bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Age on Residual Strain in the Rat Aorta
Authors: Badreck-Amoudi A. , Patel C. K. , Kane T. P. C. , Greenwald S. E.
Published: February 2008

Residual stress is observed in many parts of the cardiovascular system and is thought to reduce transmural stress gradients due to intravascular pressure. Its development is closely associated with normal growth and pathological remodeling, although there appear to be few previous reports of the relationship between aging and residual stress. We have estimated residual strain (an indicator of the magnitude of residual stress) at ten sites along the aorta of rats aged 2.5 to 56 weeks by measuring the degree to which rings of vessel spring open when cut (opening angle). At all ages the opening angle decreased along the aorta, reaching a minimum near the renal arteries and increasing toward the aorto-iliac bifurcation, a result that confirms previous studies. During growth, although the unloaded circumference of the aorta increased steadily, the wall thickness and medial surface area fell to a minimum at the age of 6 weeks before continuing a steady increase. Similarly, the opening angle decreased between the ages of 2.5 and 6 weeks, thereafter increasing with age. In the abdominal aorta, a strong correlation between opening angle and wall thickness relative to midwall radius (h/R) was seen; whereas in the thoracic segment, in which no increase in h/R with age occurred, no such relationship was found. These observations are in keeping with a recently proposed hypothesis that residual stress will change in response to growth-related changes in vessel geometry driven by a tendency to minimize the nonuniform stress distribution inevitably found in pressurized thick-walled cylinders.
A Three-Dimensional Finite Element Method for Large Elastic Deformations of Ventricular Myocardium: II—Prolate Spheroidal Coordinates
Authors: Costa K. D. , Hunter P. J. , Wayne J. S. , Waldman L. K. , Guccione J. M. , McCulloch A. D.
Published: February 2008

A three-dimensional finite element method for nonlinear finite elasticity is presented using prolate spheroidal coordinates. For a thick-walled ellipsoidal model of passive anisotropic left ventricle, a high-order (cubic Hermite) mesh with 3 elements gave accurate continuous stresses and strains, with a 69 percent savings in degrees of freedom (dof) versus a 70-element standard low-order model. A custom mixed-order model offered 55 percent savings in dof and 39 percent savings in solution time compared with the low-order model. A nonsymmetric 3D model of the passive canine LV was solved using 16 high-order elements. Continuous nonhomogeneous stresses and strains were obtained within 1 hour on a laboratory workstation, with an estimated solution time of less than 4 hours to model end-systole. This method represents the first practical opportunity to solve large-scale anatomically detailed models for cardiac stress analysis.
A Computational Pulsatile Model of the Bidirectional Cavopulmonary Anastomosis: The Influence of Pulmonary Forward Flow
Authors: Migliavacca Francesco , de Leval Marc R. , Dubini Gabriele , Pietrabissa Riccardo
Published: February 2008

The bidirectional cavopulmonary anastomosis (BCPA or bidirectional Glenn) is an operation to treat congenital heart diseases of the right heart by diverting the systemic venous return from the superior vena cava to both lungs. The main goal is to provide the correct perfusion to both lungs avoiding an excessive increase in systemic venous pressure. One of the factors which can affect the clinical outcome of the surgically reconstructed circulation is the amount of pulsatile blood flow coming from the main pulmonary artery. The purpose of this work is to analyse the influence of this factor on the BCPA hemodynamics. A 3-D finite element model of the BCPA has been developed to reproduce the flow of the surgically reconstructed district. Geometry and hemodynamic data have been taken from angiocardiogram and catheterization reports, respectively. On the basis of the developed 3-D model, four simulations have been performed with increasing pulsatile blood flow rate from the main pulmonary artery. The results show that hemodynamics in the pulmonary arteries are greatly influenced by the amount of flow through the native main pulmonary artery and that the flow from the superior vena cava allows to have a similar distribution of the blood to both lungs, with a little predilection for the left side, in agreement with clinical postoperative data.
Computational Evaluation of Intraventricular Pressure Gradients Based on a Fluid-Structure Approach
Authors: Redaelli A. , Montevecchi F. M.
Published: February 2008

