DE29724189U1 - Improved covered stent - Google Patents

Description

DE utility model from EP 9794351.3 &igr; HL-71987/UMB 27.1.2000

IMPROVED COVERED STENT

Field of the invention

The innovation relates generally to an implantable intraluminal device and in particular to a composite intraluminal device comprising a stent and a stent cover.

Background of the invention

It is well known that all kinds of body vessels are treated with various endoprostheses, one of which is widely referred to as a stent. A stent is usually an elongated piece of tube made of biocompatible material and is used to treat stenosis, strictures or aneurysms of body vessels such as blood vessels. The device is implanted to reinforce vessels in the event that they would otherwise collapse, or if they are partially occluded or generally weak, or if they have abnormally dilated areas. Stents are also commonly used after angioplasty of a blood vessel to prevent the stenosis in the diseased vessel from recurring. Although stents are mainly used in blood vessels, they can also be implanted in other body vessels, for example in the urogenital tract or in the bile duct.

Stents are usually open and flexible. This allows them to be inserted into curved vessels. This shape also allows the stent to be compressed to a center point 0 during intraluminal implantation using a catheter. Once it is installed in the damaged vessel, it expands radially and can thus support and strengthen the vessel. The radial expansion of the stent can be achieved by inflating a balloon on the catheter. The stent can also be of the self-expanding type.

and radiate out of their own accord after insertion. Examples of stent designs according to the invention are described in US patents 4,503,569 (Dotter), 4,733,665 (Palmaz), 4,856,561 (Hillstead), 4,580,568 (Gianturco), 4,732,152 (Wallsten), and 4,886,062 (Wiktor) and in WO 96/26689 and in US applications No. 08/396,569 of March 1, 1995 and No. 08/511,076 of August 3, 1995.

Although stents constructed in this way are suitable for keeping open otherwise blocked, weakened or occluded vessels, their openness makes them prone to allowing material to pass through the stent body. This includes excessive cell and tissue growth (intimal hyperplasia), thrombus formation, plaque in a vessel periphery and tumors in the bile or urogenital tract. This can cause closure or re-closure of open vessels.

The susceptibility to material passing through the walls of a deployed stent can be reduced by a composite intraluminal device comprising a stent and an outer cover that goes around the open stent structure. Although such sheaths prevent material passage through the stent walls, the sheath itself must be flexible and expandable to allow the stent to be deployed and to change from a compressed to a radially expanded state.

The following U.S. patents disclose examples of intraluminal devices.
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US Patent 5,123,916 (Lee) discloses an expandable intraluminal blood vessel implant comprising coaxial cylindrical tubes and a number of framework members therebetween. These are

stretchable, ring-shaped and they give the implant circumferential strength.

U.S. Patent 5,383,926 (Lock et al.) describes a radially expandable endoprosthesis having an elongated sleeve member in which connecting bands limit radial expansion. These bands can be individually removed for additional expansion. The sleeve can have a C-shaped cross-section to allow for even greater expansion. The sleeve member typically has an open wall structure, such as that typically found in a wire mesh tube or a slotted tube. The expandable sheet material can be placed over the open portion of the C-shaped sleeve member and can be made of Goretex®.

U.S. Patent 5,389,106 (Tower) discloses an impermeable, expandable, intravascular stent. An impermeable, deformable membrane connects sections of an inflatable frame so that the frame has an impermeable outer wall. The membrane is made of a synthetic polymer that contains neither latex nor vinyl, and the frame is made of fine filaments of annealed platinum. The inflatable frame may be an expandable stent and the membrane may be made of a hypoallergenic, biologically inert material that is free of latex rubber proteins. The membrane should be impermeable, elastic, inflatable, and it should have a retention effect. No specific materials are mentioned, except for the trade name Tactylon™. The impermeable membrane is attached to the stent by immersing the stent in a 0 membrane polymer solution and then drying the device to remove the solvent. The stent is embedded in the membrane surface.

