Elsevier

Applied Surface Science

Volume 285, Part B, 15 November 2013, Pages 664-673
Applied Surface Science

Electrophoretic deposition of nanostructured hydroxyapatite coating on AZ91 magnesium alloy implants with different surface treatments

https://doi.org/10.1016/j.apsusc.2013.08.108Get rights and content

Highlights

  • Nano-hydroxyapatite (n-HAp) particles synthesized using a sol–gel process.

  • Stable suspension of n-HAp powders was prepared in methanol medium.

  • Fluoride and MAO pre-treatments were applied on AZ91 samples as intermediate layers.

  • Electrophoretic deposition process was used for coating n-HAp particles.

  • The MAO/n-HAp/AZ91 system is highly prone to be utilized as bio-absorbable implants.

Abstract

Bio-absorbable magnesium (Mg) based alloys have been introduced as innovative orthopedic implants during recent years. It has been specified that rapid degradation of Mg based alloys in physiological environment should be restrained in order to be utilized in orthopedic trauma fixation and vascular intervention. In this developing field of healthcare materials, micro-arc oxidation (MAO), and MgF2 conversion coating were exploited as surface pre-treatment of AZ91 magnesium alloy to generate a nanostructured hydroxyapatite (n-HAp) coating via electrophoretic deposition (EPD) method. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM) techniques were used to characterize the obtained powder and coatings. The potentiodynamic polarization tests were carried out to evaluate the corrosion behavior of the coated and uncoated specimens, and in vitro bioactivity evaluation were performed in simulated body fluid. Results revealed that the MAO/n-HAp coated AZ91 Mg alloy samples with a rough topography and lower corrosion current density leads to a lower Mg degradation rate accompanied by high bioactivity.

Introduction

Metallic biomaterials compose major quota of orthopedic implants, nowadays. Prolonged therapy and possible release of undesirable ions of conventional implants such as stainless steels and cobalt–chromium alloys in body environment has demanded new bio-degradable/bio-absorbable materials [1]. In this developing field of healthcare materials, magnesium alloys are posed as potential bio-degradable/bio-absorbable metallic implants. Although, an injured tissue can consume Mg alloys implants during the healing period, the implants suffer from high corrosion rate (i.e. high mass loss and abundant rate of hydrogen evolution) in body environment [2]. Therefore, further treatments should be fulfilled to increase the corrosion resistance of Mg alloys, and consequently improve the hurt bone response to Mg alloys during post implantation [3]. Producing protective bioceramics coatings on Mg alloys would enhance their biocompatibility and decelerate their degradation rate in physiological environments.

Hydroxyapatite (HAp-Ca10(PO4)6OH2) is the main inorganic component of bones, which its application as a coating provides advantages intimately attributed to its nanoscale structure [4], [5]. Moreover, former investigations revealed that the nano-sized HAp can promote mechanical properties, bone cell adhesion and proliferation within the injured area in comparison with the micro-sized HAp [3]. Thus, it seems that applying the nanostructured HAp (n-HAp) on Mg alloys give rise to a good bioactivity and superior corrosion resistance. Many methods are utilized to coat bioceramics on metallic substrates. Among these, electrophoretic deposition (EPD) seems to be more suitable to produce a homogeneous and dense ceramic, polymer and composite coatings for biomedical applications [6]. In EPD technique, direct current (DC) is applied to suspended powder particles in a liquid medium to charge the particles and deposit them onto a conductive substrate of opposite charge [7]. Farrokhi-Rad and Ghorbani applied EPD technique to coat TiO2 nanoparticles on the 304 stainless steel and studied the electrophoretic mobility of titania nanoparticles in different suspension mediums [8]. Mehdipour et al. deposited a kind of bioactive glass-contained Mg on 316L stainless steel via EPD to improve the bioactivity of the substrate [9]. Besides, the biocompatibility of Mg-based alloy surfaces could be improved via controlling their degradation rate utilizing fluoride conversion coating and surface modification through micro arc oxidation (MAO) procedure [10], [11]. Fluoride conversion coatings are MgF2 layers produced by treating Mg substrate in hydrofluoric acid (HF) [12]. Moreover, MAO technique is able to create a thick and stable porous ceramic coating on Mg alloys [3]. The aim of this study was introduction of fluoride conversion coating and MAO-produced layer as intermediate layers for n-HAp coating; also their effect on bio-corrosion behavior of these coating systems was investigated.

Section snippets

Substrate surface preparation

AZ91 magnesium alloy with chemical composition (wt.%) of 8.63% Al, 0.59% Zn, 0.17% Mn, <0.05% Fe, <0.05% Cu and balance Mg was used as substrate. The as-cast AZ91 alloy was heat-treated according to ASTM B661. AZ91 alloy samples were cut into dimensions of 20 mm × 10 mm × 2 mm. The surface roughness of both sides of the specimens was adjusted at Ra = 0.20 ± 0.03 μm, using successive finer silicon carbide abrasive paper up to grit # 600. Then, the specimens were ultrasonically cleaned (WUC-D10H, Wisd

Zeta potential/conductivity measurements

The movement of ceramic particles in a suspension fluid is defined as the electrophoretic mobility (μ) of the charged particles in a suspension [7]. This parameter is supported by the pH, ionic strength and the viscosity of a suspension [17]. The electrophoretic mobility (μ) is defined as Eq. (2) [17].μ=ζε4πηwhere ζ, ɛ, η are the zeta potential, dielectric constant, and viscosity of the medium, respectively [17]. As the methanol is chosen as a suspension medium for n-HAp deposition, ɛ and η

Conclusion

The present literature introduced the MgF2 and MAO coatings as the inter-layers between AZ91 alloy substrate and n-HAp bioceramic top-coat. Presence of several ions within the coat layer provides promising agents necessary for enhancing the bone-forming ability of implant materials. Higher corrosion resistance of the MAO/n-HAp coated sample led to the tailored degradation kinetics of AZ91 alloy substrate as biodegradable implants. Moreover, its rough surface also plays an effective role in the

Acknowledgement

The authors are grateful for support of this research by Biomaterials Research Group of Isfahan University of Technology.

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