276 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 2, APRIL 2014

Liquid-Phase Capillary Etching ofPoly(dimethylsiloxane) MicrochannelsWith Tetra-n-Butylammonium Fluoride

Changwoo Ban, Jaewon Park, Member, IEEE, Dong-Young Jang, and Arum Han, Senior Member, IEEE

Abstract— Easy fabricability, along with other unique proper-ties, has made poly(dimethylsiloxane) (PDMS) one of the mostcommonly used materials for microfluidics-based microdevices.However, unlike other polymer materials commonly used formicrodevices, the PDMS cannot be easily etched or furtherchanged upon polymerization, and therefore, fabrication methodhas been rather limited to replication molding. Here, we demon-strated liquid-phase capillary etching of the PDMS microchannelswith tetra-n-butylammonium fluoride (TBAF) and characterizedits etching profiles depending on the initial microchannel widths,TBAF flow rates, TBAF concentrations, and substrates used forthe microfluidic channel sealing. The characterization showedthat the microchannel circumference etch rate was linearlyproportional to the TBAF flow rate and was faster at higherTBAF concentration. On the other hand, influence of the TBAFconcentration on the horizontal and vertical etch rates wasvastly different, with the vertical etch rate being much moreaffected by the concentration change. Potential applications ofthe liquid-phase capillary etching method were demonstrated byfabricating the PDMS microchannels with round-shaped crosssections, a microdevice with embedded 3-D metal electrodes withthe electrodes directly exposed to the microfluidic channel, and aglass-PDMS-glass sandwich microchannel for high-clarity opticaldetection. We believe that the characterization of the liquid-phasecapillary etching along with the applications demonstrated herewill provide new capabilities for fabricating the PDMS-basedmicrodevices that could not be easily fabricated by conventionalreplication molding processes. [2013-0298]

Index Terms— Poly(dimethylsiloxane) etching, capillarytetra-n-butylammonium fluoride (TBAF) etching, round-shapedmicrochannels, 3D metal electrode embedded microdevice,glass-PDMS-glass sandwich microchannel.

I. INTRODUCTION

POLY(DIMETHYLSILOXANE) (PDMS) is one of themost widely used materials in the field of microfluidics-

Manuscript received September 23, 2013; revised December 21, 2013;accepted January 12, 2014. Date of publication February 4, 2014; date ofcurrent version March 31, 2014. This work was supported by the NationalScience Foundation Emerging Frontiers in Research and Innovation underGrant 1240478. C. Ban and J. Park contributed equally as co-first authors.Subject Editor C. H. Ahn.

C. Ban and J. Park are with the Department of Electrical and Com-puter Engineering, Texas A&M University, College Station, TX 77843 USA(e-mail: chwban@gmail.com; jwpark@tamu.edu).

D.-Y. Jang is with the Manufacturing System and Design EngineeringProgram, Seoul National University of Technology, Seoul 139-743, Korea(e-mail: dyjang@seoultech.ac.kr).

A. Han is with the Department of Electrical and Computer Engineering, andthe Department of Biomedical Engineering, Texas A&M University, CollegeStation, TX 77843 USA (e-mail: arum.han@ece.tamu.edu).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2014.2301040

based microdevices due to its easy fabricability, biocompat-ibility, gas permeability, low cost, and optical transparency[1]–[4]. However, compared to other commonly used polymerssuch as poly(methyl methacrylate) (PMMA), cyclic olefincopolymer (COC), or polycarbonate that can be readily manip-ulated by heat or solvents [5]–[9], it is not easy to etchor further change the shape of fully polymerized PDMSmicrostructures. Since the first introduction of PDMS soft-lithography in 1998 [10], the technique has been the mostcommonly used method for fabricating PDMS microstruc-tures. Besides soft-lithography, only a few approaches such asphysical scraping, lift-off using sacrificial polymer patterns,or etching either by gas or liquid have been introduced tofabricate PDMS microstructures.

Ryu et al. introduced a method of patterning PDMSmicrostructures by physically scraping uncured PDMS pouredon top of a microstructure-patterned substrate that allowedrelatively thick PDMS layer to be patterned, however themethod had limited patterning resolution [11]. Lift-off tech-nique using a patterned parylene C layer that can be peeled offas a sacrificial layer [12] or a patterned photoresist layer thatcan be removed by solvents [13] was also utilized to patternPDMS spin-coated on top of the sacrificial layer. Fabricationof PDMS membranes with through-holes was demonstratedby spin-coating a thin layer of PDMS on top of a SU-8TM

patterned substrate having microstructures that are taller thanthe PDMS membrane, thus creating openings in the membrane[14], [15]. Approaches to use conventional plasma-based dryetching methods with oxygen plasma or O2/CF4 gas mixturehave also been reported, yet slow etch rates, rough surface afteretching, and contamination of the etching surface by residualparticles limited them from being widely used [16]–[18].

Tetra-n-butylammonium fluoride (TBAF) is a material thathas been used to chemically dissolve fully polymerized PDMS[19], [20]. Xu et al. spin-coated SU-8TM on top of a PDMSmicropattern, exposed to UV light and developed, followed bydissolving the PDMS stamp in TBAF to obtain freestandingSU-8TM microstructures. Siegel et al. used a similar schemeto create three-dimensional microelectrodes by injecting liquidsolder into PDMS microchannels and dissolving the PDMSwith TBAF [21]. Applications utilizing TBAF to dissolvePDMS residues on photoresist patterns and to pattern a PDMSlayer using metal as an etch mask have also been recentlyreported [22], [23]. However, in most cases, TBAF was simplyused to completely remove PDMS, and only few studieshave reported etching of PDMS microchannels using TBAF.

