Superelastic and flexible 3D printed waterborne polyurethane/cellulose nanofibrils structures

Waterborne polyurethane (WPU) is a type of environmental-friendly aqueous suspension that has been widely used in varied applications. However, it remains a challenge to use WPU for three-dimensional structures by additive manufacturing due to its unsatisfied rheological properties. In this study, we developed an in-situ synthesis method to modify WPU (WPUCNF) by using cellulose nanofibrils (CNF) in order to enhance its printability. The addition of CNF during emulsification reduced the WPU nanoparticles size as well as increased the suspension viscosity. To further improve the printability, additional CNFs were added as rheological modifiers. After dewatering the suspension, WPUCNF/CNF composite inks showed excellent printability, as illustrated by the printed structures of various shapes such as honeycomb, woodpile, or human ear. For these samples, heights over 10 mm could be printed with good shape fidelity at the ink concentration of as low as 2.8-7.4%, significantly lower than previously reported WPU ink for 3D printing (similar to 20-30%). The 3D printed structure can absorb 17-37% of water due to the presence of hygroscopic salt CaCl2, and demonstrated high flexibility and withstood over 20 compressive cycles. This versatile WPUCNF/CNF ink can be adapted for designing hierarchical porous 3D structures with broad emerging applications in the biomaterials field.