New cellulose 3D printing technique gets complex

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Researchers in Switzerland have developed a new method for 3D printing complex objects with a higher cellulose content than has previously been achieved.

Found in trees and plants, cellulose has excellent mechanical properties and is of particular interest to manufacturers and materials scientists looking to produce sustainably. It has previously been trialled in additive manufacturing, but printing complex geometries with cellulose has so far proved challenging.

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The researchers, from ETH Zurich and the Swiss Federal Laboratories for Materials Science and Technology (Empa), used a series of novel techniques to increase the percentage of cellulose in the 3D printed objects. They began with a water-based ink containing between 6 and 14 per cent cellulose. After printing with this ink, they put the objects in a bath containing organic solvents, causing the cellulose particles to aggregate. This made the parts shrink but also increased the concentration of cellulose.

In the next step, the objects were soaked in a solution containing a photosensitive plastic precursor. By removing the solvent by evaporation, the plastic precursors infiltrated the cellulose-based scaffold. To convert the plastic precursors into a solid plastic, they then exposed the objects to UV light, producing a composite material with a cellulose content of 27 per cent. The work is published in Advanced Functional Materials.

"The densification process allowed us to start out with a 6 to 14 per cent in volume of water-cellulose mixture and finish with a composite object that exhibits up to 27 volume per cent of cellulose nanocrystals," said research lead Michael Hausmann, from Empa.

Using different precursors, the team was able to tune the elasticity of the finished objects, creating both hard and soft cellulose-based plastics. X-ray analysis and mechanical testing showed that the cellulose nanocrystals were aligned similarly to those present in natural materials.

"This means that we can control the cellulose microstructure of our printed objects to manufacture materials whose microstructure resembles those of biological systems, such as wood," said Rafael Libanori, senior assistant in ETH Professor André Studart's research group.

So far, the cellulose 3D printing technique has been used to produce a model of a human ear and the team believes the technology could have clinical potential, as well as automotive and packaging applications.