self-operated microfluidic devices

Featuring work from the NanoBiotech Laboratory of Professor Je-Kyun Park, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea.

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

Critical review of user-friendly microfl uidic devices towards practical sample preparation in point-of-care-testing, which can facilitate mixing of multiple reagents, eff ective reaction between biomolecules, separation of target particles, and compartmentalization for high-throughput or digital analysis.

As featured in:

See Je-Kyun Park et al., Lab Chip, 2020, 20, 1191.

rsc.li/locRegistered charity number: 207890

Lab on a Chip

CRITICAL REVIEW

Cite this: Lab Chip, 2020, 20, 1191

Received 14th January 2020,Accepted 19th February 2020

DOI: 10.1039/d0lc00047g

rsc.li/loc

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

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.

Introduction

Point-of-care testing (POCT) is a simple analytical test thatcan rapidly provide the medical diagnostic results near thepatients by non-experts, even in resource-limited settings.1–5

POCT is required in various fields to prevent foodborneillnesses6 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 touse with minimal training), rapid & robust (to enabletreatment on the first visit), equipment-free, and deliverable(accessible to end-users).12

For POCT, raw samples such as saliva, blood, urine, orstool need to be prepared in a suitable form for detection,and the detection results are obtained by analyzing thecolorimetric, fluorescence, or electrical signal. For idealPOCT, a “sample-in-answer-out” system should be realized

Lab Chip, 2020, 20, 1191–1203 | 1191This journal is © The Royal Society of Chemistry 2020

Juhwan Park

Juhwan Park received his B.S.,M.S., and Ph.D. degrees in bioand brain engineering from theKorea Advanced Institute ofScience and Technology (KAIST),in 2014, 2016, and 2020,respectively. His Ph.D. thesiswas on a power-free microfluidicactuator for biological samplepreparation and analysis inpoint-of-care testing (POCT)under the supervision of Prof.Je-Kyun Park. He is alsointerested 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 theNational Research Foundation (NRF) of Korea.

Dong Hyun Han

Dong Hyun Han received his B.S.degree in chemical andbiomolecular engineering fromthe Korea Advanced Institute ofScience and Technology (KAIST)in 2020. He is currentlyundertaking his M.S. degree inthe Department of Bio and BrainEngineering at KAIST. Hiscurrent research interests includemicrofluidics, lab-on-a-chip, andbioMEMS.

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: [email protected] KAIST Institute for Health Science and Technology, 291 Daehak-ro, Yuseong-gu,

Daejeon 34141, Republic of Korea

http://crossmark.crossref.org/dialog/?doi=10.1039/d0lc00047g&domain=pdf&date_stamp=2020-03-28

http://orcid.org/0000-0002-9231-9195

http://orcid.org/0000-0003-0996-4888

http://orcid.org/0000-0003-4522-2574

1192 | Lab Chip, 2020, 20, 1191–1203 This journal is © The Royal Society of Chemistry 2020

that shows the results of the detection after the injection ofthe patient's sample. Among the POCT procedures, thesample preparation procedure is the biggest bottleneck thatmust be overcome for ideal POCT, since conventional samplepreparation steps require multistep reactions based onmanual pipetting and repeated use of equipment such as acentrifuge, which takes up the longest portion of POCT.Although various simple methods for signal detection havebeen developed using microfluidic paper-based analyticaldevices13,14 and smartphones,15,16 sample preparation stepsstill rely on conventional methods that require complicatedmanual operation or expensive automated machines. POCTwithout requiring complicated sample preparation has beenwidely used for glucose level test from whole blood andpregnancy test from urine. However, a number of medicaltests requiring complicated sample preparation have notbeen widely applied for POCT.

Meanwhile, microfluidics and lab-on-a-chip technologies,also known as micro total analysis systems (μTAS), have anumber of advantages for sample preparation in POCT dueto their high surface-to-volume ratio, rapid analysis time,closed system, ability to integrate various functions and therequirement for a small volume of samples and reagents.17,18

Hence, a number of microfluidic devices for samplepreparation have been developed to facilitate the mixing ofreagents, the separation of the target sample, an effectivereaction between samples and reagents, and thecompartmentalization for high-throughput or digital analysis.However, to operate microfluidic devices, a complicatedexternal pumping system is required, which limits thepractical and wide use of microfluidic technologies forsample preparation in POCT.19–21 To overcome thislimitation, the microfluidic operation system should beportable, easy-to-use, low cost, and working withoutelectricity so that it can be used by non-experts without anyproblems (Fig. 1). When the above requirements are satisfied,

the ideal POCT, “sample-in-answer-out”, can be developed byintegrating a practical microfluidic sample preparationsystem with a simple detection system.22

Over the decades, several methods have been developed tooperate microfluidic devices without a complicated externalpumping system for various applications, including not onlyPOCT but also microfluidic cell culture. However, themethods for microfluidic cell culture using gravity, osmoticpressure and surface tension are not suitable for POCT dueto the sophisticated operating environment.23

In this review, we focus on user-friendly methods tooperate microfluidic devices for practical sample preparationin POCT. We provide an overview of the two types of simpleoperation systems for the microfluidic devices: the pressuregenerated from inside of the device (self-operated methods)and the pressure applied manually from outside of the device(hand-operated methods). Additionally, the advantages anddisadvantages of simple operation systems, as well as theirbiomedical applications, are discussed. Although theoperating system is simplified and used for the varioussample preparation procedures in POCT, there are still somelimitations compared to conventional operating systems. Theoperation of complicated microfluidic circuits is limited dueto the issues of the integration of microfluidic valves. In thisregard, the design criteria are discussed to develop user-friendly integrated microfluidic circuits by incorporatingsimple microfluidic valves. Furthermore, we discuss thefuture perspectives of user-friendly microfluidic devices forideal POCT.

Self-operated microfluidic devicesCapillary force

In a microchannel, a capillary effect occurs due to theinterface between the surface tension of the liquid and thegeometry of its channel or solid contact.24 Capillarymicrofluidics is often known as “passive” as fluidsautonomously move along a channel without external forceby principles of the capillary effect. Conventional capillarypumps have been applied to autonomously flow polymerasechain reaction (PCR) reagents for DNA amplification.25

Nevertheless, conventional capillary pumps are vulnerable tobubble entrapment and variation in volume control.26

Additionally, as the capillary flow can only be generated inthe liquid–air interface, no more flow can be generated whenthe microfluidic channels are filled with liquid. To resolvesuch bottlenecks, microstructured capillary pumps have beendeveloped using the geometry of the microstructures.27,28

The microstructured capillary pumps overcome thelimitations of conventional capillary pumps by allowing thecontrol of the flow rate and volume over a longer time by thepredetermined guidance (Fig. 2A).

