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Managing Editor  | October 2017

Researchers create first material with shape memory behavior


By adding hollow, gas-filled micro-balloons to the silicon-based ink, a team of researchers at the Lawrence Livermore National Laboratory (LLNL) have successfully 3-D printed composite silicon materials that are flexible, stretchable, and have shape memory behavior.

 

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Ward Small, Amanda Wu and Taylor Bryson have successfully 3-D printed composite silicone materials that are flexible, stretchable and possess shape memory behavior. (Carrie Martin/LLNL)

 

This 4-D printing breakthrough, according to a report from LLNL, could lead to form-fitting cushions activated by body heat, such as for helmets or shoes.

 

“The researchers engineered the material so it can be compressed or ‘programmed’ at an elevated temperature, remaining in that state as it cools,” the report explained. “When reheated, the gas in the micro-balloons expands, causing the structures to return to their original shape.”

 

Researchers said that there was no distortion of the material when it recovered. The cross-linking of the silicon structure allowed the material to recover back to its original shape.

 

The report added, “In a stroke of serendipity, 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.”

 

Polymer micro-balloons are the key to this breakthrough.

 

“The thin polymeric shell in the micro-balloon has a glass transition temperature; below that temperature, the shell is rigid and glassy and above the temperature, the shell becomes soft and malleable,” the report explained.

 

“Therefore, by heating the composite material above the shell glass transition temperature, the spheres' polymer shells soften, allowing them to be compressed and alter their shape in a way that stays deformed and resists re-expansion of the silicone matrix when cooled. When reheated, the balloons expand, and the restoring force of the heated gas and silicone enables the structure to recover its original contour.”

 

Researchers used a direct-ink writing process at room temperature. The process is unique because of the way that the shape memory is engineered into the material, so that this process could be used with other materials, such as elastomers.

 

They have applied for a patent for the material.

 

The research was recently published in Scientific Reports. The abstract read:

 

“Direct ink writing enables the layer-by-layer manufacture of ordered, porous structures whose mechanical behavior is driven by architecture and material properties. Here, we incorporate two different gas filled microsphere pore formers to evaluate the effect of shell stiffness and Tgon compressive behavior and compression set in siloxane matrix printed structures.

 

“The lower Tg microsphere structures exhibit substantial compression set when heated near and above Tg, with full structural recovery upon reheating without constraint. By contrast, the higher Tg microsphere structures exhibit reduced compression set with no recovery upon reheating.

 

“Aside from their role in tuning the mechanical behavior of direct ink write structures, polymer microspheres are good candidates for shape memory elastomers requiring structural complexity, with potential applications toward tandem shape memory polymers.”

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