A pneumatic micropump incorporated with a normally closed valve capable of generating a high pumping rate and a high back pressure
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- Yang, Y., Hsiung, S. & Lee, G. Microfluid Nanofluid (2009) 6: 823. doi:10.1007/s10404-008-0356-7
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This study reports on a new pneumatic micropump integrated with a normally closed valve that is capable of generating a high pumping rate and a high back pressure. The micropump consists of a sample flow microchannel, three underlying pneumatic air chambers, resilient polydimethylsiloxane (PDMS) membrane structures and a normally closed valve. The normally closed valve of the micropump is a PDMS-based floating block structure located inside the sample flow microchannel, which is activated by hydraulic pressure created by the peristaltic motion of the PDMS membranes. The valve is used to effectively increase pumping rates and back pressures since it is utilized to prevent backflow. Experimental results indicate that a pumping rate as high as 900 μL/min at a driving frequency of 90 Hz and at an applied pressure of 20 psi (1.378 × 105 Nt/m2) can be obtained. The back pressure on the micropump can be as high as 85 cm-H2O (8,610.5 Nt/m2) at the same operation conditions. The micropump is fabricated by soft lithography processes and can be easily integrated with other microfluidic devices. To demonstrate its capability to prevent cross contamination during chemical analysis applications, two micropumps and a V-shape channel are integrated to perform a titration of two chemical solutions, specifically sodium hydroxide (NaOH) and benzoic acid (C6H5COOH). Experimental data show that mixing with a pH value ranging from 2.8 to 12.3 can be successfully titrated. The development of this micropump can be a promising approach for further biomedical and chemical analysis applications.
KeywordsMicrofluidicsMEMSMicropumpNormally closed microvalve
List of symbols
power of hydrogen ions
scanning electron microscope
Micro-electro-mechanical-systems (MEMS) technology has been a promising approach in a variety of fields, including biotechnology, communications, optics and many others by integrating miniaturized mechanical elements, sensors, actuators and electronics on a single chip through micromachining techniques (Angell et al. 1983; Reyes et al. 2002). Among them, the applications of MEMS technology for genetic analysis, molecular biology, and analytical chemistry have attracted considerable interest recently. It makes possible the realization of a so-called micro-total-analysis-system (μ-TAS) (Auroux et al. 2002). A μ-TAS is usually capable of performing many critical sample pre-treatment and bio-process functions, including mixing, transportation, reaction, collection, separation and detection, on a single microfluidic device and in an entirely automated system (Sato et al. 2003; Raiteri et al. 2002). The major advantages of these microfluidic devices and systems are a significant reduction of the chemical reaction time and a lower consumption of expensive chemical reagents and samples, as well as the capability of automating the entire process. More importantly, the functionality and reliability of the microfluidic systems can be enhanced if additional microfluidic functions can be integrated on a single chip.
One of the most critical issues for a microfluidic system is how to transport and precisely control a small amount of fluid. Micropumps capable of providing an appropriate pumping rate and a reasonable back pressure are usually inevitable requirements for a self-contained microfluidic system. With the advent of microfluidic technologies, miniaturized pumping devices utilizing various actuation mechanisms have being extensively investigated since 1980s. Briefly, these mechanical or membrane-based micropumps generally contain pressurized chambers and are bonded with flexible membranes. Deflections of the membranes compress the channel below and thus drive the flow. Various actuation methods for these membrane-based micropumps have been demonstrated in the literature, such as piezoelectric (Nguyen et al. 2004; Koch et al. 1998), thermopneumatic (Van de Pol et al. 1990), electrostatic (Ng et al. 2004; Bourouina et al. 1997), pneumatic (Jeong, and Konishi 2007) and electromagnetic (Zhang and Ahn 1996; Santra et al. 2002). Among them, pneumatic micropumps driven by injecting compressed air have attracted considerable interest. Using a soft lithography process involving thick-film micromachining and replica molding of elastomeric materials, pneumatic micropumps can be fabricated. For instance, Unger et al. first used an elastic material (PDMS) and standard soft-lithography processes to demonstrate a pneumatic micropump with a three-membrane layout controlled by three electromagnetic valves (EMV) (Unger et al. 2000). Furthermore, a peristaltic pneumatic micropump with an s-shape layout, which only requires one EMV, was reported by this research group (Wang and Lee 2006). The time-phased deflection of PDMS membranes along the microchannel length can generate a peristaltic effect which drives the fluid along the microfluidic channel such that it requires a simpler control circuit and less peripheral equipment.
