Jer Hsu

Relationships among carbonated apatite solubility, crystallite size, and microstrain parameters.

Abstract. The use of the metastable equilibrium solubility (MES) concept to describe the solubility properties of carbonated apatites (CAPs) and human dental enamel (HE) has been well established in previous studies using a range of CAPs with varying carbonate contents and crystallinities. It was shown in these studies that the mean value of the CAP MES is directly related to the broadening parameter full width at half maximum (FWHM) of the 002 reflection of the X-ray diffraction profile. The apparent solubility of the CAPs increased monotonically with an increase in the broadening of the diffraction peaks, and when this peak broadening was taken into account, carbonate had no additional effect upon the MES. The broadening of the diffraction peaks has been used as an indicator of crystallinity, and is generally influenced by both crystallite size and microstrain. The purpose of the present study was to extract the crystallite size and microstrain parameters separately from the X-ray diffraction peaks and then to determine their relationships to the corresponding MES values. The samples studied were CAPs synthesized by precipitation from Ca(NO3)2 and NaH2PO4 solutions in carbonate containing media at temperatures of 95, 80, and 70°C, and powdered HE. The crystallite size and microstrain parameters were determined simultaneously with the refinement of the structural parameters with the Rietveld method of whole-pattern-fitting structure-refinement. A modified pseudo-Voigt function was used to model the observed peak profiles. The MES distributions for the CAPs and HE were determined by a previously described method. The results of this study showed that the CAPs possessed an MES distribution and therefore provided further support that MES distribution is a common phenomenon, regardless of the method of CAP synthesis. The crystallite size decreased and the microstrain increased with increasing carbonate content and decreasing temperature of synthesis of the CAPs. A plot of the mean of the MES distribution versus the microstrain parameter showed that the apparent solubility of the CAPs and HE correlated very well with the microstrain parameter. On the other hand, a plot of the mean of the MES distribution versus the crystallite size parameter showed a poor correlation between MES and crystallite size. These findings support a view that microstrain, rather than crystallite size, is the dominant factor governing the effective solubility of the CAPs and dental enamel.

Mechanochemical Synthesis of Bioactive Material: Effect of Environmental Conditions on the Phase Transformation of Calcium Phosphates During Grinding

The effect of environmental conditions on the crystalline transformation of metastable calcium phosphates during grinding was investigated using X-ray diffractometry and fourier transformed infrared spectroscopy. A mixture of CoA and dicalcium phosphate anhydrate (DCPA, CaHPO4) did not transform after grinding in air. On the other hand, CaO and dicalcium phosphate dihydrate (DCPD, CaHPO4 2H2O) were converted into a noncrystalline solid. Mixtures of DCPD and Ca(OH)2 transformed into low-crystallinity hydroxyapatite after grinding in air. When ground under N2, a mixture of Ca(OH)2 and DCPD did not transform into hydroxyapatite, whereas that of DCPD: Ca(OH)2: CaCO3 = 1:0.8: 0.2 did. The results of X-ray diffraction of FT-IR spectra suggested that the presence of carbon dioxide in the grinding system was necessary for transformation from metastable calcium phosphates to hydroxyapatite.

Computer simulation of surface temperature profiles during CO2 laser irradiation of human enamel

The surface temperature profiles of human enamel blocks following laser irradiation are examined using a mathematical model. A physical model for calculating this surface temperature is developed and then solved by the Laplace transform technique. The enamel block is modeled as a homogeneous cylinder in one dimension with the entire surface exposed to laser beam irradiation. Both the photon absorption/transmission and convective heat loss are taken into consideration in the proposed model. This model allows the surface temperature to be calculated under arbitrary combinations of laser wavelength, pulse pattern, and energy density. Results indicate that (1) surface temperature depends on the wavelength, energy density, and duration of the pulse; (2) high surface temperatures (~ 300°C) can be achieved with associated inside temperature changes of less than 5°C; and (3) the surface temperature depends strongly on the environmental heat transfer coefficients. These results will facilitate the rational development of lasers as potential tools in preventive dentistry.

Metastable Equilibrium Solubility Distribution and Dissolution Kinetics of Carbonate Apatite Powders

Carbonated apatites (CAP’s) have been shown to exhibit a phenomenon that we refer to as metastable equilibrium solubility (MES). The MES behavior occurs when the kinetics of crystal growth and dissolution are such that dissolution of the crystalline material can relatively rapidly lead to a solution composition such that no further dissolution occurs, even though the degree of saturation of the solution is well below that required for growth. The solution composition at this point is indicative of the MES, and is most conveniently expressed in terms of a solution ion activity product (IAP).

Effect of Micropore Structure on Drug Release Form Self-Setting Calcium Phosphate Cement Containing Anti-Cancer Agent

The delivery of chemotherapeutic agents to bone via systemic routes of administration is problematic because of the serious side effects of these agents and the poor circulation to the bone. Hence, local delivery is an attractive option for these compounds.

Physical model approach to the design of laser irradiation regimens to improve resistance to dental caries

A set of interrelated physical models has been developed for use in the rational design of laser irradiation regimens for improving the resistance of tooth enamel to dental caries. Taken together, these models describe (1) the formation of subsurface lesions in dental enamel; (2) the dependence of this lesion formation on micro-environmental solution chemistry variables such as pH and concentrations of calcium, phosphate and fluoride; (3) the dependence of this lesion formation on the metastable equilibrium solubility; (4) the alteration of the MES of enamel by exposure to elevated temperatures and (5) the time-temperature profiles achieved by laser irradiation of enamel. Appropriate laboratory procedures have been developed and carried out to determine the values of the relevant parameters of each of the above models. Key findings are: (1) the onset of dissolution is controlled by the micro-environmental solution ion activity product (IAP) for fluorapatite; (2) the value of this threshold IAP can be shifted significantly by laser irradiation; (3) this IAP shift corresponds to a lowering of the threshold pH for onset of demineralization by about 0.7 pH units; and (4) the set of models listed above can be used together in an integrated approach for the rational design of laser irradiation regimens for improving resistance to dental caries.