Computational Investigation on the Dispersive Transport of Angiographic Contrast Injected Antegradely Into Flow in a Tube

When a solute such as angiographic contrast is introduced into a solvent such as blood analog fluid flowing in a straight circular tube, it spreads under the combined action of molecule diffusion and the variation of velocity over the cross-section [8]. If two molecules are being carried in the flow, for example, one in the center and one near the wall, the rate of separation caused by the difference in bulk velocity will greatly exceed that caused by molecule motion. Given enough time, any single molecule would wander randomly throughout the cross section of the pipe because of molecular diffusion, and would sample at random all the advective velocities [4]. Therefore, Taylor [8] adopted the Lagrangian approach to the problem, casting the equations in a coordinate system that moves with the average velocity of the flow and replacing the molecular diffusion coefficient with a dispersion coefficient, and the local concentration with the cross sectional mean concentration. Recasting Taylor’s equation in an inertial coordinate system one obtained the so called advection-dispersion equation.

Treatment of Cerebral Aneurysms With Flow Divertors: Long Term Results in an In Vivo Model

Subarachnoid hemorrhagic stroke is a devastating illness with a 30-day mortality rate of 45% and is mostly caused due to the rupture of an intracranial aneurysm. Although these aneurysms are currently treated surgically by clipping, or, endovascularly by coiling and stent-assisted coiling, the feasibility of successfully treating aneurysms solely by the placement of an intravascular flow-diverting mesh across the aneurysm neck was established more than a decade ago [1]. Flow divertors disrupt the momentum exchange between the parent artery and aneurysm and significantly reduce intraaneurysmal hydrodynamic vorticity. The resultant flow stasis promotes thrombus formation within the aneurysm sac, which eventually matures into fibrotic tissue, leading to the exclusion of the aneurysm from the circulation. With the increased use of stents in the intracranial circulation, cases where coiling is not feasible, or is staged as a secondary procedure, are providing clinical evidence of the successful treatment of aneurysms with stents alone [2,3]. Such reports are sporadic and, moreover, the devices used are not designed to be flow divertors. Methodological evidence of the performance of appropriately designed flow divertors in treating cerebral aneurysms is currently unavailable.

Advanced Material Modeling of UHMWPE for Orthopaedic Implant Design

Ultra-high molecular weight polyethylene (UHMWPE) is extensively used in orthopaedic implants, and recent constitutive modeling efforts have endeavored to rigorously capture the nonlinear, quasi-static, inelastic properties of the material. In particular, the “hybrid model” developed by Bergstrom et al. has been shown to accurately characterize the response of UHMWPE to various loading protocols including uniaxial tension to failure, cyclic uniaxial tension/compression, and small punch testing [1–2].

Development of a Discrete Finite Element Cell Model

Computational models have proven useful in the study of cell mechanics and mechanotransduction. While most finite element (FE) models of cells are commonly described in terms of the laws of continuum mechanics, a model that can accurately represent the microstructure of the filamentous network of the cytoskeleton would be required to relate mechanics to biology at the microscale level. An alternative approach to a continuum is presented here, whereby the discrete nature of the cytoskeleton of the cell is emphasized and the known structural properties of the cytoskeleton of the cell are utilized.

Culture Medium With Blood Analog Mechanical Properties

Culture of arteries has become increasingly important in studying atherosclerosis and the effect of clinical interventions [1]. Ideally, arterial culturing should imitate in vivo conditions within an ex vivo environment. Physiological wall shear stresses are important as they induce an atheroprotective endothelial phenotype [2], which is relevant for maintaining arterial wall integrity. The arteries in such ex vivo studies, however, are perfused with culture medium, which has a viscosity lower than blood. Previously, the culture medium has been supplemented with dextran to obtain physiological fluid viscosity and wall shear stresses. However, several researchers [3,4] reported side effects of dextran on the cells in the arterial wall independent of its effect on medium viscosity. Also, dextran increases medium osmolality to supraphysiological levels [5]. This suggests that dextran may not be the optimal substance to increase medium viscosity.