To better control the fluid motion inside the microchannel, it is crucial to control the transport of different samples at different times without contamination. This typically requires a microvalve. The development of microvalves has progressed rapidly in recent years. They generally fall into two major categories, namely active (Neagu et al. 1997; Baldi et al. 2003) and passive valves (Yang et al. 1996; Paul and Terhaar 2000) depending on whether an external power supply is required. Microvalves could also be classified as two types, normally open or normally closed, as determined by their initial operating states. For example, one of the most common active valves in microfluidic systems is a pneumatic valve. When injecting compressed air into the chamber, the resulting deflection of the membranes can work as a valve to stop the flow passing through the microchannel (Wang and Lee 2005). Usually, another EMV is required to control the motion of the pneumatic microvalve. Alternatively, many passive microvalves have been reported in the literature including flap valves (Feng and Kim 2004), hydrophobic valves (Ahn et al. 2004), and ball valves (Yamahata et al. 2005). Among them, a floating block structure is commonly used as a normally closed valve by using selective bonding of membranes (Duffy et al. 1999; Hua et al. 2006). For example, Baek et al. used a controllable central block in a flow channel as a normally closed valve. It could be opened and closed easily by external air pressure (Baek et al. 2005). Similarly, Hosokawa and Maeda used three membranes selectively bonded with block structures to regulate three flow channels (Hosokawa and Maeda 2000). In general, the normally closed valve is especially useful for microfluidic systems since it can prevent cross-contamination of fluids, promote the performance of micropump and also consumes less power. However, passive valve still needs an individual controller to control it open.
In this study, a normally closed microvalve is integrated into a pneumatic micropump. The microvalve is opened depending on the driving of micropump. It only requires one EMV for flow control and transport. The resulting control system is then simplified and fewer EMVs are required. The micropump can avoid cross-contamination of fluids while it is not activated. Besides, any improvement in the pumping rate can be observed since the normally closed microvalve can efficiently stop the back flow. Moreover, the normally closed microvalve can also efficiently increase the back pressure, which is an important parameter for micropumps.
2 Materials and methods
A PDMS (Sylgard® 184, Dow Corning, USA) replication process was employed to fabricate the double-layer PDMS-based micropump by replicating the inverse images on the SU-8 templates (Wang and Lee 2005). Silicone elastomer and elastomer curing agent (Sylgard 184A and Sylgard 184B, Sil-More Industrial Ltd, USA) were first mixed in a specific ratio (10:1) and then poured onto the SU-8 microstructure mold. A vacuum pump was used to remove the bubbles formed during the mixing process to prevent the formation of air pockets in the microchannels. A PDMS layer was spin-coated on the SU-8 template and then cured at 100°C for 1 h. The PDMS inverse structure was then mechanically peeled off the template. Two layers of PDMS can be formed by using the similar method. Detailed information for fabricating pneumatic micropumps can be found in our previous work (Wang and Lee 2005). The inlet and outlet reservoirs were mechanically drilled afterwards. To bond all structures together, all PDMS layers and a glass plate were treated by oxygen plasma. Notably, a shelter was utilized to cover the blocks during the oxygen plasma process such that they were not bonded with any PDMS layer. With this approach, selective binding of the PDMS layers can be achieved and a floating PDMS block can be formed as a normally closed valve. Scanning electron microscope (SEM) images of the fabricated SU-8 template and PDMS replication for the PDMS-based floating block structure are shown in Fig. 3b, c, respectively.
2.3 Experimental setup
To operate the micropumps, a custom-made controller (as shown in Fig. 4b) composed of an air compressor (MDR2-1A/11, Jun-Air Inc., Japan), EMVs (S070M-5BG-32, SMC Inc., Taiwan) and a programmable control system (ATMEGA8535, ATMEL Corp., USA) were used. A pressure gauge was also used in this custom-made controller for monitoring the supplied pressure to the actuators. The air tubing was directly connected to the pneumatic microchannel to activate the micropump. The reagents were stored in reservoirs prior to testing (see Fig. 10a). In addition, the EMVs were individually controlled by using the programmable control system for the filling and releasing processes of the compressed air. The injected air pressure and driving frequency of the EMV can be regulated by using the controller. The dimensions of the hand-held controller were measured to be 20 cm × 12 cm × 8 cm, respectively.