Capillary pumps are also widely used to facilitateimmunoassays in microchannels. Antibodies and reagentswere spotted on the surface of microfluidic channels so thatimmunoassays can be performed by loading a sample

Je-Kyun Park

Je-Kyun Park is a professor of bioand brain engineering at theKorea Advanced Institute ofScience and Technology (KAIST).He received his Ph.D. degree inbiotechnology from KAIST in1992. Prior to joining the facultyat KAIST, he worked as apostdoctoral fellow (1996–1997)at the Johns Hopkins Universityand as Chief Research Engineer(1992–2002) at the LGElectronics Institute ofTechnology. He has co-authored

more than 160 scientific papers in the field of lab-on-a-chip andmicrofluidic analytical technologies. He was also Conference Chairof μTAS 2015 and President (2016) of the Korean BioChip Society.

Lab on a ChipCritical review


solution into an inlet.29 Other substrates for additionalsample preparation functions such as plasma separation canbe integrated into a loading port.30 By controlling the velocityof the capillary flow, a chemiluminescence immunoassayrequiring multistep reactions has been demonstrated inwhich a sample solution sequentially rehydrates reagents inmicrofluidic channels.31 In another application, capillarypumps have been used to handle the mixing of multiplereagents. Multiple reagents were simultaneously loaded intoinlets and were mixed through mixing channels for enzymaticreactions32,33 and sample preparation for surface-enhancedRaman scattering (SERS).34 In addition, capillary pumps forblood analysis were introduced in which the capillary forceinduced by the microstructures separated the blood plasmafrom the whole blood,35 and the isolated plasma was furtherused for reverse ABO/Rh blood typing.36 Jose et al. used acapillary pump for antiplatelet drug assays to assess thebinding between the platelet and fibrinogen.37 A morecomplicated flow behavior such as the sequential fluiddelivery of multiple reagents can be performed by sequentiallyconnecting multiple channels to the capillary pump.38

Furthermore, microbead-packed channels have also beenused to amplify the amount of capillary force or to separatethe blood plasma from whole blood for the immunoassay.39,40

Alternatively, the capillary pumps can be replaced by a papersubstrate. Micro-sized pores of a paper substrate can applypressure into the microchannel when the reagents in themicrochannel reach the paper substrate.41 Without thefabrication of additional microstructures, a wide range ofcapillary flow can be generated depending on the pore size ofthe paper substrate and is applicable to conventionalmicrofluidic devices. As with a capillary pump, the flow relieson the capillary force induced by the microchannel before the

flow reaches the paper substrate. Therefore, to initiate thecapillary flow induced by the paper substrate, the pressuregenerated from finger actuation was used, which is calledSIMPLE (self-powered imbibing microfluidic pump by liquidencapsulation).42 SIMPLE autonomously delivers fluid to thetarget destination based on the working principle that paperabsorbs the working liquid until the paper is saturated(Fig. 2B). Dal Dosso et al. exploited the SIMPLE technology todetect creatinine in plasma using enzymatic reactions.43 Novoet al. used a paper substrate to generate a capillary flow forautonomous microfluidic immunoassays by sequential deliveryof reagents through a microfluidic circuit.44 Furthermore, thepaper substrate at the end of the microchannel was used notonly to induce capillary flow but also to store volume-defineddried plasma spots for further analysis.45

Vacuum-driven pressure

Since polyĲdimethylsiloxane) (PDMS), which is the mostcommon material for the fabrication of microfluidic devices, isgas-permeable, pressure can be generated in a microchannelwhen the device is degassed and put back into air just beforeuse. Then, the porous structures of PDMS are degassed, andthe negative pressure is applied to themicrochannel to flow thereagents in the inlet into the microchannel through thedissolving process of the gas in the microchannel into thedegassed PDMS.46 The pressure can be applied until the dead-end channels are filled with the liquid. Hosokawa et al. firstreported the injection of reagents into the microchannel byusing degassed PDMS for a sandwich immunoassay.47 Aftertaking out the microfluidic device from the vacuumenvironment, the flow is generated in the microchannel aftermanual sequential loading of the reagents into the inlets.

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

Lab on a Chip Critical review


When all sides of the microfluidic devices are exposed to air,the air both from the inside and outside of the microchanneldissolves into the degassed PDMS, thereby decreasing theamount of pressure applied into the microchannel. To preventsuch circumstances, it is advantageous to cover the device withnon-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 toenhance the reaction between biomolecules for the detectionof the target analyte. The laminar flow-assisted diffusionamong the multiple reagents was conducted for the detectionof microRNA by enabling the reaction between biomoleculesat 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 werepreloaded with reagents and a sample solution was loadedinto each microchamber driven by a degassed PDMS pump.A degassed PDMS pump was also used to mix reagents forfluorescent DNA detection with high single-nucleotidepolymorphism (SNP) discrimination.53

Blood plasma separation, another type of samplepreparation, has been performed in microfluidic devicesoperated by the degassing process of PDMS based on thegravity-assisted sedimentation of blood cells. Dimov et al. useda filter trench to collect blood cells using gravitational forceand the separated plasma was reacted with pre-spottedreagents for the detection of biomolecules.54 Yeh et al. alsoused gravity to separate blood plasma and compartmentalize

Fig. 2 Self-operated microfluidic devices. (A) A capillary pump with microstructures was integrated at the end of the microfluidic channel tomaintain a flow for a longer time. Reproduced from ref. 27 with permission from Springer Nature. (B) A paper substrate was used to apply pressureinto 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 thatenables the filling of reagents into the dead-end microchambers. Reproduced from ref. 49 with permission from Elsevier. (D) A microfluidic devicewas 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 microfluidicchannel. Reproduced from ref. 63 with permission from Springer Nature.



separated blood plasma into microchambers using a vacuumbattery system for digital quantification of DNA from blood.55

For compartmentalization for digital analysis, a monodisperseddroplet generator was developed using a degassed PDMSpump,56 and the reagent was filled into compartmentalizedmicrochambers (Fig. 2C).49 For wide applicability, the degassedPDMS was used as a “place and play” modular pump.57

