Towards practical sample preparation in point-of-care testing: user-friendly microfluidic devices

Juhwan Park a, Dong Hyun Han a and Je-Kyun Park *ab
aDepartment of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. E-mail: jekyun@kaist.ac.kr
bKAIST Institute for Health Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Received 14th January 2020 , Accepted 19th February 2020

First published on 20th February 2020


Microfluidic technologies offer a number of advantages for sample preparation in point-of-care testing (POCT), but the requirement for complicated external pumping systems limits their wide use. To facilitate sample preparation in POCT, various methods have been developed to operate microfluidic devices without complicated external pumping systems. In this review, we introduce an overview of user-friendly microfluidic devices for practical sample preparation in POCT, including self- and hand-operated microfluidic devices. Self-operated microfluidic devices exploit capillary force, vacuum-driven pressure, or gas-generating chemical reactions to apply pressure into microchannels, and hand-operated microfluidic devices utilize human power sources using simple equipment, including a syringe, pipette, or simply by using finger actuation. Furthermore, this review provides future perspectives to realize user-friendly integrated microfluidic circuits for wider applications with the integration of simple microfluidic valves.


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Juhwan Park

Juhwan Park received his B.S., M.S., and Ph.D. degrees in bio and brain engineering from the Korea Advanced Institute of Science and Technology (KAIST), in 2014, 2016, and 2020, respectively. His Ph.D. thesis was on a power-free microfluidic actuator for biological sample preparation and analysis in point-of-care testing (POCT) under the supervision of Prof. Je-Kyun Park. He is also interested in bioMEMS and lab-on-a-chip technologies for the development of cell-based assays. He received a Global Ph.D. Fellowship (2017–2019) from the National Research Foundation (NRF) of Korea.

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Dong Hyun Han

Dong Hyun Han received his B.S. degree in chemical and biomolecular engineering from the Korea Advanced Institute of Science and Technology (KAIST) in 2020. He is currently undertaking his M.S. degree in the Department of Bio and Brain Engineering at KAIST. His current research interests include microfluidics, lab-on-a-chip, and bioMEMS.

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Je-Kyun Park

Je-Kyun Park is a professor of bio and brain engineering at the Korea Advanced Institute of Science and Technology (KAIST). He received his Ph.D. degree in biotechnology from KAIST in 1992. Prior to joining the faculty at KAIST, he worked as a postdoctoral fellow (1996–1997) at the Johns Hopkins University and as Chief Research Engineer (1992–2002) at the LG Electronics Institute of Technology. He has co-authored more than 160 scientific papers in the field of lab-on-a-chip and microfluidic analytical technologies. He was also Conference Chair of μTAS 2015 and President (2016) of the Korean BioChip Society.


Introduction

Point-of-care testing (POCT) is a simple analytical test that can rapidly provide the medical diagnostic results near the patients by non-experts, even in resource-limited settings.1–5 POCT is required in various fields to prevent foodborne illnesses6 and infectious diseases,7–9 or to monitor health-related biomarkers.10,11 The World Health Organization (WHO) provides “ASSURED” guidelines for an ideal POCT, which are affordable, sensitive (avoid false-negative results), specific (avoid false-positive results), user-friendly (easy to use with minimal training), rapid & robust (to enable treatment on the first visit), equipment-free, and deliverable (accessible to end-users).12

For POCT, raw samples such as saliva, blood, urine, or stool need to be prepared in a suitable form for detection, and the detection results are obtained by analyzing the colorimetric, fluorescence, or electrical signal. For ideal POCT, a “sample-in-answer-out” system should be realized that shows the results of the detection after the injection of the patient's sample. Among the POCT procedures, the sample preparation procedure is the biggest bottleneck that must be overcome for ideal POCT, since conventional sample preparation steps require multistep reactions based on manual pipetting and repeated use of equipment such as a centrifuge, which takes up the longest portion of POCT. Although various simple methods for signal detection have been developed using microfluidic paper-based analytical devices13,14 and smartphones,15,16 sample preparation steps still rely on conventional methods that require complicated manual operation or expensive automated machines. POCT without requiring complicated sample preparation has been widely used for glucose level test from whole blood and pregnancy test from urine. However, a number of medical tests requiring complicated sample preparation have not been widely applied for POCT.

Meanwhile, microfluidics and lab-on-a-chip technologies, also known as micro total analysis systems (μTAS), have a number of advantages for sample preparation in POCT due to their high surface-to-volume ratio, rapid analysis time, closed system, ability to integrate various functions and the requirement for a small volume of samples and reagents.17,18 Hence, a number of microfluidic devices for sample preparation have been developed to facilitate the mixing of reagents, the separation of the target sample, an effective reaction between samples and reagents, and the compartmentalization for high-throughput or digital analysis. However, to operate microfluidic devices, a complicated external pumping system is required, which limits the practical and wide use of microfluidic technologies for sample preparation in POCT.19–21 To overcome this limitation, the microfluidic operation system should be portable, easy-to-use, low cost, and working without electricity so that it can be used by non-experts without any problems (Fig. 1). When the above requirements are satisfied, the ideal POCT, “sample-in-answer-out”, can be developed by integrating a practical microfluidic sample preparation system with a simple detection system.22


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Fig. 1 Microfluidic technologies are beneficial for sample preparation procedures in POCT, including mixing of multiple reagents, an effective reaction between biomolecules, separation of the target particle, and compartmentalization for digital analysis. However, microfluidic devices should be more user-friendly, portable, power-free usable, easy-to-use, and low-cost.

Over the decades, several methods have been developed to operate microfluidic devices without a complicated external pumping system for various applications, including not only POCT but also microfluidic cell culture. However, the methods for microfluidic cell culture using gravity, osmotic pressure and surface tension are not suitable for POCT due to the sophisticated operating environment.23

In this review, we focus on user-friendly methods to operate microfluidic devices for practical sample preparation in POCT. We provide an overview of the two types of simple operation systems for the microfluidic devices: the pressure generated from inside of the device (self-operated methods) and the pressure applied manually from outside of the device (hand-operated methods). Additionally, the advantages and disadvantages of simple operation systems, as well as their biomedical applications, are discussed. Although the operating system is simplified and used for the various sample preparation procedures in POCT, there are still some limitations compared to conventional operating systems. The operation of complicated microfluidic circuits is limited due to the issues of the integration of microfluidic valves. In this regard, the design criteria are discussed to develop user-friendly integrated microfluidic circuits by incorporating simple microfluidic valves. Furthermore, we discuss the future perspectives of user-friendly microfluidic devices for ideal POCT.

Self-operated microfluidic devices

Capillary force

In a microchannel, a capillary effect occurs due to the interface between the surface tension of the liquid and the geometry of its channel or solid contact.24 Capillary microfluidics is often known as “passive” as fluids autonomously move along a channel without external force by principles of the capillary effect. Conventional capillary pumps have been applied to autonomously flow polymerase chain reaction (PCR) reagents for DNA amplification.25 Nevertheless, conventional capillary pumps are vulnerable to bubble entrapment and variation in volume control.26 Additionally, as the capillary flow can only be generated in the liquid–air interface, no more flow can be generated when the microfluidic channels are filled with liquid. To resolve such bottlenecks, microstructured capillary pumps have been developed using the geometry of the microstructures.27,28 The microstructured capillary pumps overcome the limitations of conventional capillary pumps by allowing the control of the flow rate and volume over a longer time by the predetermined guidance (Fig. 2A).
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Fig. 2 Self-operated microfluidic devices. (A) A capillary pump with microstructures was integrated at the end of the microfluidic channel to maintain a flow for a longer time. Reproduced from ref. 27 with permission from Springer Nature. (B) A paper substrate was used to apply pressure into microfluidic channels, which is known as a self-powered imbibing microfluidic pump by liquid encapsulation (SIMPLE). Reproduced from ref. 42 with permission from The Royal Society of Chemistry. (C) A degassed PDMS was used to apply negative pressure into a microchannel that enables the filling of reagents into the dead-end microchambers. Reproduced from ref. 49 with permission from Elsevier. (D) A microfluidic device was encapsulated in a vacuum pouch and operated upon the piercing of the pouch in inlet ports after loading the reagents. Reproduced from ref. 58 with permission from The Royal Society of Chemistry. (E) A gas-generating biochemical reaction was utilized to generate a flow in a microfluidic channel. Reproduced from ref. 63 with permission from Springer Nature.

