Comprehensive Study of the Flow Control Strategy in a Wirelessly

Article pubs.acs.org/ac

Comprehensive Study of the Flow Control Strategy in a Wirelessly Charged Centrifugal Microfluidic Platform with Two Rotation Axes Yunzeng Zhu,†,§ Yiqi Chen,†,§ Xiangrui Meng,† Jing Wang,† Ying Lu,† Youchun Xu,*,† and Jing Cheng*,†,‡ †

Department of Biomedical Engineering, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, Tsinghua University School of Medicine, Beijing 100084, China ‡ National Engineering Research Center for Beijing Biochip Technology, Beijing 102206, China S Supporting Information *

ABSTRACT: Centrifugal microfluidics has been widely applied in the sample-in−answer-out systems for the analyses of nucleic acids, proteins, and small molecules. However, the inherent characteristic of unidirectional fluid propulsion limits the flexibility of these fluidic chips. Providing an extra degree of freedom to allow the unconstrained and reversible pumping of liquid is an effective strategy to address this limitation. In this study, a wirelessly charged centrifugal microfluidic platform with two rotation axes has been constructed and the flow control strategy in such platform with two degrees of freedom was comprehensively studied for the first time. Inductively coupled coils are installed on the platform to achieve wireless power transfer to the spinning stage. A micro servo motor is mounted on both sides of the stage to alter the orientation of the device around a secondary rotation axis on demand during stage rotation. The basic liquid operations on this platform, including directional transport of liquid, valving, metering, and mixing, are comprehensively studied and realized. Finally, a chip for the simultaneous determination of hexavalent chromium [Cr(VI)] and methanal in water samples is designed and tested based on the strategy presented in this paper, demonstrating the potential use of this platform for on-site environmental monitoring, food safety testing, and other life science applications. entrifugal microfluidic platforms manipulate fluids on “CD-like” disks using centrifugal forces generated by a rotating device.1 With the advantages of miniaturization, integration, and automation,2 such platforms have been demonstrated to be suitable for assays of nucleic acids,3,4 proteins,5,6 and small molecules.7−9 Recently, researchers have put in much efforts on proposing and establishing unit operations on centrifugal microfluidic platforms, such as liquid transport,10−13 valving,3,14−17 mixing,18,19 and metering.20 These standardized operations are validated, scalable, and standardized2 and can be easily combined to achieve complex flow control. However, there are some limitations in the conventional centrifugal platform. First, the major limitation of centrifugal platforms is related to the fact that centrifugal forces only propel liquids radially outward, restricting the flexibility of the platform.21,22 Pumping liquid inward is a challenge in the conventional centrifugal platform, which is typically achieved by compressed air generated by heat,12 centrifugation in an enclosed chamber,11 or electrolysis of water.10 However, these methods are not flexible enough to achieve reciprocating flow or liquid transfer among multiple chambers located at different distances from the device center. Second, the traditional valving techniques on the centrifugal microfluidic platform are not flexible enough for flow control.

C

© XXXX American Chemical Society

For instance, passive valves including capillary valves15 and siphon valves16,17 are often used for flow control on the centrifugal microfluidic platform. The capillary valve has a sudden expansion or constriction in the cross section of the channel to hold the fluid, while the siphon valve relies on a hydrophilic channel in the form of an inverted U, both of which can be actuated when the rotational speed reaches a high level. Both types of these passive valves are advantageous due to their simple structures. However, the major drawback of these valves is that their functions depend on the wetting properties of the channel walls, where variations are unavoidably induced by the slight differences in fabrication and surface treatment processes. Besides, the surface of the channel wall can be easily remodified after the flow of certain solutions, influencing the performance of the valves. Recently, phase-change-based valves, such as laser-irradiated ferrowax valves,3,14 have shown great flexibility for flow control, whereas external instruments are required, introducing complexity to the whole platform. Moreover, the manufacturing procedure of these valves is often laborious. Third, liquid mixing is another difficulty on centrifugal microfluidics, because the artificial gravity generated by the Received: June 1, 2017 Accepted: August 2, 2017 Published: August 2, 2017 A

DOI: 10.1021/acs.analchem.7b02080 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 1. Centrifugal microfluidic platform with two rotation axes. (a) Schematic illustration showing the components of the platform. (b) Photograph of the platform.

