ReviewCoronary stents: A materials perspective
Introduction
Percutaneous transluminal coronary angioplasty (PTCA) is an invasive procedure performed to reduce blockages in coronary arteries [1], [2]. However, restenosis follows PTCA in 30–40% of coronary lesions within 6 months [3], [4]. Although providing intra-arterial support with bare metal stents (BMS) dramatically improves the angiographic and clinical outcome of patients to a restenosis rate of 20–30% [3], [4], in-stent restenosis still remains a major limitation for this approach with exaggerated intimal hyperplasia [5]. The biology of restenosis in stents includes plaque redistribution, thrombosis, and neointimal hyperplasia [6]. The basic mechanisms [7], [8], [9] underlying thrombus formation and neointimal muscle cell proliferation, followed by extracellular expansion are understood to some extent, but the basic biology of restenosis still remains an active area of research. As a result of the inadequacies of BMS, different kinds of materials, designs, and techniques have been explored to further optimize stent design. Coronary stents developed to date can be grouped in four categories: bare metallic stents, coated metallic stents, biodegradable stents and drug-eluting stents (DES). The advent of DES, which release drugs such as sirolimus and paclitaxel for localized delivery, is a major advancement in the evolution of stents. However, there is a risk of late stent thrombosis (LST) associated with DES [10], [11].
This review evaluates the pros and cons of choosing different materials for the manufacture of coronary stents. The physical properties of each material that are relevant for this application are discussed. The influence of a material's surface characteristics on the biology of restenosis will be discussed as well. A variety of coating materials are commonly used in an attempt to improve the performance of stents; including inorganic materials, polymers, endothelial cells, and porous ceramics. The role of these different types of coatings is described in detail. The materials and the coating techniques used in commercially available DES are described. A list of ideal characteristics for coronary stents and the materials and processes that best meet these requirements are tabulated in the concluding section. The physical design of a stent, another important parameter, is not covered here as the discussion is confined to biomaterials.
Section snippets
Metallic stents
Balloon expandable stents should have the ability to undergo plastic deformation and then maintain the required size once deployed [12]. Self-expanding stents, on the other hand, should have sufficient elasticity to be compressed for delivery and then expanding in the target area [12]. The characteristics of an ideal stent have been described in numerous reviews [13], [14], [15]. In general, it should have (1) low profile—ability to be crimped on the balloon catheter supported by a guide wire;
Surface characteristics
Surface characteristics of a stent material, which influence thrombosis and neointimal hyperplasia, include surface energy, surface texture, surface potential, and the stability of the surface oxide layer [79], [80]. In many circumstances a combination of one or more of these listed factors predicts the outcome. The surface properties of a material may depend on the surface treatment of the material. For example, microblasting produced a rough Ta surface with particle contaminants [81].
Rationale for coatings
Since the basic mechanisms underlying the interaction between a metal and tissue/blood are still not completely understood, the biocompatibility and the hemocompatibility of metallic stents still remains an issue. Thrombosis and neointimal hyperplasia were commonly reported among bare metallic stents [103], [104], [105], [106], [107]. Coating the metallic surface with other materials to alter its surface characteristics without interfering with the bulk properties of the metal stent has been
Polymers
Polymers used for coating stents can be broadly classified into biostable (non-biodegrable) polymers, biodegradable polymers, copolymers, and biological polymers. Several polymers with previous medical or dental applications have been used for coating stents or for making the entire stent. Although a wide range of polymers have been used to coat the stent, only a few, like polyethylene terepthalate (PET), poly-l-lactic acid (PLLA), and poly-l-glycolic acid (PLGA) have been tested as a lone
Rationale for DES
Endothelial and smooth muscle cell damage, unavoidable in PTCA and stent placement, is a cause of restenosis [9]. The optimization of the architecture and mechanical characteristics of stents has lead to a decrease in restenosis but using drug delivery platforms remains a promising way to further reduce restenosis. The main reason for the failure of systemic pharmacological therapy is the inability to deliver an adequate drug dose at the site of injury [199]. Earlier approaches for local drug
Conclusion
From a review of the literature it is evident that the material used for making stents has to have appropriate mechanical properties, suitable surface characteristics, excellent haemocompatibility, good biocompatibility, and drug delivery capacity. Every material has its own pros and cons. Table 3 provides a list of materials which posses the ideal for a specific material property (Table 3). It may not be possible for a single material to posses all the desired requirements. So, the success
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