Earlier this summer, researchers from the Lawrence Livermore National Laboratory (LLNL) were working on using a direct ink writing (DIW) process to 3D print silicone metamaterials with shape memory behavior – which technically makes them 4D printed, as the fourth dimension is time in this instance. Now we’ve learned that, for the first time, the LLNL researchers have successfully 4D printed flexible, stretchable composite silicone materials with this shape memory behavior.

The team detailed the process in a research paper that was published in July in the online Scientific Reports journal. They added gas-filled, hollow microballoons into the silicone-based ink to create the unique material, which allows it to be compressed at an elevated temperature and remain so while it cools down. The gas in the balloons expands once it’s reheated, and the structures will then return to their original shapes.

Because the material that the LLNL researchers developed can be 3D printed into an arbitrary net shape and used to make a porous structure with open and closed cells, they believe it could be useful in making customizable, form-fitting cushioning that’s activated by body heat, which could be very helpful in the sportswear arena for items like shoes and helmets.

“You could use this for any customized mechanical energy-absorbing material. The neat thing is if the wearer grows a little bit and wants to refit the material, they just heat it up to expand it, put it on and let it cool to once again customize the fit. It’s reversible,” said Eric Duoss, a materials scientist and co-principal investigator. “It’s a completely new material really, and we’re excited about it. It’s a material that should have a lot of commercial potential and should be ripe for technology transfer to industry.”

LLNL researcher and lead author Amanda Wu explained that later, the process could even be scaled up to make larger parts for transportation and packaging applications.

Wu said, “The impressive part was how well the structures could recover their shape after they were reheated. We didn’t see a distorted structure, we saw a fully recovered structure. Because the silicone network is completely crosslinked, it holds the part together, so the structure recovers its original shape in a predictable, repeatable way.”

L-R: LLNL researchers Ward Small, Amanda Wu and Taylor Bryson. [Image: Carrie Martin, LLNL]

The researchers discovered the material by accident as they were working to develop a hierarchical porous material that would be able to recover completely after exposure to heat and then being compressed (zero compression set). But instead, the exact opposite occurred, and LLNL scientists Ward Small and Thomas Wilson, who’s also a co-principal investigator, thought that the gas in the material could cause the structure to re-expand if it was reheated, and decided to test out their theory, which turned out to be correct.

“Initially, this was an accelerated aging test to see if the material would be useful. This material took on a pretty large compression set and that made us wonder if it was permanent,” Small explained. “We weren’t really thrilled about that, but we had experimented with shape memory in the past and tried to see if it could recover its shape when heated. We tested it and it did.”

The polymer micro-balloons embedded in the silicone are the key ingredient in this shape memory material: the micro-balloon’s thin, polymeric shell has a glass transition temperature, and anything below that causes the shell to be rigid, while anything above makes it soft. So the shells soften when the composite material is heated above this temperature, causing compression and altering their shape so it remains deformed and “resists re-expansion of the silicone matrix when cooled.”

Once the balloons are heated again, they expand, allowing the structure to revert back.

LLNL researcher Taylor Bryson, who took care of the experimental work for the micro-balloon project, said, “We’d take them out hot and let them cool in the presence of a compressive force and test their thickness to measure compression set. Then to see if they’d re-expand, we’d reheat them, put them back in the ovens at the same temperatures or hotter in the absence of a compressive force, and see if they’d recover their shape. Surprisingly, we got close to 100 percent recovery.”

Bryson mixed inks that wouldn’t jam the nozzle of the team’s 3D printer, but would also be able to incorporate the micro-balloons, and then worked on setting the shape of his DIW 3D printed samples by heating, compressing, cooling, and re-expanding them. The composite ink material was extruded from the nozzle at room temperature, in order to form structures that resembled woodpiles and had “controlled porosity and architecture.”

The research team said that 3D printing was the best choice in creating the material because they were able to have more control over its composition and 3D geometry; in addition, the material is more functional and lightweight when it’s 3D printed. They explained that their approach is unique because the shape memory component is actually added into the material itself – this way, they’ve showed that micro-balloons can combine shape memory into any kind of polymeric base materials, such as stretchable elastomers.

“Historically, shape memory polymers tend to be very rigid. By incorporating micro-balloons into a rubbery matrix, we’ve created a composite that is soft and stretchy, even below the glass transition temperature of the micro-balloons, which is a shape memory material with previously unattainable qualities. It turned out to be very fortuitous,” explained Duoss.

The team has filed a patent application for its unique material; their research was funded by a Laboratory Directed Research and Development (LDRD) project, and other contributors include Stephanie Schulze of the Department of Energy’s Kansas City National Security Campus and LLNL scientists Emily Cheng and Thomas Metz.

[Source/Images: LLNL]

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