Inner Workings: 3D printer innovations tackle complexity of metamaterials, living tissue

Speed It Up

“Watching a 3D printer operate is kind of fun but it usually takes forever,” says Joseph Desimone, a chemical engineer and entrepreneur who’s on leave from his faculty position at the University of North Carolina to work as Carbon’s cofounder and CEO. Until recently, 3D printing has been too slow for mass manufacturing, requiring hours or days to make small objects. Desimone’s Carbon has made it possible to print continuously, which speeds up the process, compared with conventional technologies that deposit printed layers one at a time. Carbon’s printers can make parts in minutes.

The company has adapted a technique called stereolithography. Typically, stereolithography entails depositing material one layer at a time, in a stop-and-start process. But in Carbon’s case, researchers shine patterns of light into a polymer precursor bath to print continuously; a window at the bottom of the printer lets in not only light but a thin layer of oxygen that prevents the hardened material from adhering. It’s this oxygen barrier that obviates the need to repeatedly pry off printed layers, enabling their printers to run continuously (1). They call the process continuous liquid interface production (CLIP).

This speed boost, along with the company’s development of new kinds of printable materials, is opening up 3D printing for use in real manufacturing, not just prototyping, says Desimone. With advanced materials, 3D printing can make “real parts that are functional,” such as components for jet engines, custom car parts, and dental implants. Carbon is developing a family of printable polymers, including rubbery elastomers, hard resins, and rigid polyurethanes. Others have developed ceramics and metals that can be 3D printed.

Being able to make any arbitrary shape should free up designers, says Desimone. Plastic parts typically have had to be designed so that they can be made in molds, and pried out once they solidify. Breaking the mold and using 3D printing will allow designers and engineers to experiment. They can also start thinking about mass customization, whether that means printing a shoe sole that fits the quirks of each foot, a hearing aid that sits comfortably in the whorls of a person’s ear, or a vascular stent that’s sized to fit a particular artery just right.

Get Weird

Carbon is focused on how 3D printing can enable new kinds of manufacturing. But the method also has promise for enabling researchers—and someday companies—to make materials with entirely new properties. “We want to discover what could be better than existing manufacturing tools—or even existing materials,” says Nicholas Fang, a nanophotonics researcher at the Massachusetts Institute of Technology.

Fang and others are using 3D printing to make metamaterials. Although made from existing materials, such as metals, ceramics, and silicon, metamaterials have exotic properties. Some can, for example, bend light in ways that could be useful in optical computers and invisibility cloaks; others are light as a feather yet strong as steel. Because 3D printing methods can be used to easily pattern complex structures, they’re well suited to making metamaterials.

Metamaterial properties come not just from the materials’ chemistry, but from their structure. In the case of an optical metamaterial, its features are tailored to the length scale of the wavelength of light with which they’re designed to interact. “If you can harness both chemical and structural control, it’s quite powerful,” says Christopher “We want to discover what could be better than existing manufacturing tools—or even existing materials.”—Nicholas FangSpadaccini, a researcher at the Lawrence Livermore National Laboratory in California.

Researchers’ desire to realize theoretical metamaterials has driven printer improvements. “There’s no other way to fabricate the structures we’re making,” says Spadaccini. They’re simply too fine-grained and complex to be made in a mold or carved out by conventional means, such as lithography. Examined closely, some look like geometric lattices out of an Escher sketch, made up of small beams crisscrossing in a stellated polyhedral that consists of delicate, intertwined, geometrical lattices.

Materials that are ordinarily tough but brittle, such as ceramics, become more ductile and metal-like when they’re in the form of nanoscale structures, such as the nanorods in a latticed sheet. And because these materials are mostly empty space, they’re lightweight. Such airy materials could be used to replace heavy engine parts without compromising on durability.

That won’t happen for a few years. Today it’s only possible to print small amounts of these sorts of metamaterials. The ultimate 3D printer would print large parts made out of multiple materials, with a nanoscale resolution, and do it quickly. Creating even small bits of some ultralight, ultrastrong designer materials with conventional 3D printers takes days because of their intricately patterned structures. Some are composed of hollow struts with walls just a couple nanometers thick.

Engineers at the Hughes Research Laboratory in Malibu, California, may have a solution. Their prototype printer generates a 61-centimeter by 61-centimeter area in 20 seconds. Like Carbon, they've developed a variation on stereolithography. However, to make the kinds of patterns needed for their metamaterials, the light can’t simply be projected in a digital image; the pattern is too complex. Instead, light is shone through a patterned mask that can make patterns with any angle. Spadaccini’s group has developed a method for printing 10-micrometer resolution features of any design over a 150-millimeter diameter area. But to make that same 61-centimeter by 61-centimeter area would take 8–10 hours.

Besides speed and resolution, researchers also want to make it possible to print in a full rainbow of materials, not just one at a time. Spadaccini’s group has developed printers that can combine metals, plastics, and ceramics. They do it by first laying down a pattern of one material, then coating it with another material, and finally applying heat to sinter them together. That sounds simple, but materials like plastics and ceramics don’t always stick to each other and sinter at the same temperatures. Spadaccini’s group has to pair them carefully. These multimaterial methods have enabled him and Fang to recently print a metamaterial that shrinks instead of expanding when it’s heated (2).

Living Materials

Living tissues offer another complexity challenge for 3D printers, and tissue engineers see promise. Heart tissue, for example, is made up of aligned muscle cells and laced with blood vessels that take complex 3D paths. Blood vessels have proven particularly difficult to engineer in the laboratory.

In 2014, Harvard University 3D printing specialist Jennifer Lewis realized that one of her inorganic printing inventions, which the group called fugitive ink, was well suited for making blood vessels, so they made a cell-friendly version. The ink stays gel-like at room temperature but, counter-intuitively, becomes liquid at lower temperatures. The researchers print the fugitive ink where they want a blood vessel, then lower the temperature, flush it out, and fill the remaining gaps with blood vessel-lining cells. The result mimics a tissue with blood vessels coursing through (3).

Lewis has developed many kinds of 3D printable inks, including ones that conduct electricity, ones loaded with living biological cells, and materials for sensing motion. In each case, she takes pains to make sure all of the materials and processes are biocompatible. When living cells are exposed to solvent chemicals, or squeezed through a printhead, they could die or deteriorate.

The potential applications are many and varied. Lewis has collaborated with engineers and biologists to use her printers and inks to make an autonomously moving soft robot that looks like an octopus, as well as in vitro models of heart and kidney tissue for pharmaceutical testing. These printed devices include monitoring sensors within each organoid, eliminating the need to use a microscope to measure, for example, the heart rate of the beating tissue when exposed to a drug (4).

Lewis sees her work as more than just a series of technological tweaks and innovations. “These,” she says referring to her laboratory’s advances thus far, “can make a huge impact on society.”