Entwicklung elektroosmotischer Mikropumpen für Lab-on-Mikrochips

This work focuses on the development and characterization of electroosmotic micropumps for lab-on-chip applications. Micropumps are the most important components of labchips. Mechanical pumps with moving parts are not suitable for planar technology, especially with regard to integration into a simple standardized technology platform. The reason is their large size and complex construction. The electroosmotic effect is the most common one used for nonmechanical pumping of aqueous solutions of a wide range of conductivities and pH-values. Pumps based on this effect are easy to manufacture and offer good potential for integration into cost effective technology platforms.
AC electrokinetic micropumps, a simple version of electroosmotic pumps as known from the literature, consist of an array of asymmetric interdigitated electrodes. They are very easy to fabricate, however, they suffer from a very low working pressure and are therefore not suited for integration on labchips. For this reason this work concentrates on the development of DC electroosmotic micropumps.
The design of DC electroosmotic micropumps with a vertical arrangement of multiple narrow microchannels made of the polymer SU-8 reduces the pump dimensions, makes possible high working pressures at moderate operation voltages and is compatible with post-CMOS processing. A simple analytical model has been developed to estimate the flow velocity in the field free section of the pumping channel.
A significant problem of DC electroosmotic micropumps is water electrolysis at the metal electrodes. The drawback of bubble generation can be reduced by use of gas permeable covers, e.g. covers made of polydimethylsiloxane (PDMS). This design, however, decreases the range of operation voltages to about (4…5) V, and the hydrophobic surface of PDMS constrains the desired self priming of the fluidic channels.
A good solution to cope with bubble generation is the positioning of the metal electrodes outside the main channel in open auxiliary reservoirs. A conductive ionic path between the auxiliary reservoirs and the active pumping region can be established using a gel bridge. Therefore, micropumps using photopolymerized polyacrylamide gel electrodes have been fabricated and tested. The pumping rate of these pumps is bidirectionally linear and reaches 10 nl/min in a 1 cm long pressure-driven channel at an applied voltage of 40 V. This corresponds to a zero-flow pressure of 65 Pa. The working pressure can be further increased by reducing the distance between the vertical ribs of the pumping array.
For the measure of the pump rate a monolithically integrated mass flow sensor based on the thermal anemometric principle has been developed, fabricated and tested. For the first time, a planar programmable on-chip micropump, combining the new electroosmotic narrow channel micropump, the mass flow sensor, and external control circuitry has been operated successfully in a closed control loop for flow rates in the range from zero to up to ±30 nl/min. The programmable micropump may also be used as an active closing valve on labchips.