The effects of cytoskeleton orientation angle on strain transmission from a tissue to a cell and its internal cytoskeletons were investigated by using a model where a cell was integrated in a tissue that assumes a collagen gel construct. A cell with uni-directionally or randomly aligned cytoskeletons was embedded in a tissue, which was stretched to a strain of 0.1. When the initial orientation angle of the cytoskeleton was zero, which corresponded to the stretch direction of the tissue, cell strain was minimal and mean cytoskeletal strain was maximal. As the initial cytoskeleton orientation angle increased, mean cytoskeletal strain values decreased, while cell strain increased. Cell strain decreased at an initial cytoskeleton orientation angle of 60°. At this angle, the mean cytoskeletal strain value was nearly zero, which was absolutely minimal. Subsequent increases in the initial cytoskeleton orientation angle resulted in further decreases in cell and cytoskeletal strain. The present multi-scale model may help in achieving structural integration of the biomechanical organization and provide valuable information for understanding the mechanisms underlying cellular remodeling.

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Oscillometric noninvasive blood pressure measurement (ONBPM) is nowadays widely applied in health care. However, ONBPM has been found to exhibit large interindividual variability in accuracy. In the present study, we developed a theoretical model to describe the mechanics of the brachial arterial wall and particularly arterial stiffening dominated respectively by wall thickening and elastin degeneration. The model was incorporated into a cardiovascular-cuff model to investigate the effects of brachial arterial stiffening on the accuracy of ONBPM. It was found that wall-thickening-dominated arterial stiffening induced overestimate of the systolic pressure, whereas the elastin-degeneration-dominated arterial stiffening induced overestimate of both the systolic pressure and the diastolic pressure. The findings support the clinical observation that pseudo-hypertension is more prevalent in the elderly, and at the same time imply that arterial stiffness indices (e.g., compliance of artery, pulse wave velocity) assessed under the physiological pressure (generally between 70 and 150mmHg) conditions may be insufficient for assessing the accuracy of ONBPM.

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The mechanical behavior of articular cartilage is strongly dependent on its microstructure characterized by three-dimensional, depth-dependent densities and anisotropic arrangements of cartilage cells and collagen fibres in extracellular matrix. The inclusion of cartilage microstructure to model analysis is essential for the accurate evaluation of the mechanical property. This study proposes a model-based evaluation method, which could predict tissue microstructure (collagen fibres, proteoglycans, and cartilage cells) based on a time history of reaction force obtained from indentation test, thereby estimating the depth-dependent cartilage mechanical property that is difficult to be estimated by experiment in conventional methods. The model was used for evaluating the viscoelasticities of cartilage at medial (load-supporting) and lateral (non load-supporting) regions on bovine femoral head. The present simulation allows fitting the time-history of reaction force measured from indentation test (R² = 0.993 ± 0.0012 for lateral region and R² = 0.990 ± 0.0032 for medial region). The differences of the elastic modulus of collagen fibre and the permeability between medial and lateral regions were consistent to those observed from the chemical composition analysis.

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Both focus control and hot spot elimination applied to the High Intensity Focused Ultrasound (HIFU) treatment are addressed using three-dimensional numerical simulations, which can faithfully capture the pressure field in the HIFU system involving strong heterogeneity in material properties. The hot spot corresponds to the superficial secondary pressure peak, which originates in the reflection and refraction of the ultrasonic waves. As an extension of the Time Reversal theory (Fink, 1992, IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 39, 555), which is based on the reversibility of the linear wave equation, a heuristic, iterative and self-adaptive process is proposed to make the HIFU system mounting array piezotransducers dynamically controllable. The amplitude and phase of the driving pressure on each transducer are determined from the temporal change in the monitored pressure of diverging acoustic waves emitted from virtual acoustic sources at the hot spot as well as the target point. The primary peak is found to be strengthened through the process, while the secondary peaks are to be obliterated. The present method is applied to a transskull HIFU simulation using human body voxel data, and its effectiveness is discussed.

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We developed a numerical method for large-scale simulations of cellular flow in microvessels. We employed a particle method, where all blood components were modeled using a finite number of particles. Red blood cell deformation was modeled by a spring network of membrane particles. A domain decomposition method was used for parallel implementation on distributed memory systems. In a strong scaling test up to 64 CPU cores, we obtained a linear speedup with the number of CPU cores, and demonstrated that our model can simulate O(10³) red blood cells in vessels a few tens of micrometers in diameter. For quantitative validation, we analyzed the Fåhræus effect and the formation of a cell-depleted peripheral layer. Simulations were performed for tube hematocrit ranging from 20 to 45%, and microvessel diameters from 9 to 50 µm. Our numerical results were in good agreement with previous experimental results both for the discharge hematocrit and cell-depleted peripheral layer thickness.

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A numerical simulation is conducted for a blood flow in a capillary vessel, including not only red blood cells (RBCs)-like elastic membranes but also platelets-like elastic solids. Recently-developed full Eulerian approaches for coupling with a fluid and elastic solid/membrane are employed for dealing with dynamic interactions of the RBC, platelet and vessel wall. A pressure-induced periodic flow is imposed on a suspension of the RBCs and platelets with a hematocrit of 20 %, and then the simulation is run up to a certain time at when the flow is developed well. Numerical results indicate that a relative apparent viscosity in the suspension of the RBCs and platelets increases, and a platelet motion is strongly affected by the presence of the RBCs.

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A reduced-order model is proposed to provide effective boundary conditions accounting for a vortical fluid velocity confined in an Acoustic Boundary Layer (ABL) around a linear elastic solid object subjected to an ultrasound wave. It is validated by a priori tests of two-dimensional acoustic scatterings by a cylinder with material properties of water and bone. The viscous dissipation resulting in a heating predicted by the proposed model is in excellent agreement with that of the full model, in which the P- and S-waves in fluid are faithfully captured. The frictional heat source per area in the ABL is confirmed to be O(10⁴) higher than that in the bulk, suggesting the significance of the ABL effect in estimating overheating risks in HIFU (High Intensity Focused Ultrasound) therapy.

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