LLNL Researchers Use Direct Ink Writing to 3D Print Silicone Metamaterials with Shape Memory Behavior

A research team from Lawrence Livermore National Laboratory (LLNL) has used a direct ink writing (DIW) process to 3D print property-specific, silicone metamaterials that possess shape memory behavior, which technically makes them 4D printed. LLNL has worked with both 3D printed metamaterials and 4D printing before, but by using a DIW process, the material becomes more lightweight and functional, with a tailored, mechanical response.

This can be achieved when printed structural porosity and intrastrand porosity are combined, by adding hollow, gas-filled microspheres into the 3D printing ink. But many researchers are adding different materials to the mix, such as these shape memory polymers, in order to increase the potential applications.

The team published a paper on their work, titled “3D Printed Silicones with Shape Memory,” in the journal Scientific Reports. The co-authors of the paper include Stephanie E. Schulze, who works for the Department of Energy’s National Security Campus, and LLNL Materials Engineering Division researchers Taylor M. Bryson, Emily Cheng, Eric B. Duoss, Thomas R. Metz, Ward Small IV, Thomas S. Wilson, and Amanda S. Wu.

The abstract reads, “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 T_gon compressive behavior and compression set in siloxane matrix printed structures. The lower T_g microsphere structures exhibit substantial compression set when heated near and above T_g, with full structural recovery upon reheating without constraint. By contrast, the higher T_g 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.”

(a) Microballoon diameter size distribution, optical microscopy images of (b) Tg44 and (c) Tg113 microballoons, (d) schematic illustration of our 3D printing process, optical microscopy images of printed silicones with microballoons showing (e) x-y view, (f) x-z view and (g) high magnification image of (f) showing 25 vol% microballoons in a printed filament.

In order to produce the metamaterials, the research team extruded viscoelastic inks, which possess controlled rheological behaviors, through a 250 µm microscale nozzle. The team used an ink that was based in silicone and possessing a polymeric shell. A 3-axis, displacement-controlled 3D printing platform was used in order to create cross-ply structures, so that every layer was perpendicular to the one before it; this type of structure is called a face-centered tetragonal (FCT). The 3D printed structures were 50 x 50 mm, with eight layers, and the material was cured under nitrogen.

The LLNL research team tested the structures by evaluating the effects of glass transition temperature and shell stiffness, with respect to the metamaterials’ shape memory and compressive behavior, for the polymer-filled inks and the gas-filled inks. They used multiple instruments for testing, including optical microscopy and scanning electron microscopy. The study showed for the first time that 3D printed porous elastomers can achieve shape memory by adding polymer microspheres that have a controlled shell glass transition temperature.

The researchers used two different gas microspheres, T_g44 and T_g113, and determined through testing that the T_g44 produced the best results. The metamaterials that contain microballoons filled with T_g44 are very promising, thanks to their shape memory behavior. The team plans to use in-line mixing techniques and multi-material printing to further optimize the structure at a later time. These materials could potentially have commercial applications, if they continue to advance; for example, they could be used in wearable protective padding and cushions, and a person’s body temperature could be used to invoke a recovery response. Discuss in the LLNL forum at 3DPB.com.