bluuebunny/crossref_metadata_2025 huggingface dataset:
A Finite Helical Axis as a Landmark for Kinematic Reference of the Knee
Authors: Hart R. A. , Mote C. D. , Skinner H. B.
Published: March 2008

Reference coordinates based on the finite helical axis for flexion of the knee from 0 to 90 deg are proposed. Six degree-of-freedom tracking allows the use of such a helical axis as a kinematic landmark for knee motion representation. Data from five human subjects in vivo are presented as a path of finite helical axes for flexion of the knee from 20 to 80 deg. The finite helical axis rotates by an average of 11.4 deg, the centrode translates an average of 19.8 mm, and the total axial translation averages 0.1 mm during flexion from 20 to 80 deg. Error due to the transducer was measured on a fixed-pivot pendulum and found to be 1.0 deg and 1.9 mm rms for the helical axis orientation and position, respectively, and 0.1 mm for the axial translation. Reproducibility and soft tissue effects on the measurements were repeatable to 4.0 deg and 2.7 mm rms in orientation and position, respectively, and 0.1 mm for the axial translations. Soft tissue errors averaged 4.9 deg and 3.6 mmm in position and orientation, and 0.3 mm in the axial translations.
Cracks Emanating From Slipping Inclusions Near a Bone-Implant Interface
Authors: Gharpuray V. M. , Keer L. M. , Lewis J. L.
Published: March 2008

It has been hypothesized that mechanical fracture at the bone-cement-implant interface is the initial cause for loosening of orthopedic implants. Previous investigators have observed cracks to emanate from methacrylate beads, apparently acting as inclusions within the cement. It is believed that the bond between these inclusions and the surrounding matrix breaks prior to emanation of radial cracks from the inclusion. An analytical model is developed for radial cracks emanating from circular inclusions that allow slip along their interface. The solution to the interaction of a single dislocation and a slipping inclusion is used as a Green’s Function to model the crack. The Mode I stress intensity factors are calculated for arbitrary orientations of the crack and for varying relative stiffness of the matrix and the inclusion to test feasibility of crack growth.
Analysis of Oxygen Exchange Between Arterioles and Surrounding Capillary-Perfused Tissue
Authors: Hsu R. , Secomb T. W.
Published: March 2008

A theoretical model is used to analyze oxygen transport in a three-dimensional tissue region containing an arteriole surrounded by an array of capillaries in planes perpendicular to the arteriole. Convective removal of oxygen from the vicinity of the arteriole by nearby capillaries is shown to increase diffusive oxygen loss from the arteriole. This effect depends on the locations of the capillaries, particularly those nearest to the arteriole. The arteriolar oxygen efflux is comparable to that predicted by a previous model which used a continuum approach, but the efflux does not increase with increasing perfusion as rapidly as predicted by the continuum model. Even a small capillary flow rate strongly influences the oxygen field surrounding the arteriole.
Hemodynamic Aspects of Obliterative Processes in Peripheral Blood Vessels—Rigid and Soft Narrowing
Authors: Einav S. , Sternberg A. , Millo Z.
Published: March 2008

Hemodynamic aspects of obliterative processes in peripheral blood vessels were studied on a mechanical model built of distensible tubing, with a fixed peripheral resistance, through which citrated blood was circulated by pulsatile flow. Hemodynamics of progressive focal stenosis, elongated soft stenosis, and elongated rigid stenosis were assessed. By the use of a hydrodynamic model and a series of in vitro experiments, we have measured the pressure and flow characteristics, and calculated the pressure and energy losses for the various stenotic sites. The critical stricture was found to be larger for a rigid stenosis than a soft stenosis. The length of the stenosis was also an important factor. Increasing the length of a rigid stenosis, for example, by 50 percent resulted in an increase of 25 percent in the flow through the stenosis. The energy dissipation was determined as a preferred indication for several parameters such as: pressure drop, pulsed flow, pulse rate, and the geometry and mechanical properties of the stenosis.
Topographical Mapping of Sites of Enhanced HRP Permeability in the Normal Rabbit Aorta
Authors: Barakat A. I. , Uhthoff P. A. F. , Colton C. K.
Published: March 2008

The spatial distribution of sites of enhanced permeability to the macromolecule horseradish peroxidase (HRP) in the normal rabbit aorta after one min circulation was studied using image analysis. These sites, referred to as “HRP spots,” exhibit a nonuniform distribution that is qualitatively similar in all rabbits studied. The density of HRP spots is highest in the aortic arch, decreases distally, reaches a minimum in the lower descending thoracic aorta, and then increases again in the abdominal aorta. The region of highest spot density follows a clockwise helical pattern in the aortic arch and outside the arch occurs in streaks largely oriented in the bulk flow direction. The streaks in the abdominal aorta localize along the anatomical right lateral wall and occasionally along the left lateral wall proximal to the celiac artery and along the ventral wall between the celiac and superior mesenteric arteries. The density of spots is high in the immediate vicinity of aortic ostia with the most elevated density being distal to ostia in most cases. At a short distance from the ostium edge of the celiac and superior mesenteric branches the proximal density is comparably high, and no preferred spot orientation is observed around the brachiocephalic vessel. These results are consistent with an influence of localizing factors such as detailed hemodynamic phenomena and/or arterial wall structural and/or functional variations.
The Polyelectrolyte Properties of Elastin
Authors: Winlove C. P. , Parker K. H. , Ewins A. R. , Birchler N. E.
Published: March 2008