Furthermore, in addition to the degassing process of thePDMS, a vacuum pouch was used to operate the microfluidicdevice (Fig. 2D).58 The negative pressure can be applied to theinlets once the vacuum pouch is activated, which was used tofacilitate the mixing of reagents for the colorimetric detectionof 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 byapplying pressure into the microchannel. An effervescentreaction between sodium bicarbonate and an organic acidwas used to apply pressure into the microchannel for the firsttime.59 Qin et al. used Pt/Ag catalyzed decomposition ofdiluted H2O2 to generate oxygen.60 By sticking the Pt/Ag pininto the peroxide reservoir, the generated oxygen pumped asample solution into the microchannel for multiplexedprotein assays using whole blood. An integrated effervescentpump was also developed to measure the prothrombin timewhich generates CO2 gas in the reaction between acid powderand pre-dissolved base in water.61 The CO2 gas generated wastransferred to the microfluidic chip by tubing, and a diverserange of pressure was generated depending on theconcentration of the pre-dissolved base. Besides, the Slipchiptechnology has been integrated for the efficient use of theeffervescent pumps.62 After the injection of all the reagentsrequired for the immunoassays through a microchannel, thechips were slipped with each other to generate other fluidiccircuits and to initiate the chemical reactions for oxygengeneration between Pt and H2O2.

In addition, a volumetric bar-chart chip was reported forthe quantification of biomolecules according to theconcentration of catalase as a result of the biochemicalreactions (Fig. 2E).63 According to the concentration ofcatalase, the amount of oxygen generated from H2O2 waschanged and represented by the traveled distance of coloredinks in the microchannel. The Slipchip technology has beenintegrated to enable simultaneous reactions of multiplereagents in multiple chambers. The principle of thevolumetric bar-chart chips was used to quantify singlenucleotide variations,64 ochratoxin A,65 circulating tumorcells (CTCs),66 and cocaine.67

Hand-operated microfluidic devicesSyringe or pipette

Conventionally used simple equipment such as syringes orpipettes 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 beapplied to generate flows in microfluidic channels. Usinghand-operated syringes, reagents were injected into amicrofluidic device for nucleic acid extraction68 and magneticbead-based immunoassays.69 Chin et al. used reagent-loadedTygon tubes for multistep immunoassays in a microfluidicdevice.70 All reagents were preloaded in Tygon tubes, whichwere partitioned by air spacers in the order of reaction sothat multistep immunoassays with signal amplification weresimply performed by using a hand-operated syringe fordetection of infectious diseases. The hand-held syringe wasalso used for continuous-flow PCR.71

Similar to the syringe, a transfer pipette was used torehydrate dried reagents for PCR in a microchannel.72 Amicropipette that can handle a more precise volume ofreagents was used for uniform distribution of reagents intomultiple centrifuge tubes through a microfabricatedcartridge.73 Kim et al. used a micropipette for microflowcytometry by the injection of fluorescently-labeled cells into themicrochannel.74 As the micropipette can exert both positiveand negative pressure, it was used to generate a reciprocatingflow for enhancing immunoreaction and effective use ofantibodies in microfluidic immunohistochemistry (Fig. 3A).75

Not only the injection of reagents into the microchannel forthe reaction, but also the compartmentalization for digitalanalysis was simply carried out using a syringe or pipette thatgenerates droplets76,77 or injects reagents into thecompartmentalized microfluidic chambers.78,79 Additionally,reagents were injected into the Slipchip using a pipette, andthe chips were slipped for high-throughput digital analysisusing compartmentalized chambers.80,81

Furthermore, syringes were used to operate microfluidicdevices for particle separation using hydrophoresis82,83 orspiral microfluidic channels.84 The microfluidic devices canbe used as syringe filters for particle sorting orconcentration. However, particle separation efficiency inmicrofluidic devices is highly influenced by the differencein flow rate depending on the various end-users. To correctthe effect of different flow rates on particle separationefficiency, a smart microfluidic pipette tip was developedusing an inclined microfluidic channel and successfullyseparated blood plasma.85 The smart pipette was furtherdeveloped to generate a constant flow rate during liquiddispensing using an air chamber that can hold compressedair.86 The smart pipette and the smart microfluidic pipettetip have been used for high-purity and high-throughputblood plasma separation from whole blood.87 Additionally, aflow stabilizer was developed in which the PDMS membranewas deflected to generate a flow resistance (Fig. 3B).88–90

Differences in the actuating pressure of the syringedepending on various end-users can be corrected throughthe flow stabilizer, and the blood sample was concentratedthrough a spiral microfluidic channel using the flowstabilizer integrated syringe.

The hand-operated syringes were used to indirectly applypressure into microfluidic devices. Gong et al. used a



membrane pump to deliver fluid to downstreamcomponents in a controlled manner (Fig. 3C).91 Due to thehigh flow resistance of the microfluidic channels, thecompressed pressure generated by the syringe can be storedin a deflected PDMS membrane. Xu et al. utilized the gaspermeability of PDMS to indirectly apply pressure into themicrochannel 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 thePDMS gas chamber so that negative pressure was applied tothe microchannel. Since a large amount of the pressurechange applied by the syringe was corrected by the PDMSgas chamber, a more stable flow can be generated in themicrochannel.

Fig. 3 Hand-operated microfluidic devices. (A) A micropipette was used to enhance the reaction efficiency of immunohistochemistry in amicrofluidic device by applying both positive and negative pressure. Reproduced from ref. 75 with permission from The Royal Society ofChemistry. (B) A flow stabilizer was integrated at the end of a syringe to generate a constant flow velocity for particle concentration in a spiralmicrofluidic device. Reproduced from ref. 88 with permission from The Royal Society of Chemistry. (C) The pressure generated by a syringe wasindirectly used to operate the microfluidic device by degassing the PDMS chamber. Reproduced from ref. 91 with permission from AmericanInstitute of Physics. (D) Finger-activated blister pouches were used to flow reagents via microfluidic channels. Reproduced from ref. 94 withpermission from Springer Nature. (E) Check valves were integrated in addition to a push-button that enables repeated dispensing of reagentswithout backflow. Reproduced from ref. 102 with permission from The Royal Society of Chemistry. (F) The indirect pressurization method was usedto dispense a constant amount of reagents regardless of differences in end-users. Reproduced from ref. 111 with permission from The RoyalSociety 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.



