Time-dependent arterial wall property is an important but difficult topic in vascular mechanics. Hysteresis, which appears during the measurement of arterial pressure-diameter relationship through a cardiac cycle, has been used to indicate time-dependent mechanics of arteries. However, the cause-effect relationship between viscoelastic properties of the arterial wall and hemodynamics, particularly the viscous contribution to hemodynamics, remains challenging. Herein, we show direct comparisons between elastic (loss/storage < 0.1) and highly viscoelastic (loss/storage > 0.45) conduit structures with arterial-like compliance, in terms of their capability of altering pulsatile flow, wall shear and energy level. Conduits were made from varying ratio of vinyl- and methyl-terminated poly(dimethylsiloxane) and were fit in a mimetic circulatory system measuring volumetric flow, pressure, and strain. Results indicated that when compared to elastic conduits, viscoelastic conduits attenuated lumen distension waveforms, producing an average of 11% greater cross-sectional area throughout a mimetic cardiac cycle. In response to such changes in lumen diameter strain, pressure and volumetric flow waves in viscoelastic conduits decreased by 3.9% and 6%, respectively, in the peak-to-peak amplitude. Importantly, the pulsatile waveforms for both diameter strain and volumetric flow demonstrated greater temporal alignment in viscoelastic conduits due to pulsation attenuation, resulting in 25% decrease in the oscillation of wall shear stress. These changes in flow dynamics, due to the viscous component of viscoelastic conduit, could favor arterial health through beneficial influences on cell physiology.

Under the action of the physiological intraocular pressure the human cornea is stressed and reaches a deformed quasi-spherical configuration. If the deformed configuration is known, and the cornea is regarded as a membrane disregarding flexural behaviors, with the static analysis only is possible to estimate the distribution of the average stress across the thickness. In the cornea, the action of the intraocular pressure is supported by collagen fibrils, immersed into an elastin-proteoglycan matrix and organized in a very precise architecture to provide the necessary confinement and transparency to the light. With the goal of understanding the static consequences of shape modifications due to pathological dilatation (ectasia), we present a simplified stress analysis of the human cornea modelled as a membrane. A numerical investigation over 40 patient specific corneas (20 normal, 20 ectatic) is carried out to establish the relationship between the physiological geometry and distribution of the stresses, and to assess the possibility to obtain information on the stress state based only on topographic images. Comparative analyses reveal that, with respect to the normal corneas, in ectatic corneas the pattern of the principal stress lines is modified markedly showing a deviation from the principal orientation of the collagen fibrils. The rotation of the principal stress with respect to the fibril orientation is responsible of the transmission of a large amount of shear stresses onto the elastin-proteoglycan matrix. The anomalous loading of the matrix could be correlated to the evolution of time dependent shape modifications leading to ectasia.

Atherosclerosis develops at arterial sites where endothelial cells (ECs) are exposed to low time-averaged shear stress, in particular in regions of recirculating disturbed flow. To understand how hemodynamics contributes to EC dysfunction in atheroma development, an in vitro parallel plate flow chamber gasket was modified with protruding baffles to produce large recirculating flow regions. Computational fluid dynamics predicted that more than 60% of the flow surface area was below the 12 dynes/cm2 atheroprotective threshold. Bovine aortic endothelial cells (BAECs) were then seeded in the parallel plate flow chamber with either the standard laminar or the new disturbed flow gasket and exposed to flow for 36 hours. Cell morphology, nitric oxide, proliferation, permeability, and monocyte adhesion were assessed by phase contrast and confocal microscopy. BAEC exposed to 20 dynes/cm2 shear stress in the laminar flow device aligned and elongated in the flow direction while increasing nitric oxide, decreasing permeability, and maintaining low proliferation and monocyte adhesion. As expected, BAEC in the recirculating flow and low shear stress disturbed flow device regions did not elongate or align, produced less nitric oxide, and showed higher proliferation, permeability, and monocyte adhesion than cells in the laminar flow device. Surprisingly, cells in disturbed flow device regions exposed to atheroprotective shear stress did not consistently align or decrease permeability, and these cells demonstrated low nitric oxide levels. These results suggest that atheroprotective signaling may be inhibited by neighboring cells exposed to recirculating flow, highlighting the complex relationship between hemodynamics and atheroma.

