Abstract:
A method and apparatus for cutting a braided wire stent to a predetermined length such that a ball or sphere is formed on the end of each cut wire of the stent. These spheres are advantageous in that they provide added comfort to the patient and also act against the other wires of the stent to prevent the stent from becoming unbraided during the process of collapsing and expanding the stent such as is done when the stent is being inserted into a patient. The apparatus releasably holds and precisely positions the wires while the spheres are being formed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains generally to cutting braided stents from stock.  
           [0002]    Stents are generally metal or plastic tubes inserted into a vessel such as the urethra to keep a lumen open. A vast variety of stent materials and designs are available. A few examples of available designs include braided tubes, wire springs, and tubes having a plurality of holes formed therein to provide flexibility. It is preferable that a stent design provides a tube which can be stretched or otherwise manipulated to reduce the diameter of the tube while the stent is being inserted, and which expands to resume an original outside diameter when released. Reducing the diameter of the stent during insertion reduces the likelihood of trauma to the surrounding tissue of the lumen into which the stent is being inserted. Of the available designs, a stent braided from thin wires is particularly suited for this purpose in that, when stretched, its diameter is rapidly reduced relative to the measure of stent elongation. Furthermore, the energy stored in the stent when the stent is stretched is relatively small, so that when the stent returns to its original shape within the lumen, it does so at a safe rate in a gentle manner without damaging the surrounding tissue. Conversely, in order to reduce the diameter of a coiled spring, the spring must either be pulled, creating spaces between the coils of the spring which may potentially provide a pinch hazard, or twisted several times, setting up a potentially significant recoil force which may impart damage to soft tissue when released.  
           [0003]    Braided stents, however, have posed certain problems pertaining to their manufacture and use. The stents are cut from a length of braided tubular stent stock. The stock typically comprises a plurality of right-handed helical wires or strands interwoven with an equal number of left-handed helical wires or strands. Each wire or strand has a first end and a second end. The first ends of all the strands together generally define the first end of the stock and the second ends together generally define the second end of the stock. All of the wires or strands form helixes that have substantially equal outside diameters, twist angles, and share a common central axis. Ideally, all of the right-handed helixes are angularly spaced apart from each other by an equal angle, as are the left-handed helixes. This creates a diamond pattern formed by the intersecting strands wherein the intersections form the apexes of the diamonds and the individual strands between the intersections form the sides of the diamonds. Equally spaced apart helixes ensure that the diamond pattern further forms uniform rows of adjacent diamonds arranged so that the upper and lower apexes are substantially aligned and the side apexes are also aligned. Ideally, a line connecting the upper and lower apexes should be perpendicular to a line connecting the side apexes. The interwoven helical strands together generally define a stent periphery which is generally cylindrical.  
           [0004]    Some of the problems presented by using braided stock to form stents arise when inconsistencies are found in the individual diamond dimensions. When the angular spaces between the individual helixes are not uniform, the apexes quickly become misaligned. Attempts at cutting such a stent along a plane that is substantially perpendicular to the central axis of the stock results in free wire ends of varying lengths and angles. Moreover, devising an automated method or mechanism for cutting a braided stent is significantly complicated by pattern irregularities.  
           [0005]    For example, stent stock may be placed on a mandrel for automatic cutting by a device which provides a cutting force, whether it be heat or a mechanical force. The mandrel carrying the stock rotates around its central axis while the cutting force cuts each individual wire as they pass beneath the cutting device. This results in a cutting plane that is perpendicular to the axis of rotation. If the stent stock has an irregular diamond pattern, the cuts will occur at various positions between apexes or at the apexes themselves. This is undesirable for several reasons. The spaces between the wire ends will vary and may increase the discomfort experienced by the patient. Also, the tendency for the stent to become unraveled is significantly increased due to the varying lengths of strand portions that extend beyond the apexes adjacent the cutting plane. Additionally, the ability of the stent to be compressed and released is degraded due to the increased tendency of the stent to unravel as the wires slide relative to each other when the stent is compressed and released. If heat is used to cut the stent, and the heat source gets too close to the intersections of the strands, the adjacent strands forming the intersection may become welded together, inhibiting the ability of the braided stent to be compressed without becoming deformed.  
           [0006]    Other problems presented by braided stents pertain to the ends of the individual wires. Once the wires are cut, they tend to provide sharp edges. These edges may irritate the walls of the lumen or vessel in which the stent is being used, thereby causing discomfort to the patient, and may make removal of the stent more difficult, should removal be necessary. Additionally, the sharp edges provide little to no resistance to the unraveling problem mentioned above.  
           [0007]    Attempts at developing an automated manufacturing method, which overcomes these problems, have failed. For example, in order to present a uniform diamond pattern to the cutting device, efforts have been made to manipulate the diamond pattern by moving the individual wires into a desired formation. One effort incorporated a mandrel with helical grooves cut into the outer surface for receiving the braided stent therein. Unfortunately, these procrustean efforts resulted in creating internal stresses in the wires. Once the wires were cut, the stresses were released, and the wires “jumped” apart. This jumping action not only created additional unraveling problems, it frustrated attempts at shaping the resulting wire ends to provide a dull surface because the wire ends jumped out of operable proximity with the cutting device.  
           [0008]    Methods including visual wire location means have also been attempted with unsatisfactory results. Locating wires visually avoids some of the manipulation issues described above, but can be labor intensive and time consuming. Moreover, the stents produced contain inconsistencies due to operator inaccuracies inherent in the visual location methods.  