The pumping performance of the pneumatic micropump is mainly dependent on the movements of the pulsing membrane. Hence, a high-speed, charge-coupled device (CCD, MC1311, Mikrotron, Germany) and a microscope (TE300, Nikon, USA) were utilized to observe the motion of the pneumatic micropump at different pulsation frequencies. The frame rate of the image acquisition was set at 500 frames/s to capture the movement of the membrane pulsations. To measure the back pressure provided by the micropump, an experimental setup, as shown in Fig. 4c, was adopted (Sim et al. 2003). The height of the liquid at the outlet reservoir can be increased as the liquid was pumped continuously from the inlet reservoir. Notably, as the pressure difference (back pressure) between the inlet and outlet reservoir increased, the pumping rate decreased accordingly. By changing the frequency and air pressure using the control system, the pressure difference between the inlet and outlet tubing can be obtained such that the back pressure can be measured. A pH meter (Mettler Toledo, InLab423, Switzerland) was used to detect the pH value of the mixed sample for chemical titration applications.
3 Results and discussion
3.1 Pumping rate
Interestingly, for the case of the micropump with a normally closed valve, the pumping rate keeps increasing with the EMV driving frequency (up to 90 Hz, limited by the experimental setup). As shown in Fig. 5a, at a driving frequency of 90 Hz and a pneumatic pressure of 10 psi, the pumping rate of the Type-I micropump with a normally closed valve, is 490 μL/min, which is about 2 times higher than the one provided by the micropumps without normally closed valves. For Type-II micropumps, it can be as high as 720 μL/min at the same operation conditions. If the applied air pressure is increased to 20 psi, the pumping rates for Type-I and Type-II micropumps are 565 and 900 μL/min at a driving frequency of 90 Hz. The experimental results show that the proposed micropump with a normally closed valve can significantly increase the pumping rate. It can be clearly seen that the pumping rate of the Type-I and Type-II micropump devices increased about twofolds and threefolds higher, respectively, than the one without the integrated valve.
Ideally, the fluid delivered by micropumps with/without a microvalve is 5-cavity volumes (Wang and Lee 2006). However, the back flow will significantly reduce the pumping rate for the practical application, especially in the rage of high driving frequencies. With the integration of the normally closed microvalve, experimental data showed that the pumping rate and back pressure can be significantly improved (see Fig. 5). In Fig. 5, we used two micropumps, one with a microvalve and another without a microvalve, to compare their performance. Apparently, the pumping rate has been significantly improved for the new design. Another function of the microvalve is to stop the back flow for the prevention of cross-contamination and the increasing of the backpressure, which was also confirmed by titration experiments. The improvement of the frequency response can be also observed from Fig. 5, while the maximum driving frequency can be as high as 90 Hz, which is much higher than the one without a microvalve.
3.2 Back pressure
This study reports a new pneumatic pump integrated with a normally closed valve. The normally closed microvalve can be activated by the hydrodynamic pressure which is generated by the pneumatic micropump itself, without an additional control system. When external compressed air was applied to the pneumatic pump, the time phase among the membranes generated a peristaltic deflection to compress the liquid channel and then drive liquid forward. The normally closed valve can efficiently stop any backflow, increasing the pumping rate and back pressure as well. The location of the normally closed valve affects the performance of the micropump. When the normally closed valve was placed at the end, the pumping rate and back pressure was further improved. Experimental data showed that the pumping rate was mainly determined by the applied pressure and the driving frequency of the EMV. From the titration test, it was found that this micropump can transport two different samples by using one EMV and can prevent contamination of sample reservoirs. The development of this micropump can be crucial for a future micro-total-analysis-system and other biomedical applications.
The authors gratefully acknowledge the financial support provided to this study by the National Science Council of Taiwan (NSC 96-2120-M-006-008).