A tough graphene nanosheet/hydroxyapatite composite with improved in vitro biocompatibility

Abstract

A B S T R A C TMechanical properties and in vitro biocompatibility of graphene/hydroxyapatite (HA) composite synthesized using spark plasma sintering (SPS) are reported in this study. Raman spectroscopy corroborated that graphene nanosheets (GNSs) survived the harsh processing conditions of the selected SPS processing parameters. A 1.0 wt.% GNS/HA composite exhibits $80% improvement in fracture toughness as compared to pure HA. GNS pull-out, grain bridging by GNS, crack bridging and crack deflection are the major toughening mechanisms that resist crack propagation. In vitro osteoblast growth tests illustrate that the added GNSs contribute to the improvement of both osteoblast adhesion and apatite mineralization. Therefore, the GNS/HA composite is expected to be a promising material for load-bearing orthopedic implants.Ó 2013 Elsevier Ltd. All rights reserved. IntroductionGraphene, a monolayer of sp 2 -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 m 2 g À1 ) [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.

Effects of graphene plates’ adoption on the microstructure, mechanical properties, and in vivo biocompatibility of calcium silicate coating

“…HA ceramic containing 1.0 weight (wt) % graphene exhibited ∼80% improvement in fracture toughness. 24 The main toughening mechanisms which originated from the presence of the graphene are contributed to grain bridging, crack bridging, and crack deflection.…”

Electrochemical synthesis of nanosized hydroxyapatite/graphene composite powder

“…The results are a solid indication that Gr acts effectively as a nano-reinforcement filler, by providing frictional pull out, crack deflection, and crack bridging as the major toughening mechanisms to resist crack propagation [1]. The high specific surface area of the graphene nanosheets creates an increased contact area with the hydroxyapatite matrix, significantly enhancing the bonding strength between the graphene and hydroxyapatite grains.…”

“…Recent studies employing graphene as an ideal reinforcement material for hydroxyapatite have demonstrated the significant toughness and strength of the composites, which retain their biocompatibility without any offsetting bioactivity [1,[14][15][16]. It is worth mentioning that, for biomedical purposes, graphene has been used along with chitosan in composites that reportedly show a 200 times increase in elasticity modulus compared to undoped chitosan [17].…”

“…It has drawn attention in the composite materials field as a useful reinforcement for composites due to its unique qualities: excellent thermal and mechanical properties, large specific surface area and electrical conductivity [1][2][3]. Graphene has been utilized as a new material in applications including field-effect transistors, memory devices, photovoltaic devices, electron acceptors, various sensors [3], and composites [4][5][6][7].…”

“…In order to minimize direct metalbody fluid contact and to limit the undesired release of metallic ions into the body, biocompatible coatings on the metallic substrate, such as hydroxyapatite (Ca 10 (PO 4 ) 2 (OH) 2 , HAP), have been suggested by many researchers [10][11][12][13]. As a major component of natural bone tissues, hydroxyapatite is chemically similar to the natural mineral in bone and has excellent biocompatibility; therefore it is very suitable as an orthopedic biomaterial in various forms and shapes for bone growth and integration [1].…”