           [0009]    Shaping the wire ends to provide a dull surface may reduce the discomfort presented to the patient by sharp wire ends. Methods have been developed which form spheres on the ends of wires. These spheres are desirable because they provide a dull surface and, more importantly, because the resulting spheres generally have a diameter greater than that of the wire. This increased diameter effectively reduces the tendency of the braided stents to become unraveled. When a braided stent is stretched or compressed, the individual helical wires or strands slide relative to each other. As they slide, the positions of the intersections move relative to the wire ends. If the location of the intersection moves to the ends of the wires, there is a tendency for the wires to unravel and attempt to achieve a straighter shape. Providing spheres at the ends of the wires or strands reduces this tendency by presenting a physical barrier to wire ends passing over wires with which they intersect, thereby preventing unraveling.  
           [0010]    Unfortunately, attempts at developing an automated manufacturing process to create these spheres have heretofore been unsuccessful. Some of the reasons pertain to the inconsistencies in the braided diamond patterns, others pertain to the alternating angles presented by the interwoven helixes. Explanation of these reasons requires a brief discussion of sphere formation.  
           [0011]    It has been found that melting the ends of the strands can result in such a sphere when a focused heat source is directed to a point on the wire and then moved along a predetermined length of the wire toward the desired location of the sphere. Doing so causes molten strand material to follow the wire ahead of the heat source, accumulating to form a sphere.  
           [0012]    If a strand of meltable material, such as metal or plastic, passes through a heat source, a section of the strand will be melted away to form a gap in the strand, provided the heat source is hot enough to melt the material. The length of this gap, measured in a direction perpendicular to the direction of relative movement of the heat source, will define the effective cutting width of the heat source. The effective cutting width may be increased by providing a larger heat source, or by making multiple passes with the same heat source and laterally offsetting the path of the heat source on each subsequent pass.  
           [0013]    When a strand of meltable material under stress, such as the stress found in a wire which has been braided into a helix, is subjected to such a heat source, the molecular bonds being stretched by the stress will break and the strand will separate as the stress is relieved. Depending on the amount of tension in the strand, the newly formed ends of the strand, defining the gap, may remain subjected to the heat source and will melt and tend to move away from the heat source by following the adjacent solid portions of the strands. When the liquid cools and solidifies on the strand, the thickness of the strand is increased. This phenomenon is due to the surface tension of the liquid formed when the material melts. Surface tension causes a drop of liquid to minimize its surface area. Therefore, a drop of liquid having surface tension tends to attach itself to a solid rather than dropping off. This tendency occurs because a drop of fluid on a solid has a smaller overall surface area than a suspended drop. Similarly, surface tension also causes a body of liquid to form a sphere when the body is not acted upon by any other external forces. A sphere, geometrically, has the smallest surface area of any shape per unit volume.  
           [0014]    The magnitude of the increase in thickness will vary with the amount of liquefied material collected on the end of the strand, and, when the body is under the influence of gravity, by the strength of the surface tension relative to the weight of the material. The increase in thickness will also vary depending on the amount of heat absorbed by the liquid. The surface tension of a liquid is inversely proportional to its thermal energy. In other words, liquids become thicker as their temperatures approach freezing.  
           [0015]    If the strand is oriented such that its direction of travel is substantially perpendicular to its longitudinal axis, as the strand passes in operational proximity to the heat source, the strand will separate, as discussed above, and the newly formed ends defining the gap will spend relatively little time exposed to the heat source. The result will be insignificant increases in thickness on both newly formed ends. In order to form a significant sphere on one end, the wire is preferably oriented to approach the heat source such that an acclivitous angle is formed between the path of the wire and its longitudinal axis, with the sphere usually resulting at the top of the slope. Alternatively, the wire may be fed into the heat source along its longitudinal axis, but the heat source must be turned off when the sphere has achieved a desired size. It will become apparent that this path is not conducive to automating the process of forming spheres on the ends of the wires of braided stent stock.  
           [0016]    It is to be understood by those skilled in the art that movement between the heat source and the wire is relative. Whether the heat source is physically moved toward the wire or the wire is physically moved toward the heat source, or any combination thereof, is inconsequential for purposes of the discussion herein or when practicing the teachings of the invention. For ease of explanation of FIG. 1, the heat source will be described below as moving toward a wire or strand.  
           [0017]    [0017]FIG. 1 presents a series of sequential diagrams showing the formation of a sphere S as a focused heat source passes through a wire at an acclivitous or upwardly sloping angle. Due to the relative acclivitous angle δ between the path P, having a width w of heat source H and the wire  14 , heat source H first makes contact with wire  14  near the bottom of heat source H. Once contact is made, heat source H cuts wire  14  into two pieces, thereby creating a bottom end B and an upper end U. As heat source H continues along path P, it continues to melt upper end U and moves past bottom end B rather quickly. It can be seen that, when wire  14  is presented at an acclivitous angle 6 to heat source path P, a sphere S forms above heat source H as heat source H continues to collide with and move through wire  14 . A significant sphere S does not form on wire  14  below heat source H because the bottom end B of the wire  14  loses contact with heat source H after the initial cut and therefore, little to no strand material accumulates on end B.  
           [0018]    It should be noted that the cutting effect is due, in part, to the tension in the wire  14 , as described above. Notably, if the tension is too great, the wire  14  will spring apart quickly and take the bottom end B and the upper end U out of operably proximity with heat source H so that spheres S are not formed. Conversely, if there is little or no tension in wire  14 , the wire may not separate immediately and both upper end U and bottom end B will remain within operable proximity to the heat source H long enough to form spheres S on both ends.  
           [0019]    The size of the formed sphere S is dependent on the size of the wire  14  and the amount of energy delivered to the wire. The amount of energy delivered to the wire is dependent on the temperature of the heat source H and the amount of time the wire  14  spends in operable contact with the heat source H. The amount of time the wire  14  spends in operably contact with the heat source H may be controlled by varying the relative speed between the heat source H and the wire  14 , and is dependent on the angle 6 presented between the wire  14  and the path of the heat source H.  
