Collagen Denaturation as a Toughening Mechanism in Cortical Bone

Bone is a highly versatile tissue. It is used for protecting internal organs as ribs, supporting locomotion as long bone, as a weapon in the form of antler, and many other uses. Depending on its use or function the bone may experience repeated cyclic loads, as in leg bones, or resist sudden impact as antler. To be physiologically useful as a biomaterial, bone must be stiff and resist deformation, but also be capable of dissipating large amounts of energy while resisting failure. All bone meets these biological requirements as a composite of hydroxyapatite mineral, protein (mainly type-I collagen) and water. Together these materials form a highly complex multi-scale structure that gives rise to varied and powerful toughening mechanisms. One putative - but yet unproven - mechanism is the mechanical denaturation (unravelling) of collagen. The native form of collagen is a triple helix with internal hydrogen bonds maintaining the molecular structure. In silico experiments have suggested that collagen does denature under mechanical stresses. If the collagen does mechanically denature during fracture, then some quantity of energy is dissipated disrupting the internal hydrogen bonding. The primary objective of this work is to test the hypothesis that “collagen denatures as a toughening mechanism during stable fracture of cortical bone”. A new biotechnology, fluorescently labelled collagen hybridizing peptides (F-CHP), has seen recent successes in identifying denatured collagen in a variety of tissues. These probes are specific for denatured collagen and not native, triple helical collagen. As such they provide a unique opportunity to probe the behavior of bone collagen during fracture. A notching and staining system was devised to reproducibly image denatured collagen on bovine cortical bone fracture surfaces. This imaging showed consistent increases in staining on surfaces produced by stable crack extension during fracture. This increase in staining correlated strongly with the energy per unit area dissipated by the sample. Furthermore, the staining was confined to a visibly rough region on the fracture surface produced by stable fracture extension. This result supports the hypothesis, suggesting that the denaturation of collagen is a crucial element of how bone resists fracture during stable fracture extension

Djenné et le Ghana: deux modèles sociaux,et la question du commerce trans-saharien

　This paper puts under scrutiny the alleged link between the Trans-Saharian Arabo-Berber trade and the development of complex polities such as the Ghana empire in the West African Sahel. When did Ghana come into existence? As evidence is lacking, the city of Djenné (Jenne) could provide some clues. Meanwhile, the peculiar urbanism of Djenné-Djeno (old Jenne) can be understood as depicting alternative, auto-organizing social relations in the Inland Niger Delta. Furthermore, during the first millennium before and the first millennium after common era, the distinctive «urban cluster» pattern of Djenné-Djeno or Dia in the Macina can be found elsewhere along the Niger Bend and as far downstream as Bentia-Kukiya.　Climatic changes affecting the Sahara presumably induced a common Soninke origin for Djenné-Djeno and for the Ghana polity, in both cases via the Neolithic culture of Tichitt. Based on this and on the Wagadu myth of the snake and the rain-maker, some features of Ghana can be underlined. Specifically, the level of economical interactions clearly shows that internal West African networks dealing with copper, salt, stones and staples predated the Trans-Saharian links with the Islamic world, leading to the conclusion that the so-called Arabic stimulus explanation can be dismissed. Finally, the articulation between the two towns of Djenné is addressed

Measuring transient reaction rates from nonstationary catalysts

Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment—a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossible—catalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10–1000 times higher. We are able to record kinetic traces of CO2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions

GREGOR Fabry-Perot Interferometer - status report and prospects

The GREGOR Fabry-Perot Interferometer (GFPI) is one of three first-light instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Tenerife, Spain. The GFPI allows fast narrow-band imaging and post-factum image restoration. The retrieved physical parameters will be a fundamental building block for understanding the dynamic Sun and its magnetic field at spatial scales down to 50 km on the solar surface. The GFPI is a tunable dual-etalon system in a collimated mounting. It is designed for spectropolarimetric observations over the wavelength range from 530-860 nm with a theoretical spectral resolution of R ~ 250,000. The GFPI is equipped with a full-Stokes polarimeter. Large-format, high-cadence CCD detectors with powerful computer hard- and software enable the scanning of spectral lines in time spans equivalent to the evolution time of solar features. The field-of-view of 50" x 38" covers a significant fraction of the typical area of active regions. We present the main characteristics of the GFPI including advanced and automated calibration and observing procedures. We discuss improvements in the optical design of the instrument and show first observational results. Finally, we lay out first concrete ideas for the integration of a second FPI, the Blue Imaging Solar Spectrometer, which will explore the blue spectral region below 530 nm.Comment: 18 pages, 9 Figures, 4 Tables, "Astronomical Telescopes and Instrumentation", Amsterdam, 1-6 July 2012, SPIE Proc. 8446-276, in pres

The GREGOR Fabry-P\'erot Interferometer

The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated mounting. Thanks to its large-format, high-cadence CCD detectors with sophisticated computer hard- and software it is capable of scanning spectral lines with a cadence that is sufficient to capture the dynamic evolution of the solar atmosphere. The field-of-view (FOV) of 50" x 38" is well suited for quiet Sun and sunspot observations. However, in the vector spectropolarimetric mode the FOV reduces to 25" x 38". The spectral coverage in the spectroscopic mode extends from 530-860 nm with a theoretical spectral resolution R of about 250,000, whereas in the vector spectropolarimetric mode the wavelength range is at present limited to 580-660 nm. The combination of fast narrow-band imaging and post-factum image restoration has the potential for discovery science concerning the dynamic Sun and its magnetic field at spatial scales down to about 50 km on the solar surface.Comment: 14 pages, 17 figures, 4 tables; pre-print of AN 333, p.880-893, 2012 (AN special issue to GREGOR