Multiphase direct ink writing (MDIW) for multilayered polymer/nanoparticle composites

Additive manufacturing has advantages in freedom of design, rapid prototyping, and waste minimization. However, one bottleneck in 3D printing polymer/nanoparticle composites has been the lack of high-precision structural control, especially without sacrificing manufacturing rates. For the first time, this study demonstrated the design and development of a new additive manufacturing mechanism, the Multiphase Direct Ink Writing (MDIW). By matching the viscosity between polymer solutions/nanoparticle suspensions, an individual line composed of a desirable number of sublayers (i.e., 4, 8, 16, 32, 64, 256, 512) was printed. A thin-ply structure with continuous ink deposition showed a strong dependence upon these layer numbers per printing line or the unit layer thickness. The 64-layered structure showed the highest modulus, strength, and energy absorption at a specific strain of 30% (E_{30% strain}) (i.e., 5 times increase in Young's modulus, 3 times growth in ultimate tensile strength, and 3.5 times improvement in E_{30% strain} compared to the PVA). The enhancement in composite mechanics was due to thin layer thickness that improved the interfacial interactions and nanoparticle distribution homogeneity. The interfacial interactions between layers also facilitated the nanotube alignment and affected the crystallization behaviors. Our MDIW method is compatible with natural-, synthetic- and biopolymers as long as the feedstock rheology is well-managed, showing broad applications in structural systems, thermal insulation, electrical conductivity, optical reflectance, and biomedical scaffolds.