A new method called “reactive 4D printing” creates objects that can take on alternate forms in response to changes in temperature, electric current, or stress.
The method could bring soft robots and biomedical implants that reconfigure themselves upon demand closer to reality.
The researchers first reported their ability to make morphing structures in a mold in 2018. But using the same chemistry for 3D printing limited structures to shapes that sat in the same plane. That meant no bumps or other complex curvatures could be programmed as the alternate shape.
Overcoming that limitation to decouple the printing process from shaping is a significant step toward more useful materials, says Rafael Verduzco, an associate professor of chemical and biomolecular engineering and of materials science and nanoengineering at Rice University .
“These materials, once fabricated, will change shape autonomously,” Verduzco says. “We needed a method to control and define this shape change. Our simple idea was to use multiple reactions in sequence to print the material and then dictate how it would change shape. Rather than trying to do this all in one step, our approach gives more flexibility in controlling the initial and final shapes and also allows us to print complex structures.”
The lab’s challenge was to create a liquid crystal polymer “ink” that incorporates mutually exclusive sets of chemical links between molecules. One establishes the original printed shape, and the other can be set by physically manipulating the printed-and-dried material. Curing the alternate form under ultraviolet light locks in those links.
Once the two programmed forms are set, the material can then morph back and forth when, for instance, it’s heated or cooled.
The researchers had to find a polymer mix that could be printed in a catalyst bath and still hold its original programmed shape.
There were a lot of parameters we had to optimize—from the solvents and catalyst used, to degree of swelling, and ink formula—to allow the ink to solidify rapidly enough to print while not inhibiting the desired final shape actuation,” Barnes says.
One remaining limitation of the reactive 4D printing process is the ability to print unsupported structures, like columns. To do so would require a solution that gels just enough to support itself during printing, she says. Gaining that ability will allow researchers to print far more complex combinations of shapes.
“Future work will further optimize the printing formula and use scaffold-assisted printing techniques to create actuators that transition between two different complex shapes,” Barnes says. “This opens the door to printing soft robotics that could swim like a jellyfish, jump like a cricket, or transport liquids like the heart.”
Today’s 3D printing creates items from a wide variety of materials — plastic, ceramic, glass, metal, and even more unusual ingredients such as chocolate and living cells. The machines work by setting down layers of material just like ordinary printers lay down ink, except 3D printers can also deposit flat layers on top of each other to build 3D objects.
“Today, this technology can be found not just in industry, but [also] in households for less than $1,000,” said lead study author Dan Raviv, a mathematician at MIT. “Knowing you can print almost anything, not just 2D paper, opens a window to unlimited opportunities, where toys, household appliances and tools can be ordered online and manufactured in our living rooms.”
Now, in a further step, Raviv and his colleagues are developing 4D printing, which involves 3D printing items that are designed to change shape after they are printed. [The 10 Weirdest Things Created By 3D Printing]
“The most exciting part is the numerous applications that can emerge from this work,” Raviv told Live Science. “This is not just a cool project or an interesting solution, but something that can change the lives of many.”
In a report published online today (Dec. 18) in the journal Scientific Reports, the researchers explain how they printed 3D structures using two materials with different properties. One material was a stiff plastic, and stayed rigid, while the other was water absorbent, and could double in volume when submerged in water. The precise formula of this water-absorbent material, developed by 3D-printing company Stratasys in Eden Prairie, Minnesota, remains a secret.
The researchers printed up a square grid, measuring about 15 inches (38 centimeters) on each side. When they placed the grid in water, they found that the water-absorbent material could act like joints that stretch and fold, producing a broad range of shapes with complex geometries. For example, the researchers created a 3D-printed shape that resembled the initials “MIT” that could transform into another shape resembling the initials “SAL.”
“In the future, we imagine a wide range of applications,” Raviv said. These could include appliances that can adapt to heat and improve functionality or comfort, childcare products that can react to humidity or temperature, and clothing and footwear that will perform better by sensing the environment, he said.
In addition, 4D-printed objects could lead to novel medical implants. “Today, researchers are printing biocompatible parts to be implanted in our body,” Raviv said. “We can now generate structures that will change shape and functionality without external intervention.”
One key health-care application might be cardiac stents, tubes placed inside the heart to aid healing. “We want to print parts that can survive a lifetime inside the body if necessary,” Raviv said.
The researchers now want to create both larger and smaller 4D-printed objects. “Currently, we’ve made items a few centimeters in size,” Raviv said. “For things that go inside the body, we want to go 10 to 100 times smaller. For home appliances, we want to go 10 times larger.”
Raviv cautioned that a great deal of research is needed to improve the materials used in 4D printing. For instance, although the 4D-printed objects the researchers developed can withstand a few cycles of wetting and drying, after several dozen cycles of folding and unfolding, the materials lose their ability to change shape. The scientists said they would also like to develop materials that respond to factors other than water, such as heat and light.
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