Life-saving precision: testing memory alloy stents

High-accuracy tests in the micro range are a tough nut to crack. As well as being sensitive, the specimens are too small for contact-type measuring systems and the results must display high micro-level accuracy. Admedes Schuessler solved the problem with a laserXtens Compact HP, a Zwick laser extensometer featuring non-contact operation based on innovative technology.
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Admedes Schuessler GmbH of Pforzheim, Germany, specializes in nitinol. This nickel-titanium alloy is the best-known shape-memory alloy and is used to manufacture stents, medical implants employed for prevention of heart attacks or after a heart attack has occurred.

Stents are self-expanding; they are inserted into blood vessels and expand as they warm up to body temperature, thereby keeping the blood vessels open. To ensure that they function correctly, extremely high-accuracy tests are carried out under specified conditions.

Stents are not assembled, but are cut from solid material. The process calls for precision and experience, on top of which nitinol, being a shape-memory alloy, is very difficult to handle. As the finished stent is too complex a structure for materials testing, Admedes Schuessler developed suitable specimens in the course of comprehensive analysis. Specimens must allow a solid-body simulation which is as close to reality as possible. It has been found that cutting out a single stent strut represents the best option for enabling determination of the mechanical properties and various influences, including temperature.

It is not just the specimen which is complex; the same is true of the test which is applied to it. Minute strains in the specimen must be recorded, while contact-type measurement is ruled out by the small gage-length and the sensitivity of the specimens. Moreover, the test has to take place at body temperature, i.e. in a 37°C temperature chamber.

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Zwick's laserXtens Compact HP is a non-contact extensometer designed for exactly this type of strain determination. It requires no gage marks and employs the laser speckle method, in which a laser beam is used to illuminate the surface. Reflected rays generate what is referred as a 'speckle pattern' in the camera, which can be tracked and evaluated via a complex software algorithm. There is no contact with and therefore no influence on the specimen. In addition, the extensometer optics can compensate for off-center movement of the specimen.

Continuous testing takes place at Admedes Schuessler, with testing machine and extensometer constantly in use. The laboratory manager is content: "The laserXtens Compact HP is the only extensometer on the market capable of meeting our requirements. On top of that, we are delighted with its reliability - the system has never failed once through all the tests and equipment changes." Such attributes are essential in order to satisfy the high quality standards of Admedes Schuessler and ultimately save lives.


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