The dynamics of intraventricular blood flow, i.e. its rapid evolution, implies the rise of intraventricular pressure gradients (IPGs) characteristic of the inertia-driven events as experimentally observed by Pasipoularides (1987, 1990) and by Falsetti et al. (1986). The IPG time course is determined by the wall contraction which, in turn, depends on the load applied, namely the intraventricular pressure which is the sum of the aortic pressure (i.e., the systemic net response) and the IPG. Hence the IPGs account, at least in part, for the wall movement. These considerations suggest the necessity of a comprehensive analysis of the ventricular mechanics involving both ventricular wall mechanics and intraventricular fluid dynamics as each domain determines the boundary conditions of the other. This paper presents a computational approach to ventricular ejection mechanics based on a fluid-structure interaction calculation for the evaluation of the IPG time course. An axisymmetric model of the left ventricle is utilized. The intraventricular fluid is assumed to be Newtonian. The ventricle wall is thin and is composed of two sets of counter-rotating fibres which behave according to the modified version of Wong’s sarcomere model proposed by Montevecchi and Pietrabissa and Pietrabissa et al. (1987, 1991). The full Navier-Stokes equations describing the fluid domain are solved using Galerkin’s weighted residual approach in conjunction with finite element approximation (FIDAP). The wall displacement is solved using the multiplane quasi-Newton method proposed by Buzzi Ferraris and Tronconi (1985). The interaction procedure is performed by means of an external macro which compares the flow fields and the wall displacement and appropriately modifies the boundary conditions to reach the simultaneous and congruous convergence of the two problems. The results refer to a simulation of the ventricular ejection with a heart rate of 72 bpm. In this phase the ventricle ejects 61 cm3 (ejection fraction equal to 54 percent) and the ventricular pressure varies from 78 mmHg to 140 mmHg. The IPG show an oscillating behaviour with two major peaks at the beginning (11.09 mmHg) and at the end (4.32) of the ejection phase, when the flow rate hardly changes, according to the experimental data. Furthermore the wall displacement, the wall stress and strain, the pressure and velocity fields are calculated and reported.
Motion of Red Blood Cells in Capillaries With Variable Cross-Sections
Authors: Secomb T. W. , Hsu R.
Published: February 2008

Red blood cells undergo continual deformation when traversing microvessels in living tissues. This may contribute to higher resistance to blood flow observed in living microvessels, compared with that in corresponding uniform glass tubes. We use a theoretical model to simulate single-file motion of red cells though capillaries with variable cross-sections, assuming axisymmetric geometry. Effects of cell membrane shear viscosity and elasticity are included, but bending resistance is neglected. Lubrication theory is used to describe the flow of surrounding plasma. When a red cell encounters a region of capillary narrowing, additional energy is dissipated, due to membrane viscosity, and due to narrowing of the lubrication layer, increasing the flow resistance. Predicted resistance to cell motion in a vessel with periodic constrictions (diameter varying between 5 μm and 4 μm) is roughly twice that in a uniform vessel with diameter 4.5 μm. Effects of transient red cell deformations may contribute significantly to blood flow resistance in living microvessels.
The Effect of Diabetes on the Viscoelastic Properties of Rat Knee Ligaments
Authors: Duquette J. J. , Grigg P. , Hoffman A. H.
Published: February 2008

A series of experiments was performed to determine the effect of diabetes on the viscoelastic properties of knee joint ligaments. The experimental model was collateral ligaments from spontaneously diabetic, hyperglycemic (BBZDP/Wor) rats, and various controls including nondiabetic littermates, insulin treated diabetic rats, and alloxan treated rats. Material properties were measured using a dynamic, uniaxial loading paradigm. Ligaments were subjected to load controlled, sinusoidal tensile testing, using frequencies from 0.1 to 2.0 Hz. The resulting data were used to determine the storage and loss compliances of the ligaments Storage compliance, which reflects tissue elastic properties, did not differ between groups Loss compliance, which reflects the viscous component of the tissue response, was increased in the hyperglycemic animals. Thus, hyperglycemic diabetes affects tissue mechanical properties through the viscous rather than the elastic component of the response to dynamic loading. Rats treated with alloxan to induce diabetes did not show an increase in loss compliance.
An Evaluation of Optimization Techniques for the Prediction of Muscle Activation Patterns During Isometric Tasks
Authors: Buchanan Thomas S. , Shreeve David A.
Published: February 2008

The purpose of this study was to critically evaluate the modeling potential of proposed optimization cost functions for predicting muscle forces during isometric loading. Models of the muscles about the elbow (eleven muscles) and wrist (five muscles) were constructed. The models accounted for muscle moment arms, physiological cross-sectional area, specific tension, and percent fiber type. Five nonlinear optimization cost functions, a representative sample of those proposed to date, were analyzed: minimizing the sums of muscle force2, stress2, stress3, (normalized force)2, and minimizing fatigue. Several different protocols were implemented, including elbow models which balanced combinations of flexion-extension, supination-pronation, and varus-valgus loads. Theoretical predictions were compared with EMG data of muscle activation changes as a function of load direction and muscle coactivation relationships. Results indicate a strong dependence of muscle coordination predictions on the number of degrees of freedom balanced. The choice of cost function had little influence on the results. The cost functions examined were not able to reliably estimate muscle activation as a function of load direction. Furthermore, specific synergic relationships between muscle pairs could not be accurately represented. An error analysis indicated that the discrepancies between predicted values and actual values could not be explained by errors in physiological measurements, as the differences between these two were relatively insensitive to changes in the anatomical parameters. In short, no particular cost function was found to adequately represent actual muscle activity at the elbow, although predictions at the wrist were more favorable due to differences in the degrees of freedom at the joints.
An Instrumented Spacial Linkage for Monitoring Relative Three-Dimensional Motion Between Fracture Fragments
Authors: Gardner T. N. , Evans M. , Kyberd P. J.
Published: February 2008

