Rheological behavior of biomass-based cellulose nanomaterials for direct ink writing: A review

Direct-Ink-Writing (DIW) is one of the most commonly used three-dimensional (3D) printing methods that enables the rapid and cost-effective manufacturing of complex 3D structures. However, its application performance heavily relies on the rheological properties of printing inks. Cellulose nanomaterials (CNMs), derived from biomass, offer distinct advantages in tuning ink rheology due to their pronounced shear-thinning and thixotropic behavior. This review focuses on various strategies to modulate the rheological behavior of CNM suspensions. It outlines the influences of structural characteristics (such as morphology, concentration, and surface chemistry), external conditions (e.g., temperature and pH), and adding polymers or nanomaterials on their viscosity, thixotropy, and viscoelasticity. Building on this, the review summarizes recent progress in using CNMs as rheological modifiers for DIW printing, with emphasis on the relationship between ink rheology, printability and application performance. Furthermore, we put forward suggestions to cope with challenges of CNMs in functional applications of DIW printing inks, and present an outlook for future research efforts.