Micropumps are essential part of every microfluidic system. They are responsible for effective and reliable manipulation of fluids. Small size, low energy consumption, low reagent consumption, high speed of analysis and low price are major advantages of such microfluidics systems. In this work, design, research and fabrication of three novel types of micropumps are presented. This is a continuation of our past research on microthrottle and microcylinder pumps in LMSE at the Faculty of Electrical engineering UL. These first microcylinder pumps were successfully designed by employing of advanced 3D numerical simulations. As a result of numerical optimizations, fluid inlet was positioned in the center of pumping chamber, where membrane deformation is the largest.
In this work, an improved type of microcylinder pump with a redesigned outlet rectifying element is presented. By replacing microthrottle with step shaped outlet rectifying element, micropump performance characteristics significantly improved. In this case, fluidic resistance is decreased due to the absence of fluidic channel past the rectifying structure, which is advantageous in terms of flow rate performance. Large area of step-shaped rectifying element enables more efficient compression which is beneficial in terms of backpressure performance. Moreover, absence of fluidic channel after valve structure also reduces micropump size. With such a modification, micropump backpressure and flowrate performance characteristics can be influenced by varying the position of outlet (hole punctured into PDMS elastomer layer during micropump fabrication process) on the outlet rectifying element area without need to fabricate entirely new silicon mold. If the outlet is positioned closer to the edge of the step-shaped structure, micropump will exhibit higher flow-rate performance characteristics but lower back-pressure performance characteristic and vice versa. For micropump excitation, rectangular wave-form with amplitude of 250 V and frequency of 90 Hz was employed. Fabricated prototypes exhibited maximum flowrate performance of 2,4 ml min-1 (for deionized water at zero backpressure) and maximum backpressure performance of 520 mbar (for deionized water at closed outlet). This is a significant performance improvement in comparison to previous conventional microcylinder pump prototypes comprising throttle outlet rectifying element (with maximum flowrate performance of 1,4 ml min-1 and with maximum backpressure performance of 180 mbar at comparable excitation waveforms).
With air as pumping medium the microcylinder pumps exhibited maximum flowrate performance of 8 ml min-1, maximum backpressure performance of 60 mbar and under-pressure performance of -80 mbar at 300 Hz excitation frequency. For micropump reliability, bubble-tolerance (ability to pump two-phase medium, which is a mixture of gas bubbles and liquid) is crucial. It was determined that microcylinder pump is completely bubble tolerant for pressure loads in the range of 70 % of maximum air backpressure performance. Moreover, the microcylinder pump was found to be able to self-prime and will fill by itself with deionized water, when the reservoir is positioned up to 60 cm below the micropump.
Microcylinder pump, as most of micropumps with rectifying elements, is only capable of pumping liquids in single direction, predefined by its structure. To remove this limitation, we have taken advantage of microcylinder pump characteristic that it is normally open. We have designed and fabricated bidirectional microcylinder pump, by integrating on the same substrate in anti-series two microcylinder pumps with common outlet throttle. This bidirectional microcylinder pump has two separate pumping chambers and two actuators. The main advantage of this approach is that flow direction is selected only by exciting appropriate actuator, with no need for synchronous driving system. Therefore, complex driving system is not required. Bidirectional microcylinder pumps were excited with rectangular wave-form with amplitude of 250 V and frequency of 120 Hz. For DI-water, maximum measured flow rate performance and maximum measured backpressure performance was 1,2 ml min-1 and 200 mbar, respectively. For air, maximum measured flowrate performance was 3 ml min-1 @ 300 Hz, maximum backpressure was 35 mbar @ 180 Hz and maximum under-pressure was 48 mbar @ 180 Hz. Bidirectional microcylinder pump bubble tolerance was evaluated by introducing air bubbles of various diameters into micropump chamber. It was determined that bidirectional micropump is bubble tolerant without pressure load at the outlet. However, the flowrate decreased to approximately 50 %. With pressure load present at outlet, bidirectional micropump will fail when load exceeds 25 mbar.
Based on knowledge gained by the development of microcylinder pump, a novel structure of peristaltic micropump with single actuator was developed. Monoactuator peristaltic micropump was designed by modification of microcylinder pump, where rectifying structures were replaced by single shallow level of chamber and outlet fluidic channel. To produce this type of micropump, silicon mold with only one level of depth is required. Fabricating process for such mold is simpler and faster. It was determined that the precision of fluidic inlet and outlet has no significant impact on micropump performance and further simplify fabrication process. For deionized water, fabricated monoactuator peristaltic micropumps exhibited maximum measured flowrate of 220 μl min-1, backpressure of 300 mbar and under-pressure of 150 mbar (rectangular waveform with amplitude of 250 V and frequency of 70 Hz). Decreased flowrate performance is attributed to high fluidic resistance of shallow pumping chamber and fluidic outlet channel. With air as pumping medium, maximum flow rate, maximum backpressure and maximum under-pressure were 0,8 ml min-1, 70 mbar and 140 mbar, respectively at excitation frequency of 140 Hz. High under-pressure performance was found to improve self-priming ability. Developed single-actuator peristaltic micropumps are self-priming and bubble tolerant.
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