Pilnam Kim 1 , Keon Woo Kwon 1 , Min Cheol Park 1 , Sung Hoon Lee 1 , Sun Min Kim 2 & Kahp Yang Suh 1 1 School of Mechanical and Aerospace Engineering and the Institute of Bioengineering, Seoul National University, Seoul 151-742, Korea 2 Department of Mechanical Engineering, Inha University, Incheon 402-751, Korea Correspondence and requests for materials should be addressed to K.Y. Suh ([email protected]) Accepted 29 January 2008 Abstract Soft lithography has provided a low-expertise route toward micro/nanofabrication and is playing an important role in microfluidics, ranging from simple channel fabrication to the creation of micropatterns onto a surface or within a microfluidic channel. In this review, the materials, methods, and applications of soft lithography for microfluidics are briefly sum- marized with a particular emphasis on integrated microfluidic systems containing physical microstruc- tures or a topographically patterned substrate. Re- levant exemplary works based on the combination of various soft lithographic methods using micro- fluidics are introduced with some comments on their merits and weaknesses. Keywords: Soft lithography, Microfluidics, Micro/nanof- abrication Introduction The development of biological micro-electromech- anical (MEMS) devices comprised of microfluidic channels or a “Lab-on-a-chip”, may revolutionize biological analysis and create new ways of analyzing cells in vitro 1 . Such microdevices are advantageous in that they use tiny volumes of reagents and can be scaled-up for a high-throughput analysis. Recently, microfluidic systems integrated with embedded physical microstructures or a topographically pattern- ed substrate on the micro- or nanoscale (called an “integrated microfluidic system” in this paper, here- after) have attracted significant attention. This is because the control of surface properties and the spa- tial presentation of functional molecules within a microfluidic channel is important for the develop- ment of diagnostic assays and microreactors, and for performing fundamental studies of cell biology and tissue engineering 2 . Also, precise control over mate- rial transport and manipulation has enabled the analy- sis of intracellular parameters and the detection of cell metabolites, even on a single-cell level 1,3-5 . Soft lithography is a valuable tool for an integrated microfluidic system. Soft lithography was first intro- duced by G.M. Whitesides et al. 6,7 and includes a family of techniques involving a soft polymeric mold such as a polydimethylsiloxane (PDMS) replica from an original hard master. Mold masters are typically fabricated by photolithography in order to define a stamp pattern. Stamps are made by curing a prepoly- mer of PDMS onto a mold master. Apart from “repli- ca molding”, a well-known technique for generating a polymer channel replicated from an original silicon master, soft lithography provides many simple yet robust routes toward the fabrication of micro/nano- structures onto a surface or within a channel. Althou- gh several review articles regarding integrated micro- fluidic systems for cell analysis are available in the literature 1,3,8-11 , in this review we mainly focus on the microfluidic systems that are integrated with physical micro/nanostructures fabricated by soft lithography. Exemplary works of integrated microfluidic systems are introduced in the areas of cell docking/separation and protein or lipid membrane arrays. Soft Lithography Materials An integrated microfluidic system is usually creat- ed by combining two layers: a substrate layer con- structed with micro/nanostructures on the surface (a micro- or nanostructured layer), and a channel layer with a microchannel impression. For the channel layer, PDMS is widely used to fabricate the micro- fluidic channels because of its favorable mechani- cal/optical properties and its simple manufacturing by rapid prototyping 12 . To cure the PDMS prepolymer in general, a mixture of 10 : 1 silicon elastomer and a curing agent is poured onto the master and placed at 70-80° C for 1 h (see Figure 1A). In addition to the PDMS channels, other microfluidic devices have been introduced using different channel materials such as photocurable perfluoropolyethers, biode- gradable polymers, photosensitive polymers, and polymerized hydrogels 13-21 . However, biofouling, BIOCHIP JOURNAL, Vol. 2, No. 1, 1-11, March 2008 Soft Lithography for Microfluidics: a Review Mini Review
Transcript
Pilnam Kim1, Keon Woo Kwon1, Min Cheol Park1,Sung Hoon Lee1, Sun Min Kim2 & Kahp Yang Suh1
1School of Mechanical and Aerospace Engineering and the Instituteof Bioengineering, Seoul National University, Seoul 151-742, Korea2Department of Mechanical Engineering, Inha University, Incheon402-751, Korea Correspondence and requests for materials should be addressedto K.Y. Suh ([email protected])
Accepted 29 January 2008
AbstractSoft lithography has provided a low-expertise routetoward micro/nanofabrication and is playing animportant role in microfluidics, ranging from simplechannel fabrication to the creation of micropatternsonto a surface or within a microfluidic channel. Inthis review, the materials, methods, and applicationsof soft lithography for microfluidics are briefly sum-marized with a particular emphasis on integratedmicrofluidic systems containing physical microstruc-tures or a topographically patterned substrate. Re-levant exemplary works based on the combinationof various soft lithographic methods using micro-fluidics are introduced with some comments on theirmerits and weaknesses.