Capillary pumps are also widely used to facilitate immunoassays in microchannels. Antibodies and reagents were spotted on the surface of microfluidic channels so that immunoassays can be performed by loading a sample solution into an inlet.29 Other substrates for additional sample preparation functions such as plasma separation can be integrated into a loading port.30 By controlling the velocity of the capillary flow, a chemiluminescence immunoassay requiring multistep reactions has been demonstrated in which a sample solution sequentially rehydrates reagents in microfluidic channels.31 In another application, capillary pumps have been used to handle the mixing of multiple reagents. Multiple reagents were simultaneously loaded into inlets and were mixed through mixing channels for enzymatic reactions32,33 and sample preparation for surface-enhanced Raman scattering (SERS).34 In addition, capillary pumps for blood analysis were introduced in which the capillary force induced by the microstructures separated the blood plasma from the whole blood,35 and the isolated plasma was further used for reverse ABO/Rh blood typing.36 Jose et al. used a capillary pump for antiplatelet drug assays to assess the binding between the platelet and fibrinogen.37 A more complicated flow behavior such as the sequential fluid delivery of multiple reagents can be performed by sequentially connecting multiple channels to the capillary pump.38 Furthermore, microbead-packed channels have also been used to amplify the amount of capillary force or to separate the blood plasma from whole blood for the immunoassay.39,40

Alternatively, the capillary pumps can be replaced by a paper substrate. Micro-sized pores of a paper substrate can apply pressure into the microchannel when the reagents in the microchannel reach the paper substrate.41 Without the fabrication of additional microstructures, a wide range of capillary flow can be generated depending on the pore size of the paper substrate and is applicable to conventional microfluidic devices. As with a capillary pump, the flow relies on the capillary force induced by the microchannel before the flow reaches the paper substrate. Therefore, to initiate the capillary flow induced by the paper substrate, the pressure generated from finger actuation was used, which is called SIMPLE (self-powered imbibing microfluidic pump by liquid encapsulation).42 SIMPLE autonomously delivers fluid to the target destination based on the working principle that paper absorbs the working liquid until the paper is saturated (Fig. 2B). Dal Dosso et al. exploited the SIMPLE technology to detect creatinine in plasma using enzymatic reactions.43 Novo et al. used a paper substrate to generate a capillary flow for autonomous microfluidic immunoassays by sequential delivery of reagents through a microfluidic circuit.44 Furthermore, the paper substrate at the end of the microchannel was used not only to induce capillary flow but also to store volume-defined dried plasma spots for further analysis.45

Vacuum-driven pressure

Since poly(dimethylsiloxane) (PDMS), which is the most common material for the fabrication of microfluidic devices, is gas-permeable, pressure can be generated in a microchannel when the device is degassed and put back into air just before use. Then, the porous structures of PDMS are degassed, and the negative pressure is applied to the microchannel to flow the reagents in the inlet into the microchannel through the dissolving process of the gas in the microchannel into the degassed PDMS.46 The pressure can be applied until the dead-end channels are filled with the liquid. Hosokawa et al. first reported the injection of reagents into the microchannel by using degassed PDMS for a sandwich immunoassay.47 After taking out the microfluidic device from the vacuum environment, the flow is generated in the microchannel after manual sequential loading of the reagents into the inlets. When all sides of the microfluidic devices are exposed to air, the air both from the inside and outside of the microchannel dissolves into the degassed PDMS, thereby decreasing the amount of pressure applied into the microchannel. To prevent such circumstances, it is advantageous to cover the device with non-gas-permeable materials such as epoxy, adhesive tape, parylene C, or glass over the PDMS.48,49

A degassed PDMS pump has widely been applied to enhance the reaction between biomolecules for the detection of the target analyte. The laminar flow-assisted diffusion among the multiple reagents was conducted for the detection of microRNA by enabling the reaction between biomolecules at the interface between the flow of each reagent.50,51 Identification of the bacterial strains was performed using a degassed PDMS pumping system.52 Ten microchambers were preloaded with reagents and a sample solution was loaded into each microchamber driven by a degassed PDMS pump. A degassed PDMS pump was also used to mix reagents for fluorescent DNA detection with high single-nucleotide polymorphism (SNP) discrimination.53

Blood plasma separation, another type of sample preparation, has been performed in microfluidic devices operated by the degassing process of PDMS based on the gravity-assisted sedimentation of blood cells. Dimov et al. used a filter trench to collect blood cells using gravitational force and the separated plasma was reacted with pre-spotted reagents for the detection of biomolecules.54 Yeh et al. also used gravity to separate blood plasma and compartmentalize separated blood plasma into microchambers using a vacuum battery system for digital quantification of DNA from blood.55 For compartmentalization for digital analysis, a monodispersed droplet generator was developed using a degassed PDMS pump,56 and the reagent was filled into compartmentalized microchambers (Fig. 2C).49 For wide applicability, the degassed PDMS was used as a “place and play” modular pump.57 Furthermore, in addition to the degassing process of the PDMS, a vacuum pouch was used to operate the microfluidic device (Fig. 2D).58 The negative pressure can be applied to the inlets once the vacuum pouch is activated, which was used to facilitate the mixing of reagents for the colorimetric detection of ferrous ion concentration.

Gas-generating chemical reactions

There are chemical reactions that generate gas as a reactant, which can be used to operate a microfluidic device by applying pressure into the microchannel. An effervescent reaction between sodium bicarbonate and an organic acid was used to apply pressure into the microchannel for the first time.59 Qin et al. used Pt/Ag catalyzed decomposition of diluted H2O2 to generate oxygen.60 By sticking the Pt/Ag pin into the peroxide reservoir, the generated oxygen pumped a sample solution into the microchannel for multiplexed protein assays using whole blood. An integrated effervescent pump was also developed to measure the prothrombin time which generates CO2 gas in the reaction between acid powder and pre-dissolved base in water.61 The CO2 gas generated was transferred to the microfluidic chip by tubing, and a diverse range of pressure was generated depending on the concentration of the pre-dissolved base. Besides, the Slipchip technology has been integrated for the efficient use of the effervescent pumps.62 After the injection of all the reagents required for the immunoassays through a microchannel, the chips were slipped with each other to generate other fluidic circuits and to initiate the chemical reactions for oxygen generation between Pt and H2O2.

