Aiming to mimic the fundamental rules hidden in nature’s ‘growth patterns’, researchers from the California Institute of Technology (Caltech) showed that the new termite-inspired rules can be used to create materials with specific programmable properties.
The inside of a termite nest is a network of asymmetrical, interconnected structures. Made of sand grains, dust, dirt, saliva and dung, the disordered and irregular structures might appear arbitrary, but they are actually optimised for stability and ventilation.
“Termites are only a few millimetres in length, but their nests can stand as high as 4m – the equivalent of a human constructing a house the height of California's Mount Whitney,” said research leader Chiara Daraio.
“We thought that by understanding how a termite contributes to the nest's fabrication, we could define simple rules for designing architected materials with unique mechanical properties.”
Architected materials are foam-like or composite solids made of building blocks that are organized into 3D structures, from the nano- to the micrometre scale. Up to this point, the researchers said, the field of architected materials has primarily focused on periodic architectures, which contain a ‘uniform geometry unit cell’ such as an octahedron or cube. Those unit cells are repeated to form a lattice structure.
Focusing on ordered structures has limited the functionalities and use of architected materials, the team claimed.
“Periodic architectures are convenient for us engineers because we can make assumptions in the analysis of their properties. However, if we think about applications, they are not necessarily the optimal design choice,” said Daraio.
Disordered structures – like that of a termite nest – are more prevalent than periodic structures in nature, and often show superior functionalities. Until now engineers had not worked out a reliable way to design them, the researchers said.
“The way we first approached the problem was by thinking of a termite's limited number of resources,” said Daraio.
When it builds its nest, a termite does not have a blueprint of the overall nest design – it can only make decisions based on local ‘rules’. It might use grains of sand it finds near its nest and fit the grains together following procedures learned from other termites. A round sand grain may fit next to a half-moon shape for increased stability, for example.
These ‘rules of adjacency’ can be used to describe how to build a termite nest, Daraio said. “We created a numerical program for materials' design with similar rules that define how two different material blocks can adhere to one another.”
This algorithm, known as the ‘virtual growth program’, simulates the natural growth of biological structures. Instead of a grain of sand or speck of dust, the virtual growth program uses building blocks in a variety of 3D shapes, as well as adjacency rules for how they can attach to each other. The virtual blocks include ‘L’, ‘I’, ‘T’, and ‘+’ shapes.
The availability of each building block is given a defined limit, mimicking the limited resources a termite might have. Using these constraints, the program builds an architecture on a grid. Those architectures can then be translated into 2D or 3D physical models, before being 3D-printed from a variety of materials.
Mirroring the randomness of a termite nest, each geometry created by the virtual growth program is unique. Changing the availability of L-shaped building blocks, for example, results in a new set of structures.
Daraio and the team experimented with virtual inputs to generate more than 54,000 simulated architected samples, which were organised into groups with different mechanical characteristics that determine how a material deforms, its stiffness, or its density.
By graphing the relationship between the building block layout, the availability of resources, and the resulting mechanical features, Daraio and team can analyse the underlying rules of disordered structures.
“This represents a completely new framework for materials analysis and engineering,” the researchers claimed.
“We want to understand the fundamental rules of materials' design to then create materials that have superior performances compared to the ones we currently use in engineering,” said Daraio. “For example, we envision the creation of materials that are more lightweight but also more resistant to fracture or better at absorbing mechanical impacts and vibrations.”
She added: “This research aims at controlling disorder in materials to improve mechanical and other functional properties using design and analytical tools not exploited before.”
The work was published in Science.
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