The development of biological micro-electromech-anical (MEMS) devices comprised of microfluidicchannels or a “Lab-on-a-chip”, may revolutionizebiological analysis and create new ways of analyzingcells in vitro1. Such microdevices are advantageous inthat they use tiny volumes of reagents and can bescaled-up for a high-throughput analysis. Recently,microfluidic systems integrated with embeddedphysical microstructures or a topographically pattern-ed substrate on the micro- or nanoscale (called an“integrated microfluidic system” in this paper, here-after) have attracted significant attention. This isbecause the control of surface properties and the spa-tial presentation of functional molecules within a
microfluidic channel is important for the develop-ment of diagnostic assays and microreactors, and forperforming fundamental studies of cell biology andtissue engineering2. Also, precise control over mate-rial transport and manipulation has enabled the analy-sis of intracellular parameters and the detection ofcell metabolites, even on a single-cell level1,3-5.
Soft lithography is a valuable tool for an integratedmicrofluidic system. Soft lithography was first intro-duced by G.M. Whitesides et al.6,7 and includes afamily of techniques involving a soft polymeric moldsuch as a polydimethylsiloxane (PDMS) replica froman original hard master. Mold masters are typicallyfabricated by photolithography in order to define astamp pattern. Stamps are made by curing a prepoly-mer of PDMS onto a mold master. Apart from “repli-ca molding”, a well-known technique for generatinga polymer channel replicated from an original siliconmaster, soft lithography provides many simple yetrobust routes toward the fabrication of micro/nano-structures onto a surface or within a channel. Althou-gh several review articles regarding integrated micro-fluidic systems for cell analysis are available in theliterature1,3,8-11, in this review we mainly focus on themicrofluidic systems that are integrated with physicalmicro/nanostructures fabricated by soft lithography.Exemplary works of integrated microfluidic systemsare introduced in the areas of cell docking/separationand protein or lipid membrane arrays.
Soft Lithography Materials
An integrated microfluidic system is usually creat-ed by combining two layers: a substrate layer con-structed with micro/nanostructures on the surface (amicro- or nanostructured layer), and a channel layerwith a microchannel impression. For the channellayer, PDMS is widely used to fabricate the micro-fluidic channels because of its favorable mechani-cal/optical properties and its simple manufacturing byrapid prototyping12. To cure the PDMS prepolymer ingeneral, a mixture of 10 : 1 silicon elastomer and acuring agent is poured onto the master and placed at70-80°C for 1 h (see Figure 1A). In addition to thePDMS channels, other microfluidic devices havebeen introduced using different channel materialssuch as photocurable perfluoropolyethers, biode-gradable polymers, photosensitive polymers, andpolymerized hydrogels13-21. However, biofouling,
BIOCHIP JOURNAL, Vol. 2, No. 1, 1-11, March 2008
Soft Lithography for Microfluidics: a Review
Mini Review
weak mechanical properties, and the need for exten-sive expertise have potentially limited the versatileuse of these devices.
