CA2371964C

CA2371964C - Partial encapsulation of stents

Info

Publication number: CA2371964C
Application number: CA002371964A
Authority: CA
Inventors: Richard Layne; Sandra Cundy; Debra Bebb
Original assignee: Bard Peripheral Vascular Inc
Current assignee: Bard Peripheral Vascular Inc
Priority date: 1999-02-02
Filing date: 2000-02-02
Publication date: 2008-10-28
Anticipated expiration: 2020-02-02
Also published as: WO2000045741A1; DE60002161T3; US20110126966A1; US20040236402A1; US8617337B2; US6398803B1; JP4248151B2; EP1148843B2; EP1148843A1; DE60002161T2; US20010032009A1; EP1148843B1; CA2371964A1; US20140107763A1; US7914639B2; US10213328B2; ES2195883T3; DE60002161D1; MXPA01007790A; US20090294035A1

Abstract

Partially encapsulated stents are made using gaps cut into ePTFE covering material. Ring stents are placed over an inner ePTFE tube (e.g., supported on a mandrel) and are covered by a "lacey" graft sleev e, which is constructed by cutting apertures into an ePTFE tube so that a series of circumferential and longitudinal strips is created. This "lacey" sleeve is then laminated to the inner ePTFE tube to capture the stents. By selecting the size and position of the apertures i n the ePTFE covering, it is possible to leave critical parts of the stent unencapsulated to facilitate flexibility and expansion. Alternatively, the gaps can consist of slits cut into the ePTFE covering material. These slits can be cut in any direction including longitudinally, radially, or diagonally. In addition, the slits can be spaced at varying intervals around the covering material to maximize flexibility and expandability. Although a single stent can be used, these approaches lend themselves to use of a plurality of individual ring stents spaced apart alon g the inner ePTFE tube.

Description

PARTIAL ENCAPSULATION OF STENTS

1. Field of the Invention The present invention relates generally to the field of medical devices, and more particularly, to encapsulation of stents.

2. Description of Related Art Stents and related endoluminal devices are currently used by medical practitioners to treat portions of the vascular system that become so narrowed that blood flow is restricted. Stents are tubular structures, usually of metal, which are radially expandable to hold a narrowed blood vessel in an open configuration.
Such narrowing (stenosis) occurs, for example, as a result of the disease process known as arteriosclerosis. Angioplasty of a coronary artery to correct arteriosclerosis may stimulate excess tissue proliferation which then blocks (restenosis) the newly reopened vessel. While stents are most often used to "prop open" blood vessels, they. can also be used to reinforce collapsed or narrowed tubular structures in the respiratory system, the reproductive system, biliary ducts or any other tubular body structure. However, stents are generally mesh-like so that endothelial and other cells can grow through the openings resulting in restenosis of the vessel.
Polytetrafluoroethylene (PTFE) has proven unusually advantageous as a material from which to fabricate blood vessel grafts or prostheses used to replace damaged or diseased vessels. This is partially because PTFE is extremely biocompatible causing little or no immunogenic reaction when placed within the human body. This is also because in its preferred form, expanded PTFE (ePTFE), the material is light and porous and is potentially colonized by living cells becoming a permanent part of the body. The process of making ePTFE of vascular graft grade is well known to one of ordinary skill in the art. Suffice it to say that the critical step in this process is the expansion of PTFE into ePTFE following extrusion from a ' 1-rt1 -2001 CA 02371964 2001-07-14 US 000002884 ~

