Plasticity in nanoscale friction: Static and dynamic

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Densification of polymer glass film under combined high pressure and shear flow revealed via scanning X-ray microscopy

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Direct observation of yield in films by flat punch indentation

In regular indentation many strain states are simultaneously present in the indented region, so measured parameters such as hardness and modulus are average values over a wide range of strains. Testing of structures such as pillars, levers or film bulges enables determination of yield point and modulus with uniform strains in the sample, but requires specialised sample preparation and can be significantly affected by surface condition. Here we show how in-situ indentation with a flat punch allows direct observation of a discrete yield point in soft films on more rigid substrates. The yield point is clearly observable from the load displacement behaviour and from post indent AFM imaging. The film is in uniform uniaxial strain. Finite element simulations show that effective self-confinement by surrounding film material leads to uniformity throughout the film material down to surprisingly low aspect ratios around 4:1. This occurs for a significant range of stresses above the yield point. Eventually at even higher stresses the film material is extruded laterally. The characteristics of the yield event will be described as a function of temperature and film thickness for thin to ultrathin films. At higher aspect ratio and with sufficient stiffness of punch and substrate, quantitative, in-situ measurement of intrinsic stress vs. strain to well beyond the elastic limit becomes possible for thin films. The extent to which full constitutive relations for polymer films can be determied will be discussed, along with limitations of the technique

Myeloid-derived miR-223 regulates intestinal inflammation via repression of the NLRP3 inflammasome

MicroRNA (miRNA)-mediated RNA interference regulates many immune processes, but how miRNA circuits orchestrate aberrant intestinal inflammation during inflammatory bowel disease (IBD) is poorly defined. Here, we report that miR-223 limits intestinal inflammation by constraining the nlrp3 inflammasome. miR-223 was increased in intestinal biopsies from patients with active IBD and in preclinical models of intestinal inflammation. miR-223-/y mice presented with exacerbated myeloid-driven experimental colitis with heightened clinical, histopathological, and cytokine readouts. Mechanistically, enhanced NLRP3 inflammasome expression with elevated IL-1β was a predominant feature during the initiation of colitis with miR-223 deficiency. Depletion of CCR2+ inflammatory monocytes and pharmacologic blockade of IL-1β or NLRP3 abrogated this phenotype. Generation of a novel mouse line, with deletion of the miR-223 binding site in the NLRP3 3′ untranslated region, phenocopied the characteristics of miR-223-/y mice. Finally, nanoparticle-mediated overexpression of miR-223 attenuated experimental colitis, NLRP3 levels, and IL-1β release. Collectively, our data reveal a previously unappreciated role for miR-223 in regulating the innate immune response during intestinal inflammation

Application of geographic information systems and simulation modelling to dental public health: Where next?

Public health research in dentistry has used geographic information systems since the 1960s. Since then, the methods used in the field have matured, moving beyond simple spatial associations to the use of complex spatial statistics and, on occasions, simulation modelling. Many analyses are often descriptive in nature; however, and the use of more advanced spatial simulation methods within dental public health remains rare, despite the potential they offer the field. This review introduces a new approach to geographical analysis of oral health outcomes in neighbourhoods and small area geographies through two novel simulation methods-spatial microsimulation and agent-based modelling. Spatial microsimulation is a population synthesis technique, used to combine survey data with Census population totals to create representative individual-level population datasets, allowing for the use of individual-level data previously unavailable at small spatial scales. Agent-based models are computer simulations capable of capturing interactions and feedback mechanisms, both of which are key to understanding health outcomes. Due to these dynamic and interactive processes, the method has an advantage over traditional statistical techniques such as regression analysis, which often isolate elements from each other when testing for statistical significance. This article discusses the current state of spatial analysis within the dental public health field, before reviewing each of the methods, their applications, as well as their advantages and limitations. Directions and topics for future research are also discussed, before addressing the potential to combine the two methods in order to further utilize their advantages. Overall, this review highlights the promise these methods offer, not just for making methodological advances, but also for adding to our ability to test and better understand theoretical concepts and pathways

Global Health Governance: Analyzing China, India, and Japan as Global Health Aid Donors

10.1111/j.1758-5899.2012.00173.xGlobal Policy33336-34

Deformation and Yield of Polymer Thin Films in Confined Geometries

Polymer thin film mechanics represents a hugely promising field of research. Due to their versatility and ease of fabrication, polymer films and coatings rank among the most ubiquitous systems in all of nanotechnology. The ability to correctly characterise and mechanically pattern such small volumes of material is a significant step towards the development of the next generation of organic nanomechanical systems. In this work, the deformation and yield of polymer films is studied through a series of flat punch indentation and imprint experiments, where the film material becomes geometrically confined beneath features whose width are many times the initial thickness of the film. A new methodology is developed for the extraction of the mechanical properties of supported thin films. Indentation of a polymer film by a well-aligned cylindrical flat punch whose diameter is several times the initial thickness of the film results in a state of uniaxial strain deformation, wherein lateral displacements are suppressed by the surrounding film. This method, called the confined compression layer test, allows for extraction of Young?s modulus, Poisson?s ratio, and the bulk modulus in a single test. Further, the test leads to a distinct confined yield event throughout the volume beneath the punch, in the absence of lateral flow. This yield event occurs within a highly uniform, pressure dominated stress field that is entirely unique at the nanoscale. The confined layer compression test is characterised here via indentation of atactic polystyrene films of 190 ? 470 nm thicknesses with a 2050 nm diameter diamond flat punch and via finite element simulations. The test is also demonstrated in PMMA and amorphous selenium films. The confined layer compression test is then extended to study aspects of non- equilibrium glass mechanics in polymer thin films. The effect of thermal history on the intrinsic stress-strain behaviour of polystyrene films is characterised, with well annealed films exhibiting higher confined yield stresses and greater resistance to plastic deformation. The effect of confined plasticity on the viscoelastic properties of polystyrene is studied, with a notable increase in creep compliance observed at yield. This is linked to higher segmental mobility. An increase in yield stress for materials plastically deformed in the confined layer compression test is reported. This phenomenon is studied experimentally in polystyrene and via finite element simulations and is found to result from persistent residual stresses imparted to the material during confined yield. These residual stresses also result in an elastic densification of the confined material, with a maximum relative mass density increase of 3.4% being observed in a 203 nm polystyrene film indented to 0.84 GPa peak stress. This technique suggests the possibility of a new form of residual stress based mechanical lithography. Finally, a significant improvement to the thermal nanoimprint technique is introduced. Shear flow of resist material confined directly beneath large aspect ratio imprint mold features is enhanced by the addition of a small (~10% of the feature size), oscillating lateral strain during normal loading. This leads to greater plasticity beneath mold features and a pumping action which aids flow into the surrounding cavities. This is demonstrated to enable high fidelity imprint below the glass transition temperature in 50 ?m thick PMMA sheets, while significant improvements are also reported in 150 and 40 nm films imprinted with a 4 ?m full pitch line pattern mold of 35 nm relief. The technology, called small amplitude oscillatory shear forming, is shown to enable low temperature, high fidelity pattern transfer over macroscopic sample areas, typically on the order 1 x 1 cm with a variety of mold geometries. In summary, this work sets out to use stress and strain as control variables to extend our knowledge and understanding of glassy polymer films and find new ways of patterning and probing materials at small scales