CA2502504A1 - Improved tissue supporting devices - Google Patents

Abstract

A new multiple component stent which allows for initial self-expansion and subsequent deformation to a final enlarged size. In one embodiment, stent comprises a first relilient element and a second deformable element. In another embodiment, stent is made of a first austenite component and a second mertensite component.

Description

-I-i~'RO''PED TISSUE SUPIfORTISIG DEVICES
This invention relates to tissue supporting devices in general and most 5 particularly to vascular sterns for placement in blood vessels. A primary featuK of the devices of this invention is that they are expandable within the body.
In the past, such devices have been provided for implantation within body passageways. These devices have been characterized by the ability to be enlarged radially, often having been i~roducec! into the desired position in the body i0 as by percutaneous techniques or surgical techniques.
These devices are either expanded mechanically, such as by expansion of a balloon positioned inside the device, or are capable of releasing sfloral energy m sdf-expand themselves within. the body.
References designated as defining the general state of the art but not 15 considered to be of par;icular relevance to the invention disclosed and claimed harem are as follows. French Patent 2,617,721 appears to disclose a catheter used for permanently dilating a stenosis in a tubular organ or blood vessel.
'W09d1031~7 appears do disclose a prosthetic device for sustaiain,g a blood vessel or hollow organ lumen comprising a tubular wire frame. European Patent Application 364,787 and 20 fiuropean Pateut Apglication 335,341 appear to disclose expandable intraluminal vasrcular grafts. W092II9310 appears to disclose a tissue supporting device of a shape memory alloy. U. S. Patent 5,147, 370 appears to disclose a nitinol stmt for hollow body conduits. UK Patent 2,I75,824A appears to disclose a method of producing a composite metal materiel and a bil'kt for jet engine carbine blades, 25 armor, helicopter rotor blades, car suspension stress parts or sword blades made of said composite material.
The materials which have bean used to make up these devices have itxluded ordinary metals, shape metnory alloys, various plastics, both biodegradable and not, and the Iike.
30 . This invention is concerned with the use of these materials in a now multiple componenx arrdagrment which allows for initial self expansion and subsequent deformation to a fuial enlarged diameter in the body.

Balloon expandable std do not always expand uniformly around their circumference. As a result, healing may not take place in a consistent manner.
If the scent is coated or covered, non-uniform expansion may tear the covering or coating. Additionally, long starts of this type may require long balloons which can 5 be difficult to handle, difficult to size, and may not offer Ideal performance in tortuous passages in blood vessels and the like.
Thus, when addressing such isseus, self-cacgandabl~ stenta hava been thought to be generally more desirable. Unfortunately, one cannot control the degree of expansion and hence the degree of embedment in the vessel wall. It has IO been determined that a stmt must be embedded to some degree to be clinically satisfactory.
The stenfs of the present invention provide the best features of both of these types of scents without their drawbacks.

_J_ The tissue supporting devices of this invemion are generally cylindrical or tubular in overall shape and of such a configuration as to allow radial expansion for enlargement. They are often referred to herein in the general sense as 5 "slants" . Furthermore, the devices are comprised of at least one component, element, constituent or portion which exhibits a tendency to self-expand the device to an expanded size and at least one other component, element, constituent or portion which is deformabk so as to allow an external force, such as a balloon positioned within the body of the device, to further expand it to a final, larger 10 desned expanded size. Tile terms "compotrent", "ektnent", "constituent" and "portion" are often refa~ed to herein collectively as "portion" .
Preferably, the devices of the invention are made of metal and most preferably of shape memory alloys.
In one embodiment, a first portion is a resilient spring-Like metal for 15 self expansion and a second portion is a defortaabk metal for final sizing.
In a ~re p~gory ~bodiment, a first portion is a self-expanding austenitic one and a second is a martensidc o~ eapabk of deformation. In the case of shape manory embodiments the "portions" may be discrete or merely different phases of an alloy.
20 The most preferred embodiment of tha invention is a stunt, preferably of shape memory alloy. The most preferred shape memory alloy is Ni-'Fi, although any of tl~ other known shape memory alloys may be used as well. Such other alloys include: Au-Cd, Cu-Zn, In-Ti, Cu-Zn Al, Ti-Nb, Au-Cu-Za, Cu-ZtrSn, Cu-Zn-Si, Cu-Al-Ni, Ag-Cd, Cu-Sn, Cu-Zn-Ga, Ni-Al, Fe-Pt, U-Nb, Ti-Pd-Ni, Fo-Mn-3.S Si, and the Iike. These alloys may also be doped with small amounts of other elements for various pmperty modifications as may be desired and as is known in the art.
The invention will be specifically described hereiabelow with reference to stems, a prefcrned embodiment of the invention although it is broadly 30 applicable to tissue support devices in general.

