Cancer drugs need to be powerfully toxic to kill tumor cells. But they also can kill healthy cells, sometimes with brutal side effects. Now, scientists have designed a way to seal cancer drugs inside tiny capsules so the drugs won't harm the healthy cells while traveling through the bloodstream. They hold that medicine securely until they reach a tumor and a remote control 'switch” finally triggers the drug's release. Smaller than bacteria, the capsules are called nanoparticles because their size is measured in nanometers. A magnetic field is the invisible force generated by a magnet. Researchers use a magnetic field to work as that remote control switch. Focusing that field on the cancer site ensures that the medicine is released only where it's needed. The nanoparticles don't seek tumors out. They do, however, tend to collect at tumor sites. Tumors tend to grow so fast that the blood vessels inside them can't keep up. This causes holes to form in the blood vessels. For a nano-package carrying the medicine, those leaky spots become a doorway from the bloodstream into the tumor. The nanoparticles slip in through those leaks, then accumulate in the tumor. Nanoparticles also can pile up in unwanted places. One such unhelpful collection point is the liver. This organ acts as a filter, snagging poisons out of the blood. It will also net some nanoparticles. Those caught in the liver could damage that organ if they shed too much of an anti-cancer drug. For many years, researchers have studied how to make nanoparticles that won't drop their drug cargo at such unwanted sites. Sometimes they relied on a chemical trait of the tumor - or the enzymes it produces - to unlock the particles. But not all cancers have the same chemistry. So the medicine might still leak out to poison cells outside the tumor. The new innovation by Rinaldi's team is the creation of a nanoparticle that won't release its medicine anywhere until it gets very warm. And that warming occurs when the particle is exposed to a magnetic field.