Rewriting Life

Propellers for Microrobots

Copying bacterial propellers could give biomedical robots a boost.

Researchers have developed a novel form of propulsion for microrobots that mimics the way bacteria zip about using corkscrew-like appendages called flagella. Tests show that the tiny rotating nanocoils–just 27 nanometers thick and 40 micrometers long–are capable of spinning at 60 revolutions per minute and that it is possible to propel an object at nearly 5 micrometers per second.

Nanopropeller: A nanocoil attached to a substrate (top) is released by a probe (bottom), allowing it to corkscrew its way through a fluid. The nanocoil’s creation was inspired by bacterial flagella.

Such propulsion could be used as part of smart drug delivery systems, which are steered through the bloodstream directly to their target, says Bradley Nelson, a professor of robotics and intelligent systems at the Swiss Federal Institute of Technology, in Zurich, who led the research. And in the long term, the nanopropellers could be used to propel autonomous biomedical microrobots, he suggests.

Moving through fluids at the nanoscale can be a real challenge because of the viscosity of the liquid, says Sylvain Martel, an associate professor at the department of computer engineering at Montreal Polytechnique in Canada. As the size of an object is reduced, the force required to move it through a fluid does not reduce proportionally, Martel says.

For a bacterium trying to move around at the microscopic scale, this effect can be dramatic. “It’s like swimming through something thicker than molasses,” says Nelson, who carried out the work with graduate student Dominik Bell.

As a result, bacteria evolved the highly sophisticated flagellum. “Sperm just waggle their tails, but bacteria have a much more complicated mechanism,” says Nelson. A molecular motor that pumps protons across the cell’s membrane causes the helical filaments of the flagellum to rotate. This is carried out so efficiently that some flagella have been clocked spinning at speeds of up to 1,000 revolutions per minute.

Nelson’s nanocoils generate their motion using an external rotating magnetic field which causes them to move in much the same way as flagella. The nanocoil was made by fabricating two very thin strips of gallium arsenide on top of each other, using photolithographic techniques; the bottom layer is laced with indium. “The indium atoms in the lower layer induce a compressive stress,” says Bell. This causes it to curl up into a helix to release the stress, says Bell. “It’s like a corkscrew,” Nelson explains.

To make this helical nanocoil rotate, Nelson and his colleagues then created a set of four magnetic coils positioned so that they would produce an electromagnetic field that would rotate around a single axis. The coils were attached to small nickel plates, which acted as surrogate microrobots, and were placed in liquids to see if the coils could propel them. They could.

This is not the first time that external magnetic fields have been used to make coils rotate, says Martel. But earlier demonstrations were massive in comparison, typically in the order of millimeters, he says. “The scaling here is very impressive.”

Drug delivery systems are an attractive application for this form of propulsion. By allowing drug delivery microdevices to be directed to a very precise site, therapeutics could be used more effectively. “You can use higher-toxicity drugs that go to the source of the problem,” says Nelson.

Even so, says Martel, before such devices are made sophisticated enough to be autonomous, they will still need to be monitored, tracked, and steered through the 80,000 kilometers of blood vessels in the human body. This would have to be done using MRI scanners since ultrasound cannot pick up such small objects. This, in turn, means that, ironically, the devices would have to withstand the powerful magnetic forces produced by these machines.

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