Surgical procedures could soon be helped along with tiny robots, according to researchers.
Miniaturisation of motors has not kept pace with that of electronics, leaving such tiny robots with no means to get around in the body.
Now, research reported in the Journal of Micromechanics and Microengineering has demonstrated a motor about twice the size of a human hair.
The motors could be used to power mini robots to fly around inside the body.
Some surgical procedures are hindered by the size or inflexibility of current instruments. For example, the labyrinthine network of blood vessels in the brain prevents the use of catheters threaded through larger blood vessels.
Researchers have long envisioned that trends of miniaturisation would lead to tiny robots that could get around easily in the body.
The problem until now has been powering them.
Conventional electric motors do not perform as well as they are scaled down in size; as they approach millimetre dimensions, they barely have the power to overcome the resistance in their bearings.
This has been the significant bottleneck in the development of microtechnology such as tiny surgical robots, according to James Friend at the University of Monash in Australia.
"If you pick up an electronics catalogue, you'll find all sorts of sensors, LEDs, memory chips etc that represent the latest in technology and miniaturisation," he says.
All the other concepts to do this are very complex
Mettin Sitti Carnegie Mellon University
"Take a look however at the motors, and there are few changes from the motors available in the 1950s."
Push to turn
To address that, research in recent years has seen the use of so-called piezoelectric materials. These are typically crystals that expand and contract when a voltage is applied to them.
That makes "linear motors" - which simply move back and forth at high frequency - easy to produce, and Professor Friend published work last year about a motor the size of a grain of salt.
But for real motion within the body, the micro-motors need to be able to rotate.
Many kinds of bacteria, for instance, have tail-like structures called flagella. Rotating the flagella at their base whips them into a helical structure that propels them through fluid.
The new research leverages this same approach, by transforming the linear motion of tiny piezoelectric motors into rotation.
That is accomplished by coupling the motors to a structure with a helix-shaped cut in it. Because the structure is held in place along the helical groove, a push at one end is turned into a rotation.
The reverse case: rotation of a helical thread drives a bulb into a socket
The researchers' prototype measures is a quarter of a millimetre wide - not much more than a couple of hairs side-by-side, and 70% smaller than the previous record holder.
And the fact that it is a conceptually simple, self-contained approach is promising for future applications, says Metin Sitti, who heads the NanoRobotics Laboratory at Carnegie Mellon University.
"All the other concepts to do this are very complex," he says.
"We can already use electromagnetics and external coils to spin them inside a liquid, but then you need all these off-board coils.
"The advantage of this concept is having an on-board rotary actuation," he says. "This could make for a mobile robot with no other equipment."
However, experts in such microelectromechanical systems from QinetiQ told BBC News that while the prototype provides impressive performance in a lab, its efficiency in moving through fluids is yet to be demonstrated.
If it proves successful, however, the approach could be used in applications outside the body, according to Professor Sitti.
"These are high-frequency, lightweight motors, and those are specifications that would be advantageous for flying robots too."
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