Anh Bui

Analytical modeling and numerical simulation of forces in an endoluminal graft.

Purpose: To utilize mathematical analysis and computational fluid dynamics (CFD) to investigate the forces acting within the pressurized aorta and upon a stent-graft and how these forces may affect the ongoing performance of the stent-graft. Methods: Analytical force balance analysis and CFD simulations using the Fluent code were used to mimic blood flow through a bifurcated stent-graft in a person at rest. Steadystate blood flow was assumed in which the inlet pressure approximated the mean blood pressure (100 mm Hg) and the blood flow velocity was an approximation of the peak systolic flow rate (0.6 m/s). Two sizes of endoluminal grafts were analyzed: the larger graft had an inlet diameter of 3 cm and outlet diameters of 1 cm; the smaller graft diameters measured 2.4 cm proximally and 1.2 cm distally. The endografts were studied in 2 configurations: with the limbs straight and with one bent. Results: For the larger graft model, the normal peak blood flow induced a downward force of 7 to 9 N on the bifurcated grafts. Bending one of the limbs of the graft produced a sideways force of 1.3 N. For the smaller endograft, the downward force was in the range of 3.1 to 5.1 N and the sideways force on a curved limb was ∼1.5 N. The magnitude of the forces given by the analytical formulae and the CFD results agreed to within 2 significant figures. Conclusions: These results suggest that the downward force on a bifurcated stent-graft, which may exceed the force required to dislodge it when relying on radial attachment alone, is determined mostly by the proximal graft diameter. Curvature of the graft limbs creates an additional sideways force that works to displace the distal limbs of the graft from the iliac arteries.

A Novel Mouse Model of Atherosclerotic Plaque Instability for Drug Testing and Mechanistic/Therapeutic Discoveries Using Gene and MicroRNA Expression Profiling

Rationale: The high morbidity/mortality of atherosclerosis is typically precipitated by plaque rupture and consequent thrombosis. However, research on underlying mechanisms and therapeutic approaches is limited by the lack of animal models that reproduce plaque instability observed in humans. Objective: Development and use of a mouse model of plaque rupture that reflects the end stage of human atherosclerosis. Methods and Results: On the basis of flow measurements and computational fluid dynamics, we applied a tandem stenosis to the carotid artery of apolipoprotein E–deficient mice on high-fat diet. At 7 weeks postoperatively, we observed intraplaque hemorrhage in ≈50% of mice, as well as disruption of fibrous caps, intraluminal thrombosis, neovascularization, and further characteristics typically seen in human unstable plaques. Administration of atorvastatin was associated with plaque stabilization and downregulation of monocyte chemoattractant protein-1 and ubiquitin. Microarray profiling of mRNA and microRNA (miR) and, in particular, its combined analysis demonstrated major differences in the hierarchical clustering of genes and miRs among nonatherosclerotic arteries, stable, and unstable plaques and allows the identification of distinct genes/miRs, potentially representing novel therapeutic targets for plaque stabilization. The feasibility of the described animal model as a discovery tool was established in a pilot approach, identifying a disintegrin and metalloprotease with thrombospondin motifs 4 (ADAMTS4) and miR-322 as potential pathogenic factors of plaque instability in mice and validated in human plaques. Conclusions: The newly described mouse model reflects human atherosclerotic plaque instability and represents a discovery tool toward the development and testing of therapeutic strategies aimed at preventing plaque rupture. Distinctly expressed genes and miRs can be linked to plaque instability.

Dynamics of pulsatile flow in fractal models of vascular branching networks.

Efficient regulation of blood flow is critically important to the normal function of many organs, especially the brain. To investigate the circulation of blood in complex, multi-branching vascular networks, a computer model consisting of a virtual fractal model of the vasculature and a mathematical model describing the transport of blood has been developed. Although limited by some constraints, in particular, the use of simplistic, uniformly distributed model for cerebral vasculature and the omission of anastomosis, the proposed computer model was found to provide insights into blood circulation in the cerebral vascular branching network plus the physiological and pathological factors which may affect its functionality. The numerical study conducted on a model of the middle cerebral artery region signified the important effects of vessel compliance, blood viscosity variation as a function of the blood hematocrit, and flow velocity profile on the distributions of flow and pressure in the vascular network.

Mode of failure of rib fixation with absorbable plates: a clinical and numerical modeling study.

BACKGROUND Rib fractures as a result of trauma are a relatively common injury. There is an increasing interest in the operative stabilization of these injuries. Absorbable fixation plates are an option for improved rib fracture treatment. The aim of this study was to review our plating failures and to create a numerical model of muscle forces on fractured ribs to identify the mechanism by which these rib fixations have failed. METHODS Thirteen patients who had 58 ribs fixed with absorbable prostheses were reviewed. Finite element modeling was used to simulate the fixation of a lateral rib fracture using an absorbable plate and screw system. Internal pressure, intercostal forces, and appropriate displacement and rotational constraints were enforced at the rib ends. RESULTS Ten rib fixation failures were noted in the clinical series. The modeling results showed that stresses on the plate differ during inspiration and expiration. Failure to use the two central screws resulted in higher stresses on the plating system. During inspiration simulations, the screws on both rib parts are active in keeping the rib and plate surfaces unseparated. However, during expiration, there is a greater stress on the screws on the posterior part of the broken rib, and separation of the plate from the rib seems to be more likely to occur at this site. CONCLUSIONS This study indicates that the likely mode of failure of this absorbable plating system occurs on the posterior part of the rib, which correlates with the clinical failures seen.

