Endovascular stent

Abstract

The invention relates to an endovascular stent comprising a tubular expandable lattice structure comprising a plurality of negative Poisson' s ratio (NPR) cells in connection with at least one row of positive Poisson's ratio (PPR) cells provided along a length of the stent. Each NPR cell may have a shape of a re-entrant (RE) hexagon. The stent may be automatically bent after expansion. The invention also relates to an endovascular stent graft comprising the endovascular stent and a liner provided around the endovascular stent.

Description

ENDOVASCULAR STENTTechnical Field

[0001] The present disclosure relates to an endovascular stent.Background

[0002] Thoracic endovascular aneurysm repair (TEVAR) is a minimally invasive surgery which involves implanting a stent incorporating a liner (together known as a stent graft) within a diseased aorta site to protect the aneurysm from further damage. The liner serves as a conduit for blood flow while blocking off the aneurysm from the aorta. The stent structurally supports the graft to maintain an open lumen and anchors the stent graft in the blood vessel. The stent graft is inserted using a delivery catheter in a collapsed state. When the collapsed stent graft has reached the desired placement site, it is expanded to the correct diameter and the delivery catheter is then withdrawn, leaving the expanded stent graft in the blood vessel.

[0003] Existing stents exhibit foreshortening behaviour due to the positive Poisson’s ratio (PPR) of their constituent stent strut structures, such as those incorporating diamond- shaped repeating cells. Foreshortening refers to the decrease in length of the stent while its diameter is increased. In order to avoid foreshortening of the stent during expansion, stents made of metamaterials having auxetic structures have been developed. In this application, metamaterials refers to man-made structures that are designed expressly to manifest auxetic behaviour. Auxctic structures have a negative Poisson’s ratio (NPR) such that they expand laterally when stretched longitudinally, and vice versa. As can be seen in FIG. 1 (a), each cell 20 of an exemplary auxetic structure may comprise a re-entrant (RE) hexagon 20 that forms a basic NPR element or NPR cell 20. The RE hexagon 20 has two opposite parallel sides of length w that are separated by angled sides of length 1. The parallel sides of length w are spaced apart by a distance h. For ease of reference, the parallel directions may be referred to as the lateral direction of the RE cell 20 while the direction of the distance h may be referred to as the longitudinal direction of the RE cell 20. Two sides of length 1 together form a reflex internal angle a between the sides of length w, creating a concave vertex. In the RE cell 20, the reflex internal angles a are opposite each other. An acute internal angle 9 is formed between each pairof a side of length 1 and a side of length w, creating a convex vertex at each end of the parallel sides. In this application, the acute internal angle 9 is referred to as the re-entrant angle (RE angle) of the RE cell 20. By assembling identical RE cells 20 into a regular lattice having a close packed structure as shown in FIG. 1 (b), a metamaterial auxetic structure is thus constructed. This auxetic structure expands laterally when stretched longitudinally, as shown in FIG 1 (c), when compared to its undeformed state in FIG 1 (b). The RE angle 0 of an RE cell 20 determines the range of Poisson’s ratios the cell will assume as it deforms.

[0004] Although stents having auxetic lattice structures composed of NPR cells 20 are able to avoid foreshortening during expansion, the arch of the human thoracic aorta possesses a curvature which presents a challenge to the long-term viability of the stent graft. This is due to existing stent grafts being generally straight devices that rely on their inherent axial flexibility to conform with the aorta. Inability of the expanded stent graft to conform well with the curvature of the aorta after implantation is a contributory factor to problems such as the development of endoleaks, migration or eventual collapse of the stent graft.

[0005] Pure bending of a cylindrical tube is described by the following geometrical relation in equation (1 ) below:L = L0* ((r cosp) + R ) / R (1) where LQ is the length of the central cylindrical axis which remains constant under pure bending, L is the length after bending of each length element distributed around the circumference, is the angle of the length element from the vertical axis, and R is the radius of curvature of the bent cylindrical tube, as illustrated in FIG. 2A. FIG. 2B is an exemplary design curve showing differential extension or contraction of different length elements of a cylindrical tube required to attain a radius of curvature R of 49.71 mm, according to Equation 1. The degree of bending of the cylindrical tube increases with the magnitude of difference in extension between neighbouring length elements, which is equivalent to the slope of the curve. At p= 90 degrees, the curve crosses the x axis, meaning that length at the midsection of the stent remains the same after bending.

