Ashley A. White*'1 and Serena M. Best
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K. Ian A. Kinloch*
School of Materials, University of Manchester, Manchester Ml 7HS, U.K.
Hydroxyapatite (HA) has been used in clinical bone graft procedures for more than 25 years. However, its poor tensile strength and fracture toughness compared with bone make it unsuitable for major load-bearing devices. Carbon nanotubes (CNTs), with their high aspect ratio and excellent mechanical properties, have the potential to strengthen and toughen HA without offsetting its bioactivity, thus opening up a wider range of possible clinical uses for the material. This review discusses techniques for synthesizing and processing HA—CNT composites, as well as barriers that still remain to their successful development for clinical application.
Hydroxyapatite (HA) is a biologically active calcium phosphate ceramic that is used in surgery to replace and mimic bone. While HA's bioactivity means it has a significant ability to promote bone growth along its surface, its mechanical properties are insufficient for major load-bearing devices. To combat this problem, current devices use HA combined with other materials, such as poly-
ethylene, yttrium-doped zirconia, and Bioglass (Nova-
'This work was supported by the Marshal] Scholarship Programme, funded by the Foreign & Commonwealth Office of the United Kingdom, and administered by the Marshall Aid Commemoration Commission.
'Formerly at Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K. © 2007 Blackwell Publishing Ltd.
bone Products, Alachua, FL).3 However, significant amounts of the reinforcing phases are needed to achieve the desired properties, and as these phases are either bioinert, significantly less bioactive than HA, or bioresorbable, the ability of the composite to form a stable interface with bone is poor compared with HA.
An ideal reinforcement material would impart mechanical integrity to the composite at low loadings, without diminishing its bioactivity. Carbon nanotubes (CNTs), with their small dimensions, high aspect ratio (length to diameter), and high strength and stiffness, have excellent potential to accomplish this. Recent studies have even suggested that they may possess some bioactive properties.4-6 There is considerable interest in CNTs and their applications, with currently over 400 articles published a month in the field. In particular, their role in composite materials is being increasingly investigated. Several recent studies have shown their ability to improve the mechanical properties of ceramics.7-17 Therefore, an interest has developed in applying these findings to incorporate CNTs into HA to improve its mechanical properties, thus creating a synthetic bone graft material that can be used in major load-bearing situations. This review identifies key variables and components that should be considered and investigated to create successfully an HA/CNT composite with excellent biological and mechanical properties.
Bone is a composite material, consisting of 10% water, 20% organic material, and 70% mineral matter, by weight. The organic component consists mainly of type-I collagen fibrils a few nanometers in diameter, similar in scale to CNTs. The remainder of the organic material is made up of other proteins, a cement-like substance, and a cellular component, comprised of os-teocytes, osteoblasts, and osteoclasts, which aid in dissolution, deposition, and nourishment of the bone. The inorganic, mineral component is a calcium-deficient carbonate-substituted apatite containing calcium and phosphate ions, similar in structure and composition to HA (Ca10(PO4)6(OH)2).19
Figure 1 shows the hierarchical structure of bone. Bone mineral crystals are arranged between the ends of collagen fibrils, which are then arranged into sheets called lamellae. The morphology of the mineral crystals is generally agreed to be plate like with dimensions on the scale of lO-lOOs of angstroms. The collagen lamellae are either arranged in concentric circles, called tubular Haversian systems, or in sheets. These sheets form the spongy cancellous bone found inside the structure and at bone ends. The Haversian system configuration, on the other hand, leads to the dense, cortical form of bone, which comprises 80% of bone mass and surrounds the cancellous bone.20
When bone is lost due to injury or illness, bone grafts are introduced to perform both a mechanical and biological role, serving to restore skeletal integrity, fill voids, and enhance bone repair. Over 2 million bone graft procedures are performed annually worldwide. Ninety percent of these procedures used natural bone from autografts (from the patient's own bone) or allografts (from a cadaver), while only 10% used synthetic materials.
Natural bone is the optimal replacement choice, as it carries with it the entire bone structure, consisting of both inorganic and organic components, but there are serious drawbacks to using it in bone graft procedures. Autografts involve a second incision to harvest the replacement tissue from another area of the patient's own body. This requires additional healing at the donation site and can involve long-term postoperative pain. Al-
Hydroxyapatite míe roc rystal
Fig. 1. Hierarchical structure of bone.z lografts, as they come from a foreign body, carry with them the risk of viral infection, immune system rejection, and resorption due to immunological responses. Additionally, donor material is not always readily available.
Synthetic materials pose their own set of challenges. Hench and Wilson21 identified two key components to the clinical success of a biomaterial: (1) it must be able to form a stable interface with the surrounding natural tissue and (2) it must functionally match the mechanical behavior of the tissue to be replaced. Biomimetic approaches, using HA as the mineral component and CNTs to stand in for the collagen fibrils, are a promising method by which to create a synthetic bone graft material meeting these criteria.
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