A Confined Compression Technique for Hydraulic Conductivity Measurement in Soft Tissues

Multiphasic tissue models have been used extensively to predict the behavior of cartilaginous tissues [1]. Their application to other soft tissues, however, has often been overlooked. Unlike the more commonly used continuum model of the viscoelastic solid [2], multiphasic models allow us to infer the behaviors and properties of tissue subcomponents by observing the behavior of the tissue whole. As a great deal of tissue function and structure is related to the control and transport of fluids and fluid-borne agents, there is clearly a need for this insight in all tissues. For example, there has been a great deal of interest recently in the possibility of modifying the flow properties of solid tumors and other tissues to allow the targeted delivery of large molecular weight drugs, such as chemotherapeutic or genetic agents [3–4]. It is well known that the high interstitial fluid pressures, confused vasculature, and lack of a lymphatic system prevent the effective distribution of directly injected or systemically administered drugs into tumors [3]. Increasing the effective permeability of these tumors can ameliorate these issues and allow for more effective treatment. A handful of studies have found that the biphasic model, along with some basic experimental tools, can reasonably represent the flow properties of tumors [4–5]. In this paper, we describe a technique using a simple confined compression experiment with the biphasic model to measure the hydraulic conductivity of samples of cardiac tissue.

Biomechanical Analysis of Late Airbag Deployment in Motor Vehicle Crashes Using Computer Simulation

Motor vehicle collisions frequently result in serious or fatal inuries to occupants [1–4]. Frontal collisions are amongst the most severe types of accidents. The use of safety systems such as seat belts and airbags has been shown to reduce the severity of injuries sustained by occupants [5–10]. It is well known that frontal airbags act as supplemental restraints to seat belts in protecting occupants. Airbag deployment occurs through a reaction of chemicals in the inflator that rapidly produces gas and fills the canvas bag. The filled bag acts a cushion between the occupant and the vehicle’s interior components. The supplemental restraint provided by the airbag increases the amount of time and distance over which the occupant’s body decelerates, and accordingly reduces the potential for injury. The time at which the airbag deployment is initiated during the crash sequence can have an effect on the nature of the contact between occupant and airbag. Though properly timed, frontal airbags have been shown to reduce injuries sustained to occupants[11], it has been reported that airbags that deploy too late may cause injury[12]. To date, there have been a very limited number of studies that have addressed the biomechanical effects of late airbag deployment. The purpose of this study is to determine the biomechanical effects of late airbag deployment and restraint use on various sizes of occupants through computer simulation.

The Protective Role of Bacillus Anthracis Exosporium in Macrophage-Mediated Killing by Nitric Oxide

The ability of the endospore-forming, gram-positive bacterium Bacillus anthracis to survive exposure to antibacterial killing mechanisms by activated macrophages is key to its germination and survival. These antibacterial killing mechanisms include, but are not limited to the generation of free radicals such as nitric oxide (•NO) and superoxide (O2•−) from the upregulation of inducible nitric oxide synthase (NOS 2) along with products derived from them, e.g., peroxynitrite (ONOO−), as part of microbicidal activity. However questions still remain as to how these species are involved in microbial killing, specifically with respect to B. anthracis. In a previous study, we demonstrated that exposure of primary murine macrophages to sonicated B. anthracis endospores up-regulated NOS 2 and demonstrated a •NO-dependent bactericidal response, but unanswered in that study was which of the NOS 2-derived reactive oxygen species was responsible for the observed bactericidal response. Since NOS 2 also generates O2•−, experiments were designed to determine whether NOS 2 formed ONOO− from the reaction of •NO with O2•− and if so, was ONOO− microbicidal toward B. anthracis.

Computational Model of Interstitial Transport in the Rat Brain Using Diffusion Tensor Imaging

In spite of the high therapeutic potential of macromolecular drugs, it has proven difficult to apply them to recovery after injury and treatment of cancer, Parkinson’s disease, and other neurodegenerative diseases. One barrier to systemic administration is low capillary permeability, i.e., the blood-brain and blood-spinal cord barrier. To overcome this barrier, convection-enhanced delivery (CED) infuses agents directly into tissue to supplement diffusion and increase the distribution of large molecules in the brain [1,2]. Predictive models of distribution during CED would be useful in treatment optimization and planning. To account for large infusion volumes, such models should incorporate tissue boundaries and anisotropic tissue properties.

CFD Modeling of Transient Flow Phenomena in an Axial Flow VAD

A ventricular assist device (VAD) effectively relieves the workload from a native heart, which has been weakened by disease, and increases blood flow supplied to the body to maintain normal physiologic function. The device must be able to operate over a wide range of conditions. Designed to operate at a single, best-efficiency operating point, it must frequently perform at off-design conditions due to a fluctuating flow rate demanded by the human body and a time varying flow within the pump, due to the beating of the native heart. The design and optimization of a blood pump is a challenging and complex process. Pump design equations are used to estimate the initial dimensions of the pump regions. Computational fluid dynamics (CFD) analyses are then performed to optimize the blood flow path according to specific design criteria under steady flow conditions [1].