Fabrication of Polymersomes using Double-Emulsion Templates in Glass-Coated Stamped Microfluidic Devices

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Birgit Hausmann

Reference

J. Thiele et. al. "Fabrication of Polymersomes using Double-Emulsion Templates in Glass-Coated Stamped Microfluidic Devices", Small 6 (16), 1723–1727, (2010)

Keywords

double emulsions, glass-coatings, PDMS microfluidics, polymersomes

Overview

Since copolymer precipitates cannot be easily removed during emulsion generation a microfluidic design that combines the ability to form double emulsions with the ability to inject and mix two organic solvents is desirable. A convenient fabrication technique is soft lithography using poly(dimethylsiloxane) (PDMS) and depositing a glasslike coating using sol-gel chemistry can prevent the PDMS from swelling when it comes in contact with organic solvents. This paper presents double-emulsion-templated polymersomes in stamped microfluidic devices.

Results and Discussion

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Devices are evenly coated with a sol–gel to produce a durable glasslike layer with tailored surface properties which increases the resistance of the channel walls against organic solvents. Two streams of organic solvents can be injected to form the shell of the double emulsion in the presented device configuration which 1) prevents clogging and 2) enables controlling the ratio of the solvents having different volatilities and thus controlling the evaporation rate of the solvents. Double emulsions are produced in an array of three flow-focusing cross-junctions with different wettability. The first two junctions allow for injection of solvents enabling manipulation of the composition of the shell phase of double emulsions. Drops formed in the first two junctions enter the third junction where they are encapsulated to create double emulsions (Fig. 1). In this way formation of copolymer-stabilized double emulsions is guaranteed. Diblock copolymers are dissolved in the organic solvent stream injected at the first junction. The second organic solvent injected at the second crossjunction is miscible with the copolymer-loaded solvent. Scanning electron micrographs of the glass coated microchannels are shown in Figure 2. The glass coating provides also the ability to spatially control the wettability of the surface by functionalizing the intrinsically hydrophobic sol–gel with photoreactive silanes and subsequent conversion of the surface chemistry to hydrophilicity via using a photochemical surface treatment. The first and second cross-junctions are made hydrophobic while the third junction is made hydrophilic; this allows water drops to be dispersed in a continuous phase of organic solvents at the first and second junctions, while the continuous water phase required for the double emulsion is injected at the third, hydrophilic junction. Poly(ethylene-glycol)-<math>b</math>-poly(lactid acid),PEG<math>_{5000}-b</math>-PLA<math>_{5000}</math>, polymer vesicles are formed in this way. Diblock copolymers are first dissolved in the organic solvent chloroform (first junction). In order to lower the density of the organic phase, toluene is added to the copolymer containing chloroform as a second organic solvent. Balancing osmolarities of the inner and outer phases of the double-emulsion template is guaranteed via adding glucose to the inner phase and polyvinyl alcohol (PVA) to the outer phase of the double emulsion. Thus, toluene is added at the second droplet making junction while PVA is used for emulsifying the organic solvent phase that contains aqueous inner drops at the third junction.The three phases are fed into the device shown in Figure 2a.

Since sol–gel coatings often consist of a nanoporous structure chloroform and toluene can overcome the sol–gel barrier into the PDMS. But chloroform evaporates faster, resulting in a lower chloroform fraction in the solvent mixture. As the solubility of PEG-<math>b</math>-PLA in toluene is significantly lower than in chloroform, the diblock copolymer forms precipitates after the more volatile chloroform starts to evaporate in the microfluidic device which leads to a buildup of a thick layer of copolymers on the channel walls (Fig. 2b) and can even result in destabilization of the double-emulsion drops (Fig. 2c). Even in this new device geometry, where chloroform with PEG-b-PLA and toluene are injected separately precipitation of the copolymer can still take place at the chloroform/toluene interface if sufficient diffusive mixing is allowed. Thus, elevated flow rates are used, and the microchannel between the second nozzle, and the third cross-junction is shortened. Optimal flow rates were 1000<math> \mathrm{\mu L/h}</math> for toluene and 500<math> \mathrm{\mu L/h}</math> for chloroform . During solvent evaporation, the PEG-b-PLA-stabilized double emulsions undergo a dewetting transition as the polymersomes are formed. The organic solvent mixture initially wets the entire inner drop and is homogenously distributed on its surface, as shown in Figure 3a,b; it then dewets from the inner phase, as indicated in Figure 3c. The dewetting is driven by evaporation of the volatile organic solvents as well as by the relative high surface energy between the inner and outer phase.[33] The result is a state of partial wetting where the double emulsions adopt an acornlike, asymmetric structure. However, if the volumetric ratio of toluene and chloroform in the initial double emulsions is between 1:1 and 2:1, stable double emulsions are formed, but the drops do not undergo dewetting. If the shell of the double emulsions contains an excess of chloroform, the double emulsions are destabilized due to the density mismatch of the inner drop and surrounding shell. With the optimized volumetric ratio of toluene and chloroform, the diblock copolymer molecules at the two interfaces of the shell self-assemble into a membrane, enclosing the inner phase. Upon dewetting, the bulb of the acornlike dewetted drop which contains the excess diblock copolymer and homo polymer, remains on the surface of the polymersome. After evaporation of the organic solvents, a polymeric aggregate of these polymers remains attached to the surface of the polymersomes, as shown in Figure 3d. Occasionally, the aggregate detaches from the polymersome, as shown in Figure 3e. Since the volume of the inner drop remains unchanged during the dewetting transition, the polymersome size is only determined by the droplet size of the most inner fluid of the double-emulsion template, which can be controlled by tuning the dimension of the nozzle and theflow rate ratio of inner and middle phase.[19,34] With our microfluidic device we are able to form double-emulsion templates of approximately 100–150mm in diameter, corresponding with a polymersome diameter of approximately 50–100 mm. However, the principles of polymersome formation should be applicable down to the smallest scale as limited by the feature size of the microfluidic device. Our new geometry in stamped microfluidic devices allows us to form polymersomes from copolymer-stabilized W/O/W double emulsions. In contrast to the limited flexibility using two cross-junctions for fabricating double emulsions, our modified microfluidic device enables independent injection and mixing of two organic solvents, which form the double-emulsion shell. This is useful for maintaining the ratios of the solvents specific to the diblock copolymers used, and prevents fouling of the channel walls which would cause instantaneous failure of the device. The control over the solvent mixture is important for ensuring continuous operation of the device, and for applying the double-emulsion approach for polymersomes to a wider range of polymers. As the solvent streams do not mix before emulsification, our modified device also enables the preparation of other core–shell structures from rapidly reacting solvent streams. Our approach should also be useful for forming Januslike particles with freely tunable composition by using two curable monomer streams which can be solidified during emulsification. In addition, the ease of fabrication of stamped PDMS microfluidic devices should facilitate fabrication of highly parallelized devices for larger-scale production of polymersomes.