chosen to control the secondary rotation, which possess characteristics of light weight, low cost, and easy operation. Compared to the similar platforms,13,22,23 the basic operations on the centrifugal microfluidic platforms with two axes were comprehensively studied and realized for the first time, including liquid transport, valving, metering, and mixing. Finally, as a proof of concept, a chip for the detection of hexavalent chromium [Cr(VI)] and methanal in water is designed and tested on this platform. By the integration of the operation units demonstrated in this study, the content of these two toxic components can be simultaneously determined, demonstrating the potential use of the platform for on-site food safety testing and other life science applications.

centrifugation makes the platform inefficient in purely diffusive mixing.18 To mix fluids on the centrifugal microfluidic platform, continuous changes in the spin speed18,19 are commonly used. The Euler forces induced by the acceleration or deceleration of the device lead to layer inversions of the liquids and accelerate the mixing procedure. Nevertheless, the altering of the rotational frequency also affects the capillary valves and the siphon valves on the disk, making the chip design more difficult. To address the aforementioned limitations, especially the unidirectional propulsion, researchers proposed novel centrifugal microfluidic platforms using two degrees of freedom (2DoF), which allowed the unconstrained and reversible pumping of liquid.13,22,23 The platforms differ from traditional centrifugal systems by the fact that the device can rotate around two rotation axes in the same plane. The spinning speed of the primary rotation determines the magnitude of the centrifugal force, while the angular position of the secondary rotation regulates the direction. With such a platform, Geissler et al. demonstrated the process for timing the passage of multiple components,22 while Miao et al. achieved rapid polymerase chain reaction (PCR) by localized heating.23 Cao et al. showed more potential and versatility of the platform to achieve complex fluid control, including inward pumping, independent burst valves, and flow switching.13 However, all the platforms above achieve electrical power supply to the rotating disk through slip rings, which add friction to the rotor axis, complicate the complete setup, and might wear out over time. Furthermore, the flow control units provided in the existing researches are limited,13,22,23 and more functions need to be developed to satisfy the needs of practical applications. In this study, the strategy for flow control in a wirelessly charged centrifugal microfluidic platform with two rotation axes is comprehensively studied. Our innovative platform achieves wireless power transfer to the rotating stage via inductively coupled coils, eliminating the need for cumbersome slip rings. Inductive power transfer is stable and durable, and it is a flexible option to electrify the disk since it does not require modification of the rotation axis and takes up little additional physical space.24 Furthermore, as transmitter and receiver coils are in the coaxial configuration, rotation does not affect the power transfer efficiency. In addition, micro servo motors are

■

EXPERIMENTAL SECTION Chip Fabrication. The centrifugal microfluidic devices were made of poly(methyl methacrylate) (PMMA) and structured using a computer-numerical control (CNC) milling machine by Hongyang Laser Co., Ltd. (Beijing, China). The PMMA devices were covered by a pressure-sensitive adhesive tape (Youbisheng Adhesive Products, Hangzhou, China) to ensure the chambers and channels were sealed. The adhesive tape and the devices were laminated together using a laminating machine (Tongyi Digital Technology, Xianning, China). Standards and Reagents. All solutions were prepared using analytical grade chemicals and 18.2 MΩ deionized water from a Milli-Q water purification system (Millipore, Beijing, China). The hexavalent chromium [Cr(VI)] was measured by reaction 1 as follows. A stock 1000 mg/L Cr(VI) solution was prepared by dissolving 141.44 mg of K2Cr2O7 (Sigma-Aldrich, Shanghai, China) in 200 mL of the deionized water. Working standards of Cr(VI) were prepared by serial dilutions of the stock solution. A solution of 1,5-diphenylcarbazide was prepared by dissolving 100 mg of 1,5-diphenylcarbazide (Sigma-Aldrich) in 25 mL of ethanol (Beijing Chemical Works, Beijing, China) and diluting to 2 g/L using deionized water. Hydrogen ion was provided by 8 times diluted concentrated sulfuric acid (Beijing Chemical Works). The reagent for Cr(VI) was prepared by mixing the 1,5diphenylcarbazide solution and the diluted sulfuric acid in the proportion of 5:2. B


