Cellulose is the main building block of plant material, and it may become the main building block of many 3D printed items, as well. The all-natural, eco-friendly, abundant material has been of interest to many scientists studying 3D printing, and the latest research comes from Empa of Switzerland. Researchers Dr. Gilberto Siqueira and Dr. Tanja Zimmermann from the Laboratory for Applied Wood Materials have been working with colleagues from Harvard University and ETH Zurich to develop an environmentally friendly ink for direct ink writing methods of 3D printing, made from cellulose nanocrystals.
The cellulose biopolymer consists of glucose chains organized in long, fibrous structures. In certain places, the cellulose fibrils show a more ordered structure.
“The places with a higher degree of order appear in a more crystalline form. And it is these sections, which we can purify with acid, that we require for our research,” said Siqueira.
The result is cellulose nanocrystals, or tiny rod-like structures that are 120 nanometers long and have a diameter of 6.5 nanometers. Those crystals are what the researchers used to create a new, more environmentally friendly 3D printing ink; although cellulose inks have been created before, previous inks have been made up of a maximum of 2.5 percent cellulose nanocrystals, or CNC. The Empa team’s ink is composed of 20 percent CNC.
“The biggest challenge was in attaining a viscous elastic consistency that could also be squeezed through the 3D printer nozzles,” said Siqueira.
The first formulations the researchers tried were water-based, but they produced a very brittle material, so they tried a new polymer-based formula. Once it was 3D printed and cured with UV light, the CNC cross-linked with polymer building blocks, which gave the material a higher level of mechanical rigidity. It wasn’t easy to get to that point, though.
“Most polymers are water-repellent or hydrophobic, whereas cellulose attracts water — it is hydrophilic,” Siqueira explained. “As a result they are not very compatible.”
Therefore, the researchers had to chemically modify the cellulose nanocrystals. Then something interesting happened. After conducting X-ray analysis of the 3D printed nanostructures, the researchers discovered that the CNC had aligned itself almost perfectly in the direction it had been printed it. The mechanical strength used to extrude the material through the nozzle had pushed them into the right direction.
“It is pretty interesting that one can so easily control the direction of the nanocrystals, for example, if you want to print something that should have a specific mechanical rigidity in a certain direction,” said Siqueira.
The mechanical properties of the material, in addition to its natural base, make it highly appealing for a number of applications. The cellulose crystals, which can be obtained from numerous different sources including plants and bacteria, differ from each other morphologically and in size, but not in their properties, so they’re an abundant resource. The automobile and packaging industries could benefit from using the material, which is versatile and can be chemically modified if necessary during the 3D printing process. In Siqueira’s opinion, however, the most important applications are biomedical ones such as implants or prostheses.
Empa is further investigating the potential applications of cellulose 3D printing ink, as well as other biologically-based inks. The researchers are using a 3D-Bioplotter system from EnvisionTEC in their work on this front.
“Research in this field is only just beginning,” said Siqueira. “Printing with biopolymers is currently a very hot topic.”
Discuss in the Empa forum at 3DPB.com.[Source/Images: Empa]
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