Functionalized glass coating for PDMS microfluidic devices Adam R. Abate, Daeyeon Lee, Christian Holtze, Amber Krummel, Thao Do, and David A. Weitz* Microfluidic devices can perform multiple laboratory functions on a single, compact, and fully integrated chip. However, fabrication of microfluidic devices is difficult, and current methods, such as glass-etching or soft-lithography in PDMS, are either expensive or yield devices with poor chemical robustness. We introduce a simple method that combines the simple fabrication of PDMS with superior robustness and control of glass. We coat PDMS channels with a functionalized glass layer. The glass coating greatly increases the chemical robustness of the PDMS devices. As a demonstration, we produce emulsions in coated channels using organic solvents. The glass coating also enables surface properties to be spatially controlled. As a demonstration of this control, we spatially pattern the wettability of coated PDMS channels and use the devices to produce double emulsions with fluorocarbon oil.

Microfluidic devices consist of networks of micron scale channels that are engineered to perform specific functions. Miniaturization of the channels allows several functions to be integrated onto a single “lab-on-a-chip” microfluidic device(Whitesides 2006). This allows the devices to perform very sophisticated tasks, such as sorting analytes(Ahn et al. 2006; Fidalgo et al. 2008), cells(MacDonald et al. 2003), and worms(Chung et al. 2008), performing combinatorial chemistry(Pregibon et al. 2007), crystallizing proteins(Gerdts et al. 2006), detecting minute concentrations of DNA with ultra sensitive PCR(Cady et al. 2005; Beer et al. 2007), using bubbles for fluidic computing(Prakash and Gershenfeld 2007), as well as a host of other applications(Whitesides 2006).

One class of microfluidics that is particularly useful for analysis of chemical and biological systems is droplet microfluidics. With microfluidics, picoliter drops can be formed, merged, and sorted at kilohertz rates(Pipper et al. 2007; Shah et al. 2008; Teh et al. 2008). The drops can serve as individual compartments for chemical reactions(Teh et al. 2008). This combination of speed and containment is very useful for high-throughput screening(Warrick et al. 2007; Guo et al. 2008), useful for the as the discovery of new drugs, the selection of high efficiency chemical catalysts, and the directed evolution of enzymes and cells(Teh et al. 2008).

However, microfluidic devices can be quite complex, and their fabrication can be quite difficult. For example, fabrication of glass etched devices requires sophisticated lithographic techniques that are difficult and expensive. Fabrication of milled plastic or metal devices is simple and relatively inexpensive, but the resolution is poor and miniaturization of the fluidic components is difficult(Duffy et al. 1998; Whitesides 2006).

By contrast, soft-lithography, PDMS devices can be easily, quickly, and inexpensively fabricated with superb resolution(Duffy et al. 1998; Whitesides 2006). This ability to easily fabricate sophisticated devices has revolutionized the study of microfluidics, particularly in academic labs in which the turn around time must be short. However, PDMS devices also have several significant drawbacks that limit their usefulness for many applications. PDMS is a delicate elastomer that is degraded by common chemicals(Lee et al. 2003; Rolland et al. 2004). Even when cured, PDMS remains permeable to liquids and gases(Lee et al. 2003; Rolland et al. 2004); this limits control and can interfere with reactions in the channels. Small molecules can diffuse into PDMS walls, fouling channel surfaces, and altering device behavior(Roman et al. 2005). Organic solvents, such as toluene and chloroform, are necessary for many applications, including the formation of vesicles and the syntheses of drugs; however, these chemicals swell PDMS, collapsing microfluidic channels, and significantly degrading the device performance(Lee et al. 2003).

The poor chemical compatibility of PDMS is, therefore, a major challenge that limits its applicability to lab-on-a-chip

microfluidics. This has stimulated the development of new materials that are more chemically robust(Rolland et al. 2004). Alternatively, attempts have been made to increase the robustness of PDMS by modifying its surface properties. For example, by infusing PDMS surfaces with metal-oxide precursors, the diffusion of small molecular weight dyes can be reduced(Roman et al. 2005). By coating PDMS slabs with poly(urethaneacrylate), swelling due to organic solvents can be retarded(Lee et al. 2006).

