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Mother Knows Best
Richard Martin, 10.07.02

Imitating nature is the sincerest form of flattery.

Every summer, millions of hatchling loggerhead sea turtles emerge from their underground nests on beaches along the east coast of Florida and toddle to the sea. About the size of a child's hand, the infants have never swum. Yet they head immediately for open water and, over the next few years, make a great circular journey encompassing much of the North Atlantic before returning as adults to their natal beaches. This long-distance migration is among the most remarkable feats of animal navigation known, rivaling the north-south migrations of monarch butterflies, humpback whales, and Arctic terns. Until the past several years, not much was known about how young sea turtles navigate. Now, scientists believe the reptiles possess sophisticated guidance systems enabling them to detect subtle differences in the Earth's magnetic fields as they find their way across enormous expanses of seemingly featureless ocean.

As they swim, the baby loggerheads use the angles of inclination of regional magnetic fields to orient themselves and remain within the North Atlantic gyre, the great circular current system that includes the Gulf Stream. It's a perfect information-feedback loop: As the turtles swim, their position in relation to the magnetic field changes. As their position changes, they detect new information--i.e., the inclination angles and intensity of the magnetic field--and respond accordingly. An action (moving through the ocean) produces data (magnetic field detection) that modify future events (the turtles' migration). Exactly how the turtles detect the magnetic fields remains unknown. But it's clear that over eons of evolution, sea turtles, like many other creatures, have perfected a feedback loop that helps ensure each species' survival.

The ability to sense magnetic fields has been found in creatures from salamanders to sea slugs to spiny lobsters. Even certain bacteria tend to align themselves along magnetic field lines. But magnetism is hardly the only source of natural feedback. Nature has evolved countless models of near-perfect feedback machines. Bees, with a brain the size of a grass seed, respond to visual cues in the environment on their reconnaissance flights, while sharks can sense minute changes in underwater electrical fields with specialized organs called the ampullae of Lorenzini. Humpback whales "sing" across hundreds and possibly thousands of miles of ocean, and these eerie vocalizations give them clues to the presence and location of fellow whales.

In the past several years, scientists studying an astonishing range of phenomena, from honeybee dances to the growth of cancer cells, have discovered that sophisticated feedback loops pervade the natural world. The growing understanding of such loops, and their importance to evolution, has not only fueled advances in biology but also spawned a range of attempts to reproduce them, in applications from neural networks to cancer therapy. The new study and application of biological feedback loops form an important part of the new field of "biomimesis": imitating natural mechanisms in human technology.

At the cellular and molecular levels, feedback loops are fundamentally "the language of biology," observes Minoo Dastoor, senior adviser to NASA's Office of Aerospace Technology. "Biology at a molecular level generally monitors its environment by using tactile feedback, [such as] the shape and size of molecules and electrical charges."

From Deep Sea to Deep Space

Among nature's most remarkable feedback mechanisms is a kind of sixth sense called the lateral line system, present in fish and in some amphibians, to detect and respond to the flow of water around their bodies.

Arrayed along the length of the fish's body and sometimes clustered near the eyes, the lateral line system consists of multicellular organs called neuromasts, which in turn contain a strip of hair-cell receptors. The hair cells move back and forth in response to the water's flow, producing data that allow the fish to orient itself in relation to the current, navigate through murky waters, and detect obstacles. Some fish, such as the Antarctic Pagothenia borchgrevinki, even use their lateral line to find planktonic prey. It's easy to think of these hair cells as miniature binary circuits: Each cell is polarized so that bending in one way depolarizes the cell, and in the opposite direction hyperpolarizes it. On-off, positive-negative, one-zero: thousands of little feedback loops, distributed across the body of some of the most primitive creatures in the ocean.

Professor Fred Delcomyn, an expert on insect neurobiology at the University of Illinois at Urbana-Champaign, and his colleague Chang Liu, an assistant professor of electrical and computer engineering, are working on an artificial lateral line system that will be tested on a prototype underwater vehicle being built at MIT. Their work is an offshoot of the burgeoning field of MEMS, micro-electro-mechanical systems. MEMS developed out of integrated-circuit technology, with a crucial difference: Although integrated circuits concentrate on signal processing, MEMS devices are designed as sensors or actuators, sometimes both. In other words, they are designed to interact with and respond to the world around them, not just the closed world of a computer drive. In terms of the human body, Liu explains, "the integrated circuit would be the brain, while MEMS would be the eyes, the skin, the parts that sense and move."

MEMS technology promises rapid advances in remote sensing, robotics, and other fields in which machines need to adapt and respond to their environment. NASA's biomimetic research, for example, may one day produce space-traveling sensors that can self-assemble in a directed way. Imagine a probe that could land on another planet, sense its environment, and assemble itself to carry out the tasks for which it was programmed by earthly researchers. Says Dastoor: "Future missions of NASA will require systems that have characteristics such as evolvability, self-repair, frugality in terms of energy expenditure, and high intelligence per given mass. All of these are characteristics of biological systems."

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