In addition, a volumetric bar-chart chip was reported for the quantification of biomolecules according to the concentration of catalase as a result of the biochemical reactions (Fig. 2E).63 According to the concentration of catalase, the amount of oxygen generated from H2O2 was changed and represented by the traveled distance of colored inks in the microchannel. The Slipchip technology has been integrated to enable simultaneous reactions of multiple reagents in multiple chambers. The principle of the volumetric bar-chart chips was used to quantify single nucleotide variations,64 ochratoxin A,65 circulating tumor cells (CTCs),66 and cocaine.67

Hand-operated microfluidic devices

Syringe or pipette

Conventionally used simple equipment such as syringes or pipettes can be used to apply pressure into the microchannel. By connecting the outlet of the syringe or pipette to the microfluidic devices, positive or negative pressure can be applied to generate flows in microfluidic channels. Using hand-operated syringes, reagents were injected into a microfluidic device for nucleic acid extraction68 and magnetic bead-based immunoassays.69 Chin et al. used reagent-loaded Tygon tubes for multistep immunoassays in a microfluidic device.70 All reagents were preloaded in Tygon tubes, which were partitioned by air spacers in the order of reaction so that multistep immunoassays with signal amplification were simply performed by using a hand-operated syringe for detection of infectious diseases. The hand-held syringe was also used for continuous-flow PCR.71

Similar to the syringe, a transfer pipette was used to rehydrate dried reagents for PCR in a microchannel.72 A micropipette that can handle a more precise volume of reagents was used for uniform distribution of reagents into multiple centrifuge tubes through a microfabricated cartridge.73 Kim et al. used a micropipette for microflow cytometry by the injection of fluorescently-labeled cells into the microchannel.74 As the micropipette can exert both positive and negative pressure, it was used to generate a reciprocating flow for enhancing immunoreaction and effective use of antibodies in microfluidic immunohistochemistry (Fig. 3A).75 Not only the injection of reagents into the microchannel for the reaction, but also the compartmentalization for digital analysis was simply carried out using a syringe or pipette that generates droplets76,77 or injects reagents into the compartmentalized microfluidic chambers.78,79 Additionally, reagents were injected into the Slipchip using a pipette, and the chips were slipped for high-throughput digital analysis using compartmentalized chambers.80,81


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Fig. 3 Hand-operated microfluidic devices. (A) A micropipette was used to enhance the reaction efficiency of immunohistochemistry in a microfluidic device by applying both positive and negative pressure. Reproduced from ref. 75 with permission from The Royal Society of Chemistry. (B) A flow stabilizer was integrated at the end of a syringe to generate a constant flow velocity for particle concentration in a spiral microfluidic device. Reproduced from ref. 88 with permission from The Royal Society of Chemistry. (C) The pressure generated by a syringe was indirectly used to operate the microfluidic device by degassing the PDMS chamber. Reproduced from ref. 91 with permission from American Institute of Physics. (D) Finger-activated blister pouches were used to flow reagents via microfluidic channels. Reproduced from ref. 94 with permission from Springer Nature. (E) Check valves were integrated in addition to a push-button that enables repeated dispensing of reagents without backflow. Reproduced from ref. 102 with permission from The Royal Society of Chemistry. (F) The indirect pressurization method was used to dispense a constant amount of reagents regardless of differences in end-users. Reproduced from ref. 111 with permission from The Royal Society of Chemistry. (G) The hand-operated spinning top was applied to operate a microfluidic device by centrifugal force. Reproduced from ref. 118 with permission from The Royal Society of Chemistry.

Furthermore, syringes were used to operate microfluidic devices for particle separation using hydrophoresis82,83 or spiral microfluidic channels.84 The microfluidic devices can be used as syringe filters for particle sorting or concentration. However, particle separation efficiency in microfluidic devices is highly influenced by the difference in flow rate depending on the various end-users. To correct the effect of different flow rates on particle separation efficiency, a smart microfluidic pipette tip was developed using an inclined microfluidic channel and successfully separated blood plasma.85 The smart pipette was further developed to generate a constant flow rate during liquid dispensing using an air chamber that can hold compressed air.86 The smart pipette and the smart microfluidic pipette tip have been used for high-purity and high-throughput blood plasma separation from whole blood.87 Additionally, a flow stabilizer was developed in which the PDMS membrane was deflected to generate a flow resistance (Fig. 3B).88–90 Differences in the actuating pressure of the syringe depending on various end-users can be corrected through the flow stabilizer, and the blood sample was concentrated through a spiral microfluidic channel using the flow stabilizer integrated syringe.

The hand-operated syringes were used to indirectly apply pressure into microfluidic devices. Gong et al. used a membrane pump to deliver fluid to downstream components in a controlled manner (Fig. 3C).91 Due to the high flow resistance of the microfluidic channels, the compressed pressure generated by the syringe can be stored in a deflected PDMS membrane. Xu et al. utilized the gas permeability of PDMS to indirectly apply pressure into the microchannel using a syringe.92 The PDMS gas chamber was degassed by applying negative pressure with a syringe, and the gas in microfluidic channels permeates into the PDMS gas chamber so that negative pressure was applied to the microchannel. Since a large amount of the pressure change applied by the syringe was corrected by the PDMS gas chamber, a more stable flow can be generated in the microchannel.

Finger actuation

The simplest motion that people can perform is touching the button with a finger, and the pressure can be simply generated by pushing the deformable elastomer chamber. When the deformable elastomer is integrated into the microchannel, no further connector or equipment is required. The simplest method of operating microfluidic devices with finger actuation has a similar working principle to the pipette or syringe that can supply pressure to the microchannel manually. Air pouches activated by finger actuation have been developed to sequentially deliver reagents to the microchannel for blood cell counting93 or immunoassays (Fig. 3D).94,95 The positive pressure generated by actuating air pouches enables pre-stored reagents to flow into the microfluidic devices. The reagents can be contained in pouches or preloaded into the pushing chamber that enables the direct introduction of reagents into the microchannel by finger actuation. The negative pressure induced by finger actuation was also used to deliver a saliva sample on organic transistor arrays,96 or to deliver a urine sample to paper–plastic hybrid lab-on-a-chip devices for colorimetric analysis.97 The finger-actuated microfluidic devices are so useful to be integrated with a simple detection system such as a smartphone because sample preparation procedures can be carried out by just pushing the buttons without additional operation so that finger-actuated systems are used to fill the microfluidic chambers with reagents for diagnosis of infectious disease98 or for chemical sensing.99 Glynn et al. used a finger-actuated system to increase the capture efficiency of magnetic particle labeled CD4+ cells in the magnetic capture zone. By positioning two push-buttons on both sides of the capture zone, the capture efficiency was increased by flowing a mixture of magnetic particles and CD4+ cells back and forth.100 A finger-actuated peristaltic pump was also demonstrated using two pushing chambers, which can continuously generate flows at the microfluidic channels.101 Although the pressure induced by finger actuation can operate microfluidic devices, only the use of pushing chambers limits the use of finger-actuated microfluidic devices for various applications. The pushing chambers can generate positive or negative pressure into the microchannel only once, and it is difficult to apply the pressure repeatedly into the microchannel due to the backflow.

To overcome such limitations, Li et al. developed a check valve integrated finger-actuated microfluidic device (Fig. 3E).102 The push-button positioned between two check valves allows the repeated application of the pressure into the microchannel. For quantitative glucose assays, the sequential delivery of reagents into the detection region was demonstrated with two push-buttons. Various types of check valves have been used to realize finger-actuated microfluidic devices that enable the repeated generation of flow in microfluidic channels. Iwai et al. used a finger-actuated microfluidic device to generate single-cell encapsulated water-in-oil droplets,103 and Ball et al. reported a finger-actuated system for reverse transcription loop-mediated isothermal amplification (RT-LAMP) detection of pathogens.104

Finger-actuated systems can provide great convenience to end-users that do not require any external equipment and connection, but the application to conventional microfluidic devices is difficult because the device contains push-buttons. Recently, a modular finger-actuated pump has been introduced for more compatible use with various microfluidic devices, including plunger-type modular blocks for the injection of reagents into the microchannel105 and a PDMS suction cup for droplet generation.106

Although the finger-actuated microfluidic devices can simply handle broader applications of sample preparation, it has been difficult to handle an accurate volume of reagent due to the differences in end-users. A 3D-printed pumping lid was used to apply constant pressure into the microchannel, which was applied for compartmentalization by droplet generation or using Slipchip.107 Both positive and negative pressure can be applied, and the amount of pressure was controlled according to the pushed depth of the 3D-printed lid. Another device was designed to meter and dispense reagents using predesigned microfluidic chambers.108 The device was used to prepare a PCR mixture that required an accurate ratio of multiple reagents. In addition, the Digit Chip, with a sequence of microfluidic chambers connected by capillary valves allowing programmable dispensing of picoliter droplets, was introduced for antibiotic susceptibility testing in which the pressure of the finger actuation was corrected to control the accurate volume in a device.109 The indirect pressurization method has also been developed to reduce user-dependent variations that indirectly apply pressure into the microchannel by deflecting the PDMS membrane (Fig. 3F).110–112 Since the deflection of the PDMS membrane cannot exceed the height of the channels, the average applied pressure into the microchannel was corrected to be constant regardless of the differences in end-users. Recently, the indirect pressurization method was applied for concentration gradient generation,110 blood cross-matching test,111 and smart blood typing.112

