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The knotted materials, which were created out of polymers, exhibit a tensile toughness that far surpasses materials that are unknotted but otherwise structurally identical, including ones where individual strands are interwoven instead of knotted. Credit: Caltech

Caltech engineers have made a significant breakthrough in the field of nano- and micro-architected materials by creating a novel material composed of multiple interconnected microscale knots.

Compared to structurally identical but unknotted materials, the presence of knots in this new material significantly enhances its toughness by enabling it to absorb more energy and deform more before returning to its original shape without any damage. These new knotted materials may find applications in biomedicine as well as in aerospace applications due to their durability, possible biocompatibility, and extreme deformability.

The capability to overcome the general trade-off between material deformability and tensile toughness [the ability to be stretched without breaking] offers new ways to design devices that are extremely flexible, durable, and can operate in extreme conditions, says former Caltech graduate student Widianto P. Moestopo (MS 19, Ph.D. 22), now at Lawrence Livermore National Laboratory. Moestopo is the lead author of a paper on the nanoscale knots that was published on March 8 in Science Advances.

Moestopo helped develop the material in the lab of Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering; Fletcher Jones Foundation director of the Kavli Nanoscience Institute; and senior author of the Science Advances paper. Greer is at the forefront of the creation of such nano-architected materials, or materials whose structure is designed and organized at a nanometer scale and that consequently exhibit unusual, often surprising properties.

The tensile strength of a material constructed with microscale knots (left), compared to that of a material that lacks knots but is otherwise structurally identical (right). Credit: Caltech

Embarking on understanding how the knots would affect the mechanical response of micro-architected materials was a new out-of-the-box idea, Greer says. We had done extensive research on studying the mechanical deformation of many other types of micro-textiles, for example, lattices and woven materials. Venturing into the world of knots allowed us to gain deeper insights into the role of friction and energy dissipation, and proved to be meaningful.

Each knot is around 70 micrometers in height and width, and each fiber has a radius of around 1.7 micrometers (around one-hundredth the radius of a human hair). While these are not the smallest knots ever madein 2017 chemists tied a knot made from an individual strand of atomsthis does represent the first time that a material composed of numerous knots at this scale has ever been created. Further, it demonstrates the potential value of including these nanoscale knots in a materialfor example, for suturing or tethering in biomedicine.

The knotted materials, which were created out of polymers, exhibit a tensile toughness that far surpasses materials that are unknotted but otherwise structurally identical, including ones where individual strands are interwoven instead of knotted. When compared to their unknotted counterparts, the knotted materials absorb 92 percent more energy and require more than twice the amount of strain to snap when pulled.

The knots were not tied but rather manufactured in a knotted state by using advanced high-resolution 3D lithography capable of producing structures in the nanoscale. The samples detailed in the Science Advances paper contain simple knotsan overhand knot with an extra twist that provides additional friction to absorb additional energy while the material is stretched. In the future, the team plans to explore materials constructed from more complex knots.

Moestopos interest in knots grew out of research he was conducting in 2020 during the COVID-19 lockdowns. I came across some works from researchers who are studying the mechanics of physical knots as opposed to knots in a purely mathematical sense. I do not consider myself a climber, a sailor, or a mathematician, but I have tied knots throughout my life, so I thought it was worth trying to insert knots into my designs, he says.

Reference: Knots are not for naught: Design, properties, and topology of hierarchical intertwined microarchitected materials by Widianto P. Moestopo, Sammy Shaker, Weiting Deng and Julia R. Greer, 8 March 2023, Science Advances.DOI: 10.1126/sciadv.ade6725

The study was funded by the National Science Foundation through Moestopos Graduate Research Fellowship Program, Caltechs Clinard Innovation Fund, Greers Vannevar Bush Faculty Fellowship, and the Office of Naval Research.

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Caltech Engineers Have Developed an Unusually Tough New Material - SciTechDaily