IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 19 (2009) 045024 (9pp) doi:10.1088/0960-1317/19/4/045024

Photodefinable PDMS thin films formicrofabrication applications

Preetha Jothimuthu1, Andrew Carroll1, Ali Asgar S Bhagat1, Gui Lin2,James E Mark2 and Ian Papautsky1,3

1 Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, OH 45221,USA2 Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221, USA

E-mail: ipapauts@ececs.uc.edu

Received 18 December 2008, in final form 25 February 2009Published 26 March 2009Online at stacks.iop.org/JMM/19/045024

Abstract

In this work, direct patterning of polydimethylsiloxane (PDMS) is demonstrated by theaddition of a UV-sensitive photoinitiator benzophenone. As an improvement to our previouswork, patterns with both positive and negative features have been fabricated on the samesubstrate. Infrared spectroscopy was used to investigate photocrosslinking behavior andreaction chemistry of this new photodefinable PDMS (photoPDMS) material. Severalapplications of the photoPDMS process have been successfully demonstrated. Multi-layerstructures and multi-level microfluidic chips can be easily fabricated using thisphotopatterning process. Patterned PDMS thin films can also be removed from the underlyingsubstrates and used as shadow masks for defining patterns on both planar and non-planarsurfaces. The photopatternable PDMS was also found to be biocompatible once un-reactedbenzophenone is extracted from the cured film. Overall, photoPDMS offers a number ofcritical advantages over conventional PDMS processing, including elimination of mastertemplate fabrication, ability to process under ambient light processing conditions,positive-acting tone, low cost, and rapid and easy fabrication.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Poly(dimethylsiloxane) (PDMS) is one of the most popularsilicone elastomers used in the fabrication of microfluidicdevices in numerous lab-on-a-chip (LOC) applications [1, 2].It offers many advantages such as good flexibility, temperaturestability from −50 ◦C to +200 ◦C, chemical inertness, low costand simple fabrication. In addition, PDMS surface propertiescan be easily modified for specific applications by adsorptionof proteins or plasma processing. PDMS also has favorableoptical properties including transparency above ∼230 nm andvery low autofluorescence over a wide range of wavelengthscompared to other plastic chip materials [3]. Furthermore,PDMS is permeable to gasses, impermeable to water and non-toxic to cells, making it suitable for a variety of biological andmicrofluidic applications [1].

3 Author to whom any correspondence should be addressed.

A number of microfabrication techniques for makingPDMS devices exist; these processes have been extensivelyreviewed by Sia and Whitesides [4] and used by numerousinvestigators for fabricating PDMS-based devices. Typically,PDMS structures are obtained as negative replicas of a mastertemplate fabricated using conventional micromachiningmethods [1, 4]. To form PDMS replicas, uncrosslinkedprepolymer is mixed in a 10:1 ratio with curing agent andthermally cured at 80 ◦C for ∼2 h on the master template.Although the process is robust and a large number of PDMSreplicas can be fabricated from a single master template, thereare limitations associated with the fabrication of a master (suchas need for clean room facilities and photoresist processingequipment). This is problematic when the device is inthe early development stage, as master fabrication can betime consuming and expensive, especially when only a fewprototype devices are needed or a large number of designiterations are expected. Rapid and low-cost prototyping

0960-1317/09/045024+09$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 19 (2009) 045024 P Jothimuthu et al

(a)

(b)

(c)

(d )

(e)

(f )

Figure 1. Schematic diagram of photoPDMS fabrication. (a) Spin coat photoPDMS mixture and UV expose, (b) cure PDMS at 120 ◦C,(c) develop in toluene. Following development, photoPDMS film can be used to fabricate (d ) double-layer microfluidic chips, (e) dual-levelstructures or ( f ) freestanding shadow masks.

is therefore important in developing microsystems, andthus there is a continued interest in alternative, simplermicrofabrication methods.

