Porous metals and metallic foams are presently the focus of very active research and development activities. There are currently around 150 institutions working on metallic foams worldwide, most of them focussing on their manufacture and characterisation. Various companies are developing and producing these materials which are now being used in numerous industrial applications such as lightweight structures, biomedical implants, filters, electrodes, catalysts, and heat exchangers. This review summarizes recent developments on these materials, with particular emphasis on research presented at the latest International Conference on Porous Metals and Metallic Foams (MetFoam 2007).

EditorialPorous metals and metallic foams are presently the focus of very active research and development activities around the world, both at the academic and industrial levels. These materials are used when the combination of metal properties with the characteristics of porous structures provides significant advantages over other types of materials. Porous, cellular and foamed metals are now produced by various companies and used in numerous applications such as light-weight structures, biomedical implants, filters, heat exchangers, sound absorbers, mechanical damping devices, electrodes, sensors and catalyst substrates.

Bioactive glass 45S5 foams were produced using a powder technology process developed by The National Research Council Canada-Industrial Materials Institute. NRC-IMI's proprietary process, combining powder technology and polymer foam technique, allows the production of materials having different structures and properties. It can be used to produce components into various forms, such as fully porous bodies or coatings on solid structures. During foaming, the foaming agent is decomposed and expands the binder-bioactive glass suspension. Then, the binder is burnt out by heating the sample at 500°C and finally the bioactive glass particle network is sintered to consolidate the material. Foams sintered at various temperatures were characterized from a microstructural and mechanical point of view. The foam structure and properties are affected by the sintering temperature when it is varied between 950°C and 1025°C. Foams exhibited open porosity (64%-79%) and pore size (335-530 lm) optimal for bone ingrowth. In all cases, the glass crystallized during sintering and the material was mostly composed of Na 6 Ca 3 Si 6 O 18 and Na 2 Ca 4 (PO 4 ) 2 SiO 4 phases. The mechanical strength increased from 1.7 to 5.5 MPa while the density of the material increased from 0.56 to 0.97 g/cm 3 . P. Colombo-contributing editorManuscript No. 31380.

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Large bone defects are challenging to heal, and often require an osteoconductive and stable support to help the repair of damaged tissue. Bioglass-based scaffolds are particularly promising for this purpose due to their ability to stimulate bone regeneration. However, processing technologies adopted so far do not allow for the synthesis of scaffolds with suitable mechanical properties. Also, conventional sintering processes result in glass de-vitrification, which generates concerns about bioactivity. In this work, we studied the bioactivity and the mechanical properties of Bioglass® based scaffolds, produced via a powder technology inspired process. The scaffolds showed compressive strengths in the range of 5–40 MPa, i.e. in the upper range of values reported so far for these materials, had tunable porosity, in the range between 55 and 77%, and pore sizes that are optimal for bone tissue regeneration (100–500 μm). We immersed the scaffolds in simulated body fluid (SBF) for 28 d and analyzed the evolution of the scaffold mechanical properties and microstructure. Even if, after sintering, partial de-vitrification occurred, immersion in SBF caused ion release and the formation of a Ca-P coating within 2 d, which reached a thickness of 10–15 μm after 28 d. This coating contained both hydroxyapatite and an amorphous background, indicating microstructural amorphization of the base material. Scaffolds retained a good compressive strength and structural integrity also after 28 d of immersion (6 MPa compressive strength). The decrease in mechanical properties was mainly related to the increase in porosity, caused by its dissolution, rather than to the amorphization process and the formation of a Ca-P coating. These results suggest that Bioglass® based scaffolds produced via powder metallurgy-inspired technique are excellent candidates for bone regeneration applications.

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