[0006] To improve bendability of stents having structures composed of NPR cells, by combining cells with differing initial acute internal angles 0 in the lattice of the same stent, a stent can be constructed that will exhibit differential rates of expansion and contraction alongits circumference, thus leading to bending of the stent as it is expanded. However, the bending range of such stents is still limited due to the inherent NPR nature of the NPR cells in these stents. This can result in undesirable kinking of stents composed of NPR cells as can be seen in FIG. 2C when further bending of the stent is required to properly conform with the curvature of the aorta.

[0007] It is therefore desirable to have a stent that is able to properly conform with the curvature of the aorta when the stent is expanded while also avoiding kinking and foreshortening of the stent during expansion.Summary

[0008] Through the deliberate design of stiffness variations by making use of different RE angles to obtain difference in stiffness of the NPR cells, the present endovascular stent has a small initial diameter and a large diameter expansion potential. This enables the stent to be fabricated close to its delivery catheter size, while retaining sufficient expansion capacity (more than 3X) to reach its final deployment dimension in the thoracic aorta or other blood vessels. The stent is also better able to match operational requirements in actual practice, unlike regular uniform metamaterial and prior art stents which rely on large changes in geometry to create bending and are also limited by large, manufactured diameters and limited expansion abilities.

[0009] According to a first aspect, there is provided an endovascular stent comprising: a tubular expandable lattice structure comprising a plurality of negative Poisson’s ratio (NPR) cells in connection with at least one row of positive Poisson’s ratio (PPR) cells provided along a length of the stent.

[0010] The plurality of NPR cells may be provided as rows of NPR cells along the length of the stent; wherein each NPR cell has a shape of a re-entrant (RE) hexagon having two parallel and opposite sides that are each parallel with the length of the stent, two reflex internal angles between the two parallel sides, and four acute internal angles; wherein adjacent NPR cells between adjacent rows of NPR cells are conjoined at one of their parallel sides; and wherein all the NPR cells along each row of NPR cells have collinear first parallel sides and collinear second parallel sides.

[0011] All the NPR cells along each row of NPR cells may be spaced apart from each other, wherein adjacent NPR cells along at least some of the rows of NPR cells arc connected by a connecting bar at the concave vertices formed at their reflex internal angles.

[0012] The rows of NPR cells having adjacent NPR cells connected by connecting bars may be alternated in the circumferential direction of the stent with rows of NPR cells having unconnected NPR cells along the length of the stent.

[0013] The at least one row of PPR cells may be connected to and provided between two rows of NPR cells having unconnected NPR cells along the length of the stent.

[0014] The acute internal angles of the NPR cells may vary between rows of NPR cells in a circumferential direction around the stent from a first circumferential location on the stent to a second circumferential location on the stent, wherein the first circumferential location is diametrically opposite the second circumferential location, and wherein the row of PPR cells is located at the first circumferential location.

[0015] The NPR cells in the row of NPR cells located at the second circumferential location may have the smallest acute internal angles among the plurality of NPR cells.

[0016] The NPR cells in a row of NPR cells located at a third circumferential location on the stent and in a row of NPR cells located at a fourth circumferential location on the stent may have the largest acute internal angles among the plurality of NPR cells, wherein the third circumferential location is located midway between the first circumferential location and the second circumferential location, and wherein the fourth circumferential location is located diametrically opposite the third circumferential location.

[0017] Each of the acute internal angles of all the NPR cells before expansion of the stent may be less than 10°

[0018] Each PPR cell may comprise two centrally provided convex vertices that are opposite each other along the length of the stent, wherein the convex vertices of all the PPR cells in the row of PPR cells are collinear along the length of the stent.

