Fused deposition modeling (FDM), often simply referred to as 3D Printing, has been hailed as the future of manufacturing. Researchers at ETH Zürich have now developed a bioinspired approach to 3D print recyclable liquid crystal polymers (LCP) using conventional desktop printers that outperform state-of-the-art printed polymers and even rival the highest performance lightweight materials. Because the research has been conducted using a readily available polymer and a commercial desktop printer, it should be easy for the broader additive manufacturing and open source communities to adopt this new material and digitally design and fabricate strong and complex lightweight objects from LCPs. Thus, the technology is expected to be a game-changer in several structural, biomedical and energy-harvesting applications and finally enable complex FDM printed parts that mimic natural structural designs to be manufactured for the mass market.
Figure 1: 3D printed samples of specimens with print lines following the stress lines and the biological inspiration represented by a wood knot.
3D Printing, particularly FDM, makes it possible to produce unique complex parts quickly and at a low cost by sequentially depositing beads of a molten polymer. However, the available polymers are relatively weak and the printed parts show poor adhesion between the printed lines. Because of these limitations FDM has not yet been successfully implemented in commercial products. Traditionally, the performance of polymers was increased by including strong and stiff continuous fibres such as glass or carbon fibres into the material. Although the resulting materials exhibit very high strength and stiffness, the energy- and labour-intensive fabrication process as well as the difficulty to recycle state-of-the-art composites represent major challenges today. 3D printing with continuous fibres can create parts with more complicated geometries with good mechanical properties but this approach requires expensive specialised equipment, is still restricted to simple geometries and cannot be recycled.
For the first time, researchers from the Complex Materials group and the Soft Materials group at ETH Zürich, were able to print objects from a single recyclable material with mechanical properties that surpass all other available printable polymers and can compete even with fibre-reinforced composites. The researchers were inspired by two materials that can be found in nature – spider silk and wood – during the development of these structures. Spider silk gets its unrivalled mechanical properties from the high degree of molecular alignment of the silk proteins along the fibre directions. First, it was possible to reproduce this high alignment during the extrusion from an FDM nozzle by using a liquid crystal polymer (LCP) as an FDM feedstock material, resulting in unprecedented mechanical properties in the deposition direction. Second, the anisotropic fibre properties were utilised by tailoring the local orientation of the print path according to the specific loading conditions imposed by the environment. This design principle is inspired by the ability of living tissue like wood to arrange fibres along the stress lines developed throughout the loaded structure (illustrated in Figure 1) as it grows and adapts to its environment.
The 3D printed LCP structures are much stronger than the state-of-the-art 3D printed polymers and do not require the labour- and energy-intensive steps involved in current composite manufacturing technologies. The freedom in design that comes with using a 3D printer can be used to create more complicated geometries with complex print line architectures. Combined with the fact that these printed structures can be recycled, it should finally be possible to create FDM structures that can be used in industry as lightweight structural parts.