Artistic representation of the novel material constructed with microscale knots. Image: Caltech.
Artistic representation of the novel material constructed with microscale knots. Image: Caltech.

In the latest advance in nano- and micro-architected materials, engineers at California Institute of Technology (Caltech) have developed a new polymer material made from numerous interconnected microscale knots.

These knots produce a material that is far tougher than identically structured but unknotted materials: it can absorb more energy and is able to deform more while still being able to return to its original shape undamaged. The 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 Moestopo, who is now at Lawrence Livermore National Laboratory. Moestopo is the lead author of a paper on this work in Science Advances.

Moestopo helped develop the material in the lab of Julia Greer, a professor of materials science, mechanics and medical engineering, director of the Kavli Nanoscience Institute and senior author of the paper. Greer is at the forefront of the creation of nano-architected materials, meaning materials whose structure is designed and organized at a nanometer scale and that consequently exhibit unusual and often surprising properties.

“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 70µm in height and width, and each polymer fiber has a radius of around 1.7 µm. While these are not the smallest knots ever made – in 2017 chemists tied a knot made from an individual strand of atoms – this does represent the first time that a material composed of numerous knots at this scale has been created. Further, it demonstrates the potential value of including these nanoscale knots in a material—for example, for suturing or tethering in biomedicine.

The knotted materials 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 were able to absorb 92% more energy and required 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 at the nanoscale. The samples described in the Science Advances paper contain simple knots – an overhand knot with an extra twist that provides sufficient friction to absorb additional energy when the material is stretched. In the future, the team plans to explore materials constructed from more complex knots.

Moestopo’s 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,” he says. “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.”

This story is adapted from material from Caltech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.