Knots tied within microstructure of foam

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Sophia Chen. We tie fibers into knots all the time in everyday life. We use them to keep our shoes on our feet or secure ropes on boats. But Widi Moestopo tried creating knots in materials on an entirely different, and absolutely tiny scale. 

WIDI MOESTOPO: If you look from one end to the other end of the knots, they’re about 70 micron. So that’s around the size of the diameter of our hair. They’re pretty tiny, really hard to see it just under a normal optical microscope. We actually had to test them and kind of investigate what they look like using a scanning electron microscope. 

SOPHIA CHEN: Now a postdoc at Lawrence Livermore National Laboratory, he did this work while he was a mechanical engineering PhD student at Caltech, in Julia Greer’s research group. Moestopo wanted to see if a material made out of these knots would be more structurally robust. So he created a foam out of these knots. Foams are composite materials made of two or more materials. 

WIDI MOESTOPO: Here, one is essentially plastic and the other one is air.

SOPHIA CHEN: If you think about the foams you know— like mattress filling, or even styrofoam, you can squish and compress them easily, but you can’t pull them very hard without ripping the material. By incorporating knots into the foam, Moestopo and his team were able to create a foam that can stretch.

WIDI MOESTOPO: Now we can think of foams not just as like a punching bag, but also as like a super stretchy, tough material.

SOPHIA CHEN: They used a 3D printing technique known as two-photon lithography to create the knots and subsequently build the foam out of the knots. In two-photon lithography, you start with a block of resin. You focus a laser at points in the resin to trigger a chemical reaction, which converts those points in the resin into a different material, a polymer. Then, after scanning the resin with the laser to create the 3D object you want, you use a solvent to wash away the remaining, unconverted resin. Then, in Moestopo’s case, you’re left with your foam.

WIDI MOESTOPO: Usually, if you 3D print a structure, you kind of go like layer by layer. And then if you have like a huge overhanging structure, it would just drop down and they'll just fail. What’s unique with two-photon is that we are printing the structure inside the resin itself.

SOPHIA CHEN: To visualize Moestopo’s knots, imagine zooming in to a piece of foam and seeing a 3D lattice formed from unit cells, each consisting of three rhombuses. Each side of the rhombus consists of three strands of fiber. These fibers are woven around each other to form knots.  

WIDI MOESTOPO: You have to intersect each strand to itself to make a knot. 

SOPHIA CHEN: They compared the stretchability of the material to similar structures where the rhombuses were made of woven fibers that did not intersect each other to form knots. 

WIDI MOESTOPO: The knotted rhombus structure has 107% longer elongation than what the woven structure of the same size could have. 

SOPHIA CHEN: Compared to its woven counterpart, the knotted material could also absorb 92 percent more energy, a measure of how much you could pull it without it ripping. This work illustrates an approach to designing structures known as hierarchical ordering. It involves considering an object at different structural levels. Here, they are creating a foam that’s more stretchable by engineering its microscopic structure. You’ll find examples of hierarchical ordering in nature. 

WIDI MOESTOPO: If you take like a human femur bone or a human or thigh bone when you look under the microscope, you see that bone is actually made out of little tiny struts of biological materials.

SOPHIA CHEN: They’re looking into applications for this research. One potential application could be for a biomedical implant placed in the body to filter chemotherapy drugs. This work was published in a recent issue of Science Advances. 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 twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.