Abstract:
In accordance with embodiments of the present invention, a method for preparing a shape memory alloy endoprosthesis, displaying strain induced martensite phenomenon, for delivery includes inserting a shape memory alloy endoprosthesis into a delivery device, inducing a first strain within a first region of the shape memory alloy endoprosthesis, inducing a second strain within a second region of the shape memory alloy endoprosthesis, and sterilizing the delivery device while maintaining the first strain and the second strain induced within the shape memory alloy endoprosthesis. In accordance with other embodiments of the present invention, an apparatus for delivering a shape memory alloy endoprosthesis includes an inner core having a first diameter, an outer body having a second diameter greater than the first diameter, and a calibrated endcap attached to the inner core. The outer body surrounds the inner core, and the calibrated endcap includes a roof section having a third diameter greater than the first diameter and less than the second diameter.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a delivery method, apparatus and system for an endoprosthesis. More particularly, the present invention relates to a delivery method, apparatus and system for a shape memory alloy endoprosthesis which displays strain induced martensite phenomenon. 
       BACKGROUND OF THE INVENTION  
       [0002]    Implantable endoprostheses, such as, for example, stents, heart valves, bone plates, anchors, screws, clips, etc., must meet many requirements to be useful and safe for their intended purpose. For example, they must be chemically and biologically inert to living tissue and to be able to stay in position over extended periods of time. Furthermore, devices of the kind mentioned above must have the ability to expand from a contracted state, which facilitates insertion into body cavities, conduits, lumens, etc., to a useful expanded diameter. This expansion is either accomplished by a forced expansion, such as in the case of certain kinds of stent by the action of a balloon-ended catheter, or by self-expansion such as by shape-memory effects. 
         [0003]    A widely used metal alloy for such applications is the nickel-titanium (Ni—Ti) binary alloy generally known as “Nitinol.” Under certain conditions, Nitinols can be highly elastic such that they are able to undergo extensive deformation and yet return to their original shape. Furthermore, Nitinols possess shape memory properties such that they can “remember” a specific shape imposed during a particular heat treatment and can return to that imposed shape under certain conditions. Other shape memory alloys are also known, such as, for example, the Ni—Ti—X ternary alloy (where X may be V, Co, Cu, Fe, etc.), the Cu-AT-Ni ternary alloy, the Cu—Zn-AT ternary alloy, etc. 
         [0004]    The shape memory effect demonstrated by Nitinol alloys generally results from metallurgical phase properties. Certain Nitinol alloys are characterized by a transition temperature range, above which the predominant metallurgical phase is termed “austenite,” and below which the predominant metallurgical phase is termed “martensite.” The transformation temperature from martensite to austenite is termed as “austenitic transformation,” while the reverse transformation, from austenite (or austenitic state) to martensite (or martensitic state), is termed “martensitic transformation.” These phase transformations occur over a range of temperatures and are commonly discussed with reference to temperatures As and AF, the start and finish temperatures of the austenitic transformation, respectively, and with reference to temperatures Ms and MF, the start and finish temperatures of the martensitic transformation, respectively. The martensitic transformation temperature range is lower than the austenitic transformation temperature range, with the various temperatures related, generally, as follows: M F &lt;M S &lt;A S &lt;A F . 
         [0005]    Transformation between these two phases is reversible such that the alloys may be treated to assume different shapes or configurations in the two phases and can reversibly switch between one shape to another when transformed from one phase to the other. In the case of Nitinol medical devices, it is preferable that they remain in the austenitic state while deployed in the body because Nitinol austenite is stronger and less deformable, and thus more resistant to external forces, than Nitinol martensite. These phase transformations may be induced through changes in temperature, or, alternatively, through changes in stress or strain. For example, a Nitinol medical device may be formed in an austenitic state, and then deformed to such an extent that some or all of the austenite transforms to strain-induced martensite. 
         [0006]    A strain-induced martensitic phase transformation may alter the austenitic transformation temperatures of the Nitinol device, typically by increasing the austenitic start and finish temperatures, A S  and A F , to within several degrees below, or above, normal body temperature (37° C.). The degree to which A S  and A F  are increased depends upon the degree of the induced strain. Additionally, different regions of the Nitinol device may be subjected to different strains, resulting in different austenitic transformation start temperatures, such as, for example, A S1  and A S2 , for Regions 1 and 2, respectively. 
