CA2357650C

CA2357650C - Stent with optimal strength and radiopacity characteristics

Info

Publication number: CA2357650C
Application number: CA002357650A
Authority: CA
Inventors: Robert E. Fischell; David R. Fischell; Janet Burpee
Original assignee: Cordis Corp
Current assignee: Cardinal Health 529 LLC
Priority date: 2000-09-22
Filing date: 2001-09-19
Publication date: 2009-04-07
Anticipated expiration: 2021-09-19
Also published as: CA2651808A1; JP2011177557A; US8454677B2; CA2391624C; EP1190685A2; US20020058989A1; TW539559B; US20020072792A1; EP1399091A2; AU2005205781B2; WO2002024111A2; KR100815360B1; US6699278B2; AU773171B2; MXPA02005126A; AU8912801A; DE60123187T2; JP2004522463A; JP2002102358A; WO2002024111A3

Abstract

A stent in the form of a thin-walled, multi-cellular tubular structure is provided. The tubular structure has a longitudinal axis and the stent includes a plurality of circumferential sets of strut members. Each set of the strut members is longitudinally displaced from each other and connected to each other by longitudinally extending links. Each set of the strut members forms a closed and cylindrical portion of the stent. Further, each set of the strut members includes a plurality of connected curve sections and diagonal sections. The sets of the strut members further include end sets of strut members located at each end of the stent and central sets of strut members located between the end sets of the strut members. The diagonal sections of the end sets of the strut members have a center portion and two ends. At least one of the diagonal sections of the end sets of the strut members includes a tapered shape with width of one diagonal section is greater at the center of the diagonal section than the width at either end of the diagonal section.

Description

STENT NVITH OPTIMAL STRE_A'GTH AND RADIOPaCITY CHARACTERISTICS
FIEI_D OF JSE

This invention is in the field o~'stents foi- implantation into a vessel of a human body.
BACKGRO[:ND OF THE INti'ENTION

Stents are well known medical devices that are used for maintaining the patency of a larl-e variety of vessels of the human bodv. A more fi-equent use is for implantation into the coronai-y vasculature. Althoulgh st nts have heen used for this purpose for more than ten vears, and some current stent designs such as the CORDIS BX Velocity IRD
stent, Cordis Corporation, Miami Lakes, FL, have the required flexibility and radial rigidity to provide an excellent clinical result. they are not alwavs clearly seen under standard fluoroscopy.

Many current tubular stentS use a multiplicity of circumferential sets of stnit members eonnected by either strail-ht longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connec.ted to curved sections forming a closed-ring, zicy-zac, structure. This structure opens up as the stent expands to form the element in the stent that provides stnictural support for the arterial wa11. A sin~le strut member can be thought of as a diagonal section connected to a cur~,ed section within one of the circumferential sets of strut members. In current stent designs such as the B.X Velocity ~,.~ stent, these sets of stnit members are formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. Although a stent with uniform width of the strut mernbers will function, if the width is inereased to add strength or radiopacity, the sets of stnit members ill zxperience increaseci strain upon z.rpansion. Hi-~-,h strain can cause cracking of the metal and potential tati'~ue failure of the stent under the cyclic stress of a beating hean.

Existin,- hi~hlv radiopadut' stents, such as the ;oid plated NLROYAL stent sold bv Boston Scientific, Inc., Natick MA. can obscure the inside of the vessel due to the hi;h radiopacity over the entire lengt;1 of the stent. The BeStertt sold by Vledtronic. Inc., Vlinneapolis %1N, has small Lyold r;iarkers at the ends of the stent. Those markers only mark an end point without allo\ving visualization ot the cntire end set of stnrt members.

Fischell et al, in L~S Patent No. 6.086,604, discloses a stent with the end sets of strut members bein- gold plated. Such a stent would have ideal radiopacity but may be subject to the corrosive effects incurred tiLrough pfacement of dissimilar metals in an electroi%-tic solution such as blood. There has also been significant evidence that (yold is a poor surface material for stents because it mav increase the risk of subacute thrombosis or restenosis.
Further, Fischell et al, in US Patent No. 5,697,971 discloses in its FIG. 7, a stainless steel stent with inereased width diagonal sections in all the circumferential sets of stnit men7bers.
SUMMARY OF THE NVENTION

An ideallv radiopaque stent would have end sets of strut members that are hichly radiopaque so that they can be readily seen, everi using low power fluoroscopy, and would further contain a central section that is visible but not too bright so as to obscure the Iumen when high power cine film angio7rarns are taken. The stent should also have onlv one material on its outside surface to avoid potential corrosion; that rnaterial should not promote subacute thrombosis or restenosis.

The pi-esent inv~ntion _s a stent that iS desig-ned to have optimal strength and radiopacitv ~vith good biocompatibihtv. Unfonunately, the choices of appropriate biocompatible metals avail.ablc ,~s thir, wail tubing for stent constniction are somewhat linlited. To achieve optimai rad.iopacitv, the stent design of the present invention is adjusted to the specific radiopacitv and strenyth characteristics of the metal from whieh the stent is fabricated. What is more, coati~igs such as parylene may be needed to avoid con-osion from stents with less biocompatible materials and'or dissimilar metals on the stent's outer surface.
Of extreme importance to the prest,nt invention is the achievement of optimal radiopaciry in a stent that ideallv is only 0.004 inclles wall thickness or less. Such a stent would have a pre-deployment outer diameter (protile) that would be at least 0.003 inches less than currentlv marketed stents. Ideally, the steni described herein would have a wall thickness between 0.0025 inches and 0.004 inches.