The objective of this study was to use image-based computational fluid dynamics (CFD) techniques to analyze the impact that multiple closely spaced intracranial aneurysm (IAs) of the supra-clinoid segment of the internal carotid artery (ICA) have on each other's hemodynamic characteristics. The vascular geometry of fifteen (15) subjects with 2 IAs was gathered using a 3D digital subtraction angiography clinical system. Two groups of computer models were created for each subject's vascular geometry: both IAs present (model A) and after removal of one IA (model B). Models were separated into two groups based on IA separation: tandem (one proximal and one distal) and adjacent (aneurysms directly opposite on a vessel). Simulations using a pulsatile velocity waveform were solved by a commercial CFD solver. Proximal IAs altered flow into distal IAs (5 of 7), increasing flow energy and spatial-temporally averaged wall shear stress (STA-WSS: 3–50% comparing models A to B) while decreasing flow stability within distal IAs. Thus, proximal IAs may “protect” a distal aneurysm from destructive remodeling due to flow stagnation. Among adjacent IAs, the presence of both IAs decreased each other's flow characteristics, lowering WSS (models A to B) and increasing flow stability: all changes statistically significant (p < 0.05). A negative relationship exists between the mean percent change in flow stability in relation to adjacent IA volume and ostium area. Closely spaced IAs impact hemodynamic alterations onto each other concerning flow energy, stressors, and stability. Understanding these alterations (especially after surgical repair of one IA) may help uncover risk factor(s) pertaining to the growth of (remaining) IAs.

The impact of left subclavian artery (LSA) coverage during thoracic endovascular aortic repair (TEVAR) on the circulatory system is not fully understood. Here, we coupled a single-phase non-Newtonian model with fluid–structure interaction (FSI) technique to simulate blood flow in an acute type B aortic dissection. Three-element Windkessel model was implemented to reproduce physiological pressure waves, where a new workflow was designed to determine model parameters with the absence of measured data. Simulations were carried out in three geometric models to demonstrate the consequence of TEVAR with the LSA coverage; case A: pre-TEVAR aorta; case B: post-TEVAR aorta with the disappearance of LSA; case C: post-TEVAR aorta with virtually adding LSA. Results show that the blood flow through the compressed true lumen is only 8.43%, which may lead to ischemia in related organs. After TEVAR, the wall pressure on the stented segment increases and blood flow in the supra-aortic branches and true lumen is improved. Meantime, the average deformation of the aorta is obviously reduced due to the implantation of the stent graft. After virtually adding LSA, significant changes in the distribution of blood flow and two indices based on wall shear stress are observed. Moreover, the movement of residual false lumen becomes stable, which could contribute to patient recovery. Overall, this study quantitatively evaluates the efficacy of TEVAR for acute type B aortic dissection and demonstrates that the coverage of LSA has a considerable impact on the important hemodynamic parameters.

The phasic contractions of collecting lymphatic vessels are reduced in strength and occur at diminished frequency when a favorable pressure difference and the resulting antegrade flow create large fluid shear stresses at the luminal surface. This paper describes a minimal phenomenological model of this mechanism that is applied to a previously validated numerical model of a phasically contracting lymphangion. The parameters of the inhibition model are quantitatively matched to observations in isolated segments of rat lymphatic vessel, first for mesenteric lymphatics then for thoracic duct, and outcomes from the numerical model are then qualitatively compared with recent observations in isolated segments of rat thoracic duct.