Article


for α (α > 0°) signifies rotation in a clockwise direction, whereas a negative value (α < 0°) indicates counterclockwise rotation. For all the experiments in this paper, the spinning frequency around the primary rotation axis is set to be 1500 rpm while testing. Directional Transport of Liquid. Without the restriction in the direction of the centrifugal force, directional transport of liquid can be achieved in a direct manner. A simple validation is illustrated in Figure 2, where two chambers (A and B) are

H+

Cr(VI) + 1,5‐diphenylcarbazide ⎯→ ⎯ purple‐red complex (1) Fe

3+

methanal + MBTH → azine ⎯⎯⎯→ blue‐green compound (2)

The methanal was measured by reaction 2. The working standards were prepared by serial dilution of a stock of 36.5− 38% methanal in H2O (Sigma-Aldrich). To prepare reagent 1 for methanal, 50 mg of 3-methyl-2-benzothiazolinone hydrazone hydrochloride monohydrate (MBTH, Sigma-Aldrich) was dissolved and diluted to 50 mg/L by deionized water. Reagent 2 for methanal was prepared by dissolving 500 mg of ferric ammonium sulfate crystal (NH4Fe(SO4)2·12H2O, SigmaAldrich) in 50 mL of 0.1 M HCl (Beijing Chemical Works). Since the two reagents react with each other, the detection of methanal can only be achieved with two steps. Instrumentation. The centrifugal microfluidic platform with two rotation axes is shown in Figure 1. A servo motor with an embedded speed controller (YZ-ACSD608, Yizhi Technology, Shenzhen, China) was used to drive the main stage so that rotational frequencies and accelerations of the devices can be precisely modulated. Two micro servo motors (RB-15PG, ALSRobot Technology, Harbin, China) were integrated on the stage, adjusting the orientation of the devices with respect to the stage (α) on demand between 0° and 180° with a precision of 1°. To fix the chips, plastic carriers fabricated by a 3D printer (Replicator Fifth, MakerBot, Brooklyn, NY) were mounted onto the micro servo motors. Microcontroller (STC89C52) and communication module (NFR24L01+) were installed onto the stage as the control unit to independently adjust the micro servo motors according to the instructions wirelessly transferred from the external computer. Two control units were symmetrically placed onto the stage for balance, which is also capable for the individual control of more micro servo motors. The power supply to the rotating stage was realized through inductively coupled coils (Xinketai Electronics, Shenzhen, China). A stationary transmitter coil was placed under the main stage, while a receiver coil was attached to the stage. A printed circuit board (PCB) was connected to the receiver coil to rectify the received ac power and provide a 5 V dc supply for the microcontrollers and micro servo motors. An industrial camera (Sunway Technology, Shenzhen, China) was synchronized with the centrifugal platform to record one image per revolution. Lighting is provided by a high-intensity lightemitting diode (LED) illuminator (LED-528, Sidande Electronics, Shenzhen, China). For absorbance measurements, a spectrometer system was integrated into the platform. A white LED equipped with lens was used to illuminate the detection area, providing a broadband light source. The absorbing wavelengths used for Cr(VI) and methanal detection were 570 and 630 nm, respectively, while the nonabsorbing wavelength were 480 and 540 nm. An optic fiber was placed on the front face of the microfluidic device, which can collect the transmission light signal through a 5 mm long detection chamber. The absorbance signal was recorded by a spectrometer (USB2000+, Ocean Optics, Dunedin, FL) connected with the optic fiber.

Figure 2. Directional transport of liquid. Dyed water was used for visualization. (a) Drawing of the chip design. (b−d) The working processes. (b) Initial condition (α = −90°). The liquid is in chamber B. (c) In the process (α = 0°) of liquid transport. (d) Final condition. The liquid is transferred into chamber A. The movie that shows the working processes is available in the Supporting Information.

connected with each other by two channels (m and n) on both sides for liquid flowing and venting. The chip with 50 μL of liquid in chamber B is fixed in the carrier at the angular position of α = −90°. When the primary motor starts rotating, the liquid in chamber B would be gradually discharged to chamber A via channel n as the angular position raises to +90° in clockwise direction, and vice versa. In this way, the liquid can be transported into the destination chamber and flow backward without any “recharge” operations. Besides, the reciprocating flow in channel n can be used to improve the result of washing or binding processes in immunoassays. Valving for Sequential Release of Liquids. Most bioanalyses are based on multistep operations including washing, mixing, and wasting processes using many solutions. Hence, to release samples and reagents on demand is essential on centrifugal platforms. As is depicted in Figure 3, a chip with four loading chambers (L1, L2, L3, and L4) and a common collection chamber (C) is designed to demonstrate the capability in sequential release of liquids. Each chamber is connected to a channel in the form of an inverted U with different size. The height of the channel crest increases in turn from chamber L1 to L4. The chip is fixed onto the carrier at the original angular position (α = 0°) after 15 μL of liquid introduced into each loading chamber, respectively. The tilt of