         [0007]    In one embodiment, A S1 &lt;A S2 &lt;T body . In this embodiment, each region may individually begin the austenitic transformation as the Nitinol device reaches the corresponding austenitic transformation start temperature. However, because austenitic transformation start temperatures are different, each region will experience different transformation kinetics, with Region 1 typically experiencing austenitic transformation before Region 2. In another embodiment, A S1 &lt;T body &lt;A S2 . In this embodiment, Region 1 may complete the austenitic transformation under the influence of body temperature, while Region 2 may require another mechanism to start the austenitic transformation, such as, for example, additional heating, mechanical deformation, etc. 
         [0008]    Implantable medical devices made of Nitinol are known in the art. For example, U.S. Pat. No. 5,562,641 to Flomenblit et al. discloses a two-way shape memory alloy stent having an austenitic transformation temperature range that is above body temperature and a martensitic transformation temperature range that is below body temperature. The last conditioned state (i.e., austenite or martensite) of this two-way shape memory alloy stent is thereby retained at body temperature. In another example, U.S. Pat. No. 5,624,508 to Flomenblit et al. discloses a method for manufacturing shape memory alloy devices exhibiting thermally-induced, two-way shape memory effects. In a further example, U.S. Pat. No. 5,876,434 to Flomenblit et al. discloses an implantable shape memory alloy device which is expanded from a strain-induced martensitic state to a stable austenitic state when temperature is above increased A S ′&gt;A S °. This shape memory alloy device may, or may not, remain in the deformed martensitic, or partially martensitic, state without the use of a restraining member. Different regions of the stent may be deformed to different strains, resulting in different austenitic transformation temperature ranges, and, consequently, different shape recovery kinetics in those regions. 
         [0009]    A strain-induced martensitic stent having different deformation regions may be loaded into a delivery system and then sterilized at temperatures exceeding the different austenitic transformation temperature ranges within the stent. During the sterilization process, however, the different strains induced within the different deformation regions are equalized to a common strain provided by a restraining member of the delivery system, such as, for example, an outer body of a delivery device. Unfortunately, the common strain also provides a common austenitic transformation temperature range, thereby defeating the purpose of inducing multiple deformation regions having different strains, austenitic transformation temperature ranges and shape recovery kinetics. 
         [0010]    Devices for implanting self-expanding stents are likewise known in the art. For example, U.S. Pat. No. 5,484,444 to Braunschweiler et al. discloses a device for implanting a radially self-expanding stent that includes an outer body and an inner core element having a stamped region which complements the surface of the stent and facilitates implantation. The self-expanding stent is compressed, or folded, onto the inner core and expands immediately into the inner diameter of the body cavity, vessel, etc., as the outer body is pulled back over the inner core. Unfortunately, the sharp, leading edge of the stent may damage the internal surface of the vessel as the stent is released and immediately begins to expand. Moreover, as discussed in Braunschweiler, once the stent is partially released, it can only be pulled proximally and not pushed distally, because if the stent were to be pushed, the expanded, distal end would inevitably injure the vessel in which it was introduced. 
       SUMMARY OF THE INVENTION  
       [0011]    In accordance with embodiments of the present invention, a method for preparing a shape memory alloy endoprosthesis, displaying strain induced martensite phenomenon, for delivery includes inserting a shape memory alloy endoprosthesis into a delivery device, inducing a first strain within a first region of the shape memory alloy endoprosthesis, inducing a second strain within a second region of the shape memory alloy endoprosthesis, and sterilizing the delivery device while maintaining the first strain and the second strain induced within the shape memory alloy endoprosthesis. 
         [0012]    In accordance with other embodiments of the present invention, an apparatus for delivering a shape memory alloy endoprosthesis includes an inner core having a first diameter, an outer body having a second diameter greater than the first diameter, and a calibrated endcap attached to the inner core. The outer body surrounds the inner core, and the calibrated endcap includes a roof section having a third diameter greater than the first diameter and less than the second diameter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    FIG. i is a schematic representation of a delivery system for a shape memory alloy endoprosthesis, according to an embodiment of the present invention. 
           [0014]      FIG. 2  is a schematic representation of a delivery system depicting a partially-deployed shape memory alloy endoprosthesis, according to an embodiment of the present invention. 