One alternative rapid prototyping approach is to directlypattern PDMS by making it sensitive to UV. Lotters et al [5]were the first to successfully demonstrate the patterning ofPDMS by addition of 2,2-dimethoxy 2-phenylacetophenone(DMAP) photoinitiator. The technique, however, requiredspecial processing conditions to address the oxygen andambient light sensitivities of the photodefinable PDMSmixture. Nevertheless, Almasri et al [6] used thisphotodefinable PDMS formulation to fabricate a tunableinfrared filter based on PDMS springs. Dow Corningalso recently introduced photopatternable silicones (WL-5000series) for the electronic packaging industry [7]. The productis similar to a conventional negative photoresist in termsof processing and high costs. This material was used byHarkness et al [8] for microelectronic packaging applications,and most recently by Desai et al [9] for the fabrication ofa dielectrophoresis-based device for cell patterning. In otherrecent work, Tsougeni et al [10] demonstrated photopatterningof several types of siloxane copolymers with vinyl–methylsiloxane groups as polymerizable units by crosslinking withthree photoinitiators (4,4′-bis(diethylamino)benzophenone,99+%, thioxathen-9-one, 98%, and Igracure 651). Huck et al[11] described a method for fabricating buckles by patterninggold over a PDMS substrate soaked in benzophenone, whichwas observed to become stiffer upon UV irradiation. Wanget al [12] demonstrated a photografting surface modificationmethod in which polyacrylic acid (PAA) in contact with abenzophenone-treated PDMS layer was patterned by selectiveexposure to UV.

In our work toward a rapid prototyping process for PDMS-based devices, we recently introduced a simple and low-cost method for patterning PDMS directly by the addition ofbenzophenone [13]. Benzophenone is a photosensitizer oftenused to initiate free-radical polymerization by UV light, and

a number of investigators have reported its use with siloxanes[14, 15]. We successfully demonstrated fabrication of featureson the order of 100 μm using benzophenone concentrationof 3% (w) for a standard PDMS base to curing agent ratioof 10:1 [13]. Prototyping devices using this photodefinablePDMS (photoPDMS) offer the advantages of a conventionalPDMS elastomer, yet simplifies fabrication by eliminating theneed for a master. The fabrication process is also low costand insensitive to ambient light. By using transparency masksand a portable UV light source, devices can be prototypedultra rapidly in any lab, eliminating the need for clean roomprocessing.

In this paper, we describe improvements to thephotoPDMS process, including enhanced feature definitionand demonstration of negative freestanding patterns, aswell as increased control over feature dimensions. Whilepreviously we only speculated on the crosslinking behaviorof the photoPDMS material, herein we report on infraredspectroscopy measurements that permit us to elucidate thisbehavior. To demonstrate the versatility of the photoPDMSfabrication process, applications that normally involvecomplex microfabrication are shown with fewer and simplerprocessing steps, including a multi-level microfluidic chip andmulti-layer thin films. Another demonstrated application is thefabrication of thin film shadow masks for patterning on bothplanar and non-planar surfaces. In addition, we demonstratedbiocompatibility of the photoPDMS film using cell culture.With these improvements, the photoPDMS process is expectedto enable rapid prototyping of low-cost PDMS devices withoutclean room facilities, envisaging its numerous applications inmicrofluidics and MEMS fabrication.

2. Experimental methods

The process steps for fabricating photoPDMS devices areschematically illustrated in figure 1. Benzophenone (�99%)was purchased from Sigma Aldrich as white crystalline flakes.

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J. Micromech. Microeng. 19 (2009) 045024 P Jothimuthu et al

It is sensitive to UV in the range of 200 to 400 nm [16], andthus can be processed in ambient light. Conventional PDMSwas mixed 10:1 ratio of base to curing agent from commercialRTV615 (GE) or Sylgard 184 (Dow Corning) kits. To preparethe photoPDMS, benzophenone was dissolved in xylene in a3:5 ratio and added to the conventional PDMS mixture to yielda concentration of 3% (w). A degassing step was performed toremove air bubbles trapped during mixing. The photoPDMSmixture was spin coated at 2000 rpm to obtain a 20 μm thickfilm on a glass wafer. The thickness of the photoPDMS layercan be controlled by varying the spin speed to achieve a layerthickness ranging from 10 μm to 80 μm [13]. The spin-coatedlayer was selectively exposed to UV radiation at wavelengths<365 nm with an exposure energy of 12 mW cm−2 througha chrome mask for 10 min. Either a conventional aligner(without an I-line filter) or a portable UV lamp can be used forexposures. A proximity exposure at a distance of ∼80 μm wasused. This was followed by a soft bake step in a convectionoven at 120 ◦C for approximately 50 s for a 20 μm thick film,and a 3–5 s development in toluene or 30 s in methyl isobutylketone (MIBK) to wash off the exposed regions.

