The bright colors and iridescent features of birds, butterflies, and beetles are sometimes produced not by pigments but by microscopic structures in the materials that make up the animals’ wings and shells.
The same effect can be incorporated into 3D-printed objects with the right choice of ink: Colloidal crystals, hydrogels, and liquid crystals have all been used for printing objects with structural color.
Once those inks have dried, their structures—and therefore their colors—are fixed. But some creatures’ colors can change in response to external conditions. Inspired by that capability, Jeroen Sol and coworkers at the Eindhoven University of Technology in the Netherlands have developed an iridescent ink that changes color in response to humidity, thereby adding an effective fourth dimension to their printed objects.
The artificial beetles illustrate the transformation from green in dry conditions to red at high relative humidity and back again.
Two commercially available mesogens, molecules that form liquid crystals, are behind the ink’s iridescent appearance. Both molecules are elongated, so they tend to align and form a liquid crystalline phase. But achieving the desired color change requires a cholesteric liquid crystalline phase, that is, layers of aligned mesogens are stacked, and subsequent layers are rotated relative to each other to form a twist.
In that configuration, the mesogens’ birefringent nature gives the liquid crystal a Bragg-like reflective property. The usual chiral dopant molecule added to induce that twisting negatively marred the ink’s appearance, though, so the researchers had to develop and synthesize a new molecule to do the job.
Printing the new ink required heating it to 70 °C, hot enough that it transitioned to an isotropic liquid. After it landed on a cooler substrate, the ink slowly cooled and regained its alignment after 15 minutes.
The dried ink was then cross-linked to fix the cholesteric structure. Submerging the material in a hydrochloric-acid solution caused amine groups on the mesogens to become hygroscopic; water in the surrounding air was pulled into the liquid crystal and stretched out its layers, as illustrated in the second figure, thereby causing a color change.
Changing the rate at which the ink cooled caused variations in how much water the final polymer could hold. By fixing the print substrate at different temperatures depending on which layer of a bilayer structure was being printed, the researchers made an actuator that opened and closed like a clam shell in response to changes in humidity.
Because the single-material ink can produce both visual and dynamic responses to an external stimulus, it could be a versatile choice for optical sensors and actuators in soft robots.