Scientists from Columbia University, Arizona State University, the University of Michigan, and the California Institute of Technology (Caltech) have created a robot that’s just 4 nanometers wide. And no, it doesn’t have flashing lights, video cameras or wheels. It does, however, have four legs, and the ability to start, move, turn, and stop. Descendants of the molecular nanobot, or “spider,” could someday be used to treat diseases such as cancer or diabetes.

The team built the spider by starting with a protein called streptavidin, that conveniently has four symmetrically-placed binding pockets for a chemical called biotin. The legs were made from four strands of biotin-labeled DNA, which were bound to the pockets. Three of the legs were made from enzymatic DNA, which is a type that binds to and then dissociates (cuts away) from other particular sequences of DNA. Its fourth leg was made from what the researchers call a “start strand” of DNA - it keeps the spider tethered to its starting site, until it’s released.

Like a traditional robot, the team wanted the spider to be able to sense its environment, make a decision, and then act on that decision. Given that it was much too small to incorporate any kind of processor, however, its programming would have to come from outside its body. To accomplish this, the team had to adapt the spider’s environment to provide the instructions. They started by taking a single long strand of DNA and used shorter DNA strands to “staple” it into a two-dimensional rectangular shape. This would be the rectangular track that the spider was placed on, and had to follow.

The team proceeded to attach single-strand DNA molecules, or oligonucleotides, to the staple strands that held the track together. These molecules would tell the spider to start, walk, turn left, turn right, or stop, as it encountered them.

Once its “start strand” leg was released, the spider’s enzymatic DNA legs reached forward to seek out the oligonucleotides. Like a mouse following a trail of breadcrumbs, the spider would bind one of its legs to an oligonucleotide, dissociate from it, then be drawn forward by the next one. In this way, it walked a distance of 100 nanometers, or about 50 steps. That might not sound like much, but previous “DNA walker” nanobots have only managed about three steps.

In the future, it is hoped that such nanobots could rebuild themselves into whatever configuration is needed, build molecular structures, and that they could be used in medical applications.

“You could imagine the spider carrying a drug and bonding to a two-dimensional surface like a cell membrane, finding the receptors and, depending on the local environment, triggering the activation of this drug” said Arizona State researcher Hao Yan.

The research was published in the current issue of the journal Nature.