Sensors and Actuators B 137 (2009) 754–761

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

Novel polydimethylsiloxane (PDMS) based microchannel fabrication method forlab-on-a-chip application

Yu Hongbin !, Zhou Guangya, Chau Fook Siong, Wang Shouhua, Lee FeiwenMicro/Nano Systems Initiative, Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore

a r t i c l e i n f o

Article history:Received 22 October 2008Accepted 23 November 2008Available online 6 December 2008

Keywords:Polydimethylsiloxane (PDMS)Rounded cross-sectional contourMicrochannelLab-on-a-chipSoft lithographyMulti-depth

a b s t r a c t

A novel method is presented for fabricating PDMS-based microchannel. The mold consists of a tunablePDMS structure rather than the commonly used SU-8 mold with fixed shape. This structure is prepared bytransferring the desired microchannel pattern into one PDMS substrate via soft lithography and bondingit to a spin-coated PDMS membrane with the oxygen plasma activated technology. Through applyingproper pressure to this structure and keeping it constant during the whole molding process, the resultedmembrane deformation shape with rounded cross-sectional contour is totally replicated into the devicelayer, constituting the final main body of the microchannel. Besides the inherent feature of rounded cross-section provided by this method, the depth of the microchannel can be easily controlled by changing thepressure. At the same time, through designing different widths at different positions in one microchannel,it is straightforward to realize microchannel structure with multi-depth in one step using one photomask.Both of these characteristics have been successfully demonstrated in our experiments and it will definitelyfind wide applications in lab-on-a-chip area.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Lab-on-a-chip (LOC) is a term for devices that integrate labora-tory functions on a single chip of only millimeters to a few squarecentimeters in size. When compared with conventional systems,LOC systems can perform similar biochemical analysis with remark-ably reduced consumption of samples and reagents, size and powerrequirement, system cost and meanwhile faster analysis time andmassively parallel (high-throughput) analysis capability also makeit an attractive candidate especially for applications requiring on-site rapid assay [1–4]. With respect to these distinct advantages,a big boost in research and commercial interest in LOC has comesince the mid 1990’s.

In a typical LOC system, microchannel is one of the most com-mon and indispensable components, through which the samplepre-concentration and separation or mixing can be realized in turnand finally delivered to the desired area to execute correspondingreaction and detection tasks [5,6]. In the initial development stage,silicon and glass are the two popular substrates for microfluidicchip due to their robust surface stability (namely bio-compatibility)and the well established pre-existing fabrication techniques suchas photolithography and chemical etching (including wet etch-ing and dry etching) processes, which were mainly developed for

! Corresponding author.E-mail address: yhb hust@yahoo.com (Y. Hongbin).

the microelectronics and microelectromechanical systems (MEMS)fields [7–9]. However, the high cost and requirement on the spe-cialized tools as well as the relatively long process time associatedwith these materials present an issue. In contrast of this, more andmore research groups have chosen polydimethylsiloxane (PDMS) asan alternative substrate for rapid prototyping [10–13]. With respectto material itself, besides for the good biocompatibility, PDMS hasbetter elastic characteristic, which makes it possible to realize tun-able LOC devices thus providing more design flexibility. At the sametime, the excellent optical performance associated with PDMS canalso facilitate the commonly used optical measuring technologiesand enable the further integration of them into LOC devices [14,15].

Soft lithography is the key process for PDMS-based devices[16–20]. The inverse structures were first patterned into SU-8 soas to constitute the mother mold for the following replicationsteps. Considering the pattern transfer property of the standardlithography process, the features realized always have a flat-topsurface with nearly vertical sidewall. As a result, the cross- sec-tion of the final structure fabricated into PDMS is usually limitedto a rectangular shape [21–26]. Despite being the easiest process,the round microchannel is more desirable than rectangular onein some applications. For example, in the case of devices adopt-ing deformable membrane based valve for sealing (the simplestand the most direct method), the corner part of the rectangularmicrochannel is difficult to be sealed thoroughly, while as for therounded microchannel, owing to the good proximity to the mem-brane deformation contour, better seal of the fluid can be easily

0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.snb.2008.11.035

Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761 755

Fig. 1. Schematic of the mold structure developed in our design. (a) Original status, (b) working status.

achieved. Some methods have been developed successfully to fab-ricate rounded microchannel such as gray-scale lithography [27,28],laser-beam polymerization [29–31] and melting resist technology[32,33] etc. More recently, lithography using backside diffused-lightexposure [34] and method using the surface tension of uncured liq-uid PDMS in open vertical channel fabricated into solid PDMS [35]have also been demonstrated.