           [0020]    If the relative speed between the heat source H and the wire  14  is too fast, the wire  14  may not absorb enough heat to melt and separate or the wire  14  may separate but the amount of material melted by the heat source may be too small to a form significant sphere S. If the relative speed is created by rotating the stent around a central axis in operable proximity to a stationary heat source H, excessive angular velocity may result in a sphere S becoming radially displaced outwardly from the centerline of the wire  14  due to centrifugal force. A stent with wire ends having such radially displaced spheres S will have an increased maximum outer diameter which may provide increased discomfort and insertion and removal difficulties.  
           [0021]    If the angle δ presented is too shallow, the relative speed between the heat source H and the wire  14  must be slower because the component of the relative speed in the direction of path P will be greater. Also, sphere S will end up being larger because more wire material will be lying in path P. This may result in the loss of sphere S due to the inability of the surface tension to overcome the forces of gravity. In short, sphere S may drip off of wire  14  before it escapes path P and has a chance to cool on wire  14 . Conversely, if the angle δ is too steep, there will be insufficient wire material to form a significant sphere S.  
           [0022]    Predictably, attempts at forming a stent of braided strands with spherical ends using an automated process have struggled with presenting each wire at an appropriate angle to the heat source, ensuring that the heat source path intersects the wire between the apexes, providing an appropriate relative speed between the wire and the heat source, and manipulating the stent stock without creating internal stresses within the wire so that the wire doesn&#39;t “jump” out of the path of the heat source when initially cut. Additionally, braided stents, being formed of alternating left-handed and right-handed helixes, present alternating acclivitous and declivitous angles to a heat source travelling relative to a rotating stent. Cutting each wire in sequence would result in spheres formed on alternating sides of the cut.  
           [0023]    It can be seen that there is a need for an automated method of cutting a stent from a length of braided stock material.  
           [0024]    There is also a need for an automated method of cutting a stent from a length of braided stock material that overcomes some or all of the problems described above.  
           [0025]    More specifically, there is a need for a device for holding a length of braided stock material that does not allow the individual wire to “jump” after being cut.  
           [0026]    There is also a need for an automated method of cutting a stent from a length of braided stock material that results in a uniform plurality of wire ends.  
           [0027]    There is, more specifically, a need for an automated method of cutting a stent from a length of braided stock material that incorporates a typical laser cutting machine having a laser and an axially displaceable indexing head.  
           [0028]    There is yet a further need for an automated method of cutting a stent from a length of braided stock material that creates a sphere or similar dull surface at the end of each wire of the stent.  
           [0029]    There is an additional need for a method of cutting a stent from a length of braided stock material that results in a braided stent that is resistant to unraveling.  
           [0030]    There is a further need for a method of cutting a stent from a length of braided stock that creates a stent that provides increased comfort to the patient.  
         BRIEF SUMMARY OF THE INVENTION  
         [0031]    The present invention pertains generally to an automated method and apparatus for cutting a stent having a predetermined length from a length of braided stent stock.  
           [0032]    In a preferred form, the present invention provides a device that temporarily secures a length of stent stock so that it may be controllably moved relative to a heat source used to cut the stent. This device preferably includes an elongate mandrel having an outside diameter slightly smaller than the inside diameter of the relaxed, braided stent stock the mandrel is designed to secure. A length of stent stock is placed on the mandrel at a predetermined axial position along the length of the mandrel. The mandrel also preferably defines a central or inner channel having an inside diameter sized to receive an elongate activation dowel. An anchoring or compensating mechanism, operably attached to the mandrel, releasably fixes the stent stock to the mandrel and compensates for irregularities in the stent braiding by gently manipulating the individual wires of the stent to present the sections of the wires that are between diamond apexes, to a heat source in a predictable, repeatable manner.  
           [0033]    A preferred embodiment uses a laser as the melting source or heat source such as that found on the Eagle 500 CO 2  Laser System, manufactured by Laser Machining, Incorporated of Somerset, Wis. A laser is advantageous because it is extremely focused and emits relatively little radiant heat. In other words, the temperature gradient, as the distance from the center of the laser increases, is very steep. Lasers can also be shuttered on and off very quickly by using shutters or deflectors to block the beam from coming into operable contact with the target. It is envisioned, however, that other, similarly focusable heat sources may be used without detracting from the spirit of the invention.  
           [0034]    Preferably, the anchoring mechanism includes at least one set of two or more angularly spaced apart apertures extending radially through said mandrel. These apertures house inwardly biased, outwardly displaceable, pins or protuberances, constructed and arranged to slide in and out of the apertures when the activation dowel is inserted or engaged, and removed or disengaged.  
           [0035]    The activation dowel begins at a first end, includes a handle portion and an activation portion, and concludes at a second end. The handle portion is preferably cylindrical and has an outside diameter slightly smaller than that of the inside diameter of the channel defined by the mandrel. Preferably the handle portion slides easily in and out of the mandrel, however, is snug enough to avoid any appreciable play. The activation portion includes one or more surfaces, preferably continuous surfaces, which act on the pins in sequence causing them to protrude when the activation dowel is inserted within the mandrel. The angled portion gradually increases the diameter of the dowel from an angled portion distal end to an angled portion proximal end.  
           [0036]    It is understood that the channel and the activation dowel may be of any shape and do not necessarily have to be cylindrical. Similarly, the angle portion could be the frustum of a cone, pyramid, or any other shape of increasing or decreasing diameter. Clearly, in order to lower manufacturing costs and time, the cylindrical relationship between the dowel and the channel, herein described, is preferable.  
