Validation of Optimal Fourier Rheometry for rapidly gelling materials and its application in the study of collagen gelation

Rheological Gel Point measurements may incur errors in the case of rapid gelling systemsdue to the limitations of multiple frequency oscillatory shear techniques such as frequency sweeps and Fourier Transform Mechanical Spectroscopy, FTMS. These limitations are associated with sample mutation and data interpolation. In the present paper we consider how an alternative rapid characterisation technique known as Optimal Fourier Rheometry, OFR, can be used to study a rapidly gelling material, namely collagen at near physiological temperatures. The OFR technique is validated using a model reference gelling system whose GP characteristics have been widely reported. An analysis of the susceptibility of OFR measurements to rheometrical artefacts is made prior to its use in study of rapid gelling collagen gels formed over a range of physiologically relevant collagen concentrations. The results of this OFR are the first measurements of the stress relaxation characteristics of collagen gels in a single rheological experiment

Effects of unidirectional flow shear stresses on the formation, fractal microstructure and rigidity of incipient whole blood clots and fibrin gels

Incipient clot formation in whole blood and fibrin gels was studied by the rheometric techniques of controlled stress parallel superposition (CSPS) and small amplitude oscillatory shear (SAOS). The effects of unidirectional shear stress on incipient clot microstructure, formation kinetics and elasticity are reported in terms of the fractal dimension (df) of the fibrin network, the gel network formation time (TGP) and the shear elastic modulus, respectively. The results of this first haemorheological application of CSPS reveal the marked sensitivity of incipient clot microstructure to physiologically relevant levels of shear stress, these being an order of magnitude lower than have previously been studied by SAOS. CSPS tests revealed that exposure of forming clots to increasing levels of shear stress produces a corresponding elevation in df, consistent with the formation of tighter, more compact clot microstructures under unidirectional flow. A corresponding increase in shear elasticity was recorded. The scaling relationship established between shear elasticity and df for fibrin clots and whole blood confirms the fibrin network as the dominant microstructural component of the incipient clot in terms of its response to imposed stress. Supplementary studies of fibrin clot formation by rheometry and microscopy revealed the substantial additional network mass required to increase df and provide evidence to support the hypothesis that microstructural changes in blood clotted under unidirectional shear may be attributed to flow enhanced thrombin generation and activation. CSPS also identified a threshold value of unidirectional shear stress above which no incipient clot formation could be detected. CSPS was shown to be a valuable haemorheological tool for the study of the effects of physiological and pathological levels of shear on clot properties

The effect of sepsis and its inflammatory response on mechanical clot characteristics: a prospective observational study

Purpose: Sepsis and its progression are known to have a major influence on the coagulation system. Current coagulation tests are of limited use when assessing coagulation in sepsis patients. This study aims to assess the potential for a new functional biomarker of clot microstructure, fractal dimension, to identify changes in the mechanical properties of clot microstructure across the sepsis spectrum (sepsis, severe sepsis and septic shock). Methods: A total of 100 patients that presented acutely to a large teaching hospital were included in this prospective observational study (50 sepsis, 20 severe sepsis and 30 septic shock) against a matched control of 44 healthy volunteers. Fractal analysis was performed, as well as standard markers of coagulation, and six plasma markers of inflammation. Results: Fractal dimension was significantly higher in the sepsis and severe sepsis groups than the healthy control (1.78 ± 0.07 and 1.80 ± 0.05 respectively vs 1.74 ± 0.03) (p < 0.001), indicating a significant increase in mechanical clot strength and elasticity consistent with a hypercoagulable state. Conversely, fractal dimension was significantly lower in septic shock (1.66 ± 0.10, p < 0.001), indicating a significant reduction in mechanical clot strength and functionality consistent with a hypocoagulable state. This corresponded with a significant increase in the inflammatory response. Conclusions: This study confirms that clot microstructure is significantly altered through the various stages of sepsis. Of particular importance was the marked change in clot development between severe sepsis and septic shock, which has not been previously reported

Assessment of the stress relaxation characteristics of critical gels formed under unidirectional shear flow by controlled stress parallel superposition rheometry

Processes involving a unidirectional shear flow component are widespread in industrial manufacturing techniques such as printing and coating, or in physiological events such as blood coagulation. Standard rheometric techniques are usually employed under quiescent conditions and as such are inappropriate for the study of microstructural modification induced by the presence of a unidirectional shear flow. We demonstrate how controlled stress parallel superposition (CSPS) may be exploited to enable accurate detection of the Gel Point and analysis of Gel Point parameters for systems undergoing a viscoelastic liquid (VEL) to viscoelastic solid (VES) transition in the presence of a unidirectional flow field. Specifically, we note that certain features of the CSPS experiment, when performed near the Gel Point, may obviate previously reported concerns regarding the experiment. A biopolymer system (gelatin) which forms gels by thermoreversible gelation is employed as a model gelling material to confirm the ability of CSPS to characterise the stress relaxation characteristics of critical-gels in the presence of a) progressively decreasing and b) progressively increasing unidirectional strain rate and oscillatory strain amplitude. Additional validation of CSPS results is reported for a silicone dielectric gel used in the industrial production of printed electronic products. Finally, CSPS is used to investigate microstructural modification of fibrin-thrombin gels as a consequence of clot formation under a unidirectional shear stress. The results confirm the validity of the CSPS technique in gelation studies and the technique is used, for the first time, to directly record the thermally induced VES to VEL transition in aqueous gelatin systems

