Sep 14, 2017 | By Benedict

Researchers at Lawrence Livermore National Laboratory have 3D printed composite silicone materials that are flexible, stretchable, and possess shape memory behavior. The “4D printed” materials could be used to make form-fitting cushioning for helmets, shoes, and other products.

Since we haven’t launched 4Ders yet, you’ll have to get used to seeing a few more “4D printing” articles on these pages. And the latest study to create 4D printed structures is one of the most practical in recent times, with an LLNL research group’s 4D printed materials able to be used as protective cushioning in headgear, packaging, and other devices.

By adding hollow, gas-filled "micro-balloons" into silicone-based ink, the LLNL researchers have created materials that can be “programmed” with temperature changes. The material can be squeezed into a different shape at a slightly elevated temperature, retaining that shape even after cooling.

By reheating the material, the gas in the micro-balloons expands, returning the material to its original pre-squeezing shape.

“The impressive part was how well the structures could recover their shape after they were reheated,” said LLNL researcher Amanda Wu, lead author of the study’s research paper. “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.”

Interestingly, the researchers happened upon the 4D printable materials almost by accident. They were actually trying to engineer a material that behaved in the opposite way: a material that would recover completely after squeezing and deforming under heat.

“Initially, this was an accelerated aging test to see if the material would be useful,” said LLNL scientist Ward Small. “This material took on a pretty large compression set and that made us wonder if it was permanent. 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 material keeps its shape because of its 3D printed polymer micro-balloons. The thin polymeric shell in the micro-balloon has a glass transition temperature, meaning it is rigid above that temperature and soft and malleable below it.

All the re-shaping therefore needs to be done above the glass transition temperature. Then, after cooling down a bit, the material will keep that shape. Only when the material is heated again does it start to return to its original form, and this occurs because the micro-balloons are expanding.

Part of the challenge was finding a 3D printable ink that would carry the micro-balloons without getting stuck in the nozzle of the printer, which was a direct-ink writing 3D printer. The ink material was extruded at room temperature into “woodpile-like structures” with controlled porosity and architecture.

"We'd take [the materials] out hot and let them cool in the presence of a compressive force and test their thickness to measure compression set," said LLNL researcher Taylor Bryson. “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 per cent recovery.”

Advantages of 3D printing the material, rather than forming it in other ways, include keeping the structures lightweight and exerting greater control over their shape.

Excitingly, the researchers say the shape memory functionality of their 4D printed materials could be integrated into any polymeric base material, including stretchable materials like elastomers.

Lab researchers have also filed a patent application for the material, which could be used to make customizable thermally activated cushioning. By adjusting the transition temperature, the researchers could even make materials that can be heated with warm water, adjusted to fit one’s body shape, and then worn as protective headgear, padding etc. It could also be used in packaging and transport applications.

"You could use this for any customized mechanical energy-absorbing material," said materials scientist Eric Duoss, a co-principal investigator on the project. “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.”

The researchers’ paper, “3D Printed Silicones with Shape Memory,” has been published in Scientific Reports.



Posted in 3D Printing Materials



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