Dynamic Behavior of Healthy and Malaria

Hereditary hematological disorders and foreign organisms often introduce changes to the spectrin molecular network and membrane of human red blood cells (RBCs). These structural changes lead to altered cell shape, deformability, cytoadherence and rheology which may in turn, promote the onset of vaso-occlusive events and crises that may ultimately cause pain, stroke, organ damage and possibly death. Previous work by our group and others has shown that the RBC membrane exhibits reduced deformability as a manifestation of diseases such as malaria, spherocytosis, elliptocytosis and sickle cell anemia. However, much of this previous work has modeled the RBC membrane as a purely elastic material and experiments are typically performed within the quasistatic deformation regime. This work investigates the connection between disease, structure and function in a more physiologically relevant, dynamic context using two in-vitro experimental approaches: (1) dynamic force-displacement characterizations using advanced optical trapping techniques and (2) microfluidic flow experiments. A new set of dynamic optical trapping experiments are developed using an alternate loading configuration and a broader range of deformation rates (up to 100ptm/s) and forcing frequencies (up to 100Hz) than previously reported with optical trapping systems. Results from these experiments provide further support to recent suggestions that traditional constitutive descriptions of the viscoelastic behavior of the RBC membrane are not applicable to this wide range of deformation rates and frequencies. Initial results on RBCs infected with Plasmodium falciparum malaria suggest that the parasite and its related exported proteins act to increase the effective viscosity of the RBC membrane. The role of the temperature-dependent, viscous behavior of the RBC membrane is further explored in microfluidic flow experiments, where the flow behavior of RBCs is quantified in fluidic structures with length scales approaching the smallest relevant dimensions of the microvasculature (approximately 3pm in characteristic diameter). In particular, the role of a parasitic protein, the ring infected erythrocyte surface antigen (RESA), is investigated and determined to have a rate-dependent effect on microvascular flow behavior that has not previously been identified. Results from optical trapping and microfluidic flow experiments are used to inform and validate a collaborative effort aimed at developing a meso-scale, threedimensional model of microvascular flow using dissipative particle dynamics (DPD). This combination of modeling and experiments give new insight into the relative roles of fluid and membrane viscosity in microvascular flow. The results of this work may be used in the development of new constitutive behaviors to describe the deformation of the RBC membrane and to inform the design and optimization of microfluidic tools for blood separation and point-of-care diagnostic platforms. In addition, using the techniques developed here in further investigation of the roles of particular parasitic proteins may yield additional insight into the pathology of P.f. malaria that may, in turn, provide new avenues and approaches for treatment. Thesis Supervisor: Subra Suresh Title: Vannevar Bush Professor of Engineering Professor of Mechanical Engineering Chairman of the Committee