           [0037]    The pin protrusion sequence caused by the angled portion is advantageous. When a mandrel provides more than one set of pins, each set being displaced from a preceding set by a predetermined longitudinal distance, the angled surface causes the first set it encounters during its insertion to protrude from the outer surface of the mandrel before the next set of pins is acted upon by the angled surface. This progressive pattern of activation is advantageous in that the first set of pins functions to generally align the braided stent stock with the pins so that the second and, preferably, third sets of pins may find the appropriate respective spaces in the stent stock more easily while engaging the stent stock. In the event that the first set of protuberances should happen to abut directly against the strands of the stent stock while they are emerging from the apertures, the stent stock may be slid slightly along the axis, either forwardly or rearwardly, in order to free the stock from the interference. Alternatively, the stent stock may be rotated slightly to expose the pins or protuberances to the spaces. Subsequent sets of protuberances or pins should then be free of any interference as they are engaging the stock.  
           [0038]    It is envisioned that the present invention includes pins or protuberances that are sized to snugly fit within the diamond shaped holes defined by the strands of the braided stent stock. Sizing the pins thusly results in a more secure relationship between the stent stock and the mandrel and reduces the likelihood of manufacturing errors due to stock movement.  
           [0039]    The combination of the progressive engagement pattern described above with pins sized to snugly fit within the diamond shaped holes defined by the strands of the braided stent stock ensures that the mandrel adequately compensates for irregularities in the braided stent design.  
           [0040]    In another aspect, the present invention provides a device for securing a length of stent stock, as described above, at a predetermined axial position along the device, which includes two sets of spaced apart pins separated by a cutting groove. Providing two sets of pins, preferably including four pins per set, and a cutting groove between the sets, significantly decreases the tendency for the wires to “jump” away from the cutting tool after the cut has been made.  
           [0041]    In other aspects of the present invention, the mandrel includes three sets of pins, preferably having at least two pins per set, more preferably three pins per set, and even more preferably four pins per set, and two cutting grooves juxtaposed between each of the sets of pins such that one set of pins lies between the cutting grooves while the remaining sets are found on the outside of each groove. This arrangement is advantageous in that it facilitates a faster manufacturing process, and provides more accurate positioning of wires and intersections relative to the position of the heat source, than does the use of fewer pins.  
           [0042]    More specifically, the braided stent stock is placed on the mandrel so that numerous stents may be cut therefrom. Though each cut results in two ends of stock, usually only one end has spheres formed on the ends of the individual strands. Another cut must be made to form spheres on the other end of the stock. In other words, in order to cut a plurality of stents with spherically ended strands from a single length of stock, a certain amount of waste must be allocated between each strand. Providing two grooves, spaced apart by a distance which will result in the length of the scrap piece, allows the end of one stent to be cut, and the beginning of another stent to be cut, without adjusting the position of the stent stock on the mandrel. This is also advantageous in that it results in a predictable, repeatable length of scrap between each stent. In this embodiment, three sets of pins are provided so the strands of the stent stock are secure on either side of each cutting groove. This prevents each strand from “jumping” out of alignment after it is cut.  
           [0043]    In one aspect of the present invention, springs are provided, operably attached to each pin, thereby biasing the pins toward an inward position whereby a smooth outer mandrel surface is provided when the activation dowel is not inserted. This arrangement facilitates sliding a newly formed stent and scrap pieces off of the mandrel and also allows the remaining length of stent stock to be slid along the length of the mandrel so that another stent may be cut therefrom.  
           [0044]    In another preferred aspect of the present invention, a method of cutting a stent of a predetermined length from a length of braided stent stock is provided. This method preferably involves using a focused heat source capable of creating an area of heat sufficient heat to melt a predetermined length of one of the elongate strands of the braided stock. A length of the braided stent stock is provided and aligned with the heat source such that the heat source is aimed substantially between two adjacent rows of vertices formed by the intersections of the individual strands.  
           [0045]    Once the heat source is properly aligned with the stent stock at the desired cutting location, the stent stock is rotated relative to the heat source around the central axis of the stock. Preferably, the heat source remains between the two adjacent rows of vertices while the stock is rotating.  
           [0046]    It has been found that a preferable way to form predictable, consistent spheres on one side of the cut involves subjecting alternating strands to the heat source such that only strands having substantially acclivitous angles relative to the path of the heat source are melted, thereby forming a sphere on every other strand proximate the upper side of the area of heat. Once all of the strands presenting acclivitous angles relative to path of relative motion of the heat source are melted, the relative path of motion is reversed such that the remaining strands now present acclivitous angles to the path of the heat source. The remaining strands are then cut and spheres are formed proximate the upper side of the area of heat.  
           [0047]    This preferred aspect of the present invention preferably incorporates a turning mechanism for controllably rotating the stent stock beneath the cutting device at a controlled, predetermined angular speed. This predetermined angular speed is preferably calculated to ensure proper sphere formation at the ends of the individual strands,  
           [0048]    More preferably, the mandrel is constructed and arranged for insertion into a laser cutting machine, such as the Eagle 500 CO 2  Laser System, manufactured by Laser Machining, Incorporated of Somerset, Wis. These versatile machines include a indexing head having a chuck for receiving various tools, and a laser directed toward the axis of rotation of the indexing head. The indexing head is typically mounted on a table which is moveable, relative to the laser, in a plane generally perpendicular to the laser beam, so a work piece may be moved into and out of a cutting position by a computer controlling the movement of the table.  
           [0049]    It is thus an object of the invention to provide an automated method of cutting a stent from a length of braided stock material, which creates a sphere or similar dull surface at the end of each wire of the stent.  
           [0050]    It is also an object of the invention to provide a device for holding a length of braided stock material that does not allow the individual wire to “jump” after being cut.  
           [0051]    It is another object of the invention to provide a device that presents the individual wires of braided stent stock to a cutting device in a predictable, repeatable, accurate manner, regardless of inconsistencies present in the braids of the stent stock.  
