Patent Publication Number: US-7708925-B2

Title: Nitinol frame heating and setting mandrel

Description:
This application is a divisional of application Ser. No. 10/188,812, filed Jul. 2, 2002 now U.S. Pat. No. 7,112,055. 

   BACKGROUND OF THE INVENTION 
   This application relates to heating and setting mandrels for use in manufacturing and more particularly, a mandrel for heating and setting a stent having limb elements which provide for improved expansion characteristics. 
   The term stent generally refers to a prosthesis, which can be introduced into a corporeal lumen and expanded to support that lumen or attach a conduit to the inner surface of that lumen. Self-expanding stents are generally known in the art. During use, the self-expanding stent is compressed into a reduced size having an outer diameter substantially smaller than the stent in its expanded shape. The stent is held in its compressed state during its passage through the patient&#39;s vascular system until reaching the target treatment site, whereupon the compressed self-expanding stent may be deployed. While in its compressed state, stress is stored in the bends of the stent limbs. During deployment, the stresses in the stent limbs cause the stent to expand radially from its initially compressed state. Once in place, the radial extremities of the stent bear against the inside walls of the passageway, thereby allowing normal blood flow. 
   The processes of manufacturing self-expanding stents are also known in the art insofar as heat treating a stent upon a mandrel for purposes of setting a particular stent shape. Additionally, shape memorization processes utilizing mandrels are stent specific as each stent-type embody different design requirements. Previous attempts at heat treating simply involve mounting a stent upon a mandrel and exposing it to heat with little attention being paid to the shape that is set, other than the diameter, during the heating process. Because these previous attempts fail to control the shape of the stent limbs created during the heating process, a less effective final stent is produced. 
   Most stents known in the art change diameter through the deformation of a small percentage of a length of the limbs defining the stent. Usually, this deformation occurs only at, or near, curved apices formed in stent limbs. The length of the limb that deforms and the magnitude of the deformation has a bearing on three important and interrelated characteristics of the stent: 1) the minimum diameter to which the stent can be compressed; 2) the radial stiffness or energy required to compress the stent; and 3) the maximum value of stress/strain experienced by the stent. Many other factors are also determinative of these characteristics including stent material, resting diameter, stent length, etc.; however, these other factors are assumed to be generally constant for a given stent design. 
   A stent having curved limb members can improve the above mentioned characteristics of the stent by spreading the deformation energy over a majority of the length of the stent limbs. This is in contrast to other stent designs that concentrate the deformation at or near the apices in the stent limb. 
   For example, to maximize radial stiffness and to minimize a compressed diameter of a stent, limb elements defining the stent each can embody two curves of constant radius and opposite direction which meet at an inflection point. When such a stent is compressed, the two curved sections assume a nearly straight profile, the advantage of which is that the entire length of the curved portions store deformation energy and function to urge the stent radially outward. 
   In the event a stent having curved limb members is to be manufactured, in order to set a desired expanded configuration the stent is expanded over a cylindrical mandrel and heated. However, merely expanding the stent over a mandrel without additional controls or constraints, rarely results in limb elements having the desired profile. To wit, the end of the limbs may be provided with a smaller than desired radius of curvature whereas the portion of the limbs near an inflection point may have a much larger than desired radius of curvature. This results in producing a stent that embodies limbs which do not store stress in an optimal manner. Therefore, an expansion mandrel for heating and setting a stent which facilitates the production of a desired stent profile as well as aids in evenly dispersing stresses along limb elements defining the stent during manufacturing was developed, and is described in U.S. Pat. No. 6,279,368 B1 to Escano et al. It would be beneficial to have an improved mandrel that makes the process of loading a stent onto a mandrel easier. 
   As is known, stents are currently expanded on a mandrel through multiple shape setting steps, and at some point before the last expansion, an additional ring is placed on the stent to help protrude hooks from the stent. This process is very slow and prone to damaging the stent. Therefore, an alternative process is needed to make the hook-setting step easier, faster, and less susceptible to damage. 
   The present invention satisfies these and other needs. 
   SUMMARY OF THE INVENTION 
   Briefly, and in general terms, the present invention provides an improved heating/setting mandrel which substantially reduces the difficulty in loading stents onto the mandrel. Moreover, the mandrel construction of the present invention is relatively inexpensive to manufacture, is trouble-free and reliable in use, and attains improved and constant results in the manufacture of stents having curved limb elements. 
   Furthermore, the present invention also provides an improved mandrel and method for setting hooks of a stent. The present invention makes the hook-setting step easier, faster, and the stent is less prone to damage. 