[0019] Adjacent PPR cells along the row of PPR cells may be directly connected at their convex vertices.

[0020] Each centrally provided convex vertex of each PPR cell may be provided between a first concave vertex and a second concave vertex such that each PPR cell has two mirrored W- shaped arms, wherein the ends of each W- shaped arm are each directly connected to one end of a parallel side of a circumferentially adjacent NPR cell.

[0021] Each PPR cell may further comprise two longitudinal convex vertices that are opposite each other in a circumferential direction of the stent so that each PPR cell has a diamond shape, and wherein the longitudinal vertices of each PPR cell are each directly connected to a midpoint of a parallel side of a circumferentially adjacent NPR cell.

[0022] The endovascular stent may be automatically bent after expansion.

[0023] Diameter of the stent may be expandable by a factor of at least three.

[0024] According to a second aspect, there is provided an endovascular stent graft comprising the endovascular stent according to the first aspect and a liner provided around the endovascular stent.Brief Description of the Drawings

[0025] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.FIG. 1(a) (prior art) is a schematic illustration of a re-entrant (RE) hexagon as a RE cell.FIG. 1(b) (prior art) is a schematic illustration of an auxetic structure comprising a lattice of RE cells of FIG. 1(a) in an initial state.FIG. 1(c) (prior art) is a schematic illustration of the auxetic structure of FIG. 1(b) in an expanded state.FIG. 2A (prior art) is a schematic illustration of the geometrical relations in pure bending of a cylindrical tube.FIG. 2B (prior art) is a design curve of the cylindrical tube of FIG. 2A showing relationship between extension of length elements of the cylindrical tube at different angles from the vertical.FIG. 2C (prior art) is an illustration of kinking when a stent composed of NPR cells is bent.FIG. 3 is a perspective view of an exemplary embodiment of an endovascular stent.FIG. 4 is a surface development of the stent of FIG. 3 (stretched vertically for clarity).FIG. 5 is an illustration of improved working range of Poisson’s ratio in a hybrid structure comprising NPR cells and PPR cells.FIG. 6 is an illustration of a first exemplary embodiment of a PPR cell connected to one circumferentially adjacent NPR cell (partially shown).FIG. 7 is an illustration of a second exemplary embodiment of a PPR cell connected to one circumferentially adjacent NPR cell (partially shown).FIG. 8 shows plots of lateral strain versus longitudinal strain of RE cells with incrementally different undeformed RE angles.FIG. 9 shows plots of longitudinal load versus longitudinal deformation of RE cells with different undeformed RE angles.FIG. 10A shows a column of NPR cells and PPR cells having different undeformed RE angles with different Poisson’s ratio and loading stiffness.FIG. 1 OB is a design curve of a stent formed from five of the columns of NPR cells and PPR cells of FIG. 10A showing relationship between extension of length elements of the stent at different angles from the vertical.FIG. 11 is an illustration of an initial embodiment of a stent having NPR cells and PPR cells before and after expansion of the stent.FIG. 12 is an illustration of a surface development of a one-cell-wide circumferential portion of the lattice structure of the stent of FIG. 3 (numbers shown are exemplary dimensions in millimetres).FIG. 13 is an illustration of differential axial expansion / contraction of different rows of cells of the stent as indicated by arrows of different length.FIG. 14 is an illustration of longitudinal expansion of the circumferential portion of FIG. 12.FIG. 15 is an illustration of a surface development of a three-cell- wide circumferential portion of the lattice structure of the stent of FIG. 3 before and after expansion.FIG. 16 is an illustration of crimping and expansion of the stent of FIG. 3.FIG. 17 shows plots of lateral strain versus longitudinal strain of RE cells with different undeformed RE angles of 6° and 8°.FIG. 18 shows plots of longitudinal load versus longitudinal deformation of NPR cells with undeformed RE angles of 6° and 8°.FIG. 19 shows plots of lateral displacement versus total longitudinal displacement of NPR cells with undeformed RE angles of 6° and 8°.Detailed Description

[0026] Exemplary embodiments of an endovascular stent 10 will be described with reference to FIGS. 3 to 19 in which the same reference numerals are used across the figures to refer to the same or similar parts.

