To push through goo, use itty, bitty propellers

By stealing a trick from bacteria, tiny human-made vehicles can cruise through goo.

Researchers in Germany have designed metal and glass micropropellers that travel through mucus in part by liquefying their thick surroundings. The propellers mimic the activity of ulcer-causing bacteria, and could help inform the design of microbots or drug delivery systems, the scientists report online December 11 in Science Advances.

“It’s a really nice example of this concept of bioinspired engineering: taking what’s sort of a clever biophysical strategy from nature for overcoming a challenge,” says biophysicist Jonathan Celli of the University of Massachusetts Boston.

Inspired by the bacterium Helicobacter pylori, the micropropellers are around two micrometers long and use corkscrew-like tails to move forward. The propellers are also coated in urease, an enzyme that H. pylori releases to liquefy thick stomach mucus. Other bacteria get stuck in viscous mucus; without urease, so do the micropropellers.

Each micropropeller contains magnetic nickel in its tail, so that the corkscrew will spin forward in the presence of a rotating magnetic field. “It’s like a remote drill that we don’t need to contact,” says study coauthor Peer Fischer, a physical chemist at the Max Planck Institute for Intelligent Systems in Stuttgart. Fischer and his colleagues tested the propellers’ in a mixture of pig stomach mucus proteins and hydrochloric acid. The researchers added a couple of other ingredients to the mucus to make the propellers move, including the compound urea, which fuels urease’s liquefying power, and salts that prevent the propellers from getting tangled in chains of mucus molecules.

These micropropellers still have hurdles to overcome to be used for drug delivery. “It’s very interesting from the technological point of view, but clearly has some limitations with regard to potential application,” says Alan Mackie, a food scientist at the Institute of Food Research in Colney, England. For example, the human stomach is usually more acidic, and contains fewer disentangling salts, than the micropropellers’ test environment, he says.

“Our particular system would probably not be suitable for clinical applications,” says study coauthor Debora Walker, also a physical chemist at the Planck Institute. Instead, it’s a proof of concept, she says. “It’s the first system where such propellers actually do manipulate their surroundings in such a way.”