Hyung Kyu Huh

Particle migration and single-line particle focusing in microscale pipe flow of viscoelastic fluids

The elasto-migration of microparticles across a streamline induced by elasticity and inertia in microscale pipe flow of viscoelastic fluids was investigated using a holographic technique. The effects of blockage ratio (d/D), flow rate (Q), and entry length (L) on particle migration induced by fluid elasticity were evaluated. Single-line particle focusing was demonstrated for an elasticity-dominant fluid. Furthermore, shear-thinning effect on particle migration was evaluated. Finally, we propose the focusing number, a non-dimensional parameter, to estimate the focusing state of particles in elasticity-dominant fluids. This criterion can be used to accurately estimate the design parameters, such as rheological properties, channel length, and particle diameter, for microfluidic devices using elasto-migration.

Turbulent Kinetic Energy Measurement Using Phase Contrast MRI for Estimating the Post-Stenotic Pressure Drop: In Vitro Validation and Clinical Application

Background Although the measurement of turbulence kinetic energy (TKE) by using magnetic resonance imaging (MRI) has been introduced as an alternative index for quantifying energy loss through the cardiac valve, experimental verification and clinical application of this parameter are still required. Objectives The goal of this study is to verify MRI measurements of TKE by using a phantom stenosis with particle image velocimetry (PIV) as the reference standard. In addition, the feasibility of measuring TKE with MRI is explored. Methods MRI measurements of TKE through a phantom stenosis was performed by using clinical 3T MRI scanner. The MRI measurements were verified experimentally by using PIV as the reference standard. In vivo application of MRI-driven TKE was explored in seven patients with aortic valve disease and one healthy volunteer. Transvalvular gradients measured by MRI and echocardiography were compared. Results MRI and PIV measurements of TKE are consistent for turbulent flow (0.666 < R2 < 0.738) with a mean difference of −11.13 J/m3 (SD = 4.34 J/m3). Results of MRI and PIV measurements differ by 2.76 ± 0.82 cm/s (velocity) and −11.13 ± 4.34 J/m3 (TKE) for turbulent flow (Re > 400). The turbulence pressure drop correlates strongly with total TKE (R2 = 0.986). However, in vivo measurements of TKE are not consistent with the transvalvular pressure gradient estimated by echocardiography. Conclusions These results suggest that TKE measurement via MRI may provide a potential benefit as an energy-loss index to characterize blood flow through the aortic valve. However, further clinical studies are necessary to reach definitive conclusions regarding this technique.

Energy dissipation of graphene colloidal suspension droplets impacting on solid substrates

The impact and spreading behavior of Newtonian-fluid droplets containing 0.1 mg ml−1 of graphene nanoplatelets (GNP) are quantitatively studied using a high-speed imaging system. Impact regimes for different-sized (micrometer and millimeter) droplets are examined experimentally, and the effects of impact velocity and surface wettability on the impact regime are investigated. In addition, the maximum spreading factor of colloidal suspension droplets is compared with that of droplets without particle immanence and the additional energy dissipation by particle motion in a micrometer-scale droplet is observed. The motion of particles suppresses the spreading of the droplet, such that the maximum spreading factor is reduced by approximately 2% to 5%. Meanwhile, no significant effect of particle motion was observed for millimeter-scale droplets because of different impact regimes. The maximum spreading factor estimated for the GNP suspension droplets is well-matched with a theoretical prediction model of Newtonian-fluid droplets.

Flow characteristics around proximal and distal stenoses in a series of tandem stenosed vessels

The flow characteristics around the proximal and distal stenoses in tandem vessel models are experimentally investigated with varying flow rates (Q=0.25, 0.5, 1.0L/min), interspacing distances (L=3, 6, 10 of diameter D) and severities (S=50%, 75% reduction in diameter). When the interspacing L is larger than 10 D, no fluid-dynamic interaction is observed. The flow between the proximal and distal stenoses becomes stabilized (turbulence intensity of <3%) as the interspacing distance decreases. When the severity S is 75%, the transition from laminar to turbulent flow occurs at a flow rate higher than 0.5L/min, although the interspacing distance L is 3 D. Formation of recirculation flow is restricted by the presence of distal stenosis as the interspacing distance decreases. In this case, the flow between the stenoses is focused on the central region. The center-line velocity at the neck of the distal stenosis is approximately 10-15% higher than that of the proximal stenosis with equal severity of S=50%. When the inlet flow is center-focused, the lengths of the recirculation and the jet core behind the distal stenosis increase with decrease in interspacing distance L. When the inlet flow is turbulent, the transition from laminar to turbulent flow occurs early as the interspacing distance L is reduced. When the upstream proximal stenosis exhibits increased severity, the pressure drop is measured to be 20% compared with that when the severity of the downstream distal stenosis is increased at the flow rate of Q=1.0L/min.