           [0015]      FIG. 3  is a flow chart depicting a method for preparing a shape memory alloy medical endoprosthesis for delivery, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  is a schematic representation of a delivery system for a shape memory alloy endoprosthesis, according to an embodiment of the present invention. 
         [0017]    Referring to  FIG. 1 , delivery system  100  generally includes flexible outer body  110 , flexible inner core  120  and calibrated endcap  130 . In an embodiment, outer body  110  and inner core  120  may be generally circular in cross-section, while calibrated endcap  130  may be circular, conical, etc., in cross-section. Calibrated endcap  130  may be fixedly attached to inner core  120  (e.g., adhesive, etc.), or, alternatively, calibrated endcap  130  may be removably attached to inner core  120  (e.g., screw/thread, etc.), thereby facilitating the use of different types of removable calibrated endcaps  130  within delivery system  100 . In an embodiment, inner core  120  and calibrated endcap  130  may include an interior cavity, or lumen, in which a guide wire, fiber optic lens/cable assembly, etc., may be inserted (not shown for clarity). 
         [0018]    In an embodiment, inner core  120  may be longer than outer body  110 , and delivery system  100  may include outer handle  112 , attached to the proximal end of outer body  110 , and inner handle  122 , attached to the proximal end of inner core  120 . In this embodiment, outer handle  110  and inner handle  120  may provide convenient surfaces upon which to apply the appropriate forces necessary to slide outer body  110  over inner core  120 , in the proximal direction, during the deployment of the shape memory alloy endoprosthesis. 
         [0019]    Inner core  120  may include shoulder  126 , located near the distal end of inner core  120 . In an embodiment, shoulder  126  may be circular in cross-section. In this embodiment, the diameter of shoulder  126  may be slightly less than the diameter of outer body  110  in order to prevent lateral motion of the shape memory alloy endoprosthesis in the proximal direction during deployment, while at the same time permitting relative motion between outer body  110  and inner core  120 . In another embodiment, a gasket may be attached to the outer surface of shoulder  126  to prevent proximally-directed fluid flow, either before, during or after deployment. Additionally, the gasket may reduce the nominal coefficient of friction between outer body  110  and shoulder  126 , thereby improving the relative motion between outer body  110  and inner core  120 . In one embodiment, shoulder  126  may include x-ray opaque material, while in another embodiment, shoulder  126  may include radio-frequency opaque material. Generally, shoulder  126  may optionally include one or more materials capable of reflecting medical imaging device emissions to facilitate location of the distal end of delivery system  100  within the body. 
         [0020]    Inner core  120  may include forward section  124 , located at the distal end of inner core  120  and extending from shoulder  126  to endcap  130 . In one embodiment, the diameter of forward section  124  may be less than the diameter of inner core  120  proximal to shoulder  126 , while in another embodiment, the diameter of forward section  124  may be equal to, or greater than, the diameter of inner core  120  proximal to shoulder  126 . The diameter of forward section  124  may be constant along its length, or, alternatively, the diameter of forward section  124  may vary along its length. A shape memory alloy endoprosthesis may be fitted within payload volume  125 , generally defined by outer body  110 , shoulder  126 , forward section  124  and calibrated endcap  130 . 
         [0021]    Calibrated endcap  130  may include transition section  132  and roof section  134 , and may optionally include one or more materials capable of reflecting medical imaging device emissions to facilitate location of the distal end of delivery system  100  within the body. In an embodiment, transition section  132  may provide a reduction in diameter, generally, from the diameter of outer body  110  to the diameter of roof section  134 . As depicted in  FIG. 1 , the diameter of roof section  134  may be less than the diameter of outer body  110  but more than the diameter of forward section  124 . The distal portion of a shape memory alloy endoprosthesis may be captured by calibrated endcap  130  and deformed to a diameter smaller than the remaining, proximal portion of the shape memory alloy endoprosthesis housed within payload volume  125  and generally restrained by outer body  110 . Importantly, the reduction in diameter of the distal portion of the shape memory alloy endoprosthesis imparts an increase in strain compared to the remaining, proximal portion of the shape memory alloy endoprosthesis. Advantageously, the dimensions of calibrated endcap  130 , such as, for example, the diameter of roof section  134 , the length of roof section  134 , the length of transition section  132 , etc., may correlate to a specific increase in strain for a particular shape memory alloy endoprosthesis. 
