Interfacial electrokinetic transport phenomena and their impact on DNA electrophoresis in microfluidics

The dissertation examines two different options to separate DNA based on differences in size by utilising electric fields. Each of the techniques is based on a new approach and faces several fundamental problems concerning electrokinetics. A microfluidic environment is chosen to experimentally investigate DNA electrophoresis at a small scale. A sophisticated setup is employed that on the one hand enables a multiphase flow, while at the same time it stabilises two immiscible polymer phases in a microfluidic compartment. An aqueous two-phase system consisting of poly(ethylene glycol) and dextran provides a stable liquid-liquid interface under quiescent conditions. Such a setup allows the application of an electric field perpendicular to the liquid-liquid interface. In doing so, DNA accumulates at the interface. The parameters influencing the electrophoretic adsorption process are examined in detail. A highlight of the experimental investigations is desorption of DNA from the interface that is triggered by increasing the electric field strength. The latter phenomenon affords a separation of different sized DNA fragments across the liquid-liquid interface. Smaller DNA fragments desorb at lower field amplitudes while larger ones desorb at larger field strengths. Although liquid-liquid interfacial phenomena in aqueous two-phase systems are complex, a preliminary understanding is achieved addressing basic theoretical issues. In the following the reader is introduced into a second and alternative setup to yield a size separation of DNA. The approach is based on traditional capillary electrophoresis. The novelty is examined by combining several preconcentration techniques with a gel-based size separation of DNA in a preparative manner. The DNA migrates due to the application of an electric field. The preconcentration is accomplished by electrokinetic trapping at a charged membrane embedded into a poly(methyl methacrylate) microchip. It has been found that a fluidic counter flow supports DNA trapping at a membrane. A subsequent DNA size separation is exploited to separate free fetal DNA from maternal DNA in blood of pregnant women providing preliminary results to afford a basis for non-invasive prenatal diagnosis.