Effect of Flow on Complex Biological Macromolecules in Microfluidic Devices

Understanding the transport, orientation, and deformation of biological macromolecules by flow is important in designing microfluidic devices. In this study, epi-fluorescence microscopy was used to characterize the behavior of macromolecules in flow in a microfluidic device, particularly how the flow affects the conformation of the molecules. The microfluidic flow path consists of a large, inlet reservoir connected to a long, rectangular channel followed by a large downstream reservoir. The flow contains both regions of high elongation (along the centerline as the fluid converges from the upstream reservoir into the channel) and shear (in the channel near the walls). Solutions of λ-DNA labeled with a fluorescent probe were first characterized rheologically to determine fluid relaxation times, then introduced into the microfluidic device. Images of the DNA conformation in the device were captured through an epi-fluorescent microscope. The conformation of DNA molecules under flow showed tremendous heterogeneity, as observed by Chu [7,12] and co-workers in pure shear and pure elongational flows. Histograms of the distribution of conformations were measured along the channel centerline as a function of axial position and revealed dramatic stretching of the molecules due to the converging flow followed by an eventual return to equilibrium coil size far downstream of the channel entry. The importance of shear was probed via a series of measurements near the channel centerline and near the channel wall. High shear rates near the channel wall also resulted in dramatic stretching of the molecules, and may result in chain scission of the macromolecules.