Described herein are the novel desi-n elements for stents formed frorn the followin~
materials:

1. A highly radiopaque metal such as tantalum;

2. Metals somewhat more radiopaque than stainless steel, such as the cobalt based alloy L605;

3. Stents coated or plated %%,-ith highly radiopaque materials like gold; and

4. Layered materials such as alternative layers of tantalum and stainless steel.

~. The novel design elements tiiat are described l,,erein include:

1. Taoered stn_~.t width ror stents tormed from highlv radiopaque metals.
Althou~h reducin~T the wdth of the lon-itudinall", dia;onal section alone will reduce radiopacity without significantly affectin2 radial stren,~zth, by having a taper on the curved sections of the cii-cumferential sets of strut members, a greatly reduced level of strain upon stent esparision can be achieved without sacrificing radial strength.
This is extremelv important, as it allows a stent to he made much stronger than a stent ,,vith unifoinl %%idth of the strut rnembers while staving %vithin the same stra.in limit for the material.

Tantalum is a nletal that has been used in stents; ~vhich metal is highly radiopaque.
The optimal radiopacity for a stent design using tarttalum could have uniform width for the circumferential sets of strut members and a waJ thicl:ness of about 0.0025 iriches. To provide more radial strength and to reduce the probability of the stent ends flaring out during deployment, a wall thickness of about 0.003 inches to 0.035 inches would be hi;hly desirable. With uniform width sets of strut members, a 0.035 inches wall thiclcness tantalLun stent would be too briclht under cine an-iography. To reduce the r-adiopacity of the design without significantly impacting the raciial strength of the deployed stent, the present invention envisions curved sections and diagonal sections, either or both of which could ha~e a variable or tapered width. The curved sections should be tapered (wider at the center compared to the ends) to reduce sti-ain as previously described. 'The longitudinally diagonal sections can be thinner in the center than at the ends, to reduce radiopacity for the central sets of strut members.

~

It is envisioned that thc- ~7o%zf stent described herein mi-ght have wider diagonal sections for the end sets of stnit members as compared to the central sets of stntt members.
This feature would enharce thz -adiopacity of the ~~nd sets ofstn.it members ,vhile retainin, a moderate level of radiopacity for the central sets of stntt members. It is also envisioned to have both reduced width diagonals and.'or reduced wall thickness for the central sets of stnlt members. It should be rememberec that it is tluoroscopic visualization of the end sets of stnit members that is most important for visuaiizinc, sterits placed inside a coronary artery.

2. Thicker dia<_~:onal sections for metals with radiopacity sli2htlv better than stainless steel. The cobalt/tungsten alloy L605 is a stronger and more radiopaque metal compai-ed to stainless steel. To achieve optimal radiopacity usinc, L605 with uniform width setS of stnit members, the wall thickness is optinially equal to or greater than 0.0045 inches. 'To provide optimal radi.opacity with such a metal in stents of wall thickness 0.004 inches or less, the present invention envisions wider diagonal sections in the sets of stnit members. Thus, the tapered diagonal sections wouid be wi(der tlian the cur%-ed sections. The tapered curved section design for reduced strain may also be hi<-,hlv desirable ;:or stents made from the L605 alloy.

3. End sets of_stnit memberswith thinner curved sections. Stent deliverability into curved coronary arteries is inlproved when the diagonal sections of the end sets of strut members have a decreased length as compared to the length of the diagonal sections of the central sets or str-ut niembers. A shorter len,,)rth of the diagonal sections will also reduce outwai-d flaring upon expansion of the stent.
Decreasing end flaring of the deployed stent is of particular importance for stents having very thin walls.

>

Previous desi,ns thut desct ibe a stent ',~ ith shorter diavonal sections in the end sets of strut me-nbers are limited bv the strain limit a11o,,ved for the end sets of strut men-ibers. As a result. if the end sets of strut members are made as strong as possible ~vhile being limited by the maximum allowable strain ['(:)r that metal, the :;entral sets of strut members will not have optimized radial strength_ The present invention envisions optimizing the radial strength for all sets of stnit members, i.e., the metal in all sets of strut members just reach the maximum allowable strain at the limiting diameter for the stent's expansion. To achieve this desired atti-ibute, the stent described herein has the curved sections of the end sets of strut members bein,-, less \vide than the curved sectiorls of the central sets of strut members.

4. Good side branch arterial access .%hiie rnaintainina small cell size.
The stents described herein are tv-pically closeci cell stents, having a curved section of a central set of stnit mernbers connected to an adjacent set of stnit members by a longitudinally extending hnk. In one embo(liment of the present invention, the circumferential sets of strut members are joined by undulating longi.tudinal connectin; links with each link having a multiplicity of curved segments so as to increase the perimeter of the stent's closed cells. One aspect of the present invention is that the perimeter of each of the stent's closed cells should be at least 9 mm long.
Thi.s design parameter allo,ws each cell of the stent to be expanded to a circular diameter of approxinlately 3 mm (i.e., 9/n mm -- 3 mm). This feature allows the '`unjailing" of side branches of the artery into which the stent is placed.
The ideal desian to be radiallv strong, prevent plaque prolapse and still allow sidebranch access will have a maximum deployed cell area of less than 0.005 in.2 while havin, a cell perimeter that is at least 9 ntni in length, so as to allow unjailing of side branches. A
;ood cell for side branch access should have a perimeter lencyth between 9 mm and 11 mm. (i.e. an expandablc ;irc.u!ar (diameter bet%veen 2.S6 mm and 3.5 mm). Cell perimeters berween 9.5 and :i) n,.m are optimal.

