Abstract:
The invention generally relates to systems and methods for stress testing to failure tubular mesh devices, specifically, one or more stents or mesh grid tubes. A radial compression method of stress to fatigue is developed by placing a tubular medical implant within a lower than normal compliance test vessel. A radial expansion method of stress to fatigue is developed by placing higher than normal compliance tubes within a tubular medical implant. A method for radial expansion and radial compression is conducted by placing a higher than normal compliance tube through the mesh device and the placing the tube and device within a lower than normal compliance test vessel. Each method is tested on a tester that operates by delivering pulsatile flow with varying pressures, frequencies and testing parameters. Fracture of the test subject is visualized by high speed camera.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to U.S. Provisional Application No. 62/291,192 filed Feb. 4, 2016, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention generally relates to systems and methods for fatigue or stress testing to fracture one or more mesh grid tubes such as implantable medical stents. More specifically, the invention relates to systems and methods for stressing a stent or mesh tube to fracture or break which provides valuable data for estimating the usable life of such devices. 
         [0003]    Modern medical procedures routinely include the employment of implants into a patient&#39;s vascular system to perform various therapeutic functions. Prosthetic vascular implants, such as heart-valves, stents, grafts, mesh tubes, and stent-grafts used for human implantation are subjected to the continuous fluctuating stress of blood pressure. As an example, tubular mesh samples or stents are often inserted in an artery of a patient to maintain a flow lumen through the artery at a location that had previously been at least partially blocked or occluded. It is therefore necessary to test such implants to prove their durability over a lifetime of exposure to pulsatile blood pressure. Ideally such stents, mesh samples, or other vascular prostheses, are able to withstand the physiological dynamics that occur within the vessel or organ in which they are emplaced. For instance, in the abdominal aorta, blood pressure in the average healthy subject is 120 mm Hg/80 mm Hg, i.e. the blood pressure varies by 40 mm Hg for every pulse. Compliance of a healthy aorta can be of the order of 20-25% per 100 mm Hg so that a change in vasculature diameter of 8 to 10% can be expected at every heartbeat. In order to simulate such a change in diameter, testers employ a pulse pressure between 80 mm Hg and 100 mm Hg. Typically, in testing to success, vascular implants are tested for 400,000,000 cycles which represent approximately 10 years of implantation life at a heart rate of 80 beats per minute. 
         [0004]    Testing to success is indicative of the durability of the stent under physiological conditions of systolic/diastolic pressures encountered in accelerated radial pulsatile durability testing. However, testing to success does not predict the endurance limit or fatigue life of the stent, i.e., there is no way to know under what conditions, including conditions that may exceed physiological parameters, the stent or stent graft would fail. 
         [0005]    To address this weakness, new regulations (FDA, ASTM, ISO) are being developed that outline test requirements that are now concerned with predicting fatigue lifetime of the stent or stent graft and require stent manufacturers to test their products under a ‘testing to failure’ or ‘test to fracture’ regime, so that stents and stent grafts may be tested up to their endurance limit. An alternative method that is being rapidly pursued and evaluated is a “Fatigue to Fracture” approach. A rudimentary technique that is more akin to aerospace testing, this methodology involves a combination of Finite Element Analysis (FEA) modeling and in vitro testing to assess the durability of stents through established fracture mechanics techniques. These testing guidelines and standards are still under development, i.e. ASTM F04.30.06. 
         [0006]    Knowing when and where fracture, secondary fracture, or other failure, of the stent, mesh tubes, or other prosthesis, is most likely to occur under a variety of simulations is ideal to device development. Manufacturers can then use this information to redesign their product with the knowledge gained by fatigue to fracture analysis. Providing a stent, or other prosthesis, of suitable strength and durability for lasting implantation into a patient, to minimize the likelihood of failure is desirable. Determining the approximate fatigue and endurance location limits of the stent, or other prosthesis, helps accomplish the provision of a suitable stent, or other prosthesis. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
         [0008]    In accordance with the present invention, a vascular prosthesis tester for inducing mechanical stresses upon a vascular prosthesis having a tubular channel extending along a longitudinal axis is provided. 
