3D Printing of Liquid Crystalline Hydroxypropyl Cellulose—toward Tunable and Sustainable Volumetric Photonic Structures

Abstract

Additive manufacturing is becoming increasingly important as a flexible technique for a wide range of products, with applications in the transportation, health, and food sectors. However, to develop additional functionality it is important to simultaneously control structuring across multiple length scales. In 3D printing, this can be achieved by employing inks with intrinsic hierarchical order. Liquid crystalline systems represent such a class of self-organizing materials; however, to date they are only used to create filaments with nematic alignment along the extrusion direction. In this study, cholesteric hydroxypropyl cellulose (HPC) is combined with in situ photo-crosslinking to produce filaments with an internal helicoidal nanoarchitecture, enabling the direct ink writing of solid, volumetric objects with structural color. The iridescent color can be tuned across the visible spectrum by exploiting either the lyotropic or thermotropic behavior of HPC during the crosslinking step, allowing objects with different colors to be printed from the same feedstock. Furthermore, by examining the microstructure after extrusion, the role of shear within the nozzle is revealed and a mechanism proposed based on rheological measurements simulating the nozzle extrusion. Finally, by using only a sustainable biopolymer and water, a pathway toward environmentally friendly 3D printing is revealed.

Socially‐Directed Development of Materials for Structural Color

“…[50,63,69,70] Recent work on more scalable approaches for natural polymeric materials through printing and roll-to-roll processes points to the future potential of this field, where more benign materials could be used with existing manufacturing techniques or self-assembly approaches. [18,19,50,71] As research advances increasingly converge with scalable, environmentally benign manufacturing techniques, [12,50] this study identifies structurally colored materials development as a clear opportunity to evolve toward sustainability. These techniques range from low to high information content: more scalable, low information processes include large-area Bragg stack-based paints used for vehicles, while high information content techniques include plasmonic printing and electron beam lithography that offer high resolution and high customizability.…”

“…A socially-directed approach to material color holds potential for sustainable alternatives over existing commercialized materials. [17][18][19]63] As one example, pigmented materials incorporate multiple compounds to achieve different colors and often utilize harmful or toxic components, [74,75] may release volatile organic compounds (VOCs) during drying, involve multiple processing steps (e.g., resins or binders to impart mechanical robustness and longevity), exhibit photobleaching (fading), and are difficult to recycle. The market for nanoengineered surfaces is projected to grow to $1.7 billion by 2022, [76] so integrating sustainability as a primary consideration for this research will be impactful.…”

“…Renewable materials for optics applications include, for example, cellulose (which can be sourced from agriculture waste like fruit peels or wood pulp), chitin, algae, and silk. [18,19,33,41,42,121,122] E-LCA and S-LCA studies have been carried out on such materials, [123,124] and such approaches may be extended for use of these materials in structural color applications.…”

“…[123] By harnessing tunable mechanical and optical properties of cellulosic films, we can access cellulose-based color applications in paints and coatings, sustainable consumer packaging, security features, flexible displays, recyclable electronics, food packaging, and products, sensors, and photonic and optoelectronic materials and structures. [18,19,141,151,152,174,175] Cellulose has even been applied toward structurally colored edible glitter. [139]…”

Brighter Color-Shifting Ink for 3D Printers

“…Shang and her colleagues are not the first to use HPC-based materials to create an iridescent ink for 3D printing. For example, earlier this year, Silvia Vignolini and colleagues at the University of Cambridge, UK, demonstrated a similar iridescent ink [2]. In that case, the group used a different "flavor" of HPC, which assembles into spiral-staircase-containing filaments, rather than a layered structure.…”