A two-step approach to surface functionalize Ti dental implants

For enhancing the osseointegration aptitude of Ti dental implant, titanium oxide (TiO2) - hydroxyapatite (HA) bi-layered coatings were deposited by means of a two-step process on commercial-grade-IV Ti substrates with different pristine morphologies and topographies (machined, sandblasted, and sandblasted/acid etched). Firstly, dense, compact and crystalline titania was deposited as an inter-layer via Low Pressure Metal-Organic Chemical Vapor Deposition. Then a suitably discontinuous HA upper layer was deposited by means of spray pyrolysis. The thermal treatment at 600°C was assessed as the most suitable, both for material features and for bioactivity. The novelty of this work is that this synergic two-step approach allows the co-existence of both TiO2 and HA ceramics on the implant surface, in order to increase the surface bioactivity and improve the short(HA)- and long(TiO2)-term implant service-life. The relationship between material processing and functional properties was investigated and evaluated. The electrochemical and tribocorrosive behavior (in artificial saliva) of the functionalized surfaces as well as their metallic ion release (in lactic acid) appeared improved with respect to the uncoated ones. The (nano)hardness of the coated materials was higher than that of bare substrates. In agreement with industrial practice, HA-coating adhesion was qualitatively assessed by tape test. As expected, different results were obtained for different morphologies of substrate surfaces: machined surfaces showed reduced HA retention, while HA coatings better adhered to sandblasted and sandblasted/acid etched substrates. Useful information was obtained by wettability analysis. Freshly prepared surfaces always showed hydrophilicity, and even superhydrophilicity in the case of pure titania coatings. Exposure to air induced gradual wettability decreasing due to atmospheric hydrocarbon contamination. To maintain high-energy surfaces, two industrial-scalable strategies were positively set-up, either storing the samples in distilled water or radiating the contaminated samples by UV. Acellular in-vitro bioactivity was evaluated testing the growth rate of bone-like apatite after immersion in Dulbecco’s Phosphate Buffer Saline (DPBS) solution at 37°C. The results showed that a bone-like apatite layer was effectively formed on the TiO2 – HA functionalized substrates, giving significant results just after 30 minutes incubation in DPBS. Higher HA nucleation rate was, furthermore, detected on the solely TiO2 coated samples when compared to bare Ti substrates, confirming their improved bioactivity. Finally, according to ISO 10993-5:2009, cell viability confirmed the safety of the samples in terms of cytotoxicity.