WO 96/00103 of January 4, 1996 discloses a covered type of stent that can be radially expanded. The expandable metal stent has an outer shell made of ePTFE. The ePTFE shell has such extensibility that the shell can expand when the underlying stent expands. However, in order for the ePTFE shell to have these expansion properties, the ePTFE material must be subjected to a series of processing steps during the manufacture of the shell: stretching the material, sintering the material, radially expanding the material and re-sintering the stretched material. The ePTFE shell thus produced can be stretched so that the shell has the required expansion properties. However, the device described requires very precise manufacture and is extremely sensitive to processing. Careful processing of the material for the shell is necessary so that the shell has sufficient extensibility. The aim is to provide a covered stent in which the sheath can be radially expanded with the stent and in which the sheath is easier to manufacture and apply to the stent.

Summary of the invention

It is an object of the invention to provide a prosthetic intraluminal device such as a stent that keeps occluded, weakened or damaged vessels open.

It is a further object of the invention to provide a covered stent for intraluminal use which can hold open a damaged cavity and which prevents material from breaking through the body of the stent.

It is a further object of the invention to provide an expandable covered stent which can be placed intraluminally

and in which the sheath can expand together with the stent.

These and other objects and aims are achieved according to the invention by a composite intraluminal device comprising an elongated, radially expandable stent tube with an inner luminal surface and an opposite outer surface that extend in the longitudinal axis of the stent. The shell of the stent is made of unsintered ePTFE that can expand and is arranged around the stent in such a way that it expands when the stent is radially expanded.

In a preferred embodiment, the stent sheath includes a longitudinal segment of unsintered ePTFE, which is usually arranged in the longitudinal axis of the stent. The longitudinal segment can expand in the transverse direction upon radial expansion of the stent.

0 In a further embodiment of the invention, the stent sheath includes a longitudinal segment made of unsintered ePTFE, which has an original longitudinal extent. The segment is stretched in the transverse direction, whereby the longitudinal extent has become smaller. The sheath is generally arranged transversely to the longitudinal axis of the stent. The stretched segment can be stretched in its longitudinal direction during a radial expansion of the stent, whereby the segment can be stretched up to the original longitudinal extent and thus limits the radial expansion of the stent. In this embodiment, 0 the sheath is further arranged in relation to the stent such that the longitudinal axis of the stent is at right angles (i.e., at an acute angle offset from the axis) to the sheath.

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The intraluminal device is manufactured as follows: The method includes the step of providing an elongated, radially expandable stent tube. The elongated stent sheath is made of unsintered ePTFE and is stretchable in the transverse direction. The stent sheath is placed around the stent and is longitudinally aligned with the stent so that transverse expansion of the sheath is prevented when the stent is expanded radially.

Short description of the drawings
It shows:

Fig. 1 is a perspective view of the composite intraluminal device according to the invention;
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Fig. 2 is a perspective view of a stent as usable in the composite device of Figure 1;

Fig. 3 a representation of the stent sheath from the composite

0 direction in Figure 1;

Fig. 3A is an electron microscope image of a one-dimensionally oriented ePTFE material as used in the sheath of Figure 3;
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Fig. 4 is a cross-section of an embodiment of the covered stent according to the invention in the compressed state;

0 Fig. 5 a cross-section of the covered stent of Figure

3 in the radially expanded state;

Fig. 6 is a representation of the cover as given in another embodiment of the invention,-

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Fig. 6A is an electron microscope image of the transversely stretched ePTFE material for the shell of Figure 6 ;

Fig. 7 the casing in Figure 5 in a transversely stretched state; 5

Fig. 8 is a schematic representation of the sheath in Figure 6 when mounted on the stent;

Fig. 9 is a cross-sectional view of another embodiment of the invention in the compressed

Condition;

Fig. 10 is a cross-sectional view of the covered stent of

Figure 8 in radially expanded state;
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Fig. 11 is a graft illustrating the properties of the material forming the shell of the device according to the invention.

Detailed description of preferred embodiments
The invention provides a composite covered stent that can be implanted intraluminally in a body vessel and that, when placed adjacent to an occluded, weakened or otherwise damaged portion of the vessel, holds the vessel open. The covered stent is usually delivered intraluminally using a balloon catheter. The device is pushed in while compressed and, after proper positioning, is deployed by radial dilation. The intraluminal device will most likely be deployed by balloon dilation, but the invention also encompasses the deployment of self-expanding stents.