■

RESULTS AND DISCUSSION As a convention for this work, the original angular position of the chip (α) is defined when the connection of the positioning holes is perpendicular to the centrifugal force. A positive value C


Article


Figure 3. Valving for sequential release of liquids. Dyed water was used for visualization. (a) Drawing of the chip design. (b1−b4) Schematic illustration of the opening processes of a single valve. (b1) Initial condition. The points on the dashed curve share the same magnitude of centrifugation force; thus, the dashed curve is the contour line in the artificial field of gravity generated by the centrifugation. The liquid level in the chamber and the channel is of the same height. (b2) As the angular position of the chip rises to the critical value αC, the liquid reaches the crest of the channel. (b3) The liquid keeps on discharging to the collection chamber. (b4) Final condition. The loading chamber is empty. (c1−c5) Photographs showing the result of the sequential release of liquids. The angular position of the chip is marked on the top right corner. The movie that shows the working processes is available in the Supporting Information.

Figure 4. Valving for selective routing. Dyed water was used for visualization. (a) Drawing of the chip design. (b−g) Photographs showing the selective routing procedures. (b) Initial condition. An amount of 70 μL of blue dye is loaded into the loading chamber. (c) α = −35°. The liquid is introduced into chamber T1. (d) α = −45°. The liquid is introduced into chamber T2. (c) α = 55°. The liquid is introduced into chamber T3. (c) α = −70°. The liquid is introduced into chamber T4. (g) Reset of the angular position. The movie that shows the working processes is available in the Supporting Information.

the chip around the secondary rotation axis in clockwise direction lowers the height differences between the channel crests and the liquid levels. For each loading chamber, the liquid reaches the crest and keeps on discharging to the collection chamber until the chamber is empty at a critical angle (αC1, αC2, αC3, and αC4) determined by the crest height. In this design, the release angle of each chamber is presented in Figure S1. The data indicates that a stable and sequential release of four liquids is achieved on our platform without altering the spinning frequency and complex surface modifications. Valving for Selective Routing. Selective routing is another important function to meet the demand of automation in complex liquid manipulation, especially when a sample is loaded for the determination of multiple biomarkers. The possibility of selective routing on our platform is illustrated in

Figure 4 for a that chip possesses one common loading chamber (L) near the center of the chip and four target chambers (T1, T2, T3, and T4) around. Each dosing channel (d1, d2, d3, and d4) comprises a crest to keep the liquid in the loading chamber at the original angular position. Venting channels (v1, v2, v3, and v4) are designed to form a self-vented structure. The chip is fixed onto the carrier at the original angular position (α = 0°) after 70 μL of liquid is introduced into chamber A. With similar theories described above, as the chip tilts to critical angular positions (αC1 = −35°, αC2 = −45°, αC3 = 55°, and αC4 = 70°), liquid will flow through the crest and hence chambers B, C, D, and E will be fulfilled, respectively. The schematic illustration of the procedures is shown in Figure S2. D


Article


Figure 5. Quantitative and staged release after metering. Dyed water was used for visualization. (a) Drawing of the chip design. (b) Photographs showing the procedures of quantitative and staged release of liquid. The angular position of the chip is marked on the top right corner. (c) The volume of discharged liquid in certain angular position.

Figure 6. Mixing. Dyed water was used for visualization. (a) Drawing of the chip design. (b−f) Photographs showing the mixing procedures. (b) Initial condition. The red dye and deionized water are loaded into chambers L1 and L2, respectively. (c) The red dye and deionized water are introduced into chamber M in an uneven distribution. (d and e) The mixing procedure. The chip is tilted around the original position (α = ±15°) in a period of 1 s. (f) The red dye and deionized water are mixed together. The movie that shows the working processes is available in the Supporting Information.