The charge structure and ionic interactions of elastin prepared from the pig thoracic aorta by acid, alkali, or CNBr extraction have been investigated by potentiometric titration and radiotracer techniques. The number of charged groups was consistent with the amino acid composition, comparable to elastin from other sources and insensitive to the method of preparation. The enthalpies of ionization of the basic groups were comparable for those previously found for proteins but those of the acidic groups were higher. Ionic interactions were predominantly electrostatic although a strong affinity for chloride ions was noted. Changes in ionic interactions as the elastin was stretched had a similar effect to an increase in the apparent fixed charge density of the tissue. Mechanical strain altered the protonation of the elastin and the pK of the carboxyl groups. Conversely, the conformation of the elastin network varied with ionic strength and pH, being particularly sensitive to the degree of ionization of the more basic groups and with the ionic strength and anion composition of the medium. We speculate that strain induced changes in the conformation of elastin altering its reactivity towards lipids, ions or matrix macromolecules or changes in its mechanical properties resulting from changes in its ionic environment may be of physiological or pathological importance.
Endothelial Cell Morphometry of Atherosclerotic Lesions and Flow Profiles at Aortic Bifurcations in Cholesterol Fed Rabbits
Authors: Okano Mitsuji , Yoshida Yoji
Published: March 2008

Observations on shapes of endothelial cells both in sudanophilic and nonsudanophilic regions at bifurcations of the brachiocephalic (BC) and left subclavian (SA) arteries in hyperlipidemic rabbits were performed under a SEM. The stagnation point of flow and leading edges of flow dividers were nonsudanophilic and covered by round and long fusiform endothelial cells, respectively. The hips of flow dividers of both branchings, proven to be relatively low shear stress regions, by movement of microspheres in steady flow, were sudanophilic and covered by ellipsoidal cells. Similar studies were carried out in normolipidemic rabbits. It might be concluded that lipid deposition in hyperlipidemic rabbits occurs in relatively low shear stress regions, where endothelial cells are functionally activated, rather than in laminar high shear stress regions at the flow divider.
Convective Mass Transfer Effects on the Intracellular Calcium Response of Endothelial Cells
Authors: Nollert M. U. , McIntire L. V.
Published: March 2008

Endothelial cells, which line the vasculature, respond to specific agonists such as adenosine triphosphate (ATP) by elevating cytosolic calcium levels and increasing production of the vasoactive compounds, prostacyclin and endothelial derived relaxing factor (EDRF). Endothelial cells express ecto-enzymes which metabolize ATP. If the activity of these enzymes is sufficiently high, then the concentration of ATP near the endothelial cell surface can be substantially lower than the bulk concentration. The ATP concentration is determined by a balance between the convection of fresh ATP from upstream and the degradation of ATP by the endothelial cells. In this report, we present a parallel plate flow system for measurement of cytosolic calcium levels ([Ca2+]i) of individual bovine aortic endothelial cells with the calcium sensitive fluorescent dye, fura-2. The cells respond to increases in the flow rate by increasing [Ca2+]i if there is ATP present in the perfusing buffer, but not in the absence of ATP. The amount of agonist in the perfusing fluid near the endothelial cell surface is estimated by solving the governing differential equation for the concentration profile of ATP in the parallel plate flow geometry. The solution indicates that one mechanism endothelial cells may use to detect changes in the flow rate is to respond to the change in the local concentration of agonist.
A Force-Indentation Relationship for Gymnastic Mats
Authors: Gatto F. , Swannell P. , Neal R.
Published: March 2008

The mechanical properties of a series of gymnastic surfaces are investigated. A Force–Indentation relationship is obtained for each surface. This law can be used to simulate the ground reaction force during under-foot impact with a gymnastic surface. The law is independent of many of the properties of the striking body and when incorporated into a system of differential equations describing the motion of the impacting body, can be used to compute system responses and to investigate the importance of different properties of the various surfaces. Several examples, using both a single, rigid mass and a four link, articulated system are presented, and demonstrate that the relationship can be satisfactorily incorporated into a solution algorithm for a large–displacement dynamical analysis.
Pulsatile Flow Visualization in the Abdominal Aorta Under Differing Physiologic Conditions: Implications for Increased Susceptibility to Atherosclerosis
Authors: Moore J. E. , Ku D. N. , Zarins C. K. , Glagov S.
Published: March 2008

