Structural color originates from nanostructures that reflect or scatter light, rather than dyes or pigments that produce color by absorption. While examples of such color are abundantly found in nature, the artificial fabrication of complex materials with these properties has been challenging and restricted to ultrathin films.
Now researchers in Germany say they have discovered a way to 3D print larger objects that display structural color (Adv. Func. Mater., doi: 10.1002/adfm.202213099). The method has potential applications for anti-counterfeiting materials, smart sensors, optical coatings and display technologies.
Beyond thin films
Structural colors have certain advantages over traditional dyes and pigments, such as providing more vivid color impressions, being less harmful to the environment and not fading or bleaching over time. Methods to fabricate materials with structural color have been around since 2001 but were previously limited to processing thin polymer films less than a millimeter thick.
“The color of the film can be changed by numerous means, such as pulling the material, applying an electric voltage to it, changing the temperature or modifying the pH, to name but a few,” said study author Markus Gallei, a chemistry professor at Saarland University, Germany, in a press release accompanying the research. “You can essentially control the color of the material on demand.”
Gallei and his doctoral student Lukas Siegwardt aimed to expand the utility of structurally colored materials by finding a technique that could be used to produce 3D objects that have the characteristic. They created a scalable protocol for the design and subsequent 3D printing of polymeric core-shell particles (CSP), which consist of a hard, crosslinked core surrounded by an immobilized soft shell. CSP-based materials reflect light and can respond to external triggers from the surrounding environment, such as temperature and mechanical deformation.
Unseen 3D complexity
First, the researchers prepared tailor-made CSP, altering the composition of the interlayer, shell-monomers and core-to-shell ratio to make them fully 3D-printable. The CSP consisted of hard polystyrene cores and comparably soft polyalkylacrylate-based shells. The physical properties of the processed CSP had to be changed such that they wouldn’t clog the nozzle or become damaged from the high temperatures of the 3D printer.
“I modified the material so that it could actually be printed,” said Siegwardt. “It took me months to find the right composition and the right recipes.”
Next, Gallei and Siegwardt 3D printed objects layer-by-layer, with their baseline colors dependent on the underlying particle size. The processed CSP could be extruded easily due to the reduced viscosity under shear force and elevated temperature. The printed objects—such as models of a chimpanzee, tree house and lizard, which exhibited an unseen 3D complexity—demonstrated iridescent structural color responsive to mechanical deformation.