Processing and Sintering of Carbonate Hydroxyapatite

PhDSince the early 1970's there have been a number of investigations into the preparation of dense sintered hydroxyapatite for medical applications. However, there have been few studies reporting the production of sintered carbonate apatite, which resembles more closely the composition of human bone mineral. This work has studied the precipitation, processing and sintering of carbonate apatites. Crystallisation variables such as temperature and bicarbonate ion concentration have been investigated in order to determine some effects on the size, morphology and composition of carbonate apatite precipitates. By employing the correct conditions, nanoscale precipitates have been produced that have enabled the use of a colloidal filtration route in processing. The effect of sintering atmosphere, green density, and carbonate content were investigated isochronally over a range of temperatures. Isothermal experiments demonstrated the evolution of microstructure and changes in density with time. Results from this study indicated that translucent 99.9% relative density carbonated hydroxyapatite could be produced by sintering in an atmosphere of carbon dioxide and water. Water was found to enhance densification in carbon dioxide furnace atmospheres. The temperature at which maximum densification occurred decreased with carbonate content. Bloating was found to be related to carbonate content as larger expansions were observed in higher carbonate content materials. The partial pressure of water did not effect the composition of the carbonate apatite, whereas the green composition did, contrary to the findings of other workers.E.P.S.R.C

Pore network modeling of reaction-diffusion in hierarchical porous particles: the effects of microstructure

The final publication is available at Elsevier via https://doi.org/10.1016/j.cej.2017.07.139 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A general framework based on pore network modeling is presented for simulation of reactive transport in a porous catalyst with a hierarchy of porosity. The proposed framework is demonstrated in the context of steady state reactive transport inside a nanoporous catalyst particle interlaced with macropores that result from the use of pore-formers. A comprehensive parametric study was performed to examine the influence of structural features namely macroporosity, pore size ratio, and the particle size, as well as transport properties namely pore Damköhler number, on the net reaction rate inside the particle. The results showed that depending on the Damköhler number, increasing the macroporosity does not necessarily improve the catalytic activity of the particle. It was also shown that particles with lower pore size ratios are more kinetically active. The key finding of this work was to demonstrate and quantify how microstructure influences the reactivity of hierarchical porous catalyst particles.Natural Science and Engineering Research Council (NSERC) of Canada Ballard Power System

Elucidating the individual effects of calcium and phosphate ions on hMSCs by using composite materials

Prova tipográficaThe biological performance of bone graft substitutes based on calcium phosphate bioceramics is dependent on a number of properties including chemical composition, porosity and surface micro- and nanoscale structure. However, in contemporary bioceramics these properties are interlinked, therefore making it difficult to investigate the individual effects of each property on cell behavior. In this study we have attempted to investigate the effects of calcium and inorganic phosphate ions independent from one another by preparing composite materials with polylactic acid (PLA) as a polymeric matrix and calcium carbonate or sodium phosphate salts as fillers. Clinically relevant bone marrow derived human mesenchymal stromal cells (hMSCs) were cultured on these composites and proliferation, osteogenic differentiation and ECM mineralization were investigated with time and were compared to plain PLA control particles. In parallel, cells were also cultured on conventional cell culture plates in media supplemented with calcium or inorganic phosphate to study the effect of these ions independent of the 3D environment created by the particles. Calcium was shown to increase proliferation of cells, whereas both calcium and phosphate positively affected alkaline phosphatase enzyme production. QPCR analysis revealed positive effects of calcium and of inorganic phosphate on the expression of osteogenic markers, in particular bone morphogenetic protein-2 and osteopontin. Higher levels of mineralization were also observed upon exposure to either ion. Effects were similar for cells cultured on composite materials and those cultured in supplemented media, although ion concentrations in the composite cultures were lower. The approach presented here may be a valuable tool for studying the individual effects of a variety of soluble compounds, including bioinorganics, without interference from other material properties.TeRM Smart Mix Program of the Netherlands Ministry of Education, Culture and Science Netherlands Science Organisation TA-COAST Grant # 05321104 Portuguese Foundation for Science and Technology (FCT) Grant # SFRH/BD69962/201

The effect of amorphous pyrophosphate on calcium phosphate cement resorption and bone generation

AbstractPyrophosphate ions are both inhibitors of HA formation and substrates for phosphatase enzymes. Unlike polyphosphates their hydrolysis results simultaneously in the complete loss of mineral formation inhibition and a localised elevation in orthophosphate ion concentration. Despite recent advances in our knowledge of the role of the pyrophosphate ion, very little is known about the effects of pyrophosphate on bone formation and even less is known about its local delivery. In this work we first developed a self setting pyrophosphate based calcium cement system with appropriate handling properties and then compared its in vivo degradation properties with those of a non-pyrophosphate containing control. Contrary to expectation, the presence of the pyrophosphate phase in the cement matrix did not inhibit mineralisation of the healing bone around the implant, but actually appeared to stimulate it. In vitro evidence suggested that enzymatic action accelerated dissolution of the inorganic pyrophosphate ions, causing a simultaneous loss of their mineralisation inhibition and a localised rise in supersaturation with respect to HA. This is thought to be a rare example of a biologically responsive inorganic material and these materials seem to be worthy of further investigation. Bioceramics to date have mainly been limited to orthophosphate, silicate and carbonate salts of calcium, here we report the successful application of a pyrophosphate material as a degradable osteoconductive bone repair cement