CA2602435C - Bistable spring construction for a stent and other medical apparatus - Google Patents

Abstract

The present invention is directed to bistable cells and their use in devices, particularly medical devices such as stents, clamps and valves. An expandable stent formed of a plurality of bistable cells is described. The stent has two or more stable configurations, including a first stable configuration with a first diameter and a second stable configuration with a second, larger diameter. A valve comprising a bistable cell for use in eliminating incontinence is also disclosed.

Description

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BISTABLE SPRING CONSTRUCTYON FOR A STE_rIT AND OTHEP. ?1TEDYCAL
APPARATUS
Rack2mund Ofthhe Invention There are several kinds of stents on the market with either balloon expandable or self expanding function. Balloon expandable stents are generally made from a material that can easily be plastically deformed into two dircctions.
Before insertion, the sten[ is placed around the balloon scction at the distal end of a catheter and pressed together to reduce the outer dimensions.
As soon as the stent is brought into the body in ttie proper axial position it can be cxpanded and thereby plastically deformed by ptunping up the balloon.
In this final position, the stent is at its largest diameter and should function to support the surrounding tissue, preventing an undrsired shape change into a much smaller diameter, at least locally.
Therefore, the stent needs to have sufficient rigidity in the radial direction, but also some flexibility in the axial direction when it is in the final position_ Further, the amount of material s}iould be as small as possible and in the inner surface of the stent should not obstruct the flow through the channel (e.g., for blood) or cause too much turbulence.
Problems that generally occur with these stents are as follows: After compressing the stent to its smttllest diameter around the ba[loon, the stent will always have some elastic spring back to a slightly targer diameter, which can cause problems when the catheter is brought into the patient's body. Inaddition, the axial friction between balloon and stent can become so small that the stent slips off the catheter.
Further, a larger size stent is typically a disadvantage.
A further piobIem is the so called recoil of these stents. This means that after expansion by the balloon pressure, the outcr diameter will always become slightly stnaller as soon as the balloon is clef7ated. This dcgec of reeoil ean be as niueh as 10%, which can cause-migration of the stent.
A different type of stent is'made of a more or less elastically expanding RECTIFIED SHEET (RULE 91) ISA/EP

structure, which has to be held on the catheter by some external means. An example of this type is a stent that is held in its constrained state by a delivery sheath, that is removed at the moment that the stent should deploy to its natural form.
Some of these stents are made of shape memory material with either superelastic behaviour or temperature sensitive triggering of the expansion function.
A disadvantage of these self-expanding stents is the need for the delivery sheath, causing a larger insertion diameter. The removal of the sheath also requires a sheath retraction mechanism, which has to be activated at the proximal end.
Most stents of both types further have the disadvantage of relatively large length change during expansion and a poor hydrodynamic behaviour because of the shape of the metal wires or struts.
Another disadvantage of some stents is the positive spring rate, which means that further expansion can only be achieved by higher balloon pressure.
The construction of prior stents is typically made in such a way that the external forces, working on the stent in the radial direction, merely cause bending forces on the struts or wires of the structure.
For example, a unit cell of a Palmaz-Schatz-stent, as produced by Johnson &
Johnson Interventional Systems or the ACT One Coronary stent, produced by Progressive Angioplasty Systems, Inc. has in its collapsed condition a flat, rectangular shape and in its expanded condition a more or less diamond-shaped form with almost straight struts (Palmaz-Schatz) or more curved struts (ACT-One).
The shape of the unit cell of such stents is typically symmetrical with four struts each having the same cross section. In addition, the loading of the cell in the axial direction will typically cause an elastic or plastic deformation of all of the struts, resulting in an elongation of the unit cell in the axial direction. These unit cells have a positive spring rate. In stents based upon these unit cells the stability against radial pressure is merely dependent on the banding strength of the struts and their connections.

Summary of the Invention In one aspect, the invention provides an expandable tubular device comprising one or more bistable cells, each cell comprising a rigid segment coupled to a relatively flexible segment, each cell being capable of transitioning between only a 2a . ., first stable state and a second stable state, wherein the device is expanded by applying a force thereto, and during expansion, the force required to move between the first stable state and the second stable state decreases when the cell expands beyond an intermediate transition point.
A new type of stent is described hereafter with a unit cell, having a negative spring rate and a bistable function. Such a unit cell can also be used in other medical applications. This means that it has two configurations in which it is stable without the need for an extemal force to hold it in that shape. The unit cell is formed using at least two different sections. One section is less pliable than the other one and acts a relatively rigid support that hinders the shape change of the more pliable section in one direction. In the other direction the pliable section can be deformed, but because of the opposing force from the rigid section, the stability of the pliable or flexible section is strongly increased.
External forces in a direction perpendicular to the most pliable section are distributed to the rigid section and the cross section of the pliable section is merely loaded in compression mode. This makes the construction much stronger than prior stents. In prior stents, all struts have generally the same cross section and mechanical properties and are merely used in the bending mode.
The construction of a stent, based upon this unit cell results in an apparatus, that can easily be elastically compressed around the balloon by finger pressure.
Below a certain critical diameter, the present stent snaps further to a stable, smallest diameter, thus holding the deflated balloon firmly on to the surface of the catheter, with an insertion diameter that is as small as possible. An additional sheath is not required, but may be used for extra safety.

