Iridescent 4D-printed beetle changes color when it gets wetter

PhD student Jeroen Sol took up the challenge. He found inspiration in the world of the longhorn beetle and other animals that use iridescence and other forms of color change, writes the Eindhoven University of Technology in a press release.

Sol’s research group, led by professor Albert Schenning, has extensive experience with smart materials that respond to external stimuli, such as light, temperature or humidity. The materials use liquid crystal technology, similar to the techniques used in LCD screens, but applied in plastics.

These crystals acquire different properties depending on the direction in which they are aligned (they become anisotropic). This can be either mechanical, in which case they become stronger in one direction than in the other, or optical, in which case they have a different color depending on the angle of incidence of the light.

But Sol went a step further. He actually printed a beetle. “In the world of materials science it is very important that you make a demonstrator that proves that your invention actually works. And the choice of an animal is obvious, because there are numerous animals that make use of iridescence and other forms of color change, for example for camouflage.”

A bath of acid

But how does this beetle work? Sol explains. “First I printed a beetle from hard plastic, and applied elytra to it with photonic ink. This was also done with a printer. I then treated this layer with an acid. This causes the crystals in the ink to react to moisture. With more moisture, the material swells, so to speak, stretching the spiral structure of the crystals.

The beauty is that you can manipulate sensitivity to moisture, by making the molecules less or more charged. “That opens up all kinds of possibilities for future applications.”

Next, the researcher placed the beetle in an enclosed space where he could control the humidity. With more moisture, the green beetle slowly became redder and redder. If he brought the moisture level back down, the beetle turned green again.

“The latter is crucial,” says Sol. “That means the effect is reversible. Without the user having to reset it, it can be used again as a sensor or actuator.”