One of the most important steps in Lab-on-a-Chip systems concerns the purification and fractionation of biological samples containing constituents such as cells or proteins. Sample fractionation is done by exploiting different physico-chemical properties of the constituents such as the electrophoretic mobility or hydrophobicity. In a current project we aim at extending the space of separation criteria by considering the transport of species across a phase boundary between two immiscible liquids. By exploiting the interaction of biomolecules or cells with such interfaces it could become possible to establish new protocols for sample fractionation and purification.

For this purpose we study multilaminated microflows of aqueous two-phase systems and apply an electric field perpendicular to the fluidic interface. The figure below shows an example for the separation of leucocytes and proteins in three different frames – close to the channel inlet (left), halfway along the channel (middle) and close to the exit (right). The proteins are responsible for the diffuse background fluorescence while the cells are clearly discernible as bright spots. The electric field drives the proteins out of the liquid lamella while the cells get immobilized at the phase boundary, as can be seen in the right frame below. Currently, the studies are extended to large DNA molecules. The project is part of the DFG priority program “Nano and Microfluidics ”.