15 Dec 2017
Engineers have always been inspired by the shapes and properties of living things, how they grow and the structures that some of them create (such as spider webs and weaver bird nests). Plants are particularly fascinating for the way they respond to their environment, sensing changes and adapting in ways that human technology has always found difficult to replicate.
Until now, that is. A team of Harvard University researchers have found a way to improve on current 3D printing by adding a fourth dimension: time. Using additive manufacturing technology and special composite inks, they have planted the seed of 4D printing by learning how to mimic the natural behaviour of orchids.
3D-printed objects tend to be rather stiff and rigid, and to retain the shape they were given during manufacturing. 4D objects however can change over time, and in response to what is happening around them.
The first 4D objects can change shape when immersed in water, thanks to the properties of hydrogel composite structures. The trick is to align the cellulose fibres within the composite ink, much like the grain in a piece of wood. This gives the objects directional flexibility, just like the leaves, roots and tendrils of plants.
Think of sawing or chopping firewood. If you try to go against the grain, it’s difficult (hence the expression). With the grain, it’s much easier. This phenomenon is known as anisotropy.
The results so far are certainly beautiful and fascinating – just like the plants, which inspired them. But why is this important?
4D printing is a revolutionary idea, but doesn’t necessarily make 3D printers obsolete. Existing additive manufacturing machines could easily be adapted to use the new composite inks, effectively upgrading them to 4D printers.
By using composite inks with known qualities, it will be possible to predict how they will shift their shapes in response to the presence of water or other environmental factors. This could help overcome the so-called “inverse problem” of 3D printing. Currently, 3D printers rely on a pre-programmed process or printing toolpath. With 4D printing and the use of complex mathematical models, in the near future it should be possible to reverse engineer the required properties once the ultimate shape that the object should be has been determined.
Copying designs or qualities found in Nature is nothing new: there’s a whole field of design called biomimetics, which is devoted to this. Successful examples include wind turbine blades influenced by the fins of humpback whales, Japanese bullet trains that have a long “beak” like a kingfisher, and aircraft paints that copy the dentricle structure of shark skin and can save thousands of gallons of fuel a year thanks to reduced drag.
4D printing takes these ideas to the next level. It could have exciting applications in the manufacture of smart fabrics and biomedical devices and implants, for example. Imagine a coat that changed shape or became more water resistant when it detected rain or snow, or a heart valve that altered shape in response to changes in the patient’s blood pressure.
Orchids are notoriously difficult to grow, but they could have just ushered in the blossoming of a whole new era in printing.