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Strong, tough, and uncrushable—How Mother Nature designs structural biological materials


Nature is replete with ingenious structures to make life not just possible, but better. The bony plates of seahorse skeletons, for example, slide past each other, giving the creature incredible flexibility. Materials scientists at the University of California, San Diego, are working to unlock the secrets. Credit: Joanna McKittrick, UCSD.

The Materials Genome Initiative boosted the idea of “materials by design” to the forefront, giving it a name and a cause—reduce the timeline from material discovery to manufacture by half. Without getting into theological or philosophical arguments, Nature may be way ahead of us on executing the idea of materials by design, and the “cause” is simple: survival.

Nature has designed some ingenious materials. Spider thread, for example, has amazing tensile and stretching properties. Abalone shells resist the erosion of ocean floor environments that polish materials with similar compositions into shiny, pretty baubles prized by artists. Why don’t bird beaks break? How is the structure of seahorse spines an advantage?

University of California, San Diego, researchers Marc Meyers and Joanna McKittrick, and Po-Yu Chen, now at National Tsing Hua University, Taiwan—recently wrote a review article in Science (subscription required) on the mechanics of structural biological materials. Specifically, they were looking for the connection between the structure and properties of biological materials, with an eye toward understanding how to engineer similar structures and properties in synthetic materials. Their review focused on three properties: strength under tension, toughness, and resistance to buckling/torsion.

First, they note that there are seven distinguishing characteristics of structural biological materials: self assembly, multifunctionality, hierarchy (different structures at different scales for different purposes), hydration, mild synthesis conditions (low temperature and pressure in aqueous environments), constraints imposed by evolution and environment, and self-healing ability.

Biological materials fall into two broad structural categories: “soft” structures, which are non-mineralized, and “hard” structures, which are composites of minerals and fibrous organic biopolymers. Examples of the former include collagen, keratin, elastin, chitin, lignin, and others. Mineralized composites, the latter group, consist of a mineral reinforcement phase such as hydroxyapatite, calcium carbonate, or siica, embedded in a biopolymer matrix, such as collagen or chitin.

Examples from nature provide insights into the mechanics of structural biological materials. In a press release, McKittrick says, “Mother Nature give us templates. We are trying to understand them better so we can implement them in new materials.”

Besides properties, biological materials have secrets to reveal regarding processing. Exoskeletal animals, like abalones, grow their shells one layer at a time. McKittrick observes in the press release that 3D printing is basically the same concept. “You could build a material similar to the abalone shell using principles we learned from nature by printing layer upon layer of mineral deposits—and do it much faster than nature would.”

Like engineered materials, biological materials bring different properties to the task. “The mechanical behavior of biological constituents and composites is quite diverse,” the authors write. The stress-strain behavior of biominerals, for example, is linear. However, biopolymers behave in a nonlinear fashion, and are key contributors to the high tensile strength of biological materials. Deformation happens first with a stiffening process involving molecular uncoiling and unkinking. After the fibers are fully extended, the polymer backbone stretches and accommodates quite a lot of strain before rupture. Spider silk is a good example of a biopolymer that deforms, yet is strong.

If you are alive, you’ve got to be tough, too. According to the paper, “Toughness is defined as the amount of energy a material absorbs before it fails.” That is, tough materials can deform quite a lot and are strong. They employ several toughening mechanisms that take advantage of the nature of interfaces, for example, by interrupting crack propagation, deflecting cracks, or bridging gaps created by cracks. Examples of tough biological structures include lobster shells, antler bones, abalone nacre, and silica sponge.

Finally, Nature has tricks to share regarding structures that resist bending, torsion, and buckling and the necessary tradeoff between bending and buckling resistance. She does this with thin solid shells filled with lightweight foam or internal struts. This way structural integrity is maximized and the “weight penalty” is minimized. Examples from the plant world include bamboo and the giant bird of paradise stem. The animal world offers structures such as porcupine quills, feathers, and beaks. Skeletal bones, too, are structured with a solid external “cortical bone” sheath filled with a cellular “cancellous bone” core.

Besides structural biological materials, there are other familiar applications of bioinspired materials. For example, most are familiar with the invention of Velcro being inspired by the way plant burrs stuck to animal fur. Olympic sports fans may recall hearing about high-performance swimsuits (eventually banned from competition) that mimic the structure of shark skin and reduce drag in the water. New super-adhesive surgical tapes are designed after gecko foot structure.

“There are a tremendous number of examples of things we can’t do with traditional materials,” McKittrick says in the press release. She admits it will take time, “But they will be better.”

Who better than Mother Nature would know about genomes and design of materials!

If you are attending the PACRIM-GOMD conference in a few weeks, stop by ‘Symposium 23: Advances in Biomineralized Ceramics, Bioceramics, and Bioinstpired Designs,’ which McKittrick and Chen helped organize.

The paper is “Structural Biological Materials: Critical Mechanics-Materials Connections,” by Marc André Meyers, Joanna McKittrick, and Po-Yu Chen. Science, 15 February 2013 (DOI: 10.1126/science.1220854).