Abstract:
A tool for multi-axis mechanical testing of medical implant devices includes a plurality of pins arranged to form a contacting surface with a sample holder. The pins maintain rolling contact with the sample holder during a bending phase of a fatigue cycle and reduce the effects of friction and localized differential strain on the sample holder.

Description:
BACKGROUND 
   The present invention relates to mechanical testing of medical implant devices. 
   SUMMARY 
   A tool for multi-axis mechanical testing of medical implant devices includes a plurality of pins arranged to form a contacting surface with a sample holder. The pins maintain rolling contact with the sample holder during a bending phase of a fatigue cycle and reduce the effects of friction and localized differential strain on the sample holder. 
   One embodiment of the present invention is directed to a bend tool comprising: a first end cap; a second end cap; and an array of pins, each pin in the array of pins having a first end rotatably supported by the first end cap and a second end rotatably supported by the second end cap. In one aspect, at least one pin in the pin array supports a rotatable sheath. In one aspect, each pin of the pin array supports a rotatable sheath. In one aspect, the rotatable sheath slides along a longitudinal axis of the pin between the first and second end cap. In one aspect, the rotatable sheath is characterized by a length, the sheath length determined by a position of the pin in the pin array. In one aspect, the array of pins projected onto the first end cap approximates a desired bend curve. In one aspect, the desired bend curve is characterized by a single radius of curvature. In one aspect, the desired bend curve is characterized by a plurality of radii of curvature. In one aspect, the desired bend curve simulates an expected in-use bend curve. In another aspect, a fatigue testing device for a stent comprises the above-described bend tool. In another aspect, the fatigue testing device further comprises an upper strain relief tool, the upper strain relief tool having a first relief end cap, a second relief end cap, and an array of relief pins, each relief pin in the array of relief pins having a first end rotatably supported by the first relief end cap and a second end rotatably supported by the second relief end cap. 
   Another embodiment of the present invention is directed to a bend tool comprising: a first end cap; a second end cap; an array of pins, each pin in the array of pins having a first end held by the first end cap and a second end held by the second end cap, each pin in the array supporting a rotatably sheath. In an aspect, the rotatable sheath is sized to allow rotation of the sheath around a longitudinal axis of the supporting pin and sliding of the sheath along the longitudinal axis of the supporting pin. In an aspect, the rotatable sheath is characterized by a length, the sheath length determined by a position of the pin in the pin array. In an aspect, the pins in the array of pins are arranged along a desired bend curve, the desired bend curve simulating an expected in-use bend of a stent. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  shows a stent fatigue system. 
       FIG. 1   b  shows the fatigue system of  FIG. 1   a  at another phase of the fatigue cycle. 
       FIG. 2  is an exploded view of a bend tool. 
       FIG. 3  is a sectional side view of the bend tool of  FIG. 2 . 
       FIG. 4  is a perspective view of a bend tool assembly. 
       FIG. 5  is a sectional side view of a strain relief tool. 
       FIG. 6  is a perspective view of a strain relief assembly. 
       FIG. 7  shows the bend tool assembly of  FIG. 4  in a stent fatigue system. 
       FIG. 8  is front view of another embodiment of a bend tool. 
   

   DETAILED DESCRIPTION 
   Although more commonly known for their use in coronary arteries, stents may be implanted in peripheral arteries or other tubular structures within an organism. Peripheral arteries include, for example, renal arteries, carotid arteries, and femoral-popliteal arteries. Peripheral arteries generally experience greater bending, twisting, and stretching motions relative to coronary arteries and it is expected that stents implanted in a peripheral artery will likely experience greater stresses and strains relative to a coronary stent. 
     FIG. 1   a  illustrates a configuration used to fatigue test a stent such as, for example, a peripheral artery stent. In  FIG. 1   a , stent  105  is held within a sample holder  110 . The sample holder  110  is preferably an elastic material such as, for example, latex, silicone, and thermoplastic elastomers. The sample holder  110  preferably acts as the tubular structure during the fatigue test and is sized to accommodate the size and length of the stent being tested. The sample holder  110  is supported at one end by a grip  130 , which is supported by a fixed stage  120 . The other end of the sample holder is supported by a grip  135  mounted on a vertically movable stage  125 . In some embodiments, grips  130  and  135  are rotatably supported by stages  120  and  125 , respectively, to simulate torsion of the sample. Grips  130 ,  135  may be an inner barb fitting  132 ,  137  inserted into the end of the sample holder and a clamp (hot shown) around the outer circumference of the sample holder and over the inner barb fitting. The inner barb fittings may be configured to allow fluid flow through the sample holder and stent to simulate in-use conditions such as pulsatile flow within an artery. Similarly, grips  130 ,  135  may provide a fluid passage between the sample holder and a fluid circuit external to the sample holder. 
