Effects of incorporating nanosized calcium phosphate particles on properties of whisker-reinforced dental composites.

Clinical data indicate that secondary caries and restoration fracture are the most common problems facing tooth restorations. Our ultimate goal was to develop mechanically-strong and caries-inhibiting dental composites. The specific goal of this pilot study was to understand the relationships between composite properties and the ratio of reinforcement filler/releasing filler. Nanoparticles of monocalcium phosphate monohydrate (MCPM) were synthesized and incorporated into a dental resin for the first time. Silicon carbide whiskers were fused with silica nanoparticles and mixed with the MCPM particles at MCPM/whisker mass ratios of 1:0, 2:1, 1:1, 1:2, and 0:1. The composites were immersed for 1-56 days to measure Ca and PO4 release. When the MCPM/whisker ratio was changed from 0:1 to 1:2, the composite flexural strength (mean +/- SD; n = 5) decreased from 174 +/- 26 MPa to 138 +/- 9 MPa (p < 0.05). A commercial nonreleasing composite had a strength of 112 +/- 14 MPa. When the MCPM/whisker ratio was changed from 1:2 to 1:1, the Ca concentration at 56 days increased from 0.77 +/- 0.04 mmol/L to 1.74 +/- 0.06 mmol/L (p < 0.05). The corresponding PO4 concentration increased from 3.88 +/- 0.21 mmol/L to 9.95 +/- 0.69 mmol/L (p < 0.05). Relationships were established between the amount of release and the MCPM volume fraction v(MCPM) in the resin: [Ca]= 42.9 v(MCPM) (2.7), and [PO4] = 48.7 v(MCPM) (1.4). In summary, the method of combining nanosized releasing fillers with reinforcing fillers yielded Ca- and PO4-releasing composites with mechanical properties matching or exceeding a commercial stress-bearing, nonreleasing composite. This method may be applicable to the use of other Ca-PO4 fillers in developing composites with high stress-bearing and caries-preventing capabilities, a combination not yet available in any dental materials.

Porous tin oxide nanostructured microspheres for sensor applications.

We have sought to enhance the sensitivity of conductometric gas microsensors through the design and fabrication of porous, three-dimensional tin oxide nanoparticle structures. Electrostatically controlled layer-by-layer processing in aqueous solutions was used to decorate sacrificial latex microspheres with Sb:SnO2 nanoparticles. To evaluate their sensing performance, these structures were then deposited as films, via micropipetting, on MEMS micro-hot-plate platforms with interdigitated electrodes. Prior to gas testing, rapid heating of the micro-hot-plates was used to remove the sacrificial latex templates, thereby revealing a 3-D structure composed of interconnected spherical tin oxide nanoparticle shells with porous ultrathin walls. Changes in film conductance, caused by exposure to test gases (methanol, carbon monoxide, benzene, water) in a dry air background, were measured at different temperatures. Hollow nanoparticle microsphere films exhibited partial selectivity for these different gases, good dynamic range at different temperatures and gas concentrations, and good repeatability and stability over long runs. These films also yielded approximately 3-fold and 5-fold increases in sensitivity to methanol when compared to SnO2 polycrystalline chemical vapor deposition films and Sb:SnO2 microporous nanoparticle films, respectively. Gains in sensitivity are attributed to the multiscale porous architecture of the hollow microsphere films. This architecture promotes gas diffusion and increases the active surface area.

Properties of Nanostructured Hydroxyapatite Prepared by a Spray Drying Technique.

In previous studies nano sized hydroxyapatite (HA) particles were prepared by solgel or precipitation methods, in which the products were washed by aqueous or non-aqueous liquids to remove impurities or undesired components. The washing is know to modify the surfaces of the cystalline particles. This study evaluated properties of nano HA materials prepared by a spray drying method in which the HA product was not exposed to any liquid after its formation. The spray drying apparatus consisted of a nozzle that sprayed an acidic calcium phosphate solution in the form of a fine mist into a stream of filtered air flowing through a heated glass column. The water and volatile acid were evaporated by the time the mist reached the end of the column, and the fine particles were collected by an electrostatic precipitator. Powder x ray diffraction patterns suggested the material was amorphous, exhibiting a single broad peak at 30.5° 2θ. However, high resolution transmission electron microscopic analysis showed that the particles, some of which were 5 nm in size, exhibited well ordered HA lattice fringes. Small area diffraction patterns were indicative of HA. Fourier transfer infrared spectroscopy showed patterns of typical of HA with small amounts of HPO4 (2-). The thermodynamic solubility product of the nano HA was 3.3 × 10(-94) compared to 1 × 10(-117) for macro scale crystalline HA. These results showed that a spray drying technique can be used to prepare nanometer sized crystalline HA that have significantly different physicochemical properties than those of its bulk-scale counterpart.