Feature first published in the September/October 2021 issue of 3D ADEPT Mag. Roles, research and interviews reflect that period.
Does a nature-inspired 3D-printed product automatically have a lower environmental impact? This feature explores that question through a conversation with researcher Flavia Libonati.
Biomimicry offers ideas for new geometries and functions. Demonstrating their environmental value, however, requires evidence about the manufactured product and its life cycle.
Learning from natural material structures
Flavia Libonati was a researcher and Associate Professor at the University of Genoa at the time of this interview. Her work connects nature, materials, 3D printing and engineering, examining toughening and failure mechanisms in biological structural materials such as bone and nacre, from molecular to engineering scale, to inform new composite designs.
From the outset, Libonati said that “we are dealing with a two-way process. 3D printing can transform biomimicry and vice-versa. On the one hand, 3D printing has revolutionized everything, including the biomimicry design approach. The technology and its principles have opened the design space, broken down several manufacturing barriers and now enable designers to 3D print components with very complex shapes. On the other hand, biomimicry transforms 3D printing to the extent that it opens new perspectives and changes at a functional level. It helps for instance to explore new material possibilities, new material functions as nature does. 4D printing for instance, enables to get new functions within the printed component by adding a new dimension, so it goes beyond the simple manufacture of the part.”
In 4D printing, the additional dimension refers to a change in shape or function over time in response to a stimulus. It is not a fourth spatial dimension.
The professor goes even further to explain the principles of nature that are leveraged by engineers: “ Nature uses a few building blocks, such as proteins and minerals; and–through complex hierarchical structures–it creates a variety of materials, which are multi-functional, efficient, and sustainable. If you think about the basic materials seen in nature –nacre, bamboo, bone, etc.
They have a very complex structure and diversify a lot, and this diversity of structures and functions can also be seen within the same material. Bone for instance, is a material that features many sub-structures, which are made of minerals and proteins, to name a few composition items.
These structures are mixed together at different length scales – from the nanoscale to the macroscale –to yield different complex macrostructures able to perform different functions in the body. Seven levels of hierarchy are actually recognized for this material. Even within our body, the bone tissue differentiates a lot so, a compact tissue will definitely not have the same properties as the spongy one, but both are made from the same building blocks. We can do the same with 3D printing. We start with these building blocks and then we look at nature and how a specific local shape can affect and enhance the performance of a whole part. Thereafter, we try to combine these building blocks and diversify the various substructures to bring out for instance, composites with different properties”.
Understanding the organisation of natural materials can provide ideas for designers, materials specialists and manufacturers. Translating them into a component requires attention to its own operating and manufacturing constraints.
The example of the way materials work in nature is just one that stands out from the crowd as Libonati notes that every trade of the 3D printing value chain can see their work affected by a biomimicry approach. This includes the designer, the materials expert, or even the parts producer. “ In the designer’s work, the biomimicry approach will affect the creative part and help provide optimal solutions inspired by nature. It’s easy for designers to take inspiration from solutions found in Nature, not only because they are optimized, but they are not covered by intellectual property. A materials expert on the other hand, will need to understand how these solutions work and how to translate it into synthetic materials, bearing in mind that the latter are not living materials like biological materials.”
This analogy with nature does not remove the need to examine intellectual property covering technical solutions. The European Patent Office distinguishes a discovery as such from a practical technical application that may be patentable.
However, “the evolution of 3D printing is very related to the evolution of materials”, she said, hence this focus on materials as part of this feature. “The more we open the palette of materials we can print, the more we can develop a diverse range of structures for materials, and therefore, a wide range of material’ formulations and their printability”, she adds.
From crystal lattices to printed structures
In 3D-Printed Architected Materials Inspired by Cubic Bravais Lattices, first published online in July 2021, Libonati and colleagues examine the structure–property relationship of lattices inspired by cubic Bravais crystals. They combine 3D printing, mechanical tests and numerical models to investigate individual unit cells and lightweight arrays.
Natural porous structures offer several design ideas. A hedgehog spine contains an internal structure that supports its outer wall and helps it resist deformation. Here, “spine” refers to the animal’s quill, not its backbone. Trabecular bone provides another example of a cellular, energy-absorbing structure.
With an open-cell porous arrangement, apparently random, it is “carefully designed by Nature to bear specific local loadings and to fulfil different functional needs while keeping a low weight”, the research reads.
“Numerical models, validated based on experimental testing carried out on single unit cells and embedding manufacturing-induced defects, are used to derive the scaling laws for each studied topology, thus providing guidelines for materials selection and design, and the basis for future homogenization and optimization studies. We observe no clear effect of the layer thickness on the mechanical properties of both bulk material and lattice structures. Instead, the printing direction effect, negligible in solid samples, becomes relevant in lattice structures, yielding different stiffnesses of struts and nodes. This phenomenon is accounted for in the proposed simulation framework. The numerical models of large arrays used to define the scaling laws suggest that the chosen topologies have a mainly stretching-dominated behaviour, a hallmark of structurally efficient structures where the modulus scales linearly with the relative density.
By looking ahead, mimicking the characteristic microscale structure of crystalline materials will allow replicating the typical behaviour of crystals at a larger scale, combining the hardening traits of metallurgy with the characteristic behaviour of polymers and the advantage of lightweight architected structures, leading to novel materials with multiple functions.” the report says.
These findings apply to the materials, geometries and parameters investigated. The absence of a clear layer-thickness effect in those tests is not a universal rule for 3D printing.
What we will keep from Libonati’s explanations is that “nature does everything from the nanoscale on, with a hierarchical organization, and we need to find a way to translate the process into manufacturing, while maintaining a multiscale precision. This way, we will find better ways to develop new high-performance materials.” In addition to 3D printed architected materials, other examples of a biomimicry approach within the 3D printing sphere highlight the use of honeycomb structures in 3D printed parts.
Assessing environmental performance
Honeycomb structures can offer an attractive performance-to-mass ratio under particular loading conditions. Using less material alone does not establish an environmental advantage: production, energy, service life and end of life also matter, for an equivalent function.
“Bio-inspired” and “bio-based” describe different things. The former concerns an inspiration for design; the latter concerns the biological origin of a material. The examples in this feature explore nature-inspired architectures, without establishing that their feedstocks are bio-based or that every application has reached industrial use.
Featured image: montage combining a sunflower and an aerial view. Credit given in the original edition: Google Earth.
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