Bioactive titanium surfaces

Titanium and its alloys are the most widespread materials employed in orthopaedic and dental surgery, because of their good mechanical properties and biocompatibility. In fact titanium has a good fatigue resistance and, at the same time, its elastic modulus is the lowest between metals, so it avoids stress shielding of implants. Moreover titanium surface is naturally covered by an oxide layer preventing corrosion and it is almost inert in contact with biological fluids. Inertness avoids adverse reactions of the body, but at the same time it hampers tissue integration. Several solutions have been proposed, in order to improve titanium bioactivity: bioactive coatings based on hydroxyapatite and calcium phosphates, inorganic surface modifications (alkaline treatments, acid etchings or electrochemical oxidation) and biomolecular surface grafting. A brief commentary of recent research on these topics will be presented. Moreover an innovative treatment will be described. It is focused on inducing a bioactive behaviour on Ti6Al4V. The process includes two steps: the first one is an inorganic modification, inducing inorganic bioactivity on the surface (hydroxyapatite precipitation), while the second one is an organic treatment, promoting a biological response. In particular the first step includes an acid etching, in order to remove the natural oxide layer, and a controlled oxidation in hydrogen peroxide. The second step is the covalent surface grafting of a biomolecule (enzyme alkaline phosphatase (ALP) for instance), in order to stimulate cell response. Results show that the inorganic treatment induces hydroxyapatite precipitation on the titanium surface (it was tested in-vitro by soaking in simulated body fluid (SBF) for two weeks). The oxide layer shows a porous surface texture on a nanoscale and it is quite rich in hydroxyls groups. A thermal treatment could improve the mechanical resistance of this modified layer to the substrate, without altering its bioactive behaviour. The fatigue performances of the material are not modified by the treatment. ALP has been successfully grafted to the modified titanium surface, maintaining its activity. The results of cellular tests will be reported in order to evaluate the biological behaviour of treated samples. Finally it has been observed that there is a synergic mechanism between inorganic and biological bioactivity, one improving the other one.