Extrusion-based 3D printing of stiff materials for advanced photonic applications

Certain biological materials, such as chameleons and peacocks, display bright and vivid colors. These colors are created by the constructive interference of light that is reflected from colloidal crystals, generating the so-called structural colors. These eco-friendly and non-fading colors have fascinated researchers since their discovery in the 17th century. In synthetic materials, this phenomenon can be replicated if, for example, silica nanoparticles with diameters close to the wavelength of visible light are arranged into regular assays. However, assembling nanoparticles into well-ordered crystalline structures at the centimeter scale and above is challenging, as the high defect density reduces the quality of the structural colors. Moreover, materials composed of nanoparticle assemblies are limited to basic structures, since there is no simple technique to control colloidal assembly into 3D photonic crystals with complex macroscopic shapes. This limitation arises from the high viscosity of the photonic dispersions, their rheological properties and the stiffness of the resulting photonic materials. While direct ink writing enables the 3D printing of intricate macroscopic objects, its strict rheological requirements limit the range of materials that can be processed. This thesis introduces methods to print stiff photonic building blocks into complex macroscopic 3D structures. This is achieved by combining the favorable rheological properties of granular hydrogels with the optical properties of photonic crystals.

We introduce a high-throughput method for producing core-shell particles with a rigid core and soft shell, which can be loaded with silica nanoparticles to generate structural colors. These particles are concentrated to form granular inks suitable for direct ink writing, enabling the 3D printing of complex macroscopic structures. The resulting fragile structures are reinforced by forming a 2nd network that covalently crosslinks adjacent particles, enhancing their load-bearing capacity. We demonstrate how the microparticle composition influences the rheological properties of the inks, their spreading factor, and the mechanical properties of the reinforced granular materials. Additionally, we introduce a faster approach to 3D print rigid photonic materials by combining rigid microparticles with soft microgels, creating load-bearing structures through a 2nd hydrogel network that interpenetrates the microgels and covalently connect them. We demonstrate that the composition and volume fraction of microgels influence the inks rheological properties and the mechanical properties of the final photonic material. Additionally, we highlight the ability of these granular inks to locally vary the mechanical and optical properties within a single construct, and propose their potential use as temperature-responsive photonic materials.