5. Flexible tlndulatin~-Y lon~iitudinal links with good support between a(iiacent sets of strut mertlbers. To provide a strong bridge connection between adjacent circumferential sets of stnit members, the flexible undulating longitudinal connectin,-) links should !la~ ne-.rlv equal extension in the circumferential direction on each side of a line drativn between the attachment poirits of the flexible undulatinc, lon,gitudinal connecting linl: to the curved sections of adjacent sets of strut members.
"N" and inverted "N" sliapes for the connecting links inherently have equal cireumferential displaceme:it on each side of the line connecting their attachment points. The specially designed "M" or "'vV" shapes of the present invention also provide this desirable attribute. Nearly equal circumferential len~ths on either side of a line drawn between the attachment points of the flexible undulating longitudinal connecting links help in prcventing plaque from pushing the "M" or "W" shaped link imvard into the lumen of the stent when the stent is deployed into an artery.

The "M" and "W" shapes ar'e of particular advantage in obtaining the desired attribute of small area cells that have good side branch access capability because of an increased perimeter length. It should also be understood that the -`M" and "W" shapes each add an additional half cycle of undulatitl-, link 1enlgth to the cell perimeter as compared to an "N"
shaped link design, thus improving the stent's longitudinal tlexibility. It should also be noted that a "W" link is simply an inverted "iti1" link.

6. Variable thickness radiopaque coatings. The NIROYAL TIM stent has a tuZiform thickness of ;o1d platin~-,, which makes the center too radiopaque as compared to the radiopacit,, of the end sets of strut mennbers. Fischel[ et al.. US
Patent No. 6,0S6,6i)4, tet.ches stents ha%in- Uoid placed at the end sets of strut members. This creates a potential for corrosion frorYt dissimilar metals, namelv, ~,o1d and stainless steel. The present invention envisions a gold coating that is sufficientl%thick on the end sets ot strut m.embers to provide optimai radiopacity -with a thin coating of -old on the rest of the stent. This design prevents obscuring of the arterial lunlen while providing an exterior surface. for the stent that is a single metal, thus avoiding electrolyZic corrosion.

7. Polvmer coatinLgs for stents coated with. gold or havin2 dissimilar metal surfaces. For stents with non-biocompatible or dissinlilar metals, the present invention envisions the us~; of a polymer such as parylene to coat the entire outer surface of the stent. Thi.s would improve biocompatibility and also allow attachment of oraanic compounds such as lleparin or phospllorylcholine for reduced thrombovenicity or drugs. such as taxol or rapamycin, for reduced cell proliferation and a decreased rate of restenosis. It is also known that highl,v radiopaque rnaterials like tungsten can be mixed into polymers. A stent coatincy including a plastic with mixed in radiopaque metal could be ased to enhance both radiopacity and biocompatibility. Such a polymer coating could also be advantageous with a-old coated stent.

8. Providiu a variable wall thickness. The present invention also envisioils next generation manufacturing techniques using photo-etching, whereby a stent pattem is etched into a thin-walled metal tube. These techniques already can produce variations in wall thickness as well as strut width for any stent pattern. The present invention envisions use of these techniques to create stents with optimal ~

radiopacity. In particular to~ a stent forrneci from a single metal or alloy, thicker rnetal at eacll end of th,~ stent could increase radiopaci.ty there as compared to the central section of the ster.t. Pcrhaps more important is the use of multi-thickness etching techniques with a two- or three- layered tube where one of the layers is a highly radiopaque material such as tantalum. For example, a two-layer tube havincy one layer of stainless steel and a second layer of tantalurn could be etched to provide the end sets of strut membCrs with 0.001 inches of tantalum and 0.0025 inches of stainless steel while the i-ernainder of the stent would have less than 0.0005 inches of tantalum with a stainless steel layer of 0.003 inches. It is also envisioneci that there could be tantaluni only on the ertd sets of strut members. Thus, one could produce a stent with enhanced radiopacity at the ends with the stent having a uniform wall thickness.

One could even have a stent with increased wall thickness of a metal at the central region of the stent but still having a ciecreased radiopacity at that central region if, for example, the stent had tantalum end struts with stainless steel center struts.
Such a stent would be strongest in the center where the thickest plaque must be restrained.

It is also envisioned that any of the above optimal radiopacity stent designs may be used with plastic coatin(ys such as parylene, antithrombogenic coatings such as heparin or phosphorylcholine, or anti-proliferative coatings such as taxol or rapamycin.

Thus it is an object of the present invention to have a stent with tapered curved sections, the center of the curved sections being wider than ends of the curved sections so as to reduce plastic strain as the stent is expanded as compared to a curved section with uniform width.

~~

.another object of the prcsent invention is to have a stent with tapered diagonal sections in the sets of strut membcrs %vhere the center of the dia~ronal section is narrower than the ends to reduce the radiopaci:%, of central sets of strut membet-s of the stent as compared to a stent with diaconal sections havinu- a uniform width.

Still another object of the. invention is to have a stent with decreased wall thickness at the central stnits compared to tl-ie end struts so as to have a comparatively hio'her radiopacity foi- the end sets of stn.it members.

Still another object of the present invention is to have a stent with tapered diagonal sections for one or more of the set.s of stntt members ~vhere the center of the diagonal section is wider than the ends to increase the radiopacity of the end sets of strut niembers as compared to a stent with uniform width of the diaUonal sections.

Still another object of the present invention is to have end sets of strut members having both shorter diagonal sections and thinner width curved sections as compared to those sections in the central sets of strut rnembers.

Still another object of the present invention is to have a tantalum stent with wall thickness less than 0.035 inches having tapered sets of strut members whereby the diagonal sections are less wide than the width at the center o f the curved sections.

Still another object of the present invention is to have a closed cell stent design with maximum post-deployment cell area less than 0.005 square inches and a cell perimeter length that is edual to or greater than 9 mrr!.

I

'Still unother object of thc preSent in~'ention is to have a stent with a radiopaque metai coatina where the radiopaque n-~eta.l coating Ilas greater wall thickness on the end sets of stnit members as compared to thickncss on the szts of str-ut members at the center of the stent.

