The cellular, hierarchical nature of living materials such as wood or bone, enables their mechanical properties to vary locally. This variability grants themmulti-functionality and highmechanical performance with low resource consumption. To obtain such desirable features with man-made structures, many manufacturing restrictions need to be overcome. Currently, the most common structural anisotropic materials are fibre-reinforced composites, which allow limited control over material placement, orientation, and directional strength.

In this thesis, we explore how the additive manufacturing of Liquid Crystal Polymers (LCP) can unlock our ability to control these parameters and, therefore, expand the design space to optimize structural performance while limiting material use. To achieve this, we leverage the alignment of nematic domains in LCPs, induced by shear and elongational flow during extrusion through the 3D printer nozzle. By modulating the pressure inside the nozzle to change line width, we observed an axial stiffness range between 3 GPa and 30 GPa, associated with a failure mode shift from ductile to brittle. Demonstrating our ability to print with variable toolpathwidths and sharp curvatures,we created 3D patterns inspired by fluid dynamics. This ombination of shaping freedom and anisotropy control enabled the design of functional objects with embedded crack mitigation strategies, or improved buckling performance.

Anisotropy gradients are also present in the structure of trees. Inspired by the seamless transition of wood fibres from parallel to interlocked at the branch junctions, we manufactured sinusoidal and helical patterns with LCP. The introduction of local waviness decreased axial tensile stiffness, but significantly enhanced toughness. Tensile and interlocking performance were driven by two critical parameters: angle of maximum deviation to the axial direction, and pattern size.Moreover, resistance to normal stresses was improved, as shown by the 88% increase in load-bearing capacity compared to a nonreinforced configuration in a two-strut junction. This opens the possibility to integrate localized interlocking patterns in multi-axis loaded regions of anisotropic parts.

Manufacturing methods which utilize as littlematerial and energy as possible are needed for future space exploration missions. To assess whether the strategies mentioned previously are applicable to this context, 3D-printed LCPs were subjected to four different space environments. High-energy electron radiations created colour-centres, leading to a bright green coloration in the bulk of the specimen. However, no significant decrease in staticmechanical properties was observed, with unidirectional stiffness remaining close to 30 GPa even after intense electron irradiation and thermal cycling. These results indicate that LCPs are a promising alternative to engineering polymers like PEEK or PEI for space applications.

Future research into topology-optimization methods integrating both asymmetry between tension and compression, and mechanical property gradients, will enable to further explore the design space opened by the tunable anisotropy of LCPs.@en

Fused Filament Fabrication is a promising manufacturing technology for the circularity of space missions. Potential scenarios include in-orbit applications to maximize mission life and to support long-term exploration missions with in situ manufacturing and recycling. However, its adoption is restricted by the availability of engineering polymers displaying mechanical performance combined with resistance to space conditions. Here, a thermotropic Liquid Crystal Polymer (LCP) is reported as a candidate material with extrusion 3D printing. To expand its scope of applicability to structural parts for space applications, four different exposure conditions are studied: thermal cycling under vacuum, atomic oxygen, UV, and electron irradiations. While 1 MeV-electron irradiation leads to a green coloration due to annealable color centers, the mechanical performance is only slightly decreased in dynamic mode. It is also found that increased printing temperature improves transverse strength and resistance to thermal cycling with the trade-off of tensile stiffness and strength. Samples exposed to thermal cycling and the highest irradiation dose at lower printing temperatures still display a Young's modulus of 30 GPa and 503 MPa of tensile strength which is exceptionally high for a 3D-printed polymer. For the types of exposure studied, overall, the results indicate that LCP 3D-printed parts are well suited for space applications.@en

Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained during by utilizing directionality of the nematic flow during the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can be combined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed, offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicating intricate patterns observed in nature, such as wood or turbulent flow using 3D printing.

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Lightweight biological materials such as bone, silk and wood demonstrate complex hierarchically structured shapes with outstanding mechanical properties by utilising directed self-assembly to grow structures. Here1, we demonstrate a bioinspired 3D printing approach to create lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Using the self-assembly of liquid crystal polymer molecules combined withorienting the molecular domains with the print path, we can reinforce the polymer according to the expected mechanical stresses. The resulting material is recyclable and leads to stiffness, strength and toughness that outperform state-ofthe- art 3D printed polymers by an order of magnitude and rival even high-performance composites. Thermotropic LCPs form nematic liquid crystalline domains when molten. These domains can be aligned in the FDM nozzle along the extrusion direction. After exiting the nozzle, solidification stops the thermal reorientation of the molecules due to the cessation of flow resulting in a skin-core morphology. The maximal Young’s modulus and strength were achieved when printing parallel (0°) to the loading direction (17 GPa stiffness and 400 MPa strength). The dependence of the modulus on the print direction can successfully be predicted using a classical laminate theory approach. It is possible to adapt the local material architecture to the specific loading conditions that are applied to the part by designing the filament deposition direction during 3D printing. We show 3D printed objects with stressadapted print line architecture with mechanical properties that are much stronger than state-of-the-art 3D printed polymers and rival the highest performance lightweight materials using a readily available polymer and a commercial desktop printer. In addition, we present a new approach to combine conventional 3D printing with in-situ spinning of fibres. By controlling the volume of molten polymer at the nozzle tip and rapidly translating the printhead, we are able to spin fibres with diameters down to 20 um, reaching tensile strength and Young’s modulus as high as 2.6 GPa and 208 GPa, respectively. These values are comparable to those of carbon fibres, which require energy-intensive fabrication processes. By spin-printing complex-shaped structures with unusual fibre-reinforcing architectures, we show that this new manufacturing platform opens new opportunities for the design and fabrication of lightweight parts combining recyclability and high mechanical performance. @en

Fiber-reinforced polymers are widely used as lightweight materials in aircraft, automobiles, wind turbine blades, and sports products. Despite the beneficial weight reduction achieved in such applications, these composites often suffer from poor recyclability and limited geometries. 3D printing of liquid crystal polymers into complex-shaped all-fiber materials is a promising approach to tackle these issues and thus increase the sustainability of current lightweight structures. Here, we report a spin-printing technology for the manufacturing of recyclable and strong all-fiber lightweight materials. All-fiber architectures are created by combining thick print lines and thin spun fibers as reinforcing elements in bespoke orientations. Through controlled extrusion experiments and theoretical analyses, we systematically study the spinning process and establish criteria for the generation of thin fibers and laminates with unprecedented mechanical properties. The potential of the technology is further illustrated by creating complex structures with unique all-fiber architectures and mechanical performance.

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