Pumping Alloy: A New Way to Power Artificial Muscles May Lead to Lifelike Machines
Weiss, Peter, Science News
In a Texas laboratory, a toy mechanical arm just the length of an index finger perches, folded up, at the edge of an empty glass bowl. A young man in a lab coat squirts a volatile fluid, methanol, into the bowl. Moments later, the arm jerks and then hesitantly reaches forward. Although clumsy and slow, the gesture is a remarkable one never previously achieved in any lab: The arm moves when parts of its structure contract in response to reactions triggered by local chemical fuel--much as our own limbs do.
The toy arm's sinews, made of wire, respond to the methanol because they're coated with a fine film of platinum nanoparticles. This unique design enables the wires both to harness chemical energy and to carry out the motion, says the leader of the project, Ray H. Baughman of the University of Texas at Dallas.
That two-in-one capability could become a new design principle for scientists as they create human-like machines. "It could transform the Way complex mechanical systems are built" says John D.W. Madden of the University of British Columbia in Vancouver.
These advances may eventually lead to major improvements in prosthetic limbs and to robots that can carry out tasks ranging from repairing spaceships to assisting people in their everyday lives.
To specialists in robotics, says John A. Main of the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., the advent of artificial muscles directly powered by high-energy fuels is "a very big deal."
MOVING TARGET The human body is difficult to rival in its mechanical skill, strength, and grace. These enviable capabilities rely on muscles that extend and contract linearly, as pistons do. To power those muscles, a person consumes foods such as proteins and fats that are so densely packed with energy that a small quantity fuels long periods of hard physical work.
Achieving comparable performance from a machine today, says Main, requires an internal combustion engine plus a lot of heavy hardware--including gears, belts, pumps, and reservoirs--between the engine and the pistons it drives. Such power-hungry motors add complexity to designs and introduce heat, noise, and fumes.
While batteries offer an attractive alternative, they store little energy--only about one thirtieth as much as the same weight of methanol. To run a long time, a machine must lug around a heavy bank of batteries, as electric cars do, Main notes.
The challenge to researchers, then, is to develop an efficient way to tap a high-energy, compact power source to make artificial muscles contract and stretch.
In the now-defunct television show Futurama, the rude-talking cartoon robot Bender Bending Rodriguez was a hard drinker. But his habit was what kept him going; he and the other robots of 3000 A.D. were fueled primarily by alcohol. The idea of making robots and robotic components powered by high-energy fuel, rather than by motors or batteries, doesn't need to wait until the 31st century to become reality.
Today, some artificial muscles respond to temperature changes, as the Texas toy arm does, and others are stimulated by electrical or chemical changes. A temperature-sensitive artificial muscle appears in many products, such as automatic shut-off valves in shower-heads and teakettles, medical stents, and pipe couplings and fasteners. It typically consists of a polymer or an alloy of metals--say, nickel and titanium. Known as a shape-memory material, the polymer or alloy switches, at a threshold temperature, between two specific shapes while simultaneously changing from one crystalline structure to another.
Voltage changes activate other artificial-muscle materials, including rubberlike elastomers, electrically conductive polymers, and flat strips made of carbon nanotubes. Less mature than their shape-memory cousins, these materials still come with certain drawbacks. Some require high operating voltages, and some operate slowly, Baughman says. …