Transmission electron microscopic confirmation of the morphological predictions of the two-site model for hydroxyapatite dissolution

Hydroxyapatite (HAP) crystals were dissolved under two types of solution conditions designed to produce contrasting dissolution morphologies according to the recently proposed two-site model for HAP dissolution [Fox, J. L., Higuchi, W. I., Fawzi, M. B., and Wu, M. S., J. Colloid Interface Sci., 67, 312 (1978)]. In accordance with this model, dissolution into partially saturated buffers proceeded via formation of holes in the crystals, while dissolution of comparable amounts of material into completely unsaturated buffers showed no such holes. These results show that the two dissolution sites inferred from dissolution kinetics experiments are, in fact, physically distinct sites and that dissolution from site No. 1 results in hole formation. The physical location of site No. 2 is not yet certain.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22739/1/0000294.pd

Dissolution rate of apatite powders in acidic fluoride solutions and the relationship to hydroxyapatite disk and bovine enamel behavior

The dissolution kinetics of synthetic hydroxyapatite (HAP) and carbonate-containing HAP powders have been studied in fluoride-containing acetate buffer solutions partially saturated with respect to HAP (KFAP = 10-115 to 10-123). The experimental results indicate that in the case of HAP powders the dissolution rates become very slow when the KFAP values of the dissolution medium are larger than 10-119. For the carbonate-containing apatites, however, there is a significant dissolution in the region 10-115 >= KFAP >= 10-119 which is in the region of physiological and therapeutic significance.The present results with the HAP powder together with HAP pellet data show that the dissolution of HAP pellets in the region 10-114 > KFAP > 10-119 results from an extended transient period rising from sustained deposition of F- on the surface and at intermediate depths in the `lesion'. Similar effects have also been observed with bovine teeth dissolution in acidic F- solutions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25136/1/0000569.pd

Remineralization Of Synthetic Hydroxyapatite Powders In Fluoride Solutions (demineralization, F-treatment).

The purpose of this study was to develop a base of understanding regarding the remineralization behavior of dental enamel in acidic fluoride solutions. The approach was to investigate the reminerali- zation behavior of synthetic hydroxyapatite powders under similar experimental conditions and to relate these findings to those obtained with bovine dental enamel. Crystal suspension experiments were designed to investigate the influences of varying the hydroxyapatite (HAP) seed prepara- tions and seed concentrations (slurry densities) upon the extent of remineralization and the nature of depositing phase (chemical stoi- chiometry and transmission electron microscopy). These studies were conducted at pkFAP = 108 (where pkFAP is minus the loga- rithm of the ion activity product, a(,Ca)2+('10) a(,PO(,4))3-('6) a(,OH)-('2)), Ca/P (molar ratio) = 1.0 and fluoride (F) = 10 ppm at pH 4.5 in 0.10 M acetate buffers. These solution conditions were known to give relatively rapid and complete remineralization of artificial lesions in bovine teeth and HAP pellets. It was concluded from these experiments that, at pkFAP = 108, the remineralization behavior was comparable for the different seed preparations studied. The results were consistent with fluorapatite (FAP) deposition upon the seeds being the predominant process and all crystal dimensions were observed to grow substan- tially. In a second set of studies the kinetics of HAP powder reminer- alization provided data for developing a quantitative understanding of the tooth remineralization behavior. These data directly related the crystal suspension remineralization behavior with changes in the tooth lesion chemistry and mineral densities with time, and deductions were made regarding possible ion activity product gradients within the lesion and how these may change with time. The principal outcomes were that the HAP powder remineralization kinetics data can explain or can help explain the essentially com- plete remineralization of bovine tooth when pkFAP = 108 and the phenomenon of simultaneous demineralization/remineralization which takes place in the bovine tooth when pkFAP > 112. Finally, the influence of the presence of carbonate in apatite on the crys- tal dissolution behavior was assessed. The dissolution rate of carbonate containing HAP (C-HAP) in the presence of fluoride was controlled by an apparent solubility corresponding to an ion activity product that is 4 to 5 orders of magnitude greater than that for HAP.Ph.D.Pharmacy sciencesPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127912/2/8621307.pd

Excipient Development for Pharmaceutical, Bioteknology and Drug Delivery Systems

xvii; 436; 21 x 29 c