A tiny insect-like robot that flaps its wings 170 times per second has become the lightest drone ever to achieve sustained untethered flight.

Named Robobee, the solar-powered device is the creation of researchers at Harvard University who have spent decades trying to create a micro-drone that doesn't rely on batteries.

Robobee weighs 259 milligrams - about a quarter of a paper clip - and uses about 120 milliwatts of power.

Researchers have attached energy-generating solar cells to the device, allowing its wings to contract and relax like real muscles.

Robert Wood, a lead researcher at Harvard, said that powering flight is "something of a Catch-22", as the trade-off between mass and power becomes extremely problematic at small scales where flight is inherently inefficient.

"It doesn’t help that even the smallest commercially available batteries weigh much more than the robot," he said.

"We have developed strategies to address this challenge by increasing vehicle efficiency, creating extremely lightweight power circuits, and integrating high efficiency solar cells.”

The thrust efficiency of the RoboBee is the same as that of small insects, Noah Jafferis, postdoctoral engineer at Harvard University and a lead researcher said.

The robot is yet to be able to store energy, with current test flights lasting only a couple of seconds before falling to the ground.