Micropatterns can also be formed using PDMSmolds of various patterns in a positive or negativesense. As with a PDMS channel, a PDMS replica ispeeled off from a silicon master prepared usingphotolithography. For patterning materials, poly-ethylene glycol (PEG) is frequently used due to itseasy processability and non-biofouling properties. Todate, biofouling and a subsequent device malfunctionhave deteriorated device performance through hydro-phobic interactions between the PDMS surface andbiological samples22. When small sample quantitiesare involved, such as rare proteins, any sample lossthrough a non-specific binding may result in a criticalerror in the final analysis. To solve this challenge,silicon-based (e.g., silicon, glass, quartz, and PDMS)platforms have been surface modified using non-biofouling materials such as polyethylene glycol(PEG)12,23-28. It is believed that the resistant nature ofPEG-based polymer may be attributed to a polymerchain mobility and steric stabilization force29. Surfacemodification of silicon-based devices with PEG canbe performed using physical adsorption23, covalentimmobilization such as grafting and chemical cou-pling24-26, or a gas phase treatment (plasma or depo-sition)12,27,28. These efforts have proven to be success-ful, but they might not be able to guarantee confor-mal coating and long-term stability, i.e., modifiedPDMS surfaces slowly recover their original hydro-phobicity30.
To overcome the above-mentioned problems, atechnique has been developed to fabricate micro- andnanochannels comprised entirely of cross-linkedpolyethylene glycol (PEG) by using UV-assistedmolding31. A flat or patterned PEG substrate was us-ed for the fabrication in which a PEG channel wasbonded to a patterned PEG substrate with microwells.Further details on the fabrication and analysis of aPEG channel, shown in Figure 1B, can be found in31.
Soft Lithography Fabrication Methods
To form an integrated microfluidic system, a micro/nanostructured layer is bonded to a microchannellayer32-37. Various micro/nanofabrication techniquescan be used for manufacturing integrated micro-fluidic systems. Here, two soft lithographic methodsare introduced to fabricate micro/nanopatterns onto asurface or within a microfluidic channel: contactprinting38,39 and capillary molding40,41. Contact print-ing generates a non-structured, chemically modifiedsurface, while capillary molding fabricates a topo-graphically modified physical micro/nanostructure.
Contact printing is a direct patterning method usingan elastomeric stamp prepared by soft lithography38,39.After curing, PDMS stamps are soaked in a mole-cular “ink” and brought into conformal contact witha substrate in order to transfer the ink onto the sub-strate surface. Contact printing enables easy stampreplication, fast printing using parallelization, andlow-cost batch production. The polymer stamps also
2 Biochip Journal Vol. 2(1), 1-11, 2008
Table 1. Soft lithographic methods for the fabrication of micro/nanopatterns.
Resolution ~500 nm~50 nm with h-PDMS ~50 nm with PUA mold
1. Channel fabrication 1. Channel fabricationApplication 2. Direct printing of biological molecules 2. Fabrication of micro/nanostructures
(1D chemical modification) (2D structure)
Process
SAM solution
PDMS mold
Substrate
SAMDry of solvent
Mold removal
Contact the mold
PUA or PDMS mold
SubstratePolymer coating
Mold placement
Heat or UV
Mold removal
minimize the problems of sample carry-over and cro-ss contamination. However, contact printing hassome limitations that are mainly caused by the use ofa soft polymer stamp38. The swelling of a stampduring inking often results in an increase in the pat-tern size by diffusion of the excessive printed mole-cules on the substrate. Also, contact printing gene-rates a one-dimensional pattern within a channel.
Capillary molding is an improved version of softlithography by combining a nanoimprint and the useof an elastomeric mold40,41. When a patterned (posi-tive or negative) PDMS mold is placed on a polymersurface and heated above the polymer’s glass tran-sition temperature (Tg), capillarity forces the polymerto melt into the void space of the channels formedbetween the mold and the polymer, thereby gener-ating a negative replica of the mold. A pattern for-mation is also possible with a solvent-laden polymeror a UV-curable resin followed by solvent evapo-ration or exposure to UV light. Recently, a UV cura-ble mold made from polyurethane functionalizedwith acrylate groups has been introduced to replacePDMS molds for sub-100-nm lithography, thus ex-panding the use of capillary molding to cell biologystudies42,43.