Still another object of the present invention :s to have a stent etched from a muiti-laver metal tube having one layer signiticantlv more radiopaque than at least one other laver;
the etched stent being formed with increased wall thickness of the more radiopaque layer on the end sets of strut menibers as compared with the sets of strut members at the center of the stent.

Still another object of the present invention is to have a closed cell stent design with "M" or "W" shaped flexible undulating longitudinal connecting links wherein the circumferential extent of the flexible undulating longitudinal connectinQ
iinks is appi-oximately equal on each side i.,f a line drawn between the proximal and distal attachment points of the flexible undulating longitudinal connecting link.

Still another object of the present invention is to have the stent with optimized radiopacity formed with an outer surface that is plastic coated to improve biocompatibility.
Still another object of the present invention is to have the stent with optimized radiopacity that is coated with a plastic material and an additional organic compound to prevent thrombus formation and/or restenosis.

Still another object of the present invention is to have a stent coated with a plastic material that includes a radiopaquC riller material.

These and other objects ard advantaves o: this invention will become apparent to the person of ordinary skill in this art fieid upon reading of the detailed description of this invention includinL), the associated dra%v:nLs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a tlat layout a prior art stent having uniform strut width for the circumferential sets of strut members.

FIG. 2 is a flat layout of a prior art stent design having "M" and "W"
flexible connecting links.

FIG. j' is an enlaruement of the -ti1" link of the stent desi.~,n of FIG. 2.

FIG. 4 is an enlargement of the improved "M" liiik- design of the present invention.
FIG. 5 is a tlat layout of the present invention stent design for a highly radiopaque metal.

FIG. 6 is a flat layout of part of the present invention stent design of FIG.
5.

FIG. 7 is a flat layout of an alternate embodiment of part of the present invention stent design of FIG. 5.

FIG. S is a flat layout of the present invention stent desiorn for a somewhat t-adiopaque metal.

FIG. 9 is a flat lavout of the present inven:ion stent design for a stent coated with a radiopaque metal.

FIG. 10 is a flat layout of an alternate embodiment of the present invention stent including an "N" shaped fleYible connecting link.

FIG. 11 is a flat layout of the present invention stent design as photo-etched from a tube.

FIG. l?;a is an enlargement ot a section of tlie photo-etched stent of FIG.
11.
i FIG. 1?B is a fon~itudinal cross section a.t l:'.-1 ~ of the enlarged section of FIG. 11 shown in FIG. 12A, the stent h<ving a radiopaquc coating that is thickest on the erid sets of strut members.

FIG. 12C is a lon-itudinal cross section at 1~,'-12 of the enlarged section of FIG. 11 shown in FIG. 12A, as etched fi-om a two-laver tube where one of the tube lavers is a moderately radiopaque metal and the other layer is a highly radiopaque metal.
DETAIL.ED DESCRIPTION (DF THE DRAWINGS

FIG. I shows a flat layout of an embodiment of a prior art stent described by Fischell et al in U.S. Patent No. 6,190,403). The stent 5 of FIG. 1 is shown in its crimped, pre-deployed state as it would appear if it were cut longitudinally and then laid out into a flat, ?-dimensional conficuration. The stent 5 comprises end sets of strut members 2 located at each end of the stent 5 and three central sets of strut members 6 connected each to the other bv sets of longitudinally extendinc, undulatin-, "N" links 4. The end sets of stnit members 2 consist of alternatino- curved sections 7 and diagonal sections 9. The central sets of stnit nzembers 6 located longitudina(lv between the end sets of strut members 2 consist of alternatin- curved sections 3 and diagonal sections S. In the prior art stent 5, the longitudinally diagonal sections 9 of the end sets of strut members 2 are shorter in lencyth than the longitudinally diagonal sections 8 of the central sets of strut members 6. The shorter diagonal sections 9 will reduce the stiff longitudinal lencrth of metal at the ends of the stent 5 to improve deliverabilitv (by reducinc, "fish-scaling") and will also increase the post-expansion strength of the end sets of strut members 2 as compared with the central sets of strut members 6. In this prior art stent, the width of the curved sections 3 and 7 and the diagonal sections 8 and 9 are all the same. "There is no variation in width within any set of strut members or between the e.nc1 sets of stnit members 2 and the central sets of stnit members 6. The stent 5 is a desi,-,n w II si.ited to stainless steel having a wall thickness of 0.0045" or greater, such as found :n thc COIZDIS BX Velocity ~~ stent.

If the stent 5 wei-e fonned from a hi~-,hly radiopaque metal such as tantalum with wall thickness of 0.0030 to 0.0035 inches and with sets of strut inenZbers 6 having widths of greater than the 0.005 inches that is necessary for good radial strength, then the stent would be too radiopaque. In addition, %%ith a wall thickness of 0.003 inches or less, the end sets of sti-ut members 2 might have a tendencv to tlare outwardly into the vessel wall upon expansion. If the end sets of' strut members 2 are designed to be as strong as possible while not exceeding metal strain limits at the largest usable diameter of the stent 5, then the central sets of strut members 6 with lon,er diagonal sections 8 will not have maximized radial stren,-,th assuming the same strLlt ~%idth for both central sets of strut members 6 and end sets of strut members 2. Optimized strength at the longitudinal center of a stent is important as it is that region that must typicall~hold back a larger amount of plaque than at the ends of the stent.

One embodiment of the present invention provides that each set of stnit members should have niarimized radial strength i-ather than having the central sets of stnrt members 6 being less strong than the end sets of strt.tt rnembers as previously described. This design would be similar to the stent 5 of FIG. I with the novel improvement being that the width of the curved sections 3 of the centl-al sets of strut members 6 vvould be greater than the width of the curved sections 7 of the end sets of strut mernbers 2. The greater width of the curved sections 3 will increase the strength of the central sets of strut members 6 compensating for loss of radial strength because of the longer diagonal sections S.