Figure 1 also demonstrates how the process can berepeated to form dual-layer or dual-level devices. In dual-layer fabrication (figure 1(d )), a single layer of microchannelsis initially patterned on a glass substrate by the aforementionedprocess. The process is repeated on top of a PDMS slab to forma second layer, which is then bonded to the first layer by plasmabonding. Enclosing the upper level microchannels withanother PDMS (or glass) substrate results in a completed dual-layer chip. Additional layers can be achieved by repeatingthe process. Dual-level structures can be fabricated by spincoating a second layer directly on the previously patternedfirst layer, as shown in figure 1(e). Patterned thin films (singleor multi-level) can also be peeled off the glass substrate byimmersion in toluene for ∼2 min (figure 1( f )) and can be usedfor shadow masking applications.

Fourier transform infrared spectroscopy (FTIR) measure-ments were performed to investigate interaction among thethree components of photoPDMS, namely the PDMS base,the curing agent and benzophenone. The measurements wererecorded in the absorption intensity mode in the mid-IR range400–4000 cm−1 at a resolution of 4 cm−1 and 16 scans persample. The characterized mixtures included (a) base andbenzophenone; (b) crosslinker and benzophenone; (c) base,crosslinker and benzophenone. FTIR measurements of eachmixture were performed in the liquid phase made immediatelybefore and after UV exposure. The collected spectra werecorrected for atmospheric water and carbon dioxide from thereference scan using standard methods [20].

Biocompatibility of photoPDMS was evaluated byculturing NIH 3T3 fibroblasts and examining theirproliferation. Three PDMS formulations, two samples each,were prepared and cured in a standard six-well culture plate.One formulation was the photoPDMS material prepared asdescribed above. Two additional samples of photoPDMS wereprepared using the extraction procedure described by Lee et al[17] which swells PDMS in a strong solvent to remove anyun-reacted polymer components. The RTV615 PDMS was

used as control surfaces. NIH 3T3 fibroblasts were plateddirectly on the six PDMS substrates (∼104 cells per 2 mL ofserum per well). Prior to seeding cells, PDMS was incubatedwith IMDM (10% serum) at 37 ◦C for 4 days [18]. Two daysafter plating the cells, cell viability on each of the substrateswas examined using MTT (3-(4,5-dimethylthianzol-2-yl)-2,5-diphenyltetrazolium bromide) assay [19], which stains livecells and is used to measure their relative quantity. Absorbancemeasurements were made with a spectrophotometer at 545 nmand used to calculate relative cell viability of photoPDMSsubstrates by normalizing to the absorbance of the commercialPDMS.

3. Results and discussion

3.1. PhotoPDMS fabrication

A variety of features fabricated with photodefinable PDMS areillustrated in figure 2. Feature sizes ranging from 100 μm to2 mm were successfully fabricated. Figure 2(a) shows an SEMimage of 250 μm wide patterns with 250 μm line spacingin a 20 μm thick film. Figure 2(b) demonstrates large areapatterning; the line width of these features is 400 μm witha height of 20 μm. PhotoPDMS is only sensitive to light<365 nm due to the absorption spectrum of benzophenonethat peaks at 260 nm, with a tail at 365 nm [16]. Thus,the photoPDMS is processed under the normal ambient lightconditions in a conventional laboratory. Indeed, dust particlesseen on the surface of the device in figure 2 are due toprocessing outside the clean room. Unlike the traditionalfabrication method in which PDMS channels are the negativereplicas of a master, the PDMS features of figure 2 arethe direct replications of the mask pattern. These positivefeatures clearly demonstrate the robustness and feasibility ofthe photoPDMS process.

