Full Length ArticleNovel fluoridated hydroxyapatite/MAO composite coating on AZ31B magnesium alloy for biomedical application
Graphical abstract
Introduction
Because of their unique biodegradable and biocompatible properties, magnesium alloys have gained wide attention and interest from researchers in biomedical application. Nowadays, magnesium alloys have been successfully used as fracture internal fixation devices in some low load-bearing sites [1], [2], [3]. However, rapid corrosion rate in body fluid could result in many adverse effects [4], [5], including rapid decreased mechanical strength, serious hydrogen evolution and local alkalization, and thus affects their biomedical performance. Surface modification by bioceramic coatings is commonly used to prevent direct contact of them with body fluid and the corrosion resistance as well as the biocompatibility of magnesium alloys was also greatly enhanced [6].
Hydroxyapatite [Ca10(PO4)6(OH)2, HAp] is commonly used as a protective coating on magnesium alloys due to its excellent biocompatibility and bioactivity [7], [8], [9]. However, the different composition between pure HAp and human bone as well as the long-term stability problems in vivo can result in implant failure [10]. The incorporation of ions such as Sr2+, Mg2+, CO32− and F− in HAp has been studied and applied to enhance the corrosion resistance and osseointegration of pure HAp coating [11], [12], [13], [14], [15]. As an essential trace element of human teeth and bones, fluoride ion is more often to be incorporated in HAp to apply to dentistry and orthopedics, presenting a lower solubility in acidic medium and better bioactivity [16], [17], [18]. A recent study by Kokubo et al. has proved that the cell activity and cell proliferation on the FHAp-coated magnesium alloy surface was significantly improved due to the doping of F− [19]. FHAp coating also demonstrated improved resistance to biodegradation. Moreover, the size and shape of HAp crystals can be regulated through F− doping, and bilayer arrays of nanoneedles with micron/nano-topography can be constructed on substrate via a microwave aqueous approach [20].
Nevertheless, a single FHAp coating usually shows a low bonding strength with substrate and has long-term stability problems [21], [22], [23]. For this reason, many researchers have attempted to combine HAp coating with other coatings [24], [25], [26], [27], [28]. On the other hand, with distinguished wear resistance and high adhesion to substrates, micro-arc oxidation (MAO) is a significant technology for forming ceramic coatings on magnesium alloy [29], [30], [31], [32]. In order to further improve its biocompatibility and bioactivity, MAO is usually combined with other bioceramic coatings. Therefore, constructing a composite coating with MAO coating as an inner layer and HAp coating as an outer layer is an excellent method for enhancing anticorrosion property, bioactivity, and bonding strength of magnesium alloys. For example, Niu et al. fabricated MAO/HAp composite coating on AZ31 substrate via a sol-gel technique [33]. The results showed that the bonding strength of MAO/HAp interface was stronger than that of the AZ31/HAp interfaces due to the mechanical interlocking between HAp and porous MAO. Liu et al. prepared a Ca-Ph coating on micro-arc oxidized pure magnesium by deposition method, the composite coating obviously promoted the corrosion resistance and bonelike apatites formation of magnesium [34].
In this study, a composite ceramic coating composed of MAO coating as an inner layer and FHAp coating as an outer layer was synthesized by hydrothermal method on AZ31B magnesium alloy. AS far as we know, no research about the fabrication of FHAp/MAO composite coating on magnesium alloy has been reported before. The influence of F− concentration on phase composition as well as microstructure of the composite coating were determined in this study. The electrochemical behavior, degradation process as well as the cell response of the as-prepared composite coating were also investigated.
Section snippets
Pretreatment of substrate
Commercial AZ31B magnesium alloy sheets (chemical compositions in wt. %: Al 3.15, Zn 1.23, Mn 0.38, and balanced Mg) were used as substrate with dimension of 10 mm × 10 mm × 2 mm. Samples were mechanically polished from 800, 1200, to 2000 grits by SiC paper, ultrasonically washed in acetone and distilled water for 15 min, respectively, and then dried in cold air.
Preparation of FHAp/MAO composite coatings
The MAO process of the substrate was carried out in an alkaline electrolyte containing NaSiO3 7 g/L, Na4P2O7·10H2O 3 g/L, and NaOH
Surface morphologies of the coatings
Fig. 1 displays the surface morphologies of MAO, Ca-P/MAO and different FHAp/MAO composite coatings on AZ31B Mg alloy. The surface of MAO coating (Fig. 1a) consists of considerable amount of micropores. Fig. 1b shows that the CaP based coating is composed of many fibers with different length. And there are a lot of pores between the fibers. With the addition of F−, some spherical aggregates begin to appear in the coating (Fig. 1c). When the doping ratio of F/Ca is 0.1, the coating is made up
Conclusions
In this study, FHAp/MAO composite coating was prepared on AZ31B magnesium alloy through micro-oxide method combined with hydrothermal method. The doping of F− into HAp can change the morphology of the composite coating. When the molar ratio of F/Ca was 0.2, the FHAp/MAO composite coating was composed entirely of highly radially arrayed nanords and showed uniform and dense morphology. In the case of acidic hydrothermal solution, the addition of F− can induce the direct formation of FHAp in the
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (Project No. 51602169). The authors declare that there is no conflict of interests regarding the publication of this article.
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