Multicompartment Polymersomes from Double Emulsions

Entry by Pichet Adstamongkonkul, AP 225, Fall 2011

Reference:

Title: Multicompartment Polymersomes from Double Emulsions

Authors: Ho Cheung Shum, Yuan-jin Zhao, Shin-Hyun Kim, and David A. Weitz

Journal: Angewandte Chemie International Edition, 2011, Vol. 50, No. 7

Keyword: double emulsion, polymersome, microfluidics

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Summary

Conventional polymersomes are usually spherical in shape and consist of single compartment. This paper presented an alternative method, in which polymersomes with multiple compartments can be fabricated via water/oil/water double emulsions and the help of microfluidics technology. The polymer, PEG(5000)-b-PLA(5000), was used to form emulsion in the mixture solution of chloroform and hexanes, and stabilized with PVA solution. Upon removal of solvent in the oil shell, the water/oil and oil/water interfaces come together, forming a membrane and causing the neighboring inner droplets to adhere to one another and form multicompartment polymersomes. In addition, many aspects of the polymersomes, such as the membrane thickness and the number of compartments, can be controlled and fine-tuned by changing the flow rate of the solution from the microfluidic channels. The authors further investigated the possibility of incorporating two different encapsulants, one with a fluorescent-tagged dextran solution and another with PEG solution, into these newly developed polymersomes. The experiment confirmed the formation of multicompartment polymersomes with the two encapsulants segregated in different compartments. This technique is proven useful in many applications, including delivery and controlled reactions.

Background

Polymersomes are typically prepared by precipitating block copolymers from the solvents via the addition of a poor solvent, or rehydrating a dried film of the copolymers. For the conventional emulsion methods, the non-spherical droplets are not favored energetically, since the interfacial tension between two immiscible phases favors spherical geometry that have the smallest surface area per volume (lowest surface energy). With this current approach, multicompartment polymersomes could be generated.

Method

The glass capillary microfluidics were used to prepare the emulsion. Multiple inner droplets were made as drops of the mixture of chloroform and hexanes and the polymer, PEG(5000)-b-PLA(5000). Afterwards, the so-called "drops-in-drops" were stabilized by PVA solution. Furthermore, PEG was added to the inner droplet in order to balance the osmolalities to prevent net water diffusion across phases. The polymer adsorbed to the W/O and O/W interfaces. Its composition facilitates the dewetting of the emulsion, induced by adhesion of copolymer. When the chloroform evaporated, the polymer became less soluble and the interfaces became adhesive, eventually leading to dewetting, as the droplets stuck together and expelled solvent in the shell layer.

In the second part of the experiments, a microcapillary device consisting with a round capillary and two separate microchannels was used to inject two different solutions into separate compartments of the emulsion. The two encapsulants area fluorescein isothiocyanate-dextran solution and PEG solution. Again, the molalities of the two phases were maintained equal. Finally, the emulsion was observed under fluorescence and optical microscope.

Results and Discussion

There are several parameters that can be modulated and are suggested in this paper.

• Number of inner droplets in a W/O/W double emulsion
  • Controlled by varying the flow rates of the three phases in a microfluidic device independently.
• Thickness of the double emulsion shells
  • Adjusted by changing the flow rates, as long as the rates do not jeopardize the number of inner droplets, which implies that the shell thickness would not affect the morphology of the polymersomes (all solvents will be removed later in the process).
• Number of compartments
  • Fixed by the number of inner droplets
• Sizes of compartments
  • Controlled by the sizes of the inner droplets
• Total membrane area
  • Set by the total interfacial area of all the inner droplets
• Shape of the polymersomes
  • Dictated by the contact angle between inner droplets during dewetting, which is determined by the strength of the adhesion between the copolymer monolayer.

The spatial configuration of the compartments is not unique, and thus a few factors can be attributed to that. The authors speculated that, when sufficient amount of chloroform was removed, the reduced solubility of the diblock copolymers causes them to aggregate, which suggested that the process is kinetically driven and does not allow inner droplets rearrangement in the subsequent transition step. Secondly, the inner droplets may themselves have different relative orientations. However, there is no restriction on the number of compartments there can be in one polymersome.

Applications

First of all, non-spherical, multicompartment capsules have potential for encapsulation and delivery applications. They can be used to stored incompatible chemicals or solutions or functional components separately. This would reduce the risk of cross-contamination and allow multiple reactants to react upon triggering the release. The associated stoichiometric ratio can be manipulated by changing the number of compartments within a capsule. The polymer length, biocompatibility, functionality and degradation rates can be tailored for specific problems. The region between the two polymer interfaces can provide a hydrophobic environment suitable for the encapsulation of hydrophobic compounds within the membrane of these polymersomes.

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