Synaptic bistability due to nucleation and evaporation of receptor clusters

We introduce a bistable mechanism for long-term synaptic plasticity based on switching between two metastable states that contain significantly different numbers of synaptic receptors. One state is characterized by a two-dimensional gas of mobile interacting receptors and is stabilized against clustering by a high nucleation barrier. The other state contains a receptor gas in equilibrium with a large cluster of immobile receptors, which is stabilized from growing further by the turnover rate of receptors into and out of the synapse. Transitions between the two states can be initiated by either an increase (potentiation) or a decrease (depotentiation) of the net receptor flux into the synapse. This changes the saturation level of the receptor gas and triggers nucleation or evaporation of receptor clusters

Predictive haemodynamics in a one-dimensional human carotid artery bifurcation. Part II: application to graft design

A Bayesian surrogate modelling technique is proposed that may be able to predict an optimal bypass graft configuration for patients suffering with stenosis in the internal carotid artery (ICA). At the outset, this statistical technique is considered as a means for identifying key geometric parameters influencing haemodynamics in the human carotid bifurcation. This methodology uses a design of experiments (DoE) technique to generate candidate geometries for flow analysis. A pulsatile one dimensional Navier-Stokes solver incorporating fluid-wall interactions for a Newtonian fluid which predicts pressure and flow in the carotid bifurcation (comprising a stenosed segment in the internal carotid artery) is used for the numerical simulations. Two metrics, pressure variation factor (PVF) and maximum pressure (pm) are employed to directly compare the global and local effects, respectively, of variations in the geometry. The values of PVF and pm are then used to construct two Bayesian surrogate models. These models are statistically analysed to visualise how each geometric parameter influences PVF and pm. Percentage of stenosis is found to influence these pressure based metrics more than any other geometric parameter. Later, we identify bypass grafts with optimal geometric and material properties which have low values of PVF on five test cases with 70%, 75%, 80%, 85% and 90% stenosis in the ICA, respectively

THE DIFFERENTIAL GAS TURBINE USING ELECTRIC TRANSMISSION

ABSTRACT This paper describes studies of simple gas turbine engines integrated with electrical transmission components. Recent developments in high-speed lightweight electrical machines and compact power electronics have enabled alternators and motors to be produced which can be coupled directly to the shaft of a gas turbine without an intermediate gearbox. For applications which require a wide range of power outputs, a single-shaft gas turbine with a high speed alternator can be run at constant speed while varying the current drawn from the alternator. This combines the flexibility of operation of a separate power turbine with the simplicity of a single-shaft engine. With this arrangement, in traction use high torques are obtained at low speed, while near-constant engine efficiency is sustained to about 50% of the design power. In the differential engine, the mechanical linkage between the compressor and the turbine is replaced with an electrical linkage. The turbine drives an alternator, and part of the alternator power is taken by a highspeed motor to drive the compressor. The excess alternator power forms the output of the engine. The compressor and turbine are now able to run at different speeds, and their operating points can be separately optimised at different engine conditions. For such an engine, studies show that high efficiency can be maintained to low power levels

Hyperbolic planforms in relation to visual edges and textures perception

We propose to use bifurcation theory and pattern formation as theoretical probes for various hypotheses about the neural organization of the brain. This allows us to make predictions about the kinds of patterns that should be observed in the activity of real brains through, e.g. optical imaging, and opens the door to the design of experiments to test these hypotheses. We study the specific problem of visual edges and textures perception and suggest that these features may be represented at the population level in the visual cortex as a specific second-order tensor, the structure tensor, perhaps within a hypercolumn. We then extend the classical ring model to this case and show that its natural framework is the non-Euclidean hyperbolic geometry. This brings in the beautiful structure of its group of isometries and certain of its subgroups which have a direct interpretation in terms of the organization of the neural populations that are assumed to encode the structure tensor. By studying the bifurcations of the solutions of the structure tensor equations, the analog of the classical Wilson and Cowan equations, under the assumption of invariance with respect to the action of these subgroups, we predict the appearance of characteristic patterns. These patterns can be described by what we call hyperbolic or H-planforms that are reminiscent of Euclidean planar waves and of the planforms that were used in [1, 2] to account for some visual hallucinations. If these patterns could be observed through brain imaging techniques they would reveal the built-in or acquired invariance of the neural organization to the action of the corresponding subgroups.Comment: 34 pages, 11 figures, 2 table