The stent 60 shown in FIG. 2 is a flat layout of a prior art stent design havina "N", "A1" and "W" flexible corinecting, lir:ks. Tiie stent 60 is shown in its crimped pre-depioyed state as it would appear if it wcre cut 1on-Dinrdinally and then laid out into a flat, ?-dimensional confiauration. It should be clearly uriderstood that the stent 60 is in fact cylindrical in shape, which cvlindrical shape would be obtained by rolling the flat confiQuration of FIG. 2 into a cylinder wit11 the top points "G" joined to the bottom points "H". The stent 60 is typically fabricated by laser rnachining of a cylindrical, stainless steel tube.

A central set of strut members 62 is a cylindrical, ctosed, ring-like section of the stent 60 consisting of a multiplicity of curved sectioris 63 connected to diagonal sections 68.
Every curved section 63 of eacli central set of strut members 62 is attached to a connecting link which is either a flexible "N" link =I4, "M" link 64 or a "W" link 84.
The stent 60 also has two end sets of stn.it members 72 consisting of a multiplicity of curved sections 73 connected to diagonal sections 78. In this embodiment, half of the curved sections 73 of the end set of strut members 72 are attached to "N" links 44 with the other half of the curved sections 73 situated at the extreme ends of the stent 60. The diagonal sections 78 of the end sets of stn.it members 72 are shorter than the diagonal sections 68 of the central sets of strut members 62. Shorter diagonal sections enhance the post-expansion radial strength of the end sets of strut members 72 as compared to the central sets of strut members 62.

FIG. 3 is an enlarl-ement of the "M" link 64 of the prior art stent of FIG. 2.
One disadvantage of this design relates to the circumferential extent of the "M"
link 64 with respect to a line 65 that could be drawn between the two attachment points 68 where the "M"
link 64 attaches to the curved sections 63. Because almost all of the "M" link 64 lies above the line 65, pressure on the top of tl,e "M" link 64 from plaque in an artery could bend the top 1~

of tlie "%1" link 64 in%tiard into the ;3rterial <urnen. This vvould be hi(&ly undesirable. Ideail., an tiI" or linti should ha ~. an equa1 Circilmfei-enr_ia1 extent on either side of a Iine drawn between the attachment points to adjacent se,ts of str-ut rnembers as shown in FIG. 4.

One aspect of the present invention is an improved "M" link 14 as shovvn in FIG. 4.
The "'.vt" link 14 has a circumferential extent (i.e., len,~,th) L' above and L" below the line 15.
The line 15 is dra~~n between the attachment points 18 ~vhere the "M" link 14 attaches to adjacent curved sections 13. Such a balanced desigi1%vould diminish any likelihood of the tlexible connecting link 14 from e>:panding into the arterial lumen.

FIG. 5 is a tlat lavout view of a stent 20 that includes some embodiments of the present imention. The design of FIG. 5 is particularly applicable to stents made from a hMhlv radiopaque metat such as tantalum. The stent 20 of FIG. 5 is shown in flat, layout view based on its pre-deployed state, as it would appear before it is crimped onto a balloon catheter. The stent 20 comprises end sets of stnrt members 22 located at each end of the stent 20 and central sets of strut members 26 connecteci each to the other by sets of' individual flexible "M" links 24. The "M" links -14 are similar to the "M" linkI4 of FIG.
4. The end sets of strut members 22 consist of a multiplicity of curved sections 27 connected to diagonal sections 29. The central sets of strut members 26 located lonuitudinally between the end sets of strut members ?2 consist of a multiplicity of curved sections 23 connected to diagonal sections 28.

One can also define a strut element 25 as being composed of one adjacent curved section 23 joined to a diagonal section 28. As seen in FIG. 5, it is clear that one can describe a central set of strut members 26 as beinz a closed, circumferential, ring-like structure 1i:

comprising a multipiicity of coi:nected strut clements 25. An end set of strut members could be !ikewise detined as bein', a n~.u.tiplicity of connected strut elements 17.

The stent 20 is a ctosed cell stent having ceils 19 formed frorn portions of adjacent sets of stnit members corulected by "AI" links 24. F'or coronary arteries, prolapse of plaque into the arterial lumen will be minirnized if the area within the cell 19 does not exceed 0.005 square inches at all diameters up to the ma_Yimum deployment diameter of the stent 20. An important aspect of stent design is to be able to place a 1cuidewire through the expanded cell 19, into a side branch vessel. A balloon angioplasty catheter can then be advanced over the guidewire and intlated to enlar~e and ciretilarize the opening of the cell 19 to "unjail" the side branch vessel. By "unjailin,~" is meant removing metal from the ostium of the side brancli vessel, thus impro\ing 10lood flow to that side branch. One concept of the present invention is that the cell 19 has an interior length of the penmeter that is at least 9 mm. Since balloon dilatation of the cell 19 would cause it to be near circular, an inside perinleter ten,-,th around inside of the cell 19 would provide an inside diameter of 9!n, which is approximately 3 mm. azood cell desiQn for side branch access should have an inside perimeter length between 9 mm and I 1 n1m. (i.e., an expanded inside circular diameter between 2.86 and 3.5 mm) where cell perimeters between 9.5 and 10 mm are optimal and would be suitable for essentially anv side branch of a coronary artery.

In the stent 20, the diagorial sections 29 of the end sets of strut members 22 are shorter in length than the diagonal sections 28 of the central sets of strut members 26. The shorter dial-onal sections 29 will reduce the longitudinal extent of the metal strut at the end of the stent to improve deliverability into a vessel of the human body by decreasing fish-scaling.
In the stent 20, the width of the cucved sections 23 and 27 and the diagonal sections 28 and 29 are different as compared to the prior art stents ~ and 6 of FIGURES I and 2.

