The influence of the surface treatment and saliva on the color of two porcelains.
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The color of an object is a psychophysical phenomenon. Regarding its physical aspect, the amount of light reflected from the surface of an object is taken into consideration many times. Two brands of porcelain were used to make 10 specimens that were submitted to the following treatments: glaze, grinding, polishing and overglaze. All samples were analyzed with or without saliva. The final color differences of the ceramic were evaluated using a spectrophotometer, and reflectance curves were obtained. Results showed that there were no differences between the brands of porcelain. Saliva did not interfere with the color of the porcelain, but re-established the original one, after the grinding process had altered it. The polishing also contributed to re-established the original color. The overglaze altered the original color, with or without saliva. (+info)
Delivery of extraterrestrial amino acids to the primitive Earth. Exposure experiments in Earth orbit.
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A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50 to 100 micrometers size range, the carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon. They might have brought more carbon to the surface of the primitive Earth than that involved in the present surficial biomass. Amino acids such as "-amino isobutyric acid have been identified in these Antarctic micrometeorites. Enantiomeric excesses of L-amino acids have been detected in the Murchison meteorite. A large fraction of homochiral amino acids might have been delivered to the primitive Earth via meteorites and micrometeorites. Space technology in Earth orbit offers a unique opportunity to study the behaviour of amino acids required for the development of primitive life when they are exposed to space conditions, either free or associated with tiny mineral grains mimicking the micrometeorites. Our objectives are to demonstrate that porous mineral material protects amino acids in space from photolysis and racemization (the conversion of L-amino acids into a mixture of L- and D-molecules) and to test whether photosensitive amino acids derivatives can polymerize in mineral grains under space conditions. The results obtained in BIOPAN-1 and BIOPAN-2 exposure experiments on board unmanned satellite FOTON are presented. (+info)
An in vivo investigation into the use of resin-modified glass poly(alkenote) cements as orthodontic bonding agents.
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The aim of this study was to investigate the in vivo bonding of orthodontic brackets using two resin-modified glass poly(alkenoate) cements and to compare them with a conventional light-cured diacrylate bonding agent. Twenty consecutive patients attending for bond up appointments took part in this randomized cross-mouth control study. Alternate quadrants were bonded with either Fuji Ortho LC or 3 M Multi-Cure. Transbond [Adhesive Pre-Coated Brackets (APC)] acted as the control in the other quadrants. Failed brackets were rebonded with the same material. Bond failure rates were collected over a 1-year period. The bond failure rates over 1 year were 7.2 per cent for Transbond (APC), 5.9 per cent for 3 M Multi-Cure, and 5.8 per cent for Fuji Ortho LC. Statistically, there was no significant difference between the bond failure rates of the materials and there was no effect of time. This clinical investigation confirmed the suitability of the resin-modified glass poly(alkenoate) cements under test as orthodontic bonding agents. (+info)
An ex vivo investigation into the fluoride release and absorption profiles of three orthodontic adhesives.
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Prevention of peri-bracket decalcification is essentially mediated by effective oral hygiene, which is patient dependent. The potential to compensate for poor oral hygiene may be through the release of free fluoride ions from direct bonding agents. It is well established that glass ionomer cements (GICs) release free fluoride ions which may be replenished by exposure to fluoride containing solutions. The aim of this ex vivo study was to compare the fluoride release and absorption profiles of a recently developed low fusion, monophase GIC, Limerick glass, with two materials in common clinical use, namely, Fuji-Ortho L.C. and Concise. Ten test specimens of each material were fabricated and stored in deionized water at 37 degrees C. The concentration of fluoride release was measured daily for the first 7 days after immersion and then weekly for 5 weeks. On day 42 the test specimens were recharged in 2 per cent sodium fluoride solution and fluoride release was measured initially after 24 hours and then every 48 hours. The recharge regimen was repeated for 3 weeks. Both Limerick glass and Fuji-Ortho L.C. demonstrated the characteristic fluoride release pattern of GIC with a rapid elution of fluoride after the first 24 hours followed by a more gradual release profile over the following 2 weeks. Fuji-Ortho L.C. released the greatest quantity of fluoride but the release profile of Limerick glass was more sustained. Concise released negligible amounts of fluoride. Limerick glass and Fuji-Ortho L.C. showed an increase (P < 0.01) in fluoride release following exposure to 2 per cent sodium fluoride that decreased both with time and subsequent exposures to fluoride supplement. (+info)
Emergence of modern human behavior: Middle Stone Age engravings from South Africa.