Quantitative and Staged Release after Metering. Most applications on centrifugal microfluidic platforms require precise input volumes of liquids in order to obtain quantitatively reproducible results. Therefore, the operation units for metering of liquid volumes are widely employed. In common cases, a metering structure consists of a connection channel to an inlet, a metering chamber with a defined volume, and an overflow to a waste chamber for excess volume.2 Herein, the metering structure on the centrifugal microfluidic platform with two rotation axes is demonstrated in Figure 5a, allowing a quantitative and staged release of the metered liquid. The loading chamber (L), connecting channel (c), metering chamber (M), and waste chamber (W) form a traditional

metering structure, while the release channel (l) and target chamber (T) achieve staged release when the angular position changes. The chip is fixed onto the carrier at the original angular position (α = 0°) after 90 μL of liquid introduced into chamber L. As the main motor starts, 60 μL of the liquid is kept in chamber M while the rest overflows to chamber W. Then, as the chip tilt counterclockwise, the metered liquid is gradually discharged into chamber T (Figure 5b). For a certain angular position, a certain volume of liquid is released. As a test, the discharged liquids at the angular positions of α = −40°, −44°, −48°, −60°, and −72° are collected. Their volumes are measured by an analytical weighing balance (ME55, Mettler Toledo, Shanghai, China) and displayed in Figure 5c. The E


Article


Figure 7. Operations on our platform for simultaneous analysis of Cr(VI) and methanal. Dyed water was used for visualization. The angular position of the chip is marked on the top right corner. (a) Drawing of the chip design. (b) Photographs showing the procedures of the analysis. The angular position of the chip is marked on the top right corner. (b1) Initial condition. (b2) Metering of 60 μL of sample in chamber ME and transfer of the remainder 90 μL into chamber M2. Meanwhile, reagent for Cr(VI) and reagent 1 for methanal are transferred to chambers M1 and M2, respectively. (b3) Transfer of the metered sample into chamber M2. (b4) Mixing of samples and reagents in chambers M1 and M2. (b5) Transfer of reagent 2 for methanal into chamber M2. (b6) Mixing of reagent 2 for methanal and the mixture in chamber M2. (b7) Transfer of the mixture in chamber M1 and M2 into chamber D1 and D2, respectively. (b8) Reset of the angular position. (c) Calibration curves for Cr(VI) (c1) and methanal (c2) analysis. The correlation coefficients are both greater than 0.99 over the range of 0−0.8 mg/L. The movie that shows the working processes is available in the Supporting Information.

methanal were first performed in the tube to confirm the volume and reaction time for each reaction step (please refer to the Supporting Information). After that, the whole operation procedure of the system is determined as shown in Table S1, while the chip at different stages of the procedure is illustrated in Figure 7b. The schematic illustration of the procedure is shown in Figure S4. The whole design is a combination of the operations offered by different functional units demonstrated above. The automated analysis starts after 150 μL of sample, 30 μL of reagent for Cr(VI), 60 μL of reagent 1 for methanal, and 12 μL of reagent 2 for methanal are injected into the corresponding chambers (S, R1m, R2m, and Rc, respectively) and sealed by adhesive tapes. First, the sample is pumped into the metering chamber (ME). Part of the sample (60 μL) is kept in the chamber while the other 90 μL sample is allotted to the mixing chamber M1 for Cr(VI) analysis. At the same time, reagent for Cr(VI) and reagent 1 for methanal are transferred into their respective mixing chambers M1 and M2 (step 1, Figure 7b2). Then the chip is tilted anticlockwise to release the kept sample to chamber M2, and then mixing and reacting it with reagent 1 for methanal (steps 2−3, Figure 7, parts b3 and b4). The reaction for Cr(VI) analysis happens simultaneously in mixing chamber M1. Subsequently, reagent 1 for methanal is introduced to chamber M2 for the second step reaction (steps 4−5, Figure 7, parts b5 and b6). Finally, the liquids in mixing chamber M1 and M2 are discharged into their respective detection chambers D1 and D2 (steps 6−7, Figure 7, parts b7 and b8). The absorbance of the two chambers are both

