Niranjan Patra

Atomic force microscopy in vitro study of surface roughness and fractal character of a dental restoration composite after air-polishing

BackgroundSurface roughness is the main factor determining bacterial adhesion, biofilm growth and plaque formation on the dental surfaces in vivo. Air-polishing of dental surfaces removes biofilm but can also damage the surface by increasing its roughness. The purpose of this study was to investigate the surface damage of different conditions of air-polishing performed in vitro on a recently introduced dental restorative composite.MethodsAbrasive powders of sodium bicarbonate and glycine, combined at different treatment times (5, 10 and 30 s) and distances (2 and 7 mm), have been tested. The resulting root mean square roughness of the surfaces has been measured by means of atomic force microscopy, and the data have been analyzed statistically to assess the significance. Additionally, a fractal analysis of the samples surfaces has been carried out.ResultsThe minimum surface roughening was obtained by air-polishing with glycine powder for 5 s, at either of the considered distances, which resulted in a mean roughness of ~300 nm on a 30 × 30 μm2 surface area, whereas in the other cases it was in the range of 400-750 nm. Both untreated surfaces and surfaces treated with the maximum roughening conditions exhibited a fractal character, with comparable dimension in the 2.4-2.7 range, whereas this was not the case for the surfaces treated with the minimum roughening conditions.ConclusionsFor the dental practitioner it is of interest to learn that use of glycine in air polishing generates the least surface roughening on the considered restorative material, and thus is expected to provide the lowest rate of bacterial biofilm growth and dental plaque formation. Furthermore, the least roughening behaviour identified has been correlated with the disappearance of the surface fractal character, which could represent an integrative method for screening the air polishing treatment efficacy.

Adhesion and Proliferation of Osteoblast-Like Cells on Anodic Porous Alumina Substrates With Different Morphology

We have fabricated nanoporous alumina surfaces by means of anodization in oxalic acid in different conditions and used them as the substrates for the growth of cells from a human osteoblast-like cell line. The rough nanoporous alumina substrates have been compared both with smooth standard Petri dishes used as the control and with commercial substrates of similar material. The viability of the cells has been assessed at different culture times of 4, 11, 18, and 25 days in vitro. It turned out that the porous side of the galvanostatically fabricated alumina performed similar to the control and better than the commercial porous alumina, whereas the potentiostatically fabricated porous alumina performed better than all the other substrates at all times, and in particular at the two shortest time periods of 4 and 11 days in vitro. The best performance of the substrates is associated with intermediate surface roughness and feature spacing.

Atomic force microscopy nanoindentation of a dental restorative midifill composite

OBJECTIVES We investigated the elastic properties of one dental restoration resin composite of common use, Venus Diamond, at submicrometer spatial resolution. METHODS We performed both nanoindentation experiments with atomic force microscopy, and microindentation experiments with a traditional indenter setup. We also used scanning electron microscopy and energy dispersive X-rays spectroscopy to better understand the correlation between properties and microscopic structure and composition. RESULTS With atomic force microscopy we obtained quantitative evaluation of the elastic modulus (10.8 ± 4.3 GPa), in agreement with the microindentation value (reduced modulus of 12.7 ± 2.0 GPa), and by microindentation we also obtained an hardness value (460 ± 109 MPa) compatible in turn with the nominal value provided by the material manufacturer (H ∼ 578 MPa). The nanoindentation also revealed that no relevant difference in elasticity appears between the 5 and 10 μm diameter filler particles and the surrounding areas, showing an excellent uniformity of the composite. In support of this finding, compositional uniformity of the material was also observed by X-rays spectroscopy. We conclude that the composite contains prepolymerized particles. SIGNIFICANCE We demonstrate that, in addition to reliable quantitative analysis, the high resolution and two-dimensional mapping capability of atomic force microscopy allows for advanced insights into the microstructure of the composite that are not accessible via traditional microindentation.

Calibration issues for nanoindentation experiments: Direct atomic force microscopy measurements and indirect methods

This article discusses calibration issues for shallow depth nanoindentation experiments with Berkovich tips with respect to the accurate measurement of the diamond area function (DAF). For this purpose, two different calibration procedures are compared: (i) the direct measurement of the DAF through atomic force microscopy (AFM) imaging of the Berkovich tip at shallow depth and (ii) a novel indirect calibration method based on an iterative robust and converging scheme in which both reduced modulus and indentation hardness are simultaneously used. These results are obtained by indentation measurements on a standard specimen of fused silica, performed in the 0.5–200 mN load range with a Berkovich indenter. Direct tip shape measurements were carried out through different AFM methods. Comparisons with the standard indirect calibration procedure are also reported. For both the indirect calibration procedures a sensitivity and convergence study is presented. Microsc. Res. Tech. 73:996–1004, 2010.

Use of Ionic Liquid in Fabrication, Characterization, and Processing of Anodic Porous Alumina

Two different ionic liquids have been tested in the electrochemical fabrication of anodic porous alumina in an aqueous solution of oxalic acid. It was found that during galvanostatic anodization of the aluminum at a current density of 200 mA/cm2, addition of 0.5% relative volume concentration of 1-butyl-3-methylimidazolium tetrafluoborate resulted in a three-fold increase of the growth rate, as compared to the bare acidic solution with the same acid concentration. This ionic liquid was also used successfully for an assessment of the wettability of the outer surface of the alumina, by means of liquid contact angle measurements. The results have been discussed and interpreted with the aid of atomic force microscopy. The observed wetting property allowed to use the ionic liquid for protection of the pores during a test removal of the oxide barrier layer.

