Making tiny machines that can move through blood and other fluids that have inconsistent consistencies, or viscosities, is tough. Now scientists say that their “micro-scallop” is up to the task. The device has two shells attached at a single hinge. Magnets control how quickly the shells flap open and shut. Depending on those speeds, the viscocity of the fluid between the shells can change, propelling the 300-micrometer-wide scallop forward. The design could pave the way for similar swimmers to travel through blood and tissue and deliver therapeutic drugs to targeted areas, researchers report November 4 in Nature Communications.