circumferential strength and ~rhich has openings that pass through the thickness of the tubular member. The invenfiive self supporting endoluxninal graft preferably consists of a member formed of at least one 20 of a plurality of layers, each Iayer being comprised of a plurality of first and interconnecting members which intersect one another, as described above, to define a plurality of open regions between intersecting pairs of the first and interconnecting members. A web region subtends at least a portion of the open region to at least partially enclose each of the plurality of open regions. Successive adjacent layers of 25 the plurality of layers are positioned such that the open regions axe staggered in the Z-axis transverse through the wall of the self supporting endoluminal graft. By staggering the open regions, interlamellax spaces are created to facilitate endothelialization of the endoluminal graft.

Brief Descripfiion of fihe Fi~nres Figure 1 is a perspective view of an endoluminal scent in its expanded diameter in accordance with the present invention.
Figure 2 is a plan view of a first embodiment of the inventive endoluminal scent.
Figure 3 is a plan view of a second embodiment of tire inventive endoluminal scent.
Figure 4 is a plan view of a third embodiment of the inventive endoluminal scent.
Figure 5 is a plan view of a fourth embodiment of the inventive endoluminal stmt.
Figure 6 is a photomicrograph of an interconnecting member arid portions of cixcumferential expansion members of the inventive endoluminal stmt.
Figure 7 is a photomicrograph depicting the inventive endoluminal scent in its constricted diameter for endoluminal delivery within a constraining sheath.
Figure $ is a photomicrograph depicting the inventive endoluminal scent partially released from a constraining sheath and radially expanding.
Figure 9 is photomicrograph depicting the inventive endoluminal scent in its radialIy enlarged diameter.
Detailed Descri~~tion of the Preferred Embodiments In accordance with the present invention there is provided several preferred embodiments. In each of the preferred embodiments of the present invention, the general configuration of the inventive endoluminal stmt is substantially the same.
Specifically and with particular reference to Figure l, the inventive endoluminal stunt 10 consists generally of a tubular cylindrical element comprised of a plurality of circumfexential expansion elements I2 generally forming closed zings about the 3o circumfexential axis C' of the scent 10 and arrayed in spaced apart relationship relative to one another coaxially along the longitudinal axis L' of scent 1(3. A
plurality of interconnecting members 14 interconnects adjacent pairs of the plurality of cixcumferential expansion elements 12. Each of the plurality of circumferential expansion elements I2 have a generally sinusoidal configuration with a plurality of peaks 12p and a plurality of troughs 12t of each circumferential e;~pansion member and a plurality of struts I 6 interconnecting adjacent peaks 12p and troughs 12t. The plurality of peaks 12p and the plurality of troughs 12t in one circumferential ring member i 2 may either be in phase or out of phase with the plurality of peaks 12p and troughs 12t in adjacent circumferential ring members I2. Additionally, within each circumferential ring member I2, the peaks 12p and troughs 12t may have either regular or irregular periodicity or each of the plurality of circumferential expansion elements may have regions of regular periodicity and regions of irregular periodicity.
1 D Each of the plurality of interconnecting members 14 preferably comprise generally linear elements having a width W; that interconnect a strut 16 of a first circumferential expansion element 12 with a strut 16 of a second, adjacent circumferential element 12. Each of the plurality of interconnecting members has a generally rectangular transverse cross-sectional shape. In accordance with each t5 prefezxed embodiment of the present invention, the interconnection between each of the plurality of interconnecting members 14 and the struts 16 occurs at an approximate mid-point along the length of the strut 16. Each of the plurality of struts I6 has a width WS and is generally rectangular in transverse cross-section.
