Petros G. Koutsoukos

Effect of surface roughness of hydroxyapatite on human bone marrow cell adhesion, proliferation, differentiation and detachment strength.

Initial attachment of osteoblast cells and mineralization phenomena are generally enhanced on rough, sandblasted substrata. In the present work the effect of surface roughness of hydroxyapatite (HA) on human bone marrow cell response was investigated. Human bone marrow cells were plated onto HA disc-shaped pellets, prepared from synthetic HA powder. The pellets were sintered and polished with SiC paper 180-, 600- and 1200-grit, resulting in three surface roughness grades. Cell adhesion, proliferation and differentiation (evaluated with the expression of ALP activity) were determined following various incubation periods. Cell detachment strength was determined as the shear stress required to detach a given quantity of the adherent cells from the different substrata, using a rotating disc device that applied a linear range of shear stresses to the cells. The cells attached and grew faster on culture plastic in comparison with HA. No statistically significant differences were observed in the expression of ALP activity on all three HA surfaces and culture plastic. Cell adhesion, proliferation and detachment strength were surface roughness sensitive and increased as the roughness of HA increased. The percentage of the adherent cells decreased in a sigmoidal mode as a function of the applied shear stress. In conclusion, surface roughness of HA generally improved the short- and longer-term response of bone marrow cells in vitro. This behavior could be explained by the selective adsorption of serum proteins.

The transformation of vaterite to calcite: effect of the conditions of the solutions in contact with the mineral phase

Abstract The transformation of the thermodynamically unstable vaterite into the stable calcite was investigated during the precipitation of calcium carbonate, under conditions of constant supersaturation at 25, 35 and 45°C and pHs 8.5, 9.0 and 10.0. The calcite content in the solid precipitates was determined quantitatively by the X-ray diffraction technique (XRD) using the appropriate mixed standards of known composition. Scanning electron microscopy (SEM) and particle number and size measurements were also used for the characterisation of the solid precipitates. It was found that the transformation of vaterite into calcite does not depend on pH and temperature in the range 25–45°C. It depends only on the supersaturation, and this, only at relatively high supersaturation ratios (1.5–1.9). Specifically, the rate of transformation decreases as supersaturation increases. It was suggested on the basis of the kinetics results, that the transformation takes place through dissolution of vaterite, preferably of the small crystals followed by the crystallisation of calcite. At relatively high supersaturation ratios (1.5–1.9) the transformation is controlled by the dissolution of vaterite, whereas at lower supersaturation ratios (1.2–1.5) the rate of dissolution of vaterite is similar with that of crystallisation of calcite. Finally, a model equation which predicts a linear dependence of the rate of transformation on supersaturation ratio has been derived at relatively high supersaturation ratios.

Modern Views on Desilicification: Biosilica and Abiotic Silica Dissolution in Natural and Artificial Environments

Institute of Bioanalytical Chemistry, Dresden University of Technology, D-01069 Dresden, Germany, Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, University of Crete, Voutes Campus, GR-71003 Heraklion, Crete, Greece, Laboratory of Mechanisms and Transfer in Geology, Observatory Midi-Pyrenees (OMP), UMR 5563, CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France, and FORTH-ICEHT and Laboratory of Inorganic and Analytical Chemistry, Department of Chemical Engineering, University of Patras, GR-265 04 Patras, Greece[007f]

Crystal growth of calcium phosphates - epitaxial considerations

Abstract The growth of one crystalline phase on the surface of another that offers a good crystal lattice match, may be important in environmental, physiological and pathological mineralization processes. The epitaxial relationships and kinetics of growth of hydroxyapatite on crystals of dicalcium phosphate dihydrate, calcium fluoride and calcite have been studied at sustained low supersaturation with respect to hydroxyapatite. At the very low supersaturations, the crystallization of hydroxyapatite takes place without the formation of precursor phases. The experimental results are in agreement with theoretical predictions for epitaxial growth, while the kinetics of hydroxyapatite crystallization on the foreign substrates is the same as that for the growth of hydroxyapatite on synthetic hydroxyapatite crystals.

