Applied Surface Science 257 (2011) 9314 9317

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

Casting mold patterning for lateral capillary force migration on PDMSmicrochannel

Kim Yong-Hoona,, Tsuneo Urisuba National Academy of Agricultural Science, Department of Agro-food Biosensors, 150 suin-ro, Seodun-dong Kwansun-gu, Suwon, Republic of Koreab Institute for Molecular Science, Department of Vacuum UV Photoscience, Myodaiji, Okazaki 444-8585, Japan

a r t i c l e i n f o

Article history:Received 30 December 2010Received in revised form 3 May 2011Accepted 6 May 2011Available online 21 June 2011

Keywords:PDMSLateral capillary forceRectangular microchannelBending

a b s t r a c t

The casting molds including various shapes within axial and lateral resolution (110 !m and 5 !m) couldbe precisely fabricated and were suitable for the fabrication of PDMS microchannel possessing the largerinner volume for the purpose of the rapid capillary force migration. The tight bonding between PDMSmold and SiO2/Si surface not only was governed by the flexibility and the degree of tilted angle (TA) ofPDMS microchannel which shows the minimum value at the 0.2 weight ratio (Wr) of curing agent butalso efficiently generates the capillary migration.

Crown Copyright 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Microfluidic channel was attractive for the various researchfields including in the cell biology, biosensor, tissue engineering,bioelectronics, and bioanalytical [13]. Especially, PDMS is one ofthe versatile materials for microfluidic channel fabrications due toits merits; optical transmission up to 240 nm, nontoxic, low-costair molding, and flexibility, etc., In addition, it is easy to modify thesurface nature from hydrophobic to hydrophilic by plasma cleaningas well as the biocompatible cell living condition. Many researcheson account of the merits have been conducted.

Taylor et al. reported the unique fabrication process byemploying repeated lithographic technique on the same castingmolds surface, which makes it possible to 3-dimensional com-plex microstructure for the neuronal growth chamber [4]. Ungeret al. suggest that mechanical valve, known as Quake Valve,could be incorporated into microfluidics application [5]. A fewresearches have been performed in relation to capillary force ona microchannel system fabricated by PDMS, however, these weremainly electrophoresis [6,7], cell analysis for isolation or biosepa-rations [8], and a groove type open channel modified by polar andnon polar functional groups [9]. In addition, these researches weremainly used by syringe feeding method and mechanical methodsto inject solutions.

Corresponding author. Tel.: +82 31 290 1927.E-mail address: yhkim0420@korea.kr (K. Yong-Hoon).

The microchannel that the driving force of solution migrationis pure capillary force required a large volume analyte solution,which is a demerit, on the other hand, the channel has the meritsthat it is easy to fabricate microchannels within tens of micrometerdimension and to simultaneously adapt various transducers intomicrochannels.

Lateral capillary force of microchannels on the solid substrateis dependent of geometrical shape, surface tension of a solu-tion, and the surface nature of channel inside including charge,hydrophilicity. Especially, in relation to capillary force migrationon a solid substrate, a geometrical shape, a diameter of rectangularmicrochannel, is the most critical than any other parameter. Thevelocity of solution migration was proportional to microchanneldiameter [10].

Although syringe feeding, mechanical pumping, and capillaryforce migration are important to the injective applications ofmicrofluidics, few studies in relation to capillary force solutionmigration and microchannel fabrication have been reported yet. Inhere we reported that the casting molds including various shapeswithin axial and lateral resolution could be precisely fabricated forcapillary force PDMS microchannel and the degree of TA of PDMSmicrochannel affected lateral capillary force migration.

