Chiara Daraio, professor of mechanical engineering and applied physics at Caltech. Photo: Caltech.
Chiara Daraio, professor of mechanical engineering and applied physics at Caltech. Photo: Caltech.

Inspired by the way termites build their nests, researchers at Caltech have developed a framework for designing new materials that mimic the fundamental rules hidden in nature's growth patterns. The researchers showed that, using these rules, it is possible to create materials designed with specific programmable properties. Led by Chiara Daraio, professor of mechanical engineering and applied physics, they report their work in a paper in Science.

“Termites are only a few millimeters 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,” says Daraio.

If you peer inside a termite nest, you will see a network of asymmetrical, interconnected structures, like the interior of a loaf of bread or a sponge. Made of sand grains, dust, dirt, saliva and dung, this disordered, irregular structure appears arbitrary, but a termite nest is specifically optimized for stability and ventilation.

“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,” says Daraio. Architected materials are foam-like or composite solids made up of building blocks that are organized into three-dimensional (3D) structures, from the nano- to the micrometer scale.

Until now, the field of architected materials has primarily focused on periodic architectures – these are constructed from a uniform geometry unit cell, like an octahedron or cube, which are repeated to form a lattice structure. However, focusing on ordered structures has limited the functionalities and use of architected materials.

“Periodic architectures are convenient for us engineers because we can make assumptions in the analysis of their properties,” says Daraio. “However, if we think about applications, they are not necessarily the optimal design choice.” Disordered structures, like that of a termite nest, are more prevalent in nature than periodic structures and often show superior functionalities, but, until now, engineers had not figured out a reliable way to design them.

“The way we first approached the problem was by thinking of a termite's limited number of resources,” says 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. For example, a termite may use grains of sand it finds near its nest and fit those grains together following procedures learned from other termites: a round sand grain may fit next to a half-moon shape for increased stability.

Such basic rules of adjacency can be used to describe how to build a termite nest. “We created a numerical program for materials' design with similar rules that define how two different material blocks can adhere to one another,” explains Daraio.

This algorithm, which Daraio and her team dub the ‘virtual growth program’, simulates the natural growth of biological structures, or the fabrication of termite nests. Instead of a grain of sand or speck of dust, the virtual growth program uses unique materials' geometries, or building blocks, as well as adjacency guidelines for how those building blocks can attach to each other.

The virtual blocks used in this initial work include an L-shape, an I-shape, a T-shape and a cross shape. Additionally, the availability of each building block is given a defined limit, paralleling the limited resources a termite might encounter in nature. Using these constraints, the program builds out an architecture on a grid, and then those architectures can be translated into two-dimensional or three-dimensional physical models.

"Our goal is to generate disordered geometries with properties defined by the combinatory space of some essential shapes, like a straight line, a cross or an L-shape,” says Daraio. “These geometries can then be 3D printed with a variety of different constitutive materials depending on applications' requirements."

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 instance, results in a new set of structures. Daraio and her team experimented with the virtual inputs to generate more than 54,000 simulated architected samples.

These samples could be clustered into groups with different mechanical characteristics, which might determine a material’s stiffness, density or how it deforms. By graphing the relationship between the building-block layout, the availability of resources and the resulting mechanical features, Daraio and team can analyze the underlying rules of disordered structures. This represents a completely new framework for materials analysis and engineering.

"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," says 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."

The virtual growth program explores the uncharted frontier of disordered materials by emulating the way a termite builds its nest rather than replicating the configuration of the nest itself. "This research aims at controlling disorder in materials to improve mechanical and other functional properties using design and analytical tools not exploited before," says Daraio.

This story is adapted from material from Caltech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.