Phase-Locked loops lock-in range in Frequency Modulated-Atomic Force Microscope nonlinear control system

Since the mid 1980s the Atomic Force Microscope is one the most powerful tools to perform surface investigation, and since 1995 Non-Contact AFM achieved true atomic resolution. The Frequency-Modulated Atomic Force Microscope (FM-AFM) operates in the dynamic mode, which means that the control system of the FM-AFM must force the micro-cantilever to oscillate with constant amplitude and frequency. However, tip-sample interaction forces cause modulations in the microcantilever motion. A Phase-Locked loop (PLL) is used to demodulate the tip-sample interaction forces from the microcantilever motion. The demodulated signal is used as the feedback signal to the control system, and to generate both topographic and dissipation images. As a consequence, a proper design of the PLL is vital to the FM-AFM performance. In this work, using bifurcation analysis, the lock-in range of the PLL is determined as a function of the frequency shift (Q) of the microcantilever and of the other design parameters, providing a technique to properly design the PLL in the FM-AFM system. (C) 2011 Elsevier B.V. All rights reserved.

In Vitro Analysis of Bacterial Morphology by Atomic Force Microscopy of Low Level Laser Therapy 660, 830 and 904 nm

Objective: The objective of this study was to analyze the bacterial morphology by atomic force microscopy (AFM) after the application of low-level laser therapy (LLLT) in in vitro culture of Staphylococcus aureus ATCC 29213. Background data: Infections caused by S. aureus are among the highest occurring in hospitals and can often colonize pressure ulcers. LLLT is among the methods used to accelerate the healing of ulcers. However, there is no consensus on its effect on bacteria. Materials and methods: After being cultivated and seeded, the cultures were irradiated using wavelengths of 660, 830, and 904 nm at fluences of 0, 1, 2, 3, 4, 5, and 16 J/cm(2). Viable cells of S. aureus strain were counted after 24 h incubation. To analyze the occurrence of morphological changes, the topographical measurement of bacterial cells was analyzed using the AFM. Results: The overall assessment revealed that the laser irradiation reduced the S. aureus growth using 830 and 904 nm wavelengths; the latter with the greatest inhibition of the colony-forming units (CFU/mL) (331.1 +/- 38.19 and 137.38 +/- 21.72). Specifically with 660 nm, the statistical difference occurred only at a fluence of 3 J/cm(2). Topographical analysis showed small changes in morphological conformity of the samples tested. Conclusions: LLLT reduced the growth of S. aureus with 830 and 904 nm wavelengths, particularly with 904 nm at a fluence of 3 J/cm(2), where the greatest topographical changes of the cell structure occurred.

DNA fragmentation by gamma radiation and electron beams using atomic force microscopy

Double-stranded pBS plasmid DNA was irradiated with gamma rays at doses ranging from 1 to 12 kGy and electron beams from 1 to 10 kGy. Fragment-size distributions were determined by direct visualization, using atomic force microscopy with nanometer-resolution operating in non-tapping mode, combined with an improved methodology. The fragment distributions from irradiation with gamma rays revealed discrete-like patterns at all doses, suggesting that these patterns are modulated by the base pair composition of the plasmid. Irradiation with electron beams, at very high dose rates, generated continuous distributions of highly shattered DNA fragments, similar to results at much lower dose rates found in the literature. Altogether, these results indicate that AFM could supplement traditional methods for high-resolution measurements of radiation damage to DNA, while providing new and relevant information.

Titanium surface topography after brushing with fluoride and fluoride-free toothpaste simulating 10 years of use