A mechanical linkage with electro-magnetic sensors (a displacement transducer) is described, which may be used to measure accurately the relative motion at a bony junction such as a fracture. The linkage may be fixed to bone screws of externally-fixated fractures during routine patient activity, to measure three-dimensional inter fragmentary displacements arising from dynamic loading. Movements of the linkage are monitored by six Hall Effect devices for the six degrees of freedom (three orthogonal translations and three rotations about the translating axes). Measurements are made within error bounds of ±0.025 mm and ±0.025 deg over a range of 5 mm for the two orthogonal transverse translations, 8 mm for axial translation and 8 deg for the three rotations. Movements at the linkage, remote from the fracture, are then translated mathematically to the fracture site, assuming rigid screw contact with the bone. Displacements of the distal fragment in relation to the proximal, at the fracture center, can then be expressed anatomically through anterior, medial, and distal translations, and rotations in the sagittal, coronal, or transverse planes.
Collagen Fiber Architecture of a Cultured Dermal Tissue
Authors: Sacks M. S. , Chuong C. J. , Petroll W. M. , Kwan M. , Halberstadt C.
Published: February 2008

Advances in tissue engineering have led to the development of artificially grown dermal tissues for use in burn and ulcer treatments. An example of such an engineered tissue is Dermagraft™, which is grown using human neonatal fibroblasts on rectangular sheets of biodegradable mesh. Using small angle light scattering (SALS), we quantified the collagen fiber architecture of Dermagraft with the mesh scaffold contributions removed through the use of a structurally based optical model. Dermagraft collagen fibers were found to have a preferred direction nearly parallel to the long dimension of the kiteshaped mesh opening with small spatial variations over the mesh. This study demonstrated the utility of SALS as a rapid and inexpensive technique for the evaluation of gross collagen fiber architecture in engineered tissues.
Flow in a Simple Model Skeletal Muscle Ventricle: Comparison Between Numerical and Physical Simulations
Authors: Henry F. S. , Shortland A. P. , Iudicello F. , Black R. A. , Jarvis J. C. , Collins M. W. , Salmons S.
Published: February 2008

Flow patterns generated during ventricular filling have been investigated for three different combinations of flow rate and injection volume. The numerical solutions from a commercially available computational fluid dynamics package were compared with observations made under identical flow conditions in a physical model for the purpose of code validation. Particle pathlines were generated from the numerical velocity data and compared with corresponding flow-visualization pictures. A vortex formed at the inlet to the ventricle in both cases: During the filling phase, the vortex expanded and traveled toward the apex of the ventricle until, at the end of filling, the vortex occupied the full radial extent of the ventricle; the vortex continued to travel once the filling process had ended. The vortices in vitro were more circular in shape and occupied a smaller volume than those generated by the numerical model. Nevertheless, comparison of the trajectories of the vortex centres showed that there was good agreement for the three conditions studied. Postprocessing of velocity data from the numerical solution yielded wall shear-stress measurements and particle pathlines that clearly illustrate the mass-transport qualities of the traveling vortex structure. For the cases considered here, the vortex transit produced a time-dependent shear stress distribution that had a peak value of 20 dynes cm−2, with substantially lower levels of shear stress in those regions not reached by the traveling vortex. We suggest that vortex formation and travel could reduce the residence time of fluid within a skeletal muscle ventricle, provided that the vortex travels the complete length of the ventricle before fluid is ejected at the start of the next cycle.
A Theoretical Formulation for Boundary Friction in Articular Cartilage
Authors: Ateshian G. A.
Published: February 2008

A theoretical boundary friction model is proposed for predicting the frictional behavior of articular cartilage, including its time-dependent, velocity-dependent, and load-dependent responses. This theoretical model uses the framework of the biphasic theory for articular cartilage, and provides a mathematical formulation for the principle that interstitial fluid pressurization contributes significantly to reduction of the effective friction coefficient. Several examples of the application of this theory are provided, which demonstrate that a variety of experimentally observed cartilage frictional behaviors can now be theoretically predicted.
An Anisotropic Biphasic Theory of Tissue-Equivalent Mechanics: The Interplay Among Cell Traction, Fibrillar Network Deformation, Fibril Alignment, and Cell Contact Guidance
Authors: Barocas V. H. , Tranquillo R. T.
Published: February 2008

We present a general mathematical theory for the mechanical interplay in tissue-equivalents (cell-populated collagen gels): Cell traction leads to compaction of the fibrillar collagen network, which for certain conditions such as a mechanical constraint or inhomogeneous cell distribution, can result in inhomogeneous compaction and consequently fibril alignment, leading to cell contact guidance, which affects the subsequent compaction. The theory accounts for the intrinsically biphasic nature of collagen gel, which is comprised of collagen network and interstitial solution. The theory also accounts for fibril alignment due to inhomogeneous network deformation, that is, anisotropic strain, and for cell alignment in response to fibril alignment. Cell alignment results in anisotropic migration and traction, as modeled by a cell orientation tensor that is a function of a fiber orientation tensor, which is defined by the network deformation tensor. Models for a variety of tissue-equivalents are shown to predict qualitatively the alignment that arises due to inhomogeneous compaction driven by cell traction.
Quantitative Characterization of Vascular Endothelial Cell Morphology and Orientation Using Fourier Transform Analysis
Authors: Palmer B. M. , Bizios R.
Published: February 2008