In this paper, we present a novel fabrication method. Unlike theconventional one step molding method using SU-8 mold with fixedshape, a two steps molding is designed in our method, in which thepattern (e.g., microchannel) initially fabricated into SU-8 is thentransferred to PDMS substrate. After bonding to a PDMS mem-brane, the resultant structure is used as the mother mold for thefinal PDMS casting process under the assistance of air pressure. Thestructure realized with this method is inherently prone to havinground-like cross-section due to the deformation characteristics ofthe elastic membrane under pressure and the microchannels withthe same width but different depths can also be realized with the

same mold by only adjusting the applied pressure. In addition, bysimply designing structures with different dimension (such as thelength and width) and controlling the applied pressure during thecasting process, structures with multi-depth can also be fabricatedsimultaneously with only one step. These advantages associatedwith our suggested method will provide a new alternative for thedevelopment of lab-on-a-chip devices.

2. Design and fabrication

The design idea originates from the recently developed vari-able liquid-filled microlens technology. It is well known that thistype of microlens consists of a sealed chamber and a deformablemembrane. The membrane is deflected into a spherical-like contourunder pressure, defining the lens shape as well as the performance,such as the focal length. The same working theory is also adopted inour design. As shown in Fig. 1, the mold consists of two parts ratherthan a single fixed SU-8 pattern. Part I is a PDMS substrate incorpo-

Fig. 2. Fabrication process of the microchannel.

756 Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761

rating the device structure such as microchannel, reservoir etc. It isfabricated with the standard soft lithography process using the SU-8 mold. In the usual case, it can be directly used as the functionaldevice by bonding to another substrate. In the design presentedhere, however, its top is covered by a PDMS membrane, namely partII, forming a closed chamber. When positive air-pressure is intro-duced into this space, the suspending membrane will be forced todeform and the deflection is dependent on the membrane thick-ness, the length and width of structure and the boundary conditionof membrane as well as the applied air pressure, which can bedescribed by [36]

D

!!4w!x4 + 2

!4w!x2!y2 + !4w

!y4

"= q(x, y) (1)

where w is the membrane deflection, q(x, y)is the applied pressure,while D = Eh3/[12(1 " "2)] is the flexural rigidity of the membrane(h is the membrane thickness, E and " are the Young’s modulus andPoisson ratio of PDMS, respectively).

From the discussion presented in previous references, the edgeof the membrane under this condition is treated to be clamped tothe substrate. As a result, the boundary condition is given by

w|edge = 0,!w

!x(!y)

####edge

= 0 (2)

Combining Eqs. (1) and (2), the deflection can be eventually solved.Since this deformed structure is used as the mold for the follow-

ing PDMS casting process, the final structure parameters, such asthe depth and the cross-section contour, fabricated into PDMS sub-strate is determined by the deformation characteristics. It is obviousthat the round-like cross-section contour as well as the depth of thefabricated structure can be simply controlled with the air pressureusing the same mask. At the same time, through designing differ-ent opening dimension, the structure with multiple-height can alsobe easily fabricated with only one air pressurizing step withoutthe need for multiple photolithography process under alignmentas commonly used in SU-8 molding method. The specific discus-sion about these characteristics will be presented in the followingsection.

