Drying technique helps flat wooden objects morph into 3D shapes

In nature, plants and some animals can alter their own shapes or textures. Even after the cutting down of trees, the wood can change shape as it dries. It shrinks unevenly and warps because of variations in fibre orientation within the wood.

Doron Kam, a graduate student at the Hebrew University of Jerusalem, said despite warping being an obstacle, the research team behind the technique sought to understand this phenomenon and “harness it into a desirable morphing”.

Unlike some natural objects, artificial structures can’t typically shape themselves, said Eran Sharon, one of the project’s principal investigators. In recent years, scientists have printed flat sheets that could form themselves into 3D shapes after a stimulus, such as a change in temperature, pH or moisture content.

However, Sharon noted that such self-morphing sheets were made from synthetic materials, such as gels and elastomers. “We wanted to go back to the origin of this concept, to nature, and do it with wood,” he explained.

The team had previously developed an environmentally friendly water-based ink composed of wood-waste microparticles known as “wood flour” mixed with cellulose nanocrystals and xyloglucan, which are natural binders extracted from plants. The researchers then began using the ink in a 3D printer.

They recently discovered that the way the ink is laid down, or the “pathway,” dictates the morphing behavior as the moisture content evaporates from the printed piece.

For instance, a flat disk printed as a series of concentric circles dries and shrinks to form a saddle-like structure reminiscent of a Pringles crisp, while a disk printed as a series of rays emanating from a central point turns into a dome or cone-like structure.

According to the researchers, the ultimate shape of the object can also be controlled by adjusting print speed. This is because shrinkage occurs perpendicular to the wood fibres in the ink and print speed changes the alignment of those fibres. A slower rate leaves the particles more randomly oriented, so shrinkage occurs in all directions. Faster printing aligns the fibres with one another, so shrinkage is more directional, the team said.

The scientists learned how to program the print speed and pathway to achieve a variety of final shapes. They found that stacking two rectangular layers that are printed in different orientations yields a helix after drying.

In their latest work, the team found they can program the printing pathway, speed and stacking to control the specific direction of shape change, such as whether rectangles twist into a helix that spirals clockwise or counterclockwise.

The team said that further refinement of the technique will allow them to combine the saddles, domes, helices and other design motifs to produce objects with complicated final shapes, such as a chair.

Kam said the technique could ultimately make it possible to make wood products that are shipped flat to the end user, which could reduce shipping volume and costs: “Then, at the destination, the object could warp into the structure you want”.

Sharon added that, eventually, it may be feasible to license the technology for home use so consumers could design and print their own wooden objects with a regular 3D printer.

The team is also exploring whether they could make the morphing process reversible. “We hope to show that under some conditions we can make these elements responsive – to humidity, for example – when we want to change the shape of an object again,” Sharon concluded.