Moldable PerfluoropolyetherâPolyethylene Glycol Networks with

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Moldable Perfluoropolyether-Polyethylene Glycol Networks with Tunable Wettability and Solvent Resistance for Rapid Prototyping of Droplet Microfluidics Heon-Ho Jeong, Syung Hun Han, Sagar Yadavali, Junhyong Kim, David Issadore, and Daeyeon Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05043 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Chemistry of Materials

Moldable Perfluoropolyether-Polyethylene Glycol Networks with Tunable Wettability and Solvent Resistance for Rapid Prototyping of Droplet Microfluidics Heon-Ho Jeong,1,2 Syung Hun Han,3 Sagar Yadavali,3 Junhyong Kim,4 David Issadore3,5,*, Daeyeon Lee1,* 1

Department of Chemical and Biomolecular Engineering, University of Pennsylvania,

Philadelphia, Pennsylvania, 19104, USA 2

Department of Chemical and Biomolecular Engineering, Chonnam National University, Yeosu,

Jeonnam, 59626, Republic of Korea 3

Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, 19104,

USA 4

Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, 19104, USA

5

Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia,

Pennsylvania, 19104, USA

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Abstract Soft lithography-based droplet microfluidics has enabled production of highly uniform and complex emulsions. Although there is a significant potential to use these emulsions as templates for functional materials syntheses, conventional elastomers that are used for microfluidic device preparation are significantly deformed and swollen by various organic solvents, limiting the types of materials that can be processed using conventional soft lithography-based droplet microfluidics. In this report, we demonstrate that both water-in-oil and oil-in-water with organic solvents can be produced by using microfluidic devices that are prepared using crosslinked networks of perfluoropolyether (PFPE) and poly(ethylene glycol) diacrylate (PEGDA). We show that these PFPE-PEG networks are transparent and maintain excellent compatibility with various organic solvents. Importantly, the wettability of these devices can be systematically controlled by changing the ratio of the two macromonomers. By taking advantage of rapid prototyping and controlled surface wettability afforded by the PFPE-PEG network, we prepare three-dimensional monolithic elastomer devices for the parallel generation of oil-in-water and water-in-oil droplets. We also show that, using these devices, solid microparticles with high uniformity can be produced by using an organic solvent-based emulsion as a template. We believe the PFPE-PEG network will have broad impacts in the application of soft lithographybased elastomer microfluidic devices to a wide range of applications including drug screening and solvent-based separation processes.


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Introduction Microfluidics allows for precise control of flows of fluids at the sub-millimeter scale, which can be harnessed to produce materials with useful functionality and properties. In particular, microfluidic devices have been designed to produce highly uniform emulsion droplets by tuning the flow and interfacial phenomena of multiphasic fluids. Microfluidic droplets serve as excellent templates to form highly uniform solid microspheres with a variety of shapes and morphology in the size range of sub-micrometers to hundreds of micrometers.1-2 The uniformity of the microspheres have been particularly enabling for applications in photonics and biosensing, as well as for the encapsulation and delivery of pharmaceutical agents.3-4 One important unsolved challenge in droplet microfluidics has been the lack of a material that can be used to rapidly prototype droplet microfluidic systems that is compatible with the organic solvents necessary for the syntheses of many materials.

Soft lithography-based

techniques have been used with great success to fabricate microfluidic devices with complex geometry; however, the material of choice used for device fabrication, polydimethylsiloxane (PDMS), has poor solvent compatibility, limiting utilization of soft lithography in the preparation of organic solvent-based emulsions.5 Although approaches to modify the surface of PDMS-based devices have been introduced, such modifications require post-processing and may not provide long-term stability.6-9 Another material that has shown great promise in the fabrication of microfluidic devices based on soft lithography is perfluoropolyether (PFPE)-based elastomer, which has excellent solvent compatibility as well as moldability for use in soft lithography.10-13 PFPE, in turn, has been successfully used to make microfluidic devices. However, few reports have demonstrated



