On a crisp fall evening in 2006, Dr. Sylvain Martel held his breath as a technician slipped an anesthetized pig into a whirling fMRI machine. His eyes stared intently at a computer screen, which showed a magnetic bead hovering inside the pig’s delicate blood vessels. The tension in the room was palpable. Suddenly, the bead jumped to life, hopping effortlessly down the vessel like a microsubmarine heading to its next target destination. The team erupted in cheers. Martel and his team were testing a new way to remotely steer tiny objects inside a living animal by manipulating the magnetic forces of the machine. And for the first time, it worked.