Turbulence modeling in three-dimensional stenosed arterial bifurcations

Under normal healthy conditions, blood flow in the carotid artery bifurcation is laminar. However, in the presence of a stenosis, the flow can become turbulent at the higher Reynolds numbers during systole. There is growing consensus that the transitional k ? model is the best suited Reynolds averaged turbulence model for such flows. Further confirmation of this opinion is presented here by a comparison with the RNG k? model for the flow through a straight, nonbifurcating tube. Unlike similar validation studies elsewhere, no assumptions are made about the inlet profile since the full length of the experimental tube is simulated. Additionally, variations in the inflow turbulence quantities are shown to have no noticeable affect on downstream turbulence intensity, turbulent viscosity, or velocity in the k? model, whereas the velocity profiles in the transitional k? model show some differences due to large variations in the downstream turbulence quantities. Following this validation study, the transitional k? model is applied in a three-dimensional parametrically defined computer model of the carotid artery bifurcation in which the sinus bulb is manipulated to produce mild, moderate, and severe stenosis. The parametric geometry definition facilitates a powerful means for investigating the effect of local shape variation while keeping the global shape fixed. While turbulence levels are generally low in all cases considered, the mild stenosis model produces higher levels of turbulent viscosity and this is linked to relatively high values of turbulent kinetic energy and low values of the specific dissipation rate. The severe stenosis model displays stronger recirculation in the flow field with higher values of vorticity, helicity, and negative wall shear stress. The mild and moderate stenosis configurations produce similar lower levels of vorticity and helicity. DOI: 10.1115/1.240118

Magnetic retrieval of prosthetic heart valves for redo-TAVI

Bioprosthetic aortic heart valves are known to degenerate within 7–15 years of implantation. Currently, the options for treating a failing valve are (a) redo surgical aortic valve replacement or, increasingly, (b) valve-in-valve transcatheter aortic valve implantation (ViV-TAVI). The ViV-TAVI procedure is referred to as redo-TAVI when the failing valve is a TAVI device. Repeated procedures, such as two or three valve-in-valves, significantly reduce the effective valve flow area, putting a limit on recurrent treatments. With increasing life expectancy and the use of TAVI in younger, lower-risk patients, the demand for multiple replacement procedures will inevitably increase. Against this background, we describe a novel valve system named exchangeable-TAVI (e-TAVI) in which an electromagnetic catheter is used to remove and retrieve a failed exchangeable valve, followed by the immediate deployment of a new valve. The e-TAVI system comprises (i) an exchangeable valve, (ii) a permanent holding member that anchors the exchangeable valve and (iii) a dedicated catheter with electromagnets for removal of the exchangeable valve. Simulations have been performed to determine the forces, frame design and electromagnetic parameters required to crimp and retrieve a 26 mm diameter valve. An optimum configuration was found to comprise a 12 cell self-expanding frame with circular ferromagnetic regions of 1 mm radius and 0.5 mm thickness, along with eight electromagnets of 1 mm radius and 2 mm thickness. A force of 2.87 N and a current of 2.52 A per electromagnet were required to partially crimp the frame to an envelope radius of 11 mm. While this amount of force allowed the frame to be crimped solely through magnetic attraction, re-sheathing of the frame was not possible due to the weaker shear holding force of the magnets. Also, the current was close to the fusing current of the copper wire needed to fit sufficient windings into the available volume. These issues led to the conclusion that, in addition to the magnetic coupling, a mechanical mating between the removal catheter and the exchangeable valve is needed. This would decrease both the force that the electromagnets had to exert during crimping and the current required to generate this force

Investigating the Equivalent Plastic Strain in a Variable Ring Length and Strut Width Thin-Strut Bioresorbable Scaffold

Purpose The ArterioSorbTM bioresorbable scaffold (BRS) developed by Arterius Ltd is about to enter first in man clinical trials. Previous generations of BRS have been vulnerable to brittle fracture, when expanded via balloon inflation in-vivo, which can be extremely detrimental to patient outcome. Therefore, this study explores the effect of variable ring length and strut width (as facilitated by the ArterioSorbTM design) on fracture resistance via analysis of the distribution of equivalent plastic strain in the scaffold struts post expansion. Scaffold performance is also assessed with respect to side branch access, radial strength, final deployed diameter and percentage recoil. Methods Finite element analysis was conducted of the crimping, expansion and radial crushing of five scaffold designs comprising different variations in ring length and strut width. The Abaqus/Explicit (DS SIMULIA) solution method was used for all simulations. Direct comparison between in-silico predictions and in-vitro measurements of the performance of the open cell variant of the ArterioSorbTM were made. Paths across the width of the crown apex and around the scaffold rings were defined along which the plastic strain distribution was analysed. Results The in-silico results demonstrated good predictions of final shape for the baseline scaffold design. Percentage recoil and radial strength were predicted to be, respectively, 2.8 and 1.7 times higher than the experimentally measured values, predominantly due to the limitations of the anisotropic elasto-plastic material property model used for the scaffold. Average maximum values of equivalent plastic strain were up to 2.4 times higher in the wide strut designs relative to the narrow strut scaffolds. As well as the concomitant risk of strut fracture, the wide strut designs also exhibited twisting and splaying behaviour at the crowns located on the scaffold end rings. Not only are these phenomena detrimental to the radial strength and risk of strut fracture but they also increase the likelihood of damage to the vessel wall. However, the baseline scaffold design was observed to tolerate significant over expansion without inducing excessive plastic strains, a result which is particularly encouraging, due to post-dilatation being commonplace in clinical practice. Conclusion Therefore, the narrow strut designs investigated herein, are likely to offer optimal performance and potentially better patient outcomes. Further work should address the material modelling of next generation polymeric BRS to more accurately capture their mechanical behaviour. Observation of the in-vitro testing indicates that the ArterioSorbTM BRS can tolerate greater levels of over expansion than anticipated