A team of researchers has developed wafer-thin, stiff nanopaper that instantly becomes soft and elastic at the push of a button. The material is equipped with a mechanism so that the strength and stiffness can be modulated via an electrical switch. As soon as an electric current is applied, the nanopaper becomes soft; when the current flow stops, it regains its strength. This switching capability could be used for damping materials.
The team was inspired by sea cucumbers, which have a special defense mechanism. When attacked by predators in their habitat on the sea floor, sea cucumbers can adapt and strengthen their tissue so that their soft exterior immediately stiffens. The researchers mimicked the basic principle in a modified form using an attractive material and an equally attractive switching mechanism.
The scientists used cellulose nanofibrils extracted and processed from the cell wall of trees. Nanofibrils are even finer than the microfibers in standard paper and result in a completely transparent, almost glass-like paper. The material is stiff and strong — appealing for lightweight construction. Its characteristics are comparable to those of aluminum alloys.
The team applied electricity to these cellulose nanofibril-based nanopapers. By means of specially designed molecular changes, the material becomes flexible. The process is reversible and can be controlled by an on/off switch. This is relevant for mechanical materials, which can thus be made more resistant to fracture or for adaptive damping materials, which could switch from stiff to compliant when overloaded, for example.
At the molecular level, the process involves heating the material by applying a current and thus reversibly breaking cross-linking points. The material softens in correlation with the applied voltage: the higher the voltage, the more cross-linking points are broken and the softer the material becomes.
While currently a power source is needed to start the reaction, the next goal would be to produce a material with its own energy storage system, so that the reaction is essentially triggered internally as soon as an overload occurs and damping becomes necessary.