Fourier Transform methods were used to quantify mean elongation, mean orientation, and standard deviation of orientations of cultured vascular endothelial cells. Images of cell populations, which had been subjected to 11 and 20 hours of shear stress at 30 dynes/cm2 and 20 hours of no shear, were analyzed by Fourier Transform methods. Measurements of cell morphology and orientation characteristics were also obtained using a manual method for comparison purposes. The results of the study showed that mean cell orientation can be determined accurately with the Fourier Transform methods. Attempts to determine the standard deviation of cell orientations, however, resulted in poorer estimates of mean elongation and standard deviation of orientations except in the case of exposure of endothelial cells to 20 hours of shear, where the actual standard deviation of orientations was low. When the value for standard deviation of orientations was constrained to zero, a minimum possible mean elongation was determined reliably using the Fourier Transform methods. Use of the Fourier Transform methods in determining morphological and orientation characteristics of cell monolayers is fast and objective and may provide a basis for identifying other characteristics of cell shape.
Modeling the Tensile Behavior of the Cement-Bone Interface Using Nonlinear Fracture Mechanics
Authors: Mann K. A. , Werner F. W. , Ayers D. C.
Published: February 2008

The tensile mechanical behavior of the cement-bone interface where there was a large process (plastic) zone at the interface was modeled using a nonlinear fracture mechanics approach. A finite element method was employed, which included a piecewise nonlinear interface, to investigate the behavior of experimental cement-bone test specimens and an idealized stem-cement-bone (SCB) structure. The interface model consisted of a linear elastic region with high stiffness until the yield strength was reached, followed by an exponential softening region, until zero stress. The yield strength and rate of exponential softening after yielding at the cement-bone interface were shown to have a marked effect on the structural stiffness of the SCB model. The results indicate that both yield strength and postyield behavior should be included to characterize the mechanics of the cement-bone interface fully.
Modeling of Flow in a Straight Stented and Nonstented Side Wall Aneurysm Model
Authors: Aenis M. , Stancampiano A. P. , Wakhloo A. K. , Lieber B. B.
Published: February 2008

We investigated the changes of flow patterns in a blood vessel with a side wall aneurysm resulting from placement of a stent. Local hemodynamics can be markedly altered by placing an intravascular stent, which covers the orifice of the aneurysm. The alterations in flow patterns can lead to flow stasis in the aneurysmal pouch and promote the formation of a stable thrombus. Furthermore, a porous stent can serve as substrate for neointimal growth and subsequently induce a remodeling of the diseased arterial segment. To examine changes in local hemodynamics due to stent placement, a stented and nonstented aneurysm model was investigated computationally in a three-dimensional configuration using a finite element fluid dynamics program. The finite element model was studied under incompressible, pulsatile, viscous, Newtonian conditions. The fluid dynamic similarity parameter, i.e., the maximum/minimum Reynolds number, was set at about 240/25 based on cross-sectional average instantaneous flow. The Womersley number was set to 2.5. These values are representative of large cerebral arteries. The results of the stented versus the nonstented model show substantial differences in flow patterns inside the aneurysmal pouch. Flow activity inside the stented aneurysm model is significantly diminished and flow inside the parent vessel is less undulated and is directed past the orifice. A high-pressure zone at the distal neck and the dome of the aneurysm prior to stenting decreases after stent placement. However, elevated pressure values are found at the stent filaments facing the current. Higher shear rates are observed at the distal aneurysmal neck after stenting, but are confined to a smaller region and are unidirectional compared to the nonstented model.
Experimental Investigation of Pulsatile Flows in Tubes
Authors: Shi Z. D. , Winoto S. H. , Lee T. S.
Published: February 2008

Based on cam-piston-valve arrangement, a mechanical pulsatile flow generator is designed to investigate sinusoidal flow and other types of pulsatile flow in straight rigid tube. Measurement reveals the relation between pressure gradient and flow rate. Numerical simulation using the k-ε turbulence model are carried out to compare the pulsatile flow produced by the generator with a sinusoidal flow and a physiological flow in a rigid tube. The results show that the pulsatile flow generated has similar dynamic properties to the physiological flow. Hence, the present setup can be used for in-vitro investigation of biofluid phenomena.
The Predictive Value of Stress Shielding for Quantification of Adaptive Bone Resorption Around Hip Replacements
Authors: Kuiper J. H. , Huiskes R.
Published: February 2008

The presence of a femoral hip stem changes local mechanical signals inside the surrounding bone. In this study we examined the hypothesis that the eventual loss of bone can be estimated from the initial patterns of elastic energy deviation, as determined in FE models of the intact bone and the operated femur. For that purpose two hypothetical relations between elastic energy reduction and resorption were investigated. Their estimates of bone loss were compared to the results of iterative computer simulations. Two kinds of FE model were used, and in each stem stiffness and remodeling threshold (a measure of “biological reactivity”) were varied. Provided that reasonable values of the remodeling threshold are assumed and that the stem is firmly bonded to the bone, we found that the difference between direct estimates and simulation models was 4 percent of bone loss. It is therefore concluded that initial patterns of elastic energy deviation give a reasonable indication of expected bone loss.
Ultrasonic Wave Velocity Measurement in Small Polymeric and Cortical Bone Specimens
Authors: Kohles S. S. , Bowers J. R. , Vailas A. C. , Vanderby R.
Published: February 2008

