Nathan Ives and Dr. Aaron Hawkins, Electrical Engineering
Introduction
The purpose of this project is to test a new design for low voltage microfluidic pumps. Mechanical pumps are sometimes too large, unreliable, or difficult to integrate in lab-on-a-chip applications. DC electro-osmosis is unsuitable for many biomedical and portable applications due to the high voltages needed. AC electro-osmosis has been proposed as an alternative because of the lower voltages it requires to pump fluids1, 2. Our design uses a 3D configuration for microelectrodes with alternating DC pulses that should produce a more linear flow of fluid and prevent “eddies” that occur in other ACEO pump designs3. This should result in faster pumping velocities.
Methodology
We designed and produced a photolithography mask set using an electron beam pattern generator. With the mask set we were then able to process 4” silicon wafers in the IML lab here at BYU to produce fluid channels and 3D electrodes. Using sacrificial cores, layers of silicon dioxide, and metal deposition we produced several sets of fluid channels and fluid pumps. By attaching a laser-cut plastic reservoir using UV epoxy, we place fluid at one end of the fluid channels. Using an electric probe station with a microscope, we apply DC pulses to the electrodes. By measuring the fluid placed at one end of the reservoir we can estimate the pumping velocity of our design. A more accurate fluid velocity can be determined later by using a high-speed camera and micron-sized fluorescent glass beads.
Results
Unfortunately, we have not yet produced a working microfluidic pump. Several different fabrication changes and revisions have taken place, but so far they have been unsuccessful. A wide range of voltage levels and frequencies have been attempted. We have observed hydrolysis bubbles in the channels when the voltage levels are increased. Electrodes insulated with SiO2 were used to prevent hydrolysis when increasing the voltages. Further revisions and tests need to be conducted.
Conclusion
Although our initial background research indicated that this design should work, perhaps some of our assumptions were incorrectly assumed. There are many complexities involved with electro-osmotic pumping, especially in the electric double layer where slipping occurs, as well as interactions between fluids, metals, and dielectrics. Back-pressure in the channel can completely prevent electro-osmosis from pumping since the Stern layer in the electric double layer is what pulls the bulk fluid down the channel. Further, hydrolysis can completely strip away the metal electrodes unless a noble metal such as gold or platinum is used. Further tests and possibilities will be explored in the future.
Sources
- Ajdari, A. (1999). Pumping liquids using asymmetric electrode arrays. Physical Review E, 61(1), 45-48.
- Studer, V., Pepin, A., Chen, Y., & Adjari, A. (2002). Fabrication of microfluidic devices for AC electrokinetic fluid pumping. Microelectronic Engineering, 61-62, 915-920.
- Huang, C. C., Bazant, M. Z., & Thorsen, T. (2009). Ultrafast high-pressure AC electro-osmotic pumps for portable biomedical microfluidics. Lab on a Chip, 2010(10), 80-85.