Additionally, a plurality of terminal flange members 1 l, shown in phantom, 2o may be provided in order to provide affixation points fox mounting a graft covering (not shown) onto the scent 10. The terminal flange members 11 may be positioned at the distal end, the proximal end or both ends of the stmt I 0 and preferably are foz~xzed generally linear projections from either peak 12p or a trough 12t of a terminal cixcumferential expansion element 12 at either or both of the proximal ox distal ends 25 of the scent 10. Each of the plurality of flange members 11 may further include a rounded distal ox proximal end region to facilitate affixation of a graft covering.
With reference to Figures 2 and 6, to facilitate crimping the inventive scent to its first, smaller delivery diameter, it has been found preferable to provide at each peak 12p and trough 12t a generally U-shaped hinge element 22 that connects adjacent 3o struts along each circumferential expansion member 12. In accordance with the preferred embodiments of the invention, it is desirable that each generally U-shaped element hinge has a width Wh that is Iess than WS of the struts 16 to which it is connected. By making Wh Iess than WS, it has been found that a greater degree of compression of the angle a foamed between adjacent struts 16 interconnected by the generally U-shaped hinge element 22 may be achieved, thereby lending a greater degree of compressibility to the inventive stmt 10 than that found where the U-shaped hinge element 22 was not employed.
Additionally, it has been found desirable, in accordance with the best mode fox the present invention, to provide strain~relief sections 18 and 20 at opposing ends of each of the plurality of intercomecting members I4. The strain-relief sections 18 and 20 comprise terminal sections of the interconnecting member 14 and have a width Wt that is less than the width W; of the interconnecting member 14. In accordance with i0 one embodiment of the present invention, the strain-relief sections 18 and 20 each have a generally C-shaped configuration and traverse a radius in connecting the interconnection member 14 with the struts I6 of adjacent cixcumferential expansion members 12. Alternate geometric configurations of the C-shaped terminal strain-relief sections 18 and 20 are also contemplated by the present invention, such as S-shaped, V-shaped, M-shaped, W-shaped, U-shaped, ox merely generally z-shaped extensions projecting co-axially along the longitudinal axis of each interconnecting member 14.
Figures 2-5 depict alternative preferred embodiments of the scant I O of the present invention. Each of the preferred embodiments depicted in Figures 2-S
include 2o the same circumferential expansion elements 12, each having a plurality of peaks 12p and troughs I2t and formed of a plurality of struts 16 interconnected at the peaks 12p and troughs 12t, and the generally U-shaped elements 22 forming the peaks 12p and troughs 12t, with adjacent pairs of circun~fexential expansion elements 12 being interconnected by the plurality of interconnecting members 14. Thus, in each of 2s Figures 2-5, like elements are identified by like reference numerals. The alternafiive preferred embodiments of the inventive scent 30, 40, 50 and 60 illustrate in each of Figures 2, 3, 4 and 5, respectively, differ principally in the position and orientation of the plurality of interconnecting members 14. Tn Figures 2-5, each of the stems 30, 40, 50 and 60 are illustrated in planar views. Those skilled in the art will understand that 3o the planar view is depicted for ease of illustration and that the stems depicted are tubular with lines A-A and B-B forming division lines along the longitudinal axis L' of the scents in order to illustrate the stem geometry in a planar view, In Figure 2, scent 30 is comprised of a plurality of cixcumferential expansion members I2 and a plurality of interconnecting members 14. Each of the plurality of interconnecting members 14 joins adjacent pairs of circumferential expansion members 14. Each interconnecting member 14 forms a junction with a strut I6-of each of the adjacent circumferential expansion members 12 and intersects the strut 16 at approximately a mid-point along the length of each strut 16. The plurality of interconnecting members I4 form groupings 14a, 14b, 14c, 14d, 14e and i4f along the longitudinal axis L' of the scent 30. Because the interconnecting members 14 lie in the folding planes of the peaks I2p and troughs 12t and struts 16 about angle a, it has 1o been found desirable to offset each of the interconnecting members 14 from the a line parallel to the longitudinal axis L' of the scent 30 by an angle (i in order to enhance the folding properties of the cixcumferential expansion members I2 from a larger diameter to a smaller diameter of the stem 30. In scent 30, each of the plurality of interconnecting members 14 in groupings 14a-14f have the same offset angle (3 and all I5 of the plurality of interconnecting members I4 are parallel to each other.
