Engineers at Princeton University have developed an easy way to make composite elastic materials in a pixel-by-pixel method that's as simple as an inkjet printer. Image: Ella Maru Studio.
Engineers at Princeton University have developed an easy way to make composite elastic materials in a pixel-by-pixel method that's as simple as an inkjet printer. Image: Ella Maru Studio.

Borrowing a technique from inkjet printers, researchers at Princeton University Engineering School have rolled out a pixel-by-pixel method for programming and manufacturing soft structures for use in robotics, biomedical devices or architectural features.

The new printing technique creates pixelated sheets of soft material as easily as pressing a button. Each pixel can be programmed uniquely to create composite shapes, colors and mechanical abilities. And it works with a class of materials — curable elastic polymers — that cannot be printed with conventional inkjets or 3D printers.

“The selling point here is the simplicity of the methodology. All you need are two plates with a bunch of holes,” said Pierre Thomas Brun, assistant professor of chemical and biological engineering.

The materials start as fluids that cure into solids once they are deposited. The key is in how the fluids flow as they set. A deep understanding of this behavior led Brun’s team to print these composite materials without the need for complicated machinery. Instead, they let nature do the work.

Brun said this new approach, which applies age-old fluid dynamics to a modern materials problem, keeps costs low without cutting corners. “You will get a structure that is very precise,” he said. “This frugal approach is not a compromise on the quality.” The team’s work, reported in a paper in Advanced Materials, extends additive manufacturing into new material domains that are especially useful for biologically inspired designs.

The pixels in an inkjet printer use four droplet colors to create millions of apparent hues. Depending on how they are arranged on the paper, these dots can be made into myriad shapes, from simple letters to elaborate trees.

Scientists have wanted to do something similar with soft composite materials. “If you want a material that interacts well with humans, you want it to be soft,” Brun said. But the gooey liquids that cure into elastic solids have proven too viscous for inkjets and too pliant for 3D printers. The new approach finds a way to work with the inherent properties of curable elastic polymers such as silicone rubber, and could be extended to work with some liquid metals and molten glass.

“This simple and versatile technique opens many paths to develop ‘soft-robotic’ devices, at least for fast prototyping,” said José Bico, a physicist at ESPCI-Paris in France, who was not involved in the research. “Sometimes you think you need to make very complicated things, but what works in practice are very simple things.”

When a straw is put into a glass of water and the water rises through the straw, that’s due to capillary action, in which liquid flows into a narrow space. In this study, that narrow space is a thin layer between two acrylic plates. When the researchers dropped a sample of liquid polymer into holes in the top plate, it seeped into the space between the plates in mathematically predictable ways. The liquids spread and eventually came in contact with each other, forming the edge of each pixel.

Because of their honey-like viscosity, the pixels didn’t mix. Moreover, without air between them, the liquid polymers essentially zipped together to create geometric features. The polymers could then be cured into a solid in a few minutes at ambient temperatures, creating a sheet of soft-pixelated elastic.

As their proof of concept, the researchers used colored liquid polymers to make a variety of composite images — a Princeton-themed ‘P’ and a figure inspired by Space Invaders, the pioneering arcade game. The same principle can be applied to pixels with varied mechanical or magnetic properties, leading to vast new applications in soft robotics, medical devices and beyond.

“You could easily pick a region where you add magnetic particles so that when the sheet is composed, it has regions that could be actuated with magnets,” said co-author Christopher Ushay, a PhD candidate in Brun’s lab.

To determine where to put the holes for creating a specific design, the researchers borrowed a mathematical trick from image analysis called watershed transform. Imagine river basins on a topographic map: if you flood the area, which river basin will the water run into?

The researchers used the same idea to determine where the liquid polymers would flow before they came into contact. But to make matters more complex, the liquid polymers have different viscosities, which means they flow at different rates. “This is taking the watershed transform a little bit farther,” Ushay said. “You’re no longer doing a transform with respect to space, but also with respect to time.”

The researchers were able to create different patterns, called Voronoi tessellations, depending on how many holes they used and the spacing between them. For example, by using four holes, they could create square patterns, and by using six sources, they could create triangles.

“The surprising thing here is how well it works,” Brun said. “If we don’t start in a perfectly synchronized way, you’d think we’d get a slightly crooked pattern. But this is not the case. Things manage to catch up. This is very important for the robustness of the approach.”

Future research will look at how to stack the thin pixelated sheets to created voxels, or volumetric pixels. “We’re obtaining these very thin sheets,” Brun said. “But we’re made of 3D materials.”

The US National Science Foundation was one of the funders of the work, as part of an effort to find ecological approaches to manufacturing. By taking advantage of natural properties, this technology requires less energy because it can be performed at room temperature and needs no specific building to be designed to operate it. The printing technique could also be scaled up for architectural structures such as bridges and arches, or scaled down for microscopic structures.

This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.