The role of silica in mineralising tissues.

Caballero-Alias, A.M., 1999. The role of silica in mineralising tissues. PhD, Nottingham Trent University.

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Hydroxyapatite is the principal inorganic constituent of human bone and teeth and its formation has widely been investigated. The formation of crystalline hydroxyapatite is preceded by an amorphous calcium phosphate precipitate, the composition of which depends on the reaction solution. The effect of pH, T, precursors and ionic medium on the precipitation of calcium phosphate have been investigated. At both 23 °C and 37 °C , with the pH neutral to alkaline and at moderate supersaturation conditions, apatite is the main phase precipitated. The material is non-stochiometric (Ca/P=1.3 to 1.8) and poorly crystalline. At high supersaturation calcium phosphate dibasic and apatite form. In higher animals silicon has been shown to be an essential trace element and its involvement in bone mineralisation is well established. Since silica seems to have an important role to play in mineralisation processes, studies of the precipitation of calcium phosphate have been carried out in the presence of silica. Calcium phosphate crystals were precipitated from a solution containing 10 mM Ca(NO3)2 and 6 mM (NH4)2HPO4 in the presence of soluble silicon species (0-0.409 mM) at both 23°C and 37°C from solutions at an initial pH of 7.4. Results showed that silica promotes calcium phosphate precipitation and crystallisation. Electron microscopy analysis showed that soluble silicon species act as nucleation sites for apatite crystals. By contrast, it is believed that aluminium might play a role in the development of bone fragility and inhibit bone mineralisation. The effect of aluminium on calcium phosphate precipitation has also been investigated. Calcium phosphate was precipitated from a solution containing 10 mM Ca(NO3)2 and 6 mM (NH4)2HPO4 in the presence of freshly prepared aluminium chloride (7.5 and 75 μM) at both 23°C and 37°C from solutions at an initial pH of 7.4. The presence of aluminium decreased the crystallinity of the principal phases. Electron microscopy analysis showed that aluminium species were incorporated into apatitic material and into complex amorphous aggregates. The presence of aluminium chloride in the reaction medium appeared to inhibit the transformation of amorphous calcium phosphate and/or octacalcium phosphate to stoichiometric hydroxyapatite.

In the early 90's, Birchall hypothesised that silicon species could be involved in a mechanism to protect against aluminium poisoning. Birchall and co. workers showed that Si could lessen the toxic effect of A1 in a variety of organisms. Some studies have shown that silicic acid could release and excrete aluminium from the body stores including bone. Based on these studies, the ability of silicic acid to ameliorate the effect of aluminium on crystal composition, crystallinity and morphology have been investigated. The presence of silicon in the aluminium-containing reaction medium (up to 0.082mM) appeared to enhance the formation of hydroxyapatite at room temperature. The presence of silicon did not improve the crystallinity of dicalcium phosphate dihydrated but it seemed to improve the transformation of amorphous phosphate and/or octacalcium phosphate into hydroxyapatite. Although the hydroxyapatite crystals were not as crystalline as the control they were more perfect than the crystals formed in the presence of aluminium in the reaction media.

The bioactivity of a silica-coating was assessed using the simulated body fluid method. The osteoblast response to a silica-coating was also assessed. Primary human osteoblast cells were seeded on the surface of upright silica-coated discs. Results showed that silica-coated surfaces stimulate apatite precipitation and may promote mineralisation of osteoblast cells in vitro.

Item Type: Thesis
Creators: Caballero-Alias, A.M.
Date: 1999
ISBN: 9781369313437
Divisions: Schools > School of Science and Technology
Record created by: Jeremy Silvester
Date Added: 04 Sep 2020 08:04
Last Modified: 28 Jun 2023 08:51

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