In order to accommodate the offset angle ~3, and provide for folding of the interconnecting members I4 during compression of the scent 30 from its larger diameter to its smaller diameter, the strain relief sections I8 and 20 at terminal ends of each interconnecting member 14 have opposing orientations. Thus, when scent 30 is viewed in its tubular 20 configuration from a proximal end view P, first strain relief section I 8 has a generally C-shaped configuration that has a right-handed or clockwise orientation, while the second strain relief section 20, also having a generally C-shaped configuration has a generally left-handed or counterclockwise orientation.
In accordance with the preferred embodiment for stmt 30, it has been found 25 desirable to employ a 2: I ratio of peaks 12p or troughs 12t to interconnecting members. 'thus, as depicted, there are six peaks 12p and six peaks 12t in each of the plurality of circumferential expansion elements 12 and three interconnecting members 14 interconnect each pair of adjacent circumferential expansion elements 12, Similarly, between adjacent pairs of circumferential expansion elements I2, the 3o interconnecting members 14 are circumferentially offset one peak 12p and one trough 12t from the interconnecting members 14 in an adjacent pair of circumferential expansion elements 12. Thus, interconnecting elements in groups 14a, 14c and 14e interconnect circumferential expansion element pairs I2a-12b, I2c-12d, 12e-12f, 12g-_9_ 12h and I2i-12j, intexconnecting elements in groups I4b, I4d and 14f interconnect circumfexential expansion element pairs I2b-I2c, 12d-12f, 12f 12g, I2g-12h.
kith the interconnecting elements in group 14a, 14c and I4e each being offset by one peak 12p and one trough 12t along the circumferential axis of each cixcumferential expansion element 12.
Turning to Figure 3, stmt 40 is illustrated and has a substantially identical configuration of cixcumferential expansion elements 12 and interconnecting alexnents I4, except that instead of employing a 2:1 ratio of peaks 12p or trough's 12t to interconnecting elements, stem 40 employs a 3:1 ratio, such that each circumferential 1o expansion element 12a-12i has six peaks 12p and six troughs 12t, but adjacent pairs of circumferential elements 12 are interconnected by only two intercomiecting elements 14, Like scent 30, the interconnecting elements of a first circumferential expansion element pair are cixcumferentially offset from the intercomzecting elements of a second adjacent circumferential expansion element pair, except in stmt 40, the offset 15 is either one peak 12p and two troughs 12t or two peaks 12p and one trough 12t. Irz stent 40 there are four groups of interconnecting elements 14a, 14b, I4 c and 14d that interconnect the plurality of cixcumferenfiial expansion elements 12.
Interconnecting element groups 14a and 14c interconnects cixcmnferentiaI expansion element pairs 12b-I2c, 12d-12e, 12f 12g and 12h-I2i, and interconnecting element groups 14b and 20 I4d interconnect circumferential expansion element pairs I2a-12b, I2c-12d, 12e-12f and 12g-12h.
In stmt 40, each of the interconnecting elements I4 are also angularly offset from the longitudinal axis of the scent by an angle j3, except fihat the plurality of interconnecting elements 14 axe not all parallel relative to each other.
Rather, the 25 interconnecting elements in interconnecting element groups 14a and 14c are parallel to each other and the interconnecting elements in interconnecting elements groups 14b and 14d are parallel to each other, with the interconnecting elements in groups 14a and 14c being offset from the longitudinal axis of the scent by an angle (3-which is alternate to the angle (3, also denoted angle j3+, forming the offset from the 30 longitudinal axis L' for the interconnecting elements in groups 14b and 14d. The designation angle (3+ and angle (3- is intended to denote that these angles represent the substantially the same angular offset from the longitudinal axis L', but have alternate orientations relative to the circumferential axis of the stmt 40.

