Gantenbein et al. "Three-dimensional printing of hierarchical liquid-crystal-polymer structures", Nature, vol 561, 226–230 (2018): DOI https://doi.org/10.1038/s41586-018-0474-7
For many years (about 50+), the game changing potential of carbon fibre composite materials for lightweight structures and mobility has offered great promise. Recently, we start noticing the many applications beyond sports and aerospace in mass produced cars due to fuel-efficiency concerns. Carbon fibre composites are made by spinning polymer pre-cursor fibres at high speed to stretch them, carbonizing at circa 1000C then pre-assembling into layers that are combined with a polymer to hold them in place. This is all very energy intensive. Therefore, much research is now focusing on how to reduce the effort and cost of manufacturing these high-performance structures whilst maintaining their high properties, and how to recycle the material at the end of life and preserve the outstanding properties of carbon fibre that were so costly to achieve in the first place. The problem is that as soon as you start combining multiple materials with very different mechanical behaviours, it becomes very tricky to disassociate and recycle the composite. We believed tackling this challenge required reflection of natural constructs in the biological world.
Biological materials such as bone, wood and nacre demonstration exquisite multiscale architecture and are often made of very simple, few constituents structured across multiple length scales. Their architectures are programmed by self-assembly, which is encoded within the molecular structure of building blocks secreted by material-forming living cells. Their ability to deposit material orienting along the stress lines developed throughout a loaded structure has been a tremendous source of inspiration within the scientific community.
We decided to use 3D printing as a tool to emulate this biological design principle and realize structures with highly efficient microstructural architectures with excellent mechanical performance. We knew that to achieve high mechanical properties, we needed to have a very high reinforcing phase content, just as natural materials have, in our 3D printing ink. This needed to be of the order 50 vol% and upwards (all the way to 95% like in nacre). However, such materials are incredibly tricky to process using printing approaches required to structure into complex objects.
In the end, the answer was simple, why fight with a material that doesn’t want to structure…but rather search those that prefer to order. Liquid crystal polymers (LCPs) were developed back in the 60s with the most prominent type being Kevlar. The material consists essentially of very short polymer chains that are stiff and like to reside in an aligned manner. While there are many applications today, thermotropic LCPs were always a bridesmaid to the lyotropic type which are used to make high performance polymer fibres. The main reason for this is because they are extremely anisotropic so, when injection moulding, the mechanical properties follow the orientation of the flow within the object. While this can be very advantageous, it is also challenging to work with industrially. We imagined that if we could print this material, it would never be as good as the spun type, but maybe we could gain from the design freedom and the fact that the material can be heat treated to grow the polymer chains, which would solve a major issue of fused deposition modelling (FDM); the weak interfaces between print lines.
We started uncovering details in the microstructure which gave the material strength and resilience and noticed that we were able to tune this microstructure by choosing the right printing parameters. Finally, we had properties that blew away other polymers, with toughening that composite materials scientists like us dream of: the combination of stiffness, strength and multiscale toughness in a spatially controllable way. We could design for pre-prescribed loading situations with unparalleled mechanical performance and spatially tune the orientation.
Our research was conducted using a readily available polymer and a commercial desktop printer. Now, anyone with an FDM printer can realize the bio-inspired structural design concepts shown in the paper and create high-performance parts for a range of applications where the structure could also be recycled. The speed and new technology adoption of the open source hacker communities is unrivalled. These are the guys that we want to team up with for far reaching impact in today’s ecologically considerate society. We’re super excited to see the many future applications of 3D printed liquid crystal polymers!
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