The composite intraluminal device 1 according to the invention comprises a stent 10, such as shown in Figure 1, and a liner or sheath 12, such as shown in Figure 3. In a preferred arrangement, the sheath 12 is arranged over the stent 10.

See Figure 2. The stent 10 is typically an elongated tube with a longitudinal stent axis I ₃ . The stent 10 has two opposite open ends 10a and 10b and a central lumen 10c between them. The body of the stent 10 defines an inner surface 11 and an outer surface 13 which are opposite one another. It typically has an open structure with a plurality of body openings and perforations. These openings or perforations give the stent longitudinal flexibility and enable the stent to be expanded radially once it is inserted into the body cavity.

WO 96/03092 A1 of February 8, 1996 and US priority applications No. 282,181 of July 28, 1994 and No. 457,354 of May 31, 1995 show the stent that can be used according to the invention in more detail. The stent shown therein has first and second meander structures that are at right angles to each other.

The special meander pattern and the openings and gaps give the stent flexibility over its length and make it possible for the stent to be easily pushed through curved blood vessels. The special design of the stent IO shown therein allows the stent to be expanded radially without the length becoming shorter.

Although the invention is described with reference to a specially constructed stent 10, all open stent designs of the prior art can be used. For example, filament stents can be used according to the invention.

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Stents can be used which have a body made of spirally wound metal filaments with spaces between the helices. Stents can also be used which consist of tubes with etched or patterned slits. Such stents are known in the art and are described in the above-mentioned US patents.

The stent 10 is used together with a lining or sheath 12. See Figure 3. In a preferred embodiment, the sheath 12 is pulled over the stent tube and completely surrounds the stent. Although in the preferred embodiment the sheath 12 is pulled over the outer surface 13 of the stent, see Figure ₁₁ , the sheath 1.2 can also be applied to the inner surface 11 of the stent in the form of a lining. The sheath 12 then forms an effective barrier around the stent 10 so that excessive cells or tissue cannot grow in or thrombi can form through the expanded wall of the stent tube. However, in order for the sheath 12 to function effectively together with the stent 10, the sheath must be able to stretch sufficiently so that it can expand radially together with the stent 10.

The invention also relates to the use of a polymer material for the sheath 12 which is sufficiently stretchable when placed around the stent. Such materials include extrudable, biocompatible polymers which can be manufactured with or have a high degree of one-dimensional molecular alignment. That is, with a material which is highly oriented in one direction.

These polymers have the property of being able to stretch in a direction substantially transverse to the direction of one-dimensional orientation. In the manufacture of polymer sheets, films and the like, the direction of orientation is usually the direction in which the material

This is referred to as the machine direction (arrow M, Figure 3). Since the casing is usually manufactured in an extrusion process and the material is extruded in the long axis, this is also the machine direction. A material with a one-dimensional orientation in the machine direction can therefore be stretched in a direction substantially perpendicular to it.

In a preferred embodiment of the invention, the sheath 12 is made of a one-dimensionally oriented, stretched polytetrafluoroethylene (ePTFE). ePTFE films or sheets are known to be made by paste extrusion. Paste extrusion produces a PTFE product commonly referred to as "green". After removal of the lubricant, the material then becomes very brittle, i.e., the material crumbles easily during machining and is unsuitable for many applications requiring structural strength. However, the green material is highly one-dimensionally oriented (in the machine direction) and can be stretched or extended across the machine direction. Typically, stretching PTFE involves heating the material and stretching it lengthwise to give ePTFE. The resulting material is more stable, less brittle and easier to handle. This is due to the knotty and fibrillar structure caused by longitudinal stretching. The structure is shown in Figure 3A. However, after sintering, the ePTFE material can no longer be stretched or extended. The invention uses unsintered ePTFE which has been treated by heating only to the extent that the material is stable and not brittle. PTFE generally requires sintering at the melting point, i.e., at about 327 ⁰ C, to obtain these structural properties. The present shells are heated to a temperature which is not sufficient to sinter the product, i.e., generally slightly below 327 ⁰ C. If temperatures

at or beyond the normal sintering range, this may only be done for periods of time that are not sufficient for sintering. After such a heat treatment, however, the material still shows the property of being able to be stretched or extended in the transverse direction. However, it then has sufficient hoop strength for the purposes of the invention.

