Hydroxyapatite/graphene-nanosheet composite coatings deposited by vacuum cold spraying for biomedical applications: Inherited nanostructures and enhanced properties

Abstract

Recent exciting findings of the biological interactions of graphene materials have shed light on potential biomedical applications of graphene-containing composites. Fabrication of bulk composites in particular nanostructured coatings from nanosize particles/sheets yet remains elusive. Here we report hydroxyapatite (HA) and HA–graphene nanosheet (GN) composites synthesized by liquid precipitation approach and following coating deposition by vacuum cold spraying. The HA–GN composite coatings retained intact nano-structural features of both HA and GN. The impact of the HA–GN particles during coating formation created layered coating structures and mechanical interlocking was achieved by even distribution of GN. In vitro cell culture assessment showed that filopodia of osteoblast cells inclined to move towards and got anchored by GN. Further observation by electron microscopy of adsorption of fibronectin on GN by negative staining showed fast adsorption of fibronectin in unfolded shape with the length of ∼100–135 nm. This presumably accounts for the enhanced spreading and subsequent proliferation of the cells on the GN-containing coatings. The strategy of depositing the novel HA–GN composite coatings gives bright insight into potential biomedical applications of the composites.

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.