Elsevier

Materials Chemistry and Physics

Volume 231, 1 June 2019, Pages 138-149
Materials Chemistry and Physics

In-vitro assessment of HA-Nb coating on Mg alloy ZK60 for biomedical applications

https://doi.org/10.1016/j.matchemphys.2019.04.037Get rights and content

Highlights

  • Plasma spray method was used to coat Mg alloy ZK60 with HA, Nb and HA-Nb coatings.

  • The hardness of surface increased with the progressive increment of Nb-content HA.

  • The surface of HA and HA-Nb coatings revealed hydrophilic nature.

  • Nb-reinforcement in HA increased the degree of protection offered by the surface.

  • HA-Nb coatings revealed better hemocompatibility than Mg Alloy ZK60.

Abstract

Magnesium (Mg) and its alloys have attracted a great deal of attention as next-generation biomaterials due to their highly appealing properties. However, the high corrosion kinetics and inadequate hemocompatibility have limited their widespread acceptance as implant materials. In this study, an attempt was made to improve the corrosion resistance and hemocompatibility of Mg alloy ZK60 by coating it with niobium (Nb)-reinforced hydroxyapatite (HA). The Nb-reinforced HA (HA-Nb) coating was obtained through plasma spray technique by varying weight percent (wt%) of Nb-reinforcement in HA at three levels (10 wt%, 20 wt%, and 30 wt%). Nb-reinforcement had a more pronounced effect on the microhardness and wettability as compared to the surface roughness of the coating. The corrosion investigation revealed that the HA-Nb coatings were more effective than the pure HA and Nb coatings for inhibiting the rapid corrosion of the Mg alloy. With the progressive increment of the Nb-content in HA, protection efficiency (Pe) for HA-Nb coatings increased (∼4%, 12% and 18% for HN1, HN2, and HN3, respectively) in comparison to pure HA coating. The HA-Nb coatings exhibited no adverse effects on the erythrocytes and the hemolysis rate (HR) was within the domain of safe value (<5%) for implant materials.

Introduction

In the recent years, Mg and its alloys have been the focus and hotspot as the potential bioresorbable/biodegradable implant materials due to their attractive properties such as lightweight, high strength-to-weight ratio and almost similar elastic modulus and yield strength to human bone [1,2]. Mg and its alloys are mechanically compatible with human bone and can effectively prevent the undesirable stress shielding effect [3,4]. Moreover, Mg exists as a cation in the human body and it can improve human metabolism, cell proliferation and differentiation [5,6]. A bioresorbable implant should maintain its mechanical integrity during the healing phase of damaged tissue and physiologically degrade afterward without causing any adverse effect on the surrounding tissue. But Mg possesses very high corrosion kinetics in a physiological environment which ultimately leads to a decrease in mechanical integrity [7]. Moreover, the excessive corrosion of Mg in the human body can cause respiratory distress, hemolysis, muscular paralysis, serious local alkalization and even cardiac arrest [[8], [9], [10]]. The research related to Mg alloys for biomedical application has been primarily focused on the Mg alloys AZxx (containing aluminum and zinc), AMxx (containing aluminum and manganese) and as-cast/as-extruded alloys of Mg with alloying elements such as calcium, zinc, and lithium [2,[11], [12], [13], [14]]. The aluminum-content present in AZxx and AMxx alloys has doubtful biological safety as it adversely affects osteoblasts, accelerates dementia and has been reported neurotoxin [[15], [16], [17]]. Therefore, an aluminum-free Mg alloy ZK60 was considered as the substrate material in the present study. The Mg alloy ZK60 contains zinc and zirconium as the main alloying elements that have been characterized as human biology-friendly [18].

To control the rapid degradation of Mg alloys, calcium-phosphate coating, especially HA, has been extensively investigated [19]. HA is the major component of the human skeleton and its coating can improve the in vivo performance of the implants [20]. In most of the studies, electrodeposition, sol-gel method and sputtering technique were employed to obtain HA coating [2]. Among the deposition techniques, plasma spraying is the only permitted method by the Food and Drug Administration for HA coating of implants [21]. Plasma spraying method also has higher deposition efficiency, low processing cost and significantly higher coating thickness can be achieved with this method [22,23].