Post-stenotic Recirculating Flow May Cause Hemodynamic Perforator Infarction

Background and Purpose The primary mechanism underlying paramedian pontine infarction (PPI) is atheroma obliterating the perforators. Here, we encountered a patient with PPI in the post-stenotic area of basilar artery (BA) without a plaque, shown by high-resolution magnetic resonance imaging (HR-MRI). We performed an experiment using a 3D-printed BA model and a particle image velocimetry (PIV) to explore the hemodynamic property of the post-stenotic area and the mechanism of PPI. Methods 3D-model of a BA stenosis was reconstructed with silicone compound using a 3D-printer based on the source image of HR-MRI. Working fluid seeded with fluorescence particles was used and the velocity of those particles was measured horizontally and vertically. Furthermore, microtubules were inserted into the posterior aspect of the model to measure the flow rates of perforators (pre-and post-stenotic areas). The flow rates were compared between the microtubules. Results A recirculating flow was observed from the post-stenotic area in both directions forming a spiral shape. The velocity of the flow in these regions of recirculation was about one-tenth that of the flow in other regions. The location of recirculating flow well corresponded with the area with low-signal intensity at the time-of-flight magnetic resonance angiography and the location of PPI. Finally, the flow rate through the microtubule inserted into the post-stenotic area was significantly decreased comparing to others (P<0.001). Conclusions Perforator infarction may be caused by a hemodynamic mechanism altered by stenosis that induces a recirculation flow. 3D-printed modeling and PIV are helpful understanding the hemodynamics of intracranial stenosis.

Homocysteine-induced peripheral microcirculation dysfunction in zebrafish and its attenuation by L-arginine

Elevated blood homocysteine (Hcy) level is frequently observed in aged individuals and those with age-related vascular diseases. However, its effect on peripheral microcirculation is still not fully understood. Using in vivo zebrafish model, the degree of Hcy-induced peripheral microcirculation dysfunction is assessed in this study with a proposed dimensionless velocity parameter V¯CV/ V¯PCV, where V¯CV and V¯PCV represent the peripheral microcirculation perfusion and the systemic perfusion levels, respectively. The ratio of the peripheral microcirculation perfusion to the systemic perfusion is largely decreased due to peripheral accumulation of neutrophils, while the systemic perfusion is relatively preserved by increased blood supply from subintestinal vein. Pretreatment with L-arginine attenuates the effects of Hcy on peripheral microcirculation and reduces the peripheral accumulation of neutrophils. Given its convenience, high reproducibility of the observation site, non-invasiveness, and the ease of drug treatment, the present zebrafish model with the proposed parameters will be used as a useful drug screening platform for investigating the pathophysiology of Hcy-induced microvascular diseases.Elevated blood homocysteine (Hcy) level is frequently observed in aged individuals and those with age-related vascular diseases. However, its effect on peripheral microcirculation is still not fully understood. Using in vivo zebrafish model, the degree of Hcy-induced peripheral microcirculation dysfunction is assessed in this study with a proposed dimensionless velocity parameter V¯CV/V¯PCV, where V¯CV and V¯PCV represent the peripheral microcirculation perfusion and the systemic perfusion levels, respectively. The ratio of the peripheral microcirculation perfusion to the systemic perfusion is largely decreased due to peripheral accumulation of neutrophils, while the systemic perfusion is relatively preserved by increased blood supply from subintestinal vein. Pretreatment with L-arginine attenuates the effects of Hcy on peripheral microcirculation and reduces the peripheral accumulation of neutrophils. Given its convenience, high reproducibility of the observation site, non-invasiveness, and the ease of drug treatment, the present zebrafish model with the proposed parameters will be used as a useful drug screening platform for investigating the pathophysiology of Hcy-induced microvascular diseases.