Integraion of Soft Lithography withMicrofluidics
Arrays of Mammalian and Yeast CellsIn a microfluidic system, cells flow through a chan-
nel by a stable laminar flow, which is useful forhighly effective and accurate cell manipulation44.Controlled transport, immobilization, and manipu-lation of biological molecules and cells are importantfunctions to be incorporated into a microfluidic de-vice in order to carry out on-chip biochemical andcell biological experiments32.
Several approaches have been introduced to immo-bilize cells within particular regions of a microfluidicchannel: laminar flow patterning45, pre-patterningwith adhesive ligands46, and immobilization insidehydrogels47. However, there are potential limitationsin these approaches. Laminar flow patterning canonly pattern a limited shape of patterned regions, andhydrogel fabrication using UV radiation induces theexposure of cells to potentially toxic environments48.Also, the direct patterning of cells on a pre-patternedsubstrate of a channel could give rise to shear-drivenmodifications in cell behavior. To overcome theselimitations, integrated microfluidic systems havebeen developed to capture and localize cells withinparticular regions of a channel with the aid of softlithography32,33,48-50.
Khademhosseini et al. introduced a simple softlithographic technique for fabricating PEG micro-structures within microfluidic channels that can im-mobilize cells within specific locations48. Microwellsof various shapes were used to capture cells despite ashear flow within a channel. Immobilized cells withinthe microwells remained viable and were stained forcell surface receptors by a sequential flow of antibod-
Soft Lithography for Microfluidics 3
Figure 1. Schematic repre-sentation of microfluidic ch-annel fabrication. (A) PD-MS channels by using repli-ca molding and (B) PEG ch-annel by UV-assisted irre-versible bonding.
ies and secondary fluorescent probes. Using softlithography and reversible sealing, an advanced mic-rowell system was also fabricated for multiphenotypecell patterning within an array of reversibly sealedmicrofluidic channels49. Microfluidic channels deli-ver various fluids or cells onto specific locations andmicrowells on a substrate in order to capture andimmobilize cells within low shear stress regions (Fig-ure 2A). Alternative orthogonal placing of reversiblysealed microchannel arrays can deliver a unique setof fluids or cell types. Multiphenotype cell patterningon specific regions within a two-dimensional channelsystem can be applied to high-throughput drug scr-eening and tissue engineering.
Park et al. proposed alternative techniques to fabri-cate microwells, either by soft lithographic capillarymolding of UV curable PUA onto a glass substrate orby a direct replica molding of PDMS as shown inFigure 2B51. Cell docking within microwells inside amicrofluidic channel was induced by receding meni-scus in order to capture non-adherent yeast cells.First, cell suspension of the yeast cells was introduc-ed into the microfluidic channel by a surface-tension-driven capillary flow. One to multiple yeast cellswere then spontaneously captured onto microwells by
lateral capillary force created at the bottom of thereceding meniscus subsequently generated by naturalevaporation.
Recently, Lee et al. presented a simple method forfabricating shear-protecting cell containers integratedwithin a microfluidic channel52. A capillary moldingtechnique was used to generate hollow bottle-shapedstructures by exploiting the partial capillary risealong the slanted walls with an acute wedge angle, asshown in Figure 2C. The molded hollow micro-structure was used to capture the budding yeast cellswithin a microfluidic channel, and the shear-pro-tecting ability was evaluated by measuring the fluo-rescent intensities of docked cells upon stimulationwith a mating pheromone or high osmolarity overtime. The unique shape of the hollow cell containeroffers superior shear-protecting ability compared withprevious microwell-type structures49,53. In this appro-ach, experimental and simulation results demonstrat-ed that a higher microstructure with smaller neck di-mension offer a more stable, non-invasive micro-environment for docked cells.
The integration of cell manipulation with a channelplatform allows the measurement of biological res-ponses of cells within a confined microscale feature.
4 Biochip Journal Vol. 2(1), 1-11, 2008
Figure 2. Integrated micro-fluidic devices for cell dock-ing and manipulation. (A)Multiphenotype cell pattern-ing on specific regions with-in a two-dimensional channelsystem using reversible seal-ing of PDMS channel withpatterned surface. Reprintedwith permission from49. (B)Docking of non-adherent ye-ast cells using receding me-niscus inside a microfluidicchip with a microwell. Re-printed with permissionfrom51. (C) Molded hollowmicrostructure having shearprotecting ability within amicrofluidic channel andcapturing the budding yeastcells Reprinted with permis-sion from52.