         [0009]    In one embodiment, a method for testing fatigue to fracture by means of enhanced radial compression of a tubular mesh medical implant comprises the steps of: providing a mock vessel that has a lower than normal compliance; deploying the medical implant within the mock vessel; loading the mock vessel and implant into a testing unit; repeatedly expanding the mock vessel and medical implant therewithin using a pressurized fluid; providing a high speed camera to monitor the deflection of the test sample by direct visualization of the medical implant to ascertain the time of initial failure or fracture. It is foreseen that the testing unit controls at least one of the following parameters: temperature, pH, frequency of pulsation of the pressurized fluid, minimum pressure, and maximum pressure. 
         [0010]    The preferred compliance of the mock vessel in this method may be between 1% and 2% per 100 cmm Hg. It is foreseen that the pressurized fluid is at least one of air, a saline solution, or distilled water. The frequency of expansion of the mock vessel and medical implant may be between 1 to 150 Hz but more preferably between 20 and 40 Hz for optimal testing of a typical implantable vasculature stent. 
         [0011]    In one embodiment, the implant is a vascular graft or endovascular prosthesis or common mesh stent with an internal diameter from 2 to 50 mm 
         [0012]    In another embodiment of the invention, a method for testing fatigue to fracture by radial expansion of a medical implant comprises the steps of: providing a mock vessel that has a higher than normal compliance; positioning a tubular medical implant over the mock vessel; loading the mock vessel and implant into a testing unit; repeatedly expanding the mock vessel and medical implant from within using a pressurized fluid; providing a high speed camera to monitor the deflection of the test sample by directly visualization of the medical implant to ascertain the time of initial failure or fracture. 
         [0013]    In yet another embodiment, the medical implant fatigue is tested using a combination of enhanced radial compression and enhanced radial expansion. The combination testing comprises the steps of providing a mock vessel having lower than normal compliance; positioning a tubular medical implant within the mock vessel; placing an inner liner tube within the mock vessel to substantially capture the tubular medical implant between the outer periphery of the inner liner tube and the inner wall of the mock vessel; loading the mock vessel, implant and inner liner tube into a testing unit; repeatedly expanding the mock vessel and medical implant from within the inner liner tube from within using a pressurized fluid; providing a high speed camera to monitor the deflection of the test sample by directly visualization of the medical implant to ascertain the time of initial failure or fracture. The first inner liner tube may have an outer diameter that is from between 85% to 95% of an inner diameter of the mock vessel. 
         [0014]    In another embodiment, the inner liner tube is made from a liquid plastic, such as silicone, that has been cured within the mock vessel to substantially encapsulate the medical implant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A further understanding of the invention may be had by reference to the accompanying drawing in which: 
           [0016]      FIG. 1  is a top perspective view illustrating a durability test apparatus for fatigue to fracture according to the present invention; 
           [0017]      FIG. 2A  is an enlarged side view of  FIG. 1  with portions cut away illustrating a first embodiment of the present invention, wherein a mock artery has lower than normal compliance and a mesh stent loaded within the mock artery; 
           [0018]      FIG. 2B  is an enlarged side view of  FIG. 1  with portions cut away illustrating a second embodiment of the present invention, wherein a mock artery has a normal compliance with a mesh stent having first and second inner liners deployed within; 
           [0019]      FIG. 3  is a cross section view of  FIG. 2B  taken along the line  3 - 3 ; 
           [0020]      FIG. 2C  is an enlarged side view of  FIG. 1  with portions cut away illustrating a third embodiment of the present invention, wherein a mock artery has a normal compliance with a mesh stent loaded having an inner liner made from silicone; 
           [0021]      FIG. 2D  is an enlarged side view of  FIG. 1  with portions cut away illustrating a fourth embodiment of the present invention, wherein a mock artery has a lower normal compliance with a mesh stent loaded having an inner liner; 
           [0022]      FIG. 4  is a flow diagram of a method in a first embodiment according to the present invention; 
           [0023]      FIG. 5  is a flow diagram of a method in a first embodiment according to the present invention; 
           [0024]      FIG. 6  is a flow diagram of a method in a first embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. It is also noted that any reference to the words top, bottom, up and down, and the like, in this application refers to the alignment shown in the various drawings, as well as the normal connotations applied to such devices, and is not intended to restrict positioning of the connecting member assemblies of the application and cooperating bone anchors in actual use. 
