Fathoming the Designs of Nature
Marine-center scientists study engineering that allows sea creatures to survive harsh habitat
WAVES plunge onto the rocks of this Northern California coast, where white sands and many-hued waters have inspired artist and photographer alike. Standing on the shore, Mark Denny has a decidedly unorthodox appreciation of this setting. Where others see the stuff of Robert Wood paintings, he observes consummate examples of engineering design. An associate professor at Stanford University's Hopkins Marine Station here, Dr. Denny is a leading authority on marine biomechanics - a hybrid discipline combining marine biology and mechanical engineering. While the name may sound formidable, the field deals with the familiar - posing a basic question that might occur to someone casually strolling on the beach. How, ask biomechanics scientists, can mussels, algae, and other intertidal sea creatures withstand the constant pounding of the waves?
``It's quite remarkable,'' Denny says. ``From a biological standpoint, the intertidal zone has a greater diversity of life than any other region except coral reefs and rain forests. Yet this environment is extraordinarily stressful. The wave forces are comparable to 800 mile-per-hour wind gusts, blowing at 10-to-20-second intervals, day and night. And because of the tide, the creatures who live here must contend with an environment that is alternately submerged under water and drying in the sun.''
While the study of biomechanics can be traced back to Galileo, as a specialized field it is only now beginning to take off.
``It's growing massively, spurred on not only by new engineering techniques, but also by a new appreciation of organisms, which are much more sophisticated than we once thought,'' says Michael LaBarbera, an associate professor in the Department of Organismal Biology and Anatomy at the University of Chicago.
``Organisms have functions that engineers just drool over,'' he says. ``Bone, for example, has the same stiffness as glass yet can withstand damage much better than engineering ceramics. A squid can shoot a tentacle in the space of a few milliseconds, while making midcourse corrections. And the human tongue can make incredibly rapid positionings beyond anything we have in mechanics.''
For his part, Denny has a rich mine of engineering examples a minute's walk from his office. Stepping agilely over rocks as harbor seals bark offshore, he fetches an orange starfish from a shallow pool.
Starfish must satisfy two different requirements, Denny explains. They must be flexible enough to move over odd-shaped rocks yet rigid and strong enough to pry off mollusks that seem welded to the spot. Along with other echinoderms, starfish have some of the stiffest structural elements of any organism.
Moreover, the sucker-tipped tube feet on one species can be extended 10 times their original length, allowing them to retrieve clams buried eight inches in the sand. ``We are only now beginning to understand the mechanics of these organisms,'' Denny says.
On a nearby rock sits a colony of limpets, mollusks shaped like miniature volcanoes. Their sloping sides converge on a central apex.
``Looking at fossil records, scientists have speculated that there might be some advantage in an apex positioned toward the front of the animal,'' Denny says. ``Built this way, the Lottia gigantea, for example, is able to bulldoze competitors right off the rock. But without applying some physics to the question, we have no way of quantifying the relative advantages of one design over another.''
Denny and his graduate students have tested limpet design much the way naval engineers might go about testing the shape of a new boat. Among other techniques, they have constructed a device called an oscillating flume that simulates wave action, building it out of two wide plastic pipes, a pair of valves, and a pump. By putting a limpet in the maelstrom and connecting it to a transducer to measure resistance, they can begin to quantify the advantages of one limpet design over another. The group has also used a computer modeling technique called finite element analysis for the same purpose.
``Looking at the design of an organism in this way gives us a more complete idea of how the community works,'' Denny says. ``There may also be spinoffs for mankind, but that's not our primary focus.''
Nevertheless, the principles found in marine biomechanics, and biomechanics in general, hint at applications in industry. ``The skin on a lot of new high-tech airplanes, for example, is made out of carbon fibers imbedded in some sort of matrix,'' Denny says. ``That's exactly the same idea insects had 100 million years ago. Their exoskeleton has chitin fibers imbedded in a protein matrix.''
Denny notes that we also have much to learn from insects about the mechanics of flight, and that civil engineers might well study the 100-foot kelp beds that stay anchored to the ocean floor by ``going with the flow.'' The design of vertebrae suggests ways of creating material capable of absorbing a lot of energy when crushed, such as highway barriers and packing material.
Scientists studying marine biomechanics marvel at the ability of intertidal organisms to anchor themselves securely against the waves. At the University of Delaware in Newark, Del., associate professor of marine biochemistry Herbert Waite has isolated one of the ingredients in an adhesive used by mussels. Bio-Polymers Inc., of Farmington, Conn., is now marketing the substance to laboratories as a way to affix cells to petri dishes.
The adhesive is so sticky that it defies time-tested procedures of classical biochemistry. Pour it into a beaker partly filled with water and it quickly and inconveniently sticks to the sides.
Dr. Waite believes that potential applications are much broader. ``Even a layer of water a molecule thick can subvert most adhesives,'' he says. ``As a result, industries such as aerospace that rely on adhesive bonding must use clean rooms or even rooms in a vacuum with robot assemblers. By contrast, mussels, barnacles, oysters, and coral have all evolved ways of adhering to virtually any surface, even though it is entirely submerged.''
One of the most far-reaching applications of biomechanics is in the development of prostheses used in orthopedic surgery, including artificial joint replacements. A group headed by Dennis Carter, professor of mechanical engineering at Stanford University in Stanford, Calif., is studying bones by using a servo-hydraulic loading system. The machine is ordinarily enlisted to evaluate the strength of steel, concrete, and other material used in manufacturing and construction.
``What biomechanics is really dealing with is the relationship between form and function,'' he says. ``And what we as engineers are beginning to realize is that a lot of things in nature are beautifully designed.''