Patent Application
20130305512
Briefly and in general terms, the present invention is directed towards a stent expanding device, often referred to as an expandable mandrel, and methods for expanding the dimensions of stents. The present invention also helps to reduce stress in the processing steps during the manufacturing of a stent.
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. Stents made of shape settable material are generally known in the art. Stents are generally either balloon expandable or self-expanding devices. A balloon expandable stent is delivered within the patient's vasculature mounted on a balloon catheter and can be expanded at an interventional site to accomplish implantation. A 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 usually held in its compressed state by a restraining sheath during its passage through the patient's vascular system until reaching the target treatment site, whereupon the restraining sheath can be retracted to allow the compressed self-expanding stent to move into its deployed condition. Once in place, the radial struts of the stent bear against the inside walls of the passageway, thereby allowing normal blood flow.
One particular type class of shape settable materials that are practical for stents include Nickel-Titanium alloys (Nitinol). Previous methods to set a desired expanded Nitinol stent configuration involved forcing the stent over a cylindrical mandrel matching the desired inner diameter of the stent. The stent is then heat treated until the shape memory of the stent in its austenite phase has a diameter matching that of the mandrel. This process results in producing a stent that does not store stress in an optimal manner. Certain current approaches to stent expansion processes utilize the superelastic properties of Nitinol by creating a phase transformation in the stent as its diameter is enlarged. Alternatively, the stent may be tapered and, for example, transition from a seven millimeter (mm) diameter to a ten mm diameter over a thirty mm or forty mm length. Likewise, as with the uniform diameter stent, previous methods employed to expand a stent into a tapered stent involved forcing the stent onto a tapered mandrel and heating. Such methods suffer similar drawbacks to the methods used to uniformly expand stents.
During the expansion process, the mechanical stress in the Nitinol causes a phase transformation from austenite to martensite to accomplish a change in diameter. Stents may be chilled to lower temperatures to transform them to martensite as a way to lower the forces required by an operator to perform the expansion process. Once the stent has been shaped to the increased diameter, a heat treatment process is used to transform the atomic structure of the stent back to austenite and relieve built up internal stresses.
A variety of methods and systems are known for manufacturing stents, and for imparting a desired geometry onto the stent structure. Conventional methods of manufacturing stents required the expansion of the stents from a smaller diameter, or “as cut” position, to a larger diameter corresponding to the stent configuration as deployed in the patient. This expansion is typically performed by the intricate process of providing an initial heat treatment stage followed by the forcible sliding of the stents over a mandrel, and providing a subsequent heat treatment stage.
Some current expansion tooling consists of a cylindrical mandrel with a tapered end. In order to perform shape setting, an operator may use a push-pull technique to load the stent over the tapered mandrel. The superelastic property of Nitinol allows it to recover from up to eight to ten percent strain without deformation. The theoretical plane strain of a stent strut is up to six percent for practical expansions steps, when considering ideal radial expansion only. Additional strain provided by this technique may result in an amount of strain which exceeds the capability of the material to recover without deformation. Such a result is also associated with stents formed from materials which are not inherently superelastic. Inspection is required to determine whether further processing is needed to overcome the effects of this deformation.
Previous methods employed successive one to two millimeter expansions of stents by employing mandrels of successively larger diameters. Though so intended, these methods did not eliminate the presence of cracks and notch defects. Notch defects occur after the post expansion treatment of a cracked stent. Moreover, processing of the cut stent to an acceptable clinical size while retaining proper geometry is heavily influenced by the operator processing the stent.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. Recently, however, there is a need to reduce or eliminate the stress induced on the stent during application of the axial force required to forcibly slide the stent over the mandrel. The stresses generated within the stent material as the stent encounters radial loads and axial loads while being placed onto the mandrels can result in localized deformities such as strut fracture, kink, and flare. The presence of such deformities can jeopardize the structural integrity and performance characteristics of the stent. Further, such deformities can damage tissue in the lumen wall of the patient. Consequently, the conventional methods for expanding stents require extensive quality control and results in low product yield.
