Amorphous solids lack long-range order. Therefore identifying structural defects -- akin to dislocations in crystalline solids -- that carry plastic flow in these systems remains a daunting ...challenge. By comparing many different structural indicators in computational models of glasses, under a variety of conditions we carefully assess which of these indicators are able to robustly identify the structural defects responsible for plastic flow in amorphous solids. We further demonstrate that the density of defects changes as a function of material preparation and strain in a manner that is highly correlated with the macroscopic material response. Our work represents an important step towards predicting how and when an amorphous solid will fail from its microscopic structure.
The polyamine-inhibitory regimen difluoromethylornithine (DFMO)+sulindac has marked efficacy in preventing metachronous colorectal adenomas. Polyamines are synthesised endogenously and obtained from ...dietary sources. Here we investigate dietary polyamine intake and outcomes in the DFMO+sulindac colorectal adenoma prevention trial.
Dietary polyamine data were available for 188 of 267 patients completing the study. Total dietary polyamine content was derived by the sum of dietary putrescine, spermine and spermidine values and categorised into two groups: highest (>75-100%) vs the lower three quartiles (0-25, 25-50 and 50-75%). Baseline tissue polyamine concentration and ODC1 genotype were determined. Logistic regression models were used for risk estimation.
A significant interaction was detected between dietary polyamine group and treatment with regard to adenoma recurrence (P=0.012). Significant metachronous adenoma risk reduction was observed after DFMO+sulindac treatment in dietary polyamine quartiles 1-3 (risk ratio (RR) 0.19; 95% confidence interval (CI) 0.08-0.42; P<0.0001) but not in quartile 4 (RR 1.51; 95% CI 0.53-4.29; P=0.44). However, a lower number of events in the placebo group within dietary quartile 4 confound the aforementioned risk estimates.
These preliminary findings reveal complex relationships between diet and therapeutic prevention, and they support further clinical trial-based investigations where the dietary intervention itself is controlled.
Despite centuries of work, dating back to Galileo, the molecular basis of bone's toughness and strength remains largely a mystery. A great deal is known about bone microsctructure and the microcracks ...that are precursors to its fracture, but little is known about the basic mechanism for dissipating the energy of an impact to keep the bone from fracturing. Bone is a nanocomposite of hydroxyapatite crystals and an organic matrix. Because rigid crystals such as the hydroxyapatite crystals cannot dissipate much energy, the organic matrix, which is mainly collagen, must be involved. A reduction in the number of collagen cross links has been associated with reduced bone strength and collagen is molecularly elongated ('pulled') when bovine tendon is strained. Using an atomic force microscope, a molecular mechanistic origin for the remarkable toughness of another biocomposite material, abalone nacre, has been found. Here we report that bone, like abalone nacre, contains polymers with 'sacrificial bonds' that both protect the polymer backbone and dissipate energy. The time needed for these sacrificial bonds to reform after pulling correlates with the time needed for bone to recover its toughness as measured by atomic force microscope indentation testing. We suggest that the sacrificial bonds found within or between collagen molecules may be partially responsible for the toughness of bone.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a ...cascade of 224- and 401-keV γ rays in coincidence with ^{32}Na nuclei. This is the only known microsecond isomer (1 μs≤T_{1/2}<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2^{+} state at 885 keV and a low-lying shape-coexisting 0_{2}^{+} state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in ^{32}Na: a 6^{-} spherical shape isomer that decays by E2 or a 0^{+} deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.
In the initial stages of the biomineralization of abalone shells, a primer layer of oriented calcite crystals grows on a nucleating protein sheet. It is known that soluble shell proteins can control ...crystal morphology.
Nanoscale structural analyses of biomineralized materials can frequently help elucidate important structure–function relationships in these complex organic–inorganic composites. Atomic force ...microscope (AFM) imaging of the exterior surface of trabecular bone reveals a densely woven structure of collagen fibrils, banded with a 67-nm periodicity, and densely packed mineral plates. The mineral plates on the collagen fibrils overlap and exhibit a large range of plate diameters from 30 to 200 nm. On the collagen fibrils, small nodular features, spaced 20–30 nm, run perpendicular to the fibrils. In some cases, these nodules are also seen on filaments extending between collagen fibrils. We hypothesize that these protrusions are noncollagenous proteins such as proteoglycans and may have collapsed into compact structures when the sample was dried. AFM images of pristine fractured surfaces reveal a dense array of mineral plates. In a few isolated locations, short sections of bare collagen fibrils are visible. In other regions, the existence of the underlying collagen fibrils can be inferred from the linear patterns of the mineral plates. Fractured samples, rinsed to remove mineral plates, reveal separated collagen fibrils on the fractured surfaces. These fibrils are often covered with protrusions similar to those observed on the exterior surfaces but are less organized. In addition, as on the exterior surfaces, there are sometimes small filaments extending between neighboring collagen fibrils. These studies provide important insights into the nanostructured architecture of this complex biocomposite.
Mechanical testing of trabecular bone is mainly motivated by the huge impact of osteoporosis in post-menopausal women and the aged in society in terms of social and health care costs. Trabecular bone ...loss and impairment of its mechanical properties reduce bone strength and increase fracture risk, especially in vertebrae. It is generally accepted that in addition to bone mineral density, microarchitecture and material properties of bone also play important roles for bone strength and fracture risk. In order to overcome the limitations of standard mechanical tests delivering merely integral information about complicated samples, experiments were designed for step-wise mechanical testing with concurrent imaging of trabecular and cortical bone. In this communication we present an approach for real-time imaging of trabecular bone during compression using high-speed photography and investigate the hypothesis whether the whitening of deformed trabeculae is due to microdamage. Experiments on human trabecular bone samples from a healthy male donor revealed that failure of such samples is highly localized in fracture bands. Moreover, strongly deformed trabeculae were seen to whiten, an effect similar to stress whitening in polymers. Scanning Electron Microscopy of the same regions of interest revealed that whitened trabeculae were strongly damaged by microscopic cracks and mostly failed in delamination. Higher resolution images uncovered mineralized collagen fibrils spanning the cracks. The whitening partially faded after unloading of the samples, presumably due to partial crack closure. Overall, high-speed photography enables microdamage detection in real-time during a mechanical test and provides a correlation to recorded stress strain curves.
Natural materials are renowned for their strength and toughness,,,. Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel,, and is ...representative of many other strong natural fibres,,. The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3,000 times more fracture resistant than a single crystal of the pure mineral,. The organic component, comprising just a few per cent of the composite by weight, is thought to hold the key to nacre's fracture toughness,. Ceramics laminated with organic material are more fracture resistant than non-laminated ceramics,, but synthetic materials made of interlocking ceramic tablets bound by a few weight per cent of ordinary adhesives do not have a toughness comparable to nacre. We believe that the key to nacre's fracture resistance resides in the polymer adhesive, and here we reveal the properties of this adhesive by using the atomic force microscope to stretch the organic molecules exposed on the surface of freshly cleaved nacre. The adhesive fibres elongate in a stepwise manner as folded domains or loops are pulled open. The elongation events occur for forces of a few hundred piconewtons, which are smaller than the forces of over a nanonewton required to break the polymer backbone in the threads. We suggest that this 'modular' elongation mechanism might prove to be quite general for conveying toughness to natural fibres and adhesives, and we predict that it might be found also in dragline silk.
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