Turning now to Figure 4 in which stmt S0 is depicted. Like stents 30 and 40 described above, scent 50 shares the common elements of cixcumferential expansion elements 12, having a plurality of peaks 12p and troughs 12t interconnecting a plurality of struts I6, and U-shaped sections 22, and interconnecting elements I4. In scent 50, however, the plurality of interconnecting elements 14 form two groups of interconnecting elements I4a and interconnecting elements I4b. Each of the individual interconnecting elements 14 in interconnecting element groups 14a and 14b are also angularly offset fiom the longitudinal axis L' of the stmt 50 by angle (1.
Moreover, within each pair of adj acent cixcumferential expansion elements 12, the interconnecting element groups 34a and I4b are circumferentially offset from each other by three peaks 12p and three troughs 12t. Within each group of intercozmecting elements 14a and 14b, however, each of the plurality of individual interconnecting elements 14 axe generally aligned along a common longitudinal axis. In this manner, with the exception of the most proximal 12a and the most distal 12b eircumferential ring elements, each of the plurality of interconnecting elements form a substantially four-point junction 19 at approximately a rnid-point a strut 16 on each of circumferential expansion elements 12b-12h. The substantially four-point junction 19 is funned between a distal strain relief section 20 of one interconnecting member with a proximal side of a strut 16 and a proximal strain relief section I8 of an adjacent 2o interconnecting element I4 with a distal side of the same strut 16.
Finally, turning to Figure 5, there is illustrated scent 60 which, like stems 34, 40 and 50 is comprised of a plurality of circurnferential expansion elements 12 and interconnecting elements 14 that interconnect adjacent pairs of circumfexential expansion elements 12. Like scent 40 of Figure 3, scent 60 has groupings of interconnecting elements 14 into interconnecting element groups 14a, 14b, 14c and 14d. In stmt 60, however, intercozmecting element groups 14a and I4d interconnect identical pairs of circumferential expansion elements 12 and interconnecting element groups 14b and 14c interconnect identical pairs of circumferential expansion elements I2. Each of the interconnecting elements in interconnecting element groups 14a and 14d axe angularly offset from the longitudinal axis L' of the scent 60 by an angle ~-and are parallel to one and other. Similarly, each of the interconnecting elements in interconnecting element groups 14b and 14c are angularly offset from the longitudinal axis L' of the scent 60 by and angle ~3+ and are parallel to one and other.

For each adjacent pair of cixcuxnferential expansion elements 12, the interconnecting elements I4 have different orientations of angular offset from the longitudinal axis L' of the stmt 50. For example, for cixcumfexential expansion element pair I2a-12b, the interconnecting elements of group I4b and group I4c are offset by angle /3+ and by angle ~3-, respectively. lt~ the adjacent circumferential expansion element pair 12b-I2c, the interconnecting elements of group I4a and I4d are offset by angle ~3- and by angle ~i+, respectively. Thus, between adjacent pairs of circumferential elements I2, the interconnecting elements are out of phase, in that they have different angular orientations of angle Vii. Additionally, between adjacent 1o pairs of cixcumferential elements 12, the interconnecting elements are circumferentially offset by a single peak 12p, with interconnecting element group 14a being circumferentially offset from interconnecting element group by a single peak 12p, and interconnecting element group 14c being circumferentially offset from interconnecting element group 14d by a single peak 12p. Furthermore, there are different circurnferential offsets between interconnecting elexnerxt group pairs 14b-14c and 14a-14d within individual pairs of adjacent circumferential expansion elements 12, The circumfexential offset between interconnecting element group pair 14b-14c is two peaks 12p arid three troughs I2t, while the circurnferential offset between intercoxmecting element group pair I4a-14d is four peaks 12p and three troughs 12t.