The infrarenal abdominal aorta is a common site for clinically significant atherosclerosis. As has been shown in other susceptible locations, vessel geometry, flow division rates, and pulsatility may result in hemodynamic conditions which influence the preferential localization of disease in the abdominal aorta segment. Pulsatile flow visualization was performed in a glass model of the aorta constructed from measurements of angiograms and cadaver aortas. Flow rates and pulsatile waveforms were varied to reflect typical physiological conditions. Under normal resting conditions, the flow patterns in the infrarenal aorta were more complex than those in the suprarenal location. Time varying vortex patterns appeared at the level of the renal arteries and propagated through the infrarenal aorta into the common iliac arteries. A region of oscillating velocity direction extended from the renal arteries to the aortic bifurcation along the posterior wall. Dye became trapped along the posterior wall, requiring several cardiac cycles for clearance. In contrast, there was rapid clearance of the dye in the anterior aorta. Under postprandial conditions, the flow patterns in the aorta were basically unchanged. Simulated exercise conditions created laminar hemodynamic features very different from the resting conditions, including a decrease in dye residence time. This study reveals significant timedependent variations in the hemodynamics of the abdominal aorta under differing physiologic conditions. Hemodynamic factors such as low wall shear stress, oscillating shear direction, and high particle residence time may be related to the clinically seen preferential plaque localization in the infrarenal aorta.
A Numerical Study of the Shape of the Surface Separating Flow Into Branches in Microvascular Bifurcations
Authors: Enden G. , Popel A. S.
Published: March 2008

The shape of the separating surface formed by the streamlines entering the branches of microvascular bifurcations plays a major role in determining the distribution of red blood cells and other blood constituents downstream from the bifurcation. Using the finite element method, we determined the shape of the surface through numerical solution of three dimensional Navier-Stokes equations for fluid flow at low Reynolds numbers in a T-type bifurcation of circular tubes. Calculations were done for a wide range of daughter branch to parent vessel diameter ratios and flow ratios. The effect of Reynolds number was also studied. Our numerical results are in good agreement with previously reported experimental data of Rong and Carr (Microvascular Research, Vol. 39, pp. 186-202, 1990). The numerical results of this study will be used to predict the concentration of blood constituents downstream from microvascular bifurcations providing that the inlet concentration profile is known.
Pressure Drops Through Arterial Stenosis Models in Steady Flow Condition
Authors: Cavalcanti S. , Bolelli P. , Belardinelli E.
Published: March 2008

Measures of pressure drops were made in two different plexiglass models of axial-symmetric arterial stenoses. The stenosis models had the same are reduction (86 percent) but were of different length so as to have a different tapering degree. Pressures were measured in steady flow condition at three equidistant points of the stenosis: upstream, in the middle, and downstream. Results indicate that: the upstream-middle pressure drop is independent of tapering degree but is highly influenced by area reduction; moreover it is much greater than the middle-downstream drop. The upstream-middle pressure drop can be accurately predicted by means of a relationship deduced by the momentum equation.
Pressure Changes Induced by Whole Body Acceleration Shocks
Authors: Belardinelli E. , Ursino M. , Fabbri G. , Cevese A. , Schena F.
Published: March 2008

In the present paper pressure changes induced by sudden body acceleration are studied “in vivo” on the dog and compared to the results obtainable with a recently developed mathematical model. A dog was fixed to a movable table, which was accelerated by a compressed air piston for less than 1 s. Acceleration was varied by changing the air pressure in the piston. Pressure was measured during the experiment at different points along the vascular bed. However, only data obtained in the carotid artery and abdominal aorta are presented here. The results demonstrated that impulse body accelerations cause significant pressure peaks in the vessel examined (about + 25 mmHg in the carotid artery with body acceleration of g/2). Moreover, pressure changes are rapidly damped, with a time constant of about 0.1s. From the present results it may be concluded that, according to the prediction of the mathematical model, body accelerations such as those occurring in normal life can induce pressure changes well beyond the normal pressure value.
Passive Material Properties of Intact Ventricular Myocardium Determined From a Cylindrical Model
Authors: Guccione J. M. , McCulloch A. D. , Waldman L. K.
Published: March 2008

The equatorial region of the canine left ventricle was modeled as a thick-walled cylinder consisting of an incompressible hyperelastic material with homogeneous exponential properties. The anisotropic properties of the passive myocardium were assumed to be locally transversely isotropic with respect to a fiber axis whose orientation varied linearly across the wall. Simultaneous inflation, extension, and torsion were applied to the cylinder to produce epicardial strains that were measured previously in the potassium-arrested dog heart. Residual stress in the unloaded state was included by considering the stress-free configuration to be a warped cylindrical arc. In the special case of isotropic material properties, torsion and residual stress both significantly reduced the high circumferential stress peaks predicted at the endocardium by previous models. However, a resultant axial force and moment were necessary to cause the observed epicardial deformations. Therefore, the anisotropic material parameters were found that minimized these resultants and allowed the prescribed displacements to occur subject to the known ventricular pressure loads. The global minimum solution of this parameter optimization problem indicated that the stiffness of passive myocardium (defined for a 20 percent equibiaxial extension) would be 2.4 to 6.6 times greater in the fiber direction than in the transverse plane for a broad range of assumed fiber angle distributions and residual stresses. This agrees with the results of biaxial tissue testing. The predicted transmural distributions of fiber stress were relatively flat with slight peaks in the subepicardium, and the fiber strain profiles agreed closely with experimentally observed sarcomere length distributions. The results indicate that torsion, residual stress and material anisotropy associated with the fiber architecture all can act to reduce endocardial stress gradients in the passive left ventricle.
On a Nonlinear Theory for Muscle Shells: Part I—Theoretical Development
Authors: Taber L. A.
Published: March 2008

