A physical chemist is pleased to learn that 'microscale' swimming isn't that hard after all.

Even if small organisms perfectly mimicked gold medallist Michael Phelps's technique, they wouldn't win a microswimming Olympics. The viscosity of water is so high that these little fellows have had to develop some unusual swimming styles. In 1977, E. M. Purcell formally expressed this idea with his famous 'scallop theorem'. He showed that swimming forwards cannot be achieved at the micrometre-scale with 'time-reversible' motions such as the back-and-forth wiggling of a rigid tail. Instead, tiny organisms must use complex, asymmetrical strokes.

But this is not always the case, according to engineers at the Massachusetts Institute of Technology in Cambridge and the University of California, San Diego. In July, they proved that time-reversible tail-wiggling or wing-flapping can be a viable mode of propulsion through a fluid, provided it is done next to a deformable interface such as a soft membrane (R. Trouilloud et al. Phys. Rev. Lett. 101, 048102; 2008). The reversible motions of the swimmer couple in a nonlinear way to the deformations of the interface, producing additional flows and forces that are sufficient for locomotion.

One of the most exciting extensions of this result might be in creating 'nanosubmarines' — a much-criticized dream of nanotechnologists to have devices navigate blood vessels, finding and fixing damaged organs as they go. The idea has so far seemed implausible because such machines would need elaborate nanopropellers — which are prohibitively difficult to build — to sculpt asymmetrical swimming motions. But what about using a simpler propulsion mechanism and relying on the deformations of blood-vessel walls to move nanosubmarines along? Is there a nanoshipyard out there somewhere to put this idea to the test?

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