EuroWire May 2023

Technology News

Mechanical testing of wire stents Stents are tubular support devices that are surgically implanted into the coronary arteries to keep them open, allowing the arteries to supply blood to the heart more efficiently. They have become one of the most widely implanted medical devices, and the process of installing the stent into the body is only minimally invasive. Ensuring the safety and efficacy of every stent is critical, and consequently demands rigorous mechanical testing. ports the sequence by measuring the values, compensates for possible self-de formations, and takes account of 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.

Bespoke-design radial compression test fixture for stents

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, including medical grade stainless steel, nickel-titanium alloys, cobalt-chromium alloys and magnesium alloys. Stents must exert a radial force that is suf ficient to ensure that the device remains in the narrowed artery and prevents con striction of the blood vessels. ASTM F3067 describes the radial compression testing of balloon-expanding and self-expanding stents. Mechanical testing systems incorporat ing a 37°C temperature chamber are em ployed to simulate tests at body temper ature. Radial compression test fixtures, which measure the radial force, are spe cifically designed to test stents and are available to accommodate various diam eters 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 sup

Precise strain measurement on thin wire in a uniaxial tensile test is achieved by means of an extensometer. The probability of error is small, since measurements are taken directly at the specimen and therefore outside the force flow. Clip-on extensometers are a cost-effective way to measure values but can falsify measurements because of the direct contact they make, or they can damage the specimen. The weight of the clip on extensometer alone could lead to bending of the specimen. Non-contact extensometers incorporat- ing 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, and the movement under load is tracked with a special correlation algorithm. The evaluation of

Uniaxial tensile test in saline chamber

two sequential images shows the strain of the specimen with a resolution of less than 0.15μm. 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 only a few seconds to set the various gauge lengths. It is easy to mount and dismantle, and combined with largely automated test sequences it reduces the amount of time needed for testing. 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 simultane ously. This fixture, with an electric torsion drive, is used in combination 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 of the stents 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. Mechanical testing throughout product development and routine quality control are critical to ensure the devices will effectively and safely assist a patient. ZwickRoell Ltd www.zwickroell.com

Easy-cleaning extrusion head The Bullet® is an extrusion head with a fixed-centre, multi-port spiral flow design and gum space adjustment. With no fastening hardware, cleaning and restart are easier and faster than with conventional heads, according to Guill Tool & Engineering. is required. There is a threaded retaining ring for the die and threaded tip retainer, and tooling retainers also provide gum space adjustment.

High- and low-volume applications are suitable for this head and are accommodated with the simple changing of just one component. Various crosshead designs are available and users can specify the “calibre” – the maximum die ID. A vacuum chamber and kit for assembly and disassembly are included with the unit. Optional keyed tooling capability offers machine designers and end users quick orientation. The overall unit design enables faster disassembly, cleaning and restart. Guill Tool & Engineering Co, Inc www.guill.com

The next generation of the tool – the Bullet II – allows quick tooling changes, as the tips remove from the back and the die removes from the front of the unit. The absence of fastening hardware eliminates leaking, as does the taper body and deflector design. The new patent-pending Cam Lock® deflector retaining system offers additional benefits to extruders and machine builders. It only takes half a turn of the Cam Lock to remove and install the deflector and tip; no fastening hardware

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May 2023