With heart disease being the leading cause of death among adults in the United States, vascular stents are a critical medical device in ensuring patients live longer and healthier lives. There are a variety of stents that range in both size and strength for various applications. Mechanical testing of vascular stent involves testing the vascular stent specimens in various modes of failure including bending, torsion, tensile, crushing, abrasion, and fatigue. This blog post covers certain test types and the recommended equipment to test vascular stents.

Vascular Stents

For medical products, safety and efficacy are critical in the success of the product. Vascular stents, hollow tubular structures implanted in the native or grafted vasculature, are Class III medical devices that require premarket approval. They are intended for use as a mechanical radial support to enhance vessel patency over the intended design life of the device. Device properties and the intended use are dependent on results from extensive testing that involves mechanical tests on universal testing machines.

Types of Mechanical Tests

Failure of a loaded stent may result in loss of radial support of the stented vessel or in perforation of the vessel by the stent struts. Stress-strain analysis, combined with fatigue analysis and accelerated durability testing, provides an indication of device durability. Below is a list of mechanical tests that may be performed on vascular stents.

Bend Testing

Bend testing is required to characterize deployed stent flexibility and pre-deployment stents system flexibility in the region of the stent and the balloon. Depending on the test procedure followed, the bending flexibility of the stent is obtained from force-versus-deflection plots and/or midspan bending moment versus midspan curvature plots. ASTM F2606 provides guidelines on three-point bending procedures on balloon-expandable stents and stent systems.

Test report showing flexural strength

It is recommended to study the bending flexibility of vascular stents as well as of stent systems and delivery systems.

  • Bending flexibility of deployed stents shows the bending properties of the vascular stent and whether these properties conform to the natural curvature of the vessel.
  • For stent systems, delivery systems with pre-mounted stents, bending flexibility measures the ability of the system to track through the vascular anatomy and whether vascular trauma may occur along the delivery pathway distal to the guide catheter.
  • Finally, bending flexibility of a delivery system, which consists of a catheter that is used to deliver and deploy a stent at the target site, helps assess the separate contributions of the delivery system and how the mounted stent affects the overall flexibility of the stent system.

Bend tests may be performed using fixed span lengths or variable span lengths.

  • Fixed span length bend test method will allow force versus deflection comparisons that are independent of stent length.
  • Variable span length bend test method will allow the bending moment arm length to be maximized for any given stent length, thus minimizing the potential for non-bending deformation at a given applied load and/or deflection. Bending flexibility comparisons may be made at different span lengths by comparing midspan bending moments at given midspan bending curvatures and the bending load variation with span length may be studied.

Fatigue Testing

Fatigue testing is required to determine the stent failure due to typical cyclic blood vessel diametric distention, which is defined as the change in inner diameter of a blood vessel due to a pressure change with each pulse of blood circulation. It evaluates failure modes such as fretting, abrasion, wear, and fracture. The vascular stent specimen is exposed to physiologically relevant diametric distention levels by hydrodynamic pulsatile loading by injecting a volume of fluid into the confined test volume. The typical duration of testing per ASTM F2477 is 10 years equivalent at 72 beats per minute, or at least 380 million cycles.

Live display during fatigue testing

Radial Loading

Radial loading, directed perpendicular to the longitudinal axis of a cylinder and applied to the outer cylindrical surface of the stent, is required to determine whether the vascular stent has adequate resistance to forces acting on it and maintain vessel patency. Depending on the type of device and the clinical concern, the resistance to these loads can be presented through multiple test outputs: radial strength, collapse pressure, or chronic outward force. ASTM F3067 outlines procedures for radial testing of stents. Based on the standard, radial testing of stents will differ depending on the stent type (balloon-expandable versus self-expanding) as well as the apparatus used (segmented head, sling, or hydraulic/ pneumatic). The apparatus is selected based on clinical effects and limited by the stent type.

Tensile Testing

Tensile testing is required to determine the ultimate tensile strength, yield strength, and elongation of materials used in developing the vascular stent. If nitinol is used as a raw material in vascular stent formation, the plateau stresses and elastic strain limits should also be tested and specified. ASTM F2516 covers the tension testing of Nickel-Titanium superelastic materials. FDA also recommends reporting the stress-strain response of the stent after deployment. Other post-processing mechanical properties to report include Poisson’s ratio, elastic modulus, and endurance limit.

