Image copyright I Khalil, S Misra Image caption Sperm-inspired robots "swim" when they are made to wiggle by a moving magnetic field

Engineers have built a sperm-like robot that they can control with magnets.

The simple design has a metal-coated head and a flexible body about six times longer than a human sperm.

Using a magnetic field no stronger than a fridge magnet, the team made the robot "swim" forward and steered it towards a fixed point.

They hope the findings, published in the journal Applied Physics Letters, will be useful in medical and manufacturing applications.

"We have built a biologically inspired micro-robot that looks like a sperm cell but is completely fabricated in the lab," said Dr Sarthak Misra, a robotics engineer at the University of Twente in the Netherlands.

Sperm, like some bacteria, use a whip-like "flagellum" to propel themselves through fluid.

Other studies of this process have included coupling a magnetic tail to a red blood cell, or controlling real sperm by steering them in metal microtubes.

The new gadget is made from a strong but flexible polymer, with a metal layer painted onto its head using a technique called electron beam evaporation.

That metal element is forced in different directions when the device is placed into a moving field, produced by the coils of an electromagnet.

Precise navigation

"The magnetic head is used to orient it in a certain direction and then, just by flapping its tail, it starts to move forward," Dr Misra told the BBC. "The flapping happens because we change the current in the coils."

Image copyright I Khalil, S Misra Image caption The simple robots consist of a flexible tail and a head with an added metallic layer

For its size, the robot can hardly compete with its biological inspiration for pace: it wiggles along at up to 0.5 body lengths per second, whereas a human sperm can cover several times its body length in that time.

Importantly, however, Dr Misra and his colleagues also showed that they could steer the robot with some precision. By adjusting the magnetic field changes with a computer, they navigated their creation towards a fixed point.

This means the little robots could be useful for positioning materials in nanomanufacturing, or for medical purposes. "It opens up a lot of applications, from targeted drug delivery to in vitro fertilisation - where you want to reach precise locations," Dr Misra said.

For these ideas to work, the researchers need to test the technique in more complicated environments. They are moving on to those experiments, as well as working on making their "MagnetoSperm" smaller and faster.

Image copyright SPL Image caption Human sperm are another six times smaller than the tiny robots that were modelled on them

Dr Matthew Baker, who studies molecular motors at the Victor Chang Cardiac Research Institute in Sydney, told BBC News the work was "very cool" but commented that the tiny contraptions are not robots in the sense that most of us would imagine.

"It's just a piece of metal and it's the field that's doing the hard work," Dr Baker explained. "The clever part is the oscillating, small, tunable magnetic field."

This out-sourcing of power and navigation is what allows the gadget to be so small, according to the new study's first author Dr Islam Khalil. "As technology progresses and many products get smaller, it becomes difficult to assemble objects on nano- and micro-scales," he said in a press statement.

"MagnetoSperm can be used to manipulate and assemble objects at these scales, using an external source of magnetic field to control its motion."