GB2514074A

GB2514074A - Stents with zero poisson's ratio cells

Info

Publication number: GB2514074A
Application number: GB1117664.1A
Authority: GB
Inventors: Joseph Noel Grima; Aaron Casha; Ruben Gatt
Original assignee: Universita ta Malta UM
Current assignee: Universita ta Malta UM
Priority date: 2011-09-06
Filing date: 2011-10-13
Publication date: 2014-11-19
Also published as: GB201117664D0

Abstract

Various stents comprising cells 20 having substantially zero Poisson's ratio are disclosed. According to a disclosed embodiment, there is provided a stent, comprising: a tubular frame formed from a non-woven network of open cells 20, wherein the frame comprises a plurality of cells 20 that each have a Poisson's ratio of substantially zero, such that extension or contraction of the cell 20 in a first direction is accompanied by substantially no change in the average width of the cell perpendicular to the first direction. The cells may be substantially arrow shaped or heart shaped.

Description

STENTS WITH ZERO POISSON'S RATIO CELLS The present invention relates to stents.

Stents are typically tubular (e.g. cylindrical or near cylindrical) devices that act to hold open or expand the lumen of a blood vessel or other tubular structure. The stent may be formed from a frame comprising interconnected struts. Stents can be provided to support the lumen or hold back the lining of the vessel as in dissection. Stents are usefUl for example in the treatment of atherosclerotic disease in blood vessels after the stenosis has been compressed by an angioplasty.

A challenge in stent desiw1 is to reduce or avoid incomplete stent apposition (ISA). ISA occurs where there is the lack of' contact between stent struts that do not overlap a side branch and the underlying arterial wall. Several mechanisms nay lead to ISA and a distinction should be made between acute and late malapposition. Acute malapposition occurs at the time of stcni deployment and is more frequeni with increasing lesion complexity (e.g., calcified lesions, larger vessels, longer lesions). At follow-up, initially maiapposed struts can he welt apposed due to neointiinat hperplasia, or can remain malapposed (late persistent ISA). On the other hand, initially well-apposed struts may lose contact with the tissue because of positive remodelling of the vessel wall, or due to thrombus dissolution behind the struts (late acquired ISA).

It is expected that a larger stent diameter wilt nonnallv lead to a better apposition of the stent stmts. For most stents, it is generally the case that the zones with the best strut apposition are the diseaNed segments, which can be explained by the tendency fbr higher stent:artery dimension ratios at these locations. Consequently, increasing ihe relative stent diameter in the non-diseased segments relative to the diseased segments may improve the overall strut apposition. This finding confirms the importance of a tailored post-dilation to minimise ISA.

i'he largest strut-artery distances are observed at the plaque shoulders, suggesting that segments with strong variations in luminal diameter are more prone to ISA.

A variety of devices exist for use as stents, with different geometries that can be principally classified as helical spiral, woven or coiled. The helical spiral can have periodic peak to peak connections, minimal connections or axial spine configurations. Woven stents can be braided or knitted.

The segmentation that occurs in many stents to allow them flexibility unfortunately means that gaps open when the stent is placed in a particularly curved segment. The use of a spine in a stent design can limit placement in particularly tortuous vessels.

Another problem encountered with prior art stents is a limitation in the extent to which they can be expanded. This tends to lead to a family of different sized stents being required to provide desired functionality. It is challenging to configure a single stent to cope with significant variations in vessel diameter because uneven stresses on the stent can cause localized kinking during radial expansion.

A ifirther general problem with conventional stents is a tendency for the stent to contract along its longitudinal axis upon radial expansion of the stent. This can result in placement problems during stent deployment.

US 2011/0029063 Al describes a stent tube made of auxetic (negative Poisson's ratio) honeycomb material which has a tendency to balloon out in its middle whilst the ends have a tendency to narrow when deployed. This leads to an increased risk of strut to wall malopposition, which is a known cause of early and late stent occlusion and would also limit drug delivery in a drug eluting stent (for example to limit in-stent restenosis).

Cook Medical have produced a knitted stent comprising open cells, the "Cook ZA' stent. The Cook ZA stent is relatively bulky because the continuous knitting of the wire forming the frame of the stcnt nceds to be twisted in ordcr to form some of the stmt elements of the fiame. The interstices resulting from this twisting can act as a focus of clotting. Knitted desiws generally tend to have a higher in-stent restenosis rate compared with more modern low profile configurations, for example those that are laser cut from tubes. The knitted connection between struts in the Cook ZA design can also inhibit hinging between certain struts, which can limit both longitudinal flexibility of the stent and the ability of the stent to deploy.

It is an object of the invention to at least partially address one or more of the problems discussed

above in relation to the prior art.

For example, it is an object of the invention to provide a stent that can be deployed more reliably, that is capable of use in heavily curved vessels, that is resistant to kinking, and/or which has an enhanced degree of expandability for alleviating the need to provide a large number of stents of different nominal sizes.

According to an aspect of the invention, there is provided a stent, comprising: a tubular frame formed from a non-woven network of open cells, wherein the frame comprises a plurality of cells that each have a Poisson's ratio of substantially zero, such that extension or contraction ofthe cell in a first direction is accompanied by substantially no change in the average width of the cell perpendicular to the first direction.

