By Roland Pease
BBC radio science unit
Imagine a field of grass. Now shrink it down, a million fold.
The blades are now the size of a molecule. Imagine the insects in the grass reduced by the same amount.
Their nanofeet grip on to the blades as they move through the sward.
A team of US scientists has invented just such a creature using lengths of DNA. They're calling it a "molecular spider".
Its DNA legs - four of them - are around 10 millionths of a millimetre long (10 nanometres).
They're not powered. Instead, they waft randomly in the currents of the solution in which they're submerged (the whole lawn is kept underwater).
"Molecules at this size are constantly being hit by water molecules and are blown around," explains Milan Stojanovic, a biochemist based at Columbia University.
As they waft, the spiders' legs brush against the grass shoots. But these DNA entities have been made with feet that have molecular sticky patches, and they hook on to the molecular lawn.
They also have molecular snippers - enzyme catalysts - that cut the grass on to which they're clasped. So, while the other legs remain fixed, the newly free one waves around until it finds a new foothold.
"The whole assembly starts rolling or walking over the substrate," says Dr Stojanovic.
And as this microscopic lawnmower wanders on, it clears a sterile path behind it. (The spiders are so small they can't be seen directly by the researchers, but the growth of this path proves what's happening.)
The effect is to prevent the molecule retracing its steps; it is, in effect, a guided molecule.
The researchers are also able to control the speed of the robotic stroll.
"Because these enzymes are sensitive to substances we can add to the system, we can make them stop and start, slow down or speed up," Dr Stojanovic told the Science in Action programme on the BBC World Service.
CHEMICAL CRAWLERS - TOO SMALL TO BE SEEN
Molecular spider floats through solution trailing DNA 'legs'
Legs bind on surface contact... but induces cleavage reaction
Broken bond frees leg for new contact; spider moves forward
Old bond locations no longer grip; spider clear barren path
The team has previously used specially designed lengths of DNA to do simple molecular calculations. A molecular calculator programmed to play noughts and crosses has been one of the researchers' previous demonstrations.
Their hope is to use similar methods to exert more subtle control over their molecular spiders, steering them with chemical controls.
Dr Stojanovic also has more ambitious plans.
"Two molecules like this could communicate with each other and form more complex assemblies. So we hope we can challenge them with certain tasks and then see how they would come together to fulfil that task.
"We could have a simple predator-prey system in which one of them would try to cleave the legs of the other."
It is a proposal that sounds like a sub-microscopic robot war.
A more obviously useful application would be to exploit the lawnmower action of the automata to release drugs into the blood stream.
For example, insulin could be steadily released from the surface of a tablet: if the spiders were sensitive to blood glucose, they could be automatically activated or halted according to the diabetic patient's immediate needs.
The very fact that this degree of control and interactivity is possible with DNA is very suggestive to Dr Stojanovic.
"We are inspired by the RNA-world model [of early life], and the idea that early nucleic acid molecules were performing these complex tasks in early life," he says.
In many respects, the spiders devised by Dr Stojanovic and his colleagues are molecular robots, but the researchers realise this gets them into dangerous territory.
"Many people would say that the very mentioning of robots is purely for getting a good press; but we say a robot has to sense what's happening, it has to perform a small computation based on what it senses, it has to act... move... and it has to have a practical application.
"Many serious scientists object to this kind of language - justifiably - and it's up to us to prove that it's justifiable."