         [0022]    An exemplary shape memory alloy endoprosthesis is also depicted in  FIG. 1 , both in a deployed configuration (stent  150 ) and in an undeployed configuration (stent  155 ). In an embodiment, the shape memory alloy endoprosthesis may be constructed of Nitinol and may include residual strain e 0  (ε 0 ) when deployed in an austenitic state, generally corresponding to stent  150 . In this embodiment, the diameter of stent  150  may be greater than the diameter of outer body  110 . When inserted within delivery system  100 , however, a different configuration, generally corresponding to stent  155 , may be assumed. In this configuration, some portion of stent  155  may be deformed to a particular strain e 1  (ε 1 ) by outer body  110 , such as, for example, body  152 , while a smaller portion of stent  155  may be deformed to a particular strain e 2  (ε 2 ) by calibrated endcap  130 , such as, for example, leading edge  154 . In an embodiment, the proximal portion of leading edge  154  may be deformed to a particular strain profile by transition section  132 , while the distal portion of leading edge  154  may be deformed to a constant strain by roof section  134 . In other words, leading section  154  may include a smaller, proximal portion, in which the strain varies from el (vi) to e 2  (F 2 ) according to a particular profile (e.g., linear, parabolic, etc.), and a larger, distal portion, in which the strain is essentially constant at e 2  (ε 2 ). 
         [0023]    After deformation by delivery system  100 , stent  155  may contain regions in which the austenite transformation temperatures differ from one another, such as, for example, body  152  and leading edge  154 . In an embodiment, body  152  may experience strain e 1  (ε 1 ) producing austenitic transformation temperatures A S1  and A F1 , while the larger, distal portion of leading edge  154  may generally experience strain e 2  (ε 2 ) producing austenitic transformation temperatures A S2  and A F2 . For simplicity, the effects of the strain profile experienced by the smaller, proximal portion of leading edge  154  may be neglected. In one embodiment, e 2  (ε 2 ) may be greater than e 1  (ε 1 ), and all of the austenitic transformation temperatures may be below body temperature, i.e., A S1 &lt;A S3 , A F1 &lt;A F3 , and A S1 , A S2 , A F1 , A F3 &lt;T body . In another embodiment, e 2  (ε 2 ) may be greater than e 1  (ε 1 ), and only the austenitic transformation temperatures associated with the e 1  (ε 1 ) region may be below body temperature, i.e., A S1 &lt;A S3 , A F1 &lt;A F3 , and A S1 , A F1 &lt;T body &lt;A S3 , A F3 . In this embodiment, an alternative mechanism may be required to deploy the e 2  (ε 2 ) region after initial deployment, such as, for example, additional heating using a warm saline solution, mechanical deformation using a balloon catheter, etc. 
         [0024]    In an alternative embodiment, calibrated shoulder  140  may replace shoulder  126 , and may include a calibrated section similar In design and function to the elements of calibrated endcap  130 . For example, calibrated shoulder  140  may include transition section  142  and roof section  144 . Transition section  142  may provide a reduction in diameter, generally, from the diameter of outer body  110  to the diameter of roof section  144 , which may be less than the diameter of outer body  110  but more than the diameter of forward section  124 . In this manner, the proximal portion of a shape memory alloy endoprosthesis may be captured by calibrated shoulder  140  and deformed to a diameter smaller than the remaining, distal portion of the shape memory alloy endoprosthesis housed within payload volume  125 . Importantly, the reduction in diameter of the proximal portion of the shape memory alloy endoprosthesis imparts an Increase in strain compared to the remaining portion of the shape memory alloy endoprosthesis. Delivery system  100  may include either calibrated endcap  130  or calibrated shoulder  140 , or, alternatively, both calibrated endcap  130  and calibrated shoulder  140 . 