After the stent has been brought into the patient's body at the proper axial position, the balloon can be inflated until the stent reaches its critical elastic equilibrium diameter. Slightly above this diameter the stent automatically expands further to its final largest diameter, where it reaches its maximum stability against radial pressure. The design enables a constant length large expansiori ratio, a reliable expandability and/or a small surface ratio.
A further embodiment of this invention is the possibility of a kind of stepwise expanding stent with a range of stable diameters.
Another part of the invention is a stent with several external diameters along its length, to adapt as good as possible to the shape of the body cavity where it is placed.
Another part of the invention is the possibility to modify the stress and strain pattern in the unit cell by means of a heat treatment in such a way, that the force - ~ ... "~

displ_acement characteristic of this unit cell becomes asymar:etrical or even exhibits a rnonostable instead of a bictable function, either yArnth the expanded diarTleter or the collapsed diameter being the most stable condition. =
Another embodiment of the invention is the modification of the geometry of the cross section of some struts to achieve the symmetric or asymmetric bistable or monostable force against displacement characteristics of a unit cell. =

Another part of the invention is the use of one or more unit cells in other medical applications such as, for example, an expander or a clip, either to spread a body cavity open or to clamp or hold a body part or some body tissue.
The invention is also directed to the use of the inventive stents in conjunction with inventive expander rings to join together two vessels.

The invention is also directed to a bistable valve having a snap-action bipositiorial unit cell and uses for the same, in particular, to prevent incontinence.
The invention is also directed to multistable cells and their use in medical devices.

Description of the Construction.
The construction of the present stent includes a series of elements with an arrangement of unit cells that enable the stability in a special way. Each unit cell exists out of at least two distinct, mechanically connected sections with different mechanical behaviors. One section acts as a relatively rigid support for the more flexible counteracting section. The more flexible section is responsible for most, if not all, of the expansion of the stent. There are several ways -to manufacture a stent based upon this principle and it can be made from several materials, like polymers, composites, conventional metals or shape memory alloys with superelastic behavior or with temperature sensitive behavior.

It can be made from an arrangement of wire or strip, welded together at specific places. Another possibility is metal deposition in the desired pattern onto a substrate or the use of sintering of prealloyed powder.
A further method is making the stent from a tubular shaped starting material, with a pattern of slits or slots made in the wall by means of etching, grinding, ~-.
WO 98/32412 PCT/[JS98/01310 cutting (e.g., with a laser, water, etc.), spark erosion or any other suitable method. The pattern can also be made in a flat plate and then welded, brazed or crimped to a more or less cylindrical shape or a cylindrical mid section with two conical ends with larger diameters.

Brief Description of the Drawings Fig. I shows the principle of a bistable mechanism;

Fig. 2 shows the force-displacement characteristic of the mechanism of Fig. 1;
Fig. 3 shows another bistable mechanism with an asymmetric bistability;
Fig. 4 shows the force-displacement characteristic of the mechanism of Fig. 3;

Fig. 5a shows an inventive tubular stent in the stable, fully collapsed configuration;
Fig. 5b shows an inventive tubular stent in the stable fully expanded configuration;

Fig. 6 shows a part of a stent with one bistable unit cell, drawn in the stable expanded shape;

Fig. 7 shows the part of the stent of Fig. 6 near its elastic bistable equilibrium position;

Fig. 8 shows the part of the stent of Figs. 6 and 7 in its stable collapsed shape; and Fig. 9 shows a larger section of the stent of Figs. 6 and 8, showing some unit cells in the collapsed shape and some unit cells in the expanded shape.
Fig. 10 shows an inventive stent formed of a plurality of smaller inventive stents joined together with flexible connectors.
Fig. 11 shows a partially expanded inventive stent having more than one type of bistable unit cell;

Fig. 12 shows an inventive stent having a range of diameters along its length;

Fig. 13 shows an inventive expansion ring in expanded state;

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Fig. 14 shows the expansion ring of Fig. 13 in contracted state;
Fig. 15 shows an inventive stent joining two vessels together and further securcd with inventive expansion rings, the stent exterior to the vessels;
Fig. 16 shows a cross-sectional view of Fig. 15 along section line 16-16;
Fig. 17 shows an inventive stent joining two vessels together, the stent interior to the vessels; =
Fig. 18 shows two vesselsjoined together with an itiventive cxpansion ring and a clamp Fig. 19 shows a bistablc valve in the closed position;
Fig. 20 shows the bistable valve of Fig. 19 in the open position;
Fig. 21 a shows a multistable cell in the fully contracted state;
Fig. 21b shows the multistable cell of Fig. 21a in the fully expandcd stare;
Fig. 22a shows another multistable cell in the fully contracted state;
Fig. 22b shows the rnultistable cell of Fig. 22a in the fully expanded state;
Fig. 23 shows several unit cells as shown in Figs. 21a,b joined together and in the fully expanded state;
Fig. 24a shows several unit cells as shown in Figs. 22a,b joined together and in the contracted state;

Fig. 24b shows the interconnected cells of Fig. 24a in fully cxpanded state;
Fig. 24c shows the interconnected units cells of Fig. 24a in the process of expanding; and Fig. 24d shows several strips of interconnected cells as in Figs. 24a,b joined together and in the process of expanding.
1etail ]2 escription of the Drawings Fig. I shows the principle on whicl-s the stent is basod, Fig. la stiows a rod I with a length L, which is compressed in its axial direction unit; it reaches its buelrling stress. Then the central part of the rod will bend out in a sidewards direction, either to position 2 or-3 (dashed lines in Fig. 1b). When the axia1 displacement L of the ends of the rod is held stable by extcanal clamps 4, it is possible to move the central section of RECTIFIED SHEET (RULE 91) ISA/EP

the rod -betwe.en the two stable positions 2 and 3. This movement is in a direction X, perpendicular to the original length axis A-A ofthe rod. All positions between the stable positions 2 and 3 are unstable. In Fig. lb the eentral part of the rod has to rotate over an angle (3 before the rod can be moved in direction X. Fig. 1C shows a second order curvature in rod 1, whiah occurs when the mtation over angle P is opposed by clamping the central part of rod I and maintaining this part parallel to the axis A-A.
Fig. 2 shows the force F as a function of displaccment X, with X