paste of crystalline PTFE particles. Expansion represents a controlled longitudinal strctching in which the PTFE is stretahed up to several hundred pement of i.ts original length. During the expansion process fibrils of PTFE are drawn out of aggtegated PTFE particle (nodes), thereby creating a porous structure.
If stents could be enclosed in e,PTFE, cellular infiltration could be limited, hopefully preventing or limiting restenosis. Early attempts to produce a stent enshrouded with ePTFE focused around use of adhesives or physical attachment such as suturing (see for example TJ_S. Patent No. 5,405,377 to Cragg).
However, such methods are far from ideal, and suturing, in particular, is very labor interasive.
More recently, methods have been developed fbr encapsulating a stent between two tubular ePTFE members whereby the ePTFE of one-member contacts and bonds to the ePTFE of the other member through the openings in the stent. These methods are disclosed in recent publicatiobs WO 98/38947 and WO 96/28115. However, such a monolithically encapsulated stent tends to be rather ih,flexible. In particular, radial expansion of the stent may stress and tear the ePTFE. There is a continuing need for a stent that is encatpsulated to prevent ceilular intrusion and to provide a smooth inner surface blood flow and yet still capable of expansion without tearing or =k delaminating and is relatively more flexz-ble.

S11QMRY OF 'S"HE INVBNTZON
The present invention is directed to encapsulated stents wherein flexibility of the stent is retained, despite encapsulation.
It is aa object of this invention to provide a stent device that has improved flexibility, yet maintains its shape upotx expansion.
It is also an object of this inventi on to provide a stent encapsulated to prevent cellular infiltration wherein portion,s of the stent can move during radial expansion without stressing, or tearing the encapsulating material, These and additional objects are accomplished by an encapsulation process that leaves portions of the stent free to move duti.v,g expausion without damaging the ePTFE covering. The most basic form of this invention is produced by placing a stent over an inner ePTFE member (e.g., supported on a mandrel) and then covering the outer surface of the stent with an outer ePTFE tube into which slits have been AMENDED SHEET

.3 cut. The outer ePTFE tube is then latninated to the inner ePTFE through openings in the stent structura to capture the- steat, By selecting the size and location of the slits it is possible to leave critical parts of the stent unencapsulated to facili tate flexibility and expansion. Not only does the slit prevent capture of the uaderlying PTFE, it forms a focal point for the PTFE to fl,ex. A more complex form of the process is to place over the stent an ep'TFE sleeve into which apertures have been cut. This "lacey" outer sleeve leaves poztions of the stent exposed for increased #l=bility and for movement of the stent portions dwring expansion without damaging the ePTFE.
Although a single stent can be used, these approaches lend themselves to use of a plurality of individnal ring stents spaced apatt along an ianer ePTFE tube and covered by a"lacey" ePTFE sleeve, In the present i.nvention, individual ring stents are patlially e,ucapsuiated using the procedure outlined above. Preferably, ring stents of zigzag si.nusoidat structuze a-te placed "in phase" (e.g., peaks and valleys of one steut aligtied with those of a neighboririg stent) on the surface of a tubuIar ePTFE gra,ft supported by a mandrel. A sleeve of ePTFE is cut using COZ laser so that opaaiztgs aze created, resulting in a"la4ey ' pattera. This "lacey" sleeve is then placed over the ring stents.
The resulting stcucture is then subjected to heat and pressure so that regioas of ePTFE become I=niuated or fused together wlaere the lacey sleeve contacts the tubular gxa.fk Iu addition, the ends of the stent can be completely encapsWated, by known methods, to stabilize the overall structure.
A more complete understanding of the encapsulation process will be 'afforded to those skilled in the art, as well as a realiza.tion of additional advautages and objects thereof, by a consideration of the following detaided description of the preferred embodiment. R.eference will be made to the appended sheets of drawings which will first be descnbed briefly.

B=DESCRIl'TION OF THE DRAWIN,GS
Fig. 1 is a perspective view of a tubular e.PTFE menmber wXth individug r~g stents arranged thereon.
Fig. 2 is a perspective view of the "lacey" sleeve of the present invention.
Fig. 3 is a perspective view of the sleeve in Fig, 2 placed over the structvre AMENDED SHEET

of Fig. 1.
Fig. 4 is a perspective view of one configusation of the slitted sleeve of the present invention with longitudiaally o;riented slits.
Fig. 5 is a perspective view of a second configuration of the slitted sleeve of the present invention with circumferentially orieated slits.
Fig. 6 is a perspective view of a third confignratiion of the slitted sleeve as it is placed over the stru,cture in Fig. 1.

