shape, and including two types of particles sizes; [0025]FIG. 13 is a top view in partial cross-section of an embodiment of a side view of a microfluidic device including a gel filtration material that can be used as a flow restrictor; [0026]FIGS. 14 and 15 are top views in partial cross-section of embodiments of a microfluidic device having baffles to restrict the flow of fluid and cause a pile-