In addition to poor chemical compatibility, PDMS devices are also very difficult to functionalize to control surface chemistry, which is a significant limitation for many applications. For example, in biochemical applications, the microchannels must be coated with proteins to reduce adsorption of reagents to the channel walls. In droplet microfluidics, the interface must be functionalized to control wettability, to ensure that drops of the desired phase can be formed(Anna et al. 2003; Seo et al. 2007). In other applications, the channel properties must be controlled spatially, so that different regions of the device have different properties. For example, the fabrication of sensing devices requires spatial functionalization of the devices with specific chemical groups(Chiu et al. 2000; Rossier et al. 2002; Chen and Lahann 2005). The formation of multiple emulsions requires that the interface have distinctive wettability in different regions of the microfluidic device(Seo et al. 2007). However, spatial functionalization of PDMS is very difficult, and current methods result in channels with only marginal contrasts in wettability and of limited usefulness(Seo et al. 2007). This has stimulated the development of devices fabricated in new materials that can be more easily functionalized. Alternatively, hybrid devices can be fabricated consisting of PDMS channels bonded to a glass plate; the glass plate can be readily functionalized to control surface properties(Li et al. 2007). However, the remaining PDMS faces of the channels remain un-functionalized, so that control is limited, and the devices are still vulnerable to fouling and swelling.

Indeed, glass possesses a number of attributes that make it an optimal material for microfluidic devices. Glass is extremely chemically robust: it is resistant to corrosion and fouling, does not swell, and is compatible with a wide variety of chemicals, including organic solvents. Glass can also be functionalized to control surface properties, to graft desirable chemical groups to the surface or to spatially control wettability(Prakash et al. 2007). For example, glass capillary devices can be functionalized to spatially control wettability and can form double and triple emulsions(Utada et al. 2005; Chu et al. 2007), even using organic solvents(Chu et al. 2007). However, glass devices are difficult to fabricate. Glass capillary devices require manual tip pulling to form the drop making nozzles and hand alignment to assemble the devices, tedious processes that are difficult to automate. Glass capillary can only be made to perform a small set of functions, such as forming drops. An optimal system would combine the simple fabrication of PDMS devices with the robustness and control of glass.

Poly(dimethylsiloxane) Glass

• Ease of processing• Inexpensive• High gas permeability• Biocompatibility

• Chemical compatiblity• Extensive knowledge on surface functionalization and patterning

Photoreactive glass coatings on PDMS microfluidic channels using a sol-gel method

Poly(dimethylsiloxane) Glass

• Ease of processing• Inexpensive• High gas permeability• Biocompatibility

• Chemical compatiblity• Extensive knowledge on surface functionalization and patterning

Photoreactive glass coatings on PDMS microfluidic channels using a sol-gel method

Scheme 1. Advantages of combining the desirable properties of PDMS and glass for the fabrication of microfluidic devices.

In this chapter we combine the easy fabrication of soft-lithography in PDMS with the robustness and control of glass, as shown in scheme 1. We fabricate PDMS devices using soft lithography. We coat the devices with a glass layer using sol-gel chemistry. This glass coating greatly increases the robustness of the devices. As a demonstration of this robustness, we form emulsions with organic solvents. The glass coating is also very useful for controlling the surface chemistry of the devices. We functionalize the coating by incorporating fluorosilanes and photoinitiator-silanes into the sol-gel; these silanes allow us to tailor the surface chemistry of the microchannels and to use lithographic techniques to spatially control wettability. As a demonstration of this spatial control, we use a coated device with spatially patterned wettability to form double emulsions with fluorocarbon oil. MATERIALS

This method of coating PDMS devices with sol-gel combines the simplicity of fabrication of soft lithography with the robustness and versatility of glass. We begin by fabricating PDMS devices using the soft lithography. The advantages to this approach are that the devices are simple, quick, and inexpensive to fabricate, and the channels can be carefully engineered to perform a variety of desirable functions. After fabrication, the PDMS channels are coated with the sol-gel. The sol-gel increases the robustness of the channels and can also be functionalized to control surface properties. In this way, the PDMS channels act as a mold for the fabrication of much more useful sol-gel channels. PDMS device fabrication

1. 3” silicon wafer, Test grade P(100) SSP (University Wafer)

2. SU-8 2100 (MicroChem) 3. Spin coater, SCS G3P-12 Spincoat (Cookson

Electronics) 4. Photomask on transparency plastic (outputcity.com) 5. Borofloat UV-transparent borosilicate glass 1/8" thick,

5” x 5” (McMaster-Carr) 6. Mercury arc lamp 200 W (Optical Associates Inc.) 7. Polyethylene glycol methyl ether acetate (Sigma) 8. PDMS and cross-linker (Sylgard 184 Silicone