Additionally, the prior art method of expanding stents is disadvantageous in that the process must be performed in various discrete stages requiring numerous mandrels of differing sizes to provide incremental expansion in order to avoid damaging the stent. In many instances the requisite tooling and discrete process steps will reach a level that is too burdensome and complex to be performed in a cost effective manner. Examples of such prior art expansion techniques are disclosed in U.S. Pat. No. 6,305,436 and U.S. Pat. No. 6,402,779, each of which is hereby incorporated by reference in their entirety.
Expansion of stents from as cut to a proper clinical size while retaining proper geometry is currently heavily operator influenced. The process often requires numerous steps which can be tedious to the operator.
As evident from the related art, conventional methods often provide inadequate stent expansion techniques and cost prohibitive systems. There remains a need for an efficient and economic method and system to provide for stepwise expansion of shape memory stents, while reducing the overall stresses that the stent encounters, and thereby improving manufacturing yields due to fractured struts during expansion. Such a technique should limit the strain of individual struts to a minimal level while eliminating the presence of longitudinal forces on the stent during the process of loading the stent onto shape setting tooling. A device and method for reducing operator influence also would be beneficial. An improved method is desired that will also reduce the number of steps and will expand the stent is a more automated fashion. The present invention satisfies these and other needs.
In accordance with the purpose of the invention, as embodied and broadly described, the invention includes a method of manufacturing a medical device comprising forming a stent having an internal lumen, a proximal end, a distal end, and a longitudinal axis extending therebetween, the stent having a generally cylindrical shape defining a first stent diameter. In one aspect of the present invention, an expandable tubular braid is initially inserted into either the proximal or distal end of the stent, with the expandable tubular braid extending along the longitudinal axis of the stent and having a first expansion diameter. This stent can be radially expanded to a second stent diameter when radial force exerted by an expandable mandrel, or other expansion device, is placed into the internal lumen of the tubular braid to expand the stent to a second, larger stent diameter. This expandable tubular braid provides a sliding surface which provides a transport that reduces the amount of friction that would otherwise be present as the stent slides over the mandrel. This allows the stent to be more easily moved along the expanding diameter of the mandrel to help to uniformly stretch the stent circumferentially as it moves axially along the mandrel. The expandable tubular braid can be made from materials, such as, but not limited to, stainless steel, quartz and other high temperature materials, which can be chilled and heated along with the stent. Accordingly, the expandable tubular braid also allows the expanded stent to be easily removed from the mandrel.
In another aspect, the invention provides uniquely shaped expansion mandrels which limit the stresses applied to stent struts during the expansion process of manufacturing stents. Particularly, the expandable mandrel is made with a plurality of longitudinally extending expansion blades which cooperate to form a complete mandrel body. The expansion blades lie adjacent to one another and are capable of independently moving relative to each other in a longitudinal or axial direction. Subsets of alternating expansion blades can be moved axially relative to the other subsets. In this regard, expandable blade subsets can be moved axially to receive the stent. The expandable mandrel can be made with numerous regions having progressively larger diameters than an adjacent region. Once the stent is placed on one of the blade subsets, the stent can be progressively moved along the length of the mandrel by progressively moving the expandable blade subsets. In this regard, the mandrel changes its profile as blade subsets are axially moved to produce the desired expanded diameter for the stent. In this fashion, the stent can be incrementally “walked’ along the lengths of the expandable blades to progressively increase the stent diameter into the desired final expanded diameter. The structure of the expandable mandrel allows the stent to be progressively expanded to a larger diameter without compromising the integrity of the stent structure. Accordingly, the often thin struts of the stent should be less susceptible of breakage when a progressive increase of the stent's diameter is obtained.
