Vesicles are hollow spheres, so small that one thousand of them placed side by side would be as thick as a human hair. These microscopic structures are prepared from synthetic polymer chains, and consist of a thin "skin" of hydrophobic (water-repelling) material, out of which emanate cilia or hair-like structures of a hydrophilic (water-loving) material. Similar structures can also be prepared from naturally occurring small-molecule phospholipids, in which case they are called liposomes.
Since most polymeric systems studied to date have functioned primarily in the delivery of water insoluble drugs, work on vesicles presents a new opportunity in that they can be used to deliver hydrophilic materials to the body. According to Dr. Eisenberg, "block copolymer vehicles are much more robust than liposomes". He and his colleagues have recently succeeded in demonstrating the feasibility of attaching one type of material to the interior surface of the vesicle, with another type of material attached to the exterior surface. A three-way delivery system could be created wherein a targeting species is attached to the exterior of the vehicle, a water-soluble drug is suspended in the interior cavity and incorporated into the wall are water-repellent materials. This means that vesicles and their attachments could be used to deliver both water-soluble and water-repellent medicinal agents, with different species of drugs or targeting moieties associated with the interior or exterior surfaces of a vesicle and where each type of drug is released in the right proportion.
Currently, the simultaneous release of different materials is not easy in a pharmacological context and the preparation of multifaceted vesicles would yield a single delivery vehicle that could deliver the active agent to the specific part of the body where it is needed. Toxic side effects might thus be minimized.
In work previously published by Science, and widely cited in the scientific literature, Dr. Eisenberg and his colleagues have also shown that block polymers are capable of self-assembly into various nanosized structures including small spheres, rods, platelets and tubules - examples can be viewed at the morphology gallery: http://bravo436.