Fluorescent iron-oxide nanoparticles glow in endothelial cells in an experiment at Rice University. At left, the nanoparticles are evenly distributed among the microtubules that help give the cells their shape. At right, after a magnetic field is applied, the nanoparticles are pulled toward one end of the cells and change their shapes. The researchers believe this offers a way to make the endothelial barrier “leaky” enough to allow drug molecules to pass through to reach tissues.