           [0052]    It is further an object of the invention to provide an automated method of cutting a stent from a length of braided stock material that results in a uniform plurality of wire ends.  
           [0053]    Another object of the invention is to provide an automated method of cutting a stent from a length of braided stock material that creates a sphere or similar dull surface at the end of each wire of the stent.  
           [0054]    Yet another object of the invention is to provide a method of cutting a stent from a length of braided stock material is described which results in a braided stent that is resistant to unraveling.  
           [0055]    Still another object of the present invention is to provide a method of cutting a stent from a length of braided stock, which creates a stent that provides increased comfort to the patient.  
           [0056]    These and further objects and advantages of the present invention will become clearer in light of the following detailed description of illustrative embodiments of this invention described in connection with the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0057]    The illustrative embodiments may best be described by reference to the accompanying drawings where.  
         [0058]    [0058]FIG. 1 is a series of sequential diagrams showing the formation of a sphere as a focused heat source passes through a wire or strand of meltable material at an acclivitous angle;  
         [0059]    [0059]FIG. 2 is a perspective view of a braided stent having spherical ends, to which the present invention is directed to forming, the stent shown being greatly enlarged and loosely braided in order to show detail;  
         [0060]    [0060]FIG. 3 is a perspective view of a preferred embodiment of the stent stock retaining device of the present invention shown in a disengaged position;  
         [0061]    [0061]FIG. 4 is a perspective view of the retaining device of FIG. 3, shown in an engaged position;  
         [0062]    [0062]FIG. 5 is a section view of the retaining device of the present invention, taken generally along lines  5 - 5  of FIG. 4;  
         [0063]    [0063]FIG. 6 is a representation of the geometry of an individual diamond of the braided stent stock to which the present invention is directed, labeling the various dimensions of the stock in order to describe the geometry thereof mathematically;  
         [0064]    [0064]FIG. 7 is a representation of the geometry of an end of the braided stent stock to which the present invention is directed; and,  
         [0065]    [0065]FIG. 8 is a flowchart of the steps of a preferred embodiment of the method of the present invention. 
     
    
       [0066]    All figures are drawn for ease of explanation of the basic teachings of the preferred embodiments only. The extensions of the Figures with respect to number, position, relationship, and dimensions of the parts to form the preferred embodiments will be explained or will be within the skill of the art after the following description has been read and understood. Further, the exact dimensional proportions to conform to the specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following description has been read and understood.  
         [0067]    Where used in the various figures of the drawings, the same numerals designate the same or similar parts. Furthermore, when the terms “top,” “bottom,” “upper,” “lower,” “first,” “second,” “front,” “rear,” “end,” “edge,” “forward,” “rearward,” “upward,” “downward,” “inward,” “outward,” “inside,” “side,” “longitudinal,” “lateral,” “horizontal,” “vertical,” “acclivitous,” “declivitous,” and similar terms are used herein, it should be understood that these terms have reference only to the structure shown in the drawings as it would appear to a person viewing the drawings and are utilized only to facilitate describing the preferred embodiments. It should be further understood that the term “sphere,” as used herein, pertains to a generally curved shape at the end of a strand and does not imply the formation of a mathematical sphere. Strands having ends which are egg shaped, tear drop shaped, generally thickened, or generally rounded are considered “spherical” as the term is used herein.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0068]    Device  
         [0069]    Referring now to the figures, and first to FIG. 2, there is shown a braided stent  10  to which the various embodiments of the devices and methods of the present invention are directed to forming. Stent  10  is formed such that the cut ends  12  of the wires or strands  14  are substantially spherically shaped.  
         [0070]    Stent  10  is a segment cut from braided stent stock, which is made up of a plurality of strands  14 . Strands  14  are braided such that half of the strands  14  form left-handed helixes  16  and the other half of the strands  14  form right-handed helixes  18 . The various helixes  16  and  18  are alternately woven together to define a plurality of diamond-shaped openings  20 . Openings  20  have upper apexes  22 , lower apexes  24  and side apexes  26 , which are formed by the intersections of the individual strands  14 . The strand lengths between the intersections define the sides  28  of the diamonds  20 . It is readily apparent from the figure that any given intersection of two strands or wires  14  serves as common point for four diamonds  20  by being the upper apex  22  for one, the lower apex  24  for another, and side apexes  26  for the other two openings  20 . It should be noted that FIG. 2 shows only the upper hemisphere of a stent  10  in detail in order to preserve clarity of representation.  
         [0071]    Referring now to FIGS.  3 - 5 , there is shown a device  29  for facilitating the controlled handling of a length of stent stock while the stock is cut to a predetermined length in order to form a stent  10 . Device  29  preferably includes a mandrel  30 . Mandrel  30  has an outside diameter  32  sized to receive a given size of stent stock. Diameter  32  should be slightly smaller than the inside diameter of the corresponding stent stock, measured while the stock is in a relaxed condition, so that the stock slides easily over mandrel  30  and so no internal stresses are created within the stock due to it being placed on mandrel  30 . Mandrel  30  includes an anchoring mechanism  34  for temporarily fixing or securing a length of stent stock to mandrel  30  in such a manner that the exact location of the various intersections of strands  14  can be positioned to avoid the path of a cutting device. It is also preferable that no significant internal stresses are imparted into the strands  14 .  
         [0072]    The envisioned embodiment of anchoring mechanism  34  shown in FIGS.  3 - 5  includes a plurality of pins or protuberances  36  slideably housed within a plurality of apertures  38  defined by mandrel  30 . Preferably, pins  36  and apertures  38  are arranged in longitudinally spaced apart sets  40 . The Figures depict an embodiment having three sets,  40   a ,  40   b  and  40   c . Using three sets  40  has been found to be best suited when two cutting positions are desired, as will be discussed in more detail below. However, it is understood that if only one cutting position is needed, two sets  40  are optimal. It has been found advantageous to provide one set  40  on either side of each cutting position. This configuration ensures that stent stock on either side of a cut will be secure.  
