‘Microscale knots’ could make tougher materials for biomedicine and aerospace

Adding tiny microscale knots to a polymer material made it “far tougher” than an identically structured but unknotted material, according to its creators.

With potential applications in biomedicine and aerospace due to their durability, possible biocompatibility and ‘extreme’ deformability, the materials were developed by engineers at the California Institute of Technology (Caltech).

“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,” said former Caltech graduate student Widianto P Moestopo, lead author of a paper on the knots, now at Lawrence Livermore National Laboratory.

‘Nano-architected’ materials, whose structure is designed and organised at a nanometre scale, often exhibit unusual and surprising properties.

The new material is made from numerous interconnected microscale knots. Compared to the unknotted material, the knotted materials absorb more energy and can deform more while still being able to return to their original shape undamaged.

“Understanding how the knots would affect the mechanical response of micro-architected materials was a new out-of-the-box idea,” said senior author Julia R Greer. “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.”

The knotted materials, which were created out of polymers, exhibited a tensile toughness that “far surpasses” materials that are unknotted but otherwise structurally identical, the researchers said, including ones where individual strands are interwoven instead of knotted. When compared to their unknotted counterparts, the knotted materials absorbed 92% more energy and required more than twice the amount of strain to snap when pulled. 

Each knot is about 70 micrometres wide, and each fibre has a radius of about 1.7 micrometres, equivalent to roughly one-hundredth the radius of a human hair. 

The knots were not tied, but manufactured in a knotted state  using advanced high-resolution 3D lithography capable of producing nanoscale structures. The samples contained simple knots – an overhand knot with an extra twist that provided additional friction to absorb additional energy when the material is stretched. In the future, the team plans to explore materials constructed from more complex knots.

The research is the first time a material composed of numerous microscale knots has been created, the team said. Materials could be used for suturing or tethering in biomedicine, they added.

The work was published in Science Advances.