Comparison of radial expansion of stents within mock vessels molded with a target bent radius versus straight mock vessels bent to a target radius.

Testing of medical devices such as vascular stents generally require animal clinical studies to be conducted prior to approval for use in humans. In the recent years, several studies have shown that mock vessels serve as good alternatives or adjuncts to animal studies, having the ability to replicate some anatomical and physiological conditions [1,2]. Testing the medical device utilizing mock arteries is less time consuming and more cost effective than animal studies and is often able to predict clinical failures. Testing standards recommend that stents be tested under physiologically-relevant conditions of pulsating load. Under the pulsating load, characteristics of mock arteries play a very important role in the behavior of the stent. Distension of the mock artery controls the load applied on the stent and the shape of the mock artery ensures uniform loading of the stent [3]. The research described in this study is an effort towards determining the role played by the shape of the mock artery on distension of the stent. More specifically, does a molded curved vessel apply more uniform or less uniform loading to the stent than a straight vessel that was bent after fabrication and stent deployment?

Mock artery distension: comparison of optical, mechanical and theoretical results.

The relationship between the pressure inside a cylindrical mock vessel and the change in radius is called radial compliance. It has been postulated that it is the internal change in radius with pressure that has a significant effect on blood flow disturbance or the interaction of a graft or synthetic tube with an indwelling stent or catheter. A variety of techniques are widely used in determining this parameter. Some are direct measurements of the internal diameter and some methods measure the outer diameter along with the use of theoretical calculations in determining this parameter. It is essential to understand and evaluate all the techniques widely used in the cardiovascular industry. This research work is an effort towards making this comparison.

Linear elastic mechanics of mock arteries: empirical versus theoretically predicted pulsatile stent deflection.

Evaluation of a medical device requires human or animal studies to be conducted. In the past decade, the use of mock arteries or mock vessels has found its place in the cardiovascular industry as a best alternative to researchers. Mechanical characteristics of the mock artery play a significant role on the stent and vascular grafts being tested. It is these mechanical characteristics that determine the amount of load applied on the medical device thus deciding the validity of the fatigue test on the devices. This paper is an effort towards determining the distension of the mock artery and relating it to the distension of the vascular stent.

Single versus double ended high frequency pressurization of mock arteries: symmetry of expansion.

A series of experiments was carried out to evaluate the expansion characteristics of silicone mock arteries at frequencies from 5 to 100 Hz. Of particular interest was the magnitude of expansion along the length of the tube versus frequency. The tubes were pressurized from one end or from both ends. In general, results indicate asymmetric expansion along the length of the tube. This asymmetry, however, changed dramatically with frequency. These changes are not fully explainable by standard experimental considerations such as compliance and harmonics.

Frequency dependent hysteresis of silicone and latex mock arteries used in stent testing.

Mock arteries also called as mock vessels are one of the best alternatives available to researchers in evaluating the mechanical characteristics and durability of intravascular medical products without having to use animal and human clinical studies. The behavior of mock arteries depends on the frequency of loading. This makes it essential to evaluate and analyze the compliance and hysteresis of the mock arteries at different frequencies. Hysteresis, the difference in the pressure-volume curve between the loading cycle and the unloading cycle, plays an important role in determining the mechanical properties of the mock arteries. Six each of silicone and latex mock arteries were tested for this study. Three silicone and three latex mock arteries were tested at room temperature for dynamic internal compliance, and the remaining three each of silicone and latex mock arteries were soaked in distilled water at 37 degrees C for 36 hours and then compliance tested using a dynamic compliance tester. All arteries were tested at four different frequencies: 72, 500, 1000, and 1500 beats per minute.

High speed photographic verification of intravascular stent strains during accelerated durability testing.

Most stent durability testing does not predict resulting clinical failures. Standards committees have taken on the task of generating more reliable test protocols. Currently, the AAMI/ISO Committee has chosen an experimental direction that allows an investigator to test at ultra high frequencies (above 1000 bpm) as long as the actual strain of the stents can be verified to be moving the same amount per cycle that they do during real time testing (70 to 72 bpm). We have developed a series of techniques and protocols utilizing high-speed photographic analysis to accomplish this. The first aspect requires the utilization of ultra clear silicone mock arteries. It is important that optical anomalies do not cause aberrations in the analysis. Next is the relationship between the angle of the incident lighting and the positioning of the camera in front of the pulsing tube. Finally, a unique set of marking techniques which are used as tracking locations for the automatic measurement systems located in the software of the camera have certain characteristics that are important with respect to accurate measurements. In each case the particular setup must have independent verification of the accuracy of the measurements. We will review these variables and demonstrate how this system can be utilized to verify strains of the inside wall, outside wall, and stent.

Radial compliance of natural and mock arteries: how this property defines the cyclic loading of deployed vascular stents.

There have been a disturbing number of unexpected mechanical failures in deployed vascular stents and stent grafts. An analysis of the mechanical properties of mock arteries used to carry out standard durability testing on the aforementioned medical products suggests that some of the problems might be related to a frequency dependent change in the properties of these recipient vessels. This could lead to a lower than expected level of loading and an attendant reduction in severity of testing. A clear understanding of how a pulsating artery cyclically loads a deployed stent is important before a mock artery can be designed to carry out biologically relevant in vitro durability testing. In addition, knowledge of the time dependent response of both the vessel and the stent/stent graft is critical before any accelerated testing protocol can be properly designed. This paper will present an analysis of the loading on a stent versus pressure and diameter of the recipient vessel and how this varies with frequency. Actual compliance versus frequency data will be used from silicone mock arteries.