         [0026]    Referring to  FIG. 1 , the reference numeral  100  generally designates a fatigue to fracture test apparatus according to the present invention. The fatigue to fracture test apparatus  100  having one or more in-line conduits for attachment to mock vessels or vascular grafts  106 . The mock vessel or conduit  106  will have a compliance or measure of the distensibility of a chamber expressed as a change in volume per unit change in pressure. A normal arterial compliance varies greatly anywhere from approximately 4-5% per hundred millimeters (mm) of Mercury (Hg) in the smaller arteries and up to 20-30% per hundred millimeters (mm) of Mercury (Hg) in the ascending aorta. It is foreseen that the mock vessels  106  may be made from silicone, Teflon®, Fluorinated Ethlene Propylene, PerfluoroAlkoxy, Tygon®, PharmaPure®, Kynar, or other known fluid handling tubing. It is foreseen that these mock silicone vessels  106  can be straight, curved, undulated, or bifurcated, and are made to virtually any range of compliance for radial testing of stents. 
         [0027]    An implantable medical device, stent, or mesh tube  104  is deployed in the conduit  106  along a longitudinal axis A. A vascular stent  104  is a small tubular structure commonly made of a thin layer of one of a few biocompatible and corrosion resistant metals, such as 316L stainless steel, nitinol, or cobalt-based alloys usually in a crossed lattice like or mesh  106  pattern. Stents have unique attributes that influence the assessment of their durability. First, the overall size of the stent can be quite small; some coronary stents are less than 2.5 mm in diameter and 10 mm in length. Typical cross-sectional dimensions of a single strut, the fundamental structural unit that forms the stent, are on the order of 0.11 mm 2 . The stent  104  is deployed by compression, so that they can be delivered into the artery, for example, down to less than 1 mm in diameter. After being delivered to the location of interest in the mock artery  106 , one of two techniques can be used to expand the stent  104  to its clinically relevant diameter. The first is to use a self-expanding stent  104  made of Nitinol (Nickel Titanium) which will expand to a predefined diameter. The second technique is to use a balloon to expand the stent  104  to the indicated diameter. In either case the stents  104  are designed to expand to 110% of the inside diameter of the mock artery  106  at the point of highest blood pressure. This is done to ensure that the stents  104  do not migrate during the systolic portion of the heartbeat. 
         [0028]    A motor or drive  108  moves fluid, such as saline, from the fluid inlet  109  to the pump  110  by the action of metal bellows, rolling rubber bellows, a piston or the like. Motor  108  may include, for example, a hydraulic, electrodynamic, AC or servo drive system. The pump  110  may accelerate test conditions where, for example, when testing to success, 400 million heartbeats may be replicated at a cycle rate ranging from about 50-6000 cycles/minute. It is foreseen that testing fatigue to fracture can be done at physiological speeds between 1-2 Hz or at accelerated speeds between 40 Hz and 60 Hz, but for faster results as according to the present invention, testing can be done at accelerated speeds and high pressures of at least 50 pounds per square inch (psi). 
         [0029]    From pump  110 , fluid moves through manifold  112 , where pressures being delivered to devices  104  are monitored. To simulate blood flow, the pressurized test fluid is delivered in a pulsatile flow, with pressures modulated between specific points to mimic systolic and diastolic pressures. Manifold  112  may contain stopcocks or isolation valves (not shown) that allow conduits  106  and devices  104  to be isolated, removed, replaced, and/or inspected without draining the entire apparatus  100 . The manifold  112  allows test fluid to be pumped through all samples in a closed loop, ensuring an even temperature and pressure distribution within all mock vessels  106  and also ensuring the removal of shed particles. 
