Elsevier

Acta Biomaterialia

Volume 7, Issue 1, January 2011, Pages 395-405
Acta Biomaterialia

Responses of bone-forming cells on pre-immersed Zr-based bulk metallic glasses: Effects of composition and roughness

https://doi.org/10.1016/j.actbio.2010.08.002Get rights and content

Abstract

Bulk metallic glasses (BMGs) demonstrate attractive properties for potential biomedical applications, owing to their amorphous structure. The present work has investigated the biocompatibility of Zr-based BMGs by studying the cellular behavior of bone-forming mouse MC3T3-E1 pre-osteoblast cells. A Ti–6Al–4V alloy was used as a reference material. Pre-immersion treatment was performed on BMG samples in phosphate-buffered saline prior to cell experiments. The effects of 1 at.% yttrium alloying and surface roughness on cellular behavior were examined. The general biosafety of Zr-based BMGs for MC3T3-E1 cells was revealed as normal cell responses. Pre-immersion treatment was found to effectively reduce the surface concentrations of alloying elements. Micro-alloying with 1 at.% yttrium did not significantly affect cell adhesion and proliferation, but slightly decreased alkaline phosphatase (ALP) activity on rough surfaces. Lower cell adhesion and proliferation were found on smooth surfaces of Zr-based BMGs compared to their rougher counterparts. Higher ALP activity was detected on rougher surfaces. To obtain a mechanistic understanding surface free energy was correlated with cell adhesion.

Introduction

Despite considerable success in clinical practice, current biomedical alloys still encounter difficulties during application [1], [2], [3], such as stress shielding, particle disease, corrosion–fatigue failure and foreign body inflammation. Increasing requirements for biomedical implants has heightened the need for new materials with enhanced mechanical and chemical properties, including: a lower elastic modulus to avoid stress shielding, better wear resistance to minimize toxic wear debris and severe metal leaching, higher fatigue endurance and corrosion resistance to withstand harsh in vivo working conditions (cyclic forces and a corrosive environment) and an increase in the strength-to-weight ratio to reduce host tissue damage and immune rejection.

Bulk metallic glasses (BMGs) are a revolutionary class of alloys with an amorphous microstructure. Unlike crystalline metals/alloys, the constituent atoms of BMGs are randomly packed, instead of arranged in larger scale atomic structures. This unique microstructure yields a combination of excellent mechanical properties, good electrochemical reactions and high processability, which has inspired great interest in their biomedical applications, including as orthopedic implants [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

The unique properties of BMGs could potentially diminish problems encountered by traditional orthopedic implant alloys. A number of BMGs possess elastic moduli closer to that of human bone than commercial orthopedic implant alloys [10], [12], with a consequent reduction in stress shielding. Compared with the new generation of β-Ti alloys [17], [18], which exhibit similarly low elastic moduli, the design and synthesis of BMG foams can be considered a strategy of further modulus matching to bone, which can lower the elastic modulus while maintaining a relatively high strength and improving plasticity [19], [20], [21]. The good wear resistance of BMGs [11], [22] could prevent wear debris, reducing the risk of “particle disease”, such as osteolysis and subsequent aseptic loosening [3]. Their extremely high yield strengths and relatively low densities [12], [23] lead to high strength-to-weight ratios, so that competent implants with smaller geometries can be designed, resulting in less tissue damage. Owing to their lack of crystalline defects, BMGs exhibit equivalent or superior corrosion resistance to crystalline biomedical alloys in a simulated human body environment [24], [25]. Another distinctive characteristic of BMGs which favors their biomedical application is their unique formability/high processability. The absence of a first order phase transition during solidification reduces solidification shrinkage in BMGs by one order of magnitude over conventional cast alloys, suggesting a much higher dimensional accuracy in casting [21]. Therefore, accurately controlled surface textures/patterns on BMGs can be attained. Moreover, by introducing thermoplastic forming (TPF) into the processing of BMGs at supercooled liquid temperatures, where BMGs undergo drastic softening (super-plasticity), accurate fabrications of complex three-dimensional (3D) geometries can be achieved [15], [20], [21]. The TPF method can also be used in the nanoforming and synthesis of BMG foams [21]. The capability of BMGs to form accurate and complex surface geometries can simplify the post-processing of implant devices.

Along with their attractive properties, the main challenges for the practical uses of BMGs are to avoid crystallization and defects during processing, improve the plasticity at room temperature and further enhance the corrosion resistance. Alloys with high glass-forming ability (GFA) for different alloy systems [7], Zr-based BMGs with superhigh plasticity at room temperature [26] and BMGs without pitting [24] in chloride solution have been prepared by tailoring the composition. Moreover, appropriate processing methods (e.g. the TPF method) can be employed to reduce size limitations and improve their properties [21]. Looking to the prospective future of BMGs, continuous efforts are being made at alloy design and innovative fabrication techniques.