Modelling of flow through the circle of Willis and cerebral vasculature

The blood flow through the circle of Willis was modelled by coupling a Computational Fluid Dynamics (CFD) model of the circle of Willis with a branching tree model of the cerebral vasculature. The cerebral small vascular networks, which often cannot be accurately obtained by medical imaging, were modelled using a branching tree fractal model that accurately simulated the cerebral vasculature geometries and flow. This provided realistic mass flow boundary conditions for the outlet arteries of the circle of Willis. CFD was used to model the three-dimensional transient flow through a simplified and a patient-specific circle of Willis. The patient specific geometry was obtained directly from Computed Tomography (CT) images. A pipe network model was also used to predict the flow through the simplified circle of Willis and the predictions for the flow rates were within 4% of the CFD predictions. The coupled CFD and branching tree model provided useful insight into the variation of the flow through the circle of Willis.

Modeling of Flow Through The Circle of Willis and Cerebral Vasculature to Assess The Effects of Changes In The Peripheral Small Cerebral Vasculature on The Inflows

Abstract The cerebral hemodynamics through the Circle of Willis (CoW) was modeled by coupling a three-dimensional (3D) Computational Fluid Dynamics (CFD) model of the CoW with a one-dimensional (1D) branching tree model of the peripheral cerebral vasculature. CFD was used to model the 3D transient flow through idealized and patient-specific CoW geometries. A 1D pipe network model was also used to predict the flow through the idealized CoW. The coupled model provided useful insight into the variation of the flow through the CoW as a result of possible physiological and pathological changes (for example associated with ‘onset’ of Alzheimer’s Disease) in the peripheral networks of small cerebral vasculature. A comparison was made between the complete CoW and geometric variations with communicating arteries missing. A CoW inflow ratio showed that vasoconstriction of the peripheral vasculature of the posterior cerebral artery predominantly decreases the flow rate in the vertebral arteries, while vasoconstriction of the peripheral vasculature of the anterior and middle cerebral arteries predominantly decreases the flow rate of the internal carotid arteries. These changes in inflow in arteries that are easily monitored may be used to determine if there is vasoconstriction of the peripheral small cerebral vasculature which may indicate diseased states.

Modelling of embolus transport and embolic stroke

Cerebral microembolism may lead to the restriction of blood supply due to damaged blood vessel tissue (focal ischemia) which is increasingly seen as the cause of cognitive deterioration including Alzheimer’s disease and vascular dementia. The flow through fractal models of the peripheral vasculature of the Anterior Cerebral Arteries (ACA) and Middle Cerebral Arteries (MCA) was modelled. The multi-scale model of the cerebral vasculature was coupled with blood flow and embolus transport models. The model incorporated asymmetric bifurcation trees, embolus-vascular interactions and autoregulation. Simulations were carried out where the embolus deposition rate, embolus diameter and embolus introduction rate were varied. Increasing the embolus diameter and embolus introduction rate increased the number of blocked terminal arteries to a quasi steady-state. For a low embolus deposition rate the MCA and ACA territory had the same embolization dynamics, even though, the MCA was larger than the ACA. It was also found for a higher embolus deposition rate the MCA, due to its more expansive structure, was less prone to occlusion than the ACA. The results also showed the effect of a single blockage is expected to be less severe in asymmetric flow than symmetric flow. This model will assist in developing a better understanding into embolic stroke and effect of

Experimental measurement and mathematical modeling of pulsatile forces on a symmetric, bifurcated endoluminal stent graft model.

The objective of this study was to measure the pulsatile forces acting on a symmetric, bifurcated endoluminal stent graft to validate a computational fluid dynamics (CFD) and analytic model so that they can be used for various graft dimensions. We used a load cell to measure the force owing to the movement of an acrylic model of a bifurcated stent graft under pulsatile flow. This was then simulated with a CFD and analytic model. The main features of the experimental pulsatile force data and the CFD results were consistent. The results showed that the total force was proportional to the inlet pressure cycle. The force rose from 3.32 N at 130 mm Hg systolic to 17.5 N at 250 mm Hg systolic pressure. For the more variable regions of the flow, the experimentally measured forces lagged the computational and analytic results. The CFD and analytic models provide approximate descriptions for the forces acting on a bifurcated stent graft subjected to pulsatile flow. Such models should be of assistance to designers of endoluminal stent grafts.

Transport by pulsatile flow in a branching network of cerebral vasculature

The supply of oxygen and glucose by blood flow is vital to the normal function of the brain and the deficit of either of these metabolism elements can cause severe degradation of the brain functionality. The transport of materials in the complex multi-branching structure of the cerebral vasculature is investigated to predict brain oxygenation under normal conditions. A mathematical model of material transport due to pulsatile flow in a complex dichotomous branching tree network was developed which incorporated material-geometry interaction and diffusion across the blood vessel wall. Unlike previous work, this modelling work includes the full network structure and incorporates time-dependent flow. The predicted results indicate some effect of the flow transients on the propagation of the material introduced at the root segment in the vascular network. The effect was more pronounced in the case of constant blood viscosity. The transport model addressed the issue of oxygen transport in the cerebral vascular branching network with the inclusion of red blood cell (RBC) separation at bifurcation points. The predicted results indicate the significance of the vascular network geometry and RBC-bifurcation point interaction in defining the homogeneity of flow and oxygenation by the fractal vasculature. The simulations are found to be able to provide insights into the transport of materials by the blood circulation in the cerebral vasculature and the various factors which

Spinning CNT based composite yarns using a dry spinning process

A process for manufacturing polymer carbon nanotube (CNT) based composite yarns is reported. The aligned CNT structure was combined with a polymer resin and, after being stressed through the spinning process, the resin was cured and polymerized, with the CNT structure acting as reinforcement in the composite yarn. The present method obviates the need of special and complex treatments to align and disperse CNTs in a polymer matrix. The process allows development of polymer/CNT based composite yarns with different mechanical properties suitable for a range of applications by using various resins.