In addition, freestanding features (via exposure througha dark-field mask) have been demonstrated in this work byusing a MIBK developer and modifying the soft bake process.Figure 2(c) shows 250 μm wide line features with 250 μmspacing in a 20 μm thick film. Large area patterning is shownin figure 2(d ). In our previous work [13], freestanding featureswere washed off the glass substrate during the developmentprocess. However, by curing the wafer for longer times(∼80 s) these features could be retained, though theirdimensions were larger than that of the mask.

The difficulty in achieving freestanding features is mostlikely due to two factors. First, poor adhesion between thePDMS and glass substrate prevents small patterns from stayingon the glass after being submerged in the developer. Also,toluene quickly swells PDMS and may cause features to peeloff the substrate after expanding. In order to correct thesecomplications, an intermediate layer of cured PDMS and amilder developer can be used. Curing a thin layer (20–80 μm) of PDMS on top of the glass substrate before thenormal patterning process provides a strong bond between thesubstrate and the photoPDMS layer. The use of MIBK, amilder solvent than toluene, prevents the sudden expansion offeatures due to swelling. Together, these adjustments allowfor consistent and well-defined freestanding features.

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J. Micromech. Microeng. 19 (2009) 045024 P Jothimuthu et al

Figure 2. SEM images of (a) conventional and (c) freestanding photoPDMS features 250 μm wide at 250 μm spacing, and (b) conventionaland (d ) freestanding large area photoPDMS patterns with feature sizes varying from 200 μm to 2 mm.

Figure 3. Sidewall profile of a single-layer microchannel patternedon a glass substrate.

Two difficulties in using the photoPDMS process exist,namely sloped sidewalls and sensitivity of feature dimensionsand definition to curing time. The definition of the sidewallsshows a slope of approximately 40◦–60◦. This can be seenclearly in the cross-sectional microscope images of a positiveline feature in figure 3. The slope can be attributed to thecombination of several factors. One factor is the higherthermal conductivity of the substrate (1.13 W mK−1 for glassversus 0.17 W mK−1 for PDMS), which causes photoPDMS topreferentially cure from the bottom by conduction rather thanconvection inside a convection oven. Also, it was observed thatthe sidewall angle and the feature widening were influencedby the duration of curing and the material substrate. Anotherfactor that attributes to the sidewall angle is likely the diffusionof the photoinitiator radicals in the exposed regions during thelong exposure time of 10 min [21]. Nevertheless, the observedsidewall angles are comparable to those reported by Desai et al[9] for the Dow Corning product and are acceptable for manypackaging and microfluidic applications.

Dimensions of features created with the photoPDMSprocess can vary significantly from those of the mask. Themost influential factor for this behavior was found to be thesoft bake processing step, where under-curing causes patternwidening and over-curing reduces feature dimensions. Dueto the short (50 s) duration of the curing process, this featurevariation is difficult to control. Thus, to achieve a better controlover the photoPDMS soft bake step, several insulating layersbetween the glass wafer and the metal base of the oven wereused to reduce film curing by conduction. Using a stack ofthree 1 mm thick plastic (e.g., cyclo olefin copolymer) wafersas a spacer between the glass substrate and the conventionoven rack, or using an 80 μm thick cured intermediate layerof PDMS as insulation, the curing time was increased toapproximately 6 min for a 20 μm film. This additional timepermits the photoPDMS film to be cured accurately, givingmore consistent feature dimensions.

3.2. PhotoPDMS chemistry

In traditional PDMS fabrication, structures are formedas negative replicas by curing a two-component siliconeelastomer mixture (base monomer and curing agent) over amaster template. In terms of chemical structure, the base pre-polymer is composed of ∼60 repeating units of −OSi(CH3)2−terminating with a vinyl−CH=CH2 group (Sylgard 184, DowCorning). The curing agent is similar, but is much smaller withonly about ten repeating units and has periodic silicon hydride−OSiHCH3− units. During the curing step, the hydrosilationof olefins takes place, i.e. the base and the curing agentcrosslink forming −Si−CH2−CH2−Si− complexes. This isillustrated in equation (1). The resulting structure is insolubleand is completely cured [1, 2, 22–24]:

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J. Micromech. Microeng. 19 (2009) 045024 P Jothimuthu et al

n = ~10m = ~60

S i O

C H 3

C H 3

C H S iCH 2

C H 3

C H 3

O S i

C H 3

C H 3

C H C H 2

m+ S i O

C H 3

CH 3

C H 3

S i

H

O

C H 3

S i

C H 3

C H 3

O

C H 3

C H 3

C H 3

n

Pt

catalyst

m

n

S i

O

C H 3

CH 3

CH 3

S iC H 2

O

C H 3

S iCH 3 CH 3

O

S iCH 3 C H 3

CH 3

S i

O

CH 3

C H 3

CH 3

S i C H 2

O

CH 3

S i CH 3CH 3

O

S i CH 3CH 3

C H 3

S i O

C H 3

CH 3

C H 2 S i

CH 3

CH 3

O S i

C H 3

CH 3

CH 2

n

(1)

The crosslinking mechanism of the photoPDMS mixture,however, is different. Although there are severalpossible reaction mechanisms that could account for thebenzophenone’s inhibition of crosslinking, the most likelymechanism is based on the reduction of carbonyl groups byhydrosilanes. When benzophenone (also known as diphenylketone) is mixed with PDMS and irradiated using UV<365 nm, a benzophenone radical is formed (equation (2)):

O O

h

n

RH C

OH

+ R.

*

(2)

The benzophenone radicals react with the silicon hydridegroups present in the PDMS crosslinker (equation (3))[25–29]. This hydrogen abstraction of benzophenone preventsthe crosslinker from undergoing traditional organometalliccrosslinking with the PDMS oligomer and creates a crosslinkerradical. The radical can then undergo either disproportionationto form a stable soluble complex with a Si=C bond or radicalcoupling to form with a Si−Si bond:

OSi

CH3

CH3

CH3

Si O

H

CH3

Si

CH3

CH3

O Si CH3

CH3

CH3

n

C

OH

+

Si O

CH3

CH3

CH3

Si

CH3

O Si

CH3

CH3

O Si

CH3

CH3

CH3

n

CH

OH

+

(3a)

Si O

CH3

CH3

CH3

Si

CH3

O Si

CH3

CH3

O Si

CH3

CH3

CH3

n

C

O

+

n

Si O

CH3

CH3

CH3

Si

CH3

O Si

CH3

CH3

O Si

CH3

CH3

CH3

O

C(3b)

Alternatively, benzophenone radicals react with the basemonomer forming short complexes (equation (4)), preventingthem from crosslinking with the curing agent. This can beobserved from the post-exposure bake where the unexposedPDMS is cured and crosslinked, while the exposed PDMS iswashed away in toluene:

C

OH

+ CH CH2SiO

CH3

CH3

Si

CH3

CH3

OSi

CH3

CH3

CHCH2

n

n

Si O

CH3

CH

CH3

Si

CH3

CH3

O Si

CH3

CH3

CH2

C

HO

CH

CH2

(4)

FTIR analysis was performed to confirm the proposed reactionmechanism. Table 1 summarizes the expected and measuredpositions of IR absorption bands of the chemical groups fromthe samples taken before and after UV exposure. Figure 4(a)compares the FTIR spectra for the sample containing base

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Table 1. Expected and measured characteristic frequencies of the functional groups in photoPDMS.

Functional Chemical Expected peak Measuredgroup bond (cm−1) [23] peak (cm−1)

PDMS base monomer −Si−CH3 Si−C 1260 1260800 800

−Si−O−Si Si−O 1130–1000 1011−CH=CH2 C=C 1600 1600−CH=CH2 C−H 960 960

Crosslinker −Si−O−Si Si−O 1130–1000 1011−Si−H Si−H 2280–2080 2169−Si−CH3 Si−C 1260 1260

Benzophenone −C=O C=O 1750–1680 1664

Figure 4. FTIR spectra of the photoPDMS mixture illustrating(a) the reaction between benzophenone and PDMS monomer and(b) the reaction between benzophenone and crosslinker taken before(solid red) and after (dotted blue) UV exposure. Insets showclose-ups of the relevant regions.

and benzophenone, which indicates that the carbonyl group(C=O) peak at 1664 cm−1 reduced in intensity after UV

exposure, indicating the formation of benzophenone radicals(equation (2)). The vinyl C=C peak at 1600 cm−1 thatcorresponds to the CH=CH2 stretch decreased in intensity,while the other vinyl functionality at 960 cm−1 correspondingto the C−H deformation remained unchanged, indicatingreaction between benzophenone radicals and the PDMS base(equation (4)).