As potential biomedical materials a few studies have recently been reported regarding the biocompatibility of BMGs [15], [16], [22]. In vitro studies found that BMGs supported similar or better cell adhesion and proliferation of NIH 3T3 embryonic mouse fibroblasts compared with commercial biomedical alloys [15], [16], [22]. In vivo assessments revealed a common foreign body response and non-toxicity of BMGs in white rabbits and mice [15], [22]. BMGs were also reported to elicit better bioactivity than their crystalline counterparts [15]. This pioneer research has encouraged further efforts to exploit the biocompatibility of BMGs. In this work, the cellular behavior of bone-forming MC3T3-E1 mouse pre-osteoblast cells was studied in order to explore the bioactivity of BMGs as potential orthopedic implant materials.

Zr-based BMGs have been considered in this study, since Zr is recognized as a biocompatible element and Zr-based BMGs are among the most developed and investigated glass-forming systems, with their formation and properties widely reported. BMG Zr55Al10Cu30Ni5 was selected due to its high GFA, excellent mechanical properties and good corrosion resistance in various aqueous solutions [27], [28]. BMG (Zr0.55Al0.10Cu0.30Ni0.05)99Y1 was also included in this work, since 1 at.% yttrium addition can further improve the GFA and mechanical and corrosion properties of the Zr55Al10Cu30Ni5 BMG [23], [25]. The effects of minor Y addition on biocompatibility were be studied. Alloy Ti–6Al–4V was employed as a reference material, based on its wide clinical use as orthopedic implants and good biocompatibility [2].

The presence of “toxic” elements (e.g. Ni, Cu and Be) within glass-forming alloys is a concern when discussing their biomedical applications [29]. Immersion in phosphate-buffered saline (PBS) can alter the surface composition of Zr-based BMGs [25]. Hence, a pre-immersion treatment was performed, which could potentially reduce the concentration of toxic elements on BMG surfaces with which cells might interact.

In addition, cell responses to different levels of surface roughness are a matter of controversy [30], [31], [32], [33], [34], [35], [36]. Higher cellular activities of osteoblast/osteoblast-like cells on rougher surfaces of crystalline alloys have been reported [30], [31], [32], whereas the opposite effect or no significant effect have also been observed [33], [34]. Thus, in the present report, cell responses to different levels of surfaces roughness on BMGs have been included.

The objectives of this work were to: (1) investigate the biocompatibility of Zr-based BMGs as potential orthopedic implant materials; (2) verify the effect of pre-immersion treatment on surface modification, in order to minimize the concentration of possibly “toxic” elements; (3) study the effects of micro-alloying and surface roughness on the behavior of osteoblasts on BMGs.

Section snippets

Alloy fabrication

Alloy ingots with the nominal compositions Zr55Al10Cu30Ni5 and (Zr0.55Al0.10Cu0.30Ni0.05)99Y1 (at.%) were prepared by arc melting mixtures of pure elements in an argon atmosphere. Cylindrical rods (3 mm in diameter) were fabricated by copper mold casting. The amorphous state of the samples was verified using synchrotron radiation high energy X-ray diffraction (Fig. S1 in Supplementary information). Alloy Ti–6Al–4V (Grade 5) was commercially available from McMaster-Carr Co.

Preparation of test specimens

Disc-shaped substrates

Surface chemistry

XPS survey spectra revealed the surface compositions of the BMG samples. Elements including Zr, Al, Ni, Cu (Y) and O were found, and C was recognized as an inevitable contaminant. Additionally, P, Na and K were found on the samples pre-immersed in PBS. Representative spectra for the Y1-S samples are shown in Fig. 1. Accurate chemical compositions were calculated from the narrow scan spectra, as listed in Table 2. The existence of Ni on the pre-immersed sample surfaces could barely be observed

Discussion

The unique characteristics of BMGs have inspired their biomedical application as potential orthopedic implant materials [29]. The mechanical and electrochemical properties of the Zr-based BMGs investigated in this study have been reported in the literature [23], [25], [28]. With the emerging development of Ti alloys, especially with the discovery of the new generation β-Ti alloys, which exhibit excellent properties (e.g. moduli close to that of the bone and extremely high corrosion resistance),

Conclusions

In this study the biocompatibility of Zr-based BMGs for orthopedic applications was investigated by studying the cellular behavior of pre-osteoblasts. Before cell culture the samples were pre-immersed in PBS, which reduced the surface concentrations of biologically concerned alloying elements (e.g. Ni and Cu). The effects of micro-alloying (1 at.% Y addition) and surface roughness (600 grit and 1 μm cerium oxide polishing finishes) on the bioactivity of BMGs were studied. Generally, besides their

Acknowledgements

The authors are grateful to Drs. Scott C. Lenaghan and David A. Gerard for their kind suggestions on SEM fixation procedures and Mr. Sameer Paital for his assistance in contact angle tests. This work was financially supported by the National Science Foundation International Materials Institutes Program (DMR-0231320) and National Nature Science Foundation of China (Grants Nos. 50771005 and 50631010). Research at the Oak Ridge National Laboratory SHaRE User Facility was sponsored by the

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