Takasi
Nisisako
*,
Shingo
Okushima
and
Toru
Torii
Department of Precision Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan. E-mail: nisisako@intellect.pe.u-tokyo.ac.jp; Fax: 81-3-5841-6072; Tel: 81-3-5841-6072
First published on 15th April 2005
This paper gives an overview of our recent work on the use of microfluidic devices to formulate double emulsions. Key issues in the controlled encapsulation of highly monodisperse drops include: (a) regular periodicity in the formation of micro droplets due to the interplay between viscous shearing and interfacial tension in low Reynolds number streams; (b) serially connected hydrophobic and hydrophilic microchannels to form aqueous and organic drops consecutively. Water-in-oil-in-water emulsions and oil-in-water-in-oil emulsions can both be produced by reversing the order of hydrophobic and hydrophilic junctions. Alternating formation of aqueous droplets at a cross junction has enabled the production of organic droplets that encase two aqueous droplets of differing compositions.
Takasi Nisisako | Takasi Nisisako received his B.Eng. degree in 2000, M.Eng. degree in 2002 and Ph.D. degree in 2005 from the University of Tokyo, Japan. He is currently working as a post-doctoral researcher in the Department of Precision Engineering, Graduate School of Engineering, the University of Tokyo, Japan. His research interests include multiple-phase microfluidics, synthesis of complex colloids, micro-reactors, electronic papers and their industrial applications. |
Shingo Okushima | Shingo Okushima was born in Tokyo, Japan, in 1978. He received his B.Eng. degree in 2003 and M.Eng. degree in 2005, both from the University of Tokyo. He is currently working at Canon Inc., Japan. His research interests include bio-MEMS, microfluidic systems and other MEMS applications. |
Toru Torii | Toru Torii received a B.S. from the School of Agriculture of the University of Tokyo in 1980, and a M.S. from the School of Engineering, in 1982. From 1982 to 1984, he was a member of the technical staff of Mitsubishi Motors Corporation. From 1984 to 1989, he was an assistant professor in Tohoku Institute of Technology. From 1989 to 1999, he was an assistant professor with the Department of Agricultural Engineering in the University of Tokyo. In 1992, he received a Ph. D. from the University of Tokyo. Since 1999, he has been an associate professor with the Department of Precision Engineering, the University of Tokyo. His current research interests include microfluidic devices and lab on a chip devices. |
Various industries, including cosmetics,3 foods4 and pharmaceuticals,5 are showing increasing interest in these systems, because of their potential for the controlled release of chemical substances initially entrapped in the internal phase. Major applications studied to date include pharmaceuticals using W/O/W emulsions, which act as a carrier of water-soluble drugs for sustained release or targetable delivery.6–9 Biodegradable solid capsules loading bioactive agents such as hormones,10 peptides11 and oligonucleotides12 have also been prepared for medical use. Other applications of double emulsions include the synthesis of composite microspheres by suspension polymerization,13 and the separation of hydrocarbons,14 metals15 and organic acids16 by a liquid membrane of the intermediate phase.
The two-step emulsification process17 is now widely used in the practical production of double emulsions. For W1/O/W2 emulsions, the primary W1/O emulsion is prepared under vigorous stirring or sonication. This dispersion is then poured into the intended external aqueous phase (W2), and secondary mixing is conducted at lower shear to prevent disruption of the prepared capsules. Problems associated with this method include a broader size distribution, poor controllability and reproducibility of the droplet size, and low entrapment yield. All are due to the turbulent shear occurring in the process. Recently the membrane-emulsification technique18,19 has been applied to formulate monodisperse double emulsions by forcing single emulsions through porous media20 or microfabricated combs.21,22 This technique can produce double emulsions with narrower size distribution and higher entrapment yield, but it is still difficult to achieve good control over the internal droplet size, external droplet size and the number of droplets contained in each capsule. Moreover, the techniques stated usually consist of two separate batch processes, limiting the production efficiency. Other techniques such as phase inversion23,24 and capillary jet methods25,26 share the limitations described above.
Here, we present a new method for preparing double emulsions using microfluidic systems that have already shown promise in creating micro droplets of controlled sizes.27–34Fig. 1 shows our method for producing W/O/W emulsions.35 Aqueous droplets of uniform size are formed reproducibly within an organic flow at the upstream T-junction. This stream then flows into the second T-junction, to create monodisperse organic droplets containing aqueous droplets within an external aqueous phase. The size and breakup rate of the droplets can be flexibly controlled by varying the flow conditions at each junction. The number of encased droplets can be accurately determined by adjusting the relation between the breakup rates at the two junctions. Similarly, O/W/O emulsions can be produced when the hydrophobic junction is prepared downstream.
Fig. 1 (a) Schematic diagram of the formation of a W/O/W double emulsion; (b) one-chip module (left) and two-chip module (right). Win: internal aqueous phase. O: intermediate organic phase. Wex: external aqueous phase. |
Two configurations are proposed for the production of double emulsions: a one-chip module in which the hydrophobic junction and the hydrophilic junction are on the same plate, and a two-chip module in which these junctions are on separate chips (Fig. 1b). In a one-chip module, one of the two junctions was made hydrophobic by introducing a reagent for surface modification, whereas in a two-chip module the entire channels were modified in either of the two chips. Localized surface modification in a one-chip module for W/O/W emulsions was conducted as follows. A water-soluble siliconizing fluid was diluted with deionized water, and this solution was introduced into the microfluidic channel to make the entire internal surface hydrophobic. Sodium hydroxide solution (2 mol l−1) was then infused into the channel from the inlet for the external aqueous phase in order to remove the hydrophobic layer, so that the hydrophilic surface is produced around the downstream junction. During this process, immiscible organic fluid was infused from the inlets for the internal or intermediate phase, which filled up the channels around the upstream junction. This prevents the sodium hydroxide solution from flowing back into the intended hydrophobic upstream junction.