Finger actuation

The simplest motion that people can perform is touching thebutton with a finger, and the pressure can be simply generatedby pushing the deformable elastomer chamber. When thedeformable elastomer is integrated into the microchannel, nofurther connector or equipment is required. The simplestmethod of operating microfluidic devices with finger actuationhas a similar working principle to the pipette or syringe thatcan supply pressure to the microchannel manually. Air pouchesactivated by finger actuation have been developed tosequentially deliver reagents to the microchannel for blood cellcounting93 or immunoassays (Fig. 3D).94,95 The positivepressure generated by actuating air pouches enables pre-storedreagents to flow into the microfluidic devices. The reagents canbe contained in pouches or preloaded into the pushingchamber that enables the direct introduction of reagents intothe microchannel by finger actuation. The negative pressureinduced by finger actuation was also used to deliver a salivasample on organic transistor arrays,96 or to deliver a urinesample to paper–plastic hybrid lab-on-a-chip devices forcolorimetric analysis.97 The finger-actuated microfluidic devicesare so useful to be integrated with a simple detection systemsuch as a smartphone because sample preparation procedurescan be carried out by just pushing the buttons withoutadditional operation so that finger-actuated systems are used tofill the microfluidic chambers with reagents for diagnosis ofinfectious disease98 or for chemical sensing.99 Glynn et al. useda finger-actuated system to increase the capture efficiency ofmagnetic particle labeled CD4+ cells in the magnetic capturezone. By positioning two push-buttons on both sides of thecapture zone, the capture efficiency was increased by flowing amixture of magnetic particles and CD4+ cells back and forth.100

A finger-actuated peristaltic pump was also demonstrated usingtwo pushing chambers, which can continuously generate flowsat the microfluidic channels.101 Although the pressure inducedby finger actuation can operate microfluidic devices, only theuse of pushing chambers limits the use of finger-actuatedmicrofluidic devices for various applications. The pushingchambers can generate positive or negative pressure into themicrochannel only once, and it is difficult to apply the pressurerepeatedly into the microchannel due to the backflow.

To overcome such limitations, Li et al. developed a checkvalve integrated finger-actuated microfluidic device (Fig. 3E).102

The push-button positioned between two check valves allowsthe repeated application of the pressure into the microchannel.For quantitative glucose assays, the sequential delivery ofreagents into the detection region was demonstrated with twopush-buttons. Various types of check valves have been used torealize finger-actuated microfluidic devices that enable therepeated 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 transcriptionloop-mediated isothermal amplification (RT-LAMP) detectionof pathogens.104

Finger-actuated systems can provide great convenience toend-users that do not require any external equipment andconnection, but the application to conventional microfluidicdevices is difficult because the device contains push-buttons.Recently, a modular finger-actuated pump has beenintroduced for more compatible use with various microfluidicdevices, including plunger-type modular blocks for theinjection of reagents into the microchannel105 and a PDMSsuction cup for droplet generation.106

Although the finger-actuated microfluidic devices cansimply handle broader applications of sample preparation, ithas been difficult to handle an accurate volume of reagentdue to the differences in end-users. A 3D-printed pumpinglid was used to apply constant pressure into themicrochannel, which was applied for compartmentalizationby droplet generation or using Slipchip.107 Both positive andnegative pressure can be applied, and the amount of pressurewas controlled according to the pushed depth of the 3D-printed lid. Another device was designed to meter anddispense reagents using predesigned microfluidicchambers.108 The device was used to prepare a PCR mixturethat required an accurate ratio of multiple reagents. Inaddition, the Digit Chip, with a sequence of microfluidicchambers connected by capillary valves allowingprogrammable dispensing of picoliter droplets, wasintroduced for antibiotic susceptibility testing in which thepressure of the finger actuation was corrected to control theaccurate volume in a device.109 The indirect pressurizationmethod has also been developed to reduce user-dependentvariations that indirectly apply pressure into the microchannel bydeflecting the PDMS membrane (Fig. 3F).110–112 Since thedeflection of the PDMS membrane cannot exceed the heightof the channels, the average applied pressure into themicrochannel was corrected to be constant regardless of thedifferences in end-users. Recently, the indirect pressurizationmethod 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 afew applications of hand-operated microfluidic devices. Koriret al. used a punch card and a hand crank for programmablemicrofluidics.113 A mechanical actuator reads informationencoded in punched holes of paper tape, and the gears in thehand crank operate microfluidic devices on it, which hasbeen utilized for a multiplexed assay by generating asequence of droplets. A porous PDMS sponge was also usedas a portable pressure pump for the blood agglutinationassay in a microfluidic device.114 A working fluid wasmanually loaded into the porous PDMS pump, and it waspositioned at the inlets of the microfluidic devices andpushed manually to apply pressure to the microchannel.Thurgood et al. used latex balloons for the operation ofmicrofluidic devices to generate droplets.115 By connecting aballoon 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 varyingthe size and thickness of the balloon. Reinforced balloonswere further developed to increase the inflation pressure bycoating fabric layers over the latex balloon.116

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

Integrated user-friendly microfluidiccircuitsSimple microfluidic valves

Several user-friendly methods have been developed to operatemicrofluidic devices for practical sample preparation inPOCT, but the flow direction control by the integration ofmicrofluidic valves is needed for wider applications.Conventionally, the deformation of a PDMS membraneinduced by external pressure has been used as a commonworking principle of the microfluidic valves. Based on thisworking principle, various microfluidic valves have beendeveloped.119–122 However, due to the need for an externalpressure supplier, these are unsuitable for integration intouser-friendly microfluidic devices.

To facilitate flow direction control without the need forexternal pressure suppliers, simple microfluidic valves havebeen developed over the decades, which are classified intothree groups, including passive, active, and check valves.Passive microfluidic valves can control the flow directionwithout additional manual operation according to thehydrophobicity of the microchannel123,124 and the geometryof the microchannel (Fig. 4A).125–127 However, passive valvesare limited to single-use and they cannot withstand a largeamount of pressure. On the other hand, active valves canwithstand a large amount of pressure and can be usedrepeatedly, but they require additional manual operationssuch as screwing,128–130 alignment of holes,131,132 andactuation of magnets (Fig. 4B).133–135 Active valves aresuitable for on-demand flow direction control, but they aredifficult to use in large scale integration for high-throughputassays due to the need for manual operation. As acompromise, check valves have been introduced, which areturned on and off by the movement of microstructures in themicrochannel depending on the type of pressure applied(Fig. 4C).104,136 Check valves can withstand higher pressurethan passive valves and can be used repeatedly withoutadditional manual operations.

