3D Printing of Living and Synthetic Fiber-Based Materials

Abstract

Fibre-based materials are widespread in Nature and in man-made engineering applications due to their high specific mechanical properties. In most natural and synthetic fibrous materials these properties are a result of a high degree of molecular alignment along the fibre direction. Biological materials also use intricate hierarchical fibre architectures with locally adapted microstructures to leverage the full potential of the building blocks available in Nature. In wood, for example, cellulose microfibrils are embedded in the cell walls of elongated cells at various fibre angles. The fibre angle and direction of these cells are arranged according to the structural needs of the tree. While fibre orientation is also an important design parameter in man-made structures, synthetic fibre-based systems such as carbon-fibre reinforced composites rely mainly on the chemistry of their constituents to achieve high performance. Despite the excellent mechanical properties of these fibre-reinforced composites, only relatively simple geometries are accessible. Manufacturing approaches that combine the high-performance of fibre-composites with the complex hierarchical structures found in Nature have the potential to create the next generation of advanced functional materials. Therefore, the aim of this thesis is to develop 3D printing materials that benefit from a complex shape and a fibrous architecture to achieve mechanical properties and functionalities that are not accessible by conventional manufacturing today. A synthetic approach to directly introduce fibrous architectures in 3D printed objects and a biological approach based on the indirect self-organizing capabilities of living cells are presented in this work. In both approaches, the focus was on understanding the effect of 3D printing parameters on final material properties and utilizing these findings to create complex bioinspired structures. The synthetic approach to creating bioinspired fibrous materials is based on 3D printing of thermotropic liquid crystal polymers (LCP). Initial investigations into the structure-property relationship of print lines revealed high degrees of molecular orientation and the formation of a core-shell structure during printing. This core-shell structure was found to be a result of differences in thermal solidification times between the interior core and the shell region of the printed lines. As the shell region exhibited more molecular alignment, extruded features with large skin-to-core ratios display the highest overall orientation and mechanical properties. We showed that these findings also translate to larger geometries consisting of multiple print lines. With stiffness values up to 18 GPa and strengths up to 430 MPa, such printed laminates are more than ten times stronger and stiffer than conventional thermoplastics and can even compete with high-performance fibre-reinforced materials. The mechanical behaviour of these 3D printed liquid crystal polymer materials could be successfully described by micromechanics rules developed for fibre-composites. Because they are entirely made of a single liquid crystal polymer composition, the structures can be easily recycled through simple re-melting and printing. Finally, we harnessed the shaping freedom inherent to 3D printing of LCPs to produce bioinspired structures with load-adapted print line architectures and outstanding mechanical performance. In a second step, we present a novel technique to produce high-performance thin fibres from liquid crystal polymers directly in a 3D printer. This approach expands the LCP printing platform and makes it possible to generate fibres with diameters not accessible with standard 3D printing. Fibres produced with this in-situ fibre generation strategy – termed spin-printing – showed tailored, homogeneous diameters with stiffnesses and strengths previously only obtainable with traditional fibre-spinning processes. We explored these thin fibres to produce all-fibre materials that are efficiently reinforced along the transverse direction of the printed structures without compromising the stiffness of the bulk material in the print direction. To show the potential of the spin-printing technology beyond simple laminate geometries, we demonstrated the use of in-situ spun fibres as external reinforcement for 3D printed cylindrical tubes inspired by the structure of bamboo. Lastly, a biological approach to 3D print fibrous material was investigated and exploited for the manufacturing of living composite architectures. In this approach, we utilized fungal mycelium to grow on 3D printed hydrogel lattice structures and thus create animate materials with self-healing capabilities. In line with its biological function of exploring and exploiting porous soils, the fungal mycelium was shown to cover the lattice structure and fill the open space between the printed lines. We examined the influence of the printing ink composition on the growth rate and the mechanical properties of structures fully covered with mycelium and found optimal growth at intermediate sugar concentrations. The self-healing properties of these mycelium-covered structures were then investigated for multiple successive fracture / self-healing events. Finally, the potential of these animate materials was showcased by creating robust and self-healing skins for a robotic gripper and an untethered rolling robot. Overall, the production of fibrous 3D printed materials using both synthetic and biological approaches was shown to be a promising path towards complex materials with outstanding mechanical performance or novel living functionalities.