Getting drugs into the brain has proved to be a nanoscale puzzle: Anything bigger than 64 nanometers — about the size of a small virus — gets stuck in the space between brain cells once it gets through the blood-brain barrier. Justin Hanes of Johns Hopkins University School of Medicine and colleagues got around this rule by coating particles destined for brain cells in a dense layer of a polymer called polyethylene glycol. PEG acts like a Teflon coating for the particles, preventing them from sticking to structures within the brain and allowing them to move around more freely. When the re... (p. 10)