Integration of simple microfluidic valves into user-friendlymicrofluidic devices

Simple microfluidic valves have been integrated into user-friendly microfluidic devices to achieve complicatedmicrofluidic circuits for wider applications. For self-poweredmicrofluidic devices, passive valves are appropriate as noadditional manual operation is required after the loading ofreagents. The integration of active valves would hamper theadvantages of self-operated microfluidic devices. Capillaryburst valves have been integrated into capillary force drivenmicrofluidic devices, which do not require any manualoperation after loading the reagents. Capillary burst valvesplay an important role in holding the loaded reagents untilthey burst because it is difficult to load multiple reagentssimultaneously. Mohammed et al. used an autonomouscapillary microfluidic system for the detection of cardiactroponin I.137 Similarly, capillary trigger valves wereintegrated into capillary microfluidic devices to enable thesequential delivery of preloaded multiple reagents after theinjection of a sample solution, which was defined as an

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



autonomous capillaric circuit.126 Moreover, the autonomouscapillaric circuit was easily fabricated using a 3D printedmold and used for rapid and facile bacteria detection (Fig. 5A).138,139 Micro-weir structures in the microfluidic chamberhave also been used as a phase guide to control filling andemptying when reagents are loaded by pipettes, which applythe 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 SIMPLEthat allows complex liquid manipulations after loading thereagents and activation by the finger (Fig. 5B).142

Additionally, capillary burst valves have been integrated intoself-operated microfluidic devices operated by the degassingprocess of PDMS. Liu et al. used capillary burst valves formetering the accurate volume of reagents and reagents wereloaded in parallel by applying the degassed PDMS pump forbiochemical screening in a microfluidic reactor array.143

Capillary burst valves were further used to guide the flow inthe microchannel induced by the dead-ended vacuum pillarsfor the aptamer-based detection of thrombin after effectiveblood plasma separation.144 Zhai et al. used capillary burst

valves to stop the reagents before mixing induced by asyringe-assisted vacuum-driven flow for blood agglutinationassays (Fig. 5C).145 Furthermore, capillary burst valves wereused to guide the flow direction at a wearable softmicrofluidic device for chrono-sampling of sweat, whichdrives flow using pressure induced by sweat glands.127

On the other hand, for integrated user-friendlymicrofluidic circuits based on hand-operated methods,passive valves are of course suitable for integration as noadditional operations are required. Strachan et al. integratedcapillary burst valves into a microfluidic device operated by ascrew pump for the colorimetric detection of bacteria.146

Since the amount of pressure generated by a screw pump isquite small, capillary burst valves were able to control theflow behavior. However, it is difficult for passive valves thatrely on the surface properties of the microchannel towithstand the large amount of pressure generated by hand-operated methods. In this manner, the integration of checkvalves is more suitable for hand-operated methods. Byintegrating check valves, finger-actuated microfluidic deviceshave been developed that enable repeated dispensing of

Fig. 5 Integrated user-friendly microfluidic circuits. (A) Capillary burst valves were integrated into a capillary microfluidic device for sequentialdelivery of reagents in a programmed order.139 Published by The Royal Society of Chemistry. (B) Hydrophobic valves were applied to manipulateflow 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-actuatedmicrofluidic device for on-demand flow direction control. Reproduced from ref. 148 with permission from The Royal Society of Chemistry.



reagents as mentioned above.102,103,108,111 Additional checkvalves have been further used to control the flowdirection.102,108 Active valves are also suitable for integrationinto hand-operated microfluidic devices when the operationprinciple of each is the same. When the operation principlesof the hand-operated microfluidic device and the active valvesdo not match, the additional operation procedures for theactive valves hamper the simplicity of the device. Im et al.demonstrated the simultaneous operation of microfluidicpumps and valves based on the deformability of PDMS byrolling the bar over the PDMS for the detection of cardiactroponin T by enzyme-linked immunosorbent assay,147 butrolling the bar is less user-friendly, and it is difficult to useon-demand. Very recently, integrated microfluidic pumpsand valves were developed, which were operatedsimultaneously 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 thebuttons, which was demonstrated for microfluidic nucleicacid purification.

Summary and future perspectives

Self-operated microfluidic devices provide great convenienceto end-users as any other manual operations and equipmentare not required after loading the reagents. The flow velocityor the amount of flow can be controllable depending on thedesign of the device. However, their flow velocity is too slowso that their applications are limited, and the analysis timetakes longer. In this manner, self-operated microfluidicdevices are more suitable for applications such as theinjection of reagents for reaction into a microchannel orcompartmentalization for digital analysis. Size-basedfiltration or gravity force-assisted particle separation can behandled in self-operated microfluidic devices, but it isdifficult to perform particle separation based onhydrophoresis or inertial forces due to the limited flow rate.Additionally, self-operated microfluidic devices can onlygenerate a continuous flow in a single direction, whichmakes it difficult to handle various flow mechanisms forwide applications because the pressure can only be generatedin a single direction. With the integration of passive valves,the sequence of the fluid flow and the direction of flow werecontrolled. Since the reagents are initiated to flow into themicrochannel immediately after loading, multiple reagentsshould be loaded simultaneously, or passive valves must beintegrated to hold the loaded reagents before the flow isinitiated.

Meanwhile, hand-operated microfluidic devices cangenerate a wide range of pressure into the microchannel sothat a wider range of flow rates can be achieved compared toself-operated microfluidic devices. It is more beneficial foron-demand flow control in the microchannel because a widerange of flow rates can be achieved and both positive andnegative pressure can be applied. Simple equipment such asa 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 microfluidicdevices and additional equipment can make the overallsystem complicated. On the other hand, finger actuation hasbeen widely applied for the injection or transferring ofreagents by simply pushing the button without any externalequipment. However, it has been difficult to control aconstant amount of flow depending on the differences invarious end-users. To compensate for such limitations,several working principles have been developed to reduceuser-dependent variation in the controllable volume. Hand-operated microfluidic devices are also beneficial formicrofluidic particle separation using hydrophoresis orinertial force due to the wide range of flow rates. Syringes orpipettes can handle a set amount of volume in microfluidicdevices, but the different flow velocities in various end-usersshould be corrected for the constant performance of thedevices. To achieve this goal, additional correction systemshave been integrated into the microfluidic devices operatedby a syringe or pipette, but it is still challenging to generate aconstant flow velocity in other kinds of hand-operatedmicrofluidic devices.