[0027] In general, the endovascular stent 10 comprises a tubular expandable lattice structure as shown in FIG. 3. This lattice structure is composed of a plurality of negative Poisson’s ratio (NPR) cells 20 in connection with at least one row 31 of positive Poisson’s ratio (PPR) cells 30 provided along a length L of the stent 10, as can be seen from a 2D unrolled state or surface development 11 of the lattice structure in FIG. 4. By combining both NPR cells 20 and PPR cells 30 and integrating them into a functioning hybrid Poisson’s ratio lattice structure, the working range of Poisson’s ratios increases as illustrated in FIG. 5. In this way, an endovascular' stent 10 with enhanced curvature properties over existing stents is formed.

[0028] In a preferred exemplary embodiment of the stent 10, the plurality of NPR cells 20 are provided as rows 21 of NPR cells 20 along the length L of the stent 10 as indicated in FIG. 4. Each NPR cell 20 has a shape of a re-entrant (RE) hexagon having two parallel and opposite sides that are each parallel with the length L of the stent 10, two reflex internal angles a that form two concave vertices between the two parallel sides, and four acute internal angles or RE angles 9 that form convex vertices at the ends of the two parallel sides, as discussed above with reference to FIG. 1(a).

[0029] As can be seen in FIG. 4, adjacent NPR cells 20 between adjacent rows 21 of NPR cells 20 are conjoined at one of their parallel sides 22. All the NPR cells 20 along each row 21 of NPR cells 20 have collinear first parallel sides and collinear second parallel sides. Preferably, all the NPR cells 20 along each row 21 of NPR cells 20 are spaced apart from each other. Along some or all of the rows 24 of NPR cells 20, adjacent NPR cells 20 along such rows 24 of NPR cells are connected by a connecting bar 40 at the concave vertices 28 formed at their reflex internal angles a.

[0030] Preferably, the rows 24 of NPR cells 20 having adjacent NPR cells connected by connecting bars 40 are alternated in the circumferential direction of the stent 10 with rows 25 of NPR cells 20 having unconnected NPR cells 20 along the length of the stent 10. Further preferably, the row 31 of PPR cells 30 is connected to and provided between two rows 25 of NPR cells 20 that have unconnected NPR cells 20 along the length of the stent 10. In this way, RE cell longitudinal expansion and stent diameter can be increased during expansion of the stent 10. This improves the possible stent diameter expansion ratio to greater than 3. The reduced number of connecting bars 40 by providing rows 25 of NPR cells 20 that are not connected by connecting bars 40 among rows 24 of connected NPR cells 20 has the added advantage of improving axial flexibility of the stent 10.

[0031] FIG. 6 shows a first exemplary' (preferred) embodiment of the PPR cell 30 while FIG. 7 shows a second exemplary (alternative) embodiment of the PPR cell 30. For all embodiments, each PPR cell 30 comprises two centrally provided convex vertices 34 that are opposite each other along the length L of the stent 10. In the row 31 of PPR cells 30. the convex vertices 34 of all the PPR cells 30 arc collinear along the length L of the stent 10, as can be seen in FIG. 4. Adjacent PPR cells 30 along the row 31 of PPR cells 30 are preferably directly connected at their convex vertices 34, as also shown in FIG. 4.