   Upper strain relief tool  145  and lower strain relief tool  140  provide lateral support for the sample holder during a portion of the bending cycle and form the two outer support points of a three-point bend test configuration. The third point of the three-point bend configuration is provided by a bend tool  150 . Bend tool  150  is mechanically supported by support  155 , which may be a part of a mechanical linkage (not shown) driven by an actuator (not shown). In some embodiments, the mechanical linkage and actuator work together to laterally displace the bend tool while keeping the bend tool centered between the upper and lower strain relief tools, indicated in  FIG. 1   a  by line  199 . In other embodiments, a simple straight support may be used to link the bend tool to the actuator mounted at a fixed elevation and both the upper and lower stages are displaced vertically to keep the bend tool centered between the upper and lower strain relief tools. 
     FIG. 1   b  illustrates a configuration at another instant of the fatigue cycle where bend tool  150  is in contact with the sample holder  110  and has been displaced laterally to bend the sample holder  110  and the stent held within the sample holder  110 .  FIG. 1   b  shows the upper stage  125  displaced vertically such that the longitudinal, or axial, strain on the stent is constant. The vertical displacement of the upper stage  125  may be controlled independently from the lateral displacement of the bend tool  150  thereby enabling independent control of both the axial strain profile and bending strain profile during a fatigue cycle. 
   Stents are typically fatigue tested to simulate 10 years of normal use and, depending on the application, could mean fatigue testing a stent for up to four hundred million bending cycles. During such testing, the inventor discovered failures of the sample holder that prematurely terminated the fatigue test before the stent failed or before the test reached the desired number of fatigue cycles. Failures at locations  187 ,  181 ,  186  were in or near areas where the sample holder contacted the bend tool or strain relief tools. Without being limiting, the inventor believes these failures to be attributed to friction and shear forces exerted on the sample holder by rigid bend and relief tools. 
     FIG. 2  is an exploded view of a bend tool. In  FIG. 2 , an array of pins is held in place by end caps  210 . The end caps  210  are supported by a mounting bracket  230 . Each pin  220  in the pin array can freely rotate around the pin&#39;s longitudinal axis. In some embodiments, an over-sized sleeve or sheath  225  covers a portion of the pin  220  and can freely slide axially and circumferentially relative to the pin  220 . 
     FIG. 3  is a sectional side view of the bend tool shown in  FIG. 2 . In  FIG. 3 , like reference numbers refer to like structures. The pins in the pin array illustrated in  FIG. 3  are arranged such that the intersection of each pin&#39;s longitudinal axis with the plane of the drawing in  FIG. 3  is on a curve  350  having a common radius of curvature  301 . The radius of curvature  301  preferably simulates the curvature expected under in-use conditions of the stent after adjustment is made for the pin radius, the optional sheath thickness, and the sample holder thickness. Curve  350  preferably approximates the expected bending curve of the stent during use and may include compound curves have several radii of curvature or a continuously varying radii of curvature. 
   End cap  210  preferably comprises a low friction material such as, for example, acetal resin engineering plastic available as Delrin® acetal resin from E.I. Du Pont de Nemours and Company of Wilmington, Del. A blind hole for each pin  220  is formed in the end cap  210  and one end of the pin is inserted in the blind hole. The blind hole is sized such that the pin  220  freely rotates within the blind hole. 
   Pin  220  preferably comprises a strong and stiff material such as, for example, stainless steel that resists deformation or failure during the repeated fatigue cycle. The diameter of each pin may be selected based on the pin material properties and the sample holder material properties. For example, a smaller diameter may be selected to reduce the contact area of the pin and sample holder but a larger diameter may be desired to reduce the bending deformation of the pin when the bend tool is pushed against the sample holder. Pin spacing may be selected based on factors such as, for example, pin diameter, bend curvature, and sheathing. Complicated bend curves or small pin diameters may favor small pin spacing whereas large pin diameters and pin sheathing may favor large pin spacing. 
   A sheath or sleeve  225  may be fitted over one or more pins in the pin array. For example,  FIGS. 2 and 3  illustrate an embodiment where a sheath  225  is fitted over each pin in the pin array. The sheath  225  is sized such that the sheath can freely rotate around the pin&#39;s longitudinal axis and preferably comprises a material having a low coefficient of friction such as, for example, polytetrafluoroethylene, high-density polyethylene, and nylon. Other embodiments may include substituting the sheath with a low-friction thin film deposited onto each pin. Examples of low—friction thin films include diamond-like carbon thin films form using a chemical vapor deposition process. Other embodiments may eliminate the sheath such that the pins in the bend tool directly contact the sample holder. These embodiments enable closer pin spacing and may be favored when the desired bending curve is complex and has several radii of curvature characterizing the desired bending curve. 
   In some embodiments, a sheath or sleeve may be fitted over one or more pins in a pin array where the pins in the pin array are held in place by the end caps and do not rotate. The sheath, however, is sized such that each sheath can rotate around its corresponding pin axis and can slide in axially between the end caps as the sheath contacts the sample holder during the bending portion of the fatigue cycle. 
