J.M Oliveira

Effects of Si speciation on the in vitro bioactivity of glasses

The surface reactivity of glasses belonging to the (mol%) 31SiO2-11P2O5-(58-x)CaO-xMgO series, with x ranging from 0 to 32, was studied in Kokubos simulated body fluid (SBF). Scanning electron microscopy, inductively coupled plasma spectroscopy and Fourier transform infrared spectroscopy were used to characterise the glass surface and the SBF compositional changes. All glasses develop surface layers rich in silica and calcium phosphate. An increasing surface activity with increasing MgO/CaO ratio was observed. In a previous investigation using magic-angle spinning nuclear magnetic resonance it was found that there is an increasing abundance of Q0 species in the glass structure with increasing MgO content. The present work shows that, when immersed in SBF, Q0-rich glasses are easily leached to form a silica gel layer. It is concluded that MgO in the glass indirectly improves the early stages of mineralisation by favouring Q0 speciation. This mechanism plays an important role in glass bioactivity.

Surface modifications of a glass and a glass-ceramic of the MgO-3CaO · P2O5-SiO2 system in a simulated body fluid

A glass of nominal composition (wt%) 17.25 MgO-52.75 3CaO.P2O5-30 SiO2 and a glass-ceramic obtained from it showed surface modifications when immersed in an acellular medium having a composition similar to that of human blood plasma. A (Ca, P)-rich layer, with an approximate Ca/P atomic ratio of 1.7, identified as hydroxyapatite, developed on both samples. The precipitated film on the glassy sample was weakly bonded, whereas that formed on the glass-ceramic was strongly adherent. The apatite precipitated during the in vitro tests on both samples grew as a needle-like structure with crystals about 150-200 nm long and 50-70 nm thick, as measured on specimens soaked for 1 month in the simulated body fluid (SBF). The presence of calcium and phosphate ions in the SBF contributed to the precipitation of the (Ca, P)-rich layers on both specimens.

Influence of the CaO/MgO ratio on the structure of phase-separated glasses: a solid state 29Si and 31P MAS NMR study

Abstract 29 Si and 31 P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was used to investigate structural aspects of glasses of the series (mol%) 31SiO 2 –11P 2 O 5 –(58− x )CaO– x MgO, with x ranging from 0 to 32, in which precipitation of amorphous silica occurs. As expected for alkaline-earth silicates, a strong 29 Si MAS NMR peak was recorded, shifting to higher frequencies with decreasing CaO/MgO molar ratios. Q 2 species predominate in the alkaline-earth rich phase and the observed shift in the 29 Si spectra is attributed to a decrease in the average degree of Si polymerization in the matrix. The experimental evidence suggests that Mg 2+ replaces Ca 2+ and also promotes the disruption of Si–O–Si bonds. Substitution of CaO by MgO increases the amount of dispersed phase. 29 Si MAS NMR showed the presence of Q 4 species in the dispersed phase and also suggested the presence of Q 0 species. A disproportionation reaction 2Q 2 ⇌Q 0 +Q 4 is proposed to balance the silicate charges. 31 P MAS NMR showed that phosphorus is present in an orthophosphate environment. The shift of the 31 P MAS NMR peak was attributed to the replacement of Ca by Mg.

Role of acid attack in the in vitro bioactivity of a glass-ceramic of the 3CaO.P2O5-CaO.SiO2-CaO.MgO.2SiO2 system.

A non-bioactive glass-ceramic (GC13) that contains hydroxyapatite (Ca5(PO4)3OH), diopside (CaMg(SiO3)2) and althausite (Mg2 PO4OH) as crystalline phases has been obtained by thermal treatment of a parent bioactive glass (G13) of nominal composition (wt%) 40.0 CaO-34.5 SiO2-16.5 P2O5-8.5 MgO-0.5CaF2. To induce bioactivity, GC13 was chemically treated with 1 M HCl for different periods of time. After chemical etching the in vitro studies showed formation of an apatite-like surface layer. In this article the influence of etching time both on the surface composition of the glass-ceramic and on the growth rate of the apatite layer is studied. It is concluded that the presence of hydroxyapatite in the glass-ceramic, associated to microstructural fluctuations, can favour apatite deposition in vitro.