the use of PFPE for the fabrication of microfluidic devices that can be used to produce organic solvent-based emulsions.12 One of the key challenges that must be addressed to enable the production of such emulsions is control of the wetting properties of the polymer surface, which has a direct impact on the type and stability of the emulsion formed in microfluidic devices. In general, to form an emulsion, the surface of the channel must have more favorable wetting for the continuous phase over the dispersed phase to ensure stability of the emulsion. For PFPE, rendering its surface hydrophilic is particularly challenging due to its hydrophobicity and inertness. To use PFPE devices to generate particles that require organic solvents, it is necessary to develop a new material that can be used to prototype microfluidic devices for oil-in-water emulsion generation that is both solvent resistant and that has tunable wettability. In this report, we present the synthesis of perfluoropolyether-polyetheylene glycol (PFPE-PEG) networks that have tunable wetting properties and solvent resistance, and demonstrate that these materials can be applied to prototype three-dimensional droplet microfluidics devices for the production of polymer microspheres using an organic solvent. The wetting property of PFPE surface is controlled by introducing a hydrophilic macromonomer, polyethylene glycol diacrylate (PEGDA). The network is transparent, highly solvent resistant, and also can be readily molded using conventional soft-lithography as well as more recently developed three-dimensional soft lithography techniques.14 Although networks with similar comonomers have been studied, prior reports focused on swelling properties of such networks in organic solvents as well as their mechanical, thermal and antifouling properties.13, 15 Solvent compatibility and microfluidics applications of such networks have not been extensively explored. We demonstrate that, by combining this material with new architectures for parallelizing droplet microfluidics developed by our group and others, we can generate highly 4 ACS Paragon Plus Environment

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monodisperse organic solvent-based emulsions at a throughput of 100 mL of dispersed phase per hour. Moreover, emulsions prepared with organic solvents are used to template highly uniform polymer microspheres, demonstrating the potential of the PFPE-PEG network-based microfluidics for the large-scale synthesis of new materials.

Results and Discussion Perfluoroether (PFPE)-polyethylene glycol (PEG) networks are synthesized by photoinitiated polymerization of PFPE-urethane dimethacrylate (MW ~ 2,000 g/mol) and PEG diacrylate (MW = 575 g/mol).

This PFPE macromonomer is known to have excellent miscibility with

photoinitiatiors and also some hydrogenated acrylates due to the presence of the polar group (i.e., urethane). 2-Hydroxy-2-methylpropiophenone (Darocur 1173) is added to the mixture at 4 wt% as the photoinitiator. Upon mixing the two macromonomers and the photoinitiator, the mixture stays clear with no apparent macroscopic phase separations. Irradiating a thin slab (200 um) of the precursor mixture with 365 nm UV light for 5 min induced solidification of the macromonomers and formation of an optically transparent solid sheet. Increasing the concentration of PEGDA above 15 wt%, however, leads to formation of opaque slabs (Figure 1), indicating that there is micron-scale domain formation leading to their phase separation likely due to the lack of specific interactions between the two macromonomers. The water contact angle under hexane is measured, and it is demonstrated that the wetting property can be varied over a wide range by modifying the composition of the network. In particular, under hexane, PFPE has water contact angle of ~ 130°, whereas PFPE with 10 wt% PEG-DA network has a water contact angle of 60° as shown in Figure 1C. The water contact 5 ACS Paragon Plus Environment


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angle of the surface remains constant once it reaches a plateau value after ~7 min under hexane (Figure 1B). The advancing and receding contact angles at the steady-state (> 12 min immersion in hexane) are 79º and 32º, respectively. Importantly for droplet microfluidics, the wetting characteristics of the network can be varied over a wide range (60 – 130°) while keeping the material transparent (i.e., the concentration of PEGDA is kept below 10 wt%), as shown in Figure 1E. Also unlike polydimethylsiloxane (PDMS), which is known to recover its hydrophobicity in time even with surface treatments such as oxygen plasma treatment,16-17 a prolonged storage of the networks does not induce any changes in the wetting properties of these networks. This stability indicates that the two macromonomers have been crosslinked and the mobility of these species within the network is very low. Interestingly, if we measure the water contact angle on pure PFPE, we do not observe changes in the droplet contact angle (Supporting Information Figure S1). Thus, we believe that the change in the contact angle of hexane underwater over the first 10 min or so is likely due to slow diffusion of hexane and water into the PFPE-PEG network and their interactions with PEG. For these networks to be useful in applications involving organic solvents, especially those requiring precise shape control and geometry such as microfluidics, it is imperative that the network does not swell significantly in the presence these solvents.

