Papers by Author: Serena Best

Abstract: The aim of this work is to produce and characterise carbonate substituted hydroxyapatite (CHA) reinforced polycaprolactone (PCL) nanocomposites with a controlled degradation rate in order to match the rate of bone in-growth. The ideal degradation time for this purpose is estimated to be around 5-6 months however, in vivo, PCL degrades over a period of 2 to 3 years. It has been reported that NaOH surface treatment can accelerate the degradation of PCL [1-3]. In order to further modify the degradation rate of PCL, the effects of the incorporation of different volume fractions of CHA in samples surface treated with NaOH was investigated. CHA was produced by wet chemical synthesis. Samples comprising 8, 19, 25 wt% uncalcined CHA-PCL composites were produced by twin screw extrusion which were then injection moulded into cylinders. In order to accelerate the degradation rate of PCL, it was surface treated with 5 M NaOH for 3 days prior to PBS studies. The degradation profile was examined by % weight loss and % water uptake measurements. NaOH treatment was observed to erode the polymer surface and the polymer-filler interface. On subsequently degrading the pre-treated samples in PBS, it was observed that with increasing fraction of CHA, the degradation rate in PBS of the sample increased. Up to 8 wt % CHA filler there appeared to be little change in the degradation properties of the NaOH treated samples with the onset occurring after 60 days. However there was a marked acceleration of degradation for samples containing 19 wt% when degradation appeared to occur immediately. In conclusion, the addition of CHA significantly affects the behaviour of PCL.

Abstract: The effect of substitution of zinc into the hydroxyapatite lattice(ZnHA) was evaluated using experimental precipitation studies and with ab initio modelling data. When attempting to introduce zinc into the hydroxyapatite (HA) lattice not all of the zinc is substituted and ICP confirms its presence in the supernatant. Modelling suggests that this is likely to be because of the high formation energy introducing zinc into the pure hydroxyapatite lattice, 4.6 - 4.9eV. In the experimental study it was found that a maximum of 0.61wt% zinc could be substituted into the HA lattice at 10oC with less being substituted at room temperature »22oC. Experimental measurements revealed that the presence of zinc in the lattice decreased the a-lattice parameter and increased the c-lattice parameter. Modelling showed that this was likely to be caused by the movement of the hydroxyl ions away from the c-axis, towards the zinc atoms which were substituted in calcium type II positions. The type II position was shown to be the most stable substitution site.

Abstract: Precipitation routes for HA production result in the formation of suspensions of nano-scale hydroxyapatite particles in water. During this work, suspensions of phase-pure and carbonate-substituted hydroxyapatite were produced using a precipitation reaction with calcium hydroxide and orthophosphoric acid as reactants. The chemistry of the apatites was analysed using XRD and FTIR after heat treatment in various atmospheres and the particle morphology investigated using TEM. The thermal stability of the carbonate HA was found to be significantly less than the phase-pure HA and dependent upon the heat-treatment atmosphere. Suspensions were used directly for replication of polymer foam templates with a dip-coating technique. Good replication of the foam structure was achieved with the phase pure hydroxyapatite, but no structural integrity was achieved with the carbonate-HA structure possibly a result of limits on the sintering temperature.

Abstract: Hydroxyapatite and silicon-substituted hydroxyapatite powers were prepared in-house through a wet precipitation method and then vacuum plasma sprayed onto Ti-6Al-4V discs. Two plasma gun input powers were employed, 37 kW and 40 kW. All coatings were nearly phase pure, except small traces of impurities (TTCP, -TCP and CaO). Coatings prepared under the lower plasma gun input power had lower crystallinity. In vitro studies showed that human osteoblast-like cells attached and spread very well on all coated discs. Among the four kinds of discs, SiHAC37 was the most supportive to cell growth.

