A tough graphene nanosheet/hydroxyapatite composite with improved in vitro biocompatibility
Introduction
Graphene, a monolayer of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has drawn much attention in the composite field as reinforcement for structural composites due to its combination of excellent 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) [1], [2], [3]. In particular, the high specific surface area of graphene, inherent to its two-dimensional lattice geometry, imparts strong interfacial bonding with the matrix phase and effective load transfer from the matrix to graphene [4]. Graphene nanosheets (GNSs) with a thickness of approximately 1–10 nm, also called as graphene nanoplatelets (GNPs) or graphene platelets (GPLs), are generally composed of a few graphene layers and display compatible properties similar to that of monolayer graphene. Furthermore, it is worth to note that GNSs are much easier to produce and handle. Very recently, GNSs have been widely employed as nanofillers to polymers [5], [6], metals [7], [8] and ceramics [9], [10], [11] to produce composites with tailored mechanical properties.
Due to its chemical composition (Ca/P ratio of 1.67) and crystal structure that are similar to the apatite in human skeletal system, hydroxyapatite (HA), with excellent bioactivity and osteoconductivity, is suitable for osteoblast adhesion and proliferation, new bone growth and integration [12], and therefore is recognized as one of the most promising orthopedic biomaterials. However, the intrinsic brittleness of HA, i.e., low fracture toughness and low toughness-induced poor wear resistance, still restricts its clinical applications. Therefore, toughening of HA with a second phase such as alumina, yttria stabilized zirconia, titania and carbon nanotubes (CNTs) has been extensively explored to overcome the deficiencies of pure HA [13].
Among HA based composites, much recent attention has been devoted to the CNT/HA composites. Previous investigations have demonstrated that CNT/HA composite possesses an improved fracture toughness compared to monolithic HA [13]. This is due to the significant role of CNTs in improving fracture toughness of the composite based on basic toughening mechanisms such as CNT pull-out, crack bridging and crack deflection [13]. CNT/HA composites also display an improved osteoblast proliferation and differentiation in vitro [14], [15]. However, the biocompatibility of CNTs is still under debate due to their cytotoxic responses in organic environment, though some researchers have ascribed the cytotoxicity of CNTs to the presence of metallic catalyst particles rather than CNT itself [13]. Therefore, an ideal reinforcement material for HA should not only significantly improve the mechanical properties, but also can retain the original biocompatibility of HA [13].
Recently, Rafiee et al. [5] have illustrated that crack deflection is more effective for sheet-like reinforcement than for tubular-like reinforcement, strongly implying that GNSs exhibit more significant toughening effect on brittle materials than do CNTs. On the other hand, graphene can be synthesized in relatively pure ways such as growth by chemical vapor deposition, micromechanical exfoliation of graphite and growth on crystalline silicon carbide [16]. Hence, it is expected that graphene shows little cytotoxicity due to few metallic catalyst particles associated with its production. Recent investigations have revealed that graphene and graphene based composites possess a series of merits, e.g., no toxic for human osteoblasts [17], excellent antibacterial property [18], suitable for adhesion and proliferation of osteoblasts [18], and the ability of apatite mineralization [19]. To the best of our knowledge, few investigations have been made into a free-standing GNS/HA composite.
Spark plasma sintering (SPS) is a novel method that can simultaneously employ pressure and electrical current, and can complete the consolidation in few minutes at a relative low temperature. Due to low sintering temperature and short sintering time, SPS can inhibit the grain growth and subsequently improves the mechanical properties. Recent investigations indicate that the main SPS processing parameters for consolidation of HA and HA composites are sintering temperature of 900–1250 °C and applied pressure of 7.5–120 MPa [13]. For a GNS/HA composite, it is worth to pointed out that SPS processing parameters should be further optimized to avoid the damage of the specific structure of GNSs.
In view of the present scenario, the aim of this research is to explore the potential of using GNSs as reinforcement to HA for load-bearing orthopedic applications. HA and GNS/HA composites are fabricated using SPS. The mechanical properties and in vitro biocompatibility of the sintered samples are evaluated. The role of the GNSs and their contributions towards the mechanical response and in vitro biocompatibility of the HA composite is also analyzed.
Section snippets
Material preparation
HA nanorods (length: ∼100 nm, diameter: ∼30 nm) from Nanjing Emperor Nano Material (Nanjing, China) and GNSs (thickness: ∼0.8 nm, diameter: 0.5–2 μm) from ACS Material (MA, USA) were employed as precursor materials. Fig. 1a shows scanning electron microscopy (SEM; Hitachi S-4700, Japan) image of as-received GNSs.
GNS agglomerates were firstly ultrosonicated for 30 min in water with a concentration of about 0.1 mg/ml, in which sodium dodecyl-benzene sulfonate (SDBS) was used as dispersant. Then, HA
Microstructural characteristics and mechanical properties
Raman measurement was performed to confirm the existence of GNSs. Fig. 1b depicts the presence of D, G and 2D peaks, in which G peak is from the in-plane C–C bond stretching in graphene, D peak is related to the defects in the structure, and 2D peak is associated with few-layered graphene structure [21], [22]. The presence of G and 2D peaks in the GNS/HA composites confirms the survival of GNSs even after undergoing harsh processing conditions. Shifting of D, G and 2D peaks to higher frequency
Conclusions
A free-standing GNS/HA composite was consolidated using SPS. Raman spectra evinced that GNSs retain their original structure even after experiencing harsh processing conditions. Compared to monolithic HA, the 1.0 wt.% GNS/HA composite displays ∼80% improvement in fracture toughness. This can be attributed to a combination of toughening mechanisms including crack bridging and crack deflection, GNS pull-out and grain bridging by GNS. Grain bridging by GNS is the most effective mechanism to improve
Acknowledgements
Y.C. acknowledges financial supports from the National Natural Science Foundation of China (Grant No. 51275326), Natural Science Foundation of Jiangsu Province of China (Grant No. SBK2010212) and Opening fund of The State Key Laboratory of Nonlinear Mechanics. T.H.Z. is grateful for the financial supports from National Natural Science Foundation of China (Grant Nos. 11025212 and 11272318). Also, the authors would like to thank Dr. Meng Liu for her kindly help of biocompatibility tests.
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