Takanobu Yagi

Experimental insights into flow impingement in cerebral aneurysm by stereoscopic particle image velocimetry: transition from a laminar regime.

This study experimentally investigated the instability of flow impingement in a cerebral aneurysm, which was speculated to promote the degradation of aneurysmal wall. A patient-specific, full-scale and elastic-wall replica of cerebral artery was fabricated from transparent silicone rubber. The geometry of the aneurysm corresponded to that found at 9 days before rupture. The flow in a replica was analysed by quantitative flow visualization (stereoscopic particle image velocimetry) in a three-dimensional, high-resolution and time-resolved manner. The mid-systolic and late-diastolic flows with a Reynolds number of 450 and 230 were compared. The temporal and spatial variations of near-wall velocity at flow impingement delineated its inherent instability at a low Reynolds number. Wall shear stress (WSS) at that site exhibited a combination of temporal fluctuation and spatial divergence. The frequency range of fluctuation was found to exceed significantly that of the heart rate. The high-frequency-fluctuating WSS appeared only during mid-systole and disappeared during late diastole. These results suggested that the flow impingement induced a transition from a laminar regime. This study demonstrated that the hydrodynamic instability of shear layer could not be neglected even at a low Reynolds number. No assumption was found to justify treating the aneurysmal haemodynamics as a fully viscous laminar flow.

Optimal kissing balloon inflation after single-stent deployment in a coronary bifurcation model

AIMS To define the optimal kissing balloon inflation (KBI) after single-stent deployment in a coronary bifurcation model. METHODS AND RESULTS We deployed stents in main vessels (MV) followed by KBI in various conditions and compared the stent configurations. A) KBI at the operators discretion vs. under the guidelines of minimal balloon overlapping (MBO). Various stent configurations were observed after the former option, whereas similar maximal dilation points were observed under the MBO guidelines. B) Long balloon overlapping (LBO) vs. MBO with proximal MV dilated by a large balloon. The proximal MV was dilated to an ideal round shape with MBO versus an oval shape with LBO. C) Two-link vs. 3-link stents. Although the 2-link stent was advantageous to open the side branch, it incurred a risk of overdilatation of the proximal struts, whereas the 3-link stent preserved its structure. Computed simulations of coronary flow were analysed in the following left main coronary models: circle with a diameter of 4 and 5.5 mm, ellipse with longitudinal direction and tilt position. They revealed that the overdilated side was exposed to low shear stress regardless of its shape. CONCLUSIONS Optimal KBI can be achieved with MBO and proximal dilatation by an optimally sized balloon.

A Surgical Training Simulator for Quantitative Assessment of the Anastomotic Technique of Coronary Artery Bypass Grafting

Here we describe a personal training simulator that improves manual dexterity required for the anastomotic technique of Coronary Artery Bypass Grafting (CABG). The simulator-based surgical training is termed “DRY Lab”. The system evaluates surgical skill hydrodynamically as well as visually. This report focuses on the effects of regular DRY Lab for an inexperienced surgeon.

Biomedical Engineering Analysis of the Rupture Risk of Cerebral Aneurysms: Flow Comparison of Three Small Pre-ruptured Versus Six Large Unruptured Cases

A relationship between blood flows in cerebral aneurysms and their rupture remains obscure. In clinical practice, the size of aneurysms is one of the important factors for determining a strategy of treatment, but in our database three small aneurysms became ruptured during follow-up. Here, we aim to study their pre-ruptured hemodynamics, and differentiate them with those of six large unruptured aneurysms. All the aneurysms occurred in internal carotid artery, and their mean sizes were 6 and 10.8 mm for pre-ruptured and unruptured cases, respectively. We reproduced their replica as a patient-specific elastic model using clinical images obtained by digital subtraction angiography and a series of rapid prototyping techniques. Flows were reproduced in vitro using a cerebral flow simulator, and visualized by Time-resolved Particle Image Velocimetry. All pre-ruptured cases showed the collision of an incoming flow at a distal neck, and formed a prominent jet stream directing towards the aneurismal head. In contrast, none of unruptured aneurysms had such a marked impingement, and their flows were most likely characterized by swirling patterns. All unruptured cases occurred in a twisted vessel, or carotid siphon, whereas pre-ruptured ones were located downstream whose geometries consisted of a simple curvature. These findings suggest that internal carotid artery has a regional dependency of the risk of aneurismal rupture. Furthermore, the presence of jet streams for smallsized aneurysms may be a substantial indicator of the rupture and immediate treatments.

Controlling Sheer Stress in a Suspension Culture using Couette Flow forEfficient Proliferation of HEK 293 Cells

The suspension culture system is an increasingly popular method of culturing cells not only because of its up scaling ability, but also the non-enzymatic procurement of cells that is crucial for biomedical research, especially in the fields of pharmacology and regenerative medicine. Hypothetically, by controlling and reducing the shear stress applied to cells in a culture system, the higher viability and proliferation rates. In this study, we analyzed HEK 293 cells cultured with a commercially available spinner flask and our newly developed spinner flask which utilizes the theory of Couette flow for controlling shear stress. Fluid analysis and metabolic analysis of the cultured cells were measured at three different rotational speeds, 40, 50 and 60 rpm. It was apparent that 50 rpm was by far the best speed to proliferate the cells. A further viability test was also done in order to validate our hypothesis. Furthermore, by using the metabolic analysis results, it was observed that in the controlled stress system, the consumption of glucose doubled and lactate production was significantly higher compared to cells that were maintained in the conventional suspension method. Thus, Couette flow based suspension culture system will be a major contributor to the future biomedical and pharmacological field.