In one particular aspect of the present invention, expansion blades are arranged to provide a “walking beam” type of motion which is transferred to the mounted stent. In this particular aspect, each expansion blade is mounted within a blade holder which allows the blades to slide longitudinally or axially therein. One sequence of moving causes a first subset of blades to move upward and radially outward causing the mounted stent to expand somewhat. the subset of blades are then moved linearly to another position, while the blades are still in the outright position, to allow the stent to make contact with another subset of expansion blades which receives the now expanded stent. The second subset of blades move in a similar fashion to again expand radially outward to an expanded position which again further expands the stent. The second subset of blades then move linearly to cause the further expanded stent to make contact with the first subset of blades, albeit, that the stent contact with the first subset of blades is at a region on the first subset which has a progressively larger diameter from the position where the stent was initially mounted on the first set of blades. A motor or other actuator could be coupled to the first and second blades to achieve this walking beam” type of motion.
The methods of the present invention provide for the stepwise expansion of shape memory stents, which reduce the overall stresses that the stent encounters, and thereby improving manufacturing yields by reducing the number of fractured struts which can result during stent expansion.
The above described devices and methods have broad applicability to stents made of any shape settable material. 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.
Turning now to the figures, which are provided for example and not by way of limitation, there is shown an expandable tubular braid and expandable mandrel made in accordance with the present invention. These devices are appropriate for both open cell and closed cell stents. The methods and systems presented herein may be used for imparting a desired shape or contour to a medical endoprosthesis such as a stent. The invention is particularly suited for expanding a stent using a tapered or stepped mandrel which reduces the stresses that are generated due to axial loads applied to the stent as the stent is being loaded onto the mandrel. For purpose of explanation and illustration, and not limitation, an exemplary embodiment of the system in accordance with the invention is illustrated in the accompanying Figures.
The stents formed in accordance with the invention are preferably made from a shape memory material such as Nitinol (Ni—Ti alloy). In manufacturing the Nitinol stent, the material is first in the form of a tube. Nitinol tubing is commercially available from a number of suppliers. The tubular member is then loaded into a machine that will cut the predetermined pattern of the stent into the tube. Machines for cutting patterns in tubular devices to make stents or the like are well known to those of ordinary skill in the art and are commercially available. Such machines typically hold the metal tube between the open ends while a cutting laser, preferably under microprocessor control, cuts the pattern. The pattern dimensions and styles, laser positioning requirements, and other information are programmed into a microprocessor, which controls all aspects of the process. After the stent pattern is cut, the stent is treated and polished using any number of methods or combination of methods well known to those skilled in the art.
In one embodiment of the invention, an expandable tubular braided sleeve 12 is initially inserted within the internal lumen 14 of the stent 10, as shown in
The braided sleeve 12 includes an internal lumen 16 as well. In use, the braided sleeve 12 is inserted into either the proximal or distal end of the stent 10, with the braided sleeve 12 extending along the longitudinal axis of the stent 10 and having an initial unexpanded diameter. The initial unexpanded diameter of the braided sleeve 12 allows the braided sleeve 12 to be placed over a mandrel 18. The mandrel 18 includes a first expansion portion 20 having a constant first diameter over its length to form a tubular structure. The mandrel 18 further includes a second expansion portion 22 having a second constant diameter which also forms a tubular structure. The second diameter of the second expansion portion 22 is larger than the first diameter of the first expansion portion 20. These first and second portions 20 and 22 are coupled by a tapered portion 24 having a tapering outer diameter which starts initially at the first diameter of the first expansion portion 20 and gradually increases to the second diameter of the second expansion portion 22. In an alternative design, this portion 24 could be non-tapered.
As can be seen in
The operator will now move the braided sleeve 12 and stent to the second expansion position 22 where the mandrel 18 is at its maximum diameter. Here, the stent 10 and braided sleeve 12 will achieve their maximum diameters.
As discussed above, the stents preferred embodiment of the invention are made from Nitinol. The shape memory characteristics of such a Nitinol stent allow the stent to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics, on the other hand, generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient's body, with such deformation causing the phase transformation. Once within the body lumen, the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original un-deformed shape by the transformation back to the original phase.
Alloys having shape memory/superelastic characteristics generally have at least two phases. These phases are a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenite phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensite phase.
