Cellulose Nanocrystal Inks for 3D Printing of Textured Cellular Architectures

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 10) 1604619 exiting the nozzle, is challenging. A viscoelastic response with a finite yield stress is required for filament patterning.[3] The shear-induced alignment of anisotropic building blocks, including carbon fibers,[4,5] silicon carbide whiskers,[5] alumina platelets,[3] and nanofibrillated cellulose (NFC)[6] during direct ink writing enables the fabrication of textured 3D structured composites with enhanced mechanical properties and other functionalities. For cellulose-based materials, efforts to date have focused primarily on hydrogel-based inks that contain low NFC loading (0.8–2.5 wt%).[2,6–8] While the inherently entangled state of concentrated NFC suspensions prevent high loading, less concentrated NFC-filled inks typically require thickening agents, such as fumed silica,[3] laponite,[6] or high molecular weight polymers,[9] to achieve the desired rheological properties for direct ink writing. By contrast, the use of cellulose nanocrystals (CNC) as a reinforcing agent in inks designed for 3D printing may offer advantages over the semicrystalline NFC, since higher solid loadings may be achieved at a given viscosity and storage modulus due to the absence of physical entanglements.[10] Cellulose nanocrystals are discrete, anisotropic particles that possess a high aspect ratio (length-to-diameter ratio of 10–70) and outstanding mechanical properties. To date, they have been explored as reinforcing elements in synthetic composites.[11–13] CNC particles can be oriented in an applied electric field[14] or magnetic field,[15,16] as well as by mechanical shearing,[17] electrospinning,[18] or dry spinning.[19] However, these techniques are generally confined to producing 1D or 2D structures, such as fibers or films, respectively. Here, we create viscoelastic CNC-based inks for direct writing of textured cellular architectures. Both aqueous and monomer-based CNC inks (10–20 wt% solids) are developed, characterized, and printed into 3D structures. In both systems, shear-induced alignment of CNC particles is observed along the printing direction.