Fig. 2 (a) Photomicrograph of droplet formation at two consecutive T-junctions. The breakup rate at each junction is ∼22 drops s−1. (b, c) The generated W/O/W droplets observed in the microfluidic device. (d) Size distribution of internal aqueous droplets and external organic droplets. The scale bars are (a) 100 µm, (b, c) 50 µm. Win: deionized water, O: corn oil containing lipophilic surfactant, Wex: aqueous solution of hydrophilic surfactant. |
The internal and external drop sizes and the number of encased drops were controlled by varying the flow rates of the individual streams. As an example, Fig. 3 shows the variation of the external drop size and the number of enclosed drops as the flow rate of the external phase varies. As the flow rate of the external phase is increased, the size of the external drops decreased from 210 to 150 µm. This reduction implies an increase in the breakup rate at the second junction, reducing the number of internal drops (from 4.3 to 1.4). Similarly, the streams of the internal phase and the intermediate phase can be used to determine the drop-in-drop structure.
Fig. 3 Effect of the flow conditions of the external aqueous phase on the formation of W/O/W emulsion. (a) Variation of the internal drop size, external drop size and the number of enclosed drops. (b) W/O/W droplets formed in condition (1) in the graph. (c) W/O/W droplets formed in condition (2). The scale bars are 50 µm. Win: deionized water, O: corn oil containing lipophilic surfactant, Wex: aqueous solution of hydrophilic surfactant. |
Coalescence of the resulting capsules was prevented by surfactants added to the intermediate phase and the external phase. Fig. 4 shows various W/O/W emulsions in which the number of internal drops was precisely controlled. All of these dispersions, including non-spherical ones (Fig. 4b), were perfectly stable at room temperature (20 ± 1 °C) for at least one month. A study of stability over longer timescales is under way.
Fig. 4 Stable W/O/W emulsions with a controlled number of internal droplets (n). (a) n = 1. (b) n = 2. (c) n = 4, (d) n = 8. All are stored at room temperature (20 °C). The scale bars are 100 µm. Win: deionized water, O: decane containing lipophilic surfactant, Wex: aqueous solution of hydrophilic surfactant. |
We further developed the method to formulate capsules that contain multiple internal droplets with differing compositions. Zheng et al. used a cross-shaped channel to produce an array of droplets of alternating composition suitable for droplet-based bioassays.38,39 They characterized the fluid condition for reliable formation of stable alternating droplets by the capillary number (Ca): Ca = Uη/γ, where U is the velocity of the continuous phase stream, γ is the interfacial tension between two immiscible liquids, and η is the viscosity of the continuous phase.39 The optimal Ca range for successful alternating droplet formation was found to be 0.001 < Ca < 0.15. We arranged a hydrophobic cross-shaped geometry as the upstream junction so as to generate the array of to-be-enclosed aqueous droplets from differing sources (Fig. 5a). Red and blue aqueous drops of similar size were formed alternately when the organic stream was in the right flow condition (Ca ∼ 0.08), and the arrayed droplets flew into the second junction to be jointly packed into organic droplets (Fig. 5b,c). The droplet size and breakup rate can be varied via the flow conditions, so that multiple droplets supplied from different sources can be encapsulated with the desired volume fractions.
Fig. 5 (a) Schematic diagram of the alternating formation of aqueous droplets at the upstream junction and subsequent encapsulation at the downstream junction. (b) Photograph of formation of an oil droplet enclosing blue and red droplets. (c) Photograph of the prepared W/O/W droplets. The diameter of the external drop is 175 µm. The scale bars are (b) 100 µm, (c) 50 µm. Win: deionized water, O: corn oil containing lipophilic surfactant, Wex: aqueous solution of hydrophilic surfactant. |
An important issue in the formulation of double emulsions is the size of the external droplet. So far we have achieved droplet sizes in the range 40–200 µm, similar to the cross-section dimensions of the channels used. With smaller microchannels, capsules as small as 10 µm might be formulated, significantly widening the area of application; examples include model systems for cell-like environments40 and drug carriers for intravascular use. Such small channels have drawbacks, however. They are more susceptible to blockages, so that the raw materials to formulate double emulsions might be limited. Greater flow resistance would make it more difficult to infuse viscous fluids. Optimization of the channel geometry—the aspect ratio and channel length—is then vital. A further critical issue is the productivity scale-up needed for practical industrial applications. Some 103–104 channels would be needed for a production rate as high as 1 kg h−1, since the maximum throughput per unit is presently ∼1 g h−1. Although such large-scale integration of microchannels can be realized by conventional lithographic techniques, it remains unclear whether fluid can be supplied equally into every channel so as to produce uniform-sized droplets in parallel. This issue must be investigated together with the construction of piled up systems of multiple modules for further scale-up. In view of the parallelization of many channels that is involved, the two-chip module mentioned above is better suited, because localized surface modification as in the one-chip module is not necessary.
The methodology described here could greatly increase the range of applications of double emulsions. Droplets of defined sizes and well-controlled entrapment yield can act as ideal model systems to give a better understanding of release properties and stability conditions. Polymerization of ellipsoidal double-emulsion droplets might be able to produce monodisperse non-spherical particles. The close packing of internal drops may be valuable for structuring three-dimensional colloidal assemblies having spherical shape.41,42 Furthermore, organic drops containing multiple aqueous drops of differing compositions may be utilized in multiplexed assays for screening applications.43
This journal is © The Royal Society of Chemistry 2005 |