Elastomer Kit) 9. Automatic mixer, AR-100 conditioning mixer (Thinky) 10. Harris Uni-core, Hole 0.75~mm (Ted Pella Inc.) 11. Glass slide 75 mm x 50 mm (VWR) 12. Oxygen Plasma Treatment, PlamsaPrep2, (Gala

Instrumente) 13. Isopropanol (Sigma)

Chemically resistant glass coating

1. Tetraethoxysilane (TEOS) (Sigma) 2. Methyltriethoxysilane (MTES) (Sigma) 3. Hydrochloric acid aqueous pH 2 4. Ethanol (Sigma)

Irgacur-silane synthesis

1. Irgacure 2959 photoinitiator (Ciba) 2. Hydroquinone (Sigma) 3. Dibutyltin dilaurate (Sigma) 4. Dry chloroform (Sigma) 5. 3-(triethoxysily)propyl isocyanate (Sigma)

Photoreactive sol-gel coating

1. Tetraethoxysilane (Sigma) 2. Methyltriethoxysilane (Sigma) 3. Heptadecafluoro-1,1,2,2-

tetrahydrodecyl)triethoxysilane (Geleste) 4. Irgacur-silane (synthesized in house) 5. Trifluoroethanol (Sigma) 6. Hydrochloric acid aqueous pH 2 7. Methanol (Sigma)

Hydrophilic monomer solution

1. Acrylic acid (Sigma) 2. Sodium periodate (Sigma) 3. De-ionized Water 4. Ethanol (Sigma) 5. Acetone (Sigma) 6. Benzophenone (Sigma)

Spatial patterning of wettability

1. Exfo 100 W fiber-coupled mercury arc lamp. 2. Köhler Illumination optics to project a 100 μm

diameter UV spot of the field diaphragm onto channels. Microfluidic device operation and observation

1. Syringe 1 mL, B-D Luer-Lok™ Tip (VWR) 2. Needle 27g½, B-D PrecisionGlide® (VWR) 3. Polyetheylene tubing, 0.015” I.D. x 0.043 O.D.

(Scientific Commodities Inc.) 4. Syringe Pump, Harvard Apparatus PHD-2000 5. Computer with LabVIEW v. 8.2 and Vision v. 8. 6. Phantom V7 fast camera 7. Leica DM IRBE inverted microscope with 10x

objective. Double emulsions

1. Fluorocarbon oil, Fluorinert FC40 stabilized by 2. De-ionized water 3. Surfactant Zonyl FSN-100 (Sigma) in water 4. Surfactant Krytox 157FSL (Dupont) in oil

METHODS

PDMS device fabrication

1. Prepare photomask in Autocad and send off for printing too outputcity.com.

2. Spin coat a 3” silicon wafer with SU-8 2100 at 3000 RPM for 120 s.

3. Pre-bake coated wafer for 7 min at 65°C and then for 15 min at 95°C.

4. Place photomask above coated wafer and hold in place with UV-transparent borosilicate glass.

5. Expose to UV light with mercury arc lamp for 25 s.

6. Post-bake wafer for 1 min at 65°C and then for 10 min at 95°C.

7. Develop wafer in polyethylene glycol methyl ether acetate to dissolve uncross-linked SU-8.

8. Hard-bake wafer in 200°C for 3 min to anneal defects. 9. While wafer cools, mix PDMS and cross-linker at a

ratio of 10 to 1. 10. Place wafer in petri dish and pour over with 35 g of

PDMS mixture. 11. Evacuate uncured PDMS device in vacuum chamber

for ~5 min to remove air bubbles. 12. Cure PDMS device in 65°C oven for 90 min. 13. Slice PDMS device with scalpel around edges and peel

from master. 14. Punch holes through guide spots with Harris Uni-core. 15. Wash device and glass slide with isopropanol. 16. Plasma treat glass slide and PDMS with channel side

facing up for 10 s at power 2.5. 17. Immediately after plasma treatment, press slide and

PDMS channels together to bond. Chemically resistant glass coating

1. Combine tetraethylorthosilicate, methyltriethoxysilane, ethanol and pH 4.5 water, in an equivolumetric ratio, to make about 1mL of mixture.

