A new class of metamaterials that change its properties when a magnetic field is applied could yield the next-generation of helmets and wearable armor.
Scientists from the Lawrence Livermore National Laboratory (LLNL) have developed metamaterials that respond and stiffen when exposed to a magnetic field.
The Field-Responsive Mechanical Metamaterials (FRMMs) employs a viscous, magnetically responsive fluid that is manually injected into the hollow struts and beams of 3D-printed lattices. Unlike other 3D-printed materials, the overall structure of the FRMMs does not change. The fluid’s ferromagnetic particles in the core of the beams form chains in response to the magnetic field to stiffen the fluid and the lattice structure in less than a second.
“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,” lead author Julie (Jackson) Mancini, an LLNL engineer who has worked on the project since 2014, said in a statement. “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,” she added. “With our FRMMs, the overall form doesn’t change and the response is very quick, which sets it apart from these other materials.”
The research team injected a magnetorheological fluid into hollow lattice structures using the Large Area Projection Microstereolithography (LAPµSL) platform, which 3D prints objects with microscale features over wide areas using light and a photosensitive polymer resin.
After the magnetically responsive fluid is inside of the lattice structures, the researchers can cause the fluid to stiffen, as well as the overall 3D-printed structures by applying an external magnetic field. The change can also be easily reversed and is highly 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.”
The researchers also developed a model from single strut tests to predict how arbitrary MR fluid-filled lattice structures respond to applied magnetic fields.
“We looked at elastic stiffness, but the model [or similar models] can be used to optimize different lattice structures for different sorts of goals,” former LLNL researcher Mark Messner, now a staff engineer at Argonne National Laboratory, said in a statement. “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.”
The new technology could have a number of uses, including for development of automotive seats with fluid-responsive metamaterials integrated inside with sensors that detect a crash and seats that stiffen on impact to reduce passenger motion that can cause whiplash. They can also be applied to helmets or neck braces, housing for optical components and soft robotics.
