Biodegradable metals have been extensively studied due to their potential use as temporary biomedical devices, on non-load bearing applications. These types of implants are requested to function for the healing period, and should degrade after the tissue heals. A balance between mechanical properties requested at the initial stage of implantation and the degradation rate is required. The use of temporary biodegradable implants avoids a second surgery for the removal of the device, which brings high benefits to the patients and avoids high societal costs. Among the biodegradable metals, iron as a biodegradable metal has increased attention over the last few years, especially with the incorporation of additive manufacturing processes to obtain tailored geometries of porous structures, which give rise to higher corrosion rates. Withal by mimic natural bone hierarchical porosity, the mechanical properties of obtained structures tend to equalize that of human bone. This review article presents some of the most important works in the field of iron and porous iron. Fabrication techniques for porous iron are tackled, including conventional and new methods highlighting the unparalleled opportunities given by additive manufacturing. A comparison among the several methods is taken. The effects of the design and the alloying elements on the mechanical properties are also revised. Iron alloys with antibacterial properties are analyzed, as well as the biodegradation behavior and biocompatibility of iron. Although is necessary for further in vivo research, iron is presenting satisfactory results for upcoming biomedical applications, as orthopaedic temporary scaffolds and coronary stents.
A new generation of biodegradable metal alloys with a porous structure has been receiving growing attention as temporary bone scaffolds for tissue regeneration. The mechanical response of the scaffolds depends upon several factors including the properties of the metal itself, the amount of porosity, the geometrical topology and the immersion conditions. The purpose of this study is to evaluate the degradation behaviour and the mechanical properties of porous iron samples with porosities in the range of 20–30%. Besides the amount of porosity, the effect of topology was evaluated with the study of different arrangement of pores, as well as pore shapes. The specimens were subjected to chemical degradation by immersion of the iron samples in body fluid simulation conditions. The mechanical properties of the samples prior and after the degradation process were assessed by three-point bending tests. Numerical simulations were carried out and the results were compared with the experimental results. The degradation operated by body fluids tends to reduce the mechanical properties. In comparison with the compact structures, porous structures exhibit lower mechanical strength, but still with reasonable values for the use in temporary implants, which also allows reducing the stress shielding effect, keeping the biodegradable advantages. The present work also confirms that the topological design has a strong influence on the mechanical properties of the specimens and on the biodegradation behaviour.
Biodegradable metals have gained attention in the field of bone repair, to be used as temporary bone implants. Among those metals, iron shows good biocompatibility properties, but has high stiffness when compared to bone and exhibits a low degradation rate. There are several approaches that may be applied to overcome those disadvantages. Porous materials have become important in the design of bone substitutes, since the porosity allows for a decrease in strength and for an increase in the degradation rate, due to their high surface areas. The aim of this work is to develop iron porous structures that lead to a mechanical and corrosion performance adequate for temporary implants. Three types of structures, with different relative densities and geometries, were studied: porous graded, cellular graded truss-lattices and a sort of random distribution of pores. The mechanical properties were evaluated through a finite-element analysis using the software NX Nastran. The degradation behaviour of the iron porous samples in a simulated body fluid environment was simulated using the software COMSOL. Results show that both mechanical and corrosion properties depend on the relative density and on the arrangement of pores. Moreover, structures with low relative density exhibit compressive strength values similar to the ones of human trabecular bone, showing degradation rates in the range established for ideal bone substitutes. This means that it is possible to match the iron properties to the ones required for biodegradable devices, by choosing adequate porous arrangements that tailor the mechanical and the corrosion behaviour.
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