You can 3D print just about anything these days, from car parts to cakes. Most additive manufacturing uses plastic or metal (or sugar), because it is easy to melt these materials down and extrude them.
But, there has been blowback about the current state of 3D printing, because of its significant environmental impact. It uses considerable amounts of energy in manufacturing—50 percent more than injection molding—and creates a lot of non-biodegradable materials, which some designers and environmentalists consider unnecessary.
Printing with purely natural materials could alleviate that second environmental burden, but that has long seemed impossible. “Wood is composed of cellulose, hemicellulose and lignin. None of these components melt, and they burn when heated,” says Paul Gatenholm, a chemistry and biopolymer technology professor at Chalmers University of Technology in Sweden.
Gatenholm and his team at the Wallenberg Wood Science Center at Chalmers has, nonetheless, come up with a way to 3D print wood, making it possible to build biodegradable structures. Cellulose, the structure that gives wood its strength, is solid, sustainable and abundant, so they saw a lot of potential for printing. Plastic and metal, which are used in most additive manufacturing, melt when they’re heated, which makes for a runny, liquid material that’s conducive to printing. The researchers had to change the consistency of the wood fibers to turn cellulose into an injectable liquid.
They mixed the cellulose nanofibrils—essentially the same pulp that’s used to make paper—into a slurry that was 98 percent water. The challenge was dialing in this ratio to produce a mixture that was flexible, but that would also form solid structures, and that wasn’t temperature sensitive.
Gatenholm, who has a background in tissue engineering, had been working on similar technology in human biology, with the hope of making physical implants that would grow along with a person and adapt to their specific body chemistry. “We realized potential of this new material as bioink in 3D bioprinting,” he says. “One day we dried the sample and saw that we could produce fine structures like fabrics. We started to study the drying process of this gel and discovered that we could control it and preserve 3D architecture.”
Once they got the cellulose's consistency right for printing, the researchers started to experiment. They printed large-scale wooden structures, including chairs, and thin flexible materials, such as clothing. Gatenholm believes the technology could completely change additive manufacturing, and he is not alone in his thinking.
Other researchers, like Neri Oxman at MIT's Mediated Matter lab, have been trying to 3D print natural materials to save waste. "In the natural world, everything grows. If we can create technology that grows materials instead of subtracting them, then we can control lots of elements in that process," Oxman has said.
In addition to building structures out of the cellulose, Gatenholm and his team found a way to put carbon nanotubes into the gel, to make it conductive. This gives them the potential to build things that are both biodegradable and have built-in electrical currents, like bandages that could signal doctors about the health of wounds or clothes that could turn body heat into electricity.
"3D printing technology, which could only use metals and plastics, became suddenly green and organic,” Gatenholm says.