We test the solvent

compatibility of the network by submerging them in various solvents for 3 days. As can be seen in Figure 1c, these networks do not undergo significant swelling in hexane and toluene over all ranges of compositions, indicating that the network is compatible with these solvents. Networks with high concentrations of PEGDA (> 15 wt%) swell to a small extent (25 – 30 %) when they are submerged in chloroform. However, as long as the wt% of PEGDA is kept below 10 wt%, the range in which the network stays transparent, swelling in chloroform remains relatively small 6 ACS Paragon Plus Environment

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(< 10%). The pure PFPE network shows a very small contraction in pure water although this observation is within the uncertainty of the measurement. This small contraction could be due to the hydrophobic interactions between PFPE chains underwater. This excellent solvent compatibility of the PFPE-PEG networks can be understood by considering the enthalpy of mixing (∆Hmix) between a solvent and the two macromonomers that constitute the networks. It has been previously shown that a solvent that has a solubility parameter that is similar to that of PDMS has a higher tendency to swell PDMS due to reduced enthalpic penalty for mixing, making the free energy of mixing (∆Gmix) favorable. The reported solubility parameters of PFPE (δPFPE) and PEG (δPEG) are ~ 6 and ~ 10 (cal/cm3)1/2, respectively.18-20 The solubility parameters for hexane, toluene and chloroform, which are commonly used organic solvents for particle preparation in microfluidics, are 7.3, 8.9 and 9.2.5 With the exception of chloroform, all these values are sufficiently different from δPFPE and δPEG, making the networks with PFPE-dominant compositions highly compatible with organic solvents. Also, the relatively high crosslinking density, and thus small molecular weights between crosslinks, likely suppresses substantial swelling of the network by these solvents. Although the solubility parameters of PFPE and chloroform are quite different, the values for PEG and chloroform are similar (10 and 9.2 (cal/cm3)1/2). Thus, swelling of PFPE-PEG networks with high concentrations of PEG in chloroform is likely due to the reduced enthalpic penalty, resulting in a greater extent of swelling. We evaluate the utility of using our approach by fabricating PFPE-PEG network-based microfluidic devices and generating solvent-based emulsions. The solvent compatibility of PFPE-PEG network can be directly tested by subjecting a microfluidic device with a droplet generator orifice to hexane (Figure 2). Very small changes in the channels of a PFPE-PEG 7 ACS Paragon Plus Environment


device (10 wt% PEG) are observed (~ 3% decrease in the width of the orifice) after 48 hour exposure to hexane as shown in Figure 2A. In stark contrast, a PDMS device with the same channel dimensions undergoes significant swelling when it comes in contact with hexane; the width of the orifice in this case decreased by 80% (Figure 2B). These observations clearly indicate that PFPE-PEG is a highly solvent compatible material that can be used for microfluidic applications that require the use of organic solvents. The impact of swelling on various operations including droplet formation is likely to be minimal. Motivated by this success, we use a recently introduced double printing method to prepare a three-dimensional PFPE-PEG-based monolithic elastomer device (3D MED) with 100 parallel flow focusing droplet generators (see Figure S2 in Supporting Information).14 In addition to satisfying the channel resistance requirements for uniform distribution of fluids throughout the device,21 we employ the step-emulsification design that has been used in parallel production of emulsions in microfluidics to further enhance the uniform production of emulsions.22 A hard master that has features with two different heights is fabricated by sequential photolithography on a silicon wafer, and a soft master that contains the delivery channels for the liquids are prepared using the standard soft lithography technique. The precursor mixture with the desired ratio of PFPE and PEGDA is placed atop the hard master and the soft master is pressed onto the hard master while ensuring alignment between the features of the two masters. It is found that to facilitate the release of the PFPE-PEG network-based 3D MED from the hard master, the surface of the hard master must be treated with monoglycidyl ether-terminated PDMS. Without this surface treatment, the PFPE-PEG network exhibits fairly strong adhesion to the master and thus easily breaks during release from the master. To seal the device with top and bottom plates, we sandwich the 3-D MED in between two thin slabs of PFPE and a thin slab of PDMS is placed on 8 ACS Paragon Plus Environment

