Shape memory achieved for nano-sized objects

Alloys that can return to their original structure after being deformed have so-called shape memory. This phenomenon and the resulting forces are used in many mechanical actuation systems, for example in generators or hydraulic pumps. However, it has not been possible to use this shape memory effect at the small nanoscale. Shape memory alloy objects can only change to their original shape if they are larger than about 50 nanometers.

Researchers led by Salvador Pané, Professor of Robotics Materials at ETH Zurich, and Xiang-Zhong Chen, a senior scientist in his group, were able to circumvent this limitation by using ceramic materials. In a study published in the journal Nature communications, demonstrate the shape memory effect in a layer about twenty nanometers thick and made of materials called iron oxides. This achievement now makes it possible to apply the shape memory effect to tiny nanoscale machines.

It needs a special structure

At first glance, iron oxides do not seem very suitable for the shape memory effect: they are brittle on a bulk scale, and to produce very thin layers of them, they usually have to be fixed to a substrate, which makes them rigid. To still be able to induce the shape memory effect, the researchers used two different oxides, barium titanate and cobalt ferrite, of which they temporarily applied thin layers to a magnesium oxide substrate. The lattice parameters of the two oxides differ significantly from each other. After the researchers had peeled off the two-layer strip from the support substrate, the tension between the two oxides created a spirally twisted structure.

Such freestanding nanoscale structures made of iron oxides are highly elastic, elastic and allow flexible movements. In addition, they demonstrated a shape-memory effect: When the researchers applied mechanical tensile force to the structure, it stretched and permanently deformed. The scientists then directed a beam of electrons from a scanning electron microscope onto the deformed structure. returned to its original shape. Thus, the electricity triggered a shape memory effect. The layer thickness of about twenty nanometers is the smallest sample size in which such a phenomenon has ever been observed.

Typically, in other examples, the shape-memory effect is activated by thermal or magnetic manipulation. “The reason it works with electric radiation on iron oxides may have to do with the orientation of polarization within the oxides, we suspect,” says Chen. While the free-standing structure is stretched, the polarization within the oxides aligns parallel to the structure plane. The electron beam, however, drives the polarization to align perpendicular to the plane of the structure, causing the mechanical stress to change and contract to its original shape.

Wide range of applications

This electrical response is more suitable for a wide range of applications because precise temperature manipulations (conventionally used to induce shape memory) are not possible at the nanoscale. An application example: Thanks to their high elasticity, oxides could replace muscle fibers or parts of the spine.

“Other applications would be new nanoscale robotic systems: The mechanical motion that occurs when switching between the two structures could be used to drive tiny motors,” says Donghoon Kim. He worked as a PhD student on this study and is one of its two lead authors. “Furthermore, our approach could also facilitate the development of longer-lasting small-scale machines because the material is not only elastic but also durable,” says Minsoo Kim, postdoctoral fellow and also lead author.

The range of applications can be extended even to flexible electronics and soft robotic systems. In another study, which researchers have just published in the journal Advanced Material Technologies, were able to further develop such free oxide structures so that their magnetoelectric properties could be more precisely controlled and tuned. Such shape memory oxides could be used, among other things, to make nanorobots that are implanted in the body and can stimulate cells or repair tissues. Through external magnetic fields, the nanorobots can be activated to transform into a different shape and perform specific functions inside a human body.

“Furthermore, the magnetoelectric properties of these shape memory oxide structures could be used, among other things, to electrically stimulate cells in the body, for example to activate neuronal cells in the brain, for cardiac treatments or to speed up the healing process of the bones”. Pane says. Finally, shape memory magnetoelectric oxides could be used in nanoscale devices such as tiny antennas or sensors.