Nature-inspired 3D-printing method shoots up faster than bamboo

Researchers in Beckman’s Autonomous Materials Systems Group created “growth printing,” which mimics tree trunks’ outward expansion to print polymer parts quickly and efficiently without the molds and expensive equipment typically associated with 3D printing. Their work appears in the journal Advanced Materials.

“Humans are incredibly talented at making things. Completely new manufacturing processes are hard to find. Growth printing is entirely new, which is thrilling,” said Sameh Tawfick , a professor of mechanical science and engineering at the University of Illinois Urbana-Champaign and project lead.

Tawfick said the most common industrial manufacturing technology is injection molding, where molten polymers take shape in a metal mold. Though effective for mass production, maintaining the molds and curing ovens (where the plastic hardens) can be cost-prohibitive and unwieldy — especially for large objects like boat hulls or fan blades. Additive manufacturing, which prints 3D objects like a layer cake, is mold-less and ideal for custom parts like prosthetics.

“Polymer 3D printing equipment has matured, but there are still aspects that make it expensive and very slow,” Tawfick said. “Our goal was to increase the manufacturing speed, size and material quality while maintaining a low cost. This process that we came up with is truly fast and inexpensive.”

First, Sameh and his colleagues pour amber-colored liquid resin called dicyclopentadiene, or DCPD, into an open glass container submerged in ice water. They heat a center point in the resin to 70C. As the reaction takes over, heat radiates outward from the original point of contact at 1 mm/s, more than 100 times faster than the desktop 3D printers available for home use and 60 times faster than the world’s fastest-growing species of bamboo. Everything the heat touches hardens into a growing sphere, like if the mythical King Midas seized the Earth’s core. Self-sustained by heat’s steady release, the reaction — called frontal ring-opening metathesis polymerization and nicknamed FROMP — uses minimal energy to harden the resin into its solid form: poly- dicyclopentadiene, or p-DCPD.

As the hardened sphere grows, the researchers alter its shape by pulling it out of the resin like an apple out of gooey caramel. Since the liquid-to-solid reaction only happens below the surface, the researchers can lift, dip or spin the solid part like blown glass to manipulate its size and shape. For example: to create a corrugated, or wavy, edge, the researchers lift the resin slightly, hold it still, and repeat.

The researchers designed their process to mimic how a tree steadily expands outward, ring by ring. In nature, elements like gravity, wind and temperature complement and complicate a tree’s tendency to grow symmetrically, resulting in trees that bow in the wind or reach toward a patch of sunlight in the forest canopy.

Tawfick became enamored of living organisms’ growth patterns and resulting shapes — also known as morphogenesis — upon reading D’Arcy Wentworth Thompson’s book, “On Growth and Form.” Last August, when Tawfick was promoted from associate professor to full professor, he dedicated the book to the University Library.

Using their new method, Tawfick and his colleagues fabricated everyday items such as a pinecone, a raspberry and a squash. These are all axisymmetrical shapes, or symmetrical around a vertical axis. Non-symmetrical shapes are more difficult, but possible; for example, the researchers sculpted a kiwi bird by allowing the spherical body to expand below the surface before pulling it up just in time to create a diminutive head and minute beak.

“It is a beautiful and simple application of a reaction-diffusion process, which is found in many natural systems. The speed and energy efficiency of the growth-printing process make this process particularly attractive. On the modeling side of this collaborative project, we developed a computational tool that predicts the upward motion of the rod needed to achieve a target shape of the manufactured object,” said Philippe Geubelle, Illinois professor of aerospace engineering and co-author on the paper.

This method’s limitations are the same ones found in nature. Printing curved objects, like bananas, is theoretically possible but difficult to program mathematically, as are complex shapes “like a thorn in a rose,” Tawfick said.

“It’s hard to find a perfect cube in nature. I don’t know of any plant or organism that looks like a perfect cube. Similarly, our process cannot make a perfect cube. It’s an interesting mirror of nature,” he said.

Tawfick says the process is “simple and highly marketable” and hopes it can one day be used to create large polymer-based products like wind turbine blades. The project is funded through the U.S. Department of Energy Office of Science Basic Energy Sciences program.

“Basic energy science could lead to transformative manufacturing, meaning something with a transformative impact on our economy. This is a successful example and was made possible through collaboration here at the Beckman Institute with people from all areas of expertise,” Tawfick said.

First author and Illinois graduate student Yun Seong Kim said the project demonstrated true teamwork:

“It was really a work of true teamwork, because it required expertise in various backgrounds and we all came together to make it happen,” he said.

Coauthor Randy Ewoldt, the Alexander Rankin Professor of Mechanical Science and Engineering at Illinois, adds: “The many advances of this work resulted because of the outstanding teamwork. The Illinois culture of collaborative excellence shines bright.”