The bending modulus is an important physical constant characterizing lipid membranes. Different methods have been devised for calculating the bending modulus from simulations, and one of them, named the buckling method, is nowadays widely used due to its simplicity and numerical stability. However, questions remain on the reproducibility, finite size effects, and interpretation of results on lipid mixtures. Here we explore the dependence of simulation results on the system size and the strain. We find that the dimensions of the box have a negligible impact on the results when the system size is beyond a certain threshold. We then calculate the bending rigidity for of a series of common single-component lipid bilayers (PC, PS, PE, PG, and SM), as well as a number of binary and ternary lipid mixtures. We find that bending moduli of lipid mixtures can be predicted from the weighted average of the moduli of the individual components, as long as the mixture is homogeneous. For phase-separated mixtures, the apparent elastic modulus is closer to the value of the softer component. Predictions of the bending modulus based on the area compressibility modulus are found to be generally unreliable.

We have studied the strain rate dependence of incremental stress-strain curves of self-assembled type I collagen fibers in an effort to understand the molecular phenomena that contribute to the macroscopic mechanical behavior of tendons. Results of viscoelastic tests at strain rates between 10% and 1000% per min suggest that the slope of the elastic stress-strain curve is to a first approximation independent of strain rate while the slope of the viscous stress-strain curve increases with increased strain rate. After correction of the slope of the viscous stress-strain curve for the changes in strain rate, it is observed that the apparent viscosity decreases with increased strain rate. It is concluded that the approximate strain rate independence of the elastic spring constant of collagen is consistent with the spring-like behavior of the 12 flexible regions that make up the collagen D-period. These regions are poor in the rigid amino acid residues proline and hydroxyproline. In contrast, the thixotropy of collagen is consistent with the slippage of subfibrillar subunits during tensile deformation. It is hypothesized that at high strain rates subfibrillar subunits appear to "hydroplane" by each other on a layer of loosely bound water.

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We have studied the strain rate dependence of incremental stress-strain curves of self-assembled type I collagen fibers in an effort to understand the molecular phenomena that contribute to the macroscopic mechanical behavior of tendons. Results of viscoelastic tests at strain rates between 10% and 1000% per min suggest that the slope of the elastic stress-strain curve is to a first approximation independent of strain rate while the slope of the viscous stress-strain curve increases with increased strain rate. After correction of the slope of the viscous stress-strain curve for the changes in strain rate, it is observed that the apparent viscosity decreases with increased strain rate. It is concluded that the approximate strain rate independence of the elastic spring constant of collagen is consistent with the spring-like behavior of the 12 flexible regions that make up the collagen D-period. These regions are poor in the rigid amino acid residues proline and hydroxyproline. In contrast, the thixotropy of collagen is consistent with the slippage of subfibrillar subunits during tensile deformation. It is hypothesized that at high strain rates subfibrillar subunits appear to "hydroplane" by each other on a layer of loosely bound water.

Summary:Rapid advances in the field of interventional neurology and the development of minimally invasive techniques have resulted in a great expansion of potential therapeutic applications. We discuss therapeutic interventional neurology as applied in clinical practice in one of the two possible ways: 1) embolization leading to occlusion of blood vessels; and 2) revascularization leading to reopening of blood vessels. These procedures can be applied to a broad range of cerebrovascular diseases. In the first section of this review, we will explore the evolution of these interventions to occlude aneurysms, arteriovenous malformations, neurovascular tumors, and injuries. In the second section, revascularization in acute ischemic stroke, stenosis, and dural venous thrombosis will be discussed.