Interactions Between the Physiological Environment and Titanium-Based Implant Materials: From Understanding to Control

Titanium and titanium alloys are widely used in different biomedical applications owing to their high biocompatibility, high corrosion resistance, good mechanical properties, and good osseointegration ability. Titanium and its alloys rapidly form a surface oxide layer in air and aqueous environments. This passive and thin (a few nanometers) surface oxide hinders active corrosion and ensures a low metal ion release, enhancing biocompatibility. Compared to that of other biomedical alloys, this surface oxide is exceptionally resistant to chemical attack by halides, primarily chlorides; the presence of fluorides can, in some cases, result in localized corrosion of titanium and its alloys. However, the combination of proteins, inflammatory conditions and bacteria, which for instance generate hydrogen peroxide, can result in a reduction of the corrosion resistance of titanium-based materials. Titanium and its alloying elements, such as aluminum and vanadium, can then be released as ions, which might trigger an immune system response and reduce biocompatibility. Several surface modifications have been proposed in order to improve the bone-bonding ability of titanium and its alloys, facilitate the healing process, and enhance the success of the implant with a decreased risk of micromotions. Moreover, antimicrobial ions/nanoparticles can be added to the surface to reduce the infection risk. Surface modification of titanium (e.g., with artificially grown, micrometer-thick, titanium oxide layers) can significantly increase the corrosion resistance under critical conditions (e.g., inflammatory response and infection); however, the surfaces are not completely inert and the effect of metal ion/nanoparticle release should be carefully taken into account. This chapter reviews and discusses the current strategies for modifying and controlling the surface of titanium-based implant materials, with particular focus on corrosion resistance, bone integration, inflammatory and infection control, and interactions with the physiological environment.