3D printing, the additive manufacturing of complex objects in one piece, has also given wings to robot construction.
Instead of equipping the machines with limbs, mechanical and electronic components one after the other, as is usually the case, robots can be manufactured with all their functionalities in one step.
The most diverse shapes and structures can be realized that cannot be produced with the usual manufacturing processes.
Because almost all materials can now be processed - from plastics and metals to glass and ceramics - the mechanical, electrical and optical properties of the workpieces can be tailored during printing.
Researchers from the University of California in Los Angeles have now exhausted these possibilities for the construction of autonomous miniature robots.
Manfred Lindinger
Editor in the department "Nature and Science".
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As Xiaoyu Zheng and his colleagues report in the journal Science, their printed robots can not only move in a straight line on any surface.
They are also able to hop and spin in space.
Equipped with a kind of basket, the robots can even transport smaller loads piggyback.
Zheng's researchers use the piezoelectric effect to drive the fingernail-sized robots: Piezoelectric materials have the property that they expand, contract or otherwise deform when an electrical voltage is applied.
To print a robot, Zheng and his colleagues use a light-sensitive synthetic resin mixed with piezoelectric particles made from a lead-zirconium-titanium-oxygen compound.
During the printing itself, the liquid resin is irradiated from below in a tub with ultraviolet light pulses.
A computer program determines exactly where the light should hit the material.
Chemical reactions are triggered there, which lead to the hardening of the synthetic resin.
A platform pulls the exposed parts out of the tank layer by layer.
At the end of the printing process, the still liquid resin is removed.
The result is a cube-shaped, three-dimensional filigree framework structure.
In order to turn this into a functioning robot, the researchers deposit metallic electrodes at selected points.
Heat and a strong external electric field cause the piezoelectric lead titanate particles to align themselves along the field lines.
The material can now deform.
The areas without electrodes are not polarized.
They serve as supporting elements.
The boundary between intelligent materials and machines is blurring
When the electrodes are connected to a battery, the robot unfolds and assumes its final shape, which resembles the capital letter 'N'.
When running, the vertical bars take on the function of front and rear legs.
They alternately have contact with the ground and in this way push the robot forward – at a maximum speed of just over one centimeter per second.
The researchers can control the speed via the frequency of voltage pulses generated by a microprocessor that the robot carries along with its battery, for example.
For example, if only the hind legs are activated, the robot moves forward by hopping; if other parts are stimulated, it turns in a circle.
One specimen that Zheng and his colleagues equipped with an ultrasonic sensor was able to detect and avoid obstacles in its vicinity.
In this way, this robot could steer itself autonomously through a maze without colliding with the side walls.
What may seem like a gimmick at first glance has a serious background for Ahmad Rafsanjani from the Danish Center for Soft Robotics in Odense.
The study demonstrates that the boundary between intelligent materials and machines is becoming increasingly blurred, the researcher writes in an accompanying commentary in Science.
He suggests using the method of Zheng and colleagues to develop soft robots with soft electroactive polymers.
Soft robots, which have no rigid components, can move more smoothly and weigh less.
In-vivo robot on a test drive
The researchers led by Zheng himself can imagine that a swarm of their robots could take on scouting tasks, for example by searching for buried victims in a collapsed building.
Printed robots for biomedical tasks would also be conceivable, for example as self-steering endoscopes or tiny floating robots that can navigate independently in the body in order to deliver active substances in a targeted manner.
Researchers from Stanford University have shown that such fantastic ideas could soon become reality.
As Renee Zhao and her colleagues write in "Nature Communications", they have developed a five-millimeter robot that can move in both dry and liquid environments - rolling or swimming.
The robot can be controlled wirelessly by a magnet and navigated in any desired direction.
The five-millimeter bot is also able to release a certain amount of liquid – such as an active ingredient – in response to a magnetic impulse.
The researchers have already tested their vehicle in the gastrointestinal tract of a dead pig.