Researchers at ETH Zurich have for the first time achieved a shape memory effect in objects that are only a few nanometers thick. This can be used to create tiny machines and small nanoscale robots.
Alloys that can return to their original structure after deformation have what is known as shape memory. This phenomenon and the resulting forces are used in many machine drive systems, for example in generators or hydraulic pumps. However, this shape memory effect could not be used in the small nano range up to now: With many alloys with shape memory, objects only change back to their original shape if they are larger than around 50 nanometers.
Researchers led by Salvador Pané, Professor of Robotic Materials at ETH Zurich, and Xiang-Zhong Chen, scientist in this group, were able to circumvent this limitation with the help of ceramic materials. In a study published in the journal “Nature Communications“, they demonstrate the shape-memory effect on an approximately twenty nanometer-thin layer of materials called ferroic oxides. This achievement now enables the application of the shape-memory effect to tiny machines at the nanoscale.
A special structure is required
At first glance, ferroic oxides appear to be poorly suited for the shape memory effect: they are brittle on a large scale, and in order to produce very thin layers of them, they usually have to be applied to a substrate, which makes them inflexible. In order to be able to produce the shape memory effect anyway, the researchers used two different oxides, barium titanate and cobalt ironstone, of which they briefly applied thin layers to a carrier material and then removed them again. The lattice properties of the two oxides differ significantly from each other. After the researchers had detached the strip with the two thin layers from the carrier material, the tension between the two materials created a spirally twisted structure.
Nanoscale structures made of ferrous oxides produced in this way are highly elastic, resilient and allow flexible movements. They also showed a shape memory effect: when the researchers applied a mechanical tensile force to the structure, it expanded and permanently deformed. The scientists then directed an electron beam from a scanning electron microscope onto the deformed structure; it returned to its original shape. So the electrical energy triggered a shape memory effect. The layer thickness of around twenty nanometers is the smallest sample size on which such an effect has ever been observed.
Usually, in other examples, the shape memory effect is triggered by thermal or magnetic manipulation. “The fact that it works with electric irradiation for the ferroic oxides could have to do with the orientation of the polarization within the oxides,” explains Chen. As the strand is stretched, the polarization within the oxides aligns parallel to the plane of the strand. Due to the electron beam, the polarization is aligned perpendicular to the plane of the strand, which means that the structure returns to its original shape.
Large scope
This response to electrical energy is better suited for numerous applications, since point temperature manipulations (which otherwise produce the shape memory effect) are not possible on the nanoscale. An example of an application: Thanks to their high elasticity, the oxides could replace muscle fibers or parts of the spine. “Another application would be novel nanoscale robotic systems: The mechanical movement that occurs when switching between the two structural forms could be used to drive the smallest motors,” says Donghoon Kim. He worked on this study as a PhD student and is one of its two first authors. “In addition, our approach could also enable the development of more durable nanomachines, since our material is not only elastic but also durable,” explains Minsoo Kim, postdoc and also first author.
The area of application could even be expanded to include flexible electronics and soft robotics. In another study, which the researchers have just published in the journal “Advanced Materials Technologies” published, they were able to further develop such free-standing oxide structures in such a way that their so-called magnetoelectric properties can be precisely controlled and adjusted. Such shape-memory oxides could be used, among other things, for the production of nanorobots that are implanted in the body and can stimulate cells or repair tissue. External magnetic fields could transform such nanorobots into a different structure and, for example, perform certain functions in the human body.
“In addition, one could use the magnetoelectric properties of the shape-memory oxide structures to electrically stimulate cells within the body, for example to activate nerve cells in the brain, for heart therapies or to accelerate bone healing,” says Pané. And finally, magnetoelectric shape-memory oxides could be used in nanoscale devices such as tiny antennas or sensors.
bibliography
Kim D, Kim M, Reidt S, Han H, Baghizadeh A, Zeng P, Choi H, Puigmartí-Luis J, Trassin M, Nelson BJ, Chen XZ, Pané S: Shape-Memory Effect in Twisted Ferroic Nanocomposites. Nature Communications, 10. Februar 2023. doi: 10.1038/s41467-023-36274-in
Kim M, Kim D, Aktas B, Choi H, Puigmartí-Luis J, Nelson BJ, Pané S, Chen XZ: Strain-Sensitive Flexible Magnetoelectric Ceramic Nanocomposites. Advanced Materials Technologies, 28. Februar 2023, doi: 10.1002/admt.202202097
This article first appeared on ETH News.
