For the first time ever, researchers at Lawrence Livermore National Laboratory (LLNL) have 3D printed composite silicone materials that are flexible, stretchable, and possess shape memory behavior.
The breakthrough could lead to a new wave of innovations from body-heat-activated helmet cushioning to form-fitting shoes.
The researchers added hollow, gas-filled "micro-balloons" into silicone-based ink, letting them 3D print devices at elevated temperatures that then shrink when it cool. When reheated, the gas in the micro-balloons expands, causing the structure to return to its original shape.
When combined with 3D printing, this shape memory behavior is often referred to as "4D printing," with the fourth dimension being time.
"The impressive part was how well the structures could recovered their shape after being reheated," says LLNL researcher Amanda Wu. "We didn't see distorted structures; we saw fully recovered structures. Because the silicone network is completely crosslinked, it holds the part together, so the structure recovers its original shape in a predictable, repeatable way."
The researchers accidently discovered the material while attempting to engineer a hierarchical porous material that would completely recover after being compressed under heat, exhibiting what is known as zero compression set.
Instead, they got the opposite result. Undeterred, LLNL scientists wondered what would happen if they reheated the structures, thinking the gas trapped in the material would cause it to re-expand—and it did.
The researchers printed their samples using a direct-ink writing process in which the composite ink material was extruded at room temperature from the printer's nozzle to form woodpile-like structures with controlled porosity and architecture. By being able to 3D print the material, it became more lightweight and functional, and the researchers could exert greater control over its overall 3D geometry and composition.
Lab researchers have filed a patent application for the material. Because it can be 3D printed into an arbitrary net shape and made into a highly porous structure with both open and closed cells, researchers said it might be useful for thermally activated cushioning that is highly tunable and customizable.
For example, by modulating the micro-balloon's glass transition temperature to be below body temperature, the material could be compressed under heat and cooled, then stored at cooler than body temperature. When worn, it would expand to fit the head in a helmet or a foot in a shoe. If the glass transition temperature is slightly above body temperature, the wearer could heat the material in an oven or pot of warm water, and then customize it fit, similar to the process of form-fitting a mouth guard.
"You could use this for any customized mechanical energy-absorbing material," Duoss says. "Then, 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 to customize the fit. It's reversible.
"It's a completely new material really, and we're excited about it," he continues. "It's a material that should have a lot of commercial potential and should be ripe for technology transfer to industry."
The process could be scaled up to produce much larger parts for packaging and transportation applications, according to the researchers In addition, the material wouldn't necessarily need to be 3D printed. Micro-balloons could be incorporated into any kind of base material and molded or cast, but the resulting material might not have the same compressibility as 3D-printed porous structures.
Read the full story with engineering insights at Machine Design, an NED partner publication.