Humans are by no means alone in the search for more sustainable materials. Nature, too, has been 鈥渨orking鈥� on the problem of sustainability, and it鈥檚 been at it for a great deal longer. , researchers at the Weizmann Institute of Science show how design tricks employed by ancient creatures such as scorpions and sponges can help optimize the resilience of human-made materials, ultimately advancing sustainable design.
鈥淚n the natural world, materials have evolved over millions of years, in environments that are often defined by limited resources and harsh conditions,鈥� says Prof. of Weizmann鈥檚 Molecular Chemistry and Materials Science Department, who has been studying the mechanical aspects of natural materials for several decades. 鈥淥ur point of departure was an intuition that biological structures around us 鈥� trees, plants, bones, skeletons of various organisms 鈥� developed in a way that is, by definition, sustainable.鈥�
鈥淚n this respect, durability is key,鈥� says study coauthor Dr. Israel Greenfeld. 鈥淟iving organisms, for example, display a variety of specialized strategies to deal with outside forces while expending the least amount of energy 鈥� which is why there is so much to be learned from nature, as we try to develop stronger and longer-lasting materials of our own.鈥�
“”Increasing a material鈥檚 strength often leads to a trade-off in flexibility. But nature has developed structures that achieve both without compromise”
Improved, efficient materials offer an important avenue toward a more sustainable future because they can lead to less waste and a reduced need for fuel. But any attempt to enhance an advantageous property of a material tends to come at the expense of another of its attributes. Increasing strength, for instance, will typically lead to increased weight or decreased flexibility.
鈥淣ature, it turns out, finds amazing ways to optimize the balance,鈥� Greenfeld says. One optimization feature found in a variety of tough organic substances is laminate construction: materials composed of different substances layered or interlaced together. This type of composite material often exhibits strength and resilience, while maintaining other beneficial properties, such as being lightweight and flexible.
In the new study, Wagner and Greenfeld examined two natural laminates that show an exceptional degree of toughness: the outer shell, or cuticle, of a scorpion and the inner skeleton, or spicule, of a sea sponge. The researchers found that the secret of their resilience lies in grading, a specialized strategy that is rarely found in human-made materials: a gradual change in properties from one layer to another.
In both creatures, the different layers vary in thickness, and in the scorpion鈥檚 shell, they also decrease in stiffness from exterior to interior, so that the surface facing the harsh world the scorpion inhabits has greater resilience than its shell鈥檚 interior. In fact, the researchers鈥� study of the scorpion 鈥� , a former student on Wagner鈥檚 team 鈥� showed that the organism鈥檚 complex shell is a composite constructed from eight different structural levels.
In both the scorpion and the sponge, a subtle yet powerful 鈥渞eshuffling鈥� or rearranging of laminate layers was found to serve as a biological tradeoff between conflicting properties, helping them withstand the types of stress they are typically up against.
Cracking the code
Thanks to grading, the scorpion鈥檚 shell and the sponge鈥檚 skeleton, while being tough and strong, are particularly good at resisting cracks. Even though they differ in terms of chemical composition and structure, both optimize this resistance using the same principle: fracture deflection. This means that in both organisms, cracks are mitigated by diverting their path. As soon as a crack starts emerging in the material, it is 鈥渆ncouraged鈥� by the material鈥檚 graded structure to change course and run parallel to the surface, rather than go deeper, where it would likely cause more massive structural damage, potentially leading to catastrophic collapse.
To better understand how grading works in both organisms, the researchers adapted a model from classical fracture mechanics, the field that deals with how things break. The model showed that without grading, obtaining the same resilience in both the scorpion and the sponge would have required more wasteful measures, such as thicker components. It also showed that resilience is improved by shifting more material to structural regions that are more critical in terms of durability.
The researchers didn鈥檛 stop there. They showed how, in bioinspired materials, grading could be used in ways that nature hadn鈥檛 yet come up with. 鈥淯sing this model, we were able to shift around the grading levels in ways that the scorpion and the sponge hadn鈥檛 quite 鈥榯hought鈥� of,鈥� says Greenfeld.
Greenfeld and Wagner point out that importing concepts such as grading into human-made designs is highly challenging. 鈥淔or humans, such design is innovative,鈥� says Greenfeld. 鈥淏iological structures are created bottom-up 鈥� from tiny, nano-metric building blocks, to microscopic structures, and onward to larger and larger structures 鈥� whereas in engineering, one usually doesn鈥檛 start at the molecular level.鈥�
Still, while the scorpion鈥檚 structure is especially complex, other natural microstructures, such as that of the sea sponge, can be more readily applied in engineering. In the sponge鈥檚 skeleton, for example, apart from grading, cracks are slowed down or stopped by the fact that brittle layers are interspersed with minute amounts of softer layers. 鈥淚t鈥檚 a ceramic, it鈥檚 basically made of silica, not the type of material you usually expect to display strong fracture resistance,鈥� Wagner says.
A better understanding of the strategies found in natural composite materials, explain the researchers, could help engineers optimize our own human-made composites, a wide family of materials that ranges from the ubiquitous cement to specialized fiber-reinforced laminates used in aerospace industries.