Brushite/Hydroxyapatite Coatings obtained by galvanic deposition on 316L Stainless Steel

Abstract

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a basic calcium phosphate mineral with chemical composition similar to that of bones and teeth. Owing to this peculiarity HA is a biocompatible material of high medical interest. Unfortunately, it possesses poor mechanical properties, because is brittle, has a low fracture resistance and a poor wear resistance. For these reasons, in the last years the research was been focused on the use of HA as a coating of another biomaterial that acts as support. The best choice is that to use a substrate that must be bionert and mechanically stable, such as 316L stainless steel (316LSS). This is useful for biomedical implants, because of its excellent biocompatibility and good mechanical properties. However, since 316LSS is a bioinert material, it cannot form strong chemical bonds with bones. To improve the osteointegration, it is necessary to cover 316LSS substrates with hydroxyapatite layers. Thus, the use of bio-device based on HA coatings supported on 316LSS allows to exceed the two of principal disadvantages that these two materials present when are used individually: the brittleness of HA and the poor osteointegration of 316LSS. Many methods were developed to deposit HA on bio-inert substrate, including ion beam sputtering, sol–gel, electrophoretic deposition, pulsed laser deposition, plasma spraying and electrochemical deposition. Here, we propose a new fabrication process that permits to obtain brushite and brushite/hydroxyapatite, that is a very simple and low cost. Brushite is dicalcium phosphate dihydrate (BS, CaHPO4·2H2O) that, when placed in simulated body fluid (SBF, knowed as Hank’s solution), it converts into hydroxyapatite. In particular, brushite and brushite/hydroxyapatite coatings were deposited on 316LSS from a solution containing Ca(NO3)2·4H2O and NH4H2PO4 through a displacement reaction based on a galvanic contact between 316LSS and a sheet of zinc that acts as sacrificial anode. Driving force for the cementation reaction arises from the difference in the electrochemical standard potentials of 316LSS and Zn that were immersed in the electrolyte. This process allows to deposit Brushite/Hydroxyapatite coatings without power supply. Samples were obtained varying deposition time (24 hours, 72 h and 108 h) and temperature (6 °C, 25 °C and 50 °C). SEM, EDS, Raman and XRD were used to full characterized the coatings. SEM images showed that the morphology changes as the temperature increases passing by a spherical dispersed structure, for films obtained from a 6 °C and 25 °C, to a tetrahedral and more compact structure, in the case of the film obtained at 50 °C. Through EDS the chemical composition of coatings was identified, calculating the Ca/P ratio, to obtain the stoichiometry, and the Ca/Fe ratio, to estimate the degree of coverage of the substrate. The temperature increase allows to obtain a uniform coating with a Ca/P ratio close to the optimal value (1.67), and a very high value of Ca/Fe ratio that obviously implies the formation of a very thick coating on the substrate. By Raman spectroscopy and X-ray diffraction (XRD) the phases were identified and the characteristic peaks of BS and BS/HA were found. Besides, corrosion (open circuit potential, potentiodynamic polarization and impedence measurement) and biocompatibility (cytotoxicity assays with osteoblastic cell) test were performed. Here, we will show that galvanic deposition is an advantageous method because is able to produce BS and BS/HA coatings that enhance the corrosion and biological properties of the 316LSS substrate.