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the top surface as illustrated in Figure S2D. This stack is subsequently placed between two acrylate plates, and 5-min epoxy is used to seal the device while the two plates are compressed against each other using a 3-prong laboratory clamp. The PDMS slab is placed to relieve the pressure that may lead to damaging of the device features when pressure is applied. Using this method, 3D MED devices with up to 100 parallel flow focusing generators can be readily prepared. One of the most important applications of droplet microfluidics is the generation of highly uniform microspheres. Due to their uniformity in size and shape, these microfluidic-based microparticles can be used for advanced applications including drug delivery and photonic crystal arrays.23-28 One widely used method of preparing uniform particles is to dissolve a preformed polymer in an organic solvent and to produce uniform emulsions which are then subjected to solvent removal/evaporation to yield the final solid microparticles. We demonstrate that the PFPE-PEG network can enable the preparation of highly uniform microspheres (CV < 6%) by using a 100-flow focusing generator (FFG) 3D MED and a polystyrene solution in hexane at a throughput of 1.34 g/hr. As shown above, typical microfluidic devices made with PDMS will deform significantly in contact with hexane due to swelling, complicating the production of emulsions. Using the PFPE-PEG network-based 3D MED device made with 10 wt% PEG, we are able to create oil-in-water emulsions with high uniformity (Figure 3). By evaporating hexane from oil-water emulsions in open air, microspheres that retain the high uniformity of the parent emulsion can be produced in large quantity from the parallelized device (Figure 4). We can also produce highly uniform water-in-oil emulsion by using deionized water and hexadecane containing 2 wt% Span 80 as the water and oil phases, respectively.



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Conclusions In summary, we demonstrate systematic control over the wetting characteristics of PFPE network by incorporating a hydrophilic co-macromonomer, PEG-DA.

The change in the

wettability does not compromise the desirable properties of PFPE relevant to microfluidics such as solvent compatibility, transparency and moldability. We demonstrate that these networks can be molded into complex 3D microfluidic devices, which can be used for the large-scale production of highly uniform emulsions with organic solvents and solid microparticles. While our focus in this work has been on the application of PFPE-PEG networks in droplet microfluidics, we believe the high solvent resistance, the controlled wetting properties and the moldability of these networks make them potentially useful for coatings, separation and biomedical applications.

Experimental Methods Characterization of PFPE-PEG network. The PEG-DA (575 g/mol, Sigma-Aldrich) is added to the desired amount of the PFPE (Fluorolink MD 700, Solvay) in a range of composition weight ratios between 1:99 and 15:95. 2-Hydroxy-2-methylpropiophenone (Darocur 1173) is then added to the mixture at 4 wt% as the photoinitiator. This mixture is colorless and clear, indicating no macroscopic phase separation. To prepare the PFPE-PEG copolymer slabs, the mixture is placed between two glass substrates and then irradiated with UV light for 5 min. Contact angle measurements are performed on a goniometer (Attension, Theta) at room temperature. Typically, prepared samples are immersed in the container filled with hexane and 2 µL water droplets are used for contact angle measurement. Solvent resistance experiment is performed by measuring


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the swelling of the sample after being immersed in a solvent for 3 days. We prepare polymer cubes (3 mm × 3mm × 3mm) and measure the volume to quantify the extent of swelling. Fabrication of microfluidic device. As described in our previous report, a 3D monolithic elastomer device (3D MED) is fabricated by double-sided imprinting using a multi-height hard silicon master and a soft PDMS master.14,

29-30

To prepare the multi-height hard master,

photoresist SU-8 is first spin-coated at 4000 rpm onto a Si wafer (45 µm height). A photomask with patterns for FFGs (100 µm width for dispersed phase channel and 40 µm orifice and 200 µm width for continuous phase channel) and underpasses for the dispersed phase is used to selectively expose UV onto the spin-coated SU-8. For the second layer, 600 µm thick SU-8 of is spin-coated atop the first SU-8 layer. A second photomask with through-holes (250 µm diameter) and a collection channel (600 µm) is aligned to the first layer using a mask aligner (ABM3000HR) and then exposed to UV.