schematic illustration of the procedures is shown in Figure S3. A dilution process can be simply achieved by quantitative and staged release, extending the linear range of some detection methods.25 Mixing. To accelerate the chemical reactions on rotating disks, a sufficient mixing of samples and reagents is essential. In the design shown in Figure 6, the mixing chamber M is characterized by its curving bottoms. The chip is fixed onto the carrier at the original angular position (α = 0°) after 50 μL of different liquids are introduced into loading chambers L1 and L2, respectively. When the primary motor starts rotating, both liquids are introduced into the mixing chamber M in an uneven distribution. The steps required to achieve mixing are tilting of the chip around the original position (α = ±15°) over and over again in a period of 1 s. The centrifugal force together with the curving bottom brings fluctuations to the liquid in the chamber and accelerates the mixing procedure. The liquids are efficiently mixed after 10 periods. Compared with previously published work on traditional centrifugal microfluidic platforms,18,19 mixing on our platform is simple in structure and procedure and flexible, as the alteration of spinning frequency is unnecessary. Simultaneous Analysis of Cr(VI) and Methanal. To demonstrate the potential use of the platform in on-site life science applications, a chip was employed to determine the concentration of Cr(VI) and methanal simultaneously, as depicted in Figure 7a. The Cr(VI) and methanal detections are of great importance in environmental monitoring and food safety testing since they are both toxic to the human body. Prior to the on-chip detection, off-chip detection of the Cr(VI) and F



■

ACKNOWLEDGMENTS This study was supported by the National Key Research and Development Program of China (2016YFC0800703), the National Natural Science Foundation of China (31500691), the Beijing Municipal Science and Technology Commission (Z161100000116031), and the Beijing Lab Foundation.

detected 15 min later at the wavelengths of 570 and 630 nm, respectively. The calibration curves for Cr(VI) and methanal analyses are presented in Figure 7, parts a and b, respectively. Both of their correlation coefficients are greater than 0.99 over the range of 0−0.8 mg/L, proving the capability of our platform for reliable analyses of Cr(VI) and methanal. Since many of the biomarkers can be determined by the one-step method represented by Cr(VI) or two-step method represented by methanal, our platform has the potential for the detection of more chemicals.

■

CONCLUSION In this study, we constructed a wirelessly charged centrifugal microfluidic platform with two rotation axes and comprehensively examined its functions for flow control, including directional transport of liquid, valving for sequential release of liquids, valving for selective routing, metering, and mixing. Cr(VI) and methanal have been determined simultaneously based on the combination of aforementioned unit operations to prove the concept. Our platform is advantageous in three aspects: first, it enhances the advantages of traditional centrifugal microfluidic platforms. With the precise control of both magnitude and direction of the centrifugal force, the flow control is more flexible; thus, the integration and automation of the system might be improved. Second, the establishment of our platform is simple and inexpensive since the inductively coupled coils do not require modification of the rotation axis, and the micro servo motor is lightweight, low cost, and easy to operate. Last, the improvement of the platform simplifies the chip design. Complicated structures and surface modifications are reduced in our strategy with the help of secondary rotation. With all these features, we believe that our platform may find wide applications in the analytical arena in the near future. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b02080. Off-chip detection of Cr(VI) and methanal, the release angles of the valves for sequential release of liquids, the schematic illustration of the procedures for the experiments, and the spinning program for the determination of Cr(VI) and methanal (PDF) Movie that shows the working processes of the experiments (AVI)

