Researchers from Lawrence Livermore National Laboratory (LLNL) have announced the development of a new class of metamaterials which are designed to stiffen 3D printed structures when exposed to a magnetic field. The materials, known as “field responsive mechanical metamaterials” (or FRMMs), could be used in the production of next-generation helmets, wearable armour and various other applications.
Interestingly, the metamaterials are not themselves 3D printable, rather, they are made up of a viscous, magnetically responsive fluid and are designed to be injected into the hollow beams of 3D printed lattices. When exposed to a magnetic field, the material inside the 3D printed lattice—embedded with ferromagnetic particles—stiffens instantly, resulting in a hard lattice structure.
The research study, which was recently published in the journal Science Advances, emphasizes that this magnetically responsive metamaterial is not in the same category as other 4D printed materials (which often react to light, humidity or other environmental elements) in that its overall structure is not altered when it stiffens. When the FRMMs are exposed to a magnetic force, the ferromagnetic particles within it form chains, which results in the hardening effect.
When the magnetic force is removed, the metamaterial returns to its original state, making its change fully reversible and repeatable. Moreover, the researchers add that by varying the strength of the magnetic field applied to the FFRM-filled 3D printed scaffold, the level of stiffness can be tuned.
“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,” commented 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,” she continued. “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.”
In its research, the LLNL team 3D printed a hollow lattice structure using the lab’s Large Area Projection Microstereolithography (LAPµSL) system and injected a magnetorheological (MR) fluid into its hollow beams. According to the team, this 3D printing system played (and continues to play) an important role in the metamaterial project because of its capacity to print complex tubular lattice forms capable of containing the fluid during injection and throughout the magnetic response.
In terms of applications, the FFRMs could be used in impact absorption products, such as helmets or neck braces as well as housing for optical components and soft robotics. Mancini also suggested the 3D printed structures injected with FFRMs could be integrated into automotive seats alongside sensors to detect a crash so that the seats would stiffen on impact, protecting the passenger from whiplash.
Moving forward, the researchers will explore how to further automate the process of injecting FFRMs into the 3D printed scaffolds with the ultimate aim of turning it into a single-phase material (instead of having a fluid inside a solid). Another goal down the line is to improve and increase performance-to-weight ratios and to increase the size of the structures.