J Biomech Eng. 1986;108(4):295-300. doi:10.1115/1.3138617.

Studies have been carried out on the bio-medico-mechanical behavior in vitro of natural blood vessel (dogs) under constant and variable internal pulsatile pressure flow. The apparatus designed by us well simulated the arterial system. The studies were made for the case of pressure amplitude kept as constant, of the two-step-multi-duplicated pulsatile pressure and of the fluctuating pressure. For the case of the fluctuating pressure, the strength of the artery becomes considerably lower than those under constant amplitude and two-step-multi-duplicated pulsatile pressure. SEM observations of the inner walls of the artery shows that collagen fibers are more elongated under fluctuating pulsatile pressure flow. In conclusion, in order to avoid the mechanical deterioration of the artery strength, it is useful to keep the pulsatile blood pressure at constant amplitude. Even for the case of the blood pressure fluctuation, it is necessary to manage to keep the blood pressure as near a regular wave as possible, the total number of repeated pulse being equal.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):301-305. doi:10.1115/1.3138618.

In the present study, an analytical method is developed to deduce the constitutive equations of fibers embedded in a thick shell from the time-variant pressure volume curves obtained by experimental procedures. It is assumed that the spherical shell under consideration is composed of a fiber reinforced material and undergoes radial deflection, modeling the behavior of some biological shells such as urinary bladder. The fiber stress is expressed as a function of fiber strain, rate of strain and the degree of biochemical activation. The function form is chosen such that equations of mechanical equilibrium can be integrated analytically to yield chamber pressure as a function of chamber volume, time rate of change of volume and activation. Arbitrary coefficients appearing in the fiber stress-equation are also present in the resultant time-variant pressure-volume relation. These coefficients can be determined by curve-fitting commonly used clinical data such as cystometry measurements.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):306-311. doi:10.1115/1.3138619.

A new Adaptive Thermal Modeling (ATM) method for the measurement of local tissue blood perfusion rate is introduced. The method is based on a two-phase numerical technique. The first phase includes a fast, finite difference scheme for solution of the transient temperature field. The second phase involves iterative corrections of the perfusion until the modeled temperatures coincide with those measured by the temperature sensors. The results obtained from computer generated “data”, as well as from laboratory experiments demonstrate the potential capability of the ATM method to continuously measure local perfusion rates in heated tissues. Rigorous analysis of the technique is planned for the near future so that it can be applied to in vivo measurements of local tissue blood perfusions.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):312-316. doi:10.1115/1.3138620.

A numerical algorithm is used to estimate in-vivo segmental stiffness properties of individual spine segments based upon existing load-displacement data. A static nonlinear finite element model stimulates a pathological spine and corrective instrumentation system. A systematic procedure for establishing the model’s stiffness parameters is described, in the form of a nonlinear constrained optimal design problem. The numerical method is demonstrated using as an example a case of adolescent idiopathic scoliosis requiring corrective surgery.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):317-323. doi:10.1115/1.3138622.

The slow viscous flow in a syringe is modeled by the quasi-steady axisymmetric Stokes equation with a point sink for the needle hole. The governing equations are approximated using nonstandard finite difference formulas optimized for the boundary conditions, and solved numerically using a SOR technique. Streamlines and pressure profiles are computed for a variety of syringe configurations.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):324-331. doi:10.1115/1.3138623.

We have conducted a parametric comparison of three different vascular models for describing heat transport in tissue. Analytical and numerical methods were used to predict the gross temperature distribution throughout the tissue and the small-scale temperature gradients associated with thermally significant blood vessels. The models are: 1) an array of unidirectional vessels, 2) an array of countercurrent vessels, and 3) a set of large vessels feeding small vessels which then drain into large vessels. We show that three continuum formulations of bioheat transfer (directed perfusion, effective conductivity, and a temperature-dependent heat sink) are limiting cases of the vascular models with respect to the thermal equilibration length of the vessels. When this length is comparable to the width of the heated region of tissue, the local temperature changes near the vessels can be comparable to the gross temperature elevation. These results are important to the use of thermal techniques used to measure the blood perfusion rate and in the treatment of cancer with local hyperthermia.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):332-337. doi:10.1115/1.3138624.

An apparatus which has been developed to study the response of cultured endothelial cells to a wide range of shear stress levels is described. Controlled laminar flow through a rectangular tube was used to generate fluid shear stress over a cell-lined coverslip comprising part of one wall of the tube. A finite element method was used to calculate shear stresses corresponding to cell position on the coverslip. Validity of the finite element analysis was demonstrated first by its ability to generate correctly velocity profiles and wall shear stresses for laminar flow in the entrance region between infinitely wide parallel plates (two-dimensional flow). The computer analysis also correctly predicted values for pressure difference between two points in the test region of the apparatus for the range of flow rates used in these experiments. These predictions thus supported the use of such an analysis for three-dimensional flow. This apparatus has been used in a series of experiments to confirm its utility for testing applications. In these studies, endothelial cells were exposed to shear stresses of 60 and 128 dynes/cm2 . After 12 hr at 60 dynes/cm2 , cells became aligned with their longitudinal axes parallel to the direction of flow. In contrast, cells exposed to 128 dynes/cm2 required 36 hr to achieve a similar reorientation. Interestingly, after 6 hr at 128 dynes/cm2 , specimens passed through an intermediate phase in which cells were aligned perpendicular to flow direction. Because of its ease and use and the provided documentation of wall shear stress, this flow chamber should prove to be a valuable tool in endothelial research related to atherosclerosis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):338-341. doi:10.1115/1.3138625.