DETAILED DESCRIPTZON QF THE PREFX D EMBODIlVLENT
The present invention satisftes the need for an encapsulated stent device to prevent restenosis that is flcxible upon expansion and contraction so that the general stnuturel form is retained. This is accomplished encapsulating a stamt or a plurality of stent rings using an ePTFE covering into which openings have been cut.
Refeiring now to the drawings, in which lOce reference numbers represeat similar or identical structures tluougbout, Fig. 1 illustrates, an initial step in constructing the partially encapsulated stent of th.e present inveation. A
tubular ePTFE graft 20 is placed over a mandrel for the assembly of a device 10 (Fig.

3). A
stent is then, placed over the graft 20. In a preferred embodiment, as shown in Fig_ 1, a series of zigzag sinusoidal ring stents 30 are placed over the outer surface of the graft 20. Al,ternatively, one or more stents wherein each st:ent comprises m,ore than one ring or hoop (e,g., where the rings are helically connected) can be used.
The ring stents 30 can be made of any mater;a] but a prefeaed material is metal.lbe zigzag ring stents 30 may be assembled "in phase" with each adjacent ring stent having peaks and valleys aligned. Altermatively, the individual stents 30 can be "out of phase" to different degrees. It will be apparent that the phase relation of adjace,at stents 30 will alter the lateral flexibility as well as the lottgitudinal compressibility of the structure. The phase rela4ionship can be varied along the length of the device 10, thereby altering the physical properties in different portions of the device 10, Iiaviug individual ring stents 30, as opposed to a single tubular stent, provides the advantage that the periodicity, or the number and precise shape of the zigzags per ring, can readily be varied along the length of the graft to influence flexibility and stability properties of the structure. Also, spacing of the individual stents (number of stents AMENDED SHEET

11"01 -2001 CA 02371964 2001-07-14 US 000002884 per unit length) as well as the phase relationship of stent to stent can be varied to produce stent gra.fts with desired properties. By placing the ring stents 30 over thc outer surface of the tubular ePTFE graft 20, the resulting sttuctiae has an inner (huminal) surface that is completely smooth to facilitate the flow of blood.
However, 5 there may be instances where the ring stents 30 or other tubular stents are advantageously placed in contact with the inner graft surface or on both the inner and outer surfaces, as one of ordinary sldll in the art will readily appreciate.
Fig, 2 shows the sttucture of a"Iacey" graft 40 comprising a sleeve of ePTFE
42 into which apefiures have been cut. This 'lacey" graft 40 is placed over the ring stents 30 in the preferzed embodiment. The "lacey" graft 40 is created by cutting openings 44 in a tubular ePTFB sleeve 42. The openings 44 were cut into the sleeve by a COa laser, although any other cutting technology could readily be employed.
The "Iacey" graft 40 is slid over the ring stents 30 and the underlying tubular graft to foim the preferred device 10 shown in Fig. 3. The device 10 is then exposed to .15 heat and pressure, such as that caused by wrapping with PTFE tape followed by heating in an oven, thereby causiAg the ePTFE regions of the "lacey" gaft 40 to fuse or laminate to the tubular graft 20 wh.exever they touch each other. It should be appreciated that the circumferential sections of ePTFE 46 that are placed over the ring stents 30 can encompass many different designs. As illustrated., a sleeve 42 with 20 openings 44 cut out is one way of accomplishing the goal of flexibility and stability.
The openings 44 between the circumferential sections of ePTFE 46 can be altered to control the degree of flexibility and stability desired. In the prefeired embodiment shown in Fig. 3, the "lacey" graft 40 forms a number of circumferential sections 46, which are intended to cover a portion of the circamference of each ring stent 30, leaving the ends of the zigzags uncovered. By circwmferentially covering only a portion of each ring stent 30, the maximum amount of latexal flexibility is provided.
However, circumferentially covering the individual ring stents 30 without any longitudinal support would result in a structure with little longitudinal strength and stability that would be prone to "telescoping". Thus, the longitudinal sections 48 that connect the ciscumferential sections of ePTFB 46 are unportant, because the longitudinal sections 48 are completely laminated to the underlying graft 20 and act as "anti-compression" devices by resisting the shortening of the structure 10 (the AMENDED SHEET

