Chia Yuan Chen

Interaction between alk1 and blood flow in the development of arteriovenous malformations.

Arteriovenous malformations (AVMs) are fragile direct connections between arteries and veins that arise during times of active angiogenesis. To understand the etiology of AVMs and the role of blood flow in their development, we analyzed AVM development in zebrafish embryos harboring a mutation in activin receptor-like kinase I (alk1), which encodes a TGFβ family type I receptor implicated in the human vascular disorder hereditary hemorrhagic telangiectasia type 2 (HHT2). Our analyses demonstrate that increases in arterial caliber, which stem in part from increased cell number and in part from decreased cell density, precede AVM development, and that AVMs represent enlargement and stabilization of normally transient arteriovenous connections. Whereas initial increases in endothelial cell number are independent of blood flow, later increases, as well as AVMs, are dependent on flow. Furthermore, we demonstrate that alk1 expression requires blood flow, and despite normal levels of shear stress, some flow-responsive genes are dysregulated in alk1 mutant arterial endothelial cells. Taken together, our results suggest that Alk1 plays a role in transducing hemodynamic forces into a biochemical signal required to limit nascent vessel caliber, and support a novel two-step model for HHT-associated AVM development in which pathological arterial enlargement and consequent altered blood flow precipitate a flow-dependent adaptive response involving retention of normally transient arteriovenous connections, thereby generating AVMs.

Magnetically actuated artificial cilia for optimum mixing performance in microfluidics

Contemporary lab-chip devices require efficient, high-performance mixing capability. A series of artificial cilia with embedded magnetic particles was fabricated to achieve precise flow manipulation through magnetically driven control. These fabricated structures were actuated in a homogeneous magnetic field generated by a built-in magnetic coil system for various beating cycles inside a microchannel. Three representative trajectories, namely, circular motion, back-and-forth oscillation, and a figure-of-eight pattern, of artificial cilia were designed and generated to mimic the motion of actual cilia. Homogeneous mixing of two highly viscous (>25 centipoise) dyed solutions by using the figure-of-eight trajectory achieved a mixing efficiency of approximately 86%. The underlying relationship between ciliated structures and the induced flow fields was further elucidated by performing a hydrodynamic analysis with micro-particle image velocimetry. In addition, a numerical modeling method which used a fluid structure interaction module was applied to provide quantitative 3D illustrations of induced flow patterns, including vortical structures and vortex core locations. The results reveal that both the magnitude and distribution of induced vortices primarily affect the mixing performance of two viscous flow streams. By using magnetically controlled artificial cilia along with the presented analytical paradigms, a new active flow mixing strategy was suggested to efficiently transport/agitate flows for microfluidics and biomedical applications.

Analysis of early embryonic great-vessel microcirculation in zebrafish using high-speed confocal μPIV.

In the developing cardiovascular system, hemodynamic vascular loading is critical for angiogenesis and cardiovascular adaptation. Normal zebrafish embryos with transgenically-labeled endothelial and red blood cells provide an excellent in vivo model for studying the fluid-flow induced vascular loading. To characterize the developmental hemodynamics of early embryonic great-vessel microcirculation in the zebrafish embryo, two complementary studies (experimental and numerical) are presented. Quantitative comparison of the wall shear stress (WSS) at the first aortic arch (AA1) of wild-type zebrafish embryos during two consecutive developmental stages is presented, using time-resolved confocal micro-particle image velocimetry (μPIV). Analysis showed that there was significant WSS difference between 32 and 48 h post-fertilization (hpf) wild-type embryos, which correlates with normal arch morphogenesis. The vascular distensibility of the arch wall at systole and the acceleration/deceleration rates of time-lapse phase-averaged streamwise blood flow curves were also analyzed. To estimate the influence of a novel intermittent red-blood cell (RBC) loading on the endothelium, a numerical two-phase, volume of fluid (VOF) flow model was further developed with realistic in vivo conditions. These studies showed that near-wall effects and cell clustering increased WSS augmentation at a minimum of 15% when the distance of RBC from arch vessel wall was less than 3 μm or when RBC cell-to-cell distance was less than 3 μm. When compared to a smooth wall, the WSS augmentation increased by a factor of ~1.4 due to the roughness of the wall created by the endothelial cell profile. These results quantitatively highlight the contribution of individual RBC flow patterns on endothelial WSS in great-vessel microcirculation and will benefit the quantitative understanding of mechanotransduction in embryonic great vessel biology, including arteriovenous malformations (AVM).

Characterization of neonatal aortic cannula jet flow regimes for improved cardiopulmonary bypass.