displayed in the hnrizonal direetion. The rod is iuoved from the upper 2 to the lower 3 stable position of Fig. I. The force incrcascs rapidiv from zero to Fmax. At that moment the onset of cither the first or second order ctmature of Fig.'lb and lc is reached. Further displacemcrzt in direction X costs lcss force, because this spring svstem has a negative spring ratc. The force even becomes zero in the mid position and further movement occurs automatically. It can be seen in Fig. 2 that the system is completely symmetrical and thc forcc needed to move back from the lower to the iiPrer positioII has the same characteristic.
Fig. 3 shows rod 5, which will have an asymmetrical force displacement characteristic, because it already has a preset curvatue, even in the unload.ed position, where the length is already L-eL. This can be nchievcd bv prior plastic deformatian, heat treatment or the use of an asymmctrical geomctry of the cross section of the rod (not shown). The rod 5 in Fig. 3 can be mounted between two clamps on a length.L -dL, and if it is elastically deforined in the same way as the rod in Figs lb and lc, it Nill have a different stress distribution in the cross section in end position 2 and 3, compared to the rod of Fig. 1. This means that the rod has becomc a prefcrcnt unloaded stable position, shown in Fig. 3.
Fig, 4 shows the asynzmetrical force-displacement character.stic of the precurved rod of Fig. 3. The initial displacement from the stable upper position needs a starting forrz Fl and if the rod is in its stable lower oosition the starting force in the opposite direction is only F2, being smaller than FL Force F2 can be made as small as dcsired,. even zero or negative, but needs to. have a positive value if stability of the lower position is required.
Figs. 5d and 5b show the general appearance of an inventive tubular stent RECTIFIED SHEET (RULE 91) ISA./EP

- ~ --~ , t ;
~

in fully contracted and fully expanded configuration respectively. The stent, in its fully contracted .CittKiv Jhcns s,m generally at S/V and in itJ 1411Y Lx-GLllded state sh~, W2~ ge~~erd.liy at 60, is comprised of a plurality of interconnected bistable unit cells (shown in the =
expanded state at 64 in Fig. 5b). The bistable unit cells are formed from a first relatively rigid segment 52 (66 in Fig. 5b) and a second relatively flexible segment 54 (68 in Fig.
5b), joined together at ends 70 and 72. Second relatively flexible segments 68 are interconnected with adjacent relatively rigid members 66. Adjacent cells in the longitudinal sense (the longitudinal axis is denoted by reference numeral 75) are joined at ends 70 and 72. By applying a uniform radially outward or inward force, .the stent may be switched directly from a fully contracted to a fully expanded configuration or vice versa.
Fig. 6 (corresponding to inset 6 in Figure 5b) shows a small part of a stent such as that shown in Figs .5 which uses the bistable function of a unit cell, according to the present invention. The drawing shows a horizontal line A-A, which is parallel to the central axis of the stent. There are two series of sinusoidal segments with distinct size (see also Fig. 9 for an overview). The segments 7 and 9 have a relatively large cross section. Only segment 9 is shown entirely. The segments 9 and 10 have a relatively smatler cross section, and here only segment 8 is entirely shown. The segments are interconnected for example welded, at joints 11 and 12.

Because of the difference between the cross section of segment 8 and 9.
the deformation force of segment 8 is much lower than for segment 9.
Therefore, segment 9 can be considered as a relatively rigid clamp, like the clamps 4 in Fig. lb opposing relative displacement between the joints 12 in the axial direction, parallel to axis A-A. In contrast, segment 8 acts as a flexible rod, like rod 1, described in Fig. 1 or rod 5, described in Fig. 3. This combination of segments 7 and 8 or 9 and 10 defines a unit cell, acting as a bistable spring system with a force-displacement curve F-X like the described curves of Fig 2 and 4, depending on the unloaded condition and geometry of the segments. Alternatively, instead of using segments or struts of different diameter, the segments can have the same diameters (i.e., cross sectional area) and exhibit different strengths or rigidity and still accomplish the same effect. One way to obtain such differences in strength or rigidity would be to use different materials for the segments.

Another way would be to use the same material, like a metal, for all the segments but selectivelv strengthen (e.g., by heat treating) thuse segiixents that need to be rigid. It should be noted that heat treatment will not strengthen all materials.
Nitinol, for example becomes mare pliable as a result of heat treatment. This propeny of Nitinol can be exploited, however, to render one section of Nitinol more pliable relative to a second, non-heat-treated section of Nitinol.
Fig. 7 shows the sar.:e part of the stent (as depicted in Fig. 6) near the elastic equilibrium position. Segment 8 has been deformed into the dircction X, causcd by force F, but segment 9 has alinost its original shapc, because of its larger rigidity.
Fig. 8 shows the same unit cell of the stent of Figs. 6-7 after it has been pressed through the elastic equilibrit!rn. Fosition. It automatir.ally snaps into its stable position of Fig. 8. This snapping force can be strong enough to hold a deflated balloon tight on the catheter shaft (not shown), depending on the mechanical charaeteristics (e.g., the strength) of the material(s) used to make the segntents. With the geometry shown in these figures, the segments 8 and 9 fit close together. taking up a minimum amount of spacc whcn the stent is in its smallest stable diameter.
Fig. 9 shows a section of the st.ent of Figs. 5, flattened for illustrative purposes, showing several flexible segments in the collapsed stable shape (segments 14, 18 and 20) and one segment element 16 in the expanded stable shape. Segments 13, 15, 17,-and 19 are relatively rigid segments and substantiaily maintain tlieir original shapc:.
The distance between two relatively rigid seginents is shown as (h) in che collapscd stable shape and (H) in the expanded stable shape. The value of the displacement (H-h) in the direction X depends on the heigYtt of an cxpanded unit cell or amplitude of thc segments and the size of the connecting joints. '['he dcscribed part of the stent is showzl as a flat surface, but it may be clear that a cylindrical stent such as that shown in Pigs. 5 is shaped if segments 13 and 20 are directly connected to reach other with joints 21. In other words, the stent is shown separated along the joints 21 and in a 1lattened condition.
The rangc of stable diameters of the stent changes v&ith the value (K-h)/-;:, each ti-me that a flexiblc segment snaps from the collaPscd stable position to the expanded stable position. The resvlt is a stent with an extremely rigid -stuface at all -diameters being able to withstattd the external forces better thwt with conventional stents.
RECTIFIED SAEET (RULE 91) ISA/EP

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i.. I~
WO 98/32412 PCTIUS98ro1310 In the length direction, the flexibility of the stent can be increased by disconnecting several unit cells from thrõir nr ighbor ~~nit ce!ls, for example, by c~~tting the center of one or more joints while maintaining the several joint pieces as joints.
Another method to increase flexibility is to change the geometry of 5 several sections of the unit cells in the length direction from the relative flexible to the relative rigid shape several times along the total length of the stent. In other words, =
referring to Fig. 9 one or more or each of the segments 13 - 20 could be constructed with larger and smaller diameter (or otherwise flexible and rigid) sections which alternate after each joint 21.