Torsion Testing

Torsion testing is recommended for stent delivery systems that need to withstand torsional forces. If not demonstrated successfully, this may lead to device failure or vessel damage. The torque strength of the stent delivery system when the distal tip is not free to rotate should be measured by rotating the proximal end of the catheter until failure. The number of rotations to failure and the failure mode for each sample tested should be reported.

Depending on the type of vascular stent, its location and intended use, other tests may include crush resistance, kink resistance, corrosion, coating integrity, and more.

Testing Equipment

ADMET testing systems are versatile instruments capable of testing a wide range of materials and products.  Over the years, we’ve worked with many medical device manufacturers to develop testing solutions that will ensure their products meet the necessary specifications.  ADMET’s MTESTQuattro controller software helps to satisfy the requirements of FDA CFR 21 Part 11 by creating a documentation trail whenever there are changes to test procedures. These changes result in creation of a record which indicates what was changed, when, and by whom.

See below for a selection of testing systems used for testing vascular stents. Please contact ADMET to work with our Sales Engineers and get more information on standard and custom universal testing systems based on your testing application.

eXpert 4000 MicroTester

eXpert 4000 MicroTesters are designed to work with microscopes and can be set up with submersible load cells and fixtures in baths to test medical samples in liquid environments.

Video below shows a custom testing application where tensile fatigue loading is applied to rubber rings utilizing the eXpert 4000 frame with an attachable bath and submersible fixtures.

eXpert 5900 Fatigue Testing Systems

eXpert 5900 series table top dynamic testing machines are compact, quiet electrodynamic testing systems for determining the durability of materials and components in tension, compression or flexure. Users of ADMET’s dynamic fatigue testing systems have a choice of two controllers: the MTESTQuattro high-speed closed-loop controller, or the DC1X Dynamic Fatigue Controller.

eXpert 5900 fatigue bend testing

MTESTQuattro can be programmed to generate sine, square and triangular waveforms. Users can manually adjust control gains and end point values on the fly or activate amplitude control so that the controller automatically adjusts the end point values to ensure that the dynamic fatigue testing systems desired targets are met.

The DC1x controller is an ideal, cost-effective solution for basic cyclic fatigue testing applications. It provides basic sine, square and triangle waveforms. Amplitude control is a standard compensation program that tracks the feedback signal and automatically adjusts amplitude and setpoint control values until the desired peak and valley levels are achieved. It is ideal for cyclic tests under load control where test system or specimen characteristics vary over time. During test users can view peak/valley and number of cycles completed.

7600_pneumatic-grips_quick_change_inserts

eXpert 7600 with pneumatic grips

eXpert 7600 Single Column Universal Testing Systems

eXpert 7600 series machines are capable of performing a variety of mechanical tests on vascular stent specimens including tension, compression, puncture, and more. If your testing requires more than our standard systems offer, we can modify the stroke, speed or add a torsion actuator for biaxial tests to meet your needs. The 8 kHz servo loop rate ensures accurate, repeatable, and reliable testing.

eXpert 9000 Torsion Testing Systems

eXpert 9000 series table top torsion testing machines are available in vertical or horizontal orientations for static and fatigue testing applications. Maximum torsional stiffness and minimal axial friction is achieved with a roller slide frame design. A reaction torque transducer can be mounted to the tail stock or a rotary torque transducer can be mounted to the drive spindle to accommodate a wide range of applications. In the case of the horizontal machine, the tail stock can be left free floating or clamped during testing. A dead weight pulley system is provided with all torsion testers for applying uniaxial tensile or compressive forces. All torsion testers feature unlimited rotation in both directions.

Note: Proper calibration of all measuring devices is critical in medical device testing. ADMET follows ASTM calibration procedures for traceable calibration.

Conclusion

Full test reports for all test performed should be included for premarket approval of vascular stents as testing helps to support the safety and effectiveness of vascular stents. Regulatory approval requires including test specimen information, test protocols, protocol deviations, test parameters and acceptance criteria, raw data, test results, and data analysis. Conclusions drawn from the test results and the clinical significance of the conclusions are also recommended for final submission. Mechanical testing should be performed by accurate universal testing systems that equipped with controls and software producing repeatable and reliable results. Talk to an ADMET Sales Engineer to discuss medical device testing equipment and the best fit for your application.

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