Micrometre-and nanometre-scale surface structuring with ordered topography features may dramatically enhance orthopaedic implant integration. In this study we utilised a previously optimised micron metal injection moulding (µ-MIM) process to produce medical grade stainless steel surfaces bearing micrometre scale, protruding, hemispheres of controlled dimensions and spatial distribution. Additionally, the structured surfaces were characterised by the presence of submicrometre surface roughness resulting from metal grain boundary formation. Following cytocompatibility (cytotoxicity) evaluation using 3T3 mouse fibroblast cell line, the effect on primary human cell functionality was assessed focusing on cell attachment, shape and cytoskeleton conformation. In this respect, and by day 7 in culture, significant increase in focal adhesion size was associated with the microstructured surfaces compared to the planar control. The morphological conformation of the seeded cells, as revealed by fluorescence cytoskeleton labelling, also appeared to be guided in the vertical dimension between the hemisphere bodies. Quantitative evaluation of this guidance took place using live cytoplasm fluorescence labelling and image morphometry analysis utilising both, compactness and elongation shape descriptors. Significant increase in cell compactness was associated with the hemisphere arrays indicating collective increase in focused cell attachment to the hemisphere bodies across the entire cell population. Micrometre-scale hemisphere array patterns have therefore influenced cell attachment and conformation. Such influence may potentially aid in enhancing key cellular events such as, for example, neo-osteogenesis on implanted orthopaedic surfaces.

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Very good biocompatibility makes titanium the material of choice for permanent metallic bone implants. Work carried out in Germany and Switzerland shows that -MIM offers a promising route to designed regular micro-sized patterns on implant surfaces that enhance osteointegration..

Metal injection moulding (MIM) is an established technique for serial production of complex, mostly stainless steel parts. However, for other materials, especially for titanium parts, there is still the need for superior purity and enhanced surface quality. This paper describes recent advances at Fraunhofer IFAM to address the challenge of producing fit-for-purpose titanium MIM medical parts. Investigations have shown the effect of using very fine powders and mould surface finish: with polished moulds a very lowsurface roughness, <0.8 m, was obtainable on the sintered part. Chemical and mechanical properties of the sintered parts are determined by complex interactions between sintering conditions, purity, particle size and binder components. Tests on the feasibility of moulding high aspect ratios, wall thicknesses <200 m and special features such as a fine internal threads and hexagon headset structures are also reported

The metal injection moulding technique is already established for serial production of complex parts, mostly from various stainless steels. However, for other materials, especially for titanium parts there is still the need for superior purity and enhanced surface quality. Facing the challenge of obtaining suitable medical titanium MIM parts, advances have been made at Fraunhofer IFAM over the last few years.One strategy to overcome the high risk of carbon up-take was to adjust the sintering program. Very low Argon flow rates, 50 mbar pressure and two hours dwell time at 1350°C were found to be optimal parameters. A cleaning cycle prior to the actual sintering at 1450°C under hydrogen was also found to enhance the results.Another strategy involved the choice of binder components. Stearic acid, which is often used to improve wettability of binder to powder particle, and high polymer content affect the oxygen content of the titanium parts. Low amounts of both are beneficial for high purity parts.Other investigations were performed concerning the surface quality. It was found that the surface roughness of the mould has an effect on the surface roughness of the sintered parts. Although sintered titanium surfaces as such exhibit quite rough surfaces of about 2-3 µm (Ra value) the influence of the surface finish of the mould was detectable. Using very fine powders of only 15 µm mean particle size and a polished mould a very low surface roughness of less than 1.2 µm on the sintered part was obtainable.