Multicompartment polymersomes from double emulsions.

Polymersomes are vesicles which consist of compartments surrounded by membrane walls that are composed of lamellae of block copolymers; these are important for numerous applications in encapsulation and delivery of active ingredients such as food additives, drugs, fragrances and enzymes [2] . Polymersomes are typically prepared by precipitating block copolymers from their solvents through addition of a poor solvent for the copolymers, or by rehydrating a dried film of the copolymers. The unfavorable interactions between blocks in the copolymer and the poor solvent induce formation of aggregate structures ranging from micelles, wormlike micelles and vesicles. However, the resultant polymersomes are highly polydisperse and have poor encapsulation efficiency. Recently, a new approach has been developed to fabricate monodisperse polymersomes by using double emulsions as templates. Water-in-oil-in-water (W/O/W) double emulsions with a core-shell structure are first prepared in capillary microfluidic devices. Diblock copolymers, dissolved in the oil shell phase, assemble onto the walls of the polymersomes upon removal of the oil by evaporation 7] after adhesion of the diblock copolymeradsorbed interfaces. This approach leads to polymersomes with high size uniformity and excellent encapsulation efficiency; it also enables precise tuning of the polymersome structures. Advances in techniques for fabricating polymersomes have led to controlled spherical polymersomes with a single compartment. However, non-spherical capsules with multiple compartments also have great potential for encapsulation and delivery applications. By storing incompatible actives or functional components separately, polymersomes with multiple compartments can achieve encapsulation of multiple actives in single capsules and reduce the risk of cross contamination. Moreover, multiple reactants can be encapsulated separately to allow reaction upon triggering. By tuning the number of compartments containing each reactant, the stoichiometric ratio of the reactants for each reaction can be manipulated. These multi-compartment polymersomes will create new opportunities to deliver not only multiple functional components, but also multiple reactants for reactions on demand. In addition, with the versatility of synthetic polymer chemistry to tune properties such as polymer length, biocompatibility, functionality and degradation rates, non-spherical polymersomes with multiple compartments can be tailored for specific delivery targets. However, polymersomes that have been reported to date are almost exclusively spherical in shape, and have only one compartment; since most conventional polymersome fabrication processes rely on self-assembly of the block copolymer lamellae, little control over the size and structure of the resultant polymersomes is achieved. With the conventional emulsion-based methods, non-spherical droplets are also not favored because interfacial tension between the two immiscible phases favors spherical droplets, which have the smallest surface area for a given volume. Recent advances in microfluidic technologies enable high degree of control in droplet generation, and ease in tuning the device geometry; this offer a new opportunity to fabricate double emulsion with controlled morphology, which serve as templates for fabricating the nonspherical multi-compartment polymersomes. However, such investigations have not, as yet, been carried out. In this work, we demonstrate the generation of non-spherical polymersomes with multiple compartments. We use glass capillary microfluidics to prepare W/O/W double emulsions with different number of inner aqueous drops. These emulsions are initially stabilized by the amphiphilic diblock copolymers in the oil shells, which consist of a mixture of a volatile good solvent and a less volatile poor solvent for the copolymers. As the good solvent evaporates, the copolymers at the W/O and the O/W interfaces are attracted towards each other to form the membranes. As a result, neighboring inner droplets adhere to one another; this leads to formation of multi-compartment polymersomes, as schematically illustrated in Scheme 1. We also use a modified glass capillary device for generating double emulsions with two distinct inner phases containing different encapsulants; this process leads to the fabrication of non-spherical polymersomes with multiple compartments for separate encapsulation of multiple actives. A glass capillary microfluidic device is used to generate double emulsions with controlled morphology. (See Fig. S1 in Supporting Information) Due to the high degree of control afforded by microfluidics, the number of inner droplets in a W/O/W double emulsion system can be controlled by varying the flow rates of the three phases independently; 12] an example of the process is shown in Fig. 1A. The thickness of the double emulsion shells can be adjusted by changing the flow rates; however, as long as the flow rates are not altered enough to change the number of inner droplets of the double emulsion templates, change in shell thickness does not affect the morphology of the final polymersomes since all solvents in the shells is removed in subsequent steps. To prepare the double emulsion templates, multiple inner droplets are dispersed in drops of a mixture of chloroform and hexanes (36:65 v/v) with 10 mg/mL poly(ethylene-glycol)-b-poly(lactic acid), (PEG(5000)-bPLA(5000)); the drops-in-drops are suspended and stabilized in a poly(vinyl alcohol) (PVA) solution. A homopolymer, PEG, is added [] Prof. D. A. Weitz, Dr. H. C. Shum, Dr. S. H. Kim School of Engineering and Applied Sciences, Department of Physics and Kavli Institute for Bionano Science and Technology, Harvard University, Cambridge, Massachusetts 02138 (USA) Fax: (+1) 617-495-0426 E-mail: weitz@seas.harvard.edu Homepage: http://www.seas.harvard.edu/projects/weitzlab/