Different strokes

A single technique can produce a menagerie of tiny artificial swimmers that swim a medley of strokes, researchers report in an upcoming Physical Review E paper. Among the moves: zipping through liquid in a straight line, whirling around in tight circles and gliding in complicated loop-the-loop flower patterns.

Designing microswimmers with diverse behaviors is a step toward the ultimate goal of designing tiny, controllable machines tuned for delicate tasks in the inner space of the human body. Researchers dream about making small swimmers that can one day clear out blood clots, blast through clogged arteries or deliver chemotherapy directly to a tumor (SN: 7/04/09, p. 22).

So far, most artificial microswimmers can perform only a limited numbers of strokes. “The fact that they get such variety out of such a simple system is pretty interesting,” says physicist Greg Huber of the University of Connecticut Health Center in Farmington.

The new design comes closer than previous iterations to mimicking the flexible swimming styles of microswimmers found in the wild, such as some bacteria and algae. Real biological cells possess a vast number of behaviors. “E. coli, for example, runs in a straight line and every now and then will stop and change direction,” says study coauthor Jonathan Howse of the University of Sheffield in England.

In their approach, the Sheffield team coated half of 2-micrometer-wide polystyrene beads with platinum, like candied apples dipped halfway in caramel. (Scientists call these half-and-half beads Janus particles, after the two-faced Roman god.) In water spiked with hydrogen peroxide, these swimmers start to motor. The beads’ metallic coating burns hydrogen peroxide as a fuel: Platinum splits hydrogen peroxide into water and oxygen. As the reaction takes off, the pressure on the coated side of the bead changes, causing the beads to swim.

After the platinum coats were applied, the coated beads glommed together in groups. Swimmers made up of two beads swam in diverse patterns, the team saw. The system “can produce all kinds of trajectories,” says study coauthor Ramin Golestanian, who recently moved from the University of Sheffield to the University of Oxford in England.

The swimming path wasn’t the only thing that varied. Stroke speeds ranged from a complete standstill (some microswimmers spun their wheels in tight circles, making no forward motion) to a speedy 6 micrometers per second for those that jetted in a straight line.

The variety of speeds and directions stems from the random orientation of the conjoined beads. Since each bead has a single preferred direction, combining the two beads in random conformations results in combinations of forces, leading to different swimming styles.

While the method doesn’t yet allow researchers to specify what kinds of swimmers they get, it would be simple to pick out those with desirable strokes, says study coauthor Stephen Ebbens, also of the University of Sheffield. “If we can make this wide range of behaviors, you’ll be able to filter out the types you want later on,” he says.

Gaining precise control over all the forces that regulate swimming, including propulsion and rotation, could eventually lead to precisely calibrated swimmers. Golestanian says that once researchers know how to control these tiny swimmers, generating ideas for their “fantastic voyages” is easy. “We can let our imagination go wild and see what we can come up with.”