Hemodynamic characteristics of flow around a deformable stenosis

Clinical studies reported that some vulnerable stenoses deformed their shape in a blood vessel based on flow condition. However, the effects of shape variation on flow characteristics remain unclear. The flow characteristics are known to affect vulnerable stenosis rupture and fractional flow reserve (FFR) value which has been widely used as a diagnostic tool for stenosis. Vulnerable stenosis rupture occurs when the structural stress exerted on a fibrous cap exceeds its tolerable threshold. The stress magnitude is determined from the spatial distribution of static pressure around the stenosis. In the present study, the static pressure distribution and the FFR value in deformable stenosis were investigated with related other flow characteristics. Two phantom models were fabricated to mimic deformable and nondeformable stenoses using polydimethylsiloxane. The flow characteristics were observed under a steady-flow condition at three Reynolds numbers (Re=500, 1000, 1500) using a particle image velocimetry. The pressure drop across the stenosis models were measured using a pressure sensor to determine effects of shape deformation on FFR value. Shape variations and jet deflections were clearly observed in the deformable stenosis model, and the effective severity of the stenosis increased up to 17.2%. The shape variations of deformable stenosis model increased the static pressure difference at the upstream and downstream sides of the stenosis. The pressure drop across the deformable stenosis model was significantly higher than that of the nondeformable stenosis model. The present results substantiate that stenosis deformability should be carefully considered to diagnose the rupture of vulnerable stenosis.

Structural design of a double-layered porous hydrogel for effective mass transport

Mass transport in porous materials is universal in nature, and its worth attracts great attention in many engineering applications. Plant leaves, which work as natural hydraulic pumps for water uptake, have evolved to have the morphological structure for fast water transport to compensate large water loss by leaf transpiration. In this study, we tried to deduce the advantageous structural features of plant leaves for practical applications. Inspired by the tissue organization of the hydraulic pathways in plant leaves, analogous double-layered porous models were fabricated using agarose hydrogel. Solute transport through the hydrogel models with different thickness ratios of the two layers was experimentally observed. In addition, numerical simulation and theoretical analysis were carried out with varying porosity and thickness ratio to investigate the effect of structural factors on mass transport ability. A simple parametric study was also conducted to examine unveiled relations between structural factors. As a result, the porosity and thickness ratio of the two layers are found to govern the mass transport ability in double-layered porous materials. The hydrogel models with widely dispersed pores at a fixed porosity, i.e., close to a homogeneously porous structure, are mostly turned out to exhibit fast mass transport. The present results would provide a new framework for fundamental design of various porous structures for effective mass transport.

Effect of pannus formation on the prosthetic heart valve: In vitro demonstration using particle image velocimetry

Although hemodynamic influence of the subprosthetic tissue, termed as pannus, may contribute to prosthetic aortic valve dysfunction, the relationship between pannus extent and hemodynamics in the prosthetic valve has rarely been reported. We investigated the fluid dynamics of pannus formation using in vitro experiments with particle image velocimetry. Subvalvular pannus formation caused substantial changes in prosthetic valve transvalvular peak velocity, transvalvular pressure gradient (TPG) and opening angle. Maximum flow velocity and corresponding TPG were mostly affected by pannus width. When the pannus width was 25% of the valve diameter, pannus formation elevated TPG to >2.5 times higher than that without pannus formation. Opening dysfunction was observed only for a pannus involvement angle of 360°. Although circumferential pannus with an involvement angle of 360° decreased the opening angle of the valve from approximately 82° to 58°, eccentric pannus with an involvement angle of 180° did not induce valve opening dysfunction. The pannus involvement angle largely influenced the velocity flow field at the aortic sinus and corresponding hemodynamic indices, including wall shear stress, principal shear stress and viscous energy loss distributions. Substantial discrepancy between the velocity-based TPG estimation and direct pressure measurements was observed for prosthetic valve flow with pannus formation.

Endothelial cell monolayer-based microfluidic systems mimicking complex in vivo microenvironments for the study of leukocyte dynamics in inflamed blood vessels

In response to inflammatory signals, leukocytes circulating in blood vessels undergo dynamic interaction with endothelium lining blood vessel walls, known as leukocyte adhesion cascade, to leave the blood vessels, and infiltrate the inflamed tissue. During leukocyte extravasation, leukocytes recognize and respond to various biophysical and biochemical cues present in the complex microenvironments of inflamed blood vessels to find optimal pathways. Although advances in intravital imaging of live animals have enabled us to observe leukocyte dynamics during extravasation, in vitro model systems mimicking complex in vivo microenvironments are still needed for mechanistic studies. A parallel-plate flow chamber assembled by placing a fluidic chamber on an endothelial cell (EC) monolayer has been widely used as an in vitro model to study leukocyte dynamics in inflamed blood vessels. Although this is a simple yet powerful model providing well-defined flow conditions, a parallel-plate flow chamber lacks the complex microenvironments of inflamed blood vessels. In this article, we first describe the basic design, assembly, and operation principles of a parallel-plate flow chamber. Then, we present methods of incorporating various features of in vivo microenvironments into parallel-plate flow chambers, including EC alignment using nanogrooved surfaces, insertion of a stenotic structure for complex flow generation, and extension to 3D blood vessel/inflamed tissue models.