Materials science likes to take nature as a model and the special properties of living things, which could perhaps also be transferred to materials. A research team led by chemist Prof. Dr. Andreas Walther of Johannes Gutenberg University Mainz (JGU) has succeeded in endowing materials with a bio-inspired property: Wafer-thin, stiff nanopapers instantly becomes soft and elastic at the push of a button. "We have equipped the material with a mechanism so that the strength and stiffness can be modulated via an electrical switch," Walther explains. As soon as an electric current flows, the nanopaper becomes soft; when the current flow stops, it regains its strength. From an application perspective, this switchability could be interesting for damping materials, for example. The work, which also involved scientists from the Albert Ludwig University of Freiburg and the DFG Cluster of Excellence livMatS, was published in the journal Nature Communications.
Model from the seafloor: Mechanical switch as a protective function
In this case, the model from nature is sea cucumbers, which have a special defense mechanism: when attacked by predators in their habitat on the seafloor, the animals can adapt and strengthen their tissue so that their soft exterior immediately stiffens. "This is an adaptive mechanical behavior that is fundamentally difficult to replicate," says Andreas Walther. With the work now published, his team has succeeded in mimicking the basic principle in a modified form using an attractive material and an equally attractive switching mechanism.
For this purpose, the scientists used cellulose nanofibrils extracted and processed from the cell wall of trees. Nanofibrils are even finer than the microfibers in paper and result in a completely transparent, almost glass-like paper. The material is stiff and strong and appealing for lightweight engineering. Its properties are comparable to those of aluminum alloys. In its work, the research team applied electricity to these cellulose nanofibril-based nanopapers; via specially designed molecular changes, this makes the material flexible. The process is reversible and can be controlled by the on/off switch.
"This is extraordinary. All the materials around us are not very changeable, they don't easily switch from stiff to elastic and vice versa. Here, with the help of electricity, we can do that in a simple and elegant way," says Walther. The development is thus moving away from classic static materials toward materials whose properties can be adaptively adjusted. This is relevant for mechanical materials, which can thus be made more resistant to fracture, or for adaptive damping materials, which, for example, switch from stiff to compliant when overloaded.
Target is material with its own energy storage for autonomous switching on and off
At the molecular level, the process involves heating the material by applying a current and subsequently reversibly breaking cross-linking point. The material softens as a function of the applied voltage, i.e. the higher the voltage, the more crosslinking points break and the softer the material becomes. Andreas Walther's vision of the future also starts at the point of the power supply: While currently a power source is still needed to start the reaction, the next goal would be a material with its own energy storage system, so that the reaction is triggered practically "internally" as soon as, for example, an overload occurs and damping becomes necessary. "Now we still have to flip the switch ourselves, but our dream would be for the material system to be able to accomplish this on its own."
Andreas Walther has cooperated closely with his colleagues at the University of Freiburg on this work. He is one of the founders of the Freiburg Cluster of Excellence "Living, Adaptive and Energy-autonomous Materials Systems" (livMatS), in which he continues to be involved as an associate scientist. Since October 2020, Walther has been Professor of Macromolecular Chemistry at Johannes Gutenberg University Mainz and is also a Fellow of the Gutenberg Research College at JGU. For his project "Metabolic Mechanical Materials: Adaptation, Learning & Interactivity" (M³ALI), he received an ERC Consolidator grant, one of the EU's most highly endowed funding measures awarded to top researchers.
D. Jiao, F. Lossada, J. Guo, O. Skarsetz, D. Hoenders, J. Liu & A. Walther: Electrical switching of high-performance bioinspired nanocellulose nanocomposites, Nature Communications 12, 26. Februar 2021, DOI:10.1038/s41467-021-21599-1
Press release of the Johannes Gutenberg University Mainz