A parallel plate chamber in a flow system has been designed to study the effects of fluid shear stresses on cells. The system was applied to the study of cultured endothelial cells grown on cover slips which were accommodated in recessed wells in the base plate. Dye injection studies in the chamber indicated laminar flow over the cells. Shear rates measured over the cover slips by an electrochemical technique were found to be linear with flow rate. Laser doppler anemometry showed parabolic profiles between the plates. Endothelial cells subjected to flow showed a correlation between the time required for orientation and the magnitude of the shear stress.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):342-349. doi:10.1115/1.3138626.

Relatively inefficient heat/mass transfer is characteristic of tubular devices if the Reynolds number is low. One method of improving the heat/mass transfer efficiency of such devices is by inducing transverse laminar secondary circulations that are superimposed on the primary flow field; the resulting transverse velocity components lead to fluid mixing and hence augmented mass transfer in the tube lumen. The present work is a theoretical and experimental investigation of the enhanced transport in rotating, nonaligned, straight tubes, a method of transport enhancement that utilizes Coriolis acceleration to create transverse fluid mixing. This technique couples the transport advantages of coiled tubes with the design advantages of straight tubes. The overall mass balance equation is numerically solved for transfer into fluids flowing steadily through rotating nonaligned straight tubes. This solution, for small Coriolis disturbances, incorporates a third order perturbation solution for the primary and secondary flow fields. For sufficiently small Coriolis disturbances the bulk concentration increase is found to be uniquely determined by the value of a single similarity parameter. As the Coriolis disturbance is increased, however, two additional parameters are required to accurately characterize the mass transfer. In general, increasing the Coriolis accelerations results in an increase in mass transfer. There are solution regimes, however, in which increasing this acceleration can lead to a decrease in mass transfer efficiency. This interesting phenomena, which has important design implications, appears to result from velocity-weighting effects on the exiting sample. Experiments, involving the measurement of oxygen transferred into water and blood, produced data that agree with the theoretical predictions.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):350-354. doi:10.1115/1.3138627.

This paper discusses a computer simulation of a pneumatic portable piston-type artificial heart drive system with a linear d-c-motor. The purpose of the design is to obtain an artificial heart drive system with high efficiency and small dimensions to enhance portability. The design employs two factors contributing the total efficiency of the drive system. First, the dimensions of the pneumatic actuator were optimized under a cost function of the total efficiency. Second, the motor performance was studied in terms of efficiency. More than 50 percent of the input energy of the actuator with practical loads is consumed in the armature circuit in all linear d-c-motors with brushes. An optimal design is: the piston cross-sectional area of 10.5 cm2 cylinder longitudinal length of 10 cm. The total efficiency could be up to 25 percent by improving the gasket to reduce the frictional force.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):355-358. doi:10.1115/1.3138628.

Initial measurements of the time-varying wall shear rate at two sites in a compliant cast of a human aortic bifurcation are presented. The shear rates were derived from flow velocities measured by laser Doppler velocimetry (LDV) near the moving walls of the cast. To derive these shear rate values, the distance from the velocimeter sampling volume to the cast wall must be known. The time variation of this distance was obtained from LDV measurements of the velocity of the wall itself.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):359-364. doi:10.1115/1.3138629.

A theoretical model of oscillometric blood pressure measurement is presented. Particular emphasis is paid to the collapse behavior of the artery, and an exponential volume-pressure curve is used. The results of this study suggest that mean blood pressure can be accurately predicted from the peak of the oscillometric curve if corrections related to the cuff pressure waveform are applied. It is also shown, however, that systolic and diastolic pressure may not in general be accurately determined from fixed amplitude ratios based on the oscillometric peak due to the sensitivity of the method to variations in blood pressure waveform, pulse pressure, and arterial compliance. No simple procedures are found to correct for these effects.

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):368-371. doi:10.1115/1.3138632.

The mathematical model of the interaction between the radiation of pulsed CO2 lasers and tissue was revisited. Asymptotic calculations were employed to determine upper and lower bounds for the evaporated volume and crater depth. Dimensionless time variables for conduction, beam attenuation by tissue vapors and for damage were introduced. Optimal exposure parameters were identified through a dimensionless analysis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1986;108(4):372-381. doi:10.1115/1.3138633.

The dynamic finite deformational behavior of a biphasic model for soft hydrated tissue is examined. In the case of uni-axial confined compression the displacement and stress fields are derived for steady-state permeation, creep, and stress-relaxation. It is shown how to use the results of this analysis to obtain the constitutive relations, as well as the associated material parameters, from the corresponding experiments. It is also shown that the solutions from the theory go much farther, giving a detailed account of the deformation and iteraction of the fluid and solid phases in the tissue.

Commentary by Dr. Valentin Fuster


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