Haobo (Jack) Dong and Dr. Gregory Nordin, Department of Electrical and Computer Engineering
Introduction
Microfluidics is the science and technology of systems that process or manipulate small amounts of fluidics, using channels with dimensions of tens to hundreds of micrometers [1]. Polydimethylsiloxane (PDMS), a soft elastomeric material, is a popular choice in microfluidics research due to its tunable elastic properties, ability to seal with silicon and glass, low cost, forgiving and simple fabrication procedures [2]. To control and move fluids within the system, on-chip valves and pumps are needed. This paper reports a PDMS recirculation pumps device for MEMS-based biosensors.
Design
Fig.1 shows a basic design of the device. There are three layers of materials as marked by three colors. The blue color indicates the PDMS fluidic layer, which is physically underneath the PDMS control layer indicated by the green color. The pink color marks the glass layer at the very top of the device. The blue fluidic channel forms a circular structure and is connected to a glass hole at the top right corner from which fluidic sample is introduced. The green control channel structures are connected to the rectangular valve pads to actively control fluid flow through applying pneumatic pressure to collapse the microchannels. The large circular valve pads overlapping the fluidic reservoirs are pumps to move and circulate fluids in a counter-clock wise fashion. Specific sequence of operation is that the lower pump is first pressurized while the upper pump is relaxed. After the upper reservoir is filled with fluid, it’s pressurized and at the same time the lower pump is relaxed. Counter-clock wised flow motion is created when the two pumps are actuated alternatively.
Microfabrication
The device is microfabricated in the BYU cleanroom with soft-lithography techniques [3]. The fabrication procedure is shown in Fig.2. The glass piece was designed and then sent to an external company for fabrication. In addition to the new design, another significant improvement as a result of this research is the simplification of the device fabrication from three to two layers. The original proposal suggested an involved three-layer structure. However, the problem was that the PDMS covering of the reservoir holes was difficult to remove. Extensive research effort has been made to try out other methods, such as spinning and etching to remove the covering, but neither seemed to be a viable solution. A careful examination showed that the device could be made into two layers and the fluid sample volume of the reservoir could be compensated through increasing the reservoir diameter instead of increasing its height. This discovery significantly simplified the device fabrication and improved the turn-around time while functionality and performance were not compromised. Fig 3 shows the device sitting in a testing clamp with bended stainless steel tubing segments connected to the input holes.
Testing
The complete test setup is illustrated in Fig.4. PTFE microbore tubing was used to transport water fluid to the chip. One end of the tubing was connected to the bended steel segments which were inserted in the input PDMS holes. The other end is connected to a Harvard Apparatus Pump 33 syringe pump which is used to push the fluid into the channel. The three control holes were connected through tubing to solenoid valves on a single manifold which can regulate pressure. The fluid in the two pump control tubing provided a way for measuring flow rate by tracking its meniscus. To ensure accurate tracking, segments of capillary glass tubing were used and a high speed camera (Photron Ultima APX-RS) was set to capture meniscus movement within the capillary tubing. Several actuation periods were tested from 0.125 to 4s while 50% duty cycle was maintained for three operating pressures—10, 15, and 20 psi. A MATLAB script using edge detection was used to determine the distance the meniscus travelled. Two sample frames of one video clip were shown in Fig.5.
Experimental Results and Discussion
Fig.6 shows three plots of the distance the meniscus travelled versus time, corresponding to three actuation period. It can be seen that the fluid moved back and forth in the capillary tubing in accordance to the reservoirs emptying and filling as fluid circulates. It can also be seen that higher actuation pressure results in longer distance travelled and more volume pumped. By properly adjusting pressure and actuation period, the amount of fluid in the reservoir being pumped can be controlled for different applications. Fig. 7 shows the instantaneous volumetric flow rate (VFR) versus time for one actuation period. As seen, the instantaneous VFR was able to achieve approximately 600ul/min at 20psi, suggesting a rapid circulation flow rate on the chip.
Future Work
Future work should focus on testing more samples at more actuation periods to get a more compressive picture of the device. Variation across samples is of particular interest because reliable performance is crucially important. Future work should also focus on devising new method to more accurately capture the average volumetric flow rate on the chip.
Acknowledgement
The author gratefully acknowledges Bryan Haslam, Seunghyun Kim, and Prof. Gregory Nordin for their valuable advice.
References
- George M Whitesides, “The origins and the future of microfluidics,” ature 442, no. 7101 (July 27, 2006): 368-373.
- S R Quake and A Scherer, “From micro- to nanofabrication with soft materials,” Science (ew York, .Y.) 290, no. 5496 (November 24, 2000): 1536-1540.
- Y. Xia and G. M. Whitesides, “Soft Lithography,” Annu. Rev. Mater. Sci., vol. 28, pp. 153, 1998.
