(350e) Microchip Based Hydraulic Pumps for Performing Pressure-Driven Separations

Over the past decade, microfluidic devices actuated using electrokinetic and hydraulic forces have emerged as a powerful tool for performing better and faster liquid phase analyses. While the electrokinetic mode of operation is often preferred in these systems due to its ease of implementation and better controllability, its application in several chemical and bio-chemical assays may be limited by other factors. In an electrokinetically driven liquid chromatographic device, for example, the retention of analyte samples by the stationary phase coating the channel walls may modify the column surface characteristics thereby affecting the electroosmotic (mobile phase) flow rate. As a result, the performance of these units and the reproducibility of the assays are often compromised. To improve upon these aspects, hydraulic forces may be employed as a useful alternative to drive liquid samples in such systems. However, miniaturization of hydraulic pumping units and their efficient integration with a separation system offers a significant challenge.

In this work, we present the integration of an on-chip hydraulic pumping capability with an open-channel liquid chromatographic separation device. Pressure gradients were generated in this design by applying an electric field across two channel sections with oppositely charged surfaces, i.e., negatively charged fused silica and positively charged polyelectrolyte multilayers. The mismatch in the electroosmotic flow rates thus introduced yields a pressure-driven flow in the system, which was then guided to a field-free channel for performing liquid chromatographic separations. The hydrodynamic flow velocity in the field-free analysis channel was further maximized in this unit through appropriate depth profiling of the microchannel network. Pressure-driven velocities of about 8mm/s have been generated in a 5µm deep analysis channel using an applied electric potential of 4kV to implement pressure-driven open-channel chromatography of Coumarin dyes on the integrated device.