A system was refined for the determination of the bulk ultrasonic wave propagation velocity in small cortical bone specimens. Longitudinal and shear wave propagations were measured using ceramic, piezoelectric 20 and 5 MHz transducers, respectively. Results of the pulse transmission technique were refined via the measurement of the system delay time. The precision and accuracy of the system were quantified using small specimens of polyoxymethylene, polystyrene-butadiene, and high-density polyethylene. These polymeric materials had known acoustic properties, similarity of propagation velocities to cortical bone, and minimal sample inhomogeneity. Dependence of longitudinal and transverse specimen dimensions upon propagation times was quantified. To confirm the consistency of longitudinal wave propagation in small cortical bone specimens (<1.0 mm), cut-down specimens were prepared from a normal rat femur. Finally, cortical samples were prepared from each of ten normal rat femora, and Young’s moduli (Eii), shear moduli (Gij), and Poisson ratios (vij) were measured. For all specimens (bone, polyoxymethylene, polystyrene-butadiene, and high-density polyethylene), strong linear correlations (R2 > 0.997) were maintained between propagation time and distance throughout the size ranges down to less than 0.4 mm. Results for polyoxymethylene, polystyrene-butadiene, and high-density polyethylene were accurate to within 5 percent of reported literature values. Measurement repeatability (precision) improved with an increase in the wave transmission distance (propagating dimension). No statistically significant effect due to the transverse dimension was detected.
Inertial Loading of the Human Cervical Spine
Authors: Yoganandan N. , Pintar F. A.
Published: February 2008

While the majority of experimental cervical spine biomechanics research has been conducted using slowly applied forces and/or moments, or dynamically applied forces with contact, little research has been performed to delineate the biomechanics of the human neck under inertial “noncontact” type forces. This study was designed to develop a comprehensive methodology to induce these loads. A minisled pendulum experimental setup was designed to test specimens (such as human cadaver neck) at subfailure or failure levels under different loading modalities including flexion, extension, and lateral bending. The system allows acceleration/deceleration input with varying wave form shapes. The test setup dynamically records the input and output strength information such as forces, accelerations, moments, and angular velocities; it also has the flexibility to obtain the temporal overall and local kinematic data of the cervical spine components at every vertebral level. These data will permit a complete biomechanical structural analysis. In this paper, the feasibility of the methodology is demonstrated by subjecting a human cadaver head-neck complex with intact musculature and skin under inertial flexion and extension whiplash loading at two velocities.
A Finite Element Solution for the Anisotropic Biphasic Theory of Tissue-Equivalent Mechanics: The Effect of Contact Guidance on Isometric Cell Traction Measurement
Authors: Barocas V. H. , Tranquillo R. T.
Published: February 2008

We present a method for solving the governing equations from our anisotropic biphasic theory of tissue-equivalent mechanics (Barocas and Tranquillo, 1997) for axisymmetric problems. A mixed finite element method is used for discretization of the spatial derivatives, and the DASPK subroutine (Brown et al., 1994) is used to solve the resulting differential-algebraic equation system. The preconditioned GMRES algorithm, using a preconditioner based on an extension of Dembo’s (1994) adaptation of the Uzawa algorithm for viscous flows, provides an efficient and scaleable solution method, with the finite element method discretization being first-order accurate in space. In the cylindrical isometric cell traction assay, the chosen test problem, a cylindrical tissue equivalent is adherent at either end to fixed circular platens. As the cells exert traction on the collagen fibrils, the force required to maintain constant sample length, or load, is measured. However, radial compaction occurs during the course of the assay, so that the cell and network concentrations increase and collagen fibrils become aligned along the axis of the cylinder, leading to cell alignment along the axis. Our simulations predict that cell contact guidance leads to an increase in the load measured in the assay, but this effect is diminished by the tendency of contact guidance to inhibit radial compaction of the sample, which in turn reduces concentrations and hence the measured load.
Measurement and Simulation of Water Transport During Freezing in Mammalian Liver Tissue
Authors: Pazhayannur P. V. , Bischof J. C.
Published: February 2008