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BAN et al.: LIQUID-PHASE CAPILLARY ETCHING OF PDMS MICROCHANNELS 277

Takayama et al. utilized the laminar flow characteristics inmicrochannels to selectively etch the microchannel surfacefor cell culture application [24] and Pavesi et al. perfusedTBAF inside a microchannel for exposing polymer electrodes(PDMS and carbon nanotube composite) embedded in thePDMS device [25]. However, flow-through liquid-phase capil-lary etching characteristics of PDMS using TBAF has not yetbeen reported.

In this paper, we provide full characterization of liquid-phase capillary etching of PDMS microchannels with TBAFdepending on the initial microchannel widths, TBAF flowrates, TBAF concentrations, and substrate materials that sealthe PDMS microchannel. We also demonstrated this methodas a simple and low-cost way of fabricating PDMS microchan-nels with round-shaped cross-section without any complicatedprocesses such as gray-scale lithography [26], utilizing pneu-matic actuation [27], or reflowing patterned photoresists [28].Rectangular microfluidic channels are widely used for variouspurposes, yet show some drawbacks depending on applicationssuch as asymmetric fluidic flow, non-uniform shear stressto the channel surfaces or having stagnant flow or deadvolume at the corners of microchannels [29], [30]. In certainapplications, round-shaped cross-section microchannels canbe more suitable choice over rectangular ones. For example,round-shaped channels are widely used as an in vitro modelfor blood vascular system [31]–[33]. Round-shaped cross-sectional profile not only physically mimics blood vessels butalso enables more uniform diffusion of nutrients and gasseswithin the microchannel. Besides, they are also widely used forflow focusing applications as well as for microdevices havingpneumatic valves or pumps [34], [35]. In addition to round-shaped cross-section microchannel fabrication, a microdevicewith embedded and exposed 3D metal electrodes and a glass-PDMS-glass sandwich microchannel for high-clarity opticaldetection applications have been fabricated to further demon-strate the utility of this PDMS microfabrication method.

II. MATERIALS AND METHODS

A. Experimental Design and Fabrication

Liquid-phase capillary etching of PDMS microchannelswith TBAF was characterized by flowing TBAF solution insidea PDMS microchannel sealed against either a glass slide ora PDMS slab. First, a SU-8TM (Microchem, Inc., Newton,MA) master mold for PDMS replication was prepared by aconventional photolithography process. Briefly, SU-8TM 2015was spin-coated on a cleaned 3 inch silicon wafer at the speedof 500 rpm for 15 seconds and 2750 rpm for 35 seconds,soft-baked at 95 °C for 4 min, exposed at 210 mJ/cm2 ofUV light, post-exposure baked at 95 °C for 5 min, anddeveloped in SU-8 TM developer (Microchem, Inc., Newton,MA) for 2 min to achieve patterns with 18 μm thickness.Patterned SU-8TM master mold was then vapor-coated with(tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane (UnitedChemical Technologies, Inc., Bristol, PA) for 30 min insidea vacuum chamber, followed by a brief rinsing step with iso-propyl alcohol (IPA) to facilitate PDMS release after replica-tion. PDMS microchannels were replicated by pouring PDMS

Fig. 1. Schematic illustrations showing the fabrication process of a PDMSmicrochannel and the setup for the characterization of TBAF-based liquid-phase capillary etching of PDMS.

pre-polymer (10:1 mixture, Sylgard® 184, Dow Corning, Inc.,Midland, MI) on top of the SU-8TM master mold at thethickness of 3 mm, followed by polymerization at 85 °C forat least 3 hours. The inlet and outlet of the microchannel werepunched using a stainless steel punch (Syneo, Angleton, TX)and bonded against either a 2 mm thick PDMS slab or a1 mm thick glass slide after 2 min of oxygen plasma treatment(Harrick Plasma, Ithaca, NY). In order to prevent TBAFfrom dissolving polymer tubing connections, Teflon® FEPtubing (IDEX Health & Science, Oak Harbor, WA), stainlesssteel needles (BD, Franklin Lakes, NJ), and glass syringes(Tomopal, Inc., Sacramento, CA) were used to perfuse TBAFinto the PDMS microchannel. Additional PDMS was pouredon top of the tubing connected PDMS device to further sealthe tubing interface. The overall fabrication process is shownin Fig. 1.

The microdevice having embedded 3D metal electrodes wasfabricated by pre-defining microfluidic channels for electrodes(e.g. electrode channels) and filling the channels with metalshaving low melting temperature (Supplementary Fig. S1a).In order to facilitate the metal filling process, electrode chan-nels were designed in ‘V’-shape rather than having closed endsas typical electrodes would, with the tip of the ‘V’-shapedchannel pointing towards the flow channel. The electrode chan-nels were separated from the flow channel by a 30 μm thickPDMS wall. The PDMS device with pre-defined electrodechannels was prepared by soft-lithography as described aboveand assembled on a glass slide. The assembled device was thenplaced on top of a leveled 75 °C hot plate and a small pieceof low-temperature melting metal (Field’s metal – RotoMetals,Inc., San Leandro, CA) was placed at the inlet of the electrodechannel. Once the metal was melted, negative pressure wasapplied from the outlet of the electrode channel to fill theelectrode channel with metal.

The glass-PDMS-glass sandwich microchannel device wasprepared by placing a thin PDMS microchannel replicabetween two glass slides (Supplementary Fig. S1b). First,PDMS was spin-coated on top of a SU-8TM master moldhaving a 100 μm wide and 150 μm high ridge structure toa thickness of 300 μm, followed by polymerization at 60 °C

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for 120 min on top of a leveled hot plate. Second, holes fortubing connection were made on the top glass slide and the thinPDMS layer. The thin PDMS layer was then bonded betweenthe two glass slides with the assistance of oxygen plasma.After bonding, tubing was connected and additional PDMSwas poured to seal and secure the tubing interface.