After removing the unexposed regions of the

photoresist in the SU-8 develop, the multi-height SU-8 patterns are formed. To facilitate the removal of the PFPE-PEG 3D MED from the hard master, the master is treated with monoglycidyl ether-terminated PDMS. The hard master is silanized with 0.5 wt% aqueous solution of 3-(aminopropyl triethoxysilane) (APTES) for 10 min after a 3 min O2 plasma treatment. After washing with distilled water, the monoglycidyl ether-terminated PDMS is dropped onto the hard master and incubated at 80 °C for 4 hrs. Subsequently, unreacted PDMS is removed by rinsing with 2-propanol and acetone. For preparing the PDMS soft master, we apply the conventional single-layer photolithography. SU-8 photoresist is spin-coated onto a silicon wafer and UV exposed through a photomask and developed to obtain the desired features for delivery and supply channels. The Si masters are silanized with hexamethyldisiloxane. PDMS prepolymer mixed with cross-linker in the ratio of 10:1 is poured onto the Si master with a 11 ACS Paragon Plus Environment


single-layer SU-8 feature and cured at 95 °C for 2 hours and then peeled off to obtain the PDMS soft master mold. The PDMS soft master is subsequently silanized with tridecafluoro-1,1,2,2tetrahydrooctyl-1-trichlorosilane after a 2-min O2 plasma treatment. To fabricate a PFPE-PEG based 3D MED, a mixture with a desired ratio of PFPE and PEG-DA is poured onto both the PDMS soft master mold and the Si hard master. After removing gas bubbles in a vacuum chamber, the soft and hard masters are aligned with the aid of fiduciary features on the two masters. The PFPE-PEG network between the two masters is polymerized by UV irradiation while the two masters are aligned and brought into contact with each other. The final 3D MED is obtained by peeling off the soft master and the cured PFPE-PEG. To seal the 3D MED with a substrate, we assemble two thin slabs of PFPE (200 µm) on the top and bottom of 3D MED and a slab of PDMS (5 mm) on top of the PFPE slab (see Figure S2 in Supporting Information). This stack is placed between two hard acrylate plates to apply uniform pressure to the assembled stack. Finally, 5 min epoxy glue is used to maintain the assembled stack while pressure is applied on the top and bottom acrylate plates using a three-prong laboratory clamp. Injection holes (0.75 mm in diameter) for the fluids are punched through the top PFPE and PDMS slab using a stainless steel punch. A syringe needle (outer diameter = 0.92 mm) connected to polyethylene tubing (inner diameter = 0.86 mm and outer diameter = 1.32 mm) is inserted into the acrylate plate that has laser-cut injection ports. W/O and O/W emulsion generation using PFPE-PEG-based 3D MED. To test parallel emulsion generation using the 100-FFG 3D MED, we use hexane and 2 wt% SDS aqueous solution as the oil and water phases of an oil-in-water (O/W) emulsion, respectively. For W/O emulsion generation, we use de-ionized water as the dispersed phase and a hexadecane solution with 2 wt% Span 80 as the continuous phase. For the formation of polystyrene microparticle, we 12 ACS Paragon Plus Environment

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generate the O/W emulsion using a polystyrene-dissolved hexane solution (10 wt% PS) as the dispersed phase and 2 wt% SDS aqueous solution as the continuous phase in a PFPE-PEG (9:1 ratio) based 3D MED. The O/W emulsions are collected in a container, and the solvent is allowed to evaporate to produce PS solid microparticles. The diameter of emulsion in the microfluidic channel (Dp) is measured using optical microscopy (Nikon Diaphot 300 Inverted Microscope) and analyzed using ImageJ. The polystyrene microparticles are imaged using a scanning electron microscope (SEM, JEOL 7500F HRSEM). Supporting Information. Water contact angle in hexane on pure PFPE, design and fabrication of PFPE-PEG network-based three-dimensional monolithic elastomer device. Funding Sources: GlaxoSmithKline, National Institute of Health (5-R21-AI-124057-02), American Cancer Society - CEOs Against Cancer - CA Division Research Scholar Grant, (RSG15-227-01-CSM), The Hartwell Foundation, National Science Foundation (1554200), National Research Foundation of Korea (2017R1D1A1B0303152) Acknowledgements This work was primarily supported by GlaxoSmithKline and NIH 5-R21-AI-124057-02. D. I. was also supported by an American Cancer Society - CEOs Against Cancer - CA Division Research Scholar Grant, (RSG-15-227-01-CSM), a grant from The Hartwell Foundation, and NSF 1554200. H.-H. J. was supported by Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(2017R1D1A1B0303152). ORCID: Daeyeon Lee (0000-0001-6679-290X), David Issadore (0000-0002-5461-8653)