[0032] In the preferred embodiment of the PPR cell 30 as shown in FIG. 6, each centrally provided convex vertex 34 of each PPR cell 30 is provided between a first concave vertex 35 and a second concave vertex 37, such that each PPR cell 30 has two mirrored W-shaped arms 36. The ends of each W-shaped arm 36 are each directly connected to one end of a parallel side 22 of a circumferentially adjacent NPR cell 20 (as indicated by the black circles). The PPR cell 30 in the preferred embodiment may thus be considered diamond-shaped (as indicated by thedotted lines) but having angled extensions 39 to connect the PPR cell 30 to its circumferentially adjacent NPR cell 20. The angled extension feature allows the diamond- shaped PPR cell to interface with a circumferentially neighbouring RE or NPR cell without causing bending distortion of the parallel side 22 of the circumferentially adjacent NPR cell 20 even after expansion of the stent 10. This allows the NPR cells 20 and the PPR cells 30 in the stent 10 to be integrated while maintain a uniform repeating cell envelope and at the same time avoiding bending distortions that give rise to stress concentrations after expansion.F00331 The preferred embodiment of the PPR cell 30 as shown in FIG. 6 is preferable to the alternative embodiment of the PPR cell 30 as shown in FIG. 7, where the PPR cell 30 further comprises two longitudinal convex vertices 38 that are opposite each other in a circumferential direction of the stent 10 so that the PPR cell 30 has a diamond shape. In this configuration, the longitudinal vertices 38 would each have to be directly connected to a mid-point of a parallel side 22 of a circumferentially adjacent NPR cell 20 (as indicated by the black circle) in order to maintain a uniform cellular envelope. However, this would result in a bending distortion to the parallel side 22 of the circumferentially adjacent NPR cell 20 when the stent 10 is expanded, as can be seen in FIG. 7.

[0034] The Poisson’s ratio of a material is defined as the ratio of the lateral contractile strain to the longitudinal tensile strain for a material undergoing tension in the longitudinal direction. FIG. 8 plots the lateral strain versus longitudinal strain of RE cells with incremental undeformed re-entrant angles. The gradient at any point on the plots establishes the Poisson’s ratio of the RE cell in its undeformed and deformed states. As can be seen in FIG. 8, Poisson’s ratio of the RE or NPR cell increases with initial undeformed RE angle of the NPR cell. As the NPR cell stretches longitudinally, its deformed state Poisson’s ratio also increases. Also, the possible range of deformation of the NPR cell decreases with increase of its initial RE angle.

[0035] FIG. 9 shows the plots of longitudinal load versus longitudinal deformation of the RE or NPR cell. The gradient at any point defines the NPR cell longitudinal stiffness. Stiffness of the NPR cell increases with undeformed RE angle, but not with increasing longitudinal deformation. To create different rates of NPR cell lateral expansion along the length L of the stent 10, there are two pertinent design parameters: RE angle 9 and length 1. By changing the values of length 1 and RE angle 9, NPR cells with different Poisson’s ratio and loading stiffnesscan be created. However, once the dimensions w and h of the NPR cell are established, length 1 is also constrained, so the only remaining influencing variable is the RE angle 0 of the NPR cell 20.

[0036] Changing the RE angle of the NPR cells 20 in the stent 10 in the circumferential direction (as shown in an exemplary column of cells 20, 30 in FIG. 10A) thus results in different rates and amounts of NPR cell expansion along the length L of the stent 10, as indicated by the different lengths of the right-facing arrows where longer arrows indicate greater expansion. Contraction of the PPR cells 30 is indicated by the left-facing arrows in FIG. 10A. In the example of FIG. 10, the NPR cells 20 have RE angles of 38°, 41°, 49° and 66° while the PPR cells 30 have angles of 47° and 42°.

[0037] An initial embodiment of the stent 10 comprising five of the exemplar}' column of cells 20, 30 as shown in FIG. 10A is depicted in FIG. 11 The initial embodiment of the stent 10 has an undeformed diameter of 32.8 mm and undeformed length of 88 mm. Expanding it with a pressure of 0.65 MPa results in an expanded diameter of about 39 mm and expanded length (taken along its central longitudinal axis) of about 96.5 mm. The stent diameter expansion ratio is only 1 .2, while the stent midsection extends slightly in length by about 10 %, as predicted by the design curve of the stent as a cylindrical tube shown in FIG. 10B. As can be seen in FIG. 10B, the design curve is translated upwards by about 0.75 mm to create that slight extension of the stent 10 at its midsection (where P = 90°) upon bending, which is ideal for a stent. The initial embodiment of the stent 10 relies on RE angle directly influencing Poisson’s ratios and NPR cell stiffness. The stent 10 achieves good curvature. However, the larger RE angles result in limited deformation potential and stent balloon expansion ratio, and also result in an initial diameter too large for practical stent graft usage as it cannot be sufficiently crimped to fit the delivery catheter.