The shape memory characteristics of the invention described above are preferably imparted to the alloy under a controlled temperature environment. This temperature control serves to make the stents more ductile during the expansion process. The increase in material ductility can be achieved while exposing the stent to a temperature, for example, of approximately −40 degrees Fahrenheit. Additionally, the desired increase in material ductility can be achieved while exposing the stent to a temperature between approximately 175 and 600 degrees Fahrenheit. Consequently, the shape of the metal during this heat treatment is the shape “remembered.”
The stent 10 and braided sleeve 12 can be expanded at any operable temperature. For example, the stent 10 and braided sleeve 12 can be cooled, heated or placed at room temperature when being expanded by the mandrel 18. In one aspect, the operator can initially reduce the amount of force being applied to the stent 10 by placing the stent 10 and braided sleeve 12 in a temperature controlled environment or zone when the stent is initially placed on the first expansion portion 20 of the mandrel 18. The temperature of the ambient environment can be different depending upon the materials used to create the stent. For example, the stent 10, braided sleeve 12 and mandrel 18 could be immersed in alcohol to cool the stent 10 to approximately −10° C. when the stent is initially placed on the first expansion portion 20 of the mandrel 18. The cooling causes the Nitinol stent to transition to the martensite phase in order to reduce the forces exerted by the operator when moving the stent 10 and braided sleeve 12 along the various portions of the mandrel in order to enlarge the stent 10 to a second stent diameter. Thereafter, the stent 10 is in an expanded configuration. The stent 10 along with the braided sleeve 12 may also be heat treated (and cooled). This would allow the shape of the stent to be heat set into the stent. It is contemplated that not all stents require the same cooling or heating steps.
The use of the braided sleeve 12 allows the operator to connect the end of the braided sleeve to a mechanism which will move the stent 10 with braided sleeve along the length of the mandrel 18. Accordingly, the steps of treating the stent to expand the diameter can be implemented by a mechanism rather than by the hands of the operator. This allows prevents the need to subject the hands of the operator to the temperature controlled environment.
The braded sleeve 12 can be made from a number of materials, including, but not limited to, stainless steel, quartz and other high temperature materials. The use of such materials allows the braided sleeve 12 to be subjected to high or low temperatures during the stent enlarging process. These materials also allow the braided sleeve 12 to expand with the stent 10 as the stent 10 and braided sleeve 12 are moved along the length of the mandrel 18. These materials reduce the amount of friction that would be otherwise acting on the stent in the absence of the braided sleeve 12. Also, since the stent 10 can be made with highly fragile strut patterns, the braided sleeve acts as a suitable transport which moves the stent 10 the mandrel 18 with little risk of fracture.
The operation of inserting the mandrel into the stent can be accomplished by a myriad of manual or automatic apparatus designs. Obviously, the braided sleeve 12 could be manually advanced over the mandrel 18 by the hands of the operator. Other examples include an actuating mechanism which could be employed to provide axial motion to advance the mandrel 18 into the lumen of the braided sleeve 12. In this manner, the operator only needs to hold the braided sleeve 12 or he/she could clamp the braided sleeve 12 to a holding fixture. Additionally, various other types of mechanisms including pneumatics, hydraulics, or linear motors can be utilized to ensure that the motion of the mandrel occurs gradually and/or consistently to limit the production of stress spikes within the stent.
The stent 10 and braided sleeve 12 of
Referring now to
These longitudinally extending expansion blades 32 cooperate to form a complete mandrel body and lie adjacent to one another or, alternatively, could have space between them. These expansion blades also are capable of independently moving relative to each other in a longitudinal or axial direction. In this regard, one or more expansion blades can be advanced in front of the other expansion blades, as is shown in
Once the stent is placed on one of the blade subsets, the stent can be progressively moved along the length of the mandrel by progressively moving the various subsets 44, 46 and 48 of expandable blades. In this regard, the mandrel changes its profile as the blade subsets 44, 46 and 48 are axially moved to produce the desired expanded diameter for the stent. In this fashion, the stent can be incrementally “walked” along the length of the expandable blades to the desired expansion region where the stent will be expanded to its final diameter.