This paper presents a theory for studies of the large-strain behavior of biological shells composed of layers of incompressible, orthotropic tissue, possibly muscle, of arbitrary orientation. The intrinsic equations of the laminated-shell theory, expressed in lines-of-curvature coordinates, account for large membrane [O(1)] and moderately large bending and transverse shear strains [O(0.3)], nonlinear material properties, and transverse normal stress and strain. An expansion is derived for a general two-dimensional strain-energy density function, which includes residual stress and muscle activation through a shifting zero-stress configuration. Strain-displacement relations are given for the special case of axisymmetric deformation of shells of revolution with torsion.
A Parameter Optimization Approach for the Optimal Control of Large-Scale Musculoskeletal Systems
Authors: Pandy M. G. , Anderson F. C. , Hull D. G.
Published: March 2008

This paper describes a computational method for solving optimal control problems involving large-scale, nonlinear, dynamical systems. Central to the approach is the idea that any optimal control problem can be converted into a standard nonlinear programming problem by parameterizing each control history using a set of nodal points, which then become the variables in the resulting parameter optimization problem. A key feature of the method is that it dispenses with the need to solve the two-point, boundary-value problem derived from the necessary conditions of optimal control theory. Gradient-based methods for solving such problems do not always converge due to computational errors introduced by the highly nonlinear characteristics of the costate variables. Instead, by converting the optimal control problem into a parameter optimization problem, any number of well-developed and proven nonlinear programming algorithms can be used to compute the near-optimal control trajectories. The utility of the parameter optimization approach for solving general optimal control problems for human movement is demonstrated by applying it to a detailed optimal control model for maximum-height human jumping. The validity of the near-optimal control solution is established by comparing it to a solution of the two-point, boundary-value problem derived on the basis of a bang-bang optimal control algorithm. Quantitative comparisons between model and experiment further show that the parameter optimization solution reproduces the major features of a maximum-height, countermovement jump (i.e., trajectories of body-segmental displacements, vertical and fore-aft ground reaction forces, displacement, velocity, and acceleration of the whole-body center of mass, pattern of lower-extremity muscular activity, jump height, and total ground contact time).
An Analytical Study of Cryosurgery in the Lung
Authors: Bischof J. C. , Bastacky J. , Rubinsky B.
Published: March 2008

The process of freezing in healthy lung tissue and in tumors in the lung during cryosurgery was modeled using one-dimensional close form techniques and finite difference techniques to determine the temperature profiles and the propagation of the freezing interface in the tissue. A thermal phenomenon was observed during freezing of lung tumors embedded in healthy tissue, (a) the freezing interface suddenly accelerates at the transition between the tumor and the healthy lung, (b) the frozen tumor temperature drops to low values once the freezing interface moves into the healthy lung, and (c) the outer boundary temperature has a point of sharp inflection corresponding to the time at which the tumor is completely frozen.
Wall Shear Stress Estimates in Coronary Artery Constrictions
Authors: Back L. H. , Crawford D. W.
Published: March 2008

Wall shear stress estimates from laminar boundary layer theory were found to agree fairly well with the magnitude of shear stress levels along coronary artery constrictions obtained from solutions of the Navier Stokes equations for both steady and pulsatile flow. The relatively simple method can be used for in vivo estimates of wall shear stress in constrictions by using a vessel shape function determined from a coronary angiogram, along with a knowledge of the flow rate.
The Bleed Off Perfusion Term in the Weinbaum-Jiji Bioheat Equation
Authors: Weinbaum S. , Jiji L. M. , Lemons D. E.
Published: March 2008

The microvascular organization and thermal equilibration of the primary and secondary arteries and veins that comprise the bleed off circulation to the muscle fibers from the parent countercurrent supply artery and veins are analyzed. The blood perfusion heat source term in the tissue energy equation is shown to be related to this vascular organization and to undergo a fundamental change in behavior as one proceeds from the more peripheral tissue, where the perfusion term is proportional to the Ta - Tv difference in the parent supply vessels, to the deeper tissue layers where the bleed off vessels themselves form a branching countercurrent system for each muscle tissue cylinder and the venous return temperature can vary between the local tissue temperature and Ta. The consequences of this change in behavior are examined for the Weinbaum-Jiji bioheat equation and a modified expression for the effective conductivity of perfused tissue is derived for countercurrent bleed off exchange.
Motion of a Doublet of Two Cylinders in Contact in a Narrow Channel Flow
Authors: Sugihara-Seki Masako
Published: March 2008