The plurality of cells having zero Poisson's ratio niay be connected together to fonn regions having zero Poisson's ratio. For example, the frame may have a zero Poisson's ratio over a portion of its length or over the whole of its length. Alternatively or additionally, the fiame may comprise cells having zero Poisson's ratio that are provided in isolation, for example in between cells having positive or negative Poisson's ratio.

The provision of a plurality of cells having zero Poisson's ratio enables the stent to be expanded radially to a significant extent with a reduced or negligible corresponding contraction or expansion in the longitudinal direction. The stent can thus be made to conform effectively to the intenor geometry of the vessel in which it is to be deployed while at the same time being deployable with accuracy and reliability in the longitudinal direction. The ability of the stent to bend and conform without kinking eliminates the need to construct a stent with segments that are connected with bridges to allow for movement between adjoining segments as the stent is flexed. Such a use of bridges decreases the support that the stent offers to the wall of the vessel or other location in which it is placed. Relative to such an'angements, stents disclosed herein provide improved support.

The use of zero Poisson's ratio cells also provides the stent with the ability to provide a localized response to uneven stresses in the vessel wall where the stent is deployed. This effect is achieved because zero Poisson's ratio materials intrinsically tend to retain their dimensions in response to constricting pressure in comparison to stents having positive Poisson's ratio.

The stent may optionally consist of a combination of one or more regions having zero Poisson's ratio with one or more regions having positive Poisson's ratio and/or one or more regions having negative Poisson's ratio. It is possible in this way to tailor the stent to the particular requirements of the target site within the vessel. For example, a stent formed with a central region having negative Poisson's ratio that is surrounded on either side longitudinally by lengths of frame having zero Poisson's ratio may be provided. The central region of such a stent can bend easily by virtue of the negative Poisson's ratio behaviour whilst the zero Poisson's ratio end portions will reduce the risk of "dog-boning" found in conventional stents or in the flared ends of negative Poisson's ratio stents.

In an embodiment, zero Poisson's ratio cells are incorporated into a stent having a negative overall Poisson's ratio. In such a stent the advantages of having a negative Poisson's ratio (e.g. enhanced bending) are retained while providing for improved radial expansion properties.

The construction of the stent may be of various metals or alloys thereof, with or without memory effect, and may contain surface coatings that may be used for biocompatibility or as a depot for drugs to elute to the location where the stent is placed.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which conesponding reference symbols indicate corresponding parts, and in which: Figure 1 illustrates extension ofa material having a positive Poisson's ratio; Figure 2 illustrates extension of a material having a negative Poisson's ratio; Figure 3 illustrates extension of a material having zero Poisson's ratio; Figure 4 illustrates the behaviour of materials having a negative Poisson's ratio when pressure or force is applied to the material; Figure 5 illustrates a saddle-shaped response of a positive Poisson's ratio material to bending; Figure 6 illustrates a double-domed response of a negative Poisson's ratio material to bending; Figure 7 illustrates a single-curved response of a zero Poisson's ratio material to bending; Figure 8 depicts an example cell having a zero Poisson's ratio; Figure 9 depicts a region of stent frame fonned from a plurality of directly adjacent cells having zero Poisson's ratio; Figure 10 illustrates the behaviour under deformation of a honeycomb cell structure having positive Poisson's ratio; Figure 11 illustrates deformation of a honeycomb cell structure having negative Poisson's ratio; Figure 12 illustrates deformation of a honeycomb cell structure having zero Poisson's ratio; Figure 13 depicts a cylindrical stent comprising a honeycomb cell structure of the type illustrated in Figure 12; Figures 14 and 15 are schematic perspective views of a stent comprising a frame having an alternative configuration of cells to provide a zero Poisson's ratio; Figure lôis a schematic view of a portion of a network of cells comprising a central region having ncgativc Poisson's ratio and longitudinally outer regions having zero Poisson's ratio; Figure 17 is a schematic perspective view showing a stcnt formed from a network of the type illustrated in Figure 16; Figure 18 depicts a portion of a network of cells comprising the following longitudinal sequence: negative Poisson's ratio -zero Poisson's ratio -negative Poisson's ratio; Figure 19 depicts a portion of a network of cells having the following sequence: positive Poisson's ratio -zero Poisson's ratio -positive Poisson's ratio; Figure 20 depicts a portion of a network of cells having the following sequence: zero Poisson's ratio -negative Poisson's ratio -zero Poisson's ratio -negative Poisson's ratio -zero Poisson's ratio; Figure 21 depicts a portion ofa network of cells having the following sequence: negative Poisson's ratio -zero Poisson's ratio -negative Poisson's ratio -zero Poisson's ratio -negative Poisson's ratio; Figure 22 depicts a portion of a network of cells having the following sequence: negative Poisson's ratio -zero Poisson's ratio -positive Poisson's ratio -zero Poisson's ratio -negative Poisson's ratio; Figure 23 depicts an example cell geometry in which alternate rows of zero Poisson's ratio cells have missing arms, Figure 24 depicts an example cell geometry in which all rows have missing arms relative to a complete arrangement such as that in Figures 8 and 9; Figure 25 depicts an example cell geometry in which the concentration of missing arms is different in alternate rows (1:2 in one set of rows and 1:3 in the interleaved set of rows); Figure 26 depicts an exaniple cell geometry in which alternate rows have an increasing concentration of missing arms; Figure 27 depicts an example cell geometry of the type illustrated in Figures 8 and 9 but with less acute corners.

