The Pentagon is attempting what was, until recently, an impossible technological feat—developing a high-bandwidth neural interface that would allow people to beam data from their minds to external devices and back.

That's right—a brain modem. One that could allow a soldier to, for example, control a drone with his mind.

This seemingly unlikely piece of technology has just gotten a lot less unlikely. On Feb. 8, the Defense Advanced Research Projects Agency (DARPA)—the U.S. military's fringe-science wing—announced the first successful tests, on animal subjects, of a tiny sensor that travels through blood vessels, lodges in the brain and records neural activity.

The so-called "stentrode," a combination stent and electrode, is the size of a paperclip and flexible. The tiny, injectable machine—the invention of neurologist Tom Oxley and his team at the University of Melbourne in Australia—could help researchers solve one of the most vexing problems with the brain modem: how to insert a transmitter into the brain without also drilling a hole in the user's head, a risky procedure under any circumstances.