May 31, 2017 | By Benedict

Researchers from the Swiss Federal Laboratories for Materials Science and Technology (Empa) have developed an environmentally friendly ink for 3D printing made up of cellulose nanocrystals. The material can be used to make biomedical devices with outstanding mechanical properties.

Gilberto Siqueira 3D printing cellulose-based inks

Despite now being in the midst of an exciting biomedical research project, Empa researchers Gilberto Siqueira and Tanja Zimmerman have, for the last year or so, been focusing more on motors than medicine. The duo, whose work takes place at Empa’s Laboratory for Applied Wood Materials, have been using direct ink writing 3D printers to create automobile components and other heavy-duty parts with wood-derived printing materials.

But Siqueira and Zimmerman are now taking their 3D printing research and applying it to the most important vehicle of all: the human body.

The Empa researchers say they have developed a new, environmentally friendly 3D printing ink made from cellulose nanocrystals (CNC)—cellulose, a glucose-filled substance, being one of the main constituents of wood.

Of course, cellulose in wood doesn’t always take the form of crystals. To obtain the crystalline parts, the researchers have to find areas where the cellulose fibrils (small, slender fibers) exhibit a more ordered structure.

"The places with a higher degree of order appear in a more crystalline form,” Siqueira explained. “And it is these sections, which we can purify with acid, that we require for our research.”

Once this acid purification has taken place, the result is cellulose nanocrystals, tiny rod-shaped structures that are 120 nanometers long with a diameter of 6.5 nanometers.

Cellulose nanocrystals under the microscope

People have used these nanocrystals in 3D printing inks before, but usually in small quantities, generally bringing the total CNC content to around 2.5 percent maximum. The Empa researchers, however, were looking to develop something far more concentrated, with a CNC content of 20 percent.

But there are reasons why other researchers have thus far refrained from including this much CNC in their 3D printing inks. For one, the consistency of the CNC-laden ink makes it really hard to extrude on the laboratory’s EnvisionTEC 3D Bioplotter.

“The biggest challenge was in attaining a viscous elastic consistency that could also be squeezed through the 3D printer nozzles,” Siqueira said.

Still, just getting the material to print wasn’t even enough to show that the CNC-filled 3D printing ink could be a useful material for biomedicine: an initial, water-based ink printed beautifully, but ended up producing very brittle structures that would not have made useful implants or other biomedical devices. So the researchers tried putting the CNC in a polymer-based mix instead.

In principle, this could have been a disaster.

"Most polymers are water-repellent or hydrophobic, whereas cellulose attracts water—it is hydrophilic,” Siqueira said. “As a result they are not very compatible.”

3D printed jaw bone made from cellulose-based ink

Fortunately, after being printed and then hardened with UV radiation, the CNC “cross-linked” with the polymer, aligning itself almost perfectly in the direction it was printed in. This produced a material with excellent mechanical rigidity.

“It is pretty interesting that one can so easily control the direction of the nanocrystals,” Siqueira said. “For example, if you want to print something that should have a specific mechanical rigidity in a certain direction.”

There are several advantages to using the Empa researchers’ new 3D printing concoction for biomedical devices—and other objects too. For one, the ink is made from cellulose, a renewable and abundant material. And second, the material is biologically safe for use in bodily implants.

“The most important area of application for me is in biomedicine,” Siqueira added. “For example in implants or prostheses.”

The Empa researchers say this work is set to continue, with students now looking at developing the DIW 3D printing material for other applications.

"Research in this field is only just beginning," Siqueira said. "Printing with biopolymers is currently a very hot topic.”

A paper detailing the researchers’ findings has been published in Advanced Functional Materials.

Posted in 3D Printing Materials

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