Reportedly, the degradation rate of pure HA coating is relatively higher as compared to reinforced-HA coating [24]. In order to enhance the properties of HA-coated Mg/Mg alloys for biomedical applications, several secondary phases (reinforcements) have been investigated in HA coating, such as silicon, titania, ceria, polylactide, and polycaprolactide [[25], [26], [27], [28]]. Recently, the research community has been interested in metal-reinforced HA coatings on Mg-based materials which reveal better mechanical, corrosion and biological properties than pure HA coating [[29], [30], [31], [32]]. Nb is recognized as a value metal and it has excellent corrosion resistance [16]. However, Nb is mechanically weak in pure form which hinders its employment as a bulk-material in biomedical implantology [33]. Nb can be used as a coating constituent on a substrate material with adequate mechanical strength to overcome this limitation and take advantage of its exceptional corrosion protective effect as well. As compared to other metallic reinforcements, incorporation of Nb in HA coating is rarely reported. Therefore, utilization of Nb as a secondary phase in HA coating is a promising approach to enhance the corrosion resistance and biologic response altogether.

The in-vitro hemocompatibility analysis is indispensable in the biologic assessment of blood-contacting biomaterials, especially in case of Mg-based materials [34]. Hemocompatibility study can be considered as the cytotoxicity measure of a specific material towards the red blood cells (RBCs) [35]. Besides the degradation behavior and biologic analysis of implants, evaluation of surface properties is also essential. The body-tissue/biomolecules come into contact firstly with the implant's surface upon implantation. The surface properties, such as surface wettability, roughness and hardness influence the in-vivo response of implants [[36], [37], [38]].

As per the available literature, there are limited studies that have investigated plasma sprayed HA coating on Mg/Mg alloy for biomedical applications. To the best of authors’ knowledge, the performance of HA-Nb coating deposited via plasma spraying on an Mg alloy has not been investigated so far. Therefore, in the present investigation, Mg alloy ZK60 was coated with pure HA, pure Nb, and HA-Nb coatings. Electrochemical corrosion behavior and hemocompatibility of the samples were investigated along with the evaluation of surface properties.

Section snippets

Materials and processing

Mg alloy ZK60 pieces sized 15 × 10 × 5 mm3, with the composition as (in weight percent): 94.60 Mg, 4.80 Zn, and 0.51 Zr were used as substrate material. Prior to coating, grit blasting of the samples was performed with Al2O3 particles to generate a rough surface. Subsequently, the samples were then air blasted for the removal of residual grit. As spray feedstock, commercial HA powder of ASTM F1185 grade (Medicoat, France, particle size: 45–125 μm) and Nb powder (BGYST Co. Ltd., China, particle

Characterization

The XRD diffractogram of the feedstock powders is depicted in Fig. 2. The characteristic peaks of crystalline HA phase [Fig. 2(a), (c), 2(d) and 2(e)] and Nb [Fig. 2(b), (c), 2(d) and 2(e)] were recognized in accordance with JCPDS card 9–432 and 35–0789, respectively. The absence of any impurity phase is also evident from these XRD patterns. {{}}

With the progressive increment of the proportion of Nb in HA powder, the peak intensity of Nb-phase strengthened [Fig. 2(c), (d) and 2(e)] without

Conclusion

Mg alloy ZK60 was successfully coated with HA, Nb and HA-Nb coatings through the plasma spray technique. The surface of Nb coating exhibited microcracking, while the morphology of HA and HA-Nb coatings was relatively uneven but no microcracks were observed at their surface. With the progressive increase of Nb-content, the crystallinity of HA-based coatings enhanced. The surface hardness of Mg alloy was increased by HA coating and microhardness value further enhanced as the Nb-reinforcement was

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Acknowledgment

The authors gratefully acknowledge Medicoat, France for sponsoring HA powder. The authors are also grateful to the Mechanical Engineering Department, IIT Ropar, India for providing research facilities.

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