10 Another possibility, as shown in Figure 10 is the use of a series of short multistable stents 100 aligned lengthwise end to end and connected with flexibility joints 104 having the same or a different geometry or configuration as the joints forming individual unit cells.
The scope of the invention should include all types of material. One of the most interesting materials is superelastic Nitinol, because of its large elastic strain, well defined stress values, caused by their plateau stresses and the possibility to define the desired curvature into the metal by means of a heat treatment. A stent of Nitinol can be made by forming slits or slots in a tube, while in its collapsed or smaller stable diameter. The slotted tube is then expanded by a separate shaping tool and heat treated on this tool to define the expanded stable diameter as the unstrained shape.
In a more general sense, the present invention is directed to a device having a plurality of stable configurations. The device is comprised of a plurality of interconnected multistable cells. The cells incliide one or more relatively rigid sections and one or more relatively flexible sections interconnected so as to define a cell structure in the form of a multistable spring system having a plurality of stable configurations. In a preferred embodiment, the cells comprise a first arcuate member having first and second ends and a second arcuate member having first and second ends, the first end of the first member in communication with the first end of the second member, and the second end of the first member in communication with the second end of the second member. It should be noted, however that members need not be rigorously arcuate.
Other shaped members, including relatively straight members are contemplated'as well.

ll The invention, in particular, contemplates bistable cells, that is cells having two stable configurations. In one such cell, the distance between corresponding points on the f rst and second secd ons is larger in the first stable state of the cell than in the seeond.stable state of the cell. The cells themselves are constructed and arranged so that the device itself is at least bistable and possibly multistable. One such device, a cylindrical stent having two or mort configurations with an initial diameter size and a final larger diamcter size has been described above. However, multistable stents are also contemplated. Thus; for example, a stent may be constructed in which the cells are designed and arranged to provide a range of diameters in step-wise fashion.
One such way this may be accomplished would be to employ several different types of cells in the stcnt, each type of cell having a different spring constant so that depending on the amount of force used, the stent would assume a different diameter. Such a stent in a partially expanded state is shown schematically in Fig. 11. A partially expanded stent is shown generally at 120. The stent is comprised- of relatively rigid segments 123, 127, 131 and 135 which substantially maintain their original shape, and relativety flexible segxnents-125,129, and 133. The segments are interconnected, with joints 122.
As depicted, -first flexible elcments 125, and 133 are in an expanded configuration while sccond flexible clcment 129 is in a contracted configuration. By applying a radially outward or tangential force, flexible element 129 may be flipped to its -fiilly expanded configuration resulting in a stent (not shown) with a larger diameter. As shown in Fig.

11, ctlls 138 are larger than cells 136 even in the contracted state. First flexible elements 125 and 133 are characterized by a different degrcc of flexibility than second flexible element 129.
Another form of stcnt, as shown generally at 150 in schematic Fig. 12, has an first diameter at a first end 152, a second diameter at a second end 154 and one (or more) intermediate diameters in a reeien 156 between rrtit. crid 152 and second end 154, the intermediate diameter differing froin the first alid second diamctcrs. The interconnected cells in such a stent, as showtt gcnerally at 150 in Fig. 12 may all have the same force constant and hence be openable all at once with the application of the necessary force or there may be several different types of cells, each with their own force constant In order to achieve the multiplicity of diameters, cells of differing sizes may be RECTIFIED SHEET (RULE 91) ISA/EP

WO 98132412 PCT/US98l01310 used. In one embodiment of this type of stent, the first and second diameters are the c~.,~P , Pr}~;1A ;n ~.~nt},o~ Pmhn~;mPnt tha first and sPrnnri riiamatPrc riiffer .,c.ua.v auav ua viaav. . - , + _ -The present invention is also directed to a method of implanting an expandable stent having a plurality of stable configurations. The method comprises the steps of applying the stent to an expanding means on a catheter, delivering the stent to a desired bodily location, expanding the expanding means so as to expand the stent from a, first stable configuration to a desired second stable configuration, the second stable configuration having a larger diameter than the first stable configuration, and deploying the expanded stent at the desired bodily location. The expanding means may be a balloon, a mechanical device on or in the catheter, a heat source where the cells can be induced to change states by heating or any other suitable expanding means. The stent may be applied to the balloon in the first stable configuration or may be applied in the second stable (expanded) configuration during the applying step. In the latter case radially inward pressure may be applied to the stent so as to urge the stent into the first stable configuration to snap it onto the catheter. Where the stent has additional stable states, the stent may be applied to the balloon in an intermediate stable state in which the diameter of the stent is intermediate between the diameter in the first state and the diameter in the second state. Again, the stent may be locked on the expanding means by further applying a radially inward pressure.

A further object of the invention is the use of a single bistable unit cell as an expander (expansion ring), that can be brought into a narrow place and then triggered to snap back into its expanded stable shape. As shown in Fig. 13 an expansion ring shown generally in its expanded state at 250 coinsists of a first rigid member 254 having first 258 and second 262 ends and a second more flexible member 266 having first 270 and second 274 ends. First end 258 of first member 254 is connected to first end 270 of second member 266 and second end 262 of first member 254 is connected to second end 274 of second member 266. Fig. 14 depicts the expansion ring of Fig. 13 in its contracted state. Second member 266 is seen to be in a second stable position.

