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"ZOOM" SEMINAR
Monday, June 22, 2020 at 14:00
Biomechanical Numerical Simulations of Calcific Aortic Valve Disease in Bicuspid Valves: New Biomechanical Modeling for Treatment Planning

Karin Lavon
Under the supervision of Prof. Rami Haj-Ali and Prof. Ehud Raanani
School of Mechanical Engineering, Tel Aviv University, Tel Aviv, Israel
 karinlav@mail.tau.ac.il

Bicuspid aortic valve (BAV) is the most common type of congenital heart disease, occurring in 0.5-2% of the population, where the aortic valve has only two leaflets rather than the normal three. Calcific aortic valve disease (CAVD) is characterized in stiffened valve leaflets, which rapidly leads to aortic stenosis (AS). BAV patients constitute more than half the patients diagnosed with CAVD, which progresses rapidly compared to tricuspid aortic valves. Transcatheter aortic valve replacement (TAVR) is a treatment approach for CAVD where a stent with mounted bioprosthetic valve is deployed on the stenotic valve. Performing TAVR in calcified BAV patients has been only recently performed off-label, and raise concerns stemming from the asymmetrical structure of the BAV, which can cause partial anchoring and paravalvular leakage (PVL). This research aims to develop and utilize a new and refined numerical finite-element simulations for investigating the development of calcification in BAVs, and examine potential minimal invasive treatment approaches for those patients, such as balloon aortic valvuloplasty and TAVR deployment.
The clinical part of this study introduces a new Reverse Calcification Technique (RCT) that generates spatial calcified densities from computed tomography (CT) scans of pre-intervention AS patients, and capable of predicting the CAVD progression that leads to the current stage. The different calcification patterns of BAVs were characterized based on acquired CT scans of calcified BAV patients, taken from Sheba and Rabin medical centers. The CAVD patterns were compared to previous disease stages revealed by the RCT and compared to selected patients underwent more than one CT procedure in their past. Fluid–structure interaction (FSI) simulations were employed on calcified BAV models representing four stages of the disease, in order to investigate the contribution of hemodynamics to the calcification development.
A calcification fragmentation biomechanical model is introduced to study the balloon-valvuloplasty procedure aimed at expanding AV root and increase compliance. Towards that goal, six stenotic BAVs with varied calcification patterns were modeled by embedding their calcium deposits inside a parametric model of the BAV. The calcification fragmentation was modeled by deploying a balloon catheter inside the calcified valves, and applying a failure criterion for the calcium medium. The cracking patterns were found to be correlated to the RCT and calcification development patterns, suggesting a relation between the two. The deployments of self and balloon-expandable TAVR devices Join Zoom Meeting
inside a calcified BAV were simulated as well (Evolut and Sapien 3 devices, respectively). Their PVL was also calculated by CFD simulations. The Evolut stent was characterized in asymmetric and elliptic deployment, with lower anchoring forces compared with the Sapien 3. The Sapien 3 and Evolut PRO had comparable PVL values, reduced in half compared with the Evolut R.
The proposed clinical and biomechanical computational models in our study are shown to be effective towards future BAV patient-specific simulations to improve future treatment.

~~https://zoom.us/j/96584758181?pwd=WC9PMXdsYzJ3NFdEN2Q5ZUtOZEVjdz09
The meeting will be recorded and made available on the School’s site.

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SCHOOL OF MECHANICAL ENGINEERING SEMINAR
Monday, March 23, 2020 at 14:00 
Wolfson Building of Mechanical Engineering, Room 206
Graph Theoretical and Geometrical Investigation of Kinematics and Singularity of 3D Parallel Mechanisms

Michael Slavutin
PhD Yoram Reich

This work presents geometrical and graph theoretical methods for analysis of mechanisms in three dimensions. In particular, the singularity analysis by geometric methods of parallel mechanisms is presented. The singularity of the most common three-dimensional mechanism, the 6/6 Stewart platform is fully analyzed. The Aronhold-Kennedy theorem is used for this analysis. This analysis is based on the properties of the reciprocal product of two screws, which depends only on the relative position of the two screws. First, the singularity criterion is developed assuming that there exist two lines that cross four of the six legs of the platform. The instantaneous screw axis and the forces in the legs are found. Next, the above assumption is relaxed and the singularity criterion is developed in a most general configuration. For this purpose, the two degrees-of-freedom mechanism obtained by removal of two legs of the platform is analyzed. The line that is a locus of all possible instantaneous screw axes of such mechanism is found. It is proved that the mechanism is singular if three loci of the instantaneous screw axes of three such sub-mechanisms share a common perpendicular. This criterion is expanded to a general parallel mechanism. Examples of analyzing complex mechanisms with this criterion are shown.
The analysis of singularity of a multi-platform parallel mechanism is performed by building an equivalent simpler mechanism using the reciprocality relation. The two-platform and single- and two-circuit three-platform mechanisms are reduced to a single-platform mechanism. It is shown that the three-circuit three-platform mechanisms are irreducible except for certain cases. This method is shown to be applicable to even more complex mechanisms.
The method used for analysis of the singularity of the multi-platform mechanisms is developed formally into a graph-theoretical duality that is used for kinematic and static investigation of a mechanical system. In this duality, a node of the dual graph that corresponds to a platform of a dual mechanism is placed in a face of a graph of the original mechanism. Edges of the dual graph that correspond to the constraints of the dual mechanism cross the edges of the primal graph and are built so that the dual edge is reciprocal to any primal edge crossed by it. The algorithms for determining the geometry of the dual edges are developed in the first part of this work. Since these graphs are multigraphs, there are various possibilities of building the dual graph. Therefore, the building of dual to a dual graph results in a graph that may be different from the original one. This allows simplification of the original mechanism by successful application of the dual operation, preserving the kinematic and static properties of the original systems.

סמינר זה יחשב כסמינר שמיעה לתלמידי תואר שני, למי שמתחבר לסמינר דרך הזום.

You are invited to attend a lecture on Wednesday, Apr 22, 2020 14:00

Join Zoom Meeting

https://zoom.us/j/255986744

Tuning the Phase and Amplitude Response of Plasmonic Metasurface Etalons

By:

Danielle Ben Haim

M.Sc. student under the supervision of Prof. Tal Ellenbogen

Abstract

The research in the field of metasurfaces over the past years has shown their ability to control and manipulate light and thus form unique and novel electro-optical elements. These surfaces consist of arrays of subwavelength particles that exhibit a collective response which is governed by the particles properties, such as their material, shape and size. Understanding the physical behaviour of the particles in the subwavelength regime is what enables to control the overall effective properties of the element.

In this work we study the optical response of plasmonic metasurface etalons in reflection. The etalons consist of a metallic mirror and a plasmonic metasurface separated by wavelength-scale dielectric spacer. We explore the underlying physical mechanisms involved in their electromagnetic response and show that tuning the localized surface plasmon resonance and spacer thickness can be used to achieve both enhanced reflectivity and perfect absorption, in addition to full range phase control, and tunable regions of normal and anomalous dispersion. We characterize the spectral reflection and phase response of metasurface etalons consisting aluminum nanodisks of different radii separated from an aluminum reflector by a SiO₂ spacer. We use this approach to demonstrate a simple Hermite-Gaussian (HG) wavelength selective beam-shaping reflective mask and discuss the potential of this concept to be further extended by using multilayers to obtain multi-functional elements.