Papers by Author: J.E. Barralet

Abstract: According to the gene repertoire, distinct morphology and the organisation of extracellular matrix, osteoblast development was identified as a series of stages, proliferation, differentiation, matrix deposition, matrix maturation and mineralization. Each of these stages required tightly regulated and functionally coupled expression of genes related to the transcription factors and bone matrix. In this paper, we identify the effects of OCP to the differentiation of osteoblasts from the point of view of differentiation sequence development. Osteogenic medium (Ost MEM) mainly regulated the osteocalcin (OC) mRNA expression in the first week of culture. As culture continued to 24 days, OCP crystal assemblies became the main regulator. This shift in the role that OCP and Ost MEM played in regulation may reflect different biological functions of OC in Ost MEM induced regulation and OCP crystals induced regulation. The up-regulated OC mRNA expression by OCP crystal assemblies may function as a signal to coordinate the activities of osteoblasts and osteoclasts instead of inducing mineralisation at the end of the differentiation sequence of osteoblasts. By comparing the modified expression pattern observed on the OCP crystal particles with the patterns of differentiation sequences, it was found that BMSCs colonising OCP crystal assemblies from day 7 to day 24 matched the process of differentiation in the early stages of matrix deposition. The gene expressions of BMSC cultured in the osteogenic medium (positive control) corresponded with the process from matrix deposition to mineralisation. Hence, the differentiation process of BMSCs on OCP crystal assemblies was different from that found on the positive control. BMSCs could differentiate to osteoblasts that would function as a regulator for osteoclast activities.

Abstract: The hydrolysis of brushite in calcium phosphate cements to form hydroxyapatite is known to result in the long term stability of the material in the body. It has previously been established that this hydrolysis reaction can be influenced by implant volume, media refreshment rate and media composition. In this study, the effect of macroporosity on the rate of degradation of the material is investigated. Macroporosity was incorporated into the material using calcium alginate beads mixed into the cement paste. The inclusion of the calcium alginate beads did not influence the degree of conversion of the material and allowed the incorporation of porosity at up to maximum of 57%. The macroporosity weakened the cement matrix (from 46.5 to 3.2 MPa). When aged the brushite in the macroporous cement dissolved completely from the matrix resulting in a weight loss of 60wt% in a period of 28 days. This suggests that the controlled incorporation of calcium alginate beads into brushite cements in vivo can be used to control implant degradation rate.

Abstract: Bajpai et al. originally reported the formation of cements by the mixture of carboxylic acids and β-tricalcium phosphate (β-TCP). In the current study, we report and contrast four such cement systems formed from mixing citric, malic, 2-oxoglutaric or phosphoric acid with β-TCP. Cements formed from malic or 2-oxoglutaric appeared to contain crystalline phases and were determined to contain brushite, β-TCP and unreacted acid. In contrast, cement formed with citric acid was poorly crystalline, containing little evidence of brushite formation and was unstable in water and therefore does not appear to be a feasible cement system.

Abstract: In this study, we have shown that by incorporating pyrophosphoric acid into a brushite cement system, it is possible to produce a cement that exhibits adhesive tensile strengths with cortical bone, alumina, sintered hydroxyapatite and 316L stainless steel of 700 kPa. To our knowledge, this is the first report of a calcium phosphate cement formulation that exhibits such adhesive properties without the addition of an organic additive. The production of a bond between medical prostheses and bone may further widen the field of application of calcium phosphate cements, additionally the adhesive nature of the calcium phosphate cement may be a desirable ‘handling characteristic’ during reconstructive surgery.

Abstract: Time resolved infrared spectroscopy (FTIR) and isothermal differential scanning calorimetry (DSC) were used for the first time to monitor the chemical reaction in a fast setting brushite forming calcium phosphate cement. It was found that the reaction percentage at a given time was dependent on temperature and not powder to liquid (P/L) ratio. Both methods showed that there was, within the temperature range investigated, a single autocatalytic like setting reaction within the cement paste. Final conversion of the reactants was found to be unaffected by temperature and P/L ratio.

Abstract: Amorphous calcium carbonate (ACC) occurs with varying stabilities in different organisms. It is thought that the stability is influenced by certain ions and proteins. We describe here a study investigating the particular influence of phosphate ions on the stability of ACC. Synthesised ACC was treated with different concentrations of Na2HPO4 and molar ratios of PO4:Ca ≥ 12:1 were shown to be effective in preventing crystallisation of ACC to calcite. The nature of the resulting sample is discussed.

Abstract: Brushite cement is more soluble than apatite cement in physiological conditions and therefore may be more resorbable in vivo. Brushite cement has been formed previously by mixing β-tricalcium phosphate, water and an acidic source of phosphate ions. However, brushite cement may be formed by the mixture of H3PO4 solution and poorly crystalline precipitated hydroxyapatite (HA). Several additives have been used to alter the physicochemical properties of brushite cement. In this study sulphate ions where added to the cement system by addition of ammonium sulphate to HA during HA preperation. Sulphate ions were found to alter the structure, composition and mechanical performance of cement.

Abstract: In this study the setting times, compressive strengths and microstructures of cements formed using pyrophosphoric acid solution and b-tricalcium phosphate (β-TCP; Ca3(PO4)2) were compared with those of cement formed using orthophosphoric acid solution and b-TCP. It was found that cement formed using pyrophosphoric acid solution set more slowly than that formed using orthophosphoric acid and could be mixed to a higher powder to liquid ratio, facilitating the production of cement exhibiting compressive strengths, without pre-compaction, as high as 25 MPa. The use of pyrophosphoric acid as opposed to orthophosphoric acid resulted in a marked change in the microstructure of the cement.

Abstract: This study sought to examine the efficiency of coating cement powder reactants in order to reduce the solubility rate of reactants and thereby increase setting times of cement systems. In this investigation magnesium and sodium stearate salts were used to coat the highly soluble monocalcium phosphate monohydrate (MCPM) powder component of a hydraulic brushite forming calcium phosphate cement system with b-tricalcium phosphate (b-TCP) as other component. The results showed that stearate coating of the MCPM reactant could lead to a 100% increase in setting and working times without affecting compressive strength of the set cement when applied with the appropriate P/L-ratio.