How do you control a spermbot? Try a magnetic field

TAKE bull sperm, mix in some nanotubes, and what do you get? Why the very first spermbot, of course. Eventually, these biobots could be used to shepherd individual sperm to eggs or to deliver targeted doses of drugs.

Oliver Schmidt and colleagues at the Institute for Integrative Nanosciences in Dresden, Germany, combined individual sperm cells with tiny magnetic metal tubes to create the first sperm-based biobots.

It is far from easy to control a single cell that propels itself through fluid with its whip-like flagellum. Until now researchers had only managed to persuade groups of cells to cooperate, with the help of chemical gradients and magnetic fields. For example, a group of bacteria were used to push a tiny bead along.

To create the spermbots, the team made microtubes 50 microns long, by 5 to 8 microns in diameter from iron and titanium nanoparticles. They added the tubes to a fluid containing thawed bull sperm. Because one end of each tube was slightly narrower than the other, sperm that swam into the wider end become trapped, headfirst, with their flagella still free.

To control the orientation of the microtubes, the team used external magnetic fields. It works much as a compass needle aligns with Earth's magnetic field. This enabled the team to control the direction in which the sperm swam (Advanced Materials, doi.org/f2n46m).

Schmidt says that sperm cells are an attractive option because they are harmless to the human body, do not require an external power source, and can swim through viscous liquids.

"This type of hybrid approach could lead the way in making efficient robotic micro-systems," says Eric Diller at the University of Toronto, Canada, although it is hard to get micro-robots to swim as fast as biological cells.

This article appeared in print under the headline "How do you control a spermbot? Stick its head in a tube"