فصلنامه مهندسی پزشکی زیستی
سال دوم شماره 1 (بهار 1387)

In this paper, we studied the effect of mechanical loading on remodeling process with aging in muscular arteries. Based on the gathered experimental data, the brachial artery was selected for simulation. In this simulation, pulsatile pressure and flow waves were considered as boundary conditions to study the effect of circumferential stress and wall shear stress on the remodeling process. FSI based transient numerical simulation was used to solve the fluid and solid equations. The results of three remodeling schemes showed that inward eutrophic scheme is an optimum algorithm for brachia! Artery remodeling with aging. Such remodeling scheme causes the most optimized outcome to keep circumferential stress with minimal alteration.

Arterial stenosis and the consequent cardiovascular diseases such as atherosclerosis remain the major cause of mortality in the world. In this study, blood flow was analyzed in a three-dimensional model of stenosed carotid artery with asymmetric stenosis utilizing fluid-structure interaction method. The modeling was performed by ANSYS finite element software. To overcome the software inconsistency in FSI mode, a new code was designed in ANSYS multi-physics environment for coupling of solid and fluid domains via incremental boundary iteration method. The results indicated a considerable variation of local blood pressure, velocity and shear stress in stenosed artery, high pressure drop along stenosis, compressive stress and larger flow separation zone in the post-stenotic region as the result of increased eccentricity of stenosis. The results might be applied in evaluation of plaque severity, progression of disease, plaque growth and vulnerable regions of plaque to fracture.

Recently, considerable biomedical attention has centered on the mechanical properties of living tissues at the single cell level. Stiffness is an important parameter in determining the physical properties of living tissues. Indeed, stiffness changes of the ovum as a single cell pose a unique challenge in determining the sequence of fertilization. The ovum's extracellular layer has been reported to be altered following fertilization in a process described as Zona reaction. In the present study, the Young's modulus of Zona Pellucida of the mouse ovum was evaluated using micropipette aspiration technique. By incorporating exact engineering principles into the cell mechanics and extract appropriate formula, the Young's modulus of metaphase II (MII) and pronuclear (PN) was measured. The experimental results clearly demonstrated that the mouse Zona Pellucida hardened following fertilization. This study involves the contents of Reproductive Biology and Mechanics, and opens up a new trail of thought for evaluating the quality of mammalian oocytes and embryos.

Ureter reflux is one of the prevalent factors that causes pyelonefrit and sistit syndromes. Dilatation of ureter, renal pelves and calyx are detectable with reflux. In this paper, in order to analyze this phenomenon, an axisymmetric model was introduced. We utilized a rigid body, which is in contact with the outer ureter wall to model ureter contraction. The Navier-Stokes equations are solved for the fluid and a linear elastic model is used for ureter wall structure. The finite element equations for both the structure and the fluid were solved by the Newton-Raphson iterative method. The effect of ureter wall elasticity, pressure difference between the ureter inlet and outlet and the effect of the average velocity of peristaltic wave along the length of the ureter on the ureter outlet flow rate were analyzed. Moreover, the effect of the number of contraction waves on the pressure and flow relations in the ureter was analyzed. Increase in the number of contraction waves reduced the flow passing through the ureter. The results of investigating about the contraction wave velocity variations indicated that if average velocity the contraction wave was lower than a limited magnitude, its existence did not have any considerable effect on the ureter outlet flow rate. Finally improper function of urinary tubes junctions results in the passage of a part of back flow even in the case of low velocity beginning of the contraction wave.

Adjusting the rhythm of breath is one of the important parameters that a successful athlete must consider. In this paper, the relationship between man's activity and respiration rhythm is studied. A numerical simulation is carried out on a 2D axi-symmetric model using computational fluid dynamics (CFD) method. The model considers the oxygen uptake in the pulmonary capillaries in alveolar microcirculation system. The geometry consists of three main parts: a stationary capillary membrane, a moving plasma region and four semi-circular-shaped RBCs. Results show an inverse relationship between saturation time of RBCs and respiration rhythm. Using an inversion factor, a relationship is presented to assess the proper respiration rhythm for different exercise states.

According to mechanobilogical studies as an infrastructure for tissue engineering researches, this paper presents a triphasic finite element modeling of intervertebral discs such a hydrated porous soft tissue. First, the governmental equations were derived on the basis of the laws of continuum mechanics. Then the standard Galerkin weighted residual method was used to form the finite element model. The implicit time integration schemes were applied to solve the nonlinear equations. The formulation accuracy and convergence for one dimensional case were examined with Simon's and Sun's analytical solutions and also Drost's experimental Data. It was shown that the mathematical model is in excellent agreement and has the capability to simulate the intervertebral disc response under different types of mechanical and electrochemical loading conditions. Finally, to have a short review of the capability of the model, a homogenous two dimensional version of the model was applied to simulate the response of a simple sagittal slice of the intervertebral disc.

In this research the kinematics parameters derived from ground reaction forces were evaluated to limit the differential diagnoses and measure the degree of disabilities during the walking among neuropathic subjects. 25 neuropathic subjects affected by drop foot and 20 normal subjects were enrolled in the study. There were no differences in the age, weight and height between the patients and normal subjects (p > 0.05). Each subject was tested in average 10±2 times for calculating the kinetic parameters derived from ground reaction forces. Then time parameters and vertical components of force including three extremums of vertical forces, which state various phases in gait, anterior­-posterior component of ground reaction force, maximum propulsion force, maximum breaking force during loading stage, maximum propulsion force in the end phase of terminal stance, impact derived from the contact of the patient' foot with floor, loading rate and unloading of vertical forces during the contact' phase of the patient's foot with floor and center of pressure displacement in sole of foot and friction' coefficient between foot and floor were calculated. The results revealed that correlation between the first and second peaks of the anterior-posterior component of ground reaction forces, center of pressure displacement pattern in the sole of foot and time parameters of the vertical forces can be good indexes for differential diagnoses and measuring the degree of disabilities. This research can extend the clinical applications of ground reaction force plate, introduce suitable criteria to limit differential diagnoses and measure the degree of disabilities among the neuropathies. There is a need to replicate this research with more patients and normal subjects to confirm our findings.