What trees teach us about sustainable production of materials
Trees are excellent teachers when it comes to making materials for load-bearing structures with as little resources and energy as possible. One of their secrets? When a tree grows, the direction of the wood grain adapts itself to fit the load that part of the tree is going to bear. In other words, it can choose how much anisotropy it needs. PhD candidate Aerospace Structures and Materials, Caroline Houriet, studied different approaches to apply this smart way of utilizing anisotropy, to human-made Liquid Crystal Polymers. 3D-printing allowed her to shape the material into any tool or part and to control the orientation of the molecules during manufacturing. Her purpose: make the most of limited resources by learning from nature. Caroline Houriet defended her PhD thesis on 11 November 2024, cum laude.
Natural materials like wood are remarkably designed for the purpose of the organism they belong to. Their ‘production’ is done with minimal energy and resources. Think of trees. They use just water, light and nutrients to form structures that can be over 100 metres tall. Over the course of evolution, trees found different ways to avoid branches breaking off. One of these is a strategy to reinforce its weakest points by adapting the orientation of their wood fibres. Along the trunk, the fibres are mostly aligned in straight lines to resist compression and bending. But where the trunk branches off and the loads are complex and random, the wood fibres are arranged into interlocked patterns to sustain all sorts of loads Human-made materials normally cannot do that. They are either ‘isotropic’ like steel, where no direction is stronger than another (like the tree fork), or ‘anisotropic’ like fibre-reinforced composites, where the direction of the straight fibre is the stronger one (like the tree trunk).
In her thesis, PhD candidate Aerospace Structures and Materials Caroline Houriet, studied how these adaptations of anisotropy could be also applied to human-made materials, in her case Liquid Crystal Polymers. By 3D-printing the LCP’s she was able to shape the material into the structure she wanted, and she was able to control the molecular orientation according to what made the most sense for the future load case. Houriet used two ways to control the anisotropy of LCP. First, by tailoring the micro-structure: by changing printing parameters, she showed that you could affect the molecular orientation of the polymer, which led to variations of its stiffness. She also proposed a method to integrate this feature to 3D-print complex anisotropic patterns, such as vortices.
And second, she studied the macro-structures. Houriet: “Trees are great teachers. They use isotropy where really needed, for example at junctions between branches, and anisotropy wherever possible. So with the shaping freedom of LCP and 3D-printing, we tried manufacturing these simplified wavy patterns, and we studied them for their capability to improve toughness, create interlocking and reduce anisotropy locally. The results are quite promising: compared to no pattern at all, they increase the first failure load by 88% in curved-beam bending.”