References in this application to zero Poisson's ratio are understood to be equivalent to references to "substantially zero" Poisson's ratio. The terms zero and substantially zero Poisson's ratio are intended to cover arrangements which are zero to within relevant manufacturing tolerances, for example.

Preferably, any reference to zero or substantially zero Poisson's ratio covers arrangements of 5% or less Poisson's ratio, preferably 1% or less Poisson's ratio.

When a load is applied to an object, the Poisson's ratio may be defined as the negative ratio of transverse strain (perpendicular to the applied load) to the axial strain (in the direction of the applied load).

Some materials display zero Poisson's ratio. When subjected to positive strain along a longitudinal axis, the transverse strain in such materials will be zero (or very nearly zero). There would thus be substantially no change in the cross-sectional area.

Most materials and stents exhibit a positive Poisson's ratio, in which positive longitudinal strain is accompanied by transverse contraction. Materials and stents exhibiting a negafive Poisson's ratio, in which positive longitudinal strain is accompanied by transverse expansion, also exist and may be referred to as auxetic materials or stents.

Figure 1 illustrates extension of an element having positive Poisson's ratio, from a first state 2 to a second state 4, under an applied load represented by arrows 6. As can be seen, extension in the direction of arrows 6 is accompanied by a transverse contraction, indicated by arrows 5.

Figure 2is a corresponding illustration showing extension of a material having negative Poisson's ratio. Here, it can be seen that extension in the direction of the load 6 causes a transverse expansion (arrows 7) between the first state 2 and the second state 4.

Figure 3 illustrates the behaviour of a zero Poisson's ratio material. Here, the applied load 6 causes extension parallel to the applied load 6, but there is no change in the transverse dimension of the material from the first state 2 to the second state 4.

In materials having more complex geometries, or portions of materials, the Poisson's ratio may be defined by reference to an average change in a transverse dimension. This is discussed below with reference to Figure S. As mentioned above, a problem with many existing stent frames is that uneven stresses applied to the frame, for example due to localized variations in the geometry of the vessel, can cause localized kinking during radial expansion. This behaviour could mean that a lesion in the vessel cannot be ifilly dilated by angioplasty, for example. According to embodiments disclosed herein, this problem can be alleviated by arranging for portions of the frame to have zero Poisson's ratio behaviour or negative Poisson's ratio behaviour. In portions of the stent which have zero Poisson's ratio, any increase in inward stress caused by a protrusion in the vessel will be resisted to a greater extent than would be the case in a conventional stent having positive Poisson's ratio because the material of the stent will not be forced longitudinally away from the region of increased stress. Where the stent has a negative Poisson's ratio, the increased radial stress may actually encourage movement of stent material towards the region of enhanced stress, thus actively reinforcing the stent in the region of increased stress. This behaviour is illustrated schematically in Figure 4 which shows an object 8 being forced (arrows 10) into a material 14 having negative Poisson's ratio behaviour. The arrows 12 indicate movement of material towards the region of enhanced stress.

Stents having zero and negative Poisson's ratio behaviour also perform better in terms of their drapeability, relative to conventional stents.

Figure 5 illustrates a problem that occurs with conventional materials having positive Poisson's ratios: when a bending force is applied (arrows 16) the positive Poisson's ratio tends to encourage formation of a saddle shape. When the positive Poisson's ratio material is in the form of a tube, this tendency to fonii a saddle shape can favour kinking.

Figure 6 illustrates the response to bending 16 of a negative Poisson's ratio material. Here, it can be seen that a double-domed shape is formed, rather than a saddle shape. This behaviour tends to assist bending. In particular, a tube formed from material of this type would be able to bend more easily and resist kink formation in comparison with an otherwise equivalent tube formed from a positive Poisson's ratio material.

Zero Poisson's ratio materials behave in an intermediate manner relative to the behaviours illustrated in Figures 5 and 6. As shown in Figure 7, the bending 16 in a zero Poisson's ratio material 18 can be achieved in a single direction only without corresponding deformation in directions perpendicular to the bending direction.

As a result of the above-described behaviour, stents formed from tubes having positive Poisson's ratio will have a tendency to bend with kinking when a bending force is applied. Such kinking will tend to limit the ability of the stent to confinn to a vessel having a torturous curvature. A stent having negative Poisson's ratio can bend more easily. Furthermore, if there is an indentation in the side of the tube, caused for example by an atheroma in a vessel, the negative Poisson's ratio material will support or harden the stent at the contact point with the protuberance. Portions of the stent having zero Poisson's ratio behaviour will tend to avoid kinking and will also resist excessive bending.

h an embodiment, a stent is provided that comprises a tubular frame formed from a non-woven network of cells. This tubular frame may be constructed by using a laser to form holes in a solid cylindrical tube, for example. The frame comprises a plurality of cells that each have a Poisson's ratio of substantially zero. The frame may or may not also comprise other cells and/or structures.

Figure 8 illustrates a portion of an example network of cells. The illustrated portion comprises a cell 20 having zero Poisson's ratio. The cell 20 of this example compnses a first arm 22 extending between a first end F and a second end A. The cell 20 comprises a second arm 24 extending between a first end D and a second end C. The cell 20 thrther comprises a first linking member 26 connecting the first ends F and D of the first and second arms 22 and 24. The cell 20 further comprises a second linking member 28 connecting the second ends A and C of the first and second arms 22 and 24.

