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International audienceIn order to reinforce the clinical applications of hydroxyapatite (HAP) sol–gel coatings deposited onto 316 L stainless steel, we suggest the introduction of an intermediate thin layer of titania (TiO2) on the substrate. The titania sub-layer is introduced in order to improve both the corrosion resistance and the mechanical properties of the HAP/316 L stainless steel coated system. The two coatings, HAP and TiO2, were studied separately and afterwards, compared with the bi-layered coating. A film without any cracks is obtained under the optimum conditions in terms of annealing temperature, dipping rate and aging effect. Microstructural, morphological and profilometry analysis revealed the non-stoichiometric carbonated porous nature of the hydroxyapatite coatings, which were obtained after annealing at 500 °C during 60 min in the atmosphere. The obtained TiO2 coatings exhibit a dense and uniform surface. Addition of TiO2 as sub-layer of the HAP coating tends to increase the homogeneity and the crystallinity rate as compared to the HAP one.The mechanical properties, i.e. hardness and elastic modulus, are determined by means of nanoindentation experiments and the adhesion between the coating and substrate is estimated by scratch tests. The corrosion behavior is evaluated by potentiodynamic cyclic voltammetry tests. As a main result, the values of the elastic modulus and hardness, respectively of 30 GPa and 2.5 GPa, are relatively high for the HAP–TiO2 bilayer coating. This result allows the use of such coated material as a replacement material for hard tissues. The adhesion strength increased from 2925 mN up to 6430 mN after the addition of the TiO2 intermediate film. According to the Tafel's analysis, the 316 L stainless steel specimens coated with both HAP and titania layers (ECorr = − 234 mV, lCorr = 0.089 μA cm− 2) present a better resistance than the HAP-coated specimens (ECorr = − 460 mV, lCorr = 0.860 μA cm− 2)

Titania-Hydroxyapatite (TiO/HAP) reinforced coatings are proposed to enhance the bioactivity and corrosion resistance of 316L stainless steel (316L SS). Herein, spin- and dip-coating sol-gel processes were investigated to construct two kinds of coatings: TiO/HAP composite and TiO/HAP bilayer. Physicochemical characterization highlighted the bioactivity response of the TiO/HAP composite once incubated in physiological conditions for 7days whereas the TiO/HAP bilayer showed instability and dissolution. Biological analysis revealed a failure in human stem cells adhesion on TiO/HAP bilayer whereas on TiO/HAP composite the presence of polygonal shaped cells, possessing good behaviour attested a good biocompatibility of the composite coating. Finally, TiO/HAP composite with hardness up to 0.6GPa and elastic modulus up to 18GPa, showed an increased corrosion resistance of 316L SS. In conclusion, the user-friendly sol-gel processes led to bioactive TiO/HAP composite buildup suitable for biomedical applications.

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We report the synthesis of hydroxyapatite (HAP) powder from waste mussel shells (decomposed to CaO) and phosphoric acid at room temperature without pH control. The powder synthesized was utilized for cadmium removal from aqueous solutions using the batch technique. The effects of solution pH, adsorbent dose; initial Cd2+ concentration, contact time, and temperatures were examined. Furthermore, the adsorption process revealed a pseudo-second-order reaction model and the Langmuir isotherm is the best-fit model to predict the experimental data and adsorption capacity was found to be 62.5 mg/g. Thermodynamic analysis revealed that because of the negative values of ΔGo and the positive value of ΔHo, the adsorption process was spontaneous and endothermic. Cadmium immobilization occurs through a two step mechanism: rapid ion exchange followed by partial dissolution of hydroxapatite and precipitation of cadmium containing hydroxyapatite.