Wall shear stress (WSS) has been shown to be associated with myocardial infarction and progression of atherosclerosis. Wall elasticity is an important feature of hemodynamic modeling affecting WSS calculations. The objective of this study was to investigate the role of wall elasticity on WSS, and justify use of either rigid or elastic models in future studies. Digital anatomic models of the aorta and coronaries were created based on coronary computed tomography angiography (CCTA) in four patients. Hemodynamics were computed in rigid and elastic models using a finite element flow solver. WSS in five timepoints in the cardiac cycle and time averaged wall shear stress (TAWSS) were compared between the models at each 3mm subsegment and 4 arcs in cross sections along the centerlines of coronaries. In the left main, proximal left anterior descending, left circumflex and proximal right coronary artery of the elastic model, the mean percent radial increase were 5.95+/-1.25, 4.02+/-0.97, 4.08+/-0.94, 4.84+/-1.05%, respectively. WSS at each timepoint in the cardiac cycle had slightly different values, however when averaged over the cardiac cycle, there were negligible differences between the models. In both the subsegments (n=704) and sub-arc analysis, TAWSS in the two models were highly correlated (r=0.99)In investigation on the effect of coronary wall elasticity on WSS in CCTA-based models, the results of this study show no significant differences in TAWSS justifying using rigid wall models for future larger studies.

An experimental study is reported which investigates the wall shear stress (WSS) distribution in a transparent model of the human aorta comparing a SJM Regent bileaflet mechanical heart valve (BMHV) with the Lapeyre-Triflo FURTIVA trileaflet mechanical heart valve (TMHV) in physiological pulsatile flow. Elastic micro-pillar structures, calibrated as WSS sensors by micro-Particle-Image-Velocimetry measurement, are applied to the wall along the ascending aorta (AAo). The peak WSS values in the BMHV are observed to be almost twice that of the values seen in the TMHV. Flow field analysis illuminates that these peaks are linked to the jet-like flows generated in the valves interacting with the aortic wall. Not only the magnitude, but also the impact regions are specific for different valve designs. The side-orifice jets generated by the BMHV travel along the aortic wall in the AAo, impacting the wall throughout the AAo. Whereas the jets generated by TMHV impact further downstream in the AAo and results in a reduced WSS.

Incomplete stent apposition (ISA) is one of the causes leading to poststent complications, which can be found when an undersized or an underexpanded stent is deployed at lesions. The previous research efforts have focused on ISA in idealized coronary arterial geometry with circular cross section. However, arterial cross section eccentricity plays an important role in both location and severity of ISA. Computational fluid dynamics (CFD) simulations are carried out to systematically study the effects of ISA in arteries with elliptical cross section, as such stents are partially embedded on the minor axis sides of the ellipse and malapposed elsewhere. Overall, ISA leads to high time-averaged wall shear stress (TAWSS) at the proximal end of the stent and low TAWSS at the ISA transition region and the distal end. Shear rate depends on both malapposition distance and blood stream locations, which is found to be significantly higher at the inner stent surface than the outer surface. The proximal high shear rate signifies increasing possibility in platelet activation, when coupled with low TAWSS at the transition and distal regions which may indicate a nidus for in-stent thrombosis.

Cardiovascular diseases (CVDs) are the number one cause of death globally. Arterial endothelial cell (EC) dysfunction plays a key role in many of these CVDs, such as atherosclerosis. Blood flow-induced wall shear stress (WSS), among many other pathophysiological factors, is known to significantly contribute to EC dysfunction. The present study reports an in vitro investigation of the effect of quantified WSS on ECs, analyzing the EC morphometric parameters and cytoskeletal remodeling. The effects of four different flow cases (low steady laminar (LSL), medium steady laminar (MSL), nonzero-mean sinusoidal laminar (NZMSL), and laminar carotid (LCRD) waveforms) on the EC area, perimeter, shape index (SI), angle of orientation, F-actin bundle remodeling, and platelet endothelial cell adhesion molecule-1 (PECAM-1) localization were studied. For the first time, a flow facility was fully quantified for the uniformity of flow over ECs and for WSS determination (as opposed to relying on analytical equations). The SI and angle of orientation were found to be the most flow-sensitive morphometric parameters. A two-dimensional fast Fourier transform (2D FFT) based image processing technique was applied to analyze the F-actin directionality, and an alignment index (AI) was defined accordingly. Also, a significant peripheral loss of PECAM-1 in ECs subjected to atheroprone cases (LSL and NZMSL) with a high cell surface/cytoplasm stain of this protein is reported, which may shed light on of the mechanosensory role of PECAM-1 in mechanotransduction.