2. Experimental

2.1. Materials

All of the solvents used in this experiment (analytical grade)were obtained from SigmaAldrich and used without further

0169-4332/$ see front matter. Crown Copyright 2011 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2011.05.029

K. Yong-Hoon, T. Urisu / Applied Surface Science 257 (2011) 9314 9317 9315

purifications. Poly-dimethylsiloxane (PDMS, sylgard 184) was pur-chased from Dow Corning (Midland, MT). Silicon wafers (low dose)were obtained from Nippon Steel Co. Used water was high purityMilli-Q water (Millipore, >18 M !cm). Trimethylchlorosilane wereobtained from Gelest. Poly-ethylenimine (PEI, Mw 70,000) was pur-chased from SigmaAldrich. Negative photoresists (NPs) of SU-8series and developer was purchased from Microchem.

2.2. Silicon casting mold and PDMS channel preparation

According to the protocol of Microchem application note, cleanwafers treated by piranha solution (H2SO4:H2O2, 4:1, v/v) werecoated with SU-8 3005, 3010, 3025, and 3050 negative photoresists(NPs) by the spin coater. The wafers coated by different four NPswere cured at 95 C, 15 min (3050), 10 min (3025), 3 min (3010),and 2 min (3005) followed by 65 C, 1 min and then cooled in air,respectively. The NP coated wafer on photomask aligner, mountedwith Cr coated photomask, were exposure by UV beam of 200 mJradiant power at 60 s (3050), 50 s (3025), 35 s (3010), and 30 s(3005), respectively, followed by 65 C incubation, 1 min. The pat-terned wafers were developed by SU-8 developer at the immersingtime of 8 min (3050), 6 min (3025), 5 min (3010), and 3 min (3005),followed by rinsing the wafers with Isopropyl alchol stirred at300 rpm, 2 min.

The substrates were baked to dense the resist pattern at thecuring time of 5 min (3050), 4 min (3025), 2 min (3010), and 1 min(3005) and immersed into the trimethylchlorosilane (5 vol%) toeasily peel out the PDMS channel. The microfluidic structures weremolded by pouring thoroughly degassed PDMS mixture keeping1 h vacuum condition (10:1, wt/wt, 10 g PDMS), and sequentiallywas cured at 65 C, 3 h (Fig. 1). Indian ink diluted by PEI solution(PEI: water, 1:1000, wt/wt) was used to observe the capillary migra-tion in microchannels bonded onto the bare silicon substrate inwhich PDMS channel and silicon substrate were treated by plasmacleaning under O2 and CF4 atmosphere. High resolution ScanningElectron Microscope (SEM) images of the microfluidic structurewere obtained by employing JEOL microscope system.

3. Results and discussion

3.1. Dependence of silicon casting mold resolution varying withviscosity of NP

For the lateral capillary force migration, the microscope imagesof casting mold structure of microchannel consisted of the circularareas, lines, and inlet were shown in Fig. 2.

After fabricating PDMS mold, circle and line (shown in Fig. 2a) isto be the migration pathway of a solution and a rectangular mark

Fig. 1. Schematic illustration of the fabricating process of PDMS microchannel forthe observation of capillary force migration.

denoted in Fig. 2b) is to be the solution inlet, which is able to exten-sively inject the solution. The shape of outlet is identical to that ofinlet. To control the vertical dimensions of casting mold structure,four kinds of NPs having the different viscosity (65 (NP 3005), 340(NP 3010), 7400 (NP 3025), and 12000 (NP 3050) cSt) were usedand the results are shown in Fig. 3.

Fig. 3ad shows the SEM images of casting mold structure cor-responding to the part of Fig. 2a and 3eh were SEM images of thesame structure corresponding to an inserted rectangular mark inFig. 2b. The diameter of circular area was 25 !m. The line width wasfrom 5 !m to 20 !m. The thicknesses were 4.5 !m, 6.3 !m, 15 !m,and 110 !m, respectively. And also, the suppressed area observedin Fig. 3g was due to the contact force between the photomask andthe coated register film during UV exposure. So far as the thicknessof 110 !m and the width of 5 !m, the precise shapes of line, cir-cle, and inlet of the microstructure could be observed. Under thefabrication process explained in the experimental section, a varietyof the shapes including circle, inlet, complex structures, and evenlongish lines within a wide range of the axial and lateral dimensioncould be precisely fabricated.