Others

In addition to syringe, pipette, or finger actuation, there are a few applications of hand-operated microfluidic devices. Korir et al. used a punch card and a hand crank for programmable microfluidics.113 A mechanical actuator reads information encoded in punched holes of paper tape, and the gears in the hand crank operate microfluidic devices on it, which has been utilized for a multiplexed assay by generating a sequence of droplets. A porous PDMS sponge was also used as a portable pressure pump for the blood agglutination assay in a microfluidic device.114 A working fluid was manually loaded into the porous PDMS pump, and it was positioned at the inlets of the microfluidic devices and pushed manually to apply pressure to the microchannel. Thurgood et al. used latex balloons for the operation of microfluidic devices to generate droplets.115 By connecting a balloon and a reagent-stored syringe using a PVC tube and a valve, pressure can be applied to the microchannel on-demand. The flow rate of the pump was controlled by varying the size and thickness of the balloon. Reinforced balloons were further developed to increase the inflation pressure by coating fabric layers over the latex balloon.116

Recently, a hand-operated paper-based centrifuge was developed that can replace the conventional bench-top centrifuge, which is called “paperfuge”.117 Blood plasma was successfully separated from whole blood using a paperfuge, which showed the potential to operate centrifugal microfluidic devices without a bench-top centrifuge. In this manner, a centrifugal microfluidic device was operated using the spinning top (Fig. 3G).118 The zeolite-based purification of nucleic acids, the isothermal amplification, and the visual detection of fluorescence signals were integrated into the spinning disc using microfluidic channels and were sequentially performed for the detection of nucleic acids in bacteria.

Integrated user-friendly microfluidic circuits

Simple microfluidic valves

Several user-friendly methods have been developed to operate microfluidic devices for practical sample preparation in POCT, but the flow direction control by the integration of microfluidic valves is needed for wider applications. Conventionally, the deformation of a PDMS membrane induced by external pressure has been used as a common working principle of the microfluidic valves. Based on this working principle, various microfluidic valves have been developed.119–122 However, due to the need for an external pressure supplier, these are unsuitable for integration into user-friendly microfluidic devices.

To facilitate flow direction control without the need for external pressure suppliers, simple microfluidic valves have been developed over the decades, which are classified into three groups, including passive, active, and check valves. Passive microfluidic valves can control the flow direction without additional manual operation according to the hydrophobicity of the microchannel123,124 and the geometry of the microchannel (Fig. 4A).125–127 However, passive valves are limited to single-use and they cannot withstand a large amount of pressure. On the other hand, active valves can withstand a large amount of pressure and can be used repeatedly, but they require additional manual operations such as screwing,128–130 alignment of holes,131,132 and actuation of magnets (Fig. 4B).133–135 Active valves are suitable for on-demand flow direction control, but they are difficult to use in large scale integration for high-throughput assays due to the need for manual operation. As a compromise, check valves have been introduced, which are turned on and off by the movement of microstructures in the microchannel depending on the type of pressure applied (Fig. 4C).104,136 Check valves can withstand higher pressure than passive valves and can be used repeatedly without additional manual operations.


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Fig. 4 Simple microfluidic valves. (A) Capillary burst valves were used to guide the flow direction depending on the geometry of the microfluidic channel. Reproduced from ref. 127 with permission from John Wiley & Sons. (B) An aligning valve was utilized to open and close the microfluidic channel by controlling the alignment between the microhole in the rod and the microfluidic channel. Reproduced from ref. 131 with permission from Elsevier. (C) The movement of the microstructures in a microfluidic channel opens and closes the microfluidic channel depending on the direction of the applied pressure. Reproduced from ref. 136 with permission from Springer Nature.

Integration of simple microfluidic valves into user-friendly microfluidic devices

Simple microfluidic valves have been integrated into user-friendly microfluidic devices to achieve complicated microfluidic circuits for wider applications. For self-powered microfluidic devices, passive valves are appropriate as no additional manual operation is required after the loading of reagents. The integration of active valves would hamper the advantages of self-operated microfluidic devices. Capillary burst valves have been integrated into capillary force driven microfluidic devices, which do not require any manual operation after loading the reagents. Capillary burst valves play an important role in holding the loaded reagents until they burst because it is difficult to load multiple reagents simultaneously. Mohammed et al. used an autonomous capillary microfluidic system for the detection of cardiac troponin I.137 Similarly, capillary trigger valves were integrated into capillary microfluidic devices to enable the sequential delivery of preloaded multiple reagents after the injection of a sample solution, which was defined as an autonomous capillaric circuit.126 Moreover, the autonomous capillaric circuit was easily fabricated using a 3D printed mold and used for rapid and facile bacteria detection (Fig. 5A).138,139 Micro-weir structures in the microfluidic chamber have also been used as a phase guide to control filling and emptying when reagents are loaded by pipettes, which apply the meniscus pinning effect in the liquid–air interface.140,141 Dal Dosso et al. integrated hydrophobic valves into self-operated microfluidic devices, which are operated by SIMPLE that allows complex liquid manipulations after loading the reagents and activation by the finger (Fig. 5B).142 Additionally, capillary burst valves have been integrated into self-operated microfluidic devices operated by the degassing process of PDMS. Liu et al. used capillary burst valves for metering the accurate volume of reagents and reagents were loaded in parallel by applying the degassed PDMS pump for biochemical screening in a microfluidic reactor array.143 Capillary burst valves were further used to guide the flow in the microchannel induced by the dead-ended vacuum pillars for the aptamer-based detection of thrombin after effective blood plasma separation.144 Zhai et al. used capillary burst valves to stop the reagents before mixing induced by a syringe-assisted vacuum-driven flow for blood agglutination assays (Fig. 5C).145 Furthermore, capillary burst valves were used to guide the flow direction at a wearable soft microfluidic device for chrono-sampling of sweat, which drives flow using pressure induced by sweat glands.127
image file: d0lc00047g-f5.tif
Fig. 5 Integrated user-friendly microfluidic circuits. (A) Capillary burst valves were integrated into a capillary microfluidic device for sequential delivery of reagents in a programmed order.139 Published by The Royal Society of Chemistry. (B) Hydrophobic valves were applied to manipulate flow behavior in the microfluidic device operated by SIMPLE. Reproduced from ref. 142 with permission from American Chemical Society. (C) Capillary burst valves were used to guide the flow in a microfluidic device operated by a syringe-assisted vacuum-driven microfluidic device. Reproduced from ref. 145 with permission from The Royal Society of Chemistry. (D) Switching valves were integrated into a finger-actuated microfluidic device for on-demand flow direction control. Reproduced from ref. 148 with permission from The Royal Society of Chemistry.