B. TBAF Preparation

TBAF (Sigma-Aldrich, St. Louis, MO) was dissolved inN,N-Dimethylformamide (DMF) (Sigma-Aldrich, St. Louis,MO) at two different concentrations (0.5 M and 1.0 M) andthoroughly mixed by vortexing.

C. PDMS Etch Rate Analysis

TBAF was perfused into the PDMS microchannel at con-trolled flow rates using a syringe pump (Chemyx, Stafford,TX). Time-lapse etch profiles of the microchannel widthswere taken every 3 min at the center (20 mm from theinlet) of the 40 mm long microchannels using an uprightoptical microscope (Eclipse LV 100D, Nikon Inc., Melville,NY) equipped with a digital camera (DS-2Mv, Nikon Inc.,Melville, NY). Cross-sectional profiles were analyzed using amicroscope by slice-cutting the etched PDMS microchannelswith a razor blade.

D. TBAF Capillary Etching Characterization

Effect of initial width of microchannels on liquid-phasecapillary etching was analyzed by flowing 1.0 M of TBAFat the flow rate of 1.0 ml/h into microchannels with twodifferent dimensions (width: 50 μm and 100 μm, height:18 μm). Etching profiles of the PDMS microchannels werefurther characterized by flowing two different concentrationsof TBAF (1.0 M and 0.5 M) at various flow rates (0.3 ml/h,0.6 ml/h, and 1.0 ml/h) and measuring the change in channelsizes. Etch rate in horizontal direction (Ewidth) was analyzedevery 3 min for 30 min and the etch rate in vertical direction(Eheight) was typically analyzed after 30 min.

E. Surface Profilometry

Surface roughness of TBAF-etched PDMS microchannels atdifferent etching conditions (flow rates: 0.3-1.0 ml/h, TBAFconcentrations: 0.5-1.0 M) were analyzed with an opticalprofilometer (NT9100, Veeco, Plainview, NY) at six differentareas (500 μm2) for each condition.

F. Statistical Analysis

All data presented are mean ± standard deviation from atleast three independent experiments for each condition.

III. RESULTS AND DISCUSSIONS

A. Liquid-Phase Capillary Etching of PDMS Microchannel

Etching of PDMS microchannels with TBAF for fabri-cating microchannels with round-shaped cross-section wasinitially demonstrated by flowing TBAF through a 100 μmwide, 18 μm high, and 40 mm long PDMS microchannel

Fig. 2. Cross-sectional profile of the (a) 100 μm and (b) 50 μm widePDMS microchannels sealed against glass slides after 30 min of liquid-phasecapillary etching with 1.0 M TBAF at the flow rate of 1.0 ml/h. (c) Changesof the PDMS microchannel width during the 30 min of TBAF etching.

sealed against a glass slide. TBAF (1.0 M) perfused intothe microchannel at a flow rate of 1.0 ml/h successfullyetched the microchannel and resulted in a round-shaped cross-sectional profile after 30 min [Fig. 2(a)]. Etching of the PDMSmicrochannels on glass slides was further investigated bymeasuring the change of the microchannel widths every 3 minduring the 30 min of TBAF etching [Fig. 2(c)]. For the first6 min, microchannels were dissolved at an etch rate of 41.4 ±9.7 μm/min, however then suddenly decreased to an etch rateof 18.6 ± 2.2 μm/min. This sudden decrease in etch rateafter 6 min was consistent for multiple experiments (n = 3,Supplementary Fig. S2).

In order to investigate how the microchannel width affectsthe capillary etching, TBAF of same concentration (1.0 M) wasperfused into a microchannel with a smaller dimension (width:50 μm, height: 18 μm, length: 40 mm) at the same flow rate(1.0 ml/h). In this case, no sudden change in the etch rate wasobserved [Fig. 2(c)] and the overall etch rate over the 30 minperiod was rather constant at around 16.2 ± 4.1 μm/min,which was comparable to the etch rate of the microchannelwith 100 μm width beyond 6 min (18.6 ± 2.2 μm/min).In addition, the cross-section of the etched microchannelof the 50 μm wide channel was closer to a semi-circlecompared to that of the 100 μm wide channel (width/height =2.31 vs. 2.84) [Fig. 2(b)]. We believe that this is due tothe higher aspect ratio (50 μm width = 0.36 vs. 100 μmwidth = 0.18).

Next, microchannels (width: 100 μm, height: 18 μm,length: 40 mm) sealed against a 2 mm thick PDMS slabwas etched using the same etching parameters (1.0 M ofTBAF flown at 1.0 ml/h). The etching profile showed thatthe microchannels were etched in all directions, resulting ina microchannel with round-shaped cross-section [Fig. 3(a)],with the overall etch rate in width and in height being 19.3 ±0.2 μm/min and 14.3 ± 0.0 μm/min, respectively, after 30min of etching [Fig. 3(b)]. In order to investigate the isotropyof the liquid-phase capillary etching process, cross-sectionsof the microchannel were analyzed every 3 min by cuttingthe end of the PDMS microchannel. Ewidth was initially fasterthan Eheight, resulting in an anisotropic etching. However,Ewidth gradually decreased from 24.5 ± 4.8 μm/min to 14.8 ±2.4 μm/min, reaching a similar etch rate compared to Eheight,

BAN et al.: LIQUID-PHASE CAPILLARY ETCHING OF PDMS MICROCHANNELS 279

Fig. 3. (a) Cross-sectional profile of a PDMS microchannel sealed againsta PDMS slab and after 30 min of etching with TBAF (1.0 M, flow rate:1.0 ml/h). White arrows indicate the bonding interface between the top andbottom PDMS layer. (b) Changes of the PDMS microchannel width andheight analyzed every 3 min during TBAF etching. (c) Eheight , Ewidth, andEheight /Ewidth analyzed every 3 min. (d) Cross-sectional profile change duringthe 30 min of TBAF etching. Scale bars: 200 μm.