_~ , . -11-01 _e~001 CA 02371964 2001-07-14 ~J I i~7 G+ 000002884 double thickness of ePTFE resists telescoping of the longitudinal sections 48). The width of the circumferential sections 46 and the longitudinal sections 48 control longitudinal strength and stability versus lateral flexibility. By adjusting these parameters, grafts can be made more or less flexible with greater or lesser anti-compression strength. In the pxefe,rred embodimeat, four longitudinal sections 48 are formed and the ends of the structure 10 are completely encapsulated for greater stability. Of course, a larger number of longitudinal sections 48 could be formed.
Also the longitudinal sections 48 may themselves zigzag or may be helically az-anged depending on how the opeaings 44 are cut into the sleeve 42. Each different structure a*iIl possess different properties. Similarly, the circumferential ',- sections 46 can have different forms and may be undulating. There is nothing to preclude a covering with a more complex pattern where circumferential sections and longitudinal sections are difficu.it to discern or are even nonexistent, A second embodiment of the present invention can be seen in Figs. 4-6.
Instead of having a"lacey" graft structure, a slitted outer sleeve is used to provide partial encapsulation of the stent, the slits providing flexibility to the sttuctwre, allowing the stent to expand and retract more readily. In Fig. 4, four longitudinal slits 52 run the length of the stent, leaving 5 to 10mm of uncut sleeve at the ends.
The slits are forrned at 0 , 90 , 180 , and 270 , and are oriented to pass over a peak portion of each zigzag ring stent 30 (Fig. 6). Fig. 5 shows circumferential slits 62, wherein slits are cut circumfermtia.Ily around the sleeve 60 at spaced intervals, preferably to coincide with a stent ring. At each radial section, two slits = -eut around the circuniference at evenly spaced intervals. In a.first radial section, the slits span from 0 to 90 and from 180 to 270 . Each suecessive radial section has a pair of slits which are offset 90 from the previous pair. Thus, a second radial section will have slits spaaning from 90 to 1800 and frox,n 270 to 00. Beside the configurations shown in Figs. 4 and 5, a number of other slit con$gurations are possible, including diagonal and sinusoidal as will be appreciated by one skilled in the art. As shown in. Fig. 6, a sleeve 70 is placed over the ring stents 30 and the underlying tubular graft 20 to form a new structure 80. The longitudinal slits 7.2, which are cut into sleeve 70, differ from the slits 52 shown in Fig. 4 in that they do not span the length of the structure 80 and are staggered around the circumferexlce of AMENDED SHEET