On the other hand, for integrated user-friendly microfluidic circuits based on hand-operated methods, passive valves are of course suitable for integration as no additional operations are required. Strachan et al. integrated capillary burst valves into a microfluidic device operated by a screw pump for the colorimetric detection of bacteria.146 Since the amount of pressure generated by a screw pump is quite small, capillary burst valves were able to control the flow behavior. However, it is difficult for passive valves that rely on the surface properties of the microchannel to withstand the large amount of pressure generated by hand-operated methods. In this manner, the integration of check valves is more suitable for hand-operated methods. By integrating check valves, finger-actuated microfluidic devices have been developed that enable repeated dispensing of reagents as mentioned above.102,103,108,111 Additional check valves have been further used to control the flow direction.102,108 Active valves are also suitable for integration into hand-operated microfluidic devices when the operation principle of each is the same. When the operation principles of the hand-operated microfluidic device and the active valves do not match, the additional operation procedures for the active valves hamper the simplicity of the device. Im et al. demonstrated the simultaneous operation of microfluidic pumps and valves based on the deformability of PDMS by rolling the bar over the PDMS for the detection of cardiac troponin T by enzyme-linked immunosorbent assay,147 but rolling the bar is less user-friendly, and it is difficult to use on-demand. Very recently, integrated microfluidic pumps and valves were developed, which were operated simultaneously with just a single push-button (Fig. 5D).148 As the additional operation for active valves is not required, on-demand flow control was realized by just pushing the buttons, which was demonstrated for microfluidic nucleic acid purification.

Summary and future perspectives

Self-operated microfluidic devices provide great convenience to end-users as any other manual operations and equipment are not required after loading the reagents. The flow velocity or the amount of flow can be controllable depending on the design of the device. However, their flow velocity is too slow so that their applications are limited, and the analysis time takes longer. In this manner, self-operated microfluidic devices are more suitable for applications such as the injection of reagents for reaction into a microchannel or compartmentalization for digital analysis. Size-based filtration or gravity force-assisted particle separation can be handled in self-operated microfluidic devices, but it is difficult to perform particle separation based on hydrophoresis or inertial forces due to the limited flow rate. Additionally, self-operated microfluidic devices can only generate a continuous flow in a single direction, which makes it difficult to handle various flow mechanisms for wide applications because the pressure can only be generated in a single direction. With the integration of passive valves, the sequence of the fluid flow and the direction of flow were controlled. Since the reagents are initiated to flow into the microchannel immediately after loading, multiple reagents should be loaded simultaneously, or passive valves must be integrated to hold the loaded reagents before the flow is initiated.

Meanwhile, hand-operated microfluidic devices can generate a wide range of pressure into the microchannel so that a wider range of flow rates can be achieved compared to self-operated microfluidic devices. It is more beneficial for on-demand flow control in the microchannel because a wide range of flow rates can be achieved and both positive and negative pressure can be applied. Simple equipment such as a syringe or pipette has been utilized for the injection or transferring of reagents that can control a set amount of flow. However, the need for the connection between microfluidic devices and additional equipment can make the overall system complicated. On the other hand, finger actuation has been widely applied for the injection or transferring of reagents by simply pushing the button without any external equipment. However, it has been difficult to control a constant amount of flow depending on the differences in various end-users. To compensate for such limitations, several working principles have been developed to reduce user-dependent variation in the controllable volume. Hand-operated microfluidic devices are also beneficial for microfluidic particle separation using hydrophoresis or inertial force due to the wide range of flow rates. Syringes or pipettes can handle a set amount of volume in microfluidic devices, but the different flow velocities in various end-users should be corrected for the constant performance of the devices. To achieve this goal, additional correction systems have been integrated into the microfluidic devices operated by a syringe or pipette, but it is still challenging to generate a constant flow velocity in other kinds of hand-operated microfluidic devices.

In order to handle various applications, integrated user-friendly microfluidic circuits should be realized to operate complicated fluidic circuits. For this purpose, passive, active, and check valves have been integrated into user-friendly microfluidic devices to control the flow direction. Passive valves do not require additional manual operation, but they cannot withstand a large amount of pressure, which is suitable for integration into self-operated microfluidic devices that generate a small amount of pressure into the microchannel. The flow direction can be controllable without any manual operation, but the flexibility is poor because passive valves are limited to single-use. On the other hand, active and check valves are operated by a larger amount of pressure and are repeatedly usable, which are compatible with hand-operated microfluidic devices. Check valves do not require additional manual operation but are difficult to use on-demand, while active valves are controllable on-demand, but do require additional manual operation. Additionally, the design of fluidic circuits is freer for the integration of active valves, but the need for manual operation in addition to the manual operation for pumping can hamper the simplicity of the devices. The same working principle of the active valves with the hand-operated microfluidic device would be ideal for on-demand flow control.

Various user-friendly microfluidic devices have been developed for practical sample preparation in POCT, including the mixing of reagents, biochemical reactions, particle separation, and compartmentalization for digital analysis. Simple microfluidic valves have been further integrated to realize more complicated microfluidic circuits for handling more functions. However, it is still challenging to achieve a performance similar to that of conventional microfluidic devices. More accurate flow control should be enabled such as on-demand flow velocity, volume and direction control. As the performance of the devices can be different depending on various end-users, correction systems are required for the universality of the devices. Furthermore, large scale integration of user-friendly microfluidic devices is also a challenging aspect for high-throughput assays. In the future, we expect that user-friendly microfluidic devices may be widely used as sample preparation tools for POCT by non-experts even in resource-limited settings. With the integration with simple detection platforms, a user-friendly “sample-in-answer-out” system would be realized.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean government (MSIT) (NRF-2019R1A2B5B03070494, NRF-2015M3A9B3028685, and NRF-2017M3A7B4039936).