See Figure 3. The ePTFE sheath 12 was extruded lengthwise in the longitudinal extension I ₁ and thus has an increased extensibility in the transverse extension t _L .

Commercially available PTFE materials with such properties, such as polytetrafluoroethylene tapes, can be used.

The manufacture of such tapes is described in U.S. Patents 5,474,727 (Perez) and 5,175,052 (Tokuda). The tape resulting from these processes is porous and has little or no stretchability in the longitudinal direction. However, it can generally be stretched extremely well in the transverse direction.

In order to use such an ePTFE tape as a cover for a stent

10, a section of it must first be provided for the casing 12. See Figures 3 to 5. The casing 12 is then arranged so that its longitudinal extension

1 ₁ coincides with the longitudinal axis l _s of the stent 10. In a preferred embodiment, the sheath is wrapped around the outer surface of the stent in such a way that the opposite longitudinal edges 12a and 12b lie on top of each other and form a 0 seam 14. The edges 12a and 12b can be glued so that the seam is closed. Adhesive methods such as pressure or adhesive bonding or anchoring can also be used to close the seam 14. The longitudinal edges 12a and 12b can also be held together with the aid of weak electrostatic forces

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For a closed seam 14: no chemical bonding is required. An adhesive may also be used to bond the overlapping edges 12a and 12b together if the adhesive will wet the material to which it is applied. Although the sheath 12 is secured to the stent 10 by bonding the overlapping edges 12a and 12b to form a seam 14 as shown, the sheath 12 may also be secured to the stent 10 at one or more locations. Such an attachment is shown and described in a concurrently filed U.S. patent application (Attorney Docket No. 760-2) entitled "Stent/Membrane-Composite Device."

When placed around the compressed stent 10, the material forming the sheath 12 is capable of transverse expansion. This allows the sheath to also expand radially when the stent 10 expands radially. When the stent 10 expands radially in this manner, either by balloon dilation or by self-expansion, the sheath 12 is expanded transversely from one transverse dimension ti to a second transverse dimension t ₂ , where t ₂ is greater than t _lf . As particularly shown in Figures 4 and 5, the transverse dimension t _x of the sheath 12 forms the circular component of the sheath 12 around the compressed stent 10. When the stent 10 is expanded radially, this transverse component of the sheath 12, by being able to expand from size t ₁ to size t ₂ , allows the sheath to expand radially with the expansion of the stent 10. See Figure 5. The sheath 12 then expands to the circumference t ₂ around the expanded stent 10. The property of highly one-dimensionally oriented material such as ePTFE to be able to be stretched transversely to the extrusion direction or machine direction allows its use as a sheath according to a second embodiment of the invention.

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Figure 6 shows a further sleeve 12' - similar to the sleeve 12 of Figure 3 - with a longitudinal extent I ₁ ' and a transverse extent t-/ . Since the material is strongly one-dimensionally directed, the sleeve 12' can stretch in the transverse extent, as mentioned. In this embodiment, the sleeve 12' is stretched transversely before being applied to the stent 10. In Figure 7, the sleeve 12' is stretched in the transverse direction to a transverse extent t ₂ '. Figure 6A shows the structure of the material stretched in this way. The arrow M points in the machine direction. The direction of the transverse stretching is shown by the arrow T. Such a transverse stretching causes a corresponding reduction in the longitudinal extent of the sleeve 12' to a size I ₂ ', where I ₂ ' is smaller than I ₁ '. The stent 10 is then brought together with the sheath 12 such that the longitudinal axis l _s of the stent coincides with the transverse extension t ₂ ' of the sheath 12, as shown in Figure 8. The sheath 12' is then wrapped around the stent 10 such that the transverse edges 12c' and 12d' overlap and form a closed seam 14. The overlapping transverse edges 12c' and 12d' can be secured as described above, for example as in the aforementioned embodiments of the invention.

Although the highly one-dimensionally oriented material forming the sheath 12 has little or no stretchability in the longitudinal direction (machine direction, arrow M, see Figures 6 and 7), when the material has been stretched transversely as described above, the stretched material can return to its original length when stretched in the longitudinal direction. By using the material described above, the radial expansion of the stent can be limited.

