Elsevier

Journal of Alloys and Compounds

Volume 624, 5 March 2015, Pages 148-157
Journal of Alloys and Compounds

Bioactive hydroxyapatite/graphene composite coating and its corrosion stability in simulated body fluid

https://doi.org/10.1016/j.jallcom.2014.11.078Get rights and content

Highlights

  • Bioactive HAP/Gr coating on Ti was successfully obtained by EPD.

  • Increased fracture toughness of the HAP/Gr coating compared to pure HAP coating.

  • HAP/Gr coating exhibited superior biomimetic mineralization vs. pure HAP coating.

  • Gr improved the mechanical properties and thermal stability of HAP/Gr coating.

  • HAP/Gr coating was classified as non-cytotoxic against the targeted PBMC.

Abstract

The hydroxyapatite/graphene (HAP/Gr) composite was electrodeposited on Ti using the electrophoretic deposition process to obtain uniform bioactive coating with improved mechanical strength and favorable corrosion stability in simulated body fluid (SBF). Incorporation of Gr was verified by Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron analysis. The HAP/Gr composite coating exhibited reduced surface cracks, nearly double the hardness, and elastic modulus increased by almost 50% compared to pure HAP coating, as estimated by a nanoindentation test. The bioactive HAP/Gr composite coating provided a newly formed apatite layer in SBF with enhanced corrosion stability, as evidenced by electrochemical impedance spectroscopy. The thermal stability of the HAP/Gr coating was improved in comparison to the pure HAP coating, and the Ca/P ratio was closer to the stoichiometric value. No antibacterial activity against Staphylococcus aureus or Escherichia coli could be verified. The HAP/Gr composite coating was classified as non-cytotoxic when tested against healthy peripheral blood mononuclear cells (PBMC).

Introduction

Carbon nanomaterials with two-dimensional (2D) morphologies as a single layer of sp2-hybridized carbon atoms packed in a honeycomb form, known as graphene (Gr), have been reported recently. The extraordinary electrical, thermal, and mechanical properties (e.g., tensile strength 130 GPa and Young’s modulus 0.5–1 TPa) and very high specific surface area (up to 2630 m2 g−1) have drawn great attention as a reinforcement in the composite field of material science [1], [2], [3]. Graphene materials possess physical properties identical to those of carbon nanotubes (CNTs) but have a larger surface area. It has been reported that inclusion of Gr into polymer or ceramic matrices leads to remarkable improvements in the properties of the host materials [1]. Furthermore, graphene nanosheets (GNSs), formed by several layers of Gr with a thickness of up to 100 nm [4], are much easier to produce than other graphene materials and successfully use as nanofillers for polymers [5], metals [6], and ceramics [3], [7] to produce composites with exceptional mechanical properties.

Biomaterials used in orthopedic surgery usually encounter complex service environments and therefore require versatile performances from the materials [8]. As a major player in orthopedic surgery, synthetic hydroxyapatite (Ca10(PO4)2(OH)2, HAP), chemically similar to bone mineral, has been developed in various forms and shapes. Metallic implants, such as Ti and its alloys, have insufficient biocompatibility and lack bioactivity, which means they usually cease to function over the long term because of wear, disease, or injury [8], [9] or release metallic ions with a high potential to corrode in their biological environments [9]. HAP provides bioactivity, biocompatibility and an ability to initiate osteogenesis, but on the other side it lacks good mechanical properties. Because of its poor mechanical properties, such as an intrinsic brittleness, low fracture toughness (0.8–1.2 MPa), low flexural strength (<140 MPa), and wear resistance [10], the main focus of HAP research has been to improve its mechanical performance by combining it with various reinforcements.

