Diffusion coefficients of species in complex fluids are dependent on the dynamic condition under which the diffusion will occur. It has been observed that the apparent diffusion coefficients of bovine serum albumin and urea in a laminar flow of blood are a function of both red blood cell concentration and shear rate. When subjected to shear, red blood cells rotate, carrying fluid next to their surfaces with them. Thus, transport of other species is affected by the cells' rotation. Diffusion coefficients are measured in a microfluidic system in which three congruent streams flow under laminar conditions. The central stream contains the molecular species whose diffusion is being studied and cells at various concentrations (hematocrits). The side stream may or may not contain cells and does not contain the molecule of interest.. Cell-free side streams emulate a blood washing scheme under study, while cell-containing side streams enable the study of solute diffusion within uniform cell concentration and shear fields. The microfluidic flow channel is formed in a block of stainless steel and has an overall channel height of 300 μm and an aspect ratio of approximately 15. Syringe pumps are used to meter the flows while final concentrations are measured in the exit streams with spectrophotometric techniques. The novelties of incorporating microfluidic techniques include: 1) The opportunity to simulate a wide range of shear flows; 2) the ability to introduce sharp gradients in solute concentrations in middle parts of a flowing stream; 3) do rapid measurments consequent to the short diffusion times experienced in the small distances that characterize the microfluidic channel. Given that the ultimate purpose of this research group is to develop a new artificial kidney that will replace current end stage renal disease (ESRD) treatments, the impact of hematocrit in the transport of different species becomes an important parameter in the final design.

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