First, one layer of SU-8 (SU-8 2025, MicroChem Corp.) with80 !m thickness is spun onto a 4-in polished silicon wafer. Thepatterns of the desired structures, i.e., microchannel, inlet and out-let, are simultaneously transferred into this layer via the standardphotolithography process (Fig. 2(a)). Then the liquid PDMS prepoly-mer (Sylgard 184 silicone elastomer-a base and curing agent of DowCorning Corp-mixed in a 10:1 weight ratio) is poured onto this SU-8mold as shown in Fig. 2(b). After complete curing in the furnace, thePDMS layer with inverse structures having been transferred fromthe SU-8 mold is carefully peeled from the mold substrate with theassistance of isopropyl alcohol (IPA) solution (Fig. 2(c)). It is thenbonded to a 20 !m-thick PDMS membrane using oxygen plasmaactivation method, which had already been spun onto another 4-inpolished silicon wafer. The whole PDMS structure is then releasedfrom the hold wafer and a hole is manually drilled at the inlet of theauxiliary structure part making access to the external air pumpingsystem via the plastic tube. After performing particular treatmentto seal the inlet region, a positive air pressure is applied. As a result,the membrane covering the structures embedded into the PDMSsubstrate will be forced to deflect upward as described in Fig. 2(d).This deformed structure together with other components, i.e., thePDMS substrate and the undeformed membrane, are combined touse as the mold for the following second PDMS, namely device layer,casting step (Fig. 2(e)). It needs to be noticed that in order to obtainthe desired structure, it is important that during the whole processof this step, the air pressure should be always kept constant until thesufficient curing is achieved. Similar to the usual cases, the process

Table 1Process parameters adopted in experiment.

Item Parameters

SU-8 mold fabrication Spin speed: 2000 rpm, soft bake: 65 #C,3 min, hard bake:95 #C, 9 min,exposure: 25 s

The first PDMS curing Curing temperature: 65 #C, curingtime: 2 h

PDMS spin-coating Spin speed: 1000 rpmOxygen plasma activated bonding RF power: 20 W,exposure:30 s, curing:

60 #C, 30 min, chamber pressure:700 mTorr

The second PDMS curing Curing temperature: 65 #C, curingtime: 30 min

is also finished with the device layer being peeled off and bonded toanother substrate, such as PDMS or glass. The specific parametersinvolving in the process are listed in Table 1.

3. Experimental results

3.1. Microchannel with the same width

Considering the fact that the boundary condition in Eq. (2) isdifficult to express in a closed form for a real structure, the analyt-ical solution of membrane deformation from Eq. (1) is impractical.Owing to the powerful function provided by commercial finite ele-ment analysis (FEA) software, such as ANSYS, we can easily obtainthe numerical solution for membrane deformation under differ-ent applied pressure once the structural figure and parameters aredetermined. In our experiment, a Y-type microchannel is adoptedto demonstrate the cross-sectional contour of the microchannelfabricated with the method presented here. The schematic ofmicrochannel and corresponding structural parameters are given inFig. 3A. Fig. 3B shows the simulation results of the membrane defor-mation under 15 kPa pressure for this design. From the results, it canbe seen that the membrane is deformed into a shape with roundedcross-sectional contour and the sag height reaches 36.98 !m. Asmentioned above, this deformed membrane is used as the moldfor the following PDMS casting step. Considering the good shapereplication capability of the soft lithography process and the negli-gible influence of the liquid PDMS poured onto the membrane, theshape of the deformed membrane can be treated as the contourof the microchannel finally obtained. The optical cross-sectionalimage of the real fabricated PDMS microchannel (the same struc-tural and process parameters as adopted in simulation) is shownin Fig. 3C. It is obvious that a microchannel with rounded cross-sectional contour is successfully realized as predicted and its depthis measured to be about 35.569 !m, which agrees well with the sim-ulation result. At the same time, the microchannels fabricated undertwo other different pressures, namely 8 kPa and 5 kPa, are also pre-sented as shown in Fig. 3C. Although the microchannel’s depths arechanged to 22.85 !m and 13.586 !m, respectively, the unique char-acteristic about rounded cross-sectional contour is totally reserveddue to the nature of deformation. As a result, it is very easy to fabri-cate rounded microchannel with different depths to meet differentapplication requirements only by controlling the applied pressureusing the method presented here.