Fig. 2. Microscope image of microfludic structure (scale bar is 50 !m): (a) protruded register area (pre-migration pathway of a solution), and (b) pre-inlet of a solution (Bothinlet and outlet shapes are identical).

9316 K. Yong-Hoon, T. Urisu / Applied Surface Science 257 (2011) 9314 9317

Fig. 3. SEM image of microfludic structure (a)(h): (a)(d) are pre-migration pathways fabricated by NP 3005, 3010, 3025, and 3050, respectively and (e)(h) are pre-inletfabricated by 3005, 3010, 3025, and 3050, respectively. Inserted is the enlarged circle area (scale bar is 10 !m).

To rapidly migrate a desired solution by the lateral capil-lary force inside PDMS microchannel of rectangular cross section,the larger inner volume of PDMS microchannel was essentiallyrequired [10]. Therefore, these results reflected that the castingmolds are suitable to fabricate polymer microchannels of whichthe driving force for solution migration is the capillary force.

3.2. Bending of PDMS microchannel wall

In the fabrication process of microfluidic structure by employingPDMS, the shrinkage is inevitably accompanied. This leads to geo-metrical deformations, which cause abnormal effects such as weakbonding between the PDMS mold and the solid surface. In orderto optimize the lateral capillary force migration in PDMS channel

through the enhancement of the bonding between PDMS mold andSiO2/Si surface, among of many experimental parameters to adjustthe bending occurred at PDMS channel wall, the experiment vary-ing with curing agent was carried out, since the bending in PDMSmold denoted in the circle of Fig. 4b caused the curvature of thecontact surface, which leading to weaken the bonding between thecontacting surfaces.

For most of experiments performed under 0.1 weight ratio(Wr) of curing agent, the solution including PEI was dropped intoPDMS mold inlet to observe the lateral capillary force migrationon the SiO2 surface. It was ascertained that the solution migra-tion occurred not from the pure lateral capillary force (migrationthrough the pathway of PDMS microchannel) but from perme-ation through the interstice between the PDMS mold and the SiO2

K. Yong-Hoon, T. Urisu / Applied Surface Science 257 (2011) 9314 9317 9317

Fig. 4. Dependence of tilted angle (TA) of PDMS microstructure for curing agentweight ratio: (a) cross sectional image of tilted PDMS microchannel, (b) schematicview of TA, and (c) TA plotted as a function of Wr of curing agent.

surface. The results were summarized in Table 1 and these factswere ascribed to the bending in PDMS microchannel wall demon-strating the cross sectional image shown in Fig. 4a.

To circumvent this permeation by minimizing the bending inPDMS microchannel wall as well as to observe the lateral migra-tion of a solution by the capillary force inside PDMS microchannel.The adding ratio of curing agent was carried out to investigate thedegree of bending generated on PDMS microchannel wall in whichNP 3025 was used to fabricate silicon casting molds, and the resultsare shown in Fig. 4., The degree of tilted angle (TA) depicted inFig. 4b was gradually diminished as increasing amount of curingagent. At 0.2 Wr, the value is clearly decreased compare to thatof 0.1 Wr and the stiff decrement of TA value in the interval of0.120.14 Wr could be observed. Also, as increasing amount of cur-ing agent, TA value clearly decreased, on the other hand, PDMSchannel was to be slightly rigid (data was not shown) becauseof expediting the densification in the formation process of PDMSmold, which deteriorated the bonding between PDMS mold andSiO2/Si surface.

Considering the results summarized in Table 1 and the flow pro-file in Fig. 5 (0.14 Wr of curing agent), the bonding between twosurfaces was governed by the flexibility and the degree of TA in

Table 1Solution migration with respect to the dimension of PDMS microchannel in whichPDMS mold was fabricated at 0.1 Wr of curing agent. n is non-capillary force migra-tion (permeation), c is capillary force migration, and * is the channel width of 20 !m.

