Biomimetic Hydroxyapatite Deposition on Titanium Oxide Surfaces for Biomedical Application

Titanium is widely used as material for permanent implants in orthopedic and dental applications. It is well known that Ti shows a mechanically stable interface towards bone (osseointegration). The good biological properties are due to the beneficial properties of the native oxide (TiO2) that forms on Ti when exposed to oxygen. The native titanium oxide on Ti is usually amorphous and very thin, 2–7 nm [1-3]. In addition to being stable in the physiological environment, titanium oxides increase calcium ion interactions, which are important for protein and subsequent osteoblast adhesion [4]. Enhanced bone bonding can be achieved with bioactive materials that form a stable unit with bone through a spontaneous formation of hydroxyapatite (HA) on their surface. The biomineralized HA layer acts as a bonding layer to the bone and integration at the atomic/molecular scale can develop. For this reason HA is proposed as a suitable coating material to provide stronger early fixation of uncemented prostheses. Although hydroxyapatite coatings on implants showed long-term survival [5], there are concerns about their reliability under loads. Possible ways to overcome this lack of mechanical stability could be by reinforcing the HA with metal oxides such as zirconia and alumina [6]. Apatites, as well as other calcium phosphates (CaPs), can occur in different phases, summarized in Table 1 [7]. Most of them have been studied as biomaterials. The HA naturally occurring in bone is a multi-substituted calcium phosphate, including traces of CO32, F-, Mg2+, Sr2+, Si4+, Zn2+, Li+ etc [8, 9]. These ionic substitutions are considered to play an important role for the formation and properties of bone. Hydroxyapatite coatings can be produced by different methods [10-19]. Early attempts used plasma spraying, which however resulted in coatings with adhesion problems. Attempts have also been made with physical vapour deposition (PVD) techniques. Both of these methods suffer from the drawback that they are line-of-sight methods, which means that coating of complex implant geometries is technically difficult. The biomimetic way to