Notes and references

  1. S. Nayak, N. R. Blumenfeld, T. Laksanasopin and S. K. Sia, Anal. Chem., 2017, 89, 102–123 CrossRef CAS PubMed .
  2. T. R. Kozel and A. R. Burnham-Marusich, J. Clin. Microbiol., 2017, 55, 2313–2320 CrossRef CAS PubMed .
  3. D. C. Christodouleas, B. Kaur and P. Chorti, ACS Cent. Sci., 2018, 4, 1600–1616 CrossRef CAS PubMed .
  4. P. Wang and L. J. Kricka, Clin. Chem., 2018, 64, 1439–1452 CrossRef CAS PubMed .
  5. J. L. V. Shaw, Pract. Lab. Med., 2016, 4, 22–29 CrossRef PubMed .
  6. J. R. Choi, K. W. Yong, J. Y. Choi and A. C. Cowie, Sensors, 2019, 19, 817 CrossRef PubMed .
  7. H. Chen, K. Liu, Z. Li and P. Wang, Clin. Chim. Acta, 2019, 493, 138–147 CrossRef CAS PubMed .
  8. B. Nasseri, N. Soleimani, N. Rabiee, A. Kalbasi, M. Karimi and M. R. Hamblin, Biosens. Bioelectron., 2018, 117, 112–128 CrossRef CAS PubMed .
  9. H. Kim, D. R. Chung and M. Kang, Analyst, 2019, 144, 2460–2466 RSC .
  10. C. Dincer, R. Bruch, A. Kling, P. S. Dittrich and G. A. Urban, Trends Biotechnol., 2017, 35, 728–742 CrossRef CAS PubMed .
  11. W. He, M. You, W. Wan, F. Xu, F. Li and A. Li, Trends Biotechnol., 2018, 36, 1127–1144 CrossRef CAS PubMed .
  12. C. S. Kosack, A.-L. Page and P. R. Klatser, Bull. W. H. O., 2017, 95, 639 CrossRef PubMed .
  13. C. Carrell, A. Kava, M. Nguyen, R. Menger, Z. Munshi, Z. Call, M. Nussbaum and C. Henry, Microelectron. Eng., 2019, 206, 45–54 CrossRef CAS .
  14. T. Akyazi, L. Basabe-Desmonts and F. Benito-Lopez, Anal. Chim. Acta, 2018, 1001, 1–17 CrossRef CAS PubMed .
  15. X. Huang, D. Xu, J. Chen, J. Liu, Y. Li, J. Song, X. Ma and J. Guo, Analyst, 2018, 143, 5339–5351 RSC .
  16. A. Roda, E. Michelini, M. Zangheri, M. Di Fusco, D. Calabria and P. Simoni, TrAC, Trends Anal. Chem., 2016, 79, 317–325 CrossRef CAS .
  17. W. Jung, J. Han, J.-W. Choi and C. H. Ahn, Microelectron. Eng., 2015, 132, 46–57 CrossRef CAS .
  18. C. M. Pandey, S. Augustine, S. Kumar, S. Kumar, S. Nara, S. Srivastava and B. D. Malhotra, Biotechnol. J., 2018, 13, 1700047 CrossRef PubMed .
  19. A. K. Au, H. Lai, B. R. Utela and A. Folch, Micromachines, 2011, 2, 179–220 CrossRef .
  20. D. J. Laser and J. G. Santiago, J. Micromech. Microeng., 2004, 14, R35–R64 CrossRef .
  21. K. W. Oh and C. H. Ahn, J. Micromech. Microeng., 2006, 16, R13–R39 CrossRef .
  22. G. Comina, A. Suska and D. Filippini, Biosens. Bioelectron., 2016, 77, 1153–1167 CrossRef CAS PubMed .
  23. C. K. Byun, K. Abi-Samra, Y. K. Cho and S. Takayama, Electrophoresis, 2014, 35, 245–257 CrossRef CAS PubMed .
  24. P. G. de Gennes, Rev. Mod. Phys., 1985, 57, 827–863 CrossRef CAS .
  25. H. Tachibana, M. Saito, K. Tsuji, K. Yamanaka, L. Q. Hoa and E. Tamiya, Sens. Actuators, B, 2015, 206, 303–310 CrossRef CAS .
  26. A. Olanrewaju, M. Beaugrand, M. Yafia and D. Juncker, Lab Chip, 2018, 18, 2323–2347 RSC .
  27. R. Safavieh, A. Tamayol and D. Juncker, Microfluid. Nanofluid., 2015, 18, 357–366 CrossRef CAS .
  28. Y. Temiz and E. Delamarche, Sci. Rep., 2018, 8, 10603 CrossRef PubMed .
  29. H. J. Kim, B. H. Kim and Y. H. Seo, BioChip J., 2018, 12, 154–162 CrossRef CAS .
  30. L. Gervais and E. Delamarche, Lab Chip, 2009, 9, 3330–3337 RSC .
  31. S. Ghosh and C. H. Ahn, Analyst, 2019, 144, 2109–2119 RSC .
  32. P. B. Lillehoj, F. Wei and C. M. Ho, Lab Chip, 2010, 10, 2265–2270 RSC .
  33. R. Gao, Y. Wu, J. Huang, L. Song, H. Qian, X. Song, L. Cheng, R. Wang, L.-b. Luo, G. Zhao and L. Yu, Sens. Actuators, B, 2019, 286, 86–93 CrossRef CAS .
  34. R. Gao, Z. Lv, Y. Mao, L. Yu, X. Bi, S. Xu, J. Cui and Y. Wu, ACS Sens., 2019, 4, 938–943 CrossRef CAS PubMed .
  35. Y. C. Kim, S.-H. Kim, D. Kim, S.-J. Park and J.-K. Park, Sens. Actuators, B, 2010, 145, 861–868 CrossRef CAS .
  36. S. Karimi, P. Mehrdel, J. Farre-Llados and J. Casals-Terre, Lab Chip, 2019, 19, 3249–3260 RSC .
  37. B. Jose, P. McCluskey, N. Gilmartin, M. Somers, D. Kenny, A. J. Ricco, N. J. Kent and L. Basabe-Desmonts, Langmuir, 2016, 32, 2820–2828 CrossRef CAS PubMed .
  38. P. Novo, F. Volpetti, V. Chu and J. P. Conde, Lab Chip, 2013, 13, 641–645 RSC .
  39. J. S. Shim, A. W. Browne and C. H. Ahn, Biomed. Microdevices, 2010, 12, 949–957 CrossRef PubMed .
  40. C. Li, C. Liu, Z. Xu and J. Li, Microfluid. Nanofluid., 2012, 12, 829–834 CrossRef CAS .
  41. J. Wang, H. Ahmad, C. Ma, Q. Shi, O. Vermesh, U. Vermesh and J. Heath, Lab Chip, 2010, 10, 3157–3162 RSC .
  42. T. Kokalj, Y. Park, M. Vencelj, M. Jenko and L. P. Lee, Lab Chip, 2014, 14, 4329–4333 RSC .
  43. F. Dal Dosso, D. Decrop, E. Perez-Ruiz, D. Daems, H. Agten, O. Al-Ghezi, O. Bollen, J. Breukers, F. De Rop, M. Katsafadou, J. Lepoudre, L. Lyu, P. Piron, R. Saesen, S. Sels, R. Soenen, E. Staljanssens, J. Taraporewalla, T. Kokalj, D. Spasic and J. Lammertyn, Anal. Chim. Acta, 2018, 1000, 191–198 CrossRef CAS PubMed .
  44. P. Novo, V. Chu and J. P. Conde, Biosens. Bioelectron., 2014, 57, 284–291 CrossRef CAS PubMed .
  45. J. Hauser, G. Lenk, S. Ullah, O. Beck, G. Stemme and N. Roxhed, Anal. Chem., 2019, 91, 7125–7130 CrossRef CAS PubMed .
  46. L. Xu, H. Lee, D. Jetta and K. W. Oh, Lab Chip, 2015, 15, 3962–3979 RSC .
  47. K. Hosokawa, M. Omata, K. Sato and M. Maeda, Lab Chip, 2006, 6, 236–241 RSC .
  48. D. Y. Liang, A. M. Tentori, I. K. Dimov and L. P. Lee, Biomicrofluidics, 2011, 5, 024108 CrossRef PubMed .
  49. Q. Song, J. Sun, Y. Mu, Y. Xu, Q. Zhu and Q. Jin, Sens. Actuators, B, 2018, 256, 1122–1130 CrossRef CAS .
  50. H. Arata, H. Komatsu, A. Han, K. Hosokawa and M. Maeda, Analyst, 2012, 137, 3234–3237 RSC .
  51. H. Arata, H. Komatsu, K. Hosokawa and M. Maeda, PLoS One, 2012, 7, e48329 CrossRef CAS PubMed .
