Priya Saravanapavan

Mesoporous calcium silicate glasses. I. Synthesis

Abstract The conventional method of preparing glasses is quenching a melt. However, as with many multicomponent glasses, only a certain range of CaO–SiO2 compositions can be melt derived. Sol–gel processing allows one to produce gel-glasses with compositions lying within the liquid–liquid immiscibility dome of the CaO–SiO2 system. Crack-free silica–calcia xerogel monoliths of various shapes and sizes of compositions expressed by the following formula xCaO·(1−x)SiO2, where 0⩽x⩽0.5 mole fraction, were prepared via the sol–gel technique, starting from tetraethyl orthosilicate and calcium nitrate. In this paper the processing technique is described. The materials produced were characterised using X-ray diffraction, infrared (FTIR) spectroscopy, energy dispersive scanning electron microscopy (SEM-EDS) as well as differential thermal analysis (DTA). The gel-glasses produced are confirmed to be amorphous even after stabilisation at 600 °C, with crystallisation temperatures above 850 °C. When sintered at 1000 °C they form crystalline phases in accordance to the CaO–SiO2 phase diagram. Acid digestion analysis confirms that the compositions of the gel-glasses are similar to that of the computed values. The gel-glasses are homogeneous throughout the monolith.

Low‐temperature synthesis, structure, and bioactivity of gel‐derived glasses in the binary CaO‐SiO2 system

Glasses in the binary system CaO-SiO2 for which the molar fraction of CaO is 0 < or = x < or = 0.50, were prepared by means of a sol-gel route starting from tetraethylorthosilicate and calcium nitrate. The textural features of the monoliths obtained were characterized using N2 gas adsorption, helium ultrapycnometry, and mercury porosimetry. In vitro bioactivity tests were performed in simulated body fluid (SBF). The ionic concentration of the SBF after glass immersion was analyzed using inductively coupled plasma atomic emission spectroscopy. The surfaces of the specimens were characterized using X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray analysis before and after in vitro testing. The textural characterization revealed that the glasses were mesoporous with cylindrical pores with average diameters ranging from 25 to 663 A depending on the molar fraction of CaO. The in vitro studies showed that all binary CaO-SiO2 gel-glass compositions produced were bioactive. These results indicate that the binary gel-derived CaO-SiO2 system exhibits a level of bioactivity over a similar molar range of SiO2 content as the previously studied ternary CaO-P2O5-SiO2 system.

The structure of a bioactive calcia:silica sol-gel glass

Sol-gel derived calcium silicate glasses may be useful for the regeneration of damaged bone. The mechanism of bioactivity is as yet only partially understood but has been strongly linked to calcium dissolution from the glass matrix. In addition to the usual laboratory-based characterisation methods, we have used neutron diffraction with isotopic substitution to gain new insights into the nature of the atomic-scale calcium environment in bioactive sol-gel glasses, and have also used high energy X-ray total diffraction to probe the nature of the processes initiated when bioactive glass is immersed in vitro in simulated body fluid. The data obtained point to a complex calcium environment in which calcium is loosely bound within the glass network and may therefore be regarded as facile. Complex multi-stage dissolution and mineral growth phases were observed as a function of reaction time between 1 min and 30 days, leading eventually, via octacalcium phosphate, to the formation of a disordered hydroxyapatite (HA) layer on the glass surface. This methodology provides insight into the structure of key sites in these materials and key stages involved in their reactions, and thereby more generally into the behaviour of bone-regenerative materials that may facilitate improvements in tissue engineering applications.

Mesoporous calcium silicate glasses. II. Textural characterisation

The textural features, such as the surface area and porosity (pore structure and pore volume), of crack-free homogeneous monolith gel-glasses in the binary CaO–SiO2 compositional system are analysed using nitrogen adsorption, mercury porosimetry and helium pycnometry. The first two methods, used to study the pore morphology, are in agreement leading to the conclusion that the pore system in the gel-glasses studied consists of a three-dimensional network of cavities (pores) interconnected by constrictions (throats) in the mesopore range (75–314 A in diameter) with the pore sizes dependent on the composition of the gel-glasses. The surface area increases while the pore size decreases when the CaO content is decreased. The skeletal and bulk densities both increase when the CaO content is decreased and the gel-glasses are of approximately 30% porosity.