         [0030]    Stopcocks  114  may be disposed downstream of conduits  106  to allow the user to select which conduit(s)  106  are open and which are closed. Check valves  116 , also known as bleed valves or one-way valves, allow fluid to flow along the path shown by arrow  118 , while inhibiting or preventing backflow during diastole of pump  110  when a pulsatile flow is used for testing. Check valves  116  also help to control the rate of flow through elastomeric tubing  120 . A tank (not shown), containing implantable device  104 , conduit  106 , stopcocks  114 , check valves  116  and tubing  120 , may be filled with a heated liquid (not shown), for example, to simulate body temperature. It is also foreseen that the test fluid exits elastomeric tubing  120  through manifold  124  and is carried outward for further processing by tubing  126 . 
         [0031]    To evaluate the fatigue to fracture, high speed photographic verification of stent fatigue by means of a high speed camera  118  and external light source  130 , such as a FastCam PCI 1280, is utilized, it is foreseen that measurements are taken at 300-2000 fps, at 1280×1036 resolution or better, and at a 1/1000 to 1/100 sec range shutter speed. It is foreseen that the dimensions of the silicone mock vessel  106  are measured using a laser micrometer. The inside wall  123  of the mock vessel  106  may be marked with predetermined sets of points (not shown) and the outside surface  125  may be marked with a second predetermined set of points, to create landmarks on the vessel  106  that can be tracked by the camera  118  during pulsation. When used, the high speed camera  118  will track the first and second predetermined sets of points or landmarks on the stent  104  and the distance between the marks will change as the mock vessel  106  expands and contracts. The high speed camera  118  constitutes a method of direct observation and direct measurement of the stent. In the alternative, landmarks may be identified on the stent or test article  104  itself. 
         [0032]    It is foreseen that laser micrometer (not shown) measurements may also be added and appropriate to monitor the operating conditions of the test. 
         [0033]    It will be appreciated that  FIG. 1  teaches by way of example and not limitation. The number of system components may be increased or decreased with respect to what is shown. In one such example, sensors (not shown) providing optional alarm capabilities for temperature, pH, mean pressure, pulsatile pressure, number of cycles, and speed may be incorporated into apparatus  100 . 
         [0034]    Enhanced Radial Compression Testing: Referring now to  FIG. 2A , a mock vessel  106   a  is shown. The mock vessel  106   a  has a lower than normal compliance, which varies likewise to the purpose of the stent  104   a  to be tested. The mock vessel may have set points  141   a  marked on the outer surface  125   a  of the mock vessel, to be used with the camera to provide reference points for visualizing fracture of the stent  104   a.  It is desirable that the mock vessel  106   a  will have an inner diameter  130   a  in the range of 5%-35% greater than the outer diameter  131   a  of the mesh tube  104   a  and between 1-2% change in radius per 100 mm of mercury (Hg).Because of the very low compliance of the mock artery  106   a,  the stent  104   a  will be restrained by the artery wall from expanding to the designed amount which puts the stent  104   a  under compressive forces faster than under normal compliance conditions. Stents  104   a  are typically designed to expand to 110% of the inside diameter of an artery (not shown) at the point of highest blood pressure, herein illustrated in test situations with a mock vessel  106   a.  This is done to ensure that the stents do not migrate during the systolic portion of the heartbeat. The low compliance mock vessel  106   a  creates an overload situation wherein radial compression forces are introduced when high pressure is used to open up and on recoil deliver another high pressure load to the mesh tube  104   a.  The repetitive compressive pressure associate with the pulsatile flow through the vessel  106   a  causes greater than normal stress on the stent  104   a  and under controlled conditions the stent can be forced to fail. Testing has shown that failure typically originates at an apex of the mesh stent  104   a.  Once failure occurs at a single point of the stent, complete failure follows quickly, commonly with the mesh stent “unzipping” along a linear path originating at the original point of failure. 