~3~~~c~'g~an of the Fisures Figure 1 is a braided stcnt according to one embodiment of this invention.
Figure 2 is a graph showing the tnartensitic/austenitic temperature 5 transformation curve and the superelasdc area of a shape memory alloy.
Figures 3 is an end view of a layered slant having two discrete components according w one aspect of this invention.
Figures 4a and 4b are graphs showing the martensiticlaustenitic temperature transformation curves of the layers in the slant of Figure 3.
10 ~ Figure Sa and Sb are views of another embodiment of the invemion comprisod of alternating rings of shape memory alloy.
Figure 6 is a showing of a stem fragment of a braided version of a shape memory slant of this invention.
Figure 7 is a graph showing a temperature window for a shape 15 . memory alloy to be used in yet another scent version of this invention.
Figurc 7a is a graph showing expansion of a scent with temperature.
Figure 7b is a graph of the same type, the scent having been cold wotiCed.
Figure ~c is a gisph of the same type, the stem having had 20 pseuctoelastic prestraining.
Figure 7d is a graph of the same type, the stmt having amnesia inducement.
Figures 8-11 show various expandabk configurations (closed and open) illustrated in fragment which may be used in the stems of this invention.
25 Figures 9a acrd 9b show a preferred embodiment of an articulated scent.
Figurc I2 shows another version of an expandable slant of the invcntioa.
Figure I3 shows yet another version of a scent which may be used with the invention.
34 Figure I4 is a schematic showing of a braided stunt made up of a plurality of strands.
Figure 15 is a detail of a single strand fmm the stmt of Figure 14 showing that the strand is made up of a plurality of wires of two different types.

Figure 16 is a cross-sectional view taken along line 15-16 of Figurc 15 showing the two different types of wire.
5 Preferred embodiments of this invention are described below with particular reference to the accompanying drawing Figures.
Referring first to the embodiment shown in Figure 1, a stmt I0 is shown comprised of braided or interwoven metal strands I2 and 14. Strands 12 are of a resilient spring-like metal such as spring steel, Elgiloym for example.
I0 Pttferably, strands I2 are spirally extending in the same direction, spiraling to the right as seen in Figure I. Strands 14 are of a deformable or anncaial metal such as stainless steel and are preferably spiralai in the opposite direction as strands 12, as shown in Figure 1.
Given such a scent construction of two components 1. e. , strands I2 15 and 14, it can be seen that stmt I0 may be readily loaded on a catheter as by placing it over an uninflat&d balloon on a balloon catheter and compressing it tightly amend the balloon and then glaring a sheath over the stoat to hold it in piece during the translumvinal placement procure. Otux in place, the sheath is removed, for example slid back, to expose the scent, allowing it to self-expand by force of the 20 resilient strands 12 to substantially assume a self-expanded shape/siu.
Some self expansion may be restrained if held back by strands 14. Ta finally adjust the size of the scent, the balloon may be expanded by inflation fmm within the scent to exert an outward radial force on the stmt and fir enlsxge it by stretching and deforming the deformable metal of strands I4. This may be aided by building into strands 14, 25 a series of readily defotmable structures or means such as bends or kinks l6 as shown in Figure I. It can be sera that a pet3nanent adjustable size beyond the self expanded size may be obtained with this embodiment. It is to be noted that many configurations other. than braided may be readily devised to take advantage of this two component cotxept, including various of the subsequent configurations descn'bed 30 hereiabelow. Also, it should be notrod that, although not preferred, the runt may be initially deployed without a balloon; the balloon following on a sepatnte catheter.
R,efeiring now to subsequent faatut~es, other preferred embodiments of the invention will be described which make use of shape memory alloys and some of P~~ Pr~Y ~ sP~ h'P~ of deformation i.e., shape memory deformation in marteasite and/or superelastic deformation in austcnite.
The term "superelasticity" is used to describe the property of certain shape memory alloys to return to their original shape upon unloading after a S substantial deformation while in their austenitic state. SupereIastic alloys can be strained while in their austenitic state more than ordinary spring materials without being plastically defornud. This unusually large elasticity in the austenitic state is also called "pseudoelasticity", because the mechanisms is nonconventioual in nature, or is also sometimes referred to as "transformational superelasticity"
t~ecausc it is IO caused by a stress induced phase transformation. Alloys that show superelasticity also undergo a thermoelastic martensitic transformation which is also the prerequisite for the shape memory effect. Superelasticity and shape memory effects are therefore closely related. Superelasticity can even be considered part of the shape memory effect.
I5 The shape meamory and supezelastlcity effects are particularly pronouncxd in Ni-Ti alloys. This application will therefore focus on these alloys as the preferred shape memory alloys. The shape memory effect in Ni-Ti alloys has been described many times and is well known.
In near-equiatomic Ni-Ti alloys, martensite forms on cooling from the 20 body centered cubic high temperature phase, teamed ae~stenite, by a shear type of process. This martensitic phase is heavily twinned. In the absence of any externally applied force transformation takes place with almost no external macr~copic shape change. Tf~ martensite can be easily defornaed by a "flipping over" type of shear until a single orien~t~on is achieved. This pmcess is also celled.
"detwinning".
25 . . If a deformed martensite is now IKated, it reverts to austJenite. The crystallographic restrictions are such that it transfouns back to the initial orientation tt~roby rGStaring the original shape, Thus, if a straight piece of wire in the austenide condition is cooled to form maricasite it remains straight. If it is now deformed by bending, the twitmed mar~nsite is converted to deforaned marbensite.
30 On lxating, the transformation back to austenite occurs and the bent wire becomes straight again. This process illustrates the shape memory deformation referred to above.

