Electric field gradients during electrophoretic fractionation in microfabricated post arrays

Abstract

Electrokinetic separation methods have been used extensively in microfluidic devices due to their separation power and compatibility with miniaturization platform. Electrophoresis (EP) fractionation of DNA and proteins in microfluidic devices has been conducted successfully and incorporated toward developing diagnostic devices. Porous structures as well as microfabricated pillars are often used as separation media for separation of biomolecules. The presence of the solid structures can create electric field gradients resulting in dielectrophoresis (DEP) which most often was neglected during DNA and protein fractionation. In this study, we evaluated the effect of induced field gradients by calculating the magnitude of the three most occurring phenomena during electrophoretic fractionation of biomolecules, i.e., EP, Brownian diffusion, and DC DEP in uniformly patterned post arrays. A mathematical model of convection–diffusion–migration of these charged entities was developed and solved using finite element analysis. Non-dimensional parameters were defined as a measure to predict the particles transport mechanism within the microfabricated post arrays in the presence of field gradients caused by the posts. Based on the properties of particles and the medium, our numerical analysis provided criteria to predict when the DEP can significantly alter the fate of the charged entities through uniform post arrays and therefore cannot be neglected. In order to identify these criteria, a generalized approach based on a systematic parametric study of particle size and zeta potentials was conducted. The criteria obtained have the potential to shed light on some of the challenges such as irreversible trapping and large band broadening during fractionation of charged macromolecules.