In the embodiment shown, the first and second arms 22 and 24 are parallel to each other, but this need not be the case. In alternative embodiments, the arms 22 and 24 are provided at oblique angles relative to each other. In the embodiment shown, the arms 22 and 24 are straight, but this need not be the case. Either or both of the arms may be curved and/or may comprise one or more elbows.

In the embodiment shown in Figure 8, the cell 20 consists of a polygon having six sides.

However, this is not essential. The cell could be formed from a polygon having a different number of sides, for example 3, 4, 5, 7, or more. In the example shown, the lengths of each side of the polygon of the cell 20 are the same. However, this is not essential. The cell 20 may be formed from an irregular polygon. In the example shown, all of the sides of the cell 20 are straight, but this need not be the case.

Some or all of the sides of the polygon may be curved.

Figure 9 shows how a plurality of cells of the type illustrated in Figure 8 may be connected together to fonn a region of stent having zero Poisson's ratio. In embodiments of this type, the first linking member 26 is connected to a portion 33 of a first adjacent cell 30 or 34. The second linking member 28 of thc cell 20 is connccted to a portion 35 of a second adjacent cell 32 or 36. Tn an embodiment, elongation of the frame in the horizontal direction (as depicted) causes a deformation of the first and second linking members 26 and 28 that is such as to cause movement of the portions 33 and 35 of the first and second adjacent cells 30,34 and 32,36 by equal amounts in parallel directions. Thus, from the point of view of the size of the cell 20 in the vertical direction (as depicted) any movement of the portion 33 is compensated by a corresponding movement of the portion 35 and vice versa. The separation of the portions 33 and 35 does not change. Therefore, the average width of the cell 20 in the vertical direction (i.e. the average separation between each section of the first linking member 26 and the corrcsponding scction vertically bclow on thc second linking mcmbcr 28) is unchanged by horizontal elongation or contraction: the cell is said to have zero Poisson's ratio within the meaning of present

disclosure.

In an embodiment, the first and second linking members 26 and 28 are connected to the portions 33 and 35 of the In-st and second adjacent cells 30,34 and 32,36 at intermediate positions between the first and second arms 22 and 24. In the particular example shown, the intermediate position is halfway between the arms 22 and 24 in the horizontal direction. However, this is not essential and in alternative embodiments the intcnnediate position may be located at other positions.

in the embodiment shown, the portion 33 constitutes the second arm of the cell 30 and the first ann of the cell 34 (which are upside down relative to the cell 20). Tn altenmtive embodiments, the first linking member 26 may be attached to other portions of adjacent cells. Similarly, in the example shown, the portion 35 constitutcs thc sccond arm of cdl 32 and the first arm of cell 36. However, in alternative embodiments the second linking member may be connected to different embodiments of adjacent cells.

As mentioned above, when discussing the Poisson's ratio of a complex geometry such as that of the cell 20 in Figures 8 and 9, it is understood that reference may be made to the average length or width of the gcomchy for the purposes of dctennining sirains that result from applied loads. For example, when considering the response to extension along the horizontal direction, as depicted in Figures 8 and 9, the strain in the direction perpendicular to the applied load will be equal to the change in the average dimension of the cell 20 in the direction perpendicular to the applied load (i.e. in the vertical direction).

In an embodimel1t, the first and second linking members have the same geometry and are provided in the same orientation within a given cell. The cell 20 shown in Figures 8 and 9 is an example of this type of geometry. However, other arrangements are possible. In alternative configurations, the first and second linking members of a given cell may have different geometry and/or different orientations.

In an embodiment, the first and second linking members each compnsc an elbow. This is the case in the configurations of Figures 8 and 9, where the first linking member 26 is provided with a single elbow at position E (see Figure 8) and the second linking member 28 is provided with a single elbow at position B (see Figure 8). In alternative configurations, more than one elbow may be provided in one or both of the first and second linking members, or no elbows may be provided in one or both of the first and second linking members.

In an embodiment, the linking members may be shared between adjacent cells. This is the case in the example of Figures 8 and 9, for example. The portion of the first linking member 26 between points F and F fonns part of the first linking member of cell 34 whereas the portion of the first linking member 26 between positions E and D forms part of the first linking member of cell 30.

h an embodiment, a plurality of the zero Poisson's ratio cells may be arranged in a plurality of rows 38,40,42,44 as illustrated schematically in Figure 9. When the network of cells is formed into a cylindrical stent, the rows 3 8,40,42, 44 may be aligned longitudinally or circumferentially. Alternatively, the rows may be provided at an oblique angle to the circumferential or longitudinal axis, that is the rows can be arranged in an oblique or helical manner. Each of the cells in a given row may be aligned with each other along a predetermined axis. This is the ease in the example illustrated in Figure 9. However, this is not essential arnl in alternative configurations the cells in one or niore of the rows niay be displaced relative to at least one of the other cells in the same row in a direction perpendicular to the axis of the row.

In an embodiment, the cells in each row form a periodic stmcture. In an embodiment, the cells (and thus the periodic structure) in one row is displaced relative to the cells (and thus periodic structure) in an adjacent row. In an embodiment, the cells in one row arc displaced relative to the cells in an adjacent row by haifa period. This is the case in the example shown in Figure 9, for example. Here, the cells in rows 40 and 44 are displaced by half a period (i.e. half a cell) relative to the cells in rows 38 and 42. This displacement of adjacent rows of cells enables the cells to tessellate or interleave with each other.