The reduction of excessive, nonphysiologic shear stresses leading to blood trauma can be the key to overcome many of the associated complications in blood recirculating devices. In that regard, computational fluid dynamics (CFD) are gaining in importance for the hydraulic and hemocompatibility assessment. Still, direct hemolysis assessments with CFD remain inaccurate and limited to qualitative comparisons rather than quantitative predictions. An underestimated quantity for improved blood damage prediction accuracy is the influence of near-wall mesh resolution on shear stress quantification in regions of complex flows. This study investigated the necessary mesh refinement to quantify shear stress for two selected, meshing sensitive hotspots within a rotary centrifugal blood pump (the blade leading edge and tip clearance gap). The shear stress in these regions is elevated due to presence of stagnation points and the flow around a sharp edge. The nondimensional mesh characteristic number y + , which is known in the context of turbulence modeling, underestimated the maximum wall shear stress by 60% on average with the recommended value of 1, but was found to be exact below 0.1. To evaluate the meshing related error on the numerical hemolysis prediction, three-dimensional simulations of a generic centrifugal pump were performed with mesh sizes from 3 × 10 6 to 30 × 10 6 elements. The respective hemolysis was calculated using an Eulerian scalar transport model. Mesh insensitivity was found below a maximum y + of 0.2 necessitating 18 × 10 6 mesh elements. A meshing related error of up to 25% was found for the coarser meshes. Further investigations need to address: (1) the transferability to other geometries and (2) potential adaptions on blood damage estimation models to allow better quantitative predictions.

A fluid–structure interaction (FSI) model of a left anterior descending (LAD) coronary artery was developed, incorporating transient blood flow, cyclic bending motion of the artery, and myocardial contraction. The three-dimensional (3D) geometry was constructed based on a patient's computed tomography angiography (CTA) data. To simulate disease conditions, a plaque was placed within the LAD to create a 70% stenosis. The bending motion of the blood vessel was prescribed based on the LAD spatial information. The pressure induced by myocardial contraction was applied to the outside of the blood vessel wall. The fluid domain was solved using the Navier–Stokes equations. The arterial wall was defined as a nonlinear elastic, anisotropic, and incompressible material, and the mechanical behavior was described using the modified hyper-elastic Mooney–Rivlin model. The fluid (blood) and solid (vascular wall) domains were fully coupled. The simulation results demonstrated that besides vessel bending/stretching motion, myocardial contraction had a significant effect on local hemodynamics and vascular wall stress/strain distribution. It not only transiently increased blood flow velocity and fluid wall shear stress, but also changed shear stress patterns. The presence of the plaque significantly reduced vascular wall tensile strain. Compared to the coronary artery models developed previously, the current model had improved physiological relevance.

Computational fluid dynamics (CFD) modeling of myocardial bridging (MB) remains challenging due to its dynamic and phasic nature. This study aims to develop a patient-specific CFD model of MB. There were two parts to this study. The first part consisted of developing an in silico model of the left anterior descending (LAD) coronary artery of a patient with MB. In this regard, a moving-boundary CFD algorithm was developed to simulate the patient-specific muscle compression caused by MB. A second simulation was also performed with the bridge artificially removed to determine the hemodynamics in the same vessel in the absence of MB. The second part of the study consisted of hemodynamic analysis of three patients with mild and moderate and severe MB in their LAD by means of the developed in silico model in the first part. The average shear stress in the proximal and bridge segments for model with MB were significantly different from those for model without MB (proximal segment: 0.32 ± 0.14 Pa (with MB) versus 0.97 ± 0.39 Pa (without MB), P < 0.0001 — bridge segment: 2.60 ± 0.94 Pa (with MB) versus 1.50 ± 0.64 Pa (without MB), P < 0.0001). When all three patients were evaluated, increasing the degree of vessel compression shear stress in the proximal segment decreased, whereas the shear stress in the bridge segment increased. The presence of MB resulted in hemodynamic abnormalities in the proximal segment, whereas segments within the bridge exhibited hemodynamic patterns which tend to discourage atheroma development.

