US scientists reported on Wednesday they have created a machine that can build copies of itself. The basic principle could be extended to create robots that could replicate or repair themselves while working in space or in hazardous environments.

By now the machine is just a proof of concept, performing no useful function except to self-replicate, according to Hod Lipson, an assistant professor at Cornell University in whose lab the robots were built and tested. His group's work was reported in the May 12 issue of Nature.

These robots are made up of a series of modular cubes called " molecubes," each containing identical machinery and the complete computer program for replication. The cubes have electromagnets on their faces that allow them to selectively attach to and detach from one another, and a complete robot consists of several cubes linked together.

Each cube is divided in half along a long diagonal, which allows a robot composed of many cubes to bend, reconfigure and manipulate other cubes. For example, a tower of cubes can bend itself over at a right angle to pick up another cube.

To begin replication, the stack of cubes bends over and sets its top cube on the table. Then it bends to one side or another to pick up a new cube and deposit it on top of the first. By repeating the process, one robot made up of a stack of cubes can create another just like itself. Since one robot cannot reach across another robot of the same height, the robot being built assists in completing its own construction.

Although these experimental robots work only in the limited laboratory environment, the researchers suggest that the idea of making self-replicating robots out of self-contained modules could be used to build working robots that could self-repair by replacing defective modules.

For example, robots sent to explore Mars could carry a supply of spare modules to use for repairing or rebuilding as needed, allowing for more flexible, versatile and robust missions. Self- replication and repair also could be crucial for robots working in environments where a human with a screwdriver couldn't survive.

Self-replicating machines have been the subject of theoretical discussion since the early days of computing and robotics, but only two physical devices that can replicate have been reported. One uses Lego parts assembled in a two-dimensional pattern by moving along tracks; another uses an arrangement of wooden tiles that tumble into a new arrangement when given a shove.

The new robots in Lipson's lab are very dependent on their environment. They draw power through contacts on the surface of the table and cannot replicate unless the experimenters "feed" them by supplying additional modules.

"Although the machines we have created are still simple compared with biological self-reproduction, they demonstrate that mechanical self-reproduction is possible and not unique to biology, " the researchers said.

"This design concept could be useful for self-sustaining systems in hazardous environments, such as those encountered in space exploration, where conventional approaches to maintenance are impractical," they noted.