Abstract

Biodegradable stents are attracting the attention of many researchers in biomedical and materials research fields since they can absolve their specific function for the expected period of time and then gradually disappear. This feature allows avoiding the risk of long-term complications such as restenosis or mechanical instability of the device when the vessel grows in size in pediatric patients. Up to now biodegradable stents made of polymers or magnesium alloys have been proposed. However, both the solutions have limitations. The polymers have low mechanical properties, which lead to devices that cannot withstand the natural contraction of the blood vessel: the restenosis appears just after the implant, and can be ascribed to the compliance of the stent. The magnesium alloys have much higher mechanical properties, but they dissolve too fast in the human body. In this work we present some results of an ongoing study aiming to the development of biodegradable stents made of a magnesium alloy that is coated with a polymer having a high corrosion resistance. The mechanical action on the blood vessel is given by the magnesium stent for the desired period, being the stent protected against fast corrosion by the coating. The coating will dissolve in a longer term, thus delaying the exposition of the magnesium stent to the corrosive environment. We dealt with the problem exploiting the potentialities of a combined approach of experimental and computational methods (both standard and ad-hoc developed) for designing magnesium alloy, coating and scaffold geometry from different points of views. Our study required the following steps: i) selection of a Mg alloy suitable for stent production, having sufficient strength and elongation capability; ii) computational optimization of the stent geometry to minimize stress and strain after stent deployment, improve scaffolding ability and corrosion resistance; iii) development of a numerical model for studying stent degradation to support the selection of the best geometry; iv) optimization of the alloy microstructure and production of Mg alloy tubes for stent manufacturing; v) set up, in terms of laser cut and surface finishing, of the procedure to manufacture magnesium stents; vi) selection of a coating able to assure enough corrosion resistance and computational evaluation of the coating adhesion. In the paper the multi-disciplinary approach used to go through the steps above is summarized. The obtained results suggest that developed methodology is effective at designing innovative biomedical devices.

Highlights

  • INTRODUCTIONI n the last years stent implantations (weather coronary stent or not) have steadily grown both in terms of number and of variety of implantations (type of blood vessels involved)

  • I n the last years stent implantations have steadily grown both in terms of number and of variety of implantations

  • They prevent late vessel adaptive or expansive remodeling, and hinder surgical re-vascularization. For this reason, starting from the clinical consideration that vessel scaffolding is only required transiently [3], a new generation of stents is under study [4]. It consists of bioresorbable scaffolds (BRS): the stent should remain in the body for the time required to support the vessel tissues, it "vanishes" as it is dissolved in the human body

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Summary

INTRODUCTION

I n the last years stent implantations (weather coronary stent or not) have steadily grown both in terms of number and of variety of implantations (type of blood vessels involved). During the procedure finite element (FE) analyses were performed on two 2D models (same design but different materials) transferred from the parametric CAD: a vertical displacement was applied to the end of the curved part of the strut to simulate the expansion of the three-dimensional (3D) stent model to an outer diameter of 3.0 mm. The two main optimization objectives for a magnesium alloy stent are minimizing maximum principal strain, to avoid fracture, and maximizing mass or strut width, to extend the corrosion time and provide adequate scaffolding. They are contradictory because more material may increase the strain during expansion. The original design broke at the lower displacement than the optimized design and in the locations corresponding to stress concentrated area in the simulations, showing that the optimized design was safer than the original one during expansion (Fig. 4)

A DAMAGE MODEL FOR DESCRIBING STENT DEGRADATION
Findings
CONCLUSIONS

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