Credit: © 2009 ACS

Polymer vesicles are the high-molecular-weight synthetic counterparts of naturally occurring liposomes and show many improved properties, such as higher stability and designable permeability. Polymer vesicles have potential uses in drug delivery, as nanoreactors and as biological models. For many of these applications, controlling the size of the vesicles is important; the size is set either during formation or after it by swelling or shrinking the vesicle.

Now, Adi Eisenberg and colleagues at McGill University in Montreal have synthesized1 a block-copolymer vesicle system with a pH-induced 'breathing' feature that can be cycled repeatedly. A triblock copolymer of polyethylene oxide, polystyrene, and poly(2-diethylaminoethyl methacrylate) was formed by atom-transfer radical polymerization. Vesicles were formed by self-assembly at pH 10.4, and as the pH was lowered to 3.4, the volume of the vesicles increased by 190% and the thickness of the wall increased. This change in volume and wall thickness was shown by dynamic light scattering to be highly reversible, with relaxation times of approximately one minute.

Cryo-transmission electron microscopy images of the vesicles at low pH showed the formation of cracks in the polystyrene layers. In this swollen state, the vesicles showed an increased permeability to water and protons, which may prove important for their future applications.