Another object of the invention is the use of a single bistable loop (unit cell) as a clip, that can be used to clamp on an artery, fallopian tube or any other body part, to close or hold it for some time. For such a clip it may be desirable to define the WO 98/32412 PCTlUS98/01310 collapsed stab[e shape as the unstrained shape, because the collapsed stable shape has to be the most stable one. In the collapsed state, ihe clip would resemble the collapsed expansion ring of Fig.14: A triggering means would be used in conjunction with the clamp to switch the bistable loop from one state to another. The triggering means may be pneumatic, hydraulic, mechanicaI, therrnul or elcctromechanical means.
Examples -of such triggering means include a human hand applying force to the bistable loop, and the application of heat to the loop. dkher triggering means include pulling on the device, pushing on the device, bending the rigid section of the device or releasing a restraint holding the flexible member in place.
1 ~ Another part of the present invention involves constructions between one or more rinb-shaped elements acearding to the present invenuon, combined with a tubular sleeve that is reinforced or held open with such elements. An exnmple is a so-called graft stent made of a po137ner with one or more expansion rings. The expaiision rings may consist of the above-described bi-stable cells. The surface of'the stent comprises a skin mounted on the expansion rings. In mounting the skin, the skin may surround, be in or between the expansion rings. The skin may be human or animal slin, zi polyzncric rnatcrial or any other suitable bio-compatible matcria.l. Such a stcnt may comprise one or more expansion rings, such as a first expansion ring at a first end of the stent and a second expansion ring at a second end of the stent. The stent may be of constant diameter along its length or may have a first diameter at the first end and a second diameter at the second end.
The present invention is also directcd to a stcnt having an unCxpandcd configuration and an expanded configuration, and comprising a plurality of generally longitudinal, wave-like first members characterized by a first wavelength, and having peaks and troughs and a plurality of generally longitudinal wave-like second members chaxacterized by a second wavelength, and having peaks and troughs. The wavelengths of the tlrst and second longitudinal members are substantially equaL '1'he second members are capable of stably assuming two positions, a first position corresponding to the unexpanded configuration in which the first and second members are in phase and a 30. second position correspond"uig to the expanded configuration, in which the first and second members azt;180 out of phase. The first members are tnorc rigid than the RECTIFIED SHEET (RULE 91) ISA/EP

..+-=~ - . ' .-- ~' second members. The first and second longitudinal members are disposed on the surface of the stent such that the longitudinal fust and second members alternate. In the unexpanded state, each peak of each first member is connected to one adjacent peak of a second member in a region of attachment and each trough of each first member is attached to one adjacent tmugh of a second member in a region of attachment, as can be seen from rig. 9. The regions of attachment are separated along the longitudinal direction by one wavelength. The so described stent em be snapped from the unexpandcd configuration to the expanded configuration by applying a radially outward force and similarly can be snapped from the expanded to;he unexpanded configuration by applying a mdiatly inward force. While such stents may be used intemal to a bodily vessel, it may also be used extemal to vessels to join two vessels together.
The invention also contemplates a inethod of joining together two vessels cotnpr:sing the steps of delivering an inventive stent in an unexpanded configuration in a 'first stable state to a bodily site, expanding the stent to a second smble state, the diameter ofthe stent in the second stable state exceeding that of the vessels to be joined and placing the stent over the vessels to be joined. The stent may then be contracted to a third stable state, the stent in the third stable state having a diameter-inter.nediate between the diameters of the stent in the unexpanded state and in the second stable state.
The stent may further be secured to the vessel with the aid of one or more of the above-described expw.sion rings (a bistable loop). One or more expansion rings, such as that depicted in Figs. 13 and 14 or small clamping stents (such as that formed from the strip = shown in Fig. 23) may be delivered to each side of the stent in a contracted state and deployed so as to clamp the vessels between the ring(s). Multiple rings may be use-d for additional clamping. As shown generally at 300 in Fig. 15, a first vessel=304 and a second vesse1308 are joined together with inventive stent 312. Vessel 304 overlaps stent 312 in a first overlap region 316 while vessel 308 overlaps stent 312 in a second overlap region 320. Vessel 304 is clamped between expansion ring 324 (shown in the expanded state) and stent 312 whilc vcssel=308 is clamped between expansion ring 328 (shown in the unexpande:d state for illutrative purposes only) and stent 312. the dotted lines assooiated with expansion ring 328 illustrate expansion ring 328 in its expanded statc. It should be additionally noted that Fig. 15 provides a cut-away view of vessels 'showinb RECTIFIED SHEET (RULE 91) ISA/EP

the rings contained therein. Fig. 16 shows a cross-sectional view of Fig. 15 along section line 16-16. Vesse1304 is shown sandwiched between stent 312 and expansion ring 324.
In another embodiment, as shown in Fig. 17, a first vesse1404 and a second vessel 408 are joined together by a'stent 412. First end 416 of stent 412 rests in 5 vesse1404 while second end 420 of stent 412 rests within vesse1408. Optional clamps (such as a small portion of a collapsible inventive stent shown later in strip form in Fig. 23) 424 and 428 residing on the outside of vessels 404 and 408 clamp the stent to the vessel. Additional clamps may be used as needed.
In another embodiment, a combination of the embodiments of Figs 15 and 10 17, the first end of the stent may protrude from one of the vessels and the second end of the stent may extend over the second vessel. Again, clamps and expansion rings may be used to further secure the stent to the vessels.