The exact design of the ster,t 2U is most clearlv seen in the expanded view of the stent section 21 of FIG. 5 as shown enlar--ed in FIG. 6. FIG. 6 shows that the curved sections _23 (of the central sets of strut members 26 of FIG. 5 ) have a,vidth at the center of the curve ~V,.
The width of the curved sections 23 taper down as one moves away from the center of the curve uritil a minimum width W i is reached at the center of the section 28.
To achieve this taper, the inside arc of the curved section 23 has a center that is longitudinally displaced from the center of the outside arc. This tapered shape for the curved section 23 provides a siUni.ficant reduction in metal strain witll little eftect on the radial strength of the expanded stent as compared to a stent having sets of strut menlbers ith a uniform stnit width.

This reduced strain design has several ad%antages. First, it can allow the present invention desi(zn to have a much `;i-eater usable range of i-adial expansion as compared to a stent %\ith a uniform strut width. Second, it can ailow the width at the center of the cun-e to be increased which increases radial strenath with(Alt (-jreatl;- increasin,,, the metal strain (i.e.
one can make a stron-er stent). Finallv, the taper reduces the amount of metal in the stent and that should improve the stent tl,rombogenicity.

FIG. 6 also shows a unique design for tl.e end sets of strut members 22. The diagonal sections of the end sets of strut members 22 have a leriath L d that is shorter than the len-th L of the diaconal sections 28 of the centrat sets of strut members 26. To maximize the`-tadial strencth of a stent alonry its entire iength, each set of strut members should just reach the maximum allowable plastic strain for the metal bein- used at the largest allowable ezpanded diameter of the stent. [n the stent of FIG 1, the curved sections 7 of the end sets of strut members 2 and the curved sections 3 of the central sets of stnit members Ei have the same widths. As a result, the end sets of strut members 2(which have shorter diaUonal sections 9) will reach the maxir-num allowabie di=eter at a level of strain that is greater than the level of strain zxperien~-ed b~. ihe central sets ot strut members 6.

an optimum stren-th stent desi~m wouid have the same strain at the maYimum stent diameter for both the enct sets ot'stt-ut members 2 and the central sets of strut members 6. For the stent design of FIGS. 5 and 6. one desires to have the end sets of strut members 22 reach the maximum strain limit at the same stent diameter as the central sets of strut rriembers 26.
The present invention teaches a design with the width at the center of the cun,e W, .,, of the curved section 27 bein,, less than the width W~ of the cun'ed sections 23 of the central sets of strut members 26. This reduced width tor the curved sections 23 compensates for the shorter length L,,,,, of the end diagonal sec:tions 29 so that there is the same strain in both the central and end sets of strut nlembers ?2 and 26 respectively as the stent 20 is expanded to its maximum allowable diameter.

The end sets of stnit menibers 22 can also be tapered like the central sets of strut members 26 where the %vidth of the strut tapers down as one moves away from the center of the curve of the cur-ved sections 17 until a minimurn width Wd c- ~, is reached at the diagonal section 29. The curved sections 23, 27 each have an inside (concave) arc and an outside (convex) arc. Each arc has a center that is longitudinally displaced from the other center.

The tapered stnit design shown in FIGS. 5 and 6 also has an advantage for stents made from highly radiopaque nietals such as tantalum. If one uses uniform strut width as seen with the stent 5 of FIG. 1, ttien a properly designed thin-walled (0.0025 inches to 0.035 inches) wall tantalum stent may be too radiopaqt.ie. The reduced metal from the thinner diagonal sections 28 and 29 will decrease the radiopacity without affectinc, radial strength.
1 ~~

Nominal dimensions and dimensiwn ran-es (al1 in inches) for a tantalum stent produced usinCl the desiizn of FIG. 5 are as 60ilo,w:,:

Element Nominal Range W' --- 0.006 0.0045 to 0.007 ----__ 0.0045 0Ø35 to i).005 0.0045 0.004 to 0.005 0.0045 0.0;35 to 0.005 L 0.028 0.0?0 to 0.030 0.015 to 0.026 Wall Thickness 0_0(,-)3 0.0025 to 0.035 AIthou2h the present invention shows the "M" shaped flexible link 24 being used, the present invention stnit designs wi11 function vvith any link shape including "N", "W", "S"
"U", "V" and inverted "N", "U" and "V" designs. It should also be noted that the "N1" linl:
2-I shown in FIG. 6 has esactly tive longitudinaliy extending curved segments 2=1A, 24B, 24C, 24D and 24E.

FIG. 7 is an alternative embodiment 21' ot section 21 sho\vn in FIG. 6 of the present invention stent 20 of FIG. 5. In this embodiment, the onlv difference is the shape of the dia~zonal sections 28'. The diagonal sections 28 of FIG. 6 have uniform thickness. The dia;onaf sections 28' of FIG. 7 are tapered frotr: a width W,j'T at the end of the diagonal section 28' where it connects to the curved sections 23' to a width W.' at the center of the dia,-,onal section 28'. The advanta~!e of the inward taper of the diagonal sections 28' is that removal of more metal will reduce the radiopacity of the longitudinal center region of the stent 20 as compared to a stent with uniform width diagonal sections 28 as seen in FIG. 6.
The additional taper may also further reduce the metal strain as the stent is expanded.
Althou2h one could taper the diagonal sections 29 of the end sets of strut members 22 of FIG. 5, there is an advantage in having the end sets of strut members 22 being more radiopaque than the central sets of strut members 26. This is because visualization of the stent ends is the most important aspect of radiopacitv for a stent. Therefore, a preferred ~!J

embodiment of the prescnt invenrion is as seen in FIG. 7 to have tapered diagonal sections '_S' in the central sets of strut members 26 and rrniform thicl:ness diagonal sections ~9 (having a.zreater average widthi tor the end sets of strut members 22.

