ReviewThe evolution of cardiovascular stent materials and surfaces in response to clinical drivers: A review
Introduction
The treatment of coronary and peripheral artery disease using metallic stents has been one of the most revolutionary and most rapidly adopted medical interventions of our time. During early development much of the investigation and debate revolved around stent design, including assessment of different materials and surface treatments. In recent years, the introduction of drug-eluting stents has seen the debate, and sometimes controversy, shift to the merits of one pharmacological agent or carrier over another. The current most significant issue is concern about increased risks of late stent thrombosis when using drug-eluting stents [1], [2]. However, development of stent materials and non-pharmacological coatings has continued steadily. In fact, as we move forward, there are several reasons why the development focus is likely to return to bare metal stent technologies, including materials and coatings. This review describes some of the clinical and biological factors that drive stent material selection and presents the evolution of material developments that has resulted and the direction in which these developments may go in the future.
Section snippets
Higher-strength materials
Early in stent development, key characteristics were the ease with which a device could be tracked through to the target vessel and then cross through lesions. These features were significantly affected by strut thickness, with thinner struts leading to more flexible devices and reduced cross-sectional profiles. There was also a hypothesis that thinner struts would lead to reduced restenosis rates. However, it was not until the ground-breaking ISAR-STEREO clinical trial results were released in
Alternative inorganic coatings for improved vascular compatibility
In parallel with the developments in material strength and radio-opacity, increasing attention was also being given to optimizing the stent surface to improve compatibility with its vascular environment. While no strong scientific evidence existed, it was believed that surface optimization could help reduce restenosis rates for bare metal stents below the typical levels of 20–30%. A wide variety of surface modifications and inorganic coatings were explored, targeting a range of objectives such
Alternative stent materials for improved vascular compatibility
While the majority of efforts to improve vascular compatibility have naturally involved coatings, a brief mention needs to be given to work involving the development of new platform stent materials. Such material developments have indeed been fewer than the efforts to increase strength and radio-opacity, but this is understandable given the more tentative nature of any potential benefit and the investment already going into higher-strength and radio-opacity needs. Tantalum was explored early
Biomimetic phosphorylcholine-based coatings
While inorganic coatings were proving to have limited success in improving vascular response, polymeric surfaces were also being explored. Among the most interesting approach in this respect was the use of phosphorylcholine (PC)-based coatings to mimic the phospholipids on the outer surfaces of red blood cells, thereby aiming to provide a highly compatible implant surface [46]. Several studies and registries were performed with the BiodivYsio™ stent (Abbott Laboratories, Abbott Park, IL) and
New surfaces for direct loading of drugs
As mentioned earlier, the increased risk of late thrombosis is one of the biggest challenges facing current DES technology. It has been well demonstrated that such thrombosis is most frequently associated with poor endothelialization of stent struts [60], [61]. The exact cause of poor endothelialization has not been established, but, as indicated, the potential for the polymeric drug carriers to induce a local inflammatory reaction is considered to be one of the factors [62]. There is therefore
Stent materials for alternative imaging compatibility
This topic has already been partly addressed, at the start of this review, where radio-opacity requirements for X-ray fluoroscopy were described. For the foreseeable future, X-ray fluoroscopy will continue to be used during stent implantation to monitor location and deployment. However, for screening and follow-up procedures, alternative imaging technologies have been introduced in recent years and these are seeing increased utilization. Computed tomography (CT) has seen rapid take up,
Biodegradable stent materials – the need to disappear …
Many arguments have been put forward on the potential benefits of having the stent removed once its job is done. Most obvious amongst these is of course the fact that the stent is indeed a foreign object within the vessel and its presence is associated with the potential for inflammatory reactions, progressive neointima development, damaged endothelium and associated thrombosis risks. In addition, problems with blockage of side-branches would be reduced and difficulties with overhang at ostial
Concluding remarks
This review has covered a wide range of material science and engineering developments which have been driven by clinical needs within the cardiovascular stent field. New alloys have been developed specifically for stent applications and existing alloys have been leveraged from other fields. Surface coatings have been developed for objectives ranging from acting as a metal ion barrier through to being a carrier for storage and elution of drugs. Some of the developments have failed to achieve
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