Creating new materials by mimicking the basic rules hidden in nature’s growth patterns

Caltech researchers have developed a framework for developing new materials that mimic the fundamental rules hidden in nature’s growth patterns. Credit: Caltech

Inspired by the way termites build their nests, scientists from the California Institute of Technology (California Institute of Technology) have developed a framework for developing new materials that mimic the basic rules hidden in nature’s growth patterns. The researchers demonstrated that by using these rules, it is possible to create materials designed with specific programmable properties.

The study was published in the journal Science August 26. It was led by Chiara Daraio, the H. Bradford Jones Professor of Mechanical Engineering and Applied Physics and a researcher at the Heritage Institute for Medical Research.

“Termites are only a few millimeters long, but their nests can reach 4 meters — the equivalent of a man building a house the height of Mount Whitney in California,” says Darayo. If you look inside a termite mound, you will see a network of asymmetrical, interconnected structures, similar to the inside of a sponge or a loaf of bread. Made of grit, dirt, dust, saliva, and dung, this disordered, irregular structure appears arbitrary. However, the termite nest is specifically optimized for stability and ventilation.

A termite mound was spotted in Gaborone Game Reserve in Botswana

A termite mound in Gaborone Game Reserve in Botswana. Termites have been known to build mounds up to 30 feet high. Author: Oratile Leipego

“We thought that by understanding how the termite facilitates nest building, we could identify simple rules for designing architectural materials with unique mechanical properties,” Darayo says. Architectural materials are foam-like or composite solids composed of building blocks that are then organized into three-dimensional structures at the nano- to micrometer scale. Up to this point, the field of architectural materials has mainly focused on periodic architectures. These architectures contain a uniform 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 functionality and use 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 choices,” says Darayo. Disordered structures, such as termite mounds, are more common in nature than periodic structures and often exhibit better functionality, but until now engineers have not found a reliable way to engineer them.

Chiara Darayo

Chiara Darayo. Credit: Caltech

“We first approached the problem thinking about the limited resources of termites,” says Darayo. When a termite builds a nest, it does not have a plan for the overall structure of the nest; it can only make decisions based on local rules. For example, a termite may use grains of sand it finds near its nest and put them together by following procedures learned from other termites. A round grain of sand can fit next to a crescent shape for added stability. These basic contiguity rules can be used to describe how to build a termite mound. “We created a numerical program to design materials with similar rules that define how two different material blocks can fit together,” she says.

This algorithm, which Darayo and team call a “virtual growth program,” mimics the natural growth of biological structures or the making of termite mounds. Instead of a grain of sand or a speck of dust, a virtual growth program uses unique material geometries, or building blocks, as well as contiguous guidelines for how those building blocks can be attached to each other. The virtual blocks used in this initial work include L-form, I-form, T-form, and +-form. Furthermore, the availability of each building block has a certain limit, paralleling the limited resources that a termite may encounter in nature. Using these constraints, the program builds architectures on the mesh, and these architectures can then be translated into 2D or 3D physical models.

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

Mirroring the randomness of a termite mound, each geometry created by the virtual growth program is unique. The changing availability of L-shaped building blocks, for example, leads to a new set of structures. Darayo and her team experimented with virtual inputs to create more than 54,000 simulated architectural patterns. These samples can be grouped into groups with different mechanical characteristics that can determine the material’s stiffness, density, or how it deforms. By plotting the relationship between the arrangement of building blocks, the availability of resources, and the resulting mechanical properties, the research team can analyze the underlying rules of disordered structures. It represents a completely new framework for material analysis and development.

“We want to understand the fundamental rules of materials development, so that we can then create materials that have better properties compared to the ones we currently use in engineering,” Darayo says. “For example, we envisage creating materials that will be lighter, but also more resistant to destruction or better absorb mechanical shocks and vibrations.”

The virtual growth program explores the unexplored limits of disordered materials by simulating how 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,” Darayo says.

Reference: “Growth Rules for Irregular Architectural Materials with Programmable Properties” by Ke Liu, Rachel Sun, and Chiari Darayo, 25 Aug 2022 Science.
DOI: 10.1126/science.abn1459

In addition to Darayo, former Caltech graduate student Ke Liu and former student Rachel Sun (BS ’21) are co-authors. Sun worked on this project as a 2020 Caltech Summer Research Fellowship (SURF) student. Funding was provided by the National Science Foundation, the Caltech Carver Mead New Adventure Fund, the Caltech SURF Program, and the College of Engineering, Peking University. Creating new materials by mimicking the basic rules hidden in nature’s growth patterns

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