3.2. Microchannel with different widths

From theoretical analysis, it is obvious that the membranes withthe same thickness and length but different in width can resultin different deflection shapes as well as the sag heights underthe same pressure. This difference will finally be transferred tothe structural parameters of the fabricated microchannel, such as

Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761 757

Fig. 3. Microchannel design. (A) Schematic of the Y-type microchannel structure. (B) Simulation results of the membrane deformation. (C) Microscopy picture of the cross-section of the microchannel fabricated under different applied pressure.

the channel width, depth and the cross-sectional contour as well.Four straight microchannels with the same 20 cm length but dif-ferent widths, namely 800 !m, 400 !m, 200 !m and 100 !m, areadopted for this demonstration. The membrane thickness is chosento be 20 !m. Fig. 4A shows the simulation results about the mem-brane deformation under 5 kPa applied pressure together with thecross-sectional contour. The maximum membrane deflections (sagheight) are calculated to be 218.326 !m, 70.318 !m, 14.721 !m and3.149 !m, respectively, for these four cases. For comparison, thepictures of the cross-sectional contours of the real microchannelsfabricated under the same process parameters taken under micro-scope are also presented (Fig. 4B). With proper image processingsoftware, the microchannel depths are measured to be 209.958 !m,68.896 !m, 13.368 !m and 3.138 !m for microchannels with widthbeing 800 !m, 400 !m, 200 !m and 100 !m, respectively. Theseresults reveal that we can simultaneously fabricate microchannelswith different widths and depths on the same chip only with one

step of air pressurization process, thus largely simplifying the fab-rication and making the chip more versatile for different tasks.

3.3. Microchannel with multi-depth

As described above, different microchannel depths can beobtained at the same time only by designing different structuraldimensions. With this concept, we can design a microchannelstructure, in which different widths and/or lengths are adopted atthe different positions along the microchannel. Therefore, it canbe envisaged that a microchannel with multi-depth can also bedirectly realized in one step with only one photomask when usingthis novel method. In contrast, when using traditional method tofabricate this type of microchannel, usually at least two photomasksand photolithography steps with alignment or expensive tool suchas grayscale photolithography are needed [37]. This unique charac-teristic will largely facilitate the applications, such as cell/molecule

758 Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761

Fig. 4. Microchannel with different widths (A) simulation results about the membranes with different widths. (B) Photo images of the cross-section of the fabricatedmicrochannels with different widths: (a) 800 !m, (b) 400 !m, (c) 200 !m, (d) 100 !m.

sorting, filtering and flow mixing, etc., for immunoassay and bio-analysis purpose. For this characteristic demonstration, the mostcommonly used serpentine-type microchannel with two differ-ent configurations are designed, the details are shown in Fig. 5A.Through the simulation results about the membrane deformationas given in Fig. 5B, it is clear that when a uniform air pressure(8 kPa) is applied, the membrane will be deformed into differentshapes, including deformation contour and magnitude, at posi-tions corresponding to the varying structural dimensions. As fordesign 1, the membrane above the 300 !m width is deflectedto 40.152 !m height, while the deflection is reduced to only5.688 !m at the position above the 150 !m width. The similarphenomenon is also found in the case of design 2 except thatthe deformation is about 43.216 !m and 4.263 !m for the mem-brane 300 !m and 150 !m-wide membrane, respectively. It should

also be noted that the deflection at each turning corners is a bitlarger (nearly several micrometers) than that of the correspondingstraight part regardless the width, which is due mainly to the natureof shape-dependent deformation and has negligible effect on theoperation. With respect to the characterization for the real fabri-cated microchannel, the tomography of different parts is measuredwith ZYGO profilometer, a non-contact interference-based opticalmeasuring machine, a thin layer of gold film is first deposited ontothe whole device via evaporation to provide the required reflectivesurface. The measured results are shown in Fig. 5C, from which thecommon characteristic of multi-depth tomography for both designis clearly demonstrated as predicted by simulation. The depthsof the deeper microchannel part are 39.95 !m for design 1 and41.505 !m for design 2, while those of the shallower one are mea-sured to be 5.419 !m and 3.488 !m, respectively. At the same time,

Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761 759

Fig. 5. Multi-depth microchannel (A) schematic of the designed two types of serpentine microchannel. (B) Simulation results of the membrane deformation, namely themold shape, for design 1 (left) and design 2 (right), respectively. (C) Measurement results of the fabricated microchannel.