Plasma bonding of PDMS channeland microstructured substrate
Cell introduction
PDMS channel
Cell capturing
(a) (b)
(c)
(a)A B
C(f)
(d)
(b)
(e)
(c)
This approach can provide a potential tool for high-throughput screening of single to multiple cells oroptimization of cell-soluble signal interactions forbiological research or tissue engineering. However,many methods developed so far lack control over sur-face chemistry or topography for anchorage-depend-ent cells within the captured microstructures, whichwould limit the widespread uses to most mammaliancells, in particular, for long-term cultures. The de-velopment of simple and direct techniques for fabric-ating microstructures within microchannels with pre-cise control over their surface properties is of po-tential benefit.
Protein and Lipid Bilayer ArraysIntegration of protein or lipid bilayer arrays with
microfluidics is also important for a high-throughputanalysis of diseases and cell-to-cell communications.The roles of proteins are enormously diverse andinclude mechanical support, signaling, and sensing.Beyond their central importance to biology, proteinsare of great interest because these sub-microscalemolecules have the potential to be integrated intomicrofluidic devices. Aiming toward this application,a micro/nanopatterning technique is required that iscapable of accurately depositing proteins at pre-defined locations while retaining their native func-tionality.
To pattern microfluidic channels using soft litho-graphy, surface patterning is usually performed priorto the attachment of the PDMS mold to the substrate.Non-specific adsorption of biomolecules should besuppressed for selective binding of proteins or lipidlayers on a surface or inside a channel. Contact print-ing and capillary molding can be used to achieve thispurpose via surface modification or a physical barrierwith non-biofouling polysaccharide or PEG coat-ings46,54,55.
Khademhosseini et al. introduced a soft lithogra-phic technique to fabricate micropatterns of PEG orhyaluronic acid within a microfluidic channel46. Inthis approach, the patterned regions were protectedfrom oxygen plasma by controlling the dimensions ofthe PDMS stamp and by leaving the stamp in placeduring the plasma treatment process. The PDMSstamp was then removed, and the microfluidic moldwas irreversibly bonded to the substrate. The non-biofouling patterns were then used to fabricate arraysof fibronectin and bovine serum albumin. In addition,laminar flow patterning was used to control theadsorption of multiple proteins in various regions ofan exposed substrate. It was demonstrated that thelaminar flow of multiple proteins may be used togenerate patterned protein arrays within the channels.
The spatial patterning of multiple proteins withinindividual islands can be potentially useful in study-ing the effects of spatial organization of multipleextracellular matrix components on cell behaviorsuch as asymmetric cell division56.
Since the introduction of micropatterning of lipidbilayers by Boxer et al. in 199757, a number of met-hods have been developed for lipid membrane micro-arrays, such as deep-UV illumination through a pho-tomask under aqueous conditions58,59 and a polymerlift-off method60-63. Microcontact printing of the com-position arrays of phospholipids bilayers was firstaccomplished by printing different sized bilayers ofthe same composition onto surface patterned cor-rals64-66. Also, micromolding in capillaries (MIMIC)was used to pattern lipid membranes by utilizing alaminarly flowing stream, as shown in Figures 3Aand B67-69. The use of a laminar flow inside micro-fluidic channels is also an effective means of produc-ing composition arrays of supported phospholipidbilayers in which two distinct chemical componentscan be varied simultaneously along a one-dimensio-nal gradient67,70.
Takeuchi et al. presented a method to form a plann-er lipid bilayer in a microfluidic chip by contactingtwo monolayers that are assembled at the interfacebetween water and an organic solvent containingphospholipids71. Particularly, the bilayer was formedin a vertical direction, unlike in other approaches.The functionality of the bilayer membrane was prov-ed by the insertion of a reconstituted antibiotic pep-tide.
