Formation of planar bilayer membranes from lipid monolayers. A critique.

The formation of planar bilayer membranes from lipid monolayers as described by Montal and Mueller (Proc. Natl. Acad. Sci. 1972. 69:3561) is analyzed. Bilayers absolutely free of alkane solvents or other nonpolar hydrocarbons can be formed on polytetrafluoroethylene (PTFE) (e.g. Teflon) septa only if certain boundary conditions are satisfied. Measurements have been made of the contact angles between monolayer-coated water and PTFE in the presence and absence of alkane solvents. The measurement suggest that the boundary conditions for formation of stable bilayers can be satisfied only when a nonpolar solvent is present. We conclude that the bilayer must be surrounded by a torus of alkane solvent, petroleum jelly, or silicone grease depending upon the details of technique used to form the bilayer. The non-polar solvent used in the formation of the bilayer may or may not be present in the bilayer depending upon the water solubility and size of the solvent molecule relative to the size of the alkyl chain of the lipid. Detailed sketches describing the formation of bilayers from monolayers are presented.     

Boundary conditions at the cartilage-synovial fluid interface for joint lubrication and theoretical verifications

The objective of this study is to establish and verify the set of boundary conditions at the interface between a biphasic mixture (articular cartilage) and a Newtonian or non-Newtonian fluid (synovial fluid) such that a set of well-posed mathematical problems may be formulated to investigate joint lubrication problems. A "pseudo-no-slip" kinematic boundary condition is proposed based upon the principle that the conditions at the interface between mixtures or mixtures and fluids must reduce to those boundary conditions in single phase continuum mechanics. From this proposed kinematic boundary condition, and balances of mass, momentum and energy, the boundary conditions at the interface between a biphasic mixture and a Newtonian or non-Newtonian fluid are mathematically derived. Based upon these general results, the appropriate boundary conditions needed in modeling the cartilage-synovial fluid-cartilage lubrication problem are deduced. For two simple cases where a Newtonian viscous fluid is forced to flow (with imposed Couette or Poiseuille flow conditions) over a porous-permeable biphasic material of relatively low permeability, the well known empirical Taylor slip condition may be derived using matched asymptotic analysis of the boundary layer at the interface.     

Heat transfer simulation using energy conservative dissipative particle dynamics

Dissipative particle dynamics with energy conservation (eDPD) was used to investigate conduction heat transfer in two dimensions under steady-state condition. Various types of boundary condition were implemented to the conduction domain. Besides, 2D conduction with internal heat generation was studied and the heat generation term was used to measure the thermal conductivity and diffusivity of the eDPD system. The boundary conditions used include both the Neumann and Dirichlet boundary conditions. The Neumann boundary condition was applied via adiabatic surfaces and surfaces exposed to convection heat transfer. The DPD simulations were compared to analytical solutions and finite-difference techniques. It was found that DPD appropriately predicts the temperature distribution in the conduction regime. Details of boundary condition implementation and thermal diffusivity measurement are also described in this paper.     

A finite element model for analyzing shear wave propagation observed in magnetic resonance elastography

Magnetic resonance elastography (MRE) is a novel non-invasive approach to determine material stiffness by using a conventional magnetic resonance imaging (MRI) system incorporated with an oscillating motion-sensitizing gradient to detect nodal displacements produced by a shear excitation wave. The effects of material properties, excitation frequency, boundary conditions, and applied tension on shear wavelength measurement must be examined before MRE can become a useful diagnostic tool. We propose finite element (FE) modeling as a robust method to systematically study the effects of these parameters. An axisymmetric FE model was generated with ABAQUS to simulate agarose gel phantoms. The effects of material stiffness, density, and excitation frequency on propagating shear wavelength were examined individually. The effect of the boundary conditions on shear wavelength was also demonstrated. Results of shear wavelength from MRE measurement were compared with the results of FE model, which showed good agreement between the methods.     

« <Emphasis Type="Italic">Secondes personnes</Emphasis> », l’exemple d’un service psychiatrique d’urgence: ERIC

Phenomenological psychiatry, inspired by Binswanger and others, revolutionised traditional, objectifying psychiatry, which was based on the medical model, by pointing out and asserting the existential and intersubjective dimension in any therapeutic relationship. The strength of that revolution was the reorientation of therapy as a true face-to-face encounter between an attentive therapist and a suffering individual, rather than the mere examination of a patient by a specialist. This view of the therapeutic relationship has been proven fruitful; though it has limitations a new anthropology of human relations, based on practical ethics, is trying to overcome. Thus, we move away from a view of the therapeutic relationship as the simultaneous presence of two individuals (in the first and third person) to its view as a second-person dynamic relationship between two subjects. Yet, contrary to an interindividual perspective, the relationship takes precedence over the individuals; it is part of the ethical and practical dynamics of a second-person relationship. Such anthropology of the relationship already underlies the work of the mobile psychiatric emergency team, ERIC (Équipe Rapide d’Intervention de Crise), of Charcot Hospital in Plaisir, France. Their intervention methods are outlined in this paper through specific examples.     

