Researchers in Australia have built an electric motor just 250 µm wide that could be used to power tiny robots narrow enough to be injected into the human bloodstream, making new kinds of surgery possible.

The motors work by converting the vibrations of a piezoelectric material into rotary motion that could then be used to drive whip-like structures called flagella — mimicking how some bacteria and other micro-organisms swim.

The team claims that their piezoelectric motor is the first such device to be smaller than 1 mm and — with some improvements — could be powerful enough to drive a robot against the flow in the human bloodstream.

Propulsion is a challenge

Modern vascular surgery often involves inserting a very thin tube — or catheter — into a blood vessel in order to remove a blockage or repair damage. While this technique is often much safer than cutting open a patient, it is sometimes not possible to perform because blood vessels can be too narrow or to labyrinthine to navigate using a catheter.

Some researchers believe that surgery could be made even less invasive by using tiny, self-propelled robots that could be injected into a patient and controlled remotely by a surgeon. One challenge facing designers of such devices is how to propel them through the bloodstream to the right place in the body.

The new motor, built by James Friend and colleagues at Monash University, could do the trick. It comprises a tubular “stator” with a helical slit cut in it and mounted on a piezoelectric material (J. Micromech. Microeng. 19 022001). When an alternating voltage is applied to the material, it vibrates at about 660 kHz. This causes the stator to act like a whip, with its free end following an elliptical path. The free end of the stator is in frictional contact with a rotor, which is spun around by being whipped by the stator.

While this design is not new — much larger piezoelectric rotational motors were first developed in 1980s — Friend told physicsworld.com that the Monash design is much simpler than existing motors, making it easier to scale down to sub-millimetre dimensions.

Nearly enough power

The team was able to operate the motor at 1295 revolutions per minute at a torque of 13 nNm — which is a swimming power of about 4 μW. This power could then be used to propel the motor through a fluid by attaching a whip-like structure called a flagellum to the rotor.

Calculations done by the team suggest that the motor can only deliver about one-fifth of the power needed to drive a tiny robot against the flow in a small human artery — however, the team are hopeful that the power could be boosted in the future.

Friend added that he hopes that tiny piezoelectric motors could be commercially available by 2020.

A video of the micromotor can be seen here.