The motion of two rigid circular cylinders in contact immersed in an incompressible Newtonian fluid in a channel is examined numerically in the zero Reynolds number limit, for various values of the cylinder radius/channel width ratio. Analyses of the time courses of the lateral position and the orientation of the doublet showed that, depending on the initial condition and the doublet/channel size ratio, the doublet exhibit one of the three types of motion: a continuous rotation in the same direction during a period, and a rotation changing its direction at every half period with a large or a small variation of the orientation.
A Parametric Design Evaluation of Lateral Prophylactic Knee Braces
Authors: Daley B. J. , Ralston J. L. , Brown T. D. , Brand R. A.
Published: March 2008

Six major mechanical design variables characterizing single-upright lateral prophylactic knee braces were studied experimentally, using a generic modular brace (GMB). Impulsive valgus loading tests were conducted with the GMB applied to a surrogate leg model. The surrogate involved anatomically realistic aluminum-reinforced acrylic components to model bone, and expendable polymeric blanks to mimic the major knee ligaments. Behavior of the surrogate system reasonably reproduced that of human cadaveric knees under similar loading conditions. Load at failure of the medial collateral ligament (MCL) analog, gross knee stiffness, and MCL relative strain relief were measured for each of twelve parametric brace design permutations. Compared to the unbraced condition, bracing provided statistically significant increases in valgus load uptake at failure and in MCL strain relief. Increasing the dimensions of individual brace components (hinge length and offset; upright length, breadth, and thickness; cuff area), relative to those of a GMB baseline configuration deemed representative of current commerical products, failed to achieve statistically significant improvements in brace performance. However, most below-baseline dimensioning of individual components did significantly compromise GMB performance. These surrogate test data indicate that geometric modifications of current single-upright lateral brace designs are unlikely to substantially improve upon the fairly modest valgus load protection afforded by this class of devices.
Improved Dynamic Model of the Human Knee Joint and Its Response to Impact Loading on the Lower Leg
Authors: Engin Ali Erkan , Tu¨mer Sami Turgut
Published: March 2008

Almost a decade ago, three-dimensional formulation for the dynamic modeling of an articulating human joint was introduced. Two-dimensional version of this fomulation was subsequently applied to the knee joint. However, because of the iterative nature of the solution technique, this model cannot handle impact conditions. In this paper, alternative solution methods are introduced which enable investigation of the response of the human knee to impact loading on the lower leg via an anatomically based model. In addition, the classical impact theory is applied to the same model and a closed-form solution is obtained. The shortcomings of the classical impact theory as applied to the impact problem of the knee joint are delineated.
The Mechanics of Axially Symmetric Liposomes
Authors: Pamplona D. C. , Calladine C. R.
Published: March 2008

Hotani has filmed morphological transformations in unilamellar liposomes, starting from a spherical shape, when the interior volume decreases steadily. Hotani’s liposomes showed no evidence of general thermal fluctuations. We use a finite-deformation theory of axisymmetric, quasi-static thin shells to analyze theoretically bifurcations and changes of shape in liposomes under decreasing volume. The main structural action in a lipid bilayer is generally agreed to be its elastic resistance to bending, and it is usual to regard surface deformation as being like that of a two-dimensional liquid. We find, however, that some in-plane shear elasticity is also needed in order to produce the observed post-bifurcation behavior. Such an elasticity would be difficult to measure directly.
An Analysis of the Time-Dependent Changes in Intracellular Calcium Concentration in Endothelial Cells in Culture Induced by Mechanical Stimulation
Authors: Winston F. K. , Thibault L. E. , Macarak E. J.
Published: March 2008

When bovine pulmonary artery endothelial cells in culture are subjected to mechanical strain, their physiology is altered. Experimentally, this mechanical strain is generated by increased tension in the substrate to which the cells are attached and results in altered levels of fibronectin. Studies of the structural response of the endothelial cell suggest that this stimulus is transmitted to the cell membrane, organelles, and cytoskeleton by natural cell attachments in a quantifiable and predictable manner. This report examines altered intracellular calcium homeostasis as a possible messenger for the observed strain-induced physiologic response. In particular, using the intracellularly trapped calcium indicator dyes, Quin2 and Fura2, we observed changes in cytosolic free calcium ion concentration in response to biaxial strain of bovine pulmonary artery endothelial cells in culture. The magnitude and time course of this calcium transient resemble that produced by treatment with the calcium ionophore, Ionomycin, indicating that mechanical stimulation may alter cell membrane permeability to calcium. Additional experiments in the presence of EDTA indicated that calcium was also released from intracellular stores in response to strain. In order to explain the stretch-induced calcium transients, a first-order species conservation model is presented that takes into account both the cell’s structural response and the calcium homeostatic mechanisms of the cell. It is hypothesized that the cell’s calcium sequestering and pumping capabilities balanced with its mechanically induced changes in calcium ion permeability will determine the level and time course of calcium accumulation in the cytosol.
Aortic Pressure Estimation With Electro-Mechanical Circulatory Assist Devices
Authors: Gardner J. F. , Ignatoski M. , Tasch U. , Snyder A. J. , Geselowitz D. B.
Published: March 2008

