Study of the Biocompatibility and Corrosion Resistance of Hydroxyapatite Sol-Gel Thin Coatings on Ti6Al4V Alloy
Authors: Jiménez-Morales Antonia , El hadad Amir A. , Peón Eduardo , García-Galván Federico R. , Barranco Violeta , Galván Juan Carlos 
Published: February 2020

The aim of this work was to prepare hydroxyapatite thin films (HAp) by a sol-gel method and to study the effect of thermal treatment temperature upon the biocompatibility and corrosion protection of these coatings on Ti6Al4V alloy. The HAp sol was obtained by hydrolysis and condensation of a mixture of triethyl phosphite and calcium nitrate tetrahydrate in stoichiometric amounts (i.e. maintaining a Ca/P ratio of 1.67) adapting a water-based route.
The crystallite size and degree of crystallinity of the HAp sol-gel derived coatings strongly depends on the thermal treatment applied. Based on this knowledge, the HAp sol-gel films were annealing in air during 2h at 400ºC, 600ºC, 800ºC and 1200ºC, respectively. As an alternative to conventional thermal treatments, a set of samples was prepared by applying a modification of the Controlled Rate Thermal Analysis (CRTA) developed by Rouquerol, with the aim of obtaining HAp ceramic thin films with controlled textural characteristics at lower temperatures. The basis of this thermal treatment is to control temperature and pressure system, keeping constant the decomposition speed.
The application of Thermogravimetric/Differential Thermal Analyses (TG/DTA) and X-ray Diffraction (XRD) has provided valuable information about the phase transformation, mass loss, identification of the phases developed, crystallite size and degree of crystallinity of the obtained HAp powders. Fourier Transformer Infrared Spectroscopy (FTIR) has been utilized for studying the functional groups within the prepared structures. The surface morphology and nature of the prepared HAp coatings was studied by Scanning Electron Microscopy (SEM) with energy dispersive spectroscopy (EDS). The specific surface (BET) and porosity of the material were determined starting from isotherms of adsorption for N2 at a temperature of 77 K.
The corrosion protection behaviour of the HAp-coatings/Ti6Al4V in Kokubo’s Simulated Body Fluid (SBF) has been studied by Electrochemical Impedance Spectroscopy (EIS). Inductively Coupled Plasma (ICP) has been used to detect the concentration of Ca2+ and PO4
3- ions released and re-precipitated upon the HAp coatings submitted to soaking test in SBF. The quantification of the in vitro cytotoxicity of the HAp sol-gel coatings was made with human osteoblast cultures and a tetrazolium dye (MTT) based assay (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide)). Finally the analysis of the adhesion and cellular proliferation on the surface of the materials was also carried out. In these last studies Alamar Blue assays and human osteoblast cultures have been used.
In summary hydroxyapatite nano-crystalline coatings on Ti6Al4V alloy have been obtained by a water-based sol-gel route. The crystallite size and degree of crystallinity of the HAp sol-gel derived coatings strongly depends on the thermal treatment applied. HAp films have been successfully obtained by annealing sol-gel-derived coatings in air during 2h at 600ºC and 800ºC. As an alternative to these conventional thermal treatments it has been possible to crystallize pure hydroxyapatite at 100°C of temperature of control for the pressure and 300°C as maximum temperature by applying controlled rate thermal analysis (CRTA) conditions. The obtained results have shown that the prepared coatings are nanocrystalline HAp with little deviations from that present in the human bone. All these HAp coating/Ti6Al4V systems have shown to have good bioactivity upon immersion in SBF, good biocompatibility and corrosion protection properties.
Acknowledges

The authors would also like to acknowledge the Ministerio de Economía y Competitividad of Spain (Projects MAT2012-38541-C02-02 and MAT2015-65445-C2-1-R) and Comunidad de Madrid (Project S2013/MIT-2862-MULTIMAT-CHALLENGE) for funding this research.