Papers by Author: William Bonfield

Abstract: Human osteoclasts derived from CD14+ precursors were cultured on discs of stoichiometric hydroxyapatite (HA) and carbonate substituted hydroxyapatite (CHA) of varying carbonate contents. Resorption of the ceramic increased with increasing carbonate content up to 2.35 wt. %. Development of osteoclasts is qualitatively different on ceramics compared to dentine, occurring in discrete, confluent subpopulations, which suggests local cell signalling may be important in the process. Resorption appears to drive further development of osteoclasts. Controlling carbonate content may be one way of controlling the rate of resorption of synthetic HA ceramics.

Abstract: The object of this study was to investigate the effect of the concentration of orthosilicic acid (0, 0.5, 1, 5 and 10µM) on gene expression in human osteoblast cells isolated from trabecular bone. This was measured using reverse transcriptase-polymerase chain reaction (RT-PCR) to quantify messenger RNA (mRNA) levels for collagen type I, alkaline phosphatase and osteocalcin. Results showed that while collagen type I mRNA expression was increased by the addition of up to 10µM orthosilicic acid, ALP message was suppressed over time and osteocalcin levels were decreased.

Abstract: Silicon-substituted hydroxyapatite (SiHA) attracts particular interest due its enhanced bioactivity compared with pure hydroxyapatite. In this study we seek to clarify the effects on the lattice parameters of both composition and sintering temperature in experimentally-produced HA and 0.8wt% SiHA, 1.5wt% SiHA and 2.0wt% SiHA sintered at 800oC and 1200oC. X ray diffraction was used to determine the phase purity and crystallographic structure. We found that while the c parameter increased with increasing silicon concentration, the a parameter decreased with initial silicon incorporation then recovered with further increases in silicon incorporation. The calcium (2) channel expanded with silicon incorporation while tetrahedron distortion index (TDI) and the radius of the P channel showed a similar dependence on silicon content as the a parameter.

Abstract: For the purpose of mimicking the structure and matching mechanical properties of human cortical bone, a natural composite material, hydroxyapatite (HA) reinforced high density polyethylene (HDPE) has been developed as a bioactive, analogue material for bone replacement. This synthetic composite material is now in clinical use. To understand the deformation behaviour and determine mechanical properties of HA/HDPE composite under different loading modes and loading conditions, tensile and compression tests were performed in the current investigation. It was observed that under tension, HA/HDPE composite exhibited two types of deformation behaviour: ductile and brittle. Under compression, the composite deformed in a ductile manner and did not fracture at high compressive strains. It was found that an increase in HA content resulted in increases in Young’s modulus, compressive modulus, tensile strength and compressive yield strength of the composite. A higher strain rate led to higher modulus and strength values and lower tensile fracture strains of the composite.

Abstract: The object of this study was to investigate methods of seeding cells onto porous hydroxyapatite granules with the aim of optimising cell attachment. Two cells types were used; an osteosarcoma cell line, MG63, and human osteoblasts (HOBs) isolated from trabecular bone. Several conditions were investigated to determine their effect on cell attachment. These included varying the initial seeding concentration, pre-adsorption of the granules with the adhesion protein fibronectin and the use of mechanical agitation. Human osteoblasts and MG63 osteosarcoma cells attached to both dense and porous HA granules but with a low seeding efficiency while seeding was not significantly improved by pre-coating scaffolds with fibronectin or by introducing fluid flow.

Abstract: Electrospraying of nano-sized hydroxyapatite (HA) has been used as a technique to modify the surface of alumina in order to achieve the goal of improving bone integration. A porous HA coating on alumina was produced by heat treating electrsprayed HA. Preliminary in vitro studies shown that this porous HA coating provided a favourable surface for attachment and growth of HOB cells. The results indicate that electrospraying is a very promising technique to create thin HA coatings on a range of biomedical implants to improve interfacial bonding with the host tissue.

Abstract: Dense, polycrystalline, synthetic hydroxyapatite (HA) was incubated for 36 days in modified simulated body fluid (SBF) with increased HCO3 - and reduced Cl- ion concentrations (27 and 120 mM, respectively) closer to actual blood plasma than typical SBF. The resulting precipitated apatite layer was characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements and found to be nonstoichiometric, calcium deficient (Ca/P~1.06), non-carbonate containing, and of intermediate hydrophilicity (advancing contact angle, qa=76.5±1.3°). The nanoscale surface topography of the SBF-incubated HA sample was imaged by tapping mode atomic force microscopy (TMAFM), observed to be ≤100 nm in thickness, and composed of three distinct morphologies. These topographically distinct regions were localized within individual grains and facets of the initial HA surface and included: hemispherical, globular structures (maximum lateral dimension, d=44.7±12.7 nm, peak-tovalley height, h=3.6±2.7 nm); elongated, needle-like structures (minimum lateral dimension, w=31.0±8.5 nm, d=104.4±31.1 nm, h=5.0±3.2 nm), and regions of larger, irregularly shaped structures that were relatively smooth (d=504.9±219.1 nm, h=104.0±51.7 nm).

Abstract: Bioactive glass scaffolds with interconnected macroporous networks have been developed by foaming sol-gel derived bioactive glass of the 70S30C (70 mol% SiO2, 30 mol% CaO) composition. The effect of sintering temperature on the dissolution of the scaffolds in simulated body fluid (SBF) was investigated in 3D using x-ray micro-computer tomography (micro CT) and inductive coupled plasma (ICP) analysis. Micro-CT is non-destructive and allows observation of specific parts of the scaffold at various stages of degradation. However, data analysis is complex at present. Percentage porosity data obtained by micro-CT was compared to physical data and pore size distributions obtained from mercury intrusion porosimetry were compared to the interconnected pore diameters observed from the micro CT images.

Abstract: The adhesion of bovine collagen type I, bovine serum albumin, bovine IgG, 1 % and 10 % (v/v) human serum to hydroxyapatite (HA), silicon-substituted hydroxyapatite (Si-HA) and tissue culture plastic were studied. The materials were incubated at 37 °C for 30 minutes, after which the protein solution was removed and analyzed. The adsorbed protein was evaluated by electrophoresis and immunoassay after extraction from the materials. The degree of adhesion was higher for collagen, followed by IgG and albumin on all materials. However there was no difference in the amount of collagen adsorbed onto the surface of each material and this was also the finding with albumin and IgG. These results suggest that the increased bioactivity seen with Si-HA is not due to the degree of protein adhesion, but may possibly be due to changes in the conformation of the bound proteins.

Abstract: This paper describes the use of confocal microscopy as a new method for in situ analysis of the apatite layer which forms on bioactive substrates after immersion in a simulated body fluid (SBF). Conventional methods such as secondary electron microscopy (SEM) require drying and coating of the samples. This approach causes damage to the structure of the apatite layer of which a common example is surface cracking. With confocal microscopy samples are examined in situ and build a unique optical sectioning of the samples. As shown in this paper, using these sections, a clear image can be constructed of the reactive layer which forms on a sample both in horizontal and vertical cross-sections.