   In one aspect, the invention comprises a hollow central core cylinder made from a heat conducting material having an outer surface including a plurality of raised forms, gaps in-between the raised forms, and the central core cylinder has an outer surface diameter and a raised form diameter. The invention also includes a first and second outer cylinder, also made from a heat conducting material, and each having a curved radial end with a cut-out design similar to the shape of the plurality of raised forms. The first and second outer cylinders each have an inner diameter that is nearly equivalent to or slightly larger than the outer surface diameter of the central core cylinder, and the inner diameter is less than the raised form diameter. In operation, a stent is placed on the central core cylinder with the limb elements of the stent directed around the raised forms in the gaps in-between. Then, the first and second outer cylinders are slid onto the central core cylinder at opposite ends such that the curved radial ends of the first and second outer cylinders align with the plurality of raised forms and force the stent into the shape of the raised forms. The first and second outer cylinders may also include a plurality of hook extenders disposed at the curved radial ends which shape hooks of a stent. Once the stent and first and second outer cylinders are in place on the central core cylinder, heat is applied to the apparatus to shape set the stent. 
   In another aspect, the invention further includes a capture sleeve having a cylindrical shape with an inner diameter nearly equivalent to or slightly larger than the raised form diameter of the central core cylinder. The capture sleeve fits over the central core cylinder and the plurality of raised forms, leaving an insert space for the first and second outer cylinders to be inserted between the capture sleeve and the central core mandrel. 
   In yet another embodiment, the present invention includes a mandrel having an outer surface with a plurality of slots disposed thereon corresponding to the position of hooks on a stent. In conjunction with this mandrel, first and second stop rings may be used to engage opposite ends of the mandrel and to press against ends of the stent, holding the stent in place on the mandrel. A method of using this mandrel includes turning a stent inside-out, and positioning the stent on the mandrel such that the hooks are aligned with the plurality of slots. Once on the mandrel, the hooks are then pushed into the slots, and the apparatus and stent are heated to set the hooks. 
   Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view with some elements in the background not shown for clarity, depicting a stent having curved limb elements to be manufactured with the present invention; 
       FIG. 2  is a side view, depicting a single curved limb element of the stent of  FIG. 1 ; 
       FIG. 3  is a side view, depicting a curved limb element of a stent to be used in the present invention; 
       FIG. 4  is a side view, depicting typical joints between adjacent curved limb elements in a stent which is comprised of a multiplicity of such curved limb elements; 
       FIG. 5  is a side view, depicting an almond shaped stent cell of a stent to be used in the present invention; 
       FIG. 6  is an exploded perspective view of one embodiment of the present invention; 
       FIG. 7  is an elevational view of the first and second outer cylinders having hook extenders; 
       FIG. 8  is a partial elevational view of hook extenders shaping a hook of a stent; 
       FIG. 9  is an elevational view of another embodiment of the present invention; 
       FIG. 10  is an elevational view of the embodiment shown in  FIG. 4  with a stent in position on the mandrel; 
       FIG. 11  is a perspective view of a stent with the hooks facing outward; 
       FIG. 12  is a perspective view of a stent pushed flat against a surface; and 
       FIG. 13  is a perspective view of a stent turned inside-out with the hooks facing inward. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to an improved mandrel for use in heating and setting a stent having curved limb elements which alleviates the stresses inflicted upon the limbs of the stent during the manufacturing. The mandrel shapes the limbs of the stent during the heating process by employing the use of raised forms and outer cylinders which hold in place the limb portions of the stent to thereby produce a stent with limb portions having constant radius of curvatures. 
   Referring now to the drawings, in  FIG. 1 , there is shown an example of a stent  8  having curved limb elements to be manufactured with the present mandrel invention. Such a stent  8  may be cut from a tube or assembled from separate elements.  FIG. 2  depicts a repeating element of each limb  10  of a stent cell, having two curved elements  12  of equal radius, equal length and opposite direction. The short straight segment element  16  at the ends of each limb  10  are parallel to one another. The mid-portion or the inflection point  18  lies between the two curved segment elements  12  of each stent limb  10 . 
   Referring to  FIG. 3 , depending on the overall length of the stent, the limb element  10  may bend back and forth in a sinusoid wave pattern down the length of the stent  8 . Additionally, referring to  FIG. 4 , in the event the stent  8  is made from separate elements, the short straight segment elements  16  of adjacent limbs maybe joined, either by welding, soldering, riveting, or gluing to form joint  20 . A multiplicity of identical limb elements can be joined in this way to form the cylindrical stent structure, as seen in  FIG. 1 . 