2o Those skilled in the art will appreciate that the foregoing embodiment of stems I, 20, 30, 40 and 50 describe various geometries aII comprised of common structural elements, namely, circumferential expansion elements 12 having a plurality of peaks I2p and troughs 12t and struts 1 S interconnected by hinge elements 22.
Furthermore, those skilled in the art will understand that variations on the number of and positioning of the interconnecting members I4 between adjacent pairs of circumferential expansion elements 12 and along the circumferential axis of the stmt are also contemplated by the present invention and that the specific embodiments illustrated and described with reference to the figures is exemplary in nature.
Figure 3, however, represents a particularly preferred embodiment of the inventive stmt 40. Tnventive scent 40 was fabricated by laser-cutting the described geometry from a nickel-titanium hypotube. After laser cutting, the stem 40 was annealed to set shape memory properties for the stmt 40 with a fully expanded, enxarged outer diameter of 5.8 mm and a length of 30.6 mm. Scent 40 was capable of being crimped to a smaller, crimped outer diameter of 1.4 mm and was placed within a constraining sheath as illustrated in Figure 7. Scent 40 exhibited excellent crimpability with the struts I6 folding at the generally U-shaped hinge elements 22 through angle a without appreciable interference bett~~een the ezrcumferential expansion elements 12 and the interconnecting elements 14.
During radial expansion of the scent 40 from its first constrained smaller diameter, i.e., I.4 mm, to its second enlarged radially expanded diameter, z.e., 5.8 mm., the stmt 40 exhibited no foreshortening characteristic of many scent geometries known in the a t. in contrast to foreshortening the stem 40 unexpectedly elongated by

2.5%. Heretofore a stmt that elongates upon radial expansion is unknown in the art.
Figure 8 depicts stmt 40 radially expanding as it the constraining sheath is being withdrawn from the stmt 40. Figure 9 depicts stmt 40 in virtually its fully radially expanded enlarged diameter, with just a proximal section of the scent 40 be constrained in the constraining sheath (not pictured). Figure 6 is an enlarged section of the stmt 40 illustrating the mid-strut connection between the circumferential expansion element 12 and the interconnecting element 14 at the proximal and distal strain relief sections 18 and 20, and clearly showing the generally U-shaped hinge elements 22 a the peaks 12p and troughs 12t of each circumferential expansion element 12. Figure 6 also clearly depicts the differences in the widths Wt of the 2o proximal and distal stxain relief sections and the width W; of the body of the interconnecting member 14, as well as the difference between the width Wy, of the U-shaped hinge element 22 and the width WS of the strut 16.
The plurality of circumferential expansion elements 12 and interconnecting members 14, and components sections thereof, are preferably made of materials selected from the group consisting of titanium, vanadium, aluminum, nickel, tantalum, zirconium, chromium, silver, gold, silicon, magnesium, niobium, scandium, platinum, cobalt, palladium, manganese, molybdenum and alloys thereof, and nitinol and stainless steel. The plurality of circumferez~tial expansion elements I2 and the plurality of interconnecting members 14 may be made of the same material or of 3o different materials and have the same material properties or have different material properties. The term "material properties" is intended to encompass physical properties, including without limitation, elasticity, tensile strength, mechanical properties, hardness, bulk and/or surface grain size, grain composition, and grain boundary size, infra and inter-granular precipitates. Similarly, the materials selected for the plurality of circumferential expansion elements 12 and the plurality of interconnecting members 14 may be selected to have the same or different chemical properties. The term "chemical properties" is intended to encompass bath any chemical reaction and change of state that the material may undergo after being implanted into a body and the physiological response of the body to the material after implantation.