760 Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761

the parts with the same structural parameter but different positionon the same chip also demonstrate good shape uniformity. Exceptfor these important depth results, we can hardly obtain the infor-mation about the cross-sectional contour of the microchannel withthis measuring method. It is because PDMS is a pliable material andat the same time there is always residual stress in the depositedmetal layer. As a result, the surface quality of the gold layer isnot good enough especially on the sidewall of channel, makingit difficult to detect the required interferogram on them. There-fore, pictures revealing the cross-sectional contour of them takenunder microscope are also given in Fig. 5C, as complements forbetter demonstration. Obviously, although the large difference indepth, the rounded cross-section characteristic is totally preservedfor both of them just as described above.

4. Discussion

4.1. Release from the mold

In the fabrication method presented here, the mold is made up ofPDMS material, including the deformable membrane and the sub-strate, rather than the commonly used SU-8 negative photoresist.At the same time, the molding materials are both the liquid PDMSprepolymer. When using SU-8 as the mold structure, it is very easyto peel off the PDMS replica layer from the mold after complete cur-ing because PDMS has a fairly low surface energy [38]. But whencoping with PDMS mold, the case is quite different, since the liquidPDMS prepolymer can be fused with the PDMS mold after full cur-ing, making it impossible to release the device substrate from themold. In order to solve this problem, there are two methods. One isto passivate the PDMS mold with silanized treatment by exposure toa vapor of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane(C6F13CH2CH2SiCl3) before the casting process. It is the most com-mon method used by many groups to prevent the stiction betweentwo parts in contact. But it needs additional protective equipmentsand must be handled with much carefulness due to the toxicityof this chemical. In contrast, the other method, as adopted in ourexperiment, is more user-friendly. It can be realized through justcontrolling the curing time since partial curing can prevent entirefusion between these two parts with the same composition, thusfacilitating the release of the device layer from the mold. Mean-while, the shape of the replica agrees well with that of the moldsince the molding material has been translated from liquid intosolid status. In our experiment, the process parameters are chosento be 30 min curing under 65 #C.

4.2. Auxiliary air-connection channel

In order to apply the air pressure onto the mold region of thedesired microchannel structure without breaking its intactness,additional structures, including inlet and microchannel connectingto the main structure, are needed. Undoubtedly, this connectionpart will also be forced to deform under the applied air-pressureand then transferred into the device substrate during the mold-ing step. Since one end of it connects to the main microchannel, itmay cause leakage problem during the operation. From the aboveanalysis, membrane with different opening widths can result inlarge difference in its deformation. With respect to this consider-ation, the width of the connecting microchannel is designed to beonly 30 !m, which is much smaller than that of the main chan-nel, namely 200 !m. The final depths of these two parts realized inthe device substrate are measured to be 1.258 !m and 36.785 !m,respectively. With this small depth of the connecting microchanneland considering the fact that PDMS is a good elastomer, it is veryeasy to seal this microchannel during the bonding step by apply-

ing proper pressure onto this region. At the same time, we can alsospread some types of adhesive or even liquid PDMS prepolymeronto the end of the bonded device followed by sufficient curing(UV and/or thermal curing) for better hermetic seal.

