Imagine the effort it would take you to swim through a pool of thick honey. To bacteria and other microorganisms, even water feels just as viscous. At the micro-scales they inhabit, viscosity is the dominant force for most fluid media. So a simple, symmetric open-close motion (called a reciprocal motion) that works for a larger organism like a scallop would get them nowhere. Microbes have therefore evolved a variety of asymmetric (non-reciprocal) mechanisms to swim against the apparent tide.

Making a nanometer-sized object move through water, therefore, is no easy task. However, recent advances in nanotechnology have made it possible for scientists to design tiny artificial swimmers that will allow them to simulate and understand the swimming behavior of bacteria in fluids. Typically, these are micro- or nano-sized helices made of silica, covered in a magnetic coating. An external, rotating magnetic field can then be used to propel them forward through the fluid. These swimmers are non-reciprocal (asymmetric) swimmers, like bacteria. But bacteria are independent, self-directed swimmers, which these non-reciprocal swimmers are not – their direction of motion is controlled by the applied magnetic field.

Now, Ambarish Ghosh and his graduate student, Pranay Mandal, from the Centre for Nanoscience and Engineering at IISc, Bangalore, have shown that by using an oscillating magnetic field instead of the rotating field used before, the same helical nanostructures function as autonomous swimmers. The field provides energy to make them rotate symmetrically clockwise and counterclockwise about their axis (as shown in the schematic picture), but has no effect on the direction of the axis, which is random. This is the first time self-directed, microscopic, reciprocal swimmers have been created. These new nano-swimmers will diffuse through the fluid much like a swarm of bacteria that swim independently.

A whole slew of avenues can now be explored using these autonomous swimmers. When millions of bacteria are thrown together and their hydrodynamic flows interact, interesting collective phenomena emerge. The older non-reciprocal swimmers whose directions are controlled externally can never show such collective phenomena. But by putting millions of these new independent swimmers together, scientists can now observe the emergent collective phenomena and see whether they mimic the dynamics of biological systems. ​“It’s really a wonderful test system”, says Ghosh.

The swimmers also show promise as a therapeutic or diagnostic tool. ​“The large diffusivity of the system means that these small swimmers can sample more area in lesser time”, explains Ghosh. The researchers hope to collaborate with a cancer biologist to see whether this could have potential applications in cancer detection. The swimmers can be made to go and attach to particular type of cells like tags and can be easily imaged. Furthermore, a rotating field can subsequently be used to remove them, once they have served their diagnostic purpose.

So far, the swimmers have only been tested in water. What happens to the swimmers in biological fluids? ​“It took us a good 7 – 8 months to figure out how to make our propellers move in them”, says Ghosh. Watch this space to learn more about the challenges they faced and their ongoing exciting experiments in human blood.