Instead of connectinc-I e\"erv eurved section with a tlexible link, an alternate embodiment may use straight links connectin- onlv half of the curved sections of the sets of stnit menibers. Such a stent could also have the advantage of a reduced strain strut desi~n as shown in FIGS. 5, 6 and 7.

For the stent of FIG. 5, it Should also be understood that the wall thickness of the central set of strut members 26 could be thiriner that the wall thickness of the end set of strut nlembers 22. Also it should be ioted that the "M" links 24 also have a much narrower width as compared to the width of any strut member oi the end set of strut members.
Both these attributes of the stent 20 create the following desirable radiopacity characteristics: hi~hlv radiopaque end sets of strut mernbers and decreased radiopacity at the central region of the stent 20.

FIG. 8 is a flat layout view of another embodiment of the present invention showinz a stent 30 made from a moderatefv radiopaque metal such as the cobalt-tungsten alloy L605.
The alloy L605 has great radial strength and is approximately 20% to 30% more radiopaque than stainless steel. Therefore, with L605, the same level of radiopacity is achieved with a stent wall thickness that is 20% to 30 ' less than a stent made from stainless steel. One 'goal in the use of L605 would be to reduce the wall thickness by 30% but end up with a stent that is still more radiopaque than an equivatent stainless steel stent such as the stent 5 shown in FIG. 1.

?1 The stent 30 of FIG. 8 is ~ilown in a iavout vi~w base(i on its pre-deployed state, as it would appear before it is crimptd )nto a balloon catheter. The stent 30 comprises end sets of strut members 321 located at each nd of the stent 30 and central sets of stnrt members 36 connected each to the other by sets of tlexible '"M" links 34. The "N1" links 34 are similar to the "ti'I" links 14 of FIG. 4. Each end set of stnrti members 32 comprises alternatinu curved sections 3 7 and diagonal sections 39 connected togethei- to foml a closed circamferential structure. The central sets of stn~t members 36 iocated lon7itudinally between the end sets of stnit members 32 comprises curved sections 33 and diazonal sections 38 connected to,4ether to form a closed circumferentiai ring-like stnicture.

In the stent 30, the dia,onal sections 39 of the end sets of strut members 32 are shorter in length than the diagonal sections 33 of the central sets of strut members 36. The shorter diagonal sections 39 will reduce the lon4itudinal length of metal at the end of the stent to improve deliverability into a vessel of the Inrman body. In the stent 30, the widths of the dia~_,onal sections 38 and 39 are different as compared to the prior art stents 5 and 60 of FIGS. 1 and 2.

The novel concepts of the stent of FIG. S are shown most clearly in the expanded view of the stent section 31 shown in FIG. 9. In FIG. 9 it can be seen that the diagonal sections 38 of the central sets of strut members 36 have a width at the center T, and a width at the end T, where the width in the center T` is lar2er than the width at the end T. This allows for increased radiopacity without affecting the design of curved sections 33 that are the primary stent elements involved for stent expansion. The curved sections 33 and 37 shown in FIG. 9 are tapered similar to the curved sections 23 and 27 of FIG.
6. It is also envisioned that the curved sections 33 and 37 could have uniform width similar to the curved sections 3 and 7 of FIG. l. The dia,zonal sections 29 of the end sets of strut members 32 also have a tapered shape. The diagonal sections 3) 7 have a width in the center T, ,,d and a width at the erld T. ~ where the width i.1 the center T, _~~ is 1ar~er than the width at the end T, nu.
Because of the desire for the end <ets of strut members 32 to be the most radiopaque part of the stent 30, the diagonal section 39 center width T, _.,n, of the end sets of strut members 32 is shown in FIG. 9 to be wider than the width T, of the diagonal section 38. A
wicier piece of metal will be more radiopaque. 'Thus, the stent has curved sections with a single bend connecting the diagonal sections oi' its sets of strut rnembers, and flexible connecting links connecting the curved sections of its circumferential sets ot strut members.

The stent of FIG. 10 is an alternate embodirnent of the present invention showing central sets of strut members 46 having curved sections 43 and diagonal sections 48 with tapered shapes similar in design to the curved sections 23' and diagonal sections 28' of the stent section ? 1' shown in FIG. 7. The stent 40. of FIG. 10 is shown in a layout view in its pre-deployed state as it would appear before it is crimped onto a balloon catheter. The stent 40 comprises end sets of strut nienibers 42 located at each end of the stent 40 and central sets of strut members 46. The sets of strut members 42 and 46 are connected each to the other by sets of individual fleYible "N" links 44. The "N" links 44 are similar in shape but slightly longer than the "N" links 4 of F1G. 1. The end sets of strut members 42 consist of curved sections 47 and diagonal sections 49. The central sets of strut members 46 located longitudinally between the end sets of stn.it members 42 consist of curved sections 43 and diagonal sections 48.

The stent 40 is a closed cell stent having cells 45 formed from portions of adjacent sets of strut members connected by "N" links 44. Prolapse of plaque through the closed cells 45 is minimized if the expanded area of the cell 45 is less than about 0.005 in.'' at any diameter up to the masimum deplo1,11nent diameter of the stent 40. It is also important for an optimum stent desit-Tn that uruidetivir-:: can be pla:;ed through the expanded cell 45 into a side branch vessel. A balloon an>iopl~st~~ catheter then he advanced over the ~uide ;re, throu-h the cell 45 and inflated to "uniail" the side branch, i.e. remove anv stent strut that is blocking blood flow into that side branch. The present invention design should have an interior perimeter of the cell 45 that is at least 9 mm, thus allowing a nearly 3 mm diameter circular opening to be achieved :or unjailing.

