Hyperelastic anisotropic microplane constitutive model for annulus fibrosus

In a recent paper, Peng et al. (2006, "An Anisotropic Hyperelastic Constitutive Model With Fiber-Matrix Interaction for the Human Annulus Fibrosis," ASME J. Appl. Mech., 73(5), pp. 815-824) developed an anisotropic hyperelastic constitutive model for the human annulus fibrosus in which fiber-matrix interaction plays a crucial role in simulating experimental observations reported in the literature. Later, Guo et al. (2006, "A Composites-Based Hyperelastic Constitutive Model for Soft Tissue With Application to the Human Fibrosis," J. Mech. Phys. Solids, 54(9), pp. 1952-1971) used fiber reinforced continuum mechanics theory to formulate a model in which the fiber-matrix interaction was simulated using only composite effect. It was shown in these studies that the classical anisotropic hyperelastic constitutive models for soft tissue, which do not account for this shear interaction, cannot accurately simulate the test data on human annulus fibrosus. In this study, we show that the microplane model for soft tissue developed by Caner and Carol (2006, "Microplane Constitutive Model and Computational Framework for Blood Vessel Tissue," ASME J. Biomech. Eng., 128(3), pp. 419-427) can be adjusted for human annulus fibrosus and the resulting model can accurately simulate the experimental observations without explicit fiber-matrix interaction because, in microplane model, the shear interaction between the individual fibers distributed in the tissue provides the required additional rigidity to explain these experimental facts. The intensity of the shear interaction between the fibers can be adjusted by adjusting the spread in the distribution while keeping the total amount of the fiber constant. A comparison of results obtained from (i) a fiber-matrix parallel coupling model, which does not account for the fiber-matrix interaction, (ii) the same model but enriched with fiber-matrix interaction, and (iii) microplane model for soft tissue adapted to annulus fibrosus with two families of fiber distributions is presented. The conclusions are (i) that varying degrees of fiber-fiber and fiber-matrix shear interaction must be taking place in the human annulus fibrosus, (ii) that this shear interaction is essential to be able to explain the mechanical behavior of human annulus fibrosus, and (iii) that microplane model can be fortified with fiber-matrix interaction in a straightforward manner provided that there are new experimental data on distribution of fibers, which indicate a spread so small that it requires an explicit fiber-matrix interaction to be able to simulate the experimental data.     

Reaction sequences from light absorption to the cleavage of water in photosynthesis

The reaction sequence between the primary electron acceptor, the oxidized Chlorophyll-a_II, and the terminal electron donor, the water splitting enzyme system S, is being described in the range from nanoseconds to milliseconds. For the cleavage of water Chlorophyll-a_II ⁺ extracts four electrons in four turnovers from the enzyme system S responsible for the water oxidation. For each extraction the electron is moved step by step along the chain that connects the Chlorophyll-a_II center with that of S. Beginning with the transfer from the immediate donor, D₁, to Chl-a_II ⁺, the subsequent transfer from D₂ to D₁ ⁺ ends in the electron transfer from S to D₂ ⁺. This final act establishes in S the oxidizing equivalent, probably in the form of oxidized manganese. Coupled with these acts is an intrinsic proton release and a surplus charge formation. After the generation of the 4th oxidizing equivalent in a concerted final action the evolution of O₂ from water takes place. Correlations between the events are described quantitatively.     

Pre and postoperative assessment of regional cerebral blood flow in hydrocephalus by 99mTc-hexamethyl-propylenamine oxime SPECT

Cerebrovascular changes resulting from hydrocephalus still remain to be investigated. It has been suggested that hydrocephalus distorts the large feeding arteries and that the collapse of capillaries results in decreased cerebral blood flow. This clinical study was designed to evaluate the effect of shunting on regional cerebral blood flow in patients with obstructive hydrocephalus of varying duration. Technetium-99m hexamethyl propyleamine oxime (99mTc-HMPAO) was used to measure the cerebral perfusion, semiquantitatively, since the pattern of its distribution in brain is somewhat similar to that of regional cerebral blood flow (rCBF), and the pre and postoperative semiquantitative rCBF values of each lobe were calculated. Fifteen patients (8 F, 7 M) underwent both CT and single photon emission tomography (SPECT) using 99mTc-HMPAO examination before and 1 week after shunting. Mean percentage of all lobes were calculated by subtracting the preoperative mean rCBF of all lobes from the corresponding postoperative values. The patients were classified into 3 groups according to the mean percentage of all lobes. Group A: showed a marked increase in mean cortical blood flow (+ 16.00 +/- 2.9%), group B: a moderate increase (11.27 +/- 4.8%), and in group C: there was the least improvement in mean cortical blood flow (+ 1.17 +/- 2.7%). The mean duration of hydrocephalus of group A, group B and group C was 5 +/- 0.5 weeks, 8 +/- 1 weeks and more than 12 weeks, respectively. Psychological testing and clinical observation of the daily activities of the patients postoperatively showed some correlation with increased rCBF and clinical improvement.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nonenzymatic phosphorylation of acetate by carbamyl phosphate

It is found that carbamyl phosphate is an efficient condensing agent for acetate and hydroxylamine in the presence of Be²⁺ and Al³⁺. The reaction has an optimum at pH 4 and is completed within 30 min. The yield of hydroxamate formation reaches 30% (based on initial carbamyl phosphate). Acetylphosphate as the intermediary product of this reaction was identified by P-NMR spectroscopy.