Design and implementation of low leakage MEMS microvalves

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2011
Yıldırım, Ender
This thesis presents analysis, design, implementation, and testing of electrostatically actuated MEMS microvalves. The microvalves are specifically designed for lab-on-a-chip applications to achieve leakage ratios below 0.1 at pressure levels in the order of 101 kPa. For this purpose, two different microvalves are presented in the study. In the proposed designs, electrostatic actuation scheme is utilized to operate the microvalves in normally open and normally closed modes. Characterization of normally open microvalves show that, microvalves with radii ranging between 250 m and 450 m can be operated by applying voltages between 40 and 100 V with air and oil. It is also shown that the actuation potential becomes minimum for working fluids with dielectric constant around 3-5. During the tests, it could not be possible to operate normally open electrostatic microvalves with DI water, due to its high dielectric constant and conductivity. During flow tests with air, 17 % leakage is observed under 10 kPa inlet pressure, when actuated by applying 85 V. On the other hand, it is shown that this leakage can be controlled precisely by tuning the actuation potential with sensitivity of 10-3 V-1. To solve the problems observed in normally open microvalve, a normally closed electrostatic microvalve design is proposed. The design isolates the working fluid from electric field, hence makes it possible to operate the microvalve with any working fluid. Moreover, unique and reconfigurable valve seat design enables low leakage. Pull-in tests are carried out with air and DI water under no-flow condition. During the tests, 46-66 V pull-in voltage is observed, independent of the working fluid. Besides, during flow tests with DI water, no leakage is detected up to 20 kPa inlet pressure. Considering actuation and leakage properties of these microvalves, a multi-drug effect analysis system is proposed. The system utilizes normally closed microvalves to generate micro-droplets of different drug with cells entrapped inside. Prototypes of the system are fabricated and tested with 3 m diameter polystyrene micro-beads. The tests show that it can be possible to entrap single bead in 135 pl volume droplets. The prototypes are also tested with living yeast cells. It could also be possible to entrap yeast cells in micro-droplets using the proposed system. As an extension to the multi-drug effect analysis system, a microvalve controlled droplet metering technique is proposed. The technique uses normally open microvalves to control the flow rate of the carrier fluid in a droplet based system. Initial tests show that it can be possible to generate pl size micro-droplets with 6 % precision. During the tests, voltage sensitivity of the technique is measured as 0.4 pl/V.

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Citation Formats
E. Yıldırım, “Design and implementation of low leakage MEMS microvalves,” Ph.D. - Doctoral Program, Middle East Technical University, 2011.