Enthusiasts are building machines that can make just about anything – including their own robotic offspring. New Scientist explains:

Still, ingenious as these machines are, they merely churn out piles of parts. What about assembly? A heap of plastic and metal is not a machine, just as you don't have much in common with a pile of flesh and bones.

Greg Chirikjian, a roboticist at Johns Hopkins University in Baltimore, Maryland, agrees. "When a prototype only makes parts, the machine that made those parts wasn't reproduced," he says. A true self-replicator must handle both fabrication and assembly. Chirikjian and his colleague Matt Moses are aiming to achieve this with a kind of Lego set that doesn't need anyone to play with it.

The pair have already demonstrated key parts of such a system, using around 100 plastic blocks. Although it cannot yet fabricate these blocks itself, the machine is able to move in 3D to pick up and bind them into larger structures. Moses is currently working on having it make a complete replica of its own structure using Lego-like bricks, though the machine still relies on conventional motors - which have to be installed by hand - to drive its activity.