Silk from Crickets: A New Twist on Spinning

Raspy crickets (Orthoptera: Gryllacrididae) are unique among the orthopterans in producing silk, which is used to build shelters. This work studied the material composition and the fabrication of cricket silk for the first time. We examined silk-webs produced in captivity, which comprised cylindrical fibers and flat films. Spectra obtained from micro-Raman experiments indicated that the silk is composed of protein, primarily in a beta-sheet conformation, and that fibers and films are almost identical in terms of amino acid composition and secondary structure. The primary sequences of four silk proteins were identified through a mass spectrometry/cDNA library approach. The most abundant silk protein was large in size (300 and 220 kDa variants), rich in alanine, glycine and serine, and contained repetitive sequence motifs; these are features which are shared with several known beta-sheet forming silk proteins. Convergent evolution at the molecular level contrasts with development by crickets of a novel mechanism for silk fabrication. After secretion of cricket silk proteins by the labial glands they are fabricated into mature silk by the labium-hypopharynx, which is modified to allow the controlled formation of either fibers or films. Protein folding into beta-sheet structure during silk fabrication is not driven by shear forces, as is reported for other silks

SAXS investigation of un-etched and etched ion tracks in polycarbonate

Investigation of the ion track morphologies and track etching behaviour in polycarbonate (PC) films was carried out using synchrotron based small-angle X-ray scattering (SAXS) measurements. The tracks were induced by Au ions with kinetic energies of 1.7 and 2.2GeV with applied fluences between 1×1010 and 1×1012 ions/cm2. The average radii of the un-etched tracks were studied as a function of the irradiation fluence, indicating a general ion induced degradation of the polymer, with a simultaneous increase in ion track radius from 2.6±0.002nm to 3.4±0.03nm. Chemical etching of the ion tracks in PC leads to the formation of cylindrical pores. The pore radius increases linearly with etching time. In 3M NaOH at 55°C, a radial etching rate of 9.2nm/min is observedThe research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron. We acknowledge the DFG (HO 5722/1-1 and SCHL 384-17/1) and the Australian Research Council for financial support

The Coiled Coil Silk of Bees, Ants, and Hornets

In this article, we review current knowledge about the silk produced by the larvae of bees, ants, and hornets [Apoidea and Vespoidea: Hymenoptera]. Different species use the silk either alone or in composites for a variety of purposes including mechanical reinforcement, thermal regulation, or humidification. The characteristic molecular structure of this silk is α-helical proteins assembled into tetrameric coiled coils. Gene sequences from seven species are available, and each species possesses a copy of each of four related silk genes that encode proteins predicted to form coiled coils. The proteins are ordered at multiple length scales within the labial gland of the final larval instar before spinning. The insects control the morphology of the silk during spinning to produce either fibers or sheets. The silk proteins are small and non repetitive and have been produced artificially at high levels by fermentation in E. coli. The artificial silk proteins can be fabricated into materials with structural and mechanical properties similar to those of native silks

High-resolution X-ray diffractometry investigation of interface layers in GaN/AlN structures grown on sapphire substrates

GaInN is an important wide band gap material with applications in short wavelength optoelectronic devices. The GaInN layer is often grown on a sapphire substrate, with low-temperature-deposited AlN and thick GaN used as buffer layers. The growth regime consists of many steps, each of which contributes to the overall properties of the device. The aim of our high-resolution X-ray diffraction experiments, conducted at the Photon Factory (Tsukuba, Japan), was to investigate the structural quality of the AlN buffer layer, which affects the final properties of the device. Reciprocal space mapping was used to study samples (having various layer thicknesses) from each stage of the growth process. Analysis of the experimental data provides parameters such as mosaic block dimensions and orientation, lattice strain distribution, and layer thickness

Size characterization of ion tracks in PET and PTFE using SAXS

Ion tracks in polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) were created by swift heavy ion irradiation and subsequently characterized using small angle X-ray scattering (SAXS). Due to their reduced density compared to the surrounding matrix, cylindrical geometry, and parallel orientation, ion tracks produce a characteristic scattering pattern which allows quantitative analysis of their radius with high precision. For ion tracks in PET thermal annealing led to a gradual fading with a decrease in density difference yet a simultaneous increase in ion track radius. Such an increase in radius is the direct opposite compared to temperature induced ion track shrinking in inorganic materials, and suggests a very different thermal response of the polymer

Structural properties of nano-porous GaSb prepared by swift heavy-ion irradiation

