Inspired by the way termites build their nests, Caltech researchers have developed a framework for designing new materials that mimic the basic 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.

The research, led by Chiara Daraio, the G. Bradford Jones Professor of Mechanical Engineering and Applied Physics and a researcher at the Heritage Medical Research Institute, is published in the journal Science on August 26.

“Termites are only a few millimeters long, but their nests can be up to 4 meters high – the equivalent of a person building a house the height of Mount Whitney in California,” says Darayo. If you peer into a termite nest, you’ll see a network of asymmetrical, interconnected structures, like the inside of a loaf of bread or a mushroom. Made of grains of sand, dust, dirt, saliva and dung, this messy, irregular structure looks random, but the termite nest is specifically optimized for stability and ventilation.

“We thought that by understanding how the termite contributes to nest production, we could define simple rules for designing architectural materials with unique mechanical properties,” says Darayo. Architectural materials are foam-like or composite solids that consist of building blocks that are then organized into 3-D structures, from the nano- to micrometer scale. Until now, the field of architectural materials has focused primarily on periodic architectures—such architectures contain a geometric unit cell, 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 limited the functionalities and use of architectural materials.

“Periodic architectures are convenient for us engineers because we can make assumptions when analyzing their properties. However, if we think about applications, they are not necessarily the optimal choice for design,” says Darayo. Disordered structures, such as that of a termite nest, are more common in nature than periodic structures and often exhibit superior functionalities, but until now engineers had not come up with a reliable way to design them.

“The way we first approached the problem was by thinking about the termite’s finite number of resources,” says Darayo. When building its nest, a termite does not have a plan for the overall design of the nest; can only make decisions based on local rules. For example, a termite may use grains of sand that it finds near its nest and gather the grains together following procedures learned from other termites. A round grain of sand can fit into a crescent shape for more stability. Such basic neighborhood rules can be used to describe how to build a termite nest. “We created a digital material design program with similar rules that define how two different material blocks can fit together,” she says.

This algorithm, which Darayo and team called the “virtual growth program,” simulates the natural growth of biological structures or the production 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, as well as adjacency guidelines for how those building blocks can attach to each other. The virtual blocks used in this initial work include L shape, I shape, T shape and + shape. Furthermore, the availability of each building block has a certain limit, parallel to the limited resources that the termite can encounter in nature. Using these constraints, the program builds architecture on a grid, and these architectures can then be translated into 2-D or 3-D physical models.

“Our goal is to generate disordered geometries with properties defined by the combinatorial space of some basic shapes, such as a straight line, a cross, or an ‘L’ shape. These geometries can then be 3-D printed with a variety of different constitutive materials depending on the requirements of the applications,” says Darayo.

Reflecting 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, leads to a new set of structures. Darayo and team experimented with the virtual inputs to generate more than 54,000 simulated architectural samples; samples can be grouped into groups with different mechanical characteristics that can determine how a material deforms, its hardness or density. By graphing the relationship between the layout of the building blocks, the availability of resources, and the resulting mechanical properties, Darayo 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 basic rules of material design in order to create materials that have superior properties compared to what we currently use in engineering,” says Darayo. “For example, we envision creating materials that are lighter but also more resistant to breakage or better at absorbing mechanical shock and vibration.”

The Virtual Growth Program explores the unexplored 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 to control disorder in materials to improve mechanical and other functional properties using design and analytical tools that have not been used before,” says Darayo.

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