  52. J. Y. Ho, N. J. Cira, J. A. Crooks, J. Baeza and D. B. Weibel, PLoS One, 2012, 7, e41245 CrossRef CAS PubMed .
  53. J. Li, Y. Huang, D. Wang, B. Song, Z. Li, S. Song, L. Wang, B. Jiang, X. Zhao, J. Yan, R. Liu, D. He and C. Fan, Chem. Commun., 2013, 49, 3125–3127 RSC .
  54. I. K. Dimov, L. Basabe-Desmonts, J. L. Garcia-Cordero, B. M. Ross, Y. Park, A. J. Ricco and L. P. Lee, Lab Chip, 2011, 11, 845–850 RSC .
  55. E.-C. Yeh, C.-C. Fu, L. Hu, R. Thakur, J. Feng and L. P. Lee, Sci. Adv., 2017, 3, e1501645 CrossRef PubMed .
  56. C. Li, J. Xu and B. Ma, Microfluid. Nanofluid., 2015, 18, 1067–1073 CrossRef .
  57. G. Li, Y. Luo, Q. Chen, L. Liao and J. Zhao, Biomicrofluidics, 2012, 6, 14118–1411816 CrossRef PubMed .
  58. C. J. Lee and Y. H. Hsu, Lab Chip, 2019, 19, 2834–2843 RSC .
  59. B. T. Good, C. N. Bowman and R. H. Davis, Lab Chip, 2006, 6, 659–666 RSC .
  60. L. Qin, O. Vermesh, Q. Shi and J. R. Heath, Lab Chip, 2009, 9, 2016–2020 RSC .
  61. M. T. Guler, Z. Isiksacan, M. Serhatlioglu and C. Elbuken, Sens. Actuators, B, 2018, 273, 350–357 CrossRef CAS .
  62. J. Yang, X. Liu, Y. Pan, J. Yang, B. He, Y. Fu and Y. Song, Sens. Actuators, B, 2019, 291, 192–199 CrossRef CAS .
  63. Y. Song, Y. Zhang, P. E. Bernard, J. M. Reuben, N. T. Ueno, R. B. Arlinghaus, Y. Zu and L. Qin, Nat. Commun., 2012, 3, 1283 CrossRef PubMed .
  64. N. Shao, X. Han, Y. Song, P. Zhang and L. Qin, Anal. Chem., 2019, 91, 12384–12391 CrossRef CAS PubMed .
  65. R. Liu, Y. Huang, Y. Ma, S. Jia, M. Gao, J. Li, H. Zhang, D. Xu, M. Wu, Y. Chen, Z. Zhu and C. Yang, ACS Appl. Mater. Interfaces, 2015, 7, 6982–6990 CrossRef CAS PubMed .
  66. M. F. Abate, S. Jia, M. G. Ahmed, X. Li, L. Lin, X. Chen, Z. Zhu and C. Yang, Small, 2019, 15, e1804890 CrossRef PubMed .
  67. Z. Zhu, Z. Guan, S. Jia, Z. Lei, S. Lin, H. Zhang, Y. Ma, Z. Q. Tian and C. J. Yang, Angew. Chem., Int. Ed., 2014, 53, 12503–12507 CAS .
  68. K. Han, Y. J. Yoon, Y. Shin and M. K. Park, Lab Chip, 2016, 16, 132–141 RSC .
  69. Y. Jo, Y. K. Hahn and J.-K. Park, Microfluid. Nanofluid., 2017, 21, 74 CrossRef .
  70. C. D. Chin, T. Laksanasopin, Y. K. Cheung, D. Steinmiller, V. Linder, H. Parsa, J. Wang, H. Moore, R. Rouse, G. Umviligihozo, E. Karita, L. Mwambarangwe, S. L. Braunstein, J. van de Wijgert, R. Sahabo, J. E. Justman, W. El-Sadr and S. K. Sia, Nat. Med., 2011, 17, 1015–1019 CrossRef CAS PubMed .
  71. W. Wu, K. T. Trinh and N. Y. Lee, Analyst, 2012, 137, 983–990 RSC .
  72. D. P. Manage, J. Lauzon, A. Atrazev, R. Chavali, R. A. Samuel, B. Chan, Y. C. Morrissey, W. Gordy, A. L. Edwards, K. Larison, S. K. Yanow, J. P. Acker, G. Zahariadis and L. M. Pilarski, Lab Chip, 2013, 13, 2576–2584 RSC .
  73. P.-C. Chen, Y.-C. Chen and C.-M. Tsai, Microelectron. Eng., 2016, 150, 57–63 CrossRef CAS .
  74. B. Kim, S. Oh, S. Shin, S. G. Yim, S. Y. Yang, Y. K. Hahn and S. Choi, Anal. Chem., 2018, 90, 8254–8260 CrossRef CAS PubMed .
  75. S. Kim, S. Kwon, C. H. Cho and J.-K. Park, Lab Chip, 2017, 17, 702–709 RSC .
  76. K. Langer, N. Bremond, L. Boitard, J. Baudry and J. Bibette, Biomicrofluidics, 2018, 12, 044106 CrossRef PubMed .
  77. D. Bardin and A. P. Lee, Lab Chip, 2014, 14, 3978–3986 RSC .
  78. J. Sun, J. Hu, T. Gou, X. Ding, Q. Song, W. Wu, G. Wang, J. Yin and Y. Mu, Biosens. Bioelectron., 2019, 139, 111339 CrossRef PubMed .
  79. S. S. Bithi and S. A. Vanapalli, Sci. Rep., 2017, 7, 41707 CrossRef CAS PubMed .
  80. W. Du, L. Li, K. P. Nichols and R. F. Ismagilov, Lab Chip, 2009, 9, 2286–2292 RSC .
  81. F. Shen, W. Du, J. E. Kreutz, A. Fok and R. F. Ismagilov, Lab Chip, 2010, 10, 2666–2672 RSC .
  82. S. Yan, S. H. Tan, Y. Li, S. Tang, A. J. T. Teo, J. Zhang, Q. Zhao, D. Yuan, R. Sluyter, N. T. Nguyen and W. Li, Microfluid. Nanofluid., 2018, 22, 8 CrossRef .
  83. S. Song, M. S. Kim, J. Lee and S. Choi, Lab Chip, 2015, 15, 1250–1254 RSC .
  84. N. Xiang, X. Shi, Y. Han, Z. Shi, F. Jiang and Z. Ni, Anal. Chem., 2018, 90, 9515–9522 CrossRef CAS PubMed .
  85. S. Song, M. S. Kim and S. Choi, Small, 2014, 10, 4123–4129 CAS .
  86. B. Kim and S. Choi, Small, 2016, 12, 190–197 CrossRef CAS PubMed .
  87. B. Kim, S. Oh, D. You and S. Choi, Anal. Chem., 2017, 89, 1439–1444 CrossRef CAS PubMed .
  88. N. Xiang, Y. Han, Y. Jia, Z. Shi, H. Yi and Z. Ni, Lab Chip, 2019, 19, 214–222 RSC .
  89. X. Zhang, K. Xia, A. Ji and N. Xiang, Electrophoresis, 2019, 40, 865–872 CrossRef CAS PubMed .
  90. X. Zhang, Z. Zhu, N. Xiang and Z. Ni, Biomicrofluidics, 2016, 10, 054123 CrossRef PubMed .
  91. M. M. Gong, B. D. Macdonald, T. Vu Nguyen and D. Sinton, Biomicrofluidics, 2012, 6, 44102 CrossRef PubMed .
  92. L. Xu, H. Lee and K. W. Oh, Microfluid. Nanofluid., 2014, 17, 745–750 CrossRef CAS .
  93. S. Smith, R. Sewart, H. Becker, P. Roux and K. Land, Microfluid. Nanofluid., 2016, 20, 163 CrossRef .
  94. X. Qiu, J. A. Thompson, Z. Chen, C. Liu, D. Chen, S. Ramprasad, M. G. Mauk, S. Ongagna, C. Barber, W. R. Abrams, D. Malamud, P. L. Corstjens and H. H. Bau, Biomed. Microdevices, 2009, 11, 1175–1186 CrossRef CAS PubMed .
  95. S. W. Park, J. H. Lee, H. C. Yoon, B. W. Kim, S. J. Sim, H. Chae and S. S. Yang, Biomed. Microdevices, 2008, 10, 859–868 CrossRef CAS PubMed .
  96. A. M. Pappa, V. F. Curto, M. Braendlein, X. Strakosas, M. J. Donahue, M. Fiocchi, G. G. Malliaras and R. M. Owens, Adv. Healthcare Mater., 2016, 5, 2295–2302 CrossRef CAS PubMed .
  97. U. M. Jalal, G. J. Jin and J. S. Shim, Anal. Chem., 2017, 89, 13160–13166 CrossRef CAS PubMed .