■

REFERENCES

(1) Madou, M.; Zoval, J.; Jia, G.; Kido, H.; Kim, J.; Kim, N. Annu. Rev. Biomed. Eng. 2006, 8, 601−628. (2) Strohmeier, O.; Keller, M.; Schwemmer, F.; Zehnle, S.; Mark, D.; von Stetten, F.; Zengerle, R.; Paust, N. Chem. Soc. Rev. 2015, 44, 6187−6229. (3) Amasia, M.; Cozzens, M.; Madou, M. J. Sens. Actuators, B 2012, 161, 1191−1197. (4) Czilwik, G.; Messinger, T.; Strohmeier, O.; Wadle, S.; von Stetten, F.; Paust, N.; Roth, G.; Zengerle, R.; Saarinen, P.; Niittymaki, J.; McAllister, K.; Sheils, O.; O’Leary, J.; Mark, D. Lab Chip 2015, 15, 3749−3759. (5) Lai, S.; Wang, S. N.; Luo, J.; Lee, L. J.; Yang, S. T.; Madou, M. J. Anal. Chem. 2004, 76, 1832−1837. (6) Lee, B. S.; Lee, Y. U.; Kim, H.-S.; Kim, T.-H.; Park, J.; Lee, J.-G.; Kim, J.; Kim, H.; Lee, W. G.; Cho, Y.-K. Lab Chip 2011, 11, 70−78. (7) Kim, Y.; Jeong, S. N.; Kim, B.; Kim, D. P.; Cho, Y. K. Anal. Chem. 2015, 87, 7865−7871. (8) Kong, M. C. R.; Salin, E. D. Anal. Chem. 2012, 84, 10038−10043. (9) Xi, Y.; Templeton, E. J.; Salin, E. D. Talanta 2010, 82, 1612− 1615. (10) Noroozi, Z.; Kido, H.; Madou, M. J. J. Electrochem. Soc. 2011, 158, P130−P135. (11) Zehnle, S.; Schwemmer, F.; Roth, G.; von Stetten, F.; Zengerle, R.; Paust, N. Lab Chip 2012, 12, 5142−5145. (12) Thio, T. H. G.; Ibrahim, F.; Al-Faqheri, W.; Moebius, J.; Khalid, N. S.; Soin, N.; Kahara, M.; Madou, M. Lab Chip 2013, 13, 3199− 3209. (13) Cao, X. B.; deMello, A. J.; Elvira, K. S. Lab Chip 2016, 16, 1197−1205. (14) Park, J. M.; Cho, Y. K.; Lee, B. S.; Lee, J. G.; Ko, C. Lab Chip 2007, 7, 557−564. (15) Chen, J. M.; Huang, P. C.; Lin, M. G. Microfluid. Nanofluid. 2008, 4, 427−437. (16) Siegrist, J.; Gorkin, R.; Clime, L.; Roy, E.; Peytavi, R.; Kido, H.; Bergeron, M.; Veres, T.; Madou, M. Microfluid. Nanofluid. 2010, 9, 55−63. (17) Kitsara, M.; Nwankire, C. E.; Walsh, L.; Hughes, G.; Somers, M.; Kurzbuch, D.; Zhang, X.; Donohoe, G. G.; O’Kennedy, R.; Ducree, J. Microfluid. Nanofluid. 2014, 16, 691−699. (18) Grumann, M.; Geipel, A.; Riegger, L.; Zengerle, R.; Ducree, J. Lab Chip 2005, 5, 560−565. (19) Ren, Y.; Leung, W. W. F. Int. J. Heat Mass Transfer 2013, 60, 95−104. (20) Mark, D.; Weber, P.; Lutz, S.; Focke, M.; Zengerle, R.; von Stetten, F. Microfluid. Nanofluid. 2011, 10, 1279−1288. (21) Clime, L.; Brassard, D.; Geissler, M.; Veres, T. Lab Chip 2015, 15, 2400−2411. (22) Geissler, M.; Clime, L.; Hoa, X. D.; Morton, K. J.; Hebert, H.; Poncelet, L.; Mounier, M.; Deschenes, M.; Gauthier, M. E.; Huszczynski, G.; Corneau, N.; Blais, B. W.; Veres, T. Anal. Chem. 2015, 87, 10565−10572. (23) Miao, B. G.; Peng, N. C.; Li, L.; Li, Z.; Hu, F.; Zhang, Z. M.; Wang, C. H. Sensors 2015, 15, 27954−27968. (24) Hofflin, J.; Torres Delgado, S. M.; Suárez Sandoval, F.; Korvink, J. G.; Mager, D. Lab Chip 2015, 15, 2584−2587. (25) Meng, X.; Zhu, Y.; Chen, Y.; Lu, Y.; Xu, Y.; Cheng, J. Sens. Actuators, B 2017, 242, 710−717.

■

■

Article

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 10 62772239. Fax: +86 10 62773059. E-mail: [email protected]. *Phone: +86 10 62772239. Fax: +86 10 62773059. E-mail: [email protected]. ORCID

Youchun Xu: 0000-0003-4687-884X Author Contributions §

Y.Z. and Y.C. contributed equally to this work.

Notes

The authors declare no competing financial interest. G


Comprehensive Study of the Flow Control Strategy in a Wirelessly

Recommend Documents