Milenko Markovic

Preparation and Comprehensive Characterization of a Calcium Hydroxyapatite Reference Material

Numerous biological and chemical studies involve the use of calcium hydroxyapatite (HA), Ca10(PO4)6(OH)2. In this study detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface-area method. The contents of calcium (Ca2+), phosphate (PO43−), hydroxide (OH−), hydrogenphosphate (HPO42−), water (H2O), carbonate (CO32−), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unit-cell parameters, crystallinity, and solubility of this HA were determined. This highly pure, homogeneous, and highly crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910.

Precipitation of calcium oxalates from high ionic strength solutions: V. The influence of precipitation conditions and some additives on the nucleating phase

The influence of precipitation conditions (temperature, initial reactant concentrations and the mode of stirring) and some additives (glutamic acid (Glu), ornitine (Orn), tryptophan (Trp) and phosphate (PO4) on the morphology and composition of calcium oxalate initially pricipitating from high ionic strength solutions was investigated. The type of calcium oxalate hydrate formed strongly depends on the stirring conditions, e.g. the hydrodynamics of the system. Magnetic stirring promoted the formation of calcium oxalate trihydrate (COT), while in mechanically stirred and unstirred systems mixtures of calcium oxalate monohydrate (COM) calcium oxalate dihydrate (COD) and COT appeared. At a temperature of > 300 K significant transformation of metastable COT and COD into the thermodynamically stable form, COM, occurred within 1 h. All investigated aminoacids influenced the nucleation of the solid phase promoting the formation of mixtures of COM and COT in systems in which COT only was normally formed. Urinary concentrations of phosphate ions (PO4) did not change the nature of the solid phase.

Properties of Calcium Phosphate Cements With Different Tetracalcium Phosphate and Dicalcium Phosphate Anhydrous Molar Ratios

Calcium phosphate cements (CPCs) were prepared using mixtures of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA), with TTCP/DCPA molar ratios of 1/1, 1/2, or 1/3, with the powder and water as the liquid. Diametral tensile strength (DTS), porosity, and phase composition (powder x-ray diffraction) were determined after the set specimens have been immersed in a physiological-like solution (PLS) for 1 d, 5 d, and 10 d. Cement dissolution rates in an acidified PLS were measured using a dual constant composition method. Setting times ((30 ± 1) min) were the same for all cements. DTS decreased with decreasing TTCP/DCPA ratio and, in some cases, also decreased with PLS immersion time. Porosity and hydroxyapatite (HA) formation increased with PLS immersion time. Cements with TTCP/DCPA ratios of 1/2 and 1/3, which formed calcium-deficient HA, dissolved more rapidly than the cement with a ratio of 1/1. In conclusion, cements may be prepared with a range of TTCP/DCPA ratios, and those with lower ratio had lower strengths but dissolved more rapidly in acidified PLS.

Remineralization Effect of a Low-Concentration Fluoride Rinse in an Intraoral Model

A previous study showed that a sodium hexafluorosilicate-calcium chloride-based two-solution fluoride (F) rinse containing 6 mmol/l of F was more effective than a 12 mmol/l F sodium fluoride rinse in depositing F on tooth surfaces and increasing oral F levels. The present study compared the remineralization effects of these two rinses in an intraoral de- and remineralization model. The results showed that the 6 mmol/l F two-solution rinse produced greater remineralization in increasing lesion mineral contents and reducing lesion depths. The results demonstrated that the effectiveness of an F regimen depends less on the F dose and more on the ability of the treatment to utilize F efficiently for remineralization.

A new method to follow crystal growth by coulter counter

Abstract A reliable method for the determination of the rate of crystal growth in spontaneously precipitating systems was developed. The change of precipitated volume with time was determined from Coulter Counter data only. The main advantage of this method is taking into consideration all particles instead of median values and also particle volumes instead of diameters. The method can also be applied when the number of particles detected in the precipitating systems increases with time.

The crystal structure of calcium succinate monohydrate

The crystal structure of calcium succinate monohydrate, Ca(C4H4O4)·H2O, has been determined by single crystal X-ray diffraction. The crystals are monoclinic witha=11.952(2),b=9.691(2),c=11.606(2)Å, β=108.81(1)°, space group C2/c,Z=8,V=1272.49 Å3,dm=1.80, anddc=1.818 Mg m−3. The structure was refined by full-matrix least-squares techniques toR=0.027,Rw=0.040, for 829 reflections with1≥3δ(I). Ca is coordinated to seven oxygen atoms, and the coordination polyhedron is best described as a pentagonal bipyramid. One carboxylate group in the succinate ion is bonded to three different Ca ions, forming a four-membered chelate ring with one Ca ion is bonded to three different Ca ions, forming a four-membered chelate ring with one Ca ion and unidentate bridge bonds to two other Ca ions. The other carboxylate group is bonded to two Ca ions through unidentate bonds. The structure is highly polymeric. The general structural features are nearly identical to those of calcium adipate monohydrate.

An Octacalcium Phosphate Forming Cement.