An adaptive technique for the estimation of the time history of aortic pressure (from applied voltage and position feedback) has been designed, implemented, and bench tested using the Penn State Electric Ventricular Assist Device (EVAD). This method, known in the field of automatic control as a dynamic observer, utilizes gains which were determined using experimental data collected while the EVAD was running on a mock circulatory system. An adaptive scheme provides the observer with a method of changing its initial conditions on a stroke-by-stroke basis which improves observer performance. In both determining the feedback gains and developing the adaptation scheme, a range of beat rates and pressure loads was taken into account to yield satisfactory observer performance over a range of operating conditions. The observer was implemented, its performance was verified in vitro and results are reported. In the six experimental operating conditions, the beat rate ranged from 56-104 beats per minute (bpm) and the span of the mean systolic aortic pressure was 10.7-18.7 kPa (80–140 mmHg). For these cases, the mean deviation between the actual and estimated aortic pressure during the latter two-thirds of systole was 0.41 kPa (3.1 mmHg).
Characteristic Recovery Time of an Erythrocyte From an Extensional Deformation
Authors: Lucero J. C.
Published: March 2008

An analytical expression for the characteristic recovery time of an erythrocyte subject to an extensional deformation is derived using a previous nonlinear Kelvin-Voigt model. The recovery time thus obtained depends on the initial deformation in agreement with experimental observations, as a result of the nonlinearity of the model. The validity of the analytical results is confirmed using previous experimental data.
Bond Graph Based Analysis of Repeated Systems With Nonpotential Interactions—An Application to Fluidelastic Vibration of Tube Arrays
Authors: Samanta B. , Mukherjee A.
Published: March 2008

A procedure is presented to study the dynamics of parallel repeated systems with nonpotential interactions within the framework of bond graph technique. This type of interaction, when decoupled, gives rise to complex elements consisting of both real and imaginary parts, which cannot be used for further processing in the conventional bond graph analyzing packages like ENPORT. This shortcoming is removed by reformulating the decoupled subsystem equations to an equivalent form with all real parameters leading to a substantial reduction in size of the system to be studied at a time. The procedure is illustrated by the example of fluidelastic vibration of tube arrays in heat exchangers.
Analysis of Acoustoelastic Systems Using Modal Bond Graphs
Authors: Samanta B. , Mukherjee A.
Published: March 2008

A procedure is presented to study the dynamics of acoustoelastic systems within the framework of bond graph technique. The substructures, acoustic and structural, are modeled individually in form of bond graphs that are coupled through suitable elements satisfying the conditions at the interfaces. From this bond graph a second stage modal decomposition is performed to represent the overall system in yet another bond graph that can be analyzed to obtain the overall system dynamics. The scope for two-stage modal truncation makes the procedure suitable for analyzing acoustoelastic systems that are “stiff” in nature due to the wide range of their natural frequencies. The procedure is illustrated by suitable examples.
Stochastic Control for Systems With Faulty Sensors
Authors: Watanabe K. , Tzafestas S. G.
Published: March 2008

The problem of control of linear discrete-time stochastic systems with faulty sensors is considered. The anomaly sensors are assumed to be modeled by a finite-state Markov chain whose transition probabilities are completely known. A passive type multiple model adaptive control (MMAC) law is developed by applying a new generalized pseudo-Bayes algorithm (GPBA), which is based on an n-step measurement update method. The present and other existing algorithms are compared through some Monte Carlo simulations. It is then shown that, for a case of only measurement noise uncertainty (i.e., a case when the certainty equivalence principle holds), the proposed MMAC has better control performance than MMAC’s based on using other existing GPBA’s.
A Technique for Designing Dyadic Controllers
Authors: Chen C. F. , Freeman G.
Published: March 2008

A new technique for designing dyadic controllers is established. By the use of the recently established matrix partial fraction expansion formula, the dyadic transformation matrices are naturally formed. A direct interpretation of the parameters of the dyadic transformation matrices via the residue matrices becomes possible.
Steady-State and Stochastic Performance of a Modified Discrete-Time Prototype Repetitive Controller
Authors: Chew Kok Kia , Tomizuka Masayoshi
Published: March 2008

Perfect regulation may be too stringent a condition in repetitive control. In this paper, the rigid stability requirement is relaxed by including an appropriately chosen filter in the repetitive signal generator. Lacking an internal model, perfect regulation is not assured in the modified system. The steady-state and stochastic performances of the resulting system are analyzed. These analyses reveal that under certain conditions the dual objectives of good steady-state and stochastic performances are conflicting. A high repetitive gain may give good steady-state performance, but the variance propagation of stochastic disturbances is large (extremely large for some choice of a parameter in the modified controller). The converse is true when the repetitive gain is small. The performance of the modified scheme is evaluated by applying it to a simulated disk-file actuator system.
Nonlinear Robust Hybrid Control of Robotic Manipulators
Authors: Kuo C. Y. , Wang Shay-Ping T.
Published: March 2008