2. Set hot plate to 200ºC and alternate heating and shaking mixture for 5-10 min until it turns clear.

3. Place mixture in a 65ºC oven for 12 hrs to speed preconversion of the alkoxy silanes.

4. Use mixture within one month. 5. Soon after plasma treating the PDMS-on-glass device,

flush with the preconverted precursor mixture. 6. Heat the device glass-side down on a hot plate set to

100ºC for 10 s. 7. After then 10 s, flush out the excess precursor liquid

using 10 mL of air with a hand-held syringe. 8. Transfer device to a 200ºC hotplate while continuing to

cycle air to completely cure the coating.

Irgacur-silane synthesis 1. Combine 11.0 g Irgacure 2959 photoinitiator with 0.01

g hydroquinone, and 49.4 μL dibutyltin dilaurate in 20.0 mL of dry chloroform.

2. Stir under nitrogen until mixture is homogenous. 3. Slowly add 12.1 mL of 3-(triethoxysily)propyl

isocyanate over 30 minutes while continuing to stir. 4. Heat mixture to 50ºC and continue to stir for an

additional 3 hours to allow the reaction to complete. 5. To concentrate the reaction product, evaporate the

chloroform; this yields a yellowish solid which is used without further purification.

6. Use within two months.

Photoreactive sol-gel coating 1. Combine 0.5 g photoinitiator-silane with 2 mL

trifluoroethanol and vortex until dissolved. 2. Add to that 1 mL tetraethylorthosilicate, 1 mL

methyltriethoxysilane and 0.5 mL (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane and shake until clear.

3. Add to that 1 mL pH 2 HCl aqueous. 4. To speed mixing, set hotplate to 200 °C and alternate

heating and shaking the mixture until it turns clear. 5. Use within two days.

6. Fill freshly plasma treated device with the photoreactive sol-gel mixture and allow to sit for two minutes.

7. Place device on hotplate set to 220ºC and cover with a napkin; the high temperature of the hot plate vaporizes the solvent and deposits and cures the coating on the channel walls.

Hydrophilic monomer solution

1. Combine 0.2 mL of acrylic acid with 0.8 mL 5 mM NaIO4 H20, 1 mL ethanol, 0.5 mL acetone and 0.05 g benzophenone.

2. Use within 1 week. Spatially patterning wettability

1. Fill photoreactive coated channels with monomer solution.

2. Project an image of the field diaphragm of a Köhler Illumination setup using a mercury arc lamp as the light source onto the device where you desire the wettability to be hydrophilic.

3. Expose channels to UV-light for 2-10 min depending on amount of grafting desired. Smaller channels require longer polymerization times.

4. Flush channels with water. Double émulsion production

1. For oil-water-oil double emulsions, fill two syringes with fluorocarbon oil and Krytox surfactant, and a third syringe with water and Zonyl surfactant.

2. Mount syringes on separate syringe pumps and connect needles and tubing; the inner diameter of the tubing fits snuggly over the outer diameter of the needle.

3. Begin all pumps at 9000 μL/hr and flush out all air. After each device attains a steady drip stop the pumps.

4. Insert tubing into punch holes of device; the outer diameter of the tubing is selected so that the tubing fits snuggly into the punch hole of the Harris Uni-core. This conveniently enables the tubing and PDMS to seal without additional epoxy or curing.

5. Start inner and middle phase flow rates at 500 μL/hr and outer at 100 μL/hr. After all streams flow forward, reduce inner phase to 150 μL/hr and middle to 350 μL/hr.

6. Verify that the first nozzle drips. If not, adjust inner and middle phase flow rates until it does. Then increase outer phase flow rate 500 μL/hr, or as necessary; the middle phase will gradually narrow and eventually break off into drops.

8. Continue adjusting all flow rates until the double emulsions attain desired size and volume fraction.

PART I: Chemically resistant glass coating

Sol-gel coatings are used in industry to increase the

robustness and corrosion resistance of mechanical parts(Duran et al. 2007). Sol-gel coatings consist of a dense siloxane network that is similar to glass. However, unlike glass, they can be deposited and cured near room temperature(Brinker and Scherer 1990). This allows them to be applied to delicate substrates using simple methods. We exploit this to coat PDMS microfluidic devices. The devices are fabricated using soft lithography in PDMS. The devices are coated with the glass layer to modify the channel surfaces. The coated channels are much more robust than bare PDMS channels. We show that the glass coating prevents fouling of the channel walls by a fluorescent solute and swelling

when exposed to toluene, an organic solvent. The glass coating can also be functionalized to control surface chemistry. As a demonstration of this control, we functionalize coated devices to make a device hydrophilic and another hydrophobic. With the devices, we form direct and inverted emulsions with the organic solvent toluene.

