LLNL: Magnetically Responsive Metamaterials Instantly Stiffen 3D Printed Structures

Lawrence Livermore National Laboratory (LLNL) frequently does impressive work with 3D printing materials, including metamaterials. Now the lab has introduced a new class of metamaterial that can almost instantly respond and stiffen 3D printed structures when exposed to a magnetic field. LLNL calls the materials “field-responsive mechanical metamaterials” or FRMMs. They involve a viscous, magnetically responsive fluid that is injected into the hollow struts and beams of 3D printed lattices. Unlike other 4D printed materials, the FRMMs’ overall structure does not change. The fluid’s ferromagnetic particles located in the core of the beams form chains in response to the magnetic field, stiffening the fluid and the lattice structure. This happens in less than a second.

The research is documented in a paper entitled “Field responsive mechanical metamaterials.”

“In this paper we really wanted to focus on the new concept of metamaterials with tunable properties, and even though it’s a little more of a manual fabrication process, it still highlights what can be done, and that’s what I think is really exciting,” said lead author Julie Jackson Mancini, an LLNL engineer who has worked on the project since 2014. “It’s been shown that through structure, metamaterials can create mechanical properties that sometimes don’t exist in nature or can be highly designed, but once you build the structure you’re stuck with those properties. A next evolution of these metamaterials is something that can adapt its mechanical properties in response to an external stimulus. Those exist, but they respond by changing shape or color and the time it takes to get a response can be on the order of minutes or hours. With our FRMM’s, the overall form doesn’t change and the response is very quick, which sets it apart from these other materials.”

The researchers injected a magnetorheological fluid into hollow lattice structures built on LLNL’s Large Area Projection Microstereolithography (LAPµSL) platform, which is capable of 3D printing objects with microscale features over wide areas using light and a photosensitive polymer resin. According to Mancini, the LAPµSL machine played a big role in the development of the new metamaterials, as the complex tubular structures needed to be manufactured with thin walls and be capable of keeping the fluid contained while withstanding the pressure generated during the infill process and the response to a magnetic field.

The stiffening of the fluid and, in turn, the 3D printed structures, is reversible and tunable by varying the strength of the applied magnetic field.

“What’s really important is it’s not just an on and off response, by adjusting the magnetic field strength applied we can get a wide range of mechanical properties,” Mancini said. “The idea of on-the-fly, remote tunability opens the door to a lot of applications.”

Those applications include impact absorption, such as automotive seats that have fluid-responsive metamaterials integrated inside of them along with sensors that can detect a crash. The seats would stiffen upon impact, possibly reducing whiplash. Other applications include helmets, neck braces, housing for optical components or soft robotics.

To predict how lattice structures would respond to an applied magnetic field, former LLNL researcher Mark Messner, who now works for Argonne National Laboratory, developed a model from single strut tests. Starting with a model he developed to predict the mechanical properties of non-tunable static lattice-structured materials, he added a representation of how magenetically responsive fluid affects a single lattice member under a magnetic field and incorporated the model of a single strut into designs for unit cells and lattices. He then calibrated the model to experiments Mancini performed on fluid-filled tubes similar to the struts in the lattices. The researchers used the model to optimize the topology of the lattice, finding the structures that would result in large changes in mechanical properties as the magnetic field was varied.

“We looked at elastic stiffness, but the model (or similar models) can be used to optimize different lattice structures for different sorts of goals,” Messner said. “The design space of possible lattice structures is huge, so the model and the optimization process helped us choose likely structures with favorable properties before (Mancini) printed, filled and tested the actual specimens, which is a lengthy process.”

Mancini began the work at the University of California, Davis under her adviser, materials and engineering professor Ken Loh, who is now at the University of California, San Diego. According to Loh, the concept was partially inspired by automotive-based suspension systems. They began by investigating ways to develop flexible armor that could morph or change its mechanical properties as needed.

“One of the criteria is to achieve fast response, and magnetic fields and MR materials offer that capability,” said Loh.

He also said that the researchers will explore new ways to develop a single-phase material, instead of having a liquid embedded in a solid, and higher performance-to-weight rations. Future work, he continued, “could lead to new technologies, such as flexible armor for the warfighter that stiffen instantaneously when a threat is detected.”

Authors of the paper include Julie A. Jackson, Mark C. Messner, Nikola A. Dudukovic, William L. Smith, Logan Bekker, Bryan Moran, Alexandra M. Golobic, Andrew J. Pascall, Eric B. Duoss, Kenneth J. Loh and Christopher M. Spadaccini.

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