While the inventive stems may be fabricated by chemical, thermal ox mechanical ablative methods known in the art, such as chemical etching, laser cutting, hDM or water jet processes, it is envisioned that a preferred method fox fabricating the inventive stems is by physical vapor deposition techniques. Physical vapor deposition techniques affoxd the ability to tightly control both the tolerances of the stmt geometries as well as the physical and chemical pxopert7es of the stmt and the stmt materials. The inventive stems 10, 30, 40, SO and 60, including each of their elements, namely the plurality of circumferential expansion elements 12 and interconnecting members 14 and component sections thereof, are preferably made of a bulk material having controlled heterogeneities on the luminal surface thexeof. As is described in co-pending, commonly assigned, U.S. Patent Application Serial No.
091754,304 filed December 22, 2004, which is a divisional of U.S. Patent i~To.
6,379,383 issued April 30, 2002, which is hereby incorporated by reference, heterogeneities are controlled by fabricating the bulk material of the stmt to have defined grain sizes, chemical and infra- and intergranuiar precipitates and where the bulk and surface morphology differ, yielding areas or sites along the surface of the stmt while maintaining acceptable or optimal protein binding capability. The 2s characteristically desirable properties of the inventive stmt axe: (a) optimum mechanical properties consistent with or exceeding regulatory approval critexia, (b) minimization of defects, such as cracking ox pin hole defects, (c) a fatigue Life of 400 MM cycles as measured by simulated accelerated testing, (d) corrosion andlor corrosion-fatigue resistance, (e) biocompatibility without having biologically 3o significant impurities in the material, (f) a substantially non-frictional abluminal surface to facilitate atraumatic vasculax crossing and tracking and compatible with transcatheter techniques for scent introduction, (g) radiopaque at selected sites and MRr compatible, (h) have a luminal surface which is optimized for surface energy and microtopogxaphy, (i) minimal manufacturing and material cost consistent with achieving the desired material properfiies, and (j) high process yields.
W accordance with the present invention, the foregoing properties axe achieved by fabricating a stmt by the same metal deposition methodologies as are used and standard in the microelectronics and nano-fabrication vacuum coating arts, and which are hereby incorporated by reference. The preferred deposition methodologies include ion-beam assisted evaporative deposition and sputEering techniques. In ion beam-assisted evaporative deposition it is preferable to employ dual and simultaneous thermal electron beam evaporation with simultaneous ion bombardment of the 1 o substrate using an inert gas, such as argon, xenon, nitrogen or neon.
Bombardment with an inert gas, such as argon ions serves to reduce void content by increasing the atomic packing density in the deposited material during deposition. The reduced void content in the deposited material allows the mechanical properties of that deposited material to be similar to the bulk material properties. Deposition rates up to 15 nnalsec axe achievable using ion beam-assisted evaporative deposition techniques.
When sputtering techniques are employed, a 200-micron thick stainless steel elm may be deposited within about four hours of deposition time. With the sputtering technique, it is preferable to employ a cylindrical sputtering target, a single cixcumferential source that concentrically surrounds the substrate that is held in a 2o coaxial position within the source. Alternate deposition processes which may be employed to form the scent in accordance with the present invention axe cathodic arc, laser ablation, and direct ion beam deposition. When employing vacuum deposition methodologies, the crystalline structure of the deposited film affects the mechanical properties of the deposited film. These mechanical properties of the deposited film 25 may be modified by post-process treatmenfi, such as by, for example, annealing, high-pressure treatment or gas quenching.
Materials to make the inventive stems are chosen for their biocompatibility, mechanical properties, i.e., tensile strength, yield strength, and their ease of deposition include the following: elemental titanium, vanadium, aluminum, nickel, tantalum, 3o zirconium, chromium, silver, gold, silicon, magnesium, niobium, scandium, platinum, cobalt, palladium, manganese, molybdenum and alloys thereof, such as zirconium-titanium-tantalum alloys, nitinol, and stainless steel.
