All-aqueous core-shell droplets produced in a microfluidic device
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>d=(6q/f\pi)^(1/3)</math>. To verify this relation, the diameter of the droplets were measured for different flow rates.
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.
Key to this work is the notion that the ATPSs will form two phases a period of time after mixing. Figure 3 shows the formation of distinct phases over a period of time, as the ATPS travels down the microfluidic channel.
Connection to Soft Matter
The problem of excluding oil from a double emulsion is an interesting one. Over the course of phase formation, the authors noticed that the phases grow faster than in the case of Ostwald ripening, which is a case in which small solutes dissolve and recrystalize on larger crystals. Ostwald ripening, however, is more commonly used to describe emulsions involving an oil phase. The homogeneity of the solvent (water) in this case may provide an interesting case of double emulsion physics in which to study the effects of the solvent.