_7_ The transformation from austenite to martensite and the reverse transformation from mattansite to austenite do not take place at the same temperature. A plot of the volume fraction of austenite as a function of temperature pxovides a curve of the type shown schematically in Fig. 2. The complete 5 transformation cycle is characterized by the following temperatures:
austenite start teznperaaue (A,), austenite finfsh temperature (A~), both of which are involved in the fn~t part {1) of an increasing temperature cycle and martensite start temperature (M,) and martensite finish temperature (Mf), both of which are involved in the second part (2) of a decreasing temperature cycle.
10 Figure 2 represents the transformation cycle without applied stress.
However, if a stt~ess is applied in the temperature range betwxn A, and Ma, marGensite can be stress-induced. Stress induced martensite is deformed by detwinning as described above. Ixss energy is needed to stress induce and deform marocnsitc than to deform the austerute by conventional mechanisms. Up to about 15 896 strain can be accommodated by this process (single crystals of specific alloys can show as much as about 2S °~ pseudoelastic strain in certain directions). As austenite is the thermodynamically stable phase at tanpeTatures between A, and Md under no-load conditions, the matcrlai springs back into its original shape when the stress is no longer applied.
20 It becomes incrsasingiy diffcult to stt~,ss-induce martensite at increasing rremperauires above Af. Eventually, it is easier to deform the material by cotnrentional mechanisms (movement of the dislocation, slip) than by inducing and ~iefvrmitng martensite. The ttmptrature at which martensite can no longer be stress-inducod is called Md. Above Md, Ni Ti alloys are deformed like ordinary materials 25 by slipping.
Additional information regarding shape memory alloys is found in the following references, all of which are incorporated fully herein by reference;
"Super ;Elastic Nickel Titanium lyres" by Dieter Stsckel and Weikang Yu of Raychaar Corporation, Menlo Park, California, copy 30 receivod November 1992;
~gj,~~, Tenth Edition, Vol. 2, Propaxies and "Shape Memory Alloys" by Hodgson, Wu and 8iertnarui, pp. 897 - 902; and, _g_ In Press, T ~taryiicrn~~, ASM (1994), Section entitled "Stmerure and Properties of ?i-N Alloys by T.W. Duerig and A.R.
Pelton.
Since the most preferred shape memory alloy is Ni-Ti, the martensitic 5 state of this alloy may be used to advantage in the two component concept of this invention.
For example, with reference to Figure 3, a layered construction may be provided in a stoat 30 (shown in end view) which is generally a hollow cylindrical or tubular body in shape but which may be formed in a wide variety of 10 specific configurations or patterns to foster radial expansion of the body as exemplified in Figures 1, 5, 6 and in subsequent Figures 8-11.
Stent 30 is comprised of at least two layers 32 and 34, one of which 32 is a Ni-Ti alloy (50.8 atonaitc wt. % Ni, Ti, transition temperature of Af=0° C) and normally in the austenitic state, the other of which 34 is a Ni-Ti 15 (49.4 atomic wt. 96 Ni, b~anoe Ti, transition t~par~utro A~ = 60° C) and normally in the martensitic state. Preferably, the inner layer is 32 and the outer layer is 34. However, this may be reversed and also a plurality of layers, alternating or othe~vvise, may be utilized in this particular embodiment.
Stem 30 is made to a fabricated size and shape (parent shape) which 20 provides austenitic layer 32 its parent shape and size i. e. , its supereiastic high t~nperature shape and size. Obviously, is its as fabricated condition, the Ni-Ti alloy of austenidc layer 32 is selected so as to have a transition temperature range between its austenitic and tnartensitic states which is lower than body temperature as to ensure that in the body and at body temperawr~s the austtrritic state will always .25 prevail.
4n the other band, marrensitic layer 34 is of a Ni-Ti alloy having a transition te~etature range significarnly greater than body temperature so as to ensure that under body conditions the martensitic state will always prevail and the alloy will never transform to aust~enite in stem use. This is shown in the graphs of 30 Figure 4a atxi 4b which demonstrate tIx relative transition temperatures of layers 32 and 34, respectively far purposes of this invention. It can be seen from these graphs that the normal cotxlition of layer 32 (Figure 4a) at body temperatures and higher is the austcnitic state while the normal condition of layer 34 (Figure 4b) at body temperatuu~es is martn~sitic.
To manufacture the layered construction, one may make the austenitic portion with arty standard metallurgical technique and vapor deposit the martensitic portion on its surface. Other tpanufacairiag techniques such as diffusion bonding, welding, ion beam deposition, and various others will be apparent to those familiar with this art.
Such a stmt may be compressed or constrained (deformed to a small diameter) onto a balloon catheter as d~xibed for tlx previous embodiment and 30 captured within a sheath. During the constraintnent, austenitic layer 32 may stress induce to a marcensitic state. Tn the alternative, the scent may be cooled below the transition temperature of Layer 32 to facilitate its deformation and constrainment.
Martensitic layer 34 t~rely undergoes deformation. Thus tie scent may be "loaded"
onto a balloon catheter. However, with temperature changes occutiirtg up to body 15 temperature, layer 32 will remain marrensite until the constraint is removed. When released in place in the body, slant 30 will expand to a p~e~ge of its self expanded size and shape due to the transformation of layer 32 from martensite to austenite at which point the balloon may be used tv radially expand the stem to a greater permanent diameter by deforming ntartensitic layer 34. On the other hand, ~20 initial deployment can take plane without a balloon which may be separately inserted after deployment.
