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 researchers injected particles 100 nanometers across coated with either PEG (green) or negatively charged water-hating molecules (red) into the brain of a living mouse, the PEG particles easily penetrated the brain while the negatively charged particles got stuck. Larger nanoparticles would give doctors a more effective way to deliver drugs for brain cancers, strokes and other brain diseases, the team reports in the Aug. 29 Science Translational Medicine.