Effect of SiO2 on amorphous phase separation of CaO–P2O5–SiO2–MgO glasses

Abstract Scanning and transmission electron microscopy, differential thermal analysis and Raman spectroscopy were used to investigate glasses of the 52.75(1+x/70)3 CaO · P 2 O 5 −17.25(1+x/70) MgO −(30−x) SiO 2 (wt%) series (with x=0–10). In glasses for which the silica content is 24 wt% or larger a dispersed amorphous silica phase was present. Differential thermal analysis showed that the transition temperature and the second exothermic peak temperature are independent of silica content between 30 and 23 wt%. The first crystallization peak is independent of silica content in all the range investigated. Raman spectroscopy showed that the structure of the 20 wt% SiO2 glass differs from the others. We conclude that for a silica content 23 wt% the glasses are phase separated and the degree of polymerisation of the matrix is constant; the monophasic glass with 23 wt% SiO2 has the same composition and structure as the matrices of all other phase-separated glasses.

TiO2-induced phase separation and crystallization in SiO2–3CaO·P2O5–MgO glass

Abstract The effect of TiO 2 additions on microstructure and crystallization of a 42.6 wt% SiO 2 , 29.4 wt% 3CaO·P 2 O 5 , 28.0 wt% MgO glass was studied. TiO 2 produced amorphous separation and volume crystallization during ceramming. Heat treatments to 1100°C resulted in a simultaneous crystallization of β-3(Ca, Mg)O·P 2 O 5 and clino-enstatite (MgO·SiO 2 ) in TiO 2 -free glasses and the occurrence of a third phase, forsterite (2MgO·SiO 2 ), in all TiO 2 -containing glasses. Peak crystallization temperatures suggested that crystallization rates increase with TiO 2 contents to 3% and that further changes do not occur.

Structure and Degradation Behaviour of Calcium Phosphate Glasses

Some studies have shown a relationship between glass structure and in vitro mineralization, generally associated with the rate of glass degradation, nature of released ions and subsequent Ca-P precipitation on glass surfaces when immersed in a Simulated Body Fluid (SBF). The knowledge of the ionic species distribution in glasses and of the involved bond strengths can be used to assess the in vitro behaviour of a glass. The role of ions such as silicon or titanium is of major importance for the development of new compositions and also for the control of glass degradation behaviour. A comparative study with two calcium phosphate glasses series was performed: Both glasses series – one with Si and another with Ti – include P2O5 and alkaline earth ions in their compositions. Surface reactivity of glasses from the SiO2-containing system have been studied in SBF showing the precipitation of a Ca-P surface layer that increases with increasing MgO/CaO ratio. In glasses from the TiO2-containing series it is shown that the increase of TiO2 contributes for the stabilization of the glass network thus allowing the control of their degradation rate when immersed in SBF. The relationship between structural features of these calcium-phosphate glasses and their degradation behaviour in SBF is discussed in terms of the structural role of Si and Ti ions. It is concluded that glasses with less interconnected species favour the Ca-P surface precipitation. The understanding of this relationship in synthetic physiological fluids is expected to allow the tailoring of glass degradation rates in complex biological systems.

Effect of TiO2 and SiO2 on Surface Reactivity of Calcium Phosphate Glasses in Simulated Body Fluid

This study compares the in vitro behaviour in SBF of glasses from two different systems, TiO2-CaO-P2O5 and SiO2-CaO-P2O5 with the same TiO2 and SiO2 molar content, in order to evaluate the effect of TiO2 and SiO2 on the surface reactivity of those glasses. The glass formation regions in both systems were observed for compositions with less than 40 mol% TiO2 and 40 mol% SiO2, respectively. Four glasses with similar TiO2 and SiO2 molar contents have been selected for comparison. These glasses are equimolar in CaO and P2O5 with TiO2 or SiO2 varying from 4 to 33 mol %. Powder glasses were immersed in Simulated Body Fluid (SBF) and kept at 37°C for different times, up to 14 days. Surfaces were observed by Scanning Electron Microscopy (SEM) and specimen ion leaching to SBF was studied by Inductively Coupled Plasma (ICP) spectroscopy. Preliminary spectroscopic studies by Raman were performed to identify the structure of the glasses. For glasses of the SiO2-CaO-P2O5 system a significant dissolution of all ions was observed together with the formation of phosphoric acid. In opposition, the immersion of TiO2-CaO-P2O5 glasses produced a small Ca consumption and stable Ti and P concentrations, indicating the formation of a Ca-P rich layer on these glasses. The observed differences in the dissolution behaviour are tentatively explained in terms of the glass structures obtained by spectroscopy.