[0038] FIG. 12 shows a surface development of a one-cell-wide circumferential portion of the lattice structure of the preferred exemplary embodiment of the stent 10 as shown in FIGS. 3 and 4. In the preferred exemplary embodiment, the acute internal angles or RE angles of the NPR cells 20 vary between rows 21 of NPR cells 20 in a circumferential direction (as indicated by the white arrow) around the stent 10, from a first circumferential location CL1 on the stent 10 to a second circumferential location CL2 on the stent 10. The first circumferential locationCL1 may be at a bottom of the stent 10 where the smallest radius of curvature of the expanded stent 10 is intended to be. The first circumferential location CL1 is diametrically opposite the second circumferential location CL2 that may be at a top of the stent where a largest radius of curvature of the expanded stent 10 is intended to be. The row 31 of PPR cells 30 is located at the first circumferential location CL1 of the stent 10.

[0039] In the preferred exemplar}' embodiment of the stent 10 as shown in FIGS. 3 and 4, preferably, the NPR cells 20 in the row 21 of NPR cells 20 located at the second circumferential location CL1 have the smallest RE angles among the plurality of NPR cells 20, as can be seen in FIG. 12. Further preferably, the NPR cells 20 in a row 21 of NPR cells located at a third circumferential location CL3 on the stent 10 and also in a row 21 of NPR cells located at a fourth circumferential location (not shown) on the stent 10 have the largest RE angles among the plurality of NPR cells 20, wherein the third circumferential location CL3 is located midway between the first circumferential location CL1 and the second circumferential location CL2. The third circumferential location CL3 may thus be at a middle of the stent 10. The fourth circumferential location (not shown) is located diametrically opposite the third circumferential location CL3 and is thus also at a middle of the stent 10. In this way, when the stent 10 is expanded, self-bending of the stent 10 occurs, giving rise to a smaller inner radius of curvature where the row 31 of PPR cells 30 is provided, and a larger outer radius of curvature where the row 21 of NPR cells 20 with smallest RE angles are provided.

[0040] As illustrated in FIG. 13, this arises due to contraction (indicated by the black arrows) of the row 31 of PPR cells 30 and the differential rates of expansion along the length of the stent 10 of the rows 21 of NPR cells 20 having different RE angles (expansion indicated by grey arrows of different lengths wherein longer arrows indicate greater expansion) when the stent 10 is expanded in diameter. Shortening of the inside radius due to contraction of the row 31 of PPR cells 30 thus facilitates natural bending and reduction of compression or wrinkling of the stent 10 at the inner radius of curvature.

[0041] Preferably, each of the RE angles of all the NPR cells 20 before expansion of the stent 10 is less than 10°. Exemplary RE angles of the NPR cells 20 (before expansion of the stent 10) in a circumferential direction from the row 31 of PPR cells 30 to the row 21 of NPR cells 20 that will have the largest outer radius of curvature after expansion of the stent 10 (i.e. on onelongitudinal half of the stent) may be 7.5°, 8°, 7.5° and 6° respectively, as shown in FIG. 12. These RE angles arc preferably mirrored by the NPR cells 20 on the other circumferential half of the stent 10.

[0042] Simulating the small angle NPR cells 20 as shown in FIG. 12 under displacement control in Abaqus, it can be seen in FIG. 14 that a stent 10 having rows 24 of NPR cells 20 connected with connecting bars 40 (as shown in FIG. 4) and RE angles of 8° and 6° (as shown in FIG. 12) is able to develop sufficient difference in lateral displacements (LD) to support bending. Specifically, as shown in FIG. 14, when the stent diameter is increased by 40 mm (i.e. the cells are longitudinally deformed), the LD of the NPR cells 20 with 6° RE angles is 2.81 mm in extension, the LD of the NPR cells 20 with 8° RE angles is 1.1 mm in extension, and the LD of the PPR cells 30 is 0.62 mm in contraction. Graduated displacement of the stent 10 in the axial direction can be seen in FIG. 15 which shows a three-cell wide circumferential portion of the preferred exemplary embodiment of the stent 10 before and after expansion.

