Rapid prototyping is a concept straight out of Star Trek. Feed an RP machine a 3-D blueprint of an object and it will carve a model of that object out of metal, paper, plastic or starch, just like the replicator aboard the USS Enterprise.

Now, these RP devices, also known as 3-D printers, are about to get even better. Engineers are giving the machines the ability to build moving parts, not just block models.

University of California at Berkeley researcher Jeremy Risner is part of a team developing what are called "flextronic" devices -- or flexible mechatronics. Risner hopes to produce a proof-of-concept in six months: a small model with flexible joints and electronic parts built in. And all of it created by a 3-D printer.

"My background is bioengineering and I'd like to develop models based on living designs," Risner said. "Maybe a rather flat insect with wriggling limbs and some actuators."

Risner hopes to develop a fully functional, mechanical and electronic device within the next 18 months, using several printer heads to lay circuits, transistors, capacitors, sensors and casing. The innovation is that all these elements will be printed by one machine in one run, using a bunch of print heads spitting out each material in layers.

"Maybe we'll have to add the power afterward," said Risner.

RP technology is spreading as costs for entry-level devices come down rapidly. Fifteen years ago, they cost $300,000. Now they cost about $30,000.

Rapid prototyping and layered manufacturing are broad terms for a family of technologies that can create 3-D shapes directly from a computer screen or CAD/CAM system. They work by building up solid models layer by layer, using the same principle that's behind layered 3-D jigsaw puzzles.

The granddaddy of the RP family is stereolithography. The technique was commercialized in 1988. It uses a vat of photosensitive liquid polymer and a laser beam to trace out each layer, which drops into the vat so the next layer can be traced.

The youngest addition to the layered manufacturing family is 3-D printing, which means exactly what it says: One printer head spits out a fine powder, while another spits out a bonding agent, creating one layer. The model bed drops one layer, and the heads pass over again.

There are similar techniques with fancy names: fused deposition modeling, laminated object manufacturing and selective laser sintering. But all are variations on the same theme.

Each technology has particular strengths and weaknesses in terms of cost and finish. Each also builds models out of different materials like metals, polymers, foundry sand, starch, plaster or paper. One company, Therics, even prints pharmaceutical pills with the technology.

The concept already has revolutionized industrial engineering and prototyping, which were costly and time-consuming affairs. Product development is faster, with more models created before a final design is chosen. Best of all, these systems can print out any shape a designer can imagine. They are a godsend to product design agencies and architects, who can print out 3-D models of their latest edifices in just hours.

The future has bewildering potential. In 10 years, you might be able to fax a toy car to a favorite niece or nephew. It could even be sooner than that. The New York Times reported rumblings that Hewlett-Packard is planning to sell a 3-D printer for $1,000.

"RP is a mature industry," says Greg Gibbons of the rapid prototyping and tooling group at Britain's University of Warwick, "but the next move is to go into rapid manufacture, which requires material developments and increases in machine speed."