The two component concept of the invention in the layered embodiment of Figure 3 requires both the maroensitic and austenitic phase characteristic of shape memory alioy(s) in the two discrete components 32 and 34.
25 Preferably, the steal is fabricated in such a way that the austenitic layer 32 tends to self-expand slant 30 to a predetermined fabricated diameter (parent shape). The martensitic layer 34 feuds to hold back this self-expansion, preventing full expansion. For example, the stmt may only be able to self-expand to 759b of its ful! possible diameter (as detetmitxd by the austenitic layer). Therefore, 30 expansion beyond 75 ~b is accomglislted by an applied external force, as by the balloon inside the scent. It is suggested that the scent not be expanded beyond its normal fabricated diameter for the auste~tic layer 32 will have the tendency of making the stmt diameter smaller as it tries to recover its fabricated diameter (parent shape). If the stem is subjected to a temperature above body temperature and above the transition temperature of the marr~ensitic layer (which is clinically unlikely), the stmt will self-expand to the fabricated diameter only.
Depending on design size there are thus provided permanent stems capable of fulfiFling any needed S range of sizes with an adjustable sizing capability.
As is lrnown in the art, the desired properties of the shape memory alloys requitfed for use in this invention may be obtained by alloy composition and working and heat treatment of the alloys, in various combinations or singly.
Manufacatring techniques influence the phase characteristics of the 10 material. Alloy composition, work history, and lust ueaament all influence the final chararxeristics. At a specific operating temperature, say body temperature, the austenite phase material will have a transition temperature below body temperature {i.e., A~=0°C). Tlie material is capable of taking high strains and recovering after the load is released. The martensite phase material will have a higher transition 15 temperature than body temperature {i.e., A~=60°C), and is characteristically soft and pliable and retains the deformed shape after load removal. 'This tnartensite deformation is caused by detwinning, not the typical plastic deformation, or yielding, of crystal slip.
'With reference to Figttrts 5 and 6, other stmt constructions are shown 20 which are similar to the layered version of Figure 3 in so far as utilization of the two component concept of this invention is concerned.
Figure Sa and 5b shows a slant 50 made up of alternating expandable rings 52 and 54 of austenitic ara3 tnaroettsitic alloys, respectively, anaiogaus to layers 32 and 34 of the Figure 3 embodiment. Ringsv 52 and 54 for example are 25 interconnected by stmt members 56 which tray be of any material capable of rigidly holding the rings togettur. Other int~erc:otu~ctor nmay be used. As can be seen in Figufe'Sb, the pia~ment of strut members 56 does not require them w take part in the radial expansion of the stem and tl~y can therefore be of a relatively ordinary material such as stainless steel.
30 Referring now to Figure 6, a braided or interwoven cot>~uction is shown similar in construction to that of the embodimem of Figure 1. In this anbodiment, strat~s 62 extending to the right in Figure 6 are an alloy in the austenitic state whereas strands 64 extending to the left in Figure 6 are an a~'oy in the marrensitic state. _ Referring now to the graph of Figure 7, it is demonstrated that the two componetu concept of the invention may be embodied in two phases, i.e., components of a single shape memory alloy and need not be in the form of two discrete components such as layers, members, wir~;s, etc. Ia the graph of Figure 7, it can be seen that an alloy composition can be selected such that it has a phase transition temperature window that bounds the proposed operating temperatures of the stem, such as the normal 'body temperature range. Within this transitional window or zone, the material undergoes the phase transition and is effectively compositionally comprisai of a ratio of austenitic to n>aMCasitic phase depending on the tcrnperature of the stem. This ratio should be selected so as to provide sufficient radial force fmm the austenite phase while still allowing for further expansion of the martensite phase with a mechanical expansion means such as a balloon.
Selecting body temgeraaue as the operating temperature, a Ni-Ti alloy of about 50150 atmnic wt. 9b composition (range about 4915196) will provide an acceptable "window"
for this embodiment, the two components are the austeniu and martensite phases of the nititml.
The method of malting a scent may be descn'bed as follows. Age the shape memory material (hli Ti) until body tanperature falls somewhere within the transformation window. Therefore the stern will not fully recover to its high temperature shape at body temperature. An example of this technique is described below.
A stem of tubular 50.89b N balance Ti was prepared having a 1.5 mm diameter. It was suhstantlally alI austenite at room temperature, the Af being about 15-20°C and therefore being supereIastic at roam temperature. The stmt was cooled to below room temperature to form substantially alI marteasite and mechxnir~cally exto 4.7 mm in diameter. It was maintainal at the 4.7 mm in diameter and heat treated at S00°C for 30 minutes and water quenched.
Finally, it was again cooled to below room temperature to form substantially all tnarcensite and compressed to a diameter of 1, 5 mm. After deployment and at body temperature the scent has a diameter of 3.5 mm. At about 70~ of full expansion, i.e., about 40°C it had a diameter of 4.5 tnm and at 42°C it had a fully exdiameter of 4.7 mm.