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In the Eurasian Upper Paleolithic after about 35,000 years ago, abstract or depictional images provide evidence for cognitive abilities considered integral to modern human behavior. Here we report on two abstract representations engraved on pieces of red ochre recovered from the Middle Stone Age layers at Blombos Cave in South Africa. A mean date of 77,000 years was obtained for the layers containing the engraved ochres by thermoluminescence dating of burnt lithics, and the stratigraphic integrity was confirmed by an optically stimulated luminescence age of 70,000 years on an overlying dune. These engravings support the emergence of modern human behavior in Africa at least 35,000 years before the start of the Upper Paleolithic. (+info)
Formation of oriented molecular nanowires on mica surface.
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Molecular "nanowire" structures composed of the charge transfer complex of a bis-tetrathiafulvalene substituted macrocycle and tetrafluorotetracyanoquinodimethane were constructed on mica substrates by employing the Langmuir-Blodgett technique. The nanowires transferred from a dilute aqueous potassium chloride subphase had typical dimensions of 2.5 nm x 50 nm x 1 microm. The nanowires are oriented to specific directions, corresponding to the directions of the potassium-ion array on the mica surface having sixfold symmetry. Such correlation between the nanowires and the substrate surface was also observed when a dilute aqueous rubidium chloride subphase was used. On the other hand, the correlation completely disappeared when the subphase contained divalent cations, indicating that the molecular nanowires orient by recognizing the monocation array on the mica surface. The nanowires formed by the vertical dipping method coexist with the monolayers. Only nanowire structures are, however, observed when we apply the horizontal lifting method. Based on the crystal structure of a related complex, a possible structure of the nanowires is presented. The conductivity of the nanowires was estimated to be of the order of 10(-3) S x cm(-1). The nanowires formed specific (regular) structures such as T-shape junctions, suggesting their use in construction of future molecular nanoscale devices. (+info)
Horizontal and vertical movement of Pseudomonas fluorescens toward exudate of Macrophomina phaseolina in soil: influence of motility and soil properties.
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The role of motility and cell surface hydrophobicity in transport and dispersal of Pseudomonas fluorescens strains LAM1-hydrophilic, LAM2-hydrophobic and LAM(NM) (non-motile mutant of LAM2) under different soil conditions was studied. Maximum adhesion was recorded for LAM2 in clay loam (70%), followed by sandy loam (68%) and sandy soil (40%). Vertical migration of P fluorescens isolates in soils was recorded at 5 and 25 cm flow of wafer or M. phaseolina exudate. In all the treatments, LAM1 exhibited maximum migration followed, by LAM2 and LAM(NM). The rate of migration of such isolates was lowered in water irrigated soils compared to those irrigated with M. phaseolina exudate. In sandy soil, cells of LAM1 migrated up to 13 cm in comparison to LAM2 (11 cm) and LAN(NM) (9 cm) at 5 cm flow of fungal exudate. Population of LAM1, LAM2 and LAM(NM) was 5.7, 5.68 and 5.61 log cfu g(-1) soil at 1 cm depth, but it decreased to 2.56, 2.21 and 1.99 log cfu during migration up to 11 cm in sandy soil at 5 cm flow of fungal exudate. Greater motility was observed in sandy soil irrigated with water or fungal exudate, followed by sandy loam and clay loam. In general, filtration coefficient (lambda) of P. fluorescens was higher in soils irrigated with 5 cm of water or exudate than with 25 cm of irrigation. The horizontal movement of P. fluorescens strains in sandy soil adjusted at different psi m showed marked reduction with decrease in psi m. The non-motile LAN(NM) did not show chemotactic response and migrated up to a maximum of 3 mm in saturated soils (0 kPa). After 96 h, LAM1 and LAM2 migrated upto 35 and 29 mm respectively in sandy soil. Motile isolates had significantly greater colonization of M. phaseolina sclerotia over the non-motile mutant. (+info)
Dissecting streptavidin-biotin interaction with a laminar flow chamber.
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A laminar flow chamber was used to study single molecule interactions between biotinylated surfaces and streptavidin-coated spheres subjected to a hydrodynamic drag lower than a piconewton. Spheres were tracked with 20 ms and 40 nm resolution. They displayed multiple arrests lasting between a few tens of milliseconds and several minutes or more. Analysis of about 500,000 positions revealed that streptavidin-biotin interaction was multiphasic: transient bound states displayed a rupture frequency of 5.3 s(-1) and a rate of transition toward a more stable configuration of 1.3 s(-1). These parameters did not display any significant change when the force exerted on bonds varied between 3.5 and 11 pN. However, the apparent rate of streptavidin-biotin association exhibited about 10-fold decrease when the wall shear rate was increased from 7 to 22 s(-1), which supports the existence of an energy barrier opposing the formation of the transient binding state. It is concluded that a laminar flow chamber can yield new and useful information on the formation of molecular bonds, and especially on the structure of the external part of the energy landscape of ligand-receptor complexes. (+info)