Difference between revisions of "All-aqueous core-shell droplets produced in a microfluidic device"
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Revision as of 15:24, 29 September 2011
Entry by Max Darnell, AP 225, Fall 2011
Title: All-aqueous core-shell droplets produced in a microfluidic device
Authors: Iwona Ziemecka, Volkert van Steijn,* Ger J. M. Koper, Michiel T. Kreutzer and Jan H. van Esch
Journal: Soft Matter, 2011, Advance Article DOI: 10.1039/C1SM06517C
A current trend in bioengineering is the delivery and encapsulation of different cargoes for biological use. Relevant examples would include delivering drugs, or synthetic intracellular compartments. Although there are many technologies that address this issue, such as polymersomes or liposomes, the aqueous two-phase system (ATPS) is a simple, biocompatible model. In an ATPS, there exist two regions of water, one inside the other. Each layer contains a different solute, and the solute properties dictate the separation between these two phases. One key advantage of an ATPS from a bio-application perspective is that the structure allows for small molecules to freely diffuse between the phases, distinguishing the ATPS from membranous compartments such as liposomes.
Microfluidics is a valuable tool that can be leveraged to produce such structures, but up to this point, an oil phase or organic solvents were always used. This paper examines a microfluidic method to create ATPSs using only water and the solute.
PEG and dextran were chosen as the solutes for a proof-of-concept. A microfluidic device was fabricated using standard methods, and is given in Figure 1. To create the double emulsions, a stream of PEG solution was combined with a stream of dextran solution. If this were to occur simply by merging to streams, the streams would flow in parallel because of the low Reynold's number. A key feature of the device architecture is a piezoelectric disc, which perturbs one of the flows in a pulsed fashion, allowing the to streams to form droplets. As seen in the figure, the first junction results in mixing, the second results in a sheathing by dextran, and the third acts to sever the fluid connection between successive droplets.
The authors then quantified the volume and diameter of the ATPSs as to better understand and control their morphology. The volume is given by <math>V=q/f</math>, where q is the sum of all input flows, and f is the frequency of perturbation. This relation yields that the diameter of the ATPS is given by <math>(6q/f\pi)^1/3</math>.
Figure 2a) shows the device without the perturbation to cause droplet formation, while Figure 2b) shows the relationship between the model for droplet volume and experimental results.
Finally, the authors tested the practical application of such an approach, in this case encapsulating a fluorescent dye in one of the polymersomes created using such a method. Figure 3 shows the dyes encapsulated by the polymersomes.
Connection to Soft Matter
Wetting is one of the most important phenomena in soft matter physics. This paper is a great example of how wetting can be coupled to diffusion to achieve a desired result. It is also significant in that it shows how the phenomena of wetting is not confined only to hard surfaces and liquids, but also applies at smaller length scales. It would be interesting to see such coupling in other areas, between wetting and elasticity and other mechanical properties, for instance.