5. Conclusions

In this paper, a novel method has been presented for PDMS-based microchannel fabrication. Different from the commonly usedSU-8 mold with fixed shape, a tunable PDMS mold is adoptedfor the soft lithography process. The structure patterns are firsttransferred into one PDMS substrate using the standard soft lithog-raphy process as described in many literatures. This substrate isthen bonded to a PDMS membrane having been spin-coated ontoa silicon wafer. After making inlet to external pressure source, aproper positive air-pressure is introduced, so that the membranecovering the pattern region will be forced to deform into a shapewith rounded cross-sectional contour. This deformed structure isused as the mold for the last casting process. With respect tothe good shape replicating performance, the microchannel ulti-mately obtained will definitely possess the rounded cross-sectionalcontour. This characteristic is much desired for applications suchas membrane-based valve. At the same time, through controllingthe applied pressure and/or adopting different channel widths,the channel depth as well as its cross-sectional shape can bothbe easily adjusted. For example, the depth of the microchannelwith 200 !m-wide can be fabricated to be 36.98 !m, 22.85 !m and13.586 !m when the applied pressure value reaches 15 kPa, 8 kPaand 5 kPa, respectively. While different microchannels, namely800 !m-wide, 209.958 !m-deep; 400 !m-wide, 68.896 !m-deep;200 !m-wide, 13.368 !m-deep and 100 !m-wide, 3.138 !m-deep,have also been successfully demonstrated when keeping 5 kPaconstant pressure during the molding step. It is straightforwardto design microchannel with different widths at different posi-tions, therefore, the multi-depth characteristic can be realized withonly one air pressurization and the following casting step, largelysimplifying the process when compared with the cases of the con-ventional methods. This undoubtedly facilitates the applications ofLOC in areas, such as cell/molecule sorting, filtering and flow mix-ing, for immunoassay and bio-analysis purpose. With this concept,two types of serpentine microchannel with multi-depth (deeper:39.95 !m, shallower: 5.419 !m for design 1 and deeper: 41.505 !m,shallower: 3.488 !m for design 2) have been successfully fabri-cated.

Acknowledgements

Financial support by the Ministry of Education (MOE) SingaporeAcRF Tier 1 funding under grant no. R-265-000-235-112 and R-265-000-211-112/133 is gratefully acknowledged.

References

[1] H. Andersson, A.V.D. Berg, Lab-on-Chips for Cellomics: Micro and Nanotech-nologies for Life science, Kluwer Academic, London, 2004, pp. 171–196 (Chapter7).

[2] O. Geschk, H. Klank, P. Telleman, Microsystem Engineering of Lab-on-a-ChipDevices, Wiley-VCH, Weinheim, 2008, pp. 1–7 (Chapter 1).

[3] B.H. Weigl, R.L. Bardell, C.R. Cabrera, Lab-on-a-Chip for drug development, Adv.Drug Deliv. Rev. 55 (2003) 349–377.

[4] F. Daniel, P. Devanand, Lab-on-a-chip: a revolution in biological and medicalsciences, Anal. Chem. 72 (2000) 330–335.

[5] P.C.H. Li, Microfluidic Lab-on-a-Chip for Chemical and Biological Analysis andDiscovery, Taylor & Francis Group, LLC, 2006, pp. 1–2 (Chapter 1).

[6] G. Medoro, N. Manaresi, A. Leonardi, L. Altomare, M. Tartagni, R. Guerrieri, A lab-on-a-chip for cell detection and manipulation, IEEE Sens. J. 3 (2003) 317–325.

[7] I. Schneegaß, R. Bräutigam, J.M. Köhler, Miniaturized flow-through PCR withdifferent template types in a silicon chip thermocycler, Lab Chip 1 (2001) 42–49.

Y. Hongbin et al. / Sensors and Actuators B 137 (2009) 754–761 761

[8] Y. Kikutani, T. Horiuchi, K. Uchiyama, H. Hisamoto, M. Tokeshi, T. Kitamori, Glassmicrochip with three-dimensional microchannel network for 2 $ 2 parallel syn-thesis, Lab Chip 2 (2002) 188–192.

[9] Y. Cheng, K. Sugioka, K. Midorikawa, Microfabrication of 3D hollow structuresembedded in glass by femtosecond laser for lab-on-a-chip applications, Appl.Surf. Sci. 248 (2005) 172–176.

[10] O. Hofmann, P. Niedermann, A. Manz, Modular approach to fabrication ofthree-dimensional microchannel systems in PDMS—application to sheath flowmicrochips, Lab Chip 1 (2001) 108–114.

