A new implant technology can steer a body's nervous system from the outside, a creepy leap that started as a battlefield weapon.

• "Smart dust" — tiny, dust-sized sensors — was first dreamed up so drones could scatter them to spy on battlefields.
• Berkeley researchers built on it with "stim dust," a tiny implant that talks to the brain and nerves using ultrasound.
• In a live rat, the implant let researchers control the animal's back-leg movement.
• The worry: technology built to watch people could one day be used to control them.

Outlook: The tech is still early and animal-only, but expect more research — and growing debate over the risks of brain-linked implants.