Its precise structure and ability to bind with other molecules makes DNA an attractive scaffolding material for nanotech researchers. Scientists have already used DNA to construct two-dimensional patterns, three-dimensional objects, and simple shape-changing devices. Now two teams of researchers have separately made complex programmable machines using DNA molecules.

Researchers from Columbia University, Arizona State University, and Caltech have made a device that follows a programmable path on a surface patterned with DNA. Meanwhile, researchers from New York University, led by DNA nanoarchitecture pioneer Ned Seeman, have combined multiple DNA devices to make an assembly line. The nano contraption picks up gold nanoparticles as it tumbles along a DNA-patterned surface.

The two machines, described in today's Nature journal, are a possible step forward in making DNA nanobots that could assemble tiny electrical and mechanical devices. DNA robots could also put together molecules in new ways to make new materials, says Lloyd Smith, a chemistry professor at the University of Wisconsin-Madison. "Robots might have the ability to position one molecule in a particular way so that a reaction happens with another molecule which might not happen if they randomly collide in solution," he says.

In the past, researchers have made simple machines such as tweezers and walkers that have also been fashioned from DNA. Tweezers open and close by adding specific DNA strands to the solution. Walkers are molecules with dangling strands, or legs, that bind and detach from other DNA strands patterned on a surface, in effect moving along the surface.

The nano walker made at Columbia University is a protein molecule decorated with three legs--single-stranded DNAzymes, synthetic DNA molecules that act as enzymes and catalyze a reaction. The legs bind to complementary DNA strands on a surface. Then they catalyze a reaction that shortens one of the surface strands, so that its attachment to the leg becomes weaker. That leg lets go and moves on to the next surface strand.

The walker follows a track of strands that the researchers pattern on the surface. It can take up to 50 steps--compared to the two or three steps taken by previous walkers. It stops when it encounters a sequence that cannot be shortened. "We show how to program [the walker's] behavior by programming the landscape," says Milan Stojanovic, a biomedical engineer at Columbia University who developed the walker. "It enables us to think about adding further complexity: more than one molecule interacting and more complicated commands on the surface. What we hope to do eventually is to be able to [use nanobots to] repair tissues."

Seeman and his colleagues at New York University combine three different DNA components to make an assembly line. They have DNA path, a walker, and a machine that can deliver or hold back a cargo of a molecule of gold. The machine is a DNA structure that can be set up to either put a gold nanoparticle-laden strand in the path of the walker or away from it. The walker has four legs and three single-stranded DNA hands that can bind to the gold.

The researchers demonstrated a system in which the walker passes three machines, each carrying a different type of gold particle. Each machine can be set up to either deliver its cargo or keep it, giving a total of eight different ways in which the walker can be loaded, leading to eight different products.

The advances represent continuing success in creating nano devices with increasingly complex functions. "[We're] moving from individual entities that do something interesting to systems of entities working on something with a more complex behavior and function," Smith says.