Achieving a better understanding of cellular interactions with other critical components in physiological microenvironments is an urgent challenge due to the fact that critical cellular behaviors are delicately regulated by the complexity of the biological system. Factors influencing cellular behaviors include interactions with the surrounding cell types and biological molecules, as well as a range of biophysical factors, such as pressure, flow, and chemical gradients. With a better understanding of environmental impacts on cellular behaviors, mechanistic insights on the pathogenesis of diseases and advances in medical treatment will be provided. Because the technical difficulties of traditional cell assays have limited the systematic study of cellular interactions, novel platforms with the ability to represent cellular interactions in a quick, spatiotemporal-resolved, and biomimetic manner would be welcomed by the research community. Microfluidics, one of the novel techniques used frequently for cell biology studies, is able to introduce the cellular interactions into an in vivo-like microenvironment with high spatiotemporal resolution, also allowing the quantification of cellular behaviors at the single cell level. The aim of this thesis is to develop biomimetic microfluidic platforms to mechanistically study cellular interactions in the context of different biological processes. First, Chapter 1 of this thesis reviews the application of microfluidics in the field of cellular interactions with focus on advances of microfluidics in single cell analysis and in vivo-like microenvironment generation. The following chapters separately discuss the topics including cell-drug interactions (Chapter 2), cell migration within complex gradient patterns (Chapter 3), the interactions between iii cell migration and angiogenesis growth (Chapter 4), heterotypic cellular interactions in a biomimetic environment (Chapter 5), and the effects of shear rates on cellular adhesion behaviors (Chapter 6). In Chapter 2, we developed a microfluidic platform containing stable chemical gradients to assess the drug effects on neutrophil migration, which is the key characteristic of inflammatory diseases. By tracking the migration of single neutrophils, we achieved quantification of various parameters, including average velocity, orientation, and overall effectiveness of migration. In addition to examining neutrophil migratory behaviors, the cytotoxicity of drug candidates was also evaluated to reveal a comprehensive understanding about the drug effects on neutrophil function. In Chapter3 and 4, we continued to study neutrophil migration in more complicated in vivo-like microenvironments. To be specific, a three-dimensional endothelial cell layer was cultured in the microfluidic channel, and neutrophil transendothelial migration was monitored under various chemical gradient patterns such that the competitive and synergistic effects among different cytokine molecules were determined. Furthermore, the interactions between neutrophil migration and endothelial angiogenesis were studied by inducing angiogenic growth of the endothelial cell layer in the microfluidic channel. We found that larger endothelial cell angiogenic growth area induced significantly more neutrophil migration while the process of neutrophil migration was able to stabilize the endothelial cell structure even in the presence of an angiogenesis inhibitor that decreases the angiogenic growth of endothelial cells. After detailed evaluation of neutrophil migration in different conditions, a biomimetic microfluidic model was used in Chapter 5 to study heterotypic cellular interactions between endothelial cells and HeLa cancer cells. Three critical environmental factors, including chemical gradients, flow rate, and hypoxia, were separately introduced into the microfluidic model to determine the effect of each factor on cellular interactions. Also, all these three factors were combined together into a single microfluidic device to investigate the overall effects on cellular interactions, which provides an in vitro approach to predict the cellular behaviors in the context of cancer. In the last chapter (Chapter 6), a simple microfluidic system was established to explore the relationship between shear rates and cell adhesion behaviors. Two major blood cell types, platelets and neutrophils, were injected through the endothelial cell covered-microfluidic channels with different dimensions, and the results suggest that the expression of receptor molecules participating in the cell adhesion is selective to the dimension of microfluidic channel. This conclusion reveals the novel insights on the mechanisms of cell adhesion in various shear rate conditions and provides deeper understandings about the pathogenesis of blood-based diseases. Overall, the research presented in this thesis focuses on using microfluidic platforms to characterize cellular interactions with biological complexity, in hopes of advancing our understanding about cellular behaviors in the pathogenesis of relevant diseases. All the findings reported in this thesis indicate that the application of microfluidic platform enables the recapitulation of in vivo physiological microenvironments and predicts the cellular behaviors occurring in human body, successfully bridging the gap between current in vitro and in vivo approaches.
University of Minnesota Ph.D. dissertation.August 2016. Major: Chemistry. Advisor: Christy Haynes. 1 computer file (PDF); xx, 229 pages.
Development of Biomimetic Microfluidic Platforms for Cellular Interaction Studies.
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