In an embodiment, the adjacent rows of cells are directly adjacent, with no additional cells being located in between the adjacent rows of cells. This is the case in the example shown in Figure 9. As can be seen, this is achieved in the arrangement of Figure 9 by arranging for the first linking members 26 of the cells in row 38 for example to form the first linking members 26 of the cells in row 40 and for the second linking members 28 of the cells in row 40 to form the second linking members 28 of the cells in row 42. Rows 42 and 44 have a corresponding interrelationship.

In the case where the rows are aligned longitudinally, the rows may be arranged to alternate around the circumference such that adjacent rows are displaced by half a period and evely other row is at the same longitudinal position spatially in phase). Similarly, the rows may be arranged to alternate in a corresponding manner when they are aligned cireuinferentially. Such arrangements may be achieved by arranging for the cells in adacent rows to be oriented in opposite directions relative to each other. This can be seen in the example of Figure 9, where the cells 30, 32, 34 and 36 can be seen to be mirror images of the cell 20.

The provision of groups of cells of zero Poisson's ratio having an orientation that is opposite to other groups of cells having zero Poisson's ratio is not limited to arrangements in which the cells are arranged in rows. In alternative embodiments, cells having opposite orientations and zero Poisson's ratio may be distributed in other mariners about the stent.

In the arrangement of Figure 9, the cells in adjacent rows are arranged to interleave with each other such that there is an ovcrlap between the cells in diffcrcnt rows in the vertical direction in the page as depicted. This overlap is illustrated schematically, for rows 38 and 40, by the arrow 44. The zero Poisson's ratio Pr0l3e1ieS of the cells is such that the degree of overlap between adjacent rows increases as the frame is compressed along the direction of the rows (i.e. horizontally as depicted) such that the positions of the axes of the rows (and thus their relative separation) remains unchanged.

Figures 10 to 12 compare the deformation of cell networks having different Poissons ratio behaviour. Figure 10 depicts deformation of a positive Poisson's ratio network, Figure 11 depicts deformation of a negative Poisson's ratio network, and Figure 12 depicts deformation of a zero Poisson's ratio network (such as that illustrated in Figure 9).

In Figure 10, lateral extension (arrows 50) of the honeycomb structure of cells 52 in the left diagram leads to lateral extension and vertical compression of the honeycomb structure, as shown in the right diagram. Each cell 52 of the deformed structure can be seen to have been made wider and shorter.

In Figurc 11, thc lateral extension 50 leads to a lateral and vertical expansion of the network of negative Poisson's ratios cells 54. As can be seen from Figure 11, the vertical struts 56 are driven fUrther apart and the average separation between the coniiectiiig elements 58A and 58B connecting the vertical struts 56 of any given cell, increases.

in the arrangement of Figure 12, in contrast, while the extension caused by the applied load 50 causes the anus 22 and 24 to be driven further apart in any given cell 20, the average separation between the linking members 26 and 28 does not change.

Figurc 13 is a schcmatic illustration of a tubular frame 60 formed from a non-woven network of cells 20 of the type illustrated in Figures 8,9 and 12. As can be seen, the cells 20 are arranged in circumfereiitially aligned rows 38,40,42,44 in which adjacent rows are displaced by haifa period relative to each other circumferentially in order to allow tessellation or interleaving of the cells. Figure 13 is an example of a case where the plurality of cells that each have a Poisson's ratio of substantially zero form a circumferentially continuous region extending over at least a portion of the length of the stent, the region itself also having a Poisson's ratio of substantially zero. In this example the region extends along the whole of the portion of the stent depicted (which may or may not represent the whole stent).

Figures 14 and 15 depict a tubular frame 62 according to an alternative embodiment in which the non-woven network of cells is made up of three different types of cell 20, 64 and 66. The cells 20 and 64 are similar in that they both contain straight and parallel first and second anus and fimt and second linking members having a single elbow that connect the first and second arms together. The cells 66 also have straight and parallcl first and second arms but thc first and second linking mcmbcrs of thc cells 66 comprise three elbows, thus forming a multiple zig-zag arrangement between the first and second arms of the cells 66. Each of the cells 20, 64 and 66 has zero Poisson's ratio behaviour. Radial contraction of the frames shown in Figures 14 and 15 will cause the first and second arms of each of the cells 20, 64 and 66 to move closer together but the average separation of the first and second linking members of the cells 20, 64 and 66 in the longitudinal direction will remain unchanged. The structures of Figures 14 and 15 may be viewed as a modification of the structure of Figure 13 by the addition of the cells 64 in a circumferentially alternating sequence within every other ring of cells 20 (moving longitudinally). The cells 66 result from a corresponding adaptation of the rows of cells that are adjacent to the rows into which the cells 64 have been inserted.

In the embodiments discussed above, the portions of network or frame depicted provide an overall Poisson's ratio of zero. However, in alternative embodiments the frame may comprise portions which do not have zero Poisson's ratio behaviour.

Figure 16 illustrates a section of network from a frame according to an embodiment in which a longitudinally central region 70 is constituted from cells that provide negative Poisson's ratio behaviour and longitudinally outer regions 68 on either side of the central region 70 that are provided with cells that provide zero Poisson's ratio behaviour. Overall, the region 70 displays negative Poisson's ratio behaviour. Overall, the regions 68 display zero Poisson's ratio behaviour. The combination of regions 68 and 70 may provide a combined structure that displays negative, zero or positive Poisson's ratio behaviour, depending on the relative sizes and Poisson's ratios of the regions. Figure 17 illustrates a tubular frame fonned from a network of the type illustrated in Figure 16.