In order to handle various applications, integrated user-friendly microfluidic circuits should be realized to operatecomplicated fluidic circuits. For this purpose, passive, active,and check valves have been integrated into user-friendlymicrofluidic devices to control the flow direction. Passive valvesdo not require additional manual operation, but they cannotwithstand a large amount of pressure, which is suitable forintegration into self-operated microfluidic devices that generatea small amount of pressure into the microchannel. The flowdirection can be controllable without any manual operation,but the flexibility is poor because passive valves are limited tosingle-use. On the other hand, active and check valves areoperated by a larger amount of pressure and are repeatedlyusable, which are compatible with hand-operated microfluidicdevices. Check valves do not require additional manualoperation but are difficult to use on-demand, while activevalves are controllable on-demand, but do require additionalmanual operation. Additionally, the design of fluidic circuits isfreer for the integration of active valves, but the need formanual operation in addition to the manual operation forpumping can hamper the simplicity of the devices. The sameworking principle of the active valves with the hand-operatedmicrofluidic device would be ideal for on-demand flowcontrol.

Various user-friendly microfluidic devices have beendeveloped for practical sample preparation in POCT,including the mixing of reagents, biochemical reactions,particle separation, and compartmentalization for digitalanalysis. Simple microfluidic valves have been furtherintegrated to realize more complicated microfluidic circuitsfor handling more functions. However, it is still challengingto achieve a performance similar to that of conventionalmicrofluidic devices. More accurate flow control should beenabled such as on-demand flow velocity, volume and



direction control. As the performance of the devices can bedifferent depending on various end-users, correction systemsare required for the universality of the devices. Furthermore,large scale integration of user-friendly microfluidic devices isalso a challenging aspect for high-throughput assays. In thefuture, we expect that user-friendly microfluidic devices maybe widely used as sample preparation tools for POCT by non-experts even in resource-limited settings. With theintegration 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 Foundationof 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.

2 T. R. Kozel and A. R. Burnham-Marusich, J. Clin. Microbiol.,2017, 55, 2313–2320.

3 D. C. Christodouleas, B. Kaur and P. Chorti, ACS Cent. Sci.,2018, 4, 1600–1616.

4 P. Wang and L. J. Kricka, Clin. Chem., 2018, 64, 1439–1452.5 J. L. V. Shaw, Pract. Lab. Med., 2016, 4, 22–29.6 J. R. Choi, K. W. Yong, J. Y. Choi and A. C. Cowie, Sensors,

2019, 19, 817.7 H. Chen, K. Liu, Z. Li and P. Wang, Clin. Chim. Acta,

2019, 493, 138–147.8 B. Nasseri, N. Soleimani, N. Rabiee, A. Kalbasi, M. Karimi

and M. R. Hamblin, Biosens. Bioelectron., 2018, 117,112–128.

9 H. Kim, D. R. Chung and M. Kang, Analyst, 2019, 144,2460–2466.

10 C. Dincer, R. Bruch, A. Kling, P. S. Dittrich and G. A. Urban,Trends Biotechnol., 2017, 35, 728–742.

11 W. He, M. You, W. Wan, F. Xu, F. Li and A. Li, TrendsBiotechnol., 2018, 36, 1127–1144.

12 C. S. Kosack, A.-L. Page and P. R. Klatser, Bull. W. H. O.,2017, 95, 639.

13 C. Carrell, A. Kava, M. Nguyen, R. Menger, Z. Munshi, Z.Call, M. Nussbaum and C. Henry, Microelectron. Eng.,2019, 206, 45–54.

14 T. Akyazi, L. Basabe-Desmonts and F. Benito-Lopez, Anal.Chim. Acta, 2018, 1001, 1–17.

15 X. Huang, D. Xu, J. Chen, J. Liu, Y. Li, J. Song, X. Ma and J.Guo, Analyst, 2018, 143, 5339–5351.

16 A. Roda, E. Michelini, M. Zangheri, M. Di Fusco, D.Calabria and P. Simoni, TrAC, Trends Anal. Chem., 2016, 79,317–325.

17 W. Jung, J. Han, J.-W. Choi and C. H. Ahn, Microelectron.Eng., 2015, 132, 46–57.

18 C. M. Pandey, S. Augustine, S. Kumar, S. Kumar, S. Nara, S.Srivastava and B. D. Malhotra, Biotechnol. J., 2018, 13,1700047.

19 A. K. Au, H. Lai, B. R. Utela and A. Folch, Micromachines,2011, 2, 179–220.

20 D. J. Laser and J. G. Santiago, J. Micromech. Microeng.,2004, 14, R35–R64.

21 K. W. Oh and C. H. Ahn, J. Micromech. Microeng., 2006, 16,R13–R39.

22 G. Comina, A. Suska and D. Filippini, Biosens. Bioelectron.,2016, 77, 1153–1167.

23 C. K. Byun, K. Abi-Samra, Y. K. Cho and S. Takayama,Electrophoresis, 2014, 35, 245–257.

24 P. G. de Gennes, Rev. Mod. Phys., 1985, 57, 827–863.25 H. Tachibana, M. Saito, K. Tsuji, K. Yamanaka, L. Q. Hoa

and E. Tamiya, Sens. Actuators, B, 2015, 206, 303–310.26 A. Olanrewaju, M. Beaugrand, M. Yafia and D. Juncker, Lab

Chip, 2018, 18, 2323–2347.27 R. Safavieh, A. Tamayol and D. Juncker, Microfluid.

Nanofluid., 2015, 18, 357–366.28 Y. Temiz and E. Delamarche, Sci. Rep., 2018, 8, 10603.29 H. J. Kim, B. H. Kim and Y. H. Seo, BioChip J., 2018, 12,

154–162.30 L. Gervais and E. Delamarche, Lab Chip, 2009, 9, 3330–3337.31 S. Ghosh and C. H. Ahn, Analyst, 2019, 144, 2109–2119.32 P. B. Lillehoj, F. Wei and C. M. Ho, Lab Chip, 2010, 10,

2265–2270.33 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.

34 R. Gao, Z. Lv, Y. Mao, L. Yu, X. Bi, S. Xu, J. Cui and Y. Wu,ACS Sens., 2019, 4, 938–943.

35 Y. C. Kim, S.-H. Kim, D. Kim, S.-J. Park and J.-K. Park, Sens.Actuators, B, 2010, 145, 861–868.

36 S. Karimi, P. Mehrdel, J. Farre-Llados and J. Casals-Terre,Lab Chip, 2019, 19, 3249–3260.

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.