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(26) Lee, S. S.; Kim, S. K.; Won, J. C.; Kim, Y. H.; Kim, S. H., Reconfigurable photonic capsules containing cholesteric liquid crystals with planar alignment. Angew. Chem. Int. Ed. 2015, 127, 15481-15485. (27) Kim, S.-H.; Park, J.-G.; Choi, T. M.; Manoharan, V. N.; Weitz, D. A., Osmotic-pressurecontrolled concentration of colloidal particles in thin-shelled capsules. Nat. Commun. 2014, 5, 3068. (28) Hribar, K. C.; Lee, M. H.; Lee, D.; Burdick, J. A., Enhanced Release of Small Molecules from Near-Infrared Light Responsive Polymer−Nanorod Composites. ACS Nano 2011, 5, 29482956. (29) Jeong, H. H.; Yadavali, S.; Issadore, D.; Lee, D., Liter-scale production of uniform gas bubbles via parallelization of flow-focusing generators. Lab Chip 2017, 17, 2667-2673. (30) Jeong, H.-H.; Issadore, D.; Lee, D., Recent developments in scale-up of microfluidic emulsion generation via parallelization. Korean J. Chem. Eng. 2016, 33, 1757-1766.


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Figure 1. Characterization of the wettability and solvent resistance of PFPE-PEG networks. (A) Chemical structure of PFPE, PEG, and Darocur 1173. (B) Dynamic change of hexane-in-water contact angle value as time evolution on the 10 wt% PEG/PFPE network surface. (C) Hexane-inwater contact angel as a function of the concentration of PEG-DA. Inset optical images show wetting of water droplet. (D) Swelling ratio (Vfinal/Vinitial) of PFPE-PEG network using various organic solvents and pure water. (E) Photograph of 200 µm-thick PFPE-PEG network slabs with different ratios of PFPE and PEGDA. The dimension of each slab is 75 mm ൈ 38 mm. The



University Commnunications Office of the University of Pennsylvania provided the Penn Logo and has given the permission for its use in this image.

Figure 2. Ratio of final and initial orifice widths (Wfinal/Winitial) after exposure to hexane. (A) Optical images for shrinkage of microchannels of a PDMS and 10 wt% PEG-PFPE device upon exposure to hexane. The scale bars are 200 µm. (B) Rapid decrease in the size of the orifice width is observed upon hexane exposure as seen in the sudden decrease in the relative orifice size (orifice opening after hexane exposure/initial orifice opening) after hexane exposure.


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Figure 3. Generation of W/O and O/W emulsions. Optical images for (A) parallel W/O emulsion generation using a PFPE-based 3D MED and (B) O/W emulsion generation using a 10 wt% PEG/PFPE 3D MED. The flow rates are Qw = 7 ml/hr and QO = 10 ml/hr for W/O emulsion and QO = 7 ml/hr and Qw = 5 ml/hr for O/W emulsion. Both 3D MEDs have 100 parallel flow focusing generators. Scale bar indicate 200 µm. (C) Optical image of O/W emulsion and (D) its size distribution. The average size and CV are 115.6 µm and 5.4 %, respectively. Scale bars indicate 150 µm.



Figure 4. Production of polystyrene microparticles from an oil-in-water emulsion. (A and B) SEM images of PS particles after solvent (hexane) evaporation. The scale bars represent 200 µm. (C) Size distribution of PS particles. The average size and CV are 27.6 µm and 5.7 %, respectively.


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TOC Figure


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