         [0035]    Enhanced Radial Expansion Testing: Referring now to  FIG. 2B , a mock vessel  106   b  is shown. The mock vessel  106   b  has a higher than normal compliance, (i.e.  130   b  is greater than  130   a ) which varies likewise to the purpose of the stent  104   c  being tested. The mock vessel may have set points  141   b  marked on the outer surface  125   b  of the mock vessel, to be used with the camera to track fracture of the stent  104   b.  The mesh tube  104   b  further includes an inner liner  132   b.  The inner liner  132   b  is a thin walled tube inserted through the stent  104   b  after the stent  104   b  has been expanded. It is foreseen that the stent  104   b  may be expanded by either a balloon or by being made from Nitinol. It is preferred that the inner liner  132   b  may be made of several additional series of layers, and it is not meant to be limited. Liquid pressurization of the mock vessel  106   b  creates an inner radial expansive pressure or force that embeds the stent  104   b  into the mock vessel  106   b  as seen in  FIG. 2B . The inner liners  132   b  may be created by dipping the mock vessel  106   b  and associated stent  104   b  into silicone to create the inner liner  132   b.  The thickness of the liner  132   b  can be controlled by the length and number of “dips” into liquid silicone. When dried, the silicone substantially encapsulates the test stent  104   b  to simulate endothelialzation of the stent. 
         [0036]    The ends  135   b  of the inner liner stick out further than a length of the stent  104   b.  It is preferred that extra dips of silicone (not shown) may be formed on the ends  135   b  of the inner liner  132   b.  In the alternative, sleeves may be formed from low compliant tubing and paced over the ends  135   b  of the inner liner  132   b.    
         [0037]    Referring to  FIG. 3 , the inner liner  132   b  will have an inner diameter  140   b  in the range of ½ to ⅔ the outer diameter  142   b  of the mesh tube  104   b  or between 1-5% change in radius per 100 mm of mercury (Hg) compliance. The inner liner  132   b  will have an outer diameter  144   b  that is 90% of the working inner diameter  148   b  of the stent  104   a.  The stent  104   a  will be forced to expand to the inner diameter  130   b  of the mock vessel  106   b  because of the very high compliance of the inner liners  132   b,    134   b,  which places the stent  104   a  under expansive forces faster than under normal compliance conditions. The high compliance inner liner  132   b  creates an overload condition, as the expansion test requires high pressure or high frequency pulsatile flow to open up the mesh stent  104   b  and on recoil deliver a high load to the mesh stent tube  104   b.  The mock vessel may have set points  141   b  marked on the outer surface  125   b  of the mock vessel, to be used with the camera to track fracture of the stent  104   cb    
         [0038]    Referring now to  FIG. 2C , a mock vessel  106   c  is shown. The mock vessel  106   c  has a higher than normal compliance, which varies likewise to the purpose of the stent  104   c  being tested. The mock vessel  106   c  may have set points  141   c  marked on the outer surface  125   c  of the mock vessel, to be used with the camera to track fracture of the stent  104   c.  The mesh tube  104   c  further includes an inner liner  152  made from cured liquid silicone. The conduit tube  106   c  containing the expanded stent  104   c  is removed from the tester  100  and filled with liquid silicone. The liquid silicone sticks to the mesh structure  104   c  and inner wall  154  of the mock vessel  106   c  and may be allowed to drip out of the tube  106   c  to leave a thin layer of silicone behind. The area where the mesh tube  104   c  is deployed is cured to harden the silicone creating the inner liner  152 . It is foreseen that the inner liner  152  may be made of several additional series of layers of silicone, by repeating this process, and it is not meant to be a limiting example. The inner liner  152 , like the inner liner  132   b  explained above, creates an inner radial expansive pressure or force that embeds the stent  104   c  into the mock vessel  106   c.  The inner liner liquid form may be made from silicone, but also Teflon®, Fluorinated Ethlene Propylene, PerfluoroAlkoxy, Tygon®, PharmaPure®, Kynar, or other known fluid handling tubing. 
