A tensegrity model can be used to describe the mechanical behavior of living cells. A finite element model (FEM) was used to assess the mechanical contribution of subcellular organelles. Continuum parts like the cytoplasm and membrane were modeled as continuous elements, while the tensegrity was chosen to model the cytoskeleton and nucleoskeleton. An atomic force microscope load was implemented to simulate the external load. The cell components were loaded separately to evaluate their mechanical contributions. The analysis started with a single cytoplasm and each of the cell components was added in consecutive steps. The results showed that the cytoskeleton carried the largest part of the reaction force. The cytoplasm was the second important component of the cell’s mechanical response. It was shown that the nucleoskeleton has a stiffer structure than the membrane and cytoplasm. The cytoskeleton supported approximately 90% of the reaction force, while the cytoplasm carried 9% and the shell parts and nucleoskeleton were responsible for about 1%.

Abstract:In this study the interaction of blood flow with arterial wall has been investigated using FSI (Fluid-Structure Interaction) modeling. Computer simulation of pulsatile blood flow was carried out on the basis of the time dependent axisymmetric Navier-Stokes equations for an incompressible Newtonian fluid flow. An elastic incompressible material with large deformation was considered for the arterial wall and momentum and continuity equations of elastodynamics have been solved. The specified boundary conditions for the Navier-Stokes equations were the pulsatile pressure waveforms of the brachial artery at inflow and outflow to the given pulse wave form of a cardiac cycle. Fluid and solid equations were solved with the ALE-based loose coupling method for FSI problems. Resultant flow, wall displacement, wall shear stress and wall circumferential strain waves, and their phase differences were determined. Stiffening of the arterial wall resulted in a significant decrease in the mean values of flow and wall shear stress and altered waveforms. A tenfold increase in wall stiffness caused 33% drop in flow and negative values of shear stress in 21% of the pressure pulse. For elastic moduli corresponding to wall displacements less than 1% the blood flow and wall shear stress were not sensitive to wall stiffness. Stress phase angle was altered by stiffening of the arterial wall. It was concluded that FSI modeling with pressure boundary conditions provides a proper evaluation of hemodynamic parameters that may determine endothelial injury.

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Hemolysis and thrombus formation which are critical concerns in designing a long-term implantable ventricular assist device (VAD) have impeded the widespread use of VADs. In this study, thus, the three-dimensional fluid domain of blood flow in a small bichamber positive displacement VAD (25 ml) with a magnetically levitated moving pusher plate was simulated by the means of a finite element package called ADINA. To optimize the function of the pump for minimizing shear stress induced blood damage, three different driver patterns (linear, sinusoidal, and Guyton's pulse) were investigated. The first pattern produced a constant flow, whereas the two others created pulsatile flows. The flow pattern and the distribution of shear stress of each pattern were observed for comparison. It was revealed that the three types of motions may induce less than 0.06% red blood cell damage. Moreover, in comparison to the other patterns not only did the sinusoidal motion of the pusher plate cause less risk of hemolysis, but in comparison to the linear pattern, it produced a pulsatile flow which reduced the stagnation areas in chambers, lowering the probability of thrombosis. In addition, this motion eliminates the probability of cavitations as compared with the Guyton's pulse pattern.

The effects of age-related hypertrophic remodeling of the thoracic aortic wall on mechanical stresses are quantified using the fluid–solid interaction method. Boundary conditions include physiological flow and pressure waves. Fluid and solid governing equations are solved using the loose coupling method. The results show alteration of hemodynamic and wall mechanical parameters by the remodeling process, including reduction in maximum circumferential stress and lower shear stress fluctuation with smaller portion of negative value and smaller maximum value. Such characteristics are indicators of the reduction of risk of endothelial injury. Remodeling causes elevation of the stress phase angle, an indicator of interaction between shear and circumferential stresses that causes triggering of endothelial cell proliferation, which is necessary for coverage of extra surface required by remodeling. The improvement by remodeling is limited by age-related structural changes such as elastin dysfunction and disorganization of structural components.