The focus of the latest published research has been the fabrication of Gr or its derivatives to create reinforced HAP biocomposites because of the exciting findings regarding the biological performance of Gr [8]. Nonetheless, the mechanical properties of hydroxyapatite limit its use in the regeneration of various parts of the bone systems, especially those under significant mechanical tension. The incorporation of Gr or its derivatives as reinforcing materials in HAP composites has been studied and reported using in situ synthesis [11], [12], spark plasma sintering (SPS) [13], biomimetic mineralization [14], [15], chemical vapor deposition [16], and electrospinning [17]. The general idea of using Gr as nanofiller is to minimize the brittleness of HAP and gain an improved composite. Any reinforcement material for HAP should not only significantly improve the mechanical properties, but also retain HAP’s original biocompatibility. Latest published reports on graphene materials aimed to demonstrate that crack deflection is more effective for sheet-like reinforcement than for tubular-like reinforcement, suggesting that Gr exhibits a more pronounced toughening effect on brittle materials than do carbon nanotubes (CNTs) [18]. Also, reports on CNTs cytotoxicity in organic environments are disconcerting [19]. Unlike CNTs, Gr is synthesized in relatively pure ways and is therefore expected to show little cytotoxicity, since few metallic catalyst particles are associated with its production [20]. Also, recent reports have discussed the qualities of Gr and Gr-based composites, including low toxicity toward human osteoblasts [21], excellent antibacterial properties [22], and its potential to initialize apatite mineralization [23]. Therefore, our aim was to explore the potential of implementing Grs as HAP reinforcement for load-bearing orthopedic applications.

Electrophoretic deposition (EPD) is a special colloidal processing technique widely used to apply bioactive ceramic or composite coatings on various metal surfaces, such as Ti and its alloys, stainless steel, and Cosingle bondCr alloys, for orthopedic applications with improved osteoconductivity, bioactivity, biocompatibility, and corrosion resistance. EPD emerged as the method of choice due to its many advantages (e.g., high deposition rate) and good control of deposition parameters that affect coating thickness, crystallinity, and desirable uniformity even on substrates of complex shape [24], [25], [26], [27], [28], [29], [30]. Briefly, factors influencing the EPD process are electrical conditions (voltage and time) and parameters related to suspension (particle charging, solid loading, dispersants, suspension viscosity, particle size distribution). According to the proposed mechanism [31], the deposition process occurs in several steps. Charged particles attract oppositely charged ions (counterions) around the particles. In the case of cataphoretic deposition, positively charged particles migrate toward the cathode. The rate of migration that the particles can achieve depends on the applied electric field, suspension viscosity, particle radius and particle charge. Because the particles are close enough to the cathode, attractive forces dominate, and coagulation/deposition occurs. The primary process is OH ion generation and hydrogen evolution on the cathode by H2O discharge, followed by electrocoagulation of the ceramic particles at the cathode surface by neutralization of positively charged groups with electrochemically generated OH ions. Evolved hydrogen on the cathode goes out through the coating, leaving vacancies inside the deposited film and causing its porous structure.

In this study, the novel hydroxyapatite/graphene (HAP/Gr) composite was electrodeposited on Ti using the EPD process to obtain uniform bioactive coating with improved mechanical strength and favorable corrosion stability in simulated body fluid (SBF).

Section snippets

Materials

For synthesis of nanosized HAP powder, we used a modified chemical precipitation method that required the reaction of calcium oxide (obtained by aerobic calcination of CaCO3 for 5 h at 1000 °C) and phosphoric acid, according to our previously reported protocol [32], [33]. The final suspension was spray-dried at 120 ± 5 °C into granulated powder. Ti from Aldrich (foil, thickness 0.25 mm, 99.7% trace metals basis) was used as a substrate for electrophoretic deposition of HAP/Gr coatings. Ti samples

Surface morphology and microstructure analysis

The surface morphology of the HAP/Gr composite coating after air drying is shown in Fig. 1a. Compared to the pure HAP coating (Fig. 1b), the HAP/Gr composite coating had fewer cracks and no peeling off the Ti surface in the macroscopic observation. The microscopic view revealed that both HAP/Gr composite (Fig. 1a) and pure HAP (Fig. 1b) coatings on Ti foils displayed micro-cracks. However, these SEM images, taken under the same magnification (1000×), indisputably revealed that fewer cracks

Conclusions

Bioactive HAP/Gr composite coating was successfully produced by the EPD technique on a Ti substrate. FE-SEM images of the coating surfaces revealed indisputably fewer cracks in the HAP/Gr coating than in the pure HAP coating. According to XPS analysis, the calculated Ca/P ratio of 1.58 for the HAP/Gr coating is greater than the Ca/P ratio for the pure HAP coating of 1.50 and closer to the stoichiometric value. The total weight loss for the HAP/Gr coating was 5.28 wt.%, confirming the greater

Acknowledgements

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Project number: 2013R1A1A2A10063466). This research was also financed by the Ministry of Education, Science and Technological Development, Republic of Serbia, contract No. III 45019. The authors would like to thank Dr. Maja Vukašinović-Sekulić, Faculty of Technology and Metallurgy, University of Belgrade, for

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