Recently, Kim et al. presented capillary moldingand microcontact printing to create patterns of supp-orted lipid bilayer (SLB) membranes onto a surfaceand inside a microchannel. Micro- or nanopatterns ofa PEG random copolymer were fabricated on glasssubstrates by capillary molding to form a templatelayer against adsorption of lipid membranes72. Ascompared to microcontact printing, the molded struc-tures provided a clean interface at the patterned boun-dary, and the adhesion on the PEG surface wasstrongly restricted. The functionality of the patternedSLBs was tested by measuring the binding interac-tions between the biotin-labeled lipid bilayer andstreptavidin. SLB arrays were fabricated using aspatial resolution of down to ~500 nm on a flat sub-strate and ~1 µm inside the microfluidic channels.
To further elaborate on the performance of thelipid-based microfluidic device for analytical appli-cations, monolithic PEG microchannels were fabri-cated by the same authors utilizing UV assisted mo-lding, in which PEG microwells were located on thebottom of the channel, as shown in Figure 3C31. The
Soft Lithography for Microfluidics 5
6 Biochip Journal Vol. 2(1), 1-11, 2008
Figure 3. Microarrays of li-pid membrane within a mi-crofluidic channel. (A) Sche-matic illustration of the con-verging flow configurationused to produce the limitedmixing of two types of vesi-cles in solution. Reprintedwith permission from67. (B)Microstructuring of lipid bi-layers on gold surfaces byMIMIC employing chemi-cally modified PDMS. Re-printed with permissionfrom68. (C) Supported bilay-er membranes (SBMs) byusing capillary molding of aPEGbased polymer within aPEG channel. Reprinted wi-th permission from31.
(a)11-MUA
11-MUA-monolayer
POPC/POPG bilayer
11-MUA-monolayer
PEG polymer
Streptavidin
Biotinylated POPC liposome
(a) (b)
(c) (d)
(e) (f)
Exposed microwell
PEG
Biotin-SLB
Streptavidin
Removal of the PDMS-mold
NH3++ NH3
++NH3++NH3
++
Adding of POPC/POPG-vesicles
(b)174 nm
13.4 nm
0 nm
0 nm
0 5 10 15 20
Distance/µm
1. Seeding of biotin-liposome vesicle
2. Selective immobilization of vesicleand vesicle fusion
3. Seeding streptavidin:binding of biotin-streptavidin
PEG channel
Glass
Hei
ght /
nm
(c)
0
-5
S S S S
Captured membrane micro-array
100 µm
Flowoutlet
(a) Vesiclesolutioninjectors
(b)
Nor
mal
ized
inte
nsity
Tex
as R
ed
1.00
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0.50
0.25
0.00
1 2 3 4 5 6 7 8
1.00
0.75
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mal
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nsity
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D
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Corral (from left)
(c)(40 V/cm) 50 µm
Field-induced separationof oppositely charged dye-lipids:Texas Red (-) and DID (++)
A
B
C
lipid bilayer membranes were neatly patterned ontothe pre-defined regions of the substrate. Non-specificadsorption, which is frequently observed for mostmicrofluidic devices, was not seen. Also, streptavi-din was selectively bound to the biotinylated lipidbilayer membrane.
Other ApplicationsCells are inherently sensitive to local mesoscale,
microscale, and nanoscale topographic and molecularpatterns in an extracellular matrix (ECM) enviro-nment73-75. Integration of microfluidics and micro/nanofabrication methods can thus be employed toprecisely control the composition and topography ofthe ECM adhesion proteins on a topographicallypatterned substrate within a fluidic channel.
After the pioneering work of Chen et al., micro/nanoscale topographic features have been incor-porated into an in vitro experimental platform to mi-
mic various in vivo 3D ECM environments with stru-ctural and mechanical similarity by using advancedfabrication methods76,77. To improve the design of thebiomaterial interface within a channel, Zaari et al.created substrates with variations in mechanical com-pliance by combining microfluidics and photopoly-merization78. In this integrated platform, a well-con-trolled gradient-compliance profile on the microscalewas used to study cell migration guided by substraterigidity (called “durotaxis”). In addition to this che-mical tuning by controlling the crosslinking densityof hydrogels, one can control the wettability, adhe-sion, or contact guidance of cells by incorporatingvarious physical micro/nanostructures79,80. For exam-ple, a recent study demonstrated that superhydro-phobic surfaces are generated inside a microfluidicchannel by forming high-density arrays of tall andsharp nanoposts (“nanoturfs”) with a submicron pitchon the top and bottom substrates81. Martines et al.