The expandable mandrel can be made with numerous regions having progressively larger diameters than an adjacent region. This structure allows the stent to be progressively expanded to a larger diameter without compromising the integrity of the stent structure. Accordingly, the often thin struts of the stent will be less susceptible of breakage when a progressive increase of the stent's diameter is obtained.
As can be seen in
The subsets of expansion blades can be joined together utilizing connecting fixtures such as, for example, slip rings or other fastening devices which maintain the blades together to form the composite mandrel but which still allows the blades to move or slide relative to each other. The connecting fixtures could be attached at the ends of the mandrel. For example, the first subset 44 can be constructed such that the four individual expansion blades are connected together along the apex of the wedge shape to create a solid piece. The remaining subsets 46 and 48 can be slidingly attached along the length of the first subset 44 utilizing rings (not shown) at the ends of the blades 32 that connect the respective blades that form each subset 46 and 48. Still other fastening means could be used to maintain all of the expansion blades together.
It should be appreciated that a single blade A and single blade B could not move the stent 114 by themselves. For this reason, a set of blades A and a set of blades B are used to cooperatively contact the circular stent 114. An end view of the expansion mandrel 110 is shown in
The sets of blades can be connected to an actuating motor which is adapted to move each blade of the particular blade in the progressive movement depicted in the drawings. In this fashion, an automated walking beam can be used to expand the stent to a desired shape and diameter. As with the other disclosed embodiments, the expansion mandrel 110 can have a desired number of expansion regions and tapered regions.
It is to be recognized that number of expansion blades that can be incorporated to form an expandable mandrel described above can be varied and accordingly such blades can be spaced optimally about the mandrel. Additionally, different spacing of the blades relative to each other could also be achieved without departing from the spirit and scope of the present invention. The number of tapered regions and expansion regions of any of the described mandrels could also be varied, as desired, to obtain the desired final diameter of the stent. In this regard, the expansion mandrels can be made with numerous expansion regions. The operator only needs to advance the stent to the desired expansion region to achieve the desired diameter for the stent. If a larger diameter is needed, the operator has the ability to take the stent up to the next expansion region to obtain that diameter. Additionally, the expansion regions and tapered regions of the described mandrels have been shown having substantially uniform lengths. It should be appreciated that the length of the expansions regions could be much larger, especially when a longer stent is being processed. Long stents are used, for example, to treat Peripheral Artery Disease (PAD). Such stent are often implanted in the arteries of the legs where longer stents are needed.
The individual blades which make up the expandable mandrel can be made from suitable materials such as metals, including, but limited to stainless steel, and possible hard polymeric materials and ceramics. The blade holder could be made from similar materials.
While the present invention has been described in conjunction with a self-expanding stent, a balloon expandable stent having any configuration or pattern could also be made using the apparatus and methods described herein. The stent body can comprise metal, metal alloy, or polymeric material. Some exemplary materials include Nitinol and stainless steel. Other complimentary materials include cobalt chromium alloy, ceramics and composites abd organic and polymeric materials.
While the present invention is described herein in terms of certain preferred embodiments, those skilled in the art will recognize that various modifications and improvements may be made to the invention without departing from the scope thereof. Moreover, although individual features of one embodiment of the invention may be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment may be combined with one or more features of another embodiment or features from a plurality of embodiments.
In addition to the specific embodiments claimed below, the invention is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features presented in the dependent claims and disclosed above can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combinations. Thus, the foregoing description of specific embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to those embodiments disclosed.
Many modifications, variations, or other equivalents to the specific embodiments described above will be apparent to those familiar with the art. It is intended that the scope of this invention be defined by the claims below and those modifications, variations and equivalents apparent to practitioners familiar with this art. While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept. Accordingly, it is not intended that the invention be limited, except as by the appended claims.