Ventricular assist devices (VADs) are implanted in patients with a diseased ventricle to maintain peripheral perfusion as a bridge-to-transplant or as destination therapy. However, some patients with continuous flow VADs (e.g., HeartMate II (HMII)) have experienced gastrointestinal (GI) bleeding, in part caused by the proteolytic cleavage or mechanical destruction of von Willebrand factor (vWF), a clotting glycoprotein. in vitro studies were performed to measure the flow located within the HMII outlet cannula under both steady and physiological conditions using particle image velocimetry (PIV). Under steady flow, a mock flow loop was used with the HMII producing a flow rate of 3.2 L/min. The physiological experiment included a pulsatile pump operated at 105 BPM with a ventricle filling volume of 50 mL and in conjunction with the HMII producing a total flow rate of 5.0 L/min. Velocity fields, Reynolds normal stresses (RNSs), and Reynolds shear stresses (RSSs) were analyzed to quantify the outlet flow's potential contribution to vWF degradation. Under both flow conditions, the HMII generated principal Reynolds stresses that are, at times, orders of magnitude higher than those needed to unfurl vWF, potentially impacting its physiological function. Under steady flow, principal RNSs were calculated to be approximately 500 Pa in the outlet cannula. Elevated Reynolds stresses were observed throughout every phase of the cardiac cycle under physiological flow with principal RNSs approaching 1500 Pa during peak systole. Prolonged exposure to these conditions may lead to acquired von Willebrand syndrome (AvWS), which is accompanied by uncontrollable bleeding episodes.

Accurate cap thickness and stress/strain quantifications are of fundamental importance for vulnerable plaque research. Virtual histology intravascular ultrasound (VH-IVUS) sets cap thickness to zero when cap is under resolution limit and IVUS does not see it. An innovative modeling approach combining IVUS and optical coherence tomography (OCT) is introduced for cap thickness quantification and more accurate cap stress/strain calculations. In vivo IVUS and OCT coronary plaque data were acquired with informed consent obtained. IVUS and OCT images were merged to form the IVUS + OCT data set, with biplane angiography providing three-dimensional (3D) vessel curvature. For components where VH-IVUS set zero cap thickness (i.e., no cap), a cap was added with minimum cap thickness set as 50 and 180 μ m to generate IVUS50 and IVUS180 data sets for model construction, respectively. 3D fluid–structure interaction (FSI) models based on IVUS + OCT, IVUS50, and IVUS180 data sets were constructed to investigate cap thickness impact on stress/strain calculations. Compared to IVUS + OCT, IVUS50 underestimated mean cap thickness (27 slices) by 34.5%, overestimated mean cap stress by 45.8%, (96.4 versus 66.1 kPa). IVUS50 maximum cap stress was 59.2% higher than that from IVUS + OCT model (564.2 versus 354.5 kPa). Differences between IVUS and IVUS + OCT models for cap strain and flow shear stress (FSS) were modest (cap strain <12%; FSS <6%). IVUS + OCT data and models could provide more accurate cap thickness and stress/strain calculations which will serve as basis for further plaque investigations.

Bicuspid aortic valve (BAV) is the most common type of congenital heart disease, occurring in 0.5–2% of the population, where the valve has only two rather than the three normal cusps. Valvular pathologies, such as aortic regurgitation and aortic stenosis, are associated with BAVs, thereby increasing the need for a better understanding of BAV kinematics and geometrical characteristics. The aim of this study is to investigate the influence of the nonfused cusp (NFC) angle in BAV type-1 configuration on the valve's structural and hemodynamic performance. Toward that goal, a parametric fluid–structure interaction (FSI) modeling approach of BAVs is presented. Four FSI models were generated with varying NFC angles between 120 deg and 180 deg. The FSI simulations were based on fully coupled structural and fluid dynamic solvers and corresponded to physiologic values, including the anisotropic hyper-elastic behavior of the tissue. The simulated angles led to different mechanical behavior, such as eccentric jet flow direction with a wider opening shape that was found for the smaller NFC angles, while a narrower opening orifice followed by increased jet flow velocity was observed for the larger NFC angles. Smaller NFC angles led to higher concentrated flow shear stress (FSS) on the NFC during peak systole, while higher maximal principal stresses were found in the raphe region during diastole. The proposed biomechanical models could explain the early failure of BAVs with decreased NFC angles, and suggests that a larger NFC angle is preferable in suture annuloplasty BAV repair surgery.