During pediatric and neonatal cardiopulmonary bypass (CPB), tiny aortic outflow cannulae (2-3 mm inner diameter), with micro-scale blood-wetting features transport relatively large blood volumes (0.3 to 1.0 L/min) resulting in high blood flow velocities (2 to 5 m/s). These severe flow conditions are likely to complement platelet activation, release pro-inflammatory cytokines, and further result in vascular and blood damage. Hemodynamically efficient aortic outflow cannulae are required to provide high blood volume flow rates at low exit force. In addition, optimal aortic insertion strategies are necessary in order to alleviate hemolytic risk, post-surgical neurological complications and developmental defects, by improving cerebral perfusion in the young patient. The methodology and results presented in this study serve as a baseline for design of superior aortic outflow cannulae. In this study, direct numerical simulation (DNS) computational fluid dynamics (CFD) was employed to delineate baseline hemodynamic performance of jet wakes emanating from microCT scanned state-of-the-art pediatric cannula tips in a cuboidal test rig operating at physiologically relevant laminar and turbulent Reynolds numbers (Re: 650-2150 , steady inflow). Qualitative and quantitative validation of CFD simulated device-specific jet wakes was established using time-resolved flow visualization and particle image velocimetry (PIV). For the standard end-hole cannula tip design, blood damage indices were further numerically assessed in a subject-specific cross-clamped neonatal aorta model for different cannula insertion configurations. Based on these results, a novel diffuser type cannula tip is proposed for improved jet flow-control, decreased blood damage and exit force and increased permissible flow rates. This study also suggests that surgically relevant cannula orientation parameters such as outflow angle and insertion depth may be important for improved hemodynamic performance. The jet flow design paradigm demonstrated in this study represents a philosophical shift towards cannula flow control enabling favorable pressure-drop versus outflow rate characteristics.

Effects of Intraluminal Thrombus on Patient-Specific Abdominal Aortic Aneurysm Hemodynamics via Stereoscopic Particle Image Velocity and Computational Fluid Dynamics Modeling

The pathology of the human abdominal aortic aneurysm (AAA) and its relationship to the later complication of intraluminal thrombus (ILT) formation remains unclear. The hemodynamics in the diseased abdominal aorta are hypothesized to be a key contributor to the formation and growth of ILT. The objective of this investigation is to establish a reliable 3D flow visualization method with corresponding validation tests with high confidence in order to provide insight into the basic hemodynamic features for a better understanding of hemodynamics in AAA pathology and seek potential treatment for AAA diseases. A stereoscopic particle image velocity (PIV) experiment was conducted using transparent patient-specific experimental AAA models (with and without ILT) at three axial planes. Results show that before ILT formation, a 3D vortex was generated in the AAA phantom. This geometry-related vortex was not observed after the formation of ILT, indicating its possible role in the subsequent appearance of ILT in this patient. It may indicate that a longer residence time of recirculated blood flow in the aortic lumen due to this vortex caused sufficient shear-induced platelet activation to develop ILT and maintain uniform flow conditions. Additionally, two computational fluid dynamics (CFD) modeling codes (Fluent and an in-house cardiovascular CFD code) were compared with the two-dimensional, three-component velocity stereoscopic PIV data. Results showed that correlation coefficients of the out-of-plane velocity data between PIV and both CFD methods are greater than 0.85, demonstrating good quantitative agreement. The stereoscopic PIV study can be utilized as test case templates for ongoing efforts in cardiovascular CFD solver development. Likewise, it is envisaged that the patient-specific data may provide a benchmark for further studying hemodynamics of actual AAA, ILT, and their convolution effects under physiological conditions for clinical applications.

Microfluidics Expands the Zebrafish Potentials in Pharmaceutically Relevant Screening

The objective of this study is to enlarge the impact of microfluidics on the pharmaceutical industry by highlighting the reported scientific work on the synergistic relationship between zebrafish and microfluidics, and furthering that effort to shed light on how microfluidics can facilitate the use of zebrafish as a gene screening tool. Zebrafish is ranked the third most important animal model after rats and mice, according to a National Institutes of Health (NIH) announcement in 2003. It has become a staple for scientists to examine and subsequently begin to unravel the mystery of human diseases, and is increasingly used in toxicological studies for new drug development. The unique characteristics that this tiny fish possesses, including rapid growth rate, prodigious numbers of offspring, and eggs that develop outside the body, make it an invaluable genetic tool. Evidently, these advantages can be broadened with the addition of a properly designed microfluidic circuit. By means of the presented illustrations and demonstrated applications, the goal is to spark interest in the development of more novel microfluidic platform designs that can leverage the attributes of zebrafish and quickly come to commercial fruition.

Hemodynamics of the hepatic venous three-vessel confluences using particle image velocimetry.