New challenge for studying flow-induced blood damage: Macroscale modeling and microscale verification

Prosthetic heart valves often induce blood cell trauma, but its mechanism is not clearly understood due to the complexity of dynamical flows. We herein propose a new challenge, termed macroscale modeling and microscale verification. This report is the former, and here we aim to clarify the physical interpretation of Reynolds stress for flowing cells. One polymer and two mechanical valves are compared, and the Reynolds stress is visualized using a novel analytical technique including time-resolved particle image velocimetry and continuous wavelet transform. The method enables to analyze the dynamics of Reynolds stress in a spatial and temporal domain. As a result, it is found that the Reynolds stress should be considered as an indicator of impinging flows for circulating cells. Such flows may impulsively apply a colliding force on a membrane of flowing cells. As microscale verification based on this new hypothesis, we are currently investigating such collision phenomena of flowing cells using high-speed microfluidic techniques.

Microscale Visualization of Erythrocyte Deformation by Colliding with a Rigid Surface Using a High-Speed Impinging Jet

Erythrocyte deformation by colliding with a rigid surface using a high-speed impinging jet was studied with microfluidic techniques. We aim to investigate the relevance of colliding erythrocytes with hemolysis. A micro-channel chip was made of polydimenthyl-siloxane (PDMS), which comprised a T- and Y-shaped junction with a micro-nozzle and diffuser in order to attain a high-speed microflow with a jet velocity of m/s scale. A high-speed camera with a microscope imaged colliding erythrocytes by shadow imaging. Porcine erythrocyte with a hematocrit of 0.5 % in phosphate buffer saline was utilized. At the Y-junction, erythrocytes showed buckling due to an impulsive, longitudinal, compressive deformation. Such anomalous phenomena were not detected at the T-junction, where erythrocytes underwent sequential compressions as approaching the colliding surface. Erythrocyte after buckling showed a hazy membrane while being released from the colliding surface, suggesting the ejection of hemoglobin out of the pore on a membrane. Flow-induced hemolysis has been considered as a model of viscous shear stress and exposure time. From our data, however, it was suggested that hemolysis due to a high-speed impinging flow characterized by mechanical heart valve flows may arise as an impulsive failure of erythrocyte membrane upon collision.

Single-cell real-time imaging of flow-induced hemolysis using high-speed microfluidic technology

Understanding the mechanism of flow-induced blood cell damage, such as hemolysis and platelet activation, plays an important role for arterial diseases and artificial organs. This study for the first time demonstrates the visualization of flow-induced hemolysis in a single-cell real-time manner using high-speed microfluidic technology. Impinging microjets with a velocity of 1.5 m/s order at a nozzle exit were made in the Y- and T-shaped microchannel. The curved (r=10μm) and flat collision surface were compared. Porcine fresh erythrocytes were suspended in PBS at Ht=0.5%. Results showed that membrane failure was only observed in the Y-junction. These erythrocytes were initially elongated at the far region, and then longitudinally compressed in the near wall region due to the sharp adverse pressure gradient, whereas those in the T-junction released the membrane tension as the pressure difference per erythrocyte diminished. In the simulation of energy balance, it was found that the dominant force for the longitudinal compression was the pressure difference per erythrocyte. Such erythrocytes showed the sudden drop of elastic modulus, suggesting that the elongated spectrin network of erythrocyte was fragile for the compressive force and immediately broken by the impact force.

Three-Dimensional Pathological Analysis of Cerebral Aneurysm Initiation

Cerebral aneurysm is known to initiate at the cerebral artery bifurcation. The pathological mechanism of cerebral aneurysm awaits further understanding especially on its initiation. This study sought to elucidate the three-dimensional structure of cerebral vascular bifurcations with and without aneurysms using human cadavers. The two cases had aneurysmal initiations out of total 7 cases. The studied structure was intimal hyperplasia, tunica media and internal elastic lamina, which were recognized by elastica masson staining. The results showed that the non-existence of tunica media and internal elastic lamina was found in the lesion without aneurysm. The non-existence of intimal hyperplasia was only found in the lesion with aneurysm. These data suggest that the formation of intimal hyperplasia may be related with the initiation of aneurysm. We regarded the boundary of existence arteriosclerosis as the position for new arteriosclerosis occurs and thought the direction of new arteriosclerosis grows would influence whether the cerebral aneurysm initiates or not.

Induced Pluripotent Stem Cell Differentiation under Constant Shear Stress

Mechanotransduction in in-vitro studies has yet to be done extensively using human induced pluripotent (hiPS) cells. These external mechanical factors are important controlling factor for cell differentiation at a cellular level particularly in cardiomyocytes cells. Cells in a living body are being exposed to many kind of stresses; shear stress in particular modulates cellular function in a living body. Similarly, we are trying to understand the link between the shear stress in the bioreactor and the differentiation effects it has on iPS cells. In this study, we investigate the difference of strictly uniform laminar shear stress with a non-uniform laminar flow and its effects on the cell survival rate and differentiation. Additionally, the elimination of biochemical factors were done to satisfy the need of demand for clinically usable cells for the near future use. By eliminating cytokine induced differentiation. In addition, information pertaining to relationship between fluid shear stress, cellular deformations, cell differentiation and cell survival rate could provide a more optimised condition for cultivation specific type of cells within a shorter time period.