I In another embodiment, as shown in Fig. 18, vessel 454 and vessel 458 are held together by an expansion ring 462 internal to the vessel and a clamp 466, 15 consisting of, for example, a small section of collapsible stent, the stent chosen so that the diameter of the stent in a collapsed state affords a snug fit with vessels 454 and 458 and expansion ring 462. Either the expansion ring or the clamp, but not both, may be replaced by a suitable support such as a rigid collar.
The invention also contemplates a method ofjoining together two vessels comprising the steps of delivering an inventive stent in an unexpanded configuration in a first stable state to a bodily site, placing two bodily vessels over the stent and expanding the stent to a second stable state, the diameter of the stent in the second stable state exceeding that of the vessels to be joined. The diameter of the stent in the second stable state is preferably chosen so that the vessels fit snugly over the stent. The delivery of the stent may be accomplished by delivering the stent in an unexpanded configuration through a bodily vessel and subsequently expanding the stent to rest snugly in the vessels to be joined (where a portion of the stent resides in a vessel), or by expanding the stent to its most expanded state, placing the stent over the vessel and then contracting the stent to an intermediate state over the vessel. The collars and expansion rings mentioned above may similarly be delivered. Alternatively, the stent, collars and expansion rings may be delivered by surgically exposing the vessel in question.

~ ~ i %'~'; ~'= '' =

The present invention is also directed to a bistable valve. The valve, as shown generally at 600 in Fig. 19 includes a snap-actior bipositional unit ccII shown generally at 604 located within a conduit 606. Snap-action bipositional unit ceII 604 consists of a(substantially arcuate) tlexible member 608 having a first end 612 and a second end 616. First end 612 is in communication witt: a trigocring means 620 which is supported, in turti by a support means 624 enierging from the inner surface of conduit 606. Second end 616 oftlexible member 608 is anchored to siop surface 628 which extends across conduit 606. Suppon means 624 and stop surface 628 must be sufficiently rigid to hold flexible member 608 in place and must be more rigid than flexible naember 608. Stop surface 628 extends substantially obliquely across conduit 606 in oblique regions 630 and has a opening 632 within in longitudinal region 634 to allow the flow therel,hrough of a fluid. Although openiiig 632 is oriented along the longitudinal axis 636 of conduit 606, those of ordinary skill in the art will recognize other possible orientations of the opening and stop surface. Valve closure member 640, actuated between open and closed positions by flexible member 608, is constructed and arranged so as to block the flow of fluid through opening 632 when flexible member 608 is in the closed position. When flexible member 608 is in the open position, as depicted in Fig. 20 valve closure member 640 no longer obstructs opening 632, thereby allowing the flow of fluid therethrough.
While triggering means 620 may be any suitable inechanical, hydraulic, pneumatic, or thermal based trigger known in the art at present or in the future, in a preferred embodiment, triggering means 620 is a piezoeicctric elemcnt: In operation; if the piezoclem.ent shown in Fig. 19 at 620 is not ac;ivated, valve closure member 640 is closed. Activation of piezoelement 620, as shovkm. in Fio. 20 causes a small shorteni..-ig in the longitudinal length (denoted by Y in Fig. 15) of piezoelement 620 which in turn releases flexible member 608 from its first position. With member 608 relesised, valve closure member 640 'is frec to open under the pressure uansmitted from the fluid.
Member 608 a,ssumes a second, inverted, position, as depicted in Fig. 20.
Dv'liile the fluid pressure maintains member 608 in its second position, even in the a.hcence of any fluid.
member 608 remains in its second position, as depicted in Fig. 20 if the.triggering is tumed off and piezoelemcnt 620 assumes'its original length. Valve closure membcr.640 RECTIFIED SHEET (RULE 91) LSA/EP

may be closed again, in the absence of fluid, by a subsequent triggering of piezoelement 620 allowing member 608 to transition to its second (closed) position which is the preferred position of member 608. Member 608 has been treated to receive a preferred position as shown in Fig. 3.

The valve depicted in Figs. 19 and 20 may be applied to medical and non-medical devices. It is, in particular, an aim of the present invention to apply the inventive bistable valve to the control of urinary incontinence. In a patient with incontinence, the above described valve may be implanted in the urethra using any suitable means including the use of the above-described expansion rings to clamp the valve to the urethra. Although the valve in the default state is closed, the valve may be triggered when the bladder is full, to void the bladder. Upon voiding the bladder, the valve may be triggered again to close it. Another such application is to employ the inventive valve in conjunction with a shunt. The shunt may be activated by triggering the device and similarly may be closed by triggering the device.

Of course the valve may be used in other medical and non-medical applications as well.

In addition to the bistable unit cells disclosed above, bistable unit cells and more generally, multistable unit cells of other shapes are also contemplated by the present invention. Figs. 21 a and 21 b are schematic representations of another embodiment of an inventive hinged multistable cell in its contracted and expanded states, respectively. The contracted cell, shown generally at 700, and the expanded cell, shown generally at 705, consist of four interconnected relatively rigid members. Two side members 709 are connected to opposite ends of top member 713 via hinges 715.
Side members 709 are connected at their opposite ends to opposite ends of bottom member 717 via hinges 719. Preferably, the hinges are elastic or plastically deformable. The hinges may be fixedly attached to the side, top and bottom members or may be integral with these members. In the latter case, the hinges may be formed bv removing material from the cell in the region of the hinges so that the hinges are thinner or have a different geometry from the side, top and bottom members. In the process of transitioning from the expanded to the collapsed state, bottom member 717 opens slightly. The cell of Figs.
21 a,b also has two additional intermediate states in which one or the other (but riot both) of side members 709 and top member 713 are collapsed downward.
A hexagonai hinged ~ii~.;ltistabie unit ceii is shown schematically in Fig.
22a in the collapsed state and in Fig. 22b in the expanded state. The cell, shown generally at 750, consists of top member 754 and bottom member 758, and upper side members 762. Two upper side members 762 are connected to opposite ends of top member 754 via hinges 756. Upper side members 762 are connected to bottom member ' 758 via hinges 768. Bottom member 758 is shaped like a'U' with the two uprights of the 'U' modified to lie at oblique angles with respect to the bottom part of the'U'. As with the previously discussed inventive cells, hinges 756 and 768 may be elastic or plastically deformable and may be fixedly attached to the members or integral with the members.
The hexagonal unit cell exhibits multiple stable states. In addition to the fully expanded and fully contracted states shown in Figs. 22a and 22b, the hexagonal cell can also achieve two intermediate stable configurations in which only one of the two upper side members 762 is collapsed inward along with top member 754.
The above described hinged multistable cells may be used in any of the above discussed applications e.g. to form stents, clamps, clips, expander rings, bistable valves.
In one such application a ring or stent is formed of the hinged cells of Figs. 2 I a and 21 b. As shown in Fig. 23, a series of unit cells of the type depicted in Figs.
21 are joined together so that the top member of a cell forms a portion of the bottom member of an adjoining cell. As depicted, top member 814 of cell 810 fonns a portion of bottom element 818 of cell 820. Similarly, top member 824 of cell 828 forms a portion of bottom element 832 of cell 836. Although the ring or stent in Fig. 23 has been cut for illustrative purposes, the two ends 840 and 844 are normally joined together with a portion of lower member 848 of cell 852 serving as an upper member for cell 856. The ring so formed has a range of stable stable states including a fully expanded state and a fully contracted state. Where the individual cells are made identically, only the fully expanded states may be accessed by the application of a uniform radially outward force to the stent in the fully contracted state. It may serve as a clamp or collar, an expansion ring or a stent. Larger stents may be formed by interconnecting a plurality of such rings.
Similar products may also be formed from other multistable units cells.