This method works fairly wolf, but due to the slope of the temperature versus diameter Blot being extremely vertical at body temperature, a small change in body temperature, or nianufacruring control, can have a large impact on the actual self expansion diameter. As can be seen fmm Figure ?, the sloge of the line 5 between At and A, is rather steep with small changes in temperature Leading to large changes in percent austenite and consequently Iarge changes in diameter of a steal made of such an alloy. Figure 7a shows a temperature versus diameter plot.
Three methods of modifying the slope of the line on the ttamra versus diameter graph are cold work, pseudaelastic presuaining, and aaxnesia itxluccment, illustrated in 10 Figures 7b, 7c and 7d, respectively.
Residual cold work in nitinoi draws out or masks the point of Ar on the diameter versus the temperature curve. This is sees by the sluggish increase in diametiGr as temperature increases in the last 20-30~ of recover. By utilizing the 15 effects of cold work, the effects of temperature change on diatiuxer can be reduced in the last 20 to 30°.6 of street expansion. Shown in Figure 7b is an example of a temperature versus diameter plot for a cold worked part. Figure 7a above shows an example of a part without cold work.
20 Utilizing the effects of gsrudoclastic prestraiuing (S . Euckcn and T.W. Duerig, AGTA Metal, Vol. 37, No. 8, pp 2245-2232, 1989) one can create two distinct plateaus in the stress-strain behavior. This difference in stress strain behaviors can be directly linked to two distinct At temperatures for the two plateaus.
By placing the transition between the two plateaus at the transition from self 25 expanding to balloon expanding, i.e., 7096, one can control the characteristics of the stunt at body temperature. The goal would be to place the A,~ temperature for the first plateau (from maximum compression to 7096 expansion) below body temperature such that the stmt has self expanding charad~tristics. The .A f temperature for the second plateau would be above body temperature such that there 30 is no additional self expansion in this region (70 to 100% expansion) a mechanical device, like a balloon, ran then be used to custom size the steel between 70~
and 10096 of the high temperature shape. Results of such a technique is shown in Figure 7c.