   Referring to  FIG. 5 , a stent cell  22  may have an almond-like shape and each cell may embody four limb elements  10 . Each limb  10  essentially comprises a quarter of a full stent cell  22 . As described above, one limb  10 , making up a quarter of the stent cell  22 , starts from the midpoint of the stent cell to the end of the stent cell. The limb elements  10  are comprised of two curve elements  12 . These curve elements  12  are of equal radius, equal length and opposite direction. In a preferred stent embodiment, the limb  10  would be composed of two curves having constant radius r with an inflection point  18  in the middle where they reverse direction. 
   Under ideal conditions, it is preferred that the stress along the length of the limb  10  be as evenly distributed as possible so that there is minimal or no stress at the inflection point  18 . Along the rest of limb  10 , the stress level will be determined by the inverse of the radius r that the stent  8  limb has in its relaxed configuration. During introduction into vasculature, the stent  8  is compressed down into a catheter (not shown). In this compressed configuration, the curved limb elements  12  become generally straight. The change in radius r of curvature from the compressed state where the limbs  10  are straight to its profile in a relaxed state has a bearing on the amount of stress. The stress along the limb  10  and the amount of energy that can be stored in the stent  8  is determined by the change in the radius of curvature at any point along the limb  10 . 
   In one embodiment, as shown in  FIG. 6 , a mandrel generally designated  30 , for heating and setting elements of the stent  8 , includes a central core cylinder  32  having an outer surface  34  with a plurality of raised forms  36 , gaps  38  in-between the raised forms, and the central core cylinder also has an outer surface diameter  40  and a raised form diameter  42 . The raised forms  36  may be in any shape which is desired for the stent cell  22 , and in this embodiment the raised forms are almond-shape. The mandrel  30  also has a first outer cylinder  44  and a second outer cylinder  46 , each having a curved radial end  48  with a cut-out design  50  similar to one-half the shape of the plurality of raised forms  36 . In this embodiment, the cut-out designs  50  are one-half almond. The first and second outer cylinders  44  and  46  each have an inner diameter  52 , the inner diameter is nearly equivalent to or slightly larger than the outer surface diameter  40  of the central core cylinder  32 , and the inner diameter is lesser than the raised form diameter  42 . The first and second outer cylinders  44  and  46  can be positioned on the central core cylinder  32  such that the curved radial ends  48  of the first and second outer cylinders align with the plurality of raised forms  36 . In one embodiment, a ridge  51  is disposed on the inside surface of the first and second outer cylinders  44  and  46  following the edge of the cut-out design  50 . The ridge  51  provides a gap having a width that is nearly equivalent to a thickness of a strut of a stent. In use, the ridge  51  will hold the strut against the central core cylinder  32 . 
   The embodiment shown in  FIG. 6 , further includes a capture sleeve  54  having a cylindrical shape with an inner diameter  56  nearly equivalent to the raised form diameter  42  of the central core cylinder  32 . The capture sleeve  54  slides over the central core cylinder  32  and the plurality of raised forms  36 , leaving an insert space (not shown) for the first and second outer cylinders  44  and  46  to be inserted between the capture sleeve and the central core cylinder. In use, the capture sleeve  54  makes the shape-setting process easier by ensuring that the stent stays on the mandrel. 
   In one embodiment, the central core cylinder  62 , first and second outer cylinders  44  and  46 , and the capture sleeve  54  are all made of a heat conducting material such as aluminum or stainless steel, and all have at least one hole  58  disposed thereon. There may also be a plurality of holes  58  disposed on each piece  32 ,  44 ,  46 , and  54  of the mandrel  30 . The holes  58  may be used to orient and secure the cylinders through the use of one or more radial pins  58   a . The holes  58  may also permit improved operation if the stent is heated by immersion in hot liquid such as a molten salt. 
   A method for forming a stent using the embodiment shown in  FIG. 6  includes placing the stent on the central core cylinder  32 , and directing the limb elements of the stent around the raised forms  36 , so that the limb elements rest in the gaps  38  in-between the raised forms. Next, the capture sleeve  54  is placed over the central core cylinder  32  and stent, to retain the stent on the mandrel. The first and second outer cylinders  44  and  46  may then be inserted between the central core cylinder  32  and the capture sleeve  54  from opposite ends, such that the curved radial ends  48  force the stent into the shape of the raised forms  36  on the central core cylinder. The cylinders and sleeves may be secured by the insertion of radial pins  58   a  through the holes  58 . After this step, heat is then applied to the mandrel  30  and stent to set the shape. In another embodiment, the capture sleeve  54  may not be used, so that after the stent is placed on the central core cylinder  32  around the plurality of raised forms  36 , the first and second outer cylinders  44  and  46  are then placed on opposite ends of the mandrel such that the curved radial ends  48  force the stent into the shape of the raised forms. This shape setting method can be applied to any nitinol stent. 
