Why Is Wood So Strong? It Listened to Its Mother
It seems as though science has come full circle. More and more scientists are turning to nature for clues about how to make everything from stronger composites for aircraft parts to faster ships and improved building materials.
A “new” discipline, called biomimetics, is enjoying unprecedented growth as scientists around the world recognize that Mother Nature really does know best. At least about some things.
Biomimetics literally means the mimicking of biological systems, and while the process has been practiced for many years, it is now coming into its own as a scientific discipline. The University of Reading near London, for example, has a Centre for Biomimetics, where scientists are trying to figure out how a 200-foot-tall tree can be strong enough to withstand a hurricane.
Although he didn’t call it by the same name, one of the most notable early practitioners of biomimetics was Leonardo da Vinci, who designed flying machines patterned after birds more than 400 years ago. They didn’t work, but da Vinci was on the right track.
Modern aircraft, which flex their aerodynamic surfaces to control their flight, are not as different from birds as we might think. A Phantom jet and a sparrow both soar through the air because they employ the rules of aerodynamics.
Others have also turned to nature for the answer to their questions, believing that if they could understand more about such things as the incredible weight-to-strength ratio of a simple spider web they would be able to create far stronger materials. The silk spun by a spider has a tensile strength of 35,000 pounds per square inch, five times stronger than steel. Out of that research came the synthetic fiber nylon.
Today, through biotechnology, scientists hope to mimic the role of the spider even more by altering bacteria to produce massive quantities of silk in factories. That could be a major breakthrough in the production of such things as artificial heart valves and other replacement parts for the human body.
What it adds up to is that despite the progress in the development of synthetic threads, nothing beats the old spider. At least, not yet.
The story is being repeated often these days. The Office of Naval Research, for example, is aggressively researching the way marine organisms maximize their ability to move through water. Naval architects would like to know just how dolphins literally swim circles around their fastest ships, darting in and out of the wake as though they are trying to push the slowpoke along a little faster.
The speedy dolphin is only one of the reasons the Navy has established its own biomimetics program. The military would like to be able to mimic the ability of many biological systems to self-assemble structures that are incredibly efficient.
Scientists at the University of Reading, for example, have turned to one of nature’s grandest structures, the tree, in an effort to better understand the strength of wood. It is commonly understood that wood draws much of its strength from long, hollow cells, wound in fibers of cellulose and embedded in resin. But that structure alone could not give wood a strength-to-weight ratio similar to steel, so something else must be at work.
Scientists at Reading’s Centre for Biomimetics found that one reason wood is so strong is the way it copes with failure. When stress is too great, wood absorbs most of the energy through the propagation of tiny cracks that leave the main structure strong enough to withstand 10 times the stress that it would otherwise be able to handle.
Building on that information, other scientists at Reading are trying to mimic the structure of another biological system, the abalone, which produces mother-of-pearl. The shell of the abalone is composed of a chalk-like substance embedded in layers of sticky protein. It achieves its strength by absorbing stress through tiny, relatively harmless cracks, much the same as wood.
Scientists there are experimenting with adding graphite to the protein matrix, creating a new material similar to mother-of-pearl but able to withstand high temperatures. One possible application: turbine blades for jet engines that would not have to be cooled.
Of all of nature’s wonders, few creatures have bedeviled scientists more over the last few centuries than the honeybee. The honeycomb, where the bee deposits its sweet secretions, is far more durable than its light weight would suggest. Early scientists recognized that the strength came at least partly from the hexagonal shape of the cells in the honeycomb, because a hexagon is one of the strongest geometrical designs, but they puzzled over how the bees achieved that.
Some thought the bees chewed off the edges of the cells to give them their hexagonal shape. Others thought gravity did it.
But the answer was much simpler. You can demonstrate it yourself by placing soap bubbles between two sheets of glass. As the sheets are pressed together, the bubbles naturally assume hexagonal shapes because of the geometrical force coming in from all sides.
Nature, like engineers, recognized the strength of such a structure and repeated it often. In wood, for example. Scientists and engineers have done the same. In corrugated cardboard, for example. Nature, it turns out, can still be the best teacher.
Lee Dye can be reached via e-mail at 72040.3515@compuserve.com
