(326c) Organic-Inorganic Interactions in 3D Printed Bouligand Nanostructures

Due to the unique functionality of hierarchical structures in nature, researchers have attempted to mimic complicated biological structures. Among these complex hierarchies is the concentric cylindrical chiral nematic structure of collagen fibers in cortical bone, known as Bouligand structure, from which the remarkable compression strength arises. Cellulose nanocrystals (CNCs) can form the chiral nematic structure in aqueous suspensions, thereby can be a candidate as an organic component to resemble the concentric chiral structure. The high compression strength, however, arises from the presence of inorganic minerals, as widely observed in many biological systems. Here, we perform the mineralization of organized CNC’s chiral nematic structures in the axially aligned concentric configuration by using direct ink writing (DIW), as a 3D printing technique. The formation of such a complex structure can be expected when CNC inks with the shear-induced para-nematic structure, formed through the DIW’s nozzle, leaves the DIW’s nozzle and the chiral nematic structure start to recover within the cylindrical confinement. To immobilize the recovered chiral nematic structure during DIW, a mixture of photo-curable monomers of Acrylamide (AAm) and Acrylic acid (AAc) are incorporated into the aqueous CNC suspensions at the CNC to additive (AAm+AAc) ratio of 1:4. Polarized optical microscopy (POM) confirms that incorporating photo-curable additives at this ratio does not disturb the chiral nematic structure of CNC particles. However, the final reactive chiral nematic ink exhibits liquid-like behavior and does not provide self-sustainable 3D printed filament. Here, we employ the embedded 3D printing approach, where jammed environments, like Carbopol microgels, provide spatial support for extruded inks while UV light advances photo-polymerization. The uniform alignment of concentric chiral nematic structures in free-standing hydrogels after photo-polymerization is confirmed by POM and electron microscopy. To induce the mechanical strength, the mineralization is performed by immersing and swelling free-standing hydrogels in calcic solutions, where Calcium ions precipitate within hydrogels. The degree of mineralization, however, depends on the swelling ratio and anionic groups of CNC and AAc (sulfate, and carboxyl groups, respectively). At a constant ratio of CNC:additives, increasing the ratio of AAc over AAm results in both a higher swelling ratio and more anionic groups in the systems, leading to a higher degree of mineralization, confirmed by elemental identification by energy-dispersive X-ray spectroscopy (EDS), and thermogravimetric analysis (TGA). This experiment suggests an approach for fabricating hybrid material with axially aligned concentric chiral nematic structure to obtain outstanding mechanical properties inspired by complex biological systems.