Objectives: To conduct a controlled study contrasting titanium surface topography after procedures that simulated 10 years of brushing using toothpastes with or without fluoride. Methods: Commercially pure titanium (cp Ti) and Ti-6Al-4V disks (6 mm circle divide x 4 mm) were mirror-polished and treated according to 6 groups (n = 6) as a function of immersion (I) or brushing (B) using deionised water (W), fluoride-free toothpaste (T) and fluoride toothpaste (FT). Surface topography was evaluated at baseline (pretreatment) and post-treatment, using atomic force microscope in order to obtain three-dimensional images and mean roughness. Specimens submitted to immersion were submerged in the vehicles without brushing. For brushed specimens, procedures were conducted using a linear brushing machine with a soft-bristled toothbrush. Immersion and brushing were performed for 244 h. IFT and BFT samples were analysed under scanning electron microscope with Energy-Dispersive X-ray Spectroscopy (EDS). Pre and post-treatment values were compared using the paired Student T-test (alpha = .05). Intergroup comparisons were conducted using one-way ANOVA with Tukey post-test (alpha = .05). Results: cp Ti mean roughness (in nanometers) comparing pre and post-treatment were: IW, 2.29 +/- 0.55/2.33 +/- 0.17; IT, 2.24 +/- 0.46/2.02 +/- 0.38; IFT, 2.22 +/- 0.53/1.95 +/- 0.36; BW, 2.22 +/- 0.42/3.76 +/- 0.45; BT, 2.27 +/- 0.55/16.05 +/- 3.25; BFT, 2.27 +/- 0.51/22.39 +/- 5.07. Mean roughness (in nanometers) measured in Ti-6Al-4V disks (pre/post-treatment) were: IW, 1.79 +/- 0.25/2.01 +/- 0.25; IT, 1.61 +/- 0.13/1.74 +/- 0.19; IFT, 1.92 +/- 0.39/2.29 +/- 0.51; BW, 2.00 +/- 0.71/2.05 +/- 0.43; BT, 2.37 +/- 0.86/11.17 +/- 2.29; BFT, 1.83 +/- 0.50/15.73 +/- 1.78. No significant differences were seen after immersions (p > .05). Brushing increased the roughness of cp Ti and of Ti-6Al-4V (p < .01); cp Ti had topographic changes after BW, BT and BFT treatments whilst Ti-6Al-4V was significantly different only after BT and BTF. EDS has not detected fluoride or sodium ions on metal surfaces. Conclusions: Exposure to toothpastes (immersion) does not affect titanium per se; their use during brushing affects titanium topography and roughness. The associated effects of toothpaste abrasives and fluorides seem to increase roughness on titanium brushed surfaces. (C) 2012 Elsevier Ltd. All rights reserved.

Fragmentation of extracellular DNA by long-term exposure to radiation from uranium in aquatic environments

Persistent harmful scenarios associated with disposal of radioactive waste, high-background radiation areas and severe nuclear accidents are of great concern regarding consequences to both human health and the environment. Of particular concern is the extracellular DNA in aquatic environments contaminated by radiological substances. Strand breaks induced by radiation promote decrease in the transformation efficiency for extracellular DNA. The focus of this study is the quantification of DNA damage following long-term exposure (over one year) to low doses of natural uranium (an alpha particle emitter) to simulate natural conditions, since nothing is known about alpha radiation induced damage to extracellular DNA. A high-resolution Atomic Force Microscope was used to evaluate DNA fragments. Double-stranded plasmid pBS as a model for extracellular DNA was exposed to different amounts of natural uranium. It was demonstrated that low concentrations of U in water (50 to 150 ppm) produce appreciable numbers of double strand breaks, scaling with the square of the average doses. The importance of these findings for environment monitoring of radiological pollution is addressed.

Designing piezoresistive plate-based sensors with distribution of piezoresistive material using topology optimization

Piezoresistive sensors are commonly made of a piezoresistive membrane attached to a flexible substrate, a plate. They have been widely studied and used in several applications. It has been found that the size, position and geometry of the piezoresistive membrane may affect the performance of the sensors. Based on this remark, in this work, a topology optimization methodology for the design of piezoresistive plate-based sensors, for which both the piezoresistive membrane and the flexible substrate disposition can be optimized, is evaluated. Perfect coupling conditions between the substrate and the membrane based on the `layerwise' theory for laminated plates, and a material model for the piezoresistive membrane based on the solid isotropic material with penalization model, are employed. The design goal is to obtain the configuration of material that maximizes the sensor sensitivity to external loading, as well as the stiffness of the sensor to particular loads, which depend on the case (application) studied. The proposed approach is evaluated by studying two distinct examples: the optimization of an atomic force microscope probe and a pressure sensor. The results suggest that the performance of the sensors can be improved by using the proposed approach.