         [0025]    Advantageously, the dimensions of calibrated shoulder  140 , such as, for example, the diameter of roof section  144 , the length of roof section  144 , the length of transition section  142 , etc., may correlate to a specific increase in strain for a particular shape memory alloy endoprosthesis. In an embodiment, the strain induced by calibrated shoulder  140 , e 3  (ε 3 ), may be greater than e 1  (ε 1 ), and all of the austenitic transformation temperatures may be below body temperature, i.e., A S1 &lt;A S3 , A F1 &lt;A F3 , and A S1 , A S3 , A F1 , A F3 &lt;T body . In another embodiment, e 3  (ε 3 ) may be greater than e 1  (ε 1 ), and only the austenitic transformation temperatures associated with the e 1  (ε 1 ) region are below body temperature, i.e., A S1 &lt;A S3 , A F1 &lt;A F3 , and A S1 , A F1 &lt;T body &lt;A S3 , A F3 . In this embodiment, an alternative mechanism may be required to deploy the e 3  (ε 3 ) region after deployment, such as, for example, additional heating using a warm saline solution, mechanical deformation using a balloon catheter, etc. 
         [0026]    In a further embodiment, delivery system  100  may include cooling fluid to maintain the temperature of the shape memory alloy endoprosthesis below the various austenitic transformation finish temperature until deployment. For example, cooling fluid may be introduced into an inner lumen, extending through the entire length of inner core  120  to payload volume  125 , and may be returned through an outer lumen defined by outer body  110  and inner core  120  proximal to shoulder  126 . In this embodiment, forward section  124  may include one or more holes through which the cooling fluid may flow into payload volume  125 , and shoulder  126  may include one or more holes, cutouts, etc., to facilitate fluid flow from payload volume  125  to the outer lumen. In this manner, the shape memory alloy endoprosthesis captured within payload volume  125  may be maintained at an appropriate temperature in order to prevent instantaneous austenitic phase transformation, caused by heat transfer during advancement of delivery system  100  within the body, upon deployment. 
         [0027]      FIG. 2  is a schematic representation of a delivery system for a shape memory alloy endoprosthesis, depicted in a partially deployed state, according to an embodiment of the present invention. 
         [0028]    Referring to  FIG. 2 , delivery system  100  is depicted in a partially deployed state, in which stent  250  may be in transition from a loaded configuration within delivery system  100  to a deployed configuration within body lumen  200 . In an embodiment, stent  250  may include at least two regions of induced strain, each having a different austenitic transformation temperature range. During the deployment process, heat flow from body lumen  200  increases the temperature of stent  250 . Austenitic phase transformation may occur within each region of induced strain as the temperature of stent  250  passes through each specific austenitic transformation temperature range. Because each region of induced strain may have a different austenitic transformation temperature range, and because a temperature gradient may be established over the length of stent  250  during the deployment process, austenitic transformation may occur at different times for different regions of stent  250 . 
         [0029]    For example, stent  250  may include a region of induced strain e 1  (ε 1 ), such as body  252 , and a region of induced strain e 2  (i 2 ), such as leading edge  254 . In this example, e 1  (ε 1 ) may be less than e 2  (ε 2 ), and the austenitic transformation temperature range associated with body  252  may be less than the austenitic transformation temperature range associated with leading edge  254 . Accordingly, as stent  250  begins to deploy, heat flow from body lumen  200  may increase the temperature of stent  250  such that body  252  begins austenitic transformation before leading edge  254 . The austenitic transformation lag experienced by leading edge  254  effectively blunts the sharp edge of the expanding distal portion of stent  250 , thereby preventing damage to the walls of body lumen  200  which may occur during the initial deployment stages of a typical shape memory alloy endoprosthesis. Additionally, partially-deployed stent  250  may be repositioned within body lumen  200 , in both the proximal and distal directions, without damaging the walls of body lumen  200 . 
         [0030]      FIG. 3  is a flow chart depicting a method for preparing a shape memory alloy endoprosthesis for delivery, according to an embodiment of the present invention. 
         [0031]    In an embodiment, a shape memory alloy endoprosthesis may be inserted ( 300 ) into a delivery device. In an embodiment, inner core  120  may be fixed and outer body  110  may be advanced in the proximal direction so that the distal end of outer body  110  approaches shoulder  126 , thereby exposing at least a portion of forward section  124 . In another embodiment, outer body  110  may be fixed and inner core  120  may be advanced in the distal direction so that shoulder  126  approaches the distal end of outer core  110 , thereby exposing at least a portion of forward section  124 . Calibrated endcap  130  may be passed through the center of stent  150 , and stent  150  may then be generally aligned over forward section  124 . 