         [0073]    Similarly, the figures show four pins  36  and four apertures  38  per set, angularly spaced ninety degrees apart from each adjacent pin  36  and aperture  38 . This configuration facilitates ease of manufacturing in that two opposite apertures  38  may be drilled or machined with one tool stroke. For purposes of securing stent stock to mandrel  30 , three or five pins  36  per set  40  would also be effective.  
         [0074]    Ease of stent stock removal and readjustment is achieved by biasing pins  36  inwardly. As best seen in FIG. 5, coil springs  41  surround each pin  36  and act against mandrel  30  and against a pin flange  42  defined by each pin  36 , thereby urging each pin  36  inwardly. Preferably, spring  41  is attached at one end to mandrel  30  and at an opposite end to flange  42 , thereby preventing pins  36  from becoming unseated within apertures  38 .  
         [0075]    Apertures  38  lead into an inner channel  44  defined by mandrel  30  and preferably concentric therewith. Inner channel  44  is characterized by an inner diameter  46  which is small enough to provide the appropriate thickness between outer diameter  32  and inner diameter  46  of mandrel  30  such that pins  36  are adequately supported and long enough to protrude through the diamond shaped holes or openings  20  of the stent stock. Though any appropriate mechanism for causing pins  36  to protrude from apertures  38  would be acceptable, it is envisioned that an activation dowel  48  is provided. Activation dowel  48  preferably includes a tip  52 , a handle portion  54  and an activation portion  56 . Handle portion  54  has an outside diameter  58  sized to fit within inner channel  44  of mandrel  30 . Preferably, outer diameter  58  is only slightly smaller than inner diameter  46  such that a snug fit is provided. Handle portion  54  is of sufficient length that stability is provided to activation dowel  48  when inserted within inner channel  44 . Handle portion  54  is also preferably of sufficient length that when activation dowel  48  is fully inserted within inner channel  44  of mandrel  30  a segment of handle portion  54  remains outside of mandrel  30  such that it may be grasped for removal.  
         [0076]    Activation portion  56  is adjacent handle portion  54  and includes an angled portion  62  having a front  64  and a rear  66 . Front  64  has a smaller outside diameter than does rear  66 . In the preferred embodiment, activation dowel  48  also includes a first cylindrical segment  68  which extends from tip  52  to angled portion front  64  and a second cylindrical segment  70  extending from angled portion rear  66  to handle portion  54 . First cylindrical segment  68  functions to initially align activation dowel  48  when inserted into inner channel  44  of mandrel  30 . This can best be seen in FIG. 5. Second cylindrical segment  70  has an outer diameter  72  which is sized to cause pins  36  to fully protrude from mandrel  30  when activation dowel  48  is fully inserted. It can also be seen that the difference between outer diameter  72  and inner diameter  46  is great enough to allow sufficient space between second cylindrical segment  70  and mandrel  30  to contain pin  36  and spring  41  in a compressed state.  
         [0077]    In addition to apertures  38 , it is preferable that mandrel  30  further comprise at least one cutting groove or slot  74  for preventing damage to mandrel  30  during a cutting operation. Cutting groove  74  provides a space between the outer surface of mandrel  30  and the strands of stent stock intended to be cut. Preferably, a first slot  74  is provided between pin sets  40   a  and  40   b , and a second slot  75  is provided between pin sets  40   b  and  40   c.    
         [0078]    Optionally, mandrel  30  may also include a plurality of reference markings  76  to aid in the proper placement of a length of stent stock in determining the resulting length of cut stent  10 . Markings  76  are preferably spaced apart by a distance approximately equal to the distance between an upper apex  22  and a lower apex  24  of any given diamond opening  20  of the stent stock for which device  29  is designed. Markings  76  are also preferably aligned longitudinally as seen in FIGS.  3 - 4 .  
         [0079]    Mathematical Relationships  
         [0080]    The physical preferred embodiments having thus been described it is now important to define the mathematical relationships between the various measurements of the given stent stock and the physical locations and sizes of the pins  36  and grooves  74  of mandrel  30 . Reference is made to FIGS. 6 and 7.  
         [0081]    Pins  36  are preferably sized to have a radius r that snugly fits within any given diamond shaped opening  20  of the stent stock. It can be shown that the largest pin radius r which can fit within a diamond can be represented mathematically by the formula:  
           r =( Lh )/2( h   2   +L   2 ) 1/2    
         [0082]    where h represents the inside height of diamond opening  20  measured from its lower apex  24  to its upper apex  22  and L represents the inside length of diamond opening  20  as measured from one side apex  26  to an opposite side apex  26 .  
         [0083]    If r represents the largest possible pin radius which can fit within a diamond of height h and length L, then it can be shown that:  
           lim r=h/ 2  
         [0084]    It should be noted that, for purposes of mathematical representation and ease of calculations, some of the formulas presented herein make the assumption that diamonds  20  and strands  14  lie in a flat plane. In reality, stent  10  is cylindrical and diamonds  20  and wires  14  necessarily follow the curve of stent  10 . However, it has been found that, in practice, making the mathematical assumption that the diamonds  20  and strands  14  lie in a flat plane, has not affected the desired results and that the incremental differences between the assumed flat plane and the actual cylindrical surface are relatively inconsequential.  
