Hydroxyapatite/graphene-nanosheet composite coatings deposited by vacuum cold spraying for biomedical applications: Inherited nanostructures and enhanced properties
Introduction
Research in biomaterials has been booming in recent decades. Development of novel biomaterials holds the priority among the extensive worldwide efforts devoted to biomedical engineering. The biomaterials for orthopedic surgery usually encounter complex service environments, requiring versatile performances of the materials. As one of the major players for orthopedic surgery, hydroxyapatite (HA) has been developed in response to the deficient biocompatibility of alloys, e.g. stainless steel, Co-based alloy, Ti and Ti–6A1–4V, which usually lose their proper function in a long term due to degradation from wear, disease, or injury [1], [2], [3] or tendency to release metallic ions leading to a high potential to corrode in the biological environments [4], [5]. Yet, regardless of the successful application of HA in orthopedic surgery for promoting fast fixation of bony tissues, there are still concerns related to its long-term performance, i.e., the intrinsic brittleness and low fracture toughness of HA [6], [7], [8]. Mechanical performances of HA could be improved by incorporating second phase reinforcements like ethylene-based polymer, Ti-alloys, alumina, yttria-stabilized zirconia, carbon nanotube etc. [9], [10], [11], [12], [13]. However, few materials that have been considered for HA-based composites satisfy both favorable biocompatibility and sufficient strength. As an alternative novel material, graphene has been attracting intense attentions due to its unique structural features and exceptional mechanical properties [14], [15], [16], [17]. Exciting findings of the biological performances of graphene reported in recent years [18], [19], [20] further imply the possibility of it being used as additives in HA for load-bearing biomedical applications, even though the reason for exceptional biocompatibility of graphene remains obscure.
It is essential that any biomaterials that are implanted in the body must operate in powder or bulk form. Employment of biocompatible materials like HA as coatings on bioinert metallic implants is the key for their long term functional services [21], [22], [23]. This consequently raises the concerns of selecting appropriate techniques for depositing the coatings. Among the approaches used nowadays, of particular interest is the thermal spray technology, which offers advantages of cost-effective and environment-friendly production, controllable microstructure and good properties of the coatings [24], [25], [26], [27]. Thermal sprayed HA and HA-based composites have been successfully used in clinical surgery [3], [22]. It is well established that microstructure, crystallinity, and phase composition of HA coatings are critical in deciding behaviors of the cells attached/proliferated on them [3], [25], [26], [27]. Nanostructured HA exhibit further enhanced biocompatibility when compared to their conventional counterparts [28], [29], [30], [31], [32]. This enhanced biocompatibility is translated into better adhesion and higher reproduction of the cells on the surfaces of these materials, which is very important indication of improved bio-performances of the implants. In this regard, searching pertinent process for fabricating nanostructured biomedical coatings has been one of the major research goals in recent years. Thermal spray process is usually intrinsically associated with melting of feedstock powder, without which it is extremely difficult to make fine coatings. Certain degree of melting is usually necessary for attaining a sufficient level of particle adhesion and cohesion. To deposit coatings from nano particles, the often used agglomeration by spray-drying processing of the particles [33] introduces complexity and difficulties in controlling coating structures. Vacuum cold spray (VCS), a method based on shock-loading solidification [34], [35], is a novel and promising spray technique which enables deposition of powder with particle size range of 0.02–2 μm at room temperature on various substrates. VCS is of particular prospect for spraying nanosized particles with high deposition efficiency in coating thickness ranging from several to tens of microns per minute [34], [35], [36], [37].
In this paper, we report novel nano-HA/graphene-nanosheet (GN) composite coatings deposited by VCS, giving insights into their potential biomedical applications for repair/replacement of hard tissues. To elucidate the performances of the nanostructured coatings, we performed microstructural characterization and in vitro cell culture test. The biocompability of GN was further investigated by assessing the adsorption of fibronectin by electron microcopy. This study provides a competitive approach for processing nanostructured biomedical materials.
Section snippets
Synthesis of graphene and HA–graphene composite powder
Large-scale GN was chemically fabricated from high purity flakey graphite. HA powder in nanosizes was synthesized by the wet chemical approach using stoichiometric reaction between (NH4)2HPO4, Ca(NO3)2, and NH3·H2O. Microstructural characterization revealed that HA grains had the size of ∼20–45 nm in length and ∼10 nm in diameter. HA–GN composites were produced by adding GN to the solution prior to the synthesis of HA. The detailed procedures have been reported in another paper [38].
Deposition of HA–GN coatings
HA coatings
Microstructure of the composite coatings
The as-synthesized GN shows wrinkled-paper-like morphology (Fig. 1a). AFM measurements suggest the uniform height of the nanosheet of 0.8–1.1 nm and the lateral dimensions ranging from a few hundred nanometers to ∼4 μm (Fig. 1b). In accordance with previous reports [15] and TEM observation [38], GN is interpreted to be single-layered sheets. Further TEM characterization reveals that rod-like HA nano grains evenly attach on graphene flakes with an intimate contact (Fig. 1c). HA grains have the
Conclusions
We successfully deposited HA–GN composite coatings by VCS operated at room temperature. The coatings are uniform in tailorable thickness and showed competitive adhesive strength and fracture toughness. Comprehensive microstructural characterization showed that the physical characteristics of the starting feedstocks were completely inherited by the as-deposited coatings without detectable crystal grain growth or phase changes. GN was evenly embedded in HA matrix and plastic deformation of
Acknowledgements
This research was supported by National Natural Science Foundation of China (grant # 31271017) and 100 Talents Program of Chinese Academy of Sciences (both to H.L.).
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