The principles of electroosmosis (EO), a process whereby applying a voltage to a fluid in a small space, such as a capillary channel, causes the fluid to flow, has been known for a long time. The surfaces of many solids, including quartz, glass and the like, become variously charged, negatively or positively, in the presence of ionic materials, such as for example salts, acids or bases. The charged surfaces will attract oppositely charged (positive or negative) counterions in aqueous solutions. The application of a voltage to such a solution results in a migration of the counterions to the oppositely charged electrode, and moves the bulk of the fluid as well. The volume flow rate is proportional to the current, and the volume flow generated in the fluid is also proportional to the applied voltage.
However, the fluid flow is also dependent on the type of fluid in the channel, e.g., the concentration of salts and the like. Dasgupta and Liu, Anal. Chem. 1994, 66, 1792-1798, teach conditions for the reliable use of electroosmosis for fluid pumping. However, electroosmosis can only be used with liquids having some conductivity and cannot be used for highly resistive, non-polar solutions, such as organic solvents.
For example Pace et al, U.S. Pat. No. 4,908,112, suggests the use of electro-osmotic pumps to move fluids through channels less than 100 microns in diameter. A plurality of electrodes were deposited in the channels which were etched into a silicon wafer. An applied electric field of about 250 volts/cm was required to move a fluid to be tested along the channel. However, when the channel is very long, that means a large voltage (kV) needs to be applied to the channel, which may be impractical for highly integrated structures. Pace et al suggested that the electrodes be staggered to overcome this problem, so that only small voltages could be applied to a plurality of electrodes. However, this requires careful placement and alignment of a plurality of electrodes along the channel.
Electrohydrodynamic (EHD) pumping of fluids is also known and may be applied to small capillary channels. The principle for pumping here is somewhat different. Electrodes in contact with the fluid when a voltage is applied, transfer charge either by transfer or removal of an electron to or from the fluid, such that liquid flow occurs in the direction from the charging electrode to the oppositely charged electrode. Electrohydrodynamic (EHD) pumps can be used for pumping resistive fluids such as organic solvents.
The advantages of electroosmotic and electrohydrodynamic pump devices are the absence of moving parts, which eliminate difficulties reported in the construction or applications of micromechanical pumps, which have found only limited utility to date. However, the recent development of devices based on very small size channels, particularly for the synthesis of pharmaceuticals or for the clinical testing of DNA and the like, require integrated pumps capable of moving or holding a wide variety of fluids which may be conductive or insulating.
Further, there is a need for a configuration of these pumps to provide one or more valves in the channel to stop and hold fluids, thereby acting as an equivalent of an electronic gate or a mechanical valve.
Still further, there is a need to configure the structure for providing power to the pumps so that each pump may be operated independently. Recently suggestions have been made for synthesizing large numbers of molecules in parallel, or for testing in parallel a plurality of samples for DNA diagnostics and the like. Such processes require a plurality of wells and channels. Various reagents or samples in the channels feed into and out of the wells for these proposed applications. The well known use of an operational amplifier (driver) connected to each pump is impractical for a large array of channels. Thus it would be highly desirable to be able to control the flow of fluids in the various channels independently, without the need for an operational amplifier driver for each channel.