Surfactant-induced thermomechanical and morphological changes in TiO2-polystyrene nanocomposites.

Nanocomposites of polystyrene and TiO2 colloidal nanorods with different loadings have been prepared by mixing pre-synthesized oleic acid capped colloidal TiO2 nanorods into commercial polystyrene via solvent blending using chloroform. The microstructure and morphology of the nanocomposites was evaluated by wide angle X-ray diffraction and transmission electron microscopy. The observations revealed that the surfactant plays an important role for interactions between the polymer and the filler. Differential scanning calorimetry showed that the glass transition temperature of the nanocomposites decreased which is consistent with the surfactant acting as a plasticizer in the polystyrene matrix. Thermogravimetric analysis revealed that the nanocomposites show no significant improvement in thermal stability as compared to the bare PS up to a temperature of 400 °C. However, after 400 °C, the TGA curve shifts a little to higher temperature as compared to the bare PS. The dynamic mechanical properties of the nanocomposites indicate that the storage modulus, loss modulus, and glass transition temperature do not change with increasing nanorods content of 2 and 4 wt% but decrease afterward for 8 wt%. Transmission electron microscopy images clearly show debonding characteristics in polystyrene matrix.

AFM measurement of the stiffness of layers of agarose gel patterned with polylysine

Films of agarose gel microspotted with polylysine aqueous solution have been characterized by atomic force microscopy carried out in deionized water. Thickness and surface morphology of the layers have been checked, and the effect of polylysine impregnation on the local elasticity has been investigated. An increase in contact stiffness of the organic layer at the spotted areas has been observed, correlated with the polylysine concentration. For the considered agarose layer thickness of ∼0.9 μm in dry condition, the concentration threshold at which stiffening appears is ∼0.1 mg/mL. Above this threshold, the stiffening coefficient becomes approximately twofold and seems not to increase significantly with concentration in the range 0.3–0.7 mg/mL. For concentrations above the stiffening threshold, this effect is also accompanied by a locally lower film thickness. For quantitative determination of the stiffness, force–distance curves extracted from the regions of interest of spots and agarose substrate have been selected and processed. These curves were fitted to the Hertz model of purely elastic tip‐surface interaction, under appropriate assumptions on both tip shape and optimum indentation depth. In this way, we could determine the Youngs modulus of the agarose layer to be ∼50 kPa and quantitatively confirm the stiffening due to polylysine. Microsc. Res. Tech. 73:982–990, 2010.

Thermal and Mechanical Characterization of PMMA TiO2 Nanocomposites

Hybrid composite films made of an organic polymeric matrix and an inorganic nanosized filler, namely titania prolate nanoparticles, have been prepared and characterized both thermally and mechanically. The filler content has been varied, while still being kept in the regime of unsaturated, homogeneous nanocomposites. On increasing, the filler content there is an abrupt increase in hardness at an intermediate load, while the elastic modulus increases almost linearly. The glass transition temperature is also increased, with a tendency to saturation. The morphological characterization of the films confirms a lack of phase separation, with only a continuous, slight increase in surface roughness, and no major effects on the top film features.

RETRACTED: Local increase in stiffness of agarose gel layer by patterning with polylysine measured via atomic force microscopy

Films of agarose gel impregnated with polylysine spotted from an aqueous solution have been characterized by atomic force microscopy performed in deionized water. An increase in contact stiffness of the composite substrate on the spotted areas has been observed, for increasing polylysine concentration. For the considered agarose layer thickness of approximately 0.9 microm when dry, the polylysine concentration threshold for stiffening is as low as approximately 0.1 mg/mL. Above this threshold the stiffening coefficient increases slightly with concentration in the considered range (up to 0.7 mg/mL), reaching a highest value of approximately 2.3. For concentrations >or=0.3 mg/mL the stiffening at the polylysine spots was also accompanied by a locally lower film thickness. For accurate quantification of the stiffness, representative force-distance curves extracted from the respective regions of interest (spots and agarose substrate off the spots) have been processed. The Hertz model of purely elastic tip-surface interaction has been adopted, with appropriate hypothesis on both tip shape and optimum indentation depth. The resulting Youngs moduli of the agarose layer and of the polylysine spots are approximately 45 kPa and approximately 95 kPa, respectively, with an estimated uncertainty of approximately 15%.

Preparation and characterization of polycarbonate/multiwalled carbon nanotube nanocomposites

A polymer nanocomposite was produced by ultrasonic-assisted dispersion of multiwalled carbon nanotubes (MWCNTs) in a polycarbonate matrix using p-xylene and dichloromethane as the solvents. The filler loading was varied from 1 to 3 wt % in order to examine the effect of MWCNTs on the structure and properties of the composites. The nanocomposites were characterized by DSC, DTA, TGA, UV–vis, FTIR and Raman spectroscopy to evaluate the changes induced by the filler in the polymer matrix. UV–vis, FTIR and Raman spectroscopy measurements confirmed the presence of the dispersed phase in the composite films, while TGA and DSC analysis of the nanocomposites revealed enhanced thermal stability and decreased crystallinity, respectively, as compared to the neat polymer. The proposed composites can find application in a number of everyday products where polycarbonate is the base polymer.