The development of polymers and their diverse range of applications is a wide research field. Polymer materials became prevalent in implantable medical devices through processing capabilities in a wide variety of physical and chemical properties, as well as biocompatibility. This article describes how polymer coatings are used in the fabrication of drug-eluting coronary stents and how scanning electron microscopy (SEM) helps analyze the performance of these coatings in great detail.
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Polymers for stents
In the medical field, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open. A wide variety of stents are used for different purposes, from expandable coronary, vascular and biliary stents, to simple plastic stents used to enable the flow of urine between kidney and bladder.
Figure 1: SEM image of a stent
The clinical performance of a drug-eluting coronary stent largely depends on the properties of its polymer coating. While randomized clinical research is usually the standard for evaluating the effectiveness and safety of the coronary stent, the testing of a stent coating with a desktop electron microscope may provide valuable insights into the development of new devices.
The drug is released from the polymer coating within a few weeks or even months. By letting it function in this fashion, it prevents the inflammatory process and the associated re-narrowing, or restenosis, of the coronary artery lumen.Kapustin et al. (Complex application of instrumental analytical methods for detection and characteristics of polymer coating defects in drug-eluting stents, Kapustin et al.,Interventional-Cardiology.1000562 (2017)) argue that their combination of instrumental analytical methods might discover more information about the nature and possible causes of irregularities of the stent coating than SEM, which is conventionally used for this purpose.
One of the critical factors of polymeric coating is that it must withstand all cycles of processing (crimping, sterilization and expansion) without compromising the integrity of the coating. Therefore, in the early stages of development of polymer drug formulations and technologies — and their application within stent manufacturing — a wide range of technologies can predict clinical efficacy and safety of coronary drug-eluting stents and anticipate possible risks of their use.
Detailed analysis of polymer stent coatings
To understand more details about the coating, Kapustin et al. analyzed the elemental composition of the coating. It helped the understanding of the elemental distribution and the uniformity of the polymer layers evaluated across the surface of stents (mapping), as well as the composition of the polymer drug-containing layer through the depth.
Why SEM is a suitable technique for the analysis of polymer coatings
Irregularities in the polymeric coating of stents may result from the incorrect selection of components in the polymer composition. These determine physical-chemical and mechanical properties of the coating, from a non-optimal choice of deposition conditions, as well as from stent processing after deposition.
Any of these inconsistencies can lead to a change in morphology including cracks, wrinkles, the appearance of exposed substrate areas, lapping, craters or bubbles and conversely, coating thinning, fragmentation or delamination. Due to this, a study of texture, morphology and chemical composition of the surface layer of polymer-coated stents — as well as an assessment of the changes in the surface morphology and the stability of the stent coating — is a very important and practical task.
The comparison of various techniques lead Kapustin et al. to the conclusion that SEM is the most informative method for obtaining data on the presence and types of defects in the polymer coating. SEM provides an identification of virtually all types of defects depending on selected resolution.
As a matter of fact, scanning electron microscopy can identify features for an extremely wide range of materials. And these features or materials are often magnified beyond your imagination. For instance, would you have been able to guess that the star-shaped patterns on the surface of the leaf tissue below are just 100 µm in size?
Dr. Jasmin Zahn is an Application Engineer at Phenom-World, the world’s leading supplier of desktop scanning electron microscopes. She is highly engaged in finding out more about the possibilities for Phenom-World products in various applications. In addition, Jasmin is active in sharing best practices with the outside world to encourage them to look outside their standard scope of use and to improve in their work.