NCCR Bio-Inspired Materials researchers at Zurich’s Federal Institute of Technology (ETHZ) have developed rubbery composites that mimic the color changes of the neon tetra fish.

Animals such as squid and chameleons have evolved elegant, energy-efficient intracellular structures to dynamically control color for communication, warning, protection, and camouflage. One of these animals, the neon tetra fish changes color by varying the position of micrometer-sized structures in its scales. These lamellar structures are like mirrors, reflecting light at different intensities depending on how they tilt, moving from blue-green to indigo.

To mimic the fish scale mechanism, the researchers from NCCR PI Prof. André Studart’s Complex Materials laboratory and colleagues at the University of Southern Denmark chose ceramic platelets as synthetic mirrors. These platelets were embedded in transparent rubber, allowing for the composite to be easily stretched in different directions. The color-changing mechanism, which relies on controlling the structure of the material at different length scales, is activated according to the stretching direction, and can be programmed to yield different color patterns within the composite.

A thin coating on the micrometer-sized platelets determines the color, which is only revealed when the composite is mechanically stretched or compressed. The elongation tilts the platelets, gradually unveiling their reflective faces. The color-changing mechanism also allows for a composite to be designed with platelets aligned at different directions in different regions. Changes are fast and reversible for the whole life cycle of the rubber.

Using this technique, the researchers manufactured different composites: pneumatic actuators that become reflective upon inflation, low-power stretchable displays, tactile synthetic skin that detects pressure and shear, and a composite that optically detects the height variations of a three-dimensional object. Other potential applications could include autonomous soft robotic devices that undergo fast and reversible color changes through the mechano-optic coupling programmed within their soft composite architecture.

Reference:

Poloni, E.; Rafsanjani, A.; Place, V.; Ferretti, D.; Studart, A. R. Stretchable Soft Composites with Strain-Induced Architectured Color. Advanced Materials 2022, 34 (6), 2104874. https://doi.org/10.1002/adma.202104874.