Fig. 4. Changes of the PDMS microchannel widths and etch rates at threedifferent flow rates with (a)–(b) 1.0 M and (c)–(d) 0.5 M TBAF perfusion.

which was maintained relatively constant (15.4 ± 2.0 μm/min)throughout the 30 min of etching [Fig. 3(c)]. For the last12 min, average Eheight/Ewidth was 1.1 ± 0.1, demonstratingisotropy-like etching property. Cross-sectional image analysisalso revealed that unlike the PDMS microchannels sealedagainst the glass slides, where the etched microchannelsresulted in semi-circle like profile, the interface of the top and

Fig. 5. (a) Cross-sectional profile of the PDMS microchannels after30 min of etching with TBAF. Scale bar: 100 μm. (b)–(c) Changes inEwidth and Eheight at different TBAF concentrations and flow rates. (d) Etchrate ratio (Ewidth/Eheight) showing that 1.0 M TBAF etching resulted inmore circular cross-sectional profile regardless of the flow rate compared to0.5 M TBAF etching. (e) Characterization of PDMS microchannel etchingwith TBAF under various etching parameters analyzed by the cross-sectionalcircumgerence.

the bottom PDMS layers had sharper edges [white arrows inFig. 3(a)]. We believe that this is due to a non-perfect bondingof the two PDMS layers that resulted in relatively faster etchrate at the bonding interface.

Further analysis of the etch profile showed that for the first6 min, the bottom PDMS layer showed a more round profilecompared to the top PDMS layer [Fig. 3(d)]. This is probablydue to the fact that the microchannel was initially patternedon the top PDMS layer. However, beyond 12 min, both thetop and the bottom PDMS layer showed symmetrically round-shaped etch profile. The height of the etched area of the topPDMS layer was always 4.5-21.9 μm higher than that of thebottom PDMS layer throughout the etching process, reflectingthe initial channel depth of 18 μm.

B. Effects of TBAF Concentration and Flow Rateon PDMS Capillary Etching

We investigated how the flow rate and the concentrationof the TBAF affect the Ewidth and the Eheight. When 1.0 Mof TBAF was perfused into the PDMS microchannels (width:100 μm, height: 18 μm, length: 40 mm) at different flowrates, Ewidth increased as the flow rate increased [Fig. 4(a)].Overall Ewidth for 30 min was 10.2 ± 0.1 μm/min for 0.3 ml/hperfusion and increased approximately 60% upon increasingthe flow rate to 0.6 ml/h. At the flow rate of 1.0 ml/h, Ewidthfurther increased to approximately two times and reached an

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Fig. 6. (a) Images showing the cross-sections of a PDMS microchannel at different locations (5 mm, 20 mm, and 35 mm from the channel inlet). Scalebars: 100 μm. (b) Changes in the etch rates in width and height at various locations along the channel.

etch rate of 19.3 ± 0.2 μm/min [Fig. 4(b)]. Eheight also showeddependency on the flow rate and was 8.5 ± 0.1 μm/minat the flow rate of 0.3 ml/h, and linearly increased (R2 =0.9900) as the flow rate increased (11.9 ± 0.1 μm/min at 0.6ml/h and 14.3 ± 0.0 μm/min at 1.0 ml/h) [Fig. 4(b)]. TBAFof 0.5 M was also perfused into the microchannels at threedifferent flow rates. As expected, Ewidth increased as the flowrate increased [Fig. 4(c)], with the etch rates being 10.2 ±0.2 μm/min, 13.4 ± 0.3 μm/min, and 17.6 ± 0.2 μm/min forthe flow rates of 0.3 ml/h, 0.6 ml/h, and 1.0 ml/h, respectively[Fig. 4(d)]. Eheight again increased linearly as the flow rateincreased (R2 = 0.9981).

Although the overall etching trend at different flow rates wassimilar (higher etch rate at higher flow rate) for both TBAFconcentrations (1.0 M and 0.5 M), significant distinction couldbe observed when comparing the differences in the etchrates as well as the etch rate ratios (Ewidth/Eheight) for thetested microchannel dimensions [Fig. 5(a)]. Ewidth was notsignificantly affected by the TBAF concentration, with averagedifference between the two different TBAF concentrations atvarious flow rates being only 9.81% [Fig. 5(b)]. However,Eheight dropped significantly with the lower TBAF concen-tration (0.5 M), and was approximately 5.8 ± 0.4 μm/minslower compared to etch rate of 1.0 M of TBAF regardlessof the flow rate [Fig. 5(c)]. Due to this differences, etchrate ratio between the horizontal and the vertical directions(Ewidth/Eheight) decreased from 3.5 to 2.0 as the flow rateincreased in the case of 0.5 M TBAF perfusion, whereas theratio in the case of 1.0 M TBAF perfusion was similar for allthree flow rates with the average Ewidth/Eheight being 1.3 ±0.1, showing close to isotropic etching [Fig. 5(d)]. To bettercharacterize this etching characteristics, the etch rates wereanalyzed by measuring the circumference of the microchannelcross-section. Etch rates were more affected by the flowrates at higher TBAF concentration whereas it was linearly

dependent on the flow rate for a particular TBAF concentration(0.5 M: R2 = 0.9922; 1.0 M: R2 = 0.9973) [Fig. 5(e)].

