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This whole story is about holes

When it comes to materials that are lightweight but strong, nature's 'cellular solids' take the prize

By Pamela D. Jacobsen / April 27, 2004



Do you know what makes coral, a bird's bone, and wood so strong? Hint: You can't see it, but you really need it. The answer is holes!

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You may think that a completely solid object is always the strongest. Think again. When nature needs something to be strong and lightweight, holes are just the thing. Scientists call these hole-filled materials "cellular solids." Wood, bird (and some human) bones, coral, porcupine quills, and plant stalks are just a few examples of cellular solids.

Humans have learned nature's lesson and turned it into ways to make things like cars and packaging more lightweight.

Lorna J. Gibson is a professor of materials science and engineering at the Massachusetts Institute of Technology in Cambridge, Mass. She describes cellular solids as "a combination of solid rods, or plates and holes." Each rod or plate is connected to adjoining rods or plates.

Cellular solids are classified as "honeycomb" or "foamlike." In honeycomb cellular solids, the plates all go in the same direction. Plates are randomly scattered in foamlike materials.

Coral as strong as concrete

Coral is "just dead skeleton," says University of Buffalo (SUNY) biology professor Howard R. Lasker. In the ocean, coral is made up of many thousands of tiny animals called polyps. The cylinderlike polyps grow upward from their coral base. Think of a polyp as "a straw with long curtains inside," Professor Lasker says. The "curtains," called septa, are made of calcium carbonate. That's the same material seashells are made of. Some corals are about as strong as concrete, he says.

And because of the holes in between the calcium carbonate "curtains," a coral is able to grow larger than it would if it was completely solid, and do so without sacrificing strength, Lasker says.

Now imagine a structure hundreds of feet tall, thousands of yards wide, and as long as the distance from Florida to New York. That's about the size of the Great Barrier Reef off Australia's east coast. Coral reefs are found throughout the Pacific Ocean and the Caribbean.

To be strong, you have to bend a little

Wood's cells "are like a honeycomb," Professor Gibson says. A tree needs to be strong to support itself. When you look at wood under a microscope, you can see that its structure is like a honeycomb. The plates and hollows let the cells stretch or compress slightly under pressure. This flexibility helps a tree to be able to withstand strong winds.

Helping birds to stay up

Many (but not all) bird species have hollow bones. Bones that are strong and lightweight help birds fly, especially over long distances. According to Gary Ritchison, an ornithologist at Eastern Kentucky University in Richmond, "size determines whether birds have 'pneumatic' bones." A pneumatic bone is one that is largely hollow inside. The interior is "a series of little bone sections - like struts," he says. Struts are supports that connect opposite sides of bone to strengthen them.

Small songbirds such as bluejays and robins don't need hollow bones because they only weigh an ounce or two. They don't need extra weight-saving tactics. Larger, heavier birds - crows, for example - do need pneumatic bones.

Since the primary reason for hollow bones is flight, water birds like loons, grebes, or cormorants have more solid bone structures. Being heavier makes it easier for them to dive. Having hollow bones would be like trying to dive underwater while wearing a life preserver.

Have a slice of cellular solid with jam

It's easy to recognize the strength of coral, wood, or bird bone. But what about meringue, bread, or a Nestlé Aero candy bar? They're full of air spaces. They are cellular solids, too. But are they strong?

Get a piece of wood and press on it. Now take a piece of bread and do the same thing. How would you describe the strength of each? Gibson says that "strength depends upon a structure and how it is loaded."

"Loading" is an engineering concept. Think of someone sitting in a chair: That person is "loading" his weight onto the chair. How well the chair holds up tells us how strong the structure is for its weight. This idea also applies to cellular solids. But even though each object has its own strength, it isn't really possible to compare coral to bread. Or meringue to wood. Each cellular solid has its own unique properties that give it strength.

Okkyung Kim Chung is a cereal chemist and research leader who directs the United States Department of Agriculture's Agricultural Research Service. She says that breads, rolls, and cakes are "foamlike" cellular solids. Their plates are scattered randomly.

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