Fig. 1 Scanning electron micrographs of cross-sections of (a) uncoated and (b) coated PDMS channels. The original channel dimensions are 50x35 μm. [Abate et al. 2008] - Reproduced by permission of The Royal Society of Chemistry.

To directly observe the coating deposited on the channel walls, we use scanning electron microscopy (SEM) and image both uncoated and coating channel cross-sections. The uncoated channels have a rectangular cross-section, due to the nature of their fabrication, as shown in Fig 1a. By contrast, the coated channel has a more circular cross section, because the coating rounds off the corners, as shown in Fig 1b. From the images we estimate the coating thickness to be about 5-10 μm.

Fig. 2 Comparison of diffusion of Rhodamine B into (a) uncoated PDMS channel, in which it has diffused into the PDMS walls, and (b) coated PDMS channels, in which it has been prevented form diffusing into the PDMS walls. (c-d) Confocal images of coated channel at different orientations. [Abate et al. 2008] - Reproduced by permission of The Royal Society of Chemistry

To investigate the chemical resistance of the coated channels we perform two experiments. The first experiment tests resistance of the coated channels to surface fouling. We monitor the diffusion of a fluorescent solute from an aqueous solution into the channel walls. As the solute, we use Rhodamine B, a fluorescent dye that diffuses into cured PDMS(Roman et al. 2005), and can thus act as a visible probe of chemical resistance. We fill both uncoated and coated channels with 50 μM aqueous Rhodamine B, and store the channels in the dark for four days. During this time, the water evaporates, leaving the Rhodamine B behind. As expected, Rhodamine B diffuses into the walls of the

uncoated PDMS, fouling the channels, as shown in Fig 2a. By contrast, there is no diffusion into the walls of the coated channel, as shown in Fig 2b. Instead, all the fluorescence is contained within the channel and deposited as a thin layer on the surface of the coating. To study the coated channels in greater detail, we image them in three dimensions using a fluorescent confocal microscope. The coating is quite uniform along the channel, as shown by the lateral views in Figs. 2(c-d). The coating thickness varies over the cross-sectional perimeter of the channels, due to the circular cross-section of the coating being inscribed into the rectangular cross-section of the PDMS channel, as shown in Figs. 2(e-f), and confirmed in the SEM image.

Fig. 3 Photomicrographs of uncoated and coated channels exposed to the organic solvent toluene. Uncoated (a) and coated (b) channels at t = 0 s just prior to exposure to toluene so that the channels are empty. Uncoated (c) and coated (d) channels at t = 0.57 s. Toluene has just started flowing through the channels and, already, the uncoated channel has begun to swell. Uncoated (e) and coated (f) channels at t = 13 s. The uncoated channel has further swollen and the walls have nearly reached their final swollen state. By contrast, the coated channel remains unchanged. [Abate et al. 2008] - Reproduced by permission of The Royal Society of Chemistry

The Rhodamine B test demonstrates that the coating

prevents diffusion of the fluorescent solute into the PDMS walls; however, other chemicals, such as organic solvents, degrade PDMS channels much more aggressively. As a more stringent test, we thus expose the channels to an organic solvent. For the organic solvent we use toluene, which is commonly used in chemical synthesis, but significantly and almost instantly swells PDMS(Lee et al. 2003). For the experiment, we flow toluene