AMNESIA INDUCF.11~.NT
One of the characteristics of nitinol is cycle amnesia. This was also discussed about in the article referral to ittunodiately above. As nitinol is cycled from its heat set shape as shown in Figure Td, there is an increase in the amount of amnesia to recover to the heat set shape with each cycle. As long as this amnesia is not caused by permanent plastic deformation, the amnesia can be removed by heating the part above lvi~. This shows there is martensite left in the part after cycling which is preventing full recovery in the austenitc phase (gust above Af).
This grescace of nun recoverable tnarrensite (below M~ is what may be used for the I0 balloon expansion region of the stunt.
Figures 8-11 represent examples of various expandable configurations (a ~ closed, b = expanded) which may be incorporated inm the devices of this invention. The version shown in Figures l0a and lOb may be modified as shown in Figures lOc and IOd (closed and open, respectively) by omitting portions (indicated at 100 in Figures IOc and lOd) as to render the stmt flexible for articulation. This may be done to other of the structures as well to improve flexibility.
Yet another version of a device incorporating the two component concept of the invention is shown in Figure 12. In this embodiment, a fragment of a scent 110 is shown. The stmt includes a self-extending compornent 112 and a deformable, external force expandable component 114. Self expanding component 112 may be resilient spring like metal such a stainless steel or it may preferably be a shape memory alloy in the austenitic state. Component 114 may be any deformable metal or the like such as annealed stainless steel or preferably a shape memory alloy in the martensitic state. The two components may simply be mechanically, welded or bonded together, Functions and operations are as described hereinabove.
Referring to Figure 13 a version analogous to the embodimesu of Pigure 12 is shown in which the two component concept is again embodied as different zones or portions of a single metal material.
As slmwn in Figure 13, a stem 120 (fragment showing) is of a self expanding component 122 and a defotmable component 124, both of which may be a single metal as spring steel or austenitic Ni-Ti which has been appropriately treated with r~ect to component 124 as by localized heat treatment or the like to alter the characteristics of the material of the 122 component so as to render it deformable or martensitic, depending on whether it is merely resiliern or is austenitic.
Again, function and operation are the same as with other embodiments.
Referring now to Figures 14-16, a mufti-strand braided stem is shown in Figure 15. Each strand I50 in the stmt is a micro-cable. That is, each strand is 5 made up of a plurality of wires 152 and I54 as is seen in Figures i5 and 16.
Each of the wires 152 and I54 consists of two different nitiuol alloys as seen best in Figwre 1b, or one nitinol and one ordinary metal such as stainless steel, platinum or tantalum. The latter two would provide enhanced radiogacity. One nitinol alloy v~rire 154 has an austenitic finish (A,) temperature less than body temperature. The 10 other wire 152 could be nitinol having an A, (austenitic start) greater than body temperature. Also, it could be an ordinary rn~l. Additionally, one or more of the strands may be of a biodegradable material such as a plastic or may be of a material including an absorbable drug.
Since the two alloys are into micro-cable one does not have 15 to caga$e in selective, discrete heat treating methods to produce both shape memory and martettsitic effects.
lZadiapaque portivas or coatings may be included on any parts of these sterns as is known in the prior art.
While this invention may be embodied in many different forms, there 20 are described in detail herein specific preferred embodimants of the invention. This description is an exemplification of the principles of the invention and is not intended to Iimit the invention to the particular embodiments illustrated.
The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and 25 alternatives to one of ordinary skill in this art. All tt~sc alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may reeogniu other eduivalents to the specific embodiments described herein which equivalents are also intended to be e~ompassed by the claims attached hereto.

