Membrane-based valves and inward-pumping system for centrifugal microfluidic platforms
Introduction
In recent years, great efforts have been devoted to developing centrifugal microfluidic devices that enable miniaturized, parallel and autonomous analytical operations while significantly reducing the processing time, consumption of samples and regents, and total cost [1], [2]. These devices integrate fluid control units such as pumps, valves and mixers with other functional units into one compact disc. Their most attractive merit is to use the centrifugal force as the propulsion force for flow control and therefore eliminate external connections such as syringe pump or high voltage power supply. Thus, centrifugal microfluidics can serve as a powerful tool for automation of bio-chemical assay and point-of-care applications.
Valving and pumping are two fundamental technologies for any fluidic system to perform accurate flow control. This is especially true for the centrifugal microfluidic systems. Prior researches in the field had developed various types of valves for centrifugal microfluidic applications. The most commonly used ones are the passive valves such as hydrophobic valve [3], [4], capillary valve [5], [6] and siphon valve [7], [8], [9], [10], [11]. They usually take advantages of the capillary effect and the property of the disc material to withhold the liquid. The main drawback is that they require deliberate geometry design or surface modification. Besides, they are not suitable for the situations where sealing of reagents is necessary. The other frequently used ones are the sacrificial valves, including the laser irradiated ferrowax valves [12], film valve [13], and dissolvable membrane valve [14]. They are built on the phase transition materials and can work independently on spinning speed or valving locations. However, complex manufacturing process and external trigger for actuation are normally required. In addition, the direct contact between the substance and the sample may lead to cross-contamination. Therefore, it is still highly desired to develop a simple valve for universal applications in centrifugal microfluidic platforms.
Previously, we reported a magnetically actuated valving system incorporating with a flyball governor system [15] and a spring-plunger based pinch-valve [16]. Though they function very well, there is one significant disadvantage with both designs. Because the cross-section of the flow channel is rectangular, in order to fully pinch covering plate of the PDMS channel for complete closure, relatively large force needs to be applied to avoid leakage. To overcome this problem, a small metal ball is placed on top of covering plate of the valve channel. With the new valve design reported in this paper, the closure of the valve becomes more convenient and efficient, and requires significantly lower actuation force. The valve makes it feasible to overcome another technically challenging task in centrifugal microfluidic systems: inward-pumping. In centrifugal microfluidic systems, the intrinsic centrifugal force provides the actuation for pumping and simplifies the system design. However, the unidirectional flow of the sample fluid from the centre to the rim of the disc is a significant limitation. The flow path of the fluid sample is limited by the disc size. Inward pumping is therefore highly desirable to propel the liquid from the rim of the disc back to its centre for some specific applications. The capability for inward-pumping fluid sample to the centre of disc can provide great design and operation flexibility to microfluidic systems and for reagent storage in complex biological assays. Most of the prior researchers focused on the pneumatic expansion method to achieve inward pumping [17], [18], [19]. Its drawback lies in the inconvenient actuation method and relatively low pumping efficiency. Recently some new pumping methods had been proposed. Salin [20] utilized compressed gas to pump the liquid inward. But the external gas may cause sample contamination. Aeinehvand [21] integrated a small latex made balloon onto the disc. The balloon expanded and stored the elastic energy at high spinning speed and released the energy to pump the liquid to the centre of the disc at low spinning speed. However, the complicated structure adds complexity to the fabrication of the disc, which consisted of seven different layers. A simple, robust and non-contact pumping approach is therefore still highly desired.
In this paper, we report a new microvalve and an inward pumping system for centrifugal platform applications. The valve based on membrane actuation was first constructed and tested. The corresponding burst frequency of the valve was characterized. Next, an inward pumping system was designed based on the same actuation principle, constructed and tested. The valve provided an airtight sealing. A compression chamber was added to generate the pneumatic pressure needed for the inward pumping process. Other than the spinning motion of the disc, no external power supplies are required for the operation of the valving and inward pumping.
Section snippets
Actuation using a flyball governor
The valving and pumping systems in this paper were actuated using a specially designed flyball governor system as reported in our earlier work [13], [14]. Fig. 1 depicts the actuation principle of the system based on the proposed flyball governor. The microfluidic disc is installed on the top of the assembly. The flyball governor consists of an actuation disc with spring plungers, linkage, flyball and a supporting spring, and a DC motor (not shown) to drive it. At initial state, the spring
Fabrication of the microfluidic disc
In brief, the microfluidic disc was fabricated in four steps as shown in Fig. 6. The first step was to fabricate the SU-8 master mold with soft lithography method. A 500 μm thick layer of SU-8 100 photoresist (MicroChem, USA) was first spin-coated on a 4-inch silicon wafer. Next fluidic patterns on a mask were transferred to the photoresist by UV exposure followed by the soft bake of photoresist. After the postbake and development steps, the master mold was obtained for the PDMS casting
Characterization of the membrane valve
Fig. 8 presents the operation of a proposed valve. The valve in this study is a “normally closed” one because it stays “closed” in its initial state. In the experiment, a volume of 10 μL red food dye was first pipetted into the loading chamber. As the disc was spun at low speed, part of the sample liquid was pushed into the pneumatic chamber due to the interaction between the pressure of the trapped air and the induced centrifugal force exerted on the liquid. The valve stayed closed as the
Conclusions
This study presents a simple and effective technique for valving and inward pumping for elastic polymer based centrifugal microfluidic device. This technique utilizes a unique mechanical system to seal off the channel based on the deflection of elastic membrane. Its burst frequency can be easily manipulated by adjusting the pre-stress of the supporting spring and the mass of the flyballs of the mechanical system without changes in the fluidic pattern. An inward pump was also designed and
Acknowledgement
The research work presented in this paper was supported partially by LSU-LIFT2 Fund.
Ziliang Cai received his B.S and M.S degrees in Mechanical Engineering from Southeast University, China in 2005 and 2008, respectively. He received his PhD degree in Mechanical Engineering from Louisiana State University in 2015. His research interests include polymer-based micro fabrication, microfluidics, micro optics and computational fluid dynamics.
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Ziliang Cai received his B.S and M.S degrees in Mechanical Engineering from Southeast University, China in 2005 and 2008, respectively. He received his PhD degree in Mechanical Engineering from Louisiana State University in 2015. His research interests include polymer-based micro fabrication, microfluidics, micro optics and computational fluid dynamics.
Jiwen Xiang received his BS degree in Microelectronics Manufacturing Engineering in May 2010 and his MS degree in Mechanical Engineering in May 2013, both from Central South University of China. He is currently pursuing the PhD degree at the Department of Mechanical Engineering in Louisiana State University. His research interests include microfabrication, sensors, and actuators.
Hualing Chen received her B.S. degree in Mechanical Engineering from Xi’an Jiaotong University in China in 1982, her M.S. degree in Mechanical Engineering from Xi’an Jiaotong University in China in 1984, and her PhD degree in Mechanical Engineering from Xi’an Jiaotong University in China in 1990. Her research interests are in smart materials and structures, design and analyses of MEMS structures.
Wanjun Wang received his B.S degree in Mechanical Engineering from Xian Jiaotong University of China in 1982, his M.S. degree and PhD degree in Mechanical Engineering from The University of Texas at Austin in 1986 and 1989 respectively. He is a professor in Mechanical Engineering of LSU of United States and an adjunct professor in Xian Jiaotong University of China.