The FTIR spectra of the samples containing thecrosslinker and benzophenone (figure 4(b)) also exhibited adecrease in the C=O carbonyl group peak at 1664 cm−1,indicating benzophenone radical formation. In addition, theSi−H silicon hydride group peak at 2169 cm−1 also decreased,confirming that hydrosilanes reduce carbonyl groups by actingas hydrogen donors. This confirms interaction between thebenzophenone radicals and the crosslinker (equation (3)).The proposed possible reactions for the interaction ofbenzophenone and the crosslinker involve the silicon hydridegroup donating hydrogen to the benzophenone in its tripletstate which forms benzopinacol. However, the peak thatdenotes the hydroxyl group (OH) from the benzopinacolformation in the region 3400–3500 cm−1 was not observed.The crosslinker unit which donated hydrogen can undergo twoformations, namely disproportionation and radical couplingwhere it forms either Si=C or Si–Si bonds, respectively. In thiscase, however, the Si=C peak that occurs at ∼1000 cm−1 maybe masked by the broad band of Si−O−Si, and the changes thatmay have occurred for the Si=C bond could not be observed.

The FTIR spectra for the three-component sample mixtureof base, crosslinker and benzophenone are shown in figure 5and support the proposed reactions. From the analysis,it can be concluded that benzophenone free radicals formduring the UV irradiation of the photoPDMS mixture andreact with both the vinyl groups of the base and the siliconhydride groups of the crosslinker. Thus, benzophenoneprevents the traditional polymerization of the base and thecrosslinker through hydrosilation of olefins and instead formsa weakly crosslinked region which gets washed off duringthe development stage. The unexposed regions, wherebenzophenone did not form radicals, undergo normal curing,making the photoPDMS mixture behave as a positive toneresist.

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Figure 5. FTIR spectra of the photoPDMS mixture showing thereaction between benzophenone, PDMS monomer and crosslinkertaken before (solid red) and after (dotted blue) UV exposure. Insetsillustrate close-ups of the relevant regions.

3.3. Multi-layer and multi-level structures

To demonstrate feasibility and advantages of the photoPDMSprocess for microfluidic devices, multi-layer channels werefabricated (figure 6(a)). The double-layer microchanneldevice was fabricated in separate layers and then sandwichedtogether by plasma bonding. The first layer was patterned ona glass wafer and the second layer was patterned on a thinPDMS slab. This proves to be a much simpler fabricationmethod with fewer processing steps when compared with theconventional PDMS processing, which requires the fabricationof two separate masters [30]. The cost was therefore lowered,and the method could be employed for rapid prototyping ofbiosensor microchips. Figure 6(b) shows a cross-sectionalview of one of the channels. Both channels were ∼1 mmin width and ∼70 μm in height. Characterization of thisdouble-layered microchannel device was performed byflowing different colored dyes on each of the two levels. Theoverlapping reservoirs show blending of colors of the two dyes,demonstrating excellent channel definition as depicted in thephotograph.

Dual-level devices have also been created using thisprocess. These structures were made by first patterning aninitial photoPDMS layer using the normal processing steps.

Figure 6. (a) Photograph of a dual-layer microfluidic chip. Colordyes were added for visualization. (b) Cross-section of thedual-layer microchannel. (c) Photograph and (d ) cross-section of adual-level photoPDMS structure.

(a)

(c)

(b)

Figure 7. (a) Planar patterning of UV adhesive bumps; (b) planarand (c) non-planar patterning of gold with a photoPDMS shadowmask.

After curing and development, a second photoPDMS layerwas spin coated on top of the first, and a larger feature wasaligned with the previous pattern. Figure 6(c) illustratesa dual-level pattern made by overlapping square features,which may be used in microscale cell culture applications.Figure 6(d ) shows the cross-sectional view of these structures,with the width of the larger square being 1.5 mm and thesmaller square being 700 μm. The film thickness of eachlayer was ∼20 μm.

