Colloidal photonic crystal (CPC) ink that enables both two- and three-dimensional colored structures to be printed using additive manufacturing techniques.Many creatures from chameleons to cuttlefish can change their coloration. This phenomenon arises from structural color, which is produced by light scattering from micro- or nanoscale features or texture. Self-assembled micro or nanoparticles can be used to create colloidal photonic crystals that manipulate light in this way, creating colors that do not fade and can be changed in response to external triggers. Now researchers have devised a colloidal photonic crystal (CPC) ink that enables both two- and three-dimensional colored structures to be printed using additive manufacturing techniques [Liao et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.02.014].
“We [wanted to] address the long-time challenge of fabricating larger-scale photonic crystals that feature sophisticated volumetric structures,” explains Y. Shrike Zhang of Harvard Medical School and Brigham and Women’s Hospital, who led the work with colleagues from Harvard University, Massachusetts Institute of Technology, Southeast University, Nanjing, Suzhou University of Science and Technology, Qingdao University, Monterrey Institute of Technology and Higher Education, Vellore Institute of Technology, Emmanuel College, Singapore University of Technology and Design, and National University of Singapore. “Previous methods to produce these [structures] are difficult [to perform].”
The ink is composed of highly charged elastic nanoparticles (HENPs) dispersed in a crosslinkable pre-gel solution. The pre-gel solution contains a monomer (acrylamide), a crosslinker and a photoinitiator in water. When the ink is used in conjunction with digital light processing (DLP)-based 3D printing, the team were able to create CPC structures with long-range, highly ordered non-close-packed arrays of HENPs. The ordered arrays of nanoparticles create brilliant, vivid colors.
“DLP printing allows layer-by-layer fabrication of volumetric constructs [with] precisely designable and highly sophisticated internal structures,” points out Zhang.
The researchers created several different inks that enable the creation of a wide range of macroscale 3D objects that are simply colored, multicolored, possess temperature-dependent colors or temperature-responsive shapes and colors. Basic inks, for example, allow the formation of a spectrum of colors, depending on the type of nanoparticle and its concentration. Shape-morphing inks, meanwhile, are responsive to external stimuli such as temperature, changing color and form in response.
“We are able to produce photonic crystal constructs in large dimensions with complicated shapes and structures,” says Zhang, “unlocking additional applications for unconventionally shaped color-changing materials.”
These novel and responsive materials could find use in sensing, where color changes could signal environmental cues, information storage or even anticounterfeit devices. Optimization of the ink formulations and their printing conditions is needed now to translate the technology into practical applications.