Photometric redshifts in the Hubble Deep Fields: evolution of extinction and the star-formation rate

Photometric redshifts are studied with a template approach using data from HDF-N and -S . The problem of aliasing in photometric redshift estimates is investigated in some detail and found not to be a significant problem if at least four photometric bands are available. The performance of the approach presented here appears to exceed that of others in the literature. With reasonable restrictions, it is possible to determine the dust extinction as well as the photometric redshift, provided five or more photometric bands are available. An important result is that evolution of with redshift is seen, with higher values than locally at z = 0.5-1.5, and lower values at z > 2. This is consistent with current models for the star formation history of the universe. Deconvolving the uv-to-ir seds into an old star and young star component allows determination of M_* and \dot{M}_* for each galaxy, as well as z_{phot} and A_V, provided that infrared photometric bands are available. The expected trend of b = M_*/\dot{M}_* t_0 increasing to the past is seen. However there is a great deal of scatter in the relation between b and sed type, showing that the recent star-formation history is not very well correlated with the long-term history of a galaxy. The 2800 A luminosity function and star-formation rate are calculated for a large sample of HDF-N (2490) and HDF-S (28719) galaxies, using photometric redshifts, for the redshift range 0.2-5. The star-formation rates agree reasonably well with those from a variety of other uv, $H_{\alpha}$, infrared and submillimetre estimates, and with star-formation histories used to model optical, infrared and submillimetre source-counts.Comment: 18 pages, 29 figures, accepted for publication in MNRA

APM 08279+5255: an ultraluminous BAL quasar at a redshift z=3.87

We report on the discovery of a highly luminous, broad absorption line quasar at a redshift of $z=3.87$ which is positionally coincident, within one arcsecond, with the IRAS FSC source F08279+5255. A chance alignment of the quasar and the IRAS source is extremely unlikely and we argue that the optical and FIR flux are different manifestations of the same object. With an R-band magnitude of 15.2, and an IRAS 60\mum flux of 0.51\jy, APM 08279+5255 is (apparently) easily the most intrinsically luminous object known, with L_{Bol}\sim5\times10^{15}L_{\odot}}. Imaging suggests that gravitational lensing may play a role in amplifying the intrinsic properties of the system. The optical spectrum of the quasar clearly reveals the presence of three potential lensing galaxies, \mg absorption systems at $z=1.18$ and $z=1.81$, and a \ly absorption system at $z=3.07$. We estimate the total amplification of the optical component to be $\approx40$, but, due to the larger scale of the emitting region, would expect the infrared amplification to be significantly less. Even making the conservative assumption that all wavelengths are amplified by a factor 40, APM 08279+5255 still possesses a phenomenal luminosity of \simgt 10^{14L_{\odot}}, indicating that it belongs to a small, but significant population of high--redshift, hyperluminous objects with copious infrared emission.Comment: 15 Pages with Four figures. Accepted for publication in the Astrophysical Journa

A World Of Our Own: Climate Change Advocacy In The Anthropocene

The effects of human impacts on the environment are often not comprehensible to people and have to be given meaning through communication. Such impacts, most prominently climate change, have increased to the extent that human actions are the dominant force in planetary biophysical systems. Yet these impacts are for the most part unintentional and not subject to democratic control. A critical discourse analysis of campaign material and media content examines how three advocacy groups – 350, a climate activist organization, The Breakthrough Institute, a think-tank, and The Nature Conservancy, an established conservation organization – discursively construct climate change. The three groups acknowledge the need to more consciously or deliberatively manage environmental impacts, and yet all have very different assumptions, objectives and tactics in their advocacy. Analysis of the communication activities of the organizations and how their ideas are represented and contested in other media shows not only how they construct the particular issue of climate change but their relationship to societal power relationships. How the organizations build their case for action involves discursive acts which define or re-define the boundaries between nature and society and what (and who) is to be included or excluded from political concern. Unless directly challenged, these new formations will reproduce existing power structures and inequalities

Facts about our ecological crisis are incontrovertible: we must take action

Humans cannot continue to violate the fundamental laws of nature or science with impunity, say 94 signatories including Dr Alison Green and Molly Scott Cato MEP. Professor of Sustainability Leadership at the University of Cumbria Jem Bendell joined others in calling for a wider debate about sustainability, featured in The Guardian. We the undersigned represent diverse academic disciplines, and the views expressed here are those of the signatories and not their organisations. While our academic perspectives and expertise may differ, we are united on one point: we will not tolerate the failure of this or any other government to take robust and emergency action in respect of the worsening ecological crisis. The science is clear, the facts are incontrovertible, and it is unconscionable to us that our children and grandchildren should have to bear the terrifying brunt of an unprecedented disaster of our own making