Development of viscoelastic particle migration for microfluidic flow cytometry applications

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Bilkent University
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Advances in cell biology, quantification, and identification procedures are essential to develop novel particle characterization tools on the diagnostics, biotechnology, pharmaceutical industry, and material science. Flow cytometry is a pivotal technology and meets the need for almost a century. Increase in today’s demand for fast, precise, accurate, and low-cost point-of-care diagnostic tools and cell counting technologies necessitate further improvements for state-of-the-art flow cytometry platforms. These improvements are achievable using novel and precise particle focusing techniques, multiple detection methods, integrated fluidic, optical, and electronic units in the same workflow. Thanks to its indisputable advantages in such integrities, microfluidic flow cytometry platforms are attractive and promising tool for the future of next-generation flow cytometry technologies. In this thesis, we developed viscoelastic focusing technique compatible with optical, impedimetric, and imaging-based microfluidic flow cytometry methods. Elastic nature of the viscoelastic fluids induces lateral migration for suspended particles into a single streamline and meets the requirement for central particle focusing on flow cytometry devices. Viscoelastic focusing is a passive particle manipulation technique and eliminates the need for sheath flow or any other active actuation mechanism. Firstly, we developed viscoelastic focusing technique for optical microfluidic flow cytometry in a palm-sized glass capillary device. Optical detection was performed by fiber-coupled laser source and photodetectors. We demonstrated the detection of polystyrene (PS) cytometry calibration beads suspended in three viscoelastic solutions: Polyethylene oxide (PEO), Hyaluronic acid (HA), and Polyvinylpyrrolidone (PVP). Secondly, we investigated the viscoelastic focusing efficiency of PEO-based viscoelastic solutions at varying ionic concentrations to demonstrate their use in impedance-based microfluidic flow cytometry. We performed cytometry measurements using PS beads and human red blood cells (RBCs). We showed that elasto-inertial focusing of PS beads is possible with the combination of inertial and viscoelastic effects for high-throughput flow cytometry applications. Additionally, non-spherical shape RBCs were aligned along the channel centerline in parachute shape, which yielded to decrease the non-spherical shape-based signal variations in impedance cytometry devices consistent impedimetric signals. Our results showed that proposed flow cytometry devices give similar performance to state-of-the-art systems in terms of throughput and measurement accuracy. Optical- and impedance-based flow cytometry applications were demonstrated using only pressure-driven flow. Under the simultaneous use of pressure-driven flow and DC electric field, particles inside microfluidic channels exhibit intricate migration behavior at different particle equilibrium positions. Available experimental and analytical studies fall short in giving a thorough explanation to particle equilibrium states. Also, the understanding is so far limited to the results based on Newtonian and neutral viscoelastic fluids.Thirdly in this thesis study, a holistic approach is taken to elaborate the interplay of governing electrophoretic and slip-induced/elastic/shear gradient lift forces. Experimental studies were carried on particle migration in Newtonian, neutral viscoelastic, and polyelectrolyte viscoelastic media to provide a comprehensive understanding of particle migration. Our experiments with the viscoelastic media led to contradictory results with the existing explanations. Then, we introduced the Electro-Viscoelastic Migration (EVM) theory to provide a unifying explanation for particle migration in Newtonian and viscoelastic solutions. Additionally, we performed confocal imaging experiments with fluorescent-labeled polymer solutions to explore the underlying migration behavior in the EVM technique. We observed the formation of cross-sectionally non-uniform viscoelastic solution would pave the way for undiscovered unique applications in the microfluidic community. In summary, presented devices were emonstrated with straightforward fabrication techniques on a single straight microcapillary or microchannel. It is possible to couple fluidics, optical, and impedimetric detection units into the same workflow. Our approach in microfluidic flow cytometry applications proved that viscoelastic fluids are good candidates for the development of integrated, portable, and cost-efficient next-generation cytometry platforms and low resource settings. Additionally, the unifying EVM technique has a strong potential to precisely focusing and separating cells, polyelectrolytes, DNA fractions, and proteins according to their charge and size with a comparable resolution and measurement time as a replacement for gel electrophoresis or chromatography applications.

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Microfluidics, Flow cytometry, Viscoelastic fluids, Particle focusing
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