through both uncoated and coated PDMS channels at 100 ul/hr. As the toluene flows through the channels, we monitor the time-evolution of their width and shape. The width and shape of the initially empty channels is shown at t = 0 in Fig. 3a and 3b. Due to the cylindrical shape of the coated channel and the large index of refraction mismatch of the coating with air, an optical lensing effect can be seen in Fig 3b. By contrast, the uncoated channel has flat walls and does not show this effect. At t = 0.57 both channels have been filled with toluene and, already, the uncoated channel has begun to swell and become narrower, as shown in Fig 3c. By contrast, the coated channel remains unswollen and its shape unchanged; however, due to the smaller index of refraction mismatch of the coating with the toluene, the optical lensing effect has vanished to reveal smooth, sol-gel coated walls, as shown in Fig. 3d. At t = 13 s the uncoated channel has swollen even further and has nearly reached its final swollen state, as shown in Fig. 3e. However, again, the coated channel remains unswollen and unchanged, as shown in Fig. 3f. Even after an hour of use, the coated channel has changed very little. These experiments demonstrate the robustness to the glass coating and its utility as a chemical barrier for the PDMS channels. In addition to providing a chemical barrier against organic compounds, the glass coating can be functionalized to have a variety of chemical properties. This is particularly important for production of emulsions, which require that the surface have preferential wetting characteristics to ensure that drops are formed(Anna et al. 2003; Seo et al. 2007). To illustrate this, we produce emulsions using water and toluene. We use a flow focusing drop making device(Anna et al. 2003; Seo et al. 2007) coated with glass using our sol-gel method. We use a single inlet to fill and flush the coating mixture. We also reduce the initial time on the hotplate to 5 s, thereby depositing a thinner coating, since the channel dimensions are smaller. After coating the channels, we functionalize the device to control the wetting properties. We make one device hydrophobic by treating it with Aquapel™. This is accomplished by filling with Aquapel™ and then immediately flushing with air. This device produces inverted emulsions consisting of water drops in toluene, as shown in Fig 4a. We make an identical device hydrophilic by treating it with preconverted N-[3-(Trimethoxysilyl)propyl] ethylene diamine. The device is filled with the mixture and allowed to sit for 5 min, and then flushed out with air. This device produces direct emulsions consisting of toluene drops in water as shown in fig 4b.

Fig. 4 Photomicrographs coated PDMS channels functionalized to have different wettability. (a) Inverted emulsion consisting of water-in-toluene drops produced at flow rates 500/500 μl/hr inner/outer phase. The coated channels were made hydrophobic by application of Aquapel. (b) Direct emulsion consisting of toluene-in-water, produced at flow rates 150/500 μl/hr inner/outer phase. The channels were made hydrophilic by application of preconverted N-[3-(Trimethoxysilyl)propyl] ethylene diamine, ethanol, and pH 2 water adjusted with HCl in a equivolumetric ratio. [Abate et al. 2008] - Reproduced by permission of The Royal Society of Chemistry. The glass coating changes the shape of microchannels. The particular shape and dimensions of the coated channels can be adjusted by controlling the original channel dimensions and the thickness of the applied coating. This allows the channel dimensions to be engineered and adjusted to the desired size. Moreover, it provides a method to produce channels with cylindrical symmetry rather than square symmetry, since the glass wets the surface and collects in the corners. The glass coating significantly improves the performance of PDMS microfluidic devices. It provides a protective barrier that greatly increases the chemical resistance of the PDMS microchannels. In addition, the coating can be functionalized to have a variety of surface properties. This can be exploited, for example, to produce both direct oil-in-water and inverted water-in-oil emulsions in coated channels using organic solvents. The collection of these attributes significantly broadens the applicability of PDMS to microfluidic technology, enabling its use in a much wider class of systems and for a greater variety of chemicals. It combines the scalability and simplicity of PDMS devices with the chemical versatility and robustness of glass. PART II: Photoreactive sol-gel for high-contrast spatial patterning of microfluidic devices

In addition to making PDMS devices robust, the glass coating is also very useful for controlling the surface chemistry of the channels. As we have shown in the previous chapter, this can

be accomplished by functionalizing coated channels with functional silanes. In fact, these silanes can be incorporated directly into the sol-gel network by including them in the precursor mixture. This allows the channels to be coated and functionalized in a single step. As a demonstration, we functionalize the sol-gel with fluorosilanes and photoinitiator-silanes. Both silanes are included in the sol-gel precursor liquid and are incorporated into the sol-gel layer when it is deposited on the microchannel walls. The fluorosilanes make the coating very hydrophobic. The photoinitiator-silanes make the channels photoreactive. This allows additional functional properties to be spatially patterned onto the channels using UV-initiated graft polymerization.

Fig. 5 SEM images of channel cross-sections of (a) uncoated PDMS channel and (c) magnified view of upper right corner, and (b) coated, PAA functionalized PDMS channel and (d) magnified view of upper right corner. The corner of the coated channel is rounded off by the sol-gel and the grafted polymer. Scale bars denote 5 μm

To study the physical dimensions of the photoreactive coating deposited on the channel walls, we image channel cross-sections using SEM. The uncoated PDMS channels have a rectangular cross-section, due to the nature of their fabrication, as shown in Fig. 5a. The wavy pattern on the side walls is an artifact of the soft-lithography process, and can be clearly seen in the magnified view of Fig. 5b. By contrast, the rectangular corners are rounded off and the wavy pattern smoothed over for the coated channel, because the sol-gel liquid wets the surface and collects in regions of high curvature, as shown in Fig. 5c and in the magnified view of Fig. 5d(Abate et al. 2008). To spatially pattern wettability, we fill the channels with monomer solution and expose to patterned UV light. The photoinitiators in the exposed regions release radicals that initiate polymerization. The polymer chains grow from the interface and are tethered to the sol-gel surface through permanent covalent bonds with the photoinitiator-silanes.

