Spiral swimmers

Harvard researchers have created a new type of microscopic swimmer: a magnetised spiral that corkscrews through liquids.

Though other researchers have created similar devices in the past, Peer Fischer, a junior fellow at the Rowland Institute at Harvard, said the new nano-robot is the only swimmer that can be precisely controlled in solution.

At just two microns long and 200 to 300 nanometres wide, the corkscrew swimmer is about the size of a bacterial cell.

Fischer and Rowland Institute postdoctoral research associate Ambarish Ghosh were able to control the tiny device well enough to push a five micron bead - which had a volume more than 1,000 times that of the swimmer - and were also able to control two of the swimmers at the same time.

Fischer said the strength of his and Ghosh’s work is not just the swimmer’s performance but also its manufacturing method, which allows many swimmers to be created simultaneously.

The devices are made by exposing a silicon wafer to silicon dioxide vapour. The wafer is slowly rotated as the vapour condenses, growing the devices in a corkscrew shape. They are then shaken loose, sprayed with cobalt, and magnetised. Because they are lying on their sides when the cobalt is applied, the process provides a magnetic 'handle' to rotate the corkscrews.

'You can make hundreds of millions in a square centimetre,' Fischer added. 'Even if you use only a few per cent, that’s still a lot. You can make a lot of them very quickly.'

Fischer and Ghosh took one last step and coated the simmers with a fluorescent chemical, which didn’t improve their functionality, but allowed them to be tracked.

Once complete, the researchers surrounded the swimmers with three magnetic coils, allowing them to precisely adjust the magnetic field, and control the tiny devices in three dimensions.

The microscopic world of the nano-swimmer is different from the one we experience when going for a swim, Fischer said. Because it operates at such a tiny scale, water appears thicker to the nano-swimmers, more like honey. The swimmers meet a considerable amount of resistance to their forward motion so that they really need to drill their way forward, he added.

The devices move at about the speed of bacteria, or around 40 micrometres per second.

Though applications in drug delivery, microsurgery, and other aspects of medicine seem apparent, Fischer said it’s too early to speak about those realistically.

However, he added that the artificial swimmers can be used to test some of these ideas and could have almost immediate applications in research, being used to shuttle chemicals in and out of cells or testing the strength and properties of membranes.