A Novel Lab-on-chip System for Counting Particles/Cells Based on Electrokinetically-induced Pressure-driven Flow and Dual-wavelength Fluorescent Detection
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For the past two decades, flow cytometry has been widely used as a powerful analysis tool for the diagnosis of many diseases due to its ability to count, characterize and sort cells. However, conventional flow cytometers are often bulky, expensive and complicated because sophisticated fluidic, electronic and optical systems are required to realize the functions of flow cytometry. The high cost and the complexity in operation and maintenance associated with flow cytometers as well as the large size have limited its use. In recent years, the rapid development of microfluidics-based lab-on-a-chip technology has created a new pathway for flow cytometry. Microfluidic devices allow for the integration of multiple liquid handling processes required in the diagnostic assays, such as pumping, metering, sampling, dispensing, sequential loading and washing. These lab-on-a-chip solutions have been recognized as an opportunity to bring portable, accurate and sensitive diagnostic tests to the flow cytometry. However, most current microfluidic flow cytometry devices are micro- only in the microfluidic chip, the rest of most apparatuses are still large and costly, usually involving tubes, microscopes, lasers and mechanical pumps. Therefore, the objective of this study is to develop a novel lab-on-a-chip system based on the electrokinetically-induced pressure-driven flow and dual-wavelength fluorescent detection, which lights a promising pathway for making a real portable, compact, low-cost microfluidic flow cytometry device. In this study, the core of this microfluidic system is the custom-designed PDMS (polydimethylsiloxane) microchip. A novel method was applied to generate the electrokinetically-induced pressure-driven flow in a T-shaped microchannel using parameters settings that had been optimized by numerical study. This method combined both the electrokinetic pumping force and the pressure pumping force to eliminate their shortcomings associated with the use of each force alone. This is the fundamental of my study. By using this microchip, the size of the fluidic control subsystem is reduced significantly. Furthermore, the dual-wavelength fluorescent detection strategy is proposed in this thesis. On the optical detection side, excitation lights of two different wavelengths are provided by a single LED (light-emitting diode) from one side of the microchannel. Then the two emission lights are captured individually by two photo-detectors placed on the top and the bottom of the microchip. Compared with other microfluidic detection devices reported in the literatures that use lasers or PMTs (Photomultiplier tubes), this design allows for a significant reduction of 90% in the volume and cost. As another important part of my thesis research, a novel flow focusing method that allows the hydrodynamic focusing in a T-shaped microchannel with two sheath flows is developed. This method solves the biggest obstacle which exists in current microfluidic flow cytometry devices. In this method, no external pumps, valves and tubing are involved in the system. Although substantial progress has been made in current microfluidic flow cytometry, there is still a need for a low-cost, compact, portable microfluidic devices, especially in low-resource settings as well as the developing world for POC (point-of-care) diagnosis and analysis. This thesis work has made a great achievement towards the final goal.