Synthesis and characterisation of carbon nanotube reinforced hydroxyapatite ceramics for biomedical applications

University of Technology, Sydney. Faculty of Science.Reinforcement of nano-materials is important in many industrial processes, including the strengthening of biomedical implants for medical applications (for example artificial hip replacements). Human bone is mainly composed of collagen and hydroxyapatite (HAp) nanocrystals. HAp has been produced synthetically, with a structure and chemical composition almost identical to the HAp in human bone. When implanted, this synthetic material is accepted by the body. However, it has poor mechanical properties, making it unreliable for implant applications. The aim of this research is to combine biocompatible HAp with another biocompatible compound (carbon) to form a composite material with improved physical properties, including density, and strength. The pure HAp was chemically synthesised using a precipitation reaction between calcium nitrate and diammonium hydrogenphosphate. The precipitate was centrifuged, washed and dried. After drying, the powder was heat-treated at 650 °C for 4 hours, and then hot isostatically pressed (HIP), at 100 MPa, 900 °C, in argon gas. Carbon nanotubes (CNTs) were chosen to reinforce the HAp based on their extreme flexibility and strength. Two production methods of incorporating CNT material (between 2 wt% and 10 wt% CNTs) into the HAp have been investigated: chemical precipitation reinforcement and physical reinforcement. Full electron microscopy and diffraction characterisations of the pure and composite materials have been completed. The HIP process forms a dense pellet, with no voids between the CNT material and the HAp matrix. All CNTs imaged in the TEM had minimal degradation to the CNTs, with no visible change in the appearance. Unfortunately, the as supplied CNT material contained pockets of graphite which were non-uniformly distributed through the HAp matrix. Hence, the mixture was not homogeneous, and the CNTs were not bonding directly with the HAp. Neutron diffraction characterisation confirms that the crystal structure of the HAp was not affected by the CNT inclusion. Neutron diffraction patterns collected before and after sintering show that the CNTs must be heated in an inert atmosphere or a vacuum to prevent the CNT material from oxidising. TEM confirms no obvious visual damage to the CNTs in the material. Neutron diffraction data have enabled the positions of the hydroxide bonds to be determined. Small-angle-neutron scattering showed that the surface morphology was rough. The CNT material dominated the neutron scattering results in the composite samples, which minimised the information obtained from the HAp matrix. A range of physical properties of the pure HAp and the composite samples were measured. These included the density, porosity, surface area, hardness, fracture toughness, and Young’s modulus. Two complementary techniques have been employed to measure the hardness; the Vickers microhardness and the Berkovich nanoindentation techniques. The density of the HIP samples of all of the materials was greater than ~94% of the theoretical density, with pure HAp materials as high as ~99%. The hardness values for the material measured by micro-indentation were quite high - either equal to or greater than the literature values. Unfortunately, this resulted in a lower fracture toughness, which was not improved by the addition of the CNTs. It is possible that, if the graphite phase were removed from the material, the fracture toughness could improve. Current CNT production methods do not allow full removal of the graphite. Optical micrographs from the Vickers indentation tests of the composites show varying stages of lateral crack patterns formed, suggesting plastic deformation below the surface. This was consistent throughout all samples. The results from nanoindentation of the bulk material showed that, overall, the samples with the CNT material had a lower Young’s modulus than the pure HAp samples (for both the laboratory synthesised and the commercial material). The microhardness and nanoindentation work showed that all of the samples were influenced by an indentation size effect, where the hardness decreased with increasing load. Further work for increased fracture toughness in these composites requires the production of a pure CNT material (with no graphite impurity) for incorporation with the HAp. It is possible that, without the graphite impurity to bind the CNTs, they will spread more homogeneously throughout the HAp matrix, and bond along the CNT length. No pure CNT material was commercially available at the time of submission of this thesis

The aluminium-copper-gold ternary system

Despite Au, Al and Cu being individually very well-known elements, their ternary phase diagram has not been studied in as much detail as those of many other Au-containing ternaries. Here we review what is known, and consider the prospects for technological exploitation of some of the ternary compositions. The components of greatest interest in Al-Au-Cu may be the β-phases, at least two of which have shape memory properties. Of these, 'Spangold', which has the nominal stoichiometry Au7Cu5Al4, has received some attention for jewellery applications, while the edge compound Cu3Al is a well-known shape memory composition with corresponding specialised industrial uses. The properties of other β-phase compositions in the system have been scarcely investigated. The system also contains an extensive γ-phase, Al4AuxCu9-x, where x ranges from 0 to ~6.5, and the purple gold phase AuAl2

Observing the temperature dependent transition of the GP2 peptide using terahertz spectroscopy

The GP2 peptide is derived from the Human Epidermal growth factor Receptor 2 (HER2/nue), a marker protein for breast cancer present in saliva. In this paper we study the temperature dependent behavior of hydrated GP2 at terahertz frequencies and find that the peptide undergoes a dynamic transition between 200 and 220 K. By fitting suitable molecular models to the frequency response we determine the molecular processes involved above and below the transition temperature (TD). In particular, we show that below TD the dynamic transition is dominated by a simple harmonic vibration with a slow and temperature dependent relaxation time constant and that above TD, the dynamic behavior is governed by two oscillators, one of which has a fast and temperature independent relaxation time constant and the other of which is a heavily damped oscillator with a slow and temperature dependent time constant. Furthermore a red shifting of the characteristic frequency of the damped oscillator was observed, confirming the presence of a non-harmonic vibration potential. Our measurements and modeling of GP2 highlight the unique capabilities of THz spectroscopy for protein characterization.Yiwen Sun, Zexuan Zhu, Siping Chen, Jega Balakrishnan, Derek Abbott, Anil T. Ahuja and Emma Pickwell-MacPherso

Microstrain in hydroxyapatite carbon nanotube composites.

Synchrotron radiation diffraction data were collected from hydroxyapatite–carbon nanotube bioceramic composites to determine the crystallite size and to measure changes in non-uniform strain. Estimates of crystallite size and strain were determined by line-profile fitting of discrete peaks and these were compared with a Rietveld whole-pattern analysis. Overall the two analysis methods produced very similar numbers. In the commercial hydroxyapatite material, one reflection in particular, (0 2 3), has higher crystallite size and lower strain values in comparison with laboratory-synthesized material. This could indicate preferential crystal growth in the [0 2 3] direction in the commercial material. From the measured strains in the pure material and the composite, there was a degree of bonding between the matrix and strengthening fibres. However, increasing the amount of carbon nanotubes in the composite has increased the strain in the material, which is undesirable for biomedical implant applications. © 2008, Wiley-Blackwel

Dynamical transition in a large globular protein: macroscopic properties and glass transition.

Hydrated soy-proteins display different macroscopic properties below and above approximately 25% moisture. This is relevant to the food industry in terms of processing and handling. Quasi-elastic neutron spectroscopy of a large globular soy-protein, glycinin, reveals that a similar moisture-content dependence exists for the microscopic dynamics as well. We find evidence of a transition analogous to those found in smaller proteins, when investigated as a function of temperature, at the so-called dynamical transition. In contrast, the glass transition seems to be unrelated. Small proteins are good model systems for the much larger proteins because the relaxation characteristics are rather similar despite the change in scale. For dry samples, which do not show the dynamical transition, the dynamics of the methyl group is probably the most important contribution to the QENS spectra, however a simple rotational model is not able to explain the data. Our results indicate that the dynamics that occurs above the transition temperature is unrelated to that at lower temperatures and that the transition is not simply related to the relaxation rate falling within the spectral window of the spectrometer. © 2010, Elsevier Ltd