38 P. Novo, F. Volpetti, V. Chu and J. P. Conde, Lab Chip,2013, 13, 641–645.

39 J. S. Shim, A. W. Browne and C. H. Ahn, Biomed.Microdevices, 2010, 12, 949–957.

40 C. Li, C. Liu, Z. Xu and J. Li, Microfluid. Nanofluid.,2012, 12, 829–834.

41 J. Wang, H. Ahmad, C. Ma, Q. Shi, O. Vermesh, U. Vermeshand J. Heath, Lab Chip, 2010, 10, 3157–3162.

42 T. Kokalj, Y. Park, M. Vencelj, M. Jenko and L. P. Lee, LabChip, 2014, 14, 4329–4333.

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.



44 P. Novo, V. Chu and J. P. Conde, Biosens. Bioelectron.,2014, 57, 284–291.

45 J. Hauser, G. Lenk, S. Ullah, O. Beck, G. Stemme and N.Roxhed, Anal. Chem., 2019, 91, 7125–7130.

46 L. Xu, H. Lee, D. Jetta and K. W. Oh, Lab Chip, 2015, 15,3962–3979.

47 K. Hosokawa, M. Omata, K. Sato and M. Maeda, Lab Chip,2006, 6, 236–241.

48 D. Y. Liang, A. M. Tentori, I. K. Dimov and L. P. Lee,Biomicrofluidics, 2011, 5, 024108.

49 Q. Song, J. Sun, Y. Mu, Y. Xu, Q. Zhu and Q. Jin, Sens.Actuators, B, 2018, 256, 1122–1130.

50 H. Arata, H. Komatsu, A. Han, K. Hosokawa and M. Maeda,Analyst, 2012, 137, 3234–3237.

51 H. Arata, H. Komatsu, K. Hosokawa and M. Maeda, PLoSOne, 2012, 7, e48329.

52 J. Y. Ho, N. J. Cira, J. A. Crooks, J. Baeza and D. B. Weibel,PLoS One, 2012, 7, e41245.

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.

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.

55 E.-C. Yeh, C.-C. Fu, L. Hu, R. Thakur, J. Feng and L. P. Lee,Sci. Adv., 2017, 3, e1501645.

56 C. Li, J. Xu and B. Ma, Microfluid. Nanofluid., 2015, 18,1067–1073.

57 G. Li, Y. Luo, Q. Chen, L. Liao and J. Zhao, Biomicrofluidics,2012, 6, 14118–1411816.

58 C. J. Lee and Y. H. Hsu, Lab Chip, 2019, 19, 2834–2843.59 B. T. Good, C. N. Bowman and R. H. Davis, Lab Chip,

2006, 6, 659–666.60 L. Qin, O. Vermesh, Q. Shi and J. R. Heath, Lab Chip,

2009, 9, 2016–2020.61 M. T. Guler, Z. Isiksacan, M. Serhatlioglu and C. Elbuken,

Sens. Actuators, B, 2018, 273, 350–357.62 J. Yang, X. Liu, Y. Pan, J. Yang, B. He, Y. Fu and Y. Song,

Sens. Actuators, B, 2019, 291, 192–199.63 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.

64 N. Shao, X. Han, Y. Song, P. Zhang and L. Qin, Anal. Chem.,2019, 91, 12384–12391.

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.

66 M. F. Abate, S. Jia, M. G. Ahmed, X. Li, L. Lin, X. Chen, Z.Zhu and C. Yang, Small, 2019, 15, e1804890.

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.

68 K. Han, Y. J. Yoon, Y. Shin and M. K. Park, Lab Chip,2016, 16, 132–141.

69 Y. Jo, Y. K. Hahn and J.-K. Park, Microfluid. Nanofluid.,2017, 21, 74.

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.

71 W. Wu, K. T. Trinh and N. Y. Lee, Analyst, 2012, 137,983–990.

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.

73 P.-C. Chen, Y.-C. Chen and C.-M. Tsai, Microelectron. Eng.,2016, 150, 57–63.

74 B. Kim, S. Oh, S. Shin, S. G. Yim, S. Y. Yang, Y. K. Hahnand S. Choi, Anal. Chem., 2018, 90, 8254–8260.

75 S. Kim, S. Kwon, C. H. Cho and J.-K. Park, Lab Chip,2017, 17, 702–709.

76 K. Langer, N. Bremond, L. Boitard, J. Baudry and J. Bibette,Biomicrofluidics, 2018, 12, 044106.

77 D. Bardin and A. P. Lee, Lab Chip, 2014, 14, 3978–3986.78 J. Sun, J. Hu, T. Gou, X. Ding, Q. Song, W. Wu, G. Wang, J.

Yin and Y. Mu, Biosens. Bioelectron., 2019, 139, 111339.79 S. S. Bithi and S. A. Vanapalli, Sci. Rep., 2017, 7, 41707.80 W. Du, L. Li, K. P. Nichols and R. F. Ismagilov, Lab Chip,

2009, 9, 2286–2292.81 F. Shen, W. Du, J. E. Kreutz, A. Fok and R. F. Ismagilov, Lab

Chip, 2010, 10, 2666–2672.82 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.

83 S. Song, M. S. Kim, J. Lee and S. Choi, Lab Chip, 2015, 15,1250–1254.

84 N. Xiang, X. Shi, Y. Han, Z. Shi, F. Jiang and Z. Ni, Anal.Chem., 2018, 90, 9515–9522.

85 S. Song, M. S. Kim and S. Choi, Small, 2014, 10, 4123–4129.86 B. Kim and S. Choi, Small, 2016, 12, 190–197.87 B. Kim, S. Oh, D. You and S. Choi, Anal. Chem., 2017, 89,

1439–1444.88 N. Xiang, Y. Han, Y. Jia, Z. Shi, H. Yi and Z. Ni, Lab Chip,

2019, 19, 214–222.89 X. Zhang, K. Xia, A. Ji and N. Xiang, Electrophoresis,

2019, 40, 865–872.90 X. Zhang, Z. Zhu, N. Xiang and Z. Ni, Biomicrofluidics,

2016, 10, 054123.91 M. M. Gong, B. D. Macdonald, T. Vu Nguyen and D. Sinton,

Biomicrofluidics, 2012, 6, 44102.92 L. Xu, H. Lee and K. W. Oh, Microfluid. Nanofluid., 2014, 17,

745–750.93 S. Smith, R. Sewart, H. Becker, P. Roux and K. Land,

Microfluid. Nanofluid., 2016, 20, 163.94 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.