Figures 18 to 22 show sections of stent network illustrating alternative sequences of behaviour.

In Figure 18, a region of cells 72 having negative Poisson's ratio behaviour is followed by a region 74 having zero Poisson's ratio behaviour, which is followed by a region 76 having negative Poisson's ratio behaviour.

In Figure 19, a region 78 having positive Poisson's ratio behaviour is provided next to a region 80 having zero Poisson's ratio behaviour, which is followed by a region 82 having positive Poisson's ratio behaviour.

In Figure 20, a region 84 of zero Poisson's ratio behaviour is followed by a region 86 of negative Poisson's ratio behaviour, which is followed by a region 88 of zero Poisson's ratio behaviour, which is followed by a region 90 of negative Poisson's ratio behaviour, which is followed by a region 92 of zero Poisson's ratio behaviour.

In Figure 21, a region 94 of negative Poisson's ratio behaviour is followed by a region 96 of zero Poisson's ratio behaviour, which is followed by a region 98 of negative Poisson's ratio behaviour, which is followed by a region 100 of zero Poisson's ratio behaviour, which is followed by a region 102 of negative Poisson's ratio behaviour.

In Figure 22, a region 104 of negative Poisson's raflo behaviour is followed by a region 106 of zero Poisson's ratio behaviour, which is followed by a region 108 of positive Poisson's ratio behaviour, which is followed by a region 110 of zero Poisson's ratio behaviour, which is followed by a region 112 of negative Poisson's ratio behaviour.

In each of the arrangements shown in Figures 18 to 22, the horizontal direction may correspond to either the longitudinal or circumferential directions of the slent.

The adaptation of stent cell networks to incorporate a plurality of cells that each have a Poisson's ratio of substantially zero is not limited to frames having an overall Poisson's ratio of substantially zero.

Frames having negative overall Poisson's ratio or positive overall Poisson's ratio may also be adapted to incorporate cells having zero Poisson's ratio in order to improvc their propertics. In general, the addition of cells having zero Poisson's ratio will tend to make the performance of the frame as a whole closer to that ofa frame that has an overall Poisson's ratio ofzero. For exaniple, the addition of cells having zero Poisson's ratio may help to reduce the extent to which deployment of a stent having positive or negative Poisson's ratio is compromised due to the longitudinal contraction or expansion associated with radial expansion of the stcnt at the target site within the vessel.

In an embodiment, cells having zero Poisson's ratio, of the type used to adapt the embodiment of Figure 13 to arrive at the embodiment of Figure 15, may be added to a portion ofa stent frame that is configured to have a negative overall Poisson's ratio.

h embodiments comprising combinations of portions having zero, negative andior positive Poisson's ratio behaviour, the overall behaviour niay be arranged to provide a Poisson's ratio of zero. For example, regions of positive Poisson's ratio behaviour may be configured to compensate for regions of negative Poisson's ratio behaviour. This approach makes it possible for the behaviour of the stent to be varied longitudinally along the stent according to patient specific clinical requirements (as shown for example in the arrangements of Figure 18 to 22), while ensuring that deployment of the stent is not disturbed by any overall change in length of the stent when the stent is radially expanded at the target site within the vessel. For example, in regions of the vessel known to be particularly tortuous or to have strong local variations in geometry, the stent may be designed to have a negative Poisson's ratio behaviour in such regions, with compensating regions of positive Poisson's ratio behaviour being provided at positions in the stent that are expected to correspond to regions of the vessel which have more uniform geometry.

In the embodiments shown, the elements making up the stent networks have consistent thickness, but this is not essential. The thickness of elements may vary. For example the thickness of the first and second arms of the cells may be different from each other and/or different fiom the linking members.

Similarly, the thickness of the linking members may be different from each other or the same. The depth of the tube material (i.e. radially) may also be made to vary as a function of position.

The cross-section of the tubular frame will generally be circular or near circular. For example, the cross-section may consist of a polygon shape, which may be regular or irregular. The crOSs-SectiOl1 of the stent may be constant along the axis of the stent or may vaiy along the axis of the stent, before or after applying an axial or radial load. The centre line of the stent may be straight or curved before or afler applying an axial load.

Any of the elements described above as being aligned circumferentially or longitudinally may alternatively or additionally be aligned obliquely, for example so as to form a spiral structure. For example, the first and/or second arms and/or the first and/or second linking members may be aligned at an oblique angle rclative to the circumferential direction.

In any of the cells described above, additional struts or structures may be provided that span the cells.

Sometimes stents are placed in a location in a vessel where a side-branch or bifurcation exists.

Athcrosclcrosis has a predeliction for bifurcation lesions, i.e. the atheroma cxists at thc sitc of the bifurcation due to changes in blood flow patterns and wall support. In these cases there is a clinical requirement for a stent to be placed in the main vessel and another stent at the origin of the side-branch.

Several techniques in stent placement exist and this remains an issue with interventionalists in this field.

However there is requirement for stents to be dcsigned to handle a biffircation lesion with biffircation stenting or placement of stent-in-stent. In these eases the side-branch stent passes through an opening in the main vessel stent that is created by passing an angioplasty balloon through the cells in the main vessel stent overlying the side-branch. The geometry of the main vessel stent must be such that it can accommodate the diametcr of the side-branch stent. This is achieved by having cell sizes in the main-vessel stent that upon dilatation can reach a circumference equal to the circumfcrence of the side-branch.