Optimization of cryosurgical procedures on deep tissues such as liver requires an increased understanding of the fundamental mechanisms of ice formation and water transport in tissues during freezing. In order to further investigate and quantify the amount of water transport that occurs during freezing in tissue, this study reports quantitative and dynamic experimental data and theoretical modeling of rat liver freezing under controlled conditions. The rat liver was frozen by one of four methods of cooling: Method 1—ultrarapid “slam cooling” (≥ 1000° C/min) for control samples; Method 2—equilibrium freezing achieved by equilibrating tissue at different subzero temperatures (−4, −6, −8, −10°C); Method 3°-two-step freezing, which involves cooling at 5°C/min. to −4, −6, −8, −10 or −20°C followed immediately by slam cooling; or Method 4—constant and controlled freezing at rates from 5–400°C/min. on a directional cooling stage. After freezing, the tissue was freeze substituted, embedded in resin, sectioned, stained, and imaged under a light microscope fitted with a digitizing system. Image analysis techniques were then used to determine the relative cellular to extracellular volumes of the tissue. The osmotically inactive cell volume was determined to be 0.35 by constructing a Boyle van’t Hoff plot using cellular volumes from Method 2. The dynamic volume of the rat liver cells during cooling was obtained using cellular volumes from Method 3 (two-step freezing at 5°C/min). A nonlinear regression fit of a Krogh cylinder model to the volumetric shrinkage data in Method 3 yielded the biophysical parameters of water transport in rat liver tissue of: Lpg = 3.1 X 10−13 m3/Ns (1.86 μ/min-atm) and ELP = 290 kJ/mole (69.3 kcal/mole), with chi-squared variance of 0.00124. These parameters were then incorporated into the Krogh cylinder model and used to simulate water transport in rat liver tissue during constant cooling at rates between 5–100°C/min. Reasonable agreement between these simulations and the constant cooling rate freezing experiments in Method 4 were obtained. The model predicts that the water transport ceases at a relatively high subzero temperature (−10°C), such that the amount of intracellular ice forming in the tissue cells rises from almost none (=extensive dehydration and vascular expansion) at ≤5°C/min to over 88 percent of the original cellular water at ≥50°C/min. The theoretical simulations based on these experimental methods may be of use in visualizing and predicting freezing response, and thus can assist in the planning and implementing of cryosurgical protocols.
Pulmonary Airway Reopening: Effects of Non-Newtonian Fluid Viscosity
Authors: Low H. T. , Chew Y. T. , Zhou C. W.
Published: February 2008

This paper considers the effects of non-Newtonian lining-fluid viscosity, particularly shear thinning and yield stress, on the reopening of the airways. The airway was simulated by a very thin, circular polyethylene tube, which collapsed into a ribbonlike configuration. The non-Newtonian fluid viscosity was described by the powerlaw and Herschel-Buckley models. The speed of airway opening was determined under various opening pressures. These results were collapsed into dimensionless pressure-velocity relationships, based on an assumed shear rate γ˙ = U/(0.5 H), where U and H are the opening velocity and fluid film thickness, respectively. It was found that yield stress, like surface tension, increases the yield pressure and opening time. However, shear thinning reduces the opening time. An increased film thickness of the non-Newtonian lining fluid generally impedes airway reopening; a higher pressure is needed to initiate the airway reopening and a longer time is required to complete the opening process.
Effects of Frictional Losses and Pulsatile Flow on the Collapse of Stenotic Arteries
Authors: Downing J. M. , Ku D. N.
Published: February 2008

High-grade stenosis can produce conditions in which the artery may collapse. A one-dimensional numerical model of a compliant stenosis was developed from the collapsible tube theory of Shapiro. The model extends an earlier model by including the effects of frictional losses and unsteadiness. The model was used to investigate the relative importance of several physical parameters present in the in vivo environment. The results indicated that collapse can occur within the stenosis. Frictional loss was influential in reducing the magnitude of collapse. Large separation losses could prevent collapse outright even with low downstream resistances. However, the degree of stenosis was still the primary parameter governing the onset of collapse. Pulsatile solutions demonstrated conditions that produce cyclic collapse within the stenosis. This study predicts certain physiologic conditions in which collapse of arteries may occur for high-grade stenoses.
Experimental Study of Steady and Pulsatile Flows in Cerebral Aneurysm Model of Various Sizes at Branching Site
Authors: Liou T.-M. , Chang W.-C. , Liao C.-C.
Published: February 2008

Pulsatile and steady flow fields in cerebrovascular aneurysm models of various sizes are presented in terms of laser-Doppler velocimetry measurements and flow visualization. The bifurcation angle was 140 deg and volume flow rate ratio between the branches was 3:1. The mean, peak, and minimal Reynolds numbers based on the bulk average velocity and diameter of the parent vessel were 600, 800, and 280, respectively. It is found that among the tested sizes there exists a middle range of aneurysm sizes, above and below which the forced-vortex inside the aneurysmal model is weaker and lacking, respectively, whereas in the middle range of the tested sizes the forced vortex is stronger and the fluctuation level is higher near the dome. The present results also identify the major fluid dynamic factors of the aneurysmal promotion or rupture for the medium and larger aneurysms, respectively. Furthermore, the maximum fluctuation intensity is found to increase with aneurysm size. The locations of the maximum fluctuation intensity are found to occur in the bifurcation area or at the neck instead of intra-aneurysm.
Hemodynamic Simulations and Computer-Aided Designs of Graft-Artery Junctions
Authors: Lei M. , Kleinstreuer C. , Archie J. P.
Published: February 2008