the sleeve 70. Xdeally, the slits are aligned over the peaks in the zigzag ring stents 30.
Once the slits 72 are cut into the sleeve 70 using any of the Imown methods, tiae structure 80 is exposed to heat and pressure, such as that caused by wrapping with PTFE tape and heating in an oveu, thereby causing the ePTFE regions of the slitted graft 70 to fuse or laminate to the tubular graft 20. The slits 72 in the slitted outer sleeve 70 can be formed by using a COz laser, razor blade or any other suitable technique known in the art. The slits enhance the flexibility of the encapsulated stntcture and allow radial expaasion without tearing of the ePTFE. Iu addition, a plurality of slits help the expanded graft to grip onto the vessel wall. This is particularly important where an encapsulated stent graft is spanning a region of damaged or weakened vessel as in an anevxysm. Ftuther, during the healirxg process tissues readily grow into the slits further anchoring the graft to the vessel wall.
An advantage that cutting slits into an ePTFE sleeve offers is that it is somewhat easier to manuufacture ffian is the "lacey" graft. Because no material is removed the slee~ve is sa-mewhat stronger than a"Xacey graft". There aze a multitude of configvrations possible, including cutting the slits in asymmetric fashion to achieve desired results, such as using radial, longitudinal and diagonal cuts simultaneously. Moreover, a greater number of slits can be cut into a region of the structure in which gr4axer expansion is desired.
Although the above examples are described vrith the "Iacey ' and slitted grafts being placed over a stent which is itself placed over a tubular graft, this orientatioti can be readily reversed. That is, the 'lacey" or slitted grafts can be placed on a zuaudrel; a steat or stents can be then placed over the "lacey" or slitted grafts, and a tubular graft can be then placed over the stent or stents. This results in a structure wherein part or much of the lum,inal surface is provided by the outer graft, resulting in superior healing as only a single layer of ePTFE would separa.te body tissues from the blood. SimilarYv. a strueturP witt, twn ~s~p~--..,- Pr.+fea structure would have a smaller profile when coxnpreased because the overall amount of PTFE is reduced. Likewise, a combination of the "lacey" graft and slitted grafft could be employed.
Having thus described preferred ernboduaents of the partial encspsulation of stents, it will be apparent by those skilled in the art how certain advantages of the piroseat invention have been acttieved. It should also be appreciated that vaiious modifications, adsptations, and alternative embod'uaents thereof may be made withia the scope and spirit of the present invention Frnr example, zigzag steat xings have been illustrated, but it should be apparent t3iat the inventive coneepts described above would be equally applicable to sinusoidal and other stent designs. The descrtbed embodiments are to be considered illustrative rather than restricdve. The invention is fiutb.er deSned by the following claims.

AMENDED SHEET

Claims

WHAT IS CLAIMED IS:

1. A radially expandable reinforced vascular graft, comprising a first and a second expanded polytetrafluoroethylene layer and a radially expandable support layer comprising at least one stent, wherein the support layer is in contact with a surface of the first expanded polytetrafluoroethylene layer and is secured thereto by the second expanded polytetrafluoroethylene layer, characterized in that at least one of the expanded polytetrafluoroethylene layers includes a plurality of spaced apart apertures cut directly therein, adapted to leave at least a portion of the support layer unsecured.

2. The radially expandable reinforced vascular graft according to claim 1, wherein the second expanded polytetrafluoroethylene layer includes said plurality of spaced apart apertures.

3. The radially expandable reinforced vascular graft according to claim 1, wherein both the first and second expanded polytetrafluoroethylene layers include a plurality of spaced apart apertures wherein the apertures in the first expanded polytetrafluoroethylene layer are adapted to be aligned out of phase with the apertures in the second expanded polytetrafluoroethylene layer.

4. A radially expandable reinforced vascular graft according to any one of claims 1 through 3, wherein the apertures further comprise slits.

5. The radially expandable reinforced vascular graft according to claim 3, wherein the apertures in the first or second expanded polytetrafluoroethylene layer further comprise slits.

6. A radially expandable reinforced vascular graft as in claims 4 or 5, wherein the slits are oriented longitudinally.

7. The radially expandable reinforced vascular graft as in claims 4 or 5, wherein the slits are oriented circumferentially.

8. A radially expandable reinforced vascular graft according to any one of claims 1 through 7, wherein the radially expandable support layer comprises a plurality of ring stents.

9. The radially expandable reinforced vascular graft according to claim 8, wherein each of the ring stents is formed in a zigzag pattern of alternating peaks and valleys.

10. The radially expandable reinforced vascular graft according to claim 9, wherein the zigzag ring stents are adapted to be aligned with the alternating peaks and valleys in phase.

11. The radially expandable reinforced vascular graft according to any one of claims 1 through 10, wherein the stent is made of metal.

12. The radially expandable reinforced vascular graft according to any one of claims 1 through 11, wherein at least one end of the radially expandable reinforced vascular graft is fully encapsulated.