Fig. 6 AFM images of (a) sol-gel coated and (b) PAA grafted microchannels. The images show a 10 x 20 μm area of the channel at high magnification. The dark to light color scale maps to feature heights of -150 to 150 nm. The scale bars denote 4 μm. (e) Surface concentrations of atoms on sol-gel coated and PAA functionalized substrates, measured with XPS; Fluorine (F 1s), Oxygen (O 1s), Carbon (C 1s), and Silicon (Si 2s).

During UV initiated polymerization, the sol-gel surface

appears to roughen over time. For long polymerization times, wrinkles and bumps appear on the channel surface. To study the topography of the coated and polymerized regions of the channels at high magnification, we image the channels with atomic force microscopy (AFM). In the AFM image, the sol-gel coating appears smooth and homogenous on the hundred nanometer scale, as shown in Fig. 6a. By contrast, the polymerized region has a rough surface and complex topographical structure, with large polymer aggregates and surface depressions on the hundred nanometer scale, as shown in Fig. 6b. Graft polymerization of PAA onto the sol-gel surface significantly modifies interfacial topography.

Graft polymerization also significantly modifies interfacial chemistry, which we characterize with x-ray photoelectron spectroscopy (XPS). The x-ray beam penetrates only the upper-most portion of the substrate and provides a detailed and accurate measure of the concentrations of atoms present on the surface. The native sol-gel coating shows a pronounced concentration of fluorine and a small concentration of silica on the surface, suggesting the low surface energy fluorocarbons orient outwards during gelation, forming a fluorinated surface that masks the siloxane backbone of the sol-gel, as shown in Fig. 6c. By contrast, the polymerized region shows a reduction in fluorine and increase in carbon, indicating hydrophilic PAA on the interface, as shown in Fig. 6c. The carbon peak also shifts to higher photoelectron energy, as expected from the high energy carbonyl bonds that make up the PAA(Selli et al. 2001; Ward et al. 2003).

Fig. 7 Contact angle measurement of water droplets in air on (a) sol-gel coated substrate with contact angle 105° and (b) PAA grafted substrate with contact angle 22°.

In addition to modifying the topography and chemistry of the surfaces, the graft polymerization also significantly

modifies wettability. To illustrate this, we perform contact angle measurements with water drops. As expected, the fluorinated sol-gel is very hydrophobic(Anton 1998), having a contact angle of 105 ± 1°, as shown in Fig. 7a. By contrast, the PAA grafted surface is hydrophilic(Ward et al. 2003), so that the water droplet spreads out, forming a contact angle of 22 ± 5°, as shown in Fig. 7b. The UV initiated polymerization thus changes the contact angle of the surface by 83°, which is much larger than methods that functionalize the PDMS directly(Seo et al. 2007), and is sufficient for production of emulsions with a variety of solvents and oils. The UV-initiated polymerization thus significantly modifies the surface topography, chemistry, and wettability.

Fig. 8 (a) Photomicrograph of a sol-gel coated, PAA patterned microfluidic channel. To visualize the polymer grafting, the PAA has been stained with toluidine blue. (b) Average grayscale intensity across the channel as a function of location along the channel.

The UV initiated polymerization can also be controlled

spatially, by projecting spatially patterned UV light. To investigate this, we chemically stain a spatially patterned microfluidic channel. We flush the channel with aqueous toluidine blue, a dye that electrostatically binds to PAA(Yoo et al. 1998). Thus, after flushing the dye through the channel only the polymerized regions stain blue, enabling the grafted polymer to be visualized. We show a magnified view of a border region of a grafted PAA patch in Fig. 8a. To quantify the resolution of the grafting, we measure the average intensity across the channel as a function of location down the channel, given in Fig. 8b. From the image and the intensity profile we estimate the resolution of the grafting to be ~5 μm. With improved polymerization optics, resolution can be improved to better than a micron.