The inhibition of calcium carbonate precipitation in aqueous media by organophosphorus compounds

The formation of insoluble calcium carbonate scale is often undesirable and the use of water additives which prevent or retard the deposition of this salt is widespread as the alternative solution of water acidification would cause severe corrosion problems. In the present work, the effect of four synthetically prepared compounds, 1,2-dihydroxy-1,2-bis(dihydroxyphosphonyl)ethane (DDPE), 2-dihydroxyphosphonyl-2-hydroxypropionic acid (DHHPA), 1,3-bis[(1-phenyl-1-dihydroxyphosphonyl)methyl]-2-imidazolidinone (BPDMI), and 2,3-bis(dihydroxyphosphonyl)-1,4-butanedioic acid (BDBA), on the spontaneous precipitation of calcium carbonate in aqueous media was investigated. The precipitation process was studied in batch reactors at 25°C and pH 8.50, under conditions of sustained supersaturation. Two of the organophosphorous compounds investigated, DDPE and BDBA, contained PCCP bonds in their molecules. In a third, BPDMI, the two PC bonds were separated by a bulky five-membered ring, and the other, DHHPA, contained only one PC bond. All organophosphorus compounds examined in the present work were synthesized in the laboratory and were found to be resistant to hydrolysis even at elevated temperatures, typical of those existing in geothermal wells. In all cases, the induction periods preceding the onset of the spontaneous precipitation of calcium carbonate were significantly increased depending on the concentration of the additives examined, while the concomitant precipitation rates were markedly slower at concentration levels as low as 2 × 10−8 mol dm−3. It is possible that the inhibiting effect of the organophosphorus compounds tested is due to the blocking of the active growth sites of the crystallites formed after the induction period. The existence of the PCCP bonds in the molecules of the organophosphorus compounds resulted in the most effective inhibitors, while the separation of the two phosphonyl groups by the bulky five-membered ring resulted in the same inhibitory activity as with the compound possessing only one phosphonyl group.

The crystallization of hydroxyapatite and fluorapatite in the presence of magnesium ions

Abstract Alkaline earth cations present in biological fluids and in natural waters may play an important role in regulating the formation of calcium phosphate solid phases. The kinetics of crystallization of hydroxyapatite (HAP) and fluorapatite (FAP) has been investigated under constant composition conditions in which the supersaturation was maintained constant during the crystallization reactions. Magnesium ions markedly retard the rates of precipitation of both HAP and FAP and the fractional reduction in rate constant can be interpreted in terms of the blocking of active growth sites through adsorption of magnesium ions at the crystal surfaces. During crystallization, the magnesium ions are excluded from the crystal lattices and their adsorption at the surface may be interpreted in terms of a Langmuir isotherm.

Protein adsorption on hematite (α-Fe2O3) surfaces

Abstract Adsorption isotherms for human plasma albumin (HPA) and bovine pancreas ribonuclease (RNase) on hematite surfaces have been determined at different ionic strength and pH. Calorimetric data point to different modes of adsorption of the HPA molecule at and away from isoelectric conditions, respectively. Under most conditions the adsorption enthalpy is positive, even if the HPA molecule and the hematite surface are oppositely charged, so that the adsorption must be driven by an increase of entropy. At high surface coverage θ lateral interactions between the adsorbed HPA molecules are shown to play an important role in the adsorption mechanism, the enthalpy of those interactions becoming more endothermic with increasing distance from the isoelectric point of the protein. For HPA, additional information is obtained from infrared spectroscopy and electrophoresis, both as a function of pH and θ. At the isoelectric point of the protein the extent of reconformation of the HPA molecule is a minimum; this extent increases with increasing distance from the i.e.p. The variation of reconformation with θ is most pronounced at low θ. Adsorption is accompanied by the uptake of low-molecular-weight electrolyte. The general conclusion is that the adsorption of RNase is to a large extent determined by coulombic interactions, whereas with HPA other factors are dominant. For the latter protein, the electrical contributions to the interaction with the substrate can to a large extent be unraveled.

