Changing the availability of L-shaped building blocks, for instance, results in a new set of structures. Mirroring the randomness of a termite nest, each geometry created by the virtual growth program is unique. These geometries can then be 3-D printed with a variety of different constitutive materials depending on applications' requirements," says Daraio. "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. Using these constraints, the program builds out an architecture on a grid, and then those architectures can be translated into 2-D or 3-D physical models. Additionally, the availability of each building block is given a defined limit, paralleling the limited resources a termite might encounter in nature. The virtual blocks used in this initial work include an L shape, an I shape, a T shape, and a + shape. 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. This algorithm, which Daraio and team dub the "virtual growth program," simulates the natural growth of biological structures, or the fabrication of termite nests. "We created a numerical program for materials' design with similar rules that define how two different material blocks can adhere to one another," she says.
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Such basic rules of adjacency can be used to describe how to build a termite nest. A round sand grain may fit next to a half-moon shape for increased stability. For example, a termite may use grains of sand it finds near its nest and fit the grains together following procedures learned from other termites. 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. "The way we first approached the problem was by thinking of a termite's limited number of resources," says Daraio. 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. However, if we think about applications, they are not necessarily the optimal design choice," says Daraio. "Periodic architectures are convenient for us engineers because we can make assumptions in the analysis of their properties. However, focusing on ordered structures has limited the functionalities and use of architected materials. Up to this point, the field of architected materials has primarily focused on periodic architectures - such architectures contain a uniform geometry unit cell, like an octahedron or cube, and then those unit cells are repeated to form a lattice structure. Architected materials are foam-like or composite solids that comprise the building blocks that are then organized into 3-D structures, from the nano- to the micrometer scale. "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. 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. 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. "Termites are only a few millimeters in length, but their nests can stand as high as 4 meters - the equivalent of a human constructing a house the height of California's Mount Whitney," says Daraio. Bradford Jones Professor of Mechanical Engineering and Applied Physics and Heritage Medical Research Institute Investigator, was published in the journal Science on August 26.