Foldable like an accordion: international research team bends individual nanostructures

After bending, the tetrapods automatically retain its original shape, without suffering any damage. This makes advanced applications conceivable, both in materials science as well as in the field of regenerative medicine. The research team published their results in Nature Communications.

Regarding new materials, scientists are primarily interested in one thing: What properties do they have, and how do they behave under different conditions? This also determines the new possible uses of the materials. “In order to predict the overall mechanical behaviour of a network material, we must investigate the individual building block structures with which it is constructed,” explained Dr. Yogendra Mishra, materials scientist in the working group “Functional Nanomaterials” at the CAU.

Aerographite is constructed of tetrapods, carbon-based 3D nanostructure which consist of four hollow arms. When combined together, they form a porous, extremely lightweight network, and bring the weight of aerographite down to just 0.2 milligrams per cubic centimetre. ‘’Because of this unique structure, the material exhibits a high mechanical strength as well as a very high surface, from which interesting physical and chemical features originate,’’ says Daria Smazna, a doctoral student in the project.

The international research team led from Kiel has now managed to show that aerographite is extremely foldable. “In general, bulk materials like carbon or metal are not foldable, but due its special structure our carbon networks are highly flexible and mechanical stable too”, explained Professor Rainer Adelung, head of the Functional Nanomaterials Chair. You could imagine it much like a sheet of paper. “A flat sheet of paper offers no resistance, if you hold it on one side, it simply hangs down. However, if we roll it up or crumple, it attains a certain degree of stability,” continued the materials scientist.

It therefore depends on the geometrical arrangement within the material. The special shape of the tetrapods made the researchers suspect that they could be folded – despite the lightness of aerographite. This is because the individual arms have very thin walls and they are hollow inside. “This allows them to be bent at so many different places, even reversibly. They automatically go back to their original shape, without sustaining any damage,” explained Mishra. “Just like an accordion, the three-dimensional object can be folded into a two-dimensional form, and then unfolded again.”

The Kiel researchers imagined how aerographite behaves when it is folded – at least according to their suspicions. To characterize the material and to prove that their idea is actually true, they also had to bend the micrometer-sized objects in practice. To do so, they needed a special scanning electron microscope, which they found in Riga (Estonia). Here, the Kiel team was already working with fellow scientists on a different project. With a nanoscale measuring needle, the colleagues there were able to grasp and bend the aerographite tetrapods.

The material scientists Dr. Stefano Signetti and Prof. Pugno, co-leading author of the paper, from the Italian University of Trento, provided the final mechanical understanding and generalization, developing both the analytical and numerical models, and thus also the proof that the assumptions of the Kiel colleagues were correct. ‘’Our theoretical and numerical modelling calculations provide a general understanding for the design of aerographite materials and are in very good agreement with the assumption of the Kiel researchers as well as experimental observations from Riga machine’’ adds Nicola Pugno, Professor of Solid and Structural Mechanics.

“The calculation method which has been developed and verified because of this international cooperation, can be applied to tetrapods in various sizes. It provides a valuable basis for investigating the properties of whole tetrapod networks and aerographite even further,” elaborated Mishra. In the long term, understanding how networks of hollow tetrapods can be folded however, we like without being damaged, could help to optimise the production of highly-porous solids such as aerogels and foams, or enable their use in tissue regeneration (so-called scaffold in medical engineering).

Original publication
Raimonds Meija, Stefano Signetti, Arnim Schuchardt, Kerstin Meurisch, Daria Smazna, Matthias Mecklenburg, Karl Schulte, Donats Erts, Oleg Lupan, Bodo Fiedler, Yogendra Kumar Mishra, Rainer Adelung & Nicola M. Pugno. Nanomechanics of individual aerographite tetrapods. Nat. Commun. 8, 14982 doi: 10.1038/ncomms14982 (2017).

Video of bending a single aerographite tetrapode in a special scanning electron microscope.
https://images.nature.com/original/nature-assets/ncomms/2017/170412/ncomms14982/…
First type of in situ experiment showing the formation of a buckling-hinge at the central joint of a tetrapod with three arms fixed at a substrate and one being bent by an AFM cantilever.
Credit: Donats Erst, University of Latvia

Photos are available to download:
http://www.uni-kiel.de/download/pm/2017/2017-250-1.jpg
The orange tetrapod models are actually made of normal marker beacons, as used on sports fields. The researchers at Kiel University use them for demonstration purposes, because just like the real tetrapods, the plastic objects are hollow inside, and can therefore be easily compressed and then return to their original shape. Materials scientists Yogendra Kumar Mishra and doctoral researcher Daria Smazna demonstrate the effect.
Photo/credit: Siekmann/CAU

http://www.uni-kiel.de/download/pm/2017/2017-250-2.jpg
The black aerographite is the lightest material in the world. It is constructed from tiny tetrapod structures.
Photo/credit: Siekmann/CAU

http://www.uni-kiel.de/download/pm/2017/2017-250-3.jpg
At the Faculty of Engineering, materials scientists compress the aerographite, and measure how much force is exerted. However, in order to be able move the individual tetrapod structures from which aerographite is made, they needed a special scanning electron microscope in Riga.
Photo/credit: Siekmann/CAU

http://www.uni-kiel.de/download/pm/2017/2017-250-4.jpg
A tetrapod arm in normal shape; (b) a needle touches the arm, which slowly starts to bend; (c) the arm bends significantly, before it (d) reverts back to its original shape without being damaged.
Photo/credit: Donats Erst, University of Latvia

http://www.uni-kiel.de/download/pm/2017/2017-250-5.png
A network of four-armed carbon tetrapods interconnects to form the highly-porous material aerographite.
Photo/credit: AG Adelung

Contact:
Dr habil. Yogendra Kumar Mishra
Functional Nanomaterials Group
Faculty of Engineering
Tel.: +49 (0)431 880 -6183
E-mail: ykm@tf.uni-kiel.de

Kiel University
Press, Communication and Marketing, Dr Boris Pawlowski, Editing: Julia Siekmann
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Details, which are only a millionth of a millimetre in size: This is what the priority research area “Kiel Nano, Surface and Interface Science – KiNSIS” at Kiel University has been working on. In the nano-cosmos, different laws prevail than in the macroscopic world – those of quantum physics. Through intensive, interdisciplinary cooperation between materials science, chemistry, physics, biology, electrical engineering, computer science, food technology and various branches of medicine, the priority research area aims to understand the systems in this dimension and to implement the findings in an application-oriented manner. Molecular machines, innovative sensors, bionic materials, quantum computers, advanced therapies and much more could be the result. More information at http://www.kinsis.uni-kiel.de.

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