13. A method for making a partially encapsulated radially expandable reinforced vascular graft, comprising providing a first expanded polytetrafluoroethylene layer of material, providing a second expanded polytetrafluoroethylene layer of material, disposing a radially expandable support layer comprising at least one stent over the first expanded polytetrafluoroethylene layer, placing the second expanded polytetrafluoroethylene layer over the radially expandable support layer, and laminating the second expanded polytetrafluoroethylene layer to the first expanded polytetrafluoroethylene layer, characterized by cutting a plurality of apertures into at least one of the expanded polytetrafluoroethylene layers and positioning the apertures with respect to the support layer, leaving a portion of the support layer exposed through the apertures.

14. The method according to claim 13, wherein the radially expandable support layer comprises a plurality of ring stents formed in a zigzag pattern of alternating peaks and valleys, wherein the disposing step further comprises disposing the peaks and valleys of successive stents in phase.

15. The method as in claims 13 or 14, further comprising the step of fully encapsulating at least one end of the radially expandable reinforced vascular graft.

16. A method for making a partially encapsulated radially expandable reinforced vascular graft, comprising providing a first expanded polytetrafluoroethylene layer of material, providing a second expanded polytetrafluoroethylene layer of material, disposing a radially expandable support layer comprising at least one stent over the first expanded polytetrafluoroethylene layer, placing the second expanded polytetrafluoroethylene layer over the radially expandable support layer, and laminating the second expanded polytetrafluoroethylene layer to the first expanded polytetrafluoroethylene layer, characterized by cutting a plurality of slits into at least one of the tubular expanded polytetrafluoroethylene layers and positioning the slits to span at least a portion of the radially expandable support layer.

17. The method according to claim 16, wherein the radially expandable support layer comprises a plurality of ring stents formed in a zigzag pattern of alternating peaks and valleys, wherein the disposing step further comprises disposing said peaks and valleys of successive stents in phase.

18. The method as in claims 16 or 17, further comprising the step of fully encapsulating at least one end of the radially expandable reinforced vascular graft.

19. A radially expandable reinforced vascular graft exhibiting improved flexibility, comprising:

an expanded polytetrafluoroethylene layer;
a radially expandable support layer comprising at least one stent, wherein said support layer is in contact with a surface of the expanded polytetrafluoroethylene layer;
and an expanded polytetrafluoroethylene tube, comprising a tube wall and a plurality of openings, each of the openings extending through the tube wall comprising a longitudinal component and a circumferential component, wherein the length of the longitudinal component is less than the total length of the tube, and wherein the length of the circumferential component is less than 360°;
wherein the radial expandable support layer is secured between the expanded polytetrafluoroethylene layer and the expanded polytetrafluoroethylene tube, and wherein at least a portion of the support layer is positioned within the openings.

20. The radially expandable reinforced vascular graft of claim 19, wherein said radially expandable support layer comprises a plurality of ring stents.

21. The radially expandable reinforced vascular graft of claim 20, wherein each of said ring stents is formed in a zigzag pattern of alternating peaks and valleys.

22. The radially expandable reinforced vascular graft of claim 21, wherein said zigzag ring stents are disposed with the alternating peaks and valleys in phase.

23. The radially expandable reinforced vascular graft of claim 19, wherein said stent is made of metal.

24. The radially expandable reinforced vascular graft of claim 19, wherein an end of said radially expandable reinforced vascular graft is fully encapsulated.

25. The radially expandable reinforced vascular graft of claim 19, wherein the openings comprise similarly shaped apertures, further comprising a ring of said apertures at a distinct point along the length of the expanded polytetrafluoroethylene tube.

26. The radially expandable reinforced vascular graft of claim 25, further comprising a plurality of rings of apertures along the length of the expanded polytetrafluoroethylene tube.

27. The radially expandable reinforced vascular graft of claim 26, wherein the apertures are rectangular in shape and wherein each ring is comprised of at least three apertures.