Severe occlusion of graft–artery junctions due to restenosis, e.g., excessive tissue overgrowth and renewed plaque formation, may occur within a few months or years after bypass surgery. Our hypothesis is that nonuniform hemodynamics, represented by large sustained wall shear stress gradients, trigger abnormal biological processes leading to rapid restenosis and hence early graft failure. In turn, this problem may be significantly mitigated by designing graft-artery bypass configurations for which the wall shear stress gradient (WSSG) is approximately zero and hence nearly uniform hemodynamics are achieved. Focusing on the distal end of several femoral artery bypass junctions, a validated finite volume code has been used to compute the transient three-dimensional velocity vector fields and its first and second surface derivatives in order to test the idea. Specifically, it is shown that the Taylor patch, which generates higher patency rates than standard end-to-side anastomoses, exhibits lower WSSG levels than standard configurations, and that further geometric design improvements reduce the WSSG in magnitude and local extent even more.
Influence of Tensile Strain on Smooth Muscle Cell Orientation in Rat Blood Vessels
Authors: Liu S. Q.
Published: February 2008

Blood vessels are subject to tensile stress and associated strain which may influence the structure and organization of smooth muscle cells (SMCs) during physiological development and pathological remodeling. This study focused on the influence of the major tensile strain on the SMC orientation in the blood vessel wall. Several blood vessels, including the aorta, the mesenteric artery and vein, and the jugular vein of the rat were used to observe the normal distribution of tensile strains and SMC orientation; and a vein graft model was used to observe the influence of altered strain direction on the SMC orientation. The circumferential and longitudinal strains in these blood vessels were measured by using a biomechanical technique, and the SMC orientation was examined by fluorescent microscopy at times of 10, 20, and 30 days. Results showed that the SMCs were mainly oriented in the circumferential direction of straight blood vessels with an average angle of ~85 deg between the SMC axis and the vessel axis in all observed cases. The SMC orientation coincided with the principal direction of the circumferential strain, a major tensile strain, in the blood vessel wall. In vein grafts, the major tensile strain direction changed from the circumferential to the longitudinal direction at observation times of 10, 20, and 30 days after graft surgery. This change was associated with a decrease in the angle between the axis of newly proliferated SMCs and that of the vessel at all observation times (43 ± 11 deg, 42 ± 10 deg, and 41 ± 10 deg for days 10, 20, and 30, respectively), indicating a shift of the SMC orientation from the circumferential toward the longitudinal direction. These results suggested that the major tensile strain might play a role in the regulation of SMC orientation during the development of normal blood vessels as well as during remodeling of vein grafts.
An Implantable Transducer for Measuring Tension in an Anterior Cruciate Ligament Graft
Authors: Ventura C. P. , Wolchok J. , Hull M. L. , Howell S. M.
Published: February 2008

The goal of this study was to develop a new implantable transducer for measuring anterior cruciate ligament (ACL) graft tension postoperatively in patients who have undergone ACL reconstructive surgery. A unique approach was taken of integrating the transducer into a femoral fixation device. To devise a practical in vivo calibration protocol for the fixation device transducer (FDT), several hypotheses were investigated: (1) The use of a cable versus the actual graft as the means for applying load to the FDT during calibration has no significant effect on the accuracy of the FDT tension measurements; (2) the number of flexion angles at which the device is calibrated has no significant effect on the accuracy of the FDT measurements; (3) the friction between the graft and femoral tunnel has no significant effect on measurement accuracy. To provide data for testing these hypotheses, the FDT was first calibrated with both a cable and a graft over the full range of flexion. Then graft tension was measured simultaneously with both the FDT on the femoral side and load cells, which were connected to the graft on the tibial side, as five cadaver knees were loaded externally. Measurements were made with both standard and overdrilled tunnels. The error in the FDT tension measurements was the difference between the graft tension measured by the FDT and the load cells. Results of the statistical analyses showed that neither the means of applying the calibration load, the number of flexion angles used for calibration, nor the tunnel size had a significant effect on the accuracy of the FDT. Thus a cable may be used instead of the graft to transmit loads to the FDT during calibration, thus simplifying the procedure. Accurate calibration requires data from just three flexion angles of 0, 45, and 90 deg and a curve fit to obtain a calibration curve over a continuous range of flexion within the limits of this angle group. Since friction did not adversely affect the measurement accuracy of the FDT, the femoral tunnel can be drilled to match the diameter of the graft and does not need to be overdrilled. Following these procedures, the error in measuring graft tension with the FDT averages less than 10 percent relative to a full-scale load of 257 N.
Evaluation of Performance Criteria for Simulation of Submaximal Steady-State Cycling Using a Forward Dynamic Model
Authors: Neptune R. R. , Hull M. L.
Published: February 2008