[11] M. Svedberg, M. Veszelei, J. Axelsson, M. Vangbo, F. Nikolajeff,Poly(dimethylsiloxane) microchip: microchannel with integrated openelectrospray tip, Lab Chip 4 (2004) 322–327.

[12] L.H. Hung, R. Lin, A.P. Lee, Rapid microfabrication of solvent-resistant biocom-patible microfluidic devices, Lab Chip 8 (2008) 983–987.

[13] M. Natali, S. Begolo, T. Carofiglioc, G. Mistura, Rapid prototyping of multilayerthiolene microfluidic chips by photopolymerization and transfer lamination,Lab Chip 8 (2008) 492–494.

[14] H.B. Yu, G.Y. Zhou, F.K. Chau, F.W. Lee, Optoluidic variable aperture, Opt. Lett.33 (2008) 548–550.

[15] C.L. Bliss, J.N. McMullin, C.J. Backhouse, Rapid fabrication of a microfluidicdevice with integrated optical waveguides for DNA fragment analysis, Lab Chip7 (2007) 1280–1287.

[16] Y. Xia, G.M. Whitesides, Soft lithography, Annu. Rev. Mater. Sci. 28 (1998)153–184.

[17] S.K. Sia, G.M. Whitesides, Microfluidic devices fabricated inpoly(dimethylsiloxane) for biological studies, Electrophoresis 24 (2003)3563–3576.

[18] M. Abdelgawad, M.W.L. Watson, E.W.K. Young, J.M. Mudrik, M.D. Ungrin, A.R.Wheeler, Soft lithography: master on demand, Lab Chip 8 (2008) 1379–1385.

[19] J.P. Urbanski, W. Thies, C. Rhodes, S. Amarasinghe, T. Thorsen, Digital microflu-idics using soft lithography, Lab Chip 6 (2006) 96–104.

[20] M.A. Unger, H.P. Chou, T. Thorsen, A. Scherer, S.R. Quake, Monolithic microfab-ricated valves and pumps by multilayer soft lithography, Science 288 (2000)113–116.

[21] L. Ceriotti, K. Weible, N.F. de Rooij, E. Verpoorte, Rectangular channels for lab-on-a-chip applications, Microelectron. Eng. 67–68 (2003) 865–871.

[22] M.J. Fuerstman, A. Lai, M.E. Thurlow, S.S. Shevkoplyas, H.A. Stone, G.M. White-sides, The pressure drop along rectangular microchannels containing bubbles,Lab Chip 7 (2007) 1479–1489.

[23] R. Lima, S. Wada, S. Tanaka, M. Takeda, T. Ishikawa, K. Tsubota, Y. Imai, T.Yamaguchi, In vitro blood flow in a rectangular PDMS microchannel: experi-mental observations using a confocal micro-PIV system, Biomed. Microdevices10 (2008) 153–167.

[24] G.B. Lee, C.C. Chang, S.B. Huang, R.J. Yang, The hydrodynamic focusingeffect inside rectangular microchannels, J. Micromech. Microeng. 16 (2006)1024–1032.

[25] R. Sadr, M. Yoda, Z. Zheng, A.T. Conlisk, An experimental study of electro-osmotic flow in rectangular microchannels, J. Fluid Mech. 506 (2004) 357–367.

[26] M.T. Koesdjojo, Y.H. Tennico, J.T. Rundel, V.T. Remcho, Two-stage polymerembossing of co-planar microfluidic features for microfluidic devices, Sens.Actuators, B 131 (2008) 692–697.

[27] C.C. Chen, D. Hirdes, A. Folch, Gray-scale photolithography using microfluidicphotomasks, Proc. Natl. Acad. Sci. U.S.A. 100 (2003) 1499–1504.

[28] H.K. Wu, T.W. Odom, G.M. Whitesides, Reduction photolithography usingmicrolens arrays: applications in gray scale photolithography, Anal. Chem. 74(2002) 3267–3273.

[29] Y. Bellouard, A. Said, M. Dugan, P. Bado, Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemicaletching, Opt. Express 12 (2004) 2120–2129.