[0043] FIG. 16 shows the stages of simulation of a three-cell wide circumferential portion of the preferred exemplary embodiment of the stent 10 (half-length model) as performed in Abaqus before and after expansion (or deformation) to better illustrate the automatic bending of the stent 10 that is expanded after crimping. Initial outside diameter (OD) of the stent 10 was 1 1.8 mm (simulating the stent 10 after manufacture). The stent 10 model was then crimped to an OD of 9.6 mm using pressure of 0.093 MPa applied to the stent external surface. Finally, balloon expansion of the crimped stent was simulated by applying an internal pressure of 0.4 MPa which expanded the stent to a final OD of approximately 37.7 mm. Initial length of the simulation model of the stent 10 increased slightly from 64 mm to approximately 73.6 mm (measured at midsection of expanded stent). This confirms that the preferred exemplary embodiment of the stent 10 maintains its length after expansion without foreshortening effects. Compared with a stent composed of only NPR cells that kinks when bent (as shown in FIG. 2C), the presently disclosed stent 10 comprising NPR cells and also PPR cells demonstrates improved curvature without kinking, while avoiding foreshortening but also without unnecessary lengthening or bending distortions after stent expansion in the blood vessel.

[0044] FIGS. 17 and 18 show the plots of geometry effects on Poisson’s ratio and on NPR cell stiffness respectively for NPR cells with undeformed RE angles of 6° and 8°, while FIG. 19shows the plot of lateral displacement versus total longitudinal displacement of NPR cells with undeformed RE angles of 6° and 8°. From FIG. 19, it can be seen that the longitudinal deformation of the NPR cell with undeformed RE angle of 6° is almost twice that of the NPR cell with undeformed RE angle of 8°, as predicted by the stiffness difference. Lateral deformation of the NPR cell with undeformed RE angle of 6° is greater than that of the NPR cell with undeformed RE angle of 8°. This proves that stiffness difference overrides Poisson’s ratio differences arising from the initial geometry of the NPR cell.

[0045] By providing low initial RE angles in the NPR cells 20 of the stent 10 with minimum spread of the RE angles such that stiffness dominates, the presently disclosed stent 10 is able to harness differences in longitudinal stiffness to create differential lateral deformation. Additionally, longitudinal deformation increases the instantaneous Poisson’s ration of the NPR cell, thus amplifying lateral deformation of the NPR cell and accordingly expansion of the length L of the stent 10. This allow s use of NPR cells with small differences in their RE angles to generate bending. In particular, small initial RE angle differences substantially minimise Poisson’s ratio change as a driver for stent bending, while utilising cell stiffness as the primary agent to instigate differential lateral displacement of the NPR cells 20. Relative differences in stiffness thus provides another pathway to generating differential expansions within the same stent. Stiffness thus directly influences lateral deformation, as well as indirectly by changing Poisson’s ratio via longitudinal deformation. Following these principles, the endovascular stent 10 has been developed as described above, that can be manufactured with a small diameter that allows it to be readily crimped to less than 10 mm in diameter for mounting on a delivery catheter and passing through blood vessels, while being able to subsequently expand to its full deployment diameter with a diameter expansion ratio of greater than 3 without bending distortions that can give rise to stress concentrations.

[0046] Using the above-described endovascular stent 10, a stent graft (not shown) can be made by providing a liner around the stent 10. Such a stent graft will naturally curve when deployed, thus providing improved conformance and fitment to any similarly curved sections of the aorta when implanted.

[0047] While there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned thatmany variations in details of design, construction and / or operation may be made without departing from the present invention. It will be appreciated that many further alterations, modifications and permutations of various aspects of the described embodiments arc possible that fall within the spirit and scope of the claims.