Finally, changes in the etch rate along the microchannelswere investigated. TBAF (1.0 M and 0.5 M) was perfused atthree different flow rates (1 ml/h, 0.6 ml/h, and 0.3 ml/h) andEwidth and Eheight were analyzed at three different locations;5 mm, 20 mm, and 35 mm away from the microchannel inlet[Fig. 6(a)]. Regardless of the etching conditions, the etch rateswere lower further downstream from the inlet, with the etchrate difference being less for 0.5 M TBAF perfusion (width:E35mm/E5mm = 0.90-0.91, height: E35mm/E5mm = 0.93-0.96)compared to 1.0 M (width: E35mm/E5mm = 0.84-0.92, height:E35mm/E5mm = 0.81-0.94) as can be seen in SupplementaryTable S1. Minimum overall etch rate difference for 0.5 MTBAF was obtained at the lowest tested flow rate (0.3 ml/h).For the 1.0 M TBAF etching, the etch rate difference decreasedwith increasing flow rate and minimum overall etch rate dif-ference was observed at the fastest tested flow rate (1.0 ml/h)[Fig. 6(b)]. We believe this relatively slower etch rate at thedownstream position of the microchannels is due to dissolvedPDMS debris in the TBAF solution that accumulate whileflowing through the channel. However, the difference wasapproximately 10% for all the tested conditions repeatedly,except for the 1.0 M TBAF perfusion at 0.3 ml/h.

Other than the conditions presented above, we have alsoetched PDMS microchannels with smaller dimensions (width:20, 50 μm) sealed against PDMS slabs with TBAF, includinglower concentration (0.25 M) at broader flow rate ranges(0.1-1.0 ml/h). However, 0.25 M was not sufficient in etchingPDMS as no noticeable difference in microchannel dimensionscould be observed even after 30 min of perfusion with TBAF.Moreover, 0.1 ml/h of flow rate resulted in intermittent clog-ging of the microchannels with the dissolved PDMS debrisregardless of the microchannel dimension (SupplementaryFig. S3). Clogging was also observed for microchannels with

BAN et al.: LIQUID-PHASE CAPILLARY ETCHING OF PDMS MICROCHANNELS 281

Fig. 7. A PDMS microdevice with embedded and exposed 3D metalelectrodes as an application of the liquid-phase capillary etching of the PDMSmicrochannel with TBAF. Scale bars: 100 μm.

smaller dimensions. Approximately 60% of 50 μm widemicrochannels were clogged with dissolved PDMS debris evenat 1.0 ml/h flow rate, significantly lacking process stability.Therefore, we believe that the minimum etch rate that can beobtained by this liquid-phase capillary etching with high con-sistency and repeatability is Ewidth = 10.18 ± 0.22 μm/min,Eheight = 2.92 ± 0.12 at room temperature.

C. Surface Property of the Etched PDMS Microchannels

Etched surface of PDMS microchannels analyzed by anoptical profilometer showed overall smooth and uniformprofile for all tested conditions (Supplementary Table S2).Surfaces were rougher when perfused with higher TBAFconcentration (1.0 M) but was still smaller than 150 nmand was not noticeably affected by the flow rate changes inthe case of 1.0 M TBAF perfusion. In contrast, the surfaceroughness showed dependency on flow rates for 0.5 M TBAFperfusion and was decreased as the flow rate was increasedwith the minimum value being 60.1 ± 17.7 nm at 1.0 ml/hperfusion.

D. Microdevice With Embedded 3D Metal Electrode

As a demonstration for potential application of the liquid-phase capillary etching, a microfluidic device with embed-ded 3D metal electrodes that were directly exposed to theflow channel was fabricated. In order to directly exposethe 3D metal electrodes filled inside the electrode channels,1.0 M TBAF was perfused into the flow channel of thedevice at the flow rate of 1.0 ml/h. After 2 min of etch-ing, the 30 μm thick PDMS walls that were separating theflow channel and the electrode channels were completelydissolved and the 3D metal electrodes were successfullyexposed to the flow channel (Fig. 7). Exposure of electrodeswas validated by measuring the electrical resistance betweentwo electrodes with the flow channel filled with phosphatebuffered saline (PBS) (246 ± 11 k�, n = 6). Microflu-idic systems requiring integrated microelectrodes that aredirectly exposed to flow stream for various bio/medical appli-cations such as miniaturized electrochemical sensors, elec-trical impedance measurement microdevices, and microchipelectroporators [36]–[39] typically use planar 2D electrodesdue to the complicated fabrication processes (e.g. electro-plating) for making 3D electrodes. This method can simplyembed 3D electrodes that provide more uniform electricalfield across the microchannel compared to 2D planar elec-trodes. We believe that this liquid-phase capillary etching

Fig. 8. (a) A thin PDMS microchannel layer sandwiched between twoglass slides before and after the liquid-phase capillary etching for fabricatinga glass-PDMS-glass sandwich microchannel. (b) Fluorescence images of aPDMS microchannel and a glass-PDMS-glass sandwich microchannel initiallyfilled with a fluorescent dye (Nile red) and rinsed thoroughly with DI water.Scale bars: 200 μm. (c) Cross-sectional fluorescent intensity profile of themicrochannels in (b), taken along the red dotted lines. Red circles indicateintensity peaks from the channel side walls.

method allowing easy fabrication of exposed 3D metal elec-trodes in microfluidic channels can be widely used for suchapplications.