[30] A.A. Evstrapov, A.O. Pozdnyakov, S.G. Gornyi, K.V. Yudin, Microchannel edgeformation in laser-ablated polyimide, Techn. Phys. Lett. 31 (2005) 541–544.

[31] D. Lim, Y. Kamotani, B. Cho, J. Mazumder, S. Takayama, Fabrication of microflu-idic mixer and artificial vasculatures using a high-brightness diode-pumpedNd:YAG laser direct write method, Lab Chip 3 (2003) 318–323.

[32] G.J. Wang, C.C. Hsueh, S.H. Hsu, H.S. Hung, Fabrication of PLGA microvesselscaffolds with circular microchannels using soft lithography, J. Micromech.Microeng. 17 (2007) 2000–2005.

[33] C.B. Rohde, F. Zeng, R. Gonzalez-Rubio, M. Angel, M.F. Yanik, Microfluidic systemfor on-chip high-throughput whole-animal sorting and screening at subcellularresolution, Proc. Natl. Acad. Sci. U.S.A. 104 (2007) 13891–13895.

[34] N. Futai, W. Gu, S. Takayama, Rapid prototyping of microstructures with bell-shaped cross-sections and its application to deformation-based microfluidicvalves, Adv. Mater. 16 (2004) 1320–1323.

[35] K. Lee, C. Kim, K.S. Shin, J.W. Lee, B.K. Ju, T.S. Kim, S.K. Lee, J.Y. Kang, Fabricationof round channels using the surface tension of PDMS and its application to a3D serpentine mixer, J. Micromech. Microeng. 17 (2007) 1533–1541.

[36] E. Ventsel, T. Krauthammer, Thin Plates and Shells, Marcel Dekker, Inc., 2001,pp. 55–105 (Chapter 3).

[37] K.S. Yun, E. Yoon, Fabrication of complex multilevel microchannels in PDMSby using three-dimensional photoresist masters, Lab Chip 8 (2008) 245–250.

[38] M. Kim, K. Kim, N.Y. Lee, K. Shin, Y.S. Kim, A simple fabrication route to a highlytransparent super-hydrophobic surface with a poly(dimethylsiloxane) coatedflexible mold, Chem. Commun. 22 (2007) 2237–2239.

Biographies

Yu Hongbin received a BS in mechanical engineering, MS in electrical engineeringand PhD in optical engineering from Huazhong University of Science and Technol-ogy, China, in 1999, 2002 and 2006, respectively. He is currently a research fellowat Micro/Nano Systems Initiative Technology with the Department of MechanicalEngineering, National University of Singapore. His research interests involve thedesign, simulation and fabrication technology of microelectromechanical devicesand optofluidics.

Zhou Guangya received the B Eng and PhD degrees in optical engineering fromZhejiang Univeristy, Hangzhou, China, in 1992 and 1997, respectively. He joined theNational University of Singapore (NUS) in 2001 as a research fellow. Currently, heis an assistant professor with the Department of Mechanical Engineering, NUS. Hismain research interests include microoptics, diffractive optics, MEMS devices foroptical applications, and nanophotonics.

Chau Fook Siong is an associate professor in the Department of Mechanical Engi-neering, National University of Singapore, where he heads the Applied MechanicsAcademic Group. His main research interests are in the development and appli-cations of optical techniques for nondestructive evaluation of components and themodeling, simulation, and characterization of microsystems, particularly bio-MEMSand MOEMS.

Wang Shouhua received the B Eng from Xi’an Jiaotong University, 2005. He is cur-rently working toward the PhD degree in the Department of Mechanical Engineering,National University of Singapore. His research interests include the design, fabrica-tion and characterization of micro-spectrometer.

Lee Feiwen received the B Eng from the National University of Singapore, 2006. Heis currently a Masters student at Micro/Nano Systems Initiative Technology withthe Department of Mechanical Engineering, National University of Singapore. Hisresearch topic is on the design and fabrication of optical micro-spectrometers basedon MEMS technology.