Channel dimension (thickness*) Observed result

4.5 !m* n6.3 !m* n

15.0 !m* n, c110.0 !m* n, c

Fig. 5. Microscope image of the lateral capillary migration inside PDMS microchan-nel bonded on the SiO2/Si surface (NP 3025 was utilized for the silicon casting mold).Inserted is an enlarged flow profile.

PDMS mold. To transfer a solution in the hydrophilic microchannelby lateral capillary force, the tight bonding was essentially requiredbetween PDMS mold of microchannel and hydrophilic substrate.Tight bonding between two surfaces was accomplished by theWr (0.14), maintaining the softness of PDMS mold and minimiz-ing the curvature of contact surface in PDMS mold by decreasingthe microchannel wall bending. The PDMS microchannel shown inFig. 5 was fabricated under the condition of NP 3025 and 0.14 Wr ofcuring agent. The flow profile demonstrated a curvature shape cor-responding to hydrodynamic flow [11], which indicating that thetight bonding between PDMS mold and SiO2/Si surface efficientlygenerates the capillary migration.

4. Conclusion

The casting molds including various shapes within axial and lat-eral resolution (110 !m and 5 !m) could be precisely fabricatedunder our experimental conditions and were suitable for the fab-rication of PDMS microchannel possessing the larger inner volumefor the purpose of the rapid capillary force migration. The tightbonding between PDMS mold and SiO2/Si surface not only was gov-erned by the flexibility and the degree of TA of PDMS microchannelbut also efficiently generates the capillary migration.

Acknowledgements

This work was supported by a Grant-in-Aid Scientific Research(Nos. 13SG0016 and 17034064) from the Ministry of Education,Culture, Sports, Science, and Technology.

References

[1] A. Manz, N. Graber, H.M. Widmer, Sens. Actuators B Chem. 1 (1990) 244248.[2] A. Offenhausser, S. Bocker-Meffert, T. Decker, R. Helpenstein, P. Gasteier, J. Groll,

M. Moller, A. Reska, S. Schafer, P. Schulte, A. Vogt-Eisele, Soft Matter. 3 (2007)290298.

[3] H.Y. Tan, W.K. Loke, Y.T. Tan, N.T. Nguyen, Lab Chip 8 (2008) 885891.[4] A.M. Taylor, S.W. Rhee, C.H. Tu, D.H. Cribbs, C.W. Cotman, N.L. Jeon, Langmuir

19 (2003) 15511556.[5] M.A. Unger, H.P. Chou, T. Thorsen, A. Scherer, S.R. Quake, Science 288 (2000)

113116.[6] L.C. Mecker, R.S. Martin, Anal. Chem. 80 (2008) 92579264.[7] J.A. Vickers, M.M. Caulum, C.S. Henry, Anal. Chem. 78 (2006) 74467452.[8] J.C. McDonald, D.C. Duffy, J.R. Anderson, D.T. Chiu, H.K. Wu, O.J.A. Schueller,

G.M. Whitesides, Electrophoresis 21 (2000) 2740.[9] K. Khare, J.H. Zhou, S. Yang, Langmuir 25 (2009) 1279412799.

[10] E. Delamarche, A. Bernard, H. Schmid, A. Bietsch, B. Michel, H. Biebuyck, J. Am.Chem. Soc. 120 (1998) 500508.

[11] D.A. Skoog, F.J. Holler, T.A. Nieman, Principles of Instrumental Analysis, 5thed., Saunders College Pub./Harcourt Brace College Publishers, Philadelphia,Orlando, FL, 1998, p. 782.

Casting mold patterning for lateral capillary force migration on PDMS microchannel1 Introduction2 Experimental2.1 Materials2.2 Silicon casting mold and PDMS channel preparation

3 Results and discussion3.1 Dependence of silicon casting mold resolution varying with viscosity of NP3.2 Bending of PDMS microchannel wall

4 ConclusionAcknowledgementsReferences