The success of a robot force and position (hybrid) control scheme relies extensively upon its robustness against uncertainties such as unknown external disturbance or modeling errors in the description of robot, sensor and environment. In this paper we propose a new nonlinear robust hybrid control scheme for robot motion control. The control input consists of a nonlinear and a linear part. The nonlinear input decouples a robot dynamics and gives a set of position and force equations in the hand or cartesian coordinates. The linear part applies the servomechanism theory to suppress position or force tracking error due to uncertainties. This nonlinear robust hybrid control scheme is applied to a two-joint SCARA type robot, and simulation results demonstrate excellent robustness properties and satisfactory hybrid control under severe modeling errors.
Nonlinear Compensation for Flexible Manipulators
Authors: Castelazo I. A. , Lee H.
Published: March 2008

This paper presents an analysis on the use of nonlinear feedback to control the tip position of a single-link flexible manipulator. This form of feedback uses products of state variables to obtain a lightly damped system in the beginning of a step response and a more heavily damped system when it is close to the steady state position. This scheme results in faster rise and settling times than those obtained with an optimal linear controller while maintaining a comparable amount of control effort. The model used in the analysis is a single Bernouli-Euler beam moving in a horizontal plane, considering only a rigid mode plus two vibrating modes. A heuristic method to tune the nonlinear feedback gains is proposed. This method results in a design that, compared to a linear optimal controller, reduces the settling time by 32 percent without increasing the percent overshoot.
Closed-Loop Digital Control of an Impact Printer Hammer
Authors: Ryan L. E. , Stengel R. F.
Published: March 2008

This paper explores closed-loop digital control of a high-speed impact printer hammer in the presence of sensor measurement noise and disturbances. Conventional control of high-speed impact printers is accomplished without feedback; hence disturbances, such as mechanical and magnetic interaction between hammers and hammer settle-out period, force long delays between hammer firings to assure proper time and velocity of impact. By computer simulation, we show the feasibility of using the solenoid actuator to control both time and velocity of impact. This is accomplished by setting an initial trajectory and introducing mid-course guidance based on state estimates derived from hammer position measurements and a Kalman filter estimator. To overcome difficulties in performing necessary computations in a short time, and in synchronizing the digital system to the “real world,” the controller allows the solenoid to be commanded “on” and “off” between samples. Although the solenoid actuator produces a force in one direction only, the same control law is used to decelerate the hammer on the return trip, bringing it to a soft landing that provides satisfactory initial conditions for the next hammer command. Attention is given to computation time required, and a general scenario for real-time implementation is given.
Preshaping Command Inputs to Reduce System Vibration
Authors: Singer N. C. , Seering W. P.
Published: March 2008

A method is presented for generating shaped command inputs which significantly reduce or eliminate endpoint vibration. Desired system inputs are altered so that the system completes the requested move without residual vibration. A short move time penalty is incurred (on the order of one period of the first mode of vibration). The preshaping technique is robust under system parameter uncertainty and may be applied to both open and closed loop systems. The Draper Laboratory’s Space Shuttle Remote Manipulator System simulator (DRS) is used to evaluate the method. Results show a factor of 25 reduction in endpoint residual vibration for typical moves of the DRS.
Rear Wheel Steering Dynamics Compared to Front Steering
Authors: Whitehead J. C.
Published: March 2008

The lateral dynamics of rear wheel steering vehicles are examined using low order linear mathematical models. The response to rear steer angle inputs differs significantly from the front wheel steering response at low speeds. However, both the transient and steady state responses become less dependent on which wheels are steered as vehicle speed increases. This fact indicates that the unusual fixed control response does not alone cause rear wheel steering vehicles to be unsafe at high speeds. The free control instability unique to rear wheel steering vehicles is analyzed using a torque input model which treats steer angle as a degree of freedom. The cause of this unstable weave mode and the stable front wheel steering weave mode is a ratio of tire slip angle to steer angle in excess of unity during high speed cornering.
Control of an Active Suspension System Subject to Random Component Failures
Authors: de Benito C. D. , Eckert S. J.
Published: March 2008

Two fault-tolerant control algorithms based on certainty-equivalence and dual control approximations, are described in this paper. These algorithms maintain the system performance in the presence of component failures. Their performance is demonstrated by simulations of an active suspension system subjected to sensor and actuator failures. Both algorithms provide viable solutions to the problem of faulttolerant control in automotive systems. However, the certainty-equivalence control algorithm is simpler to implement.
Biaxial Testing of Membrane Biomaterials: Testing Equipment and Procedures
Authors: Nielsen P. M. F. , Hunter P. J. , Smaill B. H.
Published: March 2008

A testing facility for measuring the biaxial mechanical properties of highly deformable membranes is described. Forces are applied, via strain-gauge force transducers, to four points on each side of an initially square 12 to 25 mm membrane sample to produce biaxial extensions of up to 80 percent of undeformed length. Strain is estimated from the displacement of markers bounding a 1 to 2 mm central square. The accuracy of stress and strain field measurements has been assessed by finite element analysis of a biaxially-loaded isotropic elastic membrane. Major advantages of the present system over those previously described in the literature are that 1) sample mounting procedures are simplified, 2) there is provision for independent adjustment of stress field uniformity and measurement of the applied point forces and 3) faster strain rates can be imposed on the relatively small samples tested.
Convective Heat Transfer Coefficients in the Circulation
Authors: Barozzi G. S. , Dumas A.
Published: March 2008