Antimicrobial Macroporous Gel-Glasses: Dissolution and Cytotoxicity

The incidence of biomaterial-centered infections underlies the need to improve the properties of existing biomaterials. Combining the bioactive properties of calcia-silicate gel-glasses with that of the silver would prevent infections without the use of antibiotic drugs. Inclusion of silver into bioactive gel-glass foam scaffolds is explored using in vitro characterization techniques. The amt. of silver released from Ag-doped S70C30 foams is well above the min. bactericidal concn. (0.1 ppm) but below the cytotoxic concn. (1.6 ppm) for human cells. Primary human osteoblasts proliferate on the silver-doped gel-glasses. [on SciFinder (R)]

Indirect Cytotoxicity Evaluation of Silver Doped Bioglass Ag-S70C30 on Human Primary Keratinocytes

Bioactive gel-glasses, such as the silver-doped Ag-S70C30 glass, can be used to modify the inflammatory response in a local body compartment such as in acne lesions and in nonhealing dermal wounds. In this study, the cytotoxicity of soluble silver, calcium and silica ions on human epidermal keratinocytes was investigated by measurements of mitochondrial activity (MTT assay) and neutral red dye uptake (NR assay). Ag-S70C30 extracts were prepared by soaking glass powder in complete culture medium at concentrations of 1 mg/ml and 2 mg/ml (mg of glass powder per ml of culture medium). Silver concentrations for both concentrations of approximately 1 ppm were detected by inductive coupled plasma analysis (ICP). No negative effect on the cell viability was measured for an initial gel-glass concentration of 1 mg/ml and for the two shortest extraction times at a concentration of 2 mg/ml. Based on the results from MTT/ NR assays, a pH rise of approximately one unit had no negative effect on the NHEK-A cell viability. This preliminary study on keratinocyte viability merits future investigations on silver bioglass as a novel antimicrobial wound healing agent.

Antimicrobial Treatment of Dental Osseous Defects with Silver Doped Bioglass: Osteoblast Cell Response

In dentistry, chronic periodontitis often leads to bone resorption together with an increasing risk of bacteremia. Bioactive glass has found extensive application as dental graft material. A successful antimicrobial bactericidal effect has been shown from the introduction of Ag2O into the glass composition. In this study, the cytotoxicity of soluble silver, calcium and silica ions on primary human osteoblasts was investigated by measurements of mitochondrial activity and neutral red dye uptake. Silver concentrations of 4 - 6 ppm (1 mg/ml conc.) and 6 - 9 ppm (2 mg/ml conc.) have been measured in complete culture medium. It was found that the bioactive gel-glass extract with an initial concentration of 1 mg/ml (1mg glass per ml of culture medium) has no negative effect, whereas increased gel-glass concentration of 2 mg/ml seemed to have a toxic effect on the cell viability of human osteoblasts. It might be concluded that a reduction of the rate of silver dissolution from the bioactive gel-glass might preserve a maximum cell viability.

The Atomic-Scale Interaction of Bioactive Glasses with Simulated Body Fluid

The formation of a carbonate-containing hydroxyapatite, HCAp, layer on bioactive calcium silicate sol-gel glass of the formula (CaO)0.3(SiO2)0.7 has been studied in-vitro in Simulated Body Fluid (SBF). Extended X-ray Absorption Fine Structure (EXAFS), X-ray Absorption Near Edge Structure (XANES), X-ray diffraction (XRD), and solid state nuclear magnetic resonance (NMR) measurements have been performed with results showing the formation of a significantly amorphous HCAp layer after less than 5 hours in solution.

Dissolution of Bioactive Gel-Glass Powders in the SiO2-CaO System

Glass powders containing 70 mol% SiO2 and 30 mol% CaO have been prepared using the sol-gel technique and their in vitro bioactivity was studied by immersing them in simulated body fluid at 37 °C. The formation of the hydroxy carbonate apatite layer on the powder surface was analysed using Fourier transform infrared spectroscopy and X-ray diffraction techniques while the dissolution of the gel-glass was followed by inductively coupled plasma – optical emission spectroscopy. The results show that the deposition of amorphous calcium phosphate, takes place within the first 30 minutes of reaction in SBF indicating that the S70C30 gel-glass powder is a ‘Class A’ bioactive material and is comparable in bioactivity to the previously studied 45S5 Bioglass® and 58S gelglass compositions.

Effect of Composition and Texture on Controlled Rate Release of an Antibacterial Agent from Bioactive Gel-Glasses

Bioactive glasses are not only biocompatible they elicit a cellular response from the host tissue resulting in an interfacial bond to implants. However, the incidence of biomaterial-centred infections underlies the need to improve the properties of existing biomaterials. The broad-spectrum antimicrobial action of silver has been well exploited since ancient times. Combining the bioactive properties of glasses with that of the silver would prevent infections without the use of antibiotic drugs. In this paper the in vitro bioactivity of two bioactive gel-glass compositions (58S and S70C30) and their silver containing equivalents are reported. This study leads to the following conclusions: 1) The rate of formation of hydroxy apatite on the silver doped powders is slower than their un-doped counterparts. 2) The presence of silver does not compromise the in vitro bioactivity of the gel-glasses