E. Glass-PDMS-Glass Sandwich Microchannel forHigh-Clarity Optical Imaging and Detection

One of the biggest drawbacks of PDMS-based microfluidicdevices compared to glass devices are the adsorption orabsorption of chemicals to the PDMS microchannel surfacesthat often hinders accurate detection or measurement of tar-get materials (e.g. background noise of fluorescent signals)as well as the optical clarity. Typically, glass-PDMS-glasssandwich devices have been made by clamping a PDMSmembrane having through holes between two glass slides.Membrane with through holes are usually prepared by pressinga PDMS poured patterned substrate against a flat substrateduring polymerization [40], [41] or by etching [17]. Howeverthese methods involve complicated preparation steps and it isalso not easy to handle thin PDMS membrane with throughholes. Utilizing the liquid-phase capillary etching character-ized here, a glass-PDMS-glass sandwich microchannel was

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fabricated by simply flowing 1.0 M TBAF into the PDMSmicrochannel sandwiched between two glass slides. After30 min of etching, the upper part of the PDMS microchannelwas completely dissolved, directly exposing the channel to theglass [Fig. 8(a)]. In order to demonstrate that the glass-PDMS-glass sandwich microchannel fabricated here does minimizebackground fluorescent signal that results from fluorescentdye adsorption/absorption in PDMS, a fluorescent dye (Nile-red, Sigma-Aldrich, St. Louis, MO) was perfused into themicrochannel at 1.0 ml/h followed by thorough rinsing withdeionized (DI) water [Fig. 8(b)]. Fluorescent intensity profilemeasured after the rinsing [Fig. 8(c)] showed significantlylower background noise after the PDMS etching (i.e. PDMSmicrochannel vs. glass-PDMS-glass sandwich microchannel).The background noise measured through fluorescent intensitywas approximately 40 fold less. Although strong fluorescentbackground noise was still obvious at the microchannel side-walls [peaks indicated with red circles in Fig. 8(c)], this canalso be seen in glass microdevices and more importantlywill not affect the monitoring or detection of samples inthe main channel region. The simplicity of fabricating aglass-PDMS-glass sandwich microchannel by the liquid-phasecapillary etching method presented here will allow developingPDMS-glass hybrid microdevices for applications that canbenefit from the advantages of both PDMS devices and glassdevices.

IV. CONCLUSION

Liquid-phase capillary etching of PDMS microchannelswith TBAF by different channel sealing substrates, channelwidths, TBAF concentrations, and TBAF flow rates has beensystematically characterized. Microchannels showed similarround-shaped etching profile regardless of the sealing sub-strate, although the microchannel sealed against a PDMSslab showed slight faster etching at the bonding interface.The etch rate was linearly proportional to the flow rate ata given TBAF concentration and was faster when TBAFof higher concentration was perfused. In addition, etchedchannel width was not significantly affected by the TBAFconcentration, yet etched channel height showed noticeabledrop as TBAF concentration was decreased from 1.0 M to0.5 M. Microchannels with round-shaped cross-sections couldbe easily fabricated by the capillary etching method withoutany special equipment or complicated processes, with higherTBAF concentration at slower flow rate resulting in morecircular cross-sectional profile. Potential applications of thisetching method beyond fabricating circular-shaped microchan-nels were demonstrated by fabricating a microdevice withembedded and exposed 3D metal electrode and a glass-PDMS-glass sandwich microchannel for applications requiring highoptical clarity. We expect that the full characterization of theliquid-phase TBAF capillary etching of PDMS microchan-nels provide alternative routes for PDMS microfabricationbeyond the limitation of conventional replicate moldingmethod and be widely utilized for making PDMS-glass hybridmicrodevices.

REFERENCES

[1] C. Kothapalli, E. van Veen, S. de Valence, S. Chung,I. Zervantonakis, F. Gertler, et al., “A high-throughput microfluidicassay to study neurite response to growth factor gradients,” Lab Chip,vol. 11, no. 3, pp. 497–507, Feb. 2011.

[2] J. Park, H. Koito, J. Li, and A. Han, “Multi-compartment neuron-gliaco-culture platform for localized CNS axon-glia interaction study,” LabChip, vol. 12, no. 18, pp. 3296–3304, Sep. 2012.

[3] J. Kim, D. Taylor, N. Agrawal, H. Wang, H. Kim, A. Han, et al., “A pro-grammable microfluidic cell array for combinatorial drug screening,”Lab Chip, vol. 12, no. 10, pp. 1813–1822, Apr. 2012.

[4] W. Longsine-Parker, H. Wang, C. Koo, J. Kim, B. Kim, A. Jayaraman,et al., “Microfluidic electro-sonoporation: A multi-modal cell porationmethodology through simultaneous application of electric field andultrasonic wave,” Lab Chip, vol. 13, no. 11, pp. 2144–2152, Jun. 2013.

[5] M. T. Koesdjojo, Y. H. Tennico, and V. T. Remcho, “Fabrication ofa microfluidic systems for capillary electrophoresis using a two-stageembossing technique and solvent welding on poly(methylmethacrylate)with water as a sacrificial layer,” Anal. Chem., vol. 80, no. 7,pp. 2311–2318, Feb. 2008.

[6] Y. Cho, J. Park, H. Park, X. Cheng, B. Kim, and A. Han, “Fabricationof high-aspect-ratio polymer nanochannels using a novel Si nanoimprintmold and solvent-assisted sealing,” Microfluid. Nanofluid., vol. 9, nos.2–3, pp. 163–170, Aug. 2010.

[7] P. W. Leech, “Hot embossing of cyclic olefin copolymers,” J. Micromech.Microeng., vol. 19, no. 5, pp. 055008-1–055008-5, May 2009.

[8] J. Park, J. Li, and A. Han, “Micro-macro hybrid soft-lithographymaster (MMHSM) fabrication for lab-on-a-chip applications,” Biomed.Microdevices, vol. 12, no. 2, pp. 345–351, Apr. 2010.