The objectives of this study were twofold. The first was to develop a forward dynamic model of cycling and an optimization framework to simulate pedaling during submaximal steady-state cycling conditions. The second was to use the model and framework to identify the kinetic, kinematic, and muscle timing quantities that should be included in a performance criterion to reproduce natural pedaling mechanics best during these pedaling conditions. To make this identification, kinetic and kinematic data were collected from 6 subjects who pedaled at 90 rpm and 225 W. Intersegmental joint moments were computed using an inverse dynamics technique and the muscle excitation onset and offset were taken from electromyographic (EMG) data collected previously (Neptune et al., 1997). Average cycles and their standard deviations for the various quantities were used to describe normal pedaling mechanics. The model of the bicycle-rider system was driven by 15 muscle actuators per leg. The optimization framework determined both the timing and magnitude of the muscle excitations to simulate pedaling at 90 rpm and 225 W. Using the model and optimization framework, seven performance criteria were evaluated. The criterion that included all of the kinematic and kinetic quantities combined with the EMG timing was the most successful in replicating the experimental data. The close agreement between the simulation results and the experimentally collected kinetic, kinematic, and EMG data gives confidence in the model to investigate individual muscle coordination during submaximal steady-state pedaling conditions from a theoretical perspective, which to date has only been performed experimentally.
A Triphasic Analysis of Corneal Swelling and Hydration Control
Authors: Bryant M. R. , McDonnell P. J.
Published: February 2008

Physiological studies strongly support the view that hydration control in the cornea is dependent on active ion transport at the corneal endothelium. However, the mechanism by which endothelial ion transport regulates corneal thickness has not been elaborated in detail. In this study, the corneal stroma is modeled as a triphasic material under steady-state conditions. An ion flux boundary condition is developed to represent active transport at the endothelium. The equations are solved in cylindrical coordinates for confined compression and in spherical coordinates to represent an intact cornea. The model provides a mechanism by which active ion transport at the endothelium regulates corneal hydration and provides a basis for explaining the origin of the “imbibition pressure” and stromal “swelling pressure.” The model encapsulates the Donnan view of corneal swelling as well as the “pump-leak hypothesis.”
Heat-Induced Changes in the Mechanics of a Collagenous Tissue: Isothermal, Isotonic Shrinkage
Authors: Chen S. S. , Wright N. T. , Humphrey J. D.
Published: February 2008

We present data from isothermal, isotonic-shrinkage tests wherein bovine chordae tendineae were subjected to well-defined constant temperatures (from 65 to 90°C), durations of heating (from 180 to 3600 s), and isotonic uniaxial stresses during heating (from 100 to 650 kPa). Tissue response during heating and “recovery” at 37°C following heating was evaluated in terms of the axial shrinkage, a gross indicator of underlying heat-induced denaturation. There were three key findings. First, scaling the heating time via temperature and load-dependent characteristic times for the denaturation process collapsed all shrinkage data to a single curve, and thereby revealed a time-temperature-load equivalency. Second, the characteristic times exhibited an Arrhenius-type behavior with temperature wherein the slopes were nearly independent of applied load—this suggested that applied loads during heating affect the activation entropy, not energy. Third, all specimens exhibited a time-dependent, partial recovery when returned to 37°C following heating, but the degree of recovery decreased with increases in the load imposed during heating. These new findings on heat-induced changes in tissue behavior will aid in the design of improved clinical heating protocols and provide guidance for the requisite constitutive formulations.
Factors Influencing Accuracy of Screw Displacement Axis Detection With a D.C.-Based Electromagnetic Tracking System
Authors: Bottlang M. , Marsh J. L. , Brown T. D.
Published: February 2008

Recent technical improvements and cost reductions in electromagnetic motion tracking systems invite their application to motion axis determination in the surgical setting. After evaluation of the accuracy of a state-of-the-art D. C. electromagnetic tracking system, which generates complete three-dimensional kinematic outputs from just a single receiver, we calculated screw displacement axes (SDA’s) from its source data. The accuracy of SDA determination from such source data was evaluated for various rotational increment sizes around a revolute joint. A novel smoothing procedure, customized for this type of source data, was developed, enabling SDA detection from incremental rotations of less than 1 deg, at an accuracy appropriate for intra-operative measurement of human joint motion. Examples of SDA determination are given for motion tracking of a ball joint and of the elbow articulation.
Performance Analysis of a Cardiac Assist Device in Counterpulsation
Authors: Chesler N. C. , Kamm R. D.
Published: February 2008

Performance of a cardiac assist device pumping chamber in counterpulsation was evaluated using numerical simulations of the unsteady, three-dimensional flow inside the chamber and an analytical model of the force required to eject and fill the chamber. The wall shear stress within the device was similarly computed and modeled. The analytical model was scaled to match the numerical results and then used to predict performance at physiological operating conditions. According to these models for a stroke volume of 70 ml, between 0.4 and 1.0 W is required for counterpulsation at a frequency of 1.33 Hz against a restorative spring, depending on the spring constant chosen. The power and the maximum force calculated are within the ranges a trained skeletal muscle is capable of providing. Shear stress predictions show that platelet activation in the absence of surface effects and hemolysis due to high shear are unlikely to occur with this design. Furthermore, vortices that develop in the chamber during filling are predicted to increase blood mixing and provide favorable washing of the chamber walls. A computational-analytical approach such as this may have potential to aid rapid performance evaluation of new devices and streamline the design optimization process.