In its own image: simple robot replicates itself block by block.

No robot can reproduce the way that algal cells, begonias, and people can. However, an automaton that's little more than a stack of blocks has shown that it, too, can make more of its own kind.

Self-replicating robots could be a boon for space exploration and for jobs such as clearing minefields, says Hod Lipson of Cornell University, who led the new research. Ideally, he says, self-copying robots would build coworkers from materials on hand and also repair themselves.

The researchers describe their robotic-block constructions, composed of modules they call "molecubes," in the May 12 Nature. Harboring a microprocessor, a motor, and electromagnets, each fist-size cube consists of plastic halves that swivel along a diagonal plane. As an assemblage of cubes copies itself, the modules execute a sequence of rotations programmed into their microprocessors. At the same time, the modules use their electromagnets to release cubes or pick up others that researchers place within the robot's reach.

The parent assumes a variety of shapes, such as L's and upside-down US, as it twists and bends to build a copy of its original shape. Lipson says that his team plans to develop millimeter-to-micrometer-size modules.

Jordan B. Pollack of Brandeis University in Waltham, Mass., says the Cornell work "is a tour de force of elegant engineering ... and a precursor to some tremendously flexible microrobotic machinery of the future."

Scientists and engineers have pursued self-replicating constructions since the 1940s, when theoreticians began working out concepts of mechanical replication, notes Moshe Sipper of Ben-Gurion University in Beer Sheva, Israel. A few researchers have made mechanical gadgets of materials such as wood or plastic blocks that made copies of themselves using components in their surroundings.

The new molecubes may be better suited to self-replicating complex devices, says Mark Yim of the University of Pennsylvania in Philadelphia. Lipson notes that his group has already simulated on a computer an F-shaped self-replicating robot made of the blocks, but not yet built it.

Ultimately, more-complex structures may be possible. Think of a car made of many identical modules, adds Sipper. "It hits a wall maybe, but it's able to self-repair."

Not all scientists developing self-replicating machines are impressed with the Cornell research. Gregory S. Chirikjian of Johns Hopkins University in Baltimore faults the work for assembling complex starting materials in a trivial manner, rather than the other way around. More impressive, he says, would be a robot that starts with simpler parts and assembles them into a more-complex result.

Although Sipper also points out such limitations of the molecube approach, he says that it "opens up nice avenues."