     FIG. 4  is a partially exploded perspective view of a bend tool assembly  400 . The bend tool assembly  400  includes at least one bend tool  300  mounted on an assembly bracket  410 . The assembly bracket  410  may be mounted to one or more actuators through a mechanical linkage such that the one or more actuators control the lateral displacement of each bend tool. The bend fool assembly  400  enables simultaneous testing of more than one sample thereby reducing the time required for testing multiple samples. 
     FIG. 5  is a sectional side view of a strain relief tool used in some embodiments of the present invention. In  FIG. 5 , pin  520  is rotatably supported between end caps  510  and end caps  510  are supported by a mounting bracket  530 . Each pin  520  may be supported by placing an end of each pin into a corresponding blind hole in the end caps  510 . Each blind hole preferably is sized to allow the pin to rotate freely within the hole. Each pin  520  of the strain relief pin array is shown in  FIG. 5  having a sleeve or sheath  525  over each pin. The selection of materials for the end cap  530 , pin  520 , and sheath  525  for the strain relief tool  500  may use the same criteria described above for the bend tool and preferably use the same corresponding materials for both the bend tool and the strain relief tool. In  FIG. 5 , the pin axes are disposed along a curve  550  having a constant radius of curvature  501  but it is understood that other curves may be used and are within the scope of the present invention. 
     FIG. 6  is a partially exploded perspective view of a strain relief tool assembly  600 . The strain relief tool assembly  600  includes at least one strain relief tool  500  mounted on an assembly bracket  610 . A strain relief tool assembly  600  may be mounted to, for example, the stationary stage of a fatigue test machine such as that shown in  FIG. 1   a  and provides one of the outer points of the three-point bend configuration. A second strain relief tool assembly  600  may be mounted to the movable stage and provides another of the outer points of the three-point bend configuration. The strain relief tool assemblies enable simultaneous testing of more than one same thereby reducing the time required for testing multiple samples. 
     FIG. 7  illustrates a multi-sample fatigue test machine incorporating a bend tool assembly  754  and a strain relief tool assembly  740  and  745 .  FIG. 7  illustrates a configuration capable of testing up to eight samples, each sample supported by a sample holder  701 . Each sample holder  701  is contacted by a corresponding bend tool  750  in the bend tool assembly  754 . The two outer points of the three-point bend configuration are provided by an upper strain relief tool assembly  745  and a lower strain relief fool assembly  740 . An inlet port  710  enables the user to install the sample stent for testing. An upper sample grip  735  provides support for the sample holder  701  and allows flow to exit the sample holder. A lower sample grip  730  provides support for the sample holder  701  and allows flow to enter the sample holder. 
     FIG. 8  is a front view of a bend tool  800  that allows the sample holder to be rotated around the longitudinal axis of the sample holder during a bending portion of a fatigue cycle. In  FIG. 8 , an array of pins is supported at either end by end brackets  810 . Each pin  820  in the pin array is partially covered by a sleeve or sheath  827 ,  825 . Each sheath is sized such that the sheath can freely rotate around the corresponding pin and can also slide axially along the pin&#39;s longitudinal axis between the end brackets. The selection of materials and sizes of the pin  820 , end cap  810 , and sheath  827  preferably use the same criteria as the corresponding components of the bend tool shown in  FIG. 3 . In contrast to  FIG. 3 , however, sheath length in  FIG. 8  varies according to the position of the pin in the pin array. In  FIG. 8 , sheath  825  covering the center pin in the pin array has the longest length and sheath  827  covering an outer pin in the pin array has the shortest length. 
   The shorter length sleeves allow for a greater range of axial motion, which may be useful when one or both ends of the sample holder are rotated while being bent. Complicated multi-axis motions such as simultaneous axial strain, bending, and rotation can occur, for example, in the femoral-popliteal artery during walking and it is desirable to test a peripheral artery stent under those expected use conditions. During a combined bend-rotation, the center pin of the bend tool pin array is expected to contact the sample holder first and as the bend fool is laterally displaced to bend the sample holder, the other pins in the pin array make contact progressively from the inner pins to the outer pins in the pin array. If a rotation is applied to the sample holder as the bend tool is bending the sample holder, a tangential shear force on the sample holder may be generated from contact with the pins in the pin array. The contact shear force caused by the rotation of the sample holder may be reduced by allowing the sheath to move along the longitudinal axis of the pin. If the rotation is applied after the bend tool has contacted the sample holder, the outer pins of the pin array may need a larger axial displacement to relieve the contact shear stress on the sample holder and may required a shorter length sleeve than the center pin sleeve. 
   Having thus described illustrative embodiments of the invention, various modifications and improvements will readily occur to those skilled in the art and are intended to be within the scope of the present invention. For example, although a peripheral artery stent has been used as an illustrative example, other embodiments of the present invention may be applied to coronary artery stents or other implantable devices that experience in-use cyclic strains and are intended to be within the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.