Despite rapid advancements in the patient-specific hemodynamic analysis of systemic arterial anatomies, limited attention has been given to the characterization of major venous flow components, such as the hepatic venous confluence. A detailed investigation of hepatic flow structures is essential to better understand the origin of characteristic abnormal venous flow patterns observed in patients with cardiovascular venous disease. The present study incorporates transparent rapid-prototype replicas of two pediatric hepatic venous confluence anatomies and two-component particle image velocimetry to investigate the primary flow structures influencing the inferior vena cava outflow. Novel jet flow regimes are reported at physiologically relevant mean venous conditions. The sensitivity of fluid unsteadiness and hydraulic resistance to multiple-inlet flow regimes is documented. Pressure drop measurements, jet flow characterization, and blood damage assessments are also performed. Results indicate that the orientation of the inlets significantly influences the major unsteady flow structures and power loss characteristics of this complex venous flow junction. Compared to out-of-plane arranged inlet vessel configuration, the internal flow field observed in planar inlet configurations was less sensitive to the venous inlet flow split. Under pathological flow conditions, the effective pressure drop increased as much as 77% compared to the healthy flow state. Experimental flow field results presented here can serve as a benchmark case for the surgical optimization of complex anatomical confluences including visceral hemodynamics as well as for the experimental validation of high-resolution computational fluid dynamics solvers applied to anatomical confluences with multiple inlets and outlets.

Microscale flow propulsion through bioinspired and magnetically actuated artificial cilia

Recent advances in microscale flow propulsion through bioinspired artificial cilia provide a promising alternative for lab-on-a-chip applications. However, the ability of actuating artificial cilia to achieve a time-dependent local flow control with high accuracy together with the elegance of full integration into the biocompatible microfluidic platforms remains remote. Driven by this motive, the current work has constructed a series of artificial cilia inside a microchannel to facilitate the time-dependent flow propulsion through artificial cilia actuation with high-speed (>40 Hz) circular beating behavior. The generated flow was quantified using micro-particle image velocimetry and particle tracking with instantaneous net flow velocity of up to 10(1 ) μm/s. Induced flow patterns caused by the tilted conical motion of artificial cilia constitutes efficient fluid propulsion at microscale. This flow phenomenon was further measured and illustrated by examining the induced flow behavior across the depth of the microchannel to provide a global view of the underlying flow propulsion mechanism. The presented analytic paradigms and substantial flow evidence present novel insights into the area of flow manipulation at microscale.

Manipulation of zebrafish's orientation using artificial cilia in a microchannel with actively adaptive wall design

The zebrafish is a powerful genetic model organism especially in the biomedical chapter for new drug discovery and development. The genetic toolbox which this vertebrate possesses opens a new window to investigate the etiology of human diseases with a high degree genetic similarity. Still, the requirements of laborious and time-consuming of contemporary zebrafish processing assays limit the procedure in carrying out such genetic screen at high throughput. Here, a zebrafish control scheme was initiated which includes the design and validation of a microfluidic platform to significantly increase the throughput and performance of zebrafish larvae manipulation using the concept of artificial cilia actuation. A moving wall design was integrated into this microfluidic platform first time in literature to accommodate zebrafish inside the microchannel from 1 day post-fertilization (dpf) to 6 dpf and can be further extended to 9 dpf for axial orientation control in a rotational range between 0 to 25 degrees at the minimum step of 2-degree increment in a stepwise manner. This moving wall feature was performed through the deflection of shape memory alloy wire embedded inside the microchannel controlled by the electrical waveforms with high accuracy.

Simultaneous real-time quantification of blood flow and vascular growth in the chick embryo using optical coherence tomography

Experimental and clinical data indicate that hemodynamic forces within the embryo provide critical biomechanical cues for cardiovascular morphogenesis, growth, and remodeling and that perturbed flow is a major etiology of congenital heart disease. However, embryonic flow-growth relationships are largely qualitative and poorly defined. In this work, we provide a quantitative analysis of in vivo flow and growth trends in the chick embryo using optical coherence tomography (OCT) to acquire simultaneous velocity and structural data of the right vitelline artery continuously over a ten hour period beginning at stage 16 (hour 54). We obtained 3D vessel volumes (15 μm lateral, 4.3 μm axial resolutions, 6 μm slice spacing) at 60 minute intervals, taking a B-scan time series totaling one cardiac cycle at each slice. Embryos were maintained at a constant 37°C and 60% humidity during the entire acquisition period through an inhouse built chamber. The 3D vessel lumen geometries were reconstructed manually to assess growth. Blood flow velocity was computed from the central B-scan using red blood cell particle image velocimetry. The use of extended OCT imaging as a non-invasive method for continuous and simultaneous flow and structural data can enhance our understanding of the biomechanical regulation of critical events in morphogenesis. Data acquired will be useful to validate predictive finite-element 3D growth models.