The osteoconductive and possibly osteoinductive characteristics of OCP increased the interest in preparation of bone graft materials that contain OCP in its composition. Calcium phosphate cements (CPCs) were prepared using a mixture of α-tricalcium phosphate (α-TCP) and dicalcium phosphate anhydrous (DCPA), with α-TCP/DCPA molar ratio of 1/1 and distilled water or 0.5 mol/L phosphate aqueous solution (pH = 6.1 ± 0.1) as the cement liquid. Hardening time was (30 ± 1) min for the CPC mixed with water and (5 ± 1) min for the CPC mixed with phosphate solution. Diametral tensile strength (DTS), porosity (P), and phase composition (powder x-ray diffraction) were determined after the hardened specimens had been immersed in a physiological-like solution (PLS) for 1 d, 3 d, and 7 d. In CPC specimens prepared with water, calcium hydroxyapatite (HA) was formed and DTS and P were (9.03 ± 0.48) MPa and (37.05 ± 0.20) vol % after 1 d, respectively, and (9.15 ± 0.45) MPa and (37.24 ± 0.63) vol % after 3 d, respectively. In CPC specimens prepared with phosphate solution OCP and HA were formed and DTS and P were (4.38 ± 0.49) MPa and (41.44 ± 1.25) vol % after 1 d, respectively,(4.38 ± 0.29) MPa and (42.52 ± 2.15) vol % after 3 d, respectively, and (4.30 ± 0.60) MPa and (41.38 ± 1.65) vol % after 7 d, respectively. For each group DTS and P did not change with PLS immersion time. DTS was significantly higher and P was significantly lower for CPCs prepared with water. HA formation slightly increased with immersion time from 40 mass % after 1 d to 50 mass % after 3 d in CPCs prepared with water. OCP + HA formation increased with immersion time from 30 mass % after 1 d to 35 mass % after 3 d and to 45 mass % after 7 d in CPCs prepared with 0.5 mol/L phosphate solution.

Calcium Fluoride Precipitation and Deposition From 12 mmol/L Fluoride Solutions With Different Calcium Addition Rates

The effects of different Ca-addition rates on calcium fluoride (CaF2) precipitation and deposition were investigated in 12 mmol/L sodium fluoride solutions to which 0.1 mol/L calcium chloride solution was continuously added at average rates of (5, 7.5, 10, 12.5, 15 or 20) mmol L−1 min−1. The changes in ionic fluoride and calcium concentrations, as well as turbidity, were continuously recorded by F and Ca electrodes, and a fiber optic based spectrophotometer, respectively. The F− concentration decreased and turbidity increased with time indicating precipitation of CaF2. For the systems with Ca-addition rates of (5, 7.5, 10, 12.5, 15, and 20) mmol L−1 min−1, the 1 min CaF2 depositions in the model substrate (cellulose filter paper, pores 0.2 µm) expressed as mean ± SD of deposited F per substrate surface area were (3.78 ± 0.31, 11.45 ± 0.89, 9.31 ± 0.68, 8.20 ± 0.56, 6.63 ± 0.43, and 2.09 ± 0.28) µg/cm2, respectively (n = 10 for each group). The 1-min F depositions did not show positive correlation to Ca-addition rates. The lowest 1-min F deposition was obtained in the systems with the highest Ca-addition rate of 20 mmol L−1 min−1 for which CaF2 precipitation rate reached the maximum value of 0.31 mmol L−1 s−1 almost immediately after beginning of reaction (6 s). The largest 1-min F depositions were obtained from the systems with Ca addition rates of (7.5 to 12.5) mmol L−1 min−1 in which CaF2 precipitation rates continuously increased reaching the maximum values of (0.13 to 0.20) mmol L−1 s−1 after (18 to 29) s, respectively. The 1-min F depositions were greatly enhanced in comparison with the control F solutions that did not have continuous Ca-addition. This indicates that continuous Ca addition that controls the rate of CaF2 formation could be a critical factor for larger F depositions from F solutions. The efficacy of conventional F mouthrinses could be improved with addition of a substance that continuously releases Ca.

Precipitation of calcium oxalates from high-ionic-strength solutions. Part 6.—Kinetics of precipitation from solutions supersaturated in calcium oxalates and phosphates

The kinetics of precipitation of calcium oxalate trihydrate (COT) and calcium hydrogenphosphate dihydrate (DCPD) from high-ionic-strength solutions (I= 0.26 mol dm–3, made up with NaCl) supersaturated in both solid phases [Si(COT)=Pi/Ksp= 6.86 – 47.20; Si(DCPD)= 2.10 – 3.57] have been investigated. Under the experimental conditions employed [e.g. initial total reactant concentrations c(Ca)=c(PO4)=(2.6 – 2.8)× 10–2 mol dm–3, c(C2O4)=(1.5 – 10)× 10–4 mol dm–3; pHi 5; temperature 298 K; magnetic stirring alone or followed by mechanical stirring] COT precipitated first and effectively initiated the precipitation of DCPD. Under the conditions at which the induction periods ti(COT) and ti(DCPD) differed by more than 1 h, previously defined quantitative criteria were employed to determine the influence of excess phosphate and calcium ions on the kinetics of crystal growth and aggregation of COT. It is shown that phosphate ions inhibit crystal growth, but have no influence on aggregation. A comparison with previous results obtained at equimolar total calcium and oxalate concentrations [e.g. c(Ca)=c(C2O4)] shows that in the presence of excess calcium ions the efficiency of the inhibition of crystal growth by aggregation is enhanced, while the collision rate coefficient and rate-controlling mechanism of aggregation are not significantly affected.

Physicochemical Properties of Fluorapatite

Fluoride (F) is a subject of great interest in dental caries research because F, applied in various ways, is the most effective factor in reducing caries. The cariostatic mechanisms of F are complex and have been studied extensively for several decades, and the large body of information in the literature on this subject was reviewed recently.1,2