Fig. 9 (a) Expanded view of a diagram of the double emulsion formation device with spatially patterned wettability. (b) Photomicrographs of double emulsions being formed (R1) and flowing out of the microfluidic

device (R2 and R3). (c) Magnified view of the double flow-focusing junction in R1. The scale bars for all figures denote 100 μm.

Fig. 10 Photomicrographs of double emulsion production in a microfluidic device. The number of core drops encapsulated can be controlled by adjusting flow rate: (a) two, (b) three, (c) five, and (d) six oil cores in each O/W/O double emulsion drop.

Fig. 11 Photomicrograph of O/W/O double emulsion drops produced in a PDMS device with spatially patterned wettability. The double emulsion drops order into a hexagonal array due to their high monodispersity. The scale bar denotes 100 μm.

The sol-gel coating allows the surface chemistry of the interface to be tailored and the wettability of the device to be spatially patterned. We take advantage of this by incorporating fluorosilanes and photoinitiator-silanes into the coating. The fluorosilanes produce a fluorinated interface that is both hydrophobic and fluorophilic, optimal for forming of water-in-fluorocarbon oil emulsions(Seo et al. 2007). The photoinitiator-silanes allow us to spatially pattern hydrophilic patches of polyacrylic acid onto the interface. Together, these silanes allow us to spatially pattern high contrasts in the wettability of the microfluidic channels, which we us to form double emulsions with fluorocarbon oil. For the double emulsions we use water and fluorocarbon oil (Fluorinert FC40) stabilized by surfactants Zonyl FSN-100 (Sigma-Aldrich) and Krytox 157FSL (Dupont), respectively. We coat a device consisting of two flow-focus drop makers arranged in series(Seo et al. 2007) and functionalize the first to make it hydrophilic; the default fluorination of the sol-gel make the rest of the device both hydrophobic and fluorophilic, as shown in Fig 9a. Syringes are filled with the water and oil used to form the double emulsions. The syringes are amounted on

computer controlled syringe pumps, enabling the fluids to be injected into the device at controlled rates.

Due to its hydrophilic wettability, the first drop maker produces a direct fluorocarbon oil-in-water emulsion, as shown in Fig. 9b and the magnified view in Fig. 9c. By contrast, the hydrophobic second drop maker produces water drops, which encapsulate the oil drops from the first stage, producing O/W/O double emulsions, as shown in Fig. 9b and the magnified view in Fig. 9c. To control the morphology of the double emulsion, we adjust the inner phase flow rate and control the number of encapsulated drops. At low inner phase flow rates of ~100 μl/hr, only one or two drops are encapsulated, as shown in Fig. 10a. However, at higher flow rates of ~500 μl/hr, three, five, and six drops are encapsulated, as shown in Figs. 10(b-d). The drops can be produced with high monodispersity, as illustrated by the hexagonal ordering of the monodisperse double emulsion drops in Fig. 11.

III. CONCLUSION

Sol-gel coatings increase the usefulness of PDMS microfluidic devices. Sol-gel coatings make PDMS device more chemically robust, useful for a variety of applications, including those with organic solvents. Sol-gel coatings are also useful for controlling the surface chemistry of PDMS channels. The sol-gel coating can be functionalized with silanes to tailor the surface chemistry of the channels and to spatially pattern wettability. This is useful for a many applications, including, as we have shown, the formation of multiple emulsions. Sol-gel coated PDMS

channels combine the simplicity of fabrication of soft-lithography in PDMS with the robustness and control of glass.

IV. FUTURE TRENDS

The sol-gel coatings described in this chapter significantly improve the chemical compatibility of PDMS microfluidic devices. This makes the devices available for many applications, including performing bioassays that require organic solvents. The ability to spatially control the surface functionality of channels should also be useful for patterning of proteins, enzymes, antibodies, cells, and DNA, onto the channel surfaces, for applications in biosensing and separation. Due to the precision fabrication, the devices should also be useful for forming more complex high order emulsions than can be formed by other methods. Moreover, the simple fabrication also readily lends itself to parallelization, which will be essential for scale up, and on which we are currently working. This should aid large-scale production of monodisperse emulsions, which could be useful for encapsulation of active ingredients in many industrial applications. [ACKNOWLEDGEMENTS] This work was supported by a Human Frontiers Grant (RGP0004/2005-C102), the NSF (DMR-0602684) and (DBI-0649865), the Harvard MRSEC (DMR-0213805), and the Deutsche Forschungsgemeinschaft (DFG).

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