Brighter color-changing ink for 3D printers
&ball; Physics 15, 91
Researchers have developed a more vibrant iridescent ink for 3D printers, bringing increased luster and shine to otherwise dull objects created by these devices.
Spot a soap bubble on a sunny day and the hollow, normally transparent sphere will sparkle like a rainbow, changing color as it moves across your field of vision. The changing hue of the bubble comes from a phenomenon known as iridescence, where color is produced by a material’s structure rather than pigments. Today, Luoran Shang from Fudan University, China, and his colleagues have developed a new iridescent ink for 3D printing that is brighter than existing ones. . The ink changes color when heated or hydrated, which Shang says could allow its use in making sensors to monitor a person’s temperature or hydration level.
Most structurally colored materials are thin-film systems that derive their changing hue from patterns etched into the films or particles embedded therein. When light strikes one of these film systems, the interference effects cause a single color of visible light to be reflected. For a given system, this selected color changes depending on the angle at which incoming light hits the material and the angle at which the material is viewed, resulting in its shimmering behavior.
Although iridescent materials have been manufactured for decades, their use has been limited due to traditional thin film geometry. To broaden their application, researchers have developed massive iridescent materials that use the inherent patterns formed by the molecules they contain to reflect and refract light. Recently, these materials have started to enter the field of 3D printing, but some challenges, including low reflection intensity and poor flow behavior, have hindered their adoption.
For their ink, Shang and his colleagues used a biological liquid crystal called hydroxypropyl cellulose (HPC), which consists of long-chain molecules that self-assemble in a spiral staircase within an overall layered structure. The “step heights” of the staircase, which determine the wavelength of light that the material strongly reflects, can be tuned by changing the temperature and hydration of the HPC, allowing the color of the material to be fine-tuned. material. It’s a very versatile system for generating iridescence, says Shang.
On its own, HPC has poor properties for printing, so the team mixed it with gelatin and a polymer called poly(acrylamide-co-acrylic acid) or PACA. The gelatin helped the mixture flow properly in a 3D printer, while the PACA ensured the material retained its shape after printing. They also added carbon nanotubes, broadband light absorbers, to improve the vividness of the ink.
To demonstrate the 3D ink, the team printed a variety of objects, including a butterfly and a pyramid. After printing, they heated, cooled, hydrated and dried the objects, showing that they could induce all colors of the visible spectrum. By shining UV light on the objects, they were also able to polymerize the PACA into the ink, permanently locking in a single shade.
Shang and his colleagues aren’t the first to use HPC-based materials to create iridescent ink for 3D printing. For example, earlier this year, Silvia Vignolini and her colleagues at the University of Cambridge, UK, demonstrated a similar iridescent ink . In this case, the group used a different “flavor” of HPC, which assembles into filaments containing spiral staircases, rather than a layered structure. The group also omitted the carbon nanotubes, which reduced the color intensity of Vignolini’s ink compared to Shang’s ink.
But Vignolini thinks his ink has other properties that could increase the likelihood of its absorption. “The HPC we use is much more widely available in industry, while the one that [Shang] used is kind of a niche,” she says. She also notes that since they only mix their HPC with water, their ink is fully biodegradable. “It’s much more sustainable.”
Due to HPC’s green credentials, Vignolini thinks it won’t be long before it starts to see widespread use in one form or another in 3D printing. “You have a material that is inherently colorful – you don’t need to add pigment to the plastic – and it has the benefit of being environmentally friendly,” she says. Shang also hopes that these materials will soon be widespread. “Iridescent 3D-printed objects are very beautiful things,” she says. “It gives me hope that this breakthrough could enable iridescent materials to transition from the laboratory to everyday life and commodity products.”
Katherine Wright is associate editor of physics review.
- Z.Zhang et al.“Cholesteric Cellulose Liquid Crystal Ink for Three-Dimensional Structural Coloring,” proc. Natl. Acad. Science. UNITED STATES 11923 (2022).
- CLC Chan et al.“3D Printing of Liquid Crystalline Hydroxypropyl Cellulose – Towards Tunable and Durable Volumetric Photonic Structures”, Adv. Function Mater. 322108566 (2022).