ILLUSTRATIONS: BRYAN CHRISTIE; PHOTOS: DALE HIGGINS

ELECTRONIC MUSCLE: Passive dielectrics are a variant of artificial muscle activated by the movement of electrons. In an actuator, two flexible conducting plates form a sandwich [illustration, right] with the passive dielectric, a springy, insulating plastic, as a filling. When the plates are given opposite charges, their mutual attraction flattens and expands the filling. A bending actuator, built by SRI International, Menlo Park, Calif., is made from two C-shaped strips of such an artificial muscle that surround a spring-loaded tube [bottom, left]. When one strip is charged, it expands, bending the actuator. SRI used multiple passive dielectric artificial muscles to build an insectlike robot called Flex. [top, left]

I found two other electroactive polymers that had already been reported to show a significant actuation strain, and although they were not strong enough to amplify an astronaut's body movements, I was still able to develop other robotic applications, including an actuator for a miniature lens wiper. This wiper was selected for use in a tiny vehicle called Nanorover, which was to have been sent to an asteroid in 2002 in a joint Japanese-U.S. mission. Even though the mission was canceled and the wiper never got a chance to brush aside any space dust, NASA's imprimatur gave impetus to research into electroactive materials.

Meanwhile, I discovered that though several research groups around the world were already working on this subject, they scarcely communicated with one another. I therefore sent out a flurry of e-mails, cadged over 50 papers, and in 1999 held the first annual international conference in the field.

I inaugurated the conference with a challenge to develop a robotic arm powered by artificial muscles that could beat the world heavyweight champion arm-wrestler. The International Society for Optical Engineering (SPIE), in Bellingham, Wash., the society that organizes the conference each year, is seeking prize money for the competition.

The first stage in the arm-wrestling series requires that the machine beat even a weakling. This stage in the competition will take place at SPIE's Smart Structures/Non-destructive Evaluation symposium next March in San Diego. Later competitions will face ever better arm-wrestlers, using ever more humanlike mechanical designs. The ultimate goal is to have the machine arm-wrestle John Brzenk Jr., considered to be the best arm-wrestler in the history of the sport.

Real human skeletal muscles remain the touchstone of success. They achieve as much as 50 percent strain, contracting to half their length, although their force peaks at about 35 percent strain, where they can generate up to 350 kilopascals (kilonewtons per square meter). At that point they deliver their maximum power density of 150-225 watts per kilogram. Brzenk, for his part, can lift 55 kg in the one-armed dumbbell curl, an exercise that requires few of the muscles that come into play in arm-wrestling.

Beating Brzenk would be quite a technological feat. It is just the beginning, though, of a biomimetic project, in which art imitates life. Engineers would love to make machines with the abilities of an octopus, which can squeeze its body through a small hole, come out the other side, and revert to its normal shape—all without a single drive shaft, gear, or bearing.

I LIKE TO GROUP ARTIFICIAL MUSCLES into two categories, classified by their main means of activation: electronic and ionic. Electronic materials shuttle electrons about, ionic ones move ions around. Within each category are many subtypes, each with its own strengths and weaknesses. The first artificial muscle, by the way, made of natural rubber, was of the electronic kind, demonstrated by Wilhelm Konrad Röntgen 15 years before he discovered X-rays.

Because electrons move more easily than ions, the electronic polymers react in mere microseconds. They also have a greater energy density and can be operated in the open air, whereas ionic materials must be bathed in liquid solvents. The electronic polymers, however, had long required much stronger electric fields to cause them to contract—more than 150 volts per micrometer, dangerously close to the level at which these materials break down. But advances made less than a year ago have reduced the required field strength by about an order of magnitude.

Within the electronic category are five subcategories: passive dielectrics, piezoelectric polymers, graft elastomers, liquid crystals, and electrostrictive paper.

Of this group, the passive dielectrics are the simplest and also the most robust. Two conducting plates form a sandwich with a filling of springy, insulating plastic. When the plates are given opposite charges, as in a capacitor, their mutual attraction squeezes the filling, extruding it at the sides to do work [see illustration, “Electronic Muscle”]. SRI has a material that can expand by 380 percent, the record for an artificial muscle, but because the sandwich filling is very soft, the force it develops is limited.

Using a clever design and a stiffer filling, SRI managed to make an actuator that can exhibit a pressure of about 8 megapascals, which is about 30 times stronger than human muscle, gram for gram. The actuator, about the size of a finger, can bend sideways and lift a weight of about a kilogram.

Piezoelectric polymers work similarly to the more familiar piezoelectric ceramics found in inkjet printers and ultrasound transducers, by changing their crystalline structure—bending chemical bonds to, for instance, turn rectangles in the lattice into parallelograms. The increase or decrease in volume creates mechanical pressure. Piezoelectric polymers were discovered back in the 1920s, but because they showed a relatively small strain and force, they remained a curiosity until the development of a subclass, called ferroelectrics.