Scientists have discovered a remarkably simple way to create continuous rotary motion using nothing more than an elastic polymer fibre, its carefully designed geometry, and heat. The new concept could eventually contribute to the development of intelligent materials, artificial muscles, soft robots and self-propelled mechanical systems.
Researchers from Heidelberg University in Germany and the University of Strasbourg in France have demonstrated how a prestrained polymer fibre, closed into a ring, can spontaneously rotate when exposed to a temperature difference. Unlike conventional motors, the system does not depend on a rigid wheel, axle, gears or complicated machinery.
The findings, published in Nature Materials, demonstrate how the geometry and mechanical properties of a material can themselves perform functions normally associated with machines.
A Motor Without a Conventional Wheel and Axle
Most rotary machines depend on a familiar combination: a wheel rotates around an axle. From electric motors to engines and mechanical devices, this arrangement has been used for centuries.
But researchers have been exploring a very different question: Can a material generate useful rotary motion simply because of the way it is shaped and mechanically deformed?
Responsive materials have already been used to create structures that can change shape, move linearly and even act as microswimmers. These developments suggest that it may eventually be possible to construct machines in which the material itself provides both the structure and the movement.
Rotary motion, however, has remained particularly challenging.
The new research offers a possible solution.
Instead of placing a rigid wheel around an axle, the scientists created what they describe as a “wheel within.” It consists of an elastic polymer fibre that has been mechanically prestrained and then closed into a ring.
When the right energy conditions are applied, the ring can begin rotating continuously.
The Secret Is Hidden in the Geometry
At first glance, the device appears extremely simple. There are no conventional gears, motors or sophisticated electronic components.
Its unusual behaviour comes from the combination of elastic deformation, prestrain, geometry and heat flow.
The polymer fibre is first mechanically deformed before being formed into a closed ring. This creates stored elastic energy within the material.
The ring is then positioned between two heat baths, creating a temperature difference across the system.
When heat flows through the material, the polymer undergoes thermal expansion and contraction. But because the fibre has already been mechanically prestrained and arranged into a particular geometry, this thermal deformation does not simply make the ring expand or contract uniformly.
Instead, it interacts with the ring's existing deformation.
This interaction produces an elastic deformation wave that travels around the structure.
And that wave drives the rotation.
How Heat Turns Into Motion
The basic idea can be compared to processes in nature.
When different parts of a material experience different temperatures, they can expand by different amounts. In the Earth's atmosphere, differences in heating contribute to convection and large-scale movements of air.
The researchers use a similarly fundamental physical principle, but in a carefully designed elastic structure.
The temperature difference causes thermal deformation in the polymer. Because the ring has a prescribed mechanical deformation, the thermal changes interact with that geometry.
This breaks the rotational symmetry of the system around the fibre's axis.
As a result, the deformation does not remain stationary. Instead, it can propagate around the ring, producing continuous rotary motion.
Importantly, the ring does not need a rigid axle around which it rotates.
The shape-changing material itself becomes the mechanism that generates movement.
A Motor and Energy Storage Device in One
One of the particularly interesting aspects of this concept is that the same component can perform multiple functions.
The polymer fibre stores mechanical energy because it has been prestrained. At the same time, its deformation can be converted into movement when an external energy source—in this case, a temperature difference—is supplied.
This gives the system characteristics of both an energy storage element and an actuator.
Rather than assembling a machine from separate components such as a spring, motor, axle and gears, researchers are investigating whether carefully engineered materials could combine several of these functions into one structure.
That could be particularly valuable in soft robotics and microscale devices, where traditional mechanical components can become difficult to manufacture or integrate.
Why the Simplicity Matters
The researchers emphasize that their approach is deliberately minimalistic.
Instead of depending on expensive or highly sophisticated smart materials, the concept relies primarily on geometry and topology—the way the material is shaped, connected and mechanically constrained.
This is an important shift in thinking.
Engineers often make machines more capable by adding components, sensors, electronics and control systems. But another approach is to design the material itself so that its physical structure naturally produces the desired behaviour.
In this case, the geometry effectively acts as part of the machine's control mechanism.
The researchers believe this principle could provide a foundation for developing materials that perform specific functions without requiring complicated assemblies.
Potential Applications in Soft Robotics
The technology is still at an early research stage, but the underlying principle could inspire future applications.
One possibility is the development of artificial muscles capable of producing controlled movement through thermal or other forms of energy flow.
Such systems could potentially be incorporated into soft robots, adaptive structures and miniature mechanical devices.
For example, instead of using conventional motors and rigid gears, future soft robotic systems might use elastic structures that deform and move naturally when supplied with heat or another energy source.
This could make certain machines lighter, simpler and more mechanically integrated.
The concept may also inspire new approaches to self-propelled materials, where energy flowing through a material is directly converted into mechanical motion.
What Comes Next?
The researchers are now investigating different geometries, materials and methods of transferring energy through such structures.
The polymer fibre used in the demonstration is only one possible material. Similar principles could potentially be explored using other elastic materials, including polymer fibres, rubber-like materials and engineered soft structures.
The major challenge will be translating the elegant laboratory demonstration into systems that can deliver useful amounts of force, speed and controlled motion.
Researchers will also need to understand how efficiently different geometries convert thermal energy into mechanical movement and how reliably these structures can operate over long periods.
Nevertheless, the underlying idea is powerful because it demonstrates that complex mechanical behaviour does not always require complex machinery.
A New Way to Think About Machines
The “wheel within” represents more than an unusual rotating ring. It demonstrates a broader design philosophy: use the material's own geometry and physical properties to create functionality.
A simple elastic fibre, when prestrained and arranged correctly, can store energy, respond to heat and generate continuous rotary motion—all without a conventional wheel-and-axle mechanism.
The work could therefore mark an interesting step toward machines in which structure, energy storage and movement are integrated into the same material.
In the future, this approach could help scientists create artificial muscles, soft robotic components and intelligent materials that move not because they contain a traditional motor, but because their geometry has been designed to make movement emerge naturally.
The machine of the future may not always look like a machine.
Sometimes, the material itself could be the motor.
Reference: Baumann, A., Sánchez-Ferrer, A., Jacomine, L. et al. Motorizing fibres with geometric zero-energy modes. Nature Mater 17, 523–527 (2018). https://doi.org/10.1038/s41563-018-0062-0
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