Comprised of biocompatible metal or biodegradable polymers, stents bear a complex geometry, enabling them to act as effective scaffolds. As they must be able to push against the internal walls of the blood vessel or other conduit into which they are placed, their mechanical integrity is of the utmost importance. An insufficient level of flexibility can result in tissue damage while insufficient rigidity inhibits the device's capacity to support natural flow. 

Designed to address specific applications, stents are available in a wide range of sizes, diameters, mesh patterns, and strengths.An intraluminal coronary artery stent is a small, self-expanding, metal mesh tube placed inside a coronary artery following a balloon angioplasty procedure. This particular type of stent is designed to prevent the artery from re-closing. As it is placed within an artery, it is subjected to rather large forces that must be thoroughly characterized during the product development cycle and as part of quality management initiatives.

Drug-eluting stents are among the most recent types of stents approved for use. Coated with time release pharmaceutical compounds, drug-eluting stents are also utilized in cardiovascular procedures to maintain blood flow. According to the New England Journal of Medicine, more than 500,000 patients in the United States are implanted with drug-eluting stents annually. A chief benefit is the reduced risk of repeat revascularization, a condition in which the patient requires additional cardiovascular procedures.  

Cook Medical has introduced a self-expanding stent designed to treat diseased veins near the hip and to correct vein stenosis in patients with obstructed vessels. Medtronic has introduced the Resolute Integrity drug-eluting stent produced from a single strand of wire that has been molded into a continuous sinusoidal pattern, increasing range of motion and thereby aiding deliverability.  

According to STI Laser Industries, Ltd., a leading supplier of machined stent assemblies, typical stent fabrication methods include: wire braiding or knitting, laser sheet cutting, and laser tube cutting. The Or Akiva, Israel-based manufacturer states that manufacturing begins with the selection of the optimal raw materials, then moves to high-precision laser cutting of complex geometries, and ultimately to final finishing procedures that include both surface and heat treatments. Surface treatments include polishing, honing, micro-blasting, pickling, electro polishing, passivation and ultrasonic cleaning. These steps result in a biocompatible product that bears a high degree of quality, a surface free of defects, and improved corrosion resistance.

While biocompatible metals have been the primary class of materials for stents, recent developments have seen the introduction of bioresorbable polymeric stents for a specific suite of applications. Metallic stents are typically comprised of the following biocompatible alloys:  Stainless Steel (316LVM), Nickel Titanium (Nitinol), Cobalt (L605), Cobalt Chromium (CoCr) or Nickel-Cobalt (MP35N). Some stents, such as those used for grafts, are made of fabric for use in larger arteries. Reliable characterization of these materials is necessary to meet surgical standards and to assure patient safety.

Balloon-expandable stents comprised of stainless steel and cobalt chrome alloys undergo an annealing process.  According to STI, annealing relieves internal stresses, softens the metal, improves the elongation rate, lowers the risk of strut breakage, and improves fatigue resistance. Self-expanding stents utilize the elastic properties of Nitinol and require a different process called shape setting to fix the final shape of the stent and the transition temperature.

Visual and dimensional inspections of the stents are undertaken throughout the manufacturing process utilizing high-resolution optical microscopes and video inspection systems. Such inspections are conducted to identify defects, to comply with customer specifications and to support adherence to ISO 13485:2003 and ISO 9001:2008 quality standards.

Advances in imaging technology and stent design have led to the manufacture of smaller delivery systems and thinner stent profiles. Fluoroscopic visibility of smaller stents decreases with their size and therefore requires the integration of radiopaque markers into the stent design. Markers are comprised of industrial-grade precious and semi-precious metals, including Gold, Tantalum and Platinum-Iridium.

Research and development engineers perform a range of tests on the materials used to manufacture stents, according to Erik Berndt, medical industry manager for Zwick/Roell, an Ulm, Germany-based manufacturer of material and component testing equipment.

"This is an extremely precise form of testing that calls for platforms capable of performing measurements with a high degree of accuracy," he said.

One of the primary challenges facing test engineers involves the gripping of test specimens that may be only a few millimeters in length and the measurement of elongations that are often in the range of just a few percentiles, Berndt said.

Sample preparation is key to accurate measurement. Because it is difficult to test a complete stent, small dog-bone shaped specimens comparable to stent parts are produced and tested. The stent sections are laser-milled to create the dog-bone shaped test coupons.

"Tests on new stent structures usually include analysis of radial compression force as well as standard tensile properties," Berndt said.

Radial compression tests are performed with machines such as the zwicki-Line system with a temperature chamber capable of elevating temperature up to 37 degrees C and specialized tooling.  While a test standard has not yet been finalized for radial compression tests, an ASTM task group is working to establish one and is engaged in the development of a draft.  While the standard undergoes development, ASTM F2081 and ISO 25539 are often employed as substitutes.  

"We offer an assortment of specialized tooling to measure radial compression in stent samples," Berndt said.

Tensile tests on stent samples are conducted with a Zwick PrecisionLine Vario system in combination with a temperature chamber, a laser extensometer and specialized fixtures that are designed to properly grip the samples.

"With this instrumentation we are able to grip specimens that are quite small, starting from 3mm length and width of only 0.2mm," Berndt said.

While gripping the test specimen presents one set of challenges, measuring strain presents a different set of issues.  Laser extensometers offer high precision, non-contact measurement of strain for delicate specimens undergoing extremely small changes in elongation. The latest in extensometer technology employs a non-contacting device that does not require measurement marks on the sample.

The laser extensometer utilizes the unique structure of a specimen's surface as a fingerprint to generate a virtual measurement mark.  A laser directed at these measurement positions is reflected in various directions corresponding to the surface structure and creates a specific pattern of speckles. Selected measurement points are tracked and converted to direct extension values. The change in the surface structure, which is the basis for the speckle pattern, is continuously evaluated during specimen deformation.

The laser interferometer-based method of non-contact extensometry allows test labs to characterize materials, components and subassemblies in quality control and research and development applications. Additionally, this approach to extensometry supports tests on micro-specimens with small gage lengths that require exceptional accuracy in strain measurement.

"Such tests would not be possible with traditional extensometry. Elevating throughput levels and delivering the utmost accuracy in strain measurement offers significant value for high volume test labs," Berndt said.

Management and control of the test process and equipment is achieved by dedicated software.  Zwick's laserXtens laser extensometer is directly integrated into the company's testXpert II measurement and control software.

Further streamlining of testflows may be realized by incorporating temperature as an external channel directly in testXpertII software. Making temperature available as a recorded channel places all test data within a single file structure, assisting manufacturers in recordkeeping procedures and supporting compliance with FDA 21 CFR Part 11.

Additional tests performed on stents include horizontal track equipment that can precisely measure the force required to guide a stent or catheter down a tortuous pathway simulating insertion in an artery or blood vessel. Zwick offers a horizontal test platform to conduct such simulations.                                   

Figure 1

Stents differ greatly in their design, dimensions and the optimal material for their fabrication, based on their intended application. The following table enumerates common stent applications and the range of possible dimensions:

Erik Wittenzellner is a senior applications engineer for Zwick USA. Applying deep skills in the support of test system control electronics, Wittenzellner works in concert with customers to develop novel test procedures for a wide range of medical applications. He has been integral to concept development for fixtures that support complex testing requirements for medical device manufacturers.  Understanding the importance of applying built-in platform flexibilities, Wittenzellner frequently lends his expertise in test flow development to address the unique needs of individual customers.