Claims

Claims1. An endovascular stent comprising: a tubular expandable lattice structure comprising a plurality of negative Poisson’s ratio (NPR) cells in connection with at least one row of positive Poisson’s ratio (PPR) cells provided along a length of the stent.

2. The endovascular stent according to claim 1 , wherein the plurality of NPR cells are provided as rows of NPR cells along the length of the stent; wherein each NPR cell has a shape of a re-entrant (RE) hexagon having two parallel and opposite sides that are each parallel with the length of the stent, two reflex internal angles between the two parallel sides, and four acute internal angles; wherein adjacent NPR cells between adjacent rows of NPR cells are conjoined at one of their parallel sides; and wherein all the NPR cells along each row of NPR cells have collinear first parallel sides and collinear' second parallel sides.

3. The endovascular stent of claim 2, wherein all the NPR cells along each row of NPR cells arc spaced apart from each other, and wherein adjacent NPR cells along at least some of the rows of NPR cells are connected by a connecting bar at the concave vertices formed at their reflex internal angles.

4. The endovascular stent according to claim 3, wherein the rows of NPR cells having adjacent NPR cells connected by connecting bars are alternated in the circumferential direction of the stent with rows of NPR cells having unconnected NPR cells along the length of the stent.

5. The endovascular stent according to claim 3 or claim 4, wherein the at least one row of PPR cells is connected to and provided between two rows of NPR cells having unconnected NPR cells along the length of the stent.

6. The endovascular stent according to any one of claims 2 to 5, wherein the acute internal angles of the NPR cells vary between rows of NPR cells in a circumferential direction around the stent from a first circumferential location on the stent to a second circumferential location on the stent, wherein the first circumferential location is diametrically opposite thesecond circumferential location, and wherein the row of PPR cells is located at the first circumferential location.

7. The endovascular stent according to claim 6, wherein the NPR cells in the row of NPR cells located at the second circumferential location have the smallest acute internal angles among the plurality of NPR cell .

8. The endovascular stent according to claim 7, wherein the NPR cells in a row of NPR cells located at a third circumferential location on the stent and in a row of NPR cells located at a fourth circumferential location on the stent have the largest acute internal angles among the plurality of NPR cells, wherein the third circumferential location is located midway between the first circumferential location and the second circumferential location, and wherein the fourth circumferential location is located diametrically opposite the third circumferential location.

9. The endovascular stent according to any one of claims 2 to 8, wherein each of the acute internal angles of all the NPR cells before expansion of the stent is less than 10°10. The endovascular stent according to any one of claims 1 to 9, wherein each PPR cell comprises two centrally provided convex vertices that are opposite each other along the length of the stent, and wherein the convex vertices of all the PPR cells in the row of PPR cells are collinear along the length of the stent.

11. The endovascular stent according to claim 10, wherein adjacent PPR cells along the row of PPR cells are directly connected at their convex vertices.

12. The endovascular stent according to claim 10 or claim 11, wherein each centrally provided convex vertex of each PPR cell is provided between a first concave vertex and a second concave vertex such that each PPR cell has two mirrored W- shaped arms, and wherein the ends of each W- shaped arm are each directly connected to one end of a parallel side of a circumferentially adjacent NPR cell.

13. The endovascular stent according to claim 10 or claim 11, wherein each PPR cell further comprises two longitudinal convex vertices that are opposite each other in a circumferential direction of the stent so that each PPR cell has a diamond shape, and wherein the longitudinal vertices of each PPR cell are each directly connected to a mid-point of a parallel side of a circumferentially adjacent NPR cell.

14. The endovascular stent according to any one of claims 1 to 13, wherein the stent is automatically bent after expansion.

15. The endovascular stent according to any one of claims 1 to 14, wherein diameter of the stent is expandable by a factor of at least three.

16. An endovascular stent graft comprising the endovascular stent according to any one of claims 1 to 15 and a liner provided around the endovascular stent.

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