FIG. 1 1 is a flat layout view of another embodiment of' the present invention in the fomi of a stent 50 that is photo-etched fron7 a metal tube. The stent 50 is shown in its pre-deployed state as it wouid appear before it is crimped onto a balloon catheter. Tlie stent 50 comprises end sets of stnit members 52P and 52D located respectively at the proxinlal and distal ends of the stent 50. The sternt 50 also has central sets of strut members 56 connected each to the other by sets of flexible "NI" links 54 The "M" links 54 are similar to the "y-1"
links 14 of FIG. 4. The end sets of stnit members 52P and 52D each consists of cun-ed sections 57 and diagonal sectior.s 59. The central sets of stntt members 56 located longitudinally between the end se:s of strut members 52 consist of curved sections 53 and diagonal sections 58.

The section 55 of the photo-etched stent 50 is sho~vn enlarged in FIG. 12A.
The FIGS. 12B and 12C show two etnbodiments of the present invention that can provide a stent with enhanced radiopacity at the stent ends.

FIG. 12A shows diagonal sections 58 and 59 and an "M" link 54 conrlecting the curved sections 53 and 57.

FIG. 12B is a[on-,ituciinal cross section at 11 2-1? of the stent section 55 shown in FIG.
1?A. The stent desi,-,n sl:own in F[G. ':21B has a ilighlv radiopaque coating that is thicker on the end sets of strut menlbers as compared to the thickness on either the flex links 54 or the central sets of strut members 56. FIG. :?B shows the coating 57C on the curved section 57 of the end set of strut members 521 being, thicker than the coating 54C on the flex link 54 and also thicker than the coatin~ 5~C on the cun~ed section 53. The most likely coating for the stent 50 would be ~2old oLting although p1=:.tinum, tantalum or any other highlv radiopaque metal could be used.

The present invention llas the entire stent coated to provide an exterior surface for the stent 50 that is formed from a sin~~le metal. This reduces the potential for corrosion that can occur with dissimilar metals on rhe stent's exterior sui-face when the stent is placeci in a saline solution such as blood.

It is also envisioned that: even with the entire stent coated with a highly radiopaque metal, an additional coating of a flexible plastic such as parylene may be desirable. Such an organic coating has the additional advantage of allowing the attachment of drugs such as taxol or rapamycin to reduce resterlosis. Techniques for 'gold plating metals such as stainless steel and controlling the thickness of the plating are well known in the art of metal plating.

FIG. 12C is the longitudinal cross section at 12-12 of yet another alternate embodiment of the enlarued section 55 of FIG. 11 shown in FIG. 12A. The stent design shown in FIG. 12C is etched frorn a two-layer tube where one of'the tube layers is a metal of conventional radiopacity such as stainless steel and the other layer is a highly radiopaque metal such as tantalum. AlthouIYh the total wall thickness of the stent of this embodiment remains nearly constant, the end sets of strut members 52' have a thicker layer of the -J

radiopaque metal than the tlex lin.l:s -54' or the centrai sets of strn.lt members 56'. The curved section 57' of the end set of strut rnembers 5`'' izas conventional metal Iave~ ~7iti~' and radiopaque metal laver ~7R'. I}:e t1ex Iink 54' (llas a standard metal laver 54N' and a radiopaque metal layer 54R'. The central sets of stnit members 56' have cuned sections 53' with conventional metal iavers ~~N' and radiopaque metal layers 53R'.

It can be seen from FIG. 12C that the radiopaque metal layer 57R' of the end sets of strut members 52' is thicker than the radiopaque rrietal layers 54R' and 53R'.
In recent years, multi-layer photo-etchin,-, processes for rrmetals that can control the thiclcness of individual lavers have been developed so that the embodiment of FIG. 12C can be produced within the current state of the art of phot:o-etchin2. Using this approach, tvvo and three layer tubin'- is now available from several manufacture,s and can be photo-etched to make a stent with an optimal design which is high radiopacitv :`or the end set of strut members and reduced radiopacit,v for the central sets of stnrt rnembers. Specifically, a sterit with the characteristics as seen in FIG. 11-B or FIG. 12C would have the desirable attribute of end sets of strut menibers with ~-,reater radiopacitv than the remainder of the stent.

Various other modifications, adaptations, and alternative designs are of course possible in light of the above teachings. Thereforz, it should be understood at this time that within the scope of the appended claims the invention may be practiced otherwise than as specifically described lierein.

~'6

Claims

What is claimed is:

1. A stent in the form of a thin-walled, multi-cellular, tubular structure having a longitudinal axis, the stent comprising a multiplicity of circumferential sets of strut members, each set of strut members being longitudinally separated each from the other and each set of strut members forming a closed, cylindrical portion of the stent, each set of strut members comprising a multiplicity of connected curved sections and diagonal sections, the sets of strut members including end sets of strut members located at each end of the stent and central sets of strut members positioned between the end sets of strut members, the curved sections of the central sets of strut members having a generally greater width than the width of the curved sections of the end sets of strut members and the diagonal sections of the central sets of strut members having a greater length as compared to the length of the diagonal sections of the end sets of strut members so as to provide approximately matched radial strength for the central sets of strut members and the end sets of strut members.

2. The stent of claim 1 wherein the width of the curved sections of the central sets of strut members is at least 0.0005 inch greater than the width of the curved sections of the end sets of strut members.

3. The stent of claim 1 wherein the length of the diagonal sections of the central sets of strut members is at least 0.001 inch greater than the length of the diagonal sections of the end sets of strut members.