Figs. 24a and 24b illustrate one such possibility schematically in which hexagonal unit cells such as those shown in Figs. 22a, b may be joined together to form. a rino: The top member 884 of each cel1880 is joined with a the bottom portion 886 or modified'U' shaped bottom member 890. Although shown in strip form in Figs. 24a and 24b, end 894 can be joined to end 898 to ibrm a ring. The strip of Fig. 24a is shown in fully expanded state in Fig. 24b. Adjacent cells 880 are seen in their expanded state. For the sake of completeness, the hinges are designated 902. Fig. 24c shows or.e cell 920 in the process of expanding and one atlready expandcd cei1924. The cells 920 and 924 are joined at bottom member 92$ and top member 932. 1-Tinaes are shown at 936. Multiple strips may also be joined togcther so as to form a stent whose length is a multiple of the length of the unit cell. In such a casc, upper side members of adjacent cells would be joined togcther. This is illustrated in Fig. 24d which, like Fig. 24c shows cells 940 in the expanded state and eells 944 in the process of expanding. Upper side members 948 are shown by dashed lines_ Adjacent strips of interconnected eclls 952 are joined together by tipper side members 948 as well as by oblique regions 956 of bottom members 960.
It should be noted that the inventive devices of the present application may be used on a temportuy basis or on a permauent basis in the body. i hus, for example, permanent stents atid clamps are contempiated, as are removable stents and clamps.
It should further be noted that in expanding some of the inventive multistable cells, there rnay be components of expansion/contraction in u direction perpendictilar to the direction of ttie fo= applitd to expand thc eells.
Finally, for the purposes of this application, the terrn 'multistable' is intended to include 'bistable'.
In the described drawings and text only some examples of different embodiments have been given_ While the stents of the present invention can appear similar to prior stents, the mechruiical results are eotnpletely different due to the special combination of a rigid section and a.tnore flexible section in the same unit cell. Of course there are, beside the illustrated sinusoidal shape many other possible basic shapes for the unit cells, with similar characteristic behavior.
From the above disclosure of the general principles of the present RECTIFIED SHEET (RULE 91) ISA/EP

-WO 98/32412 19a PCT/US98/01310 invention and the preceding detailed description, those skilled in this art will readily RECTHUD SHEET (RULE 91) ISA/EP

WO 9W2412 PCTlUS98101310 comprehend the various modifications to which the present invention is susceptible. It is intended for the coverage of the present application to include different geometries, different constructions and different combinations of one or more materials to obtain the same basic mechanical behavior as exhibited by the above described examples.

Claims (40)

1. A stent comprising:
one or more unit cells, at least one of which has one or more stable collapsed configurations and one or more stable expanded configurations;
wherein:
at least one of the one or more unit cells comprises a relatively rigid portion and a relatively flexible portion interconnected so as to define a cell structure having a plurality of stable configurations; and at least one of the one or more unit cells passes at least one transition point that allows force to be decreased either during expansion from the stable collapsed configuration to the stable expanded configuration or during collapse from the stable expanded configuration to the stable collapsed configuration.

2. The stent of Claim 1, wherein the stent has more than two stable configurations.

3. The stent of Claim 1, wherein the relatively rigid portion comprises a first arcuate member having first and second ends and the relatively flexible portion comprises a second arcuate member having first and second ends, the first end of the first member being coupled to the first end of the second member, and the second end of the first member being coupled to the second end of the second member.

4. The stent of Claim 1, wherein the stent comprises more than one type of cell, each type of cell having a different spring constant, each spring constant causing the stent to assume a different diameter according to the amount of the applied uniform radially directed force.

5. The stent of Claim 4, wherein the stent has two or more stable configurations.

6. The stent of Claim 1, wherein one or more of the unit cells has an equilibrium center position and an asymmetrical force-displacement characteristic around the equilibrium center position, and wherein the expanded configuration is the most stable configuration.

7. The stent of Claim 1, wherein the stent comprises nitinol.

8. A stent comprising one or more unit cells, at least one of the one or more unit cells having at least a first stable state and a second stable state, wherein:
at least one of the one or more unit cells comprises a relatively rigid portion and a relatively flexible portion interconnected so as to define a cell structure having a plurality of stable states;
the second stable state encompasses a larger area than the cell in the first stable state; and at least one of the one or more unit cells is characterized by a negative spring constant and is constructed such that the stent has a plurality of stable states.

9. The stent of Claim 8, having only one unit cell.

10. The stent of Claim 8, wherein at least one of the one or more unit cells is capable of assuming only the first stable state or the second stable state.