   In another embodiment of the apparatus as shown in  FIG. 7 , the first and second outer cylinders  44  and  46  have a plurality of hook extenders  60  disposed at the curved radial ends  48 , in order to shape set hooks  62  found on a stent  64 . This embodiment is for the expansion of a nitinol stent on the last stage of expansion.  FIG. 8  shows a partial view of a top hook extender  60   a  on the first or top outer cylinder  44 , and a bottom hook extender  60   b  on the second or bottom outer cylinder  46 , shaping a hook  62  located in-between the two hook extenders. The top and bottom hook extender  60   a  and  60   b  each have a curved tip  66   a  and  66   b  respectively, and the curved tip  66   a  faces curved tip  66   b , so that when the first and second outer cylinders  44  and  46  are in position on the central core cylinder  32 , the curved tips  66   a  and  66   b  are complementary to each other. In this embodiment, the top and bottom outer cylinders  44  and  46  are used in conjunction with the central core cylinder  32 , but not the capture sleeve  54 . The first and second outer cylinders  44  and  46  are made of a heat conducting material such as aluminum, but it is preferred to use  300  series stainless steel. All or some of the cylinders and sleeves may be coated or plated with substances such as titanium nitride to improve operation or extend the service life of the cylinders and sleeves. 
   A method of forming a stent using this embodiment, includes placing the stent  64  on the central core cylinder  32  and directing limb elements  68  of the stent around the raised forms  36 . Next, the first and second outer cylinders  44  and  46  are placed on opposite ends of the central core cylinder  32  such that the curved radial ends  48  force the stent into the shape of the raised forms  36 , and the hook extenders  60  shape the hooks  62  of the stent  64 . Once the cylinders  44  and  46  are in place, heat is then applied to set the shape of the stent. This method makes the shape-setting process easier, and helps achieve uniformity of stent cells and hooks after expansion. 
   Now referring to  FIGS. 9 and 10 , another embodiment is shown of a mandrel  100  used for heating and setting hooks  122  of a stent  120  (shown in  FIGS. 11 ,  12  and  13 ). The mandrel  100  has an outer surface  102  with a plurality of slots  104  disposed thereon corresponding to the position of hooks  122  on the stent  120 . The mandrel  100  may also include first and second stop rings  106  and  108  constructed to engage a top end  110  and a bottom end  112  of the mandrel  100 , and to press against a top end  124  and a bottom end  126  of the stent  120 , so that the first and second stop rings keep the stent from moving and hold the hooks inside the plurality of slots  104 . In this embodiment, there are no additional rings placed over the stent to shape the hooks or barbs, and therefore the stent is less prone to damage. 
   A method for setting the hooks  122  of a stent  120  using the mandrel  100  with a plurality of slots  104 , includes turning the stent inside-out. As shown in  FIG. 11 , the stent  120  has the hooks  122  pointed toward the outside, and once the stent is turned inside-out, the hooks are pointed toward the inside of the stent as shown in  FIG. 13 . The stent  120  may be turned inside-out manually by pressing the top end  124  downward so the stent becomes flattened with the top end forming an inner diameter, and the bottom end  126  forming an outer diameter as shown in  FIG. 12 . From this position, the top end  124  is held down, while the bottom end  126  is pulled upward, so that the hooks  122  face inward as shown in  FIG. 13 . After the stent  120  is turned inside-out, the stent is positioned on the mandrel  100  such that the hooks  122  are aligned with the plurality of slots  104 . The hooks  122  are then pushed into the slots  104 , and heat is applied to set the hooks. 
   This method may further include engaging the first and second stop rings  106  and  108  at the top and bottom ends  110  and  112  of the mandrel respectively, such that the first and second stop rings press against the top and bottom ends  124  and  126  of the stent, thereby holding the stent in place during heating. This method makes the hook-setting step much easier, faster, and the stent is less likely to be damaged. This method can apply to any super elastic stent (at or below 37° C.) with hooks or barbs, which require thermal shape setting. 
   Heating and setting procedures vary for particular stent designs and stent materials but are in general, conventionally known in the art. 
   While several particular forms of the invention have been illustrated and described, it will also be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.