Claims (13)

1. A stent comprising:
a plurality of annular elements, each annular element having a compressed state and an expanded state, wherein each annular element has a longitudinal dimension which is smaller in the radially expanded state than in the compressed state; and connecting members connecting adjacent annular elements;
wherein the annular elements and connecting members are made of Nitinol, with each connecting member preset with an elasticity which causes the connecting member to elongated longitudinally when the annular elements are in their expanded state to compensate for the smaller longitudinal dimension of the annular elements in the expanded state.

2. The stent of claim 1, wherein each annular element comprises a plurality of alternating struts and apices connected to each other to form a substantially annular configuration.

3. The stent of claim 2, wherein the connecting members are connected to the apices of the adjacent annular members.

4. The stent of claim 2, wherein the plurality of struts comprises left and right struts, with each pair of left and right struts connected to each other at an apex.

5. The stent of claim 2, wherein each strut has a longitudinal dimensional which is smaller when the annular elements are in the expanded state than in the compressed state.

6. The stent of claim 2, wherein each strut has a longitudinal dimensional which is larger when the annular elements are in the compressed state than in the expanded state.

7. The stent of claim 2, wherein at least one of the annular elements is closed such that the plurality of alternating struts and apices are connected to each other to form a closed annular element.

8. The stent of claim 1, wherein at least one of connecting member has a plurality of alternating segments.

9. The stent of claim 8, wherein the at least one connecting member has a plurality of alternating and angled straight segments.

10. The stent of claim 1, wherein each connecting member has a larger longitudinal dimension when each annular element is in the expanded state than in the compressed state to compensate for the smaller longitudinal dimension of the annular element in the expanded state.

11. The stent of claim 1, wherein each connecting member has a smaller longitudinal dimension when each annular element is in the compressed state than in the expanded state to compensate for the larger longitudinal dimension of the annular element in the compressed state.

12. The stent of claim 1, wherein the annular elements and connecting members define an alternating longitudinal pattern of annular elements and connecting members.

13. The stent of claim 1 comprising, at about normal body temperatures, a shape memory, superelastic, austenitic alloy portion and a shape memory, martensitic alloy portion, the superelastic austenitic alloy portion having a transition temperature from martensite to austenite less than body temperature while the martensitic alloy portion has a transition temperature from martensite to austenite greater than body temperature, the martensitic alloy portion and superelastic austenitic alloy portion being constructed, arranged and associated with respect to each other in comprising the stent such that the two alloy portions act in combination to allow, upon transformation of the austenitic alloy portion to martensite at a temperature below the transition temperature, constraint of the stent to a deployment diameter smaller than the predetermined fabricated diameter and upon transformation of the austenite alloy portion from martensite back to austenite to self-expand the stent back to about the predetermined fabricated diameter at temperatures in excess of the transition temperature of the austenitic superelastic portion, the shape memory of the superelastic austenitic portion tending to form the stent to a larger diameter due to its shape memory but being restrained therefrom by the martensitic alloy portion whereby the austenitic alloy portion can be deformed by external force without plastic deformation along with the martensitic portion to an enlarged stent diameter beyond that of the self-expanded diameter.