Abstract: In this paper, we present a modified density-dependent Drucker-Prager Cap (DPC) model with a nonlinear elasticity law developed to describe the compaction behavior of pharmaceutical powders. The model is implemented in ABAQUS with a user subroutine. Using microcrystalline cellulose (MCC) Avicel PH101 as an example, the modified DPC model is calibrated and used for finite element simulations of uniaxial single-ended compaction in a cylindrical die. To validate the proposed model, finite element simulation results of powder compaction are compared with experimental results. It was found that finite element analyses gave a good prediction of both the loading-unloading curves during powder compaction and the compaction force required for making a tablet with a specified density. Further, the failure mechanisms of chipping, lamination and capping during tabletting are investigated by analysing the stress and density distributions of powders during the three different phases of the tabletting processes, i.e. compression, decompression and ejection. The results indicate that the model has excellent potential to describe the compaction process for generic pharmaceutical powders.

Abstract: Dense and porous HA and Si-HA discs and granules with varying percentages of silicon substitution have been produced and physically and chemically characterised using scanning electron microscopy, surface area analysis, porosimetry, density measurement, image analysis, Xray diffraction, X-ray fluorescence, FT-infrared spectroscopy and in-vitro and in-vivo testing. Results have shown that cell adhesion in-vitro and bone apposition in-vivo are enhanced by the presence of silicon substitution in the hydroxyapatite structure. The biological response to the materials appears to indicate an optimum outcome for levels of silicon substitution of 0.8wt%.

Abstract: Fine nanoapatite relics were deposited on glass substrates by electrohydrodynamic atomisation, using nanohydroxyapatite (nHA), nano-carbonated hydroxyapatite (nCHA) and nanosilicon- substituted hydroxyapatite (nSiHA) suspensions. These electrosprayed nanoapatites were evaluated in-vitro using simulated body fluid (SBF) and human osteoblast (HOB) cells. The SBF study revealed that newly-formed apatite layers were observed on the surface of the relics. Furthermore, enhanced HOB cell growth was observed on each of the nanoapatites at all time points. Hence, this work demonstrated that electrosprayed nanoapatites offer considerable potential as biomaterials.

Abstract: In this study, a biocomposite comprising nanostructured α-tricalcium phosphate (α-TCP) in a poly(D,L-lactic-co-glycolic acid) (PLGA) matrix was fabricated by a modified solution evaporation method. As a potential temporary bone fixation and substitution material, its bioactivity was evaluated by its ability to form bone-like apatite layer in simulated body fluid (SBF). Owing to the increased surface area covered by the osteoconductive bioceramic of α-TCP, rapid apatite formation was observed. After 7 days of immersion, enhanced nucleation of apatite was observed on the nanocomposite. At day 14, dense lamellar-like apatite was formed on the nanocomposite whilst apatite nucleation had only just started to develop on the surface of pure PLGA. At the same time, a preliminary in-vitro cell culture study was conducted using human osteoblast-like (HOB) cells. A significant increase in cell number with culturing time was observed for the nanocomposite. After 9 days incubation, a confluent lamellar-like apatite layer was formed on the composite surface. This apatite layer was also shown beneath the proliferating HOB cells at Day 16.

Abstract: Novel hybrid gels in the system gelatin-GPTMS-TEOS were prepared via a sol-gel route, and their ability to release Si(IV) was examined using MG63 osteoblast-like cell culture. The amount of Si released and the release rate were controllable by changing the mixing ratio of GPTMS and TEOS. In addition, the hybrids had biocompatible surfaces. It is expected that the hybrids will be utilized for the investigation of the effect of Si on cell differentiation and tissue regeneration.

Abstract: Composites of hydroxyapatite (HA) and multiwalled carbon nanotubes (CNTs) have been prepared and characterised for potential application in major load-bearing medical devices. We have studied the effect of nanotube surface chemistry, composite preparation methods, and heat treatment conditions on the microstructure of the composites, dispersion of CNTs, and interaction between the HA and CNTs. The samples were characterised using SEM, XRD, FTIR, and BET surface area. It was found that, compared with pure HA, the composites had lower densities and higher surface areas. Additionally, functionalisation improved the dispersion of CNTs in the HA matrix and the interaction between the two phases.