Development of Microfluidic Chips to Study Shear Flow Effects on Cell Functions
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Flow shear stress has profound effect on cell physiology, including proliferation, migration, transport, and apoptosis. In kidney tubule, the epithelial cells are subject to the urine flow that modulates ion transport and kidney physiology. In vascular system, endothelial cells are constantly exposed to hemodynamic forces that affect cell remodeling and other cell functions. There is increasing interest in studying flow sensory and force transduction mechanisms that convert the mechanical force into biochemical signals, and further alter cell physiological functions and phenotypes. Microfluidic system provides precise control of flow field in microchannels, provides a convenient platform for application of various shear stresses and monitoring the response of living cells. This thesis aims at development of such microfluidic systems capable of applying various flow fields to living cells, and studying the cell response under flow. The thesis consists of two parts. The first part is to develop a novel electrolytic bubble-based sensing mechanism that would enable accurate and sensitive measurement of fluid flow characteristics within the microfluidic channels. The pressure measurement was achieved by generating a gas bubble in the fluid channel and measures the channel resistance across the bubble. By simultaneously generating two gas bubbles along a microfluidic channel, we are able to measure the pressure difference, and thus the flow rate. The fabrication, characterization and calibration of the flow sensor are discussed in detail. The second part is to develop a microfluidic system capable of generating variable magnitudes, gradients, and different modes of shear flow, to study sensory and force transduction mechanisms in cells. Utilizing this microfluidic system, we studied the sensing and shear stress transduction pathways in MDCK cells. The results show that MDCK cells use primary cilia to sense flow shear stress. Flow shear stress induces a transient intracellular calcium increase, causing extracellular calcium uptake followed by a calcium-induced calcium release from intracellular calcium stores. Our results demonstrate that shear stress induced calcium entry is via stretch activated ion channels. This transport may be modified by reorganization of actin cytoskeleton under chronic flow perfusion. In addition, MDCK cells may use different Ca 2+ pathways to promote Ca 2+ entry in response to different mechanical stimuli.