  98. T. Laksanasopin, T. W. Guo, S. Nayak, A. A. Sridhara, S. Xie, O. O. Olowookere, P. Cadinu, F. Meng, N. H. Chee, J. Kim, C. D. Chin, E. Munyazesa, P. Mugwaneza, A. J. Rai, V. Mugisha, A. R. Castro, D. Steinmiller, V. Linder, J. E. Justman, S. Nsanzimana and S. K. Sia, Sci. Transl. Med., 2015, 7, 273re271 Search PubMed .
  99. G. Comina, A. Suska and D. Filippini, Angew. Chem., Int. Ed., 2015, 54, 8708–8712 CrossRef CAS PubMed .
  100. M. T. Glynn, D. J. Kinahan and J. Ducree, Lab Chip, 2014, 14, 2844–2851 RSC .
  101. X. Li and C. O. Chui, Microfluid. Nanofluid., 2018, 22, 14 CrossRef .
  102. W. Li, T. Chen, Z. Chen, P. Fei, Z. Yu, Y. Pang and Y. Huang, Lab Chip, 2012, 12, 1587–1590 RSC .
  103. K. Iwai, K. C. Shih, X. Lin, T. A. Brubaker, R. D. Sochol and L. Lin, Lab Chip, 2014, 14, 3790–3799 RSC .
  104. C. S. Ball, R. F. Renzi, A. Priye and R. J. Meagher, Lab Chip, 2016, 16, 4436–4444 RSC .
  105. Y. Lee, B. Kim, I. Oh and S. Choi, Small, 2018, 14, e1802769 CrossRef PubMed .
  106. S. Lee, H. Kim, W. Lee and J. Kim, Micro Nano Syst. Lett., 2018, 6, 1 CrossRef CAS .
  107. S. Begolo, D. V. Zhukov, D. A. Selck, L. Li and R. F. Ismagilov, Lab Chip, 2014, 14, 4616–4628 RSC .
  108. K. Xu, M. R. Begley and J. P. Landers, Lab Chip, 2015, 15, 867–876 RSC .
  109. A. Mepham, J. D. Besant, A. W. Weinstein, I. B. Burgess, E. H. Sargent and S. O. Kelley, Lab Chip, 2017, 17, 1505–1514 RSC .
  110. J. Park, H. Roh and J.-K. Park, Micromachines, 2019, 10, 174 CrossRef .
  111. J. Park and J.-K. Park, Lab Chip, 2018, 18, 1215–1222 RSC .
  112. J. Park and J.-K. Park, Anal. Chem., 2019, 91, 11636–11642 CrossRef CAS PubMed .
  113. G. Korir and M. Prakash, PLoS One, 2015, 10, e0115993 CrossRef PubMed .
  114. K. J. Cha and D. S. Kim, Biomed. Microdevices, 2011, 13, 877–883 CrossRef CAS PubMed .
  115. P. Thurgood, J. Y. Zhu, N. Nguyen, S. Nahavandi, A. R. Jex, E. Pirogova, S. Baratchi and K. Khoshmanesh, Lab Chip, 2018, 18, 2730–2740 RSC .
  116. P. Thurgood, S. A. Suarez, S. Chen, C. Gilliam, E. Pirogova, A. R. Jex, S. Baratchi and K. Khoshmanesh, Lab Chip, 2019, 19, 2885–2896 RSC .
  117. M. S. Bhamla, B. Benson, C. Chai, G. Katsikis, A. Johri and M. Prakash, Nat. Biomed. Eng., 2017, 1, 9 CrossRef CAS .
  118. L. Zhang, F. Tian, C. Liu, Q. Feng, T. Ma, Z. Zhao, T. Li, X. Jiang and J. Sun, Lab Chip, 2018, 18, 610–619 RSC .
  119. T. Thorsen, S. J. Maerkl and S. R. Quake, Science, 2002, 298, 580–584 CrossRef CAS PubMed .
  120. K. Hosokawa and R. Maeda, J. Micromech. Microeng., 2000, 10, 415 CrossRef CAS .
  121. D. Irimia and M. Toner, Lab Chip, 2006, 6, 345–352 RSC .
  122. J. Y. Baek, J. Y. Park, J. I. Ju, T. S. Lee and S. H. Lee, J. Micromech. Microeng., 2005, 15, 1015 CrossRef .
  123. L. Riegger, M. M. Mielnik, A. Gulliksen, D. Mark, J. Steigert, S. Lutz, M. Clad, R. Zengerle, P. Koltay and J. Hoffmann, J. Micromech. Microeng., 2010, 20, 045021 CrossRef .
  124. Y. Ouyang, S. Wang, J. Li, P. S. Riehl, M. Begley and J. P. Landers, Lab Chip, 2013, 13, 1762–1771 RSC .
  125. H. Cho, H.-Y. Kim, J. Y. Kang and T. S. Kim, J. Colloid Interface Sci., 2007, 306, 379–385 CrossRef CAS PubMed .
  126. R. Safavieh and D. Juncker, Lab Chip, 2013, 13, 4180–4189 RSC .
  127. J. Choi, D. Kang, S. Han, S. B. Kim and J. A. Rogers, Adv. Healthcare Mater., 2017, 6, 1601355 CrossRef PubMed .
  128. Y. Zheng, W. Dai and H. Wu, Lab Chip, 2009, 9, 469–472 RSC .
  129. S. E. Hulme, S. S. Shevkoplyas and G. M. Whitesides, Lab Chip, 2009, 9, 79–86 RSC .
  130. D. A. Markov, S. Manuel, L. M. Shor, S. R. Opalenik, J. P. Wikswo and P. C. Samson, Biomed. Microdevices, 2010, 12, 135–144 CrossRef PubMed .
  131. M. T. Guler, P. Beyazkilic and C. Elbuken, Sens. Actuators, A, 2017, 265, 224–230 CrossRef CAS .
  132. B. Hu, J. Li, L. Mou, Y. Liu, J. Deng, W. Qian, J. Sun, R. Cha and X. Jiang, Lab Chip, 2017, 17, 2225–2234 RSC .
  133. A. Gholizadeh and M. Javanmard, J. Microelectromech. Syst., 2016, 25, 922–928 CAS .
  134. C. Y. Chen, C. H. Chen, T. Y. Tu, C. M. Lin and A. M. Wo, Lab Chip, 2011, 11, 733–737 RSC .
  135. J. C. Harper, J. M. Andrews, C. Ben, A. C. Hunt, J. K. Murton, B. D. Carson, G. D. Bachand, J. A. Lovchik, W. D. Arndt, M. R. Finley and T. L. Edwards, Lab Chip, 2016, 16, 4142–4151 RSC .
  136. J. Hyeon and H. So, Biomed. Microdevices, 2019, 21, 19 CrossRef PubMed .
  137. M. I. Mohammed and M. P. Desmulliez, Biosens. Bioelectron., 2014, 61, 478–484 CrossRef CAS PubMed .
  138. A. O. Olanrewaju, A. Ng, P. DeCorwin-Martin, A. Robillard and D. Juncker, Anal. Chem., 2017, 89, 6846–6853 CrossRef CAS PubMed .
  139. A. O. Olanrewaju, A. Robillard, M. Dagher and D. Juncker, Lab Chip, 2016, 16, 3804–3814 RSC .
  140. P. Vulto, S. Podszun, P. Meyer, C. Hermann, A. Manz and G. A. Urban, Lab Chip, 2011, 11, 1596–1602 RSC .
  141. S. Hakenberg, M. Hugle, M. Weidmann, F. Hufert, G. Dame and G. A. Urban, Lab Chip, 2012, 12, 4576–4580 RSC .
  142. F. Dal Dosso, L. Tripodi, D. Spasic, T. Kokalj and J. Lammertyn, ACS Sens., 2019, 4, 694–703 CrossRef CAS PubMed .
  143. Y. Liu and G. Li, Sci. Rep., 2018, 8, 13664 CrossRef PubMed .
  144. S. Shin, B. Kim, Y. J. Kim and S. Choi, Biosens. Bioelectron., 2019, 133, 169–176 CrossRef CAS PubMed .
  145. Y. Zhai, A. Wang, D. Koh, P. Schneider and K. W. Oh, Lab Chip, 2018, 18, 276–284 RSC .
  146. B. C. Strachan, H. S. Sloane, E. Houpt, J. C. Lee, D. C. Miranian, J. Li, D. A. Nelson and J. P. Landers, Analyst, 2016, 141, 947–955 RSC .
  147. S. B. Im, M. J. Uddin, G. J. Jin and J. S. Shim, Lab Chip, 2018, 18, 1310–1319 RSC .
  148. J. Park and J.-K. Park, Lab Chip, 2019, 19, 2973–2977 RSC .

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