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.



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.

97 U. M. Jalal, G. J. Jin and J. S. Shim, Anal. Chem., 2017, 89,13160–13166.

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.

99 G. Comina, A. Suska and D. Filippini, Angew. Chem., Int.Ed., 2015, 54, 8708–8712.

100 M. T. Glynn, D. J. Kinahan and J. Ducree, Lab Chip,2014, 14, 2844–2851.

101 X. Li and C. O. Chui, Microfluid. Nanofluid., 2018, 22, 14.102 W. Li, T. Chen, Z. Chen, P. Fei, Z. Yu, Y. Pang and Y.

Huang, Lab Chip, 2012, 12, 1587–1590.103 K. Iwai, K. C. Shih, X. Lin, T. A. Brubaker, R. D. Sochol and

L. Lin, Lab Chip, 2014, 14, 3790–3799.104 C. S. Ball, R. F. Renzi, A. Priye and R. J. Meagher, Lab Chip,

2016, 16, 4436–4444.105 Y. Lee, B. Kim, I. Oh and S. Choi, Small, 2018, 14, e1802769.106 S. Lee, H. Kim, W. Lee and J. Kim, Micro Nano Syst. Lett.,

2018, 6, 1.107 S. Begolo, D. V. Zhukov, D. A. Selck, L. Li and R. F.

Ismagilov, Lab Chip, 2014, 14, 4616–4628.108 K. Xu, M. R. Begley and J. P. Landers, Lab Chip, 2015, 15,

867–876.109 A. Mepham, J. D. Besant, A. W. Weinstein, I. B. Burgess, E. H.

Sargent and S. O. Kelley, Lab Chip, 2017, 17, 1505–1514.110 J. Park, H. Roh and J.-K. Park, Micromachines, 2019, 10, 174.111 J. Park and J.-K. Park, Lab Chip, 2018, 18, 1215–1222.112 J. Park and J.-K. Park, Anal. Chem., 2019, 91, 11636–11642.113 G. Korir and M. Prakash, PLoS One, 2015, 10, e0115993.114 K. J. Cha and D. S. Kim, Biomed. Microdevices, 2011, 13,

877–883.115 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.

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.

117 M. S. Bhamla, B. Benson, C. Chai, G. Katsikis, A. Johri andM. Prakash, Nat. Biomed. Eng., 2017, 1, 9.

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.

119 T. Thorsen, S. J. Maerkl and S. R. Quake, Science, 2002, 298,580–584.

120 K. Hosokawa and R. Maeda, J. Micromech. Microeng.,2000, 10, 415.

121 D. Irimia and M. Toner, Lab Chip, 2006, 6, 345–352.122 J. Y. Baek, J. Y. Park, J. I. Ju, T. S. Lee and S. H. Lee,

J. Micromech. Microeng., 2005, 15, 1015.

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.

124 Y. Ouyang, S. Wang, J. Li, P. S. Riehl, M. Begley and J. P.Landers, Lab Chip, 2013, 13, 1762–1771.

125 H. Cho, H.-Y. Kim, J. Y. Kang and T. S. Kim, J. ColloidInterface Sci., 2007, 306, 379–385.

126 R. Safavieh and D. Juncker, Lab Chip, 2013, 13,4180–4189.

127 J. Choi, D. Kang, S. Han, S. B. Kim and J. A. Rogers, Adv.Healthcare Mater., 2017, 6, 1601355.

128 Y. Zheng, W. Dai and H. Wu, Lab Chip, 2009, 9, 469–472.129 S. E. Hulme, S. S. Shevkoplyas and G. M. Whitesides, Lab

Chip, 2009, 9, 79–86.130 D. A. Markov, S. Manuel, L. M. Shor, S. R. Opalenik, J. P.

Wikswo and P. C. Samson, Biomed. Microdevices, 2010, 12,135–144.

131 M. T. Guler, P. Beyazkilic and C. Elbuken, Sens. Actuators, A,2017, 265, 224–230.

132 B. Hu, J. Li, L. Mou, Y. Liu, J. Deng, W. Qian, J. Sun, R. Chaand X. Jiang, Lab Chip, 2017, 17, 2225–2234.

133 A. Gholizadeh and M. Javanmard, J. Microelectromech. Syst.,2016, 25, 922–928.

134 C. Y. Chen, C. H. Chen, T. Y. Tu, C. M. Lin and A. M. Wo,Lab Chip, 2011, 11, 733–737.

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.

136 J. Hyeon and H. So, Biomed. Microdevices, 2019, 21, 19.137 M. I. Mohammed and M. P. Desmulliez, Biosens.

Bioelectron., 2014, 61, 478–484.138 A. O. Olanrewaju, A. Ng, P. DeCorwin-Martin, A. Robillard

and D. Juncker, Anal. Chem., 2017, 89, 6846–6853.139 A. O. Olanrewaju, A. Robillard, M. Dagher and D. Juncker,

Lab Chip, 2016, 16, 3804–3814.140 P. Vulto, S. Podszun, P. Meyer, C. Hermann, A. Manz and

G. A. Urban, Lab Chip, 2011, 11, 1596–1602.141 S. Hakenberg, M. Hugle, M. Weidmann, F. Hufert, G. Dame

and G. A. Urban, Lab Chip, 2012, 12, 4576–4580.142 F. Dal Dosso, L. Tripodi, D. Spasic, T. Kokalj and J.

Lammertyn, ACS Sens., 2019, 4, 694–703.143 Y. Liu and G. Li, Sci. Rep., 2018, 8, 13664.144 S. Shin, B. Kim, Y. J. Kim and S. Choi, Biosens. Bioelectron.,

2019, 133, 169–176.145 Y. Zhai, A. Wang, D. Koh, P. Schneider and K. W. Oh, Lab

Chip, 2018, 18, 276–284.146 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.

147 S. B. Im, M. J. Uddin, G. J. Jin and J. S. Shim, Lab Chip,2018, 18, 1310–1319.

148 J. Park and J.-K. Park, Lab Chip, 2019, 19, 2973–2977.


self-operated microfluidic devices

Documents