Convective heat transfer in the vessels of the circulatory system is investigated numerically. In the modeling, account is taken of the non-Newtonian rheological properties of blood and the presence of a cell-depleted plasma layer at the vessel wall. The latter is found to produce a remarkable enhancement of the heat transfer rate in the small vessels, while the effects due to the rheological behavior of blood are comparatively low. A comparison with experimental data available in the open literature is finally attempted.
A New View of Convective-Diffusive Transport Processes in the Arterial Intima
Authors: Yuan F. , Chien S. , Weinbaum S.
Published: March 2008

In this paper a new theoretical framework is presented for analyzing the filtration and macromolecular convective-diffusive transport processes in the intimal region of an artery wall with widely dispersed macromolecular cellular leakage sites, as proposed in the leaky junction-cell turnover hypothesis of Weinbaum et al. [11]. In contrast to existing convection-diffusive models, which assume that the transport is either 1-D, or convection is primarily in a direction normal to the endothelial surface, the present model considers for the first time the nonuniform subendothelial pressure field that arises from the different hydraulic resistances of normal and leaky endothelial clefts and the special role of the internal elastic lamina (IEL) in modulating the horizontal transport of macromolecules after they have passed through the leaky clefts of cells that are either in mitosis or demonstrate IgG labeling. The new theory is able to quantitatively explain the growing body of recent experiments in which an unexpectedly rapid early-time growth of the leakage spot has been observed and the longer time asymptotic behavior in which the leakage spot appears to approach an equilibrium diameter. The new theory also predicts the observed doubling in macromolecular permeability between EBA labeled blue and white areas when the frequency of leakage sites is doubled. This frequency for doubling of permeability, however, is an order of magnitude smaller than predicted by the author’s previous model, Tzeghai et al. [10], in which only convection normal to the endothelial surface was considered and the pressure was uniform in the intima. The longer time model predictions are used to explain the time scale for the formation of liposomes [4] in subendothelial tissue matrix in animal feeding experiments where it has been observed that the extracellular lipid concentration rises sharply prior to the entry of monocytes into the intima [45].
Fracture Prediction for the Proximal Femur Using Finite Element Models: Part I—Linear Analysis
Authors: Lotz J. C. , Cheal E. J. , Hayes W. C.
Published: March 2008

Over 90 percent of the more than 250,000 hip fractures that occur annually in the United States are the result of falls from standing height. Despite this, the stresses associated with femoral fracture from a fall have not been investigated previously. Our objectives were to use three-dimensional finite element models of the proximal femur (with geometries and material properties based directly on quantitative computed tomography) to compare predicted stress distributions for one-legged stance and for a fall to the lateral greater trochanter. We also wished to test the correspondence between model predictions and in vitro strain gage data and failure loads for cadaveric femora subjected to these loading conditions. An additional goal was to use the model predictions to compare the sensitivity of several imaging sites in the proximal femur which are used for the in vivo prediction of hip fracture risk. In this first of two parts, linear finite element models of two unpaired human cadaveric femora were generated. In Part II, the models were extended to include nonlinear material properties for the cortical and trabecular bone. While there was poor correspondence between strain gage data and model predictions, there was excellent agreement between the in vitro failure data and the linear model, especially using a von Mises effective strain failure criterion. Both the onset of structural yielding (within 22 and 4 percent) and the load at fracture (within 8 and 5 percent) were predicted accurately for the two femora tested. For the simulation of one-legged stance, the peak stresses occurred in the primary compressive trabeculae of the subcapital region. However, for a simulated fall, the peak stresses were in the intertrochanteric region. The Ward’s triangle (basicervical) site commonly used for the clinical assessment of osteoporosis was not heavily loaded in either situation. These findings suggest that the intertrochanteric region may be the most sensitive site for the assessment of fracture risk due to a fall and the subcapital region for fracture risk due to repetitive activities such as walking.
Nonlinear Dynamic Behavior of the Human Knee Joint—Part II: Time-Domain Analyses: Effects of Structural Damage in Postmortem Experiments
Authors: Dortmans  , Jans H. , Sauren A. , Huson A.
Published: March 2008

A description is given of the results obtained for step excitation for two human knee joint specimens using a time-domain analysis technique. As was expected from the results of a previous study, the magnitude of the dynamic load applied has a marked influence upon the stiffness and damping values for the two observed vibration modes. Deliberate damaging of selected joint elements also yields a well observable change in the dynamic behavior of the joint although these changes are difficult to interpret. Here the use of a nonlinear dynamic numerical model of the knee joint seems indispensable. An important observation is, however, that the experimental method discussed here enables to quantify the behavior of the joint and therefore may provide a valuable tool for validation of such a model.