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Figure 8. Microscope images of cell culture on (a) a conventional PDMS control surface, (b) an extracted photoPDMS surface and (c) aphotoPDMS surface. (d ) Absorbance measurement and cell viability for the three surfaces.

3.4. Thin-film applications

Patterned freestanding thin films for shadow maskingapplications were also fabricated using the photoPDMSprocess. To fabricate thin films, RTV615 formulation ofPDMS was used since it is more elastic than Sylgard 184.Following patterning, the films were peeled from the substrateby immersing in toluene for 2 min. The resulting flexible thinfilms were used as shadow masks for patterning both planarand non-planar surfaces.

Figure 7(a) illustrates a pattern of UV adhesive bumpsmade using a thin film photoPDMS shadow mask ∼20 μmthick, with through square pattern features ranging from500 μm to 2 mm in size. A photoPDMS thin film wasalso used as a shadow mask for patterning gold on a planar(figure 8(b)) and a non-planar (figure 8(c)) surface. Thepatterned gold squares had slightly rounded edges (figure 8(b),inset) and approximately 5% reduction in the pattern size ofthe 1.2 mm features was shown. This is most likely dueto poor contact between the shadow mask and the substrateduring the deposition process. Patterning on planar surfacesis traditionally done by metal masking or by other well-established methods such as physical vapor deposition of themetal layer followed by patterning by photolithography andchemical etching techniques. However, patterning on a non-planar surface is not trivial and requires special processingsuch as stereolithography, which is complex, time consumingand requires specialized equipment. Using this simple process

of directly patterning PDMS, the above-mentioned limitationscan be easily overcome. Also, this method can be potentiallyused for patterning proteins and cells on a non-planar 3Dstructure.

3.5. PhotoPDMS biocompatibility

PDMS is commonly used in a wide variety of biologicalapplications. It is thus important for an alternativematerial to be tested for biocompatibility before it canbe considered a reasonable substitute. In this work,biocompatibility of photoPDMS was assessed by culturingNIH 3T3 fibroblast cells on the photoPDMS and comparingwith the conventional PDMS substrates used as controls. Inaddition, two photoPDMS samples were prepared using theextraction process to remove any un-reacted crosslinker andphotoinitiator.

Figure 8 illustrates bright field microscope images of thethree types of substrates following cell culture. It was foundthat cells proliferated readily on both the conventional PDMS(control) and the extracted photoPDMS surfaces. However,little to no cells grew on the plain photoPDMS substrate.Cell viability was calculated to be 76.1% for the extractedphotoPDMS and ∼0% for the plain photoPDMS (figure 8(d )).This result suggests that benzophenone, even in very smallconcentrations, is toxic to cells. Nevertheless, using the simpleextraction procedure following device fabrication, cytotoxicity

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due to benzophenone can be addressed, allowing photoPDMSto be used for biological applications.

4. Conclusions

Photopatterning of PDMS using benzophenonephotosensitizer by UV irradiation was demonstrated inthis work. Thin films 20–80 μm in thickness with featuresranging from 100 μm to a few millimeters in size weresuccessfully demonstrated. This innovative photosensitivepolymer material can be used for rapid prototyping ofmulti-layered microfluidic chips, thus introducing a simplifiedfabrication process for complex microstructures. Theapplication of thin films as shadow masks for patterningmetal on planar and non-planar surfaces was also successfullydemonstrated. FTIR spectroscopy was used to elucidatethe photocrosslinking behavior of photoPDMS, and bio-compatibility of photoPDMS was confirmed by cell culture,although an extraction process is needed to remove anyun-reacted benzophenone. Overall, the photoPDMS processeliminates the need for a master, permits processing underambient light conditions and is expected to enable rapidprototyping of low-cost devices without clean room facilities,envisaging its numerous applications in microfluidics andMEMS fabrication.

Acknowledgments

The authors would like to thank Vidhyasagar Jayaseelan forhelp with FTIR analysis and Girish Kumar for help with cellcultures and biocompatibility testing (Department of Chemicaland Materials Engineering, University of Cincinnati). Thiswork was supported by the National Science Foundation (BES-0428600 and DMR-0314760) and the University of CincinnatiInstitute for Nanoscale Science and Technology.

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