[9] Z. K. Wang, H. Y. Zheng, R. Y. H. Lim, Z. F. Wang, and Y. C. Lam,“Improving surface smoothness of laser-fabricated microchannels formicrofluidic application,” J. Micromech. Microeng., vol. 21, no. 9,pp. 095008-1–095008-8, Sep. 2011.

[10] Y. N. Xia and G. M. Whitesides, “Soft lithography,” Annu. Rev. Mater.Sci., vol. 28, no. 1, pp. 153–184, 1998.

[11] K. S. Ryu, X. F. Wang, K. Shaikh, and C. Liu, “A method for precisionpatterning of silicone elastomer and its applications,” J. Microelectro-mech. Syst., vol. 13, no. 4, pp. 568–575, Aug. 2004.

[12] J. Tong, C. A. Simmons, and Y. Sun, “Precision patterning of PDMSmembranes and applications,” J. Micromech. Microeng., vol. 18, no. 3,pp. 037004-1–037004-5, Mar. 2008.

[13] J. Park, H. S. Kim, and A. Han, “Micropatterning ofpoly(dimethylsiloxane) using a photoresist lift-off technique forselective electrical insulation of microelectrode arrays,” J. Micromech.Microeng., vol. 19, no. 6, pp. 065016-1–065016-18, Jun. 2009.

[14] E. Ostuni, R. Kane, C. S. Chen, D. E. Ingber, and G. M. Whitesides,“Patterning mammalian cells using elastomeric membranes,” Langmuir,vol. 16, no. 20, pp. 7811–7819, Oct. 2000.

[15] R. J. Jackman, D. C. Duffy, O. Cherniavskaya, and G. M. Whitesides,“Using elastomeric membranes as dry resists and for dry lift-off,”Langmuir, vol. 15, no. 8, pp. 2973–2984, Apr. 1999.

[16] D. Eon, L. de Poucques, M. C. Peignon, C. Cardinaud, G. Turban,A. Tserepi, et al., “Surface modification of Si-containing polymersduring etching for bilayer lithography,” Microelectron. Eng., vols. 61–62,pp. 901–906, Jul. 2002.

[17] S. R. Oh, “Thick single-layer positive photoresist mold andpoly(dimethylsiloxane) (PDMS) dry etching for the fabrication ofa glass–PDMS–glass microfluidic device,” J. Micromech. Microeng.,vol. 18, no. 11, pp. 115025-1–115025-13, Nov. 2008.

[18] J. Garra, T. Long, J. Currie, T. Schneider, R. White, and M. Paranjape,“Dry etching of polydimethylsiloxane for microfluidic systems,” J. Vac.Sci. Technol., vol. 20, no. 3, pp. 975–982, May 2002.

[19] B. Xu, F. Arias, S. T. Brittain, X.-M. Zhao, B. Grzybowski, S. Torquato,et al., “Making negative Poisson’s ratio microstructures by soft lithog-raphy,” Adv. Mater., vol. 11, no. 14, pp. 1186–1189, Oct. 1999.

[20] B. Xu, F. Arias, and G. M. Whitesides, “Making honeycomb microcom-posites by soft lithography,” Adv. Mater., vol. 11, no. 6, pp. 492–495,Apr. 1999.

[21] A. C. Siegel, D. A. Bruzewicz, D. B. Weibel, and G. M. Whitesides,“Microsolidics: Fabrication of three-dimensional metallic microstruc-tures in poly(dimethylsiloxane),” Adv. Mater., vol. 19, no. 5,pp. 727–733, Mar. 2007.

[22] A. Plecis and Y. Chen, “Fabrication of microfluidic devices based onglass–PDMS–glass technology,” Microelectron. Eng., vol. 84, nos. 5–8,pp. 1265–1269, May/Aug. 2007.

BAN et al.: LIQUID-PHASE CAPILLARY ETCHING OF PDMS MICROCHANNELS 283

[23] B. Balakrisnan, S. Patil, and E. Smela, “Patterning PDMS using acombination of wet and dry etching,” J. Micromech. Microeng., vol. 19,no. 4, pp. 047002-1–047002-7, Apr. 2009.

[24] S. Takayama, E. Ostuni, X. P. Qian, J. C. McDonald,X. Y. Jiang, P. LeDuc, et al., “Topographical micropatterning ofpoly(dimethylsiloxane) using laminar flows of liquids in capillaries,”Adv. Mater., vol. 13, no. 8, pp. 570–574, Apr. 2001.

[25] A. Pavesi, F. Piraino, G. B. Fiore, K. M. Farino, M. Moretti, andM. Rasponi, “How to embed three-dimensional flexible electrodes inmicrofluidic devices for cell culture applications,” Lab Chip, vol. 11,no. 9, pp. 1593–1595, Mar. 2011.

[26] C. Chen, D. Hirdes, and A. Folch, “Gray-scale photolithography usingmicrofluidic photomasks,” Proc. Nat. Acad. Sci. USA., vol. 100, no. 4,pp. 1499–1504, Feb. 2003.

[27] H. B. Yu, G. Y. Zhou, C. F. Siong, S. H. Wang, and F. W. Lee, “Novelpolydimethylsiloxane (PDMS) based microchannel fabrication methodfor lab-on-a-chip application,” Sens. Actuators B, Chem., vol. 137, no. 2,pp. 754–761, Apr. 2009.

[28] M. A. Unger, H.-P. Chou, T. Thorsen, A. Scherer, and S. R. Quake,“Monolithic microfabricated valves and pumps by multilayer soft litho-graphy,” Science, vol. 288, no. 5463, pp. 113–116, Apr. 2000.

[29] P. M. Gerhart, R. J. Gross, and J. I. Hochstein, Fundamentals of FluidMechanics. Reading, MA, USA: Addison-Wesley, 1985.

