Down to the wire: The importance of stent testing

Alan Thomas, marketing at ZwickRoell, provides some insight into the firm’s mechanical testing of wire stents.

When arteries in the heart become blocked due to coronary artery disease, one way to effectively treat the condition is to implant a coronary stent. Stents are surgically implanted into the coronary arteries to keep them open, allowing the arteries to supply blood to the heart more efficiently. Stents have become one of the most widely implanted medical devices, and the process of installing the stent into the body is only minimally invasive. Therefore, ensuring the safety and efficacy of every stent is critical and consequently demands rigorous mechanical testing.  

A variety of stents are manufactured by braiding or knitting thin metal wires. This is commonly done on a metal caliper called a mandrel. Several materials can be used for the wires and common ones are medical grade stainless steel, nickel-titanium alloys, cobalt-chromium alloys, and magnesium alloys.

Stents are subjected to heavy loads when inserted and left in blood vessels and these loads must be simulated before the stents can be safely used in patients. Along with obtaining accurate material characteristic values, determining the radial compression strength is the most important test for stents. Stents must exert a radial force that is sufficient to ensure that the device remains in the narrowed artery and prevents constriction of the blood vessels.

Mechanical testing systems incorporating a 37°C temperature chamber, are employed to simulate tests at body temperature. Radial compression test fixtures are specifically designed to test stents and are available to accommodate various diameters and lengths. The fixture simulates the pressure placed by the artery on the stent. The stent is inserted, compressed radially to a minimum target diameter, and then released. Testing software supports the sequence by measuring the values, compensates for possible self-deformations, and accounts for the very slight frictional and inertial forces that arise during measurement.

Along with tests for the entire system, components such as single wires and stent struts are also mechanically tested. This includes the tensile strength and strain at break, as well as the minimum yield strength. It defines the force at which a material under a single-axis tensile load demonstrates no permanent deformation.

Precise strain measurement on thin wire in a uniaxial tensile test is best achieved by means of an extensometer. The probability of error is much smaller since measurements are taken directly at the specimen and therefore outside the force flow.

Selecting the most suitable extensometer is essential. The difference is whether the extensometer contacts the specimen during measurement. Clip-on extensometers are cost-effective but can falsify measurements because of the direct contact they make, or they can damage the specimen. This is the danger with specimens made of thin wire. The weight of the clip-on extensometer alone could lead to bending of the specimen. Furthermore, there is a risk that the knife edges slip and damage the wire. A safe, accurate way to measure strain is to use a non-contact extensometer.

Non-contact extensometers incorporating lasers, are designed for tensile, compression, and flexure tests on various materials. They create a speckle pattern on the surface of the specimen, which is recorded by a full image digital camera. This pattern creates a virtual gauge mark on the specimen, whose movement under load is tracked with a special correlation algorithm. The evaluation of two sequential images shows the strain of the specimen with a resolution of less than 0.15 μm. This non-contact strain measurement is also used on stents to obtain accurate material characteristics for the finite element method (FEM) simulation, from the beginning of deformation until strain at break.

Using non-contacting extensometry, the operator needs a few seconds to set the various gauge lengths. It is easy to mount and dismantle, and is combined with largely automated test sequences, which significantly reduce the amount of time needed for testing. This in turn increases the integrity of the tests because subjective influences are minimised, which is particularly efficient and useful in routine testing or for tests with integrated production chains. Measuring specimen strain inside a temperature chamber is also possible using a non-contacting instrument.

Another important consideration is the ability to test the fatigue strength of a stent under a periodically changing force. To investigate the durability of stents a fixture is available that allows up to 30 stents to be accommodated simultaneously. This fixture, with an electric torsion drive, is used with a low force capacity servo-hydraulic testing machine and allows both separate and superimposed loading of the stents with compression and torsion. The gripped area can also be equipped with a fluid chamber to facilitate testing under physiological conditions.

With stents or any other implantable device, the cost of failure can be extremely high. Performing mechanical testing throughout product development and routine quality control is critical to ensure safety and effectiveness.