The increase in cell size can be achieved by joining 2 or niore cells siniilar to Figure 8, by the removal of the common arm (strut or rib) FA or CD between the adjacent cells. Similarly, in Figure 9 this can be achieved by the deletion of either arm 22 or 24 to produce a larger sized cell.

With conventional and re-entrant (auxetic) honeycombs, the Poisson's ratio tends to zero as vertical ribs are removed. Removal of vertical ribs (arms) in zero-Poisson's ratio honeycombs does little to alter the basic geometry whilst resulting in larger cell sizes for the purpose of biffircation stenting or stent-in-stent placement.

With zero-Poisson's ratio honeycombs, the basic arrow-shaped' unit as shown in Figure 8 is balanced with a row of opposite facing or mirror image units. This is illustrated in Figure 9 by rows 40 and 42; or by rows 42 and 44. Removing arms or elements from the geometry can remove this balance, leading for example to wcakly auxetic behaviour.

Figure 23 illustrates an example geometry in which every other row 120 has missing arms 122 (ribs or struts) in comparison with a regular geometry of the type shown in Figures 8 and 9. The result is a geometry consisting of two types of cell: a smaller cell 20 similai-to that of Figures 8 and 9 and a larger cell 124..

Figure 24 illustrates an alternative arrangement in which all rows have missing arms relative to a complete arrangement such as that of Figures 8 and 9. The total honeycomb in this example remains balanced, and exhibits exactly zero Poisson's ratio (within manufacturing tolerances). The geometry is made up of identical cells 124.

Figure 25 illustrates a further example comprising a zeroPoisson's ratio honeycomb in which the number of missing arms in adjacent rows 126 and 128 is different. Rows 126 have one in three arms missing while rows 128 have one in two arms missing..

Figure 26 shows an anangement where altcrnatc rows 130 have an increasing ratio of niissing ribs. The ratios of present:missing vertical arms are given in the right part of the Figure.

The geometries illustrated in Figures 23, 24, 25 and 26 can be used to produce stents that are basically cylinder shaped or conical section shaped for example.

Figure 27 illustrates a further example zero Poisson's ratio geometry. The geometry is similar to that in Figures 8 and 9. The only difference is that the corners 132 arc less acute and are more amenable to industrial production by laser cutting of hollow tubes. It also diminishes the risk of fracture occurring at the hinges. This geometry is being demonstrated to show that it behaves similarly to that shown iii Figures 8 and 9 and discussed above.

More generally, it is noted that any of the example geometries discussed above could be altered to suit manufacturing, medical or other requirements, for example by smoothing edges and/or rounding corners, without departing from the scope of the invention as defined in the appended claims.

The cross-sections of the arms and/or elements described above may have various forms, including circular, square, rectangular, or other shape. The arms and/or elements may have a longitudinally uniform cross-section or the cross-section may vary, for example in a tapered or conical form. TI-ic sides of the arms and/or elements may thus be parallel or non-parallel. A single arm or element could have one or more pairs of parallel sides and one or more pairs of non-parallel sides in its cross-section.

