Biodegradable metals for cardiovascular stent application: interests and new opportunities

Abstract

During the last decade, biodegradable metallic stents have been developed and investigated as alternatives for the currently-used permanent cardiovascular stents. Degradable metallic materials could potentially replace corrosion-resistant metals currently used for stent application as it has been shown that the role of stenting is temporary and limited to a period of 6-12 months after implantation during which arterial remodeling and healing occur. Although corrosion is generally considered as a failure in metallurgy, the corrodibility of certain metals can be an advantage for their application as degradable implants. The candidate materials for such application should have mechanical properties ideally close to those of 316L stainless steel which is the gold standard material for stent application in order to provide mechanical support to diseased arteries. Non-toxicity of the metal itself and its degradation products is another requirement as the material is absorbed by blood and cells. Based on the mentioned requirements, iron-based and magnesium-based alloys have been the investigated candidates for biodegradable stents. This article reviews the recent developments in the design and evaluation of metallic materials for biodegradable stents. It also introduces the new metallurgical processes which could be applied for the production of metallic biodegradable stents and their effect on the properties of the produced metals.

Keywords: biodegradable stents; coronary stents; electroforming; metallurgical processes.

Figure 1

Biodegradable iron stents: (a) NOR-I stent expanded to 3.5 mm diameter and (b) X-ray photograph of iron stent (Biotronik, Erlangen, Germany ) after implantation in porcine coronary artery, adapted from References [16] and [15], respectively.

Figure 2

Lekton Magic coronary stent: (a) non-expanded and (b) expanded, adapted from Reference [16].

Figure 3

Fabrication process of cardiovascular stents.

Figure 4

Fabrication process of Fe-35Mn alloy by powder metallurgy for biodegradable stents, adapted from Reference [20].

Figure 5

(a) Microstructure of PM Fe-35Mn alloy after PM processing and (b) Cross-section of the corroded alloy after dynamic degradation testing, adapted from Reference [21] and [28], respectively.

Figure 6

Microstructure of (a) annealed electroformed iron and (b) Armco^® iron.

Figure 7

Microstructure of iron stents at different processing steps: (a–c) as ground minitube; (d–f) as laser cut minitube; (g–i) final stent; (j) high magnification cross section of as laser cut minitube; and (k) high magnification cross section of final stent.