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
In biology, the cell utilizes a number of structures to encapsulate aqueous solutions. For example, liposomes and vesicles can encapsulate proteins and other cargo, as well isolate the surrounding environment from deleterious internal conditions such as low pH. Such methods are effective in biology in allowing enzymes to function at varied conditions without impacting the rest of the cell, as well as providing a means to transport cargoes within and outside the cell.
The artificial leveraging of such structures for bioengineering uses holds great promise in a number of areas. For instance, polymersomes could be used for drug delivery, intra-cell bioreactors for enzymatic reactions, and could be used in artificial cells. one of the main issues, however, with developing artificial vesicles is that there is very little control over vesicle formation, as the process is mainly mediated by self-assembly, which is poorly understood. One alternative method has involved a stream of polymer containing suspension directed at the cargo of choice, but such a method has showed poor efficacy due to poor control over all of the polymer in the stream.
In the past, water-in-oil emulsions have been used to aggregate emulsions, but an approach involving water-in-oil emulsions formed via capillary microfluidics is a promising technique to create polymersomes with a high degree of control. The key to this technique is choosing different solvents, such as the disparity between solvation drives the attraction of the two phases of the double-emulsion.
Poly(ethylene glycol)-b-poly(lactic acid) [PEG(5000)-b-PLA(5000)] was chosen as the copolymer that would ultimately form the polymersome membrane. It was dissolved into a mixture of chloroform and hexane, and a double-emulsion was created via a previous method in a glass capillary microfluidic device. In this setup, the PEG is in the middle-phase while the PLA is in the solvent-phase. A relation yielding a negative spreading coefficient is given as follows:
A negative spreading coefficient implies that there is an attractive force between the outer-middle and middle-inner phases, thus yielding the micelle. Due to the difference in solubility involving hexane and chloroform and the negative spreading coefficient, it becomes energetically favorable for dewetting to take place as shown in Figure 1:
In other words, the dewetting and subsequent attraction of the two phases is partially driven by the diffusion of chloroform out of the polymersome. The resulting degree of attraction between the two phases was quantified by measuring the contact angle between the two, given in Figure 2:
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.