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

The aim of this study was to investigate and quantify contributions of kinetic energy and viscous dissipation to airway resistance during inspiration and expiration at various flow rates in airway models of different bifurcation angles. We employed symmetric airway models up to the 20th generation with the following five different bifurcation angles at a tracheal flow rate of 20 L/min: 15 deg, 25 deg, 35 deg, 45 deg, and 55 deg. Thus, a total of ten computational fluid dynamics (CFD) simulations for both inspiration and expiration were conducted. Furthermore, we performed additional four simulations with tracheal flow rate values of 10 and 40 L/min for a bifurcation angle of 35 deg to study the effect of flow rate on inspiration and expiration. Using an energy balance equation, we quantified contributions of the pressure drop associated with kinetic energy and viscous dissipation. Kinetic energy was found to be a key variable that explained the differences in airway resistance on inspiration and expiration. The total pressure drop and airway resistance were larger during expiration than inspiration, whereas wall shear stress and viscous dissipation were larger during inspiration than expiration. The dimensional analysis demonstrated that the coefficients of kinetic energy and viscous dissipation were strongly correlated with generation number. In addition, the viscous dissipation coefficient was significantly correlated with bifurcation angle and tracheal flow rate. We performed multiple linear regressions to determine the coefficients of kinetic energy and viscous dissipation, which could be utilized to better estimate the pressure drop in broader ranges of successive bifurcation structures.

We present a new approach to designing three-dimensional, physically realizable porous femoral implants with spatially varying microstructures and effective material properties. We optimize over a simplified design domain to reduce shear stress at the bone-prosthetic interface with a constraint on the bone resorption measured using strain energy. This combination of objective and constraint aims to reduce implant failure and allows a detailed study of the implant designs obtained with a range of microstructure sets and parameters. The microstructure sets are either specified directly or constructed using shape interpolation between a finite number of microstructures optimized for multifunctional characteristics. We demonstrate that designs using varying microstructures outperform designs with a homogeneous microstructure for this femoral implant problem. Further, the choice of microstructure set has an impact on the objective values achieved and on the optimized implant designs. A proof-of-concept metal prototype fabricated via selective laser melting (SLM) demonstrates the manufacturability of designs obtained with our approach.

The feasibility of implementing magnetic struts into drug-eluting stents (DESs) to mitigate the adverse hemodynamics which precipitate stent thrombosis is examined. These adverse hemodynamics include platelet-activating high wall shear stresses (WSS) and endothelial dysfunction-inducing low wall shear stresses. By magnetizing the stent struts, two forces are induced on the surrounding blood: (1) magnetization forces which reorient red blood cells to align with the magnetic field and (2) Lorentz forces which oppose the motion of the conducting fluid. The aim of this study was to investigate whether these forces can be used to locally alter blood flow in a manner that alleviates the thrombogenicity of stented vessels. Two-dimensional steady-state computational fluid dynamics (CFD) simulations were used to numerically model blood flow over a single magnetic drug-eluting stent strut with a square cross section. The effects of magnet orientation and magnetic flux density on the hemodynamics of the stented vessel were elucidated in vessels transporting oxygenated and deoxygenated blood. The simulations are compared in terms of the size of separated flow regions. The results indicate that unrealistically strong magnets would be required to achieve even modest hemodynamic improvements and that the magnetic strut concept is ill-suited to mitigate stent thrombosis.