[30] J. P. Camp, T. Stokol, and M. L. Shuler, “Fabrication of a multiple-diameter branched network of microvascular channels with semi-circularcross-sections using xenon difluoride etching,” Biomed. Microdev.,vol. 10, no. 2, pp. 179–186, Apr. 2008.

[31] G.-J. Wang, C.-C. Hsueh, S.-H. Hsu, and H.-S. Hung, “Fabricationof PLGA microvessel scaffolds with circular microchannels using softlithography,” J. Micromech. Microeng., vol. 17, no. 10, pp. 2000–2005,Oct. 2007.

[32] L. K. Fiddes, N. Raz, S. Srigunapalan, E. Tumarkan, C. A. Simmons,A. R. Wheeler, et al., “A circular cross-section PDMS microfluidicssystem for replication of cardiovascular flow conditions,” Biomaterials,vol. 31, no. 13, pp. 3459–3464, May 2010.

[33] J. T. Borenstein, M. M. Tupper, P. J. Mack, E. J. Weinberg, A. S. Khalil,J. Hsiao, et al., “Functional endothelialized microvascular networks withcircular cross-sections in a tissue culture substrate,” Biomed. Microdev.,vol. 12, no. 1, pp. 71–79, Feb. 2010.

[34] D. Lee and Y. Cho, “High-radix microfluidic multiplexer with pressurevalves of different thresholds,” Lab Chip, vol. 9, no. 12, pp. 1681–1686,Jun. 2009.

[35] L. Yobas, S. Martens, W.-L. Ong, and N. Ranganathan, “High-performance flow-focusing geometry for spontaneous generation ofmonodispersed droplets,” Lab Chip, vol. 6, no. 8, pp. 1073–1079,Aug. 2006.

[36] K.-H. Han, A. Han, and A. B. Frazier, “Microsystems for isolationand electrophysiological analysis of breast cancer cells from blood,”Biosensors Bioelectron., vol. 21, no. 10, pp. 1907–1914, Apr. 2006.

[37] M. Khine, C. Ionescu-Zanetti, A. Blatz, L.-P. Wang, and L. P. Lee,“Single-cell electroporation arrays with real-time monitoring and feed-back control,” Lab Chip, vol. 7, no. 4, pp. 457–462, Feb. 2007.

[38] K.-S. Huang, Y.-C. Lin, C.-C. Su, and C.-S. Fang, “Enhancement ofan electroporation system for gene delivery using electrophoresis witha planar electrode,” Lab Chip, vol. 7, no. 1, pp. 86–92, Feb. 2007.

[39] C.-C. Wu, T. Saito, T. Yasukawa, H. Shiku, H. Abe, H. Hoshi, et al.,“Microfluidic chip integrated with amperometric detector array for insitu estimating oxygen consumption characteristics of single bovineembryos,” Sens. Actuators B, Chem., vol. 125, no. 2, pp. 680–687,Aug. 2007.

[40] C. Carlborg, T. Haraldsson, M. Cornaglia, G. Stemme, andW. van der Wijngaart, “A high-yield process for 3-D large-scale inte-grated microfluidic networks in PDMS,” J. Microelectromech. Syst.,vol. 19, no. 5, pp. 1050–1057, Oct. 2010.

[41] C.-H. Hsu, C. Chen, and A. Folch, “‘Microcanals’ for micropipetteaccess to single cells in microfluidic environments,” Lab Chip, vol. 4,no. 5, pp. 420–424, Apr. 2004.

Changwoo Ban received the B.S. and M.S. degreesin information & industrial engineering in 2002 and2004, respectively, and the Ph.D. degree in Nano &IT Engineering from Seoul National University ofScience and Technology, Seoul, Korea, in 2012.

He is currently a Postdoc researcher in the Elec-trical and Computer Engineering Department atTexas A&M University, College Station, TX, USA.His current research interests include reliability ofMEMS structures, micro system packaging, andapplication of nanofluidic devices.

Jaewon Park (S’09–M’13) received the B.S. degreein electrical engineering from Korea University,Seoul, Korea, in 2004, and the Ph.D. degree inelectrical engineering from Texas A&M University,College Station, TX, USA, in 2011.

He is currently a Postdoctoral Research Associateat Texas A&M University. His current researchinterests include the development and application oforgan-on-a-chip microdevices, and high- throughputmicrobial mutant screening platforms.

Dong-Young Jang received the B.S. degree innuclear engineering, and the M.S. degree in mechan-ical and design engineering from Seoul NationalUniversity, Seoul, Korea, in 1979 and 1981, respec-tively, and the Ph.D. in mechanical engineering fromthe University of Florida, Gainesville, FL, USA, in1990.

From 1990 to 1997, he worked in the MechanicalEngineering Department at Missouri State Univer-sity, Springfield, MO, USA, as an Assistant Profes-sor. He is currently a Professor at Seoul National

University of Science and Technology, Seoul, Korea, and President of SeoulTechnopark, Seoul. His current research interests include design, nanotech-nology, and packaging.

Arum Han (S’98–M’05–SM’13) received the B.S.degree from Seoul National University, Seoul,Korea, in 1997; the M.S. degree from the Univer-sity of Cincinnati, Cincinnati, OH, USA, in 2000;and the Ph.D. degree from the Georgia Institute ofTechnology, Atlanta, GA, USA, in 2005.

In 2005, he joined the Department of Electricaland Computer Engineering, Texas A&M University,College Station, TX, USA, as an Assistant Professor,where he has also been holding a joint facultyposition in the Biomedical Engineering Department

since 2006. He became an Associate Professor, in 2011. His research interestis in solving grand challenge problems in the broad area of health and energythrough the use of micro/nano systems technology.