         [0085]    It is important to provide a heat source, preferably a laser beam, having an effective cutting area small enough to cut strands  14  while avoiding intersections of left-hand helixes  16  and right-hand helixes  18 . In order to determine the appropriate position of a heat or melting source H emanating an energy field having an effective width w, it is necessary to define and determine the relationships between the heat source width w, the length a of any given side of diamond  20 , the length m which represents the length of the strand which will be melted by the heat source H, the angle α which is the inner angle between the strands of the upper apex  22  or lower apex  24 , and the outer diameter D of the stent stock. These variables having been defined, it can be shown that  
           m=w /(cos(α/2)) and, 
           m/a= 7.6 (w/D) tan (α/12)  
         [0086]    where m/a represents the portion of wire material of a given side of a diamond  20  which will be melted and displaced by heat source H to form a gap and a sphere S.  
         [0087]    Having established these relationships, an appropriate axial separation between the center of the cutting path of the heat source H and the upper apex  22  or lower apex  24  of a given diamond  20  can be determined. Referring to FIGS. 6 and 7, this distance is represented by t. It can be seen that t may fall within a range of values. The range varies, depending on the desired sphere S size, the width w of the heat source, and the desired strand length k between the sphere S and the intersection of the strands  14 .  
         [0088]    In a preferred embodiment of the present invention it is desired to cut alternating strands to more predictably form significant spheres S on one side of a stent, and also to protect the mandrel from damage due to repeated exposure to the heat source H. This can be accomplished by turning the heat source H on while cutting and turning the heat source H off while the stent is being rotated to the next cutting position. More preferably, when the heat source H is a laser beam, the beam may be alternately directed toward and away from the strand by using a reflector or by blocking and unblocking the beam using a shutter. In order to determine the appropriate timing of the activation and deactivation of heat source H, it is necessary to determine the angle β of stent rotation during which a heat source should be turned on. In other words, an angle β needs to be defined, which represents the angular length of the melted portion m of wire arm a. Angle β may be represented by the following formula:  
         β=114.6°(w/D)tan(α/2)  
         [0089]    This formula holds true for stent stock having twelve left-hand helixes  16  interwoven with twelve right hand helixes  18  for a total of twenty-four strands  14 . Furthermore, it has been found that in order to create spheres S on the same side of the heat source H, the left-hand helix  16  should be cut during one stent rotation direction while the right-hand helix  18  should be cut while the stent stock is rotating in an opposite direction. The preferred method of forming spheres S on the ends of the strands  14  will be discussed further below. But having this in mind, the angle during which heat source H should be deactivated, defined herein as γ, is related to β in the following manner:  
         γ=(360°−12β)/12  
         [0090]    An example of a preferred embodiment is now provided. Given stent stock having an outside diameter D of 14 millimeters, strand diameters of 0.17 millimeters and braid angle α of 145 degrees, favorable results have been obtained using a mandrel having an outer diameter 32 on the order of 13.7 millimeters and defining an inner channel  44  with an inner diameter 46 on the order of 9.53 millimeters, more preferably 9.53±0.03 millimeters. Pins  36  preferably have a radius r of 0.5 millimeters. Furthermore, pertaining to activation dowel  48 , outer diameter 72 of second cylindrical segment  70 , is on the order of 4.01±0.05 millimeters, while outer diameter  58  of handle portion  54  is on the order of 9.37-9.50 millimeters to fit nicely within inner channel  44 .  
         [0091]    It should be noted that the acclivitous angle δ at which a strand  14  relatively approaches oncoming heat source H was not necessary for purposes of explaining the above mathematical relationships. However, it is related to angle α in the following manner:  
         δ=α/2+90° 
         [0092]    δ is preferably between 130 and 175 and more preferably on the order of 162, for best results.  
         [0093]    Method  
         [0094]    The physical embodiments and mathematical relationships having thus been described, attention can now be drawn to FIG. 8, a flow chart detailing the preferred steps of the method of the present invention. The process starts at step  100 . Here, the assumption is made that stent stock is being used that has not yet been cut to form spheres S on one end. In the event that the stent stock already has spheres S formed on one end, the method of the present invention should start at step  150 .  
         [0095]    First, the mandrel  30  is attached to a rotation device, preferably the indexing head of a laser cutting machine, at  105  in preparation for cutting. It is understood that mandrel  30  could already have been attached to the indexing head and that certain steps of the sequence described herein could be rearranged as would be seen by one skilled in the art. The mandrel is then positioned below the laser of the laser cutting machine so that the beam is aimed at second cutting groove  75  at 110. This is preferably accomplished by moving the indexing head relative to the laser after mandrel  30  has been placed within the chuck of the indexing head. It has been found that it is preferable to use the center of one of the pins  36  as a target when aligning the mandrel  30  under the laser. The laser can then be offset from the pin  36  to the second cutting groove  75  by entering the known distance between the groove  75  and the pin  36  into a computer controlling the movement of the table on which the indexing head is mounted. This is preferable because the pin  36  provides a more precise point on the mandrel  30 . The groove  75  is a relatively wide area designed just to protect the mandrel  30  against over exposure to the heat source H.  
         [0096]    The activation dowel  48  is then removed from the mandrel  30  at 115. Once the activation dowel  48  has been removed, the stent stock is slid onto the mandrel  30  at  120 . The stent stock is then adjusted axially along the length of the mandrel  30  at 125 such that all three pin sets  40  are able to engage diamonds  20 . The activation dowel  48  is then inserted at  130 . The dowel  48  is inserted slowly such that the first pin set  40   a  protrudes first and finds diamond openings  20  in the stent stock. The subsequent pin sets  40   b  and  40   c  then protrude sequentially, also finding diamond openings  20  in the stent stock.  
