Engineering news
Using cellulose for additive manufacturing is not a new idea - but the MIT team says that it has been able to overcome major drawbacks associated with the naturally occurring material by using cellulose acetate, a natural plastic made from purified cellulose.
For instance, when heated to create a “flowable” material to be used in 3D printers, cellulose decomposes. Also, hydrogen bonds between cellulose molecules usually make solutions too thick to easily print with.
However, because the number of hydrogen bonds are reduced in acetate groups, cellulose acetate can be dissolved in the natural compound acetone – commonly used for removing nail polish – and easily extruded through a nozzle, say the researchers.
Another benefit to using cellulose acetate for 3D printing, they add, is that it is already widely produced for a range of products, from coatings to photographic film, and is readily available. It also “much less” expensive than the typical plastic filament materials used for 3D printing.
A further optional treatment replaces the acetate groups and makes the printed parts even stronger than if they had been created using typical 3D printing plastics used in industry.
"After we 3D print, we restore the hydrogen bonding network through a sodium hydroxide treatment," said MIT postdoc Sebastian Pattinson, lead author of a paper. "We find that the strength and toughness of the parts we get ... are greater than many commonly used materials for 3D printing, including acrylonitrile butadiene styrene and polylactic acid."
Boosting the speed
While most existing 3D printers rely on heating the polymer to make it flow, their production speed is limited by the amount of heat that can be delivered to the polymer without damaging it.
But the MIT team says that their room-temperature cellulose process, which relies on evaporation of the acetone to solidify the part, could potentially be faster. And various methods could speed it up even further, such as laying down thin ribbons of material to maximise surface area, or blowing hot air over it to speed evaporation.
A production system would also seek to recover the evaporated acetone to make the process more cost-effective and environmentally friendly.
Pattinson and co-author John Hart also built in extra functions to the products they 3D printed from cellulose acetate. For example, by adding a small amount of antimicrobial dye to the cellulose acetate ink, they printed a pair of surgical tweezers with antimicrobial functionality.
The researchers demonstrated that the tweezers could kill bacteria when a fluorescent light was shone on them. Such custom-made tools "could be useful for remote medical settings where there's a need for surgical tools but it's difficult to deliver new tools as they break, or where there's a need for customised tools,” said Pattison.
'Not very green'
However, despite these advantages, a scientist not involved in the study says that "the route MIT researchers use is not very 'green'" - because of their use of acetone as a solvent. It's a "no go industrially," thinks Paul Gatenholm, a bioengineer at Chalmers University of Technology in Gothenburg, Sweden, who published his own work on 3D printing of cellulose nanofibrils last year.
Also, "they need to convert cellulose acetate into cellulose by hydrolysis, and it does not work very well for more thick structures," he says.
But "it is very positive that more researchers are looking at the challenge of 3D Printing of wood-based materials," adds Gatenholm.
The MIT research appears in the journal
Advanced Materials Technologies.