_15_ During deposition, the chamber pressure, the deposition pressure and the partial pressure of the process gases are controlled to optimize deposition of the desired species onto the substrate. As is known in the microelectronic fabrication, nano-fabrication and vacuum coating axis, both the reactive and non-reactive gases axe controlled and the inert or non-reactive gaseous species introduced into the deposition chamber are typically argon and nitrogen. The substrate may be either stationary or moveable, either rotated about its longitudinal axis, or moved in an X-Y plane within the reactor to facilitate deposition or patterning of the deposited material onto the substrate. The deposited material maybe deposited either as a uniform solid film onto l0 the substrate, or patterned by (a) imparting either a positive or negative pattern onto the substrate, such as by etching or photolithography techniques applied to the substrate surface to create a positive or negative image of the desired pattern or (b) using a mask or set of masks which axe either stationary or moveable relative to the substrate to define the pattern applied to the substrate. Patterning may be employed to 15 achieve complex finished geometries of the resultant stmt, both in the context of spatial orientation of the pattern as well as the material thickness at diffexent regions of the deposited film, such as by varying the wall thicl~~ess of the material aver its length to thicken sections at proximal and distal ends of the stent to prevent flaring of the scent ends upon radial expansion of the stmt.
24 The stem may be removed from the substrate aftex scent formation by any of a variety of methods. For example, the substrate may be removed by chemical means, such as etching or dissolution, by ablation, by machining or by ultrasonic energy.
Alternatively, a sacrificial layer of a material, such as carbon or aluminum, may be deposited intermediate the substrate and the stem and the sacrificial layer removed by 25 melting, chemical means, ablation, machining or other suitable means to free the stmt from the substrate.
The resulting stmt may then be subjected to post-deposition processing to modify the crystalline structure, such as by annealing, or to modify the surface topography, such as by etching to affect and control the heterogeneities on the blood 3o flow surface of the stmt.
A plurality of micragrooves may be imparted onto the Iuminal and/or abluminal surface of the stem 10, as is more fully described in International Publication No. WO 99/23977, published 20 May 1999, which is commonly assigned with the present application and is hereby incorporated by reference. The plurality of microgrooves may be formed either as a post-depasition process step, such as by etching, or during deposition, such as by depositing the scent-forming material onto a mandre2 which has a zr~icrotopography on the surface thereof which causes the metal to deposit with the microgroove pattern as part of the deposited material.
Each of the preferred embodiments of the present invention are preferably fabricated by employing a vapor deposition technique which entails vapor depositing a stmt-forming metal onto a substrate. The substrate may be planar or cylindrical and is either pre-patterned with one of the preferred geometries of first and interconnecting 1 o members, in either positive or negative image, or the substrate may be un-patterned.
Where the substrate is un-patterned, the deposited stmt-forming metal is subjected to post-deposition patterning to pattern the deposited stmt-forming metal into one of the preferred geometries of the first and interconnecting members. In all embodiments of the present invention fabricated by vapor deposition techniques, the need for post-deposition processing of tl~e patterned endoluminal stmt, e.g., modifying the surface of the scent by mechanical, electrical, thermal or chemical machining or polishing, is eliminated or minimized.
Vapor deposition fabrication of the inventive endoluminal stems offers many advantages, including, for example, the ability to fabricate stems of complex geometries, ultrafine dimensional tolerances on the order of Angstroms, the ability to control fatigue life, corrosion resistance, corrosion fatigue, inter- and infra-granular precipitates and their effect on corrosion resistance and corrosion fatigue, bulk material composition, bulk and surface material properties, radioopacity, and the ability to vary the transverse profiles, Z-axis thickness and X-Y-axis surface area of 2s the scent structural elements in manners that affect the longitudinal flexibility, hoop strength, and radial expansion behavior and profile of the scent. Bulk material composition may be adjusted to employ elemental fractions in alloy compositions that are not feasible when using conventionally formed metals. This results in achieving the ability to tailor the alloy compositions in a manner that optimizes the alloy composition for a desired material or mechanical property. For example, nickel-titanium tubes exhibiting shape memory and/or superelastic properties were made employing in excess of S 1.S atomic percent nickel, which is riot achievable using coxiventional working techniques due to high plateau stresses exhibited by the material. Specifically, the present inventors have fabricated nickel-titanium alloy tubes employing the method of the present invention that contain between 51.5 and 55 atomic percent nickel.
