3D printing is now child’s play, but making an object in 4D is a lot less easy. Nevertheless, Eindhoven University of Technology managed to print a beetle in 4D. The thing changed color due to moisture.

With 4D printing you have to think of three-dimensional objects that change because they come into contact with light, a certain temperature or humidity. PhD student Jeroen Sol from Eindhoven University of Technology focused on humidity. “We humans consist largely of water, and a sensor that you can easily and cheaply print with a 3D printer and that reacts to moisture, can have all kinds of applications in healthcare. Think of a ring that measures perspiration, or maybe your blood sugar level in the future.”

He found his inspiration in nature. “And especially in the Tmesisternus isabellae, a longhorn beetle whose elytra changes color under the influence of humidity.” Longhorn beetles make use of iridescence, a physical phenomenon of angle-dependent color. Think of the colors of oil on water, but also of soap bubbles.

Glittering Crystals

Sol wanted to achieve a similar effect with the 4D-printed beetle. “Many 3D printing materials (filaments or resins) are sold in a wide variety of colors and types, but in all cases the final product is a static object that does not change shape or color without requiring human action,” explains Sol to Scientias.nl. Nevertheless, to make the beetle four-dimensional, he used liquid crystal technology. These crystals acquire different properties depending on their alignment.

“The aim of our research was to develop a 3D printing ink based on liquid crystals that can remedy both of these ‘defects’.” The material had to be able to change shape and color. “As it is now developed, it reacts to water; liquid or from the atmosphere. This can translate into a color change, as with the beetle, or a shape change. For example, a shape change could be used to print a small water-sensitive element in a larger whole. Think of a valve that closes or opens when the humidity becomes too high or too low.”

houseplants

There are even more applications for color changes. “An example from my immediate environment is houseplants: some species are very sensitive to humidity. Then it could be visually appealing to have an object that fits into the interior of your home that can indicate at a glance whether the humidity is good enough.”

The material could also be useful for electronics, the researcher thinks. “Increasingly, electronic elements such as printed circuit boards or circuits are printed with extrusion techniques† An element could then also be made in the printed object with my ink that indicates whether or not moisture has entered the electronics.”

A short stroke and a long stroke

His material has the great advantage that the color change is caused by molecular ordering and not by dyes. As a result, the color is firstly easy to adjust and secondly it is just as firm as the material. That’s nice, because there’s no arguing about taste, says the researcher. “And dyes are often the most sensitive part of inks, paints and other materials (think of clothing or garden furniture whose color is no longer there, such as when purchased). In the case of my ink, the color is ‘baked in’ into the material, so as long as the material itself remains intact, the color will remain just as bright.”

How the ink is made? “During the ‘synthesis’ of the ink, a ‘chiral dopant’ enters it, a molecule that makes the molecular order what it is: like a spiral. The amount of dopant influences the ‘stroke’ of this spiral. A lot of dopant provides a short stroke, i.e. a reflection of a short wavelength, and thus the color blue. And vice versa: less dopant leads to a longer stroke, a longer reflection wavelength, and thus the color red. This can be controlled entirely at your own discretion during the synthesis of the ink.”

Image: Jeroen Sol

The colorful beetle

To prove that the technology works, Sol created a beetle in 4D. “First I printed a beetle from hard plastic and applied a shield with iridescent, photonic ink. That also went 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, as it were, stretching the spiral structure of the crystals. This also changes the color.” The great thing is that you can manipulate the sensitivity to moisture by making the molecules less or more charged. “That offers all kinds of possibilities for future applications.”

To test his technique, the researcher placed the beetle in an enclosed space where he could monitor the humidity. With more moisture, the green beetle slowly became redder and redder. If he lowered the moisture level, the beetle turned green again. “The latter is crucial,” says Sol. “That means the effect is reversible. The object can then be used again as a sensor, for example.”

enchanting

Sol is impressed by the resulting materials. “To see a molecule that I made in a fume hood, a molecule that is itself colorless, then cause color formation in my ink will always be magical to me. That after the curing of the ink, a normally corrosive acid treatment, my material became what it is—water-sensitive and dynamic—is perhaps just as enchanting.”

He was genuinely surprised that it was possible to create a colored structure by means of 3D printing. “This requires some background: liquid crystals are liquids in which the molecules preferably all point in the same direction. This direction is easy to steer, for example by flow fields that arise during 3D printing† “That the ‘chiral dopant’ was nevertheless strong enough to break the influence of this flow field and organize a light-reflecting structure was a big surprise. Without this find, this work would never have been accomplished. Then the 3D printing of colored objects would probably have required a static dye or pigment for much longer. Not to mention a moisture-sensitive color change…”

The future possibilities for his ink are endless, the scientist thinks. “This material would also be suitable for wearables, such as bracelets, watches or rings, where the recognizable substance is released through the skin.”

But for the time being, a beetle that changes color is also very special: