After 3D printing now comes 4D printing
In their development, the scientists from the University of Freiburg and the University of Stuttgart (both Germany) took their cue from the movement mechanisms of a twining climbing plant, the air potato (dioscorea bulbifera). Using the method, the team produced the prototype of a forearm splint that adapts to the wearer and can be further developed for medical applications.
3D printing (aka additive manufacturing) has established itself as a manufacturing process for a wide range of applications. It can also be used to create intelligent materials and material systems that are movable after production and change their shape independently by means of an external stimulus such as light, temperature or humidity. This so-called 4-D printing, with which a shape change triggered by a stimulus can be specified, greatly expands the application potential of the systems. Such shape changes are made possible by the chemical composition of the materials, which consist of stimuli-responsive polymers. However, the printers and starting materials used to produce such materials have so far been highly specialised, expensive custom-made products.
But now – as a result of the research teams from Stuttgart and Freiburg – materials can be produced with commercially available 3-D printers that react to changes in humidity. They consist of a swelling and a stabilising layer. Due to their structure, these material systems can undergo changes in the shape of the entire system as well as individual parts. The researchers combined two reactive material systems and were thus able to realise a complex movement mechanism: A sinuous structure that tightens when pressure pockets are unfolded and that can release on its own when the pressure pockets collapse and the sinuous structure returns to an open state.
For the process, the scientists used a mechanism from nature: the air potato climbs up trees by itself applying pressure force against the stem of the host plant. To do this, the plant first loosely winds itself around a tree trunk and then sprouts stipules, so-called stipulae, which increase the distance between the climbing plant and the trunk of the host plant. In the process, the twining trunk of the air potato is put under tension. To mimic these mechanisms, the researchers have built the material system in a modular way: its layers are structured so that it can bend in different directions and to different degrees, forming a helix structure. Pockets on the surface ensure that the helix is pressed outwards and comes under tension, whereupon the entire material system contracts.
So far, the process is still limited to existing starting materials that react to moisture. However, the researchers hope that in the future, inexpensive materials will be available for 3D printing that react to other stimuli and that can also be used for the process.