Soft Lithography for Microfluidics 7
PDMS microfluidic mould (a) (f) (g)
(i)(h)
(a)
(c) (d)
(b)
(b)
(d) (e)
(c)
PUA nanostructure
Plasma bonding
Silicon substrate
Lift off agarose mold
Bond agarose mold to thin agarose surface
Silicon substrate Silicon substrate
Cast melted agarosesolution containingcells
Cast melted agarosesolution
Agarose Agarose
Figure 4. Other applications of integrated microfluidic devices. (A) PUA nanostructures included inside a microfluidic channelfor study on cell separation by adhesion difference. Reprinted with permission from83. (B) Encapsulation of mammalian cellswithin the bulk material of microfluidic channels. Reprinted with permission from84.
A
B
10 µm
presented a microfluidic device having nanopits in amicrochannel in order to investigate their response tocell adhesion under dynamic conditions by means ofa shear flow82. Dynamic cell adhesion was quanti-fied and compared on flat and nanopitted polymethylmethacrylate (PMMA) substrates with a cell suspen-sion flow.
Kwon et al. recently developed a label-free micro-fluidic method for the separation and enrichment ofhuman breast cancer cells using controlled cell adhe-sion as a physical marker83. As shown in Figure 4A,the nanostructured polymer surfaces (400 nm pillars,400 nm perpendicular, or 400 nm parallel lines) wereconstructed on the bottom of polydimethylsiloxane(PDMS) microfluidic channels in a parallel fashionusing a UV-assisted capillary molding technique tomaximize the adhesion difference between humanbreast epithelial cells (MCF10A) and cancer cells(MCF7). The normal cells showed higher adhesionthan the cancer cells regardless of culture time andsurface nanotopography at all flow rates, resulting inlabel-free separation and an enrichment of the cancercells. The separation efficiency was increased on the400 nm perpendicular line pattern followed by flow-induced detachment.
Ling et al. demonstrated the encapsulation of mam-malian cells within a bulk material of microfluidicchannels for applications ranging from tissue engin-eering to cell-based diagnostic assays, shown in(Figure 4B)84. Channels of different dimensions weregenerated, and it was shown that agarose, thoughhighly porous, is a suitable material for performingmicrofluidics. Cells embedded within the micro-fluidic molds were well distributed, and media pump-ed through the channels allowed the exchange ofnutrients and waste products.
Conclusions
Soft lithography has been proven useful in micro-fluidics under a wide range of applications from cha-nnel fabrication to pattern generation. In particular,the fabrication of precise micro/nanostructures on asurface or within a fluidic channel can not only offersimple routes toward cell docking and manipulation,but can also serve as a template for protein or lipidbilayer arrays. In addition, controlled adhesion usingmicro/nanostructures in a microfluidic device can beused as a label-free method for the separation andenrichment of cancer cells from a body fluid con-taining a mixed population of normal and cancerouscells. To fabricate physical structures for mechanicaltopography and surface patterning, the materials used
need to be biocompatible and compatible with fluidicapplications, and they should provide rigid, smoothsurfaces with dimensions relevant to the biologicalsample, such as a cell, bacteria, and yeast. It is en-visioned that along with other advanced fabricationmethods, soft lithography will continuously find usesin integrated microfluidic systems due to its simple,cheap, and low-expertise route toward micro/nano-fabrication.
Acknowledgements
This work was supported by Korea Science andEngineering Foundation (KOSEF) grant funded bythe Korea government (MOST) (R01-2007-000-20675-0) and the Grant-in-Aid for Next-GenerationNew Technology Development Programs from theKorea Ministry of Commerce, Industry and Energy(No.10030046). This work was also supported by theKorea Research Foundation Grant funded by theKorean Government (MOEHRD, Basic ResearchPromotion Fund)(KRF-2007-331-D00064) for SunMin Kim.
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