Li-Yu Tseng

Investigation of Pulsatile Flowfield in Healthy Thoracic Aorta Models

Cardiovascular disease is the primary cause of morbidity and mortality in the western world. Complex hemodynamics plays a critical role in the development of aortic dissection and atherosclerosis, as well as many other diseases. Since fundamental fluid mechanics are important for the understanding of the blood flow in the cardiovascular circulatory system of the human body aspects, a joint experimental and numerical study was conducted in this study to determine the distributions of wall shear stress and pressure and oscillatory WSS index, and to examine their correlation with the aortic disorders, especially dissection. Experimentally, the Phase-Contrast Magnetic Resonance Imaging (PC-MRI) method was used to acquire the true geometry of a normal human thoracic aorta, which was readily converted into a transparent thoracic aorta model by the rapid prototyping (RP) technique. The thoracic aorta model was then used in the in vitro experiments and computations. Simulations were performed using the computational fluid dynamic (CFD) code ACE+® to determine flow characteristics of the three-dimensional, pulsatile, incompressible, and Newtonian fluid in the thoracic aorta model. The unsteady boundary conditions at the inlet and the outlet of the aortic flow were specified from the measured flowrate and pressure results during in vitro experiments. For the code validation, the predicted axial velocity reasonably agrees with the PC-MRI experimental data in the oblique sagittal plane of the thoracic aorta model. The thorough analyses of the thoracic aorta flow, WSSs, WSS index (OSI), and wall pressures are presented. The predicted locations of the maxima of WSS and the wall pressure can be then correlated with that of the thoracic aorta dissection, and thereby may lead to a useful biological significance. The numerical results also suggest that the effects of low WSS and high OSI tend to cause wall thickening occurred along the inferior wall of the aortic arch and the anterior wall of the brachiocephalic artery, similar implication reported in a number of previous studies.

CFD-Based Optimization of a Diamond-Obstacles Inserted Micromixer with Boundary Protrusions

Abstract: Effective mixing is vitally important to many microfluidic devices with applications in the areas of biotechnical industries, analytic chemistry and medical industries. In practice, passive micromixers are dependent on the proper layout of channel geometric configurations with obstacles deposited in microchannels to break-up and recombine the flow and reduce the diffusion path to improve the mixing performance. This study aims to investigate the mixing behavior of two different fluids in a passive micromixer with protruded boundary structures. The predicted mixing indexes at different longitudinal locations and Reynolds numbers achieved a good approximation of the experimental data in the literature for numerical simulations. The simulation results also indicated that intense vortices and secondary flows in spanwise planes were induced near the boundary protrusion regions to augment mixing efficiency along the flow course. In order to attain the optimal micromixer design, numerical calculations were attempted for the first time to thoroughly examine the effects of shape, length, width and placement of boundary protrusion structures on the mixing efficiency of a diamond-obstacles inserted micromixer.

Investigation of a piezoelectric valveless micropump with an integrated stainless-steel diffuser/nozzle bulge-piece design

To meet a growing need in biological and medical applications, innovative micro-electro-mechanical system (MEMS) technologies have realized important progress on the micropump as one of the essential fluid handling devices to deliver and control precise amounts of fluid flowing along a specific direction. This research proposes a piezoelectric (PZT) valveless micropump adopting an integrated diffuser/nozzle bulge-piece design. The pump mainly consisted of a stainless-steel structured chamber with dimensions of 8 mm in diameter and 70 μm in depth to enhance its long-term reliability, low-cost production, and maximized liquid compatibility. A PZT diaphragm was also used as a driving source to propel the liquid stream under actuation. As commonly used indices to describe pump operation, the delivered volumetric flow rates and pressures were determined at bulge-piece diameters of 2, 4 and 6 mm, with a driving voltage of 160 Vpp and frequency ranging from 50 to 550 Hz. Measurements and simulations have successfully shown that this micropump is capable of operating at a greater volumetric flow rate of up to 1.2 ml min−1 with a maximum back pressure of 5.3 kPa. In addition, the time-recurring flow behavior in the chamber and its relationship to the pumping performance were examined in detail.

An innovative numerical approach to resolve the pulse wave velocity in a healthy thoracic aorta model

Aortic dissection and atherosclerosis are highly fatal diseases. The development of both diseases is closely associated with highly complex haemodynamics. Thus, in predicting the onset of cardiac disease, it is desirable to obtain a detailed understanding of the flowfield characteristics in the human cardiovascular circulatory system. Accordingly, in this study, a numerical model of a normal human thoracic aorta is constructed using the geometry information obtained from a phase-contrast magnetic resonance imaging (PC-MRI) technique. The interaction between the blood flow and the vessel wall dynamics is then investigated using a coupled fluid–structure interaction (FSI) analysis. The simulations focus specifically on the flowfield characteristics and pulse wave velocity (PWV) of the blood flow. Instead of using a conventional PC-MRI method to measure PWV, we present an innovative application of using the FSI approach to numerically resolve PWV for the assessment of wall compliance in a thoracic aorta model. The estimated PWV for a normal thoracic aorta agrees well with the results obtained via PC-MRI measurement. In addition, simulations which consider the FSI effect yield a lower predicted value of the wall shear stress at certain locations in the cardiac cycle than models which assume a rigid vessel wall. Consequently, the model provides a suitable basis for the future development of more sophisticated methods capable of performing the computer-aided analysis of aortic blood flows.

A vacuum-pumped microfluidic device for automated in-line mixing and focusing processes

Microfluidic devices with multi-functional features have been a highly promising tool to implement mixing, reaction, transport, separation, and detection of bio-samples on a solitary microchip for applications in the biochemistry, biophysics and medical fields. This work presents a unified vacuum-pumped microfluidic device consisting of a micromixer module and a microflow cytometer module to complete an automated in-line sequence of mixing and focusing procedures for possible rapid screening of the marker cell detection. For the former module, an in-plane passive micromixer with optimum Tesla structures was adopted to achieve good mixing performance. We also devised a microcytometer as the latter module to attain planar symmetric hydrodynamic focusing in microchannels. Using the coupled mixing and hydrodynamic focusing analysis to predict the suction-driven flow behavior for guiding the chip design, the microfluidic device was fabricated through SU-8 lithography and a PDMS replica molding technique to construct reverse structures from the patterned SU-8 molds on a silicon substrate. Numerical and experimental results have demonstrated the capability of the developed microfluidic device to realize a 93% mixing index and to control the focused stream within a width of 20 μm in tests.

A new swappable fluidic module for applications of capillary convective polymerase chain reaction

A new swappable fluidic module (SFM) is demonstrated in this research. We designed and fabricated two basic modular fluidic components including pumps and mixers, which can be easily swapped and integrated into a variety of modular devices for ready assembly of a fully-portable, disposable fluidic system. An integrated SFM utilizes finger-driven electricity free pumps to transport the fluids. Using the swirling mechanism, the vortex mixer, simultaneously actuated by two pumps, has established a mixing index of 93% in a one-shot mixing event. The assembled SFM, working as a sample loader, was serially jointed a DNA application zone to establish a capillary convective polymerase chain reaction (ccPCR) test kit for performing the experiments of rapid DNA amplification. The successful results indicate the amplified DNA sequences from Hepatitis B Virus (HBV) in 30 min using ccPCR. Besides, the current SFM in conjunction with ccPCR show great potential for many bio-applications.