Indirect Electro-Osmotic Pumping

Abstract

The manipulation of liquids within a microcapillary network remains a considerable challenge in the development of miniaturized total chemical analysis systems (μTAS). Fluid manipulation can be achieved using (micro) mechanical pumps connected or integrated into the device, and by using an electric field (E) for generation of electro-osmotic flow (EOF). For glass microdevices, electro-osmotic pumping (EOP) is most attractive, since no moving parts and/or valves are required. In its simplest embodiment, EOP in microfluidic devices involves imposing an E along the full length of the channel by immersing electrodes into open solution reservoirs situated at both ends of the channel. Electrolytically generated gases at the electrodes drift to the surface of the solution reservoirs and escape into the air. In more complex situations, however, EOP in a subsection of a microchannel may be required. For sampling, for example, from brain tissue in living organisms, the presence of electrodes in the ‘sample reservoir’ (i.e., the brain), and thus outside the microdevice is undesirable, since potentials applied to external electrodes interfere with the sampling environment. In these cases, electrodes need to be integrated into the microfluidic device. The use of electrodes in a microchannel, however, is not trivial. Electrolytic gases get caught in the sealed microchannel and hence effectively interrupt the electric field, and thus fluid movement. A number of approaches to avoid bubble formation during spatially localized application of voltages in microfluidic networks have been reported. In one example, a 1-mm-thick poly(dimethylsiloxane) (PDMS) substrate containing the microchannel was sealed with a glass cover plate containing the electrodes.1 Electrolytic gases formed at the electrodes dissipated through the highly gas-permeable PDMS film into the air. An alternative method for application of the electric field is the use of a conducting barrier between the electrodes and the channel. A Nafion membrane has been presented as an interface between an open reservoir containing the electrode and a microchannel.2 Electrolytic gases dissipate into the air via the open reservoir, while the electrical contact afforded by the membrane ensured that an E was applied to the closed microchannel. A similar approach involves the use of adjacent side channels, which are electrically connected, via porous barriers, but where fluid exchange is strongly limited.3,4 Either the porous membrane was formed using a thin layer of potassium silicate, in or the contact was directly over the glass wall separating adjacent channels. The three approaches mentioned above allow the creation of field-free zones in addition to regions where the field is applied. In the field-free regions, charge-independent fluid transport can be controlled by EOP elsewhere in the microfluidic system, an effect we term “electro-osmotic indirect pumping” (EOIP) to distinguish between EOP in- and outside the electric field. In this paper, a glass microdevice for both EOP and EOIP using electrically connected side channels is presented. Electrical contact between the main and side channels is achieved by electrical breakdown of the glass barrier between these channels. Electrical breakdown for initiating liquid contact between disconnected channels has been demonstrated in PDMS devices.5 To our knowledge, this is the first time that electrical breakdown for initiation of electrical contact between glass microchannels is presented. Cross injection by a combination of EOP and EOIP is demonstrated.