Development of biodegradable magnesium alloy stents with coating

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.