See Figure 9. The stent 10 is aligned with the sheath 12' as in Figure 8. A radial expansion of the stent 10 from the compressed state (see Figure 9) to the expanded state (see Figure 10) causes a corresponding

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Stretching of the stretched sleeve 12'. Such stretching occurs in the longitudinal extension of the sleeve 12'. Since the transversely stretched sleeve 12' can only be stretched in the longitudinal extension up to the original length I ₁ ', this limits the radial expansion of the stent 12 located underneath. When the stent is expanded radially, the stent only expands to the extent that the sleeve 12' can be stretched to the length I ₁ '. Further expansion of the stent 10 is not possible since the sleeve 12 has reached the maximum expansion size. By controlling the stretching properties of the sleeve 12 in this way, the expansion of the stent 10 can be limited.

As the sheath 12' returns to its original length I ₁ ' when the stent 10 is stretched, the transverse extension t ₂ ' will also return to the transverse extension t _x ' . This leads to a shortening of the sheath over the stent 10. Such shrinkage effects can be reduced by arranging the stent 10 with the stent axis l _s ' slightly orthogonal to the transverse extension t ₂ '. There is then still an expansion limit for the stent 10, but the disadvantageous shrinkage effects are thereby reduced.

In a preferred embodiment of the invention, ePTFE is used as the material for the shells 12 and 12'. Since the ePTFE is strongly one-dimensionally oriented in its processing direction, it exhibits the desirable stretch properties described here. However, the invention is not limited exclusively to ePTFE. Other stretchable biocompatible polymer materials such as polyurethane can also be used according to the invention, provided that they have a high degree of one-dimensional orientation in the extrusion direction of the material. These materials are suitable according to the invention if they have different tensile properties in the direction of the

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longitudinal axis and the transverse direction. The curve in Figure 11 shows properties of materials suitable for the invention in relation to one another. The y-axis shows the load on the material. The x-axis, on the other hand, shows the deformation. Curve 20 shows the relationship between load and deformation of the material along the machine direction. Curve 30, on the other hand, shows the load and deformation characteristics of the material at an angle of 90° to it. Materials with such properties are suitable for the composite device according to the invention. Other such materials include, for example, polyesters such as polyethylene terephthalate (PET), polypropylenes, polyamides, nylon materials and copolymers and mixtures thereof.

The invention also includes the incorporation of biological materials into the shell. Agents such as collagen and/or heparin or other active ingredients and agents can be embedded in the shell 12 for various known therapeutic purposes. Such combinations are described in WO 95/29647, filed November 9, 1995, and in priority U.S. applications No. 235,300, filed April 29, 1994, and No. 350,233, filed December 6, 1994.

Changes and modifications may be made to the invention. Such changes and modifications are intended to be encompassed by the claims.

Claims (9)

1. An intraluminal device comprising a radially expandable tubular stent having inner and outer surfaces extending in the direction of the longitudinal axis of the stent; and a stent cover, the stent cover being arranged around the stent, being made of a substantially one-dimensionally oriented polymer, being oriented in a first direction and stretched in a second direction transverse to the first, the length of the stent cover being reduced from the original length, and the longitudinal axis of the stent being aligned with the second direction such that upon radial expansion of the stent, the stent cover is expandable in the first direction in the original length and limits the radial expansion of the stent. 2. The intraluminal device of claim 1, wherein the stent is expandable from a compressed state allowing intraluminal deployment to a second, expanded state allowing intraluminal deployment. 3. The intraluminal device of claim 1 or 2, wherein the stretched stent cover is expandable in the first direction to the original length. 4. An intraluminal device according to any preceding claim, wherein the one-dimensionally directed polymer comprises unsintered ePTFE. 5. An intraluminal device according to any preceding claim, wherein the stent covering consists of a longitudinal segment of unsintered ePTFE. 6. An intraluminal device according to any preceding claim, wherein a longitudinal segment is disposed around the stent and joined at a seam formed by abutting overlapping edges of the segment. 7. An intraluminal device according to any preceding claim, wherein the suture is formed by compressing and securing overlapping edges. 8. An intraluminal device according to any preceding claim, wherein overlapping edges are glued. 9. An intraluminal device according to any preceding claim, wherein the segment is wrapped externally around the stent.