Nanoscale assembly in biological systems: from neuronal cytoskeletal proteins to curvature stabilizing lipids

The review will describe experiments inspired by the rich variety of bundles and networks of interacting microtubules (MT), neurofilaments, and filamentous-actin in neurons where the nature of the interactions, structures, and structure-function correlations remain poorly understood. We describe how three-dimensional (3D) MT bundles and 2D MT bundles may assemble, in cell free systems in the presence of counter-ions, revealing structures not predicted by polyelectrolyte theories. Interestingly, experiments reveal that the neuronal protein tau, an abundant MT-associated-protein in axons, modulates the MT diameter providing insight for the control of geometric parameters in bio- nanotechnology. In another set of experiments we describe lipid-protein-nanotubes, and lipid nano-tubes and rods, resulting from membrane shape evolution processes involving protein templates and curvature stabilizing lipids. Similar membrane shape changes, occurring in cells for the purpose of specific functions, are induced by interactions between membranes and proteins. The biological materials systems described have applications in bio-nanotechnology.     

Synchronized External Pulsation for Improved Tolerance to Acceleration Stress: Model Studies and Preliminary Experiments

Synchronized external pulsation is proposed as a method to improve tolerance to acceleration stress. This technique uses a modified anti-G suit which is pressurized and depressurized synchronously with the heart cycle. The feasibility of the procedure has been studied using a computer model of the cardiovascular system which includes the effects of Gz stress, and contains simulations of baroreceptor control of heart rate and venous tone. Model predictions indicate that for unprotected subjects, carotid pressure at eye level (ophthalmic artery pressure) decreases to 20 mmHg (beginning of central light loss) at approximately +3.6 Gz. Applying standard anti-G suit pressure to the model increases this level to 5.3 Gz. When synchronized external pulsation of 2 psi is superimposed on the standard anti-G suit pressure, the tolerance to acceleration stress is further augmented by at least 0.9 G above the protection afforded by the standard anti-G suit alone. A set of preliminary experiments on human subjects to test the feasibility of using the technique in the high-G environment has also been carried out. The results under various protection modes compare favorably to the model predictions. Our results suggest that the computer model presented here is a useful tool for studying cardiovascular responses under +GZ stress. It also indicates that using synchronized external pulsation pressure superimposed on the standard anti-G suit pressure may offer extra protection to acceleration stress.     

Efficient non‐linear reanalysis of skeletal structures using combined approximations

Non‐linear reanalysis of large‐scale structures usually involves much computational effort, because the set of non‐linear equations must be solved repeatedly during the solution process. Various approximations that are often used for linear reanalysis are not sufficiently accurate for non‐linear problems. In this study, solution procedures based on the combined approximations approach are developed and compared in terms of efficiency and accuracy. Various path‐independent non‐linear analysis and reanalysis problems are considered, including material non‐linearity, geometric non‐linearity and buckling analysis. Numerical examples demonstrate the effectiveness of the procedures presented. It is shown that in various cases accurate results can be achieved efficiently. Copyright © 2007 John Wiley & Sons, Ltd.     

Interfuel substitution in the primary metals industries

This paper applies a translog price possibility frontier to a pooled sample in order to measure the extent of interfuel substitution effects in the primary metals industries. The results clearly indicate that relative changes in fuel prices across primary metals industries have significant effects on energy consumption. This, in turn, has important implications for public policy. In particular, the market system appears better able to deal with exogenous shifts in energy supplies than has frequently been assumed in the formulation of public policies toward the energy crisis.     

Synthesis of elastic structures with controlled forces

An analytical model is presented for synthesis of elastic structural systems under external loads and desired “controlled forces”. In the first stage, the optimal force distribution and the final cross sectional dimensions are chosen, solving an optimization problem with elastic compatibility temporarily excluded. In the second stage, the force due to the external loads is determined for the optimal structure by elastic analysis, and the desired controlled force needed for the optimal cross sections and force distribution is obtained by subtracting the above result from the optimal force of the first stage. The final design which has been determined in the first stage, satisfies both conditions of equilibrium and compatibility. It is a lower-bound solution for a similar incontrolled elastic system. In some cases the first-stage optimization problem may be cast in linear programming form. Numerical examples of continuous beam and indeterminate truss demonstrate application of the proposed method.     

Pressure–volume–temperature relationships of solid and molten polypropylene and poly(butene-1)

Experimental data on the dependence of the specific volume on temperature and pressure (PVT data) to 2000 kg/cm² of isotactic polypropylene and isotactic poly(butene-1) are reported and discussed. The temperature range covered is 30–297°C for polypropylene and 30–246°C for poly(butene-1), thus encompassing the solid and molten states of both materials. An empirical equation of state based on the Tait equation can be fitted to the melt data of both materials. The coefficients reported reproduce the measured specific volumes with a standard deviation of less than 0.001 cm³/g.     

Conversion of work of deformation to heat in polymers

The time dependence of the conversion of mechanical work of deformation to heat in polymers is investigated using viscoelastic models. The deformation chosen is a constant strain rate, $\text{?}\text{?}{}_0$, in tension for t?t₀ and a constant strain $\text{?}{}_0\text{=}\text{?}\text{?}{}_0\text{t}{}_0$ for t?t₀. For a simple Maxwell model with only one relaxation time, τ, the ratio of heat produced to work done (‘conversion fraction’) is calculated as a function of time for different values of $\text{t}{}_0\text{τ}$. At t=t₀ the conversion fraction is high only when τ?₀. For a continuous Maxwell model with a distribution of relaxation times H(τ) we obtain the result that the conversion fraction is near 100% at t=t₀ if H(τ) is not peaked strongly in the region τ>0·1t₀ but extends with reasonable values into the region τ < 0·1t₀. A formula is given which allows the calculation of the conversion fraction at t=t₀ from the distribution of relaxation times H(τ).