Engineers from the Massachusetts Institute of Technology (MIT) and Northeastern University have developed a new class of metamaterials that could offer improved strength, flexibility, and impact resistance by incorporating irregular internal structures.
The materials, described as “scale-rich metamaterials”, take inspiration from naturally occurring networks such as leaf veins and dragonfly wing structures. Unlike conventional metamaterials, which rely on repeating identical building blocks, the new designs contain interconnected beams of different sizes.
Published in the journal Nature, the research demonstrates how introducing controlled structural variation can improve a material’s ability to withstand damage and absorb energy. Potential applications include protective helmets, packaging, body armour, and acoustic lenses.
Moving Beyond Conventional Metamaterial Designs
Metamaterials are engineered materials whose internal structures give them physical properties that are difficult to achieve with conventional materials.
Traditionally, researchers create them using a repeated structural unit, known as a unit cell. These building blocks are arranged in regular patterns, including square lattices and honeycomb structures, to produce specific mechanical properties.
However, conventional designs can have limitations when exposed to localised damage. If one section becomes deformed or cracked, neighbouring sections with similar structures may respond in the same way, potentially causing the material to buckle or fail.
The MIT and Northeastern University team explored an alternative approach based on network science, a field that examines how interconnected structures behave across complex systems.
The researchers drew inspiration from biological networks, including leaf vasculature and dragonfly wing veins, as well as connections found in social networks.
“This is a completely new way of thinking for metamaterials,” said study author Carlos Portela, associate professor of mechanical engineering at MIT. “Unlocking this network design for materials really frees us to think of new microstructures and mechanical benefits that haven’t been possible before.”
Algorithm Generates Networks of Interconnected Beams
To develop the new materials, the researchers created an algorithm that generates networks of beams with different lengths and thicknesses while accounting for the physical constraints of a material.
The process begins by placing a point at a random location within a virtual space. The algorithm then draws a line through that point at a random angle, extending it until it reaches the boundary of the space.
The process is repeated with additional points and lines, with successive beams becoming progressively shorter and thinner. This creates a dense network of intersecting structures at multiple scales.
Researchers can adjust two key parameters: the thickness of the initial beam and the rate at which the thickness decreases with each successive beam. By modifying these parameters, they can generate different network configurations and control properties such as stiffness.
The team used computer simulations to predict how these structural changes would affect the material’s mechanical performance before producing physical samples.
3D-Printed Materials Demonstrate Greater Impact Resistance
After developing the virtual designs, the researchers used 3D-printing techniques to deposit polymer into patterns of interconnected beams.
Each sample was a large square approximately the size of a credit card, containing internal structures ranging from micrometres to centimetres in scale.
The team tested the materials by dropping heavy weights onto them and applying increasing compressive forces. Their performance was compared with conventional lattice and honeycomb metamaterials.
The network-inspired designs demonstrated greater resistance to compression and sudden impacts.
The researchers attributed this improvement to the presence of interconnected structures at multiple length scales. Instead of relying on identical repeating units, the network distributes damage across a wider region, reducing the likelihood of catastrophic failure.
“Because there is such a richness of length scales in these networked materials, if one part fails, there is a strong web of structures around it that keeps the entire material from failing catastrophically,” Portela said. “It’s more efficient at distributing damage and absorbing energy.”
Pomelo-Inspired Design Could Protect Fragile Objects
The researchers also developed a separate design inspired by the thick rind of a pomelo, which has a density-graded internal structure.
They created a material with a denser network of beams around its outer edges and a less dense structure towards the centre. Tests demonstrated that the design could cushion and protect a fragile object embedded within it.
This approach could be useful in developing protective packaging and other materials designed to absorb external impacts while safeguarding delicate objects.
Beyond impact protection, the algorithm could also support the development of acoustic lenses, which use engineered internal structures to focus sound or vibrations.
The research was supported in part by the US National Science Foundation’s Designing Materials to Revolutionize and Engineer our Future (DMREF) programme.
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