Background
Past Methodology for Stent Endurance Testing
The Bose Approach to Stent Testing
System Operation

New stent and stent/graft designs are typically post-amendment Class III devices which require the approval of a Food & Drug Administration (FDA) Pre-Market Approval application prior to commercial marketing. The FDA requires medical device manufacturers of stents to submit animal, clinical and in-vitro test data to support the safety and efficacy of the permanent implant device. One critical in-vitro test is the accelerated durability testing which requires the equivalent of ten years of data, or approximately 400 million cycles of fatigue stress. This testing requires a highly reliable test system.

Actual Stent in Service

Copyright 1995 Vital Images, Inc. and Stanford University Dept. of Radiology

In the past, stent testing was performed by placing the stent(s) inside compliant tubes with physiological compliance characteristics which were filled with saline or other fluids. The tubes were then pulsated with a cam-driven pump assembly. The system was calibrated prior to the dynamic test by statically adjusting the pump assembly to produce cyclic pressures comparable to physiological pressures (50 to 150 mm Hg). It was believed the diameter of the stent would increase and decrease with the pulsation of the tubes, thereby subjecting the stent to fatigue stress/strains.

In order to simulate physiological pressure and compliance conditions, the test developer selected elastic tubes that approximated the physiological behavior of the blood vessel (3-5% dilation for 100 mm Hg pressure change). The stent/tube assembly then interacted with the cyclic pressure as a third-order system (spring/mass/dashpot). As a system, the stent/tubes displaced radially and the test was considered valid if the test speed stayed below the frequency where viscoelastic properties and fluid mass resonances changed the performance dramatically. If speeds were increased, the tubes consumed the pressure load meant to test the stent and consistent dilation could not be maintained across the tube length.

This method relegated the test frequency to somewhere in the 10 to 15 Hz range, which dictated a ten-month duration for a 400 million cycle test. To maintain data consistency, the operator also had to continually monitor and adjust the pump assembly to ensure required pressure levels were achieved. For time-to-market reasons, users wanted tests done more quickly, which called for a new approach to testing.

The Bose approach to stent testing

The ElectroForce Systems Group of the Bose Corporation met these challenges by using the patented high-bandwidth, low distortion Bose linear actuator with our WinTest control system and a laser-based compliance measuring device to perform controlled radial dilation testing of stents. This test instrument generates hydraulic pressure to load the tube (with the stent deployed) while controlling the radial displacement of the tube's exterior. This system can perform the required in-vitro tests at simulated heart rates from 50 to 100 beats per second and at tube compliances from zero to seven percent.

Since WinTest measures and controls vessel dilation directly, the problems stemming from the response of the tube have been eliminated. Our approach has been proven to the satisfaction of the FDA and has been the basis of successful IDE/PMA submissions by client companies.

System operation

1. In stent testing, the objective is to ensure the stent is able to last approximately 400 million cycles in a vessel that is dilating radially anywhere from 2 to 7%. For this reason, the dilation is selected as the control mode.
2. A transducer is selected to measure the vessel dilation at the test speed. For stents, Bose uses a high-performance laser to measure the change in the outside diameter of the simulated vessel.
3. Two Bose linear actuators act as a system to pressurize a saline test solution ported to test samples. Pump assemblies convert the actuator displacements to test pressures.
4. The laser feedback is compared to the desired value within the controller, and if the values differ from the programmed levels, the current which drives the electrodynamic motor is adjusted in proportion to the desired value, causing the system to compensate.
5. Over time, the software monitors changes in test conditions and has the ability to modify the original setup parameters and adjust the output accordingly.

Bose systems are designed to directly measure and control stent deformation to follow desired test protocols without continuous operator intervention.

System setup and calibration is straightforward and automated. Once the test is set up, the user can add additional supervisory commands to automate amplitude control of the system. This is especially useful in stent testing, because minor changes occur over 400 million cycles, including the dynamic response of the stent/tube system. Amplitude control capability maintains the desired test parameters during the test.

Data acquisition can be set up according to individual test needs. All of the load and displacement information can be acquired and then presented in documented graphical reports. The data is acquired in an ASCII text format for manipulation with Microsoft Excel or any word processor.

Safeguards are included to shut the system down in the case of power failure or an emergency stoppage. The Uninterruptible Power Supply (UPS) maintains enough power to ramp the program down to a safe level while data is acquired and saved prior to system shutdown. This feature is valuable for long-term time-critical tests.

Several test configurations are available to optimize system performance for the range of sample sizes that require testing.