Claims

CLAIMS1. A stent, comprising: a tubular frame formed from a non-woven network of open cells, wherein the frame comprises a plurality of cells that each have a Poisson's ratio of substantially zero, such that extension or contraction of the cell in a first direction is accompanied by substantially no change in the average width of the cell perpendicular to the fir st direction.
2. A stent according to claim 1, wherein the plurality of cells that each have a Poisson's ratio of substantially zero fonn a region comprising a length of frame that has a Poisson's ratio of substantially zero.
3. A stent according to claim 1 or2, wherein each of one or more of the plurality of cells comprises: a first ann and a second arm; a first linking member for connecting a first end of the first arm to a first end of the second arm; a second linking member for connecting a second end of the first arm to a second end of the second ann
4. A stent according to claim 3, wherein: the first linking member is connected to a portion of a first adjacent cell and the second linking member is connected to a portion of a second adjacent cell; and extension or contraction of the frame parallel to the first direction causes a deformation of the first and second linking members that is such as to cause movement of the portions of the first and sccond adjacent cells by equal amounts in parallel directions.
5. A stent according to claim 3 or 4, wherein: the first and second linking members are connected to the portions of the first and second adjacent cells at intermediate positions between the first and second anus.
6. A stent according to claim 4 or 5, wherein: the portion of the first adjacent cell to which the first linking member is connected is the first or second aiim of the first adjacent cell.
7. A stent according to any one of claims 4 toô, wherein: thc portion of the second adjacent cell to which the second linking membcr is connected is thc first or second arm of the second adjacent cell.
8. A stent according to any one of claims 3 to 7, wherein the first linking member has the same geometry and orientation as the second linking member.
9. A stent according to any onc of claims 3 to 8, wherein the first linking member compnses an elbow.
10. A stent according to claim 9, wherein the first or second arm of an adjacent cell is connected to said elbow in the first linking member.
11. A stent according to any one of claims 3 to 10, wherein the second linking member comprises an elbow.
12. A stent according to claim 11, wherein the first or second arm of an adjacent cell is connected to said elbow in the second linking member.
13. A stent according to any one of claims 3 to 12, wherein a first ami of one cell is the second arm of an adjacent cell.
14. A stent according to any one of claints 3 to 13, wherein a portion of a first linking member of one cell forms part of a first linking member of an adjacent cell.
15. A stent according to any one of claims 3 to 14, wherein said first and second arms are substantially parallel.
16. A stent according to any one of the preceding clainis, comprising a plurality of cells that each have a Poisson's ratio of substantially zero that are arranged in a plurality of rows.
17. A stent according to claim 16, wherein the cells in each of said rows are aligned with each other along a predetennined axis.
18. A stent according to claim 16 or 17, wherein the cells in each of said rows form a periodic structure.
19. A stent according to claim 18, wherein the cells in one row are displaced parallel to the axis of the row relative to the cells in an adjacent row.
20. A stent according to claim 19 wherein the cells in one row are displaced parallel to the axis of the row relative to the cells in an adjacent row by half a period and rotatcd by I 8() dcgrccs.
21. A stent according to ally one of claims 16 to 20, wherein no cells are provided in between any two adjacent rows.
22. A stent according to any one of claims 16 to 21, wherein the cells having Poisson's ratio of substantially zero are all identical.
23. A stent according to any one of claims 16 to 22, wherein a degree of overlap between adjacent rows increases as the frame is compressed along the direction of the rows such that the positions of axes of the rows remains unchanged.
24. A stent according to any one of claims 16 to 23, wherein each of said plurality of rows extends in a circumferential direction.
25. A stent according to any one of claims 16 to 23, wherein each of said plurality of rows extends in a direction parallel to the longitudinal axis of the stent.
26. A stent according to any one of the preceding claims, comprising a first group of cells that all have a Poisson's ratio of substantially zero and are oriented in a first direction, and a second group of cells that all have a Poisson's ratio of substantially zero and are oriented in a second direction opposite to the first direction.
27. A stent according to claim 26, wherein the first group of cells comprises a first row of cells and the second group of cells comprises a second row of cells, the second row being directly adjacent to the first row and positioned and oriented so that the cells of the first row interleave with the cells of the second row.
28. A stent according to any one of the preceding claims, wherein the magnitude of the Poisson's ratio of substantially zero is less than 5%.
29. A stent according to ally one of the preceding claims, wherein: the frame comprises a region of negative Poisson's ratio.
30. A stent according to claim 29, wherein: the region of negative Poisson's ratio extends over a length of the frame that is surrounded on either side by lengths of frame having a Poisson's ratio of substantially zero.
31. A stent according to any one of the preceding claims, wherein: the frame comprises a region of positive Poisson's ratio.
32. A stent according to any one of the preceding claims, wherein: a region consisting of a lcngth of fiame having a Poisson's ratio of substantially zero is formed from a combination of one or more lengths of frame having a positive Poisson's ratio and one or more lengths of frame having a negative Poisson's ratio.
33. A stent according to any onc of the preceding claims, wherein a plurality of cells having a Poisson's ratio of substantially zero are incorporated into regions of thc frame that have a negative Poisson's ratio.
34. A stcnt according to any one of the preceding claims, wherein: the frame comprises a plurality of lcngths of frame having Poisson's ratios in one or more of the following sequences: negative-zero-negative; positive-zero-positive; zero-negative-zero-negative-zero; negative-zero-negative-zero-negative; negative-zero-positive-zero-negative.
35. A stent according to any one of the preceding claims that is configured to deliver a drug to a deployment site.
36. A stent according to any one of the preceding claims, wherein the tubular frame is formed by laser machining holes into a solid tube.
37. A stent according to any one of the preceding claims comprising a missing arm region formed fl-om a geometiy that would exhibit substantially zero Poisson's ratio which has been modified so that it is missing one or more arms.
38. A stent according to claim 37 whercin the gcomehy that would exhibit substantially zero Poisson's ratio comprises a plurality of rows of identical cells.
39. A stent according to claim 38, wherein the arms are missing in every other row.
40. A stent according to claim 39, wherein the proportion of arms that are missing is the same in every other row.
41. A stent according to claim 39, wherein the proportion of arms that are missing is different in at least three different rows.
42. A stent according to any one of claims 37 to 41, wherein the locations of missing arms is balanced so that the missing ann region has substantially zero Poisson's ratio.
43. A stent according to any onc of claims 37 to 41, wherein the locations of missing arms is unbalanced so that the missing arm region exhibits weakly negative Poisson's ratio.
44. A stent according to any one of claims 38 to 41, wherein the proportion of missing arms per row of cells increases in a dircction perpendicular to the rows' axes so as to provide a gradient of increasingly negative Poisson's ratio behaviour.
45. A stent according to any one of the preceding claims, wherein: the stent comprises honeycomb elements of substantially zero Poisson's ratio that arc located within the stent whether in transverse, longitudinal, oblique, checkerboard or other pattern whether repetitive or not.
46. A stent according to any one of the preceding claims, wherein: one or more of the anus or elements defining the plurality of cells has circular, square, rectangular or other cross-section; and/or one or more of the arms or elements defining the plurality of cells are stTaight, curved or zig-zag.
47. A stent according to claim 46, wherein one or more of the anus or elements defining the plurality of cells have parallel sides.
48. A stent according to claim 46 or 47, wherein one or more of the anus or elements defining the plurality of cells have non-parallel sides.
49. A stent according to any one of the preceding claims, wherein: two or more of the plurality of cells having substantially zero Poisson's ratio are arranged in a helical manner.
50. A stent constructed and arranged to operate as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.