       Combination Enhanced Radial Compression and Enhanced Radial Expansion 
       [0039]    Referring now to  FIG. 2D , is a combination of compressive and expansive forces are imparted on the stent to be tested  104   d.  A mock vessel  106   d  is shown with a lower than normal compliance, which varies likewise to the purpose of the stent  104   d.  The mock vessel may have set points  141   d  marked on the outer surface  125   d  of the mock vessel, to be used with the camera to track fracture of the stent  104   d.  It is preferred that the mock vessel  106   d  will have an inner diameter  130   d  in the range of ½ to ⅔ the outer diameter  131   d  of the mesh stent tube  104   d  or between 1-2% change in radius per 100 mm of mercury (Hg), and because of the very low compliance of the mock artery  106   d,  the stent  104   d  will not always expand to the designed amount which places the stent  104   d  under compressive forces faster than under normal compliance conditions. After the stent  104   d  is positioned within the test vessel  106   d  an inner liner  132   d  is placed therewithin to capture the stent  104   d  between the outer wall of the inner liner  132   d  and the inner wall of the test vessel. The inner liner  132   d  may be a thin walled tube inserted through the stent  104   d.  The inner liner  132   d  will have an inner diameter  140   d  in the range of ½ to ⅔ the outer diameter  142   d  of the mesh tube  104   d  or between 1-5% change in radius per 100 mm of mercury (Hg) compliance. The outer diameter  144   d  of the inner liner  132   d  may between 85-95% of the working inner diameter  148   d  of the stent  104   d.  The stent  104   d  will be forced to expand to the inner diameter  130   d  of the mock vessel  106   d  because of the very high compliance of the inner liner  132   d  which places the stent  104   d  under expansive forces upon application of the pressurized test liquid. The high compliance inner liner  132   d  in combination with the low compliance mock vessel  106   d  create an overload condition, as the expansion/compression test requires high pressure or high frequency pulsatile flow to open up the mesh stent  104   d  and on recoil deliver a high compression load to the mesh tube  104   d.  Repeated compression and expansive force imparted on the mesh stent  104   a  during pulsatile flow may embed the stent  104   d  to the mock vessel  106   d,  as is seen in  FIG. 2D . 
         [0040]      FIG. 4  is a flow diagram illustrating a method  200  of fatigue to fracture testing using radial compression. At step  210 , the inside wall of the mock vessel is marked with predetermined sets of points and the outside wall is marked with a predetermined set of points, to create landmarks on the vessel that can be tracked by the camera during testing. 
         [0041]    At step  212 , a mesh tube, stent, or implantable sample is deployed in a mock vessel. The stent may be balloon expanded or made from Nitinol and is expanded as it is deployed. The mock vessel has a lower than normal compliance, i.e. 1% to 2% per 100 mm Hg. At step  214 , the mesh tube and mock vessel as a unit are loaded onto a tester unit. 
         [0042]    At step  216 , the tester is set to deliver pulsatile flow including a predetermined: temperature, pH, pulsation frequency, and maximum and minimum pressure for each cycle. 
         [0043]    At step  218 , a high speed or still camera is set to monitor the deflection of the test sample. The high speed camera will track pairs of landmarks on the stent and the distance between the marks will change as the mock vessel expands and contracts. It is foreseen that a multitude of cameras may be necessary to give a full 360 degree view of the mesh tube. 
         [0044]    At step  220 , the tester is set to run at the predetermined parameters. 
         [0045]    At step  222 , when a break occurs, then at step  224 , the number of cycles to break is determined, and the tester continues to run. When a predetermined number of breaks have been met, then process is ended at step  226 . The whole method is repeated with adjusted parameters to generate the data necessary to determine fatigue properties of the tested stent. 
         [0046]      FIG. 5  is a flow diagram illustrating a method  300  of fatigue to fracture testing using radial expansion. 
         [0047]    At step  312 , a mesh tube, stent, or implantable sample is deployed onto a mock vessel. The stent may be balloon expanded or made from Nitinol and is expanded as it is deployed. The mock vessel has a higher than normal compliance for the test stent deployed. 