11. The stent of Claim 8, wherein the relatively rigid portion comprises a first arcuate member having first and second ends and the relatively flexible portion comprises a second arcuate member having first and second ends, the first end of the first member coupled to he first end of the second member, and the second end of the first member coupled to the second end of the second member.

12. The stent of Claim 8, wherein the one or more unit cells are constructed and arranged so that the stent is switchable between at least two stable states by applying a uniform radially directed force to the stent.

13. The stent of Claim 8, wherein the one or more unit cells comprises more than one type of cell, each type of cell having a different spring constant, each spring constant causing the stent to assume a different diameter according to the amount of the applied uniform radially directed force.

14. The stent of Claim 8, wherein the stent has three or more stable states.

15. The stent of Claim 8, wherein one or more of the unit cells has an equilibrium center position and an asymmetrical force-displacement characteristic around the equilibrium center position, and wherein the second stable state is the most stable state.

16. The stent of Claim 8, wherein one or more of the unit cells has an equilibrium center position and an asymmetrical force-displacement characteristic around the equilibrium center position, and wherein the first stable state is the most stable state.

17. The stent of Claim 1, wherein one or more of the unit cells has an equilibrium center position and an asymmetrical force-displacement characteristic around the equilibrium center position, and wherein the collapsed configuration is the most stable configuration.

18. The stent of Claim 1, wherein the relatively flexible portion comprises a plurality of members interconnected by one or more hinges.

19. A vascular device, comprising a first circumferential support including the stent of Claim 1, a second circumferential support, and a flexibility joint coupled at a first end to the first circumferential support and at a second end to the second circumferential support.

20. A vascular device, comprising a first circumferential support including the stent of Claim 8, a second circumferential support, and a flexibility joint coupled at a first end to the first circumferential support and at a second end to the second circumferential support.

21. A tubular device comprising:
one or more unit cells, at least one of which has more than two stable configurations, at least one of the stable configurations being a stable collapsed configuration and at least one of the stable configurations being a stable expanded configuration;
wherein at least one of the one or more unit cells passes at least one transition point that allows force to be decreased either during expansion from the stable collapsed configuration to the stable expanded configuration or during collapse from the stable expanded configuration to the stable collapsed configuration.

22. The tubular device of Claim 21, wherein at least one of the one or more unit cells comprises a relatively rigid portion interconnected with a relatively flexible~portion to define a cell structure having a plurality of stable configurations.

23. The tubular device of Claim 22, wherein the relatively rigid portion comprises a first arcuate member having first and second ends and the relatively flexible portion comprises a second arcuate member having first and second ends, the first end of the first member being coupled to the first end of the second member, and the second end of the first member being coupled to the second end of the second member.

24. The tubular device of Claim 22, wherein the relatively flexible portion comprises a plurality of members interconnected by one or more hinges.

25. The tubular device of Claim 21, wherein the tubular device comprises more than one type of cell, each type of cell having a different spring constant, each spring constant causing the device to assume a different diameter according to the amount of the applied uniform radially directed force.

26. The tubular device of Claim 21, wherein one or more of the unit cells has an equilibrium configuration and an asymmetrical force-displacement characteristic around the equilibrium configuration, and wherein the expanded configuration is the most stable configuration.

27. The tubular device of Claim 21, wherein the device is selected from the group consisting of a stent, a bistable valve, an expander, a clip, a loop, a collar, and a ring.

28. The tubular device of Claim 21, wherein the device comprises nitinol.

29. The tubular device of Claim 21, wherein one or more of the unit cells has an equilibrium configuration and an asymmetrical force-displacement characteristic around the equilibrium configuration, and wherein the collapsed configuration is the most stable configuration.

30. A vascular device, comprising a first circumferential support including the tubular device of Claim 21, a second circumferential support, and a flexibility joint coupled at a first end to the first circumferential support and at a second end to the second circumferential support.

31. A tubular device comprising one or more unit cells, at least one of the one or more unit cells having more than two stable states, including a first stable state and a second stable state, wherein:
at least one of the one or more unit cells comprises a relatively rigid portion and a relatively flexible portion interconnected so as to define a cell structure having a plurality of stable states;
the second stable state encompasses a larger area than the cell in the first stable state; and at least one of the one or more unit cells is characterized by a negative spring constant and is constructed such that the device has a plurality of stable states.

32. The tubular device of Claim 31, having only one unit cell.

33. The tubular device of Claim 31, wherein at least one of the one or more unit cells is capable of assuming only the first stable state or the second stable state.

34. The tubular device of Claim 31, wherein the relatively rigid portion comprises a first arcuate member having first and second ends and the relatively flexible portion comprises a second arcuate member having first and second ends, the first end of the first member coupled to the first end of the second member, and the second end of the first member coupled to the second end of the second member.

35. The tubular device of Claim 31, wherein the one or more unit cells are constructed and arranged so that the device is switchable between two stable states by applying a uniform radially directed force to the device.

36. The tubular device of Claim 31, wherein the one or more unit cells comprises more than one type of cell, each type of cell having a different spring constant, each spring constant causing the device to assume a different diameter according to the amount of the applied uniform radially directed force.

37. The tubular device of Claim 31, wherein one or more of the unit cells has an equilibrium configuration and an asymmetrical force-displacement characteristic around the equilibrium configuration, and wherein the second stable state is the most stable state.

38. The tubular device of Claim 31, wherein the device is selected from the group consisting of a stent, a bistable valve, an expander, a clip, a loop, a collar, and a ring.

39. A vascular device, comprising a first circumferential support including the tubular device of Claim 31, a second circumferential support, and a flexibility joint coupled at a first end to the first circumferential support and at a second end to the second circumferential support.

40. The tubular device of Claim 31, wherein one or more of the unit cells has an equilibrium configuration and an asymmetrical force-displacement characteristic around the equilibrium configuration, and wherein the first stable state is the most stable state.