3D-Printing Liquid Crystal Polymers to Replicate The Anisotropic Complexity of Wood

Anisotropic materials formed by living organisms such as cellulose fibres in wood grain and fibre bundles in osteons of bone can readily be found in Nature. Their microstructures can be shaped into any direction, with spatially tuneable gradients and sharp orientation changes. In contrast, engineered materials such as composite materials cannot be shaped with similar levels of anisotropy and directionality freedom. While the latter can be achieved with 3D printing, compatible anisotropic materials are typically fiber-filled. Paradoxically, these fibers restrict directionality freedom due to their intrinsic stiffness. Problems such as fiber breakage have been reported and often result in setting curvature constraints in the design space. Here, we present a new approach to replicate complex microstructures such as wood using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs). The LCPs can be reliably extruded to produce lines whose widths vary from half to three times the nozzle diameter, with stiffness ranging from 5 GPa to 35 GPa. This method allows shaping of anisotropic microstructures with tuneable stiffness and failure modes within a single material. By using a distance-aware toolpath generation algorithm, we can generate print lines of varying widths and curvatures that cover the shape domain homogeneously. We successfully 3D-print infills with no curvature constraint. By increasing allowed curvature, our method offers new design possibilities for composites, such as preventing crack propagation, or spatially distributing stress. Furthermore, this method creates the opportunity to study mechanical responses of natural anisotropic materials of intricate microstructures such as wood or bone.