         [0048]    At step  314 , the test vessel has a radial compliance of 5% per 100 mm Hg and that has an outer diameter from between 85% to 95% a working inner diameter of the stent. The ends of the inner tube extending beyond the positioned stent may be strengthened by placing low compliance tubing over the ends or by applying a liquid silicone material to the ends of the test vessel, and then curing the silicone to harden. 
         [0049]    At step  316 , the mesh tube and mock vessel as a unit are loaded onto a tester unit. 
         [0050]    At step  318 , the tester is set to deliver pulsatile flow including a predetermined: temperature, pH, pulsation frequency, and maximum and minimum pressure for each cycle. 
         [0051]    At step  320 , a high speed or still camera is set to monitor the deflection of the test sample. The stent may have set points  141  marked on the outer surface or specific landmarks may be identified to be used with the camera to track fracture of the stent  104   b.  The high speed camera will track landmarks  141  on the stent and the distance between the marks  141  will change as the mock vessel expands, contracts, and fractures. A plurality of cameras may be necessary to give a full 360 degree view of the mesh tube. 
         [0052]    At step  322 , the tester is set to run at the predetermined parameters. 
         [0053]    At step  324 , when a break occurs then at step  326 , the number of cycles to break is determined, and the tester continues to run. When a predetermined number of breaks have been met, then process is ended at step  328 . The whole method is repeated with adjusted parameters to generate the data necessary to determine fatigue properties of the tested stent. 
         [0054]      FIG. 6  is a flow diagram illustrating a method  400  of fatigue to fracture utilizing a combination of radial expansion and compression. At step  410 , the inside wall of the mock vessel is marked with predetermined sets of points and the outside wall is marked with a predetermined of points, to create landmarks on the vessel that can be tracked by the camera during testing. 
         [0055]    At step  412 , a mesh tube, stent, or implantable sample is deployed within a mock vessel. The stent may be balloon expanded or made from Nitinol and is expanded as it is deployed. The mock vessel has a lower than normal compliance for the test stent deployed, i.e. 1% to 2% per 100 mm Hg. 
         [0056]    At step  414 , insert a higher than normal compliance inner tube that has a radial compliance of 5% per 100 mm Hg and that has an outer diameter between 85-95% of the working inner diameter of the stent deployed. The inner tube may be formed by applying a liquid silicone material to the inner wall of the test vessel, and then curing the silicone to harden into an inner layer. The stent becomes encapsulated within the silicone-formed wall of the vessel. 
         [0057]    At step  416 , the mesh tube, inner liner, and mock vessel as a unit are loaded onto a tester unit. 
         [0058]    At step  418 , the tester is set to deliver pulsatile flow including a predetermined: temperature, pH, pulsation frequency, and maximum and minimum pressure for each cycle. Also, a predetermined number of breaks in the stent are set. 
         [0059]    At step  420 , a high speed or still camera is set to monitor the deflection of the test sample. The high speed camera will track landmarks on the stent and the distance between the marks will change as the mock vessel expands and contracts. A plurality of cameras may be necessary to give a full 360 degree view of the mesh tube. 
         [0060]    At step  422 , the tester is set to run at the predetermined parameters. 
         [0061]    At step  424 , when a break occurs then at step  426 , the number of cycles to break is determined, and the tester continues to run. When a predetermined number of breaks have been met, then process is ended at step  428 . The whole method is repeated with adjusted parameters to generate the data necessary to determine fatigue properties of the tested stent. 
         [0062]    Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present invention. Embodiments of the present invention have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present invention. Further, it will be understood that certain features and subcombinations may be of utility and may be employed within the scope of the disclosure. Further, various steps set forth herein may be carried out in orders that differ from those set forth herein without departing from the scope of the present methods. This description shall not be restricted to the above embodiments. 
         [0063]    Further, it is understood that this testing methodology will work for virtually any mesh tubular device or any tube shaped device having expansion or compression characteristics, for example, automobile fluid tubes, garden hoses, flexible water piping and the like. The parameters and characteristics of a suitable testing vessel are selected for compatibility with the tube shaped device to be tested. These parameters and characteristics include compliance, size, length, thickness and whether the test to be performed is expansion, compression or both. 
         [0064]    It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.