The crystallization of calcite in the presence of orthophosphate

The kinetics of crystal growth of calcite seed crystals was investigated at pH 8.50, 25°C, in stable calcium carbonate supersaturated solutions. An apparent rate order of 2 suggests a spiral growth mechanism. The presence of inorganic orthophosphate in the supersaturated solutions at levels as low as 8 × 10−8 mole dm−3 completely inhibited crystal growth. Application to a kinetic Langmuir-type model suggested that adsorption of phosphate at the active growth sites is responsible for the reduction in the crystal growth rates. Adsorption studies revealed that phosphate adsorbs on calcite and the Langmuir equation was found to give a satisfactory fit of the data at ionic strength 0.1 mole dm−3. At lower solution ionic strengths, adsorption was enhanced, and the plateau attained still corresponded to a surface coverage lower than that anticipated for a monolayer of inorganic orthophosphate anions. The ionic strength dependence, however, suggests that the calcite—phosphate interaction is mainly electrostatic in its nature. Electrokinetic measurements yielded an isoelectric point for calcite at pH 10.4 ± 0.3. Adsorption of orthophosphate at the calcite/water interface resulted in more negative electrokinetic charges, as a consequence of the adsorption of negative phosphate species in the solution.

Principles of demineralization: modern strategies for the isolation of organic frameworks. Part I. Common definitions and history.

In contrast to biomineralization phenomena, that are among the most widely studied topics in modern material and earth science and biomedicine, much less is systematized on modern view of demineralization. Biomineralized structures and tissues are composites, containing a biologically produced organic matrix and nano- or microscale amorphous or crystalline minerals. Demineralization is the process of removing the inorganic part, or the biominerals, that takes place in nature via either physiological or pathological pathways in organisms. In vitro demineralization processes, used to obtain mechanistic information, consist in the isolation of the mineral phase of the composite biomaterials from the organic matrix. Physiological and pathological demineralization include, for example, bone resorption mediated by osteoclasts. Bioerosion, a more general term for the process of deterioration of the composite biomaterials represents chemical deterioration of the organic and mineral phase followed by biological attack of the composite by microorganisms and enzymes. Bioerosional organisms are represented by endolithic cyanobacteria, fungi, algae, plants, sponges, phoronids and polychaetes, mollusks, fish and echinoids. In the history of demineralization studies, the driving force was based on problems of human health, mostly dental caries. In this paper we summarize and integrate a number of events, discoveries, milestone papers and books on different aspect of demineralization during the last 400 years. Overall, demineralization is a rapidly growing and challenging aspect of various scientific disciplines such as astrobiology, paleoclimatology, geomedicine, archaeology, geobiology, dentistry, histology, biotechnology, and others to mention just a few.

The crystallization of calcium carbonate on polymeric substrates

Abstract Sulfonated polystyrene and polystyrene divinylbenzene polymers were found to be substrates, suitable for the growth of calcium carbonate monohydrate, slowly converting into the thermodynamically stable calcite. The nature of the calcium carbonate polymorph forming was confirmed as being calcium carbonate monohydrate. Kinetics analysis has shown that the apparent order of the crystal growth reaction is 2.0, pointing out a surface controlled mechanism. Analysis of the initial rates of reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 29 mJ/cm 2 for the surface energy of the growing phase and a size of 4 ions in the ion cluster forming the critical nucleus. In all cases, a well-defined induction period preceded the onset of calcium carbonate monohydrate overgrowth, reflecting the time required for the formation of the critical nucleus. The induction times were inversely proportional with respect to the solution supersaturation while the rates of the concomitant crystal growth increased with supersaturation.