Episode 14: Mechanical metamaterials reprogrammable via magnetic interactions

In this podcast episode, MRS Bulletin’s Sophia Chen interviews Daniel Garcia-Gonzalez from Universidad Carlos III de Madrid in Spain about his research group’s reprogrammable metamaterial. The researchers use a soft polymer, mixed with magnetic particles. By rotating the orientation of the magnets, they tune the softness or compressibility of the material. This work was published in a recent issue of Advanced Materials. 

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Picture a refrigerator magnet. Typical magnets like these are hard, rigid materials, and they are difficult to integrate into biomedical applications, such as a device that goes inside the human body, or in the emerging field of soft robotics. Daniel Garcia-Gonzalez and his team have come up with a new material that might work better.

DANIEL GARCIA-GONZALEZ: We managed to design super soft and flexible magnets.

SOPHIA CHEN: Garcia is a mechanical engineer at Universidad Carlos III de Madrid in Spain. Not only has his team made soft magnets, their magnets can actually change stiffness on demand. You can make it softer or more rigid. He refers to this as a “programmable” material. To understand how it works, let’s first talk about what it’s made of. It’s made of a soft polymer, where some of it is mixed with magnetic particles. 

DANIEL GARCIA-GONZALEZ: The more particles you add, the stronger the magnetic properties you get, but the stiffer the resulting material is. So you need to find a balance.

SOPHIA CHEN: But that’s not the whole picture. The other essential element of their design is the material’s architecture. Picture it like this. They take small octagon-shaped pieces of their soft magnet and place them into a square frame made of just the polymer, without magnetic particles. Each square is about the size of the face of a dice.  Each square is a unit in a larger rectangular matrix that looks like a checkerboard that alternates between squares and square-shaped gaps. So the material looks like this rectangular checkerboard, where each square has an octagonal soft magnet embedded in it, and each square connects to its neighbors via flexible hinges. The whole thing is soft and compressible.

DANIEL GARCIA-GONZALEZ: Once you compress it, these local cell units are going to rotate because these hinges, like these flexible beams that are connecting the different the different squares are just bending or deforming, and then the whole thing is going to collapse in something that you can think like you are removing the holes, so you are creating a compact material.

SOPHIA CHEN: You can tune how soft, or compressible the material by basically rotating the orientation of the magnets through compressing the material. As the magnets change orientation, the repulsive or attractive force between them changes.

DANIEL GARCIA-GONZALEZ: Depending on how the distribution of these magnetic nodes is designed, and then during the evolution, these magnetic interactions are evolving, you are changing the effective resist and the effective stiffness, or mechanical properties of the structure.

SOPHIA CHEN: They can program the material to be harder or softer in two main ways. There’s what Garcia calls the “passive” way, where you take out the octagonal soft magnet from the frame and rotate it. Then there’s the active way, where you put the whole structure in an external magnetic field. This can compress or expand the material, and changes its stiffness. Garcia’s material qualifies as a metamaterial. This is a design paradigm where you change the internal architecture and geometry of the material to change its properties. This is in contrast with conventional materials design where scientists achieve materials properties by altering chemical composition and changing the material’s phase through heating and cooling.

DANIEL GARCIA-GONZALEZ: This concept of metamaterials means that now is not about the chemistry of the material, but about finding a given microstructure.

SOPHIA CHEN: In future work, Garcia would like to scale down the structure of the material, so that it could be used in microfluidic devices. This work was published in a recent issue of Advanced Materials. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.