Heart stents have revolutionized treatment for atherosclerosis by reducing the possibility of the narrowing of the artery after it is treated. These devices consist of small stainless-steel mesh tubes designed for insertion into diseased arteries, typically in patients receiving angioplasty or an atherectomy (see figure 1). When spread open with an inflatable balloon, heart stents remain enlarged to permanently maintain an open cavity, enabling blood flow. Stents can range in size from just larger than a millimeter in diameter unopened to as much as a centimeter when expanded and inserted into larger blood vessels, such as those in the leg. Lengths range from just a few millimeters to several centimeters.
Figure 1. Arterial stents consist of metal-mesh structures that can be opened up to support the artery and ensure blood flow.
Initially, surgeons used bare metal stents. Arteries im-planted with these stents had high rates of restenosis, or scar-tissue-induced re-closing, with up to 25% of patients requiring follow-on procedures to ensure proper blood flow. Stents coated in anticoagulant drugs, called drug-eluting stents, greatly reduce the formation of scar tissue in the artery and lower the rate of restenosis to less than 5%; they are the most common type of stent in use currently.
Because heart stents are implanted medical devices crucial to the long-term health of a patient, quality control of these devices must be extremely high. Quality-control procedures require inspection of the entire interior and exterior stent surfaces, sometimes with multiple human and machine inspection steps to ensure that all devices will be safe and effective for patients. Critical parameters for stents include surface roughness, widths, feature shape, and radius of curvature of the device. Potential defects include broken struts, scratches, pits, improperly curved junctions, con-tamination, and either too much or too little material removed in the manufacturing process.
With the addition of drug coatings on stents, not only is the surface quality of the bare metal important, as it constitutes the final implanted piece, but the uniformity and thickness of the drug coating must be ensured to guarantee the proper duration and coverage area of the drug delivery. This further increases the quality-control burden on the manufacturer.
With more than 10 million stents currently produced each year worldwide, manufacturers require high throughput without sacrificing device quality. Various optical techniques are used to meet this challenge. The most common technique to this point has been visual inspection through both high- and low-magnification microscopes, enabling examination of both inside and outside surfaces; fortunately, the voids in the stent structure allow examination of the inside surface through the opposing wall of the device. A combination of human operators and sophisticated software packages looks for lateral defects and errors in the devices, with automation enabling rapid rotation and translation to minimize inspection time. Optical Profiling
One critical shortcoming of bright-field imaging is that it does not provide quantitative, 3-D height or roughness information about the stent surface or the drug-coating thickness; therefore, many critical parameters for heart stents are not measurable using bright-field imaging alone. For this reason, 3-D optical inspection techniques such as optical profiling (white-light interferometry) are increasingly being used for quality control of both coated and uncoated stents.
An optical profiler is a microscope-based system fitted with a broadband source (typically a tungsten-halogen bulb or white LED), a high signal-to-noise ratio camera, various zoom optics, and specialized interferometric microscope objectives that split the light between a high-quality reference surface and the test sample to recombine the light after it reflects from each. The system is translated vertically using a precision scanner, varying the distance from the objective to the sample. During this process, the beams from the reference surface and test sample interfere, producing dark and bright fringes. The intensity frames are stored and analyzed using various algorithms to provide surface information with sub-nanometer vertical resolution. Lateral resolution of approximately 0.5 µm can be obtained, and practical fields of view for stent measurements can exceed 2 mm on a side. The base systems are highly versatile. Customization for stent measurements primarily involves staging and handling mechanisms for part placement, rotation, and translation to get 100% measurement coverage.
Optical profiling enables manufacturers to accurately measure stent radius of curvature on a global and local level, and allows them to map out any large-scale shape variations along the stent such as could occur through mishandling. In addition, the noncontact, nondestructive technology can determine various surface-roughness parameters of both the inside and outside of the stent. Specialized pattern-recognition software calculates feature widths and relative positions, and identifies deviations from the ideal shape. Finally, the user can screen for scratches, pits, and other defects at user-specified lateral and vertical thresholds so that part rejection becomes automatic.
The systems present data visually and can automatically store critical parameters to a database with appropriate pass/fail criteria and part-tracking information, improving process control. Each field takes several seconds to measure, with a full stent taking several minutes, depending on part size and magnification. Automation routines can allow for full stent coverage automatically, with no operator intervention during the measurement process (see figure 2).
Figure 2. A white-light interferometer with the proper software can automatically locate pitted areas, quantifying depth and area.
The thickness and uniformity of the drug coating determine the location and amount of the drug that will be applied to the surrounding artery. Uncoated or thinly coated areas may increase the risk of restenosis, while thickly coated areas may lead to other complications. Recent advances in white-light interferometry now enable stent manufacturers to simultaneously measure coating rough-ness and thickness, as well as the roughness of the stainless-steel substrate. Interference signals are produced from the air-to-coating interface as well as the coating-to-stent interface, essentially wherever light is reflected. These interference patterns can be intelligently separated through the software such that each surface is independently profiled. The separation between the interference patterns, combined with knowledge of the coat-ing index of refraction, allows the thickness to be calculated.
Coatings are typically less than 20 µm in thickness, so even with high-magnification interference, fringe con-trast is substantially unaffected by dispersion. The greater challenge, however, is that as film thickness decreases, the interference patterns overlap and must be intelligently separated through modeling. The ability to measure the coatings in 3-D is a great improvement for stent manufacturing over point-wise film-thickness measurement systems; for the first time, the coating can be mapped with sub-micrometer lateral resolution across the entire stent surface, defects identified, and manufacturing procedures improved to enhance uniformity.
If we examine a coated part measured at high magnification, we can see the coating helps smooth the underlying surface (see figure 3). The data also indicates several pits toward the bottom of the measurement area where the coating is thin or nonexistent that might have been missed by 2-D bright-field inspection or single-point thickness measurements alone. Acceptable ranges for the average thickness, standard deviation, and maximum/minimum acceptable levels can all be evaluated on the stent to screen for proper drug coverage.
Figure 3. A white-light interferogram of a coated part shows top and bottom surfaces as well as coating thickness. Pits in the coating can be seen in the lower half of the image.
Arterial stenting constitutes a more than $3.5 billion dollar industry, with millions of devices implanted each year. Inspection of these devices must be rapid, accurate, and nondestructive. Hardware and software must be flexible enough to handle stents of various designs and dimensions, and inspection must be performed not only on the outside surface of the stents but on the inside to ensure proper quality. Only optical metrology techniques offer the required combination of flexibility, speed, and high-confidence results. 3-D surface and film-thickness-measurement techniques provide a more accurate, complete picture of the stent surface and the drug-coating coverage than traditional bright-field imaging and point-wise film thickness techniques. These improvements promise higher quality and lower cost as manufacturers refine their processes based on the metrology feedback, which, in turn, will hopefully enable even greater use of these devices to improve the health of patients worldwide. oe
Erik Novak is director of R&D at Veeco Instruments Inc., Tucson, AZ.