A simplified in vivo approach for evaluating the bioabsorbable behavior of candidate stent materials.

Metal stents are commonly used to revascularize occluded arteries. A bioabsorbable metal stent that harmlessly erodes away over time may minimize the normal chronic risks associated with permanent implants. However, there is no simple, low-cost method of introducing candidate materials into the arterial environment. Here, we developed a novel experimental model where a biomaterial wire is implanted into a rat artery lumen (simulating bioabsorbable stent blood contact) or artery wall (simulating bioabsorbable stent matrix contact). We use this model to clarify the corrosion mechanism of iron (≥99.5 wt %), which is a candidate bioabsorbable stent material due to its biocompatibility and mechanical strength. We found that iron wire encapsulation within the arterial wall extracellular matrix resulted in substantial biocorrosion by 22 days, with a voluminous corrosion product retained within the vessel wall at 9 months. In contrast, the blood-contacting luminal implant experienced minimal biocorrosion at 9 months. The importance of arterial blood versus arterial wall contact for regulating biocorrosion was confirmed with magnesium wires. We found that magnesium was highly corroded when placed in the arterial wall but was not corroded when exposed to blood in the arterial lumen for 3 weeks. The results demonstrate the capability of the vascular implantation model to conduct rapid in vivo assessments of vascular biomaterial corrosion behavior and to predict long-term biocorrosion behavior from material analyses. The results also highlight the critical role of the arterial environment (blood vs. matrix contact) in directing the corrosion behavior of biodegradable metals.

A platinum-chromium steel for cardiovascular stents.

The desire to reduce the strut thickness of cardiovascular stents has driven the development of a new high strength radiopaque alloy, based on additions of platinum to a chromium-rich iron based matrix. This paper reports on initial development of the alloy and the rationale for selection of the composition. Data is presented for tensile and microstructural characterization, surface oxide analysis, corrosion resistance and endothelial cell response of the alloy. The results demonstrate the solid solution strengthening effect of the platinum, with an average yield strength of 480 MPa achieved. The material surface consists of primarily chromium oxide which contributes to the high corrosion resistance observed. The cell assay result suggests that surfaces of this Pt-enhanced alloy endothelialize in a manner comparable to stainless steel.

Characterization of an NbTaWZr alloy designed for magnetic resonance angiography compatible stents.

The majority of stent materials are not fully compatible with magnetic resonance imaging due to their ferromagnetic or paramagnetic compositions. This leads to image artifact which can obscure clinical data in the vicinity of the stent. An Nb-28Ta-3.5W-1.3Zr alloy has been developed specifically to provide reduced magnetic susceptibility and therefore reduce image artifact. This study reports on initial surface characterization, corrosion behaviour, endothelial cell response and MR image performance. Surface analysis confirms the presence of a niobium oxide with some tantalum oxide also present. Electrochemical corrosion testing demonstrates the oxide to be stable with no evidence of film breakdown. Leaching of metallic ions during a 60-day immersion test shows low levels of release, comparable to cobalt-chromium L605. A short term endothelial cell adhesion study shows that the Nb-28Ta-3.5W-1.3Zr may be similar to stainless steel for supporting cell growth. The MR artifact assessment shows that the material has significantly reduced artifact compared to stainless steel. In summary, results from this initial study show that the Nb-28Ta-3.5W-1.3Zr meets many on the criteria expected of a stent material and that improved MR imaging behaviour is also obtained.

Initial exploration of Ti-Ta, Ti-Ta-Ir and Ti-Ir alloys: Candidate materials for coronary stents.

The objective of this study was to explore titanium alloys with increased elastic modulus and improved radiopacity, with a view to utilizing titanium in balloon-expandable coronary stents. Ti-50Ta, Ti-45Ta-5Ir and Ti-17Ir alloys were prepared using arc-melting techniques. Microstructural and tensile properties were evaluated in solution-treated conditions for each alloy, and also in aged conditions for the Ti-17Ir. An elastic modulus of 128GPa was recorded for the Ti-17Ir alloy and this high value is attributed to the stiff Ti(3)Ir phase present in the eutectoid structure observed. The mechanical properties recorded, in addition to improved radiopacity, make the Ti-17Ir alloy more suitable for stent applications than commercially available titanium materials. Corrosion resistance and biocompatibility have not been assessed but the noble characteristics of iridium suggest that these aspects will be acceptable.

Development of a new niobium-based alloy for vascular stent applications.

This study was performed in order to develop a new stent material that would provide reduced MR image artifact compared to current stent materials. Alloy design rationale is initially presented and following this the development of a Nb-28 Ta-3.5 W-1.3 Zr alloy is described, including the manufacture of stent tubing. Tensile testing of this new alloy showed that it had approximately twice the yield strength of current Nb-1 Zr material with a 25% higher elastic modulus. The new alloy was also confirmed to have suitably low magnetic susceptibility. Mechanical testing of demonstration coronary stents made from the new alloy were shown to have acceptable compression strength and elastic recoil performance. It is concluded that this new Nb-28 Ta-3.5 W-1.3 Zr alloy is a practical candidate stent material for both coronary applications and peripheral uses such as carotid or intracranial stenting, where reduced MR image artifact would be beneficial.