         [0032]    In one embodiment, stent  150  may be deformed to a smaller diameter and then inserted ( 300 ) into delivery system  100 . The distal portion of stent  150  may be inserted into calibrated endcap  130  and advanced to roof section  134 . The proximal portion of stent  150  may be inserted, generally, towards shoulder  126  and then the distal portion of delivery system  100  may be closed, for example, by fixing outer body  110  and advancing inner core  120  in proximal direction, by fixing inner core  120  and advancing outer body  110  in a distal direction, etc. As noted above, stent  155  represents the undeployed, or loaded, configuration of stent  150 . In an alternative embodiment, the proximal portion of stent  150  may be inserted into calibrated shoulder  140  and advanced to roof section  144 . 
         [0033]    A first strain, having a first austenitic transition temperature range, may be induced ( 310 ) within a first region of the shape memory alloy endoprosthesis. In an embodiment, outer body  110  of delivery system  100  may induce a particular strain e 1  (ε 1 ) within a proximal portion of stent  155 , such as, for example, body  152 . This strain may produce an austenitic transformation temperature range generally denoted by start and finish temperatures, A S1  and A F1 , respectively. In one embodiment, this austenitic transformation temperature range may be below normal body temperature. 
         [0034]    A second strain, having a second austenitic transition temperature range, may be induced ( 320 ) within a second region of the shape memory alloy endoprosthesis. In an embodiment, roof section  134  of delivery system  100  may induce ( 320 ) a particular strain e 2  (ε 2 ), greater than e 1  (ε 2 ), within a distal portion of stent  155 , such as, for example, leading edge  154 . This strain may produce an austenitic transformation temperature range generally denoted by start and finish temperatures, A S2  and A F2 , respectively. In one embodiment, this austenitic transformation temperature range may be below normal body temperature, while in another embodiment, this austenitic transformation temperature range may be above normal body temperature. 
         [0035]    In an alternative embodiment, roof section  144  of delivery system  100  may induce ( 320 ) a particular strain e 3  (ε 3 ) within a proximal portion of stent  155 , such as, for example, the trailing edge of body  152 . This strain may produce an austenitic transformation temperature range generally denoted by start and finish temperatures, A S3  and A F3 , respectively. 
         [0036]    The delivery device may be sterilized ( 330 ) at a temperature above the first austenitic transition temperature range and second austenitic transition temperature range while maintaining the first strain and the second strain. In an embodiment, delivery system  100 , containing stent  155 , may be sterilized ( 330 ) at a temperature above the austenitic transformation temperature ranges associated with the various regions of induced strain, such as, for example, e 1  (ε 1 ), e 2  (ε 2 ), etc. Due to the constraining effects of delivery system  100 , and, in particular, outer body  110  and calibrated endcap  130 , stent  155  may not undergo strain equalization normally experienced during high-temperature sterilization. Rather, after the sterilization process concludes, the various regions of induced strain within stent  155 , such as, for example, e 1  (ε 1 ), e 2  (ε 2 ), etc., may be preserved by delivery system  100 . Importantly, the austenitic transformation temperature ranges associated with each region of induced strain will also be preserved. Accordingly, each region of induced strain may experience different kinetics upon deployment within the body. For sterilization processes occurring below these austenitic transformation temperature ranges, delivery system  100  also preserves the various regions of induced strain within stent  155 . 
         [0037]    In a further embodiment, the shape memory alloy endoprosthesis may be deployed ( 340 ) from the delivery device. Generally, delivery system  100  may be introduced into a body lumen, cavity, etc., and advanced to the deployment location. In an embodiment, inner core  120  of delivery system  100  may be fixed during deployment while outer body  110  may be advanced in a proximal direction, as indicated, generally, by directional arrow  210 . This relative motion between inner core  120  and outer body  110  gradually exposes stent  250  to body lumen  200 , as well as to any fluid which may be present therein. Heat flow between body lumen  200  and stent  250  may depend, generally, upon various factors, including, for example, the temperature different between body lumen  200  and stent  250 , the heat conductivity coefficient a, etc. As the temperature of stent  250  increases due to this heat flow, austenitic phase transformation may occur and stent  250  may then assume the deployed configuration within body lumen  200 . 
         [0038]    Several embodiments of the present invention are specifically illustrated and described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.