Inspired by the way termites build their nests, researchers at Caltech have developed a framework to design new materials that mimic the basic rules hidden in nature’s growth patterns. The researchers showed that with these rules it is possible to create materials with specific programmable properties.
The study, led by Chiara Daraio, G. Bradford Jones Professor of Mechanical Engineering and Applied Physics and a researcher at the Heritage Medical Research Institute, was published Aug. 26 in the journal Science.
“Termites are only a few millimeters long, but their nests can grow up to 4 meters tall — the equivalent of a human building a house the height of Mount Whitney, California,” says Daraio. When you look inside a termite nest, you see a network of asymmetrical, interconnected structures, like the inside of a loaf of bread or a sponge. This disordered, irregular structure of grains of sand, dust, dirt, saliva and dung appears random, but a termite nest is specifically optimized for stability and ventilation.
A termite mound in the Gaborone Game Reserve in Botswana. Termites have been known to build mounds up to 30 feet tall.
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A termite mound in the Gaborone Game Reserve in Botswana. Termites have been known to build mounds up to 30 feet tall.
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“We thought that by understanding how a termite helps create the nest, we could define simple rules for designing architectural materials with unique mechanical properties,” says Daraio. Architectural materials are foam-like or composite solids that comprise the building blocks, which are then organized into 3-D structures, from the nano to micron scale. Up to this point, the field of architectural materials has mainly focused on periodic architectures—such architectures contain a unit cell of uniform geometry, such as an octahedron or a cube, and then these unit cells are repeated to form a lattice structure. However, the focus on ordered structures has limited the functionalities and uses of architectural materials.
“Periodic architectures are convenient for us engineers because we can make assumptions when analyzing their properties. However, when we think about applications, they are not necessarily the optimal design choice,” says Daraio. Disordered structures like that of a termite’s nest are more common in nature than periodic structures and often exhibit superior functionalities, but until now engineers had not found a reliable way to design them.
“Initially, we approached the problem by considering the limited resources of a termite,” says Daraio. When building its nest, a termite does not have a blueprint of the entire nest design; it can only make decisions based on local rules. For example, a termite can take grains of sand it finds near its nest and stitch the grains together using methods learned from other termites. A round grain of sand can fit next to a crescent shape for stability. Such basic rules of neighborhood can be used to describe how to build a termite nest. “We created a material design numerical program with similar rules that define how two different blocks of material can stick together,” she says.
This algorithm, which Daraio and his team call a “virtual growth program,” simulates the natural growth of biological structures or the creation of termite nests. Instead of a grain of sand or a speck of dust, the virtual growth program uses unique material geometries, or building blocks, and proximity guidelines for how those building blocks can stick together. The virtual blocks used in this first work include an L-shape, an I-shape, a T-shape and a +-shape. In addition, the availability of each building block is defined, paralleling the limited resources that a termite may encounter in nature. Using these constraints, the program builds an architecture on a grid, and these architectures can then be translated into 2D or 3D physical models.
“Our goal is to generate disordered geometries with properties defined by the combination space of some essential shapes like a straight line, a cross or an ‘L’ shape. These geometries can then be 3D printed with a variety of different materials depending on the needs of the application,” says Daraio.
Mirroring the randomness of a termite nest, each geometry created by the virtual growth program is unique. For example, changing the availability of L-shaped building blocks results in a new set of structures. Daraio and his team experimented with the virtual inputs to generate more than 54,000 simulated architectural examples; The samples could be grouped into groups with different mechanical properties that could determine how a material deforms, its stiffness or its density. By graphically depicting the relationship between device layout, resource availability, and the resulting mechanical features, Daraio and his team can analyze the underlying rules of disordered structures. This represents an entirely new framework for materials analysis and engineering.
“We want to understand the basic rules of material design in order to then create materials that have superior performance compared to those that we currently use in engineering,” says Daraio. “For example, we envision the creation of materials that are lighter but also more break-resistant or absorb mechanical shocks and vibrations better.”
The virtual growth program explores the uncharted frontier of disordered materials by mimicking the way a termite builds its nest rather than replicating the configuration of the nest itself. “This research aims to control disorder in materials to improve mechanical and other functional properties by using design and analysis tools not previously exploited,” says Daraio.
The paper is titled “Growth Rules for Irregularly Shaped Materials with Programmable Properties”. Alongside Daraio, former Caltech postdoc Ke Liu and former student Rachel Sun (BS ’21) are co-authors. Sun worked on this project as a student in the 2020 Caltech Summer Undergraduate Research Fellowship (SURF) program. Funding was provided by the National Science Foundation, the Caltech Carver Mead New Adventures Fund, the Caltech SURF program, and Peking University College of Engineering .