The durability of silicone versus latex mock arteries.

Latex mock arteries used in medical device testing allow researchers to evaluate mechanical characteristics of intravascular medical products without using animal or human clinical studies for this data. Such intravascular situations include determining properties such as drag and steerability of catheters, recoil of vascular stents, and clinician training. In fatigue testing, the latex mock arteries are used to receive deployed products and are then repeatedly pressurized at biologically relevant pressures to determine the long term durability of the product. By matching dimensions and pressure-volume relationships (compliance) of these latex tubes, researchers have a reliable means to evaluate and predict product lifetimes. The problem with latex mock arteries is two-fold: First, they are opaque so the product inside the artery cannot be seen during evaluation of the integrity of the product or during clinical training sessions. Second, latex tubes fatigue; therefore, the loading that they place on the internalized products varies with time. During long term durability studies, latex tubes may have to be replaced as often as every 100 million cycles. This can be problematic with products that are difficult to redeploy. We have developed a clear silicone mock artery system that allows us to fabricate three-dimensional objects, including tubes with precise geometric and mechanical properties. Our evaluations show that the mock arteries can be stressed up to 400 million cycles with little or no change in mechanical properties. We are in the process of continuing evaluations to determine long term durability.

Evaluating the in situ loading and accelerated durability of barbs located on bifurcated aorto-iliac stent-grafts.

A special three-part protocol has been generated for the isolated durability testing of stent barbs. This triple protocol includes an initial evaluation on a cardiovascular duplicator to determine the loading per barb that occurs during normal flow through the stent-graft. The next stage of this protocol determined the frequency response characteristics of the bending point where the barb attaches to the stent to allow for the determination of the appropriate frequency to carry out the accelerated testing. The final part of the test includes high speed bend testing at frequencies determined in the second part of the methods to determine the long term durability of the isolated barb stent unit. The results of this testing indicated that under normal cardiovascular conditions each barb is experiencing a loading of 20 grams peak during maximum forward flow. Loading the isolated barb/stent segment at 600 beats per minute (bpm) for 400 million cycles indicated no tendency for these barbs to experience a change in physical properties. During this testing there were no barbs that broke.

The high frequency testing of vascular grafts and vascular stents: influence of sample dimensions on maximum allowable frequency.

There are over four dozen companies developing new products for the lucrative vascular graft and vascular stent markets. We have been studying the procedures used to pre-test vascular grafts, vascular stents, and mock arteries into which vascular stents are placed, so that appropriate high frequency durability/fatigue studies can be done in as short a time as possible, but also in a manner in which the data are highly reliable and dependable. In the past, we have evaluated the testing of the natural frequency response of the grafts, mock arteries, and stents in order to determine the frequencies that can be used for long-term life testing. In this paper, we present experiments that evaluate how product geometry affects product frequency response at various loading pressures. Results show that changing the dimensions of the device to be tested such that less fluid has to be injected (in the case of hydrodynamically driven experiments) results in the ability to test these products at a higher frequency.

Sources of error in monitoring high speed testing of vascular grafts.

Mechanical compliance issues are increasingly important in the design, testing, and manufacture of vascular grafts. This has to do with the observed relationship between long-term patency of implanted grafts and accurate compliance matching of those devices with the recipient natural vessels. Another important concern in this type of investigation has to do with the use of mock arteries during the testing of implantable medical products such as intravascular stents, stent/grafts, etc. At issue here are not simply the techniques used to monitor the static compliance of the vessels, but the dynamic properties which are in effect during the in vivo utilization of these devices. Perhaps of even more importance is understanding the high speed dynamic properties of these vessels so that a proper and reliable high-speed durability experiment might be designed. There is a natural tendency to adopt procedures that monitor outside dimensions of the graft or artery. The theoretical problem with this approach is the fact that compliance matching is a phenomenon associated with the inner lumen of the tube, whether it is hydrodynamic considerations in vascular graft testing or loading considerations in stent testing. Optical techniques such as lasers and ultrasound are encumbered by two physical motion phenomena unique to this approach. The first is simple jumping or movement of the tube due to vibrations. This problem can be overcome by high sampling rates. The second is more problematic and results in lengthening of the tube that has longitudinal as well as radial compliance. Both quantitative and qualitative examples will be examined. A comprehensive understanding of the sources of error encountered in various monitoring techniques is reviewed. These techniques will include lasers, dynamic internal compliance, ultrasound, and cantilevered beams.

Frequency dependent radial compliance of latex tubing.

Custom latex tubing is often used in medical device evaluation. Examples include thin-walled devices used to reduce leakage of porous vascular grafts, and thicker-walled prototypes used as mechanically equivalent synthetic arteries. Medical devices such as stents and balloons are introduced into these for mechanically comparable in vitro testing. The three-dimensional mechanical properties of these tubes are critically important, particularly in accelerated testing, since they are primarily designed to replicate the mechanical rather than biological properties of in vivo arteries. This paper explores the instrumentation and protocols necessary to evaluate the frequency dependent radial compliance of precision built latex tubing. Five cm long samples of custom dipped latex tubing 6 mm in diameter with wall thickness from 0.015" to 0.033" were kept dry or soaked in 37 degrees C phosphate buffered saline for 48 or 96 hours before being mounted on a dynamic internal compliance tester. Each tube was tested initially at 70 bpm to establish the internal radial compliance at the physiologically relevant rate. The frequency of the test was then increased incrementally and the radial compliance re-checked. In the most extreme case, tubes were tested up to 2700 bpm. In each case, the volume, pressure, and length of the tube was monitored continuously.