The effects of tidally-driven temporal variation on measuring intertidal cohesive sediment erosion threshold

Accurate measurement of intertidal sediment erodibility is essential for the development of meaningful and accurate models of sediment dynamics. Despite considerable advances in technology and methodology, the measurement of cohesive intertidal sediment erosion remains problematic. Sediment erodibility varies according to both physical and biological properties and processes. These cannot be considered in isolation, as they can interact to create both positive and negative feedbacks, resulting in seemingly idiosyncratic responses in the system. If working models of estuarine sediment dynamics are to be made, it is essential that the influence of these processes on both the measurements and the system itself be considered. Recent developments in measurement technology enable rapid measurement of sediment stability allowing temporal and spatial variability to be measured on a time scale of minutes. This paper reports temporal variability in cohesive intertidal sediment erosion threshold related to immersion and emersion, and the concomitant responses in selected sediment properties (carbohydrates, water content and chlorophyll). Erosion threshold tended to increase over emersion and decrease over immersion, although the patterns of change varied depending upon local conditions, and in one case there was no temporal trend. Temporal changes resulted in a range of measured erosion threshold, dependant upon the erosion device used. Modifications to existing methodology, in order to account for this variation, are proposed and implications for modelling erosion processes are considered.     

The implementation of slab geometry for membrane-channel molecular dynamics simulations

Slab geometric boundary conditions are applied in the molecular dynamics simulation of a simple membrane-channel system. The results of the simulation were compared to those of an analogous system using normal three-dimensional periodic boundary conditions. Analysis of the dynamics and electrostatics of the system show that slab geometric periodicity eliminates the artificial bulk water orientational polarization that is present while using normal three-dimensional periodicity. Furthermore, even though the water occupancy and volume of our simple channel is the same when using either method, the electrostatic properties are considerably different when using slab geometry. In particular, the orientational polarization of water is seen to be different in the interior of the channel. This gives rise to a markedly different electric field within the channel. We discuss the implications of slab geometry for the future simulation of this type of system and for the study of channel transport properties.     

How Subdiffusion Changes the Kinetics of Binding to a Surface

Under molecular crowding conditions, biopolymers have been reported to subdiffuse, ?r²(t)? ? t^α, with 0 < α < 1. Here we study the exchange dynamics of such a subdiffusing particle with a reactive boundary using a continuous time random walk approach. We derive the generalized boundary condition and consider the unbinding from the boundary. An ensuing weak ergodicity breaking has profound consequences for material exchange between the boundary and bulk. We discuss the effects in biological contexts such as gene regulation or membrane-bulk exchange processes. We also suggest various methods to experimentally probe the subdiffusive behavior.     

On the behaviour of fluidic material at molecular dynamics boundary conditions used in hybrid molecular-continuum simulations

This paper presents an investigation of the behaviour of the density profile of fluidic material confined by a force field, as it occurs across molecular-continuum mechanics boundaries in multiscale, hybrid molecular-continuum simulations. A theoretical model for the density profile across the boundary is derived. Furthermore, numerical experiments to validate the density profile and thickness of the relaxation zone are performed using molecular dynamics for a Lennard-Jones fluid in gaseous, liquid and supercritical state conditions. The simulation results show excellent agreement with the theoretical model.     

Technique for chestband contour shape-mapping in lateral impact

The chestband transducer permits noninvasive measurement of transverse plane biomechanical response during blunt thorax impact. Although experiments may reveal complex two-dimensional (2D) deformation response to boundary conditions, biomechanical studies have heretofore employed only uniaxial chestband contour quantifying measurements. The present study described and evaluated an algorithm by which source subject-specific contour data may be systematically mapped to a target generalized anthropometry for computational studies of biomechanical response or anthropomorphic test dummy development. Algorithm performance was evaluated using chestband contour datasets from two rigid lateral impact boundary conditions: Flat wall and anterior-oblique wall. Comparing source and target anthropometry contours, peak deflections and deformation-time traces deviated by less than 4%. These results suggest that the algorithm is appropriate for 2D deformation response to lateral impact boundary conditions.     

Extending a spectrin repeat unit. I: linear force-extension response

Nonequilibrium molecular dynamics simulations were used to calculate the elastic properties of a spectrin repeat unit. A contiguous alpha-helical linker was constructed by employing periodic boundary conditions, allowing a novel scheme for evaluating the thermodynamic force as a function of extension. By measuring the force-extension response under small extensions, spectrin was observed to behave primarily as an elastic material with a spring constant of 1700 +/- 100 pN/nm. The implications of this spring constant, in terms of the properties of the spectrin tetramer, are also discussed.