These Micro-Robots Do the Breaststroke

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Du said experiments and simulations showed swimmers with multiple torsos and arms could be controlled, though their speed varied depending on the strength of the field and – in the simulations – on Brownian motion, the omnipresent, random push and pull of molecules in gases and liquids.

In tests with multiple-particle swimmers, Du said, some arms would drift a bit further from the torso than others. Because this “arm fragmentation” influenced the swimmer’s speed, it helped Du shake up theories about how particles respond to Brownian motion.

“Only when there is Brownian motion do we see this fragmentation,” he said. “With Brownian motion our simulations match experimental results; sometimes fragmentation drives the swimmers to swim slower, and sometimes faster. Without Brownian motion, there’s a huge difference.”

Previous studies about the “scallop theorem” showed Brownian motion can influence the movement of things with reciprocating motion, like a scallop that simply opens and closes without propelling itself but still moves randomly. The arms in Du’s swimmers move in a nonreciprocal way — the driving stroke is longer than the return stroke — but he showed their velocity is also influenced by Brownian motion.

Du said it will be possible to attach ligands or proteins to the large particles for delivery to cells or other biological locations, and the entire vehicle could be moved with two magnetic coils at 90-degree angles.

“In that way, swimmers could serve as micro-robots,” he said.

Rice graduate student Elaa Hilou is co-author of the paper. Du is a research statistical analyst at the University of Texas MD Anderson Cancer Center. Biswal is an associate professor of chemical and bimolecular engineering and of materials science and nanoengineering.

The National Science Foundation supported the research.

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