Vapor deposition of the inventive endoluminal stent, in accordance with a preferred embodiment of the present invention, significantly reduces or virtually eliminates inter- and infra-granular precipitates in the bulk material. Tt is common practice in the nickel-titanium endoluminal device industry to control transition temperatuxes and resulting mechanical properties by altering local granular nickel-titanium ratios by precipitation xegimens. Zn the present invention, the need to 1 o contxol precipitates for mechanical properties is eliminated. Where nickel-titanium is employed as the stmt-forming metal in the present invention, local nickel-titanium ratios will be the same or virtually identical to the nickel-titanium ratios in the bulk material, while still allowing for optimal morphology and eliminating the need for employing precipitation heat treatment. The resulting deposited stmt-forming metal 15 exhibits superior corrosion resistance, and hence, resistance to corrosion fatigue, when compared to conventional wrought nickel-titanium alloys.
The plurality of circumferential expansion elements 12 and the plurality of interconnecting members I4 may be conformationally configured during vapor deposition to impart a generally rectangular, ovular ox elliptical transverse cross-20 sectional profile with eithex right angled edges or with chamfered or curved leading and trailing luminal and abluminal surface edges in the longitudinal axis of the stmt in order to provide better blood flow surface profiles.
While the present inventions have been described with reference to their preferred embodiments, those of ordinary skill in the art will understand and 25 appreciate that a multitude of variations on the foregoing embodiments are possible and within the skill of one of ordinary skill in the vapor deposition and scent fabrication arts, and that the above-described embodiments are illustrative only and are not limiting the scope of the present invention which is limited only by the claims appended hereto.

Claims (9)

1. An endoluminal stent comprising:
a. a plurality of circumferential expansion elements co-axially spaced to form a generally tubular configuration and each having a generally undulating pattern of peaks and valleys interconnected by struts; and b. a plurality of generally linear interconnecting elements interconnecting adjacent pairs of circumferential expansion elements and joined at approximate mid points of adjacent struts along a longitudinal axis of the endoluminal stent.

2. The endoluminal stent according to Claim 1, wherein each of the plurality of circumferential expansion elements further comprises a generally zig-zag configuration along a circumferential axis of the endoluminal stent wherein the struts form generally linear sections and are interconnected at the peaks and valleys by hinge elements having a width narrower than a width of the struts.

3. The endoluminal stent according to Claim 2, wherein the plurality of generally linear interconnecting elements further comprise generally curvilinear first and second terminal sections at opposing ends of each interconnecting element that join with the struts.

4. The endoluminal stent according to Claim 3, wherein each of the plurality of circumferential expansion elements are integral and monolithic with each of the plurality of interconnecting members.

5. The endoluminal stent according to Claim 4, wherein the generally curvilinear first and second terminal sections of the plurality of generally linear interconnecting elements further comprise generally C-shaped sections.

6. The endoluminal stent according to Claim 5, wherein the generally C-shaped sections have a width narrower than a width of the remainder of the interconnecting member.

7. The endoluminal stent according to Claim 1, wherein the plurality of generally linear interconnecting members are all parallel to each other.

8. The endoluminal stent according to Claim 1, wherein the plurality of generally linear interconnecting members are arrayed as at least two groups of interconnecting members along a longitudinal axis of the endoluminal stent, a first of the at least two groups having a different angular orientation relative to the longitudinal axis of the endoluminal stent than a second of the at least two groups.

9. The endoluminal stent according to Claim 1, wherein the endoluminal stent elongates along the longitudinal axis of the endoluminal stent as it expands from a smaller diameter to a larger diameter.