A new process for producing movable, self-adjusting materials systems with standard 3D printers has been developed in Germany.

The systems can undergo complex shape changes, by being pre-programmed to contracting and expanding under the influence of moisture.

Researchers at the University of Freiburg and the University of Stuttgart modelled their development on the movement mechanisms of the climbing plant known as the air potato (Dioscorea bulbifera).

A forearm brace that adapts to the wearer and which can be further developed for medical applications has been produced as a first prototype.

3D printing has established itself as a manufacturing process for a wide range of applications and can even be used to produce intelligent materials and material systems that remain in motion after printing, autonomously changing shape in response to external stimuli such as light, temperature or moisture.

4D printing – in which predetermined shape changes can be triggered by a stimulus – immensely expands the potential applications of material systems. The changes in shape are made possible by the chemical composition of the materials, which consist of stimuli-responsive polymers.

However, the printers and base materials used to produce such materials systems have until now been highly specialised, custom-made and expensive.

Multiple layers

To produce materials systems that react to changes in moisture, the researchers combined multiple swelling and stabilizing layers to realize a complex movement mechanism – a coiling structure that pulls tighter by unfolding ‘pockets’, and which can loosen up again on its own when the pockets release and the coiled structure returns to the open state.

The air potato climbs trees by applying pressure to the trunk of the host plant. To do this, the plant first winds loosely around a tree trunk. Then it sprouts ‘stipules’ – basal outgrowths of the leaves – which increase the space between the winding stem and the host plant. This creates tension in the winding stem of the air potato. To imitate these mechanisms, the researchers constructed a modular material system by structuring its layers so that it can bend in different directions and to different degrees, coiling and forming a helix structure. Pockets on the surface cause the helix to be pushed outwards and put under tension, causing the entire material system to contract.

“So far, our process is still limited to existing base materials that respond to moisture,” said Professor Achim Menges of University of Stuttgart. “We’re hoping that in the future, inexpensive materials that also respond to other stimuli will become available for 3D-printing and can be used with our process.”

The research has been presnted in the journal Advanced Science.

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