         [0097]    At 135, the mandrel  30  and the stent stock are rotated in a first direction, at least one revolution, preferably at a speed of less than 10 revolutions per minute, more preferably on the order of 6 revolutions per minute. While the mandrel  30  is rotating in this first direction, the laser beam is shuttered on and off. The shuttering of the laser is timed such that the laser is shuttered on and cutting whenever an acclivitous strand  14  is below the laser. Once a strand  14  has angularly passed completely beyond the laser beam, the laser is shuttered off until another acclivitous strand  14  is presented. This will result in the laser being shuttered on twelve times during one revolution. At 140, after the laser has rotated at least one revolution in a first direction and all acclivitous strands  14  have been cut and spheres S formed thereon, the mandrel  30  is rotated in a second, opposite direction in order that the remaining strands  14  may present acclivitous angles relative to the laser beam. While the mandrel  30  is rotating in the second direction, the laser is again shuttered on and off, cutting and forming spheres S on the remaining strands  14 . The laser is preferably shuttered off whenever a strand  14  is not present to avoid unnecessary heating of mandrel  30 , declivitous strands  14 , and any spheres S that were formed during the first rotation. The activation dowel  48  is then removed from mandrel  30  at 145, thereby allowing springs  41  to urge pins  36  inwardly, disengaging pins  36  from the stent stock.  
         [0098]    At this point, the stent stock has been given an end that is complete with spheres S on the ends of each of the wires  14 . This end may then be used to form the end of a cut stent  10 . At 150, it is necessary to slide the stent stock along the length of the mandrel  30  an appropriate distance such that when cutting takes place along the first cutting groove  74 , a stent  10  of a desired length results with spheres S formed at both ends. It is understood, however, that it may be desirable to form a length of stent stock with spheres S on only one end and that the method herein described may be easily modified to do so. Reference markings  76  may aid in sliding the stent stock the appropriate distance to form a stent of a desired length. Once the stock is in a desired position, the activation dowel  48  is reinserted within mandrel inner channel  44  at 155. While the activation dowel  48  is being inserted into inner channel  44 , thereby causing pins  36  to protrude from apertures  38 , it may be desired to adjust the position of the stent stock on mandrel  30  so pins  36  do not encounter any interference with the intersections of strands  14 . Care should be taken while adjusting the stent stock such that an undesired length is not achieved.  
         [0099]    The mandrel is then moved under the laser so that the first cutting groove  74  is aligned under the laser at  160 . This is preferably accomplished by entering an appropriate command into the computer which then moves the table on which the indexing head is mounted, obviating the need to retarget the laser at a pin  36 . Again, the mandrel is rotated in a first direction at 165 and the laser is appropriately shuttered on and off to cut acclivitous strands  14 . After at least one revolution is completed, the stock and mandrel  30  are then rotated in a second direction at 170 while the laser is again shuttered on and off to cut remaining strands  14 .  
         [0100]    One complete stent  10  has now been cut. However, in a preferred embodiment, two cutting grooves  74  and  75  are provided such that spheres S may be formed using second cutting groove  75  on the newly cut end of the stent stock without having to move the stent stock along the length of mandrel  30 . Therefore, at 175, the mandrel is aligned so that second groove  75  is beneath the laser. As the relative position of the mandrel and the laser has already been established, it is not necessary to target the laser to a pin  36 , rather, the computer may be used to move the table an appropriate distance to align the second groove  75  below the laser. The stent stock is then again rotated in a first direction, not necessarily the same direction as the first direction of the first cut, at least one revolution at 180; and while this is happening, the laser is again shuttered on and off to cut acclivitous strands  14  in this first direction. Again, at 185, the stock is rotated in a second direction while the laser is shuttered on and off to cut remaining strands  14 .  
         [0101]    At this point, on mandrel  30 , there exists a cut stent  10 , a piece of scrap stent stock having no spheres S on the strands  14  of either end, and a length of stent stock having spheres S formed on the ends of the wires  14  making up the stent stock. Activation dowel  48  is then removed at 190, and the cut stent  10  is slid off the mandrel  30 , along with the scrap, at 195.  
         [0102]    At 200, a decision is made as to whether more stents  10  are desired to be cut from this length of stent stock. If more stents  10  are desired, the process is repeated starting at step  150 . If no further stents  10  are desired, either because the desired number of stents  10  have been formed or because there is not enough remaining length of the stent stock to form another stent  10 , the process is finished at 205.  
         [0103]    Results  
         [0104]    It has been found that by practicing the preferred embodiments of the present invention, namely, using the structures taught herein and following the above method to acquire the disclosed mathematical relationships, stents can be formed with ends having spheres S that are uniquely uniform in size and shape. Moreover, an extremely predictable length of braid material is melted to form the spheres S and a desired resulting stent length can be achieved with surprising consistency.  
         [0105]    For example, when cutting a length of braided stent stock made of braids having diameters of 0.17 millimeters and helixes which present acclivitous angles 6 on the order of 162.5 degrees to a laser found on an Eagle 500 CO 2  Laser System, at an angular speed of 6 rotations per minute, in accordance with the preferred embodiments of the present invention, a repeatable sphere S size of 0.012-0.013 millimeters can be attained.  
         [0106]    Those skilled in the art will further appreciate that the present invention may be embodied in other specific forms without departing from the spirit or central attributes thereof. In that the foregoing description of the present invention discloses only exemplary embodiments thereof, it is to be understood that other variations are contemplated as being within the scope of the present invention. For example, it would be foreseeable, using the teachings of the present invention, to create a similar device having more pins and grooves such that two lasers could be used simultaneously, one cutting each end of a stent. It is also foreseeable, and within the envisioned embodiments, to utilize a laser system or other heat source which moves the laser beam while keeping the planar position of the indexing head fixed. A third example of an alternate specific form is using multiple passes of a heat source across a predetermined length of wire to create effective energy field width w, as opposed to using a single pass, to form a sphere thereon. This may be desired when using an energy field having an extremely small effective heating area. These are merely three examples of other specific forms in which the present invention may be embodied. Accordingly, the present invention is not limited in the particular embodiments, which have been described in detail herein. Rather, reference should be made to the appended claims as indicative of the scope and content of the present invention.