The influence of calcium and magnesium on the growth of the lichens Parmelia saxatilis and Xanthoria parietina on slate substrates

In South Gwynedd, Wales, U.K., the calcicole lichen Xanthoria parietina occurs not only on alkaline substrates at inland sites but also on siliceous rock at coastal martimie sites while the calcifuge species Parmelia saxatilis occurs only at inland sites and on slate rocks. Samples of maritime and inland slate did not differ significantly in their calcium or magnesium content. Thalli of X. parietina on pieces of slate did not survive when transplanted from maritime rocks to a site inland. Thalli of maritime X. parietina and P. saxatilis on slate were then transplanted to a site inland and were treated at intervals during 1 year either with calcium carbonate applied as a thick paste or a 0.25 mM solution of calcium chloride. Treatment of X. parietina with calcium carbonate enabled the thalli to survive and grow. However, addition of calcium carbonate to P. saxatilis resulted in low growth rates and fragmentation of the centres of the thalli. The calcium chloride solution had no statistically significant effects on the growth of either species. In addition, thalli of both species were treated with calcium or magnesium carbonates or wetted with an alkaline buffer at intervals over 12-14 months. Thalli of X. parietina survived and grew rapidly when treated with either carbonate but the growth of the buffer-treated thalli gradually declined over the experimental period. Thalli of P. saxatilis fragmented and disappeared after 8-10 months after treatment with either carbonate but normal growth occurred in the buffer treatment. Xanthoria parietina may occur on siliceous maritime rocks at the site because of the presence of calcium or magnesium in sea spray combined with the spray’s alkaline pH. By contrast, P. saxatilis may be confined to siliceous rocks inland because the thalli grow poorly in the presence of calcium and magnesium.

Effect of calcium source on structure and properties of sol-gel derived bioactive glasses

The aim was to determine the most effective calcium precursor for synthesis of sol-gel hybrids and for improving homogeneity of sol-gel bioactive glasses. Sol-gel derived bioactive calcium silicate glasses are one of the most promising materials for bone regeneration. Inorganic/organic hybrid materials, which are synthesized by incorporating a polymer into the sol-gel process, have also recently been produced to improve toughness. Calcium nitrate is conventionally used as the calcium source, but it has several disadvantages. Calcium nitrate causes inhomogeneity by forming calcium-rich regions, and it requires high temperature treatment (>400 C) for calcium to be incorporated into the silicate network. Nitrates are also toxic and need to be burnt off. Calcium nitrate therefore cannot be used in the synthesis of hybrids as the highest temperature used in the process is typically 40-60 C. Therefore, a different precursor is needed that can incorporate calcium into the silica network and enhance the homogeneity of the glasses at low (room) temperature. In this work, calcium methoxyethoxide (CME) was used to synthesize sol-gel bioactive glasses with a range of final processing temperatures from 60 to 800 C. Comparison is made between the use of CME and calcium chloride and calcium nitrate. Using advanced probe techniques, the temperature at which Ca is incorporated into the network was identified for 70S30C (70 mol % SiO, 30 mol % CaO) for each of the calcium precursors. When CaCl was used, the Ca did not seem to enter the network at any of the temperatures used. In contrast, Ca from CME entered the silica network at room temperature, as confirmed by X-ray diffraction, Si magic angle spinning nuclear magnetic resonance spectroscopy, and dissolution studies. CME should be used in preference to calcium salts for hybrid synthesis and may improve homogeneity of sol-gel glasses.

In situ mineralization by the release of calcium and phosphate ions from nanogels

Biomimetic hydroxyapatite was synthesized by the controlled release of calcium and phosphate ions from poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-co-AA)) nanogels. Mixing nanogels containing calcium chloride (CaCl2 ·2H2O) and nanogels containing sodium hydrogen phosphate (Na2HPO4·2H2O) in simulated body fluid (SBF) at physiological conditions of 37 °C and pH 7.4, biomimetic hydroxyapatite was obtained. By studying separately the loading and controlled release of the salts from the nanogels, adequate conditions were chosen to synthesize the hydroxyapatite: Calcium loaded (Ca-loaded) nanogels (1000 mg/ml; 400:3) and inorganic phosphate loaded (Pi-loaded) nanogels (90 mg/ml; 12:1) in a ratio of 2:1 were placed in SBF solution. The obtained powders characterization showed that a low crystalline and substituted hydroxyapatite similar to bone apatite was formed. Such a strategy could be used in medical and dental procedures to induce rapid inorganic mineral formation from a nanogel-containing biomaterial. © 2012 American Scientific Publishers. All rights reserved.