We are investigating the structural and optical properties of nano-porous GaSb formed by swift heavy ion irradiation using a combination of high resolution structural characterization techniques including synchrotron based small- and wide-angle X-ray scattering (SAXS/WAXS) and extended X-ray absorption fine structure (EXAFS), as well as optical measurements like Raman and Fourier transform infrared spectroscopy. In contrast to nano-porous GaSb prepared by low energy ion irradiation, the swift heavy ion irradiated samples do not show any sign of homo-polar bonding typical for amorphous materials in EXAFS measurements despite the observed significant structural disorder. Furthermore, X-ray diffraction and Raman-spectroscopy reveal the presence of small (<10 nm) crystallites inside the pore walls

Studying metamorphic phase transformations and nano-porosity evolution in situ with Synchrotron X-ray scattering:the example of gypsum dehydration

We demonstrate how nano-scale metamorphic processes can be unravelled with small- and wide-angle transmission scattering of Synchrotron radiation (SAXS and WAXS, respectively) in concert with a novel loading cell. We studied the dehydration of Volterra alabaster, a polycrystalline gypsum rock, at four temperatures (128, 144, 152, and 173˚C) and two axial strains. Cylindrical samples with a diameter of 13 mm and a thickness of 1 mm were confined axially by the Beryllium (Be) windows of the loading cell. Prior to heating, the Be windows were screwed into the cell up to hand-tightness ("low strain") or up to 54 Nm ("high strain") using a torque wrench, thus keeping the sample discs in place. Radially, the discs remained unconfined with respect to the cell interior. A high-powered electric heater clamped around the cell supplied heat conductively. Numerical models of the thermo-elastic response of the cell and the sample suggest that dehydration onset occurred at temperatures above 105 ˚C. Hence, the transient heating period of the sample was so short compared to the observed dehydration kinetics that the experiments were essentially isothermal. During each experiment, three or five 100x100 micron interrogation points along the disc radius were irradiated with Synchrotron radiation (12 or 16 KeV) for 5 s per scan. These points were distributed equidistantly between disc centre and margin. WAXS and SAXS detectors collected patterns over a scattering vector length range between 1.17 to 4.13 per Å and 0.003 to 0.22 per Å, respectively. The dehydration product was always hemihydrate. The WAXS data provide the dehydration kinetics. The conversion curves are well described by an Avrami model. However, at the two intermediate temperatures of the low-strain experiments, a clear transient "bump" in the conversion curves suggests a switch in mechanism, possibly from nucleation-and-growth to diffusion control. An Arrhenius fit to low- and high-strain data yields activation energies of 74 and 50 kJ/mol, respectively, lower than those for powders at T ≤ 120˚C (≥ 90kJ/mol). However, a single low-strain experiment at 120˚C converted ten times more slowly than extrapolated from the aforementioned Arrhenius fit and powders dehydrated at the same temperature. This observation probably indicates another change in dehydration mechanism at lower temperatures. The SAXS data yield novel insight into the evolution of pores in the size range 1 to 80 nm. Due to instrument geometry, the Guinier region could not be observed. Thus, absolute porosity could not be measured. Nevertheless, the SAXS signal experienced its most significant modification well after the phase conversion was completed. Therefore, pore-space geometry and porosity on the nano-scale kept on evolving well past reaction completion, indicating two different characteristic timescales for both processes. This observation contradicts the general assumption that phase conversion and porosity evolution proceed hand-in-hand during gypsum dehydration. Our experiments highlight the tremendous impact of microstructure (powder versus polycrystal) and mechanical loading on dehydration. These insights are only possible because the experiments allowed the coeval monitoring of phase changes and nano-porosity evolution under geological loading conditions

Local symmetry predictors of mechanical stability in glasses

The mechanical properties of crystals are controlled by the translational symmetry of their structures. But for glasses with a disordered structure, the link between the symmetry of local particle arrangements and stability is not well established. In this contribution, we provide experimental verification that the centrosymmetry of nearest-neighbor polyhedra in a glass strongly correlates with the local mechanical stability. We examine the distribution of local stability and local centrosymmetry in a glass during aging and deformation using microbeam x-ray scattering. These measurements reveal the underlying relationship between particle-level structure and larger-scale behavior and demonstrate that spatially connected, coordinated local transformations to lower symmetry structures are fundamental to these phenomena. While glassy structures lack obvious global symmetry breaking, local structural symmetry is a critical factor in predicting stability. </p