Biodegradable stents made of pure Mg and AZ91 alloy through SPS sintering

The implantation of stents is an effective procedure to unblock the arteries of patients with serious heart problems. Traditionally, stents are made of inert materials such as stainless steel and titanium alloys. It has been shown that the traditional stents can cause restenosis or thrombosis. In recent years the proposal of biodegradable stents is attracting the interest of the industry and the research, since the stent is mechanically needed only in the first year, eliminating the problems caused by the long duration of the implant. Magnesium (Mg) alloys are of increasing interest because of their engineering properties, including the high strength to density ratio. Recently, they have been also proposed as biomaterials for the production of bioabsorbable stents and for other medical devices due to its harmless effect to human body when compared with other structural materials. In this work, the possibility to produce biodegradable stents made of magnesium starting from the powder is investigated. Pure Mg and the AZ91 Mg powders were used in the present study. Pure Mg powder was sintered by Spark Plasma Sintering at 400 and 470 °C, and the AZ91 powder was sintered at 400 °C without homogenization and at 470 °C after homogenization. The preforms produced by sintering were then submitted to hot compression, rod extrusion and tube extrusion at 330 and 380 °C with different strain rates. During all the process was not possible to obtain recrystallization or grain refinement on pure Mg, and after the tube extrusion it has shown a high brittleness and sever defects, which led to the decision of proceed only with AZ91 alloy. The AZ91 presented good recrystallization in al process, always following the Zener-Hollomon relation. The grain size obtained was as small as 1.5 μm. The AZ91 tube was then submitted to manual machining and laser cutting and it was possible to obtain the stent precursors. The results of the present investigation have demonstrated the suitability of the proposed route for producing Mg-based stents. It is clear, however, that the process has to be further optimized, investigating also the possibility of using different types of powder with a tailored composition.