Metamaterial based on 3D liquid-crystal elastomers structures can change properties (Poisson’s Ratio and twist per strain) in response to light.
Metamaterial based on 3D liquid-crystal elastomers structures can change properties (Poisson’s Ratio and twist per strain) in response to light.

Researchers have designed and constructed metamaterials that can change their mechanical properties in response to light [Münchinger et al., Materials Today (2022),]. Metamaterials are artificial materials that derive their properties from their designed microstructure rather than their constituent materials.

“Many applications in everyday life depend on responsive materials whose properties can be manipulated from the outside via a certain stimulus. However, the palette of available responsive materials is limited,” points out first author, Alexander Münchinger of Karlsruhe Institute of Technology.

Ideally, say the researchers, materials should have a ‘knob’ that allows their properties to be changed at will like a digital switch. Now, thanks to advances in the manufacturing of three-dimensional liquid-crystal elastomers, the researchers from the Karlsruhe Institute of Technology and Ruprecht-Karls-Universität Heidelberg have created metamaterials that can flip between different mechanical behaviors upon exposure to light.

Using a novel two-photon 3D laser microprinting technique they developed themselves, the researchers fabricated two different metamaterial microstructures. One structure is based on a bow-tie-shaped element within tetragonal unit cell and the other on chiral elements on the surface of connected cubes in a similar unit cell. By infusing the liquid-crystal-elastomers with a dye, the metamaterials are rendered responsive to light. When exposed to a blue LED, the structures significantly change their geometry – in one case the sign of the Poisson’s ratio flips back and forth between positive and negative values, while in the second case the direction of the chiral twist is altered. The changes in structure and associated behavior arise from the fact that beams making up the metamaterial unit cells respond anisotropically to light.

“To our knowledge, the presented metamaterials represent the first 3D optomechanical metamaterials – where the effective mechanical properties are different from the constituent material and can be tuned via light,” says Münchinger.

The mechanical behavior of the metamaterials can be tuned reversibly and repeatedly, over 100 cycles, in response to switching the LED light source off and on. Such light-stimulated metamaterials overcome one of the major hurdles to the adoption of these materials for useful applications. A metamaterial that can change its mechanical properties in response to sunlight, for example, could automatically adjust to different lighting conditions, change appearance, or be used to control room temperature.

“We believe that many more responsive metamaterials can be designed [in this way],” adds Münchinger.

The team are now working on improved liquid-crystal elastomer compositions to lower the required optical intensity.